A method and device for correcting laser frequency noise of an interferometric fiber-optic hydrophone

Abstract

The application provides a laser frequency noise correction method and device for an interferometric fiber-optic hydrophone, the interferometric fiber-optic hydrophone comprising a reference probe and a plurality of sensing probes, the correction method comprising: setting the echo power ratio of the reference probe and the sensing probes to a specific value by using a coupler and an adjustable optical attenuator; and calculating and correcting the phase noise introduced by laser frequency drift based on the reference probe arm length difference and the sensing probe arm length difference, and combining the demodulated reference phase signal and the sensing phase signal. The application can suppress the laser frequency noise in the interferometric fiber-optic hydrophone system based on linear frequency modulation, correct the additional white noise introduced by the correction algorithm, improve the tolerance of the interferometric fiber-optic hydrophone system to the laser line width, reduce the detection cost, and improve the performance of the fiber-optic hydrophone.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and in particular to an interferometric fiber optic hydrophone. Background Technology

[0002] Fiber optic hydrophones possess advantages such as high sensitivity, wide response bandwidth, good frequency response characteristics, and strong resistance to electromagnetic interference and crosstalk, and are widely used in various fields. Among them, phase interferometric fiber optic hydrophones have the highest detection sensitivity, enabling long-distance repeater-free detection. One of the important indicators for evaluating hydrophone performance is the noise floor of the detection system. In interferometric measurement systems, the power drop caused by long-distance transmission leads to a significant decrease in the signal-to-noise ratio of the received signal. However, simply increasing the optical power will excite severe fiber nonlinearity, further worsening the noise floor of the detection.

[0003] Interferometric fiber optic hydrophone systems employing linearly frequency-modulated (LFM) light sources can improve the power utilization of the light source while suppressing fiber nonlinearity. However, LFM-based interferometric fiber optic hydrophones require interferometers with unequal arm lengths as sensors. The frequency variation of the light source is converted into phase noise after passing through the interferometer with a non-zero arm length difference, which further deteriorates the background noise. In this case, the light source frequency noise dominates the background noise of the detection system. Summary of the Invention

[0004] In view of this, embodiments of the present invention provide a laser frequency noise correction method and apparatus for an interferometric fiber optic hydrophone, in order to eliminate or improve one or more defects existing in the prior art, and to solve the problems of fiber nonlinearity and background noise degradation caused by frequency changes of the light source in the prior art.

[0005] One aspect of the present invention provides a laser frequency noise correction method for an interferometric fiber optic hydrophone. The method is performed on a linear frequency modulated interferometric fiber optic hydrophone, which includes a reference probe and multiple sensing probes. The reference probe and the sensing probes have the same structure. Each sensing probe receives acoustic wave modulation through an internal signal arm and interferes with the signal of the reference arm to pick up acoustic wave information. The reference probe is disposed in a sound-insulating and vibration-damping encapsulation layer to isolate the effect of acoustic waves on its signal arm and reference arm. The reference probe and each sensing probe are connected by a coupler. Adjustable optical attenuators are provided at the input terminals of the reference probe and each sensing probe. The method includes the following steps:

[0006] The echo power ratio of the reference probe and each sensor probe is controlled by the coupler and the adjustable optical attenuator to make the echo power of each sensor probe equal. The echo power of the reference probe is a first set multiple of each sensor probe, and the first set multiple is 2 to 6.

[0007] Obtain the reference arm length difference between the signal arm and the reference arm within the reference probe, and obtain the sensor arm length difference between the signal arm and the reference arm within each sensing probe.

[0008] Acquire the reference phase signal demodulated by the reference probe, and the sensing phase signal demodulated by each sensing probe;

[0009] The reference phase signal is used as the reference additional phase noise introduced by the laser frequency drift. The additional phase noise generated by each sensor probe due to the laser frequency drift is calculated by combining the ratio of the reference arm length difference to the length difference of each sensor arm. The sensor phase signal is corrected according to the additional phase noise corresponding to each sensor probe to obtain the final demodulated phase output of each sensor probe.

[0010] In some embodiments of the present invention, the first set multiple is 4.

[0011] In some embodiments of the present invention, the reference phase signal is used as the reference additional phase noise introduced by the laser frequency drift, and the additional phase noise generated by each sensing probe due to the laser frequency drift is calculated by combining the ratio of the reference arm length difference to the length difference of each sensing arm. The sensing phase signal is then corrected according to the additional phase noise corresponding to each sensing probe, and the calculation formula is as follows:

[0012]

[0013] in, This represents the final demodulated phase output after correction by the k-th sensor at time t. This represents the sensing phase signal of the k-th sensor probe at time t. The reference additional phase noise at time t, τ k τ represents the difference in sensing arm length of the k-th sensing probe. r This indicates the difference in the length of the reference arm.

[0014] Another aspect of the present invention provides an interferometric fiber optic hydrophone, the interferometric fiber optic hydrophone comprising:

[0015] Laser;

[0016] A phase modulator, connected to the laser, modulates the beam emitted by the laser to obtain a linear frequency modulated optical pulse signal; the linear frequency modulated optical pulse signal is then input into the sensor array after passing through a first filter, a first fiber amplifier, a first single-mode fiber, and a first optical attenuator in sequence.

[0017] The output of the sensor array is sequentially connected to a second single-mode optical fiber, a second optical fiber amplifier, a second filter, a photodetector, an analog-to-digital converter, and a data processing module.

