An Isolator is set for maximum isolation
at a center wavelength. If the laser deviates from this wavelength,
then the isolation will decrease. In order to regain maximum isolation,
it is necessary to adjust the Isolator. Narrow Range Isolators are
readjusted by rotating the Output Polarizer. At the extreme ends
of this range of adjustment, transmission through the Isolator will
decrease, but by no more than 3% from the peak wavelength value.
It is possible to tune a small amount beyond the range and still
retain maximum isolation; however, transmission will decrease by
more than 3%.
For example, an isolator centered at 815 nm rotates
exactly 45°. If its isolation and transmission are, say, 40
dB and 94%, what are these values when it is retuned for maximum
isolation at a new wavelength, say 780 nm?
Over a small wavelength range, Faraday rotation is
approximately proportional to l-X. Therefore, rotation at 780 nm
is ~49°. In order to satisfy the condition for extinction (two
planes of polarization are perpendicular), the output polarizer
is set at 41° (90°- 49° = 41°), thus retaining 40
Finally, the Law of Malus shows that transmittance
through two polarizers is proportional to the cosX of the angle
between their axes of polarization. This angle is 8° (49°-41°)
and the cosX is 0.98. Thus, transmission is reduced by 2%, nominally
from 94% to 92%.
Characteristics and Features
These Isolators are set at OFR for any center wavelength
within the adjustment range specified at time of order. Adjustment
range is a nominal 5% of the center wavelength.
Unless otherwise specified at time of order, these
Isolators are set for horizontal input polarization. Output polarization
is in the +/- 45° quadrant. The output polarization plane can
be rotated to horizontal or vertical with an OFR ½-Wave Retarder.
See IO-15 Double-dB Isolators, the IOT Series, have output polarization
in the same plane as the input, available as indicated. These are
two Isolators in tandem on a common base, sharing a common center
Polarizer. Isolation is doubled and transmittance is reduced to
approximately the squared value. Net rotation can be ordered to
be 0° or 90°. All surfaces are AR-coated for maximum transmittance.