Frequency selection and stabilization of semiconductor laser diode systems

• Main Author: Ahmed, H. H. I. S.
• Published: Swansea University 2004
• Subjects: 535
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.635861

Different types of semiconductor diode laser sources were tested in a range of spectroscopic and metrological applications to demonstrate the versatility of our laser set-up implementations. Two main topics were pursued in this study: (a) experiments on absorption spectroscopy were carried out using external cavity laser diode modules in the wavelength range (410 – 1550 nm) while vertical cavity surface emitting lasers (VCSELs) were used in the experiments involving opto-galvanic spectroscopy and laser frequency stabilization at 800.6 nm. Absorption experiments were performed for the quantitative detection of (atmospheric) trace gases such as H₂O, CO_, and CO₂. The Tuneable Diode Laser Absorption Spectroscopy (TDLAS) was realised without the need for sophisticated detection electronics (e.g. lock-in amplifiers, etc.). A notebook-data acquisition system in conjunction with dedicated software developed during this study was fully adequate and allowed us to generate “any” desired shape of the modulation signal; and – after the data had been acquired – post-collection processing could be carried out (like averaging, noise removal, signal normalization, trend plots, etc.). In order to implement the opto-galvanic (OG) spectroscopy experiment, the opto-galvanic driver, control and detection unit was designed and built in-house. This electronic device was one of the centrepieces of the set-up for locking the laser diode frequency to an atomic transition in a hollow cathode discharge lamp. An integrated device such as the one required in this study is not available commercially. The observed OG signal revealed a series of sub-structures in the form of doublets (0.0074 nm or 4 GHz apart). The doublet frequency splitting results from the fact that VCSEL light is composed of two clearly-resolved spectral components, both linearly polarized, associated with transverse mode oscillation. Locking the laser frequency in the OG signal of the Argon transition at 800.6 nm was achieved using modulation of the injection current of the laser diode. This was done by feeding the OG signal into the data acquisition and control card to generate the suitable error signal (in magnitude and sign) to the laser current driver. This process was entirely controlled by a software program written during the course of this work. Incomplete attempts were made to couple the Fabry-Perout (FP) laser radiation to a fibre Bragg grating (FBG) due to some technical problems. Specifically, the fabricated FB didn’t match fully to any diodes available to us during the time of the experiments. Thus, only very weak feedback on FBG side-bands was observed which was insufficient to push the laser into single-wavelength oscillation.