CN110677196A - Bias control method based on double parallel Mach-Zehnder modulator - Google Patents

Bias control method based on double parallel Mach-Zehnder modulator Download PDF

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CN110677196A
CN110677196A CN201910899181.9A CN201910899181A CN110677196A CN 110677196 A CN110677196 A CN 110677196A CN 201910899181 A CN201910899181 A CN 201910899181A CN 110677196 A CN110677196 A CN 110677196A
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bias
modulator
control
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dpmzm
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李如玮
代凯璇
杨锋
杨登才
孙兴武
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Beijing University of Technology
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/5165—Carrier suppressed; Single sideband; Double sideband or vestigial
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H04B10/54—Intensity modulation
    • H04B10/541—Digital intensity or amplitude modulation

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Abstract

A bias control method based on a double parallel Mach-Zehnder modulator relates to the field of microwave photonics, optical fiber sensing or optical communication. According to the invention, low-frequency disturbance voltage with fixed frequency is applied to three bias voltage control ends of the DPMZM, a modulator output signal is fed back to a bias voltage control module in a certain proportion, and after being processed by hardware circuits such as photoelectric conversion, filtering, amplification, sampling and the like, the bias point voltage of a branch corresponding to the DPMZM is obtained through calculation of a bias point control algorithm in the MCU. And (3) superposing and inputting the bias point voltage and three different frequency disturbance signals without overlapping the fundamental wave and the second harmonic wave to the DPMZM bias voltage control port. Because the three control ports use disturbance voltages with different frequencies, feedback information of each control port is separated in a frequency domain, independent closed-loop control of the three control ports is achieved, voltages of three bias points of the DPMZM are adjusted at the same time, and stable control of the DPMZM on any bias point is achieved.

