Abstract: Plasma enhanced chemical vapor deposition(PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin filmsystems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasmasystems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Abstract: This paper proposes two kinds of novel 2/spl times/2 circuit configuration for finite-impulse response (FIR) half-band filters. These configurations can be transformed into each other by a symmetric transformation and their power transmittance is identical. The configurations have only about half the elements of conventional FIR lattice-form filters. We derive a design algorithm for achieving desired power transmittance spectra. We also describe 2/spl times/2 circuit configurations for infinite-impulse response (IIR) half-band filters. These configurations are designed to realize arbitrary-order IIR half-band filter characteristics by extending the conventional half-band circuit configuration used in millimeter-wave devices. We discuss their filter characteristics and confirm that they have a power half-band property. We demonstrate design examples including FIR maximally flat half-band filters, an FIR Chebyshev half-band filter, and an IIR elliptic half-band filter.

Abstract: By properly adjusting the spacing between ring resonators in linear parallel arrays, it is possible to selectively depress modes of the transmission spectra and effectively increase the free spectral range of the individual rings. This enables the synthesizing of a transmission response with modal spacing that otherwise would require much smaller ring diameters. Examples of filters with narrowband and flat-top passbands, which are desirable for all-optical wavelength division multiplexed networks, are given.

Abstract: IPCavelength monitor is provided based on the transmission response of an optical filter (50). The monitor (52a, 52b) provides feedback to the laser (10) enabling it to lock to any given wavelength within its tuning range. The invention is also a process for integrating the wavelength monitor directly on chip with a variety of tunable semiconductor lasers. The invention also comprises a method for controlling the wavelength of a tunable laser by using a wavelength monitor to measure the output light and provide feedback to a control system (62). The laser and wavelength monitors are integrated together on a single indium phosphide chip. The wavelength monitor comprises a filter (50) with a wavelength dependent transmission function and a pair of detectors (52a, 52b). One detector (52a) is illuminated with light that has passed through the filter and the other provides a reference to measure the input intensity. Taking the ratio of the filtered light level to the unfiltered light provides a wavelength dependent signal. The filter (50) is designed such that the transmission function is monotonic and varies from a minimum at one extent of the laser's tuning range to a maximum at the other extent.

Abstract: Multiple micro-optical four-port resonators, such as ring resonators, can be combined to obtain response functions not attainable with a single element A matrix propagator method for analysis of multi-element filters is presented which takes into account bidirectional coupling among elements of a linear resonator array Examples of filters attaining flat-top passbands, desirable for all-optical wavelength-division-multiplexed networks, are given

Abstract: This paper studies the influence of random phase and amplitude fabrication errors on the performance of optical filters based on fiber Bragg gratings (FBG's). In particular, we analyze two effects of particular importance for optical communications: the excess crosstalk induced in apodized gratings commonly used in wavelength-division-multiplexing (WDM) systems, and the time-delay fluctuations that appear in chirped gratings employed to compensate the fiber dispersion. A statistical model is presented to explain these effects in terms of the coherence length of the grating fabrication procedure.

Abstract: In photonic crystals, light propagation is forbidden in a certain wavelength range, the bandgap. In a two-dimensional crystal composed of parallel high-refractive index rods in a low-index background a line defect can be formed by removing a row of these rods, which can act as a waveguide for frequencies in the bandgap of the crystal. In order to get more insight into the main features of such waveguides we have studied a number of properties, using simulation tools based on the finite difference time domain method and a finite element Helmholtz solver. We show conceptually simple methods for determining the bandgap of the crystal as well as the dispersion of a waveguide for wavelengths in this bandgap. For practical applications, it is also important to know how much light can be coupled into the waveguide. Therefore, the coupling of light from a dielectric slab waveguide into the photonic crystal waveguide has been examined, showing that a coupling efficiency of up to 83% can be obtained between a silicon oxide slab and a waveguide in a crystal of silicon rods. Finally, calculations on an ultra-compact filter based on reflectively terminated side-branches of waveguides (similar to tuned stubs in microwave engineering) are shown and discussed.

