CN111258092A - MZ electro-optic modulator bias point control method and system - Google Patents
MZ electro-optic modulator bias point control method and system Download PDFInfo
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Abstract
The invention relates to a method and a system for controlling a bias point of an MZ electro-optic modulator. The control method comprises the following steps: adding a disturbance signal of a kHz level to a Mach-Zehnder (MZ) electro-optic modulator, and acquiring output light intensity and bias voltage of the MZ electro-optic modulator; determining an output light intensity-bias voltage relation between the output light intensity and the bias voltage according to the disturbance signal; preprocessing the output light intensity-bias voltage relation, and determining the processed output light intensity-bias voltage relation; determining the harmonic ratio of the first harmonic component and the second harmonic component according to the processed output light intensity-bias voltage relation; and controlling the bias point of the MZ electro-optic modulator according to the harmonic ratio. The MZ electro-optic modulator bias point control method and system provided by the invention can improve the accuracy of the detection result.
Description
Technical Field
The invention relates to the field of MZ electro-optic modulator bias point control, in particular to a MZ electro-optic modulator bias point control method and system.
Background
The development of information technology has raised higher requirements for Analog-to-Digital converters (ADCs) with larger bandwidth, faster speed and higher precision, and in order to solve the problem that the sampling rate of the conventional electronic ADC is difficult to achieve further improvement, the optical ADC is attracting attention, wherein the optical sampling-based electro-quantization ADC is highly concerned because it can simultaneously utilize the superior performance of optical information processing and the mature technology of electronic quantization. The electro-optical modulator is used as a core device of the optical sampling electric quantization ADC, has very important influence on the performance of the whole ADC, and has very important application significance and practical value for solving the problem of offset drift of the modulator.
Optical power monitoring techniques, perturbation signal monitoring techniques, are the most commonly used methods for bias point control in existing electro-optic modulators. In the optical power monitoring method, whether the working point of the modulator drifts or not is judged by monitoring the optical output power of the modulator according to the change of the optical output power, and the bias voltage of the modulator is adjusted according to the change amplitude of the optical output power so as to realize the stability and control of the working point. In the disturbance signal monitoring method, a low-frequency disturbance signal is added at a direct current offset end of a modulator, Fast Fourier Transform (FFT) is carried out on an output modulation signal, a power spectrum of a first harmonic frequency signal and a second harmonic frequency signal is extracted from the output modulation signal, the position of a working point of the modulator is obtained by analyzing a harmonic component of the power spectrum, and the working point of the modulator is stably controlled.
The method for detecting the output optical power is simple, but the output optical power of the modulator is influenced by bias voltage and is also easily influenced by instability of the output optical power of the light source, so that the error is large, the accuracy of a method for detecting the first harmonic or the second harmonic of the modulation disturbance signal is improved greatly, and the harmonic component of the modulation signal is also influenced by the input optical power.
Kenro Sekine et al, 2007 proposed a method for controlling the bias point of an electro-optical modulator based on the optical power method, in which a continuous wave was placed at the output of an MZ modulator toward a light source, and the output light of the light source passed through the MZ modulator in the opposite direction to a modulated signal light. And arranging a power monitor at the input end of the MZ modulator, and controlling the bias voltage of the target MZ modulator to ensure that the monitoring direct-current power of the backlight is minimum to stabilize the working point of the modulator. While this technique does have the advantage of the ability to lock to any bias position, it requires the addition of an additional optical source in the system, increasing the complexity of the system, and the output optical power signal is strongly dependent on MZM input optical power fluctuations and optical path loss variations, resulting in a limited control accuracy of the system.
The kochang sword et al in 2017 put forward a method based on the partial derivative of the output optical power of a modulator to realize the control of the working point of the modulator, and judge and control the bias voltage of the modulator by comparing the average optical power partial derivative corresponding to the bias voltage in a small range with a set theoretical partial derivative value.
