Patent Description:
To determine a range of a scene, traditional systems rely on bulky mechanical devices such as, articulated and rotating mirrors, and gimbals. However, these devices are costly and potentially sensitive to mechanical failures and damage. Also, these devices limit the scan rate, decrease reliability, and increase the system cost. Hence, solid-state systems are being developed to replace these traditional systems.

In the solid-state systems, light detection and ranging (lidar) is a key technology which is widely used in different applications such as, autonomous driving and unmanned aerial vehicles. Also, lidar enables a host of other emerging technologies such as, aerial mapping and robotics. Typically, a lidar system includes a light source and an optical receiver. Further, the light source emits light having a predetermined wavelength towards a surrounding scene which then scatters the light. In response to emitting the light, some of the scattered light from the scene is received back at the optical receiver. Further, the lidar system determines the ranging of the scene based on one or more characteristics associated with the received scattered light. The ranging information may be further used to produce a three-dimensional (3D) image of at least a part of the scene, which in-turn is used for different applications such as, 3D mapping and autonomous decision making.

In the conventional lidar systems, two well-known methods such as, direct time-of-flight (TOF) method and frequency modulated continuous wave (FMCW) method are employed for determining the ranging of the scene. In the time-of-flight (TOF) method, a beam of pulsed light is scanned over the scene or the light is flashed at once over the entire scene. Further, range of the scene is determined by measuring the time between transmission and reception of the light signal. However, in this method, ambient light increases the noise floor of the receiver. As a result, the light scattered from weakly reflecting or distant targets can be too weak to detect, which in-turn may lead to corrupted measurements. Although the TOF method can achieve high frame rates, the TOF method has low sensitivity and poor resilience to ambient noise.

On the other hand, in the FMCW method, the FMCW light beam is frequency modulated hence, the range and the velocity of the scene can be determined by coherent detection of the frequency shift. However, due to the complexity of coherent detection, the light beam is scanned point-by-point by a beam scanner to cover the entire scene. By using frequency modulation and coherent detection, sensitivity is improved, and the impact of ambient noise is greatly reduced. Although the FMCW method can improve sensitivity and resilience to ambient noise, the FMCW method has low frame rate due to scanning of the scene.

<NPL>, disclose a nanophotonic coherent imager. The imager includes a laser that is used to illuminate the target object that is being imaged. Also, a part of the laser output that is used to illuminate the target object is coupled into the input grating coupler through an optical fiber. Further, it is guided to Y-junction splitter network through nano-waveguides, to be used as a reference signal. On the other hand, in response to illuminating the target object, the incoming/reflected light from the target object is coupled into nano-waveguides through an array of grating couplers serving as pixels. Thereafter, the received light from each pixel is then combined with a part of the references in a Y-junction combiner and is converted to electrical current using a photo-detector.

<NPL>, discloses a LiDAR system that includes reception ports (SN1) and emission ports (SN2). Further, emission channels from the emission ports are connected via optical fiber circulators (FC) to the corresponding photodiodes (BPD) at the reception ports. The emission ports (SN2) emit the laser power (E) via the collimation lenses. In response, the backscattered light (R) is collected by optical fiber circulators (FC) and redirects it to the reception ports (SN2). In parallel, another switching network (SN1) routes the local oscillator to the photodiodes on which the back-reflected signal or backscattered light (R) is incoming. Further, the photodiodes sum these photocurrents and are routed to the RF output.

Thus, there is a need for an improved system and method that aid in determining the ranging of the scene with high frame rate, high sensitivity, and strong resilience to ambient noise.

In accordance with aspects of the present specification, a system for determining a range of a scene is presented. The system includes an optical source configured to generate an input signal, wherein the input signal is a frequency modulated coherent signal. Further, the system includes a first optical coupler coupled to the optical source and configured to tap a predetermined portion of the input signal as a local oscillator (LO) signal. Also, the system includes an emitting unit coupled to the first optical coupler and configured to receive and transmit a remaining portion of the input signal as an output signal onto the scene, and wherein the emitting unit (<NUM>) is configured to flash the scene (<NUM>) with the output signal (<NUM>). In addition, the system includes an imaging unit arranged to receive a plurality of return signals from the scene in response to the output signal. Further, the imaging unit includes an array of detectors optically and directly coupled to at least one lens, wherein a position of each detector is associated with a unique direction of the return signals received from the scene, wherein the at least one lens is configured to receive and direct the return signals onto the array of detectors, and further wherein each detector of the array is configured to receive and mix the local oscillator signal with a corresponding return signal of each detector thereby generating a radio frequency (RF) beat signal, wherein the RF beat signal is fit for determining the range of the scene), and wherein; each detector of the array of detectors comprises: a photodetector site configured to directly receive one of the return signals; and at least one waveguide evanescently coupled to the photodetector site and configured to receive the LO signal and distribute the local oscillator signal uniformly over the photodetector site thereby the local oscillator signal interferes with the one of the return signals.

