Ultrafast pulse dynamics in low noise Tm/Ho doped mode-locked fiber lasers

Mode-locked fiber lasers have attracted significant scientific and commercial interest since they offer a compact and highly stable platform with straightforward operation for exploiting ultrafast and nonlinear phenomena. They have enabled a vast range of applications that span from distinct disciplines such as medical diagnostics, molecular spectroscopy, and high-power precise mechanical cutting, to optical metrology. Various gain media have been utilized to achieve laser emission at different wavelengths. We have developed unique thulium/holmium (Tm/Ho) doped mode-locked fiber laser systems to address the needs of low-noise ultrafast optical sources in the wavelength vicinity of 2 μm at higher repetition rates. Since the 2 μm wavelength regime has recently attracted more attention with the emergence of thulium gain fibers, the rich underlying cavity dynamics, novel pulse operation regimes and nonlinear phenomena in compact fiber configurations have not been fully explored yet. In this thesis, research is conducted on novel Tm fiber laser cavity configurations and on the formation of unique, polarization-based pulsing regimes. Particularly, this research is focused on the exploration of novel ultrafast and nonlinear phenomena, and the development of optical sources emitting unprecedented ultrafast pulse trains beyond conventional equal-intensity distribution using Tm/Ho doped gain media. The research presented features four main results: 1) development of a high repetition rate and low-noise Tm/Ho doped mode-locked fiber laser platform as an attractive optical source for a wide variety of applications 2) investigation of a novel mode-locked state in which the ultrafast pulse train is composed of co-generated, consecutive, equal intensity and orthogonally polarized pulses in order to achieve dual RF comb generation for dual-comb spectroscopy applications, 3) exploration of controllable ultrafast waveform generation utilizing vector soliton and harmonic mode-locking mechanisms for optical telecommunication applications, and 4) demonstration of unique transitional mode-locked states showing exceptional features such as powerful irregular bursts of ultrafast pulses and rogue wave behavior without damaging the laser elements. The aim of these projects has been to explore the novel optical properties of Tm/Ho co-doped fiber lasers in order to achieve advanced functionalities in commonly practiced applications such as telecommunication, metrology and spectroscopic applications.2019-10-22T00:00:00

Subgross and macroscopic investigation of the coeliac artery in the chinchilla (chinchilla lanigera)

The knowledge of branching and variations of the coeliac artery is clinicallyimportant, especially in the surgical operations and non-surgical treatments.Moreover, the chinchillas abdominal region have been used as a model in somesurgical experimental researches. In this frame, we have aimed to explain thebranching of this artery in the chinchillas detailedly. A total of 10 adult, healthy,male chinchillas (chinchilla lanigera) were used to investigate the origin and thecourse of the coeliac artery and its branches. Coloured latex was injected intothe carotid arteries, following conventional anatomical applications. The resultsindicated that the coeliac artery was divided into 4 branches such as left gastricartery, hepatic artery, splenic artery and gastrolienal artery. The left gastric arterywas a continuity of the coeliac artery and the main vessel of the stomach. Thehepatic artery was divided into the left lateral branch, the left medial branch andthe right branch. The splenic artery was covered by the pancreas tissue and sentbranches to the pancreas. The gastrolienal artery was supplying the fundus ofthe stomach and the dorsal extremity of the spleen. We believe that the findingswill be of help to the researchers interested in the anatomical area, surgeons andexperimental researches

