A noise digital cancellation method for an optical frequency comb receiving system

Abstract

The application discloses a kind of optical frequency comb receiving system noise digital cancellation methods, specifically related to digital signal processing technical field, by beam splitter, femtosecond laser is divided into signal path and noise reference path and synchronous acquisition. After Fourier transform to two-way data, the effective spectrum is divided into initial frequency band and the projection coefficient of each section is calculated. According to the relative difference of adjacent frequency band projection coefficient, the frequency band division is dynamically adjusted, and the adaptive frequency band is formed. In each adaptive frequency band, the projection of signal path on reference path is subtracted, the noise cancellation is realized, and finally the time domain signal is obtained by spectrum reorganization and inverse transform;The application is synchronous acquisition and dynamic frequency band division by signal path and noise reference path, effectively suppresses light source relative intensity noise, and improves signal quality;Digital processing mode is used, without complex hardware adjustment, to reduce system cost;Through adaptive frequency band division, environmental changes are adapted, and noise cancellation stability is improved.

Description

Technical Field

[0001] This invention relates to the field of digital signal processing technology, and more specifically, to a method for digital noise cancellation in an optical frequency comb receiving system. Background Technology

[0002] With the development of electronic technology, the electromagnetic environment is becoming increasingly complex, placing higher demands on the rapid and effective acquisition of electromagnetic field signals over a wide frequency range. Traditional superheterodyne receivers are limited by the sampling rate of analog-to-digital converters, resulting in a limited instantaneous frequency coverage. Expanding the receiving bandwidth requires local oscillator frequency sweeping, which creates a trade-off between receiving accuracy, response time, and frequency coverage, especially reducing the probability of intercepting transient signals such as pulses.

[0003] Optical frequency comb undersampling technology provides a new technical solution for broadband rapid measurement. This technology uses an optical frequency comb generated by a femtosecond pulsed laser as a sampling light source, which is input into an optical electric field sensor to sample the spatial electric field. Because the optical frequency comb generated by the femtosecond pulsed laser has pulse amplitude flatness reaching the terahertz level at radio frequency, the high-frequency signal is shifted to a pulse repetition frequency range for measurement, thereby significantly reducing the sampling rate requirements of the back-end receiver and effectively resolving the contradiction between test sensitivity and test time.

[0004] However, while optical frequency comb receiving systems address broadband reception issues, they also introduce relative intensity noise from femtosecond pulsed lasers. This noise exhibits variations across different frequency bands, and in some bands, it even becomes the primary contributor to noise, affecting the system's signal reception quality. To suppress this noise, existing technologies typically employ hardware methods, such as introducing differential photodetectors in conjunction with differential optical electric field sensors to achieve common-mode suppression of light source noise in the optoelectronic link. These methods require fine-tuning of parameters such as time delay, phase, and amplitude in the optical path, making them highly sensitive to environmental changes and difficult to adapt to. Furthermore, differential optical electric field sensors are complex and costly. Therefore, this invention proposes a digital noise cancellation method for optical frequency comb receiving systems to address the aforementioned problems. Summary of the Invention

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for digital noise cancellation in an optical frequency comb receiving system includes the following steps: An optical frequency comb receiving system was constructed, comprising a femtosecond pulsed laser, a polarization-maintaining fiber beam splitter, an optical electric field sensor, a first photodetector, a second photodetector, a multi-channel data acquisition card, and a personal computer. The optical frequency comb excitation emitted by the femtosecond pulsed laser was split into two paths by the polarization-maintaining fiber beam splitter. The first path of optical frequency comb excitation entered the optical electric field sensor, and after spatial electric field modulation, entered the first photodetector, defined as the signal path. The second path of optical frequency comb excitation directly entered the second photodetector, defined as the noise reference path. The radio frequency voltage signals output by the first and second photodetectors were connected to the first and second channels of the multi-channel data acquisition card, respectively, for synchronous data acquisition. The personal computer processes the data acquired by the multi-channel data acquisition card, defining the data acquired by the first channel as signal path data and the data acquired by the second channel as reference path data; it performs fast Fourier transform on the signal path data and the reference path data respectively to obtain the signal path spectrum and the reference path spectrum, and sets a predetermined length; The effective frequency range from the first frequency point to half the predetermined length in the signal path spectrum and the reference path spectrum is divided into multiple initial frequency bands. The signal path spectrum segment corresponding to the i-th initial frequency band is denoted as the i-th signal path segment, and the corresponding reference path spectrum segment is denoted as the i-th reference path segment. Calculate the projection coefficient for each initial frequency band. The projection coefficient is the projection of the i-th signal path segment onto the i-th reference path segment. The frequency band division is dynamically adjusted based on the relative difference between the projection coefficients of adjacent initial frequency bands, and then all the divided frequency bands are used as adaptive frequency bands. For each adaptive frequency band, the projection result of the signal path spectrum segment of the adaptive frequency band onto the reference path spectrum segment of the adaptive frequency band is calculated. The projection result is subtracted from the signal path spectrum segment to obtain the noise-cancelled signal path spectrum segment. The noise-cancelled signal path spectrum segments corresponding to all adaptive frequency bands are recombined into a complete canceled spectrum according to frequency order. An inverse fast Fourier transform is performed on the canceled spectrum, the real part is taken and multiplied by two to compensate the transform coefficients, and the time-domain signal after the relative intensity noise is canceled is obtained.

