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Kerr-odaklamalı kip-kilitli Cr:LiSAF lazerinden kazanç-anahtarlamalı çıkış aynası yardımıyla 15 femtosaniye altı darbe üretimi
Generation of ultrashort laser pulses with long?term stability and robustness is very important for many applications such as material processing, biomedical imaging, and spectroscopy. Kerr?lens mode?locking (KLM) is one of the well?known techniques used to generate ultrashort pulses. One drawback of the KLM method is that the modulation depth resulting from the Kerr nonlinearities is typically weak especially in lasers with low nonlinear refractive index. This makes it very difficult to apply the KLM technique to colquiriite type gain media such as Cr:LiCAF and Cr:LiSAF which have low nonlinear refractive indices. Recently, a novel approach has been proposed to overcome this drawback by using what can be referred to as gain?matched output couplers (GMOC). The spectral transmission characteristics of a GMOC are designed in such a way as to provide a higher effective gain for a broadband pulse as opposed to a continuous wave beam, which favors the mode?locked operation of a laser oscillator with much higher temporal stability. In this thesis work, this approach which utilizes the GMOC technology has been applied to a low?cost, directly diode pumped Cr:LiSAF laser around 850 nm. In the experiments, we used a low?cost red diode around 660 nm which could pump the Cr:LiSAF laser without any cooling requirement. An astigmatically compensated x?fold cavity was constructed to house a 6 mm long, 1.5 % Cr doped LiSAF crystal. The GMOC mirror used in the experiments had a transmission of 0.7 % near the center wavelength of the emission band. To obtain the shortest pulses, dispersion compensation was used with chirped mirrors and fused silica prism pairs. In this case, the net cavity dispersion was around ~?50fs2. When KLM operation was initiated with the GMOC mirror, this configuration resulted in the generation of nearly transform?limited pulses as short as 13 fs, with 25 mW of average output power, at a repetition rate of 126 MHz near 855 nm. Other cases with different levels of cavity dispersion and the tunability of the femtosecond laser were also investigated. The experimental results clearly demonstrated that the use of a GMOC mirror enables a very robust and stable operation of a Cr:LiSAF laser despite the fact that the nonlinear refractive index of the medium is low.
Yüzey destekli sıvı mikrodamlacıklarda ışımayan erk dönüşümlü optoakışkan mikrolazerler
Optofluidics is a recently established research field combining the microfluidic chip technology with optics. It allows for a broad range of applications aiming to develop new photonic devices whose functions are defined by the liquid or to build new integrated photonic devices in which light is used for excitation, analysis, or manipulation of the fluid and the particles in the fluid. In particular, optofluidic lasers hold great promise for developing biological sensors thanks to the high sensitivity of stimulated emission to small perturbations in the laser cavity and gain medium. In an optofluidic dye laser, the gain medium can be directly excited by tuning the pump laser into dye absorption band or non-radiative Förster resonance energy transfer (FRET) can be employed exciting donor molecules which transfer their energy efficiently to acceptor molecules. Optofluidic dye lasers based on FRET enable the extension of the lasing wavelength range and provide new ways of biological sensing thanks to the high sensitivity of FRET efficiency to the donor-acceptor pair separation. In this thesis, the behavior of energy-transfer dye lasers using a mixture of saturable absorber and laser dye as an active medium is investigated theoretically by using rate-equation model. The rate equation model describing the working principle of such dye laser consists of coupled differential equations for the population of energy levels of the dye and the absorber and photon density inside the cavity. Here, we study the temporal characteristics and power of the output lasing pulses as a function of the fluence of the pulsed excitation beam. In the second part, we demonstrate optofluidic microlasers using highly efficient non-radiative FRET for pumping of the gain medium placed within glycerol/water microdroplets situated on a superhydrophobic surface. These droplets can serve as natural microcavities hosting whispering gallery modes (WGMs). Droplets are doped with the FRET donor-acceptor dye pair Rhodamine 6G-Rhodamine 700. FRET lasing is characterized for different acceptor and donor concentrations, and low threshold pump fluences of acceptor lasing are demonstrated. We also verify the nature of the energy transfer (non-radiative vs. radiative) for the range of parameters studied in the experiments.
