CN108963725B - Device and method for generating broadband microwave frequency comb - Google Patents

Device and method for generating broadband microwave frequency comb Download PDF

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CN108963725B
CN108963725B CN201810980277.3A CN201810980277A CN108963725B CN 108963725 B CN108963725 B CN 108963725B CN 201810980277 A CN201810980277 A CN 201810980277A CN 108963725 B CN108963725 B CN 108963725B
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frequency comb
semiconductor laser
comb
seed
optical
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CN108963725A (en
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樊利
吴正茂
夏光琼
钟祝强
邓涛
林晓东
唐曦
高子叶
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Southwest University
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S1/00—Masers, i.e. devices using stimulated emission of electromagnetic radiation in the microwave range
    • H01S1/02—Masers, i.e. devices using stimulated emission of electromagnetic radiation in the microwave range solid

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Abstract

The invention discloses a device and a method for generating a broadband microwave frequency comb, and relates to the technical field of optics.A signal source outputs a modulation signal with adjustable modulation frequency and modulation power, the modulation signal directly modulates the working current of a first semiconductor laser DFB-SL1, a regular pulse signal is output as a seed frequency comb, the regular pulse signal is injected into a second semiconductor laser DFB-SL2 in a single direction through an optical pulse injection system and an optical circulator, and the second semiconductor laser DFB-SL2 outputs a broadband microwave frequency comb signal with balanced power, pure comb line and stable frequency by utilizing the spectrum spreading effect caused by optical injection. Since the comb pitch of the seed frequency comb can be continuously adjusted simply by changing the frequency of the modulation signal, the comb pitch of the microwave frequency comb output from the second semiconductor laser DFB-SL2 based on the optical pulse injection can also be continuously adjusted over a wide range. The method is suitable for the fields of radar ranging, broadband wireless communication, satellite communication, military reconnaissance and the like.

Description

Device and method for generating broadband microwave frequency comb
Technical Field
The invention relates to the technical field of optics, in particular to a microwave frequency comb generation technology.
Background
The microwave frequency comb is composed of a series of comb-shaped microwave signals with equal frequency intervals, can simultaneously provide dozens of microwave signals with different frequencies in a frequency band, and therefore has wide application in the fields of radar ranging, broadband wireless communication, satellite communication, military reconnaissance and the like.
At present, the generation method of the microwave frequency comb mainly comprises electricity and optics. The traditional electrical method is limited by the bandwidth of the used electronic device, only a microwave frequency comb with the bandwidth of tens of GHz can be generated, and the microwave frequency comb generated by the electrical method has the defects that the comb pitch cannot be flexibly adjusted, the comb line power is extremely unbalanced and the like.
In the generation of a microwave frequency comb using optical methods, one way is to convert the optical frequency comb into a microwave frequency comb via a photodetector. However, due to the physical mechanism of the acquisition method, the system for acquiring the microwave frequency comb based on the optical frequency comb is complex and high in cost, and the microwave frequency comb generated by the method has unbalanced comb line power, large comb distance (more than several GHz) and is difficult to adjust. In addition to the above-described method of generating a microwave frequency comb, it has been proposed to generate a microwave frequency comb using the nonlinear characteristics of a semiconductor laser. For example, the chinese patent entitled "an optical generation method of high-quality microwave frequency comb" (publication No. CN 106159640 a), discloses an acousto-optic frequency shift optical loop based on a semiconductor laser to generate a high-quality microwave frequency comb; the invention discloses a Chinese patent named 'a double-path microwave frequency comb based on a photoelectric feedback VCSEL' (CN 105006727A), discloses a Chinese patent named 'all-optical broadband microwave frequency comb generator' (CN 104577648A) which generates a double-path microwave frequency comb based on a photoelectric feedback semiconductor laser, discloses a Chinese patent named 'a microwave frequency comb generating method and device' (CN 106816802A) which generates a broadband microwave frequency comb based on an incoherent light feedback semiconductor laser, and discloses a Chinese patent named 'a microwave frequency comb generating method and device' (CN 106816802A) which generates a microwave frequency comb with high-fine comb tooth spacing based on light injection and the photoelectric feedback semiconductor laser. The microwave frequency comb generation technology adopts an optical feedback loop or an optoelectronic feedback loop, and because the performance and the comb pitch of the microwave frequency comb are closely related to the length of the feedback loop and limited by a system structure, the length of the feedback loop is often limited and difficult to flexibly adjust, so that the microwave frequency comb with the continuously adjustable comb pitch in a larger range is difficult to obtain by a microwave frequency comb generation scheme based on the feedback loop. For example, CN 106159640 a discloses an optical generation method of high-quality microwave frequency comb, the comb pitch of which can only be adjusted in the range of tens of MHz to 1 GHz. In addition, the purity of the microwave frequency comb obtained by the feedback loop is not high, the line width of the comb line is large, generally in the magnitude of several kHz to several hundred kHz, and the phase noise of the microwave frequency comb is high, so that the microwave frequency comb is greatly limited to radar detection and military reconnaissance with high precision and high accuracy. The name of the method is a tunable ultra-wideband microwave frequency comb generation method based on a semiconductor laser (publication number CN 106981814A), and a pure microwave frequency comb with a bandwidth as high as 59GHz (within +/-5 dB amplitude flatness) and a phase noise lower than-86 dBc/Hz @10kHz can be obtained in two discontinuous adjustable comb distance ranges of 1.1 GHz-1.2 GHz and 3.3 GHz-8.0 GHz. However, the low frequency comb line power of the microwave frequency comb generated by this method is significantly higher than other high frequency comb line powers, and the power of the microwave frequency comb is greatly unbalanced if the flatness of the entire microwave frequency comb is considered from a conventional point of view (i.e., generally ranging from the low frequency comb line to the high frequency comb line). CN 106981814A is therefore a microwave frequency comb that excludes 0GHz to 8GHz low frequency comb lines and achieves a 59GHz bandwidth (within + -5 dB amplitude flatness) over two non-continuously adjustable comb pitch ranges.
