CN114485907B - Device and method for eliminating parasitic interference signals in fiber grating hydrophone array - Google Patents
Device and method for eliminating parasitic interference signals in fiber grating hydrophone array Download PDFInfo
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Abstract
The invention relates to the technical field of optical fiber sensing and oceans, in particular to a device and a method for eliminating parasitic interference signals in a fiber grating hydrophone array, wherein the device comprises a laser, a common-mode phase modulator, an acousto-optic modulator, a matching interferometer, a polarization modulation modulator, a fiber circulator, a photoelectric conversion and acquisition module, a main control board, an upper computer and the fiber grating hydrophone array; the invention has the technical effects that: according to the invention, by designing the common-mode phase modulation frequency and modulation amplitude, the non-common-mode phase modulation frequency and the interrogation pulse frequency parameter in the device, the effect of eliminating the influence of parasitic interference signals of the fiber grating hydrophone array on the premise of not changing the optical structure advantage of the wet-end array and the signal demodulation algorithm is achieved.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing and oceans, in particular to a device and a method for eliminating parasitic interference signals in an optical fiber grating hydrophone array.
Background
The strategic position of the ocean in the national development is more prominent at present, and the importance of maintaining the ocean safety, developing the ocean and passing through the ocean reaches unprecedented height. Because the sound wave is the only information carrier which can be remotely transmitted underwater, the hydrophone which can acquire underwater information through the sound wave becomes important equipment for supporting the national ocean strategy. Among hydrophones with various mechanisms, the fiber grating hydrophone has the characteristics of high sensitivity of optical phase detection and good reusability of grating, and has the advantages of high sensitivity, wide dynamic range, easy multiplexing and the like. In particular, compared with the early fiber optic hydrophone, the wet end of the fiber grating hydrophone only contains a single fiber component, namely, an online fiber grating. The rapid reduction of the number of the underwater components and the number of the fusion points of the optical fibers brings outstanding application advantages of high reliability, light weight, miniaturization and the like, and the fiber grating hydrophone becomes an optimal design scheme of a next-generation large-scale hydrophone with high integration level.
In order to detect underwater weak acoustic signals, the fiber grating hydrophone usually adopts a matching interference scheme. As shown in fig. 1, the basic element of the array is a pair of fiber bragg gratings whose reflection centers are completely coincident. The wet end is interrogated with a pair of laser pulses whose wavelength corresponds exactly to the reflection center of the grating. The time interval of the pulse pair is exactly the same as the time delay of one round trip transmission of the fiber between the two gratings. Therefore, the tail pulse reflected by the first grating and the head pulse reflected by the second grating are completely overlapped in time, so that interference occurs, and the interference result is determined by the transmission phase of the optical fiber between the two gratings. By detecting the interference light intensity, the optical fiber phase information between the two gratings can be acquired, so that the underwater acoustic information can be acquired. N +1 gratings with the same central wavelength are written on one optical fiber at equal intervals, and two adjacent gratings and the sensing optical fiber between the two adjacent gratings form a sensing channel, so that an n-fold hydrophone time division multiplexing array can be formed.
In consideration of the problems of coherence of interference, array optical power balance, array time division channel crosstalk and the like, the reflectivity of the grating in the fiber grating hydrophone is low and generally does not exceed 5%, and even can reach about 0.5% under the condition of inhibiting the time division channel crosstalk without adopting other technical means. When other reflection points exist in the fiber grating hydrophone array, such as rayleigh scattering points and backward enhanced scattering points caused by fiber fusion, parasitic interference pulses, parasitic reflection and grating weak reflection coupling pulses and the like are generated, as shown in fig. 2. Because the fiber grating hydrophone uses monochromatic laser with long coherence length, parasitic interference can be formed after the pulses are superposed with grating reflection pulses. The spurious channel crosstalk and noise level increase problems of the hydrophone array caused by the aliasing of the spurious interference and the normal interference of the hydrophone.
The influence of parasitic interference on the optical fiber interference result is generally studied more in the field of optical fiber gyros. Studies such as "progress of semiconductor light source for fiber optic gyro system" (laser technology ", 1989, 04) and" Rayleigh scattering study in interferometric fiber optic gyro "(optical technology, 2007, 04) show that a semiconductor light source with a low coherence length is generally used in a fiber optic gyro system in order to eliminate the influence of Rayleigh scattering back scattering on the fiber optic gyro. This is also a solution commonly applied in the field of fiber optic gyroscopes. In the field of optical fiber hydrophones, because the background noise of a system is required to be ensured, the self laser frequency noise and the intensity noise of a semiconductor light source with low coherence length are high and cannot be adopted generally. In the document "research on key technology of ultra-long-range optical transmission low-noise optical amplification chain of optical fiber hydrophone array (doctor's academic paper of national defense science and technology university, 2013.11), the influence of rayleigh scattering on the background noise of the optical fiber hydrophone is analyzed, but no solution is proposed. In addition, research on distributed fiber optic hydrophones using rayleigh scattering in optical fibers is relatively extensive, and there is a related research on enhancing rayleigh scattering effect using a fiber grating with lower reflectivity (document "high signal-to-noise ratio distributed vibration sensing system based on ultra-weak fiber bragg grating" (proceedings of optics, 13 th 2021)). In this type of research, the rayleigh scattered light is the information carrier and not the parasitic interference. In summary, no solution is currently available for the parasitic interference effect in fiber grating hydrophone arrays.
