Laser system, lidar system, and method for improving linearity of frequency chirp

Abstract

According to an aspect of the present inventive concept, there is provided a laser system for generating a frequency modulated continuous wave (FMCW) optical signal.SOLUTION: The system (100) includes a tunable laser (110) for generating an FMCW optical signal (112), an optical hybrid coupler (122) for receiving a portion (114) of the FMCW optical signal (112) and for, based on a difference between first and second signals (114a, 114b) formed by splitting the portion (114) of the FMCW optical signal (112): And a control unit (130) configured to estimate, based on the at least two angular diversity signals (124), a compensation required to adjust a non-linearity of the frequency chirp of the tunable laser (110) and to output a corresponding control signal (132) to the tunable laser (110), wherein the control signal (132) is configured to improve the linearity of the frequency chirp.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This specification relates to the field of frequency modulated continuous wave (FMCW) laser systems. More particularly, the present invention relates to a method and system for achieving linear chirp in laser frequency modulation. [Background technology]

[0002] Frequency-modulated continuous-wave (FMCW) laser systems rely on rapid laser frequency chirps to function effectively. The linearity of the frequency chirp is crucial to determining the performance of the laser system. In particular, FMCW LIDAR systems calculate the distance between the transmitter and the target by utilizing the beat frequency of different time-delayed linear chirp signals. If the laser chirp is not linear, the beat spectrum will be broadened, reducing the LIDAR range accuracy. Additionally, the peak frequency is directly proportional to the target distance, requiring a light source with high phase linearity, especially for long detection distances such as hundreds of meters.

[0003] High-quality linear chirps are typically achieved using external modulators and fast-swept RF sources. However, this method is complex, incurs high optical losses, and is expensive. An alternative approach is to achieve linear laser chirp by directly modulating the laser, known as direct modulation. However, the chirp linearity of direct modulation is not as high as that achieved with external modulation. The nonlinearity in direct modulation is affected by various laser conditions, including temperature, chirp rate, chirp range, and laser drive voltage.

[0004] In applications where environmental conditions can vary significantly, a control loop is required for chirp linearization in FMCW LIDAR systems. However, known methods require large systems with long fibers and involve complex digital signal processing (DSP).

[0005] Therefore, a more efficient solution is needed to achieve high-quality linear chirp in FMCW laser systems. Summary of the Invention

[0006] It is an object of the present specification to provide a method and system for achieving high quality linear chirp in a laser system configured to generate an FMCW optical signal.

[0007] Another object is to enable simplification of the system architecture of FMCW laser systems, such as FMCW LIDAR systems.

[0008] A further objective is to enable improved robustness and reliability in laser systems for generating FMCW optical signals by minimizing or at least reducing the impact of changing environmental conditions on chirp linearity.

[0009] These and other objects are at least partly achieved by the invention as defined in the independent claims. Preferred embodiments are set out in the dependent claims.

[0010] According to a first aspect, there is provided a laser system for generating a frequency modulated continuous wave (FMCW) optical signal, the system comprising: a tunable laser for generating an FMCW optical signal; an optical measurement unit configured to receive a portion of the FMCW optical signal and output at least two angle diversity signals based on a difference between first and second signals formed by splitting the portion of the FMCW optical signal via an optical hybrid coupler, wherein the second signal is delayed relative to the first signal, and a pair of signals of the at least two angle diversity signals have a fixed phase shift relative to each other; A control unit, receiving at least two angular diversity signals; Estimating a compensation required to adjust for the nonlinearity of the frequency chirp of the tunable laser based on the at least two angle diversity signals; a control unit for outputting a corresponding control signal to the tunable laser; Equipped with The control signal is configured to improve the linearity of the frequency chirp.

[0011] The laser system includes an optical measurement unit configured to output at least two angular diversity signals. The optical measurement unit determines the angular diversity signals based on a portion of the FMCW signal and a delayed version of the portion of the FMCW signal. This means that the angular diversity signals may represent a beat signal formed by interference between the first signal and the second signal.

[0012] Thanks to the measurement of the angle diversity signal, simple processing can be performed to determine the adjustment of the frequency chirp nonlinearity. The angle diversity signal can be easily processed to determine the frequency or phase of the beat signal, which represents the frequency change in the chirp. This allows the linearity of the frequency chirp to be determined quickly and accurately.

[0013] The high accuracy of determining the frequency / phase of the beat signal allows for the use of a short delay in the second signal, which can be achieved by physically guiding the second signal along an optical path. Thanks to the laser system's ability to use a short delay, the laser system can also be compact, as it does not need to provide a long optical path.

[0014] Therefore, a laser system for generating FMCW optical signals using a simplified system architecture is enabled. The laser system further reduces optical losses and minimizes the effects of chirp linearity due to changes in environmental conditions. Furthermore, because the system ensures that chirp nonlinearities are mitigated, the system has a high ability to use long chirp periods, which are typically associated with relatively large nonlinearities.

[0015] The nonlinearity of the frequency chirp can be due to factors such as driver bandwidth limitations and the nonlinear characteristics of the tunable laser, among other causes.

[0016] The term "tunable laser" as used herein refers to any unit, tuning device, and / or element capable of emitting at least partially coherent light. A tunable laser may include, for example, a laser diode configured to generate an optical carrier signal. The optical carrier signal is generated in the form of laser light (i.e., an FMCW optical signal).

[0017] In this disclosure, the term "light" is not limited to visible electromagnetic radiation, but should be broadly interpreted. Rather, the term "light" can also include, for example, ultraviolet and infrared light.

[0018] The tunable laser may include a frequency modulation module configured to modulate the frequency of the optical carrier signal. This indicates that the frequency modulation module may be external to the laser light source so that it can perform frequency modulation on the laser light output from the laser light source. Alternatively, the frequency modulation module may form part of the laser light source so that frequency modulation can be performed within the laser light source. As an example, frequency modulation may be performed by direct frequency modulation of the laser light source.

[0019] Furthermore, the laser system may be integrated on a single semiconductor chip. The laser system enables a compact (e.g., on-chip integratable) solution for laser linear chirp generation with reduced digital signal processing (DSP) complexity. The estimation and output of the necessary compensation of the control signal configured to improve the linearity of the frequency chirp may be realized using a compact architecture that can be integrated, for example, on a photonic integrated circuit (PIC). In other words, some or all of the components of the laser system may be integrated on the PIC.

[0020] The DSP complexity is reduced by phase accumulation, for example, by splitting a portion of the FMCW optical signal so that the second signal is delayed relative to the first signal. In particular, the use of angle diversity signals and their detection and processing by the control unit can avoid complex DSP architectures. Therefore, avoiding complex Hilbert transform-based techniques in the frequency domain can reduce DSP complexity.

[0021] Additionally, the laser system enables adaptive control loops that can run in real time, enabling chirp linearization of FMCW optical signals. In particular, adaptive nonlinearity monitoring and control can address the changing frequency chirp nonlinearity due to changing (external) conditions. Real-time chirp tracking enables fast control loops that may require sample-by-sample chirp calculations so that the instantaneous phase of the beat signal can be determined.

[0022] Here, "samples" may refer to data collected at a particular interval, compared to a frame, which may be associated with one or more chirps. The number of samples in a frame may be related to the frequency at which the time-domain waveform is sampled to derive the chirp (the rate of sampling may be well above the Nyquist limit). In general, chirps may be thought of as frames, with, for example, one up-chirp representing one frame and one down-chirp representing another frame.

[0023] Therefore, calculation of a sample-by-sample metric may require sequential processing of samples, compared to a frame-by-frame metric where a complete set of samples (frame) is processed together.

[0024] The number of samples in a frame can depend on the sampling rate. For example, in FMCW LIDAR applications, the chirp rate can range from 3 to N x 10 GHz within 50 microseconds, where N is an integer representing a multiple of 10 GHz. In high-speed data communications applications, the chirp rate can range from 1 to N x 100 GHz within 50 to 100 picoseconds. This allows for a compact, adaptive, and integrated laser linear chirp control loop.

