System and method for providing chirped electromagnetic radiation

A system and method for controllably chirping electromagnetic radiation from a radiation source includes an optical cavity arrangement. The optical cavity arrangement enables electromagnetic radiation to be produced with a substantially linear chirp rate and a configurable period. By selectively injecting electromagnetic radiation into the optical cavity, the electromagnetic radiation may be produced with a single resonant mode that is frequency shifted at the substantially linear chirp rate. Producing the electromagnetic radiation with a single resonant mode may increase the coherence length of the electromagnetic radiation, which may be advantageous when the electromagnetic radiation is implemented in various applications. For example, the electromagnetic radiation produced by the optical cavity arrangement may enhance a range, speed, accuracy, and/or other aspects of a laser radar system.

FIELD OF THE INVENTION

The invention relates to electromagnetic radiation sources and more particularly to systems and methods for providing chirped electromagnetic radiation.

BACKGROUND OF THE INVENTION

Various measuring devices for measuring linear distances using one or more laser radars are known. Such measuring devices may generate information related to a distance or range of a target from the measuring device and/or a velocity, or range rate, of the target relative to the measuring device. This range and range rate information may be useful in a variety of settings. For the purposes of this application the term range rate refers to the rate of change in the range between the target and the measuring device.

A typical measuring device may include, for example, a frequency modulated laser radar system. The system may include a laser source that emits a beam of electromagnetic radiation. The beam may be emitted at a frequency that is continuously varied, or chirped. In some instances, chirping the frequency may include sweeping the frequency between a lower frequency and an upper frequency (or vice versa) in a periodic manner (e.g. a sawtooth waveform, a triangle waveform, etc.). The beam may be divided into a target beam and a reference beam.

In conventional embodiments, the system may include a target interferometer and a reference interferometer. The target interferometer may receive the target beam, and may generate a target signal corresponding to a frequency difference between one portion of the target beam directed towards, and reflected from, the target, and another portion of the target beam directed over a path with a known or otherwise fixed path length. The frequency difference may be determined by the target interferometer based on an interference signal derived from the two portions of the target beam. The reference interferometer may receive the reference beam and may generate a reference signal corresponding to a frequency difference between two portions of the reference beam that may be directed over two separate fixed paths with a known path length difference. The frequency difference may be determined by the reference interferometer based on an interference signal derived from the two portions of the reference beam.

Generally, the system may include a processor. The processor may receive the target signal and the reference signal and may process these signals to determine the range between the target interferometer and the target. Range information determined based on the target signal and the reference signal may be used to determine a range rate of the target with respect to the target interferometer.

Conventional systems may be built, for example, as described in U.S. Pat. No. 5,114,226, entitled “3-DIMENSIONAL VISION SYSTEM UTILIZING COHERENT OPTICAL DETECTION,” which is incorporated herein by reference in its entirety.

Conventional systems are typically limited in various aspects of operation. For example, these conventional systems are not able to provide range and/or range rate information instantaneously based on the target signal and reference signal, or unambiguously determine distance and velocity. These conventional systems are limited in other ways as well. These limitations may be exacerbated by various operating conditions such as, for example, target acceleration toward or away from the target interferometer, using an actuated optical element (e.g. a mirror or lens) to scan the target at high speeds, or other operating conditions.

In some configurations, beams produced by two laser sources may be combined to provide a beam of electromagnetic radiation that may then be divided into a reference beam and a target beam. In these configurations, the frequencies of the two laser sources may be counter chirped, or, in other words, the two frequencies may be chirped such that while a frequency of one of the laser sources is ascending toward an upper frequency, the other is descending toward a lower frequency, and vice versa. Systems utilizing such a configuration may suffer some or all of the drawbacks associated with single laser source systems, as well as other drawbacks unique to two laser source systems. Additionally, conventional systems may not enable sufficient control over the frequency of emitted electromagnetic radiation to suitably manipulate the chirp rate of the radiation, may not be capable of chirping the frequency of emitted electromagnetic radiation in a sufficiently linear manner, or include other drawbacks.

SUMMARY

One aspect of the invention may relate to a system and method for controllably chirping electromagnetic radiation from a radiation source. The system and method may include an optical cavity arrangement that enables electromagnetic radiation to be produced with a substantially linear chirp rate and a configurable period. By selectively injecting electromagnetic radiation into the optical cavity, the electromagnetic radiation may be produced with a single resonant mode that is frequency shifted at the substantially linear chirp rate. Producing the electromagnetic radiation with a single resonant mode may increase the coherence length of the electromagnetic radiation, which may be advantageous when the electromagnetic radiation is implemented in various applications. For example, the electromagnetic radiation produced by the optical cavity arrangement may enhance a range, speed, accuracy, and/or other aspects of a laser radar system.

In some embodiments of the invention, a system may include radiation source, one or more optical elements that form an optical cavity, a frequency shifter, an optical switch and an optical amplifier. The system may be implemented to provide chirped electromagnetic radiation to a coherent laser radar device, a spectral analysis device, an interferometer, a remote sensing device, or another device.

In some embodiments, the frequency shifter may be disposed within the optical cavity to receive electromagnetic radiation from the optical cavity, and to output a frequency shifted portion of the received electromagnetic radiation back to the optical cavity. The optical switch may be disposed within the optical cavity to receive electromagnetic radiation from the optical cavity. The optical switch may be controllable to either dump the received electromagnetic radiation away from the optical cavity, or to return the received electromagnetic radiation back to the optical cavity. In some instances, the optical switch may be controllable to couple radiation from the radiation source to the optical cavity while dumping the received electromagnetic radiation away from the optical cavity, the radiation from the source being received at the optical switch at an initial frequency. Dumping the electromagnetic radiation received from the optical cavity while coupling radiation from the radiation source to the optical cavity may reset the frequency of the electromagnetic radiation within the optical cavity to the initial frequency.

