CN117848525A - Frequency measurement system for injection locking laser single-period loop self-excitation subharmonic modulation - Google Patents
Frequency measurement system for injection locking laser single-period loop self-excitation subharmonic modulation Download PDFInfo
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Abstract
The invention relates to the technical field of microwave photons, and provides a frequency measurement system which is adjustable in broadband and is based on injection locking laser single-period loop self-excited subharmonic modulation. The device consists of an adjustable laser light source, a modulator, a polarization controller, an optical circulator, a DFB slave laser, a long optical fiber, a dispersion compensation optical fiber, an optical amplifier, a photoelectric detector, an electric amplifier, a high-pass electric filter, a narrow-band electric filter, an arbitrary waveform generator and other elements, and detection equipment such as a spectrometer, an oscilloscope and the like. The invention provides a novel microwave photon filter system for measuring broadband microwave signal frequency, which is used for periodically and quickly tuning the frequency or optical power of a main laser in a single-period locking mode of a main laser and a slave laser, realizing the quick tuning of the center frequency of the microwave photon filter system and realizing the measurement of the signal frequency to be measured by utilizing a frequency conversion and frequency time mapping method.
Description
Technical Field
The invention belongs to the technical field of microwave photons, and particularly relates to a frequency measurement system based on injection locking laser single-period loop self-excitation subharmonic modulation.
Background
Microwave Photon Filters (MPF) have wide application in millimeter wave communications, high performance radars, optical wireless communications systems, and wireless local area networks. Compared with the traditional electronic filter, the microwave photon filter has the advantages of low loss, large bandwidth, electromagnetic interference resistance, tunability, reconfigurability and the like, and gradually develops into a key technology for controlling and processing high-frequency broadband signals. With the increasing requirements of the front-edge technical fields such as high-purity frequency spectrum microwave signal generation, high-sensitivity microwave photon sensing, high-resolution microwave photon radar and the like on the frequency selectivity of the filter, the broadband adjustable MPF gradually becomes a hot spot and a difficult point of research in the microwave photon technical field in recent years.
Heretofore, researchers have proposed a variety of tunable, narrow bandwidth MPFs (Wang Wenxuan. Tao Ji. Yellow dragon. Narrow band tunable microwave photon filters based on optical injection fabry perot lasers [ J ]. Chinese lasers, 2017, 44 (10): 1006002.) (Hu Zonghua, nie Kuiying, ruan Yi, etc.. Tunable bandpass microwave photon filters based on dispersive fiber optic ring cascade structures [ J ]. Semiconductor photovoltaics, 2019, 40 (2): 189-192.199.). For example, a narrow-band, tunable MPF implemented using a phase modulator and a super-Structured Fiber Bragg Grating (SFBG) has a 3dB bandwidth of 143MHz and a tunable range of 0.4 to 6.4GHz (GAO L.CHEN X F.YAO J P.tunable microwave photonic filter with a flarrow and fiat-top passband [ J ]. IEEE Microwave and Wireless Components Letters,2013, 23 (7): 362-364.). However, in the scheme, the reflection bandwidth and the notch bandwidth of the SFBG determine the frequency adjustable range and the passband width, and the manufacturing process is relatively complicated; in 2016, zhang (ZHANT T, XIONG J T, ZHENG J L, et a1.. Wideband tunable single bandpass microwave photonic filter based on FWM dynamics of optical-injected DFB laser [ J ]. Electronics Letters,2016, 52 (1): 57-59.), et al used four-wave mixing based on optical injection lower distributed feedback semiconductor lasers to achieve broadband tunable MPF with 3dB bandwidth and out-of-band rejection ratio of 61.2MHz and 25dB, respectively, and by varying the optical injection parameters, a frequency tuning range of 12-40 GHz was achieved; recently, a 2×2 fiber coupler-based MPF with a ring resonator has been reported, which achieves a 40GHz tuning range through a coherent detection link, and has a filtering bandwidth as low as 1.2MHz compared to the conventional non-cavity MPF. (Zhang Ziping, niu Xiaochen, huang Jie, etc.) high performance microwave photon filters based on fiber optic ring resonators [ J ]. J.Ind., 2020, 40 (21): 2106001.). The