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- Title
- APPLICATIONS OF LINEAR AND NONLINEAR OPTICAL EFFECTS IN LIQUID CRYSTALS.
- Creator
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Sarkissian, Hakob, Zeldovich, Boris, University of Central Florida
- Abstract / Description
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Liquid crystals have been a major subject of research for the past decades. Aside from the variety of structures they can form, they exhibit a vast range of optical phenomena. Many of these phenomena found applications in technology and became an essential part of it. In this dissertation thesis we continue the line to propose a number of new applications of optical effects in liquid crystals and develop their theoretical framework. One such application is the possibility of beam combining...
Show moreLiquid crystals have been a major subject of research for the past decades. Aside from the variety of structures they can form, they exhibit a vast range of optical phenomena. Many of these phenomena found applications in technology and became an essential part of it. In this dissertation thesis we continue the line to propose a number of new applications of optical effects in liquid crystals and develop their theoretical framework. One such application is the possibility of beam combining using Orientational Stimulated Scattering in a nematic liquid crystal cell. Our numerical study of the OSS process shows that normally this possibility does not exist. However, we found that if a number of special conditions is satisfied efficient beam combining with OSS can be done. These conditions require a combination of special geometric arrangement of incident beams, their profiles, nematic material, and more. When these conditions are fulfilled, power of the beamlets can be coherently combined into a single beam, with high conversion efficiency while the shape and wave-front of the output beam are still of good quality. We also studied the dynamics of the OSS process itself and observed (in a numerical model) a number of notorious instabilities caused by effects of back-conversion iv process. Additionally, there was found a numerical solitary-wave solution associated with this back-conversion process. As a liquid crystal display application, we consider a nematic liquid crystal layer with the anisotropy axis modulated at a fixed rate in the transverse direction with respect to light propagation direction. If the layer locally constitutes a half-wave plate, then the thinscreen approximation predicts 100% -efficient diffraction of normal incident wave. If this diffracted light is blocked by an aperture only transmitting the zero-th order, the cell is in dark state. If now the periodic structure is washed out by applying voltage across the cell and light passes through the cell undiffracted, the light will pass through the aperture as well and the cell will be in its bright state. Such properties of this periodically aligned nematic layer suggest it as a candidate element in projection display cells. We studied the possibility to implement such layer through anchoring at both surfaces of the cell. It was found that each cell has a thickness threshold for which the periodic structure can exist. The anchored periodic structure cannot exist if thickness of the cell exceeds this threshold. For the case when the periodic structure exists, we found the structure distortion in comparison with the preferable ideal sinusoidal profile. To complete description of the electromechanical properties of the periodic cell, we studied its behavior at Freedericksz transition. Optical performance was successfully described with the coupled-mode theory. While influence of director distortion is shown to be negligibly small, the walk-off effects appear to be larger. In summary, there are good prospects for use of this periodically v aligned cell as a pixel in projection displays but experimental study and optimization need to be performed. In the next part we discuss another modulated liquid crystal structure in which the director periodically swings in the direction of light propagation. The main characteristic of such structure is the presence of bandgap. Cholesteric liquid crystals are known to possess bandgap for one of two circular polarizations of light. However, unlike the cholesterics the bandgap of the proposed structure is independent of polarization of normally incident light. This means that no preparation of light is needed in order for the structure to work in, for example, liquid crystal displays. The polarization universality comes at the cost of bandgap size, whose maximum possible value ∆ωPTN compared to that of cholesterics ∆ωCh is approximately twice smaller: ∆ωPTN ≈ 0.58∆ωCh if modulation profile is sinusoidal, and ∆ωPTN ≈ 0.64∆ωCh if it is rectangular. This structure has not yet been experimentally demonstrated, and we discuss possible ways to make it.
Show less - Date Issued
- 2006
- Identifier
- CFE0001164, ucf:46856
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0001164
- Title
- PROPAGATION EFFECT IN INHOMOGENEOUS MEDIA, INCLUDING MEDIA WITH LIGHT-INDUCED AND FIXED GRATINGS.
- Creator
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TSAI, CHANG-CHING, Zeldovich, Boris, University of Central Florida
- Abstract / Description
-
Optical waves propagation in various types of volume gratings, materials with constant impendence and optical fibers are studied. Instability of crossphase modulation and of Energy transfer via GRON-type (Grating-type Orientational Nonlinearity in Liquid Crystal) Stimulated Scattering is numerically observed. Two diffractive optical elements made of volume gratings are suggested and analyzed. A transmission hologram based on the analogy with Stimulated Raman Adiabatic Passage (STIRAP)...
Show moreOptical waves propagation in various types of volume gratings, materials with constant impendence and optical fibers are studied. Instability of crossphase modulation and of Energy transfer via GRON-type (Grating-type Orientational Nonlinearity in Liquid Crystal) Stimulated Scattering is numerically observed. Two diffractive optical elements made of volume gratings are suggested and analyzed. A transmission hologram based on the analogy with Stimulated Raman Adiabatic Passage (STIRAP) in nonlinear optics is proposed. This transmission hologram demonstrates high diffraction efficiency and low sensitivity to polarization and hologram strength. The other is a reflection hologram with two crossed-gratings. It features good angular selectivity in comparison with the poor angular selectivity of conventional Bragg grating mirror. This defense also contains the approximation of Maxwell equations for the description of depolarized light sources and polarization-insensitive detectors. A scalar wave equation, Z-Helmholtz equation, is proposed and discussed in the approximation of constant impedance media. As examples, this equation successfully describes a) Fresnel transmission coefficient, and b) Goose-Hanschen shift in total internal reflection, for depolarized incident light and, at the same time, polarization-insensitive detectors. Evolution of polarization during light propagation in an inhomogeneous locally isotropic medium, and also in a single-mode fiber is described by Rytov's non-rotation equation. With arbitrary chosen real unit vector, the complete description of polarization change can be described in a single rotation angle obtained from the integral of rotation rate. Based on introduction of this reference frame, a device is suggested as rigid body's rotation sensor due to polarization change in a twisted fiber.
Show less - Date Issued
- 2006
- Identifier
- CFE0001198, ucf:46861
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0001198
- Title
- THEORETICAL STUDY OF BEAM TRANSFORMATIONS BY VOLUME DIFFRACTION.
- Creator
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Mokhov, Sergiy, Zeldovich, Boris, University of Central Florida
- Abstract / Description
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Laser beams can be manipulated by volume diffractive elements in addition to conventional optical elements like mirrors, lenses, and beam splitters. Conventional optical elements can be described by applying the basic laws of reflection and refraction at the surfaces of the elements. Even diffraction by surface gratings utilizes relatively simple mathematics. This is to be contrasted with the volume diffraction, which requires coupled wave theory in the slowly varying envelope approximation ...
Show moreLaser beams can be manipulated by volume diffractive elements in addition to conventional optical elements like mirrors, lenses, and beam splitters. Conventional optical elements can be described by applying the basic laws of reflection and refraction at the surfaces of the elements. Even diffraction by surface gratings utilizes relatively simple mathematics. This is to be contrasted with the volume diffraction, which requires coupled wave theory in the slowly varying envelope approximation (SVEA) to obtain accurate results. Efficient spatially distributed diffraction of laser beams is possible due to the high coherence of laser light, and it occurs at specific resonant Bragg conditions. This research work is inspired and driven by the successful development of recording technology for robust, high-efficiency volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass. Mostly VBGs of the reflective type are discussed in this dissertation. Starting with an analysis of electro-magnetic wave propagation in layered media, we have reformulated Fresnel and volume reflection phenomena in terms of a convenient parameter S - strength of reflection. The influence that the different non-uniformities inside a VBG have on its spectral properties has been examined. One important result of this work is the proposal of moire VBG and the derivation of an analytical expression for its bandwidth. A multiplexed VBG used as a coherent combiner is discussed as well. Beam distortion via transmission through and/or reflection by a heated VBG due to residual absorption is analyzed.
