Know about the principle of optical isolator
Semiconductor lasers, optical amplifiers and optical fiber lasers from the connector, fusion point, filter the reflection light is very sensitive, and may cause performance deterioration and even damaged, requiring a optical isolator to prevent the reflection of light. The optical isolator is permitted only light along one direction through and in the opposite direction blocks light through the optical passive devices. In the optical fiber communication, optical fiber reflection light through the optical isolator can be a good isolation. In the fiber laser applications, optical isolators are usually used in the optical path to avoid the light path of the light source, the echo on the pumping source and other light emitting device causes interference and damage. Isolators’s isolation represents the optical isolator to echo the isolation (blocking) ability.
Optical isolator using magnetic optical crystal Faraday effect ( also known as the Faraday effect ). In 1845, Faraday first observed with optical material under the action of magnetic field to make the material in the direction of polarization rotation, therefore often called the Faraday effect. In Faraday effect, the rotation of the polarization direction direction and magnetic field, and the orientation of the light transmitting is independent of the forward and reverse, and we usually in the index of refraction, reflection phenomena seen in the reversibility of optical path difference. Along the magnetic field direction of transmission line polarized, the polarization direction rotating angle and magnetic field strength of B and L is proportional to the product of the length of the material, the proportion coefficient is what we often say that the Wilde constant. Optical isolator based on polarization characteristics can be divided into polarization-independent and polarization dependent type. These two kinds of isolators are used with the Faraday effect in magneto-optic crystal, Faraday magnetic medium in 1~2m wavelength range usually adopts the optical loss low yttrium iron garnet ( YIG ) single crystals. Model of input and output of the fiber optical isolator has fairly good performance, the minimum insertion loss of approximately 0.5 dB, the isolation of up to 35-60 dB, a maximum of 70 dB. The optical isolator using most still is polarization independent type, its principle is shown in Figure 1, using the forward and reverse transmission optical path is inconsistent, it is this time signal transmission is not reversible, thereby forming isolation. The typical structure of only four major components: the magnetic ring, a Faraday rotator, two pieces of LiNbO3 wedge angle piece, with a pair of fiber collimator, can be made into an in-line optical isolators.
Positive transmission: the parallel light beam from the collimator, into the first wedge angle piece P1, beam is divided into o light and e light, the polarization direction perpendicular to the propagation direction, forming an included angle. When they pass through 45o Faraday rotator, emitted by the o light and e light polarizing surfaces of respective to the same direction of rotation 45o, because the second wedge-shaped plate P2 crystal axis relative to the first wedge angle piece is just in a 45o angle, so o light and e light is refracted into a small space, synthesis. Parallel light, and then by another collimator is coupled to the optical fiber core. In this case, the input optical power only a very small fraction of outage, this loss is called isolator insertion loss.
Reverse transmission: when a beam of parallel light reverse transmission, first with a P2 crystal, divided into the polarization direction and P1 crystal axis respectively in 45o angle o light and E light. Due to the Faraday effect non reciprocity, O Light and e light through the Faraday rotator, the polarization direction to the same direction of rotation 45 °, so the original o light and e light in the second wedge-shaped plate ( P1 ) later became e and O light. Because the refractive index differences, the two light beam in the P1 no longer possible synthesis of a parallel beam of light, but in different directions to the refraction of light, e and o are further separated from a larger perspective, even after a GRIN lens coupling, can not enter the fiber core to, from and achieved reverse isolation purposes. The transmission loss is bigger, this loss is called isolators isolation..
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