Disclosure of Invention
The invention provides a device and a method for eliminating parasitic interference signals in a fiber grating hydrophone array aiming at the technical background. On the premise of not changing the optical simple structure advantage of the wet end of the fiber grating hydrophone array, the influence of parasitic interference on signal demodulation is eliminated by a modulation and demodulation method, and the accuracy of underwater sound signal extraction is ensured.
In order to achieve the technical purpose, the technical scheme of the invention is as follows:
a device for eliminating parasitic interference in a fiber grating hydrophone array comprises a laser 1, a common mode phase modulator 2, an acousto-optic modulator 3, a matching interferometer 4, a polarization modulation modulator 5, a fiber circulator 6, a photoelectric conversion and acquisition module 7, a main control board 8, an upper computer 9 and a fiber grating hydrophone array 10;
the laser 1 is used for emitting monochromatic continuous laser with the wavelength completely consistent with the central wavelength of grating reflection in the fiber grating hydrophone array 10, and the laser 1 is connected to the common-mode phase modulator 2 through a polarization maintaining fiber;
the common-mode phase modulator 2 is used for carrying out common-mode phase modulation on monochromatic continuous laser emitted by the laser 1 to generate continuous laser carrying a common-mode phase modulation signal, and the common-mode phase modulator 2 is connected to the acousto-optic modulator 3 through a polarization maintaining optical fiber;
the acousto-optic modulator 3 is used for modulating continuous laser carrying common-mode phase modulation signals into pulse laser, the output time interval between the pulse laser is tau, and the acousto-optic modulator 3 is connected to the matching interferometer 4 through a polarization maintaining optical fiber;
the matching interferometer 4 is used for converting input pulse laser into a laser pulse pair and simultaneously carrying out secondary phase modulation on one pulse in the laser pulse pair; the matching interferometer 4 comprises an input polarization-maintaining fiber coupler 401, an output polarization-maintaining fiber coupler 402, an interferometer short-arm polarization-maintaining fiber 403 and an interferometer long-arm polarization-maintaining fiber 404, wherein the input end of the input polarization-maintaining fiber coupler 401 is connected to the output end of the acousto-optic modulator 3 through the polarization-maintaining fiber, and the output end of the output polarization-maintaining fiber coupler 402 is connected to the input end of the polarization modulation modulator 5 through the polarization-maintaining fiber; one output end of the input polarization-maintaining fiber coupler 401 is connected with the interferometer short arm polarization-maintaining fiber 403, and the other end of the interferometer short arm polarization-maintaining fiber 403 is connected with one input end of the output polarization-maintaining fiber coupler 402; the long-arm polarization-maintaining fiber 404 of the interferometer comprises a section of polarization-maintaining fiber 404A and a non-common mode phase modulator 404B, the polarization-maintaining fiber 404A is connected to the other output end of the input polarization-maintaining fiber coupler 401, and the non-common mode phase modulator 404B is connected to the other input end of the output polarization-maintaining fiber coupler 402 through the polarization-maintaining fiber;
the polarization modulator 5 is used for respectively modulating the polarization states of two pulses in the input laser pulse pair and outputting a polarized laser pulse pair meeting the modulation requirement, and the polarization modulator 5 is connected to the optical fiber circulator 6 through a single-mode optical fiber;
the optical fiber circulator 6 is used for injecting a polarized laser pulse pair into the fiber grating hydrophone array 10 and injecting an interference pulse sequence returned by the fiber grating hydrophone array 10 into the photoelectric conversion and acquisition module 7; the fiber circulator 6 includes three ports: the polarization grating hydrophone comprises a first port 601, a second port 602 and a third port 603, wherein the first port 601 is connected to the polarization modulator 5 through a single mode fiber, the second port 602 is connected to the fiber grating hydrophone array 10 through a single mode fiber, and the third port is connected to the photoelectric conversion and acquisition module 7 through a single mode fiber;
the photoelectric conversion and acquisition module 7 is used for performing photoelectric conversion and discretization sampling on an interference pulse sequence returned by the fiber grating hydrophone array 10, and the photoelectric conversion and acquisition module 7 is connected to the main control board 8 through a cable;
the main control board 8 includes a modulation module 801 and a demodulation module 802, the modulation module 801 includes a first modulation signal output terminal 801A, a second modulation signal output terminal 801B, a third modulation signal output terminal 801C and a fourth modulation signal output terminal 801D, the first modulation signal output terminal 801A outputs a common mode phase modulation signal, which is connected to the common mode phase modulator 2 through a cable, the second modulation signal output terminal 801B outputs a square wave pulse modulation signal, which is connected to the acousto-optic modulator 3 through a cable, the third modulation signal output terminal 801C outputs a non-common mode phase carrier signal, which is connected to the phase modulator 404B through a cable, and the fourth modulation signal output terminal 801D outputs a polarization modulation signal, which is connected to the polarization modulator 5 through a cable; the demodulation module 802 is used for completing demodulation of an optical pulse sequence returned by the fiber grating hydrophone array 10, and the main control board 8 is connected to the upper computer 9 through a cable;
the upper computer 9 is used for setting modulation and demodulation parameters and displaying a demodulation result;
the fiber grating hydrophone array 10 is a wet end of the fiber grating hydrophone array, and is used for generating an optical interference result carrying an external signal, and the optical interference result comprises a parasitic interference excitation point 1001 and a fiber grating hydrophone element 1002: the parasitic interference excitation point 1001 is a backscattering point formed by factors such as the rayleigh scattering effect in the optical fiber, the optical fiber fusion splice and the like, and the fiber grating hydrophone element 1002 comprises a first grating 1002A, a second grating 1002B and a sensing optical fiber 1002C; the fiber grating hydrophone array 10 is connected with the second port 602 of the fiber circulator 6 through a single-mode fiber, after the fiber circulator 6 injects the polarized laser pulse pairs into the fiber grating hydrophone array 10, the polarized laser pulse pairs are respectively reflected back to a pair of laser pulse pairs through a parasitic interference excitation point 1001, a first grating 1002A and a second grating 1002B, a second pulse in the laser pulse pair reflected back by the first grating 1002A and a first pulse in the laser pulse pair reflected back by the second grating 1002B coincide in time to form an interference signal, an interference phase difference is a phase when the laser passes through the sensing fiber 1002C, the interference is main interference, and the generated interference pulse is a main interference pulse; when the first pulse in the laser pulse pair reflected back by the spurious interference excitation point 1001 is temporally coincident with the main interference pulse, the first pulse in the laser pulse pair reflected back by the first grating 1002A and the first pulse in the laser pulse pair reflected back by the second grating 1002B will interfere with each other, and two new interference results, that is, spurious interference, are generated; the interference pulse sequence formed by the parasitic interference pulse and the main interference pulse is simultaneously transmitted to the photoelectric conversion and collection module 7 through the third port 603 of the optical fiber circulator 6.
