Optical sampling signal holding method for photonic analog-to-digital conversion system

Abstract

A method for holding an optical sampling signal of a photon analog-to-digital conversion system is provided. Based on the frequency response principle of sampling and holding, the photoelectric conversion process after photon sampling is controlled to be equivalent to the signal holding effect in switch sampling, and the sampled optical pulse is converted into a special holding waveform, which directly eliminates the time mismatch between the electronic analog-to-digital converter at the back end and the optical pulse. The photoelectric conversion frequency response of the present invention does not increase additional active devices and software overhead, greatly improves the performance of the photon analog-to-digital conversion system, and is not limited by the number of channels, and can provide a more reliable technical solution for future photon analog-to-digital conversion systems that realize high sampling rates.

Description

Technical Field

[0001] The invention relates to photon information processing, in particular to a method for maintaining an optical sampling signal of a photon analog-to-digital conversion system. Background Art

[0002] Photonic analog-to-digital conversion technology is an effective means to overcome the performance bottleneck of traditional electronic analog-to-digital converters. Among the many types of photonic analog-to-digital conversion technologies, optical sampling and quantization photonic analog-to-digital conversion technology combines the advantages of photon sampling (ultra-low jitter of mode-locked lasers and ultra-large bandwidth of modulators, etc.) and electronic quantization (high precision and mature technology, etc.), has made great progress and has moved towards practical application. The photon multi-channel demultiplexing technology enables multiple parallel electronic analog-to-digital converters to operate at a lower clock frequency, while still achieving a high sampling rate as a whole. In this scheme, photodetectors are important devices that combine the photon sampling part and the electronic quantization part, and their frequency response has been explored by many researchers. Based on the low-pass filtering model, researchers studied the influence of photodetection bandwidth on the frequency response of the entire photonic analog-to-digital conversion system and the influence on the electronic sampling clock jitter, and pointed out that the bandwidth of the photodetector can be set to half of the single-channel sampling rate [F.Su, et al., "Effects of the photonic sampling pulse width and thephotodetection bandwidth on the channel response of photonic ADCs." Opticsexpress.Vol.24, No.2, 924-934, 2016.]. On this basis, some researchers equated the sampled optical pulse to a digital code element, and corrected the inter-symbol interference and other problems generated by the sampling pulse after low-pass filtering through digital compensation, etc., to improve the flatness of the system frequency response [Z.Jin, et al., "Equflization based inter symbolinterference mitigation for time-interleaved photonic analog-to-digitalconverters." Optics Express.Vol.26, No.26, 34373-34383, 2018.]. However, they all ignore the role of photodetectors in signal retention of a photonic analog-to-digital conversion system, which can directly eliminate the delay mismatch between photon sampling and electronic quantization.

[0003] Traditional electronic analog-to-digital converters contain at least one sample-and-hold circuit to ensure that the input signal is constant during the analog-to-digital conversion period, thereby ensuring the accuracy of the output data. The sample-and-hold circuit enables the analog-to-digital converter to process rapidly changing high-frequency signals, and the subsequent encoding device can complete the accurate analog-to-digital conversion process at any time within the holding window. However, the existing photonic analog-to-digital conversion technology lacks the sample-and-hold device. The photon clock used for sampling is a series of extremely narrow pulse sequences in the time domain. Even if it can be decomposed into multiple low-speed pulses using technologies such as time wavelength interleaving or channel interleaving, the original sampling point still exists at the apex of a single pulse. Usually, the back-end electrical quantization analog-to-digital converter is difficult to accurately match the position of the pulse apex, resulting in mismatched spurs. In solutions with higher rates and more channels, the digital quantization accuracy is often difficult to further improve. This is largely due to the deterioration of the digital quantization results caused by the time mismatch introduced by the electrical quantization process in each channel. On the other hand, most of the current time mismatch compensation methods draw on the channel mismatch compensation principle used in electronic analog-to-digital conversion technology, and require a large amount of software and hardware resources for mismatch extraction and compensation, which is not conducive to the further development of the advantages of high speed and high precision of photonic analog-to-digital conversion technology. Therefore, in order to achieve high-speed and high-precision photonic analog-to-digital conversion, a low-complexity method is needed to efficiently eliminate time mismatch. Summary of the invention

