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 makes it possible to reduce or completely suppress the frequency shift induced by the movement of objects in the scene relative to the LIDAR, an effect known as Doppler frequency shift. [Background technology]

[0002] Light detection and ranging (LIDAR) devices generate distance maps to targets 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, geoinformation, archaeology, geography, geology, geomorphology, 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 the light reflected off the sample is mixed with a local oscillator that is coherent with the reflected light. This approach has several advantages, such as optical gain that allows 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 appears when using 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 may be large relative to the bandwidth of the signal used to measure the relevant properties of the objects, complicating the extraction of such relevant data. This problem becomes very important when the relative velocity of the objects is large, as is the case for vehicles, aircraft or satellites.

[0006] This frequency shift is variable, often unknown, and can very significantly widen the bandwidth of the detected signal. For ground vehicles, the relative speed can reach more than 300 km / h. This relative speed corresponds to a Doppler frequency shift of 54.0 MHz for a 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 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 exploiting the 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 may then be split at a splitter positioned subsequent to 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 desired second intermodulation product having its Doppler shift scaled or completely eliminated.

[0018] The system described above is one possible embodiment. However, the system can 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 can 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 generation of intermodulation products occurs directly at the photodetector without the need for electronic mixing. [Brief description of the drawings]

[0020] To complement the description given herein and to aid towards 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 said description and are represented below, by way of example and non-limiting nature:

[0021] [Figure 1]FIG. 1 illustrates an exemplary LIDAR system for imaging an object in 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. 13 illustrates an alternative implementation using 2×4 MMI for an optical IQ receiver in one embodiment.

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

[0025] [Diagram 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 of 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] [Diagram 5] FIG. 1 shows two versions of a Gilbert cell, one including a photodetector that allows direct multiplication of the differential photocurrents.

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

[0029] [Figure 7]FIG. 2 illustrates a signal filter arrangement used on a reference channel of a LIDAR system in one embodiment.

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

[0031] [Figure 9] FIG. 2 illustrates another signal filter arrangement used on the reference channel of a LIDAR system in one embodiment.

[0032] [Figure 10] 10A-B show exemplary reference sampling periods and intermediate reference sample periods obtained using the signal filter arrangement 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 PREFERRED EMBODIMENTS

[0034] A preferred embodiment of the present disclosure is described below with reference to FIGS.

[0035] Embodiments herein relate to a LIDAR system, such as the LIDAR system (100) illustrated in FIG. 1, which comprises 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 the receiving unit (104). More specifically, as will be 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 may represent 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 a Doppler frequency shift.

[0037] As can be seen in Figure 1A, the reference input aperture (103) enables the LIDAR system (100) to generate a reference interference signal between the 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 the reflected light (112) coming from the object (108) and the imaging oscillator (111) 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, for example as shown in the embodiment of FIG. 1A, 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).

[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 interference signal formed can be processed at electrical frequencies.

[0040] In one example shown in Figure 1A, the system comprises a single input aperture (101, 103). In this case, the system comprises a light source (102) that emits light towards an object (108). The light reflects off the object, and the reflected light (112) that enters the system by 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 manner, while the non-Doppler information-bearing modulation remains different between them. This allows 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 into a first imaging optical IQ receiver (5) and the reference channel (4) is fed into 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 receiver (5, 6), both oscillators (111, 113) are fed through a 90° hybrid that creates 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 an 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 ensuring 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 obtain 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 obtain 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] As can be seen in Fig. 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 the ranging and amplitude information into the baseband, thus minimizing the sampling frequency and the readout complexity of the 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 possible relative phase shift if the imaging input aperture (101) is part of an array. For equal illumination of the scene with two light sources of two wavelengths (with associated wave numbers and angular frequencies, k1, k2 and ω1, ω2, respectively) and equal amplitudes A, the optical signal at a distance x from the light sources 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 reflectance ρ j , and the relative velocity v in the direction between the input aperture (101) and the object. j then 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 by multiplying the Doppler shift by 2k1v in the equation. j and 2k2v j This is visible in terms of the frequency of the reflected light.

[0051] For the calculation of the interference signal at 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 beating products resulting from the difference in optical angular frequency, when detected, are at very high frequencies relative to electrical standards. For example, assuming that the two wavelengths of light in the source are 1.55 μm in wavelength 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 products are demodulated to a lower frequency that is the frequency difference between the emitted and received phase modulation frequencies plus or minus the Doppler shift.

