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.
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.
A multiplication method of the Sagnac effect scale factor in ring laser gyroscopes is presented based on the Vernier effect of a dual-coupler passive ring resonator coupled to the active ring. The multiplication occurs when the two rings have comparable lengths or integer multiples and their scale factors have opposite signs. In this case, and when the rings have similar areas, the scale factor is multiplied by ratio of their length to their length difference. The scale factor of the presented configuration is derived analytically and the lock-in effect is analyzed. The principle is demonstrated using optical fiber rings and semiconductor optical amplifier as gain medium. A scale factor multiplication by about 175 is experimentally measured, demonstrating larger than two orders of magnitude enhancement in the Sagnac effect scale factor for the first time in literature, up to the authors’ knowledge.
In recent years, the development of a reliable, low cost, miniaturized optical gyroscope has been a subject of research and development in the scientific and industrial communities. Since the sensitivity of the optical gyroscopes is reduced with the sensor footprint, multi-ring configurations, such as the coupled resonator optical waveguide (CROW) and the triple ring resonator configuration (TRR), were suggested to increase the sensitivity of the small foot print optical gyroscopes. In this work, a dual coupler coupled cavities configuration is suggested to be used in optical gyroscopes to enhance their performance. The configuration consists of two resonators directly coupled to each other. The passive cavity is a dual coupler ring resonator and is inserted in the main cavity such that the circulating light is fed back to the main cavity by the drop port of the coupled cavity. This leads to a smaller full width half maximum (FWHM) and hence better sensitivity. The passive resonator is studied to find analytical expressions for the output electric field and the effect of rotation on this field. The shot noise-limit sensitivity is calculated and compared to the conventional passive gyroscope in two cases: for a constant source power and for a constant detected power. The effect of propagation loss is also studied. An order of magnitude potential enhancement in the sensitivity is shown for silica-on-silicon technology.
Portable and handheld spectrometers are being developed and commercialized in the late few years leveraging the rapidly-progressing technology and triggering new markets in the field of on-site spectroscopic analysis. Although handheld devices were commercialized for the near-infrared spectroscopy (NIRS), their size and cost stand as an obstacle against the deployment of the spectrometer as spectral sensing components needed for the smart phone industry and the IoT applications. In this work we report a chip-sized microelectromechanical system (MEMS)-based FTIR spectrometer. The core optical engine of the solution is built using a passive-alignment integration technique for a selfaligned MEMS chip; self-aligned microoptics and a single detector in a tiny package sized about 1 cm3. The MEMS chip is a monolithic, high-throughput scanning Michelson interferometer fabricated using deep reactive ion etching technology of silicon-on-insulator substrate. The micro-optical part is used for conditioning the input/output light to/from the MEMS and for further light direction to the detector. Thanks to the all-reflective design of the conditioning microoptics, the performance is free of chromatic aberration. Complemented by the excellent transmission properties of the silicon in the infrared region, the integrated solution allows very wide spectral range of operation. The reported sensor’s spectral resolution is about 33 cm-1 and working in the range of 1270 nm to 2700 nm; upper limited by the extended InGaAs detector. The presented solution provides a low cost, low power, tiny size, wide wavelength range NIR spectral sensor that can be manufactured with extremely high volumes. All these features promise the compatibility of this technology with the forthcoming demand of smart portable and IoT devices.
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.
Optical filters with relatively large FSR and narrow linewidth are simultaneously needed for different applications. The ratio between the FSR and the 3-dB linewidth is given by finesse of the filter, which is solely determined by the different energy loss mechanisms limited by the technology advancement. In this work, we present a novel coupled-cavity configuration embedding an optical filter and a gain medium; allowing an overall finesse enhancement and simultaneous FSR and 3-dB linewidth engineering beyond the technological limits of the filter fabrication method. The configuration consists of two resonators. An active ring resonator comprises an optical gain medium and a passive resonator. In one configuration, the optical filter is the passive resonator itself. In a second configuration, the passive resonator is another ring resonator that embeds the optical filter. The presented configurations using a semiconductor optical amplifier are applied one time to a mechanically Fabry-Perot filter in the first presented configuration; and a second time to a fiber ring filter in the second presented configuration. The mechanical filter has an original 3-dB linewidth of 1nm and an FSR that is larger than 100nm while the enhanced linewidth is about 0.3nm. The fiber ring filter length is 4 m and directional coupler ratios of 90/10corresponding to a 3-dBlinewidth of about 4MHz and an FSR of 47 MHz. The enhanced 3- dBlinewidth of the overall filter configuration is 200kHz, demonstrating finesse enhancement up to20 times the original finesse of the filter.
The characterization of long fiber cavities is essential for many systems to predict the system practical performance. The conventional techniques for optical cavity characterization are not suitable for long fiber cavities due to the cavities’ small free spectral ranges and due to the length variations caused by the environmental effects. In this work, we present a novel technique to characterize long fiber cavities using multi-longitudinal mode fiber laser source and RF spectrum analyzer. The fiber laser source is formed in a ring configuration, where the fiber laser cavity length is chosen to be 15 km to ensure that the free spectral range is much smaller than the free spectral range of the characterized passive fiber cavities. The method has been applied experimentally to characterize ring cavities with lengths of 6.2 m and 2.4 km. The results are compared to theoretical predictions with very good agreement.
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