Brillouin filter pumping laser frequency automatic tuning device

Abstract

The invention discloses a Brillouin filter pumping laser frequency automatic tuning device, and belongs to the technical field of information transmission and processing. The frequency-locked loop comprises a pump laser, a Brillouin filter, a coupler, a combiner, a photoelectric converter, a filtering detection module, a frequency discriminator, a loop filter, a frequency-locked monitoring module and the like, and the frequency-locked loop is used for maintaining the frequency difference between the pump laser frequency and the Brillouin filter output working signal laser frequency to be Brillouin frequency shift. Therefore, the center frequency of the Brillouin filter automatically tracks the center frequency of the signal needing to be filtered, and the drift of the frequency of the pump laser and the change of the laser frequency of the input working signal along with time are eliminated. The device integrates the advantages of a microwave technology and a photon technology, can be used for filtering various continuous single-frequency and modulated laser signals, has the remarkable advantages of low cost, small size and low power consumption, and is an important improvement on the prior art.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of information transmission and processing, and particularly relates to a Brillouin filter pump laser frequency automatic tuning device. BACKGROUND

[0002] The Brillouin filter is a laser filter with adjustable center frequency, controllable filter bandwidth and amplification gain, and has wide application in the fields of laser filtering, optoelectronic oscillator and laser amplification. In order to make the Brillouin filter have a good filtering effect on the working signal laser, the center frequency of the Brillouin filter needs to be equal to the center frequency of the working signal laser. The center frequency of the Brillouin filter is determined by the pump laser frequency of the Brillouin filter, and the change of the pump laser center frequency will lead to the change of the center frequency of the Brillouin filter. In use, the pump laser center frequency needs to follow the change of the working signal laser center frequency and maintain a fixed frequency difference (i.e. Brillouin frequency shift).

[0003] There are mainly two kinds of technologies for realizing the Brillouin filter filtering out the working signal laser at present.

[0004] The first kind is to perform electro-optical modulation on the working signal laser input into the Brillouin filter, filter out the pump frequency laser, and then pump into the Brillouin filter after amplification, wherein the frequency of the electro-optical modulation electric signal is equal to the fixed frequency difference between the pump laser frequency and the center frequency of the Brillouin filter. This technology has the advantage of automatically tracking the signal frequency of the Brillouin filter center frequency, but the power of the pump laser signal frequency filtered out after modulation is required to be much greater than the power of other frequency noise, which has certain requirements for the input laser of the Brillouin filter, the modulator and the filter, resulting in high cost and limited application scenarios. For example, this technology cannot filter out a specified comb spectrum from an optical frequency comb or filter out one of multiple signals with similar power.

[0005] The second kind is to use a low-frequency drift reference laser and a pump laser, and the pump laser output is used to drive the Brillouin filter after being modulated to increase the bandwidth. As long as the bandwidth of the pump laser is greater than the sum of the signal bandwidth and the drift range of the input laser and the pump laser, a satisfactory filtering effect can be obtained. However, this method has high requirements for the frequency stability of the laser and the power of the pump laser. For example, to filter out a specified comb spectrum from an optical frequency comb, the long-term frequency difference drift of the two lasers needs to be less than 30 MHz, and the pump laser frequency drift needs to be corrected frequently. In fact, the long-term frequency drift of the low-cost DFB laser that can be used is as high as 1 GHz or more, and the pump laser power needs to be increased by 25 times or more after being modulated to increase the bandwidth, which limits the application range of this technology. SUMMARY

[0006] In view of the above, the present application provides a Brillouin filter pump laser frequency automatic tuning device. The present application uses a frequency-locked loop to maintain the frequency difference between the pump laser frequency and the output laser signal frequency of the Brillouin filter as the Brillouin frequency shift, so that the center frequency of the Brillouin filter automatically tracks the center frequency of the signal to be filtered, and eliminates the drift of the pump laser frequency and the change of the input laser signal frequency over time. The present application combines the advantages of existing microwave technology and photon technology, and can be used for filtering various continuous single-frequency and modulated laser signals, and has the significant advantages of low cost, small size and low power consumption.

[0007] In order to achieve the above-mentioned purpose, the technical scheme provided by the present application is:

[0008] A Brillouin filter pump laser frequency automatic tuning device, comprising a frequency-locked loop composed of a Brillouin filter, a first optical coupler, a second optical coupler, an optical combiner, an optical-electric converter, a filter detection module, a frequency discriminator, a frequency adjustment module, a loop filter, a pump laser and a frequency-locked monitoring module. The automatic tuning of the pump laser frequency of the Brillouin filter is completed when the frequency-locked loop is locked.

