Fiber Optic Isolator, which is one of the most applied optical componnet in fiber amplifier, laser system, fiber optic sensor, it’s a fiber passive component which allow signal light pass the isolator and avoid the backward signal, the backward light may damage the laser seed source, pump laser, or cause it to laser mode hop, amplitude modulate, or frequency shift.

For example, a Semiconductor Laser or Laser Diode is very sensitive to the back reflection light, even a very low level optical reflection from an the optical circuit will affect the optical system. Therefore an optical isolator is usually required at above such application which need a low optical noise and stable optical frequency.

The Optical Isolator function is based on the Faraday Effect. Faraday Effect is Faraday first observed in 1845 that a non-optically active material rotates the polarization direction of the light passing through the material under the action of a magnetic field. It is also called the magneto-optical rotation effect. The polarization direction of transmission in the magnetic field, whose direction of polarization rotation angle θ and the magnetic field intensity B of the material is proportional to the product of the length L.

For optical isolator principle, Polarization Independent Fiber Optic Isolator as example, the input forward singal light, will be transformed to a line polarization light after passing the first polarizer, after the Faraday Rotator, the line polarization light will be turned over 45°, the 45° line polarization light will finally output after it pass the second polarizer, the backward light will be also 45° line polarization light, it will be rotated another 45°after passing the Faraday Rotator on its backward reflection optical path, it will be blocked at the first polarizer effectively, the forward and backward light path for reference as below:

      The Faraday Rotator is a key componnet for the optical isolator, for 1310nm, 1480nm, 1550nm fiber optic isolator, Bismuth-substitute yttrium iron garnet (Bi-YIG) crystals are often used. The specification of insertion loss less than 1dB and isolation more than 30dB is available for the optical isolator, if higher isolation is required, dual stage or tripple stage optical isolator can reach more than 50dB isolation. For such double or tripple stage optical isolation, the isolation will have an apparent growth, the insertion loss will also grow some. The inner configuration difference between single stage and double or tripple stage fiber optic isolator is isolator core, double stage optical isolator has two isolator core, tripple stage has three. The double or tripple stage is only for long wavelength optical isolator, like 1310nm, 1480nm, 1550nm.