Testing method and device for measuring nonlinearity of phase modulator

Abstract

The invention discloses a test method and device for measuring nonlinearity of a phase modulator, and belongs to the technical field of measurement, the device comprises a laser source, a polarization controller, a beam splitter, the phase modulator, a coupler, a photoelectric detector, a microwave source I, an acousto-optic frequency shifter, a microwave amplitude phase detection module and a microwave source II; the laser source is used for outputting a fixed-frequency optical signal to the polarization controller; the polarization controller is used for compensating the polarization state of the optical signal; the beam splitter is used for equally dividing an optical signal into two paths, the first path of optical signal is transmitted to the phase modulator, and the second path of optical signal is transmitted to the acousto-optic frequency shifter; by means of the mode, the problem that the phase modulator directly enters the photoelectric detector, photoelectric conversion cannot be conducted, and nonlinear parameters cannot be detected is solved.

Description

Technical Field

[0001] The present invention relates to the field of measurement technology, and particularly relates to a test method and device for measuring the nonlinearity of a phase modulator. Background Art

[0002] Microwave photonics technology is increasingly widely used in modern communication, radar, quantum communication and other fields, and the improvement of its performance is crucial for the development of the entire technology field. The importance of nonlinear problems in microwave photon devices, links and systems is becoming increasingly prominent, and the demand for measurement technology is increasing.

[0003] The nonlinear characteristics in microwave photon devices and systems are inevitable. This may be ignored in the case of small signals, but in large-signal or wide-band operation, the nonlinear effects become significant and affect the system performance.

[0004] Existing measurement technologies have limitations. Traditional S-parameter measurement methods are mainly designed for linear systems. When the system enters the nonlinear working region, the accuracy of these methods will decrease, and they cannot meet the requirements of modern microwave photon systems for precise measurement and performance characterization.

[0005] Therefore, there is a greater need for nonlinear models and more accurate measurement technologies. In order to accurately measure and characterize the frequency response of microwave photon devices, links and systems, new nonlinear models and broadband measurement technologies need to be developed. These technologies should be able to handle multi-dimensional nonlinear parameters and be applicable to broadband signals.

[0006] Under nonlinear conditions, the input signal will generate harmonics (second harmonic, third harmonic, etc.) and intermodulation components, and these additional frequency components need to be accurately measured and characterized.

[0007] Currently, the nonlinearity in the phase modulator cannot directly convert the optical signal into current through the detector, and an additional way to break the phase symmetry is required. At the same time, there is a problem of beat frequency cancellation when the phase modulator directly passes through the photodetector.

[0008] Based on this, the present invention designs a test method and device for measuring the nonlinearity of a phase modulator to solve the above problems. Summary of the Invention

[0009] In view of the above-mentioned disadvantages of the prior art, the present invention provides a test method and device for measuring the nonlinearity of a phase modulator.

[0010] To achieve the above objectives, the present invention is realized through the following technical solutions: A test device for measuring the nonlinearity of a phase modulator, comprising a laser source, a polarization controller, a beam splitter, a phase modulator, a coupler, a photodetector, a microwave source I, an acousto-optic frequency shifter, a microwave amplitude-phase detection module, and a microwave source II; The laser source is used to output an optical signal with a fixed frequency to a polarization controller; The polarization controller is used to compensate for the polarization state of the optical signal; The beam splitter is used to evenly divide the optical signal into two paths, the first path of the optical signal is transmitted to a phase modulator, and the second path of the optical signal is transmitted to an acousto-optic frequency shifter; The microwave source 1 and the microwave source 2 respectively output a radio frequency signal 1 and a radio frequency signal 2 with fixed frequencies; The radio frequency signal 1 and the radio frequency signal 2 respectively act on the phase modulator and the acousto-optic frequency shifter; The phase modulator is used to use the radio frequency signal 1 as a modulation signal to control the phase modulation of the first path of the optical signal and output a phase-modulated optical signal; The acousto-optic frequency shifter is used for the frequency-shifted optical signal of the superposition of the radio frequency signal 2 and the second path of the optical signal; The coupler is used to synthesize the phase-modulated optical signal and the frequency-shifted optical signal and output a coupled optical signal to a photodetector; The photodetector is used to convert the non-linear optical signal of the phase-modulated optical signal of the coupled optical signal into an electrical signal and send it to a microwave amplitude-phase detection module; The microwave amplitude-phase detection module is used to extract the amplitude and phase information of the DC, fundamental frequency, and harmonic components corresponding to the electrical signal, and obtain the signal response term of the phase modulator under different input electrical signal powers.

