CN112129491B - Optical fiber time delay measuring method and device based on single-optical-frequency comb interference - Google Patents
Optical fiber time delay measuring method and device based on single-optical-frequency comb interference Download PDFInfo
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- CN112129491B CN112129491B CN202011145214.XA CN202011145214A CN112129491B CN 112129491 B CN112129491 B CN 112129491B CN 202011145214 A CN202011145214 A CN 202011145214A CN 112129491 B CN112129491 B CN 112129491B
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- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
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- G01M11/331—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face by using interferometer
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Abstract
The invention discloses an optical fiber time delay measuring method based on single optical frequency comb interference, which is characterized in that an optical carrier output by a narrow linewidth laser is divided into two paths, wherein one path of optical carrier is modulated into an optical frequency comb signal by an optical frequency comb modulator and then is sent into an interferometer connected with an optical fiber to be measured, and the other path of optical carrier is subjected to frequency shift processing; coupling the optical carrier after frequency shift processing with an optical signal output by an interferometer, and then carrying out photoelectric detection to obtain a microwave signal carrying time delay information; and extracting the frequency spectrum information of the microwave signal, extracting the free frequency spectrum range of the interference frequency spectrum, and calculating the time delay of the optical fiber to be measured according to the free frequency spectrum range of the interference frequency spectrum. The invention also discloses an optical fiber time delay measuring device based on single optical frequency comb interference. Compared with the prior art, the invention has the advantages of high measurement stability, high measurement speed, simple structure and low realization cost.
Description
Technical Field
The invention relates to an optical fiber time delay measuring method, and belongs to the technical field of optical measurement.
Background
The time delay is a basic parameter in signal generation, transmission, control and processing, and along with the rapid development of optical fiber electronic information systems, the rapid and stable measurement and control of the time delay in an optical fiber link become the key of research and application of high-performance information systems such as light-operated phased arrays, distributed radar networks, optical fiber time-frequency transmission and the like.
The commonly used optical fiber time delay measuring methods mainly include three methods, namely a pulse method, a phase-push method and a frequency-sweep interference method. The pulse method is also called as a time domain method, and the time delay of the optical fiber to be measured is obtained by directly recording the time interval between the emission of the optical pulse signal and the reception of the optical pulse signal. The pulse method generally takes a backscattering signal of a pulse in an optical fiber as a receiving signal, but the backscattering signal is weaker, the signal-to-noise ratio of a system is not high, the precision is limited, the nonlinear effects such as chromatic dispersion and the like limit, and the pulse energy cannot be infinitely improved; meanwhile, the pulse transmission is gradually widened, so that the pulse method is low in precision and is usually meter-level. The phase-push method utilizes phase change in transmission to solve the optical fiber time delay, has high precision, but when the microwave signal with high frequency is utilized to carry out measurement, because of the shortage of the high-frequency phase discriminator, the up-conversion and the down-conversion are needed, extra noise is easily introduced, and errors are brought. In addition, in order to obtain the integer ambiguity to realize the absolute time delay measurement, the frequency sweep of the microwave signal is needed, so that the system cost and the complexity are improved, and the measurement speed of the method is limited. The frequency sweep interference method is also called as a frequency domain method, and can obtain higher precision by utilizing a laser and an interference structure which are continuously scanned, but is limited by the line width of the laser and the linearity of frequency sweep, the precision is in the centimeter magnitude and the price is expensive; in addition, the optical fiber time delay measurement based on the interference structure can also be realized by using a spectral domain white light interferometry and a coherent light interference fringe counting method, the spectral domain white light interferometry usually adopts a wavelength scanning light source, a photoelectric detector is used at the output end of the optical fiber interferometer to detect the interference light intensity, and the optical fiber time delay (Xu F, Ren D, Shi X, et al.high-sensitivity Fabry-Perot interferometer based on a nano-logic filter pilot [ J ]. Optics Letters,2012,37(2):133 and 135.) is obtained by analyzing the interference spectrum, but the method is not suitable for the measurement of the time delay of some optical fiber systems containing narrow-band filters, and the algorithm processing is more complex; the interference fringe counting method measures the time delay by observing the distance between the interference fringes, and the method has a complex structure and high difficulty in realization and is commonly used for time delay measurement of space light.
