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Tuneable Diode Laser Absorption Spectroscopy with Optical Fiber: Noise Analysis and Noise Reduction
Norwegian University of Science and Technology, Faculty of Information Technology, Mathematics and Electrical Engineering, Department of Electronics and Telecommunications.
2006 (English)MasteroppgaveStudent thesis
Abstract [en]
A prototype of a fiber based absorption spectroscopy instrument is built. A single mode fiber pigtail Distributed Feedback laser (DFB) is used to scan a known $mbox{NH}_3$ absorption line near $1512nm$ ($6614cm^{-1}$). $2mm$ diameter InGaAs PIN photodiodes are used to convert the light signal to electrical signals by use of special designed transimpedance amplifiers. Laser modulation, coherent detection at twice the modulation frequency (second harmonic detection), other electronics, and digital signal processing are provided by Norsk Elektrooptikk. bigskip The DFB laser from NEL (NLK1S5EAAA) delivers 10mW fiber output and has maximum output around 30mW at maximum injection current of 200mA. This laser was found to be very quiet at dc operation. However, in wavelength modulation spectroscopy operation it exhibit large modulation noise that was found to stem from the fiber coupling inside the laser module. A detection limit of $6times10^{-5}$ to $1times10^{-4}$ in absorbance unit with 0.712m optical path length and an effective measurement bandwidth of 52Hz was achieved by this laser. This is significantly above the detection limit with free air lasers which achieve $3times10^{-6}$ relative absorbance (0.15ppm $mbox{NH}_3$ at room temperature and atmospheric pressure). The acquisition time for each concentration measurement takes 3.2s. Hence, the response time is very fast which is a common property of wavelength modulation spectroscopy. bigskip The modulation noise problem was attacked by an electronic noise cancellation scheme. The idea was that since the noise source is inside the laser module and no external optical component can be used to reduce it. With great linearity and reliability properties of fused biconical taper (FBT) couplers the laser beam is split into two identical beams with nearly equal amount of the same noise. One beam, signal beam, is used to induce second harmonic signals in the gaseous path. The other beam goes directly to an identical photodetector and is used as a reference signal. Two special designed noise canceller circuits were built to remove common signals. Both the circuits, based on current subtraction (not optimized) or voltage subtraction, give comparable results. The modulation noise was reduced by more than 12dB. A detection limit of $5times10^{-6}$ was achieved with this prototype. Even after transmitting the laser beam through a 3200 meter single mode fiber a signal to noise ratio (relative to 100ppm signal) of $180times$. Hence, a sensitivity is $1.1times10^{-5}$ in absorbance unit was achieved with the basic noise canceller (not optimized). This corresponds to a detection limit of 0.55ppm ammonia (12.5ppm without cancellation). The optimized noise canceller could reduce this by a factor of 3 or more. bigskip The basic noise canceller based on current domain subtraction was investigated in detail and optimized. This new circuit suppressed the intensity modulation signal by more than 70dB. The 2f noise was reduced to approximately 50ppb or $1.1times10^{-6}$ in absorbance unit with an effective noise bandwidth of 52.3Hz and 1m optical path length. This is a record sensitivity with such simple electronics. The sensitivity of our fiber based prototype is limited by the fluctuating etalon fringing in the signal beam. Under normal operation the prototype achieves approximately $pm20$ppb zero point drift in the concentration with an acquisition time of 48s (3.5Hz noise bandwidth). By our knowledge these results are the best available and are competitive with the much more complex frequency modulation spectroscopy technique. bigskip These balanced transimpedance amplifiers also have the ability to measure the absorption spectra directly without modulation. Determining the absorption linewidth and its shape is now a straight forward task. By use of dual beam configuration and noise cancellation thermal effects that caused background fluctuations were greatly suppressed. The laser delivers high intensity light, $sim22mW$. The dominating noise is the photocurrent shot noise and the etalon fringing from optical components in the signal path. Remote sensing, in-situ, noninvasive measurements can be performed with this instrument. In some cases it's flexibility, and remote sensing possibility are of great attractiveness for the industry. The sensitivity is also improved by a factor of two with the fiber based tuneable diode laser absorption spectroscopy instrument. bigskip In the end of this thesis we demonstrate the ability to measure multiple absorption lines simultaneously. The number of measurement paths is limited by the laser output power.
Place, publisher, year, edition, pages
Institutt for elektronikk og telekommunikasjon , 2006. , 158 p.
Keyword [no]
ntnudaim:1161, MTEL elektronikk, Fotonikk og mikroteknologi
URN: urn:nbn:no:ntnu:diva-13089Local ID: ntnudaim:1161OAI: diva2:432280
Available from: 2011-08-02 Created: 2011-08-02

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