Device and method for testing nonlinear characteristic of photoelectric detector

Abstract

The invention discloses a device and method for testing the nonlinear characteristic of a photoelectric detector, and the device mainly consists of a Mach-Zehnder interference structure which consists of a laser, an optical splitter, an electro-optic phase modulator, a frequency shifter, an optical combiner and an optical power regulator. One path passes through an electro-optic phase modulator modulated by a signal source, the other path passes through a frequency shifter, and the two paths of optical signals are combined and then sent into a photoelectric detector to be detected through an optical power regulator; the signal source is electrically connected with the electro-optic phase modulator, and the photoelectric detector to be detected is electrically connected with the signal receiving module; the control and data processing module is in data connection with the signal source and the signal receiving module. The control and data processing module is used for controlling the output signal frequency and amplitude of the signal source and controlling the signal receiving module to measure the amplitude of a light current signal output by the photoelectric detector to be measured, and the third-order intermodulation component of the photoelectric detector is extracted by combining the modulation coefficient of electro-optic phase modulation.

Description

Technical Field

[0001] The invention belongs to the field of optoelectronic devices, and in particular relates to linearity characterization and nonlinear characteristic testing of photoelectric detectors, and specifically is a device and method for testing the nonlinear characteristics of photoelectric detectors. Background Art

[0002] Photodetectors are important devices responsible for photoelectric conversion in analog optical links and microwave photonic applications, such as radar beamforming, photonic signal processing, antenna remote control, and fiber-optic radio. In these applications, the nonlinear characteristics of photodetectors are key parameters affecting their dynamic range. Especially in some analog applications that require high optical power, high-linearity photodetectors are advantageous devices for minimizing signal distortion and improving spurious-free dynamic range. Therefore, quantitatively and accurately measuring the nonlinear characteristics of photodetectors is crucial for both device optimization and link evaluation. Among all the nonlinear components of photodetectors, third-order intermodulation is one of the most important intermodulation components in high-power operation and is difficult to eliminate by filtering. Therefore, the output third-order intermodulation intercept point (OIP3) of photodetectors is usually used to characterize and quantify the nonlinear characteristics of photodetectors.

[0003] The OIP3 of a photodetector is usually defined as the extrapolated intercept point of the fundamental frequency power and the third-order intermodulation power. Its test methods mainly include the all-optical wavelength beat method and the electro-optic intensity modulation method. The method based on the wavelength beat principle requires four high-performance tunable lasers, which are combined with beat frequencies to generate the all-optical excitation required for the photodetector test without any electrical drive. However, the wavelength beat frequency often has problems with wavelength instability and polarization alignment, which brings great challenges to fast and precise sweep analysis. In contrast, the electro-optic intensity modulation method can effectively realize sweep analysis by generating the electro-optic excitation required for the photodetector test through electro-optic modulation, such as: dual laser dual modulator method, three laser three modulator method and improved dual laser dual modulator method. The dual laser dual modulator method uses two lasers, two intensity modulators and two microwave signal sources to construct dual parallel optical links. At the same time, the two intensity modulators need to be biased at the orthogonal point and the working conditions of the small signal approximation, which will result in limited test signal-to-noise ratio. In order to avoid the influence of the nonlinear modulation of the intensity modulator, the three-laser three-modulator method uses three lasers cascaded with three intensity modulators to indirectly extract the two-tone OIP3 from the three-tone OIP3 through a more complex three-parallel optical link. The test setup of the improved dual-laser dual-modulator method is similar to that of the dual-laser dual-modulator method, but the improved method achieves the decoupling between the nonlinearity of the intensity modulator and the nonlinearity of the photodetector, avoiding the need for small signal operation and significantly improving the signal-to-noise ratio. However, this method still requires two lasers and two intensity modulators with orthogonal transmission point bias, and the setup is relatively complex.

[0004] Although many devices and methods have been proposed for testing the nonlinear characteristics of photodetectors, there are still problems such as the complexity of the test devices and methods, and the test methods are easily affected by modulator nonlinearity and bias. Therefore, it is very necessary to develop low-cost and low-complexity devices and methods to realize frequency sweeping, bias-free and modulator-independent testing of the nonlinear characteristics of photodetectors. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a testing device and method suitable for nonlinear characteristics of photodetectors, which has the characteristics of low cost, frequency sweeping, no bias and no influence of modulator nonlinearity.