[0009] Further, the Brillouin filter, driven by the pump laser transmitted by the second optical coupler, performs band-pass filtering and amplification on the externally input laser signal, suppresses other frequency noise, and outputs the laser working signal to the first optical coupler;

[0010] The first optical coupler divides the laser working signal into two paths, one of which enters the optical combiner, and the other of which is externally output;

[0011] The second optical coupler divides the pump laser output by the pump laser into two paths, one of which enters the optical combiner, and the other of which enters the Brillouin filter;

[0012] The optical combiner combines the outputs of the first optical coupler and the second optical coupler into one path and outputs to the optical-electric converter;

[0013] The optical-electric converter converts the frequency difference between the outputs of the pump laser and the Brillouin filter into a beat frequency electric signal and outputs to the filter detection module;

[0014] The filter detection module performs band-pass filtering on the beat frequency electric signal output by the optical-electric converter and outputs a radio frequency signal to the frequency discriminator; at the same time, the power of the radio frequency signal is detected and output to the frequency-locked monitoring module;

[0015] The frequency discriminator converts the frequency difference between the center frequency of the input radio frequency signal and the center frequency of the frequency discriminator into a proportional direct current voltage and outputs to the frequency adjustment module;

[0016] The frequency adjustment module adds the output voltage of the frequency discriminator to the DC offset voltage output by the frequency lock monitoring module to obtain a frequency discrimination signal, and adjusts the DC offset voltage so that the center frequency of the frequency discrimination signal is equal to the Brillouin frequency shift f Ω The frequency discrimination signal is output to the loop filter and the frequency lock monitoring module.

[0017] The loop filter is used to implement a negative feedback loop filter of the frequency lock loop, and under the control of the frequency lock monitoring module, the frequency lock loop is opened or closed, and a loop control voltage is output to the frequency lock monitoring module and the pump laser.

[0018] The pump laser is used to output pump laser corresponding to the frequency of the set voltage and the loop control voltage output by the frequency lock monitoring module and the loop filter to the second optical coupler under the control of the set voltage and the loop control voltage.

[0019] The frequency lock monitoring module is used to monitor and control the frequency lock loop, output a control signal to the loop filter, output a set voltage to the pump laser, and output a DC offset voltage to the frequency adjustment module.

[0020] When the frequency lock loop is locked, the center frequency of the pump laser output by the pump laser is f c +f Ω , and the center frequency of the laser operating signal output by the Brillouin filter is f c , wherein f Ω is the Brillouin frequency shift.

[0021] Further, the filter detection module includes a band-pass filter, a first splitter, and a first detector, wherein:

[0022] The band-pass filter performs band-pass filtering on the beat frequency electrical signal output by the photoelectric converter, and the filtered signal is output to the first splitter; the center frequency of the band-pass filter is the Brillouin frequency shift, and the bandwidth of the band-pass filtering is greater than the sum of the signal bandwidth, the pump laser frequency drift range per second, and the operating signal laser frequency drift range;

[0023] The first splitter divides the radio frequency signal into two paths with the same power, one of which is output to the frequency discriminator, and the other of which is output to the first detector;

[0024] The first detector converts the power of the input radio frequency signal into a direct current voltage, and outputs the signal power to the frequency lock monitoring module.

[0025] Further, the frequency discriminator includes a second splitter, a first filter, a second filter, a second detector, a third detector, and a frequency difference module, wherein:

[0026] The second splitter divides the input signal into two paths, one of which is output to the first filter, and the other of which is output to the second filter;

[0027] The first filter band-pass filters the input electrical signal, and the center frequency of the band-pass filtering is f1, and the attenuation of the frequency f Ω is δ dB on the upper transition band, and the filtered signal is output to the second detector, wherein, f Ω is the Brillouin frequency shift, and δ is a constant value between 2 and 7;

[0028] The second filter band-pass filters the input electrical signal, and the center frequency of the band-pass filtering is f2, and the attenuation of the frequency f Ω is δ dB on the lower transition band, and the filtered signal is output to the third detector;

[0029] The second detector power detects the input radio frequency signal, and outputs a direct current voltage V1 proportional to the signal power to the frequency difference module; the third detector power detects the input radio frequency signal, and outputs a direct current voltage V2 proportional to the signal power to the frequency difference module;

[0030] Alternatively, the second detector logarithmically power detects the input radio frequency signal, and outputs a direct current voltage k*lnV1 to the frequency difference module; the third detector logarithmically power detects the input radio frequency signal, and outputs a direct current voltage k*lnV2 to the frequency difference module; wherein, k is a fixed value for adjusting the frequency discrimination slope;

[0031] The frequency difference module calculates the two-way direct current input voltage: Lim[k*ln(V1 / V2)], and outputs a frequency difference signal to the frequency adjustment module, wherein Lim[] is an amplitude limiting operation;

[0032] The first filter and the second filter are band-pass filters of the same normalized prototype, and satisfy f2-f Ω = f Ω -f1, and the filtering characteristics satisfy that k*ln(V1 / V2) calculated within the application bandwidth of the frequency discriminator has a linear frequency discrimination characteristic.

