Patent Publication Number: US-2022236393-A1

Title: Techniques for increasing effective power in multi-beam lidar systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/586,737, filed on Sep. 27, 2019. The disclosure the above-referenced application is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to light detection and ranging (LIDAR) systems and methods using multi-laser transmitters to increase range and detection sensitivity. 
     BACKGROUND 
     Increasing LIDAR detection range and detection sensitivity has been limited to designing more powerful lasers, but this approach is difficult and costly in applications such as autonomous vehicle navigation systems. 
     SUMMARY 
     The present disclosure describes example LIDAR systems and methods for increasing the effective power and sensitivity of a LIDAR transceiver through the use of multiple coherent lasers with constructive interference. 
     In one example, a LIDAR system includes a number of optical sources to emit a corresponding number of optical beams with synchronized chirp rates and chirp durations, where the optical beams provide a comb of coherent optical beams in the frequency/wavelength domain with a fixed frequency separation between frequency adjacent optical beams. 
     In one example, the fixed frequency separation between the frequency adjacent optical beams is selected to produce periodic regions of constructive and destructive interference when the optical beams are combined, where the separation between the periodic regions of constructive interference is less than the range resolution of the system. 
     In one example, a first set of optical components is coupled with the optical sources to amplify and combine the optical beams into a combined optical beam. 
     In one example, a second set of optical components is coupled to the first set of optical components to transmit the combined optical beam toward a target environment and to receive a target return signal. 
     In one example, a third set of optical components is coupled to the second set of optical components to downconvert the target return signal to a number of fixed frequency downconverted target return signals corresponding to the number of optical beams, and to coherently combine the downconverted target return signals. 
     In one example, a fourth set of optical components is coupled to the first set of optical components to sample the optical beams and to generate control signals to synchronize the optical sources. 
     In one example, a method in a LIDAR system includes, generating, from a number of optical sources, a corresponding number of optical beams with synchronized chirp rates and chirp durations, where the optical beams provide a comb of coherent optical beams with a fixed frequency offset between frequency adjacent optical beams. In one example, the method also includes combining the plurality of optical beams into a combined optical beam, transmitting the combined optical beam toward a target environment, downconverting a target return signal to a number of fixed frequency downconverted target return signals corresponding to the number of optical beams, and coherently combining the downconverted target return signals. 
     In one example, the fixed frequency separation between the frequency adjacent optical beams is selected to produce periodic regions of constructive and destructive interference in the combined optical beam, wherein a separation between the periodic regions of constructive interference is less than a range resolution capability of the system. 
     In one example, the method also includes generating control signals from the optical beams to synchronize the optical sources. 
     These and other aspects of the present disclosure will be apparent from a reading of the following detailed description together with the accompanying figures, which are briefly described below. The present disclosure includes any combination of two, three, four or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined or otherwise recited in a specific example implementation described herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosure, in any of its aspects and examples, should be viewed as combinable unless the context of the disclosure clearly dictates otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of various examples, reference is now made to the following detailed description taken in connection with the accompanying drawings in which like identifiers correspond to like elements: 
         FIG. 1  is a block diagram illustrating an example LIDAR system according to the present disclosure; 
         FIG. 2  is a block diagram illustrating an example LIDAR system in a first configuration according to the present disclosure; 
         FIG. 3  is a time-frequency diagram illustrating one example of LIDAR waveforms according to the present disclosure; 
         FIG. 4  is a time-frequency diagram illustrating another example of LIDAR waveforms according to the present disclosure; 
         FIG. 5  is a time-frequency diagram illustrating another example of LIDAR waveforms according to the present disclosure; 
         FIG. 6  is a time-frequency diagram illustrating another example of LIDAR waveforms according to the present disclosure; 
         FIG. 7  is a block diagram illustrating an example LIDAR system in a second configuration according to the present disclosure; and 
         FIG. 8  is a flow diagram illustrating an example method in a LIDAR system according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a LIDAR system  100  according to example implementations of the present disclosure. The LIDAR system  100  includes one or more of each of a number of components, but may include fewer or additional components than shown in  FIG. 1 . The LIDAR system  100  may be implemented in any sensing market, such as, but not limited to, transportation, manufacturing, metrology, medical, and security systems. For example, in the automotive industry, the described beam delivery system becomes the front-end of frequency modulated continuous-wave (FMCW) devices that can assist with spatial awareness for automated driver assist systems, or self-driving vehicles. As shown, the LIDAR system  100  includes optical circuits  101  that may be implemented on a photonics chip. The optical circuits  101  may include a combination of active optical components and passive optical components. Active optical components may generate, amplify, or detect optical signals and the like. In some examples, the active optical circuit includes optical beams at different wavelengths, one or more optical amplifiers, one or more optical detectors, or the like. Passive optical components may transmit, reflect, combine, separate, sample and polarize optical beams, for example. Passive optical components, for example, may include optical waveguides, circulators, wavelength division multiplexers and demultiplexers, polarization wave plates, interferometers, samplers, couplers and the like. 