[0006] A test device for nonlinear characteristics of a photodetector, comprising a Mach-Zehnder interference structure composed of a laser, an optical splitter, an electro-optical phase modulator, a frequency shifter, an optical combiner, and an optical power regulator, a photodetector to be tested, a signal receiving module, a signal source, and a control and data processing module; wherein the electro-optical phase modulator and the frequency shifter are respectively located in two interference arms of the Mach-Zehnder interference structure; the laser, the optical splitter, the electro-optical phase modulator, the frequency shifter, the optical combiner, the optical power regulator, and the photodetector to be tested are optically connected, the signal source is electrically connected to a driving electrode of the electro-optical phase modulator, and the photodetector to be tested is electrically connected to a driving electrode of the electro-optical phase modulator. The device is electrically connected to the signal receiving module, and the control and data processing module is data-connected to the signal source and the signal receiving module respectively; the optical carrier output by the laser is divided into two paths through the optical splitter, one path outputs a phase-modulated optical signal through the electro-optical phase modulator, and the other path outputs a frequency-shifted optical carrier through the frequency shifter. The two optical signals are combined by the optical combiner and then sent to the photoelectric detector to be tested through the optical power regulator. The control and data processing module is used to control the signal source output signal frequency and amplitude, and the signal receiving module is controlled to measure the amplitude of the photocurrent signal output by the photoelectric detector to be tested, and the third-order intermodulation component of the photoelectric detector is extracted in combination with the modulation coefficient of the electro-optical phase modulation.

[0007] A method for testing nonlinear characteristics of a photoelectric detector comprises the following steps: Step 1: construct a test device for nonlinear characteristics of a photodetector as described in claim 1; Step 2: Set the wavelength of the optical carrier output by the laser, and use the optical power regulator to adjust the optical power incident on the photodetector to be tested so that it works in the nonlinear region; Step 3: Use the control and data processing module to control the signal source to output a sinusoidal signal with a frequency of f1, and load it on the RF driving electrode of the electro-optic phase modulator, and set the frequency shift of the frequency shifter to f s ; Step 4: Use the signal receiving module to measure the frequency f1+f in the output photocurrent of the photodetector to be tested. sor f1-f s The amplitude and f1+3f s or f1-3f s The amplitudes are recorded as i(f1±f s ; m) and i P (f1±3f s ; m), change the power P1 of the signal source output, and record the i(f1±f s ; m) and i P (f1±3f s ; m) data; Step 5: Fix the wavelength of the laser light and change the light power incident on the photodetector to be tested by using the optical power regulator to make it work in the linear region. Corresponding to the signal source output power P1 in step 4, use the signal receiving module to measure the frequency f of the photocurrent output by the photodetector under different P1. s The signal amplitude i L (f s m), the modulation coefficient m of the electro-optic phase modulator at different powers P1 is calculated according to the following formula: where J0(·) is the zero-order Bessel function of the first kind, and They represent the modulation coefficient corresponding to the signal source output power P1' and the frequency measured by the signal receiving module f s The signal amplitude of Step 6: Use the signal source measured in steps 4 and 5 to obtain a set of i corresponding to different output powers P1 P (f1±3f s m) data and the modulation coefficient m data, the third-order intermodulation signal f1±3f of the photodetector to be tested is calculated according to the following formula under different P1. s The amplitude is denoted as i W (f1±3f s ; m): Step 7: Compare the data i(f1±f s ; m) and i W (f1±3f s m) perform linear fitting, the ordinate value of the intersection point determined by the two linear fitting curves is the photoelectric detector to be tested at a frequency of f1±f s The output third-order intercept point (OIP3) at Step 8: Repeat steps 2-7 to obtain the photodetector under test at different frequencies f1±f s OIP3 at.

[0008] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention only requires a laser, a modulator and a microwave signal source, which greatly reduces the test cost and simplifies the test device and test method; 2. Compared with the all-optical wavelength beat frequency method, the present invention uses electro-optical modulation to test the third-order intermodulation of the photoelectric detector, and has fast and precise frequency sweep test capabilities; 3. Compared with the electro-optic intensity modulation method, the present invention adopts electro-optic phase modulation to realize the test without bias control requirements, and can completely eliminate the influence of electro-optic modulation nonlinearity on the test without the need for small signal approximation conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 It is a schematic diagram of the device of the present invention.

[0010] Figure 2 It is a test effect diagram of the present invention. DETAILED DESCRIPTION

[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 the scope of protection of the present invention should not be limited thereto.