For many practical applications, people are eager for broadband and pure microwave frequency combs with continuously adjustable comb pitches and flat comb lines from low frequency to high frequency. Therefore, in order to overcome the drawbacks of the microwave frequency comb generation scheme and obtain a broadband microwave frequency comb with balanced power, pure comb line, stable frequency and continuously adjustable comb distance in a large range, a more in-depth technical exploration is needed.
Disclosure of Invention
The invention aims to solve the technical problem that aiming at the defects in the prior art, the device for nonlinear dynamic acquisition of the microwave frequency comb based on the semiconductor laser is designed, and the broadband microwave frequency comb with balanced power, pure comb lines, stable frequency and continuously adjustable comb distance in a large range can be generated.
The technical scheme for solving the technical problems is to provide a broadband microwave frequency comb generating device with balanced power, adjustable comb distance and pure comb lines, which comprises: the device comprises a seed frequency comb module, a first optical beam splitter, a second semiconductor laser, an optical pulse injection module, an optical circulator OC and a microwave frequency comb detection module.
The seed frequency comb module is used for directly modulating the first semiconductor laser to generate a seed frequency comb and providing a seed source for subsequently obtaining a broadband high-quality microwave frequency comb signal with balanced power, adjustable comb distance and pure comb lines; the first optical beam splitter is used for splitting the seed frequency comb signal; the second semiconductor laser is used for generating a broadband high-quality microwave frequency comb signal and comprises a radio frequency modulation port, and the temperature and the current of the second semiconductor laser are controlled by a high-precision temperature control and current source; the optical pulse injection module is used for injecting the seed frequency comb into the second semiconductor laser in a one-way mode, and the input end and the output end of the optical pulse injection module are respectively connected with the first optical beam splitter and the optical circulator; the optical circulator is used for injecting the seed frequency comb into the second semiconductor laser, and simultaneously outputting a signal of the second semiconductor laser to the microwave frequency comb detection module for detection, and the optical circulator is a three-port device; and the microwave frequency comb detection module is used for detecting the performance of the seed frequency comb and the broadband high-quality microwave frequency comb output by the second semiconductor laser.
Specifically, an apparatus for generating a broadband microwave frequency comb, comprising: the optical pulse laser device comprises a seed frequency comb module, a first optical beam splitter, an optical pulse injection module, an optical circulator and a second semiconductor laser, wherein a signal source in the seed frequency comb module directly modulates the first semiconductor laser DFB-SL1 to generate a seed frequency comb signal; the first optical beam splitter FC1 divides the seed frequency comb signal into two paths, wherein one path of signal enters the optical pulse injection module, the other path of signal enters the microwave frequency comb detection module, the output signal of the optical pulse injection module unidirectionally injects the seed frequency comb signal into the second semiconductor laser DFB-SL2 through the optical circulator OC, and the second semiconductor laser DFB-SL2 outputs a broadband and pure microwave frequency comb signal with continuously adjustable comb pitch and very flat comb line from low frequency to high frequency.
Further, the apparatus further includes a microwave frequency comb detection module for detecting the performance of the seed frequency comb and the microwave frequency comb output by the second semiconductor laser, including: the microwave frequency comb signals output by the first semiconductor laser DFB-SL1 or the second semiconductor laser DFB-SL2 are divided into two paths through the third optical beam splitter FC3, the two paths of microwave frequency comb signals are converted into electric signals through the first photodetector PD1 and the second photodetector PD2 and then input into the digital oscilloscope OSC and the spectrum analyzer ESA, the digital oscilloscope detects the time sequence of the microwave frequency comb, and the spectrum analyzer detects the power spectrum and the single-sideband phase noise of the microwave frequency comb.
