A half-wave voltage measurement system and method for a high-frequency electro-optic phase modulator

Abstract

The present invention provides a high-frequency electro-optic modulator half-wave voltage measurement system and method. The system includes a single-frequency laser, a microwave drive signal source, an electro-optic modulator, a dispersion spectroscopy module, a beam splitting device, a detector array, a signal acquisition module, and a data processing module. The microwave drive signal source is connected to the electro-optic modulator and is used to apply a drive voltage to the electro-optic modulator. The single-frequency laser generates single-frequency laser light and inputs it into the electro-optic modulator, and multi-frequency composite light is output. After the multi-frequency composite light passes through the dispersion spectroscopy module, the sideband light of different frequencies is spatially separated into a plurality of light beams with a spacing of the order of millimeters. Then, the target carrier light and the ±1 order sideband light are separated by the beam splitting device and simultaneously input into the detector array respectively, and the light intensity signal is converted into an electrical signal and input into the signal acquisition module, and the collected voltage value is output. The collected voltage value is input into the data processing module, and the half-wave voltage value of the electro-optic modulator is output.

Description

Technical Field

[0001] The present invention relates to the technical field of laser modulation, and specifically to a half-wave voltage measurement system and method for a high-frequency electro-optic modulator using VIPA dispersion spectroscopy detection. Background Art

[0002] An electro-optic modulator (EOM) is an integrated device that combines the electro-optic interaction of loading radio frequency electrical signals onto optical signals and has wide applications in the fields of optical communication, microwave photonics, fiber optic sensing, quantum information technology, etc. The basic principle of an electro-optic modulator is an optical modulator that works based on the electro-optic effect of a crystal. By changing the refractive index of the crystal, the characteristics of light waves are changed, and ultimately the control of the phase, amplitude, intensity, and polarization state of the input light is achieved. A typical electro-optic modulator is a phase modulator. By applying radio frequency modulation to an optical carrier, equally spaced sidebands outside the carrier are generated in the optical frequency domain. The frequency interval between each sideband is equal to the frequency of the modulation signal, and the power magnitude of each sideband conforms to a predetermined function model, representing the modulation efficiency of the phase modulator. The half-wave voltage is one of the most important parameters of a phase modulator. It represents the change in the bias voltage corresponding to a phase delay of π caused by the phase modulator. The half-wave voltage characterizes the modulation efficiency and modulation power consumption of the phase modulator and largely determines the performance of the phase modulator. However, due to the drift problem of the half-wave voltage due to environmental changes and the fact that for high-speed radio frequency drive signals, the half-wave voltage is also related to the specific frequency magnitude, it is difficult to accurately obtain the half-wave voltage of a high-frequency electro-optic modulator.

[0003] At present, there are mainly three common measurement methods for the half-wave voltage of electro-optic modulators, namely the frequency doubling modulation method, the extreme value measurement method, and the spectral analysis method. The basic principle of the frequency doubling modulation method is to simultaneously apply a DC voltage and an AC signal. When the DC voltage is adjusted to the voltage value corresponding to the extreme value of the output optical intensity, the output AC signal will exhibit frequency doubling distortion, and the difference between the DC voltages corresponding to the occurrence of frequency doubling distortion is the half-wave voltage. The measurement method is relatively accurate, but the frequency doubling modulation method has high requirements for the adjustment point and it is difficult to capture the optimal state, so it is not suitable for high-frequency measurement. The basic principle of the extreme value measurement method is not to apply a modulation signal to the phase modulator, but only to apply a DC voltage. When gradually changing the magnitude of the applied DC voltage, the extreme value point can be judged by the magnitude of the output optical intensity of the designed interferometer optical path. The difference between the DC voltages corresponding to adjacent maximum and minimum values is the half-wave voltage. This measurement method is relatively simple, but it has high requirements for system factors such as the stability of the light source, which limits the measurement accuracy of this method. In addition, it is sensitive to the optical path difference and vulnerable to the external environment. The method for measuring the half-wave voltage based on the Sagnac effect fiber interferometer solves the problem of instability sensitive to the optical path difference of the optical path. Since the optical paths of the two beams of light are the same, the system is stable. However, it is difficult to build this system due to its high asymmetry, and this method also cannot solve the problem of accurately measuring the half-wave voltage under different high-frequency drives. The basic principle of the spectral analysis method is to modulate the light wave of the phase modulator to be measured with a sine signal, and input the output optical signal of the phase modulator into a spectral analyzer for spectral analysis to obtain the relative intensities of the sidebands and carriers of the light wave, and calculate the half-wave voltage of the phase modulator from the relationship between the ratio of the relative intensities and the modulation depth. This method can measure the half-wave voltage under different high-frequency drives online. However, since the frequency of the high-frequency drive signal of the EOM is generally between 1 GHz and 20 GHz, it is difficult to spatially separate the frequency components using traditional dispersion spectroscopy elements such as gratings and prisms in traditional spectral detection methods, resulting in low frequency resolution and limited accuracy in measuring the half-wave voltage, and the system requires relatively complex calibration. Summary of the Invention

[0004] The present invention provides a high-frequency electro-optic modulator half-wave voltage measurement system and method, which are used to overcome the problem of accurately measuring the half-wave voltage of existing high-frequency electro-optic modulators at different driving frequencies. At the same time, for the first time, a VIPA high-dispersion spectroscopy device is applied to the spectral method for measuring the half-wave voltage, improving the spectral resolution and real-time measurement ability, and having strong technological innovation and practical value.

