CN114777901A - Interference type optical fiber hydrophone system and linear frequency modulation method thereof - Google Patents

Abstract

The invention provides a linear frequency modulation system and a linear frequency modulation method applied to an optical fiber hydrophone system, which belong to the technical field of transducers, and comprise a narrow linewidth laser connected with a phase modulator, wherein the phase modulator is connected with a first filter, and the first filter is connected with a first optical fiber amplifier; the first optical fiber amplifier is connected with the input end of the sensor array, the output end of the sensor array is connected with the second optical fiber amplifier, the second optical fiber amplifier is connected with the second filter, the second filter is connected with the photoelectric detector, and the photoelectric detector is connected with the data processing module; in a sensor array, each sensor is composed of a michelson interferometer, and a pair of faraday rotating mirrors is used to cancel polarization dependent attenuation. The invention can effectively inhibit the stimulated Brillouin scattering phenomenon in the optical fiber, thereby improving the incident light power; different from the prior SBS suppression means, the linear frequency modulation is completely compatible with the existing seabed hydrophone probe; the unrepeatered transmission distance can be obviously prolonged, and the network scale is enlarged.

Description

Interference type optical fiber hydrophone system and linear frequency modulation method thereof

Technical Field

The invention relates to the technical field of transducers, in particular to an interference type optical fiber hydrophone system and a linear frequency modulation method thereof.

Background

In recent years, microwave photonics has been widely studied and applied in many fields including communications, radar, sensing, and computing. Microwave photonic sensors are one of the active sub-areas that use microwave photonic technology to achieve high speed and high resolution measurements. Many technologies have been implemented on the basis of microwave photon sensing, for example, high speed and high resolution sensing based on the beat frequency of two optical wavelengths.

It has been reported that the use of an optoelectronic oscillator (OEO) converts optical wavelength shifts into microwave frequency variations. Wavelength-time mapping is used to convert the sensing information from the optical wavelength domain to the microwave frequency domain, thereby increasing interrogation speed and resolution.

The paper, "a plurality of key technical researches of interference type optical fiber hydrophone array system based on heterodyne detection", of phoma discloses that light emitted by a light source is divided into two paths after passing through a coupler C1, and the two paths of light respectively pass through two Acousto-optic modulators (AOMs), wherein the AOMs are frequency shifters and also switch modulators, and the two AOMs respectively shift two paths of light into pulsed light at the same time in frequency of f1 and f 2. The purpose of frequency shift is to output heterodyne interference signals for a subsequent interferometer; the purpose of the pulsing is to meet the sensing array time division multiplexing requirements. One of the two AOM output optical pulses passes through a delay line L, and two optical pulses with time interval τ ═ Nl/C are output from the coupler C2, and the pair of optical pulses is input into the fiber optic hydrophone array. Because the carrier frequencies of the two light pulses are different, the pulses of different carrier frequencies are superposed after the array, and then the beat frequency of the photoelectric detector is obtained.

The prior art does not consider matching of the wavelength of the laser and the center frequency and the bandwidth of the filter, and the maximum tolerable loss cannot meet the use requirement, so that the signal receiving and conversion is not stable enough, and the transmission distance without relay is short.

Disclosure of Invention

The invention aims to provide an interference type optical fiber hydrophone system which improves incident light power and can prolong unrepeatered transmission distance and a linear frequency modulation method thereof, so as to solve at least one technical problem in the background technology.

In order to achieve the purpose, the invention adopts the following technical scheme:

in one aspect, the present invention provides an interferometric fiber optic hydrophone system comprising:

the narrow-linewidth laser is connected with a phase modulator, the phase modulator is connected with a first filter, and the first filter is connected with a first optical fiber amplifier; the LFM optical pulse is obtained by carrying out carrier suppression single sideband modulation on a narrow linewidth laser source;

the first optical fiber amplifier is connected with the input end of the sensor array, the output end of the sensor array is connected with a second optical fiber amplifier, the second optical fiber amplifier is connected with a second filter, the second filter is connected with a photoelectric detector, and the photoelectric detector is connected with the data processing module; in the sensor array, each sensor consists of a Michelson interferometer, and a pair of Faraday rotation mirrors is used for eliminating attenuation related to polarization;

wherein, in the data processing module, by reading the positions and their changes from the time domain interference fringes, the sound modulation conditions experienced by the corresponding remote sensor can be known.

In a second aspect, the present invention provides a linear frequency modulation method for an interferometric fiber optic hydrophone system as described above, wherein in the michelson interferometer, each optical pulse is decomposed into two pulses of equal power, which are then combined to produce interference fringes in the time domain; linear frequency modulation ensures that the beat frequency does not change within a single pulse duration; converting amplitude oscillations generated by the two pulse beats with equal power into an electric signal by using incoherent detection; a small change in the michelson interferometer arms will cause a phase shift of the light of the passing light pulse under phase modulation of the acoustic signal, which then shifts into a beat at a constant magnitude, eventually demodulating at the receiver.

