Patent Publication Number: US-11378691-B2

Title: Generation of LIDAR data from optical signals

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/671,913, filed on May 15, 2018, entitled “Optical Sensor Chip;” and to U.S. Provisional Patent Application No. 62/726,101, filed on Aug. 31, 2018, entitled “Optical Sensor System;” each of which is incorporated herein in its entirety. 
    
    
     FIELD 
     The invention relates to optical devices. In particular, the invention relates to LIDAR systems. 
     BACKGROUND 
     There is an increasing commercial demand for 3D sensing systems that can be economically deployed in applications such as ADAS (Advanced Driver Assistance Systems) and AR (Augmented Reality). LIDAR (Light Detection and Ranging) systems are an example of a technology that can be used to construct these 3D images. LIDAR systems build these images by illuminating a scene with light and measuring the light signals that return to the LIDAR system from the illuminated scene. 
     Frequency Modulated Continuous Wave (FMCW) is an example of a coherent detection method that can be used for LIDAR applications. The FMCW technique is capable of determining both distance and radial velocity between an object and a LIDAR system in a single measurement period. Additionally, FMCW techniques have reduced sensitivity to ambient light and light from other LIDAR systems. 
     Many FMCW techniques resolve distance and radial velocity by performing a Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) on an electrical signal that is a function of the distance and radial velocity between an object and the LIDAR system. However, for a given object, these Fourier transforms provide more than one possible solution. When the source of the LIDAR signal and reflecting object are not moving relative to one another or are moving slowly relative to one another, it is well-known which of the solutions to select. However, as the relative speed of the source and the reflecting object increases, it becomes unclear which of the solutions is the correct solution. This lack of clarity is particularly apparent for LIDAR as opposed to FMCW applications that use other forms of electromagnetic radiation, such as radar, because of the high carrier frequency of the light signal and the associated high Doppler shift frequency. As a result, there is a need for a LIDAR system that overcomes this ambiguity. 
     SUMMARY 
     A LIDAR system includes optics that generate a light signal that carries LIDAR data that indicates a distance and/or radial velocity between the optics and an object located outside of the LIDAR system. Electronics are configured to convert light from the light signal to a first data electrical signal and a second data electrical signal. The electronics perform a Complex Fourier transform on a complex signal such that the first data electrical signals acts as a real component of the complex signal and the second data electrical signals acts as an imaginary component of the complex signal. The electronics determine the LIDAR data from an output of the Complex Fourier transform. 
     A method of operating a LIDAR system includes generating multiple data electrical signals from a light signal that carries the LIDAR data. The method also includes performing a Complex Fourier transform on a complex signal such that a first one of the data electrical signals is a real component of the complex signal and a second one of the data electrical signals is an imaginary component of the complex signal. The method also includes using an output from the Complex Fourier transform to determine the distance and/or radial velocity between the source and the object. 
     An embodiment of a LIDAR system includes a utility waveguide that carries an outgoing LIDAR signal to a facet through which the outgoing LIDAR signal exits from the LIDAR system. An optical component combines a first portion of a reference signal with a first portion of a comparative signal so as to form a first composite signal. The reference signal includes a portion of the light from the outgoing LIDAR signal before the outgoing LIDAR signal exits from the LIDAR system and the comparative signal includes light from the outgoing LIDAR that was reflected off an object located remote from the LIDAR system. A second optical component combines a second portion of the reference signal with a second portion of the comparative signal so as to form a second composite signal. The first portion of the reference signal in the first composite signal being out of phase with the second portion of the reference signal in the second composite signal. In some instances, the first portion of the comparative signal in the first composite signal is in-phase with the second portion of the comparative signal in the second composite signal. 
     An embodiment of a LIDAR system includes a utility waveguide that carries an outgoing LIDAR signal to a facet through which the outgoing LIDAR signal exits from the LIDAR system. An optical component combines a first portion of a reference signal with a first portion of a comparative signal so as to form a first composite signal. The reference signal includes a portion of the light from the outgoing LIDAR signal before the outgoing LIDAR signal exits from the LIDAR system and the comparative signal includes light from the outgoing LIDAR that was reflected off an object located remote from the LIDAR system. A second optical component combines a second portion of the reference signal with a second portion of the comparative signal so as to form a second composite signal. The first portion of the comparative signal in the first composite signal being out of phase with the second portion of the comparative signal in the second composite signal. In some instances, the first portion of the reference signal in the first composite signal is in-phase with the second portion of the reference signal in the second composite signal. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a top view of a LIDAR chip. 
         FIG. 2  is a cross-section of a LIDAR chip according to  FIG. 1  constructed from a silicon-on-insulator wafer. 
         FIG. 3  illustrates the LIDAR chip of  FIG. 1  used with an off-chip scanning mechanism. 
         FIG. 4  illustrates the LIDAR chip of  FIG. 1  used with another embodiment of an off-chip scanning mechanism. 
         FIG. 5  is a cross section of the LIDAR chip of  FIG. 1  having an integrated scanning mechanism. 
         FIG. 6A  illustrates the chip of  FIG. 1  modified to include multiple different balanced detectors for further refining data generated by the chip. 
         FIG. 6B  provides a schematic of electronics that are suitable for use with the chip of  FIG. 6A . 
         FIG. 6C  is a graph of magnitude versus frequency. A solid line on the graph shows results for a Complex Fourier transform performed on output generated from the LIDAR chip of  FIG. 6B . 
     
    
    
