Novel Frequency Chirp Measurement Techniques

Abstract

Optical communication technologies have particular interest in noise metrology. Coherent transmission systems are substantially formed by distributed feedback laser diodes whose radiation spectral width is smaller than 0.1 nm. Their other desirable features are reduced susceptibility to temperature and driving current variations and they are robust, small in size and low cost. Significant noise of various types in coherent transmission systems is generated by the laser itself. The direct modulation of the laser injection broaden the line width which is commonly known as the frequency chirp. The purpose of this paper is to describe and compare different techniques for measuring this kind of noise. Double feedback lasers and origins of chirp noise are first explained. The novel chirp measurement techniques of Fabry Perot interferometer, heterodyne, homodyne and temporal chirp are then described and compared. It is explained that Fabry Perot's strongly reflect when not in resonance so a problem often encountered is the reflection back into the laser source. The use of optical isolation in Fabry Perot interferometers in thus essential. The Birefringence Insensitive Interferomter naturally compensates for any fibre birefringence and eliminates the need for a polarisation state controller. Homodyning techniques provide a resolution capability of 100kHz which has proved appropriate and also suitable for the majority of double feedback lasers when measuring line width. The self-homodyning in particular offers the best combination of performance verses cost and it has a useful feature of having a self tracking local oscillator which compensates proportionally any changes in the device under test. The temporal chirp measurement techniques are either direct or indirect and they are based on the pulse response of laser diodes. The direct techniques are easier but require a fast detector and rely on a reasonable estimate of the pulse shape.