Evolution of modes in double-clad Raman fiber amplifier

Stimulated Raman scattering in a double cladding optical fiber is studied with a continuous wave laser used as a pump source. Under various launch conditions, pump modes are differently excited. Considering the mode coupling effect among the pump modes, the evolution of the power in the Stokes modes is studied. The results show that the scattered waves （the Stokes waves） in the fiber core with 9%tm diameter and 0.14 NA could propagate predominantly in the fundamental mode of the fiber by carefully adjusting the pump light launching conditions.

Acoustic Response and Micro-Damage Mechanism of Fiber Composite Materials under Mode-Ⅱ Delamination

Realizing the accurate characterization for the dynamic damage process is a great challenge. Here we carry out testing simultaneously for dynamic monitoring and acoustic emission （AE） statistical analysis towards fiber composites under mode-Ⅱ delamination damage. The load curve, AE relative energy, amplitude distribution, and amplitude spectrum are obtained and the delamination damage mechanism of the composites is investigated by the microscopic observation of a fractured specimen. The results show that the micro-damage accumulation around the crack tip region has a great effect on the evolutionary process of delamination. AE characteristics and amplitude spectrum represent the damage and the physical mechanism originating from the hierarchical microstructure. Our finding provides a novel aud feasible strategy to simultaneously evaluate the dynamic response and micro-damage mechanism for fiber composites.

Analysis of Radio over Fiber system for mitigating four-wave mixing effect

In this paper,an efficient 8-channel 32Gbps RoF(Radio over Fiber)system incorporating Bessel Filter(8/32 RoFBF)has been demonstrated to reduce the impact of non-linear transmission effects,specifically Four-Wave Mixing(FWM).The simulation results indicate that the proposed 8/32 RoF-BF system provides an optimum result w.r.t.channel spacing(75 GHz),input source power(0 dBm)and number of input channels(8).In comparison with the existing RoF system,the proposed 8/32 RoF-BF system has been validated analytically and it is found that the performance of the proposed system is in close proximity particularly in FWM sideband power reduction of the order of 4 dBm for the 8-channel 32Gbps RoF system.

Optical Pulse Compression Based on High-doped Erbium Fiber Amplifier and Standard Single-mode Fiber

Proposed is a novel optical pulse compression technique based on high-doped erbium fiber amplifier and standard single-mode fiber(SMF). We used the amplifier with the erbium ion concentration of 6.3×10-3 to amplify a hyperbolic secant pulse from a regeneratively mode-locked fiber laser. The central wavelength, pulsewidth and peak power of the pulse are 1 550 nm, 12.5 ps and 3 mW, respectively. Then the amplified pulse with peak power level corresponding to a higher-order soliton is compressed when it propagates through a 3-km-long single-mode fiber. Studied are the compressed pulses under different pump powers and fiber lengths. The results show that it can get a narrower pulse, and solve the difficulty that pulses at low power can not be compressed directly in the fiber. And the construct is compact.

Experimental investigation on flow modes of electrospinning

Electrospinning experiments are performed by using a set of experimental apparatus, a stroboscopic system is adopted for capturing instantaneous images of the cone- jet configuration. The cone and the jet of aqueous solutions of polyethylene oxide （PEO） are formed from an orifice of a capillary tube under the electric field. The viscoelastic con- stitutive relationship of the PEO solution is measured and discussed. The phenomena owing to the jet instability are described, five flow modes and corresponding structures are obtained with variations of the fluid flow rate Q, the electric potential U and the distance h from the orifice of the cap- illary tube to the collector. The flow modes of the cone-jet configuration involves the steady bending mode, the rotat- ing bending mode, the swinging rotating mode, the blurring bending mode and the branching mode. Regimes in the Q-U plane of the flow modes are also obtained. These results may provide the fundamentals to predict the operating conditions expected in practical applications.

RESEARCH ON TRANSVERSE STRESS CHARACTER OF SINGLE MODE OPTICAL FIBER

This paperdetails experimental work done to quantify stress measurements made optically utilizing ordinary single mode optical fibers. Strain-induced changes of birefringence for ordinary single mode optical fiber responses are characterized against standard stress measurements in a well understand configuration. The experimental scheme for this work and the results are presented in detaial. In this paper, POssible applications for this transverse stress character of single mode fibers are also proposed.

Derivation of General Formula for Coupling Loss of Single-mode Fiber Collimators with Gradient-index Rod Lenses

Gradient-index rod lens (GRIN-lens) whose pitch is ordinary value with bevel ferrule coupling system is analyzed, an equivalent method which can be used to analyze this system is put forward, and a general formula for determining the coupling loss with axes mismatch, radial mismatch, and angular mismatch is derived by use of the Gaussian field approximation and mode-field coupling theory. The experimental results are in good agreement with the theory prediction. It indicates that these formulas are suitable to analyze the gradient-index rod lens coupling system with pigtail fiber.

