Function and basic principle of optoisolator
The function of optical isolator is to let the light transmitted in the forward direction pass through and isolate the light transmitted in the reverse direction, so as to prevent the reflected light from affecting the stability of the system, similar to the function of diode in electronic devices. Optical isolators are divided into two types according to the polarization correlation: polarization correlation type and polarization independent type, the former is also called free space type (Freespace), because there is no fiber input and output at both ends; the latter is also called in-line type (in-Line), because there is fiber input and output at both ends. Freespace type optical isolators are generally used in semiconductor lasers, because the light from semiconductor lasers has a very high linearity, so you can use this polarization-dependent optical isolators and enjoy the advantages of low cost; in communication lines or EDFA, generally use in-line optical isolators, because the polarization characteristics of the light on the line is very unstable, requiring the device to have a small polarization-dependent loss.
The basic principle of the optical isolator is Marius' law of polarized light and Farady's magneto-optical effect. The basic structure and principle of the free-space type optical isolator is shown in the figure below, which consists of a magnetic ring, a Faraday rotor and two polarizers, the optical axes of which are at an angle of 45°. The direction of polarization of the forward-incident line polarized light is along the direction of the transmission axis of the polarizer, and then rotates 45° counterclockwise to the direction of the transmission axis of the polarizer when passing through the Faraday rotator, and then transmits smoothly; the direction of polarization of the reverse-incident line polarized light is along the direction of the transmission axis of the polarizer, and then rotates 45° counterclockwise to be perpendicular to the transmission axis of the polarizer when passing through the Faraday rotator, and then is isolated without transmitting light. The free-space optoisolator is relatively simple, and the polarizer and rotor are tilted at a certain angle (e.g., 4°) during assembly to reduce the reflected light from the surface.
The earliest in-line optoisolators were made with Displacer crystals and Faraday spinners, which were replaced by Wedge optoisolators due to their large size and high cost; in-line optoisolators introduced PMD due to the use of birefringent crystals, so PMD-compensated Wedge isolators emerged accordingly; some applications demanded higher isolation, so dual-stage optoisolators emerged to obtain higher isolation within a wider bandwidth. The in-line optical isolators are introduced in the following order.
The structure and principle of these in-line optoisolators are introduced in the following order.
1) Displacer type optical isolator
The structure and optical path of the Displacer type optical isolator is shown in the figure below, which consists of two collimators, two Displacer crystals, a half-wave plate, a Faraday rotor and a magnetic ring. The forward light is incident from collimator 1 on Displacer1. divided into o-light and e-light transmission, and after passing through the half-wave and Faraday rotator, the conversion of o-light and e-light occurs counterclockwise by rotating 45 +45 =90. and the coupling is synthesized into collimator 2 by Displacer2; the reverse light is incident from collimator 2 on Displacer2. divided into o-light and e-light transmission, and after passing through Faraday After the Faraday rotator and half-wave, the counterclockwise rotation of 45 -45 = 0. no conversion of o and e light occurs, and after the Displacer1 both beams deviate from the collimator 1 and are isolated.
The disadvantage of the Displacer type optical isolator is that, in order to meet the isolation requirements, the two beams in the reverse optical path need to be offset by a large distance, and the birefringent characteristics of yttrium vanadate Displacer crystal, its length and offset ratio can only do 10:1. which requires the Displacer crystal size is very large, resulting in large device size and high cost.
2) Wedge type optical isolator
The structure and optical path of the Wedge-type optical isolator is shown in the figure below, which consists of two collimators, a magnetic ring, a Faraday rotor and two wedge-shaped birefringent crystals, with the optical axes of the two wedges at an angle of 45°. The forward light from the input collimator is divided into o-light and e-light by Wedge1. and the polarization direction rotates counterclockwise (against the direction of forward light propagation, same as below) by 45° when it passes through the spinner, and the conversion between o-light and e-light does not occur when it enters Wedge2. so the polarization states of the two beams in the two wedge polarizers are o→o and e→e respectively. A parallel plate, the direction of the forward light through the unchanged, coupled into the output collimator; the reverse light from the output collimator is Wedge2 divided into o and e light respectively, after the spin plate polarization direction is still counterclockwise rotation 45 °, into the wedge1 when the conversion of o and e light, so the two beams in the two wedge polarization state is o → e and e → o, the two wedge The combined pair of reverse light is equivalent to a Wollaston prism, and the reverse light passes through and deviates from its original direction and cannot be coupled into the input collimator.
The package design should take Offset into consideration; Walkoff is generally about 10um, which will introduce a little PDL, but it does not matter much; for PMD, compensation will be made as needed. The PMD compensation method is to add a birefringent crystal plate at the back, whose optical axis is perpendicular to the optical axis of Wedge2. and the thickness is calculated by optical path tracing, which is not repeated here.
Compared with the Displacer type optical isolator, the Wedge type optical isolator has a much different isolation mechanism for the reverse light, as the former causes the reverse light to shift laterally with respect to the input collimator, and the latter causes the reverse light to deviate angularly with respect to the input collimator, and the latter has a better isolation effect. The Wedge crystal is small in size, so the device is small in size and low in cost, and has replaced the Displacer type.
3) Two-stage optoisolator
The following figure shows a two-stage optoisolator scheme, two single-stage optoisolator cores are connected in series, the direction of the optical axis of each wedge, the forward light in the first stage and the second stage are o-light and e-light, so the PMD generated by the two stages compensate each other, the disadvantage of this scheme is that the assembly accuracy requirements are very high, otherwise the isolation index is worse than the single-stage optoisolator.
The disadvantage of this scheme is that it is difficult to adjust the isolation and PMD to the best state at the same time when rotating, so the PMD of the two stages are compensated separately first, and then assembled by rotating relative to each other, which can make qualified dual-stage optoisolators, but still lead to low yield and low efficiency due to the complicated process.
