Distributed Feedback Fiber Laser Research Articles

Design of resistant static pressure probe for DFB fiber laser hydrophone

大深度工作环境下，静水压会引起分布反馈式光纤激光水听器反射中心波长漂出解调系统的复用窗口，使水听器无法解调目标声压信号；不同的静水压也会引起分布反馈式光纤激光水听器灵敏度的变化，影响光纤激光水听器阵列的一致性。基于电力声类比理论，提出带静压补偿的分布反馈式光纤激光水听器探头，建立结构的声压传递函数，分析各结构参数对传递函数的影响，为分布反馈式光纤激光水听器探头频率响应平坦化设计提供理论依据。加工制作了耐静压分布反馈式光纤激光水听器样品进行测试，在0 kHz～10 kHz频率范围内声压灵敏度波动范围不大于±2.4 dB，2.3 MPa内出射激光中心波长漂移量不大于0.06 nm，对深水光纤激光水听器的研究具有指导意义。

Short-cavity random distributed feedback fiber laser with ultra-low threshold

We experimentally demonstrated a random distributed feedback fiber laser (RDFB-FL) with an ultra-low threshold, utilizing a highly nonlinear optical fiber (HNLF). A piece of HNLF was used as the random laser gain medium to resolve the contradiction between the lasing threshold and cavity length. Consisting of a half-open cavity structure, the proposed RDFB-FL had a lasing threshold as low as 0.408 W (record low) with a relatively short cavity length of 850 m. In a ring cavity, the lasing threshold could be further reduced to 0.284 W with a cavity length of 950 m by the combined action of distributed feedback and cavity feedback.

Random distributed feedback fiber lasers: Impact of third-order dispersion

In this paper, one of the most modern random lasers known as random distributed feedback fiber laser is investigated when the third-order dispersion (TOD) is taken into account. The laser characteristics are simulated based on the nonlinear Schrödinger equations (NLSEs) where the power evolution of three interacting waves: the pump, the forward and the backward Stokes waves, are investigated as they propagate down the fiber. The results show that due to TOD, the output characteristics of the laser are changed and particularly, the output power becomes asymmetrical. Moreover, the impacts of fiber nonlinear coefficient and input power on the output power and the output spectrum are studied.

Symmetric Step-Apodized Distributed Feedback Fiber Laser With Improved Efficiency

We propose and demonstrate a symmetric step-apodized distributed feedback (DFB) fiber laser, which has an improved efficiency and a narrow linewidth. This special DFB laser cavity is based on a unique symmetric step-apodized π-phase-shifted fiber Bragg grating (FBG) directly inscribed in a heavily erbium-doped fiber. The symmetric step-apodized phase-shifted grating is equivalent to a weak uniform phase-shifted FBG embedded in a pair of strong FBGs, and this design could increase the effective cavity length of a DFB fiber laser. Moreover, the symmetric step-apodized DFB laser cavity proposed in this work has a phase shift located in the cavity center. It could simplify the design process significantly due to the symmetry in the laser cavity. Experimental results show that the slope efficiency of a symmetric step-apodized DFB fiber laser could be increased to 1.06%, which is significantly higher than that of a uniform DFB fiber laser (i.e., 0.45%) under the same cavity length and pump conditions. In addition, the proposed DFB fiber laser exhibits a high stability. The fluctuations in lasing wavelength and output power were less than 12 pm (corresponding to a frequency shift of 1.49 GHz) and 0.13 dB within 24 h, respectively. Moreover, the full-width at half-maximum linewidth of the symmetric step-apodized DFB fiber laser was ∼2.6 kHz, measured by the delayed self-heterodyne method with a 50-km fiber delayed line. As such, the proposed symmetric step-apodized DFB fiber laser could potentially be used as high-performance light source for fiber-optic sensors or coherent optical communication systems.

