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Publication Metadata only 1200 nm pumped Tm3+:Lu2O3 ceramic lasers(Optical Soc Amer, 2018) Özharar, Sarper; N/A; Department of Physics; Toker, Işınsu Baylam; Sennaroğlu, Alphan; PhD Student; Faculty Member; Department of Physics; N/A; College of Sciences; N/A; 23851We report on an experimental demonstration of a 1200-nm pumped Tm3+:Lu2O3 ceramic laser. By using a gain-switched, tunable Cr4+:forsterite laser, the excitation spectrum was measured, with optimum pumping bands centered near 1198 nm, 1204 nm, and 1211 nm. The highest slope efficiency of 21.5% was obtained at the pump wavelength of 1204 nm. Comparative energy efficiency measurements performed near 1200-nm and 800-nm pumping further showed that nearly 40% improvement was obtained in slope efficiency measured with respect to the incident pump energy for 1200-nm pumping. A transition was further observed from single-wavelength operation at 2066 nm to dual-wavelength operation near 2066 nm and 1967 nm for absorbed pump energies above 50 mu J. In this regime, two consecutive output pulses were observed in the time domain. The shortest temporal duration of the first pulse was 1.1 mu s at the incident pulse energy of 105 mu J. The duration and build-up time of the second pulse remained around 5.9 mu s and 18.5 mu s. We believe that the improved energy efficiency demonstrated for the 1.5% Tm3+:Lu2O3 ceramic with 1200-nm pumping can be used as an alternative scheme for the excitation of Tm3+:Lu2O3 ceramic lasers.Publication Metadata only 2.3-μm Tm3+: YLF laser passively Q-switched with a Cr2+: ZnSe saturable absorber(Optical Soc Amer, 2017) N/A; N/A; Canbaz, Ferda; Yorulmaz, İsmail; Sennaroğlu, Alphan; PhD Student; PhD Student; Faculty Member; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; 23851We report, what is to our knowledge, the first passively Q-switched operation of a 2.3-mu m Tm3+ : YLF laser by using a Cr2+ : ZnSe saturable absorber. In the experiments, a tunable Ti3+ : sapphire laser was used to end pump the Tm3+ : YLF gain medium inside an x cavity. A Cr2+ : ZnSe saturable absorber was also included in the cavity to initiate passive Q switching. At all pump power levels above lasing threshold, passively Q-switched operation of the Tm3+ : YLF laser could be obtained at 2309 nm with pulse durations and repetition frequencies in the ranges of 1.21.4 mu s and 0.3-2.1 kHz, respectively. Analysis of power dependent repetition rate data further gave an estimated value of 3.1% for the round-trip saturable loss of the Cr2+ : ZnSe saturable absorber.Publication Metadata only 21 fs Cr:LiSAF laser mode locked with a single-walled carbon nanotube saturable absorber(Optical Soc Amer, 2019) Bae, Ji Eun; Rotermund, Fabian; Demirbaş, Ümit; N/A; N/A; N/A; Department of Physics; Tanısalı, Gökhan; Toker, Işınsu Baylam; Taşçı, Mısra; Sennaroğlu, Alphan; PhD Student; PhD Student; Undergraduate Student; Faculty Member; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; School of Medicine; College of Sciences; N/A; N/A; N/A; 23851We report the shortest femtosecond pulses directly generated from a solid-state laser that is mode locked by using a single-walled carbon nanotube saturable absorber (SWCNT-SA). In the experiments, we used a 660 nm diode-pumped, low-threshold extended-cavity Cr:LiSAF laser operating around 850 nm with a repetition rate of 47.9 MHz. The SWCNT-SA mode-locked Cr:LiSAF laser produced 21 fs pulses with a time-bandwidth product of 0.56 by using only 210 mW of pump power. Pump-probe spectroscopy measurements showed that the SWCNT-SA exhibited saturable absorption with slow and fast decay times of 2.7 ps and 0.4 ps. The single-pass modulation depth and saturation fluence of the SWCNT-SA were further determined as 0.3% and 45 mu J/cm(2) at the pump wavelength of 850 nm.Publication Metadata only Advanced imaging with dynamic focus and extended depth using integrated FR4 platform(Optical Society of America (OSA), 2009) Varghese, Samuel; Abdullah, Fahd; Augustine, Robin; Sprague, Randy B.; Andron, Voytek; N/A; Department of Electrical and Electronics Engineering; Işıkman, Serhan Ömer; Ürey, Hakan; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 8579A two-degrees-of-freedom scanned beam imaging system with large dynamic range and dynamic focusing is demonstrated. The laser diode, photo-detector and the optical components are integrated on a moving platform that is made of FR4 (Flame-Retardant 4), a common polymeric substrate used in printed circuit boards. A scan angle of 52 degrees is demonstrated at 60Hz resonant frequency while the laser is moved 250um in the out-of-plane direction to achieve dynamic focusing. The laser is scanned by physically rotating the laser diode and the collection optics to achieve high signal-to-noise ratio and good ambient light rejection. The collection optics is engineered such that the collection efficiency decreases when collecting light from close distances to avoid detector saturation. The detection range is extended from contact distance up to 600mm while the collected power level varies only by a