Preferably, by increasing the repetition rate of the LFM pulse, a linear frequency modulation is achieved, comprising:

the LFM pulse sequence used can be viewed as a constant repetition of a single LFM pulse in the time domain, with a period T; the LFM pulse sequence x (t) is expressed by the following equation:

where α represents the chirp rate and q (t) represents the envelope of the pulse, i.e. excluding the amplitude and phase evolution of the LFM;

focusing only on the interferometric process within a single probe, the optical pulses x (t) will be backed up with their delays

Interfere and produce a beat frequency in the receiver, where τ is the arm length difference of the michelson interferometer, and

modulating the optical phase difference generated to the interferometer arms for the sound field;

hilbert transform is performed on the beat signal and its positive frequency component y (t) is determined as follows

Since the time length of a single LFM light pulse is controlled so that there is no overlap between different interfering light pulses, y (t) can be rewritten as:

wherein the pulse p ═ q (t) q⁺(t-τ)；

Expressing y (t) from the sum of the plurality of pulses to the sum of the plurality of carriers, one obtains:

wherein P (-) represents the Fourier transform of the pulse P (-);

then the signal under test

Carried in the form of phase modulation on a plurality of carrier frequencies having a frequency of

I.e. its frequency is proportional to the repetition frequency of the transmitter light pulses.

The invention has the beneficial effects that: the interferometric fiber hydrophone scheme based on the high repetition rate linear frequency modulation optical pulse sequence is realized, and a light source of the interferometric fiber hydrophone scheme is generated on the basis of a new microwave photon technology, so that the stimulated Brillouin scattering phenomenon in an optical fiber can be effectively inhibited, and the incident light power is improved; different from the prior SBS suppression means, the linear frequency modulation is completely compatible with the existing submarine hydrophone probe; the unrepeatered transmission distance can be remarkably prolonged, and the network scale is enlarged.

Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.

Fig. 1 is a functional schematic block diagram of an interferometric fiber optic hydrophone system according to an embodiment of the present invention.

Fig. 2 is a schematic diagram of an internal structure of a michelson interferometer according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of an interference process inside a michelson interferometer using LFM pulses according to an embodiment of the present invention.

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by way of the drawings are illustrative only and are not to be construed as limiting the invention.

It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

As used herein, the singular forms "a", "an", "the" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, the various embodiments or examples and features of the various embodiments or examples described in this specification can be combined and combined by those skilled in the art without contradiction.

In the description of the present specification, the terms "first", "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of the feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.

In the description of the present specification, the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, and are only for convenience in describing the present technology and simplifying the description, but do not indicate or imply that the device or element referred to must have a specific orientation, be constructed in a specific orientation, and operate, and thus, should not be construed as limiting the present technology.

Unless expressly stated or limited otherwise, the terms "mounted," "connected," and "disposed" are to be construed broadly and can include, for example, fixed connections, arrangements, detachable connections, arrangements, or the like. Specific meanings of the above terms in the present technology can be understood according to specific situations by those of ordinary skill in the art.

For the convenience of understanding, the present invention will be further explained by the following embodiments with reference to the drawings, and the embodiments are not to be construed as limiting the embodiments of the present invention.

It should be understood by those skilled in the art that the drawings are merely schematic representations of embodiments and that the elements shown in the drawings are not necessarily required to practice the invention.

Examples

This embodiment provides an interferometric fiber optic hydrophone system that uses Linear Frequency Modulation (LFM) light pulses as a new light source for the hydrophone system, and uses microwave photon methods to improve the performance of long-range, unrepeatered fiber optic hydrophone systems. The microwave photon sensing uses broadband analog optical signal processing, so that the optical sensor realizes higher detection speed, sensitivity and resolution.

The interferometric fiber optic hydrophone system comprises: the narrow-linewidth laser is connected with a phase modulator, the phase modulator is connected with a first filter, and the first filter is connected with a first optical fiber amplifier; the LFM optical pulse is obtained by carrying out carrier suppression single sideband modulation on a narrow linewidth laser source; the first optical fiber amplifier is connected with the input end of the sensor array, the output end of the sensor array is connected with a second optical fiber amplifier, the second optical fiber amplifier is connected with a second filter, the second filter is connected with a photoelectric detector, and the photoelectric detector is connected with the data processing module; in the sensor array, each sensor consists of a Michelson interferometer, and a pair of Faraday rotating mirrors is used for eliminating attenuation related to polarization; wherein, in the data processing module, by reading the positions and their changes from the time domain interference fringes, the sound modulation condition experienced by the corresponding remote sensor can be known.

