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.

Gain Flattened Raman Fiber Amplifier Using Parameter Optimization and DCF Fiber

We investigate two configurations of Raman fiber amplifier in the 1530-1570 nm region. The gain spectrum variation is decreased from 8.31 dB to 2.48 dB when appropriately adjust the individual two pumps and optical fiber.

Raman Suppression in a Kilowatt Narrow-Band Fiber Amplifier

A novel technique to suppress stimulated Raman scattering in a high power narrow-band fiber amplifier is reported. By seeding with a comSination of a broadband amplified spontaneous emission seed and a narrowband master oscillator seed, the Raman Stokes components can be reduced about 16dB at a total output power of i kW. Ratnan suppression results are depicted in a different wavelengths seeding case and the same wavelength seeding ease, respectively, with different seed power ratios.

Amplification Effect on Rayleigh Scattering and SBS in 25 km Distributed Fiber Raman Amplifier

The amplification effect on stimulated Brillouin scattering （SBS） and Rayleigh scattering in the backward pumped G652 fibers Raman amplifier have been researched. The signal source is a tunable narrow spectral bandwidth （〈10 MHz） ECL laser and is pumped by the tunable power 1427.2 nm fiber Raman laser. The Rayleigh scattering lines are amplified by fiber Raman amplifier, and Stokes stimulated Brillouin scattering lines are amplified by fiber Raman amplifier and fiber Brillouin amplifier. The SBS lines total gain is a production of the gain of Raman and the gain of Brillouin amplifier. In experiment, the gain of SBS is about 42 dB and the saturation gain of 25 Ion G652 backward FRA is about 25 dB, so the gain of fiber Brillouin amplifier is about 17 dB.

Effect of stimulated Brillouin scattering on the gain saturation of distributed fiber Raman amplifier and its suppression by phase modulation

For distributed fiber Raman amplifiers（DFRAs）, stimulated Brillouin scattering（SBS） can deplete the pump once occurring and consequently generate gain saturation. On the basis of such a theory, theoretical gain saturation powers in DFRAs with various pump schemes are obtained by calculating SBS thresholds in them, and the experimental results show that they are in excellent agreement with the calculation results. The saturation power of the DFRA with a 300 m W forward pump is as low as 0 d Bm, which needs to be enhanced by phase modulation, and the effect is quantitatively studied. A simple model taking both modulation frequency and index into consideration is presented by introducing a correction factor to evaluate the effect of phase modulation on the enhancement of saturation power. Experimentally, it is shown that such a correction factor decreases as the modulation frequency increases and approaches zero when the modulation frequency becomes high enough. In particular, a phase modulation with a modulation frequency of 100 MHz and a modulation index of 1.380 can enhance the saturation power by 4.44 d B, and the correction factor is 0.25 d B, in which the modulation frequency is high enough. Additionally, the factor is 1.767 d B for the modulation frequency of 25 MHz. On this basis,phase modulations with various indexes and a fixed frequency of 25 MHz are adopted to verify the modified model, and the results are positive. To obtain the highest gain saturation power, the model is referable. The research results provide a guide for the design of practical DFRAs.

Theoretical Analysis of Rayleigh Backscattering Noise in Fiber Raman Amplifiers

In this paper, a new theoretical model for Rayleigh backscattering (RB) analysis of fiber Raman amplifiers is proposed. The model includes all the interactions among the pumps, signals, and all orders of RB. The results show that the higher order RB has a negligible influence on the performance of the amplifier. The co-propagating and counterpropagating RB power of the signal grow quadratically with the net-gain of the amplifier. The signal to double Rayleigh backscattering noise ratio (OSNRDRB ) of backward-pumped FRAs is better than that of the forward-pumped ones at high net-gain level (＞ 13 dB), while at low net-gain level the OSNRDrb of the forward-pumped FRAs is slightly better than that of the backward-pumped ones.

