Patent ID: 12210098

DETAILED DESCRIPTION

Example implementations of the present disclosure are based on a different type of LiDAR that uses frequency modulation (FM) and coherent detection to overcome the shortcomings of traditional LiDAR systems and the limitations of prior FM LiDAR systems. Historically, FM LiDAR systems suffer from significant losses in the beam's return path; thus, such systems, which are often quite bulky, require a higher beam output power to measure distances comparable to TOF LiDAR systems. Alas, the range is limited by the operating distance for eye-safe output powers.

The present disclosure proposes a method to measure the range and velocity simultaneously, a process that requires coherent detection and has the added benefit of immunity to crosstalk from other LiDAR systems. Also, this method minimizes optical losses in the beam's return path, thereby increasing the system's measurement range. Additionally, by using nondegenerate optical sources, the proposed design can leverage mature wavelength division multiplexing (WDM) techniques often used in integrated silicon photonics, a desired platform due to its compactness and relative stability in varying environmental conditions. Finally, the methods described in the present disclosure allow for fewer polarization discrimination components, which are currently more elusive in integrated silicon photonics platforms; this can reduce the number of input and output facets that are additional sources of optical loss and back reflection, two of the main problems that limit the performance of an FM LiDAR system.

Moreover, the present disclosure allows for the detection and estimation of different material properties depending on its reflectivity/absorption at different wavelengths.

As introduced above, example implementations of the present disclosure are directed to a LiDAR system that uses nondegenerate optical sources and is able to simultaneously measure range and velocity. The non-degeneracy of the sources enables the use of components that mitigate optical loss, thereby making this design a compact, mass-manufacturable, and power efficient LiDAR system with greater and more accurate awareness of its environment. Example implementations have application in a number of different contexts, including in sensing contexts such as those in transportation, manufacturing, metrology, medical, security, and the like. For example, in the automotive industry, such a device can assist with spatial awareness for automated driver assist systems, or self-driving vehicles. Additionally, it can help with velocity calibration of a moving vehicle without the need for a separate inertial movement unit (IMU).

FM LiDAR systems, such as that described in this disclosure, keep the power of the optical beam constant while modulating the frequency of the optical beam.FIG.1shows an example of a sawtooth modulation waveform wherein the optical source's frequency (inversely related to the wavelength) is periodically swept between a low value and a high value about a central frequency, fc. Such frequency sweeps are called “chirps.” The span between the lower and higher frequencies is our sweep bandwidth, BS; this is sometimes referred to as the “frequency excursion” or more concisely “excursion.” The sweep time, TS, is also the period of the sawtooth modulation waveform. The delay between the transmitted waveform (solid line) and the return waveform (dashed line), also called the “echo,” is Δt. The phase difference between these two waveforms yields a beat frequency, Δf.

One can calculate the range, R, to a target, or velocity, V, of said target with the following equations:

R=Δ⁢fRange⁢cTS2⁢BS(1)V=Δ⁢fDoppler⁢λc2(2)
where λc=c/fc, and the total beat frequency, Δf, is the sum of the range and Doppler beat frequencies, corresponding to stationary and moving objects, respectively.

The primary difficulty with the aforementioned sawtooth modulation scheme is differentiating between the range and Doppler beat frequencies. In order to resolve this challenge, one can apply a triangle modulation waveform to the input optical source, as shown inFIG.2. This scheme has up-sweep and down-sweep regions. When detecting a moving object, the Doppler effect shifts the center frequency from fcto fd=fc+ΔfDoppler. The Doppler shift enables us to calculate the up-sweep and down-sweep beat frequencies using:
Δfup=ΔfRange−ΔfDoppler(3)
Δfdn=ΔfRange+ΔfDoppler(4)
By solving this system of equations for ΔfRangeand ΔfDopplerand substituting those values into equations (1) and (2) respectively, one sees that the range and velocity, shown in equations (5) and (6), are proportional to the average and difference, respectively, of the up-sweep and down-sweep beat frequencies.

