A testing device and method for nonlinearity of a photodetector

Abstract

The application discloses a kind of photodetector nonlinear characteristic testing device and method, testing device mainly by laser, optical splitter, electro-optic phase modulator, frequency shifter, optical combiner and optical power regulator constitute Mach-Zehnder interference structure, the optical carrier output by laser is divided into two ways, one way passes through the electro-optic phase modulator modulated by signal source, another way passes through frequency shifter, two optical signals are combined and sent into the photodetector to be measured by optical power regulator;Signal source is electrically connected with electro-optic phase modulator, and the photodetector to be measured is electrically connected with signal receiving module;Control and data processing module are connected with signal source, signal receiving module data respectively.With control and data processing module control signal source output signal frequency and amplitude, and control signal receiving module measures the amplitude of the output photocurrent signal of the photodetector to be measured, combined with the modulation coefficient of electro-optic phase modulation, the third-order intermodulation component of photodetector is extracted.

Description

Technical Field

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

[0002] Photodetectors are essential components 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 nonlinearity of the photodetector is a key parameter affecting its dynamic range. In particular, in analog applications requiring high optical power, highly linear photodetectors are advantageous for minimizing signal distortion and improving spurious-free dynamic range. Therefore, quantitatively and accurately measuring the nonlinearity of photodetectors is crucial for both device optimization and link evaluation. Among all nonlinear components in a photodetector, third-order intermodulation (OIM) is one of the most significant in high-power operation and is difficult to eliminate through filtering. Therefore, the output third-order intercept (OIP3) of a photodetector is often used to quantify its nonlinearity.

[0003] The OIP3 of a photodetector is typically defined as the extrapolated intercept point between the fundamental frequency power and the third-order intermodulation power. Testing methods primarily include the all-optical wavelength beat method and the electro-optical intensity modulation method. The wavelength beat method requires four high-performance tunable lasers, which beat each other to generate the all-optical excitation required for photodetector testing. This method, without any electrical drive, often suffers from wavelength instability and polarization alignment issues, posing significant challenges for fast and precise swept-spectrum analysis. In contrast, the electro-optical intensity modulation method, which uses electro-optical modulation to generate the electro-optical excitation required for photodetector testing, can effectively implement swept-spectrum analysis. Examples include the dual-laser dual-modulator method, the three-laser three-modulator method, and the 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. Furthermore, the two intensity modulators must be biased at orthogonal points and operate under conditions close to small signal levels, resulting in limited signal-to-noise ratio (SNR). To avoid the influence of the intensity modulator's nonlinear modulation, the three-laser three-modulator method uses a more complex three-parallel optical link to indirectly extract the two-tone OIP3 from the three-tone OIP3 at the expense of three lasers cascaded with three intensity modulators. The test setup of the improved two-laser two-modulator method is similar to that of the two-laser two-modulator method, but the improved method achieves decoupling between the intensity modulator nonlinearity and the photodetector nonlinearity, 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, making the setup more 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 the swept-frequency, bias-free and modulator-unaffected testing of the nonlinear characteristics of photodetectors. Summary of the Invention

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

[0006] A device for testing the nonlinear characteristics of a photodetector, comprising a Mach-Zehnder interference structure consisting 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 by the optical splitter, one path outputs a phase-modulated optical signal by the electro-optical phase modulator, and the other path outputs a frequency-shifted optical carrier by the frequency shifter. The two optical signals are combined by the optical combiner and then sent to the photodetector to be tested through the optical power regulator. The control and data processing module is used to control the frequency and amplitude of the signal output signal of the signal source, and to control the signal receiving module to measure the amplitude of the photocurrent signal output by the photodetector to be tested, and the third-order intermodulation component of the photodetector is extracted in combination with the modulation coefficient of the electro-optical phase modulation.

[0007] A method for testing nonlinear characteristics of a photodetector comprises the following steps:

[0008] Step 1: Build a test device for the nonlinear characteristics of the photodetector;

[0009] Step 2: Set the wavelength of the laser output optical carrier and use the optical power regulator to adjust the optical power incident on the photodetector to be measured so that it operates in the nonlinear region.

