In this paper, we propose a compact optical gas sensor based on the widespread silicon-on-insulator (SOI) technology, operating in the near-infrared (NIR) region around the 1.55 µm wavelength. The sensor employs a loop-terminated Mach–Zehnder interferometer (LT-MZI) with a slot waveguide and a strip waveguide for the sensing arm and the reference arm, respectively. For the same arm length, the LT-MZI can achieve a detection limit two times lower than that of the conventional MZI. Different sensor components were designed, and the optimum dimensions were obtained using finite-difference eigenmode (FDE) and finite-difference time-domain (FDTD) solvers. With a sensing arm length of only 150 μm, our sensor achieves a device sensitivity of 1070 nm/RIU and a figure-of-merit (FOM) as high as 280.8 RIU−1 at the 1.55 μm wavelength. Higher values of FOM can be attained by employing a longer sensing arm. The whole sensor is subjected to air cladding; thus, there is no need for oxide deposition and a further lithography step for sensing-area patterning. The sensor is well suited for low-cost fabrication and large-scale production. Finally, the same LT-MZI device with strip and slot arms but with oxide cladding was fabricated and characterized. The measurements were in good agreement with the electromagnetic (EM) simulation results, ensuring the reliability of our proposed design.

In this work, a highly sensitive sensor based on silicon nitride (SiN) waveguide is proposed that can be used for gas sensing using visible light. The whole sensor waveguide uses a silicon dioxide (SiO2) cladding while the sensing arm uses a fluidic cladding such as water. The proposed device is based on loop-mirror terminated (LMT) Mach-Zehnder interferometer (MZI), where the reference arm is exposed to the reference SiO2 medium, while the sensing arm is exposed to the sensing medium leading to a change in the refractive index of the waveguide cladding. The sensor performance is overall optimized by optimizing the design performance of all the components of the structure one by one. The waveguide sensitivity of both strip and slot types is analyzed for gaseous medium in order to compare their sensitivities and select the dimensions of the waveguide that results in the highest device sensitivity. Transverse-electric (TE) polarization is considered in this study for strip waveguide, where a higher sensitivity is founded with respect to the transverse-magnetic (TM) polarization. The field confinement in the slot waveguide in the sensing region is investigated and verified with a mode solver, where the optimum dimensions are obtained using finite difference eigenmode and finite difference time domain solvers. With a sensing arm length of 150 μm only, the proposed sensor achieves a device sensitivity of about 1,320 nm/RIU and a figure-of-merit (FOM) as high as 641 RIU−1 at the wavelength of 650 nm, which is the highest reported FOM up to the author’s knowledge. Higher values of the FOM are possible by employing a longer sensing arm.

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In this work, a highly sensitive sensor made of SiN is proposed that can be used in gas or biological sensing, where the choice depends on whether a marker is used or not. The whole sensor is subjected to water cladding. The proposed device is based on a Mach-Zehnder Interferometer (MZI), while the sensing arm is used for sensing the change in the refractive index of the analyte. Both polarizations (TE, TM) are considered in this study, where a higher sensitivity is achieved for the TE-polarized light. The field confinement in the strip waveguide in the sensing region is investigated and verified with a mode solver, whereas the optimum dimensions are obtained using finite difference eigenmode and finite difference time domain solvers. With a sensing arm length of only 180 μm, the proposed sensor achieves a device sensitivity of about 1942 nm/RIU and a figure-of-merit (FOM) as high as 2284 RIU −1 at the wavelength of 1.55 μm. Higher values of FOM can even be attained by employing a longer sensing arm.