[0025] Additionally, the laser system enables and facilitates direct tracking of both positive and negative chirps, as the angular diversity signal provides information on whether the first or second signal has a higher frequency.

[0026] The control unit may be further configured to estimate the instantaneous frequency of the FMCW optical signal. The estimation of the instantaneous frequency may be a phase-based estimation. In other words, the instantaneous frequency may be estimated based on the phase of the angle diversity signal. Thus, the frequency at any given moment may be determined by analyzing the phase change of the angle diversity signal over time. By monitoring how the phase changes, the frequency variation of the FMCW optical signal may be calculated.

[0027] The optical measurement unit may include an interferometer structure, such as an asymmetric Mach-Zehnder interferometer (AMZI), configured to split a portion of the FMCW optical signal into a first signal and a second signal and delay the second signal relative to the first signal.

[0028] In other words, an interferometer structure may be configured to split a portion of an FMCW optical signal into two separate signals, one of which is delayed relative to the other. An asymmetric Mach-Zehnder interferometer is a particular configuration of such an interferometer structure that allows precise control over the splitting and delay of the optical signal.

[0029] Therefore, creating a controlled delay between the first and second signals can result in accurate measurements. Additionally, the use of AMZI can reduce noise and increase the clarity of the measurements.

[0030] Finite-difference methods can be used to estimate the instantaneous frequency of an optical signal. Such techniques have the advantage of a small delay between the first and second signals, which results in greater accuracy because the finite-difference approximation corresponds closely to the derivative (instantaneous frequency) of the optical signal. The delay can be adjusted based on the rate of change of the laser frequency. However, if the frequency change is slow, an extremely small delay (e.g., nanoseconds to microseconds) is not necessary to maintain good accuracy.

[0031] For high speed applications such as frequency modulation in data communication systems (eg, 50 GBaud / s on-off modulation), the delay may be less than 20 picoseconds (eg, about 10 picoseconds).

[0032] A smaller delay can result in a more accurate instantaneous frequency estimation, but may also result in a lower signal-to-noise ratio (SNR). Therefore, selecting an appropriate delay can balance accuracy and performance to ensure optimal results for various applications. Therefore, the delay induced by the interferometer structure can be configured differently depending on whether the optical signal from the chirped laser source has slow or rapid frequency changes.

[0033] In one example, the second signal can be configured to be delayed by a relatively short delay relative to the first signal. The second signal can be delayed relative to the first signal by a delay line having a length in the range of 1 cm to 150 cm, preferably 5 cm to 50 cm, and more preferably 10 cm to 20 cm. Thus, a relatively short delay line can be used.

[0034] For example, a 10 cm path length difference between the first and second signals may allow for accurate measurements, whereas a 25 cm path length difference may provide approximately the same accuracy as a 200 cm difference.

[0035] For on-chip designs, a path length difference of less than 25 cm can be considered to maintain compactness and integration efficiency, thus enabling a compact form factor with delay line lengths in the range of, for example, tens of centimeters.

[0036] A path length difference of 10 cm can correspond to a time delay of about 0.3 nanoseconds in free space (or 0.5 nanoseconds in fiber), and a difference of 25 cm can result in a time delay of about 0.8 nanoseconds in free space (or 1.2 nanoseconds in fiber).

[0037] In general, the optical measurement unit may be configured to split a portion of the FMCW optical signal into a first optical signal propagating along a first path and a second optical signal propagating along a second path. The second path may have a preset delay relative to the first path. As an example, the preset delay may be provided by the first path and the second path having different optical path lengths. The preset delay may be, for example, a known preset delay. The preset delay may be provided by one of the first path and the second path being longer than the other. Longer paths can be achieved by guiding the light in a loop or spiral, thereby providing long path lengths in a compact solution.

[0038] After passing through the first and second paths, respectively, the first and second optical signals are combined again. When the first and second optical signals are combined, interference between the optical signals may occur. As a result, the combined signal may form a beat frequency. The beat frequency is linked to a preset delay of the optical measurement unit. For example, the beat frequency may be proportional to the preset delay.

[0039] The optical measurement unit may further include an optical hybrid coupler configured to output at least two angle diversity signals based on interference between the first signal and the second signal.

[0040] The optical hybrid coupler may be, for example, a 120-degree optical hybrid coupler or a 90-degree optical hybrid coupler.

[0041] A 120-degree optical hybrid coupler is a device that splits an input optical signal into three output signals (i.e., three angle-diversity signals) with a fixed phase difference of 120 degrees between each pair of outputs. The 120-degree phase shift ensures that the signal phases are evenly spaced.

[0042] A 90-degree optical hybrid coupler, also known as a quadrature coupler, splits an input signal into two output signals with a 90-degree phase difference (i.e., two angle diversity signals).

[0043] Furthermore, the light measurement unit may comprise at least two photodiodes for detecting at least two angle diversity signals.

[0044] A photodiode may generally be any unit or device that includes a photosensitive element configured to detect the intensity of light incident on the photosensitive element, generate an electrical signal in response thereto, and allow the electrical signal to be read out.

[0045] The photodiodes may be single-ended and / or balanced photodiodes (BPDs). A balanced photodiode here refers to a device comprising two photodiodes connected in series. When the two photodiodes detect the same level of light, i.e., when their generated electrical signals are equal, the electrical signals cancel each other. This arrangement allows for the detection of small differences in light levels on the two photodiodes.

[0046] In particular, the single-ended photodiode detects the strength of the angle diversity signal directly from the output of the optical hybrid coupler. In contrast, the balanced photodiode measures the difference in strength between the two output angle diversity signals (i.e., a pair of outputs), which can help cancel common-mode noise and improve the signal-to-noise ratio.

[0047] In general, the optical measurement unit may include three components: an optical AMZI with a short arm length difference (e.g., 20 cm or tens of cm to support on-chip structures), an optical hybrid coupler, and an optical detection (e.g., a single-ended photodiode or BPD).

[0048] Regarding optical hybrid couplers and angle diversity signal detection, a 120-degree hybrid coupler-based design may include, but is not limited to, a 3x3 MMI followed by three single-ended photodiodes. Alternatively, a 90-degree hybrid coupler may be used, for example, by implementing a 2x4 or 4x4 MMI followed by two BPDs. In general, an optical hybrid coupler may include at least one MMI (or directional coupler).

[0049] Incorporating at least one MMI or directional coupler into an optical hybrid coupler provides precise control over the splitting and combining of optical signals. Additionally, the use of such couplers may enable compact and integratable designs (e.g., suitable for on-chip integration), i.e., reducing the size and complexity of the optical measurement unit.

[0050] A 90-degree hybrid coupler structure may reduce phase noise by using balanced detection, while a 120-degree hybrid coupler-based approach may have a less complex MMI structure, fewer manufacturing errors, and / or a smaller footprint. A 120-degree optical hybrid coupler design may use a single-ended photodiode (compared to a BPD), for example, which may reduce the number of components required. Fewer components means fewer radio-frequency connections are required.

[0051] Additionally, using a single-ended photodiode can enable a simpler transimpedance amplifier (TIA) structure. A single-ended photodiode generates a current that is directly proportional to the incident light, and this current can be converted to a voltage using a single TIA. Therefore, this conversion process requires fewer components and connections compared to a BPD configuration with differential signals.

[0052] The fixed phase shift may be the same between any pair of at least two angular diversity signals.

[0053] In other words, the fixed phase shift between any two of the at least two angular diversity signals can be the same, such as 90 degrees or 120 degrees, meaning that the phase difference between each pair of angular diversity signals remains constant and equal regardless of whether it is 90 degrees, 120 degrees, or another phase angle.