In accordance with another aspect of the present specification, a method for determining a range of a scene is presented. The method includes generating, by an optical source, an input signal, wherein the input signal is a frequency modulated coherent signal. Further, the method includes tapping, by a first optical coupler, a predetermined portion of the input signal as a local oscillator (LO) signal. Also, the method includes transmitting, by an emitting unit configured to flash the scene (<NUM>), a remaining portion of the input signal as an output signal onto the scene. In addition, the method includes receiving, by an imaging unit comprising at least one lens and an array of detectors, a plurality of return signals from the scene in response to the output signal. Furthermore, the method includes directing, by the at least one lens directly coupled to the array of detectors, the return signals onto the array of detectors. Finally, the method includes mixing, by each of the array of detectors, the local oscillator signal with a corresponding return signal of each detector thereby generating a radio frequency (RF) beat signal, wherein the RF beat signal is fit for determining the range of the scene, wherein mixing the local oscillator signal with the return signals comprises: receiving, by the photodetector site in each detector, one of the return signals; distributing uniformly, by at least one waveguide evanescently coupled to the photodetector site, over the photodetector site whereby the local oscillator signal interferes with the one of the return signals.

As will be described in detail hereinafter, various embodiments of systems and methods for determining a range of a scene is presented. In particular, a frequency modulated coherent optical signal is flashed over the scene. In response, return signals from different objects in the scene are received. Further, these return signals are processed to determine the range of the scene. Also, this systems and methods aid in determining the range of the scene with high frame rate, high sensitivity and resilience to ambient noise. This in-turn improves the detection range and resolution of objects in the scene.

Turning now to the drawings and referring to <FIG>, a diagrammatical representation of a system <NUM> for determining a range of a scene <NUM>, in accordance with aspects of the present specification, is depicted. The scene <NUM> may include but not limited to surrounding structures or objects <NUM>, <NUM>, <NUM> such as, trees, plants, vegetation, buildings, and landscape. Further, the range of the scene <NUM> may be referred to as one or more parameters of the scene <NUM>. In one example, the parameters may include velocity and distance of the scene <NUM>. The velocity is defined as the relative speed and direction of travel of the objects <NUM>-<NUM> with respect to the system <NUM>. Similarly, the distance is defined as the distance between the system <NUM> and the objects <NUM>-<NUM> in the scene <NUM>.

In a presently contemplated configuration, the system <NUM> may be referred to as a light detection and ranging (lidar) system. It may be noted that the terms "system" and "lidar system" may be used interchangeably in the below description. The exemplary lidar system <NUM> is used as a remote sensing system for sensing and mapping surrounding structure or objects <NUM>-<NUM> of the scene <NUM>. Also, the lidar system <NUM> may aid in ranging the scene in multiple directions. Moreover, the lidar system <NUM> employs lidar technology that has high potential to achieve long range and high resolution of the scene <NUM>. In particular, the lidar system <NUM> transmits a frequency modulated continuous wave (FMCW) signal <NUM> in multiple directions towards the scene <NUM>. The FMCW signal <NUM> is referred to as a continuous laser beam where the frequency is increased or decreased periodically by a modulating signal. In one example, the FMCW signal <NUM> may be a triangular wave having an up-chirp portion and a down-chirp portion. The up-chirp portion exhibits a linear increase in frequency versus time while, the down-chirp portion exhibits a linear decrease in frequency versus time. It may be noted that, in the exemplary lidar system <NUM>, the FMCW signal <NUM> is flashed at once over the entire scene <NUM>.

In response to transmitting the FMCW signal <NUM>, a plurality of return signals <NUM>-<NUM> resulting from reflected scattering of the transmitted signal <NUM> by the scene <NUM> is received. It may be noted that the return signals <NUM>-<NUM> are referred to as echoed, scattered, or reflected signals of the transmitted FMCW signal <NUM> onto the scene <NUM>. Further, the lidar system <NUM> determines the range of the scene <NUM> based on optical coherence between the transmitted FMCW signal <NUM> and the return signals <NUM>-<NUM>. More specifically, the frequency shift in the return signals <NUM>-<NUM> as compared with the transmitted signal <NUM> is determined. Further, this frequency shift information is used to compute the range such as, the distance, the relative speed and the direction of travel of the objects <NUM>-<NUM> in the scene. Moreover, the lidar system <NUM> is extensively used in different applications such as, autonomous vehicles, atmospheric measurements, aerial mapping, and robotics.

