Design and spectral characteristics of gain-flattened tellurite-based fiber Raman amplifiers

The design and the spectral gain and noise figure (NF) characteristics of tellurite-based fiber Raman amplifiers (FRAs), which are tellurite FRAs (T-FRAs) or hybrid tellurite/silica FRAs (hybrid FRAs), are described. Propagation equations for a multiwavelength (multi-/spl lambda/)-pumped T-FRA that include the pump interaction terms are presented, with which the gain and NF of the T-FRA can be calculated. Tellurite fiber (TF) length dependences of gain and NF of a T-FRA are clarified experimentally and theoretically. Numerical calculations on the gain and NF spectra of the tellurite-based FRAs show that a T-FRA with a two-stage configuration and a hybrid FRA with a three-stage configuration can provide seamless gain bands with widths of more than 130 nm over the S-, C-, and L-bands. The two-stage T-FRA has a couple of two-/spl lambda/-pumped T-FRA stages, and a gain equalizer (GEQ) and a dispersion-compensation fiber (DCF) between the T-FRA stages, while the three-stage hybrid FRA has a couple of two-/spl lambda/-pumped T-FRA stages, and a GEQ and a two-/spl lambda/-pumped DCF Raman amplifier (DCF-RA) stage between the T-FRA stages. The numerical calculations also show that the two-stage T-FRA and the three-stage hybrid FRA achieve top gains with regard to their flattened gain spectra of 9.7 and 24 dB, and maximum NFs in their gain bands of 11.8 and 8.2 dB, respectively. Measured gain and NF spectra of the tellurite-based FRAs coincide well with the corresponding calculated spectra.

I. Introduction

Fiber Raman amplifiers (FRAs) are attractive devices in high-capacity and wideband optical transmission systems/photonic networks, because they can provide wide gain bands in any wavelength region required by properly setting their pump wavelengths [1]–[4]. In particular, the FRAs can be used as post-, pre-, and inline amplifiers in wavelength-division-multiplexing (WDM) systems, and can support new gain bands beyond the gain bands offered by rare-earth-doped fiber amplifiers such as erbium-doped fiber amplifiers (EDFAs) and thulium-doped fiber amplifiers (TDFAs) [5]–[13]. Two types of FRAs with different active fibers have been reported to date, namely: 1) silica FRAs (S-FRAs) [1]; and 2) tellurite FRAs (T-FRAs) [14]. Hybrid tellurite/silica FRAs (hybrid FRAs), which basically are a serial combination of a T-FRA and an S-FRA, were also reported [15], [16]. Note that S-FRAs can yield, in a distinct way, distributed Raman amplification (DRA) in deployed transmission fibers [1], [17]. Some S-FRAs achieved seamless gain bandwidths of up to 100 nm in the configurations of discrete (or lumped), distributed, and hybrid discrete/distributed amplification [18]–[21].