Faraday circulators (or less specifically optical circulators) are a kind of non-reciprocal optical devices. They are technically related to Faraday isolators, and on a broader scale similar to electronic circulators. Typically, a circulator has three or four optical ports (inputs / outputs), although there could in principle be more. These devices typically have three or four optical ports, although more can be incorporated. Light entering one port exits from the next port, or from the first port if injected into the last. For instance, in a three-port device, the light paths can be illustrated as:
- 1 → 2
- 2 → 3
- 3 → 1
Depending on the type of circulator, this may work only for specific polarization directions of input light, or in a polarization-independent manner.
In diagrams of optical setups, a circular symbol with an arrow is used to denote the direction of circulation (refer to Figure 1).
The non-reciprocal behavior is evident when light enters the device at port 1 and exits at port 2, but the reverse is not possible: light entering port 2 exits at port 3, not port 1.
Faraday circulators are magneto-optic devices based on the Faraday effect – rotation of the polarization – as explained in the article on Faraday rotators. Typically, one uses a Faraday rotator with 45° rotation of the polarization, often in combination with a <$\lambda/2$> waveplate oriented such that it compensates for the effect of the rotator in one propagation direction, while for the other propagation direction one obtains a 90° change of polarization direction.
As an example, a four-port circulator can be constructed as shown in Figure 2. As explained above, the combination of Faraday rotator and waveplate in the middle will maintain the linear polarization of light moving from left to right, while rotating the polarization of light moving from right to left by 90°.
That device would not work, for example, with s polarization sent into port 1; that light would be sent to the bottom by the first polarizer, where no port is installed.
More advanced designs can accommodate any input polarization to any port, a feature often desired in fiber optics. This is achieved by creating different geometric paths through the Faraday rotator and waveplate, based on the polarization direction.
Some Faraday isolators are already equipped with an additional output port, where back-reflected light from the output emerges. However, what is missing to a circulator is that light sent into that output port gets back to the original input.
Fiber-optic and Other Faraday Circulators
Although Faraday circulators are usually bulk-optical devices, where light beams travel through homogeneous optical media (the rotator crystal, polarizers, and air), they can be equipped with optical fiber ports, each of which has a collimating/focusing lens. That makes them suitable for applications in fiber optics, such as optical fiber communications. Indeed, fiber-optic circulators are used more widely than bulk-optical variants, which are not common in bulk laser technology, but used in some areas such as LIDAR.
In numerous instances, fiber-optic Faraday circulators are equipped with single-mode fibers for use in the 1.5-μm telecom band. They are fairly compact and lightweight devices. They are available in polarizing and polarization-independent variants, and are typically suitable for some hundreds of milliwatts of optical power.
The above-described operation applies to a hypothetical ideal circulator. A real circulator exhibits various kinds of imperfections, which are the subject of technical product specifications:
- Insertion loss: there is some loss of optical power of light going through it. (It can be well below 1 decibel.)
- Incomplete isolation: some (usually rather small) non-intended optical powers emerging from other output ports, or reflected back to the input port (finite extinction ratio or return loss, typically several tens of decibels).
- Beam distortions: for bulk-optical devices, the wavefronts of transmitted light may be somewhat deformed.
These performance figures depend on the optical wavelength; circulators work well only within a limited optical bandwidth. One may specify lower limits of the mentioned performance factors within the whole bandwidth.
Typical Applications of Faraday Circulators
A few typical applications of fiber-optic Faraday circulators are briefly explained in the following:
- One can integrate a reflective fiber-optic component, e.g. a fiber Bragg grating, into a fiber-optic light path (see Figure 3). That is used, for example, in some fiber ring lasers and for signal waves in optical fiber communications systems.
- Similarly, one can realize a double pass through some device, such as a fiber amplifier.
- There are also various applications in the area of fiber-optic sensors.
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