[0004] The purpose of the present invention is to propose a photoelectric detection design and sampling and holding method in a photon analog-to-digital conversion system in view of the deficiencies of the prior art. The specific operation of the method is to selectively retain and filter the high-frequency components of the signal generated by the optical pulse sampling by utilizing the frequency response of the photoelectric detector. These high-frequency components will be folded back to the signal frequency position when quantized by an electronic analog-to-digital converter of the same rate, equivalently creating a time offset opposite to the time mismatch and offsetting the time mismatch existing in the system, and finally obtaining the correct output when there is no time mismatch. It is only necessary to control the frequency response of the photoelectric conversion to create a time offset that is always opposite to the time mismatch before quantization, thereby fundamentally suppressing the generation of time mismatch. The method is only related to the optical pulse rate and has nothing to do with the number of channels used in the system, so there is no need to perform complex error extraction and compensation operations on each channel, so that the photon analog-to-digital conversion technology can improve the system sampling rate while ensuring that the digital quantization accuracy is not deteriorated by the time mismatch. In the design of the next generation of high-speed and high-precision photon analog-to-digital conversion systems, this method is expected to become the most easily implemented and practical solution.

[0005] The principle of the present invention is demonstrated as follows:

[0006] In a photon analog-to-digital conversion system using optical pulses as the sampling medium, whether there is a demultiplexing process or not, in one channel, assuming that the sampled signal is a signal with a frequency of fin The signal is repeated at a frequency of F. s The direct sampling of the optical pulse causes the periodic extension of the spectrum, generating many high-frequency components in addition to the original signal. These high-frequency components exist symmetrically around the multiples of the optical pulse sampling rate, and the frequency can be expressed as kF s ±f in , after an impulse response of h OE (t) The output v of the photodetector after out (t) is shown in the following formula, and its frequency domain is Figure 2 As shown in (a),

[0007]

[0008] Where k is a positive integer greater than or equal to 1, and the phase signs of each pair of high-frequency components are opposite. Ideally, the back-end electronic analog-to-digital converter samples the pulse apex at the same sampling rate, that is, specify t = nT s , n is an integer greater than or equal to zero. In the final sampled data, the high-frequency components in the analog signal are folded back into the Nyquist interval without any phase shift. The expression of the folding process is shown in the following formula:

[0009]

[0010] where f d is the digital frequency within the Nyquist bandwidth. However, in reality, there is usually an arbitrary delay mismatch τ between the electronic analog-to-digital converter and the optical pulse. After sampling, the phase of the high-frequency component folded back to the Nyquist interval will contain a time mismatch τ, as shown in the following formula. The frequency domain process is as follows: Figure 2 (b)

[0011]

[0012] The result of superimposing these high-frequency components with the original signal in the baseband is a signal vector with the same frequency but a phase change, which is equivalent to an electronic analog-to-digital converter with time mismatch collecting a new signal vector. The final digital sampling sequence y out (n) is shown in the following formula. The calculation process of vector superposition is as follows: Figure 2 (c)

[0013]

[0014] The original signal is shifted by an equivalent delay τ eff It can be calculated based on the operation rules of vector addition, such as Figure 2 As shown in (c), the amplitude change of the superimposed signal can be removed by simple calibration or normalization, and the signal equivalent delay τ effAs shown below

[0015]

[0016] Among them, H OE (f) is a photodetector with controlled frequency response. By controlling the amplitude of these high-frequency components, the equivalent delay τ can be controlled. eff The original time mismatch τ is offset or reduced to a level that is not enough to affect the system performance.

[0017] Formula (5) indicates that high-frequency components exist in pairs outside the Nyquist bandwidth. In the traditional low-pass filter response, high-frequency components are either completely filtered out or retained in pairs. The equivalent delay τ eff The present invention proposes a type of photoelectric conversion response, whose amplitude-frequency response function can be regarded as a combination of a rectangular window function and a triangular window function. The frequency response is 0.5F. s Keep constant within 0.5F s The outside of the sample decays to zero in a triangular shape, and its 3dB bandwidth is the sampling rate F s The larger the bandwidth, the more high-frequency components are retained. This type of frequency response is shown in the following formula:

[0018]

