Generation of coaxial local oscillators in conjugate focal planes of FMCW-LIDAR systems

Abstract

According to one aspect of the present invention, a LIDAR system includes a light source that emits a light beam and a free-space optical system coupled to the light source and configured to focus the light beam onto a first focal plane. A local oscillator (LO) signal is generated by partial reflection of the light beam from a partially reflective surface positioned proximate to the first focal plane, and a transmitted portion of the light beam is directed toward a scanned target environment to generate a target return signal. The free-space optical system focuses the LO signal and the target return signal onto a second focal plane that is a conjugate focal plane of the first focal plane. The LIDAR system also includes a photodetector having a receiving surface positioned proximate to the second focal plane, the receiving surface mixing the LO signal with the target return signal to generate target information.

Description

[Technical field]

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S. patent application Ser. No. 17 / 031,515, filed Sep. 24, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates generally to light detection and ranging (LIDAR) systems, and more particularly to the generation of coaxial local oscillator (LO) signals at conjugate focal planes of free space optics. [Background technology]

[0003] Frequency-modulated continuous wave (FMCW) LIDAR systems use a tunable laser to project frequency-modulated light toward a target, and a coherent receiver with a local copy of the transmitted signal (LO signal) to detect the backscattered or reflected light from the target, where it is mixed with a return signal delayed by the round-trip time to the target, thereby producing a beat frequency proportional to the distance to each target within the system's field of view.

[0004] In such LIDAR systems, an optical scanner with a fast mirror is used to scan the field of view (FOV) and then back-scan the target return signal from the FOV. As the mirror speed increases, the movement of the mirror during the round trip time to the target introduces a spatial misalignment between the LO signal and the target return signal, which in turn reduces the spatial mixing efficiency of the photodetector used to mix the signals. Summary of the Invention [Means for solving the problem]

[0005] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Below, we will discuss aspects of the present invention, namely, LIDAR systems and methods for improving the spatial mixing efficiency degradation of coaxial LO signals and target return signals.

[0006] According to one aspect of the present invention, a LIDAR system includes a light source that emits a light beam, and a free space optical system associated with the light source that focuses the light beam onto a first focal plane. A local oscillator (LO) signal is generated by partial reflection of the optical beam from a partially reflective surface located proximate to the first focal plane, and a transmitted portion of the optical beam is directed toward a scanned target environment to generate a target return signal. The free space optics focuses the LO signal and the target return signal onto a second focal plane that is a conjugate focal plane of the first focal plane. The LIDAR system also includes a photodetector having a receiving surface (detection surface) positioned adjacent to the second focal plane, which is the conjugate focal plane, and mixing the LO signal with the target return signal at the receiving surface to generate target information.

[0007] In one aspect of the invention, the free space optical system includes a polarizing beam splitter (PBS) that transmits the light beam to a first lens system and focuses the light beam through the first lens system onto the first focal plane. The free space optical system also includes an optical window having the partially reflective surface that generates the LO signal from the light beam and that reflects the LO signal through the first lens system toward the polarizing beam splitter (PBS).

[0008] In one aspect of the invention, the free space optical system includes a second lens system for directing the LO signal directed from the PBS, the second lens system focusing the LO signal and the target return signal at the second focal plane.

[0009] In one aspect of the invention, the free space optical system includes a third lens system that collimates a transmitted portion of the light beam, and an optical scanner that cooperates with the third lens system to scan a target environment with the transmitted portion of the light beam and to back-scan the target return signal. The third lens system focuses the target return signal onto a first focal plane, the first lens system collimates the LO signal and the target return signal, and the PBS directs the LO signal and the target return signal to a second lens system.

[0010] In one aspect of the invention, the partially reflective surface is located at a position away from the first focal plane.

[0011] In one aspect of the present invention, the light receiving surface of the photodetector is disposed at a position away from the second focal plane.

[0012] According to one aspect of the present invention, a method for a LIDAR system includes: focusing the light beam at a first focal plane; generating a local oscillator (LO) signal by reflecting a portion of the optical beam from a partially reflective surface located proximate to the first focal plane and directing a transmitted portion of the optical beam toward a scanned target environment; focusing the LO signal and a target return signal from the transmitted portion of the optical beam onto a second focal plane conjugate to the first focal plane; and mixing the LO signal with the target return signal by a photodetector positioned proximate to the second focal plane to generate target information.

[0013] In one aspect of the present invention, the method in the LIDAR system comprises: generating said light beam with a coherent light source; The method includes transmitting the light beam through a polarizing beam splitter (PBS) to the first lens system, focusing the light beam through the first lens system onto a first focal plane, and passing the light beam through an optical window having the partially reflective surface while reflecting the LO signal, which is a portion of the light beam, through the first lens system at the partially reflective surface.

[0014] In one aspect of the present invention, the method in the LIDAR system comprises: The polarizing beam splitter (PBS) reflects the LO signal and the target return signal to a second lens system, which focuses the LO signal and the target return signal on a second focal plane of the second lens system.

[0015] In one aspect of the present invention, the method in the LIDAR system comprises: collimating a transmitted portion of the light beam using a third lens system; scanning a target environment with the transmitted portion of the light beam; backscanning a target return signal from said scan; and focusing the target return signal onto the first focal plane using the third lens system.

