• 비파괴검사학회지
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• 제32권4호
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• pp.347-354
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• 2012

Using a single-line pulsed laser beam is well known as a useful noncontact method to generate a directional surface acoustic wave. In this method, different laser beam energy profiles produce different waveforms and frequency characteristics. In this paper, we considered two typical kinds of laser beam energy profiles, Gaussian and square-like, to find out a difference in the frequency characteristics. To achieve this, mathematical models were proposed first for Gaussian laser beam profile and square-like respectively, both of which depended on the laser beam width. To verify the theoretical models, experimental setups with a cylindrical lens and a line-slit mask were respectively designed to produce a line laser beam with Gaussian spatial energy profile and square-like. The frequency responses of the theoretical models showed good agreement with experimental results in terms of the existence of harmonic frequency components and the shift of the first peak frequencies to low.

• Journal of the Optical Society of Korea
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• 제13권1호
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• pp.146-151
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• 2009

A Gaussian beam of single mode fiber laser was changed into a ring-shaped pattern after it was transmitted through the hollow optical fiber. The ring-shaped beam was focused on a plane by an f-. lens and it was scanned by a Galvano-mirror. The spatial profile of laser energy incident on a plane had two peaks at both sides of the scanned linear track. The profile was compared with the result obtained when the Gaussian beam was dithered transversely by an acousto-optic modulator. It is found that hollow optical fiber beam shaper can replace acousto-optic beam dithering device which is employed in a laser induced thermal printing system.

• Journal of Welding and Joining
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• 제25권3호
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• pp.85-91
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• 2007

The conventional study on the laser surface transformation hardening has been carried out with a beam of the specified shape and uniform power-intensity distribution in order to ensure the uniformity of the hardening depth. Two types of beams - the circular gaussian beam and rectangular beam of the uniform power-intensity distribution were used in this study. we were supposed to optimize the process parameters and to compare the hardening results with two optics respectively. As a result, the hardness distribution of the hardened zone was similar in both cases and the hardened phase by the rectangular beam was denser than that by the circular gaussian beam.

• 비파괴검사학회지
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• 제33권3호
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• pp.257-263
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• 2013

Acoustic nonlinearity of surface waves is an effective method to evaluate the micro damage on the surface of materials. In this method, the $A_1$ (magnitude of the fundamental wave) and $A_2$ (magnitude of the second-order harmonic wave) are measured for evaluation of acoustic nonlinearity. However, if there is another source of second-order harmonic wave other than the material itself, the linear relationship between $A_1{^2}$ and $A_2$ will not be guaranteed. Therefore, the second-order harmonic generation by another source should be fully suppressed. In this paper, we investigated the initial second-order harmonic generation in narrowband surface waves by multi-line laser beams. The spatial profile of laser beam was considered in the cases of Gaussian and square-like. The temporal profile was assumed to be Gaussian. In case of Gaussian spatial profile, the generation of the initial second-order harmonic wave was inevitable. However, when the spatial profile was square-like, the generation of the initial second-order harmonic wave was able to be fully suppressed at specific duty ratio. These results mean that the multi-line laser beams of square-like profile with a proper duty ratio are useful to evaluate the acoustic nonlinearity of the generated surface waves.

• 한국정밀공학회지
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• 제27권8호
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• pp.84-90
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• 2010

In focused ion beam (FIB) fabrication processes the ion beam intensity with Gaussian profile has a drawback for high resolution machining. In this paper, the fabrication method to modify the beam profile at substrate using silt mask is proposed to increase the machining resolution at high current. Slit mask is utilized to block the part of beam and transmit only high intensity portion. A nano manipulator is utilized to handle the silt mask. Geometrical analysis on fabricated profile through silt mask was conducted. By utilizing proposed method, improvement of machining resolution was achieved.

• 비파괴검사학회지
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• 제26권3호
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• pp.198-205
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• 2006

The necessity of nondestructively inspecting austenitic steels, fiber-reinforced composites, and other inherently anisotropic materials has stimulated considerable interest in developing beam models for anisotropic media. The properties of slowness surface playa key role in the beam models based on the paraxial approximation. In this paper, we apply a modular multi-Gaussian beam (MMGB) model to study the effects of material anisotropy on ultrasonic beam profile. It is shown that the anisotropic effects of beam skew and excess beam divergence enter into the MMGB model through parameters defining the slope and curvature of the slowness surface. The overall beam profile is found when the quasilongitudinal(qL) beam propagates in the symmetry plane of transversely isotropic austenitic steels. Simulation results are presented to illustrate the effects of these parameters on ultrasonic beam diffraction and beam skew. The MMGB calculations are also checked by comparing the anisotropy factor and beam skew angle with other analytical solutions.