Preferably, the signal source in the seed frequency comb module outputs a modulation frequency fmAnd modulating the power PmThe modulation signals which are all adjustable directly modulate the working current of the first semiconductor laser DFB-SL1 by current, and the first semiconductor laser DFB-SL1 outputs a comb distance along with fmThe altered microwave seed frequency combs the signal.
Further, the optical pulse injection module includes: the optical power meter comprises an erbium-doped fiber amplifier EDFA, a polarization controller PC, a variable optical attenuator VA, a second optical splitter FC2 and an optical power meter PM, wherein the erbium-doped fiber amplifier EDFA amplifies a seed frequency comb signal generated by a first semiconductor laser DFB-SL 1; the polarization controller PC matches the polarization states of the first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL 2; the variable optical attenuator VA adjusts the signal power of the injected seed frequency comb and then the input optical circulator OC unidirectionally injects the seed frequency comb to the second semiconductor laser DFB-SL 2.
The invention also provides a method for generating the broadband microwave frequency comb, which comprises the step that a signal source MFS outputs a modulation frequency fmAnd modulating the power PmThe adjustable modulation signals directly modulate the working current of the first semiconductor laser DFB-SL1, so that the first semiconductor laser DFB-SL1 outputs a microwave seed frequency comb with adjustable comb pitch, and the microwave seed frequency comb is input into the first optical beam splitter FC 1; the FC1 divides the seed frequency comb into two paths, wherein one path of signal enters the microwave frequency comb detection module, the other path of signal enters the optical pulse injection module, the optical pulse injection module amplifies, polarizes and shunts the microwave seed frequency comb, and then the microwave seed frequency comb is injected into the second semiconductor laser DFB-SL2 in a single direction through the optical circulator, so that the second semiconductor laser DFB-SL2 outputs a broadband and pure microwave frequency comb signal with continuously adjustable comb distance and very flat comb line from low frequency to high frequency.
By adjusting the frequency f of the modulating signalmAnd power PmDirectly current-modulating the first semiconductor laser DFB-SL1 to output a regular pulse signal and generating a seed frequency comb; first semiconductor laser DFB-SL1 outputThe output regular pulse signal is divided into two paths, one path is input into a microwave frequency comb detection system to monitor the performance of a seed frequency comb, the other path is injected into a second semiconductor laser DFB-SL2 in a one-way mode through an optical pulse injection system and an optical circulator, the bandwidth of a microwave frequency comb generated by the second semiconductor laser DFB-SL2 is changed by adjusting the variation range of optical pulse injection parameters by utilizing the spectrum spreading effect caused by optical injection, the second semiconductor laser DFB-SL2 can output broadband microwave frequency comb signals with balanced power, pure comb lines and stable frequency, and the high-quality frequency comb signals can be observed from the microwave frequency comb detection system. At the same time, the comb distance of the seed frequency comb can be changed simply by changing the frequency f of the modulation signalmContinuous adjustment is performed, which results in that the comb pitch of the microwave frequency comb output from the second semiconductor laser DFB-SL2 based on the optical pulse injection can also be continuously changed over a wide range. Further, by adjusting the control temperature and the bias current of the first semiconductor laser DFB-SL1, the frequency detuning between the free-running first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL2 can be changed.
The invention adopts a frequency spectrum optimization structure injected by optical pulses to generate a broadband and pure microwave frequency comb with continuously adjustable comb pitch in a large range and very flat comb lines from low frequency to high frequency. The broadband microwave frequency comb with frequency covering 67GHz high frequency and very balanced comb line power in the range from 0GHz to 33.6GHz can be obtained; the comb lines of the obtained microwave frequency comb are pure and stable in frequency, the line width of the comb lines is as low as below 1Hz, and the phase noise of all the comb lines in the range from low frequency 0GHz to high frequency 33.6GHz is lower than-90.9 dBc/Hz @10 kHz; the system has simple structure and convenient operation. By simply changing the frequency f of the modulated signalmThe comb pitch of the microwave frequency comb can be continuously adjusted, the comb pitch of the microwave frequency comb can be continuously changed within a large range from 250kHz to 20GHz, and the broadband pure microwave frequency comb with the bandwidth exceeding 21.0GHz can be obtained within a continuously adjustable comb pitch range from 1.0GHz to 5.5 GHz.
Drawings
FIG. 1 is a schematic diagram of the basic structure of a microwave frequency comb generating device according to the present invention;
FIG. 2 is a schematic structural diagram of a preferred embodiment of the microwave frequency comb generating device of the present invention;
FIG. 3 is a graph of the spectrum of a seed frequency comb output by the first semiconductor laser DFB-SL1 (a) and the spectrum of a broadband microwave frequency comb output by the second semiconductor laser DFB-SL2 (b);
FIG. 4 is a graph of single sideband phase noise at 10kHz frequency offset for the microwave frequency comb output by the second semiconductor laser DFB-SL 2;
FIG. 5 shows four different modulation frequencies fmNext, the spectrum of the microwave frequency comb output by the second semiconductor laser DFB-SL 2.