[0006] In a preferred embodiment, the projection coefficient is calculated as follows: Multiply the conjugate of the i-th signal path segment and the i-th reference path segment, take the modulus value, and sum them over all frequency points to obtain the first accumulated value; sum the square of the modulus of the i-th reference path segment over all frequency points to obtain the second accumulated value; divide the first accumulated value by the second accumulated value to obtain the projection coefficient.

[0007] In a preferred embodiment, the predetermined length refers to the fact that the number of Fast Fourier Transform points of the two data paths, the signal path spectrum and the reference path spectrum, is the same, and is denoted as the predetermined length.

[0008] In a preferred embodiment, dynamically adjusting the frequency band allocation specifically involves: Calculate the relative difference of the projection coefficients of all adjacent initial frequency bands. The relative difference is the absolute value of the difference between two adjacent projection coefficients divided by the larger of the two, and then multiplied by 100%. Set a preset threshold, and merge two adjacent initial frequency bands with a relative difference greater than the preset threshold into one frequency band to be adjusted. For each frequency band to be adjusted, redivide it into multiple sub-frequency bands, and recalculate the projection coefficient of each sub-frequency band and the relative difference of the projection coefficients between adjacent sub-frequency bands. If the relative difference between sub-frequency bands is still greater than the preset threshold, continue to merge the adjacent sub-frequency bands and subdivide them again until the relative difference of the projection coefficients of all adjacent sub-frequency bands is no greater than the preset threshold.

[0009] In a preferred embodiment, the preset threshold is set to 5% to 20%.

[0010] In a preferred embodiment, the frequency band to be adjusted is re-divided into four to eight sub-bands.

[0011] In a preferred embodiment, the method for calculating the projection result for each adaptive frequency band is as follows: The projection result is obtained by multiplying the reference path spectrum segment of the adaptive frequency band by the projection coefficient of the adaptive frequency band.

[0012] In a preferred embodiment, after obtaining the time-domain signal after canceling the relative light intensity noise, the time-domain signal is subjected to digital filtering processing, specifically as follows: The comb tooth spacing of the optical frequency comb is determined based on the repetition frequency of the femtosecond pulse laser. The time-domain signal after relative intensity noise cancellation is divided into multiple sub-bands in the frequency domain, with each sub-band corresponding to an optical frequency comb tooth channel. A digital bandpass filter bank is used to filter each sub-band separately to extract the effective signal component carried by each comb tooth channel. The effective signal component refers to the original electric field signal that has been down-converted to baseband or intermediate frequency after modulation by an optical electric field sensor.

[0013] The technical effects and advantages of this invention are as follows: This invention divides an optical frequency comb into a signal path and a noise reference path, and processes the synchronously acquired data from both paths in a personal computer. After dividing the initial frequency bands within the effective frequency range, the projection coefficients of each frequency band are calculated. The frequency band division is dynamically adjusted based on the relative differences between the projection coefficients of adjacent initial frequency bands to obtain adaptive frequency bands. Then, within each adaptive frequency band, the projection result of the signal path spectrum segment onto the reference path spectrum segment is calculated and subtracted. Finally, the time-domain signal after relative intensity noise cancellation is obtained through spectrum reconstruction and inverse fast Fourier transform. This invention can effectively suppress the influence of relative intensity noise of the light source on the signal reception quality in the optical frequency comb receiving system and improve the system's ability to sense weak electric field signals.

