In this paper, we present results of numerical investigation of quantum walks (QWs) in different type arrays of single-mode waveguides: (i) regular arrays of identical waveguides (AIW), (ii) periodic arrays of two different waveguides (PAW), (iii) quasi-periodic arrays of waveguides with Fibonacci sequences (FAW), (iv) quasi-periodic arrays of waveguides with Thue-Morse sequences (TMAW). Our simulations show in contrast with randomly disordered systems, localized QWs in quasi-periodic arrays of waveguides FAWs and TMAWs are predictable and controllable due to the deterministic disorder nature of the quasi-periodic systems.
Laser writing of waveguides in bulk glasses opens the opportunity for creating three-dimensional photonic devices. In
order to become practical, the numerical aperture (NA) of these waveguides should be significantly higher than currently
achievable of 0.1 - 0.15. One reason is that with higher NAs one can decrease the bending radii of the embedded
photonic devices without significant loss penalty and make them compact. Thus, femtosecond-laser-written waveguides
in glasses do not allow bending radii smaller than 15 - 20 mm. In order to overcome this limitation, we propose to
fabricate waveguides in phase-separable and leachable glass where the index contrast is determined by the difference
between the refractive indices of the unprocessed glass and of the leached porous glass. We show that we can achieve
the NA = 0.25 prior to optimization. Surface and sub-surface treatment with a nanosecond ultraviolet (UV) laser
produces a similar effect with even higher NA = 0.35. Applications may include a range of tightly packed embedded and
three-dimensional photonic devices in bulk glass like directional couplers, splitters, interferometers, etc.
Optical and electron confinement are utilized to tailor the optical characteristics of active materials and photonic devices. A technique to incorporate semiconductor quantum dots into planar glass waveguides with low propagation loss is demonstrated. The waveguides are fabricated by potassium-sodium and silver-sodium ion exchange processes in glasses that contain PbS quantum dots with radii of a few nanometers. The unique optical properties of the quantum dots are preserved throughout the waveguide fabrication process. We also demonstrate novel compact fiber lasers based on active, highly doped fibers with photonic crystal cladding. The flexibility provided by microstructuring the fiber enables improved fiber laser performance and several Watts of laser output are generated from few centimeters of active fiber.
Fused silica, when exposed to excimer laser light, exhibits permanent anisotropic birefringence and wavefront changes. These laser-induced changes depend on the silica composition and processing conditions. The optical anisotropy is most clearly observed in samples that are exposed with linear polarization. This polarization-induced effect has been known for several years, but has become much more important with the advent of immersion lithography and its associated very high numerical apertures. High numerical aperture optics require controlled polarization, notably linear polarization, in order to maintain phase contrast at the image. When birefringence and wavefront changes are induced by laser exposure, the image contrast at the wafer deteriorates. We interpret the changes in optical properties in terms of permanent anisotropic strain induced by laser damage, and the associated strain-induced optical effects. This is accomplished using the mathematics of tensors to account for anisotropic strain and optical anisotropy, and using finite element analysis to calculate the strain fields taking the sample and exposure geometries into account. We report the relations between underlying density and strain anisotropy changes and the induced birefringence and wavefront for a given experimental sample geometry. We also report some examples of the different degree of laser damage from silica with different compositions and processing conditions.
In this invited paper, we will discuss the use of quantum dots as
nonlinear optical elements in fiber laser sources. Furthemore, a
review of the fabrication of the first low-loss (< 0.5 dB/cm)
ion-exchanged waveguides in a quantum-dot-doped glass will be
presented. We will discuss the coupling, propagation, absorption,
and scattering losses in these waveguides. The near-field mode
profile along with the refractive index profile of these waveguides will be presented. This PbS quantum-dot-doped glass was chosen due to its attractive optical gain and bleaching characteristics at wavelengths throughout the near infrared. This bleaching of the ground-state optical transition has been utilized for passive modelocking of a variety of lasers in the near infrared. In addition, we will discuss some of the potential integrated and fiber optics applications of our quantum-dot-doped waveguides.
Silica glass exposed to pulsed UV excimer laser irradiation undergoes optical changes that can include either an optical path increase or a decrease. During a given exposure the sign of the induced optical path change can reverse as a function of pulse count. The reduced optical path and sign reversal are only observed in H2-containing glasses, and at high exposure fluence only optical path increase is observed. In past work we proposed an induced density change model invoking a dynamic equilibrium density to explain the high fluence experiments. Here we present a model that extends the density model to the low fluence regime by allowing the equilibrium density to be a function of the time-dependent break-up of the silica network during exposure. The network break-up is tracked by calculation of the induced SiH concentration in the glass. The agreement of optical path change obtained from experimental data with that deduced from the kinetic approach covers a wide range of exposure fluence and molecular hydrogen concentration. Using the model one can predict the change in optical path that arises from the excimer laser exposure.
We briefly review recent progress in the fabrication and characterization of air-core photonic band-gap fibers. These are silica fibers with an hexagonal array of air holes in the cladding, and a larger air hole creating the core. Improved structural uniformity transverse to the fiber axis and down the fiber axis has yielded fibers with better transmission characteristics. We have measured a minimum loss of 13 dB/km at 1500 nm for a 100 m length of our fiber. This is a marked improvement over previous loss measurements for air-core fibers of any kind. A comparison of observed spectra and calculated gap modes suggests that coupling between surface modes and core modes may be an important contributor to the remaining loss. We present a detailed analysis of the expected losses associated with mode crossings between the fundamental core mode and surface modes, showing that Lorentzian-shaped loss peaks are predicted.