[0018] The sensor array includes a reference probe and multiple sensing probes. The reference probe has the same structure as the sensing probes. Each sensing probe receives acoustic wave modulation through its internal signal arm and interferes with the signal of the reference arm to pick up acoustic wave information. The reference probe is housed in a sound-insulating and vibration-damping encapsulation layer to isolate the acoustic waves from its signal arm and reference arm. The reference probe and each sensing probe are connected by a coupler. The input terminals of the reference probe and each sensing probe are equipped with adjustable optical attenuators. The echo power ratio of the reference probe and each sensing probe is controlled by the coupler and the adjustable optical attenuator to make the echo power of each sensing probe equal. The echo power of the reference probe is a first set multiple of each sensing probe, where the first set multiple is between 2 and 6.

[0019] The data processing module acquires the reference arm length difference between the signal arm and the reference arm within the reference probe, and the sensing arm length difference between the signal arm and the reference arm within each sensing probe; it acquires the demodulated reference phase signal of the reference probe, and the demodulated sensing phase signal of each sensing probe; it uses the reference phase signal as the reference additional phase noise introduced by the laser frequency drift, and calculates the sensing additional phase noise generated by each sensing probe due to the laser frequency drift by combining the ratio of the reference arm length difference to the length difference of each sensing arm; it corrects the sensing phase signal according to the sensing additional phase noise corresponding to each sensing probe to obtain the final demodulated phase output of each sensing probe.

[0020] In some embodiments of the present invention, the reference probe and the sensing probe are Michelson interferometers.

[0021] In some embodiments of the present invention, the first fiber amplifier and the second fiber amplifier are erbium-doped fiber amplifiers.

[0022] In some embodiments of the present invention, the reference probe and the sensing probe are combined and transmitted in a time-division multiplexing manner.

[0023] In some embodiments of the present invention, the interferometric fiber optic hydrophone further includes a data acquisition module for acquiring signals detected by the reference probe and the sensing probe.

[0024] In some embodiments of the present invention, the interferometric fiber optic hydrophone further includes a display module for displaying data from the data processing module.

[0025] Another aspect of the present invention provides a computer-readable storage medium having a computer program stored thereon, characterized in that the program, when executed by a processor, implements the steps of the method described above.

[0026] The beneficial effects of the present invention are at least as follows:

[0027] The present invention discloses a laser frequency noise correction method and apparatus for an interferometric fiber optic hydrophone, which improves upon linear frequency modulation-based interferometric fiber optic hydrophones. The interferometric fiber optic hydrophone includes a reference probe and multiple sensing probes with identical structures. The method calculates and corrects phase noise introduced by laser frequency drift based on the difference in arm length between the reference probe and the sensing probes, combined with the demodulated reference phase signal and the sensing phase signal. Simultaneously, the invention utilizes a coupler and an adjustable optical attenuator to set the echo power ratio of the reference probe and the sensing probes to a specific value, thereby mitigating white noise degradation caused by the correction algorithm. This invention reduces the impact of background noise using a common laser that does not have narrow linewidth and low-frequency noise characteristics, lowering detection costs and improving the performance of the fiber optic hydrophone.

[0028] Additional advantages, objects, and features of the invention will be set forth in part in the description which follows, and will also become apparent in part to those skilled in the art upon studying the description, or may be learned by practice of the invention. The objects and other advantages of the invention can be realized and obtained by means of the structures specifically pointed out in the description and drawings.

[0029] Those skilled in the art will understand that the objectives and advantages achievable with the present invention are not limited to those specifically described above, and that the above and other objectives achievable with the present invention will become clearer from the following detailed description. Attached Figure Description

[0030] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, are not intended to limit the scope of the invention. The components in the drawings are not drawn to scale but are merely illustrative of the principles of the invention. For ease of illustration and description of certain parts of the invention, corresponding portions in the drawings may be enlarged, i.e., may appear larger relative to other components in an exemplary device actually manufactured according to the invention. In the drawings:

[0031] Figure 1 This is a connection structure diagram of an interferometric fiber optic hydrophone system based on linear frequency modulation according to an embodiment of the present invention.

[0032] Figure 2 The graph shows the phase noise of a laser as a function of frequency when driven by white noise with an amplitude ranging from 0V to 2V.

[0033] Figure 3 Experimental data and fitting plots of phase noise in an interferometric fiber optic hydrophone system when the laser is driven by white noise with an amplitude of 0V to 2V.

[0034] Figure 4The demodulated phase noise and the corrected phase noise at laser noise drive off and drive amplitude of 1V, respectively, are plotted in another embodiment of the interferometric fiber optic hydrophone system based on linear frequency modulation of the present invention.

[0035] Figure 5 The diagram shows the corrected phase noise results obtained in the experiment when the laser noise drive amplitude was adjusted from 0V to 2V in the linear frequency modulation-based interferometric fiber optic hydrophone system described in another embodiment of the present invention, with echo power ratios of 1 and 4 respectively. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the embodiments and accompanying drawings. Here, the illustrative embodiments and descriptions of this invention are used to explain the invention, but are not intended to limit the invention.

[0037] It should also be noted that, in order to avoid obscuring the invention with unnecessary details, only the structures and / or processing steps closely related to the solution according to the invention are shown in the accompanying drawings, while other details that are not closely related to the invention are omitted.

