Atalla El-Taher

Effect of Rayleigh-scattering distributed feedback on multiwavelength Raman fiber laser generation

We experimentally demonstrate a Raman fiber laser based on multiple point-action fiber Bragg grating reflectors and distributed feedback via Rayleigh scattering in an ~22-km-long optical fiber. Twenty-two lasing lines with spacing of ~100 GHz (close to International Telecommunication Union grid) in the C band are generated at the watt level. In contrast to the normal cavity with competition between laser lines, the random distributed feedback cavity exhibits highly stable multiwavelength generation with a power-equalized uniform distribution, which is almost independent on power.

Tunable random fiber laser

An optical fiber is treated as a natural one-dimensional random system where lasing is possible due to a combination of Rayleigh scattering by refractive index inhomogeneities and distributed amplification through the Raman effect. We present such a random fiber laser that is tunable over a broad wavelength range with uniquely flat output power and high efficiency, which outperforms traditional lasers of the same category. Outstanding characteristics defined by deep underlying physics and the simplicity of the scheme make the demonstrated laser a very attractive light source both for fundamental science and practical applications.

Experimental and theoretical study of longitudinal power distribution in a random DFB fiber laser.

We have measured the longitudinal power distribution inside a random distributed feedback Raman fiber laser. The observed distribution has a sharp maximum whose position depends on pump power. The spatial distribution profiles are different for the first and the second Stokes waves. Both analytic solution and results of direct numerical modeling are in excellent agreement with experimental observations.

High efficiency supercontinuum generation using ultra-long Raman fiber cavities

Supercontinuum generation in a multi-fibre ultra-long Raman fibre laser cavity is investigated experimentally. By using this cavity configuration we demonstrate improved spectral flatness and supercontinuum generation efficiency.

Optical turbulence and spectral condensate in long fibre lasers

We study numerically optical turbulence using the particular example of a recently created, ultra-long fibre laser. For normal fibre dispersion, we observed an intermediate state with an extremely narrow spectrum (condensate), which experiences instability and a sharp transition to a fluctuating regime with a wider spectrum. We demonstrate that the number of modes has an impact on the condensates lifetime. The smaller the number of modes, the more resistant is the condensate to perturbations. Experimental results show a good agreement with numerical simulations.

Gain bandwidth optimisation and enhancement in ultra-long Raman fibre laser based amplifiers

We show experimentally a 57nm gain bandwidth for an ultra-long Raman fiber laser based amplification technique using only a single pump wavelength. The enhanced gain bandwidth and gain flatness is investigated for single and multi-cavity designs.

Long-distance soliton transmission through ultralong fiber lasers

We present the first experimental demonstration (to our knowledge) of long-distance unperturbed fundamental optical soliton transmission in conventional single-mode optical fiber. The virtual transparency in the fiber required for soliton transmission, over 15 complete periods, was achieved by using an ultralong Raman fiber laser amplification scheme. Optical soliton pulse duration, pulse bandwidth, and peak intensity are shown to remain constant along the transmission length. Frequency-resolved optical gating spectrograms and numerical simulations confirm the observed optical soliton dynamics.

Ultra-flat wideband single-pump Raman-enhanced parametric amplification

We experimentally optimize a single pump fiber optical parametric amplifier in terms of gain spectral bandwidth and gain variation (GV). We find that optimal performance is achieved with the pump tuned to the zero-dispersion wavelength of dispersion stable highly nonlinear fiber (HNLF). We demonstrate further improvement of parametric gain bandwidth and GV by decreasing the HNLF length. We discover that Raman and parametric gain spectra produced by the same pump may be merged together to enhance overall gain bandwidth, while keeping GV low. Consequently, we report an ultra-flat gain of 9.6 ± 0.5 dB over a range of 111 nm (12.8 THz) on one side of the pump. Additionally, we demonstrate amplification of a 60 Gbit/s QPSK signal tuned over a portion of the available bandwidth with OSNR penalty less than 1 dB for Q2 below 14 dB.

Ultra-long raman laser with a feedback based on the Rayleigh scattering

Rayleigh scattering (RS) is a fundamental optical effect that determines losses in optical fibres induced by micro-inhomogeneities of refractive index. Though the effect of Rayleigh back scattering in conventional fibre lasers with high-Q cavity is weak, it might be important in very long fibre cavities. Recently, lasing in a 165-km long fibre span has been obtained utilizing distributed Raman gain and fibre Bragg gratings (FBGs) as cavity reflectors [1]. It was noted that the RS effects remain observable in such ultra-long Raman fibre laser (URFL), though the operating Raman laser wavelength (∼1.55 µm) was chosen to minimize fibre losses and to reduce the amount of the backscattered radiation, accordingly. In this paper we report on the detailed study of the RS effects in URFLs. It has been found that in a long fibre cavity (∼100 km) the distributed feedback due to Rayleigh back scattering at propagation of light between FBG reflectors may be comparable with the lumped feedback provided by the FBG itself. As a result, Raman lasing in the fibre span limited by lumped (FBG) reflector at one side only appears possible due to significant reflection from the RS-based “random” distributed mirror at the other side.

RIN-penalty mitigation and transmission performance improvement using forward propagated broadband first order Raman pump

We demonstrate that using a broadband pump enables forward-propagated first order distributed Raman amplification by mitigating RIN-associated penalty. This extends the reach of 10 × 120 Gb/s DP-QPSK WDM transmission up to 7499 km, compared with other commercially available pumps. Moreover, using this Raman scheme maintains uniform/symmetric signal power distribution and requires low pump power.