[0011] To better achieve the purpose of the present invention, the present invention also provides a test method for measuring the non-linearity of a phase modulator, including the following steps: Step 1, input the output optical signal output by the laser source into the polarization controller and then input it into the beam splitter by the polarization controller. At this time, the output optical signal of the laser source is expressed as: ; Where, is the amplitude of the output optical signal, is the angular frequency of the output optical signal, is the output optical signal; Step 2, the output optical signal passes through the first path of the beam splitter and passes through the phase modulator. Under the action of the external electric field of the input signal of the phase modulator, the radio frequency signal 1 with a frequency of ω e output by the microwave source 1, and the optical signal at this time is phase-modulated; the output optical wave electric field expression can be expressed as: ; In the formula, E0 represents the amplitude of the optical carrier, J n (·) represents the nth-order Bessel function of the first kind, β represents the modulation coefficient of the phase modulator, is the base of the natural logarithm, the content in the parentheses behind is the exponent, i is the imaginary unit, and t is time. is the angular frequency of the output optical signal. is the part of the phase modulation by the phase modulator, and β is the modulation coefficient of the phase modulator. The second path of the output optical signal passes through the acousto-optic frequency shifter via the beam splitter. The frequency output by the second microwave source is of the optical signal. The optical wave electric field output by the beam splitter The expression can be represented as: ; Where is the angular frequency of the acousto-optic frequency shift.

[0012] Step Three: The coupled optical signal output by the coupler can be represented as: ; Step Four: The coupled optical signal output by the coupler passes through the photodetector. The nonlinearity of is converted from the optical signal to the electrical signal through the photodetector. At this time, the electrical signal output in the link The expression can be represented as: ; Where is the response coefficient of the photodetector, is taking the conjugate, represents of order harmonic; The amplitude and phase information of the direct current, fundamental frequency, and harmonic components corresponding to the output of the photodetector are extracted through the microwave amplitude-phase detection module, and the signal response terms of the phase modulator under different input electrical signal powers are obtained. Step Five: The harmonic and intermodulation components of the phase modulator are measured, and the complete nonlinear parameters of the phase modulator to be measured under different input electrical signal powers are obtained. By scanning the frequency of the electrical signal input to the phase modulator and repeating Steps One to Four, the complete nonlinear parameters of the phase modulator at different frequencies can be obtained. Step Six: After the coupler couples the optical signal output by the acousto-optic frequency shifter and the optical signal output by the phase modulator, it outputs to the photodetector. The coupled optical signal is converted into an electrical signal through the photodetector, and the electrical signal is input to the microwave amplitude-phase detection module to obtain the nonlinear parameters.

[0013] Step Six: The coupler couples the optical signal output by the acousto-optic frequency shifter and the optical signal output by the phase modulator, then outputs to the photodetector. The coupled optical signal is converted into an electrical signal through the photodetector, and the electrical signal is input to the microwave amplitude-phase detection module to obtain the nonlinear parameters.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: It avoids the problem that the phase modulator directly enters the photodetector and cannot perform photoelectric conversion to detect the non-linear parameter. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0016] Figure 1 It is a block diagram of a test device for measuring the non-linearity of a phase modulator according to the present invention.