In some embodiments, a quality factor of the optical cavity may be degraded by various losses within the optical cavity. For example, radiation output from the optical cavity to a device may constitute a loss. Other losses may also be present, such as losses to imperfections in the optical elements, or other parasitic losses. To combat the degradation of the quality factor, system components may be selected and/or the system configuration may be designed to reduce cavity losses. Cavity losses may also reduce the energy stored within the optical cavity and/or the power output from the optical cavity. To combat cavity losses, an optical amplifier may be disposed within the optical cavity. The optical amplifier may be selected to provide enough gain to radiation within the optical cavity to overcome the sum of the cavity losses so that an intensity of radiation output from the optical cavity may be maintained, forming an optical oscillator or laser. The optical amplifier may also be selected based on one or more other specifications, such as, for example, homogeneous line width, gain bandwidth, or other specifications.

One aspect of various embodiments of the invention may relate to a laser radar system that unambiguously detects a range of a target and a range rate at which the target is moving relative to the laser radar system. Another aspect of various embodiments of the invention may relate to a laser radar system that uses multiple laser radar sections to obtain multiple simultaneous measurements (or substantially so), whereby both range and range rate can be determined without various temporal effects introduced by systems employing single laser sections taking sequential measurements. In addition, other aspects of various embodiments of the invention may enable faster determination of the range and rate of the target, a more accurate determination of the range and rate of the target, and/or may provide other advantages.

In some embodiments of the invention, the laser radar system may emit a first target beam and a second target beam toward a target. The first target beam and the second target beam may be reflected by the target back toward the laser radar system. The laser radar system may receive the reflected first target beam and second target beam, and may determine at least one of a range of the target from the laser radar system, and a range rate of the target. In some embodiments of the invention, the laser radar system may include a first laser radar section, a second laser radar section, and a processor.

In some embodiments of the invention, the first laser radar section may generate a first target beam and a first reference beam. The first target beam and the first reference beam may be generated by a first laser source at a first frequency that may be modulated at a first chirp rate. The first target beam may be directed toward a measurement point on the target. The first laser radar section may combine one portion of the first target beam that may be directed towards, and reflected from, the target with another portion of the first target beam, referred to as a local oscillator beam, directed over a path with a known or otherwise fixed path length. This may result in a combined first target beam.

According to various embodiments of the invention, the second laser radar section may be collocated and fixed with respect to the first laser radar section. More particularly, the relevant optical components for transmitting and receiving the respective laser beams are collocated and fixed. The second laser radar section may generate a second target beam and a second reference beam. The second target beam and the second reference beam may be generated by a second laser source at a second frequency that may be modulated at a second chirp rate. The second chirp rate may be different from the first chirp rate. This may facilitate one or more aspects of downstream processing, such as, signal discrimination, or other aspects of downstream processing. The second target beam may be directed toward the same measurement point on the target as the first target beam. The second laser radar section may combine one portion of the second target beam directed towards, and reflected from, the target, and another portion of the second target beam directed over a path with a known or otherwise fixed path length. This results in a combined second target beam.

According to various embodiments of the invention, the processor receives the first and second combined target beams and measures a beat frequency caused by a difference in path length between each of the respective reflected target beams and its corresponding local oscillator beam, and by any Doppler frequency shift created by target motion relative to the laser radar system. The beat frequencies may then be combined linearly to generate unambiguous determinations of the range and the range rate of the target, so long as the beat frequencies between each of the respective local oscillator beams and the its reflected target beam correspond to simultaneous (or substantially simultaneous) temporal components of the reflected target beams. Simultaneous (or substantially simultaneous) temporal components of the reflected target beams may include temporal components of the target beams that: 1) have been incident on substantially the same portion of the target, 2) have been impacted by similar transmission effects, 3) have been directed by a scanning optical element under substantially the same conditions, and/or 4) share other similarities. The utilization of beat frequencies that correspond to simultaneous (or substantially simultaneous) temporal components of the reflected target beams for linear combination may effectively cancel any noise introduced into the data by environmental or other effects (see e.g. Equation (1)).

Since the combined target beams may be created by separately combining the first local oscillator beam and the second local oscillator beam with different target beams, or different portions of the same target beam, the first combined target beam and the second combined target beam may represent optical signals that would be present in two separate, but coincident, single source frequency modulated laser radar systems, just prior to final processing. For example, the combined target beams may represent optical signals produced by target interferometers in single source systems.

According to various embodiments, the target beams may be directed to and/or received from the target on separate optical paths. In some embodiments, these optical paths may be similar but distinct. In other embodiments the first target beam and the second target beam may be coupled prior to emission to create a combined target beam directed toward the target along a common optical path. In some embodiments, the target beam may be reflected by the target and may be received by the laser radar system along a reception optical path separate from the common optical path that directed the target beam toward the target. Such embodiments may be labeled “bistatic.” Or, the combined target beam may be received by the laser radar system along the common optical path. These latter embodiments may be labeled “monostatic.” Monostatic embodiments may provide advantages over their bistatic counterparts when operating with reciprocal optics. More particularly, monostatic embodiments of the invention are less affected by differential Doppler effects and distortion due to speckle, among other things. Differential Doppler effects are created, for example, by a scanning mirror that directs the target beam to different locations on a target. Since different parts of the mirror are moving at different velocities, different parts of the target beam experience different Doppler shifts, which may introduce errors into the range and or range rate measurements. These effects have been investigated and analyzed by Anthony Slotwinski and others, for example, in NASA Langley Contract No. NAS1-18890 (May 1991) Phase II Final Report, Appendix K, submitted by Digital Signal Corporation, 8003 Forbes Place, Springfield, Va. 22151, which is incorporated herein by reference in its entirety.

