CN115833953A - MZ modulator bias point control system and method - Google Patents
MZ modulator bias point control system and method Download PDFInfo
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Abstract
The invention discloses a MZ modulator bias point control system and method, which relates to the technical field of optical fiber communication, and the system comprises: the bias voltage output module is connected with the target modulator and used for applying a disturbance signal and bias voltage to the target modulator; the photoelectric detector is arranged on an optical path of an optical signal output by the target modulator and used for converting the optical signal output by the target modulator into a current signal; the signal acquisition module is connected with the photoelectric detector and is used for converting the current signal into a voltage signal within a set threshold range; and the control module is respectively connected with the signal acquisition module and the bias voltage output module and is used for controlling the bias voltage output module to output a disturbance signal and bias voltage, adjusting the bias voltage according to the voltage signal and determining the position of the bias point of the target modulator according to the voltage signal and the bias voltage. The invention can ensure that the MZ modulator reliably works at the set bias point, thereby not only ensuring the control precision, but also improving the control efficiency of the bias point.
Description
Technical Field
The invention relates to the technical field of optical fiber communication, in particular to a system and a method for controlling a bias point of an MZ modulator.
Background
The MZ electro-optical modulator is used as an external modulator and widely applied to modern optical fiber communication systems. Before the MZ modulator is used, the bias voltage needs to be set at different bias points according to different application fields, but the bias point of the MZ modulator drifts along with the change of external conditions, which greatly influences the normal use of the MZ modulator. To solve this problem, the most common control technique at present is to design a bias point control circuit to adjust the bias voltage, so as to stably control the MZ modulator at a predetermined bias point. With the increasing requirements of modern information war and high-speed communication, it is also very significant to develop a better bias point control technology of the MZ modulator.
At present, various countries have a lot of researches on the problem of stable control of the bias point of the MZ modulator. These studies can be broadly divided into two types, among others, the first being based on optical power monitoring techniques and the second being based on disturbance signal monitoring techniques. The realization of the optical power monitoring-based technology is to collect the output optical power of the MZ modulator (some technologies also need to collect the input optical power), and the drift condition of the bias point of the MZ modulator can be obtained by analyzing the collected optical power change condition, so that the magnitude of the direct current bias voltage can be correspondingly adjusted to stabilize the bias point of the MZ modulator. The technical scheme for realizing the monitoring based on the disturbing signals is that a weak disturbing signal is superposed on a direct current voltage of a bias end of the MZ modulator, when the bias point is at different positions, the disturbance brought by the disturbing signal to the output optical power also has different changes, and the position of the current bias point can be obtained by detecting and analyzing the changes.
In the current technology based on optical power monitoring, a relatively mature control technology is a control scheme proposed by x.yuan and the like of beijing postal and telecommunications university in 2021, which collects the output optical power of an MZ modulator, calculates a first derivative and a second derivative of a bias voltage, and finds that the ratio of the first derivative and the second derivative is irrelevant to the input optical power and the insertion loss of the MZ modulator and is in a one-to-one correspondence with the bias voltage.
In the current technology based on disturbance signal monitoring, a relatively mature control technology is proposed by l.l.wang of the company rocheydate martin space system, 2010, and the technology applies a method of adding a disturbance signal of a kHz level to a bias end of an MZ modulator and analyzing a harmonic component of the signal in output optical power of the disturbance signal. In addition, the authors verify that the ratio of the first harmonic component to the second harmonic component of the signal is independent of the input optical power and the insertion loss of the device, and that the ratio corresponds to the bias point at which the MZ modulator is located. Therefore, only the first harmonic ratio and the second harmonic ratio of the bias point to be controlled need to be calculated, the current drift condition can be analyzed by utilizing the harmonic ratio, and the bias voltage is controlled to realize the effect of stabilizing the bias point. The scheme successfully solves the problem that the control effect of the bias point is influenced by the instability of the light source and the optical link, and simultaneously realizes the control of any bias point.
A great deal of research has been done on the control technique of the bias point of the MZ modulator, and the control technique has become more and more mature, but each solution still has certain limitations and disadvantages. The technology based on the optical power detection method is susceptible to some external conditions, so that the control accuracy is poor, and therefore the technology based on the disturbance signal monitoring is selected to be relatively higher in control accuracy, but the scheme requires higher hardware conditions and is more complex in design, and a disturbance signal is introduced, so that the modulation signal is affected. And based on the technology of disturbance signal monitoring, the larger the amplitude of the loaded disturbance signal is, the larger the modulation error is, but reducing the amplitude of the disturbance signal will affect the control accuracy of the bias point, and the problem is mainly caused by the problem that small signals are easily interfered, so the requirements on circuits and control algorithms will be higher, and it is necessary to research the control technology of small-amplitude disturbance signals.
Disclosure of Invention
The invention aims to provide a bias point control system and a method of an MZ modulator, which can enable the MZ modulator to reliably work at a set bias point.
