High-resolution extraction method of electro-optical modulation spectrum

Abstract

The invention discloses a high-resolution extraction method of an electro-optical modulation spectrum, belongs to the technical field of photoelectrons, and aims to realize high-resolution extraction of the electro-optical modulation spectrum. According to the method, a frequency shift heterodyne structure is established, heterodyne beat frequency is carried out on an electro-optical modulation spectrum to be detected and a frequency shift optical signal, and an optical domain spectrum is converted into a radio frequency domain and is received and detected by an electric spectrometer; the optical frequency shift module performs up-shift on the optical carrier, the frequency shift amount is fs, and the frequency shift amount is equal to the output frequency of the driving source. By analyzing the distribution characteristics of spectral lines in the measured electric spectrum, a spectrum-electric spectrum frequency mapping relation is established, and the frequency information of the electro-optical modulation spectrum to be measured can be solved; through back-end calibration processing, spectrum-electric spectrum power mapping is in a linear relation, and power information of the electro-optical modulation spectrum to be measured can be recovered. In conclusion, through inverse solution of the spectrum-electric spectrum mapping relation, the electro-optical modulation spectrum to be measured can be extracted from the electric spectrum, and benefited from the sub-Hz-level resolution of the electric spectrometer, the extraction method has the characteristic of high resolution.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronic technology, and particularly relates to a high-resolution extraction method for electro-optic modulation spectra. Background Art

[0002] Electro-optic modulation is a core link in fiber optic communication systems and microwave photonics systems. It loads an electrical signal onto an optical carrier through an electro-optic modulator, thereby achieving the key step of electro-optic conversion. Precise measurement and analysis of electro-optic modulation spectra are common methods for evaluating and optimizing the performance of electro-optic modulators. With the continuous development of fiber optic communication systems towards high-capacity and high spectral efficiency, increasingly complex modulation formats pose high requirements for the high-precision analysis of electro-optic modulation spectra, that is, high-resolution modulation spectra need to be obtained. Existing commercial spectrometers based on diffraction gratings can cover a large measurement bandwidth, but the resolution can only reach the order of dozens of picometers, far from meeting the requirements for high-precision analysis of electro-optic modulation spectra. In recent years, many new high-resolution spectral analysis techniques have emerged, mainly including spectral analysis methods based on stimulated Brillouin scattering effects, spectral analysis methods based on Brillouin dynamic gratings, and coherent spectral analysis methods based on heterodyne interference.

[0003] Spectral analysis method based on stimulated Brillouin scattering effect (Domingo J M S, Pelayo J, Villuendas F, et al. Very high resolution optical spectrometry by stimulated Brillouin scattering[J]. IEEE photonics technology letters, 2005, 17(4): 855 - 857; Preuβler S, Schneider T. Bandwidth reduction in a multistage Brillouin system[J]. Optics Letters, 2012, 37(19): 4122 - 4124.) uses the electrostriction effect to form refractive index modulation inside the optical fiber and amplifies the part of the incident spectrum that falls within the Brillouin gain interval. Since the bandwidth of the Brillouin gain interval is only about 10 MHz and its central frequency can vary with the laser scanning, a narrowband optical filter with adjustable central wavelength is constructed to achieve time-division extraction of different frequency components of the original spectrum. The spectral analysis method based on Brillouin dynamic grating (Song K Y, Zou W, He Z, et al. All-optical dynamic grating generation based on Brillouin scattering in polarization-maintaining fiber[J]. Optics letters, 2008, 33(9): 926 - 928; Song K Y. Operation of Brillouin dynamic grating in single-mode optical fibers[J]. Optics letters, 2011, 36(23): 4686 - 4688.) uses two counter-propagating pump lights with a frequency difference equal to the Brillouin frequency shift to form a periodic change in the refractive index inside the optical fiber. This periodic change is similar to a fiber Bragg grating, and the part of the incident light that satisfies the phase condition is strengthened and reflected into the receiver. However, both of these methods require several kilometers of optical fiber to generate the stimulated Brillouin scattering effect, making them extremely vulnerable to external vibrations and environmental temperatures, and the stability of the measurement results is poor.Heterodyne-interference-based coherent spectroscopy (Feng K, Cui J, Dang H, et al. Investigation and development of a high spectral resolution coherent optical spectrum analysis system [J]. Optics Express, 2016, 24(22): 25389-25402.) uses a scanned local oscillator optical signal to heterodyne beat with the optical signal to be measured, down-converts the optical signal to be measured into a radio frequency signal, and uses a narrowband filter to receive and detect. Although this method can achieve high resolution, there is a trade-off between its power measurement uncertainty and resolution, and it is impossible to distinguish the magnitude relationship between the local oscillator optical frequency and the optical frequency to be measured during the heterodyne beat process. In addition, additional calibration of the scanned wavelength of the local oscillator optical signal is required.

