Browsing by Author "Midilli, Yakup"
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Item Open Access 915 nm pumped 1018 nm Yb-doped all-fiber high power fiber laser system(Institute of Electrical and Electronics Engineers Inc., 2019) Midilli, Yakup; Efunbajo, O. Benjamin; Şimşek, Bartu; Ortaç, BülendFibers lasers have attracted great attention in the last decades and the power scaling has reached tens of kW levels. Especially with the tandem pump configuration, pumping the active medium with a pump laser light instead of a diode laser, has made a breakthrough and so many research has been conducted about 1018 nm fiber laser systems [1]. Decreasing the quantum defect, the slope efficiency could be increased up to 90 % levels; on the other hand, due to the emission cross section of the Ytterbium (Yb), to operate the laser in the 1018 nm wavelength region is very challenging because of the presence of the ASE about the 1030 nm region. However, in the literature by using 976 nm pump diodes multi-hundred watts level 1018 nm fiber lasers could be demonstrated [2, 3].Item Open Access An all-fiber ultra-low numerical aperture high power fiber MOPA system with an output power above 500 W(Institute of Electrical and Electronics Engineers Inc., 2019) Midilli, Yakup; Ortaç, BülendOptical fiber technology has been developed dramatically in the last two decades. Especially, the invention of the Large Mode Area (LMA) fibers has made a great impact, and consequently the power scale of the fiber lasers started to increase exponentially [1]. However, at a certain point this increase has been saturated due to the non-linear effects such as Stimulated Raman Scattering (SRS) and Thermal Modal Instability (TMI). Therefore, a new approach has been proposed to mitigate these so called problems by decreasing the numerical aperture (NA) of the LMA active fiber so that it behaves like an intrinsically single mode fiber. In the literature, an active fiber having ultra-low (<; 0.04) NA, is proposed [2, 3] in 2009; and finally, the highest power, which is 4.3 kW, was demonstrated in 2017 [4]. However, all of these works based on free space orientation. In this letter, we demonstrate all-fiber and monolithic version of the high power low NA fiber laser system based on an Yb-doped active fiber having 26 μm/410 μm core/cladding diameters respectively with a NA of 0.032 which has also been verified experimentally.Item Open Access A combinatorial buffered oxide etching method for high-power cladding light stripper(Institute of Electrical and Electronics Engineers Inc., 2019) Yapar-Yıldırım, Elif; Karatutlu, Ali; Balk, Ekin Teslime; Midilli, Yakup; Ortaç, BülendHigh-power fiber lasers have been applied in many areas due to their advantages such as high beam quality, compact structure, flexibility and high efficiency. However, the absorption of the pump light is limited and a residual pumped light keeps propagating in the cladding. This residual pumped light affects the beam quality and can damage the whole system. Therefore, an efficient removal of excess high-power cladding light is critical for the safe operation of the high-power fiber lasers and high beam quality [1]. Adding a new structure to the cladding of the fiber leading to interrupt total internal reflection in the clad and scatter away the unwanted pump light is the generalized approach for a cladding light stripper (CLS) device fabrication. Also, it is important to scatter all the unwanted light uniformly along the CLS. Etching the fiber for surface damage and recoating the fiber [2] are two general methods for stripping the cladding light [3-5].Item Open Access Combined method for the fabrication of high-power cladding light stripper using a buffered oxide etchant(OSA - The Optical Society, 2019-08) Yıldırım, Elif Yapar; Karatutlu, Ali; Balk, Ekin Teslime; Midilli, Yakup; Ortaç, BülendCladding light strippers (CLSs) are vital and one of the critical components for high-power fiber laser applications. In this study, we show the first studies of the formation mechanisms and optimum conditions of a CLS device using a buffered oxide etchant by a combined method of stain (wet) etching and vapor-phase etching. This high-power CLS was shown to result in a stripping performance of ∼17.2 dB∼17.2 dB at the launched power of 333 W (pump limited). The thermal imaging demonstrates that the maximum temperature reached when operating the device at maximum launched power was ∼75°C∼75°C. The atomic force microscopy (AFM) results show that the combined method yields crystal-like structures with the height in microscales, whereas other conventional methods give only nanoscale roughness. The method also preserves the diameter of the CLS device close to the bare fiber with about 10 μm tapering leads to a