Advisory Notes on Optical Isolators for Infrared Wavelengths

“It’s the IR that makes isolators difficult.”

An Optical Isolator consists of a Faraday Rotator and two Polarizers, all contained in a Body.

A Faraday Rotator consists of a Magneto-Optic Material mounted in a magnetic field.

(Request separate paper on the Operation of an Optical Isolator).

The Magneto-Optic Material must have:

  • low absorption at the wavelength of interest,
  • ability to rotate 45º at the wavelength of interest.

This narrows down our options.

The materials for the infrared are specially formulated so as to achieve design goal performance at the wavelength of operation.

Thus, the wavelength of operation determines the choice of material and its formulation.

Even then, the optimum material for one wavelength may perform very differently at a nearby wavelength. For example, transmission at λ1 can be different at λ2, even though the two wavelengths are close together.

Because it is not feasible to have a laser for every isolator wavelength ordered, we assemble and test such isolators using the closest wavelength laser we have. With performance data at many wavelengths, accumulated over many years, we can interpolate or extrapolate to wavelengths that are close to our lasers.

In other words, we cannot always test at our customer’s wavelength, but usually we are able to predict the isolator performance (transmission) based on our accumulated data.

Still, there are some wavelengths that will have low transmission, and there is nothing we can do about this. At all times, we inform our customers of the situation at the very start. No one likes an unpleasant surprise.

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