System and method for measuring instantaneous frequency of light signal

Abstract

To provide a system for measuring frequency of a light signal from a chirped laser source.SOLUTION: A system 100 comprises: an optical measurement unit 120 configured to receive at least a portion 114 of a light signal 112 and to output at least two angle diversity signals 124 based on a difference between first and second signals 114a and 114b formed by splitting the light signal via an optical hybrid coupler 122, the second signal being delayed relative to the first signal and a pair of the at least two angle diversity signals having a fixed phase shift relative to each other; and a control unit 130 configured to receive the at least two angle diversity signals, generate a complex signal based on the at least two angle diversity signals, and determine an instantaneous phase of the complex signal to determine an instantaneous frequency of the light signal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The inventive concept relates to the field of optical signal processing, and more particularly to real-time instantaneous frequency measurement of optical signals. [Background technology]

[0002] High-speed linear frequency-modulated continuous-wave (FMCW) lasers are essential in a variety of applications, including absolute distance, velocity, and vibration measurement, light detection and ranging (LIDAR), and optical coherence tomography (OCT). These and other applications depend on precise laser frequency measurements to function properly. However, accurately measuring instantaneous frequency has been a significant challenge due to the lack of suitable methods for high-speed, precise frequency tracking.

[0003] In the prior art, techniques employed to estimate laser frequency include the use of a delayed self-heterodyne interferometer and the Hilbert transform. However, the use of the Hilbert transform has limitations, such as ambiguity in distinguishing between positive and negative chirps and reduced accuracy at low-frequency beats. In addition, methods based on the Hilbert transform require long delay lines, typically using one to several meters of fiber.

[0004] Therefore, there is a need for an improved system and method for accurately and efficiently measuring the instantaneous frequency of a laser. Summary of the Invention

[0005] The objective of the inventive concept is to provide a method and system for enabling accurate and efficient measurement of the instantaneous frequency of a chirped laser source.

[0006] Another object of the inventive concept is to enable on-chip integration of a system for measuring the instantaneous frequency of a chirped laser source.

[0007] Yet another object is to improve the accuracy and reliability of instantaneous frequency estimation in various applications, including, but not limited to, absolute distance measurement, velocity and vibration measurement, LIDAR, and optical coherence tomography (OCT).

[0008] A further object is to enable determination of the instantaneous frequency during the frequency chirp of a chirped laser source.

[0009] A further object is to be able to determine a compensation signal for improving the linearity of an optical signal.

[0010] Additionally, the goal is to enable a compact, integrated laser linear chirp enabler with reduced digital signal processing (DSP) complexity.

[0011] 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.

[0012] In accordance with a first aspect of the inventive concept, there is provided a system for measuring the frequency of an optical signal from a chirped laser source, the system comprising: an optical measurement unit configured to receive at least a portion of an optical signal and output at least two angle diversity signals based on a difference between first and second signals formed by splitting at least a portion of the 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; generating a complex signal based on the at least two angular diversity signals; a control unit configured to determine the instantaneous phase of the complex signal to determine the instantaneous frequency of the optical signal.

[0013] This enables a real-time instantaneous frequency estimation system for optical signals. In particular, this system can improve the accuracy of determining the instantaneous frequency of chirped laser sources, surpassing any need for Hilbert transform-based techniques.

[0014] In certain applications, it may be desirable to determine the instantaneous frequency relative to a reference. However, some applications may require the determination of the absolute frequency. In such cases, calibration may be performed. Calibration may involve comparing the determined instantaneous frequency to a known standard or reference to ensure that the absolute frequency is accurately determined.

[0015] Calibration can convert the relative frequency measurement into an absolute frequency value. By comparing the relative instantaneous frequency to a known reference frequency module, the absolute frequency can be determined.

[0016] In other words, the calibration may map relative frequency measurements to absolute frequencies (eg, by a linear mapping).

[0017] In one example, the linear mapping can be expressed as y=Kx+B, where y represents the absolute frequency, x represents the relative frequency, K represents the frequency change slope, and B represents the offset.

[0018] To perform the calibration, the absolute frequencies of at least two measured samples can be determined. Knowing these samples, a linear mapping function can be calculated. The frequency change slope (K) can be a fixed, known value, determined by the physical characteristics of the system, such as the length of the delay line. For example, K can be specified as 50 GHz per 1π phase shift.

[0019] However, the measured frequency curve may exhibit an offset (B) that varies between different measurements. This offset may arise because the phase calculation is constrained within a 2π range, resulting in ambiguity in accurately identifying the 2π range to which the reference sample (or the first of at least two measurement samples) belongs. As a result, a calibration must be performed for each measurement of instantaneous frequency to accurately determine the instantaneous absolute frequency.

[0020] In other words, the calibration recalibrates the offset (B) for each measurement, ensuring precise and reliable instantaneous frequency measurements.

[0021] The system may utilize an angle-diversity optical delayed self-heterodyne structure for direct complex-domain DSP, thereby effectively mitigating or reducing ambiguities associated with positive and negative chirps and errors related to low beat frequency signals.

[0022] The DSP complexity is reduced by phase accumulation, for example, by splitting a portion of the 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, the DSP complexity can be reduced by avoiding complex techniques based on the Hilbert transform in the frequency domain.

[0023] Generally, the 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 optical signal and a delayed version of the portion of the optical signal. This means that the angular diversity signals may represent a beat signal formed by interference between the first signal and the second signal.

[0024] Thanks to the measurement of the angle diversity signal, simple processing can be performed to measure the instantaneous frequency of the optical signal. The angle diversity signal can be easily processed to determine the frequency or phase of the beat signal. This allows for highly accurate estimation of the instantaneous frequency, even for chirped laser sources with fast frequency chirps. The system can be configured to determine the real-time instantaneous frequency of a fast chirped laser. In particular, the system can improve the accuracy of measuring or estimating the instantaneous frequency of a fast chirped laser source, for example, compared to conventional approaches.

[0025] The high accuracy of determining the frequency / phase of the beat signal allows for the use of a short delay of the second signal, which can be achieved by physically guiding the second signal along an optical path. The ability to use a short delay also allows the system to be compact.

[0026] Furthermore, as described herein, the system's control unit architecture reduces algorithmic complexity, thereby reducing potential DSP-induced delays.

[0027] In general, the system enables ultrafast, low-cost, and accurate instantaneous frequency measurement or estimation of optical signals.

[0028] This system may be referred to as a direct complex domain optical phase finite difference (OPFD) system. In particular, the system may be a direct complex domain OPFD system for efficiently and instantaneously measuring laser frequency.

[0029] The system operates in the time domain, providing a comprehensive and versatile model. Furthermore, the system is not constrained by relying on frequency analysis that assumes a constant frequency during a single measurement period. Therefore, the system is not limited in its ability to manage scenarios where the frequency changes over time.

[0030] Furthermore, the system supports both fast and wide laser frequency drift, providing robust performance over a wide spectrum of operating conditions. In other words, the system enables precise and reliable frequency estimation even under conditions of rapidly changing frequencies and a wide frequency range.

