We demonstrate a new scheme of cantilever-enhanced photoacoustic spectroscopy, combining a sensitivity-improved fiber-optic cantilever acoustic sensor with a tunable high-power fiber laser, for trace gas detection. The Fabry-Perot interferometer based cantilever acoustic sensor has advantages such as high sensitivity, small size, easy to install and immune to electromagnetic. Tunable erbium-doped fiber ring laser with an erbium-doped fiber amplifier is used as the light source for acoustic excitation. In order to improve the sensitivity for photoacoustic signal detection, a first-order longitudinal resonant photoacoustic cell with the resonant frequency of 1624 Hz and a large size cantilever with the first resonant frequency of 1687 Hz are designed. The size of the cantilever is 2.1 mm×1 mm, and the thickness is 10 μm. With the wavelength modulation spectrum and second-harmonic detection methods, trace ammonia (NH<sub>3</sub>) has been measured. The gas detection limits (signal-to-noise ratio = 1) near the wavelength of 1522.5 nm is achieved to be 3 ppb.
We demonstrate a high-sensitivity fiber-optic Fabry-Perot acoustic sensor based on a thin Parylene-C diaphragm. The vacuum thermal evaporation deposition method is used to fabricate the Parylene-C nanofilm, which possesses strong adhesion and good compactness. Based on these characteristics, the Parylene-C diaphragm is fabricated with 9 mm in diameter and 500 nm in thickness. The noise limited equivalent acoustic signal level is 33.5 μPa/Hz<sup>1/2</sup> at the frequency of 30 Hz. The pressure sensitivities of the acoustic sensor are more than 1000 mV/Pa at the frequency from 10 Hz to 30 Hz. The fundamental resonance frequency of the Parylene-C diaphragm is about 13 Hz. The acoustic sensor is applied in a multiple trace gas detection system based on photoacoustic spectroscopy. The detection limits of acetylene (C<sub>2</sub>H<sub>2</sub>), carbon monoxide (CO) and carbon dioxide (CO<sub>2</sub>) are achieved to be 0.25, 0.32 and 1.1 parts-per-million, respectively.
Optical loss of fiber-optic connectors has a vital impact on fiber-optic-related systems. We analyze the contact loss caused by the endface geometry and the contact force. Based on analytical and simulated results, the analytical equations of the insertion loss (IL) and return loss (RL) as a function of the endface geometry and the contact force are derived. Then four fiber-optic connectors are experimentally tested. The experimental results are well consistent with the theoretical results, which show that there is an optimum relationship between the endface geometry and the contact force to ensure a low IL and a high RL.
A novel evanescent field refractometer based on a two-core photonic crystal fiber (TWPCF) sandwiched between multimode fibers(MMFs) is demonstrated. Through splicing a short piece of TWPCF between two MMFs, a simple structure and high sensitivity RI sensor can be constructed. Instead of using wavelength information as sensor signal, we focus more on the light intensity signal different from most PCF based RI sensor. The TWPCF section functions as a tailorable bridge between the excited high order modes and the surrounding refractive index (SRI). With a light filter inserting in the front of white light, the transmission spectrum of the light through the sensing region occurs in a welldefined wavelength bands. As a result, the peak power of the transmission light is tailored with the SRI perturbation via the MMF-TWPCF–MMF structure. The experiment result shows a quadratic relation between the light intensity and samples within RI range of 1.33-1.41 while a linear response can be achieved from the 1.33-1.35 which is a most used RI range for biologically sensing.
We report two fiber multiple-mode interferometers formed in photonic crystal fiber (PCF). The interference between the core and the cladding modes of a PCF is utilized. We use two methods to form a coupling point, and the cladding modes are excited from the fundamental core mode. One method is blowing compressed gas into the air holes and discharging at the coupling point; the air holes will expand due to gas expansion in the discharge process. Similarly, the other is discharging at the coupling point after the air is exhausted from the air holes, and the holes will contract during the process. By making another coupling point at a different location along the fiber, the proposed PCF interferometers are implemented. Experimental results show that the sensitivities of the two devices can achieve 1.54 and 1.45 nm for a 0.01 refractive index change.
We report a type of multimode fiber interferometers (MMI) formed in photonic crystal fiber (PCF). To excite the cladding modes from the fundamental core mode of a PCF, a coupling point is formed. To form the coupling point, we used the method that is blowing compressed gas into the air-holes and discharging at one point, and the air-holes in this point will expand due to gas expansion in the discharge process. By placing two coupling points in series, a very simple all-fiber MMI can be implemented. The detailed fabrication process is that the one end of the PCF is tightly sealed by a short section of single mode fiber (SMF) spliced to the PCF. The other end of the PCF is sealed into a gas chamber and the opened air holes are pressurized. The PCF is then heated locally by the fusion splicer and the holes with higher gas pressure will expand locally where two bubbles formed. We tested the RI responses of fabricated sensors at room temperature by immersing the sensor into solutions with different NaCl concentration. Experimental results show that as refractive-index (RI) increases, the resonance wavelength of the MMI moves toward longer wavelengths. The sensitivity coefficients are estimated by the linear fitting line, which is 46nm/RIU, 154mn/RIU with the interferometer lengths (IL) of 3mm and 6mm. The interferometer with larger IL has higher RI sensitivity. The temperature cross-sensitivity of the sensor is also tested. The temperature sensitivity can be as low as -16.0pm/°C.