**Figure 1.**

*(a) Spontaneous Raman scattering: incident photons inelastically scatter off spontaneously from vibrationally excited molecules, behaving independently. (b) Stimulated Raman scattering: two incident lights, a pump and a Stokes laser beams, whose energy difference matches a particular vibrational energy, drive the molecule at ων = ω<sup>S</sup> − ωP, producing coherent Raman signals at ωS = ωP + ων. SRS modalities are: SRG, stimulated Raman gain; SRL, stimulated Raman loss. (c) Stimulated Raman scattering: inelastic scattering of probe photons off from vibrationally excited molecules that interfere coherently.*

Due to its Raman-shifted output, SRS is a workable method for generating coherent radiation at new frequencies. SRS permits, in principle, the amplification in a wide interval of wavelengths, from the ultraviolet to the infrared. Since the Raman frequency of a medium is usually fixed, the tunability can be achieved by using a tunable pump laser. Raman lasing occurs when the Raman-active gain medium is placed between mirrors, reflecting the first Stokes wavelength. This is analogous to lasers, where the gain medium must be placed inside a cavity to achieve laser threshold. Raman lasers and traditional lasers differ in the wavelength of light required for pumping. In the case of Raman laser, it does not depend on the electronic structure of the medium, so the wavelength of pump laser can be chosen to minimize absorption.

In order to tailor Raman laser characteristics and performances, there are two main basic configurations. The first one, external-resonator Raman laser, the Raman crystal is placed inside a cavity, resonating the Stokes field (**Figure 2(a)**). This configuration is used for pump pulses that are longer than the transit time through the Raman crystal. The second one, the intracavity Raman laser (**Figure 2(b)**) combines both a Raman medium and the laser medium inside a single cavity, so that the fundamental and Stokes fields are both resonating within the cavity [3].

Raman amplification, demonstrated in the early 1970s, is a feasible approach for fiber optics amplification, being only restricted by the pump wavelength and Raman active modes of the gain medium [4]. In this case, optical fiber is used as Raman gain medium and both pump and signal waves are launched into it (**Figure 2(c)**). In the past century, fused silica has been the main material used for transmission of optical signals, because of its good optical properties and attractive trade-off between Raman gain and losses. The main disadvantage of the current silica fiber amplifiers is the limited usable bandwidth for Raman amplification (5 THz, approx. 150 cm<sup>−</sup><sup>1</sup> ). A development in fiber optics communications was achieved opening the communication range to span from 1270 to 1650 nm, corresponding to about 50 THz bandwidth [5]. For future amplification requirements, due to this significant increase in bandwidth, the use of existing Er-doped fiber amplifiers is kept out, while Raman gain becomes the key mechanism.

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**Figure 2.**

*Stimulated Raman Scattering in Micro- and Nanophotonics*

Silicon photonics is an important player in the low-cost optical interconnect technology, as silicon-based optical components could be manufactured using the existing silicon fabrication techniques [6]. Silicon on insulator (SOI) waveguides allows to limit the optical field into an area 100 times smaller than the modal area of a typical single-mode optical fiber. In addition, the Raman gain in silicon is much stronger than in glass (≈10,000 times), therefore allowing to reduce the length required from kilometers of fiber to centimeters of silicon waveguides [2]. The waveguide approach, schematically reported in **Figure 2(d)**, led to the demonstration of pulsed Raman silicon laser [7] and continuous-wave (CW) lasing [8]. The merit of this approach is the ability to use pure silicon without the need for Er doping; i.e. it is fully compatible with silicon microelectronics manufacturing. On the other hand, there are three main limitations. The first, Raman laser cannot be electrically excited and it requires an off-chip pump. The second, the narrowband (105 GHz) of stimulated Raman gain makes it unsuitable for its use in WDM applications, unless expensive multi-pump schemes are implemented. The third,

*Basic configurations of Raman laser: (a) external-resonator Raman laser; (b) intracavity Raman laser;* 

*(c) fiber Raman amplifiers; and (d) silicon on insulator waveguide Raman laser.*

*DOI: http://dx.doi.org/10.5772/intechopen.80814*

*Stimulated Raman Scattering in Micro- and Nanophotonics DOI: http://dx.doi.org/10.5772/intechopen.80814*