optoelectronic oscillator system is a classical model that can create steady-state single-mode oscillations from noise, low phase noise being one of its core advantages (HAO T, CEN Q, GUAN S, et al optoelectronics parametric oscillator [ J ] Light: science & Applications,2020,9,102). The mathematical models of the current light injection semiconductor lasers are developed based on the theory proposed by Lang and Kabayshi in 1980, and the light injection semiconductor laser system mainly has four typical nonlinear dynamics, among which the simplest steady-state locking state has been widely studied and applied in the fields of wavelength synchronization, coherence improvement and the like. In addition, the steady-state locking state of the light injection semiconductor laser is also applied to improvement of modulation characteristics such as enhancement of modulation bandwidth, chirp, noise reduction, and the like. The double period oscillation is mainly applied to frequency multiplication and frequency division of the microwave signal. The chaotic oscillation state is widely applied to the fields of chaotic secret communication, high-speed random number generation, chaotic radar and the like. In recent years, the application of a single-period oscillation state of a light injection semiconductor laser is paid attention to, and researchers are sequentially researching the application of the light injection semiconductor laser in the fields of signal generation (Zhou Pei, li Nianjiang, pan Shilong. Wideband radar signal generation and application [ J ]. Semiconductor photoelectric, 2022,43 (01): based on the light injection semiconductor laser, photon microwave amplification, single-sideband modulation, optical frequency conversion and the like.
Disclosure of Invention
The invention mainly realizes a system capable of measuring the frequencies of a plurality of signals to be measured simultaneously in a single system. The frequency measurement system based on injection locking laser single-period loop self-excitation subharmonic modulation, provided by the invention, can realize measurement of microwave signals with unknown broadband frequency by changing the center wavelength or optical power of a main laser and combining with a high-linearity frequency time mapping technology.
The technical scheme of the invention is as follows:
the frequency measuring system for injection locking laser single-period loop self-excitation subharmonic modulation comprises an adjustable laser light source, a modulator, a polarization controller, an optical circulator, a DFB slave laser, a long optical fiber, a dispersion compensation optical fiber, an optical amplifier, a photoelectric detector, an electric amplifier, a high-pass electric filter, a narrow-band electric filter and an arbitrary waveform generator.
The output end of the adjustable laser light source is connected with the light input end of the modulator, light modulated by the radio frequency input end of the loop signal modulator is output by the light output end of the modulator and is input into the polarization controller, light after polarization control is input into the No. 1 port of the optical circulator and is input into the DFB through the No. 2 port of the optical circulator to be injected and locked, then the locked laser is input into the No. 2 port of the optical circulator and is output into the long optical fiber and the dispersion compensation optical fiber through the No. 3 port of the optical circulator, then the optical signal is input into the light input end of the photoelectric detector to perform beat frequency, the electric signal after beat frequency is input into the electric amplifier to be amplified, the amplified radio frequency signal is respectively output into the high-pass electric filter to filter out low-frequency clutter signals and is input into the radio frequency port of the modulator to be modulated after being combined with the signal to be detected, and is input into the narrow-band-pass electric filter to be subjected to frequency time mapping and detection in the oscilloscope, and the system can be respectively subjected to frequency division and frequency spectrum detection at the positions of the optical detector and the electric power divider.
Furthermore, the arbitrary waveform generator is used for tuning the output optical power or the optical wavelength of the adjustable laser light source or adjusting the output optical power or the optical wavelength of the adjustable laser light source and synchronizing a periodic tuning signal to the oscilloscope, and the DFB is kept in the P1 state from the final operation state of the laser in the tuning process.
