Gas weak absorption in near-infrared is considered as inherent limit hinder the achievement of the miniaturized gas sensor. Thus, cavity enhanced absorption spectroscopy (CEAS) techniques are used to enhance the sensitivity by improving light-matter interaction length. In this work, a novel cavity enhanced technique is proposed based on the detection of the optical signal beating on RF analyzer to enhance the signal to noise performance by eliminating the flicker noise. Besides, enhancing the effective interaction length. The source consists of a fiber drum, a directional coupler, a tunable filter with FWHM of 1 nm and a semiconductor optical amplifier pumped above the laser threshold. The ring length is 1004 m leading to an FSR of 199 kHz. The gas cell is inserted into another ring, which consists of two directional couplers. The ring length is 6 m leading to an FSR of 33 MHz. The large ratio between the lengths of the two rings eliminates the need for a mode-locking technique. The acetylene gas cell is measured around 1535 nm. The novel technique capable of enhancing measurement sensitivity, providing experimental sensitivity and flicker noise immunity.
In this work, we report the usage of multi-longitudinal mode laser as an input source to achieve a larger enhancement in the interaction length in cavity enhanced absorption spectroscopy. The MM laser source is constructed in the form of a ring laser using a long fiber coil, a directional coupler, a tunable filter with FWHM of 1 nm and a semiconductor optical amplifier pumped above the laser threshold. The ring has a length of about 1004 m with an FSR of 199.2 kHz. The gas cell is inserted into another ring cavity, which consists of two directional couplers and a gain medium pumped below threshold. The gain medium is used to compensate for the losses and boost the ring cavity finesse. The cavity ring has a length of about 6 m which gives an FSR of 33.3 MHz. The large ratio between the lengths of the two cavities (the MM laser cavity and the gas cell ring cavity) eliminates the need for a mode-locked technique. The acetylene gas cell is measured around 1535 nm. The interaction length is improved by a factor of about 37 compared to the direct absorption of the gas cell.
In this work, we present the realization of a novel configuration of dual-coupler nested coupled ring resonator on silicon photonics technology. The waveguide height and width are 220 nm and 500 nm, respectively, surrounded by air from the top and by silicon-oxide on the three other sides. The design assumes TM mode with group refractive index of 3.17 at 1550 nm. The design consists of two mini-racetrack resonators of the smallest lengths of 150.8 and 182.13 μm corresponding to resonance, and bending radius of 25 μm, to minimize bending losses. The directional couplers are designed for a coupling ratio of 97/3. The proposed configuration and the single cavity ring resonator are fabricated using IMEC- ePIXfab passive technology. The measured response shows that the new configuration enhances the finesse by up to 10 times. The proposed high finesse resonator can boost the performance in many applications such as gas sensing, rotation sensing and optical filters.
Incoherent broadband cavity enhanced spectroscopy can significantly increase the effective path length of light-matter interaction to detect weak absorption lines over broad spectral range, for instance to detect gases in confined environments. Broadband cavity enhancement can be based on the decay time or the intensity drop technique. Decay time measurement is based on using tunable laser source that is expensive and suffers from long scan time. Intensity dependent measurement is usually reported based on broadband source using Fabry-Perot cavity, enabling short measurement time but suffers from the alignment tolerance of the cavity and the cavity insertion loss. In this work we overcome these challenges by using an alignment-free ring cavity made of an optical fiber loop and a directional coupler, while having a gain medium pumped below the lasing threshold to improve the finesse and reduce the insertion loss. Acetylene (C2H2) gas absorption is measured around 1535 nm wavelength using a semiconductor optical amplifier (SOA) gain medium. The system is analyzed for different ring resonator forward coupling coefficient and loses, including the 3-cm long gas cell insertion loss and fiber connector losses used in the experimental verification. The experimental results are obtained for a coupler ratio of 90/10 and a fiber length of 4 m. The broadband source is the amplified spontaneous emission of another SOA and the output is measured using a 70pm-resolution optical spectrum analyzer. The absorption depth and the effective interaction length are improved about an order of magnitude compared to the direct absorption of the gas cell. The presented technique provides an engineering method to improve the finesse and, consequently the effective length, while relaxing the technological constraints on the high reflectivity mirrors and free-space cavity alignment.