CN114526891A - Method and device for measuring electrostriction coefficient of optical fiber - Google Patents
Method and device for measuring electrostriction coefficient of optical fiber Download PDFInfo
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Abstract
The invention relates to a method and a device for measuring an optical fiber electrostriction coefficient, which are characterized in that firstly, the electrostriction coefficient is theoretically modeled based on the stimulated Brillouin scattering effect of the optical fiber to obtain the relation between the electrostriction coefficient and the gain peak value and the peak intrinsic line width of a stimulated Brillouin gain spectrum, then the gain peak value is solved and the Brillouin frequency shift value is determined based on the envelope detection technology, then the peak intrinsic line width of the stimulated Brillouin gain spectrum is determined through microwave frequency sweep by utilizing a phase modulation-direct demodulation link, and finally, the electrostriction coefficient value of the optical fiber to be measured is calculated according to the gain peak value and the peak intrinsic line width. Different from the traditional method, the invention is based on the stimulated Brillouin scattering effect, adopts the microwave photon technology to measure the electrostriction coefficient of the optical fiber, avoids high environmental sensitivity and instability caused by coherent interference, and simultaneously realizes the measurement process in the electric domain, thereby further improving the measurement precision and having better application value.
Description
Technical Field
The invention relates to the crossing field of microwave technology and optical communication technology, in particular to a method and a device for measuring an optical fiber electrostriction coefficient based on microwave photon technology.
Background
The electrostrictive capabilities of different types of optical fibers tend to be different. The electrostriction coefficient is an important parameter for representing electrostriction capability, reflects the length change capability of the material along the direction of an external electric field, is an important parameter in optical fiber application, and is an important basis for the optical fiber application if the electrostriction coefficient can be accurately measured.
The traditional method for measuring the electrostriction coefficient of the optical fiber mainly comprises the steps of applying an electric field to the optical fiber to change the length of the optical fiber, and analyzing the interference phenomenon of an optical signal transmitted through the optical fiber by constructing a Mach-Zehnder interferometer to measure the electrostriction coefficient of the optical fiber. The traditional measuring method has the following defects: first, the conventional measurement method is based on the interference principle of light, and has to be improved in environmental sensitivity and stability. Secondly, the traditional measurement method is completed in the optical domain, and the measurement precision is not high due to the limitation of spectral resolution.
Disclosure of Invention
One of the purposes of the invention is to provide a method for measuring the electrostrictive coefficient of an optical fiber by adopting a microwave photon technology based on the stimulated Brillouin scattering effect, so as to overcome the defects of low precision, high environmental sensitivity and poor stability of the traditional measuring method.
In order to achieve the purpose, the invention adopts the following technical scheme: a method for measuring the electrostrictive coefficient of an optical fiber comprises the following steps:
determining the relationship between the optical fiber electrostriction coefficient and the gain peak value and the peak intrinsic line width of the stimulated Brillouin gain spectrum based on the stimulated Brillouin scattering effect of the optical fiber and the nonlinear optical principle;
pump light is utilized to excite nonlinearity of an optical fiber to be detected so as to generate stimulated Brillouin scattering, an envelope detection technology is combined to calculate a stimulated Brillouin gain spectrum gain peak value and determine a Brillouin frequency shift value; the method comprises the steps that a microwave frequency sweep signal is utilized to carry out phase modulation on optical carriers which are in the same source with pump light, conversion from microwave frequency sweep to optical frequency sweep is achieved, and the peak intrinsic line width of a stimulated Brillouin gain spectrum is calculated by extracting the characteristics of the stimulated Brillouin gain spectrum and combining the square law detection principle;
and obtaining the electrostriction coefficient of the optical fiber to be tested according to the relationship among the electrostriction coefficient of the optical fiber, the gain peak value of the stimulated Brillouin gain spectrum and the intrinsic line width of the peak value.
The relationship among the optical fiber electrostriction coefficient, the gain peak value of the stimulated Brillouin gain spectrum and the intrinsic line width of the peak value is as follows:
in the above formula, γeIs the optical fiber electrostrictive coefficient, n is the optical fiber refractive index, lambdapIs the wavelength of the pump light, p0Is the density of the fiber, c is the speed of light, VaAt the speed of sound, Δ νBIs the intrinsic line width, g, of the gain spectrumBIs the stimulated brillouin gain spectral gain peak.
