Graphene based widely-tunable and singly-polarized pulse generation with random fiber lasers

Supercontinuum generated in an all-polarization-maintaining random fiber laser structure

We demonstrated a linearly-polarized supercontinuum (SC) directly generated in an all-polarization-maintaining random fiber laser (RFL) structure. Owing to the comparatively high Raman gain of the polarization-maintaining germanium doped fiber (GDF), the spectrum of the output SC shows an enhanced bandwidth and improved spectral flatness compared to the unpolarized counterpart. The output SC has an average output power of 4.43 W with a spectrum covering from 600 nm to 1900nm. The polarization extinction ratio (PER) is measured to be greater than 18 dB from 800 nm to 1700nm at the highest output power level. To the best of our knowledge, this is the first demonstration of a linearly-polarized SC generated directly from a RFL. This work is meaningful to help further expand the bandwidth of SC generated from a RFL and provides a simple and cost-effective method of generating linearly-polarized SC for practical applications.

Mode locking of a coherent random fiber laser with selectable repetition rates

Controlling emission of light in random structures/disordered systems, e.g., implementing mode-locked pulses in a laser system with a random structures/disordered systems, is a complex task. Usually, the generation of laser pulse by mode locking needs a stable fixed-length cavity that determines a specific repetition rate of the mode-locked pulses. Here, mode-locking laser pulses with selectable repetition rates are achieved in a typical one-dimensional disordered laser by passive mode locking. The laser includes disordered reflectors to provide multiple resonant modes associated with different cavity length. The regular pulses with adjustable repetition rates can be generated and selected by a nonlinear polarization rotator and a semiconductor saturable absorber mirror. The proposed work utilizing the advantages of multiple resonances in random lasers could pave a new way for regulating emission of light in the random structures/disordered system. And it displays an effective and realistic technical route to study ultrafast pulses generation and optical soliton dynamics in random structures/disordered systems.

Phosphosilicate fiber-based dual-wavelength random fiber laser with flexible power proportion and high spectral purity

Phosphosilicate fiber has the inherent advantage of generating dual-wavelength output owing to the two Raman gain peaks at the frequency shifts of ∼13.2 THz (silica-related) and 39.9 THz (phosphorus-related), respectively. The frequency shift of 39.9 THz is often adopted to obtain long wavelength laser, while the control of Stokes light at 13.2 THz has attracted much attention currently. In this paper, a dual-wavelength random distributed feedback Raman fiber laser (RDFL) with over 100 nm wavelength interval and continuously tunable power proportion was presented based on phosphosilicate fiber for the first time. Through using the filtered amplified spontaneous emission (ASE) source as the pump source, the spectral purity of the Stokes light could be as high as 99.8%. By tuning two manual variable optical attenuators (VOAs), the power proportion of the silica-related Stokes light could range from ∼0% to 99.0%, and the maximum value is limited by the generation of second order Stokes light. Although the power handling capability of the VOA is merely 2 W, over 23 W total output power of the Stokes light was obtained thanks to the particular power distribution property of RDFL. This experiment demonstrates the potential to achieve a flexible high-power and high-spectral purity dual-wavelength RDFL output.

Weak feedback assisted random fiber laser from 45°-tilted fiber Bragg grating

We have demonstrated the realization of a high-polarization random fiber laser (RFL) output based on the hybrid Raman and Erbium gain with the tailored effect provided by a 45°-tilted fiber Bragg grating (45°-TFBG), revealing an improvement in the polarization extinction ratio (PER) and achieving a PER of ~15.3 dB. The hybrid RFL system incorporating the 45°-TFBG has been systematically characterized. The random lasing wavelength can be fixed under the extremely weak feedback effect of the 45°-TFBG with reflectivity of 0.09%. In addition, numerical simulation has verified that the weak feedback can boost the random lasing emission with fixed wavelength using a power balance model, which is in good accordance with the experiment results.

Random phase-shift Bragg grating-based random fiber laser with a half-open cavity

A random fiber laser based on a 25 mm long random phase-shift fiber Bragg grating with a half-open cavity is experimentally demonstrated in this paper. The random phase-shift grating is fabricated in single-mode fiber with the beam-scanning method. The random feedback and light localization are achieved by introducing 20 phase shifts with random amplitudes along the grating length. Different couplers and different directions are adopted to investigate the output characteristics under multiple conditions. The stable single-wavelength laser operation with the pump threshold of only 25 mW is realized using a 980 nm pump laser. The optical signal-to-noise ratio up to 67 dB and the 3 dB bandwidth of 17.7 pm are obtained at the pump power of 350 mW. It is found that the emitted wavelength varies linearly with the surrounding refractive index within a certain range by immersing the grating in different concentrations of NaCl solutions, which provides a potential method for lasing wavelength tuning and refractive index detection.