Description

Bias control method based on double parallel Mach-Zehnder modulator
Technical Field
The invention relates to the technical fields of microwave photon technology, optical fiber sensing technology and optical communication technology, in particular to a bias control method based on a double-parallel Mach-Zehnder modulator.
Background
The Mach-Zehnder modulator is made of LiNbO3The (lithium niobate) material is made of LiNbO3 material, which is easily affected by the temperature of the environment, the optical power of the input device, the aging of the device and other factors, thereby affecting the stability of the modulator, causing the drift of the bias point of the modulator, and affecting the waveform of the output signal, so that the control of the automatic bias point can ensure that the modulator can stably play a role.
The single sideband modulation is a unique modulation mode of the DPMZM, the microwave modulation is carried out on incident laser, the working point is adjusted to inhibit other-order optical frequency components in output, only a required first-order specific sideband is reserved, and finally a single-frequency signal with high sideband inhibition after frequency shift is obtained. The use of DPMZM to produce single sideband modulation is actually a modulator operating point control problem.
At present, a time-sharing circulation control method is adopted, which is divided into three time slots, and the upper arm, the lower arm and the main arm are sequentially controlled by the bias points, but in the time-sharing control method, the working point of the main arm needs to be stabilized firstly, and then the working point of the upper arm and the working point of the lower arm need to be stabilized, so that the working point of the main arm can drift in the process of stabilizing the upper arm and the lower arm, and the accurate stable working point can not be reached, particularly in the control of the working state of any point of a modulator.
Disclosure of Invention
The invention provides a bias control method based on double parallel Mach-Zehnder modulators, which realizes bias control of a working point by adjusting different frequency signals added into three Mach-Zehnder modulators, and realizes simultaneous control of three frequency signals in the process.
The invention provides a bias control method based on a double parallel Mach-Zehnder modulator, and the whole system framework is as follows: the laser provides a light source signal to be connected with the double parallel Mach-Zehnder modulator, the radio frequency signal is connected with a radio frequency input port of the DPMZM, three different disturbance frequency signals are connected to bias ports of three sub-modulators of the DPMZM, an output end of the DPMZM is connected with the optical splitter, the optical splitter inputs 5% of the signal to the bias control module, the output of the bias control module is connected with the bias input port of the DPMZM, and the other path of signal of the optical splitter is connected to the spectrometer to observe the stable condition of the modulated signal.
The invention comprises the following steps during working:
1. firstly, a laser signal is input into an input port of the DPMZM, an output port of the DPMZM is connected with an optical splitter, and 1% -10% of the output signal is selected to be sent into a control module for subsequent processing and feedback control.
2. Initializing, carrying out direct current scanning on the bias voltages of the upper arm and the lower arm and the main arm of the Mach- delta modulator to obtain respective conversion curves, analyzing output signals of an output end part to obtain fundamental waves and second harmonics, judging the drift condition by using the ratio of the fundamental waves and the second harmonics, and obtaining the offset condition of a bias point by detecting the numerical value of a specific moment of a conversion function and comparing the numerical value with the numerical value of an initial conversion function.
3. Knowing the offset condition of the offset point, adding disturbance signals at the offset input ends of three Mach- delta modulators respectively, wherein the frequencies of the three disturbance signals meet specific conditions, frequently selecting kHz level frequency to prevent mutual overlapping crosstalk phenomenon of fundamental waves and second harmonics of three signals with different frequencies, outputting a disturbance signal at the output end of the modulator, analyzing the fundamental waves and the second harmonics of the output disturbance signal, judging the drift condition of the offset point according to the ratio of the fundamental waves and the second harmonics, and adjusting the offset point to a normal value through control feedback.
4. And (3) judging: after three different disturbance signals are added to three Mach- delta modulators of the DPMZM, the conversion function, namely the output power, of each Mach- delta modulator is as follows:
Po＝(Pi/2)[1+cos(Vd(t)+π(Vb+Vpiao)/Vπ)]in which P isoIs the output intensity of the modulator, PiIs the input intensity of the modulator, VbIs an input bias voltage, VπIs a half-wave voltage, VpiaoFor the drift voltage generated, Vd(t) is the added perturbation signal, let Vd(t)＝V0cosωt，V0Is a constant which is 1% -5% of half-wave voltage, and the frequency is about 1% of the frequency of the input signal.
We normalize its transfer function to:
Figure BDA0002211274780000021
wherein
Figure BDA0002211274780000031
The above formula is expanded by taylor series as:
Figure BDA0002211274780000032
because the amplitude of the disturbance signal is about 1% -5% of the half-wave voltage, the signal is modulated by a small signal, namely the amplitude from the wave crest to the wave trough of the disturbance signal is far less than VπIn the time, the components of the third order and above are weak, so that the components can be ignored, the first order and the second order components in the above Taylor expansion are selected, and the ratio of the first order component and the second order component is calculated.
The first and second order components are:
Figure BDA0002211274780000033
Figure BDA0002211274780000034
the ratio is as follows:
wherein
Figure BDA0002211274780000036
V0As a constant, we can see a second order component ratio R andthe correlation is obtained, the bias working point of the DPMZM is related to the ratio of the first-order harmonic component to the second-order harmonic component, so that R satisfies the cotangent relation, and therefore, the R can be known to be in the half-wave voltage VπMonotonous in the range, and therefore R can be taken as a feedback variable.
5. Feedback control
In the control circuit, each of the three sub-modulators is feedback-controlled, and each sub-modulator is respectively associated with a different conversion function at the time of initialization, and the corresponding relationship between the conversion function and the R value is known at the time of initialization, so that the R value corresponding to the offset operating point is known and compared with the R value at the previous time in each cycle control. The voltage drift condition of each path can be obtained by analyzing according to the change of the R value, and V can be obtained through an R value calculation formulapiaoThe obtained drift voltage is input into a bias voltage control port to compensate the voltage, and the bias point is controlled to be in a state before the offset, and the bias operating point is stabilized by continuously and circularly comparing the magnitude of the R value and compensating the voltage.