In order to achieve the purpose, the invention provides the following scheme:
a MZ modulator bias point control system, the system comprising: the device comprises a bias voltage output module, a photoelectric detector, a signal acquisition module and a control module;
the bias voltage output module is connected with the target modulator and used for applying a disturbance signal and bias voltage to the target modulator;
the photoelectric detector is arranged on an optical path of an optical signal output by the target modulator and used for converting the optical signal output by the target modulator into a current signal;
the signal acquisition module is connected with the photoelectric detector and is used for converting the current signal into a voltage signal within a set threshold range;
the control module is respectively connected with the signal acquisition module and the bias voltage output module and is used for controlling the bias voltage output module to output the disturbance signal and the bias voltage, adjusting the bias voltage according to the voltage signal and determining the position of the bias point of the target modulator according to the voltage signal and the bias voltage.
Optionally, the system further comprises a power conversion module; the power supply conversion module is respectively connected with the bias voltage output module, the signal acquisition module and the control module and is used for providing corresponding power supply voltage for the bias voltage output module, the signal acquisition module and the control module.
Optionally, the power conversion module includes a positive and negative 12V voltage conversion circuit, a positive and negative 5V voltage conversion circuit, a 3.3V voltage conversion circuit, and a 1.8V voltage conversion circuit;
the positive and negative 12V voltage conversion circuit outputs +/-12V voltage by adopting a DCDC circuit and is used for supplying power to the bias voltage output module;
the positive and negative 5V voltage conversion circuit outputs +/-5V voltage by adopting a DCDC circuit and is used for supplying power to the signal acquisition module;
the 3.3V voltage conversion circuit outputs 3.3V voltage by adopting an LDO voltage stabilizer;
the 1.8V voltage conversion circuit outputs 1.8V voltage by adopting an LDO voltage stabilizer;
the 3.3V voltage conversion circuit and the 1.8V voltage conversion circuit are used for supplying power to the control module.
Optionally, the system further comprises a beam splitter; the optical splitter is arranged on an optical path of the optical signal output by the target modulator and is used for splitting the optical signal output by the target modulator into an optical signal for work and an optical signal for analysis;
and the optical signal output by the target modulator enters the photoelectric detector after passing through the optical splitter.
Optionally, the bias voltage output module includes a bias voltage generating circuit, a disturbing signal generating circuit, a superimposing circuit, and a low-pass filter;
the bias voltage generating circuit is connected with the control module and used for outputting bias voltage according to the bias control signal output by the control module;
the disturbance signal generating circuit is connected with the control module and used for outputting a disturbance signal according to the disturbance control signal output by the control module;
the superposition circuit is respectively connected with the bias voltage generating circuit and the disturbing signal generating circuit and is used for combining the bias voltage and the disturbing signal and outputting a combined signal;
and the low-pass filter is connected with the superposition circuit and is used for filtering the combined signal.
Optionally, the signal acquisition module includes a conversion circuit, a band-pass filter circuit, and an analog-to-digital converter matching circuit;
the conversion circuit is connected with the photoelectric detector and is used for converting the current signal into a first voltage signal;
the band-pass filter circuit is connected with the conversion circuit and is used for filtering the first voltage signal to obtain a second voltage signal;
and the analog-to-digital converter matching circuit is connected with the band-pass filter circuit and is used for converting the second voltage signal into a voltage signal within a set threshold range.
Optionally, the conversion circuit comprises a T-type feedback network and a current-to-voltage conversion circuit;
the current-voltage conversion circuit is connected with the photoelectric detector and is used for converting the current signal into an initial voltage signal;
the T-shaped feedback network is connected with the current-voltage conversion circuit and used for amplifying the initial voltage signal to obtain a first voltage signal.
Optionally, the band pass filter circuit is a fourth order butterworth band pass filter.
An MZ modulator bias point control method applied to the MZ modulator bias point control system includes:
establishing a harmonic ratio table according to a function curve of the modulated harmonic ratio of the disturbance signal and the bias voltage;
acquiring a voltage signal output by a signal acquisition module;
applying a signal cutting algorithm to the voltage signals to obtain signal sequences with a set number of cycles;
scaling the signal sequences with the set number of cycles to obtain signals with set sequence length;
applying a linear interpolation algorithm to the signal with the set sequence length to obtain a reconstructed signal;
applying a radix-2 FFT to the reconstructed signal to obtain a first harmonic component and a second harmonic component;
taking a ratio of the first harmonic component and the second harmonic component to obtain an initial harmonic ratio;
and searching the harmonic ratio table according to the initial harmonic ratio and the bias voltage, and determining the position of a bias point.
Optionally, the searching for a corresponding harmonic ratio table according to the initial harmonic ratio and the bias voltage to determine a position of the bias point specifically includes:
adjusting the bias voltage according to the initial harmonic ratio to obtain an updated bias voltage and target harmonic ratio;
and searching a corresponding harmonic ratio table according to the monotonicity of the target harmonic ratio and the updated bias voltage, and determining the position of the bias point.
