Temperature-dependent gain and noise in fiber Raman amplifiers

An experimental investigation of the temperature dependence of the gain and noise performance of a silica-fiber Raman amplifier is described. A decrease in the Raman scattering cross section in a fiber amplifier cooled from a temperature of 300 K to 77 K was measured and found to be in agreement with theoretical values. No difference between the Raman gain coefficients at these two temperatures was observed.     

Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers

We demonstrate coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using a feedback loop based on an electro-optic phase modulator, an average power of 7.2?W before compression is obtained with a combining efficiency of 90%. The spatial and temporal qualities of the oscillator are retained, with a recombined pulse width of 325?fs. This experiment opens up a way to scale the peak/average power of ultrafast fiber sources.     

Theoretical analysis of gain characteristics of Dy-doped fiber amplifiers

The rate equations and power evolution equations of Dy³⁺-doped fiber amplifiers (DDFA) are proposed, and numerically analyzed. Based on the numerical analysis, several major characteristics of the fiber amplifier are calculated and compared with those of Pr³⁺-doped fiber amplifier (PDFA). The results show that DDFA can achieve better efficiency and wider gain bandwidth than PDFA.     

Investigation of gain ripple in two-pump fiber optical parametric amplifiers

By using the four-sideband theory, we analyze the gain spectrum in wideband two-pump fiber optical parametric amplifiers and predict gain ripples over the flat gain region. We derive an approximation of their pseudo-periods and discuss methods for reducing their amplitudes.     

Passive coherent beam combining of two femtosecond fiber chirped-pulse amplifiers

We propose and demonstrate an architecture that achieves passive coherent combination of two femtosecond fiber chirped-pulse amplifiers. The setup consists in the use of a well-balanced amplifying Sagnac interferometer. The experiment shows that the temporal, spectral, and spatial qualities of each beam are retained, with the generation of 250?fs pulses at 35?MHz repetition rate, an uncompressed average power of 10?W, and a combining efficiency of 96%. The behavior of this architecture in the presence of high accumulated nonlinear phase is investigated.     

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.     

Theoretical study of gain distortions in dual-pump fiber optical parametric amplifiers

We study analytically and numerically the small signal gain in dual-pump fiber optical parametric amplifiers by including the phase modulation of the pump waves needed for practically increasing the stimulated Brillouin scattering threshold. As for the single-pump case, we show that large signal gain distortions are generated under co-phase modulation, which depend on the rise/fall time of the phase modulation and on the fiber dispersion slope. However, it is clearly confirmed that the counter-phase modulation scheme allows to efficiently suppress these gain distortions over the whole flat gain region. In addition, we demonstrate through realistic numerical simulations that this useful technique overcomes the additional impact of pump-phase modulation to amplitude modulation conversion and zero-dispersion wavelength variations.     

Modeling polarization-dependent gain in fiber Raman amplifiers with randomly varying birefringence

We report a new stochastic model for studying the polarization-dependent gain in Raman fiber amplifiers with randomly varying birefringence as a function of the pump state of polarization and the polarization mode dispersion parameter. Our theoretical results agree with the previously obtained experimental data.     

光纤喇曼放大器偏振相关增益研究进展

光纤喇曼放大器(FRA)系统中的偏振相关增益(PDG)降低光纤通信系统的传输性能,因此研究FRA系统中PDG的特性和如何降低FRA系统中的PDG成为FRA系统应用中的一项重要内容。论述了FRA的基本原理,总结了有关PDG的理论研究和实验研究,详细分析了降低FRA中偏振相关增益的几种方法,对FRA偏振相关增益的进一步研究方向进行了展望。     

Modeling the gain and amplified spontaneous emission spectra of erbium-doped fiber amplifiers

We have used a modified “black box” model to calculate the gain and amplified spontaneous emission spectra for an erbium-doped fiber amplifier. Based on this and using an occupation number model, we determined the limiting spectral efficiency of information transfer in such an amplifier. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 273–276, March–April, 2006.     

Adaptive femtosecond pulse shaping to control supercontinuum generation in a microstructure fiber

Efficient confinement of laser radiation in the core of a photonic crystal fiber increases the nonlinear processes resulting in supercontinuum generation. The technique of adaptive pulse shaping using an evolutionary algorithm provides a method to gain control over such highly nonlinear processes. Adaptive pulse shaping of the driving laser radiation passing through the photonic crystal fiber is employed to modify the shape and composition of the output supercontinuum. Amplitude and phase shaping are used to optimize the broadband emission between 500 and 700 nm, as well as a soliton centered at 935 nm. The intensities of the emission and of the soliton driven by a shaped laser pulse increase in comparison to an unshaped pulse by factors of 4 and 3, respectively. The spectral width in the range of 500-600 nm is increased by approximately 40%. In addition, the suppression of self-steepening effects in supercontinuum spectra is demonstrated.     

Influences of finite gain bandwidth on pulse propagation in parabolic fiber amplifiers with distributed gain profiles

The evolutions of the pulses propagating in decreasing and increasing gain distributed fiber amplifiers with finite gain bandwidths are investigated by simulations with the nonlinear Schr¨odinger equation. The results show that the parabolic pulse propagations in both the decreasing and the increasing gain amplifiers are restricted by the finite gain bandwidth. For a given input pulse, by choosing a small initial gain coefficient and gain variation rate, the whole gain for the pulse amplification limited by the gain bandwidth may be higher, which is helpful for the enhancement of the output linearly chirped pulse energy. Compared to the decreasing gain distributed fiber amplifier, the increasing gain distributed amplifier may be more conducive to suppress the pulse spectral broadening and increase the critical amplifier length for achieving a larger output linearly chirped pulse energy.     

