Spectrum testing method for electro-optical modulation signal

Abstract

The invention provides a spectrum testing method for electro-optical modulation signals. The device is composed of a laser, a microwave signal source, a to-be-measured electro-optical modulation signal generation module, an optical frequency shifter, an electro-optical phase modulator, a photoelectric detector and a spectrum analysis module, optical signals output by the laser are divided into two paths, the upper path is in optical connection with the to-be-measured electro-optical modulation signal generation module, and the lower path is in optical connection with the to-be-measured electro-optical modulation signal generation module. The lower path is sequentially and optically connected with the optical frequency shifter and the electro-optic phase modulator and is optically connected with the photoelectric detector after being combined, the microwave signal source is electrically connected with the radio frequency end of the electro-optic phase modulator, and the output end of the photoelectric detector is electrically connected with the spectrum analysis module. Heterodyne mixing signals f1-pf2 (p is a positive integer) of an electro-optical modulation spectrum signal f1 to be detected and an electro-optical phase modulation signal f2 are detected, mixing signal power ratios when output signal voltages of a microwave signal source are V2 and kV2 respectively are analyzed, the p value and the frequency f1 are determined by utilizing the Bessel function relation, meanwhile, the frequency response of a photoelectric detector is calibrated, and the frequency response of the photoelectric detector is calibrated. The relative amplitude of each order of light modulation sideband of the electro-optical modulation spectrum to be tested is obtained, and spectrum testing of the electro-optical modulation signal to be tested is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optoelectronic technology, and in particular to a spectrum testing method of an electro-optical modulation signal. BACKGROUND

[0002] Spectrum analysis is to realize qualitative and quantitative analysis of the measured object by detecting the composition, shape and amplitude information of the measured spectrum, and is widely used in biological medicine, food detection, environmental monitoring, optical communication and device testing fields. Especially in optical communication systems, through spectrum analysis of the link transmission optical signal, it can effectively support the establishment and optimization of system link and optoelectronic device model.

[0003] Currently, spectral analysis methods mainly include: spectral analysis techniques based on diffraction gratings, spectral analysis techniques based on Fourier transform, and spectral analysis techniques based on stimulated Brillouin scattering. Spectral analysis techniques based on diffraction gratings mainly involve splitting incident light into a series of parallel beams by using an entrance slit and a lens to be incident on the grating. Light of different wavelengths diffracts and interferes on the grating, producing different diffraction angles, thereby separating light of different wavelengths. The intensity of light of different wavelengths is then detected by a detector to obtain the incident light spectrum (YXZhang, WZLi, WHDuan, ZHHuang, and HDYang, “Echelle Grating Spectroscopic Technology for High-Resolution and Broadband Spectral Measurement,” Applied Sciences, 2022, 12(21), 11042). However, the resolution limit of commercial spectrometers is currently 2.5 GHz (0.02 nm @ 1550 nm), which greatly limits the application of spectrometers in the field of high resolution. The Fourier transform-based spectral analysis technique generates an interference signal by passing incident light through a Michelson interferometer. The detector detects the interference signal and transforms the interferogram from the time domain to the frequency domain through Fourier transform to obtain the spectral distribution of the light. The current resolution limit of this scheme is 2.9 pm (JYChai, K.Zhang, Y.Xue, WGLiu, T.Chen, Y.Lu, and GMZhao, “Review of MEMS Based Fourier Transform Spectrometers”, Micromachines, 2020, 11(2), 214). Due to the presence of the Michelson interferometer, this method is quite sensitive to vibration and has poor environmental adaptability.The spectral analysis technique based on stimulated Brillouin scattering (SBS) filters the light signal under test by introducing an SBS filter, retaining only the light under test with the center frequency of the SBS scattering. This light is then mapped to the electrical spectrum by beating the local oscillator light, and the spectral information of the light under test is obtained by sweeping the light source. This method has the potential for high resolution (JWCui,SWZhang,H.Dang,YQTian,ZBXu,ZYWang,andX.Sun,“High resolution coherent spectral analysis method based on brillouin scattering in an optical fiber”,OpticsExpress,2021,29(15),23258). However, it is affected by the stability of the sweeping light source. In addition, the introduced SBS filter is prone to large measurement errors in spectral amplitude and is sensitive to ambient temperature.

