CN111342892B - System and method for measuring high-frequency half-wave voltage parameters of electro-optical intensity modulator - Google Patents
System and method for measuring high-frequency half-wave voltage parameters of electro-optical intensity modulator Download PDFInfo
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- CN111342892B CN111342892B CN202010159681.1A CN202010159681A CN111342892B CN 111342892 B CN111342892 B CN 111342892B CN 202010159681 A CN202010159681 A CN 202010159681A CN 111342892 B CN111342892 B CN 111342892B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
- H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
- H04B10/077—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using a supervisory or additional signal
- H04B10/0779—Monitoring line transmitter or line receiver equipment
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/54—Intensity modulation
Abstract
The invention discloses a system and a method for measuring high-frequency half-wave voltage parameters of an electro-optical intensity modulator, which comprises a microwave signal source, a high-frequency photoelectric detector, a frequency spectrograph, a 5:95 polarization maintaining coupler, a direct-current bias voltage control circuit, a polarization controller and a laser light source; and fitting a corresponding curve of the output radio frequency power of the photoelectric detector relative to the input radio frequency power value of the electro-optic modulator, and calculating the half-wave voltage of the electro-optic intensity modulator under the high-frequency modulation through a nonlinear inflection point of the curve. The invention has simple light path structure, is easy to build and can accurately measure the half-wave voltage under any frequency point.
Description
Technical Field
The invention relates to a system and a method for measuring high-frequency half-wave voltage parameters of an electro-optical intensity modulator, belonging to the technical field of measurement (optical communication) of photoelectric devices and microwave photonics.
Background
A common electro-optic intensity modulator is one made of lithium niobate (LiNbO3) crystal material. The basic principle of the electro-optical modulator is an electro-optical effect, namely, the refractive index of an optical transmission medium is controlled by the change of loading voltage, so that the intensity of an output optical signal is changed, and electro-optical intensity modulation is realized. With the rapid development of the application fields of high-speed optical communication, microwave electronic countermeasure, radar systems, satellite remote sensing, broadband wireless communication, astronomical detection and the like, the method for processing microwave signals by an optical method is more and more important, and the electro-optical intensity modulator is a key device for realizing the conversion from the microwave signals to optical signals. The electro-optical intensity modulator is used as a most basic electro-optical conversion device and has the advantages of low cost, simple structure, low insertion loss, high linearity and the like. The half-wave voltage of the electro-optical intensity modulator is one of important parameters, which represents the modulation efficiency of the electro-optical intensity modulator, however, the half-wave voltage of the electro-optical intensity modulator changes with the increase of the modulation frequency, so in order to optimize the dynamic performance of the analog electro-optical modulation system, the half-wave voltage of the electro-optical intensity modulator under different radio frequency modulation frequencies needs to be accurately measured, which is the basis for knowing the modulation characteristics of the high-speed electro-optical intensity modulator and optimizing the link performance. The conventional half-wave voltage testing method of the electro-optical modulator comprises a spectrum analysis method, an extreme value measuring method and a frequency multiplication modulation method.
The extreme value measuring method of the spectral analysis method comprises the steps of loading a radio frequency signal at a radio frequency end of an electro-optical intensity modulator to modulate an optical wave of the electro-optical intensity modulator to be measured, inputting the optical signal output by the modulator into a spectral analyzer for analysis, obtaining the relative intensity of a sideband and a subcarrier of the optical wave by modulating a direct current voltage at a direct current offset end, and calculating the half-wave voltage of the electro-optical intensity modulator according to the spectral power intensity. However, the resolution of the spectroscopic analysis method is low, and the frequency and power resolution of the spectrometer is low, so that half-wave voltages of power points of not all test points cannot be directly measured.
The basic method of the extreme value measuring method is that a direct current voltage is loaded at the offset end of the electro-optical intensity modulator, the output light power of the modulator is tested by using an optical power meter through changing the direct current voltage so as to judge the extreme value point of the output light intensity, and the difference between the direct current voltages corresponding to the adjacent maximum value and the extreme value is the half-wave voltage.
