CN113503955A - Optical fiber hydrophone based on optical frequency domain reflection technology - Google Patents
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Abstract
The invention relates to the technical field of optical fiber sensing, in particular to an optical fiber hydrophone based on an optical frequency domain reflection technology, which comprises a linear frequency-sweeping laser, an interferometer, a signal sensing detection device, a photoelectric detector and a signal processing unit, wherein the linear frequency-sweeping laser is connected with the interferometer; the output end of the linear frequency-sweeping laser is connected with the input end of an optical fiber coupler in the interferometer, the output end of the optical fiber coupler divides the frequency-sweeping laser into two paths, one path is signal light, the other path is reference light, and the reference light is transmitted to the Faraday rotating mirror through an optical fiber and reflected back to the optical fiber coupler by the Faraday rotating mirror; the signal light enters the signal sensing detection device through the optical fiber, receives modulation and returns a Rayleigh signal to the optical fiber coupler; the interference occurs in the optical fiber coupler, the output end of the interferometer is connected with the input end of the photoelectric detector, and the output end of the photoelectric detector is connected with the input end of the signal processing unit for signal processing.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an optical fiber hydrophone based on an optical frequency domain reflection technology.
Background
With the application of various advanced technologies in submarine manufacturing technology, the noise of modern submarines is continuously reduced when the modern submarines run underwater, which brings huge challenges to anti-submarine battles, the sensitivity of the traditional piezoelectric hydrophones equipped in large quantities at present can not meet the actual combat requirements of underwater acoustic detection, and the performance is difficult to be greatly improved due to the physical nature of piezoelectric materials, the development of the optical fiber sensor technology provides technical possibility for developing high-performance hydrophones, an array system for searching, early warning, detecting and identifying underwater targets can be formed through the combination of optical fiber sensing and transmission, the optical fiber hydrophone adopts optical fibers as information carriers, is suitable for long-distance and large-range monitoring, is an advanced detection means of modern navy anti-submarine battles and underwater weapon tests, and simultaneously, the acoustic propagation, noise, reverberation and noise of the optical fiber hydrophone in the marine acoustic environment, The monitoring of the submarine acoustic characteristics, the target acoustic characteristics and the like has wide application prospect in the fields of marine resource exploration, submarine observation networks, marine homeland security and the like.
At present, optical fiber sensors in most optical fiber hydrophones are point-type sensing, the measurement range is limited in discrete areas, and a plurality of sensing units are generally added to expand the measurement range. The cost, complexity and vulnerability limit the wide application of this sensing technology, and thus a "distributed sensing technology" capable of continuous sensing covering the entire length of the optical fiber is created, and most distributed sensors are based on the Optical Time Domain Reflectometry (OTDR), which has the basic principle of detecting and analyzing the reflected short pulse light, but it usually cannot solve the contradiction between dynamic distance and spatial accuracy, and therefore, there is a need for improving the prior art to solve the above technical problems.
Disclosure of Invention
In order to solve the technical problems, the invention provides an optical fiber hydrophone based on an optical frequency domain reflection technology, which is specifically realized by the following technical scheme:
the invention comprises a linear frequency-sweeping laser, an interferometer, a signal sensing and detecting device, a photoelectric detector and a signal processing unit; the output end of the linear frequency-sweeping laser is connected with the input end of an optical fiber coupler in the interferometer, the output end of the optical fiber coupler divides the frequency-sweeping laser into two paths, one path is signal light, the other path is reference light, and the reference light is transmitted to the Faraday rotating mirror through an optical fiber and reflected back to the optical fiber coupler by the Faraday rotating mirror; the signal light enters the signal sensing detection device through the optical fiber, receives modulation and returns a Rayleigh signal to the optical fiber coupler; the signal is interfered in the interferometer, the output end of the interferometer is connected with the input end of the photoelectric detector, and the output end of the photoelectric detector is connected with the input end of the signal processing unit for signal processing.
