With the proper choice of laser parameters focused femtosecond laser light creates long-range self-assembled planar nanocracks inside and on the surface of fused silica glass. The orientation of the crack planes is normal to the laser polarization direction and can be precisely controlled. The arrays of cracks when properly oriented and combined with chemical etching produce high aspect ratio micro- and nanofluidic channels. Direct femtosecond laser writing without any chemical etching can be used to fabricate embedded nanoporous capillaries in bulk fused silica for biofiltering and electrophoresis applications. The morphology of the porous structures critically depends on the laser polarization and pulse energy and can be used to control the transmission rates of fluids through the capillaries. Finally high aspect ratio, polarization-dependent, self-ordered periodic nanoslots can be fabricated from nanocracks produced on the surface of fused silica wafers. Control of the surface slot width from 10 to 60 nm is achieved through selective chemical etching. This technique, which may be useful for Surface Enhanced Raman Scattering (SERS) applications, has sub-diffraction limited resolution and features high throughput writing over centimeters.
Self-organized nanostructures have been recently observed when femtosecond laser pulses were focused inside fused silica glass. We have shown that these nanostructures extend throughout the focal volume and their order is preserved over macroscopic distances when the focus is scanned. We discuss the present understanding of the formation of the nanostructures including a model based on transient nanoplasmonics. The model predicts the periodicity of nanoplanes to scale as λ/2 in the medium. This is experimentally verified at 800 nm and 400 nm light with which we obtain nanoplane spacing of 250 ± 20 nm and 140 ± 20 nm respectively, which scale as predicted. Another requirement of the model is that ionization occurs preferentially at regions that have previously been ionized. This allows an initially inhomogeneous plasma to develop into an ordered nanoplasma array. Using transmission measurements we show that the required "memory" exists in the case of fused silica.
A study of the group delay responses in fibre Bragg dispersion compensators due to chromatic dispersion and first-order
PMD is characterised at different optical modulation frequencies using a system based on a modified phase-shift method. A
simple theoretical model has been constructed to account for the interdependency of the chromatic dispersion and PMD and
to explain the effect of the modulation frequency on the group delay responses and the PDL. Measurement results and
conclusions are presented.
We present a measurement setup for the complete characterization of fiber Bragg gratings and wavelength selective fiber-optic devices. We can measure the spectral response of these devices both in transmission and in reflection, the wavelength dependency of the group delays due to chromatic and Polarization Mode Dispersion (PMD), as well as the wavelength dependency of the Polarization Dependent Loss (PDL). Experimental results are presented and sources of error are discussed. Comparisons with the Jones matrix method for the measurement of PDL and group delay due to PMD are made.
We present an optical setup for complete characterization of fiber Bragg gratings and wavelength selective fiber-optic devices. We measure the spectral response of these devices either in transmission or in reflection geometry, and the wavelength dependency of the group delays due to chromatic and polarization mode dispersion, as well as the wavelength dependency of the polarization dependent loss.
We have measured Polarization Mode Dispersion (PMD) and Polarization Dependent Loss (PDL) in chirped fiber Bragg gratings for dispersion compensation and in long-period fiber gratings for gain flattening. PMD and PDL were measured in reflection and in transmission by using a polarization analyzer and a tunable laser diode. Typical experimental results are presented for two grating devices.
In this paper, we demonstrate that eigen mode expansion can be used successfully to find the propagation constants and field distributions for the modes in an optical fiber. Instead of using sine-functions, we use Bessel functions as the basis for the expansion of the modes. Comparison between the results of this method and the exact analytical results for a single-mode, step-index optical fiber is shown. Also, theoretical and experimental results for a commercial boron co-doped germano-silicate fiber are compared.
This paper describes an application of holographic to product diffusers with different optical properties. We report on holographic diffusers for making uniform the infrared source radiation patterns with a variable dispersion angle. The holographic recording geometry provides control over the angular dispersion characteristics of the diffuser. Multiple-lobe and multiple-spot holographic diffusers with 3D scattering patterns have been fabricated. The scattering pattern is achieved by recording a diffuser. We provide some details on the recording set-up and procedure. We discuss some holographic properties of DuPont holographic recording films used in this application. Measurements of the scattering properties are presented for diffusers made in DuPont holographic recording films. For a holographic multiple-spot diffuser, the optical channel frequency response was measured and compared with an opal glass diffuser.
The spectral response of a chirped sinusoidal surface relief grating in an optical waveguide has been studied by means of Local Normal Mode Expansion theory. A matrix technique has been applied to account for the varying grating period. The grating is divided into segments with constant periods and coupling coefficients. Nonslanted grating is considered in a first-order diffraction. Also, contradirectional coupling between guided modes in phase synchronism at an arbitrary angle of incidence is considered. Performance considerations include TE-TE, TE- TM, and TM-TM mode coupling for different grating geometry and waveguide parameters. We show the effect of geometry and waveguide parameters on the performance of the chirped grating as a wavelength demultiplexer. Also, we demonstrate how the filtering and demultiplexing characteristics and the spectral shift between different polarization modes can be controlled.
The aim of this work is a proof-of-concept demonstration of an optical CDMA system based on spectral amplitude encoding of incoherent broadband sources. A Light Emitting Diode is employed as a source in a multi-mode fiber-optics system, and simple pseudorandom sequences such as m-sequences are used for optical encoding. We propose a multi-pass configuration with a Littrow grating arrangement for the optical encoder and decoder. Since this is a power-limited system, the multi-pass arrangement allows us to greatly reduce the number of lossy optical components and to increase compactness and power throughput. A differential receiver with a simplified design is employed. Measurement results are presented and discussed.