Preferably, the modulation signal of the common mode phase modulator 2 is a sinusoidal phase signal.
Preferably, the modulation signal of the non-common mode Phase modulator 404B is a Phase Generated Carrier (PGC) signal or a heterodyne modulation signal.
Preferably, the common-mode phase modulator 2 and the non-common-mode phase modulator 404B may be electro-optical crystal phase modulators or PZT phase modulators.
The invention also provides a signal modulation method based on the device, which comprises the following steps:
s1: common mode phase modulation
Monochromatic continuous laser output by the laser 1 and having the wavelength completely consistent with the grating reflection center wavelength in the fiber grating hydrophone array 10 is modulated into continuous laser carrying a common-mode phase modulation signal after passing through the common-mode phase modulator 2; the phase modulation being of the form beta m sin(2πf m t) wherein β m To modulate amplitude, f m Is the common mode modulation frequency. Beta is a m Is selected such that J is 0 (2β m ) Value of 0, i.e. beta m ＝1.2025，2.25，4.327......。
S2: pulse modulation
The continuous laser carrying the common mode phase modulation signal output by the common mode phase modulator 2 is modulated into pulse laser after passing through the acousto-optic modulator 3. The acousto-optic modulator 3 adopts a square wave modulation mode, the period of the modulated square wave signal is tau, and the inquiry frequency is
And is provided with f a And common mode modulation frequency f in S1 m Satisfy the requirement of
n is a positive integer
S3: non-common mode phase modulation
Acousto-optic modulationThe pulse laser output by the generator 3 is changed into a laser pulse pair after passing through the matching interferometer 4, and the first pulse E in the laser pulse pair 1 (t) the second pulse E in the laser pulse pair is generated by the short arm polarization maintaining fiber 403 of the interferometer 2 (t) generated by long arm polarization maintaining fiber 404 of interferometer and introduced into non-common mode phase modulation signal by non-common mode phase modulator 404B
Modulating signals for phase carriers, i.e.
C is the modulation amplitude, f 0 Is a non-common mode modulation frequency. Non-common mode modulation frequency f 0 Common mode modulation frequency f m And frequency of interrogation f a The three components need to meet the following requirements:
at this time
And is
S4: polarization modulation and output
Carrying a common-mode phase modulation signal beta m sin(2πf m t) and non-common mode phase modulation signal
The laser pulse pair is injected into the polarization modulator 5, the polarization states of two laser pulses in the laser pulse pair are modulated respectively, and the generated polarization laser pulse pair is output to the first port 601 of the optical fiber circulator 6 after the polarization modulation is completed.
S5: dominant and parasitic interference light field generation
The fiber circulator 6 injects the polarized laser pulse pair into the fiber grating hydrophone array 10, and the polarized laser pulse pair is respectively reflected by the parasitic interference excitation point 1001, the first grating 1002A and the second grating 1002BShoot back a pair of laser pulses, wherein the second pulse E of the laser pulse pair reflected back by the first grating 1002A res (t) first pulse E of the laser pulse pair reflected back from the second grating 1002B sig (t) overlapping in time to form an interference signal, wherein the interference phase difference is the phase of the laser passing through the sensing optical fiber 1002C, the interference is main interference, and the generated interference pulse is a main interference pulse; the first pulse E in the laser pulse pair reflected back by the spurious interference excitation point 1001 resd (t) the second pulse E of the laser pulse pair, which is also reflected back from the first grating 1002A when temporally coincident with the primary interference pulse res (t) first pulse E of the laser pulse pair reflected back by the second grating 1002B sig (t) interference occurs, producing two new interference results, i.e. parasitic interference;
E res (t)、E sig (t) and E resd (t) after aliasing, the three pulsed lights simultaneously generate main interference and parasitic interference, and an interference result I (t) is expressed as:
I(t)＝[E res (t)+E sig (t)+E resd (t)] * [E res (t)+E sig (t)+E resd (t)]
＝|E res | 2 +|E sig | 2 +|E resd | 2 +[E res (t)*E sig (t)+E res (t)E sig (t)*]+[E resd (t)*E sig (t)+E resd (t)E sig (t)*]+[E resd (t)*E res (t)+E resd (t)E res (t)*]
wherein E res (t)*E sig (t)+E res (t)E sig (t) is the dominant interference term, E resd (t)*E sig (t)+E resd (t)E sig (t) and E resd (t)*E res (t)+E resd (t)E res (t) is two parasitic interference terms.