[0010] 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-optical phase modulator, and set the frequency shift of the frequency shifter to f s ;

[0011] Step 4: Use the signal receiving module to measure the frequency of the output photocurrent of the photodetector to be tested, which is f1+f s or f1-f s The amplitude and f1+3f s or f1-3f s The amplitude is recorded as i(f1±f s ; m) and i P (f1±3f s ; m), change the power P1 of the signal source output, record the i(f1±f s ; m) and i P (f1±3f s ; m) data;

[0012] Step 5: Fix the laser wavelength and change the incident light power of the photodetector to be measured by using the optical power regulator so that it works 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), calculated according to the following formula, the modulation coefficient m of the electro-optical phase modulator at different powers P1 is obtained:

[0013]

[0014] where J0(·) is the zero-order Bessel function of the first kind, and They represent the modulation coefficient corresponding to the output power of the signal source P1' and the frequency measured by the signal receiving module f s The signal amplitude;

[0015] 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, according to the following formula to calculate the third-order intermodulation signal f1±3f of the photodetector to be tested under different P1 s The amplitude, denoted as i W (f1±3f s ; m):

[0016]

[0017] Step 7: For the data i(f1±f s ; m) and i W (f1±3f sm) linear fitting, the ordinate value of the intersection of the two linear fitting curves is the photoelectric detector to be tested at a frequency of f1 ± f s Output third-order intercept point (OIP3) at

[0018] Step 8: Repeat steps 2-7 to obtain the photodetector under test at different frequencies f1±f s OIP3 at.

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

[0020] 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;

[0021] Second, compared to the all-optical wavelength beat frequency method, the present invention uses electro-optical modulation to test the third-order intermodulation of photodetectors, and has fast and precise sweep frequency testing capabilities;

[0022] 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. It does not require small signal approximation conditions and can completely eliminate the influence of electro-optic modulation nonlinearity on the test. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the device of the present invention.

[0024] Figure 2 This is a test effect diagram of the present invention. DETAILED DESCRIPTION

[0025] 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 thereby.

[0026] 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 optical carrier 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 In the frequency shifter, the frequency f c +f sThe 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 regulator is used to adjust the optical power incident on the photoelectric detector to be tested so that it works in the nonlinear region, and the control and data processing module is used to control the signal source to output different powers P1; under different powers P1, the signal receiving module is used to obtain the frequency f1+f in the photocurrent output by the photoelectric detector to be tested. s or f1-f s The amplitude information i(f1±f s ; m) and f1+3f s or f1-3f s Amplitude information i P (f1±3f s m); using an optical power regulator to change the incident light power to the photodetector to be measured so that it operates in the linear region, at the above corresponding different powers P1, the use of a signal receiving module to measure the photodetector output photocurrent frequency f s The amplitude information of the signal at i L (f s m), the modulation coefficient m of the electro-optical phase modulator is calculated using 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 measured 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.

[0027] In order to better explain the present invention, the measurement principle of the present invention is briefly introduced below:

[0028] 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 after one optical carrier passes through the electro-optical phase modulator is expressed as follows:

[0029] E1(t)=E c exp(j2πf c t+jmsin(2πf1t+θ)) (1)

[0030] Among them E c and f care 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-optical phase modulator, which is determined by the half-wave voltage V π Defined as:

[0031]

[0032] 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:

[0033] E2(t)=E c exp(j2πf c t+j2πf s t) (3)

[0034] The optical field after the phase modulated optical signal and the frequency shifted optical carrier are combined by the optical combiner is expressed as:

[0035]

[0036] where γ and are the relative amplitude ratio and phase difference of the upper and lower paths after they are combined by the optical combiner. Using Jacobi-Anger expansion, the intensity of the combined optical signal can be expressed as:

[0037]

[0038] 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 measured, so that the photodetector operates in the nonlinear region. At this time, the photocurrent generated by the photodetector is expressed by the Taylor series expansion as follows:

[0039]

[0040] 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:

[0041] i P (f1±3f s ; m) = 2c3γ 3 E c 6 J1(3m) (7)

[0042] Due to the third-order intermodulation product f1±3f of the photodetector sis composed only of the signal f1±f s The photodetector is in a nonlinear state and is mixed with each other. From formula (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:

[0043] i W (f1±3f s ; m) = 6c3γ 3 E c 6 J1 3 (m) (8)

[0044] 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-optical phase modulator and the signal receiving module. s The signal amplitude i P (f1±3f s ; m) directly calculated:

[0045]

[0046] Formula (9) shows that once the modulation coefficient m of the electro-optical 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.

[0047] In order to obtain the modulation coefficient m of the electro-optical phase modulator, the optical power regulator is used to change the optical power incident on the photodetector to ensure that the photodetector operates in the linear region. In this case, it can be seen from formula (6) that the frequency of the photocurrent signal generated by the photodetector is f s The signal can be expressed as:

[0048] i L (f s ; m) = 2c1γE c 2 J0(m) (10)

[0049] 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:

[0050]

[0051] in and They represent the modulation coefficient corresponding to the output power of the signal source P1' and the measured output frequency of the photodetector 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) to obtain the third-order intermodulation component f1±3f to be measured. s Amplitude information i W (f1±3f s Repeat the above operation and 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 photodetector to be measured.

[0052] Example

[0053] The test device block diagram 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 of 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.

