CN104901746B - A kind of method that any bias point stabilization is realized according to any bias point stabilising arrangement of external modulator - Google Patents

Abstract

The invention relates to a device and method for stabilizing an arbitrary bias point of an external modulator. The device includes: a laser, a variable optical attenuator, a lithium niobate modulator, an optical coupler, a signal source, a drive amplifier, and a feedback control Part; the device completes the collection of feedback signals and the setting of bias voltage through the data acquisition board in the feedback control part, so as to realize the stability of any bias point. The invention solves the problem that the optimum bias point of the modulator drifts and the output signal deteriorates with the change of factors such as time, ambient temperature, laser light power, fiber insertion and coupling loss. The device and method can be applied in the field of optical communication, especially high-speed long-distance communication in the field of optical communication.

Description

An Arbitrary Bias Point Stabilization Device Based on External Modulator Arbitrary Bias Point Stabilization Methods

technical field

The invention relates to the field of optical communication, in particular to a device and method for stabilizing an arbitrary bias point of an external modulator.

Background technique

The most commonly used in high-speed long-distance communication is LiNbO3 (lithium niobate) Mach-Zehnder (M-Z) external modulator, which has many advantages: using traveling wave electrodes, it can obtain a high working rate; the frequency chirp of the modulated signal is very Small; the wavelength dependence of the performance is small; the optical loss is low; the electro-optical coefficient is high, and it is suitable for a variety of code types, etc. The transfer function of the M-Z external modulator is a nonlinear function, generally a periodic function. In order to avoid signal distortion, the modulator must work at the best bias point. However, the optimal bias point is not stable, and it will drift with time, ambient temperature, laser optical power, and fiber insertion and coupling loss, which will cause the output signal of the modulator to deteriorate. Therefore, it is very important to study the optimal bias point control technology of M-Z external modulator.

Usually the external modulator is stable at four commonly used operating points, but in practical applications, the best bias point for the external modulator is not limited to these four operating points. In the field of digital communication and microwave photonics, sometimes it depends on The external modulator needs to be biased at any point on the transmission curve to optimize the performance of the system.

Traditional external modulator bias point stabilization schemes are mainly divided into two categories: (1) technology based on power detection; (2) technology based on dithering signal.

The technology based on power detection uses the input optical power of the external modulator, the output optical power, or the ratio of the two as the feedback signal, and the feedback signal is closely related to the input optical power. Because in practical applications, the input optical power signal of the external modulator Fluctuations will occur, which will affect the feedback signal and cause the operating point of the external modulator to be unstable. How to overcome the influence of input optical power fluctuations on the feedback signal is a problem to be solved.

The technique based on the dithering signal mainly utilizes the harmonic signal of the dithering signal recovered from the output signal of the external modulator as the feedback signal. Compared with the technology based on power detection, the technology based on the jitter signal is relatively mature, and some merchants have launched commercial modules. However, due to the use of its harmonic signal as feedback, the external modulator can only be stabilized at four commonly used operating points. , it is impossible to offset any point.

Contents of the invention

(1) Technical problems to be solved

The technical problem to be solved by the present invention is how to make the external modulator realize the stability of any bias point.

(2) Technical solutions

In order to solve the above technical problems, the present invention provides a device for stabilizing any bias point of an external modulator, including: a laser; a variable optical attenuator, used to receive the laser light emitted by the laser, and generate a suitable optical power signal; a signal source, It is used to generate electrical signals; the drive amplifier is used to amplify the electrical signals generated by the signal source and input them into the radio frequency electrodes to generate radio frequency signals; the lithium niobate modulator is used to receive the optical power signal output by the variable optical attenuator, and The radio frequency signal output by the radio frequency electrode is modulated into an optical signal after receiving; the optical coupler is used to divide the optical signal modulated by the lithium niobate modulator into two parts, and a part of the optical signal is output through the main circuit; the feedback control part is used for Receive another part of the optical signal split by the optocoupler, including a low-pass filter, a low-frequency optical receiver, a data acquisition board with ADC (analog-to-digital converter) and DAC (digital-to-analog converter), and a computer;

