ISSN 2310-4090 An introduction to Stimulated Raman Scattering and its Applications in Optical Fiber Communications Saimunur Rahman1 1 Dept. of Computer Science and Engineering, International Islamic University Chittagong Chittagong City Campus, Chittagong, Bangladesh Keywords: Fiber nonlinearities; Stimulated Raman Scattering; Optical fiber Communications; SRS Applications; Inelastic-scattering. Abstract The nonlinear scattering effects in optical fiber occur due to inelastic-scattering of a photon to a lower energy photon. This Correspondence: Saimunur Rahman. D e p t . o f C o m p u t e r Science and Engineering, International Is lamic University Chittagong, Chittagong City Campus, Chittagong, Bangladesh . E-mail: saimun1992@gmail.com review presents the stimulated Raman scattering and some of its applications in fiber optic communications. Funding Information: No funding information provided. Received: June 2014; Accepted: July 2014 International Journal of Scientific Footprints 2014; 2(3): 3 1 –45 I. Introduction The nonlinear Raman phenomenon was distributed or discrete signal amplification. observed by C. V. Raman in 1928. In 1971, Even if discovered many years ago [43] and the stimulated Raman scattering (SRS) in highly glass fiber was observed by Stolen et al. [41]. applications of Stimulated Raman Scattering The same group in 1972 measured the Raman (SRS) presented a renewed interest for gain in single-mode fiber [42]. More recently, compensation of optical losses in fibers the optical transmissions [45], for the development of amplification in optical telecommunications in new tunable laser sources [46] or for low SRS has been used for investigated in the 2014. The Authors, International Journal of Scientific Footprints This is an open access article which permits use, distribution and reproduction in any medium, with the condition that original work is properly cited. past [44], Int. j. sci. footpr. Rahman, S. (2014) noise amplification of optically carried radio implementable frequency Raman providing narrowband gain or lasing in amplification in optical fibers started early in silicon-on-insulator waveguide devices at the the 1970s [41]. The advantages from Raman wavelengths for telecommunications [54]. amplification in the transmission fiber were The first experiment of spontaneous Raman studied since the mid-1980s [47]. But, around emission in silicon waveguides in 2003 [52] 1995, when the maturity of suit-able high was followed by the demonstration of power pump lasers was achieved [48] new stimulated interest in Raman amplification emerged. parametric Raman wavelength conversion Researchers have showed some of the [55]. signals. Research on advantages that Raman amplifiers have over EDFAs, particularly when the transmission fiber itself is used as a Raman amplifier [49, 50]. This enabled to increase the advances in Raman amplifier technology [51]. Some of these advances are the novel Raman pumping schemes recently used in transmission experiments. and Raman attractive result scattering [53] for and The Raman Effect in silicon is advantageous since it does not need rare earth dopants and its spectrum is widely tunable through the pump laser wave-length. The use of germanium in the nonlinear Raman processes in silicon presents new possibilities for adjusting the device characteristics. Recently, the first GeSi optical Raman amplifier and Recently, there has been much investigation laser were demonstrated [56]. The results in order to obtain devices to amplify or indicate that the spectrum of Raman scattering generate light using stimulated Raman scatter- can be tailored using the GeSi material ing in silicon [52]. The Raman coefficient of system. Therefore, GeSi Raman devices silicon is several orders of magnitude larger represent a stimulating subject for future than silica [53], thus reducing the needed research and development. interaction lengths for stimulated Raman scattering and optical gain to practical lengths for planar waveguides [54]. As the first order Raman scattering shift in silicon is 15.6 THz and the 1400-1500 nm wavelength range high In this work, the author presents an overview of stimulated Raman scattering and its applications. Stimulated Raman Scattering power pump lasers are already commercially available, Raman amplification is a possibly The Raman scattering effect is the inelastic scattering [1] of a photon with an optical Int. j. sci. footpr. Rahman, S. (2014) phonon, which originates from a finite slightly reduced energy response time of the third order nonlinear that: (Figure 1) such polarization [20] of the material. When a monochromatic light beam propagates in an optical fiber, spontaneous Raman scattering occurs. It transfers some of the photons to new frequencies. The scattered photons may lose energy (Stokes shift) or gain energy (antiStokes shift). If the pump beam is linearly polarized, the polarization of scattered photon may be the same (parallel scattering) or orthogonal (perpendicular scattering). If photons at other frequencies are already present then the probability of scattering to those frequencies is enhanced. This process is known as stimulated Raman scattering. The modulation in refractive index is taken into account through discussion of polarizability of material in case of Raman scattering process. To understand this, the classical model of Raman scattering may be a simple way. In this model, it is assumed that electrons are attached to an atom through a spring, and the strength of the spring is assumed to depend on the position of the atom. If atom is in vibrational motion with angular frequency , then spring constant is modulated at angular frequency . If a light wave of angular frequency propagates In stimulated Raman scattering, a coincident through the material, the motion of electron photon at the downshifted frequency will will be amplitude modulated sinusoidal receive a gain. This feature of Raman motion. Therefore the radiation