Power scalability of linearly polarized random fiber laser through polarization-rotation-based Raman gain manipulation

Random fiber laser based on Raman gain and random distributed feedback has drawn great attention in recent years. One of the most widely-studied fields is to improve the optical efficiency and the output power. However, the power scaling of a random fiber laser is instinctively restricted by the high order Stokes generation. In this manuscript, we propose a simple yet effective method, which employs a homemade all-fiber Lyot filter to manipulate the polarization dependent Raman gain, thus increasing the threshold of the 2nd-order Stokes wave and enhancing the maximum output power of the linearly polarized random fiber laser. Through reliable theoretical analysis, we optimize the design of the wavelength dependent Lyot filter. Moreover, the performance of the filter and the power scaling capability of the linearly polarized random fiber laser are investigated in detail. A proof-of-principle experiment is carried out by inserting the homemade Lyot filter into a half-opened random fiber laser. The experimental results indicate that the 2nd-order Stokes wave can be effectively suppressed, and the maximum output power of the 1st-order Stokes wave is significantly increased with a range of ~50% (from 43.6 to 63.2 W).

Optical Graphene Gas Sensors Based on Microfibers: A Review

Graphene has become a bridge across optoelectronics, mechanics, and bio-chemical sensing due to its unique photoelectric characteristics. Moreover, benefiting from its two-dimensional nature, this atomically thick film with full flexibility has been widely incorporated with optical waveguides such as fibers, realizing novel photonic devices including polarizers, lasers, and sensors. Among the graphene-based optical devices, sensor is one of the most important branch, especially for gas sensing, as rapid progress has been made in both sensing structures and devices in recent years. This article presents a comprehensive and systematic overview of graphene-based microfiber gas sensors regarding many aspects including sensing principles, properties, fabrication, interrogating and implementations.

Flat amplitude and wide multiwavelength Brillouin/erbium fiber laser based on Fresnel reflection in a micro-air cavity design

In this report, we demonstrate a wide multiwavelength Brillouin-erbium fiber laser (MBEFL) with improved flatness that integrates a micro-air cavity. This air-gap introduces a cavity loss to overcome the gain saturation as well as providing efficient pump recycling scheme through Fresnel back-reflection. In addition, the efficient four-wave mixing in the highly nonlinear fiber contributes to the self-flattening of the output spectra. During operation, the optimized pumping values are set at 13 dBm Brillouin power and 600 mW erbium-ytterbium doped fiber amplifier when the air-gap length is fixed at 10 µm. A total of 180 Stokes lines are produced with a channel spacing of 0.08 nm. The flat lasing bandwith is 14 nm that consists of 1557 to 1571 nm wavelengths within 3-dB span. The average optical signal-to-noise ratio is 18 dB, having high peak power of -8 dBm. To our knowledge, this is the best result attained in MBEFLs with respect to the spectral flatness. In fact, the power stability of 0.76 dB order over 45 minute durations merits it applications in optical fiber sensing and communications.

Graphene Q-switched distributed feedback fiber lasers with narrow linewidth approaching the transform limit

A compact all-in-line graphene-based distributed feedback Bragg-grating fiber laser (GDFB-FL) with narrow linewidth of hundreds kHz is demonstrated and investigated in this study. Performing as an optical saturable absorber, graphene oscillates the initially kHz linewidth DFB-FL, and generates high-quality passively Q-switched pulses. Pumped with a 980 nm continuous-wave laser, the Q-switched GDFB-FL observes ~1 μs pulse durations, with pulse energies up to ~10 nJ and approaching the transform limit. The peak power is ~600 times higher than the original DFB-FL laser. By optimizing the cavity design and the graphene material, it is predicted that fast Q-switched pulses with more than MHz repetition rates and sub-100 ns pulse durations are achievable. Such transform-limited Q-switched GDFB-FLs with narrow linewidth of sub-MHz have long coherence length, good tunability, stability, compactness and robustness, with potential impact in optical coherent communications, metrology and sensing.

Incoherently pumped high-power linearly-polarized single-mode random fiber laser: experimental investigations and theoretical prospects

We present a hundred-watt-level linearly-polarized random fiber laser (RFL) pumped by incoherent broadband amplified spontaneous emission (ASE) source and prospect the power scaling potential theoretically. The RFL employs half-opened cavity structure which is composed by a section of 330 m polarization maintained (PM) passive fiber and two PM high reflectivity fiber Bragg gratings. The 2nd order Stokes light centered at 1178 nm reaches the pump limited maximal power of 100.7 W with a full width at half-maximum linewidth of 2.58 nm and polarization extinction ratio of 23.5 dB. The corresponding ultimate quantum efficiency of pump to 2nd order Stokes light is 86.43%. To the best of our knowledge, this is the first demonstration of linearly-polarized high-order RFL with hundred-watt output power. Furthermore, the theoretical investigation indicates that 300 W-level linearly-polarized single-mode 1st order Stokes light can be obtained from incoherently pumped RFL with 100 m PM passive fiber.