The basic method of the frequency multiplication modulation method is that direct current voltage is loaded at the offset end of the electro-optical intensity modulator, a radio frequency signal is added at the radio frequency end, the output of the electro-optical intensity modulator is connected with the photoelectric detector and the oscilloscope, frequency multiplication distortion occurs through an observation signal, and the difference of the direct current voltage corresponding to the frequency multiplication distortion is half-wave voltage.
The methods can only test the half-wave voltage of the offset end or the half-wave voltage of the electro-optical intensity modulator under direct current or low frequency modulation, but cannot test the half-wave voltage under high frequency radio frequency modulation.
Disclosure of Invention
1. Objects of the invention
The invention provides a system and a method for measuring a half-wave voltage of an electro-optic intensity modulator under high-frequency modulation, aiming at solving the defect that the existing method cannot measure the half-wave voltage of the electro-optic intensity modulator under the high-frequency modulation.
2. The technical scheme adopted by the invention
The invention adopts a measuring system consisting of a microwave signal source, a high-frequency photoelectric detector, a frequency spectrograph, a 5:95 polarization-maintaining coupler, a direct-current bias voltage control circuit, a polarization controller and a laser light source; the method for measuring the high-frequency half-wave voltage of the electro-optical intensity modulator comprises the steps of firstly connecting the output end of a laser light source to the optical input port of the electro-optical intensity modulator through a polarization controller, and adjusting the polarization controller to enable the output optical power of the electro-optical intensity modulator to be maximum; the light output port of the electro-optical intensity modulator is output by a 5:95 polarization maintaining coupler in a shunt way, a 5% end of the polarization maintaining coupler is input into a direct current bias voltage control circuit, the direct current bias voltage output of the control circuit is connected to a direct current bias end of the electro-optical intensity modulator, and the direct current bias voltage control circuit is powered on to enable the electro-optical intensity modulator to work in a linear region; the 95% end of the polarization-maintaining coupler is input into the high-frequency photoelectric detector and connected to the frequency spectrograph; the microwave signal source and the frequency spectrograph are connected to the upper computer through a USB or GPIB interface. The frequency of a radio frequency signal output by a microwave signal source is adjusted to be a frequency point to be tested through upper computer test software, a group of microwave modulation signals with different radio frequency powers are loaded to the electro-optic intensity modulator to be tested, the radio frequency power values of fundamental waves and second harmonics output by a photoelectric detector under different radio frequency modulation powers are tested, finally, the upper computer software fits a corresponding curve of the radio frequency power output by the photoelectric detector relative to the radio frequency power input by the electro-optic modulator, and the half-wave voltage of the electro-optic intensity modulator under the high-frequency modulation can be calculated through a nonlinear inflection point of the curve, so that the purpose of the invention is achieved.
The laser light source, the polarization controller, the electro-optical intensity modulator to be measured, the 5:95 polarization-maintaining coupler and the high-frequency photoelectric detector are sequentially connected through optical fibers.
The microwave signal source and the electro-optical intensity modulator to be tested are connected through a radio frequency cable.
The 5:95 polarization maintaining coupler is connected with the bias voltage control circuit through an optical fiber to provide feedback information for the bias control circuit; the bias voltage control circuit is connected with the electro-optical intensity modulator to be tested through a cable.
The high-frequency photoelectric detector and the frequency spectrograph are connected through a high-frequency cable.
The microwave signal source and the frequency spectrograph are connected through a USB communication cable.