[0004] In summary, although there are many spectral analysis methods available at present, there are still relatively few mature solutions that can achieve high-resolution spectral analysis, and the existing high-resolution spectral analysis methods often have the above-mentioned various restrictive factors. Summary of the Invention

[0005] The present invention provides a high-resolution extraction method for electro-optic modulation spectra, aiming to achieve high-resolution extraction of electro-optic modulation spectra.

[0006] A high-resolution extraction method for electro-optic modulation spectra includes the following operating steps:

[0007] Step 1: Build a frequency-shifted heterodyne structure. The frequency-shifted heterodyne structure includes a laser, a Mach-Zehnder interferometer, the electro-optic modulation spectrum to be measured, an optical frequency-shifting module, a photodetector, an electrical spectrometer, and a driving source; the output end of the laser is optically connected to the input end of the Mach-Zehnder interferometer. The upper branch of the Mach-Zehnder interferometer is the electro-optic modulation spectrum to be measured, and the lower branch of the Mach-Zehnder interferometer is the optical frequency-shifting module. The output end of the Mach-Zehnder interferometer is optically connected to the optical port of the photodetector; the output end of the driving source is electrically connected to the electrical port of the optical frequency-shifting module, and the electrical port of the photodetector is electrically connected to the input end of the electrical spectrometer; assume that the spectral line interval of the electro-optic modulation spectrum is f1 and the maximum order of the modulation sidebands is N;

[0008] Step 2: Set the frequency of the radio frequency signal output by the driving source to f s , up-frequency shift the optical carrier of the lower branch by f s , use the electrical spectrometer to measure the electrical spectral information of the electrical signal output by the photodetector, and analyze the spectral line distribution characteristics of the measured electrical spectrum.

[0009] Step 3: Observe the intervals between all adjacent spectral lines in the electro-spectrum (including the interval between the leftmost spectral line and 0 when counting from left to right). If the values of these electro-spectrum line intervals are f s / k or 2f s / k, where k is any positive integer, it indicates that the following two situations may occur: f s = kf1, 1 ≤ k ≤ 2N, or f s = 0.5kf1, k is odd and satisfies 1 ≤ k ≤ 2N - 1. In these two cases, there is a problem of indistinguishable spectral line overlap in the electro-spectrum, and the spectro-electro-spectrum does not satisfy one-to-one mapping. Therefore, change the frequency of the driving source to increase the frequency shift amount f s by Δf, measure a new set of electro-spectra, and then execute Step 4; if it is determined that there is no such problem of spectral line overlap, directly execute Step 4;

[0010] Step 4: Observe the spectral line f s and the spectral lines on its right. If they are equally spaced, and each spectral line on the right of f s can find another corresponding spectral line on the left of f s and their positions are symmetric about f s , it indicates that at this time Nf1 < f s . Denote the frequency of the first spectral line on the right of f s as x1. From this, it can be known that the spectral line interval f1 of the electro-optic modulation spectrum to be measured is f1 = x1 - f s ;

[0011] Step 5: When the distribution of the electro-spectrum lines does not conform to Step 4, observe the number of spectral lines on the left of the spectral line f s in the electro-spectrum. If the number of spectral lines is 0, denote the frequency of the first spectral line on the right of f s as x1, then the spectral line interval f1 of the electro-optic modulation spectrum to be measured is f1 = x1 + f s ; if the number of spectral lines is 1, denote the frequency of this spectral line as x1, then the spectral line interval f1 of the electro-optic modulation spectrum to be measured is f1 = x1 + f s ; if the number of spectral lines is at least 2, denote the frequencies of the first and second spectral lines when counting from left to right as x1 and x2, then the spectral line interval f1 of the electro-optic modulation spectrum to be measured is f1 = x1 + x2;

[0012] Step 6: According to Steps 3, 4, and 5, the spectral line interval f1 of the electro-optic modulation spectrum to be measured can be solved. Find the spectral lines in the electro-spectrum whose frequencies satisfy |kf1 - f s |, where k = 0, ±1, ±2, …. According to the mapping relationship between the spectro-electro-spectrum, the frequency values of the optical sidebands kf1 in the electro-optic modulation spectrum to be measured can be restored;

[0013] Step 7: Denote the amplitude of the electro-spectrum sideband |kf1 - f s | as A(|kf1 - fs |), subtract the cascaded response R(|kf1 - f s |) of the photodetector, cable, and spectrometer, that is, calculate according to the following formula:

[0014]

[0015] Step 8: Denote A cal (|kf1 - f s |) as the maximum value A cal,max (|kf1 - f s |), then the normalized spectrometer is:

[0016]

[0017] Step 9: In summary, the complete normalized electro-optic modulation spectrum can be obtained, and the normalized power value corresponding to the optical sideband kf1 is:

[0018] 20lg[A cal . norm (|kf1 - f s |)].

