Optoisolator Basics: Working Principles and Specifications

An optoisolator is an isolator used for optical communication. It’s essentially a passive, non-reciprocal device. An opto-isolator allows light to pass in only one direction and blocks it in the other.

Figure 1: Simple schematic of opto-isolator

Figure 1 depicts a simple schematic of an opto-isolator consisting of an LED, a dielectric barrier in between, and a phototransistor (sensor). Optical isolators are often used at the output of amplifiers or lasers to prevent light reflections from entering these optical devices backwards. This helps improve the performance of the amplifier and other optical components.

The key parameters to consider when selecting an isolator are insertion loss and isolation. Ideally, insertion loss should be low, and isolation should be high. Insertion loss refers to the signal loss in the forward direction, while isolation refers to the loss in the backward direction, preventing light from degrading the performance of preceding devices. Typically, isolation is in the range of 40 dB to 50 dB, and insertion loss is around 1 dB.

Figure 2: Optical isolator

Let’s examine the workings of a polarized type of isolator as shown in Figure 2. There are two modes of polarization, defined by the orientation of the electric field vector relative to the direction of light propagation. If the vector is perpendicular to the direction, it’s referred to as vertical SOP (State Of Polarization) of the light.

As shown in the figure, assume that the incoming light signal has vertical SOP and the polarizer passes light only having vertical SOP and blocks the light having horizontal SOP. The rotator in the middle rotates the light signal by 45 degrees clockwise irrespective to the input signal direction. The polarizer on the right passes the light signal with a 45-degree orientation, allowing the light signal to pass from left to right without significant attenuation. Light entering from right to left is shifted by 45 degrees and will have horizontal SOP which is blocked by the polarizer on the left, as it only allows vertical SOP. Since we don’t have control over the SOP of the input light signal, this type of isolator isn’t ideal.

Figure 3: Polarization independent isolator fig1

For this purpose, polarization-independent isolators have been designed. The schematic is depicted in Figure 3. First, the signal is passed through a SWP (Spatial Walk-off Polarizer) made of two birefringent crystals with two different refractive indexes. When light signal having random SOP is applied as input to these crystals, two orthogonal components are refracted at two different angles. These two light components pass through a rotator and a half-wave plate, each providing a 45-degree drift. Remember that a half-wave plate provides a 45-degree phase shift clockwise for signals travelling from left to right, but anticlockwise for signals from right to left. This results in a total drift of 90 degrees from left to right. Hence, the combination of a Faraday rotator along with a half-wave plate converts vertical polarization into horizontal polarization and vice versa.

Figure 4: Polarization independent isolator fig2

In the reverse direction (right to left), the SOP of the light signal won’t change, as the rotator and half-wave plate cancel each other’s phase changes, with one introducing a phase shift clockwise and the other anticlockwise.

Typical Specifications for Optical Isolators

Here are some typical specifications to consider for optical isolators:

• Number of Ports: 2 ports
• Single Grade or Dual Grade: P or A
• Operating Wavelength: 1310/1480/1550/1585/1590nm
• Bandwidth: +/- 15 nm or +/-30 nm
• Peak Isolation: Min. 40 dB
• Insertion Loss: Typically about 0.4dB
• Return Loss: About 60dB
• PDL: About 0.05 to 0.15 dB
• PMD: About 0.05 to 0.25 ps
• Power Handling: About 300mW