Temperature Compensated Solution Concentration Measurement Based on a Cascaded SMS/LPFG Fiber Structure

In this paper, a hybrid optical fiber structure for solution concentration measurement with the temperature compensation is proposed. The structure consists of long period fiber grating (LPFG) and single mode-multimode-single mode (SMS) fiber structures. The sensing mechanism of the device is studied and verified by experiments. LPFG is sensitive to solution concentration and is affected by temperature crosstalk. SMS structure is not affected by solution concentration, but sensitive to ambient temperature. It can be used as a temperature compensation system. The sensitivity coefficients of LPFG and SMS on temperature and concentration were measured experimentally, and a dual-wavelength matrix was established to realize simultaneous measurement of solution temperature and concentration.

Design of PANDA ring-core fiber with 10 polarization-maintaining modes

We present a polarization-maintaining PANDA ring-core fiber(PM-PRCF) characterized by the combination of a ring-core structure with two stress-applying rods. This special fiber design separates the adjacent modes and avoids the cutoff of the higher-order modes, which is a common problem in elliptical core polarization-maintaining few-mode fibers. Using a high-contrast index ring and stress-induced birefringence, the PM-PRCF features support for 10 vector modes, with effective refractive index separations from their adjacent modes >10^(-4).Broadband performance is investigated subsequently over a wide wavelength range from 1500 to 1630 nm.The proposed fiber is targeted at applications in space-division multiplexing while eliminating the complex multiple-input multiple-output signal processing.

Guided Modes of Hollow Optical Fiber Based Components: Analytical Solutions

Guided modes in a hollow optical fiber are investigated using both scalar approximation and exact vectorial analysis. Effective indices of modes are seen to exhibit "nearly degenerate" groups. Besides providing an insight of modal characteristics, the analysis would prove to be useful to define design parameters for realizing components based on these fibers, and to explore new possibilities.

All-fiber linear polarization and orbital angular momentum modes amplifier based on few-mode erbium-doped fiber and long period fiber grating

A few-mode erbium-doped fiber（FM-EDF） is fabricated using modified chemical vapor deposition in combination with liquid solution. The core and cladding diameters of the fiber are approximately 19.44 and 124.12 μm,respectively. The refractive index difference is 0.98%, numerical aperture（NA） is 0.17, and normalized cut-off frequency at 1550 nm is 6.81. Therefore, it is a five-mode fiber, and can be used as a higher-order mode gain medium. Furthermore, a long period fiber grating（LPFG） is fabricated, which can convert LP〈01 mode to LP11 mode, and its conversion efficiency is up to 99%. The first-order orbital angular momentum（OAM） is also generated by combining the LPFG and polarization controller（PC）. Then, an all-fiber amplification system based on the FM-EDF and LPFG, for LP11 mode and first-order OAM beams, is built up. Its on-off gain of the LP11 mode beam is 37.2 d B at 1521.2 nm. The variation, whose transverse mode field intensity of first-order OAM is increased with the increase of pumping power, is obvious. These show that both the LP11 mode and first-order OAM beams are amplified in the all-fiber amplification system. This is a novel all-fiber amplification scheme,which can be used in the optical communication fields.

Chromatic Dispersion Monitoring Method Based on Phase Shift Technique

The modulation phase shift method was used to measure chromatic dispersion in a standard single mode fiber for telecommunication. The modulation phase difference of the transmitted light at the wavelength of 1532.16 nm modulated by a radio frequency signal was measured, relative to the transmitted light at the wavelength of 1549.33 nm modulated by the same signal. By introducing a reference light at the wavelength of 1310 nm, a 1310/1550 nm wavelength division multiplexing was used instead of the high cost dense wavelength division multiplexing. In the experiment, two testing lights were coupled with the reference light to the fiber spools of different lengths, respectively. By finite difference method, the chromatic dispersion between the two testing lights was measured, and the fixed errors generated during transmission were less than 0.5 ps/(nm·km).

DISPERSION COMPENSATION CFBG IN 100km 10Gbit/s RECIRCULATING SYSTEMS

A 10 Gbit/s recirculating system is configured with Chirped Fiber Bragg Grating (CFBG) for the dispersion compensation. For the first time, the transmission distance in the loop reaches 1000km with bit error rate of 10-9. The effect of the group delay ripple of the fiber grating is also investigated in the recirculating systems, and it is shown that the transmission distance is limited to 4 cycles (4× 167.1km ) in the loop with the power penalty fluctuation below 1.0dB. Thus the group delay ripple should be reduced to allow for the wavelength drift of ±5GHz.At the end of this letter, the principles are given for designing long haul recirculating systems with dispersion compensation CFBG.

Polarization-maintaining few mode fiber composed of a central circular-hole and an elliptical-ring core

We propose a novel waveguide design of polarization-maintaining few mode fiber(PM-FMF) supporting ≥10non-degenerate modes, utilizing a central circular air hole and a circumjacent elliptical-ring core. The structure endows a new degree of freedom to adjust the birefringence of all the guided modes, including the fundamental polarization mode. Numerical simulations demonstrate that, by optimizing the air hole and elliptical-ring core,a PM-FMF supporting 10 distinctive polarization modes has been achieved, and the effective index difference Δn_(eff) between the adjacent guided modes could be kept larger than 1.32 × 10^(-4) over the whole C +L band. The proposed fiber structure can flexibly tailored to support an even larger number of modes in PM-FMF(14-mode PM-FMF has been demonstrated as an example), which can be readily applicable to a scalable mode division multiplexing system.

