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”.

Light IN Light

OUT

Pin

Pin/2

0

Primary

Secondary

0 š/2 3š/2š 2š

P(z) Optical Power in each waveguide

Coupling Length

Cz

Corresponding optical amplitude E(z) in coupler*

Light IN

Light OUT

Photonic Devices - Couplers

Photonic Devices - Couplers

Light input to Port 1 Evanescent coupling

No light into Port 4 50% output

on Port 3

50% output on Port 2

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.

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.