     DESCRIPTION 
     The LIDAR system is configured to generate multiple data electrical signals and to perform a Complex Fourier transform on a complex signal where one of the data electrical signals is a real component of the complex signal and another one of the data electrical signals is an imaginary component of the complex signal. The LIDAR system uses output from the Complex Fourier transform to generate LIDAR data (the distance and/or radial velocity between the LIDAR system and an object that reflects from the LIDAR system). The use of the Complex Fourier transform provides a non-ambiguous solution for the LIDAR data. As a result, there is no need to identify the correct LIDAR data solution. 
       FIG. 1  is a topview of a LIDAR chip that includes a laser cavity. The laser cavity includes a light source  10  that can include or consist of a gain medium (not shown) for a laser. The chip also includes a cavity waveguide  12  that receives a light signal from the light source  10 . The light source can be positioned in a recess  13  so a facet of the light source is optically aligned with a facet of the cavity waveguide  12  to allow the light source and cavity waveguide  12  to exchange light signals. The cavity waveguide  12  carries the light signal to a partial return device  14 . The illustrated partial return device  14  is an optical grating such as a Bragg grating. However, other partial return devices  14  can be used; for instance, mirrors can be used in conjunction with echelle gratings and arrayed waveguide gratings. 
     The partial return device  14  returns a return portion of the light signal to the cavity waveguide  12  as a return signal. For instance, the cavity waveguide  12  returns the return signal to the light source  10  such that the return portion of the light signal travels through the gain medium. The light source  10  is configured such that at least a portion of the return signal is added to the light signal that is received at the cavity waveguide  12 . For instance, the light source  10  can include a highly, fully, or partially reflective device  15  that reflects the return signal received from the gain medium back into the gain medium. As a result, light can resonate between the partial return device  14  and the reflective device  15  so as to form a Distributed Bragg Reflector (DBR) laser cavity. A DBR laser cavity has an inherently narrow-linewidth and a longer coherence length than DFB lasers and accordingly improves performance when an object reflecting the LIDAR output signal from the chip is located further away from the chip. 
     The partial return device  14  passes a portion of the light signal received from the cavity waveguide  12  to a utility waveguide  16  included on the chip. The portion of the light signal that the utility waveguide  16  receives from the partial return device  14  serves as the output of the laser cavity. The output of the laser cavity serves as an outgoing LIDAR signal on the utility waveguide  16 . The utility waveguide  16  terminates at a facet  18  and carries the outgoing LIDAR signal to the facet  18 . The facet  18  can be positioned such that the outgoing LIDAR signal traveling through the facet  18  exits the chip and serves as a LIDAR output signal. For instance, the facet  18  can be positioned at an edge of the chip so the outgoing LIDAR signal traveling through the facet  18  exits the chip and serves as a LIDAR output signal that includes or consists of light from the outgoing LIDAR signal. 
     The LIDAR output signal travels away from the chip and is reflected by objects in the path of the LIDAR signal. The reflected signal travels away from the objects. At least a portion of the reflected signal returns to the facet  18  of the utility waveguide  16  as a LIDAR input signal. The LIDAR chip is configured to receive the LIDAR input signal through the facet  18 . The portion of the LIDAR input signal that enters the utility waveguide  16  through the facet  18  serve as an incoming LIDAR signal that is guided by the utility waveguide  16 . Accordingly, the incoming LIDAR signal includes or consists of light from the LIDAR input signal, the LIDAR output signal, and the outgoing LIDAR signal. 
     The utility waveguide  16  can include a tapered portion before the facet  18 . For instance, the utility waveguide  16  can include a taper  20  that terminate at the facet  18 . The taper  20  can relax the alignment tolerances required for efficient coupling of the utility waveguide  16  to the LIDAR input signal and the outgoing LIDAR signal. Accordingly, the taper  20  can increase the percentage of the LIDAR input signal that is successfully returned to the chip for processing. In some instances, the taper  20  is constructed such that the facet  18  has an area that is more than two, five, or ten times the area of a cross section of a straight portion of the utility waveguide  16 . Although  FIG. 1  shows the taper  20  as a horizontal taper, the taper  20  can be a horizontal and/or vertical taper. The horizontal and/or vertical taper can be linear and/or curved. In some instances, the taper  20  is an adiabatic taper. 
     The chip includes a data branch  24  where LIDAR data (the distance and/or radial velocity between a LIDAR chip and an object) is added to optical signals. The data branch includes an optical coupler  26  that moves a portion of the light signals from the utility waveguide  16  into the data branch. For instance, an optical coupler  26  couples a portion of the outgoing LIDAR signal from the utility waveguide  16  onto a reference waveguide  27  as a reference signal. The reference waveguide  27  carries the reference signal to a light-combining component  28 . 
     The optical coupler  26  also couples a portion of the incoming LIDAR signal from the utility waveguide  16  onto a comparative waveguide  30  as a comparative signal. As a result, the comparative signal includes or consists of at least a portion of the light from the incoming LIDAR signal. The comparative signal can exclude light from the reference light signal. The comparative waveguide  30  carries the comparative signal to the light-combining component  28 . 
     The illustrated optical coupler  26  is a result of positioning the utility waveguide  16  sufficiently close to the reference waveguide  27  and the comparative waveguide  30  that light from the utility waveguide  16  is coupled into the reference waveguide  27  and the comparative waveguide  30 ; however, other signal tapping components can be used to move a portion of the of the light signals from the utility waveguide  16  onto the reference waveguide  27  and the comparative waveguide  30 . Examples of suitable signal tapping components include, but are not limited to, y-junctions, multi-mode interference couplers (MMIs), and integrated optical circulators. 
     The light-combining component  28  combines the comparative signal and the reference signal into a composite signal. The reference signal includes light from the outgoing LIDAR signal. For instance, the reference signal can serve as a sample of the outgoing LIDAR signal. The reference signal can exclude light from the LIDAR output signal, the LIDAR input signal and the incoming LIDAR signal. In contrast, the comparative signal includes or consists of at least a portion of the light from the incoming LIDAR signal, the LIDAR input signal, the LIDAR output signal, and the outgoing LIDAR signal. Additionally, the comparative signal can exclude light from the reference light signal. As a result, the comparative signal can serve as a sample of the LIDAR input signal and/or of the incoming LIDAR signal. Accordingly, the comparative signal has been reflected by an object located off of the chip while the reference signal has not been reflected. When the chip and the reflecting object are moving relative to one another, the comparative signal and the reference signal have different frequencies due to the Doppler effect. As a result, beating occurs between the comparative signal and the reference signal. 
     The resulting composite sample signal is received at a light sensor. For instance, in the LIDAR system of  FIG. 1 , the light-combining component  28  also splits the resulting composite sample signal onto a first detector waveguide  36  and a second detector waveguide  38 . The first detector waveguide  36  carries a first portion of the composite sample signal to a first light sensor  40  that converts the first portion of the composite sample signal to a first electrical signal. The second detector waveguide  38  carries a second portion of the composite sample signal to a second light sensor  42  that converts the second portion of the composite sample signal to a second electrical signal. Examples of suitable light sensors include germanium photodiodes (PDs), and avalanche photodiodes (APDs). 
     The light combining component  28 , the first light sensor  40  and the second light sensor  42  can be connected as a balanced photodetector that outputs an electrical data signal. For instance, the light combining component  28 , the first light sensor  40  and the second light sensor  42  can be connected such that the DC components of the signal photocurrents cancel, improving detection sensitivity. Suitable methods for connecting the first light sensor  40  and the second light sensor  42  as balanced photodetectors includes connecting the first light sensor  40  and the second light sensor  42  in series. In one example, the first light sensor  40  and the second light sensor  42  are both avalanche photodiodes connected in series. Balanced photodetection is desirable for detection of small signal fluctuations. 
     An example of a suitable light-combining component  28  is a Multi-Mode Interference (MMI) device such as a 2×2 MMI device. Other suitable light-combining components  28  include, but are not limited to, adiabatic splitters, and directional coupler. In some instances, the functions of the illustrated light-combining component  28  are performed by more than one optical component or a combination of optical components. 
     A single light sensor can replace the first light sensor  40  and the second light sensor  42  and can output the data signal. When a single light sensor replaces the first light sensor  40  and the second light sensor  42 , the light-combining component  28  need not include light-splitting functionality. As a result, the illustrated light light-combining component  28  can be a 2×1 light-combining component rather than the illustrated 2×2 light-combining component. For instance, the illustrated light light-combining component can be a 2×1 MMI device. In these instances, the chip includes a single detector waveguide that carries the composite sample signal to the light sensor. 
     The data branch includes a data optical attenuator  44  positioned along the comparative waveguide  30  such that the data optical attenuator  44  can be operated so as to attenuate the comparative signal on the comparative waveguide  30 . The chip also includes an output optical attenuator  46  positioned along the utility waveguide  16  such that the output optical attenuator  46  can be operated so as to attenuate the outgoing LIDAR signal on the utility waveguide  16 . Suitable attenuators for the data optical attenuator  44  and/or the output optical attenuator  46  are configured to attenuate intensity of a light signal. Examples of a suitable attenuator configured to attenuate intensity of a light signal include carrier-injection based PIN diodes, electro-absorption modulators, and Mach-Zehnder (MZ) modulators. 
     The chip also includes a sampling directional coupler  50  that couples a portion of the comparative signal from the comparative waveguide  30  onto a sampling waveguide  52 . The coupled portion of the comparative signal serves as a sampling signal. The sampling waveguide  52  carries the sampling signal to a sampling light sensor  54 . Although  FIG. 1  illustrates a sampling directional coupler  50  moving a portion of the comparative signal onto the sampling waveguide  52 , other signal tapping components can be used to move a portion of the comparative signal from the comparative waveguide  30  onto the sampling waveguide  52 . Examples of suitable signal tapping components include, but are not limited to, y-junctions, and MMIs. 
     The chip includes a control branch  55  for controlling operation of the laser cavity. The control branch includes a directional coupler  56  that moves a portion of the outgoing LIDAR signal from the utility waveguide  16  onto a control waveguide  57 . The coupled portion of the outgoing LIDAR signal serves as a tapped signal. Although  FIG. 1  illustrates a directional coupler  56  moving portion of the outgoing LIDAR signal onto the control waveguide  57 , other signal-tapping components can be used to move a portion of the outgoing LIDAR signal from the utility waveguide  16  onto the control waveguide  57 . Examples of suitable signal tapping components include, but are not limited to, y-junctions, and MMIs. 
     The control waveguide  57  carries the tapped signal to an interferometer  58  that splits the tapped signal and then re-combines the different portions of the tapped signal with a phase differential between the portions of the tapped signal. The illustrated interferometer  58  is a Mach-Zehnder interferometer; however, other interferometers can be used. 
     The interferometer  58  outputs a control light signal on an interferometer waveguide  60 . The interferometer waveguide  60  carries the control light signal to a control light sensor  61  that converts the control light signal to an electrical signal that serves as an electrical control signal. The interferometer signal has an intensity that is a function of the frequency of the outgoing LIDAR signal. For instance, a Mach-Zehnder interferometer will output a sinusoidal control light signal with a fringe pattern. Changes to the frequency of the outgoing lidar signal will cause changes to the frequency of the control light signal. Accordingly, the frequency of the electrical control signal output from the control light sensor  61  is a function of the change in frequency of the outgoing lidar signal. Other detection mechanisms can be used in place of the control light sensor  61 . For instance, the control light sensor  61  can be replaced with a balanced photodetector arranged as the light combining component  28 , the first light sensor  40  and the second light sensor  42 . 