Further, in the frequency measurement system, the relation between the loop delay and the tuned period of the output signal of the adjustable laser source is an integer multiple, so that the DFB slave laser is in a subharmonic modulation state. Wherein the long fiber is used for adjusting the loop delay, and the dispersion compensation fiber is used for adjusting the loop dispersion to 0.
Further, wherein the optical amplifier and the electrical amplifier are configured to cause the loop open loop gain to be greater than 1 to generate periodic self-oscillation to generate a periodic frequency modulated signal. The high-pass filter is used for filtering low-frequency clutter signals so as to improve the purity of the frequency spectrum and the frequency measurement precision.
Further, the injection coefficients ζ of the tunable laser source and the DFB from the optical injection system of the laser are adjusted by adjusting the optical power output by the tunable laser source through the arbitrary waveform generator or by adjusting through the polarization controller.
Analysis of a mathematical model of light injection semiconductor laser technology shows that: the property of the optical signal injected into the slave semiconductor laser changes the working characteristics of the slave laser, and the resonant cavity mode of the semiconductor laser generates an optical gain region when red-shifted, and the property of the injected optical signal itself can be mainly determined by the injection coefficient ζ and the detuning frequency f i These two parameters, located in the power and frequency dimensions, respectively, are characterized. By changing the external light injection parameters, the slave laser can display rich nonlinear dynamic output, such as single-period oscillation (P1 state), double-period oscillation, chaos state, injection locking and the like. When the system is in the P1 state, the red shift of the laser center wavelength output from the laser will follow ζ and f i Is changed by a change in (a). The schematic diagram of the sideband of the output spectrum at this time is shown in FIG. 2, where lambda m 、λ s And lambda (lambda) c Respectively, the center wavelength of the main laser, the center wavelength of the free state of the slave laser and the center wavelength of the red-shifted slave laser, lambda m And lambda is s The frequency difference of (f) i 。
When the signal f to be measured is loaded into the master laser through the modulator to output laser light and then the laser light is injected into the slave laser, a spectral sideband diagram as shown in fig. 3 is formed when the system is operated in the P1 state. Then, the spectrum generates frequency f after beat frequency c =λ c -λ m And lambda (lambda) c -(λ m +f)=f c -beat signal of f. From the P1 state spectrum variation characteristics, zeta and f of the injected light signal i When the change occurs, f c And will vary accordingly. Thus byThe arbitrary waveform generator periodically tunes the output light power or wavelength of the adjustable laser source to obtain the f of periodic sweep frequency c And f c -f. And filtering out the corresponding frequency signal by a narrow band-pass electric filter to obtain the pulse signal twice in each period. Finally, by measuring the time interval of two pulse signals in a sweep period and comparing with f c The frequency sweep characteristics of the signals are matched, and the frequency of the signal to be measured can be measured by a frequency-time mapping method.
Meanwhile, due to the characteristic that an optical gain region is generated when the resonant cavity mode of the semiconductor laser is red shifted, the change period time of an adjustable laser light source in the system is regulated to be completely equal to the running time of a loop signal or to be in an integral multiple relation, and therefore signals circularly loaded on the modulator at any moment through the loop can be just the wavelength of the DFB after the red shift of signals output from the laser. This is consistent with the subharmonic oscillation principle of laser injection locking (i.e., the characteristic that the N-order sidebands modulated on the injected light are closer to the wavelength red-shifted from the laser, where n=1) can make the periodic operation of the system more stable.
According to the invention, the frequency measurement of the frequency conversion thought is realized through the narrow band-pass electric filter, only the frequency modulation signal with periodical change is generated in the loop, the difference frequency signal after the beat frequency of the signal and the signal to be detected is filtered into the pulse signal by the narrow band-pass electric filter, and the frequency-time mapping is completed.
The invention can complete the tuning of the frequency measuring range of the system by tuning the output light power or the wavelength range of the adjustable laser light source through the arbitrary waveform generator.
