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.

Partially reduced graphene oxide based FRET on fiber-optic interferometer for biochemical detection.

Fluorescent resonance energy transfer (FRET) with naturally exceptional selectivity is a powerful technique and widely used in chemical and biomedical analysis. However, it is still challenging for conventional FRET to perform as a high sensitivity compact sensor. Here we propose a novel 'FRET on Fiber' concept, in which a partially reduced graphene oxide (prGO) film is deposited on a fiber-optic modal interferometer, acting as both the fluorescent quencher for the FRET and the sensitive cladding for optical phase measurement due to refractive index changes in biochemical detection. The target analytes induced fluorescence recovery with good selectivity and optical phase shift with high sensitivity are measured simultaneously. The functionalized prGO film coated on the fiber-optic interferometer shows high sensitivities for the detections of metal ion, dopamine and single-stranded DNA (ssDNA), with detection limits of 1.2 nM, 1.3 µM and 1 pM, respectively. Such a prGO based 'FRET on fiber' configuration, bridging the FRET and the fiber-optic sensing technology, may serve as a platform for the realization of series of integrated 'FRET on Fiber' sensors for on-line environmental, chemical, and biomedical detection, with excellent compactness, high sensitivity, good selectivity and fast response.

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.

Local expression and role of BMP-2/4 in injured spinal cord.

We investigated local changes in BMP-2/4 expression in rat spinal cords 1 week following injury to study the damage effects of BMP-2/4 in spinal cord injury (SCI). Sprague Dawley rats (45, 4 months old) were randomized into three groups comprising 15 rats each: a SHAM group, an SCI without noggin group (SCIO), and an SCI with noggin group (SCID). The SCIO and SCID groups were subjected to spinal cord hemisection, and motor activity was assessed using the BBB score. Expression of BMP-2/4 in each injured spinal cord section was examined by hematoxylin and eosin staining, immunohistochemistry, and western blot. There were no significant differences in BBB scores among the three groups (P > 0.05). Following hemisection, the BBB score in the SHAM group was significantly higher than in the other two groups on the 1st day after modeling (P < 0.05), and the BBB scores in the SCIO and SCID groups were not significantly different (P > 0.05). Seven days after modeling, the BBB score in the SHAM group was significantly higher than in the other two groups (P < 0.05), and the BBB score in the SCID group was obviously higher than in the SCIO group (P < 0.05). The expression of BMP-2/4 was highest in the SCIO group and lowest in the SHAM group (P < 0.05). SCI can cause severe impairment of motor activity in rats. Seven days after SCI, the local expression of BMP-2/4 had obviously increased; noggin can effectively inhibit the expression of BMP-2/4 and reduce impairment.

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 14 mW 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.

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 laser's 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.

Stark effect induced microcavity polariton solitons.

This paper proposes a way of generating polariton solitons (PSs) in a semiconductor microcavity using Stark effect as the trigger mechanism. A Stark pulse performing as the writing beam is used to excite non-resonant fluctuations of polariton, which finally evolves into bright PSs. It is found that a branch of PS solutions versus pump parameters could be found through optimizing parameters of the Stark pulse, and polarization of the generated PS is dependent on the writing beam.

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.

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 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 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.

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 100 km is experimentally demonstrated as an example. FBA significantly enhances the probe pulse signal, especially at the second half of the sensing fiber, with only 6.4 dBm pump power. It is confirmed that its amplification efficiency is much higher than 28.0 dBm 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.

Effectiveness of olfactory ensheathing cell transplantation for treatment of spinal cord injury.

The aim of this study was to determine the effectiveness and safety of transplantation of olfactory ensheathing cells for functional repair of the spinal cord. An olfactory bulb was obtained from a 4- to 5-month-old aborted fetus, and it was digested into single olfactory ensheathing cells and then cultured and purified for 1 to 2 weeks. Under general anesthesia, these single-cell suspensions of olfactory ensheathing cells were injected into the corresponding spinal injury site with 0.45-mm-diameter injections. The American Spinal Injury Association (ASIA) Impairment Scale was used to evaluate spinal function. A total of 15 patients (12 men, 3 women; age range, 18-56 years; mean age, 40) were admitted for obsolete spinal injuries. Spinal functions of the 15 patients were observed and followed postoperatively for a period ranging from 2 weeks to 1 month. All the 15 patients exhibited improvements in spinal function, and the improvement tendencies continued. Twelve patients had obvious spinal function improvement, and three had slight improvement according to the ASIA scale, with an obvious difference between preoperation and postoperation measures (P < 0.05). No fevers, infections, functional deteriorations, or deaths were seen. Thus, transplantation of olfactory ensheathing cells promoted spinal and neurofunctional recovery in patients with malignant spinal injuries, and this therapeutic method was safe.

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.

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.

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.

All-fiber bandwidth-tunable band-rejection filter based on a composite grating induced by CO2 laser pulses.

We propose an all-fiber band-rejection filter with a tunable bandwidth, which is realized by putting a normal long-period fiber grating in series with a rotary long-period fiber grating written in a twisted single-mode fiber by CO(2) laser pulses. Bandwidth tuning is achieved by applying torsion to the composite grating. Our experimental filter shows a bandwidth tuning of approximately 16.3 nm at a rejection level of approximately 15 dB and a polarization-dependent loss lower than approximately 0.9 dB.

Laser-micromachined Fabry-Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index.

We propose and demonstrate a Fabry-Perot (F-P) optical fiber tip sensor for high-resolution refractive-index measurement fabricated by using 157-nm laser micromachining, for the first time to our knowledge. The sensor head consists of a short air F-P cavity near the tip of a single-mode fiber and the fiber tip. The external refractive index is determined according to the maximum fringe contrast of the interference fringes in the reflective spectrum of the sensor. Such a sensor can provide temperature-independent measurement of practically any refractive index larger than that of air and offers a refractive-index resolution of ~4 x 10(-5) in its linear operating range. The experimental data agree well with the theoretical results.

Miniature in-line photonic crystal fiber etalon fabricated by 157 nm laser micromachining.

A miniature in-line fiber-optic Fabry-Perot etalon is fabricated on a photonic crystal fiber (PCF) by using 157 nm laser micromachining for the first time to our knowledge. Experimental results show that such a PCF-based etalon has an excellent fringe visibility of up to approximately 26 dB due to the mirror-finish quality of the two cavity surfaces inside the PCF. This etalon can be used as an ideal sensor for precise strain measurement under high temperature of up to 800 degrees C. It can also offer some other outstanding advantages, such as fast and easy fabrication, high reproducibility, capacity of mass production, low cost, low temperature-strain cross-sensitivity, and high signal-to-noise ratio.