Most gas molecules have vibration characteristic absorption peaks in the mid-infrared spectral range (2.5~20 μm), which can be used for trace gas detection and quantitative analysis. On-chip gas sensing that enables waveguide interconnection can improve system portability. Because quantum cascade laser and quantum cascade detector material structures can be epitaxially grown on the InGaAs-InP platform, it can be used for the preparation of on-chip integrated gas sensors. On the InGaAs-InP platform, the refractive indices of nInGaAs = ~ 3.4 and nInP = ~ 3.1 at λ = 4.602 5 μm are insufficient to realize the total internal reflection constraint between core and substrate. We can etch the InP substrate by using hydrochloric acid so that the refractive index between the core and cladding is approximately 2.4. At this point, the suspension structure allows gas to flow through the upper and lower cladding, allowing stronger interaction between gas molecules and light. In order to study the sensing performance of on-chip waveguide in the mid-infrared band, a suspended photonic crystal waveguide and a suspended ridge waveguide sensor based on InGaAs-InP platform are proposed for targeting the 2 172.75 cm-1 absorption line of carbon monoxide. Based on Lambert-Beer law, Rsoft software and COMSOL software are used to optimize the waveguide structure. The lattice constant, hole radius, central hole radius, ridge width, strip width, ridge height and strip height of the photonic crystal waveguide are optimized, and the performances of the two sensors are calculated.Firstly, a mid-infrared suspended InGaAs-InP photonic crystal waveguide sensor is proposed. The InGaAs photonic crystal waveguide adopts a hexagonal lattice with a lattice constant of a. The width of Holey Photonic Crystal Waveguide (HPCW) is , and the thickness of InGaAs layer hInGaAs is 1.15 μm. The lattice constant, hole radius and central hole radius parameters of the photonic crystal waveguide are optimized by constructing a defect row in the Γ-K direction. The central defect row radius of the constructed waveguide is rs = 0.6r, where r = 0.27a is the aperture radius of the photonic crystal. The relationship between the lattice constant of the photonic crystal waveguide and the group index (ng) is obtained by simulation. When the lattice constant a = 1 018 nm and the photonic crystal waveguide ng is 43.97, the optimal waveguide structure is obtained at λ = 4.602 5 μm. It is found that most of the light is confined to the center hole region, and the peak electric field intensity of the center hole cross-section is increased by ~ 3.41 times. At this time, the power confinement factor fPC of the photonic crystal waveguide is 250.69%, the optimal waveguide length Lopt = 72 μm, and the limit of detection CLoD = 9.13 × 10-6.Then, a mid-infrared suspended InGaAs-InP ridge waveguide sensor is proposed. The total thickness of the core layer is set to 780 nm. In order to obtain a large fPC under guided mode condition, COMSOL Multiphysics software based on finite element method is used to optimize the waveguide structural parameters, including ridge width w1, plate width w2, ridge height h1, plate thickness h2 and waveguide length L. The optimum waveguide structure and the optical mode field distribution of TM0 mode of the mid-infrared suspended ridge waveguide are obtained. Most of the light is concentrated around the ridge. fPC = 115.65%, Lopt = 162 μm, and CLoD = 8.51 × 10-6.Comparing the two sensors designed, since HPCW can only guide TE mode, a polarization rotator is required to integrate it with the quantum cascade device with TM polarization. Ridge Waveguide (RWG) can support TM polarized light and does not require a polarization rotator for integration, which reduces total transmission loss and integrated device area, and simplifies the preparation process. The advantage of suspended HPCW is that it has a high group refractive index, so it can effectively reduce the light absorption path length. But HPCW has a higher propagation loss, which is not conducive to improving the sensitivity of integrated gas sensors. The lower propagation loss of the suspended RWG allows for longer waveguide lengths, which in turn improves gas sensor sensitivity. The influence of propagation loss on the performance of the two sensors is analyzed and calculated, and the variation curves of Lopt and CLoD versus αint are obtained. Lopt decreases and CLoD increases with the increase of propagation loss. If the waveguide propagation loss can be effectively reduced, the lower detection limit can be further reduced.With CO as the target gas, the InGaAs-InP platform is used to simulate the design of suspended photonic crystal waveguide and ridge waveguide. The waveguide parameters are optimized to achieve single-mode transmission and higher fPC. After optimization, fPC of the suspended photonic crystal waveguide and ridge waveguide are 250.69% and 115.65%, respectively. When the waveguide loss is 27.5 dB/cm and 3 dB/cm, respectively, the optimum waveguide lengths of the two sensors are determined, respectively. When SNRmin = 10, CLoD of the two sensors are 9.13 × 10-6 and 8.51 × 10-6, respectively.