In a conventional lidar system, the FMCW signal is transmitted to scan point-by-point to cover the entire scene. In particular, the FMCW signals are transmitted sequentially by a beam steerer over a plurality of objects in the scene. Further, the return signals from each object is processed sequentially to build an image of the scene. In other words, the conventional lidar system using the FMCW method is not designed or equipped to concurrently interrogate the scene in multiple directions. As a result, the conventional lidar system may have a low frame rate to image the scene, which in-turn delays imaging of the scene and provides low resolution.

To avoid the above shortcomings or problems, the exemplary lidar system <NUM> includes an emitting unit <NUM> and an imaging unit <NUM> that aid in imaging the scene <NUM> with high sensitivity and strong resilience to ambient noise and at a high frame rate. In particular, the emitting unit <NUM> flashes the FMCW signal <NUM> over the entire scene <NUM>. Further, the objects <NUM>-<NUM> in the scene <NUM> may scatter or reflect at least a portion of the FMCW signal <NUM>. Some of the scattered or reflected signals may return toward the lidar system <NUM>. These reflected/scattered signals are then received concurrently by the imaging unit <NUM> as the return signals <NUM>-<NUM>.

In the presently contemplated configuration, the imaging unit <NUM> includes an array of detectors <NUM> and one or more lenses <NUM> that are arranged in a way to concurrently receive the return signals <NUM>-<NUM> from the scene <NUM>. More specifically, multiple detectors <NUM> are arranged to simultaneously interrogate the scene in multiple directions. These lenses <NUM> are optically coupled to the array of detectors <NUM> without a waveguide or any other coupling element. Also, these lenses <NUM> are directly coupled to the array of detectors <NUM>. Further, the lenses <NUM> are configured to direct the return signals <NUM>-<NUM> received from the scene <NUM> onto the array of detectors <NUM>.

Furthermore, the array of detectors <NUM> is arranged in such a way that each detector is dedicated to receive the signals <NUM>-<NUM> returning in a particular direction. Thus, different directions of the scene correspond to different detectors or pixels of the imaging unit. More specifically, the detectors <NUM>, <NUM><NUM> are arranged in a predefined pattern as depicted in <FIG>. Further, the position of each detector is associated with a unique direction of the scattered/return signals <NUM>-<NUM> received from the scene <NUM>. For example, the first detector <NUM> is positioned to receive the signal <NUM> returning in a first direction <NUM>. In a similar manner, the second detector <NUM> is positioned to receive the signal <NUM> returning in a second direction <NUM>, and the third detector <NUM> is positioned to receive the signal <NUM> returning in a third direction <NUM>. It may be noted that the detectors <NUM>-<NUM> may be positioned in any pattern and is not limited to the pattern depicted in <FIG>. Also, the array of detectors <NUM> may be a two-dimensional (2D) array or a three-dimensional (3D) array.

Further, each of the detectors <NUM>-<NUM> is configured to receive and mix the corresponding return signal with a local oscillator (LO) signal (shown in <FIG>) to generate a radio frequency (RF) beat signal. The LO signal may be referred to as the signal having the frequency information of the FMCW signal <NUM>. Thereafter, the RF beat signal may be processed and analyzed electronically to determine or calculate the range, such as the velocity and the distance of the scene. The range of the scene may aid in producing a three-dimensional (3D) image of parts of the surrounding scene, which can be used for applications such as 3D mapping and autonomous decision making. The aspects of positioning the array of detectors <NUM> and processing the return signals <NUM>-<NUM> will be explained in greater detail with references to <FIG>.

Thus, by employing the exemplary lidar system <NUM>, the FMCW signal <NUM> is flashed at once over the entire scene and the return signals <NUM>-<NUM> are received concurrently from the scene <NUM>. As a result, the frame rate to scan the entire scene <NUM> is substantially increased. This in-turn reduces the delay in imaging the scene <NUM>. Also, as the FMCW signals are used, the lidar system <NUM> has high sensitivity and is resilient to the ambient noise, resulting in obtaining uncorrupted measurements of the scene <NUM>. In addition, the speed of the lidar system <NUM> is substantially improved by performing ranging in multiple directions at once.

Referring to <FIG>, a block diagram of the lidar system <NUM> for determining the range of the scene, in accordance with aspects of the present specification, is depicted. It may be noted that the emitting unit <NUM> and the imaging unit <NUM> are similar to the emitting unit <NUM> and the imaging unit <NUM> of <FIG>, respectively. In addition to the emitting unit <NUM> and the imaging unit <NUM>, the lidar system <NUM> includes an optical source <NUM>, a first optical coupler <NUM>, a variable delay unit <NUM>, and a second optical coupler <NUM>. It may be noted that the lidar system may include other components and is not limited to the components depicted in <FIG>. Also, the lidar system <NUM> may be built in a single chip. Although most attractive when implemented entirely on the single chip, the system may be implemented wholly or partially with off-chip components such as the first optical coupler <NUM>, the variable delay unit <NUM>, and the emitting unit <NUM>.

