Interferometric optical fiber hydrophone system based on white light source

Abstract

The invention provides an interference type optical fiber hydrophone system based on a white light source. The interference type optical fiber hydrophone system comprises the white light source, a transmission optical fiber, a sensing probe, a matching interferometer, a balance photoelectric detector and a data processing unit. Wherein the white light source generates a white light pulse signal; the transmission optical fiber transmits the white light pulse signal; the sensing probe converts the underwater acoustic signal into the change of an optical signal, and returns the optical signal carrying underwater acoustic information to the transmission optical fiber; the matching interferometer is matched with the time delay of the sensing probe, so that optical signals are subjected to interference, and heterodyne detection is completed; the balanced photoelectric detector detects and receives an optical signal, converts the optical signal into an electric signal and eliminates common-mode noise; and the data processing unit processes the electric signal and demodulates the background noise and the underwater acoustic signal of the evaluation system. The interference type optical fiber hydrophone system provided by the invention can realize long-distance underwater sound sensing under the condition of no underwater relay.

Description

Technical Field

[0001] The invention relates to the technical field of underwater acoustic wave signal detection, and in particular to an interference type optical fiber hydrophone system based on a white light source. Background Art

[0002] Interferometric fiber optic hydrophone is an underwater acoustic wave sensor that uses optical fiber as a transmission medium and converts water acoustic signals into optical signals based on the interference effect of light. It has the advantages of high sensitivity, strong anti-electromagnetic interference ability, and wide operating frequency band, and is widely used in military and civilian fields, such as long-range military target detection, marine resource development, and marine physics research. In order to achieve underwater relay-free transmission, reduce deployment costs, and improve long-term stability, research has mainly focused on improving link tolerance loss and reducing background noise. However, increasing the optical power entering the transmission fiber will stimulate the nonlinear effect of the fiber, resulting in an increase in background noise, which in turn reduces the detection sensitivity. This problem is particularly significant in systems using narrow linewidth lasers as light sources.

[0003] Existing interferometric fiber optic hydrophone systems usually use narrow linewidth lasers as light sources. In order to increase the optical power entering the transmission fiber and improve the link loss tolerance without relaying, the existing technology has tried a variety of methods, but these methods all have certain defects:

[0004] 1. Use normal dispersion optical fiber to replace ordinary single-mode optical fiber as the transmission optical fiber: The cost is too high and it is not feasible in actual projects.

[0005] 2. Suppress modulation instability (MI) by reducing background noise before entering the transmission fiber: This requires high-performance, high-cost optical filters, which increases system complexity and cost.

[0006] 3. Distribute the optical power entering the transmission fiber to orthogonal polarization states of different wavelengths: Although modulation instability can be avoided, the background noise of the system will be increased.

[0007] 4. Use coherent seeds to suppress modulation instability: The conditions for achieving optimal suppression are complex, and the suppression effect weakens as the optical power increases. Summary of the invention

[0008] In view of this, an embodiment of the present invention provides an interference type fiber optic hydrophone system based on a white light source to eliminate or improve one or more defects existing in the prior art.

[0009] The present invention provides an interferometric fiber optic hydrophone system based on a white light source, the system comprising:

[0010] A white light source, used to generate a white light pulse signal;

[0011] A transmission optical fiber, used for transmitting the white light pulse signal;

[0012] A sensing probe, used to convert the hydroacoustic signal into a change in the optical signal, and return the optical signal carrying the hydroacoustic information to the transmission optical fiber;

[0013] A matching interferometer is used to match the delay of the sensor probe so that the optical signal interferes and completes heterodyne detection;

[0014] A balanced photodetector, used for detecting and receiving the optical signal, converting the optical signal into an electrical signal, and eliminating common mode noise;

[0015] The data processing unit is used to process the electrical signal and demodulate the background noise and hydroacoustic signal of the evaluation system.

[0016] In some embodiments of the present invention, the white light source includes a broadband light source, an optical filter and an acousto-optic modulator connected in sequence; the broadband light source is used to output white light; the optical filter is used to adjust the spectral bandwidth of the white light; and the acousto-optic modulator is used to generate light pulses.

[0017] In some embodiments of the present invention, an optical fiber amplifier is provided between the white light source and the transmission optical fiber to amplify the white light pulse signal.