[0033] Further, the frequency-locked loop monitoring module has a setting voltage table corresponding to the laser signal frequency and a direct current offset voltage table stored in advance, and the setting voltage table and the direct current offset voltage table are constructed in the following way:

[0034] (P1) setting the laser signal frequency to the lowest working frequency;

[0035] (P2) controlling the frequency-locked loop to be open-loop, adjusting the setting voltage to make the signal power detected by the filter maximum, recording the setting voltage at this time, then controlling the frequency-locked loop to be closed-loop, fine-tuning the direct current offset voltage until the radio frequency signal power detected by the filter is maximum, and recording the direct current offset voltage at this time;

[0036] (P3) increase a fixed step frequency on the current laser signal frequency, the step frequency is a fixed value between 5GHz and 10GHz, repeat step P2 until the laser signal frequency is greater than or equal to the highest working frequency, obtain the setting voltage table and the direct current offset voltage table corresponding to each laser signal frequency;

[0037] The working mode of the frequency-locked monitoring module is as follows:

[0038] (1) in the stage of starting working or changing the working signal frequency of the frequency-locked loop, the control loop filter makes the frequency-locked loop open-loop, the setting voltage corresponding to the working signal frequency is found and interpolated from the setting voltage table and output to the pump laser, the direct current offset voltage corresponding to the working signal frequency is found and interpolated from the direct current offset voltage table and output to the frequency adjustment module, and the frequency-locked loop enters the locking-in stage;

[0039] (2) in the locking-in stage of the frequency-locked loop, the frequency-locked loop remains open-loop, and a cyclic triangular wave sweep voltage is generated, the triangular wave sweep voltage is superimposed on the setting voltage and then output to the pump laser, if the sweep frequency is greater than the frequency drift range of the input laser signal and the frequency drift range of the pump laser, and the frequency adjustment discriminator output is within the frequency offset threshold, the sweep frequency is stopped, and the frequency-locked loop is closed, and the frequency-locked loop enters the locking stage; wherein the sweep frequency range is greater than the sum of the frequency drift range of the input laser signal and the frequency drift range of the pump laser, the signal power threshold is a fixed value between 2 and 4 times the noise power, and the frequency offset threshold is 1-1.5 times the frequency offset corresponding to the Brillouin filter bandwidth;

[0040] (3) in the locking stage of the frequency-locked loop, the loop control voltage and the radio frequency signal power are detected, if the loop control voltage is greater than 0.8 times the maximum value, a slow-changing compensation voltage is superimposed on the setting voltage until the loop control voltage is less than 0.1 times the maximum value, if the radio frequency signal power is detected to be lower than the signal power threshold, the frequency-locked loop is opened, and the frequency-locked loop enters the locking-in stage.

[0041] From the above description, it can be seen that the present application has the following beneficial effects:

[0042] 1. The present application realizes the automatic compensation of the signal carrier laser frequency drift and the signal frequency change by realizing the automatic alignment of the Brillouin filter center frequency to the signal center frequency based on the frequency-locked loop, and can realize high-precision signal filtering.

[0043] 2. The present application uses the signal filtered by the Brillouin filter as the reference frequency of the frequency-locked loop, can amplify the selected signal and filter out other signals in the wideband input laser containing multiple signals, can be used as a selected filter of a specified comb tooth in an optical frequency comb, or can filter and select a certain signal from the wideband input optical signal and then amplify and output.

[0044] 3. This invention provides a method for implementing a flexibly designed radio frequency discriminator that can operate on single-frequency signals, analog modulation signals, or digital modulation signals, and can be widely used in the discriminator units of various electronic devices.

[0045] 4. This invention can be implemented using mature optical and electrical devices. Equipment manufactured based on this principle has advantages such as low cost, good manufacturability, and high reliability, and can be used as a key component of microwave photonic systems. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a block diagram illustrating the principle of an automatic tuning device for a Brillouin filter-pumped laser frequency according to an embodiment of the present invention. The input laser signal in the diagram contains a center frequency f. c Useful signals and various interference signals ∑f ip After filtering and amplification by the Brillouin filter, the output frequency is f. c The useful signal. After the frequency-locked loop composed of the components in the figure is locked, the pump laser frequency of the Brillouin filter is f. Ω +f c f Ω It is Brillouin shift.

[0048] Figure 2 This is a block diagram illustrating the composition principle of a filter detection, frequency discriminator, and frequency adjustment module according to an embodiment of the present invention. The frequency discriminator shown in the diagram is particularly suitable for use in... Figure 1 The frequency-locked loop can also be used as a frequency discrimination unit in other electronic devices. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0050] An automatic tuning device for the pump laser frequency of a Brillouin filter, such as Figure 1 As shown, it includes a frequency-locking loop consisting of a Brillouin filter, a first optical coupler, a second optical coupler, an optical combiner, a photoelectric converter, a filter detection module, a frequency discriminator, a frequency adjustment module, a loop filter, a pump laser, and a frequency-locking monitoring module. When the frequency-locking loop is locked, it completes the automatic tuning of the pump laser frequency of the Brillouin filter.

[0051] The main reason for using a frequency-locked loop instead of a phase-locked loop is that the frequency-locked loop is easier to implement and has better environmental stability, and a small frequency deviation (less than 1 MHz) of the pump laser frequency does not affect the filtering performance of the Brillouin filter.

[0052] The working modes of the modules in the frequency-locked loop are as follows:

[0053] The Brillouin filter, driven by the pump laser transmitted by the second optical coupler, performs band-pass filtering and amplification on the externally input laser signal, suppresses other frequency noise, and outputs the laser working signal to the first optical coupler.