In order to further optimize the optical power method, heuchong et al proposed a composite control algorithm based on an average optical power slope value and a cotangent value in 2017, and implemented stable control of the operating point using the FPGA technology, and the design block diagram is shown in fig. 1. The scheme solves the defect that the output optical power of the modulator is influenced by the input optical power, but a first derivative and a second derivative of the optical power are required to be obtained, and other tangent values are required to be obtained.
In 2017, an Integrated Circuit (IC) for MZ modulator bias control is realized by adopting a 28-nm CMOS technology based on an average power monitoring method in Min-Hyeong Kimz, and the architecture is shown in FIG. 2. The circuit firstly searches for the modulator bias voltage with the maximum light modulation amplitude by scanning the bias voltage in a preset range, and then controls the bias voltage by monitoring the average modulation output power to keep the average power in a certain state, thereby providing the optimal bias voltage. Although the performance of the chip still needs to be further improved, the technology provides a greater possibility for realizing the monolithic integration of the silicon modulator and the control circuit in the future.
In order to realize high extinction ratio and high control precision, people also do a lot of research work on a control algorithm, and in 2016, the Zhang Wenqi of Beijing postal and telecommunications university proves that the overall performance of a modulator control system can be improved by selecting a high-quality algorithm through researching and testing several offset point drift reset algorithms. Hofer in 2017 devised a feedback algorithm that interleaved a set of diagnostic pulses between the main pulses of the laser to map the transfer function, and after mapping, utilized curve fitting to maintain the minimum of the modulator transfer function accurately and maintain the highest extinction ratio of the modulator. Tests prove the effectiveness of the algorithm in maintaining a high extinction ratio in response to temperature and time drifts of the transfer function, but the method requires an additional pulse signal and is complex to implement. Shijiu et al proposed a loop iteration locking algorithm without disturbance signal in 2015, and approached the optimum bias voltage through multiple iterations, the method did not need to transmit disturbance signal and has simple structure, but the modulation precision that can be realized at present was not high.
The summary of the current research situation of the bias point control scheme of the electro-optical modulator shows that a great deal of research work has been done by the predecessors aiming at the bias point drift phenomenon of the electro-optical modulator, and a plurality of research schemes are also provided, which have advantages, but the problems of low control precision, complex control mode or excessively complex control algorithm still exist.
Disclosure of Invention
The invention aims to provide a method and a system for controlling a bias point of an MZ electro-optic modulator, which aim to solve the problem that the detection result accuracy is low due to the influence of input optical power on the bias point of the existing electro-optic modulator.
In order to achieve the purpose, the invention provides the following scheme:
a MZ electro-optic modulator bias point control method comprises the following steps:
adding a disturbance signal of a kHz level to a Mach-Zehnder (MZ) electro-optic modulator, and acquiring output light intensity and bias voltage of the MZ electro-optic modulator;
determining an output light intensity-bias voltage relation between the output light intensity and the bias voltage according to the disturbance signal;
preprocessing the output light intensity-bias voltage relation, and determining the processed output light intensity-bias voltage relation;
determining the harmonic ratio of the first harmonic component and the second harmonic component according to the processed output light intensity-bias voltage relation;
and controlling the bias point of the MZ electro-optic modulator according to the harmonic ratio.
Optionally, the output light intensity-bias voltage relationship between the output light intensity and the bias voltage is determined according to the disturbance signal;
using formulas
Determining a phase shift caused by the perturbation signal; wherein Vsin (ω t + θ (ω)) is a disturbance signal;
a phase shift induced for the perturbation signal; θ (ω) is the modulation signal phase response delay due to microwave-optical velocity mismatch; vπ(ω) is a function of the half-wave voltage of said MZ electro-optic modulator with respect to the input signal frequency;
using formulas
Determining an output light intensity-bias voltage relationship between the output light intensity and the bias voltage according to the phase shift; wherein, PoutIs the output optical power of the modulator; t isDIs the inherent loss of the modulator; p0Is the input power of the input signal;
is the inherent phase difference of the two branch arms of the MZ electro-optic modulator.