[0005] The technical solution of the present invention is as follows:

[0006] A high-frequency electro-optic phase modulator half-wave voltage measurement system includes a single-frequency laser, a microwave drive signal source, an electro-optic modulator, a dispersion spectroscopy module, a beam splitting device, a detector array, a signal acquisition module, and a data processing module;

[0007] The dispersion spectroscopy module includes a laser collimator, a cylindrical lens, a virtual imaging phase array, and a focusing lens;

[0008] The microwave drive signal source is connected to the electro-optic modulator and is used to apply a drive voltage to the electro-optic modulator;

[0009] The single-frequency laser generates single-frequency laser as a laser source and inputs it into the electro-optic modulator. The single-frequency laser is output from the electro-optic modulator as multi-frequency composite light composed of a carrier wave and sidebands. After the multi-frequency composite light sequentially passes through the laser collimator, the cylindrical lens, the virtual imaging phase array, and the focusing lens in the dispersion spectroscopy module, the sideband lights of different frequencies are spatially separated into multiple beams with a spacing on the order of millimeters. The multiple beams with a spacing on the order of millimeters pass through the beam separation device to separate the target carrier light and the ±1st order sideband lights, and at the same time, they are respectively input into the detector array. The detector array converts the light intensity signal into an electrical signal and inputs it into the signal acquisition module, and outputs the acquired voltage value. The acquired voltage value is input into the data processing module. After the data processing module processes and corrects the acquired voltage value and its corresponding drive voltage, it outputs the half-wave voltage value of the electro-optic modulator at the preset drive frequency.

[0010] Further, the virtual imaging phase array is composed of two mutually parallel optically coated flat plates; the flat plate close to the incident light is the front panel. A window area is provided at the bottom of the front panel. The surface of the window area is coated with an antireflection film, and the non-window area is coated with a 100% reflectivity reflective film; the flat plate far from the incident light is the rear panel, and the rear panel is coated with a partial transmission film with a reflectivity of 95% - 98%.

[0011] Further, there is an incident angle between the virtual imaging phase array and the incident light. The incident light enters through the window area of the virtual imaging phase array and undergoes multiple back-and-forth reflections to generate multiple parallel beams with different output angles on the transmission side of the rear panel; the parallel beams with different output angles are spatially separated into multiple beams with a spacing on the order of millimeters by the focusing lens.

[0012] Further, the distance between the focusing lens and the beam separation device is the focal length of the focusing lens.

[0013] Further, the beam separation device includes a diaphragm and a knife-edge prism; the multiple beams with a spacing on the order of millimeters first pass through the diaphragm to block the unnecessary beams, and then pass through a pair of knife-edge prisms to completely separate the target carrier light and the ±1st order sideband lights. The knife-edge formed by the intersection of the two reflecting surfaces of the knife-edge prism is arranged on the side close to the incident light and is used to separate the light containing two different frequency components.

[0014] The present invention also provides a method for measuring the half-wave voltage of a high-frequency electro-optic phase modulator, comprising the following steps:

[0015] Step A: Connect the phase modulator to be measured to the above-mentioned high-frequency electro-optic phase modulator half-wave voltage measurement system;

[0016] Step B: Set the detection frequency and detection wavelength of the phase modulator to be measured, as well as the frequency value and amplitude change increment value of the microwave drive signal source amplitude value;

[0017] Step C: Sequentially load the microwave modulation signals corresponding to the amplitude values set in Step B onto the phase modulator to be measured through the microwave drive signal source. Stop loading when the carrier light intensity decreases to 0, and obtain the light intensity values of the carrier wave and the ±1st order sidebands under the modulation of each microwave modulation signal; calculate the modulation ratio of the ±1st order sideband light intensity relative to the carrier light intensity and correct the data, and fit to obtain the amplitude V of the EOM modulation signal when the carrier wave is equal to the corrected ±1st order sideband light intensity ±1 ; Utilize the proportional relationship between the modulation signal amplitude and the phase shift to calculate and output the half-wave voltage of the electro-optic modulator to be measured at the microwave drive frequency set in Step B.