Show less - Date Issued
- 2011
- Identifier
- CFE0004054, ucf:49152
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004054
- Title
- DENSE SPECTRAL BEAM COMBINING WITH VOLUME BRAGG GRATINGS IN PHOTO-THERMO-REFRACTIVE GLASS.
- Creator
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Andrusyak, Oleksiy, Zeldovich, Boris, University of Central Florida
- Abstract / Description
-
Beam combining techniques have become an important tool in the design of high-power high-brightness laser systems. Spectral beam combining (SBC) is an incoherent combining technique that does not require phase control of sources, allowing for a stable and robust system. Using SBC, beams from an array of lasers with each element operated at a different wavelength are combined into a single near-diffraction-limited beam with the same aperture using dispersive optical elements. SBC by means of...
Show moreBeam combining techniques have become an important tool in the design of high-power high-brightness laser systems. Spectral beam combining (SBC) is an incoherent combining technique that does not require phase control of sources, allowing for a stable and robust system. Using SBC, beams from an array of lasers with each element operated at a different wavelength are combined into a single near-diffraction-limited beam with the same aperture using dispersive optical elements. SBC by means of volume Bragg gratings (VBGs) utilizes unique spectral response of VBGs: diffraction efficiency is close to unity when the Bragg condition is satisfied and is close to zero at multiple points corresponding to particular wavelength offsets from Bragg condition. High-efficiency VBGs can be recorded in UV-sensitive photo-thermo-refractive (PTR) glass. Narrow-band reflecting VBGs allow multi-channel SBC with high spectral density of channels. In this dissertation, experimental results of SBC with high spectral density of combined channels in two spectral regions of interest (1064 and 1550 nm) are reported. The behavior of narrow-band VBGs under high-power laser radiation is investigated. A laser system with kW-level output power and near-diffraction-limited divergence of spectrally-combined output beam is demonstrated. The system combines five randomly-polarized Yb-doped fiber lasers with 0.5 nm spectral separation in central wavelengths using narrow-band reflecting VBGs with absolute efficiency of combining > 90%. A novel design of a multi-channel high-power SBC system is suggested. In this approach, a common-cavity is created for all channels such that wavelengths of the sources are passively controlled by the combination of a common output coupler and intra-cavity VBGs which also act as combining elements. Laser wavelengths are automatically selected to match resonant wavelengths of respective VBGs. We report successful demonstration of a passively-controlled SBC system consisting of two amplifiers in a common cavity configuration. A compact and rugged monolithic SBC module based on multiplexed VBGs is introduced. Experimental results of a four-channel implementation of such module are discussed. Modular design of high-power laser systems is suggested with multiple modules arranged in a series. We show that with basic combining parameters achieved up to date, laser systems with 10 kW output power can be constructed using this arrangement. Further scaling to 100 kW power level is discussed.
Show less - Date Issued
- 2009
- Identifier
- CFE0002662, ucf:48189
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0002662
- Title
- Image degradation due to surface scattering in the presence of aberrations.
- Creator
-
Choi, Narak, Harvey, James, Zeldovich, Boris, Moharam, M., Eastes, Richard, University of Central Florida
- Abstract / Description
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This dissertation focuses on the scattering phenomena by well-polished optical mirror surfaces. Specifically, predicting image degradation by surface scatter from rough mirror surfaces for a two-mirror telescope operating at extremely short wavelengths (9nm~30nm) is performed. To evaluate image quality, surface scatter is predicted from the surface metrology data and the point spread function in the presence of both surface scatter and aberrations is calculated.For predicting the scattering...
Show moreThis dissertation focuses on the scattering phenomena by well-polished optical mirror surfaces. Specifically, predicting image degradation by surface scatter from rough mirror surfaces for a two-mirror telescope operating at extremely short wavelengths (9nm~30nm) is performed. To evaluate image quality, surface scatter is predicted from the surface metrology data and the point spread function in the presence of both surface scatter and aberrations is calculated.For predicting the scattering intensity distribution, both numerical and analytic methods are considered. Among the numerous analytic methods, the small perturbation method (classical Rayleigh-Rice surface scatter theory), the Kirchhoff approximation method (classical Beckman-Kirchhoff surface scatter theory), and the generalized Harvey-Shack surface scatter theory are adopted. As a numerical method, the integral equation method (method of moments) known as a rigorous solution is discussed. Since the numerical method is computationally too intensive to obtain the scattering prediction directly for the two mirror telescope, it is used for validating the three analytic approximate methods in special cases. In our numerical comparison work, among the three approximate methods, the generalized Harvey-Shack model shows excellent agreement to the rigorous solution and it is used to predict surface scattering from the mirror surfaces.Regarding image degradation due to surface scatter in the presence of aberrations, it is shown that the composite point spread function is obtained in explicit form in terms of convolutions of the geometrical point spread function and scaled bidirectional scattering distribution functions of the individual surfaces of the imaging system. The approximations and assumptions in this formulation are discussed. The result is compared to the irradiance distribution obtained using commercial non-sequential ray tracing software for the case of a two-mirror telescope operating at the extreme ultra-violet wavelengths and the two results are virtually identical. Finally, the image degradation due to the surface scatter from the mirror surfaces and the aberration of the telescope is evaluated in terms of the fractional ensquared energy (for different wavelengths and field angles) which is commonly used as an image quality requirement on many NASA astronomy programs.
Show less - Date Issued
- 2012
- Identifier
- CFE0004289, ucf:49492
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004289
- Title
- High Performance Liquid Crystals for Displays and Spatial Light Modulators.
- Creator
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Peng, Fenglin, Wu, Shintson, Moharam, Jim, Zeldovich, Boris, Fang, Jiyu, University of Central Florida
- Abstract / Description
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Liquid crystals (LCs) are an amazing class of soft materials which have been widely used in the visible, infrared (IR), millimeter wave, and terahertz spectral regions. Both amplitude modulation (e.g. displays) and phase modulation (e.g. spatial light modulators (SLMs) for adaptive optics and adaptive lens) have been investigated extensively. Thin-film-transistor liquid crystal displays (TFT-LCDs) have become ubiquitous in our daily lives. Its widespread applications span from TVs, monitors,...