[0031] The chirped laser source may be any suitable type of laser, including, but not limited to, external cavity lasers (ECLs) and distributed feedback lasers (DFBs). The system is designed to handle optical signals from lasers with continuous frequency shifts, whether fast or slow. The frequency variation can take a variety of forms, including linear, triangular, and pulse-shaped modulation.

[0032] In one example, some or all of the steps of the control unit may be replaced or assisted by one or more neural networks. In other words, the neural networks may be trained to perform the steps. Thus, the control unit of the system may be configured to receive at least two angular diversity signals and determine the instantaneous frequency of the optical signal.

[0033] This can result in improved performance and efficiency. Neural networks can enable accurate and fast processing compared to conventional systems and methods. The integration of neural networks further reduces computational complexity and enhances the system's ability to adapt to changing input conditions. Furthermore, neural networks can improve the robustness and reliability of the system, making it more effective in, for example, real-time applications.

[0034] 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.

[0035] The term "laser light source" as used herein refers to any unit, device, and / or element capable of emitting at least partially coherent light. A chirped laser light source 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 optical signal).

[0036] In this context, a "chirped laser source" refers to a laser source whose frequency changes rapidly over time (i.e., the frequency changes rapidly). Several types of lasers are capable of chirping, allowing their frequency to be changed quickly. Therefore, the present incentive concept allows for the measurement of the instantaneous frequency of a chirped or frequency-changing laser source.

[0037] Conventional methods for frequency measurement are typically only effective for lasers with very slowly varying frequencies. These conventional techniques lack the theoretical basis to support instantaneous optical frequency measurements for rapidly varying frequencies.

[0038] The light measurement unit may comprise an interferometer structure configured to split at least a portion of the light signal into a first signal and a second signal and to delay the second signal relative to the first signal.

[0039] In other words, the interferometer structure may be configured to split a portion of the optical signal into two separate signals, one of which is delayed relative to the other.

[0040] The interferometer structure may be, for example, a Mach-Zehnder interferometer (MZI) or a self-heterodyne interferometer. The MZI configuration may enable precise phase difference measurements by splitting a portion of light into two paths, introducing a controlled phase shift, and then recombining the optical signals to generate an interference pattern. The interferometer structure may be, for example, an asymmetric MZI.

[0041] A self-heterodyne interferometer splits a portion of light into two parts, one of which is frequency-shifted and time-delayed before being recombined with the other. Self-heterodyne interferometers, in particular, facilitate laser linewidth measurements and can provide high accuracy in analyzing beat frequencies generated by interference of delayed and frequency-shifted signals.

[0042] Therefore, creating a controlled delay between the first and second signals can result in accurate measurements. Additionally, the use of an asymmetric MZI can reduce noise and increase the clarity of the measurements.

[0043] 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.

[0044] 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).

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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).

[0050] It will be further appreciated that the system may utilize relatively long delay lines within the interferometer structure. By employing longer delay lines, the system can accommodate applications where extended delay is beneficial or necessary.

[0051] In general, the optical measurement unit may be configured to split a portion of the 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 first or second optical signal in a loop or spiral, thereby providing long path lengths in a compact solution.

[0052] 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.

[0053] 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.

[0054] The optical hybrid coupler may be, for example, a 120-degree optical hybrid coupler or a 90-degree optical hybrid coupler.

[0055] 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.

[0056] 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).

[0057] Furthermore, the light measurement unit may comprise at least two photodiodes for detecting at least two angle diversity signals.

[0058] 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. The photodiode may be, for example, a photodetector.

[0059] 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.

[0060] 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.

[0061] In general, the optical measurement unit may include three components: an optical MZI 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).

[0062] Regarding optical hybrid couplers and angle diversity signal detection, a design based on a 120-degree hybrid coupler 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.

[0063] 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.

[0064] However, it will be appreciated that alternative configurations may be utilized, such as directional couplers or coupler-based systems. For example, a 3x3 coupler may be utilized to facilitate interference and combining of the desired optical signals.

[0065] 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.

[0066] 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.

[0067] The system may be integrated on a single semiconductor chip, or the system may be fiber-based.

[0068] This system allows for short delay lines, enabling more compact on-chip integration. The structures or components of the system may be scaled down in size to accommodate photonic integration, including, for example, delay lines, photodiodes, and / or other components.

[0069] The system thus enables a compact (e.g., on-chip integratable) solution for frequency measurement with reduced DSP complexity. Estimation of the frequency of the optical signal may be achieved using a compact architecture that may be integrated, for example, on a photonic integrated circuit (PIC). It will be appreciated that some or all of the system components may be integrated on a PIC.

[0070] Additionally, the system allows for adaptive frequency measurements that can be performed in real time, enabling real-time instantaneous frequency measurements of optical signals. In particular, adaptive measurements can handle nonlinearities in frequency chirps, e.g., nonlinearities that change due to changing (external) conditions.

[0071] Thus, a compact, adaptive and integrated system for frequency measurement is possible.

[0072] However, it is understood that the system may be fiber-based. Some components of the system may be implemented using optical fiber, for example, as an alternative to on-chip integration. In one example, certain components may be integrated on-chip (e.g., to take advantage of the benefits of miniaturization and high-speed processing), while other components may be fiber-based (e.g., to facilitate long-distance signal transmission and reduce electromagnetic interference).

[0073] Fiber-based can also refer to system configurations that utilize optical fiber for signal transmission, including, but not limited to, systems in which optical fiber is used as the primary medium for transmitting light or other signals.

[0074] The system may further comprise an analog-to-digital converter (ADC), which 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.

[0075] Therefore, a complex signal can be generated based on the digital signal. This allows for phase calculation in the complex domain compared to the real domain. The use of complex signals can enable real-time positive / negative chirp measurement. In a sense, the phase information obtained from the phase calculation of the (extracted) beat signal is the estimated instantaneous frequency of the optical signal. The use of complex signals also allows for a smaller time delay without sacrificing the accuracy of the estimation performance (e.g., chirp edges between up-ramp and down-ramp, and vice versa).

[0076] 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.

[0077] Additionally, the system may include a TIA configured to amplify the detected signals (e.g., one TIA per detected signal), so that the amplified signals can be sent to the ADC.

[0078] The ADC may be associated with at least two photodiodes configured to detect at least two angular diversity signals, for example. For example, each diversity signal may be converted to a digital signal by a respective ADC. The amplitude of the detected angular diversity signals may be altered after detection. For example, because different photodiodes may have different responses, the amplitude may be determined and / or adjusted so that all angular diversity signals have the same or similar amplitude.

[0079] 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.

[0080] In other words, a complex signal can be synthesized using multiple real (digital) signals, for example, at least two real signals. The complex signal can be represented, for example, by the sum of digital angle diversity signals in the complex domain. In one example, the complex signal can be represented as a vector in complex space.

[0081] Based on the complex signal, the phase of the beat signal can be extracted, e.g., real-time tracking chirps can be captured over time.

[0082] The phase information of the extracted beat signal then corresponds to an estimate of the instantaneous frequency, thus obtaining the instantaneous frequency (e.g., after calibration).