Optionally, the preprocessing the output light intensity-bias voltage relationship, and determining the processed output light intensity-bias voltage relationship specifically includes:
processing the output light intensity-bias voltage relation by using a Taylor formula, and determining the expanded output light intensity-bias voltage relation;
and reordering the high-order frequency terms of the expanded output light intensity-bias voltage relation, and determining the processed output light intensity-bias voltage relation.
Optionally, the determining a harmonic ratio of the first harmonic component and the second harmonic component according to the processed output light intensity-bias voltage relationship specifically includes:
using formulas
Determining a harmonic ratio of the first harmonic component to the second harmonic component; wherein, I1stFor the first harmonic component of the first harmonic,
a＝πV/Vπ(ω), ε is the responsivity of the photodetector; i is2ndFor the first harmonic component of the first harmonic,
r is the harmonic ratio.
An MZ electro-optic modulator bias point control system, comprising: the device comprises a photoelectric detector, a current I-voltage V conversion and amplification circuit, a band-pass filter, a signal amplitude regulation circuit, an analog-to-digital conversion circuit, a Field Programmable Gate Array (FPGA), a digital-to-analog conversion circuit, a signal amplification circuit and a low-pass filter;
the photoelectric detector, the I-V conversion and amplification circuit, the band-pass filter, the signal amplitude regulation circuit, the analog-to-digital conversion circuit, the field programmable gate array FPGA, the digital-to-analog conversion circuit, the signal amplification circuit and the low-pass filter are sequentially connected; the photoelectric detector is used for outputting a photoelectric detector signal, and the photoelectric detector signal is a current signal which is converted by the photoelectric detector from an optical signal output by the MZ electro-optic modulator; the I-V conversion and amplification circuit is used for converting the current signal into an amplified voltage signal; the band-pass filter is used for filtering direct current signals and high-frequency interference signals in the voltage signals; the signal amplitude regulating circuit is used for proportionally reducing the input voltage of-5V- +5V to the range of 1V-3V; the analog-to-digital conversion circuit is used for converting the reduced voltage signal into a digital signal; the FPGA is used for analyzing the first harmonic component and the second harmonic component and outputting a control voltage signal; the digital-to-analog conversion circuit is used for converting the output control voltage signal from a digital voltage signal into an analog voltage signal; the signal amplification circuit is used for amplifying the analog voltage signal, and the amplified analog voltage signal is the bias voltage of the MZ electro-optic regulator; the low-pass filter is used for filtering out high-frequency components; the disturbance signal is a 1kHz low-frequency sinusoidal signal generated by a digital control oscillator NCO module in the FPGA, and the disturbance signal and the bias voltage are loaded to a direct current bias end of the MZ electro-optic modulator together to control a bias point of the MZ electro-optic modulator.
Optionally, the I-V conversion and amplification circuit specifically includes: a first operational amplifier and a second operational amplifier;
the first operational amplifier and the second operational amplifier are connected in series; the first operational amplifier is used for converting the current signal output by the photoelectric detector into a voltage signal, performing primary amplification on the voltage signal and determining a voltage signal after the primary amplification; the second operational amplifier is used for carrying out secondary amplification on the voltage signal after the primary amplification.
Optionally, the frequency of the disturbance signal is 1 kHz; the frequency of the first harmonic component of the disturbance signal is 1 kHz; the frequency of the second harmonic component of the disturbing signal is 2 kHz.
Optionally, the passband of the bandpass filter is in a range of 500Hz to 2.5 kHz.
Optionally, the method further includes: an adder;
the adder is arranged between the digital-to-analog conversion circuit and the low-pass filter and used for adding the disturbance signal generated by the NCO module and the bias voltage.
Optionally, the method further includes: a power supply and a reference voltage circuit;
the power supply and the reference voltage circuit are used for supplying power to the MZ electro-optic modulator bias point control system; the power supply and reference voltage circuit comprises six levels of 1.2V, 2.5V, 3.3V, 5V, 6V, -5V and-6V.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention provides a method for controlling a bias point of an MZ electro-optic modulator, which realizes the control of the working point of the modulator by detecting the harmonic ratio of first harmonic and second harmonic, eliminates the influence of factors except bias voltage on a detection result by the first harmonic and the second harmonic, and can further improve the control precision, thereby improving the accuracy of the detection result.
