CN113251937B - Method and device for measuring diameter of cladding of polarization maintaining optical fiber - Google Patents
Method and device for measuring diameter of cladding of polarization maintaining optical fiber Download PDFInfo
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- CN113251937B CN113251937B CN202110395743.3A CN202110395743A CN113251937B CN 113251937 B CN113251937 B CN 113251937B CN 202110395743 A CN202110395743 A CN 202110395743A CN 113251937 B CN113251937 B CN 113251937B
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Abstract
A method and a device for measuring the cladding diameter of a polarization maintaining optical fiber belong to the field of distributed optical fiber sensing, and aim to solve the problem that the existing method for measuring the diameter of the polarization maintaining optical fiber cannot give consideration to nondestructive, high-precision and distributed measurement. The method of the invention comprises the following steps: sequentially injecting a plurality of groups of frequency intervals of f to one end of the polarization maintaining optical fiber m The linear polarization excitation pulse light and the frequency interval of f m The linear polarization reading pulse light is vertical to the polarization direction of the linear polarization reading pulse light; the other end of the polarization maintaining fiber is injected with the frequency of
Or alternatively
And a frequency of
Or
Wherein v is B The backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber is obtained; f is measured in a preset frequency range containing a forward Brillouin frequency shift theoretical value of the polarization-maintaining optical fiber m Scanning, measuring and reading the optical power of high-frequency components and low-frequency components in the pulsed light, and obtaining forward stimulated Brillouin signal spectral lines corresponding to a preset frequency range; and obtaining the cladding diameter of the polarization maintaining fiber according to the spectral line.
Description
Technical Field
The invention relates to a distributed measurement technology of optical fiber basic parameters, belonging to the field of distributed optical fiber sensing.
Background
The optical fiber is used as a sensing unit for sensing the change of the external environment, and the sensing signal of each position of the optical fiber is obtained through scattering in the optical fiber, so that distributed sensing is realized. In recent years, with the great demand for sensors in modern industry and agricultural production, distributed optical fiber sensing is widely applied to the fields of bridge and road structure health monitoring, pipeline safety monitoring, gas measurement and the like.
In fact, the optical fiber is not only an optical waveguide with good performance, but also an acoustic waveguide, and the sensing of the properties of the external environment substances is completed by utilizing the characteristic that the acoustic wave can be in direct contact with the outside. The forward Brillouin scattering means that pumping light and scattered light which are transmitted along the axial direction of an optical fiber in the same direction interact to generate transverse sound waves, a transverse sound wave field is reflected back through interfaces such as a cladding layer, a coating layer or a coating layer, the external environment and the like to form a stable sound wave mode, and the transverse sound wave field carrying external information returns to the fiber core of the optical fiber to perform photoacoustic coupling with the optical wave, so that the transmission of the scattered light is influenced. In recent years, distributed fiber sensing based on Forward Stimulated Brillouin Scattering (FSBS) has been proposed by researchers and has become a hot spot in the sensing field. The basic principle is to detect a forward stimulated Brillouin scattering spectrum through Rayleigh scattering or backward stimulated Brillouin scattering, and then demodulate distributed matter information through data processing. Using this principle, distributed measurements of the fiber cladding diameter can also be made.