In addition, the invention also relates to a device for measuring the electrostriction coefficient of the optical fiber, which comprises a light source supply module, a measuring main body module and a signal processing and analyzing module which are sequentially connected;
the light source supply module is used for providing a single-frequency point signal light source, shunting an optical signal into a carrier optical signal, a pumping optical signal and a reference optical signal and correspondingly inputting the carrier optical signal, the pumping optical signal and the reference optical signal into an upper link, a middle link and a lower link of the measurement main body module;
the upper link is used for carrying out phase modulation on a carrier optical signal, reversely inputting the carrier optical signal into the middle link through an optical fiber to be tested, and changing the wavelength of a modulated optical signal sideband by tuning and modulating microwave frequency to realize conversion from microwave frequency sweep to optical frequency sweep; the middle-layer link is used for inputting the pump light signals into the optical fiber to be tested after power tuning, exciting nonlinearity, generating stimulated Brillouin scattering, exciting a gain spectrum, and transmitting backward transmitted Stokes light to the signal processing and analyzing module; the lower link is used for transmitting the reference optical signal to the signal processing and analyzing module;
the signal processing and analyzing module is used for combining the optical signals input from the middle link and the lower link, converting the optical signals into electric signals through envelope detection, performing frequency-selective filtering on the electric signals, and performing spectrum and power analysis on the filtered signals to obtain a gain peak value and a peak intrinsic line width of the stimulated Brillouin gain spectrum, and obtaining the gain peak value and the peak intrinsic line width of the stimulated Brillouin gain spectrum according to the relationship between the optical fiber electrostriction coefficient and the gain peak value and the peak intrinsic line width of the stimulated Brillouin gain spectrum
Calculating the electrostriction coefficient of the optical fiber to be detected;
in the above formula, γeIs the optical fiber electrostrictive coefficient, n is the optical fiber refractive index, lambdapIs the wavelength of the pump light, p0Is the density of the fiber, c is the speed of light, VaIs the speed of sound, Δ νBIs the intrinsic line width, g, of the gain spectrumBIs the stimulated brillouin gain spectrum gain peak.
In an embodiment of the present invention, the light source supply module includes a laser, an isolator and a coupler 1, which are connected in sequence, a single-frequency point optical signal generated by the laser enters the coupler 1 through the isolator and is divided into three branches, which are respectively transmitted to an upper link, a middle link and a lower link of the measurement main module, the isolator is used for isolating return of the optical signal, and the coupler 1 is used for splitting the optical signal.
In an embodiment of the present invention, the signal processing and analyzing module includes a coupler 2, a detector, a narrow band filter and a signal analyzer, which are sequentially connected, where the coupler 2 is configured to combine waves of optical signals input from the middle link and the lower link, the detector converts the optical signals into electrical signals through envelope detection, the narrow band filter is configured to perform frequency-selective filtering on signals output by the detector, and the signal analyzer is configured to perform spectrum and power analysis on the filtered signals.
In an embodiment of the present invention, the upper link includes a polarization controller, a phase modulator, and an adjustable microwave signal source, an input end of the polarization controller is connected to one output end of the coupler 1, an output end of the polarization controller and the adjustable microwave signal source are both connected to the phase modulator, the polarization controller is configured to control a polarization state of a carrier optical signal so as to match the polarization state of the phase modulator, and the optical signal input to the phase modulator is phase-modulated by an electrical signal sent by the adjustable microwave signal source and then reversely transmitted to the middle link through the optical fiber to be measured.
Furthermore, the middle link comprises an amplifier, an adjustable attenuator and a circulator which are connected in sequence, the amplifier is connected with one output end of the coupler 1, the pump optical signal entering the middle link enters the adjustable attenuator after being amplified by the amplifier, and the power of the optical signal entering the circulator is adjusted by the adjustable attenuator; and pumping light signals flow into the right port from the left port of the circulator and then enter the optical fiber to be detected, the nonlinear effect of the optical fiber to be detected is excited to generate stimulated Brillouin scattering after the power value of the pumping light signals exceeds the threshold value power, and then the pumping light signals reversely transmit the Stokes light to the right port of the circulator and flow into a lower link from the lower port of the circulator.
Further, the lower link includes an optical fiber transmission line connecting the coupler 1 and the coupler 2, and the optical fiber transmission line is used for transmitting the reference optical signal.