Abstract: An apparatus for enhanced light transmission through a perforated metal film is provided. The apparatus comprises a metal film having a first surface and a second surface, at least one aperture provided in the metal film and extending from the first surface to the second surface, and first and second dielectric layers. The first dielectric layer is provided substantially adjacent to the first metal film surface, and the second dielectric layer is provided substantially adjacent to the second metal film surface. The respective refractive indices of the first and second dielectric layers are substantially equal. Light incident on one of the surfaces of the metal film interacts with a surface plasmon mode on at least one of the surfaces of the metal film thereby enhancing transmission of light through the at least one aperture in the metal film. The apparatus may have a single aperture or a plurality of periodically arranged apertures, and the metal film surface may be provided with or without a periodic surface topography for further enhanced transmission. Wavelength-selective optical filters, spatial optical filters, light collectors, near-field scanning optical microscope probes and photolithographic masks are also provided.

Abstract: The invention pertains to wavelength-agile optical filters suitable for wavelength-division-multiplexed (WDM) optical communications networks. More particularly, the invention pertains to optical filters with a wavelength reference that can be remotely switched to arbitrarily selectable channels on a standard grid, and to re-configurable optical communications networks employing same. The present invention provides a communication apparatus with a tunable filter which may be used in a wide range of applications including tuning an external cavity laser (ECL), selecting a wavelength for an add/drop multiplexer and providing channel selection and feedback for a wavelength locker. The filter may be utilized as a discrete component or in combination with circulators, wavelength lockers and gain medium. The filter may be implemented in whole or in part as part of a gain medium. The tunable filter exhibits a compact form factor and precise tuning to any selected wavelength of a predetermined set of wavelengths comprising a wavelength grid. The tunable filter may thus be utilized in telecom applications to generate the center wavelengths for any channel on the ITU or other optical grid.

Abstract: Low-loss dichroic filters, a subgroup of frequency-selective components, have been characterized by terahertz time-domain spectroscopy in the region from 0.1 to 3 THz and with Fourier transform spectroscopy. The two data sets are fully consistent. The time-domain spectrometer is used to investigate the phase velocity behavior of dichroic filters. The dichroic filters have various applications in frequency mixing, multiplying, and diplexing experiments. In a novel application, cascaded filters were used to limit the terahertz pulse bandwidth and to monitor molecular transitions of atmospheric water vapor in a selected frequency band.

Abstract: The present invention provides a wavelength selective optical filter device (240) for receiving input radiation and outputting corresponding filtered output radiation, characterized in that the filter device (240) includes a plurality of at least partially mutually coupled Fabry-Perot optical resonators (330, 340, 360; 360, 400, 430) for filtering the input radiation to generate the output radiation, the filter device (240) being tunable from a first radiation wavelength to a second radiation wavelength by mutually detuning the resonators in a period where the resonators are being retuned from the first wavelength (λ1) to the second wavelength (λ2) so that the filter device (240) is substantially in a non-responsive state during the period. The resonators incorporate freely suspended mirrors (360, 430) which are electrostatically actuated to affect tuning of the resonators (330, 340, 360; 360, 400, 430). The filter device (240) is thereby capable of tuning between different wavelengths without tuning through wavelengths therebetween. The filter device (240) can be included into an add-drop filter (10) for providing channel add and drop functions when the filter (10) is incorporated in a multichannel WDM communication system (100).

Abstract: PROBLEM TO BE SOLVED: To provide a semiconductor laser device capable of keeping the density of recording material constant even when the spectroscopic sensitivity characteristic of a photodetector and the recording material is not flat. SOLUTION: This device is equipped with the photodetector 59 irradiating the recording material A with a light beam L emitted from a semiconductor laser element 50 and detecting one part of the emitted light beam L, and constituted so that output from the photodetector 59 is fed back to the element 50 so as to control light quantity. In such a case the light quantity is adjusted by using an optical filter 58a having the spectroscopic transmissivity characteristic corresponding to the spectroscopic sensitivity characteristic of the material A and that of the photodetector 59.