high surface area to strip unwanted light, which is good for heat dissipation. The combined method possesses the outcome of two methods, including both the crystal-like structures and nanosized hillocks, resulting in high-power stripping performance and robustness.Item Open Access Demonstration of a novel cladding light stripper fabrication method based on poly (Chloro-P-Xylene) polymer material(IEEE, 2021-09-30) Midilli, Yakup; Liman, Görkem; Demirel, Gökhan; Ortaç, BülendCladding light stripper (CLS) is a key component for high power fiber laser systems to remove the unwanted light sources out of the system effectively. If they cannot be removed out of the system, the quality of the laser beam is affected severely. For that purpose, the fabrication of CLS has been investigated for years based on two main approaches. One of them is etching method [1] , [2] and the other one is the coating method [3] , [4] . The CLS fabrication based on the recoated polymer has a problem of power scaling whereas the other one is more fragile since the fiber is deformed even though they are more capable of handling power. However, in this work we present a new method to coat a special polymer material on top of the fiber with Chemical Vapor Deposition (CVD) method with a high precision level in the range of nm scale.Item Open Access Demonstration of an all-fiber ultra-low numerical aperture ytterbium-doped large mode area fiber in a master oscillator power amplifier configuration above 1 kW power level(IEEE, 2020) Midilli, Yakup; Ortaç, BülendWe demonstrate an all-fiber ultra-low numerical aperture high power fiber laser system operating in the continuous-wave regime at a central wavelength of 1080 nm. A special Ytterbium-doped fiber preform has been designed and fabricated by using modified chemical vapor deposition technique with the deposition percentages of 0.03mol% of Yb2O3, 1.8mol% of Al2O3, and 2.1mol% of P2O5. Then it has been drawn to obtain an active fiber having core/cladding diameters of 26 μm/410 μm respectively. The numerical aperture of the fiber has been first predicted as 0.034 from refractive index profile of the preform. Afterward, this number has been verified with a simple test setup by altering the bending diameter of the fiber and the excitation conditions of it. To test the high power performance, a laser system has been constructed in a master oscillator power amplifier configuration, and ~ 80 W seed signal power is amplified to 1.05 kW. The quality of the laser output beam has been measured in terms of M 2 value along both x and y coordinates as 1.11 and 1.16 respectively.Item Open Access Development of high-beam quality high power Ytterbium-doped fiber lasers(Bilkent University, 2022-01) Midilli, YakupHigh power fiber laser (HPFL) systems have drawn considerable interest for the last decades in health, industry, and especially defense applications due to their compactness, robustness, and high directionality. In this respect, the defense industry is currently in high demand for HPFL systems in the naval, air force, and ground operations. As an example, they have been implemented to the battleship, armored vehicles, and most currently to the drones. Outstanding features of these systems allow us to utilize them in various applications; however, this great demand brings some shortcomings. For example, power scaling of highpower fiber lasers has been impeded by non-linear interactions such as Stimulated Raman Scattering (SRS) and Transverse Mode instability (TMI). Regarding these non-linear interactions, I have built high-power fiber laser oscillators and amplifier systems based on both commercial and homemade selffabricated Ytterbium (Yb)-doped large mode area active (LMA) fibers. Amplifier systems have been built based on the Master Oscillator Power Amplifier (MOPA) configuration, and the average power reaches up to 1 kW power level. Besides, the fiber oscillator system has been built with a power level up to 2 kW power level and M2 value of 1.2, the beam quality parameter of the fiber laser system. To understand and investigate the TMI effect on the fiber laser system and the fiber itself, I have intended to observe the intensity change of the probe lasers and the color center formation inside a homemade active fiber in the presence of TMI. Then, I have rebuilt the system to eliminate the TMI effect and repeated the same experiments to ensure that the TMI effect was responsible for the difference. For that purpose, I have installed a fiber laser system whose fiber has been coiled in a large bending diameter to ensure the existence of the TMI effect. I have utilized two different probe lasers with 645 nm and 520 nm central wavelengths, respectively. I have coupled these probe lasers to the fiber laser system via