The optical frequency comb includes many frequency components, and the intervals of the components are the same and stable, so the optical frequency comb is favored in the field of high-precision measurement. The common method is to use the phase difference of a double-optical comb with small measurement weight frequency difference after passing through an interferometer to realize high-precision optical time delay measurement (Coddington I, Swann W C, Nenadovic L, et al, Rapid and precision interference measurement at long range [ J ]. Nature Photonics,2009,3(6): 351-. The measuring method needs to lock the two optical combs to avoid drift caused by environmental influence, has a complex structure and higher cost, can only be applied to space optical absolute distance measurement, and has a smaller measuring range. The single optical comb also has a good advantage in spatial ranging, and korean science and technology institute (KAIST) utilizes a Sagnac interferometer structure in combination with phase-locked feedback, utilizes a mode-locked laser to generate an optical frequency comb signal, measures a delay difference between a microwave signal and the optical frequency comb signal, and realizes sub-femtosecond level delay measurement accuracy (Na Y, Jeon C-G, Ahn C, et al. ultra fast, sub-nanometer-precision and multi-functional time-of-flight detection [ J ]. Nature Photonics,2020,14(6):355-360.) because of the periodicity of the optical frequency comb signal and the microwave signal, the measurement range of the measurement method is small and is only in millimeter level. In addition, due to the adoption of a feedback mechanism, a slide and a Faraday rotation mirror are required to be added in a Sagnac interference loop to realize pi/2 phase difference, the position of the slide is easy to change, the slide needs to be adjusted for calibration before measurement, and the operation difficulty is high; the interferometric loops must also use high cost polarization maintaining fibers; in addition, in order to achieve stable measurement results, a high-quality voltage-controlled oscillator and a mode-locked laser are used in the scheme. In conclusion, the single optical comb time delay measurement system has the advantages of complex structure, high requirement on equipment, large operation difficulty and small measurement range, can only be used for space optical measurement at present, and is not suitable for time delay measurement in optical fibers.
Disclosure of Invention
The invention aims to solve the technical problem of overcoming the defects of the prior art, provides the optical fiber time delay measuring method based on single-optical-frequency comb interference, avoids the complex phase-locked structure required in the double-optical-frequency comb interference scheme, and has the advantages of high measuring stability, high measuring speed, simple structure and low implementation cost.
The invention specifically adopts the following technical scheme to solve the technical problems:
the optical fiber time delay measuring method based on single optical frequency comb interference divides an optical carrier output by a narrow linewidth laser into two paths, wherein one path of the optical carrier is modulated into an optical frequency comb signal by an optical frequency comb modulator and then is sent to an interferometer connected with an optical fiber to be measured, and the other path of the optical carrier is subjected to frequency shift processing; coupling the optical carrier after frequency shift processing with an optical signal output by an interferometer, and then carrying out photoelectric detection to obtain a microwave signal carrying time delay information; and extracting the frequency spectrum information of the microwave signal, extracting the free frequency spectrum range of the interference frequency spectrum, and calculating the time delay of the optical fiber to be measured according to the free frequency spectrum range of the interference frequency spectrum.
Further, the measurement range and the measurement precision of time delay measurement are adjusted by adjusting the comb tooth interval of the optical frequency comb signal.
Preferably, the time delay of the optical fiber to be measured is calculated according to the free spectrum range, specifically adopting the following formula:
wherein, Delta tau is the time delay of the optical fiber to be measured, tau0Is the initial time delay difference of the two arms of the interferometer, fFSRIs the free spectral range of the interference spectrum.
Preferably, the extracting of the frequency spectrum information of the microwave signal is performed by firstly collecting the time domain information of the microwave signal by using an oscilloscope or an acquisition card, and then converting the time domain information into the frequency domain information by using fast fourier transform.
