Scattering losses in fiber are caused due to microscopic fluctuations in material density, variations in composition, and structural heterogeneities or defects that may occur during the manufacturing of the fiber. There are mainly two types of scattering losses: linear scattering loss and nonlinear scattering loss.

Nonlinear scattering losses result in uneven attenuation of the light while traveling through the fiber at high optical power levels. In optical fibers, modes refer to different spatial and polarization distributions of light waves that can propagate through the fiber. The transfer of optical power from one mode to other modes at a different frequency in either the forward or backward direction result in non-linear scattering losses. It depends on the optical power density inside the fiber and becomes significant only above threshold power levels. 

Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS) are the two different types of nonlinear scattering. Both are mainly observed at high optical power densities in long single-mode fibers because the interaction length between the light and acoustic waves increases, leading to a larger amount of energy loss.

These scattering mechanisms lead to optical gain accompanied by a frequency shift. As a result, they contribute to attenuation in the transmission of light at a particular wavelength. 

Stimulated Brillouin Scattering

Figure 1: Stimulated Brillouin Scattering

Stimulated Brillouin Scattering is a nonlinear scattering process that happens when light interacts with the acoustic waves in a medium. Due to this interaction, the energy is transferred from the light incident to the acoustic wave leading to the creation of a density grating or diffraction grating, which is a periodic modulation of the refractive index of the fiber core. SBS results in the modulation of light due to molecular vibrations induced by heat inside the fiber. This results in scattered light that appears as upper and lower sidebands, separated from the incident light by the modulation frequency. The photon incident in this process will produce a phonon of acoustic frequency and a scattered photon. This creates a frequency shift between the incident and the scattered light beams due to the movement of the diffraction grating formed by the acoustic wave known as Stoke’s shift. This shift varies with the scattering angle since the sound wave frequency changes with the acoustic wavelength. SBS is mainly a backward process as the frequency shift is highest in the backward direction and gradually reduces to zero in the forward direction.

The threshold power for stimulated brillouin scattering, P_B, is given by:

where d represents the diameter of the fiber core in micrometers, λ denotes the operating wavelength in micrometers, α_dB gives the fiber attenuation in decibels per kilometer and ν represents the source bandwidth in gigahertz. This equation determines the threshold optical power that must be launched into a single-mode optical fiber before the occurrence of SBS.

The threshold power for SBS in optical fibers typically ranges from a few watts to tens of watts, depending on the fiber's properties and the specific application. 

The intensity of the scattered light is proportional to the square of the incident light intensity. SBS is important only at high optical powers, where the nonlinear effects dominate.

Stimulated Raman Scattering

Figure 2: Stimulated Raman Scattering

Stimulated Raman Scattering is a type of nonlinear scattering that occurs when the incident light interacts with the vibrational modes of the molecules in a medium. This causes energy transfer from the incident light to the vibrational mode of the medium, resulting in scattering losses. The incident light will be a high intensity laser beam with a specific wavelength. This highly intense laser beam causes energy transfer from the laser beam to the molecules of the medium and creates a scattered light beam. The light scattered has lower frequency than the incident light frequency. Stimulated raman scattering is similar to SBS, but distinguishes itself by generating a high-frequency optical phonon instead of an acoustic phonon during the scattering process.

The stimulated raman scattering is shown in figure 1. Here the orange line depicts the incident high frequency photon transition. It gets excited and then re-emits photons with a lower frequency. These new downshifted photons are called Stokes photons. They propagate in both the forward and backward directions in an optical fiber, and it may require a significantly higher optical power threshold, up to three orders of magnitude higher than the Brillouin threshold, in a specific fiber.

The threshold optical power for stimulated raman scattering, P_R, in a long single-mode fiber is given by:

where d is the fiber core diameter, λ is the operating wavelength and α_dB is the attenuation of fiber in decibels per kilometer.

Applications of Stimulated Brillouin Scattering

Stimulated Brillouin scattering can be used in fiber-optic sensing applications to detect temperature, strain, pressure, and other physical parameters. By measuring the shift in the Brillouin frequency, the change in the physical parameter can be determined.

It can be used for signal processing applications such as narrowband filtering, pulse compression, and phase conjugation. By using SBS to generate a stimulated Brillouin scattering mirror, the phase and amplitude of a signal can be reversed.

SBS is used for laser frequency stabilization by using the Brillouin shift to provide a reference frequency for the laser. This technique is commonly used in high-precision spectroscopy applications.

It can also be used in optical communications to compensate for signal distortion caused by fiber dispersion. A backward-propagating wave in the fiber is generated using SBS and the distortion can be corrected.

SBS is used to study a variety of nonlinear effects. For example, SBS can be used to study the dynamics of solitons in optical fibers.

Applications of Stimulated Raman Scattering

Stimulated Raman scattering is widely used in Raman spectroscopy to identify the chemical composition of a material. By analyzing the frequency and intensity of the emitted photons, the chemical bonds and functional groups in the sample can be identified.

SRS is used in optical microscopy to image biological tissues and cells. By selectively exciting the Raman-active molecules in the sample, SRS can provide label-free images with high spatial and temporal resolution.

It is an important phenomenon in nonlinear optics and has been used to study different nonlinear effects. It is used to generate ultrashort laser pulses with high spectral bandwidth.

SRS can be used in optical fiber communication systems to extend the transmission distance and increase the data rate. The wavelength of the transmitted light is shifted using SRS and the signal can be transmitted over longer distances without significant signal degradation.

SRS can also be used to convert the frequency of laser light to a higher or lower frequency. This technique is widely used in laser-based applications such as laser cooling, frequency doubling, and frequency shifting.