Model and Simulation of a Tunable Birefringent Fiber Using Capillaries Filled with Liquid Ethanol for Magnetic Quasiphase Matching In-Fiber Isolator

A technique to tune a magnetic quasi-phase matching in-fiber isolator through the application of stress induced by two mutually orthogonal capillary tubes filled with liquid ethanol is investigated numerically. The results show that it is possible to “tune” the birefringence in these fibers over a limited range depending on the temperature at which the ethanol is loaded into the capillaries. Over this tuning range, the thermal sensitivity of the birefringence is an order-of-magnitude lower than conventional fibers, making this technique well suited for magnetic quasi-phase matching. 1. Introduction Optical isolators play a critical role in the fabrication of high-power fiber amplifiers. Their purpose is to protect optical sources from light traveling in the backward direction which can cause damage to the optical source and instabilities in the output spectra. Optical isolators come in two flavors: polarization-dependent and polarization-independent. The polarization dependent isolator consists of an input polarizer, a Faraday rotator, and an output polarizer. Polarization independent isolators consist of an input birefringent wedge (with its ordinary polarization direction vertical and its extraordinary polarization direction horizontal), a Faraday rotator, and an output birefringent wedge oriented at 45° to the first. Ideally the birefringent crystals and Faraday rotator should have low absorption coefficients at the wavelengths of interests, low nonlinear refractive indices, and high damage thresholds. Also, the Faraday rotator should have a high Verdet constant to achieve the highest degree of rotation with minimal length to prevent self-focusing inside the material and other thermal effects, as well as nonreciprocal nonlinear polarization coupling at high power. The major sources of backward traveling light in high-power fiber lasers and amplifiers are reflections from the fiber output facet, spontaneous Raleigh scattering (SRS), stimulated Brillouin scattering (SBS), and amplified spontaneous emission (ASE). While certain techniques may be employed to minimize backward-traveling light, it is often a safe practice to incorporate isolators capable of suppressing backward power of at least 1–5% of the output power of the laser or amplifier. Current (free space) isolators can handle average powers up to the order of 100？W with limited beam distortions. For fiber-coupled devices, the power levels are currently limited to about 50？W. Recent demonstrations show continuous wave (CW) diffraction-limited power in fiber amplifiers scalable to the kW level