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Demirbaş, Ümit

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Master Student

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Ümit

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Demirbaş

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Demirbaş, Ümit

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2005 - 2009142010 - 20182

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    Continuous-wave broadly tunable Cr2+:ZnSe laser pumped by a thulium fiber laser
    (Elsevier Science Bv, 2006) Vermeulen, Nathalie; Ottevaere, Heidi; Thienpont, Hugo; Department of Physics; N/A; Sennaroğlu, Alphan; Demirbaş, Ümit; Faculty Member; Master Student; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; 23851; 20328
    We describe a compact, broadly tunable, continuous-wave (cw) Cr2+:ZnSe laser pumped by a thulium fiber laser at 1800 nm. In the experiments, a polycrystalline ZnSe sample with a chromium concentration of 9.5 x 10(18) cm(-3) was used. Free-running laser output was around 2500 urn. Output couplers with transmissions of 3%, 6%, and 15% were used to characterize the power performance of the laser. Best power performance was obtained with a 15% transmitting output coupler. In this case, as high as 640 mW of output power was obtained with 2.5 W of pump power at a wavelength of 2480 nm. The stimulated emission cross-section values determined from laser threshold data and emission measurements were in good agreement. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster cut MgF2 prism and a single set of optics.
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    Luminescent Nd3+ doped thermoplastic silicone-urea copolymers
    (Amer Chemical Soc, 2006) Department of Chemistry; N/A; Department of Physics; Department of Physics; Department of Chemistry; Yılgör, İskender; Demirbaş, Ümit; Sennaroğlu, Alphan; Kurt, Adnan; Yılgör, Emel; Faculty Member; Master Student; Faculty Member; Teaching Faculty; Researcher; Department of Physics; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; 24181; 20328; 23851; 194455; N/A
    A simple method for the preparation of rare earth ion-doped polymers, which display luminescence, is reported. For this purpose silicone–urea copolymers were doped with Nd(NO3)3·6H2O. Various structural and physicochemical properties of the resultant materials were investigated. FTIR studies indicated strong interaction of Nd3+ ions with urea groups, but no interaction with siloxane backbone, which is expected. Absorption measurements in the visible and near infrared region were performed and the radiative decay rates and branching ratios for the meta-stable 4F3/2 level were determined by using Judd–Ofelt theory. The samples were also excited at 800nm and emission spectra were observed in the near infrared at 905, 1059, and 1331nm. In Nd3+ doped silicone–urea systems the highest emission cross section at 1059nm was determined to be 60.7×10−21cm2. Spectroscopic parameters determined in this study suggest that Nd3+ doped silicone–urea copolymers are promising candidates for the development of fiber lasers or amplifiers near 1.06 and 1.3μm. 
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    Synthesis and characterization of diffusion-doped Cr2+: ZnSe and Fe2+:ZnSe
    (Elsevier Science Bv, 2006) N/A; Department of Physics; Department of Chemistry; Demirbaş, Ümit; Sennaroğlu, Alphan; Somer, Mehmet Suat; Master Student; Faculty Member; Faculty Member; Department of Physics; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; 20328; 23851; 178882
    This paper provides a detailed description of the preparation and characterization of diffusion-doped Cr2+ :ZnSe and Fe2+ :ZnSe. In the experiments, Cr2+:ZnSe samples with peak absorption coefficients (1775 nm) of as high as 74 cm(-1) and fairly good spatial uniformity were obtained. In the case of Fe2+:ZnSe, samples with a maximum absorption coefficient of 11.6 cm(-1) at 2400 nm were synthesized. A three-dimensional analytical diffusion model was further developed to determine the diffusion coefficient D from absorption data. At 1000 degrees C, D was determined to be 5.4 x 10(-10) cm(2)/s and 7.95 x 10(-10) cm(2)/s for Cr2+:ZnSe and Fe2+:ZnSe, respectively. In the case of Cr2+:ZnSe, we employed an alternative technique to determine the diffusion coefficient from position-dependent absorption data taken with a Cr4+:YAG laser. With this method, the temperature variation of D was further measured in the 800-1100 degrees C temperature ranges. (c) 2005 Elsevier B.V. All rights reserved.