The diameter measurement of the optical fiber is important for the fields of ultra-high sensitivity sensing, high-speed optical communication and the like. Currently, the measurement of the diameter of the optical fiber relies on a point-type measurement scheme. For example, in industry, scanning electron microscopes or optical microscopes are widely used due to their good spatial resolution, i.e., by acquiring cross-sectional images to measure the fiber diameter. However, cutting the fiber during the measurement causes irreversible damage to it, and the resolution and field of view of the microscopic measurement cannot be optimized simultaneously. Since the 70's of the 20 th century, non-destructive fiber diameter measurement methods including the analysis of coherent edge patterns based on back-scattering (Presby, H.M. "reflective index and diameter measurements of non-polar Optical fibers". Optic Society of America.64,280-284(1974)) and forward-scattering (Smithgarl, D.H., Watkins, L.S.and zee, R.E. "High-speed non-polar characteristics fiber-diameter measurement with optimal precision of 250nm in the 50 μm-150 μm fiber diameter range have been used for commercially prepared fiber diameter measurements. However, the diameter resolution achieved by the above method is not sufficient for more demanding applications. The introduction of methods such as spectroscopic interferometry (Jasapara, j., monoberg, e., dimancello, f.and Nicholson, j.w. "Accurate non-optical fiber measurement with spectral interferometry." Optics letters.28,601-603(2003)) can achieve accuracies above 10 nm. Measurements with an accuracy of more than 10nm can be made by backscattering light by near field resonance (Ashkin, A., Dziedzic, J.M. and Stolen, R.H. "Outer diameter measurement of low birefringence optical fibers by a new resonator technique". Applied optics.20,2299-2303 (1981)). Another high precision approach utilizes whispering gallery modes created by the auxiliary microfibers to achieve 1/10 4 (iii) resolution (T.A. bits, J.C. Knight, and T.E. Dimmick, "High-resolution measurement of the fiber diameter using High-resolution modules and no optical alignment" IEEE Photonic technologies Letters 12,182-183 (2000)).
The solutions currently used for measuring the diameter of optical fibers are based on the above-mentioned point measurement. In order to achieve diameter measurements over the entire fiber length, flexural wave-based Time-resolved acousto-optic techniques have achieved centimeter-scale spatial resolution and quasi-distributed fiber diameter measurements (e.p. accusa-S a, A.D a, m.gonz a lez-her a, and m.v. andres, "Time-resolved optical interface in single-mode optical fibers: diffraction of an axial non-terminals at the nano-meter scale" optics.letters.39,1437-1440 (2014))). However, both the whispering gallery mode technique and the acousto-optic technique are applied to detect relative diameter changes along the fiber, and do not obtain absolute values of the diameter.
Disclosure of Invention
The invention aims to solve the problem that the existing polarization maintaining optical fiber diameter measuring method cannot give consideration to nondestructive, high-precision and distributed measurement, and provides a polarization maintaining optical fiber cladding diameter measuring method and device.
The method for measuring the cladding diameter of the polarization maintaining optical fiber comprises the following steps:
injecting a plurality of groups of frequency intervals of f to one end of the polarization maintaining optical fiber to be tested m The excitation pulse light is linearly polarized light, the polarization direction of the excitation pulse light is matched with the fast axis or the slow axis of the polarization maintaining optical fiber to be tested, and the frequency interval is f m The excitation pulse light passes through the pair frequency v 0 Obtaining the laser by frequency shift;
injecting a group of frequency intervals f into one end of the polarization maintaining optical fiber to be tested m The reading pulse light is linearly polarized pulse light, the polarization direction of the reading pulse light is perpendicular to the polarization direction of the excitation pulse light, and the frequency interval is f m Reading pulse light pass through pair frequency v 0 The laser is obtained by frequency shift;
injecting the frequency of the other end of the polarization maintaining optical fiber to be tested into the optical fiber sequentially
Or
And a frequency of
Or alternatively
Wherein v is B The backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested is obtained;
f is processed in a preset frequency range containing the forward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested m Scanning, and measuring the optical power of a high-frequency component and a low-frequency component in the reading pulse light to obtain a forward stimulated Brillouin signal spectral line corresponding to the preset frequency range;
determining a forward Brillouin frequency shift test value f of the polarization maintaining optical fiber to be tested according to the forward stimulated Brillouin signal spectral line m '；
According to the formula
Calculating the cladding diameter d of the distributed polarization maintaining fiber to be measured, wherein V L Is the longitudinal wave sound velocity in the polarization maintaining fiber to be measured, k is the wave vector, y m As a boundary equation
Intrinsic solution of (V) T For the transverse wave sound velocity, J, in the polarization-maintaining fiber to be measured i Is a bessel equation of order i.
Optionally, the number of the excitation pulsed light is two.
Optionally, the excitation pulsed light is completely separated from the reading pulsed light in the time domain.
