Showing papers on "Optical filter published in 2000"

Imaging spectroscopy using tunable filters: a review

Abstract: Major spin-offs from NASA's multi- and hyper spectral imaging remote sensing technology developed for Earth resources monitoring, are creative techniques that combine and integrate spectral with spatial methods. Such techniques are finding use in medicine, agriculture, manufacturing, forensics, and an e er expanding list of other applications. Many such applications are easier to implement using a sensor design different from the pushbroom or whiskbroom air- or space-borne counterparts. This need is met by using a variety of electronically tunable filters that are mounted in front of a monochrome camera to produce a stack of images at a sequence of wavelengths, forming the familiar 'image cube'. The combined spectral/spatial analysis offered by such image cubes takes advantage of tools borrowed form spatial image processing, chemometrics and specifically spectroscopy, and new custom exploitation tools developed specifically for these applications. Imaging spectroscopy is particularly useful for non homogeneous samples or scenes. examples include spatial classification based on spectral signatures, use of spectral libraries for material identification, mixture composition analysis, plume detection, etc. This paper reviews available tunable filters ,system design considerations, general analysis techniques for retrieving the intrinsic scene properties from the measurements, and applications and examples.

Plasma deposition of optical films and coatings: A review

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.

Higher order filter response in coupled microring resonators

Abstract: Second- and third-order filter response is experimentally shown in filters constructed using mutually coupled microring resonators. The higher order filters have passbands with sharper rolloff and flatter tops, and greater out-of-band rejection than previously demonstrated microring filters, which were first order. The effect of varying interesonator coupling on filter passband shape was studied.

An efficient photolysis system for fast-response NO2 measurements

Abstract: A new photolytic converter for NO2 measurement is described and its performance assessed using laboratory, ground-, and aircraft-based field data. Focusing the output of a 200-W short-arc Hg lamp into a photolytic cell attains conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO3. Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO2 inlet line and photolysis cell residence times. NO and NO2 sample residence times are matched in a multichannel instrument so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the new design. This configuration permits simple and accurate retrieval of NO2 concentrations in aircraft transects of power plant plumes, where ambient NO concentrations can change over several orders of magnitude in seconds. At lower concentrations found in the planetary boundary layer, agreement between calculated and observed NO2 is within ±(40 pptv+7%) for a 10-s average. The new converter consumes less power, is more efficient, and is less expensive to operate than previous photolysis designs.

Optical half-band filters

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.

High-performance microwave transversal filter using fiber Bragg grating arrays

Abstract: Measured results from a microwave photonic traversal filter exploiting fiber Bragg grating (FBG) arrays and achieving a high-performance tunable bandpass microwave response are presented. Tuning is affected by changing the optical carrier wavelength to select the operating gratings, and the high-quality frequency response is obtained by weighting the reflections of the gratings. This experiment demonstrates that sophisticated passive signal processing functionality can be achieved practically using FBGs.

A tunable laser source with an integrated wavelength monitor and method of operating same

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.

Vernier effect in asymmetrical ring resonator arrays

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.

Synthesis of optical filters using ring resonator arrays

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

Effects of random phase and amplitude errors in optical fiber Bragg gratings

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.

Enhanced optical transmission apparatus with improved inter-surface coupling

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.

Numerical studies of 2D photonic crystals: Waveguides, coupling between waveguides and filters

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.

Method and apparatus for filtering an optical beam

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.

Wavelength selective optical filter

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).

Experimental and theoretical study of filtered optical feedback in a semiconductor laser

Abstract: We report on the systematical investigation of the steady-state regime and the dynamical behavior of a semiconductor laser subject to delayed filtered optical feedback. We study a Fabry-Perot (FP) interferometer type of filter placed in the external feedback loop of a diode laser. The effects of the filter on the locking of the diode laser frequency to the external cavity modes are described. We report and observe hysteresis, bistability, and multistability and show that all these are well described by a set of rate equations for the coupled laser and FP cavity system. We also present an experimental stability diagram that summarizes the dynamical behavior of the system.

Semiconductor laser device

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.

Very narrow spectral filters with multilayered grating-waveguide structures

Abstract: Promising configurations for grating-waveguide structures are presented. In these structures, the grating layer, which is normally adjacent to the waveguide layer, is displaced by means of an intermediate layer, resulting in significant reduction of losses and weaker coupling. This leads to very narrow spectral bandwidths and high contrast ratios. Experimental results reveal that the spectral bandwidths can be as low as 0.1 nm with contrast ratios greater than 1000, suggesting that these grating-waveguide structures could be useful for optical communication networks.

All-optical analog fm optical receiver

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.

Characterization and application of dichroic filters in the 0.1-3-THz region

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.

Compact all-fiber acoustooptic tunable filters with small bandwidth-length product

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.

High-selectivity single-chip spectrometer in silicon for operation in visible part of the spectrum

Abstract: A microspectrometer has been realized based on an array of Fabry-Perot optical thin-film filters. The 16-channel microspectrometer is compatible with IC fabrication methods and operates in the visible spectral range with an interchannel shift of 6 nm. Each of the channels is sensitive in a single peak with full-width-half-maximum (FWHM) of 16 nm. Also a FWHM below 2 nm and finesse of 40 for narrow band operation is demonstrated. The device can easily be tuned during fabrication to cover a different spectral band only by adjusting the etching times without affecting the device layout. Such a device is extremely suitable for applications in microsystems because of its small size, high spectral selectivity, and low cost. Microspectrometers for the UV and IR regions are also feasible using this technique.

A new and simple concept of tunable two-chip microcavities for filter applications in WDM systems

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.

12 Gsample/s wavelength division sampling analogue-to-digital converter

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.

Variable optical filter

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.

FMCW multiplexing of fiber Bragg grating sensors

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.

High-efficiency micro-optical color filter for liquid-crystal projection system applications.

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.

Loss-less pulse intensity repetition-rate multiplication using optical all-pass filtering

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.