[0016] In one aspect of the present invention, the method in the LIDAR system comprises: collimating the LO signal and the target return signal with the first lens system; and The method includes directing the collimated LO signal and the target return signal to the second lens system using the PBS, and focusing the LO signal and the target return signal through the second lens system onto the second focal plane.

[0017] In one aspect of the invention, the method in the LIDAR system includes positioning the partially reflective surface away from the first focal plane.

[0018] In one aspect of the present invention, the method in the LIDAR system includes disposing a light receiving surface of the photodetector at a position away from the second focal plane.

[0019] A LIDAR system according to one aspect of the present invention comprises: A processor; and a non-transitory computer-readable medium storing instructions that, when executed by the processor, cause the LIDAR system to: Focusing the light beam on a first focal plane; generating a local oscillator (LO) signal by reflecting a portion of the optical beam from a partially reflective surface located proximate to the first focal plane and directing a transmitted portion of the optical beam toward a scanned target environment; focusing the LO signal and a target return signal from the transmitted portion of the optical beam onto a second focal plane conjugate to the first focal plane; and A photodetector positioned proximate to the second focal plane mixes the LO signal with the target return signal to generate target information.

[0020] In one embodiment of the invention, the actions commanded to the LIDAR system also include: generating said light beam with a coherent light source; The light beam is transmitted through a polarizing beam splitter (PBS) to the first lens system, the light beam is focused through the first lens system onto a first focal plane, and the light beam is passed through an optical window having the partially reflective surface, while the LO signal, which is a portion of the light beam, is reflected through the first lens system by the partially reflective surface.

[0021] In one aspect of the invention, the actions commanded to the LIDAR system also include reflecting the LO signal and the target return signal off the polarizing beam splitter (PBS) and directing them to a second lens system and focusing the LO signal and the target return signal at a second focal plane of the second lens system.

[0022] In one embodiment of the invention, the actions commanded to the LIDAR system also include: collimating a transmitted portion of said light beam using a third lens system; scanning a target environment with a transmitted portion of said light beam; backscanning a target return signal from said scan; and The third lens system is used to focus the target return signal to the first focal plane.

[0023] In one embodiment of the invention, the actions commanded to the LIDAR system also include: collimating the LO signal and the target return signal using the first lens system; and The collimated LO signal and the target return signal are directed to the second lens system using the PBS, and the LO signal and the target return signal are focused through the second lens system onto the second focal plane.

[0024] In one aspect of the invention, the partially reflective surface of the optical window is located away from the first focal plane.

[0025] In one aspect of the present invention, the light receiving surface of the photodetector is disposed at a position away from the second focal plane. [Brief description of the drawings]

[0026] In order to clarify various aspects of the present invention, reference is made to the drawings which are referred to in the following detailed description (embodiments), in which the same reference numerals designate the same components.

[0027] [Figure 1] FIG. 1 is a block diagram showing an FMCW-LIDAR system according to an embodiment of the present invention.

[0028] [Diagram 2]FIG. 2 is a time-frequency diagram illustrating an example FMCW-LIDAR waveform according to an embodiment of the present invention.

[0029] [Diagram 3] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0030] [Figure 4] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0031] [Diagram 5] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0032] [Figure 6] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0033] [Figure 7] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0034] [Figure 8] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0035] [Figure 9] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0036] [Figure 10] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0037] [Figure 11] FIG. 1 is a block diagram illustrating an example of a lens system of a LIDAR system according to an embodiment of the present disclosure.

[0038] [Figure 12] FIG. 1 is a block diagram illustrating an example of an optical system of a LIDAR system according to an embodiment of the present invention.

[0039] [Figure 13] 1 illustrates a method for a LIDAR system according to an embodiment of the present invention, and is a flowchart showing an example of a method for setting a coaxial local oscillator at a conjugate focal plane.

[0040] [Figure 14] FIG. 2 is a block diagram showing a configuration example of a coaxial local oscillator on a conjugate focal plane, illustrating a processing system of a LIDAR system according to an embodiment of the present invention.

[0041] [Figure 15] FIG. 1 is a block diagram illustrating an example of a multi-beam optical system of a LIDAR system in accordance with an embodiment of the present invention.

[0042] [Figure 16] FIG. 1 is a block diagram illustrating an example of a multi-beam optical system of a LIDAR system in accordance with an embodiment of the present invention.

[0043] [Figure 17] FIG. 1 is a block diagram illustrating an example of a multi-beam optical system of a LIDAR system in accordance with an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a LIDAR system and method according to an embodiment of the present invention will be described, that is, a LIDAR system and method for improving the degradation of spatial mixing efficiency of a coaxial LO signal and a target return signal. The LIDAR system of the embodiments of the present invention can be implemented in any sensing market, including, but not limited to, transportation, manufacturing, metrology, medical, and security systems. Additionally, the LIDAR system described in the embodiments is implemented as part of the front end of a frequency modulated continuous wave (FMCW) device that aids in the spatial awareness of autonomous driving assistance systems and autonomous vehicles. FIG. 1 illustrates a LIDAR system 100 according to one embodiment. LIDAR system 100 may include any one or more of numerous components, but may include fewer or additional components than those shown in FIG. 1. As shown in Figure 1, the LIDAR system 100 has an optical circuit 101 implemented on a photonics chip. The optical circuit 101 includes a combination of active and passive optical components. The active optical components generate, amplify, and / or detect optical signals, etc. In some examples, the active optical components have light beams of different wavelengths and include one or more optical amplifiers, one or more photodetectors, etc.

