Optical fiber gas sensor based on thermal lens spectroscopy

Abstract

This article describes how to design an optical fiber gas sensor based on a thermal lens (TL) spectroscopy instrument composed of spherical lensed fibers. This sensor features a detection head composed of two spherical lensed fibers assembled across from each other. The distance between the two fibers is just under 100 μm, with the gas in between. In other words, the optical absorption path length for our gas sensor is extremely short compared with that for other optical spectroscopic techniques (e.g., 10–20 mm∼1–2 m). In addition, our gas sensor is extremely simple, small (e.g., 20 mm in length), adjustment-free and robust. These points are extremely important for actual use in the field. The measurement principle and theoretical analysis of the design are first described. Acetylene (12C2H2) gas is used as a sample gas to evaluate the sensitivity of our sensor. The limit of detection was estimated to be approximately 5200 Pa in our experimental condition. In the conclusion, the capability of our technique is discussed, and a technique for enhancing the sensitivity in our TL technique is proposed.

A new technique for measuring viscosity of liquid with pico-liter volume based on Doppler shift by photothermal effects

“…Note that our sensing head for the sample liquid is composed of two spherical lensed fibers (SLFs) assembled across from each other. This is our original technique, recently developed with other works [3]. Due to using this sensing head, the volume of the specimen liquid is no more than the order of pico-liter.…”

Optical fiber sensor for humidity monitoring based on thermal lens detection technique

“…The relative humidity (RH) is hereinafter labeled as the RH. Here, the modulation frequency fm of the pumping beam was set to 10 Hz, which is the optimum value determined with our previous work [7]. Besides, the spherical radius R of the SLF was set to 10 µm, which is the optimum value for maintaining sensitivity as well as for adjusting the optical alignment.…”

“…Note that even though the beam locus (b) in Fig. 1 is not the actual shape, it is still very useful for deriving theoretical formulas and enabling an optical coupling efficiency determination [7]. Furthermore, the pumping beam locus is not exactly the same as the probe beam because of the wavelength dispersion of the refractive index of quartz.…”

“…They pass through the air and are captured by Fiber B. The optical coupling efficiency between the two SLFs is changed when the refractive index of the air changes due to the TL effect [6,7,8]. Figures 1 (a) and (b) show the locus of the beam corresponding to the non-excited and excited TL, respectively.…”

“…In addition, the fairly low-power source (normally under 100 mW) is applicable as the pumping laser in our sensor in spite of a certain optical absorption path-length under 50 µm. The sensing principle in our proposed sensor is based on the technique originally developed in our earlier works [6,7,8]. First, the measurement principle is briefly described and the measurement system is shown.…”