Meanwhile, the invention also provides a MZ electro-optic modulator bias point control system, the invention researches the bias point control technology aiming at the working point drift phenomenon of the MZ electro-optic modulator, designs the MZ electro-optic modulator bias point control system based on the FPGA + DSP technology, the invention collects the output signal of the ADC in real time and searches the FFT conversion and harmonic point frequency amplitude, the data volume needing to be processed is larger during collection, the FPGA has larger parallelism degree compared with the traditional processor, a plurality of modules in the FPGA can simultaneously and independently calculate, but not limited to simultaneously execute the same function, the operation speed can meet the high requirement, thereby improving the operation speed; digital Signal Processing (DSP) technology includes a digital-to-analog conversion circuit and an analog-to-digital conversion circuit; the DSP is a special microprocessor specially used for digital signal processing, and higher precision and faster operation speed can be obtained by using the DSP.
Before the bias voltage and the disturbance signal are added to the bias end, the low-pass filter is used for filtering out the high-frequency interference signal in the circuit, so that the noise of the circuit is weakened, the bias voltage signal and the disturbance signal are better reserved, and the more accurate output optical signal can be obtained, so that the higher-precision operation is realized.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without inventive exercise.
FIG. 1 is a block diagram of a bias operating point control of a Hao Chong proposed MZ modulator provided by the present invention;
FIG. 2 is a block diagram of a MZ modulator bias point control IC provided by the present invention;
FIG. 3 is a flow chart of a method for controlling the bias point of the MZ electro-optic modulator provided by the invention;
FIG. 4 is a block diagram of an MZ modulator provided by the present invention;
FIG. 5 is a graph of the transmission characteristic of the MZ modulator provided by the present invention;
FIG. 6 is a graph of quadrature point output provided by the present invention;
FIG. 7 is a graph of the operating point drift of the MZ electro-optic modulator provided by the present invention;
FIG. 8 is a graph comparing the relationship between the first harmonic component and the second harmonic component and the harmonic ratio provided by the present invention;
FIG. 9 is a block diagram of a bias point control system of the MZ electro-optic modulator provided by the present invention;
FIG. 10 is a diagram of a bias point control architecture of an MZ electro-optic modulator provided by the present invention;
FIG. 11 is a general hardware block diagram of a MZ electro-optic modulator bias point control system provided by the invention;
FIG. 12 is a block diagram of the overall architecture of the process provided by the present invention;
FIG. 13 is a timing diagram of the sampling of the A/D module provided by the present invention;
fig. 14 is a flowchart of a procedure for determining a control voltage according to the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments 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 given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention aims to provide a method and a system for controlling a bias point of an MZ electro-optic modulator, which can improve the accuracy of a detection result.
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in further detail below.
Fig. 3 is a flowchart of a method for controlling a bias point of an MZ electro-optic modulator provided by the present invention, and as shown in fig. 3, the method for controlling the bias point of the MZ electro-optic modulator includes:
step 301: adding a disturbance signal of a kHz level to a Mach-Zehnder electro-optic modulator, and obtaining output light intensity and bias voltage of the MZ electro-optic modulator.
The structure of MZ modulator is shown in FIG. 4, usually, a voltage is applied to one branch arm of the modulator, the other branch arm is not applied, the phase difference of two points is adjusted, the inherent phase difference of two branch arms is considered
The relationship between the output light intensity of the modulator and the applied electric field voltage (the applied voltage is the so-called bias voltage) is expressed as follows:
wherein, TDIs the inherent loss of the modulator, P0Is the input power of the signal, VπIs the half-wave voltage of the modulator, namely the voltage value required for enabling the phase difference of two branch signals to reach pi,
is the phase shift caused by the branch circuit after being applied with voltage, and the relation with the applied voltage V is as follows:
thus, the transmission characteristic curve of the MZ modulator can be obtained as shown in fig. 5, and the quadrature point can realize linear modulation on the input signal, as shown in fig. 6, the input modulation signal is shown below, the output signal is shown on the right, and it can be seen that the input and output are all complete sine waves without distortion, so that the bias point is locked to the quadrature point.