[0045] The free space optics 115 includes one or more optical waveguides for transmitting optical signals and for routing and manipulating the optical signals to the appropriate input / output ports of the active optical circuit. The free space optics 115 includes one or more optical components such as taps, wavelength division multiplexers (WDMs), splitters / combiners, polarizing beam splitters (PBSs), collimators, couplers, etc. In one aspect, the free space optics 115 includes components for converting the polarization state and directing the received polarized light to a photodetector, for example, using a PBS. The free space optics 115 may also include a diffractive element that deflects light beams having different frequencies at different angles along an axis (e.g., a fast axis).

[0046] The LIDAR system 100 of this embodiment includes an optical scanner 102 having one or more scanning mirrors that are rotatable along an axis orthogonal or substantially orthogonal to the fast axis (e.g., a slow axis) of the diffractive element to direct an optical signal that scans the environment according to a scanning pattern. For example, the scanning mirrors can be rotated by one or more galvanometers. For example, the return light beam may be directed to a photodetector by a polarizing beam splitter. Note that the optical scanner 102 may include quarter wave plates, lenses, anti-reflective coated optical windows, etc., in addition to mirrors and galvanometers.

[0047] The LIDAR system 100 is provided with a LIDAR controller 110 to control and support the optical circuit 101 and the optical scanner 102. The LIDAR controller 110 contains the processing equipment required for the LIDAR system 100. In one aspect, the processing device is one or more general-purpose processing devices, such as a microprocessor, a central processing device, and in particular a complex instruction set computing (CISC) microprocessor, a reduced instruction set computing (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets, or a combination of instruction sets. Additionally, the processing device may be one or more of special purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), a network processor, or the like.

[0048] In one embodiment, the LIDAR controller 110 is provided with a signal processing unit 112, such as a DSP, which outputs a digital control signal for controlling the optical driver 103. The digital control signal is converted to an analog signal via the signal conversion unit 106. For example, the signal conversion unit 106 includes a digital-to-analog converter. The optical driver 103 provides drive signals to the active optical components of the optical circuit 101 to drive light sources such as lasers and amplifiers. In one embodiment, multiple optical drivers 103 and signal conversion units 106 may be provided to drive multiple light sources.

[0049] The LIDAR controller 110 is also configured to output digital control signals to the optical scanner 102. The motion controller 105 can control the galvanometers of the optical scanner 102 based on the control signals received from the LIDAR controller 110. In particular, a digital-to-analog converter can be used to convert the coordinate routing information from the LIDAR controller 110 into signals that can be processed by the galvanometers of the optical scanner 102. In one embodiment, the motion controller 105 may also send information regarding the position or movement of components of the optical scanner 102 back to the LIDAR controller 110. In particular, an analog-to-digital converter may be used to convert the information regarding the galvanometer position into a signal that the LIDAR controller 110 can process.

[0050] The LIDAR controller 110 is further configured to analyze the input digital signal. In this regard, the LIDAR system 100 is provided with an optical receiver 104 for measuring one or more beams received by the optical circuit 101. In particular, the optical receiver 104 as a reference beam receiver measures the amplitude of a reference beam from the active optical components and converts the signal from the reference beam receiver by an analog-to-digital converter into a signal that can be processed by the LIDAR controller 110. The target receiver, as the optical receiver 104, also measures an optical signal carrying information about the range and velocity of the target in the form of a beat frequency modulated optical signal, where the reflected beam of the optical signal may be mixed with a local oscillator (LO) signal. The optical receiver 104 may include a high speed analog-to-digital converter to convert the signal from the target receiver into a signal that can be processed by the LIDAR controller 110. In one aspect, the signal from the optical receiver 104 may be subject to signal conditioning by the signal conditioning unit 107 before being received by the LIDAR controller 110. For example, the signal from the optical receiver 104 may be provided to an operational amplifier in the signal conditioning unit 107 for amplification of the return signal, and the amplified signal provided by the operational amplifier to the LIDAR controller 110.

[0051] In some applications, the LIDAR system 100 may additionally be provided with one or more imagers 108 configured to capture images of the environment, a global positioning system (GPS) 109 configured to provide a geographic location of the system, or other sensor inputs. The LIDAR system 100 may also include an image processor 114 that may be configured to receive images and geographic locations from the imager 108 and Global Positioning System (GPS) 109, and to transmit the images and location or related information to the LIDAR controller 110 or other systems connected thereto.

[0052] In operation according to some embodiments, the LIDAR system 100 is configured to simultaneously measure distance and velocity in two dimensions using a non-degenerate optical source. This capability enables real-time long-range measurements of distance, velocity, azimuth, and height (elevation) of the surrounding environment.

[0053] In some examples, the scanning process is initiated by the optical driver 103 and the LIDAR controller 110. The LIDAR controller 110 instructs the optical driver 103 to modulate one or more light beams, respectively, and these modulated signals are transmitted through the passive optical circuitry of the optical circuitry 101 to a collimator of the free-space optics 115. The collimator directs the modulated signals to the optical scanner 102, which scans the environment in a pattern preprogrammed by the motion controller 105. The optical circuit 101 may include a polarizing waveplate (PWP) that transforms the polarization state of the light as it exits the optical circuit 101. In this case, the polarizing waveplate may employ a quarter waveplate or a half waveplate, or a non-reciprocal polarization rotator such as a Faraday rotator. A portion of the polarized light beam may be reflected back into the optical circuit 101. For example, a lens or collimating system used in the LIDAR system 100 may have natural reflective properties or a reflective coating that causes a portion of the light beam to be reflected back into the optical circuit 101.