     Free space optics  115  may include one or more optical waveguides to carry optical signals, and route and manipulate optical signals to appropriate input/output ports of the active optical circuit. The free space optics  115  may also include one or more optical components such as taps, wavelength division multiplexers (WDM), splitters/combiners, polarization beam splitters (PBS), collimators, couplers, circulators, interferometers, mixers, or the like. In some embodiments, the free space optics  115  may include components to transform the polarization state and direct received polarized light to optical detectors using a PBS. The free space optics  115  may further include a diffractive element to deflect optical beams having different frequencies at different angles along an axis (e.g., a fast-axis). 
     In embodiments, the LIDAR system  100  includes an optical scanner  102  that includes one or more scanning mirrors that are rotatable along an axis (e.g., a slow-axis) that is orthogonal or substantially orthogonal to the fast-axis of the diffractive element to steer optical signals to scan an environment according to a scanning pattern. For instance, the scanning mirrors may be rotatable by one or more galvanometers. In other embodiments, the optical scanner  102  may be realized as an optical phased-array or an array of MEMS (micro electro-mechanical systems) mirrors. The optical scanner  102  also collects light incident upon any objects in the environment into a return optical beam that is returned to the passive optical circuit component of the optical circuits  101 . For example, the return optical beam may be directed to an optical detector by a polarization beam splitter or a circulator. In addition to the mirrors and galvanometers, optical phased arrays, and MEMS mirror arrays, the optical scanning system may include components such as a quarter-wave plate, lens, anti-reflective coated window or the like. 
     To control and support the optical circuits  101  and optical scanner  102 , the LIDAR system  100  includes a LIDAR control systems  110 . The LIDAR control systems  110  may include a processing device for the LIDAR system  100 . In embodiments, the processing device may be one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processing device may be complex instruction set computing (CISC) microprocessor, reduced instruction set computer (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or processor implementing other instruction sets, or processors implementing a combination of instruction sets. The processing device may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. 
     In some embodiments, the LIDAR control systems  110  may include a signal processing unit  112  such as a digital signal processor. The LIDAR control systems  110  are configured to output digital control signals to control optical drivers  103 . In some embodiments, the digital control signals may be converted to analog signals through signal conversion unit  106 . For example, the signal conversion unit  106  may include a digital-to-analog converter. The optical drivers  103  may then provide drive signals to active components of optical circuits  101  to drive optical sources such as lasers and amplifiers. In some embodiments, several optical drivers  103  and signal conversion units  106  may be provided to drive multiple optical sources. 
     The LIDAR control systems  110  are also configured to output digital control signals for the optical scanner  102 . A motion control system  105  may control the galvanometers of the optical scanner  102  based on control signals received from the LIDAR control systems  110 . For example, a digital-to-analog converter may convert coordinate routing information from the LIDAR control systems  110  to signals interpretable by the galvanometers in the optical scanner  102 . In some embodiments, a motion control system  105  may also return information to the LIDAR control systems  110  about the position or operation of components of the optical scanner  102 . For example, an analog-to-digital converter may in turn convert information about the galvanometers&#39; position to a signal interpretable by the LIDAR control systems  110 . 
     The LIDAR control systems  110  are further configured to analyze incoming digital signals. In this regard, the LIDAR system  100  includes optical receivers  104  to measure one or more beams received by optical circuits  101 . For example, a reference beam receiver may measure the amplitude of a reference beam from the active optical circuit, and an analog-to-digital converter converts signals from the reference receiver to signals interpretable by the LIDAR control systems  110 . Target receivers measure the optical signal that carries information about the range and velocity of a target in the form of a beat frequency, modulated optical signal. The reflected beam may be mixed with a second signal from a local oscillator. The optical receivers  104  may include a high-speed analog-to-digital converter to convert signals from the target receiver to signals interpretable by the LIDAR control systems  110 . In some embodiments, the signals from the optical receivers  104  may be subject to signal conditioning  107  prior to receipt by the LIDAR control systems  110 . For example, the signals from the optical receivers  104  may be provided to an operational amplifier for amplification of the received signals and the amplified signals may be provided to the LIDAR control systems  110 . 
     In some applications, the LIDAR system  100  may additionally include one or more imaging devices  108  configured to capture images of the environment, a global positioning system  109  configured to provide a geographic location of the system, or other sensor inputs. The LIDAR system  100  may also include an image processing system  114 . The image processing system  114  can be configured to receive the images and geographic location, and send the images and location or information related thereto to the LIDAR control systems  110  or other systems connected to the LIDAR system  100 . 
     In operation according to some examples, the LIDAR system  100  is configured to use nondegenerate optical sources to simultaneously measure range and velocity across two dimensions. This capability allows for real-time, long range measurements of range, velocity, azimuth, and elevation of the surrounding environment. In some example implementations, the system points multiple modulated optical beams to the same target. 
     In some examples, the scanning process begins with the optical drivers  103  and LIDAR control systems  110 . The LIDAR control systems  110  instruct the optical drivers  103  to independently modulate one or more optical beams, and these modulated signals propagate through the passive optical circuit to the collimator. The collimator directs the light at the optical scanning system that scans the environment over a preprogrammed pattern defined by the motion control subsystem. The optical circuits may also include a polarization wave plate to transform the polarization of the light as it leaves the optical circuits  101 . In embodiments, the polarization wave plate may be a quarter-wave plate or a half-wave plate. A portion of the polarized light may also be reflected back to the optical circuits  101 . For example lensing or collimating systems may have natural reflective properties or a reflective coating to reflect a portion of the light back to the optical circuits  101 . 
     Optical signals reflected back from the environment pass through the optical circuits  101  to the receivers. Because the polarization of the light has been transformed, it may be reflected by a polarization beam splitter along with the portion of polarized light that was reflected back to the optical circuits  101 . Accordingly, rather than returning to the same fiber or waveguide as an optical source, the reflected light is reflected to separate optical receivers. These signals interfere with one another and generate a combined signal. Each beam signal that returns from the target produces a time-shifted waveform. The temporal phase difference between the two waveforms generates a beat frequency measured on the optical receivers (photodetectors). The combined signal can then be reflected to the optical receivers  104 . 
     The analog signals from the optical receivers  104  are converted to digital signals using analog-to-digital converters (ADCs). The digital signals are then sent to the LIDAR control systems  110 . A signal processing unit  112  may then receive the digital signals and interpret them. In some embodiments, the signal processing unit  112  also receives position data from the motion control system  105  and galvanometer (not shown) as well as image data from the image processing system  114 . The signal processing unit  112  can then generate a 3D point cloud with information about range and velocity of points in the environment as the optical scanner  102  scans additional points. The signal processing unit  112  can also overlay a 3D point cloud data with the image data to determine velocity and distance of objects in the surrounding area. The system also processes the satellite-based navigation location data from GPS  109  to provide a precise global location. 
       FIG. 2  is a block diagram illustrating an example LIDAR system  200 . Some of the optical components illustrated in  FIG. 2  may be integrated components within a photonics integrated circuit and may be included in the optical circuits  101  illustrated in  FIG. 1 , for example. Other optical components illustrated in  FIG. 2  may be free space optical components and may be included in the free space optics  115  illustrated in  FIG. 1 . 
     System  200  includes a plurality of optical sources  201 - 1  through  201 - n  that each emit a corresponding coherent optical beam (e.g., laser beam)  202 - 1  through  202 - n . The optical beams ( 202  collectively) are frequency modulated with synchronized linear “chirp” waveforms, which may be sawtooth waveforms or triangle waveforms, for example.  FIG. 3  illustrates an example of sawtooth modulation. As illustrated in  FIG. 3 , the optical beams  202  are tuned to have a fixed frequency separation or offset Δf O  between frequency adjacent beams so that the optical beams  202  form a “comb” of equally spaced signals in the frequency domain.  FIG. 3  also illustrates that, because the chirps are synchronized, having the same timing, chirp bandwidth (Δf C ), and chirp period (T C ), the frequency offset between any two adjacent beams is maintained throughout each chirp. This is illustrated in the time-frequency diagram  300  of  FIG. 3  for the first four optical beams ( 202 - 1 ,  202 - 2 ,  202 - 3 , and  202 - 4 ), where the separation Δf O  between frequencies f 1 , f 2 , f 3  and f 4  is the same as the separation between frequencies f 5 , f 6 , f 7  and f 8 . 
     Returning to  FIG. 2 , each optical beam  202  is amplified in optical amplifiers  203  and then combined by optical multiplexer (MUX)  204 , which may be, for example, a wavelength division multiplexer. The output of MUX  204  is a combined optical beam  205  including optical beams  202 - 1  through  202 - n . In one example, the combined optical beam  205  may be sampled by a sampler  206 , which may be for example, a tapped isolator (Iso-Tap) or optical coupler. In one example, the sample  207  of the combined optical beam  205  may be applied to an interferometer  217 , which may be, for example, a Mach Zehnder Interferometer (MZI). The output of the interferometer  217  will be n beat frequency signals  218  corresponding to the n optical beams  202 - 1  through  202 - n . Each beat frequency signal in the n beat frequency signals  218  will be a constant frequency if the chirp modulation of the corresponding optical beam  202 - 1  through  202 - n  is linear. Any deviation from chirp linearity will result in a frequency deviation in a corresponding beat frequency signal. 
     In one example, the output  218  of the interferometer  217  may applied to an optical demultiplexer (DEMUX)  219 , which may be a wavelength division demultiplexer, for example. DEMUX  219  may be configured to separate the n beat frequency signals  218 . The n beat frequency signals  218  may be applied to a bank of photodetectors (PDs)  220 - 1  through  220 - n , (collectively photodetectors  220 ), to detect each of the beat frequency signals. 
     The outputs of photodetectors  220 , collectively  221 , may be fed back to optical sources  201  in a phase-locked loop (not shown) to correct any nonlinearities in the chirp modulation of the optical beams  202 . 
     After passing through sampler  206 , the combined optical beam may be sampled a second time by sampler  208 , to generate a local oscillator (LO) signal  209  for further processing described in detail below. After passing through sampler  208 , the combined optical beam is routed by optical circulator  210  to scanner  211 . Scanner  211  is configured to scan a target environment in azimuth and elevator with the combined optical beam  205  and to de-scan a target return signal  212  from a target  213  in the target environment. 
     The target return signal  212  from the scanner  211  is then routed by optical circulator  210  to optical mixer  214 , where it is mixed with LO signal  209  from sampler  208 , which includes samples of optical beams  202 - 1  through  202 - n  in the combined optical signal  205 . 
     The target return signal  212  will be a delayed version of the combined optical beam  205  (and of the LO signal  209 ) due to the round-trip time to and from the target  213 . Due to the chirp modulation, this delay will result in a range-related frequency shift (Δf R ) between each outgoing component optical beam  202 - 1  through  202 - n  of combined optical beam  205 , and its corresponding return signal  212 - 1  through  212 - n  in the target return signal  212 . The target return signal  212  (and its component return signals  212 - 1  through  212 - n ) may also include a Doppler frequency shift MD due to the velocity of the target. 
     These frequency shift effects are illustrated in  FIG. 4 , using optical beams  202 - 1  and  202 - 1 , and their corresponding target return signals  212 - 1  and  212 - 2 , as an example. In  FIG. 4 , the target return signals are delayed by Δt R . If the chirp bandwidth is Δf C  and the chirp period is T C , then the range-related frequency shift Δf R  can be expressed as equation (1): 
       Δ f   R   =Δt   R (Δ f   C   /T   C )  (1)
 
     If there is a Doppler shift on the return signals  212 , the net frequency shift between the outgoing optical beams  202  and the target return signals  212  will be Δf R −Δf D . If the Doppler shift is positive (approaching target), the net frequency shift will be Δf R −Δf D  (as illustrated in the example of  FIG. 4 ). If the Doppler shift is negative (receding target), the net frequency shift will be ΔfR+Δf D . 
       FIG. 5  is a time-frequency diagram  500  illustrating the frequency difference between optical beam  202 - 1  and target return signal  212 - 1 , which is a constant value (assuming target range and velocity do not change during the measurement interval). Similarly,  FIG. 6  is a time-frequency diagram  600  illustrating the frequency difference between optical beam  202 - 2  and target return signal  212 - 2 , which is also a constant value (under the same constraints). It can be seen that even though the absolute frequencies of the signal pairs are different, their difference frequencies are the same. 
     Returning to  FIG. 2 , as a result of first-order mixing between each sampled optical beam  202 - 1  through  202 - n  in LO signal  209 , and its corresponding target return signals  212 - 1  through  212 - n  in the target return signal  212 , there will be n downconverted fixed frequency target return signals  215  at the output of optical mixer  214 , all at the same intermediate frequency f I =Δf R −Δf D . As noted above, these n signals will all be at the same phase-locked frequency, and may be applied to photodetector (PD)  216 , where they may be coherently combined to generate a combined downconverted return signal with potentially n times the amplitude of each individual signal. 
     However, to realize the full advantage of the multiple optical beams in the system  200  described above, the signals in the transmit and return paths should combine constructively in the photodetector  216 . The total current in the photodetector  216  due to the n fixed frequency downconverted target return signals  215  can be expressed as: 
     
       
         
           
             
               
                 
                   
                     
                       
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     where G is the gain of each optical amplifier  203 , Γ Tx  is the forward transmission coefficient (including component losses and free space path loss), η is the transmission coefficient of the return path, R is the reflectively of the target  213 ,   is the responsivity of the photodetector  216 , A LO   ω     i    and A TR   ω     i    are the respective amplitudes of the LO signal  209  and the target return signal  212  associated with optical beam frequency ω i  (where ω i =2πƒ i ), ϕ i (t)−ϕ i (t−τ) is the phase noise of the i-th optical beam  202 , γ i  ≡dω i /dt is the chirp rate (equal to 2πΔf/T C ), and τ is the time of flight (τ=2d/c, where d is the distance to the target and c is the speed of light). 
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     must be greater than 1. Ideally, the summation can be as large as the number of optical beams (n) when the cosines constructively superpose. Assuming the linewidth of each optical beam is sufficiently narrow for this application (i.e., ϕ i (t)≅0), two conditions must be satisfied to maximize the photodetector current: 
     1. γ i τ=γ i τ for all optical beams, meaning that the chirp rate for all lasers should be equal, and 
     2. ω i τ−ω j τ≅2πn for all lasers and all target distances (times of flight). 
     As described above, the first condition can be satisfied using an interferometer (such as MZI 217) in a phase-locked loop. The second condition can be restated as: 
       (ω i −ω j )2 d/c≅ 2π m→d   cons   =m ×( c/ 2Δ f   ij ),
 
     where m is any integer and d cons  are the multiple ranges where the two optical beams combine constructively. For example, for two optical beams separated by 500 GHz (gigahertz), constructive interference will occur at multiples of 0.3 mm (millimeters). That is, the combined beams will alternate between constructive interference and destructive interference every 1.5 mm. In most applications, such as automotive applications, these distances are much smaller than the range resolution associated with the chirp rate and detection interval of the LIDAR system; so that the integrated energy of the downconverted target return signals will be greater than 1-times and less than 2-times the energy available from a single beam system. It can be seen that non-adjacent signals, separated by multiples of 500 GHz in this example, will have a more fine-grained periodicity in constructive interference. For example, a 1000 GHz separation yields a 1.5 mm periodicity; a 1500 GHz separation yields a 1.0 mm periodicity, and so on. When all of the n signals are integrated, the integrated signal energy will be greater than 1-times and less than n-times the energy available from a single beam system. As n increases, and the average spatial periodicity of the combined signals decreases, the integrated energy of the downconverted target return signals will approach n times the energy of a single beam system. 
       FIG. 7  is a block diagram illustrating another example LIDAR system  300 . System  300  is similar to system  200  in many respects, and some details will not be repeated here. As for system  200 , some of the optical components illustrated in  FIG. 3  may be integrated components within a photonics integrated circuit and may be included in the optical circuits  101  illustrated in  FIG. 1 , for example. Other optical components illustrated in  FIG. 7  may be free space optical components and may be included in the free space optics  115  illustrated in  FIG. 1 . 
     System  300  includes a plurality of optical sources  201 - 1  through  201 - n  that each emit a corresponding coherent optical beam (e.g., laser beam)  202 - 1  through  202 - n . The optical beams ( 202  collectively) are frequency modulated with synchronized linear “chirp” waveforms, which may be sawtooth waveforms or triangle waveforms, for example.  FIG. 3 , described above with respect to system  200 , illustrates an example of sawtooth modulation. As illustrated in  FIG. 3 , the optical beams  202  are tuned to have a fixed frequency separation or offset Δf O  between frequency adjacent beams so that the optical beams  202  form a “comb” of equally spaced signals in the frequency domain.  FIG. 3  also illustrates that, because the chirps are synchronized, having the same timing, chirp bandwidth (Δf C ), and chirp period (T C ), the frequency offset between any two adjacent beams is maintained throughout each chirp. This is illustrated in the time-frequency diagram  300  of  FIG. 3  for the first four optical beams ( 202 - 1 ,  202 - 2 ,  202 - 3 , and  202 - 4 ), where the separation Δf O  between frequencies f 1 , f 2 , f 3  and f 4  is the same as the separation between frequencies f 5 , f 6 , f 7  and f 8 . 
     Returning to  FIG. 3 , each optical beam  202 - 1  through  202 - n  is sampled by a corresponding sampler  206 - 1  through  206 - n  (collectively samplers  206 ), which may be the same as sampler  206  in system  200 . The samples of the optical beams  207 - 1  through  207 - n  (collectively samples  207 ) are combined in optical multiplexer (MUX)  222  to generate a combined sample signal  222 . An interferometer, such as MZI 217 receives the combined sample signal  222 . The output of the interferometer  217  will be n beat frequency signals  218  corresponding to the n optical beams  202 - 1  through  202 - n . Each beat frequency signal in the n beat frequency signals  218  will be a constant frequency if the chirp modulation of the corresponding optical beam  202 - 1  through  202 - n  is linear. Any deviation from chirp linearity will result in a frequency deviation in a corresponding beat frequency signal. 
     The output  218  of interferometer  217  may be applied to optical demultiplexer (DEMUX)  219 . DEMUX  219  is configured to separate the n beat frequency signals  218 . The n beat frequency signals  218  are then applied to a bank of photodetectors  220 - 1  through  220 - n  (collectively photodetectors  220 ), to detect each of the beat frequency signals. 
     The outputs of photodetectors  220 , collectively  221 , may be fed back to optical sources  201  in a phase-locked loop (not shown), to correct any nonlinearities in the chirp modulation of the optical beams  202 . 
     After being sampled by samplers  206 , each optical beam  202  is amplified in an optical amplifier  203  and then combined by optical multiplexer (MUX)  204 . The output of MUX  204  is a combined optical beam  205  including optical beams  202 - 1  through  202 - n.    
     The combined optical beam may then be sampled by sampler  208  to generate a local oscillator (LO) signal  209  for further processing described in detail below. After passing through sampler  208 , the combined optical beam  205  is routed by optical circulator  210  to scanner  211 . Scanner  211  is configured to scan a target environment in azimuth and elevator with the combined optical beam  205  and to de-scan a target return signal  212  from a target  213  in the target environment. 
     The target return signal  212  from the scanner  211  is then routed by optical circulator  210  to optical mixer  214 , where it is mixed with LO signal  209  from sampler  208 , which includes samples of optical beams  202 - 1  through  202 - n  in the combined optical signal  205 . 
     As described above with respect to system  200 , the target return signal  212  will be a delayed version of the combined optical beam  205  (and of the LO signal  209 ) due to the round-trip time to and from the target  213 . Due to the chirp modulation, this delay will result in a range-related frequency shift (Δf R ) between each outgoing component optical beam  202 - 1  through  202 - n  of combined optical beam  205 , and its corresponding return signal  212 - 1  through  212 - n  in the target return signal  212 . The target return signal  212  (and its component return signals  212 - 1  through  212 - n ) may also include a Doppler frequency shift MD due to the velocity of the target. These frequency shift effects are discussed above with respect to system  200  in conjunction with  FIGS. 4, 5 and 6 , and are not repeated here. 
     Returning to  FIG. 7 , as a result of first-order mixing between each sampled optical beam  202 - 1  through  202 - n  and its corresponding target return signals  212 - 1  through  212 - n  in the target return signal  212 , there will be n downconverted target return signals  215  at the output of optical mixer  214 , all at the same intermediate frequency f I =Δf R −Δf D . As noted above, these n signals will all be at the same phase-locked frequency, and may be applied to photodetector  216 , where they may be coherently combined to generate a combined downconverted target return signal with potentially n times the amplitude of each individual signal. As discussed above with respect to system  200 , the mixing process may produce range-dependent interference patterns between the transmitted and target return signals, with periodicities that are smaller than the range resolution capability of the LIDAR system  300 , based on chirp rate and target detection time. As a result, periodic zones of constructive and destructive interference are integrated to yield an average detected signal energy that is greater than 1-times and less than n-times the energy available from a single beam system. As described above, as n increases, the integrated energy of the downconverted target return signals approaches n times the energy available from a single beam system. 
       FIG. 8  is a flow diagram  800  illustrating one example of a method for increasing the effective power and sensitivity of a LIDAR system, such as systems  200  and  300 . Method  800  begins at block  802 , where a plurality of optical beams (e.g., optical beams  202 ) are generated with synchronized chirp rates (e.g., Δf C /T C ) and chirp durations (e.g., T C ), where the plurality of optical beams provide a comb of coherent optical beams with a fixed frequency offset (e.g., Δf O ). 
     In block  804 , the plurality of optical beams is combined in an optical multiplexer (e.g., MUX  204 ) to form a combined optical beam (e.g., combined optical beam  205 ). 
     In block  806 , the combined optical beam is transmitted toward a target environment. For example, combined optical beam  205  is transmitted through sampler  208 , circulator  210 , and scanner  211 . 
     In block  808 , a target return signal (e.g., target return signal  212 ) is downconverted into a plurality of fixed frequency downconverted target return signals (e.g., signals  215 ) corresponding to the plurality of optical beams. 
     And, in block  810 , the downconverted target return signals are coherently combined in a photodetector (e.g., photodetector  216 ). 
     The preceding description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present disclosure. It will be apparent to one skilled in the art, however, that at least some embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present disclosure. Thus, the specific details set forth are merely exemplary. Particular embodiments may vary from these exemplary details and still be contemplated to be within the scope of the present disclosure. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiments included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. In addition, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.” 
     Although the operations of the methods herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent or alternating manner. 
     The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The words “example” or “exemplary” are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X includes A or B” is intended to mean any of the natural inclusive permutations. That is, if X includes A; X includes B; or X includes both A and B, then “X includes A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. Moreover, use of the term “an embodiment” or “one embodiment” or “an implementation” or “one implementation” throughout is not intended to mean the same embodiment or implementation unless described as such. Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. as used herein are meant as labels to distinguish among different elements and may not necessarily have an ordinal meaning according to their numerical designation.