The method comprises the following steps that by disconnecting the upper link, only the middle link and the lower link are connected, a pumping light signal provided by a light source supply module enters the middle link, is amplified by an amplifier and then enters a variable optical attenuator to be subjected to power tuning, then flows into a right port through a left port of a circulator and then enters a to-be-measured optical fiber to be subjected to nonlinear excitation, stimulated Brillouin scattering is generated, generated Stokes light is reversely transmitted to a right port of the circulator and then enters a coupler 2 together with a reference light signal transmitted by the lower port and the lower link, two phase dry light signals are subjected to beat frequency by a detector and then are reduced into electric signals to be output to a narrow-band filter, the output signal frequency and the output signal power after passing through the narrow-band filter are read by the signal analyzer, and the value of a gain peak value of a stimulated Brillouin gain spectrum is calculated by combining the following formula:
in the above formula, gBIs the value of the gain peak of the stimulated Brillouin gain spectrum, poutFor the output signal power, p is the responsivity of the detector, A1For the amplitude of the pump light entering the circulator, A4For the amplitude of the reference optical signal, l and α are the length and loss coefficient of the optical fiber to be measured, respectively.
Further, by simultaneously connecting the upper link, the middle link and the lower link, the middle link excites the nonlinearity of the optical fiber to be detected to generate stimulated Brillouin scattering, the carrier optical signal is subjected to phase modulation by the upper link, the wavelength of the optical signal sideband after modulation is changed by tuning and modulating the microwave frequency, the conversion from microwave frequency sweep to optical frequency sweep is realized, so that the modulated optical signal output by the upper link passes through the optical fiber to be detected, the negative first-order sideband of the modulated optical signal is positioned in a Brillouin amplification passband and amplified, the first-order sideband is suppressed, the modulated microwave signal is restored and modulated by the detector after envelope detection to realize the extraction of the amplified passband characteristic of the stimulated Brillouin gain spectrum, and the reference optical signal transmitted by the lower link compensates the carrier loss of the upper link to improve the microwave signal power value output by the detector, and finally, reading the frequency corresponding to the maximum gain and the frequency corresponding to the position reduced to the quarter of the maximum gain through the signal analyzer, and calculating the peak intrinsic line width of the stimulated Brillouin gain spectrum by combining the following formula:
ΔνB＝2|fmax-f-6dB|；
in the above formula, Δ νBIs the peak intrinsic linewidth, f, of the stimulated Brillouin gain spectrummaxFor maximum gain corresponding to frequency, f-6dBTo reduce to the frequency corresponding to the quarter maximum gain.
The method is completely different from the traditional measuring method, the microwave photon technology is adopted to measure the electrostriction coefficient of the optical fiber, the physical essence of the measuring method is based on the stimulated Brillouin effect rather than the coherent interference, and the method overcomes the defects of high environmental sensitivity and instability in the traditional measuring method when the electrostriction coefficient of the optical fiber is measured. It is particularly worth mentioning that the signal processing process of the present invention is performed in the "electrical domain" rather than the "optical domain", and the spectral resolution of the "electrical domain" is much higher than the wavelength resolution of the "optical domain" (for 1550nm wavelength, the resolution of 0.1nm corresponds to 2.5GHz), so the test accuracy is higher than that of the conventional method. In addition, compared with the traditional method, the test scheme involved in the invention is simpler and more feasible, and has better practical value.
Drawings
Fig. 1 is a simple structure and principle analysis diagram of stimulated brillouin scattering.
Fig. 2 is a simple structure and principle analysis diagram based on the stimulated brillouin scattering amplifier.
Fig. 3 is a schematic analysis diagram of microwave frequency sweep in the embodiment.
FIG. 4 is a block diagram of an optical fiber electrostrictive coefficient measuring apparatus according to an embodiment.
Detailed Description
To facilitate a better understanding of the present invention as compared to the prior art by those skilled in the art, the present invention is further described below in conjunction with the accompanying drawings, it being understood that the following detailed description is provided for illustration only and not for the purpose of limiting the invention specifically.
In this embodiment, first, theoretical modeling is performed on electrostrictive coefficients based on a stimulated brillouin scattering effect in an optical fiber, and a physical relationship between the electrostrictive coefficients and a gain peak value and an intrinsic line width of a stimulated brillouin gain spectrum is solved; then solving peak gain and determining Brillouin frequency shift value based on envelope detection technology; determining the peak intrinsic line width of the stimulated Brillouin gain spectrum by microwave frequency sweep by using the constructed phase modulation-direct demodulation link; and finally solving the electrostriction coefficient value. The contents of each part will be described in detail below.