The optical source <NUM> is optically coupled to the first optical coupler <NUM>. Further, the optical source <NUM> is configured to generate a frequency modulated continuous wave (FMCW) signal having a predetermined operating wavelength. In one embodiment, the FMCW signal is a continuous laser beam with a prescribed and continuous change in the frequency. This change in frequency information is later used for determining the distance and the velocity of the scene. Also, the generated FMCW signal is a coherent signal, that is a signal from a laser source with a coherence time larger than two times the longest expected round trip time. This ensures that the return signal remains coherent with the LO. In one example, the optical source <NUM> may be a single laser source that transmits the FMCW signal in multiple directions to illuminate the scene. Also, it may be noted that the frequency modulation of the light may be performed on-chip or off-chip to generate the FMCW signal.

Further, it may be noted that the operating wavelength of the FMCW signal may be in the ultraviolet, infrared, or visible portions of the electromagnetic spectrum. In one embodiment, the operating wavelength of the FMCW signal is selected from a range that is less sensitive to the ambient noise such as the sunlight. For example, the light produced by Sun may act as background noise which can obscure signal light detected by the lidar system. Also, this solar background noise can corrupt measurement of the lidar system <NUM>. Thus, the operating wavelength and the power level of the FMCW signal is selected in such a way that the FMCW signal is less sensitive to the ambient or background noise. In one example, the operating wavelength may be in a range from about <NUM> to <NUM>. Also, the power level of the FMCW signal is in a range from about <NUM> mW to about <NUM> W.

Upon generating the FMCW signal, the optical source <NUM> transmits the generated frequency modulated and coherent signal as an input signal <NUM> to the first optical coupler <NUM>. As illustrated in <FIG>, the first optical coupler <NUM> is electrically coupled to the emitting unit <NUM> and the variable delay unit <NUM>. Also, the first optical coupler <NUM> is configured to receive the input signal <NUM> from the optical source <NUM>. Thereafter, the first optical coupler <NUM> splits the input signal <NUM> into a local oscillator (LO) signal and an output signal. In particular, the first optical coupler <NUM> may tap the predetermined portion of the input signal <NUM> by first frequency chirping the input signal <NUM> and then transmit the remaining portion of the input signal <NUM> to the emitting unit <NUM>. In one example, the first optical coupler <NUM> may include a directional coupler or a local oscillator tap or an MMI. Upon splitting the input signal <NUM>, the first optical coupler <NUM> may transmit the tapped LO signal <NUM> to the variable delay unit <NUM>.

The variable delay unit <NUM> is configured to reduce the decoherence between the LO signal <NUM> and the return signals <NUM>-<NUM> and manage frequency of the RF beat signal. In particular, the variable delay unit <NUM> is electrically coupled to the first optical coupler <NUM> and the imaging unit <NUM>. Further, the variable delay unit <NUM> is configured to delay the transmission of the LO signal <NUM> to the imaging unit <NUM> so as to reduce decoherence between the LO signal <NUM> and the return signals <NUM>-<NUM>. In one example, the variable delay unit <NUM> is included. The delay can be set to match the expected average range of the scene so that the LO and scattered signals have approximately the same time delay and the requirements for coherence time for the laser source is reduced. The delay can also be set so that the beat frequency is of an order in which the measurement is most sensitive. Thereafter, the variable delay unit <NUM> transmits the LO signal to the imaging unit <NUM>.

Moving back to the first optical coupler <NUM>, after tapping the input signal <NUM>, the remaining portion of the input signal <NUM> is transmitted from the first optical coupler <NUM> to the emitting unit <NUM>. Further, the emitting unit <NUM> is configured to transmit this portion of the input signal as an output signal <NUM> onto the scene <NUM>. Typically, in the conventional lidar systems, the FMCW signal is scanned point-by-point by a beam scanner/steerer to cover the entire scene. Consequently, the frame rate is low in such conventional lidar systems. However, in the exemplary lidar system, the FMCW signal or the output signal <NUM> is flashed at once over the entire scene <NUM>. Hence, the exemplary lidar system <NUM> has high frame rate. As depicted in <FIG>, the emitting unit <NUM> includes an emitting aperture154 through which the output signal <NUM> is transmitted onto the scene <NUM>. In one embodiment, the emitting unit <NUM> includes an optical phased array, a leaky wave antenna array, a grating coupler, an edge coupler, or any other suitable aperture that facilitates to transmit the output signal <NUM> in multiple directions towards the scene <NUM>.

In response to transmitting the output signal <NUM> onto the scene <NUM>, the imaging unit <NUM> is arranged to receive a plurality of return signals <NUM>-<NUM> from the scene <NUM>. In particular, when the output signal <NUM> reaches the scene <NUM>, the objects <NUM>-<NUM> in the scene <NUM> may scatter or reflect at least a portion of the output signal <NUM>. Further, some of the scattered or reflected signals may return toward the lidar system <NUM>. These scattered or reflected signals are then received by the imaging unit <NUM> of the lidar system <NUM> as the return signals <NUM>-<NUM>.