The offset point may drift due to external conditions, the drift curve is shown in fig. 7, the solid line is an un-drift curve, and the dotted line and the line with a triangle represent the drift curve. If the bias voltage is unchanged, the output modulation signal is distorted. All that is required is to control the bias voltage to vary together, for example: if the drift is a dashed line, the bias voltage is controlled to the quadrature point of the dashed line.
The theoretical scheme is that a disturbance signal of a kHz level is added at the offset end, and first harmonic component and second harmonic component are detected. If the perturbation signal is Vsin (ω t + θ (ω)), the phase shift caused by the perturbation signal is expressed as:
Step 302: and determining the output light intensity-bias voltage relation between the output light intensity and the bias voltage according to the disturbance signal.
Into equation (1) (i.e., into
Substituted into formula (1)
) Obtaining:
step 303: and preprocessing the output light intensity-bias voltage relation, and determining the processed output light intensity-bias voltage relation.
PoutIs the output optical power of the modulator, TDIs the inherent loss of the modulator, P0Is the input power of the signal, let a ═ π V/Vπ(ω), when a < 1, equation (4) can be expanded with a trigonometric function as:
taylor expansion:
the higher order frequency terms are rearranged:
step 304: and determining the harmonic ratio of the first harmonic component and the second harmonic component according to the processed output light intensity-bias voltage relation.
The first and second harmonic components and the harmonic ratio of the respective photodiode output currents can be obtained from equation (7):
where epsilon represents the responsivity of the photodetector. As can be seen from equation (10), for a given MZ modulator and a fixed input signal, the harmonic ratio R of the output signal is related to the bias voltage of the modulator.
Step 305: and controlling the bias point of the MZ electro-optic modulator according to the harmonic ratio.
The specific method for controlling the bias point according to the MZ electro-optic modulator bias point control system comprises the following steps:
if the original operating point should be 5V (actually determined according to the modulator), and the operating point drifts, for example, to 4V, and at this time, the ratio of the first harmonic to the second harmonic of the output signal of the modulator changes under the action of the disturbing signal, and the relationship between the first harmonic component and the second harmonic component and the harmonic ratio is shown in fig. 8, and the specific implementation method is two parts of circuit and program combination; the circuit has two functions of signal acquisition and voltage output control, the specific function of the program of the FPGA chip is to perform Fourier transform on the acquired signal (laser-MZ modulator-coupler-photodetector-FPGA) to obtain a first harmonic and a second harmonic, find the amplitude of the first harmonic through an amplitude searching module, and transmit the amplitude into the DSP chip; in the DSP chip, the magnitude of the two wave amplitude values is calculated, then the working point of the modulator after drift can be found out because the bias voltage (working point) corresponds to the ratio one by one, the value of the voltage is transmitted back to the FPGA after the working point is found out, and then the voltage is output and is added to the bias end of the modulator together with the disturbance signal generated by the NOC module.