[0055] For typical speeds of ground vehicles, the Doppler shift is equal to or lower than 100 MHz, so that according to some embodiments, the higher frequency mixing terms (including the difference 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, it is observed 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. The Doppler shift, however, remains independent and depends on the scene characteristics. Since the Doppler shift can go up to frequencies of tens of MHz, which typically require fast acquisition electronics, 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] But the difference between these two frequencies is:

number

[0060] According to some embodiments, when the two wavelengths of light emitted by the two light sources are selected to be close to each other (e.g., 0.1 nm apart at wavelengths of 1.55 μm), 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 separated from the FMCW modulation to be measured. One exemplary way to achieve this is to vary K in the FMCW frequency sweep over time (e.g., by alternating its sign) 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 above four multiplication terms is a first intermodulation product (low frequency) with a frequency difference Δf, and a Doppler frequency

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 of the multiplication terms are filtered out using a second set of low pass filters (23) such that only the 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 a first quadrature component (8) and a 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 perform baseband conversion and demodulation by working with the individual components of the interference signal, namely the first in-phase component (7), the first quadrature component (8) and a derivative of the reference interference signal provided by a time differentiation module (15) which generates a time derivative of a reference in-phase component (90) and a time derivative of a reference quadrature component (91).

[0069] This can be particularly useful in embodiments where both the imaging oscillator and the reference oscillator are the same, because 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 leaving 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, for example 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, in the case where 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 FIG. 2).

[0075] Regarding the use of a direct FM demodulation approach, Fig. 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 Fig. 2 provides a demodulation scheme that simultaneously includes both frequency demodulation and amplitude demodulation.

[0076] FIG. 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 Fig. 3 is that the different imaging channels (3) maintain their relative phase difference (contained in the IQ data) in the electrical domain after demodulation, which allows coherent combination of the demodulated signals coming from said imaging channels (3) to recover the different directions.

[0078] It is possible to use different construction schemes for the various mixers (collectively designated as 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 the 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, considering that the input to the cell is differential and voltage-based, the photocurrent coming from the optical IQ receivers (5, 6) is amplified to a voltage by a transimpedance amplifier (14) and, if appropriate, may be obtained in the analog domain.

[0079] To simplify the Gilbert cell, it may be possible to use the photocurrents 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 to achieve 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 integration 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 manner to a normal imaging 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 circuits.

[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, no electronic mixing is required after photodetection, thereby providing an advantage in terms of system complexity. According to some embodiments, as shown in FIG. 4, an optical modulator (17) is used to modulate the amplitude of the optical local oscillator. In one embodiment, the optical modulator (17) is an optical amplitude modulator, whether based on electro-optic absorption, Mach-Zehnder interferometer, or other.

[0082] If the amplitude modulation leaves some phase modulation, a phase modulator can be added in series to ensure constant phase operation and avoid undesired frequency shifts in the reference channel. Amplitude modulation can also be obtained in various ways, e.g., via optical amplifiers, modulation of the 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 and shifted by 90° with respect to each other to directly achieve the desired mathematical result. To physically achieve this, a 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 FIG. 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, but can also be replaced by other multiplexing schemes (frequency domain multiplexing, code multiplexing, etc.). The switching of the demodulation signals in both the imaging channel and the reference channel 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 by 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 an optical modulator 17), the different optical light source channels are modulated to provide modulated light source beams of illumination directed toward one or more targets. In this way, the 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 modulation 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 a corresponding optical signal, such that each of the light source channels provides an optical output (1110) that may 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-optical modulator. According to some embodiments, any of the one or more SOAs (1106) and / or one or more optical modulators (1108) 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, the various optical outputs (1110) are transmitted towards one or more targets and received over the imaging channels (3) from one or more targets as illustrated in Figure 3 or Figure 4. According to some embodiments, the light received across the various imaging channels (3) may be mixed with the imaging oscillator (111) in the various imaging receivers (5) without the need for mixers (12) or optical modulators (17) since modulation has already been performed on the source light. The imaging oscillator (111) may represent light generated from a laser source (1102).

[0089] When the Doppler shift is large (e.g., due to high relative velocity of the object being imaged), demodulation of the individual signals from the array to baseband results in a highly scalable but slow electronics readout. This achieves the desired effect, but 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 originating from the discrete nature of photon detection. If the reference channel does not have any SNR advantage over the other inputs to the mixer in the array, the array gain from coherent combining of the array outputs can be nullified. Furthermore, at low input signal SNR per element, there is an 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 arrangement is provided above the reference channel to provide a clean set of tones and minimize the effect of noise on the mixer. The sampling period of the camera reading out the imaging array is typically around 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 illustrates one example of a signal filter arrangement (700) provided above the reference channel to improve the SNR of the reference signal. According to some embodiments, the signal filter arrangement (700) is provided after the reference optical IQ receiver (6) but before the signal is mixed with the imaging channel, for example via a mixer (12). According to some embodiments, the signal filter arrangement (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 arrangement (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) as seen above the reference channel from any of FIGS. 2-4. Following these elements, the signal filter arrangement 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 number 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, which is 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 affect the modulation provided by the light modulator (17) to the imaging oscillator (111) that is mixed with the various imaging channels (3) of the imaging array 706. According to some other embodiments, the Ref1 signal is used to affect the modulation provided to the 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 the sampling rate of the camera and the higher sampling rate generated on the reference channel using a signal filter arrangement (700), according to some embodiments.