[0054] The fixed phase relationship can ensure that the angular diversity signals maintain uniform phase alignment. The fixed phase shift can be achieved by an optical hybrid coupler. For example, a 90-degree hybrid coupler can provide a fixed phase shift of 90 degrees, while a 120-degree hybrid coupler can provide a fixed phase shift of 120 degrees between the angular diversity signals output to the control unit.

[0055] The control unit may be configured to repeatedly output a control signal to the tunable laser. Conversely, the control signal may be configured to repeatedly improve the linearity of the frequency chirp. In other words, the control unit may periodically output a control signal based on an estimated compensation required to adjust for the nonlinearity of the frequency chirp of the tunable laser.

[0056] Thus, the tunable laser can be continuously updated, ie, the nonlinearity of the frequency chirp of the tunable laser can be continuously linearized or compensated for.

[0057] The repeat rate may be relatively slow, for example, every 10 seconds. However, in the case of an unstable laser or one that is sensitive to, for example, changes in chirp regime and / or environmental temperature, the nonlinearity of the frequency chirp of the tunable laser may be monitored or detected, for example, when the nonlinearity exceeds a threshold. Thus, the control signal may be configured to improve the linearity of the frequency chirp as needed to optimize the performance of the tunable laser.

[0058] The threshold may be defined by a figure of merit (i.e., a key performance indicator (KPI) value). For example, the figure of merit (r) may be: It can be extracted from the formula like TIFF2026000856000002.tif10150, where: TIFF2026000856000003.tif6150 [Hz] is the root mean square of the nonlinear component of the frequency chirp (ramp up or ramp down), TIFF2026000856000004.tif6150 [Hz] is the frequency modulation width of the wavelength tunable laser.

[0059] In one example, if the figure of merit exceeds 1e-5, the linearity of the frequency chirp may be considered not to be sufficiently linear. The threshold may indicate that the linearity of the frequency chirp needs adjustment or improvement.

[0060] The monitoring of nonlinearity may be adaptive, in other words, if nonlinearity is detected, the linearity may be improved at a faster rate.

[0061] The control unit may be further configured to calculate frequencies and / or phases corresponding to the at least two angular diversity signals by creating a vector and calculating the (phase) angle of the vector, where the frequency and / or phase may be an instantaneous frequency or phase (angle), respectively.

[0062] The vector may be created by the sum of at least two angular diversity signals, for example, the angular diversity signals may be rotated relative to each other in the complex plane.

[0063] For a 120-degree optical hybrid coupler, first (S1), second (S2), and third (S3) angular diversity signals may be provided, so that a vector may be formed as S1 + S2 * exp(j2π / 3) + S3 * exp(j4π / 3), where j is a complex number. Thus, the second angular diversity signal is rotated 120 degrees in the complex plane relative to the first angular diversity signal, and the third angular diversity signal is rotated 120 degrees in the complex plane relative to the second angular diversity signal.

[0064] Similarly, for a 90-degree hybrid coupler, two detected angular diversity signals (S1, S2) are formed as the angular diversity signals are detected by the BPD, and the vector can be expressed as S1 + S2 * exp(jπ / 2). Therefore, the second angular diversity signal is rotated 90 degrees in the complex plane with respect to the first angular diversity signal.

[0065] The resulting vector angles (i.e., phase angles) may be extracted using, for example, a vector angle calculator. Furthermore, the phase angles may be unwrapped using a method involving the addition or subtraction of multiples of 2π to eliminate discontinuities and maintain smooth transitions between successive phase values. Such unwrapping processes may convert phase angles that are initially outside the standard range of -π to π into a continuous sequence.

[0066] The laser system may further include an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). The ADC may be configured to convert an analog input signal to the control unit into a digital signal based on the at least two angle diversity signals. Furthermore, the control unit may be configured to convert the digital signal into the complex domain, calculate a phase angle, estimate nonlinearity, perform pre-distortion calculations, and / or output a processed digital signal, and the DAC may be configured to convert the processed digital signal into a control signal.

[0067] Therefore, phase calculation and accumulation in the complex domain is possible compared to the real domain. The use of complex signals can enable real-time positive / negative chirp measurements. Also, time delays can be reduced without sacrificing accuracy in estimation performance (e.g., chirp edges between up-ramp and down-ramp, and vice versa).

[0068] The at least two angular diversity signals may be analog signals. The at least two angular diversity signals may then be converted into digital signals by an ADC. In other words, the ADC may be configured to convert the detected signals from analog detected signals to digital detected signals and send the digital detected signals to the control unit.

[0069] Additionally, the laser system may include TIAs configured to amplify the detected signals (e.g., one TIA per detected signal), so that the amplified signals can be sent to the ADC.

[0070] The ADC may be associated with at least two photodiodes configured to detect, for example, at least two angular diversity signals, each of which may be converted to a digital signal by a respective ADC.

[0071] The digital (i.e., converted) angular diversity signal may then be sampled by the control unit, which may then convert the digital signal to the complex domain.

[0072] In other words, a complex signal may be synthesized using multiple real (digital) signals, for example at least two real signals. A complex signal may be represented, for example, by a vector, as described above.

[0073] Based on the complex signal, the (phase) angle may be calculated.

[0074] It is understood that the wavelength of the angle diversity signal may be nonlinearly mapped to the driving voltage of the tunable laser, and the mapping between voltage and time is a linear curve. Therefore, the relationship between (phase) angle and time is a nonlinear curve. However, a linear frequency chirp requires a linear angle-time (i.e., angle-voltage) mapping. Therefore, the measured nonlinear angle-time mapping can be used to estimate the nonlinearity of the frequency chirp. In one example, the nonlinearity can be estimated by unwrapping the phase angle and comparing it to a linear response (i.e., by measuring the deviation of the actual output from the output predicted by a linear model).

[0075] Based on the estimated nonlinearity, a predistorted voltage ramp can be obtained. The predistorted voltage ramp can be obtained from the estimated nonlinearity, for example, by least mean squares or linear interpolation. For example, chirp linearization can be implemented iteratively to obtain a higher linear phase. For this purpose, the calculated voltage ramp (up and down) can be stored and applied to the next iteration.

[0076] The calculated predistortion curve can then be used to update the drive voltage of the tunable laser. In particular, the output processed digital signal can comprise the predistortion curve, and thus can be converted into a control signal via a DAC.

[0077] The control unit may further comprise a reference signal, and the control unit and the measurement unit form an optoelectronic phase-locked loop (OEPLL) configured to stabilize the frequency and phase of the FMCW optical signal.

[0078] In other words, the OEPLL is a high-speed, real-time control loop that can lock the laser chirp to a reference linear signal. The reference signal can be, for example, a reference ramp signal, and can change slower (e.g., 100 times slower) than the feedback rate. The estimated error between the measured nonlinear angle-time mapping and the reference ramp signal is used for further calculations, such as proportional amplification, integral calculation, and summation for nonlinear compensation.

[0079] Therefore, the OEPLL allows for faster feedback control and can be used, for example, in fast-changing lasers.

[0080] According to a second aspect, there is provided a LIDAR system comprising a laser system according to the first aspect and a LIDAR detection unit, the LIDAR detection unit configured to receive a reflected optical signal based on an FMCW optical signal.

[0081] This embodiment may generally offer the same or corresponding advantages as the first embodiment.

[0082] Thus, a system for direct laser frequency chirp nonlinearity estimation (and predistortion) in LIDAR systems is enabled. In particular, the LIDAR system enables adaptive frequency chirp nonlinearity monitoring and control to manage the nonlinearity, which may change in response to changing conditions such as external temperature.

[0083] The LIDAR system may further be realized using a miniaturized architecture that can be integrated on a chip such as a PIC. The optical (nonlinearity) measurement unit may be integrated into one and the same LIDAR PIC, such as being compactly co-packaged on the same LIDAR main board as the control unit.

[0084] As described in relation to the first aspect, simplification of the digital signal processing complexity is enabled, for example, by angle diversity detection and phase accumulation. In particular, frequency domain processing techniques may be avoided, i.e., high digital signal processing complexity based Hilbert transform techniques may be avoided.