The invention has the beneficial effects that:
the frequency measurement system can maintain higher frequency measurement accuracy under longer continuous working time. The system stability is directly related to the stability of the loop oscillation signal, and the stability of the self signal can be continuously maintained by the optical injection locking mode under subharmonic modulation; the expected value of the frequency measurement error is close to the bandwidth of the narrow band-pass electric filter, and the frequency measurement error can reach 8MHz in the prior art.
Drawings
FIG. 1 is a schematic diagram of a design apparatus for a broadband signal frequency measurement system provided by the present invention;
FIG. 2 is a schematic diagram of spectral sidebands in the P1 state of an injection locked laser;
FIG. 3 is a schematic diagram of an injection-locked P1 state spectral sideband after signal under test is input;
fig. 4 is a graph of the measurement results of the frequency measurement MATLAB program of example 2.
Detailed Description
In order to make the technical problems solved by the invention, the technical scheme adopted and the technical effects achieved clearer, the invention is further described in detail below with reference to the accompanying drawings and the embodiments.
Example 1:
fig. 1 is a link diagram of a frequency measurement system based on injection locking laser monocycle state loop free subharmonic modulation. The output optical power or wavelength of the tunable laser source 1 is tuned by the arbitrary waveform generator 13, while the output signal of the arbitrary waveform generator 13 is connected to an oscilloscope 17 for clock locking for subsequent frequency measurement. By adjusting the tuning range of the output optical power or wavelength of the tunable laser source 1, the DFB is operated from the laser 5 always in the P1 state, at which time f c And f c F will vary periodically with the variation of the output signal of the tunable laser source 1.
The beat signal frequency is now assumed to be between 12 and 18GHz. When the system has a signal to be measured with a frequency of 10GHz input, the modulator will have a frequency of lambda m Lambda is respectively modulated at two sides m -10GHz and lambda m Positive and negative sideband signals at +10 GHz. Wherein lambda is m +10GHz and lambda c The beat frequency after passing through the photoelectric detector can obtain the frequency lambda c -λ m -10GHz and likewise varies with the periodic variation of the central wavelength of the primary laser. Known lambda c -λ m The frequency sweep range of (2) is 12-18GHz, the lambda is c -λ m The signal variation range of-10 GHz is 2-8GHz. When the signal frequency is the same as the center frequency of the narrow band-pass filter (assumed to be 6 GHz), it is obtainedThe primary pulse peak is easy to obtain, and two pulse peaks appear in one sweep frequency period. The frequency-time mapping can be completed by measuring the time interval between pulse peaks and the sweep frequency characteristic curve, and the frequency of the signal to be detected is calculated.
Example 2:
fig. 4 is a diagram of primary frequency measurement data based on the system, wherein the system stably operates in the range of about 12-24GHz, the corresponding frequency to be measured is 6-18GHz, the sweep characteristic curve of the system is shown in the left diagram in fig. 4, and the sweep characteristic curve is obtained by performing short-time fourier transform calculation on sweep signals acquired by an oscilloscope 17. The 14GHz signal generated by the microwave signal source 14 is input into the system, and is filtered by the loop beat frequency and narrow band-pass electric filter 12, and then a double-pulse time domain pulse envelope shown in the right diagram in fig. 4 is obtained on the oscilloscope 17, and the time interval between the pulse peaks is the corresponding parameter of the frequency-time mapping. Comparing the left and right images in FIG. 4 through MATLAB program, the frequency value is 14.005GHz, namely the measurement error is 5MHz.
The above description is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto. Any person skilled in the art, within the technical scope of the present invention described in the present invention, shall cover the scope of the present invention by equally replacing or changing the technical scheme and the inventive concept thereof.