Preferably, the free spectrum range of the interference spectrum is, specifically, a frequency difference between adjacent peaks or adjacent troughs in the frequency domain information of the microwave signal.
Based on the same inventive concept, the following technical scheme can be obtained:
optical fiber time delay measuring device based on single frequency comb interference includes:
the optical carrier module is used for dividing the optical carrier output by the narrow linewidth laser into two paths;
the optical frequency comb modulator is used for modulating one path of optical carrier into an optical frequency comb signal and sending the optical frequency comb signal into the interferometer connected with the optical fiber to be detected;
the frequency shift module is used for carrying out frequency shift processing on the other path of optical carrier;
the optical detector is used for performing photoelectric detection on the optical carrier wave after the frequency shift processing and a coupling optical signal of an optical signal output by the interferometer to obtain a microwave signal carrying time delay information;
the frequency spectrum extraction module is used for extracting frequency spectrum information of the microwave signal and then extracting a free frequency spectrum range of an interference frequency spectrum;
and the resolving module is used for resolving the time delay of the optical fiber to be measured according to the free spectrum range of the interference spectrum.
Further, the measurement range and the measurement precision of the measuring device are adjusted by adjusting the comb tooth interval of the optical frequency comb signal.
Preferably, the calculating module calculates the time delay of the optical fiber to be measured according to the free spectrum range, specifically adopting the following formula:
wherein, Delta tau is the time delay of the optical fiber to be measured, tau0Is the initial time delay difference of the two arms of the interferometer, fFSRIs the free spectral range of the interference spectrum.
Preferably, the extracting of the frequency spectrum information of the microwave signal is performed by firstly collecting the time domain information of the microwave signal by using an oscilloscope or an acquisition card, and then converting the time domain information into the frequency domain information by using fast fourier transform.
Preferably, the free spectrum range of the interference spectrum is, specifically, a frequency difference between adjacent peaks or adjacent troughs in the frequency domain information of the microwave signal.
Compared with the prior art, the technical scheme of the invention has the following beneficial effects:
(1) the measuring speed is high, frequency sweeping is not needed, and the measurement can be completed at one time. The invention adopts the optical frequency comb modulator to simultaneously generate optical signals with a large number of frequency components, and the frequency interval can be adjusted by the modulation signals, thereby solving the problem of slow measurement speed caused by the need of continuously sweeping frequency to obtain the free frequency spectrum range of the interferometer in the traditional interferometric length measuring system.
(2) The system stability is high. In the interferometer provided by the invention, the signals in the reference arm and the measuring arm are from the same optical frequency comb, so that the influence of frequency drift caused by the influence of the environment on the optical frequency comb is avoided.
(3) The flexibility is high. The invention realizes the optical frequency comb with adjustable frequency comb interval by using an external modulation mode, and realizes the flexible control of the optical frequency comb interval by controlling the modulated microwave signal, thereby achieving the purpose of controlling the measurement range and precision of the system.
(4) And the expansibility is strong. The interferometer may be a structure of a mach-zehnder (MZI) interferometer, or a structure of a michelson interferometer.
Drawings
FIG. 1 is a schematic structural diagram of an embodiment of the present invention;
fig. 2 is a schematic diagram of a conventional optical interference delay measurement system.
Detailed Description
Aiming at the defects in the prior art, the solution idea of the invention is to use the optical frequency comb signal as the input light source of the interferometer, and to replace the frequency sweeping process in the traditional interferometer system by using abundant frequency components in the optical frequency comb, thereby greatly improving the measurement speed; signals entering the two arms of the interferometer are from the same optical frequency comb signal, so that the influence of frequency drift caused by the influence of the environment on the optical frequency comb is avoided; simultaneously, heterodyne reception is adopted to map the frequency spectrum information of the interference signal from the optical domain to the electrical domain, and the free frequency spectrum range f of the interference signal is accurately obtained by utilizing a relatively low-frequency and high-precision frequency spectrum measurement mode on the electrical domainFSRAnd then the time delay of the optical fiber to be measured is calculated.