[0085] The tunable laser of the laser system may include an optical beam splitter disposed in the path of the FMCW optical signal and configured to split the FMCW optical signal into a LIDAR emission signal that can be emitted from the LIDAR system and a portion of the FMCW optical signal that is provided to an optical measurement unit of the laser system. Thus, the "portion of the FMCW optical signal" is the portion of light that is separated from the FMCW optical signal and directed to the optical measurement unit. The remainder of the FMCW optical signal then forms a LIDAR emission signal that can be emitted from the LIDAR system, for example, toward a target.

[0086] In other words, the LIDAR system may split the FMCW optical signal from the tunable laser into two parts: a local oscillator (LO) signal and a transmission (Tx) signal. The FMCW optical signal may be split, for example, by a beam splitter. Typically, the LO signal may constitute a small portion (e.g., 1%) of the optical signal, and the Tx signal may constitute a large portion (e.g., 99%) of the optical signal. The Tx signal is used to detect distance and is reflected back from the target to the LIDAR detection unit.

[0087] In the detection unit, the LO signal and the reflected optical signal may be combined together for coherent detection. This processing may include, for example, the use of receiver optics to collect the reflected optical signal, a 2x2 coupler to combine the LO and reflected signals, a balanced photodetector to detect the combined signal and convert it to an electrical signal, and / or a signal processing unit to analyze the electrical signal, for example to determine the range, velocity, and / or other characteristics of the target.

[0088] Additionally, the LIDAR system may include an optical phased array to facilitate the LIDAR system dynamically steering the Tx signal across the field of view.

[0089] The LIDAR system may be integrated into a vehicle and configured to provide real-time distance and speed data about the vehicle's surroundings.

[0090] In other words, a LIDAR system can be incorporated into a vehicle to continuously monitor and measure the distance and velocity of objects around the vehicle in real time. LIDAR systems may be implemented in devices or systems such as robotics, drones, and various industrial applications to provide real-time distance and / or velocity data, for example.

[0091] Real-time distance and speed data allows vehicles to quickly detect and respond to obstacles and other vehicles. Additionally, when implemented in other devices such as robotics and drones, LIDAR systems improve their ability to navigate their environment and avoid obstacles.

[0092] Furthermore, linear frequency chirp in LIDAR systems can improve range resolution, for example, allowing for more accurate measurement of distances to closely spaced objects. Additionally, linear frequency chirp can improve the signal-to-noise ratio, making it easier to detect weak reflections from distant objects.

[0093] According to a third aspect, there is provided a method for improving frequency chirp linearity of a frequency modulated continuous wave (FMCW) optical signal, the method comprising: generating an FMCW optical signal via a tunable laser; receiving a portion of the FMCW optical signal with an optical measurement unit; outputting, via an optical hybrid coupler, at least two angle diversity signals based on a difference between first and second signals formed by splitting a portion of the FMCW optical signal, wherein the second signal is delayed relative to the first signal, and a pair of signals of the at least two angle diversity signals have a fixed phase shift relative to each other; receiving at least two angular diversity signals at a control unit; estimating a compensation required to adjust for nonlinearity of frequency chirp of the tunable laser based on the at least two angle diversity signals; outputting a corresponding control signal to the tunable laser; Including, The control signal improves the linearity of the frequency chirp.

[0094] This embodiment may generally offer the same or corresponding advantages as the first and second embodiments.

[0095] The step of estimating the compensation required to adjust the nonlinearity of the frequency chirp of the tunable laser may include converting an analog signal to a digital signal based on the at least two angle diversity signals, converting the digital signal to a complex domain, calculating a phase angle, estimating the nonlinearity, performing a pre-distortion calculation, outputting the processed digital signal, and / or converting the processed digital signal to a control signal.

[0096] The analog signal may be converted to a digital signal via an ADC, which may be multiple ADCs, and a DAC may convert the processed digital signal into a control signal.

[0097] The digital signal may be transformed into the complex domain by forming a vector in the complex plane based on the angular diversity signal.

[0098] The phase angle may be calculated by calculating the angle of the vector in the complex plane.

[0099] The phase angle can be mapped to time so that the nonlinearity of the frequency chirp can be estimated based on the (nonlinear) curve of the angle-time mapping. For example, the nonlinearity can be estimated by unwrapping the phase angle and comparing it to a linear response (i.e., by measuring the deviation of the actual output from the output predicted by a linear model).

[0100] Based on the estimated nonlinearity, the pre-distortion calculation can be performed, for example, by forming a pre-distorted voltage ramp with least mean squares or linear interpolation.

[0101] The calculated pre-distortion curve can then be used to update the drive voltage of the tunable laser, thereby improving the linearity of the frequency chirp.

[0102] The method may further comprise repeating the steps of the method described in the second aspect.

[0103] In other words, the control signal may be repeatedly output to the tunable laser to repeatedly improve the linearity of the frequency chirp. The control unit may continuously output the control signal based on the estimated compensation required to adjust for the nonlinearity of the frequency chirp of the tunable laser.

[0104] The method may further include, in the optical measurement unit, splitting a portion of the FMCW optical signal into a first signal and a second signal in an interferometer structure, delaying the second signal relative to the first signal, and detecting the at least two angle diversity signals via at least two photodiodes.

[0105] In other words, the optical measurement unit may comprise an interferometer structure for splitting off a portion of the FMCW optical signal.

[0106] A portion of the FMCW optical signal can be split, for example, so that 50% of the portion of the FMCW optical signal is delayed relative to the remaining 50%. However, other splitting ratios can also be used. In principle, the first and second signals (after the delay line) should be equal. If the delay line has high losses, the delay line can be used to allocate more power to the second signal to compensate for the losses. If the power is not split equally, the signal-to-noise ratio (SNR) of the recombined signal is reduced. However, this method can work effectively with unequal splitting, but performance degrades as the SNR decreases.

[0107] The effects and features of the first, second, and third aspects are generally similar. Examples mentioned for the first, second, and third aspects are generally interchangeable. It should be further noted that the present disclosure relates to all possible combinations of features unless expressly stated otherwise. [Brief explanation of the drawings]

[0108] The above, as well as additional objects, features, and advantages of the present specification, will be better understood through the following illustrative and non-limiting detailed description, taken in conjunction with the accompanying drawings, in which like reference numerals are used for like elements unless otherwise noted.

[0109] [Figure 1] 1 illustrates a schematic diagram of a laser system for generating an FMCW optical signal, where a control signal is configured to improve the linearity of the frequency chirp of the FMCW optical signal. [Figure 2] 10 illustrates a schematic diagram of an optical measurement unit forming a 120-degree optical hybrid coupler. [Figure 3] 10A and 10B show schematic diagrams of an optical measurement unit forming a 90-degree optical hybrid coupler. [Figure 4] FIG. 1 shows a block diagram of the signal processing steps. [Figure 5A] 1 illustrates a schematic representation of a LIDAR system comprising a laser system. [Figure 5B] 1 illustrates a schematic graph of linear and nonlinear frequency chirp. DETAILED DESCRIPTION OF THE INVENTION

[0110] 1 illustrates a laser system 100 for generating a frequency-modulated continuous wave (FMCW) optical signal 112. The system 100 includes a tunable laser 110 for generating the FMCW optical signal 112, and an optical measurement unit 120 configured to receive a portion 114 of the FMCW optical signal 112 and output at least two angle-diversity signals 124 based on a difference between first and second signals 114a, 114b formed by splitting the portion 114 of the FMCW optical signal 112 via an optical hybrid coupler 122, where the second signal 114b is delayed relative to the first signal 114a, and where the pair of signals of the at least two angle-diversity signals 124 have a fixed phase shift relative to each other.

