On-chip gas sensor based on infrared absorption spectroscopy is useful for environmental detection because of its small size and low power consumption. Direct absorption spectroscopy is a commonly used detection technique for on-chip gas sensors, but the noise of this detection method is high. The wavelength modulation spectroscopy technique can suppress noise. The combination of the wavelength modulation spectroscopy technique with the on-chip gas sensor can improve the performance of the sensor. However, the waveguide parameters including external confinement factor, loss, and length influence the second harmonic signal. A slot waveguide can increase the external confinement factor by using the mode field distributed in the slot for sensing. We provide guidance for the design of on-chip gas sensors based on wavelength modulation spectroscopy.

The optical field distribution results and external confinement factor are obtained by COMSOL Multiphysics with electromagnetic waves and frequency domain module. The optical parameters of the waveguide are set at the wavelength of 3291 nm. The chalcogenide rectangular waveguide is fabricated by the lift-off method. The process of the lift-off method includes spinning photoresist, lithography, development, thermal evaporation, and removal of photoresist. The noise of the waveguide sensing system is used for simulation analysis. The second harmonic signal amplitude of the on-chip gas sensor is simulated by MATLAB. The important parameters of the simulation model include gas absorption parameters at 3291 nm, waveguide parameters, and laser parameters. The simulated limit of detection is calculated based on the signal-to-noise ratio.

The trapezoid waveguide morphology is shown in Fig. 2, and the external confinement factor of the waveguide is about 8%. The CH₄ sensing results based on wavelength modulation spectroscopy at 3291 nm show that the response result is linear (Fig. 5). The slot waveguide structure with magnesium fluoride as the lower cladding layer and chalcogenide glass as the core layer is optimized, and the external confinement factor reaches 42% (Fig. 6). Based on the experimental results, the effects of waveguide loss and waveguide length on the second harmonic signal amplitude are studied (Fig. 7). Decreasing waveguide loss and selecting an appropriate waveguide length can increase the sensing performance. The influence of the change of environmental pressure on the slot waveguide sensor can be ignored (Fig. 8). The influence of fabrication errors on slot waveguide sensor performance is analyzed (Fig. 9).

In this paper, an optical waveguide CH₄ sensor with a lower cladding of magnesium fluoride and a core layer of chalcogenide glass is fabricated. With the combination of wavelength modulation spectroscopy technique and on-chip optical waveguide gas sensor, the CH₄ sensing performance is analyzed. The performance of the slot waveguide CH₄ sensor combined with the wavelength modulation spectroscopy technique is studied. Decreasing waveguide loss and choosing an appropriate waveguide length can increase the amplitude of the second harmonic signal and improve the performance of the waveguide gas sensor. When the waveguide loss is <3 dB/cm, the limit of detection can be <1×10^-3. Further reducing the noise of the system can also reduce the limit of detection. The influence of the change of environmental pressure on the slot waveguide sensor can be ignored. The influence of fabrication errors on slot waveguide sensor performance is analyzed. We provide guidance for the design of an on-chip gas sensor based on wavelength modulation spectroscopy.