NUSIRALS · Novel Ultra-Sensitive Infra-Red Absorption Laser Sensors

Novel ultra-sensitive infrared absorption laser sensors

The overall aim of the project is to develop and demonstrate a concept of a novel technique and methodology for optical gas sensors based upon frequency modulation (FM) and optical feedback locking (OFL) in cavity-enhanced absorption spectroscopy (CEAS). A simple optical and electronic set-up of an ultra-sensitive gas sensor should be based on three basic principles: long-path lengths arranged in smallest volumes by using a high-finesse optical cavity with an excitation of the TEM00 mode; use of an OFL of the laser to a high-finesse cavity; minimisation of photodetection noise by means of high FM of light and photodetection at MHz-GHz frequencies, whilst FM of the laser optical frequency is identical to the cavity free spectral range. The V-shaped cavity (Fox-Smith mirror design) was selected as most suitable for stable and reliable locking of the laser to the cavity TEM00 in FM-OFL-CEAS. The laser current was modulated at high frequency of 121 MHz identical to the free spectral range of the V-shaped cavity with two equal 62-cm long arms. For the distance of 62 cm between the laser and the first mirror cavity an excitation of the odd and even modes of the cavity was observed. It was successfully demonstrated that the frequency modulated triplet could pass through the V-shaped cavity and heterodyne signal could be detected by a high-frequency bandwidth InGaAs photodetector. We found that the average mirror reflectivity of 99.97 % in the V-shaped cavity resulted in a cavity finesse of about 10 465, a cavity path length enhancement gain of 3 333 and an effective absorption path length of 4 133 m. In order to estimate best absorption sensitivity that could be achieved for FM signals in FM-OFL-CEAS a short 1.5-cm long reference cell filled with acetylene (C2H2) at 40 mbar was inserted between the V-shaped cavity output mirror and the photodetector PHD1. The FM light triplet emerged from the cavity experienced the absorption losses and phase shifts after propagation through the C2H2 absorber. The recorded and processed (maximum value of each cavity mode) cavity output intensity and the FM signals are shown. For 1-s measurement over 40 successive cavity modes an absorbance baseline noise (standard deviation) of 0.0039 was estimated from the measured maximum mode intensities observed on the V-shaped cavity output within the laser frequency tuning over C2H2 line at 1529.18 nm. For the effective beam path length of 4 133 m in the V-cavity an absorption sensitivity of 9.4×10-9 cm-1 was derived for the absorption spectra. The observed signal-to-noise ratios of the recorded FM in-phase components were by a factor of 20 higher. Hence, a minimum detectable absorption coefficient of 4.7×10-10 cm-1 for the 40 mode points of the in-phase absorption spectra were observed for 1-s averaging of 10 successive in-phase signal scans and a noise-equivalent detection bandwidth of 2 kHz. The demonstrated FM-OFL-CEAS noise-equivalent absorption sensitivity of 1.05×10-11 cm-1 /Hz1/2 was only 5.8 times worse than a theoretical shot-noise limited absorption sensitivity of 1.8×10-12cm-1/Hz1/2 calculated for the FM laser beam with an FM index of 1.1, the effective path length of 4 133 m and the power of 10 µW incident onto a InGaAs photodetector. The minimum detectable C2H2 number density was approximately 2.6×107 molecule / cm3 or approximately 10 ppt at a pressure of 100 mbar. For stronger absorbing molecules such as, for instance, CO2 and N2O absorption sensitivities demonstrated in FM-OFL-CEAS may lead to detection of an even smaller number density of molecules down to tens of ppq (part per quadrillion). Detection of H2O, HF, NH3 at ppt level may find application in nanotechnology for monitoring purity of components in production of three-dimensional (3D) nanostructures and production of high purity gases. Monitoring radio-labelled 11CO2, 11CO, H11CN, 11CH4 molecules at ppt using FM-OFL-CEAS technology may find application in optimisation of production of radio-labelled minute quantities of drugs for positron emission tomography (PET) and other medical research. In conclusion, we demonstrated a novel technique FM-OFL-CEAS for ultra-sensitive gas detection with the noise equivalent absorption sensitivities of 1.05 ×10-11 cm-1 Hz-1/2 and a near-infrared distributed feedback (DFB) laser at 1 529 nm. This absorption sensitivity in FM-OFL-CEAS can be further improved by a factor of 10 - 100 using higher scan rates, higher reflectivity mirrors and laser power. The full potential of FM-OFL-CEAS with near-infrared and mid-infrared lasers with this high sensitivity and the detection of other molecules than acetylene is yet to be realised, but the theory and technical principles are now developed, demonstrated and established. Small gas volumes of 10 ml and shot-noise limited absorption sensitivity of the FM-OFL-CEAS sensor will allow to analyse minute volumes of samples with a fast response time and ultra-high sensitivity at ppt level. The successful implementation of the proposed improvements for the developed FM-OFL-CEAS sensor enables routine measurements with noise-equivalent absorption sensitivities at 10-14-10-11cm-1Hz-1/2. This represents a breakthrough in the art of gas detection and will open the door to new approaches in ultra-sensitive detection and new applications of laser sensors in nanotechnology, industry, chemistry, physics, life and environmental sciences.

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