As depicted in <FIG>, the imaging unit <NUM> includes a second optical coupler <NUM>, an array of detectors <NUM>, and one or more lenses <NUM> (see <FIG>). The array of detectors <NUM> is optically coupled to the one or more lenses <NUM> and electrically coupled to the second optical coupler <NUM>. Further, the lenses <NUM> are configured to receive and direct the return signals onto the array of detectors <NUM>.

Furthermore, the array of detectors <NUM> includes a plurality of detectors <NUM>, <NUM>, <NUM> that are configured to receive the return signals <NUM>-<NUM> via the one or more lenses <NUM>. In one example, the array <NUM> may be one-dimensional or two-dimensional (2D) array. The 2D array of detectors may allow ranging to be performed along two axes without any beam steering. In particular, the detectors <NUM>-<NUM> are arranged in a desired pattern to receive the return signals <NUM>-<NUM> from the scene <NUM>. More specifically, as depicted in <FIG>, the position of each detector is associated with a unique direction of the scattered signals received from the scene. For example, the return signal <NUM> from the object <NUM> is received by the first detector <NUM>. In a similar manner, the return signal <NUM> from the object <NUM> is received by the second detector <NUM>. Further, the return signal <NUM> from the object <NUM> is received by the third detector <NUM>. Thus, each detector is dedicated to receiving return signals in a particular direction from the scene <NUM>. It may be noted that the array <NUM> may include any number of detectors and is not limited to the detectors shown in <FIG>. Also, these detectors may be arranged in any pattern.

In addition, each detector is configured to receive the LO signal <NUM> via the second optical coupler <NUM>. More specifically, the second optical coupler <NUM> is configured to split the LO signal <NUM> into multiple LO signals with equal power and transmit each LO signal to a corresponding detector. In one example, the second optical coupler <NUM> includes directional couplers to distribute the LO signal to the array of detectors. In another example, the second optical coupler may include MMIs, star-couplers, and splitter trees. The received LO signal <NUM> and the corresponding return signal may be orthogonal to each other and temporally coherent. Further, the received LO signal <NUM> is mixed with the corresponding return signal to generate a radio frequency (RF) beat signal that is further processed to determine the range of the scene <NUM>.

In particular, each detector includes a photodetector site <NUM> and one or more waveguides <NUM>. The photodetector site <NUM> is configured to receive one of the return signals via the one or more lenses <NUM>. In one example, the width of the photodetector site <NUM> is in a range from about <NUM> to about <NUM>. Further, the length of the photodetector site <NUM> is in a range from about <NUM> to about <NUM> for a 2D array from <NUM> to about <NUM> for a 1D array.

Further, the one or more waveguides <NUM> are coupled to the second optical coupler <NUM> and the photodetector site <NUM>. Also, the one or more waveguides <NUM> are configured to receive the LO signal <NUM> and distribute uniformly over the photodetector site <NUM> so that the LO signal <NUM> interferes with the one of the return signals <NUM>-<NUM>. This LO signal <NUM> and the return signal <NUM>-<NUM> interfere to generate an RF beat signal. In particular, since the return signal <NUM>-<NUM> and the LO signal <NUM> are coherent, the two signals interfere on the photodetector site, generating a RF beat signal. This RF beat signal has a frequency which is equal to the difference between the optical frequency of the return signal <NUM>-<NUM> and the LO signal <NUM>. This frequency is referred to as the beat frequency between the LO signal <NUM> and the return signal <NUM>-<NUM>. Further, the signal associated with this beat frequency may be referred to as the RF beat signal. Thereafter, the RF beat signal is processed electronically for determining the range of the scene <NUM>. More specifically, the return signal <NUM>-<NUM> will have a different time delay compared to the LO signal <NUM>. Further, as the input signal <NUM> is frequency modulated and the LO signal <NUM> interfere with the return signal <NUM>-<NUM>, the RF beat signal frequency will encode the distance travelled by the return or scattered signal <NUM>-<NUM>. The frequency of the RF beat signal is measured by an analog or digital electronic circuit to determine the distance of the scene. Further, if the target or objects in the scene is moving, a Doppler shift will occur in the return signal received from these objects or target. This Doppler shift is further detected in the RF beat signal to determine the velocity of the scene. In one example, the RF beat signal may be processed at a speed of <NUM>. It may be noted the RF beat signal may be processed internally within the imaging unit <NUM> or by an external processing unit that is away from the imaging unit <NUM>. The aspect of positioning the waveguide <NUM> on the photodetector site <NUM> and mixing the LO signal <NUM> with the return signals <NUM>-<NUM> will be explained in greater detail with reference to <FIG>.

