Investigation On Resonant Degenerate Four Wave Mixing Technique And Its Application In Gas-media Spectrscopy

Degenerate four wave mixing ( DFWM ) has been developed as a new spectroscopic technique for species trace and investigating on the spectroscopy of atom, molecule and radical. Comparison with others, DFWM spectroscopic technique has its features: (1)DFWM spectroscopic technique has the advantages of sub-Doppler , good timely and spatial spectroscopy resolution. Because of having background free and high sensitivity, it could be used for remote detection.(2)DFWM spectroscopy obtained by measuring absorption in media suits for weak florescence and short florescence lifetime species. Especially the buffer gas rather affects on DFWM spectroscopy than great enhances the signal intensity ,but it will decrease laser-induced fluorescence (LIF) detecting sensitivity because of collision quenching in the high pressure environment;(3)besides ordinary single photon resonant transition, two-photon resonant DFWM spectroscopic technique can be used for investigating forbidden transition from ground level to excited level in molecule. So DFWM technique could be investigated on atmospheric pollution measurement, combustion diagnostics, drugs detection and other regimes. Generally speaking, gas media has little nonlinear susceptibility. DFWM signal intensity is proportional to the square ofχ(3), so weaker DFWM signal intensity in gas state could not be detected because of the ambient scattering light. The strict managing procedure for DFWM technique applied in gas-phase media spectroscopy limits it's widely uses. There are three reasons: 1) laser beam excursion make DFWM experiment more difficult.2) no laser output wavelength band for stimuli measuring species in high resolution. 3) Organic molecular DFWM spectroscopy has not obtained so far.To solve the problems, we have designed a self-stability spilt-beam system and image focusing method, which effective manage the difficulty and unstability caused by laser beam excursion and environmental variation. Also the availability of tunable ultraviolet sources has made possible the investigation of gas-phase molecules DFWM spectroscopy. For example, high resolution visible and ultraviolet DFWM experiments have been done on iodine, naphthalene and OH radical in the flame via resonant DFWM technique. Based on the resonant DFWM technique, we can enhance the weak behavior of the nonlinear susceptibilityχ(3) by orders of magnitude, so that the DFWM signal can be easily detected in gas-phase media. Furthermore, we obtain DFWM spectroscopy in organic naphthalene vapor under the condition of ordinary temperature and atmospheric pressure, in which quantity of the organic naphthalene vapor concentration is 1ppm. This is the best outcome reported so far.This dissertation systematically reviewed the development of DFWM technique in species trace and laser combustion diagnostics. Then the theory about DFWM is introduced and the elimination of laser beam excursion in DFWM spectroscopy and the stability of tunable dye laser source is analyzed and discussed. Based on this analysis, we investigate DFWM spectroscopy in iodine vapor and Polycyclic-Aromatic Hydrocarbons (PAHs) naphthalene vapor. Finally, we obtain OH radical DFWM spectroscopy and its distribution of concentration in the flame of CH4/O2/N2 pre-mixtured gas.Self-stability spilt-beam system and image focusing method are designed. Any little laser beam excursion and weak stray light decrease the DFWM signal intensity, so as to the signal can not be detected. The two techniques of Self-stability spilt-beam system and image focusing method are designed to solve the above problems, and the techniques are effectively tested in visible and ultraviolet DFWM spectroscopy respectively. The techniques are not reported until now.The continuous tunable laser source is discussed. We reported the performance of binary mixture—dyes PM-597 and DCM, in a 10-Hz, Nd:YAG pumped dye-laser system. The mixture-dyes demonstrated broader and more efficient tuning curves which used for degenerate four wave mixing (DFWM) studies of OH in combustion diagnostics. This enhanced efficiency should improve the signal-to-noise ratio in multi-spices DFWM measurements with the continuous tuning dye laser. The performance of the continuous tuning dye laser is investigated in chapter 4. The availability of tunable ultraviolet sources has made possible the investigation of multi-species measurement in high resolution ultraviolet DFWM experiments by PM580&DCM, PM597&DCM are dissolved respectively in nearly 100% ethanol. The stability is effectively tested in ultraviolet DFWM spectroscopy in the solution of LDS698, Rh610 and naphthalene respectively. The continuous tuning laser source for measuring species stimuli in DFWM experiments is not reported until now.The vapor of iodine and naphthalene are studied in the DFWM experiments. The signals and line shapes of iodine DFWM spectra versus with total input intensity and the pressure of buffer gas are measured respectively under normal temperature. Also, photostability of dye laser on the DFWM signal and ambient noise is detailed investigated in saturation regime. Finally, DFWM spectroscopy in organic naphthalene vapor is obtained under the condition of ordinary temperature and atmospheric pressure. The outcome by DFWM method is not reported so far.OH radical DFWM spectroscopy in the flame of CH₄/O₂/N₂ mixtured gas is studied via self-stability spilt-beam system and image focusing method. The stronger absorption wavelength at 309.760nm is used to probe OH radical DFWM spectroscopy in the different positions of the flame in the pre-mixtured gas, and the relative distribution of OH radical concentration is measured.