SMC - 18 Optical Isolator

Topics

Basic Concepts
Optical Diode
Beam Splitting Cube
Quarter Wave Plate
Faraday Rotator

 

Basic Concept

The carrier of information in optical fibre telecommunications is laser light which is generated by separate highly stabilised laser sources. For each of the optical channels, such a source must be provided to deliver a stable wavelength. It is a well known phenomena that the stabilistaion process is severly disturbed when even very small fractions of laser light is coupled back to the source. However, along a fibre optical transmission line many reflecting surfaces, like conectors or other optical omponents exist. Therefore, it is a must to protect the laser sources against back reflected light. This is done by so-called optical isolators. The most commonly used isolator is still a combination of a Faraday rotator operated between two polarisers based on the magneto-optical effect.


Experimental Set-up

The concept of using optical diodes formed by a polarising beamsplitter and a quarter wave plate will be treated within the theoretical part of this experiment, since it is not applied to such an extent as the magneto optical isolators. Therfore, this experiment focuses on optical isolators with their operation wavelengths in the 1.30-1.55 um range used widely in optical fiber communication. The basic of operation, its structures, mechanisms, and applications will be demonstrated. To understand this extremely important issue for optical data transmission the set-up provides a free space Farady isolator in conjunction with a 1.5 µm diode laser.
The light of the laser diode leaves the fibre telescope (FT-1) almost parallel. The polariser, as well as the Farady rotator, are mounted in rotaional holders with a 360° scale. The light which passes the optical isolator is collimated by the fibre telescope (FT-2). The task of the experiment is to find the best settings for maximum suppression and transmission. Especially for low intensities, it is recommended to modulate the diode laser and monitor the AC signal using an oscilloscope.