[0018] Further, in Step A, connecting the phase modulator to be measured to the half-wave voltage measurement system specifically includes: Connect the laser signal input port of the phase modulator to be measured to a single-frequency laser through an optical fiber to input a laser signal; connect the RF interface of the phase modulator to be measured to the output port of the microwave drive signal source through a cable to receive a modulation signal; connect the laser signal output port of the phase modulator to be measured to the input port of a laser collimator through an optical fiber to output a collimated optical signal into the dispersion spectroscopy module.

[0019] Further, in Step C, sequentially load the microwave modulation signals corresponding to the amplitude values set in Step B onto the phase modulator to be measured through the microwave drive signal source. Stop loading when the carrier light intensity decreases to 0, and obtain the light intensity values of the carrier wave and the ±1st order sidebands under the modulation of each microwave modulation signal; calculate the relative modulation ratio of the ±1st order sideband light intensity relative to the carrier light intensity and correct the data, and fit to obtain the amplitude V of the EOM modulation signal when the carrier wave is equal to the corrected ±1st order sideband light intensity ±1 Specifically, it is as follows:

[0020] In the calculation processing module, control the amplitude of the microwave drive signal source to increase from 0 with a fixed step size. At the same time, obtain the drive signal amplitude and the corresponding carrier wave and ±1st order light intensity signals. According to the intensity ratio of the +1st order light and the -1st order light, determine the relative modulation ratio k1 of the +1st order sideband light to the -1st order sideband light, and deduce the modulation ratio values k0 and k2 of the +1st order light and the -1st order light relative to the carrier wave, and use the relative modulation ratio to compensate and correct the ±1st order light;

[0021] Divide the corrected +1 level light by the carrier intensity to obtain the corresponding relationship between the ratio and the modulation voltage. When the intensity ratio is P′ +1 / P0 = 1, record the drive voltage value V at this time +1 , and at the same time use the -1 level light to verify the measurement result. Fit the corrected -1 level light and the carrier intensity in the same way to obtain the drive voltage value V -1 .

[0022] Further, in step C, the selected measurement point is the position where the intensities of the 0 level light and the 1 level light are equal. The phase shift amount m here is 1.4347; the half-wave voltage of the electro-optic modulator to be measured output is

[0023] V π = π / 1.4347 · (V +1 + V -1 ) / 2.

[0024] Further, in step C, if there is more than one place where the intensities of the 0 level light and the 1 level light are equal, there are multiple sets of values {V +1，i , i = 1, 2,...}, {V -1，j , j = 1, 2,...}. Select the pair of values with the smallest absolute value of the absolute error between V +1 and V -1 , that is, min{|V +1，i - V -1，j |} as the drive voltage value V ±1 .

[0025] The present invention has the following beneficial effects:

[0026] The present invention provides a high-frequency electro-optic phase modulator half-wave voltage measurement system. By splitting the carrier and sidebands of the EOM modulation output light in space using a VIPA high-dispersion element and directly detecting the light intensity, collecting and calculating the detection results, it can achieve real-time and accurate measurement of the half-wave voltage of a high-speed EOM, and calibrate the half-wave voltage at different drive frequencies. The laser measurement technology involved in the present invention can quickly, accurately, and real-time measure the half-wave voltage of a high-frequency EOM at different modulation frequencies, and has important engineering practical value for the fields of optoelectronic modulation technology and microwave photonics.

[0027] In addition to the purposes, features, and advantages described above, the present invention has other purposes, features, and advantages. The following will refer to the drawings to further elaborate on the present invention in detail. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0029] Figure 1 It is a schematic structural diagram of a half - wave voltage measurement system for a high - frequency electro - optic modulator using VIPA dispersion spectroscopy detection.

[0030] Figure 2 It is a diagram showing the relationship between the modulation sideband power and the modulation depth of the EOM; among them, Figure 2 In (a), it is a schematic diagram of a single - frequency light generating coherent multi - frequency composite light after passing through the EOM modulation. Figure 2 In (b), it is the change of the relative power values (expressed as a percentage) corresponding to the carrier and each sideband component as the modulation phase shift (corresponding to the modulation depth) increases.

[0031] Figure 3 It is the schematic diagram of the VIPA dispersion spectroscopy module in the present invention.

[0032] Figure 4 It is the modulation envelope diagram of the VIPA for the input light; among them, Figure 4 In (a), it is a schematic diagram of the output relative light intensity and position on the focal plane of a single - frequency light with the same intensity and a fixed frequency interval after passing through the VIPA dispersion system. Figure 4 In (b), it is to Figure 4 The data scatter and Gaussian fitting curve diagram in (a) with the optical frequency as the horizontal axis and the relative light intensity as the vertical axis. Figure 4 In (c), it is the effect diagram of amplifying and linearly fitting the best - linearity section in (b). Figure 4 In (b).

[0033] Figure 5 It is the calculation and processing flow chart of the data processing module.

[0034] Figure 6 It is the system diagram for measuring the half - wave voltage of the phase modulator in the preferred embodiment of the present invention. Specific embodiments

[0035] The following will describe the embodiments of the present invention in detail with reference to the accompanying drawings. However, the present invention can be implemented in many different ways defined and covered by the claims.