[0038] It should be emphasized that the term "including / comprises" as used herein refers to the presence of a feature, element, step, or component, but does not exclude the presence or addition of one or more other features, elements, steps, or components.

[0039] It should also be noted that, unless otherwise specified, the term "connection" in this article can refer not only to a direct connection, but also to an indirect connection involving an intermediary.

[0040] In the following description, embodiments of the invention will be illustrated with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar parts, or the same or similar steps.

[0041] Fiber optic hydrophones are sensors that use optical fibers as the transmission medium to convert underwater acoustic signals into optical signals for detecting underwater sound waves. They offer advantages such as high sensitivity, wide response bandwidth, good frequency response characteristics, strong resistance to electromagnetic interference and crosstalk, and the ability to form large-scale, large-area arrays using multiplexing technology, making them widely used in various fields. Among them, phase interferometric fiber optic hydrophones have the highest detection sensitivity and can achieve long-distance, repeater-free detection.

[0042] In fiber optic hydrophone systems, reducing noise floor and improving the signal-to-noise ratio (SNR) are crucial goals for enhancing hydrophone performance. Noise floor originates from various random interference signals, such as electronic noise, thermal noise, and optical noise. These noises mix with the signal during transmission or detection, making accurate signal extraction or detection difficult. Employing appropriate noise suppression techniques, filter design, and signal processing algorithms can effectively reduce the impact of noise floor on the SNR and improve the reliable detection capability of the signal.

[0043] Compared to traditional heterodyne fiber optic hydrophones, interferometric fiber optic hydrophones based on linear frequency modulation (LFM) can improve the power utilization of the light source and suppress fiber nonlinearity, thereby mitigating noise floor degradation. However, LFM-based interferometric fiber optic hydrophones require interferometers with unequal arm lengths as sensors. The frequency noise of the laser manifests as the change in its instantaneous frequency over time. This frequency change of the light source is converted into phase noise after passing through the interferometer with a non-zero arm length difference, further degrading the noise floor. Therefore, correcting the light source frequency noise is crucial for improving the performance of LFM-based interferometric fiber optic hydrophones.

[0044] On one hand, embodiments of the present invention provide a method for correcting laser frequency noise in an interferometric fiber optic hydrophone. For example... Figure 1 As shown, this method is implemented on a linear frequency modulated interferometric fiber optic hydrophone, which includes a reference probe and multiple sensing probes. The reference probe and each sensing probe have the same structure and are connected using a Michelson interferometer. Each sensing probe receives acoustic wave modulation through its internal signal arm and interferes with the signal of the reference arm to pick up acoustic wave information. The reference probe is housed in a sound-insulating and vibration-damping encapsulation layer to isolate the effect of acoustic waves on its signal arm and reference arm. The reference probe and each sensing probe are connected by a coupler, and the input terminals of the reference probe and each sensing probe are equipped with adjustable optical attenuators. The method includes the following steps S101 to S104:

[0045] Step S101: Control the echo power ratio of the reference probe and each sensor probe through the coupler and the adjustable optical attenuator to make the echo power of each sensor probe equal. The echo power of the reference probe is a first set multiple of each sensor probe, and the first set multiple is 2 to 6.

[0046] Step S102: Obtain the reference arm length difference between the signal arm and the reference arm inside the reference probe, and obtain the sensor arm length difference between the signal arm and the reference arm inside each sensor probe.

[0047] Step S103: Obtain the reference phase signal demodulated by the reference probe, and the sensing phase signal demodulated by each sensing probe.

[0048] Step S104: Use the reference phase signal as the reference additional phase noise introduced by the laser frequency drift, and combine the ratio of the reference arm length difference to the length difference of each sensor arm to calculate the sensing additional phase noise generated by each sensor probe due to the laser frequency drift. Correct the sensing phase signal according to the sensing additional phase noise corresponding to each sensor probe to obtain the final demodulated phase output of each sensor probe.

[0049] Specifically, in an interferometric fiber optic hydrophone based on linear frequency modulated (LFM) signals, the LFM optical pulse signal received by the coupler is represented as follows:

[0050]

[0051] Where α is the chirp rate, q(t) is the pulse envelope, i represents the imaginary unit, k represents an integer specifying the time offset of q(t), t represents the pulse signal transmission time, and T is the period of the pulse sequence. θ(t)=∫2πf ins (t)d t This represents the time-varying phase of the laser, including phase noise caused by laser frequency drift, f ins (t) represents the instantaneous frequency of the light source.

[0052] Ignoring factors other than laser phase noise, such as various non-ideal linear and nonlinear processes in fiber optic transmission, array losses, and noise from transceiver amplification, in each interferometer, the linearly frequency-modulated optical pulse interferes only with its delay, and a beat frequency pulse is obtained at the receiving end. The positive frequency part of the beat frequency signal is expressed as the sum of multiple charge carriers, as follows:

[0053]

[0054] Where τ represents the time delay difference between the two paths in the interferometer. The optical phase difference generated by the acoustic modulation of the two arms has a bandwidth much smaller than 1 / T. P(·) is q(t)·q + The Fourier transform of (t-τ) shows that p(t) is the same for different time slots. The phase noise Δθ(t) = θ(t) - θ(t-τ) caused by the laser frequency drift can be approximated as follows when the laser frequency drift range is small or the difference in interferometer arm length is small:

[0055] Δθ(t)≈2πτf ins (t),

[0056] As can be seen from the above equation, the phase noise introduced by the light source is positively correlated with τ. Therefore, the phase noise introduced by the light source can be corrected by the difference in arm length of the interferometer.

