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Dear Ann,

An isolator is simply a Faraday rotator sandwiched with two polarizers.
Without those polarizers, your feedback into the laser cavity (i.e.
back-reflection) will be orthogonally polarized to the output beam. It is
likely that the laser will be little disturbed in this case, because the
TiSap laser cavity favors one polarization. However complete (>30dB)
suppression of feedback will be achieved only with an isolator.

I would recommend the following companies for an isolator. Please also keep
in mind the group-velocity dispersion, if you are using <30fs pulses.
http://www.isowave.com/
http://www.ofr.com/ 

Best, 
Hyungsik

-- 
Hyungsik Lim, Ph.D.
Postdoctoral Associate, Webb Group
Applied Physics, Cornell Univ.
607-255-8034