[0017] The reference numerals in the figure respectively represent: 1. Laser source; 2. Polarization controller; 3. Beam splitter; 4. Phase modulator; 5. Coupler; 6. Photodetector; 7. Microwave source 1; 8. Acousto-optic frequency shifter; 9. Microwave amplitude-phase detection module; 10. Microwave source 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] In order to make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0019] Embodiment 1: In some embodiments, please refer to Figure 1 of the specification drawings. A test device for measuring the non-linearity of a phase modulator includes a laser source 1, a polarization controller 2, a beam splitter 3, a phase modulator 4, a coupler 5, a photodetector 6, a microwave source 1 7, an acousto-optic frequency shifter 8, a microwave amplitude-phase detection module 9, and a microwave source 2 10; The laser source 1 is used to output a fixed-frequency optical signal to the polarization controller 2; The polarization controller 2 is used to compensate the polarization state of the optical signal; The beam splitter 3 is used to equally divide the optical signal into two paths. The first path of the optical signal is transmitted to the phase modulator 4, and the second path of the optical signal is transmitted to the acousto-optic frequency shifter 8; The microwave source 1 7 and the microwave source 2 10 are respectively a radio frequency signal 1 and a radio frequency signal 2 with fixed frequencies; The radio frequency signal 1 and the radio frequency signal 2 respectively act on the phase modulator 4 and the acousto-optic frequency shifter 8; The phase modulator 4 is used to control the phase modulation of the first optical signal based on a radio frequency signal as a modulation signal and output a phase-modulated optical signal; The acousto-optic frequency shifter 8 is used for the frequency-shifted optical signal of the superposition of the second radio frequency signal and the second optical signal; The coupler 5 is used to synthesize the phase-modulated optical signal and the frequency-shifted optical signal and output a coupled optical signal to the photodetector 6; The photodetector 6 is used to convert the non-linear optical signal of the phase-modulated optical signal of the coupled optical signal into an electrical signal and send it to the microwave amplitude-phase detection module 9; The microwave amplitude-phase detection module 9 is used to extract the amplitude and phase information of the DC, fundamental frequency, and harmonic components corresponding to the electrical signal, and obtain the signal response items of the phase modulator 4 under different input electrical signal powers.

[0020] The present invention solves the problem that the phase modulator will cause beat cancellation directly through the photodetector. The solution principle of this problem is: for example, the function of the acousto-optic frequency shifter 8 in this process is to shift the frequency of the optical signal from the beam splitter 3; The function of the beam splitter 3 is to split the optical signal from the laser source 1 into two paths; The function of the coupler 5 is to couple the modulation signal output by the phase modulator 4 and the signal after frequency shifting by the acousto-optic frequency shifter 8.

[0021] The present invention realizes the extraction of various non-linear parameters such as the 1 dB compression current, intermodulation components, harmonic components, and spurious-free dynamic range of the electro-optic phase modulator by solving the current information.

[0022] Embodiment 2: In some embodiments, please refer to the Figure 1 , a test method for measuring the non-linearity of a phase modulator, includes the following steps: Step 1, input the output optical signal output by the laser source 1 into the polarization controller 2 and then input it into the beam splitter 3 by the polarization controller 2. At this time, the output optical signal of the laser source 1 is expressed as: ; Among them, is the amplitude of the output optical signal, is the angular frequency of the output optical signal, is the output optical signal; Step 2, the output optical signal passes through the first path of the beam splitter 3 and passes through the phase modulator 4. Under the action of the external electric field of the input signal of the phase modulator 4, the radio frequency signal 1 with a frequency of ω e output by the microwave source 1, and the optical signal at this time is phase-modulated; the optical wave electric field output by the phase modulator 4 can be expressed as: ; where \(E_0\) represents the amplitude of the optical carrier, \(J\) n \(\cdot\) represents the Bessel function of the first kind of order \(n\), \(\beta\) represents the modulation coefficient of the phase modulator 4, is the exponential with base \(e\) and the content in the subsequent parentheses as the exponent, \(i\) is the imaginary unit, \(t\) is time, \(\omega\) is the angular frequency of the output optical signal, is the part of the phase modulation of the phase modulator, and \(\beta\) is the modulation coefficient of the phase modulator; The output optical signal passes through the second path of the beam splitter 3 through the acousto-optic frequency shifter, and the frequency of the optical signal output by the microwave source two 10 is The optical wave electric field output by the beam splitter 3 The expression can be represented as: ; where \(\omega_{ao}\) is the angular frequency of the acousto-optic frequency shift.

[0023] Step three, the coupled optical signal output by the coupler 5 can be represented as: ; Step four, the coupled optical signal output through the coupler 5 passes through the photodetector 6, and the photodetector 6 converts the non-linearity of into an electrical signal. At this time, the electrical signal output in the link The expression can be represented as: ; where \(R\) is the response coefficient of the photodetector, taking the conjugate, represents of the \(m\)-th harmonic; The microwave amplitude and phase detection module 9 extracts the amplitude and phase information of the direct current, fundamental frequency, and harmonic components corresponding to the output of the photodetector 6, and obtains the signal response terms of the phase modulator 4 under different input electrical signal powers; Step five, measure the harmonic and intermodulation components of the phase modulator 4, and obtain the complete non-linear parameters of the phase modulator 4 to be measured under different input electrical signal powers; scan the frequency of the electrical signal input to the phase modulator 4, and repeat steps one to four to obtain the complete non-linear parameters of the phase modulator 4 at different frequencies.

