On-off Gain Research Articles

Femtosecond Laser Amplification Based on Stimulated Raman Scattering in Optical Fibers

The amplification of femtosecond laser has been experimentally investigated around 1.060 m m wavelength through an optical fiber based on stimulated Raman scattering (SRS) . A passively mode locked Nd:glass laser (model Timebandwidth GLX 200) giving a pulse duration of about 200 fsec at 1.060 m m wavelength and 120 mW average optical power with 100 MHz repetition rate is used as seed signal . Whereas the high repetition rate 88MHz Nd YLF laser operating at 1.053 m m with pulse duration of 120 Psec is used as pump laser of the 15 and 25 m long single mode fiber. The broadening in the time of the pulse is done in the first few meters of the optical fiber and then amplified due to the energy transfer from pump laser source to the seed laser pulse . The results showed that the peak power conversion efficiency exceeds 45% and the on-off peak gain is about 2 dB. The peak wavelength shift is about 0.3 nm when the optical fiber is forward pumping by Nd-YLF laser. The self phase modulation (SPM) leads to compress the laser spectral width from 6.96 nm to 5 nm with small effect on the maximum nonlinear phase shift.

Multipath Interference in Pulse-Pumped Fiber Raman Amplifiers: Analytical Approach

We present an analytical study of multipath interference (MPI) in pulse-pumped fiber Raman amplifiers (FRAs), caused by the internal reflections from the fiber facets and the effect of Rayleigh backscattering. For the first time, to the best of our knowledge, we derive a generalized expression for the MPI-induced noise on signal in counter-pumped FRAs. The obtained result takes into account four major sources of MPI, suggests their physically transparent classification, and enables us to predict the FRA's noise performance for an arbitrary modulated pump. Using the derived expression, we characterize the MPI-induced noise in a typical 80-km span of single-mode optical fiber. Specifically , we examine the noise enhancement in the pulse-pumped amplifier as compared to the amplifier operating in the continuous-wave regime, and analyze its dependence on the on-off gain and the pump-pulse duty cycle. We also estimate the impact of the MPI-induced noise transfer on the amplifier's performance, and discuss the optimal choice of pump modulation for different values of Q-penalty. The results of our study are important for the design optimization of FRAs with time-division-multiplexed pumping.

Saturation of the Raman amplification by self-phase modulation in silicon nanowaveguides

We experimentally show that the self-phase modulation of picosecond pump pulses, induced by both the optical Kerr effect and free-carrier refraction, has a detrimental effect on the maximum on-off Raman gain achievable in silicon on insulator nanowaveguides, causing it to saturate. A simple calculation of the Raman gain coefficient from the measured broadened output pump spectra perfectly matches the saturated behavior of the amplified Raman signal observed experimentally at different input pump powers.

Pump scheme for gain-flattened Raman fiber amplifiers using improved particle swarm optimization and modified shooting algorithm

An effective pump scheme for the design of broadband and flat gain spectrum Raman fiber amplifiers is proposed. This novel approach uses a new shooting algorithm based on a modified Newton-Raphson method and a contraction factor to solve the two point boundary problems of Raman coupled equations more stably and efficiently. In combination with an improved particle swarm optimization method, which improves the efficiency and convergence rate by introducing a new parameter called velocity acceptability probability, this scheme optimizes the wavelengths and power levels for the pumps quickly and accurately. Several broadband Raman fiber amplifiers in C+L band with optimized pump parameters are designed. An amplifier of 4 pumps is designed to deliver an average on-off gain of 13.3 dB for a bandwidth of 80 nm, with about +/-0.5 dB in band maximum gain ripples.

Optical gain measurements in porous silicon planar waveguides codoped by erbium and ytterbium ions at 1.53μm

The on-off optical gain measurements as a function of the pump power were performed on porous silicon planar waveguides codoped by erbium and ytterbium ions. These measurements were obtained for different ratios of Yb concentration to Er concentration. The highest value of the gain was reached when the Yb concentration is three times higher than that of Er at a moderate 980nm pump power value equal to 70mW. Optical losses measurements have been performed on these waveguides and were equal to 2.1dB∕cm and an internal gain of about 6.4dB∕cm was obtained.

Robust, compact, and flexible neural model for a fiber Raman amplifier

In this paper, a novel robust, compact, and flexible neural-network model for a fiber Raman amplifier (FRA) is presented. The model can be used in various applications with promising accuracy and low requirement for memory. Analytical expressions are derived in order to make the optimal pump-power configuration much easier, and the computational time is reduced dramatically in comparison with other gain-design methods in real-time pump-power adjustment. The calculated on-off gain spectrum and the noise figure using the proposed model agree well with the experimental results. The model has a potential value in simulation and pump-power dynamic control.

