CN117268442A - Sensing device based on optomechanical high-order harmonic mode-locked laser and working method thereof - Google Patents
Sensing device based on optomechanical high-order harmonic mode-locked laser and working method thereof Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/06—Construction or shape of active medium
- H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
- H01S3/067—Fibre lasers
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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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08018—Mode suppression
- H01S3/08022—Longitudinal modes
- H01S3/08031—Single-mode emission
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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
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/30—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects
- H01S3/302—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range using scattering effects, e.g. stimulated Brillouin or Raman effects in an optical fibre
Abstract
The invention relates to a sensing device based on an optomechanical high-order harmonic mode-locked laser and a working method thereof, comprising the following steps: the system comprises a mode-locked fiber laser, a sensing fiber and a beat frequency demodulation device; the sensing optical fiber is arranged in the annular laser resonant cavity; the invention fully utilizes the optical mechanical effect of forward Brillouin scattering (FSBS), designs a cavity length structure meeting the output of optical mechanical harmonic laser, enables a longitudinal mode matched with acoustic resonance frequency to oscillate, inhibits other longitudinal modes, obtains single longitudinal mode beat frequency signal output with high side mode inhibition ratio (SMSR) and high SNR in multi-longitudinal mode beat frequency signals as a sensing signal, and is convenient for monitoring and identifying external parameters.
Description
Technical Field
The invention relates to a sensing device based on an opto-mechanical high-order harmonic mode-locked laser and a working method thereof, belonging to the technical field of sensing.
Background
The fiber laser sensor has the advantages of narrow linewidth, high signal-to-noise ratio, high resolution, electromagnetic interference resistance and the like, and is currently researched and applied to measurement of parameters such as temperature, strain, pressure, current, magnetic field, sound pressure and the like. Fiber laser sensors can be divided into two categories according to sensing principles: one is a wavelength coded sensor similar to a Fiber Bragg Grating (FBG) sensor, and the other is a Beat Frequency (BF) sensor that converts the measured value into a beat frequency change between the two modes. Compared with a wavelength coding sensor, the beat frequency sensor does not need expensive wavelength detection equipment, has the advantages of convenience in inquiry, low cost, high resolution, real-time monitoring and the like, and is more suitable for practical sensing application.
Currently, most beat fiber laser sensors are constructed with a Distributed Bragg Reflector (DBR) structure comprising two cascaded wavelength matched FBGs, and the sensing signal may be a single longitudinal mode polarized beat signal between two polarization modes, or a specific beat signal selected from multiple longitudinal mode beat signals generated between different longitudinal modes. Comparing the above sensing device, it can be found that the existing beat frequency fiber laser sensor has the following defects: (1) The DBR fiber laser sensor needs to write a wavelength matching grating, so that the manufacturing difficulty and cost are increased; (2) The mode of single longitudinal mode polarization beat frequency detection has high signal readability, but the laser cavity length needs to be controlled in cm magnitude, the manufacturing difficulty is high, and mode jump is easy to occur; (3) The multi-longitudinal mode beat signal has a plurality of longitudinal mode signal outputs and each longitudinal mode sensitivity and signal-to-noise ratio (SNR) are mutually restricted, and how to balance the sensitivity and SNR and select an appropriate longitudinal mode beat signal as a sensing signal is a difficult problem. The existing temperature sensing device based on the mode-locked fiber laser is also a multi-longitudinal mode beat frequency output mode, and it is also difficult to balance the constraint relation between sensitivity and SNR to select a proper beat frequency signal as a sensing signal, or an additional radio frequency band-pass filter device and microwave scanning are needed to perform signal demodulation. Therefore, how to explore a new sensing demodulation mechanism, and to realize a beat frequency fiber laser sensor with low cost, simple structure and single longitudinal mode signal output is a key problem to be solved.
Disclosure of Invention
In order to solve the defects in the prior art, the invention provides a sensing device based on an optical mechanical harmonic mode-locked fiber laser, which aims to solve the technical problems that: the sensing device is simple in structure, low in cost, has a single longitudinal mode beat frequency sensing signal, and is convenient for measuring parameters to be measured in real time.
The invention fully utilizes the optical mechanical effect of forward Brillouin scattering (FSBS), designs a cavity length structure meeting the output of optical mechanical harmonic laser, enables a longitudinal mode matched with acoustic resonance frequency to oscillate, inhibits other longitudinal modes, obtains single longitudinal mode beat frequency signal output with high side mode inhibition ratio (SMSR) and high SNR in multi-longitudinal mode beat frequency signals as a sensing signal, and is convenient for monitoring and identifying external parameters.