[0111] The laser system 100 further comprises a control unit 130 configured to receive the at least two angle diversity signals 124, estimate a compensation required to adjust for nonlinearity of the frequency chirp of the tunable laser 110 based on the at least two angle diversity signals 124, and output a corresponding control signal 132 to the tunable laser 110. The output control signal 132 is configured to improve the linearity of the frequency chirp of the FMCW optical signal 112 generated by the tunable laser 110.

[0112] The tunable laser 110 that generates the FMCW optical signal 112 may be a laser whose output frequency can be adjusted or changed over a range. The tunability may enable the laser to be adjusted to generate the FMCW optical signal 112 with a linearized frequency chirp.

[0113] The portion 114 of the FMCW optical signal 112 received by the optical measurement unit 120 may be relatively small compared to the remaining propagating portion 116 of the FMCW optical signal 112. The portion 114 may be, for example, a small portion (e.g., approximately 1%) of the FMCW optical signal 112. Similarly, the remaining propagating portion 116 may represent approximately 99% of the FMCW optical signal 112. However, these percentages are exemplary and may vary depending on the specific requirements of the laser system 100. The division of the FMCW optical signal 112 into portions may be such that a sufficient amount of light is available for both measurement for linear chirp improvement and propagation to a target. The portion 114 may be separated from the FMCW optical signal 112 using, for example, an optical beam splitter or similar device configured to direct a small portion of the light to the optical measurement unit 120 while allowing the majority of the FMCW optical signal 112 to continue propagating.

[0114] The optical measurement 120 unit may be, for example, a nonlinearity measurement module based on an asymmetric Mach-Zehnder interferometer (AMZI) structure.

[0115] An AMZI structure may be an optical device configured to split an optical signal into two separate paths, each with a different optical path length, that eventually recombine. The AMZI can detect changes in the optical path length difference by analyzing the interference pattern that results when the optical paths merge.

[0116] 1, the optical path of the second signal 114b is shown as being longer than the optical path of the first signal 114a (see the dashed jagged line). However, the optical path of the first signal 114a may be longer than the optical path of the second signal 114b. Alternatively, the paths may include different materials. The different materials may affect the propagation speeds of the first and second signals 114a, 114b such that one of the signals 114a, 114b is delayed relative to the other signal.

[0117] The optical paths of the first signal 114 a and the second signal 114 b may be provided by waveguides for guiding the first signal 114 a and the second signal 114 b, respectively. Thus, the light measurement unit 120 may include a first waveguide for guiding the first signal 114 a and a second waveguide for guiding the second signal 114 b.

[0118] An optical hybrid coupler 122 is a device that can be used to combine or split optical signals. It typically consists of multiple input and output ports, allowing signals from different sources to be mixed or a single signal to be distributed to multiple destinations. Thus, the optical hybrid coupler 122 operates based on the principle of interference, where input optical signals are combined such that their phase and amplitude are manipulated to achieve a desired output. In FIG. 1, the input signals to the optical hybrid coupler 122 are the first and second signals 114a, 114b, and the output is the angle diversity signal 124.

[0119] Thus, optical interference occurs within the optical hybrid coupler 122, such as within a multi-mode interference coupler. When the two optical signals 114a, 114b that have traveled different optical path lengths recombine in the optical hybrid coupler 122, they interfere with each other. The interference pattern is a result of a phase difference between the two signals 114a, 114b. In one example, the optical hybrid coupler 122 may be a multi-mode interferometer (MMI). The optical hybrid coupler 122 may, for example, comprise at least one MMI.

[0120] The resulting combined optical signal, i.e., the resulting angle diversity signal 124, may then be directed to a photodiode (not shown in FIG. 1). Each signal detected by the photodiode may represent an interference product.

[0121] 1, the optical measurement unit 120 includes an optical hybrid coupler 122 configured to output at least two angle-diversity signals 124 based on the interference (i.e., difference) between the first signal 114a and the second signal 114b. However, it is understood that the optical hybrid coupler 122 may be located separately from, for example, the waveguides used to guide the first and second signals 114a, 114b. Thus, the first and second signals 114a, 114b may be output from the waveguides and received by the optical hybrid coupler 122, which then outputs the at least two angle-diversity signals 124 to the control unit 130.

[0122] The control unit 130 may repeatedly output the control signal 132 to the tunable laser 110. Thus, the control signal 132 may repeatedly improve the linearity of the frequency chirp. The repeatedly output control signal 130 may correspond to an iteratively estimated compensation required to adjust the nonlinearity of the frequency chirp of the tunable laser 110.

[0123] The control unit 130 may be a microcontroller, for example a simple microcontroller, in other words, the control unit 130 may be low in complexity and / or easy to integrate on-chip.

[0124] It should be understood that the control unit 130 may be implemented as a general-purpose processing unit, e.g., a central processing unit (CPU), capable of executing instructions of one or more computer programs to implement the functions of the control unit 130. The control unit 130 may also, or alternatively, be implemented as firmware located in, e.g., an embedded system, or as a specially designed processing unit, such as an application specific integrated circuit (ASIC) or field programmable gate array (FPGA). It should be understood that the control unit 130 may be implemented as a combination of hardware and software components.

[0125] Although not shown in detail in FIG. 1 , the laser system 100 may be integrated on a single semiconductor chip. In particular, all or some of the components of the laser system 100 may be integrated on a single chip. For example, the light measurement unit 120 and the controller 130 may be integrated on the same chip (such as a PIC further provided with electronic circuitry). The tunable laser 110 may be located, for example, outside the chip. However, it will be understood that the tunable laser may be further located or integrated on the same chip as the light measurement unit 120 and the controller 130.

[0126] The control unit 130 may further include a reference signal (not shown in FIG. 1), and the control unit 130 and the measurement unit 120 form an optoelectronic phase-locked loop (OEPLL) configured to stabilize the frequency and phase of the FMCW optical signal 112.

[0127] The OEPLL can be used to lock the laser frequency chirp to a ramp signal in real time. The phase angle of the detected beat signal can then be compared to a reference ramp signal that can change slower (e.g., at a rate of 1 kHz) than the feedback rate (e.g., 100 kHz). The corresponding difference (i.e., error) based on the comparison can be used for further calculations, such as proportional amplification, integral calculations, and / or summation for nonlinearity compensation.

[0128] FIG. 2 shows a schematic representation of the light measurement unit 120 .

[0129] A portion 114 (also denoted λ) of the FMCW optical signal is received by an interferometer structure 121 configured to split the portion 114 of the FMCW optical signal into a first signal 114a and a second signal 114b. The portion 114 of the FMCW optical signal is split into the first and second signals 114a, 114b by a splitter 121s or a 1×2 coupler. The splitter may be, for example, a beam splitter. In one example, the portion 114 of the FMCW optical signal is split 50-50, with the first signal 114a representing 50% of the portion 114 and the second signal 114b representing 50% of the portion 114 of the FMCW optical signal.

[0130] The interferometer structure 121 further comprises an optical path configured to delay the second signal 114b relative to the first signal 114a. Here, the optical path corresponding to the second signal 114b is shown as having a looped optical path (e.g., a fiber loop), i.e., a longer optical path than the optical path of the first signal 114a, thereby forming a delay line. The longer optical path may be, for example, about 20 cm longer than the shorter optical path. However, it will be understood that the delay may be achieved by any suitable delay line or signal delay technique, such as by propagating the signals 114a, 114b through materials with different refractive indices.

[0131] A portion 114 of the FMCW optical signal 112 is an electric field having unit amplitude, TIFF2026000856000005.tif6150, and the first signal 114a is an electric field, TIFF2026000856000006.tif6150, and the second signal 114b is an electric field, These signals can be represented as follows: TIFF2026000856000008.tif946, TIFF2026000856000009.tif957, TIFF2026000856000010.tif865 where, TIFF2026000856000011.tif6150 is the phase of the FMCW optical signal, TIFF2026000856000012.tif6150 is the time delay on the longer optical path.