In the feedback control part: a low-pass filter, used to filter out the RF (radio frequency signal) high-frequency signal in the optical signal; a low-frequency optical receiver, used to convert the optical signal output by the low-pass filter into an electrical signal, the The low-frequency optical receiver has two output ports of DC and AC, which respectively correspond to the average optical power value output by the external modulator and the low-frequency jitter signal waveform; ADC is used to collect the average optical power value output by the external modulator output by the low-frequency optical receiver and low-frequency jitter signal waveform; computer, used to process the information collected by ADC, and then set a bias voltage value according to the processing result; DAC, DAC, used to send the bias voltage value set by the computer into niobium after digital-to-analog conversion The bias voltage electrode (3) of the Lithium-Oxide modulator is used to set the bias voltage.

Preferably, the DFB laser (distributed feedback laser) is a DFB laser with a wavelength of 1550 nm.

Preferably, the signal source can generate a pseudo-random code and a clock signal.

Preferably, the low-frequency optical receiver is an optical receiver with a bandwidth of 8MHz.

Preferably, the optical coupler is a 90:10 optical coupler, 10% of the optical signal is sent to the feedback control part for feedback control, and the other 90% of the signal is output through the main circuit.

The present invention also provides a method for stabilizing an arbitrary bias point of an external modulator, comprising the following steps:

Step 1: Initialize the data acquisition board, control the DAC output bias voltage to scan the external modulator in the entire cycle, corresponding to each bias voltage value V, and obtain the DC component output by the optical receiver through the first ADC—corresponding to the external The average optical power value P _D output by the modulator is used to obtain the AC component output by the optical receiver through the second ADC—corresponding to the low-frequency jitter signal waveform S(t); the collected data is sent to the computer, and the computer records each offset The P _D value corresponding to the voltage, and the first harmonic component I _1st of S(t) is calculated by the fast algorithm of discrete Fourier transform (FFT algorithm);

Step 2: After scanning the entire cycle, the minimum value of the data collected by the first ADC is P _Dmin , and the output data of the first ADC corresponding to each bias voltage V is updated to be P _D '=P _D -P _Dmin , calculated once The ratio R of the harmonic component I _1st to P _D ';

Step 3: Find the adjacent bias voltages V1 and V2 corresponding to the two R values of zero, set V1<V2, and V1 and V2 respectively correspond to adjacent peak points on the transmission curve of the external modulator; set V1, The voltage value between V2 is equally divided according to the scan step size, and the bias voltage V and the corresponding R value corresponding to each point on the transmission curve can be obtained;

Step 4: Find the bias voltage corresponding to the preset bias point as V _D , set the bias voltage of the external modulator as V _D , and record the R value corresponding to this bias voltage;

Step 5: After a period of time, use the data acquisition board to re-acquire the first ADC and second ADC values, and send them to the computer to calculate the ratio R' of the first harmonic component to the average optical power at this time;

Step 6: Compare whether R' and R are equal, if not, change the DC bias voltage to make R'=R;

Step 7: Repeat steps 5 and 6.

(3) Beneficial effects

The device and method for stabilizing an arbitrary bias point of an external modulator in the present invention make the external modulator no longer limited to the traditional four optimal bias points, and can realize the stability of any bias point.

Description of drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. Those skilled in the art can also obtain other drawings based on these drawings without creative work.

Figure 1a: Schematic diagram of the external modulator structure;

Figure 1b: Schematic diagram of the transfer function curve of the external modulator;

Figure 2: The corresponding value P _D ' of the average power output by the external modulator, the first harmonic component I _1st of the low-frequency jitter signal waveform S(t) corresponding to each bias voltage V, and the first harmonic component I1 _st and P _D The curve schematic diagram of the ratio R of ';

Figure 3: A schematic block diagram of an arbitrary bias point stabilization device for an external modulator provided by the present invention;

In the figure: 1. Optical waveguide; 2. RF electrode; 3. Bias voltage electrode;

detailed description

Embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but should not be used to limit the scope of the present invention.

Figure 1a is a schematic diagram of the structure of the external modulator, and Figure 1b is a schematic diagram of the transfer function curve of the external modulator, and its output function is:

Where P _i is the input optical power, k is the insertion loss of the external modulator, φ ₀ is the phase shift caused by the DC bias voltage, φ ₁ is the phase shift caused by the RF signal, and Δφ is the phase shift caused by the dithering signal.