generated by scattering is exploited in Raman amplifiers for the electron will also be amplitude modulated. signal amplification. This radiation has components corresponding to Stokes and anti- A. Basic Theory Stokes Raman scattering. Raman scattering is a weak effect in comparison to Rayleigh scattering. It occurs When a light wave with angular frequency ω due to slight modulation of the refractive is incident on the material, the electric field index through molecular vibration of material vector will induce a dipole moment p such [2, 15]. A photon with energy that: travelling through a material can excite a vibrational (1) transition of the material forming optical Where α is molecular polrizability and E is phonon with energy electric field vector. The α measures the and a photon with Int. j. sci. footpr. Rahman, S. (2014) resistance of the particle to the displacement dipoles per unit volume then, of its electron cloud. Fig. 1: Schematic Representation of Raman This expression consists of two parts. The first Scattering. part corresponds phenomenon, and to linear relative to optical incident radiation, it remains un-shifted. The second part is nonlinear because the output frequency is different from input one. Fig. 2(a): Stokes scattering process For harmonic electric field , the variation of α with time can be written as (2) Here dx(t) is the displacement from the equilibrium molecular length x_0 such that (ħωs = ħωp – ħωv) Fig. 2(b): Anti-Stokes scattering process (3) Now, (4) Using Equations (2) and (3), p(t) can be obtained as (ħωA= ħωp + ħωv) The polarization vector P is defined as dipole moment per unit volume. If there are N The scattered light with lower energy Int. j. sci. footpr. Rahman, S. (2014) ) corresponds to Stokes scattering ( (Figure 2(a)) and with higher energy the forward Raman process (Figure 2(a)) and the energy conservation for the process is ) one has anti-Stokes scattering ( phenomenon (Figure 2(b)). In thermal Where and are ground state and final equilibrium situation, because of greater population of the ground state in comparison state energies respectively. to vibrational state, the Stokes scattering The absorption of incident photon, the dominates. At low illumination levels, the emission of scattered photon and transition of spontaneous Raman scattering occurs because the in this situation molecules contributing to the simultaneously in one step. Therefore, Raman process are vibrating independently and hence process may be considered as a single step scattered light is non-directional. But when process, which makes stimulated Raman the intensity level becomes high the molecules effect possible whenever sufficient numbers may be considered as an array of vibrating of Stokes photons are created. At this juncture oscillators and the generated photons aligned it is worth to mention that, in step wise in phase or behave coherently. This results in transitions, the absorption and emission of stimulated Raman scattering (SRS). photons occur through two consecutive single B. The Raman Process molecule to excited state occurs quantum transitions via a third molecular energy level. Such transitions are associated In quantum mechanical picture, Raman effect with complete disruption of the phase of a is a process, which involves double quantum molecule after each act of absorption and molecular transition. In most frequent Stokes emission of a single quantum. scattering process, the energy of incident photon is reduced to lower level C. SRS Spectrum and difference energy is transferred to With classical electromagnetic concepts, the molecule of silica in form of kinetic energy, growth of stimulated Raman scattered signal inducing stretching, bending or rocking of the intensity [1] is proportional to the product of molecular bonds [21]. The Raman shift the pump is dictated by the vibrational energy levels of silica. The Stokes Raman process is also known as that and signal intensities such Int. j. sci. footpr. Rahman, S. (2014) [26] is exploited in broadband Raman amplifiers. Here is known as Raman-gain coefficient. D. Threshold Power In order to generate stimulated emission, Stokes and pump waves must overlap The initial growth in stokes wave is given by spatially and temporally. The Raman-gain Equation (6). Considering the fiber losses, the coefficient g_R is related to cross-section of net growth in Stokes wave is written as spontaneous probability Raman of a scattering. Raman scattering The is proportional to the number of photons in Where pump wave per cross-sectional area and Fig. 4: Spectrum of Raman gain for silica Raman cross-section. The material properties at pump wavelength 1µm is attenuation coefficient. determine almost entirely the frequency spectrum of Raman cross-section because the Raman process is related to vibrational modes of the molecules of material. In crystalline materials, the Raman scattered light has a narrow bandwidth. The silica, which is main constituent of optical fiber, is amorphous in nature. The vibrational energy levels of such materials are not sharp but merge together and form a band [24]. In such a situation the Stokes frequency frequency may differ from pump For pump wave the coupled equation can be written as over a wide range. Two major peaks occur at 13 THz and 15 THz for Raman . For this shift, some miner Equations (7) and (8) are known as coupled peaks are also present in spectrum [25]. wave equations for forward Raman scattering Therefore, the amorphous nature of silica is process [6]. In case of backward SRS process, responsible for large bandwidth and multipeak Equation (8) remains same but in Equation (7) nature of spectrum (Figure 3). This extension a minus sign must be added to of Raman-gain over broad range in silica fiber set of equation is similar to SBS process. The shift . This Int. j. sci. footpr. Rahman, S. (2014) coupled equations for forward and backward Raman threshold [11]. Therefore first term on SRS right hand side of