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.

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.

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

Pulse generation often requires a stabilized cavity and its corresponding mode structure for initial phase-locking. Contrastingly, modeless cavity-free random lasers provide new possibilities for high quantum efficiency lasing that could potentially be widely tunable spectrally and temporally. Pulse generation in random lasers, however, has remained elusive since the discovery of modeless gain lasing. Here we report coherent pulse generation with modeless random lasers based on the unique polarization selectivity and broadband saturable absorption of monolayer graphene. Simultaneous temporal compression of cavity-free pulses are observed with such a polarization modulation, along with a broadly-tunable pulsewidth across two orders of magnitude down to 900 ps, a broadly-tunable repetition rate across three orders of magnitude up to 3 MHz, and a singly-polarized pulse train at 41 dB extinction ratio, about an order of magnitude larger than conventional pulsed fiber lasers. Moreover, our graphene-based pulse formation also demonstrates robust pulse-to-pulse stability and wide-wavelength operation due to the cavity-less feature. Such a graphene-based architecture not only provides a tunable pulsed random laser for fiber-optic sensing, speckle-free imaging, and laser-material processing, but also a new way for the non-random CW fiber lasers to generate widely tunable and singly-polarized pulses.

Graphene enhanced evanescent field in microfiber multimode interferometer for highly sensitive gas sensing

Graphene based new physics phenomena are leading to a variety of stimulating graphene-based photonic devices. In this study, the enhancement of surface evanescent field by graphene cylindrical cladding is observed, for the first time, by using a graphene-coated microfiber multi-mode interferometer (GMMI). It is found theoretically and experimentally that the light transmitting in the fiber core is efficiently dragged by the graphene, hence significantly enhancing the evanescent fields, and subsequently improving the sensitivity of the hybrid waveguide. The experimental results for gas sensing verified the theoretical prediction, and ultra-high sensitivities of ~0.1 ppm for NH(3) gas detection and ~0.2 ppm for H(2)O vapor detection are achieved, which could be used for trace analysis. The enhancement of surface evanescent field induced by graphene may pave a new way for developing novel graphene-based all-fiber devices with compactness, low cost, and temperature immunity.

Graphene-based D-shaped fiber multicore mode interferometer for chemical gas sensing

In this Letter, a graphene-coated D-shaped fiber (GDF) chemical gas sensor is proposed and demonstrated. Taking advantage of both the graphene-induced evanescent field enhancement and the in-fiber multimode interferometer, the GDF shows very high sensitivity for polar gas molecule adsorptions. An extinction ratio of up to 28 dB within the free spectrum range of ~30  nm in the transmission spectrum is achieved. The maximum sensitivities for NH₃ and H₂O gas detections are ~0.04 and ~0.1  ppm, respectively. A hybrid sensing scheme with such compactness, high sensitivity, and online monitoring capabilities may pave the way for others to explore a series of graphene-based lab-on-fiber devices for biochemical sensing.

Graphene Bragg gratings on microfiber

Graphene Bragg gratings (GBGs) on microfiber are proposed and investigated in this paper. Numerical analysis and simulated results show that the mode distribution, transmission loss, and central wavelength of the GBG are controllable by changing the diameter of the microfiber or the refractive index of graphene. Such type of GBGs with tunability may find important applications in optical fiber communication and sensing as all-fiber in-line devices.