Sığır veziküler bezinin morfometrik ve stereolojik olarak değerlendirilmesi

The vesicular gland is an important genital gland related to the fertility of bovine. In this study, the morphometric and stereological methods were sought to adapt to the bovine vesicular gland. For this aim, 8 healthy Holstein bull’s paired vesicular glands were collected from the slaughterhouse. The weight and the dimensions of the glands were measured before the fixation. After the formalin fixation, all the glands were divided into 3 subgroups for the total volume estimation of the gland, cell counting and the volume estimation of the cell nucleus. The cell counting and the volume estimation of the cell nucleus were performed on the principal cells. The average weight, width, length, height, the volume of the glands, the mean number and nucleus volume of the principal cells were 30.9 g., 2.62 cm, 8.86 cm and 1.86 cm, 29.7 cm3 , 3.58.109 and 108 µm3 , respectively. The researchers believe that the findings will contribute to the literature and in particular, facilitate experimental and toxicological researches performed in the future.Veziküler bez sığır fertilitesinde önemli bir üreme bezidir. Sunulan bu çalışmada sığır veziküler bezinin morfometrik ve stereolojik olarak incelenmesi amaçlanmıştır. Bu amaçla 8 sağlıklı Holstein boğasının çift olan veziküler bezleri mezbahadan toplanmış ve ağırlığı ile boyutları ölçülmüştür. Formaldehit tespitinden sonra tüm bezler bezin toplam hacminin, hücre sayısının ve çekirdek hacminin ölçülmesi için 3 alt gruba ayrılmıştır. Hücre sayımı ve hücre çekirdek hacmi ölçümü bezin en çok bulunan ana hücresi olan principal hücreler üzerinde gerçekleştirilmiştir. Bezin ortalama ağırlığı, eni, uzunluğu, yüksekliği, ortalama hacmi, principal hücrelerin ortalama sayısı ve ortalama çekirdek hacmi sırasıyla 30,9 gr., 2,62 cm, 8,86 cm, 1,86 cm, 29,7 cm3 , 3,58,109 ve 108 µm3 olarak bulunmuştur. Araştırmacılar bu bulguların literature katkı sağlayacağına ve gelecekte yapılacak olan deneysel ve toksikolojik çalışmalara ışık tutacağına inanmaktadır

Asymmetric light propagation in chirped photonic crystal waveguides

Cataloged from PDF version of article.We report numerical and experimental investigations of asymmetric light propagation in a newly designed photonic structure that is formed by creating a chirped photonic crystal (PC) waveguide. The use of a non-symmetric distribution of unit cells of PC ensures the obtaining of asymmetric light propagation. Properly designing the spatial modulation of a PC waveguide inherently modifies the band structure. That in turn induces asymmetry for the light's followed path. The investigation of the transmission characteristics of this structure reveals optical diode like transmission behavior. The amount of power collected at the output of the waveguide centerline is different for the forward and backward propagation directions in the designed configuration. The advantageous properties of the proposed approach are the linear optic concept, compact configuration and compatibility with the integrated photonics. These features are expected to hold great potential for implementing practical optical rectifier-type devices. (C) 2012 Optical Society of America

Estimation of Purkinje cell quantification and volumetry in the cerebellum using a stereological technique

Systematic sampling using the Cavalier’s principle provides accurate, efficient, cheap, and simple quantitative estimates of objects within neuroanatomical structures like the cerebellum. We identified and isolated the Purkinje cell layer and used this information to extrapolate the stereological technique to estimate the total number of Purkinje cells and volume on light microscope in rabbits’ cerebella. Volumes of the cell populations in the cerebellum have not been estimated previously. Using this method, we counted the Purkinje cells in the two right and three left hemicerebella of five male healthy rabbits and found the total number of Purkinje cells to have a mean of 671,597. The mean volume of the Purkinje cell was estimated at 2207 μm3 for the entire cerebella. The contribution of the stereological method to cell quantification and volumetry was emphasised by the neuroanatomical experimental animal study. The method proved to be an excellent tool for evaluating the Purkinje cell numbers and volumes in the rabbits. The data may also support the significance of considering anatomical data when evaluating pathological changes in cerebella. (Folia Morphol 2011; 70, 4: 240–244

Resveratrol ve n-asetilsistein kombinasyonunun MK-801’ le i̇ndüklenen lokomotor hiperaktivite karşısında koruyucu etkisi

