Development of a 25-TW, 60-fs Ti:sapphire laser with frequency-doubled erbium-fiber oscillator

Sung Jae Hee,Kim Seongsu,Jang Jeong Hyun,Lee Seong Ku,Kim Hyung Taek 한국물리학회 2024 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.84 No.1

High-power Ti:sapphire femtosecond laser systems have contributed signifcantly to advancements in various aspects of science; however, the stability of the laser systems limits the application of intense laser pulses. This study presents the development of a stable high-power femtosecond laser system. To this end, 25-TW laser pulses were generated at 10-Hz repetition rate from a chirped-pulse amplifcation Ti:sapphire laser seeded with a frequency-doubled erbium-fber laser. Laser pulses with 54-fs duration and 780-nm central wavelength from the stable fber laser were amplifed to 2.3 J after sufcient temporal stretching. To shorten the temporal duration of the amplifed pulse, the spectral bandwidth was broadened by controlling its spectral amplitude, and the spectral phase error was minimized by precisely controlling the pulse compressor. Finally, the compressed laser pulse had a high energy of 1.6 J, good energy stability of 0.8%, and a short duration of 60 fs, which is close to the seed pulse duration. Thus, the stable high-power femtosecond laser system developed in this study can facilitate various femtosecond laser pulse applications.

펨토초레이저 백내장수술 도입 초기의 단기 임상경과 및고식적 백내장수술과의 비교

최재환(Jae Hwan Choi),양찬민(Chan Min Yang),정나연(Na Yeon Chung),임동희(Dong Hui Lim),정태영(Tae-Young Chung) 대한안과학회 2018 대한안과학회지 Vol.59 No.5

Purpose: To evaluate the short-term clinical outcome of femtosecond laser-assisted cataract surgery at the initial phase and to compare it with results from conventional phacoemulsification. Methods: This retrospective study was performed on 50 eyes who underwent femtosecond laser-assisted cataract surgery and 50 eyes who underwent conventional phacoemulsification. The short-term outcomes included uncorrected and corrected distance visual acuity and spherical equivalent. The complications of femtosecond laser-assisted cataract surgery were evaluated, and the prevalences of complications between the initial 25 eyes and latter 25 eyes of the cases were compared to evaluate the importance of the learning curve. Results: There were no significant differences in uncorrected (p = 0.78) and corrected (p = 0.89) distance visual acuity between groups 1 month postoperatively. There was no difference in spherical equivalent (p = 0.09). The phacoemulsification time was not different between groups (p = 0.15). However, there was a significant decrease in cumulative dissipated energy in the femtosecond laser-assisted cataract surgery group (p = 0.04). There were complications during femtosecond laser-assisted cataract surgery, such as subconjunctival hemorrhage, miosis, early entry corneal incision, and incomplete corneal incision. The prevalence of complications in the latter 25 eyes was less than that of the first 25 eyes treated with femtosecond laser-assisted cataract surgery; the prevalence of subconjunctival hemorrhage was reduced significantly (p = 0.02). Conclusions: The short-term clinical outcomes of the femtosecond laser-assisted cataract surgery group at the initial phase were not different, and in some cases, better than those of the conventional phacoemulsification group. A learning period is therefore required during the initial phase of femtosecond laser-assisted cataract surgery. J Korean Ophthalmol Soc 2018;59(5):419-427

매뉴얼 트레파인과 펨토초레이저를 이용한 전층각막이식의 장기 임상결과 비교

소하림(Ha-Rim So),김선주(Seon-Joo Kim),주천기(Choun-Ki Joo),황유숙(You-Sook Hwang),변용수(Yong-Soo Byun),정소향(So-Hyang Chung),김현승(Hyun-Seung Kim) 대한안과학회 2020 대한안과학회지 Vol.61 No.12

