Researches On The Molecular Spectroscopy Measurement Techniques Based On NICE-OHMS

Spectroscopy information can reflect the energy level structure of atoms and molecules,thus providing macroscopic or microscopic details such as composition,concentration,and structure.This has wide applications across fields such as physics,chemistry,biology,medicine,and materials science.To accommodate the requirements of high sensitivity and rapid detection in these fields,there is a need to research and develop ultra-sensitive real-time spectroscopy methods and technologies.Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy（NICE-OHMS）is a laser absorption spectroscopy that effectively combines Frequency Modulation Spectroscopy（FMS）and Cavity-Enhanced Absorption Spectroscopy（CEAS）by utilizing the Pound-Drever-Hall（PDH）and De Voe-Brewer（DVB）locking techniques.This combination enables both the FMS carrier and the modulation sidebands to resonate with the optical cavity simultaneously,reducing the influence of 1/f noise and lengthening the interaction path of the laser with the gas,thereby enhancing the detection sensitivity for trace gases.The advantages of NICE-OHMS include:（1）The combination of FMS and CEAS provides NICE-OHMS with immunity to frequency-amplitude noise induced by laser-cavity frequency locking,making NICE-OHM the most sensitive techniques in laser absorption spectroscopy;（2）The optical cavity can passively amplify the laser,establishing a strong laser power within the cavity,which easily saturates molecular absorption,enabling observation of sub-Doppler saturation spectroscopy of molecules.However,as of now,the factors limiting the further development of NICE-OHMS include:（1）The detection sensitivity of NICE-OHMS is still sensitive to PDH locking performance.To improve the locking performance of the laser-cavity,it is common to feedback control the internal frequency actuator of the laser（such as the piezoceramic inside the fiber laser）while adding an external laser frequency actuator（such as an acoustic-optic modulator）.This increases the complexity of the system and reduces its versatility;（2）In traditional NICE-OHMS,the modulation frequency of FMS is locked to the free spectral range（FSR）of the optical cavity.Any drift in the cavity length can lead to shifts in the modulation frequency and laser frequency,affecting the long-term stability of spectroscopy measurement;（3）In NICE-OHMS,demodulation is required to obtain the final absorption spectroscopic information.However,the demodulation process introduces an unknown instrument factor.To obtain accurate concentration information,a known concentration of standard gas is required to calibrate this instrument factor.This increases the complexity of the operation and introduces additional errors.This thesis focuses on the NICE-OHMS technique and addresses the aforementioned limitations through a series of theoretical and experimental studies,concentrating on the following three aspects:（1）Measurement and application of sub-Doppler NICE-OHMS spectroscopy based on a fiber-optic single-sideband phase modulator（f-SSM）;（2）Precise control of laser frequency based on NICE-OHMS technology and its high-accuracy measurement of Direct Absorption Spectroscopy（DAS）;（3）A proposed method for measuring asymmetric NICE-OHMS signals,with theoretical analysis on its generation mechanism and calibration-free characteristics,and experimental verification of its self-calibration properties.The main content of the thesis is as follows:1.A brief introduction of the development,current research,advantages,and limitations of the NICE-OHMS technology is presented,ending with the significance of this study.2.The basic principle of NICE-OHMS technology is explained,which mainly includes the PDH and DVB locking mechanisms and the generation mechanisms of Doppler and sub-Doppler NICE-OHMS signals.3.A sub-Doppler NICE-OHMS spectroscopy measurement system based on f-SSM is proposed and set up and is applied to lock the laser frequency to the weak absorption line sub-Doppler spectroscopy.The system uses the f-SSM as the sole frequency actuator in the NICE-OHMS device to control the laser frequency.The erbium-doped fiber laser and the s Whispering Gallery Mode laser are used as fixed-frequency light sources.The sub-Doppler signal of C₂H₂ gas is measured,verifying the universality of this technique for different laser sources.Facing the application demand of atmospheric laser radar,it is necessary to achieve high-precision locking of the laser frequency to the center of the weak absorption line,and the sub-Doppler NICE-OHMS technique is the optimal choice.Therefore,we use the sub-Doppler spectroscopy generated by this system as an error signal and feedback to the piezoceramic on the optical cavity to stabilize the laser frequency.The relative frequency stability of the two lasers is 8.3×10^-13 and 7.5×10^-13 respectively.4.By analyzing the basic principle of NICE-OHMS technology,a scheme to link radio frequency and light frequency using an optical cavity is proposed based on PDH and DVB frequency locking techniques,achieving precise control of optical frequency through radio frequency.The frequency stability within 2.5 hours is maintained within 110 k Hz based on the experimental setup.The system is then applied to the high-precision measurement of DAS,and a detection sensitivity of 9.4×10^-6 is obtained with the balanced detection mechanism.5.A measurement method for asymmetric NICE-OHMS signals based on RF scanning is proposed.Theoretically,by analyzing the frequency shift of cavity modes caused by gas dispersion in the cavity,it is verified that gas absorption information is reflected not only in the NICE-OHMS signal but also transferred to the laser frequency,thereby generating an asymmetric signal lineshape.This asymmetric signal can be used for high-sensitivity calibration-free trace gas concentration detection.A series of NICE-OHMS dispersion signals under different absorption conditions are measured based on the experimental setup.The results show that the experiment is consistent with the theory,and the method has good self-calibration characteristics,with a fitting accuracy of the partial pressure of up to 0.65%.The innovative points of this thesis are as follows:1.The use of the f-SSM as the sole frequency actuator in the PDH locking system reduces the complexity of the NICE-OHMS system.At the same time,the feedback object of the PDH error signal is the output frequency of the f-SSM,not the output frequency of the laser.Therefore,a single PDH locking system designed for the f-SSM can be compatible with different lasers,improving the universality of the system.2.A new type of NICE-OHMS technology is proposed,wherein the feedback object of the DVB error signal is the cavity length,linking the cavity mode frequency with the radio frequency,thus improving cavity length stability.Additionally,this technique establishes a connection between radio frequency and light frequency,allowing precise control of light frequency by scanning the radio frequency.3.Based on the above new NICE-OHMS technology,a measurement method for asymmetric NICE-OHMS signals is proposed.By analyzing its generation mechanism,the theoretical line shape of the asymmetric signal can be obtained.Based on this,highly sensitive,calibration-free trace gas concentration detection can be achieved.