Show moreLiquid crystals (LCs) are an amazing class of soft materials which have been widely used in the visible, infrared (IR), millimeter wave, and terahertz spectral regions. Both amplitude modulation (e.g. displays) and phase modulation (e.g. spatial light modulators (SLMs) for adaptive optics and adaptive lens) have been investigated extensively. Thin-film-transistor liquid crystal displays (TFT-LCDs) have become ubiquitous in our daily lives. Its widespread applications span from TVs, monitors, tablets, smartphones, augmented reality, virtual reality, to vehicle displays. LCD shows advantages in 1) high resolution, 2) long lifetime, 3) vivid colors using quantum dots backlight, and 4) high dynamic contrast ratio employing local dimming technology. However, LCD exhibits a serious problem, which is slow response time. Therefore, it is commonly perceived that LCD exhibits a more severe image blur than organic light emitting diode (OLED) displays. Indeed, the response time of LCD is ~100x slower than that of OLED. To evaluate image blurs, Motion Picture Response Time (MPRT) has been proposed to quantify the visual performance of a moving object. MPRT is jointly governed by three factors: the sample and hold effect of an active matrix display, motion pursuing, and human vision system. It is a complicated problem and is difficult to obtain analytical solution. In this thesis, we analyze the sample-and-hold effects and derive a simple equation to correlate MPRT with LC response time, TFT frame rate, and duty ratio. From our analytical equation, we find that as long as an LCD's response time is less than 2 ms, its MPRT would be comparable to that of OLED at the same frame rate, even if the OLED's response time is assumed to be zero. To further reduce MPRT, we could boost the frame rate to 144 Hz or reduce the duty ratio through backlight modulation. This discovery sheds new physical insights for LCDs to achieve CRT-like displays with negligible image blurs. In addition to displays, LCs are widely employed in SLMs for modulating the phase and polarization of an incident light. This is because LCs possess high birefringence and relatively low absorption from the visible, IR, to terahertz regions. The useful applications include adaptive lens, adaptive optics, fiber-optic communication, antenna, and phase shifter. Fast response time is a common requirement for the abovementioned photonic devices. To achieve fast response time while maintaining 2-pi phase change, polymer-stabilized blue phase liquid crystal (BPLC) and polymer-network liquid crystal (PNLC) are promising candidates for the visible and IR SLMs, respectively. However, the operation voltage of present BPLC and PNLC devices is too high. To reduce operation voltage while keeping fast response time, we developed a new device configuration for BPLC SLM to work in the visible region. The new device structure allows the incident laser beam to traverse the BPLC layer four times before exiting the reflective SLM. As a result, the 2-pi phase change voltage is reduced to below 24V, which is the maximum attainable voltage for a high resolution liquid-crystal-on-silicon device. On the other hand, PNLC is a better candidate for the IR SLM because several high birefringence LC materials can be used. To reduce the operation voltage of a PNLC, we have investigated following three approaches: 1) developing large dielectric anisotropy and high birefringence (?n) LC materials, 2) optimizing polymer concentration, and 3) optimizing UV curing conditions. In the visible and near IR regions, most LCs are highly transparent. However, to extend the electro-optic application of LCs into MWIR and LWIR, absorption loss becomes a critical issue. In the MWIR region, several fundamental molecular vibration bands and overtones exist, which contribute to high absorption loss. The absorbed light turns to heat and then alters the birefringence locally, which in turns causes spatially non-uniform phase modulation. To suppress the optical loss, we have taken following approaches: (1) Designing high birefringence to minimize the LC layer thickness; (2) Shifting the absorption bands outside the spectral region of interest by deuteration, fluorination, or chlorination; (3) Reducing the overtone absorption by using a short alkyl chain. As a result, we have developed several low loss and high birefringence chlorinated LCs for the first time. To achieve fast response time, we demonstrated a PNLC with 2-pi phase change at MWIR and response time less than 5 ms. Molecular tailoring strategies for extending liquid crystal SLM into long-wavelength infrared (LWIR) are also explored.
Show less - Date Issued
- 2017
- Identifier
- CFE0006636, ucf:51230
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0006636
- Title
- Metrology of Volume Chirped Bragg Gratings Recorded in Photo-Thermo-Refractive Glass for Ultrashort Pulse Stretching and Compressing.
- Creator
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Lantigua, Christopher, Glebov, Leonid, Zeldovich, Boris, Schulzgen, Axel, University of Central Florida
- Abstract / Description
-
Chirped Bragg gratings (CBGs) recorded in photo-thermo-refractive (PTR) glass provide a very efficient and robust way to stretch and compress ultra-short laser pulses. These gratings offer the ability to stretch pulses from hundreds of femtoseconds, to the order of 1 ns and then re-compress them. However, in order to achieve pulse stretching of this magnitude, 100 mm thick CBGs are needed. Using these CBGs to both stretch, and re-compress the pulse thus requires propagation through 200 mm of...
Show moreChirped Bragg gratings (CBGs) recorded in photo-thermo-refractive (PTR) glass provide a very efficient and robust way to stretch and compress ultra-short laser pulses. These gratings offer the ability to stretch pulses from hundreds of femtoseconds, to the order of 1 ns and then re-compress them. However, in order to achieve pulse stretching of this magnitude, 100 mm thick CBGs are needed. Using these CBGs to both stretch, and re-compress the pulse thus requires propagation through 200 mm of optical glass. This therefore demands perfect control of the glass homogeneity, as well as the holographic recording process of the CBG. In this thesis, we present a study of the CBG parameters that lead to distortions in the quality of diffracted beams. We first present the challenges associated with measuring the quality of these beams and we show that such measurements are not easily achieved using commercial systems that rely on the ISO standard M2 method. Thus, we introduce a new metric of beam quality, which we have coined S2, that is a combination of both the M2 and power in the bucket metrics. Subsequently, we investigate the influence of the CBG parameters on the quality of diffracted beams. In particular, we examine the impact of small optical heterogeneities known as striae, as well as the impact of the optically and thermally induced distortions in the grating. We then use this data to improve the fabrication and characterization of 100 mm long CBGs.Finally, we characterize the performance of CBGs recorded in PTR for stretching and compression of femtosecond pulses using a custom autocorrelation system. We present data on high quality 100 mm long CBGs and an analysis on the correlation between beam quality and the final pulse duration after stretching and re-compressing the pulse.
Show less - Date Issued
- 2013
- Identifier
- CFE0004876, ucf:49680
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004876
- Title
- High-efficiency Blue Phase Liquid Crystal Displays.
- Creator
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Li, Yan, Wu, Shintson, Saleh, Bahaa, Zeldovich, Boris, Wu, Xinzhang, University of Central Florida
- Abstract / Description
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Blue phase liquid crystals (BPLCs) have a delicate lattice structure existing between chiral nematic and isotropic phases, with a stable temperature range of about 2 K. But due to short coherent length, these self-assembled nano-structured BPLCs have a fast response time. In the past three decades, the application of BPLC has been rather limited because of its narrow temperature range. In 2002, Kikuchi et al. developed a polymer stabilization method to extend the blue-phase temperature range...