FIG. 1 showsThe simple structure and the principle analysis of the stimulated Brillouin scattering are provided. Because the circulator is a nonreciprocal device, according to the clockwise direction, the input optical signals of the left port can only be output from the right port, the input optical signals of the right port can only be output from the lower port, the input optical signals of the lower port can only be output from the left port, and other transmission paths are forbidden. Pump light (frequency f)p) And the optical fiber enters the optical fiber ring after entering the left part and exiting the right part, and the nonlinearity of the optical fiber is excited when the optical power is large enough and the optical fiber is long enough, so that the stimulated Brillouin scattering phenomenon is generated. Physically, brillouin scattering is the scattering of an optical signal propagating in a medium by acoustic vibrations of the lattice of the material of the medium. The acoustic vibrations doppler shift the optical signal resulting in a new optical signal component. The scattering process is an inelastic process, and there are two possible generation forms: during stokes scattering, one photon annihilates with the production of one photon at a lower frequency and one phonon; in the anti-stokes scattering process, annihilation of a photon and a phonon is accompanied by the generation of a photon of higher frequency. The stokes process, which scatters light in the opposite direction to the input optical signal and at a frequency lower than the frequency of the pump light, i.e., f, is more likely to occurp-fs＝fbWherein f issAnd fbRespectively the stokes optical frequency and the brillouin frequency shift. According to the law of conservation of energy, the power of the pump light is reduced after the pump light is converted into the Stokes light, and an amplification passband, namely a stimulated Brillouin gain spectrum, is generated at the center frequency of the Stokes light, wherein the center frequency is generally 9-10GHz, and the bandwidth is in the order of MHz; while an attenuation passband is created at the pump light center frequency. The gain peak whose amplification passband is known from nonlinear optics principles can be expressed as
Wherein, γeI.e. the coefficient of optical expansion and contraction to be determined, n is the refractive index of the optical fiber, lambdapIs the wavelength of the pump light, p0Is the density of the fiber, c is the speed of light, VaIs the speed of sound, Δ νBThe intrinsic linewidth of the gain spectrum. Solving the above formula can not easily obtainCoefficient of photo-induced expansion
Except for the gain peak gBAnd intrinsic line width DeltavBThe other parameters are constants, i.e. by solving for the gain peak gBAnd intrinsic line width DeltavBThe optical fiber magnetostriction coefficient gamma can be obtainedeThe value of (c).
Fig. 2 shows a simple structure based on a stimulated brillouin scattering amplifier and principle analysis thereof. Based on the stimulated Brillouin scattering effect analysis in FIG. 1, after the pump light is input into the optical fiber ring, the stimulated Brillouin nonlinearity of the optical fiber is excited, and the pump light is shifted in frequency by fbAn amplification gain region is generated when the light is incident in the reverse direction (frequency f)in) The optical fiber ring equivalently combines a narrow-band-pass filter and a narrow-band amplifier when passing through the optical fiber ring from right to left, the narrow-band filtering is realized, signals in a pass band can be amplified, the signals are finally input into a lower port and output through a right port of the circulator, the frequency of output signals is foutWherein f isin＝foutAnd f isp-fin＝fb。
Fig. 4 shows the structure of the apparatus for fiber electrostrictive coefficient measurement. The device mainly comprises a light source supply module, a measurement main body module and a signal processing and analyzing module.
The light source supply module provides a single-frequency point signal light source, shunts an optical signal into a carrier optical signal, a pump optical signal and a reference optical signal, and injects the carrier optical signal, the pump optical signal and the reference optical signal into an upper link, a middle link and a lower link of the measurement main body module respectively. Specifically, the light source supply module includes laser, isolator and coupler 1 that connect gradually, and the single frequency point light signal that the laser produced gets into coupler 1 behind the isolator and is divided into three branch road and transmit respectively to measuring the upper link, middle level link and the lower floor link of main part module, and the isolator is used for keeping apart the passback of light signal, and coupler 1 is used for realizing 1 of light signal: 3, splitting flow.
The measurement subject module is a core structure generating the stimulated brillouin effect. Wherein the upper link functions to implement phase modulationSo as to complete the conversion from microwave frequency sweep to optical frequency sweep; the middle link is used for realizing the stimulated Brillouin scattering effect, exciting the gain spectrum and facilitating the solution of the gain peak value gBAnd intrinsic line width DeltavB(ii) a The lower link mainly carries a reference optical signal and provides the reference optical signal for the signal processing and analyzing module. Specifically, the upper link comprises a polarization controller, a phase modulator and an adjustable microwave signal source, the input end of the polarization controller is connected with one output end of the coupler 1, the output end of the polarization controller and the adjustable microwave signal source are both connected with the phase modulator, the polarization controller is used for controlling the polarization state of the carrier optical signal to be matched with the polarization state of the phase modulator, and the optical signal input to the phase modulator is subjected to phase modulation by an electric signal sent by the adjustable microwave signal source and then is reversely transmitted to the middle link through the optical fiber to be detected. The middle link comprises an amplifier, an adjustable attenuator and a circulator which are sequentially connected, the amplifier is connected with one output end of the coupler 1, a pumping optical signal entering the middle link enters the adjustable attenuator after being amplified by the amplifier, and the power of an optical signal entering the circulator is adjusted by the adjustable attenuator; and pumping light signals flow into the right port from the left port of the circulator and then enter the optical fiber to be detected, the nonlinear effect of the optical fiber to be detected is excited to generate stimulated Brillouin scattering after the power value of the pumping light signals exceeds the threshold value power, and then the pumping light signals reversely transmit the Stokes light to the right port of the circulator and flow into a lower link from the lower port of the circulator. The lower link is mainly composed of an optical fiber transmission line connecting the coupler 1 and the coupler 2, and the reference optical signal is transmitted to the signal processing and analyzing module through the optical fiber transmission line.