A method for measuring high-frequency half-wave voltage parameters of an electro-optical intensity modulator comprises the following steps:
step A, a laser light source sends out a wavelength optical signal with fixed wavelength, the optical signal enters a polarization controller, and the output of the polarization controller is connected to the input end of an electro-optical intensity modulator to be detected and is used as an optical carrier of the modulator; inputting a high-frequency microwave signal sent by a microwave signal source into a radio frequency port of the electro-optical intensity modulator to be tested; and meanwhile, the direct-current bias voltage of the electro-optic intensity modulator to be tested is controlled through the bias voltage control circuit, so that the linear region modulation of the electro-optic intensity modulator to be tested is realized. The modulated signal is passed through 5: the optical signal of the 5% port enters the bias voltage control circuit as a feedback signal; the optical signal of the 95% port passes through the high-frequency photoelectric detector, convert the optical signal into the electrical signal and connect to the frequency spectrograph;
b, adjusting the polarization state of the laser signal by using a polarization controller to ensure that the laser power at the output end of the electro-optical intensity modulator to be detected is maximum so as to match the polarization state of the optical signal of the laser light source with the polarization state of the electro-optical intensity modulator to be detected; electrifying a direct current bias voltage control circuit to enable a direct current bias point of the electro-optical intensity modulator to work in a linear region;
step C, setting working parameters:
setting the detection frequency of the electro-optical intensity modulator to be detected, determining according to the typical value of low-frequency half-wave voltage Vpi L given in the technical parameters of the electro-optical intensity modulator to be detected or by using a power method, converting the half-wave voltage value into a radio frequency power value, setting a group of power values of a high-frequency microwave modulation signal according to the power values, outputting the fundamental wave power to be nonlinear relative to the input radio frequency power when the modulator is close to the half-wave voltage, and calculating the corresponding half-wave voltage value under the high frequency according to the definition of 3dB bandwidth of the electro-optical intensity modulator
Therefore, the range of the selected test voltage value should be larger than
Selecting 0-1.65V pi L, wherein the smaller the stepping is, the better the stepping is, and then recording the set detection frequency and the power value of the group of microwave modulation signals into the upper computer software for standby;
d, reading the output of the high-frequency photoelectric detector through a frequency spectrograph, detecting the power value of a fundamental wave microwave signal related to frequency, and recording the microwave power of the fundamental wave microwave signal by upper computer software;
step E, changing the modulation power of different microwave signal sources through upper computer software, testing all the loaded radio frequency voltages in the step C, and repeatedly testing and recording according to the step D; testing data under different microwave powers for many times until the high-frequency signal power applied by a microwave signal source is close to the maximum radio-frequency power allowed by the modulator;
step F, comprehensively processing the microwave power output by the high-frequency photoelectric detector and the modulation power of different microwave signal sources through upper computer software, so as to obtain the half-wave voltage of the electro-optical intensity modulator to be detected under the set detection frequency; when the electro-optic intensity modulator operates in the linear region, the input-output function is:
in the above formula Pout、PinRespectively the output and input optical power of the electro-optical intensity modulator, VbTheta is the initial phase value of the modulator, and V (t) is the RF modulation voltage. Let the RF modulation voltage V (t) be V0The radio frequency signal of frequency f is expressed as
V(t)＝V0cos(2πft) (2)
By substituting formula (2) into formula (1), can be obtained
Wherein the content of the first and second substances,
when the direct current bias control circuit is used for closed-loop control, the working point drift of the modulator is compensated, and the radio frequency voltage modulation is ensured to be in a linear region.
Electric field expression of input light
Omega 2 pi f, and the output optical electric field expression is Eout(t) can be written by Bessel expansion
Wherein
To modulate depth, Jn(. cndot.) is a first class of nth order Bessel functions.
From the equation (4), the fundamental frequency component in the output electric field strength is J0And is only related to the modulation depth m, when m is less than 1, the modulator outputs a first-order component which tends to be constant and takes a fundamental frequency component as a main component; when m is 1, the first order component starts to increase, i.e. when the modulator input rf voltage is equal to the half wave voltage, the non-linear effect increases significantly. Therefore, the modulation signals with different radio frequency powers are loaded, the fundamental wave signal power in the output electric field is tested, the input and output optical electric field function relationship of the electro-optical modulator is fitted, the modulator input radio frequency power corresponding to the curve inflection point is found, and the half-wave voltage under the test frequency can be calculated.