[0083] The estimated instantaneous phase, i.e., frequency, may be output from the control unit (e.g., via a digital-to-analog converter (DAC)), such that the output signal may indicate the instantaneous frequency of the optical signal.

[0084] A complex signal can be represented in the digital domain based on the sum of at least two angular diversity signals, provided that each angular diversity signal is assigned a matching phase term.

[0085] In other words, the digital representation of each angle diversity signal may be assigned or given a matched phase shift term. The matched phase term may be related to the corresponding phase shift of the respective angle diversity signal. In one example, for a 120-degree optical hybrid coupler, the matched phase shift terms may correspond to 0 degrees, 120 degrees, and 240 degrees. Meanwhile, for a 90-degree optical hybrid coupler, the matched phase shift terms may correspond to, for example, 0 degrees and 90 degrees.

[0086] Furthermore, the complex signal can be represented in the complex domain by the sum of the angular diversity signals, if each angular diversity signal is assigned a respective matched phase term. Thus, for 120 degrees, the second angular diversity signal will be 120 degrees ( TIFF2025186164000002.tif6150), so that the third angular diversity signal is shifted an additional 120 degrees ( TIFF2025186164000003.tif6150) where j is a complex number.

[0087] Similarly, in the case of 90 degrees, the second angular diversity signal is 90 degrees ( TIFF2025186164000004.tif6150) can be shifted.

[0088] The instantaneous phase of the complex signal may be determined based on a sample-by-sample calculation.

[0089] Sample-by-sample processing allows input samples to be processed individually and immediately, minimizing latency and enabling real-time performance, eliminating delays associated with accumulating and processing large amounts of data, allowing for immediate feedback and improved responsiveness.

[0090] If the beat phase is extracted on a sample-by-sample basis, the instantaneous laser frequency is estimated on a sample-by-sample basis. In particular, real-time chirp tracking may require a sample-by-sample basis chirp calculation so that the instantaneous phase of the beat signal can be determined in real time.

[0091] Sample-by-sample calculations (i.e., not frame-by-frame) enable real-time applications. For example, sample-by-sample instantaneous frequency tracking can support real-time applications such as ultra-wideband and high-precision laser wavelength lockers (WLLs), digital high-speed feedback control loops (e.g., optical-electrical phase-locked loops (OPLLs)), and / or optical spectrum analysis-based optical sensing (e.g., high-speed gas sensing).

[0092] Here, "samples" may refer to data collected at a particular interval, as compared to a "frame," which may relate to one or more chirps. The number of samples in a frame may relate 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.

[0093] Therefore, calculation of a sample-by-sample metric may require sequential processing of samples, as compared to a frame-by-frame metric where a complete set of samples (ie, a frame) is processed together.

[0094] 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.

[0095] Furthermore, by comparison, frame-by-frame processing requires the collection and processing of many samples, often in large batches (e.g., 100,000 samples). This introduces significant delays as the system must wait for the entire frame to be collected before processing can begin. As a result, frame-by-frame processing is less suitable for applications where low latency and real-time responsiveness are desired.

[0096] The determined instantaneous phase of the complex signal may be adjusted based on a phase unwrap calculation to account for instantaneous phase values ​​greater than 2π. In particular, by utilizing a phase unwrap algorithm, 2π phase ambiguities may be avoided.

[0097] In other words, the phase may be unwrapped by using a method involving the addition or subtraction of multiples of 2π to eliminate discontinuities and maintain smooth transitions between successive phase values. From TIFF2025186164000005.tif6150 Phases that fall outside the standard range of TIFF2025186164000006.tif6150 can be converted into a continuous sequence.

[0098] At least a portion of the optical signal represented by TIFF2025186164000007.tif6150 The first signal is represented by TIFF2025186164000008.tif6150 and The second signal is represented by TIFF2025186164000009.tif6150. TIFF2025186164000010.tif6150 is the second signal TIFF2025186164000011.tif6150 as the first signal Time delay for TIFF2025186164000012.tif6150 TIFF2025186164000013.tif6150 may be configured to pass through a delay line to delay the image with different time delays. The first and second signals having the same wavelength may be configured to be split and combined into at least two angle diversity signals via an optical hybrid coupler. Each angle diversity signal may be further configured to be detected by a respective photodiode. The phase of the first detected angle diversity signal may be determined by a delay of the optical signal. tif6150。 Phase finite differencing can be used to estimate the instantaneous frequency of an optical signal.

[0099] Time Delay It is understood that when TIFF2025186164000016.tif6150 is small, the phase finite difference represents the laser instantaneous frequency.

[0100] Extracting the phase from a real signal would typically require a complex Hilbert transform. However, the phase can be converted by the system herein in a similar manner (e.g., TIFF2025186164000017.tif6150 and TIFF2025186164000018.tif6150), which may represent the same signal except with a fixed phase difference (e.g., 120 degrees) between themselves.

[0101] First Angle Diversity Signal TIFF2025186164000019.tif6150 represents the real signal, and each angular diversity signal is assigned a matched phase term ( TIFF2025186164000020.tif13131) to form a newly generated angular diversity signal, and then all the newly generated angular diversity signals are summed together. Thus, the generated complex signal is (e.g., the first angular diversity signal The complex representation of a real signal (corresponding to TIFF2025186164000021.tif6150) can then be extracted sample by sample.

[0102] According to a second aspect of the inventive concept, there is provided a laser system comprising a chirped laser source and a system according to the first aspect, wherein the control unit is configured to determine a compensation signal for improving linearity of the optical signal based on a plurality of determined instantaneous frequencies during the chirp of the chirped laser source.

[0103] Therefore, the chirped laser light source may be a wavelength-tunable chirped laser light source.

[0104] This aspect may generally offer the same or corresponding advantages as the previous aspect.

[0105] The compensation signal may be configured to adjust the frequency and / or nonlinearity of a frequency chirp of the optical signal of the chirped laser source. The compensation signal may be configured, in particular, to improve the linearity of the frequency chirp.

[0106] The laser system thereby reduces optical losses and minimizes the impact of changing environmental conditions on chirp linearity. Furthermore, because the laser 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.

[0107] The nonlinearity of the frequency chirp can be due to factors such as driver bandwidth limitations and the nonlinear characteristics of the chirped laser source, among other causes.

[0108] The compensation signal may further be used to wavelength lock the chirped laser source.

[0109] Thus, the laser system may provide ultra-fast, ultra-wideband, and ultra-precise laser wavelength locking, for example, in optical communication systems.

[0110] Direct complex-domain OPFD according to the first aspect enables fast frequency measurements with real-time response in laser systems. Conventional WLLs are based on optical frequency analysis, where the frequency is maintained during each measurement, limiting measurement speed. Naturally, for fast-shifting lasers, e.g., due to rapidly changing conditions, a fast-response WLL is advantageous.

[0111] Furthermore, high-speed real-time tracking allows for both unlimited locking range and high locking accuracy. However, in conventional WLL, a trade-off between locking range and accuracy is unavoidable.