The signal processing and analyzing module is the core of generating the measurement data. As shown in the figure, the signal processing and analyzing module includes a coupler 2, a detector, a narrow band filter and a signal analyzer, which are connected in sequence, the coupler 2 is used for combining the optical signals input from the middle link and the lower link, the detector converts the optical signals into electric signals through envelope detection, the narrow band filter is used for performing frequency-selective filtering on the signals output by the detector, and the signal analyzer is used for performing spectrum and power analysis on the filtered signals.
The process of measuring the electrostriction coefficient of the optical fiber by using the device mainly comprises the following steps: the first step is to disconnect the upper link and only let the middle link and the lower link work jointly, aiming at solving the gain peak gBWhile simultaneously determining Brillouin frequency shift fb. The second step, linking the upper link to realize the combined work of the upper, middle and lower links, aiming at solving the intrinsic line width DeltavBThe value is obtained. Thirdly, according to the gain peak value gBAnd intrinsic line width DeltavBCalculating the optical fiber's optical expansion coefficient gammae。
The specific process and principle of the first step are as follows: the pump light signal provided by the light source supply module enters the middle-layer link, is amplified by the amplifier and enters the variable optical attenuator for power tuning, then enters the optical fiber to be tested through the circulator to excite nonlinearity, stimulated Brillouin scattering is generated, Stokes light is reversely transmitted to the right port of the circulator and then simultaneously enters the coupler 2 through the lower port and the reference light signal in the lower-layer link, the two dry light signals are subjected to beat frequency by the detector and then are reduced into electric signals, and then the electric signals are filtered by the narrow-band filter and enter the signal analyzer to realize frequency spectrum and power analysis, so that a gain peak value g is obtainedBWith Brillouin frequency shift fb. Specifically, the method comprises the following steps: as shown in FIG. 4, the left end of the circulator is marked A1And with A1Representing the amplitude of the pump light entering the circulator; the lower end of the ring is marked A2And with A2Representing the light amplitude of the Stokes light transmitted reversely after passing through the optical fiber and the circulator; marking the tail end of the optical fiber to be tested as A3With A3The amplitude of the stokes light representing the reverse transmission at the end of the fiber (i.e., the origin of the reverse transmission); the lower link is marked as A4And with A4Representing the amplitude of the reference optical signal. Neglecting the insertion loss of the circulator (which can be deducted by "calibration" in engineering applications) to facilitate theoretical derivation and modeling, then A1I.e. the amplitude of the pump light entering the fiber to be measured, A3I.e. amplitude value and gain peak value of Stokes light signal transmitted reversely
A1And A4The optical power meter can directly measure the optical power, and the measured value is a known value; a. the2And A3Cannot be measured directly and is unknown (supplementary note that the spectrum at the lower end of the circulator is more complex, and A is2Only the stokes light amplitude and cannot be measured directly). Solving for the gain peak gBThe procedure of (2) is as follows. Let the reference optical signal entering the coupler 2 be
The Stokes light signal is
ωpAnd omegasAngular frequencies of the reference light signal and the Stokes light signal, respectively, andp＝2πfp，ωs＝2πfs. The two optical signals are subjected to envelope detection after being subjected to beat frequency by a detector, and finally are restored into electric signals, and the electric signals output by the electric signals have current Ip＝ρ|E2+E4|2Is solved to obtain
Where p is the responsivity of the detector, ωbIs Brillouin frequency shift angular frequency with a value of omegab＝2πfb＝ωp-ωs. The current value of an output signal after passing through a narrow-band filter is I-2 rho A2A4cosωbt. Final reading of its output signal frequency f by signal analyserb(i.e., the aforementioned Brillouin frequency shift) and output signal power pout. According to
I0For the effective current of the signal, p can be solvedout＝2(ρA2A4)2And R is shown in the specification. When the length of the optical fiber is l and the loss coefficient is alpha, the optical fiber can be obtained
Finally, A is determined from the measured power values3Has a value of
While
Further, it can be found that:
finally obtaining the gain peak value gBThe value of (c).