Bidirectional higher order cascaded Raman amplification benefits for 10-Gb/s WDM unrepeated transmission systems

Benefits provided by higher order bidirectional Raman pumping schemes in 10-Gb/s unrepeated wavelength-division-multiplexing transmission systems are experimentally quantified in terms of BER performances at 10 Gb/s. By keeping under control double-Rayleigh-scattering-noise-induced transmission penalties, which can degrade system performance at very high ON-OFF Raman gain, as well as nonlinear propagation effects such as Brillouin scattering, self- and cross-phase modulations, four-wave-mixing, and Raman-induced crosstalks, we show a total unrepeated system reach enhancement up to 3.5 dB with respect to first-order bidirectional pumping. As confirmed by theory, the maximum reach enhancement is mainly limited by pump-to-signal relative intensity noise transfer induced by higher order copumping.

Wide-Band Tuning of the Gain Spectra of One-Pump Fiber Optical Parametric Amplifiers

By suitably choosing the fiber properties, and by tuning the pump wavelength near the fiber zero-dispersion wavelength, one can in principle generate a wide variety of one-pump fiber optical parametric amplifier gain spectra. These can range from a very wide single region to two symmetric narrow gain regions far away from the pump. We have experimentally verified these predictions. With a highly nonlinear fiber, we have inferred the existence of gain over a single 400-nm region and measured a maximum on-off gain of 65 dB. With a common dispersion-shifted fiber, we have obtained tunable gain regions less than 1 nm wide, up to 200 nm from the pump; we have also shifted these by several nanometers by lowering the fiber temperature to 0/spl deg/C.

Optimal Design of Fiber Raman Amplifier Based on Hybrid Genetic Algorithm

A hybrid genetic algorithm (GA) based on the clustering fitness sharing and the elite replacement is proposed. Our proposed GA can effectively solve the multimodal optimization in the distributed multipump Raman amplifier (DMRA). The optimal results show that, under the conditions of the ON-OFF gain of > 10 dB, the relative gain flatness of < 0.1 and the fiber length of 80 km, the bandwidth of > 100 nm can be designed from DMRA with five pumps after the optimization of our GA. There are four schemes to realize the required setup parameters in our simulation.

Impact of double Rayleigh scattering noise in distributed higher order Raman pumping schemes

We analyze the equivalent optical noise figure (NF) performances and double Rayleigh-scattering-induced noise characteristics of distributed Raman amplifiers based on higher order pumping schemes. We confirm that higher order counterpumping configurations improve the amplifier equivalent NF but also increase multipath interference noise, inducing system penalties at high ON-OFF Raman gain. A Q-factor analysis allows a quantitative evaluation of the real benefits provided by higher order pumping schemes.

Phase-matched four-wave mixing between pumps and signals in a copumped Raman amplifier

We observe four-wave mixing (FWM) between copropagating pumps and signals in a Raman amplifier when the zero-dispersion wavelength of the transmission fiber lies symmetrically between the pump and the signal wavelengths. The resultant FWM products, which grow as they experience Raman gain along the fiber, can degrade the signal's optical signal-to-noise ratio by as much as 10 dB for a Raman ON-OFF gain of 15 dB.

Time-division multiplexed Raman pump experiment using a tunable C-band laser

In this letter, we report on the first experimental demonstration of a time-division multiplexed frequency-swept Raman pump used to adaptively flatten the gain of a Raman amplifier. By means of a single, but tunable Raman pump laser operating in the C-band, an ON-OFF Raman gain flatness of 0.6 dB over 15 nm (or 1.5 dB over 30 nm) at an average ON-OFF Raman gain of 22 dB could be achieved for a pump wavelength range of 22 nm.

Continuous-wave-pumped Raman-assisted fiber optical parametric amplifier and wavelength converter in conventional dispersion-shifted fiber

We present a continuous-wave-pumped fiber optical parametric amplifier, operating near 1539 nm in conventional dispersion-shifted fiber, with maximum on-off gain and wavelength-conversion efficiency of 13.7 and 13.1 dB, respectively. In addition, we show a novel configuration based on Raman amplification assistance in the parametric gain fiber that further increases the gain and wavelength-conversion efficiencies to 16.7 and 16.2 dB, respectively.