Optical feedback resonance enhanced forward Brillouin demodulation structure

Abstract

The invention provides an optical feedback resonance enhanced forward Brillouin demodulation structure, which comprises a tunable laser, a first polarization controller, an erbium-doped optical fiber amplifier, an optical coupler, an optical isolator, a forward Brillouin gain optical fiber, a band-pass filter and a photoelectric detector, a second polarization controller; and a spectrum analyzer. The first polarization controller is connected with a port a of the optical coupler, a port b of the optical coupler is connected with an input end of the erbium-doped optical fiber amplifier, an output end of the erbium-doped optical fiber amplifier is connected with the optical isolator, the optical isolator is connected with the band-pass filter, the band-pass filter is connected with the forward Brillouin gain optical fiber, and the optical fiber is fed back to a port c of the coupler through the second polarization controller. The output power of the erbium-doped optical fiber amplifier is controlled, and the polarization states of the pump light and the feedback laser are controlled by the first polarization controller and the second polarization controller, so that the forward Brillouin effect is amplified.

Description

Technical Field

[0001] This application relates to the field of fiber laser technology, and in particular to an optical feedback resonant enhancement forward Brillouin demodulation structure. Background Technology

[0002] Phononic lasers can be viewed as mechanical analogues of optical lasers, such as phononic lasers powered by stimulated Brillouin scattering (SBS). The SBS process generates acoustic waves, which interact with light waves to ultimately form a laser. Similar coupling has been demonstrated in silicon-based photonic platforms. In cylindrical waveguides, such as optical fibers, transverse resonant acoustic waves can reach the cladding, and the lifetime of these transverse acoustic waves is used for acoustic imaging, sensing, and matter identification. The first forward Brillouin fiber laser was implemented in polarization-maintaining fiber, with laser feedback from Bragg gratings at both ends of the fiber, and an output power of only 250 µW. A challenge with forward Brillouin lasers is that forward SBS in a single spatially guided light mode leads to phase modulation of the continuously input pump wave, without providing stimulated amplification of the laser signal. Therefore, it is difficult to realize FSBS lasers in standard single-mode fiber. Utilizing whispering-gallery modes to detect perturbations in the dielectric constant and geometry of fiber cross-sections caused by transverse acoustic mode resonances is a novel approach. Coupled with a tapered fiber probe beam to a WGM excited by a cylindrical waveguide, the narrowest linewidth and optimal efficiency for detecting low-order TAMRs were achieved. Another method involves cascaded forward stimulated Brillouin scattering excited in a chalcogenide microcavity, which excited a 25th-order acoustic mode at sub-mw thresholds.

[0003] Phonons can be amplified through the SBS process, and a phonon laser effect with a frequency of 23 MHz was observed in a coupled microcavity. Based on the SBS between two higher-order modes in the coupled microring, its operation is close to that of a two-level laser system. Literature reports the simultaneous excitation of photonic and phonon lasers by a single pump source in a resonant cavity composed of two-mode fibers, realizing a laser system composed of coupled oscillators from two different physical domains. This dual-domain laser is based on forward intermodal SBS excitation, demonstrating that phonon and photonic lasers simultaneously enhance the output power of both the phonon and photonic lasers. By injecting a single-frequency electronic signal into this suspended microsphere to achieve higher-order harmonic locking of the fundamental mode phonon laser, a significant enhancement of higher-order harmonics of the nonlinear phonon laser effect in the microsphere can also be achieved. The brightness of the phonon laser is increased by three orders of magnitude, and the linewidth is compressed by five orders of magnitude. A demonstration was conducted using a mode-locked fiber laser to select the transverse acoustic mode of a standard single-mode fiber; this approach requires successful mode-locking using a long single-mode fiber. Acoustic oscillators are implemented in multi-core optical fibers using Sagnac rings and photoelectric feedback, with the coupling between the pump and probe based entirely on internuclear optomechanical cross-phase modulation: no direct optical feedback is provided. Recently, a method was proposed to amplify the FSBS signal by utilizing back-brillouin scattering to amplify a sideband, thereby increasing the FSBS signal gain.

[0004] However, the above methods all rely on traditional pumping and detection schemes to demodulate transverse acoustic signals, and the direct amplification of transverse acoustic modes in optical waveguides has not yet been explored. This severely hinders the development and application of forward Brillouin lasers. Summary of the Invention

[0005] To address the aforementioned issues, this application discloses an optical feedback resonance-enhanced forward Brillouin demodulation structure, which provides effective support for the controllable generation of high-intensity, high-quality FSBS signals.

