Patent Publication Number: US-2023160681-A1

Title: Method and apparatus for mapping and ranging based on coherent-time comparison

Description:
TECHNOLOGICAL FIELD 
     The present invention is generally in the field of distance measurements using electromagnetic (EM) signals reflected from a target object, e.g., usable for 3D mapping and ranging for autonomous vehicles and airplanes, biological tissue imaging, and thin layer measurement in fabrication processes. 
     BACKGROUND 
     Accurately measuring the distance to an object plays a key role in many applications, such as navigation, object detection, biomedical research and operations, thin layer measurements, and more. Different methods and applications based on exploiting the electromagnetic radiation can be roughly divided into two groups—long and short distance measurements. For example, long-distance measurements including navigation and detection can be used for autonomous vehicles, and/or airplane, light detection and ranging (LiDAR) and/or RADAR, applications, while short distance measurements are usually used for biomedical operations and thin layer characterization. There are well-known techniques for carrying out active distance measurement using electromagnetic waves—usually utilizing Radio Frequency (RF), optical, and infrared regime, signals. Some of these techniques exploit the fact that the propagation speed of electromagnetic radiation in free space is constant, for determining distance of a target object by measuring travel time of electromagnetic signals reflected from the target, while other related techniques use white light, or broadband source interferometry, exploiting short coherence length to measure distance by means of zero Optical Path Difference (OPD). 
     The former techniques are usually used for long-distance measurements, while the latter techniques are usually used for short-distance measurements. For example, one of the former techniques utilizes the time delay incurred due to the distance of the target object from the measurement setup. Typically, a sharp electromagnetic interrogating pulse signal is directed towards the target object, and a portion of the electromagnetic pulse signal reflected back from the target object is directed toward a detection unit. Due to the round-trip time delay Δt of the electromagnetic pulse signal (toward the object and backwardly toward the detection unit), and the constant speed of the electromagnetic pulse signal c, the distance of the target object can be calculated using the following expression: 
     
       
         
           
             
               
                 
                   
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     This technique is usually termed time-of-flight (ToF). As the speed of electromagnetic radiation is eminently fast, for accurate, high-resolution distance measurement, the round-trip time delay Δt of the electromagnetic pulse signal needs to be measured with high precision. Moreover, typically this requires high-speed detector and fast acquisition electronics (measurement bandwidth) for achieving the task. Due to the requirement for high-speed and sensitive detector unit tailored with fast acquisition electronics and high-power electromagnetic source, systems based on ToF techniques tend to be very expensive, or alternatively cheap but with poor performances, especially when using an electromagnetic source in the optical/IR regime. In addition, ToF based measurement systems are also very sensitive to backlight caused by different sources, for example, the sun. 
     A different technique, known as frequency-modulated continuous wave (FMCW), enables to filter the backlight noise through coherent detection and hence improves the imaging range and performance due to higher signal-to-noise-ratio (SNR). In this technique, the transmitted electromagnetic wave is in the form of a linearly increasing (and/or decreasing) frequency signal, known as “chirp”. A part of the returned signal reflected from the target is superposed with a reference (“chirp”) signal. Due to the time delay, caused by the distance of the target object, the frequency of the returned signal has a different frequency, shifted compared to the reference signal, and a beating pattern is detected. Additionally, the frequency of the reflected signal may change by the movement of the target object causing Doppler shift. The beating pattern can be analyzed to measure the distance of the object, together with its velocity. In the RF regime, for high lateral resolution, a large antenna (phased array) and/or small wavelength needs to be used. This leads to a high acquisition rate, leading to high costs. In the optical/IR regime, a light source with a long coherence length and decent bandwidth integrated with high-speed and sensitive detectors need to be used. As a result, this technique is considerably expensive to utilize. Additionally, the resolution is highly dependent on the bandwidth and the acquisition time. 
     Another technique, known as optical coherence tomography (OCT), is frequently used in bio-medical examinations and thin layers profiling. In this technique the electromagnetic source being used is usually in the optical/IR regime and exploits the short coherence length of the light source for “white light interferometry” in order to measure the cross-section of the examined sample. 
     In such OCT techniques, signals generated by, usually broadband, short coherence light source, is split into two arms using a beam splitter. One portion of the split signal is directed towards the examined sample, along an optical path termed the sample arm, and the second portion of the split signal is directed towards a reference mirror, along a path termed the reference arm. The signals reflected in both the sample and the reference arms are superposed on a detector. Due to the low coherence length of the light source, light reflected only from a specific depth inside the examined sample, defined by the reference arm position, will interfere with the reference signal. By repositioning the reference mirror of the reference arm, the reflectance amplitude at different depths is measured, and cross-sectional information of the examined sample is therefrom determined. The resolution in this method is limited by the coherence length of the light source, and the imaging range is limited by the scanning range of the reference arm mirror. Another related technique, utilizing different hardware and analysis schemes, used for this task, is known as Fourier domain optical coherence tomography (FDOCT), wherein the reference mirror is fixed in place and spectrometry is used, whereby similar results can be achieved. 
     International Patent Publication No. WO 2019/234752 describes a method of detecting objects in a region that comprises transmitting to the region a radiofrequency signal characterized by a coherence range and receiving an echo signal from the region. The method comprises processing the transmitted signal and the echo signal to provide a processed signal, determining at least an existence of objects within the coherence range based on the processed signal. The method dynamically varies the coherence range, and repeats the transmission, receiving, and processed to determine at least an existence of objects within other coherence ranges. 
     US Patent Publication No. 2018/299255 describes a method of inspecting a multilayer sample, comprising: receiving, at a beam splitter, light and splitting the light into first and second portions; combining, at the beam splitter, the first portion of the light after being reflected from a multilayer sample and the second portion of the light after being reflected from a reflector; receiving, at a computer-controlled system for analyzing Fabry-Perot fringes, the combined light and spectrally analyzing the combined light to determine a value of a total power impinging a slit of the system for analyzing Fabry-Perot fringes; determining an optical path difference (OPD); recording an interferogram that plots the value versus the OPD for the OPD; performing the previous acts of the method one or more additional times with a different OPD; and using the interferogram for each of the different OPDs to determine the thicknesses and order of the layers of the multilayer sample. 
     Chinese Patent Publication No. 108964778 describes a decoding device used for time bit-phase encoding. The decoding device comprises an unequal-arm Michelson interferometer, two photoelectric detectors, a decoding unit and a circulator, wherein the unequal-arm Michelson interferometer comprises a beam splitter and two reflectors, the two reflectors are connected with the beam splitter to form a long arm and a short arm of the interferometer respectively, a time difference value corresponding to an arm length difference of the unequal-arm Michelson interferometer is identical with a time interval between two time mode optical pulses of a phase basis vector; the two photoelectric detectors are connected with two output ports of the unequal-arm Michelson interferometer respectively; the decoding unit is connected with the two photoelectric detectors, and performs decoding under the phase basis vector and/or time basis vector according to outputs of the photoelectric detectors; and the circulator is connected with an input port of the decoding device, an output port of the unequal-arm Michelson interferometer and one of the photoelectric detectors. 
     GENERAL DESCRIPTION 
     The present application provides a distance measurement technique utilizing coherent detection and finite coherence electromagnetic signals. 
     Generally, for an electromagnetic source with a gaussian spectrum, the intensity of the interference signal obtained from interrogating (I 1 ) and reference (I 2 ) beams transmitted from the same source and experiencing a spatial delay, is measured by a detector (I det ) and can be expressed by: 
     
       
         
           
             
               
                 
                   
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     where I 1  and I 2  are the optical intensities of the interrogating and reference beams, φ is the relative phase between the interrogating and reference arms/paths at the detector, l c  is the coherence length of the signal source, and Δx is the OPD between paths of the interrogating and reference signals. In this connection, it should be noted that the present technique is described herein in connection with optical radiation for simplicity. It should be understood that the present technique may be used with optical and non-optical electromagnetic radiation with the appropriate modification as described herein below. It should further be understood that the term optical path is used herein for describe effective length of propagation of an electromagnetic signal being of optical or non-optical frequency. Such effective length may be affected by physical length of path of propagation as well as by effective speed of the radiation (e.g., refractive index and/or dielectric constant and permeability along the path) 
     The relation between the coherence length (l c ) of the electromagnetic signal source and the bandwidth (Δk) for a Gaussian spectrum can be expressed by: 
     
       
         
           
             
               
                 
                   
                     
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     where c is the speed of light, τ c  is the coherence time, Δν is the linewidth of the source, λ is the central wavelength of the source, and Δλ is the width of the wavelengths range of the source. 
     As indicated, ToF techniques measure the distance directly from the time delay of a reflected signal; FMCW techniques use frequency analysis for this task; in OCT approach finding the zero OPD, or Fourier analysis, is used to extract the distance. The present technique, however, utilizes coherence factor (e.g., coherence length and/or coherence time) in interference of an electromagnetic signal between reference and interrogating paths, to determine the time delay between a reference beam and the reflected interrogating beam. This enables to determine the distance to an object from which the interrogating beam is reflected. 
     The present technique utilizes an optical arrangement comprising two or more reference arms providing corresponding two or more delay lines. This configuration enables extract depth information by measuring directly to coherence factor and fundamentally eliminating the ambiguity issues associated with variation of intensity such as the inverse square law 
     
       
         
           
             
