Z. N. Wang

Long-distance fiber-optic point-sensing systems based on random fiber lasers

We find that the random fiber laser (RFL) without point-reflectors is a temperature-insensitive distributed lasing system for the first time. Inspired by such thermal stability, we propose the novel concept of utilizing the RFL to achieve long-distance fiber-optic remote sensing, in which the RFL offers high-fidelity and long-distance transmission for the sensing signal. Two 100 km fiber Bragg grating (FBG) point-sensing schemes based on RFLs are experimentally demonstrated using the first-order and the second-order random lasing, respectively, to verify the concept. Each sensing scheme can achieve >20 dB optical signal-to-noise ratio (OSNR) over 100 km distance. It is found that the second-order random lasing scheme has much better OSNR than that of the first-order random lasing scheme due to enhanced lasing efficiency, by incorporating a 1455 nm FBG into the lasing cavity.

Ultra-long phase-sensitive OTDR with hybrid distributed amplification

A phase-sensitive optical time-domain reflectometry (Φ-OTDR) with 175 km sensing range and 25 m spatial resolution is demonstrated, using the combination of co-pumping second-order Raman amplification based on random fiber lasing, counter-pumping first-order Raman amplification, and counter-pumping Brillouin amplification. With elaborate arrangements, each pumping scheme is responsible for the signal amplification in one particular segment of all three. To the best of our knowledge, this is the first time that distributed vibration sensing is realized over such a long distance without inserting repeaters. The novel hybrid amplification scheme in this work can also be incorporated in other fiber-optic sensing systems for extension of sensing distance.

Recent advances in fundamentals and applications of random fiber lasers

Random fiber lasers blend together attractive features of traditional random lasers, such as low cost and simplicity of fabrication, with high-performance characteristics of conventional fiber lasers, such as good directionality and high efficiency. Low coherence of random lasers is important for speckle-free imaging applications. The random fiber laser with distributed feedback proposed in 2010 led to a quickly developing class of light sources that utilize inherent optical fiber disorder in the form of the Rayleigh scattering and distributed Raman gain. The random fiber laser is an interesting and practically important example of a photonic device based on exploitation of optical medium disorder. We provide an overview of recent advances in this field, including high-power and high-efficiency generation, spectral and statistical properties of random fiber lasers, nonlinear kinetic theory of such systems, and emerging applications in telecommunications and distributed sensing.

Phase-sensitive optical time-domain reflectometry with Brillouin amplification

We propose a phase-sensitive optical time-domain reflectometry (Φ-OTDR) scheme with counterpumping fiber Brillouin amplification (FBA). High-sensitivity perturbation detection over 100xa0km is experimentally demonstrated as an example. FBA significantly enhances the probe pulse signal, especially at the second half of the sensing fiber, with only 6.4xa0dBm pump power. It is confirmed that its amplification efficiency is much higher than 28.0xa0dBm counterpumping fiber Raman amplification. The FBA Φ-OTDR scheme demonstrated in this work can also be incorporated into other distributed fiber-optic sensing systems for extension of sensing distance or enhancement of sensing signal level.

Random fiber laser formed by mixing dispersion compensated fiber and single mode fiber

Taking advantage of relatively strong Rayleigh scattering and Raman gain of dispersion compensated fiber (DCF), three configurations to form efficient random fiber lasers (RFL) are proposed in this paper. Compared with the reported RFL formed by single-mode fiber (SMF) solely, lasing threshold and length of the proposed RFL are effectively reduced through combination of DCF and SMF. In addition, FBGs with central wavelengths at the 1st and 2nd -order Raman Stokes wavelengths are also added to the hybrid SMF/DCF cavity to further reduce the lasing threshold, leading to realization of a new kind of 2nd-order RFL.

Broadband flat-amplitude multiwavelength Brillouin-Raman fiber laser with spectral reshaping by Rayleigh scattering

In this letter, we propose a novel configuration for generating multiwavelength Brillouin-Raman fiber laser (MBRFL). The spectral reshaping effect introduced by Rayleigh scattering in a 50 km single mode fiber unifies the generated Brillouin comb in terms of both power level and linewidth. As a consequence, we are able to obtain a 40 nm flat-amplitude MBRFL with wide bandwidth from 1557 nm to 1597 nm covering >500 Stokes lines. This is, to the best of our knowledge, the widest flat-amplitude bandwidth of MBRFL with uniform Stokes combs using just a single Raman pump laser. The channel-spacing is 0.08 nm and the measured OSNR is higher than 12.5 dB. We also demonstrate that the output spectrum of the MBRFL is nearly unaffected over 14 dB range of Brillouin pumping power.

Hybrid lasing in an ultra-long ring fiber laser

In this paper, we reported the realization of an ultra-long ring fiber laser (RFL) with hybrid emission related to both random lasing and cavity resonance. Compared with a linear random fiber laser (LRFL), the Rayleigh scattering (RS) inducting distributed feedback effect and the cavity inducting resonance effect exist simultaneously in the laser, which reduces the lasing threshold considerably and provides a hybrid way to form random lasing (RL). The laser output can be purely modeless RL when pump power is high enough. It is also discovered that the laser is insensitive to temperature variation and mechanical disturbance, this is unique and quite different from conventional RFLs which are environmentally unstable due to existence of the cavity modes.

Distributed Raman amplification using ultra-long fiber laser with a ring cavity: characteristics and sensing application

Distributed Raman amplification (DRA) based on ultra-long fiber laser (UL-FL) pumping with a ring cavity is promising for repeaterless transmission and sensing. In this work, the characteristics (including gain, nonlinear impairment and noise figure) for forward and backward pumping of the ring-cavity based DRA scheme are fully investigated. Furthermore, as a typical application of the proposed configuration, ultra-long-distance distributed sensing with Brillouin optical time-domain analysis (BOTDA) over 142.2 km fiber with 5m spatial resolution and ± 1.5 °C temperature uncertainty is achieved, without any repeater. The key point for the significant performance improvement is the system could offer both of uniform gain distribution and considerably suppressed pump-probe relative intensity noise (RIN) transfer, by optimized design of system structure and parameters.

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 900u2009ps, a broadly-tunable repetition rate across three orders of magnitude up to 3u2009MHz, and a singly-polarized pulse train at 41u2009dB 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.

All optical mode controllable Er-doped random fiber laser with distributed Bragg gratings

An all-optical method to control the lasing modes of Er-doped random fiber lasers (RFLs) is proposed and demonstrated. In the RFL, an Er-doped fiber (EDF) recoded with randomly separated fiber Bragg gratings (FBG) is used as the gain medium and randomly distributed reflectors, as well as the controllable element. By combining random feedback of the FBG array and Fresnel feedback of a cleaved fiber end, multi-mode coherent random lasing is obtained with a threshold of 14xa0mW and power efficiency of 14.4%. Moreover, a laterally-injected control light is used to induce local gain perturbation, providing additional gain for certain random resonance modes. As a result, active mode selection of the RFL is realized by changing locations of the laser cavity that is exposed to the control light.