Abstract: Method and apparatus for an analog FM optical link having a low noise figure and a high spurious-free dynamic range. In one embodiment, the apparatus includes a FM DFB laser and a balanced receiver. The balanced receiver includes an optical splitter to split the received beam into two optical paths. Each of the two paths includes an optical filter and a photodetector. A differentiator coupled to the electrical output of the photodetectors produces a demodulated electrical signal in response to the RF signal used to modulate the DFB laser. This configuration can eliminate the laser relative intensity noise and second order harmonics. In addition, third order distortion is eliminated when there is no intensity modulation or greatly reduced when intensity modulation is present.

Abstract: All-fiber acoustooptic tunable filters (AOTFs) with bandwidth as small as 2.8 nm were fabricated from highly uniform, tapered optical fiber. These filters have typical excess loss of 0.02 dB, greater than 20 dB extinction, are polarization insensitive, and have the smallest bandwidth-length products reported to date.

Abstract: A new and simple two-chip concept for microcavities with application in dense wavelength-division-multiplexed systems as tunable optical filters is presented. The Fabry-Perot cavity is created without any costly distance holder or sacrificial layer by simply utilizing the stress of the dielectric Bragg-mirrors against the semiconductor substrate. Electrostatic tunability is achieved by making use of the high capacitance of the metallized cantilevers holding the actuable membrane.

Abstract: We have demonstrated a 12Gsample/s continuous time ADC system using wavelength division sampling. Three 4Gsample/s optical pulse trains operating at different wavelengths are interleaved in time to achieved a rate of 12Gsample/s. The interleaving and demultiplexing of the samples is performed using only passive optical filters. Three 4Gsample/s electronic ADCs are then used to digitise the data, the complete signal being reconstructed in the digital domain.

Abstract: We report on the use of frequency-modulated continuous wave (FMCW) techniques for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a linear swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency domain and demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A three-sensor FMCW multiplexed FBG array of parallel topology and a six-sensor hybrid FMCW/WDM system were experimentally demonstrated with -30 dB crosstalk between sensors and 2 /spl mu//spl epsi/ resolution in terms of root mean square (RMS) strain value.

Abstract: The present invention relates to a variable optical filter that can be used to filter an incoming signal, attenuate an incoming signal or in one configuration switch an incoming signal from one path to another. The present invention has found that an accurate and economical variable optical filter can be created by using an elastomeric material having a high coefficient of expansion in cooperation with a means for locally varying the temperature of the elastomeric material as an actuator for moving a reflective surface within the optical filter. The actuator can be operated in a controlled manner for example, to effect a tilt of the reflective surface for switching or attenuating an optical signal, or to vary the resonant wavelengths of a resonant cavity between partially reflective surfaces. In accordance with the invention there is provided, a variable optical filter comprising an input port and an output port; a first at least partially reflective disposed to receive a beam of light launched from the input port; an elastomeric material for supporting and varying the position of the at least partially reflective surface with respect to the input port; a heater for applying variable amounts of heat to the elastomeric material to move or pivot the at least partially reflective surface relative to the input port; and, control means for controlling the heater and for providing a signal to apply variable amounts of heat.

Abstract: Schemes based on an optical all-pass filtering techniques for multiplying the intensity repetition-rate of an optical pulse train are examined. These methods are in principle without energy loss and the multiplication factor can be any given integer. Practical implementation using cascaded side-coupled ring resonators or multireflection filters is proposed and analyzed for the first time.

Abstract: High-efficiency color filters composed of a microprism array, optical interference color filters, and a microlens array light compressor were developed to increase the optical throughput of liquid-crystal projection systems. The new devices utilize the energy of whole spectra by taking full advantage of a light compressor and interference dichroic filters to distribute the energy of spectra to the respective color pixel area. Thus high-efficiency micro-optical color filters allow efficient utilization of the energy of incident light and maximize the optical throughput of the projection system.