Silikon mikrokürelerde fısıldayan galeri kiplerinin optik uyarım ile kontrolu
Geçtiğimiz yüzyılda silikon maddesi, yarı-iletken endüstrisi için temel bir malzeme haline gelmiş ve fotonik alanında da CMOS teknolojisi ile birlikte kullanılacağı ön görülmüştür. Geçtiğimiz yıllarda ise fısıldayan galeri kipleri (WGM) optik çınlaçları, optik filtreler, modülatörler, yükselticiler ve faz yöneticileri gibi birçok farklı fotonik uygulamasında yer bulmuştur. Buna göre, WGM çınlaçları CMOS teknolojisi ile birlikte kullanılarak tümleşik optik cihazların üretiminde yer alabilir. Bu tezde, 1427 nm dalga boyunda çalışan 500 µm yarı çaplı silikon WGM çınlaçlarının ürettiği WGM çınlamaları, 405 nm dalga boyunda çalışan bir pompa lazeri kullanılarak kırmızıya doğru kaydırılmıştır. Elde edilen WGM kırmızıya kaymaları, silikon kürelerdeki pompa lazerinin etkisi ile ortaya çıkan termo-optik etkiye (TOE) dayanmaktadır. Silikon küresel çınlaçların karakterize edilmesi amacıyla bir ölçüm düzeneği kurulmuş ve kip aralığı ∆λ = 0.23 nm ve kalite faktörü Q ≈ 60000 olan WGM kipleri gözlemlenmiştir. Bu adımdan sonra pompa-ölçüm deney düzeneğine geçilmiştir. Bu düzenekte, 405 nm dalga boyunda çalışan odaklanmamış sürekli dalga (CW) pompa uyarımı kullanılarak silikon küre WGM izgesinde 13.5 pm kırmızıya kayma ve odaklanmış pompa uyarımı ile 24.5 pm kırmızıya kayma gözlemlenmiştir. Ek olarak, pompa lazeri 8 Hz frekansında % 80 çevrim ile uyarım yaptığında WGM kiplerinde 40 pm'ye kadar WGM kayması da elde edilmiştir. Sonuçlara göre, silikon kürelerde WGM optik çınlamalarının yine optik yoluyla kontrol edilebileceği ortaya konulmuştur. Deney düzeneği ve analizlere göre silikon mikro kürelerin ultra hızlı optik anahtarlama alanında uygulama bulacağı öngörülmektedir.
X-band uygulamaları tasarım ve geniş bant meandering anten imalatı
Novel meandering antenna designs have been proposed for broadband communication applications in X-band spectral region. Broad band antennas are becoming more popular as a result of growing demand for bandwidth, for example, video conferencing; direct sequence spread spectrum (DSSS) communication system and frequency hopping. Meanders introduce longer current paths and hence add multiple staggered resonating modes. This design technique can be used to achieve multiband designs using resonating modes far from each other. However, if the design is engineered to let these resonant modes come closer to each other, they will complement the effect of closely spaced resonances, forming staggered resonating behavior, and broadband antennas are achieved. Current in the adjacent arms of the meandering antennas cancel out each other due to symmetry of the design, and radiation efficiency is compromised as a result. Antisymmetric meandering arms are presented in this work, to achieve higher radiation efficiency. A relationship between physical dimensions of the meandering loops, that is, height, thickness, mutual coupling and material properties are presented and resonant frequencies are mathematically modeled. The physical dimensions are selected to obtain set of resonant frequencies, which are closely spaced in the frequency spectrum. The model is simulated using finite element method (FEM) numerical modeling technique. A good agreement between mathematically calculated resonant response and simulated results is obtained for different designs geometries. Six designs are modeled and fabricated on a single sided FR4 substrate of dielectric constant 4.4 and thickness 1.6 mm. In order to evaluate the performance for suitable comparison, each antenna is identically implemented using a 50-Ohm coplanar waveguide (CPW) as feeding probe. Antennas are independently assessed using a Network Analyzer. Both S11 and S12, VSWR, efficiency, gain and bandwidth (BW) parameters are measured. The overall performance of the antennas has been found good.