[0012] like Figure 1 As shown, the optical carrier signal f output by the laser c After passing through the optical splitter, it is divided into two paths with the frequency of f c One of the optical carriers is sent to the electro-optical phase modulator and modulated by the microwave signal with a frequency of f1 output by the signal source to generate a frequency of f c ±nf1 (n=0,1,2,3,…) phase modulated optical signal; another optical carrier is sent to the phase modulated optical signal with a frequency shift of f s The frequency shifter generates a frequency f c +f s The phase modulated optical signal and the frequency shifted optical carrier are combined by the optical combiner and sent to the photoelectric detector to be tested for photoelectric conversion to generate a photocurrent signal; the wavelength of the optical carrier output by the laser is set, and the optical power incident on the photoelectric detector to be tested is adjusted by the optical power regulator to make it work in the nonlinear region, and the signal source is controlled to output different powers P1 by the control and data processing module; under different powers P1, the frequency f1+f in the photocurrent output by the photoelectric detector to be tested is obtained by the signal receiving module s or f1-f s The amplitude information i(f1±f s ; m) and f1+3f s or f1-3fs The amplitude information i P (f1±3f s m); using the optical power regulator to change the incident light power of the photodetector to be tested so that it works in the linear region, at the above corresponding different powers P1, the signal receiving module is used to measure the photocurrent output of the photodetector at a frequency of f s The amplitude information of the signal at i L (f s m), the modulation coefficient m of the electro-optic phase modulator is calculated by the control and data processing module; and then i P (f1±3f s ; m) and m calculate the amplitude information i of the third-order intermodulation signal of the photoelectric detector to be tested W (f1±3f s ; m); use the control and data processing module to analyze the data i(f1±f s ; m) and i W (f1±3f s m) perform linear fitting to obtain the OIP3 of the photodetector to be tested, and repeat the above steps to achieve the OIP3 frequency sweep test by changing the output frequency of the signal source.

[0013] In order to better explain the technical invention solution, the measurement principle of the present invention is briefly introduced below:

[0014] The optical carrier output by the laser is divided into two optical carrier signals after passing through the optical splitter. The optical field of the phase modulated optical signal formed by one optical carrier after passing through the electro-optical phase modulator is expressed as: E1(t)=E c exp(j2πf c t+jmsin(2πf1t+θ)) (1) Where E c and f c are the amplitude and frequency of the optical carrier, f1 and θ are the frequency and initial phase of the output signal of the signal source, and m is the modulation depth of the electro-optic phase modulator, which is determined by the half-wave voltage V π Defined as: Where P1 is the output power of the signal source, Z L The characteristic impedance is 50Ω. The optical field expression of the frequency-shifted optical carrier generated by the other optical carrier after passing through the frequency shifter is: E2(t)=E c exp(j2πf c t+j2πf s t) (3) The optical field after the phase modulated optical signal and the frequency shifted optical carrier are combined by the optical combiner is expressed as: where γ and They are the relative amplitude ratio and phase difference of the upper and lower paths after being combined by the optical combiner. Using Jacobi-Anger expansion, the intensity of the combined optical signal can be expressed as: Among them J n (·) is the first-order Bessel function of the first kind. The optical power regulator is used to adjust the optical power incident on the photodetector to be tested, so that the photodetector works in the nonlinear region. At this time, the photocurrent generated by the photodetector is expressed by the Taylor series expansion as follows: where c i represents the nonlinear coefficient of the photodetector. From formula (6), it can be seen that when n+p+q=±1, the frequency that can be directly measured by the signal receiving module is f1±3f s The amplitude information of the signal is expressed as: i P (f1±3f s ; m) = 2c3γ 3 E c 6 J1(3m) (7) Due to the third-order intermodulation product f1±3f of the photodetector s is only composed of the signal f1±f s The photodetector is in a nonlinear state and is mixed with each other. From equation (6), we can know that the third-order intermodulation component f1±3f of the photodetector is s The extraction of not only needs to satisfy n+p+q=±1, but also needs to satisfy n, p, q=±1, so the third-order intermodulation component f1±3f of the photodetector is s The amplitude information is expressed as: i W (f1±3f s ; m) = 6c3γ 3 E c 6 J1 3 (m) (8) From equations (7) and (8), we can see that the third-order intermodulation f1±3f of the photodetector is s The frequency f1±3f can be measured by the modulation coefficient m of the electro-optic phase modulator and the signal receiving module. s The signal amplitude i P (f1±3f s ; m) directly calculated: Formula (9) shows that once the modulation coefficient m of the electro-optic phase modulator is determined, the third-order intermodulation product of the photodetector to be tested can be measured from the signal receiving module at a frequency of f1±3f s The signal amplitude i P (f1±3f s ; m) can be directly calculated.