According to the specific embodiment provided by the invention, the invention discloses the following technical effects:
the invention provides a MZ modulator bias point control system, which comprises: the device comprises a bias voltage output module, a photoelectric detector, a signal acquisition module and a control module; the bias voltage output module is connected with the target modulator and used for applying a disturbance signal and bias voltage to the target modulator; the photoelectric detector is arranged on an optical path of the optical signal output by the target modulator and used for converting the optical signal output by the target modulator into a current signal; the signal acquisition module is connected with the photoelectric detector and used for converting the current signal into a voltage signal within a set threshold range; and the control module is respectively connected with the signal acquisition module and the bias voltage output module and is used for controlling the bias voltage output module to output a disturbance signal and bias voltage, adjusting the bias voltage according to the voltage signal and determining the position of the bias point of the target modulator according to the voltage signal and the bias voltage. The control module controls the output of the bias voltage output module according to the disturbance signal and the data fed back by the initial bias voltage, so that the closed-loop control of the MZ modulator bias point control system 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 structural diagram of an MZ electro-optic modulator of the present invention;
FIG. 2 is a schematic diagram of the modulation principle at the positive Q point in the present invention;
FIG. 3 is a schematic diagram illustrating the modulation principle of minimum point modulation in the present invention;
FIG. 4 is a schematic diagram illustrating the modulation principle of the present invention when modulating at any point;
FIG. 5 is a schematic diagram of bias point drift in the present invention;
FIG. 6 shows the harmonic values according to the present invention
A trend graph of changes;
FIG. 7 is a graph of the ratio of the first harmonic to the second harmonic of the present invention
A trend graph of changes;
FIG. 8 is a block diagram of the overall hardware system of the present invention;
FIG. 9 is a schematic diagram of the connection of modules in the present invention;
FIG. 10 is a flow chart of a method for controlling the bias point of the MZ modulator provided by the present invention;
FIG. 11 is a block diagram of the overall algorithm and process of the present invention;
FIG. 12 is a signal sequence of the present invention comprising exactly 32 complete cycles;
FIG. 13 is a signal sequence of the present invention comprising more than 32 complete cycles;
FIG. 14 is a block diagram of a signal clipping algorithm of the present invention;
FIG. 15 is a schematic diagram of a linear interpolation algorithm of the present invention;
FIG. 16 is a graph of the ratio of the first harmonic to the second harmonic of the present invention as a function of V DC A trend graph of changes;
FIG. 17 is a block diagram illustrating a process for determining the position of a bias point according to the present invention;
FIG. 18 is a block diagram illustrating a process for locating offset point locations by a lookup table method according to the present invention;
fig. 19 is a block diagram of the positive Q feedback control flow in 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 bias point control system and a method of an MZ modulator, which can enable the MZ modulator to reliably work at a set bias point.
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.
As shown in fig. 1, in most of the push-pull MZ modulators on the market, the rf terminal and the dc bias terminal are designed as one, i.e. the voltage signals V applied to the two arms 1 And V 2 If they are the same, let V 1 ＝-V 2 ＝V/2＝[V 0 sin(ω 0 t+φ 0 )+V DC ]2, the normalized transfer function T of such a push-pull MZ modulator can be expressed as:
the modulation principle of the MZ modulator can be derived from the above formula, and when the bias point is at Q point, minimum point (maximum points are similar), and arbitrary point, the modulation principle is as shown in fig. 2, fig. 3, and fig. 4.
The phase difference between the two arms of the MZ modulator is affected by the applied electrical signal, such as the schematic diagrams of the modulation shown in fig. 2, fig. 3 and fig. 4, and also affected by the external environment (most importantly, temperature), which may cause the MZ modulator to have a drift of the bias point. The drift phenomenon is derived as follows:
suppose V = V RF (t)+V DC Then T can be expressed as:
the drifting condition of the MZ modulator can be approximately regarded as that a DC bias voltage V is additionally added to the bias end of the MZ modulator D Then, the normalized transfer function at this time is:
wherein the content of the first and second substances,
representing the portion of the phase difference between the two arms that is caused by the occurrence of phase drift. When the tuning signal is an analog signal, the bias point can be set at the positive Q point, the added rf voltage signal is a sine wave, and the schematic diagram of the bias point drift is shown in fig. 5. In fig. 5, normal Q-point modulation and Q-point modulation after drift are shown, and it can be seen from fig. 5 that when the bias point is not located at Q-point, the signal modulation is distorted.
The relationship between the output optical power and the input optical power of the MZ modulator is as follows:
in practical applications, the output optical power can be expressed as:
Will be provided with
Taken into the output optical power formula, the output optical power expression can be simplified as:
suppose that the disturbance signal V added at this time LF (t) is: v LF (t)＝V DB sin (ω t + θ (ω)); wherein, V DB To perturb the signal amplitude, θ (ω) is the modulation signal phase response delay caused by the microwave-optical velocity mismatch.