Claims (8)
1. The frequency measurement system for injection locking laser single-period loop self-excitation subharmonic modulation is characterized by comprising a tunable laser light source (1), a modulator (2), a polarization controller (3), an optical circulator (4), a DFB slave laser (5), a long optical fiber (6), a dispersion compensation optical fiber (7), an optical amplifier (8), a photoelectric detector (9), an electric amplifier (10), a high-pass electric filter (11), a narrow-band-pass electric filter (12) and an arbitrary waveform generator (13);
the output end of the adjustable laser light source (1) is connected with the optical input port of the modulator (2), light modulated by the radio frequency input end of the loop signal modulator is output by the optical output port of the modulator (2) and is input into the polarization controller (3), light after polarization control is input into the No. 1 port of the optical circulator (4) and is input into the DFB from the laser (5) by the No. 2 port of the optical circulator (4) for injection locking, then the locked laser is input by the No. 2 port of the optical circulator (4) and is output into the long optical fiber (6) and the dispersion compensation optical fiber (7) by the No. 3 port, then the optical signal is input into the optical input port of the photoelectric detector (9) for beat frequency, the electric signal after beat frequency is input into the electric amplifier (10), the amplified radio frequency signal is respectively output into the high-pass electric filter (11) for filtering off the low-frequency clutter signal and mixing with the signal to be detected and then is input into the radio frequency port of the modulator (2) for modulation, and is input into the narrow-pass electric filter (12) for mapping in the optical splitter (17) and the optical splitter for spectral power detection in the optical splitter and the spectral system respectively.
2. The frequency measurement system of injection locked laser single-period loop self-excited subharmonic modulation according to claim 1, wherein the arbitrary waveform generator (13) is used for tuning the output optical power or optical wavelength or both of the tunable laser source (1) and synchronizing the periodic tuning signal to the oscilloscope (17), and the DFB is kept in P1 state from the final operation state of the laser (5) during the tuning process.
3. The frequency measurement system of injection locking laser single-period loop self-excited subharmonic modulation according to claim 1 or 2, wherein the relation between the loop delay and the period of the tunable laser source (1) output signal is integer multiple in the frequency measurement system, so that the DFB is in subharmonic modulation state from the laser (5); wherein the long optical fiber (6) is used for adjusting the loop delay, and the dispersion compensation optical fiber (7) is used for adjusting the loop dispersion to 0.
4. The frequency measurement system of injection locked laser single periodic state loop self-excited subharmonic modulation according to claim 1 or 2, wherein the optical amplifier (8) and the electrical amplifier (10) are configured to make the loop open loop gain greater than 1, and generate periodic self-oscillation to generate periodic frequency modulation signal; the high-pass electric filter (11) is used for filtering low-frequency clutter signals so as to improve the frequency spectrum purity and the frequency measurement precision.
5. A frequency measurement system for injection locked laser single period loop free running subharmonic modulation as claimed in claim 3 wherein said optical amplifier (8) and electrical amplifier (10) are adapted to provide a loop open loop gain greater than 1 to produce periodic free running oscillations to produce periodic frequency modulated signals; the high-pass electric filter (11) is used for filtering low-frequency clutter signals so as to improve the frequency spectrum purity and the frequency measurement precision.
6. The frequency measurement system of the injection locking laser single-period state loop self-excitation subharmonic modulation as claimed in claim 1, 2 or 5, wherein the injection coefficients ζ of the tunable laser light source (1) and the DFB from the light injection system of the laser (5) are adjusted by adjusting the light power output by the tunable laser light source (1) through an arbitrary waveform generator (13) or by adjusting through a polarization controller (3).
7. A frequency measurement system for injection locking laser single periodic state loop self-excited subharmonic modulation as claimed in claim 3 wherein said tunable laser source (1) and DFB are adjusted by adjusting the optical power output from the tunable laser source (1) by means of an arbitrary waveform generator (13) or by means of a polarization controller (3) from the injection coefficient ζ of the optical injection system of the laser (5).
8. The frequency measurement system of injection locking laser single-period loop self-excited subharmonic modulation as claimed in claim 4, wherein the injection coefficients ζ of the tunable laser light source (1) and DFB from the optical injection system of the laser (5) are adjusted by adjusting the optical power output by the tunable laser light source (1) by an arbitrary waveform generator (13) or by adjusting by a polarization controller (3).
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