Step G, measuring half-wave voltage under different frequencies: the half-wave voltage of the electro-optical intensity modulator to be measured at different frequencies can be measured through the adjusting step C, D, E, F.
3. Advantageous effects adopted by the present invention
The invention adopts a measuring system consisting of a microwave signal source, a high-frequency photoelectric detector, a frequency spectrograph, a 5:95 polarization-maintaining coupler, a direct-current bias voltage control circuit, a polarization controller and a laser light source; when the electro-optical intensity modulator to be measured is connected into the system, the resolution ratio of the required measuring frequency depends on the frequency resolution ratio of the microwave signal source and the frequency spectrograph, so that the half-wave voltage of the electro-optical intensity modulator under any different frequencies can be measured, the number of frequency points which can be measured by the same electro-optical intensity modulator to be measured is large, the measuring precision is high, the measuring system and the measuring cost are low, and the like. The system and the method have the advantages of simple light path structure, easy construction and capability of accurately measuring the half-wave voltage at any frequency point.
Drawings
Fig. 1 is a schematic structural diagram of a measuring system according to the curve of the output intensity of a modulator along with the loaded radio frequency power.
FIG. 2 shows the microwave power test values and the fitting curve output by the high-frequency photodetector under different microwave modulation powers.
Detailed Description
The technical solutions in the examples of the present invention are clearly and completely described below with reference to the drawings in the examples of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without inventive step, are within the scope of the present invention.
The present invention will be described in further detail with reference to the accompanying drawings.
Example 1
The invention is further described with reference to figures 1, 2 and mathematical derivations:
FIG. 1 is a structural diagram of a system for measuring high-frequency half-wave voltage parameters of an electro-optic intensity modulator, wherein a laser light source adopts a KG-TLS-C-17-P-FA tunable laser light source, the wavelength tunable range of the laser light source is 1527-1563 nm, and the output power is 7-17 dBm and is adjustable; the model of the polarization controller is FPc 030; the model of the direct-current bias voltage control circuit is KG-ABC-MZ-Q-15-P01-R00; 5, the model of the 95 polarization-maintaining coupler is KG-PB-S-15-0595-1; the high-frequency photoelectric detector has a model KG-PD-50G, the conversion gain of the detector is 40V/W, and the 3dB analog bandwidth is more than or equal to 43 GHz; the model of the microwave signal source is Agilent,8257D, the signal frequency range is 250 kHz-40 GHz, and the maximum radio frequency power is output by 13 dBm; the microwave amplifier is KG-RF-40G-PA in model, the maximum gain of the amplifier is more than 25dB, and the maximum power output is 30 dBm; the model of the frequency spectrograph is Keysight N9030B, and the signal range is 2 Hz-50 GHz; the implementation employs a conventional desktop computer.
The electro-optical intensity modulator to be tested used in the embodiment is a Beijing Kangguan photoelectric KG-AM-15-10G-PP-FA intensity modulator, the working waveband of the electro-optical intensity modulator is 1525nm to 1565nm, the central wavelength of the electro-optical intensity modulator is 1550nm, the low-frequency half-wave voltage is 4.8V, the theoretical high-frequency half-wave voltage is about 6.7V, the input optical power is less than 20dBm, and the input radio-frequency power is less than 28 dBm.