Optionally, the method for obtaining the excitation pulse light and the reading pulse light includes:
controlling a first electro-optical modulator by using an arbitrary waveform generator to enable the first electro-optical modulator to carry out frequency v 0 The continuous laser carries out frequency shift to obtain the frequency v 0 -f 1 -f m 、ν 0 +f 1 +f m 、ν 0 +f 1 V and v 0 -f 1 The excitation pulsed light of (1);
controlling a second electro-optic modulator to make the second electro-optic by using an arbitrary waveform generatorModulator pair frequency v 0 Is frequency shifted to obtain a frequency of
And
the reading pulse light of (1).
Optionally, the method for obtaining continuous probe light includes:
controlling a single-sideband modulator by a microwave source to enable the single-sideband modulator to have a v frequency 0 Is frequency shifted to obtain a frequency of
Or
And a frequency of
Or
The continuous probe light of (1).
The invention relates to a device for measuring the diameter of a cladding of a polarization maintaining optical fiber, which comprises: the device comprises a laser, a beam splitter, a first frequency shift device, a second frequency shift device, a photoelectric detector and a data acquisition card;
the linearly polarized continuous laser output by the laser is divided into three beams by a beam splitter, wherein the first beam outputs a plurality of groups of frequency intervals of f after being frequency-shifted by the first frequency shifting device m Excited pulsed light of, said f m The laser pulse light enters a fast axis/a slow axis of the polarization maintaining optical fiber to be measured from one end of the polarization maintaining optical fiber to be measured as a forward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be measured, and a second beam is subjected to frequency shift by the first frequency shift device and then outputs a frequency interval of f m The reading pulse light enters the slow axis/fast axis of the polarization maintaining optical fiber to be detected from one end of the polarization maintaining optical fiber to be detected, and the third beam is subjected to frequency shift by the second frequency shift deviceOutput successively at a frequency of
Or
And a frequency of
Or
Wherein v is B The continuous probe light enters the polarization maintaining optical fiber to be tested from the other end of the polarization maintaining optical fiber to be tested as a backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested;
the photoelectric detector is used for measuring the power of two frequency components in reading pulse light output from the polarization-maintaining optical fiber.
Optionally, the first frequency shifting device includes an arbitrary waveform generator, a first electro-optic modulator, and a second electro-optic modulator, where the arbitrary waveform generator controls the first electro-optic modulator and the second electro-optic modulator, the first electro-optic modulator modulates the first beam of light into pulsed light and shifts the frequency, and the second electro-optic modulator modulates the second beam of light into pulsed light and shifts the frequency.
Optionally, the second frequency shifting device includes a microwave source and a single-sideband modulator, and the microwave source controls the single-sideband modulator to shift the frequency of the third beam of light.
Optionally, the polarization maintaining fiber further comprises two fiber amplifiers, and the excitation pulse light and the reading pulse light output by the first frequency shift device enter the polarization maintaining fiber to be measured after being amplified by the two fiber amplifiers respectively.
Optionally, the device further includes a polarization beam combiner and three polarization controllers, the excitation pulse light and the reading pulse light respectively pass through the two polarization controllers and then enter the polarization beam combiner, light emitted from the polarization beam combiner enters the polarization maintaining fiber to be measured, and the continuous probe light enters the polarization maintaining fiber to be measured after passing through the third polarization controller.
The method and the device for measuring the cladding diameter of the polarization maintaining optical fiber have the following advantages:
1. the nondestructive measurement of the diameter of the distributed optical fiber cladding can be realized;
2. the method has high spatial resolution, adopts coherent forward stimulated Brillouin scattering which is excited and then read, and time domain separation enables the read pulse width to be compressed to ten nanometers, so that meter-level spatial resolution is obtained;
3. with a high signal-to-noise ratio: (1) the excitation pulse light and the reading pulse light are subjected to polarization separation, so that the signal-to-noise ratio of FSBS measurement is improved, and the window length of differentiation in the subsequent data processing process is reduced, so that the spatial resolution is further improved, and a high-precision result is obtained; (2) the excitation pulse light adopts the action of two pairs of FSBS (frequency selective base stations), the excitation action of a transverse acoustic wave field is enhanced, a stable transverse acoustic wave field is formed, and the coherent FSBS can realize the effect of enhancing the acoustic wave field under the condition of matching the phase relation, so that the signal-to-noise ratio is improved.
