Hi.
Polarization-Maintaining (PM) Optic Circulators are non-reciprocating, unidirectional, three-port devices that are used in a wide range of optical setups. Available with a center wavelength of 1064, 1310 (O-Band), or 1550 nm (C-Band), these circulators are designed to work with light that is launched into the slow axis. Choose from versions with no connectors, FC/PC connectors, or FC/APC connectors.An optical circulator is analogous to an electronic circulator and both perform similar functions. An optical circulator is a three-port device that allows light to travel in only one direction. A signal entering Port 1 will exit Port 2 with minimal loss, while a signal entering Port 2 will exit Port 3 with minimal loss. Light entering port 2 experiences a large amount of loss at port 1, and light entering port 3 experiences a large amount of loss at ports 2 and 1. Optical circulators are non-reciprocal devices. This means that any changes in the properties of the light caused by passing through the device are not reversed by traveling in the opposite direction.
You may refer to these links for the design of PM circulator. I hope it helps.
https://www.agiltron.com/pdfs/1500%20pm%20circulator.pdf
http://www.phoenixphotonics.com/website/products/documents/PolarizationMaintainingCirculator_v01401.pdf
http://www.acphotonics.com/products/Product%20files/Short%20Wavelength%20Circulator/980nm%20PM%20Circulator.htm
Thanks