A fiber optic isolator lets light passing through in one direction with a low loss while blocking the light in the opposite direction with a high loss.
Isolators are placed in output circuits of devices with a high output light level such as laser diode transmitter and EDFAs, as shown in this picture.
Their function is to reduce the level of reflected light back into the laser diode or EDFA.
Most fiber optic isolators use the Faraday effect to achieve their function. Faraday effect governs the rotation of the polarization plane of the optical beam in a magnetic field. The rotation is in the same direction for light propagating either parallel or antiparallel to the magnetic field direction.
Optical isolators consist of a rod of Faraday material such as yttrium iron garnet (YIG), whose length is selected to provide 45° rotation. The Faraday material is sandwiched between two polarizers whose axes are tilted by 45° with respect to each other.
Light propagating in one direction passes through the second polarizer because of the Faraday rotation. By contrast, light propagating in the opposite direction is blocked by the first polarizer.
There are a few critical parameters that determine the performance of isolators.
Wavelength-dependence, especially for so-called narrowband isolators that are designed to operate in a spectral range narrower than 20nm. Isolators are described by peak reverse direction attenuation and by the bandwidth for which the isolation is within 3 dB of the peak value
Low insertion loss. The insertion loss should be less than 1 dB in the forward direction, and in excess of 35 dB (single-stage isolator) or 60 dB (double-stage isolator) in the reverse direction.
Polarization mode dispersion (PMD). Isolators are constructed using high birefringent elements; and they are very prone to PMD – typically 50 to 100 fs, especially for single-stage designs. Double-stage isolators can be designed so that the PMD induced by the first stage is largely cancelled by the second stage.
Polarization-dependent loss (PDL). This degrades the performance of an optical isolator.