The invention provides an optical fiber time delay measuring method based on single-optical-frequency comb interference, which comprises the following steps:
dividing optical carriers output by a narrow linewidth laser into two paths, wherein one path of optical carriers is modulated into optical frequency comb signals by an optical frequency comb modulator and then is sent to an interferometer connected with an optical fiber to be detected, and the other path of optical carriers is subjected to frequency shift processing; coupling the optical carrier after frequency shift processing with an optical signal output by an interferometer, and then carrying out photoelectric detection to obtain a microwave signal carrying time delay information; and extracting the frequency spectrum information of the microwave signal, extracting the free frequency spectrum range of the interference frequency spectrum, and calculating the time delay of the optical fiber to be measured according to the free frequency spectrum range of the interference frequency spectrum.
The invention relates to an optical fiber time delay measuring device based on single-optical-frequency comb interference, which comprises:
the optical carrier module is used for dividing the optical carrier output by the narrow linewidth laser into two paths;
the optical frequency comb modulator is used for modulating one path of optical carrier into an optical frequency comb signal and sending the optical frequency comb signal into the interferometer connected with the optical fiber to be detected;
the frequency shift module is used for carrying out frequency shift processing on the other path of optical carrier;
the optical detector is used for performing photoelectric detection on the optical carrier wave after the frequency shift processing and a coupling optical signal of an optical signal output by the interferometer to obtain a microwave signal carrying time delay information;
the frequency spectrum extraction module is used for extracting frequency spectrum information of the microwave signal and then extracting a free frequency spectrum range of an interference frequency spectrum;
and the resolving module is used for resolving the time delay of the optical fiber to be measured according to the free spectrum range of the interference spectrum.
The measuring range and the precision of the measuring device can be flexibly controlled by changing the interval of the comb teeth of the optical frequency comb: in measuring large-range time delay, f of interferometerFSRThe comb tooth interval of the optical frequency comb needs to be reduced at the moment, so that the free frequency spectrum range of the interferometer is obtained; because the number of the comb teeth is limited, when the comb tooth interval is too small, the frequency spectrum measurement range cannot cover the whole free frequency spectrum range, and therefore, the comb tooth interval cannot be too small; while the free frequency spectrum range of the interferometer can be obtained, the comb tooth interval can be reduced to obtain more accurate fFSRAnd the time delay measurement precision is improved.
For the public to understand, the technical scheme of the invention is explained in detail by a specific embodiment and the accompanying drawings:
the structure of the measuring device in this example is shown in fig. 1, and includes a narrow linewidth laser, a microwave source, an optical frequency comb modulator, an acousto-optic frequency shifter, an interferometer (a measuring arm is connected with an optical fiber to be measured), a photodetector, a spectrum extraction module, and a plurality of optical couplers. The optical carrier output by the narrow linewidth laser is divided into two paths, wherein one path of optical carrier is modulated into an optical frequency comb signal (the comb tooth interval of the optical frequency comb depends on the microwave frequency output by a microwave source) by an optical frequency comb modulator and then is sent to an interferometer connected with an optical fiber to be detected, and the acousto-optic frequency shifter carries out frequency shift processing on the other path of optical carrier; after the optical carrier wave after frequency shift processing and an optical signal output by the interferometer are coupled into a path, performing beat frequency through a photoelectric detector to obtain a microwave signal carrying time delay information; the spectrum extraction module extracts the spectrum information of the microwave signal, extracts the free spectrum range of the interference spectrum from the spectrum information, and finally, the time delay of the optical fiber to be measured can be calculated according to the free spectrum range of the interference spectrum (the calculation module is not shown in fig. 1).