[0054] The optical signal reflected from the environment is sent to the receiver (optical receiver 104) through the optical circuit 101. At this time, the polarization state of the light has been converted, so it is reflected by the polarizing beam splitter along with a portion of the polarized light that was reflected back to the optical circuit 101. As a result, the reflected optical signal does not return to the same optical fiber or waveguide as the light source, but is reflected to different optical receivers. These signals interfere with each other and generate a mixed (combined) signal. Each beam signal returning from the target produces a time-shifted waveform, and the time phase difference between these two waveforms produces a beat frequency that is measured by an optical receiver (photodetector). The mixed (combined) signal can then be reflected to the optical receiver 104 .

[0055] The analog signal received by the optical receiver 104 is converted to a digital signal by an ADC (analog-to-digital converter), which is then sent to the LIDAR controller 110. A signal processing unit 112 of the device receives the digital signals and processes them. In one embodiment, the signal processing unit 112 receives position data from the motion controller 105 and a galvanometer (not shown) and image data from the image processor 114. This enables the signal processing unit 112 to generate a 3D point cloud with information about the distance and velocity of points in the environment as the optical scanner 102 scans additional points. The signal processing unit 112 may also overlay the 3D point cloud with image data to determine the speed and distance of surrounding objects. The LIDAR controller 110 may also process satellite-based navigation position data to provide precise global position information.

[0056] 2 is a time-frequency diagram 200 of an FMCW scanning signal 201 that a LIDAR system, such as LIDAR system 100, can use to scan a target environment in one embodiment. FM The scanning signal 201, labeled (t), has a chirp bandwidth Δf C and chirp period T C It is a sawtooth waveform (sawtooth "chirp") with The inclination of the sawtooth is k=(Δf C / T C ). Also shown in FIG. 2 is a target return signal 202 in one embodiment. FM The target return signal 202, denoted (t-Δt), is a time delayed version of the scanning signal 201, where Δt is the round trip time to and from the target illuminated by the scanning signal 201. This round trip time is given by Δt=2R / v, where R is the range of the target and v is the speed of light, c, which is the speed of the light beam. Therefore, the range R of the target can be calculated as R = c(Δt / 2). When the target return signal 202 is optically mixed with the scanning signal, a range-dependent difference frequency ("beat frequency") Δf R (t) is generated. Beat frequency Δf R (t) is linearly related to the time delay Δt by the sawtooth slope k. That is, Δf R (t) = kΔt. Since the target distance R is proportional to Δt, the target distance R is R = (c / 2)(Δf R (t) / k) That is, the distance R can be calculated as the beat frequency Δf R It has a linear relationship with (t). Beat frequency Δf R (t) is generated as an analog signal, for example, in optical receiver 104 of LIDAR system 100. This beat frequency is digitized, for example, by an analog-to-digital converter (ADC) in signal conditioning unit 107 of LIDAR system 100. The beat frequency signal thus digitized is then digitally processed in a signal processing unit (e.g., signal processing unit 112) in LIDAR system 100. However, it should be noted that the target return signal 202 typically contains a frequency offset (Doppler shift) if the target has a relative velocity with respect to the LIDAR system 100. For simplicity and ease of illustration, the Doppler shift is not shown in FIG. 2 because it is detected separately and used to correct the frequency of the return signal. It is also important to note that the sampling frequency of the ADC is determined by the highest beat frequency that the system can handle without aliasing. Typically, the highest frequency that can be handled is half the sampling frequency (i.e., the "Nyquist limit"). For example, and not by way of limitation, if the sampling frequency of the ADC is 1 GHz, then the highest beat frequency that can be handled without aliasing (Δf R max) is 500 MHz. This limit is the system's maximum target range Rmax = (c / 2)(Δf R max / k), which can be adjusted by changing the sawtooth inclination k. In one example, the data samples from the ADC may be continuous, but the subsequent digital processing described below may be divided into "time segments" that may be associated with a predetermined periodicity of the LIDAR system 100. For example, and without limitation, the time segments may be divided into "time segments" that are equal to or longer than the chirp period T C or the number of azimuth rotations made by the optical scanner described above.

[0057] 3 is a two-dimensional diagram of an optical element system 300 according to one embodiment, which may include one or more components of the optical circuit 101, the free space optics 115, and the optical scanner 102 of the LIDAR system 100 shown in FIG.

[0058] The optical element system 300 includes a light source 301 that generates a coherent light beam 302 having a selected polarization (e.g., s-polarized or p-polarized). The light beam 302 has an approximately circular or elliptical cross-section, as illustrated by cross section AA (303). The light source 301 directs the light beam 302 to a polarizing beam splitter (PBS) 304, which transmits the selected polarization of the light beam 302 to a first lens system 305. In one embodiment, a polarizing plate or Faraday rotator can be used to adjust the reflected polarization from the optical window 307 . Additionally, in one embodiment, the optical window 307 includes a wedge glass configured to eliminate spatial interference of front and back reflections, where the optical window 307 can be configured based on the scan speed and / or region of interest. In this way, changing the orientation of the wedge can induce a shift in the LO signal on one or more photodetectors to compensate for the overlap mismatch between the return and LO signals due to reverse scan delays in high speed unidirectional scanners such as spin polygons.