freespace arrangements. Afterward, I have repeated the same experiment only with the 520 nm probe laser ensuring the absence of the TMI effect by rebuilding the laser structure. Finally, I have taken data about the intensity change of the probe lasers for both cases and compared them. Having benefited from the experience of these studies, to suppress the SRS and TMI, I have fabricated a new type of generation Yb-doped LMA active fiber having an ultra-low numerical aperture (NA) around 0.034. Then I have built a monolithic MOPA system based on this fiber with a 1 m bending diameter. In addition, I have obtained 1 kW maximum power with a diffraction-limited beam quality with an M2 value of 1.16. Additionally, I have studied the side-pump combining technique, which is one of the mitigation methods for TMI. It allows us to pump the active fiber from both sides, thus decreasing the thermal load on fiber. Finally, I have studied the side pump combiner on both homemade self-fabricated Photonic Crystal Fiber (PCF) and ultra-low NA active fiber in a (1 + 1) x 1 pumping configuration with 95% and 89% pump coupling e ciencies, respectively.Item Open Access Effects of gamma radiation on Yb-doped Al–P–silicate optical fibers(Springer, 2022-09) Kendir Tekgül, Esra; Midilli, Yakup; Çamiçi, Hüseyin Can; Yapar Yıldırım, Elif; Karatutlu, Ali; Ortaç, BülendYtterbium (Yb)-doped optical fibers are mainly used in the fiber laser resonator and amplifier systems. These systems have been widely utilized for applications in air and space, the defense industry, and the medical field. Particularly for the applications yielding operation in harsh environments consisting of radiation, it is essential to determine the radiation hardness of the Yb-doped optical fibers and their long-term performance in such environments. This study analyzed the optical properties of four different Yb-doped aluminophosphosilicate fibers before and after gamma irradiation. For each fiber, the effect of different total dose values including 0.5, 1, 10, and 50 kGy were determined at different operation wavelengths, such as 495 nm, 590 nm, 685 nm, and 730 nm, using radiation-induced attenuation (RIA) analysing curves. The total dose values of 10 kGy and 50 kGy were studied to demonstrate the results under extreme environmental conditions such as large hadron colliders (LHCs). Our findings reveal that the formation of radiation-induced color centers (e.g. AlOHC, POHC, and NBOHC) are highly dependent on the Yb-concentration, the amount of excess alumina (Al22O33) compared to the phosphorous pentoxide (P22O55), total irradiation dose and wavelength at which the respective RIA is recorded.Item Open Access Fabrication of biaxial polarization-maintaining optical fiber with ultra-low bending-dependent polarization extinction ratio deterioration(Academic Press, 2022-02-07) Karatutlu, Ali; Yapar Yıldırım, Elif; Midilli, Yakup; Akçimen, Samet; Ortaç, Bülend; Kendir, EsraDifferent applications, including interferometers, gyroscopes, and frequency combs, require a single polarized light transmission by maintaining this property against the environmental perturbation. As a new type of polarization-maintaining (PM) fiber, a biaxial PM fiber was fabricated over 30 dB of high polarization extinction ratio (PER) values among two orthogonal axes over a fiber length of 110 m. The PM fiber was manufactured with a combination of an elliptical core and Panda-type geometries. The PER values were demonstrated to be preserved at harsh temperature conditions from −55 °C to +85 °C. The bending loss measurements indicate the biaxial PM fiber has ultra-low bending dependent PER deterioration (<0.5 %) at various bending diameters from 12 mm to 5 mm. The biaxial PM property was also confirmed by the group beat length measurements and found to be less than 1.5 mm at 1550 nm. The optical time-domain reflectometer (OTDR) measurements show that the optical loss of the biaxial PM fiber was below 1.5 dB/km over 3 km length. The mode-field diameter (MFD) and the numerical aperture (NA) values were also reported along two polarized axes. This novel PM fiber offers a single solution for the elliptical and Panda-type PM fibers utilized for integrated fiber-based sensors and instruments.Item Open Access Performance evaluation of fiber-based ballistic composites against laser threats(Elsevier, 2019) Candan, C.; Seymen, A. Aytaç; Karatutlu, Ali; Tiken, M.; Midilli, Yakup; Orhan, E.; Berberoğlu, H.; Ortaç, BülendThe interaction between a laser and a composite material has been an intense subject within the past decade and become an emerging field for the defense and manufacturing industry since high-power lasers were initiated to be utilized for the directed-beam applications. In this study, a specially developed