[0006] An optical feedback resonant enhanced forward Brillouin demodulation structure includes a tunable laser and a first optical coupler, with the pump light output from the tunable laser feeding the first optical coupler. The structure further includes an erbium-doped fiber amplifier, an optical isolator, a bandpass filter, a forward Brillouin gain fiber, a second polarization controller, and a feedback resonant cavity. Port a of the optical coupler is connected to the first polarization controller; port b of the optical coupler is connected to the input of the erbium-doped fiber amplifier; the output of the erbium-doped fiber amplifier is connected to the input of the optical isolator; the output of the optical isolator is connected to the bandpass filter; the output of the bandpass filter is connected to the input of the forward Brillouin gain fiber; the output of the forward Brillouin gain fiber is connected to the second optical coupler; the output port of the second optical coupler is connected to the input of the second polarization controller; and the output port of the second polarization controller is connected to port c of the first optical coupler. By adjusting the output power of the erbium-doped fiber amplifier, the first polarization controller, and the second polarization controller, the forward Brillouin scattering effect of the fiber is enhanced through photoacoustic resonance.

[0007] Furthermore, the tunable laser emits pump light with a center wavelength of 1550nm and a power of 10dBm, which enters the input terminal of the first polarization controller and then enters the ring resonant cavity. Port a of the optical coupler is connected to the first polarization controller, and port b is connected to the input terminal of the erbium-doped fiber amplifier.

[0008] Furthermore, the laser beam within the ring cavity is amplified by an erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to the input of an optical isolator, which suppresses backscattered light from the fiber.

[0009] Furthermore, a bandpass filter is connected to the output of the optical isolator, which suppresses the spontaneous emission of light from the erbium-doped fiber amplifier.

[0010] Furthermore, the output of the bandpass filter is connected to the input of the forward Brillouin gain fiber. The laser excites transverse acoustic resonance in the FUT, and the acoustic mode forms a standing wave in the cross section of the fiber, which modulates the phase of the incident laser. The output of the forward Brillouin gain fiber is connected to the second optical coupler.

[0011] Furthermore, the 90% output port of the second optical coupler is connected to the input port of the second polarization controller, and the output port of the second polarization controller is connected to the port c of the first optical coupler, forming a ring structure, which converts phase modulation into intensity modulation through interference.

[0012] Furthermore, the 10% output port of the second optical coupler is connected to a photodetector, and the intensity modulation is detected by an ESA (Electronic Stability Spectrum Analyzer).

[0013] Furthermore, by controlling the output power of the erbium-doped fiber amplifier and the orientation of the first and second polarization controllers, the forward Brillouin scattering effect of the fiber is enhanced through photoacoustic resonance.

[0014] This application discloses an optical feedback resonant enhanced forward Brillouin demodulation structure, comprising a tunable laser, a coupler, and other components, forming a ring structure. The 1550nm pump light, after amplification and filtering, excites acoustic resonance in the gain fiber to achieve FSBS enhancement. The photoacoustic resonance is controlled by a polarization controller, and finally, the signal is detected by a detector and a spectrum analyzer.

[0015] The beneficial effects of this application are:

[0016] 1. The innovative optical feedback resonant-enhanced forward Brillouin demodulation scheme proposed in this application provides a completely new technical path for signal demodulation and amplification of forward stimulated Brillouin scattering. It improves upon traditional forward Brillouin demodulation schemes by designing a ring structure composed of components such as a tunable laser and a coupler, thus forming an innovative optical feedback resonant-enhanced forward Brillouin demodulation scheme.

[0017] 2. Optimized optical signal processing and modulation: The 1550nm pump light, after amplification and filtering, excites acoustic resonance in the gain fiber to enhance FSBS. Simultaneously, the photoacoustic resonance is precisely controlled by a polarization controller, effectively strengthening the FSBS process. When used as a gain module, it can amplify traditional FSBS signals to 1.145×10⁻⁶. 4 This significantly improves signal strength and detection sensitivity by 10 times (equivalent gain of 40.59 dB).