[0019] Where f is the independent variable of the frequency response expression, m is the number of high-frequency component pairs retained, and the 3dB bandwidth of this type of frequency response is (m+1)*F s / 2, the more high-frequency component pairs are retained, the greater the equivalent signal delay τ eff The closer to the delay mismatch τ, the larger the bandwidth is, and the more difficult it is to achieve. In addition, in actual situations, the delay mismatch is usually a very small amount at the ps level. Therefore, in general, m can be selected as 1 to achieve a certain retention effect. You can also select a positive integer greater than 1 according to actual conditions. The method of using this frequency response in the system includes but is not limited to directly designing the photodetector response or adding a filter that conforms to the response after a large-bandwidth photodetector. The photodetector with this controlled frequency response will widen the sampled narrow pulse into an approximately ideal triangular waveform, and its vertex amplitude is equal to the amplitude of the sampling point. At this time, any time mismatch τ will only cause the original pulse vertex amplitude to shift up and down, which will appear as an easily eliminated DC component in the final recovered data, achieving the effect of signal retention, thereby having a counteracting effect on the time mismatch.

[0020] A method for maintaining an optical sampling signal in a photonic analog-to-digital conversion system comprises the following steps:

[0021] ①Assume that the sampled signal has a frequency of f in The signal is repeated at a frequency of F.s The optical pulse sampling causes the periodic extension of the spectrum, generating many high-frequency components in addition to the original signal. These high-frequency components exist symmetrically around the multiples of the optical pulse sampling rate, with a frequency of kF s ±f in ;

[0022] ② Control the frequency response of the photodetector |H OE (f)|, as shown below:

[0023]

[0024] Where f is the independent variable of the frequency response expression, m is the number of high-frequency component pairs retained, and the 3dB bandwidth of this type of frequency response is (m+1)*F s / 2, the more high-frequency component pairs are retained, the greater the equivalent signal delay τ eff The closer to the delay mismatch τ, the larger the bandwidth and the more difficult it is to achieve. In addition, in actual situations, the delay mismatch is usually a very small amount at the ps level. Therefore, in general, m can be selected as 1 to achieve a certain holding effect. You can also select a positive integer greater than 1 according to actual conditions. The method of using this frequency response in the system includes but is not limited to directly designing the photodetector response or adding a filter that meets the response after a large-bandwidth photodetector.

[0025] ③ Input the light pulse into a photodetector with controlled frequency response to obtain a retained electrical signal, and then obtain an electrical digital signal through an electronic analog-to-digital converter.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. Based on the principle proposed by the present invention, the amplitude-frequency response of the photoelectric conversion can be controlled to achieve an approximate signal retention effect after sampling, so that the signal after optical sampling is more accurate when it is electrically quantized, and the original signal can be restored even if there is a certain time mismatch between sampling and quantization.

[0028] 2. The operation of controlling the photoelectric conversion amplitude-frequency response proposed by the present invention does not add additional active devices and software overhead, and can be achieved by selecting a suitable photodetector or adding a filter after the original photodetector without adding additional noise.

[0029] 3. When the number of channels of the channel-interleaved photonic analog-to-digital conversion system continues to increase, the sampling rate of a single channel decreases, and the bandwidth of the photodetector required by the method proposed in the present invention decreases accordingly, making it easier to realize a device with the same amplitude-frequency response as the ideal one. In addition, the method proposed in the present invention is only related to the sampling rate of a single channel and is not limited by the number of channels. It can provide a more reliable technical solution for realizing high-sampling-rate photonic analog-to-digital conversion systems in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 The overall architecture diagram of the photon analog-to-digital conversion architecture embodiment applied by the method of the present invention;

[0031] Figure 2 The calculation process of the sampling and holding effect in the frequency domain proposed by the present invention, wherein (a) represents the signal spectrum diagram of the photodetector with controlled frequency response, A 0 is the original signal vector, A 1 A 2 is the high-frequency component that is retained, and (b) represents the high-frequency component A of the signal during the quantization process of the electronic analog-to-digital converter. 1 A 2 (c) Schematic diagram of vector superposition of the original signal and the folded-back high-frequency component signal. 0 ' is the signal vector after equivalent time shift.

[0032] Figure 3 This is a schematic diagram of the frequency response set proposed by the present invention, that is, formula (6), where m is a positive integer greater than or equal to 1.

[0033] Figure 4 This is a simulation result of a specific embodiment of the present invention. A two-channel channel-interleaved optical analog-to-digital conversion system is added with time mismatch. (a) shows the change of the equivalent delay with time, and (b) shows the improvement effect of the spurious-free dynamic range. In the two figures, 1 is a photonic analog-to-digital conversion system using a low-pass filtered frequency response photodetector, 2 is a photonic analog-to-digital conversion system using the controlled frequency response photodetector, and 3 is a photonic analog-to-digital conversion system in which the time mismatch is completely suppressed under ideal conditions. DETAILED DESCRIPTION

[0034] A specific embodiment of the present invention is given below in conjunction with the accompanying drawings. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and process, but the protection scope of the present invention is not limited to the following embodiment.