[0036] The Virtually Imaged Phased Array (VIPA) is a high-dispersion etalon that can separate spectra with a certain frequency difference. It has the advantages of being insensitive to polarization, having a simple structure, and high spectral resolution, and is mainly applied in fields such as dispersion compensation, optical communication, pulse compression and shaping, and spectral detection. In this invention, the high-dispersion spectral splitting of VIPA is applied to an EOM or a similar electro-optic modulation device, which can directly separate the high-frequency carrier and sidebands (with a spacing on the order of GHz) into different light beams in space, and perform real-time light intensity detection on the carrier light and the ±1st order sideband lights. This method can achieve accurate calibration of the half-wave voltage of the high-frequency EOM at different modulation frequencies.

[0037] As Figure 1 shown, this invention provides a half-wave voltage measurement system for a high-frequency electro-optic phase modulator, which includes a single-frequency laser, a microwave drive signal source, an electro-optic modulator, a dispersion spectral splitting module, a beam separation device, a detector array, a signal acquisition module, and a data processing module;

[0038] The dispersion spectral splitting module includes a laser collimator, a cylindrical lens, a virtually imaged phased array, and a focusing lens;

[0039] The microwave drive signal source is connected to the electro-optic modulator and is used to apply a drive voltage to the electro-optic modulator;

[0040] The single-frequency laser generates single-frequency laser light as a laser source and inputs it into the electro-optic modulator. The single-frequency laser light outputs multi-frequency composite light composed of a carrier and sidebands after passing through the electro-optic modulator. After the multi-frequency composite light sequentially passes through the laser collimator, cylindrical lens, virtually imaged phased array, and focusing lens in the dispersion spectral splitting module, the sideband lights with different frequencies are separated into multiple light beams with a spacing on the order of millimeters in space. The multiple light beams with a spacing on the order of millimeters separate the target carrier light and the ±1st order sideband lights through the beam separation device and simultaneously input them into the detector array respectively. The detector array converts the light intensity signal into an electrical signal and inputs it into the signal acquisition module, and outputs the acquired voltage value. The acquired voltage value is input into the data processing module. After the data processing module processes and corrects the acquired voltage value and its corresponding drive voltage, it outputs the half-wave voltage and modulation depth of the electro-optic modulator at a preset drive frequency.

[0041] The specific working process is as follows: First, after the single-frequency input light passes through the electro-optic modulator (EOM), certain sidebands will be generated according to the frequency and amplitude of the set EOM drive signal (the signal frequency is in the GHz range). Then, the light beam passes through a dispersion spectroscopy module with a VIPA device as the core, and the sideband lights of different frequencies will be spatially separated by millimeters (this parameter is related to the device selection of the dispersion spectroscopy module). After that, through an aperture and a pair of knife-edge prisms, the required carrier and ±1 order sidebands are respectively input into the photodetectors. The detectors convert the optical intensity signals into electrical signals and input them into the signal acquisition circuit. The processor controls the amplitude of the EOM drive signal to increase from 0 with a fixed step size, and at the same time records the corresponding relationship between the sideband signal intensity and the amplitude of the EOM modulation signal, performs data correction fitting, and processes and calculates the half-wave voltage value of the EOM at this drive signal frequency.

[0042] The schematic diagram of the principle of generating sideband light using the EOM phase modulator is as Figure 2 shown, where Figure 2 (a) in it is the schematic diagram of the single-frequency light generating coherent multi-frequency composite light after being modulated by the EOM. The frequency of the carrier light is f0, and the corresponding relative power at a certain modulation depth is P0; the frequency of the sideband light is fi, and the corresponding relative power at a certain modulation depth is Pi, and i is ±1, ±2, and ±3. Figure 2 (b) in it is the change (expressed as a percentage) of the relative power values corresponding to the carrier and each sideband component as the modulation phase shift (corresponding to the modulation depth) increases. The relative intensity of each component sideband follows the Bessel function with the modulation depth (corresponding to the phase shift). Different modulation depths will result in different intensities of each component sideband light. When the phase shift is m = π, the intensity ratio of the carrier to the 1st order sideband of the EOM output light is P0 / P1 = 1.1427.

[0043] Let the optical frequency corresponding to the 0th order light be ν0, and the frequency of the microwave drive signal of the phase modulator be f m be the interval of the sideband light, then the optical frequency corresponding to the kth order light is ν ±k , and its magnitude is ν ±k = ν0 ± k·f m . The half-wave voltage refers to the voltage required when the optical path difference of the two perpendicular components of the light wave is half a wavelength (the corresponding phase difference is 180 degrees). This parameter is a reference voltage and is used as a reference value when setting the EOM parameters. Therefore, to achieve efficient, fast, and accurate control of the optical powers of each order output by the EOM, first, it is necessary to calibrate the modulation depth of the EOM, especially the half-wave voltage parameter, and it is necessary to correspond the modulation signal voltage to Figure 2 the positions of each order of light in (b).