     Electronics  62  can operate one or more components on the chip. For instance, the electronics  62  can be in electrical communication with and control operation of the light source  10 , the data optical attenuator  44 , output optical attenuator  46 , the first light sensor  40 , the second light sensor  42 , the sampling light sensor  54 , and the control light sensor  61 . Although the electronics  62  are shown off the chip, all or a portion of the electronics can be included on the chip. For instance, the chip can include electrical conductors that connect the first light sensor  40  in series with the second light sensor  42 . 
     During operation of the chip, the electronics  62  operate the light source  10  such that the laser cavity outputs the outgoing LIDAR signal. The electronics  62  then operate the chip through a series of cycles. In some instances, each cycle corresponds to a region in a field of view. For instance, the LIDAR system can be configured to steer the LIDAR output signal to different regions in the field of view and to generate the LIDAR data for all or a portion of the regions. 
     The cycles can include one or more different time periods. In some instances, different periods are associated with a different waveform for the outgoing LIDAR signal. For instance, the electronics can add chirp to the frequency of the outgoing LIDAR signal and accordingly to the LIDAR output signal(s). The chirp can be different during adjacent periods in a cycle. For instance, the cycles can include one, two, three, or three or more periods that are each selected from a group consisting of a period where the frequency of the outgoing light signal is increased during the period, the frequency of the outgoing light signal is decreased during the period, and the frequency of the outgoing light signal is held constant during the period. In some instances, the increase or decrease in the frequency during a period is a linear function of time. In some instances, the periods are configured such that the waveform of the outgoing LIDAR signal is different in adjacent periods. For instance, when two periods are adjacent to one another in a cycle and the frequency of the outgoing LIDAR signal is increased in each of the two periods, the rate of increase can be different in the two adjacent periods. As will be described in more detail below, the electronics can employ output from the control branch in order to control the frequency of the outgoing LIDAR signal such that the frequency of the outgoing LIDAR signal as a function of time is known to the electronics. 
     In one example, a cycle includes at least a first period and a second period. During the first period, the electronics  62  can increase the frequency of the outgoing LIDAR signal and during the second period the electronics  62  can decrease the frequency of the outgoing LIDAR signal. For instance, the laser cavity can be configured to output an outgoing LIDAR signal (and accordingly a LIDAR output signal) with a wavelength of 1550 nm. During the first period, the electronics  62  can increase the frequency of the outgoing LIDAR signal (and accordingly a LIDAR output signal) such that the wavelength decreases from 1550 nm to 1459.98 nm followed by decreasing the frequency of the outgoing LIDAR signal such that the wavelength increases from 1459.98 nm to 1550 nm. 
     When the outgoing LIDAR signal frequency is increased during the first period, the resulting LIDAR output signal travels away from the chip and then returns to the chip as the incoming LIDAR signal. A portion of the incoming LIDAR signal becomes the comparative signal. During the time that the LIDAR output signal and the LIDAR input signal are traveling between the chip and a reflecting object, the frequency of the outgoing LIDAR signal continues to increase. Since a portion of the outgoing LIDAR signal becomes the reference signal, the frequency of the reference signal continues to increase. As a result, the comparative signal enters the light-combining component with a lower frequency than the reference signal concurrently entering the light-combining component. Additionally, the further the reflecting object is located from the chip, the more the frequency of the reference signal increases before the LIDAR input signal returns to the chip. Accordingly, the larger the difference between the frequency of the comparative signal and the frequency of the reference signal, the further the reflecting object is from the chip. As a result, the difference between the frequency of the comparative signal and the frequency of the reference signal is a function of the distance between the chip and the reflecting object. 
     For the same reasons, when the outgoing LIDAR signal frequency is decreased during the second period, the comparative signal enters the light-combining component with a higher frequency than the reference signal concurrently entering the light-combining component and the difference between the frequency of the comparative signal and the frequency of the reference signal during the second period is also function of the distance between the chip and the reflecting object. 
     In some instances, the difference between the frequency of the comparative signal and the frequency of the reference signal can also be a function of the Doppler effect because relative movement of the chip and reflecting object can also affect the frequency of the comparative signal. For instance, when the chip is moving toward or away from the reflecting object and/or the reflecting object is moving toward or away from the chip, the Doppler effect can affect the frequency of the comparative signal. Since the frequency of the comparative signal is a function of the speed the reflecting object is moving toward or away from the chip and/or the speed the chip is moving toward or away from the reflecting object, the difference between the frequency of the comparative signal and the frequency of the reference signal is also a function of the speed the reflecting object is moving toward or away from the chip and/or the speed the chip is moving toward or away from the reflecting object. Accordingly, the difference between the frequency of the comparative signal and the frequency of the reference signal is a function of the distance between the chip and the reflecting object and is also a function of the Doppler effect. 
     The composite sample signal and the data signal each effectively compares the comparative signal and the reference signal. For instance, since the light-combining component combines the comparative signal and the reference signal and these signals have different frequencies, there is beating between the comparative signal and reference signal. Accordingly, the composite sample signal and the data signal have a beat frequency related to the frequency difference between the comparative signal and the reference signal and the beat frequency can be used to determine the difference in the frequency of the comparative signal and the reference signal. A higher beat frequency for the composite sample signal and/or the data signal indicates a higher differential between the frequencies of the comparative signal and the reference signal. As a result, the beat frequency of the data signal is a function of the distance between the chip and the reflecting object and is also a function of the Doppler effect. 
     As noted above, the beat frequency is a function of two unknowns; the distance between the chip and the reflecting object and the relative velocity of the chip and the reflecting object (i.e., the contribution of the Doppler effect). The change in the frequency difference between the comparative signal and the reference signal (Δf) is given by Δf=2Δvf/c where f is the frequency of the LIDAR output signal and accordingly the reference signal, Δv is the relative velocity of the chip and the reflecting object and c is the speed of light in air. The use of multiple different periods permits the electronics  62  to resolve the two unknowns. For instance, the beat frequency determined for the first period is related to the unknown distance and Doppler contribution and the beat frequency determined for the second period is also related to the unknown distance and Doppler contribution. The availability of the two relationships allows the electronics  62  to resolve the two unknowns. Accordingly, the distance between the chip and the reflecting object can be determined without influence from the Doppler effect. Further, in some instances, the electronics  62  use this distance in combination with the Doppler effect to determine the velocity of the reflecting object toward or away from the chip. 
     In instances where the relative velocity of target and source is zero or very small, the contribution of the Doppler effect to the beat frequency is essentially zero. In these instances, the Doppler effect does not make a substantial contribution to the beat frequency and the electronics  62  can take only the first period to determine the distance between the chip and the reflecting object. 
     During operation, the electronics  62  can adjust the frequency of the outgoing LIDAR signal in response to the electrical control signal output from the control light sensor  61 . As noted above, the magnitude of the electrical control signal output from the control light sensor  61  is a function of the frequency of the outgoing LIDAR signal. Accordingly, the electronics  62  can adjust the frequency of the outgoing LIDAR signal in response to the magnitude of the control. For instance, while changing the frequency of the outgoing LIDAR signal during one of the periods, the electronics  62  can have a range of suitable values for the electrical control signal magnitude as a function of time. At multiple different times during a period, the electronics  62  can compare the electrical control signal magnitude to the range of values associated with the current time in the period. If the electrical control signal magnitude indicates that the frequency of the outgoing LIDAR signal is outside the associated range of electrical control signal magnitudes, the electronics  62  can operate the light source  10  so as to change the frequency of the outgoing LIDAR signal so it falls within the associated range. If the electrical control signal magnitude indicates that the frequency of the outgoing LIDAR signal is within the associated range of electrical control signal magnitudes, the electronics  62  do not change the frequency of the outgoing LIDAR signal. 
     During operation, the electronics  62  can adjust the level of attenuation provided by the output optical attenuator  46  in response to the sampling signal from the sampling light sensor  54 . For instance, the electronics  62  operate the output optical attenuator  46  so as to increase the level of attenuation in response to the magnitude of the sampling signal being above a first signal threshold and/or decrease the magnitude of the power drop in response to the magnitude of the sampling signal being below a second signal threshold. 
     In some instances, the electronics  62  adjust the level of attenuation provided by the output optical attenuator  46  to prevent or reduce the effects of back-reflection on the performance of the laser cavity. For instance, the first signal threshold and/or the second signal threshold can optionally be selected to prevent or reduce the effects of back-reflection on the performance of the laser cavity. Back reflection occurs when a portion of the incoming LIDAR signal returns to the laser cavity. In some instances, on the order of 50% of the incoming LIDAR signal returns to the laser cavity. The back reflection can affect performance of the laser cavity when the power of the incoming LIDAR signal entering the partial return device  14  does not decrease below the power of the outgoing LIDAR signal exiting from the partial return device  14  (“power drop”) by more than a minimum power drop threshold. In the illustrated chip, the minimum power drop threshold can be around 35 dB (0.03%). Accordingly, the incoming LIDAR signal can affect the performance of the laser cavity when the power of the incoming LIDAR signal entering the partial return device  14  is not more than 35 dB below the power of the outgoing LIDAR signal exiting from the partial return device  14 . 
     The electronics  62  can operate the output optical attenuator  46  so as to reduce the effect of low power drops, e.g. when the target object is very close or highly reflective or both. As is evident from  FIG. 1 , operation of the output optical attenuator  46  so as to increase the level of attenuation reduces the power of the incoming LIDAR signal entering the partial return device  14  and also reduces the power of the outgoing LIDAR signal at a location away from the partial return device  14 . Since the output optical attenuator  46  is located apart from the partial return device  14 , the power of the outgoing LIDAR signal exiting from the partial return device  14  is not directly affected by the operation of the output optical attenuator  46 . Accordingly, the operation of the output optical attenuator  46  so as to increase the level of attenuation increases the level of the power drop. As a result, the electronics can employ the optical attenuator  46  so as to tune the power drop. 
     Additionally, the magnitude of the sampling signal is related to the power drop. For instance, the magnitude of the sampling signal is related to the power of the comparative signal as is evident from  FIG. 1 . Since the comparative signal includes light from the incoming LIDAR signal and the LIDAR input signal, the magnitude of the sampling signal is related to the power of the incoming LIDAR signal and the magnitude of the LIDAR input signal. This result means the magnitude of the sampling signal is also related to the power of the incoming LIDAR signal. Accordingly, the magnitude of the sampling signal is related to the power drop. 
     Since the magnitude of the sampling signal is related to the power drop, the electronics  62  can use the magnitude of the sampling signal to operate the output optical attenuator so as to keep the magnitude of the comparative signal power within a target range. For instance, the electronics  62  can operate the output optical attenuator  46  so as to increase the magnitude of the power drop in response to the sampling signal indicating that the magnitude of power drop is at or below a first threshold and/or the electronics  62  can operate the output optical attenuator  46  so as to decrease the magnitude of the power drop in response to the sampling signal indicating that the magnitude of power drop is at or above a second threshold. In some instances, the first threshold is greater than or equal to the minimum power drop threshold. In one example, the electronics  62  operate the output optical attenuator  46  so as to increase the magnitude of the power drop in response to the magnitude of the sampling signal being above a first signal threshold and/or decrease the magnitude of the power drop in response to the magnitude of the sampling signal being below a second signal threshold. The identification of the value(s) for one, two, three, or four variables selected from the group consisting of the first threshold, the second threshold, the first signal threshold, and the second signal threshold can be determined from calibration of the optical chip during set-up of the LIDAR chip system. 