Equation (8) can be process may be understood phenomenologically by keeping in mind the neglected. processes through which photons appear in or disappear from each beam. In absence of losses due to fiber, Equations (7) and (8) can Solution of this equation can be written as be reduced to With Equation (7) and (11) we may have, This equation dictates the conservation law on total number of photons in pump and Stokes . Where, effective length, waves during the SRS process. Practically, SRS builds up from spontaneous The stimulatation occurs in Raman process Raman scattering occurring throughout the when pump power exceeds a certain power fiber length. The Stokes power can be level known as threshold power. In order to calculated by considering amplification of grow the stimulated scattering, the stimulated each frequency component of energy ħω gain must exceed linear loss. In fact this is the according to Equation (12) and integrating origin of threshold power. over the whole range of Raman-gain spectrum, i.e., SRS can occur in both directions i.e., forward and backward direction in optical fibers. The beat frequency drives the molecular oscillations. These oscillations are responsible The main contribution to the integral comes for increment in amplitude of scattered wave from narrow region around the gain peak. So which in turn enhances the molecular oscillations. In this way a positive feedback using , above equation can be written as loop is setup. It results in SRS process. The feedback process is governed by coupled Equations (7) and (8). In terms of power, the Equation (11) may be In case of forward SRS process the pump written as under depletion can be neglected for estimating the Int. j. sci. footpr. Rahman, S. (2014) The Equation (17) is derived by using many approximations, but it is able to predict the Where is input pump power and Raman threshold quite accurately. For a is effective core area. The Raman typical optical communication system at 1550 threshold is also defined as the input pump nm, , and power at which the Stokes power becomes . With these values equal to the pump power at the fibre output. Equation (17) predicts So, . As channel powers in optical communication systems are typically below 10 mW, SRS With assumption , the threshold process is not a limiting factor for single- condition may be approximated [11] by using channel light wave systems. However it Equation (14) and (16) we can write, affects the performance of WDM systems considerably. E. Threshold Power Exactly a similar analysis can be carried out for backward SRS, and threshold power can Many schemes can be applied for reduction of power penalty in SRS process [14, 15], such be approximated as as, Presence of dispersion reduces the SRS penalty. In presence of dispersion, signals in different channels travel at different velocities Clearly the threshold for forward SRS is and hence reducing chances of overlap reached first at a given pump power. The between pulses propagating at different wave backward SRS is generally not observed in lengths. fibers. By decreasing channel spacing SRS penalty can be reduced. Int. j. sci. footpr. Rahman, S. (2014) The power level should be kept below Perot cavity. Inside the cavity a piece of threshold level which requires the reduction in single mode fiber is placed in which SRS distance between amplifiers [27]. The SRS process occurs due wave length-selective imposed limitations on the maximum transmit feedback for the Stokes light. This results in power per channel is shown in Figure 5. intense output. The spatial dispersion of various Stokes wavelengths allows tuning of Fig. 5: SRS produced limitation on maximum transmit power per channel. Channel spacing = 0.8 nm, and amplifiers are spaced 80 km apart. the laser wavelength through an intra-cavity prism. The Raman amplification during a round trip should be as large as to compensate the cavity losses, and this determines the Raman threshold power. Fig. 6: Schematic Representation of A Tunable Raman Laser III. Applications of SRS Phenomenon Higher-order Stokes wavelengths are generated inside the fiber at high pump An easy way to comply with the conference powers. paper formatting requirements is to use this dispersed spatially by the intra-cavity prism in document as a template and simply type your association with separate mirrors for each text into it. Stokes beam. Such kind of Raman laser can The SRS process is exploited in many applications, which includes, Raman Fiber Laser: Fiber based Raman lasers [28, 29] are developed by employing the SRS phenomenon. The Figure 11 shows a schematic of Raman laser. The partially reflecting mirrors M1and M2form a Febry- be Again operated these at wavelengths several are wavelengths simultaneously. Raman Fiber Amplifier: The SRS phenomenon may be applied to provide optical amplification within optical fibers. The SRS process in fiber causes energy transfer from the pump to the signal. The Raman Int. j. sci. footpr. Rahman, S. (2014) amplification may occur at any wavelength as fundamental radiation resonator. Such eye- long as appropriate pump laser is available. safe laser has the highest output energy and There are three basic components of Raman shortest pulse width among the Nd:KGW amplifier: pump laser, wavelength selective lasers. coupler and fiber gain medium. A schematic diagram is shown in Figure 7. Raman IV. Conclusion amplification exhibits advantages of self- Stimulated phase matching and broad gain-bandwidth phenomenon is discussed in this paper. which is advantageous in wavelength division Normally SRS phenomenon put limitation on multiplexed systems [30]. optical systems. But with suitable system Fig. 7: Schematic of Raman fiber Amplifier Raman Scattering or SRS arrangement it can be exploited in many applications. Typical threshold power for SRS is about 570 mW. The typical value of channel power in optical systems is below 10 mW. 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