[0132] The optical hybrid coupler 122 in Figure 2 is a 120-degree optical hybrid coupler 122. The 120-degree optical hybrid coupler 122 splits an input optical signal into three output angular diversity signals 124 with a fixed phase difference of 120 degrees between each pair of outputs. Notably, as can be seen in Figure 2, the fixed phase shift (α) is the same between any pair of angular diversity signals 124. Specifically, in Figure 2, the fixed phase shift is 120 degrees (i.e., α = 120°).

[0133] Although not explicitly shown in FIG. 2, the 120-degree hybrid coupler 122 may be based on a directional coupler or a 3×3 MMI, such as a 3×3 multimode interference coupler. A 3×3 MMI coupler can split and combine input optical signals using multimode interference. A 3×3 MMI coupler may include a multimode waveguide section in which multiple modes interfere constructively and destructively, resulting in a desired power distribution between the output ports of the 120-degree hybrid coupler 122.

[0134] In the case of a 3x3 MMI coupler, the first and second signals 114a, 114b may be input to any two of the three input ports of the 3x3 MMI coupler. Therefore, one input port of the 3x3 MMI coupler may remain unused or may not receive a signal. In one example, a 2x3 MMI coupler may be utilized.

[0135] 2, the optical hybrid coupler 122 is designed to achieve a 120-degree phase shift between its output signals (i.e., angular diversity signals 124). This can be achieved by selecting dimensions such as the width and length of the multimode waveguide to ensure the correct / desired interference pattern. Thus, the 120-degree hybrid coupler 122 provides three output angular diversity signals 124 with equal amplitude signals that are 120 degrees phase shifted relative to each other.

[0136] 2, the angle diversity signal 124 is detected by three photodiodes 126. The light measurement unit 120 including the 120-degree optical hybrid coupler 122 may utilize, for example, three single-ended photodiodes 126. Each photodiode 126 is positioned to detect the intensity of light from a respective output of the 120-degree optical hybrid coupler 122 (i.e., a respective angle diversity signal 124).

[0137] In this configuration, the 120-degree optical hybrid coupler 122 splits the input signals 114a and 114b into three separate paths, each with a 120-degree phase difference. The photodiode 126 is then used to form three detected angle diversity signals S1, S2, and S3 based on the signals from the three separate paths. The angle diversity signals can be expressed as follows: TIFF2026000856000013.tif1296, TIFF2026000856000014.tif13107, TIFF2026000856000015.tif14106 where, TIFF2026000856000016.tif6150 is the response of the photodiode 126.

[0138] In a particular example, the optical measurement unit 120 with the 120-degree optical hybrid coupler 122 may utilize three photodiodes and / or three TIAs (not shown).

[0139] Figure 3 illustrates another optical measurement unit 120. To avoid undue repetition, it should be understood that Figure 3 illustrates an optical measurement unit 120 similar to that described in connection with Figure 2. However, in Figure 3, the optical hybrid coupler 122 is a 90-degree optical hybrid coupler 122. This type of hybrid coupler is designed to produce a 90-degree phase shift between the output signals.

[0140] The 90-degree optical hybrid coupler 122 splits the input signals 114a, 114b into four output signals (ie, four angle diversity signals 124).

[0141] The 90-degree optical hybrid coupler may use the principles of multimode interference to achieve this splitting. In particular, although not explicitly shown in FIG. 3, the 90-degree hybrid coupler 122 may comprise a 2×4 MMI, a 4×4 MMI, or a directional coupler.

[0142] The four output signals from the 90-degree optical hybrid coupler 122 are then detected by four photodiodes 124, respectively. These photodiodes 124 are arranged to form two balanced photodiodes 128. Each balanced photodiode 128 measures the difference in optical intensity between two of the output angular diversity signals 124 (this differential measurement can help cancel common-mode noise and increase the accuracy of the detected signal).

[0143] A balanced photodiode 128 processes the four output angular diversity signals 124 to generate two detected angular diversity signals S1 and S2. These detected angular diversity signals S1 and S2 have a phase difference of 90 degrees between them (i.e., α=90°). The conversion from four signals to two signals is achieved by the balanced photodiode 128, which combines intensity information from the pairs of output angular diversity signals to generate the detected angular diversity signals S1 and S2.

[0144] In a particular example, the optical measurement unit 120 with the 90-degree optical hybrid coupler 122 may utilize four photodiodes forming two balanced photodiodes and / or two TIAs (not shown).

[0145] FIG. 4 illustrates a block diagram of the signal processing steps of the laser system 100.

[0146] 4, it can be seen that the laser system 100 includes an ADC and a DAC. The ADC may be located inside or outside the control unit 130, and the DAC may be located inside or outside the control unit 130.

[0147] As shown in FIG. 4, the ADC converts the analog input signal to the control unit 130 into a digital signal based on the at least two angle diversity signals 124 .

[0148] Furthermore, the control unit 130 converts the digital signal into the complex domain, where the digital signal can be represented as a vector in a complex plane. Based on the detection angle diversity signal described above, in the case of a 120-degree optical hybrid coupler, the complex signal TIFF2026000856000017.tif6150 can be expressed as follows: TIFF2026000856000018.tif13139

[0149] The phase component of the beat signal (e.g., S1) can be extracted. In particular, as described herein, the angle diversity signal (i.e., the phase diversity amplitude signal) is used to calculate the phase signal of the beat signal. This is in contrast to methods where the phase is extracted using a Hilbert transform and the beat signal is a real signal.

[0150] In other words, angle (i.e., phase) diversity detection is used to obtain angle (i.e., phase) diverse beat signals S1, S2, and S3. Two or more detections that provide angle (phase) information are provided, resulting in angle detection diversity, which can be used for simple angle determination based on the detection.

[0151] Real signal ( TIFF2026000856000019.tif6150 ) instantaneous phase ( The signal (TIFF2026000856000020.tif6150) can be extracted using mathematical calculations to convert it into the complex domain (IQ without DC). IQ stands for the in-phase (I) and quadrature (Q) components of the signal, and "DC without" means that the direct current component has been removed or is not considered.

[0152] In a sense, the IQ signal is another type of S1 signal, but in the complex domain (i.e., with the same phase). By creating an IQ signal, the phase of S1 can be easily extracted (e.g., because complex signals make it easier to extract the phase accurately in real time).

[0153] The phase of the signal S1 then corresponds to the instantaneous frequency of the laser. TIFF2026000856000021.tif6150) is understood to correspond to finite differencing of the laser phase and thus to an estimation of the instantaneous laser frequency.

[0154] Therefore, the phase of the beat signal can be extracted by: TIFF2026000856000022.tif13105

[0155] The interference is It is measured with a fixed delay, denoted as TIFF2026000856000023.tif6150, which is small but not zero. When TIFF2026000856000024.tif6150 approaches zero, the signals are identical with no delay, so there is no beat signal. If TIFF2026000856000025.tif6150 is relatively large (or too large), TIFF2026000856000026.tif6150 and The difference between the phases measured in TIFF2026000856000027.tif6150 is taken into account, which affects the accuracy of the measurement.

[0156] moreover, If TIFF2026000856000028.tif6150 is zero, the result essentially corresponds to a direct current (DC) component, and therefore there is no beat signal. Therefore, the method may use phase finite differences to estimate the phase derivative, and therefore the instantaneous frequency. From this, TIFF2026000856000029.tif6150 must not be zero. However, TIFF2026000856000030.tif6150 can be appropriately selected based on the rate of change of the laser frequency. A smaller TIFF2026000856000031.tif6150 may increase the accuracy of the method, but at the cost of a lower signal-to-noise ratio (SNR).

[0157] In a scenario where the laser frequency changes rapidly, TIFF2026000856000032.tif6150 is small enough (i.e., smaller than the rate of change of frequency) that these changes can be tracked accurately. Conversely, if the laser frequency changes slowly, a larger TIFF2026000856000033.tif6150 can be used.