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     effective numerical aperture, and/or target reflectivity. 
     The term coherence length used herein should be broadly interpreted as the coherence length of an electromagnetic signal source as described in Salamon, T. “ Michelson and Fabry - Perot interferometers with light sources of Gaussian and Lorentzian spectral distribution ”, Acta Physica 36, 269-286 (1974). Specifically, coherence length relates to propagation distance over which a coherent wave maintains a specific degree of coherence. 
     Thus, according to a broad aspect, the present invention provides a system comprising at least one light source arrangement configured to provide illumination of a selected wavelength range and certain coherence length, an optical arrangement, and a detection arrangement comprising at least one detector unit; 
     said optical arrangement comprised optical elements forming at least first and second interferometer arrangements, each of said first and second interferometer arrangements is formed of a reference path and an interrogating path, said interrogating path comprises optical arrangement adapted to direct light portions toward a target object and collect light portions reflected from said target object; said reference path and interrogating path generate an interference signal on at least one detector of the detection arrangement, thereby generating detection data comprising at least first and second detected signals indicative of interference signal of said first and second interferometer arrangements; 
     wherein said first and second interferometer arrangements are configured with respective first and second different coherence factors being associated with at least one of optical path of reference paths of the first and second interferometer arrangements, and coherence length of light passing in said first and second interferometer arrangements; 
     and wherein a relation between said at least first and second detected signals is indicative of a distance to said target object. 
     According to some embodiments, interrogating paths of said least first and second interferometer arrangements may be at least partially overlapping. 
     According to some embodiments, the at least partially overlapping portion of said interrogating paths of said least first and second interferometer arrangements may comprise confocal optical elements for illuminating said target object and for collecting light reflected from said target object. 
     According to some embodiments, the at least partially overlapping portion of said interrogating paths further may comprise a circulator unit configured to receive the interrogating signal in a first port thereof and direct it towards the target object via a second port thereof and receive the at least one return signal via said second port and direct it towards the detection arrangement via a third port thereof. 
     According to some embodiments, the system may further comprise a control unit configured for receiving and processing said detection data and to determine distance of said target. 
     According to some embodiments, the processing may comprise determining a relation between coherence terms in said first and second detected signals. 
     According to some embodiments, the processing may further comprise utilizing data on intensity of constant terms associated with intensity of light portions of said reference paths and said interrogating paths. 
     According to some embodiments, the optical arrangement comprises optical fibers or waveguides. 
     According to some embodiments, the detection arrangement may comprise at least two detector arrays, said interrogating paths is configured for illuminating a field of view and for collecting light reflected from said field of view to form image data on detection plane of said detection unit, thereby enabling detection of distance map of said field of view. 
     According to some embodiments, the detection arrangement may be configured to provide balanced detection of said interference signal, said at least one detector unit comprises respective one or more additional detector units, thereby enhancing signal to noise ratio and filtering out at least one of background noise and distance independent components from the measurement data. 
     According to some embodiments, the light source arrangement may comprise at least one broadband light source. 
     According to some embodiments, the light source arrangement is adapted to emit light of at least first and second wavelength ranges to propagate in corresponding said at least first and second interferometer arrangements. The light source arrangement may be adapted for sequentially emit light of said first and second wavelength ranges, and wherein said first and second wavelength ranges are associated with corresponding first and second different linewidths, thereby affecting coherence length of light of said first and second wavelength ranges. 
     According to some embodiments, the at least first and second interferometer arrangement may be overlapping along interrogating path and at least a portion of reference paths. 
     According to some embodiments, the at least portion of overlapping reference path overlapping between said at least first and second interferometer arrangement, comprise one or more wavelength selective delay lines thereby differentiating optical path of light portions of at least first and second wavelength ranges. 
     The at least portion of overlapping reference path may comprise one or more fiber Bragg grating array (FBGA) elements configured to reflect selected one or more wavelength ranges, thereby varying optical path of said one or more wavelength selective delay lines. 
     According to some embodiments, the system may be formed within a photonic integrated circuit. 
     According to one other broad aspect, the present invention provides a system comprising: light source arrangement configured for emitting at least one light beam of certain coherence length, an optical arrangement comprising at least one reference arm, transmitting optics for directing at least one interrogating beam toward a target and collection optics for collecting reflected signal from said target, and a detection arrangement comprising at least two detectors configured for detecting interfered signals of said reflected signal and corresponding reference signal, said detection unit is configured for generating at least first and second detected signal being together indicative of a distance of said target; wherein the light source arrangement is configured to emit at least first and second light beams having first and second different coherence lengths, said interfered signals being formed of combined reference signal with corresponding one of reflected signal associated with said first and second light beams. 
     The system may further comprise a control unit configured for processing said first and second detected signals and determine distance of said target. The system may also further comprise any additional element or configurations as described above. 
     According to yet another broad aspect, the present invention provides a system comprising: light source arrangement configured for emitting at least one light beam of certain coherence length, an optical arrangement comprising at least one reference arm, transmitting optics for directing at least one interrogating beam toward a target and collection optics for collecting reflected signal from said target, and a detection arrangement comprising at least two detectors configured for detecting interfered signals of said reflected signal and corresponding reference signal, said detection unit is configured for generating at least first and second detected signal being together indicative of a distance of said target; wherein the optical arrangement comprises at least first and second reference arms having first and second different lengths and configured for directing light portions forming corresponding at least first and second reference beams, said interfered signals being formed of combined reflected signal with corresponding one of the first and second reference beams. 
     The system may further comprise a control unit configured for processing said first and second detected signals and determine distance of said target. The system may also further comprise any additional element or configurations as described above. 
     According to yet another broad aspect, the present invention provides a method for determining distance to a target, the method comprising: 
     generating at least one electromagnetic beam having certain coherence length; 
     directing a first reference portion of the beam along a first reference path having first length; 
     directing a second reference portion of the beam along a second reference path having a second length different than the first length; and 
     directing a third portion of the beam toward the target; 
     collecting reflection of said third portion from the target; 
     combining a first portion of collected reflected radiation with said first reference portion to form a first combined signal and a second portion of the collected radiation with said second reference portion to form a second combined signal and detecting intensity of said first and second combined signals to generate corresponding first and second detected signals; and 
     processing said first and second detected signals and determining data on distance of said target. 
     According to some embodiments, said processing of said first and second detected signal comprises determining a relation between coherence terms in said first and second detected signals. 
     According to some embodiments, said processing further comprises determining constant terms in said first and second detected signals. 
     Further additional aspects of the present invention provide a distance measurement apparatus comprising: one or more signal sources configured to generate one or more electromagnetic signals each having a defined different coherence length (linewidth); a splitting arrangement configured to split the one or more electromagnetic signals into at least one interrogating signal directed towards a target object and one or more reference signals, each propagating along a respective reference path, and combine each of the one or more reference signals with respective at least one return signal of the at least one interrogating signal reflected back from the target object; a detection arrangement comprising one or more detector units each configured to measure intensity of a respective combined signal from the splitting arrangement and generate measurement data indicative thereof; and at least one processing unit configured and operable to determine data on distance between the apparatus and the target object based on the measurement data and at least one of the following: difference between at least two coherence lengths (linewidths) of respective at least two electromagnetic signals from the one or more signal sources, and difference between lengths of the reference paths of respective at least two reference signals from the splitting arrangement. 
     In some embodiments the respective reference paths are of different length. 
     In some additional embodiments, at least one of the one or more signal sources is configured to generate first and second electromagnetic signals having defined different coherence lengths (linewidths). The at least one processing unit may be configured and operable to determine the target distance based on a ratio of coherence terms of the measurement data generated by the detection arrangement for the combined signals obtained for the first and the second electromagnetic signals. 
     In some embodiments the splitting arrangement is configured to form for each electromagnetic signal at least two reference signals, each associated with a different reference path length. The at least one processing unit can be configured to determine the distance based on a ratio of the coherence terms of the measurement data generated by the detection arrangement for the combined signals of each of the at least two reference signals with the at least one return signal from the target object. 
     Optionally, the different reference signals are obtained utilizing waveguide (e.g., fiber optic) paths of different lengths. The apparatus can comprise directing optical arrangements for directing the at least one interrogating beam towards the target object through free space medium, and/or for directing the at least one return signal towards the detection arrangement. At least one optical phase modulating unit can be used to control an optical phase of the at least one return signal. Optionally, an additional detection arrangement comprising respective one or more detector units configured to implement a balanced detection arrangement is used for cancelling out noise and the constant (the distance independent terms) components from the measurement data. 
     The apparatus comprises in some embodiments: one or more signal sources, each configured to produce electromagnetic signals having a different or variable coherence length. The splitting arrangement can be configured to split the electromagnetic signals from each of the one or more signal sources into interrogating signal portions directed towards the target object and respective reference signal portions propagating along a reference path, combine the return signals of the interrogating signal portions with the respective reference signal portions, and direct the combined signals towards a respective detector unit of the detection arrangement. 
     The at least one processing unit can be configured and operable to determine the distance based on ratios of the coherence terms of the data/signals measured by the detection arrangement. 
     The different reference signals are obtained in some embodiments utilizing a wavelength disperser. The wavelength disperser comprises in some embodiments a fiber Bragg grating array (FBGA) element. Optionally, at least one of the one or more signal sources is a broadband light source. Alternatively, at least one of the one or more signal sources is a swept signal source configured to successively produce two or more electromagnetic signals, each having a defined linewidth centered about a different central wavelength. The at least one processing unit can be configured and operable to determine the distance based on a ratio of the coherence terms of the measurement data generated by the detection arrangement for combined signals obtained for the two or more electromagnetic signals. 
     The apparatus comprises in some embodiments a circulator unit configured to receive the interrogating signal in a first port thereof and direct it towards the target object via a second port thereof and receive the at least one return signal via the second port and direct it towards the detection arrangement via a third port thereof. The splitting arrangement can comprise a switching unit configured to select for each electromagnetic signal from the one or more signal sources a respective propagation path having a defined length. A circulator unit can be used to receive one or more reference signals in a first port thereof and direct them to the switching unit via a second port thereof and receive respective back-reflected reference signals from the switching unit via the second port and direct them towards the detection arrangement via a third port thereof. 
     In some embodiments the apparatus comprises at least one detector unit that is implemented by a detector array consisting of a plurality detector elements, wherein each detector element is configured and operable to collect a portion of the return signal from a field of view associated with a different direction of arrival combined with a portion of the at least one reference beams. According to certain embodiments the apparatus comprises at least two detector arrays, wherein detector elements in one of the detector arrays have conjugate detector elements in the other detector arrays, collecting portions of the return signal from the same certain field of view. According to specific embodiments the apparatus comprises collection imaging optics configured to collect a plurality of return beams that are reflections of the interrogating beam. 
     In some possible embodiments the apparatus is implemented in a photonic integrated circuit. 
     The apparatus comprises in some embodiments one or more transmitting antenna elements configured to transmit the at least one interrogating signal towards the target object, one or more receiving antenna elements configured to receive the at least one return signal reflected back from the target object, a mixer arrangement configured to mix the at least one return signal from each one of the receiving antenna elements with two or more reference signals from the splitting arrangement, wherein the reference paths are implemented by waveguide elements. A filtering unit can be used to remove constant distance-independent components from the combined signals and generate respective filtered signals. The apparatus can comprise a divider arrangement configured to determine ratio signals indicative of a ratio of coherence terms of the filtered signals. A phase control arrangement can be used to control relative phase between the transmitting antenna elements. 
     In some possible embodiments the one or more signal sources are configured to generate the one or more electromagnetic signals with different coherence lengths. In such possible embodiments the mixer arrangement can be configured to mix the at least one return signal received from each one of the receiving antenna elements with one single reference signal from the splitting arrangement. 
     A coherence length measurement arrangement can be used to receive portions of the one or more electromagnetic signals generated by the one or more signal sources, identify changes in a coherence length of the one or more electromagnetic signals, and generate coherence length measurement data/signals indicative thereof for adjusting the coherence length of the one or more electromagnetic signals generated by the one or more signal sources. The coherence length measurement arrangement comprises in some embodiments a feedback loop for each of the one or more signal sources. Each one of the feedback loops can comprise a splitter configured to split a respective one of the one or more electromagnetic signal portions into long and short arms signal portions having different path lengths, a combiner configured to combine the long and short arms signal portions, and a detector configured to measure intensity of the combined signal from the combiner and generate the coherence length measurement data/signals based hereon. A phase modulator can be used for controlling phase of the short arm signal portion. 
     The apparatus comprises in possible embodiments a phase correction arrangement configured to generate measurement signals for monitoring relative phase of the two or more of the reference signals, or of split portions of the return signal. For this purpose, the apparatus can comprise a splitter configured to split the one or more reference signals into long and short reference arm signal portions, a phase modulator for controlling a phase of the short reference arm signal portion, a combiner configured to combine a portions of the long and short reference arm signal portions, and a detector configured to measure intensity of the combined signal from the combiner and generate measurement data/signals indicative thereof. 
     Another inventive aspect of the subject matter disclosed herein relates to a method of determining distance of a target object. The method comprising: splitting one or more source electromagnetic signals, each having a defined different coherence length (linewidth), into at least one interrogating signal directed towards a target object, and one or more reference signals each propagating along a respective reference path having a different length; combining each of the one or more time delayed reference signals with at least one return signal of the at least one interrogating signal reflected back from the target object; measuring intensity of the combined signals and generating measurement data indicative thereof; and processing the measurement data and determining the distance to the target object based on a difference between at least two coherence lengths (linewidths) of at least two of the source electromagnetic signals, or based on a difference between the path lengths of respective at least two reference signals. 
     The splitting can comprise splitting first and second electromagnetic signals having defined different coherence lengths (linewidths), and the determining of the distance of the target object can be based on a ratio of coherence terms of the measurement data generated for the combined signals obtained for the first and second source electromagnetic signals. 
     The method comprises in some possible embodiments collimating and/or focusing and/or a directing mechanism (e.g., Galvo or MEMS mirror) the at least one interrogating beam, and/or the at least one return signal. Optionally, but in some embodiments preferably, the method comprises a phase modulator to the interrogating beam or to return signals. 
     The method may comprise utilizing a balanced detection arrangement for measuring intensity portions of the combined signals and cancelling out noise and DC (distance independent) components from the measurement data. 
     The splitting comprises in some embodiments splitting two or more signal sources, each having a different coherence length, and each is split into an interrogating signal portion directed towards the target object and a respective reference signal portion propagating along a reference path, the combining comprises combining the return signals of the interrogating signal portions with the respective reference signal portions, the measuring comprises measuring intensity of each combined signal and generating measurement indicative thereof, and the processing comprises determining the distance based on a ratio of coherence terms of measurement data from the detection arrangement. 
     The splitting can be successively applied to two or more source electromagnetic signals. The method may comprise selecting for each successively split reference signal a respective propagation path having a defined different path length. The combining may comprise successively combining each reference signal passed through the selected different path length, and the processing may be applied to the successively combined signals obtained. 
     The method comprises in some embodiments transmitting by a transmit array antenna the at least one interrogating signal towards the target object and receiving by a receive array antenna the return signal reflected back from the target object. The method may comprise controlling relative phase between antenna elements of the transmit array antenna. The processing may comprise filtering constant distance-independent components from the combined signals. The method comprises in some embodiments determining ratio of coherence terms of the filtered signals. 
     In some embodiment the method comprises identifying changes in a coherence length of the one or more source electromagnetic signals, generating coherence length measurement data/signals indicative thereof, and adjusting the coherence length of the one or more source electromagnetic signals based on the coherence length measurement data/signals. The identifying can comprise splitting each one of the one or more source electromagnetic signals into long and short arms signal portions having different path lengths, combining the long and short arms signal portions, and measuring intensity of the combined signals and generating the coherence length measurement data/signals based on the combined signals. The method may comprise controlling phase of the short arm signal portion. 
     The method comprises in some embodiments monitoring the relative phase of the one or more reflected interrogating beam and/or of the reference signals. 
     According to further additional broad aspect, the present invention provides a distance measurement apparatus comprising: 
     one or more signal sources configured to generate one or more electromagnetic radiation beams having certain coherence length; 
     an optical arrangement configured for directing one or more interrogating beams toward a target and for collecting at least a portion of said interrogating beam reflected from the target; 
     a splitting arrangement comprising at least one reference arm and configured to split said one or more electromagnetic radiation beams into said one or more interrogating beams, and one or more reference beams propagating along a respective reference path of said at least one reference arm, and combine said one or more reference beams with respective at least portion of said interrogating beam reflected from the target thereby forming one or more interfered combined signals; 
     a detection arrangement comprising one or more detector units configured for collecting said one or more interfered combined signals and generated data on intensity thereof; and 
     at least one control unit configured to receive data on intensity of said one or more interfered combined signals and to utilize said data to determine a distance between said apparatus and said target object. 
     In some additional broad aspects, the present invention further provides a system comprising one or more signal sources comprise one or more transmitting antenna elements configured to transmit the at least one interrogating signal towards a target object, one or more receiving antenna elements configured to receive the at least one return signal reflected back from said target object, at least one mixer arrangement configured to mix the at least one return signal from said one of said receiving antenna elements with one or more reference signals thereby generating at least two mixed signals associated with two or more different coherence factors, wherein a relation between said at least two mixed signals is indicative of a distance to said target object. 
     According to some embodiments, said one or more transmitting antenna elements being configured to transmit at least two interrogating signals being different in at least coherence length between them. 
     According to some embodiments, said at least two interrogating signals being distinguishable in at least one of frequency range, polarization, and time of transmission. 
     According to some embodiments, said one or more reference signals comprise two or more reference signals having two or more respective different coherence factors. 
     According to some embodiments, the system may comprise at least one local oscillator for generating said one or more reference signals. 
     According to some embodiments, the system may comprise a phase control arrangement configured to control relative phase between the one or more transmitting antenna elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings. Features shown in the drawings are meant to be illustrative of only some embodiments of the invention, unless otherwise implicitly indicated. In the drawings like reference numerals are used to indicate corresponding parts, and in which: 
         FIG.  1    schematically illustrated a system for range detection according to some embodiments of the invention; 
         FIG.  2    schematically illustrates a range detection apparatus according to some possible embodiments; 
         FIGS.  3 A and  3 B  graphically illustrate coherence term amplitudes as a function of the object&#39;s distance for short and long reference arms/paths, respectively; 
         FIGS.  4 A and  4 B  graphically illustrate the coherence term amplitudes for the short and the long reference arms/paths as a function of the object&#39;s distance, and an AC ratio parameter Γ, respectively; 
         FIG.  5    schematically illustrates a fiber-based range detection apparatus, according to some possible embodiments; 
         FIG.  6    schematically illustrates a fiber-based range detection apparatus, and a free space detection unit thereof, according to some possible embodiments; 
         FIG.  7    schematically illustrates a range detection apparatus utilizing a microelectromechanical system (MEMS) switch for implementing a variable delay line with a single detector, according to some possible embodiments; 
         FIG.  8    schematically illustrates a range detection apparatus using variable coherence length source, according to some possible embodiments; 
         FIG.  9    schematically illustrates a range detection apparatus using two different coherence length sources, according to some possible embodiments; 
         FIG.  10    schematically illustrates a range detection apparatus using broadband source, a FBGA, and a spectrometer, according to some possible embodiments; 
         FIG.  11    schematically illustrates a range detection apparatus using a swept source, a fiber Bragg grating array (FBGA), and a single detector, according to some possible embodiments; 
         FIG.  12    schematically illustrates a three-dimensional (3D) implementation of a range detection apparatus using an array of detectors, according to some possible embodiments; 
         FIG.  13    schematically illustrates a range detection apparatus using source in the RF regime and conventional RF components, according to some possible embodiments; 
         FIG.  14    schematically illustrates a range detection apparatus using a phased antenna array, a signal source in the RF regime, and conventional RF components, according to some possible embodiments; 
         FIG.  15    schematically illustrates a continuous coherence length measurement apparatus, according to some possible embodiments; 
         FIGS.  16 A and  16 B  schematically illustrate relative phase correction apparatuses, according to some possible embodiments; 
         FIG.  17    schematically illustrates a distance measurement/imaging apparatus according to possible embodiments implementation in a photonic integrated circuit (PIC) in a simplified manner e.g., using a hybrid III-V/Silicon platform; and 
         FIG.  18    schematically illustrates rotation measurement apparatus according to some possible embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     One or more specific embodiments of the present disclosure are described below with reference to the drawings, which are to be considered in all aspects as illustrative only and not restrictive in any manner. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. Elements illustrated in the drawings are not necessarily to scale, or in correct proportional relationships, which are not critical. Emphasis instead being placed upon clearly illustrating the principles of the invention such that persons skilled in the art will be able to make and use the embodiments, once they understand the principles of the subject matter disclosed herein. This invention may be provided in other specific forms and embodiments without departing from the essential characteristics described herein. 
     In a broad aspect, there are provided methods and apparatuses for distance measurement (also referred to herein as range detection) utilizing the coherence term of a transmitted electromagnetic beam combined with a smart delay line, or by varying the coherence length of the electromagnetic signal source. 
     For an overview of several example features, process stages, and principles of the invention, the range detection examples illustrated schematically and diagrammatically in the figures are generally intended for a long-distance, or short-distance, measurements. These range detection apparatuses are shown as one example implementation that demonstrates a number of features, processes, and principles used to provide distance measurement, but they are also useful for other applications and can be made in different variations. Therefore, this description will proceed with reference to the shown examples, but with the understanding that the invention recited in the claims below can also be implemented in myriad other ways, once the principles are understood from the descriptions, explanations, and drawings herein. All such variations, as well as any other modifications apparent to one of ordinary skill in the art and useful in coherence-based applications may be suitably employed and are intended to fall within the scope of this disclosure. 
     Reference is made to  FIG.  1    exemplifying a system according to some embodiments of the invention. The system includes a light source arrangement  101 , detection arrangement  127  and optical arrangement  3000 . The system may also include a control unit  115 . The optical arrangement includes one or more optical elements as described further below in more details. The optical arrangement  3000  defined at least first and second interferometer arrangements. In this exemplary illustration, beam splitter BS 1  splits light beam  3001  to propagate along reference path  3005  an along interrogating path  3003  toward a target object  105 . Portion of the beam  3007  reflected from the object  105  is directed to interfere with the reference beam  3005  to generate a detected interference signals, detected by detection arrangement  127 . Similarly beam splitter BS 2  splits light beam  3002  to reference beam  3006  and interrogating path  3004 , followed by collection of reflected light portion  3008  generating a second detected signal. Coherence factor of the first and second interferometer arrangements is different by at least one of optical path, or length, of reference paths, or by coherence length of light beams  3001  and  3002  relative to the reference beams  3005  and  3006 . According to the present technique, and as described below in more detail, a relation between the detected signals along the different interferometer arrangements provides data distance to said target object  105 . The control unit  115  is generally configured to receive the detected signals and utilizes them to determine distance to the object  105 . The following describes further arrangement and configurations of the present technique in more detail. As described hereinbelow, the technique may generally be used with electromagnetic radiation, being optical illumination, infrared, UV as well as Radio Frequency radiations or others electromagnetic sources. 
     Reference is made to  FIG.  2   , schematically illustrating a range detection system  100  according to some embodiments, wherein an electromagnetic interrogating beam  104  is used to determine a distance to a target object (also referred to herein as object)  105 . The apparatus  100  includes a signal source  101 , a splitting arrangement  125 , a detection arrangement  127 , and a processing unit  115  (also referred to herein as DAQ). In this non-limiting example, the splitting arrangement  125  includes first and second beam splitters,  102  and  103  respectively, and first and second reflectors  110  and  112  respectively. The detection arrangement  127  includes first and second detectors,  113  (A) and  114  (B) respectively. Optionally, but in some embodiments preferably, the signal source  101  may be a finite coherence length electromagnetic signal source configured to generate a source beam  119  of selected bandwidth (linewidth or wavelength range). The source beam  119  is directed at the splitting arrangement and is split into three portions using the first and second beam splitters,  102  and  103 , respectively. The signal source  101  may generally be a CW or pulsed laser unit having selected bandwidth, distributed feedback (DFB) laser, External cavity laser (ECL), semiconductor lasers, microwave source and/or RF sources. In some configurations, the signal source  101  may include, or be associated with, a coherence length controller (e.g., phase modulator) for stabilizing the coherence length of the source beam  119 . In some configurations, coherence factor (e.g., coherence length) of source beam  119  provided by the signal source  101  may be selectively varied, e.g., by bandwidth selection and/or by applying phase modulation to the source beam  119 . Such variation of the coherence factor may be used for tunning of measurement sensitivity providing tunable dynamic range. 
     Particularly, the first beam splitter  102  is configured to split the source beam  119  into a first reference beam portion  109  directed to a first reflector (e.g., mirror)  110 , and a first residual beam portion  122  directed towards the second beam splitter  103 . The second beam splitter  103  is configured to split the first residual beam portion  122  into a second reference beam portion  111  directed towards a second reflector (e.g., mirror)  112 , and a second residual beam portion acting as interrogating beam  104 . The interrogating beam  104  is directed to illuminate a spot on the target object  105  (e.g., by a directing optics module—not specifically shown), and a portion  106  of the interrogating beam  104  is reflected back from the target object  105  to be collected by the apparatus  100 , e.g., using the directing optics module. 
     The reflected interrogating beam portion  106  is directed back to the splitting arrangement (e.g., by a collection optics module, which is not specifically shown and in general may be the same or different from the directing optics module) wherein it is split into a first reflected interrogating beam portion  107 , and a second interrogating reflected beam portion  108 , thereby directed onto detectors,  113  (A) and  114  (B), respectively. Particularly, the reflected interrogating beam portion  106  is directed towards the second beam splitter  103 , whereby it is split into the second reflected interrogating beam portion  108  directed towards the second detector  114 , and a residual reflected interrogating beam portion  118 . The residual reflected interrogating beam portion  118  is directed towards the first beam splitter  102 , wherefrom it is directed as the first reflected interrogating beam portion  107  towards the first detector  113 . 
     The first reference beam portion  109  of the source beam  119  is directed by the first beam splitter  102  towards a first reflector (e.g., mirror)  110 , wherefrom it is reflected back ( 121 ) to the first beam splitter  102 , which outputs therefrom the first reference beam  116  directed to the first detector  113  (A). The first reference beam  116  outputted from the first beam splitter  102  is combined at the first detector  113  (A) with the first reflected interrogating beam signal portion  107 . 
     The second reference beam  111  portion, generated from the first residual beam portion  122 , is directed by the second beam splitter  103  towards the second reflector (e.g., mirror)  112 , wherefrom it is reflected ( 120 ) back to the second beam splitter  103 , which outputs therefrom the second reference beam  117  directed to the second detector  114  (B). The second reference beam  117  outputted from the second beam splitter  103  is combined at the second detector  114  (B) with the second reflected interrogating beam portion  108 . 
     The optical paths corresponding to the path of the first reference beam ( 109 ,  121 ,  116 ) and the second reference beam ( 122 , 111 ,  120 ,  117 ), are referred to herein as a first reference arm and a second reference arm, and collectively as reference arms. In this specific and non-limiting example, the optical paths of the beams propagating towards, and back from, the reflectors  110  and  112 , are substantially perpendicular to the path of the beams propagating from the signal source  101  towards the target object  105 . However, in possible embodiments, the path of the first and/or second reference arm is parallel (or angled with respect) to the path of the interrogating beam. The first reference beam  116  is delayed along the first reference arm by a first-time delay, and the second reference beam  117  is delayed along the second reference arm by a second time delay, whereas the difference in the delay between the first-time delay and the second time delay corresponds to the additional distance 2L between the first and second reference arms. Thus, the reference arm with the shorter time delay (i.e., that produces the first reference beam  116 ) is referred to herein as the short reference arm, and the reference arm with the longer time delay (i.e., that produces the second reference beam  117 ) is referred to herein as the long reference arm. 
     As explained hereinabove, the first reflected interrogating beam  107 , and the second reflected interrogating beam  108 , are superposed with the first and second reference beam portions  116  and  117 , on the detectors  113  (A) and  114  (B) respectively. This way, one of the reflected interrogating beam portions is superposed with the reference beam propagating along the short reference arm at the first detector  113 , and the second reflected interrogating beam portion is superposed with the reference beam propagating along the long reference arm at the second detector  114 . Detector signals I short  and I long , generated by the first and second detectors  113  and  114 , corresponding to these beams superposition, are directed into data acquisition module DAQ  115  (e.g., including one or more processors (CPU) and memories (MEM)), and thereby processed, as described hereinbelow, to form a detection result  126  for estimating the distance of the target object  105  from the apparatus  100 . 
     In general, directing optics (not shown) can be used to direct the interrogating beam  104  towards the target object  105 . The interrogating beam  104  may be a collimated or focused beam, and in such case may illuminate the target object  105 , and a reflected interrogating beam  106  can be thus reflected from the object  105  towards the apparatus  100  and collected by collection optics (not shown), which may be same as (or different from) the directing optics. 
     To explain how the detection result can be calculated in some possible embodiments we define x 0  as the distance corresponding to the delay along the optical path length (OPL) of the short reference arm. The distance measured to the target object is defined as x, which may be measured with respect to x 0 . The detector signal I short  measured at the detector  113  corresponding with the short reference arm may be expressed as follows (assuming the signal source  101  is of a Gaussian spectrum): 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       s 
                       ⁢ 
                       h 
                       ⁢ 
                       o 
                       ⁢ 
                       r 
                       ⁢ 
                       t 
                     