Silisyum mikro-yuvarlarda optik anahtarlama
Silicon is a cheap and naturally abundant material and has contributed to the electronics revolution, which in turn has led to the foundation of separate fields known as silicon electronics and silicon photonics. In silicon photonics optical resonances are utilized, which are useful for various applications, i.e., optical components such as Fabry-Pérot resonators. However, with the passage of time focus has shifted to three dimensional optical resonators with microspheres being the prime example. Nowadays, optical resonances excited in microspheres are called whispering gallery modes (WGMs) and they can be beneficial for various photonics applications. In this work, a silicon microsphere, with a 1 mm diameter and a refractive index of 3.48, is used to excite the WGMs of the silicon microsphere by evanescent coupling with a silica optical fiber half coupler (OFHC). A near infrared CW laser with the central wavelength of 1472 nm is used for the excitation of the WGMs. Additionally, these WGMs are tuned with the help of a pulsed 405 nm Fabry-Pérot laser, which acts as pump laser to modify the refractive index of the microsphere. As the pump energy changes, an average red shift of 0.37 nm in the WGMs spectrum of the silicon microsphere is observed for 50% duty cycle of the pump laser, which leads to a refractive index change of ΔN = 8.75x10‑4 and a temperature change of ΔT = 4.70 K. This behavior can be explained by the thermo-optic effect (TOE) in silicon. All-optical switching in time domain is also carried out for the WGMs of the silicon sphere, as the pump modulates the refractive index of silicon, the resonant peaks are observed to be shifting in time domain. Overall, TOE tuning of the resonant spectra is shown to be beneficial for the all optical control of silicon resonators. This technique can be useful for applications in silicon photonics.
Kristal ve amorf sıvılarda elastik ve elastik olmayan saçılma ile desteklenmiş yuvarlak ve düz yüzeyli silisyum ışık algılayıcıları
When light interacts with matter, absorption, scattering and fluorescence of light can take place depending on the wavelength and energy of the light together with the size of the interacting particle. In this work, we present light scattering enhancement of spherically shaped silicon photodetectors immersed in liquid environments. Our results indicate higher detection performance for spherical surface detectors as compared to conventional flat surface photodiodes. In order to characterize the photodiodes, we devised a measurement setup with a diode pumped solid state laser (DPSS) operating at 532 nm wavelength to induce light scattering in the liquid solutions. The solutions used were pure ethanol, 0.1 mM Rhodamine 640 perchlorate in ethanol, pure 5CB nematic liquid crystal, and 0.1 mM Rhodamine 640 perchlorate in 5CB, Rhodamine solution gives the ability of fluorescence in addition to the light scattering. We obtained elastic light scattering in ethanol and liquid crystal solutions and inelastic light scattering in fluorescent solutions with the laser dye. The silicon photodiodes were immersed in the solutions and characterized for their responsivity for the detection of scattering. Positional tomography for the voltage response of the photodetectors was recorded. It is seen that the flat photodetector responded as directional, whereas the sphere's responsivity pattern showed omnidirectional detection angle. Inelastic scattering, i.e. fluorescence increases the response. Elastic scattering by the LC also increases the response. Dye in LC has decreased response. Different solutions of laser dye in LC need to be studied for optimum performance. The results indicate higher performance for spherically shaped silicon photodetectors and these results can be applied to optics of highly scattering media.
Düzlemsel genişlemeli akış içinde hidrodinamik tuzaklanmış mikrodamlaların ışıması ve çözülmesi
Dissolution of one phase into an immiscible phase is of great importance to food and drug industry. Dissolution process is mainly affected by diffusion coefficient, solubility, density and viscosity of the dissolving material in the immiscible phase. This thesis reports dye lasing in hydrodynamically manipulated microdroplets and proposes the hydrodynamic trap as a new method to observe dissolution of microdroplets. In the first part of this thesis, utilizing previously available hydrodynamic trapping method, we trap and manipulate the position of a dye doped liquid microdroplets in a microfluidic chip. By manipulating the position of the trapped droplets along the outlet channels of the microfluidic chip, we show lasing can be achieved for different positions of the droplet. When a trapped microdroplet is excited at a fixed position, lasing modes in the consecutive spectra of the droplet show a blue shift for all modes. The shift in modes is an indicator of the dissolution. Therefore, the next part focuses on dissolution of liquid microdroplets. Within a microfluidic chip, a trapped liquid microdroplet is exposed to planar extensional flow which increases the rate of the mass transfer out of the microdroplet. The static dissolution, that is diffusion, of droplets is modelled by Epstein and Plesset. However, the hydrodynamic trap induces flow around a trapped particle, therefore besides diffusion, the effect of convection should also be considered. The change in particle's radius as a function of time in the presence flow is numerically investigated by Zhang et al. To fit the experimental data we use the model provided by Zhang. The liquids used in the microdroplet dissolution experiments were n-octanol, n-decanol, undecanol and benzyl benzoate. Benzyl Benzoate and n-octanol showed good agreement with the Zhang model. In addition, experiments revealed that concentration of surfactant is an important parameter affecting dissolution rate which requires further investigation.
