Depth sensing using multiple coherent transmitters

Abstract

A range sensing device includes a transmitter including an array of emitters configured to emit respective beams of coherent optical radiation and a switching circuit coupled to the emitters. The optical assembly is configured to: split each beam of the coherent optical radiation into a transmitted beam and a local oscillator beam; projecting the local oscillator light beam towards an optical detector; projecting the transmitted light beam towards a corresponding position on a target; and directing the optical radiation reflected from the respective location onto the optical detector so as to optically interfere with the local oscillator beam. A controller is coupled to: apply an amplitude chirped electrical drive pulse to the transmitters while controlling the switching circuit to multiplex the electrical drive pulse in time between the transmitters; and receiving and processing an electrical beat signal output by the optical detector in response to interference between the reflected optical radiation and the local oscillator beam.

Description

Technical Field

[0001] The present invention relates generally to systems and methods for depth sensing, and particularly to frequency modulated continuous wave LiDAR. Background Art

[0002] In some depth sensing arrangements, radio frequency (RF) chirping is applied to modulate the frequency of a light beam (typically a single-mode laser beam) directed toward a target. The light reflected from the target is mixed with a sample of the transmitted light (called a local oscillator beam or local oscillator) and detected by a photodetector (such as a balanced photodiode pair). The photodetector outputs an RF signal with a beat frequency that is proportional to the distance to the target.

[0003] When a target is moving, the resulting Doppler shift of the reflected light will cause the beat frequency to increase or decrease, depending on the direction of motion. By comparing the beat frequencies obtained from chirps with positive and negative slopes, it is possible to extract both the range and velocity of the target. In an ideal case, if the beat frequency due to Doppler shift is d, and the beat frequency due to chirp and range is r, the measured beat frequency for an upward chirp would be f u = d + r and the beat frequency on the down chirp will be f d = dr. Thus, the sum of the measured up-chirp and down-chirp frequencies shows the Doppler shift, and the difference shows the range. Summary of the Invention

[0004] Embodiments of the present invention described below provide improved methods and apparatus for depth sensing.

[0005] Therefore, according to an embodiment of the present invention, a range sensing device is provided, comprising: a transmitter including an array of emitters configured to emit corresponding beams of coherent optical radiation and switching circuitry coupled to the emitters; and an optical detector. The optical assembly is configured to: split each beam of the coherent optical radiation into a transmitted beam and a local oscillator beam; project the local oscillator beam toward the optical detector; project the transmitted beam toward a corresponding location on a target; and direct optical radiation reflected from the corresponding location onto the optical detector so as to optically interfere with the local oscillator beam. A controller is coupled to: apply amplitude-chirped electrical drive pulses to the transmitter while controlling the switching circuitry to time-multiplex the electrical drive pulses between the emitters; and receive and process an electrical beat signal output by the optical detector in response to interference between the reflected optical radiation and the local oscillator beam.

[0006] In a disclosed embodiment, the emitter array and the switching circuit are provided on a single integrated circuit. Additionally or alternatively, the apparatus comprises a lenslet array disposed on the emitter array, wherein each lenslet is aligned with a corresponding emitter. Further additionally or alternatively, the emitter comprises a vertical cavity surface emitting laser (VCSEL).

[0007] In one embodiment, the apparatus includes a digital-to-analog converter configured to generate the electrical drive pulses in response to a digital input from the controller, and the controller is configured to vary the digital input during each of the drive pulses so as to linearize the frequency chirp of the beam of optical radiation.

[0008] In a disclosed embodiment, the optical detector includes a pair of balanced photodiodes.

[0009] In some embodiments, the optical detector comprises a single detector, and the optical assembly is configured to direct the optical radiation reflected from the corresponding light beams of all emitters in the array onto the single detector. Alternatively, the optical detector comprises a detector array.

[0010] In a disclosed embodiment, the controller is configured to analyze the beat signal to find the range of the corresponding position on the target.

[0011] In some embodiments, the optical assembly includes: a polarizing beam splitter cube configured to split each beam of the coherent optical radiation into the transmitted beam and the local oscillator beam; and one or more polarization rotators. In one embodiment, the polarizing beam splitter cube includes an X cube.

[0012] Additionally or alternatively, the optical assembly comprises a monolithic assembly of a prism. In one embodiment, the optical detector comprises a first photodiode and a second photodiode, the first photodiode and the second photodiode being positioned on different first and second sides of the monolithic assembly of the prism and configured to receive respective first and second portions of the reflected optical radiation and respective first and second portions of the local oscillator light beam.

[0013] In an alternative embodiment, the optical component includes an optical waveguide including an input coupler and an output coupler, wherein the transmitted light beam and reflected radiation pass through the waveguide to the target and from the target, and the local oscillator light beam is directed from the transmitter to the optical detector within the waveguide between the input coupler and the output coupler.

[0014] In one embodiment, the emitter and the optical detector are interleaved on a common substrate. Additionally or alternatively, the emitter is disposed above the optical detector.