S6 Signal reception and demodulation
The third port 603 of the optical fiber circulator 6 returns the main interference and parasitic interference aliased signal i (t) generated by the fiber grating hydrophone array 10 to the photoelectric conversion and acquisition module 7, and completes signal demodulation in the demodulation module 802 after completing signal acquisition.
Further, in S3, the non-common mode phase modulation signal
It may also be a heterodyne modulated signal, i.e.
f 0 Is a non-common mode modulation frequency.
Further, in S6, the demodulation algorithm is based on the modulation signal applied in S3
In practice, if
For phase carrier modulation signals, a phase carrier demodulation algorithm is used, I (t) is first compared with cos (2 π f) 0 t) and cos (4 π f) 0 t) separately mixing frequencies; the mixed signal has a cut-off frequency of 1/f 0 The low pass filter of (1). Due to the inclusion of the common-mode phase modulation signal beta m sin(2πf m t) and β) m ＝1.2025，2.25，4.327......，
sin(2πf m τ) ± 1 thus J 0 [β m sin(2πf m τ)]0, parasitic interference term E resd (t)*E res (t)+E resd (t)E res (t) all spectral components in the result of the mixing are greater than f 0 /2. Parasitic interference term E resd (t)*E sig (t)+E resd (t)E sig (t) does not contain
After aliasing all spectral components are larger than f 0 . Both terms are eliminated after passing through a low pass filter; and finally, extracting a main interference item E by adopting DCM or arc tangent algorithm res (t)*E sig (t)+E res (t)E sig (t) phase signal in (t).
Further, in S6, the demodulation algorithm is based on the modulation signal applied in S3
In practice, if
For heterodyne modulation, heterodyne demodulation algorithm is used, I (t) is first compared with cos (2 π f) 0 t) and sin (2 π f) 0 t) are mixed separately. The mixed signal has a cut-off frequency of 1/f 0 The low pass filter of (1). Due to the common-mode phase modulation signal beta m sin(2πf m t) and β) m ＝1.2025，2.25，4.327......，
sin(2πf m τ) ± 1 therefore J 0 [β m sin(2πf m τ)]0, parasitic interference term E resd (t)*E res (t)+E resd (t)E res (t) all spectral components in the result of the mixing are greater than f 0 /2. Parasitic interference term E resd (t)*E sig (t)+E resd (t)E sig (t) does not contain
After aliasing all spectral components are larger than f 0 . Both terms are eliminated after passing through a low pass filter; finally, a DCM or an arc tangent algorithm is adopted to extract a main interference item E res (t)*E sig (t)+E res (t)E sig (t) phase signal in (t).
The invention has the technical effects that: the invention provides a device for jointly using common-mode phase modulation and non-common-mode phase modulation, and achieves the effect of eliminating the influence of parasitic interference signals of an optical fiber grating hydrophone array on the premise of not changing the optical structure advantage of a wet end array and the signal demodulation algorithm by designing the common-mode phase modulation frequency and modulation amplitude, the non-common-mode phase modulation frequency and the inquiry pulse frequency parameter in the device.
Drawings
FIG. 1 shows the single-element interference principle of a fiber grating hydrophone;
FIG. 2 is a schematic diagram of the cause of parasitic interference in a fiber grating hydrophone array;
FIG. 3 is a schematic structural diagram of the apparatus for eliminating parasitic interference in a fiber grating hydrophone array according to the present invention;
the system comprises a laser 1, a common-mode phase modulator 2, an acoustic-optical modulator 3, a matching interferometer 4, a polarization modulator 5, an optical fiber circulator 6, a photoelectric conversion and acquisition module 7, a main control board 8, an upper computer 9 and an optical fiber grating hydrophone array 10, wherein the common-mode phase modulator is a laser, the optical fiber circulator 6 is an optical fiber circulator, and the photoelectric conversion and acquisition module is an optical fiber grating hydrophone array;
801A is a first modulation signal output terminal, 801B is a second modulation signal output terminal, 801C is a third modulation signal output terminal, and 801D is a fourth modulation signal output terminal;
and 10 is a fiber grating hydrophone array. Wherein 1001 is a parasitic interference excitation point, 1002 is a fiber grating hydrophone element 1002, 1002A is a first grating of the element 1002, 1002B is a second grating of the element 1002, and 1002C is a sensing fiber of the element 1002;
FIG. 4 is a schematic diagram of the generation of the main interference and parasitic interference optical field according to the present invention, which contains the parasitic interference signal and the fiber grating hydrophone underwater acoustic signal.
Detailed Description
The invention will be further explained with reference to the drawings.
Fig. 3 is a schematic structural diagram of the apparatus for eliminating parasitic interference in a fiber grating hydrophone array according to the present invention, which includes a laser 1, a common mode phase modulator 2, an acousto-optic modulator 3, a matching interferometer 4, a polarization modulation modulator 5, an optical fiber circulator 6, a photoelectric conversion and collection module 7, a main control board 8, an upper computer 9, and a fiber grating hydrophone array 10;
the invention discloses a method for eliminating parasitic interference signals in a fiber grating hydrophone array, which comprises the following steps:
s1: common mode phase modulation
The output of the laser 1 is ANDMonochromatic continuous laser with completely consistent grating reflection center wavelength in the fiber grating hydrophone array 10 is modulated into continuous laser carrying a common-mode phase modulation signal after passing through the common-mode phase modulator 2; the phase modulation being of the form beta m sin(2πf m t) wherein β m To modulate amplitude, f m Is the common mode modulation frequency. Beta is a beta m Is selected such that J is 0 (2β m ) Value of 0, i.e. beta m ＝1.2025，2.25，4.327......。
The expression for the laser at this time can be written as:
where E is the electric vector amplitude term, upsilon 0 Is the frequency of the laser light and is,
is the laser initial phase.