The invention further provides a working method of the optical mechanical harmonic mode locking fiber laser sensing device based on the temperature sensing as a specific embodiment.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
a sensing device based on an opto-mechanical harmonic mode-locked fiber laser, comprising: the system comprises a mode-locked fiber laser, a sensing fiber and a beat frequency demodulation device;
the sensing optical fiber is arranged in the annular laser resonant cavity;
the round-trip frequency f of the ring laser resonator is equal to the spacing FSR between the different longitudinal modes, which are determined by the length of the resonator of the mode-locked fiber laser, expressed as formula (1):
f＝FSR＝c/nL (1)
in the formula (1), c is the light speed in vacuum, n is the effective refractive index of the sensing optical fiber, and L is the total cavity length of the mode-locked optical fiber laser;
when the laser cavity has a frequency f which is N times the round trip frequency f N Near a certain forward Brillouin acoustic wave mode R of sensing optical fiber in annular laser resonant cavity 0,m Is of resonant frequency omega m When, i.e. f N ＝N×f≈Ω m Frequency f N The corresponding longitudinal mode signal is amplified by oscillation; wherein m is an integer, m=1, 2,3 …, representing the order of the forward brillouin acoustic wave mode and the order of its corresponding resonant frequency; n is the multiple value of f at the moment, namely the order of harmonic mode locking;
the frequency interval FSR between different longitudinal modes is greater than or equal to R 0,m Forward Brillouin scattering spectrum linewidth gamma guided by acoustic wave mode m Half of (F), i.e. FSR.gtoreq.Γ m At/2, only one longitudinal mode frequency f is ensured N Is in the forward Brillouin scattering spectrum and is amplified by oscillation, while other frequency longitudinal modes are outside the forward Brillouin scattering spectrum or at the edge of the forward Brillouin scattering spectrum and are effectively inhibited, so that only the frequency f is finally realized N Single longitudinal mode signal output of (a);
when external temperature or strain is applied to the sensing fiber of the laser cavity, the length l and the refractive index n of the sensing fiber are changed, so that the frequency f is caused N The corresponding longitudinal mode is shifted by the frequency shift delta f N The relation between the temperature change amount Δt and the applied strain amount Δε is expressed as formula (2):
wherein Δf is the frequency offset of the fundamental frequency of the cavity, P e Is the strain optical coefficient, ζ is the thermal expansion coefficient of the sensing optical fiber, and f is N 、L、l、P e The 5 laser sensor parameters of xi are brought into the formula (2) to obtain the frequency offset delta f N And the variation relation between the variation amounts delta epsilon and delta T, thereby estimating the frequency f N Sensing sensitivity v corresponding to longitudinal mode T、Δε The method comprises the steps of carrying out a first treatment on the surface of the Frequency offset delta f of longitudinal mode sensing signal by beat frequency demodulation device N Real-time monitoring is carried out to enable delta f to be N /v T、Δε I.e. the change to be measured is demodulated.
According to the invention, the sensing optical fiber can be used for single measurement of different parameters of temperature, strain, humidity and sound pressure, and different types of optical fibers can be selected as the sensing optical fiber for different to-be-measured parameters.
According to the invention, the sensing optical fiber is preferably any one of commercial standard single mode optical fiber, high nonlinearity optical fiber, polyimide coated optical fiber or photonic crystal optical fiber.
According to the invention, the mode-locked fiber laser comprises a pump laser, a wavelength division multiplexer, a rare earth doped active fiber, an NPR mode locking structure and a fiber coupler, which are sequentially arranged along an optical path;
the NPR mode locking structure comprises a first polarization controller, a polarization dependent isolator and a second polarization controller;
the output light of the pumping laser enters the annular laser resonant cavity through the wavelength division multiplexer, and the rare earth doped active optical fiber emits light with wavelength within the gain bandwidth after absorbing the pumping light and continuously circulates in the optical fiber annular cavity until stable laser pulse output is realized; the NPR mode locking structure is used for causing the pulse modulation of the gain in the annular laser resonant cavity and realizing the stable mode locking pulse output under the specific wavelength in the gain bandwidth of the rare earth doped active optical fiber.
According to the invention, preferably, the beat frequency demodulation device comprises a photoelectric detector and a spectrum analyzer; the photoelectric detector is used for converting an output optical signal of the laser sensor into an electric signal and transmitting the electric signal to the spectrum analyzer; the spectrum analyzer realizes real-time acquisition and display of beat frequency sensing signals.
According to the invention, the detection bandwidth of the spectrum analyzer and the photodetector is preferably higher than the frequency of the beat frequency sensing signal.
Further preferably, the wavelength response range of the photodetector is 800-1700nm, and the bandwidth is more than 15GHz;
the bandwidth of the spectrum analyzer is 9kHz-6.2GHz, and the sampling resolution and the video resolution are adjustable and are smaller than 1Hz.
According to a preferred embodiment of the invention, the output power of the pump laser is adjustable.