[0059] In one embodiment, a beam splitter (BS) can be used in place of the PBS 304. A first lens system 305 is configured to focus the light beam 302 at a first focal plane 306. Note that the lens system may be any multi-element lens design including a combination of spherical and aspherical lenses, or may be a single aspherical lens.

[0060] The optical window 307 has a partially reflective surface 308 located at the first focal plane 306. The partially reflective surface 308 reflects a portion of the light beam 302 as a polarization-altered local oscillator (LO) signal 309 towards the first lens system 305. Because the optical window 307 is substantially perpendicular to the main optical axis 310 of the first lens system 305, the return path of the LO signal 309 is substantially the same as the forward path of the light beam 302. Although not shown, the optical window 307 may also have a partially reflective surface at its rear surface. Thus, an additional LO signal from the light beam 302 may be reflected from the partially reflective surface at the rear surface of the optical window 307. In one embodiment, either the LO signal 309 from the front surface or the additional LO signal from the rear surface may be used as the LO signal. However, of the two LO signals, one may be in focus and the other out of focus, which may result in competing LO signals and increased shot noise. Therefore, in one embodiment, the optical window 307 may be a wedged glass so that the LO signal reflected from the rear surface of the optical window 307 does not interfere with the front reflection (e.g., LO signal 309) and vice versa.

[0061] The LO signal 309 is collimated by the first lens system 305 and reflected towards the PBS 304, where the LO signal 309 with its polarization changed by the PBS 304 is directed towards the second lens system 311. As shown in cross section BB 312, the LO signal 309 has a substantially circular or elliptical cross section that is the same or similar to cross section AA 303. The second lens system 311 focuses the LO signal 309 onto a second focal plane 313 that is a conjugate focal plane of the first focal plane 306. The photodetector 314 has a receiving surface located at the second focal plane 313 and receives the energy of the LO signal 309. Ideally, the LO signal 309 would be focused to a point on the second focal plane 313, but practical limitations on the alignment of optical components cause the LO signal 309 to be out of focus and, as a result, projected 316 onto the surface (receiving surface 315) of the photodetector 314 with a non-zero diameter. The other optical elements of the optical element system 300, including the third lens system 317 and the optical scanner 318, are described below.

[0062] 4 illustrates an isolated view of the entire path of the light beam 302 towards the target environment 320 according to one embodiment. After the light beam 302 converges at the first focal plane 306, the portion of the light beam 302 that is not reflected by the optical window 307 diverges towards a third lens system 317. In this embodiment, the third lens system 317 has the same focal length as the first lens system 305, and the light beam 302 is collimated by the third lens system 317. As shown in cross section CC (319) in FIG. 4, the collimated light beam 302 has a substantially circular cross section.

[0063] The collimated light beam 302 is received by an optical scanner 318 that scans in azimuth and elevation over a target environment 320. Objects in the target environment 320 reflect a portion of the light beam 302 as a return signal 321, as shown in FIG. 5, a return signal 321 having a different polarization than the light beam 302 is counter-scanned by the optical scanner 318. Ignoring the effect of counter-scanning errors in the optical scanner 318 (described in more detail below), the counter-scanned return signal 321 is a collimated beam with a circular cross-section that is approximately parallel to the main optical axis 310 of the third lens system 317 and the first lens system 305, as shown by cross-section DD (322) in FIG. As a result, the return signal 321 converges at the first focal plane 306 and diverges towards the first lens system 305 where it is recollimated. The recollimated return signal 321 is reflected by the PBS 304 due to its changed polarization and directed towards the second lens system 311.

[0064] The second lens system 311 focuses the return signal 321 reflected by the PBS 304 onto the second focal plane 313, as described for the LO signal 309. For example, in one embodiment, the return signal 321 may be focused to a point on the second focal plane 313.

[0065] FIG. 6 illustrates the operation of optical element system 300 when the optical elements are arranged to minimize optical aberrations. As an example, the optical beam, LO signal, and return signal (e.g., optical beam 302, LO signal 309, and return signal 321 of FIGS. 3 and 4) are all symmetrically arranged. This arrangement is shown in FIG. 6 using composite beam notation. For example, the beam 322 between the first focal plane 306 and the target environment 320 includes both the emitted light beam 302 and the incident return signal 321 (e.g., the return signal 321 shown in FIG. 5). The beam 323 between the first focal plane 306 and the PBS 304 includes the emitted light beam 302, the incident return signal (e.g., the return signal 321 shown in FIG. 5), and the LO signal (e.g., the LO signal 309 in FIG. 3). The beam 324 between the PBS 304 and the photodetector 314 includes both the LO signal (e.g., the LO signal 309 in FIG. 3) and the return signal (e.g., the return signal 321 in FIG. 5). In this example with the optical element system 300, the LO signal and the return signal (e.g., the LO signal 309 and the return signal 321 in FIG. 3 and FIG. 5) will be focused to a point on the second focal plane 313.