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     [ 
                     
                       
                         I 
                         
                           R 
                           ⁢ 
                           1 
                         
                       
                       + 
                       
                         
                           η 
                           
                             S 
                             ⁢ 
                             1 
                           
                         
                         ⁢ 
                         
                           I 
                           s 
                         
                       
                       + 
                       
                         2 
                         · 
                         
                           
                             
                               I 
                               
                                 R 
                                 ⁢ 
                                 1 
                               
                             
                             ⁢ 
                             
                               η 
                               
                                 S 
                                 ⁢ 
                                 1 
                               
                             
                             ⁢ 
                             
                               I 
                               s 
                             
                           
                         
                         · 
                         
                           cos 
                           ⁡ 
                           ( 
                           
                             φ 
                             1 
                           
                           ) 
                         
                         · 
                         
                           e 
                           
                             
                               
                                 - 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       k 
                                       2 
                                     
                                   
                                   8 
                                 
                               
                               · 
                               Δ 
                             
                             ⁢ 
                             
                               x 
                               2 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where I R1  is the power intensity of the first reference beam  116  of the short reference arm; I s  is the power intensity of the reflected interrogating beam  106  collected by the system  100 ; η S1  defines an effective collection efficiency associated with intensity loss resulting from different reasons such as the collection optics, target&#39;s reflectivity, and speckles intensity variation of the reflected interrogating beam  106  from the target object  105 . Thus, the power intensity of the first reflected interrogating beam  107  is I s ·η S1 ; φ 1  is the relative phase between the short arm reference beam  116  and the first reflected interrogating beam  107  as the beam are superimposed at the first detector  113  (A); Δk is the bandwidth of the electromagnetic signal source  101 ; and Δx=x−x 0  is the distance between the apparatus  100  and the target object  105 . Alternatively, in some embodiments, I s  can be used to define the interrogating beam  104 , and η s1  is used to define the coupling efficiency of the interrogating beam  104  to the first detector (A)  113 , including the reflectivity of the target object  105 . 
     The third term on the right-hand-side (RHS) of equation (4), which depends on the distance to be measured Δx i.e., 
     
       
         
           
             
               2 
               · 
               
                 
                   
                     I 
                     
                       R 
                       ⁢ 
                       1 
                     
                   
                   ⁢ 
                   
                     η 
                     
                       S 
                       ⁢ 
                       1 
                     
                   
                   ⁢ 
                   
                     I 
                     s 
                   
                 
               
               · 
               
                 cos 
                 ⁡ 
                 ( 
                 
                   φ 
                   1 
                 
                 ) 
               
               · 
               
                 e 
                 
                   
                     
                       - 
                       
                         
                           Δ 
                           ⁢ 
                           
                             k 
                             2 
                           
                         
                         8 
                       
                     
                     · 
                     Δ 
                   
                   ⁢ 
                   
                     x 
                     2 
                   
                 
               
             
             , 
           
         
       
     
     is sometimes referred to herein as the coherence term. The first and second terms on the right-hand-side (RHS) of equation (4), which are considered independent of the distance to be measured Δx i.e., I R1 +η s1 I s , are sometimes mutually referred to herein as the constant term or the distance independent term. 
       FIG.  3 A  illustrates absolute value of envelope amplitude of the coherence term  201  in equation (4) above, as a function of the distance Ax of the target object  105  from the apparatus  100 . In this non-limiting example, the maximum amplitude is obtained when the target object  105  is positioned at Δx=L 1 =0 where L1 relates to length of the short reference arm. 
     The signal I long  measured at the second detector  114  (B), corresponding to the long reference arm, can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     long 
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     [ 
                     
                       
                         I 
                         
                           R 
                           ⁢ 
                           2 
                         
                       
                       + 
                       
                         
                           η 
                           
                             S 
                             ⁢ 
                             2 
                           
                         
                         ⁢ 
                         
                           I 
                           s 
                         
                       
                       + 
                       
                         2 
                         · 
                         
                           
                             
                               I 
                               
                                 R 
                                 ⁢ 
                                 2 
                               
                             
                             ⁢ 
                             
                               η 
                               
                                 S 
                                 ⁢ 
                                 2 
                               
                             
                             ⁢ 
                             
                               I 
                               s 
                             
                           
                         
                         · 
                         
                           cos 
                           ⁡ 
                           ( 
                           
                             φ 
                             2 
                           
                           ) 
                         
                         · 
                         
                           e 
                           
                             
                               - 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     k 
                                     2 
                                   
                                 
                                 8 
                               
                             
                             · 
                             
                               
                                 ( 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     x 
                                   
                                   - 
                                   L 
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where I R2  is the power intensity of the second reference beam  117  from the long reference arm, and η S2  is the effective collection efficiency of the reflected interrogating beam  106  collected by the system  100  arriving at the second detector  114  (B). Thus, the power intensity of the second reflected interrogating beam  108  is I s *η S2 . φ 2  is the relative phase between the second reference beam  117  from the long reference arm and the reflected interrogating beam  108  arriving at detector  114  (B), and L is the half of the OPD between the long and short reference arms. Here, as well, the third term on the RHS of equation (5), is sometimes referred to as the coherence term, and the first and second terms on the RHS of equation (5) are sometimes mutually referred to herein as the constant term. 
       FIG.  3 B  illustrates the absolute value of envelope amplitude of the coherence term  202  in equation (5), as a function of the distance Δx of the target object  105  from the apparatus  100 . In this specific and non-limiting example, the maximum amplitude is obtained when the object is positioned at Δx=L 2 =L. 
     In certain embodiments, the following assumptions and estimations may be made in order to extract the distance Δx of the target object  105  from the apparatus  100 : (i) the intensities of the reference beams I R1 , and I R2  from the reference arms are well-known and constant in time, or can be independently measured/determined; (ii) the ratio between the power intensities of the reflected interrogating signals  107  and  108  that arrive at the first and second detectors  113  (A) and  114  (B) can be found and used, their power intensity may be equal, but their relation is always constant in time, typically due to characteristics of the beams splitting arrangement  125 ; and (iii) the phases φ 1 , and φ 2  can be adjusted actively or passively and the relative phase Δφ=φ 1 −φ 2  may be tracked if needed. 
     By filtering/subtracting the constant (distance independent) terms of Equations (4) and (5) (e.g., using hardware techniques such as balanced detector, electrical DC rejecting filter, or using software techniques) and dividing the two coherence (Coh) terms of Equations (4) and (5) a coherence ratio term Γ, is determined as follows: 
     
       
         
           
             
               
                 
                   Γ 
                   
                     = 
                     Δ 
                   
                   
                     
                       
                         C 
                         ⁢ 
                         
                           oh 
                           ⁡ 
                           ( 
                           
                             I 
                             long 
                           
                           ) 
                         
                       
                       
                         C 
                         ⁢ 
                         
                           oh 
                           ⁡ 
                           ( 
                           
                             I 
                             
                               s 
                               ⁢ 
                               h 
                               ⁢ 
                               o 
                               ⁢ 
                               r 
                               ⁢ 
                               t 
                             
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         
                           χ 
                           eff 
                         
                         · 
                         
                           
                             e 
                             
                               
                                 - 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       k 
                                       2 
                                     
                                   
                                   8 
                                 
                               
                               · 
                               
                                 
                                   ( 
                                   
                                     
                                       Δ 
                                       ⁢ 
                                       x 
                                     
                                     - 
                                     L 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                           
                             e 
                             
                               
                                 - 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       k 
                                       2 
                                     
                                   
                                   8 
                                 
                               
                               · 
                               
                                 
                                   ( 
                                   
                                     Δ 
                                     ⁢ 
                                     x 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                           
                         
                       
                       = 
                       
                         
                           χ 
                           eff 
                         
                         · 
                         
                           e 
                           
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                   k 
                                   2 
                                 
                                 ⁢ 
                                 L 
                               
                               4 
                             
                             · 
                             
                               [ 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   x 
                                 
                                 - 
                                 
                                   L 
                                   2 
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where χ eff  is representing the constants parameters of the coherence terms, that are well-known a and can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     χ 
                     eff 
                   
                   
                     = 
                     Δ 
                   
                   
                     
                       
                         
                           
                             η 
                             
                               s 
                               ⁢ 
                               2 
                             
                           
                           ⁢ 
                           
                             I 
                             
                               R 
                               ⁢ 
                               2 
                             
                           
                         
                         
                           
                             η 
                             
                               s 
                               ⁢ 
                               1 
                             
                           
                           ⁢ 
                           
                             I 
                             
                               R 
                               ⁢ 
                               1 
                             
                           
                         
                       
                     
                     ⁢ 
                         
                     
                       { 
                       
                         
                           
                             assuming 
                             : 
                             
                               
                                 cos 
                                 ⁡ 
                                 ( 
                                 
                                   φ 
                                   2 
                                 
                                 ) 
                               
                               
                                 cos 
                                 ⁡ 
                                 ( 
                                 
                                   φ 
                                   1 
                                 
                                 ) 
                               
                             
                           
                           ≈ 
                           1 
                         
                         , 
                         
                           or 
                           ⁢ 
                               
                           otherwise 
                           ⁢ 
                               
                           known 
                         
                       
                       } 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Accordingly, there is one to one relation between the coherence terms ratio Γ to the distance Δx of the target object  105  from the apparatus  100 , which can be determined as follows: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     x 
                   
                   = 
                   
                     
                       
                         4 
                         
                           Δ 
                           ⁢ 
                           
                             k 
                             2 
                           
                           ⁢ 
                           L 
                         
                       
                       ⁢ 
                       
                         ln 
                         [ 
                         
                           Γ 
                           
                             χ 
                             eff 
                           
                         
                         ] 
                       