Show moreBlue phase liquid crystals (BPLCs) have a delicate lattice structure existing between chiral nematic and isotropic phases, with a stable temperature range of about 2 K. But due to short coherent length, these self-assembled nano-structured BPLCs have a fast response time. In the past three decades, the application of BPLC has been rather limited because of its narrow temperature range. In 2002, Kikuchi et al. developed a polymer stabilization method to extend the blue-phase temperature range to more than 60 K. This opens a new gateway for display and photonic applications.In this dissertation, I investigate the material properties of polymer-stabilized BPLCs. According the Gerber's model, the Kerr constant of a BPLC is linearly proportional to the dielectric anisotropy of the LC host. Therefore, in the frequency domain, the relaxation of the Kerr constant follows the same trend as the dielectric relaxation of the host LC. I have carried out experiments to validate the theoretical predictions, and proposed a model called extended Cole-Cole model to describe the relaxation of the Kerr constant. On the other hand, because of the linear relationship, the Kerr constant should have the same sign as the dielectric anisotropy of the LC host; that is, a positive or negative Kerr constant results from positive or negative host LCs, respectively. BPLCs with a positive Kerr constant have been studied extensively, but there has been no study on negative polymer-stabilized BPLCs. Therefore, I have prepared a BPLC mixture using a negative dielectric anisotropy LC host and investigated its electro-optic properties. I have demonstrated that indeed the induced birefringence and Kerr constant are of negative sign. Due to the fast response time of BPLCs, color sequential display is made possible without color breakup. By removing the spatial color filters, the optical efficiency and resolution density are both tripled. With other advantages such as alignment free and wide viewing angle, polymer-stabilized BPLC is emerging as a promising candidate for next-generation displays.However, the optical efficiency of the BPLC cell is relatively low and the operating voltage is quite high using conventional in-plane-switching electrodes. I have proposed several device structures for improving the optical efficiency of transmissive BPLC cells. Significant improvement in transmittance is achieved by using enhanced protrusion electrodes, and a 100% transmittance is achievable using complementary enhanced protrusion electrode structure.For a conventional transmissive blue phase LCD, although it has superb performances indoor, when exposed to strong sunlight the displayed images could be washed out, leading to a degraded contrast ratio and readability. To overcome the sunlight readability problem, a common approach is to adaptively boost the backlight intensity, but the tradeoff is in the increased power consumption. Here, I have proposed a transflective blue phase LCD where the backlight is turned on in dark surroundings while ambient light is used to illuminate the displayed images in bright surroundings. Therefore, a good contrast ratio is preserved even for a strong ambient. I have proposed two transflective blue phase LCD structures, both of which have single cell gap, single gamma driving, reasonably wide view angle, low power consumption, and high optical efficiency. Among all the 3D technologies, integral imaging is an attractive approach due to its high efficiency and real image depth. However, the optimum observation distance should be adjusted as the displayed image depth changes. This requires a fast focal length change of an adaptive lens array. BPLC adaptive lenses are a good candidate because of their intrinsic fast response time. I have proposed several BPLC lens structures which are polarization independent and exhibit a parabolic phase profile in addition to fast response time.To meet the low power consumption requirement set by Energy Star, high optical efficiency is among the top lists of next-generation LCDs. In this dissertation, I have demonstrated some new device structures for improving the optical efficiency of a polymer-stabilized BPLC transmissive display and proposed sunlight readable transflective blue-phase LCDs by utilizing ambient light to reduce the power consumption. Moreover, we have proposed several blue-phase LC adaptive lenses for high efficiency 3D displays.
Show less - Date Issued
- 2012
- Identifier
- CFE0004787, ucf:49725
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004787
- Title
- Theoretical Study of Laser Beam Quality and Pulse Shaping by Volume Bragg Gratings.
- Creator
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Kaim, Sergiy, Zeldovich, Boris, Flitsiyan, Elena, Leuenberger, Michael, Likamwa, Patrick, University of Central Florida
- Abstract / Description
-
The theory of stretching and compressing of short light pulses by the chirped volume Bragg gratings (CBG) is reviewed based on spectral decomposition of short pulses and on the wavelength-dependent coupled wave equations. The analytic theory of diffraction efficiency of a CBG with constant chirp and approximate theory of time delay dispersion are presented. Based on those, we performed comparison of the approximate analytic results with the exact numeric coupled-wave modeling. We also study...
Show moreThe theory of stretching and compressing of short light pulses by the chirped volume Bragg gratings (CBG) is reviewed based on spectral decomposition of short pulses and on the wavelength-dependent coupled wave equations. The analytic theory of diffraction efficiency of a CBG with constant chirp and approximate theory of time delay dispersion are presented. Based on those, we performed comparison of the approximate analytic results with the exact numeric coupled-wave modeling. We also study theoretically various definitions of laser beam width in a given cross-section. Quality of the beam is characterized by the dimensionless beam propagation products (?x???_x)?? , which are different for each of the 21 definitions. We study six particular beams and introduce an axially-symmetric self-MFT (mathematical Fourier transform) function, which may be useful for the description of diffraction-quality beams. Furthermore, we discuss various saturation curves and their influence on the amplitudes of recorded gratings. Special attention is given to multiplexed volume Bragg gratings (VBG) aimed at recording of several gratings in the same volume. The best shape of a saturation curve for production of the strongest gratings is found to be the threshold-type curve. Both one-photon and two-photon absorption mechanism of recording are investigated. Finally, by means of the simulation software we investigate forced airflow cooling of a VBG heated by a laser beam. Two combinations of a setup are considered, and a number of temperature distributions and thermal deformations are obtained for different rates of airflows. Simulation results are compared to the experimental data, and show good mutual agreement.
Show less - Date Issued
- 2015
- Identifier
- CFE0005638, ucf:50210
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005638
- Title
- Advanced liquid crystal materials for display and photonic applications.
- Creator
-
Chen, Yuan, Wu, Shintson, Zeldovich, Boris, Schoenfeld, Winston, Fang, Jiyu, University of Central Florida
- Abstract / Description
-
Thin-film-transistor (TFT) liquid crystal display (LCD) has been widely used in smartphones, pads, laptops, computer monitors, and large screen televisions, just to name a few. A great deal of effort has been delved into wide viewing angle, high resolution, low power consumption, and vivid color. However, relatively slow response time and low transmittance remain as technical challenges. To improve response time, several approaches have been developed, such as low viscosity liquid crystals,...
Show moreThin-film-transistor (TFT) liquid crystal display (LCD) has been widely used in smartphones, pads, laptops, computer monitors, and large screen televisions, just to name a few. A great deal of effort has been delved into wide viewing angle, high resolution, low power consumption, and vivid color. However, relatively slow response time and low transmittance remain as technical challenges. To improve response time, several approaches have been developed, such as low viscosity liquid crystals, overdrive and undershoot voltage schemes, thin cell gap with a high birefringence liquid crystal, and elevated temperature operation. The state-of-the-art gray-to-gray response time of a nematic LC device is about 5 ms, which is still not fast enough to suppress the motion picture image blur. On the other hand, the LCD panel's transmittance is determined by the backlight, polarizers, TFT aperture ratio, LC transmittance, and color filters. Recently, a fringe-field-switching mode using a negative dielectric anisotropy (??) LC (n-FFS) has been demonstrated, showing high transmittance (98%), single gamma curve, and cell gap insensitivity. It has potential to replace the commonly used p-FFS (FFS using positive ?? LC) for mobile displays.With the urgent need of submillisecond response time for enabling color sequential displays, polymer-stabilized blue phase liquid crystal (PS-BPLC) has become an increasingly important technology trend for information display and photonic applications. BPLCs exhibit several attractive features, such as reasonably wide temperature range, submillisecond gray-to-gray response time, no need for alignment layer, optically isotropic voltage-off state, and large cell gap tolerance. However, some bottlenecks such as high operation voltage, hysteresis, residual birefringence, and slow charging issue due to the large capacitance, remain to be overcome before their widespread applications can be realized. The material system of PS-BPLC, including nematic LC host, chiral dopant, and polymer network, are discussed in detail. Each component plays an essential role affecting the electro-optic properties and the stability of PS-BPLC.In a PS-BPLC system, in order to lower the operation voltage the host LC usually has a very large dielectric anisotropy (??(>)100), which is one order of magnitude larger than that of a nematic LC. Such a large ?? not only leads to high viscosity but also results in a large capacitance. High viscosity slows down the device fabrication process and increases device response time. On the other hand, large capacitance causes slow charging time to each pixel and limits the frame rate. To reduce viscosity, we discovered that by adding a small amount (~6%) of diluters, the response time of the PS-BPLC is reduced by 2X-3X while keeping the Kerr constant more or less unchanged. Besides, several advanced PS-BPLC materials and devices have been demonstrated. By using a large ?? BPLC, we have successfully reduced the voltage to (<)10V while maintaining submillisecond response time. Finally we demonstrated an electric field-indeced monodomain PS-BPLC, which enables video-rate reflective display with vivid colors. The highly selective reflection in polarization makes it promising for photonics application.Besides displays in the visible spectral region, LC materials are also very useful electro-optic media for near infrared and mid-wavelength infrared (MWIR) devices. However, large absorption has impeded the widespread application in the MWIR region. With delicate molecular design strategy, we balanced the absorption and liquid crystal phase stability, and proposed a fluoro-terphenyl compound with low absorption in both MWIR and near IR regions. This compound serves as an important first example for future development of low-loss MWIR liquid crystals, which would further expand the application of LCs for amplitude and/or phase modulation in MWIR region.