[0054] In this embodiment, the Brillouin filter is used for band-pass filtering and amplification by default, and the frequency-locked loop is in a locked state. The Brillouin filter usually uses multiple series to enhance the band-pass filtering performance. After the Brillouin fiber and the filtering series are determined, the center frequency and the bandwidth of the Brillouin filter are determined by the center frequency and the bandwidth of the pump laser. The bandwidth of the pump laser can be changed by electro-optical modulation. In this embodiment, the electro-optical modulation and the pump laser power amplifier are integrated into the Brillouin filter, and only the center frequency, the Brillouin frequency shift f Ω and the working signal are concerned. c The Brillouin frequency shift f Ω is mainly determined by the type of optical fiber and is an approximate fixed value in the range of 9 GHz to 12 GHz, which changes slightly (within tens of MHz) with the working laser frequency and the environment. For a wideband (bandwidth greater than 100 MHz) Brillouin filter, the Brillouin frequency shift f Ω can be considered as a fixed value, and for a narrowband Brillouin filter, the change of f Ω should be corrected.

[0055] The first optical coupler divides the laser working signal into two paths, one of which enters the optical combiner, and the other of which is externally output.

[0056] The second optical coupler divides the pump laser output by the pump laser into two paths, one of which enters the optical combiner, and the other of which enters the Brillouin filter.

[0057] The optical combiner combines the outputs of the first optical coupler and the second optical coupler into one path and outputs to the photoelectric converter.

[0058] The photoelectric converter converts the frequency difference between the outputs of the pump laser and the Brillouin filter into a beat frequency electrical signal and outputs to the filtering detection module.

[0059] The use of the optical combiner and the photoelectric converter has low cost; a 180° balanced detector can also be used to achieve the same function, but the cost is higher.

[0060] The filter detection module performs band-pass filtering on the beat frequency electrical signal output by the photoelectric converter, and outputs the radio frequency signal to the frequency discriminator; meanwhile, the filter detection module completes power detection of the radio frequency signal, and outputs the signal power to the frequency lock monitoring module.

[0061] Further, as shown in Figure 2 the filter detection module includes a band-pass filter, a first shunt and a first detector, wherein:

[0062] The band-pass filter performs band-pass filtering on the beat frequency electrical signal output by the photoelectric converter, and outputs the filtered signal to the first shunt; the center frequency of the band-pass filter is the Brillouin frequency shift, and the bandwidth of the band-pass filtering is greater than the sum of the signal bandwidth, the pump laser frequency drift range per second and the working signal laser frequency drift range.

[0063] The first shunt divides the radio frequency signal into two paths with the same power, one of which is output to the frequency discriminator, and the other of which is output to the first detector.

[0064] The first detector converts the power of the input radio frequency signal into a direct current voltage, and outputs the signal power to the frequency lock monitoring module.

[0065] The first detector can be implemented by a microwave detector or a logarithmic detector, and the signal power needs to be corrected for the influence of noise power when the signal-to-noise ratio is low. The optical power detection of the working signal can also be performed after the output of the first optical coupler by optical branching and photoelectric conversion, but the radio frequency branching and detection can be integrated with the band-pass filter to greatly reduce the cost and volume.

[0066] The frequency discriminator converts the frequency difference between the center frequency of the input radio frequency signal and the center frequency of the frequency discriminator into a proportional direct current voltage, and outputs the voltage to the frequency adjustment module.

[0067] There are many methods to implement the frequency discriminator, since the Brillouin frequency shift is an approximate fixed value between 9GHz and 12GHz, a common method is to frequency down-convert the radio frequency signal and then frequency discriminate at low frequency, but using frequency down-conversion increases the complexity and cost of the frequency discriminator, therefore the present application provides a low-cost method of directly implementing the frequency discriminator at a radio frequency of 9GHz to 12GHz.

[0068] Further, as shown in Figure 2 the frequency discriminator includes a second shunt, a first filter, a second filter, a second detector, a third detector and a frequency difference module, wherein:

[0069] The second shunt divides the input signal into two paths, one of which is output to the first filter, and the other of which is output to the second filter.

[0070] The first filter performs band-pass filtering on the input electrical signal, and the center frequency of the band-pass filtering is f1, and the frequency f ΩThe filtered signal is output to the second detector, where f Ω is the Brillouin shift, and δ is a constant between 2 and 7.

[0071] The second filter band-pass filters the input electrical signal, and the center frequency of the band-pass filtering is f2, and the frequency f Ω is attenuated by δ dB, and the filtered signal is output to the third detector.

[0072] The frequency range of the Brillouin shift f Ω is 9 GHz to 12 GHz, and two band-pass filters of the same normalized prototype have the same filtering characteristics, order and type, and can be implemented by a dielectric filter, a microstrip filter or a cavity filter, etc. Ideally, the filtering characteristic curves of the two band-pass filters intersect at the frequency f Ω .

[0073] The second detector power detects the input radio frequency signal, and outputs a direct current voltage V1 proportional to the signal power to the frequency difference module; the third detector power detects the input radio frequency signal, and outputs a direct current voltage V2 proportional to the signal power to the frequency difference module.