The invention provides a method for controlling bias of any working point of a DPMZM (dual-band Mach Zehnder interferometer), which judges the drift condition of a bias point by using a ratio of a disturbance frequency signal added at an input end to a harmonic signal after the DPMZM and controls the bias point to be at the optimal position in real time.
Compared with the prior art, the invention has the following advantages:
(1) compared with the existing method for controlling the offset point by time slots, the method avoids the drift between each time slot during time slot by simultaneously controlling the disturbance frequency signals added to the three Mach- delta modulators;
(2) the method is only influenced by the added disturbance frequency signal and is not influenced by other factors.
(3) The method enables simultaneous control of three mach- delta modulators, and control of each mach- delta modulator is not associated with the others relative to slotted control, so control of one mach- delta modulator does not cause drift in the other two modulators.
Drawings
FIG. 1 is a block diagram of the bias control module of the present invention;
FIG. 2 is a flow chart of bias control for a dual parallel Mach-Zehnder modulator;
fig. 3 is a graph showing the relationship between the conversion curve of the mach-zehnder modulator and the R value in the normal case and in the case where the drift occurs.
Detailed Description
Fig. 1 is a schematic diagram of Bias control performed by a Bias control circuit according to the present invention, and it can be seen that a feedback closed loop system is adopted, in which a Bias control voltage and a disturbance frequency signal are input from a DC Bias input end and modulated onto 1550nm laser, a small part of signals are separated from an output end of a modulator by an optical splitter, and then demodulated by a photodetector, optical signals are converted into electrical signals, which are used as input signals of a Bias point control circuit module.
As shown in fig. 1, the apparatus of the present invention comprises: a laser source, a Mach-Zehnder modulator, a radio frequency signal source, an optical splitter, a photoelectric detector and a bias control module, wherein the radio frequency signal is loaded on laser and output through the modulator, then 5% of output signals are converted into electric signals through the photoelectric detector, and then subsequent bias control module processing is carried out, FIG. 2 is a bias control module, in the bias control circuit, because the feedback signal contains interference components such as direct current, high frequency and the like, the signal is firstly isolated and filtered, then amplified, then enters a digital signal processor after analog-to-digital conversion, the signals are analyzed and processed, then the processed digital signals are converted into analog voltage signals through a digital-to-analog conversion circuit, and the analog voltage signals are loaded on the two sub-modulators of the double parallel Mach-Zehnder modulator and the DC Bias input port of the main modulator respectively to control and realize the stability of the three working points.
In this example, the method is implemented by the following steps:
the method comprises the following steps: an optical carrier is loaded into the modulator by a laser while an input radio frequency signal is modulated.
Step two: and initializing, and carrying out direct current scanning on bias voltages of the upper arm, the lower arm and the main arm to record the conversion functions of the upper arm, the lower arm and the main arm of the DPMZM in an initial state.
Step two: three different disturbance frequency signals with specific frequencies are input at DC Bias input ends of three Mach- delta modulators of a DPMZM, the frequency of an upper arm disturbance frequency signal is 1.15kHz, the frequency of a lower arm disturbance frequency signal is 1.55kHz, the frequency of a main arm disturbance frequency signal is 1.9kHz, fundamental waves of the three disturbance signal frequencies and second harmonics do not interfere with each other, and the purity of each component of an error signal in a frequency domain is guaranteed.
Step three: the method comprises the following steps of obtaining modulated optical domain disturbance signals through a modulator, separating 5% of optical signals through an optical fiber coupler, converting the optical signals into electric signals through a photoelectric detector, filtering high-frequency components through an amplifying and low-pass filter, controlling three sub-modulators through a digital processing unit part, and performing simultaneous feedback control on three input disturbance signals through the obtained feedback signals to stabilize a bias point, wherein the specific steps are as follows:
(1) first, initialization is performed to scan the bias voltages of the three sub-modulators, and the transfer function during initialization is as follows:
Po＝(Pi/2)[1+cos(πVb/Vπ)]
normalization is as follows: p ═ 1+ cos (pi V)b/Vπ) A corresponding initialization transfer curve can thus be obtained, as in the upper half of fig. 3.
(2) Three disturbance signals with different frequencies are respectively added to three sub-modulators of the DPMZM, the frequency of the upper arm disturbance signal is 1.15kHz, the frequency of the lower arm disturbance signal is 1.55kHz, and the frequency of the main arm disturbance signal is 1.9kHz, so that the fundamental wave and the second harmonic do not overlap. 5% of the output optical signal enters the DSP part of the control module through subsequent processing to obtain first-order and second-order harmonic components of the disturbance signal.
(3) The ratio of the fundamental wave to the second harmonic component of the output signal corresponding to the sub-modulator is:
Figure BDA0002211274780000061
it can be obtained that this ratio is a cotangent function, R being related to
Figure BDA0002211274780000062
Cotangent function of, therefore
Figure BDA0002211274780000063
Is in a monotonically decreasing state between 0 and pi, in a state where no drift occurs, i.e. when V ispiao＝0，
Figure BDA0002211274780000071
Then, can obtain
Figure BDA0002211274780000072
And a bias voltage VbThe conversion function P and the R value function are in positive correlation, so that the one-to-one correspondence relationship of the conversion function P and the R value function can be obtained, namely the R value function is in a monotonous state in a half-wave voltage range.
The R value is continuously measured and known:
when the value of R changes, according to R and the bias voltage VbThe drift condition is calculated through the algorithm of the control module, and a feedback voltage, namely the drift voltage generated by the sub-modulator, is obtained to automatically control the bias point so as to keep the bias point in a stable state.
(3) The other two sub-modulators realize stable control of the bias point according to the change of the ratio R of the fundamental wave to the second-order harmonic component as in the previous step.
(4) And feeding back the feedback voltage obtained in the process to three bias input ports of the DPMZM so as to perform reciprocating cycle control, and controlling any working point.
Step four: the modulated signal is connected to a spectrometer, the change condition of the signal is observed, and the modulated signal can be kept in a stable state by simultaneously controlling three disturbance frequency signals added to the bias port.

