Fiber Amplifiers and Fiber Lasers Based on Stimulated Raman Scattering: A Review

Abstract

Nowadays, in fiber optic communications the growing demand in terms of transmission capacity has been fulfilling the entire spectral band of the erbium-doped fiber amplifiers (EDFAs). This dramatic increase in bandwidth rules out the use of EDFAs, leaving fiber Raman amplifiers (FRAs) as the key devices for future amplification requirements. On the other hand, in the field of high-power fiber lasers, a very attractive option is provided by fiber Raman lasers (FRLs), due to their high output power, high efficiency and broad gain bandwidth, covering almost the entire near-infrared region. This paper reviews the challenges, achievements and perspectives of both fiber Raman amplifier and fiber Raman laser. They are enabling technologies for implementation of high-capacity optical communication systems and for the realization of high power fiber lasers, respectively.

Keywords: amplifiers; fiber optics; lasers; optical communication systems; stimulated raman scattering.

Figure 1

Stimulated Raman-Scattering (SRS) principle. (a) Pump–probe modalities associated with the SRS process are pointed out: SRG, stimulated Raman gain; SRL, stimulated Raman loss. (b) Stimulated Raman scattering occurs through inelastic scattering of probe photons off from vibrationally excited molecules that interfere coherently.

Figure 2

Cascaded Raman scattering principle. An input frequency ω_P (green) can cause the spontaneous emission of frequencies ω_P ± ω_ν, [Stokes (blue) and anti-Stokes (orange)]. This initial spontaneous event provides the seed photons necessary for subsequent first-order SRS (left). The first-order Stokes and anti-Stokes photons then can cause second-order Raman scattering (right) to produce new frequencies ω_P ± 2ω_ν (red and violet) and so on through propagation, resulting in a broadband ladder of frequencies ω_P ± nω_ν.

Figure 3

Schematic representation of multiplexing/demultiplexing based distributed optical Raman amplifier.

Figure 4

Ideal distributed amplifier: the loss is counterbalanced at every point along the span, leading to an improvement of the signal to noise ratio (SNR) respect to (ideal) discrete amplification.

Figure 5

Discrete fiber Raman amplifier (FRA) scheme with pump power confined to the lumped element.

Figure 6

Multi-wavelength pumping technique for wideband Raman amplifiers.

Figure 7

Schematic representation of a Raman fiber laser with one wavelength shift. (HR—high-reflectivity fiber Bragg grating, OC—output coupler—a low-reflectivity fiber Bragg grating).

Figure 8

Schematic representation of a Raman fiber laser with multiple wavelength shifts by using a cascaded Raman resonator (RIG—Raman input grating set, ROG—Raman output grating set).

Figure 9

Schematic illustration of a soliton communication system. Solitons are amplified through SRS by injecting continuous wave (CW) pump radiation periodically.

Figure 10

Schematic representation of the ring-cavity geometry implemented for Raman soliton lasers. BS is a dichroic beamsplitter, M₁ and M₂ are mirrors of 100% reflectivity, L₁ and L₂ are microscope objective lenses.