On Chip Infrared Gas Sensing Technology Based On Silicon Based Optical Waveguides

Traditional discrete optical gas sensing systems have been challenging to apply in scenarios that require lightweight solutions,such as airborne gas inspection,planetary atmosphere exploration,and detection of gas leaks in narrow pipelines,due to their large size and sensitivity to vibrations.Consequently,on-chip infrared gas sensors based on optical waveguides,with their small size,low power consumption,and potential for optoelectronic integration,have emerged as the preferred solution for these applications.However,gas sensing technology based on optical waveguides currently faces several issues:Firstly,optical waveguide gas sensors suffer from transmission losses,resulting in weak signals that are also susceptible to on-chip noise,such as mode interference,leading to low signal-to-noise ratios.Secondly,in the sensing method that involves external coupling of lasers and detectors,wavelength modulation spectroscopy（WMS）technology is vulnerable to vibrations,causing fluctuations in coupled power and reducing device stability.Thirdly,existing optical waveguide sensors predominantly employ inorganic material systems,such as silicon（Si）,silicon nitride（Si₃N₄）,and chalcogenide glass（Ch G）.Compared to these materials,polymer materials offer simpler processing and lower costs.However,the lower refractive index of polymers and larger bending radius result in lower integration density for the devices.Lastly,most reported optical waveguide sensors utilize externally coupled lasers,which does not favor high integration of the devices.To address the aforementioned issues and enhance the sensitivity of on-chip gas detection,this paper proposes a method combining WMS with Kalman filtering;to improve the resistance of the sensor to power fluctuations and its stability,a self-calibration on-chip gas sensing technique based on the second harmonic/first harmonic（2f/1f）WMS is presented;to increase the integration density of polymer waveguide sensors,a polymer sensing waveguide structure combining Euler bends with Archimedean spirals is proposed;to improve device integration,a monolithic integrated gas sensor combining a silicon-based external cavity laser with a polymer sensing waveguide is theoretically studied and designed.Moreover,to detect gas molecules across different absorption bands and to verify the universality of the on-chip sensing technology,materials systems with varying transparent windows were employed in this thesis,including polymer SU8（0.4～5μm）,Si（1.1～8μm）,and Ge Sb Se（1～12μm）.Specifically,the research content and conclusions of this thesis are as follows:Firstly,a silicon-on-insulator（SOI）based optical waveguide gas sensor was fabricated,and an on-chip methane（CH₄）sensing system was established.The waveguide’s loss was measured at 0.71 d B/cm,with a confinement factor of 23%.By employing quasi-synchronous time-division multiplexing of direct absorption spectroscopy（DAS）and WMS techniques,the sensor’s CH₄ absorption characteristics and sensing performance at a wavelength of 3.291μm were tested,and the noise characteristics of the optical waveguide sensor under both detection techniques were analyzed.To suppress noise,the Kalman filtering algorithm was introduced.Experimental results showed that after applying the Kalman filter,with an averaging time of 0.2 s,the limit of detection（Lo D）for DAS and WMS at 1 cm waveguide length were 347 ppm（parts per million）and 85 ppm,respectively,while for 2 cm waveguide length,the Lo Ds were 155 ppm and 78 ppm,respectively.Without the Kalman filter,the Lo D for DAS at 2 cm waveguide length was 1319 ppm.Thus,by using both WMS and the Kalman filter,the gas detection limit was reduced by approximately 17 times compared to using DAS without the filter.Secondly,in response to the problem that WMS technology is susceptible to power fluctuations leading to decreased stability,a self-calibration on-chip gas sensing technique based on 2f/1f WMS was proposed.Using the lift-off technique,Ch G（Ge Sb Se）sensing waveguides were fabricated,and an on-chip mid-infrared CH₄sensing system was established.At a wavenumber of 3038.5 cm^-1,the performance of a 1-cm-long optical waveguide sensor was tested using DAS,2f WMS,and 2f/1f WMS.With an averaging time of 0.2 s,the Lo D for the three methods were 2.17%,0.54%,and 0.504%,respectively.The effect of optical power fluctuations on the on-chip gas sensing performance was analyzed by adjusting the input power to the waveguide through changing the aperture of an iris,thus simulating fluctuations in