[0015] According to an embodiment of the present invention, a method for range sensing is also provided, the method comprising: providing a transmitter comprising an array of emitters configured to emit corresponding beams of coherent optical radiation. Applying amplitude-chirped electrical drive pulses to the transmitter, while temporally multiplexing the electrical drive pulses between the emitters so that the emitters sequentially emit the corresponding beams. Each beam of the coherent optical radiation is divided into a transmitted beam and a local oscillator beam. The local oscillator beam is projected toward an optical detector, while the transmitted beam is projected toward a corresponding position on a target. Optical radiation reflected from the corresponding position is directed to the optical detector so as to optically interfere with the local oscillator beam. An electrical beat signal output by the optical detector is received and processed in response to the interference between the reflected optical radiation and the local oscillator beam.

[0016] The present invention will be more fully understood from the following detailed description of embodiments of the present invention when taken in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic cross-sectional view of a range sensing device according to an embodiment of the present invention; and

[0018] Figures 2 to 8 is a schematic cross-sectional view of a range sensing device according to other embodiments of the present invention. DETAILED DESCRIPTION

[0019] Overview

[0020] Frequency modulated continuous wave (FMCW) depth sensors are commonly used to map the topography of a target. Such depth sensors include: a scanner that scans a light beam from one or more optical radiation emitters across the target; or an emitter array, the individual light beams of which are projected onto the target. Scanners are costly in terms of power consumption and size, and they are sensitive to shock and vibration. When an array of emitters is used, each emitter typically requires its own driver circuit to drive the emitter. The radiation reflected from the target is typically detected by an array of optical detectors, each of which has an analog-to-digital (A / D) converter. The emission frequency of each emitter is modulated by an external modulation circuit. Although various arrangements for FMCW depth sensors are currently used, their complexity and size reduce their practicality in mobile use cases.

[0021] To address these issues in embodiments of the invention described herein, a range sensing device includes an emitter array and fewer photodetectors than emitters, typically a single photodetector or a balanced photodetector pair. A drive circuit is time-multiplexed to drive the emitters of the emitter array, where the drive signal is sequentially switched between the emitters by a switching circuit. The drive circuit and the switching circuit can be integrated with the emitter array on a single chip, thereby reducing packaging costs and electromagnetic interference. Similarly, the detection circuit is simplified to a single A / D converter coupled to a single photodetector (or alternatively, two A / D converters coupled to a balanced pair).

[0022] In some embodiments, rather than having a separate frequency modulation circuit coupled to each of the emitters, the inherent Joule heating of each emitter due to the excitation current is exploited: amplitude-chirped electrical drive pulses are applied to the emitters, i.e., the instantaneous drive voltage is ramped up over the duration of each excitation pulse. The chirped electrical pulses induce a temperature rise during each pulse, which shifts the emission wavelength, thereby causing a chirp in the emission frequency.

[0023] In a disclosed embodiment, a range sensing device includes a transmitter comprising an array of emitters that emit respective beams of coherent radiation and switching circuitry coupled to the emitters. The device also includes an optical detector, an optical assembly, and a controller. The optical assembly separates each beam of coherent radiation into a transmitted beam and a local oscillator beam, projects the local oscillator beam toward the optical detector, and projects the transmitted beam toward a respective location on a target. The respective beam from each emitter impinges upon a different respective location. The optical assembly also directs optical radiation reflected from a location on the target toward the optical detector to optically interfere with the local oscillator beam.

[0024] The controller applies amplitude-chirped electrical drive pulses to the transmitters while controlling the switching circuit to apply time-multiplexed electrical drive pulses between the transmitters, causing the transmitters to fire sequentially. The controller also receives and processes electrical beat signals output by the optical detector in response to interference between the reflected optical radiation and the local oscillator beam, thereby enabling calculation of the range and velocity of all locations on the target where the transmitted light beam impinges.

[0025] System Description

[0026] Figure 1is a schematic cross-sectional view of a range sensing device 10 according to an embodiment of the present invention. The device 10 includes a transmitter 12, an optical assembly 14, an optical detector 16, a digital / analog (D / A) converter 18, an analog / digital (A / D) converter 20, and a controller 22. The transmitter 12 includes a two-dimensional VCSEL (vertical cavity surface emitting laser) array 24, a lenslet array 26 in which each lenslet is aligned with a corresponding VCSEL of the VCSEL array, a switching circuit 28, and a lens 30. The optical assembly 14 includes a polarization beam splitter 32, lenses 34 and 36, one or more polarization rotators (such as quarter wave plates 38 and 40), and a mirror 42. The controller 22 typically includes a microprocessor or microcontroller having a suitable interface for communicating with the other elements of the system 10 and programmed with software and / or firmware to perform the functions described herein. Additionally or alternatively, at least some of the functions of the controller 22 may be performed by suitable hardwired or programmable logic circuitry and may be integrated with the D / A converter 18 and / or the transmitter 12 .

[0027] To map the topography of target 44 (i.e., to find the range and likely velocity of various points on the target), controller 22 transmits a series of digital signals to D / A converter 18, which converts the series of digital signals into a pulse train 46 of analog drive pulses. In one embodiment, each pulse in train 46 has a duration of 10 μs and an average current of 3 mA, but other pulse characteristics may alternatively be used. Figure 1 As illustrated, the amplitude of each pulse is ramped over the duration of the pulse, typically by ramping a digital signal and, therefore, the pulse's drive current. This ramping of the pulse amplitude is referred to herein as amplitude chirp. The amplitude-chirped analog drive pulses in string 46 produce a frequency-chirped optical signal through Joule heating of the individual VCSELs.