[0158] To achieve this, the delay line described herein may be a small delay line, for example to record the instantaneous frequency. However, this delay cannot be made too small, as otherwise the beat signal will disappear. The length of the delay line may be adapted or optimized based on these constraints. Specifically, TIFF2026000856000034.tif6150 may be selected according to the rate of change of the laser frequency.

[0159] In some cases, a small fixed delay is added to the delay line to derive the instantaneous frequency by analyzing how the angle of the complex signal changes over time. It may be feasible to use a delay of 1.5 cm. For applications where the frequency changes relatively slowly, such as LIDAR, the delay can be selected based on the fastest changing scenario. Such a delay may be appropriate for low velocities and still provide a good or sufficient SNR. For example, a 5 cm delay is appropriate for chirp rates in the kHz to MHz range.

[0160] In one example, the beat signals of the three angle diversity signals 124 output by the 120-degree optical hybrid coupler-based approach are offset by 120 degrees, as shown in Figure 2. The three signals detected from the three single-ended photodiodes are converted from the analog domain to the digital domain through ADCs (e.g., three ADCs). On the other hand, in the 90-degree optical hybrid coupler-based approach, two beat signals with a 90-degree phase offset are extracted from two balanced photodiodes (as shown in Figure 3).

[0161] The control unit 130 samples three signals S1, S2, S3 for 120 degrees and two signals S1, S2 for 90 degrees.

[0162] Thus, for 120 degrees, the resulting three-dimensional vector in the complex domain can be formed by S1 + S2 * exp(j2π / 3) + S3 * exp(j4π / 3), where j is a complex number. Thus, the second angle diversity signal S2 is rotated 120 degrees (j2π / 3) in the complex plane relative to the first angle diversity signal S1, and the third angle diversity signal S3 is rotated another 120 degrees (j4π / 3) in the complex plane relative to the second angle diversity signal S2.

[0163] Similarly, for 90 degrees, the two detected angular diversity signals S1 and S2 are formed into a two-dimensional vector in the complex domain, which is expressed as S1 + S2 * exp(jπ / 2). Therefore, the second angular diversity signal S2 is rotated by 90 degrees (jπ / 2) in the complex plane relative to the first angular diversity signal S1.

[0164] The instantaneous phase angle of the signal in the complex domain can then be calculated as follows: This phase angle corresponds to the instantaneous frequency of the FMCW optical signal.

[0165] The phase angle can be calculated by determining the angle of the vector formed using equation (1) for 120 degrees or equation (2) for 90 degrees. TIFF2026000856000036.tif16143 TIFF2026000856000037.tif15125

[0166] In equations (1) and (2), j is a complex number, ANGLE is a calculator that finds the angle of a vector, and UNWRAP is a calculator that unwraps a radian phase angle by adding multiples of ±2π.

[0167] This allows the phase angles to be calculated. In other words, the control unit 130 now calculates the frequencies / phases (i.e., phase angles) corresponding to the at least two angular diversity signals 124 by creating a vector and calculating the angle of the vector.

[0168] Furthermore, the ANGLE and UNWRAP calculator may, by way of example, be a calculator having the corresponding name Matlab, a programming and numerical calculation program offered by MathWorks, Inc., Natick, Massachusetts, USA.

[0169] Specifically, the UNWRAP calculator adds a multiple of 2π to successive phase angles of a phase angle ramp, provided that the phase difference between the phase angle (of the phase angle ramp) and the successive phase angle (of the phase angle ramp) is greater than or equal to π.

[0170] The operation of adding multiples of 2π to successive phase angles of a phase angle ramp, particularly when the phase difference between the phase angle of the phase angle ramp and the successive phase angle is π or greater, is sometimes referred to as an "unwrap" operation. The purpose of performing the unwrap operation on an extracted phase angle ramp is to eliminate 2π or 360° jumps in the extracted phase angle ramp.

[0171] Note that in equations (1) and (2), signals S1, S2, and S3 are real signals. As seen in equations (1) and (2), signals S1, S2, and S3 combine to form a vector in the complex domain. In other words, by synthesizing a complex signal with multiple real beat signals, measurement of the instantaneous frequency / phase of the resulting beat signal can be achieved.

[0172] This allows the phase angle ramp of the FMCW optical signal to be extracted over time, as can be seen in the lower part of FIG. 4 (i.e., the zoom in on the processing step "Phase Calculation and Accumulation").

[0173] It is understood that the wavelength of the FMCW optical signal is nonlinearly mapped to the drive voltage (i.e., power) of the tunable laser, and the mapping between drive voltage and time is a linear curve. Therefore, the mapping between phase angle and time (similarly, angle vs. voltage) is a nonlinear curve, as shown in Figure 4.

[0174] On the other hand, a linear angle-to-time (angle-to-voltage) mapping corresponds to a linear frequency chirp, so the measured nonlinear mapping can be used to estimate the nonlinearity.

[0175] As shown in Figure 4, after the nonlinearity is estimated, a predistortion calculation is performed. The predistortion calculation may include determining a predistortion curve (i.e., a predistorted voltage ramp) based on the nonlinearity estimation. The predistortion curve is then used to generate the control signal 132 via conversion by a DAC. In other words, the predistortion curve is used to update the drive voltage of the tunable laser 110 via the DAC.

[0176] The predistorted voltage ramp can be calculated from the estimated phase nonlinearity, for example, by least mean squares or linear interpolation. Frequency chirp linearization can be realized, for example, iteratively. Thus, the calculated voltage ramps (up and down) can be stored and applied to subsequent iterations. This iterative approach can also be implemented in the control unit 130.

[0177] In FIG. 4, the illustrated graphic in control unit 130 schematically illustrates the process of using input angle diversity signals (here three) to derive phase angle curves (solid curves), which are used to form pre-distortion curves (dashed curves).

[0178] Although not shown in the figure, a reference phase angle ramp may be subtracted from the extracted phase angle ramp. Therefore, real-time locking of the frequency chirp to a ramp signal can be achieved. In other words, an optoelectronic phase-locked loop (OEPLL) may be provided. The OEPLL may stabilize the frequency and phase of the FMCW optical signal 112, for example.

[0179] The term "reference phase angle ramp" herein means a phase angle ramp that changes much slower than the feedback rate (eg, 100 times slower).

[0180] Subtracting the reference phase angle ramp from the extracted phase angle ramp leaves an estimated error that may be used for further calculations such as, but not limited to, proportional amplification, integral calculations, and / or summation for nonlinearity compensation.

[0181] 5A schematically illustrates a LIDAR system 200 comprising a laser system 100 (e.g., as described in connection with FIGS. 1-4) and a LIDAR detection unit 202. The LIDAR detection unit 202 receives a reflected optical signal 118 based on the FMCW optical signal 112.

[0182] In particular, the LIDAR detection unit 202 of the LIDAR system 200 detects the LIDAR response signal (i.e., the reflected optical signal 118). Thus, the LIDAR response signal may be based on the propagating portion 116 (i.e., the LIDAR emission signal) of the FMCW optical signal 112. The LIDAR detection unit 202 may be, for example, a photodiode, such as a balanced and / or unbalanced photodiode, a photomultiplier tube (PMT), and / or an image detector.

[0183] The LIDAR emitted light signal is typically separated from the propagating portion 116 of the FMCW optical signal 112 before the LIDAR emitted light signal is emitted from the LIDAR system 200. The portion separated from the propagating portion 116 may be referred to as a local oscillator signal. The LIDAR emitted light signal may then be emitted toward the target 204. When the LIDAR emitted light signal reaches the target 204, at least a portion of the LIDAR emitted light signal may reflect back toward the LIDAR system 200. The reflected LIDAR emitted light signal (i.e., the reflected optical signal 118) and the local oscillator signal may then be recombined at the LIDAR system 200. When the two optical signals are combined, interference between the optical signals may occur. As a result, the combined signal may form a beat frequency. This beat frequency is related to the distance between the LIDAR system 200 and the target 204. In the present LIDAR system 200, the combined signal may be detected by a LIDAR detection unit 202 to detect the reflected light signal 118 (i.e., the LIDAR response signal).