Show less - Date Issued
- 2014
- Identifier
- CFE0005314, ucf:50531
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005314
- Title
- Inversion of the Broken Ray Transform.
- Creator
-
Krylov, Roman, Katsevich, Alexander, Tamasan, Alexandru, Nashed, M, Zeldovich, Boris, University of Central Florida
- Abstract / Description
-
The broken ray transform (BRT) is an integral of a functionalong a union of two rays with a common vertex.Consider an X-ray beam scanning an object of interest.The ray undergoes attenuation and scatters in all directions inside the object.This phenomena may happen repeatedly until the photons either exit the object or are completely absorbed.In our work we assume the single scattering approximation when the intensity of the raysscattered more than once is negligibly small.Among all paths that...
Show moreThe broken ray transform (BRT) is an integral of a functionalong a union of two rays with a common vertex.Consider an X-ray beam scanning an object of interest.The ray undergoes attenuation and scatters in all directions inside the object.This phenomena may happen repeatedly until the photons either exit the object or are completely absorbed.In our work we assume the single scattering approximation when the intensity of the raysscattered more than once is negligibly small.Among all paths that the scattered rays travel inside the object we pick the one that isa union of two segments with one common scattering point.The intensity of the ray which traveled this path and exited the object can be measured by a collimated detector.The collimated detector is able to measure the intensity of X-rays from the selected direction.The logarithm of such a measurement is the broken ray transform of the attenuation coefficientplus the logarithm of the scattering coefficient at the scattering point (vertex)and a known function of the scattering angle.In this work we consider the reconstruction of X-ray attenuation coefficient distributionin a plane from the measurements on two or three collimated detector arrays.We derive an exact local reconstruction formula for three flat collimated detectorsor three curved or pin-hole collimated detectors.We obtain a range condition for the case of three curved or pin-hole detectors and provide a special caseof the range condition for three flat detectors.We generalize the reconstruction formula to four and more detectors and find anoptimal set of parameters that minimize noise in the reconstruction.We introduce a more accurate scattering model which takes into accountenergy shifts due to the Compton effect, derive an exact reconstruction formula and develop an iterativereconstruction method for the energy-dependent case.To solve the problem we assume that the radiation source is monoenergeticand the dependence of the attenuation coefficient on energy is linearon an energy interval from the minimal to the maximal scattered energy. %initial radiation energy.We find the parameters of the linear dependence of the attenuation on energy as a function of a pointin the reconstruction plane.
Show less - Date Issued
- 2014
- Identifier
- CFE0005514, ucf:50324
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005514
- Title
- Low Voltage Blue Phase Liquid Crystal Displays.
- Creator
-
Rao, Linghui, Wu, Shintson, Vanstryland, Eric, Zeldovich, Boris, Wu, Xinzhang, University of Central Florida
- Abstract / Description
-
From cell phones, laptops, desktops, TVs, to projectors, high reliability LCDs have become indispensable in our daily life. Tremendous progress in liquid crystal displays (LCDs) has been made after decades of extensive research and development in materials, device configurations and manufacturing technology. Nowadays, the most critical issue on viewing angle has been solved using multidomain structures and optical film compensation. Slow response time has been improved to 2-5 ms with low...
Show moreFrom cell phones, laptops, desktops, TVs, to projectors, high reliability LCDs have become indispensable in our daily life. Tremendous progress in liquid crystal displays (LCDs) has been made after decades of extensive research and development in materials, device configurations and manufacturing technology. Nowadays, the most critical issue on viewing angle has been solved using multidomain structures and optical film compensation. Slow response time has been improved to 2-5 ms with low viscosity LC material, overdrive and undershoot voltage, and thin cell gap approach. Moving image blur has been significantly reduced by impulse driving and frame insertion. Contrast ratio in excess of one million-to-1 has been achieved through local dimming of the segmented LED backlight. The color gamut would exceed 100% of the NTSC (National Television System Committee), if RGB LEDs are used. Besides these technological advances, the cost has been reduced dramatically by investing in advanced manufacturing technologies. Polymer-stabilized blue phase liquid crystal displays (BPLCDs) based on Kerr effect is emerging as a potential next-generation display technology. In comparison to conventional nematic devices, the polymer-stabilized BPLCDs exhibit following attractive features: (1) submillisecond response time, (2) no need for molecular alignment layers, (3) optically isotropic dark state when sandwiched between crossed polarizers, and (4) transmittance is insensitive to cell gap when the in-plane electrodes are employed. However, aside from these great potentials, there are still some tough technical issues remain to be addressed. The major challenges are: 1) the operating voltage is still too high (~50 Volts vs. 5 Volts for conventional nematic LCDs), and the transmittance is relatively low (~65% vs. 85% for nematic LCDs), 2) the hysteresis effect and residual birefringence effect are still noticeable, 3) the mesogenic temperature range is still not wide enough for practical applications (?40 oC to 80 oC), and 4) the ionic impurities in these polymer-stabilized nano-structured LC composites could degrade the voltage holding ratio, which causes image sticking.In this dissertation, the BPLC materials are studied and the new BPLC device structures are designed to optimize display performances. From material aspect, the electro-optical properties of blue phase liquid crystals are studied based on Kerr effect. Temperature effects on polymer-stabilized blue phase or optically isotropic liquid crystal displays are investigated through the measurement of voltage dependent transmittance under different temperatures. The physical models for the temperature dependency of Kerr constant, induced birefringence and response time in BPLCs are first proposed and experimentally validated. In addition, we have demonstrated a polymer-stabilized BPLC mixture with a large Kerr constant K~13.7 nm/V2 at 20 oC at 633 nm. These models would set useful guidelines for optimizing material performances. From devices side, the basic operation principle of blue phase LCD is introduced. A numerical model is developed to simulate the electro-optic properties of blue phase LCDs based on in-plane-switching (IPS) structure. Detailed electrode dimension effect, distribution of induced birefringence, cell gap effect, correlation between operation voltage and Kerr constant, and wavelength dispersion are investigated. Viewing angle is another important parameter. We have optimized the device configurations according to the device physics studied. With proper new device designs, the operating voltage is decreased dramatically from around 50 Volts to below 10 Volts with a reasonably high transmittance (~70%) which enables the BPLCDs to be addressed by amorphous silicon thin-film transistors (TFTs). Moreover, weak wavelength dispersion, samll color shift, and low hysteresis BPLCDs are achieved after their root causes being unveiled. Optimization of device configurations plays a critical role to the widespread applications of BPLCDs.In addition to displays, blue phase liquid crystals can also be used for photonic applications, such as light modulator, phase grating, adaptive lens and photonic crystals. We will introduce the application of blue phase liquid crystal as a modulator to realize a viewing angle controllable display. The viewing angle can be tuned continuously and precisely with a fast response time. The detailed design and performance are also presented in this dissertation.
Show less - Date Issued
- 2012
- Identifier
- CFE0004625, ucf:49930
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004625
- Title
- Optically isotropic liquid crystals for display and photonic applications.
