Photonic Devices - Couplers

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. Photonic Devices - Couplers 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 l0 / 2, the grating reflects wavelength l0 selectively, very useful in filtering communication channels in or out.