The measurement method of the present embodiment is:
a, firstly, an electro-optical intensity modulator to be tested is accessed into a measurement system, namely, an input end optical fiber of the electro-optical intensity modulator to be tested is accessed into an output port of a polarization controller, an output end of the electro-optical intensity modulator to be tested is connected with an output port of a polarization maintaining coupler, and an optical signal of a 5% port of the polarization maintaining coupler is used as a feedback signal to enter a bias voltage control circuit; the optical signal of the 95% port passes through the high-frequency photoelectric detector, convert the optical signal into the electrical signal and connect to the frequency spectrograph; the radio frequency port of the modulator is connected with the output port of the microwave signal source through a radio frequency cable so as to be connected with a modulation signal;
b, setting the output light wavelength of the laser light source to be 1550nm and the output light power to be 13dBm, and adjusting the polarization state of the laser signal by using a polarization controller to ensure that the laser power of the output end of the electro-optical intensity modulator to be tested is the maximum so as to match the polarization state of the optical signal of the laser light source with the polarization state of the electro-optical intensity modulator to be tested; electrifying a direct current bias voltage control circuit, enabling a direct current bias point of the electro-optical intensity modulator to work in a linear region, and waiting for the electro-optical intensity modulator to work stably;
step C, setting working parameters: setting the wavelength of a test light source to be 1550nm, setting the frequency to be tested of the electro-optical intensity modulator to be tested to be 5GHz, setting the gain of a microwave amplifier to be 20dB, converting the typical value of half-wave voltage given in technical parameters of the electro-optical intensity modulator to be tested to be 5V, wherein the value is a radio frequency power value corresponding to 23.9dBm, and setting a group of power values of high-frequency microwave modulation signals according to the power values, wherein the selected test radio frequency voltage values are 0.3V pi L, 0.5V pi L, 0.7V pi L, 0.8V pi L, 0.85V pi L, 0.9V pi L, 0.95V pi L, V pi L, 1.05V pi L, 1.1V pi L, 1.15V pi L, 1.2V pi L, 1.25V pi L, 1.3V pi L, 1.35V pi L, 1.4V pi L, 1.45V pi L, 1.5V pi L, 1.55V pi L, 1.6V pi L and 1.65V pi L, and the radio frequency power values are respectively corresponding to radio frequency power values: 13.52dBm, 17.96dBm, 20.88dBm, 22dBm, 22.56dBm, 23dBm, 23.53dBm, 24dBm, 24.4dBm, 24.8dBm, 25.19dBm, 25.56dBm, 25.91dBm, 26.26dBm, 26.58dBm, 26.9dBm, 27.2dBm, 27.5dBm, 27.79dBm, 28dBm and 28.33dBm, because the gain of 20dB exists in the microwave amplifier, the microwave signal source sets the power to the power minus 20dB, and then records the set detection frequency and the power value of the set of microwave modulation signals into the upper computer software for standby;
d, reading the output of the high-frequency photoelectric detector through a frequency spectrograph, detecting the power value of a fundamental wave 5GHz microwave signal related to frequency, and recording the microwave power of the fundamental wave by upper computer software;
e, changing the modulation power of different microwave signal sources through upper computer software, and repeatedly testing and recording the radio frequency power according to the step C and the step D; testing data under different microwave powers for many times until the high-frequency signal power applied by a microwave signal source is close to the maximum radio-frequency power allowed by the modulator;
f, performing curve fitting on the modulation power of different microwave signal sources and the microwave power output by the high-frequency photoelectric detector through upper computer software, and finding out an inflection point through data processing so as to obtain the half-wave voltage of the electro-optic intensity modulator to be detected under the set detection frequency; as shown in fig. 2, the modulation power at the inflection point is 27dBm, and the half-wave voltage is 6.67V, which corresponds to the theoretical value.
And G, changing the modulation frequency through upper computer software to obtain the half-wave voltage under different modulation frequencies.