X-bant aralığında çalışan 2 boyutlu altıgen fotonik kristaller
Photonic crystals have been a hot area of scientific and technological research, since their proposal in 1987. Photonic crystals are generally known as the "semiconductors" of photonics, and found applications in photonic circuits, optical imaging, and telecommunications. The scope of this research is to study microwave transmission response of 2D hexagonal photonic crystals of graphite rods. We used graphite rods to fabricate a photonic crystal structure and stacked the rods together to form a hexagonal lattice. In order to characterize the structure, we measured the transmission of the photonic crystal in the microwave X-band. No structure is observed in the transverse electric (TE) polarization transmission spectrum, as the graphite rods absorb more efficiently for that geometry. A photonic band gap with a width of 0.5 GHz is observed for transverse magnetic (TM) polarization at 10.8 GHz. As the angle of incidence is scanned, we observed the shifting of the bandgap frequency. A similar bandgap frequency was observed at 00 and 600 coinciding with the symmetry of the photonic crystal. We induced defect states in the photonic crystal, and observed a mode in the band gap, for a point defect. As Maxwell's equations are scalable, we can scale down our hexagonal photonic crystal to operate in micro to nano length scales. Our results show that, 2D graphite photonic crystals can be useful in microwave engineering.
Laser taramalı konfokal mikroskobu denetleyicisinin açık tasarım yaklaşımı ile donanımsal/yazılımsal olarak tasarlanması ve uygulanması
Laser scanning confocal microscope (LSCM) is a powerful electro-optic instrument in biological imaging and material science, compared to traditional wide-field microscopy methods. This stems from the fact that confocal microscopy enables optical imaging with a better spatial resolution. However, high cost and complexity of commercially available confocal systems hinder their wider usage. In this thesis, a new LSCM hardware/software is studied and developed using an open design approach. This device is made more convenient by adapting low cost design techniques, widely adapted components and well supported open source software. For this reason, a CDAQ (Control and Data Acquisition) unit is designed and implemented to perform essential electrical input/output (I/O) operations. This unit consists of a 2-channel 16-bit ultralow glitch Digital to Analog Converter (DAC) for driving the galvanometer scanning mirrors, and interference reduction circuitry for a 3-channel on-board 12-bit Analog to Digital Converter (ADC) which is used for sampling photomultiplier tube and motor position feedback signals. These I/O peripherals are controlled by an ARM CortexM based microcontroller which runs a multithreaded firmware to accomplish given tasks. Components of the CDAQ are all pluggable I/O interfaces and they are open to modification for the desired application. Along with these operations, CDAQ provides power supply and gain control units. CDAQ unit is controlled as a device from the user interface named "Konfokal" which is developed using Python programming language along with PyQt framework. "Konfokal" is an open source software which aims at increasing the productivity of researchers by providing environment where LSCM image acquisition and modification can be done under one single program. Also, the opportunity to tailor or modify the source for the application is possible. Capabilities of this complete working device are shown with acquired images of test samples and it is offered as a more convenient and modern device for use of researchers. "Konfokal" program was also extended in order to incorporate a Digital Micromirror Device (DMD) as a scanning unit in place of the galvanometer scanning mirrors.