                     
                     + 
                     
                       L 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
       FIG.  4 A  illustrates the absolute values of the amplitude of the envelope of the coherence terms  201  and  202  of the short and the long reference arms respectively (of equations (4) and (5)). Additionally,  FIG.  4 B  illustrates the coherence ratio term Γ  303  as a function of the distance Δx of the target object  105  from the apparatus  100 . 
     Any backlight noise can be considered in equations (4) and (5) as a constant distance and time independent component, such that its interference has a low influence on the measurement due to the filtering/subtraction of the constant components and the coherent detection scheme utilized. 
     Accordingly, the present technique utilizes illumination of a target by light of predetermined coherence length and determining data on first interference signal of light reflected from the target with first reference beam and of second interference signal of the reflected light with a second reference beam. The first and second reflected beams have corresponding first and second different optical paths. A ratio between interference terms of the first and second interference signal is indicative of optical path of the interrogating beams illuminating the target, thereby indicating a distance to the target. 
     Further, the system according to the present technique includes one or more light sources, configured to direct light of selected one or more coherence lengths toward a target. The system also includes one or more reference arms of selected different optical paths (lengths), and a detection unit including two or more detectors, each positioned for detecting intensity of interference signals between light reflected from the target and light of a corresponding one of the two or more reference arms. A relation between in intensities of the interference signals is indicative of a distance between the system and the target object. 
     Generally, according to some embodiments, the present technique utilizes one of the following configurations: at least one interrogating beam having selected coherence factor and two or more reference beams, or two or more interrogating beams and at least one reference beam. The detection unit is configured for detection of combined signals of reflected interrogating beam and a reference arm, to provide detection data indicative of a relation between coherence terms of the combined signals to determine coherence ratio term F. In some configuration, the technique may utilize prestored data on intensity of light source output and efficiency of beam splitters (e.g., the first and second beam splitters  102  and  103 ) for determining coherence ratio term FAs indicated, the present distance measurement technique may be implemented with more than two reference arms, as the reference and the returned signal can be split to any number of portions, as long as each reference portion is delayed in a different amount/length. For example, the reflected interrogating beam  106  may be split into a plurality of reflected beams (not shown), and by additional splitting of the source beam  119  a respective plurality of reference beams of different optical paths may be generated. The plurality of reflected interrogating beams may be combined respectively with the plurality of reference beams of different optical paths and directed onto a plurality of respective detectors to form a plurality of respective detected signals. The respective detected signals are detected providing information indicative of the distance Δx of the target object  105  from the apparatus  100 . Thus, the target distance Δx may be determined based on ratios between respective pairs of coherence terms. 
     The simplified technique for determining target distance as shown in equation (8) above can be carried out assuming the signal source ( 101  in  FIG.  2   , or the other signal sources shown in the figures) has a gaussian spectrum. However, this distance calculation can be similarly carried out for other signal sources having any arbitrary spectrum, as long as the coherence ratio term Γ is a function of the distance Δx. 
     Another embodiment of a distance detection apparatus  400  according to the present disclosure is schematically illustrated in  FIG.  5   . The apparatus  400  includes a signal source  401 , a splitting arrangement  434  including beam splitters  402 ,  415  and  411 , beam combiners  423  and  424 , and circulator  409 , a detection arrangement  435  formed by at least first and second detectors,  420  and  431  respectively, and a processing unit  421  (also referred to herein as DAQ). In this distance detection apparatus  400  embodiment, electromagnetic beam(s) used to determine data on distance of the target object  405  are propagating in a waveguide e.g., an optical fiber. 
     In apparatus  400  the electromagnetic signal source  401  is configured to generate a beam having a finite coherence length for passage through a waveguide element  425 . In some possible embodiments the waveguide elements shown in  FIG.  5    can be implemented by one or more optical fibers. The electromagnetic beam passing through the waveguide element  425  is split into interrogating beam and two or more reference beams. Specifically, the initial beam is split by first beam splitter  402  into an interrogating beam portion passed through waveguide element  403 , and a reference beam portion passed through the waveguide element  404 . The reference beam portion passing through the waveguide element  404  is split by a second beam splitter  415  into a second reference beam portion passed through the waveguide element  417 , and a second reference beam portion passed through the waveguide element  416 . Waveguide elements  416  and  417  are configured with different length, generally having length difference L. Thus, the second reference beam portion passed through the waveguide element  417  is delayed by an additional distance L obtained by the waveguide element  418  configured to introduce the additional time delay to the reference beam ( 432 ). 
     The interrogating beam portion passed through the waveguide element  403  is directed to the target object, typically using optical arrangement, e.g., lens  406  and filter  408 . To enable transmission and collection of light using common optics, the apparatus may include circulator  409 , e.g., a three-port circulator element, configured to permit passage of the interrogating beam portion introduced thereinto only through its second port b, that is coupled to output waveguide element  433 . As seen, the interrogating beam portion  403  is introduced via port a into the circulator element  409 , which permits passage thereof only through port b i.e., passage thereof from port a to port c is substantially prevented. The output waveguide element  433  is coupled to the second port b of the circulator element  409  provide output beam directed toward the target. The optical arrangement in this example includes a collimator  406 , from which the interrogating beam  426  is directed towards the target object  405 . 
     The electromagnetic interrogating beam  426  is thus directed towards the target object  405  substantially collimated, and it may be then passed through a narrowband filter  408  configured corresponding to the bandwidth of the electromagnetic signal source  401 . Filter  408  is typically used to filter collected light enabling collection of light associated with reflection of the interrogating beam and thus reduce background noise. Accordingly, in this non-limiting example, the electromagnetic beam  426  illuminating the target object  405  is substantially collimated. In some possible embodiments directing optical arrangements may be used for directing the substantially collimated interrogating beam  426  towards the target object  405 , and/or for collecting interrogating beam portions reflected from the target object  405 . The directing optics, and/or additional collecting optics, may be used to collect a reflected interrogating beam portion  407 , which may include reflection of the substantially collimated and band-filtered beam  426  from the target object  405 . 
     Accordingly, a portion of the interrogating beam  426  illuminating the target object  405  is reflected back towards the collimator  406  to form a reflected interrogating beam portion  407 , that is collected by the system  400  after being filtered by the narrowband filter  408  to remove ambient light components therefrom. In this example the reflected beam  407  is collected by common optics and diverted to collection arm using a circulator  409 . More specifically, the return interrogating beam portion  407  is passed through the collimator  406  and the waveguide element  433 , into the circulator  409  via its second port b. The circulator  409  is configured to permit passage of beam introduced thereinto via its second port b only through its third port c, which is thereby passed into the waveguide element  427  i.e., the passage from the third port c to the first port a is substantially prevented. 
     In some embodiments the reflected interrogating beam coupled into the waveguide element  427  from the second port b of the circulator  409  may be phase modulated by phase modulator  410  to form phase modulated reflected interrogating beam coupled to the waveguide element  428 . The phase modulation of the reflected interrogating beam coupled to waveguide element  428  may be generated using, for example, but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based phase modulation provided in the phase modulator  410 . 
     The phase modulated reflected interrogating beam passing through the waveguide  428  is split by the beam splitter  411  into a first reflected interrogating beam portion passed through the waveguide element  412 , and a second reflected interrogating beam portion passed through the waveguide element  413 , which are both coupled to the beam splitter  411 . The first reflected interrogating beam passing through the waveguide element  412  is combined at the beam combiner  423  with the second reference beam portion passed through the waveguide element  432 , and therefrom it is projected onto the second detector (B)  420 , to thereby generate the measured signal I long . The second reflected interrogating beam portion passing through the waveguide element  413  is combined at the beam combiner  424  with the first reference beam portion passing through the waveguide element  416 , and therefrom it is projected onto the first detector (A)  431 , to thereby generate the measured signal I short . 
     The measured signals I short  and I long  from the detectors  431  and  420 , are transmitted to be processed by the data acquisition module DAQ  421  (e.g., including one or more processors (CPU) and memories (mem) and/or formed by electronic circuit), as described hereinabove, to form a detection result  440  used for determining the distance of the target object  405  from the distance detection apparatus  400 . By analyzing and comparing the interferences received in each detector, the distance of the target object  405  from the distance detection apparatus  400  can be determined, as described hereinabove, with the required adjustments. 
     As indicated above, this distance measurement technique can be implemented utilizing two or more reference arms (e.g., arms  416 , 417 ), used as paths for the reference beam from the waveguide element  404 . For example, additional splitting of the reference beam from the waveguide element  404  can be used to form a plurality of reference arms associated with a plurality of reference beams of different time delays. The reflected interrogating signal from the waveguide element  427  can thus be split into a corresponding number of beam portions, and thereafter combined with the respective plurality of split reference beam portions, as long as each split reference beam portion is delayed in each reference arm by a different amount (time delay). 
     Additionally, the phase modulator  410  can be alternatively used to modulate the reference beam portion  404  and/or the source electromagnetic beam  425 . 
     A respective plurality of superimposed/combined beams may be respectively produced and directed onto a respective plurality of detectors, to thereby form a plurality of respective measurement signals. The respective measurement signals can be analyzed and processed to provide information indicative of the distance of the target object  405  from the apparatus  400 , and the distance may be determined based on ratios between respective pairs of coherence components of the measurement signals. 
     In some embodiments the apparatus may utilize an additional collection collimator lens, typically in addition to collimator  406 . the additional collection lens may be used to collect the reflected interrogating signal portion  407 , and to couple the reflected interrogating beam portion  407  directly into the waveguide element  427 . This configuration provides parallel collection path. In other embodiments disclosed herein a circulator is used for sake of simplicity (e.g.,  FIG.  7   ), but a different collection mechanism can be similarly used to avoid the need of circulator elements, e.g., using a polarization beam splitter, etc. 
     Another possible distance detection apparatus  500  is schematically illustrated in  FIG.  6   , wherein an electromagnetic beam is used to measure the distance of a target object  505  from the apparatus  500 . The apparatus  500  includes a signal source  501 , a splitting arrangement  543  including beam splitters  502 ,  514  and  533 , beam combiners  511  and  542 , reflector  538 , a detection arrangement  545  formed by at least first and second detectors,  520  (A) and  521  (B) respectively, and a processing unit  539  (also referred to herein as DAQ). 
     In this embodiment the reference and interrogating beams partially propagate in waveguide elements, such as optical fibers, while the reflected interrogating beam is collected and controlled in free space. As seen, output signal of the electromagnetic signal source  501  is coupled to a waveguide element  523  e.g., optical fiber, to form an electromagnetic source beam. Waveguide element  523  direct the source beam to beam splitter  502  splitting the beam into two beam portions, forming an interrogating beam portion passing through the waveguide element  503 , and a reference beam portion passing through the waveguide element  504 . 
     The interrogating beam portion passing through the waveguide element  503  may be substantially collimated by a collimator  506  and directed towards the target object  505 . In some possible embodiments, directing optic arrangements (not shown) may be used for directing the substantially collimated interrogating beam portion  522  towards the target object  505 . The directing optics, and/or additional collecting optics, is used to collect a reflected interrogating beam portion  507 , formed by reflection of the interrogating beam  522  from the target object  505 . 
     As seen, the electromagnetic interrogating beam portion  522  propagates in free space medium (e.g., air) onto the target object  505 . The reflected interrogating beam portion  507  also propagates through the free space medium, but in the reverse direction, and as it is collected by the apparatus  500  it is filtered by a narrowband filter  508  configured corresponding to the bandwidth of the electromagnetic signal source  501  to remove ambient light components therefrom, to thereby form the reflected interrogating band-filtered beam  532 . The reflected interrogating beam  507  is passed through the optical system  509  configured to control (focus and/or collimate) the reflected interrogating beam  507 . The reflected interrogating beam portion  532  may be phase modulated using, for example, but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, of the phase modulator  510 , to thereby form a reflected interrogating phase modulated beam  534  propagating through free space medium. 
     The reflected interrogating phase modulated beam  534  is split by the beam splitter  533  into two portions: (i) a first reflected interrogating phase modulated beam portion  536 ; and (ii) a second reflected interrogating phase modulated beam portion  535 , both of which propagate through free space medium in different (e.g., orthogonal) directions. The first portion  536  of the reflected interrogating phase modulated and band-filtered beam is directed into the beam combiner  511 , wherefrom it is reflected as the beam  525  through free space medium towards the first detector (A)  520 . 
     In this particular and non-limiting example illustrated in  FIG.  6   , the first reflected interrogating phase modulated and band-filtered beam portion  536  is directed by the reflector (e.g., mirror)  538 , thereby forming the reflected beam portion  537  directed by the reflector  538  through free space into the beam combiner element  511 , to thereby form the beam  525  projected onto the first detector (A)  520 . The second reflected interrogating phase modulated and band-filtered beam portion  535  also propagates through free space medium, and it is reflected by the beam combiner  542  through free space medium as the beam  527  projected onto the second detector (B)  521 . 
     The reference beam portion passing through waveguide element  504  is used as a reference to the interrogating beam  507  reflected from the target object  505 . The reference beam from the wave guide element  504  is split by the beam splitter  514  into a first reference beam portion passed through the waveguide element  516 , and a second reference beam portion passed through the waveguide element  515 . The first and second reference beams, passing through the waveguides  516  and  515  respectively, are set to propagate along two reference arms, each having a different time delay, a short delay reference arm and a long delay reference arm, respectively. 
     According to this non-limiting example the first reference beam portion passing through the waveguide element  516  is set to propagate through the short reference arm and to be substantially collimated by a collimator  518 , to form the first collimated reference beam  530  passing through free space medium. The first collimated reference beam  530  passes through the beam splitter  511  and directed therefrom as the first collimate reference beam  528  projected onto the first detector (A)  520 . The first reflected interrogating phase modulated beam portion  525 , and the first collimated reference beam  528  are superposed on detector (A)  520 , to thereby generate the detector signal I short . 
     The second reference beam portion passing through the waveguide element  515  is delayed by the additional distance L using a waveguide element  517 , and thereafter it is collimated by a collimator  519 . The collimated second reference beam from the collimator  519  is passed through free space medium as the collimated second reference beam  531 , and thereafter it is passed through the beam combiner  523 . The collimated second reference beam propagates from the combiner  523  as the beam  529  is projected onto the second detector (B)  521 . The second reflected interrogating phase modulated beam portion  527 , and the collimated second reference beam  529  are superposed on the second detector (B)  521 , to thereby generate the detector signal I long . 
     The superposed signals, I short  and I long , can be measured electrically using the first and second detectors  520  and  521 , respectively, and analyzed by DAQ  539 . The DAQ  539  may include one or more processors (CPU) and memories (MEM) configured and operable for analyzing and comparing the interferences received in each detector and determine the distance of the target object  505  from the range detection apparatus  500 , as described above with reference to  FIGS.  2 ,  3  and  4   . 
     In some possible embodiments known balanced detection techniques may be implemented using a balanced detectors arrangement e.g., as described in Stierlin, R. et al,  Excess - noise suppression in a fibre - optic balanced heterodyne detection system , Opt Quant Electron 18, 445-454 (1986). According to such embodiments, an additional detection arrangement  544  including detectors, (A′)  540  and (B′)  541 , is used, wherein detectors (A)  520  and (A′)  540  form a first balanced detector, and detectors (B)  540  and (B′)  541  form a second balanced detector. In this case the short arm reference beam  530  is split by the beam splitter/combiner  511  into first short arm signal portion  528  and second short arm signal portion  530 ′, and the long arm reference beam  531  is split by the beam splitter/combiner  542  into first long arm signal portion  529  and second short arm signal portion  531 ′. The beam portions  525  and  528  are superposed on the detector (A)  520 , the beam portions  527  and  529  are superposed on the detector (B)  521 , the beam portions  525 ′ and  530 ′ are superposed on the detector (A′)  540 , and the beam portions  527 ′ and  531 ′ are superposed on the detector (B′)  541 . The signals detected by detectors  540  and  520  are of opposite phase and respectively, the signals detected by detectors  541  and  521  are also of opposite phase, thus, the cross-correlation term for each pair of detectors is of opposite sign. The outputs of the detectors (A′)  540  and (B′)  541  are electrically connected to the electrical signal outputs of the detectors (A)  520  and (B)  521 , respectively, in a balanced configuration to form a differential detection signal (e.g., detected signal in detector  541  minus detected signal in detector  521 ). Thus, the detection of the I short  signal includes detection of the cross-correlation term (with a factor of two) while the noise and the constant (distance independent) terms are filtered. In the same manner for the I long  signal associated with detectors (B′)  541  and (B′)  521 , the detected signal includes detection of the cross-correlation term (with a factor of two) while the noise and the constant (distance independent) terms are filtered out. This way the constant (distance independent) terms of Equations (4) and (5) and the backlight/ambient noise components can be efficiently filtered/cancelled out from the superposed signals, I short  and I long  received by the DAQ  539 . It should be noted that such additional detection arrangement  544  can be similarly used in all of the other embodiments disclosed hereinabove and below. 
     In possible embodiments the range detection apparatus  500  is implemented with more than two reference arms e.g., in addition to the reference arms formed using the waveguide elements  516  and  515 , to provide a plurality of reference signals (not shown) split from the reference beam passed through the waveguide element  504 . The reflected interrogating beam  507  is thus split into a respective plurality of reflected interrogating beam portions (not shown). Each of the plurality of reference beam portions can be delayed along a respective reference arm by a different time delay. Accordingly, the plurality of split interrogating beams reflected from the target object  505  may be combined respectively with the plurality of reference beam portions of different time delays and directed onto a plurality of respective detectors to form a plurality of respective detected signals. The respective detected signals can be analyzed and processed to provide information indicative of the distance of the target object  505  from the apparatus  500 , and the distance may be determined based on ratios between respective pairs of coherence components of the detected signals. 
     In some possible embodiments additional phase modulators (not shown) can be alternatively, or additionally, used to modulate the reference beam portion from the waveguide element  504  and/or the electromagnetic source beam from the waveguide element  523 . 
     Turning to  FIG.  7   , a further exemplary embodiment of a distance measurement apparatus  600  is schematically illustrated.  FIG.  7    schematically illustrates a configuration in which a single detector  617  is used, with a plurality of reference arms having different lengths, and a reference arm selection\switch mechanism  613 . The apparatus  600  includes a signal source  601 , a splitting arrangement  629  formed by at least beam splitter  602 , beam combiner  616 , and circulators  609  and  615 , a detection arrangement including detector  617 , and a processing unit  618  (also referred to herein as DAQ). In this specific and non-limiting example two (long and short) reference arms are used, and the shifting between these different delay reference arms is implemented using a Micro-Electro-Mechanical-System (MEMS e.g., optical MEMS) based switch. 
     The distance detection apparatus  600  utilizes an electromagnetic signal source  601 , configured to generate beams having a finite coherence length, coupled to a waveguide element  619  for passage of the source beam therethrough. The waveguide elements used in this non-limiting example may be implemented by optical fibers. The electromagnetic beam produced by the signal source  601  passed through the waveguide element  619 , is split by the beam splitter  602  into two portions: an interrogating beam portion passed through the waveguide element  603 , and a reference beam portion passed through the waveguide element  604 . The interrogating beam portion from the waveguide element  603  is passed through a three-port circulator element  609  to the collimator  606 . The three-port circulator element  609  is configured to receive the reference beam from the waveguide element  603  via its first port a, and enable passage thereof via its second port b to the waveguide element  620 , while substantially preventing its passage through the third port c. 
     The interrogating beam passed through the waveguide element  620  is then collimated by the collimator  606 . The collimated interrogating electromagnetic beam from the collimator  606  is passed through a narrowband filter  608  to form a substantially collimated interrogating beam  621 . The collimated interrogating beam  621  propagates through free space medium towards the target object  605  to measure its distance from the apparatus  600 . 
     A portion of the collimated interrogating beam  621  illuminating the object  605 , is reflected towards the apparatus  600  to form reflected beam  607 . In some embodiments the reflected interrogating beam  607  is collected by collection optics (not shown). The reflected interrogating beam  607  collected by the system  600  can be filtered using the narrowband filter  608  configured to remove ambient light components therefrom, and it is then introduced into the waveguide element  620  by the collimator  606 . The reflected interrogating beam passed through the waveguide element  620  is directed into the second port b of the three-port circulator  609 . The three-port circulator  609  is configured to permit the passage of the reflected interrogating beam introduced thereinto via the second port b through its third port c, while substantially preventing its passage through its first port a, to thereby pass the reflected interrogating beam into the waveguide element  622 . The reflected interrogating beam passed through the waveguide element  622  may be phase modulated using, for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, applied by phase modulator  610  positioned along the waveguide  622 . The reflected interrogating beam is passed through the waveguide element  623  to the beam combiner  616 , which output is projected onto the detector (A)  617 . 
     The reference beam portion is directed by the waveguide element  604  into the first port a of the three-port circulator element  615 . The three-port circulator  615  is configured to permit passage of the reference beam introduced thereinto via its first port a through its second port b, while substantially preventing its passage via its third port c, to thereby pass the reference beam into the waveguide element  625 , from which it is directed to the switch mechanism  613 . In this specific and non-limiting example, the switch mechanism  613  has two states, 1 and 2, configured to selectively couple the waveguide element  625  with a short reference arm ( 612 ), or with a long reference arm ( 611 ), respectively. 
     When the switch mechanism  613  is in state 1, the reference beam is guided into the short reference arm, wherein the reference beam portion is passed through the waveguide element  612 , wherefrom it is backreflected by a reflector (e.g., mirror or retroreflector)  627 . The reference beam thereby propagates from the reflector  627  backwardly through the waveguide element  612  and the switch mechanism  613 , and therefrom it is redirected backwardly though the waveguide element  625  as a short-delay reference beam portion into the third port c of the circulator  615 . The circulator  615  is configured to receive the short-delay reference beam portion introduced thereinto via its third port c and permit passage thereof via its second port b, while substantially preventing its passage via its first port a. The short-delay reference beam portion is directed by the circulator  615  along the waveguide element  624  to the beam combiner  616 . The short-delay reference beam portion passed through the waveguide element  624  is combined in the beam combiner  616 , which projects its output onto the detector (A)  617  to generate the short-delay detection signal I short . 
     When the state of the switch mechanism  613  is changed into state  2 , the reference beam portion passing through the waveguide element  625  from the circulator  615  is introduced into the long reference arm  611 , wherein the reference beam portion is delayed by the additional distance L formed by the waveguide element  626  of the long reference arm. The reference beam portion is backreflected by a reflector (e.g., mirror or retroreflector)  614 . The back-reflected reference beam portion propagates backwardly through the waveguide element  628  to the delay line  626  and the switch mechanism  613 , wherefrom it is directed as long-delay reference beam portion into the waveguide element  625 . The long-delay reference beam portion is directed by the waveguide element  625  into the third port c of the circulator  615 , which permits passage thereof via its second port b into the waveguide element  624 , while substantially preventing its passage through the first port a. The long-delay reference beam is directed by the waveguide element  624  to the beam combiner  616 , which projects its output beam onto the detector (A)  617  for generating the long-delay detection signal I long . 
     The state of the switch mechanism may be controlled by a control signal, e.g., the transistor-transistor logic (TTL) signal, and/or by control signals generated by the DAQ  618 . 
     The short-delay, or long-delay, reference beam portion supplied by the waveguide element  624  is combined on the beam combiner  616  with the returned interrogating beam portion passed by the waveguide element  623  to the combiner  616 . In states (1 and 2) of the switch mechanism  613  the back-reflected reference beam portion passed through the waveguide element  624 , and the returned interrogating beam portion passed through the waveguide element  623 , are superposed by the beam combiner  616 . The superposed beam portions are directed by the combiner  616  onto the detector (A)  617  to generate the detection signal (I short  or I long ) supplied to the DAQ  618  for processing and analysis. The DAQ  618  may include one or more processors (CPU) and memories (MEM) configured and operable to analyze and compare the interferences in the received I short  and I long  detection signals corresponding to the short ( 612 ) and long ( 611 ) reference arms, respectively, and determine therefrom the distance of the target object  605  from the apparatus  600 , as described hereinabove with reference to  FIGS.  2 ,  3  and  4   . More specifically, the detector  617  generates temporal detection data having periods associated with I short  and periods associated with I long  in accordance with temporal switching of switch mechanism  613 . The DAQ  618  utilizes time switching pattern of the switch mechanism  613  for processing of detector data to determine I short  and I long  measures and to determine distance to the target as described above. 
     As indicated above, in this embodiment as well as in other embodiments the apparatus  600  can be implemented with more than two reference arms,  612  and  611 , utilizing a modified switch mechanism  613  having more than two states. In such embodiments the reference signal portion passed through the waveguide element  604 , can be split into any number of beam reference portions, as long as each split reference signal portion is delayed by a different amount\time delay. Using suitable control signals (TTL), the modified switch mechanism can be used to interchange between a plurality of reference arms, each having the different delay time, and to collect the back-reflected beams from each of the plurality of the reference arms. This way, back-reflected reference beams can be sequentially introduced via the waveguide element  625 , the circulator  615  and the waveguide element  624 , to the combiner  616 , for combining them therein with the return interrogating beam  607  from the target object  605 . The detector  617  correspondingly generates a respective sequence of detection signals indicative of the superposition of each of the back-reflected reference beams from the different reference arms (according to the state of the switch mechanism) with the reflected interrogating beam. The DAQ  618  can be used to process the sequence of signals measured by the detector  617  and determine therefrom the distance of the target object  605  from the apparatus  600  based on ratios between pairs of separated signal components of the measured signals, as described hereinabove with reference to  FIGS.  2 ,  3  and  4   . 
     Additionally or alternatively, one or more phase modulators may be used to modulate the reference beam portion passed through the waveguide element  604 , and/or through the waveguide element  625 . This may be implemented over both waveguide and free-space medium, or combinations thereof, such as illustrated in  FIGS.  2 ,  5 , and  6    and described hereinabove. 
       FIG.  8    schematically illustrates a distance measurement apparatus  700  utilizing variation of coherence length of the interrogating beam for determining distance of a target. As shown in  FIG.  8   , apparatus  700  includes an electromagnetic signal source  701  having a variable coherence length integrated with an interferometric system, used to measure a distance between the target object  704  and the apparatus  700 . The apparatus  700  includes the signal source  701 , a splitting arrangement  720  formed by at least beam splitter  702  and reflector  707 , a detection arrangement including detector  708 , and a processing unit  709  (also referred to herein as DAQ). In this specific and non-limiting example, a finite (and variable) coherence length signal source  701  (or  701 + 719 ) is used to generate a source electromagnetic beam  710  propagating therefrom through free space medium, which may have a variable coherence length that may be stabilized at a plurality of selected coherence lengths. The variable coherence length setting unit  719  may be integrated in the electromagnetic signal source  701 , or alternatively, it may be an external component configured to provide the required coherence length control over the beams produced by the electromagnetic signal source  701 . 
     The apparatus  700  includes a beam splitter  702  configured to split the source beam  710  into an interrogating beam portion  703 , and a reference beam portion  706 . The interrogating beam portion  703  is directed (e.g., using beam directing collimating/focusing optics) through free space medium towards the target object  704 , and the reflected interrogating beam portion  711 , reflected from the target object  704 , is collected by the system  700  (e.g., utilizing collection optics that may be similar to, or different from, the directing optics). The reflected interrogating beam  711  received in the apparatus  700  may be phase modulated using, for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, of the phase modulator  705 . The modulated reflected interrogating beam  712  is directed by the beam splitter  702  as interrogating beam  715  onto the detector (A)  708 . 
     The reference beam portion  706 , split by the beam splitter  702 , is used as a reference beam as it is back-reflected by the reflector (e.g., mirror)  707  to form the back-reflected reference beam  713 . The back-reflected reference beam  713  is directed through the beam splitter  702  to form the reference beam  714  directed onto the detector (A)  708 . The reference beam  714  from the splitter  702  is superposed in the detector  708  with the reflected interrogating beam  715 , to generate the detector signal I c  received in the DAQ  709  for analysis and processing. 
     In some possible embodiments the variable electromagnetic signal source  701 / 719  may be stabilized/set consecutively about two different coherence lengths c 1  and c 2 , to thereby generating corresponding detector signals, I c1  and I c2  at the detector (A)  708  at selected time pattern. The detector signals are transmitted to the DAQ  709 . In some embodiments the DAQ  709  may include one or more processors (CPU) and memories (MEM) configured and operable to process and analyze the detector signals, I c1  and I c2 , and determine therefrom the distance Δx′ between the target object  704  and the apparatus  700 . 
     With a stabilized coherence length lc 1 , a signal I c1  measured at the detector A  708  can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       c 
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     [ 
                     