[0014] This invention uses digital signal processing to achieve noise cancellation, eliminating the need for complex hardware such as differential optical electric field sensors and fine adjustments to parameters such as time delay, phase, and amplitude in the optical path. It is compatible with ordinary optical electric field sensors and electro-optic modulator receiving systems, reducing the hardware complexity and manufacturing cost of the receiving system, while simplifying the system deployment and maintenance process.

[0015] This invention dynamically adjusts the frequency band division based on the relative difference between the projection coefficients of adjacent initial frequency bands, enabling the frequency band division to adaptively match the optical link delay and phase drift caused by environmental changes. This maintains the stability of the noise cancellation effect under different operating conditions and improves the system's adaptability to changes in environmental factors such as temperature and vibration. Attached Figure Description

[0016] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings; Figure 1 This is a schematic diagram of a noise digital cancellation method for an optical frequency comb receiving system according to the present invention.

[0017] Figure 2 This is a schematic diagram of the noise cancellation result after the implementation of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] Reference Figures 1-2 The following examples were obtained: Example 1: A method for digital noise cancellation in an optical frequency comb receiving system includes the following steps: An optical frequency comb receiving system is constructed, comprising a femtosecond pulsed laser, a polarization-maintaining fiber beam splitter, an optical electric field sensor, a first photodetector, a second photodetector, a multi-channel data acquisition card, and a personal computer. The optical frequency comb excitation emitted by the femtosecond pulsed laser is split into two paths by the polarization-maintaining fiber beam splitter. The first path of optical frequency comb excitation enters the optical electric field sensor, and after spatial electric field modulation, enters the first photodetector, defined as the signal path. The second path of optical frequency comb excitation directly enters the second photodetector, defined as the noise reference path. The radio frequency voltage signals output by the first and second photodetectors are respectively connected to the first and second channels of the multi-channel data acquisition card for synchronous data acquisition. This step constructs a photoelectric detection platform capable of simultaneously acquiring the signal path carrying electric field information and the reference path containing only source noise. Synchronous acquisition ensures that the two signals are strictly aligned in time, providing a hardware foundation for subsequent digital processing using the correlation of noise from the same source.

[0020] The personal computer processes the data acquired by the multi-channel data acquisition card, defining the data acquired from the first channel as signal path data and the data acquired from the second channel as reference path data. Fast Fourier Transform (FFT) is performed on both the signal path data and the reference path data to obtain their respective spectra, with predetermined lengths set. This step converts the time-domain signal to the frequency domain, ensuring a one-to-one correspondence between the two spectra on the frequency axis through the same number of FFT points (predetermined length), providing a unified frequency resolution for subsequent frequency band analysis of noise correlation.

[0021] The effective frequency range from the first frequency point to half the predetermined length in both the signal path spectrum and the reference path spectrum is evenly divided into multiple initial frequency bands. The signal path spectrum segment corresponding to the i-th initial frequency band is denoted as the i-th signal path segment, and the corresponding reference path spectrum segment is denoted as the i-th reference path segment. This step of evenly dividing the effective frequency band into several initial frequency bands is to examine the noise correlation between the signal path and the reference path within a local range, laying the foundation for subsequent dynamic adjustment of the frequency band division. Furthermore, the even division method is simple, efficient, and convenient for initial processing. The value of i is an integer greater than or equal to 1.

[0022] The projection coefficients for each initial frequency band are calculated. The projection coefficients are the projections of the i-th signal path segment onto the i-th reference path segment. This step quantifies the amplitude ratio between the relative intensity noise of the light source in the signal path and the noise in the reference path within each initial frequency band through projection operations. The magnitude of the projection coefficients directly reflects the correlation between the two noise sources in that frequency band, providing an accurate estimate for subsequently subtracting noise components from the signal path.

[0023] The frequency band division is dynamically adjusted based on the relative differences between the projection coefficients of adjacent initial frequency bands, and then all the resulting frequency bands are used as adaptive frequency bands. This step identifies frequency regions where noise correlation changes abruptly due to environmental variations by comparing the relative differences in the projection coefficients of adjacent frequency bands. This allows for the merging or refinement of frequency bands, ensuring that the final frequency band division adaptively matches the actual optical link state and maintains stable noise correlation within each frequency band, thereby improving the accuracy of projection estimation.