Elmas mikroyuvarın elastik ışık saçılması, fotoışıma ve Raman izge ölçümleri
Over the past decades, morphology dependent resonances, the optical resonances occurring by the interaction of light with microspheres of various materials, were vastly investigated. Spherical microcavities have diverse applications due to their high concentration of modes in small volumes at optical frequencies and high Q factors. The localization of the electromagnetic waves inside the microsphere is realized by total internal reflection. Microspheres provide the concave surface for light to circumnavigate around the microcavity, giving rise to whispering gallery modes. Because of its unique lattice structure, robust physical properties, as well as its distinctive optical properties, different forms of diamond were vastly investigated for various applications. As is well known, nitrogen vacancy (NV) centers inside the diamond lattice give rise to such applications by replacing two carbon atoms by one nitrogen atom and a vacancy, that yields a floating electron at the NV- center. In this thesis, Raman and photoluminescence spectroscopy is performed with various diamond samples including a diamond microsphere. The measurement setup is presented for both spectroscopies. NV centers exhibit 575 nm or 637 nm zero phonon lines along with their corresponding phonon side bands depending on the charge state of the point defect, NV0 and NV-, respectively. A 572 nm inelastic Raman scattering (1332.25 cm-1) is observed with 532 nm excitation laser due to the vibrational state of carbon-carbon bonds with sp3 configuration, whereas sp2 hybridization (1444.56 cm-1) yields inelastically scattered photons with 576 nm wavelength. It is possible to identify the main impurities within the diamond lattice, and confirm the carbon-carbon bonds with spectroscopic techniques. In addition, the demonstration of a diamond microsphere whispering gallery modes resonator in the standard telecom wavelengths between 1426.10 nm and 1427.42 nm in the 90° elastic light scattering for both transverse magnetic and transverse electric polarizations is realized. The highest measured whispering gallery mode Q-factor is on the order of 104, and the mode spacing 0.332 nm for both transverse polarizations. The coupling of the continuous wave tunable infrared excitation laser to the diamond microsphere is achieved by a single mode silica optical fiber half coupler. All in all, diamond is a unique material with particular optical properties. Utilizing such properties in a spherical morphology can give rise to distinctive applications. It is essential to observe the aspects of the whispering gallery modes created by the light circumnavigating a diamond microsphere such as mode spacing and Q-factor to classify the properties of a 1 mm synthetically grown spherical diamond microresonator. Such a diamond resonator can further be used as stable optical frequency comb generators or lasing microcavities by exploiting the nitrogen vacancy centers present within the lattice of the diamond.
Silisyum ve elmas mikroyuvarların fısıldayan dalga kiplerinin femtosaniye lazer ile yazılmış cam ve elmas dalga kılavuzları ve cam fiber bağlayıcı ile uyarılması
Whispering gallery modes (WGMs) are the optical resonances, where light is trapped inside a dielectric and symmetric cavity by total internal reflection (TIR) on the boundary facets. Due to the unique optical properties, such as very high quality factors, and very narrow linewidths, due to the long photon lifetime inside the cavity, and confinement of the field in very small mode volumes, resulting in high power densities, cavities of different geometries, supporting WGMs, have been extensively studied for more than four decades now. However, the efficient coupling to WGMs mostly rely on experimentally challenging schemes including prism coupling, optical fiber half coupling (OFHC) or free space coupling. This thesis consists of the excitation of silicon microsphere WGMs using an alternative coupling method: shallow waveguides fabricated by femtosecond laser inscription. The evanescent field coupling from the guided modes of the femtosecond micromachined glass and diamond waveguides has been used to excite microsphere WGMs. Furthermore, this thesis includes the excitation of a diamond sphere using an OFHC. Employment of diamond material in photonic applications is substantial, because among all materials, diamond exhibits unique optical properties such as its color centers, large band-gap, high Raman gain, as well as a unique crystal lattice, and robust physical properties. Utilizing diamond as an excitation waveguide material or the cavity material opens the way for combination of both diamond's and WGMs' unique properties for applications in electronics, quantum information processing, and sensing.
Femtosaniye lazer ile yazılmış sığ optik cam ve elmas dalgakılavuzlarına bağlaştırılmış silisyum ve elmas mikroyuvarların esnek saçılması
Elastic light scattering of silicon and diamond microspheres excited by femtosecond (fs)-laser written glass and diamond waveguides is studied in order to assemble an all diamond optical system towards integrated diamond photonics applications. Transverse electrically (TE) and transverse magnetically (TM) polarized light from a wavelength tunable laser operating in the near-infrared region is coupled to a 1 mm silicon sphere by using the Gorilla and Eagle2000™ glass optical waveguides. Silicon sphere elastic scattering characteristics are gathered on both glass and diamond optical waveguides, in order to examine diamond optical waveguide characteristics. After which, an all-diamond optical system is established with diamond sphere on fs-laser written diamond shallow waveguide. The assembled diamond system integrates a Type-Ib (nitrogen impurity > 5 ppm) diamond microsphere with a fs-laser written Type-IIa (nitrogen impurity of 100 ppb) diamond waveguide. The diamond waveguide is fabricated by exploiting type II fabrication method to achieve stress induced waveguiding. TE and TM polarized light from a wavelength tunable laser operating in the near-infrared region is coupled to a 1 mm diamond sphere by using the diamond optical waveguide. By carefully engineering the microsphere's high quality factor resonances, and further exploiting the nonlinear properties of existing nitrogen-vacancy (NV) centers in diamond microspheres and/or diamond waveguides in such configurations, it is possible to realize various applications in integrated diamond photonics.