[0018] In some embodiments of the present invention, after the white light pulse signal is converted into an optical signal and then returned to the transmission optical fiber, multiple sets of optical fiber amplifier-optical filter combinations are used to improve the signal-to-noise ratio of the system.

[0019] In some embodiments of the present invention, an optical signal processed by a combination of multiple fiber amplifiers and optical filters is input into a circulator to achieve fixed-direction transmission; port 1 of the circulator is used to receive the optical signal, port 2 is used to output the optical signal to the matching interferometer, and port 3 is used to receive the optical signal returned from the matching interferometer and output it to the balanced photodetector.

[0020] In some embodiments of the present invention, the sensing probe uses an unbalanced Michelson interferometer, and the arm length difference thereof is greater than half the optical pulse width.

[0021] In some embodiments of the present invention, the matching interferometer adopts an unbalanced Michelson interferometer, and an acousto-optic modulator is respectively arranged on two arms of the matching interferometer to perform frequency shift to realize heterodyne detection.

[0022] In some embodiments of the present invention, the data processing unit is provided with a digital-to-analog converter to convert the analog electrical signal into a digital signal.

[0023] In some embodiments of the present invention, the minimum background noise of the system is calculated as:

[0024]

[0025] Wherein, D represents the pulse duty cycle of the white light pulse signal; B0 represents the spectral bandwidth of the white light pulse signal.

[0026] The present invention provides an interference type fiber optic hydrophone system based on a white light source, comprising: a white light source, a transmission optical fiber, a sensor probe, a matching interferometer, a balanced photodetector and a data processing unit. The white light source generates a white light pulse signal; the transmission optical fiber transmits the white light pulse signal; the sensor probe converts the hydroacoustic signal into a change in the optical signal, and returns the optical signal carrying the hydroacoustic information to the transmission optical fiber; the matching interferometer matches the delay of the sensor probe to interfere the optical signal and complete heterodyne detection; the balanced photodetector detects the received optical signal and converts it into an electrical signal, and eliminates the common mode noise; the data processing unit processes the electrical signal, and demodulates the background noise and hydroacoustic signal of the evaluation system. The interference type fiber optic hydrophone system provided by the present invention can realize long-distance hydroacoustic sensing without underwater relaying. The nonlinear effect of optical fiber has little effect on white light. By using white light as a light source, the optical power entering the transmission optical fiber can be greatly increased while maintaining a low background noise, thereby correspondingly improving the link tolerance loss.

[0027] Additional advantages, purposes, and features of the present invention will be described in part in the following description, and will become apparent to those skilled in the art after studying the following, or may be learned from the practice of the present invention. The purposes and other advantages of the present invention may be achieved and obtained by the structures specifically indicated in the specification and the accompanying drawings.

[0028] Those skilled in the art will appreciate that the objectives and advantages that can be achieved with the present invention are not limited to the above specific description, and the above and other objectives that can be achieved by the present invention will be more clearly understood from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute a limitation of the present invention. In the drawings:

[0030] Figure 1 FIG. 4 is a schematic diagram of the structure of an interferometric fiber optic hydrophone system based on a white light source in one embodiment of the present invention.

[0031] Figure 2 Schematic diagram of the change of background noise with the optical power entering the transmission optical fiber in one embodiment of the present invention.

[0032] Figure 3Schematic diagram of the change of background noise with link margin loss in one embodiment of the present invention.

[0033] Figure 4 FIG. 1 is a schematic diagram of a frequency-background noise curve in an embodiment of the present invention. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments and the accompanying drawings. Here, the illustrative embodiments of the present invention and their descriptions are used to explain the present invention, but are not intended to limit the present invention.

[0035] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the solutions according to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0036] It should be emphasized that the term “include / comprises” when used herein refers to the presence of features, elements, steps or components, but does not exclude the presence or addition of one or more other features, elements, steps or components.

[0037] It should also be noted that, unless otherwise specified, the term “connection” herein may refer not only to a direct connection but also to an indirect connection involving an intermediate.

[0038] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the accompanying drawings, the same reference numerals represent the same or similar components, or the same or similar steps.

[0039] It should be emphasized here that the step marks mentioned below are not intended to limit the order of the steps, but it should be understood that the steps can be executed in the order mentioned in the embodiment, or in a different order from the embodiment, or several steps can be executed simultaneously.