Drawings
Fig. 1 is a schematic structural diagram of a device for measuring a cladding diameter of a polarization-maintaining optical fiber according to a first embodiment.
Detailed Description
Example one
The present embodiment provides a method for measuring the cladding diameter of a polarization maintaining optical fiber, which may generally include the following steps S1 to S6.
Step S1, injecting a plurality of groups of frequency intervals f to one end of the polarization maintaining optical fiber to be tested m The excitation pulse light is linearly polarized light, and the polarization direction of the excitation pulse light is matched with the fast axis or the slow axis of the polarization maintaining optical fiber to be tested;
in this embodiment, the excitation pulse light can be obtained by modulating laser light with an arbitrary waveform generator and an electro-optical modulator, for example, a continuous laser is used to generate a laser light with a frequency v 0 Then under the precondition of carrier suppression, the arbitrary waveform generator is utilized to control the electro-optical modulator, so that the electro-optical modulator can adjust the frequency of the laser beam to the frequency ofν 0 Is continuously laser-processed 1 And f 1 +f m The continuous light is modulated into pulse light, and the electro-optical modulator shifts the frequency in two directions, so that v is output 0 -f 1 -f m 、ν 0 +f 1 +f m 、ν 0 +f 1 V and v 0 -f 1 Four frequency component pulsed laser, where one set of frequencies is v 0 -f 1 -f m V and v 0 -f 1 And the other set of frequencies is v 0 +f 1 +f m V and v 0 +f 1 Two frequency intervals of each group are both f m ，f 1 The setting of be used for avoiding arousing pulsed light and reading the pulsed light influence each other, every group arouses the pulsed light and can produce an acoustic wave field, and four frequency components can arouse stronger horizontal acoustic wave field.
Step S2, after the step S1 is finished, the exciting pulse light forms a stable acoustic wave field in the polarization maintaining optical fiber to be detected, and before the acoustic wave field disappears, a group of frequency intervals f is injected into the same end of the polarization maintaining optical fiber to be detected m The reading pulse light is linear polarization pulse light, the polarization direction of the linear polarization pulse light is perpendicular to the polarization direction of the excitation pulse light, and a coherent stimulated process, namely a Forward Stimulated Brillouin Scattering (FSBS) process, is formed between the dual-frequency reading pulse laser light and the transverse acoustic wave field, and the energy is transferred from a high-frequency component to a low-frequency component in the dual-frequency reading pulse light along with the energy.
The strong long excitation pulse can generate a Kerr effect, and if the excitation pulse and the reading pulse are injected into the polarization maintaining fiber to be measured at the same time, the Kerr effect can influence the FSBS (frequency shift keying) action of the reading pulse, so that the finally obtained forward Brillouin scattering spectrum type is distorted. And when no excitation pulse light exists, the kerr effect disappears immediately, but the generated acoustic wave field still has about 1 microsecond time (acoustic wave life), so in the embodiment, the excitation pulse and the reading pulse are controlled to be separated in the time domain, the reading pulse is injected into the polarization maintaining fiber to be tested after the excitation pulse is injected into the polarization maintaining fiber to be tested and before the acoustic wave life is ended, and the influence of the kerr effect is removed by utilizing the separation in the time sequence.
The reading pulse light can be generated in the same manner as the excitation pulse light, for example, by generating a frequency v with a continuous laser 0 The linear polarization continuous laser is controlled by an arbitrary waveform generator to enable the electro-optical modulator to adjust the frequency v 0 Is continuously laser-processed
The continuous light is modulated into pulsed light, and the electro-optical modulator is frequency-shifted in two directions, so that the total output frequency is
And
two frequency components of pulsed laser light.