Fig. 9 is a structural diagram of a bias point control system of an MZ electro-optic modulator provided by the present invention, fig. 10 is a structural diagram of a bias point control system of an MZ electro-optic modulator provided by the present invention, fig. 11 is a general hardware block diagram of a bias point control system of an MZ electro-optic modulator provided by the present invention, and as shown in fig. 9 to fig. 11, a bias point control system of an MZ electro-optic modulator includes: the system comprises a photoelectric detector 1, an I-V conversion and amplification circuit 2, a band-pass filter 3, a signal amplitude adjusting circuit 4, an analog-to-digital conversion circuit 5, a field programmable gate array FPGA6, a digital-to-analog conversion circuit 7, a signal amplification circuit 8 and a low-pass filter 9; the photoelectric detector 1, the current I-voltage V conversion and amplification circuit 2, the band-pass filter 3, the signal amplitude adjustment circuit 4, the analog-to-digital conversion circuit 5, the field programmable gate array FPGA6, the digital-to-analog conversion circuit 7, the signal amplification circuit 8 and the low-pass filter 9 are connected in sequence; the photoelectric detector 1 is used for outputting a signal of the photoelectric detector 1, and the signal of the photoelectric detector 1 is a current signal converted from an optical signal output by the MZ electro-optical modulator through the photoelectric detector 1; the I-V conversion and amplification circuit 2 is used for converting the current signal into an amplified voltage signal; the band-pass filter is used for filtering direct current signals and high-frequency interference signals of the voltage signals; the signal amplitude regulating circuit 4 is used for proportionally reducing the input voltage of-5V- +5V to the range of 1V-3V; the analog-to-digital conversion circuit 5 is used for converting the reduced voltage signal into a digital signal; the FPGA6 is used for analyzing the first harmonic component and the second harmonic component and outputting a control voltage signal; the digital-to-analog conversion circuit 7 is used for converting the output control voltage signal from a digital voltage signal to an analog voltage signal; the signal amplifying circuit 8 is configured to amplify the analog voltage signal, where the amplified analog voltage signal is a bias voltage of the MZ electro-optic regulator; the low-pass filter 9 circuit is used for filtering out high-frequency components; the disturbance signal is a 1kHz low-frequency sinusoidal signal generated by a digital control oscillator NCO module in the FPGA6, and the disturbance signal and the bias voltage are loaded to a direct current bias end of the MZ electro-optical modulator together to control the bias point of the MZ electro-optical modulator. Wherein the coupler functions to split light, transmitting 10% of the light to the photodetector 1.
Programming:
in the MZ electro-optic modulator bias point control system, most of signal processing is realized based on the FPGA6, a hardware frame diagram generated by a program is shown in fig. 12, a clock module provides a clock signal for the whole system, the operating frequencies of the devices of the system are synchronized, and the clocks required by the modules are satisfied by a frequency division manner; the program operation of the circuit needs clock control, and then the clock frequency needed by each module is different, so that a clock module with fixed frequency is generated firstly, and the frequency division can change the clock frequency and then meet the clock frequency needed by each module.
The disturbing signal mentioned in the hardware part is generated by an IP core of a Numerical Control Oscillator (NCO); as shown in fig. 13, the sampling control module of the analog-to-digital converter (a/D) controls the sampling speed of the a/D and receives the sampling data of the a/D, the chip can perform data acquisition and conversion only by providing a proper clock signal to the chip, and output the converted digital signal in a parallel manner, and the OTR pin of the chip also provides an input voltage range detection function, and determines whether the input voltage exceeds the measurement range where the chip is designed by the output of the pin.
In order to ensure that all the acquired data can enter an FFT (Fourier transform) module for spectrum conversion, a First-in First-out (FIFO) queue is arranged between the data obtained by A/D sampling and the FFT module to buffer the A/D data.
The FFT module is used for converting the digital signal obtained by A/D conversion from time domain to frequency domain. The real part and the imaginary part of the spectrum data obtained by conversion are separated, so that a square root conversion program is required to be designed to obtain real spectrum data, and a sampling clock required by A/D conversion is obtained by frequency division of a system clock through the program, so that the required frequency division frequency can be directly input from the outside when the sampling rate is required to be changed, and the program is not required to be set every time.
After the harmonic frequency amplitude search module searches for the value of the required frequency point, the data is transmitted to a DSP for processing through a Serial Peripheral Interface (SPI), and the DSP performs algorithm processing on the data, where the process of searching is shown in fig. 14.
When searching, firstly, the range of the data to be searched is found, then the difference value between the value and two numbers in the range is compared, the number with smaller difference value is taken as the searching result, then the address of the value is returned, and the address plus M is the storage address of the required control voltage. After finding the control voltage that needs to be input to the bias terminal, the DSP transmits the data back to the FPGA6 again through the SPI protocol. The FPGA6 inputs the control signal to the D/A module for output, thereby completing the control of the whole system.