In some instances, the first laser source and the second laser source may generate electromagnetic radiation at a first carrier frequency and a second carrier frequency, respectively. The first carrier frequency may be substantially the same as the second carrier frequency. This may provide various enhancements to the laser radar system, such as, for example, minimizing distortion due to speckle, or other enhancements.

In some embodiments, the first laser source and the second laser source may rely on, or employ, highly linearized components to generate their respective laser beams. To this end, the first laser source and the second laser source may be linearized on a frequent basis (e.g. each chirp), or in some embodiments continuously (or substantially so). This linearization may provide enhanced range measurement accuracy, or other enhancements, over conventional systems in which linearization may occur at startup, when an operator notices degraded system performance, when the operator is prompted to initiate linearization based on a potential for degraded performance, or when one or more system parameters fall out of tolerance, etc. Frequent and/or automated linearization may reduce mirror differential Doppler noise effects during high speed scanning and may maximize the effectiveness of dual chirp techniques for canceling out these and other noise contributions to range estimates.

In some embodiments of the invention, the laser radar system may determine the range and the range rate of the target with an increased accuracy when the range of the target from the laser radar system falls within a set of ranges between a minimum range and a maximum range. When the range of the target does not fall within the set of ranges, the accuracy of the laser radar system may be degraded. This degradation may be a result of the coherence length(s) of the first laser source and the second laser source, which is finite in nature. For example, the distance between the minimum range and the maximum range may be a function of the coherence length. The longer the coherence length of the first laser source and the second laser source, the greater the distance between the minimum range and the maximum range. Thus, increasing the coherence length of the first laser source and the second laser source may enhance range and range rate determinations by the laser radar system by providing the ability to make determinations over an enhanced set of ranges.

In some embodiments of the invention, one or both of the first laser source and the second laser source may implement a system and method for controllably chirping electromagnetic radiation from a radiation source, as described herein. The system and method may enable electromagnetic radiation to be produced at a substantially linear chirp rate with a configurable period. In some embodiments, the radiation may include a single, frequency shifted, resonant mode.

In some embodiments of the invention, one of the chirp rates may be set equal to zero. In other words, one of the laser sources may emit radiation at a constant frequency. This may enable the laser source emitting at a constant frequency to be implemented with a simpler design, a small footprint, a lighter weight, a decreased cost, or other enhancements that may provide advantages to the overall system. In these embodiments, the laser radar section with chirp rate set equal to zero may be used to determine only the range rate of the target.

In some embodiments of the invention, the processor may linearly combine the first combined target beam and the second combined target beam digitally to generate the range signal and the range rate signal. For example, the processor may include a first detector and a second detector. The first detector may receive the first combined target beam and may generate a first analog signal that corresponds to the first combined target beam. The first analog signal may be converted to a first digital signal by a first converter. The processor may include a first frequency data module that may determine a first set of frequency data that corresponds to one or more frequency components of the first digital signal.

The second detector may receive the second combined target beam and may generate a second analog signal that corresponds to the second combined target beam. The second analog signal may be converted to a second digital signal by a second converter. The processor may include a second frequency data module that may determine a second set of frequency data that corresponds to one or more of frequency components of the second digital signal.

The first set of frequency data and the second set of frequency data may be received by a frequency data combination module. The frequency data combination module may generate a range rate signal and a range signal derived from the first set of frequency data and the second set of frequency data.

In other embodiments of the invention, the processor may mix the first combined target beam and the second combined target beam electronically to generate the range signal and the range rate signal. For example, the processor may include a modulator. The modulator may multiply the first analog signal generated by the first detector and the second analog signal generated by the second detector to create a combined analog signal. In such embodiments, the processor may include a first filter and a second filter that receive the combined analog signal. The first filter may filter the combined analog signal to generate a first filtered signal. The first filtered signal may be converted by a first converter to generate a range rate signal. The second filter may filter the combined analog signal to generate a second filtered signal. The second filtered signal may be converted by a second converter to generate a range signal.

According to other embodiments of the invention, the processor may mix the first combined target beam and the second combined target beam optically to generate the range signal and the range rate signal. For example, the processor may include a detector that receives the first combined target beam and the second combined target beam and generates a combined analog signal based on the detection of the first combined target beam and the second combined target beam. In such embodiments, the processor may include a first filter and a second filter that receive the combined analog signal. The first filter may filter the combined analog signal to generate a first filtered signal. The first filtered signal may be converted by a first converter to generate a range rate signal. The second filter may filter the combined analog signal to generate a second filtered signal. The second filtered signal may be converted by a second converter to generate a range signal.

These and other objects, features, benefits, and advantages of the invention will be apparent through the detailed description of the preferred embodiments and the drawings attached hereto. It is also to be understood that both the foregoing general description and the following detailed description are exemplary and not restrictive of the scope of the invention.

DETAILED DESCRIPTION

FIG. 1illustrates a conventional system110for producing electromagnetic radiation at a frequency that is chirped at a substantially linear chirp rate. System110may include a radiation source112, one or more optical elements114(illustrated as optical elements114a-114d), and a frequency shifter116. System110may be implemented to provide chirped electromagnetic radiation to a coherent laser radar device, a spectral analysis device, an interferometer, a remote sensing device, or another device.

