The light will then pass through the FaradayRotator and experience an additional 45° of nonreciprocal rotation. As Figure 4 shows, light traveling in the reverse direction willpass through the output polarizer and become polarized at 45°. The output polarizer, which has been aligned 45° relative to the inputpolarizer will allow the light to pass unimpeded. Upon passing through the Faraday Rotator, the plane of polarizationwill have been rotated 45° on axis. As shown in Figure 3, light traveling in the forward direction passes through the input polarizer andbecomes polarized in the vertical plane. Operation of a Faraday IsolatorA Faraday Isolator consists of three main components, an input polarizer, a Faraday Rotator and an outputpolarizer. The amount of polarization rotation is, θ = V∫Hzdzwhere: θ = polarization rotation angle V = Verdet constant of optical element Hz = longitudinal component of magnetic intensity vector z = length of optical length of optical element Electro-Optics Technology, Inc. Faraday Rotators utilize high strength, rare earthpermanent magnets in conjunction with a high damage threshold optical element to provide a uniform 45°polarization rotation to light passing through the device. Figure 2 illustrates theeffectiveness of EOT's Faraday Isolators in eliminating this problem.Figure 1 Figure 2At the heart of a Faraday Isolator is a Faraday Rotator. The noise emissionscorrespond to relaxation oscillations in the laser being driven by residual feedback. Figure 1 illustrates noise emission displaced 60kHz from the actual laserfrequency of a mode-locked laser due to reflections from an uncoated glass surface. A Faraday Isolator’s ability to allow light to pass unimpeded in one direction, while stronglyattenuating light traveling in the opposite direction, eliminates the negative effects of optical feedback. As lasers have improved, the need to protect laser oscillators and laser diodes from optical feedbackhas increased. Problemsinclude frequency instability, relaxation oscillations, amplified stimulated emission, and in some cases, opticaldamage.

IntroductionThe negative effects of optical feedback on laser oscillators and laser diodes have long been known.

935 optical isolator