     Light sensors can become saturated when the power of the composite light signal exceeds a power threshold. When a light sensor becomes saturated, the magnitude of the data signal hits a maximum value that does not increase despite additional increases in the power of the composite light signal above the power threshold. Accordingly, data can be lost when the power of the composite light signal exceeds a power threshold. During operation, the electronics  62  can adjust the level of attenuation provided by the data optical attenuator  44  so the power of the composite light signal is maintained below a power threshold. 
     As is evident from  FIG. 1 , the magnitude of the sampling signal is related to the power of the comparative signal. Accordingly, the electronics  62  can operate the data optical attenuator  44  in response to output from the sampling signal. For instance, the electronics  62  can operate the data optical attenuator so as to increase attenuation of the comparative signal when the magnitude of the sampling signal indicates the power of the comparative signal is above an upper comparative signal threshold and/or can operate the data optical attenuator so as to decrease attenuation of the comparative signal when the magnitude of the sampling signal indicates the power of the comparative signal is below a lower comparative signal threshold. For instance, in some instances, the electronics  62  can increase attenuation of the comparative signal when the magnitude of the sampling signal is at or above an upper comparative threshold and/or the electronics  62  decrease attenuation of the comparative signal when the magnitude of the sampling signal is at or below an upper comparative signal threshold. 
     As noted above, the electronics  62  can adjust the level of attenuation provided by the output optical attenuator  46  in response to the sampling signal. The electronics  62  can adjust the level of attenuation provided by the data optical attenuator  44  in response to the sampling signal in addition or as an alternative to adjusting the level of attenuation provided by the output optical attenuator  46  in response to the sampling signal. 
     Suitable platforms for the chip include, but are not limited to, silica, indium phosphide, and silicon-on-insulator wafers.  FIG. 2  is a cross-section of portion of a chip constructed from a silicon-on-insulator wafer. A silicon-on-insulator (SOI) wafer includes a buried layer  80  between a substrate  82  and a light-transmitting medium  84 . In a silicon-on-insulator wafer, the buried layer is silica while the substrate and the light-transmitting medium are silicon. The substrate of an optical platform such as an SOI wafer can serve as the base for the entire chip. For instance, the optical components shown in  FIG. 1  can be positioned on or over the top and/or lateral sides of the substrate. 
     The portion of the chip illustrated in  FIG. 2  includes a waveguide construction that is suitable for use with chips constructed from silicon-on-insulator wafers. A ridge  86  of the light-transmitting medium extends away from slab regions  88  of the light-transmitting medium. The light signals are constrained between the top of the ridge and the buried oxide layer. 
     The dimensions of the ridge waveguide are labeled in  FIG. 2 . For instance, the ridge has a width labeled w and a height labeled h. A thickness of the slab regions is labeled T. For LIDAR applications, these dimensions are more important than other applications because of the need to use higher levels of optical power than are used in other applications. The ridge width (labeled w) is greater than 1 μm and less than 4 μm, the ridge height (labeled h) is greater than 1 μm and less than 4 μm, the slab region thickness is greater than 0.5 μm and less than 3 μm. These dimensions can apply to straight or substantially straight portions of the waveguide, curved portions of the waveguide and tapered portions of the waveguide(s). Accordingly, these portions of the waveguide will be single mode. However, in some instances, these dimensions apply to straight or substantially straight portions of a waveguide while curved portions of the waveguide and/or tapered portions of the waveguide have dimensions outside of these ranges. For instance, the tapered portions of the utility waveguide  16  illustrated in  FIG. 1  can have a width and/or height that is &gt;4 μm and can be in a range of 4 μm to 12 μm. Additionally or alternately, curved portions of a waveguide can have a reduced slab thickness in order to reduce optical loss in the curved portions of the waveguide. For instance, a curved portion of a waveguide can have a ridge that extends away from a slab region with a thickness greater than or equal to 0.0 μm and less than 0.5 μm. While the above dimensions will generally provide the straight or substantially straight portions of a waveguide with a single-mode construction, they can result in the tapered section(s) and/or curved section(s) that are multimode. Coupling between the multi-mode geometry to the single mode geometry can be done using tapers that do not substantially excite the higher order modes. Accordingly, the waveguides can be constructed such that the signals carried in the waveguides are carried in a single mode even when carried in waveguide sections having multi-mode dimensions. The waveguide construction of  FIG. 2  is suitable for all or a portion of the waveguides selected from the group consisting of the cavity waveguide  12 , utility waveguide  16 , reference waveguide  27 , comparative waveguide  30 , first detector waveguide  36 , second detector waveguide  38 , sampling waveguide  52 , control waveguide  57 , and interferometer waveguide  60 . 
     The light source  10  that is interfaced with the utility waveguide  16  can be a gain element that is a component separate from the chip and then attached to the chip. For instance, the light source  10  can be a gain element that is attached to the chip using a flip-chip arrangement. 
     Use of flip-chip arrangements is suitable when the light source  10  is to be interfaced with a ridge waveguide on a chip constructed from silicon-on-insulator wafer. Examples of suitable interfaces between flip-chip gain elements and ridge waveguides on chips constructed from silicon-on-insulator wafer can be found in U.S. Pat. No. 9,705,278, issued on Jul. 11, 2017 and in U.S. Pat. No. 5,991,484 issued on Nov. 23 1999; each of which is incorporated herein in its entirety. The constructions are suitable for use as the light source  10 . When the light source  10  is a gain element, the electronics  62  can change the frequency of the outgoing LIDAR signal by changing the level of electrical current applied to through the gain element. 
     The attenuators can be a component that is separate from the chip and then attached to the chip. For instance, the attenuator can be included on an attenuator chip that is attached to the chip in a flip-chip arrangement. The use of attenuator chips is suitable for all or a portion of the attenuators selected from the group consisting of the data attenuator and the control attenuator. 
     As an alternative to including an attenuator on a separate component, all or a portion of the attenuators can be integrated with the chip. For instance, examples of attenuators that are interfaced with ridge waveguides on a chip constructed from a silicon-on-insulator wafer can be found in U.S. Pat. No. 5,908,305, issued on Jun. 1 1999; each of which is incorporated herein in its entirety. The use of attenuators that are integrated with the chip are suitable for all or a portion of the light sensors selected from the group consisting of the data attenuator and the control attenuator. 
     Light sensors that are interfaced with waveguides on a chip can be a component that is separate from the chip and then attached to the chip. For instance, the light sensor can be a photodiode, or an avalanche photodiode. Examples of suitable light sensor components include, but are not limited to, InGaAs PIN photodiodes manufactured by Hamamatsu located in Hamamatsu Ciy, Japan, or an InGaAs APD (Avalanche Photo Diode) manufactured by Hamamatsu located in Hamamatsu City, Japan. These light sensors can be centrally located on the chip as illustrated in  FIG. 1 . Alternately, all or a portion the waveguides that terminate at a light sensor can terminate at a facet  18  located at an edge of the chip and the light sensor can be attached to the edge of the chip over the facet  18  such that the light sensor receives light that passes through the facet  18 . The use of light sensors that are a separate component from the chip is suitable for all or a portion of the light sensors selected from the group consisting of the first light sensor  40 , the second light sensor  42 , the sampling light sensor  54 , and the control light sensor  61 . 
     As an alternative to a light sensor that is a separate component, all or a portion of the light sensors can be integrated with the chip. For instance, examples of light sensors that are interfaced with ridge waveguides on a chip constructed from a silicon-on-insulator wafer can be found in Optics Express Vol. 15, No. 21, 13965-13971 (2007); U.S. Pat. No. 8,093,080, issued on Jan. 10 2012; U.S. Pat. No. 8,242,432, issued Aug. 14 2012; and U.S. Pat. No. 6,108,8472, issued on Aug. 22, 2000 each of which is incorporated herein in its entirety. The use of light sensors that are integrated with the chip are suitable for all or a portion of the light sensors selected from the group consisting of the first light sensor  40 , the second light sensor  42 , the sampling light sensor  54 , and the control light sensor  61 . 
     Construction of optical gratings that are integrated with a variety of optical device platforms are available. For instance, a Bragg grating can be formed in a ridge waveguides by forming grooves in the top of the ridge and/or in the later sides of the ridge. 
     In some instances, it is desirable to scan the LIDAR output signal to different regions in a field of view and to generate the LIDAR data for all or a portion of the regions. The above chip construction is suitable for use with various scanning mechanisms used in LIDAR applications. For instance, the output LIDAR signal can be received by one or more reflecting devices and/or one more collimating devices. The one or more reflecting devices can be configured to re-direct and/or steer the LIDAR output signal so as to provide scanning of the LIDAR output signal. Suitable reflecting devices include, but are not limited to, mirrors such mechanically driven mirrors and Micro Electro Mechanical System (MEMS) mirrors. The one or more collimating devices provide collimation of the LIDAR output signal and can accordingly increase the portion of the LIDAR input signal that is received in the utility waveguide  16 . Suitable collimating devices include, but are not limited to, individual lenses and compound lenses. 
       FIG. 3  illustrates the above chip used with a reflecting device  90  and a collimating device  92 . For instance, a lens serves as a collimating device that receives the LIDAR output signal and provides collimation of the LIDAR output signal. A mirror serves as a reflecting device  90  that receives the collimated LIDAR output signal and reflects the collimated LIDAR output signal in the desired direction. As is illustrated by the arrow labeled A, the electronics can move the mirror so as to steer the collimated LIDAR output signal and/or scan the collimated LIDAR output signal. The movement of the mirror can be in one dimension or two dimensions. Suitable mirrors include, but are not limited to, mechanically driven mirrors and Micro Electro Mechanical System (MEMS) mirrors. 
       FIG. 4  illustrates the above chip used with a reflecting device  90  and a collimating device  92 . For instance, a mirror serves as a reflecting device  90  that receives the LIDAR output signal and reflects the LIDAR output signal in the desired direction. As is illustrated by the arrow labeled A, the electronics can move the mirror so as to steer the LIDAR output signal and/or scan the LIDAR output signal. A lens serves as a collimating device  92  that receives the LIDAR output signal from the mirror and provides collimation of the LIDAR output signal. The lens can be configured to move with the movement of the mirror so the lens continues to receive the LIDAR output signal at different positions of the mirror. Alternately, the movement of the mirror can be sufficiently limited that the lens continues to receive the LIDAR output signal at different positions of the mirror. The movement of the mirror can be in one dimension or two dimensions. Suitable mirrors include, but are not limited to, mechanically driven mirrors and Micro Electro Mechanical System (MEMS) mirrors. 
     Technologies such as SOI MEMS (Silicon-On-Insulator Micro Electro Mechanical System) technology can be used to incorporate a reflecting device such as a MEMS mirror into the chip. For instance,  FIG. 5  is a cross section of a portion of the chip taken through the longitudinal axis of the utility waveguide  16 . The illustrated chip was constructed on silicon-on-insulator waveguide. A mirror recess extends through the light-transmitting medium to the base. The mirror is positioned in the mirror recess such that the mirror receives the LIDAR output signal from the utility waveguide. A lens serves as a collimating device  92  that receives the LIDAR output signal from the mirror and provides collimation of the LIDAR output signal. The lens can be configured to move with the movement of the mirror so the lens continues to receive the LIDAR output signal at different positions of the mirror. Alternately, the movement of the mirror can be sufficiently limited that the lens continues to receive the LIDAR output signal at different positions of the mirror. The electronics can control movement of the mirror in one dimension or two dimensions. 