In various conventional embodiments, radiation source112may provide a beam118of coherent electromagnetic radiation to system110. Optical elements114may form an optical cavity120, such as a ring cavity, for example. Beam118may be coupled to the optical cavity120to introduce the electromagnetic radiation that forms beam118into optical cavity120. Frequency shifter116may be disposed in optical cavity120to receive the electromagnetic radiation, and may include a diffraction element (or elements) that diffract the electromagnetic radiation. Electromagnetic radiation that is zero-order diffracted by frequency shifter116may pass through frequency shifter116without being frequency shifted, and may form an output beam122of electromagnetic radiation that may be provided for use in one of the devices listed above. Diffracted electromagnetic radiation of an order other than the zero-order (e.g., the first order) may be frequency shifted by a predetermined (and in some cases adjustable) amount to form a frequency shifted beam124of electromagnetic radiation. Beams124and118may then be combined within optical cavity120, and again be directed to frequency shifter116. In this manner, frequency shifter116may incrementally shift the frequency of the resonant modes of electromagnetic radiation within optical cavity120at each pass through frequency shifter116. These incremental shifts may cause the frequency of the electromagnetic radiation within optical cavity120(and output beam122) to be chirped at a substantially linear rate.

In conventional embodiments, a quality factor of optical cavity120(defined as the ratio of energy stored to energy dissipated in the cavity) may be degraded by various losses within optical cavity120. For example, radiation output from system110in output beam122may constitute a loss. Other losses may also be present, such as losses to imperfections in optical elements114, or other parasitic losses. To combat the cavity losses, an optical amplifier126may be disposed within optical cavity120. The optical amplifier126may be selected to provide enough gain to beam124to overcome the sum of the cavity losses so that an intensity of resonant modes contained within output beam122may be maintained. Optical amplifier126may also be selected based on one or more other specifications, such as, for example, homogeneous line width, gain bandwidth, or other specifications. Source112may be selected to emit electromagnetic radiation at a frequency that falls within a gain bandwidth of optical amplifier126.

In conventional embodiments, the chirp rate at which the frequency of output beam122may be chirped may be controlled by a length of optical cavity120, which may be adjusted by adjusting a configuration of optical elements114. Another mechanism for controlling the chirp rate may include controlling the frequency shift applied to electromagnetic radiation within optical cavity120by frequency shifter116. In some embodiments, frequency shifter116may include an acousto-optic Bragg cell that may be driven to apply a selectable frequency shift to electromagnetic radiation within optical cavity120. An example of some conventional embodiments of a system for producing electromagnetic radiation at a frequency that is chirped at a substantially linear chirp rate including an optical cavity and a frequency shifter that includes an acousto-optic Bragg cell may be found in U.S. Pat. No. 4,697,888 to Schmadel et al., which is incorporated herein by reference.

In conventional embodiments, a mode of electromagnetic radiation within optical cavity120may be linearly chirped until the frequency of the mode is shifted so far that the frequency no longer falls within the gain bandwidth of optical amplifier126. Once the frequency of the mode is outside of the gain bandwidth of optical amplifier126, optical amplifier126may not provide a gain to the mode, so that losses within optical cavity120may cause the mode to die out. As modes die out in this manner, electromagnetic radiation introduced into optical cavity120in beam114may form new modes whose frequencies may then be linearly chirped by frequency shifter116, until these modes also die out.

FIG. 2is an exemplary illustration of a system210for producing electromagnetic radiation, in accordance with some embodiments of the invention. The electromagnetic radiation may be emitted by system210at a single mode, the frequency of which may be chirped at a substantially linear chirp rate. System210is illustrated with a configuration similar in some respects to system110ofFIG. 1, and similar components may be labeled with the same reference numbers. For example, system210may include radiation source112, one or more optical elements114(illustrated as optical elements114a-114d) that form optical cavity120, frequency shifter116, and optical amplifier126. As with system110, system210may be implemented to provide chirped electromagnetic radiation to a coherent laser radar device, a spectral analysis device, an interferometer, a remote sensing device, or another device.

In some embodiments of the invention, radiation source112may provide beam118of coherent electromagnetic radiation to system210; and optical elements114forming optical cavity120, frequency shifter116, and optical amplifier126may interact with the electromagnetic radiation therefrom. Beam118may be coupled to optical cavity120. Frequency shifter116may be disposed in optical cavity120to receive the electromagnetic radiation, and may include, for example, an acousto-optic Bragg cell that may be driven by an RF source212to apply a configurable frequency shift to radiation within optical cavity120. As may be the case with system110, in system210, zero-order diffracted electromagnetic radiation from frequency shifter116may pass through frequency shifter116without being frequency shifted, and may form output beam122of electromagnetic radiation that may be provided for use in one of the devices listed above. Diffracted electromagnetic radiation of an order (or orders) other than the zero-order (e.g., the first order) may be frequency shifted by a predetermined (and in some cases adjustable) amount to form frequency shifted beam124of electromagnetic radiation. Beam124may then again be directed to frequency shifter116. In this manner, frequency shifter116may incrementally shift the frequency of one or more resonant modes present in the electromagnetic radiation within optical cavity120at each pass through frequency shifter116. These incremental shifts may cause the frequency of the electromagnetic radiation within optical cavity120(and output beam122) to be chirped at a substantially linear rate. To combat the degradation of the quality factor of optical cavity120, optical amplifier126may be disposed within optical cavity120to provide a gain to electromagnetic radiation within optical cavity120. Optical amplifier126may be selected based on one or more of the criteria provided above.

In some embodiments of the invention, system210may include an optical switch214. Optical switch214may be disposed within optical cavity120to receive electromagnetic radiation within optical cavity120(e.g. beam124), and from source112, and may be selectively controllable to direct beams118and124such that one of beams118and124may be dumped away from optical cavity120while the other one of beams118and124may be coupled into optical cavity120. This configuration may enable a single mode of linearly chirped electromagnetic radiation to be stored within, and emitted from, optical cavity120. More particularly, optical switch214may enable electromagnetic radiation to be introduced to optical cavity120from source112at the emission frequency of source112. For instance, optical switch214may enable beam118to be coupled into optical cavity120for a period of time that may correspond to an optical length of optical cavity120. During this same period of time, switch214may dump energy from the cavity, replacing it with energy from radiation source112. After an appropriate amount of radiation has been coupled into optical cavity120, optical switch214may dump beam118away from optical cavity120, and may couple electromagnetic radiation within optical cavity120(e.g., electromagnetic radiation included in beam124) back into optical cavity120. Provided that the amount of time that radiation was received from source112into optical cavity120was substantially equal to, or less than, the optical length of optical cavity120, this may create a single resonant mode of radiation with optical cavity120. As the mode of electromagnetic radiation contained within optical cavity120circulates about optical cavity120through frequency shifter116, optical amplifier126, and optical switch214, the frequency of the mode is incrementally shifted by frequency shifter116, causing a linear chirp of the frequency of the mode within optical cavity120.