- Creator
-
Yan, Jin, Wu, Shintson, Zeldovich, Boris, Schoenfeld, Winston, Fang, Jiyu, University of Central Florida
- Abstract / Description
-
For the past few decades, tremendous progress has been made on liquid crystal display (LCD) technologies in terms of stability, resolution, contrast ratio, and viewing angle. The remaining challenge is response time. The state-of-the-art response time of a nematic liquid crystal is a few milliseconds. Faster response time is desirable in order to reduce motion blur and to realize color sequential display using RGB LEDs, which triples the optical efficiency and resolution density. Polymer...
Show moreFor the past few decades, tremendous progress has been made on liquid crystal display (LCD) technologies in terms of stability, resolution, contrast ratio, and viewing angle. The remaining challenge is response time. The state-of-the-art response time of a nematic liquid crystal is a few milliseconds. Faster response time is desirable in order to reduce motion blur and to realize color sequential display using RGB LEDs, which triples the optical efficiency and resolution density. Polymer-stabilized blue phase liquid crystal (PS-BPLC) is a strong candidate for achieving fast response time because its self-assembled cubic structure greatly reduces the coherence length. The response time is typically in the submillisecond range and can even reach microsecond under optimized conditions. Moreover, it exhibit several attractive features, such as no need for surface alignment layer, intrinsic wide viewing angle, and cell gap insensitivity if an in-plane-switching (IPS) cell is employed. In this dissertation, recent progresses in polymer-stabilized blue phases, or more generally optically-isotropic liquid crystals, are presented. Potential applications in display and photonic devices are also demonstrated.In Chapter 1, a brief introduction of optically isotropic liquid crystals is given. In Chapter 2, we investigate each component of polymer-stabilized blue phase materials and provide guidelines for material preparation and optimization. In Chapter 3, the electro-optical properties of PS-BPLCs, including electric-field-induced birefringence and dynamic behaviors are characterized. Theoretical models are proposed to explain the physical phenomena. Good agreements between experimental data and models are obtained. The proposed models also provide useful guidelines for both material and device optimizations. Four display and photonic devices using PS-BPLCs are demonstrated in Chapter 4. First, by red-shifting the Bragg reflection and using circular polarizers, we reduce the LCD driving voltage by 35% as compared to a short-pitch BPLC while maintaining high contrast ratio and submillisecond response time. Second, a turning film which is critically needed for widening the viewing angle of a vertical field switching (VFS) BPLC mode is designed. With this film, the viewing angle of VFS is widened to (&)#177; 80(&)deg; in horizontal direction and (&)#177; 50(&)deg; in vertical direction. Without this turning film, the viewing angle is only (&)#177;30(&)deg;, which is too narrow for most applications. Third, a reflective BPLC display with vivid colors, submillisecond response time, and natural grayscales is demonstrated for the first time. The proposed BPLC reflective display opens a new gateway for 3D reflective displays; it could make significant impact to display industry. Finally, we demonstrate a tunable phase grating with a high diffraction efficiency of 40% and submillisecond response time. This tunable grating exhibits great potential for photonic and display applications, such as optical interconnects, beam steering, and projection displays.
Show less - Date Issued
- 2013
- Identifier
- CFE0005279, ucf:50551
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005279
- Title
- Transverse mode selection and brightness enhancement in laser resonators by means of volume Bragg gratings.
- Creator
-
Anderson, Brian, Glebov, Leonid, Zeldovich, Boris, Schulzgen, Axel, Rahman, Talat, University of Central Florida
- Abstract / Description
-
The design of high power lasers requires large mode areas to overcome various intensity driven nonlinear effects. Increasing the aperture size within the laser can overcome these effects, but typically result in multi-transverse mode output and reduced beam quality, limiting the brightness of the system. As one possible solution, the angular selectivity of a diffractive optical element is proposed as a spatial filter, allowing for the design of compact high brightness sources not possible...
Show moreThe design of high power lasers requires large mode areas to overcome various intensity driven nonlinear effects. Increasing the aperture size within the laser can overcome these effects, but typically result in multi-transverse mode output and reduced beam quality, limiting the brightness of the system. As one possible solution, the angular selectivity of a diffractive optical element is proposed as a spatial filter, allowing for the design of compact high brightness sources not possible with conventional methods of transverse mode selection. This thesis explores the angular selectivity of volume Bragg gratings (VBGs) and their use as spatial transverse mode filters in a laser resonator. Selection of the fundamental mode of a resonator is explored using transmission Bragg gratings (TBGs) as the spatial filter. Simulations and experimental measurements are made for a planar, 1 cm long resonator demonstrating near diffraction limited output (M2 (<) 1.4) for aperture sizes as large as 2.0 mm. Applications to novel fiber laser designs are explored. Single mode operation of a multi-mode Yb3+ doped ribbon fiber laser (core dimensions of 107.8 ?m x 8.3 ?m) is obtained using a single transmission VBG as the filter in an external cavity resonator. Finally, a novel method of selecting a pure higher order mode to oscillate within the gain medium while simultaneously converting this higher order mode to a fundamental mode at an output coupler is proposed and demonstrated. A multiplexed transmission VBG is used as the mode converting element, selecting the 12th higher order mode for amplifications in an Yb3+ doped ribbon fiber laser, while converting the higher order mode of a laser resonator to a single lobed output beam with diffraction limited divergence.
Show less - Date Issued
- 2015
- Identifier
- CFE0005754, ucf:50103
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005754
- Title
- Applications of Volume Holographic Elements in High Power Fiber Lasers.
- Creator
-
Jain, Apurva, Glebov, Leonid, Zeldovich, Boris, Schulzgen, Axel, Likamwa, Patrick, Rahman, Talat, University of Central Florida
- Abstract / Description
-
The main objective of this thesis is to explore the use of volume holographic elements recorded in photo-thermo-refractive (PTR) glass for power scaling of narrow linewidth diffraction-limited fiber lasers to harness high average power and high brightness beams. Single fiber lasers enable kW level output powers limited by optical damage, thermal effects and non-linear effects. Output powers can be further scaled using large mode area fibers, however, at the cost of beam quality and...
Show moreThe main objective of this thesis is to explore the use of volume holographic elements recorded in photo-thermo-refractive (PTR) glass for power scaling of narrow linewidth diffraction-limited fiber lasers to harness high average power and high brightness beams. Single fiber lasers enable kW level output powers limited by optical damage, thermal effects and non-linear effects. Output powers can be further scaled using large mode area fibers, however, at the cost of beam quality and instabilities due to the presence of higher order modes. The mechanisms limiting the performance of narrow-linewidth large mode area fiber lasers are investigated and solutions using intra-cavity volume Bragg gratings (VBG) proposed. Self-pulsations-free, completely continuous-wave operation of a VBG-stabilized unidirectional fiber ring laser is demonstrated with quasi single-frequency ((<) 7.5 MHz) output. A method for transverse mode selection in multimode fiber lasers to reduce higher order mode content and stabilize the output beam profile is developed using angular selectivity of reflecting VBGs. By placing the VBG output coupler in a convergent beam, stabilization of the far-field beam profile of a 20 ?m core large mode area fiber laser is demonstrated.Beam combining techniques are essential to power scale beyond the limitations of single laser sources. Several beam combining techniques relevant to fiber lasers were compared in this study and found to be lacking in one or more of the following aspects: the coherence of the individual sources is compromised, the far-field beam quality is highly degraded with significant power in side lobes, spectrally broad and unstable, and uncertainty over scaling to larger arrays and higher power. Keeping in mind the key requirements of coherence, good far-field beam quality, narrow and stable spectra, and scalability in both array size and power, a new passive coherent beam combining technique using multiplexed volume Bragg gratings (M-VBGs) is proposed.In order to understand the mechanism of radiation exchange between multiple beams via these complex holographic optical elements, the spectral and beam splitting properties a 2nd order reflecting M-VBG recorded in PTR glass is experimentally investigated using a tunable single frequency seed laser. Two single-mode Yb-doped fiber lasers are then coherently combined using reflecting M-VBGs in both linear and unidirectional-ring resonators with (>)90% combining efficiency and diffraction-limited beam quality. It is demonstrated that the combining bandwidth can be controlled in the range of 100s of pm to a few pm by angular detuning of the M-VBG. Very narrow-linewidth ((<) 210 MHz) operation in a linear cavity and possibility of single-frequency operation in a unidirectional ring cavity of the coherently combined system is demonstrated using this technique. It is theoretically derived and experimentally demonstrated that high combining efficiency can be achieved even by multiplexing low-efficiency VBGs, with the required diffraction efficiency of individual VBGs decreasing as array size increases. Scaling of passive coherent beam combining to four fiber lasers is demonstrated using a 4th order transmitting M-VBG. Power scaling of this technique to 10 W level combined powers with 88% combining efficiency is demonstrated by passively combining two large mode area fiber lasers using a 2nd order reflecting M-VBG in a unidirectional ring resonator. High energy compact single-frequency sources are highly desired for several applications (-) one of which is as a seed for high power fiber amplifiers. Towards achieving the goal of a monolithic solid-state laser, a new gain medium having both photosensitive and luminescence properties is investigated (-) rare-earth doped PTR glass. First lasing is demonstrated in this new gain element in a VBG-stabilized external cavity.