     The chips can be modified so that data branch includes one or more secondary branches and one or more secondary balanced detectors that can be employed to refine the optical data provided to the electronics. The reference signal and the comparative signal can be divided among the different balanced detectors. For instance,  FIG. 6A  illustrates the above chip modified to include two different balanced detectors. A first splitter  102  divides the reference signal carried on the reference waveguide  27  onto a first reference waveguide  110  and a second reference waveguide  108 . The first reference waveguide  110  carries a first portion of the reference signal to the light-combining component  28 . The second reference waveguide  108  carries a second portion of the reference signal to a second light-combining component  112 . 
     A second splitter  100  divides the comparative signal carried on the comparative waveguide  30  onto a first comparative waveguide  104  and a second comparative waveguide  106 . The first comparative waveguide  104  carries a first portion of the comparative signal to the light-combining component  28 . The second comparative waveguide  108  carries a second portion of the comparative signal to the second light-combining component  112 . 
     The light-combining component  28  combines the first portion of the comparative signal and the first portion of the reference signal into composite signal. The light-combining component  28  also splits the composite signal onto the first detector waveguide  36  and the second detector waveguide  38 . As noted above, the first detector waveguide  36  carries the first portion of the composite sample signal to the first light sensor  40  which converts the first portion of the composite sample signal to a first electrical signal. The second detector waveguide  38  carries the second portion of the composite sample signal to the second light sensor  42  which converts the second portion of the composite sample signal to a second electrical signal. 
     The second light-combining component  112  combines the second portion of the comparative signal and the second portion of the reference signal into a second composite signal. The second light-combining component  112  also splits the second composite signal onto a first auxiliary detector waveguide  114  and a second auxiliary detector waveguide  116 . The first auxiliary detector waveguide  114  carries a first portion of the second composite signal to a first auxiliary light sensor  118  that converts the first portion of the second composite signal to a first auxiliary electrical signal. The second auxiliary detector waveguide  116  carries a second portion of the second composite signal to a second auxiliary light sensor  120  that converts the second portion of the second composite signal to a second auxiliary electrical signal. Examples of suitable light sensors include germanium photodiodes (PDs), and avalanche photodiodes (APDs). 
     The first reference waveguide  110  and the second reference waveguide  108  are constructed to provide a phase shift between the first portion of the reference signal and the second portion of the reference signal. For instance, the first reference waveguide  110  and the second reference waveguide  108  can be constructed so as to provide a 90° phase shift between the first portion of the reference signal and the second portion of the reference signal. Accordingly, one of the reference signal portions can be a sinusoidal function and the other reference signal portion can be a cosinusoidal function. In one example, the first reference waveguide  110  and the second reference waveguide  108  are constructed such that the first reference signal portion is a cosine function and the second reference signal portion is a sinusoidal function. Accordingly, the portion of the reference signal in the first composite signal is phase shifted relative to the portion of the reference signal in the second composite signal, however, the portion of the comparative signal in the first composite signal is not phase shifted relative to the portion of the comparative signal in the second composite signal. As a result, the first composite signal and the second composite signal can act as the components of a complex composite signal. For instance, the first composite signal can be an in-phase component of the complex composite signal and the second composite signal can be the quadrature component of the complex composite signal. In these examples a particular relationship is not required between the phase of the portion of the reference signal in the first composite signal and the portion of the comparative signal in the first composite signal and/or between the phase of the portion of the reference signal in the second composite signal and the portion of the comparative signal in the second composite signal. 
     In one example, the portion of the reference signal in the first composite signal is phase-shifted relative to the portion of the reference signal in the second composite signal but the portion of the comparative signal in the first composite signal is in-phase with the portion of the comparative signal in the second composite signal. 
     The desired phase shift can be a result of a length differential between the length of the first reference waveguide  110  and the second reference waveguide  108 . In some instances, the length differential is on the order of 0.05-0.3 microns in order to achieve the desired phase shift such as a 90° phase shift. The desired length differential can be a function of wavelength. 
     In some instances, depending on the layout of the LIDAR chip, there are one or more intersections that are each between two waveguides selected from the waveguide group consisting of the first comparative waveguide  104 , the second comparative waveguide  106 , the first reference waveguide  110  and the second reference waveguide  108 . For instance,  FIG. 6A  illustrates an intersection between the second comparative waveguide  106  and the first reference waveguide  110 . The waveguide intersection can affect the effective index of refraction of the intersection waveguides. As a result, one or more dummy waveguides  119  can optionally be added to one or more of the waveguides in the waveguide groups so as to bring each waveguide to the same number of intersections. For instance, in the layout of  FIG. 6A , the first comparative waveguide  104  and the second reference waveguide  108  do not have an intersection. As a result, a dummy waveguide  119  is added to the first comparative waveguide  104  and the second reference waveguide  108  so as to provide each waveguide in the waveguide groups with the same number of intersections. Providing each waveguide in the waveguide group with the same number of intersections allows the phase difference between the light signals carried in these waveguides to be controlled. The dummy waveguides can each be constructed such that any light guided within the dummy waveguide is not guided to other components on the LIDAR chip and/or to other waveguides on the LIDAR chip. Accordingly, light guided within the dummy waveguides is not processed by the LIDAR chip. As a result, the dummy waveguides can include two terminal ends as illustrated in  FIG. 6A . 
     The first light sensor  40  and the second light sensor  42  can be connected as a balanced detector and the first auxiliary light sensor  118  and the second auxiliary light sensor  120  can also be connected as a balanced detector. For instance,  FIG. 6B  provides a schematic of the relationship between the electronics, the first light sensor  40 , the second light sensor  42 , the first auxiliary light sensor  118 , and the second auxiliary light sensor  120 . The symbol for a photodiode is used to represent the first light sensor  40 , the second light sensor  42 , the first auxiliary light sensor  118 , and the second auxiliary light sensor  120  but one or more of these sensors can have other constructions. 
     The electronics connect the first light sensor  40  and the second light sensor  42  as a first balanced detector  124 . In particular, the first light sensor  40  and the second light sensor  42  are connected in series. The first balanced detector  124  acts as a light sensor that converts the first composite signal to an electrical signal (a first data signal) carried on a first data line  128 . Additionally, the electronics connect the first auxiliary light sensor  118  and the second auxiliary light sensor  120  as a second balanced detector  126 . In particular, the first auxiliary light sensor  118  and the second auxiliary light sensor  120  are connected in series. The second balanced detector  124  acts as a light sensor that converts the second composite signal to an electrical signal (a second data signal) carried on a second data line  132 . 
     The first data line  128  carries the first data signal to a transform module  136  and the second data line  132  carries the second data signal to the transform module  136 . The transform module is configured to perform a complex transform on a complex signal so as to convert the input from the time domain to the frequency domain. The first data signal can be the real component of the complex signal and the second data signal can be the imaginary component of the complex signal. The transform module can execute the attributed functions using firmware, hardware and software or a combination thereof. 
     The solid line in  FIG. 6C  provides an example of the output of the transform module when a Complex Fourier transform converts the input from the time domain to the frequency domain. The solid line shows a single frequency peak. The frequency associated with this peak is used by the electronics as the frequency of the LIDAR input signal and/or as the frequency of the incoming LIDAR signal. 
     The electronics use this frequency for further processing to generate the LIDAR data (distance and/or radial velocity between the reflecting object and the LIDAR chip or LIDAR system).  FIG. 6C  also includes a second peak illustrated by a dashed line. Prior methods of resolving the frequency of the LIDAR input signal made use of real Fourier transforms rather than the Complex Fourier transform technique disclosed above. These prior methods output both the peak shown by the dashed line and the solid line. As noted above, when using LIDAR applications, it can become difficult to identify the correct peak. Since the above technique for resolving the frequency generates a single solution for the frequency, the inventors have resolved the ambiguity with the frequency solution. 
     The electronics use the single frequency that would be present in  FIG. 6C  to determine the distance of the reflecting object from the chip and/or the relative speed of the object and the chip. For instance, the following equation applies during a period where electronics linearly increase the frequency of the outgoing LIDAR signal during the period: +f ub =f d +ατ 0  where f ub  is the frequency provided by the transform module, f d  represents the Doppler shift (f d =2vf c /c) where f c  is the frequency of the LIDAR output signal at the start of the period (i.e. t=0), v is the radial velocity between the reflecting object and the LIDAR chip where the direction from the reflecting object toward the chip is assumed to be the positive direction, and c is the speed of light, a represents the rate at which the frequency of the outgoing LIDAR signal is increased or decreased during the period, and τ 0  is the roundtrip delay (time between the LIDAR output signal exiting from the LIDAR chip and the associated LIDAR input signal returning to the LIDAR chip) for a stationary reflecting object. The following equation applies during a period where electronics decrease the frequency of the outgoing LIDAR signal: −f db =−f d −ατ 0  where f db  is the frequency provided by the transform module. In these two equations, v and τ 0  are unknowns. The radial velocity can then be determined from the Doppler shift and the separation distance can be determined from c*τ 0 /2. 
     Above, the complex composite signal is described as having an in-phase component and a quadrature component that include out-of-phase portions of the reference signal; however, the unambiguous LIDAR data solution can be achieved by generating other complex composite signals. For instance, the unambiguous LIDAR data solution can be achieved using a complex composite signal where the in-phase component and the quadrature component include out-of-phase portions of the comparative signal. For instance, the first comparative waveguide  104  and the second comparative waveguide  106  can be constructed so as to provide a 90 degree phase shift between the first portion of the comparative signal and the second portion of the comparative signal but the first reference waveguide  110  and the second reference waveguide  108  are constructed such that the first portion of the reference signal and the second portion of the reference signal are in-phase in the composite signals. Accordingly, the portion of the comparative signal in the first composite signal is phase shifted relative to the portion of the comparative signal in the second composite signal, however, the portion of the reference signal in the first composite signal is not phase shifted relative to the portion of the reference signal in the second composite signal. 
     Example 1 
     The mathematics for a LIDAR system where the portion of the reference signal in the first composite signal is out of phase by 90° relative to the portion of the reference signal in the second composite signal and the portion of the comparative signal in the first composite signal is in-phase with the portion of the comparative signal in the second composite signal are set forth in this example. The electronics operate the LIDAR system with cycles that have a first period and a second period. During the first period, the frequency of the LIDAR output signal is ramped up. During the second period, the frequency of the LIDAR output signal is ramped down. 
     The roundtrip time can be mathematically expressed as the following equation 1: 
               τ   ⁡     (   t   )       =           2   ⁢   R   ⁢           ⁢   0     c     -         2   ⁢   v     c     ⁢   t       =       τ   0     -         2   ⁢   v     c     ⁢   t               
where RO represents the initial distance of the object from the LIDAR system, c represents the speed of light, τ 0 =2RO/c represents the roundtrip delay for a stationary target in seconds, v is the radial velocity, and t represents time where the time is zero when at the start of the period. Assuming the direction from the object to the LIDAR system as the positive direction, v will be positive when the target and transmitter move towards each other and negative when the target and transmitter move away from each other.
 