[0018] 3. This invention provides effective support for the controllable generation of high-intensity, high-quality FSBS signals, thereby demonstrating important application value and broad industrialization prospects in fields such as mechanical sensing and biomedical ultrasound imaging. Attached Figure Description

[0019] Figure 1(a) shows the transmission response of the all-fiber ring resonator under dynamic conditions. Figure 1(b) shows the relationship between phase modulation amplitude and fiber length. Figure 1(c) is a diagram of the periodic transmission response caused by multi-beam interference. Figure 1(d) is a spatiotemporal gain distribution diagram of FSBS in single-mode fiber;

[0020] Figure 2(a) is a schematic diagram of the principle of the optical feedback resonance enhanced forward Brillouin demodulation scheme according to an embodiment of this application. Figure 2(b) is a schematic diagram of the optical feedback resonance-enhanced forward Brillouin demodulation scheme according to an embodiment of this application;

[0021] Figure 3(a) shows the FSBS gain spectrum of the HNLF demodulated by the feedback loop resonant cavity in the embodiment; Figure 3(b) shows the FSBS gain spectrum of the demodulated SMF in the feedback loop resonant cavity in the embodiment;

[0022] Figure 4(a) shows the FSBS gain spectra of 960m and 20m HNLF in the example. Figure 4(b) shows the FSBS gain spectra of the 10m, 20m, 30m, 490m, 980m, 5km and 10km SMF in the embodiments;

[0023] Figure 5(a) shows the response characteristics of FSBS with a fiber length of 490 m. Figure 5(b) shows the response characteristics of FSBS with a fiber length of 30 m. Figure 5(c) shows the response characteristics of FSBS with a fiber length of 20 m. Figure 5(d) shows the response characteristics of FSBS with a fiber length of 10m.

[0024] Figure 6(a) shows the R(0,7) mode and its harmonic spectrum based on the demodulation scheme of this application. Figure 6(b) shows the R(0,7) mode and its harmonic spectrum based on the Sagnac ring demodulation scheme. Figure 6(c) is a schematic diagram of the R(0,7) mode after Lorentz fitting. Figure 6(d) is a comparison of the R(0,7) mode enhancement before and after using the optical feedback resonance-enhanced forward Brillouin demodulation scheme and the Sagnac ring demodulation scheme based on the embodiments of this application.

[0025] Figure 7-1 This is a schematic diagram of the oscillation of different acoustic modes in the SMF in the embodiment (where (a) is a schematic diagram of the oscillation of R(0,4) and R(0,8) modes, (b) is a schematic diagram of the oscillation of R(0,5) mode, and (c) is a schematic diagram of the oscillation of R(0,6) mode). Figure 7-2 This is a schematic diagram of the oscillation of different acoustic modes 2 in the SMF in the embodiment (where (d) is the oscillation diagram of the R(0,7) mode, (e) is the oscillation diagram of the R(0,8) mode, and (f) is the oscillation diagram of the R(0,5), R(0,7) and R(0,10) modes).

[0026] Figure 8(a) shows the R(0,17) mode and its harmonic spectrum based on the demodulation scheme of the example in this application; Figure 8(b) shows the R(0,17) mode and its harmonic spectrum based on the Sagnac ring demodulation scheme. Figure 8(c) is a schematic diagram of the R(0,17) mode fitted by Lorentz. Figure 8(d) is a comparison of the signal-to-noise ratio before and after enhancement in the R(0,17) mode under the optical feedback resonance enhanced forward Brillouin demodulation scheme and the Sagnac ring demodulation scheme in the embodiments of this application.

[0027] Figure 9(a) is a schematic diagram showing the effect of EDFA output power on peak power and signal-to-noise ratio of HNLF R(0,17) mode; Figure 9(b) is a schematic diagram showing the effect of EDFA output power on peak power and signal-to-noise ratio of SMF R(0,7) mode;

[0028] Figure 10(a) is a schematic diagram of the frequency stability analysis of the SMF in the embodiment. Figure 10(b) is a schematic diagram of the frequency stability analysis of HNLF in the embodiment. Figure 10(c) is a schematic diagram of the fluctuation of EDFA power within 2 minutes in the embodiment.