[0004] Although many methods have been proposed for measuring the frequency and amplitude of spectra, few methods simultaneously achieve high-precision measurement of both frequency and amplitude. Meanwhile, low-cost, high-resolution, high-precision, and low-frequency detection methods are expected to be researched and developed. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a spectral testing method for electro-optic modulated signals, achieving simple structure, high resolution, and wide spectral range electrical domain measurement of electro-optic modulated spectral signals.

[0006] A method for spectral testing of electro-optic modulated signals, comprising the following steps: Step 1: The test system includes a laser, a microwave signal source, a module for generating the electro-optic modulation signal under test, an optical frequency shifter, an electro-optic phase modulator, a photodetector, and a spectrum analysis module. The optical signal output from the laser is split into two paths: the upper path is optically connected to the module for generating the electro-optic modulation signal under test, and the lower path is optically connected to the optical frequency shifter and the electro-optic phase modulator in sequence, and then combined and optically connected to the photodetector. The microwave signal source is electrically connected to the radio frequency terminal of the electro-optic phase modulator; the output terminal of the photodetector is electrically connected to the spectrum analysis module. Step 2: The DC optical signal output by the laser, with a center wavelength of λ0 and a frequency of f0, is split into two paths. The upper path generates the electro-optic modulation spectrum with a repetition rate of f1 using the electro-optic modulation signal generation module. The lower path optical signal undergoes a frequency shift of f... s After passing through an optical frequency shifter, the signal is modulated by a sinusoidal signal with amplitude V2 and frequency f2 output from a microwave signal source via an electro-optic phase modulator to generate an optical frequency comb (at which point its modulation coefficient is m). lThe upper and lower optical signals are combined and then enter a photodetector for photoelectric conversion to generate a beat frequency electrical signal. The amplitude and frequency of the beat frequency electrical signal within the frequency range of f2 are measured using a spectrum analysis module and denoted as A(|f1-pf2-f). s |), where p is a non-negative integer; Step 3: Change the output amplitude of the microwave signal source to kV2, and record its amplitude and frequency information, denoted as A'(|f1-pf2-f s |); Step 4: Calculate the ratio of the mixed signals from the two tests, and use the Bessel function relationship to solve for the p-value and the repetition frequency f1 of the electro-optic modulation spectrum under test. Step 5: Using the known detector frequency response R and phase modulator modulation coefficient m l The center wavelength λ0 of the DC optical signal, the frequency f2 of the microwave signal source output, and the repetition frequency f1 of the electro-optic modulated spectrum signal measured in step 4 are used to calculate and recover the wavelength and amplitude information of the spectrum to be measured according to the following formula:

[0007] Compared with the prior art, the present invention has the following beneficial effects: I. This invention uses heterodyne mixing with a wide-range electro-optic modulated optical frequency comb to map the spectrum to be tested onto the electric domain for testing, thereby achieving high-resolution spectral analysis and testing, with a theoretical resolution on the order of Hz. Second, this invention uses high-order harmonic heterodyne mixing of an electro-optic modulated optical frequency comb to downconvert broadband spectral components, enabling broadband spectral testing that only requires narrowband signal reception, and can recover unknown modulated signals. Attached Figure Description

[0008] Figure 1 This is a schematic diagram of the device of the present invention.

[0009] Figure 2 For J when k = 0.5, and p from 1 to 10 p (m l ) / J p (km l A schematic diagram of the function curve.

[0010] Figure 3 The secondary drive measures the spectrum and the OSA measures the spectral image. Detailed Implementation

[0011] The following is a further description of the present invention in conjunction with the accompanying drawings and embodiments. It should be noted that the scope of protection claimed by the present invention is not limited to the scope described in the embodiments, and should not be construed as limiting the scope of protection of the present invention.