Referring to <FIG>, a perspective view <NUM> of a waveguide <NUM> coupled to a photodetector site <NUM>, in accordance with one embodiment of the present specification, is depicted. The waveguide <NUM> may be similar to the waveguide <NUM> shown in <FIG>. In the embodiment of <FIG>, the waveguide <NUM> may be a tubular structure having a length in a range from about <NUM> to about <NUM> and a width in a range from about <NUM> to about <NUM> The waveguide <NUM> is evanescently coupled to the photodetector site <NUM>. In one example, the waveguide <NUM> may be positioned at a height of about <NUM> to about <NUM> above the photodetector site <NUM>.

Further, the waveguide <NUM> includes a first portion <NUM> that is positioned on a top surface <NUM> of the photodetector site <NUM>. The top surface <NUM> may be referred to as the surface that is used for receiving the return/scattered signals <NUM>-<NUM> from the scene <NUM>. Further, the first portion <NUM> of the waveguide <NUM> may have a tapering structure as depicted in <FIG>. In one example, the tapering structure may have a narrow end at a side farther away from the second optical coupler <NUM>. Also, the tapering structure may have apertures on both sides of the waveguide <NUM>. Further, the LO signal <NUM> may pass through these apertures and spread over the top surface <NUM> of the photodetector site <NUM>. The LO signal <NUM> may propagate in a direction that is perpendicular to the length of the waveguide <NUM>, while the return signal <NUM>-<NUM> may be directed perpendicular to the top surface of the photodetector site <NUM>. As the waveguide <NUM> is tapered and placed on the surface <NUM> of the photodetector site <NUM>, the LO signal <NUM> is distributed uniformly across the top surface <NUM> of the photodetector site <NUM>, which in-turn increases mixing efficiency of the LO signal <NUM> and the return signals <NUM>-<NUM>. In one embodiment, the waveguide <NUM> may be a straight waveguide.

In one embodiment, the waveguide <NUM>, particularly the first portion <NUM> may be positioned or coupled to a bottom surface <NUM> of the photodetector site <NUM>. Further, the LO signal <NUM> may mix with the return/scattered signals <NUM>-<NUM> within the photodetector site <NUM> to generate a RF beat signal. In one example, the photodetector site may have a thickness of <NUM>.

Moving now to <FIG>, a perspective view <NUM> of a waveguide <NUM> coupled to a photodetector site <NUM>, in accordance with another embodiment of the present specification. The waveguide <NUM> is similar to the waveguide of <FIG> except that the first portion <NUM> of the waveguide <NUM> may have a grating structure. Similar to the tapering structure, the grating structure may aid in uniformly distributing the LO signal <NUM> across the top surface <NUM> of the photodetector site <NUM>. The advantage of this structure is the ability to engineer the radiation of light from the waveguide. This enables more uniform distribution of the LO signal on the photodetector site, which in-turn may improve mixing efficiency of the return and LO signals. The grating can be realized by designs such as sidewall corrugations, top surface corrugations, and periodic pillars or islands in a plane parallel to the waveguide plane.

<FIG> is a perspective view <NUM> of waveguides <NUM>, <NUM> coupled to a photodetector site <NUM>, in accordance with yet another embodiment of the present specification. In this embodiment, two waveguides <NUM>, <NUM> are positioned on the top surface <NUM> of the photodetector site <NUM>. Also, these two waveguides <NUM>, <NUM> are separated by a predetermined distance <NUM>. In one example, the predetermined distance <NUM> may be in a range from about <NUM> to about <NUM>. Further, it may be noted that the first portion <NUM> of these two waveguides <NUM>, <NUM> may include flat structure, tapering structure, and/or grating structure that are used for distributing uniformly the LO signal <NUM> across the top surface <NUM> of the photodetector site <NUM>. In addition, these two waveguides <NUM>, <NUM> along with the photodetector site <NUM> may form a balanced detector for cancelling the noise embedded in the return signals <NUM>-<NUM>. The aspect of cancelling the noise will be described in greater detail with reference to <FIG>.

Referring to <FIG>, a cross-sectional view of a detector <NUM> depicting flow of return signal and local oscillator (LO) signal, in accordance with one embodiment of the present specification, is depicted. It may be noted that the cross-sectional view of <FIG> is the representation of the waveguide and the photodetector site shown in <FIG>. The photodetector site <NUM> includes p-i-n junctions <NUM> and electrodes <NUM> coupled to metal terminals <NUM>. The electrodes <NUM> are positioned on two ends <NUM>, <NUM> on the top surface <NUM> of the photodetector site <NUM> for receiving the mixed LO and the scattered signals on the surface of the photodetector site. Further, the mixed LO and scattered signals are transmitted to a processor or controller via the metal terminals <NUM> for generating the RF beat signal which is further processed to determine the range of the scene. It may be noted that the positioning of the electrodes <NUM> is not limited to the top surface <NUM> as depicted in <FIG>. In one embodiment, the electrodes may be positioned on the bottom surface of the photodetector site <NUM>. In another embodiment, the photodetector site <NUM> may be in-built electrodes that may not protrude on the surface of the photodetector site <NUM>.