[0024] For each adaptive frequency band, the projection result of the signal path spectrum segment of that adaptive frequency band onto the reference path spectrum segment of that adaptive frequency band is calculated. The projection result is subtracted from the signal path spectrum segment to obtain the noise-cancelled signal path spectrum segment. The noise-cancelled signal path spectrum segments corresponding to all adaptive frequency bands are reassembled in frequency order to form a complete canceled spectrum. An inverse fast Fourier transform is performed on the canceled spectrum, and the real part is taken and multiplied by two to compensate the transform coefficients, resulting in the time-domain signal after relative intensity noise cancellation. This step completes noise cancellation within each adaptive frequency band using projection coefficients. Then, the cancellation results of all frequency bands are stitched together to form a complete spectrum, and finally, the time-domain signal is recovered through an inverse transform. The entire process achieves adaptive suppression of relative intensity noise of the light source, effectively cancels the noise component in the output signal, and significantly improves the quality of the electric field signal.

[0025] A femtosecond pulsed laser is an ultrashort pulse laser capable of outputting pulses with pulse widths in the femtosecond range. Femtosecond lasers are typically achieved through mode-locking technology, which locks the phase relationship between different longitudinal modes in the laser, thereby generating extremely narrow pulse outputs. Femtosecond lasers possess extremely high time resolution and peak power, and have virtually no thermal effects, making them suitable for applications such as ultrafast process research and ultra-precision machining. Application in this invention: The femtosecond pulsed laser of this invention serves as the excitation source for an optical frequency comb, generating an optical frequency comb signal with wide spectral coverage and a stable repetition frequency. This optical frequency comb is split into two paths: one path enters an optical electric field sensor to perform optical undersampling of the electric field under test, shifting the high-frequency electric field signal to the low-frequency baseband; the other path is directly used as a noise reference. The characteristics of the optical frequency comb output by the femtosecond laser (repetition frequency, spectral flatness) directly affect the system's receiving bandwidth and noise cancellation effect.

[0026] A polarization-maintaining fiber beam splitter is an optical power distribution device based on polarization-maintaining fiber. It can split an input optical signal into two or more outputs at a certain ratio, while preserving the polarization state of the input light at the output end. Polarization-maintaining fiber introduces stress regions on both sides of the fiber core, causing two orthogonal polarization modes to have different propagation constants, thereby suppressing polarization mode coupling and maintaining the stability of the polarization direction of linearly polarized light. Application in this invention: The polarization-maintaining fiber beam splitter of this invention splits the optical frequency comb excitation output from a femtosecond pulsed laser into two paths: the first path enters an optical electric field sensor to form a signal path, and the second path directly enters a second photodetector to form a noise reference path. Since the two signals originate from the same source and have the same polarization state, the relative intensity noise of the light source in the signal path and the reference path is strongly correlated, providing a physical basis for subsequent digital cancellation.

[0027] An optical electric field sensor is a sensing device based on the Pockels effect. When an electric field is applied to an electro-optic crystal, the crystal's refractive index changes, causing modulation of the phase or polarization state of the light wave passing through the crystal. By detecting this modulation, the intensity, frequency, and phase information of the electric field to be measured can be retrieved. Optical electric field sensors have advantages such as wide bandwidth, fast response, and strong resistance to electromagnetic interference. In this invention: The optical electric field sensor of this invention is located in the signal path. The electric field signal to be measured is modulated onto an optical frequency comb carrier wave through the sensor. Because the optical frequency comb generated by the femtosecond pulsed laser has wide spectrum characteristics, the high-frequency electric field signal will be down-converted to the baseband or intermediate frequency range, thereby realizing the reception of broadband electric field signals. The modulation signal introduced by the sensor is the effective signal component that this invention needs to retain.

[0028] A photodetector is a device that converts optical signals into electrical signals. Its core component is a photodiode (such as a PIN diode or avalanche photodiode), which converts incident photons into photogenerated carriers through the photoelectric effect, forming a photocurrent. Key parameters of a photodetector include responsivity, bandwidth, noise equivalent power, and dynamic range. In an optical frequency comb receiving system, the photodetector converts a sequence of optical pulses into a radio frequency voltage signal for subsequent data acquisition and processing. In this invention: The first photodetector receives the optical signal from the signal path modulated by an optical electric field sensor and converts it into a radio frequency voltage signal containing the electric field signal and various types of noise. The second photodetector directly receives the unmodulated reference optical signal and converts it into a radio frequency voltage signal containing only relative intensity noise, shot noise, and thermal noise from the light source. The signals output from the two photodetectors are acquired synchronously, providing a data basis for subsequent noise correlation cancellation.