Continuous-Wave Pumping Supercontinuum Generation in Random Distributed Feedback Laser Cavity

In this paper, we demonstrated a continuous-wave pumping supercontinuum generation in a random distributed feedback fiber laser with a completely open laser cavity. A broadband wavelength conversion was obtained by pumping a 36-km long TrueWaveREACH fiber in anomalous dispersion regime with a high power continuous-wave Raman laser. The spectral broadening was assisted via nonlinear mechanism such as modulation instability and stimulated Raman scattering. An extended 10-dB flat supercontinuum with 129-nm bandwidth spanning over C-, L-, and U-band wavelengths was generated in the forward direction of lasing cavity under 3.65-W pump power. The super-continuum exhibited excellent bandwidth stability in 60 minutes of lasing operation. A simultaneous generation of random Raman laser operating in the backward direction of cavity was also demonstrated within the same gain fiber. The simple laser cavity presented significant versatility in its generation of novel light sources for both telecommunication and applied science applications.

Demodulation of Polarimetric Fiber Laser Ultrasonic Sensor With Intensity Noise Cancellation

In this paper, the influences of the relative intensity noise, in particular the relaxation oscillation noise, of an erbium distributed feedback (DFB) fiber laser has been investigated theoretically and experimentally for use as a polarimetric ultrasonic sensor. We show that the relaxation oscillation noise and the ultrasonic signal both induce modulation sidebands on the polarization beat frequency of the DFB laser ultrasonic sensor. We present a novel demodulation algorithm for the polarimetric DFB fiber laser ultrasonic sensor, which can effectively eliminate the influences of the intensity noise and slowly varying drift of the polarization beat frequency.

Pico-Strain Resolution Multiplexed Fiber Grating Sensor Array Interrogated With Mode-Locked Laser

A multiplexed fiber grating sensor system based on injection-locking and high-order Pound–Drever–Hall techniques is presented. In the sensor system, two distributed feedback fiber lasers and a mode-locked laser are employed as the sensor array and probe laser, respectively. The weak comb teeth of the mode-locked laser are injection locked to corresponding sensing elements, to suppress the free running noise of the laser sensors and thus interrogate the sensor array. A high strain resolution of 72 p $\boldsymbol {\varepsilon }$ in 100 s and 4 p $\varepsilon$ / ${\surd }$ Hz above 10 Hz is proved. The proposed method allows the direct use of optical frequency comb as the light source in fiber grating sensor system. We believe this achievement is of great significance for fiber grating sensors in both sensing resolution and multiplexing scale.

Realization of Sub-Nano-Strain Static Resolution With Injection-Locking Between Two Fiber Laser Sensors

Fiber Bragg grating (FBG) based sensors have achieved great performance in dynamic strain sensing, while static strain measurement is still very challenging because it requires extra reference for temperature compensation. In this paper, a high-resolution FBG laser sensor system is presented, which is capable of both static and dynamic strain measurements. In the proposed sensor, a distributed feedback fiber laser (DFB-FL) works as the strain sensing element, and a distributed Bragg reflector fiber laser (DBR-FL) works as the reference. The DFB-FL is injection-locked to the DBR-FL with closed loop control using Pound–Drever–Hall (PDH) technique and an acousto-optic frequency shifter. No independent narrow-linewidth probe laser source is required. In experiments, a frequency fluctuation range of 31.6 kHz in 1000 s between the DFB-FL and the DBR-FL is obtained, corresponding to a static strain resolution of 248 pico-strain. Meanwhile, a dynamic resolution of 580 femto-strain at l kHz is achieved. The limitation on strain resolution and dynamic range of the sensor is also discussed. Due to the high strain resolution and moderate cost, the proposed sensor system provides a powerful tool for various applications of strain measurement.

Random Distributed Feedback Fiber Laser Generating Cylindrical Vector Beams

The invention discloses a random distributed feedback fiber laser generating cylindrical vector beams. Because of the existence of random distributed feedback in the laser, the output cylindrical vector beam has the characteristic of low coherence. The laser mainly includes a highly reflective mirror, a mode output module, an amplifier module and a random distributed feedback module. According tothe invention, random distributed feedback is provided by a fiber resonant ring, and a cylindrical vector beam is output by a mode selection coupler. The random distributed feedback fiber laser of theinvention has the advantages of simple structure, high integration degree and low cost, and is suitable for special fields requiring low-coherence cylindrical vector laser sources.

Spatial location of correlations in a random distributed feedback Raman fiber laser.