Let the frequency of the optical carrier be f and the frequency of the microwave source be fmThe frequency shift generated by the acousto-optic modulator is fa. In the upper light path, a comb modulator with center frequency f and comb teeth spacing f is generatedmThe spectrogram of the signal of (a) is as shown at a; in the lower optical path, after passing through the acousto-optic modulator, the frequency of the optical carrier becomes f + faThe spectrogram is shown at b.
The link light is coupled to a path and enters the photoelectric detector for beat frequency to form a heterodyne structure for frequency spectrum sampling, and finally, only f is lefta，fm±fa，2fm±fa，3fm±fa…, the electrical signal frequency components are equalized to convert the interference spectrum from the high frequency optical domain to the low frequency electrical domain, the spectrum of which is shown at c.
Let the optical signal input to the interferometer be:
E(t)＝E1ejωt (1)
the optical fiber is divided into two paths with equal power through a coupler, one path enters a measuring arm with an optical fiber to be measured, the other path enters a reference arm, and the two paths of optical signals can be respectively expressed as follows:
where v is the speed of light in the fiber, and the subscripts "ref", "FUT" denote the reference arm, the measurement wall, respectively; and then two paths of signals are coupled into one path, and the optical signal power is output at the moment:
the photocurrent is in direct proportion to the power of the optical signal, the light intensity of the obtained interference signal changes with the frequency cycle of the signal, the cycle is determined by the optical path difference of the two arms of the interferometer, and the changed frequency cycle is the free frequency spectrum range f of the interferometerFSR。
In the conventional method, a frequency shift is performed on single-frequency light by using a modulation mode to realize frequency sweeping, and as shown in fig. 2, the light intensities of light with different frequencies passing through the interferometer are measured to obtain the free spectrum range of the interferometer. The invention utilizes abundant frequency components in the optical frequency comb, and can obtain the light intensity of light with different frequencies by acquiring data once, obtain the free frequency spectrum range of the interferometer, and solve the time delay of the optical fiber to be measured.
Meanwhile, signals in the reference arm and the measuring arm in the interferometer structure are from the same optical frequency comb, so that the influence of frequency drift caused by the influence of the environment on the optical frequency comb on the measurement is avoided, and the measurement stability is greatly improved.
And obtaining the time domain waveform of the interference signal by using a real-time oscilloscope or a signal acquisition card, and obtaining the frequency domain information of the photocurrent after Fourier transformation, namely a spectrum extraction module. After extracting the spectrum information of the photocurrent, the frequency intensity can be found to show periodic variation with the frequency of the signal, and the period is equal to the reciprocal of the time delay difference between the measuring arm and the reference arm of the interferometer, and the initial time delay difference tau between the two arms0(i.e. without access to the fiber under test, the interferometerTime delay difference of the two arms; the method provided by the invention can be used for measuring the time delay difference of two arms of the interferometer as a calibration value when the optical fiber to be measured is not accessed, and the tau is obtained0) Can be obtained by calibration, subtracting τ from the measured value0And then the time delay delta tau of the optical fiber to be measured can be obtained by calculation. Meanwhile, the change of the measurement precision of the free frequency spectrum range can be realized by changing the frequency of the microwave source, so that different time delay measurement precisions can be realized.
The calculation method is specifically as follows:
and acquiring the time domain waveform of the beat frequency electric signal by using a real-time oscilloscope or a high-speed acquisition card, and obtaining a spectrogram of the electric signal after fast Fourier transform. Sequentially finding out adjacent wave troughs or wave crests on the frequency intensity according to the sequence of the frequency of the electric signal from small to large, and recording the frequency interval, namely fFSR. Because of the heterodyne reception, the smaller frequency spectrum component in the output signal of the interferometer can still be detected, so that the acquired frequency spectrum information is more complete.
Therefore, the time delay of the fiber to be measured can be obtained as follows:
in conclusion, the measuring device has the advantages of simple structure, strong system stability and extremely simple and convenient resolving process, so that the rapid and high-stability optical fiber time delay measurement can be realized.