[0074] The power detection can weaken the influence of noise through the calculation of Vi=S i -N i , where i is 1 or 2, S i is the detection power of the signal with noise corresponding to Vi, and N i is the noise power of the branch detected in advance corresponding to Vi, so that the frequency discriminator works in the application with low signal-to-noise ratio.

[0075] Alternatively, the second detector logarithmically power detects the input radio frequency signal, and outputs a direct current voltage k*lnV1 to the frequency difference module; the third detector logarithmically power detects the input radio frequency signal, and outputs a direct current voltage k*lnV2 to the frequency difference module; where k is a fixed value for adjusting the frequency discrimination slope.

[0076] In the application with high signal-to-noise ratio, the influence of noise can be ignored, and the subsequent processing of logarithmic power detection is simple. In the application with low signal-to-noise ratio, the processing for eliminating noise after logarithmic power detection is relatively complex.

[0077] The frequency difference module calculates the two direct current input voltages: Lim[k*ln(V1 / V2)], and outputs a frequency difference signal to the frequency adjustment module, where Lim[] is a limiting operation.

[0078] The use of limiting can prevent the pump laser frequency from having large jitter when V1 / V2 has an abnormal large value. The use of V1 / V2 makes the frequency discrimination characteristic only related to the frequency of the input signal, and not related to the power of the input signal. The center frequencies of the two filters and the frequency difference calculation in the embodiment can be flexibly configured, such as using V2 / V1, and the sign of the frequency difference is opposite.

[0079] If the signal sample that needs to be frequency discriminated is a digital signal, the frequency discriminator of the present application can be implemented entirely using digital processing. The signal can also be detected and then sampled by AD, and the calculation of the frequency difference, the frequency adjustment, the loop filtering, and the setting of the pump laser frequency can all be implemented using digital processing.

[0080] The first filter and the second filter are bandpass filters of the same normalized prototype, and satisfy f2-f Ω Ω f1, and the filtering characteristic satisfies that k*ln(V1 / V2) calculated in the application bandwidth of the frequency discriminator has a linear frequency discrimination characteristic.

[0081] Obviously, the wider the bandpass filter, the greater the influence of input noise and the higher the possibility of being disturbed.

[0082] In the embodiment, one of the first filtering and the second filtering can use a bandpass filter, and the other can use a fixed attenuation or pass-through. The use range of the frequency discrimination can be selected to be the upper transition band or the lower transition band of the bandpass filter, and the selected transition band filtering characteristic satisfies that k*ln(V1 / V2) calculated in the application frequency range of the frequency discriminator has a linear frequency discrimination characteristic. Obviously, such processing of the filter design is simple, but the linearity of the obtained frequency discrimination characteristic is slightly worse and asymmetric.

[0083] The frequency adjustment module adds the output voltage of the frequency discriminator to the direct current offset voltage output by the frequency lock monitoring module to obtain a frequency discrimination signal, and adjusts the direct current offset voltage so that the center frequency of the frequency discrimination signal is equal to the Brillouin frequency shift f Ω The frequency discrimination signal is output to the loop filter and the frequency lock monitoring module.

[0084] In the embodiment of the present application, the frequency discrimination can be performed by the amplitude-frequency characteristics of the first filtering and the second filtering, or the center frequency of the frequency discriminator can be adjusted by adjusting the slope of the detector. Figure 2 In the embodiment of the present application, the frequency discrimination can be performed by the amplitude-frequency characteristics of the first filtering and the second filtering, or the center frequency of the frequency discriminator can be adjusted by adjusting the slope of the detector.

[0085] The loop filter is used to implement the negative feedback loop filter of the frequency lock loop, and makes the frequency lock loop open-loop or closed-loop under the control of the frequency lock monitoring module, and outputs a loop control voltage to the frequency lock monitoring module and the pump laser. ​

[0086] Pump laser, for outputting pump laser corresponding to the frequency of the set voltage and the loop control voltage under the control of the set voltage and the loop control voltage output by the frequency locking monitoring module and the loop filter.

[0087] As Figure 1 The frequency of the pump laser is determined by the set voltage and the loop control voltage.

[0088] (1) The set voltage of the pump laser is composed of the preset voltage and the sweep voltage output by the frequency locking monitoring module, wherein:

[0089] The pump laser of the embodiment uses a DFB laser, the preset voltage is divided into a temperature preset voltage and a driving current preset voltage, the temperature preset voltage can control the frequency variation of the DFB laser in a large range (more than 100 GHz), in the application of the signal working frequency in a small range (within 100 GHz), the temperature preset voltage can be fixed, only the driving current preset voltage is used, the corresponding set voltage table is looked up according to the working signal frequency, the preset voltage is calculated after interpolation from the table value, the driving current preset voltage is obtained by adding the preset voltage and the drift compensation voltage, and the DFB laser is driven to obtain the preset frequency with coarse and fine accuracy.