Specifically, the signal sensing and detecting device comprises an armored optical fiber and a hydrophone probe. The armored optical fiber plays a role in protection, signals are modulated through the hydrophone probes, and the hydrophone probes are connected in a distributed mode.
Specifically, the exterior of the armored optical fiber is made of one of polyurea or polyurethane composite materials.
Specifically, the hydrophone probe is subjected to sensitization treatment by adopting a mandrel type elastic structure, the sensing optical fiber inside the hydrophone probe is wound on the sound pressure elastic body, and the outer layer of the hydrophone probe is packaged by adopting an acoustic sensor.
Specifically, the length of the reference arm optical fiber is determined according to the frequency of the linear laser sweep and the interval between the hydrophone probes.
Specifically, the insertion loss of the Faraday rotator mirror is not higher than 0.3dB, the diameter of the Faraday rotator mirror is 2.5mm, and the length of the Faraday rotator mirror is 10 mm.
Specifically, the linear frequency-swept laser is a linear tunable laser.
Specifically, the optical fiber coupler is a 2 x 2 optical fiber coupler, the insertion loss is not higher than 0.2dB, and the splitting ratio is 1: 1.
Specifically, the interferometer is a michelson interferometer.
Specifically, the photoelectric detector is of a type with high photoelectric conversion efficiency and high noise resistance, and is packaged by an aluminum shell, the bandwidth of the photoelectric detector is not lower than 300MHz, the gain is 700A/W, and the equivalent noise pressure is not higher than 100pw/Hz 1/2.
The invention has the beneficial effects that:
the invention creatively uses the optical frequency domain reflection technology, in the interferometer, the sweep light is divided into two paths by the 2 x 2 optical fiber coupler, one part of the sweep light is used as reference light and is reflected back to the optical fiber coupler by the Faraday rotating mirror, the other part of the sweep light enters the signal sensing detection device, then the signal light is converted into signal light through modulation, the Rayleigh signals are returned to the optical fiber coupler and then mutually interfered, and the signal light is output to a photoelectric detector, and then the information is processed and demodulated by the signal processing unit, and the information is restored, so that the optical fiber hydrophone has the advantages of the traditional optical fiber hydrophone, namely the characteristics of high sensitivity, wide frequency band response, electromagnetic interference resistance, severe environment resistance, light structure, easiness in remote measurement, large-scale array formation and the like, and also effectively solves the problem that the measurement precision and the measurement range of the hydrophone are restricted, and other beneficial effects of the optical fiber hydrophone are further explained by combining with the following specific embodiments.
Drawings
The invention is further described below with reference to the following figures and examples:
FIG. 1 is a signal flow diagram of the present invention;
fig. 2 is a detailed view of the structure of the present invention.
Detailed Description
As shown in fig. 1-2: an optical fiber hydrophone based on an optical frequency domain reflection technology comprises a linear frequency-sweeping laser 1, an interferometer 2, a signal sensing and detecting device 3, a photoelectric detector 4 and a signal processing unit 5; the output end of the linear frequency-sweeping laser 1 is connected with one input end of an optical fiber coupler 21 in the interferometer 2, the output end of the optical fiber coupler 21 divides the frequency-sweeping laser into two paths, one path is signal light, the other path is reference light, and the reference light is transmitted to a Faraday rotation mirror 22 through an optical fiber and reflected back to the optical fiber coupler 21; the signal light enters the signal sensing detection device 3 through the optical fiber, the signal light modulated by the signal sensing detection device 3 returns to a Rayleigh signal and is interfered with reference light which is not modulated in the interferometer 2, and the two signals become mixing signals; the output end of the optical fiber coupler 21 is connected with the input end of the photoelectric detector 4, the output end of the photoelectric detector 4 is connected with the input end of the signal processing unit 5 for signal processing, and the mixed frequency signal is processed by the photoelectric detector 4 and the signal processing unit 5 to restore information and obtain the position of a specific signal-emitting object. The core technology of the invention is OFDR, and the OFDR is based on Rayleigh backscattering and optical heterodyne detection.