[0057] In some embodiments of the present invention, the first set multiple is 4.

[0058] In some embodiments of the present invention, a reference phase signal is used as the reference additional phase noise introduced by laser frequency drift. The additional phase noise generated by each sensing probe due to laser frequency drift is calculated by combining the ratio of the reference arm length difference to the length difference of each sensing arm. The sensing phase signal is then corrected based on the additional phase noise corresponding to each sensing probe, as calculated by the following formula:

[0059]

[0060] in, This represents the final demodulated phase output of the k-th sensor probe after correction at time t. This represents the sensing phase signal of the k-th sensor probe at time t. τ represents the reference additional phase noise at time t. k τ represents the difference in sensing arm length between the k-th sensing probes. r This indicates the difference in reference arm length.

[0061] On the other hand, embodiments of the present invention provide an interferometric fiber optic hydrophone, such as... Figure 1 As shown, the interferometric fiber optic hydrophone includes:

[0062] Laser.

[0063] A phase modulator, connected to a laser, modulates the laser beam to obtain a linearly frequency-modulated (LFM) optical pulse signal. The LFM optical pulse signal is then sequentially passed through a first filter, a first fiber amplifier, a first single-mode fiber, and a first optical attenuator before being input into the sensor array.

[0064] The sensor array output is sequentially connected to a second single-mode fiber, a second fiber amplifier, a second filter, a photodetector, an analog-to-digital converter, and a data processing module.

[0065] The sensor array is combined and transmits signals using a time-division multiplexing method. The sensor array includes a reference probe and multiple sensing probes. The reference probe and the sensing probes have the same structure and are connected using a Michelson interferometer. Each sensing probe receives acoustic wave modulation through its internal signal arm and interferes with the signal of the reference arm to pick up acoustic wave information. The reference probe is housed in a sound-insulating and vibration-damping encapsulation layer to isolate the sound waves from its signal arm and reference arm. The reference probe and each sensing probe are connected via couplers, and each sensing probe has an adjustable optical attenuator at its input. The echo power ratio of the reference probe and each sensing probe is controlled by the couplers and adjustable optical attenuators to ensure that the echo power of each sensing probe is equal. The echo power of the reference probe is a first set multiple of each sensing probe, with the first set multiple ranging from 2 to 6.

[0066] Among them, the first fiber amplifier and the second fiber amplifier are erbium-doped fiber amplifiers.

[0067] The data processing module acquires the reference arm length difference between the signal arm and the reference arm within the reference probe, and the sensing arm length difference between the signal arm and the reference arm within each sensing probe. It acquires the demodulated reference phase signal from the reference probe, and the demodulated sensing phase signal from each sensing probe. The reference phase signal is used as the reference additional phase noise introduced by the laser frequency drift. Combined with the ratio of the reference arm length difference to the length difference of each sensing arm, the module calculates the sensing additional phase noise generated by each sensing probe due to the laser frequency drift. The sensing phase signal is then corrected based on the corresponding sensing additional phase noise for each sensing probe to obtain the final demodulated phase output for each sensing probe.

[0068] In some embodiments of the present invention, the interferometric fiber optic hydrophone further includes a data acquisition module for acquiring signals detected by the reference probe and the sensing probe.

[0069] In some embodiments of the present invention, the interferometric fiber optic hydrophone further includes a display module for displaying data from the data processing module.

[0070] Optionally, embodiments of the present invention provide a laser frequency noise correction method and apparatus for an interferometric fiber optic hydrophone, the specific implementation method of which is as follows:

[0071] This invention aims to suppress laser frequency noise and additional white noise in linear frequency modulation (LFM)-based interferometric fiber optic hydrophone systems, thereby improving the system's tolerance to laser linewidth. The basic approach is as follows: First, a reference probe shielding external signals is introduced into the LFM-based interferometric fiber optic hydrophone system. This reference probe is given a higher echo power ratio than other sensing probes, forming a novel time-division multiplexing (TDM) array with unbalanced echo power. Then, the reference probe signal is used to cancel the phase noise introduced by the laser in other sensing probes, effectively improving the white noise degradation caused by traditional algorithms.

[0072] Linear frequency modulated (LFM) signals are signals whose instantaneous frequency changes linearly with time. Fiber optic hydrophones use optical fibers as the transmission medium to convert underwater acoustic signals into optical signals for detecting underwater sound waves. Laser frequency noise refers to the noise introduced by laser center frequency jitter and phase fluctuations. Background noise refers to the total noise excluding the useful signal.