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    Intracavity-pumped Cr2+: ZnSe laser with ultrabroad tuning range between 1880 and 3100 nm
    (Optical Soc Amer, 2006) N/A; Department of Physics; Demirbaş, Ümit; Sennaroğlu, Alphan; Master Student; Faculty Member; Department of Physics; Graduate School of Sciences and Engineering; College of Sciences; 20328; 23851
    We obtained, what is to our knowledge, record tuning from an intracavity-pumped gain-switched Cr2+:ZnSe laser. In the experiments, a polycrystalline Cr2+: ZnSe sample with an absorption of about 43% at 1570 nm was used to minimize reabsorption losses at the lasing wavelengths below 2000 nm. By placing the gain medium inside the cavity of a pulsed KTP optical parametric oscillator (OPO) operating at 1570 nm, smooth, continuous tuning was achieved in the 1880-3100 nm rang with four different sets of cavity optics. As high as 145 mW of average laser output power was obtained at 2365 nm with 1.2 W of intracavity OPO power. (c) 2006 Optical Society of America
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    Analysis of solid-state saturable absorbers with temperature dependent absorption cross-sections
    (Elsevier, 2009) Department of Physics; N/A; N/A; Sennaroğlu, Alphan; Coşkun, Ahmet Faruk; Demirbaş, Ümit; Faculty Member; PhD Student; Master Student; Department of Physics; College of Sciences; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 23851; N/A; 20328
    We describe a comprehensive model which can be used in the continuous-wave (cw) analysis of solid-state transition metal-doped saturable absorbers, which, in addition to the commonly observed temperature dependence of the refractive index and fluorescence lifetime, also exhibit a temperature dependent absorption cross-section. By using a rate equation analysis, two coupled differential equations are derived for the beam power and the q-parameter of the pump beam. To test the validity of the model, we analyzed the cw and pulsed saturation data taken with Cr4+:forsterite crystals which are subject to these three thermal effects at 1064 nm. Data for electric field parallel to the crystal b-axis (E parallel to b) and c-axis (E parallel to c) were analyzed. Results indicate that when thermal loading effects are taken into account, better agreement is obtained between the cross-section results of the cw, and pulsed saturation data. In particular. inclusion of thermal effects reduced the fractional deviation between the average cw and pulsed cross-section values from 23% to 2% and from 34% to 29% for the E parallel to c and E parallel to b cases, respectively. In the E parallel to b case, the average absorption cross-section and the normalized strength of excited-state absorption were further determined to be 5.98 x 10(-19) cm(2) and 0.45, respectively. The presented model can be readily extended to analyze other thermal effects or other media with different energy-level schemes.
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    Preparation and spectroscopic investigation of diffusion-doped Fe2+:ZnSe and Cr2+:ZnSe
    (The Optical Society (OSA) Publishing, 2005) N/A; Department of Physics; Department of Physics; Department of Chemistry; Demirbaş, Ümit; Kurt, Adnan; Sennaroğlu, Alphan; Somer, Mehmet Suat; Master Student; Teaching Faculty; Faculty Member; Faculty Member; Department of Physics; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; 20328; 194455; 23851; 178882
    We describe the preparation and spectroscopic characterization of diffusion-doped Fe2+:ZnSe and Cr2+:ZnSe. Diffusion coefficients were determined at 1000 0C. In gain-switched operation, 80-mJ, 1-kHz pulses were produced with Cr2+:ZnSe samples near 2600 nm.
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    POLY 708-Photoluminescent rare-earth ion containing silicone copolymers
    (Amer Chemical Soc, 2006) Department of Chemistry; Department of Chemistry; N/A; Department of Physics; Yılgör, İskender; Yılgör, Emel; Demirbaş, Ümit; Sennaroğlu, Alphan; Kurt, Adnan; Tezgel, Arife Özgül; Faculty Member; Researcher; Master Student; Faculty Member; Teaching Faculty; Undergraduate Student; Department of Chemistry; Department of Physics; College of Sciences; College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; 24181; N/A; 20328; 23851