[0184] 5A further illustrates a block diagram of a LIDAR system 200, which includes a tunable laser 110 (e.g., a fast frequency-chirp laser tunable by phase or gain), an optical measurement unit 120 (e.g., for real-time chirp tracking), and a control unit (for complex-domain nonlinearity estimation and pre-distortion calculation). Here, the angle diversity signal 124 output by the optical measurement unit 120 is converted by an ADC before entering the control unit 130. A pre-distortion curve (as described in connection with FIG. 4) is then used to compensate for the inherent nonlinearity in the frequency chirp via a control signal 132. The control signal 132 is output from the control unit 130 via a DAC 136.

[0185] In FIG. 5A, the LIDAR system 200 further comprises an optical phased array or optical antenna 206 for dynamically steering the propagating portion 116 of the FMCW optical signal 112 across the field of view.

[0186] In the ideal case where the frequency chirp is linear, the range resolution of the LIDAR system 200 (based on the FMCW optical signal 112) is given by equation (3). TIFF2026000856000038.tif1489 where, TIFF2026000856000039.tif6150 [m / s] is the speed of light in a vacuum, TIFF2026000856000040.tif6150 is the frequency modulation width of the wavelength tunable laser 110.

[0187] However, if the frequency chirp is nonlinear, the width of the corresponding peak in the beat spectrum increases, degrading the distance accuracy (see the bottom of Figure 5B). The broadening or broadening of the peak caused by the nonlinearity of the frequency chirp can be quantified by Equation (4). TIFF2026000856000041.tif1396 where D [m] is the distance to the target 204, TIFF2026000856000042.tif6150 [Hz] is the root mean square of the nonlinear component of the frequency chirp (ramp-up or ramp-down). From equation (4), the nonlinearity of the laser If TIFF2026000856000043.tif6150 is not suppressed, the target distance The longer TIFF2026000856000044.tif6150, the better the axial resolution. TIFF2026000856000045.tif16153 becomes higher. In other words, the effects of chirp nonlinearity are more pronounced for farther away targets 204. High chirp linearity is thereby beneficial, for example, in LIDAR systems 200 that may measure long target distances (e.g., on the order of hundreds of meters).

[0188] The LIDAR system 200 may be incorporated into, for example, an automobile, and thus may provide accurate real-time distance and speed data (even at long ranges) about the automobile's surroundings.

[0189] In conclusion, the methods and systems described herein achieve high quality linear chirp in FMCW laser systems with reduced complexity architecture and enhanced robustness against environmental variations.

[0190] The inventive concept has been primarily described above with reference to a limited number of examples. However, as will be readily understood by those skilled in the art, other examples than those disclosed above are equally possible within the scope of the inventive concept as defined by the appended claims.

Claims

1. 1. A laser system (100) for generating a frequency modulated continuous wave (FMCW) optical signal (112), the system (100) comprising: a tunable laser (110) for generating said FMCW optical signal (112); an optical measurement unit (120) configured to receive a portion (114) of the FMCW optical signal (112) and output at least two angle diversity signals (124) based on a difference between first and second signals (114a, 114b) formed by splitting the portion (114) of the FMCW optical signal (112) via an optical hybrid coupler (122), wherein the second signal (114b) is delayed relative to the first signal (114a), and the pair of signals of the at least two angle diversity signals (124) have a fixed phase shift relative to each other; A control unit (130) comprising: receiving the at least two angular diversity signals (124); estimating a compensation required to adjust for nonlinearity of frequency chirp of the tunable laser (110) based on the at least two angular diversity signals (124); outputting a corresponding control signal (132) to said tunable laser (110); a control unit (130) configured as follows: Equipped with The laser system (100), wherein the control signal (132) is configured to improve the linearity of the frequency chirp.

2. The light measurement unit (120) an interferometer structure configured to split the portion (114) of the FMCW optical signal (112) into the first signal (114a) and the second signal (114b), and to delay the second signal (114b) relative to the first signal (114a); the optical hybrid coupler (122) configured to output the at least two angle diversity signals (124) based on interference between the first signal (114a) and the second signal (114b); and / or at least two photodiodes for detecting said at least two angular diversity signals (124); The laser system (100) of claim 1, comprising:

3. 3. The laser system (100) of claim 1 or 2, wherein the fixed phase shift is the same between any pair of the at least two angular diversity signals (124).

4. 4. The laser system of claim 1, wherein the control unit is configured to repeatedly output a corresponding control signal to the tunable laser, the control signal being configured to repeatedly improve the linearity of the frequency chirp.

5. The laser system (100) of any one of claims 1 to 4, wherein the laser system (100) is integrated on a single semiconductor chip.

6. The laser system (100) of any one of claims 1 to 5, wherein the optical hybrid coupler (122) comprises at least one multimode interferometer (MMI).

7. 7. The laser system of claim 1, wherein the control unit is further configured to calculate frequencies and / or phases corresponding to the at least two angular diversity signals by creating a vector and calculating an angle of the vector.

8. 8. The laser system (100) of any one of claims 1 to 7, further comprising an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC), wherein the ADC is configured to convert an analog input signal to the control unit (130) into a digital signal based on the at least two angle diversity signals (124), the control unit (130) is configured to convert the digital signal into the complex domain, calculate a phase angle, estimate nonlinearity, perform pre-distortion calculations, and / or output a processed digital signal, and the DAC is configured to convert the processed digital signal into the control signal (132).

9. 9. The laser system (100) of claim 1, wherein the control unit (130) further comprises a reference signal, and wherein the control unit (130) and the measurement unit (120) form an optoelectronic phase-locked loop (OEPLL) configured to stabilize the frequency and phase of the FMCW optical signal (112).

10. 10. A LIDAR system comprising the laser system (100) of any one of claims 1 to 9 and a LIDAR detection unit, the LIDAR detection unit configured to receive a reflected optical signal based on the FMCW optical signal (112).

11. 11. The LIDAR system of claim 10, wherein the LIDAR system is integrated into a motor vehicle and configured to provide real-time distance and speed data about a surrounding of the motor vehicle.

12. 1. A method for improving frequency chirp linearity of a frequency modulated continuous wave (FMCW) optical signal (112), the method comprising: generating said FMCW optical signal (112) via a tunable laser (110); receiving a portion (114) of the FMCW optical signal (112) at an optical measurement unit (120); outputting at least two angle diversity signals (124) based on a difference between first and second signals (114a, 114b) formed by splitting the portion (114) of the FMCW optical signal (112) via an optical hybrid coupler (122), the second signal (114b) being delayed relative to the first signal (114a), and the pair of signals of the at least two angle diversity signals (124) having a fixed phase shift relative to each other; receiving, at a control unit (130), the at least two angular diversity signals (124); estimating a compensation required to adjust for nonlinearity of frequency chirp of the tunable laser (110) based on the at least two angular diversity signals (124); outputting a corresponding control signal (132) to said tunable laser (110); Including, The method of claim 1, wherein the control signal (132) improves the linearity of the frequency chirp.

13. The step of estimating the compensation required to adjust the nonlinearity of the frequency chirp of the tunable laser (110) comprises: converting an analog signal to a digital signal based on the at least two angular diversity signals (124); converting the digital signal to the complex domain; calculating a phase angle; estimating the nonlinearity; performing a pre-distortion calculation; outputting the processed digital signal; and / or converting the processed digital signal into the control signal (132); 13. The method of claim 12, comprising:

14. 14. The method of claim 12 or 13, further comprising repeating the steps of claim 12.

15. In the light measurement unit, splitting the portion of the FMCW optical signal into the first signal and the second signal with an interferometer structure; delaying the second signal relative to the first signal; detecting the at least two angular diversity signals via at least two photodiodes; The method of any one of claims 12 to 14, further comprising:

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