[0066] 7 shows an optical system where the optical window 307 is misaligned by an angle Φ. This misalignment can cause the focus of the LO signal 309 to be misaligned on the second focal plane 313 from the focus of the return signal 321. The spatial mixing efficiency (γ) of the optical element system 300 is a function of the overlap integral between the spot size of the LO signal 309 and the spot size of the return signal 321, given by: TIFF0007645365000001.tif32141 Here, E s and E Lo is the electric field profile of the return signal and the LO signal on the photodetector, and x 0 andy 0 represents the displacement (shift) between the spot position of the return signal and the spot position of the LO signal, and det is the radius of the circular detector. As shown in FIG. 7, if the LO and return signal spots do not overlap, the spatial mixing efficiency will decrease to zero.

[0067] Even if the optical window 307 is misaligned, mixing efficiency can be maximized by positioning the light receiving surface 315 of the photodetector 314 in front of the second focal plane 313 (or behind the second focal plane 313 to achieve a similar defocusing effect), as shown in FIG. 8. In FIG. 8, the projection of composite beam 324 (including coaxial LO signal 309 and return signal 321) onto receiving surface 315 has a larger diameter than the spot focus of composite beam 324 in the arrangement shown in FIG.

[0068] 9 shows the effect of relocating the photodetector 314 in front of (away from) the second focal plane 313 (the conjugate focal plane) when the LO signal 309 and the return signal 321 are misaligned. Instead of having two non-overlapping focal points, the LO signal 309 and the return signal 321 now have an overlapping region on the receiving surface 315 of the photodetector 314. This causes spatial mixing in this region, resulting in a baseband signal (at the beat frequency) Δf R (t) can be generated.

[0069] By placing the photodetector 314 (light receiving surface 315) in front of the second focal plane 313, the spatial mixing ratio can be increased to address misalignment due to error scan lag between the LO signal 309 and the return signal 321. At angular velocities of approximately 20,000 degrees / second or greater, the time delay of the return signal from a target at a long distance becomes long, causing the scanning mirror to rotate at a very small angle in the optical scanner 318 due to the reverse scan lag of the return signal 321. As a result, the return signal 321 may be reflected by the optical scanner 318 to the third lens system 317 with a deviation (skew) in its angle.

[0070] The effect of such a misalignment (skew) is illustrated in Figure 10. In this example, return signal 321 is misaligned from light beam 302 as shown in cross section GG. As shown in Figure 11, such a skew angle is propagated through the combination of the third lens system 317 and the first lens system 305. Note that in Figure 11, the lens system is represented as an equivalent thin double convex lens. When the return signal 321 enters the third lens system 317 as a plane wave at an angle θ relative to the primary optical axis 310, the return signal 321 is focused to the first focal plane 306 at a point determined by the focal length of the third lens system 317 and the angle θ. Furthermore, as previously mentioned, the distance between the first lens system 305 and the third lens system 317 is equal to the sum of their respective focal lengths, so the return signal 321 is recollimated by the first lens system 305 at an angle θ relative to the primary optical axis 310.

[0071] 10 above, the skewed return signal 321 and the LO signal 309 are reflected by the PBS 304 and directed to a second lens system 311 as overlapping signals as shown in cross section HH. Both signals are focused at a second focal plane 313 (a conjugate focal plane), but at different points. The LO signal 309 without any skew is focused on the main optical axis of the second lens system 311, while the return signal 321 with a skew is focused on a point shifted from the main optical axis in the same way that the third lens system 317 focuses the return signal 321 with a shift on the first focal plane 306. However, in this embodiment, since the photodetector 314 (light receiving surface 315) is disposed in front of the second focal plane 313, the LO signal 309 and the return signal 321 largely overlap on the light receiving surface 315 of the photodetector 314.

[0072] FIG. 12 shows a variation of the optical system 300 in which the LO signal 309 and the return signal 321 are configured to completely overlap on the light receiving surface 315 of the photodetector 314 . This variation is achieved by moving the partially reflective surface 308 of the optical window 307 a distance δ (away from the first focal plane) from the first focal plane 306 towards the third lens system 317. Note that a similar effect can also be achieved by moving the optical window 307 from the first focal plane 306 towards the first lens system 305. This movement causes light beam 302 to diverge from the focal point before being reflected from partially reflective surface 308. As a result, LO signal 309 reflected from partially reflective surface 308 has a significantly larger diameter than if it had been reflected from first focal plane 306, as shown in cross section JJ.

[0073] 12, the second lens system 311 focuses the LO signal 309 and the return signal 321 towards a second focal plane 313 (a conjugate focal plane). The placement of the photodetector 314, combined with the increased diameter of the LO signal 309, allows the LO signal 309 and the return signal 321 to fully overlap, maximizing spatial mixing efficiency.

[0074] FIG. 13 illustrates a flow chart illustrating a method 400 in a LIDAR system for generating coaxial local oscillator signals at conjugate focal planes, according to an embodiment. Method 400 begins with operation 402 of focusing a light beam (e.g., light beam 302) at a first focal plane (e.g., focal plane 306). Then, in operation 404, a local oscillator (LO) signal (e.g., LO signal 309) is generated by reflecting a portion of the light beam off a partially reflective surface (e.g., light receiving surface 308 on optical window 307) proximate to the first focal plane, and directing the transmitted portion of the light beam towards a target environment to be scanned (e.g., by optical scanner 318). Then, in step (operation) 406, the LO signal and the target return signal (eg, return signal 321) are focused onto a second focal plane (eg, second focal plane 313) that is conjugate to the first focal plane. Method 400 is then completed by mixing the LO signal and the target return signal at a photodetector proximate to the second focal plane (eg, surface 315 of photodetector 314) in step 408 to generate target information.

