Generation of cavity-birefringence-dependent multi-wavelength bright-dark pulse pair in a figure-eight thulium-doped fiber laser

We experimentally demonstrated a stable multi-wavelength bright-dark pulse pair in a mode-locked thulium-doped fiber laser(TDFL).The nonlinear polarization rotation(NPR)and nonlinear optical loop mirror(NOLM)were employed in a figure-eight cavity to allow for multi-wavelength mode-locking operation.By incorporating different lengths of high birefringence polarization-maintaining fiber(PMF),the fiber laser could operate stably in a multi-wavelength emission state.Compared with the absence of the PMF,the birefringence effect caused by PMF resulted in rich multi-wavelength optical spectra and better intensity symmetry and stability of the bright-dark pulse pair.

2-μm mode-locked nanosecond fiber laser based on MoS_2 saturable absorber

We demonstrated a 2-μm passively mode-locked nanosecond fiber laser based on a MoS2 saturable absorber（SA）.Owing to the effect of nonlinear absorption in the MoS2 SA, the pulse width decreased from 64.7 to 13.8 ns with increasing pump power from 1.10 to 1.45 W. The use of a narrow-bandwidth fiber Bragg grating resulted in a central wavelength and 3-dB spectral bandwidth of 2010.16 and 0.15 nm, respectively. Experimental results show that MoS2 is a promising material for a 2-μm mode-locked fiber laser.

Research on the Surface Subsidence Monitoring Technology Based on Fiber Bragg Grating Sensing

In order to monitor the process of surface subsidence caused by mining in real time, we reported two types of fiber Bragg grating （FBG） based sensors. The principles of the FBG-based displacement sensor and the FBG-based micro-seismic sensor were described. The surface subsidence monitoring system based on the FBG sensing technology was designed. Some factual application of using these FBG-based sensors for subsidence monitoring in iron mines was presented.

Research on a New Fiber-Optic Axial Pressure Sensor of Transformer Winding Based on Fiber Bragg Grating

Based on the principle of the fiber Bragg grating, a new type of fiber-optic pressure sensor for axial force measurement of transformer winding is designed, which is designed with the structure of bending plate beam, the optimization of the packaging process, and material of the sensor. Through the calibration experiment to calibrate the sensor, the field test results of the Taikai transformer factory show that the sensitivity of the sensor is 0.133 pm/kPa and the repeatability error is 2.7%FS. The data of the fiber-optic pressure sensor in different positions maintain consistent and repeatable, which can meet the requirement of the real-time monitoring of the axial force of transformer winding.

Research on Optical Fiber Flow Test Method With Non-Intrusion

In the field of oil well logging, real-time monitoring of the fluid flow parameter provides a scientific basis for oil and gas optimization exploration and increase in reservoir recovery, so a non-intrusive flow test method based on turbulent vibration was proposed. The specific length of the sensor fiber wound tightly around the outer wall of the pipe was connected with the optical fiber gratings at both ends, and the sensor fiber and the optical fiber gratings composed the flow sensing unit. The dynamic pressure was generated by the turbulence when fluid flows through the pipe, and the dynamic pressure resulted in the light phase shift of the sensor fiber. The phase information was demodulated by the fiber optic interferometer technology, time division multiplexing technology, and phase generated carrier modulation and demodulation techniques. The quadratic curve relationship between the phase change and flow rate was found by experimental data analysis, and the experiment confirmed the feasibility of the optical fiber flow test method with non-intrusion and achieved the real-time monitoring of the fluid flow.

All-in-fiber amplification and compression of coherent frequency-shifted solitons tunable in the 1800–2000 nm range

We report an all-fiber, all-polarization maintaining(PM) source of widely tunable(1800–2000 nm) ultrashort pulses based on the amplification of coherent self-frequency-shifted solitons generated in a highly nonlinear fiber pumped with an Er-doped fiber laser. The system delivers sub-100 fs pulses with energies up to 8.6 nJ and is built entirely from PM optical fibers, without any free-space optics. The all-fiber alignment-free design significantly increases the suitability of such a source for field deployments.

Generation of sub-100 fs pulses tunable from 1700 to 2100 nm from a compact frequency-shifted Er-fiber laser

We report generation of sub-100 fs pulses tunable from 1700 to 2100 nm via Raman soliton self-frequency shift.The nonlinear shift occurs in a highly nonlinear fiber, which is pumped by an Er-doped fiber laser. The whole system is fully fiberized, without the use of any free-space optics. Thanks to its exceptional simplicity, the setup can be considered as an alternative to mode-locked Tm-and Ho-doped fiber lasers.

