Spatial Profile Of Laser Beam Research Articles

Hydroxyl radical concentration measurements near the deposition substrate in low-pressure diamond-forming flames

OH radical profiles were measured in the near-deposition-surface region in low-pressure, stagnation-flow diamond-forming flames using laser-induced fluorescence (LIF). The objective of these measurements was to explore the chemistry of the diamond chemical-vapor deposition (CVD) process by resolving the OH profile near the deposition substrate. Detailed OH LIF measurements were performed in stagnation-flow diamond-forming C 2H 2/O 2 flames with equivalence ratios of 1.80, 2.10, and 2.32. The flame with an equivalence ratio of 1.80 exhibited no diamond film growth, while the flame with an equivalence ratio of 2.32 exhibited significant diamond film growth. The LIF measurements were performed in the linear LIF regime and the laser frequency was scanned across the spectral profile of the P 1(3) line in the (1,0) band of the A 2 Σ +– X 2 Π electronic transition at each spatial location. In earlier H 2 CARS measurements, the temperature gradients near the deposition substrate were resolved and good agreement between the measured temperature profiles and the numerical modeling results was demonstrated. The measured LIF profiles were corrected for electronic quenching and the variation of the Boltzmann fraction with temperature. Major species profiles were calculated using the results of the numerical flame code. The laser beam spatial profile was accounted for by convolving the theoretical OH profiles from the numerical model with the measured laser beam profile. The experimental OH profiles for all three cases are compared with theoretical results calculated using a stagnation-flame model that includes the diamond-formation surface chemistry. The measured OH concentrations near the deposition surface are substantially lower than theoretical results from a flame model that does not include the surface chemistry, but somewhat higher than for the model that does include the diamond-formation surface chemistry, especially for the flames with equivalence ratios of 1.80 and 2.10. The measured OH profile for the flame with an equivalence ratio of 2.32 is in good agreement with the profile calculated using a theoretical model with diamond-formation surface chemistry.

Theoretical Modelling of Oxide Thin Film Growth by Pulsed Laser Sources

This paper considers theoretical modelling and experimental investigation of TiO2 film growth on Ti films previously deposited on glass substrates. The Ti oxidation occurs due to film surface heating by a moving beam of a pulsed Nd:YAG laser that sweeps the surface at a constant speed of 2 mm/s, under atmospheric environment. The model takes into account the self-consistent solutions of the three-dimensional heat diffusion and oxidation rate equations. Numerical results obtained for 100 and 200 pulses of 290 ns and 3.17 MW/cm2 with a repetition frequency of 100 and 200 Hz, showed that maximum temperatures of approximately 850 and 1200 K are reached after the action of the fiftieth pulse. The occurrence of surface cooling between two consecutive pulses is observed up to the repetition frequency of 100 Hz, and the surface temperature and the film thickness profiles closely match the Gaussian shape of the spatial laser beam profile. This strongly indicates that heat diffusion may be neglected during the laser pulse duration. Theoretical predictions for the TiO2 film thickness dependence on laser intensity are in close agreement with the experimental data in the range between 2.4 and 3.2 MW/cm2.

Theoretical Modelling of Oxide Thin Film Growth by Pulsed Laser Sources

This paper considers theoretical modelling and experimental investigation of TiO2 film growth on Ti films previously deposited on glass substrates. The Ti oxidation occurs due to film surface heating by a moving beam of a pulsed Nd:YAG laser that sweeps the surface at a constant speed of 2 mm/s, under atmospheric environment. The model takes into account the self-consistent solutions of the three-dimensional heat diffusion and oxidation rate equations. Numerical results obtained for 100 and 200 pulses of 290 ns and 3.17 MW/cm2 with a repetition frequency of 100 and 200 Hz, showed that maximum temperatures of approximately 850 and 1200 K are reached after the action of the fiftieth pulse. The occurrence of surface cooling between two consecutive pulses is observed up to the repetition frequency of 100 Hz, and the surface temperature and the film thickness profiles closely match the Gaussian shape of the spatial laser beam profile. This strongly indicates that heat diffusion may be neglected during the laser pulse duration. Theoretical predictions for the TiO2 film thickness dependence on laser intensity are in close agreement with the experimental data in the range between 2.4 and 3.2 MW/cm2.