The invention converts the photocurrent signal into the voltage signal and amplifies the voltage signal, and uses the band-pass filter, the circuit can filter the direct current signal and the high-frequency interference signal in the circuit, and can reduce the interference of other signals while keeping the information of the disturbance signal, thereby obtaining more accurate first harmonic component and second harmonic component and further calculating more accurate harmonic ratio.
Compared with the traditional processor, the FPGA adopted by the invention has higher parallelism, a plurality of internal modules can simultaneously and independently calculate without simultaneously executing the same function, and the calculation speed can meet very high requirements. Meanwhile, the logic inside the FPGA can be changed according to design requirements, development is very convenient, and the processing performance of the FPGA is more excellent than that of the DSP on the digital processing function of high-speed signals. In the system, the output signal of the ADC needs to be acquired in real time, FFT conversion and searching of the harmonic point frequency amplitude are carried out, the data volume needing to be processed is large during acquisition, the requirement on the processing speed of hardware is high, the operation process is relatively simple, and the part is particularly suitable for being processed by the FPGA.
The DSP adopted by the invention is a unique microprocessor specially used for digital signal processing, and compared with the FPGA, the DSP is mainly used for calculation, has the advantages of strong data processing capacity, higher operation speed and capability of easily realizing high operation precision, needs division processing related to floating point operation for converting to obtain harmonic spectrum data, usually consumes a plurality of display lookup tables (Look-Up-tables, LUTs), registers and multipliers when the floating point operation is carried out in the FPGA, greatly increases the development cost, and has low precision of the result obtained by operation although the resources are consumed, so the DSP can obtain higher precision and faster operation speed.
Before the bias voltage and the disturbance signal are added to the bias end, the low-pass filter is used for filtering out the high-frequency interference signal in the circuit, the noise of the circuit is weakened, the bias voltage signal and the disturbance signal are better reserved, and the output optical signal can be more accurately obtained so as to realize the operation with higher precision.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other.
The principles and embodiments of the present invention have been described herein using specific examples, which are provided only to help understand the method and the core concept of the present invention; meanwhile, for a person skilled in the art, according to the idea of the present invention, the specific embodiments and the application range may be changed. In view of the above, the present disclosure should not be construed as limiting the invention.
Claims (10)
1. A method for controlling a bias point of an MZ electro-optic modulator is characterized by comprising the following steps:
adding a disturbance signal of a kHz level to a Mach-Zehnder (MZ) electro-optic modulator, and acquiring output light intensity and bias voltage of the MZ electro-optic modulator;
determining an output light intensity-bias voltage relation between the output light intensity and the bias voltage according to the disturbance signal;
preprocessing the output light intensity-bias voltage relation, and determining the processed output light intensity-bias voltage relation;
determining the harmonic ratio of the first harmonic component and the second harmonic component according to the processed output light intensity-bias voltage relation;
and controlling the bias point of the MZ electro-optic modulator according to the harmonic ratio.
2. The method of claim 1, wherein the determining an output light intensity-bias voltage relationship between the output light intensity and the bias voltage according to the disturbance signal specifically comprises:
using formulas
Determining a phase shift caused by the perturbation signal; wherein Vsin (ω t + θ (ω)) is a disturbance signal;
a phase shift induced for the perturbation signal; θ (ω) is the modulation signal phase response delay due to microwave-optical velocity mismatch; vπ(ω) is a function of the half-wave voltage of said MZ electro-optic modulator with respect to the input signal frequency;
using formulas
Determining an output light intensity-bias voltage relationship between the output light intensity and the bias voltage according to the phase shift; wherein, PoutIs the output optical power of the modulator; t isDIs the inherent loss of the modulator; p0Is the input power of the input signal;
is the inherent phase difference of the two branch arms of the MZ electro-optic modulator.
3. The method of claim 2, wherein the preprocessing is performed on the output light intensity-bias voltage relationship, and the determining of the processed output light intensity-bias voltage relationship specifically comprises:
processing the output light intensity-bias voltage relation by using a Taylor formula, and determining the expanded output light intensity-bias voltage relation;
and reordering the high-order frequency terms of the expanded output light intensity-bias voltage relation, and determining the processed output light intensity-bias voltage relation.