[0112] For WLLs, calibration may be desired to convert relative frequency measurements into absolute frequency values. This calibration may be performed by a calibration module that can provide a known reference frequency. By comparing the relative instantaneous frequency to this reference, the laser system can accurately determine the absolute frequency.

[0113] The control unit may further comprise a reference signal, the control unit and the measurement unit forming an optoelectronic phase-locked loop (OEPLL) configured to stabilise the frequency and phase of the optical signal.

[0114] 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 calculations, and summation for nonlinear compensation.

[0115] Therefore, the OEPLL allows for faster feedback control and can be used, for example, with fast-varying chirped laser sources.

[0116] The control unit of the laser system may be configured to estimate the nonlinearity of the optical signal by processing the complex signal, perform a pre-distortion calculation, and output a compensation signal, and a digital-to-analog converter (DAC) is configured to convert the processed complex signal into the compensation signal.

[0117] 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).

[0118] 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.

[0119] Based on the complex signal, the (phase) angle of the optical signal can be calculated.

[0120] It is understood that the wavelength of the angle diversity signal may be nonlinearly mapped to the driving voltage of the chirped laser light source, 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).

[0121] 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.

[0122] The calculated predistortion curve can then be used to update the driving voltage of the chirped laser light source. In particular, the output processed digital signal can comprise the predistortion curve. Thus, the predistortion curve can be converted into a compensation signal via a DAC.

[0123] In one example, the laser system may be integrated into a LIDAR system, which may include the laser system according to the second aspect and a LIDAR detection unit, which may be configured to receive a reflected optical signal based on the optical signal.

[0124] 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.

[0125] 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.

[0126] 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.

[0127] The chirped laser light source of the laser system may include an optical beam splitter disposed in the path of the optical signal and configured to split the optical signal into a LIDAR emission signal that can be emitted from the LIDAR system and a portion of the optical signal that is provided to a light measurement unit of the laser system. Thus, the "portion of the optical signal" is the portion of light that is separated from the optical signal and directed to the light measurement unit. The remainder of the optical signal then forms a LIDAR emission signal that can be emitted from the LIDAR system, for example, toward a target.

[0128] In other words, the LIDAR system may split the optical signal from the chirped laser light source into two parts: a local oscillator (LO) signal and a transmit (Tx) signal. The optical signal may be split, for example, by a beam splitter. Generally, 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 reflects from the target to the LIDAR detection unit.

[0129] 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.

[0130] 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.

[0131] The LIDAR system may be integrated into a vehicle and configured to provide real-time distance and speed data about the vehicle's surroundings.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] According to a third aspect of the inventive concept, there is provided a method for measuring the frequency of an optical signal from a chirped laser source, the method comprising: In the light measurement unit, receiving at least a portion of an optical signal; outputting, via an optical hybrid coupler, at least two angle diversity signals based on a difference between first and second signals formed by splitting at least a portion of the 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; In the control unit, receiving at least two angular diversity signals; generating a complex signal based on the at least two angular diversity signals; determining an instantaneous phase of the complex signal to determine an instantaneous frequency of the optical signal; Includes.

[0136] This aspect may generally offer the same or corresponding advantages as the previous aspect.

[0137] The method may further include, in the optical measurement unit, splitting at least a portion of the 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 at least two angle diversity angles via at least two photodiodes.

[0138] In other words, the light measurement unit may comprise an interferometer structure for splitting off a portion of the optical signal.

[0139] A portion of the optical signal can be split, for example, so that 50% of the portion of the 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.

[0140] It should be noted that any example or exemplary configuration or feature referred to herein (e.g., "for example" or "eg," etc.) is intended as a non-limiting example. These examples are provided to illustrate the described concepts and should not be construed as limiting the scope of the invention unless otherwise stated.

[0141] 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]

[0142] The above, as well as additional objects, features, and advantages of the inventive concept 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.

[0143] [Figure 1] 1 is a schematic diagram of a system for measuring the frequency of an optical signal. [Figure 2a] FIG. 2 is a schematic diagram of a light measurement unit. [Figure 2b]1 shows a graph of intensity over time of three angular diversity signals that are phase shifted relative to one another by 120°. [Figure 3] FIG. 10 is an alternative schematic diagram of a light measurement unit. [Figure 4] 1 illustrates a flow diagram of a method for measuring instantaneous frequency based on an angular diversity signal. [Figure 5a] 1A and 1B illustrate schematically an exemplary system for simulating tracking of a fast chirped laser and corresponding measurement graphs. [Figure 5b] 1A and 1B illustrate schematically an exemplary system for simulating tracking of a fast chirped laser and corresponding measurement graphs. [Figure 5c] 1A and 1B illustrate schematically an exemplary system for simulating tracking of a fast chirped laser and corresponding measurement graphs. [Figure 5d] 1A and 1B illustrate schematically an exemplary system for simulating tracking of a fast chirped laser and corresponding measurement graphs. [Figure 5e] 1A and 1B illustrate schematically an exemplary system for simulating tracking of a fast chirped laser and corresponding measurement graphs. [Figure 6] 1 illustrates a schematic diagram of a laser system for determining a compensation signal for adjusting the frequency of a laser signal. DETAILED DESCRIPTION OF THE INVENTION

[0144] 1 illustrates a system 100 for measuring the frequency of an optical signal 112 from a chirped laser source 110. The system includes an optical measurement unit 120 configured to receive at least a portion 114 of the 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 at least a portion 114 of the optical signal 112 via an optical hybrid coupler 122, where 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.

[0145] The system 100 further comprises a control unit 130 configured to receive the at least two angular diversity signals 124, generate a complex signal based on the at least two angular diversity signals 124, and determine an instantaneous phase of the complex signal to determine the instantaneous frequency of the optical signal 112. Furthermore, an output signal 132 outputs the determined instantaneous frequency, e.g., for further processing.

[0146] Therefore, system 100 directly forms the complex domain OPFD, allowing for reduced delay lines and facilitating further integration into external systems.

[0147] The portion 114 of the optical signal 112 received by the light measurement unit 120 may be relatively small compared to the remaining propagating portion 116 of the optical signal 112. The portion 114 may be, for example, a small portion (e.g., approximately 1%) of the optical signal 112. Similarly, the remaining propagating portion 116 may represent approximately 99% of the 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 optical signal 112 into portions may ensure that a sufficient amount of light is available for both measurement and propagation to the target. The portion 114 may be separated from the optical signal 112 using, for example, an optical beam splitter or similar device configured to direct a small portion of the light to the light measurement unit 120 while allowing the majority of the optical signal to continue propagating.

[0148] The optical measurement 120 unit may be based on a Mach-Zehnder interferometer (MZI) structure, for example an asymmetric MZI.

[0149] An MZI 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. MZIs can be used to detect changes in the optical path length difference by analyzing the interference pattern that results when the optical paths merge.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] Although not shown in detail in FIG. 1 , the laser system 100 may be integrated on a single semiconductor chip. 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 control device 130 may be integrated on the same chip (such as a PIC further provided with electronic circuitry). The chirped laser source 110 may be located outside the chip, for example. However, it is understood that the chirped laser source 110 may be further located or integrated on the same chip as the light measurement unit 120 and the control device 130.