[0044] Since the frequency of the high-frequency driving signal of the EOM is generally between 1 GHz and 20 GHz, it is very difficult for traditional dispersion spectroscopy elements such as gratings and prisms to separate the equally spaced frequency components in space. How to separate and measure the carrier and sidebands is a major difficulty. The VIPA device exactly makes up for this shortcoming. It provides an ability to spatially separate the composite light with different frequencies. By using the dispersion spectroscopy component based on VIPA, the separation measurement of the optical power of each frequency component can be achieved.

[0045] The VIPA (Virtual Imaging Phase Array) is a new type of spectral dispersion device. It consists of two mutually parallel coated high-quality optical flats, which can be called the front panel and the rear panel. Among them, the flat plate close to the incident light (i.e., the front panel) is the incident surface, and the flat plate far from the incident light (i.e., the rear panel) is the exit surface. The inner side or the reflective surface of the rear panel is coated with a partial transmission film (the reflectivity is above 95%, preferably 95% - 98%). The purpose is to make a small part of the light incident on the rear panel can be transmitted parallelly out of the VIPA; the inner side or the light-reflecting side of the front panel is coated with a reflective film with a reflectivity almost of 100%. At the same time, an incident window (also called the window area) is designed at the lower end of the bottom of the front panel. The surface of the window (coated with an antireflection film) is almost flush with the front panel. A narrow slit is also specially designed on the surface. Only the incident light that is compressed can pass through smoothly and irradiate the inner side of the rear panel with a small incident angle to enter the VIPA, and then enter the state mode of circulating reflection between the two optical flats. The dispersion spectroscopy module based on VIPA includes: a collimator, a cylindrical lens, a VIPA, and a focusing lens, as Figure 3 shown. First, the collimator collimates the light beam to propagate with a small spot diameter, and then the cylindrical lens further compresses the beam size in the direction of the VIPA etalon window, so that it is easier to enter the narrow window of the VIPA; the laser beam will experience multiple back-and-forth reflections. Each reflection will generate a weak output beam from the non-totally reflecting optical flat at the back. After multiple reflections like this, multiple parallel beams will be output from the transmission side of the rear panel. At the same time, due to the different frequencies of the sideband components, the angles of the parallel beams output by the light with different frequencies are different. Therefore, parallel beams with different output angles will be formed; finally, a focusing lens is placed at the rear end of the VIPA output beam. At a position near the focal length of the lens, the light of different frequency components can be seen distributed at different positions in space. The spatial distance after the splitting of the composite light with a GHz frequency interval can reach the millimeter level.

[0046] After passing through the VIPA and converging through the focusing lens, the output optical field on the focal plane can be expressed as:

[0047]

[0048]

[0049] Among them, Iout is the output optical intensity, E out is the electric field intensity of the laser, Δ represents the variable of the optical path difference, t is the VIPA thickness (i.e., the distance between the front panel and the rear panel), W is the radius of the collimated beam before the cylindrical lens, R and r are the reflectivities of the front and rear panels of the VIPA respectively, f and F are the focal lengths of the cylindrical lens and the focusing lens respectively, θ is the incident angle of the light entering the VIPA, x F is the coordinate perpendicular to the optical path direction at the focal length of the focusing lens, and λ is the laser wavelength.

[0050] When light of a certain frequency passes through the EOM to generate sidebands, taking the second-order sidebands as an example, when the light intensities of the three wavelength components are equal, through simulation, the output optical field intensity distribution is as Figure 3 shown at the focal length of the rightmost lens. After passing through the VIPA system, the central light and each order of sidebands can be separated by a certain distance in space. Through simulation experiment analysis, the separation interval of the three component lights at the strongest part of the optical field can be optimized by adjusting the system parameters. The adjustable parameters of the system include the relative incident angle of the beam to the VIPA, the focal length of the focusing lens, the focal length of the cylindrical lens, the beam diameter, and the VIPA thickness. Among them, the two parameters that have an obvious impact on the sideband separation distance are the VIPA incident angle and the focal length of the focusing lens. And the smaller the incident angle (there is a minimum angle limit, too small an angle will cause light attenuation), and the longer the focal length of the focusing lens, the larger the separable distance.

[0051] Specifically, the beam separation device includes a diaphragm and a pair of knife-edge prisms. There is an extremely thin intersection surface (less than a millimeter) between the two mutually perpendicular reflecting surfaces of the knife-edge prism, like a knife edge. The incident light shines on the knife-edge prism against this knife edge, and the light is separated by the knife edge and hits the left and right reflecting surfaces. After the dispersion spectroscopy module with VIPA as the core used in the previous step separates the sideband light by a certain distance, the diaphragm is used to block the unwanted higher-order sidebands, and then a pair of knife-edge prisms are used to separate the carrier and the +1st-order sideband and the -1st-order sideband light by a larger distance, and then they are respectively injected into the photodetectors, and the real-time relative optical intensity is obtained through the signal processor.

