An optical circulator is an apparatus that has typically three or four ports for accepting optical signals. In a three-port optical circulator, light that enters the circulator through the first port exits through the second port, and light that enters the circulator through the second port exits through the third port. An optical circulator is an inherently non-reciprocal apparatus, since if light enters through the first port it exits through the second port, but if that light is subsequently reflected back from the second port, it does not retrace its path back to the first port, but exits through the third port instead.
Optical circulators are necessary, for example, when using the same fiber for both receiving and transmitting data. The first port may be connected to a data transmitter, and the second port to an end of a long distance optical fiber. Data can be sent from the transmitter to a distant receiver connected to the distant end of the fiber. At the same time, incoming data from a distant transmitter connected to the same fiber enters the circulator through the second port and is directed to the third port where a local receiver may be connected. By installing an optical circulator at each end of a fiber link, a single fiber can support bi-directional communication. An optical circulator can also be used in devices such as wavelength division multiplexers (WDMs), Erbium-doped fiber amplifiers (EDFAs), Raman amplifiers, add-drop multiplexers, dispersion compensators and optical time domain reflectometers (OTDRs).
It should be clear to those skilled in the art that an optical circulator can be readily used as an optical switch with no moving parts if the rotating assembly(s) used can be externally controlled, e.g. by an electrical field or a magnetic field. It is also clear to those skilled in the art that the rotating assembly used in an optical switch can be a Faraday rotator, an electro-optic crystal, a liquid crystal or their equivalents. For example, if a switching electrical field can externally control the rotations of Faraday rotator(s) of a three-port apparatus, the light that enters the second port of the apparatus can be alternatively switched between exiting the first port and exiting the third port.
Optical circulators are now key elements in modern optical networks. However, they have not been widely adopted due to high costs of materials and manufacturing. A typical polarization independent optical circulator usually has many optical elements and a large optical footprint. The manufacturing of a conventional optical circulator requires precise alignment of each separate optical element, which leads to low productivity and high production costs. It is an existing challenge to simplify not only the design of an optical circulator but also the manufacturing procedure.
U.S. Pat. Nos. 5,930,039, 6,052,228, and 6,226,115 disclose recent attempts to improve optical circulator designs. Each of these circulators, however, still suffers from one or more of the following disadvantages: high material cost, large optical footprint, alignment difficulty, and complicated structural configuration. In view of the above, it would be an advance in the art to provide a compact optical circulator (or switch) that has fewer and less expensive optical components, easier manufacturability, less sensitivity to alignment, and improved performance.