[0029] A multi-channel data acquisition card is a hardware device capable of simultaneously acquiring analog signals from multiple channels and converting them into digital signals. Its core components include an input amplifier, an anti-aliasing filter, an analog-to-digital converter, a clock synchronization circuit, and a high-speed data transmission interface. For high-precision measurements requiring maintaining the phase relationship between channels, the acquisition card must employ a synchronous sampling architecture—that is, each channel is equipped with an independent sample-and-hold circuit and an analog-to-digital converter to ensure that the signals from each channel are sampled at the same time. Application in this invention: The multi-channel data acquisition card of this invention has at least two synchronous sampling channels. The first channel acquires the RF voltage of the signal path of the first photodetector, and the second channel acquires the RF voltage of the reference path of the second photodetector. Synchronous sampling ensures that the two signals are strictly aligned in time, which is a prerequisite for subsequent frequency domain projection cancellation. The sampling rate of the acquisition card must meet the requirements of the photodetector's output bandwidth (e.g., 400 MSaps corresponds to 100 MHz bandwidth in the embodiment), and the data length determines the spectral resolution.

[0030] A personal computer, serving as a platform for data processing and algorithm execution, receives digital signals transmitted from a multi-channel data acquisition card, executes digital signal processing algorithms such as Fast Fourier Transform, projection coefficient calculation, dynamic frequency band division, noise cancellation, and inverse transform, and outputs the canceled time-domain signal. Its core performance indicators include processor computing power, memory capacity, and high-speed data transmission interfaces (such as PCIe, USB 3.0 / 4.0, Thunderbolt, etc.). Application in this invention: The personal computer of this invention runs a specially developed noise cancellation algorithm, including spectrum transformation, initial frequency band division, projection coefficient calculation, dynamic frequency band adjustment, noise cancellation, and time-domain reconstruction. Adaptive noise suppression is achieved through software algorithms, eliminating the need to adjust the hardware optical path, thus improving the system's environmental adaptability and deployment convenience.

[0031] The projection coefficient is calculated as follows: Multiply the conjugates of the i-th signal path segment and the i-th reference path segment, take the modulus, and sum these values ​​over all frequency points to obtain the first accumulated value. Then, sum the squares of the modulus of the i-th reference path segment over all frequency points to obtain the second accumulated value. Divide the first accumulated value by the second accumulated value to obtain the projection coefficient. This step mathematically quantifies the noise correlation between the signal path and the reference path within the same frequency band. Specifically, multiplying the conjugates of the two signals extracts their phase alignment. Summing the modulus values ​​reflects the total correlation strength of the two signals across all frequency points in that frequency band; the first accumulated value represents the magnitude of the component in the signal path that is correlated with the reference path. Summing the squares of the reference path segment moduli yields the second accumulated value, representing the total energy of the reference path itself. The ratio of the two is the projection coefficient, which represents the proportion of the component in the signal path that is proportional to the noise in the reference path. Since the relative intensity noise of the light sources in the two paths originates from the same laser and has a strong correlation, the projection coefficient can accurately estimate the amplitude of the noise component in the signal path, providing a precise numerical basis for subtracting the component from the signal path.

[0032] The predetermined length refers to the fact that the number of Fast Fourier Transform (FFT) points for both the signal path spectrum and the reference path spectrum is the same, and this is recorded as the predetermined length. This setting ensures that the signal path spectrum and the reference path spectrum have completely consistent frequency resolution and frequency point correspondence on the frequency axis. The Fast Fourier Transform (FFT) is an algorithm that converts a time-domain signal into a frequency-domain signal. The number of transform points determines the frequency interval of the output spectrum: the more points, the higher the frequency resolution, and the finer the frequency details that can be distinguished. When both data paths use the same number of transform points, their spectra represent the same actual frequency at each frequency point, allowing the k-th frequency point of the signal path spectrum to be directly compared with the k-th frequency point of the reference path spectrum. This is the basis for subsequent frequency band processing, calculation of projection coefficients, and noise cancellation, ensuring that all frequency domain operations are performed on the same frequency reference, avoiding errors introduced by frequency point misalignment.