Step S2 and step S1 may share an arbitrary waveform generator.
Step S3, injecting two continuous detecting lights with frequency to the other end of the polarization maintaining fiber to be detected, wherein one frequency is
Or
Another frequency is
Or
ν B And obtaining a backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested.
The probe light may be obtained by modulating laser light with a microwave source, such as a Brillouin frequency shift (v) of the microwave source, at a frequency of about the back of the fiber under test B 11GHz) to a microwave signalOn a single-sideband modulator at a frequency v 0 The down-conversion continuous detection light under the carrier suppression is generated on the continuous optical wave, and the sideband frequency of the continuous detection light falls within the Brillouin amplification range of the reading pulse. Then, the continuous detection light passes through a polarization scrambler to remove polarization noise, then passes through an isolator to avoid the influence on the counter light wave, and then is injected into the optical fiber to be detected. When the frequency of the microwave is equal to
When the microwave frequency is equal to the microwave frequency, the amplifying part of the continuous probe light reads the forward Brillouin signal corresponding to the low-frequency light in the pulse, and similarly, when the microwave frequency is equal to the microwave frequency
In this case, the amplification section of the continuous probe light is a forward brillouin signal corresponding to the high-frequency light in the read pulse.
Step S4, calculating the forward Brillouin frequency shift theoretical value f of the polarization maintaining optical fiber to be measured 0 Then setting a scanning range, said f 0 Is contained in the scanning range, and then f is controlled m And changing according to a preset step length in the scanning range, simultaneously measuring the light power of the high-frequency component and the low-frequency component in the reading pulse light, and calculating the forward stimulated Brillouin signal spectral line corresponding to the scanning range according to the measured light power value.
Step S5, determining the forward Brillouin frequency shift test value f of the polarization maintaining optical fiber to be tested according to the forward stimulated Brillouin signal spectral line m '；
Step S6, according to the formula
Calculating the cladding diameter d of the distributed polarization maintaining fiber to be measured, wherein V L Is the longitudinal wave sound velocity in the polarization maintaining fiber to be measured, k is the wave vector, y m As a boundary equation
Intrinsic solution of (V) T For the transverse wave sound velocity, J, in the polarization-maintaining fiber to be measured i Be bezier equation of order i.
The embodiment also provides a device for measuring the cladding diameter of the polarization maintaining optical fiber, which has the structure shown in FIG. 1. The laser 1 is a narrow linewidth continuous polarization-maintaining laser, the output linear polarization continuous laser wavelength is around 1550nm, and is divided into three beams by the polarization-maintaining beam splitter 2, wherein the first beam is frequency-shifted by the first frequency shifter 3 and then outputs a plurality of groups of frequency intervals of f m The laser pulse light enters the fast axis/slow axis of the polarization maintaining fiber 16 to be measured from one end of the polarization maintaining fiber 16 to be measured, and the second beam is subjected to frequency shift by the first frequency shift device 3 and then has an output frequency interval of f m The reading pulse light enters the slow axis/fast axis of the polarization maintaining fiber 16 to be measured from one end of the polarization maintaining fiber 16 to be measured, and the third beam is subjected to frequency shift by the second frequency shift device 4 and then output with the frequency of
Or
And a frequency of
Or
Wherein v is B The continuous probe light enters the polarization maintaining optical fiber 16 to be tested from the other end of the polarization maintaining optical fiber 16 to be tested, which is the backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber 16 to be tested;
the photoelectric detector 5 is used for measuring the power of two frequency components in the reading pulse light output from the polarization-maintaining fiber 16.
In a preferred embodiment of the present invention, the first frequency shifter 3 includes an arbitrary waveform generator that controls the first electro-optical modulator and the second electro-optical modulator, respectively, the first electro-optical modulator modulating the first beam of light into pulsed light and shifting the frequency, and the second electro-optical modulator modulating the second beam of light into pulsed light and shifting the frequency.