R=cTS4⁢BS⁢(Δ⁢fdn+Δ⁢fup)(5)V=λc4⁢(Δ⁢fdn-Δ⁢fup)(6)
This triangle modulation scheme makes the assumption that the velocity is constant over twice the period of the waveform. In a dynamic environment, this condition may not hold yielding inaccurate measurements.

By employing a counter-chirp mechanism, shown inFIG.3, one can achieve more accurate measurements for range and velocity since the up-sweep and down-sweep beat frequencies are measured simultaneously. This modulation scheme requires two transmitted beams (solid line and long-dashed line) pointed at the same target; each beam yields its respective echo (short-dashed line and dotted line). As a result, the system simultaneously measures both beat notes (Δfup, and Δfdn) from which it can calculate the range and velocity using equations (5) and (6).

Another example modulation scheme according to the present disclosure is to have a counter chirp scheme but with a different center frequency as shown inFIG.4. Having such scheme allows us to measure the reflectivity/absorption of different material surfaces.

Shown in the remainder of this disclosure are methods to construct a system capable of achieving long range 4D measurements using the counter-chirp modulation or similar schemes.

FIG.5shows the first such implementation wherein two nondegenerate optical beams (Source A and Source B) are launched into the optical circuit. Suitable examples of optical sources include laser sources, light-emitting diodes (LEDs) and the like. One can mount these optical sources on the same sub-mount or on separate one, and then tune each optical beam with a triangle wave. In order to achieve the counter-chirp modulation, shown inFIG.3, one should sweep the two optical beams exactly out of phase, such that an upsweep for Source A is co-temporal with the down-sweep of Source B and vice versa. In some examples, more than two optical beams are used. The two or more optical beams may be from the same or separate and distinct optical sources.

A subsequent tap splits each beam into a high-power path and a low-power path. For each optical beam, the latter becomes the optical local oscillator (LO) signal that generates the beat frequency when it is mixed with the return signal on the respective splitter/combiner (labeled coupler A and coupler B inFIG.5).

An optical frequency multiplexer (labeled as WDM) combines the high-power paths from both optical beams into a single spatial mode. The multiplexer can be of many types and designs including, but not limited to, arrayed waveguides, dense or coarse wavelength division multiplexers, optical add-drop multiplexers, and fiber Bragg gratings. Similarly, the output spatial mode can be in free space, various types of optical fiber, or more exotic waveguides. Any suitable optical amplifier, such as an xDFA, SOA, or booster amplifies the high-power beam thereby increasing the signal-to-noise (SNR) ratio at the output. Note that xDFA refers to any doped fiber amplifiers for use with the appropriate wavelength optical source, e.g., an erbium-doped fiber amplifier (EDFA) is necessary for a 1550-nm source. The amplifier output routes to one or more optical devices such as a polarizing beamsplitter (PBS), followed by a polarization wave plate such as a quarter-wave plate (QWP), and lensing optics which launch the output of Port2into free space towards the target. Other examples of optical devices suitable to route the amplifier output to the lensing optics include an optical circulator, optical splitter/combiner or the like.

The same lensing optics and optical device(s) (e.g., QWP) collect the emitted light that was incident upon a target into the return path. Since the PBS linearizes the polarization of the outgoing light, the collected light in the return path is the opposite linear polarization, and thus output at Port3. A frequency demultiplexer, of any suitable mechanism or design, splits the light into two separate spatial modes. Each demultiplexed signal mixes with its respective LO signal at the coupler to generate the detected beat frequency. Note thatFIG.5shows balanced detection in an effort to not discard any optical power at the output of the mixer, although single channel detection can also work.

