Two optical waveguide sensors based on SOS (silicon-on-sapphire) for detecting CO2 are theoretically proposed. The operational wavelength is 4.23 μm, which is the maximum absorption line of CO2. The power confinement factor (η) value is over 40% and 50%, the propagation loss is 0.98 dB/cm and 2.99 dB/cm, respectively, in the slot waveguide and SWGS (subwavelength grating slot) waveguide. An inverted tapered structure is used for the transition from strip waveguide to slot waveguide and constitutes the sensing absorption region, with the coupling efficiency that can reach more than 90%. When the optimal absorption length of the slot waveguide and SWGS waveguide is 1.02 cm and 0.33 cm, respectively, the maximum sensitivity can reach 6.66 × 10−5 (ppm−1) and 2.60 × 10−5 (ppm−1). Furthermore, taking the slot waveguide as an example, spiral and meander structures enable the long-distance sensing path to integrate into a small area.
Integrated narrow-linewidth lasers are the key devices in compact coherent optical systems of metrology, sensing, and optical microwave generation. Here, we demonstrate a hybrid integrated laser based on an optical negative feedback scheme. The laser is composed of a commercial distributed feedback (DFB) laser diode and an on-chip micro-resonator with a Q-factor of 0.815 million. The feedback optical field is coupled back to the laser cavity through the back facet. Therefore, the laser can maintain the lasing efficiency of the DFB laser diode. The linewidth of the DFB laser diode is compressed from 2 MHz to 6 kHz, corresponding to the linewidth reduction factor of 25.2 dB. The theoretical result shows that the laser performance still has a huge improvement margin through precise control of the detuning between laser frequency and the micro-resonator, as well as the phase delay of the feedback optical field. The hybrid narrow-linewidth laser diode has wide application prospects in coherent optical systems benefitting from the low cost and volume productivity.
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