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.

Abstract: An optical isolator includes an optical isolator unit including a main polarization discriminator in concatenation with a Faraday rotator, and an auxiliary polarization discriminator disposed in series with the optical isolator unit. One of the polarization discriminators initially divides an applied optical signal propagating in a forward direction along a reference axis into two polarized signals subsequently experiencing polarization dispersion and differential transverse deflection, while the other polarization discriminator eventually eliminates the polarization dispersion. The auxiliary discriminator includes a plate of a uniform thickness inclined at a tilt angle with respect to the reference axis selected so that the transverse deflection difference incurred during the passage through the one polarization discriminator is eliminated in the other polarization discriminator. This facilitates achieving very low polarization dependent loss in the isolator.

Abstract: An optical isolator utilizes a pair of polarization selective elements, as for example birefringent wedges, and all integral Faraday rotator aligned therewith to perform optical signal isolation without introducing the polarization mode dispersion inherent in conventional polarization independent optical isolators. Optical isolation is accomplished by passing a forward-directed optical signal through a first birefringent element which separates the optical signal into two orthogonal states. The two orthogonal polarization states exchange identities upon entering the second birefringent element from the Faraday rotation element and are again deflected by the second birefringent element so that they emerge from the second element parallel to each other and, having traveled the same optical path length, witllout any polarization mode dispersion. Both polarization states of the reverse propagating optical signal are sufficiently angularly deflected to avoid coupling with the optical signal path.

Abstract: Disclosed is means for driving a laser whereby a dc bias current is supplied to keep the laser normally above threshold, and a modulation current brings the laser below threshold when a pulse appears. In one embodiment, a feedback controlled bias current source (11) and modulation current source (15) are coupled to the p-side terminal of the laser (10) while a constant voltage source (27) is coupled to the n-side terminal of the laser in a way which results in subtraction of the modulation current from the dc bias current. This mode of operation essentially eliminates laser overshoot and more nearly equalizes rise and fall times of the light pulses.