Show less - Date Issued
- 2012
- Identifier
- CFE0004553, ucf:49230
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004553
- Title
- Ultra High Density Spectral Beam Combining By Thermal Tuning of Volume Bragg Gratings in Photo-Thermo-Refractive Glass.
- Creator
-
Drachenberg, Derrek, Zeldovich, Boris, Bass, Michael, Schulzgen, Axel, Likamwa, Patrick, Glebov, Leonid, University of Central Florida
- Abstract / Description
-
High power lasers with diffraction limited beam quality are desired for many applications in defense and manufacturing. A lot of applications require laser beams at the 100 kW power level along with divergence close to the diffraction limit. The figure of merit for a beam used in such applications should be radiance which determines the laser power delivered to a remote target. One of the primary limiting factors is thermal distortion of a laser beam caused by excessive heat generated in the...
Show moreHigh power lasers with diffraction limited beam quality are desired for many applications in defense and manufacturing. A lot of applications require laser beams at the 100 kW power level along with divergence close to the diffraction limit. The figure of merit for a beam used in such applications should be radiance which determines the laser power delivered to a remote target. One of the primary limiting factors is thermal distortion of a laser beam caused by excessive heat generated in the laser media. Combination of multiple laser beams is usually considered as a method to mitigate these limitations. Spectral beam combining (SBC) by volume Bragg gratings (VBGs) is a very promising method for the future of high radiance lasers that needs to achieve 100 kW-level power. This work is dedicated to development of methods to increase spectral density of combined beams keeping their divergence at an acceptably low level.A new figure of merit for a beam combining system is proposed, the Beam Combining Factor (BCF), which makes it possible to distinguish the quality of the individual beams from the quality of beam combining. Also presented is a method of including the effect of beam divergence and spectral bandwidth on the performance of VBGs, as well as a method to optimize VBG parameters in terms of thickness and refractive index modulation for an arbitrary number of beams.A novel thermal tuning technique and apparatus is presented with which the SBC system can be tuned for peak efficiency from low to high power without the need for mechanical re-alignment. Finally, a thermally tuned SBC system with five beams, with a spectral separation between beams of 0.25 nm at a total power of 685 W is presented. The results show the highest power spectral density and highest spectral radiance of any SBC system to date. Recent demonstrations in SBC by multiplexed VBGs and the use of super Gaussian beams for beam quality improvement are also discussed.
Show less - Date Issued
- 2011
- Identifier
- CFE0004104, ucf:49089
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004104
- Title
- Third Order Nonlinearity of Organic Molecules.
- Creator
-
Hu, Honghua, Vanstryland, Eric, Hagan, David, Zeldovich, Boris, Hernandez, Florencio, University of Central Florida
- Abstract / Description
-
The main goal of this dissertation is to investigate the third-order nonlinearity of organic molecules. This topic contains two aspects: two-photon absorption (2PA) and nonlinear refraction (NLR), which are associated with the imaginary and real part of the third-order nonlinearity (?(3)) of the material, respectively. With the optical properties tailored through meticulous molecular structure engineering, organic molecules are promising candidates to exhibit large third-order nonlinearities....
Show moreThe main goal of this dissertation is to investigate the third-order nonlinearity of organic molecules. This topic contains two aspects: two-photon absorption (2PA) and nonlinear refraction (NLR), which are associated with the imaginary and real part of the third-order nonlinearity (?(3)) of the material, respectively. With the optical properties tailored through meticulous molecular structure engineering, organic molecules are promising candidates to exhibit large third-order nonlinearities. Both linear (absorption, fluorescence, fluorescence excitation anisotropy) and nonlinear (Z-scan, two-photon fluorescence, pump-probe) techniques are described and utilized to fully characterize the spectroscopic properties of organic molecules in solution or solid-state form. These properties are then analyzed by quantum chemical calculations or other specific quantum mechanical model to understand the origins of the nonlinearities as well as the correlations with their unique molecular structural features. These calculations are performed by collaborators. The 2PA study of organic materials is focused on the structure-2PA property relationships of four groups of dyes with specific molecular design approaches as the following: (1) Acceptor-?-Acceptor dyes for large 2PA cross section, (2) Donor-?-Acceptor dyes for strong solvatochromic effects upon the 2PA spectra, (3) Near-infrared polymethine dyes for a symmetry breaking effect, (4) Sulfur-squaraines vs. oxygen-squaraines to study the role of sulfur atom replacement upon their 2PA spectra. Additionally, the 2PA spectrum of a solid-state single crystal made from a Donor-?-Acceptor dye is measured, and the anisotropic nonlinearity is studied with respect to different incident polarizations. These studies further advance our understanding towards an ultimate goal to a predictive capability for the 2PA properties of organic molecules. The NLR study on molecules is focused on the temporal and spectral dispersion of the nonlinear refraction index, n2, of the molecules. Complicated physical mechanisms, originating from either electronic transitions or nuclei movement, are introduced in general. By adopting a prism compressor / stretcher to control the pulsewidth, an evolution of n2 with respect to incident pulsewidth is measured on a simple inorganic molecule (-)carbon disulfide (CS2) in neat liquid at 700 nm and 1064 nm to demonstrate the pulsewidth dependent nonlinear refraction. The n2 spectra of CS2 and certain organic molecules are measured by femtosecond pulses, which are then analyzed by a 3-level model, a simplified (")Sum-over-states(") quantum mechanical model. These studies can serve as a precursor for future NLR investigations.
Show less - Date Issued
- 2012
- Identifier
- CFE0004387, ucf:49400
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0004387
- Title
- White Light Continuum for Broadband Nonlinear Spectroscopy.
- Creator
-
Ensley, Trenton, Hagan, David, Vanstryland, Eric, Zeldovich, Boris, Christodoulides, Demetrios, Schulte, Alfons, University of Central Florida
- Abstract / Description
-
Supercontinuum (SC) generation, oftentimes referred to as white-light continuum (WLC), has been a subject of interest for more than 40 years. From the first observation of WLC in condensed media in the early 1970s to the first observation of WLC in gases in the mid-1980s, much work has been devoted to developing a framework for understanding the complex nature of this phenomenon as well as discovering its utility in various applications. The main effort of this dissertation is to develop a...