     When an unmodulated LIDAR output signal is represented by a sinusoid such as cos(2πf e t), with f e  representing the frequency of the sinusoid and t representing time, the LIDAR input signal can be represented by cos(2πf e t+Φ(t)), where Φ(t) is the phase of the LIDAR input signal relative to the transmitted signal and is given by the following equation 2: 
               Φ   ⁡     (   t   )       =         -   2     ⁢   π   ⁢           ⁢     f   c     ⁢     τ   ⁡     (   t   )         =         -   2     ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       +     2   ⁢     π   ⁡     (         2   ⁢   v     c     ⁢     f   c       )       ⁢     t   .                 
Substituting this phase into the equation for the LIDAR input signal provides the following Equation 3: S rx (t)∝ cos(2π(f c +f d )t−2πf c T 0 ) where f d =2vf c /c is known as the Doppler frequency shift and can be positive or negative depending on the radial velocity of the object. The roundtrip delay can now be rewritten in terms of f d  and τ 0  as the following Equation 4:
 
               τ   ⁡     (   t   )       =           2   ⁢   R   ⁢           ⁢   0     c     -         2   ⁢   v     c     ⁢   t       =       τ   0     -         f   d       f   c       ⁢     t   .                 
When τ 0  and f d  are measured, this equation can be used to evaluate RO and v. The above LIDAR system is used to measure τ 0  and f d .
 
