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.

Polarization-maintaining single-mode optical fiber isolator

1. Definition
The single mode optical isolator is a passive magneto-optical device which uses the Faraday effect of magneto-optic crystal to isolate the reflected light and only allows the light to transmit in a single direction. The optical fiber isolators are used to protect light sources from adverse effects caused by back-reflection or signal.
2. Characteristic

• 1). Minimize Feedback into Optical Systems.
• 2). Low insertion loss and high-power handling capability.
• 3). Polarization independent structure.

3. Description
1). Introduction to the working principle of polarization dependent isolator
Polarization-Dependent Isolators-Displacer type polarization dependent 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 the unpolarized light is divided into o light and e light transmission. But this application is usually the transmission of linearly polarized light, and the direction of incidence of the collimator can be rotated to travel along the e-light path. After passing through the half-wave plate and the Faraday rotator and rotating counterclockwise, the conversion of o light and e light occurs. By flipping Displacer2, the e light (if any) is refracted out of the crystal from the side, and the o light is normally transmitted and coupled into the collimator 2 ; The reverse light is incident on Displacer2 from the collimator 2, and the o light is transmitted. After passing through the Faraday rotator and half-wave plate, it rotates counterclockwise, and there is no o-light and e-light conversion. At the transition after Displacer1, the beam are deviated from collimator 1 and 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.

Figure 2. Structure schematic diagram of displacer type polarization dependent isolator