As a preferred embodiment of the present invention, the second frequency shifting device 4 comprises a microwave source and a single sideband modulator, and the microwave source controls the single sideband modulator to shift the frequency of the third beam of light.
As a preferred embodiment of the present invention, the apparatus further includes two fiber amplifiers, and the excitation pulse light and the reading pulse light output by the first frequency shift device 3 enter the polarization maintaining fiber 16 to be measured after being amplified by the first fiber amplifier 7 and the second fiber amplifier 8, respectively.
As a preferred embodiment of the present invention, the apparatus further includes a polarization beam combiner 11 and three polarization controllers, the excitation pulse light and the reading pulse light enter the polarization beam combiner 11 through the first polarization controller 9 and the second polarization controller 10, respectively, light emitted from the polarization beam combiner 11 enters the polarization maintaining fiber 16 to be tested, and the continuous probe light enters the polarization maintaining fiber 16 to be tested after passing through the third polarization controller 15.
As a preferred embodiment of the present invention, the apparatus further includes a circulator 12, the light output by the first polarization controller 9 enters a first port of the circulator 12, and exits from a second port of the circulator 12, and then enters the polarization beam combiner 11; the signal emitted from the polarization maintaining fiber 16 to be measured enters the second port of the circulator 12 after passing through the polarization beam combiner 1, and is emitted from the third port of the circulator 12, and then is received by the photoelectric detection 5 after being filtered by the filter 13.
As a preferred embodiment of the present invention, the apparatus further includes an isolator 14, and the light modulated by the second frequency shift device 4 enters the isolator 14, exits from the isolator 14, and enters the third polarization controller 15.
As a preferred embodiment of the present invention, the beam splitter 2 may be implemented using two couplers, as shown in fig. 1.
The principle of the polarization maintaining optical fiber cladding diameter measuring device is as follows:
a narrow-linewidth continuous laser is used as a light source, the narrow-linewidth continuous laser is divided into two branches through a coupler, one branch successively generates a four-frequency long pulse and a double-frequency short pulse to be respectively used as an acoustic wave field excitation pulse light and a reading pulse light, and the other branch generates continuous probe light to be used for measuring the condition that high-frequency light energy in the reading pulse is transferred to low-frequency light.
In one branch, an arbitrary waveform generator is used to drive an electro-optical modulator to generate the excitation pulse and the reading pulse involved in the FSBS process, and a radio frequency pulse signal is applied to the light wave through the electro-optical modulator under the condition of inhibiting the carrier wave. Wherein the RF pulse consists of two parts, firstly a long excited electrical pulse is injected, which comprises two RF frequencies f 1 And f 1 +f m Thereby generating light waves of four frequencies; then injecting a shorter frequency of
Thereby generating a frequency difference of f m The dual-frequency light wave of (1). The power of the optical pulse is amplified by the erbium-doped fiber amplifier and is injected into the polarization maintaining fiber to be tested through the second port of the circulator.
It should be noted that in this embodiment, the excitation pulse and the read pulse need to be generated separately, that is, two channels of the arbitrary waveform generator respectively control the two electro-optical modulators to shift the carrier frequency, and the polarization controller of each sub-branch controls the polarization state of the pulse light injected into the polarization-maintaining fiber to be measured, so that the excitation pulse enters the fast axis (or slow axis) of the polarization-maintaining fiber to be measured, and the read pulse enters the slow axis (or fast axis) of the polarization-maintaining fiber to be measured. Since the acoustic wave generated by the excitation pulse is not bound by the fiber core, the read pulse can read the acoustic wave in a polarization direction orthogonal to the excitation pulse.
