Distributed Feedback Fiber Laser Research Articles

Study on vibration signals identification method for pipeline leakage detection based on deep learning technology

For natural gas pipeline leakage systems based on distributed feedback fiber laser vibration sensors (DFB-FL), a novel neural network model (mCNN-LFLBs) integrating multi-scale convolutional neural networks (mCNN) with local feature extraction blocks (LFLBs) is proposed in this study. To address the problem of feature loss in the traditional signal image transfer process, the one-dimensional original signal is directly used as the input of the model, effectively maintaining the time-domain correlation of the leaked signal. The time-domain correlation of the leakage signal is preserved. To address the problem that traditional pattern recognition algorithms have a weak ability to extract deep features and local features, mCNN is established by this model, which realizes the use of convolution kernels of different scales to extract features of different frequency bands in one-dimensional signals. Additionally, LFLBs are introduced, which enhances the local expression ability of the model, further improves the feature mining quality of the model, and better presents the deep features of leaked signals. Experimental results demonstrate that the mCNN-LFLBs model has a classification accuracy of up to 98.6%, with a testing response time of approximately 10 ms, and outstanding performance in terms of convergence speed. These results highlight the potential practical application of the mCNN-LFLBs model in natural gas leak detection.

Construction of trace hydrogen detection system based on distributed feedback fiber laser

Construction of trace hydrogen detection system based on distributed feedback fiber laser

Implementation of Er-doped random fiber laser self-mixing sensor with ultra-limit sensitivity

This study first demonstrates that the random distributed feedback fiber laser (RDFL) can be implemented for sensing detection by using the self-mixing effect as a sensing mechanism. By constructing a compact self-mixing velocimeter based on Er-doped RDFL with the integration of a laser, sensing element, and transmission platform, we successfully measured the minimum detectable feedback intensity of 38.65 fW for the velocity signal, corresponding to 0.55 photons per Doppler cycle, exhibiting ultra-high sensitivity dynamics characteristics. In addition, the velocity measurement of a non-cooperative target at a single-channel distance of 100 km is accomplished because of the natural feature of long-distance transmission for the random distributed feedback fiber lasers, which greatly improves the ultra-long detection range in the field of self-mixing sensing. The proposed sensing scheme not only unveils a fresh perspective on the exploration of random fiber laser sensing but also showcases its diverse and wide-ranging applications within the realm of remote sensing measurements.

Study on the mechanism of fiber-optic hot-wire sensing based on DFB-FL thermally induced chirp effect

In this paper, we propose an intensity demodulated fiber-optic hot-wire sensing method based on the thermally induced chirp effect of distributed feedback fiber lasers. The cobalt-doped fiber is fused at the rear end of the erbium-doped distributed feedback fiber laser to constitute a hot fiber wire with a gradient temperature distribution. The cobalt-doped fiber is heated up under the excitation of the residual pump light. Then the thermally chirp of the erbium-doped phase-shifted fiber grating brings out the change of the lasing threshold condition of the distributed feedback cavity and a significant reduction of the output power. When this structure is further altered by thermal induced chirp under external thermal field, the lasing output power will be further changed. The experimental results of the dynamic fluid thermal field and the static thermal field shows that, the fiber-optic hot-wire structure can sensitively detect the weak airflow and the thermal field of the environment by monitoring the laser power.

Near infrared ammonia sensing system based on on‐axis cavity‐enhanced absorption spectroscopy using board‐level mode‐locked circuit

AbstractWe proposed a near‐infrared gas sensing system based on mode‐locked cavity‐enhanced absorption spectroscopy. A distributed feedback fiber laser (DFB‐FL) with a central wavelength of 1550 nm was employed as the light source, coupling with a Fabry–Perot (F‐P) cavity with an effective optical path length of 42.7 m served as the gas cell. By utilizing a homemade mode‐locked electric circuit and Pound–Drever–Hall (PDH) technique, the mode‐locking between the DFB‐FL and F‐P cavity was achieved with long‐term fluctuation of 3.2%. System performance was verified using ammonia as the analyte gas with a target line at 1550.17 nm. Allan variance analysis revealed that the minimum detection limit was 15.8 ppm with an averaging time of 216 s, corresponding to an absorption coefficient of 2.3 × 10−7 cm−1.

