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OPTIMIZATION OF A REWRITABLE NARROWBAND FILTER IN A SBN:75 CRYSTAL
Abstract
We propose a rewritable optical frequency filter based on a volume Bragg grating recorded by holography on an SBN:75 photorefractive crystal. The theoretical results show the possibility of implementing a narrow-band filter whose reflectance is total for the characteristic wavelength of the third harmonic of the infrared for both TE and TM polarizations by optimizing the size of the interference fringes and the angle of incidence of the beam to be filtered, which must be close to 80 degrees.
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LaF<sub>3</sub>/MgF<sub>2</sub> reflection filters with a high spectral-discrimination capacity of the atomic-oxygen lines at 130.4 and 135.6 nm, which were employed in vacuum ultraviolet imagers, were prepared by molybdenum-boat thermal evaporation. The optical properties of reflection filters were characterized by a high-precision vacuum ultraviolet spectrophotometer. The vulnerability of the filter's microstructures to environmental contamination and the recovery of the optical properties of the stored filter samples with ultraviolet ozone cleaning were experimentally demonstrated. For reflection filters with the optimized nonquarter-wave multilayer structures, the reflectance ratios R<sub>135.6 nm</sub>/R<sub>130.4 nm</sub> of 92.7 and 20.6 were achieved for 7° and 45° angles of incidence, respectively. On the contrary, R<sub>135.6 nm</sub>/R<sub>130.4 nm</sub> ratio of 12.4 was obtained for a reflection filter with a standard π-stack multilayer structure with H/L=1/4 at 7° AOI.
Highly-tolerant distributed Bragg reflectors (DBRs) based on the same materials consisting of nanoporous/dense titanium dioxide (TiO2) film pair structures with wide-angle and broadband highly-reflective properties at visible wavelengths are reported. For a high refractive index contrast, the two dense and nanoporous TiO2 film stacks are alternatingly deposited on silicon (Si) substrates by a oblique angle deposition (OAD) method at two vapor flux angles (θα) of 0 and 80° for high and low refractive indices, respectively. For the TiO2 DBRs at a center wavelength (λc) of 540 nm, the maximum level in reflectance (R) band is increased with increasing the number of pairs, exhibiting high R values of > 90% for 5 pairs, and the normalized stop bandwidth (∆λ/λc) of ~17.8% is obtained. At λc = 540 nm, the patterned TiO2 DBR with 5 pairs shows an uniform relative reflectivity over a whole surface of 3 inch-sized Si wafer and a large-scalable fabrication
Narrow notch and multi-notch thin-film filters have applications in many fields. Raman spectroscopy and laser-based fluorescence instruments both desire filters that can remove one or more narrow spectral bands while maintaining high transmittance for light at adjacent wavelengths. Key figures of merit for notch filters include the width of the notch as a fraction of the blocked wavelength (narrower is better), the degree of suppression, and the overall transmittance of the filter. Design approaches for narrowband notch and multi-notch filters are well-known, and include rugate or "quasi-rugate" designs. The manufacturing of these filters has proven to be challenging. The filters have to be very thick to achieve high suppression, and typically involve the deposition of gradient index layers or many very thin, discrete layers. Accurate spectral placement of the notches often requires extreme process control or post-deposition tuning of the filter. JDSU has recently developed a design and manufacturing capability for single and multi-notch filters in the visible wavelength region where the notch width is less than 2% and the blocking levels are greater than OD 6. Designs for these types of filters can be 20 - 60 mum thick and consist of more than 1,000 layers. Our Ucp-1 high-rate magnetron sputtering platform with load-lock provides an inherently stable deposition process. This enables us to coat these challenging designs. In this paper, we present examples of both single and multi-notch filters that have 612 to 4410 layers and are 31 to 127 mum thick.