In the other branch, the microwave source makes the frequency about Brillouin frequency shift (v) of the polarization maintaining optical fiber to be measured B 11GHz) is applied to a single sideband modulator to produce a down-converted continuous probe light under carrier suppression on an optical wave, the sideband frequency of which falls within the brillouin amplification range of the read pulse. Then, the light wave passes through an isolator and a polarization controller and then is injected into a polarization maintaining optical fiber to be detected, wherein the polarization controller is used for controlling and detecting continuous lightI.e. when the reading pulse enters the slow axis of the polarization maintaining fiber to be measured, the continuous probe light should also be injected into the slow axis of the polarization maintaining fiber to be measured. When the microwave frequency is equal to
When the microwave frequency is equal to the microwave frequency, the amplified part of the continuous probe light reads the Brillouin signal corresponding to the low-frequency light in the pulse, and similarly, when the microwave frequency is equal to the microwave frequency
In this case, the amplified portion of the continuous probe light is the brillouin signal corresponding to the high-frequency light in the read pulse. Because a fixed frequency point v is selected B And the power measurement result of the frequency point is used in the presentation of a forward spectrum, so that the measurement time is shortened, the interference caused by temperature change and system stability is inhibited, and higher signal-to-noise ratio and better spatial resolution are obtained.
Finally, brillouin scattering light is output from the third port of the circulator, and a filter is used to remove noise such as rayleigh scattering signals and amplified spontaneous emission. Selecting a mode with the strongest transverse acoustic field, and adjusting f near the forward Brillouin frequency shift m And (4) carrying out frequency sweeping, wherein the corresponding Brillouin signal is received by the photoelectric detector and recorded by the data acquisition card.
By adopting the polarization maintaining optical fiber as the sensing optical fiber, the interference of polarization noise on the result of Brillouin Optical Time Domain Analysis (BOTDA) process (BOTDA between the down light and the reading pulse) can be avoided, so that the line type is smoother, and the polarization maintaining optical fiber can enable the measuring result to achieve higher spatial resolution. However, since high spatial resolution means that the read pulse width needs to be smaller, and the BOTDA signal becomes weaker due to the narrowing of the pulse width of the pump light (here, the read pulse), the interference of the backward brillouin signal spontaneous by the excitation pulse on the scattering signal result increases, thereby reducing the signal-to-noise ratio of the result. Therefore, when the excitation pulse and the reading pulse are respectively injected into the fast axis and the slow axis, the signal-to-noise ratio of the FSBS result can be effectively enhanced, the window length of differentiation in the subsequent demodulation process is further reduced, and the higher FSBS spatial resolution is achieved.
The spatial resolution of the BOTDA process is calculated by the formula
T denotes a pulse width of a read pulse; the final spatial resolution of the FSBS should be the sum of the differential window length and Δ z. For example, if the pulse width of the read pulse is 10ns and the core refractive index n is 1.5, the spatial resolution corresponding to the BOTDA process is 1m, and if the window length of 1m is used for moving average in the data processing process, the total FSBS spatial resolution is 2 m.
Claims (10)
1. A method for measuring the diameter of a cladding of a polarization maintaining optical fiber is characterized by comprising the following steps:
injecting a plurality of groups of frequency intervals of f to one end of the polarization maintaining optical fiber to be tested m The excitation pulse light is linearly polarized light, the polarization direction of the excitation pulse light is matched with the fast axis or the slow axis of the polarization maintaining optical fiber to be tested, and the frequency interval is f m The excitation pulse light passes through the pair frequency v 0 The laser is obtained by frequency shift;
injecting a group of frequency interval f into one end of the polarization maintaining optical fiber to be tested m The reading pulse light is linearly polarized pulse light, the polarization direction of the reading pulse light is perpendicular to the polarization direction of the excitation pulse light, and the frequency interval is f m Reading pulse light pass through pair frequency v 0 Obtaining the laser by frequency shift;
injecting the frequency of the other end of the polarization maintaining optical fiber to be tested into the optical fiber sequentially
Or
And a frequency of
Or
Wherein v is B The backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested is obtained;
f is processed in a preset frequency range containing the forward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested m Scanning, and measuring the optical power of a high-frequency component and a low-frequency component in the reading pulse light to obtain a forward stimulated Brillouin signal spectral line corresponding to the preset frequency range;
determining a forward Brillouin frequency shift test value f of the polarization maintaining optical fiber to be tested according to the forward stimulated Brillouin signal spectral line m '；
According to the formula
Calculating the cladding diameter d of the distributed polarization maintaining fiber to be measured, wherein V L Is the longitudinal wave sound velocity in the polarization maintaining fiber to be measured, k is the wave vector, y m As a boundary equation
Intrinsic solution of (V) T For the transverse wave sound velocity, J, in the polarization-maintaining fiber to be measured i Be bezier equation of order i.