The sensing method based on the opto-mechanical harmonic mode-locked fiber laser is realized by the sensing device and comprises the following steps:
step 1: according to f N ＝N×f≈Ω m And FSR is greater than or equal to Γ m 2, constructing an optical fiber laser meeting the opto-mechanical harmonic mode locking condition; f (f) N A frequency N times the laser cavity round trip frequency f; omega shape m For a certain forward Brillouin acoustic wave mode R of a sensing optical fiber in a ring-shaped laser resonant cavity 0,m Is a resonant frequency of (a); FSR is the interval between different longitudinal modes; Γ -shaped structure m Is R 0,m Forward brillouin scattering spectrum linewidth guided by acoustic wave mode;
as known from the opto-mechanical harmonic mode locking condition f=fsr=c/nL, the laser cavity round trip frequency f and FSR depend on the total cavity length L of the mode-locked fiber laser; whereas omega m And Γ m Depending on the type of fiber used in the laser resonator;
when the type of the optical fiber used by the laser resonant cavity is determined, the corresponding omega of the optical fiber is obtained m And Γ m Adding a sensing optical fiber with the length of L, so that f and FSR corresponding to the length L of the resonant cavity meet the opto-mechanical harmonic mode locking condition; raising the power of pumping light, regulating the optical fiber polarization controller to realize stable single longitudinal mode N-order harmonic mode locking laser output signal, and setting the frequency value corresponding to the single longitudinal mode sensing signal as f at room temperature N ；
Step 2: monitoring the frequency change of the single longitudinal mode sensing signal in real time by using a beat frequency demodulation device;
according to the formula Deltaf N /v T、Δε Namely, demodulating the change condition of the external sensing parameters; f (f) N The frequency value corresponds to a single longitudinal mode sensing signal at room temperature; Δf N The frequency offset of the single longitudinal mode sensing signal caused by the change of the external sensing parameters is monitored in real time through a beat frequency demodulation device; v T The sensitivity of the single longitudinal mode sensor signal obtained by the formula (2).
The frequency resolution of the beat frequency demodulation device is adjustable, the higher the frequency resolution is, the higher the corresponding demodulation precision is, and the high-precision measurement of the external parameter change can be realized by improving the frequency resolution.
The beneficial effects of the invention are as follows:
1. the invention can obtain the high signal-to-noise ratio single longitudinal mode beat frequency sensing signal for high-precision sensing. According to the invention, a wavelength matching grating and an ultra-short laser cavity are not required to be manufactured, and the laser cavity length is designed, so that the cavity round-trip frequency meets the opto-mechanical high-order harmonic mode locking condition, a single longitudinal mode beat frequency sensing signal with high signal to noise ratio and narrow line width can be easily obtained for sensing measurement, and the ultra-high side mode rejection ratio of the beat frequency sensing signal is beneficial to the signal identification and monitoring precision of the sensor. Compared with the existing laser sensing scheme, the scheme has the characteristics of simple structure, high sensitivity, good stability and the like.
2. The sensing method can realize ultra-high-precision parameter measurement by combining a spectrum analyzer with high frequency resolution. The sensor has the advantages of low cost, high sensitivity, simple structure and the like, and has great application potential in monitoring various parameters in the fields of agriculture, food storage, medicine and the like.
Drawings
FIG. 1 is a schematic diagram of a sensing device based on an opto-mechanical harmonic mode-locked fiber laser;
FIG. 2 is R 0,7 Schematic diagram of sensing principle of harmonic mode-locked laser guided by acoustic wave mode;
FIG. 3 is a diagram showing the comparison of the output states of two mode locks at 1.785MHz and 182 order harmonic frequencies of the fundamental frequency;
FIG. 4 is a graph of the spectrum of the sensing signal at different temperatures and a frequency shift-temperature response;
FIG. 5 is a graphical representation of real-time temperature test results of the example apparatus over 30 minutes;
1, pumping a laser; 2. a wavelength division multiplexer; 3. rare earth doped active optical fiber; 4. a first polarization controller; 5. a polarization dependent isolator; 6. a second polarization controller; 7. a sensing optical fiber; 8. an optical fiber coupler; 9. a temperature control box; 10. a beat frequency demodulation device; 10a, a photodetector; 10b, spectrum analyzer.
Detailed Description
For a better understanding of the technical solution of the present invention, the present invention will be described in more detail below with reference to the drawings and specific examples. The present exemplary embodiments are intended to facilitate an understanding of the invention and should not be construed as limiting the invention.