[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 with a stable frequency during the extended reference sample collection window, meaning that the object has a stable distance and relative velocity at least over the integration time. A stable frequency may not be generated, however, for example, if the object is accelerated by ±1g or more and the camera integration time is 0.1 ms or more. However, it is possible to mathematically compensate for the chirp in the filtering stage. This can be done through parallel application of multiple chirps, corresponding to different object accelerations, to the digitized reference signal, finding the maximum 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 more and more complicated 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, which may negate the coherent combining of signals and result in an SNR performance that only improves with the square root of the elements in the array. One way to deal with 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 light modulator 1108). In another example, it is possible to perform a spatial Fourier transform of the incoming light signal through a lens that focuses the light into a subarray corresponding to a particular direction. When this is done using a cylindrical lens, each subarray becomes a 1D coherent receiver array, and the number of directions (and corresponding number of targets) imaged becomes significantly smaller.

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

[0097] According to some embodiments, the signal filter arrangement (900) includes a temporal filtering unit (704) as discussed above with reference to FIG. 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 filter arrangement (900) also generates a set of additional reference outputs (collectively referred to as Ref2 in FIG. 9) to be mixed with the imaging channels from the imaging array (706). According to some embodiments, each of the additional reference outputs (Ref2) corresponds to a rough direction of the received light from a scene containing multiple objects. A number of 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 directional discrimination capability when all of them are combined to generate a set of additional reference outputs (Ref2). The output from the mixer (904) is received by a second A / D and then a fast Fourier transform (FFT) is performed on the signal using an FFT element (906) to more easily distinguish noise from signal peaks and convert the secondary reference channel (902) back to the channel domain. A 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, a phase adjustment stage (910) is used to coherently accumulate the signal and resolve the reference signal into intermediate sample periods to compensate for acceleration or deceleration of the imaged object. 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 affect the modulation provided to different light source channels of the light source modulation scheme (1100). FIG. 10 illustrates the sampling rate of the camera and the higher sampling rate generated on the reference channel for both the Ref1 and Ref2 signals, where the Ref2 signal sampling rate is an intermediate sampling rate, according to some embodiments.

[0098] For example, unless specifically stated otherwise, terms such as "processing," "computing," "calculating," "determining," or similar terms may be understood to refer to 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 registers and / or memory units of a computer system into other data similarly represented as physical quantities in registers, memory units, or other such information storage transmission or display devices of a computer system. Embodiments are not limited in this context.

[0099] The term "circuit" or "circuitry", as used in any embodiment herein, may include, for example, alone or in any combination, hardwired circuitry, programmable circuitry such as a computer processor with one or more individual instruction processing cores, state machine circuitry, and / or firmware that stores instructions executed by a programmable circuit. 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, which may further be embodied or implemented to include any number of threads, etc. in a hierarchical manner. Firmware may be embodied as code, instructions or instruction sets, and / or data hard-coded (e.g., non-volatile) in a memory device. The circuits, together or individually, may be embodied as circuits that form 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.

[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). Thus, 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 said at least one reference aperture, configured to receive a reference reflected light reflected by said moving object illuminated by said light source; at least one imaging oscillator; at least one first imaging light receiver associated with the imaging input aperture and the imaging oscillator and configured to obtain 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 with 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 and 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 cancelled 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 having a component that is coherent with a component of the reference reflected light that corresponds 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 orthogonal component and said reference orthogonal 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 said 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, further comprising:

7. The at least one mixer comprises: a third mixer intended to mix said first in-phase component and said 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, further comprising:

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. The LIDAR system of claim 1, 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 the samples.

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 said at least one reference aperture, configured to receive a reference reflected light reflected by said moving object illuminated by said light source; at least one imaging oscillator; at least one first imaging light receiver associated with the imaging input aperture and the imaging oscillator and configured to obtain 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 with 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 a Doppler frequency shift caused by the moving object is cancelled 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 having a component that is coherent with a component of the reference reflected light that corresponds 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 claim 18, 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 said at least one reference aperture, configured to receive a reference reflected light reflected by said moving object illuminated by said light source; at least one imaging oscillator; at least one first imaging light receiver associated with the imaging input aperture and the imaging oscillator and configured to obtain 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 with 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 an 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 a Doppler frequency shift caused by the moving object is 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 that corresponds 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 a Doppler frequency shift caused by the moving object is cancelled or reduced; 1. A method for suppressing Doppler frequency shift in a LIDAR system, comprising:

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