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 connectors or other optical components exist. Therefore it is necessary to protect the laser sources against back reflected light by optical isolators. The most commonly used isolator is 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. Therefore, the experiment focuses on optical isolators with their operational wavelengths in the 1.30-1.55 µm range used widely in optical fiber communication. The basics of operation, its structures, mechanisms, and applications will be demonstrated. To understand the concept of optical data transmission the set-up provides a free space Faraday isolator in connection 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 Faraday rotator, are mounted in rotational 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.