Direct air–water communication by using an optical-acoustic method

The Air-Water Cross-Medium Communication (AWCMC) is significant for ocean exploration. This work presents an approach to direct AWCMC based on the optoacoustic conversion technique, which utilizes a pulsed laser and a distributed feedback fiber laser (DFB-FL) hydrophone. On-off keying (OOK) is used to modulate the pulsed laser, and ASCII is selected as the signal coding format. The light signal after modulation is transmitted to the air–water interface, where it is converted into underwater sound signals by the optoacoustic effect. Next, the underwater acoustic signal containing the encoded information is detected and demodulated by the DFB-FL hydrophone, which proves the feasibility of the proposed method used for the AWCMC. The experimental results indicate that the ASCII-encoded OOK signals with varying repetition frequencies can be generated by modulating the laser. Then, the performance of laser-generated sound (LGS) communication is evaluated by measuring the data rate and bit error rate (BER) with the repetition frequency in the range of 1–500 Hz. It is found that both the data rate and BER exhibit a positive correlation with the repetition frequency of the pulsed laser. Therefore, when optoacoustic technology is applied to the AWCMC, optimization design is required for the indicators of data rate and BER.

Monolithic 2-µm single-frequency linearly-polarized gain-switched distributed feedback fiber laser by femtosecond laser direct-writing.

We report a single-frequency, linearly polarized gain-switched, distributed feedback (DFB), 2-µm thulium doped silica fiber laser (TDFL), with an effective cavity length of 2.5 mm. The cavity is based on a heavily thulium doped non-polarization-maintaining silica fiber and composed of a π-phase-shifted fiber Bragg grating (FBG) with a total FBG length of 35 mm. The DFB FBG was written by femtosecond-laser point-by-point (PbP) method. In-band pumping scheme is chosen with a 1550 nm nanosecond pulsed erbium-doped silica fiber laser pump. Single-longitudinal, linearly polarized, gain-switched TDFL at 2002 nm, with a recorded shortest pulse duration of 4.7 ns, a repetition rate of 20 kHz, a maximum peak power of 170 W, and single pulse energy of 0.8 µJ, has been obtained, benefitting from the ultrashort DFB cavity made by the femtosecond laser direct-writing method.

Single-frequency pulsed fiber laser based on an electro-optic modulator with injection seeding technique.

A single-frequency linearly polarization pulsed fiber laser based on an electro-optic modulator with injection seeding technique is demonstrated. The single-frequency performance of the fiber ring-cavity laser is guaranteed by the seed source, which is a distributed-feedback fiber laser based on the π-phase-shifted fiber Bragg grating. The electro-optic modulator triggers active Q-switching of the laser for pulse generation. The devices used in the fiber laser are all polarization-maintaining to ensure linear polarization laser output. Through parameter optimization, the laser generates a single-frequency linearly polarization pulsed laser with a central wavelength of 1064.22nm, linewidth of 35MHz, and polarization extinction ratio of better than 40dB. This type of fiber laser can be applied in lidar, beam combining, nonlinear frequency conversion, and other fields.

Switchable and tunable multi-wavelength random distributed feedback fiber laser based on parallel Lyot filter

A switchable and tunable multi-wavelength random distributed feedback fiber laser based on a parallel Lyot filter is designed, and the transmission characteristics of the filter are deduced in detail by using the transmission matrix theory. The randomly distributed backward Rayleigh scattering generated in single-mode fiber (SMF) serves as a feedback mechanism. The number of output wavelength increases linearly with the increase of pump power. By adjusting the polarization controller (PC) to control the polarization state of the beam into the polarization maintaining fiber (PMF), laser channels with wavelength interval of 0.88 nm or 0.46 nm can be output. Where the wavelength interval depends on the wavelength phase difference caused by the corresponding branch PMF. The intensity-dependent loss (IDL) caused by the nonlinear optical loop mirror (NOLM) in the experimental structure is not only used to suppress the intense mode competition caused by EDF, but also to control the spectral range of the output wavelength to achieve tunable laser output. When the wavelength interval is 0.88 nm, the wavelength tuning ranges of the three and four laser channels are 4.4 nm and 3.7 nm, respectively. When the wavelength interval is 0.46 nm, the wavelength tuning ranges of the three and four laser channels are 3.7 nm and 4 nm, respectively. Subsequently, the influence of using different lengths of SMF on the output wavelength is analyzed. It is found that longer SMF can increase the linewidth of the laser channel, and the side mode suppression ratio (SMSR) will be improved to 39 dB. In addition, the output power fluctuation of the laser is less than 0.32 dB within 30 min, and the power amplitude and SMSR are stable at −4.47dBm and 34.5 dB, respectively, indicating that the laser can achieve flat and stable multi-wavelength output at room temperature.