Claims (10)
1. The optical fiber time delay measuring method based on single-optical-frequency comb interference is characterized in that an optical carrier output by a narrow-linewidth laser is divided into two paths, wherein one path of optical carrier is modulated into an optical-frequency comb signal by an optical-frequency comb modulator and then is sent to an interferometer connected with an optical fiber to be measured, and the other path of optical carrier is subjected to frequency shift processing; coupling the optical carrier after frequency shift processing with an optical signal output by an interferometer, and then carrying out photoelectric detection to obtain a microwave signal carrying time delay information; and extracting the frequency spectrum information of the microwave signal, extracting the free frequency spectrum range of the interference frequency spectrum, and calculating the time delay of the optical fiber to be measured according to the free frequency spectrum range of the interference frequency spectrum.
2. The single-optical-frequency-comb-interference-based optical fiber time delay measurement method as claimed in claim 1, wherein the measurement range and the measurement accuracy of the time delay measurement are adjusted by adjusting the comb tooth interval of the optical frequency comb signal.
3. The method for measuring time delay of optical fiber based on single-optical-frequency comb interference as claimed in claim 1, wherein the time delay of the optical fiber to be measured is calculated according to the free spectrum range, specifically using the following formula:
wherein, Delta tau is the time delay of the optical fiber to be measured, tau0Is the initial time delay difference of the two arms of the interferometer, fFSRIs the free spectral range of the interference spectrum.
4. The single-optical-comb interference-based optical fiber time delay measuring method as claimed in claim 1, wherein the extracting of the spectral information of the microwave signal is performed by first collecting the time domain information of the microwave signal with an oscilloscope or a collection card, and then converting the time domain information into frequency domain information with fast fourier transform.
5. The method for measuring time delay of optical fiber based on single-optical-frequency comb interference as claimed in claim 1, wherein the free spectrum range of the interference spectrum is specifically the frequency difference between adjacent peaks or adjacent valleys in the frequency domain information of the microwave signal.
6. Optical fiber time delay measuring device based on single optical frequency comb interference, its characterized in that includes:
the optical carrier module is used for dividing the optical carrier output by the narrow linewidth laser into two paths;
the optical frequency comb modulator is used for modulating one path of optical carrier into an optical frequency comb signal and sending the optical frequency comb signal into the interferometer connected with the optical fiber to be detected;
the frequency shift module is used for carrying out frequency shift processing on the other path of optical carrier;
the optical detector is used for performing photoelectric detection on the optical carrier wave after the frequency shift processing and a coupling optical signal of an optical signal output by the interferometer to obtain a microwave signal carrying time delay information;
the frequency spectrum extraction module is used for extracting frequency spectrum information of the microwave signal and then extracting a free frequency spectrum range of an interference frequency spectrum;
and the resolving module is used for resolving the time delay of the optical fiber to be measured according to the free spectrum range of the interference spectrum.
7. The single-optical-frequency-comb-interference-based optical fiber time delay measuring device as claimed in claim 6, wherein the measuring range and measuring accuracy of the measuring device are adjusted by adjusting the comb tooth spacing of the optical-frequency comb signal.
8. The single-optical-frequency-comb-interference-based optical fiber time delay measuring device as claimed in claim 6, wherein the calculating module calculates the time delay of the optical fiber to be measured according to the free spectrum range, specifically adopting the following formula:
wherein, Delta tau is the time delay of the optical fiber to be measured, tau0Is the initial time delay difference of the two arms of the interferometer, fFSRIs the free spectral range of the interference spectrum.
9. The single-optical-comb interference-based optical fiber time delay measuring device as claimed in claim 6, wherein the extracting of the spectrum information of the microwave signal is performed by first collecting the time domain information of the microwave signal with an oscilloscope or a collection card, and then converting the time domain information into the frequency domain information with fast fourier transform.
10. The single-optical-comb-interference-based optical fiber time delay measuring device as claimed in claim 6, wherein the free spectral range of the interference spectrum is a frequency difference between adjacent peaks or adjacent valleys in the frequency domain information of the microwave signal.
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