composite material for the ballistic applications was shined to a continuous wave (CW) laser beam at 915 nm. The ballistic material was composed of 77 layers of the single sheet of the SR-3136 by Spectra Shield® from Honeywell consisting of ultra-high molecular weight polyethylene (UHMW-PE) fibers reinforced with low-density PE (LD-PE) fibers and a polyurethane-based thermoplastic resin. At the instant of the exposure, the region of interest was completely evaporated and punctured with a slight swelling around the hole where the temperature was over 450 °C. The composite material was drilled completely upon exceeding 20 kJ of laser energy. The chemical and physical changes on the composite material after the laser exposure were extensively studied by a combination of techniques including High-Resolution Scanning Electron Microscopy (HR-SEM), Energy Dispersive X-ray Spectroscopy (EDX) and X-ray Photoelectron Spectroscopy (XPS). The physical properties of a single layer of the SR-3136 were also studied using HR-SEM, UV-VIS-NIR Absorption Spectroscopy, Thermogravimetric analysis (TGA), Differential Scanning Calorimetry (DSC) and XPS. The research presented here reveals the first study on the effects of the high-power laser beam irradiance on the fiber-reinforced composite materials utilized for the ballistic protection.Item Open Access Side pump combiner fabrication on a photonic crystal fiber in (1 + 1) x 1 configuration(IEEE, 2021-09-30) Midilli, Yakup; Şimşek, Bartu; Ortaç, BülendPump combiners have been utilized to combine the power of the pump diodes in the high power fiber laser oscillator and amplifier systems. The common way of the fabrication of the pump combiners with signal feed-through are fused taper fiber bundle (TFB) which is based on end pump technique in which the signal fiber are also tapered along with the pump fibers [1] . Therefore, with these type of combiners pumping can only be achieved in one direction; however, recently it has been demonstrated that counter-pumping or bi-directional pumping mechanisms allow the power scaling up with lower non-linear interaction thresholds such as Stimulated Raman Scattering and most recently and importantly Transverse Mode Instability (TMI), such an effect that reduces the beam quality of the laser beam drastically [2] . Alternative one to the end pumping technique making both counter and bi-directional pumping possible is side coupler technique in which the fiber core remains uninterrupted and pump fiber points can be increased more freely than the other one. The most popular side pump combiner fabrication technique is the direct fusion method due to the suitability for high power laser operation [3] , [4] . With this motivation, we have also intended to fabricate a side pump combiner for the first time on a photonic crystal fiber (PCF) in order to open a way to all-fiber monolithic systems with PCFs. For that purpose, we have used a home-made PCF having 45/360 µm core/cladding diameters respectively and a pump fiber having 130 µm cladding diameter. PCF has air holes with 12 µm diameters and a lattice constant of 25 µm and so the ratio of them would be, dΛ=0.48 before the CO 2 laser operation.Item Open Access A twisted shaped cladding light stripper by CO2 laser processing for high power fiber lasers and amplifiers(IEEE, 2017) Şimşek, Bartu; Şimşek, Elif Uzcengiz; Midilli, Yakup; Ortaç, BülendApplications of high power lasers and amplifiers have been increasing because of its superior properties such as high wall-plug efficiency, excellent beam quality, and reliability [1]. Despite advances in high power fiber components, there is still component reliability based challenges on extraction process of unwanted cladding light [2]. There are several techniques to extract the cladding light from the fiber laser system such as high index polymer coating that of working principle is violating the total internal reflection [2], roughened cladding surface which uses the scattering to eliminate the cladding light [3], soft metal coating to absorb the cladding light [4] and CO2 laser processing of cladding to disturb the light path with the structural manipulations and strip the unwanted light from these structures. There are some limitations for each case. The high index polymer CLS are limited by the thermal degradation of recoated polymer [4]. For the roughened or etching CLS, even though very high attenuations levels were achieved, the roughing or etching process decrease the fiber strength. This could create undesired problems such as heat localization and microcrack formation on cladding wall. Here, we present novel method for practical, robust, compact, and all glass cladding light stripper fabrication on Ytterbium (Yb) doped octagonally shaped double clad fiber (DCF).