목적: 펨토초레이저를 이용한 전층각막이식을 시행하여 임상결과를 매뉴얼 트레파인을 이용한 전층각막이식과 비교하여 보고하고자 한다. 대상과 방법: 매뉴얼 트레파인을 이용한 전층각막이식을 받은 21안, 펨토초레이저를 이용한 전층각막이식을 받은 29안에 대해 최대교정시력, 굴절난시, 각막난시, 각막내피세포수, 이식 거부, 기능부전 등의 술 후 임상결과를 비교하였다. 결과: 수술 후 1, 3, 6, 12, 18개월에 측정한 최대교정시력은 매뉴얼군은 logMAR 1.21, 1.28, 1.25, 1.14, 1.43, 펨토초레이저군은 logMAR 0.82, 0.71, 0.78, 0.49, 0.56으로 모든 기간에서 펨토초레이저군이 매뉴얼군에 비해 높은 시력을 유지하였다. 굴절난시는 매뉴얼군은 5.13, 6.35, 5.28, 5.18, 6.36 diopters (D), 펨토초레이저군은 6.09, 6.33, 5.14, 5.23, 4.89 D, 각막난시는 매뉴얼군은 5.40, 6.45, 6.30, 5.64, 5.78 D, 펨토초레이저군은 6.17, 6.24, 5.72, 5.02, 4.83 D이며 굴절난시는 수술 후 18개월째, 각막난시는 수술 후 12, 18개월째에 펨토초레이저군에서 유의하게 난시 도수가 낮았다. 각막내피세포수, 이식 거부 및 기능부전은 두 군에서 유의한 차이를 보이지 않았다. 결론: 펨토초레이저를 이용한 전층각막이식은 매뉴얼 트레파인을 이용한 전층각막이식과 비교하여 시력회복이 좋고 굴절난시와 각막난시가 적으며, 창상부의 안정성이 더 좋은 장점이 있으므로 전층각막이식 시에 고려할 수 있는 하나의 수술 방법이다. Purpose: We compare the clinical outcomes of femtosecond-laser penetrating keratoplasty and penetrating keratoplasty performed using a manual trephine. Methods: The clinical outcomes of 21 eyes that underwent penetrating keratoplasty using a manual trephine and 29 eyes that underwent femtosecond-laser penetrating keratoplasty were compared in terms of best-corrected visual acuity, refractive and corneal astigmatisms, endothelial cell counts, and graft rejection and failure. Results: The best-corrected visual acuities measured 1, 3, 6, 12, and 18 months after surgery were logMAR 1.21, 1.28, 1.25, 1.14, and 1.43 for the manual trephine group and logMAR 0.82, 0.71, 0.78, 0.49, and 0.56 for the femtosecond-laser group; the latter group thus exhibited better visual acuity at all times. The refractive astigmatism values were 5.13, 6.35, 5.28, 5.18, and 6.36 diopters (D) for the manual trephine group and 6.09, 6.33, 5.14, 5.23, and 4.89 D for the femtosecond-laser group. The corneal astigmatism values were 5.40, 6.45, 6.30, 5.64, and 5.78 D for the manual trephine group and 6.17, 6.24, 5.72, 5.02, and 4.83 D for the femtosecond-laser group. The refractive astigmatism was significantly lower at 18 months after surgery and the corneal astigmatism was significantly lower at both 12 and 18 months after surgery in the femtosecond-laser group. None of endothelial cell count, graft rejection or failure rate, differed significantly between the two groups. Conclusions: Femtosecond-laser-enabled keratoplasty affords better visual recovery, less refractive and corneal astigmatism, and better wound stability than does penetrating keratoplasty using a manual trephine.

Enhanced thermal conductivity of alumina nanoparticle suspensions by femtosecond laser irradiation

Ha, J.,Seo, Y.,Choi, T.-Y.,Kim, D. Elsevier 2017 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.107 No.-

<P><B>Abstract</B></P> <P>This work reports significant enhancements over the Hashin-Shtrikman upper bound, by means of femtosecond laser irradiation, of thermal conductivity of alumina nanoparticle suspensions dispersed in water. By adjusting laser parameters, the applied femtosecond laser irradiation could enhance the colloidal stability of the suspension and reduce the size of nanoparticles. The thermal conductivity and zeta potential of the suspensions were measured before and after the laser-induced stabilization and fragmentation processes. When the laser stabilization and laser fragmentation processes were combined, the thermal conductivity of the suspension increased up to about 40% compared to the thermal conductivity of the base fluid at 1wt%. This laser technique demonstrates a strong potential to produce nanofluids with high thermal conductivity and colloidal stability.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The thermal conductivity, <I>k</I> of alumina nanofluid was enhanced by laser irradiation. </LI> <LI> The laser stabilization process (low laser fluence) enhances the colloidal stability. </LI> <LI> The laser fragmentation process (high laser fluence) reduces the particle size. </LI> <LI> By combining the two processes, <I>k</I> was significantly enhanced up to 38% at 1 wt%. </LI> <LI> The enhancement of <I>k</I> is due to the enhanced stability and reduced particle size. </LI> </UL> </P>

Original Articles : Factors Influencing Corneal Flap Thickness in Laser In Situ Keratomileusis with a Femtosecond Laser

( Chan Young Kim ),( Ji Hye Song ),( Kyoung Sun Na ),( So Hyang Chung ),( Choun Ki Joo ) 대한안과학회 2011 Korean Journal of Ophthalmology Vol.25 No.1