More specifically, then,FIG.5illustrates a LiDAR system500according to example implementations of the present disclosure. As shown, the LiDAR system includes at least a first optical source502aand a second optical source502b(e.g., laser sources, LEDs) configured to emit respectively a first optical beam504aand a second optical beam504bthat are nondegenerate and are chirped antiphase. The LiDAR system includes a first tap506aand a second tap506bconfigured to split respectively the first optical beam and the second optical beam into a first high-power path optical beam508aand a first low-power path optical beam510a, and a second high-power path optical beam508band a second low-power path optical beam510b.

The LiDAR system500includes an optical frequency multiplexer512configured to combine the first high-power path optical beam508aand the second high-power path optical beam508binto a single spatial mode optical beam514. The LiDAR system includes lensing optics516configured to launch the single spatial mode optical beam towards a target518, and collect light incident upon the target into a return path, the light being collected into a return optical beam520. The LiDAR system includes an optical frequency demultiplexer522configured to split the return optical beam into a first spatial mode optical beam524aand a second spatial mode optical beam524b.

As also shown, the LiDAR system500includes a first mixer526aconfigured to mix the first spatial mode optical beam524aand the first low-power path optical beam510ato produce an optical beam528ahaving a first beat frequency, and a second mixer526bconfigured to mix the second spatial mode optical beam524band the second low-power path optical beam510bto produce an optical beam528bhaving a second beat frequency. The LiDAR system includes a first optical detector530aand a second optical detector530bconfigured to detect respectively the optical beam528ahaving the first beat frequency and the optical beam528bhaving the second beat frequency. In some examples, the first optical detector and the second optical detector are each a balanced optical detector. As explained above, the range and velocity of the target are determinable from the first beat frequency and the second beat frequency.

In some examples, The LiDAR system500further include an optical amplifier532between the optical frequency multiplexer512and lensing optics516, the optical amplifier configured to amplify the single spatial mode optical beam514. And in some examples, the LiDAR system further includes at least one optical device configured to route the single spatial mode optical beam514from the optical frequency multiplexer512to the lensing optics516, and route the return optical beam520from the lensing optics to the optical frequency demultiplexer522. As shown, the optical device(s) include a polarizing beamsplitter534and polarization wave plate536. Other examples of suitable optical device(s) include an optical circulator or an optical splitter/combiner.

As mentioned earlier, the amplifier increases the SNR at the output. It also increases the potential range of the system. However, the amplifier also can create a strong back-reflection signal from the end of the fiber-air or waveguide-air facet. Such a strong signal would increase the noise floor thereby reducing the SNR. Fortunately, the back-reflected signal is co-polarized with the outgoing signal, so the PBS essentially filters out the undesired signal by passing it back through Port1. Moreover, if any of the undesired signal leaks through the PBS into Port3, the detected beat note from that signal should be reduced due to a mismatch with the mixing LO polarization.

Another variation of this design is to replace the PBS with an optical circulator or an optical splitter/combiner. In that concept there is no need for a QWP.

The prior design, albeit sufficient, can suffer from back-reflection related problems arising from any intermediate component between the LO tap and the LO mixer, through either the low-power or high-power paths. Hence, an alternate implementation, shown inFIG.6, reduces the noise and removes additional beat frequencies that are generated by the back-reflections from such components. It achieves this by moving the first WDM before the taps, and the other WDMs after the LO mixer. This design also benefits by having fewer components since only one tap and one mixer are necessary. The reduction in the number of components minimizes the transmission loss through the system, lowers the cost of the design, and simplifies the production steps.

FIG.6therefore illustrates a LiDAR system600according to other example implementations. Similar to before, the LiDAR system includes first and second optical sources502a,502bconfigured to emit respectively a first optical beam504aand a second optical beam504bthat are nondegenerate and are chirped antiphase. The LiDAR system includes an optical frequency multiplexer606configured to combine the first optical beam and the second optical beam into a single spatial mode optical beam608. The LiDAR system includes at least one tap610configured to split the single spatial mode optical beam into a high-power path optical beam612and a low-power path optical beam614.