                       
                         I 
                         R 
                       
                       + 
                       
                         
                           η 
                           s 
                         
                         ⁢ 
                         
                           I 
                           s 
                         
                       
                       + 
                       
                         2 
                         · 
                         
                           
                             
                               I 
                               R 
                             
                             ⁢ 
                             
                               η 
                               s 
                             
                             ⁢ 
                             
                               I 
                               s 
                             
                           
                         
                         · 
                         
                           cos 
                           ⁡ 
                           ( 
                           
                             φ 
                             1 
                           
                           ) 
                         
                         · 
                         
                           e 
                           
                             
                               
                                 - 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       k 
                                       1 
                                       2 
                                     
                                   
                                   8 
                                 
                               
                               · 
                               Δ 
                             
                             ⁢ 
                             
                               x 
                               ′2 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     where I R , I s  are the power intensities of the reference beam portion  714  and of the interrogating beam portion  711 , respectively, η s  is the effective collection efficiency of the reflected interrogating beam  711 , thus the power intensity of the reflected interrogating beam  715  received from the object  704 , and collected at the detector A  708 , is I s ·η S , φ is the relative phase between the reference arm  714  and the reflected interrogating signal arm  715  arriving at detector (A)  708 , Δk 1  is the linewidth of the signal source  701  with coherence length lc 1 , and Δx′=x−x 0 . Alternatively, in some possible embodiments I s  may be used to define the interrogating beam portion  703 , and in this case, η s  is used to define the reflectivity of the target object  704 , the backpropagation in the free space medium, the coupling efficiency of the apparatus  700 , and the coupling ratio to the detector  708  (as also mentioned hereinabove with respect to Equation (4)). 
     The coherence length of the signal source  701  may be changed by the coherence length setting unit  719  to a second coherence length lc 2 , for measurement of the respective detector signal I c2 , while the target is substantially at the same position. In this operational condition the signal measured at the detector  708  corresponding to the second coherence length may be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     
                       c 
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     [ 
                     
                       
                         I 
                         R 
                       
                       + 
                       
                         
                           η 
                           s 
                         
                         ⁢ 
                         
                           I 
                           s 
                         
                       
                       + 
                       
                         2 
                         · 
                         
                           
                             
                               I 
                               R 
                             
                             ⁢ 
                             
                               η 
                               s 
                             
                             ⁢ 
                             
                               I 
                               s 
                             
                           
                         
                         · 
                         
                           cos 
                           ⁡ 
                           ( 
                           
                             φ 
                             2 
                           
                           ) 
                         
                         · 
                         
                           e 
                           
                             
                               
                                 - 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     
                                       k 
                                       2 
                                       2 
                                     
                                   
                                   8 
                                 
                               
                               · 
                               Δ 
                             
                             ⁢ 
                             
                               x 
                               ′2 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     where Δk 2  is the linewidth of the signal source  701 / 719  with coherence length of lc 2 . 
     According to this specific and non-limiting example, the following assumptions and estimations may be made in order to determine the distance Δx′ of the target object  704  from the apparatus  700 : (i) the intensity of the reference arm I R1  is well-known and relatively constant in time, or can be measured/determined, and (ii) the phases φ 1 , φ 2  can be adjusted actively, or passively, assuming the phase does not change between the measurements lc 1  and lc 2  and that the phase is constant in time. 
     By eliminating/filtering from the signals the part that is associated with the first two terms of the RHS of equations (9) and (10), and computing the ratio between the coherence terms of the two respective measurements (e.g., by the DAQ  709  or using an analog divider unit), the value Γ′ indicative of the ratio between the respective coherence terms may be determined as follows: 
     
       
         
           
             
               
                 
                   
                     Γ 
                     ′ 
                   
                   
                     = 
                     Δ 
                   
                   
                     
                       
                         C 
                         ⁢ 
                         o 
                         ⁢ 
                         
                           rr 
                           ⁡ 
                           ( 
                           
                             Ic 
                             1 
                           
                           ) 
                         
                       
                       
                         C 
                         ⁢ 
                         o 
                         ⁢ 
                         
                           rr 
                           ⁡ 
                           ( 
                           
                             Ic 
                             2 
                           
                           ) 
                         
                       
                     
                     = 
                     
                       
                         
                           e 
                           
                             
                               - 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     k 
                                     1 
                                     2 
                                   
                                 
                                 8 
                               
                             
                             ⁢ 
                             
                               
                                 ( 
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     x 
                                     ′ 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                         
                           e 
                           
                             
                               - 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     k 
                                     2 
                                     2 
                                   
                                 
                                 8 
                               
                             
                             ⁢ 
                             
                               
                                 ( 
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     x 
                                     ′ 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                       = 
                       
                         e 
                         
                           
                             
                               Δ 
                               ⁢ 
                               
                                 x 
                                 ′2 
                               
                             
                             8 
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 Δ 
                                 ⁢ 
                                 
                                   k 
                                   2 
                                   2 
                                 
                               
                               - 
                               
                                 Δ 
                                 ⁢ 
                                 
                                   k 
                                   1 
                                   2 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     Here again, there is one to one relation between the calculated value Γ′ and the distance Δx between the target object  704  and the apparatus  700 , which can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     Δ 
                     ⁢ 
                     
                       x 
                       ′ 
                     
                   
                   = 
                   
                     
                       
                         8 
                         · 
                         
                           ln 
                           ⁡ 
                           ( 
                           
                             Γ 
                             ′ 
                           
                           ) 
                         
                       
                       