Sub-kHz-linewidth laser generation by self-injection locked distributed feedback fiber laser

We presented an integrated all-fiber sub-kHz-linewidth distributed feedback fiber laser (DFB-FL) assisted by self- injection locking. A π phase-shifted fiber Bragg grating (π-FBG) was employed to achieve single longitudinal mode operation, which was UV-inscribed in Er/Yb co-doped fiber with super-Gaussian apodization profile. The experiment results have shown that the apodized π-FBG based laser cavity has got the higher slope efficiency and the narrower linewidth, compared with the normal grating, which are consistent with the simulation one very well. To further compress the linewidth, we have employed the self-injection locking mechanism, which the linewidth of output laser could reach to around 350 Hz with 10-m delay fiber. After integrated encapsulation, the relative intensity noise (RIN) peak of the DFB-FL was reduced to below −125 dB/Hz. The beat frequency fluctuation was less than 15 MHz within 1000 s and power fluctuation was less than 1% in 7 h.

Characterization of optical phase-locked two distributed-feedback fiber lasers for 87Rb atom interferometry

We report the optical phase-locking of two 1560 nm distributed-feedback (DFB) fiber lasers converted to 780 nm with a frequency difference of 6.834 GHz for 87Rb. The fiber laser exhibits a narrow linewidth and low-intensity noise, which are advantageous for a Raman pulse laser in an atomic gravimeter. However, the piezoelectric actuator of the DFB laser causes a low feedback rate in an optical phase-locking loop (OPLL). To overcome this limitation, we used an acousto-optic modulator as an additional actuator to increase the modulation bandwidth of the OPLL and achieved a residual phase noise of approximately -127 dBrad2/Hz in the offset frequency range from 10 Hz to 1 kHz. The sensitivity limit estimated using the sensitivity function and experimental parameters of 87Rb atomic gravimeter shows that our OPLL system can be used in state-of-the-art gravimeters.

Research on Very-Low-Frequency Hydroacoustic Acoustic Velocity Sensor Based on DFB Fiber Laser

We have conducted a study on a very-low-frequency acoustic-velocity sensor which is based on a cantilever of distributed-feedback (DFB) fiber laser immersed in castor oil. A mathematical model of the frequency dependent response of the proposed sensor to the acoustic pressure signal influenced by the fluid viscosity is established. We have fabricated the proposed sensor and conducted experimental measurements in the standing wave tube. The results show that the sensor has an average phase sensitivity of −179.5 dB (0 dB = 1 rad/μPa) with ±1.45 dB fluctuation over the frequency range of 20–38 Hz. It has good cosine directivity with a directivity index of 32.5 dB and axial maximum asymmetry of 0.4 dB. The sensor presents promising applications for detecting very-low-frequency underwater acoustic signals.

Extremely Efficient DFB Lasers with Flat-Top Intra-Cavity Power Distribution in Highly Erbium-Doped Fibers.

High-performance erbium-doped DFB fiber lasers are presently required for several sensing applications, whilst the current efficiency record is only a few percent. Additionally, a flat-top intra-cavity power distribution that is not provided in traditional DFB lasers is preferred. Moreover, cavity lengths of <20 cm are attractive for fabrication and packaging. These goals can be achieved using highly erbium-doped fiber (i.e., 110 dB/m absorption at 1530 nm), providing high gain with proper engineering of coupling coefficients. In this paper, for a given background fiber loss, first the optimum intra-cavity signal powers for various pump powers are numerically calculated. Then, for a fully unidirectional laser, optimum coupling profiles are determined. Design diagrams, including contour maps for optimum cavity lengths, maximum output powers, maximum intra-cavity signal powers, and quality factors considering various pump powers and background fiber losses, are presented. The laser pump and intra-cavity signal distribution are also calculated for a realistic, feasible modified coupling profile considering a strong unidirectionality. The DFB laser is finally simulated using generalized coupled-mode equations for such modified profiles. The efficiency of more than 22% can be realized, which is the highest reported for DFB lasers based only on erbium-doped fiber.