As also shown, the LiDAR system600includes at least one optical arrangement616. This optical arrangement includes lensing optics618configured to launch the high-power path optical beam612towards a target620, and collect light incident upon the target into a return path, the light being collected into a return optical beam622. The optical arrangement includes a mixer624configured to mix the return optical beam622and the low-power path optical beam614, and thereby produce a mixed optical beam626.

The optical arrangement616also includes an optical frequency demultiplexer628configured to split the mixed optical beam into an optical beam630having a first beat frequency, and an optical beam632having a second beat frequency. And the optical arrangement includes a first optical detector634aand a second optical detector634bconfigured to detect respectively the optical beam630having the first beat frequency and the optical beam632having the second beat frequency. Similar to before, in some examples, the first optical detector and the second optical detector are each a balanced optical detector. Again, the range and velocity of the target are determinable from the first beat frequency and the second beat frequency.

In some examples, the LiDAR system600further includes an optical amplifier634between the at least one tap610and lensing optics618, the optical amplifier configured to amplify the high-power path optical beam612. And in some examples, the optical arrangement616further includes at least one optical device configured to route the high-power path optical beam612from the at least one tap610to the lensing optics618, and route the return optical beam622from the lensing optics to the mixer624. As shown, the optical device(s) include a polarizing beamsplitter636and polarization wave plate638, but other optical device(s) may also be used such as an optical circulator or an optical splitter/combiner.

Shown inFIG.7is the implementation of a bi-static design for the nondegenerate 4D FM LiDAR system. This implementation decouples the outgoing beam path from the returning collection path. An additional QWP and lens system replaces the PBS from the previous design. This design has the benefit that any potential crosstalk that previously leaked through the PBS is entirely removed. Note that the bi-static design can be employed in any of the prior or following designs at the cost of additional components.

In particular,FIG.7illustrates a LiDAR system700similar to the LiDAR system600ofFIG.6, although with optical arrangement712instead of optical arrangement616. The optical arrangements are similar but in the optical arrangement712ofFIG.7, the lensing optics618include distinct first lensing optics618aand second lensing optics618bconfigured to respectively launch the high-power path optical beam612towards the target, and collect the light incident upon the target into the return path, the light being collected into the return optical beam622. In some examples, the optical arrangement712further includes a first optical device (e.g., first polarization wave plate740a) configured to route the high-power path optical beam612from the at least one tap610to the first lensing optics618a, and a second optical device (e.g., second polarization wave plate740b) configured to route the return optical beam622from the second lensing optics618bto the mixer624.

An additional method to improve the versatility and usefulness of such a 4D LiDAR system is to illuminate one's surroundings using multiple beams. One way to achieve this is by building many separate systems and synthesizing their data streams. Alas, such a design is cost and resource inefficient from a hardware perspective since every system requires its own optical sources and amplifiers.FIGS.8and9both show schematics of implementations that use two outgoing beams in an effort to pool resources. The addition of a couple splitters generates the two separate beam paths and LiDAR collection systems. Employing similar strategies using splitters with more outputs can increase the number of beams.FIG.8shows a design that optimizes for number of components including one fewer amplifier. The design shown inFIG.9optimizes for back-reflections that may reduce the SNR for each individual beam path. Note that for short ranges, one can remove the amplifiers from these designs entirely. In this manner, one can combine a short-range system with a long-range system for various applications including wider environmental field-of-view, velocity calibration, or more precise local resolution for positional awareness. Essentially, this implementation has the benefits of having multiple 4D LiDAR systems with various parameters, while reducing the size and cost of such a system by combining resources wherever possible.

FIGS.8and9in particular illustrate LiDAR systems800,900that similar to the other implementations, includes first and second optical sources502a,502bconfigured to emit respectively a first optical beam504aand a second optical beam504bthat are nondegenerate and are chirped antiphase. The LiDAR system includes an optical frequency multiplexer606configured to combine the first optical beam and the second optical beam into a single spatial mode optical beam608. The LiDAR system includes at least one tap610configured to split the single spatial mode optical beam into a high-power path optical beam612and a low-power path optical beam614. And the LiDAR system includes at least one optical arrangement616.