[0090] The pump laser of the embodiment uses a DFB laser, the frequency drift when the frequency locking loop is out of lock is in the order of 1 GHz, and the signal cannot pass through the Brillouin filter, which will cause the frequency discriminator to fail, so the sweep frequency near the preset frequency is needed to speed up the locking of the frequency locking loop. The frequency range corresponding to the sweep voltage needs to be greater than the frequency drift range, and the DFB laser is driven by the voltage obtained by adding the sweep voltage and the driving current preset voltage to complete the cyclic sweep.

[0091] (2) The loop control voltage output by the loop filter.

[0092] When the frequency locking loop is open, the loop control voltage output by the loop filter is a fixed bias voltage; when the frequency locking loop is closed and locked, the loop control voltage output by the loop filter changes with the signal and the frequency of the pump laser. The DFB laser is driven by the voltage obtained by adding the loop filter output voltage and the driving current preset voltage.

[0093] The embodiment can use an analog circuit to realize the control from the photoelectric converter to the pump laser at low cost; or digital sampling can be performed after the detector, and then the frequency difference calculation, frequency adjustment, loop filtering and pump laser frequency setting value can be realized by digital processing, and then the DAC is used to output an analog voltage to control the frequency of the pump laser. In this way, the cost is slightly higher, but digital processing has the advantages of high precision, flexible use, easy upgrading, etc.

[0094] The frequency locking monitoring module is used for monitoring and controlling the frequency locking loop, outputting a control signal to the loop filter, outputting a setting voltage to the pump laser, and outputting a direct current offset voltage to the frequency adjustment module.

[0095] The frequency locking monitoring module of the embodiment can be composed of a CPU such as an ARM or a DSP, an ADC, a DAC, a communication module, and the like. The frequency locking monitoring module completes basic monitoring functions, including: communicating with the outside, obtaining information such as the working signal frequency; collecting the working states and necessary parameters of various modules of the system such as the pump laser, the driving current amplifier, the electro-optical modulation, and the Brillouin filter, and making corresponding controls, and the like.

[0096] The filter detection module output voltage represents the power of the output signal of the Brillouin filter, the frequency adjustment module output voltage represents the frequency deviation between the center frequency of the Brillouin filter and the center frequency of the working signal, and the loop filter output voltage after the frequency locking loop is locked represents the frequency deviation between the setting frequency of the frequency locking monitoring module and the actual required setting frequency.

[0097] Further, the frequency locking monitoring module has a setting voltage table corresponding to the laser signal frequency and a direct current offset voltage table stored in advance, and the setting voltage table and the direct current offset voltage table are constructed in the following manner:

[0098] (P1) setting the laser signal frequency to the lowest working frequency;

[0099] (P2) controlling the open loop of the frequency locking loop, adjusting the setting voltage to make the signal power detected by the filter maximum, recording the setting voltage at this time, then controlling the closed loop of the frequency locking loop, and fine-tuning the direct current offset voltage until the radio frequency signal power detected by the filter is maximum, and recording the direct current offset voltage at this time;

[0100] (P3) increasing a fixed step frequency on the current laser signal frequency, the step frequency being a fixed value between 5 GHz and 10 GHz, repeating step P2 until the laser signal frequency is greater than or equal to the highest working frequency, and obtaining the setting voltage table and the direct current offset voltage table corresponding to each laser signal frequency.

[0101] The frequency of the pump laser is corrected in correspondence with the working signal frequency before the frequency locking loop works, and a preset parameter table of the pump laser corresponding to the stepped working signal frequency is established and stored. When the working signal frequency changes in a large range or the environment changes, the Brillouin frequency shift changes within tens of MHz, and the Brillouin frequency shift needs to be corrected when a narrow-band Brillouin filter is used.

[0102] The working mode of the frequency locking monitoring module is as follows:

[0103] (1) During the initial operation or change of the working signal frequency of the frequency-locked loop, the control loop filter opens the frequency-locked loop, finds and interpolates the set voltage corresponding to the working signal frequency from the set voltage table, outputs it to the pump laser, finds and interpolates the DC offset voltage corresponding to the working signal frequency from the DC offset voltage table, outputs it to the frequency adjustment module, and enters the frequency-locked loop locking stage.

[0104] Once the system is stable, the preset operating parameters of the pump laser can ensure that the pump laser's frequency operates near the sum of the signal center frequency and the Brillouin shift (with an error of less than 1 GHz).

[0105] (2) During the frequency-locked loop locking stage, the frequency-locked loop is kept open, and a cyclic triangular wave frequency sweep voltage is generated. The triangular wave frequency sweep voltage is superimposed with the set voltage and output to the pump laser. If the frequency sweep reaches the point where the power of the radio frequency signal detected by the filter exceeds the signal power threshold and the frequency adjustment frequency discrimination output is within the frequency deviation threshold, the frequency sweep stops, and the frequency-locked loop is closed, entering the frequency-locked loop locking stage. The frequency sweep range is greater than the sum of the frequency drift range of the input laser signal and the frequency drift range of the pump laser. The signal power threshold can be a fixed value between 2 and 4 times the noise power, and the frequency deviation threshold can be the frequency deviation corresponding to 1 to 1.5 times the bandwidth of the Brillouin filter.