The principle analysis and the beneficial effect analysis of the optical heterodyne detection are as follows:
signal light E assuming that a coherence condition is satisfieds(t) and reference light ErThe light fields of (t) are respectively:
the coherent light field e (t) of the two beams can be expressed as:
wherein A iss、ArIs amplitude, ws、wrIn order to be the angular frequency of the frequency,
is the phase.
From the square-law characteristics of the photodetector 4, the photocurrent output by the photodetector 4 is: i (t) ═ α [ Es(t)+Er(t)]2Where α represents the photoelectric conversion factor of the photodetector 4, considering that the photodetector 44 does not respond to the part above the cut-off frequency and the dc component is filtered out, the final output photocurrent signal is:
the above equation contains the amplitude, frequency and phase information of the optical heterodyne detection output signal. The amplitude of the output signal is in direct proportion to the amplitudes of the two optical signals, and the frequency is the difference frequency of the two optical signals. As can be seen from the above formula, the magnitude of the photocurrent signal output by the photo detector 4 depends on the optical power of the signal light and the reference light, and a larger photocurrent signal can be obtained by reasonably adjusting the optical power of the two optical signals, because the photo detectors 4 in the optical heterodyne detection system have a filtering function and only allow optical signals within the intermediate frequency to enter, the detection system also has a better filtering performance.
For the conversion gain of the optical heterodyne detection method, let the resistance of the photodetector 4 be RThe power of the output electrical signal obtained by the optical heterodyne detection mode and the direct detection mode is P1And P2Then, the conversion gain of the optical heterodyne detection method can be expressed as:
wherein P isRAnd PSThe power of reference light and signal light in the optical heterodyne detection system are respectively represented, and the formula shows that the conversion gain of the optical heterodyne detection output is determined by the power ratio of the reference light and the signal light.
The principle of the optical frequency domain reflection technology is analyzed as follows:
let γ be the linear sweep speed of the light source, L be the length of the sensing fiber, and τ be the delay time between the reference light and the Rayleigh scattered light returning from the scattering point at a certain position along the fiber. For simplicity, assuming that the maximum optical lengths 2L of the reference optical path and the test optical path are based on the optical heterodyne detection principle, in the OFDR system, the signal light and the reference light can be respectively expressed as follows:
wherein f is0Is the initial frequency of the tunable laser and,
which represents the random phase of the reference light,
representing the reflectivity at the time delay tau,
representing the random phase of the back rayleigh scattered signal at time t.
Therefore, the expression formula of the light intensity signal of beat frequency interference in the system is deduced:
wherein
The term represents the phase difference of the two beams at time t, which varies non-linearly with time. f. ofbt is the linear beat phase over time. It varies linearly with time and is a function of the position of the scattering point on the sensing fiber, i.e.
fb(x)＝γ2x/Vg
x corresponds to the position of a scattering point on the sensing optical fiber, and x is more than or equal to 0 and less than or equal to L.
According to the formula, when the laser sweeps frequency linearly, scattering points at different positions on the optical fiber correspond to different frequencies, and the positions of the scattering points on the sensing optical fiber can be reversely deduced according to the frequency of the frequency spectrum, so that the optical fiber is positioned, and meanwhile, the OFDR technology can also realize the function of distance measurement. The OFDR original signal is transformed to a frequency domain by Fourier transform, and the position of a scattering point of the optical fiber can be reversely deduced by the frequency on the frequency spectrum. The minimum frequency corresponds to the position where the sensing fiber x is equal to 0, the maximum frequency corresponds to the position where the sensing fiber x is equal to L, and the frequency difference corresponds to the distance between two scattering points.
Specifically, in the present invention, when disturbance occurs from the outside, it is assumed that the optical fiber corresponding to the measured optical fiber is the optical fiberThere is a disturbance at the time delay tau, whose Asin 2 pi fkt is the phase change of the optical signal due to the perturbation, where A is the modulation amplitude of the phase, fkIs the frequency magnitude of the disturbance.