Show moreSupercontinuum (SC) generation, oftentimes referred to as white-light continuum (WLC), has been a subject of interest for more than 40 years. From the first observation of WLC in condensed media in the early 1970s to the first observation of WLC in gases in the mid-1980s, much work has been devoted to developing a framework for understanding the complex nature of this phenomenon as well as discovering its utility in various applications. The main effort of this dissertation is to develop a WLC for the purpose of broadband nonlinear spectroscopy and use it in spectroscopic measurements. The ability to generate a high-quality, high-spectral-irradiance source of radiation confined in a single beam that spans the visible and near-infrared spectral regimes has great utility for nonlinear measurement methods such as the Z-scan technique. Using a broadband WLC instead of conventional tunable sources of radiation such as optical parametric generators/amplifiers has been shown to increase the efficiency of such measurements by nearly an order of magnitude. Although WLC generation has many complex processes involved, and complete models of the process involve highly complex numerical modeling, simple models can still guide us in the optimization of systems for WLC generation. In this dissertation the effects of two key mechanisms behind WLC generation in gaseous media are explored: self-phase modulation (SPM) and ionization leading to plasma production. The effects of SPM are largely dependent upon the third-order nonlinear refractive index, n2, of the gaseous medium whereas the effects of plasma production are dependent upon many parameters including the initial number density, ionization potential/energy, and the rate of ionization production. It is found that in order to generate a stable WLC suitable for nonlinear spectroscopy, the phase contributions from SPM and plasma production should be nearly equal. This guided our experiments in inert gases using mJ level, 150 fs-FWHM (full-width at half-maximum) pulses at 780 nm as well as 40 fs-FWHM pulses primarily at 1800 nm to create a stable, high-spectral-irradiance WLC. The generated WLC is shown to have sufficient spectral energy and spatial quality suitable for nonlinear spectroscopic measurements. In addition to extending the WLC bandwidth by using a long wavelength (1800 nm) pump source, it is found that by using a secondary weak seed pulse with a peak irradiance three orders of magnitude less than the main pulse, the spectral energy density is enhanced by more than a factor of 3 in Krypton gas for a WLC spectrum that spans over 2 octaves. Numerical simulations are presented which qualitatively describe the experimental results. The spectral enhancement of the WLC by seeding is also demonstrated for other inert gases and condensed media. Other efforts described in this dissertation include the development of the Dual-Arm Z-scan technique and its extension to measuring thin film nonlinearities in the presence of large substrate signals as well as predicting the n2 spectra of organic molecules (where we can approximate their behavior as if they were centrosymmetric) from knowledge of the one-photon and two-photon absorption spectra using a simplified sum-over-states quantum perturbative model by utilizing a quasi 3-level and quasi 4-level system.
Show less - Date Issued
- 2015
- Identifier
- CFE0005608, ucf:50264
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005608
- Title
- Holographic Recording and Applications of Multiplexed Volume Bragg Gratings in Photo-thermo-refractive Glass.
- Creator
-
Ott, Daniel, Glebov, Leonid, Zeldovich, Boris, Moharam, Jim, Rahman, Talat, University of Central Florida
- Abstract / Description
-
Recent developments in holographic recording of volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass have demonstrated their utility as components in high power laser systems for spectral narrowing, transverse mode control, beam combining, and pulse stretching/compression. VBG structures are capable of diffracting incident light into a single diffraction order with high efficiency given the Bragg condition is met. The Bragg condition depends on both the wavelength and angle of...
Show moreRecent developments in holographic recording of volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass have demonstrated their utility as components in high power laser systems for spectral narrowing, transverse mode control, beam combining, and pulse stretching/compression. VBG structures are capable of diffracting incident light into a single diffraction order with high efficiency given the Bragg condition is met. The Bragg condition depends on both the wavelength and angle of the incident light making VBGs useful for filtering and manipulating both the wavelength and angular spectrum of a source. This dissertation expands upon previous research in PTR VBGs by investigating multiplexed VBGs and their applications in laser systems. Multiplexing involves the integration of several VBGs into the same volume of PTR glass. This process enables the fabrication of splitting and combining elements which have been used for high power beam combining with significantly reduced complexity as compared to other combining schemes. Several configurations of multiplexed beam combiners were demonstrated for both spectral and coherent combining systems with high power results yielding a combined power of 420 W with 96% efficiency. Multiplexing was also used to produce unique phase structures within VBGs. This effect was exploited to create extremely narrowband spectral filters called moir(&)#233; Bragg gratings. The technical challenges of producing moir(&)#233; gratings in bulk glass have revealed new insights into the use of PTR glass as a recording medium and produced devices capable of narrowband filtering of only 15 pm in the near infrared. Experiments were performed using such devices as intra-cavity laser elements for longitudinal mode selection. Investigations have also been made into increasing the level of multiplexing possible within PTR glass. These explorations included scaling the number of beam combining channels, fabrication of integrated multi-notch filters, and generated several other potentially interesting devices for future research. The summation of this work indicates a promising future for multiplexed VBGs in PTR glass.
Show less - Date Issued
- 2014
- Identifier
- CFE0005392, ucf:50446
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005392
- Title
- Volume Phase Masks in Photo-Thermo-Refractive Glass.
- Creator
-
Segall, Marc, Glebov, Leonid, Zeldovich, Boris, Dogariu, Aristide, Rahman, Talat, Bass, Michael, University of Central Florida
- Abstract / Description
-
In many applications such as beam shaping, mode conversion, and phase encoding it is necessary to alter the spatial phase profile of a beam via a phase mask. Conventional techniques to accomplish this either involve surface relief profiling in thin films such as PMMA or refractive index modulation in bulk photorefractive crystals such as lithium niobate. These materials have been used extensively for the past several decades and perform admirably in low power conditions. However, in high...
Show moreIn many applications such as beam shaping, mode conversion, and phase encoding it is necessary to alter the spatial phase profile of a beam via a phase mask. Conventional techniques to accomplish this either involve surface relief profiling in thin films such as PMMA or refractive index modulation in bulk photorefractive crystals such as lithium niobate. These materials have been used extensively for the past several decades and perform admirably in low power conditions. However, in high power systems these materials will be destroyed, requiring a new means of producing phase masks. In this dissertation a method for producing robust phase masks in the bulk of photo-thermo-refractive glass is developed and successfully demonstrated. Three main applications of phase masks were studied in detail. The first is mode conversion, where binary phase masks convert a Gaussian beam to higher order modes. The second is beam shaping, where phase masks are used as focusing elements and for optical vortex generation. Near-theoretical conversion efficiency was achieved for all elements in these cases. The third application is aberration analysis and correction. Here the degradation of volume Bragg gratings recorded in an aberrated holographic system was modeled, with the simulations indicating that correcting elements are generally necessary for high-quality production of gratings. Corrective phase masks are designed which can selectively correct one or multiple aberrations of varying magnitudes are shown. A new type of optical element is also developed in which a phase mask is encoded into a transmitting Bragg grating. This technique combines the local phase modulation of a phase mask with the multiplexing ability of transmitting Bragg gratings, allowing for multiple phase masks to be recorded in a single element. These masks may be used at any wavelength satisfying the Bragg condition, increasing the useful wavelength regime of a single element by orders of magnitude.
Show less - Date Issued
- 2013
- Identifier
- CFE0005414, ucf:50431
- Format
- Document (PDF)
- PURL
- http://purl.flvc.org/ucf/fd/CFE0005414