A polarisation diversity circuit in lithium niobate for coherent optical transmission

The authors describe an integrated optical circuit for polarisation diversity reception, in which the incoming signals from the link are resolved into TE and TM polarised components. These two signals are independently mixed with the TE and TM components of the LO laser output in a pair of 3 dB couplers. A fully balanced design has been adopted, with complementary output ports for each polarisation. Besides making full use of the available signal and local oscillator power, this arrangement provides for the suppression of intensity fluctuations in the LO laser. They implement the circuit IN lINbO₃ using titanium indiffused waveguides, since this material system offers low waveguide loss and good coupling efficiency to standard optical fibres and is now at a relatively advanced stage of development. The operating wavelength is in the 1550 nm optical fibre transmission window