Example 1
A sensing device based on an opto-mechanical harmonic mode-locked fiber laser, for example, temperature measurement, as shown in fig. 1, includes: a mode-locked fiber laser, a sensing fiber 7 and a beat frequency demodulation device 10;
the sensing optical fiber 7 is arranged in the annular laser resonant cavity;
the round-trip frequency f of the ring laser resonator is equal to the spacing FSR between the different longitudinal modes, which are determined by the length of the resonator of the mode-locked fiber laser, expressed as formula (1):
f＝FSR＝c/nL(1)
in the formula (1), c is the light speed in vacuum, n is the effective refractive index of the sensing optical fiber 7, and L is the total cavity length of the mode-locked fiber laser;
when the laser cavity has a frequency f which is N times the round trip frequency f N Near a certain forward Brillouin acoustic wave mode R of sensing optical fiber 7 in annular laser resonant cavity 0,m Is of resonant frequency omega m When, i.e. f N ＝N×f≈Ω m Frequency f N The corresponding longitudinal mode signal is amplified by oscillation; as shown in FIG. 2, when the frequency f of the sensing signal N At omega m In the vicinity, the frequency is effectively amplified, and other frequencies are suppressed, so that single longitudinal mode output is realized. When the temperature changes, the sensing signal f N An offset occurs, and temperature sensing can be achieved by detecting its offset. Wherein m is an integer, m=1, 2,3 …, representing the order of the forward brillouin acoustic wave mode and the order of its corresponding resonant frequency; taking a standard commercial single-mode fiber as an example, a 7-order forward Brillouin acoustic wave mode R 0,7 Is of resonant frequency omega 7 About 319.8MHz; n is the multiple value of f at the moment, namely the order of harmonic mode locking; f (f) N As the sensing signal frequency of the mode-locked laser temperature sensor, the corresponding omega is obtained after the type and the length of the optical fiber in the laser are determined m And f value, and further determines NValue sum f as a sensor signal N A value;
the frequency interval FSR between different longitudinal modes is greater than or equal to R 0,m Forward Brillouin scattering spectrum linewidth gamma guided by acoustic wave mode m Half of (F), i.e. FSR.gtoreq.Γ m At/2, only one longitudinal mode frequency f is ensured N Is effectively oscillated and amplified in the forward Brillouin scattering spectrum, while other frequency longitudinal modes (… f N-1 、f N+1 、f N+2 …) is effectively suppressed outside the forward brillouin spectrum or at the edges of the forward brillouin spectrum, and only the frequency f is finally achieved N Single longitudinal mode signal output of (a);
the change of the length L of the sensing fiber 7 is caused when the external temperature changes, the influence of the temperature on the refractive index n is negligible, so that the total cavity length L of the whole mode-locked fiber laser is influenced, and the frequency f is caused N The corresponding longitudinal mode is shifted by the frequency shift delta f N The relational expression between the temperature change amount Δt is expressed as formula (3):
where Δf is the frequency offset of the fundamental cavity frequency, ζ is the thermal expansion coefficient of the sensing fiber 7, and f N The 4 laser sensor parameters of L, l and ζ are brought into the formula (3) to obtain the frequency offset Δf N And the temperature change amount DeltaT, thereby estimating the frequency f N Temperature sensitivity corresponding to longitudinal modeFrequency offset deltaf of longitudinal mode sensing signal by beat frequency demodulation device 10 N Real-time monitoring is carried out to enable delta f to be N /v T I.e. the temperature change situation deltat is demodulated.
The principle and the condition for realizing the opto-mechanical harmonic mode locking are as follows: when the laser pulse is transmitted in the sensing optical fiber 7, sound waves transmitted along the transverse direction of the optical fiber are generated due to electrostriction effect, the sound wave vibration changes the density of the material to cause the refractive index of the optical fiber to change, and the laser pulse is reversely processedModulating. When the driving pulse energy reaches the threshold E p When the pulse duration is much shorter than the acoustic lifetime and the acoustic resonance frequency Ω in the fiber is an integer multiple of the mode-locked laser repetition frequency f, the pulse frequency corresponding to the acoustic resonance frequency will be amplified by oscillation and suppress other frequencies. In the invention, in order to realize a temperature sensing device based on an optomechanical high-order harmonic laser, a certain forward Brillouin scattering acoustic wave mode R of the sensing optical fiber 7 needs to be ensured 0,m Is of resonant frequency omega m Is an integral multiple of the laser cavity round trip frequency f, i.e. Ω m =n×f, N is the order of the final harmonic mode locking. The cavity round trip frequency f is equal to the spacing FSR between the different longitudinal modes, denoted f=fsr=c/nL, where c is the speed of light in vacuum, the effective refractive index of the n-fiber, and L is the total cavity length of the laser. In order to realize single longitudinal mode beat signal output, the frequency interval between different longitudinal modes is more than or equal to R 0,m Half of the line width of the forward Brillouin scattering spectrum guided by the acoustic wave mode ensures that only one longitudinal mode is in the forward Brillouin scattering spectrum and the other longitudinal modes are outside or at the edges of the scattering spectrum. Therefore, according to the principle of the optomechanical high-order harmonic mode-locked laser, the two polarization controllers can be adjusted to enable the laser to reach the phase matching condition, so that a single longitudinal mode beat frequency sensing signal with narrow line width and high signal-to-noise ratio for temperature sensing is realized.
Example 2
A temperature sensing device based on an opto-mechanical harmonic mode-locked fiber laser according to embodiment 1, which is different in that:
the sensing optical fiber 7 is placed in the temperature control box 9 and is used for detecting the temperature change condition in the temperature control box 9.
