29.03.2016 Views

Nonlinear Fiber Optics - 4 ed. Agrawal

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

Chapter 8<br />

Stimulat<strong>ed</strong> Raman Scattering<br />

Stimulat<strong>ed</strong> Raman scattering (SRS) is an important nonlinear process that can turn optical<br />

fibers into broadband Raman amplifiers and tunable Raman lasers. It can also<br />

severely limit the performance of multichannel lightwave systems by transferring energy<br />

from one channel to the neighboring channels. This chapter is devot<strong>ed</strong> to a thorough<br />

study of SRS phenomenon in optical fibers. Section 8.1 presents the basic theory<br />

behind SRS with emphasis on the pump power requir<strong>ed</strong> to reach the Raman threshold.<br />

SRS under continuous-wave (CW) and quasi-CW conditions is consider<strong>ed</strong> in Section<br />

8.2, where we also discuss the performance of fiber-bas<strong>ed</strong> Raman lasers and amplifiers.<br />

Ultrafast SRS occurring for pulses of 100-ps width or less is consider<strong>ed</strong> in Sections 8.3<br />

and 8.4 for normal and anomalous group-velocity dispersion (GVD), respectively. In<br />

both cases, attention is paid to the walk-off effects together with those resulting from<br />

self-phase modulation (SPM) and cross-phase modulation (XPM). Section 8.5 focuses<br />

on the polarization effects.<br />

8.1 Basic Concepts<br />

In any molecular m<strong>ed</strong>ium, spontaneous Raman scattering can transfer a small fraction<br />

(typically ∼10 −6 ) of power from one optical field to another field, whose frequency<br />

is downshift<strong>ed</strong> by an amount determin<strong>ed</strong> by the vibrational modes of the m<strong>ed</strong>ium.<br />

This process was discover<strong>ed</strong> by Raman in 1928 and is known as the Raman effect [1].<br />

It can be describ<strong>ed</strong> quantum mechanically as scattering of a photon of energy ¯hω p<br />

by a molecule to a lower-frequency photon with energy ¯hω s , as the molecule makes<br />

transition to a vibrational state (see Figure 8.1). From a practical perspective, incident<br />

light acts as a pump and generates the frequency-shift<strong>ed</strong> radiation, the so-call<strong>ed</strong> Stokes<br />

wave, that serves as a spectroscopic tool. It was observ<strong>ed</strong> in 1962 that, for intense<br />

pump fields, the nonlinear phenomenon of SRS can occur in which the Stokes wave<br />

grows rapidly inside the m<strong>ed</strong>ium such that most of the pump energy is transferr<strong>ed</strong> to<br />

it [2]. Since then, SRS has been studi<strong>ed</strong> extensively in a variety of molecular m<strong>ed</strong>ia<br />

[3]–[7]. This section introduces the basic concepts, such as the Raman gain and the<br />

274

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!