S2: pulse modulation
The continuous laser carrying the common mode phase modulation signal output by the common mode phase modulator 2 is modulated into pulse laser after passing through the acousto-optic modulator 3. The acousto-optic modulator 3 adopts a square wave modulation mode, the period of the modulated square wave signal is tau, and the inquiry frequency is
And is provided with f a And the mode modulation frequency f in S1 m Satisfy the requirement of
n is a positive integer. At this time, the laser still has the expression shown in formula 1, and is in a front-back sequence output state in time sequence.
S3: non-common mode phase modulation
The pulse laser output by the acousto-optic modulator 3 is changed into a laser pulse pair after passing through the matching interferometer 4, and the first pulse E in the laser pulse pair 1 (t) generated by interferometer short arm polarization maintaining fiber 403, the second of the laser pulse pairA pulse E 2 (t) generated by long arm polarization maintaining fiber 404 of interferometer and introduced into non-common mode phase modulation signal by non-common mode phase modulator 404B
Modulating signals for phase carriers, i.e.
C is the modulation amplitude, f 0 Is a non-common mode modulation frequency. Non-common mode modulation frequency f 0 Common mode modulation frequency f m And frequency of interrogation f a The three components need to meet the following requirements:
at this time
And is
The output laser light at this time is two pulses, which can be expressed as:
wherein
And
respectively, the optical phase delays generated by the interferometer short arm polarization maintaining fiber 403 and the interferometer long arm polarization maintaining fiber 404.
S4: polarization modulation and output
Carrying a common-mode phase modulation signal beta m sin(2πf m t) and non-common mode phase modulated signal
The laser pulse pair is injected into the polarization modulator 5, the polarization states of two laser pulses in the laser pulse pair are modulated respectively, and the generated polarization laser pulse pair is output to the first port 601 of the optical fiber circulator 6 after the polarization modulation is completed. The expression for the laser pulse pair at this time is still as shown in (equation 2).
S5: dominant and parasitic interference light field generation
The fiber circulator 6 injects the polarized laser pulse pairs into the fiber grating hydrophone array 10, and the polarized laser pulse pairs are respectively reflected back to a pair of laser pulse pairs through the parasitic interference excitation point 1001, the first grating 1002A and the second grating 1002B. Referring to FIG. 4, the second pulse E of the laser pulse pair reflected back by the first grating 1002A res (t) first pulse E of the laser pulse pair reflected back from the second grating 1002B sig (t) overlapping in time to form an interference signal, wherein the interference phase difference is the phase of the laser passing through the sensing optical fiber 1002C, the interference is main interference, and the generated interference pulse is a main interference pulse; first pulse E of the laser pulse pair reflected back from the spurious interference excitation point 1001 resd (t) the second pulse E of the laser pulse pair, which is also reflected back from the first grating 1002A when it coincides in time with the main interference pulse res (t) first pulse E of the laser pulse pair reflected back by the second grating 1002B sig (t) interference occurs, producing two new interference results, i.e., parasitic interference. Expressions for three lasers can be written as
Wherein E is res (t) denotes E reflected by the first grating 100A2 2 (t)，E sig (t) denotes E reflected by the second grating 1002B 1 (t)，E resd (t) is E in the next query pulse pair 1 (t + τ) backscattered light at the spurious interference excitation point A. τ is the interrogation time interval between two adjacent interrogation pulse pairs.
For the phase delay of the fiber passing from the third port 603 of the circulator to the first grating 1002A,
is the phase delay through the third port 603 of the circulator to the parasitic interference excitation point a.
Is the phase delay inherent to the sensing fiber 1002C. Because the optical path length of the sensing fiber 1002C is half of the optical path difference between the long-arm polarization maintaining fiber 403 and the short-arm polarization maintaining fiber 404 of the interferometer 4, the inherent phase delay generated after the light is transmitted back and forth in the sensing fiber 1002C once satisfies the requirement
Is the acoustic signal perceived on the hydrophone 1002.
The result of the above three lights being superimposed can be expressed as
The (formula 4) contains six terms in total, wherein the first three terms are direct-current terms and do not contain any phase information, and the direct-current terms do not have any influence on the conventional PGC or heterodyne demodulation method. The fourth term is the signal term of the fiber grating hydrophone, and the carrier term is
The signal term being
The fifth item, although an alternating item, does not contain a carrier item therein
The PGC or heterodyne demodulation method is not affected. The sixth term is a parasitic interference term, and the phase contains a carrier term
While the phase delay of the transmission fiber between the parasitic interference excitation point A and the first grating 1002A
The term and the fourth term both contain carrier terms
When the PGC or heterodyne demodulation method is adopted, the system can simultaneously respond to the PGC or heterodyne demodulation method, and two results of simultaneously demodulating the superposition interference result are formed.
If beta is not considered m sin(2πf m t+2πf m τ)-β m sin(2πf m t), the sixth term produces effects that include three: (1) when the transmission fiber of the fiber grating hydrophone array is disturbed,
random disturbance can occur, so that noise appears in a demodulation result; (2) when the two superimposed interference results are simultaneously demodulated by a PGC or heterodyne demodulation method, the amplitude of a demodulation signal of each interference result is unstable; (3) if there are other fiber grating hydrophone elements before the first grating 1002A, all the elements at the front end are equivalent to the transmission fiber of the element, and the phase information is contained in
When signals are applied to these elements, the signals are passedThe sixth term is passed into the following primitives, forming channel crosstalk. The above analysis reveals the cause of the parasitic interference term, the physical model and the effect on the fiber grating hydrophone.