Specifically, as shown in fig. 1, at the transmitting end, the LFM optical pulses are obtained by carrier-suppressed single sideband (CS-SSB) modulation of a narrow linewidth Continuous Wave (CW) source. The modulation includes a Phase Modulator (PM) followed by a narrow band optical filter aligned with the upper band of the modulated spectrum. According, a transmitter converts a Radio Frequency (RF) LFM pulse output from an Arbitrary Waveform Generator (AWG) into an optical LFM pulse. After amplification, the optical pulse enters the long-distance optical fiber. In a sensor array, each sensor consists of a michelson interferometer, and a pair of Faraday Rotating Mirrors (FRMs) are used to cancel polarization-dependent attenuation. In an interferometer, each light pulse is decomposed into two pulses of equal power, which are then combined to produce interference fringes in the time domain. The sound modulation is then recorded by the phase of the fringes. The multiple sensors form multiple independent interference fringes that are combined in a Time Division Multiplexed (TDM) fashion in response to respective vibration signals to support large scale networking. The optical coupling ratio between the sensors is optimized to ensure that the interference fringes have equal power. Over TDM, a single interrogation will result in N interference fringes spaced at time intervals T/N, where N is the number of sensors and T is the period of the LFM pulses output by the transmitter. Note that TDM requires the LFM pulse to be transmitted for a duration less than T/N. After the light pulse returns to the receiver, the light is amplified, bandpass filtered, and then converted to an electronic signal by a Photodetector (PD). In data processing, by reading the positions and their changes from the time domain interference fringes, the sound modulation experienced by the corresponding remote sensor can be understood.

Figure 1 shows a single wavelength system. The greatest advantage of the fiber optic system is that it supports Wavelength Division Multiplexing (WDM) technology, so that the sensor array can be easily extended with multiple wavelengths. In the downlink, the optical pulses of each wavelength occupy different time slots and do not overlap with each other. In the uplink, the optical power is very weak. Thus, nonlinear crosstalk between multiple wavelengths in the fiber can be neglected. Only single wavelength systems have been investigated, but the conclusions can be extended to WDM systems.

In this embodiment, the method for performing linear frequency modulation on the interferometric optical fiber hydrophone system improves the sensitivity of the sensing receiver and the optical power of the transmitter, effectively improves the excitation threshold of the optical fiber nonlinearity, and makes the hydrophone system have a larger maximum tolerable loss.

Specifically, as shown in fig. 2 and 3, the principle of detecting an acoustic signal using LFM light pulses is that, since there is a slight difference in length between the two arms of the michelson interferometer, two decomposed LFM pulses participating in interference have a relative displacement in the time domain. The frequency of the LFM pulse changes with time, and the optical carrier frequency of the two pulses at the same time is no longer the same, and their beating produces an amplitude oscillation at a frequency much lower than the optical frequency. This amplitude oscillation can be converted into an electrical signal using non-coherent detection. Linear frequency modulation ensures that the beat frequency does not change within a single pulse duration. A small change in the interferometer arms under modulation of the acoustic signal will cause an optical phase shift of the passing optical pulses, which phase modulation will then be transferred into the jitter with a constant magnitude, which can eventually be demodulated at the receiver. In our LFM-based hydrophones, these difficulties are addressed by increasing the repetition rate of the LFM pulses. In contrast to ranging applications, the echo-beat frequency at high repetition rates no longer follows the above-mentioned direct proportional relationship, but is a constant, integer multiple of the repetition rate.

The following is an analytical derivation of beat frequency at high repetition rates. The optical LFM pulse sequence x (t) to be generated by the transmitter is expressed by the following equation:

where α is the chirp rate and q (t) is the envelope of the pulse, i.e. the amplitude and phase evolution of the LFM is not included. This equation shows that the LFM pulse sequence we use can be viewed as a continuous repetition of a single LFM pulse in the time domain, with a period T. In the analysis, only the interference process in a single probe is concerned by neglecting various non-ideal linear and non-linear processes of optical fiber transmission, loss of an array, noise introduced by amplification of a transceiver and the like, and then the optical pulse x (t) is delayed from the optical pulse x (t) for backup

Interfere and produce a beat frequency in the receiver, where τ is the arm length difference of the michelson interferometer, and

the optical phase difference generated by the interferometer double arms is modulated by the sound field, and the bandwidth of the optical phase difference is far less than 1/T. Performing Hilbert transform on the beat frequency signal, and solving a positive frequency part y (t) of the beat frequency signal, wherein the positive frequency part y (t) comprises the following components:

since τ is small, controlling the duration of a single LFM light pulse allows no overlap between different interfering light pulses, and the above equation can be rewritten as:

where the pulse p ═ q (t) q⁺(t-T), the p pulses of different time slots are identical

