Temporal evolution of photorefractive double phase-conjugate mirrors

We present wave-optics calculations of the temporal and spatial evolution from random noise of a double phase-conjugate mirror in photorefractive media that show its image exchange and phase-reversal properties. The calculations show that for values of coupling coefficient times length greater than two the process exhibits excellent conjugation fidelity, behaves as an oscillator, and continues to operate even when the noise required for starting it is set to zero. For values less than two, the double phase-conjugation process exhibits poor fidelity and disappears when the noise is set to zero

Double phase conjugation

We model the double phase-conjugate mirror (DPCM) as a function of time, the average direction of propagation of the two beams forming the DPCM, and one transverse coordinate. Calculations show that the conjugation fidelity and reflectivity have different dependencies on the photorefractive coupling coefficient times length; the fidelity turns on abruptly with a threshold, whereas the reflectivity increases smoothly. The DPCM behaves as an oscillator at and above threshold: the time required for the reflectivity to reach the steady state dramatically slows down near threshold (like critical slowing down in lasers); above threshold the DPCM is self-sustaining even if the random noise terms used to start the process are set to zero. A decrease in the noise level improves the fidelity but increases the response time. The use of unbalanced input beam ratios results in asymmetric conjugation such that the fidelity obtained on the side of the weaker input beam is significantly reduced. The slowing down diminishes with increasing noise level or unbalanced input intensities

Tios ve çevresinde kültler

Antik Tios kenti, Zonguldak İli, Çaycuma İlçesi’ne bağlı Filyos Beldesi sınırları içerisinde yer almaktadır. Tez kapsamında, Tios kökenli ya da yakın çevresinden ele geçen arkeolojik ve epigrafik bulgular, bölgesel ölçekte değerlendirilerek MÖ 7. yüzyıldan MS 4. yüzyıla, Roma Dönemi’nin sonlarına kadar Tios kentinde kabul görmüş olan, göçler ve kültürel ilişkiler ile bölgeye dışarıdan gelen, çevre kültür bölgelerine özgü kültler ve yerel kültler ele alınmıştır. Son yıllarda Tios’da açığa çıkartılan Akropolis Tapınağı, sikkeler, kurşun ağırlıklar üzerindeki tanrısal betimlemeler, Tios ve yakın çevresinden yazıtlar kentteki kültler ile ilgili ipuçları sunmaktadır. Buna göre Tios ve yakın çevresinde Grek tanrı ve tanrıçaları, yerel tanrı ve tanrıçalar, Frig, Suriye ve Mısır kökenli tanrı ve tanrıçalar ile Dea Roma (Roma Tanrıçası) kültünün tapınım gördüğü anlaşılmaktadır.The ancient city of Tios is located within the boundaries of the town of Filyos, in the Çaycuma district of Zonguldak. In this thesis, archaeological and epigraphical findings recovered from or near Tios were evaluated on a regional scale and the local and surrounding regional cults which came from outside the region with migrations and cultural relations, accepted in Tios from the 7 th century BC to the 4t century BC, until end of the Roman Period were discussed. In the recent years, the Temple of Acropolis, which was unearthed in Tios, the divine descriptions on coins, lead weights, inscriptions from Tios and its close surroundings provide important clues about the cults in the city. According to these findings, Greek gods and goddesses, local gods and goddesses, gods/goddesses of Phrygian, Syrian and Egyptian origin and the Cult of Dea Roma (Roman Goddess) are seen to have been worshiped

Fixing of photorefractive volume holograms in K1-yLiyTa1-xO3

We report the f ixing of photorefractive holographic gratings with high eff iciency in a sample of K1-yLiyTa1-xNbxO3 doped with Cu, V, and Ti. Holograms are thermally fixed through the screening of a photorefractive space-charge field by a nonphotoactive species at elevated temperatures. Fixed holograms are revealed by illumination at lower temperatures. Diffraction efficiencies of 25% in a 0.54-cm-thick sample are measured. Holograms undergo thermal decay with a 0.67-eV activation energy

Self-trapping of optical beams in photorefractive media

We study the possibility of self-trapping of an optical beam in a photorefractive medium under the combined influence of diffraction and self-scattering (two-wave mixing) of its spatial frequency components. We investigate the spectrum of solutions for the resulting photorefractive spatial solitons and discuss their unique properties. Design considerations and material requirements for experimental realization of these solitons, together with specific examples, are given