The UV-photosensitivity effect in germania-doped optical waveguides has become an important area of research because of the ease and utility of making Bragg gratings. In this study we report on the finding of a large UV-induced refractive index change in a conventionally melted an alkali-alumino-boro-germano- silicate composition that has been loaded with molecular hydrogen. The exposure was done with either CW 244-nm light, or a pulsed KrF excimer laser at 248-nm. A modulated refractive index of the order of 2-3 X 10-4 has been measured.
Laser irradiation of fused silica produces compaction. Irradiation at photolithographic wavelengths of 248nm or 193nm produces density increase of order parts per millon. While these changes are small, the accompanying increase in index of refraction may be large enough to degrade the performance of photolithographic exposure optics. This paper reports experimental of compaction by interferometry and theoretical analysis of the elastic response of the glass samples to extract the sample-independent unconstrained compaction. The power law behavior of compaction vs. exposure dose is presented, along with a Monte Carlo analysis of error bars on this curve.
The use of silica lenses in a photolithographic system employing a 193 nm excimer laser has been proposed. It is desirable to determine if, at the low intensity to be used in the system (approximately equals 0.1 mJ/cm2), the glass will withstand about ten years of use without objectionable induced absorption. At a pulse frequency of 1 Khz, this length of time corresponds to about 1011 pulses. Because of the long time involved, an accelerated test is needed to determine the susceptibility of silica to induced absorption. The mechanism of darkening must be understood in order that the behavior of the glass under use conditions be predicted with confidence from the results of the accelerated test. The most important processes in the mechanism of induced absorption are: (1) Two photon absorption creating an exciton; (2) Trapping of the exciton by a localized state; (3) Dissociation of the trapped exciton to form an E' center and a NBOHC; (4) Reaction of these centers with hydrogen to form SiH and SiOH bonds; and (5) Photolysis of the SiH bonds to produce more E' centers. The mechanism will be discussed in detail and the agreement with experimental results over a range of intensities and hydrogen levels will be presented.
The compaction (densification) of fused silica under low fluence (< 1 mJ/cm2/pulse), long term (hundreds of millions of pulses) 193 nm irradiation has been studied. With the use of a finite element analysis, the unconstrained densification, (delta) (rho) /(rho) , is extracted from the experimentally determined wavefront distortion. We find that the densification of silica in the low fluence exposure regime corresponds to what is predicted from the behavior described by: (delta) (rho) /(rho) equals 0.000117 (NI2)0.53 where N equals number of pulses, I equals intensity (mJ/cm2/pulse). Results are presented of high and low intensity studies that establish the utility of the above form as a predictive tool for densification in fused silica.
We report the ground-state and excited-state optical absorption spectra in the visible and near infrared for several substituted fullerenes and higher fullerenes in toluene solutions. Based on these measurements, broadband predictions of the optical limiting performance of these molecules can be deduced. These predictions are then tested in the wavelength range from 532 nm to 700 nm in intensity-dependent transmission measurements. We observe optical limiting in all fullerenes measured; higher fullerenes show the greatest potential for limiting in the near infrared (650 - 1000 nm), while substituted C60 shows optimal limiting in the visible (450 - 700 nm). We observe dramatically reduced limiting for solid forms of C60 (thin films and C60-doped porous glasses), indicating that efficient optical limiting in fullerenes requires true molecular solutions.
Glasses containing a post-thermally developed CuClBr microcrystalline phase were stretched under an applied stress at a temperature above the strain-point. The resulting glass was optically transparent and birefringent. The stretched glass was heated under reducing conditions to effect the reduction of the Cu-halide particles to Cu metal thereby rendering the glass polarizing. The polarizing behavior is compared to that of Ag/Ag-halide containing glasses which are made in the same manner. The polarizing behavior of the two glasses is very similar except for the wavelength region below 500 nm. The absence of a crossover of the parallel and perpendicular polarization transmittances in the Cu/Cu-halide glass suggests a visible polarizer application.
The physical elongation of a phase-separated glass by the application of a pulling stress to the glass above its softening temperature leads to a material with a number of interesting optical properties. This study is concerned with silver halide as the separated phase in an alkali-alumino- borosilicate glass. The initially spherically shaped halide particle is elongated into an elliptical shape by the action of the stretching process. As a consequence, the composite body acquires optical anisotropy exhibited as birefringence and dichroism. Experimental results show how the magnitude of the optical anisotropy can be related to shape anisotropy of the particles and, in turn, to the pulling stress. These results are discussed in terms of a simple polarizability model which adequately explains the results.
The properties of zero-order retardation plates fabricated from mechanically stretched phase- separated glass are presented and discussed. The dissolved stretched phase is silver halide. The values of the birefringence as a result of the elongated silver halide phase are studied as a function of the average particle aspect ratio and wavelength. The (lambda) /4 and (lambda) /2 thicknesses required for the wavelength interval 633 - 1520 nm were 0.5 - 3.0 mm. Advantages in the use of these glass waveplates in devices such as circular polarizers are discussed.
Spectral changes induced by UV irradiation were studied in a variety of glasses. These include photosensitive glasses in which colloidal silver is formed, glasses in which the formation of V+ ions produces violet color, and glasses in which irradiation within a charge transfer band leads to the formation of Fe+2 ions and holes in the valence band. In all of these glasses, hole trapping by non-bridging oxygen atoms is required for the occurence of spectral changes.
Microlens arrays which image a full size document onto a photoreceptor for copying or transmitting purposes are currently of much interest. In particular those arrays which have total conjugate distances less than 10 mm are important because of the compactness they afford in the contact image sensor (CIS) unit design. In this paper the fabrication of such a one-to-one array utilizing the photosensitive glass-based SMILE tm process is described. The performance of the array in terms of contrast vs. spatial frequency for a facsimile document reader application is tested in a commercial CIS unit. The overall measured performance is examined in terms of the process parameters involved in the lens fabrication. Advantages and disadvantages of the microlens array relative to other methods are considered.