Detailed Description
The invention will be further described with reference to the accompanying drawings and specific examples,
the method comprises the steps that a first semiconductor laser DFB-SL1 is directly modulated by current to output a regular pulse signal, a seed frequency comb is generated, the comb line power of the seed frequency comb is unbalanced, the bandwidth is small, the regular pulse signal is divided into two paths by a first optical beam splitter FC1, one path of the regular pulse signal is input into a microwave frequency comb detection system to monitor the performance of the seed frequency comb, the other path of the regular pulse signal is injected into a second semiconductor laser DFB-SL2 in a single direction through an optical pulse injection system and an optical circulator, and the second semiconductor laser DFB-SL2 outputs a broadband microwave frequency comb signal which is balanced in power, pure in comb line and stable in frequency by utilizing the spectrum spreading effect caused by optical injection. Since the comb pitch of the seed frequency comb can be changed simply by changing the frequency f of the modulation signalmThe comb pitch of the microwave frequency comb output from the second semiconductor laser DFB-SL2 based on the optical pulse injection can be continuously adjusted over a wide range.
By adjusting the frequency f of the modulating signalmAnd power PmThe first semiconductor laser DFB-SL1 is caused to output a regular pulse signal to generate a seed frequency comb. The second semiconductor laser DFB-SL2 outputs a bandwidth-dependent injection of the microwave frequency comb when the injection power varies between predetermined thresholds (160 muW to 2710 muW) by adjusting the adjustable optical attenuator VA in the optical pulse injection module to increase the injection optical power by a fixed step (e.g., 10 muW) each timeThe input power is changed. The wavelength of the free running first semiconductor laser DFB-SL1 is caused to change by adjusting the temperature and current of the driving source of the high precision, low noise first semiconductor laser DFB-SL1, thereby causing a change in the frequency detuning between the first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL 2.
Fig. 1 is a schematic diagram showing a basic structure of a microwave frequency comb generating apparatus according to the present invention, which includes: the device comprises a seed frequency comb module, a first optical beam splitter, an optical pulse injection module, an optical circulator and a second semiconductor laser. In order to test the performance of the microwave frequency comb generated by the microwave frequency comb generating device, a microwave frequency comb detection module can be added to detect the microwave frequency comb output by the device.
The seed frequency comb module provides a seed frequency comb source for obtaining broadband high-quality microwave frequency comb signals with balanced power, adjustable comb distance and pure comb lines, the modulation signal output by the signal source MFS directly current-modulates the working current of the first semiconductor laser DFB-SL1, and the modulation frequency f of the modulation signal is changedmAnd modulating the power PmDriving the first semiconductor laser DFB-SL1 to output a comb pitch with fmA seed source (seed frequency comb) of a varying microwave frequency comb, the seed frequency comb having a smaller bandwidth and an unbalanced power; the seed frequency comb is divided into two paths after passing through the first optical beam splitter, one path of seed frequency comb signals output by the first optical beam splitter enters the optical pulse injection module, the other path of seed frequency comb signals are output to the microwave frequency comb detection module, the output end of the optical pulse injection module is connected with the 1 st port of the optical circulator, the seed frequency comb is injected into a second semiconductor laser (DFB-SL2) in a single direction through the optical circulator, and the second semiconductor laser DFB-SL2 is enabled to output broadband and pure microwave frequency comb signals with continuously adjustable comb pitch and very flat comb lines from low frequency to high frequency by utilizing the bandwidth enhancement effect caused by optical injection. The second semiconductor laser includes a radio frequency modulation port, the temperature and current of which are controlled by a high precision temperature control and current source, for generating a high quality microwave frequency comb signal.
The temperature and current of the first semiconductor laser DFB-SL1 are controlled by high-precision temperature controlModulated power P output from signal source MFS and controlled by current sourcemUnder the action of larger current modulation, the first semiconductor laser DFB-SL1 outputs a regular pulse state for generating a seed frequency comb with unbalanced power and small bandwidth.
The optical circulator comprises a 1 st port to a 3 rd port which are sequentially and adjacently arranged along the optical transmission direction, wherein the 1 st port is connected with an optical pulse injection module, the 2 nd port is connected with a second semiconductor laser DFB-SL2, the 3 rd port is connected with a microwave frequency comb detection module, an optical pulse signal output by the optical pulse injection module injects a seed frequency comb into the second semiconductor laser DFB-SL2 through the 2 nd port of the optical circulator, and generates a high-quality microwave frequency comb signal from the second semiconductor laser DFB-SL2, and the microwave frequency comb detection module is further arranged to detect the performance of the seed frequency comb and the high-quality microwave frequency comb output by the second semiconductor laser DFB-SL 2.
The optical pulse injection module may include: the device comprises an erbium-doped fiber amplifier EDFA, a polarization controller PC, a variable optical attenuator VA, an optical power meter PM and a second optical beam splitter FC 2.