Temporal evolution of fanning in photorefractive materials

We present detailed calculations of the temporal and spatial evolution of beam fanning in photorefractive crystals that is initiated by scattering from noise. We show that fanning starts from beam coupling between the incident radiation and part of the incident radiation scattered by noise at or near the input plane. We show that scattering within the volume of the crystal has negligible effect on fanning, that absorption affects the time response but not the spatial pattern of the fanning, and that the difference between calculations including only phase-matched terms and those including non-phase-matched terms is negligible

High Energy, Narrow Linewidth 1572nm Eryb-Fiber Based MOPA for a Multi-Aperture CO2 Trace-Gas Laser Space Transmitter

Accurate global measurements of tropospheric CO2 mixing ratios are needed to study CO2 emissions and CO2 exchange with the land and oceans. NASA Goddard Space Flight Center (GSFC) is developing a pulsed lidar approach for an integrated path differential absorption (IPDA) lidar to allow global measurements of atmospheric CO2 column densities from space. Our group has developed, and successfully flown, an airborne pulsed lidar instrument that uses two tunable pulsed laser transmitters allowing simultaneous measurement of a single CO2 absorption line in the 1570 nm band, absorption of an O2 line pair in the oxygen A-band (765 nm), range, and atmospheric backscatter profiles in the same path. Both lasers are pulsed at 10 kHz, and the two absorption line regions are sampled at typically a 300 Hz rate. A space-based version of this lidar must have a much larger lidar power-area product due to the x40 longer range and faster along track velocity compared to airborne instrument. Initial link budget analysis indicated that for a 400 km orbit, a 1.5 m diameter telescope and a 10 second integration time, a 2 mJ laser energy is required to attain the precision needed for each measurement. To meet this energy requirement, we have pursued parallel power scaling efforts to enable space-based lidar measurement of CO2 concentrations. These included a multiple aperture approach consists of multi-element large mode area fiber amplifiers and a single-aperture approach consists of a multi-pass Er:Yb:Phosphate glass based planar waveguide amplifier (PWA). In this paper we will present our laser amplifier design approaches and preliminary results

Fiber-Based Laser Transmitter at 1.57 Micrometers for Remote Sensing of Atmospheric Carbon Dioxide from Satellites

Over the past 20 years, NASA Goddard has successfully developed space-based lidar for remote sensing studies of the Earth and planets. The lidar in all missions to date have used diode pumped Nd:YAG laser transmitters. Recently we have been concentrating work on developing integrated path differential absorption (IPDA) lidar to measure greenhouse gases, with the goal of measurements from space. Due to the absorption spectrum of CO2 a fiber-based master oscillator power amplifier (MOPA) laser with a tunable seed source is an attractive laser choice. Fiber-based lasers offer a number of potential advantages for space, but since they are relatively new, challenges exist in developing them. In order to reduce risks for new missions using fiber-based lasers, we developed a 30- month plan to mature the technology of a candidate laser transmitter for space-based CO2 measurements to TRL-6. This work is also intended to reduce development time and costs and increase confidence in future mission success

Nonlinear spatial dynamics of double phase conjugation in photorefractive crystals and holographic dynamics of photopolymerization

This thesis explores spatial nonlinear optical effects in photorefractive crystals and photopolymers. In these materials upon exposure with spatially varying light, large refractive index changes occur. In the first part of the thesis Double Phase Conjugation in photorefractive crystals is studied both theoretically and experimentally. Various processes effecting the conjugation fidelity, such as fanning are quantified through a coupled multiple mode model which is an extension of the coupled mode theory. Predictions of the model such as phase conjugation is confirmed experimentally. Critical slowing down near the threshold is also predicted and experimentally confirmed. Lastly the amplitude equation formalism is carried out for the wave mixing phenomenon. This approach unifies the optical phenomenon with a large class of other physical phenomena referred to as pattern formation outside of equilibrium. Through this formalism the instability is identified as a convective instability and the possibility of a transition to absolute instability is studied. In the second part of the thesis, photopolymerization dynamics is studied with holography. A theoretical model is developed for the holographic configuration starting from a standard chain polymerization model. A holographic characterization method is developed. The method is especially powerful in measuring diffusion constants. Various multifunctional polymers is characterized using the novel technique