[0075] FIG. 14 illustrates a processing unit for a LIDAR system according to an embodiment, and is a block diagram illustrating an example of a processing unit 500 for generating coaxial local oscillator signals at conjugate focal planes. The processing device 500 includes a processor 501, which is for example incorporated in the signal processing unit 112 of the LIDAR control device 110 in the LIDAR system 100. As another example, the processor 501 can be one or more general-purpose processing devices, such as a microprocessor, a central processing unit, or the like. More specifically, the processor 501 may be a complex instruction set computing (CISC) microprocessor, a reduced instruction set computer (RISC) microprocessor, a very long instruction word (VLIW) microprocessor, or a processor implementing other instruction sets, or a processor implementing a combination of instruction sets. The processor 501 may also be one or more special purpose processing devices, such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), a network processor, or the like.

[0076] The processing device 500 also includes a computer readable memory 502 (computer readable medium) associated with the processor 501. The memory 502 may be, for example, a read only memory (ROM), a random access memory (RAM), a programmable read only memory (PROM), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM), a flash memory, a magnetic disk memory such as a hard disk drive (HDD), an optical disk memory such as a compact disk read only (CD-ROM) or a compact disk read write memory (CD-RW), or any other type of non-transitory memory.

[0077] As an example, the memory 502 includes instructions that, when executed by the processor 501, cause a LIDAR system (e.g., the LIDAR system 100) to generate a coaxial local oscillator signal (e.g., the LO signal 309) at a conjugate focal plane (e.g., the second focal plane 313) in accordance with an embodiment of the present disclosure.

[0078] In FIG. 14, memory 502 includes instructions 504 for focusing a light beam (e.g., light beam 302) at a first focal plane (e.g., focal plane 306), instructions 506 for reflecting a portion of the light beam off a partially reflective surface (e.g., receiving surface 308 of optical window 307) proximate the first focal plane to generate a local oscillator (LO) signal (e.g., LO signal 309) and directing a transmitted portion of the light beam to a scanned target environment by optical scanner 318, instructions 508 for focusing the LO signal and a target return signal (e.g., return signal 321) at a second focal plane conjugate to the first focal plane (e.g., second focal plane 313), and instructions 510 for mixing the LO signal and the target return signal at a photodetector (e.g., receiving surface 315 of photodetector 314) proximate the second focal plane to generate target information.

[0079] FIG. 15 is a block diagram of a multi-beam LIDAR system 600 for generating coaxial local oscillator signals in conjugate planes according to an embodiment of the present disclosure. LIDAR system 600 is functionally similar to optical element system 300 described above and includes FMCW light sources 601-1 to 601-n, each operating at a different frequency and / or bandwidth and emitting a corresponding light beam 602-1 to 602-n (collectively light beam 602). Each light beam 602 passes through a polarizing beam splitter (PBS) 604 and is focused to a focal point corresponding to a corresponding first lens system 605-1 through 605-n (collectively, first lens systems 605). Each of the light beams 602 is partially reflected by a partially reflective surface 608 on an optical window 607, and the reflected portion of each light beam 602 becomes a corresponding local oscillator (LO) signal 609-1 through 609-n (collectively, LO signals 609).

[0080] The LO signals 609 are collimated by a respective first lens system 605 and reflected by the PBS 604 towards a corresponding second lens system 611-1 to 611-n (collectively, second lens systems 611). Each second lens system 611 focuses each corresponding LO signal 609 onto a second focal plane 613 that is conjugate to the first focal plane 606. Each LO signal 609 is received by a corresponding photodetector 611-1 to 611-n (collectively, photodetectors 611) with a non-zero diameter.

[0081] The transmitted portion of each light beam 602 that is not reflected from the optical window 607 (partially reflective surface 608) is collimated by a third lens system 617 and transmitted to an optical scanner 618. The optical scanner 618 scans a target environment 620 with the light beams 602 and back-scans corresponding target return signals 621-1 through 621-n (collectively return signals 621). The return signals 621 are focused by the third lens system 617 to the first focal plane 606, then diverge and are incident on a corresponding first lens system 605. Each first lens system 605 collimates the corresponding return signal 621 and reflects it off the PBS 604 towards a corresponding second lens system 611. Each second lens system 611 focuses its corresponding return signal 621 onto a second focal plane 613. Each return signal 621 is received by a corresponding photodetector 614 overlapped with a corresponding LO signal 609, where the return signal and the LO signal are spatially mixed to generate target information.

[0082] FIG. 16 is a block diagram of a multi-beam LIDAR system 700 for generating coaxial local oscillator signals in conjugate planes according to an embodiment of the present disclosure. LIDAR system 700 is similar in all respects to LIDAR system 600 described above, but system 700 has independent optical windows 607-1 to 607-n, each of which can be independently adjusted for offset from first focal plane 606.

[0083] FIG. 17 is a block diagram of a multi-beam LIDAR system 800 for generating coaxial local oscillator signals in conjugate planes according to an embodiment of the present disclosure. The LIDAR system 800 has a configuration similar to the previously described LIDAR system 700 in all respects, but the system 800 is provided with independent third lens systems 617-1 to 617-n.