     As noted above, the outgoing LIDAR signal can be modulated such that chirp is added to the outgoing LIDAR signal during one or more periods in a cycle. When chirp is added to the outgoing LIDAR signal, the frequency of the outgoing LIDAR signal can be represented by the following Equation 5: f(t)=f c +αt where f c  is the frequency of the outgoing LIDAR signal without the added chirp, t is time and t=0 at the start of the period, and a is the rate at which the frequency of the outgoing LIDAR signal is changed during the period. The phase of the outgoing LIDAR signal (in radians per second) may be obtained by integrating equation 5 as shown in the following Equation 6: Φ(t)=2π∫f(t)·dt=2πf c t+παt 2 . As a result, the outgoing LIDAR signal can be expressed as the following Equation 7: S tx (t)=√{square root over (P)} cos(2πf c  t+παt 2 ) where √{square root over (P)} is the power of the LIDAR output signal. The resulting LIDAR input signal upon return to the LIDAR system (i.e. at the facet or after undergoing the roundtrip delay τ defined above by Equation 1) and propagation attenuation β can be represented by the following equation 8: S rx (t)=√{square root over (BP)} cos(2πf c (t−τ)+πα(t−τ) 2 ). 
     As described above, a portion of the LIDAR input signal is mixed with an in-phase version of a reference signal and another portion of the LIDAR input signal is mixed with the quadrature version of the reference signal (i.e., complex mixing). The resulting reference signal may then be represented by adapting Equation 6 as shown in the following Equation 8: S ref (t)=(√{square root over (Pγ/ 2 )} cos(2πf c t+παt 2 ), (√{square root over (Pγ/ 2 )} sin(2πf c t+παt 2 )) where γ represents a ratio of the power of the reference signal power to the power of the LIDAR output signal. In Equation 8, the first term represents the in-phase portion of the reference signal and the second term represents the quadrature portion of the reference signal. 
     As described above, the reference signal and the comparative signal (the LIDAR input signal) are combined to provide a complex and beating composite signal. During the first period, the complex composite signal can be represented by the following Equation 9: 
                 I   ub     +     jQ   ub       ≈         P   4     ⁢     γβ     ⁢     cos   ⁡     (       2   ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       -     2   ⁢   π   ⁢           ⁢     f   d     ⁢   t     +     2   ⁢     πατ   0     ⁢   t       )         +     j   ⁢     P   4     ⁢     γβ     ⁢     sin   ⁡     (       2   ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       -     2   ⁢   π   ⁢           ⁢     f   d     ⁢   t     +     2   ⁢     πατ   0     ⁢   t       )                 
where I ub  represents the first composite signal and/or the first data signal and Q ub  represents the second composite signal and/or the second data signal. When written in exponential format, the complex beat signal during the first period can be represented by the following Equation 10:
 