[0024] ​Step 6: The coupler 5 couples the optical signal output by the acousto-optic frequency shifter 8 and the optical signal output by the phase modulator 4, and then outputs the coupled optical signal to the photodetector 6. The photodetector 6 converts the coupled optical signal into an electrical signal and inputs the electrical signal to the microwave amplitude-phase detection module 9 to obtain the nonlinear parameter.

[0025] Embodiment 3: In some embodiments, please refer to the Figure 1 accompanying drawings of the specification. A test method for measuring the nonlinearity of a phase modulator includes the following steps: This embodiment is a further specific implementation of the measurement principle.

[0026] The measurement principle includes the following steps: Step 1: First, input the optical signal output by the laser source 1 into the phase modulator 4 to be measured. At this time, the optical signal output by the laser source 1 can be expressed as:

[0027] where is the amplitude of the input optical signal, is the angular frequency of the signal output by the laser source 1; Step 2: The output optical signal passes through the beam splitter 3. One path passes through the phase modulator 4 to be measured. Under the action of an external electric field, the radio frequency signals with frequencies ω1 and ω2 output by the microwave source 7 modulate the optical signal. The expression of the output optical wave electric field can be expressed as:

[0028] In the formula, E0 represents the amplitude of the optical carrier, Jn(·) represents the nth-order Bessel function of the first kind, β represents the modulation coefficient of the phase modulator, ω1 and ω2 are the angular frequencies of the two radio frequency signals, Epm is the output of the phase modulator, i is the imaginary unit, n and m are the subscripts of the summation symbol, and Jn(·) represents the nth-order Bessel function of the first kind; The output optical signal passes through the beam splitter 3. The other path passes through the acousto-optic frequency shifter 8. The expression of the output optical wave electric field can be expressed as: ; where is the angular frequency of acousto-optic frequency shifting; Step 3: One path of the two optical signals is phase-modulated, and the other path is acousto-optic frequency-shifted and then coupled through the coupler 5. At this time, the optical signal output by the coupler 4 can be expressed as:

[0029] where is the angular frequency of the signal output by the laser source 1, is the angular frequency of acousto-optic frequency shifting; Step 4: The optical signal output by the coupler 5 passes through the photodetector 6. The photodetector 6 converts the nonlinearity of the phase modulator 4 from the optical domain to the electrical domain. At this time, the expression of the electrical signal output in the link can be expressed as: ; where is the response coefficient of the photodetector.

[0030] The microwave amplitude and phase detection module 9 extracts the amplitude and phase information of the DC, fundamental frequency, and harmonic components corresponding to the electrical signal output by the photodetector 6, and obtains the signal response terms of the phase modulator 4 under different input electrical signal powers.

[0031] Step 5: Measure the harmonic and intermodulation components of the corresponding electro-optic phase modulator 4 to obtain the complete nonlinear parameters of the phase modulator 4 to be measured under the input electrical signal power; scan the frequency of the electrical signal input to the phase modulator 4 to be measured, and repeat Steps 1 to 4 to obtain the complete nonlinear parameters of the phase modulator 4 at different frequencies.

[0032] The fundamental difference between Embodiment 3 and Embodiment 2 is that the input RF signal changes from a single signal to two signals with the same amplitude and a very small frequency interval; When a single RF signal is input, the signal output by the photodetector only enters the microwave amplitude and phase detection module 9 to obtain DC, fundamental frequency signals, and harmonic signals; When two RF signals with the same amplitude and a very small frequency interval are input, the signal output by the photodetector 6 enters the microwave amplitude and phase detection module 9, and DC, fundamental frequency signals, harmonic signals, and intermodulation signals can be obtained.

[0033] Thus, the nonlinear measurement is more complete (nonlinearity includes harmonics and intermodulation).