Wherein m is LF ＝πV DB /V π . The amplitude of the moving signal is several tens of mV, while the half-wave voltage of the MZ modulator is generally at the level of several V, i.e. m LF Is a very small number, much less than 1. The output optical power expression is expanded by a trigonometric function as:
due to m LF Much less than 1, so the taylor stage evolves as:
converting output light power into photocurrent by a photoelectric detector, extracting a first harmonic component and a second harmonic component in the photocurrent by assuming that the responsivity is eta, and solving the ratio R of the second harmonic component to the first harmonic component 2/1 And then:
accordingly, the ratio R of the first harmonic component to the second harmonic component 1/2 The method comprises the following steps:
the first harmonic and the second harmonic and their ratio and phase
The relationship (which may represent a bias point) is shown in fig. 6 and 7. As can be seen from FIG. 7, the ratio R of the first harmonic to the second harmonic in the interval from the maximum point to the minimum point of the bias point and in the interval from the minimum point to the maximum point of the bias point 2/1 And R 1/2 And offset point position
All the relations of (1) are monotonous, that is, in a certain interval, only the current R needs to be judged 2/1 Or R 1/2 The position of the current bias point can be judged, and the control can be correspondingly carried out on any specific bias point.
FIG. 8 is a block diagram of the overall hardware system of the present invention; FIG. 9 is a schematic diagram of the connection of modules in the present invention; as shown in fig. 8 and 9, the present invention provides an MZ modulator bias point control system, the system comprising: the device comprises a bias voltage output module, a photoelectric detector, a signal acquisition module and a control module.
The bias voltage output module is connected with the target modulator and used for applying a disturbance signal and bias voltage to the target modulator; specifically, the bias voltage output module comprises a bias voltage generating circuit, a disturbing signal generating circuit, a superposition circuit and a low-pass filter.
The bias voltage generating circuit is connected with the control module and used for outputting bias voltage according to a bias control signal output by the control module; the disturbance signal generating circuit is connected with the control module and used for outputting a disturbance signal according to the disturbance control signal output by the control module; the superposition circuit is respectively connected with the bias voltage generation circuit and the disturbance signal generation circuit and is used for combining the bias voltage and the disturbance signal and outputting a combined signal; and the low-pass filter is connected with the superposition circuit and is used for filtering the combined signal.
In practical application, the bias voltage output module has the function of transmitting bias voltage and disturbance signals to a bias end of the MZ modulator in a superposition manner, so that the module mainly comprises three parts, wherein the first part is a bias voltage generating circuit which changes a bias voltage value calculated by a DSP into an Analog value by using a Digital to Analog Converter (DAC), and the DAC can adopt parallel communication and has higher transmission speed; the second part is a disturbing signal generating circuit which is mainly responsible for generating a disturbing signal meeting the requirement, the adopted chips are a direct digital frequency synthesizer (DDS) and a program-controlled potentiometer, the DDS chip generates a sine wave signal with adjustable frequency, the program-controlled potentiometer is used for dividing the voltage to control the amplitude of the sine wave signal, and if the frequency amplitude is adjustable, the disturbing signal can be adjusted to an optimal value when MZ modulators of different types are used; the third part is a superposition circuit, and since the bias voltage generated by the DAC and the disturbance signal are added to the bias end of the MZ modulator together, the two signals need to be combined by using the superposition circuit, and the signal-to-noise ratio of the combined signal can be higher by adding a low-pass filter at the last.
The photoelectric detector is arranged on an optical path of the optical signal output by the target modulator and used for converting the optical signal output by the target modulator into a current signal.
The signal acquisition module is connected with the photoelectric detector and used for converting the current signal into a voltage signal within a set threshold range. Specifically, the signal acquisition module comprises a conversion circuit, a band-pass filter circuit and an analog-to-digital converter matching circuit.
The conversion circuit is connected with the photoelectric detector and is used for converting the current signal into a first voltage signal; specifically, the conversion circuit comprises a T-shaped feedback network and a current-voltage conversion circuit; the current-voltage conversion circuit is connected with the photoelectric detector and is used for converting the current signal into an initial voltage signal; the T-shaped feedback network is connected with the current-voltage conversion circuit and used for amplifying the initial voltage signal to obtain a first voltage signal. The conversion circuit is designed into a T-shaped feedback network, the network can realize high-magnification times by using a low-resistance feedback resistor, and the temperature drift of the low-resistance feedback resistor is small, so that the conversion circuit can have higher precision; the band-pass filter circuit is connected with the conversion circuit and is used for filtering the first voltage signal to obtain a second voltage signal; the band-pass filter circuit is a fourth-order Butterworth band-pass filter. The part has the function of reserving the component of the output optical power changed by the disturbing signal, adopts a 4-order Butterworth band-pass filter, has a flatter pass band and a higher interference suppression level, and better reserves the component of a second harmonic; and the analog-to-digital converter matching circuit is connected with the band-pass filter circuit and is used for converting the second voltage signal into a voltage signal within a set threshold range. Since the ADC of the DSP has a limitation on the input sampling voltage range, the obtained second voltage signal needs to be processed into a signal satisfying the input voltage range of the ADC.