[0029] List of reference numerals in the attached diagram:

[0030] Wherein: 1-Tunable laser, 2-First polarization controller, 3-First optical coupler, 4-Erbium-doped fiber amplifier, 5-Optical isolator, 6-Bandpass filter, 7-Forward Brillouin gain fiber, 8-Second optical coupler, 9-Second polarization controller, 10-Photodetector, 11-Spectrum analyzer. Detailed Implementation

[0031] The present application will be further explained below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the present application. It should be noted that the terms "front", "rear", "left", "right", "up" and "down" used in the following description refer to the directions in the accompanying drawings, and the terms "inner" and "outer" refer to the directions toward or away from the geometric center of a specific component, respectively.

[0032] An optical feedback resonant enhanced forward Brillouin demodulation scheme includes a tunable laser 1, a first polarization controller 2, a first optical coupler 3, an erbium-doped fiber amplifier 4, an optical isolator 5, a bandpass filter 6, a forward Brillouin gain fiber 7, a second optical coupler 8, a second polarization controller 9, a photodetector 10, and a spectrum analyzer 11. Port a of the first optical coupler 3 is connected to the first polarization controller 2; port b of the first optical coupler 3 is connected to the input of the erbium-doped fiber amplifier 4; the output of the erbium-doped fiber amplifier 4 is connected to the input of the optical isolator 5; the output of the optical isolator 5 is connected to the bandpass filter 6; the output of the bandpass filter 6 is connected to the input of the forward Brillouin gain fiber 7; and the output of the forward Brillouin gain fiber 7 is connected to the second optical coupler 8. The output port of the second optical coupler 8 is connected to the input port of the second polarization controller 9, and the output port of the second optical polarization controller 9 is connected to port c of the first optical coupler 3; the output power of the erbium-doped fiber amplifier and the polarization states of the pump light and feedback light are controlled by the first polarization controller and the second polarization controller to amplify the forward Brillouin effect.

[0033] First optical coupler 3: serves as the input and feedback beam combining node of the ring cavity. Its port a is connected to the first polarization controller 2 to receive the initial pump light; its port b serves as the formal entrance of the ring cavity, leading the light to the amplification link; and its port c serves as the entrance for the feedback light, used to receive the optical signal returned from the loop.

[0034] Erbium-doped fiber amplifier 4: Its input end is connected to port b of the first optical coupler 3; the EDFA provides power compensation and gain for the optical signal circulating in the cavity, and the optical power level in the cavity can be controlled by adjusting its driving current, thereby directly dominating the excitation intensity of forward Brillouin scattering.

[0035] Optical isolator 5: Its input is connected to the output of erbium-doped fiber amplifier 4. It ensures that the optical signal is transmitted strictly in one direction in the ring cavity, effectively suppresses backscattered light, and ensures stable system operation.

[0036] Bandpass filter 6: Its input is connected to the output of optical isolator 5. This filter is used to filter out broadband spontaneous emission noise generated by erbium-doped fiber amplifier 4, purify the optical field inside the cavity, and improve the signal-to-noise ratio of the output signal.

[0037] The input of the forward Brillouin gain 7 is connected to the output of the bandpass filter 6. High-power pump light in this fiber excites transverse acoustic modes through electrostriction, causing periodic modulation of the refractive index and thus phase modulation of the light wave.

[0038] Second optical coupler 8: Its input end is connected to the output end of the forward Brillouin gain fiber 7, and it is used to split the outgoing light into two paths.

[0039] The second polarization controller 9 has its input connected to the 90% output port of the second optical coupler 8. It is used to adjust the polarization state of the feedback light so that it achieves optimal interference conditions with the pump light newly injected through port a within the first optical coupler 3, thereby completing the conversion from phase modulation to intensity modulation.

[0040] Optical feedback loop: The output of the second polarization controller (9) is connected back to port c of the first optical coupler 3 via an optical fiber. This connection closes the optical path, forming a complete optical feedback ring resonant cavity.

[0041] This optical feedback resonant-enhanced forward Brillouin demodulation scheme amplifies the forward Brillouin effect by controlling the output power of the erbium-doped fiber amplifier and the polarization states of the pump light and feedback laser using a first and a second polarization controller. This scheme effectively enhances the FSBS process and, when used as a gain module, can amplify traditional FSBS signals to 1.145 × 10⁻⁶. 4The signal strength and detection sensitivity are significantly improved by 10 times (equivalent gain 40.59 dB), providing effective support for the controllable generation of high-intensity, high-quality FSBS signals. It has important application value and broad industrialization prospects in fields such as mechanical sensing and biomedical ultrasound imaging.