[0012] like Figure 1 As shown, a spectral testing method for electro-optic modulated signals comprises a laser, a microwave signal source, a module for generating the electro-optic modulated signal under test, an optical frequency shifter, an electro-optic phase modulator, a photodetector, and a spectrum analysis module. The optical signal output from the laser is divided into two paths: the upper path is optically connected to the module for generating the electro-optic modulated signal under test, and the lower path is optically connected to the optical frequency shifter and the electro-optic phase modulator in sequence. After being combined, the signals are optically connected to the photodetector. The microwave signal source is electrically connected to the radio frequency terminal of the electro-optic phase modulator, and the output terminal of the photodetector is electrically connected to the spectrum analysis module.

[0013] To better explain the technical invention, the following is an example. Figure 2 Taking k=0.5 in the secondary drive shown as an example, the measurement principle of the present invention will be briefly introduced:

[0014] After the DC optical signal with a center wavelength of λ0 and a frequency of f0 output from the laser is split, the upper branch generates the electro-optic modulation spectrum with a repetition rate of f1 by the electro-optic modulation signal generation module. This spectrum has the following form: Among them, A n Let f be the amplitude of the nth-order modulation sideband of the spectrum to be measured. The optical signal in the lower branch undergoes a frequency shift of f. s After passing through an optical frequency shifter, the signal is modulated by a sinusoidal signal with amplitude V2 and frequency f2 output from a microwave signal source via an electro-optic phase modulator to generate an optical frequency comb (at which point its modulation coefficient is m). l The optical frequency comb has the following form: E l (t)=E0 expj[2π(f0+f s )t+m l sin(2πf2t)] (2) Where E0 is the amplitude of the optical carrier signal. After the combined optical signal enters the photodetector for photoelectric conversion, it generates a beat frequency electrical signal, which is expanded by the Jacobi-Anger method as follows: Where R is the frequency response of the photodetector, J p (·) represents the p-th order Bessel function of the first kind. From formula (3), it can be seen that the frequency of the beat frequency signal output by the photodetector is |nf1-pf2-f|. s The frequency component amplitude is: A(|nf1-pf2-f s |)=2RE0A n J p (m l (4) As can be seen from Equation (4), when the frequency f1 satisfies the condition pf2 < f1 < (p + 1)f2 (where p is a positive integer), the following frequency relationship exists: That is, f1 - pf2 - f can be found through Equation (5). s Set the output amplitudes of the microwave signal source to be sinusoidal signals of V2 and 0.5V2 respectively, and load them onto the electro-optic phase modulator. By analyzing the amplitude and frequency of the frequency component f1 - pf2 - f s the value of p and the repetition frequency f1 of the electro-optic modulation spectrum to be measured are obtained: From Equation (6) and Figure 2 it can be seen that the H p (m l ) curves corresponding to different p values are different, and the H p (m l ) curves corresponding to adjacent p values are clearly distinguishable, thereby solving for p and the repetition frequency f1 of the electro-optic modulation spectrum to be measured.

[0015] The repetition frequency f1 of the electro-optic modulation spectrum to be measured can be obtained from Equation (6). According to the beat frequency signal information recorded by the previous spectrum analysis module, it can be obtained that: In the formula, λ n and A n are respectively the wavelength and relative amplitude of the nth-order modulation sideband of the modulation spectrum to be measured. The repetition frequency f1 of the electro-optic modulation spectrum to be measured can be solved according to the relational expression in Equation (6), and the wavelength and relative amplitude of the nth-order modulation sideband of the electro-optic modulation spectrum to be measured can be calculated according to Equation (7). Embodiment

[0016] The block diagram of the test device of the present invention is as shown in Figure 1 . The DC optical signal output by the laser with a frequency f0 = 193.1 THz (wavelength about 1550 nm) is divided into two paths. The upper path generates the electro-optic modulation spectrum to be measured by the electro-optic modulation signal generation module to be measured, and the lower path is sent into an optical frequency shifter with a frequency shift amount f s = 80 MHz to generate a frequency-shifted optical signal, and then enters the electro-optic phase modulator, and is modulated by the sinusoidal signal with a frequency f2 = 4 GHz output by the microwave signal source to generate an optical frequency comb; the optical signals of the upper and lower paths are combined and then enter the photodetector for photoelectric conversion, and the generated optical current signal enters the spectrum analysis module to be recorded and analyzed.

