A Faraday mirror (or Faraday rotator mirror) is made by combining a Faraday rotator (for 45° rotation) with a mirror. Light hitting such a reflector device will thus first pass through the Faraday rotator, then be reflected at the mirror, and finally pass the Faraday rotator a second time (see Fig. 1).
Assuming that the Faraday rotator applies a single-pass rotation by 45° (which can be approximately achieved within a limited wavelength range), the polarization state at the output will always be orthogonal to that at the input:
- If the initial polarization is linear, the output polarization will also be linear, but with an orthogonal direction: rotated twice by 45°, in total 90°.
- If the initial polarization is circular, the output polarization will be circular with the opposite sense of rotation. (In that situation, the same would occur even without the Faraday rotator.)
The major imperfection of those devices is a double-pass rotation which deviates from exactly 90°, apart from the insertion loss.
Application in Double-pass Configurations
Interestingly, the orthogonality between input and output polarization is still obtained when another optical element is added to the Faraday mirror, which may introduce birefringence with an arbitrary amount and birefringence axis. For example, that can be a laser crystal for amplifying the beam (see Figure 2). One then obtains a double-pass amplifier where the output can be separated from the input with a polarizer, accepting only a minimum amount of loss. This works better than using a Faraday isolator (with an output port for reflected light), an amplifier and an ordinary mirror, where birefringence in the amplifier may modify the polarization state such that some of the light will consequently be lost at the polarizer.
Note, however, that the perfect polarization orthogonality may be lost if we have a substantial influence of diffraction in the optical setup. Therefore, one should work with collimated beams, having a low beam divergence. Nonlinear polarization rotation can also not be compensated.
Fiber-optic Faraday Mirrors
There are also Faraday mirrors in fiber optics, where the input and output light goes through an optical fiber – usually, a single-mode fiber. It can be a “fiber-optic pigtail”, but versions with a fiber connector (e.g. FC/PC or FC/APC) are also available.
Another important specification is the precision of polarization rotation. Some devices achieve a rotation angle accuracy of e.g. 1° over a 30-nm wavelength range.
Fiber-optic Faraday mirrors are used for similar purposes as bulk-optic devices, often for exploiting the compensation of random birefringence in some length of passive or active fiber. Applications are in different fields of fiber optic, for example in optical fiber communications, fiber amplifiers, measurement technology with fiber interferometers and fiber-optic sensors.
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