In one embodiment, the photodetector site <NUM> includes germanium (Ge) material for near infrared (NIR) communication. In another embodiment, the photodetector site <NUM> includes silicon (Si) material for receiving the FMCW signal or return signal have operating wavelength less than <NUM>. In yet another embodiment, the photodetector site <NUM> includes III/V materials or other suitable semiconductor material for a multitude of wavelengths. In a similar manner, the waveguide <NUM> includes silicon nitride (SiN) material. Further, a transparent, low-index material such as silicon dioxide material may be positioned between the waveguide <NUM> and the photodetector site <NUM>.

As depicted in <FIG>, the scattered or return signals <NUM> are directed onto the surface <NUM> of the photodetector site <NUM>. In one example, the direction of the scattered or returned signals <NUM> may be substantially perpendicular to the surface <NUM> of the photodetector site <NUM>. Further, the photodetector site <NUM> includes an occlusion free portion <NUM> for receiving the return signals <NUM>. In one embodiment, the occlusion free portion <NUM> may be referred to as the space on the photodetector site <NUM> that is between the electrodes <NUM> for receiving the scattered/return signals <NUM> without any hinderance from the electrodes <NUM>. The occlusion free portion <NUM> includes a width in a range from about <NUM> to about <NUM> and a length in a range from about <NUM> to about <NUM>.

Similarly, the waveguide <NUM> uniformly distributes the LO signal <NUM> onto the surface <NUM> of the photodetector site <NUM>. In one example, the direction of the LO signal <NUM> may be substantially parallel to the surface <NUM> of the photodetector site <NUM>. Further, the return signal <NUM> and the LO signal <NUM> are substantially orthogonal to each other on the photodetector site <NUM>. Also, the return signal <NUM> and the LO signal <NUM> are coherent and interfering on the photodetector site <NUM> to generate the radio frequency (RF) beat signal.

In one embodiment, the waveguide <NUM> may be coupled to a bottom surface <NUM> of the photodetector site <NUM> while, the scattered/return signals <NUM> are directed onto the top surface <NUM> of the photodetector site <NUM>. Further, the LO signal <NUM> may mix with the return signal <NUM> within the photodetector site <NUM> to generate a RF beat signal.

Referring to <FIG>, a cross-sectional view of a detector <NUM> having a noise cancellation structure, in accordance with one embodiment of the present specification, is depicted. It may be noted that the cross-sectional view of <FIG> is the representation of the waveguides and the photodetector site shown in <FIG>. The photodetector site <NUM> includes two p-i-n junctions <NUM>, <NUM> that are connected in series, and a pair of electrodes <NUM> are positioned on two sides of each p-i-n junction for communicating the mixed LO and scattered signals to an external unit, such as a processing unit or a control unit (not shown).

Further, the detector <NUM> includes two waveguides <NUM>, <NUM> that are coupled to the photodetector site <NUM> as depicted in <FIG>. One of the waveguides <NUM> may be similar to the waveguide depicted in <FIG>. Also, this waveguide <NUM> is configured to receive the LO signal <NUM> without any phase shift. However, the other waveguide <NUM> is referred to as a supporting waveguide that is configured to receive the LO signal <NUM> with a phase shift of <NUM> degrees. This phase shifted LO signal <NUM> may be used to cancel or suppress noise in the received scattered/return signals <NUM>. In particular, the two p-i-n junctions <NUM>, <NUM> generate two photocurrent signals, one signal is obtained by mixing the LO signal <NUM> with the return signals <NUM> and the other signal is obtained by mixing the phase shifted LO signal <NUM> with the return signals <NUM>. Hence, the two photocurrent signals cancel each other when they are equal in magnitude and <NUM> degrees out of phase with each other. The difference between these two photocurrent signals may be referred to as the signal with no or suppressed noise. This signal is further processed to generate the range of the scene. As the noise rejection is inherent to coherent detection, the exemplary lidar system may consume less laser power compared to the conventional lidars.

<FIG> is a flow chart illustrating a method <NUM> for determining the range of the scene, in accordance with aspects of the present specification. For ease of understanding, the method <NUM> is described with reference to the components of <FIG>. The method <NUM> begins with a step <NUM>, where an optical source generates an input signal. It may be noted that the input signal is a frequency modulated coherent signal. The input signal may be referred to as a frequency modulated continuous laser beam with a prescribed and continuous change in the frequency. The input signal may also be referred as the FMCW signal.