                         ( 
                         
                           
                             Δ 
                             ⁢ 
                             
                               k 
                               2 
                               2 
                             
                           
                           - 
                           
                             Δ 
                             ⁢ 
                             
                               k 
                               1 
                               2 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     It is noted that although this specific and non-limiting example utilizes a variable coherence length source propagating in a free-space medium, in other possible embodiment, however, with the required changes, the apparatus  700  can be similarly implemented utilizing waveguides (e.g., optical fibers) for propagating the beams in accordance with the above examples. 
     Additionally, in some possible embodiments one or more phase modulators can be also placed in the reference arm  706 , and/or the in the path of the source electromagnetic beam  710 . 
     This technique method can be similarly implemented utilizing a plurality different electromagnetic beam sources configured (e.g., Source 1, Source 2 . . . , Source n where n&gt;2 is an integer) for creating a full 3D image of one or more target objects (not shown), for example, but not limited to, utilizing a vertical cavity surface emitting laser (VCSEL) array, a full field reference mirror  707  and a respective array of detection units. This way, multiple points can be imaged utilizing a proper imaging system (using the full field mirror) and a respective array of detection units for each beam of the apparatus. 
     Reference is now made to  FIG.  9    schematically illustrating another possible embodiment of a distance measurement apparatus  800 , using multiple signal sources. In this example the apparatus  800  includes two signal sources  801 ,  802 , however two or more signal sources may be used. The signal sources  801  and  802  have different coherence lengths . The apparatus  800  further include a splitting arrangement  830 , generally formed by beam splitters  803  and  804 , configured to split the source beams to respective interrogating portions  805  and  806  and reference portions  810  and  811 . The reference beams  810  and  811  are directed to propagate in corresponding reference arms respectively defined by reflectors  812  and  813 . A detection arrangement  829  including detectors  814  and  815  is used for detecting intensity of interfered (superpose) signals as described above and provide detection data to a processing unit  816  (also referred to herein as DAQ). 
     In this exemplary configuration, apparatus  800  utilizes two or more interrogating beams having corresponded two or more coherence lengths, and corresponding reference beams, typically of similar length. In some configurations, the reference beams may be configured to propagate within a common reference arm and utilize a wavelength selective filter for splitting the collected reflected beams and reference beams to the respective detectors  814  and  815 . 
     In this specific and non-limiting example two electromagnetic signal sources,  801  and  802 , with two different coherence lengths, lc 1  and lc 2 , respectively, are generating two respective electromagnetic beam,  818  and  817  propagating through free space medium, each of which is split into two portions by the beam splitters,  803  and  804 , respectively, to form respective two interrogating beam portions,  805  and  806 , and two respective reference beam portions,  810  and  811 . The first interrogating beam portion  805 , and the second interrogating beam portion  806 , are directed through the free space medium (e.g., using separate, or common, directing optics—not shown) towards the target object  809 . Respective portions of the first and second interrogating beams,  819  and  820 , are received by the apparatus  800  e.g., utilizing separate collection optics (not shown), and/or by a common optical arrangement, in which case the signal sources may be optionally operated separately in time. The reflected interrogating beams,  819  and  820 , may optionally be phase modulated using, for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, of the phase modulators,  807  and  808 , respectively. The collected and optionally phase modulated reflected interrogating beams,  821  and  823 , are reflected by the beam splitters,  803  and  804 , respectively, and the reflected beams portions,  822  and  824 , are projected onto the detectors,  814  and  815 , respectively. 
     The reference beam portions,  810  and  811 , propagate towards reflectors (e.g., mirrors)  812  and  813 , respectively, thereby forming respective reflected reference beam portions,  825  and  827 . The reflected reference beam portions,  825  and  827 , pass through the beam splitters,  803  and  804 , to form the back-reflected reference beam portions,  826  and  828 , that are projected onto the detectors,  814  and  815 , respectively. 
     The reflected interrogating beam portions,  822  and  824 , are superposed with the back-reflected reference beam portions,  826  and  828 , at the beam splitters,  803  and  804 , and detected by the detectors, (A)  814  and (B)  815 , respectively. The electrical signals, I a  and I b , measured by the detectors, (A)  814  and (B)  815 , respectively, are provided to the DAQ  816  for processing and analysis. The DAQ  816  may include, in some embodiments one or more processors (CPU) and memories (MEM) configured and operable to analyze and compare the interferences in the received electrical signals, I a  and I b , corresponding to different coherence lengths lc 1 ,lc 2 , for determining the distance between the target object  809  and apparatus  800 , as described hereinabove using Equation (9) to equation (12). 
     It is noted that embodiments according to this specific and non-limiting example can be implemented with more than two electromagnetic signal sources, as long as the coherence length of each electromagnetic signal source is different, and this example can duplicate many times for forming a full 3D image (e.g., utilizing a suitable scanning mechanism). It is also noted that though the apparatus in this example is implemented using free space medium to propagate the beams. In other possible embodiment, however, a similar apparatus including a plurality of electromagnetic signal sources having different coherence lengths may be similarly implemented utilizing waveguides (e.g., optical fibers), or combinations of waveguides and free space medium, to propagate the beams. Additionally, in yet another configuration the apparatus  800  can be implemented using a single detector, and an optical switch that may be used to switch between a plurality of signal sources having different coherence lengths, each source used to generate measurement at a dedicated time slot. Using pairs of such measurements performed in different time slots corresponding to respective different coherence lengths can be used to calculate the distance between the target object  809  and the apparatus  800 , as described hereinabove. 
     Additionally, in possible embodiments, additional phase modulators can be alternatively, or additionally, used to modulate the reference beam portions,  810  and  811 , or the electromagnetic source beams,  818  and  817 . In this example, the lengths of the reference arms can be of equal (or not equal) lengths, depending on the specific design requirements. 
     In other possible embodiments, the distance measurement (or cross-sectional object characterization) apparatus can be configured to include a wavelength disperser, for example but not limited to, fiber Bragg grating array (FBGA), as illustrated in  FIG.  10   . The wavelength disperser can be used in the optical/IR regime, but a similar implementation can be made in the RF regime utilizing frequency-dependent transmitting or reflecting components. 
     The distance measurement apparatus  900  in  FIG.  10    includes a broadband signal source  901  configured to transmit an electromagnetic beam, which passes through an isolator  915 , the output of which is coupled to the waveguide element  913 . The apparatus  900  includes the signal source  901 , a splitting arrangement formed by at least one beam splitter  902  and wavelength disperser  907 , a detection arrangement including spectrometer  908 , and a processing unit  909  (also referred to herein as DAQ). The waveguide elements shown in  FIG.  10    may be implemented by, but not limited to, optical fibers. The apparatus  900  includes a beam splitter  902 , configured to split the broadband beam passed through the waveguide element  913  into two portions; a broadband interrogating beam portion passing through the waveguide element  903 , and a broadband reference beam portion passing through the waveguide element  906 . 
     The broadband interrogating beam portion passed through the waveguide element  903 , can be directed through free space medium by a directing optics  905  (e.g., including collimation/focusing elements) towards the target object  904 , thereby forming collimated broadband interrogating beam  912 . The collimated broadband interrogating beam  912  illuminates the target object  904 , and broadband reflected interrogating beam portion  911  is received in the apparatus  900  e.g., by the same directing optics  905 . Alternatively, the collimated broadband reflected interrogating beam portion  911  may be collected by specific collection optics (not shown). 
     In this specific and non-limiting example, the collimated broadband reflected interrogating beam  911  is coupled into the waveguide element  903  using directing optics (e.g., collimator  905 ), that propagates towards the beam splitter  902 . A portion of the interrogating reflected beam that passes the beam splitter  902  is directed through the waveguide element  910  towards the spectrometer  908 . The broadband reference beam portion passed through the waveguide element  906 , is optionally phase modulated using, for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, of the phase modulator  913 . The modulated broadband reference beam portion from the phase modulator  913  can be passed through waveguide element  914  into a number of wavelength-dependent reflectors, where each group of the wavelength is time delayed by different amount i.e., due to the broad spectrum of the signal source  900 , wavelength-sensitive reflectors can be placed in the optical path such that each wavelength-sensitive reflector is positioned at a different location (e.g., as can be achieved utilizing Bragg grating element. This way, each group of wavelengths is reflected from a different position and thereby accumulates a different OPL. 
     This process can be carried out using a fiber Bragg grating array (FB GA) element to implement the wavelength disperser  907 , where each group of wavelengths is reflected back by a specific Bragg grating located at a different position along the fiber. Thus, N Bragg gratings along the FBGA element form N wavelength groups/beam portions associated with a specific wavelength band Δλ i , (i=1, . . . , N) (where N&gt;1 is an integer). Each wavelength beam portion introduced into the FBGA element  907  experiences a different time delay corresponding to a different optical path L i  caused by the i-th FB GA. The reflected beam that is backreflected by the FBGA element  907  into the waveguide element  914  constitutes a combined reference beam formed of N reflected, separately delayed beam portions (wavelength groups). The combined reference beam backreflected through the waveguide element  914  is optionally additionally modulated using the phase modulator  913 . The back-reflected and optionally modulated combined reference beam is passed through the waveguide element  906 , in which it propagates back towards the beam splitter  902 . 
     The beam splitter  902  receives and combines the broadband reflected interrogating beam  911  (passed via waveguide element  903 ) and the broadband combined reference beam (passed via waveguide element  906 ) and directs them, superimposed, towards the spectrometer module  908 . Specifically, the broadband combined reference beam passed through the waveguide element  910  is formed by N superimposed beam portions, each having a separate wavelength sub-band Δλ i , (i=1, . . . , N) and a different time delay due to length difference L i , (i=1, . . . , N) associated therewith. The broadband reflected interrogating beam portion is superimposed with the combined broadband reference beam passed through the waveguide element  910 . 
     The spectrometer  908  may include a wavelength dependent beam splitter (not shown e.g., a grating) and wavelength dependent signal detector (not shown), configured to separately measure each of the wavelength groups. The detected signals I i  generated by the spectrometer  908  are transferred to the DAQ  909  for processing and analysis. In some embodiments, the DAQ  909  includes one or more processors (CPU) and memories (MEM) configured and operable to determine the distance ΔX between the apparatus  900  and the target object  904 , using any of the techniques described herein. 
     This way, splitting the combined beam into N wavelength bands facilitates identification of N wavelength dependent wave groups in the reflection of the interrogating beam that are passed through the waveguide element  910 . The reflected wave groups are respectively combined with N different reference wavelengths (and time delay) dependent optical arms of the reference beam. The effective different wavelength dependent optical arms form N wavelength dependent interferences, each interference is the interference of the superimposed i-th component of the reflected interrogating beam portion with the i-th reference beam portion, forming each of the wavelength bands. 
     In accordance with Equation (4) hereinabove, with the required changes, the detected signal power at the i-th detected wavelength band I i , (i=1, . . . , N) can be expressed as follows: 
     
       
         
           
             
               
                 
                   
                     I 
                     i 
                   
                   = 
                   
                     
                       1 
                       4 
                     
                     [ 
                     
                       
                         I 
                         
                           R 
                           ⁢ 
                           i 
                         
                       
                       + 
                       
                         
                           η 
                           
                             s 
                             ⁢ 
                             i 
                           
                         
                         ⁢ 
                         
                           I 
                           
                             s 
                             ⁢ 
                             i 
                           
                         
                       
                       + 
                       
                         2 
                         · 
                         
                           
                             
                               I 
                               
                                 R 
                                 ⁢ 
                                 i 
                               
                             
                             ⁢ 
                             
                               η 
                               s 
                             
                             ⁢ 
                             
                               I 
                               
                                 s 
                                 ⁢ 
                                 i 
                               
                             
                           
                         
                         · 
                         
                           cos 
                           ⁡ 
                           ( 
                           
                             φ 
                             i 
                           
                           ) 
                         
                         · 
                         
                           e 
                           
                             
                               - 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                     k 
                                     i 
                                     2 
                                   
                                 
                                 8 
                               
                             
                             · 
                             
                               
                                 ( 
                                 
                                   
                                     Δ 
                                     ⁢ 
                                     x 
                                   
                                   - 
                                   
                                     L 
                                     i 
                                   
                                 
                                 ) 
                               
