Optical gas sensors using laser diodes have received attention because they satisfy air-monitoring requirements. Various sensor configurations have been proposed and developed that are basically applications of laser spectroscopy methods. Because of this, it is necessary to extend the length of the optical absorption path to enhance the sensitivity. To overcome this problem, our earlier work proposed an optical fiber gas sensor based on a thermal lens (TL) spectroscopy technique. One feature of our technique is that the optical absorption path is much shorter (usually 20-50 μm) compared with other optical spectroscopy techniques. In our previous apparatus, it is necessary to use a wavelength-tunable laser diode as the light source for pumping the TL. However, this laser diode was not only extremely expensive but also troublesome because it required accurate wavelength adjustments. Additionally, the coverage of the tunable wavelength was extremely narrow in commercial laser diodes, limiting the ability to sense typical gases. To overcome these problems, we propose the TL spectroscopy gas-sensing apparatus based on an etalon-stabilized wavelength sweep technique for fiber ring laser. Figure 1 shows a block diagram of our measurement apparatus. The configuration of this apparatus is basically a fiber ring laser containing our TL sensor head and wavelength-tunable devices on the inside of the ring loop. A fiber Bragg grating (FBG; λ{sub}o = 1532.8 nm, FWHM = 1 nm) is the coarse-tuning device of the oscillation wavelength, and an etalon plate (FWHM = 0.02 nm, FSR = 50 GHz) is the fine-tuning device. The etalon plate is mounted on a high-precision rotation stage, which is driven with a piezo actuator so that its rotation angle can be varied, resulting in the optical route distance changing. The rotation angle was estimated to be approximately 10 arc-minutes, which corresponds to the sweep span of approximately 0.1 nm. This span is sufficient for gas spectroscopic detection. Here, the beam is an ordinary continuous wave (CW). The optical coupling efficiency of TL detection head is significantly changed, depending on the optical absorption spectrum of the gas, when the wavelength of the pumping beam is swept smoothly. The TL detection signal could be obtained as an alternate signal by sweeping, enabling synchronous detection with a sweep signal using a lock-in amplifier.
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