Browsing by Subject "Amplifiers"
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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 pulse duration on magnetostimulation thresholds(Wiley-Blackwell Publishing, Inc., 2015-06) Saritas, E. U.; Goodwill, P. W.; Conolly, S. M.Purpose: Medical imaging techniques such as magnetic resonance imaging and magnetic particle imaging (MPI) utilize time-varying magnetic fields that are subject to magnetostimulation limits, which often limit the speed of the imaging process. Various human-subject experiments have studied the amplitude and frequency dependence of these thresholds for gradient or homogeneous magnetic fields. Another contributing factor was shown to be number of cycles in a magnetic pulse, where the thresholds decreased with longer pulses. The latter result was demonstrated on two subjects only, at a single frequency of 1.27 kHz. Hence, whether the observed effect was due to the number of cycles or due to the pulse duration was not specified. In addition, a gradient-type field was utilized; hence, whether the same phenomenon applies to homogeneous magnetic fields remained unknown. Here, the authors investigate the pulse duration dependence of magneto stimulation limits for a 20-fold range of frequencies using homogeneous magnetic fields, such as the ones used for the drive field in MPI. Methods: Magnetostimulation thresholds were measured in the arms of six healthy subjects (age: 27±5 yr). Each experiment comprised testing the thresholds at eight different pulse durations between 2 and 125 ms at a single frequency, which took approximately 3040 min/subject. A total of 34 experiments were performed at three different frequencies: 1.2, 5.7, and 25.5 kHz. A solenoid coil providing homogeneous magnetic field was used to induce stimulation, and the field amplitude was measured in real time. A pre-emphasis based pulse shaping method was employed to accurately control the pulse durations. Subjects reported stimulation via a mouse click whenever they felt a twitching/tingling sensation. A sigmoid function was fitted to the subject responses to find the threshold at a specific frequency and duration, and the whole procedure was repeated at all relevant frequencies and pulse durations. Results: The magnetostimulation limits decreased with increasing pulse duration (Tpulse). For Tpulse < 18 ms, the thresholds were significantly higher than at the longest pulse durations (p < 0.01, paired Wilcoxon signed-rank test). The normalized magnetostimulation threshold (BNorm) vs duration curve at all three frequencies agreed almost identically, indicating that the observed effect is independent of the operating frequency. At the shortest pulse duration (Tpulse ≈ 2 ms), the thresholds were approximately 24% higher than at the asymptotes. The thresholds decreased to within 4% of their asymptotic values for Tpulse > 20 ms. These trends were well characterized (R2 = 0.78) by a stretched exponential function given by BNorm = 1+αe?(Tpulse/β)γ, where the fitted parameters were α = 0.44, β = 4.32, and γ = 0.60. Conclusions: This work shows for the first time that the magnetostimulation thresholds decrease with increasing pulse duration, and that this effect is independent of the operating frequency. Normalized threshold vs duration trends are almost identical for a 20-fold range of frequencies: the thresholds are significantly higher at short pulse durations and settle to within 4% of their asymptotic values for durations longer than 20 ms. These results emphasize the importance of matching the human-subject experiments to the imaging conditions of a particular setup. Knowing the dependence of the safety limits to all contributing factors is critical for increasing the time-efficiency of imaging systems that utilize time-varying magnetic fields. © 2015 American Association of Physicists in Medicine.