[0028] Simultaneously, while delivering the amplitude-chirped electrical drive pulses in pulse train 46 to VCSEL array 24, controller 22 drives switching circuitry 28 to temporally multiplex the drive signals so as to sequentially direct each analog pulse to a different VCSEL in array 24. Consequently, each of the emitters in VCSEL array 24 emits a beam of coherent radiation in a sequence determined by controller 22, while using only a single D / A converter to drive the array's VCSELs.

[0029] As an example, VCSEL 24a in array 24 emits light beam 48 in response to pulse 46a in pulse train 46. Light beam 48 is collimated by lenslet 26a of lenslet array 26 and directed by lens 30 as light beam 50 toward lens 34 and polarizing beam splitter 32 of optical assembly 14. (Light beams are depicted as simple arrows, without regard to their divergence or power.) A portion of light beam 50 is transmitted through polarizing beam splitter 32, forming local oscillator (LO) beam 52. The intensity of LO beam 52 is typically about 10% of the intensity of beam 50, determined by the polarization of beam 50 or by the coating of beam splitter 32, although stronger or weaker LO beams may alternatively be used. LO beam 52 impinges upon optical detector 16.

[0030] A major portion of beam 50 is reflected by polarizing beam splitter 32 and is projected onto location 56a on target 44 as beam 54. Radiation reflected from location 56a returns toward beam splitter 32 as beam 58. Due to the double passage through quarter-wave plate 38 and the accompanying 90° rotation of its polarization, beam 58 now passes through beam splitter 32 and is projected onto mirror 42. After further reflection from mirror 42 into beam 60 and due to the double passage through quarter-wave plate 40, beam 60 is reflected by beam splitter 32 toward optical detector 16 as beam 62.

[0031] At the optical detector 16, the local oscillator beam 52 and the beam 62 that has arrived from the position 56a on the target 44 optically interfere, thereby generating a beat signal 64a in the signal train 64 as the output from the optical detector 16. The beat signal 64a is converted into a digital signal by the A / D converter 20 and forwarded to the controller 22, which analyzes the signal to find the range of the position 56a on the target 44 and, if necessary, the speed of the target. The pulse 46a can be repeated to improve the signal-to-noise ratio of the measurement of the range of the position 56a on the target.

[0032] Pulses 46b and 46c in pulse train 46 are directed by switching circuitry 28 to different, corresponding VCSELs 24b and 24c in VCSEL array 24. Due to the lateral offset of VCSELs 24a to 24c, each VCSEL illuminates a different location on target 44. For example, VCSELs 24b and 24c illuminate corresponding locations 56b and 56c, which are offset from each other and from location 56a. Radiation reflected from locations 56b and 56c, similar to radiation reflected from location 56a, generates corresponding beat signals 64b and 64c. These signals, similar to signal 64a, are used by controller 22 to find the ranges of locations 56b and 56c. By sequentially energizing the VCSELs in VCSEL array 24 in this manner, a single photodetector 16 can be used to map the topography on target 44. Thus, the resources of D / A converter 18, detector 16, and A / D converter 20 are temporally multiplexed among the VCSELs.

[0033] The time intervals between the peaks in the beat signal (i.e., the frequency of the beat signal) are determined by the rate of Joule heating in the VCSELs of the VCSEL array 24 and the range to the target. The amplitude-chirped electrical drive pulses in the pulse train 46 can be tailored to produce constant Joule heating and, therefore, evenly spaced peaks in the beat signal, i.e., to linearize the frequency chirp of the beat signal. This, in turn, will improve the accuracy of finding the range to the target 44. The inventors have discovered that a linear ramp of the pulses, combined with a small amount of curvature added to the ramp shape, produces evenly spaced peaks in the beat signal, i.e., a constant frequency of the signal.

[0034] Figure 2 is a schematic cross-sectional view of a range sensing device 100 according to another embodiment of the present invention. This device includes a greater number of prisms than the device 10, but reduces optical power loss and eliminates the need for the reflector 42.

[0035] Device 100 includes optoelectronic assembly 101 and electronic assembly 129. Optoelectronic assembly 101 includes optical assembly 102, which includes a monolithic prism assembly 103, which includes prisms 104a, 104b, 104c, 104d, 104e, 104f, and 104g. (A monolithic prism assembly is an assembly of prisms glued together.) The following beam splitters and reflectors are provided between the prisms:

[0036] A reflector 106 is provided between the prisms 104a and 104b, which has 100% reflectivity for all polarizations;

[0037] A polarizing beam splitter 108 is provided between the prisms 104b and 104c, which has a 100% reflectivity for TE polarized radiation and a 100% transmittance for TM polarized radiation;

[0038] A reflector 110 is provided between prisms 104d and 104e, which has 100% reflectivity for all polarizations;

[0039] A beam splitter 112 is provided between prisms 104e and 104f, which has 50% reflectivity and transmittance for all polarizations; and

[0040] • A polarizing beam splitter 114 is provided between prisms 104f and 104g, which has a 100% transmission for TE polarized radiation and a 100% reflection for TM polarized radiation.

[0041] Optical assembly 102 also includes a collimating lens 116 , a quarter wave plate 118 , and a projection lens 120 .

