Silicon-on-sapphire waveguides design for mid-IR evanescent field absorption gas sensors

A Miniature On-Chip Methane Sensor Based on an Ultra-Low Loss Waveguide and a Micro-Ring Resonator Filter

“…When the incident angle is above the critical angle, light travelling through the high refractive index waveguide undergoes total internal reflection and most of the energy is confined within the high refractive index waveguide core. However, there will be an evanescent field extending to the cladding region formed by the surrounding media [15]. …”

“…Then, the intensity of the absorbed light depends on the concentration of the gas. This can be described by the Beer-Lambert Law [9]:$I\left(\lambda \right)=I_o\left(\lambda \right)\exp \left(-\eta {\epsilon }_{\lambda }cl-{\alpha }_{\mathrm{base}}l\right)$ where $I_o\left(\lambda \right)$ is the output optical power without the target gas, $I\left(\lambda \right)$ is the output optical power with the target gas, $c$ is the concentration of the target gas, ${\epsilon }_{\lambda }$ is the absorption coefficient of the gas at wavelength $\lambda$ (for methane, the range of ${\epsilon }_{\lambda }$ is from 0.035 cm −1 to 0.3 cm −1 in 1650 nm band) [11], $l$ is the length of the optical path, ${\alpha }_{\mathrm{base}}$ is the waveguide loss, and $\eta$ is the evanescent field ratio (EFR), which can be calculated by the following expression [15]:$\eta =I_{\mathrm{gas}}/I_{\mathrm{all}}=true{\iint }_{\mathrm{gas}}\stackrel{true\to }{S}·\stackrel{true\to }{n}dxdy÷{\displaystyle {\iint }_{\mathrm{all}}\overrightarrow{S}·\overrightarrow{n}dxdy}$ where $\overrightarrow{S}$ is the Poynting Vector of the mode field in the waveguide, and $\overrightarrow{n}$ is the normal vector to the waveguide cross-section.…”

“…The sensitivity of the sensor S is considered as the change in optical power with gas concentration, and can be described by the following [15]:$S=-\eta {\epsilon }_{\lambda }l{I}_o\left(\lambda \right)\exp \left(-\eta {\epsilon }_{\lambda }cl-{\alpha }_{\mathrm{base}}l\right)$…”

“…H. Mohseni [16] developed a detector in which the NEP could decrease to 2 × 10 −16 W/Hz 1/2 . The base concentration of methane was chosen to be the approximate atmospheric level of 2 ppm; the absorption coefficient of methane is 0.3 cm −1 [15]; coupling loss is the most important additional loss and is approximately 2 dB. Ultra-low-loss waveguides have been successfully manufactured previously [17,18], and the lowest waveguide loss has been reported to be 1.2 dB/m.…”

Integratable all-optical spectral quantization scheme based on chalcogenide–silicon slot waveguide

“…However, more than 1 km highly nonlinear fiber (HNLF) and single mode fiber were used in the previous work, which is not easy to be integrated. CMOS-compatible silicon-based photonics technologies are developed to satisfy the on-chip all-optical signal processing [12][13][14][15]. Thus, the silicon-on-insulator (SOI) waveguides can be used as the ideal nonlinear medium of SSFS-based quantization for the integratable AOADC with high analog bandwidth and resolution.…”