High efficiency volume Bragg gratings (VBGs) in photo-thermo-refractive (PTR) glass provide unmatched optical filtering capabilities with optical densities as high as 50 dB and linewidths as narrow as 1 cm -1 . In this work we review recent advances in VBG technologies that enabled key improvements of high efficiency grating properties and led to development of unique VBG based optical filters for Raman spectroscopy and other applications. Such narrow band notch and bandpass filters make ultra-low frequency Raman measurements possible with single stage spectrometers, therefore, largely improving optical throughput of high end Raman instruments while reducing complexity of the measurements. In this work we also present novel volume multiplexed ultra-narrow band VBG filters with high reflection at multiple wavelengths. Such multiband holographic optical elements are formed by overlapping of several high efficiency VBGs in a single glass plate. Raman spectra obtained with multiband VBG filters and single stage spectrometers, show unmatched capability of the filters to provide simultaneous access to Stokes and anti-Stokes Raman modes with frequencies as low as 5 cm -1 at different wavelengths.
An experimental demonstration of a moiré reflecting Bragg grating in photo-thermo-refractive glass is carried out. This narrowband filter is obtained by the recording of two reflecting Bragg gratings with different periods. Filters with central wavelength at 1550 nm, bandwidth of 50 pm, and transmission higher than 95% are demonstrated. The methods to decrease bandwidth to 1 pm are finally investigated.
Cr/Sc multilayer mirrors, synthesized by ion-assisted magnetron sputter deposition, are proved to have a high near-normal reflectivity of R = 14.5% at a grazing angle of 87.5 degrees measured at the wavelength lambda = 3.11 nm, which is an improvement of more than 31% compared with previously published results. Elastic recoil detection analyses show that the mirrors contained as much as 15 at. % of N and traces of C and O. Soft x-ray reflectivity simulations reveal interface widths of sigma = 0.34 nm and an exceptionally small layer thickness drift of approximately 1.6 x 10(-5) nm/multilayer period throughout the stack. Simulations show that a reflectivity of R = 25.6% is attainable if impurities and layer thickness drift can be eliminated. The abrupt interfaces are achieved with ion assistance with a low ion energy of 24 eV and high ion-to-metal flux ratios of 7.1 and 23.1 during Cr and Sc sputter deposition, respectively. In addition, a near-normal incidence reflectivity of 5.5% for the C VI emission line (lambda = 3.374 nm) from a laser plasma source was verified.
We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45 degrees angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm(2) with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.
A tunable narrowband (FWHM=2.5 GHz) 1.55 μm reflective drop
filter is realised in an iron doped indium phosphide crystal. The index
grating results from the photorefractive effect via an interference
pattern. The grating period is determined by the wavelength of the laser
diode which provides the two counter-propagating writing beams
Written by a world-renowned authority of optical coatings, Thin-Film Optical Filters, Fourth Edition presents an introduction to thin-film optical filters for both manufacturers and users. The preeminent author covers an assortment of design, manufacture, performance, and application topics. He also includes enough of the basic mathematics of optical thin films to enable readers to carry out thin-film calculations. This new edition of a bestseller retains most of the descriptions of older design techniques because of their importance in understanding how designs work. However, this edition includes a substantial amount of new material as well. A new chapter on color takes into account the increasing importance of color in optical coatings. In addition, a new section discusses the effects of gain in optical coatings. This comprehensive yet accessible book continues to offer valuable insight into the principles, techniques, and processes of successful coating design. It provides the sound foundation required to make further advances in the field.
1. The Phenomenon of Photorefraction.- 2. Holography, and Optical Information Processing Systems.- 3. Salient Features of Photorefractive Crystals for Holography.- 4. Formation of an Electric-Field Grating During Holographic Recording in PRCs.- 5. Light Diffraction from Anisotropic Volume Phase Gratings.- 6. Fundamentals of Dynamic Holography.- 7. The Physics of Electro-Optic Spatial Light Modulators.- 8. Spatial Light Modulators.- 9. Applications of Photorefractive Crystals.- Appendix: Photorefractive Crystals.- A10 Semiconductor Photorefractive Crystals.- A 11 Other Photorefractive Crystals and Media.- References.
