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Photoacoustic Hydrocarbon Spectroscopy Using a Mach-Zehnder · PDF file 2018. 8. 28. · Photoacoustic Hydrocarbon Spectroscopy Using a Mach-Zehnder Modulated cw OPO Henry Bruhns,

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  • Sensors & Transducers, Vol. 188, Issue 5, May 2015, pp. 40-47

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    Sensors & Transducers © 2015 by IFSA Publishing, S. L.

    http://www.sensorsportal.com

    Photoacoustic Hydrocarbon Spectroscopy Using a Mach-Zehnder Modulated cw OPO

    Henry Bruhns, Yannick Saalberg, Marcus Wolff

    Hamburg University of Applied Sciences, Heinrich Blasius Institute for Physical Technologies, Berliner Tor 21, 20099 Hamburg, Germany

    Tel.: +49 40 42875-8664, fax: +49 40 42875-8626 E-mail: [email protected]

    Received: 2 April 2015 /Accepted: 5 May 2015 /Published: 29 May 2015 Abstract: This paper describes a new photoacoustic spectrometer for the investigation of hydrocarbons based on a continuous wave optical-parametric oscillator (OPO). Two modulation methods for the generation of the photoacoustic signal are compared. In addition to the traditional modulation by a mechanical chopper a Mach- Zehnder modulator was set up for this wavelength range and used to shape the OPO beam. Spectra of three hydrocarbon test gases (methane, ethane and propane in nitrogen) were measured between 3200 nm and 3500 nm. The differences between the two modulation methods are explained and the advantages of the newly introduced Mach-Zehnder modulator are elaborated. In particular the frequency fluctuation and complexity of both methods are set in contrast with each other. The measured methane spectrum is compared against data from the HITRAN database. Copyright © 2015 IFSA Publishing, S. L. Keywords: Chopper, Mach-Zehnder interferometer, Optical-parametric oscillator, Hydrocarbons, Methane, Ethane, Propane, Photoacoustic spectroscopy. 1. Introduction

    Developing a gas detector based on the

    photoacoustic effect is a different approach than realizing a gas spectrometer. Usually, a gas detector is designed to detect a unique sort of gas molecules. In this case it is often sufficient to pick a strong and distinct absorption line of the selected molecule. Choosing a light source with exactly the center- frequency of this absorption line would make tuning of the emission wavelength expendable. In case of a laser diode with that frequency available, no additional frequency or wavelength measurement is necessary if constant operational conditions can be maintained. Additionally, the laser diode can be electronically modulated with a repetition rate that matches the resonance frequency of the used

    photoacoustic cell. The measured photoacoustic signal of such a detector is linearly proportional to the selected molecule's gas concentration, provided that the absorption is not saturated. At best, the optical emission power does not change over time. Then, optical power measurement and a corrective calculation resulting from power changes are unneeded. If, on the other hand, a gas spectrometer shall be realized, the light source should be tunable over a broad wavelength range. The spectral tuning should cover as many absorption lines as possible. Therefore, the light source's spectral emission linewidth must be narrow enough to allow a clear discrimination of spectrally close absorption lines. An optical-parametric oscillator (OPO) can be such a light source. However, the OPO's optical output power differs with changes of the operation

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    parameters. Therefore, a permanent power measurement is unavoidable, because the power affects the optoacoustic signal strength. Also, the emission wavelength must be observed while spectrally tuning the OPO. However, if the light wave is modulated with frequencies above the wavelength meter's measurement frequency, problems will arise. Therefore, it is best to supply the wavelength meter with continuous wave (cw) radiation. Consequently, a cw OPO is needed to satisfy the requirements of the wavelength meter. On the other hand, no photoacoustic signal is generated if the light intensity is not modulated. Thus, a light intensity modulator is necessary. In this study, two different modulators are compared in the experimental setup: the chopper wheel modulator, representing a mechanical solution and the electro-optical Mach-Zehnder modulator, a purely electronic solution.

    2. Experimental Setup

    The spectrometer can be divided in the building blocks: OPO, sample cell, wavemeter, power meter, signal path and modulator. A block diagram is presented in Fig. 1.

    Fig. 1. Experimental setup block diagram.