optical power.Introducing a 30%CH₄ sample into the gas chamber,the maximum and minimum CH₄ concentrations measured using the 2f/1f WMS technique across five different aperture settings were 33.8%and 26.4%,respectively,with a relative error range of～12.0%to 12.7%.The maximum relative error obtained using the 2f WMS technique reached 70.7%.Therefore,after applying the self-calibration method,the maximum relative error was reduced by about 5.6 times.Additionally,by changing the waveguide temperature from 15°C to 45°C,the sensor’s temperature stability was tested.The results showed that the relative error caused by temperature changes（3%）was six times smaller than that of discrete sensors（18%）,indicating that the on-chip sensor has good temperature stability.Thirdly,to address the issue of the large sensor size due to the low refractive index of the polymer SU8 material and large waveguide bending radius,a waveguide structure combining Euler bends with Archimedean spirals was designed,which reduced the sensor footprint by more than 50%compared to arc-shaped SU8 bent waveguides.SU8 spiral waveguide sensors were fabricated and a near-infrared acetylene（C₂H₂）sensing system was established,with sensor performance tested using WMS technology.The measured optical power confining factorΓwas 0.0172,which is in close agreement with the calculated value of 0.016,and waveguide losses measured by the cut-back method were 3 d B/cm.At a wavelength of 1532.83 nm and an averaging time of 0.2 s,the Lo D of C₂H₂ for 7.4 cm and 13 cm long SU8 spiral waveguides were 2197.1 ppm and 425.5 ppm,respectively.The rise and fall times of the sensor were 2.05 s and 3.27 s,respectively.To verify the broadband sensing capabilities of the SU8 waveguide,an 8-cm-long,fiber-packaged SU8 waveguide was tested for its C₂H₂ sensing performance using a broadband amplified spontaneous emission（ASE）source as the light source.Within the wavelength range of 1528.8-1533.4 nm,the acquired absorption spectrum signal was fitted with a Fourier series,from which absorbance was extracted,and the relationship between gas concentration and absorbance was calibrated.Allan variance results showed that the detection limit of the sensor was 3929.3 ppm with a single sample averaging and 1256.5 ppm with an averaging of 12 samples.Finally,to improve the integration of optical waveguide sensors,an SU8 optical waveguide was taken as an example to design a monolithically integrated gas sensor combining an SU8 sensing waveguide with a silicon-based external cavity laser.Due to factors such as mode mismatch and substrate incompatibility,the gain waveguide of the laser could not be directly butt-coupled with the SU8 sensing waveguide.Therefore,a three-stage mode transition structure was designed,transitioning first from the gain waveguide to a ridge Si waveguide,then from the ridge Si waveguide to a rectangular Si waveguide,and finally to an SU8 rectangular waveguide.The coupling structure parameters were optimized,achieving a coupling efficiency of over90%at each stage.As a vital component of the silicon-based external cavity laser,a distributed Bragg reflector（DBR）designed for cavity surface feedback was optimized,with a reflection bandwidth of over 50 nm.A dual microring resonator utilizing the Vernier effect for tuning the laser wavelength was also optimized,with radius of 18.5and 16.6μm,a free spectral range of 54 nm,and a wavelength tuning range of over 50nm.Innovations in this thesis include:（1）In the measurement approach involving off-chip coupling of lasers/detectors,to suppress the impact of vibration-induced power fluctuations on sensing performance,a self-calibrating on-chip gas sensing technique based on 2f/1f WMS was proposed.Experimental results with chalcogenide waveguides demonstrated that,in comparison to the 2f WMS technique,this method could reduce the maximum relative error by approximately 5.6 times.（2）To address the issue of large sensor size caused by the low refractive index of the polymer SU8 material and the large bending radius of waveguides,a sensing waveguide structure combining Euler bends with Archimedean spirals was designed.Compared to the arc-shaped SU8 bent waveguides,the sensor area has been reduced from 9 mm² to 2.1 mm².（3）In order to address the issue of low integration of optical waveguide sensors,a single-chip integrated gas sensor combining SU8 sensing waveguides and silicon-based external-cavity lasers has been designed.Through optimization,the three-stage vertical coupling efficiency between the laser gain waveguide and the sensing waveguide has achieved over 90%..