Browsing by Author "Kesim, Denizhan Koray"

Now showing 1 - 9 of 9

Open Access
10 μJ 150 fs all-fiber Yb laser amplifier system
(IEEE, 2015) Akçaalan, Önder; Kalaycıoğlu, Hamit; Kesim, Denizhan Koray; İlday, F. Ömer
Femtosecond laser pulse sources have become increasingly popular in the last decade as a result of their practical features, such as insensitivity to environmental variations, versatile designs, high-power outputs. However, much of the progress is with non-integrated specialty fibers, which involve some compromise on these practical features. Monolithic fiber chirped pulse amplification (CPA) systems are very attractive for industrial and scientific applications due to the features such as compactness, reliability and robustness [1].
Open Access
All-fiber burst mode femtosecond laser system integrated with OCT for cataract surgery
(IEEE, 2015) Kesim, Denizhan Koray; Kalaycıoglu, Hamit; Kerse, Can; İlday, F. Ömer
Cataract condition is responsible for two thirds of preventable visual impairments afflicting nearly 190 million all over the world [1]. Every year, 19 million cataract surgeries are performed worldwide [2]. Femtosecond laser-assisted cataract surgery has entered clinical use in recent years as an efficient and safe alternative to the traditional method. However, this laser-assisted surgery is in its infancy period and would benefit from lower pulse energies (to minimize collateral effects), shorter operation durations, and more compact laser systems. Fiber lasers can address these requirements with their robustness, compact size and minimal-alignment requiring structure. Further, burst mode operation [3], where high repetition rate pulses are delivered in packets repeated at a relatively low rate, have been shown to produce efficient ablation with minimal collateral thermal effects [4]. Here, we demonstrate the first fiber based burst-mode femtosecond laser device for cataract surgery. The laser is coupled to an optical coherence tomography (OCT) system with computerized controls (Fig. 1(a)). A home built all fiber Yb laser amplifier is seeded by a 109-MHz fiber oscillator, followed by a double-clad preamplifier, acusto optic modulator (AOM) pulse picker and a double-clad power amplifier. An FPGA based electronic system triggered by the oscillator is used to impose the desired pulse train on the laser beam. The system is able to produce pulses in the 5-10 µJ range compressible to sub-500 fs. The imaging and tissue processing functions are executed with OCT monitoring, using its galvo scanner and a common beam path. Maximum imaging depth is 1.6 mm with 8 µm axial resolution. The galvo scanner can reach a scan speed of 1 m/s with any desired scanning pattern. In our preliminary experiments, bursts comprising of 8 pulses with 3 µJ per pulse at 50 kHz burst repetition rate were applied to agar jells (Fig. 1 (b)), which have optical response similar to that of the cornea, as well as cornea obtained from cow eyes. Smooth cuts on ex vivo cow cornea by raster scan pattern with 70% overlapping spots (Fig 1. (c)) and on agar jells with non-overlapping spots pattern (Fig 1. (d)) were obtained. Agar and ex vivo cow eye trials show that we can create incisions with different patterns below the surface, in addition to dotted patterns for corneal incision and smooth lines for capsulorhexis.
Open Access
All-fiber laser systems that can operate in burst mode
(OSA, 2016) Kesim, Denizhan Koray; Kalaycıoğlu, Hamit; Akçaalan, Önder; İlday, Fatih Ömer
Fiber lasers which operate in burst-mode where densely spaced pulses occur inside bursts repeated at much lower repetition rates can be valuable tool for sensing and imaging. We introduce such lasers and propose possible applications.
Open Access
Application of ablation cooling to cataract surgery technique using all-fibre burst-mode laser
(2016-01) Kesim, Denizhan Koray
Cataract is the most common cause of preventable blindness in the world. Each year more than 19 million operations are performed to treat cataract. While history of cataract surgery goes as far as 2000 BC, beginning with the last quarter of the 20th century, cataract surgery procedures has benefited from advancements in ophthalmology such as phaco-emulsification, intra-ocular lens (IOL) and precision tools. There was another transformation of special interest to this thesis: In 2009, the cataract surgery was performed on a human being using a femtosecond laser for the first. Use of lasers has since gathered much attention for their high precision and repeatability in cataract surgery, but there are still challenges that their prevent wide-spread adaptation. The leading challenges can roughly be group into technical and fundamental. The technical challenges include the high cost, complexity and bulk of the associated laser technology. The more fundamental challenges relates to the interaction of ultrafast pulses with tissue. Namely, there is always interest in further reducing collateral damage and post-operation complications, which can potentially be done by reducing required pulse energies or average powers. Such an advance would also simplify the required laser technology indirectly. The principle aim of thesis has been to help overcome these challenges. In this thesis, a Yb-doped fibre laser system generating femtosecond pulses was designed to exploit recently discovered ablation-cooled laser-material removal technique. The laser system was then integrated with optical coherence tomography (OCT) for in-situ imaging during cataract surgery. In order to keep required average powers low, ablation-cooled regime is accessed through burst-mode operation of the laser. This custom-built system aims to enhance further the procedure with lower collateral tissue damage, cleaner, efficient cuts with a compact and robust structure. Preliminary experiments have been conducted on blood agar, plexiglas and extracted bovine eyes comparing the new ablation-cooled regime with the traditional regime. All experiments indicate that this regime achieves ablation with smaller pulse energies and reduced thermal effects to nearby tissue. Specically, the pulse uency required for corneal incision is decreased by a factor of 15 compared to previous publications. The system has been developed into a transportable laboratory prototype, ready to be used by medical doctors through custom-developed computer control software.