                 I   ub     +     jQ   ub       ≈       P   4     ⁢     γβ     ⁢       exp   ⁡     (     j   ⁡     (       2   ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       -     2   ⁢   π   ⁢           ⁢     f   d     ⁢   t     +     2   ⁢     πατ   0     ⁢   t       )       )       .             
As the beat signal is complex and no longer real, the frequency of its spectral peak is uniquely expressed by the following Equation 11: F ub =−f d +ατ 0  where F ub  can be positive, negative or zero.
 
     During the second period, the complex composite signal can be represented by the following Equation 12: 
                 I   db     +     jQ   db       ≈         P   4     ⁢     γβ     ⁢     cos   ⁡     (       2   ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       -     2   ⁢   π   ⁢           ⁢     f   d     ⁢   t     -     2   ⁢     πατ   0     ⁢   t       )         +     j   ⁢     P   4     ⁢     γβ     ⁢     sin   ⁡     (       2   ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       -     2   ⁢   π   ⁢           ⁢     f   d     ⁢   t     -     2   ⁢     πατ   0     ⁢   t       )                 
where I db  represents the first composite signal and/or the first data signal and Q db  represents the second composite signal and/or the second data signal. When written in exponential format, the complex beat signal during the second period can be represented by the following Equation 13:
 
                 I   db     +     jQ   db       ≈       P   4     ⁢     γβ     ⁢     exp   ⁡     (     j   ⁡     (       2   ⁢   π   ⁢           ⁢     f   c     ⁢     τ   0       -     2   ⁢   π   ⁢           ⁢     f   d     ⁢   t     -     2   ⁢     πατ   0     ⁢   t       )       )               
with the corresponding beat frequency given by the following Equation 14: F db =−f d −ατ 0 . The unambiguous solutions to Equation 11 and Equation 14 are represented by:
 
               f   d     =         -         F   ub     +     F   db       2       ⁢           ⁢     τ   0       =     +           F   ub     -     F   db         2   ⁢   α       .               
The LIDAR data (radial distance and velocity) can be calculated without ambiguity as follows:
 
     
       
         
           
             
               R 
               ⁢ 
               
                   
               
               ⁢ 
               0 
             
             = 
             
               
                 
                   
                     c 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       τ 
                       0 
                     
                   
                   2 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
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     Suitable electronics can include, but are not limited to, a controller that includes or consists of analog electrical circuits, digital electrical circuits, processors, microprocessors, digital signal processors (DSPs), computers, microcomputers, or combinations suitable for performing the operation, monitoring and control functions described above. In some instances, the controller has access to a memory that includes instructions to be executed by the controller during performance of the operation, control and monitoring functions. Although the electronics are illustrated as a single component in a single location, the electronics can include multiple different components that are independent of one another and/or placed in different locations. Additionally, as noted above, all or a portion of the disclosed electronics can be included on the chip including electronics that are integrated with the chip. 
     A single light sensor can replace the second balanced detector first light sensor  40  and the second light sensor  42  and/or a second light sensor can replace the first auxiliary light sensor  118  and the second auxiliary light sensor  120 . When a single light sensor replaces the first light sensor  40  and the second light sensor  42 , the light-combining component  28  need not include light-splitting functionality. As a result, the illustrated light light-combining component  28  can be a 2×1 light-combining component rather than the illustrated 2×2 light-combining component. For instance, the illustrated light light-combining component can be a 2×1 MMI device. In these instances, the chip includes a single detector waveguide that carries the composite signal to the light sensor. 
     When a single light sensor replaces the first auxiliary light sensor  118  and the second auxiliary light sensor  120 , second light-combining component  112  need not include light-splitting functionality. As a result, the illustrated second light-combining component  112  can be a 2×1 light-combining component rather than the illustrated 2×1 light-combining component. For instance, the illustrated light light-combining component can be a 2×1 MMI device. In these instances, the chip includes a single detector waveguide that carries the composite signal from the second light-combining component  112  to the light sensor. 
     Suitable electronics can include, but are not limited to, a controller that includes or consists of analog electrical circuits, digital electrical circuits, processors, microprocessors, digital signal processors (DSPs), computers, microcomputers, or combinations suitable for performing the operation, monitoring and control functions described above. In some instances, the controller has access to a memory that includes instructions to be executed by the controller during performance of the operation, control and monitoring functions. Although the electronics are illustrated as a single component in a single location, the electronics can include multiple different components that are independent of one another and/or placed in different locations. Additionally, as noted above, all or a portion of the disclosed electronics can be included on the chip including electronics that are integrated with the chip. 
     Although the laser cavity is shown as being positioned on the chip, all or a portion of the laser cavity can be located off the chip. For instance, the utility waveguide  16  can terminate at a second facet through which the outgoing LIDAR signal can enter the utility waveguide  16  from a laser cavity located off the chip. 
     The chip can include components in addition to the illustrated components. As one example, optical attenuators (not illustrated) can be positioned along the first detector waveguide  36  and the second detector waveguide  38 . The electronics can operate these attenuators so the power of the first portion of the composite sample signal that reaches the first light sensor  40  is the same or about the same as the power of the second portion of the composite sample signal that reaches the second light sensor  42 . The electronics can operate the attenuators in response to output from the first light sensor  40  which indicates the power level of the first portion of the composite sample signal and the second light sensor  42  which indicates the power level of the second portion of the composite sample signal. 
     Other embodiments, combinations and modifications of this invention will occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawings.