LIDAR system with suppressed Doppler frequency shift.

Abstract

A LIDAR system for reducing or suppressing a frequency shift induced by movement of an object in a scene relative to the LIDAR, the LIDAR system comprising a light source, an input aperture (101), a splitter (2) configured to split the reflected light into a reference channel (4) and a first imaging channel (3), a first imaging light IQ receiver (5) configured to obtain a first interference signal, a reference light IQ receiver (6) configured to obtain a reference interference signal, an imaging oscillator (111) configured to be temporally coherent with the reflected light, and at least one mixer (12) connected to the first imaging light IQ (5) and the reference light IQ (6) configured to obtain a first intermodulation product having a higher frequency and a target intermodulation product scaled by its Doppler shift.

Description

[Technical Field]

[0001] The subject of this disclosure is a LIDAR system that allows for reducing or completely suppressing the frequency shift induced by the movement of objects in a scene relative to the LIDAR, an effect known as Doppler frequency shift. [Background technology]

[0002] Light detection and ranging (LIDAR) devices generate a distance map to a target by illuminating the target with laser light and measuring the reflected light with a sensor. Differences in the properties of the laser light, including total round-trip time, phase, or wavelength, can then be used to create a digital 3D representation of the target.

[0003] LIDAR is commonly used to create high-resolution maps and has applications in geodesy, geoinformatics, archaeology, geography, geology, topography, seismology, forestry, atmospheric physics, laser guidance and airborne laser swath mapping (ALSM), laser altimetry, and is also used in the control and navigation of some autonomous vehicles.

[0004] Some LIDARs utilize what is known as coherent detection, in which light reflected off a sample is mixed with a local oscillator that is coherent with the reflected light. This approach has several advantages, including optical gain that allows for single-photon sensitivity, and allows changes in the phase and wavelength of the light to be used to measure distance.

[0005] A common problem that arises when utilizing this type of LIDAR is the frequency shift induced by the movement of objects in the scene relative to the device, an effect known as Doppler frequency shift. Such frequency shifts can be large relative to the bandwidth of the signal used to measure the relevant characteristics of the objects, complicating the extraction of such relevant data. This problem becomes particularly important when the relative velocity of the objects is large, as is the case with vehicles, aircraft, or satellites.

[0006] This frequency shift is variable and often unknown, and can very significantly broaden the bandwidth of the detected signal. For ground vehicles, the relative speed can reach 300 km / h or more, which corresponds to a Doppler frequency shift of 54.0 MHz for λ=1.55 μm illumination. This variable frequency shift complicates the electronic readout and signal processing chains of systems that rely on coherent detection of the signal of interest.

[0007] Even if the signal chain may still be manageable for a small number of channels, it increases the cost, size, and complexity of the final LIDAR system, which is a major obstacle to the practical implementation of multi-channel coherent LIDAR systems with a large number of inputs.

[0008] To solve the problem described, several approaches exist, one of which involves using non-uniform sampling or other compressed sensing schemes to reduce the overall data rate of the signal.

[0009] Generally, all approaches developed suffer from the same drawback of complex electronic readout circuitry and overall signal processing chains, which make them expensive, large in size, and generally difficult to implement and scale to multi-channel architectures with a large number of channels. Summary of the Invention

[0010] The LIDAR system object of this disclosure describes a modification of a coherent LIDAR system that utilizes one or more input apertures and is simple in its implementation, with the goal of reducing or completely eliminating the frequency shift induced by the movement of objects in the scene relative to the LIDAR, an effect known as Doppler frequency shift.

[0011] According to some embodiments, the reduction or elimination of frequency shift is achieved by measuring the Doppler-shifted signal in a reference channel and then using mathematical properties of signal mixing in the time domain to shift the frequency of one or more imaging channels to cancel or reduce the Doppler shift.

[0012] According to some embodiments, a Light Detection and Ranging (LIDAR) system with suppressed Doppler frequency shift comprises at least one light source configured to emit a first light toward an external object, the first light being diffusely or specularly reflected on the object and then received at at least one input aperture, thus resulting in a reflected light.

[0013] The reflected light can then be split at a splitter positioned following the at least one input aperture, the splitter configured to split the reflected light into a reference channel and at least one first imaging channel.

[0014] A portion of the split reflected light is then directed through at least one first imaging channel to a first imaging light IQ (In-phase and Quadrature) receiver associated with the first imaging channel, the first imaging light IQ receiver configured to obtain a first interference signal including a first in-phase component and a first quadrature component.

[0015] Furthermore, another portion of the reflected light is guided through a reference channel to a reference optical IQ receiver associated with the reference channel, the reference optical IQ receiver being configured to obtain a reference interference signal including a reference in-phase component and a reference quadrature component.

[0016] At least one local optical oscillator is associated with the first imaging light IQ receiver and the reference light IQ receiver and configured to be temporally coherent with the reflected light.

[0017] Finally, in one embodiment, the system comprises at least one mixer connected to the first imaging light IQ receiver and the reference light IQ receiver and configured to obtain a first intermodulation product having a higher frequency and a second intermodulation product of interest having its Doppler shift scaled or completely eliminated.

[0018] The system described above is one possible embodiment. However, the system may include a reference aperture and multiple input apertures, or multiple imaging channels associated with a reference channel and one or more input apertures. The system may also include a single local optical oscillator associated with all optical IQ receivers, or a reference local optical oscillator associated with the reference optical IQ receiver and an imaging local optical oscillator associated with the imaging optical IQ receiver, or a reference local optical oscillator associated with the reference optical IQ receiver and multiple imaging local optical oscillators each associated with one or more imaging optical IQ receivers.

[0019] The system may also include an optical amplitude modulator and / or an optical phase modulator applied to the imaging local optical oscillator so that the generation of intermodulation products occurs directly at the photodetector without the need for electronic mixing. [Brief explanation of the drawings]

[0020] To complement the description given herein and to aid in a better understanding of the characteristics of the LIDAR system, a set of drawings, relating to a preferred example of a practical embodiment thereof, are attached as an integral part of the description and are represented by way of example and non-limiting nature, in which:

[0021] [Figure 1]FIG. 1 illustrates an exemplary LIDAR system for imaging an object, according to one embodiment.

[0022] [Figure 1A] FIG. 1 illustrates the input aperture, optical IQ receiver, and reference and imaging local optical oscillator scheme in one embodiment.

[0023] [Figure 1B] FIG. 10 illustrates an alternative implementation using 2×4 MMI for an optical IQ receiver in one embodiment.

[0024] [Figure 2] FIG. 1 illustrates a scheme for a LIDAR system in one embodiment, comprising a reference channel and an imaging channel.

[0025] [Figure 3] FIG. 1 illustrates a scheme for a LIDAR system in one embodiment having a reference channel and an array of imaging channels with directional information encoded in the relative phase between the array of imaging channels.

[0026] [Figure 4] FIG. 1 illustrates a scheme for a LIDAR system in one embodiment having multiple input apertures and an amplitude modulator for direct mixing of a reference signal on a photodetector.

[0027] [Figure 5] FIG. 1 shows two schemes of a Gilbert cell, one including a photodetector that allows direct multiplication of the differential photocurrent.

[0028] [Figure 6] FIG. 1 illustrates an integration scheme for a Gilbert cell using switched capacitors.

[0029] [Figure 7]FIG. 1 illustrates a signal filter device used on a reference channel of a LIDAR system in one embodiment.

[0030] [Figure 8] 8 illustrates an exemplary reference sampling period obtained using the signal filtering device of FIG. 7.

[0031] [Figure 9] FIG. 10 illustrates another signal filter arrangement used on the reference channel of a LIDAR system in one embodiment.

[0032] [Figure 10] 10A and 10B illustrate exemplary reference sampling periods and intermediate reference sampling periods obtained using the signal filtering apparatus of FIG. 9.

[0033] [Figure 11] FIG. 2 illustrates an exemplary light source modulation scheme for providing multiple light source channels in one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0034] A preferred embodiment of the present disclosure is described below using FIGS.

[0035] Embodiments herein relate to a LIDAR system, such as the LIDAR system (100) illustrated in FIG. 1 , that includes at least one light source (102) configured to emit light (110) toward an external target (108). The light is reflected from the target (108) and the reflected light (112) is received by a light receiving unit (104). More specifically, as discussed in more detail with reference to subsequent figures, in a first embodiment, the light is received at a reference input aperture (103) and at an imaging input aperture (101). The light source (102) may represent a single light source or multiple light sources having different wavelengths. In some embodiments, the light source (102) includes one or more laser light sources.

[0036] The LIDAR system 100 also includes a processor 106 configured to receive electrical signals from the receiving unit 104 and perform one or more operations using the received electrical signals. For example, the processor 106 may use the received electrical signals to reconstruct a 3D image including the object 108. As discussed above, movement of the object 108 (identified by the upward arrow) while attempting to capture the reflected light 112 causes a frequency shift induced by the movement of the object 108 relative to the LIDAR system 100, an effect known as Doppler frequency shift.

[0037] 1A, the reference input aperture 103 enables the LIDAR system 100 to generate a reference interference signal between reflected light coming from the object 108 and a reference oscillator 113. This reference interference signal is then used to modulate an interference signal formed between reflected light 112 coming from the object 108 and the imaging oscillator 111, which is collected by the imaging input aperture 101. According to some embodiments, both the reference oscillator 113 and the imaging oscillator 111 are generated from the same light source, such as the light source 102.

[0038] In any given implementation, one or more of the reference input aperture (103) and the imaging input aperture (101) may overlap, as may the reference oscillator (113) and the imaging oscillator (111), for example as shown in the embodiment of FIG. 1A.

[0039] According to some embodiments, the reference oscillator (113) and the imaging oscillator (111) exhibit a degree of temporal coherence with the reflected light (112) so that the formed interference signal can be processed at electrical frequencies.

[0040] In one example shown in Figure 1A, the system includes a single input aperture (101, 103). In this case, the system includes a light source (102) that emits light toward an object (108). The light reflects off the object, and the reflected light (112) that enters the system through the input aperture (101, 103) is split by a splitting element (2), which may be a 1x2 splitter, into a first imaging channel (3) and a reference channel (4).

[0041] According to some embodiments, at least two channels (e.g., the reference channel (4) and the first imaging channel (3)) are affected by object motion via Doppler frequency shift in substantially the same way, while non-Doppler information-bearing modulations remain different between them, allowing the signals on both channels to be combined in a way that eliminates or significantly reduces the Doppler frequency shift while restoring the information-bearing modulation.

[0042] As shown in Figure 1A, the first imaging channel (3) is fed to a first imaging optical IQ receiver (5), and the reference channel (4) is fed to a reference optical IQ receiver (6). The first imaging optical IQ receiver (5) is associated with an imaging oscillator (111), and the reference optical IQ receiver (6) is associated with a reference oscillator (113). According to some embodiments, within the optical IQ receivers (5, 6), both oscillators (111, 113) are fed through a 90° hybrid that generates a phase shift between the in-phase (7, 9) and quadrature (8, 10) components of each channel.

[0043] In other embodiments, the IQ receivers (5, 6) are implemented by 2x4 MMI couplers designed to provide a phase shift between each of the four outputs (7, 8, 9, 10) and two inputs (3, 4). In Figure 1B, one embodiment is shown in which the first imaging optical IQ receiver (5) is a 2x4 MMI coupler fed by the first imaging channel (3) and the imaging oscillator (111), and the reference optical IQ receiver (6) is a 2x4 MMI coupler fed by the reference channel (4) and the reference oscillator (113).

[0044] In one embodiment, the imaging oscillator (111) has its wavelength swept according to a standard FMCW (Frequency Modulated Continuous Wave) scheme, while the reference oscillator (113) keeps its wavelength unchanged. According to some embodiments, the reflected light (112) has a component that is coherent with both components in the oscillators (111, 113). Thus, either the illumination is derived from a combination of both components, or both components share a common origin with the illumination, which ensures mutual coherence.

[0045] According to some embodiments, a first imaging optical IQ receiver (5) is associated with the first imaging channel (3) and is configured to acquire a first interference signal having a first in-phase component (7) and a first quadrature component (8). A reference optical IQ receiver (6) is associated with the reference channel (4) and is configured to acquire a reference interference signal having a reference in-phase component (9) and a reference quadrature component (10).

[0046] Both interference signals will be affected by Doppler in substantially the same way (with small differences due to different wavelengths in some embodiments), but only the first interference signal associated with the imaging oscillator 111 carries information about the distance between the object 108 and the LIDAR system 100 at its interference frequency.

[0047] 2, mixing the first interfering signal and the reference signal in at least one mixer 121-124 (sometimes also collectively identified as 12) results in the generation of two intermodulation products. For example, the mixing generates a first intermodulation product having a higher frequency that can be discarded, and an output intermodulation product (16) having a lower frequency where the Doppler shift is significantly scaled and which offers the possibility of bringing ranging and amplitude information into baseband, thus minimizing sampling frequency and readout complexity of electronics.

[0048] For illustrative purposes, the first interference signal and the reference interference signal are obtained for this implementation as discussed herein. The imaging input aperture (101) and the reference aperture (103) are assumed to be in substantially the same position, except for a relative phase shift that may occur if the imaging input aperture (101) is part of an array. For equal illumination of a scene using two light sources of two wavelengths (with associated wavenumbers and angular frequencies, k1, k2 and ω1, ω2, respectively) and equal amplitudes A, the optical signal at a distance x from the light source is:

number

[0049] Here, it is assumed that the first wavelength of the first light source of the LIDAR system is linearly frequency modulated with a constant K. The object reflecting the light emitted by the first light source is a single diffuse reflector, and the object is located at a distance x j and has an intensity reflectivity ρ j , and the relative velocity v in the direction between the input aperture (101) and the object. j , the reflected field of light collected at the input aperture (101) is:

number

[0050] where i is the index of the input aperture in the case where an array of apertures is present. The Doppler shift is calculated as 2k1v in the equation j and 2k2v j Visible in terms of wavelength and changes the frequency of reflected light.

[0051] For the calculation of the interference signal in the optical IQ receiver (5, 6), it is assumed for simplicity that the two wavelength components of the reference and imaging oscillators (111, 113) have an amplitude of 1:

number

[0052] After the imaging light IQ receiver (5) and the reference light IQ receiver (6), the first interference signal and the reference interference signal are respectively:

number

number

[0053] In these, the difference in optical angular frequency results in beating products that, when detected, have frequencies that are very high relative to electrical standards. For example, if the two wavelengths of light in the source are 1.55 μm and 0.1 nm apart, the intermodulation products have a frequency of 12.5 GHz:

number

[0054] Conversely, when the frequencies of the local oscillator and the reflected light are equal, the beating product is demodulated to a lower frequency obtained by adding or subtracting the Doppler shift to the frequency difference between the emitted and received phase modulation frequencies.

[0055] For typical speeds of ground vehicles, the Doppler shift will be equal to or lower than 100 MHz, so that, according to some embodiments, higher frequency mixing terms (including differences in optical angular frequency) can be suppressed by low pass filters. Thus, as shown in Figure 2, a first set of low pass filters (13) can be associated with the optical IQ receivers (5, 6) to filter the first in-phase component (7), the first quadrature component (8), the reference in-phase component (9), and the reference quadrature component (10).

[0056] The low frequency component of the interference signal is given by:

number

number

[0057] Depth and velocity information is encoded in the frequency (and phase) of both photocurrents. Looking only at the frequency information, we observe that the frequencies of I1(t) and I2(t) are:

number

[0058] These two frequency shift components scale differently with line rate. The modulation constant K has a direct effect on the range-derived frequency. However, the Doppler shift remains independent and is determined by the scene characteristics. Because the Doppler shift can go up to frequencies of tens of MHz, it typically requires fast acquisition electronics, which can increase the cost of the system. Also, these video frequencies can be problematic when scaling up scene detection using multiple parallel imaging channels (3).

[0059] However, the difference between these two frequencies is:

number

[0060] According to some embodiments, if the two wavelengths of light emitted by the two light sources are selected to be close to each other (e.g., 1.55 μm wavelengths separated by 0.1 nm), the difference in Doppler frequency shift is significantly reduced (v of 50 m / s). j 2kHz for

[0061] However, it is worth noting that both wavelengths can be equal. In this case, the Doppler shift can be completely suppressed while the frequency shift due to FMCW is maintained. This approach simplifies the optical system and the associated electro-optical circuitry.

[0062] If both wavelengths are equal, the Doppler shift may be completely suppressed, and the signal frequency is moved to baseband. This lower Doppler frequency allows for significant reductions in line speed, data throughput, and hardware complexity in systems where a large number of input apertures (101) are desired. If the Doppler frequency is maintained, the Doppler shift should be measured and separated from the FMCW modulation. One exemplary way to accomplish this is to vary K over time (e.g., by alternating its sign) in the FMCW frequency sweep and compare the resulting electrical frequency shift between both modulation slopes.

[0063] One exemplary way to subtract the frequencies obtained from the optical IQ receivers (5, 6) above is to multiply one current by the complex conjugate of the other. Standard frequency mixing techniques can be applied. This can be done in the digital or analog domain, and potentially based on an interfering signal, as shown below:

number

[0064] In one embodiment, this can be implemented using one or more mixers 121-124 connected to the first imaging light IQ (5) and reference light IQ (6) outputs or to the first low pass filter set (13) output, as shown in Figure 2. Each of the four multiplication terms above is a first intermodulation product (low frequency) with a frequency difference Δf, and a Doppler frequency Δf.

number

[0065] When the four multiplication terms are combined, the terms related to the addition of Doppler frequencies cancel, leaving only the low frequency intermodulation products, which contain depth information at their own frequency (according to Δf above), as the output intermodulation products (16).

[0066] According to some embodiments, the higher frequency components of each multiplication term are filtered out using a second set of low pass filters (23) so that only low frequency intermodulation products are retained. These low frequency intermodulation products contain depth information at their own frequency (according to Δf above) as output intermodulation products (16). According to some embodiments, the output intermodulation products (16) are amplified using one or more non-linear amplifiers (25).

[0067] In the embodiment shown in FIG. 2, one or more mixers 121-124 mix the first quadrature component (8) and the reference in-phase component (9) to produce a multiplication term (I 1q ×I 2i ), and a first mixer 121 designed to mix the first in-phase component (7) and the reference in-phase component (9) to give a multiplication term (I 1i ×I 2i ) is included.

[0068] In an alternative demodulation technique, an FM demodulation technique can be adapted to simultaneously convert to baseband and demodulate by treating the individual components of the interference signal, namely the first in-phase component (7), the first quadrature component (8), and the derivative of the reference interference signal provided by a time differentiation module (15) which generates a time derivative of the reference in-phase component (90) and a time derivative of the reference quadrature component (91).

[0069] This can be particularly useful in embodiments where both the imaging oscillator and the reference oscillator are the same, as in that situation the frequency difference in the multiplication term expressed above will be Δf=0, and the time derivative can be used to extract depth information that is frequency encoded in the amplitude of the time-differentiated signal.

[0070] For example, in this case where the imaging oscillator and the reference oscillator are the same, the operations that may be performed in one or more mixers (121)-(124) are as follows:

number

number

[0071] Similar to the direct frequency mixing approach, in this case one can generate the four multiplication terms above and combine them to leave only the DC component, or alternatively, a second set of low pass filters (23) can be used to filter out the higher frequency components of each of the multiplication terms, retaining only the DC component, whose amplitude contains the depth and Doppler information.

[0072] To separate the Doppler and depth information, K can be varied over time in the FMCW frequency sweep, e.g., alternating its sign, and the resulting shift in the DC component compared between both modulation gradients.

[0073] The drawback of direct FM demodulation is that the target reflectivity (ρ j ) and frequency shift are mixed. According to some embodiments, this can be addressed by demodulating the amplitude separately:

number

[0074] Alternatively, when the imaging oscillator and the reference oscillator are the same, the target reflectivity can also be obtained from a multiplication term between the signal component and the reference component before time differentiation (e.g., as provided by the first mixer (121) and the second mixer (122) from Figure 2).

[0075] Regarding the use of a direct FM demodulation approach, Figure 2 illustrates a time differentiation module (15) and one or more mixers (121)-(124), including a third mixer (123) designed to mix the first in-phase component (7) and the time-differentiated reference quadrature component (91), and a fourth mixer (124) designed to mix the first quadrature component (8) and the time-differentiated reference quadrature component (91). Thus, the embodiment in Figure 2 provides a demodulation scheme that simultaneously includes both frequency demodulation and amplitude demodulation.

[0076] Figure 3 shows an implementation in which multiple imaging channels (3) are combined with a common reference channel (4) obtained from reflected light (112) coming from the same scene but mixed with a separate light source (of a different wavelength, but coherent with at least a portion of the power collected from the scene).

[0077] The advantage of the scheme shown in Figure 3 is that the different imaging channels (3) maintain their relative phase differences (contained in the IQ data) in the electrical domain after demodulation, which allows for coherent combination of the demodulated signals coming from said imaging channels (3) to recover the different directions.

[0078] Different construction schemes can be used for the various mixers (collectively designated 12 in FIG. 3). For example, the mixers may be implemented in the analog domain based on circuits relying on translinear techniques. One of these circuits may be a Gilbert cell, an example of which is depicted in FIG. 5. This circuit has the advantage of working in all four quadrants of the interference signal. According to some embodiments, given that the input to the cell is differential and voltage-based, the photocurrent coming from the optical IQ receivers (5, 6) may be amplified to a voltage by a transimpedance amplifier (14) and, if appropriate, obtained in the analog domain.

[0079] To simplify the Gilbert cell, it may be possible to use the photocurrent of a balanced differential pair as the source of both the input signal and the current bias. This would reduce the need for intermediate transimpedance amplifiers, making the cell more amenable to replication for achieving large-scale integration. According to some embodiments, the imaging oscillator (111) mixed with the different imaging channels (3) can be generated and distributed as voltage signals onto the detector array (e.g., imaging channels) from a single imaging input aperture (101) without significant scalability issues.

[0080] To simplify the readout of the cells, an integrated scheme with switched capacitors and multiplexed video outputs can be applied, for example as shown in Figure 6. The readout of such switched capacitors can be configured in a similar way to a normal image sensor. For example, the switched capacitors can be organized by column and a multiplexing scheme can be used to route the analog values ​​to the appropriate ADC circuitry.

[0081] Finally, it is also possible to modulate the amplitude of the optical local oscillator going to each of the imaging channels to provide the desired mixing function. If this is done, electronic mixing after optical detection is not required, thereby providing an advantage in terms of system complexity. According to some embodiments, an optical modulator (17) is used to modulate the amplitude of the optical local oscillator, as shown in FIG. 4. In one embodiment, the optical modulator (17) is an optical amplitude modulator, whether based on electro-optic absorption, a Mach-Zehnder interferometer, or others.

[0082] If the amplitude modulation leaves some phase modulation, a phase modulator can be added in series to ensure constant phase operation and avoid unwanted frequency shifts in the reference channel. Amplitude modulation can also be obtained in various ways, e.g., via optical amplifiers, modulation of laser current, etc.

[0083] In some embodiments, the first in-phase component (7), the first quadrature component (8), the reference in-phase component (9), and the reference quadrature component (10) are multiplied with different versions of the signals, shifted by 90° relative to each other, to directly achieve the desired mathematical result. To physically achieve this, distribution of separately modulated reference signals to each output mixer (12) may be used. Considering the fact that the modulation applied to these two channels is also orthogonal in the electrical domain, in some embodiments it is possible to add them together in a modulated signal, as shown in Figure 4.

[0084] According to some embodiments, products between the first in-phase component (7) and the first quadrature component (8) or between the reference in-phase component (9) and the reference quadrature component (10) generate high frequency intermodulation products that can be filtered out.

[0085] According to some embodiments, to separate the amplitude and distance information, the modulation signal applied to the optical modulator (17) can be switched between different modes (with or without time differentiation) to selectively recover the depth information and / or the signal amplitude. This time-domain multiplexing may be suitable for implementations with an integrator synchronized with the switching of the demodulation signal, and can also be replaced by other multiplexing schemes (frequency-domain multiplexing, code multiplexing, etc.). The switching of the demodulation signals in both the imaging and reference channels can be performed using a switch (27).

[0086] According to some embodiments, FIG. 4 shows a combination of the two implementation options described above for the single wavelength case: Doppler frequency demodulation with amplitude modulation of the optical reference signal, and time multiplexed amplitude / frequency demodulation.

[0087] According to some embodiments, rather than modulating an optical local oscillator signal (e.g., by using optical modulator 17), different optical light source channels are modulated to provide modulated light source beams of illumination directed toward one or more targets. In this manner, light is modulated at the light source before being transmitted to one or more targets. FIG. 11 illustrates a light source modulation scheme (1100) that can provide different modulations to any number of optical light source channels. A laser light source (1102) has its output split among any number of different channels using any number of 1×2 optical splitters (1104). The laser light source (1102) may be the same as the light source (102) used to generate the imaging light (110). In some other embodiments, the light source (102) represents all of the light source modulation scheme (1100).

[0088] According to some embodiments, each of the different light source channels of the light source modulation scheme (1100) may have its optical signal amplified using a semiconductor optical amplifier (SOA) 1106 and subsequently modulated using an optical modulator (1108). In some arrangements, the optical modulator (1108) precedes the SOA (1106) on one or more of the light source channels. Any of the optical modulators (1108) may be configured to modulate the phase, frequency, or both the phase and frequency of the corresponding optical signal, such that each of the light source channels provides an optical output (1110) that can be modulated independently relative to the optical outputs (1110) of the other light source channels. The optical modulator (1108) may be any type of electro-optic modulator. According to some embodiments, any of the one or more SOAs (1106) and / or one or more optical modulators (1108) may receive a signal from a reference channel to affect the amplitude, phase, and / or frequency modulation being performed on a given light source channel. According to some embodiments, various optical outputs (1110) are transmitted toward one or more targets and received from one or more targets over imaging channels (3) as shown in Figures 3 or 4. According to some embodiments, light received across various imaging channels (3) can be mixed with an imaging oscillator (111) in various imaging receivers (5) without the need for mixers (12) or optical modulators (17) because modulation has already been performed on the source light. The imaging oscillator (111) can represent light generated from a laser source (1102).

[0089] When the Doppler shift is large (e.g., due to a high relative velocity of the object being imaged), demodulation of individual signals from the array to baseband results in a highly scalable but slow electronics readout. While this achieves the desired effect, it can suffer from significant signal-to-noise ratio (SNR) degradation, especially when performance is considered against the potential array gain resulting from mixing. This can be particularly relevant at optical wavelengths where the signals collected by different elements of the array are, in the ideal case, dominated by shot noise resulting from the discrete nature of photon detection. If the reference channel does not have any SNR advantage over other inputs to the mixer in the array, the array gain from coherent combining of the array outputs can be negated. Furthermore, at low input signal SNRs per element, there is additional degradation characteristic of incoherent demodulation. This can reduce the range achieved using such a structure in a typical LIDAR system.

[0090] Therefore, according to some embodiments, an additional signal filter Device is placed on top of the reference channel to provide a clean set of tones and minimize the impact of noise on the mixer. The sampling period of the camera reading out the imaging array is typically on the order of 100 μs to 20 ms, orders of magnitude longer than is possible for single-channel reference sampling (which can exceed 1 GSPS), but can be faster than the frame update rate of many other applications (typically around 50 ms). Therefore, according to some embodiments, additional filtering is applied to the reference signal, for example via a long acquisition window and a narrow digital filter centered around the signal peak in the spectrum.

[0091] FIG. 7 shows a signal filter placed above the reference channel to improve the SNR of the reference signal. Device 7 illustrates an example of a signal filter (700). Device(700) is provided after the reference light IQ receiver (6) but before the signal is mixed with the imaging channel, for example via mixer (12). According to some embodiments, the signal filter Device (700) is provided after the reference optical IQ receiver (6) in the system illustrated in FIG. 4, where the amplitude of the optical local oscillator going to each of the imaging channels is modulated so that electronic mixing is not required (e.g., mixer 12 is not required). According to some embodiments, the signal filter Device (700) includes a transimpedance amplifier (14) and a low pass filter (13), which may be the same as the transimpedance amplifier (14) and low pass filter (13) seen on the reference channel from any of Figures 2-4. Following these elements is a signal filter. Device 700 includes an analog-to-digital converter (A / D) 702 and a temporal filtering unit 704. The A / D 702 may be any standard analog-to-digital converter that converts the analog voltage output from the transimpedance amplifier 14 into a digital signal.

[0092] According to some embodiments, the temporal filtering unit (704) comprises a plurality of accumulators and filters that accumulate samples of the reference channel signal and average the samples to improve the SNR of the reference signal. Frequency bands with low amplitudes or amplitudes below a given threshold are suppressed to reduce noise and maximize the clean part of the signal.

[0093] The filtered reference signal with improved SNR is identified as Ref1 and output from the temporal filtering unit (704). According to some embodiments, the Ref1 signal is mixed with one or more of the imaging channels (represented as the imaging array (706)) using a mixer (12). According to some other embodiments, the Ref1 signal is used to influence the modulation provided by the optical modulator (17) to the imaging oscillator (111), which is mixed with the various imaging channels (3) of the imaging array (706). According to some other embodiments, the Ref1 signal is used to influence the modulation provided to different light source channels of the light source modulation scheme (1100). In any case, a clean carrier can be generated for each object in the field of view, which can be further used to optimize the output SNR, even for low input SNR levels per channel. A longer sample integration time for the reference channel relative to the camera gives that channel an intrinsic SNR advantage from averaging under additive white Gaussian noise (AWGN) conditions, while subsequent thresholding and filtering can optimize low SNR performance levels. FIG. 8 illustrates a method for controlling the camera sampling rate and signal filter according to some embodiments. Device (700) is used to illustrate the higher sampling rate generated on top of the reference channel.

[0094] According to some embodiments, the temporal filtering unit (704) includes a series of phase-locked loops (PLLs), assuming that a single tone can be expected for each reference channel input. This scheme works when the imaged object generates a carrier wave with a stable frequency during the extended reference sample collection window, meaning that the object has a stable range and relative velocity over at least the integration time. A stable frequency, however, may not be generated, for example, if the object is experiencing accelerations of ±1 g or more and the camera integration time is 0.1 ms or longer. However, it is possible to mathematically compensate for chirp in the filtering stage. This can be done through the parallel application of multiple chirps, corresponding to different object accelerations, to the digitized reference signal, finding the maximum value for each peak, then filtering, and applying the filtered signal with the corresponding chirp as output to the digital processor. In some cases with large integration windows, compensation becomes increasingly complex as the phase error becomes larger over time and the potential gain from integration increases.

[0095] The situation changes when multiple targets are being imaged simultaneously, as the presence of multiple received tones increases the noise bandwidth of the demodulated output, thus affecting the output of the array. This can negate coherent combining of signals and result in SNR performance that only improves with the square root of the number of elements in the array. One way to address multiple targets is to combine the detection and demodulation scheme discussed above with suitable illumination control in such a way that only one or a few targets generate reflections at a given time. In one example, the light source can be implemented using an optical phased array (OPA) to scan the scene. The OPA can be implemented using a light source modulation scheme (1100) with phase modulation applied to each of the light source channels (e.g., using optical modulator 1108). In another example, it is possible to perform a spatial Fourier transform of the incoming optical signal through a lens that focuses the light into subarrays corresponding to specific directions. When this is done using a cylindrical lens, each subarray becomes a 1D coherent receiver array, and the number of directions (and corresponding targets) imaged is significantly smaller.

[0096] As illustrated in FIG. 9, according to some embodiments, different signal filters Device (900) may be placed above the reference channel (e.g., any of the systems illustrated in one of Figures 2-4) to generate an intermediate array with a mixer that allows for faster acquisition after mixing. This staged approach allows for better tolerance to shifts in frequency because it allows the downmix frequency to be matched with a higher rate. Considering that the sampling rate will be higher than that for the camera array, this intermediate array may have a smaller number of elements and therefore a lower angular resolution. However, this intermediate array can resolve the direction of different tones and apply both directional and frequency filtering using different demodulation outputs, which may be useful for reducing clutter and improving SNR in multi-target situations.

[0097] According to some embodiments, a signal filter Device (900) includes a temporal filtering unit (704) as discussed above with reference to Figure 7. The output (Ref1) from the temporal filtering unit (704) is still mixed with each of the imaging channels from the imaging array (706). However, the signal filters Device(900) generates a set of additional reference outputs (collectively referred to as Ref2 in FIG. 9) to mix with the imaging channels from the imaging array (706). According to some embodiments, each of the additional reference outputs (Ref2) corresponds to the general direction of received light from a scene containing multiple objects. Multiple secondary reference channels (902) are mixed with the Ref1 signal using a series of mixers (904). According to some embodiments, each of the multiple secondary reference channels (902) represents a smaller version of the imaging array (706) that has some degree of direction discrimination capability when all of them are combined to generate the set of additional reference outputs (Ref2). The outputs from the mixers (904) are received by a second A / D, and a fast Fourier transform (FFT) is then performed on the signal using FFT element (906) to more easily distinguish noise from signal peaks and convert the secondary reference channels (902) back to the channel domain. The thresholding / filtering stage (908) is used to filter out frequency components with low amplitude or amplitude below a given threshold (e.g., remove noise components). According to some embodiments, the phase adjustment stage (910) is used to coherently accumulate signals to compensate for acceleration or deceleration of the imaged object and to resolve the reference signal into intermediate sample periods. According to some embodiments, each of the generated additional reference outputs (Ref2) can be mixed with the signal of a particular imaging channel of the imaging array 706. According to some other embodiments, the Ref2 signal is used to influence the modulation provided to different light source channels in the light source modulation scheme (1100). FIG. 10 illustrates the camera sampling rate and the higher sampling rate generated on the reference channel for both the Ref1 and Ref2 signals, with the Ref2 signal sampling rate being the intermediate sampling rate, according to some embodiments.

[0098] Unless specifically stated otherwise, terms such as "processing," "computing," "calculating," "determining," or similar terms may be understood to refer to the actions and / or processes of a computer or computing device, or similar electronic computing device, manipulating and / or transforming data represented as physical quantities (e.g., electrons) in the registers and / or memory units of a computer system into other data similarly represented as physical quantities in the registers, memory units, or other such information storage, transmission, or display device of a computer system. Embodiments are not limited in this context.

[0099] The terms “circuit” or “circuitry,” as used in any embodiment herein, may include, for example, hardwired circuitry, programmable circuitry such as a computer processor with one or more discrete instruction processing cores, state machine circuitry, and / or firmware that stores instructions executed by a programmable circuit, singly or in any combination. A circuit may include a processor and / or controller configured to execute one or more instructions to perform one or more operations described herein. The instructions may be embodied, for example, as an application, software, firmware, etc. configured to cause a circuit to perform any of the operations described above. Software may be embodied as a software package, code, instructions, instruction sets, and / or data recorded on a computer-readable storage device. Software may be embodied or implemented to include any number of processes, and processes may further be embodied or implemented to include any number of threads, etc., in a hierarchical manner. Firmware may be embodied as code, instructions, instruction sets, and / or data hard-coded (e.g., non-volatile) in a memory device. The circuits may be embodied, collectively or individually, as circuitry that forms part of a larger system, such as an integrated circuit (IC), an application specific integrated circuit (ASIC), a system on a chip (SoC), a desktop computer, a laptop computer, a tablet computer, a server, a smartphone, etc. Other embodiments may be implemented as software executed by a programmable control device. As described herein, various embodiments may be implemented using hardware elements, software elements, or any combination thereof.Examples of hardware elements may include processors, microprocessors, circuits, circuit elements (e.g., transistors, resistors, capacitors, inductors, etc.), integrated circuits, application specific integrated circuits (ASICs), programmable logic devices (PLDs), digital signal processors (DSPs), field programmable gate arrays (FPGAs), logic gates, registers, semiconductor devices, chips, microchips, and chipsets, etc.

[0100] Any of the various electro-optical or electrical elements discussed with reference to any of the systems disclosed herein may be components disposed on a planar lightwave circuit (PLC) or optical integrated circuit (OIC). Accordingly, the PLC or OIC may include any number of integrated waveguide structures for guiding light around the PLC or OIC. The PLC or OIC may include a silicon-on-insulator (SOI) substrate using silicon waveguides. In some other embodiments, the PLC or OIC includes III-V semiconductor materials with waveguides including gallium nitride (GaN), silicon nitride (Si3N4), indium gallium arsenide (InGaAs), gallium arsenide (GaAs), indium phosphide (InP), indium gallium phosphide (InGaP), or aluminum nitride (AlN), to name a few.

Claims

1. 1. A light detection and ranging (LIDAR) system having suppressed Doppler frequency shift, the LIDAR system comprising: at least one light source configured to emit a first light; at least one imaging input aperture and one imaging channel associated with said at least one imaging input aperture, configured to receive input reflected light reflected by a moving object illuminated by said light source; at least one reference aperture and one reference channel associated with the at least one reference aperture, configured to receive reference reflected light reflected by the moving object illuminated by the light source; at least one imaging oscillator; at least one first imaging light receiver associated with the imaging input aperture and the imaging oscillator, configured to acquire an interference signal between the input reflected light and the imaging oscillator; a reference oscillator, a reference light receiver associated with the reference aperture and the reference oscillator and configured to obtain a reference interference signal between the reference reflected light and the reference oscillator; a signal filter device positioned subsequent to the reference optical receiver, the signal filter device having a temporal filtering unit configured to accumulate samples of the reference interference signal and to combine the samples to improve the SNR of the reference interference signal; and at least one mixer connected to the at least one first imaging optical receiver and to the signal filtering device, the mixer configured to generate intermodulation products between the interference signal and the reference interference signal such that a Doppler frequency shift caused by the moving object is canceled or reduced; Equipped with the first light includes a first component and a second component; the at least one imaging oscillator has a component that is coherent with a component of the input reflected light corresponding to the first component; the reference oscillator has a component that is coherent with a component of the reference reflected light corresponding to the second component.

2. 2. The LIDAR system of claim 1, wherein the at least one first imaging optical receiver is an optical IQ receiver configured to acquire an interference signal between the input reflected light and the imaging oscillator including a first in-phase component and a first quadrature component, and the reference optical receiver is an optical IQ receiver configured to acquire a reference interference signal including a reference in-phase component and a reference quadrature component.

3. 3. The LIDAR system of claim 2, further comprising a time differentiation module associated with the reference light receiver and intended to time differentiate the reference in-phase component and the reference quadrature component.

4. The at least one mixer comprises: a first mixer intended to mix said first quadrature component and said reference in-phase component; a second mixer intended to mix said first in-phase component and said reference in-phase component; 4. The LIDAR system of claim 2 or 3, comprising:

5. The at least one mixer comprises: a first mixer intended to mix said first quadrature component and said reference quadrature component; a second mixer intended to mix said first in-phase component and said reference quadrature component; 4. The LIDAR system of claim 2 or 3, comprising:

6. The at least one mixer comprises: a third mixer intended to mix said first in-phase component and the time-differentiated reference quadrature component, and a fourth mixer intended to mix said first quadrature component and said time-differentiated reference quadrature component; 6. The LIDAR system of claim 2, wherein

7. The at least one mixer comprises: a third mixer intended to mix said first in-phase component and the time-differentiated reference in-phase component, and a fourth mixer intended to mix said first quadrature component and said time-differentiated reference in-phase component; 6. The LIDAR system of claim 2, wherein

8. 8. The LIDAR system of claim 4, further comprising a low pass filter associated with each mixer.

9. 9. The LIDAR system of claim 1, wherein the reference oscillator and the imaging oscillator share a common origin.

10. 10. The LIDAR system of claim 1, wherein the reference aperture is the same as the imaging input aperture, and the reference channel and the imaging channel are derived therefrom by a splitter.

11. 11. A LIDAR system according to any one of claims 1 to 10, wherein the wavelength of the reference oscillator remains unchanged and the wavelength of the imaging oscillator is swept according to a standard FMCW (Frequency Modulated Continuous Wave) scheme.

12. 12. The LIDAR system of claim 1, further comprising one or more low pass filters associated with the first imaging light receiver and the reference light receiver and configured to filter the interference signal and the reference interference signal.

13. 3. The LIDAR system of claim 2, further comprising a transimpedance amplifier positioned subsequent to the reference light receiver and the first imaging light receiver and configured to amplify the reference in-phase component, the reference quadrature component, the first in-phase component, and the first quadrature component.

14. The LIDAR system of claim 1 or any one of claims 4 to 7, wherein the mixer is a Gilbert cell.

15. 15. The LIDAR system of claim 1, wherein the signal filter device mixes the reference interference signal with a plurality of other reference signals.

16. 16. The LIDAR system of claim 1, wherein the temporal filtering unit is configured to combine the samples by averaging them.

17. 17. The LIDAR system of claim 1, wherein the temporal filtering unit is configured to combine the samples by using a series of phase-locked loops (PLLs).

18. at least one light source configured to emit a first light; at least one imaging input aperture and one imaging channel associated with said at least one imaging input aperture, configured to receive input reflected light reflected by a moving object illuminated by said light source; at least one reference aperture and one reference channel associated with the at least one reference aperture, configured to receive reference reflected light reflected by the moving object illuminated by the light source; at least one imaging oscillator; at least one first imaging light receiver associated with the imaging input aperture and the imaging oscillator, configured to acquire an interference signal between the input reflected light and the imaging oscillator; a reference oscillator, a reference light receiver associated with the reference aperture and the reference oscillator and configured to obtain a reference interference signal between the reference reflected light and the reference oscillator; a signal filter device positioned subsequent to the reference optical receiver, the signal filter device having a temporal filtering unit configured to accumulate samples of the reference interference signal and to combine the samples to improve the SNR of the reference interference signal; and an optical modulator connected to the at least one imaging oscillator and configured to apply amplitude or phase modulation to the at least one imaging oscillator based on a signal obtained from the reference channel, such that intermodulation products between the interference signal and the reference interference signal appear at an output of the at least one first imaging optical receiver, such that Doppler frequency shifts caused by the moving object are canceled or reduced; Equipped with the first light includes a first component and a second component; the at least one imaging oscillator has a component that is coherent with a component of the input reflected light corresponding to the first component; the reference oscillator has a component that is coherent with a component of the reference reflected light corresponding to the second component.

19. 20. The LIDAR system of claim 18, wherein the at least one first imaging optical receiver is an optical IQ receiver configured to acquire an interference signal between the input reflected light and the imaging oscillator including a first in-phase component and a first quadrature component, and the reference optical receiver is an optical IQ receiver configured to acquire a reference interference signal including a reference in-phase component and a reference quadrature component.

20. 20. The LIDAR system of claim 19, further comprising a transimpedance amplifier positioned subsequent to the reference light receiver and the first imaging light receiver and configured to amplify the reference in-phase component, the reference quadrature component, the first in-phase component, and the first quadrature component.

21. 21. The LIDAR system of claim 18, wherein the reference oscillator and the imaging oscillator share a common origin.

22. 22. The LIDAR system of claim 18, wherein the reference aperture is the same as the imaging input aperture, and the reference channel and the imaging channel are derived therefrom by a splitter.

23. 23. The LIDAR system of any one of claims 18 to 22, wherein the wavelength of the reference oscillator remains unchanged and the wavelength of the imaging oscillator is swept according to a standard FMCW (Frequency Modulated Continuous Wave) scheme.

24. 24. The LIDAR system of claim 18, further comprising one or more low pass filters associated with the first imaging light receiver and the reference light receiver and configured to filter the interference signal and the reference interference signal.

25. 25. The LIDAR system of claim 18, wherein the signal filter device mixes the reference interference signal with a plurality of other reference signals.

26. 26. The LIDAR system of claim 18, wherein the temporal filtering unit is configured to combine the samples by averaging the samples.

27. 27. The LIDAR system of any one of claims 18 to 26, wherein the temporal filtering unit is configured to combine the samples by using a series of phase-locked loops (PLLs).

28. at least one light source configured to emit a first light; at least one imaging input aperture and one imaging channel associated with said at least one imaging input aperture, configured to receive input reflected light reflected by a moving object illuminated by said light source; at least one reference aperture and one reference channel associated with the at least one reference aperture, configured to receive reference reflected light reflected by the moving object illuminated by the light source; at least one imaging oscillator; at least one first imaging light receiver associated with the imaging input aperture and the imaging oscillator, configured to acquire an interference signal between the input reflected light and the imaging oscillator; a reference oscillator, a reference light receiver associated with the reference aperture and the reference oscillator and configured to obtain a reference interference signal between the reference reflected light and the reference oscillator; a signal filter device positioned subsequent to the reference optical receiver, the signal filter device having a temporal filtering unit configured to accumulate samples of the reference interference signal and to combine the samples to improve the SNR of the reference interference signal; Equipped with wherein the at least one light source has a light source modulation scheme configured to apply amplitude or phase modulation to the emitted first light based on a signal obtained from the reference channel, such that intermodulation products between the interference signal and the reference interference signal appear at the output of the at least one first imaging light receiver, such that Doppler frequency shifts caused by the moving object are cancelled or reduced; the first light includes a first component and a second component; the at least one imaging oscillator has a component that is coherent with a component of the input reflected light corresponding to the first component; The reference oscillator has a component that is coherent with a component of the reference reflected light corresponding to the second component. LIDAR system.

29. Using a LIDAR system according to any one of claims 1 to 28, - Emitting a first light toward a moving object; receiving reflected light coming from said moving object; obtaining a first interference signal between the reflected light and an imaging oscillator; - obtaining a reference interference signal between the reflected light and a reference oscillator; accumulating samples of the reference interference signal and averaging the samples to improve the SNR of the reference interference signal; and obtaining intermodulation products between the first interference signal and the reference interference signal such that Doppler frequency shifts caused by the moving object are cancelled or reduced; 1. A method for suppressing Doppler frequency shift in a LIDAR system, comprising:

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