A new construction method of LDPC codes for optical transmission systems

Based on the construction method of system- atically constructed Gallager （SCG）（4, k） code, a new improved construction method of low density parity check （LDPC） code is proposed. Compared with the construction method of SCG（4, k） codes improved before, the proposed construction method has some advantages, such as saving storage space and reducing computation complexity in the hardware implementation. And then LDPC（5929, 5624） code with 5.42% redundancy is constructed by the proposed method. The simulation results and analysis show that the constructed LDPC（5929, 5624） code has better error-correction performance, lower redundancy and lower decoding complexity than those of a classic Reed- Solomon （RS）（255, 239） code. Therefore, LDPC（5929, 5624） code, constructed by the proposed construction method on LDPC codes, can better suitable for optical transmission systems.

2μm noise-like mode-locked fiber laser based on nonlinear optical loop mirror

We report a Tm-doped noise-like mode-locked(NLML)pulsed fiber laser with a compact linear cavity which consists of dual nonlinear optical loop mirrors(NOLMs).The design of dual-NOLM shows both exceptional compactness in construction and distinct flexibility.In this laser,mode-locking can be realized through the nonlinear optical loop mirror technique.Stable noise-like mode-locked pulses with spectral bandwidth of 29.18 nm and pulse energy of 46 nJ are generated at a central wavelength of 1999.7 nm.Our results indicate that such a simple and inexpensive structure can pave the way for the development of generating supercontinuum with desirable performance.

A Precision Fiber Bragg Grating Interrogation System Using Long-Wavelength Vertical-Cavity Surface-Emitting Laser

This paper presents the development of a cost-effective precision fiber Bragg grating （FBG） interrogation system using long-wavelength vertical-cavity surface-emitting laser （VCSEL）. Tuning properties of a long-wavelength VCSEL have been studied experimentally. An approximately quadratic dependence of its wavelength on the injection current has been observed. The overall design and key operations of this system including intensity normalization, peak detection, and quadratic curve fitting are introduced in detail. The results show that the system achieves an accuracy of 1.2 pm with a tuning range of 3 nm and a tuning rate of 1 kHz. It is demonstrated that this system is practical and effective by applied in the FBG transformer temperature monitoring.

Fiber Bragg Grating （FBG） Sensors Used in Coal Mines

The fiber Bragg grating （FBG） strain sensors were used for on-line monitoring of the stress variation of the lined wall in the gateway retained along the goaf of No. 3203 coal mining face in Dongtan Mine. The results showed that the FBG strain sensor with the wide measuring range could measure the stress variation accurately during the support process of the gateway retained along the goaf and could provide the basis to further optimize the support structure and to determine the support plan of the gateway retained along the goaf. The FBG micro-seismic sensors were used in Xinglong Mine to detect the micro-seismic signal. The signals were well received and analyzed to determine the location and energy level of the source of the micro-seismic event warning. The FBG sensors and detecting system show a significant potential for micro-seismic detection and geological disasters detection.

Interferometric Distributed Sensing System With Phase Optical Time-Domain Reflectometry

We demonstrate a distributed optical fiber sensing system based on the Michelson interferometer of the phase sensitive optical time domain reflectometer （q0-OTDR） for acoustic measurement. Phase, amplitude, frequency response, and location information can be directly obtained at the same time by using the passive 3 ×3 coupler demodulation. We also set an experiment and successfully restore the acoustic information. Meanwhile, our system has preliminary realized acoustic-phase sensitivity around -150 dB （re rad/μPa） in the experiment.

Ultrabroadband wavelength-swept source based on total mode-locking of an Yb:CaF_(2)laser

We present an ultrabroadband, high-speed wavelength-swept source based on a self-modulated femtosecond oscillator. Photonic crystal fiber was pumped by a mode-locked Yb:CaF2 laser, resulting in a strong spectral broadening from 485 to 1800 nm. The pump laser cavity could be realigned in order to achieve total mode-locking of the longitudinal and transverse TEM00 and TEM01 electromagnetic modes. This led to spatial oscillations of the output beam, which induced modulation of the coupling efficiency to the fiber. Due to the fact that nonlinear spectral broadening was intensity dependent, this mechanism introduced wavelength sweeping at the fiber output. The sweeping rate could be adjusted between 7 and 21.5 MHz by changing the geometry of the pump cavity. By controlling the ratio of the transverse mode amplitudes, we were able to tune the sweeping bandwidth, eventually covering both the 1300 nm and 1700 nm bioimaging transparency windows. When compared with previously demonstrated wavelength-swept sources, our concept offers much broader tunability and higher speed. Moreover,it does not require an additional intensity modulator.