[0042] Figure 1 is a schematic diagram of the working principle of this application, wherein (a) is the transmission response diagram of the all-fiber ring resonator under dynamic conditions, and the transmission of the ring resonator can be described by a theoretical model: ,in, = 0.99 represents the on / off coefficient of the fiber optic coupler. = 0.99 represents the transmission coefficient of the fiber optic loop. The cumulative phase in the loop. Figure 1(b) shows the relationship between phase modulation amplitude and fiber length, illustrating the variation of phase modulation amplitude with fiber length in the co-directional propagation scheme. Compared to the reverse propagation scheme constrained by the zero point of the sinc function, the co-directional propagation scheme can generate a detectable phase modulation signal even with short fiber lengths. The modulation frequency is... The periodic oscillations indicate that the resonant characteristics of the ring cavity introduce a frequency-dependent gain enhancement during the transition from phase modulation to intensity modulation. Figure 1(c) shows the periodic transmission response induced by multi-beam interference, the period of which is determined by the round-trip time of the intracavity photons. The decision is made. Figure 1(d) is a spatiotemporal gain distribution of FSBS in single-mode fiber, which reveals a characteristic time constant of about 1 μs, directly reflecting the underlying phonon dynamics.

[0043] Figure 2 is a schematic diagram of the optical feedback resonant enhancement forward Brillouin demodulation scheme of this application. Among them, (a) is a schematic diagram of the principle of the optical feedback resonant enhancement forward Brillouin demodulation scheme of this application embodiment. Its core is the optomechanical demodulation and amplification mechanism based on the feedback ring resonator. The laser emitted by the tunable laser is used as the pump light. After entering the feedback ring resonator composed of EDFA, FUT, etc., the laser amplified by EDFA excites transverse acoustic resonance in FUT. The acoustic mode forms a standing wave in the cross section of the optical fiber and generates phase modulation on the incident laser. The optical coupler connected to FUT feeds part of the light back to the ring cavity. The phase modulation is converted into intensity modulation through interference. At the same time, the optical feedback directly enhances the transverse acoustic mode in the optical fiber under the transverse acoustic mode signal excited by the pump light. In this example, the pump light and the feedback signal are generated by a single laser. No additional probe laser is required, which avoids the problem of cross-phase modulation. The whole presents FSBS. The core principle of signal enhancement and demodulation is illustrated in Figure 2(b), which is a schematic diagram of the optical feedback resonant enhancement forward Brillouin demodulation scheme according to an embodiment of this application. A tunable laser emits pump light with a center wavelength of 1550 nm and a power of 10 dBm, which enters the input of the first polarization controller and then the ring resonant cavity. Optical coupler port a is connected to the first polarization controller, and port b is connected to the input of the erbium-doped fiber amplifier. The laser light inside the ring cavity is amplified by the erbium-doped fiber amplifier. The output of the erbium-doped fiber amplifier is connected to the input of an optical isolator, which suppresses backscattered light from the fiber. The output of the optical isolator is connected to a bandpass filter, which suppresses the spontaneous emission light from the erbium-doped fiber amplifier. The filter output is connected to the input of the forward Brillouin gain fiber. The laser excites transverse acoustic resonance in the FUT, and the acoustic mode forms a standing wave in the fiber cross-section, which modulates the phase of the incident laser. The output of the forward Brillouin gain fiber is connected to the second optical coupler. The 90% output port of the second optical coupler is connected to the input of the second polarization controller, and the output of the second polarization controller is connected to port c of the first optical coupler, forming a ring structure. The phase modulation is converted into intensity modulation through interference. The 10% output port of the second optical coupler is connected to a photodetector, and the intensity modulation is detected by an ESA. The output power of the erbium-doped fiber amplifier and the two polarization controllers are controlled to enhance the forward Brillouin scattering effect of the fiber through photoacoustic resonance.

[0044] Figure 3 shows the FSBS gain spectrum of the optical feedback resonant enhanced forward Brillouin demodulation scheme based on the embodiments of this application. Figure 3(a) shows the FSBS gain spectrum of the HNLF demodulated by the feedback loop resonator in the embodiment, illustrating the demodulated FSBS signal when the FUT is a 960-meter-long HNLF and the EDFA output power is 15 dBm. The frequency corresponding to the R(0,17) mode is 832 MHz, consistent with the Sagnac loop demodulation result. Figure 3(b) shows the FSBS gain spectrum of the SMF demodulated by the feedback loop resonator in the embodiment, with a 490-meter-long SMF. The frequency of 320.44 MHz, indicated by the arrow, corresponds to the R(0,7) mode of the SMF, a result also consistent with literature reports.