Purpose: To evaluate factors responsible for the variability between intended and achieved corneal-flap thickness during femtosecond laser-assisted laser in situ keratomileusis (LASIK). Methods: A prospective, nonrandomized, case study was performed on 35 eyes of 18 consecutive patients who underwent LASIK surgery using the 60 kHz femtosecond laser microkeratome. Eyes were assigned to three different thickness groups, with 110-, 120-, or 130-μm cut depths. Anterior segment optical coherence tomography was used to assess the morphology of 35 LASIK flaps at postoperative one week postoperatively. The flap thickness was assessed at seven measuring points across each flap. Patient age, preoperative spherical equivalent, manual keratometry, preoperative central pachymetry, and regional variability of the cornea were evaluated to determine where they influenced the achieved corneal flap thickness. Results: Cuttings of all flaps were easily performed without any intraoperative complications. Flap-thickness measurements had a mean of 115.21 ± 4.98 μm (intended thickness, 110 μm), 121.90 ± 5.79 μm (intended, 120 μm), and 134.38 ± 5.04 μm (intended, 130 μm), respectively. There was no significant difference between the 110-μm and 120-μm groups when compared with the 130-μm group (one-way analysis of variance test, p>0.05). Patients` age, preoperative spherical equivalent, manual keratometry, and preoperative central pachymetry did not affect the achieved flap thickness (Pearson correlations test, p>0.05). The reproducibility of flap thickness in the central 1.5-mm radius area was more accurate than that in the peripheral 3.0 to 4.0-mm radius area (paired samples t-test, p<0.05). Conclusions: Femtosecond laser-assisted LASIK is likely to reproduce a reliable thickness of the corneal flap, which is independent of corneal shape factors or refractive status. Future studies should focus on variations in corneal biomechanical factors, which may also play an important role in determining flap thickness.

Fabrication of Internal Gratings in PDMS Using a Femtosecond Laser

Park, Jung-Kyu,Cho, Sung-Hak,Yu, Jae-Yong,Kim, Jae-Gu,Sugioka, Koji,Hong, Jong-Wook,Heo, Won-Ha,Hwang, Kyung-Hyun Korean Society of Laser Processing 2011 한국레이저가공학회지 Vol.14 No.1

Photo-induced gratings m flexible PDMS (polydimethly siloxane) film are directly written by a high-intensity femtosecond (130fs) Ti: Sapphire laser (${\lambda}_p$ = 800nm). The refractive index modifications with $4\;{\mu}m$ diameters were photo-induced after the femtosecond irradiation with peak intensities of more than $1{\times}10^{11}W/cm^2$. The graded refractive index profile was fabricated to be a symmetric around the center of the point at which femtosecond laser by controlling both laser power and focused depth. The change on refractive index in the laser-modified regions was estimated to be approximately $10^{-3}$. The internal flexible symmetric diffraction gratings in PDMS film was successfully fabricated using a femtosecond laser.

펨토초 레이저 유도 나노 및 마이크로 구조물을 활용한 금속 표면 기능성 제어

박태훈,이효수,이해중,황택용 한국금형공학회 2023 한국금형공학회지 Vol.17 No.2

With femtosecond (fs) laser pulse irradiation on metals, various types of nano- and micro-scale structures can be naturally induced at the surface through laser-matter interaction. Two notable structures are laser-induced periodic surface structures (LIPSSs) and cone/spike structures, which are known to significantly modify the optical and physical properties of metal surfaces. In this work, we irradiate fs laser pulses onto various types of metals, cold-rolled steel, pickled & oiled steel, Fe-18Cr-8Ni alloy, Zn-Mg-Al alloy coated steel, and pure Cu which can be useful for precise molding and imprinting processes, and adjust the morphological profiles of LIPSSs and cone/spike structures for clear structural coloration and a larger range of surface wettability control, respectively, by changing the fluence of laser and the speed of raster scan. The periods of LIPSSs on metals used in our experiments are nearly independent of laser fluence. Accordingly, the structural coloration of the surface with LIPSSs can be optimized with the morphological profile of LIPSSs, controlled only by the speed of the raster scan once the laser fluence is determined for each metal sample. However, different from LIPSSs, we demonstrate that the morphological profiles of the cone/spike structures, including their size, shape, and density, can be manipulated with both the laser fluence and the raster scan speed to increase a change in the contact angle. By injection molding and imprinting processes, it is expected that fs laser-induced surface structures on metals can be replicated to the plastic surfaces and potentially beneficial to control the optical and wetting properties of the surface of injection molded and imprinted products.

