High-Sensitivity Cavity-Enhanced Absorption Spectroscopy For OH Radicals’ Detection In Mid-Infrared

The hydroxyl radical（OH radical）is one of the most important oxidizing substances in the tropospheric atmosphere and it is very crucial for studying the formation of ozone and secondary organic aerosols.The accurate measurement of OH radical is of great significance to the response and prevention for atmospheric composite pollution.However,due to the low concentration of OH radicals in the atmosphere（～10⁶ molecule/cm³）,high reactivity and short lifetime（<1 s）,the current technical methods that can realize practical atmospheric measurements are extremely limited.Therefore,it is really necessary to develop new techniques for OH radical measurement.Cavity enhanced absorption spectroscopy techniques can achieve an ultra-long optical path on the order of kilometers utilizing a high-finesse optical resonator with a limited optical base length,which has high detection sensitivity and technical application potential.In this paper,the cavity enhanced absorption spectroscopy techniques were carried out for the detection of OH radicals in the mid-infrared 2.8μm(3568.52 cm^-1).The main contents are as follows:An off-axis integrated cavity output spectroscopy experimental setup was developed to complete in-situ detection of OH radicals.It can achieve an effective optical path of 512 m at the base length of 35.8 cm.The influences of the amplified spontaneous emission of the DFB laser on absorption measurements was suppressed by a narrow-band filter.Then the wavelength modulation technique combined with the off-axis integrated cavity technique was used to measure the OH radicals.A detection limit of～1.2×10⁸ molecule/cm³（1σ,100 s）was obtained at an acquiring time of 100 s.A mid-infrared V-shaped optical feedback cavity enhanced device with an arm length of 48.8 cm and an optical path of 823 m was established.The effects of optical feedback rate and feedback phase on cavity mode and laser linewidth narrowing were investigated.According to the two methods of cavity mode symmetry analysis and wavelength modulation,the corresponding error signal is generated and fed back to the PID loop.And the expansion and contraction of the PZT is controlled in time to control the distance from the laser to the V-shaped cavity.Then it can realize phase matching and the auto-locking of the laser to the cavity mode.The performance of the OF-CEAS system was evaluated.And it was found that the wavelength modulation method to control the phase was about 3 times better than the symmetry analysis method.Then the technique was applied to the detection of OH radicals in situ and finally achieved the minimum detectable absorption of the system of 1.7×10^-9 cm^-1 with 25 s acquiring time,corresponding to the detection limit of the concentration of OH radicals of about 2×10⁸ molecule/cm³（1σ,25 s）.A novel optical feedback-magnetic rotation OH radical measurement device was constructed.The optical feedback cavity enhanced spectroscopy combined with magnetic rotation spectroscopy technique studied by simulation.The magnetic effect of OH radical absorption line and the influence of magnetic circular birefringence,magnetic circular dichroism,and average absorption on the line shape of the magnetic rotation spectral signal were analyzed.The model of the optical feedback cavity mode combined with the magnetic rotation spectral signal was simulated.It can reveal the possibility of theoretical combination of the two techniques.Then the preliminary design and construction of the experimental device has been completed,which has laid the foundation for the following research.