4. The method as claimed in claim 3, wherein the determining the harmonic ratio of the first harmonic component to the second harmonic component according to the processed output light intensity-bias voltage relationship comprises:
using formulas
Determining a harmonic ratio of the first harmonic component to the second harmonic component; wherein, I1stFor the first harmonic component of the first harmonic,
a＝πV/Vπ(ω), ε is the responsivity of the photodetector; i is2ndIn order to be the second harmonic component,
r is the harmonic ratio.
5. An MZ electro-optic modulator bias point control system comprising: the device comprises a photoelectric detector, a current I-voltage V conversion and amplification circuit, a band-pass filter, a signal amplitude regulation circuit, an analog-to-digital conversion circuit, a Field Programmable Gate Array (FPGA), a digital-to-analog conversion circuit, a signal amplification circuit and a low-pass filter;
the photoelectric detector, the I-V conversion and amplification circuit, the band-pass filter, the signal amplitude regulation circuit, the analog-to-digital conversion circuit, the field programmable gate array FPGA, the digital-to-analog conversion circuit, the signal amplification circuit and the low-pass filter are sequentially connected; the photoelectric detector is used for outputting a photoelectric detector signal, and the photoelectric detector signal is a current signal which is converted by the photoelectric detector from an optical signal output by the MZ electro-optic modulator; the I-V conversion and amplification circuit is used for converting the current signal into an amplified voltage signal; the band-pass filter is used for filtering direct-current signals and high-frequency interference signals in the disturbance signals; the signal amplitude regulating circuit is used for proportionally reducing the input voltage of-5V- +5V to the range of 1V-3V; the analog-to-digital conversion circuit is used for converting the reduced voltage signal into a digital signal; the FPGA is used for analyzing the first harmonic component and the second harmonic component and outputting a control voltage signal; the digital-to-analog conversion circuit is used for converting the output control voltage signal from a digital voltage signal into an analog voltage signal; the signal amplification circuit is used for amplifying the analog voltage signal, and the amplified analog voltage signal is the bias voltage of the MZ electro-optic regulator; the low-pass filter is used for filtering out high-frequency components; the disturbance signal is a 1kHz low-frequency sinusoidal signal generated by a digital control oscillator NCO module in the FPGA, and the disturbance signal and the bias voltage are loaded to a direct current bias end of the MZ electro-optic modulator together to control a bias point of the MZ electro-optic modulator.
6. The MZ electro-optic modulator bias point control system of claim 5, wherein said I-V conversion and amplification circuit comprises: a first operational amplifier and a second operational amplifier;
the first operational amplifier and the second operational amplifier are connected in series; the first operational amplifier is used for converting the current signal output by the photoelectric detector into a voltage signal, performing primary amplification on the voltage signal and determining a voltage signal after the primary amplification; the second operational amplifier is used for carrying out secondary amplification on the voltage signal after the primary amplification.
7. The MZ electro-optic modulator bias point control system of claim 5, wherein said perturbation signal has a frequency of 1 kHz; the frequency of the first harmonic component of the disturbance signal is 1 kHz; the frequency of the second harmonic component of the disturbing signal is 2 kHz.
8. The MZ electro-optic modulator bias point control system of claim 5, wherein said band pass filter has a passband in the range of 500Hz-2.5 kHz.
9. The MZ electro-optic modulator bias point control system of claim 5, further comprising: an adder;
the adder is arranged between the digital-to-analog conversion circuit and the low-pass filter and used for adding the disturbance signal generated by the NCO module and the bias voltage.
10. The MZ electro-optic modulator bias point control system of claim 5, further comprising: a power supply and a reference voltage circuit;
the power supply and the reference voltage circuit are used for supplying power to the MZ electro-optic modulator bias point control system; the power supply and reference voltage circuit comprises six levels of 1.2V, 2.5V, 3.3V, 5V, 6V, -5V and-6V.
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