N spectroscopy, whose principle has been described in quite a few literatures [7,224], is
N spectroscopy, whose principle has been described in several literatures [7,224], is equivalent for the direct absorption spectroscopy. By straight measuring the transmitted light intensity from the resonant cavity and combining the BeerLambert law of gas absorption, the concentration from the absorbed substance within the cavity is determined by Equation (1): I (v) = I0 (v) exp[-(v) L], (1)where I(v) represents the transmitted light intensity; v represents the light frequency; I0 (v) represents the incident light intensity; denotes the absorption coefficient, which can be connected for the concentration on the absorbing substance, the intensity from the absorption lines, the line function, etc.; and L denotes the powerful absorption path. The high finesse corresponds to the path length L enhancement induced by the cavity, which improves the detection sensitivity of program. As shown in Figure 1, the distributed feedback laser functions under the manage of higher precision temperature driver and existing driver. In this experimental setup, the light from DFB laser diode (SWLD-151210P22-09, Allwave Compound 48/80 Activator Lasers, Xi’an, China) is coupled into a single-mode polarization preserving (PM) fiber, plus the pigtail output is connected having a 30 db fiber optical isolator. Then the optical isolator connected to a fiber coupler (50:50, 1550 nm Center Wavelength), along with the two output ports in the fiber coupler are connected to an adjustable fiber collimator and also a photo-detector PDA2 (PDA10CS-EC, Thorlabs, Newton, NJ, USA), respectively. The transmitted beam is transformed into a Gaussian beam by the collimator with adjustable optical waist radius and a divergence angle of about 0.074 , and its optical waist MAC-VC-PABC-ST7612AA1 custom synthesis position might be adjusted at any distance from 0.3 m to infinity. The optical cavity features a special U-shaped configuration in which 3 arms form two folded angles of 45 . The cavity is created of extremely steady glass ceramics for ultralow expansion coefficient, the length of L1 and L2 is 170 mm, along with the length of L3 is 132 mm. The two finish mirrors M1 and M4 are spherical mirrors together with the same radius of curvature of 8 m, along with the two folded mirrors M2 and M3 are plane mirrors. The ultra-low loss thin film coats within the central location from the mirrors possesses a diameter of 10 mm (R 99.99 at 1550 nm). The transmittance is significantly less than 0.05 in its functioning band from 1500 to 1600 nm. A piezoelectric ceramic transducer (PZT) cavity length modulator is mounted on the finish mirror M1 to adjust the cavity length within a tiny region. Ultimately, the cavity physical length is 472 mm. In the course of the measurement of spectrum and optical path alignment, the PZT isSensors 2021, 21, x FOR PEER REVIEW3 ofSensors 2021, 21,film coats within the central region in the mirrors possesses a diameter of 10 mm (R 99.99 at 1550 nm). The transmittance is less than 0.05 in its working band from 1500 to 1600 nm. 3 of 11 A piezoelectric ceramic transducer (PZT) cavity length modulator is mounted around the end mirror M1 to adjust the cavity length in a tiny area. Lastly, the cavity physical length is 472 mm. In the course of the measurement of spectrum and optical path alignment, the PZT is driven by a triangular wave signal with amplitude of 200 V along with a frequency of 20 of driven by a triangular wave signal with an an amplitude of 200 V and also a frequency Hz20 Hz generated by the driver circuit modulate the cavity length. generated by the driver circuit toto modulate the cavity length.Figure Figure 1. Structure diagram ofof a folded-cavity enhanced abso.
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