From equation (3), it appears that the phase modulation

Is carried on a carrier e^-i(2πατ)tThe above. But in practice the presence of the final sum term makes this expression no longer correct. Since the width of the pulse p is small (duty cycle is about 1/N), the last two terms of (3) contain many harmonic components, the phase shift within each period T by 2 pi α τ · kT makes α τ no longer one of the harmonics, unless 2 pi α τ T is exactly an integer multiple of 2 pi (this condition is also called Talbot condition, here the article hwh can be cited). Further, expressing y (t) from the sum of the plurality of pulses to the sum of the plurality of carriers, one can obtain:

where P (-) is the Fourier transform of the pulse P (-). The formula clearly shows that the signal to be measured

Carried in the form of phase modulation on a plurality of carrier frequencies having a frequency of

I.e. its frequency is just higher than the repetition frequency of the transmitter light pulses.

The above theoretical analysis shows that with high repetition rate LFM pulses, the echo beat frequency is insensitive to interferometer arm length difference, transmitter chirp coefficient, etc. This means that the transmitter is synchronized to the repetition frequency of the transmitter as long as it remains constant with very low phase noiseThe receive demodulation process of (a) can avoid the additional phase demodulation result drift caused by the non-idealities. In theory, the phase demodulation of any carrier in the formula (4) can realize the demodulation of the sensing signal; in practice, however, the carrier frequency position where the signal-to-noise ratio is highest should be selected, and the power of each carrier is determined by the spectral envelope P. In the generally linear or quasi-linear case, the pulse envelope function p is a baseband signal, the energy of which is concentrated at zero frequency. Therefore, the maximum of the spectral envelope P should be satisfied

I.e., the carrier with the frequency closest to the value of α τ, will have the highest signal-to-noise ratio. This conclusion is consistent with ranging applications that use low repetition rate LFM pulses, for example.

Although the embodiments of the present invention have been described with reference to the accompanying drawings, it is not intended to limit the scope of the invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive changes in the embodiments of the present invention.

Claims (3)

1. An interferometric fiber optic hydrophone system, comprising:

the narrow linewidth laser is connected with a phase modulator, the phase modulator is connected with a first filter, and the first filter is connected with a first optical fiber amplifier; the LFM optical pulse is obtained by carrying out carrier suppression single sideband modulation on a narrow linewidth laser source;

the first optical fiber amplifier is connected with the input end of the sensor array, the output end of the sensor array is connected with a second optical fiber amplifier, the second optical fiber amplifier is connected with a second filter, the second filter is connected with a photoelectric detector, and the photoelectric detector is connected with the data processing module; in the sensor array, each sensor consists of a Michelson interferometer, and a pair of Faraday rotation mirrors is used for eliminating attenuation related to polarization;

wherein, in the data processing module, by reading the positions and their changes from the time domain interference fringes, the sound modulation condition experienced by the corresponding remote sensor can be known.

2. A method of linear frequency modulation of an interferometric fiber optic hydrophone system as claimed in claim 1, characterized in that in a michelson interferometer each light pulse is split into two pulses of equal power, which are then combined to produce interference fringes in the time domain; linear frequency modulation ensures that the beat frequency does not change within a single pulse duration; converting amplitude oscillations generated by the two pulse beats with equal power into an electric signal by using incoherent detection; a small change in the michelson interferometer arms will cause a phase shift of the light of the passing light pulse under phase modulation of the acoustic signal, which then shifts into the jitter with a constant magnitude, eventually being demodulated at the receiver.

3. The linear frequency modulation method according to claim 2, wherein the linear frequency modulation is achieved by increasing a repetition rate of the LFM pulse, comprising:

the LFM pulse sequence used can be seen as a constant repetition of a single LFM pulse in the time domain, with a period T; the LFM pulse sequence x (t) is expressed by the following equation:

where α represents the chirp rate and q (t) represents the envelope of the pulse, i.e., excluding the amplitude and phase evolution of the LFM;

focusing only on the interference process within a single probe, the light pulse x (t) will be backed up with its delay

Interfere and generate beat frequency in the receiver, where τ is Michelson interferenceDifference in arm length of the instrument, and

modulating the resulting optical phase difference for the interferometer arms for the acoustic field;

hilbert transform is performed on the beat signal and its positive frequency component y (t) is determined as follows

Since the time length of a single LFM light pulse is controlled so that there is no overlap between different interfering light pulses, y (t) can be rewritten as:

wherein the pulse p ═ q (t) q⁺(t-τ)；

Expressing y (t) from the sum of the plurality of pulses to the sum of the plurality of carriers, one obtains:

wherein P (-) represents the Fourier transform of the pulse P (-);

then the signal under test

Carried in the form of phase modulation on a plurality of carrier frequencies having a frequency of

I.e. its frequency is proportional to the repetition frequency of the transmitter light pulses.

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