We report on metalorganic vapor phase epitaxy (MOVPE) of distributed Bragg reflectors (DBR) applying a periodic modulation of the GaN doping concentration only. The doping modulation changes the refractive index of GaN via the Burstein-Moss-effect. MOVPE growth of highly doped GaN:Ge and modulation of the dopant concentration by at least two orders of magnitude within few nanometers is required to achieve a refractive index contrast of 2-3%. Such modulation characteristic is achieved despite the presence of Ge memory effects and incorporation delay. We realized DBRs with up to 100 layer pairs by combining GaN:Ge with a nominal doping concentration of 1.6×1020 cm−3 as low-refractive index material with unintentionally doped GaN as high-refractive index layer. Scanning transmission electron microscope images reveal DBR structures with abrupt interfaces and homogenous layer thicknesses in lateral and vertical direction. Reflectance measurements of DBRs designed for the blue and near UV-spectral region show a narrow stopband with a maximum reflectivity of 85% at 418 nm and even 95% at 370 nm. InGaN/GaN multi-quantum well structures grown on top of such DBRs exhibit narrow emission spectra with linewidths below 3 nm and significantly increased emission intensity.
The Sr//1// minus //xBa//xNb//2O//6 (SBN) and Ba//2// minus //xSr//xK//1// minus //yNa//yNb//5O//1//5 (BSKNN) tungsten bronze solid-solution systems are shown to be promising photorefractive materials. Because of the versatility of the bronze structure, both the response time and spectral response can be controlled by altering the type of dopant and its crystallographic site preference. This paper reviews the current status of the tungsten bronze crystals SBN and BSKNN for photorefractive applications in terms of their growth, electro-optic character, and the role of cerium dopants. Ferroelectric morphotropic phase boundary (MPB) bronze materials are also discussed as important for future development.
This chapter examines different aspects chirped fiber Bragg gratings. Gratings that have a non-uniform period along their length are known as chirped. Chirp in gratings take different forms. The period varies symmetrically, either increasing or decreasing in period around a pitch in the middle of a grating. The chirp is linear and the period varies linearly with length of the grating, may be quadratic, or may even have jumps in the period. The performance of chirped gratings as a dispersive filter specifically for the compensation of chromatic dispersion is analyzed. An important feature of a dispersion-compensating device is the figure of merit. There are several parameters that affect the performance of chirped fiber Bragg gratings for dispersion compensation. These are the insertion loss, dispersion, bandwidth, polarization mode-dispersion, and deviations from linearity of the group delay and group delay ripple. It is found that the transfer matrix method is ideally suited to chirped gratings, since the grating is broken up into smaller sections of uniform period and/or refractive index profile. It is observed that the role played by the rear end of the grating is apparent when the coupling constant is apodized asymmetrically.
In this article, light reflection spectrums for different one-dimensional multi-layer structures are obtained. Optical reflectivities for periodic, superperiodic, and quasiperiodic structures have been calculated using the transfer matrix method. PBGs of each structure have been obtained using the optical reflectivity pattern. From a comparison of reflectivity and standard dense wavelength division multiplexing grids, it is demonstrated that narrow and dense band filters can be built from generalized Fibonacci quasiperiodic structures. Reflectivity of these structures contains bands that meet the ITU-T dense wavelength division multiplexing standard. Reflectivity of other multi-layer structures has poor or less potential to be used in dense wavelength division multiplexing systems.
Introduction.- Light propagation in nonlinear optical media.- The photorefractive nonlinearity.- Spatial photorefractive solitons in anisotropic, saturable media.- Interaction of spatial solitons in saturable and photorefractive media.- Self-organized pattern formation in single-feedback systems.- Selection, manipulation and control of self-organized patterns.- Transverse patterns in active photorefractive oscillators.
We proposed an analytical method to design optical minus filters by the thickness modulation of discrete, homogeneous thin-film layers of a two-material multilayer coating. The main stack provides the narrow, second-order rejection band, and the correct thickness-modulation apodization and match layers can effectively suppress the sidelobes of the passband. Using this approach, we can design minus filters with layer thicknesses close to half-wave of the rejection wavelength, making this method well suited for accurate monitoring during the deposition.