[0033] The relative difference of the projection coefficients of all adjacent initial frequency bands is calculated. The relative difference is the absolute value of the difference between two adjacent projection coefficients divided by the larger of the two, and then multiplied by 100%. A preset threshold is set, and two adjacent initial frequency bands with a relative difference greater than the preset threshold are merged into a single frequency band to be adjusted. For each frequency band to be adjusted, it is redivided into multiple sub-frequency bands, and the projection coefficients of each sub-frequency band and the relative difference of the projection coefficients between adjacent sub-frequency bands are recalculated. If the relative difference between sub-frequency bands is still greater than the preset threshold, the adjacent sub-frequency bands are merged and subdivided again until the relative difference of the projection coefficients of all adjacent sub-frequency bands is no greater than the preset threshold. This step identifies regions where noise correlation changes abruptly in the frequency domain by monitoring the relative changes in the projection coefficients of adjacent frequency bands. Environmental factors (such as temperature changes and vibrations) can cause slow drift in fiber optic link delay and phase. The correlation changes caused by this drift at different frequency points are non-uniform, manifested as large differences in the projection coefficients of adjacent frequency bands. By calculating the relative difference, the degree of this abrupt change can be quantified. Merging adjacent frequency bands with significant differences is necessary because they may have originally been located in a region of continuously varying correlation, resulting in a wider band to be adjusted. Further finer-grained subdivision of this band aims to find more precise correlation boundaries within a local area. This merging and subdivision process iteratively continues until the relative differences between all adjacent sub-bands do not exceed a preset threshold. The resulting band division ensures that the noise correlation within each band is essentially consistent, thus providing a more accurate local linear model for subsequent projection estimation.

[0034] The preset threshold is set between 5% and 20%, a range chosen based on a balance between the analysis of the noise characteristics of the optical frequency comb system and engineering practice. If the threshold is set too low (below 5%), even minor noise fluctuations or calculation errors will trigger frequency band adjustments, resulting in overly fragmented frequency band divisions. This not only increases the computational burden but may also introduce unnecessary frequency band boundaries due to overfitting the noise. If the threshold is set too high (above 20%), it may fail to capture correlation shifts caused by environmental changes in a timely manner, leaving significant changes in the projection coefficients within some frequency bands and reducing the noise cancellation effect. The 5% to 20% range effectively filters out normal noise fluctuations while also responding to correlation drift caused by environmental changes, making it suitable for the actual operating conditions of most optical frequency comb receiving systems.

[0035] The frequency band to be adjusted is redivided into four to eight sub-bands. This number strikes a balance between frequency domain resolution and computational efficiency. Dividing into four sub-bands ensures that the subdivision can initially reveal the details within the band, while eight sub-bands represent a finer granularity achievable without significantly increasing computational complexity. If the number of sub-bands is less than four, the subdivision effect is not obvious and may not accurately capture changes in local correlations within the band. If the number of sub-bands is more than eight, multiple rounds of iterative merging may be required, multiplying the computational load. Furthermore, overly fine divisions are susceptible to random noise, which can reduce stability. Four to eight sub-bands provide sufficient frequency domain detail in a single subdivision while ensuring rapid convergence of the iterative process, representing a reasonable balance between effectiveness and efficiency in practical applications.

[0036] For each adaptive frequency band, the projection result is calculated as follows: the reference path spectrum segment of the adaptive frequency band is multiplied by the projection coefficient of that adaptive frequency band to obtain the projection result. This step uses the determined projection coefficient and the reference path spectrum segment to reconstruct the component in the signal path contributed by the relative intensity noise of the light source. The projection coefficient is a scaling factor obtained through correlation analysis within the corresponding frequency band, which accurately reflects the amplitude ratio between the noise in the signal path and the noise in the reference path within that frequency band. Since the reference path spectrum segment contains the relative intensity noise of the light source that originates from the same source as the signal path, and this noise is the only correlated component between the two signals, directly multiplying the reference path spectrum segment by the projection coefficient yields an accurate estimate of this noise component in the signal path. This estimate is called the projection result, and it represents the noise portion that needs to be subtracted from the signal path. Through this linear scaling method, the specific waveform or phase details of the noise do not need to be known; the noise component can be reconstructed using only the correlation, providing an accurate subtraction term for subsequent noise cancellation.

[0037] After obtaining the time-domain signal with noise cancellation due to relative light intensity, digital filtering is performed on it. This step further refines and extracts the signal based on noise cancellation. Although noise cancellation removes relative light intensity noise, the signal may still contain out-of-band noise or other interference components. Furthermore, the optical frequency comb receiving system shifts the broadband electric field signal to multiple comb channels, each carrying electric field information in a different frequency band. Therefore, digital filtering is needed to separate the effective signals from each channel for subsequent analysis and demodulation.