Claims (1)

1. A bias point voltage control method based on double parallel Mach-Zehnder modulators is characterized in that three disturbance signals with different frequencies are respectively added to an upper sub-modulator, a lower sub-modulator and a main sub-modulator of a DPMZM, fundamental waves and second harmonics of the three disturbance signals are enabled to meet the condition of no overlapping, a part of signals at an output end are subjected to photoelectric conversion, filtering, amplification and digital signal processing to obtain first-order frequency information and second-order frequency information of respective disturbance frequencies of the three sub-modulators, the drift condition is judged according to the ratio of the first order to the second order, a corresponding feedback voltage value, namely a drift voltage value generated by the modulator, is added to bias voltage input ports of the three sub-modulators, the working state of each sub-modulator is fed back at the same time, and the modulators are controlled to work at any working point;
in the bias voltage control circuit module, according to the relation between the ratio R of the first-order component and the second-order component of the disturbance frequency output signal and the initial conversion functions and the corresponding conversion curves of the three sub-modulators of the DPMZM, the respective drift conditions are judged, the feedback voltage is obtained and fed back to the bias input port of the sub-modulator, and the voltage feedback control of the sub-modulator is realized through the compensation of the offset voltage.
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CN112925122A (en) * 2021-01-28 2021-06-08 华中科技大学 Silicon-based Mach-Zehnder modulator bias control device and system based on pilot frequency method
CN113109951A (en) * 2021-03-22 2021-07-13 南京航空航天大学 Parallel electro-optical modulator bias point control method and device based on random signal
CN113395111A (en) * 2021-07-16 2021-09-14 中国计量大学 Quick and accurate bias voltage calibration device for double-parallel Mach-Zehnder modulator
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CN114499684A (en) * 2020-10-26 2022-05-13 青岛海信宽带多媒体技术有限公司 Method and system for controlling stability of working point of MZ modulator
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CN113109951A (en) * 2021-03-22 2021-07-13 南京航空航天大学 Parallel electro-optical modulator bias point control method and device based on random signal
CN113109951B (en) * 2021-03-22 2022-05-03 南京航空航天大学 Parallel electro-optical modulator bias point control method and device based on random signal
CN113395111A (en) * 2021-07-16 2021-09-14 中国计量大学 Quick and accurate bias voltage calibration device for double-parallel Mach-Zehnder modulator
CN115882963A (en) * 2022-12-09 2023-03-31 华中科技大学 Hidden communication device and system
CN115882963B (en) * 2022-12-09 2024-07-02 华中科技大学 Hidden communication device and system

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