Periodic multilayer structures of quarterwave and multiple quarterwave stacks with shifted ratios of high and low index layers in the half-wave pairs are considered. Analytical dependencies of the reference wavelength reflectance and the width of high reflectance zone on the number of layers, fraction quarterwave and layer refractive indices are obtained. The structures are used as starting designs for notch filters. Obtained dependencies allow one to estimate in advance parameters required to achieve target spectral characteristics.
A general analytical model is developed for analysis of the diffraction properties of layered structures of photorefractive gratings. With this model a simple closed-form expression is derived in both the transmission and the reflection geometry for the diffraction efficiency that describes the Bragg selectivity properties of these structures. We then propose and analyze a novel optical device, a dynamic multiple-wavelength filter, that combines the diffraction properties of such layered structures with the photorefractive effect and the electrically controlled diffraction effect.
Sylvester's theorem is often applied to problems involving light propagation through periodic optical systems represented by unimodular 2 × 2 transfer matrices. We extend this theorem to apply to broader classes of optics-related matrices. These matrices may be 2 × 2 or take on an important augmented 3 × 3 form. The results, which are summarized in tabular form, are useful for the analysis and the synthesis of a variety of optical systems, such as those that contain periodic distributed-feedback lasers, lossy birefringent filters, periodic pulse compressors, and misaligned lenses and mirrors. The results are also applicable to other types of system such as periodic electric circuits with intracavity independent sources, high-energy particle accelerators, and periodic computer graphics manipulations that may include object translation. As an example, we use the 3 × 3 form of Sylvester's theorem to examine Gaussian beam propagation in a misaligned resonator.
Bragg gratings yield a single diffracted order when irradiated by a coherent beam at the appropriate Bragg angle. In many cases, nearly all of the energy of the incident beam can be coupled to the diffracted beam. Hence these gratings can form many useful optical elements, and this has been realized in 1-D, 2-D, and 3-D photonic crystals. Bragg gratings made with liquid crystals offer the added dimension of dynamic properties through the large electro-optical effect in liquid crystals. Applications for spatial light modulators are numerous, including optical switches, modulators, active optical elements (e.g., lenses), laser sources, and tunable filters. We have been exploring a number of approaches for making liquid crystal Bragg gratings, including holographic polymer-dispersed liquid crystals, cholesteric liquid crystals, and homogenous nematic liquid crystals in hybrid devices. We have studied the dynamic properties of these Bragg gratings by electrical, thermal, and optical stimulation. Modification and control of optical and dynamic properties have been obtained through combinations of liquid crystals with polymers, combinations of various dopant materials, and interactions of liquid crystals with organic and inorganic interfaces. We discuss the materials, fabrication, characterization, and physics of liquid crystal Bragg gratings and present the results of various devices we have studied in our lab. We will also discuss potential applications.
A coupled wave analysis is given of the Bragg diffraction of light by thick hologram gratings, which is analogous to Phariseau's treatment of acoustic gratings and to the dynamical theory of X-ray diffraction. The theory remains valid for large diffraction efficiencies where the incident wave is strongly depleted. It is applied to transmission holograms and to reflection holograms. Spatial modulations of both the refractive index and the absorption constant are allowed for. The effects of loss in the grating and of slanted fringes are also considered. Algebraic formulas and their numerical evaluations are given for the diffraction efficiencies and the angular and wavelength sensitivities of the various hologram types.
A narrow-bandwidth interference filter has been developed using volume holography in photorefractive LiNbO3 crystals. The device works as a Bragg reflector using a volume hologram recorded, fixed and developed in a LiNbO3 sample. With a 2 mm thick LiNbO3 plate the following performances have been achieved: a peak reflectivity above 32%, a full-width at half-maximum bandwidth below 0.05 nm, an angular field of view of about 3 degrees and a peak wavelength of 518.45 nm (for green Ar laser writing wavelength). These performances compare well with that exhibited by the conventional Lyot-Ohman and Solc filters. A complete explanation of the fabrication procedure is given, including practical details needed for producing such an efficiency with fixed holograms in LiNbO3.