[0159] The control unit 130 may further comprise 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, for example, stabilize the frequency and phase of the optical signal 112.

[0160] The OEPLL can be used to lock the laser frequency and / or chirp to a ramp signal in real time. The determined instantaneous frequency can be compared to a reference 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.

[0161] In FIG. 2a, the optical measurement unit 120 of the system 100 for measuring the frequency of the optical signal 112 is shown schematically in more detail.

[0162] Here, a chirped laser source 110 directly illuminates an optical signal 112 (also denoted as λ) onto an optical measurement unit 120. The optical signal 112 is then received by an interferometer structure 121 configured to split the optical signal 112 into a first signal 114a and a second signal 114b. The optical signal 112 is split into the first and second signals 114a, 114b by a splitter 121s or a 1×2 coupler. The splitter 121s may be, for example, a beam splitter. In one example, the optical signal 112 received by the optical measurement unit is split 50-50, with the first signal 114a representing 50% of the received optical signal 112 and the second signal 114b representing 50% of the received optical signal 112.

[0163] The interferometer structure 121 further comprises an optical path configured to delay the second signal 114b relative to the first signal 114a, where 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.

[0164] The longer optical path may be, for example, about 10 cm or 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 signals 114 a, 114 b through materials with different refractive indices.

[0165] As an example, the second signal 114b travels through a delay waveguide and recombines with the local oscillator (LO), ie, the first signal 114a, at an optical hybrid coupler 122.

[0166] The optical hybrid coupler 122 in Figure 2a is a 120-degree optical hybrid coupler 122. The 120-degree optical hybrid coupler 122 splits two input optical signals into three output angle diversity signals 124 with a fixed phase difference of 120 degrees between each pair of outputs (see Figure 2b, the three angle diversity signals 124 are labeled PD1, PD2, and PD3, respectively).

[0167] The 120-degree hybrid coupler 122 may be based on a directional coupler or a 3x3 MMI, such as a 3x3 multimode interference coupler. A 3x3 MMI coupler can split and combine input optical signals using multimode interference. A 3x3 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.

[0168] 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.

[0169] In the configuration shown in Figure 2a, 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.

[0170] 2a, 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).

[0171] In a particular example, the optical measurement unit 120 with the 120-degree optical hybrid coupler 122 may utilize three photodiodes 126 and / or three TIAs (not shown).

[0172] In a sense, the system 100 of FIG. 2a utilizes a 120° angular diversity self-heterodyne detection structure.

[0173] The received optical signal 112 and the first and second signals 114a, 114b are coupled to an electric field having a unit amplitude, TIFF2025186164000022.tif6150, and the first signal 114a is an electric field, TIFF2025186164000023.tif6150, and the second signal 114b is an electric field, TIFF2025186164000024.tif6150 These signals can be represented as follows (the second signal 114b is delayed with respect to the first signal 114a): TIFF2025186164000025.tif994 TIFF2025186164000026.tif11101 TIFF2025186164000027.tif10107 where, TIFF2025186164000028.tif6150 is the phase of the optical signal 112 from the chirped laser light source 110, TIFF2025186164000029.tif6150 is the time delay on the longer optical path.

[0174] The 120° optical hybrid coupler 122 is TIFF2025186164000030.tif6150 and TIFF2025186164000031.tif6150 into a 120° phase-shifted equal amplitude angular diversity signal 124. In other words, the 120° optical hybrid coupler 122 combines the first TIFF2025186164000032.tif6150 and the second The angular diversity signals 124 are split into three separate paths, each with a 120-degree phase difference. Each angular diversity signal 124 is then detected by a respective photodiode 126. In other words, the photodiodes 126 are used to form three detected angular diversity signals I1, I2, and I3 based on the respective angular diversity signals 124.

[0175] Further, in FIG. 2a, the three detected angular diversity signals 124 (i.e., three photocurrents) are sampled by an analog-to-digital converter (ADC) and analyzed in the digital signal processing (DSP) domain, i.e., control unit 130 (see further in connection with the description of FIG. 4).

[0176] The high beat frequency components may be outside the bandwidth of the photodiode 126 because the photodiode may act as a low pass filter.

[0177] Responsivity of photodiode 126 Considering TIFF2025186164000034.tif6150, the detected angle diversity signal 124 (i.e., photocurrent) can be expressed as: TIFF2025186164000035.tif15108 TIFF2025186164000036.tif16108 TIFF2025186164000037.tif16108 where, TIFF2025186164000038.tif6150 represents the detected real photocurrent (i.e., real angular diversity signal 124).

[0178] Figure 3 illustrates another optical measurement unit 120. To avoid excessive repetition, it should be understood that Figure 3 illustrates a similar optical measurement unit 120 to that described in connection with Figure 2a. 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.

[0179] The 90-degree optical hybrid coupler 122 splits the input signals 114a, 114b into four output signals (ie, four angle diversity signals 124).

[0180] The 90-degree optical hybrid coupler may use the principles of multimode interference to achieve this splitting. Although not explicitly shown in Figure 3, the 90-degree hybrid coupler 122 may comprise a 2x4 MMI, a 4x4 MMI, or a directional coupler.

[0181] 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).

[0182] A balance photodiode 128 processes the four output angle diversity signals 124 to produce two detected angle diversity signals TIFF2025186164000039.tif6150, Generate TIFF2025186164000040.tif6150. These detected angular diversity signals TIFF2025186164000041.tif6150, TIFF2025186164000042.tif6150 have a 90 degree phase difference between them. The four-signal to two-signal conversion is performed by combining the intensity information from the output angular diversity signal pair to generate the detected angular diversity signal. TIFF2025186164000043.tif6150, This is achieved by a balanced photodiode 128, which produces TIFF2025186164000044.tif6150.

[0183] 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).

[0184] In general, the angle diversity structures described herein may be 120° hybrid-based three-angle diversity structures as shown in FIG. 2a, or 90° hybrid-based four-angle diversity architectures as shown in FIG. 3. The 90° hybrid coupler produces four outputs with phase shifts of 0°, 90°, 180°, and 270°. Thus, in the electrical domain, the terms combining 0° and 180° are in phase ( TIFF2025186164000045.tif6150) components, and the combined terms of 90° and 270° are orthogonal ( TIFF2025186164000046.tif6150) components can be represented.

[0185] 4 is a block diagram illustrating digital signal processing steps of a method for determining the instantaneous frequency of an optical signal based on at least two angle diversity signals 124 (e.g., received as described according to any of FIGS. 1-3). The steps illustrated in FIG. 4 may be performed within or by the control unit 130. However, it will be understood that the ADC may be located internal or external to the control unit 130.

[0186] The at least two (analog) angular diversity signals 124 now undergo ADC conversion: the angular diversity signals 124 are thus received by an ADC that converts them into the digital domain, i.e., into digital signals.