[0054] 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 these P1s. The frequency is 10.08 GHz (f1+f s ) signal amplitude information i(f1+f s ; m) were -12.79, -10.75, -8.71, -6.74dBm, and the frequency was 10.24GHz (f1+3f s ) of the signal amplitude information iP (f1+3f s m) were -20.44, -18.46, -16.48, -14.57dBm respectively; the optical power regulator was used to change the incident optical power of 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 at a frequency of 80MHz (f s ) amplitude information i L (f s m), combined with formula (11), the modulation coefficient m of the electro-optical phase modulator is 0.18, 0.23, 0.29, and 0.37 rad when P1 is 4, 6, 8, and 10 dBm, respectively. Then, using formula (9), the third-order intermodulation component f1+3f to be measured of the photodetector is calculated to be 0.18, 0.23, 0.29, and 0.37 rad when P1 is 4, 6, 8, and 10 dBm. s Amplitude information i W (f1+3f s ; m) are -61.81, -55.63, -49.44, -43.11dBm respectively; for data i(f1+f s ; m) and i W (f1+3f s m) linear fitting, the intersection of the two fitting lines determined by the ordinate value of 20.77dBm is the photoelectric detector to be tested at f1 + f s = OIP3 at 10.08GHz, such as Figure 2 shown.

Claims

1. A method for testing the nonlinear characteristics of a photodetector, comprising the following steps: Step 1: Build a test device for the nonlinear characteristics of a photodetector. The test device includes a Mach-Zehnder interferometer structure consisting of a laser, an optical splitter, an electro-optical phase modulator, a frequency shifter, an optical combiner, and an optical power regulator; the photodetector to be tested; a signal receiving module; a signal source; and a control and data processing module. The electro-optical phase modulator and frequency shifter are respectively located in the two interferometer arms of the Mach-Zehnder interferometer structure; The laser, optical splitter, electro-optical phase modulator, frequency shifter, optical combiner, optical power regulator, and photodetector to be tested are optically connected; the signal source is electrically connected to the driving electrode of the electro-optical phase modulator; the photodetector to be tested is electrically connected to the signal receiving module; and the control and data processing module are data-connected to the signal source and the signal receiving module, respectively. The optical carrier output by the laser is divided into two paths by the optical splitter, one path outputs a phase-modulated optical signal via the electro-optical phase modulator, and the other path outputs a frequency-shifted optical carrier via the frequency shifter. The two optical signals are combined by the optical combiner and then sent to the photodetector to be tested via the optical power regulator. Step 2: Set the wavelength of the laser output optical carrier and use the optical power regulator to adjust the optical power incident on the photodetector to be measured so that it operates 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-optical 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 of the output photocurrent of the photodetector to be tested, which is f1+f s or f1-f s The amplitude and f1+3f s or f1-3f s The amplitude is recorded as i(f1±f s ; m) and i P (f1±3f s ; m), change the power P1 of the signal source output, record the i(f1±f s ; m) and i P (f1±3f s ; m) data; Step 5: Fix the laser wavelength and change the incident light power of the photodetector to be measured by using the optical power regulator so that it works 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), calculated according to the following formula, the modulation coefficient m of the electro-optical phase modulator at different powers P1 is obtained: where J0(·) is the zero-order Bessel function of the first kind, and They represent the modulation coefficient corresponding to the output power of the signal source P1' and the frequency measured by the signal receiving module f s The signal amplitude; 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, according to the following formula to calculate the third-order intermodulation signal f1±3f of the photodetector to be tested under different P1 s The amplitude, denoted as i W (f1±3f s ; m): Step 7: For the data i(f1±f s ; m) and i W (f1±3f s m) linear fitting, the ordinate value of the intersection of 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.

2. The method for testing the nonlinear characteristics of a photodetector according to claim 1, wherein: 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 operates in the nonlinear region or the linear region.

3. The method for testing the nonlinear characteristics of a photodetector according to claim 1, wherein: The frequency shifter is an acousto-optic frequency shifter or an electro-optic frequency shifter.

4. The method for testing the nonlinear characteristics of a photodetector according to claim 1, wherein: 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-optical phase modulator is obtained by using the signal amplitude of , wherein the signal source output power P1' is set to zero.

5. The method for testing the nonlinear characteristics of a photodetector according to claim 1, wherein: The third-order intermodulation signal i of the photodetector is extracted using the modulation coefficient m of the electro-optical phase modulator. W (f1±3f s ; m).

6. The method for testing the nonlinear characteristics of a photodetector according to claim 1, wherein: The output frequency f1 of the signal source in step 3 is proportional to the frequency shift amount f s Satisfying the relationship f1≠f s .

7. The method for testing the nonlinear characteristics of a photodetector according to claim 1, wherein: In step 7, the discrete data point i(f1±f s ; m) and i W (f1±3f s ; m) is fitted by the least squares method or the gradient descent method.

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