The erbium-doped fiber amplifier EDFA amplifies an optical pulse signal which is branched from one port of a first optical beam splitter FC1, a polarization controller PC is matched with the polarization states of a first semiconductor laser DFB-SL1 and a second semiconductor laser DFB-SL2, a variable optical attenuator VA adjusts the optical power injected into the second semiconductor laser DFB-SL2, the second optical beam splitter FC2 divides the injected optical pulse into two parts, one part of the optical pulse enters an optical power meter PM, the optical power meter PM measures the optical power injected into the second semiconductor laser DFB-SL2, the other part of the optical pulse enters an optical power circulator OC, and the unidirectional injection seed frequency is combed to the second semiconductor DFB-SL 2.
The microwave frequency comb detection module includes: the output of the second semiconductor laser DFB-SL2 is divided into a left path and a right path by a third optical beam splitter FC3, a first photoelectric detector PD1, a second photoelectric detector PD2, a digital oscilloscope OSC and a spectrum analyzer ESA, wherein the left path of optical signal is input into the first photoelectric detector PD1 to be converted into an electric signal, the right path of optical signal is input into the second photoelectric detector PD2 to be converted into an electric signal, the electric signals output by the PD1 and the PD2 are respectively input into the digital oscilloscope OSC and the spectrum analyzer ESA, and the time sequence, the power spectrum and the single-sideband phase noise of the microwave frequency comb are detected.
For the convenience of understanding, the technical solution of the present invention will be further described in detail with reference to a preferred embodiment of the microwave frequency comb generating device of the present invention. Fig. 2 shows a detailed structure of a preferred embodiment of the microwave frequency comb generating device of the present invention, which comprises: the device comprises a signal source MFS, a first semiconductor laser DFB-SL1, a first optical beam splitter FC1, an erbium-doped fiber amplifier EDFA, a polarization controller PC, a variable optical attenuator VA, a second optical beam splitter FC2, an optical power meter PM, an optical circulator OC, a second semiconductor laser DFB-SL2, a third optical beam splitter FC3, a first photodetector PD1, a second photodetector PD2, a digital oscilloscope OSC and a spectrum analyzer ESA.
Modulating the current of a first semiconductor laser DFB-SL1 by a modulation signal provided by a signal source MFS to generate a seed frequency comb, wherein the seed frequency comb output by DFB-SL1 enters a first optical beam splitter FC1, one part of the output of FC1 enters an erbium-doped fiber amplifier EDFA, the other part of the output of FC1 is branched from a link 1Line1 through a third optical beam splitter FC3, one part of the output of FC1 enters a first photodetector PD1 and is converted into an electric signal, and then the electric signal is input into a digital oscilloscope OSC to detect a time sequence, and the other part of the output of FC1 enters a second photodetector PD2 and is converted into the electric signal and then is input into a spectrum analyzer ESA to perform spectrum analysis and phase noise analysis;
the output of the erbium-doped fiber amplifier EDFA enters a polarization controller PC, the optical power injected into the seed frequency comb is adjusted by a variable optical attenuator VA and then is input into a second optical beam splitter FC2, a part (such as 10%) of the output of FC2 enters an optical power meter PM to measure the optical power injected into a second semiconductor laser DFB-SL2, the rest (such as 90%) of the output of FC2 enters a 1 st port of an optical circulator OC, the seed frequency is injected into the second semiconductor laser DFB-SL2 through the 2 nd port of the OC in a one-way mode, the output signal of the 3 rd port of the OC enters the third optical beam splitter FC3 from the link 2Line2 to be branched, one part of the branched output signal enters the first photoelectric detector PD1 to be converted into an electric signal and then is input into the digital oscilloscope OSC to carry out time series detection, and the other part of the branched output signal enters the second photoelectric detector PD2 to be converted into an electric signal and then is input into the spectrum analyzer ESA to carry out spectrum measurement and phase noise measurement.
Preferably, the following devices can be adopted in each part of the device in the specific implementation, and other types and components with similar functions can also be adopted, wherein the first semiconductor laser DFB-SL1 adopts a high-speed direct modulation distributed feedback semiconductor laser with the modulation bandwidth of 10GHz, and the center wavelength of the first semiconductor laser is 1550 nm; the signal source MFS adopts an E8257C type analog signal generator, the frequency range of the signal source MFS is adjustable from 250kHz to 20GHz, and the power range of the signal source MFS is adjustable from-135 dBm to 25 dBm; the first optical splitter FC1 used a 10:90 fiber coupler; the erbium-doped fiber amplifier EDFA adopts a common commercial erbium-doped fiber amplifier with selectable power amplification or gain amplification; the polarization controller PC adopts a common commercial polarization controller; the variable optical attenuator VA adopts a variable optical attenuator with 1550nm wavelength; the optical circulator OC adopts a three-port optical circulator; the second optical splitter FC2 used a 10:90 fiber coupler; the optical power meter PM adopts an S155C optical fiber power sensor with a PM100D gauge head; the second semiconductor laser DFB-SL2 adopts a high-speed direct modulation distributed feedback semiconductor laser with the modulation bandwidth of 10GHz, and the central wavelength of the second semiconductor laser is 1550 nm; the third optical splitter FC3 used a 10:90 fiber coupler; the first photoelectric detector PD1 adopts a New Focus 1544-B photoelectric detector with a bandwidth of 12 GHz; the second photodetector PD2 adopts a U2T-XPDV2150R photodetector with a bandwidth of 50 GHz; the digital oscilloscope OSC adopts an Agilent X91604A digital real-time oscilloscope with the bandwidth of 16 GHz; the ESA of the spectrum analyzer adopts 67GHz bandwidth
Figure BDA0001778362940000071
And a FSW spectrum analyzer.
