Gas Sensing Via Widely Tunable Semiconductor Lasers
The infrared absorption spectrum of a molecule (e.g. Figure 1) is analogous to a fingerprint, providing a means of species
identification and molecular concentration determination.
Figure 1. Absorption spectrum of the R rotational branch of the A vibrational band in the b1g('=0) ← X3g(''=0) electronic transition band of O2 obtained using a GaAs-AlGaAs DFB laser diode.
Fortuitously, the emission wavelengths (in the range 0.7 m to 1.7 m) of laser diodes, developed for telecommunications, coincide with the overtone/combination absorption spectra of most gases of environmental and industrial interest. These devices can be used to probe the molecular rotational absorption lines in the vibrational overtone/combination or electronic band features of the target gases. At near infrared wavelengths, molecular absorption band strengths are significantly weaker than in the fundamental absorption region, hence high sensitivity detection techniques such as Frequency Modulation Spectroscopy (FMS) and Wavelength Modulation Spectroscopy (WMS) are employed. High sensitivity detection can also be achieved by using specific microstructures such as Hollow-Core Photonic Bandgap Fibres (HC-PBFs) or high Q cavities that provide long absorption path lengths through the gas.
We have investigated the application to multi-gas sensing of widely tuneable, single frequency lasers, such as Sampled Grating (SG) Distributed Bragg Reflector (DBR) laser diodes, Modulated Grating Y (MGY) lasers and Strongly Gain Coupled (SGC) cascaded DFB laser diodes, all recently developed for the telecommunications industry. Other applicative studies relate to the investigation of the modification of gas absorption rotational line shapes and wavelength shifts, within the vibrational bands, as a function of gas temperature and pressure.
We have also shown that the tuning behaviour of widely tuneable lasers can be characterised using a combination of reference gases, for example (acetylene, hydrogen cyanide, ammonia) that provide unique and accurate wavelength identifiers.
We are also investigating the application of Novel Photonic Microstructures such as optical micro-spheres and Hollow-Core Photonic Bandgap Fibres (HC-PBFs) to Gas Sensing. This latter microstructure technology has the potential to achieve ultra-high sensitivity in a miniaturized gas-sensing platform, which can operate with very small gas volumes.
In micro-sphere based optical cavities, light can be confined by total internal reflection in a so-called Whispering Gallery Mode (WGM). As the light is guided along the spherical surface of the glass-gas interface, a significant part of the optical field (called the evanescent field) exists outside the micro-sphere and so can be used to interact with and measure the optical absorption of any gases present (Figure 2). Q factors in glass micro-spheres can be as high as 10 million giving a huge increase in the effective path length and hence absorption sensitivity. Application of these high-quality WGMs to laser noise characterization and measurement has been also investigated for DFB laser diodes (Figure 3).
Figure 2. Overview of laser-excited Whispering Gallery Modes (WGM) in spherical microresonators and their main properties.
Figure 3. Whispering Gallery Modes (WGM) coupled into Si spherical microresonators with a Q-factor of Q≈107 using a DFB laser diode of linewidth ≈6MHz. Laser frequency noise is significant compared to the WGM width and becomes transformed into amplitude noise as observed. The noise pattern can be used to estimate the original laser linewidth, and becomes negligible when a narrow linewidth (≈300kHz) laser is used to excite the microsphere modes
Photonic bandgap fibres
Hollow-Core Photonic Bandgap Fibres (HC-PBF) fibres use periodic structures along their length to confine and guide selected wavelengths in the hollow core. Gas introduced into the hollow fibre may be detected by direct absorption of coupled laser light using a transmission measurement. The high intensity of light in the fibre due to tight confinement (10m core diameter) can give rise to a non-linear optical effect where the molecular transition is saturated and a hole or dip is seen in the gas absorption profile. While un-saturated absorption lines at low gas pressures can have linewidths on the order of 500 MHz, the saturated profile in the absorption line is often only tens of MHz wide. In this “saturated” regime a novel application of a wavelength modulation technique using HC-PBF’s to achieve high accuracy laser frequency stabilisation is possible. Essentially narrow linewidth saturated absorption transitions, of Acetylene (12C2H2) contained in HC-PBF, are used to achieve high accuracy laser frequency locking utilising a pump-probe technique. Moreover line-locking of the pump laser (Figure 4) is accomplished in a modulation-free format using non-resonant saturation of an absorbing transition. Non-resonant saturation of 12C2H2 overtone rotational transitions in the v1+v3 vibrational band around 1.5 mm is possible in this case due to an allowed energy transfer mechanism between the two rotational absorption states targeted.