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] First, the present invention uses a photodetector to transfer the spectral measurement in the optical domain to the electrical domain measurement, taking advantage of the high-resolution advantage of spectrometer measurement, and greatly improving the equivalent resolution of spectral measurement.

[0021] Second, the present invention also adopts a heterodyne interference structure. Compared with the traditional coherent spectroscopy analysis method based on heterodyne interference, it does not require a frequency-scanned local oscillator light source and a wavelength calibration module, saving measurement time and having high wavelength measurement accuracy. In addition, the self-heterodyne structure ensures the coherence of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the frequency-shifted heterodyne structure of the present invention.

[0023] Figure 2 It is an example of a spectrometer diagram.

[0024] Figure 3 It is from Figure 2 An example of the electro-optic modulation spectrum diagram extracted. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be further explained and described below in conjunction with embodiments. The described embodiments are only one of the implementation manners involved in the present invention. Other implementation manners obtained by those skilled in the art without creative efforts all fall within the protection scope of the present invention.

[0026] Embodiment

[0027] In this embodiment, the following Figure 1 test structure is adopted. The frequency of the optical carrier output by the laser is f0 = 193.1 THz. The Mach-Zehnder interferometer consists of two 1*2 polarization-maintaining couplers. Moreover, the pigtails of the laser, the electro-optic modulator, and the frequency-shifted optical module are all polarization-maintaining optical fibers to ensure the power stability of the interference system. The electro-optic modulator selects a dual-drive Mach-Zehnder intensity modulator. The spectral line interval of the electro-optic modulation spectrum is f1 = 100 MHz, and the modulation power is 10 dBm. It is loaded on the upper arm of the dual-drive Mach-Zehnder intensity modulator. The frequency of the radio frequency signal output by the drive source is set to f s = 80 MHz, and the power is set to -3.8 dBm. Under this condition, the frequency-shifting efficiency of the acousto-optic frequency shifter reaches the highest. The selected photodetector bandwidth is greater than 15 GHz, and the electrical spectrometer bandwidth is 26.5 GHz.

[0028] The electrical spectrum measured by the electrical spectrometer is as Figure 2 shown. It can be seen that the distribution characteristics of the electrical spectrum do not conform to Step 3 and Step 4, and only conform to the case where the number of spectral lines on the left side of f s in Step 5 is 1. From the measured electrical spectrum data, it can be read that the frequency of this spectral line is 20 MHz, and this value plus f s is equal to f1, which conforms to the solution formula of f1 in this case in Step 5. The electro-optic modulation spectrum extracted according to Steps 6, 7, 8, and 9 is as Figure 3 shown. Figure 3 (a) is a comparison diagram of the extracted electro-optic modulation spectrum, the spectrum measured by the spectrometer, and the theoretically calculated spectrum. Figure 3 (b) is the magnified part in the center. For the convenience of power comparison, the Figure 3 theoretically calculated spectrum in (b) is uniformly shifted in frequency. It can be seen that due to the spectrometer resolution being in the GHz order of magnitude, it is completely impossible to measure the electro-optic modulation spectrum with a spectral line interval of 100 MHz, and its measurement result is only the carrier spectrum; the electro-optic modulation spectrum extracted by the method of the present invention is highly consistent with the theoretically calculated result in terms of frequency and power, reflecting the high-resolution characteristics of the extraction method.

