Download Photonic Devices - Couplers

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”.
Photonic Devices - Couplers
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”.
Light input
to Port 1
No light
into Port 4
Evanescent coupling
50% output
on Port 2
50% output
on Port 3
• 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.
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.
1 at Port 1
1 at Port 3
2 at Port 1
2 at Port 2
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.