Figure 4. Modulation-free DFB laser stabilization results implementing an active feedback loop based on non-resonant saturation of acetylene transitions in a HC-PBF. A pump-probe set up is used in conjunction with Wavelength Modulation techniques and laser drifts of ≈10 Mhz are reduced to a RMS value of ≈180 kHz by locking the laser frequency to the centre of a saturated absorption line. Small temperature perturbations are compensated as observed in the corresponding trace (scaled up by a factor of 50 for clarity). Whispering Gallery Modes (WGM) coupled into Si spherical microresonators with a Q-factor of Q≈107 using a DFB laser diode of linewidth ≈6MHz. Laser frequency noise is significant compared to the WGM width and becomes transformed into amplitude noise as observed. The noise pattern can be used to estimate the original laser linewidth, and becomes negligible when a narrow linewidth (≈300kHz) laser is used to excite the microsphere modes
References:1. “Non-Resonant Wavelength Modulation Saturation Spectroscopy in Acetylene-Filled Hollow-Core Photonic Bandgap Fibres Applied to Modulation-Free Laser Stabilization”, submitted to Optics Express, 2009
2. David McInerney, , Michael Lynch, John Donegan, Vincent Weldon, “Mode referencing of an external cavity diode laser for continuous frequency stabilization”, Optical Engineering, February 2008, Volume 47(2).
3. Vincent Weldon, David McInerney, Richard Phelan, Michael Lynch, John Donegan, “Characteristics of several NIR tuneable diode lasers for spectroscopic based gas sensing: A comparison.” Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy Volume 63, Issue 5 , April 2006, Pages 1013-1020
4. David McInerney, John Donegan, Michael Lynch, and Vincent Weldon, “Spectral Linewidth and Tuning Requirements of Sources for Gas Sensing in Space based Applications” ”, IEE Proc.-Optoelectron., Vol. SIOE 2005.
5. Vincent Weldon, “ Spectroscopic based Gas Sensing using Tuneable Diode Lasers”, Encyclopedia of Sensors 2005, edited by Craig A. Grimes, Elizabeth C. Dickey and Michael V. Pishko, published by American Scientific Publishers, 25650 North Lewis Way, Stevenson Ranch, California 91381-1439, USA.
6. R. Phelan, M. Lynch, J. F. Donegan, V. Weldon, “Multi-species gas sensing using monolithic widely tuneable laser diodes” Proc. SPIE Vol. 5826, pp 449-459, Opto-Ireland June 2005: Optical Sensing and Spectroscopy; Hugh J. Byrne, Elfed Lewis, Brian D. MacCraith, Enda McGlynn, James A. McLaughlin, Gerard D. O'Sullivan, Alan G. Ryder, James E. Walsh; Eds.
7. R. Phelan, M. Lynch, J.F. Donegan and V. Weldon, “Simultaneous multi-species gas sensing using a sampled grating-DBR and modulated-grating Y laser diode”, Appl. Optics, 20 September, 2005, Vol.44, No. 27, pp5824-5831.
8. R. Phelan, M. Lynch, J.F. Donegan and V. Weldon, “Absorption Line Shift with Temperature and Pressure: Impact on Laser Diode based H2O Sensing at 1.393m” Applied Optics. -LP 2003, Volume 42, Issue 24, pp4968-4974.
9. Richard Phelan, Michael Lynch, John Donegan and Vincent Weldon, “Investigation of a Strongly Gain Coupled DFB Laser Cascade for Simultaneous Multigas Sensing”, IEE Proc.-Optoelectron., Vol. 150, No.2, 2003, pp182-186.