In summary, the present invention provides a system and a method for measuring a high-frequency half-wave voltage parameter of an electro-optical intensity modulator, wherein the frequency resolution of a point to be measured depends on the frequency resolution of a microwave signal source, so that the half-wave voltage of the electro-optical intensity modulator under any different frequencies can be measured, and the system and the method have the characteristics of a plurality of measurable frequency points for the same electro-optical intensity modulator to be measured, high measurement accuracy, low measurement system and measurement cost, and the like.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (7)
1. The utility model provides a measurement system of electro-optical intensity modulator high frequency half-wave voltage parameter which characterized in that: the device comprises a microwave signal source, a high-frequency photoelectric detector, a frequency spectrograph, a 5:95 polarization-maintaining coupler, a direct-current bias voltage control circuit, a polarization controller and a laser light source;
the output end of the laser light source is connected to the optical input port of the electro-optic intensity modulator through the polarization controller, and the polarization controller is adjusted to enable the output optical power of the electro-optic intensity modulator to be maximum;
the light output port of the electro-optical intensity modulator is output by a 5:95 polarization maintaining coupler in a shunt way, a 5% end of the polarization maintaining coupler is input into a direct current bias voltage control circuit, and the direct current bias voltage output of the control circuit is connected to a direct current bias end of the electro-optical intensity modulator; electrifying a direct current bias voltage control circuit to enable the electro-optical intensity modulator to work in a linear region;
the 95% end of the polarization-maintaining coupler is input into the high-frequency photoelectric detector and connected to the frequency spectrograph; the microwave signal source and the frequency spectrograph are connected to the upper computer; adjusting the frequency of a radio frequency signal output by a microwave signal source to be a frequency point to be tested by an upper computer, loading a group of microwave modulation signals with different radio frequency powers to an electro-optical intensity modulator to be tested, and testing the radio frequency power values of fundamental waves and second harmonics output by a photoelectric detector, which are measured by a frequency spectrograph under different radio frequency modulation powers;
and the upper computer simulates a corresponding curve of the output radio frequency power of the photoelectric detector relative to the input radio frequency power value of the electro-optic modulator, and calculates the half-wave voltage of the electro-optic intensity modulator under high-frequency modulation through a nonlinear inflection point of the curve.
2. The system for measuring the voltage parameter of the high-frequency half-wave of the electro-optic intensity modulator according to claim 1, wherein: the laser light source, the polarization controller, the electro-optical intensity modulator to be tested, the 5:95 polarization-maintaining coupler and the high-frequency photoelectric detector are sequentially connected through optical fibers.
3. The system for measuring the voltage parameter of the high-frequency half-wave of the electro-optic intensity modulator according to claim 1, wherein: the microwave signal source and the electro-optical intensity modulator to be tested are connected through a radio frequency cable.
4. The system for measuring the voltage parameter of the high-frequency half-wave of the electro-optic intensity modulator according to claim 1, wherein: the 5:95 polarization maintaining coupler is connected with the direct current bias voltage control circuit through an optical fiber to provide feedback information for the bias control circuit; the bias voltage control circuit is connected with the electro-optical intensity modulator to be tested through a cable.
5. The system for measuring the voltage parameter of the high-frequency half-wave of the electro-optic intensity modulator according to claim 1, wherein: the high-frequency photoelectric detector and the frequency spectrograph are connected through a high-frequency cable.
6. The system for measuring the voltage parameter of the high-frequency half-wave of the electro-optic intensity modulator according to claim 1, wherein: the 95% end of the polarization-maintaining coupler is input into the high-frequency photoelectric detector and connected to the frequency spectrograph; the microwave signal source and the frequency spectrograph are connected to the upper computer through a USB or GPIB interface.