An investigation was made of the photoelectric and photorefractive properties of barium strontium niobate crystals having the composition (SrxBa1–x)1–y(Nb2O6)y where x =0.61 and y =0.4993, doped with 0.05 and 0.10 wt.% CeO2. The crystals have the following characteristics: a sensitivity of 2×10–3 cm2/J at λ = 0.44 μ with spatial frequencies of 700–2000 lines/mm, a data storage time exceeding 9 months, a maximum diffraction efficiency exceeding 50%, and an unlimited number of read-write cycles. It was established that the holograms were recorded by a diffusion mechanism.
Fine tuning of narrow-bandwidth interference filters based on volume holographic gratings fixed in photorefractive LiNbO3 has been demonstrated. A filter with a peak wavelength at 518.45 nm, peak reflectivity 32.5% and bandwidth of 50 pm was used. Filter tuning has been achieved both by varying the filter temperature and by applying an electric field. By varying the filter temperature between 35 and 60 degrees C a temperature tuning coefficient of 5.1 pm K-1 was obtained. The effect is induced by the temperature-dependent change of refractive index and by the lattice thermal expansion. By applying a direct current electric field, an electrical tuning coefficient of 5.9 pm kV-1 has been measured too. In this case the tuning mechanisms are the electro-optic and piezoelectric effects. On the other hand, with an applied alternating current electric field, a beam intensity modulation from 90% to 20% has been demonstrated.
Cerium-ion-doped barium-strontium niobate crystals are considered as a medium for information holographic recording. A brief account is given of the technology of crystal growth and of their optical and electrophysical properties. The laws of photoconductivity drop are established and the products of phenomenological parameters μβτ are determined. The photorefractive characteristics of the crystal are investigated; the dependences of efficient sensitivity and amplification coefficient on spatial frequency are established. A new effect of light anisotropic scattering by crystals is observed and its interpretation is given. Finally holographic characteristics of the crystal are presented.Cer-dotierte Barium-Strontiumniobatkristalle als Medien für holographische Informationsauf-zeichnung werden untersucht. Es wird iiber die Technologie des Kristallwachstums sowie üiber deren optischen und elektrophysikalischen Eigenschaften berichtet. Die Gesetzmäßigkeit des Photoleitungsabfalls und die Produkte der phanomenologischen Parameter μβτ werden bestimmt. Die Lichtbrechungscharakteristiken des Kristalls werden untersucht; dabei werden die Ab-hängigkeiten der wirksamen Empfindlichkeit und des Verstärkungskoeffizienten von der Ortsfrequenz bestimmt. Ein neuer Effekt der anisotropen Lichtstreuung durch Kristalle wird beobachtet und seine Erklärung gegeben. Schließlich werden die holographischen Charakteristiken des Kristalls mitgeteilt.
We described theoretical and experimental investigations of a dynamic optical filter based on a reflective holographic Bragg grating recorded in BaTiO3 and consisting of a number of phase-shifted sections. The reconfiguration from a pass- to a stop-band transfer function, the reconfiguration of the transfer function providing reflection of waves at one, two, or three wavelengths and the synthesis of a flat-top transfer function in real time are demonstrated. All these transformations were performed by phase-shift keying of sections of the holographic grating.
Permanent refractive-index gratings in waveguide devices are of high potential for optical communication systems that make use of the high spectral selectivity of holographic filters, e.g. dense wavelength division multiplexing (DWDM), optical sensors, or narrow-bandwidth mirrors for integrated waveguide lasers. In this contribution we review our results on holographically recorded refractive-index gratings in Cu-doped LiNbO 3 channel waveguides. Elementary holograms are recorded with green light and read in reflection geometry in the insensitive infrared wavelength region around 1.5 µm. To enable long-term stability of the Bragg gratings a thermal fixing technique is applied. In this way strong and almost permanent refractive-index gratings are obtained and their application as narrow-bandwidth filters for DWDM applications is demonstrated. In comparison with Bragg gratings in silica fibres, the electro-optic effect in LiNbO 3 allows for a direct wavelength tuning and a fast, reliable electrical switching of these gratings.