factor of 30 within this long range. Slight modifications will allow increasing the detection range up to one meter. This is the first demonstration of a laser scan engine with such a high degree of integration of electronics, optoelectronics, optics and micromechanics on the same platform.Publication Open Access Applications of augmented reality in ophthalmology [invited](Optical Society of America (OSA), 2021) Artal, Pablo; Department of Physics; Department of Electrical and Electronics Engineering; Aydındoğan, Güneş; Kavaklı, Koray; Ürey, Hakan; Şahin, Afsun; Faculty Member; Faculty Member; Department of Physics; Department of Electrical and Electronics Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; College of Engineering; School of Medicine; N/A; N/A; 8579; 171267Throughout the last decade, augmented reality (AR) head-mounted displays (HMDs) have gradually become a substantial part of modern life, with increasing applications ranging from gaming and driver assistance to medical training. Owing to the tremendous progress in miniaturized displays, cameras, and sensors, HMDs are now used for the diagnosis, treatment, and follow-up of several eye diseases. In this review, we discuss the current state-of-the-art as well as potential uses of AR in ophthalmology. This review includes the following topics: (i) underlying optical technologies, displays and trackers, holography, and adaptive optics; (ii) accommodation, 3D vision, and related problems such as presbyopia, amblyopia, strabismus, and refractive errors; (iii) AR technologies in lens and corneal disorders, in particular cataract and keratoconus; (iv) AR technologies in retinal disorders including age-related macular degeneration (AMD), glaucoma, color blindness, and vision simulators developed for other types of low-vision patients.Publication Open Access Bidirectional optical neuromodulation using capacitive charge-transfer(The Optical Society (OSA) Publishing, 2020) Department of Electrical and Electronics Engineering; N/A; Department of Chemical and Biological Engineering; Department of Molecular Biology and Genetics; Melikov, Rustamzhon; Srivastava, Shashi Bhushan; Karatüm, Onuralp; Nizamoğlu, Sedat; Doğru-Yüksel, Itır Bakış; Dikbaş, Uğur Meriç; Kavaklı, İbrahim Halil; PhD Student; Researcher; PhD Student; Faculty Member; Master Student; Faculty Member; Department of Electrical and Electronics Engineering; Department of Chemical and Biological Engineering; Department of Molecular Biology and Genetics; Graduate School of Sciences and Engineering; College of Engineering; College of Sciences; N/A; N/A; N/A; 130295; N/A; N/A; 40319Artificial control of neural activity allows for understanding complex neural networks and improving therapy of neurological disorders. Here, we demonstrate that utilization of photovoltaic biointerfaces combined with light waveform shaping can generate safe capacitive currents for bidirectional modulation of neurons. The differential photoresponse of the biointerface due to double layer capacitance facilitates the direction control of capacitive currents depending on the slope of light intensity. Moreover, the strength of capacitive currents is controlled by changing the rise and fall time slope of light intensity. This approach allows for high-level control of the hyperpolarization and depolarization of membrane potential at single-cell level. Our results pave the way toward advanced bioelectronic functionalities for wireless and safe control of neural activity.Publication Metadata only Broadly tunable continuous-wave orange-red source based on intracavity-doubled Cr4+:forsterite laser(Optica Publishing Group, 2002) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851The operation of a room-temperature, continuous-wave, intracavity frequency-doubled Cr4+:forsterite laser capable of producing broadly tunable output in the orange-red region of the electromagnetic spectrum is described. Intracavity doubling was achieved in a periodically poled lithium niobate crystal that had gratings with different periods. Tunable second-harmonic output could be obtained between 613 and 655 nm. At a wavelength of 630 nm, intracavity doubling yielded as much as 45 mW of continuous-wave output. To the author's knowledge, this represents the highest second-harmonic-power generation obtained to date with a continuous-wave Cr4+:forsterite laser.Publication Open Access Coagulation measurement from whole blood using vibrating optical fiber in a disposable cartridge(Society of Photo-optical Instrumentation Engineers (SPIE), 2017) Çivitci, Fehmi; Barış, İbrahim; Yaralıoğlu, Göksenin; Department of Electrical and Electronics Engineering; Yaras, Yusuf Samet; Gündüz, Ali Bars; Sağlam, Gökhan; Ölçer, Selim; Ürey, Hakan; Other; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; N/A; N/A; N/A; N/A; 8579In clinics, blood coagulation time measurements are performed using mechanical measurements with blood plasma. Such measurements are challenging to do in a lab-on-a-chip (LoC) system using a small volume of whole blood. Existing LoC systems use indirect measurement principles employing optical