1. Photonic Devices - Couplers Optical fibre couplers A basic photonic device used to split or combine light to or from different fibres. A building block of “passive optical networks”.

2. Photonic Devices - Couplers

3. How a four-port 50:50 splitter works Light entering Port 1 propagates into the ‘coupling region’ where the waveguides are close together. The evanescent tail of the field of the first waveguide overlaps into the second and light leaks across - This is “evanescent coupling”. The splitting ratio for light leaving Ports 2 and 3 can be adjusted to any desired value by adjusting the amount of coupling, or the length of the coupling region- the most common split is 50:50. The splitting ratio also depends on the wavelength of the light.

4. Photonic Devices - WDM couplers A 2 channel Wavelength Division Multiplexer (WDM) We aim to split two wavelengths from one fibre into two outputs, to separate two channels of information, or combine two into one fibre. We contrive to make a coupler with a splitting ratio at one desired wavelength of zero (the light all comes out of the primary waveguide) while at the other desired wavelength, total cross coupling occurs.

5. Photonic Devices - Bragg Gratings A Bragg grating is a periodic refractive index variation written along the fibre core. If the optical period is 0 / 2, the grating reflects wavelength 0 selectively, very useful in filtering communication channels in or out.