Nonlinear interactions between different components of multiwavelength radiation are one of the main processes shaping properties of quasi-CW fiber lasers. In random fiber lasers, nonlinear influence may be more complicated, as there are no distinct longitudinal modes in radiation because of the random nature of the feedback. In this Letter, we experimentally characterize internal correlations in the radiation of a multiwavelength random distributed feedback fiber laser. An analysis of Pearson correlation functions allows us to spatially locate the area over the fiber laser length in which correlations are more likely to occur. This, in turn, leads us to the conclusion about the main mechanism of spectral correlations-the relative intensity noise transfer from the pump wave.

Open Cavity Hybrid Raman-Erbium Random Fiber Laser With Common Pump

A symmetrical 80-km open cavity erbium-integrated hybrid random distributed feedback fiber laser (HRFL) was proposed and experimentally demonstrated. A variation of pumping schemes and cavity lengths was first investigated prior to the integration of the EDF. The impact of Raman and EDF hybrid amplification was then investigated through EDF length variation. The proposed scheme used a single common pump to incite both Raman and erbium gain to produce a single peak at a 1567-nm wavelength with maximum OSNR of 62.37 dB. A maximum total output power generation of 1420 mW was achieved with high-slope efficiency of 38%. The proposed hybrid setup has shown improved performance despite using open-ended cavity sustained by only a single pump in contrast to previous more complex hybrid schemes. Prolonged chaotic regime manifesting spontaneous pulse burst was also observed before the stable regime. The simple operation with the high performance of the proposed configuration offers a great potential for long distance or remote access applications such as heavy metals sensing or even for biological hazard sensing.

Beat frequency tuning in dual-polarization distributed feedback fiber laser using side polishing technique.

We propose and demonstrate a method for tuning the beat frequency of a dual-polarization distributed feedback (DFB) fiber laser via fiber side polishing. This process can significantly alter the birefringence in DFB fiber lasers. Beat frequency evolutions in DFB fiber lasers were investigated, and the experimental results showed that the beat frequency tuning was dependent on polished thickness, roughness, and direction. The abrasive paper with a grain size of 1.8 µm was adopted to fine-tune the beat frequency. It was found that the beat frequency of DFB fiber lasers shifted toward higher frequencies with increasing polished thickness. However, the beat frequency shifted toward lower frequencies using a secondary side polishing process in the direction orthogonal to the first polished surface. As a result, the beat frequency of the DFB fiber laser was tuned in a wide frequency range from 475.5 MHz to 2080.4 MHz, which corresponds to a birefringence change of 1.2 × 10-5. Side-polished DFB fiber lasers could provide a novel approach to frequency division multiplexing for a large number of fiber laser sensors.

Theoretical and experimental study of artificially controlled backscattering fiber using femtosecond laser fabrication

An artificially controlled backscattering (ACBS) fiber is theoretically proposed and experimentally verified using a femtosecond (fs) laser micromachining system. The backscattering power of the ACBS fiber per unit length is similarly defined as the one of the single mode fiber (SMF). We have calculated the relative reflecting power per unit length from a fiber with a reflector fabricated on an SMF core at 5576 m by the fs laser beam. The impact of the pulse energy of the fs laser and the reflector position on the performance of the ACBS fiber are then compared via observing the backscattering trace. The results show that the proposed ACBS fiber can enhance the backscattering of the SMF. High backscattering fibers (HBSFs) can be achieved via appropriate selection of fabrication parameters, offering an efficient way to shorten the fiber length and form a random distributed feedback fiber laser.

Ultra-long (290 km) remote interrogation sensor network based on a random distributed feedback fiber laser.

In this work, an interferometric sensor has been interrogated 290 km away from the monitoring station, reaching the longest distance in fiber optic sensing up to date. This has been attained by employing a double-pumped random distributed feedback fiber laser as the light source for a fiber optic low-coherence interferometry scheme. Additionally, the capability of the system to achieve coherence multiplexing for ultra-long range measurements (up to 270 km) has been proved, without presenting crosstalk between the sensors. The use of coherence multiplexing together with a random distributed feedback fiber laser addresses two of the main limitations of long-range sensing setups: their limited multiplexing capability and the need to reach the maximum monitoring distance.

Watt-level narrow-linewidth fibered laser source at 852 nm for FIB application

We realize a 1W all-fibered polarized compact and robust laser source at 852nm for laser cooling of cesium atoms. The architecture is based on the sum-frequency generation of 1540 and 1908nm lasers, realized through a periodically poled lithium niobate waveguide with a conversion efficiency of 40%. A linewidth of 20kHz is achieved with the development of a distributed feedback fiber laser at 1908nm. The operation of this laser source is demonstrated on a focused ion beam (FIB) experiment based on cold cesium atoms.