Subsequently, at step <NUM>, a first optical coupler taps a predetermined portion of the input signal as a local oscillator (LO) signal. In particular, the first optical coupler may tap the predetermined portion of the input signal by first frequency chirping the input signal and then transmit the remaining portion of the input signal to the emitting unit. In one example, the first optical coupler may include one or more directional couplers. Thereafter, the first optical coupler may transmit the tapped LO signal to the imaging unit via the variable delay unit.

In addition, at step <NUM>, an emitting unit transmits a remaining portion of the input signal as an output signal onto the scene. In particular, the output signal which is frequency modulated continuous wave is flashed at once over the entire scene. Hence, the exemplary lidar system has high frame rate. As depicted in <FIG>, the emitting unit includes an emitting aperture through which the output signal is transmitted onto the scene.

Furthermore, at step <NUM>, an imaging unit receives a plurality of return signals from the scene in response to the output signal. In particular, when the output signal reaches the scene, the objects in the scene may scatter or reflect at least a portion of the output signal. Further, some of the scattered or reflected signals may return toward the lidar system. These reflected/scattered signals are then received by the imaging unit of the lidar system as the return signals.

Further, at step <NUM>, at least one lens directs the return signals onto the array of detectors. The at least one lens is optically coupled to the array of detectors without any waveguide. In other words, these lenses are directly coupled to the array of detectors. Further, the lens is positioned in such a way that the return signals that are received on one side of the lens are directed onto the array of detectors positioned on the other side of the lens.

Thereafter, at step <NUM>, each detector is configured to mix the LO signal with a corresponding return signal thereby generating a radio frequency (RF) beat signal. The RF beat signal is fit for determining the range of the scene. In particular, the detectors are arranged in a desired pattern to receive the return signals from the scene. More specifically, as depicted in <FIG>, the position of each detector is associated with a unique direction of the return signals received from the scene. In addition, each detector is configured to receive the LO signal via the second optical coupler. Further, the received LO signal is mixed with the corresponding return signal to generate the radio frequency (RF) beat signal that is further processed to determine the range of the scene.

The various embodiments of the exemplary systems and methods presented hereinabove aid in determining the range of the scene with high frame rate and high sensitivity and strong resilience to the ambient noise. Also, the exemplary system may be included in a single chip FMCW that can support ranging in multiple directions without any beam steering. This in-turn improves long range and high resolution of the scene. Also, the exemplary lidar system requires less laser power compared to the conventional lidars due to inherent noise rejection feature in the exemplary lidar system, which in-turn reduces the cost of the lidar system. Moreover, the exemplary lidar system helps in reaching longer distance compared to the conventional lidars for the same laser power. In addition, automation of the automobiles and robotic systems may be substantially increased by using this low cost and low power lidar system.

Claim 1:
A system (<NUM>) for determining a range of a scene (<NUM>), the system comprising:
an optical source (<NUM>) configured to generate an input signal (<NUM>), wherein the input signal (<NUM>) is a frequency modulated coherent signal;
a first optical coupler (<NUM>) coupled to the optical source (<NUM>) and configured to tap a predetermined portion of the input signal (<NUM>) as a local oscillator (LO) signal;
an emitting unit (<NUM>) coupled to the first optical coupler (<NUM>) and configured to receive and transmit a remaining portion of the input signal (<NUM>) as an output signal (<NUM>) onto the scene (<NUM>), and wherein the emitting unit (<NUM>) is configured to flash the scene (<NUM>) with the output signal (<NUM>);
an imaging unit (<NUM>) arranged to receive a plurality of return signals (<NUM>, <NUM>, <NUM>) from the scene (<NUM>) in response to the output signal; the imaging unit (<NUM>) comprises:
an array of detectors (<NUM>) optically coupled to at least one lens (<NUM>), wherein a position of each detector is associated with a unique direction of the return signals (<NUM>, <NUM>, <NUM>) received from the scene (<NUM>), wherein the at least one lens (<NUM>) is configured to receive and direct the return signals (<NUM>, <NUM>, <NUM>) onto the array of detectors (<NUM>), and further wherein
each detector of the array is configured to receive and mix the local oscillator signal with a corresponding return signal of each detector thereby generating a radio frequency (RF) beat signal, wherein the RF beat signal is fit for determining the range of the scene (<NUM>), characterised in that:
the array of detectors (<NUM>) is directly coupled without any waveguide to the at least one lens (<NUM>); and
each detector of the array of detectors (<NUM>) comprises:
a photodetector site (<NUM>) configured to directly receive one of the return signals (<NUM>, <NUM>, <NUM>); and
at least one waveguide (<NUM>) evanescently coupled to the photodetector site (<NUM>) and configured to receive the LO signal and distribute the local oscillator signal uniformly over the photodetector site (<NUM>) thereby the local oscillator signal interferes with the one of the return signals (<NUM>, <NUM>, <NUM>).