257 nm Deep UV Femtosecond Laser Ablation with Minimized Crack and Chipping on Display Ultra-Thin Glass

Kwangwoo Cho,Junha Choi,Changdeok Ko,Muhyun Kim,Joohan Lee,Eunhyang Eom,Sung-Hak Cho 한국정밀공학회 2024 International Journal of Precision Engineering and Vol.25 No.2

Glass is widely used for various applications, including flat panel displays, solar panels, architectural windows, and exterior materials. These applications exhibit increasing complexity and improved functionality. In particular, glass substrates used in display panels require diverse forms of processing, prompting the exploration of laser applications to enhance processing quality, yield, and efficiency. This study aims to investigate the feasibility of using a high absorption, 257 nm femtosecond laser for processing glass substrates. The goal is to minimize damage and eliminate the need for post-processing, and ensuring superior quality and cross-sectional features. The analysis focuses on the influence of energy density and laser beam overlap ratio on processing variations. Point, line, and area processing were conducted within the achievable energy density range of 2.4–10.8 J/cm2. The results indicate that as the overlap ratio increases, processing depth, influenced by heat accumulation, exhibits a non-linear growth pattern. Moreover, the phenomenon of excessive processing width, surpassing design specifications, is mitigated by utilizing burst pulses that induce heat accumulation of ultra-short pulse lasers, thereby promoting increased processing depth while restraining width expansion. By comparing the outcomes of glass substrate processing using different laser wavelengths and pulse durations, it is confirmed that employing a 257 nm femtosecond laser minimizes damage, cracks, and chipping in the processed areas, obviating the need for post-processing. This paper presents the pioneering research on glass processing using deep ultraviolet femtosecond lasers. Results indicate that clear glass ablation is achieved without cracks.

Femtosecond laser-assisted selective reduction of neovascularization in rat cornea

Sidhu, Mehra S.,Choi, Min-Yeong,Woo, Suk-Yi,Lee, Hyun-Kyu,Lee, Heung-Soon,Kim, Kyu-Jin,Jeoung, Sae Chae,Choi, Jun-Sub,Joo, Choun-Ki,Park, Il-Hong Springer London 2014 LASERS IN MEDICAL SCIENCE Vol.29 No.4

<P>Nonlinear multiphoton absorption induced by focusing near infrared (NIR) femtosecond (fs) laser pulses into a transparent cornea allows surgery on neovascular structures with minimal collateral damage. In this report, we introduce an fs laser-based microsurgery for selective treatment of rat corneal neovascularizations (in vivo). Contiguous tissue effects are achieved by scanning a focused laser pulse below the corneal surface with a fluence range of 2.2–8.6 J/cm<SUP>2</SUP>. The minimal visible laser lesion (MVL) threshold determined over the corneal neovascular structures was found to be 4.3 J/cm<SUP>2</SUP>. Histological and optical coherence tomography examinations of the anterior segment after laser irradiations show localized degeneration of neovascular structures without any unexpected change in adjacent tissues. Furthermore, an approximately 30 % reduction in corneal neovascularizations was observed after 5 days of fs laser exposure. The femtosecond laser is thus a promising tool for minimally invasive intrastromal surgery with the aid of a significantly smaller and more deterministic photodisruptive energy threshold for the interaction between the fs laser pulse and corneal neovascular structures.</P>

A Study of Femtosecond Laser Ablation Threshold on Several Materials Using Fiber Optic Beam Delivery System

최준하(Junha Choi),신영관(Young Gwan Shin),김훈영(Hoon Young Kim),최원석(Wonsuk Choi),김재구(Jae-Gu Kim),장원석(Won-Seok Chang),윤재성(Jae-sung Yoon),조성학(Sung-Hak Cho) Korean Society for Precision Engineering 2020 한국정밀공학회지 Vol.37 No.12

A femtosecond laser is used in various fields such as microscale machining, OLED repair, micro 3D structure fabrication, and eye surgery. Particularly, because of non-thermal property, ablation and ablation threshold are the most representative characteristics of femtosecond laser. The ablation system is accompanied by many optics, stage, or gantry. In the case of the gantry, an ordinary optic system delivers a beam where mirrors and lens are required. If the gantry moves to the sample, external stimulation such as vibration will occur. Vibration has an influence on optics such as transforming beam path and becomes an error that decreases accuracy, precision, and spatial resolution. Generally, Fiber Optic Beam Delivery System (FOBDS) is used to solve this issue. But in the case of the femtosecond laser, FOBDS is incompatible. Recently, another FOBDS model that is compatible with the femtosecond laser has been developed. In this paper, the ablation threshold was obtained by FOBDS and femtosecond laser. The results present a possibility of ablation without external stimulation.