Polarisation state insensitive optical discriminator

An optical transmission system comprising at least one polarisation state insensitive optical discriminator, said at least one optical discriminator including a Mach Zehnder interferometer with two birefringent limbs which interferometer is optically interconnected with first and second polarisation beam splitters, wherein the Mach Zehnder interferometer has first and second ports at one end of its birefringent limbs and third and fourth ports at the other, wherein each of the first and second polarisation beam splitters has first and second ports and third and fourth ports, wherein the first, third, second, and fourth ports of the Mach Zehnder interferometer are respectively optically interconnected with the fourth and third ports of the first polarisation beam splitter and the second and first ports of the second polarisation beam splitter with relative alignments such that an optical signal of arbitrary state of polarisation (SOP) launched into one port of one of the polarisation beam splitters selected from the group consisting of the first and second ports of the first polarisation beam splitter and the third and fourth ports of the second polarisation beam splitter is separated into two orthogonally polarised components and launched into the Mach Zehnder interferometer to propagate with substantially the same SOP in opposite directions through said two limbs, which SOP is substantially matched with one of the birefringence eigenstate SOP's of said limbs of the Mach Zehnder interferometer.