[0073] Fiber optic hydrophones possess advantages such as high sensitivity, wide response bandwidth, good frequency response characteristics, strong resistance to electromagnetic interference and crosstalk, and the ability to form large-scale, large-area arrays using multiplexing technology, leading to their widespread application in various fields. Phase interferometric fiber optic hydrophones exhibit the highest detection sensitivity, enabling long-distance, repeater-free detection; therefore, they have become an important research topic. Noise floor is one of the core performance indicators of a hydrophone. In interferometric systems, power degradation caused by long-distance transmission leads to a significant decrease in the signal-to-noise ratio (SNR) of the received signal. However, simply increasing the optical power can excite severe fiber nonlinearity, further worsening the detection noise floor. Compared with other heterodyne schemes, the LFM-based interferometric fiber optic hydrophone system effectively solves this contradiction, improving the power utilization of the light source and suppressing fiber nonlinearity. Experiments show that using a linearly frequency-modulated (LFM) light source increases the maximum allowable loss of the fiber optic hydrophone by 7.1 dB. However, the LFM scheme requires interferometers with varying arm lengths as sensors, thus placing higher demands on the frequency noise of the laser. The frequency noise of the laser manifests as the change in its instantaneous frequency over time. Traditional heterodyne fiber optic hydrophones use equal-arm interferometers for sensing, meaning the two interfering light pulses travel equal optical distances, thus theoretically overcoming the difficulty of requiring high-quality light sources. However, in LFM (Light Filtering Mechanism) schemes, the frequency variation of the light source is converted into phase noise after passing through an interferometer with a non-zero arm length difference, leading to a deterioration of the noise floor. In this case, the light source frequency noise dominates the noise floor of the detection system, thus requiring effective measures to minimize its negative impact.

[0074] In an LFM-based interferometric fiber optic hydrophone, the linear frequency modulated (LFM) optical pulse signal received by the coupler is represented as follows:

[0075]

[0076] Where α is the chirp rate, q(t) is the pulse envelope, and R is the period of the pulse sequence. θ(t)=∫2πf ins (t)d t It is the time-varying phase of the laser, and includes phase noise caused by laser frequency drift, f ins (t) represents the instantaneous frequency of the light source. In the analysis, various non-ideal linear and nonlinear processes, such as those in fiber optic transmission, are neglected, except for laser phase noise, as well as array losses and noise from transceiver amplification. In each interferometer, the optical LFM pulse interferes only with its delay, and a beat frequency pulse is obtained at the receiving end. The positive frequency portion of the beat frequency signal is expressed as the sum of multiple charge carriers, yielding:

[0077]

[0078] Where τ is the time delay difference between the two paths in the Michelson interferometer. The optical phase difference generated by the acoustic modulation of the two arms has a bandwidth much smaller than 1 / T. P(·) is q(t)·q + The Fourier transform of (t-τ) shows that p(t) is the same for different time slots. The phase noise Δθ(t) = θ(t) - θ(t-τ) caused by the laser frequency drift can be approximated as follows when the laser frequency drift range is small or the difference in interferometer arm length is small:

[0079] Δθ(t)≈2πτf ins (t),

[0080] As shown in the above equation, the phase noise introduced by the light source is positively correlated with τ, which provides a solution for canceling the phase noise introduced by the light source. In an embodiment of this invention, a novel time-division multiplexing (TDM) array for a large-scale interferometric fiber optic hydrophone system is proposed. An additional reference probe, isolated from the environment, is introduced into the sensor array and used as a reference for calibrating the demodulation results of other actual sensors. However, during calibration, the reference probe also introduces a significant amount of white noise. In the array of this embodiment, the echo power of the reference probe is set higher than that of other sensors, thus solving this problem. This novel array design is applied to an LFM-based fiber optic hydrophone system to suppress laser frequency noise and the additional white noise introduced. Experiments show that in the new system of this embodiment, a laser with a linewidth of 338.06 MHz can replace a 1.417 kHz laser, achieving the same demodulation noise floor, significantly relaxing the linewidth requirements of the interferometric hydrophone. Simultaneously, the degradation of the phase noise floor caused by the added white noise is improved from at least 3 dB initially to within 1 dB.

[0081] like Figure 1As shown, in a novel time-division multiplexing (TDM)-based linear frequency modulated interferometric fiber optic hydrophone system, the transmitter uses a conventional laser without narrow linewidth and low-frequency noise characteristics. The laser's light source is modulated by carrier-suppressed single-sideband modulation to obtain LFM optical pulses. These LFM pulses are amplified by an erbium-doped fiber amplifier (EDFA) before entering the transmission fiber and sensor array. A reference probe is added to the sensor array; unlike the sensing probes, the reference probe effectively shields external signals. The sensor array is assembled in a TDM manner, supporting large-scale networking. The power of the reference probe can be adjusted by controlling the ratio of couplers in the array to ensure that the interference fringes of the reference probe and the sensing probes have different echo powers, while maintaining equal power for each sensing probe. After passing through the array and transmission fiber, the optical pulses enter the receiver for optical amplification, filtering, and photoelectric conversion, and are received by an analog-to-digital converter (ADC). At this point, the interference signals from the reference probe and the sensing probes are read together, preparing for subsequent data processing.

