Multi-species gas sensing using monolithic widely tuneable laser diodes. R. Phelan * , M. Lynch, J.F. Donegan and V. Weldon. Semiconductor Photonics Group, Physics Department, Trinity College, Dublin 2, Ireland ABSTRACT Widely tuneable laser diodes operating in the 1520 ≤ λ ≤ 1570 nm are characterised and compared for use as sources for tuneable laser diode gas absorption spectroscopy. Three gases hydrogen cyanide, ammonia and acetylene with overlapping absorption features within the 50 nm tuning range of the devices were targeted employing wavelength modulation spectroscopy with second harmonic detection. Keywords: Tuneable laser, wavelength modulation spectroscopy, gas sensing 1. INTRODUCTION Widely tuneable single frequency lasers, such as sampled grating distributed Bragg reflector and modulated grating Y- branch laser diodes recently developed for optical communications, present exciting opportunities for applications in absorption based multi-gas sensing regimes. Such wide wavelength tuning is not possible with conventional single frequency distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers, and hence their use is limited to the detection of one gas. Widely tuneable laser diodes are more complex than standard DFB lasers used for single-species gas sensing and have undesirable artefacts in their operating characteristic. The tuning ranges of the widely tuneable lasers have been characterised and look-up tables, which yield high wavelength accuracy and high SMSR across the tuning range. The use of widely tuneable laser diodes as sources in a multi-gas analysing system using wavelength modulation spectroscopy and second harmonic detection of acetylene hydrogen cyanide and ammonia have been investigated. The critical issues relevant to the application of such widely tuneable diode lasers to spectroscopic based high selectivity multi-gas sensing are outlined. The general emphasis of the work described here is not on detection limits but to selectively detect three gases with overlapping absorption bands. 2. EXTENDING THE TUNING RANGE OF DBR-TYPE LASERS The tuning range of conventional DFB and DBR lasers is approximately 5-10nm which is significantly smaller than the available gain bandwidth of multiple quantum well semiconductor lasers (more than 100nm) and Erbium doped fibre amplifiers (about 40nm in the C or L band) [1]. Consequently, much research has targeted the development of integrated lasers with extended tuning ranges beyond the refractive index limit [2-4]. The basic principle behind all schemes that have been developed for wide tuning is that a refractive index difference is changed rather than the index itself. Therefore, the relative wavelength change is equal to a relative change in index difference, which can be significantly larger for similar absolute refractive index variations. In the following sections we will describe the most common scheme for achieving the broad tuning range. 2.1 Vernier effect between two comb reflectors The Vernier caliper is a well-known instrument for high-resolution length measurement. The same principle can be applied to a tuneable laser (Fig. 1) if the laser has two mirrors with a comb-shaped reflectivity spectrum. The mirrors are designed such that the peak reflectivity spacing of the front mirror (δ f ) and the rear mirror (δ r ) differ by a small amount. Opto-Ireland 2005: Optical Sensing and Spectroscopy, Byrne, Lewis, MacCraith, McGlynn, McLaughlin, O'Sullivan, Ryder, Walsh, Eds., Proc. of SPIE Vol. 5826 (SPIE, Bellingham, WA, 2005) • 0277-786X/05/$15 • doi: 10.1117/12.605312 449 Downloaded from SPIE Digital Library on 04 Feb 2010 to 134.226.1.229. Terms of Use: http://spiedl.org/terms