The control module is respectively connected with the signal acquisition module and the bias voltage output module and is used for controlling the bias voltage output module to output the disturbance signal and the bias voltage, adjusting the bias voltage according to the voltage signal and determining the position of the bias point of the target modulator according to the voltage signal and the bias voltage. Specifically, the control module is a minimum system module of a DSP. The minimum system module of the DSP is mainly to set a corresponding peripheral circuit for the DSP to meet the normal operation of each function of the DSP, for example, a crystal oscillator provides a clock, a Joint Test Action Group (JTAG) interface is responsible for downloading a program, a serial port is responsible for sending data to a computer, and the like.
In addition, the system further comprises a power conversion module; the power supply conversion module is respectively connected with the bias voltage output module, the signal acquisition module and the control module and is used for providing corresponding power supply voltage for the bias voltage output module, the signal acquisition module and the control module.
Specifically, the power conversion module comprises a positive and negative 12V voltage conversion circuit, a positive and negative 5V voltage conversion circuit, a 3.3V voltage conversion circuit and a 1.8V voltage conversion circuit.
The positive and negative 12V voltage conversion circuit outputs +/-12V voltage by adopting a DCDC circuit and is used for supplying power to the bias voltage output module; the positive and negative 5V voltage conversion circuit outputs +/-5V voltage by adopting a DCDC circuit and is used for supplying power to the signal acquisition module; the 3.3V voltage conversion circuit outputs 3.3V voltage by adopting an LDO voltage stabilizer; the 1.8V voltage conversion circuit outputs 1.8V voltage by adopting an LDO voltage stabilizer; the 3.3V voltage conversion circuit and the 1.8V voltage conversion circuit are used for supplying power to the control module.
In practical application, the system further comprises a power conversion module, and each part of the module needs to be driven by a different voltage value, so that the power supply voltage of the hardware system needs to be converted into a different voltage value for each module to use. The power supply of the power supply conversion module is 15V, and the power supply voltage needs to be converted into +/-12V (power supply of the bias voltage output module), +/-5V (power supply of the signal acquisition module), 3.3V (power supply of the DSP and power supply of an active crystal oscillator in the disturbing signal generating circuit) and 1.8V (power supply of the DSP). Wherein, the voltages of +/-12V and +/-5V are larger, a DCDC chip is used, and the voltages of 3.3V and 1.8V are smaller, an LDO chip is used. Because the noise of the DCDC chip is too large, a multi-stage filtering mode needs to be designed to suppress the power supply noise, pi-type LC filtering is added at the output port of the power supply voltage, 22uF electrolytic capacitor and 100nF ceramic capacitor are used at the input end of the DCDC chip for filtering, and magnetic beads and 22uF electrolytic capacitor are used at the output end of the DCDC chip for filtering.
The system further includes a beam splitter; the optical splitter is arranged on an optical path of the optical signal output by the target modulator and is used for splitting the optical signal output by the target modulator into an optical signal for work and an optical signal for analysis; and the optical signal output by the target modulator enters the photoelectric detector after passing through the optical splitter.
In practical application, a sine wave disturbing signal with the amplitude of 50mV and the frequency of 1kHz is added to a bias point of the modulator, so that the disturbing signal influences the output optical power of the modulator (the process is similar to the modulation principle). Then an optical splitter was used to split the output light into one 99% (for normal operation) and one 1% (for analysis of the optical signal for control); then, converting 1% of a beam of optical signals into current signals by using a photoelectric detector, processing the current signals by using a circuit, converting the current signals into voltage signals, and acquiring the signals into a DSP through an ADC (analog to digital converter) of a DSP chip; the signal in the DSP is a digital signal, the first harmonic and the second harmonic can be obtained by FFT conversion, the ratio and the phase of the harmonic are in one-to-one correspondence, and the current position of the offset point can be obtained by analysis; finally, by means of the position information, the drift of the bias point can be corrected by correspondingly changing the bias voltage, and the bias point is locked to the set bias point.
As shown in fig. 9, the design of the hardware system of the present invention is also important in the overall layout design of the PCB, which also affects the accuracy of the hardware system. According to the functions of all parts of the whole hardware system and the difference of all parts on the noise suppression requirements, the signal acquisition module, the DSP minimum system module, the bias voltage output module and the power supply conversion module are separated and processed respectively during the layout of the PCB, so that the modules are prevented from interfering with each other. And the mode of isolating each part of the module ground adopts a mode of connecting single points. The connection between the grounds is through a 0 Ω resistor and the connection between the power supplies is through magnetic beads. The 0 omega resistor can effectively limit the loop current, so that the noise is suppressed. And the voltage output by the power supply conversion module is transmitted to each part of module through the magnetic beads, and the magnetic beads are equivalent to band-stop wave limiters and can also play a role in inhibiting noise.