[0042] The optoelectronic component 101 also includes a silicon substrate 122 on which is disposed a transmitter 123 comprising a two-dimensional VCSEL array 124 and a switching circuit 125; and photodiodes 126 and 128. The photodiodes in this example are configured as a balanced pair, stacked anode to cathode, and output a difference signal to a transimpedance amplifier 127. Components of the electronic component 129 may also be integrated into the silicon substrate 122 as integrated circuits, as described in further detail below. The VCSELs in the VCSEL array 124 are oriented so that each emitted radiation beam has 90% of its power in the TE orientation and 10% of its power in the TM orientation. The VCSEL array 124 may also include a VCSEL array similar to the VCSEL array 124. Figure 1 A lenslet array (not shown) is provided in the lenslet array 26 in order to modify the angular range of the light beams emitted by the individual VCSELs of the array. Similar lenslet arrays can be added to the VCSEL arrays described in the following figures.

[0043] Electronic assembly 129 includes a controller 130, a D / A converter 132, and an A / D converter 133, which are coupled to digitize the signal output by transimpedance amplifier 127. These components of electronic assembly 129 may be discrete components or portions of a single integrated circuit in silicon substrate 122. Controller 130 is coupled to D / A converter 132, to A / D converter 133, and to switching circuit 125. D / A converter 132 is also coupled to switching circuit 125.

[0044] To find the range of the position on the target (not shown in this figure), the controller 130, together with the D / A converter 132 and the switching circuit 125 (similar to Figure 1 The controller 22 in the VCSEL array 124, along with the D / A converter 18 and the switching circuit 28, sequentially transmits a current pulse to each of the VCSELs in the VCSEL array 124, thereby causing the VCSELs to sequentially emit beams of coherent optical radiation. As an example, VCSEL 124a in the VCSEL array 124 emits a light beam 136, which is collimated by the collimating lens 116 and directed toward the polarizing beam splitter 114. Due to the orientation of the VCSELs, the light beam 136 is split at the polarizing beam splitter 114 into a local oscillator light beam 138 having TM polarization (comprising 10% of the power of the light beam 136) and a light beam 140 having TE polarization (comprising 90% of the power of the light beam 136).

[0045] Light beam 140 is reflected by reflector 106 and polarizing beam splitter 108 into quarter-wave plate 118 and further projected by projection lens 120 toward a location on a target. The radiation reflected by the target returns to apparatus 100 as light beam 142, which traverses projection lens 120 and quarter-wave plate 118. Double passage through quarter-wave plate 118 (first as light beam 140 and then as light beam 142) rotates the polarization by 90°, and light beam 142 passes through polarizing beam splitter 108 and is split by beam splitter 112 into two light beams 144 and 146 of equal power. After light beam 146 has been reflected by reflector 110, light beams 144 and 146 impinge on respective photodiodes 126 and 128.

[0046] The LO beam 138 is split by the beam splitter 112 into two beams 148 and 150 of equal power, which impinge on respective photodiodes 126 and 128 after the beam 150 has been reflected by the reflector 110 .

[0047] The optical beam pairs 144 and 148 and the optical beam pairs 146 and 150 optically interfere with each other on the corresponding photodiodes 126 and 128, where each photodiode emits an analog electrical beat signal in response to the optical interference signal. The electrical signals are converted to digital signals by the A / D converter 133 and processed by the controller 130 to find the range of the position on the target illuminated by the optical beam 140.

[0048] Electronic component 129 is below Figures 3 to 8 It is used in a similar manner in , both for driving the VCSELs in a VCSEL array and for receiving and processing analog signals from a single photodiode or from a balanced pair.

[0049] Figure 3is a schematic cross-sectional view of a range sensing device 200 according to another embodiment of the present invention.

[0050] Device 200 includes optoelectronic assembly 202 and electronic assembly 129. Optoelectronic assembly 202 includes optical assembly 203, which includes a planar optical waveguide 212 having an input coupler 214 and an output coupler 216 disposed on a lower surface 217 of the waveguide, for example as a surface relief grating or a volume phase grating; and a collimating lens 218. Alternatively, input coupler 214 and output coupler 216 may be disposed on an upper surface 219 of waveguide 212, respectively. Further alternatively, input coupler 214 and output coupler 216 may comprise other types of couplers, such as refractive index modulation couplers.

[0051] The optoelectronic component 202 further includes a silicon substrate 220 on which are disposed: a transmitter 221 including a two-dimensional VCSEL array 222 and a switch circuit 223; and a photodiode 224. The VCSEL array 222 may also include a similar Figure 1 A lenslet array (not shown) is provided in the lenslet array 26 for modifying the angular extent of the light beams emitted by the individual VCSELs of the array.

[0052] Including the above reference Figure 2 The depicted subassembly's electronic assembly 129 is coupled to a switching circuit 223 and to a photodiode 224 .

[0053] To find the range of a location on a target (not shown in this figure), the electronic assembly 129 drives the VCSELs of the VCSEL array 222 to sequentially emit beams of coherent optical radiation. As an example, VCSEL 222a centered around collimating lens 218 emits a beam 226, which is collimated by the collimating lens and projected onto input coupler 214. Input coupler 214 splits the beam into: a beam 228, which exits the waveguide 212 as a continuation of beam 226; and a local oscillator beam 230, which propagates within the waveguide as a guided beam reflected by waveguide surfaces 217 and 219 through total internal reflection.