S6 Signal reception and demodulation
The third port 603 of the optical fiber circulator 6 returns the signal generated by the fiber grating hydrophone array 10 and mixed with the parasitic interference to the photoelectric conversion and acquisition module 7, and completes signal demodulation in the demodulation module 802 after completing signal acquisition. The signal acquired by the photoelectric conversion and acquisition module 7 is a photocurrent corresponding to (formula 4), and can be expressed as:
wherein, V DC K is a photoelectric conversion coefficient. Taking into account the modulation term beta introduced in the present invention m sin(2πf m t+2πf m τ)-β m sin(2πf m t), and the fourth term in (equation 5) is defined as V resd (t)，V resd (t) is further developed by a Bessel function to obtain
Wherein
β＝2β m sin(f m τ). Due to beta m Values of 1.2025, 2.25, 4.327 … etc.,
sin (2 π f) in this case m τ)＝±1，J 0 (β) 0. Thus (equation 6)
All loaded at f m And around its frequency multiplication terms of various orders.
In using PGC or heterodyne demodulationIn the method, a signal is mixed with a specific signal, and the mixed signal is higher than f after being mixed by a low-pass filter 0 The frequency component of/2 is filtered out. This means that all signal terms in (equation 6) will be removed during the filtering process. Specifically, if
For PGC demodulation, a PGC demodulation algorithm is used. First, the interference result is compared with cos (2 π f) 0 t) and cos (4 π f) 0 t) mixing, as can be seen, because
Other terms and cos (2 π f) have been eliminated by system modulation parameter design 0 t) and cos (4 π f) 0 t) after mixing, the lowest frequency term is f 0 -f m Due to the fact that
Thus, the device
Will be filtered out in the following low-pass filter. If it is
For heterodyne demodulation, a heterodyne demodulation algorithm is adopted. First, the interference result is compared with cos (2 pi f) 0 t) and sin (2 π f) 0 t) mixing. Also, since
Other terms and cos (2 π f) have been eliminated by system modulation parameter design 0 t) and sin (2 π f) 0 t) the lowest frequency after mixing is still f 0 -f m Due to the fact that
Thus, it is possible to provide
Parasitic in the following low-pass filterAll components of the interference are likewise filtered out.
Based on the analysis, the method based on the combined use of the common-mode modulation and the non-common-mode modulation can eliminate the influence of parasitic interference on the fiber grating hydrophone, and does not change the optical structure advantage of the wet end of the array and add an additional demodulation algorithm process.
Claims (7)
1. The method for eliminating the parasitic interference signals in the fiber grating hydrophone array is based on a device comprising a laser (1), a common-mode phase modulator (2), an acoustic-optical modulator (3), a matching interferometer (4), a polarization modulator (5), a fiber circulator (6), a photoelectric conversion and acquisition module (7), a main control board (8), an upper computer (9) and the fiber grating hydrophone array (10), and is characterized by comprising the following steps of:
s1: common mode phase modulation
Monochromatic continuous laser output by the laser (1) and completely consistent with the central wavelength of grating reflection in the fiber grating hydrophone array (10) is modulated into continuous laser carrying a common-mode phase modulation signal after passing through the common-mode phase modulator (2); the phase modulation being of the form beta m sin(2πf m t) wherein β m To modulate amplitude, f m Is the common mode modulation frequency; beta is a m Is selected such that J is 0 (2β m ) A value of 0;
s2: pulse modulation
Continuous laser carrying a common-mode phase modulation signal and output by the common-mode phase modulator (2) passes through the acousto-optic modulator (3) and is modulated into pulse laser; the acousto-optic modulator (3) adopts a square wave modulation mode, the period of a modulated square wave signal is tau, and the inquiry frequency is
And set f a And the common mode modulation frequency f in S1 m Satisfy the requirements of
n is a positive integer;
s3: non-common mode phase modulation
The pulse laser output by the acousto-optic modulator (3) is changed into a laser pulse pair after passing through the matching interferometer (4), and the first pulse E in the laser pulse pair 1 (t) is generated by a short arm polarization maintaining fiber (403) of the interferometer, the second pulse E of the laser pulse pair 2 (t) generated by interferometer long arm polarization maintaining fiber (404) and introduced into non-common mode phase modulation signal by non-common mode phase modulator (404B)
For modulating signals for phase carriers, i.e.
