Polarization scrambler/modulator

Abstract

A modulator and scrambler for utilization in lightwave transmission systems which utilizes an electrically controlled Faraday rotator to modulate the polarization of an optical signal. The modulator rotates the polarization of light by a controlled or time-varying amount due to the application of a variable magnetic field to a Faraday rotator material via an external electric coil. To permit high frequency excitation of the Faraday rotator a non-conducting, non-magnetic material such as glass or a composite is used for the casing of the Faraday rotator. A Faraday rotator material requiring a low intensity magnetic field, such as the Bismuth Iron Garnet films, is preferred. Several films may be stacked to increase the rotation range for a given amplitude of induced magnetic field. The modulator may include one or more optical reflectors to achieve multiple passes through the Faraday rotator to multiply the total rotation with modest additional insertion loss. The modulator modulates or scrambles the polarization of the optical signal in fiber amplifier applications to avoid the cumulative depreciation in signal to noise in long-range systems with many cascaded amplifiers (e.g., undersea) where the fixed polarization signal experiences lower gain than does the unpolarized noise in the system. The modulation frequency required to achieve these results in Erbium Doped Fiber Amplifiers may be in the tens of Kilohertz range which is well within the frequency response capability of garnet materials.