The mode-locked fiber laser comprises a 980nm wavelength pump laser 1, a 980/1550nm wavelength division multiplexer 2, a rare earth doped active fiber 3, an NPR mode-locked structure and a 10:90 fiber coupler 8 which are sequentially arranged along a light path; the NPR mode-locking structure comprises a first polarization controller 4, a polarization dependent isolator 5 and a second polarization controller 6;
the output power of the 980nm wavelength pump laser 1 is adjustable, the pump laser 1 is used as the pump light of the laser sensor, and the output power of the pump laser 1 is adjustable. The specific threshold power needs to be adjusted by combining with the actual light path loss, cavity length, type of sensing optical fiber 7 and the like in the device; the invention can select different pumping sources, rare earth doped active optical fibers 3 and optical fiber passive devices corresponding to the matched wavelength, and realize 1550nm and 1060nm and other different wave band mode locking optical fiber laser sensors. The output power used in this example was 400mW; the length of the rare earth doped active optical fiber 3 is 20cm, the mode field diameter at 1550nm is 6.5+/-0.5 nm, and the peak absorption power at 1530nm is 110+/-10 dB/m;
the output light of the pumping laser 1 enters the annular laser resonant cavity through the wavelength division multiplexer 2, and the rare earth doped active optical fiber 3 emits light with wavelength within the gain bandwidth after absorbing the pumping light and continuously circulates in the optical fiber annular cavity until stable laser pulse output is realized; the two polarization controllers and the polarization dependent isolator 5 together form a conventional Nonlinear Polarization Rotation (NPR) saturable absorber, and the NPR mode locking structure is used for causing the pulse modulation of the gain in the annular laser resonant cavity to realize the stable mode locking pulse output under the specific wavelength within the gain bandwidth of the rare earth doped active optical fiber 3.
FIG. 2 is R 0,7 An acoustic wave mode guided harmonic mode locking laser temperature sensing schematic.
First, the principles of an opto-mechanical harmonic mode-locked laser will be described. When a laser pulse propagates in the optical fiber, an acoustic wave is generated, which is transmitted transversely along the optical fiber due to the electrostrictive effect, and the acoustic wave vibration changes the material density, which causes the refractive index of the optical fiber to change, which in turn modulates the pulse. When the driving pulse energy reaches the threshold E p When the pulse duration is much shorter than the acoustic lifetime and the resonant frequency Ω of the acoustic wave in the fiber is an integer multiple of the repetition frequency of the mode-locked laser, the resonant frequency will be amplified by oscillation and suppress other frequencies. The conditions for implementing an opto-mechanical higher order harmonic mode-locked laser can be expressed as:
wherein, delta epsilon r (z, t, r, θ) is the modulation of the dielectric constant of the material due to photoacoustic interactions, γ e Q is the overlap integral between the light field fundamental mode and the forward stimulated Brillouin scattering acoustic wave mode, E p Is pulse energy ρ a (R, θ) is the i-th order (i=1, 2,3 …) acoustic mode R 0,i Q is the propagation constant of the acoustic wave along the single mode fiber axis, n and a are the effective refractive index of the single mode fiber and the mode field area of the fundamental optical mode, respectively, c is the speed of light in vacuum, ρ 0 Is the density of silicon dioxide Γ B Is the brillouin line width and,is the phase offset between the sound wave and the pulse train driving it. From equation (4), the amplitude of the generated sound wave is equal to (4δ) 2 +Γ B 2 ) 1/2 Inversely proportional. According to the phase matching condition, when->The acoustic gain will occur in the (0, pi) interval and at +.>The maximum value is obtained.
Specifically, the total cavity length l=116.7m of the laser corresponds to the cavity round-trip frequency f= 1.7563MHz, and the 7 th-order acoustic wave mode R of forward brillouin scattering in the single-mode fiber is adopted 0,7 Corresponding resonant frequency omega 0,7 = 319.9MHz, about 182 times the laser cavity round trip frequency f. According to the principles of the optomechanical high-order mode-locked laser and the phase matching conditions, when the pumping power of the 980nm wavelength pumping source reaches a threshold value, an automatic mode-locked state with the resonant frequency of 319.63MHz can be obtained by carefully adjusting the polarization controller 4 and the polarization controller 6. At this time, the corresponding longitudinal mode of 319.63MHz oscillates, and other longitudinal modes are effectively suppressed, so that stable single-longitudinal-mode output with the longitudinal mode frequency of 319.63MHz can be finally obtained.
Then, a temperature sensing principle of single longitudinal mode output of the optomechanical high-order harmonic mode-locked laser is introduced. When the external temperature changes, the length L of the sensing optical fiber is changed, so that the cavity length L of the whole mode-locked optical fiber laser is affected, the frequency of a sensing signal to be detected is shifted, and the relation between the frequency shift deltaf and the temperature change deltaT can be expressed by a formula (3).
Where ζ=7.5×10 -6 ℃ -1 Is the thermal expansion coefficient of the optical fiber, f N N×f=182× 1.7563 = 319.63MHz. In this embodiment, l=100m, l=116.7m, and bringing the above laser sensor parameters into equation (3) can estimate that the temperature sensitivity of the 319.63MHz beat sensor signal is-2.06 kHz/°c. The temperature change condition can be demodulated by real-time monitoring of the frequency offset of the sensing signal by the beat frequency sensing demodulation device 10.
The beat frequency demodulation device 10 includes a photodetector 10a and a spectrum analyzer 10b; the photodetector 10a is configured to convert an output optical signal of the laser sensor into an electrical signal and transmit the electrical signal to the spectrum analyzer 10b; the spectrum analyzer 10b realizes real-time acquisition and display of beat frequency sensing signals. After the temperature sensitivity is determined, the higher the frequency resolution, the higher the corresponding temperature demodulation accuracy.
