ID: 
2219
Maker's Name: 
Coherent Components Group
Where made: 
USA
Dimensions: 
11 × 6.3 × 4.1 cm

Probably an electro-optic modulator that rotates the plane of polarisation of light when a voltage is applied. Very likely used as a Q-switch or mode-locker.

It is attached to a liquid cooled heat sink.