（90° optical hybrids）

1The Description

The COH24is a

single polarization 90° optical hybrid that enables to extract phase andamplitude fromasingle polarization signal by performing four 90° phase stepped

interferences between the signal and aLocalOscillator (LO). The COH24-X is a

new generation optical hybrid with improved performances (among otherthings, a

wider operating wavelength range and muchbetter dual uniformities).

The COH28

and

COH28-X are based on two single polarization 90° optical hybrids that enable to

extract phase, amplitude and polarization from a signal with any polarization.

These

products are purely passive and athermal. The COH24-X and COH28-X also feature a

phase tunability option in order to perfectly adjust the 90° ofthehybrid

depending on the wavelength used.

2 – Block

diagram

Below is

a block diagram showing how the four 90° phase stepped. Interferences are done

in the opticalhybrid.

The

graphs above compare the dual uniformities, a key parameter of 90° optical

hybrids, for the SignalandLO of a standard hybrid (COH) and a new generationone

(COH-X). We can clearly see that COH-X exhibits much better dual uniformities

over a wider range.

7 – Phase

tuning option

The 90° phase shift of the hybrids depends

slightlyon the wavelength. For example, it increases of about 2°to 3° over the

C-band. That means that if at 191THzthe phase shift is 88.5° it will be

approximately91.5°at197THz (see next paragraph for some typical measurements).

For some

applications it can be useful to have a very precise 90°phase shift whatever

the wavelengthyouare working at. That is what the phase tunability option on

COH-X enables. With this option, two voltages(V1 and V2) between 0V and 4V can

be applied on thehybrid. One will increase the phase shift when thesecond one

will decrease it. Hence for any wavelength in the OWR the phase shift can be