In some embodiments of the invention, optical switch214may enable the frequency of the electromagnetic radiation within optical cavity120to be reset. For example, an existing mode of radiation may be effectively extinguished by controlling optical switch214to dump radiation that has-been circulating within optical cavity120, or the existing mode of radiation having a shifted frequency, out of optical cavity120. At the same time (or substantially so), a new mode of radiation may be begun by controlling optical switch214to couple beam118from source112into optical cavity120as the existing, or old, mode (beam124) gets dumped. This may be conceptualized as emptying optical cavity120of the old mode of electromagnetic radiation having a shifted frequency, and introducing a new mode of electromagnetic radiation at the emission frequency of source112into optical cavity120. When electromagnetic radiation from source112has been allowed to enter optical cavity120for an appropriate amount of time (e.g., the optical length of optical cavity120), optical switch214may again be controlled to dump radiation included in beam118from source112away from optical cavity120, and the new mode of radiation may be enabled to circulate through optical cavity120.

For demonstrative purposes,FIGS. 3A and 3Billustrate an optical switch310, according to some embodiments of the invention. For example, optical switch310may include a micro electromechanical system (MEMS) switch. In such embodiments, optical switch310may include a cavity input312where optical switch310may receive electromagnetic radiation from an optical cavity, such as optical cavity120, and a source input314where optical switch310may receive electromagnetic radiation from a radiation source, such as source112. Optical switch310may include a plurality of movable optical members (e.g., micro-mirrors)316(illustrated as316aand316b). Movable optical members316may be controllably actuated into and out of the optical paths of radiation within optical switch310in the manner illustrated inFIGS. 3A and 3Bto selectively guide one or the other (or both) of the electromagnetic radiation received at cavity input312or source input314to a cavity output318, at which electromagnetic radiation may be guided into the optical cavity. Radiation not guided by movable optical members316to cavity output318may be dumped by optical switch310away from the optical cavity. In other embodiments of the invention, optical switch310(and214) may include a non-mechanical, solid-state, optical switch, a Mach-Zender interferometer switch, an optical-electrical-optical switch, or other optical switches.

It may be appreciated that the configuration of system210is shown for illustrative purposes only, and that various alternatives and/or substitutions may be included without departing from the scope of the invention. For example, although frequency shifter116is illustrated as a diffractive acousto-optic Bragg cell, in number of frequency shifting components may be implemented. Similarly, optical members114, illustrated inFIG. 2as mirrors, may include an optical fiber, a mirror, a prism, or any other optical member capable of guiding electromagnetic radiation. In some embodiments, electromagnetic radiation may be output from system210at a point other than frequency shifter116. For instance, one of optical elements114may include a half mirror that may enable output of radiation from optical cavity120for use in a device.

In some embodiments of the invention, electromagnetic radiation from source112may be coupled to optical cavity120without being received at optical switch214, and source112may be configured to only provide radiation to optical cavity120when optical switch214dumps electromagnetic radiation received from optical cavity120out of optical cavity120. For example, radiation may be received into optical cavity120from source112via a blocking member, or optical switch separate from optical switch214, that may only enable radiation emitted from source112to be coupled to optical cavity120at appropriate times. In other embodiments, source112may only emit radiation when optical switch214dumps radiation out of optical cavity120.

In some embodiments of the invention, system210may include one or more additional elements and/or components to provide additional enhancements to the system. For example, an optical diode may be incorporated into optical cavity120to insure that radiation propagates in a single direction within optical cavity120. In some embodiments, optical filtering devices may be added that may restrict an amount of longitudinal modes in which source112may operate. One or more polarization elements may also be added to enhance an optical stability of system210.

FIG. 4illustrates a frequency modulated laser radar system410. System410typically includes a laser source412that emits a beam414of electromagnetic radiation. Beam414may be emitted at a frequency that is continuously varied, or chirped. In some instances, chirping the frequency may include sweeping the frequency between a lower frequency and an upper frequency (or vice versa) in a periodic manner (e.g. a sawtooth waveform, a triangle waveform, etc.). Beam414may be divided by an optical coupler416into a target beam418and a reference beam420.

According to various embodiments of the invention, laser source412may include system210described above. Providing system210in laser source412may enhance the operation of laser radar system410by increasing the coherence length of electromagnetic radiation used by laser radar system410to determine range and/or range rate information. For example, increased coherence length of the electromagnetic radiation may enhance a range, speed, accuracy, and/or other aspects of laser radar system410.

In some embodiments, system410may include a target interferometer422and a reference interferometer424. Target interferometer422may receive target beam418, and may divide the target beam at an optical coupler426. Target interferometer422is typically used to generate a target signal that may depend upon a range of a target430from target interferometer422. Target interferometer may accomplish this by directing one portion428of target beam418toward target430, and the other portion432of target beam418to a target frequency difference module434over an optical path with a fixed path length. Portion428of target beam418may be reflected by target430and may be transmitted to target frequency difference module434via optical coupler426and an optical fiber436. Based on interference between portions436and432at coupler448, target frequency difference module434may generate the target signal corresponding to a beat frequency of portions436and432of target beam418due to the difference between their path lengths.