We demonstrate transmissive holographic stars based on multiple-beam planar configuration and multiple-exposure with Bragg degeneracy effect fabricated in Du Pont photopolymers HRF-610 with He-Ne laser light. With the latter approach the required number of gratings is reduced significantly. The stars can operate in a relatively large window of replay wavelengths only by adjusting the Bragg angles. Experimental results are presented.
An optically tunable filter device is considered based on a Bragg reflection grating in a nonlinear waveguide. An external light beam of high intensity controls the refractive index via nondegenerate optical Kerr effect and tunes the grating to a selected wavelength. The signal wavelengths are different from the excitation, and typical values of the signal power do not affect the refractive index of the material. Assuming an instantaneous response of the Kerr material and neglecting absorption, we apply the general predictions of the local normal mode expansion theory (LNME) for an arbitrary angle of incidence. A design example is given in a waveguide based on InGaAs on a AlGaAs substrate. The spectral response and tuning range of the waveguide filter device is examined.
In this work, we present a study on the effect of four external electron donors in the formulation of dichromated polyvinyl alcohol (DC-PVA) films on the photosensitivity and real-time diffraction efficiency of volume holographic transmission gratings. The studied amines were found to have a detrimental effect on both holographic characteristics, even in small concentration. We found that higher relative humidity significantly improves the photospeed and the diffracted efficiency in DC-PVA films with and without an electron donor in almost all cases. The effect of the environmental humidity on the real-time diffraction efficiency in DC-PVA films with and without electron donors has been studied. Measurements of holographic characteristics are discussed. The photochemical recording mechanism in DC- PVA in the presence of external electron donors is briefly discussed as well. The amines may form a strong chelate ring structure with the chromium ions and, possibly, interfere with the crosslinking action of the photoreduced chromium. Crosslinking of the PVA matrix may occur either by PVA radicals or complexation of PVA with chromium. In both cases, hologram formation includes an electron transfer reaction, and the amines, good electron donors hinder the process by making chelate with the chromium.
We demonstrate a 4 X 4 holographic star coupler fabricated at He-Ne laser light in silver- halide gelatin. A new approach has been tested for reducing the required number of gratings. It requires a simple recording geometry and a 3D arrangement of the replay beams based on Bragg degeneracy. This element can operate in a large window of replay wavelengths by adjusting the Bragg angles. Experimental results are presented.
We demonstrate 4 X 4 holographic star-couplers fabricated at He-Ne laser light in Silver- Halide Sensitized Gelatin derived from Agfa-Gevaert plates. A modified processing formula is discussed. Two approaches are tested: multiple-beam in a planar configuration and multiple- exposure in a 3D arrangement. Some discussions and experimental results are presented.
Star couplers are fanin-fanout devices with each input beam split into several output beams with arbitrary intensities traveling at different angles. We demonstrate a 4 x 4 holographic star coupler fabricated as multiplexed holographic gratings in silver-halide sensitized gelatin (SHSG) derived from Agfa-Gevaert plates. The processing formula is modified to achieve high diffraction efficiency and an acceptable modulation transfer function within the region of spatial frequencies of interest. The holographic characteristics of the processed SHSG are measured. The required gratings are recorded simultaneously to avoid reciprocity law failure, which occurs in SHSG as well. The gratings are recorded to ensure equal grating strength, which gives an acceptable intensity ratio between the output beams. The star coupler is examined for use as a broadcast medium in fiber-based local area networks (LAN5), where relatively high loss and inhomogeneity between the outputs could be tolerated.
Fourier transform fringe-pattern analysis is a wide-spread technique with many applications to optical metrology. Introducing a spatial carrier into a holographic moiré pattern is also a well-known technique to obtain observable moiré fringes. Preprocessing of the holographic moiré pattern includes optical or digital Fourier filtering to remove the carrier. The pattern is then processed by employing time-consuming and less accurate fringe-counting algorithms. This work presents a new application of the Fourier transform method to automated analysis of holographic moiré fringe patterns. The method has the advantage of instantaneous determination of both phase functions in the fringe pattern. Investigations are carried out on the frequency limitation imposed on the spatial carrier. The technique is demonstrated on computer-generated noisy holographic moiré patterns. The technique does not complicate the conventional holographic moiré arrangement.
Spatial-carrier fringe pattern analysis is a wide-spread technique and has found a lot of applications to optical metrology. Introducing a spatial-carrier into a holographic moire pattern is also a well-known technique to obtain observable moire fringes. Preprocessing of the holographic moire pattern includes optical or digital Fourier filtering to remove the carrier. The fringe pattern is then processed by using time-consuming fringe-counting algorithms. The current work presents a new application of the Fourier transform method to automated analysis of holographic moire fringe patterns. The method owns the advantage of instantaneous determination of both phase functions in the pattern. Investigations are carried out on the frequency restrictions imposed on the phase functions. The technique is demonstrated on computer generated noisy holographic moire patterns. The technique does not complicate the conventional experimental holographic moire arrangement.
Holographic moire method was applied to in-plane displacement measurement in the near- and far-field in single-edged notch specimens in plane stress state under static uniform tension. The holographic set-up and the loading rig are shown. Image-plane hologram recording and real-time observation were adopted to obtain optimum fringe visibility. To avoid fringe visibility degradation because of the localization problem the carrier fringe technique was used. Improving the holographic recording characteristics and signal-to-noise ratio were done through selection of the light-sensitive recording medium and its chemical processing. Silver- halide holographic plates HP-650 with hardened carrying matrix combined with optimized physical and chemical processing was used. Two-dimensional finite element calculation was performed to be compared with the experimental values. Agreement between experimental and numerical results was analyzed and discussed.
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