![]() In the single-stage reflective isolator, a Faraday rotator for 22.5-degree rotation is used instead of the common 45-degree Faraday rotator due to the fact that a beam is passed through the Faraday rotator twice, providing a total rotation of 45 degrees. With the reflectivedesign, the material usage can be reduced almost by a factor of 2. Rongqing Hui, Maurice O'Sullivan, in Fiber Optic Measurement Techniques, 2009 3.5.5.1 Optical IsolatorsĪn optical isolator is a device that only allows unidirectional transmission of the optical signal. It is often used in optical systems to avoid unwanted optical reflections. For example, a single-frequency semiconductor laser is very sensitive to external optical feedback. Even a very low level of optical reflection from an external optical circuit, on the order of –50 dB, is sufficient to cause a significant increase in laser phase noise, intensity noise, and wavelength instability. Therefore an optical isolator is usually required at the output of each laser diode in applications that require low optical noise and stable optical frequency. ![]() ![]() Another example is in optical amplifiers, where unidirectional optical amplification is required. In this case, bidirectional optical amplification provided by the optical gain medium would cause self-oscillation if the external optical reflections from, for example, connectors and other optical components are strong enough. The traditional optical isolator is based on a Faraday rotator sandwiched between two polarizers, as shown in Figure 3.5.23. In this configuration, the optical signal coming from the left side passes through the first polarizer whose optical axis is in the vertical direction, which matches the polarization of the input optical signal. Then a Faraday rotator rotates the polarization of the optical signal by 45° in a clockwise direction. The optical axis of the second polarizer is oriented 45° in respect to the first polarizer, which allows the optical signal to pass through with little attenuation. If there is a reflection from the optical circuit on the right side, the reflected optical signal has to pass through the Faraday rotator from right to left. Since the Faraday rotator is a nonreciprocal device, the polarization state of the reflected optical signal will rotate for an additional 45° in the same direction as the input signal, thus becoming perpendicular to the optical axis of the first polarizer.
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