The detection bandwidth of the spectrum analyzer 10b and the photodetector 10a is higher than the frequency of the beat frequency sensing signal. The higher the frequency resolution of the spectrum analyzer 10b, the higher the temperature demodulation accuracy corresponding thereto.
The wavelength response range of the photodetector 10a is 800-1700nm, and the bandwidth is more than 15GHz;
the bandwidth of the spectrum analyzer 10b is 9kHz-6.2GHz, and the sampling resolution and the video resolution are adjustable and are smaller than 1Hz. The method has the advantages of realizing the display of the frequency spectrum and the three-dimensional spectrogram, and realizing the long-term real-time display record of the frequency spectrum.
The sensing optical fiber 7 is a standard commercial single-mode optical fiber, the length l=100deg.M, and the total cavity length L=of the whole mode-locked fiber laser116.7m, according to equation (1), the corresponding cavity round trip frequency f of the laser is 1.7563MHz; f=c/nL; wherein c is the light velocity in vacuum of 3×10 8 m/s, n=1.468 is the effective refractive index of a single mode fiber.
FIG. 3 is a diagram showing the comparison of the output states of two mode locks at 1.785MHz and 182 order harmonic frequencies of the fundamental frequency; wherein (a) in fig. 3 is a time domain pulse sequence test result diagram of a mode-locked laser with a fundamental frequency of 1.785MHz, and the corresponding pulse interval is 582ns; fig. 3 (b) is a spectrum diagram of a mode-locked laser with a fundamental frequency of 1.785MHz within a 9MHz bandwidth; fig. 3 (c) is a graph of the test result of the time domain pulse sequence of the 182 th harmonic mode-locked laser, and the pulse interval is 3.2ns; fig. 3 (d) shows a spectrum diagram of a 182 order harmonic mode-locked laser within a 9MHz bandwidth; the frequency scan width corresponding to the spectrum analyzer 10b is 9MHz, and the resolution bandwidth and the video bandwidth are both 100Hz. As can be seen from comparison of FIG. 3, when the 182-order harmonic self-mode locking state is reached, the signal-to-noise ratio of the 319.63MHz beat signal is obviously increased by 80dB, the SNR of the beat signal with other frequencies is effectively inhibited, and the side mode inhibition ratio (SMSR) of the 319.63MHz beat signal is 42dB, so that the beat signal has good single longitudinal mode output characteristic. The advantages of the optical mechanical harmonic mode locking laser sensor are fully demonstrated by the comparison, namely, a single longitudinal mode beat frequency sensing signal for temperature sensing can be easily obtained without manufacturing a wavelength matching grating, the structure is simple, and the cavity round trip frequency meets the optical mechanical high-order harmonic mode locking condition by designing the laser cavity length.
FIG. 4 is a graph of the spectrum of the sensing signal at different temperatures and a frequency shift-temperature response; winding a 100m temperature sensing single-mode fiber into a ring, placing the ring into a temperature control box 9 to sense temperature change, adjusting the temperature control box 9, and gradually heating the temperature from 20 ℃ to 65 ℃ with a heating gradient of 5 ℃. Fig. 4 (a) shows the spectrum of the sensing signal at different temperatures; it can be seen that as the temperature increases, the beat sense signal shifts to a small frequency. FIG. 4 (b) is a frequency shift versus temperature response plot of the sensed signal at a temperature between 20-65 ℃; it can be found that the frequency is linearly shifted along with the temperature, and the linear fitting is performed to obtain a frequency shift-temperature linear response coefficient, namely the temperature sensitivity of the sensing signal is-2.18 MHz/DEG C. When the frequency resolution is 100Hz, the corresponding temperature demodulation accuracy is about 0.0487 ℃.
FIG. 5 is a graphical representation of real-time temperature test results of the example apparatus over 30 minutes; the temperature-raising program of the temperature control box 9 was adjusted to raise the temperature stepwise from 20 ℃ to 65 ℃, wherein the temperature-raising gradients were 5 ℃, and each temperature gradient was maintained for 2 minutes. Recording the time-varying condition of the beat signal in real time by using the spectrum analyzer 10b, wherein (a) in fig. 5 is a 3D frequency waterfall diagram measured by the spectrum analyzer 10b within 30 minutes; it can be found that the 319.63MHz beat sense signal is significantly shifted. Reading the beat frequency sensing signal can obtain each time frequency value, and combining the temperature sensitivity obtained in (b) of fig. 4, the obtained signal frequency and the time-dependent change result of the test temperature are shown in (b) of fig. 5. The implementation test result shows that the temperature test result corresponds well to the actual applied temperature, and the sensing device can realize real-time measurement of the temperature.
Example 3
A temperature sensing device based on an opto-mechanical harmonic mode-locked fiber laser according to embodiment 2, which is different in that: the sensing fiber 7 is a highly nonlinear fiber, a polyimide coated fiber or a photonic crystal fiber. According to the different selected sensing optical fibers 7, the length, pumping power and total cavity length of the laser of the corresponding sensing optical fibers 7 are selected in combination with the opto-mechanical high-order harmonic mode locking condition.