Performance of DRA with different pumping schemes based on coupling equation

After the coupling equations of co-pumping,counter-pumping and bidirectional-pumping distributed Raman amplifiers(DRAs) are theoretically analyzed,the power distributions of these three DRAs are acquired with numerical analysis.The performance factors,such as net gain(NG),amplified spontaneous emissions(ASE) noise,double Rayleigh backscattering(DRB) noise,noise figure(NF) and optical signal noise ratio(OSNR),are discussed in depth.It can be found that the NGs of the three DRAs are almost equal.The counter-pumping scheme will induce higher ASE.And the co-pumping DRA has greater DRB noise,but its NF and OSNR are both better than the others'.Furthermore,the relations between signal power and NG,NF,OSNR are also investigated.It is shown that the NF and OSNR of the co-pumping scheme are superior to those of the other two schemes at small input signal power.The above conclusions can play a very important role in guiding the optimum design of DRA.

Short distributed feedback fiber laser with unidirectional output for sensing applications

A short distributed feedback fiber laser with a nearly unidirectional output is fabricated and tested. The short fiber laser is made of a polarization-dependent phase-shift grating fabricated with a vertically polarized 244-nm ultraviolet （UV） laser. A single π phase-shift is introduced to a 2-cm grating at a specified position by directly moving the phase mask during UV beam scanning. Test results show that the laser has a single polarization longitudinal mode with 2.6-mW pump threshold. The backward-to- forward output power ratio is approximately 30：1. The relative intensity noise is -88 dB/Hz, and the linewidth is approximately 10 kHz at 75-mW 980-nm pumping. The unidirectional output and short dimension of this short fiber laser make it very useful in sensing applications, especially in multiplexed sensing aoolications.

Experimental Study on Dual-Wavelength Distributed Feedback Fiber Laser

Four distributed feedback fiber lasers with dual-wavelength operations were studied experimentally in detail. The laser characteristics of these distributed feedback fiber lasers （DFB-FLs） were investigated. The operation conditions for the dual-wavelength output are analyzed and presented. Through this study, the guidance of the design and fabrication of some dual-wavelength distributed feedback fiber lasers are provided.

Characteristics Research on Self-Amplified Distributed Feedback Fiber Laser

A distributed feedback （DFB） fiber laser with a ratio of the backward to forward output power of 1：100 was composed by a 45-ram-length asymmetrical phase-shifted fiber grating fabricated on the 50-mm erbium-doped photosensitive fiber. Forward output laser was amplified using a certain length of Nufern EDFL-980-Hp erbium-doped fiber to absorb the surplus pump power after the active phase-shifted fiber grating and get population inversion. By using OptiSystem software, the best fiber length of the EDFL to get the highest gain was simulated. In order to keep the amplified laser with the narrow line-width and low noise, a narrow-band light filter consisting of a fiber Bragg grating （FBG） with the same Bragg wavelength as the laser and an optical circulator was used to filter the amplified spontaneous emission （ASE） noise of the out-cavity erbium-doped fiber. The designed laser structure sufficiently utilized the pump power, and a DFB fiber laser with the 32.5-mW output power, l l.5-kHz line width, and -87-dB/Hz relative intensity noise （R1N） at 300 mW of 980 nm pump power was brought out.

Multi-Wavelength Narrow Linewidth Fiber Laser Based on Distributed Feedback Fiber Lasers

A narrow linewidth laser configuration based on distributed feedback fiber lasers （DFB-FL） with eight wavelengths in the international telecommunication union （ITU） grid is presented and realized. In this laser configuration, eight phase-shifted gratings in series are bidirectionally pumped by two 980-nm laser diodes （LDs）. The final laser output with over 10-mW power for each wavelength can be obtained, and the maximum power difference within eight wavelengths is 1.2 dB. The laser configuration with multiple wavelengths and uniform power outputs can be very useful in large scaled optical fiber hydrophone fields.

A novel RS BTC coding scheme for optical communications

A novel Reed Solomon （RS） block turbo code （BTC） coding scheme of RS（63,58）xRS（63,58） for optical communications is proposed. The simulation results show that the net coding gain （NCG） of this scheme at the sixth iteration is more than that of other coding schemes at the third iteration for the bit error rate （BER） of 10~2. Furthermore, the novel RS BTC has shorter component code and rapider encoding and decoding speed. Therefore, the novel RS BTC coding scheme can be better used in high-speed long-haul optical communication systems, and the novel RS BTC can be regarded as a candidate code of the super forward error correction （super-FEC） code~ Moreover, the encoding/decoding design and implementation of the novel RS BTC are also presented.

Single-polarization distributed feedback fiber laser with multiple phase shifts

Distributed feedback fiber lasers with multiple phase shifts are fabricated and investigated. Single longitudinal mode lasers with single polarization are obtained by the structure design with two phase shifts. No obvious differences in laser performance, including pump threshold, slope efficiency, relative intensity noise, spectral linewidth, and polarization state, are observed for the presented distributed feedback fiber laser structures with different phase shift locations.