N-Methyl-D-Aspartate (NMDA) receptors are one of the most important elements of the glutamatergic system. The hypofunction of this receptor causes locomotor hyperactivity. The chemical agent MK-801, which is an NMDA receptor antagonist, also causes locomotor hyperactivity in rodents. In the present study, it is aimed to find the lowest protective dose of neuroprotective resveratrol and N-acetyl-cysteine (NAC) combination on increased locomotor activity using MK-801 in mice. For this purpose, 84 female mice were used and 14 groups of equal number of mice were formed. Locomotor hyperactivity was created in two parts as acute (1 day drug administration) and sub-acute (4 days drug administration) phases. After drug administrations, animals were subjected to open field testing. According to the results, the drug combination was successful in reducing locomotor hyperactivity in the acute phase than in the sub-acute phase. It was observed that the intraperitoneal administration of both low doses of the combination, 40mg/kg resveratrol + 20mg/kg NAC and 20mg/kg resveratrol + 10mg/kg NAC, successfully prevented the locomotor hyperactivity in the acute phase. As a result, it was concluded that the combination of antioxidants has an effect on acutely formed locomotor hyperactivity.N-Methyl-D-Aspartate reseptörleri glutamaterjik sistemin önemli bir öğesidir. Bu reseptörün hipofonksiyonu lokomotor hiperaktiviteye sebep olur. NMDA reseptörlerinin antagonisti olan MK-801 isimli kimyasal da lokomotor hiperaktiviteye sebep olmaktadır. Sunulan bu çalışmada nöroprotektif özellikleri olduğu bilinen resveratrol ve N-asetilsistein isimli iki antioksidan kombinasyonunun farelerde MK-801’le indüklenen lokomotor hiperaktivite üzerinde etkin olan en düşük dozu araştırılmıştır. Bu amaçla 84 dişi fare eşit olarak 14 gruba bölünmüştür. Lokomotor hiperaktivite akut (1 gün ilaç uygulanması) ve sub-akut (4 gün ilaç uygulanması) faz olmak üzere iki bölümde oluşturuldu. İlaç uygulamalarından sonra hayvanların katettikleri mesafe açık alan test düzeneğinde 10 dakika boyunca kaydedilmiştir. Ölçüm sonuçlarına göre antioksidan kombinasyonunun akut fazda sub-akut faza göre başarılı olduğu gözlenmiştir. Kombinasyonun her iki düşük dozu olan 40mg/kg resveratrol + 20mg/kg NAC ve 20mg/kg resveratrol + 10mg/kg NAC’ın intraperitoneal uygulamalarının akut fazda lokomotor hiperaktiviteyi başarıyla engellediği gözlendi. Sonuç olarak antioksidan kombinasyonunun akut oluşan lokomotor hiperaktivite karşısında etkisinin olduğu sonucuna varıldı

Design of compact optical devices based on periodic meta-structures

Ankara : The Department of Electrical and Electronics Engineering and the Graduate School of Engineering and Science of Bilkent Univ., 2013.Thesis (Master's) -- Bilkent University, 2013.Includes bibliographical references leaves 115-133.Manipulation of the flow of light is demanded for several applications such as communication, data storage, sensor, photovoltaic cells, microscopy, lasers and light emitting diodes for the purpose of designing compact, high-throughput and high efficiency optical devices. Nevertheless, the control of the propagation of the light becomes much harder in devices with smaller geometries mostly because of diffractions, loss mechanisms and fabrication difficulties. Furthermore, materials that are already available in the nature do not provide unprecedented optical properties for nanoscale optical applications. Due to this fact that fabrication of artificial materials is needed for utilizing novel and intriguing optical devices. For this purpose, some relatively new research fields have emerged like photonic crystals, metamaterials and high contrast gratings. We propose several designs based on aforementioned meta-structures to achieve compact and practically realizable optical devices. We presented compact optical demultiplexer, diode-like device and electro-optic modulator designs that are based on photonic crystals. We also proposed two circular polarizer designs based on metamaterials and high contrast gratings. Further, we investigated unidirectional transmission and polarization manipulation properties in chiral metamaterials. For most of the proposed designs, we also experimentally verified the numerical and theoretical findings. In conclusion, we can claim that the utilization of artifically structured materials give opportunity to realize the control of light much more easily in nanoscale designs.Akosman, Ahmet EminM.S

Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides

Cataloged from PDF version of article.A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths. (c) 2012 Elsevier B.V. All rights reserved

Compact wavelength de-multiplexer design using slow light regime of photonic crystal waveguides

Cataloged from PDF version of article.We demonstrate the operation of a compact wavelength de-multiplexer using cascaded single-mode photonic crystal waveguides utilizing the slow light regime. By altering the dielectric filling factors of each waveguide segment, we numerically and experimentally show that different frequencies are separated at different locations along the waveguide. In other words, the beams of different wavelengths are spatially dropped along the transverse to the propagation direction. We numerically verified the spatial shifts of certain wavelengths by using the two-dimensional finite-difference time-domain method. The presented design can be extended to de-multiplex more wavelengths by concatenating additional photonic crystal waveguides with different filling factors. (C) 2011 Optical Society of Americ

Asymmetric chiral metamaterial circular polarizer based on four U-shaped split ring resonators

Cataloged from PDF version of article.An asymmetric chiral metamaterial structure is constructed by using four double-layered U-shaped split ring resonators, which are each rotated by 90 degrees with respect to their neighbors. The peculiarity of the suggested design is that the sizes of the electrically and magnetically excited rings are different, which allows for equalizing the orthogonal components of the electric field at the output interface with a 90 degrees phase difference when the periodic structure is illuminated by an x-polarized wave. As a result, left-hand circular polarization and right-hand circular polarization are obtained in transmission at 5: 1 GHz and 6: 4 GHz, respectively. The experiment results are in good agreement with the numerical results. (C) 2011 Optical Society of Americ