[0045] Figure 4 illustrates the effect of fiber length on the FSBS gain spectrum in the embodiment. Fiber length plays a crucial role in the intensity of acousto-optic interaction. Figure 4(a) shows the FSBS gain spectra of the 960m and 20m HNLFs in the embodiment. When the HNLF is shortened to 20 meters, no significant FSBS is observed at an EDFA output power of 15 dBm. Therefore, the EDFA power is increased to 19 dBm, and the FSBS intensity of the 960m and 20m HNLFs is compared at this power. The FSBS intensity of the 960m HNLF is significantly enhanced compared to the 20m HNLF. Fiber length is a key parameter determining the acoustic mode excitation efficiency: the 960m HNLF, with its sufficient interaction length, can effectively detect weaker higher-order acoustic modes; while the 20m HNLF, due to insufficient phase accumulation, cannot excite such modes. Furthermore, in the ring resonator structure, the amplified FSBS can further excite nonlinear harmonic processes. The 960-meter HNLF exhibits a significantly greater number of gain peaks than the 20-meter HNLF, reflecting its enhanced capabilities in acoustic mode excitation and harmonic generation. Figure 4(b) shows the FSBS gain spectrum for SMFs of different lengths in the embodiment. When the SMF is used as the acoustic oscillation medium, the output power of the EDFA is fixed at 15 dBm. Fiber length can enhance the FSBS signal strength within a specific range, and the signal-to-noise ratio of the FSBS signal shows a gradual upward trend in the range of 10m to 5km. However, when the fiber length increases to 10km, the FSBS signal attenuates significantly. Excessive extension of the FUT within the cavity introduces significant power attenuation, which is extremely detrimental to the demodulation of the FSBS signal. Therefore, during the experiment, the EDFA power and fiber length must be strictly controlled.

[0046] Figure 5 illustrates the effect of EDFA power on FSBS in a relatively short fiber in the embodiment. The ring resonator not only achieves FSBS demodulation but also significantly improves FSBS generation efficiency. Due to the electrostriction effect, the intensity of photoacoustic interaction within the fiber is directly related to the laser power penetrating the fiber. Therefore, the gain of the EDFA within the resonant cavity is crucial for achieving acoustic mode amplification. When a relatively short single-mode fiber is used as the Fourier transform unit, the gradual increase in EDFA output power leads to a corresponding increase in the intensity of acoustic modes within the fiber. (a), (b), (c), and (d) correspond to the FSBS response characteristics when the fiber length is 490 m, 30 m, 20 m, and 10 m, respectively. As the EDFA power gradually increases, the intensity of transverse acoustic modes in the fiber continuously increases. As shown in Figure 5(a), when a 490-meter SMF is connected to the ring cavity, obvious acoustic mode characteristics are observed under the condition of EDFA output power of 16 dBm (signal-to-noise ratio SNR = 4.981 dB). Comparative analysis shows that when the fiber length is shortened to 30 meters, 20 meters, and 10 meters, respectively, the system needs to increase the EDFA power to 19 dBm to detect a weak acoustic response (SNR < 3.1 dB). Furthermore, the marginal effect of increasing the EDFA output power (in 1 dBm increments) on acoustic mode enhancement gradually weakens. The longer the fiber length, the more significant the phase accumulation, and the more pronounced the TAW enhancement effect.

