Analysis of acoustic modes induced by backward stimulated Brillouin scattering in acoustic wave-guided single mode optical fibers

In this work, the acoustic Helmholtz equation is derived, and its analytical solution and the characteristic equation of the uniform guide mode in single mode fibers are obtained by the method of separation of variables. The normalized frequency of the acoustic mode is defined. By combining the argument approximation of the Bessel function are analyzed the eigenvalue range of the acoustic mode, the cut-off frequency, far from the cutoff state of the acoustic mode induced by backward stimulated Brillouin scattering, the dispersion and the multi-peak Brillouin gain spectrum. The research results indicate that the longitudinal acoustic fundamental mode L-01 cannot be cut-off and is mainly confined in the fiber core, which is coupled with the optical fundamental mode LP01 to form the main peak of the Brillouin gain spectrum. The other higher-order acoustic modes all have low cut-off frequencies and are distributed more in the fiber cladding than mode L-01 which couples with the optical fundamental mode LP01 to form the subpeaks of the Brillouin gain spectrum. The transverse normalized phase constant and effective refractive index of the acoustic mode increase with normalized frequency increasing. Only longitudinal acoustic modes L-0n contribute to backward Brillouin gain spectrum in single mode fiber. When the GeO2 concentration is less than 4% and core radius is 4.5 mu m, the single mode characteristics of the fiber remain unchanged, but the maximum number of acoustic L-0n modes is 4. With the increase of GeO2 concentration in the fiber core, the Brillouin gain spectrum is red-shifted and the number of acoustic modes increases, the Brillouin gain peak value of L-01 mode gradually increases, and the contributions of higher-order modes decrease. The single-mode fiber with a core's germanium doped concentration of 3.65% and core radius of 4.3 mu m has 4 L-0n modes and 16 L-mn (m>0) modes at a wavelength of 1.55 mu m, with one main peak and two subpeaks in the Brillouin gain spectrum appearing due to the acousto-optic coupling of the acoustic L-01, L-03, and L-04 modes with the optical LP01 mode. The single-mode fiber with a core's germanium doped concentration of 15% and core radius of 1.3 mu m has 3 L-0n modes and 7 L-mn (m>0) modes, with the Brillouin gain spectrum having 3 main peaks due to the acousto-optic coupling of the L-01, L-02, and L-03 modes with the LP01 mode. These conclusions are well consistent with the reported experimental phenomena and provide theoretical references for studying and utilizing the SBS acoustic waveguide in optical fibers.