B. The Raman Process In quantum mechanical picture, Raman effect is a process, which involves double quantum molecular transition. In most frequent Stokes scattering process, the energy of incident photon (ћωP) is reduced to lower level (ћωS) and difference energy is transferred to molecule of silica in form of kinetic energy, inducing stretching, bending or rocking of the molecular bonds [1]. The Raman shift ωR (= ωp − ωs) is dictated by the vibrational energy levels of silica. The Stokes Raman process is also known as the forward Raman process and the energy conservation for the process is Eg + ћωP = Ef + ћωS Where Eg and Ef are ground state and final state energies respectively. The absorption of incident photon, the emission of scattered photon and transition of the molecule to excited state occurs simultaneously in one step. Therefore, Raman process may be considered as a single step process, which makes stimulated Raman effect possible whenever sufficient numbers of Stokes photons are created. At this juncture it is worth to mention that, in step wise transitions, the absorption and emission of photons occur through two consecutive single quantum transitions via a third molecular energy level. Such transitions are associated with complete disruption of the phase of a molecule after each act of absorption and emission of a single quantum. C. Threshold Power The incident photon produces a phonon of acoustic frequency & scattered photon. This produces an optical frequency shift varies with scattering angle. The frequency shift in maximum in backward direction & reduces zero in forward direction. The Threshold power is given by, Similar to the case of SBS, the threshold power P th is defined as the incident power at which half of the pump power is transferred to the Stokes field at the output end of a fiber of length L. It is estimated from [3]. R g P A Where g R is the peak value of the Raman gain. As before, L eff can be approximated by 1/α. If we replace A eff by πw 2 , where w is the spot size, P th for SRS is given by, th P If we use πw 2 = 50 μm 2 and α = 0.2 dB/km as the representative values, P th is about 570 mW near 1.55 μm. It is important to emphasize that Eq. (4) provides an order of magnitude estimate only as many approximations are made in its derivation. As channel powers in optical communication systems are typically below 10 mW, SRS is not a limiting factor for single-channel light wave systems.
CITATION STYLE
Kale, R., Ingale, P., & Murade, R. (2013). Comparison of SRS & SBS (Non Linear Scattering) In Optical Fiber. International Journal of Recent Technology & …, (1), 118–122. Retrieved from http://www.ijrte.org/attachments/File/v2i1/A0525032113.pdf
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