According to various embodiments of the invention, reference interferometer424may receive reference beam420and may generate a reference signal corresponding to a frequency difference between two portions of reference beam424that may be directed over two separate fixed paths with a known path length difference. More particularly, reference beam420may be divided by an optical coupler440into a first portion442and a second portion444. First portion442may have a fixed optical path length difference relative to second portion444. Based on interference between portions442and444at coupler446, reference frequency difference module450may generate the reference signal corresponding to a beat frequency of portions442and444of reference beam420caused by the fixed difference between their path lengths.

As will be appreciated, target interferometer422and reference interferometer424have been illustrated and described as Mach-Zehnder interferometers. However other interferometer configurations may be utilized. For example, target interferometer422and reference interferometer424may include embodiments wherein Michaelson-Morley interferometers may be formed.

In some embodiments, system410may include a processor438. Processor438may receive the target signal and the reference signal and may process these signals to determine the range of target430. Range information determined based on the target signal and the reference signal may be used to determine a range rate of target430with respect to target interferometer422.

FIG. 5illustrates an exemplary embodiment of a laser radar system510that employs two or more laser radar sections, each of which emits a target beam toward a target. For example, a first laser radar section574emits a first target beam512and a second laser radar section576emits a second target beam514toward a target516. In some embodiments of the invention, first target beam512and second target beam514may be chirped to create a dual chirp system. According to various embodiments of the invention, laser section574may include a laser source controller536, a first laser source518, a first optical coupler522, a first beam delay544, a first local oscillator optical coupler530, and/or other components. Second laser radar section576may include a laser source controller538, a second laser source520, a second optical coupler524, a second beam delay550, a second local oscillator optical coupler532and/or other components. For example, some or all of the components of each of laser radar sections574and576may be obtained as a coherent laser radar system from MetricVision™. Coherent laser radar systems from MetricVision™ may provide various advantages, such as enhanced linearity functionality, enhanced phase wandering correction, and other advantages to laser radar system510in determining the range and the range rate of target516.

According to various embodiments of the invention, one or both of first and second laser sources518and520may include system210described above. Providing system210in first and/or second laser source518and520may enhance the operation of laser radar system510by increasing the coherence length of electromagnetic radiation used by laser radar system510to determine range and/or range rate information. For example, increased coherence length of the electromagnetic radiation may enhance a range, speed, accuracy, and/or other aspects of laser radar system510.

In some embodiments of the invention, first target beam512and second target beam514may be reflected by target516back toward laser radar system510. Laser radar system510may receive first target beam512and second target beam514, and may determine at least one of a range of target516from laser radar system510, and a range rate of target516.

According to various embodiments of the invention, first laser source518may have a first carrier frequency. First laser source518may emit a first laser beam540at a first frequency. The first frequency may be modulated at a first chirp rate. The first frequency may be modulated electrically, mechanically, acousto-optically, or otherwise modulated as would be apparent. First laser beam540may be divided by first optical coupler522into first target beam512and a first local oscillator beam542. First local oscillator beam542may be held for a first delay period at a first beam delay544.

In some embodiments of the invention, second laser source520may emit a second laser beam546at a second frequency. The second frequency may be modulated at a second chirp rate different from the first chirp rate. The second frequency may be modulated electrically, mechanically, acousto-optically, or otherwise modulated. The first chirp rate and the second chirp rate may create a counter chirp between first laser beam540and second laser beam546.

In some instances, the second carrier frequency may be substantially the same as the first carrier frequency. For example, in some embodiments the percentage difference between the first baseline frequency and the second baseline frequency is less than 0.05%. This may provide various enhancements to laser system510, such as, for example, minimizing distortion due to speckle, or other enhancements. Second laser beam546may be divided by second optical coupler524into a second target beam514and a second local oscillator beam548. Second local oscillator beam548may be held for a second delay period at a second beam delay550. The second delay period may be different than the first delay period.

In some embodiments, the output(s) of first laser source518and/or second laser source520(e.g. first laser beam540and/or second laser beam546) may be linearized using mechanisms provided in, for example, METRICVISION™ Model MV200. Phase wandering of the output(s) of first laser source518and/or second laser source520may corrected using mechanisms provided in, for instance, METRICVISION™ Model MV200.

In some embodiments of the invention, laser radar system510may determine the range and the range rate of target516with an increased accuracy when the range of target516from laser radar system510falls within a set of ranges between a minimum range and a maximum range. When the range of target516does not fall within the set of ranges, the accuracy of laser radar system510may be degraded.

According to various embodiments of the invention, first beam delay544and second beam delay550may be adjustable. Adjusting first beam delay544and second beam delay550may enable laser radar system510to be adjusted to bring the set of ranges over which more accurate determinations may be made closer to, or further away from, laser radar system510. First beam delay544and the second beam delay550may be adjusted to ensure that the range of target516falls within the set of ranges between the minimum range and the maximum range so that the range and the range rate of target516may be determined accurately. First beam delay544and second beam delay550may be adjusted by a user, or in an automated manner.

The degradation of determinations of range and range rate when the range of target516is outside of the set of ranges may be a result of the finite nature of the coherence length of first laser source518and second laser source520. For example, the distance between the minimum range and the maximum range may be a function of the coherence length. The longer the coherence length of first laser source518and second laser source520, the greater the distance between the minimum range and the maximum range may be. Thus, increasing the coherence length of first laser source518and second laser source520may enhance range and range rate determinations by laser radar system510by providing the ability to make determinations over an enhanced set of ranges.

In some embodiments of the invention, first local oscillator beam542may be divided into a plurality of first local oscillator beams and second local oscillator beam548may be divided into a plurality of second local oscillator beams. In such instances, laser radar system510may include a plurality of beam delays that may apply delays of varying delay periods to the plurality of first local oscillator beams and the plurality of second local oscillator beams. This may ensure that one of the plurality of first local oscillator beams and one of the plurality of second local oscillator beams may have been delayed for delay periods that may enable the range and range rate of the target to determined accurately.