The specific process and principle of the second step are as follows: and the upper link is communicated, so that the three links work simultaneously. As described above, the middle link constructs the stimulated brillouin scattering architecture and excites the brillouin gain spectrum of the optical fiber to be measured, as long as the frequency of the communication optical signal is within the passband of the gain spectrum, the signal is amplified, and the signal outside the passband is suppressed. The upper link builds a phase modulation framework, and the wavelength of the optical signal sideband after modulation is changed by tuning and modulating the microwave frequency, so that the conversion from microwave frequency sweep to optical wave frequency sweep is realized. Fig. 3 is a diagram illustrating the principle analysis of microwave frequency sweep in this step. As shown in the figure, the carrier optical signal of the upper link and the pump optical signal of the middle link are from the same light source supply module, and the two signals have the same frequency, i.e. f0＝fpI.e. the exact alignment shown in the figures. When modulating microwave signal (frequency f) of upper linkm) When phase modulating the carrier, only the first-order sidebands (the frequencies of the negative and first-order sidebands are f, respectively) are considered-1And f1) According to the modulation principle, the relation f between the negative first-order sideband frequency and the carrier frequency can be obtainedm＝f0-f-1If the modulated microwave frequency is close to the Brillouin frequency shift, the frequency is amplified, namely the continuous change of the optical frequency of the negative first-order sideband is realized by continuously adjusting the frequency of the microwave modulation signal, the transition from the electric domain microwave frequency sweep to the optical domain sideband frequency sweep is realized, and the successful extraction of the stimulated Brillouin scattering gain spectrum characteristic is finished. The characteristic extraction of the optical domain amplification passband is demodulated after the envelope detection of a detector, and finally the demodulation is embodied in an electric domain, specifically: the microwave frequency sweep signal of the upper link carries out phase modulation on the optical carrier, and the modulated optical signal is carried out after passing through the optical fiber to be measuredThe first-order sideband is positioned in the Brillouin amplification passband and is further amplified, and the first-order sideband is suppressed, so that the relation of equal amplitude and phase difference pi between the first-order sideband and the Brillouin amplification passband is broken (the amplitude is unequal), the signal can be subjected to envelope detection through a detector and then can be restored and modulated to obtain a microwave signal, and the frequency of the microwave signal is tuned to present the amplification passband characteristics of the stimulated Brillouin gain spectrum; meanwhile, the reference optical signal input by the lower link can compensate the carrier loss of the upper link, so that the microwave power value output by the detector is effectively improved. Through microwave frequency sweeping, according to the principle that the optical domain 3dB bandwidth corresponds to the electrical domain 6dB bandwidth (square law detection principle), if the maximum gain corresponds to the frequency fmaxDown to one quarter of the maximum gain corresponds to a frequency f-6dBSo as to obtain the intrinsic line width Deltav of the stimulated Brillouin gain spectrumBValue of DeltavB＝2|fmax-f-6dBI.e. reading f by a signal analyzermaxAnd f-6dBThe value of (a) is solved to obtain delta vB。
Finally, the third step is executed, and the gain peak value g is obtained based on the first step and the second stepBAnd intrinsic line width DeltavBValue according to formula
The optical expansion coefficient gamma of the optical fiber to be measured can be calculatede。
As can be seen from the above description of the working principle and the operation steps of the optical fiber electrostrictive coefficient measuring device in this embodiment, the measuring device has a simple structure and is easy to implement, and the physical nature for realizing the measurement of the optical fiber electrostrictive coefficient is based on the stimulated brillouin effect rather than the coherent interference, so that the defects of high environmental sensitivity and instability in the conventional measuring method can be overcome when the electrostrictive coefficient of the optical fiber is measured; in addition, since the signal processing is performed in the "electrical domain" rather than the "optical domain", the spectral resolution of the "electrical domain" is much higher than the wavelength resolution of the "optical domain" (for 1550nm wavelength, 0.1nm resolution corresponds to 2.5GHz), so the test accuracy of the scheme is higher than that of the conventional method.
The above embodiments are preferred implementations of the present invention, and the present invention can be implemented in other ways without departing from the spirit of the present invention.