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A test device for measuring nonlinearity of a phase modulator, characterized in that: The device comprises a laser source (1), a polarization controller (2), a beam splitter (3), a phase modulator (4), a coupler (5), a photodetector (6), a microwave source 1 (7), an acousto-optic frequency shifter (8), a microwave amplitude and phase detection module (9) and a microwave source 2 (10); The laser source (1) is used to output a fixed frequency optical signal to the polarization controller (2); The polarization controller (2) is used to compensate the polarization state of the optical signal; The beam splitter (3) is used to equally split the optical signal into two paths, wherein the first optical signal is transmitted to the phase modulator (4), and the second optical signal is transmitted to the acousto-optic frequency shifter (8); The microwave source 1 (7) and the microwave source 2 (10) respectively generate a radio frequency signal 1 and a radio frequency signal 2 of fixed frequencies; The radio frequency signal 1 and the radio frequency signal 2 act on the phase modulator (4) and the acousto-optic frequency shifter (8) respectively; The phase modulator (4) is used to control the phase modulation of the first optical signal based on the radio frequency signal 1 as a modulation signal, and output a phase modulated optical signal; The acousto-optic frequency shifter (8) is used for the frequency-shifted optical signal formed by superimposing the frequency of the second radio frequency signal and the second optical signal; The coupler (5) is used to synthesize the phase modulated optical signal and the frequency shifted optical signal, and output the coupled optical signal to the photodetector (6); The photoelectric detector (6) is used to convert the nonlinear optical signal of the phase modulated optical signal of the coupled optical signal into an electrical signal and send the electrical signal to the microwave amplitude and phase detection module (9); The microwave amplitude and phase detection module (9) is used to extract the amplitude and phase information of the direct current, fundamental frequency and harmonic components corresponding to the electrical signal, and obtain the signal response items of the phase modulator (4) under different input electrical signal powers.

2. The test method for measuring nonlinearity of a phase modulator according to claim 1, characterized in that: The following steps are involved: Step 1: input the output optical signal output by the laser source (1) into the polarization controller (2) and then input the output optical signal from the polarization controller (2) into the beam splitter (3). At this time, the output optical signal of the laser source (1) is expressed as: ; in, is the amplitude of the output optical signal, is the angular frequency of the output optical signal, To output an optical signal; Step 2: The output optical signal passes through the first path of the beam splitter (3) and passes through the phase modulator (4). Under the action of the external electric field of the input signal of the phase modulator (4), the frequency of the output of the microwave source 1 (7) is ω e The radio frequency signal is 1, and the optical signal is phase modulated at this time; the phase modulator (4) outputs the light wave electric field The expression can be expressed as: ； Where, E0 represents the amplitude of the optical carrier, J n (·) represents the nth-order Bessel function of the first kind, β represents the modulation coefficient of the phase modulator (4), The base is e, the content in the brackets is the exponent, i is the imaginary unit, t is the time, is the angular frequency of the output optical signal, is the phase modulated part of the phase modulator, and β is the modulation coefficient of the phase modulator; The output optical signal passes through the second path of the beam splitter (3) and passes through the acousto-optic frequency shifter. The frequency output by the microwave source 2 (10) is The beam splitter (3) outputs the light wave electric field The expression can be expressed as: ； in, is the angular frequency of the acousto-optic frequency shift; Step 3: Coupled optical signal output by the coupler (5) It can be expressed as: ； Step 4: Output the coupled optical signal through the coupler (5) and pass it through the photodetector (6). The nonlinearity is converted from optical signals to electrical signals; At this time, the output in the link electrical signal The expression can be expressed as: ； in is the photodetector responsivity, is to take the conjugate, express of order harmonics; The output of the photodetector (6) is extracted by a microwave amplitude and phase detection module (9) Corresponding to the amplitude and phase information of the DC, fundamental frequency and harmonic components, the signal response items of the phase modulator (4) under different input electrical signal powers are obtained; Step 5: measuring the harmonics and intermodulation components of the phase modulator (4) to obtain the complete nonlinear parameters of the phase modulator (4) under different input electrical signal powers; scanning the frequency of the electrical signal input to the phase modulator (4), and repeating steps 1 to 4 to obtain the complete nonlinear parameters of the phase modulator (4) under different frequencies; Step six: the coupler (5) couples the optical signal output by the acousto-optic frequency shifter (8) and the optical signal output by the phase modulator (4), and outputs the coupled optical signal to the photodetector (6). The photodetector (6) converts the coupled optical signal into an electrical signal, and the electrical signal is input into a microwave amplitude and phase detection module (9) to obtain nonlinear parameters.

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