[0082] In embodiments of the present invention, the acquired data is demodulated. After demodulation, the signals introduced by sound modulation and laser frequency drift can be obtained from the corresponding time-domain interference fringes of each probe. In the TDM sequence, since the input of each sensor comes from the same laser, both the reference probe and the actual sensing probe will obtain signals from the same laser drift frequency f. ins The additional phase noise introduced by (t). Therefore, it is assumed that the phase signal demodulated by the Kth actual sensor in the TDM sequence is The phase signal after demodulation by the reference probe is Where φ s (t) represents the desired acoustic signal, and the remainder is the additional phase noise introduced by the laser frequency drift. Since the reference probe is isolated from the environment, the signal from the reference probe only considers the phase noise introduced by the laser frequency drift. The signal from the reference probe can be used to correct the sensor probe signal, achieving cancellation of the light source phase noise and obtaining the desired acoustic signal from the sensor probe. As analyzed previously, the additional demodulation phase noise introduced by the light source frequency noise is proportional to the difference in the interferometer arm length. Due to manufacturing errors or environmental interference, differences inevitably exist between TDM interferometers. To more accurately correct the phase noise, this difference must be considered; therefore, the correction algorithm should be:

[0083]

[0084] Where, τ k and τ r These represent the difference in arm length between the Kth real sensor and the reference probe, respectively. This is the final demodulated phase output after correction. Therefore, by knowing the ratio of the arm length difference between the sensor probe under test and the reference probe, and substituting this ratio of arm length difference and the phase signals previously demodulated from the sensor probe under test and the reference probe into the correction algorithm, the light source phase noise of the sensor probe under test can be suppressed, and the system's requirements for the light source can be relaxed.

[0085] The above analysis shows that the reason for introducing a reference probe and thus eliminating the influence of laser frequency noise is the correlation between the additional phase noise of the sensing probe and the reference probe. However, some uncorrelated noise still exists between the interferometers during demodulation. Especially in long-distance repeater-free hydrophone systems, fiber optic transmission and TDM arrays introduce significant losses, resulting in very weak echoes generated by each interferometer when reaching the receiver. Therefore, the echo beat frequency signal will include a large amount of broadband white noise, mainly caused by amplifier spontaneous emission noise (ASE) of the EDFA. This white noise will dominate when laser frequency noise cancellation is achieved using the above correction algorithm. Since the white noise from each interferometer is uncorrelated, the subtraction in the correction algorithm will not cancel them out; instead, random noise will be superimposed after the algorithm. When the echo power of the sensing probe and the reference probe are the same, their signal-to-noise ratio (SNR) is also the same. Therefore, the correction algorithm will cause the white noise in the sensing signal to be superimposed to twice its original value, resulting in a 3dB deterioration in SNR. Therefore, the optimal phase noise correction algorithm should consider the influence of the above uncorrelated noise and minimize its impact.

[0086] To mitigate the white noise degradation caused by the correction algorithm, with a fixed LFM optical pulse power entering the TDM array, the echo power of the reference probe is appropriately increased by controlling the coupler ratio. This ensures that the SNR of the echo beat frequency signal generated by the reference probe is greater than the SNR of other interferometers. The selection of each coupler ratio must meet the following conditions: First, the echo power of each sensing probe should be the same; second, the echo power of the reference probe should be M times that of the sensing probes; and finally, the overall loss of the TDM array should be as low as possible. The correction algorithm is then applied at the receiver. After the algorithm, the white noise of the sensor will primarily originate from itself, effectively avoiding the superposition of white noise and thus effectively improving the signal-to-noise ratio.

[0087] Through simulation comparison, it was determined that when the echo power ratio of the reference probe and the sensing probe is M=4, the additional loss introduced by the sensor is only 1.1dB, and the background noise of the corrected system is minimized. Therefore, the aforementioned non-uniform power distribution does not affect the suppression of laser frequency noise. The solution proposed in this embodiment can successfully eliminate the influence of laser frequency noise and reduce the deterioration of white noise.

[0088] This invention effectively reduces additional white noise while suppressing laser frequency noise and relaxing the laser linewidth requirements of the interferometric fiber optic hydrophone. The effectiveness of this invention is demonstrated experimentally below. The transmitter outputs an LFM pulse sequence with a pulse duration of 312.5 ns, a bandwidth of 1.5 GHz, a repetition frequency of approximately 195.31 kHz, and a duty cycle of 1 / 16, supporting demodulation of a 16-TDM array. The pulse sequence is amplified to 8 dBm using an EDFA. Tunable optical attenuators (VOAs) are placed before and after the TDM array to simulate the downlink and uplink transmission fibers, respectively. The total loss of the two attenuators is 16 dB, used to simulate 100 km of linear transmission (assuming a loss of 0.16 dB / km for G.654D fiber), to avoid the influence of optical nonlinearity on the experimental results. In practical applications, only the reference probe should be isolated from the environment. However, to more accurately measure the invention's ability to correct for additional phase noise caused by the light source, two identical, environmentally isolated Michelson interferometers were used to simulate a 16-TDM array. Each interferometer used a pair of Faraday rotating mirrors (FRMs) to eliminate polarization-dependent fading. These two pseudo-interferometers were used to simulate the reference probe and the sensor array, respectively, and their insertion losses could be set independently. In the experiment, the sensor array loss was fixed at 27 dB, corresponding to the loss introduced by a conventional 16-TDM, while the reference probe loss was adjustable to simulate different echo power ratios between the sensor array and the reference probe.

[0089] To better compare the effectiveness of the proposed solution in correcting phase noise introduced by different levels of laser frequency noise, a frequency-tunable laser was used in the experiment to avoid inaccurate evaluation results due to different light sources. A white noise source was also used to drive the narrow-linewidth laser. The laser's linewidth was 1.417 kHz, and the driving noise was output from an arbitrary waveform generator (AWG) with a bandwidth of 100 kHz and adjustable amplitude.

[0090] Figure 2 and Figure 3 The phase noise of the laser and hydrophone system under white noise with amplitudes ranging from 0V to 2V is shown respectively.