[0040] In order to solve the problems of high cost, high system complexity, and inability to effectively suppress background noise and modulation instability in the improvement of the existing interferometric fiber optic hydrophone system, the present invention provides an interferometric fiber optic hydrophone system based on a white light source, such as Figure 1 As shown, the system includes:

[0041] The white light source is used to generate a white light pulse signal.

[0042] Transmission optical fiber, used to transmit white light pulse signals.

[0043] The sensing probe is used to convert the white light pulse signal into an optical signal and then return it to the transmission optical fiber.

[0044] The matching interferometer is used to match the delay of the sensor probe so that the optical signal interferes and completes heterodyne detection.

[0045] Balanced photodetector, used to detect received optical signals, convert optical signals into electrical signals, and eliminate common mode noise.

[0046] The data processing unit is used to process the electrical signals and demodulate the background noise and hydroacoustic signals of the evaluation system.

[0047] Based on the background technology, it can be known that the interferometric fiber optic hydrophone system provided by the present invention is different from the traditional interferometric fiber optic hydrophone system in that the light source adopts a white light pulse light source instead of a narrow linewidth laser. Among them, the narrow linewidth laser (Narrow-Linewidth Laser) has a higher coherence, and its spectrum width is about the kHz level, while the white light (White-Light) has a lower coherence, and its spectrum width is in the GHz and higher levels.

[0048] In some embodiments, the white light source includes a broadband light source, an optical filter, and an acousto-optic modulator (AOM) connected in sequence. The broadband light source outputs white light, and the optical filter is used to adjust the spectral bandwidth of the white light. The optical filter can be of fixed bandwidth or adjustable bandwidth, such as a wavelength division multiplexer or a wavelength selection switch; the acousto-optic modulator is used to modulate a continuous optical signal into an optical pulse, and the acousto-optic modulator is controlled by a square wave signal to specify the repetition frequency and duty cycle of the pulse.

[0049] In some embodiments, after the white light pulse signal is generated by the white light source part, the signal is amplified by an erbium-doped fiber amplifier, so that it enters the transmission optical fiber with higher optical power.

[0050] In some embodiments, the transmission optical fiber is a common single-mode optical fiber.

[0051] In some embodiments, the sensor probe uses an unbalanced Michelson interferometer, that is, a delay loop is provided on one arm to introduce a fixed optical path difference, and the arm length difference is greater than half the optical pulse width. This unbalanced design helps to improve the sensitivity and detection capability of the system. Faraday rotators are provided at the ends of the two arms to eliminate polarization state dependence and improve system stability. Exemplarily, the interferometer can adopt a Mach-Zehnder structure, a Michelson structure, etc.

[0052] In the present invention, the sensing probe is used to convert the optical path difference caused by the hydroacoustic signal into the phase change of the optical signal, thereby realizing the sensing function. The optical signal carrying the hydroacoustic information is then returned to the transmission optical fiber and enters the receiving end.

[0053] In some embodiments, after the optical signal carrying the hydroacoustic information is returned to the transmission optical fiber, multiple sets of erbium-doped fiber amplifier-optical filter combinations are used to improve the signal-to-noise ratio of the system, wherein all optical filters have the same performance.

[0054] In some embodiments, the optical signal processed by multiple sets of erbium-doped fiber amplifier-optical filter combinations is input into a circulator to achieve fixed-direction transmission. Exemplarily, port 1 of the circulator is used to receive the optical signal, port 2 is used to output the optical signal to a matching interferometer, and port 3 is used to receive the optical signal returned from the matching interferometer and output it to a balanced photodetector.

[0055] The optical signal is input into a matching interferometer and matched with a sensor probe to realize heterodyne detection.

[0056] In some embodiments, the matching interferometer also uses an unbalanced Michelson interferometer, that is, a delay loop is provided on one arm to introduce a fixed optical path difference, and the arm length difference is greater than half the optical pulse width. This unbalanced design helps to improve the sensitivity and detection capability of the system. Faraday rotators are provided at the ends of the two arms to eliminate the polarization state dependence and improve the stability of the system. Unlike the sensor probe, acousto-optic modulators are respectively provided on the two arms of the matching interferometer for frequency shift to achieve heterodyne detection. Among them, heterodyne detection is a technology that uses two optical signals of different frequencies to interfere and extract information by detecting their difference frequency signal. The core idea is to mix the signal to be measured with a reference signal to generate an intermediate frequency signal (difference frequency signal), and then extract the information of the signal to be measured by detecting this intermediate frequency signal.