[0035] See also Figure 1 As can be seen from the figure, the present invention is based on the interconnection mode of the traditional channel interleaved photonic analog-to-digital conversion architecture, including an optical sampling clock source 1, a sampled signal source 2, a photon sampling gate 3, a demultiplexer array 4, a photodetector array 5, an electronic analog-to-digital converter array 6 and a data integration and processing module 7. In determining the sampling rate N*F of the entire channel interleaved photonic analog-to-digital conversion system sAfter the number of channels N is calculated, the sampling rate of a single channel is Fs. In the traditional architecture, the 3dB bandwidth of the photodetector is half of the single channel sampling rate, that is, Fs / 2. In this embodiment, based on the above-mentioned formula (5), the key to obtaining the time mismatch cancellation effect is to retain the high-frequency components generated by the sampled signal in a single channel. In this example, a photodetector amplitude-frequency response as shown in the following formula is used, and its 3dB bandwidth is set to the single channel sampling rate, that is, F s In the spectrum of a single channel, in addition to the signal within the Nyquist bandwidth, two high-frequency components symmetrical to the sampling rate are retained, that is, m = 1 in equation (6).

[0036]

[0037] Based on the basic principle of the channel-interleaved photonic analog-to-digital converter, its actual implementation is based on the expansion of the two-channel photonic analog-to-digital conversion system. The photodetector with controlled frequency response proposed above is used in the two-channel photonic digital converter, and its signal retention effect is verified by simulation. In the two-channel channel-interleaved analog-to-digital converter, when the input signal is a single-frequency signal, the time mismatch will cause the spectrum of the output data to contain a large spurious signal in addition to the original input signal. The power of the spurious signal is proportional to the square of the time mismatch, so the signal-to-noise ratio of the final spectrum can reflect the size of the remaining time mismatch, thereby reflecting the sampling and holding effect. In the simulation of this embodiment, the number of channels of the photonic analog-to-digital conversion system is set to N=2, and the single-channel sampling rate F s =5GSPS, the total sampling rate is 10GSPS, the spectrum response of the photodetector used can be obtained by the guidance of formula (7), the system noise floor is set to 60dBm, the input signal power is 0dBm, it can be seen that the upper limit of the signal-to-noise ratio is 60dB, a time mismatch of -50ps-50ps and a step of 1ps is added to one of the channels, and the size of the signal-to-noise ratio is calculated respectively. Under the same conditions, it is compared with the photodetector with a rectangular amplitude-frequency response in the prior art. The results are as follows Figure 4 As shown, (a) shows the change of the equivalent delay size with time, (b) shows the improvement effect of the spurious-free dynamic range, and Figure 1 in the two figures is a photon analog-to-digital conversion system using a low-pass filter frequency response photodetector, 2 is a photon analog-to-digital conversion system using the controlled frequency response photodetector, and 3 is a photon analog-to-digital conversion system in which the time mismatch is completely suppressed under ideal conditions. It can be seen that a smaller time mismatch in the prior art can lead to a significant reduction in the signal-to-noise ratio. Controlling the frequency response of the photodetector based on the method proposed in the present invention can make the photon analog-to-digital conversion system unaffected by the time mismatch within a certain range, which is equivalent to maintaining the optical sampling pulse point at ±20ps, about 0.2 sampling cycles in the time domain.

[0038] The method for implementing the above-mentioned photon analog-to-digital conversion architecture based on photon parallel sampling includes the following steps:

[0039] 1) Determine the sampling rate N*F according to the channel interleaved photonic analog-to-digital conversion system architecture s and the number of channels N;

[0040] 2) According to equation (6) and the actual bandwidth requirement, an achievable frequency response is proposed, which should meet two conditions: the high-frequency components generated by pulse sampling should be retained in pairs; the 3dB bandwidth should be equal to an integer multiple of the single-channel Nyquist bandwidth, that is, in equation (6), m is a positive integer greater than or equal to 1, and the corresponding photodetector array is used in the system.