Measurement and modelling of intensity dependent absorption and transit relaxation on the cesium line

The intensity dependent absorption was measured on the line (6 transition) in atomic cesium. The magnetic field applied to the vapour and the spatial cross section of the laser beam were controlled and varied during data collection. A three-level rate equation model is presented in an attempt to explain the results. We show that this well known approach does not sucessfully model the data obtained in the absence of a magnetic field. Hence, a more complex and complete model that explicitly includes all of the hyperfine magnetic sublevels (a multilevel model) is presented. This approach accurately models all of the data collected. The good agreement betweeen this model and the data allows the determination of the transit relaxation rate (due to atomic time of flight through the laser beam), , where is the two-dimensional root-mean-squared speed of the atom and D is the FWHM of the Gaussian laser beam spatial profile.

レーザビーム集光形状制御光学系の開発

A new optical system for controlling spatial profile of laser beam has been developed. The principle of the system controls independently transverse and longitudinal diameters of the laser beam. The system consists of cylindrical mirrors and integration mirrors which focus the beam in directions perpendicularly intersecting with each other. In order to investigate the focusing property of the optical system, a calculating model for predicting beam paths through the optical system has beendeveloped using the ray tracing method. According to the model, three typical optical systems for a line shape focused beam have been designed-(1), two cylindrical mirrors, (2) one cylindrical mirror and one integration mirror, and (3) one cylindrical mirror and one integration mirror with convex segments. After laser heating experiments of steel by using these optical systems, it has been proved that they are useful in actual industrial applications.

Flattening of the spatial laser beam profile with low losses and minimal beam divergence

A new type of Nd:YAG laser is described that uses two spatial filters that are placed against the mirrors in the focal planes of a converging lens (Fourier planes). With the appropriate filters, we experimentally obtained a uniphase beam with a flat spatial profile and minimal beam divergence in the free-running regime.

Use of digital filtering techniques in the spatial profiling of pulsed laser beams

A method of spatially profiling pulsed laser beams using a digital filtering technique is described. This technique has the advantage of requiring fewer experimental scans of the laser beam to produce usable lownoise replicas of the laser beam's spatial profile than previously described methods. The technique is simple and can be implemented on a small laboratory computer.

Circuit for studying the spatial profile of a laser beam using a closed-circuit television camera

A circuit for studying the laser beam spatial intensity profile using a closed-circuit television camera (CCTV) has been developed. The camera gives an output proportional to the laser beam intensity in the form of a composite video signal. The circuit can select any scan line and the intensity variation along that line can be displayed on an oscilloscope. The circuit also generates a trigger pulse for firing the laser synchronized with the selected scan line. It can be operated in either a single-shot or repetitive mode. The circuit can be used for both pulsed and cw lasers. The dynamic range, resolution, linearity, and accuracy of measurement all depend on the CCTV camera and the oscilloscope used. The dynamic range and resolution for our particular vidicon is limited to 5 and 12 lp/mm (line pairs/mm), respectively, for a 15-ns excitation pulse and to 10 and 18 lp/mm for cw and long pulse signal.

Thermal blooming of single pulse CO2 laser radiation

An experimental investigation of the thermal blooming of a focused 3 mu s single pulse CO2 laser beam is described. An absorption cell, containing a propane-nitrogen gas mixture at atmospheric pressure was placed with its exit aperture in the focal plane of the laser beam. Measurements were made over a range of absorption coefficients from 2.4*10-4 cm-1 to 2.3*10-3 cm-1 and laser energies from 1.5 J to 2.4 J using an eight element linear array of detectors positioned across the beam diameter. Thermal blooming results are shown for times up to 2 mu s after the start of the laser pulse. This spans the focal plane hydrodynamic time of approximately 1.7 mu s. These results are compared with theoretical predictions obtained using a full wave, full hydrodynamic computer code, in which care was taken to accurately model the laser beam. The theoretical calculations consistently overpredicted the changes in the laser profile due to blooming. It is concluded that the most likely explanation of this discrepancy is that more accurate modelling of the laser beam spatial profile is required.