[0038] The spacing of the optical frequency comb is determined based on the repetition frequency of the femtosecond pulsed laser. The time-domain signal, after relative intensity noise cancellation, is divided into multiple sub-bands in the frequency domain, each corresponding to a single optical frequency comb channel. The optical frequency comb generated by the femtosecond pulsed laser appears in the frequency domain as a series of equally spaced comb teeth, with the frequency interval between adjacent teeth equal to the laser's repetition frequency. Therefore, the repetition frequency determines the comb tooth spacing, which in turn determines the bandwidth occupied by each comb channel. After converting the time-domain signal to the frequency domain, the entire frequency band is divided into multiple sub-bands according to the comb tooth spacing, with each sub-band corresponding to the frequency range of a single comb channel. This division ensures that the signal within each sub-band originates from the sampling action of the same optical frequency comb tooth, facilitating subsequent separate processing of each channel.

[0039] A digital bandpass filter bank is used to filter each sub-band separately, extracting the effective signal components carried by each comb channel. The digital bandpass filter bank is a set of filters with center frequencies and bandwidths matched to each sub-band. Each filter allows only the frequency components within its corresponding sub-band to pass through, while suppressing noise and interference from other frequency bands. After filtering, the effective signal components in each comb channel are separated. These signal components contain the information of the original electric field signal after down-conversion. Since different comb channels correspond to different original frequency ranges, the filtering process achieves channelized reception, enabling the system to simultaneously acquire electric field signals from multiple frequency bands.

[0040] The effective signal component refers to the original electric field signal after being down-converted to baseband or intermediate frequency by an optical electric field sensor. In an optical frequency comb receiving system, the high-frequency electric field signal is shifted to the low-frequency baseband or intermediate frequency range through optical sampling. The specific shifted frequency depends on the difference between the original signal frequency and the frequency of the nearest optical frequency comb tooth. These shifted signals carry the amplitude and phase information of the original electric field and are the target signals that the system ultimately needs to acquire. Through the above filtering process, these effective signal components are separated from noise and interference, providing a clean signal basis for subsequent measurements and analysis.

[0041] Before synchronous data acquisition, time delay calibration can be performed on the signal path and the noise reference path to align the two signals in time. Time delay calibration can be achieved through various techniques, which users can choose based on system conditions and accuracy requirements. A common method is to use an adjustable fiber delay line connected to the shorter optical path (usually the noise reference path). The fiber length is adjusted mechanically or electrically to compensate for the optical path difference with the signal path, ensuring both optical signals arrive at the photodetector simultaneously. Another method is electrical domain delay calibration, which involves digitally delaying the two digitized signals after data acquisition and before digital processing. Time alignment is achieved in the digital domain through interpolation or shifting operations. This method requires no hardware adjustment and offers high flexibility. Higher precision time delay calibration can employ a phase detection-based feedback control scheme. By monitoring the phase difference of common-source noise in the two signals, the delay is adjusted in real time until the phase difference is zero, achieving automatic closed-loop calibration. In the field of fiber optic time and frequency transfer, high-precision time delay calibration techniques based on bidirectional comparison and round-trip delay measurement have also been developed, achieving femtosecond-level time alignment accuracy. Users can choose one or more combinations of the above methods for time delay calibration based on the system's calibration accuracy requirements, cost budget, and whether hardware adjustments are permissible. Alternatively, other existing technologies that achieve the same purpose and effect can be used, as long as the signal path and noise reference path are precisely aligned in time. However, it should be noted that time delay calibration is actually only an optional operation; its absence does not affect the specific implementation of this invention. Figure 2 The noise cancellation results in the data fully demonstrate that differences in fiber length (delay) do not affect the noise cancellation effect.

[0042] The above algorithms or formulas are all dimensionless and numerical calculations, and the results are obtained by software simulation based on a large amount of collected data to obtain the most recent real-world results. The preset parameters are set by those skilled in the art according to the actual situation.