Low-Threshold Broadband Spectrum Generation by Amplification of Cascaded Stimulated Raman Scattering in an Ytterbium-Doped Fiber Amplifier

We present an experimental study on low-threshold broadband spectrum generation mainly due to the amplirfication of the cascaded stimulated Raman scattering （SRS） effect in a four-stage fiber master oscillator power amplifier system. The cascaded SRS is achieved by using a long passive fiber pumped by a pulsed fiber laser cen： tered at wavelength 1064 nm. The amplified spontaneous emission during the amplification process is efficiently suppressed by cutting the length of the passive fiber and by using a double-clad ytterbium-doped fiber amplifier. The generated broadband spectrum spans from 960nm to 1700nm with maximum average output 13.6 W and average spectral power density approximately 17. 7 mW/nm.

Influence of double Rayleigh backscatter in CATV systems with Raman amplifiers

The double Rayleigh backscatter （DRB） effect in long distance CATV systems using fiber Raman amplifiers （FRAs） is investigated theoretically in the paper. As a comparison, performance of a system with erbium doped fiber amplifier （EDFA） is also evaluated. According to the simulation results, it is found that, in case of FRA, the increase of carrier-to-noise ratio （CNR） due to the DRB effect will not impair the performance more than that using EDFA.

An improved calculation method for fiber Raman amplifier equations with multi-wavelength pumping

A novel numerical method for fiber Raman amplifier (FRA) from standard propagation equations is presented and derived based on the one-step method for ordinary differential equation (ODE). The proposed algorithm is effective in solving FRA equations including all the interactions among pumps, signals, and noises. Applications of the numerical analysis to practical FRA-based systems show a great reduction in computation time in comparison with the average power method and the fourth-order Runge-Kutta (RK) method, under the same condition. Also the proposed method can decrease the computing time over three orders of magnitude with excellent accuracy promises in comparison with the direct integration method.

Investigation of Fiber Design for Discrete Raman Amplifiers

Aeff,R and gR of the optical fibers for discrete Raman amplifiers are theoretically and experimentally investigated. It is demonstrated that Gaussian approximation is adaptable for Aeff,R of these fibers, and gR does not depend on only core materials.

On the Mitigation of Polarization Dependency of Distributed Raman Amplifier Gain by Transmission Fiber PMD

We show theoretically and experimentally that Raman PDG can be formulated as a function of the pump light DOP and the transmission fiber PMD. Raman PDG is sufficiently reduced thanks to the inevitable fiber PMD.

A novel control method for on-off gain and gain tilt of fiber Raman amplifiers

Considering spectrum tilt due to signal-to-signal Raman scattering (SSRS) in backward distributed fiber Raman amplifiers (B-DFRA),an inverse tilted on-off gain profile is adopted to achieve flat net gain.A simple approximate linear relationship of pump power at each wavelength versus on-off gain level and tilt was derived numerically and experimentally so that a novel control method was established.Since there are only 3 pre-determinable constants required for individual pump wave,it is easy to be realized.As an example,maximum errors less than 0.2 and 0.4 dB respectively for average gain and gain tilt were achieved over C+L band in 100-km back-pumped standard single-mode fiber (SMF) experimentally.

A Low Noise L-band Dispersion-Compensating Hybrid Fiber Amplifier with 1500 nm Raman Pumping

This report presents a low noise L-band dispersion-compensating hybrid fiber amplifier with 1500-nm Raman pumping. It describes the pre-stage optimization, Raman pump selection, and possible nonlinearity problems to achieve a practical low noise L-band optical amplifier.

Sodium guide star laser generation by single-pass frequency doubling in a periodically poled near-stoichiometric LiTaO_3 crystal

We report in this work a continuous wave(CW) narrowband 589 nm light generation for the purpose of laser guide assisted adaptive optics.A 39 mm long 1 mm thick periodically poled near stoichiometric LiTaO3 crystal with duty cycle near 50% was fabricated using electrical poling at room temperature and pumped by a Raman fiber amplifier.We tested two temperature control ovens,and a maximum conversion efficiency of about 14.3%,corresponding to 4 W of yellow light with 28 W of fundamental power,and bandwidth less than 0.18 GHz was achieved.