In addition, in addition to the need to solve the interference of the hardware system itself, various electromagnetic wave interferences other than the hardware system also need to be handled. Since the effective signal component transmitted in the signal acquisition module is small, the signal acquisition module is most easily interfered by external electromagnetic waves, and therefore the module needs to be processed. The signal processing module is arranged on the upper layer of the PCB, the ground of the module should avoid the elements and the wires of the module when the module is paved with copper, a shielding cover connected to the ground is designed to cover the whole module, and the lower layer of the PCB should be paved with copper completely to cover the whole signal processing module.
Example two
In order to implement a corresponding system of the above-mentioned embodiments to achieve the corresponding functions and technical effects, the following provides a MZ modulator bias point control method, as shown in fig. 10, the method comprising:
step S1: and establishing a harmonic ratio table according to a function curve of the modulated harmonic ratio of the disturbance signal and the bias voltage.
Step S2: and acquiring a voltage signal output by the signal acquisition module.
And step S3: and applying a signal cutting algorithm to the voltage signals to obtain a signal sequence with a set number of cycles.
And step S4: and scaling the signal sequences with the set number of cycles to obtain signals with set sequence lengths.
Step S5: and applying a linear interpolation algorithm to the signals with the set sequence length to obtain reconstructed signals.
Step S6: and applying a basis 2FFT (fast Fourier transform) to the reconstructed signal to obtain a first harmonic component and a second harmonic component.
Step S7: and taking a ratio of the first harmonic component and the second harmonic component to obtain an initial harmonic ratio.
Step S8: and searching the harmonic ratio table according to the initial harmonic ratio and the bias voltage, and determining the position of a bias point.
S8 specifically comprises the following steps:
step S81: and adjusting the bias voltage according to the initial harmonic ratio to obtain an updated bias voltage and target harmonic ratio.
Step S82: and searching a corresponding harmonic ratio table according to the monotonicity of the target harmonic ratio and the updated bias voltage, and determining the position of the bias point.
The control chip selected by the invention is a DSP chip, so the control chip is programmed by adopting C language. The overall architecture of the designed algorithm and program is shown in fig. 11. The MZ modulator bias point control method provided by the invention is based on programming, and the programming is divided into 4 main modules, wherein the main modules comprise a hardware initialization module, a sampling and signal preprocessing module, an FFT conversion module and a feedback control algorithm.
The hardware initialization module is mainly used for resource initialization of the DSP, initialization of the bias voltage generation module and initialization of the sine wave generation module, wherein the resource initialization of the DSP is realized by directly calling a system function, the initialization of the bias voltage generation module mainly needs to control the DAC, the voltage of the DAC is directly adjusted to 0 by pulling up and pulling down the level of a DAC data input pin, the chips needing to be controlled by the sine wave generation module are a DDS chip and a program control potentiometer chip, and the chips are controlled through an SPI protocol.
The sampling and signal preprocessing algorithm module is used to set the sampling interval time of the ADC, and ensure that the normal function of sampling signals can be completed, but because there may be a problem that the frequency of the disturbing signal is inaccurate or the sampling frequency is inaccurate, the sampled signal cannot contain a complete signal period, as shown in fig. 12 and 13, and when the signal sequence cannot just contain a complete signal period, the FFT may have a spectrum leakage problem, that is, the extracted values of the first and second harmonics may be inaccurate, and therefore an algorithm is required to correct the problem. To correct for this problem, the present invention employs a signal reconstruction algorithm. The flow of the signal reconstruction algorithm is as follows: 1. firstly, a sampled signal sequence needs to be elongated, then a signal cutting algorithm is used for cutting out the signal for 32 complete cycles, and the starting point and the end point of the interval are recorded. 2. The signal sequence is cut out, the signal can be reconstructed, and the length of the signal sequence is scaled by a linear interpolation algorithm to the length of the signal sequence required by the FFT.
Fig. 14 is a block diagram of a signal clipping algorithm process in the present invention, as shown in fig. 14, the process can be briefly described as follows: the maximum point in the first period is found and is used as the starting point (also the starting point of the first period), then 32 points (the length of one period when stable) are behind the point, and the maximum value is found in 7 points before and after the point, which is the starting point of the second period, and the like according to the scheme, so that the starting point of each period can be found. And when the starting point of the last period is found, finding a period from the beginning to the end point of the whole cutting sequence.
The process of reconstructing the new sequence can be briefly described as follows: and mapping the clipped signal to a null signal sequence with a set length, namely multiplying each sequence point by a scaling length coefficient, wherein the probability of the mapped point is not an integer, and a linear interpolation algorithm is needed to estimate the value of an integer point near the point. The process of the linear interpolation algorithm is shown in fig. 15. In fig. 15, (x, y) is the integer point to be calculated, (x 0, y 0), (x 1, y 1) is the two points mapped by the clipping sequence, and the integer point can be calculated by the following formula:
the FFT module needs to perform time-frequency domain conversion on the clipped signal sequence, and finds signal amplitudes of 1kHz and 2kHz in the frequency domain, which are components of the first harmonic and the second harmonic. When the Discrete Fourier Transform (DFT) is used for processing, the calculation amount is relatively large, and when the DFT is operated in a hardware device such as a DSP, not only too many resources are occupied, but also the speed of the whole control program is slowed down, so that the FFT algorithm needs to be used for calculation more quickly. Therefore, the radix-2 FFT transform employed by the FFT module has a faster speed than the DFT.