[0017] Set the amplitude of the sinusoidal signal output by the microwave signal source to V2 = 2.18 V. Through the spectrum analysis module and Equation (5), f1 - pf2 - f s=0.22GHz; The 0.22GHz (f1-pf2-f) value in the photocurrent signal was measured using a spectrum analysis module. s The amplitude A(f1-pf2-f) s = -32.50dBm; Set the microwave signal source to output a sinusoidal signal amplitude of 0.5V² = 1.09V, and use the spectrum analysis module to measure the 0.22GHz (f1-pf2-f) in the photocurrent signal. s The amplitude A'(f1-pf2-f) s = -37.46dBm; p = 1 and frequency f1 = 4.3GHz were obtained by formula (6); the wavelength and amplitude of the modulation sideband of the spectrum to be measured were calculated by formula (7) and the relative spectrum image was plotted. Then the spectrum to be measured was measured by a spectrometer and the relative spectrum result measured by the spectrometer was plotted and compared with the measurement result of this method. Figure 3 It can be seen that this method can accurately measure the spectrum.

Claims

1. A method for testing the spectrum of an electro-optical modulation signal according to claim 1, comprising the following steps: Step 1: The testing system comprises a laser, a microwave signal source, a module for generating the electro-optical modulation signal to be tested, an optical frequency shifter, an electro-optical phase modulator, a photodetector and a spectrum analysis module; the optical signal output by the laser is divided into two paths, one of which is optically connected to the module for generating the electro-optical modulation signal to be tested, and the other is optically connected to the optical frequency shifter and the electro-optical phase modulator in sequence, and then optically connected to the photodetector; the microwave signal source is electrically connected to the radio frequency end of the electro-optical phase modulator; and the output end of the photodetector is electrically connected to the spectrum analysis module. Step 2: The direct current light signal with central wavelength λ0 and frequency f0 outputted by the laser is divided into two paths, the upper path generates the to-be-tested electro-optical modulation spectrum with repetition frequency f1 by the to-be-tested electro-optical modulation signal generation module, and the light signal of the lower path is modulated by the sinusoidal signal with amplitude V2 and frequency f2 outputted by the microwave signal source after passing through the optical frequency shifter with frequency shift amount f s , and then passes through the electro-optical phase modulator to generate the optical frequency comb (at this time, the modulation coefficient is m l ). The upper and lower light signals are combined and then enter the photoelectric detector to generate the beat frequency electric signal through photoelectric conversion. The amplitude and frequency of the beat frequency electric signal in the f2 frequency range are measured by using the spectrum analysis module, and are recorded as A(|f1-pf2-f s |), wherein p is a non-negative integer; Step 3: change the output amplitude of the microwave signal source to kV2, record its amplitude and frequency information, denoted as A'( |f1-pf2-f s ) ; Step 4: The ratio of the two tested mixing signals is calculated, and the value p and the repetition frequency f1 of the electro-optical modulation spectrum to be tested are solved by using the Bessel function relationship: Step 5: using the known probe frequency response R, the phase modulator modulation coefficient m l , the center wavelength of the DC light signal λ0, the frequency f2 output by the microwave signal source, and the measured repetition frequency f1 of the spectrum signal to be measured in step 4, the wavelength and amplitude information of the spectrum to be measured is recovered according to the following formula:

2. The method of claim 1, wherein: The ratio k of the two output signal voltages of the microwave signal source satisfies 0 < k < 1.

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