The Principle And Function Of Optical Isolator

- Oct 17, 2020-

Semiconductor lasers and optical amplifiers are very sensitive to reflected light from connectors, splices, filters, etc., and cause performance degradation. Therefore, an optical isolator is required to block the reflected light. An optical isolator is an optical passive device that only allows light to pass in one direction and blocks light in the opposite direction. The light reflected by the optical fiber echo can be well isolated by the optical isolator, and the isolation represents the ability of the optical isolator to isolate (block) the echo. The optical isolator is a very useful device, which is usually used in the optical path to avoid interference and damage to the light source, pump source and other light-emitting devices caused by the echo in the optical path. Including polarization-independent in-line optical isolator and polarization-dependent miniaturized optical isolator.

The working principle of optical isolator

The optical isolator mainly uses the Faraday effect of the magneto-optical crystal. The Faraday effect is the first observation by Faraday in 1845 that a non-optically active material rotates the polarization direction of light passing through the material under the action of a magnetic field. It is also called the magneto-optical rotation effect. The polarized light transmitted in the direction of the magnetic field has a rotation angle θ of the polarization direction and the product of the magnetic field strength B and the length L of the material in proportion. The working principle of the optical isolator is shown in Figure 1.

For the signal light incident in the forward direction, it becomes linearly polarized light after passing through the polarizer. Together with the external magnetic field, the Faraday gyromagnetic medium rotates the polarization direction of the signal light by 45 degrees to the right, and makes the low loss pass and the polarizer 45 degrees. Degree of analyzer placed. For reverse light, when the linearly polarized light exiting the analyzer passes through the placement medium, the deflection direction is also rotated 45 degrees to the right, so that the polarization direction of the reverse light is orthogonal to the direction of the polarizer, completely blocking the transmission of reflected light .

Faraday magnetic medium usually uses yttrium iron garnet (YIG) single crystal with low optical loss in the wavelength range of 1μm~2μm. The optical isolator of the new pigtail input and output has quite good performance, the lowest insertion loss is about 0.5dB, the isolation is 35~60dB, and the highest can reach 70dB.

The role of optical isolator
Its function is to prevent the adverse effects of the backward transmitted light in the optical path due to various reasons on the light source and the optical path system. For example, installing an optical isolator between the semiconductor laser source and the optical transmission system can greatly reduce the adverse effects of reflected light on the stability of the spectral output power of the light source. In the high-speed direct modulation and direct detection optical fiber communication system, the backward transmission light will generate additional noise, which will degrade the performance of the system. This also requires an optical isolator to eliminate. Installing optical isolators on both ends of the doped fiber in the fiber amplifier can improve the working stability of the fiber amplifier. Without it, the retro-reflected light will enter the signal source (laser), causing severe fluctuations in the signal source. In the coherent optical long-distance optical fiber communication system, an optical isolator is installed at intervals to reduce the power loss caused by stimulated Brillouin scattering. Therefore, optical isolators play an important role in optical fiber communications, optical information processing systems, optical fiber sensing, and precision optical measurement systems.

Features of optical isolator

The characteristics of optical isolator are high isolation, low insertion loss; high reliability, high stability; extremely low polarization dependent loss and polarization mode dispersion.

Type of optical isolator

There are many types of optical isolators, including in-line optical isolators, free space optical isolators, etc. We provide various specifications of optical isolators to meet the needs of different application fields. The internal design of the 1310/1480/1550nm polarization-independent optical isolator treats the two orthogonal polarization states in the single-mode fiber separately to ensure the polarization-independent characteristics of the entire device. Unipolar devices have low insertion loss, and dual-stage devices have extremely high optical isolation. They are suitable for different applications. They are mainly used in fiber amplifiers, fiber lasers, fiber CATV networks and satellite communications.

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