Effects of group-velocity mismatch and cubic-quintic nonlinearity on cross-phase modulation instability in optical fibers

The synthetic effects of group-velocity mismatch and cubic-quintic nonlinearity on cross-phase modulation induced modulation instability in loss single-mode optical fibers have been numerically investigated. The results show that the quintic nonlinearity plays a role similar to the case of neglecting the group-velocity mismatch in modifying the modulation instability, namely, the positive and negative quintic nonlinearities can still enhance and weaken the modulation instability, respectively. The group-velocity mismatch can considerably change the gain spectrum of modulation instability in terms of its shape, peak value, and position. In the normal dispersion regime, with the increase of the group-velocity mismatch parameter, the gain spectrum widens and then narrows, shifts to higher frequencies, and the peak value gets higher before approaching a saturable value. In the abnormal dispersion regime, two separated spectra may occur when the group-velocity mismatch is taken into account. With the increase of the group-velocity mismatch parameter, the peak value of the gain spectrum gets higher and shorter before tending to a saturable value for the first and second spectral regimes, respectively.

Linearly polarized operation of Yb-doped fiber laser by Brewster's angle-polished fiber end

A new method to obtain linear polarization operation by Brewster＇s angle-polished fiber end is demonstrated. By using the special polarization operation technique together with introducing a narrow-linewidth fiber grating into the laser cavity, a cladding-pumped linear polarization and single-transverse-mode （M2 〈 1.1） Yb-doped fiber laser with narrow linewidth whose full-width at half-maximm （FWHM） is less than 0.2 nm, is obtained in a simple configuration. The output power is up to 10 W which is continuous- wave output at 1085 nm, and the slope efficiency is 63% with respect to the coupled pump power and 75% with respect to the absorbed pump power, respectively. The measured 21-dB polarization extinction ratio does not degrade with the output power. The simplicity of such an approach is highly beneficial for a number of applications, including the use of a fiber laser for the nonlinear wavelength conversion （especially for the intracavity frequency doubling） and for the coherent and spectral beam combination.

High-temperature sensor using a Fabry-Perot interferometer based on solid-core photonic crystal fiber

A micro Fabry-Perot interferometer （M-FPI） is constructed by splicing a short section of polarization- maintaining photonic crystal fiber （PM-PCF） to an end-cleaved single-mode fiber with controllable offset. Due to the high effective optical path difference induced by the solid core of the PCF, the M-FPI has an ultrasmall cavity of approximately 110 μm. The temperature sensitivity within a range from 33 ℃ to approximately 600 ℃ is measured to be 13.8 pm/℃, which shows good agreement with the theoretical result. This proposed sensor has the advantages of ultracompact size and high stability. Therefore, it is suitable for various space-limited sensing applications in harsh environments.

Weak laser pulse signal amplification based on a fiber Brillouin amplifier

We propose a high-gain and frequency-selective amplifier for a weak optical signal based on stimulated Brillouin scattering in a single mode fiber. To be able to satisfy the needs of high gain and high signal-to-noise ratio laser pulse amplification, different fiber lengths and core diameters are used to fulfill this experiment. In the experiment, a 430 nW （peak power） pulsed signal is amplified by 70 dB with a signal-to-noise ratio of 14 dB. The small size, high gain, low cost, and low noise of the fiber Brillouin amplifier make it a promising weak signal amplification method for practical applications such as lidar.

Compact fiber optic accelerometer

A compact fiber optic accelerometer based on a Michelson interferometer is proposed and demonstrated. In the proposed system, the sensing element consists of two single-mode fibers glued together by epoxy, which then act as a simple supported beam. By demodulating the optical phase shift, the acceleration is determined as proportional to the force applied on the central position of the two single-mode fibers. This simple model is able to calculate the sensitivity and the resonant frequency of the compact accelerometer. The experimental results show that the sensitivity and the resonant frequency of the accelerometer are 0.42 rad/g and 600 Hz, respectively.

Mach-Zehnder Interferometer for High Temperature(1000℃)Sensing Based on a Few-Mode Fiber

A Mach-Zehnder interferometer(MZI)for high temperature(1000°C)sensing based on few mode fiber(FMF)was proposed and experimentally demonstrated.The sensor was fabricated by fusing a section of FMF between two single-mode fibers(SMFs).The structure was proven to be an excellent high temperature sensor with good stability,repeatability,and high temperature sensitivity(48.2 pm/C)after annealing process at a high temperature lasting some hours,and a wide working temperature range(from room temperature to 1000 C).In addition,the simple fabrication process and the low cost offered a great potential for sensing in high temperature environments.