[0057] It should be noted that when both the sensing probe and the matching interferometer use unbalanced Michelson interferometers, the difference in arm lengths between the two must be reduced to within the coherence length of white light to ensure stable interference.

[0058] Finally, the balanced photodetector converts the interfered optical signal into an electrical signal and eliminates the common mode noise. The data processing unit processes the electrical signal to demodulate the background noise and underwater acoustic signal of the evaluation system.

[0059] Among them, the balanced photodetector is a photoelectric device used to detect light signals and convert them into electrical signals. Its core feature is that it can eliminate common-mode noise and improve the signal-to-noise ratio. A balanced photodetector usually consists of two photodiodes and a differential amplifier. The two light signals are input into the two photodiodes respectively. The photodiodes convert the light signals into current signals. The differential amplifier receives the current signals of the two photodiodes and calculates their difference. Since the common-mode noise is the same in the two light signals, it will be offset after differential amplification, and the useful signal (differential-mode signal) will be amplified. The electrical signal output by the differential amplifier is the useful signal with the common-mode noise eliminated.

[0060] In some embodiments, the data processing unit is provided with a digital-to-analog converter, and processing the electrical signal includes converting the analog electrical signal into a digital signal. After the analog-to-digital conversion, the data is processed to evaluate the background noise of the system.

[0061] In some embodiments, the theoretical minimum low noise of the system can be expressed as formula (1):

[0062]

[0063] Wherein, D represents the pulse duty cycle of the white light pulse signal; B0 represents the spectral bandwidth of the white light pulse signal.

[0064] In summary, the interferometric fiber optic hydrophone system based on a white light source provided by the present invention overcomes the challenge of typical fiber nonlinear effects faced by the current solution using narrow linewidth laser as a light source, and does not require additional system design.

[0065] Among them, the challenges of typical fiber nonlinear effects are mainly stimulated Brillouin scattering and modulation instability with the lowest and second lowest excitation thresholds. The excitation threshold of stimulated Brillouin scattering can be expressed as formula (2):

[0066]

[0067] Where K represents the polarization factor; A eff Represents the optical effective area; g B represents the peak Brillouin gain; L eff Indicates the effective length of the optical fiber; Δv S Indicates the bandwidth of the light source; Δv B represents the Brillouin gain bandwidth.

[0068] According to formula (2), based on the parameter calculation in one embodiment of the present invention, the excitation threshold of the interference type fiber optic hydrophone system based on the white light source provided by the present invention is 8.62W, which is much higher than the typical value of narrow linewidth laser 4.3mW.

[0069] The effect of modulation instability on the interferometric fiber optic hydrophone system is mainly manifested in the introduction of large relative intensity noise into the system. At the receiving end, they will cause the background noise to rise. However, the present invention relies on white light for sensing, which can be regarded as a kind of noise light, and the noise introduced by modulation instability can also achieve the sensing function, which is the same as the nature of white light, so modulation instability will not affect the performance of the system.

[0070] In order to better understand the present invention and its beneficial effects, further description is given below in conjunction with specific embodiments.

[0071] Traditional interference fiber optic hydrophones use narrow linewidth lasers as light sources, which are affected by fiber nonlinearity, and the optical power entering the transmission fiber is low. However, the fiber nonlinear effect has little effect on white light. Therefore, the present invention uses white light as a light source, which can greatly increase the optical power entering the transmission fiber.

[0072] like Figure 2 As shown, the change of the background noise with the optical power entering the transmission fiber. By comparison, it can be seen that the link tolerance loss is fixed at 57dB, and the narrow linewidth laser solution has an obvious inflection point when the optical power entering the transmission fiber is about 8 to 9dBm. Before this inflection point, the background noise decreases with the increase of optical power, however, with the excitation of the nonlinear effect of the optical fiber, the noise rises rapidly. Therefore, the optimal value of the optical power entering the transmission fiber of the narrow linewidth laser solution is 9dBm, and the background noise is -96.25dB ref 1rad / √Hz. In contrast, the background noise of the present invention continues to decrease when the optical power entering the transmission fiber is within 3 to 15dBm, the maximum optical power entering the transmission fiber increases by 11dB to 20dBm, and the background noise is lower than -101dBref 1rad / √Hz.

