Mengqiu Fan

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.

High power random fiber laser with short cavity length: theoretical and experimental investigations

In this paper, we make a comprehensive study on a highly efficient half-open cavity design for high power random fiber laser (RFL). With the theoretical analysis, we optimize the cavitys fiber length for getting higher output power within the scheme, i.e., shorter fiber length is preferred for efficiently harvesting the first order random lasing at the open end of the cavity. As the verification of the theory, we experimentally demonstrate a high output power (7 W), highly efficient (70% optical conversion efficiency) RFL working at 1140 nm, using 10 W 1090 nm laser as the pump source and only 1 km standard single-mode fiber as the distributed cavity.

Third-order random lasing via Raman gain and Rayleigh feedback within a half-open cavity

Third-order random lasing operating in 1670 nm spectral band is experimentally demonstrated for the first time to the best of our knowledge, with only 2.45 W pump threshold. The lasing cavity is formed by G.652 fiber and fiber loop mirrors (FLMs), while the former acts as the distributed reflector and the latter acts as the point reflector. The G.652 fiber and the FLMs are connected via a multi-band wavelength-division-multiplexer, which ensures each of the three Raman Stokes components generated in the long fiber is routed to one FLM and then reflected back with minimum loss. Unlike existing half-open random lasing cavities using fiber Bragg gratings, the reflection bandwidth of FLMs is wide enough to preserve the intrinsic spectral features of each lasing bands, providing a valuable platform to study the mechanism of high-order random lasing in fibers. Also, the reflection efficiency can be treated as an invariant as the pump power grows, significantly reducing the threshold of high-order random lasing. The stationary model is used to calculate the output power, and the results fit the experimental data well.

Coherent Φ-OTDR based on I/Q demodulation and homodyne detection.

We demonstrate a novel distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR). Both the phase and the amplitude of the Rayleigh scattering (RS) light can be demodulated in real-time. The technique is based on I/Q demodulation and homodyne detection using a 90° optical hybrid. The theoretical analysis is given, and as a proof of the concept, the dynamic strain sensing is experimentally demonstrated, with a sensing range of 12.566 km and a spatial resolution of 10 m.

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.

Random distributed feedback Raman fiber laser with polarized pumping

In this letter, the polarization properties of a random fiber laser operating via Raman gain and random distributed feedback owing to Rayleigh scattering are investigated for the first time. Using polarized pump, the partially polarized generation is obtained with a generation spectrum exhibiting discrete narrow spectral features contrary to the smooth spectrum observed for the depolarized pump. The threshold, output power, degree of polarization and the state of polarization (SOP) of the lasing can be significantly influenced by the SOP of the pump. Fine narrow spectral components are also sensitive to the SOP of the pump wave. Furthermore, we found that random lasings longitudinal power distributions are different in the case of polarized and depolarized pumping that results in considerable reduction of the generation slope efficiency for the polarized radiation. Our results indicate that polarization effects play an important role on the performance of the random fiber laser. This work improves the understanding of the physics of random lasing in fibers and makes a step forward towards the establishment of the vector model of random fiber lasers.

Role of the mirror’s reflectivity in forward-pumped random fiber laser

In this paper, we thoroughly analyze the role of the point reflectors reflectivity in the performance of forward-pumped random fiber laser, in both the long- and short-cavity cases. The results show that the power performance is sensitive to the small reflection added on the pump side of the fiber end, whereas both the power distribution and threshold tend to be stable when the reflectivity reaches a relatively high level (>0.4). Moreover, for the short cavity case (e.g. 500m), the maximum achievable 1st-oder random lasing output can even increase when the reflectivity decreases from 0.9 to 0.01, due to the different lasing power distributions with different reflectivity values. This work reveals a new and unique property of random fiber lasers and provides insights into their design for the applications such as distributed amplification and high power sources.

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.

Polarization-modulated random fiber laser

In this letter, we propose and experimentally demonstrate a polarization-modulated random fiber laser (RFL) for the first time. It is found that the output power of the half-opened RFL with polarized pumping is sensitive to the state of polarization (SOP) of the Stokes light in a fiber loop acting as a mirror. By inserting a polarization switch (PSW) in the loop mirror, the state of the random lasing can be switched between on/off states, thus such a polarization-modulated RFL can generate pulsed output with high extinction ratio.

Long-range and high-precision correlation optical time-domain reflectometry utilizing an all-fiber chaotic source

We propose a long range, high precision optical time domain reflectometry (OTDR) based on an all-fiber supercontinuum source. The source simply consists of a CW pump laser with moderate power and a section of fiber, which has a zero dispersion wavelength near the lasers central wavelength. Spectrum and time domain properties of the source are investigated, showing that the source has great capability in nonlinear optics, such as correlation OTDR due to its ultra-wide-band chaotic behavior, and mm-scale spatial resolution is demonstrated. Then we analyze the key factors limiting the operational range of such an OTDR, e. g., integral Rayleigh backscattering and the fiber loss, which degrades the optical signal to noise ratio at the receiver side, and then the guideline for counter-act such signal fading is discussed. Finally, we experimentally demonstrate a correlation OTDR with 100km sensing range and 8.2cm spatial resolution (1.2 million resolved points), as a verification of theoretical analysis.