Seung Nam Park

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.

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.

Realization of a nickel carbon eutectic fixed point with an optimized filling process monitored by the melting–freezing temperature difference measurement

A nickel carbon eutectic point was realized by a radiation thermometer for two geometrically different fixed point cells made by an optimized filling process. The process was based on monitoring the variation of the melting?freezing temperature difference as a function of the filling ratio of the cell. The integrity of construction and the filling endpoint could be confirmed by observing a clear convergence of the melting?freezing temperature difference as the filling ratio increases. From 14 sets of realizations with two different cells and two different spectral bands of the radiation thermometer, the eutectic temperature is determined to be 1328.73??C with an expanded uncertainty of 0.22??C (k = 2).

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.

Optical design and illumination simulation of Fresnel lenses for marine signal lanterns

Providing marine signal lanterns, Fresnel lens has been adopted to transfer the beam from the lanterns up to 10 nautical miles (18.53 km). The Fresnel lens with diameters of 250 mm was designed by a ray tracing program and optimized by adjusting the groove parameters of the lens. Each optical sag element which is a part of a lens was independently designed by using the analytical method. The angular luminous intensity distributions (ALID) of this lens were calculated by the illumination analysis program considering the ALID of a light bulb. The ALID of a C-8 type bulb (24 W) was measured with a goniophotometer and its luminous flux was measured by an integrating sphere to be 397 lm. At the best alignment of the bulb, the maximum luminous intensity of the lantern was more than 1000 cd for the 250 mm lens. The ALID was investigated as a function of distance from the lens focus to determine the tolerance margin of the alignment. Horizontal deviations of the light bulb from the focus along the optical axis widened the angular FWHM of the vertical ALID. However, vertical deviations caused shifts of the vertical ALID without spreading the angular FWHM. The designed 250 mm aspherical lens of marine signal lantern was made by the injection molding with single peace acryl. We measured the luminous intensity distribution of acryl lens and found that the MLI of the lens was 827 cd. And the full width at half maximum of the diverging angle of the diverging beam was 3.5 deg. Although the measured MLI was 83% of the calculated result, it would be increased with surface polishing of prototype molding pattern.

Polychromatic modulation transfer function analysis of color LCD

The analysis of the polychromatic modulation transfer function (PMTF) for evaluating the image quality of the color LCD monitor is presented. The PMTF is easily calculated from the values of the MTF weighted by the overall spectral response of the system. To obtain the spectral response of the monitor, the chromaticity (x, y) and luminance are measured with a spectroradiometer. The variations of the luminance, chromaticity (x, y), and MTF of the color CRT and LCD monitors for different viewing angles are measured. We designed a synthetic equipment that could obtain the chromaticity (x, y), luminance, and MTF of the color monitor simultaneously. The experimental results of several color monitors made in Korea are presented.

Development of certificated reference materials for polarization mode dispersion

We constructed a standard measurement setup for polarization mode dispersion (PMD) based on Jones matrix eigen-analysis method. We measured a differential group delay of a 1 m-long polarization maintaining fiber (PMF) and evaluated the measurement uncertainty to be less than 0.6 %. As a transfer standard for PMD, we fabricated mode-coupled PMD artifacts by concatenating the 50 PMF sections with random birefringent axis orientation. Using the standard setup, we certificated the PMD values of the three PMD artifacts to be 0.0884 ps, 0.977 ps, 1.541 ps with the standard uncertainty of 0.4 fs, 16 fs, 23 fs, respectively.

An argon arc source for UV radiometry initiated by laser-induced gas breakdown

By utilizing arc discharge, we have constructed an argon arc radiation source for ultraviolet (UV) radiometry, and have studied how to initiate the source using laser-induced gas breakdown (LIGB) without any contamination or degradation of its components. Investigation of the source of the arc discharge characteristics and spectral radiance characteristics of the arc yielded the optimum operating conditions. The spectral radiance of the arc source was stable to within 0.3%, and could be adjusted by controlling the arc current and argon pressure during normal operation.

Structure of counterflow flames: comparison of CARS CO and temperature measurements with numerical calculation

The structure of methane and propane flames in a counterflow burner is studied. The method used for the concentration measurement of CO, which is a reaction intermediate and maintains a low concentration in the flames, is from the modulation dip in the spectrum of broadband coherent anti-Stokes Raman spectroscopy (CARS) arising from the cold-band Q-branch resonant signal of CO superimposed on the nonresonant background. The measured CO profiles together with the temperature and the velocity, using nitrogen CARS and laser Doppler velocimetry, are compared with the numerical results accounting for the detailed kinetics suggested by Peters [1993] with plug flow assumption.