[0015] In order to obtain the modulation coefficient m of the electro-optic phase modulator, the optical power regulator is used to change the optical power incident on the photodetector to ensure that the photodetector works in the linear region. In this case, it can be seen from equation (6) that the frequency of the photocurrent signal generated by the photodetector is f s The signal can be expressed as: i L (f s ; m) = 2c1γE c 2 J0(m) (10) Set the signal source to output different powers P1 and P1', and use the signal receiving module to test the frequency f s The amplitude information of the electro-optic phase modulator can be obtained by comparing the two: in and They represent the modulation coefficient corresponding to the signal source output power P1' and the measured photodetector output frequency f s The signal amplitude, i L (f s ; m) indicates that the output frequency of the photodetector measured when the output power of the signal source is P1 is f s The signal amplitude of the photoelectric detector can be calculated using formulas (9) and (11): s The amplitude information i W (f1±3f s ; m). Repeat the above operation, set the signal source to test at different output powers P1 to obtain a set of i W (f1±3f s ; m) and i(f1±f s ; m) data, and perform linear fitting on P1 in the same coordinate system. The ordinate value of the intersection of the two fitting lines is the OIP3 of the photoelectric detector to be tested. Example

[0016] The block diagram of the test device of the present invention is as follows Figure 1 As shown. Laser output frequency f c=193.4THz optical carrier is divided into two paths by the optical splitter. One path is sent to the electro-optical phase modulator and modulated by the microwave signal with the output frequency of the signal source f1=10GHz; the other path is shifted by f s =80MHz frequency shifter performs frequency shifting to generate frequency-shifted optical carrier; the two optical signals are combined by the optical combiner and sent to the photoelectric detector to be tested for photoelectric conversion, and the generated photocurrent signal enters the signal receiving module to be recorded and analyzed.

[0017] The optical power regulator is used to adjust the optical power incident on the photodetector to 21.44 dBm, so that it can work in the nonlinear region. The output power P1 of the signal source is controlled to 4, 6, 8, and 10 dBm by the control and data processing module. The signal receiving module is used to measure and record the corresponding photocurrent signal output by the photodetector to be tested at P1. The frequency is 10.08 GHz (f1+f s ) of the signal amplitude information i(f1+f s ; m) were -12.79, -10.75, -8.71, -6.74dBm respectively, and the frequency was 10.24GHz (f1+3f s ) is the amplitude information of the signal i P (f1+3f s m) were -20.44, -18.46, -16.48, -14.57dBm respectively; the optical power regulator was used to change the optical power incident on the photodetector to be tested to 5.30dBm, so that it could work in the linear region. When P1 was 4, 6, 8, and 10dBm, the signal receiving module was used to measure the photocurrent signal output by the photodetector with a frequency of 80MHz (f s ) of the amplitude information i L (f s m), combined with formula (11), the modulation coefficient m of the electro-optic phase modulator is 0.18, 0.23, 0.29, and 0.37rad respectively when P1 is 4, 6, 8, and 10dBm, and then the third-order intermodulation component f1+3f to be measured of the photodetector is calculated by formula (9) when P1 is 4, 6, 8, and 10dBm. s The amplitude information i W (f1+3f s ; m) are -61.81, -55.63, -49.44, -43.11dBm respectively; for the data i(f1+f s ; m) and i W (f1+3f s m) for linear fitting, the ordinate value of the intersection of the two fitting lines, 20.77 dBm, is the value of the photoelectric detector to be tested at f1 + f s= OIP3 at 10.08GHz, such as Figure 2 shown.

Claims

1. A test device for nonlinear characteristics of a photodetector, comprising a Mach-Zehnder interference structure composed of a laser, an optical splitter, an electro-optical phase modulator, a frequency shifter, an optical combiner, and an optical power regulator, a photodetector to be tested, a signal receiving module, a signal source, and a control and data processing module; The electro-optic phase modulator and the frequency shifter are respectively located in the two interference arms of the Mach-Zehnder interference structure; The laser, optical splitter, electro-optical phase modulator, frequency shifter, optical combiner, optical power regulator, and photoelectric detector to be tested are optically connected; the signal source is electrically connected to the driving electrode of the electro-optical phase modulator; the photoelectric detector to be tested is electrically connected to the signal receiving module; the control and data processing module is data-connected to the signal source and the signal receiving module respectively; the optical carrier output by the laser is divided into two paths through the optical splitter, one path outputs a phase-modulated optical signal through the electro-optical phase modulator, and the other path outputs a frequency-shifted optical carrier through the frequency shifter; the two paths of optical signals are combined by the optical combiner and then sent to the photoelectric detector to be tested through the optical power regulator; the control and data processing module is used to control the signal source output signal frequency and amplitude, and the signal receiving module is controlled to measure the amplitude of the photocurrent signal output by the photoelectric detector to be tested, and the third-order intermodulation component of the photoelectric detector is extracted in combination with the modulation coefficient of the electro-optical phase modulation.