[0091] like Figure 2 As shown, with the increase of noise-driven amplitude, the laser linewidth increases from a minimum of 1.417 kHz to 338.06 MHz. Simultaneously, the laser's phase noise also increases across the entire frequency band, indicating that simulating the laser's frequency noise in this manner is reasonable. Next, the demodulation results of the hydrophone system under different linewidth conditions are observed. The measured phase noise and its fitting are shown below. Figure 3As shown, the demodulated phase noise increases across the entire offset frequency band as the laser linewidth increases. When the linewidth exceeds 1 MHz, the phase noise deteriorates by 14 dB. The above comparative experiments only varied the laser linewidth; all other parameters remained constant to ensure the reliability of the experimental results.

[0092] Next, the effectiveness of this invention in suppressing laser frequency noise and improving white noise degradation was verified by increasing the echo power of the reference probe. Before implementing the correction, the arm length difference between the two interferometers was measured; the reference probe and the sensing probe were 0.252m and 0.247m, respectively. Therefore, the coefficient in the correction algorithm was 0.98. When the laser noise drive was off, the laser linewidth was 1.417kHz. The measured system noise floor of the demodulated sensing probe signal was -98.8dBc / Hz, and the phase noise at a 1kHz frequency offset was -97.6dBc / Hz. These results, when the laser was undisturbed, represent the optimal performance achievable by the system and can serve as a benchmark for subsequent comparative experiments.

[0093] Figure 4 The figure shows the phase noise results before and after correction when the white noise drive amplitude is 1V and the echo power ratios of the reference probe and the sensor probe are 1 and 4, respectively. The linewidth is widened to 25MHz, and the demodulated phase noise is improved by 20dB. The figure shows that the influence of laser frequency noise can be suppressed by simple subtraction, but the effects vary. At a 1kHz frequency offset, when the echo power ratio is 1, the phase noise is suppressed by 16.76dB, and the corrected noise floor is 3dB higher than that of the narrow linewidth source. When the echo power ratio is 4, the phase noise is suppressed by 19.72dB, and the white noise degradation is limited to within 1dB. This improvement in correction effect can be seen across the entire frequency band.

[0094] Figure 5 The results show the phase noise suppression for different laser linewidths when the echo power ratios of the reference probe and the sensing probe are 1 and 4, respectively. The laser noise drive amplitude varies from 0V to 2V. The yellow horizontal line represents the phase noise when the laser is undisturbed, indicating the optimal performance achievable by the system. When the echo power ratio is 1, the corrected phase noise ranges from -96 to -95 dBc / Hz, with an average of 95.3 dBc / Hz, approximately 3 dB lower than the optimal value. This result further verifies the conclusion that the traditional approach increases white noise. When the echo power ratio is 4, the corrected phase noise ranges from -99 to -98 dBc / Hz, with an average of -98.6 dBc / Hz. This result demonstrates that even with severe laser frequency noise (maximum linewidth of 338.06 MHz), increasing the echo power of the reference probe can effectively suppress phase noise degradation while reducing white noise superposition.

[0095] This invention effectively suppresses laser frequency noise, enabling a 338.06 MHz linewidth laser to replace a 1.417 kHz laser in an LFM-based interferometric fiber optic hydrophone system while achieving the same demodulation noise floor. Furthermore, the increased reference probe echo power in this invention improves the system noise floor degradation caused by white noise superposition from at least 3 dB initially to less than 1 dB.

[0096] In summary, this invention provides a laser frequency noise correction method and apparatus for an interferometric fiber optic hydrophone. The interferometric fiber optic hydrophone includes a reference probe and multiple sensing probes. The correction method includes: using a coupler and an adjustable optical attenuator to set the echo power ratio of the reference probe and the sensing probes to a specific value; calculating and correcting the phase noise introduced by laser frequency drift based on the difference in reference arm length and the difference in sensing arm length, combined with the demodulated reference phase signal and the sensing phase signal. This invention can suppress laser frequency noise in linear frequency modulation (LFM) based interferometric fiber optic hydrophone systems, as well as the additional white noise introduced by the correction algorithm, improve the tolerance of the interferometric fiber optic hydrophone system to laser linewidth, reduce detection costs, and improve the performance of the fiber optic hydrophone.

[0097] This invention also provides a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the steps of the aforementioned edge computing server deployment method. The computer-readable storage medium can be a tangible storage medium, such as random access memory (RAM), main memory, read-only memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, floppy disks, hard disks, removable storage disks, CD-ROMs, or any other form of storage medium known in the art.

[0098] Those skilled in the art will understand that the exemplary components, systems, and methods described in conjunction with the embodiments disclosed herein can be implemented in hardware, software, or a combination of both. Whether implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this invention. When implemented in hardware, it can be, for example, electronic circuits, application-specific integrated circuits (ASICs), appropriate firmware, plug-ins, function cards, etc. When implemented in software, the elements of this invention are programs or code segments used to perform the desired tasks. The programs or code segments can be stored in a machine-readable medium or transmitted over a transmission medium or communication link via data signals carried in a carrier wave.

[0099] It should be clarified that the present invention is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of the present invention is not limited to the specific steps described and shown. Those skilled in the art can make various changes, modifications, and additions, or change the order of steps, after understanding the spirit of the present invention.