Two high-speed direct modulation distributed feedback semiconductor lasers with the modulation bandwidth of 10GHz are respectively used as a first semiconductor laser DFB-SL1 and a second semiconductor laser DFB-SL2, the current and the temperature of the two high-speed direct modulation distributed feedback semiconductor lasers are controlled by two high-precision low-noise laser driving sources (ILX-Lightwave, LDC-3724C), the current control precision is 0.01mA, and the temperature control precision is 0.01 ℃. In the implementation process, for example, the temperatures of the first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL2 are respectively stabilized at 20.86 ℃ and 21.19 ℃, the currents are respectively stabilized at 18.70mA and 25.00mA, under the conditions of the temperatures and the currents, the relaxation oscillation frequencies of the first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL2 which are free to operate are respectively 7.60GHz and 8.20GHz, and the frequency detuning deltaf between the two is 0 GHz.
Firstly, an external signal source MFS is adopted to modulate the driving current of the first semiconductor laser DFB-SL1, and the frequency f of the output signal of the signal source MFS is adjustedmAnd power PmThe first semiconductor laser DFB-SL1 is operated in a regular pulse state, and a seed frequency comb is generated. Then, the optical pulse signal output by the first semiconductor laser DFB-SL1 is divided into two parts by a first optical beam splitter FC1 with the ratio of 10:90, 90% of the part enters a microwave frequency comb detection system through a line1 to test the performance of a seed frequency comb, and the other 10% of the part is injected into a second semiconductor laser DFB-SL2 in a single direction after passing through an erbium-doped fiber amplifier EDFA, a polarization controller PC, a variable optical attenuator VA, a second optical beam splitter FC2 and an optical circulator OC. Wherein an in-line variable optical attenuator VA is used for adjusting the injected light power PiMeasured by an optical power meter PM. The output of the second semiconductor laser DFB-SL2 enters the microwave frequency comb detection system after passing through the optical circulator OC and the line 2. In the microwave frequency comb detection system part, after optical signals output by the first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL2 are converted into electric signals by the first photo detector PD1 and the second photo detector PD2, a spectrum analyzer ESA observes a power spectrum and single-sideband phase noise output by the optical signals, and a digital oscilloscope OSC records a time sequence output by the optical signals.
The invention is further illustrated below with reference to examples.
FIG. 3 is a spectrum diagram (a) of a seed frequency comb output by the first semiconductor laser DFB-SL1 and a spectrum diagram (b) of a broadband microwave frequency comb output by the second semiconductor laser DFB-SL2, and FIG. 4 is a single-sideband phase-phase comb of the microwave frequency comb output by the second semiconductor laser DFB-SL2 at a frequency offset of 10kHzBit noise figure, fig. 5 shows four different modulation frequencies fmNext, the spectrum of the microwave frequency comb output from the second semiconductor laser DFB-SL 2. In order to facilitate understanding of the technical scheme of the invention, the following relevant parameters are taken as examples to be analyzed and explained in the embodiments.
The temperature of the first semiconductor laser DFB-SL1 is stabilized at 20.86 ℃, and the current is controlled at 18.70 mA; the temperature of the second semiconductor laser DFB-SL2 is stabilized at 21.19 ℃, the current is controlled at 25.00mA, under the conditions of the temperature and the current, the output power of the first semiconductor laser DFB-SL1 which runs freely is 1.27mw, the output wavelength is 1549.106nm, and the relaxation oscillation frequency is 7.60 GHz; the output power of the second semiconductor laser DFB-SL2 is 0.71mw, the output wavelength is 1549.106nm, and the relaxation oscillation frequency is 8.20 GHz; at the same time, the frequency detuning Δ f between the two lasers is 0 GHz.