Light propagation in crystals Tensorial Analysis Electro-optic effect Concluding Remarks
Single‐crystal Sr 0.75 Ba 0.25 Nb 2 O 6 when used in conjunction with an externally applied electric field is shown to be a sensitive volume holographic medium capable of high‐resolution information storage. Optically induced refractive index changes of 5 × 10<sup>-4</sup> have been observed with laser exposure of 14 J/cm<sup>2</sup>. A simple relation between the applied electric field and optical exposure is determined which allows electrical control of optically induced refractive index change. In addition to control of the optically induced effect it is found that the applied electric field can also be used to control the diffraction efficiency of the holographic reconstruction process.
The ferroelectric phase Sr1−xBaxNb2O6 has been grown as large single crystals by the Czochralski technique. Single crystals have been grown over the composition range Sr0.75Ba0.25Nb2O6 to Sr0.25Ba0.75Nb2O6. The crystals are yellow-amber in color, transparent from 0.4 μm to 6.0 μm, show a room temperature tetragonal symmetry and appear to be in the point group 4mm. The optical character is uniaxial negative and the indices of refraction are n0 = 2.31 and ne = 2.27. A large dielectric anomally occurs at 70 °C for the composition Sr0.75Ba0.25Nb2O6 and extends to 220 °C for the barium rich member. Ferroelectric hysteresis loops have been observed at room temperature as well as exceptionally large linear electro-optic effects.
Holographic storage experiments using crystal Sr(0.75)Ba(0.25)Nb(2)O(6) (SBN 75/25) as a volume-phase holographic medium show that this material is the most sensitive crystalline storage medium yet discovered. An exposure level of 0.003 J/cm(2) at 0.488 microm produces a 1% diffraction efficiency in a 5-mm length of crystal. In addition to high recording sensitivity, SBN 75-25 exhibits interesting electric-field induced effects that include electric-field enhanced recording sensitivity and voltage-switchable latent-to-active holographic reconstruction efficiency. These effects are explained in terms of drift and diffusion of photoionized carriers coupled with the nonlinear electrooptic behavior characteristic of this low Curie temperature ferroelectric crystal. Detailed measurements of the holographic recording and reconstruction properties of thick phase-volume holograms stored in SBN 75/25 are reported. A mechanism is proposed that correlates the observed electrically controlled holographic response with the dielectric behavior of the crystal. These interesting effects are used to implement a novel, layered optical memory having no moving parts and having electrical access for writing or reading selected layers. An experimental model of the layered memory was constructed to demonstrate the interrelation of the several phenomena involved in the layered concept.
The propagation of electromagnetic radiation in periodically stratified media is considered. Media of finite, semi-infinite, and infinite extent are treated. A diagonalization of the unit cell translation operator is used to obtain exact solutions for the Bloch waves, the dispersion relations, and the band structure of the medium. Some new phenomena with applications to integrated optics and laser technology are presented.
The angular properties of various wideband high reflectors are investigated. The theory is developed for the design of high reflectors based on contiguous quarter-wave stacks for use at one oblique angle or for a range of angles of incidence. Numerical results are presented for several high reflectors designed to have a high reflectance in the 0.4-0.8-mum spectral region for use at 50 degrees and with angles of incidence ranging between 0 degrees and 50 degrees . A random error perturbation analysis shows that such layer systems can be produced experimentally.
A combination of stepwise graded refractive-index profiles and a cavity structure is used for designing narrow-bandpass filters of TiO(2)/SiO(2) multilayer films upon BK7 glass substrates. Symmetrical profiles of stepwise graded refractive indices result in high transmittance of passbands for the designed filters. The bandwidth of the narrow-bandpass filter is controlled by adjustment of parameters such as the thickness and the number of layers in the multilayer stack. This design is proposed as a new and simple method for coating synthesis of optical filters.
Light-induced dynamic gratings and photorefraction
- H J Eichler
- A Hermerschmidt
Eichler, H. J. and A. Hermerschmidt, "Light-induced dynamic gratings and photorefraction,"
Photorefractive Materials and Their Applications 1, Basic Effects, P. Günter and J.-P. Huignard,
Eds., 7-28, Springer, New York, 2006.