or electrochemical methods. We developed an LoC system using mechanical measurements with a small volume of whole blood without requiring sample preparation. The measurement is performed in a microfluidic channel where two fibers are placed inline with a small gap in between. The first fiber operates near its mechanical resonance using remote magnetic actuation and immersed in the sample. The second fiber is a pick-up fiber acting as an optical sensor. The microfluidic channel is engineered innovatively such that the blood does not block the gap between the vibrating fiber and the pick-up fiber, resulting in high signal-to-noise ratio optical output. The control plasma test results matched well with the plasma manufacturer's datasheet. Activated-partial-thromboplastin-time tests were successfully performed also with human whole blood samples, and the method is proven to be effective. Simplicity of the cartridge design and cost of readily available materials enable a low-cost point-of-care device for blood coagulation measurements.Publication Open Access Comparison of surgically induced astigmatic changes after cataract surgery in post-Laser in situ keratomileusis corneas versus virgin eyes(Conselho Brasileiro de Oftalmologia, 2022) Karslıoğlu, Melisa Zişan; Kesim, Cem; Taş, Ayşe Yıldız; Hasanreisoğlu, Murat; Şahin, Afsun; Doctor; Teaching Faculty; Faculty Member; Faculty Member; School of Medicine; Koç University Hospital; N/A; N/A; N/A; 182001; 171267Purpose: postoperative refraction in modern microincision cataract surgery gained extra importance in patients with the previous laser-assisted in situ keratomileusis (LASIK) surgery. The surgically induced astigmatic changes in those eyes may differ not only in magnitude but also in direction compared to virgin corneas. This study aimed to compare the surgically induced astigmatic changes after microscopic cataract surgery between post-LASIK corneas and virgin eyes. Methods: cases that underwent microincision cataract surgery in eyes with and without previous LASIK surgery were reviewed. The demographics, the axial length at cataract surgery, the central corneal thickness, spheric and cylindric values, keratometry readings, and postoperative posterior corneal astigmatism were retrospectively evaluated. A modified Alpins method was used for astigmatic vector analysis, and baseline astigmatism, surgically induced astigmatism, difference vector, flattening effect, and torque were assessed. Results: a total of 42 eyes from 24 subjects was evaluated. Group I consisted of 14 eyes with the previous LASIK, and Group II included 28 eyes without any refractive surgery. Preoperative mean central corneal thickness in Group I was significantly thinner (p=0.012). There was no significant difference in baseline astigmatism between the groups regarding magnitude and power vectors. After microincision cataract surgery, there were no significant differences in mean spheric and cylindric values and mean keratometry readings (all p>0.05). However, surgically induced astigmatism and difference vector were significantly higher on J45 vector component in post-LASIK eyes and microincision cataract surgery steepening effect on post-LASIK corneas was significantly higher than those in virgin eyes (p=0.001, p=0.002 and p=0.018, respectively). Conclusions: cataract surgery has steepened the corneas in both groups with a significantly higher steepening effect in post-LASIK eyes. Certainly, corneal topography cataract surgery is particularly helpful to provide more precise surgically induced astigmatism interpretations.Publication Metadata only Continuous wave thermal loading in saturable absorbers: theory and experiment(Optical Society of America (OSA), 1997) Department of Physics; Sennaroğlu, Alphan; Faculty Member; Department of Physics; College of Sciences; 23851Theoretical and experimental results of a study that investigates cw thermal loading in solid-state saturable absorbers with low heat conductivities are presented. In addition to the temperature dependence of the refractive index, the proposed model considers the temperature dependence of the fluorescence lifetime to account for the local variations in the saturation intensity resulting from thermal gradients. In the calculations an iterative scheme is employed to calculate first the temperature distribution produced by the pump beam subject to saturable absorption with a constant saturation intensity and then the resulting modifications in the propagation parameters that are due to the presence of the calculated temperature distribution. Excellent agreement is obtained between the numerically calculated results and experimentally measured cw transmission data obtained with use of a Cr:YAG saturable absorber. Because the absorption cross section of the medium is used as one of the fitting parameters to yield the best fit between theory and experiment, the model further offers an accurate method whereby the cw power transmission data can be used to determine the absorption cross section of a saturable absorber subject to thermal loading. (C) 1997 Optical Society of America.