After the values of the first harmonic and the second harmonic are obtained, the ratio of the values can be used for judging the position of the current bias point. Since the harmonic value calculated in the DSP is not a negative number, and since the phase is correlated with the dc bias voltage, the relationship between the harmonic ratio and the bias voltage is as shown in fig. 16. Since a harmonic ratio may correspond to a plurality of bias voltage values, it is necessary to partition the harmonic ratio first and determine which partition the bias point is in. The maximum point to the negative Q point is recorded as an AQ section, the negative Q point to the minimum point is recorded as a QI section, the minimum point to the positive Q point is recorded as an IQ section, the positive Q point to the maximum point is recorded as an QA section, and through the subareas, a harmonic ratio on each subarea only corresponds to one bias voltage value. And the judgment of the partition where the current bias point is located also needs the strength information of the current average optical power, and here, the judgment can be known only by sampling the output result of the IV conversion circuit.
When only the approximate subarea where the bias point is located currently, the bias voltage can be adjusted to the set bias point only by gradually increasing or decreasing the magnitude of the bias voltage through a voltage scanning method if the bias point is controlled. Assuming that the preset bias point is now the point Q, the flow of the determination is shown in fig. 17. However, such a scheme is slow and requires multiple adjustments to stabilize the bias point. In order to realize rapid control, the accurate position of the bias point needs to be located, and harmonic information of the whole interval from the first maximum point to the second maximum point is needed. After the harmonic information is established into the table, the position of the offset point can be accurately positioned only by looking up the harmonic ratio table, and the difference value of the offset voltage relative to the preset offset point is correspondingly increased or decreased, so that the offset point is quickly pulled to the preset offset point.
Specifically, the process of establishing the harmonic ratio table is as follows: 1. the bias voltage is controlled to be increased from small to large by adopting a voltage scanning mode, the average optical power intensity at the moment and the current bias voltage are recorded in the process, and the recorded average optical power intensity needs to contain two maximum values, namely two maximum points. 2. After the average optical power intensity and the current bias voltage are obtained, the transmission function of the MZ modulator can be obtained through a data fitting mode. 3. And simulating a disturbance signal on a computer, simulating a harmonic ratio obtained by adding the disturbance signal to each bias point, and storing the harmonic ratio in sections to obtain the relation between the harmonic ratio and the relative bias voltage. 4. Storing this relationship in the DSP chip creates a harmonic ratio table.
After the harmonic ratio table is established, the position of the current bias point can be judged by looking up the harmonic ratio table, and then the voltage is increased or decreased correspondingly, so that the bias voltage can be quickly pulled to be close to the preset bias point, and then fine tuning is carried out by means of voltage fine scanning. As shown in fig. 18, since the harmonic ratio values in each segment table are monotonous, the harmonic ratio tables are arranged from small to large and can be searched by bisection, or taking the positive Q point control as an example, assuming that the length of each segment table is N, a certain harmonic ratio table is represented by T, and R is represented by R 2/1 [x](0. Ltoreq. X. Ltoreq.N-1) represents the harmonic ratio at the x point, V DC [x]Representing the relative bias voltage magnitude at point x.
Fig. 19 is a block diagram of the positive Q feedback control flow in the present invention, and as shown in fig. 19, the positive Q feedback control flow is as follows:
(1) Determination of Q Point R Q ＝R 2/1 And the average optical power P Q ＝P out 。
(2) Looking up the harmonic ratio table to obtain V Q 。
(3) The bias voltage code is initialized.
(4) Calculating the current R 2/1 And P out 。
(5) Judgment of P out -P Q Whether greater than 0.2.
(6) If P is out -P Q If the value is greater than 0.2, the table look-up obtains V P 。
(7) Bias voltage code = V Q -V P + the current bias voltage code. And (4) is executed.
(8) If P is out -P Q And if not more than 0.2, judging the position of the current offset point.
(9) And judging whether the current offset point position is in the QA section or not.
(10) And if the current bias point position is in the QA section, changing the bias voltage code = the current bias voltage code-1-1. And (4) executing.
(11) And if the current bias point position is not in the QA section, changing the bias voltage code = the current bias voltage code-1+1. And (4) is executed.