Accordingly, in some embodiments of the invention, first laser source518and second laser source520may emit chirped electromagnetic radiation with an enhanced coherence length. For example, first laser source518and/or second laser source520may include system210as illustrated inFIG. 5and described above.

According to various embodiments, first target beam512and second target beam514may be directed and/or received from target516on separate optical paths. In some embodiments, these optical paths may be similar but distinct. In other embodiments, first target beam512and second target beam514may be coupled by a target optical coupler526into a combined target beam552prior to emission that may be directed toward target516along a common optical path. In some embodiments, combined target beam552(or first target beam512and second target beam514, if directed toward target516separately) may be reflected by target516and may be received by laser radar system510along a reception optical path separate from the common optical path that directed combined target beam552toward target516. Such embodiments may be labeled “bistatic.” Or, combined target beam552may be received by laser radar system510as a reflected target beam556along the common optical path. These latter embodiments may be labeled “monostatic.” Monostatic embodiments may provide advantages over their bistatic counterparts when operating with reciprocal optics. In monostatic embodiments, the common optical path may include optical member528that may provide a common port for emitting combined target beam552and receiving reflected target beam556. Optical member528may include an optical circulator, an optical coupler or other optical member as would be apparent.

In some embodiments, the common optical path may include a scanning element557. Scanning element557may include an optical element such as, for instance, a mirror, a lens, an antennae, or other optical elements that may be oscillated, rotated, or otherwise actuated to enable combined target beam552to scan target516. In some instances, scanning element557may enable scanning at high speeds. In conventional systems, scanning elements may be a source of mirror differential Doppler noise effects due to speckle or other optical effects that may degrade the accuracy of these systems. However, because various embodiments of laser radar system510use simultaneous measurements (or substantially so) to unambiguously determine range and range rate, inaccuracies otherwise induced by high speed scanning may be avoided.

In some embodiments of the invention, a target optical coupler554may divide reflected target beam556into a first reflected target beam portion558and a second reflected target beam portion560. First local oscillator optical coupler530may combine first local oscillator beam542with first reflected target beam portion558into a first combined target beam562. Second local oscillator optical coupler532may combine second local oscillator beam548with second reflected target beam portion560into a second combined target beam564. In some embodiments not shown in the drawings, where, for example first target beam512and second target beam514may be directed to and/or received from target516separately, first local oscillator optical coupler530may combine first target beam512that is reflected with first local oscillator beam542to create first combined target beam562, and second target beam514that is reflected may be combined with second local oscillator beam548to create second combined target beam564.

Because first local oscillator beam542and second local oscillator beam548may be combined with different target beams, or different portions of the same target beam (e.g. reflected target beam556), first combined target beam562and second combined target beam564may represent optical signals that would be present in two separate, but coincident, single laser source frequency modulated laser radar systems, just prior to final processing. For example, laser source controller536, first laser source518, first optical coupler522, first beam delay544, and first local oscillator optical coupler530may be viewed as a first laser radar section574that may generate first combined target beam562separate from second combined target beam564that may be generated by a second laser radar section576. Second laser radar section576may include laser source controller538, second laser source520, second optical coupler524, second beam delay550, and second local oscillator optical coupler532.

In some embodiments, laser radar system510may include a processor534. Processor534may include a detection module566, a mixing module568, a processing module570, and/or other modules. The modules may be implemented in hardware (including optical and detection components), software, firmware, or a combination of hardware, software, and/or firmware. Processor534may receive first combined target beam562and second combined target beam564. Based on first combined target beam562and second combined target beam564, processor534may generate the range signal and the range rate signal. Based on the range signal and the range rate signal, the range and the range rate of target516may be unambiguously determined.

In some embodiments of the invention, processor534may determine a first beat frequency of first combined local oscillator beam562. The first beat frequency may include a difference in frequency, attributable to a difference in path length, of first local oscillator beam542and the component of reflected target beam556that corresponds to first target beam512that has been reflected from target516. Processor534may determine a second beat frequency of second combined local oscillator beam564. The second beat frequency may include a difference in frequency, attributable to a difference in path length, of second local oscillator beam548and the component of reflected target beam556that corresponds to second target beam514that has been reflected from target516. The first beat frequency and the second beat frequency may be determined simultaneously (or substantially so) to cancel noise introduced by environmental or other effects. One or more steps may be taken to enable the first beat frequency and the second beat frequency to be distinguished from other frequency components within first combined target beam562, other frequency components within second combined target beam564, and/or each other. For example, these measures may include using two separate chirp rates as the first chirp rate and the second chirp rate, delaying first local oscillator beam542and second local oscillator beam550for different delay times at first beam delay544and second beam delay550, respectively, or other measures may be taken.

It will be appreciated that whileFIG. 5illustrates an exemplary embodiment of the invention implemented primarily using optical fibers and optical couplers, this embodiment is in no way intended to be limiting. Alternate embodiments within the scope of the invention exist in which other optical elements such as, for example, prisms, mirrors, half-mirrors, beam splitters, dichroic films, dichroic prisms, lenses, or other optical elements may be used to direct, combine, direct, focus, diffuse, amplify, or otherwise process electromagnetic radiation.

