Jae Won Hahn

Broadband coherent anti-Stokes Raman spectroscopy with a modeless dye laser

We develop a modeless dye laser for broadband coherent anti-Stokes Raman spectroscopy (CARS) and investigate the operational characteristics of the modeless laser. The energy efficiency of the modeless laser is 6%, and the beam divergence is 0.65 mrad. We construct a compact movable CARS system with the modeless laser and a graphite tube furnace to assess the accuracy of the CARS temperature. It is found that the difference between the averaged CARS temperature and the radiation temperature measured with an optical pyrometer is <2% at a temperature range from 1000 to 2400 K. We also measure the averaged CARS temperature drift owing to the variation of the spectral distribution of the modeless laser, which is <1.5% during 5 h of operation.

Reference wavelength method for a two-color pyrometer

The reference wavelength method is used for a two-color pyrometer and, with the reference wavelength method, an analytical formula of the ratio temperature for the two-color pyrometer is derived. For one channel of the two-color pyrometer, with a triangular spectral response of 0.1-micro m FWHM and 2.0-micro m peak wavelength, the effective wavelength and the correction factors with several reference wavelengths are determined. By fitting the curves of the effective wavelength and the correction factor to simple functional forms of temperature, the radiance errors for both cases are calculated. Also, it is found that the correction factor determined in a single-color pyrometer can be used directly in one channel of the two-color pyrometer without additional calculation.

Measuring the Concentration of Minor Species from the Modulation Dip of the Nonresonant Background of Broad-Band CARS Spectra

A new technique for measuring the concentration of species from the modulation dip of the nonresonant background of broad-band CARS spectra has been proposed. To reduce the mode noise superimposed on the CARS spectra, we used a modeless laser generating amplified spontaneous emission as the Stokes beam of the broad-band CARS. The modulation dip induced by a minor species, CO in Ar, was numerically calculated and fitted as a function of CO concentration and temperature. We applied this technique in measuring CO concentration in a static cell and also the profile of CO concentration in a CH4/air premixed flame of a counterflow burner.

Fabry–Perot wavemeter for shot-by-shot analysis of pulsed lasers

A Fabry-Perot wavemeter for analyzing a pulsed laser operating in a single longitudinal mode with an injection-seeding technique has been constructed with an array detector. This wavemeter permits the real-time measurement of both the wavelength and the spectral effectiveness of the laser pulse produced at 10 Hz. The performance of the wavemeter is checked with a frequency-stabilized He-Ne laser and a double Nd:YAG laser that operates in the single longitudinal mode. The precision of the wavemeter is found to be < 10 MHz. Also, we calculated the uncertainties in determining the wave number by processing a Fabry-Perot fringe pattern imaged on a linear-array detector. The calculation is done by changing the number of pixels of the array detector, the finesse of the Fabry-Perot étalon, the waist of the incident laser beam, and the magnitude of random noise.

Relative phase control between two successive coherent anti-Stokes Raman-scattering signals for the recovery of spectral lines

We have controlled the relative phase between two successively generated coherent anti-Stokes Raman-scattering signals with a homemade phase-shifting unit, the thickness of which could be automatically adjusted as the wavelength of Stokes beam scanned in a wide spectral range. With this technique we have recovered the Q-branch spectrum of carbon monoxide distorted by a large nonresonant contribution from propane. Also, we have selectively recorded the spectrum of hydrogen chloride buried in the spectrum of propane.

Broadband D 2 coherent anti-Stokes Raman spectroscopy for single-shot pressure and temperature determination with a Fabry-Perot etalon

A method is demonstrated that employs a Fabry-Perot etalon to modulate a broadband coherent anti-Stokes Raman spectroscopy signal beam spatially to obtain enhanced resolution and spectral information for single-shot measurements of pressure and temperature. Resulting images are analyzed by; first, fits to Fabry-Perot patterns for single rovibrational lines; second, a line-shape analysis for a single rovibrational line; and third, a mapping of the Fabry-Perot channel spectra to a linear spectrum. Measurements of the D(2) Raman Q-branch lines were made for a D(2) in Ar mixture to take advantage of the large pressure shift and rovibrational line spacing. Peaks are located to better than 0.5% of the free spectral range of the etalon (approximately 0.01 cm(-1)) and a quantitative analysis of the pressure shifting and broadening is determined for the 1-10-MPa range. Finally, temperature and pressure determination using a band-fitting analysis is demonstrated.

Suppression of amplified spontaneous emission from a four-pass dye laser amplifier

We demonstrate a new design for a four-pass dye laser amplifier that can be used to reduce amplified spontaneous emission (ASE) in the amplifier output beam. Compared with the results obtained from normal operation of the four-pass dye laser amplifier without a frequency-selective device, by using a diffraction grating in the amplifier we can reduce the ASE ratio by a factor in excess of 10 and increase output energy by ~4%. The obtained ASE ratio of the four-pass amplifier system is <1.5%.

Effect of the Slit Function of the Detection System and a Fast-Fitting Algorithm on Accuracy of CARS Temperature

The effect of the erroneous functional forms of the slit functions on the best-fit temperature in CARS (coherent anti-Stokes Raman spectroscopy) thermometry has been investigated. When the shape of the slit function is described as a Lorentzian or a Gaussian function and its width adjusted with the room-temperature CARS spectrum, the maximum systematic errors due to the slit functions reach 1.5–2.5% in the temperature range from 300 to 2100 K. A fast-fitting algorithm exponentially interpolating the CARS spectrum at an intermediate temperature with two precalculated spectra has been proposed, and its validity has been assessed by calculating the error due to the fitting algorithm.

End-gas temperature measurements in a DOHC spark-ignition engine using CARS

We adopted a broadband CARS technique to measure end-gas temperatures in a combustion chamber of a conventional 4-cylinder DOHC spark-ignition engine. A mobile CARS system measured a full CARS spectrum on a shot by shot basis. The system consisted of a frequency doubled Nd:YAG laser (Continuum Surlite II), broadband modeless dye laser and a monochromator equipped with an optical multi-channel analyzer (RG&G 1420).

Measurement of OH radical in nonthermal plasma for NO/NO/sub 2/ reduction

NO in exhaust gas is the major source of air pollutants, causing photochemical smog and acid rain. The corona discharge plasma process for removal of NO has been studied intensively for increasing its removal efficiency. Therefore, the development of diagnostic tools measuring major species and minor radicals produced in the plasma is important to characterize the removal process. OH is one of the oxidizing radicals that can initiate the NO removal process. The end product (HNO/sub 3/) of the NO oxidation process is produced by the OH and NO/sub 2/ reaction. Therefore the OH concentration is closely related to the removal efficiency of NO and NO/sub 2/. In the study we measure the oxidizing radical OH emission spectrum in a nonthermal plasma reactor and investigated H/sub 2/O, H/sub 2/O/sub 2/ and C/sub 2/H/sub 4/ additive effects on NO/NO/sub 2/ reduction and OH emission intensity. For the measurement of the low intensity of OH emission in a corona discharge, we made a cylinder-wire type plasma reactor that concentrated the corona emission in the center of the reactor.