[0106] The frequency sweep range caused by the frequency drift between the input laser and the pump laser is on the order of 1 GHz, while the bandwidth of the Brillouin filter can be as small as about 20 MHz. Therefore, frequency sweeping is required to lock the frequency-locked loop.

[0107] (3) During the frequency lock loop locking phase, the loop control voltage and the radio frequency signal power are detected. If the loop control voltage is greater than 0.8 times its maximum value, a slowly changing compensation voltage is superimposed on the set voltage until the loop control voltage is less than 0.1 times its maximum value. If the radio frequency signal power is detected to be lower than the signal power threshold, the frequency lock loop is controlled to open and enter the frequency lock loop locking phase.

[0108] Due to long-term frequency drift of the system, the loop control voltage output by the loop filter may reach its limit, causing the frequency-locked loop to lose lock. Therefore, it is necessary to adjust the loop control voltage to a smaller range before it reaches its limit. The 0.8 times and 0.1 times the maximum value of the loop control voltage mentioned are two thresholds that can be flexibly changed according to the actual situation. For example, compensation can start when the voltage exceeds 0.6 times the maximum value and stop when it falls below 0.2 times the maximum value.

[0109] When the frequency-locked loop is locked, the center frequency of the pump laser output by the pump laser is f. c +f Ω The center frequency of the laser working signal output by the Brillouin filter is f. c , where fΩ is the Brillouin frequency shift.

[0110] In summary, the present application uses a frequency-locked loop to maintain the frequency difference between the pump laser frequency and the output working signal laser frequency of the Brillouin filter as the Brillouin frequency shift, so that the center frequency of the Brillouin filter automatically tracks the center frequency of the signal to be filtered, eliminating the drift of the pump laser frequency and the change of the input working signal laser frequency over time. The present application combines the advantages of microwave technology and photon technology, and can be used for filtering various continuous single-frequency and modulated laser signals, and has the significant advantages of low cost, small size and low power consumption, which is an important improvement over the prior art.

[0111] It should be understood by those of ordinary skill in the art that the above discussion of any of the embodiments is merely exemplary and is not intended to suggest the scope of the disclosure (including claims) is limited to these examples. Any omission, modification, equivalent replacement, improvement, etc. of the above embodiments within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A device for automatic tuning of a Brillouin filter pumped laser frequency, characterized in that, The frequency-locked loop comprises a Brillouin filter, a first optical coupler, a second optical coupler, an optical combiner, an optical-electric converter, a filter detection module, a frequency discriminator, a frequency adjustment module, a loop filter, a pump laser and a frequency-locked monitoring module.

2. The apparatus according to claim 1, wherein the apparatus is characterized by: The Brillouin filter performs band-pass filtering and amplification on the externally input laser signal under the driving of the pump laser from the second optical coupler, suppresses other frequency noise, and outputs a laser working signal to the first optical coupler. The first optical coupler divides the laser working signal into two paths, one of which enters the optical combiner and the other of which is externally output. The second optical coupler divides the pump laser output by the pump laser into two paths, one of which enters the optical combiner and the other of which enters the Brillouin filter. The optical combiner combines the outputs of the first optical coupler and the second optical coupler into one path and outputs the combined path to the optical-electric converter. The optical-electric converter converts the frequency difference between the outputs of the pump laser and the Brillouin filter into a beat electric signal and outputs the beat electric signal to the filter detection module. The filter detection module performs band-pass filtering on the beat electric signal output by the optical-electric converter, outputs a radio frequency signal to the frequency discriminator, and simultaneously performs power detection on the radio frequency signal and outputs a signal power to the frequency-locked monitoring module. The frequency discriminator converts the frequency difference between the center frequency of the input radio frequency signal and the center frequency of the frequency discriminator into a proportional direct current voltage and outputs the direct current voltage to the frequency adjustment module. The frequency adjustment module adds the output voltage of the frequency discriminator to the direct current offset voltage output by the frequency lock monitoring module to obtain a frequency discrimination signal, and adjusts the direct current offset voltage so that the center frequency of the frequency discrimination signal is equal to the Brillouin frequency shift f Ω The frequency discrimination signal is output to the loop filter and the frequency lock monitoring module. The loop filter is used to realize negative feedback loop filtering of the frequency-locked loop and makes the frequency-locked loop open-loop or closed-loop under the control of the frequency-locked monitoring module and outputs a loop control voltage to the frequency-locked monitoring module and the pump laser. The pump laser is used to output pump laser with a frequency corresponding to the set voltage and the loop control voltage output by the frequency-locked monitoring module and the loop filter to the second optical coupler under the control of the set voltage and the loop control voltage. The frequency-locked monitoring module is used to realize monitoring and control of the frequency-locked loop, outputs a control signal to the loop filter, a set voltage to the pump laser and a direct current offset voltage to the frequency adjustment module. When the lock-in loop is locked, the center frequency of the pump laser output by the pump laser is f c +f Ω , the center frequency of the laser working signal output by the Brillouin filter is f c , and f Ω is the Brillouin frequency shift.

