Upconversion Detector for Remote Gas Sensing

Lichun Meng*

*Corresponding author for this work

    Research output: Book/ReportPh.D. thesis

    212 Downloads (Pure)


    An intracavity-enhanced upconversion detector (UCD) for range-resolved atmospheric CH4 sensing over a long distance is demonstrated. The UCD detects an IR signal at 1645 nm by mixing with a 150W 1064nm laser inside a bulk periodically poled LiNbO3 to produce an upconverted signal with a maximum efficiency of∼ 40%. A photomultiplier tube is used to measure the upconverted signal yielding a noise equivalent power of 127fW/√Hz for the whole detection system. The UCD outperforms a conventional APD in an atmospheric CH4 DIAL measurement, and the differential absorption optical depths given by the UCD have less than 11% error at ranges between 3 km and 9 km. In addition, the UCD is based on a ring cavity with the ability of single-longitudinal mode pumping, which makes the detector free of frequency beat noise. In an experimental demonstration of high-speed IR detection, a UCD with a bandwidth larger than 1 GHz is demonstrated. Performing both experimental and theoretical investigation of the noise property of the UCD is another essential content of this thesis. Two additional UCDs (one is a singlepass, short-wavelength pumped UCD, and the other is an intracavity-enhanced, long-wavelength pumped UCD) are built in order to study the noise sources (USPDC, SHG-SPDC, upconverted thermal radiation and upconverted SRS processes) systematically.
    The following conclusions are obtained: 1) for the short-wavelength pumped UCD, the USPDC noise due to the RDC error of PPLN crystal is the primary noise source when the IR signal wavelength is smaller than 2µm, and the upconverted thermal radiation becomes the primary noise source when the wavelength is larger than 3µm. 2) As a newly identified noise source, the SHG-SPDC process can be avoided by choosing a proper combination of PPLN poling period, operating temperature and bandpass filter. 3) For the long-wavelength pumped, intracavity enhanced UCD, the USPDC process is avoided fundamentally, but the noise count rate can be larger than 5×104/s due to the upconverted SRS process, especially when the Raman frequency shift is small. During the process of noise investigation, the radially modulated pattern of the USPDC noise is first observed. Based on that discovery, a noise reduction method for the short-wavelength pumped UCD is proposed: collecting the upconverted signal at the local minimum of the noise pattern, and the noise can be reduced with the use of noncollinear QPM condition and a proper spatial filter. This simple technique results in a 14 dB reduction of dark-count rate with only 2.2 dB signal loss.
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherTechnical University of Denmark
    Number of pages109
    Publication statusPublished - 2018


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