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. 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. Then a Faraday rotator rotates the polarization of the optical signal by 45° in a clockwise direction. 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. The traditional optical isolator is based on a Faraday rotator sandwiched between two polarizers, as shown in Figure 3.5.23. 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. Another example is in optical amplifiers, where unidirectional optical amplification is required.
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. 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. For example, a single-frequency semiconductor laser is very sensitive to external optical feedback.
It is often used in optical systems to avoid unwanted optical reflections. 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. With the reflectivedesign, the material usage can be reduced almost by a factor of 2. 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. The operating principles of the reflective isolators are the same as those of the transmissive designs, except for the sharing of common elements and the utilization of a mirror for beam folding. Typical designs of single-stage and two-stage reflective isolators are shown in Figs. In the reflective design, all ports of the device are coming out from one side, further providing the advantage of easy installation in the applications. To further simplify design and reduce materials usage and cost, a reflective design concept has been introduced based on the fact that most of the transmissive designs have an image plane and all elements are symmetric in respect of the image plane. Optical isolators and circulators mentioned in Sections I and II are so-called transmissive devices that is, the light is propagated along one direction and the input and output ports are on the opposite side of the devices.
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