[0100] In this invention, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or in place of features of other embodiments.

[0101] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and variations of the embodiments of the present invention are possible. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for correcting laser frequency noise in an interferometric fiber optic hydrophone, characterized in that, The method is performed on a linear frequency modulated interferometric fiber optic hydrophone, which includes a reference probe and multiple sensing probes. The reference probe and the sensing probes have the same structure. Each sensing probe receives acoustic wave modulation through its internal signal arm and interferes with the signal of the reference arm to pick up acoustic wave information. The reference probe is housed in a sound-insulating and vibration-damping encapsulation layer to isolate the effect of acoustic waves on its signal arm and reference arm. The reference probe and each sensing probe are connected by a coupler. The input terminals of the reference probe and each sensing probe are equipped with adjustable optical attenuators. The method includes the following steps: The echo power ratio of the reference probe and each sensor probe is controlled by the coupler and the adjustable optical attenuator to make the echo power of each sensor probe equal. The echo power of the reference probe is a first set multiple of each sensor probe, and the first set multiple is 2 to 6. Obtain the reference arm length difference between the signal arm and the reference arm within the reference probe, and obtain the sensor arm length difference between the signal arm and the reference arm within each sensing probe. Acquire the reference phase signal demodulated by the reference probe, and the sensing phase signal demodulated by each sensing probe; The reference phase signal is used as the reference additional phase noise introduced by the laser frequency drift. The additional phase noise generated by each sensor probe due to the laser frequency drift is calculated by combining the ratio of the reference arm length difference to the length difference of each sensor arm. The sensor phase signal is corrected according to the additional phase noise corresponding to each sensor probe to obtain the final demodulated phase output of each sensor probe.

2. The laser frequency noise correction method for the interferometric fiber optic hydrophone according to claim 1, characterized in that, The first set multiple is 4.

3. The laser frequency noise correction method for the interferometric fiber optic hydrophone according to claim 1, characterized in that, The reference phase signal is used as the reference additional phase noise introduced by the laser frequency drift. Combined with the ratio of the reference arm length difference to the length differences of each sensor arm, the sensing additional phase noise generated by each sensor probe due to the laser frequency drift is calculated. The sensing phase signal is then corrected based on the sensing additional phase noise corresponding to each sensor probe. The calculation formula is as follows: in, This represents the final demodulated phase output after correction by the k-th sensor at time t. This represents the sensing phase signal of the k-th sensor probe at time t. The reference additional phase noise at time t, τ k τ represents the difference in sensing arm length of the k-th sensing probe. r This indicates the difference in the length of the reference arm.

4. An interferometric fiber optic hydrophone, characterized in that, include: Laser; A phase modulator, connected to the laser, modulates the beam emitted by the laser to obtain a linear frequency modulated optical pulse signal; the linear frequency modulated optical pulse signal is then input into the sensor array after passing through a first filter, a first fiber amplifier, a first single-mode fiber, and a first optical attenuator in sequence. The output of the sensor array is sequentially connected to a second single-mode optical fiber, a second optical fiber amplifier, a second filter, a photodetector, an analog-to-digital converter, and a data processing module. The sensor array includes a reference probe and multiple sensing probes. The reference probe has the same structure as the sensing probes. Each sensing probe receives acoustic wave modulation through its internal signal arm and interferes with the signal of the reference arm to pick up acoustic wave information. The reference probe is housed in a sound-insulating and vibration-damping encapsulation layer to isolate the acoustic waves from its signal arm and reference arm. The reference probe and each sensing probe are connected by a coupler. The input terminals of the reference probe and each sensing probe are equipped with adjustable optical attenuators. The echo power ratio of the reference probe and each sensing probe is controlled by the coupler and the adjustable optical attenuator to make the echo power of each sensing probe equal. The echo power of the reference probe is a first set multiple of each sensing probe, where the first set multiple is between 2 and 6. The data processing module obtains the reference arm length difference between the signal arm and the reference arm in the reference probe, and obtains the sensor arm length difference between the signal arm and the reference arm in each sensing probe. The reference phase signal after demodulation of the reference probe and the sensing phase signal after demodulation of each sensing probe are acquired. The reference phase signal is used as the reference additional phase noise introduced by the laser frequency drift. The additional phase noise generated by each sensing probe due to the laser frequency drift is calculated by combining the ratio of the reference arm length difference and the length difference of each sensing arm. The sensing phase signal is corrected according to the additional phase noise corresponding to each sensing probe to obtain the final demodulated phase output of each sensing probe.

5. The interferometric fiber optic hydrophone according to claim 4, characterized in that, The reference probe and the sensing probe are both Michelson interferometers.

6. The interferometric fiber optic hydrophone according to claim 4, characterized in that, The first fiber amplifier and the second fiber amplifier are erbium-doped fiber amplifiers.

7. The interferometric fiber optic hydrophone according to claim 4, characterized in that, The reference probe and the sensing probe are combined and transmit signals using a time-division multiplexing method.

8. The interferometric fiber optic hydrophone according to claim 4, characterized in that, The interferometric fiber optic hydrophone also includes a data acquisition module for acquiring signals detected by the reference probe and the sensing probe.

9. The interferometric fiber optic hydrophone according to claim 4, characterized in that, The interferometric fiber optic hydrophone also includes a display module for displaying data from the data processing module.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the steps of the method as described in any one of claims 1 to 3.

Images