Research On Infrared Gas Sensing Technique Based On Photonic Crystal Slow Light Waveguide

Gas sensors based on infrared absorption spectroscopy technique have been widely used in environmental monitoring,industrial safety,and other fields due to their high selectivity and sensitivity.Most of these infrared gas sensors use hollow-core optical fiber chambers or free-space chambers,which have drawbacks such as large volume,complex optical alignment,and susceptibility to vibration.Waveguide sensors can integrate with light sources and detectors on a single chip,significantly reducing the size and weight of the sensing system,thus enabling portable and lightweight gas detection applications,such as planetary atmospheric exploration and gas detection in manned spacecraft cabins.The refractive index guided non-slow-light waveguide sensor has a small proportion of evanescent field,resulting in weak gas absorption.Photonic crystal waveguide（PCW）utilizes the slow light effect to reduce the group velocity of light and enhance gas absorption.Therefore,the development of highly sensitive infrared gas sensors using photonic crystal slow light waveguides has important theoretical value and application prospects.On this basis,a near-infrared gas sensing technology based on odd even mode multiplexing and polarization multiplexing is proposed to address the problems of narrow detection bandwidth and single gas detection types in existing slow light waveguide sensors.This expands the detection bandwidth and achieves multi-component gas detection.In response to the problem of slow light wavelength being greatly affected by manufacturing errors and easily deviating from the target gas absorption peak,thermal optical tuning techniques is used to compensate for errors,so as to increase the group index at the gas absorption peak,and improve on-chip detection sensitivity.The main research contents are as follows:Firstly,to address the issue of low interaction factor in refractive index guided non-slow-light waveguide sensors,a 1D PCW for near-infrared acetylene（C₂H₂）sensing is designed and fabricated,and a near-infrared gas sensing system based on intensity modulation is established.The limit of C₂H₂ detection for a 1mm long 1D PCW sensor is 120 ppm（parts per million）.The interaction factor of 1D PCW calculated based on absorbance reaches 2.88,and the group index is 18.The interaction factor is 11 times that of a rectangular waveguide.A group index testing system based on Mach-Zehnder interferometer is established using single-mode optical fibers and the sensing waveguide.At 1532.83 nm,the group index of 1D PCW is 16.3.This is very close to the group index calculated based on absorbance,verifying the accuracy of the sensing results.Secondly,in response to the reported issues of narrow spectral bandwidth and limited types of detected gases in PCW sensors,broadband near-infrared gas sensing technoiques based on mode multiplexing are proposed.By exciting multiple slow-light modes of the 1D PCW,the spectral bandwidth is expanded,enabling on-chip multi gas detection.（1）Using a mode converter to thansform fundamental mode of a rectangular waveguide to higher order mode,both odd and even modes of the 1D PCW are excited.The odd mode covers the spectral range of 1520–1560 nm,and the even mode has the wavelength range of 1610–1660 nm.Taking C₂H₂ and methane（CH₄）as examples,the performance of the broadband near-infrared multi-component gas sensor is experimentally validated.At 1532.83 nm,the interaction factor for the odd mode is0.836,while at 1653.73 nm,the interaction factor for the even mode is 1.308.For a waveguide length of only 2 mm,the C₂H₂ detection limit for the sensor based on the even mode is 166 ppm,and for CH₄,the detection limit is 660 ppm.A Mach-Zehnder interferometer is employed to verify the group indices of the odd and even modes.（2）Furthermore,a slow light-enhanced polarization-multiplexed broadband gas sensor for infrared absorption spectroscopy is proposed.The transverse magnetic（TM）polarized mode has a transmission spectrum in the range of 1500–1500 nm,and the transverse electric（TE）polarized mode has a transmission spectrum within 1610–1660 nm.Compared to previously reported slow light waveguides,the detection bandwidth is increased by 2-3 times.With a waveguide length of 1 mm and an averaging time of17.6 s,the sensor achieves detection limits of 1800 ppm for C₂H₂ and 550 ppm for CH₄.Thirdly,to further enhance the performance of on-chip gas sensors in the near-infrared,the research explores both non-suspended and suspended structures of 2D PCW.（1）A non-suspended 2D PCW with length of 1 mm is designed and fabricated on silicon-on-insulator（SOI）platform with a top silicon thickness of 220 nm.Through structural optimization,the non-suspended 2D PCW achieved an interaction factor of6.84,which is lareger than the reported 2D PCW sensors.With an averaging time of33.6 s,the non-suspended 2D PCW sensor realizes a detection limit of 26.4 ppm for C₂H₂.（2）Besides,the study proposes a near-infrared gas sensor based on suspended2D PCW.By optimizing the defect rows of the suspended 2D PCW,four even modes and four odd modes are generated within the photonic bandgap.The 1st and 3rd order even modes can cover the near-infrared absorption band of C₂H₂.To address the issue of slow light wavelength deviation from gas absorption peak and decrease in group index,the thermal optical tuning technique is employed to compensate for the influence caused by fabrication errors.This compensation leads to a 7-fold increase in the group index at 1532.83 nm.Experimental results demonstrate that the interaction factor of the suspended 2D PCW sensor reached 9.5,with a spectral bandwidth of 130 nm.With an averaging time of 44.4 s,the detection limit of C₂H₂ was determined to be 18 ppm.The performance of the suspended 2D PCW sensor surpassed that of non-suspended 2D PCW sensors and 1D PCW sensors.Finally,considering that the absorption coefficients of gas molecules in the mid-infrared are 1-3 orders of magnitude larger than that in the near-infrared,a theoretical study is conducted on mid-infrared gas sensing technique based on holes-in-slab PCW（HPCW）and rods-in-air PCW（RPCW）.Theoretical results indicate that HPCW sensor and RPCW sensor achieve interaction factors of 2.6 and 5.78 at 3291 nm,respectively.With a propagation loss of 15 d B/cm,the optimal waveguide lengths for HPCW and RPCW are 0.26 cm and 0.24 cm,and the detection limits for CH₄ are 11.3 ppm and 4.5ppm,respectively.Due to the fact that HPCW can only guide TE polarization mode,while RPCW can guide TM polarization mode,the slow light mode of RPCW matches with the mode of mid infrared light source,eliminating the loss of mode conversion and reducing the complexity of system design.Mid-infrared PCW sensors achieve an order of magnitude enhancement in absorption compared to near-infrared PCW sensors,resulting in lower detection limit.The novel aspects of the thesis are as follows:（1）In response to the issue of small interaction factors in non-slow-light waveguide sensors,sensors based on 1D PCW and non-suspended 2D PCW are designed.The interaction factors for these sensors reached 2.88 and 6.84,representing an improvement of 11 times and 27 times compared to non-slow-light waveguide sensors.（2）Two broadband near-infrared multi gas sensing techniques based on odd-even mode multiplexing and polarization multiplexing are proposed to address the issues of narrow spectral bandwidth and single gas detection types in slow light waveguide sensors.Both the gas sensors cover bandwidth of near 100 nm,which can detect various gases such as C₂H₂ and CH₄.Compared with reported photonic crystal slow light waveguide sensors,the bandwidth has increased by 2-3 times.（3）To address the issues of the significant impact of fabrication errors on the group index at gas absorption peaks in slow-light waveguides,the thermo-optical tuning technique is employed to compensate for the interference caused by fabrication errors.As a result,the suspended 2D PCW sensor gets a 7-fold increase on the group index.The interaction factor reached 9.5,demonstrating superior performance compared to non-suspended 2D PCW and 1D PCW sensors.