                               2 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     where Δx is the distance to the target, and where, with respect to the i-th wavelength band, I Ri  denotes the power intensity directed to the respective reference arm with respect to the i-th wavelength band, I Si  denotes the power intensity directed to the reflected interrogating beam  911  with respect to the i-th wavelength band, η s  denotes the effective coupling efficiency of the reflected interrogating beam  911  at the spectrometer  908 , φ i  denotes the relative phase between the respective reference signal I Ri , and the reflected interrogating beam I si  with respect to the i-th wavelength band, L i  denotes the relative length delay of the respective reference signal for the i-th wavelength band with respect to the position of the i-th FBGA element, and Δk i  denotes the bandwidth reflected by the ith FBG element. 
     Though the apparatus  900  is illustrated using a fiber-based system for simplicity it can also be implemented in a free-space based system e.g., by utilizing diffraction gratings instead of the FBGA. 
     In some possible embodiments a distance measurement apparatus  1000  disclosed herein and illustrated in  FIG.  11   , is operated utilizing a swept-source  1001  instead of a broadband source (like  901  in  FIG.  10   ). The apparatus  1000  includes the signal source  1001 , a splitting arrangement formed by at least one beam splitter  1002  and wavelength disperser  1007 , a detection unit  1008 , and a processing unit  1009  (also referred to herein as DAQ). The detection unit may utilize a spectrometer or wavelength sensitive detector enabling selective detection of light of the N wavelength bands used. 
     Each beam generated by the swept-source  1001  has a narrow-bandwidth centered about a central wavelength that can be changed at different time instances (swept). The distance measurement apparatus  1000  may further include a wavelength disperser, for example, but not limited to, fiber Bragg grating array (FBGA), to be used in the optical/IR regime. If the signals produced by the swept source are RF signals, a frequency-dependent transmitter (not shown) coupled to a frequency dependent reflector component can be used instead of the FBGA. This way, the time delay obtained for each beam produced by the swept source  1001  is dependent on the instantaneous central wavelength of the produced beam (due to the FBGA wavelength sensitivity). 
     In this specific and non-limiting example, a plurality of signals, each associated with a certain distance between the apparatus  1000  and a target object  1004 , are sequentially generated by the swept signal source  1001 . A corresponding plurality of references beams signals having different wavelength bands and respective time delays are generated and superimposed with interrogating beams signals of respective central wavelengths reflected from the target object  1004  to produce wavelength dependent interference signals indicative of the distance between the target object  1004  and the apparatus  1000 . 
     The swept-source  1001  may have a relatively narrow linewidth for transmitting a wavelength-swept electromagnetic beam passed into the waveguide element  1014  after passing through the isolator  1015 . The waveguide elements of apparatus  1000  may be implemented by, for example, optical fibers. The wavelength-swept beam passed through the waveguide element  1014  is split by the beam splitter  1002  into two portions: a wavelength-swept interrogating beam portion passed into waveguide element  1003 ; and a wavelength-swept reference beam portion passed into waveguide element  1006 . The wavelength-swept interrogating beam portion from the waveguide element  1003  may be directed by directing optics (e.g., using a collimator/focusing means)  1005  through free space medium towards the target object  1004 , thereby forming a wavelength-swept interrogating collimated beam  1012 . The wavelength-swept electromagnetic interrogating collimated beam  1012  illuminates the target object  1004 , and a reflected portion thereof  1011  is received in the apparatus  1000  e.g., by the same directing optics  1005 . Alternatively, or additionally, the reflected wavelength-swept interrogating beam  1011  may be collected by specific collection optics (not shown). 
     In this specific and non-limiting example, the reflected wavelength-swept interrogating beam  1011  is coupled into the waveguide element  1003  using the collimator  1005 , to form the reflected interrogating beam propagated towards the beam splitter  1002 . A portion of the wavelength-swept reflected interrogating beam  1011  that passes through the beam splitter  1002  is passed through the waveguide element  1017  to form the beam projected therefrom onto the detector (Det)  1008 . The wavelength-swept reference beam portion passed through the waveguide element  1006  is optionally phase modulated using, for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, of the phase modulator  1013 . The modulated reference beam from the phase modulator  1013  is passed through the waveguide element  1016  and directed into, and backreflected by, the wavelength disperser (FBGA element)  1007 . The reflection obtained from the FBGA element  1007  is dependent on the instantaneous central wavelength of the beam produced by the swept-source  1001 , and therefore the time delay thereby obtained is wavelength-dependent, as it is reflected back by a specific Bragg grating located at a different position L i  along the optical fiber element. 
     The time delay of the wavelength-swept reference beam back-reflected from the FBGA element  1007  through the waveguide element  1016  depends on the instantaneous central wavelength λ s (t) of the swept signal source  1001 . In case the central wavelength λ s (t) of the swept signal source  1001  is found to be in the reflective-responsivity-range of the i-th FBGA located at L i , the delay will be 2L i  relatively to the entrance point of the FBGA element  1007  at L 0 . Knowing the instantaneous central wavelength λ s (t), the effective instantaneous reference arm length and its respective length different 2L i  is also well-known. 
     The wavelength-swept reference beam back-reflected from the FBGA element  1007  through the waveguide element  1016  is again, optionally, modulated using the phase modulator  1013 , and passed therefrom through the waveguide element  1006  back towards the beam splitter  1002 . A portion of the wavelength-swept back-reflected reference signal that passes the beam splitter  1002  is directed through the waveguide element  1017  to the detector  1008 . The wavelength-swept reflected interrogating beam portion that passes through the waveguide element  1003 , and the swept back-reflected reference signal portion that passes through the waveguide element  1006 , are superposed by the beam splitter  1002 , and passed therefrom through the waveguide element  1017  and thereby projected onto the detector  1008 . The beam superimposed at the splitter  1002  are electrically measured by the detector  1008 , and the measured signal is processed and analyzed using one or more processors (CPU) and memories (MEM) of the DAQ  1009 . 
     The distance between the apparatus  1000  and the target object  1004  can be determined in similar technique as described hereinabove with reference to  FIGS.  2 ,  3 ,  4  and  10   , where each FB GA element acts as a delay line with a different length enabling the apparatus  1000  to determine different distances of multiple target objects  1004  (like OCT). The distances can be determined in the same manner as carried out utilizing the apparatuses  900  in  FIG.  10   . However, in apparatus  1000  the detected signals L are not the outputs of i-th detectors in the spectrometer, but rather time-dependent signals sequentially produced by the swept source  1001  i.e., a single detector  1008  is used instead of a detector array (each detector for each FBGA), and the measurement data L is time-dependent (each reference signal is reflected by different FB GA imparting a different time delay). In some embodiments the apparatus  1000  is implemented using a fiber-based (optic) waveguides for simplicity, but it can also be implemented in a free-space medium to communicate the beam therein. 
     In embodiments disclosed hereinabove, an interrogating beam having a specific field of view (FOV) (determined by the beam width e.g., having certain direction and angular deviation/range) is used, and in such case, it may illuminate a specific FOV from which the interrogating beam portion is reflected back towards the apparatus to provide information indicative of the direction and distance of the target object located in the specific FOV. In order to receive information from other fields of view determined by other directions (e.g., for 3D mapping/imaging), embodiments such as those disclosed hereinabove may implement a scanning mechanism is typically used in the transmission of the interrogating beam (e.g., one beam with a scanning mechanism, which may be configured to scan and successively illuminate a large field of view, or by multiple transmuting transmission arrays wherein each interrogating beam in the array is configured to illuminate a small field of view, or by a very wide beam—such as a flash source). In such embodiments, the scanning mechanism may also be used in the collection/acquisition of reflections of the interrogating signal from the fields of view. 
     The apparatus  1100  illustrated in  FIG.  12    exemplifies a 3D mapping/imaging technique that uses imaging of a field of view on at least one detector array and does not necessarily require scanning mechanisms in the transmission/acquisition portions of the apparatus. The apparatus  1100  includes signal source  1101 , a splitting arrangement  1139  comprising splitters  1103 ,  1124  and  1113 , combiners  1116  and  1121 , a detection arrangement  1139  comprising array detectors  1131  (A) and  1130  (B), and a processing unit  1137  (also referred to herein as DAQ). For each specific field of view covered by the general field of view of the apparatus  1100  there is a dedicated detector (in the detector array) that collects the reflected interrogating beam from its specific field of view, which thereby facilitates construction of a full 3D image. 
       FIG.  12    schematically illustrates an apparatus  1100  utilizing a detector array A, B, to image a full scene without scanning the reflected signal, wherein the transmitting signal can be for example a “flash”, a wide beam illuminating the general FOV at once (typically not in the visible wavelength e.g., about 0.7 um to 2 um) or a beam directed to illuminate a specific FOV. More particularly, a region of interest is being illuminated by an interrogating beam  1109  (e.g., a very wide beam, or narrow beam directed by a scanning mechanism) generated by the apparatus  1100 , the interrogating beam  1109  having an angular field of view (FOV) that may illuminate target object (not shown) located at some distance from the apparatus  1100 . In this specific and non-limiting example, the collection optics  1111  is implemented by imaging optics configured to collect a plurality of reflected beams  1110 , 1138  . . . , that are the reflections of the interrogating beam  1109  from the FOV of the apparatus  1100 . This configuration enables pixel-by-pixel range detection within an image of a field of view, enabling to determine distances of different objects within the field of view. 
     The reflected beams,  1110 , 1138  . . . , are directed to the imaging optics  1111  from more than one direction, defined by the fields of view associated with the different detector cells such as FOV(i), FOV(j). The imaging optics  1111  is configured to image/direct reflection beams from different directions onto two detector arrays, A and B, wherein for each FOV such as  1110 , there are respective conjugated detector elements  1118 , 1123  in each of the detector arrays A and B, i.e., each detector in each array is associated with a different direction/angle of arrival from which a reflected beam may originate. Coupling the direction of each reflection beam and the distance from which the reflection beam originates enables construction of a 3D map of the illuminated FOV. In general, it should be noted that a distance mapping system according to some possible embodiments may alternatively combine different mechanisms (not shown), such as but not limited to, scanning mechanism with a focused/collimated beam, or with a narrow field of view, or using an interrogating beam having a wide field of view. 
     In some embodiments, apparatus  1100  includes an electromagnetic signal source  1101  configured to generate an electromagnetic beam having a finite coherence length for passage through a waveguide element  1102 . The waveguide elements of the apparatus  1100  may be implemented by optical fibers. The beam passed through the waveguide element  1102  is split by the beam splitter  1103  to form an interrogating beam portion passed through the waveguide element  1104 , and a reference beam portion passed through the waveguide element  1105 . The interrogating beam portion passed through the waveguide element  1104 , is optionally phase modulated using, for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulation, of the phase modulator  1106 , to thereby form the optionally modulated interrogating beam passed through the waveguide element  1107 . 
     The source output signal, being modulated or not by phase modulator  1106 , is transmitted through wave guide element  1107  and directed by the directing optics (e.g., collimator)  1108  to form the interrogating beam  1109  that propagates through free space medium to illuminate a selected field of view FOV. In this connection, the field of view FOV, is typically determined by the directing optics  1108  and may in some configurations be the same as FOV of the collection optics  1111 . The interrogating electromagnetic beam  1109  propagates towards the FOVs such as  1110 , 1138 , and some portion thereof may be reflected from an object(s) located within the FOVs  1110 , 1138 . A portion of the reflected beam is received by the collection optics  1111  from the respective direction in the apparatus  1100  to form the received reflected interrogating beam portion  1112 . It should be noted that generally the FOV of illumination determined by the directing optics  1108  may be different than the FOV of light collection determined by the collection optics  1111 . For example, in some embodiments, the directing optics  1108  may be configured to illuminate an entire field in “flash like” illumination, while the collection optics  1111  may be configured to scan the FOV pixel by pixel. In some embodiments the collection optics may be configured for collection of light from a region of the FOV. 
     The received reflected interrogating beam portions, such as portion  1112 , are split by the beam splitter  1113  into first reflected interrogating beam portions  1114 , and second reflected interrogating beam portions  1115 . The first reflected interrogating beam portions  1114  are further directed onto an additional beam combiner  1116 , to thereby form reflected beam portions  1117  projected onto respective detectors DA(i)  1118 , located in the detector array (A)  1131 . It is noted that the specific detector onto which the beam is projected depends on the direction from which the reflected beam  1110  is reaching the apparatus  1100 , and therefore it provides information required to build a 3D map of the interrogated FOV. As will be elaborated below, the beam combiner  1116  forms a combined reflected beam  1117  that is superimposed with a portion of the collimated long arm reference beam  1136 . 
     The second reflected interrogating beam portions  1115  are directed onto a reflector (e.g., mirror)  1119 , and reflected therefrom to form the reflected beam portions  1120 , which passes an additional beam combiner  1121 . As will be elaborated below, a portion of the reflected beam portions  1120  from the reflector  1119  is superimposed by the beam combiner  1121  with a portion of the substantially uniform short arm reference beam  1129  to form the combined beam. The combined beam is projected from the combiner  1121  onto the detector DB(i)  1123 , located in the detector array (B)  1130 . As noted above with regard to beam portions  1117 ,  1122  projected onto the detectors  1118 ,  1123  is projected depends on the direction from which the reflected beam  1110  is reaching the apparatus  1100 , and therefore it provides information required to build a 3D map of the interrogated FOV. 
     The reference beam portion passed through the waveguide element  1105  is split by the beam splitter  1124  and directed into a short reference arm and a long reference arm forming a short arm reference beam portion passed through the waveguide element  1125 , and a long arm reference beam portion passed through the waveguide element  1126 . 
     The short arm reference beam portion passed through the waveguide element  1125  is collimated by the collimator  1127  to form the short reference arm collimated beam  1128  which is combined by the beam combiner  1121  with the reflected beams  1120  . . . , to thereby form the short arm combined beams  1129  . . . . The short arm combined beam  1129  is directed from the beam combiner  1121  onto the detector array (B)  1130 . 
     The long arm reference beam portion from the waveguide element  1126  is passed through delay line  1132 , thereby forming the delayed reference beam passed through the waveguide element  1133 , which is time delayed by the additional length L of the delay line  1132 , with respect to the short reference arm. The time delayed reference beam passed through the waveguide element  1133  is then collimated by the collimator  1134 , to thereby form the long arm collimated reference beam  1135 , which is directed through the free space medium to the beam combiner  1116 . The beam combiner  1116  combines the long arm collimated reference beam  1135  with the first reflected interrogating beam portions  1114 , which are thereby directed onto the detector array (A)  1131 . 
     The reflected interrogating beam portions,  1117  and  1122 , and the reference beam portions,  1136  and  1129 , are superposed on the detectors DA(i)  1118  and DB(i)  1123  in the detector arrays,  1131  and  1130  respectively, and the signals, I A  and I B , thereby measured electrically are transferred to the DAQ  1137  for analysis and processing e.g., by one or more processor (CPU) and memories (MEM) thereof. Thus, the measured signal I A  transferred to the DAQ  1137  is indicative of the interference of the reflected interrogating beam portions  1117  . . . with the collimated long reference arm beam portion  1136 , which is substantially localized at detector DA(i)  1118 , and the measured signal I B  transferred to the DAQ  1137  is indicative of the interference of the reflected interrogating beam portion  1122  with the collimated short reference arm beam portion  1129 , which is substantially localized at detector DB(i)  1123 . In particular, in accordance with equations (6) and (7), with the required changes, the detected signals, I A  and I B , can be used to determine the distance between the apparatus  1100  and one or more of the target objects in the interrogated FOV. 
     Since for every reflected beam reaching the imaging optics  1111  from a corresponding direction/FOV the interaction with the substantially uniform short (or long) reference arm beam is substantially localized, the detected signal is localized at, or at the vicinity of a corresponding specific detector in the detector array. Consequently, the detected signals provide information from which a 3D map of the interrogated FOV may be derived by the DAQ  1137  which combines the information from the detector arrays  1130  and  1131 , e.g., using equation corresponding to equations (4) to (8) for comparing the detected signals generated by corresponding detectors. This way, by using all detection elements (pixels) in the detector arrays, an image of the full scene/FOV can be established. 
     It should be noted that for the clarity and simplicity of the explanation of the example the effect of one reflected beam  1110 , indicative of a target object at FOV(i), is elaborated, however, a plurality of reflections from different directions may reach the imaging optics, either simultaneously or non-simultaneously. This is illustrated in the figure by reflected beam  1138  impinging at the imaging optics  1111 , from a different direction/different field of view FOV(j) and subsequently at detector array (A)  1131  and detector array (B)  1130  at different detector positions, which are indicative of the direction of the reflected beam  1138 . It is noted that although the apparatus  1100  is exemplified in  FIG.  12    without any beam scanning mechanism(s) (for the beam illumination and/or collection), any combination of scanning mechanism(s), and scanning-free variants, for the signal beam illumination and collection can be used in possible embodiments of the apparatus  1100 . 
       FIG.  13    schematically illustrates a distance measurement apparatus  1200 , wherein a radio frequency (RF) signal source  1201  is used to measure the distance between the apparatus  1200  and a target object  1204 . The apparatus  1200  comprises the signal source  1201 , a splitting arrangement  1228  comprising mixers  1208  and  1209  and splitters implemented by interconnections of the waveguide elements described below, a detection arrangement comprising Rx antenna  1205 , and a processing unit  1211  (also referred to herein as DAQ). In this specific and non-limiting example RF signal generated by the RF signal source  1201  is passed through a waveguide element  1214 . The RF signal passed through the waveguide element  1214  is split into first, second and third, RF signals portions passed through the waveguide elements  1202 ,  1224 ,  1225 , respectively. 
     The first RF signal portion from the waveguide element  1202 , used as the interrogating signal, is transmitted towards the target object  1204  using the Tx antenna element  1203 . The electromagnetic interrogating RF beam  1216  transmitted from the Tx antenna element  1203  propagates through free space medium towards the target object  1204 , and a reflected portion thereof  1217  is received by the Rx antenna element  1205  of the apparatus  1200 . The received reflected interrogating RF signal portion  1217  received by the Rx antenna element  1205  and is passed through the waveguide element  1218 , wherefrom it is split into first and second reflected RF interrogating signal portions passed through the waveguide elements  1226  and  1227 , respectively. 
     The second RF signal portion passed through the waveguide element  1224 , and the third RF signal portion passed through the waveguide element  1225  are used as reference signals. The second RF reference signal portion from the waveguide element  1224  is time delayed by the delay line  1207  of the long reference arm configured to affect an additional length L with respect to the length of the short reference arm at  1225 , through which the third RF reference signal portion is passed. The phase of the second RF reference signal portion from the waveguide element  1225  can be controlled by a Variable Phase Shifter (VPS)  1212 , wherefrom the third RF reference signal portion is passed to the waveguide element  1206  of the short reference arm. 
     The reflected interrogating RF signal portions from the waveguide elements  1226  and  1227  are mixed by the mixer units,  1208  and  1209  respectively, with the reference RF signal portions from the waveguide elements  1206  and  1215 , of the short and long reference arms, respectively. The short and long mixed RF signals are passed from the mixer units  1208  and  1209  through the waveguide elements  1219  and  1220 , to the filtering units  1213  and  1214 , respectively. 
     The DC components of the mixed short and long RF signals passed through the waveguide elements  1219  and  1220  (the constant/distance-independent terms) are filtered by the DC blockers (DCB)  1213  and  1214  respectively. The filtered signals from the DC blockers  1213  and  1214  are passed via the waveguide elements  1222  and  1221  respectively, to the signal dividing unit  1210  configured to determine the ratio of the coherence terms of the mixed short and long RF signals. The output signal  1223  from the divider unit  1210  is used as the ratio Γ parameter by the DAQ  1211 , for determining the distance between the apparatus  1200  and target object  1204  e.g., using one or more processors (CPU) and memories (MEM) configured and operable to compute the distance according to Equation (8). This specific and non-limiting example can be implemented by a different hardware and analysis means. While described herein using delay lines with different lengths similar to the apparatuses described herein with reference to  FIGS.  2  and  5    etc., this technique can be implemented using multiple signal sources with different coherence lengths, or using one signal source with varying coherence lengths, such as illustrated in  FIGS.  8  and  9   . 
       FIG.  14    schematically illustrates an imaging apparatus  1300 , wherein radio frequency (RF) signals are used to measure a distance between the apparatus  1300  and a target object  1308 , and a phase array detection unit is used to construct a 3D image for the interrogated target object  1308 . According to different embodiments illustrated with respect to the figure the direction of the interrogating beam may be controlled during transmission by controlling a transmission added phase, added to the transmitted signal in each element in a transmission phase array (Tx antenna elements of the transmitter  1306 ), forming an RF interrogating signal  1307 , and the direction from which signal is collected may be controlled by control of a reception added phase added to a collected signal in each reception element in a reception array (i.e. in Rx antenna elements of the array antenna of the receiver  1310 ), forming a directed reception providing sensitivity and directionality in sensing a reflected portion of the interrogating signal  1309 . As described in more detail below, the collected reflected portion of the interrogating signal  1309  is mixed with at least two reference signals having different paths or different coherence length and further processed to estimate the distance of the target object. The apparatus  1300  comprises signal source  1301 , a splitting arrangement  1336  comprising mixers  1323 / 1324  . . . and splitters implemented by interconnections of the waveguide elements described below, a detection arrangement comprising receiver  1310 , and a processing unit  1334  (also referred to herein as DAQ). 
     The apparatus comprises an RF signal source  1301  configured to generate and propagate RF signals into a waveguide element  1302  coupled thereto. The RF beam passed through the waveguide element  1302  is split into first, second, and third RF signal portions passed through the waveguide elements  1305 ,  1304 , and  1303 , respectively. The third RF portion from the waveguide element  1303 , used as RF interrogating signal  1307 , is transmitted through free space medium towards the target object  1308  by the Tx array antenna of the transmitter  1306 . The transmitter  1306  includes a phase control unit (PCU)  1312  configured to control the relative phase between the Tx antenna elements of the transmitter  1306 , thereby enabling to electrically control the transmitted direction of the interrogating signal through free space medium. As shown, the electromagnetic interrogating beam  1307  propagates towards a specific FOV. The reflected portion of the interrogating signal  1309  is received by one or more Rx antenna elements of the array antenna of the receiver  1310  of the apparatus  1300 . 
     An implementation for an i-th antenna element  1311  (1&lt;i&lt;N, where i is an integer) is described below, which also pertains to the other antenna elements of the Rx array antenna of the receiver  1310 . The reflected interrogating RF signal  1309  received by the ith antenna element  1311  is passed through the waveguide element  1314 , wherefrom it is split into first and second reflected interrogating RF signal portions passed through the waveguide elements  1316  and  1317 , respectively. 
     The first RF signal portion passed through the waveguide element  1305 , and the second RF signal portion passed through the waveguide element  1304  are used as first and second reference RF signals, respectively. The second reference RF signal portion from the waveguide element  1304  is passed through a long reference arm in which it is time delayed by the delay waveguide line  1335  by an additional length L, by which its length is greater than the length of the short reference arm used for the passage of the first reference RF signal portion passed through the waveguide element  1305 . In some embodiments the phase of the first reference RF signal portion from the waveguide element  1305  is controllably shifted in the short reference arm by a variable phase shifter (VPS)  1318 . The first reference RF signal portion is passed through the waveguide element  1319 , and the time delayed second reference RF signal from the delay waveguide line  1335  is passed through the waveguide element  1321 . 
     The optionally phase shifted reference RF signal from the waveguide element  1319  of the short reference arm, and the time delayed reference RF signal from the waveguide element  1321  of the long reference arm, are each split into N RF signal portions of respective RF signal propagation channels of the apparatus  1300 , each one of these N signal propagation channels corresponds to a respective one of the N Rx antenna elements in the array antenna of the receiver  1310 . In the i-th RF signal propagation channel, the optionally phase shifted RF reference signal from the short reference arm is passed to waveguide element  1320 , and the time delayed RF reference signal from the long reference arm is passed to the waveguide element  1322 , which are associated with the i-th Rx antenna element  1311 . 
     The i-th RF signal propagation channel comprises first and second mixer units  1323  and  1324 , configured for respectively mixing the first and second reflected interrogating RF signal portions from the waveguide elements  1316  and  1317 , with the first (optionally phase shifted) and the second (time delayed) reference RF signal portions from the waveguide elements  1320  and  1322  i.e., that are associated with the short and long reference arms, respectively. The first mixed RF signal from the first mixer unit  1323  (associated with the short reference arm) is passed through the waveguide element  1325 , and the second RF mixed signal from the second mixer unit  1324  (associated with the long reference arm) is passed through the waveguide element  1326 . 
     The DC component of the first mixed reference RF signal from the waveguide element  1325  (constant/distance-independent term) is filtered by the DC blocker unit  1327 , and the second DC component of the mixed reference signal from the waveguide element  1326  (constant/distance-independent term) is filtered by the DC blocker unit  1328 . The filtered first mixed reference RF signal from the DC blocker unit  1327  is passed through the waveguide element  1329 , and the filtered second mixed reference RF signal from the DC blocker unit  1328  is passed through the waveguide element  1330 . The i-th RF signal propagation channel comprises a divider unit  1331  configured to determine the ratio signal  1332  indicative of the ratio of the coherence terms of the filtered first and second mixed reference RF signals passed through the waveguide elements  1329  and  1330 , respectively. 
     The ratio signal  1332  is used by the DAQ  1334  to determine for the reflected interrogating signal  1309  received in each Rx antenna of the receiver  1310  a respective F parameter denoted by Equation (6), and to thereby determine the distance (depth) Δx(T)between the apparatus  1300  and the target object  1308  using Equation ( 8 ). The DAQ  1334  comprises in some embodiments one or more processors (CPU) and memories (MEM), configured and operable to compute a respective distance from the N RF signal propagation channels corresponding to the N Rx antenna elements in the antenna array of the receiver  1310 , while the horizontal position and the height position of the target object  1308  being determined by relative phase between the Rx antenna elements of the receiver  1310 . 
     In this specific and non-limiting example, the horizontal and vertical position of the target object  1308  is related to the angel of arrival of the reflected interrogating beam portion  1309  from the FOV. This angel of arrival causes a relative phase shift to the reflected portion of the interrogating signal  1309  detected by the different antennas in the antenna array elements of the receiver  1310 . Using this information, reconstruction of the 3D image is done by processing and analyzing the different amplitudes (caused by the phase shift&#39;s) of the received RF signals corresponding to the different Rx antenna element in the array antenna of the receiver  1310 , by the DAQ units  1334 . 
     In this specific and non-limiting example, the transmitting beam angle can be broad or narrow, and it can be directed with a reorienting mechanism (not shown). In some possible embodiments, the reorienting mechanism and the Tx antenna element of the receiver  1306  can be part of the Rx array antenna of the receiver  1310 . 
     In some embodiments a continuous coherence length measurement apparatus  1400 , as schematically illustrated in  FIG.  15   , is used. For each of the previously described distance measurement techniques, a change in the coherence length source might occur in some cases, hence a continuous measurement of coherence length e.g., as exemplified in  FIG.  15   , might ease calculations, and increase performance. 
     The apparatus  1400  comprises a signal source  1401  configured to generate an electromagnetic beam that is passed through the waveguide element  1403 . The waveguide elements used in apparatus  1400  may be implemented by, for example, but not limited to, optical fibers. The beam passed through the waveguide element  1403  is split by beam splitter  1402  into two portions: an interrogating beam portion passed through the waveguide element  1404 ; and a feedback beam portion passed through the waveguide element  1405  to a feedback loop unit  1406 . The interrogating beam portion passed through the waveguide element  1404  is used to illuminate a target object (not shown), and/or a FOV, for determining a distance to said target object using any of the techniques/apparatuses described herein. The feedback beam portion passed through the waveguide element  1405  is used to evaluate the instantaneous coherent length of the signal source  1401  by the feedback loop unit  1406 . 
     The feedback beam portion from the waveguide element  1405  is split by beam splitter  1407  into two portions: a short arm portion passed through the waveguide element  1408 ; and a long arm portion passed through the waveguide element  1409 . The short arm beam portion from the waveguide element  1408  is optionally modulated by a phase modulator  1410 , comprising for example but not limited to, Electro-Optic, Acousto-Optic, or piezoelectric based modulator. The long arm beam portion from the waveguide element  1409  is time delayed by an additional length L of a delay waveguide line  1411 , where L is the difference between the lengths of the long and short beam arms. The beam portions from the long and short beam arms ( 1419 ,  1420 ) are superposed by beam combiner  1412 , and the superposed signal from the beam combiner  1412  is passed through the waveguide element  1421  to the detector  1413 . The signal  1416  measured by the detector  1413  can be used in a feedback loop for stabilizing the coherence length of the signal source  1401 . Additionally, or alternatively, the signal  1417  measured by the detector  1413  can be used by the DAQ  1414  for later analysis e.g., by one or more processors (CPU) and memories (MEM) thereof. 
     This coherence length stabilizing technique can also be implemented using the delay lines existing in the distance measuring apparatuses described hereinabove (e.g.,  418 ,  517 ,  626 , in  FIGS.  5 ,  6 ,  7   ). 
     For example, with reference to  FIG.  6   , beam splitters may be introduced to act on beams  530  and  531  e.g., after collimators  518  and  519  and before beam splitter  511  and  542 , for respectively splitting a first small portion and a second small portion of the respective short-arm and long-arm reference beams to be used for the feedback loop unit  1406 . A phase modulator corresponding to phase modulator  1410  may be introduced in the first small portion and/or the second small portion of the respective beams. Combining the first small portion and second small portion of the respective beams allows for direct measurement of the coherence length stability by a dedicated detector (not shown). 
     In other possible embodiments a phase correction apparatus  1500 , as illustrated in  FIG.  16 A , is used to ease calculations, and improve measurement speed in the distance detection techniques/apparatuses described hereinabove and hereinbelow. 
     The apparatus  1500  comprises a signal source  1501  configured to generate an electromagnetic beam passed therefrom through the waveguide element  1506 . The waveguide elements in the apparatus  1500  can be implemented by, for example, but not limited to, optical fibers. The beam from the waveguide element  1501  is split by the beam splitter  1502  into two portions: (i) an interrogating beam portion passed through the waveguide element  1503 ; and (ii) a reference beam portion passed through the waveguide element  1504 . 
     The interrogating beam portion from the waveguide element  1503  is used to illuminate a target object (not shown), and/or a FOV, and measure distance to said object by any of the techniques/apparatuses described herein. The reference beam portion from the waveguide element  1504  is split by the beam splitter  1505  into a long arm beam portion passed through the waveguide element  1507 , and a short arm beam portion passed through the waveguide element  1509 . The long arm reference beam portion from the waveguide element  1507  is delayed by an additional length L of the delay line  1508  i.e., the difference between the lengths of the long and short reference arms equal to L. 
     In order to control the relative phase between the short and long reference arm signals ( 1513  and  1512 ), for example, but not limited to, a phase modulator  1510  can be used in a closed, or open, loop configuration with the previously described detection unit, or with an additional detection unit (i.e., the relative phase can be determined from measurements obtained from the detection units used in the distance detection apparatuses described herein). 
     Alternatively, as exemplified in  FIG.  16 B , each one of the reference beams passes through the short and long waveguide elements,  1512  and  1513  respectively, can be split by the respective splitters  1514  and  1515  into two portions for this purpose. First split signal portions of the long and short reference beam arms are passed from the splitters  1515  and  1514  to respective waveguide element  1521  and  1520  can be used for determining the distance between the apparatus  1500  and the target object (not shown) using the techniques disclosed herein (as shown in  FIGS.  5  and  6   ), and the second split portions of the long and short reference beam arms from the splitters  1515  and  1514  can be combined by the combiner  1516  and directed therefrom towards an additional detection unit  1518  configured to generate measurement signals  1519  for monitoring the relative phase (the relative phase between the two delay lines  1520 ,  1521  can be determined based on the amplitude measured by the detector  1519 ). 
     Though apparatus  1500  exemplifies use of two reference delay lines having different lengths, it can be similarly implemented employing any of the other techniques previously described herein involving reference signals. 
       FIG.  17    schematically illustrates a distance measurement/imaging apparatus according to possible embodiments disclosed herein, implemented as a photonic integrated circuit (PIC). The use of a hybrid III-V/Silicon platform in PICs is now commercially available and can be used to implement all of the passive and active components required for implementing the distance measurement/imaging apparatuses disclosed herein. It is noted that the PIC implementation exemplified in  FIG.  17    is not limited to III-V platforms, as it can be similarly implemented in any other suitable material platform. 
     The PIC&#39;s chip  1600  can be formed/patterned on, but not limited to, Silicon on Isolator (SOI) platform, or a Silicon Nitride (SNi) substrate. A narrow-linewidth laser module with a phase modulator for coherent length control and amplifier  1601  is used as a light signal source which is split into two portions. The first portion is guided into the amplifiers array  1602  and therefrom transmitted through the free space medium by the transmitters array  1603 . The transmitted interrogating beam  1609  is directed by the transmitters array  1603  towards one or more target objects in the FOV of the apparatus  1600 . The transmitter elements of the array  1603  can be optionally implemented utilizing, but not limited to, etched grating coupler(s). An additional optical component (not shown) can be used in some embodiments to control the transmitted beam  1609 , passively or actively. 
     The second beam portion of the light source  1601 , is passed to a short reference arm  1610 , and a long reference arm  1611 . An additional time delay of the long reference arm is implemented using a delay-line  1608  configured to add an additional length L thereto. Correction unit  1606 , for relative phase control, may be used to stabilize the relative phase between the long and short reference lines. 
     The interrogating beam reflected back from the target objects that corresponds to specific FOV is received by a respective detector that corresponds to the same FOV in the detection array unit  1604 . The reflected interrogating beam (not shown) is guided into the unit  1605  wherein the reflected interrogating signal portion is split and superposed with the short and the long reference beam independently on a balanced detector, and the electrical measurement signal from the balanced detector is then analyzed and processed for distance estimation. Additionally, an amplification unit (not shown) can be added also at the receiver detection array unit  1604  in order to increase the SNR. For example, one or more signal amplifiers can be added in some of the waveguide elements (at any location) of the apparatus  1600  for increasing the strength of the signals passed therethrough. 
     In this specific and non-limiting example, the detection array unit  1604  can be implemented in site, and the received interrogating beam reflected back from the target object does not need to be guided. The fabrication process of the apparatus  1600  may include lithography, e-beam, ion implementation, epitaxial bonding, and/or other methods that can be used for this purpose. In the same manner, this platform can be used for implementing the distance detection/imaging apparatuses disclosed herein on-chip in the RF regime. Additionally, any of the previously described apparatuses illustrated in  FIGS.  5  to  14    can be implemented in a similar manner on a PIC&#39;s. 
     It is noted that this distance measurement technique is free of bandwidth (in contrast to all other active remote sensing techniques known to the inventor hereof), and that the long reference arm comprising the delay line e.g.,  1608  can have a dual use. Namely, in addition to the use of the long reference arm\delay for distance estimation, it can be also used for measurement of the rotation (angular velocity) of the apparatus i.e., it can act as an optical-gyroscope. 
     In other possible embodiments a rotation measurement apparatus  1700 , as illustrated in  FIG.  18   , is used to measure the rotation of the apparatus, and thus acts as an optical gyroscope, in addition to the distance detection functionality described herein. Particularly, in apparatus  1700  the long reference arm comprising the delay line  1713  can have a dual use, namely, in addition to the regular use of the long reference arm\delay disclosed herein for distance estimation, it can be also used for measuring angular velocity of the apparatus. 
     In apparatus  1700  the electromagnetic signal source (A)  1701  is configured to generate a beam having a finite coherence length for passage through a waveguide element  1702 . In some possible embodiments the waveguide elements shown in  FIG.  18    can be implemented by optical fibers. The electromagnetic beam passing through the waveguide element  1702  is split by the beam splitter  1703  into two portions: an interrogating beam portion passed through waveguide element  1704 ; and a reference beam portion passed through the waveguide element  1705 . The interrogating beam portion propagates towards the target object (not shown). The reference beam portion passing through the waveguide element  1705  is split by the beam splitter  1706  into two reference signal portions; the first reference beam portion is passed through the waveguide element  1707  (used as a short reference arm for source (A)  1701 ); and a second reference beam portion passed through the waveguide element  1708  (used as a long reference arm for source (A)  1701 ). The second reference beam portion passed through the waveguide element  1708  is directed and passed through the FBGA element  1711  (that acts as a transparent waveguide only for signal source (A)  1701 ). The second reference beam portion passed through the FBGA element  1711 , and thereafter through the waveguide element  1710 , is directed towards three-port circulator element  1709 . 
     The second reference beam portion from the waveguide element  1710  is introduced via the first port a into the circulator element  1709 , which permits passage thereof only through its second port b i.e., passage thereof from port a to port c is substantially prevented. 
     The second reference beam portion passing through the waveguide element  1712  is time delayed by an additional distance L affected by the waveguide element  1713  configured to introduce the additional time delay to the second reference beam. The time delayed second reference beam portion passed from the waveguide element  1713  through the waveguide element  1714  is passed through an additional three-port circulator element  1726 . The additional three-port circulator element  1726  is configured to permit passage of the delayed second reference beam portion introduced thereinto via its first port a only through its second port b, that is coupled to the waveguide element  1715  i.e., passage thereof from port a to port c is substantially prevented. The delayed second reference beam portion passed through the circulator element  1726  is passed through the waveguide element  1715  and the FBGA element  1716  (that acts as a transparent waveguide only for signal source (A)  1701 ). The delayed second reference beam portion passed through the waveguide element  1717  is used as a long delay reference signal for distance detection employing any of the techniques/apparatuses described herein. 
     The second electromagnetic source signal (B)  1718 , the detector  1724 , and the ring interferometer  1770  formed by the waveguide element  1721 , FBGA element  1711 , waveguide element  1710 , circulator  1709 , waveguide element  1712 , delay line  1713 , waveguide element  1714 , circulator  1726 , waveguide element  1715 , FB GA element  1716 , and the waveguide element  1722 , can be used to detect “Sagnac interference” effect usable for angular velocity measurements. 
     In apparatus  1700  the second electromagnetic signal source (B)  1718  is configured to generate a beam, passing through a waveguide element  1719 . The electromagnetic beam passing through the waveguide element  1719  is split by the beam splitter  1720  into first and second interference beam portions that are passed through waveguide elements  1721 , and  1722  respectively. The first interference beam portion passed through the waveguide element  1721  is introduced via the third port c into the circulator element  1709 , which permits passage thereof only through its first port a i.e., passage thereof from port c to port b is substantially prevented. The first interference beam portion is passed from the first port a of the circulator element  1709  into the waveguide element  1710 . 
     The first interference beam portion passed through waveguide element  1710  is backreflected by the FBGA element  1711  which acts as a mirror only for the signal source (B)  1718 . The first interference beam portion reflected by the FBGA element  1711 , propagates backwards through the waveguide element  1710  towards the three-port circulator element  1709 , and introduced thereinto via the first port a of the circulator element  1709 . 
     The first interference beam portion is passed from the second port b of the circulator element  1709  into the waveguide element  1712 . 
     The first interference beam portion passing through the waveguide element  1712  is delayed by an additional distance L affected by the waveguide element  1713  configured to introduce the additional time delay to the first interference beam. After passage through the waveguide element  1713 , the first delayed interference beam portion is passed through the waveguide element  1714  into the first port a of the additional three-port circulator element  1726 . The additional circulator element  1726  is configured to permit passage of the first delayed interference beam portion introduced thereinto via its first port a only through its second port b, that is coupled to the waveguide element  1715  i.e., the passage from port a to port c is substantially prevented. 
     The first delayed interference beam portion passed through waveguide element  1715  coupled to the second port b of the circulator element  1726  is backreflected by the second FBGA element  1716 , which acts as a mirror only for signal source (B)  1718 . The first delayed interference beam portion reflected by the FB GA element  1716  through the waveguide element  1715  propagates backwards into the second port b of the three-port circulator element  1726 . The circulator element  1726  is configured to permit passage of the first delayed reference beam portion received via its second port b through its third port c, into the waveguide element  1722 , while substantially preventing passage thereof via its first port a. 
     The first delayed interference beam portion is passed from the third port c of the three-port circulator element  1726  into the waveguide element  1722  towards the beam splitter/combiner  1720 . 
     The second interference beam portion propagates in the waveguide element  1722  and introduced via the third port c into the circulator element  1726 , which permits passage thereof only through its first port a i.e., the passage from port c to port b is substantially prevented. 
     The second interference beam portion is passed from the first port a of the circulator element  1726  into the waveguide element  1714 . 
     The second interference beam portion passing through the waveguide element  1714  is delayed by an additional distance L obtained by the waveguide element  1713  configured to introduce the additional time delay to the second interference beam. After passage through the waveguide element  1713 , the second delayed interference beam portion is passed through the waveguide element  1712  into the second port b of the three-port circulator element  1709 . The circulator element  1709  is configured to permit passage of the second delayed interference beam portion introduced thereinto via its second port b only through its third port c, that is coupled to the waveguide element  1721  i.e., the passage from port b to port a is substantially prevented. 
     The second delay interference beam portion propagates in the waveguide  1721  towards the beam splitter/combiner  1720 . 
     The first and second delayed interference beam portions that propagates backwardly in the waveguides  1722  and  1721  are superposed by the beam splitter/combiner  1720 , wherefrom the superposed signal propagates through the waveguide element  1723  towards the detector  1724 . The superposed signal is projected onto the detector  1724 , to thereby generate the measured signal  1725 . 
     The measured signal  1725  can be analyzed in the frequency domain and the detected angular velocity ω is related to the rotation angle Δφ indicative by the Sagnac effect, which can be expressed as follows: 
     