The LiDAR system800inFIG.8further includes optical splitters834between the at least one tap610and the at least one optical arrangement616. The optical splitters are configured to split the high-power path optical beam612and the low-power path optical beam614into multiple respective high-power path and low-power path optical beams612,614. The optical arrangement616includes multiple optical arrangements for multiple targets620. Each of the multiple optical arrangements is configured to receive respective high-power path and low-power path optical beams, and detect respective optical beams630,632having respective first and second beat frequencies from which the range and velocity of a respective target of the multiple targets is determinable.

In some examples, the LiDAR system800further includes an optical amplifier634between the at least one tap610and one of the optical splitters, shown as optical splitter834, the optical amplifier configured to amplify the high-power path optical beam612. And in some examples, similar to earlier described example implementations, each of the multiple optical arrangements616further includes at least one optical device (e.g., polarizing beamsplitter636and polarization wave plate638) configured to route the high-power path optical beam612from the at least one tap610to the lensing optics618, and route the return optical beam622from the lensing optics to the mixer624.

The LiDAR system900inFIG.9further includes an optical splitter934between optical frequency multiplexer606and the at least one tap610. The optical splitter is configured to split the single spatial mode optical beam608into multiple single spatial mode optical beams608. Also, in this example implementation, the at least one tap includes multiple taps configured to split a respective single spatial mode optical beam608of the multiple single spatial mode optical beams into respective high-power path and low-power path optical beams612,614.

Similar to LiDAR system800inFIG.8, the LiDAR system900inFIG.9includes multiple optical arrangements616for multiple targets620. In LiDAR system900, each of the multiple optical arrangements is configured to receive respective high-power path and low-power path optical beams612,614, and detect respective optical beams630,632having respective first and second beat frequencies from which the range and velocity of a respective target of the multiple targets is determinable.

In some examples, The LiDAR system900further includes an optical amplifier634between each of the multiple taps610and lensing optics618, the optical amplifier configured to amplify the high-power path optical beam612. And in some examples, each of the multiple optical arrangements616further includes at least one optical device (e.g., polarizing beamsplitter636and polarization wave plate638) configured to route the high-power path optical beam612from one of the multiple taps610to the lensing optics618, and route the return optical beam622from the lensing optics to the mixer624.

An extension of the two-beam implementation is to have an auxiliary range arm, as shown inFIG.10. This implementation differs from the two-beam design in that the second beam is not launched toward a target, but rather sent to a local reference interferometer via a tap. The reference interferometer enables the system to compensate for drifts in the optical frequency, ensure a well-tuned modulation waveform, and calibrate the beat frequency to range measurement. Essentially, the auxiliary arm aids the system in correcting for any nonlinear fluctuations to the optical sources. Note that althoughFIG.10shows the auxiliary arm added to the second implementation (FIG.6), this auxiliary arm can be added to any of the aforementioned implementations. Keeping that in mind, a single auxiliary arm could vastly improve the resource efficiency of a multiple-beam design, like that shown inFIG.9, since only one auxiliary arm is necessary for all the separate beams.

FIG.10in particular illustrates a LiDAR system1000according to example implementations of the present disclosure. Again, similar to the other implementations, the LiDAR system1000includes first and second optical sources502a,502bconfigured to emit respectively a first optical beam504aand a second optical beam504bthat are nondegenerate and are chirped antiphase. The LiDAR system includes an optical frequency multiplexer606configured to combine the first optical beam and the second optical beam into a single spatial mode optical beam608. The LiDAR system includes at least one tap610configured to split the single spatial mode optical beam into a high-power path optical beam612and a low-power path optical beam614. And the LiDAR system includes at least one optical arrangement616.