FIG. 3 shows the modulation frequency fm1.2GHz modulated power PmAt 22dBm, the first semiconductor laser DFB-SL1 outputs a spectral diagram (a) of the seed frequency comb and the injection intensity PiA spectrogram (b) of a broadband microwave frequency comb output by the second semiconductor laser DFB-SL2 at 2060 μ W. Wherein the resolution bandwidth of the ESA of the spectrum analyzer is 100 kHz. As can be seen from FIG. 3(a), at (f)m,Pm) The first semiconductor laser DFB-SL1 with direct current modulation can generate a seed frequency comb with a low-frequency comb amplitude significantly higher than the other high-frequency comb amplitudes at modulation parameters of (1.2GHz,22dBm), which results in a very unbalanced power and a small bandwidth of only 14.4GHz (within an amplitude variation range of ± 5 dB). In FIG. 3(b), a seed frequency comb is further injected into the second semiconductor laser DFB-SL2, at PiUnder the injection power of 2060 muW, the performance of the microwave frequency comb output by the second semiconductor laser DFB-SL2 is obviously improved, the comb line of the microwave frequency comb is flatter, the power is more balanced, and the bandwidth is expanded to 33.6GHz (from low-frequency 0GHz to high-frequency 33.6GHz (within the amplitude variation range of +/-5 dB)).
FIG. 4 shows the modulation frequency fm1.2GHz modulated power Pm22dBm, injection strength PiThe microwave frequency output by the second semiconductor laser DFB-SL2 combs for single sideband phase noise at 10kHz frequency offset, 2060 μ W. Fig. 4 shows the single sideband phase noise at 10kHz offset for the first, fifth, tenth, fifteenth, twenty-fifth, and thirtieth combs of the microwave frequency comb obtained in fig. 3 (b). As can be seen from fig. 4, the single-sideband phase noise of the microwave frequency comb output from the second semiconductor laser DFB-SL2 after the optical pulse injection is lower than-90.9 dBc/Hz @10kHz, which indicates that the present invention can generate a microwave frequency comb signal with pure comb lines and stable frequency.
FIG. 5 shows the injection strength Pi2060 μ W, modulation power Pm22dBm, four different modulation frequencies fmNext, the spectrum of the microwave frequency comb output from the second semiconductor laser DFB-SL 2. As can be seen from FIG. 5, with the modulation frequency fmThe comb pitch, the number of comb lines and the amplitude flatness of the microwave frequency comb vary with the microwave frequency comb, and the comb pitch of the microwave frequency comb is equal to the modulation frequency fmThe size of (2). In FIG. 5(a), fmWhen the frequency is equal to 0.5GHz, the comb pitch of the microwave frequency comb is 0.5GHz, and the low-frequency comb line power of the microwave frequency comb is obviously higher than that of other high-frequency comb lines, so that the microwave frequency comb has extremely small bandwidth which is only 1.5 GHz. In FIGS. 5(b), 5(c) and 5(d), when f ismThe comb pitch of the microwave frequency comb is 1.2GHz, 2.2GHz and 3.9GHz respectively, and the high frequency comb line of the microwave frequency comb has stronger power at the moment, so that the microwave frequency comb has flatter amplitude and larger bandwidth which are 33.6GHz, 30.8GHz and 31.2GHz respectively. This shows that the present invention can obtain a broadband microwave frequency comb within a continuously adjustable comb pitch range.
In conclusion, the test fully verifies the excellent effect of the technical scheme of the invention, and the broadband microwave frequency comb which is very flat from the low-frequency comb line to the high-frequency comb line can be generated; the generated microwave frequency comb has the pure comb line characteristic, the comb line width of the microwave frequency comb is lower than 1Hz, and the single-side band phase noise is lower than-90.9 dBc/Hz @10 kHz; meanwhile, the obtained microwave frequency comb has a large-range continuously adjustable comb distance area, and in the comb distance area, the ultra-wideband high-quality pure microwave frequency comb signal can be obtained. The microwave frequency comb generated by the technical scheme of the invention can be suitable for common low-frequency-band (0.5-3 GHz) dense microwave communication, can also meet the requirement of high-frequency-band high-speed microwave communication, and solves the problem of the existing microwave technology.