Open Access
Applying the principle of orthogonality of high dimensional random vectors to obtain high-density, large-volume 3D holographic display
(OSA, 2018) Makey, Ghaith; Yavuz, Özgün; Kesim, Denizhan Koray; Turnalı, Ahmet; Elahi, Parviz; Toumi, J.; El-Daher, M. S.; Ilday, Serim; Tokel, Onur; İlday, F. Ömer
The efforts toward truly 3D displays are far from exploiting the full potential of holography. Here, we apply the principle of orthogonality of high dimensional random vectors to obtain unprecedented dense, large volume holograms.
Open Access
Buried waveguides written deep inside silicon
(OSA, 2017) Turnalı, Ahmet; Tokel, Onur; Kesim, Denizhan Koray; Makey, Ghaith; Elahi, Parviz; İlday, Fatih Ömer
Summary form only given. Silicon waveguides are widely used as optical interconnects and they are particularly important for Si-photonics. Si-based devices, along with other optical elements, are entirely fabricated on the top surface of Si wafers. However, further integration of photonic and electronic devices in the same chip requires a new approach. One alternative is to utilize the bulk of the wafer for fabricating photonic elements. Recently, we reported a direct-laser-writing method that exploits nonlinear interactions and can generate subsurface modifications inside silicon without damaging the surface. Using this method, we fabricated several functional optical elements including gratings, lenses, and holograms. In this work, we demonstrate optical waveguides entirely embedded in Si.
Open Access
Discrete similariton and dissipative soliton modelocking for energy scaling ultrafast thin-disk laser oscillators
(Institute of Electrical and Electronics Engineers, 2018) İlday, Fatih Ömer; Kesim, Denizhan Koray; Hoffmann, M.; Saraceno, C. J.
Since their first demonstration, modelocked thin-disk lasers have consistently surpassed other modelocked oscillator technologies in terms of achievable pulse energy and average power by several orders of magnitude. Surprisingly, state-of-the-art results using this technology have so far only been achieved in modelocking regimes where soliton pulse shaping is dominant (i.e., soliton modelocking with semiconductor saturable absorber mirrors or Kerr lens modelocking), in which only small nonlinear phase shifts are tolerable, ultimately limiting pulse energy scaling. Inspired by the staggering success of novel modelocking regimes applied to overcome these limitations in modelocked fiber lasers, namely the similariton (self-similarly evolving pulses) and dissipative soliton regimes, here, we explore these nonlinearity-resistant regimes for the next generation of high-energy modelocked thin-disk lasers, whereby millijoule pulse energies appear to be realistic targets. In this goal, we propose two possible implementations. The first is based on a passive multipass cell and designed to support dissipative solitons in an all-normal dispersion cavity. The second incorporates an active multipass cell and is designed to support similaritons. Our numerical investigations indicate that this is a very promising path to increase the pulse energy achievable directly from modelocked oscillators toward the millijoule level while additionally simplifying their implementation by eliminating the need for operation in cumbersome vacuum chambers.
Open Access
Tailoring nonlinear temperature profile in laser-material processing
(2019-03) Kesim, Denizhan Koray
Ablation cooled material removal opened up great opportunities for understanding nonlinear processes. Especially using lasers as a tool to tailor nonlinear temperature gradient of material and engineering them to achieve effects such as high ablation efficiency, speed, and low collateral damage. Numerical simulations showed such engineering of the temperature gradient of material is possible for any repetition rate falling inside the ablation cooling regime. Two temperature model is used to investigate the effects of repetition rate, pulse energy, and burst duration. Simulations suggest ablation can continue indefinitely as burst duration increases. They also suggest there is an optimum pulse energy for any repetition rate in terms of efficiency of ablation regarding the material. The results of the simulations are confirmed by experiments using lasers with 1.6 GHz, 1.46 GHz, and 13 GHz repetition rate on biological and technical materials. The ablation threshold for a single pulse is lowered 100 times compared to our previous publication. Finally, related studies that can build upon the shown results are presented. A new thin-disk laser oscillator scheme is proposed that implements mode-locking regimes already established in fiber lasers. Dissipative soliton and similariton simulation results are promising for further studies. They can achieve high energy pulses with the help of nonlinear effects instead of limited by it. Then, a new computer generated hologram algorithm is explained where hundreds of layers can be generated from a single hologram. The algorithm utilizes diffusion as a tool to increase the degree of freedom which in turn decreases the cross-talk between layers.
Open Access
Two-photon excitation of quantum dots in 3D via stacked fresnel hologram algorithm
(OSA, 2017) Kesim, Denizhan Koray; Makey, Ghaith; Yavuz, Özgün; Tokel, Onur; İlday, Fatih Ömer
Quantum dots are engineered to have nanometers dimensions. The ability to specify the diameter and the material of the quantum dots allow us tune the absorption and emission properties. This results in extensive capabilities for imaging and display applications. Further, with two photon absorption and high peak power laser they can be excited at any point in 3D locally.