3. The apparatus according to claim 2, wherein the apparatus is characterized by: The filter detection module comprises a band-pass filter, a first shunt and a first detector. The band-pass filter performs band-pass filtering on the beat electric signal output by the optical-electric converter, and outputs the filtered signal to the first shunt. The first shunt divides the radio frequency signal into two paths with the same power, one of which is output to the frequency discriminator and the other of which is output to the first detector. The first detector converts the power of the input radio frequency signal into a direct current voltage and outputs the signal power to the frequency-locked monitoring module.

4. The apparatus according to claim 2, wherein the apparatus is characterized by: The frequency discriminator comprises a second shunt, a first filter, a second filter, a second detector, a third detector and a frequency difference module. The second shunt divides the input signal into two paths, one of which is output to the first filter and the other of which is output to the second filter. The second shunt divides the input signal into two paths, one of which is output to the first filter and the other of which is output to the second filter. The first filter performs band-pass filtering on the input electrical signal, the center frequency of the band-pass filtering being f1, and the frequency f Ω is attenuated by δdB on the upper transition band, and the filtered signal is output to the second detector, wherein, f Ω is the Brillouin frequency shift, and δ is a constant value between 2 and 7. The second filter band-pass filters the input electric signal, the center frequency of the band-pass filtering is f2, and the frequency f Ω is attenuated by δdB on the lower transition band, and the filtered signal is output to the third detector. The second detector performs power detection on the input radio frequency signal and outputs a direct current voltage V1 proportional to the signal power to the frequency difference module; the third detector performs power detection on the input radio frequency signal and outputs a direct current voltage V2 proportional to the signal power to the frequency difference module; Alternatively, the second detector performs logarithmic power detection on the input radio frequency signal and outputs a direct current voltage k*lnV1 to the frequency difference module; the third detector performs logarithmic power detection on the input radio frequency signal and outputs a direct current voltage k*lnV2 to the frequency difference module; wherein k is a fixed value for adjusting the frequency discrimination slope; The frequency difference module performs calculation on the two direct current input voltages: Lim[k*ln(V1 / V2)], and outputs a frequency difference signal to the frequency adjustment module, wherein Lim[] is an amplitude limiting operation; The first filter and the second filter are bandpass filters of the same normalized prototype, satisfying f2-f Ω = f Ω f1, and the filter characteristic satisfies a linear frequency discrimination characteristic of k*ln(V1 / V2) calculated within an application bandwidth of the frequency discriminator.

5. The apparatus according to claim 2, wherein the apparatus is characterized by: The frequency-locked loop monitoring module has a table of setting voltages corresponding to laser signal frequencies and a table of direct current offset voltages stored in advance, and the tables are constructed in the following way: (P1) set the laser signal frequency to the lowest working frequency; (P2) control the frequency-locked loop to be open-loop, adjust the setting voltage to make the signal power detected by the filter maximum, record the setting voltage at this time, then control the frequency-locked loop to be closed-loop, fine-tune the direct current offset voltage until the radio frequency signal power detected by the filter is maximum, and record the direct current offset voltage at this time; (P3) increase a fixed step frequency on the current laser signal frequency, the step frequency is a fixed value between 5GHz and 10GHz, repeat step P2 until the laser signal frequency is greater than or equal to the highest working frequency, and obtain the table of setting voltages corresponding to each laser signal frequency and the table of direct current offset voltages; The working mode of the frequency-locked loop monitoring module is as follows: (1) in the initial working or working signal frequency changing stage of the frequency-locked loop, control the loop filter to make the frequency-locked loop open-loop, find and interpolate the setting voltage corresponding to the working signal frequency from the table of setting voltages, output to the pump laser, find and interpolate the direct current offset voltage corresponding to the working signal frequency from the table of direct current offset voltages, output to the frequency adjustment module, and enter the frequency-locked loop locking-in stage; (2) in the frequency-locked loop locking-in stage, keep the frequency-locked loop open-loop, start to generate a cyclic triangular wave sweep voltage, superimpose the triangular wave sweep voltage and the setting voltage and output to the pump laser, if the radio frequency signal power detected by the filter exceeds the signal power threshold and the frequency discrimination output of the frequency adjustment is within the frequency deviation threshold during the sweep, stop the sweep, control the frequency-locked loop to be closed-loop, and enter the frequency-locked loop locking stage; wherein the sweep range is greater than the sum of the frequency drift range of the input laser signal and the frequency drift range of the pump laser, the signal power threshold is a fixed value between 2 and 4 times the noise power, and the frequency deviation threshold is 1 to 1.5 times the frequency deviation corresponding to the Brillouin filter bandwidth. (3) In the lock-in stage of the frequency-locked loop, the loop control voltage and the radio frequency signal power are detected, if the loop control voltage is greater than 0.8 times of the maximum value, a slow-changing compensation voltage is superimposed on the basis of the set voltage until the loop control voltage is less than 0.1 times of the maximum value; if the radio frequency signal power is detected to be lower than the signal power threshold, the frequency-locked loop is controlled to be open-loop, and enters the lock-in stage of the frequency-locked loop.

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