According to various embodiments of the invention, processor534may mix first combined target beam562and second combined target beam564to produce a mixed signal. The mixed signal may include a beat frequency sum component that may correspond to the sum of the first beat frequency and the second beat frequency, and a beat frequency difference component that may correspond to the difference between the first beat frequency and the second beat frequency. For a target having constant velocity, first laser beam540and second laser beam546beat frequencies may be described as follows:

f1⁡(t)-f2⁡(t)=2⁢⁢π⁢⁢R⁡(γ1-γ2)-2⁢⁢π⁡(γ1⁢RO1-γ2⁢RO2)(3)R=(f1⁡(t)-f2⁡(t))2⁢⁢π⁡(γ1-γ2)+(γ1⁢RO1-γ2⁢RO2)(γ1-γ2)(4)
as the corrected range measurement. Similarly, we may combine (1) and (2) to obtain the expression,

v=λ4⁢⁢π⁢(f1⁡(t)-γ1γ2⁢f2⁡(t)1-γ1γ2)+λ⁢⁢γ12⁢(RO1-RO21-γ1γ2),(5)
which provides a measure of the target velocity.

According to various embodiments of the invention, the beat frequency sum component, described above in Equation 4, may be filtered from the mixed signal to produce a range signal. From the beat frequency sum component included in the range signal (e.g. f1(t)+f2(t)), a determination of the distance from laser radar system510to target516may be made. The determination based on the range signal may be unambiguous, and may not depend on either the instantaneous behavior, or the average behavior of the Doppler frequency shift (e.g. v/λ).

In some embodiments, the beat frequency difference component, described above in Equation 4, may be filtered from the mixed signal to produce a range rate signal. From the beat frequency difference component included in the range rate signal, a determination of the range rate of target516may be unambiguously made. To determine the range rate of target516,

f1⁡(t)-γ1γ2⁢f2⁡(t)
may be represented as a value proportional to a chirp rate difference between the first chirp rate and the second chirp rate. This may enable the Doppler shift information to be extracted, which may represent an instantaneous velocity of target516.

In some embodiments of the invention, the second chirp rate may be set to zero. In other words, second laser source518may emit radiation at a constant frequency. This may enable second laser source518to be implemented with a simpler design, a small footprint, a lighter weight, a decreased cost, or other enhancements that may provide advantages to the overall system. In such embodiments, laser radar system510may include a frequency shifting device. The frequency shifting device may include an acousto-optical modulator572, or other device. Acousto-optical modulator572may provide a frequency offset to second local oscillator beam548, which may enhance downstream processing. For example, the frequency offset may enable a stationary target beat frequency between second local oscillator beam548and second reflected target beam portion560representative of a range rate of a stationary target to be offset from zero so that the a direction of the target's movement, as well as a magnitude of the rate of the movement, may be determined from the beat frequency. This embodiment of the invention has the further advantage that it may allow for continuous monitoring of the target range rate, uninterrupted by chirp turn-around or fly-back. Chirp turn-around or fly-back may create time intervals during which accurate measurements may be impossible for a chirped laser radar section. In these embodiments, laser radar section576may only determine the range rate of target516while laser radar system510retains the ability to measure both range and range rate.

FIG. 6illustrates a processor534according to one embodiment of the invention. Processor534may mix first combined target beam562and second combined target beam564digitally. For example, processor534may include a first detector610and a second detector612. The first detector610may receive first combined target beam562and may generate a first analog signal that corresponds to first combined target beam562. The first analog signal may be converted to a first digital signal by a first converter614. Processor534may include a first frequency data module616that may determine a first set of frequency data that corresponds to one or more frequency components of the first digital signal. In some instances, the first digital signal may be averaged at a first averager module618. In such instances, the averaged first digital signal may then be transmitted to first frequency data module616.

Second detector612may receive second combined target beam564and may generate a second analog signal that corresponds to second combined target beam564. The second analog signal may be converted to a second digital signal by a second converter620. Processor534may include a second frequency data module622that may determine a second set of frequency data that corresponds to one or more of frequency components of the second digital signal. In some instances, the second digital signal may be averaged at a second averager module624. In such instances, the averaged second digital signal may then be transmitted to second frequency data module622.

The first set of frequency data and the second set of frequency data may be received by a frequency data combination module626. Frequency data combination module626may linearly combine the first set of frequency data and the second set of frequency data, and may generate a range rate signal and a range signal derived from the mixed frequency data.

FIG. 7illustrates a processor534according to another embodiment of the invention. Processor534may include a first detector710and a second detector712that may receive first combined target beam562and second combined target beam564, respectively. First detector710and second detector712may generate a first analog signal and a second analog signal associated with first combined target beam562and second combined target beam564, respectively. Processor534may mix first combined target beam562and second combined target beam564electronically to generate the range signal and the range rate signal. For example, processor534may include a modulator714. Modulator714may multiply the first analog signal generated by first detector710and the second analog signal generated by second detector712to create a combined analog signal. In such embodiments, processor534may include a first filter716and a second filter718that receive the combined analog signal. First filter716may filter the combined analog signal to generate a first filtered signal. In some instances, first filter716may include a low-pass filter. The first filtered signal may be converted by a first converter720to generate the range rate signal. Second filter718may filter the combined analog signal to generate a second filtered signal. For instance, second filter718may include a high-pass filter. The second filtered signal may be converted by a second converter722to generate the range signal.

FIG. 8illustrates a processor534according to yet another embodiment of the invention. Processor534may mix first combined target beam562and second combined target beam564optically to generate the range signal and the range rate signal. For example, processor534may include a detector810that receives first combined target beam562and second combined target beam564and generates a combined analog signal based on the detection. In such embodiments, processor534may include a first filter812and a second filter814that receive the combined analog signal. First filter812may filter the combined analog signal to generate a first filtered signal. First filter812may include a low-pass filter. The first filtered signal may be converted by a first converter816to generate the range rate signal. Second filter814may filter the combined analog signal to generate a second filtered signal. Second filter14may include a high-pass filter. The second filtered signal may be converted by a second converter818to generate the range signal.

While the invention has been described herein in terms of various embodiments, it is not so limited and is limited only by the scope of the following claims, as would be apparent to one skilled in the art.