The average power output by the external modulator is

Its value is related to the amplitude of the RF signal and the jitter signal, but for a certain RF signal and jitter signal, this value is only related to the DC bias voltage. which is:

For a sinusoidal jitter signal, the resulting phase shift is:

where V _π is the modulator half-wave voltage,

Into the formula to get:

Passing the result through a low-pass filter to remove the RF signal yields

To simplify the expression, let A=(πV)/(V _π )

Trigonometric expansion:

Taylor series expansion, reserved to quadratic terms:

Tidy up

The average power output by the external modulator, the first harmonic component I _1st of S(t), and the ratio relationship between them are:

The relationship between the three is shown in Figure 2, where the triangular curve is the first harmonic curve, the circular curve is the output average optical power curve, and the smooth curve is the ratio of the two.

It can be seen that the ratio R is only a function of φ ₀ with a period of 360°. Although we can't get the exact R value in advance, because we don't know the output average optical power curve, we can adjust the DC bias voltage at the beginning of the experiment, and scan the entire cycle, and then we can get the corresponding Ratio R curve. We first find two adjacent points where the ratio R is zero, these two points correspond to the adjacent peak points on the transfer function, and then by equally dividing the two DC voltage values, we can get the The R value corresponding to any point of . Then for the R value we want to lock, we can adjust the DC bias voltage so that the R value obtained by the feedback signal is equal to our preset value, so that the lock at any point can be achieved.

In this embodiment, the laser is a DFB laser with a wavelength of 1550 nm. The laser emits 1550nm laser, and the variable optical attenuator generates a suitable optical power signal and sends it to the lithium niobate modulator. The electrical signal generated by the signal source is amplified by the drive amplifier and becomes a radio frequency signal, and the radio frequency signal enters the lithium niobate modulator. The radio frequency electrode 2 is used for modulation, the lithium niobate modulator outputs the modulated optical signal, and 10% of the optical signal is separated by the 90:10 optical coupler and sent to the feedback control part for feedback control, and the other 90% of the signal is passed through the main output.

In the feedback control part, the acquisition of the feedback signal and the setting of the bias voltage are completed by the data acquisition board. The feedback signal light first passes through a low-pass filter to filter out the RF high-frequency signal, and passes through a low-frequency optical receiver to convert the The optical signal is converted into an electrical signal. In this embodiment, an optical receiver with a bandwidth of 8MHz is used. The optical receiver has two output ports of direct current (DC) and alternating current (AC), corresponding to the average optical power value and The low-frequency jitter signal waveform is collected by two ADC acquisition modules after output, the collected data is sent to the computer, and the calculation is performed according to the method mentioned above, and then a bias voltage value is set according to the calculation result, and the DC voltage is output by the DAC module , sent to the bias voltage electrode 3 of the lithium niobate modulator for bias voltage setting.

The method for stabilizing any bias point of the external modulator proposed by the present invention comprises the following steps:

Step 1: Initialize the data acquisition board, control the DAC output bias voltage to scan the external modulator in the entire cycle, corresponding to each bias voltage value V, and obtain the DC component output by the optical receiver through the first ADC—corresponding to the external The average optical power value P _D output by the modulator is used to obtain the AC component output by the optical receiver through the second ADC—corresponding to the low-frequency jitter signal waveform S(t); the collected data is sent to the computer, and the computer records each offset The P _D value corresponding to the voltage, and the first harmonic component I _1st of S(t) is calculated by the fast algorithm of discrete Fourier transform (FFT algorithm);

Step 2: After scanning the entire cycle, the minimum value of the data collected by the first ADC is P _Dmin , and the output data of the first ADC corresponding to each bias voltage V is updated as P _D '=P _D -P _Dmin , calculated once The ratio R of the harmonic component I _1st to P _D ';

Step 3: Find the adjacent bias voltages V1 and V2 corresponding to the two R values of zero, set V1<V2, and V1 and V2 respectively correspond to adjacent peak points on the transmission curve of the external modulator; set V1, The voltage value between V2 is equally divided according to the scan step size, and the bias voltage V and the corresponding R value corresponding to each point on the transmission curve can be obtained;