2. The method according to claim 1, wherein the number of the excitation pulsed light is two groups.
3. The method according to claim 1 or 2, characterized in that the excitation pulsed light is completely separated from the reading pulsed light in the time domain.
4. The method according to claim 1, wherein the obtaining of the excitation pulsed light and the reading pulsed light comprises:
controlling a first electro-optical modulator by using an arbitrary waveform generator to enable the first electro-optical modulator to have a frequency v 0 Is continuously laser-operatedFrequency shifting to obtain a frequency v 0 -f 1 -f m 、ν 0 +f 1 +f m 、ν 0 +f 1 V and v 0 -f 1 The excitation pulsed light of (1);
controlling a second electro-optical modulator by using an arbitrary waveform generator to enable the second electro-optical modulator to have a frequency v 0 Is frequency shifted to obtain a frequency of
And
the reading pulse light of (1).
5. The method according to claim 1, wherein the method of obtaining the continuous probe light comprises:
controlling a single-sideband modulator by a microwave source to enable the single-sideband modulator to have a v frequency 0 Is frequency shifted to obtain a frequency of
Or
And a frequency of
Or
The continuous probe light of (1).
6. A polarization maintaining optical fiber cladding diameter measuring device, comprising: the device comprises a laser, a beam splitter, a first frequency shift device, a second frequency shift device, a photoelectric detector and a data acquisition card;
the linearly polarized continuous laser output by the laser passes through the beam splitterDividing into three beams, wherein the first beam outputs several groups of frequency intervals of f after frequency shift by the first frequency shift device m Excitation pulsed light of, said f m The excitation pulse light enters a fast axis or a slow axis of the polarization maintaining optical fiber to be measured from one end of the polarization maintaining optical fiber to be measured as a forward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be measured, and the second beam is subjected to frequency shift by the first frequency shift device and then outputs a frequency interval of f m The reading pulse light enters the polarization maintaining optical fiber to be tested from one end of the polarization maintaining optical fiber to be tested, the polarization direction of the reading pulse light is vertical to the polarization direction of the exciting pulse light, and the third beam passes through the second frequency shifting device and then the output frequency is
Or alternatively
And a frequency of
Or
Wherein v is B The continuous probe light enters the polarization maintaining optical fiber to be tested from the other end of the polarization maintaining optical fiber to be tested as a backward Brillouin frequency shift theoretical value of the polarization maintaining optical fiber to be tested;
the photoelectric detector is used for measuring the power of two frequency components in reading pulse light output from the polarization-maintaining optical fiber.
7. The apparatus according to claim 6, wherein the first frequency shift means includes an arbitrary waveform generator that controls the first electro-optical modulator and the second electro-optical modulator, respectively, the first electro-optical modulator modulating the first beam of light into pulsed light and shifting the frequency, and the second electro-optical modulator modulating the second beam of light into pulsed light and shifting the frequency.
8. The apparatus of claim 6, wherein the second frequency shifting means comprises a microwave source and a single sideband modulator, the microwave source controlling the single sideband modulator to frequency shift the third beam of light.
9. The device according to claim 6, 7 or 8, further comprising two fiber amplifiers, wherein the excitation pulse light and the reading pulse light output by the first frequency shift device enter the polarization maintaining fiber to be tested after being amplified by the two fiber amplifiers, respectively.
10. The device according to claim 6, 7 or 8, further comprising a polarization beam combiner and three polarization controllers, wherein the excitation pulse light and the reading pulse light respectively enter the polarization beam combiner after passing through the two polarization controllers, light emitted from the polarization beam combiner enters the polarization maintaining fiber to be tested, and the continuous probe light enters the polarization maintaining fiber to be tested after passing through the third polarization controller.
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