[0054] Light beam 228 impinges on a target at a given location. Radiation reflected from the target returns to apparatus 200 as light beam 232, which is projected through waveguide 212 and output coupler 216, impinging on photodiode 224. Local oscillator light beam 230, propagating within waveguide 212, is coupled out of the waveguide via output coupler 216 as light beam 234, which propagates collinearly with light beam 232 and overlaps with photodiode 224. Light beams 232 and 234 optically interfere with each other, with the photodiode emitting an analog electrical beat signal in response to the optical interference signal. The electrical signal is coupled via amplifier 127 to electronics assembly 129, where controller 130 determines the range of locations on the target impinged by light beam 228.

[0055] To find the range of another location on the target, a different VCSEL (e.g., VCSEL 222b) is driven by electronics 129 to emit a beam 236. Similar to beam 226 above, beam 236 is collimated and split into beam 238, which is projected onto the target (to a different location than beam 228), and local oscillator beam 240, which propagates in waveguide 212. Radiation from beam 238 is reflected by the target and returned to device 200 as beam 242. Similar to the beam originating from VCSEL 222a above, local oscillator beam 240 is coupled out of waveguide 212 as beam 244. Beam 242 is transmitted through the waveguide, and both beams 242 and 244 impinge on photodiode 224 and optically interfere. As detailed above, the optical interference signal is used to find the range of the location on the target illuminated by beam 238.

[0056] Figure 4 is a schematic cross-sectional view of a range sensing device 300 according to another embodiment of the present invention. The device 300 uses a simpler beam splitting arrangement than the previous embodiments, but is in a bistatic configuration which may be susceptible to inaccuracies at short distances due to parallax.

[0057] Device 300 includes optoelectronic assembly 302 and electronics assembly 129. Optoelectronic assembly 302 includes an optical assembly 304, which includes a monolithic prism assembly 312, a collimating lens 314, a projection lens 316, and a collection lens 318. Prism assembly 312 includes prisms 320, 322, and 324, with neutral (non-polarizing) beam splitters 326 and 328 disposed between prisms 320 and 322, and between prisms 322 and 324, respectively. Both beam splitters 326 and 328 typically transmit a majority of the impinging radiation and reflect a smaller portion.

[0058] The optoelectronic component 302 further includes a silicon substrate 330 , on which are disposed: a transmitter 331 including a two-dimensional VCSEL array 332 and a switching circuit 333 ; and a photodiode 334 .

[0059] To find the range of a location on a target (not shown in this figure), electronics assembly 129 drives the VCSELs to sequentially emit beams of coherent optical radiation. As an example, VCSEL 332a emits beam 336, which is collimated by collimating lens 314 and projected toward prism assembly 312, where it is split by beam splitter 326 into beam 338 and into local oscillator beam 340. Beam 338 is projected by projection lens 316 onto the location on the target. A portion of the radiation reflected by the location on the target strikes collection lens 318 as beam 342 and is projected by the lens toward prism assembly 312, where it passes through beam splitter 328 and onto photodiode 334. Local oscillator beam 340 is reflected by beam splitter 328 onto photodiode 334, where it optically interferes with beam 342. The photodiode 334 converts the optical interference signal into an analog electrical beat signal, which is coupled to the electrical component 129 via the amplifier 127 for use in finding the range of the position on the target.

[0060] Figure 5 is a schematic cross-sectional view of a range sensing device 400 according to another embodiment of the present invention.

[0061] Device 400 includes optoelectronic assembly 402 and electronic assembly 129. Optoelectronic assembly 402 includes an optical assembly 403 including a collimating lens 404 and a partial retroreflector 406.

[0062] Optoelectronic assembly 402 also includes a silicon substrate 408 on which is disposed a transmitter-detector assembly 409. Transmitter-detector assembly 409 includes a sparse array of VCSELs 410, photodiodes 412, and switching circuitry 411. Transmitter-detector assembly 409 is shown in a front view in inset 414. The VCSELs of VCSEL array 410 are interleaved with photodiodes 412 on substrate 408 such that a portion of photodiode 412 is visible around and behind each VCSEL.

[0063] To find the range of a location on a target (not shown in this figure), electronic assembly 129 drives the VCSELs to sequentially emit beams of coherent optical radiation. As an example, VCSEL 410a emits beam 416, which is collimated by collimating lens 404 and projected onto partial retroreflector 406. Partial retroreflector 406 splits beam 416 into: beam 418, which is projected toward the location on the target; and local oscillator beam 420, which is returned to photodiode 412 through collimating lens 404.

[0064] A portion of the beam 418 reflected by the target returns to the device 400 as beam 422. This beam is projected by the collimating lens 404 onto the photodiode 412, where it optically interferes with the local oscillator beam 420, thereby generating an optical interference signal. This signal is converted by the photodiode 412 into an analog electrical beat signal, which is coupled to the electronics assembly 129 via the amplifier 127. At this electronics assembly, the controller 130 analyzes the signal to find the range of the position on the target.