[0187] The digital signal is then used to generate the complex signal Generate or create TIFF2025186164000047.tif6150. Digital signals may be represented, for example, by vectors in the complex domain.

[0188] Complex signals ( TIFF2025186164000048.tif6150) is TIFF2025186164000049.tif7150 For example, based on the detected angle diversity signals described above for the 120-degree optical hybrid coupler in equations (4)-(6), the complex signal TIFF2025186164000050.tif6150 can be expressed as follows: TIFF2025186164000051.tif14132Here, j is a complex number.

[0189] Beat signals (e.g., The phase component of the angular diversity signal (i.e., the phase diversity amplitude signal) can be extracted. As described herein, the angular 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.

[0190] In other words, phase (i.e., angle) diversity detection is performed by detecting the beat signal of phase (i.e., angle) diversity. TIFF2025186164000053.tif6150 is used to obtain the phase information. Two or more detections providing angle (phase) information are provided, resulting in angle detection diversity, which can be used for simple detection-based phase determination.

[0191] Real signal ( Instantaneous phase of TIFF2025186164000054.tif6150 ( TIFF2025186164000055.tif6150) is calculated using mathematical calculations in the complex domain ( It can be extracted by converting it to TIFF2025186164000056.tif6150DC). TIFF2025186164000057.tif6150 shows the in-phase ( TIFF2025186164000058.tif6150) components and orthogonal ( TIFF2025186164000059.tif6150) component, and "DC-less" means that the direct current component has been removed or is not considered.

[0192] In a sense, TIFF2025186164000060.tif6150 signal, TIFF2025186164000061.tif6150 Another type of signal, but in the complex domain (i.e., with the same phase). By creating a TIFF2025186164000062.tif6150 signal, TIFF2025186164000063.tif6150 The phase of a complex signal can be easily extracted (e.g., because it is easier to extract the phase accurately in real time from a complex signal).

[0193] And the signal The phase of TIFF2025186164000064.tif6150 corresponds to the instantaneous frequency of the laser light source. TIFF2025186164000065.tif6150) is understood to correspond to finite differencing of the laser phase and thus to an estimate of the instantaneous laser frequency.

[0194] Therefore, the phase of the beat signal can be extracted by: TIFF2025186164000066.tif1699

[0195] The phase information of the extracted beat signal is the estimated instantaneous frequency. For example, If TIFF2025186164000067.tif6150 is small enough, TIFF2025186164000068.tif6150 represents the instantaneous frequency of the optical signal from a laser light source.

[0196] Furthermore, the interference It is measured at a fixed delay, denoted as TIFF2025186164000069.tif6150, which is small but not zero. When TIFF2025186164000070.tif6150 approaches zero, the signals are identical with no delay, so there is no beat signal. If TIFF2025186164000071.tif6150 is relatively large (or too large), the observed phase may be averaged over time, which may affect the accuracy of the measurement.

[0197] moreover, When TIFF2025186164000072.tif6150 is zero, the beat signal essentially corresponds to a direct current (DC) component. Therefore, the method may use phase finite differences to estimate the phase derivative, i.e., the instantaneous frequency. From this, TIFF2025186164000073.tif6150 must not be zero. However, TIFF2025186164000074.tif6150 can be appropriately selected based on the rate of change of the laser frequency. A smaller TIFF2025186164000075.tif6150 may increase the accuracy of the method, but at the cost of a lower signal-to-noise ratio (SNR).

[0198] In a scenario where the laser frequency changes rapidly, Conversely, if the laser frequency is varied slowly, a larger gain can be achieved without loss of accuracy. TIFF2025186164000077.tif6150 can be used.

[0199] 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, TIFF2025186164000078.tif6150 may be selected according to the rate of change of the laser frequency.

[0200] 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.

[0201] 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. 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).

[0202] The control unit sends three signals for 120 degrees. TIFF2025186164000080.tif6150 is sampled, and two signals are obtained at 90 degrees. TIFF2025186164000081.tif6150 Sample the following.

[0203] Therefore, for 120 degrees, the resulting three-dimensional vector in the complex domain is TIFF2025186164000082.tif7150. Therefore, the second angular diversity signal TIFF2025186164000083.tif6150 is the first angle diversity signal 120 degrees in the complex plane for TIFF2025186164000084.tif6150 ( TIFF2025186164000085.tif6150) rotated and the third angular diversity signal TIFF2025186164000086.tif6150 is the second angle diversity signal TIFF2025186164000087.tif6150 is shifted by another 120 degrees ( TIFF2025186164000088.tif6150) Rotating.

[0204] Similarly, for the 90 degree case, the two detected angular diversity signals TIFF2025186164000089.tif6150 is formed into a two-dimensional vector in the complex domain, and this vector is TIFF2025186164000090.tif7150. Therefore, the second angular diversity signal TIFF2025186164000091.tif6150 is the first angle diversity signal 90 degrees in the complex plane ( TIFF2025186164000093.tif9150) Rotating.

[0205] Next, the complex signal in the complex domain The instantaneous phase of TIFF2025186164000094.tif6150 (and hence the instantaneous frequency of the optical signal), i.e., the instantaneous angle of the vector in the complex domain, can be calculated as described below.

[0206] The phase can be calculated by determining the angle of the vector formed using equation (9) for 120 degrees and equation (10) for 90 degrees. TIFF2025186164000095.tif25140 TIFF2025186164000096.tif23130

[0207] In equations (9) and (10), ANGLE is a calculator that finds the angle of a vector, and UNWRAP is a calculator that unwraps a radian phase angle by adding a multiple of ±2π.

[0208] This allows the phase (angle) to be calculated. In other words, the phase calculation step here calculates the frequency / phase (i.e., phase angle) corresponding to the at least two angle diversity signals 124 by creating a vector and calculating the angle of the vector.

[0209] Additionally, 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. However, the ANGLE and UNWRAP calculator may correspond to any suitable angle calculator or unwrap algorithm, respectively.

[0210] 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.

[0211] Adding multiples of 2π to successive phase angles of a phase angle ramp when the phase difference between the phase angle of the phase angle ramp and the successive phase angle is greater than or equal to π is sometimes referred to as an "unwrap" operation. The purpose of performing the unwrap operation on the extracted phase angle ramp is to eliminate 2π or 360° jumps in the extracted phase angle ramp. In other words, the phase unwrapping step is concerned with avoiding 2π phase ambiguities.

[0212] In addition, in equations (9) and (10), the signal TIFF2025186164000097.tif6150 and TIFF2025186164000098.tif6150 itself is a real signal. As can be seen from equations (9) and (10), the signal TIFF2025186164000099.tif6150 and TIFF2025186164000100.tif6150 are combined to form a vector in the complex plane. In other words, by synthesizing a complex signal with multiple real beat signals, we can achieve measurement of the instantaneous frequency / phase of the resulting beat signal.

[0213] Also in Figure 4, calibration is performed after phase unwrapping, which may include comparing the determined instantaneous frequency to a known standard or reference.

[0214] Thus, after calibration, the instantaneous frequency can be output via output signal 132. As a result, real-time tracking chirps plotted over time can also be captured. For example, the phase angle ramp of the optical signal can be extracted over time.