Step 4: Find the bias voltage corresponding to the preset bias point as V _D , set the bias voltage of the external modulator as V _D , and record the R value corresponding to this bias voltage;

Step 5: After a period of time, use the data acquisition board to re-acquire the first ADC and second ADC values, and send them to the computer to calculate the ratio R' of the first harmonic component to the average optical power at this time;

Step 6: Compare whether R' and R are equal, if not, change the DC bias voltage to make R'=R;

Step 7: Repeat steps 5 and 6.

The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications or equivalent replacements of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all should cover Within the scope of the claims of the present invention.

Claims (1)

1. it is a kind of that the stable method of any bias point is realized according to any bias point stabilising arrangement of external modulator, wherein, the investigation mission outside the city or town System has a high regard for meaning bias point stabilising arrangement, including：Laser；Variable optical attenuator, for receiving the laser that laser sends, produces Suitable optical power signals；Signal source, for producing electric signal；Driving amplifier, the electric signal for signal source to be produced is put Input radio frequency electrode (2) after big, generates radiofrequency signal；Lithium niobate modulator, the light work(for receiving variable optical attenuator output Rate signal, and the radiofrequency signal exported by radio-frequency electrode (2), are modulated into optical signal after reception；Photo-coupler, for by niobium Optical signal after sour lithium modulators modulate is divided into two parts, and a portion optical signal is exported by main road；Feedback-system section, For receiving another part optical signal that photo-coupler is separated, the feedback-system section includes low pass filter, and low frequency light connects Receipts machine, a data collecting plate card with ADC and DAC, a computer；

In feedback-system section：Low pass filter, for filtering the RF high-frequency signals in optical signal；Low frequency photoreceiver, is used for The optical signal that low pass filter is exported is converted into electric signal, the low frequency photoreceiver has direct current and exchanges two delivery outlets, The average optical power value and low frequency dither waveform of external modulator output are corresponded to respectively；ADC, for gathering low frequency photoreceiver The average optical power value and low frequency dither waveform of the external modulator output of output；Computer, for processing what ADC was collected Information, then sets a bias voltage value according to result；DAC, for by the bias voltage value of computer installation through number The bias voltage electrode (3) that lithium niobate modulator is sent into after modular transformation is biased voltage setting；

Characterized in that, the described method comprises the following steps：

Step one：Initialization data analog input card, control DAC output bias voltages are swept to external modulator in whole cycle Retouch, correspond to each bias voltage value V, the DC component that photoreceiver is exported is obtained by an ADC --- correspondence external modulation The average optical power value P of device output_D, the AC compounent that photoreceiver is exported is obtained by the 2nd ADC --- correspondence low-frequency jitter Signal waveform S (t)；The data feeding computer that will be collected, the corresponding P of each bias voltage of computer recording_DValue, by from The fast algorithm for dissipating Fourier transform is calculated the first harmonic component I of S (t)_1st；

Step 2：After scanning through whole cycle, the data minimum value for obtaining ADC collections is P_Dmin, update each bias voltage The corresponding ADC output datas of V are P_D'=P_D-P_Dmin, it is calculated first harmonic component I_1stWith P_D' ratio R；

Step 3：Corresponding adjacent bias voltage V1, V2 when two R values are zero are found, if V1<V2, this V1, V2 is corresponded to respectively Adjacent peak point on external modulator transmission curve；V1, the magnitude of voltage between V2 are carried out decile, obtained according to scanning step The corresponding bias voltage V of every bit and corresponding R values above transmission curve；

Step 4：The corresponding bias voltage of default bias point is found for V_D, setting outer modulator bias voltage is V_D, record this inclined Put the corresponding R values of voltage；

Step 5：Through after a period of time, Usage data collection board resurveys an ADC and the 2nd ADC values, feeding is calculated Machine calculates the ratio R of now first harmonic component and average light power '；

Step 6：Compare R ' whether equal with R, if unequal, changing DC offset voltage makes R '=R；

Step 7：Repeat step five and step 6.