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 dB isolation.

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.

The adjustment mechanism moves the Faraday rotator rod in the axial hole in the magnet, thus controlling the length of the Faraday rod that is exposed to the magnetic field. Faraday rotation is directly proportional to the product of length times field (LxH).

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 the horizontal or vertical with an OFR Polarization Rotator. See Retarders on page IO-15