[0047] Figure 6 is a comparison of the FSBS gain spectra of a 490mSMF using an optical feedback resonance-enhanced forward Brillouin demodulation scheme and a Sagnac ring demodulation scheme based on embodiments of this application. Figure 6(a) shows the R(0,7) mode and its harmonic spectra based on the demodulation scheme of this application. In the ring cavity, the R(0,7) mode exhibits excellent signal-to-noise ratio (SNR>40 dB). Figure 6(b) shows the R(0,7) mode and its harmonic spectra based on the Sagnac ring demodulation scheme. Experiments show that when demodulated using a Sagnac ring interferometer under the same excitation power condition (22 dBm), the SNR of this mode drops sharply to 9.874 dB, a reduction of 80.3% compared to the ring cavity configuration. Figure 6(c) shows the Lorentz fitting spectrum of the enhanced R(0,7) mode. The frequency of the R(0,7) mode is around 320 MHz, where the amplification effect occurs first. It can be inferred that, due to the relationship between the overlapping regions of the optical and acoustic fields caused by electrostriction and elasto-optic effects, different order resonant modes exhibit different intensities of FSBS, with their intensity reaching its peak in the optimal region of photoacoustic overlap. In the SMF, the R(0,7) mode achieves the maximum gain. The gain coefficients of each mode are given by the following equations. Figure 6(d) compares the enhancement of the R(0,7) mode before and after the two demodulation schemes. The intensity of the amplified acoustic mode in the feedback loop cavity is about 40.59 dB higher than that of the Sagnac loop demodulation. In the traditional Sagnac loop demodulation scheme, directly increasing the pump power cannot effectively enhance the acoustic mode within the fiber. Another conventional method is to introduce an electrical amplifier after the detector to amplify the RF signal. However, both methods introduce stimulated emission noise (ASE), which raises the signal noise floor and ultimately fails to achieve effective amplification of the RF signal. In contrast, the optical feedback loop cavity used in this embodiment has significant advantages, as it can directly obtain a high-quality acoustic signal without additional normalization processing.

[0048] Figure 7 is a schematic diagram of the oscillation of different acoustic modes in the SMF in the embodiment. (a) shows the oscillation of modes R(0,4) and R(0,8), (b) shows the oscillation of mode R(0,5), (c) shows the oscillation of mode R(0,6), (d) shows the oscillation of mode R(0,7), (e) shows the oscillation of mode R(0,8), and (f) shows the oscillation of modes R(0,5), R(0,7), and R(0,10). The 490m SMF suffers from insufficient phase accumulation, leading to mode competition between the R(0,7) acoustic mode and neighboring modes. These acoustic modes can be enhanced by adjusting the polarization coupler. The observed R(0,5), R(0,6), R(0,7), and R(0,8) modes all exhibit single-mode oscillation capability, which can be attributed to their relatively high gain coefficients. It should be noted that additional acoustic modes exist in both the low-order acoustic mode R(0,4) and the high-order acoustic mode R(0,10) oscillations. The stability of the intensity-modulated signal can be enhanced by increasing the fiber length. When the single-mode fiber length reaches 5 km, the phase modulation intensity of the R(0,7) acoustic mode on the laser is sufficiently significant.

[0049] Figure 8 is a comparison of the FSBS gain spectra of a 960mHNLF using an optical feedback resonance-enhanced forward Brillouin demodulation scheme and a Sagnac ring demodulation scheme based on embodiments of this application. Figure 8(a) shows the R(0,17) mode and its harmonic spectra of the demodulation scheme based on this application. The R(0,17) mode of the HNLF has the highest acoustic gain, while other acoustic modes are suppressed due to mode competition. Figure 8(b) shows the R(0,17) mode and its harmonic spectra based on the Sagnac ring demodulation scheme. Figure 8(c) is a schematic diagram of the R(0,17) mode after Lorentz fitting, with its gain spectrum center frequency located at approximately 832 MHz. Figure 8(d) is a comparison of the R(0,17) mode before and after enhancement under the two demodulation schemes. The R(0,17) acoustic mode intensity enhanced by the feedback ring cavity is 43.17 dB higher than the intensity of the traditional Sagnac ring interference demodulation.

[0050] Figure 9 illustrates the effect of EDFA output power on the peak power and signal-to-noise ratio (SNR) of the HNLF R(0,17) and SMF R(0,7) modes in this embodiment. The TAW intensity in the optical fiber is proportional to the optical intensity in the fiber core; therefore, the SNR and peak power of the FSBS are directly affected by the output power of the EDFA within the cavity. In this embodiment, when the EDFA output power is low (13.6 dBm), the feedback loop cavity cannot reach the oscillation threshold. Therefore, mode enhancement cannot be achieved, and the SNR of the HNLF R(0,17) mode is approximately 5.78 dB, with a peak power of approximately -66.47 dBm. As the EDFA power increases, the peak power of the HNLF R(0,17) mode also increases. However, excessively high EDFA power introduces excessive ASE noise, leading to a decrease in the mode SNR. Initially, the EDFA power can boost the SNR of the R(0,17) mode to 35.73 dB. However, when the optical power is too high in the later stages, the rate of increase in the floor noise is faster than that of the peak power, and the signal-to-noise ratio of this mode gradually decreases to 18.05 dB. The acoustic mode of SMF shows the same trend, reaching a maximum signal-to-noise ratio of 42.46 dB when the EDFA power is 21 dBm.