       
         
           
             
               
                 
                   Δφ 
                   ≈ 
                   
                     
                       
                         8 
                         ⁢ 
                         π 
                       
                       
                         λ 
                         c 
                       
                     
                     ⁢ 
                     ω 
                     ⁢ 
                     A 
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
           
         
       
     
     where λ c  is the central wavelength of the electromagnetic signal source (B)  1718 , and where, A represent the effective area (given by: A=Nα, where α is the area within the delay line  1713  and N is the number of windings of the delay line  1713 ). 
     The equations mentioned hereinabove (for distance measurement) are particularly usable for beam sources having Gaussian spectrums. In other cases, these equations can take a different form, but the same distance determination principles are substantially maintained. 
     The embodiments disclosed herein provide some examples of implementations, but they can be used partially, or with other components for other embodiments. Furthermore, the core basis of the subject matter disclosed herein is the measurement and distance/imaging techniques which can be widely used in many applications, and which can be implemented by a variety of hardware components and analyzed by different methods of signal processing. 
     It is noted that though the array detection configurations are exemplified and illustrated utilizing a one-dimensional (1D) array for simplicity, but in a similar manner they can be implemented by a two-dimensional (2D) array. 
     It is further noted that in possible embodiments disclosed herein the coherence length of the electromagnetic signal source can be controlled by an external unit/device. 
     As will be appreciated by those skilled in the art, the embodiments disclosed herein enable to detect signals of considerably low magnitudes due to the coherent detection techniques employed. Furthermore, by increasing the power of the reference arm signal, the power of the signals that can be detected using the disclosed embodiments is very low (even a single photon can be detected), by this method noted as heterodyne detection. With these techniques the velocity of the target object can be also detected due to the beating frequency of the returned signal and the reference signal by utilizing the Doppler effect. These techniques can be also used for cross-section analyses and not only for distance measurement. 
     In contrast to the techniques of the prior art mentioned in the background section e.g., ToF, FMCW and OCT, with the techniques of this application the accuracy of the distance measurement is not time-dependent or bandwidth-dependent, but rather depends on the coherence stability of the electromagnetic signal source, and the ability to measure the Γ parameter with high precision. In addition, the techniques disclosed herein offers inherent noise filtering via coherent detection. By combining these properties, lower signals can be detected, and the SNR can be improved significantly at a low cost. 
     In some of the configurations disclosed herein, polarized beams can be used, and polarizing beam splitters, for advanced designs. 
     As described hereinabove and shown in the associated figures the present invention provides implementations for distance measurement/imaging and related methods. While particular embodiments of the invention have been described, it will be understood, however, that the invention is not limited thereto, since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, all without exceeding the scope of the claims.