Claims (3)

1. An apparatus for generating a broadband microwave frequency comb, the apparatus comprising: the optical pulse laser device comprises a seed frequency comb module, a first optical beam splitter, an optical pulse injection module, an optical circulator and a second semiconductor laser device DFB-SL2, wherein a signal source in the seed frequency comb module directly modulates the first semiconductor laser device DFB-SL1 to generate a seed frequency comb signal; signal source output modulation frequency in seed frequency comb modulef mAnd modulating powerP mThe modulation signals are all adjustable, the working current of the first semiconductor laser DFB-SL1 is directly modulated by current, and the first semiconductor laser DFB-SL1 outputs a microwave seed frequency comb signal with adjustable comb distance; the first optical beam splitter FC1 divides the seed frequency comb signal into two paths, wherein one path of signal enters the optical pulse injection module, and the other path of signal enters the microwave frequency comb detection module; the optical pulse injection module includes: the optical power meter comprises an erbium-doped fiber amplifier EDFA, a polarization controller PC, a variable optical attenuator VA, a second optical splitter FC2 and an optical power meter PM, wherein the erbium-doped fiber amplifier EDFA amplifies a seed frequency comb signal generated by a first semiconductor laser DFB-SL 1; the polarization controller PC matches the polarization states of the first semiconductor laser DFB-SL1 and the second semiconductor laser DFB-SL 2; after the variable optical attenuator VA adjusts the signal power of the injected seed frequency comb, the input optical circulator OC unidirectionally injects the seed frequency comb to a second semiconductor laser DFB-SL 2; the output signal of the optical pulse injection module unidirectionally injects a seed frequency comb signal into a second semiconductor laser DFB-SL2 through an optical circulator OC, and the second semiconductor laser DFB-SL2 outputs a broadband microwave frequency comb signal with continuously adjustable comb pitch and very flat comb line from low frequency to high frequency; the microwave frequency comb detection module is used for detecting a seed frequency comb and a semiconductor laser outputThe properties of the microwave frequency comb include: a third optical beam splitter FC3, a first photodetector PD1, a second photodetector PD2, a digital oscilloscope OSC, and a spectrum analyzer ESA, wherein a seed frequency comb signal output by the first semiconductor laser DFB-SL1 and a microwave frequency comb signal output by the second semiconductor laser DFB-SL2 are respectively divided into two paths by the third optical beam splitter FC 3; after the two paths of microwave frequency comb signals are converted into electric signals through a first photoelectric detector PD1 and a second photoelectric detector PD2 respectively, the electric signals are input into a digital oscilloscope OSC and a spectrum analyzer ESA, the digital oscilloscope detects the time sequence of a microwave frequency comb, and the spectrum analyzer detects the power spectrum and single-sideband phase noise of the microwave frequency comb; the two paths of seed frequency comb signals respectively enter a first photoelectric detector PD1 and a second photoelectric detector PD2 to be converted into electric signals, and then the electric signals are input into a digital oscilloscope and a spectrum analyzer ESA, the digital oscilloscope detects the time sequence of the seed frequency comb signals, and the spectrum analyzer detects the power spectrum and the single-sideband phase noise of the seed frequency comb signals.
2. A method for generating a broadband microwave frequency comb, characterized in that a signal source MFS outputs a modulation frequencyf mAnd modulating powerP mThe modulation signals are all adjustable to directly modulate the working current of the first semiconductor laser DFB-SL1, so that the first semiconductor laser DFB-SL1 outputs a comb pitchf mA comb of varying microwave seed frequencies input to a first optical splitter FC 1; the FC1 divides the seed frequency comb into two paths, wherein one path of signal enters the microwave frequency comb detection module, the other path of signal enters the optical pulse injection module, the optical pulse injection module amplifies, polarizes and shunts the microwave seed frequency comb, and then the microwave seed frequency comb is injected into the second semiconductor laser DFB-SL2 in a single direction through the optical circulator, so that the second semiconductor laser DFB-SL2 outputs a broadband and pure microwave frequency comb signal with continuously adjustable comb pitch and very flat comb line from low frequency to high frequency;
an erbium-doped fiber amplifier EDFA in the optical pulse injection module amplifies a seed frequency comb signal generated by a first semiconductor laser DFB-SL1, a polarization controller PC is matched with the polarization states of the first semiconductor laser DFB-SL1 and a second semiconductor laser DFB-SL2, and a variable optical attenuator VA is used for inputting the light into an optical circulator OC after adjusting the power of the injected seed frequency comb signal and unidirectionally injecting the seed frequency comb to the second semiconductor laser DFB-SL 2;
the microwave frequency comb detection module detects microwave frequency comb signals, and specifically comprises that microwave frequency comb signals output by a second semiconductor laser DFB-SL2 are divided into two paths through a third optical beam splitter FC 3; after signals of the two paths of microwave frequency combs enter a first photoelectric detector PD1 and a second photoelectric detector PD2 respectively and are converted into electric signals, the electric signals are input into a digital oscilloscope OSC and a spectrum analyzer ESA, the digital oscilloscope OSC detects a time sequence of the microwave frequency combs, and the spectrum analyzer ESA detects power spectrums and single-sideband phase noises of the microwave frequency combs;
the microwave frequency comb detection module detects a seed frequency comb signal, and specifically comprises that a path of signal, which is output by a first semiconductor laser DFB-SL1 and is subjected to FC1 branching, of a seed frequency comb is divided into two paths through a third optical beam splitter FC 3; the two paths of seed frequency comb signals respectively enter a first photoelectric detector PD1 and a second photoelectric detector PD2 to be converted into electric signals, and then the electric signals are input into a digital oscilloscope and a spectrum analyzer ESA, the digital oscilloscope detects the time sequence of the seed frequency comb signals, and the spectrum analyzer detects the power spectrum and the single-sideband phase noise of the seed frequency comb signals.
3. The method of claim 2 wherein the signal source MFS output is adjusted such that the direct current modulated first semiconductor laser DFB-SL1 outputs regular pulses to produce a seed frequency comb with unequal power and a small bandwidth.
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