[0065] In an alternative embodiment, rather than utilizing beam 420 reflected by partial retroreflector 406 as the LO beam, backscattered light from VCSEL 410 a may be used as the LO beam for interfering with beam 422 on photodiode 412 .

[0066] To find the range of another location on the target, the electronics assembly 129 drives another VCSEL (e.g., VCSEL 410b) to emit a light beam 424. Similar to the light beam 416 emitted by VCSEL 410a, light beam 424 is split at the partial retroreflector 406 into a light beam 426 that is projected toward another location on the target and a local oscillator light beam 428. A portion of light beam 426 is reflected back to the device 400 as light beam 430. Light beams 428 and 430 impinge on the photodiode 412 and optically interfere, thereby generating an optical interference signal. The optical interference signal is converted into an analog electrical beat signal by the photodiode 412 and coupled to the electronics assembly 129 for use in finding the range of another location on the target.

[0067] In alternative embodiments, the single photodiode 412 may be replaced by an array of photodiodes interleaved with the VCSELs of the VCSEL array 410 or even below the VCSEL array 410 .

[0068] Figure 6 is a schematic cross-sectional view of a range sensing device 500 according to another embodiment of the present invention.

[0069] Device 500 includes an optoelectronic assembly 502 and an electronics assembly 129. Optoelectronic assembly 502 includes an optical assembly 503, which includes a collimating lens 504, a monolithic prism assembly 506, and a quarter-wave plate 508. Prism assembly 506 includes prisms 510, 512, and 514, with half-polarizing beam splitters 516 and 518 disposed between prisms 510 and 514 and between prisms 512 and 514, respectively. Beam splitters 516 and 518 have transmittances and reflectances as shown in Table 1 below, although other ratios of transmittance and reflectance may alternatively be used, particularly for TM polarization.

[0070] Table 1 - Reflectivity R and Transmittance T of Beam Splitters 516 and 518

[0071] polarization R(％) T(％) TE 0 100 TM 50 50

[0072] The optoelectronic component 502 further includes a silicon substrate 520 on which a transmitter 521 is disposed. The transmitter includes a two-dimensional VCSEL array 522 and a switching circuit 523. The optoelectronic component 502 further includes two photodiodes 524 and 526.

[0073] To find the range of a location on a target (not shown in this figure), the electronics assembly 129 drives the VCSELs of the array 522 to sequentially emit beams of coherent optical radiation. As an example, VCSEL 522a emits a divergent beam 528 of optical radiation into an angle θ, which is collimated by the collimating lens 504. For clarity, the emitted beams are represented by discrete beams 530 and 532, where beam 530 represents those beams impinging on beam splitter 516 and beam 532 represents those impinging on beam splitter 518. The VCSELs of the VCSEL array 522 are oriented so that 90% of their radiation intensity is in TE polarization and 10% is in TM polarization.

[0074] Light beam 530 impinges on beam splitter 516, which transmits 100% of its TE component into light beam 534 and reflects 50% of its TM component into local oscillator light beam 536. Light beam 534 is transmitted through quarter-wave plate 508 and projected toward a location on the target. A portion of light beam 534 reflected by the target returns to apparatus 500 as light beam 538 and is transmitted through quarter-wave plate 508. Due to the double passage through quarter-wave plate 508, the polarization of light beam 538 is rotated to the TM direction, and 50% of light beam 538 is reflected from beam splitter 516 as light beam 540 to photodiode 524. Local oscillator light beam 536, in which the intensity is reduced by 50%, is transmitted by beam splitter 518 to photodiode 526.

[0075] Light beam 532 follows a path symmetrical to the path of light beam 530, resulting in light beam 542 being projected onto the target at the same location as light beam 534, returning as light beam 544, and impinging on photodiode 526 as light beam 546 (with a 50% reduction in intensity). Local oscillator light beam 548 impinges on photodiode 524.

[0076] Light beams 540 and 548 optically interfere with each other on photodiode 524, which converts the optical interference signal into an analog electrical beat signal. Similarly, light beams 546 and 536 optically interfere with each other on photodiode 526, thereby generating an analog electrical beat signal. The two electrical signals from photodiodes 524 and 526 are coupled to electronics 129 via amplifier 127, where controller 130 analyzes the two electrical signals to find the range of the position on the target.

[0077] Figure 7 is a schematic cross-sectional view of a range sensing device 600 according to another embodiment of the present invention. This embodiment is based on a device similar to device 500 ( Figure 6 ) but with reduced light losses.

[0078] Device 600 includes optoelectronic assembly 602 and electronic assembly 129. Optoelectronic assembly 602 includes optical assembly 603, which includes collimating lens 604, a monolithic prism assembly 606 in the form of an X-cube, and a quarter-wave plate 608. Prism assembly 606 includes prisms 610, 612, 614, and 616, with half-polarizing beam splitters 618, 620, 622, and 624 disposed between adjacent prisms. Beam splitters 618, 620, 622, and 624 have the same transmittance and reflectance as beam splitters 516 and 518 ( Figure 6 , Table 1).

[0079] The optoelectronic component 602 further includes a silicon substrate 626 on which a transmitter 627 is disposed. The transmitter includes a two-dimensional VCSEL array 628 and a switching circuit 629. The optoelectronic component 602 further includes two photodiodes 630 and 632.