[0051] Figure 10 is a schematic diagram of the frequency stability test in this embodiment. Frequency stability is a key parameter for laser output performance. Figure 10(a) shows the frequency stability analysis of the SMF in this embodiment, and Figure 10(b) shows the frequency stability analysis of the HNLF in this embodiment. In this embodiment, the mode fluctuations of the R(0,7) mode in the SMF and the R(0,17) mode in the HNLF were monitored under constant temperature conditions (26°C) in the laboratory. The total observation time was 90 minutes, with data collected every 30 seconds. The EDFA output power was adjusted every 30 minutes throughout the experiment. For this embodiment, increasing the output power improved the stability of the acoustic modes of the SMF and HNLF, but the degree of improvement differed. During any 30-minute observation period, the maximum fluctuation amplitude of the R(0,7) mode of the SMF was ±1.208 MHz, while the fluctuation amplitude of the R(0,17) mode of the HNLF was ±0.716 MHz. Even at higher power levels, instability still exists, which is attributed to the inherent fluctuations of the EDFA output. Furthermore, when the EDFA output power stabilizes at 17.6 dBm, Figure 10(c) is a schematic diagram of the fluctuation of the EDFA power within 2 minutes in the embodiment.

[0052] The technical means disclosed in this application are not limited to those disclosed in the above embodiments, but also include technical solutions composed of any combination of the above technical features.

Claims

1. An optical feedback resonant enhancement forward Brillouin demodulation structure, comprising a tunable laser (1), a first polarization controller (2), and a first optical coupler (3), wherein the tunable laser (1) outputs pump light to the first optical coupler (3), characterized in that: It also includes an erbium-doped fiber amplifier (4), an optical isolator (5), a bandpass filter (6), a forward Brillouin gain fiber (7), a second optical coupler (8), a second polarization controller (9), a photodetector (10), and a spectrum analyzer (11); port a of the first optical coupler (3) is connected to the first polarization controller (2), and port b is connected to the input of the erbium-doped fiber amplifier (4); the output of the erbium-doped fiber amplifier (4) is connected to the input of the optical isolator (5); the output of the optical isolator (5) is connected to the input of the bandpass filter (6); the output of the bandpass filter (6) is connected to the input of the forward Brillouin gain fiber (7); the forward Brillouin gain fiber (7) is connected to the input of the forward Brillouin gain fiber (8). The output end of the erbium-doped fiber (7) is connected to the input end of the second optical coupler (8); the first output port of the second optical coupler (8) is connected to the input end of the second polarization controller (9), and the output end of the second polarization controller (9) is connected to port c of the first optical coupler (3), forming a ring structure; the second output port of the second optical coupler (8) is connected to the input end of the photodetector (10), and the output end of the photodetector (10) is connected to the spectrum analyzer (11); by adjusting the output power of the erbium-doped fiber amplifier (4) and the polarization states of the first polarization controller (2) and the second polarization controller (9), the photoacoustic resonance enhancement of the forward Brillouin scattering signal is realized.

2. The optical feedback resonance-enhanced forward Brillouin demodulation structure according to claim 1, characterized in that: The ring structure is a photoacoustic resonance enhancement cavity.

3. The optical feedback resonance-enhanced forward Brillouin demodulation structure according to claim 1, characterized in that: The pump light output by the tunable laser (1) has a center wavelength of 1550nm and a power of 10dBm.

4. The optical feedback resonance-enhanced forward Brillouin demodulation structure according to claim 1, characterized in that: The forward Brillouin gain fiber (7) is a single-mode fiber or a highly nonlinear fiber.

5. The optical feedback resonance-enhanced forward Brillouin demodulation structure according to claim 1, characterized in that: The second optical coupler (8) has a splitting ratio of 90:10, in which 90% of the light is fed back to the ring cavity and 10% of the light is output to the photodetector (10).

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