[0052] In addition, it can be known from prior knowledge that the VIPA has a certain modulation effect on the light passing through it, and the modulation scale factor is related to the light frequency, specifically as Figure 4As shown in (a), after light with the same intensity but different frequencies passes through the VIPA dispersion system, the output light intensity distribution is different. Therefore, it is necessary to first compensate the received sideband light in the calculation and processing system. In order to calibrate the relative modulation magnitude of the VIPA for light with different frequencies, a series of lights with the same output intensity and a fixed frequency interval are used as the input of the VIPA dispersion system respectively, and the output light intensity within the same free spectral range (FSR) is measured to obtain the relationship between the output relative light intensity and the input light frequency, as Figure 4 shown in (b). Through data fitting, this envelope presents a Gaussian type. The free spectral range (FSR) of the VIPA device is generally dozens of GHz, while the driving signal frequency of the EOM to be measured is within 10 GHz. For the convenience of compensation, a 20-GHz approximate linear region in the envelope curve is selected, as Figure 4 shown in (c).

[0053] Through measurement and calibration, the modulation of the VIPA on the input light in this frequency band can be approximately expressed by the following relational expression:

[0054] I(f) = a·(f - f0) + b

[0055] where f and I(f) are respectively the frequency and light intensity of the optical signal input to the VIPA dispersion system, the parameter a is the slope of the light intensity change relative to the frequency change, f0 is the central position of the selected linear region, that is, the frequency at which the intermediate value of the light intensity is located in the above curve, and b is the relative light intensity at the central frequency. The parameter values are determined through experimental calibration.

[0056] According to the above formula, the following relationship can be obtained:

[0057]

[0058] where k0, k1, and k2 are respectively the relative modulation ratios of the +1st-order sideband light to the 0th-order light, the +1st-order sideband light to the -1st-order sideband light, and the -1st-order sideband light to the 0th-order light. a, b, and f0 have the same meanings as in the previous formula, f m is the frequency of the EOM driving signal, I(f0), I(f0 + f m ), and I(f0 - f m ) are respectively the intensities of the 0th-order light, the +1st-order sideband light, and the -1st-order sideband light, and I0, I +1 , and I -1 are their corresponding abbreviations.

[0059] To compensate for the modulation of VIPA on lights of different frequencies, it is necessary to adjust the modulation magnitudes of the sideband light intensity and the carrier light intensity to the same ratio, that is, it is necessary to process the light intensity using the above relative modulation ratios k0, k1, and k2. The processing method is as follows: when the VIPA dispersion system and the driving signal frequency are determined, k0 and k1 are fixed values. Therefore, first calculate the relative ratio k1 of the detected ±1-order sideband light intensity, derive the value of k0 from the value of k1, and then divide the +1-order sideband light intensity by k0, that is, the correction of the sideband light relative to the carrier light intensity is completed.

[0060] The present invention calculates the half-wave voltage value by an indirect measurement method. Instead of directly scanning to obtain the driving voltage value when the phase shift is π, it obtains the phase shift amount corresponding to the sine driving signal with an amplitude of V applied to the EOM, and indirectly derives the half-wave voltage value using the following formula. m when the sine driving signal is applied, and indirectly derives the half-wave voltage value using the following formula.

[0061]

[0062] where: V m is the amplitude of the driving signal, m is the modulation phase corresponding to the amplitude of the driving signal, V π is the half-wave voltage, and π is the pi.

[0063] To perform the measurement more quickly, conveniently, and accurately, the selected indirect measurement point is Figure 2 the position where the intensity of the 0-order light and the 1-order light is equal for the first time in (b), and the phase shift amount m = 1.4347 here. As Figure 5 shown, in the calculation and processing module, control the amplitude of the microwave driving signal to increase from 0 with a fixed step size, and at the same time obtain the amplitude of the driving signal and the corresponding carrier and ±1-order light intensity signals. According to the intensity ratio of the +1 and -1-order lights, determine the relative modulation ratio k1, derive k0 and k2, and then compensate and correct the ±1-order lights. Divide the corrected +1-order light by the carrier intensity to obtain the corresponding relationship between the ratio and the modulation voltage. When the intensity ratio is P' +1 / P0 = 1, record the driving voltage value V +1 at this time, and at the same time use the -1-order light to verify the measurement result. Divide the corrected -1-order light by the carrier intensity in the same way to obtain the driving voltage value V -1 If the intensity is equal at more than one place, select a pair with the smallest absolute value of the absolute error among them, and then indirectly obtain the half-wave voltage value using the above formula.