[0084] In the above description, a number of specific examples of specific systems, components, methods, etc. are shown to facilitate understanding of the embodiments of the present invention, but those skilled in the art may practice the present invention without the description of these specific examples. In addition, details of known components and methods may be omitted or shown in the form of simple block diagrams, but this is for the purpose of facilitating understanding of the present invention. Therefore, the contents disclosed are merely examples, and even if one example differs from other examples, it is considered to be included within the scope of the present invention.

[0085] When the phrase "one embodiment" or "an embodiment" is used in this specification, it means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase "in one embodiment" or "in an embodiment" in various places in this specification do not necessarily refer to the same embodiment.

[0086] Although the operations of the methods described herein are shown in a particular order, the order of operations of each method may be changed, certain operations may be performed in reverse order, at least some operations may be performed simultaneously with other operations, and instructions for different operations or auxiliary operations may be performed intermittently or alternately.

[0087] The above-described description of the embodiments of the invention (including those described in the Abstract) is not intended to be detailed or exhaustive, nor is it intended to limit the invention to the specific forms disclosed. While specific embodiments and examples of the invention are described herein for illustrative purposes, various equivalent modifications will occur to those skilled in the art. The word "example" or "exemplary" is used herein to mean serving as an example, illustration, or illustration. Any aspect or design described herein as "example" or "exemplary" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word "example" or "exemplary" is intended to illustrate concepts in a concrete manner. The term "or" as used herein is intended to be interpreted as an inclusive "or" rather than an exclusive "or." That is, unless otherwise specified or clear from the context, the phrase "X contains A or B" refers to any of the natural inclusive permutations: if X contains A, if X contains B, or if X contains both A and B, then in any of the above cases, the condition "X contains A or B" is satisfied. Furthermore, the articles "a" and "an" as used in this specification and the appended claims are to be construed to mean "one or more" unless otherwise specified and unless the singular form is clear from the context. Furthermore, when terms such as "first," "second," "third," and "fourth" are used in this specification, these terms are used as identifiers to distinguish different elements and do not necessarily indicate an order according to the numerical designation.

Claims

1. 1. A light detection and ranging (LIDAR) system comprising: a light source emitting a light beam; a free space optical system associated with the light source for focusing the light beam onto a first focal plane, generating a local oscillator (LO) signal by partial reflection of the light beam from a partially reflective surface located proximate to the first focal plane, and generating a target return signal by directing a transmitted portion of the light beam towards a scanned target environment, and focusing the LO signal and the target return signal onto a second focal plane that is a conjugate focal plane of the first focal plane; a photodetector having a light receiving surface disposed at a position adjacent to the second focal plane, the conjugate focal plane, and mixing the LO signal with the target return signal at the light receiving surface to generate target information; The free space optical system includes: a polarizing beam splitter (PBS) that transmits the light beam to a first lens system and focuses the light beam through the first lens system onto the first focal plane; an optical window having the partially reflective surface that generates the LO signal from the light beam and that reflects the LO signal through the first lens system toward the polarizing beam splitter (PBS).

2. 2. The system of claim 1, The free space optical system includes: a second lens system that directs the LO signal directed from the PBS, the second lens system focusing the LO signal and the target return signal at the second focal plane.

3. 3. The system of claim 2, The free space optical system includes: a third lens system for collimating a transmitted portion of the light beam; an optical scanner in communication with the third lens system to scan a target environment with a transmitted portion of the light beam and to reverse-scan the target return signal and focus it through the third lens system to a first focal plane, and to collimate the LO signal and the target return signal with the first lens system and direct the LO signal and the target return signal with the PBS to a second lens system.

4. 2. The system of claim 1, The partially reflective surface is positioned at a distance from the first focal plane.

5. 2. The system of claim 1, The light detector is positioned away from the second focal plane.

6. 1. A method in a light detection and ranging (LIDAR) system, comprising: generating a light beam with a coherent light source and focusing the light beam at a first focal plane; generating a local oscillator (LO) signal by reflecting a portion of the light beam from a partially reflective surface located proximate to the first focal plane and directing a transmitted portion of the light beam toward a scan target environment, comprising transmitting the light beam through a polarizing beam splitter (PBS) to the first lens system, focusing the light beam through the first lens system to the first focal plane, and passing the light beam through an optical window having the partially reflective surface while reflecting the LO signal, which is a portion of the light beam, through the first lens system at the partially reflective surface; focusing the LO signal and a target return signal from the transmitted portion of the optical beam onto a second focal plane conjugate to the first focal plane; and mixing the LO signal with the target return signal by a photodetector positioned proximate to the second focal plane to generate target information.

7. 7. The method of claim 6, The method in a LIDAR system, wherein the polarizing beam splitter (PBS) reflects the LO signal and the target return signal to a second lens system and focuses the LO signal and the target return signal at a second focal plane of the second lens system.

8. 8. The method of claim 7, collimating a transmitted portion of the light beam using a third lens system; scanning a target environment with the transmitted portion of the light beam; backscanning a target return signal from said scan; and focusing the target return signal onto the first focal plane using the third lens system.

9. 9. The method of claim 8, collimating the LO signal and the target return signal with the first lens system; and directing the collimated LO signal and the target return signal to the second lens system using the PBS and focusing the LO signal and the target return signal through the second lens system onto the second focal plane.

10. 7. The method of claim 6, 13. A method in a LIDAR system, wherein the partially reflective surface is positioned at a distance from the first focal plane.

11. 7. The method of claim 6, 13. A method in a LIDAR system, wherein the photodetector is positioned away from the second focal plane.

Images