    N/A 
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    Concentration dependence of fluorescence and lasing efficiency in Cr2+ : ZnSe lasers
    (Elsevier Science Bv, 2007) Department of Physics; N/A; Department of Physics; Department of Chemistry; Sennaroğlu, Alphan; Demirbaş, Ümit; Kurt, Adnan; Somer, Mehmet Suat; Faculty Member; Master Student; Teaching Faculty; Faculty Member; Department of Physics; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; 23851; 20328; 194455; 178882
    We investigated the effect of Cr2+ concentration on the fluorescence lifetime, fluorescence efficiency, and lasing performance of Cr2+:ZnSe lasers. In the experiments, polycrystalline Cr2+:ZnSe samples with concentrations of 0.34 x 10(18)-66 x 10(18) ion/cm(3) were prepared by diffusion doping. Spectroscopic measurements performed with Cr4+:YAG and Tm3+:fiber lasers show that the fluorescence efficiency decreases monotonically with increasing active ion concentration. Lifetime measurements and lasing characterization were also performed with a pulsed 1570-nm KTP optical parametric oscillator. In gain-switched operation, the highest output energy was obtained with the Cr2+:ZnSe sample having a Cr2+ ion concentration of 14 x 10(18) cm(-3). The power performance degrades with increasing concentration due to larger passive losses at the lasing wavelength. Furthermore, the pulse build-up time and the output pulsewidth decrease with pump energy and ion concentration. Finally, with a single set of optics, tunable output could be obtained in the 2520-3050 nm wavelength range.
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    Luminescent Nd3+ doped silicone-urea copolymers
    (Elsevier Sci Ltd, 2006) N/A; Department of Physics; Department of Physics; Department of Chemistry; Department of Chemistry; Demirbaş, Ümit; Kurt, Adnan; Sennaroğlu, Alphan; Yılgör, Emel; Yılgör, İskender; Master Student; Teaching Faculty; Faculty Member; Researcher; Faculty Member; Department of Physics; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; College of Sciences; 20328; 194455; 23851; N/A; 24181
    A simple method for the preparation of rare earth ion-doped polymers, which display luminescence, is reported. For this purpose silicone-urea copolymers were doped with Nd(NO3)(3)(.)6H(2)O. Various structural and physicochemical properties of the resultant materials were investigated. FTIR studies indicated strong interaction of Nd3+ ions with urea groups, but no interaction with siloxane backbone, which is expected. Absorption measurements in the visible and near infrared region were performed and the radiative decay rates and branching ratios for the meta-stable 4 level were determined by using Judd-Ofelt theory. The samples were also excited at 800 nm and emission spectra were observed in the near infrared at 905, 1059, and 1331 nm. In Nd3+ doped silicone-urea systems the highest emission cross section at 1059 nm was determined to be 60.7 X 10(-21) cm(2). Spectroscopic parameters determined in this study suggest that Nd3+ doped silicone-urea copolymers are promising candidates for the development of fiber lasers or amplifiers near 1.06 and 1.3 mu m. (c) 2006 Elsevier Ltd. All rights reserved.
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    Graphene mode-locked femtosecond Alexandrite laser
    (2018) Kocabaş, Coşkun; N/A; N/A; Department of Physics; Cihan, Can; Demirbaş, Ümit; Sennaroğlu, Alphan; PhD.Student; Master 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; College of Sciences; N/A; 20328; 23851
    We report for the first time, to the best of our knowledge, graphene mode-locked operation of a femtosecond Alexandrite laser at 750 nm. A multipass-cavity configuration was employed to scale the output energy and to eliminate spectral/Q-switching instabilities. By using a monolayer graphene saturable absorber, mode locking could be obtained. With 5W of pinup at 532 nm, nearly transform-limited, 65 fs pulses with a time-bandwidth product of 0.319 were generated. The mode-locked laser operated at a pulse repetition rate of 5.56 MHz and produced 8 mW output power, corresponding to a pulse energy and peak power of 1.4 nJ and 22 kW, respectively. These experiments further show that graphene can be used to initiate mode locking at wavelengths as low as 750 nm. (C) 2018 Optical Society of America