Decrease in Photosensitivity of Erbium-Doped Fiber Under Tension: Implications for Distributed Feedback Fiber Lasers

Applying strain to a fiber prior to grating inscription is often used to provide some flexibility in the design wavelength for phase mask based grating fabrication systems. Although earlier studies have characterized germanosilicate fibers under strain, an analysis has never been performed on photosensitive erbium-doped fiber (EDF). For this experiment, arrays of 10-mm-long gratings were written in EDF under various strains and exposure conditions. As with studies performed in passive fiber, a decrease in the photosensitivity of the EDF was observed with increasing strain. The reduction in grating strength with strain was characterized and utilized to model the threshold of distributed feedback fiber lasers inscribed at various strains. Multiplexed arrays of fiber lasers were then fabricated to verify the simulations.

Pump distribution effect in dual-wavelength Raman-erbium random distributed feedback fiber laser.

This work demonstrates a simple dual-wavelength random distributed feedback fiber laser via excitation of the Raman-erbium hybrid gain by a single pump source. Lasing wavelengths at 1568 nm and 1595 nm with 48.48 dBm maximum OSNR were generated without the need for physical reflectors. Enhancements were performed using pump power distribution and a seeded feedback to reduce the peak disparity to only 0.16 dB. The long cavity hybrid random laser with its balanced and broadly spaced dual-wavelength output offers immense potential for long distance dual laser applications.

Numerical modeling and optimization of mid-infrared random distributed feedback fiber lasers

For the first time ever, we numerically demonstrated a random distributed feedback fiber laser in the mid-infrared regime by using fluoride fibers pumped at 2.05 µm. The relation of the total loss for the Rayleigh scattering coefficient in fluoride fibers at the 2.05 µm wavelength band was derived theoretically. The numerical model was established to optimize the fiber length and pump power to maximize the first-Stokes output power. A fiber length of 68 m with a maximum available 200 W pump power is recommended to obtain a relatively large output power. The maximum obtained first-Stokes power was 137.6 W, and the conversion efficiency was 71%. In addition, the influences of the reflectance of the fiber Bragg grating on the output properties were also investigated.

Ultrahigh resolution optic fiber strain sensor with a frequency-locked random distributed feedback fiber laser.

This Letter presents an ultrahigh resolution optic fiber strain sensor. A random distributed feedback fiber laser (RDFL) is locked by the proposed frequency-shift Pound-Drever-Hall technique, which tracks the resonant frequency change of a π-phase-shifted fiber Bragg grating. The random distributed feedback for the RDFL gives a Lorentzian envelope over the original laser frequency noise, which can suppress the thermally induced frequency noise. The frequency noise of the laser is reduced from about 100 to 20Hz/√Hz at 1kHz. An ultrahigh dynamic strain resolution of 140 fε/√Hz at 1 kHz is achieved.

Novel fabrication technique for phase-shifted fiber Bragg gratings using a variable-velocity scanning beam and a shielded phase mask.

A new method is proposed and demonstrated for fabricating phase-shifted fiber Bragg gratings (FBGs) using a variable-velocity scanning UV laser beam in combination with a shielded phase mask. The transmission properties of phase-shifted FBGs were analyzed based on coupled-mode theory and a transfer matrix method. The grating is divided into three parts to allow for easier analysis of FBG properties. These segments included a uniform FBG1 and FBG2 which were separated by a shielded section. A novel phase-shifted FBG was fabricated using this method, in which the refractive indices of FBG1 and FBG2 were different. Transmission properties of these phase-shifted FBGs were simulated numerically using MATLAB, and the experimental results and simulated results are in good agreement. In addition to the length and effective refractive index of the shielded section, the phase shift value of a phase-shifted FBG was also found to be dependent on the lengths and effective refractive indices of FBG1 and FBG2. Moreover, we predicted that changing the scanning velocity for fabricating FBG2 would adjust phase shift value, which exhibits a positive linear relationship with the scanning velocity. These results can provide guidelines for fabricating any phase shift value FBGs. This technique is simple, convenient, and may be developed further for use in fabricating novel tunable fiber filters or DFB fiber lasers.