2. The device for testing the nonlinear characteristics of a photodetector according to claim 1, characterized in that: The optical power regulator is composed of an optical attenuator or an optical amplifier or a combination of an optical amplifier and an optical attenuator that can change the optical power. Its purpose is to change the optical power incident on the photodetector to be tested so that the photodetector to be tested works in a nonlinear region or a linear region.

3. The device for testing the nonlinear characteristics of a photodetector according to claim 1, characterized in that: The frequency shifter may be an acousto-optic frequency shifter or an electro-optic frequency shifter.

4. A method for testing the nonlinear characteristics of a photodetector, comprising the following steps: Step 1: construct a test device for nonlinear characteristics of a photodetector as described in claim 1; Step 2: Set the wavelength of the optical carrier output by the laser, and use the optical power regulator to adjust the optical power incident on the photodetector to be tested so that it works in the nonlinear region; Step 3: Use the control and data processing module to control the signal source to output a sinusoidal signal with a frequency of f1, and load it on the RF driving electrode of the electro-optic phase modulator, and set the frequency shift of the frequency shifter to f s ; Step 4: Use the signal receiving module to measure the frequency f1+f in the output photocurrent of the photodetector to be tested. s or f1-f s The amplitude and f1+3f s or f1-3f s The amplitudes are recorded as i(f1±f s ; m) and i P (f1±3f s ; m), change the power P1 of the signal source output, and record the i(f1±f s ; m) and i P (f1±3f s ; m) data; Step 5: Fix the wavelength of the laser light and change the light power incident on the photodetector to be tested by using the optical power regulator to make it work in the linear region. Corresponding to the signal source output power P1 in step 4, use the signal receiving module to measure the frequency f of the photocurrent output by the photodetector under different P1. s The signal amplitude i L (f s m), the modulation coefficient m of the electro-optic phase modulator at different powers P1 is calculated according to the following formula: where J0(·) is the zero-order Bessel function of the first kind, and They represent the modulation coefficient corresponding to the signal source output power P1' and the frequency measured by the signal receiving module f s The signal amplitude of Step 6: Use the signal source measured in steps 4 and 5 to obtain a set of i corresponding to different output powers P1 P (f1±3f s m) data and the modulation coefficient m data, the third-order intermodulation signal f1±3f of the photodetector to be tested is calculated according to the following formula under different P1. s The amplitude is recorded as i W (f1±3f s ; m): Step 7: Compare the data i(f1±f s ; m) and i W (f1±3f s m) perform linear fitting, the ordinate value of the intersection point determined by the two linear fitting curves is the photoelectric detector to be tested at a frequency of f1±f s The output third-order intercept point (OIP3) at Step 8: Repeat steps 2-7 to obtain the photodetector under test at different frequencies f1±f s OIP3 at.

5. A method for testing nonlinear characteristics of a photodetector according to claim 4, characterized in that: The signal receiving module is used to measure the frequency f of the photodetector output photocurrent at different output powers P1 and P1' of the signal source. s The modulation coefficient m of the electro-optic phase modulator is obtained by using the signal amplitude, wherein the signal source output power P1' can also be set to zero.

6. The method for testing the nonlinear characteristics of a photodetector according to claim 4, characterized in that: The third-order intermodulation signal i of the photodetector is extracted using the modulation coefficient m of the electro-optic phase modulator W (f1±3f s ; m).

7. The method for testing the nonlinear characteristics of a photoelectric detector according to claim 4, characterized in that: The output frequency f1 of the signal source in step 3 is proportional to the frequency shift amount f s Satisfy the relationship f1≠f s .

8. The method for testing the nonlinear characteristics of a photoelectric detector according to claim 4, characterized in that: In step 7, the discrete data point i(f1±f s ; m) and i W (f1±3f s The fitting method of m) can be the least square method, or other straight line linear fitting methods such as the gradient descent method.

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