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. An MZ modulator bias point control system, the system comprising: the device comprises a bias voltage output module, a photoelectric detector, a signal acquisition module and a control module;
the bias voltage output module is connected with the target modulator and used for applying a disturbance signal and bias voltage to the target modulator;
the photoelectric detector is arranged on an optical path of an optical signal output by the target modulator and used for converting the optical signal output by the target modulator into a current signal;
the signal acquisition module is connected with the photoelectric detector and is used for converting the current signal into a voltage signal within a set threshold range;
the control module is respectively connected with the signal acquisition module and the bias voltage output module and is used for controlling the bias voltage output module to output the disturbance signal and the bias voltage, adjusting the bias voltage according to the voltage signal and determining the position of the bias point of the target modulator according to the voltage signal and the bias voltage.
2. The MZ modulator bias point control system of claim 1, further comprising a power conversion module; the power supply conversion module is respectively connected with the bias voltage output module, the signal acquisition module and the control module and is used for providing corresponding power supply voltage for the bias voltage output module, the signal acquisition module and the control module.
3. The MZ modulator bias point control system of claim 2, wherein the power conversion module comprises a positive-negative 12V voltage conversion circuit, a positive-negative 5V voltage conversion circuit, a 3.3V voltage conversion circuit, and a 1.8V voltage conversion circuit;
the positive and negative 12V voltage conversion circuit outputs +/-12V voltage by adopting a DCDC circuit and is used for supplying power to the bias voltage output module;
the positive and negative 5V voltage conversion circuit outputs +/-5V voltage by adopting a DCDC circuit and is used for supplying power to the signal acquisition module;
the 3.3V voltage conversion circuit outputs 3.3V voltage by adopting an LDO voltage stabilizer;
the 1.8V voltage conversion circuit outputs 1.8V voltage by adopting an LDO voltage stabilizer;
the 3.3V voltage conversion circuit and the 1.8V voltage conversion circuit are used for supplying power to the control module.
4. The MZ modulator bias point control system of claim 1, further comprising an optical splitter; the optical splitter is arranged on an optical path of the optical signal output by the target modulator and is used for splitting the optical signal output by the target modulator into an optical signal for work and an optical signal for analysis;
and the optical signal output by the target modulator enters the photoelectric detector after passing through the optical splitter.
5. The MZ modulator bias point control system of claim 1, wherein the bias voltage output module comprises a bias voltage generating circuit, a disturbing signal generating circuit, a superimposing circuit, and a low-pass filter;
the bias voltage generating circuit is connected with the control module and used for outputting bias voltage according to the bias control signal output by the control module;
the disturbance signal generating circuit is connected with the control module and used for outputting a disturbance signal according to the disturbance control signal output by the control module;
the superposition circuit is respectively connected with the bias voltage generation circuit and the disturbance signal generation circuit and is used for combining the bias voltage and the disturbance signal and outputting a combined signal;
and the low-pass filter is connected with the superposition circuit and is used for filtering the combined signal.
6. The MZ modulator bias point control system of claim 1, wherein the signal acquisition module comprises a conversion circuit, a band-pass filter circuit and an analog-to-digital converter matching circuit;
the conversion circuit is connected with the photoelectric detector and is used for converting the current signal into a first voltage signal;
the band-pass filter circuit is connected with the conversion circuit and is used for filtering the first voltage signal to obtain a second voltage signal;
and the analog-to-digital converter matching circuit is connected with the band-pass filter circuit and is used for converting the second voltage signal into a voltage signal within a set threshold range.
7. The MZ modulator bias point control system of claim 6, wherein said conversion circuit comprises a T-type feedback network and a current to voltage conversion circuit;
the current-voltage conversion circuit is connected with the photoelectric detector and is used for converting the current signal into an initial voltage signal;
the T-shaped feedback network is connected with the current-voltage conversion circuit and used for amplifying the initial voltage signal to obtain a first voltage signal.
8. The MZ-modulator bias point control system of claim 6, wherein said band-pass filter circuit is a fourth order Butterworth band-pass filter.
9. A method of MZ modulator bias point control, the method comprising:
establishing a harmonic ratio table according to a function curve of the modulated harmonic ratio of the disturbance signal and the bias voltage;
acquiring a voltage signal output by a signal acquisition module;
applying a signal cutting algorithm to the voltage signals to obtain signal sequences with a set number of cycles;
scaling the signal sequences with the set number of cycles to obtain signals with set sequence length;
applying a linear interpolation algorithm to the signals with the set sequence length to obtain reconstructed signals;
applying a radix-2 FFT to the reconstructed signal to obtain a first harmonic component and a second harmonic component;
taking a ratio of the first harmonic component and the second harmonic component to obtain an initial harmonic ratio;
and searching the harmonic ratio table according to the initial harmonic ratio and the bias voltage, and determining the position of a bias point.
10. The MZ modulator bias point control method of claim 9, wherein the finding a corresponding harmonic ratio table according to the initial harmonic ratio and the bias voltage to determine a bias point position specifically comprises:
adjusting the bias voltage according to the initial harmonic ratio to obtain an updated bias voltage and target harmonic ratio;
and searching a corresponding harmonic ratio table according to the monotonicity of the target harmonic ratio and the updated bias voltage, and determining the position of the bias point.
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