[0064] The measurement method of this embodiment is as follows:

[0065] Step A. First, connect the phase modulator to be measured as shown in Figure 6In the shown half-wave voltage measurement system, that is: the laser signal input port of the phase modulator to be measured is connected to a single-frequency laser through an optical fiber to input a laser signal; the RF interface of the phase modulator to be measured is connected to the output port of a microwave drive signal source through a cable to receive a modulation signal; the laser signal output port of the phase modulator to be measured is connected to the input port of a collimator through an optical fiber to output a collimated optical signal into the VIPA system;

[0066] Step B: Determination of the detection system working parameters and adjustment of the working state, that is: setting the frequency f for detecting the phase modulator to be measured m , the detection wavelength f0, the amplitude value of the microwave modulation signal starts from 0, and increases gradually to the maximum voltage value V that can be applied to the EOM with a step of u max ;

[0067] Step C: Data comprehensive processing: The microwave modulation signals corresponding to the respective amplitude values set in Step B are successively loaded onto the phase modulator to be measured through the microwave drive signal source. When the carrier optical intensity decreases to 0, the loading stops, and the optical intensity values of the carrier and the ±1st order sidebands modulated by each microwave modulation signal are obtained and input into the processor. By calculating a series of intensity ratios of the +1st order light and the -1st order light, the relative modulation ratio k1 of the VIPA system is determined by fitting, and further the modulation ratio values k0 and k2 of the +1st order light and the -1st order light relative to the carrier are deduced. The original optical intensity is corrected using the relative modulation ratio, that is

[0068]

[0069]

[0070] The driving voltage value V applied when the corrected ±1st order light and the carrier intensity are equal can be obtained by fitting calculation +1 and V -1 . If there is more than one place where the intensities are equal and there are multiple sets of values {V +1，i , i = 1, 2,...}, {V -1，j , j = 1, 2,...}, select a pair with the smallest absolute value of the absolute error, that is, min{|V +1，i - V -1，j |}, as the driving voltage value V ±1 for the next calculation.

[0071] Calculate and output the final result:

[0072] V π = π / 1.4347·(V +1 + V -1 ) / 2.

[0073] In summary, the half-wave voltage measurement system for a high-frequency electro-optic phase modulator provided by the present invention can perform spectral splitting of the carrier and sidebands of the EOM-modulated output light in space by using a VIPA high-dispersion element and directly detect the optical intensity. By collecting and calculating the detection results, it can achieve real-time and accurate measurement of the half-wave voltage of a high-speed EOM, as well as calibration of the half-wave voltage at different driving frequencies. The laser measurement technology involved in the present invention can quickly, accurately, and real-time measure the half-wave voltage of a high-frequency EOM at different modulation frequencies, which has important engineering practical value in the fields of photoelectric modulation technology and microwave photonics.

[0074] The foregoing are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A half-wave voltage measurement system for a high-frequency electro-optic phase modulator, characterized in that, It includes a single-frequency laser, a microwave drive signal source, an electro-optic modulator, a dispersion spectroscopy module, a beam splitting device, a detector array, a signal acquisition module, and a data processing module; The dispersion spectroscopy module includes a laser collimator, a cylindrical lens, a virtual imaging phase array, and a focusing lens; The microwave drive signal source is connected to the electro-optic modulator and is used to apply a drive voltage to the electro-optic modulator; The single-frequency laser generates single-frequency laser as a laser source and inputs it into the electro-optic modulator. The single-frequency laser is output from the electro-optic modulator as a multi-frequency composite light composed of a carrier wave and sidebands. The multi-frequency composite light sequentially passes through the laser collimator, cylindrical lens, virtual imaging phase array, and focusing lens in the dispersion spectroscopy module, and then the sideband lights of different frequencies are spatially separated into multiple beams with a spacing of the order of millimeters. The multiple beams with a spacing of the order of millimeters pass through the beam splitting device to separate the target carrier light and the ±1st order sideband lights, and at the same time, they are respectively input into the detector array. The detector array converts the light intensity signal into an electrical signal and inputs it into the signal acquisition module, and outputs the collected voltage value. The collected voltage value is input into the data processing module. After processing and correcting the collected voltage value and its corresponding drive voltage, the data processing module outputs the half-wave voltage value of the electro-optic modulator at a preset drive frequency.

2. The half-wave voltage measurement system of a high-frequency electro-optic phase modulator according to claim 1, wherein The virtual imaging phase array is composed of two mutually parallel optically coated flat plates; the flat plate close to the incident light is the front panel. A window area is provided at the bottom of the front panel. The surface of the window area is coated with an antireflection film, and the non-window area is coated with a reflection film with a reflectivity of 100%. The flat plate far from the incident light is the rear panel, and the rear panel is coated with a partial transmission film with a reflectivity of 95% - 98%.

3. The half-wave voltage measurement system of a high-frequency electro-optic phase modulator according to claim 2, wherein There is an incident angle between the virtual imaging phase array and the incident light. The incident light enters through the window area of the virtual imaging phase array and undergoes multiple back-and-forth reflections, generating multiple parallel beams with different output angles on the transmission side of the rear panel; the parallel beams with different output angles are spatially separated into multiple beams with a spacing of the order of millimeters through the focusing lens.