C is the modulation amplitude, f 0 A non-common mode modulation frequency; non-common mode modulation frequency f 0 Common mode modulation frequency f m And frequency of interrogation f a The three components need to meet the following requirements:
at this time
And is
S4: polarization modulation and output
Carrying a common-mode phase modulation signal beta m sin(2πf m t) and non-common mode phase modulation signal
The laser pulse pair is injected into a polarization modulator (5), the polarization states of two laser pulses in the laser pulse pair are respectively modulated, and the generated polarization laser pulse pair is output to a first port (601) of an optical fiber circulator (6) after the polarization modulation is finished;
s5: dominant and parasitic interference light field generation
The fiber circulator (6) injects the polarized laser pulse pair into the fiber grating hydrophone array (10), and the polarized laser pulse pair is respectively reflected back to a pair of laser pulse pairs through a parasitic interference excitation point (1001), a first grating (1002A) and a second grating (1002B), wherein the second pulse E in the laser pulse pair reflected back by the first grating (1002A) res (t) first pulse E of the laser pulse pair reflected back from the second grating (1002B) sig (t) overlapping in time to form an interference signal, wherein the interference phase difference is the phase of the laser passing through the sensing optical fiber (1002C), the interference is main interference, and the generated interference pulse is a main interference pulse; the first pulse E of the laser pulse pair reflected back by the spurious interference excitation point (1001) resd (t) the second pulse E of the laser pulse pair, which is also reflected back from the first grating (1002A), when it coincides in time with the main interference pulse res (t) the first pulse E of the laser pulse pair reflected back by the second grating (1002B) sig (t) interference occurs, producing two new interference results, i.e. parasitic interference;
E res (t)、E sig (t) and E resd (t) after aliasing, the three pulsed lights simultaneously generate main interference and parasitic interference, and an interference result I (t) is expressed as:
I(t)＝[E res (t)+E sig (t)+E resd (t)] * [E res (t)+E sig (t)+E resd (t)]
＝|E res | 2 +|E sig | 2 +|E resd | 2 +[E res (t)*E sig (t)+E res (t)E sig (t)*]+[E resd (t)*E sig (t)+E resd (t)E sig (t)*]+[E resd (t)*E res (t)+E resd (t)E res (t)*]
wherein E res (t)*E sig (t)+E res (t)E sig (t) is a main interference term, E resd (t)*E sig (t)+E resd (t)E sig (t) and E resd (t)*E res (t)+E resd (t)E res (t) is two parasitic interference terms;
s6 Signal reception and demodulation
A third port (603) of the optical fiber circulator (6) returns a signal I (t) of aliasing of main interference and parasitic interference generated by the fiber grating hydrophone array (10) to the photoelectric conversion and acquisition module (7), and signal demodulation is completed in the demodulation module (802) after signal acquisition is completed;
the demodulation algorithm is based on the modulated signal applied in S3
In practice, if
For phase-carrier modulation of the signal, a phase-carrier demodulation algorithm is used, I (t) first being compared with cos (2 π f) 0 t) and cos (4 π f) 0 t) separately mixing frequencies; the mixed signal has a cut-off frequency of 1/f 0 The low-pass filter of (1); due to the common-mode phase modulation signal beta m sin(2πf m t)，
sin(2πf m τ) ± 1 thus J 0 [β m sin(2πf m τ)]0, parasitic interference term E resd (t)*E res (t)+E resd (t)E res (t) all spectral components in the result of the mixing are greater than f 0 2; parasitic interference term E resd (t)*E sig (t)+E resd (t)E sig (t) does not contain
After aliasing all spectral components are larger than f 0 (ii) a Both terms are eliminated after passing through a low pass filter; and finally, extracting a main interference item E by adopting DCM or arc tangent algorithm res (t)*E sig (t)+E res (t)E sig (t) phase signal in (t).
2. The method for eliminating parasitic interference signals in fiber grating hydrophone array based on claim 1The method is characterized in that: in S3, non-common mode phase modulation signal
It may also be a heterodyne modulated signal, i.e.
f 0 Is a non-common mode modulation frequency.
3. A method for eliminating parasitic interference signals in a fiber grating hydrophone array according to claim 1, wherein the method comprises the following steps: in S6, the demodulation algorithm is based on the applied modulation signal in S3
In practice, if
For heterodyne modulation, heterodyne demodulation algorithm is used, I (t) is first compared with cos (2 π f) 0 t) and sin (2 π f) 0 t) separately mixing; the mixed signal has a cut-off frequency of 1/f 0 The low-pass filter of (1); due to the common-mode phase modulation signal beta m sin(2πf m t)，
sin(2πf m τ) ± 1 thus J 0 [β m sin(2πf m τ)]0, parasitic interference term E resd (t)*E res (t)+E resd (t)E res (t) all spectral components in the result of the mixing are greater than f 0 2; parasitic interference term E resd (t)*E sig (t)+E resd (t)E sig (t) does not contain
After aliasing all spectral components are larger than f 0 (ii) a Both terms are eliminated after passing through a low pass filter; finally adopting DCM or arc tangentAlgorithm extraction of dominant interference term E res (t)*E sig (t)+E res (t)E sig (t) phase signal in (t).