• 한국광학회지
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• 제35권4호
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• pp.150-156
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• 2024

자유 공간 광 통신 또는 레이저 전력 전송과 같은 장거리 레이저 빔 전파 연구의 핵심 요소 중 하나는 레이저 빔의 전송 효율이다. 본 논문에서는 레이저 에너지 전송의 효율을 개선하기 위해 레이저 빔의 공간적 분포를 변형하는 방법을 제안하였으며, 특히 수퍼 가우시안 빔을 대상으로 수치 해석을 진행해 그 유효성을 확인하였다. 송신기의 광학 시스템에 의해 결정된 회절 한계 반지름 이내로 수신되는 전력의 양을 기준으로, 2차 수퍼 가우시안 레이저 빔이 1 km 거리를 전파할 때 단일 채널과 다중 채널에서 모두 전송 효율이 개선되는 것을 확인하였다. 또한 1 km 전파거리를 전제로 기본 가우시안 빔과 2차 수퍼 가우시안 빔을 사용하는 상황을 비교하였을 때, 전송 효율 개선률이 단일 채널 레이저에서 1.2% 이상, 3채널 및 7채널에서는 각각 4,2% 및 4.6% 이상이라는 결과를 얻었다. 레이저 빔의 전파거리가 750 m에서 1,250 m 사이인 경우, 2차 수퍼 가우시안 빔의 전송효율 개선률은 단일, 3채널, 7채널일 때 각각 1.2%, 4.1%, 4.0% 이상으로 전송 효율의 우위가 유지되었다.

• 한국레이저가공학회지
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• 제15권2호
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• pp.11-14
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• 2012

In this paper, we have made a proposal concerning the beam shaping mask(BSM) using random-array slits to control intensity distribution profile of laser beam and demonstrated its proprieties experimentally. When a lot of slits are set out irregularly, diffraction patterns of light does not appear but granularity patterns as a bundle of fibers appear. Intensity distribution profile is controlled by densities distribution of circular slits arrayed randomly because the number of slits and its area means amount of light energy through BSM. Namely as the number of slits in high intensity area is increased and that in low intensity area decreased, amount of light energy is same over all local parts. So gaussian intensity distribution could be changed to flat-top.

• 한국광학회지
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• 제16권2호
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• pp.152-158
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• 2005

출력거울로 GRM(Gaussian reflectivity mirror)을 채택한 불안정 공진기를 구성하여 그 Q-스위치 출력의 특성을 분석하였다. 입력 전기에너지가 55J 일 때 1064 nm의 기본파장에서 470 mJ, 532 nm의 제2고조파 파장에서 280 mJ의 출력 에너지를 얻었다. 출력빔의 발산각은 1.7 mrad 이었으며 근접장 영역에서 Top-Hat 형태의 빔을 얻을 수 있었다.

• Journal of the Optical Society of Korea
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• 제10권1호
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• pp.16-22
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• 2006

Wavefront sensing using a Shack-Hartmann sensor has been widely used for estimating wavefront errors or distortions. The sensor combines the local slopes, which are estimated from the centroids of each lenslet image, to give the overall wavefront reconstruction. It was previously shown that the pupil-plane irradiance profile effects the centroid estimation. Furthermore, a previous study reported that the reconstructed wavefront from a planar wavefront with a Gaussian pupil irradiance profile contains large focus and spherical aberration terms when there is a focus error. However, it has not been reported yet how seriously the pupil irradiance profiles, which can occur in practical applications, effect the sensing errors. This paper considered two cases when the irradiance profiles are not uniform: 1) when the light source is Gaussian and 2) when there is a partial interference due to a double reflection by a beam splitting element. The images formed by a Shack-Hartmann sensor were simulated through fast Fourier transform and were then supposed to be detected by a noiseless CCD camera. The simulations found that sensing errors, due to the Gaussian irradiance profile and the partial interference, were found to be smaller than RMS ${\lambda}/50$ when ${\lambda}$ is $0.6328\;{\mu}m$, which can be ignored in most practical cases where the reference and test beams have the same irradiance profiles.