[0215] Once the beat phase is extracted sample by sample, the instantaneous laser frequency is also estimated sample by sample, thus enabling high-speed measurements. Furthermore, because the digital signal processing operates directly in the complex domain, there is no ambiguity between ±chirp, enabling accurate measurements.

[0216] In Figures 5a-5e, an exemplary simulation of tracking a high-speed chirped laser with an integrated photonics-based Lumerical Interconnect is investigated.

[0217] Figure 5a shows a schematic of a simulation system, which includes a directly modulated laser (LD) driven by a typical sinusoidal signal to generate a sinusoidal chirp with a large amount of chirp variation (in the simulation, sinusoidal drive can produce sinusoidal frequency drift).

[0218] Here, the chirped laser signal is fed into a delayed self-heterodyne interferometer structure with two delay waveguides of different lengths (with no or minimal loss) to investigate the effect of different delays. In this simulation, a 5 cm delay waveguide and a 10 cm delay waveguide were used (with no or minimal loss). The 120° hybrid coupler in the angle diversity detection is represented by an optical 3 × 3 multimode interferometer (3 × 3 MMI) followed by three photodiodes (PDs). The three sampled photocurrents are processed in the digital signal processing (DSP) domain.

[0219] For reference, the dashed line shows a Hilbert transform based system.

[0220] As shown in Figure 5b, the sinusoidal chirp signal is set to 1 MHz.

[0221] As shown in Figure 5c, the chirp frequency range is approximately 5.6 GHz.

[0222] Figures 5d and 5e illustrate the performance of the methods and systems described herein (e.g., in connection with Figures 1-4) compared to a method / system based on the Hilbert transform (sampling rate 64 GSa / s). The estimated frequency chirp in these two methods is compared to the transmitted (Tx) reference signal by fitting the Tx sinusoidal chirp with a reconstruction curve and normalizing it to unit amplitude.

[0223] It is observed that methods based on the Hilbert transform cannot perfectly track sinusoidal or triangular chirps (such as those used in FMCW LIDAR) because they suffer from ±chirp ambiguity. In particular, the estimation accuracy decreases as the delay waveguide becomes shorter, which is mainly due to the shorter time window of the Hilbert transform.

[0224] At a fixed chirp rate (1 MHz), the beat frequency decreases with shorter delays, so the Hilbert transform requires a longer time window to obtain better spectral resolution.

[0225] However, the methods and systems described herein do not have such accuracy limitations and provide real-time (as opposed to frame-by-frame) tracking.

[0226] 6 illustrates a laser system 100 for generating a frequency-modulated continuous wave (FMCW) optical signal 112. The laser 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 the pair of signals of the at least two angle-diversity signals 124 have a fixed phase shift relative to each other.

[0227] 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 the 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.

[0228] 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. The rapid frequency change of the tunable laser can be triggered by rapidly changing environmental conditions or rapidly changing operating parameters.

[0229] 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.

[0230] 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 to continue propagating.

[0231] The optical measurement 120 unit may be, for example, a nonlinearity measurement module based on an asymmetric Mach-Zehnder interferometer (AMZI) structure.

[0232] 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.

[0233] In block 1 of Figure 6, the optical path of the second signal 114b is shown as being longer than the optical path of the first signal 114a (see the loop 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.

[0234] 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.

[0235] 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. 6, the input signals to the optical hybrid coupler 122 are the first and second signals 114a and 114b, and the output is the angle diversity signal 124.

[0236] 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.

[0237] The resulting combined optical signal, i.e., the resulting angular diversity signal 124, may then be directed to a photodiode. Each signal detected by the photodiode may represent an interference product.

[0238] 6, 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.

[0239] 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.

[0240] 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 110 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.

[0241] 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 an expression like TIFF2025186164000101.tif10150, where: TIFF2025186164000102.tif6150 [Hz] is the root mean square of the nonlinear component of the frequency chirp (ramp up or ramp down), TIFF2025186164000103.tif6150 [Hz] is the frequency modulation width of the wavelength tunable laser.

[0242] 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.

[0243] Therefore, an ultrafast, ultrawideband, ultrahigh precision wavelength locker (WLL) can be realized. The WLL may comprise two main parts: a high speed instantaneous frequency measurement and a high speed control loop.

[0244] The systems and methods described herein can be used to track laser frequency drift in real time with high accuracy (by using an angle-diversity optical delayed self-heterodyne structure). Digital signal processing then performs the subsequent calculation of the instantaneous frequency. The system can be broadly referred to as an optical instantaneous frequency estimator.

[0245] The fast control loop can be a DSP-based control loop (e.g., a controller with an ADC / DAC, as shown in block 2 of Figure 6). Thus, the fast control loop can be used to correct the center frequency of the laser in real time. The speed of the control loop can be adjusted depending on the drift speed of the laser.

[0246] The control unit 130 may be a microcontroller, such as a simple microcontroller. In other words, the control unit 130 may be low in complexity and / or facilitate on-chip integration. For example, the system 100 and / or the laser system 100 may be an on-chip system, enabling a compact and stable industrial product.

[0247] 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.

[0248] Although not shown in detail in FIG. 6 , 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 control device 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 control device 130.

[0249] The control unit 130 may further include a reference signal (not shown in FIG. 6), 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.

[0250] 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.

[0251] As shown schematically in block 1 of FIG. 6, the optical hybrid coupler 122 of the laser system may correspond, for example, to any one of the optical hybrid couplers 122 described in connection with FIGS.

[0252] Additionally, block 2 of FIG. 6 illustrates a block diagram of the signal processing steps of laser system 100.

[0253] 6, it can also be seen that the laser system 100 includes an ADC and a DAC. The ADC may be located internal or external to the control unit 130, and the DAC may be located internal or external to the control unit 130.

[0254] 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 .

[0255] Furthermore, the control unit 130 converts the digital signal into the complex domain, where the digital signal can be represented as a vector in the complex domain. Based on the detected angle diversity signals S1, S2, and S3, in the case of a 120-degree optical hybrid coupler, the complex signal TIFF2025186164000104.tif6150 can be expressed as follows: TIFF2025186164000105.tif15151

[0256] Therefore, the phase of the beat signal can be extracted by: TIFF2025186164000106.tif13139

[0257] The three signals detected from the three single-ended photodiodes are converted from the analog domain to the digital domain through an ADC (e.g., three ADCs), while in the 90-degree optical hybrid coupler-based approach, two beat signals with a 90-degree phase offset are extracted from two balanced photodiodes.

[0258] The control unit 130 samples three signals S1, S2, S3 for the 120 degree angle and two signals S1, S2 for the 90 degree angle.

[0259] The instantaneous phase angle of the signal in the complex plane can then be calculated as follows: This phase angle corresponds to the instantaneous frequency of the FMCW optical signal.

[0260] The phase angle can be calculated by determining the angle of the vector formed using equation (9) for 120 degrees or equation (10) for 90 degrees, as described in connection with FIG. 4.