Claims (10)
1. The method for measuring the electrostrictive coefficient of the optical fiber is characterized by comprising the following steps of:
determining the relationship between the optical fiber electrostriction coefficient and the gain peak value and the peak intrinsic line width of the stimulated Brillouin gain spectrum based on the stimulated Brillouin scattering effect of the optical fiber and the nonlinear optical principle;
pump light is utilized to excite nonlinearity of an optical fiber to be detected so as to generate stimulated Brillouin scattering, an envelope detection technology is combined to calculate a stimulated Brillouin gain spectrum gain peak value and determine a Brillouin frequency shift value; the method comprises the steps that a microwave frequency sweep signal is utilized to carry out phase modulation on optical carriers which are in the same source with pump light, conversion from microwave frequency sweep to optical frequency sweep is achieved, and the peak intrinsic line width of a stimulated Brillouin gain spectrum is calculated by extracting the characteristics of the stimulated Brillouin gain spectrum and combining the square law detection principle;
and obtaining the electrostriction coefficient of the optical fiber to be tested according to the relationship among the electrostriction coefficient of the optical fiber, the gain peak value of the stimulated Brillouin gain spectrum and the intrinsic line width of the peak value.
2. The method for measuring the optical fiber electrostrictive coefficient according to claim 1, wherein the relationship between the optical fiber electrostrictive coefficient and the stimulated brillouin gain spectrum gain peak value and the peak intrinsic line width is as follows:
in the above formula,. gamma.eIs the optical fiber electrostrictive coefficient, n is the optical fiber refractive index, lambdapIs the wavelength of the pump light, p0Is the density of the fiber, c is the speed of light, VaIs the speed of sound, Δ νBIs the intrinsic line width, g, of the gain spectrumBFor stimulated Brillouin increaseA spectrum gain peak is benefited.
3. The device for measuring the electrostriction coefficient of the optical fiber is characterized in that: the device comprises a light source supply module, a measurement main body module and a signal processing and analyzing module which are connected in sequence;
the light source supply module is used for providing a single-frequency point signal light source, shunting an optical signal into a carrier optical signal, a pumping optical signal and a reference optical signal and correspondingly inputting the carrier optical signal, the pumping optical signal and the reference optical signal into an upper link, a middle link and a lower link of the measurement main body module;
the upper link is used for carrying out phase modulation on a carrier optical signal, reversely inputting the carrier optical signal into the middle link through an optical fiber to be tested, and changing the wavelength of a modulated optical signal sideband by tuning and modulating microwave frequency to realize conversion from microwave frequency sweep to optical frequency sweep; the middle-layer link is used for inputting the pump light signals into the optical fiber to be tested after power tuning, exciting nonlinearity, generating stimulated Brillouin scattering, exciting a gain spectrum, and transmitting backward transmitted Stokes light to the signal processing and analyzing module; the lower link is used for transmitting the reference optical signal to the signal processing and analyzing module;
the signal processing and analyzing module is used for combining the wave of the optical signals input from the middle link and the lower link, converting the optical signals into electric signals through envelope detection, performing frequency-selective filtering on the electric signals, performing frequency spectrum and power analysis on the filtered signals to obtain a gain peak value and a peak intrinsic line width of the stimulated Brillouin gain spectrum, and obtaining the gain peak value and the peak intrinsic line width of the stimulated Brillouin gain spectrum according to the relationship between the optical fiber electrostriction coefficient and the stimulated Brillouin gain spectrum gain peak value and the peak intrinsic line width
Calculating the electrostriction coefficient of the optical fiber to be detected;
in the above formula, γeIs the optical fiber electrostrictive coefficient, n is the optical fiber refractive index, lambdapIs the wavelength of the pump light, p0Is the density of the fiber, c is the speed of light, VaIs the speed of sound, Δ νBIs the intrinsic line width, g, of the gain spectrumBFor stimulated Brillouin gain spectrum increaseThe peak value is benefited.
4. The apparatus for measuring the electrostrictive coefficient of optical fibers according to claim 3, wherein: the light source supply module comprises a laser, an isolator and a coupler 1 which are sequentially connected, a single-frequency point light signal generated by the laser enters the coupler 1 after passing through the isolator and is divided into three branches to be respectively transmitted to an upper link, a middle link and a lower link of the measurement main body module, the isolator is used for isolating the return of the light signal, and the coupler 1 is used for realizing the shunting of the light signal.
5. The apparatus for measuring the electrostrictive coefficient of optical fibers according to claim 3, wherein: the signal processing and analyzing module comprises a coupler 2, a detector, a narrow-band filter and a signal analyzer which are sequentially connected, wherein the coupler 2 is used for combining optical signals input from a middle-layer link and a lower-layer link, the detector converts the optical signals into electric signals through envelope detection, the narrow-band filter is used for carrying out frequency-selective filtering on the signals output by the detector, and the signal analyzer is used for carrying out frequency spectrum and power analysis on the filtered signals.