[0080] To find the range of a location on a target (not shown in this figure), the electronics assembly 129 drives the VCSELs to sequentially emit beams of coherent optical radiation. As an example, VCSEL 628a emits a diverging beam 634 of optical radiation into an angle θ, which is collimated by collimating lens 604. For clarity, the emitted beam 634 is represented by two discrete beams 636 and 638. The VCSELs of VCSEL array 628 are oriented so that 90% of their radiation intensity is in TE polarization and 10% is in TM polarization.

[0081] Beam 636 impinges on beam splitter 624, which transmits 100% of its TE component and 50% of its TM component into beam 640 and reflects 50% of its TM component into local oscillator beam 642. Beam 640 impinges on beam splitter 618, which transmits 100% of its TE component into beam 643 and reflects the remaining 50% of the TM component into local oscillator beam 644. Beam 642 is transmitted through quarter-wave plate 608 and projected toward a location on a target. A portion of beam 643 reflected by the target is captured by apparatus 600 as beam 646, which is TM polarized after passing through quarter-wave plate 608. This beam impinges on two beam splitters 618 and 624, which each reflect 50% of the impinging beam as beams 648 and 650 onto photodiodes 632 and 630, respectively, with beam 648 experiencing additional reflection losses at beam splitter 620. The local oscillator beams 642 and 644 are incident on the photodiodes 632 and 630 , respectively, wherein the local oscillator beam 642 has additional reflection loss at the beam splitter 622 .

[0082] Light beam 638 follows a path that is symmetrical to the path of light beam 636. For clarity, only the light beams that impinge on photodiodes 630 and 632 have been labeled: from a position on the target, light beam 652 impinges on photodiode 630, and light beam 654 impinges on photodiode 632. Local oscillator beam 656 impinges on photodiode 630, and local oscillator beam 658 impinges on photodiode 632. At photodiode 630, light beams 650 and 652 reflected from the target and local oscillator beams 644 and 656 optically interfere. Similarly, at photodiode 632, light beams 648 and 654 reflected from the target and local oscillator beams 642 and 658 optically interfere. (Although Figure 7 The paths of two discrete light beams 636 and 638 are shown, but in reality, the continuum of emitted light beams in the diverging light beam 634 produces a continuum of both light beams reflected by the target and the local oscillator beam.) Photodiodes 630 and 632 convert the detected optical interference signal into an analog electrical beat signal, which is coupled to the electronic assembly 129 via amplifier 127 for use in finding the range of the position on the target.

[0083] Figure 8 is a schematic cross-sectional view of a range sensing device 700 according to another embodiment of the present invention.

[0084] Device 700 includes optoelectronic assembly 702 and electronics assembly 129. Optoelectronic assembly 702 includes optical assembly 703, which includes collimating lens 704, monolithic prism assembly 706, and quarter-wave plate 708. Prism assembly 706 includes prisms 710, 712, 714, 716, 718, and 720. Beam splitters and reflectors are disposed between the prisms, as detailed in Table 2 below.

[0085] Table 2 - Beam splitters and reflectors

[0086]

[0087] The optoelectronic component 702 further includes a silicon substrate 730 on which is disposed a transmitter 731 comprising a two-dimensional VCSEL array 732 and a switching circuit 733. The optoelectronic component 702 further includes two photodiodes 734 and 736 in a balanced configuration.

[0088] To find the range of a location on a target (not shown in this figure), the electronics assembly 129 drives the VCSELs of the array 732 to sequentially emit beams of coherent optical radiation. As an example, the VCSEL 732a emits a diverging beam 738 of optical radiation into an angle θ, which is collimated by the collimating lens 704. For clarity, the emitted beam 738 is represented by two discrete beams 740 and 742.

[0089] Light beam 740 impinges on polarizing beam splitter 722, which splits the beam into a transmitted beam 744 with TE polarization and a reflected local oscillator beam 746 with TM polarization. Light beam 744 is reflected by reflector 724 and polarizing beam splitter 726, and is projected onto a location on a target through quarter-wave plate 708. Local oscillator beam 746 is split by neutral beam splitter 728 into local oscillator beams 748 and 750, which impinge on photodiodes 734 and 736, respectively. A portion of light beam 744 reflected by the target is captured by apparatus 700 as light beam 752. After passing through quarter-wave plate 708, light beam 752 is TM polarized, passes through polarizing beam splitter 726, and is split by neutral beam splitter 728 into light beams 754 and 756. Light beams 754 and 756 impinge on photodiodes 734 and 736, respectively. Light beam 742 travels along a path symmetrical to that of light beam 740.

[0090] like Figure 7, only the beams impinging on photodiodes 734 and 736 have been labeled for clarity: from a position on the target, beam 758 impinges on photodiode 734, and beam 760 impinges on photodiode 736. Local oscillator beam 762 impinges on photodiode 734, and local oscillator beam 764 impinges on photodiode 736. At photodiode 734, beams 754 and 758 reflected from the target and local oscillator beams 748 and 762 optically interfere. Similarly, at photodiode 736, beams 756 and 760 reflected from the target and local oscillator beams 750 and 764 optically interfere. (As shown in FIG. Figure 7 In, although Figure 8 Only two discrete beams 740 and 742 are shown in FIG, but the continuum of emitted beams in the diverging beam 738 actually produces a continuum of beams reflected by both the target and the local oscillator beam.) Photodiodes 734 and 736 convert the detected optical interference signal into an analog electrical beat signal. Figure 2 The photodiodes in this example are configured as a balanced pair, stacked anode to cathode, and output a difference signal to a transimpedance amplifier 127, the output of which is coupled to electronics 129 for use in finding the range of the position on the target.