Spatial Hole Burning in Distributed-Feedback Fiber Laser and its Reduction by Grating Apodization

Spatial Hole Burning in Distributed-Feedback Fiber Laser and its Reduction by Grating Apodization

Research Progress on Packaging and Application of Distributed Feedback Fiber Laser Hydrophone Probe

Research Progress on Packaging and Application of Distributed Feedback Fiber Laser Hydrophone Probe

Improved demodulation method for fibre optic sensors applied in transformer partial discharge detection

AbstractThe 2 × 2 couplers, as a demodulation core of the fibre optic sensing system, have been extensively employed in fibre optic sensors for partial discharge (PD) detection of power equipment. However, the unstable demodulation sensitivity and low signal‐to‐noise ratio (SNR) of such a system are still unsatisfactory in engineering. This work uses a distributed feedback fibre laser as the sensor and employs a common asymmetric 2 × 3 coupler instead of a 2 × 2 coupler as the demodulation core to design a PD detection system. Then, we propose a new demodulation method for PD acoustic emission signals based on the Naval Postgraduate School method. On the PD detection platform, the performance of the conventional detection system and the proposed system is compared. The experimental results show that the proposed system has better reliability and can identify the PD signals in all instances. Furthermore, the average SNR of the new demodulation method is 6.11 dB higher than the traditional 2 × 2 coupler system and can be extended to most PD detection systems using fibre optic sensors.

Smart composite using fibre optic sensors for fluid flow characterization and temperature measurement

The paper presents the results obtained in simulation of how air flow type over a wing surface, laminar or turbid, is detected using a Distributed Feedback Fibre Laser (DFB-FL) sensor or a Long Period Grating Fibre Sensor (LPGFS). A comparison of the two sensors is made. The air flow type over a wing surface is important for improving the “green flight” to a large as possible extent. The “green flight” with a reduced fuel consumption is assured by laminar air flow. The DFB-FL or LPGFS constitutes the feedback loop of the aircraft automatization having the optimisation of wing “attack angle” as a main parameter. The DFB-FL or LPGFS can be used for simultaneous system temperature measurements. The simulation results are compared with experimental ones and a good agreement between the two kinds of data being observed.

Extreme Narrowing of the Distributed Feedback Fiber Laser Linewidth Due to the Rayleigh Backscattering in a Single-Mode Fiber: Model and Experimental Test

We present our theoretical model and experimental results on the extreme linewidth narrowing of an erbium-doped fiber distributed feedback (DFB) laser due to an additional random DFB via Rayleigh backscattering in a long single-mode fiber connected to the laser. The relative narrowing of instantaneous (&lt;1 μs) linewidth in such a hybrid cavity configuration predicted by the theoretical model may exceed six orders of magnitude, whereas the linewidth estimated from the phase noise measurements narrows by four orders of magnitude from 15 Hz to 10–3 Hz in the hybrid configuration, with the lower limit defined by the background electrical noise. Significant narrowing was also observed for the long-term ≥10 μs) linewidth directly measured by the self-delay heterodyne technique: the values for the DFB laser and hybrid configuration amount to 6 kHz and 160 Hz, respectively.

Low-spatial coherence vortex beam generation by random distributed feedback fibre laser

Vortex beams with low spatial coherence have great potential in applications such as imaging, medical diagnosis, optical communication, and manipulation. In this study, we experimentally demonstrated a radially polarised vortex beam with extremely low spatial coherence using a random distributed feedback fibre laser (RDFB-FL). The results show that the spatial coherence of vortex beams can be reduced in an RDFB-FL and further reduced by introducing radial polarisation. In addition, the evolution of the spatial coherence of a RDFB-FL-generated vortex beam with varying pump power was clearly revealed for the first time. The spatial coherence first decreases sharply up to a point as the pump power increases and then remains constant, and this behaviour is quite different from that of vortex beams generated by conventional fibre lasers. The present study provides an effective method for achieving a low-spatial coherence vortex beam source for application in telecommunications and imaging systems.

Low-Noise Frequency and Phase Locking of Yb-Doped Fiber Ring Cavity via Cavity Modulation

The control bandwidth of the injection-locked system utilizing a DFB fiber laser and fiber ring cavity is analyzed to estimate system stability. Within the bandwidth of the injection lock, an electrical feedback control loop is adopted to better suppress the phase noise of the fiber ring cavity. The entire system operates in low noise conditions of lower than −60 dBc/Hz from 1 Hz to 100 kHz. The output power of the fiber ring cavity is amplified stably to 0.346 W with a polarization extinction ratio of 31 dB based on an all-polarization-maintaining ring structure. The output laser from the single-mode fiber has the near diffraction-limited beam quality for wide applications.