Freespace Optical Isolators Eliminate Return Path Light Interference

Opto Isolator
Opto Isolator – Credit: Finisar

An overview of the principles of freespace optical isolators, and offerings by Finnisar and Thorlabs.

In the most basic terms, a free space isolator is an optical component that permits light transmission in a single direction. Optical isolation is often required in a laser cavity to prevent undesirable feedback into an optical oscillator component.

Functionally, a magnetic-optical effect, characterized by the Faraday effect, is how a free space isolator passes light forward while reducing or eliminating light in a return path.

This was described in 1842 by physicist Michael Faraday. His experiments showed that exposing light to a magnetic field caused the plane of polarized light to rotate while transmitting through glass (or other permissive material) . The direction of rotation is dependent not on the direction of light but rather on the direction of the magnetic field.

This led him to the discovery that the light rotation is non-reciprocal. Mathematically, The amount of rotation Θ equals ν * L * H, where ν, L, and H are as defined below.

Faraday Rotation Θ = ν * L * H

ν: the Verdet Constant, a property of the optical material, in minutes/Oersted-cm.

L: the path length through the optical material in cm.

H: the magnetic field intensity in Oersted (Oe).

Depending on whether one is using the CGS (centimeter-gram-second) system above, or the SI (Systeme Internationale) which is equivalent to the MKS (meter-kilogram-second) system, the conversions are:

1 Gauss = 1 Oersted in a vacuum or in non-magnetic substances

10,000 Guass = 1 Tesla

10,000 Oersted = 1 Tesla 1 A/m = 4*pi Gauss

As a brief aside, to put these values into perspective: The magnetic field of the Earth is 1 ~ 2 Gauss Magnetic recording for video, cassette or hard disk drive is 15 to 45 Gauss MRI for humans is 25,000 to 75,000 Gauss

Two companies that produce many of the highest quality free-space optical isolators are

Thorlabs
and
Finnisar .

Thorlabs offers a variety of free-space optical isolators designed for use in the visible spectral range (395 – 690 nm). Depending on the application, there are two types of isolators. The first type, Fixed Narrowband Isolators, contains fixed, factory-aligned optics, for which peak isolation and peak transmission occurs at a pre-defined center wavelength. Any deviation from this wavelength will cause a dip in isolation and transmission. The second type, Adjustable Narrowband Isolators, offers the user the ability to adjust the alignment of the input and output polarizers, allowing tuning of the center wavelength within a 20 – 30 nm range, depending upon the exact isolator.

Finisar’s Free Space Isolators are used in various TOSA (Transmitter Optical Sub Assembly) designs. These are available in single and double stage designs. The Free Space Isolators offer excellent performance with low insertion loss and high isolation. Bare isolator chips survive more than 5,000 hours of damp heat testing without delamination.

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