[0091] It should be understood that the embodiments described above are cited by way of example, and the present invention is not limited to what has been particularly shown and described above. On the contrary, the scope of the present invention includes the various features described above, as well as combinations and sub-combinations thereof, variations and modifications not disclosed in the prior art, which may occur to those skilled in the art after reading the above description.

Claims

1. A range sensing device, comprising: a transmitter comprising an array of emitters configured to emit respective beams of coherent optical radiation and switching circuitry coupled to the emitters; Optical detectors; an optical assembly configured to: split each beam of the coherent optical radiation into a transmitted beam and a local oscillator beam; and project the local oscillator beam toward the optical detector; projecting the transmitted light beam toward a corresponding position on a target; and directing the optical radiation reflected from the corresponding position onto the optical detector so as to optically interfere with the local oscillator beam; and a controller coupled to: apply amplitude-chirped electrical drive pulses to the transmitters while controlling the switching circuitry to time-multiplex the electrical drive pulses among the transmitters; and receiving and processing an electrical beat signal output by the optical detector in response to interference between the reflected optical radiation and the local oscillator light beam.

2. The apparatus of claim 1, wherein the transmitter array and the switching circuit are provided on a single integrated circuit.

3. The apparatus of claim 1 , comprising an array of lenslets disposed over the emitter array, wherein each lenslet is aligned with a respective emitter.

4. The apparatus of claim 1, wherein the emitter comprises a vertical cavity surface emitting laser (VCSEL).

5. The apparatus of claim 1 , comprising a digital-to-analog converter configured to generate the electrical drive pulses in response to a digital input from the controller, and wherein the controller is configured to vary the digital input during each of the drive pulses so as to linearize the frequency chirp of the beam of optical radiation.

6. The apparatus of claim 1, wherein the optical detector comprises a pair of balanced photodiodes.

7. The apparatus of claim 1 , wherein the optical detector comprises a single detector, and the optical assembly is configured to direct the optical radiation reflected from the corresponding light beams of all emitters in the array onto the single detector.

8. The device of claim 1, wherein the optical detector comprises a detector array. 9 . The apparatus of claim 1 , wherein the controller is configured to analyze the beat signal to find a range of the corresponding position on the target.

10. The device according to any one of claims 1 to 9, wherein the optical assembly comprises: a polarizing beam splitter cube configured to split each beam of the coherent optical radiation into the transmitted beam and the local oscillator beam; and one or more polarization rotators.

11. The apparatus of claim 10, wherein the polarizing beam splitter cube comprises an X-cube.

12. The device of any one of claims 1 to 9, wherein the optical assembly comprises a monolithic assembly of prisms.

13. The apparatus of claim 12 , wherein the optical detector comprises a first photodiode and a second photodiode, the first photodiode and the second photodiode being positioned on different first and second sides of the monolithic assembly of the prism and being configured to receive respective first and second portions of the reflected optical radiation and respective first and second portions of the local oscillator light beam.

14. The apparatus of any one of claims 1 to 9, wherein the optical component comprises an optical waveguide comprising an input coupler and an output coupler, wherein the transmitted light beam and the reflected radiation pass through the waveguide to and from the target, and the local oscillator light beam is guided within the waveguide between the input coupler and the output coupler from the transmitter to the optical detector.

15. The device of any one of claims 1 to 9, wherein the emitters and the optical detectors are interleaved on a common substrate.

16. The apparatus of any one of claims 1 to 9, wherein the emitter is disposed above the optical detector.

17. A method for range sensing, the method comprising: providing a transmitter comprising an array of emitters configured to emit respective beams of coherent optical radiation; applying amplitude-chirped electrical drive pulses to the transmitters while temporally multiplexing the electrical drive pulses between the transmitters such that the transmitters sequentially transmit the respective light beams; splitting each beam of the coherent optical radiation into a transmitted beam and a local oscillator beam; Projecting the local oscillator light beam toward an optical detector and simultaneously projecting the transmitted light beam toward a corresponding position on a target; directing the optical radiation reflected from the corresponding position onto the optical detector so as to optically interfere with the local oscillator beam; as well as An electrical beat signal output by the optical detector in response to interference between the reflected optical radiation and the local oscillator light beam is received and processed.

18. The method of claim 17, wherein applying the amplitude-chirped electrical drive pulse to the transmitter comprises: The shape of the drive pulse is varied so as to linearize the frequency chirp of the beam of optical radiation.

19. The method of claim 17, wherein projecting the local oscillator light beam toward an optical detector comprises: The local oscillator light beam is projected toward a single detector, and wherein directing the optical radiation reflected from the corresponding location onto the optical detector includes directing the reflected optical radiation onto the single optical detector.

20. The method according to any one of claims 17 to 19, wherein processing the electrical beat signal comprises: The beat signal is analyzed to find the range of the corresponding position on the target.

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