[0261] In other words, by synthesizing a complex signal with multiple real beat signals, measurements of the instantaneous frequency / phase of the resulting beat signal can be achieved.

[0262] 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, so the mapping between phase angle and time (and similarly angle vs. voltage) is a nonlinear curve.

[0263] 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.

[0264] The nonlinearity is estimated and a predistortion calculation can be performed. The predistortion calculation may include determining a predistortion curve (i.e., a predistorted voltage ramp) based on the nonlinearity estimate. 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.

[0265] 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.

[0266] 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.

[0267] The term "reference phase angle ramp" herein means a phase angle ramp that changes much slower than the feedback rate (eg, 100 times slower).

[0268] 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.

[0269] 6 may be a LIDAR system 200 including the laser system 100 and a LIDAR receiver 202. The LIDAR receiver 202 receives a reflected optical signal 118 based on the FMCW optical signal 112.

[0270] In particular, the LIDAR receiver 202 of the LIDAR system 200 detects the LIDAR response signal (i.e., the reflected optical signal 118). The LIDAR response signal may therefore be based on the propagating portion 116 (i.e., the LIDAR emission signal) of the FMCW optical signal 112. The LIDAR receiver 202 may be, for example, a photodiode, such as a balanced and / or unbalanced photodiode, a photomultiplier tube (PMT), and / or an image detector.

[0271] 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 a target. When the LIDAR emitted light signal reaches the target, at least a portion of the LIDAR emitted light signal may be reflected 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 in 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. In the present LIDAR system 200, the reflected optical signal 118 (i.e., the LIDAR response signal) may be detected by detecting the combined signal with the LIDAR receiver 202.

[0272] Further, in the block diagram illustrating the LIDAR system 200, the LIDAR system 200 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. The pre-distortion curve 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.

[0273] 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 (11). TIFF2025186164000107.tif1593 where, TIFF2025186164000108.tif6150 [m / s] is the speed of light in a vacuum, TIFF2025186164000109.tif6150 is the moving frequency of the wavelength tunable laser 110.

[0274] However, a nonlinear frequency chirp leads to a worsening of the precision to the extent that the width of the corresponding beat spectrum peak increases. The broadening or broadening of the peak caused by the nonlinearity of the frequency chirp can be quantified by Equation (12). TIFF2025186164000110.tif16101Here, D[m] is the distance to the target, TIFF2025186164000111.tif6150 [Hz] is the root mean square of the nonlinear component of the frequency chirp (ramp-up or ramp-down). From equation (14), the laser nonlinearity If TIFF2025186164000112.tif16153 is not suppressed, the target distance The longer TIFF2025186164000113.tif16153 is, the better the axial resolution. TIFF2025186164000114.tif16153 becomes higher. In other words, the effect of chirp nonlinearity is more pronounced for targets that are farther away. Therefore, high chirp linearity is beneficial, for example, in LIDAR systems 200 that may measure long target distances (e.g., on the order of hundreds of meters).

[0275] 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 system (100) for measuring the frequency of an optical signal (112) from a chirped laser light source (110), said system (100) comprising: an optical measurement unit (120) configured to receive at least a portion (114) of the 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 at least a portion (114) of the optical signal (112) via an optical hybrid coupler (122), wherein the second signal (114b) is delayed with respect to the first signal (114a), and the pair of signals of the at least two angle diversity signals (124) have a fixed phase shift with respect to each other; A control unit (130) comprising: receiving the at least two angular diversity signals (124); generating a complex signal based on the at least two angular diversity signals (124); determining the instantaneous phase of the complex signal to determine the instantaneous frequency of the optical signal (112); a control unit (130) configured to: A system (100) comprising:

2. 2. The system of claim 1, wherein the optical measurement unit comprises an interferometer structure configured to split the at least a portion of the optical signal into the first signal and the second signal, and to delay the second signal relative to the first signal.

3. 3. The system (100) of claim 1 or 2, wherein the light measurement unit (120) comprises at least two photodiodes for detecting the at least two angular diversity signals (124).

4. The system (100) of any one of claims 1 to 3, wherein the system (100) is integrated on a single semiconductor chip or the system (100) is fiber-based.

5. The system (100) of any one of claims 1 to 4, wherein the optical hybrid coupler comprises at least one multimode interferometer (MMI).

6. 6. The system (100) of claim 1, further comprising an analog-to-digital converter (ADC), the ADC configured to convert an analog input signal to the control unit (130) into a digital signal based on the at least two angular diversity signals (124).

7. 7. The system (100) of claim 1, wherein the complex signal is represented in the digital domain based on a sum of the at least two angular diversity signals (124), where each angular diversity signal (124) is assigned a matching phase term.

8. The system (100) of any one of claims 1 to 7, wherein the instantaneous phase of the complex signal is determined based on a sample-by-sample calculation.

9. The system (100) of any one of claims 1 to 8, wherein the determined instantaneous phase of the complex signal is adjusted based on a phase unwrap calculation to take into account instantaneous phase values ​​greater than 2π.

10. The at least a portion (114) of the optical signal (112) represented by the first signal (114a) represented by and the second signal (114b) represented by The second signal (114b) is is time delayed relative to the first signal (114a). configured to pass through a delay line for delaying the signal with Different time delays are configured to be split and combined into the at least two angle diversity signals (124) via the optical hybrid coupler (122); Each angular diversity signal (124) is configured to be detected by a respective photodiode; The phase of the first detected angular diversity signal is determined by the delay of the optical signal (112). corresponds to a phase finite difference with the phase finite differencing is used to estimate the instantaneous frequency of the optical signal (112); The system (100) according to any one of claims 1 to 9.

11. 10. A laser system comprising: a chirped laser light source; and the system of claim 1, wherein the control unit is configured to determine a compensation signal for improving the linearity of the optical signal based on a plurality of determined instantaneous frequencies during the chirp of the chirped laser light source.

12. 12. The laser system of claim 11, wherein the control unit (130) is configured to estimate nonlinearity of the optical signal (112) by processing the complex signal, perform pre-distortion calculations, and output the compensation signal (132), and a digital-to-analog converter (DAC) is configured to convert the processed complex signal into the compensation signal (132).

13. A method (1000) for measuring the frequency of an optical signal (112) from a chirped laser light source (110), the method (1000) comprising: In the light measurement unit (120), receiving at least a portion (114) of the optical signal (112); outputting at least two angular diversity signals (124) based on a difference between first and second signals (114a, 114b) formed by splitting the at least a portion (114) of the 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 angular diversity signals (124) having a fixed phase shift relative to each other; In the control unit (130), receiving the at least two angular diversity signals (124); generating a complex signal based on the at least two angular diversity signals (124); determining the instantaneous phase of said complex signal to determine the instantaneous frequency of said optical signal (112); A method (1000) comprising:

14. In the light measurement unit, splitting the portion (114) of the optical signal into the first signal (114a) and the second signal (114b) in an interferometer structure; delaying the second signal (114b) relative to the first signal (114a); detecting the at least two angular diversity signals (124) via at least two photodiodes; 14. The method of claim 13, further comprising:

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