Example 4
The temperature sensing method based on the opto-mechanical harmonic mode-locked fiber laser is realized by the temperature sensing device and comprises the following steps:
step 1: according to f N ＝N×f≈Ω m And FSR is greater than or equal to Γ m 2, constructing an optical fiber laser meeting the opto-mechanical harmonic mode locking condition; f (f) N A frequency N times the laser cavity round trip frequency f; omega shape m For a certain forward Brillouin acoustic wave mode R of the sensing optical fiber 7 in the annular laser resonant cavity 0,m Is a resonant frequency of (a); FSR is the interval between different longitudinal modes; Γ -shaped structure m Is R 0,m Forward brillouin scattering spectrum linewidth guided by acoustic wave mode;
from opto-mechanical harmonic mode locking condition f=fsr=cIt is known that the laser cavity round trip frequency f and FSR depend on the total cavity length L of the mode-locked fiber laser; whereas omega m And Γ m Depending on the type of fiber used in the laser resonator;
when the type of the optical fiber used by the laser resonant cavity is determined, the corresponding omega of the optical fiber is obtained m And Γ m A sensing optical fiber 7 with the length of L is added, so that f and FSR corresponding to the length L of the resonant cavity meet the opto-mechanical harmonic mode locking condition; raising the power of pumping light, regulating the optical fiber polarization controller to realize stable single longitudinal mode N-order harmonic mode locking laser output signal, and setting the frequency value corresponding to the single longitudinal mode sensing signal as f at room temperature N ；
Step 2: monitoring the frequency change of the single longitudinal mode sensing signal in real time by using the beat frequency demodulation device 10;
according to the formula Δt=Δf N /v T Namely, demodulating the change condition of the external temperature; f (f) N The frequency value corresponds to a single longitudinal mode sensing signal at room temperature; Δf N The frequency offset of the single longitudinal mode sensing signal caused by the external temperature change is monitored in real time through the beat frequency demodulation device 10;the temperature sensitivity of the single longitudinal mode sensor signal obtained by the formula (2).
The beat frequency demodulation device 10 has adjustable frequency resolution, and the higher the frequency resolution is, the higher the demodulation accuracy is, and the higher the frequency resolution is, the high-accuracy measurement of the external temperature change can be realized.
The foregoing has shown and described the basic principles, principal features and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the foregoing embodiments, which have been described in the foregoing embodiments and description merely illustrates the principles of the invention, and various changes and modifications may be made therein without departing from the spirit and scope of the invention, the scope of which is defined in the appended claims, specification and their equivalents.
Claims (10)
1. A sensing device based on an opto-mechanical harmonic mode-locked fiber laser, comprising: the system comprises a mode-locked fiber laser, a sensing fiber and a beat frequency demodulation device;
the sensing optical fiber is arranged in the annular laser resonant cavity;
the round-trip frequency f of the ring laser resonator is equal to the spacing FSR between the different longitudinal modes, which are determined by the length of the resonator of the mode-locked fiber laser, expressed as formula (1):
f＝FSR＝c/nL (1)
in the formula (1), c is the light speed in vacuum, n is the effective refractive index of the sensing optical fiber, and L is the total cavity length of the mode-locked optical fiber laser;
when the laser cavity has a frequency f which is N times the round trip frequency f N Near a certain forward Brillouin acoustic wave mode R of sensing optical fiber in annular laser resonant cavity 0,m Is of resonant frequency omega m When, i.e. f N ＝N×f≈Ω m Frequency f N The corresponding longitudinal mode signal is amplified by oscillation; wherein m is an integer, m=1, 2,3, representing the order of the forward brillouin acoustic wave mode and the order of the corresponding resonant frequency thereof; n is the multiple value of f at the moment, namely the order of harmonic mode locking;
the frequency interval FSR between different longitudinal modes is greater than or equal to R 0,m Forward Brillouin scattering spectrum linewidth gamma guided by acoustic wave mode m Half of (F), i.e. FSR.gtoreq.Γ m At/2, only one longitudinal mode frequency f is ensured N Is effectively oscillated and amplified in the forward Brillouin scattering spectrum, and other frequency longitudinal modes are effectively inhibited outside the forward Brillouin scattering spectrum or at the edge of the forward Brillouin scattering spectrum, so that only the frequency f is finally realized N Single longitudinal mode signal output of (a);
when external temperature or strain is applied to the sensing fiber of the laser cavity, the length l and the refractive index n of the sensing fiber are changed, resulting in a frequency f N The corresponding longitudinal mode is shifted by the frequency shift delta f N The relation between the temperature change amount Δt and the applied strain amount Δε is expressed as formula (2):
wherein Δf is the frequency offset of the fundamental frequency of the cavity, P e Is the strain optical coefficient, ζ is the thermal expansion coefficient of the sensing optical fiber, and f is N 、L、l、P e The 5 laser sensor parameters of xi are brought into the formula (2) to obtain the frequency offset delta f N And the variation relation between the variation amounts delta epsilon and delta T, thereby estimating the frequency f N Sensing sensitivity v corresponding to longitudinal mode T、Δε The method comprises the steps of carrying out a first treatment on the surface of the Frequency offset delta f of longitudinal mode sensing signal by beat frequency demodulation device N Real-time monitoring is carried out to enable delta f to be N /v T、Δε I.e. the change to be measured is demodulated.
