In the forward direction, the first wedged crystal splits the input beam into e and o beams. Though most diode lasers emit linearly polarized beams with about a 20-dB extinction ratio, regular singlemode fibers do not maintain the input polarization status-which explains why most passive components, including isolators, are polarization-independent (see Fig. The birefringent crystal-based structure makes a polarization-independent isolator, and the polarizer-based module makes a polarization-dependent isolator. A pair of microlenses such as gradient-index lenses usually couples the optical beam from one fiber to the other. The core of a typical commercially available isolator contains a Faraday rotator with 45° rotation and a pair of birefringent crystals or a pair of polarizers. There have been numerous designs of different types of optical isolators such as all-fiber isolators 1,2, fiber-embedded isolators, fiber Faraday rotator isolators 3, and waveguide-based isolators 4. Whereas most passive components are reciprocal, isolators are normally nonreciprocal-that is, they allow the passage of an optical beam in the forward direction with minimal losses (<0.5 dB) while blocking the transmission in the backward direction with 40- to 70-dB losses. Isolators are playing an increasingly important role in fiber-optic systems. These components typically fall into four categories-connecting devices such as connectors and splices branching devices such as couplers and splitters filtering devices such as fiber Bragg gratings, add/drop filters, and wavelength-division multiplexers (WDMs)/demultiplexers, and performance-improving devices such as attenuators, polarizers, dispersion compensators, and isolators. In a modern optical transmission system many types of passive fiber-optic components are used to perform various functions. Optical reflection can degrade performance of lasers or amplifiers unless blocked by isolators.
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