Optical Isolator Tutorial

Function

An optical isolator is a passive magneto-optic device that only allows light to travel in one direction. Isolators are used to protect a source from back reflections or signals that may occur after the isolator. Back reflections can damage a laser source or cause it to mode hop, amplitude modulate, or frequency shift. In high-power applications, back reflections can cause instabilities and power spikes.

An isolator's function is based on the Faraday Effect. In 1842, Michael Faraday discovered that the plane of polarized light rotates while transmitting through glass (or other materials) that is exposed to a magnetic field. The direction of rotation is dependent on the direction of the magnetic field and not on the direction of light propagation; thus, the rotation is non-reciprocal. The amount of rotation β equals V x B x d, where V, B, and d are as defined below.

Figure 1. Schematic diagram of Faraday effect

Faraday Rotation

β = V x B x d

• V: the Verdet Constant, a property of the optical material, in radians/T • m.
• B: the magnetic flux density in teslas.
• d: the path length through the optical material in meters.

Single Mode optical Isolator

Displacer type polarization independent isolator.

The structure and optical path of the Displacer optical isolator are shown in Figure 2, which consists of two collimators, two Displacer crystals, a half-wave plate, a Faraday rotator, and a magnetic ring. The forward light is incident on Displacer1 from collimator 1 and is divided into o light and e light for transmission. After passing through the half-wave plate and Faraday rotator, it rotates counterclockwise, and the conversion of o light and e light occurs, and is synthesized into a beam by Displacer 2 Coupling into the collimator 2; the reverse light is incident on the Displacer 2 from the collimator 2, and is divided into o light and e light for transmission. After passing through the Faraday rotator and half-wave plate, it rotates counterclockwise, and no o light and e light occur. The conversion of, after Displacer1, the two beams deviate from the collimator 1 and are isolated.

The disadvantage of the Displacer optical isolator is that in order to meet the isolation requirements, the two beams of light in the reverse optical path need to be shifted by a large distance,  and the yttrium vanadate Displacer crystal with better birefringence , the ratio of the length to the offset can only be 10:1, which requires the Displacer crystal to be very large, resulting in a large device volume and high cost. But this kind of crystal usually has a higher Damage Threshold and is more suitable for using high power, such as 1030~1080nm TGG optical isolator.