Claims

1. A high-resolution extraction method of electro-optical modulation spectrum, comprising the following steps: Step 1: Build a frequency-shift heterodyne structure, which includes a laser, a Mach-Zehnder interferometer, an electro-optical modulation spectrum to be measured, an optical frequency-shift module, a photodetector, an electric spectrometer, and a driving source; the output end of the laser is optically connected to the input end of the Mach-Zehnder interferometer, the upper branch of the Mach-Zehnder interferometer is the electro-optical modulation spectrum to be measured, the lower branch of the Mach-Zehnder interferometer is the optical frequency-shift module, and the output end of the Mach-Zehnder interferometer is optically connected to the optical port of the photodetector; the output end of the driving source is electrically connected to the electrical port of the optical frequency-shift module, and the electrical port of the photodetector is electrically connected to the input end of the electric spectrometer; it is assumed that the spectral line interval of the electro-optical modulation spectrum is f1, and the maximum order of the modulation sideband is N; Step 2: Set the frequency of the RF signal output by the driving source to f s , shift the frequency of the lower branch optical carrier up by f s , using an electric spectrometer to measure the electric spectrum information of the electric signal output by the photoelectric detector, and analyzing the spectral line distribution characteristics of the measured electric spectrum; Step 3: Observe the intervals between all adjacent spectral lines in the spectrum (including the interval between the first spectral line and 0 from left to right). If the value of the interval between these spectral lines is f s / k or 2f s / k, where k is any positive integer, indicating that the following two situations may occur at this time: f s = kf1, 1≤k≤2N, or f s =0.5kf1, k is an odd number and satisfies 1≤k≤2N-1. In these two cases, there is a problem of spectral line overlap in the electric spectrum that cannot be distinguished. The optical spectrum and the electric spectrum do not satisfy a one-to-one mapping. Therefore, the frequency of the driving source is changed to make the frequency shift amount f s Increase Δf, re-measure a set of electric spectra, and then execute step 4; if it is determined that there is no problem of spectral line overlap, directly execute step 4; Step 4: Observe the spectral line f s and the spectral lines to its right, if they are equally spaced and f s Each spectral line on the right can be s Find another spectral line on the left side that corresponds to it. Their positions are about f s Symmetric, indicating that Nf1 <f s , let f s The frequency of the first spectrum line measured on the right is x1, from which we can know that the spectrum line interval of the electro-optic modulation spectrum to be measured is f1 = x1-f s ; Step 5: When the distribution of the spectrum lines does not match that of step 4, observe the spectrum line f in the spectrum. s The number of spectral lines on the left. If the number of spectral lines is 0, record f s The frequency of the first spectrum line measured on the right is x1, so the spectrum line interval of the electro-optic modulation spectrum to be measured is f1=x1+f s ; If the number of spectral lines is 1, let the frequency of this spectral line be x1, then the spectral line interval of the electro-optical modulation spectrum to be measured is f1 = x1 + f s ; If the number of spectral lines is at least 2, the frequencies of the first and second spectral lines from left to right are recorded as x1 and x2, then the spectral line interval f1 of the electro-optical modulation spectrum to be measured is f1 = x1 + x2; Step 6: According to steps 3, 4, and 5, the spectral line interval f1 of the electro-optical modulation spectrum to be measured can be solved, and the frequency satisfying |kf1-f can be found from the spectrum. s | spectrum, where k = 0, ±1, ±2, ..., according to the mapping relationship between the spectrum and the electric spectrum, the frequency value of the optical sideband kf1 in the electro-optical modulation spectrum to be measured can be restored; Step 7: Record the spectrum sideband |kf1-f s The amplitude of | is A(|kf1-f s |), deducting the cascade response R(|kf1-f s |), that is, calculated according to the following formula: Step 8: Remember A cal (|kf1-f s |) is the maximum value in A cal,max (|kf1-f s |), the normalized electric spectrum is: Step 9: In summary, the complete normalized electro-optic modulation spectrum can be obtained. The normalized power value corresponding to the optical sideband kf1 is: 20lg[A cal.norm (|kf1-f s |)]。 2. The high-resolution extraction method of electro-optical modulation spectrum according to claim 1, characterized in that: The electro-optic modulation spectrum to be measured includes, but is not limited to, spectrum generated by various types of electro-optic modulators such as a phase modulator, a push-pull Mach-Zehnder intensity modulator, and a dual-drive Mach-Zehnder intensity modulator.

3. The high-resolution extraction method of electro-optical modulation spectrum according to claim 1, characterized in that: Even if spectral line aliasing occurs in the electrical spectrum, the frequency shift can be modified to s It can distinguish and extract the spectrum, which is suitable for electro-optic modulation spectrum extraction with arbitrary spectral line interval.

4. The high-resolution extraction method of electro-optical modulation spectrum according to claim 1, characterized in that: The typical value of Δf in step 3 is 1 MHz, which separates the overlapping electric spectrum lines. The parameter value can be reasonably set according to actual conditions.

5. The high-resolution extraction method of electro-optical modulation spectrum according to claim 1, characterized in that: In steps 7, 8, and 9, the optical sideband kf1 in the electro-optical modulation spectrum to be measured and its corresponding electric spectrum sideband |kf1-f s | has a linear relationship, so it can be based on A cal.norm (|kf1-f s |) to obtain the normalized optical sideband power.

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