2. The sensing device based on the optical mechanical harmonic mode-locked fiber laser according to claim 1, wherein the sensing fiber is used for single measurement of different parameters of temperature, strain, humidity and sound pressure.
3. The sensing device based on the opto-mechanical harmonic mode-locked fiber laser according to claim 1, wherein the mode-locked fiber laser comprises a pump laser, a wavelength division multiplexer, a rare earth doped active fiber, an NPR mode-locked structure and a fiber coupler which are sequentially arranged along an optical path;
the NPR mode locking structure comprises a first polarization controller, a polarization dependent isolator and a second polarization controller;
the output light of the pumping laser enters the annular laser resonant cavity through the wavelength division multiplexer, and the rare earth doped active optical fiber emits light with wavelength within the gain bandwidth after absorbing the pumping light and continuously circulates in the optical fiber annular cavity until stable laser pulse output is realized; the NPR mode locking structure is used for causing the pulse modulation of the gain in the annular laser resonant cavity and realizing the stable mode locking pulse output under the specific wavelength in the gain bandwidth of the rare earth doped active optical fiber.
4. The sensing device based on the optical mechanical harmonic mode-locked fiber laser according to claim 1, wherein the beat frequency demodulation device comprises a photoelectric detector and a spectrum analyzer; the photoelectric detector is used for converting an output optical signal of the laser sensor into an electric signal and transmitting the electric signal to the spectrum analyzer; the spectrum analyzer realizes real-time acquisition and display of beat frequency sensing signals.
5. The optical mechanical harmonic mode-locked fiber laser based sensing device according to claim 4, wherein the detection bandwidth of the spectrum analyzer and the photodetector is higher than the frequency of the beat frequency sensing signal.
6. The optical mechanical harmonic mode-locked fiber laser based sensing device according to claim 4, wherein the wavelength response range of the photodetector is 800-1700nm, and the bandwidth is greater than 15GHz.
7. The optical mechanical harmonic mode-locked fiber laser based sensing device according to claim 4, wherein the bandwidth of the spectrum analyzer is 9kHz-6.2GHz, and the sampling resolution and the video resolution are adjustable and are both smaller than 1Hz.
8. A sensing device based on an opto-mechanical harmonic mode-locked fiber laser according to claim 3, characterized in that the output power of the pump laser is adjustable.
9. The optical mechanical harmonic mode-locked fiber laser based sensing device according to any one of claims 1-8, wherein the sensing fiber is a commercial standard single mode fiber, a highly nonlinear fiber, a polyimide coated fiber or a photonic crystal fiber.
10. A sensing method based on an opto-mechanical harmonic mode-locked fiber laser, realized by the sensing device according to any one of claims 1-9, comprising the steps of:
step 1: according to f N ＝N×f≈Ω m And FSR is greater than or equal to Γ m 2, constructing the optical mechanical harmonic lockA fiber laser in a mode condition; f (f) N A frequency N times the laser cavity round trip frequency f; omega shape m For a certain forward Brillouin acoustic wave mode R of a sensing optical fiber in a ring-shaped laser resonant cavity 0,m Is a resonant frequency of (a); FSR is the interval between different longitudinal modes; Γ -shaped structure m Is R 0,m Forward brillouin scattering spectrum linewidth guided by acoustic wave mode;
as known from the opto-mechanical harmonic mode locking condition f=fsr=c/nL, the laser cavity round trip frequency f and FSR depend on the total cavity length L of the mode-locked fiber laser; whereas omega m And Γ m Depending on the type of fiber used in the laser resonator;
when the type of the optical fiber used by the laser resonant cavity is determined, the corresponding omega of the optical fiber is obtained m And Γ m Adding a sensing optical fiber with the length of L, so that f and FSR corresponding to the length L of the resonant cavity meet the opto-mechanical harmonic mode locking condition; raising the power of pumping light, regulating the optical fiber polarization controller to realize stable single longitudinal mode N-order harmonic mode locking laser output signal, and setting the frequency value corresponding to the single longitudinal mode sensing signal as f at room temperature N ；
Step 2: monitoring the frequency change of the single longitudinal mode sensing signal in real time by using a beat frequency demodulation device;
according to the formula Δt=Δf N /v T Namely, demodulating the change condition of the external temperature; f (f) N The frequency value corresponds to a single longitudinal mode sensing signal at room temperature;
Δf N the frequency offset of the single longitudinal mode sensing signal caused by the change of the external temperature is monitored in real time through a beat frequency demodulation device;the temperature sensitivity of the single longitudinal mode sensor signal obtained by the formula (2).
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