The LiDAR system1000inFIG.10further includes a first tap1040between the optical frequency multiplexer606and the at least one tap610. The first tap is configured to split the single spatial mode optical beam608into multiple single spatial mode optical beams608, and each of the at least one optical arrangement616is configured to receive a respective single spatial mode optical beam of the multiple single spatial mode optical beams. In addition, a local reference interferometer1042is configured to receive another respective single spatial mode optical beam608of the multiple single spatial mode optical beams.

As shown, the local reference interferometer1042includes an optical splitter1044configured to split the other respective single spatial mode optical beam608into a first part optical beam1046aand a second part optical beam1046bfor propagation along respectively a first path and a second path. The first path has a propagation medium1048such as an optical fiber spool to give the first path a greater length than the second path. The local reference interferometer includes an optical combiner1050configured to combine the first part optical beam and the second part optical beam from the first path and the second path into a reference optical beam1052. And an optical frequency demultiplexer1054is configured to split the reference optical beam into a first reference optical beam1052aand a second reference optical beam1052b.

In some examples, the LiDAR system1000further includes an optical amplifier634between the at least one tap610and lensing optics618, the optical amplifier configured to amplify the high-power path optical beam612. And in some examples, similar to other examples, the optical arrangement616further includes at least one optical device (e.g., polarizing beamsplitter636and polarization wave plate638) configured to route the high-power path optical beam612from the tap610to the lensing optics618, and route the return optical beam622from the lensing optics to the mixer624.

Another variation of the system is to adjust the center wavelength of both optical beams since the performance of these designs is relatively wavelength agnostic. This tuning can provide a different understanding of the target's material by looking at the difference in the return intensity between the two wavelengths with reasonably different absorption or reflection spectra. Again, by employing the multiple beam implementation, once can obtain such characterizations of the local environment immediately.

As remarked briefly earlier, this system can be built in a variety of propagation mediums including optical fibers, crystalline waveguides, or free-space air. This versatility implies that any 2D optical scanning system can be added at the output of any of the implementations mentioned and their extensions or combinations to provide a true, complete 4D map of the sensor's surrounding environment.

FIG.11illustrates a flowchart with various steps in a method1100of light detection and ranging, according to some example implementations of the present disclosure. As shown at blocks1102and1104, the method includes emitting respectively a first optical beam and a second optical beam that are nondegenerate and are chirped antiphase, and combining the first optical beam and the second optical beam into a single spatial mode optical beam. The method includes splitting the single spatial mode optical beam into a high-power path optical beam and a low-power path optical beam, as shown at block1106.

The method1100includes launching the high-power path optical beam towards a target, and collecting light incident upon the target into a return path, the light being collected into a return optical beam, as shown at block1108. The method includes mixing the return optical beam and the low-power path optical beam, and thereby producing a mixed optical beam, as shown at block1110. The method includes splitting the mixed optical beam into an optical beam having a first beat frequency, and an optical beam having a second beat frequency, as shown at block1112. And the method includes detecting respectively the optical beam having the first beat frequency and the optical beam having the second beat frequency, a range and velocity of the target being determinable from the first beat frequency and the second beat frequency, as shown at block1114. And in some examples, the method may further include determining the range and velocity of the target from the first beat frequency and the second beat frequency.

The preceding description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present disclosure. It will be apparent to one skilled in the art, however, that at least some embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present disclosure. Thus, the specific details set forth are merely exemplary. Particular embodiments may vary from these exemplary details and still be contemplated to be within the scope of the present disclosure.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiments included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. In addition, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.”

Although the operations of the methods herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent or alternating manner.

The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The words “example” or “exemplary” are used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X includes A or B” is intended to mean any of the natural inclusive permutations. That is, if X includes A; X includes B; or X includes both A and B, then “X includes A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. Moreover, use of the term “an embodiment” or “one embodiment” or “an implementation” or “one implementation” throughout is not intended to mean the same embodiment or implementation unless described as such. Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. as used herein are meant as labels to distinguish among different elements and may not necessarily have an ordinal meaning according to their numerical designation.