7. A measuring method using a measuring system according to any of claims 1-6, characterized by the following procedure:
step A, a laser light source sends out a fixed wavelength optical signal to enter a polarization controller, and the polarization controller outputs the optical signal to the input end of an electro-optical intensity modulator to be detected as an optical carrier of the modulator; inputting a high-frequency microwave signal sent by a microwave signal source into a radio frequency port of the electro-optical intensity modulator to be tested;
meanwhile, the direct current bias voltage of the electro-optic intensity modulator to be tested is controlled through a direct current bias voltage control circuit, so that the linear region modulation of the electro-optic intensity modulator to be tested is realized; the modulated signal is passed through 5: the optical signal of the 5% port enters the bias voltage control circuit as a feedback signal; the optical signals of 95 percent of ports are converted into electric signals through a high-frequency photoelectric detector and are input into a frequency spectrograph;
b, adjusting the polarization state of the laser signal by using a polarization controller to ensure that the laser power at the output end of the electro-optical intensity modulator to be detected is maximum so as to match the polarization state of the optical signal of the laser light source with the polarization state of the electro-optical intensity modulator to be detected; electrifying a direct current bias voltage control circuit to enable a direct current bias point of the electro-optical intensity modulator to work in a linear region;
step C, setting working parameters: setting the detection frequency of the electro-optical intensity modulator to be detected, determining according to the typical value of low-frequency half-wave voltage Vpi L given in the technical parameters of the electro-optical intensity modulator to be detected or by using a power method, converting the half-wave voltage value into a radio frequency power value, setting a group of power values of a high-frequency microwave modulation signal according to the power values, when the modulator has the radio frequency voltage close to the half-wave voltage, outputting the fundamental wave power which is nonlinear relative to the input radio frequency power, and calculating the corresponding half-wave voltage value under the high frequency to be the half-wave voltage value under the high frequency according to the definition of 3dB bandwidth of the electro-optical intensity modulator
Therefore, the range of the selected test voltage value should be larger than
Selecting 0-1.65V pi L, the smaller the stepping is, the better the stepping is, and then recording the set detection frequency and the power value of the group of microwave modulation signals into the upper computer software to be waitedUsing;
d, reading the output of the high-frequency photoelectric detector through a frequency spectrograph, detecting the power value of a fundamental wave microwave signal related to frequency, and recording the microwave power of the fundamental wave microwave signal by upper computer software;
step E, changing the modulation power of different microwave signal sources through upper computer software, testing all the loaded radio frequency voltages in the step C, and repeatedly testing and recording according to the step D; testing data under different microwave powers for many times until the high-frequency signal power applied by a microwave signal source is close to the maximum radio-frequency power allowed by the modulator;
step F, comprehensively processing the microwave power output by the high-frequency photoelectric detector and the modulation power of different microwave signal sources through upper computer software, so as to obtain the half-wave voltage of the electro-optical intensity modulator to be detected under the set detection frequency; when the electro-optic intensity modulator operates in the linear region, the input-output function is:
in the above formula Pout、PinRespectively the output and input optical power of the electro-optical intensity modulator, VbIs the loaded DC bias voltage, theta is the initial phase value of the modulator, and V (t) is the RF modulation voltage; let the RF modulation voltage V (t) be V0The radio frequency signal of frequency f is expressed as
V(t)＝V0cos(2πft) (2)
By substituting formula (2) into formula (1), can be obtained
Wherein the content of the first and second substances,
when closed-loop control is performed by using a DC bias control circuit to compensate the modulatorThe working point of the radio frequency voltage modulator is shifted to ensure that the radio frequency voltage modulation is in a linear region;
electric field expression of input light
Omega 2 pi f, and the output optical electric field expression is Eout(t) can be written by Bessel expansion
Wherein
To modulate depth, Jn(. cndot.) is a first class of nth order Bessel function;
from the equation (4), the fundamental frequency component in the output electric field strength is J0And is only related to the modulation depth m, when m is less than 1, the modulator outputs a first-order component which tends to be constant and takes a fundamental frequency component as a main component; when m is 1, the first-order component starts to increase, namely when the input radio frequency voltage of the modulator is equal to the half-wave voltage, the nonlinear effect obviously increases; therefore, the modulation signals with different radio frequency powers are loaded, the fundamental wave signal power in the output electric field is tested, the input and output optical electric field function relationship of the electro-optical modulator is fitted, the modulator input radio frequency power corresponding to the curve inflection point is found, and the half-wave voltage under the test frequency can be calculated;
step G, measuring half-wave voltage under different frequencies: the half-wave voltage of the electro-optical intensity modulator to be measured at different frequencies can be measured through the adjusting step C, D, E, F.
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