6. The apparatus for measuring electrostrictive coefficient of optical fiber according to claim 3, wherein: the upper link comprises a polarization controller, a phase modulator and an adjustable microwave signal source, the input end of the polarization controller is connected with one output end of the coupler 1, the output end of the polarization controller and the adjustable microwave signal source are both connected with the phase modulator, the polarization controller is used for controlling the polarization state of a carrier optical signal to be matched with the polarization state of the phase modulator, and the optical signal input to the phase modulator is subjected to phase modulation by an electric signal sent by the adjustable microwave signal source and then reversely transmitted to the middle link through an optical fiber to be measured.
7. The apparatus for measuring the electrostrictive coefficient of optical fibers according to claim 6, wherein: the middle-layer link comprises an amplifier, an adjustable attenuator and a circulator which are sequentially connected, the amplifier is connected with one output end of the coupler 1, a pumping optical signal entering the middle-layer link enters the adjustable attenuator after being amplified by the amplifier, and the power of an optical signal entering the circulator is adjusted by the adjustable attenuator; and pumping light signals flow into the right port from the left port of the circulator and then enter the optical fiber to be tested, the nonlinear effect of the optical fiber to be tested is excited to generate stimulated Brillouin scattering when the power value of the pumping light signals exceeds the threshold power, then the pumping light signals reversely transmit the Stokes light to the right port of the circulator, and then the pumping light signals flow into a lower link from the lower port of the circulator.
8. The apparatus for measuring the electrostrictive coefficient of optical fibers according to claim 7, wherein: the lower link comprises an optical fiber transmission line connecting the coupler 1 and the coupler 2, and the optical fiber transmission line is used for transmitting the reference optical signal.
9. The apparatus for measuring the electrostrictive coefficient of optical fibers according to claim 8, wherein: by disconnecting the upper link, only connecting the middle link and the lower link, the pump light signal provided by the light source supply module enters the middle link, enters the variable optical attenuator for power tuning after being amplified by the amplifier, then enters the right port through the left port of the circulator and enters the fiber to be tested to excite nonlinearity, stimulated Brillouin scattering is generated, the generated Stokes light is reversely transmitted to the right port of the circulator and then enters the coupler 2 together with the reference light signal transmitted by the lower port and the lower link through the lower port, the two-phase dry light signal is subjected to beat frequency by the detector and then is reduced into an electric signal to be output to the narrow-band filter, the output signal frequency and the output signal power after passing through the narrow-band filter are read by the signal analyzer, and the value of the gain peak value of the stimulated Brillouin gain spectrum is calculated by combining the following formula:
in the above formula, gBIs stimulated BrillouinValue of the gain peak of the gain spectrum, poutFor output signal power, ρ is the responsivity of the detector, A1For the amplitude of the pump light entering the circulator, A4For the amplitude of the reference optical signal, l and α are the length and loss coefficient of the optical fiber to be measured, respectively.
10. The apparatus for measuring the electrostrictive coefficient of optical fibers according to claim 9, wherein: by simultaneously connecting the upper link, the middle link and the lower link, exciting the nonlinearity of the optical fiber to be detected by the middle link to generate stimulated Brillouin scattering, modulating the phase of a carrier optical signal by the upper link, changing the wavelength of a sideband of the modulated optical signal by tuning and modulating the microwave frequency to realize the conversion from microwave frequency sweep to optical frequency sweep, so that a negative first-order sideband of the modulated optical signal output by the upper link is amplified and a first-order sideband is inhibited after passing through the optical fiber to be detected, the modulated optical signal is subjected to envelope detection by the detector to reduce the modulated microwave signal to realize the amplification characteristic extraction of the stimulated Brillouin gain spectrum, and the carrier loss of the passband of the upper link is compensated by a reference optical signal transmitted by the lower link to improve the microwave signal power value output by the detector, and finally, reading the frequency corresponding to the maximum gain and the frequency corresponding to the position reduced to the quarter of the maximum gain through the signal analyzer, and calculating the peak intrinsic line width of the stimulated Brillouin gain spectrum by combining the following formula:
ΔνB＝2|fmax-f-6dB|；
in the above formula, Δ νBIs the peak intrinsic linewidth, f, of the stimulated Brillouin gain spectrummaxFor maximum gain corresponding to frequency, f-6dBTo reduce to the frequency corresponding to the quarter maximum gain.
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