[0073] The present invention enhances the maximum optical power entering the transmission optical fiber, which directly translates into improved link margin loss while maintaining a lower system noise floor. Figure 3 As shown, the change of background noise with link tolerance loss. This embodiment selects -95dB ref 1rad / √Hz as the threshold of background noise, and the optical power entering the transmission fiber is fixed to the optimal value. For the narrow linewidth laser solution, the background noise approaches the threshold when the link tolerance loss is 58dB. In contrast, the present invention shows greater tolerance to increased link tolerance loss. Before 63dB, the background noise increases slightly with the increase of link tolerance loss. After 63dB, although the background noise of the system rises linearly with the increase of loss, it does not approach the threshold until the link tolerance loss reaches 70dB, which results in the maximum link tolerance loss increasing by 12dB.

[0074] like Figure 4 As shown, it is a frequency-background noise curve diagram, which shows the detection capability of underwater acoustic signals of different frequencies. Considering that the frequencies of actual underwater acoustic signals are mostly concentrated at lower frequency positions, the background noise near the frequency of 1kHz is shown, which is -101.69dB ref 1rad / √Hz when the link tolerance loss is 57dB, and -95.08dB ref 1rad / √Hz when the link tolerance loss is 70dB. This shows that the present invention can meet the requirements for detecting weak underwater acoustic signals of low frequency at long distances without underwater relay.

[0075] It should be clear that the present invention is not limited to the specific configuration and processing described above and shown in the figures. For the sake of simplicity, a detailed description of the known method is 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, and those skilled in the art can make various changes, modifications and additions, or change the order between the steps after understanding the spirit of the present invention.

[0076] In the present 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 features of other embodiments or replace features of other embodiments.

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the embodiments of the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An interferometric fiber optic hydrophone system based on a white light source, characterized in that: The system comprises: A white light source, used to generate a white light pulse signal; A transmission optical fiber, used for transmitting the white light pulse signal; A sensing probe, used to convert the hydroacoustic signal into a change in the optical signal, and return the optical signal carrying the hydroacoustic information to the transmission optical fiber; A matching interferometer, used to match the delay of the sensing probe, so that the optical signal interferes and completes heterodyne detection; A balanced photodetector, used for detecting and receiving the optical signal, converting the optical signal into an electrical signal, and eliminating common mode noise; The data processing unit is used to process the electrical signal and demodulate the background noise and hydroacoustic signal of the evaluation system.

2. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: The white light source comprises a broadband light source, an optical filter and an acousto-optic modulator which are connected in sequence; the broadband light source is used to output white light; the optical filter is used to adjust the spectral bandwidth of the white light; and the acousto-optic modulator is used to generate optical pulses.

3. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: An optical fiber amplifier is provided between the white light source and the transmission optical fiber to amplify the white light pulse signal.

4. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: After the white light pulse signal is converted into an optical signal and then returned to the transmission optical fiber, a plurality of sets of optical fiber amplifier-optical filter combinations are used to improve the signal-to-noise ratio of the system.

5. The interferometric fiber optic hydrophone system based on a white light source according to claim 4, characterized in that: The optical signal processed by multiple groups of optical fiber amplifier-optical filter combinations is input into the circulator to achieve fixed-direction transmission; port 1 of the circulator is used to receive the optical signal, port 2 is used to output the optical signal to the matching interferometer, and port 3 is used to receive the optical signal returned from the matching interferometer and output it to the balanced photodetector.

6. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: The sensing probe adopts an unbalanced Michelson interferometer, and the arm length difference thereof is greater than half the optical pulse width.

7. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: The matching interferometer adopts an unbalanced Michelson interferometer, and acoustic-optic modulators are respectively arranged on two arms of the matching interferometer to perform frequency shift so as to realize heterodyne detection.

8. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: The data processing unit is provided with a digital-to-analog converter to convert the analog electrical signal into a digital signal.

9. The interferometric fiber optic hydrophone system based on a white light source according to claim 1, characterized in that: The minimum background noise of the system is calculated as follows: Wherein, D represents the pulse duty cycle of the white light pulse signal; B0 represents the spectral bandwidth of the white light pulse signal.

Images