Influences of finite gain bandwidth on pulse propagating in parabolic fiber amplifiers with distributed gain profiles

The evolutions of the pulses propagating in decreasing and increasing gain distributed fiber amplifiers with finite gain bandwidths are investigated by simulating with the nonlinear Schrödinger equation. The results show that the parabolic pulse propagations in both the decreasing and the increasing gain amplifiers are restricted by the finite gain bandwidth. For a given input pulse, by choosing small initial gain coefficient and gain variation rate, the whole gain for the pulse amplification limited by the gain bandwidth may be higher, which is helpful to the enhancement of the output linearly chirped pulse energy. Compared to the decreasing gain distributed fiber amplifier, the increasing gain distributed amplifier may be more conducive to suppress the pulse spectral broadening and increase the critical amplifier length for achieving a larger output linearly chirped pulse energy.     

Stimulated Brillouin scattering effect on gain saturation of distributed fiber Raman amplifiers

<正>Gain saturation is a significant phenomenon of fiber Raman amplifiers(FRAs).Gain figures versus signal power are well explained.For the small signal,the coupled ordinary differential equations are used,and for the large signal,the Raman gain coefficient is modified.It is shown that the saturation power of FRAs decreases with the pump power,and gain saturation is easier to occur in the forward pump scheme than in the backward pump scheme.These phenomena are well explained by the stimulated Brillouin scattering (SBS) effect.This research provides a guide to the fabrication of practical FRAs.     

Shooting algorithm and particle swarm optimization based Raman fiber amplifiers gain spectra design

An initial value determination method with a contraction factor for the counter-pumped Raman coupled equations is proposed. This method is used in conjunction with initial guess correction mechanism of Newton's method to construct a new efficient shooting algorithm for the solution of counter-pumped Raman coupled equations. The particle'swarm optimization is used to find the optimal wavelengths and powers for the pumps. By combining the new shooting algorithm and particle swarm optimization a powerful approach to the design of gain spectra for Raman fiber amplifiers is developed. Using this approach a counter-pumped broadband Raman fiber amplifier in C + L-band is designed and optimized. An average on-off gain of 9.3 dB for a bandwidth of 95 nm is obtained using only 4 pumps, with an in-band ripple level of ± 0.7 dB.     

88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers

The generation of 0.5 mJ femtosecond laser pulses by coherent combining of two high power high energy fiber chirped-pulse amplifiers is reported. The system is running at a repetition frequency of 175 kHz producing 88 W of average power after the compressor unit. Polarizing beam splitters have been used to realize an amplifying Mach-Zehnder interferometer, which has been stabilized with a H?nsch-Couillaud measurement system. The stabilized system possesses a measured residual rms phase difference fluctuation between the two branches as low as λ/70 rad at the maximum power level. The experiment proves that coherent addition of femtosecond fiber lasers can be efficiently and reliably performed at high B-integral and considerable thermal load in the individual amplifiers.     

Spectral efficiency of information transfer by erbium-doped fiber amplifiers in the unsaturated gain regime

The results of numerical simulation of the ultimate spectral efficiency of information transfer by erbium-doped fiber amplifiers in the S and C bands in the regime of unsaturated gain are presented. The cases of co-and counterpropagating pumps are analyzed in terms of the three-level and quasi-two-level schemes. It is shown that, for the peak gain in the range 12–24 dB and a signal-to-noise ratio at the entrance equal to 20 dB, the ultimate efficiency lies in the range 5.5–7.5 bit s^?1 Hz^?1 for the S band and 5.6–7.2 bit s^?1 Hz^?1 for the C band.     

Suppression of dynamic instabilities in erbium-doped fiber amplifiers with a combined gain control system

We report an improved transient response of an Er(3+) -doped fiber-optic amplifier with combined electronic feedforward (EFF) and optical feedback (OFB) control systems. We find, theoretically and experimentally, that the time delay of an EFF response leads to control instabilities in the form of erbium-doped fiber amplifier gain auto-oscillations, which can be suppressed by an appropriate choice of OFB wavelength and control system design.     

Linearity and nonlinearity of quantum-dot semiconductor optical amplifiers

Linearity and nonlinearity of quantum-dot semiconductor optical amplifiers (QD-SOAs) are analyzed in this paper using numerical and analytical solutions. The basic formulas are derived from rate equations of multiple-states QD-SOAs. The solutions reveal that linearity and nonlinearity of QD-SOAs, which is quantificationally characterized by the maximal linear output power and the nonlinear factor, are strongly dependent on injected current, modal gain and spontaneous radiative lifetime. Additionally, the solutions also provide some useful suggestions in designing and using of QD-SOAs, either be used in linear or nonlinear operational region.     

Interplay between nonlocality and nonlinearity in nematic liquid crystals

Spatial nonlocality has a crucial role in the optical response of bulk liquid crystals. With specific reference to one-dimensional modulation instability, we demonstrate the tuning of transverse nonlocality in the nonlinear response of nematics.