Abstract: In recent years, microspectrometer systems fabricated by the LIGA technology for use in the visible wavelength range increasingly made their way to the market. For spectral analysis in the infrared range, a highly transmissive hollow waveguide has now been developed and demonstrated successfully. In combination with linear detector arrays, the hollow waveguide microspectrometer allows to design portable near infrared spectrometer systems. Such a system has now been applied to evaluate the spectrum from 0.9 micrometer to 1.15 micrometer in second order and from 1.15 micrometer to 1.75 micrometer in first order. Due to its outer dimensions of 54 X 36 X 7 mm3 and a low power consumption, it can be integrated in portable spectral analysis systems. It exhibits a high sensitivity (NEP below 11 pW) due to a fiber-optical entrance with a fiber core diameter of 300 micrometer. An indium-gallium-arsenide linear detector array based on a novel setup concept is incorporated in the system. Furthermore, units for preamplification of the signal and 16-bit AD conversion are contained. In the present paper, the setup and fabrication of the whole microspectrometer system, its optical features and the detector-specific solutions are described. The fabrication process which is based on molded polymer parts is presented. Using the experimental results of the electro- optical tests and the polymer spectra measured, the performance of the system will be demonstrated.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Abstract: The invention relates to an image representation system comprising a projector (1) for emitting projection light (4) so as to generate an image on a light-reflecting projection surface (7). According to the invention the wavelengths of the projection light (4) are situated in one or more discrete wavelength ranges in the visible spectrum and the projector-side of the projection surface (7) is covered with an optical filter (11) which is permeable only to light at wavelengths corresponding to those of the projection light (4).

Abstract: We present a theoretical investigation of the spectral properties of spontaneous emission in semiconductor optical amplifiers. We use an extended (3/spl times/3) transfer matrix formalism to derive in the spectral domain an expression for the total longitudinally averaged internal field, which is valid regardless of the levels of optical input and bias current. The material parameters are saturated not only by the monochromatic signal, but also by the amplified spontaneous emission, filtered into the resonance modes of the structure, and integrated over its whole spectral range.

Abstract: This paper gives results for using distortion tolerant filters to improve performance of fingerprint correlation matching Three types of distortion tolerant filters were tested: summation, weighted, and MINACE A set of 55 fingers were used from NIST Special Database 24 to evaluate the filters Our results show performance was improved from 49% correct, using one training fingerprint, to 100% correct, using multiple training fingerprints, and a distortion- tolerant MINACE filter, with no false alarms© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering Downloading of the abstract is permitted for personal use only

Abstract: In a WDM optical communication system, at least one optical tunable filter is placed along an optical fiber provided as an optical transmission path between a transmitting station and a receiving station. The optical tunable filter has a controllable transmission factor versus wavelength characteristic. In the receiving station, the transmission characteristics (for example, optical signal to noise ratios and Q factors) for optical signals of different wavelengths propagated over the optical fiber are measured. The measurements are sent to the transmitting station. The transmitting station then properly controls both the amounts of pre-emphasis in the transmitting station and the wavelength characteristic of the optical filter on the basis of the measurements to thereby equalize the transmission characteristics for the optical signals. In this case, it is also possible to calculate the optimum proportion of allocation of control between the pre-emphasis control and the filter control and then perform the pre-emphasis control and the filter control according to the optimum proportion of allocation. In this control, the main controller may be either the transmitting station or the receiving station.

Abstract: The invention comprises a system and method of calibrating a reflected spectral imaging apparatus used for analysis of living tissue. In addition to the reflected spectral imaging apparatus itself, the calibration apparatus comprises an optical filter that is placed between the light source used in the imaging apparatus and the object under analysis, and a calibration module. The filter is fabricated such that when the light is passed through the filter, an image is projected onto the focal plane where imaging is to take place within the object. The image projected by the filter comprises a plurality of areas, each having a different known optical density. For each area, the calibration module measures the intensity of the light reflected from the area and maps the light intensity measurement to the optical density known to be present at the area. This correspondence of light intensity measurements and known optical densities is then used to calibrate the reflected spectral imaging apparatus.