A Faraday rotator in a magnet, which turns polarization by 45 degrees in the same sense regardless of direction, placed between input and output polarizers at 45 degrees to each other. Forward light passes; returning light is rotated a further 45 degrees and blocked, giving 30 to 40 dB of isolation per stage. Isolators protect diodes, seeds and amplifiers from destabilizing feedback and from damaging back-reflected pulses.
Put the isolator right after the seed, those reflections from the fiber launch were making it mode hop every few seconds.
Free-space isolators have a power rating and a magnet: keep steel tools and screwdrivers away, and never run them above their power spec or the TGG crystal thermally lenses.
Also heard as
- isolator
- Faraday isolator
- optical diode
Related terms
Waveplate
A birefringent plate that shifts the phase between two polarization components to rotate or convert polarization.
Mode hop
A sudden jump of a laser's output frequency from one cavity mode to a neighboring one.
SBS
Stimulated Brillouin Scattering: narrow-linewidth light scattering backward off sound waves it creates in fiber.
MOPA
Master Oscillator Power Amplifier: a low-power seed laser followed by amplifier stages that bring it to full power.