[0043] It should be understood that in the various embodiments of this application, the order of the above-mentioned processes does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0044] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0045] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the devices and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0046] The above are merely specific embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for digital noise cancellation in an optical frequency comb receiving system, characterized in that, Includes the following steps: An optical frequency comb receiving system was constructed, comprising a femtosecond pulsed laser, a polarization-maintaining fiber beam splitter, an optical electric field sensor, a first photodetector, a second photodetector, a multi-channel data acquisition card, and a personal computer. The optical frequency comb excitation emitted by the femtosecond pulsed laser was split into two paths by the polarization-maintaining fiber beam splitter. The first path of optical frequency comb excitation entered the optical electric field sensor, and after spatial electric field modulation, entered the first photodetector, defined as the signal path. The second path of optical frequency comb excitation directly entered the second photodetector, defined as the noise reference path. The radio frequency voltage signals output by the first and second photodetectors were connected to the first and second channels of the multi-channel data acquisition card, respectively, for synchronous data acquisition. The personal computer processes the data acquired by the multi-channel data acquisition card, defining the data acquired by the first channel as signal path data and the data acquired by the second channel as reference path data; Perform Fast Fourier Transform on the signal path data and the reference path data respectively to obtain the signal path spectrum and the reference path spectrum, and set a predetermined length; The effective frequency range from the first frequency point to half the predetermined length in the signal path spectrum and the reference path spectrum is divided into multiple initial frequency bands. The signal path spectrum segment corresponding to the i-th initial frequency band is denoted as the i-th signal path segment, and the corresponding reference path spectrum segment is denoted as the i-th reference path segment. Calculate the projection coefficient for each initial frequency band. The projection coefficient is the projection of the i-th signal path segment onto the i-th reference path segment. The frequency band division is dynamically adjusted based on the relative difference between the projection coefficients of adjacent initial frequency bands, and then all the divided frequency bands are used as adaptive frequency bands. For each adaptive frequency band, the projection result of the signal path spectrum segment of the adaptive frequency band onto the reference path spectrum segment of the adaptive frequency band is calculated. The projection result is subtracted from the signal path spectrum segment to obtain the noise-cancelled signal path spectrum segment. The noise-cancelled signal path spectrum segments corresponding to all adaptive frequency bands are recombined into a complete canceled spectrum according to frequency order. An inverse fast Fourier transform is performed on the canceled spectrum, the real part is taken and multiplied by two to compensate the transform coefficients, and the time-domain signal after the relative intensity noise is canceled is obtained.

2. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 1, characterized in that, The projection coefficient is calculated as follows: Multiply the conjugate of the i-th signal path segment and the i-th reference path segment, take the modulus value, and sum them over all frequency points to obtain the first accumulated value; The square of the modulus of the i-th reference path segment is summed over all frequency points to obtain the second accumulated value; the first accumulated value is divided by the second accumulated value to obtain the projection coefficient.

3. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 1, characterized in that, The predetermined length refers to the fact that the number of Fast Fourier Transform points of the two data paths, the signal path spectrum and the reference path spectrum, is the same, and both are recorded as the predetermined length.

4. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 3, characterized in that, The dynamic adjustment of frequency band allocation is specifically as follows: Calculate the relative difference of the projection coefficients of all adjacent initial frequency bands. The relative difference is the absolute value of the difference between two adjacent projection coefficients divided by the larger of the two, and then multiplied by 100%. Set a preset threshold, and merge two adjacent initial frequency bands with a relative difference greater than the preset threshold into one frequency band to be adjusted. For each frequency band to be adjusted, redivide it into multiple sub-frequency bands, and recalculate the projection coefficient of each sub-frequency band and the relative difference of the projection coefficients between adjacent sub-frequency bands. If the relative difference between sub-frequency bands is still greater than the preset threshold, continue to merge the adjacent sub-frequency bands and subdivide them again until the relative difference of the projection coefficients of all adjacent sub-frequency bands is no greater than the preset threshold.

5. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 4, characterized in that, The preset threshold is set to 5% to 20%.

6. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 4, characterized in that, The frequency band to be adjusted will be redivided into four to eight sub-bands.

7. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 1, characterized in that, The method for calculating the projection results for each adaptive frequency band is as follows: The projection result is obtained by multiplying the reference path spectrum segment of the adaptive frequency band by the projection coefficient of the adaptive frequency band.

8. The method for digital noise cancellation in an optical frequency comb receiving system according to claim 7, characterized in that, After obtaining the time-domain signal with canceled relative light intensity noise, digital filtering is performed on the time-domain signal, specifically as follows: The comb tooth spacing of the optical frequency comb is determined based on the repetition frequency of the femtosecond pulse laser. The time-domain signal after relative intensity noise cancellation is divided into multiple sub-bands in the frequency domain, with each sub-band corresponding to an optical frequency comb tooth channel. A digital bandpass filter bank is used to filter each sub-band separately to extract the effective signal components carried by each comb tooth channel.

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