4. A half-wave voltage measurement system for a high-frequency electro-optic phase modulator according to claim 3, characterized in that, The distance between the focusing lens and the beam splitting device is the focal length of the focusing lens.

5. The half-wave voltage measurement system of a high-frequency electro-optic phase modulator according to claim 1, characterized in that, The beam splitting device includes a diaphragm and a pair of knife-edge prisms; the multiple beams with a spacing of the order of millimeters first pass through the diaphragm to block the unnecessary beams, and then pass through a pair of knife-edge prisms to completely separate the target carrier light and the ±1st order sideband lights.

6. A method for measuring the half-wave voltage of a high-frequency electro-optic phase modulator, characterized in that, It includes the following steps: Step A: Connect the phase modulator to be measured to a half-wave voltage measurement system for a high-frequency electro-optic phase modulator according to any one of claims 1 - 5; Step B: Set the detection frequency and detection wavelength of the phase modulator to be measured, as well as set the frequency value and amplitude change increment value of the amplitude of the microwave drive signal source; Step C: The microwave modulation signals corresponding to the amplitude values set in Step B are successively loaded onto the phase modulator under test via a microwave drive signal source. Stop loading when the carrier light intensity decreases to 0, and obtain the light intensity values of the carrier and the ±1st order sidebands under the modulation of each microwave modulation signal. Calculate the modulation ratio of the ±1st order sideband light intensity relative to the carrier light intensity and correct the data, and fit to obtain the amplitude V of the EOM modulation signal when the carrier and the corrected ±1st order sideband light intensities are equal. ±1 ; Using the proportional relationship between the modulation signal amplitude and the phase shift, calculate and output the half-wave voltage of the electro-optic modulator under test at the microwave drive frequency set in Step B.

7. A method for measuring the half-wave voltage of a high-frequency electro-optic phase modulator according to claim 6, characterized in that, In step A, connecting the phase modulator to be measured to the half-wave voltage measurement system specifically means: connecting the laser signal input port of the phase modulator to be measured to the single-frequency laser through an optical fiber to input the laser signal; connecting the RF interface of the phase modulator to be measured to the output port of the microwave drive signal source through a cable to receive the modulation signal; The laser signal output port of the phase modulator to be measured is connected to the input port of the laser collimator through an optical fiber to output the collimated optical signal into the dispersion spectroscopy module.

8. A method for measuring the half-wave voltage of a high-frequency electro-optic phase modulator according to claim 6, characterized in that, In step C, the microwave modulation signals corresponding to the amplitude values set in step B are successively loaded onto the phase modulator under test through a microwave drive signal source. When the carrier optical intensity decreases to 0, the loading stops, and the optical intensity values of the carrier and the ±1st order sidebands modulated by each microwave modulation signal are obtained. By calculation, the relative modulation ratio of the ±1st order sideband optical intensity relative to the carrier optical intensity is obtained, and the data is corrected. By fitting, the modulation signal amplitude V of the EOM when the optical intensities of the carrier and the corrected ±1st order sidebands are equal is obtained. ±1 , specifically: In the calculation and processing module, the amplitude of the microwave drive signal source is controlled to increase from 0 with a fixed step size. At the same time, the drive signal amplitude and the corresponding carrier and ±1st order optical intensity signals are acquired. According to the intensity ratio of the +1st and -1st order lights, the relative modulation ratio k1 of the +1st order sideband light to the -1st order sideband light is determined, and the modulation ratio values k0 and k2 of the +1st order light and -1st order light relative to the carrier are deduced. The ±1st order lights are compensated and corrected using the relative modulation ratio. Divide the corrected +1 order light by the carrier intensity to obtain the corresponding relationship between the ratio and the modulation voltage. When the intensity ratio is P′ +1 / P0 = 1, record the drive voltage value V at this time +1 , and at the same time use the -1 order light to verify the measurement result. Similarly, fit the corrected -1 order light and the carrier intensity to obtain the drive voltage value V -1 .

9. A method for measuring the half-wave voltage of a high-frequency electro-optic phase modulator according to claim 6, characterized in that, In step C, the selected measurement point is the position where the intensities of the 0th order light and the 1st order light are equal, and the phase shift amount m here is 1.4347; the half-wave voltage of the electro-optic modulator to be measured output is V π = π / 1.4347·(V +1 + V -1 ) / 2。 10. A method for measuring the half-wave voltage of a high-frequency electro-optic phase modulator according to claim 9, characterized in that, In step C, if there are more than one place where the intensity of the 0th-order light is equal to that of the 1st-order light, and there are multiple sets of values {V +1，i , i = 1, 2,...}, {V -1，j , j = 1, 2,...}, select a pair of values with the smallest absolute value of the absolute error between V +1 and V -1 , that is, min{|V +1，i - V -1，j |} as the driving voltage value V ±1 .

Images