4. An apparatus for eliminating parasitic interference signals in a fiber grating hydrophone array based on the method of any one of claims 1-3, comprising: the device comprises a laser (1), a common-mode phase modulator (2), an acoustic-optical modulator (3), a matching interferometer (4), a polarization modulator (5), an optical fiber circulator (6), a photoelectric conversion and acquisition module (7), a main control board (8), an upper computer (9) and an optical fiber grating hydrophone array (10);
the laser (1) is used for emitting monochromatic continuous laser with the wavelength completely consistent with the central wavelength of grating reflection in the fiber grating hydrophone array (10), and the laser (1) is connected to the common-mode phase modulator (2) through a polarization maintaining fiber;
the common-mode phase modulator (2) is used for carrying out common-mode phase modulation on monochromatic continuous laser emitted by the laser (1) to generate continuous laser carrying a common-mode phase modulation signal, and the common-mode phase modulator (2) is connected to the acousto-optic modulator (3) through a polarization-maintaining optical fiber;
the acousto-optic modulator (3) is used for modulating continuous laser carrying common-mode phase modulation signals into pulse laser, the output time interval between the pulse laser is tau, and the acousto-optic modulator (3) is connected to the matching interferometer (4) through a polarization-maintaining optical fiber;
the matching interferometer (4) is used for converting input pulse laser into a laser pulse pair and simultaneously carrying out secondary phase modulation on one pulse in the laser pulse pair; the matching interferometer (4) comprises an input polarization-maintaining fiber coupler (401), an output polarization-maintaining fiber coupler (402), an interferometer short-arm polarization-maintaining fiber (403) and an interferometer long-arm polarization-maintaining fiber (404), wherein the input end of the input polarization-maintaining fiber coupler (401) is connected to the output end of the acousto-optic modulator (3) through the polarization-maintaining fiber, and the output end of the output polarization-maintaining fiber coupler (402) is connected to the input end of the polarization modulator (5) through the polarization-maintaining fiber; one output end of the input polarization-maintaining fiber coupler (401) is connected with the interferometer short-arm polarization-maintaining fiber (403), and the other end of the interferometer short-arm polarization-maintaining fiber (403) is connected with one input end of the output polarization-maintaining fiber coupler (402); the long-arm polarization maintaining fiber (404) of the interferometer comprises a section of polarization maintaining fiber (404A) and a non-common mode phase modulator (404B), the polarization maintaining fiber (404A) is connected to the other output end of the input polarization maintaining fiber coupler (401), and the non-common mode phase modulator (404B) is connected to the other input end of the output polarization maintaining fiber coupler (402) through the polarization maintaining fiber;
the polarization modulator (5) is used for respectively modulating the polarization states of two pulses in the input laser pulse pair and outputting a polarization laser pulse pair meeting the modulation requirement, and the polarization modulator (5) is connected to the optical fiber circulator (6) through a single-mode optical fiber;
the optical fiber circulator (6) is used for injecting the polarized laser pulse pair into the fiber grating hydrophone array (10) and injecting an interference pulse sequence returned by the fiber grating hydrophone array (10) into the photoelectric conversion and acquisition module (7); the fiber optic circulator (6) includes three ports: the device comprises a first port (601), a second port (602) and a third port (603), wherein the first port (601) is connected to the polarization modulator (5) through a single-mode fiber, the second port (602) is connected to the fiber grating hydrophone array (10) through a single-mode fiber, and the third port is connected to the photoelectric conversion and collection module (7) through a single-mode fiber;
the photoelectric conversion and acquisition module (7) is used for performing photoelectric conversion and discretization sampling on an interference pulse sequence returned by the fiber grating hydrophone array (10), and the photoelectric conversion and acquisition module (7) is connected to the main control board (8) through a cable;
the main control board (8) comprises a modulation module (801) and a demodulation module (802), the modulation module (801) comprises a first modulation signal output end (801A), a second modulation signal output end (801B), a third modulation signal output end (801C) and a fourth modulation signal output end (801D), the first modulation signal output end (801A) outputs a common-mode phase modulation signal, is connected to the common-mode phase modulator (2) through a cable, the second modulation signal output terminal (801B) outputs a square wave pulse modulation signal, is connected to the acousto-optic modulator (3) through a cable, a third modulation signal output end (801C) outputs a non-common mode phase carrier signal, the phase modulator (404B) is connected through a cable, the fourth modulation signal output end (801D) outputs a polarization modulation signal, and the polarization modulator (5) is connected through a cable; the demodulation module (802) is used for completing demodulation of an optical pulse sequence returned by the fiber grating hydrophone array (10), and the main control board (8) is connected to the upper computer (9) through a cable;
the upper computer (9) is used for setting modulation and demodulation parameters and displaying a demodulation result;
the fiber grating hydrophone array (10) is a wet end of the fiber grating hydrophone array, is used for generating an optical interference result carrying an external signal, and comprises a parasitic interference excitation point (1001) and a fiber grating hydrophone element (1002): the parasitic interference excitation point (1001) is a backscattering point formed by Rayleigh scattering effect and fiber fusion factors in the optical fiber, and the fiber grating hydrophone element (1002) comprises a first grating (1002A), a second grating (1002B) and a sensing fiber (1002C); the fiber grating hydrophone array (10) is connected with a second port (602) of the fiber circulator (6) through a single-mode fiber, after the fiber circulator (6) injects a polarized laser pulse pair into the fiber grating hydrophone array (10), the polarized laser pulse pair is respectively reflected back to a pair of laser pulse pairs through a parasitic interference excitation point (1001), a first grating (1002A) and a second grating (1002B), a second pulse in the laser pulse pair reflected back by the first grating (1002A) and a first pulse in the laser pulse pair reflected back by the second grating (1002B) coincide in time to form an interference signal, the interference phase difference is the phase when the laser passes through the sensing fiber (1002C), the interference is main interference, and the generated interference pulse is a main interference pulse; when the first pulse in the laser pulse pair reflected back by the parasitic interference excitation point (1001) is coincident with the main interference pulse in time, the first pulse in the laser pulse pair reflected back by the first grating (1002A) and the first pulse in the laser pulse pair reflected back by the second grating (1002B) are interfered with the same, and two new interference results, namely parasitic interference, are generated; the interference pulse sequence formed by the parasitic interference pulse and the main interference pulse is simultaneously transmitted to the photoelectric conversion and collection module (7) through a third port (603) of the optical fiber circulator (6).
5. An apparatus for eliminating parasitic interference signals in a fiber grating hydrophone array as recited in claim 4, further comprising: the modulation signal of the common mode phase modulator (2) is a sine phase signal.
6. An apparatus for eliminating parasitic interference signals in a fiber grating hydrophone array as recited in claim 4, wherein: the modulation signal of the non-common mode phase modulator (404B) generates a carrier signal or a heterodyne modulation signal for the phase.
7. An apparatus for eliminating parasitic interference signals in a fiber grating hydrophone array as recited in claim 4, further comprising: the common-mode phase modulator (2) and the non-common-mode phase modulator (404B) may be electro-optic crystal phase modulators or PZT phase modulators.
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