The basic principle of optical isolator Polarization-insensitive fiber isolator (Polarization Insensitive Fiber Isolator) can be divided into polarization-independent (Polarization Insensitive) and polarization-dependent (Polarization Sensitive) according to polarization characteristics. Since the optical power passing through the polarization-dependent optical fiber isolator depends on the polarization state of the input light, it is required to use a polarization maintaining fiber as a pigtail. This optical fiber isolator will be mainly used in coherent optical communication systems. At present, the most widely used optical fiber isolator is still polarization-independent, and we only analyze this type of optical fiber isolator
1 Typical structure of polarization-independent fiber isolator A relatively simple structure is shown in Figure 1. This structure only uses four main elements: magnetic ring (Magnetic Tube), Faraday rotator (Faraday Rotator), two LiNbO3 wedge pieces (LN Wedge), and a pair of fiber collimators (Fiber Collimator), you can Make an in-line optical fiber isolator. 2 Basic working principle The following is a detailed analysis of the two conditions of optical signal forward and reverse transmission in the optical fiber isolator.
2.1 Forward transmission As shown in (Figure 2), the parallel light beam emitted from the collimator enters the first wedge plate P1, the light beam is divided into o light and e light, the polarization directions of which are perpendicular to each other, and the propagation direction is one Angle. When they pass through the 45° Faraday rotator, the polarization planes of the emitted o light and e light rotate in the same direction by 45°, because the crystal axis of the second LN wedge plate P2 is exactly relative to the first one. The angle is 45°, so the o light and e light are refracted together to combine two parallel light beams with a small spacing, and then are coupled into the fiber core by another collimator. In this case, only a small part of the input optical power is lost. This loss is called the insertion loss of the isolator. ("+" in the figure indicates e light direction)
2 Reverse transmission As shown in (Figure 3), when a beam of parallel light is transmitted in the reverse direction, it first passes through the P2 crystal and is divided into o light and e light whose polarization direction and the crystal axis of P1 are at an angle of 45°. Due to the non-reciprocity of the Faraday effect, after the o light and e light pass through the Faraday rotator, the polarization direction is still rotated in the same direction (counterclockwise in the figure) by 45°, so that the original o light and e light are entering The second wedge (P1) becomes e-light and o-light. Due to the difference in refractive index, the two beams of light can no longer be combined into a parallel beam in P1, but refracted in different directions. The e-light and o-light are further separated by a larger angle, even after passing through the self-focusing lens. The coupling can not enter the fiber core, thus achieving the purpose of reverse isolation. The transmission loss at this time is called isolation.
3 Technical parameters For optical fiber isolators, the main technical indicators are Insertion Loss, Isolation, Return Loss, Polarization Dependent Loss, Polarization Mode Dispersion (Polarization). Mode Dispersion), etc., will be explained one by one under .
3.1 Insertion Loss (Insertion Loss) In the polarization-independent fiber isolator, the insertion loss mainly includes the loss of the fiber collimator, Faraday rotator, and birefringent crystal. For a detailed analysis of the insertion loss caused by the fiber collimator, please refer to " Principles of Collimator. The isolator core is mainly composed of a Faraday rotator and two LN wedge pieces . The higher the extinction ratio of the Faraday rotator, the lower the reflectivity, and the smaller the absorption coefficient, the smaller the insertion loss. Generally, the loss of a Faraday rotator is about 0.02～0.06dB. It can be seen from (Figure 2) that after a beam of parallel light passes through the isolator core, it will be divided into two parallel beams of o and e. Due to the inherent characteristics of birefringent crystals, no¹ne, o light and e light cannot be completely converged, causing additional loss.
3.2 Reverse isolation (Isolation) Reverse isolation is one of the most important indicators of an isolator, which characterizes the attenuation ability of the isolator to the reverse transmission light. There are many factors that affect the isolation of an isolator, and the specific discussion is as follows.
(1) The relationship between the isolation and the distance between the polarizer and the Faraday rotator (2) The relationship between the isolation and the surface reflectivity of the optical element The greater the reflectivity of the optical element in the isolator, the worse the reverse isolation of the isolator. In the actual process, R must be less than 0.25% to ensure that Iso is greater than 40dB.
(3) The relationship between the isolation and the wedge angle and spacing of the polarizer. The birefringent crystal is an optical isolator with yttrium vanadate (YVO4). When the wedge angle is less than 2°, the isolation increases rapidly with the increase of the angle. When the wedge angle is greater than 2°, the change is much smaller, and is approximately stable at about 43.8dB. For optical isolators made of different materials, the isolation varies with the wedge angle. The optical isolation varies little with the increase of the distance, because the isolation mainly depends on the angle between the reverse output light and the optical axis.
(4) The relationship between the isolation and the relative angle of the crystal axis The relative angle of the two polarizers and the crystal axis of the rotator has the greatest impact on the isolation. When the angle difference is greater than 0.3 degrees, the isolation cannot be greater than 40dB. There are many other factors, mainly the extinction ratio of the two polarizers, crystal thickness, etc. To make the isolation greater than 40dB , must also make: R1 and R2 equal, less than 0.25%; the beam splitter crystal axis clamp The angle error is less than 0. 57°, etc. In addition, because in the Faraday effect, θ=VBL, V is not only a function of wavelength, but also a function of temperature, so the Faraday rotation angle will also change with the temperature, which is also one of the factors.
3.3 Return loss The return loss RL of an optical isolator refers to the ratio of the optical power incident on the isolator in the forward direction and the optical power returning to the input port of the isolator along the input path. This is an important indicator because the return is strong, Isolation will be greatly affected. The return loss of the isolator is caused by the mismatch of the refractive index of the components and the air and the reflection. Usually the return loss caused by planar components is 14dB
On the left and right, the echo can be lost to more than 60dB through antireflection coating and bevel polishing. The return loss of an optical isolator mainly comes from its collimated optical path (ie, the collimator part). According to theoretical calculations, when the slope angle is 8°, the return loss is greater than 65dB. The return loss of the collimator has been analyzed in the principle of collimator, please refer to "Principle of Collimator".
3.4 Polarization-dependent loss PDL PDL is different from insertion loss. It refers to the maximum change in the insertion loss of the device when the polarization state of the input light changes while other parameters remain unchanged. It is an indicator that measures the degree of polarization of the insertion loss of the device. For polarization-independent optical isolators, due to the presence of some components that may cause polarization, it is impossible to achieve zero PDL. Generally, the acceptable PDL is less than 0.2dB.
3.5 Polarization Mode Dispersion PMD
Polarization mode dispersion PMD refers to the phase delay of the signal light passing through the device in different polarization states. In optical passive devices, different polarization modes have different propagation trajectories and different propagation speeds, resulting in corresponding polarization mode dispersion. At the same time, because the spectrum of the light source has a certain bandwidth, it will also cause a certain dispersion. In high-speed optical communication systems, PMD is very important. In the polarization-independent optical isolator, the two beams generated by the birefringent crystal polarized light is transmitted at different phase and group speeds, that is, PMD, and its main source is the birefringent crystal used to separate and condense o-light and e-light . It can be approximated by the path difference ΔL of the two linearly polarized light beams. Polarization mode dispersion: In a polarization-independent isolator: Of course, the PMD of the entire device can be obtained by calculating the optical path length L of each component. PMD is mainly affected by the refractive index difference between e-light and o-light, and therefore has a greater relationship with wavelength.