Quasi mode-locking of coherent feedback random fiber laser

Mode-locking is a milestone in the history of lasers that allows the generation of short light pulses and stabilization of lasers. This phenomenon is known to occur only in standard ordered lasers for long time and until recently it is found that it also occurs in disordered random lasers formed by nanoscale particles. Here, we report the realization of a so-called quasi mode-locking of coherent feedback random fiber laser which consists of a partially disordered linear cavity formed between a point reflector and a random distributed fiber Bragg grating array with an inserted graphene saturable absorber. We show that multi-groups of regular light pulses/sub-pulses with different repetition frequencies are generated within the quasi mode-locking regime through the so-called collective resonances phenomenon in such a random fiber laser. This work may provide a platform to study mode locking as well as pulse dynamic regulation of random lasing emission of coherent feedback disordered structures and pave the way to the development of novel multi-frequency pulse fiber lasers with potentially wide frequency tuning range.

Powerful linearly-polarized high-order random fiber laser pumped by broadband amplified spontaneous emission source

A great deal of attention has been drawn to Random fiber lasers (RFLs) for their typical features of modeless, cavity-less and low coherence length. However, most previously reported high power RFLs employ narrowband fiber lasers as the pump source, thus inducing the self-pulsing transferring from pump source to output Stokes. In this contribution, linearly-polarized RFL pumped by broadband amplified-spontaneous-emission (ASE) is demonstrated and continuous-wave (CW) high order Stokes can be obtained.With 30.6 W pump injected into the half-opened cavity, 23.51 W the 2^nd order Stokes centered at 1178 nm with a full width at half-maximum linewidth of 1.73 nm and polarization extinction ratio of about 25 dB can be obtained. The standard deviation and peak-vale value of the 2^nd order Stokes light at maximal output power is just 0.47% and 4.10%, which indicates the good power stability. Significantly, the corresponding quantum efficiency of the 1^st and 2^nd order Stokes light is about 87% and 85%, and almost all pump photons are converted into Stokes photons. As far as we know, it is the highest power ever reported from linearly polarized RFL, and further power scaling is available in the case of more powerful pump source and optimization of system parameters.

Narrow-linewidth Q-switched random distributed feedback fiber laser

A narrow-linewidth Q-switched random fiber laser (RFL) based on a half-opened cavity, which is realized by narrow-linewidth fiber Bragg grating (FBG) and a section of 3 km passive fiber, has been proposed and experimentally investigated. The narrow-linewidth lasing is generated by the spectral filtering of three FBGs with linewidth of 1.21 nm, 0.56 nm, and 0.12 nm, respectively. The Q switching of the distributed cavity is achieved by placing an acousto-optical modulator (AOM) between the FBG and the passive fiber. The maximal output powers of the narrow-linewidth RFLs with the three different FBGs are 0.54 W, 0.27 W, and 0.08 W, respectively. Furthermore, the repetition rates of the output pulses are 500 kHz, and the pulse durations are about 500 ns. The corresponding pulse energies are about 1.08 μJ, 0.54 μJ, and 0.16 μJ, accordingly. The linewidth of FBG can influence the output characteristics in full scale. The narrower the FBG, the higher the pump threshold; the lower the output power at the same pump level, the more serious the linewidth broadening; and thus the higher the proportion of the CW-ground exists in the output pulse trains. Thanks to the assistance of the band-pass filter (BPF), the proportion of the CW-ground of narrow-linewidth Q-switched RFL under the relative high-pump-low-output condition can be reduced effectively. The experimental results indicate that it is challenging to demonstrate a narrow-linewidth Q-switched RFL with high quality output. But further power scaling and linewidth narrowing is possible in the case of operating parameters, optimization efforts, and a more powerful pump source. To the best of our knowledge, this is the first demonstration of narrow-linewidth generation in a Q-switched RFL.

Statistical properties of radiation of multiwavelength random DFB fiber laser

In the presented paper, the temporal and statistical properties of a Lyot filter based multiwavelength random DFB fiber laser with a wide flat spectrum, consisting of individual lines, were investigated. It was shown that separate spectral lines forming the laser spectrum have mostly Gaussian statistics and so represent stochastic radiation, but at the same time the entire radiation is not fully stochastic. A simple model, taking into account phenomenological correlations of the lines' initial phases was established. Radiation structure in the experiment and simulation proved to be different, demanding interactions between different lines to be described via a NLSE-based model.

Narrowband random lasing in a Bismuth-doped active fiber

Random fiber lasers operating via the Rayleigh scattering (RS) feedback attract now a great deal of attention as they generate a high-quality unidirectional laser beam with the efficiency and performance comparable and even exceeding those of fiber lasers with conventional cavities. Similar to other random lasers, both amplification and random scattering are distributed here along the laser medium being usually represented by a kilometers-long passive fiber with Raman gain. However, it is hardly possible to utilize normal gain in conventional active fibers as they are usually short and RS is negligible. Here we report on the first demonstration of the RS-based random lasing in an active fiber. This became possible due to the implementation of a new Bi-doped fiber with an increased amplification length and RS coefficient. The realized Bi-fiber random laser generates in a specific spectral region (1.42 μm) exhibiting unique features, in particular, a much narrower linewidth than that in conventional cavity of the same length, in agreement with the developed theory. Lasers of this type have a great potential for applications as Bi-doped fibers with different host compositions enable laser operation in an extremely broad range of wavelengths, 1.15–1.78 μm.