    2.1. Optical-Parametric Oscillator

    An OPO is a source of coherent radiation based on nonlinear optical effects. The energy of a pump photon is split into two photons with lower energies, of which the one with the higher wavelength is called idler and the other one signal. The relationship between the wavelengths follows the law of optical parametric generation (OPG)

    , (1)

    where λp, λi and λs are the wavelengths of pump, idler and signal radiation [1]. A commercially available OPO (Type Argos Model 2400-BB-5 Module C from Lockheed Martin Aculight in Bothell, WA) is used. Its configuration is illustrated in Fig. 2. The complete OPO system comprises of a pump laser with its controller, the OPO cavity with its control unit and a positioning unit for the lithium niobate (LiNbO3) crystal located inside the OPO cavity.

    The pump laser for the OPO is a high power diode-pumped ytterbium fiber laser, model YLR-10-

    1064-LLP-SF from IPG Photonics in Oxford, MA. The laser is mounted together with all control elements in a 19-inch rack mount enclosure. It is a cw laser with linear wave polarization. The measured maximum optical output power is 10.5 W with a measured instability of 1.2 % over eight hours. The measured central wavelength is 1063.9 nm. This laser was chosen for three main requirements: the output power and wavelength needed for the OPO as well as the narrow output linewidth of the laser beam. According to the specified optical characteristics, the output linewidth expressed in the frequency domain, lies between 70 kHz and 100 kHz full width at half maximum (FWHM). The pump laser’s optical output IP is connected with the OPO cavity via a fiber cable of 1.0 m length and a collimator. Due to the very high sensitivity to back reflections, the laser is equipped with an in-line isolator, type ISO1060-05LP-M-20W. This component provides 25 dB isolation introducing a power reduction of approximately 8 %. The pump laser can be remotely controlled via General Purpose Interface Bus (GPIB) or Electronic Industries Association Radio Sector - 232 (EIA RS-232) interfaces. The EIA RS-232 interface is used in this application. The EIA RS-232 interface is transformed to the Universal Serial Bus 2.0 (USB2.0) interface with a USB2.0 to Serial Adapter from LogiLink in Schalksmühle. The USB2.0 interface side of the adapter is connected via a USB Hub to the experiment's control computer.

    Spectral fine tuning of the OPO is realized by adjusting the etalon angle. This results in a variation of resonance frequencies and, therefore, the wavelength of maximum gain. Fine tuning in steps of 0.5 nm or better is possible. The Argos OPO allows etalon angles between -2° and 2°. Changing the etalon angle allows tuning over a total range of approximately 16 nm for the idler wave (4 nm/°). If the nonlinear crystal of an OPO is fan-poled or different periodicities are applied, changing its position causes a change of phase matching conditions, which results in a shift of wavelength of maximum gain. The crystal of the Argos OPO can be moved manually only. With a spectral shift of 22.6 nm per screw turn the idler wave can be tuned approximately from 3200 nm to 3900 nm achieving a total range of ca. 700 nm.

    Changing the temperature of the nonlinear crystal causes a variation of its index of refraction. Consequently, the phase matching condition and wavelength of maximum gain change accordingly. With increasing temperature idler wavelength decreases while signal wavelength increases. The Argos OPO can be operated in the temperature range from 45°C to 65°C. This results in a total spectral range of approximately 40 nm for the idler wave. With the etalon installed the continuous tuning of 2 nm/°C cannot be observed. The wavelength changes in leaps of approximately 13 nm.

    The OPO controller unit allows manual setting of the etalon angle and the crystal temperature as well as remote control. Also the block temperature of the

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    OPO's cavity is controlled and settable. The EIA RS-232 interface of the OPO controller is transformed to USB2.0 and connected to the control computer.

    As described before, the PPLN crystal translation tuning of the OPO can be achieved manually only. For the sake of complete automation and reproducibility of measurements it was decided to automatize the crystal translation. Therefore, a positioning unit was designed and manufactured. The PPLN crystal translation inside the OPO cavity is realized by a spindle drive. A translation of 11.4 mm is the crystal's maximum positioning range, corresponding to 31 spindle turns. Instead of manually turning the spindle with a screw driver through a hole in the OPO's enclosure, the screw driver is replaced by an automatized tool, consisting of a stepper motor driven chuck in which a screwdriver blade is inserted. To allow reproducible positioning and spindle axis calibration,