[0041] On the other hand, the phase-frequency response is kept as a linear phase, and a corresponding photodetector array is used in the system, and the obtained N optical pulse sequences are input into N PD units with controlled frequency response to obtain N maintained electrical signals, and the N maintained electrical signals are then passed through N synchronously sampled electronic analog-to-digital converters to obtain N electrical digital signals;

[0042] 3) The obtained N electrical signals are passed through N electronic analog-to-digital converters to obtain N electrical digital signals, and the N electrical digital signals are input into a data integration and processing module, which reconstructs and interleaves the received N electrical digital signals and processes them to obtain information of the original electrical analog signals.

[0043] In the above process, the basic architecture setting of the channel-interleaved photonic analog-to-digital converter is retained, and an appropriate photoelectric conversion frequency response is selected according to the proposed equivalent sampling and holding principle, so as to achieve the holding effect on each signal after demultiplexing, thereby offsetting the sampling error caused by time mismatch. Experiments have shown that the present invention can enable the photonic analog-to-digital conversion system to offset the time mismatch between sampling and quantization in any sub-channel. At the same time, the controlled photoelectric conversion frequency response does not increase additional active devices and software overhead, greatly improving the performance of the photonic analog-to-digital conversion system. The present invention is based on a channel-interleaved photonic analog-to-digital conversion architecture, simplifies the compensation method for time mismatch, and is not limited by the number of channels. It can provide a more reliable technical solution for future photonic analog-to-digital conversion systems that achieve high sampling rates.

Claims

1. A method for maintaining an optical sampling signal of a photon analog-to-digital conversion system, wherein the photon analog-to-digital conversion system comprises a photodetector and an electronic analog-to-digital converter, characterized in that: By controlling the frequency response of the photodetector, the high-frequency components of the signal generated by the light pulse sampling are selectively retained and filtered out. These high-frequency components will be folded back to the signal frequency position when quantized by the electronic analog-to-digital converter of the same rate, which is equivalent to creating a time offset opposite to the time mismatch, which offsets the time mismatch in the system, and finally obtains a signal output without time mismatch.

2. The optical sampling signal holding method of a photonic analog-to-digital conversion system according to claim 1, characterized in that: The method specifically comprises the following steps: ①Assume that the sampled signal has a frequency of f in The signal is repeated at a frequency of F. s The optical pulse sampling causes the periodic extension of the spectrum, generating many high-frequency components in addition to the original signal. These high-frequency components are multiples of the optical pulse sampling rate kF s It is centrosymmetric and has a frequency of kF s ±f in , where k is a positive integer greater than or equal to 1 and less than or equal to the number of harmonics of the optical sampling clock; ② Control the frequency response of the photodetector |H OE (f)|, as shown below: Wherein, f is the independent variable of the frequency response expression, m is the number of high-frequency component pairs retained, and m is a positive integer greater than or equal to 1; ③ Input the light pulse into a photodetector with controlled frequency response to obtain a retained electrical signal, and then obtain an electrical digital signal through an electronic analog-to-digital converter.

3. The optical sampling signal holding method of a photonic analog-to-digital conversion system according to claim 2, characterized in that: The frequency response |H proposed by this method OE (f)|The m in the expression is 1.

4. The method for maintaining an optical sampling signal of a photonic analog-to-digital conversion system according to claim 2, characterized in that: The method also includes adding a filter matching the frequency response after the photodetector.

5. The optical sampling signal holding method of the photonic analog-to-digital conversion system according to claim 2 or 3, characterized in that: The method also includes ④ removing direct current and reconstructing data through interleaving processing of the electrical digital signal to obtain a quantization result of the original electrical analog signal without time mismatch.

6. The optical sampling signal holding method of the photonic analog-to-digital conversion system according to any one of claims 1 to 3, characterized in that: The photon analog-to-digital conversion system further includes a light sampling clock source, a sampled signal source, a photon sampling gate, a demultiplexer array, and a data integration and processing module. The photodetector is composed of N PD units in parallel, and the electronic analog-to-digital converter is composed of N electronic analog-to-digital converters in parallel. The output end of the optical sampling clock source is connected to the input end of the photon sampling gate, the output end of the photon sampling gate is connected to the input end of the demultiplexer to which it belongs, the N output ends of the demultiplexer are connected to the input ends of the N PD units, the output ends of the N PD units are respectively connected to the input ends of the N electronic analog-to-digital converter units, and the output ends of the N electronic analog-to-digital converter units are respectively connected to the N input ends of the data integration and processing module, wherein N is a positive integer greater than or equal to 1.

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