Thereby obtaining the light intensity formula under the condition of disturbance signals:
in the subsequent processing process, beat frequency is solved through modes of deskew filtering, cross-correlation operation and the like.
Specifically, the signal sensing detection device 3 includes a hydrophone probe 31 and an armored optical fiber 32, signal light enters the hydrophone probe 32 through optical fiber transmission, optical signals are subjected to external modulation through the probe, the hydrophone probe 31 is provided with a plurality of groups which are connected in a distributed manner and arranged on the armored optical fiber 32 at intervals, and the signal light returns a rayleigh signal after modulation and enters the optical fiber coupler 21 so as to complete interference with reference light.
Preferably, the exterior of the armored optical fiber 32 is one of polyurea or polyurethane composite material, which has better protection effect.
Preferably, the hydrophone probe 31 is sensitized by a mandrel type elastic structure, the sensing optical fiber inside the hydrophone probe 32 is wound on the sound pressure elastic body, and the method is a simpler sensitizing method, and the outer layer of the hydrophone probe 32 is packaged by an acoustic sensor.
Specifically, the insertion loss of the Faraday rotator mirror is not higher than 0.3dB, the diameter of the Faraday rotator mirror is 2.5mm, and the length of the Faraday rotator mirror is 10 mm.
Preferably, the linear frequency-swept laser 1 is a linear tunable laser, has the characteristics of narrow line width, wide frequency sweep and high frequency sweep speed, and can achieve the purpose of avoiding the nonlinear frequency sweep effect and further optimize the characteristics of the hydrophone.
Specifically, the optical fiber coupler 21 is a 2 × 2 optical fiber coupler 21, the insertion loss is not higher than 0.2dB, and the splitting ratio is 1: 1.
Specifically, the interferometer 2 is a michelson interferometer 2, in the embodiment, the output signal of the michelson interferometer is interference of reference light and a rayleigh scattering signal of signal light, and two light waves meeting a coherence condition are respectively signal light and reference light; the signal light comes from the back scattering signal in the optical fiber, the reference light comes from the reflection signal of the incident light, the two beams of light pass through the optical fiber coupler 21 and become a mixing signal, and are converted into an electric signal on the photosensitive surface of the balanced photodetector 4, and then a beat signal is obtained after filtering and amplification, the frequency of the beat signal corresponds to the frequency difference between the signal light and the reference light, when the tuning frequency of the tunable laser source is fixed, the size of the beat frequency is in direct proportion to the length of the optical fiber to be tested, and when the parameter of a certain point in the optical fiber to be tested changes, the size of the beat frequency also changes correspondingly, so that the frequency shift of the beat signal to be tested can detect the change of the parameter of the certain point in the optical fiber, and then the position of the probe is obtained. The hydrophone positioning is based on the vector space positioning principle, and the position of an object sending a signal is determined according to the space vector angle of a plurality of probes for receiving external sound signals.
Specifically, the photoelectric detector 4 is of a type with high photoelectric conversion efficiency and high noise resistance, and is packaged by an aluminum shell, the bandwidth of the photoelectric detector 4 is not lower than 300MHz, the gain is 700A/W, and the equivalent noise sound pressure is not higher than 100pw/Hz1/2, so that the interference of external noise can be effectively eliminated, the system is particularly suitable for detecting Rayleigh scattering weak signals, and the capability of the whole equipment for resisting electromagnetic interference and harsh environment is improved.
When the optical fiber sensor is actually used, the linear frequency-sweeping laser 1 is connected with the 2 x 2 optical fiber coupler 21, the linear frequency-sweeping laser 1 emits frequency-sweeping light, the frequency-sweeping laser is divided into two paths of optical signals in the optical fiber coupler 21, one part of the optical signals is a signal, the signal enters the signal sensing and detecting device 3 along a signal arm, and the Rayleigh signal is returned to the optical fiber coupler 21 after modulation; the other optical signal is a reference signal which is directly reflected back to the optical fiber coupler 21 by the faraday rotating mirror 22 arranged at the output end of the reference signal without modulation, the signal light and the reference light interfere in the optical fiber coupler 21 in the interferometer and output a mixing signal, the mixing signal enters the photoelectric detector 4 through the output end of the interferometer and then is converted into an electric signal by the photoelectric detector 4 for being analyzed and demodulated by the subsequent signal processing unit 5, and the object position information is restored through some demodulation based on the above-mentioned OFDR technology, rayleigh backscattering and optical heterodyne detection principles The frequency band response is wide, the hydrophone has the characteristics of electromagnetic interference resistance, severe environment resistance, light structure, easiness in remote measurement, large-scale array formation and the like, the problem that the measurement accuracy and the measurement range of the hydrophone are restricted is effectively solved, and meanwhile, the hydrophone is lower in production cost, simple in structure and more beneficial to production and use of a large number of devices.
Finally, the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting, although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all of them should be covered in the claims of the present invention.
Claims (10)
1. An optical fiber hydrophone based on optical frequency domain reflection technology is characterized in that: the device comprises a linear frequency-sweeping laser, an interferometer, a signal sensing and detecting device, a photoelectric detector and a signal processing unit; the output end of the linear frequency-sweeping laser is connected with the input end of an optical fiber coupler in the interferometer, the output end of the optical fiber coupler divides the frequency-sweeping laser into two paths, one path is signal light, the other path is reference light, and the reference light is transmitted to the Faraday rotating mirror through an optical fiber and reflected back to the optical fiber coupler by the Faraday rotating mirror; the signal light enters the signal sensing detection device through the optical fiber, receives signal modulation and returns a Rayleigh signal to the optical fiber coupler; the output end of the interferometer is connected with the input end of the photoelectric detector, and the output end of the photoelectric detector is connected with the input end of the signal processing unit for signal processing.
2. The fiber optic hydrophone based on optical frequency domain reflectometry as in claim 1, wherein: the signal sensing detection device comprises an armored optical fiber and a hydrophone probe, and signal light enters the hydrophone probe through the armored optical fiber and is modulated by the hydrophone probe to return Rayleigh signals.
3. The fiber optic hydrophone based on optical frequency domain reflectometry as in claim 2, wherein: and the exterior of the armored optical fiber is made of one of polyurea or polyurethane composite materials.
4. The fiber optic hydrophone based on optical frequency domain reflectometry as in claim 3, wherein: the hydrophone probe is subjected to sensitization treatment by adopting a mandrel type elastic structure, the sensing optical fiber inside the hydrophone probe is wound on the sound pressure elastic body, and the outer layer of the hydrophone probe is packaged by adopting an acoustic sensor.
5. The fiber optic hydrophone based on optical frequency domain reflection technology of claim 4, wherein: the insertion loss of the Faraday rotator mirror is not higher than 0.3dB, the diameter of the Faraday rotator mirror is 2.5mm, and the length of the Faraday rotator mirror is 10 mm.
6. The fiber optic hydrophone based on optical frequency domain reflection technology of claim 5, wherein: the linear frequency-sweeping laser is a linear tunable laser.
7. The fiber optic hydrophone based on optical frequency domain reflectometry as in claim 6, wherein: the optical fiber coupler is a 2-by-2 optical fiber coupler, the insertion loss is not higher than 0.2dB, and the splitting ratio is 1: 1.
8. The fiber optic hydrophone based on optical frequency domain reflection technology of claim 7, wherein: the interferometer is a Michelson interferometer.
9. The fiber optic hydrophone of claim 8, wherein the length of the reference fiber is not arbitrary and is related to the frequency of the laser sweep and the spacing between the hydrophone probes.
10. The fiber optic hydrophone based on optical frequency domain reflection technology as recited in any of claims 1-8, wherein: the photoelectric detector is of a type with high photoelectric conversion efficiency and high noise resistance, and is packaged by an aluminum shell, the bandwidth of the photoelectric detector is not lower than 300MHz, the gain is 700A/W, and the equivalent noise pressure is not higher than 100pw/Hz 1/2.
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