Showing papers on "Optical filter published in 1999"

Coupling of modes analysis of resonant channel add-drop filters

Abstract: The operation principle of resonant channel add-drop filters based on degenerate symmetric and antisymmetric standing-wave modes has been described elsewhere using group theoretical arguments. In this paper, the analysis is carried out using coupling of modes in time. A possible implementation of such a filter is a four-port system utilizing a pair of identical single-mode standing wave resonators. The analysis allows a simple derivation of the constraints imposed on the design parameters in order to establish degeneracy. Numerical simulations of wave propagation through such a filter are also shown, as idealized by a two-dimensional geometry.

Optical Filter Design and Analysis: A Signal Processing Approach

Abstract: From the Publisher: A Unique, Cutting-Edge Approach to Optical Filter Design With more and more information being transmitted over fiber-optic lines, optical filtering has become crucial to the advanced functionality of today’s communications networks. Helping researchers and engineers keep pace with this rapidly evolving technology, this book presents digital processing techniques for optical filter design. This higher-level approach focuses on filter characteristics and enables readers to quickly calculate the filter response as well as tackle larger and more complex filters. The authors incorporate numerous theoretical and experimental results from the literature and discuss applications to a variety of systems—including the new wavelength division multiplexing (WDM) technology, which is fast becoming the preferred method for system upgrade and expansion. Special features of this book include: *The theory underlying various architectures that can approximate any filter function *Filter design techniques applicable to a broad range of materials systems—from silica to fiber to microelectromechanical (MEM) systems *Design examples relevant to filters for WDM systems and planar waveguide devices *250 figures as well as problem sets for use in graduate-level studies

Control of optical transmission through metals perforated with subwavelength hole arrays

Abstract: The transmission spectrum of a metal that is perforated with a periodic array of subwavelength holes exhibits well-defined maxima and minima resulting from, respectively, a transmission enhancement by surface plasmons and Wood's anomaly, a diffraction effect. These features occur at wavelengths determined by the geometry of the hole arrays, the refractive index of the adjacent medium, and the angle of incidence. We demonstrate control of the transmission through variation of these parameters and show that perforated metal films may form a novel basis for electro-optic devices such as flat-panel displays, spatial light modulators, and tunable optical filters.

An eight-channel add-drop filter using vertically coupled microring resonators over a cross grid

Abstract: An eight-channel add-drop cross-grid vertically coupled microring resonator (VCMRR) filter is proposed and demonstrated. The cross grid comprises a grid-like array of buried channel waveguides which perpendicularly cross through each other, VCMRRs at each of the cross-grid nodes serve as the wavelength selective add-drop filters. Measured crosstalk levels at the crossings are typically less than -30 dB. Rings with a nominal radius of 10 /spl mu/m are used to achieve a free-spectral range of 20 nm and optical bandwidths of 1 nm, while changes of the radii in increments of 50 nm lead to a nominal channel spacing of 5.7 nm.

Fiber-optic millimeter-wave downlink system using 60 GHz-band external modulation

Abstract: In this paper, a fiber-optic millimeter-wave (mm-wave) downlink system using 60 GHz-band external modulation is investigated. We prepare the fiber-optic 60 GHz-band mm-wave downlink testbed. It consists of an optical modulation section with a mm-wave signal generator, an optical single sideband (SSB) filter, a standard single-mode fiber (SMF), an optical detection section with a 60 GHz-band radio transmitter and a 60 GHz-band radio receiver. To modulate the laser output with 60 GHz-band mm-wave signals directly, a specially designed electro-absorption modulator with high-efficiency at around 60 GHz is used. The use of this modulator makes the simpler system configuration possible. Using the downlink testbed, the 5 m-long free-space propagation of subcarrier multiplexed 156 Mb/s-DPSK 60 GHz-band mm-wave signals recovered by optical direct detection is successfully demonstrated. The transmission of the mm-wave signals over 85 km-long standard SMF is also successfully demonstrated, using an optical SSB filtering technique to overcome the fiber dispersion. The BER of 10/sup -9/ is achievable at the optical received power of -7.0 dBm.

General optical all-pass filter structures for dispersion control in WDM systems

Abstract: All-pass filters (APFs) are devices that allow phase correction or equalization without introducing any amplitude distortion. An optical implementation of such devices is very attractive since they can be used for dispersion compensation. In contrast to other dispersion control devices, optical APFs can correct any order of dispersion. This can be achieved by careful design of multistage APFs to approximate a target phase profile. However, large dispersion is usually narrow band or requires many filter stages. These performance tradeoffs and the general phase properties of optical APFs are reviewed and clarified in the first part of this paper. In the second part, a general design methodology of optical APFs is introduced. We show that any all-pass structure may be constructed from simple N-port devices (such as directional couplers or Mach-Zehnder interferometers) with N-1 outputs fed back to any of the N-1 inputs. The feedback paths may contain delays or further APFs (recursive design). This set of design rules allows for constructing complex all-pass filters of any number of stages starting with very simple elements. We use this technique to demonstrate a number of optical all-pass structures that may be implemented in planar waveguide or using thin-film filter technology.

Optical transmission control apparatus utilizing metal films perforated with subwavelength-diameter holes

Abstract: Generally speaking, in accordance with the invention, an optical transmission modulation apparatus is provided for modulating light transmitted through the apparatus. The apparatus comprises a metal film having a periodic array of subwavelength-diameter holes provided therein, and a supporting layer. At least a portion of the supporting layer has a selectively variable refractive index, the selectively variable refractive index portion being substantially adjacent to the metal film such that the metal film and the supporting layer form a perforated metal film unit. Selective variation of the refractive index of the selectively variable refractive index portion modulates the intensity of the light transmitted through the perforated metal film unit without substantially changing the direction of the light. Flat panel displays, spatial light modulators and tunable optical filters based on the optical transmission control apparatus are also provided.

Surface micromachined fabry-perot tunable filter

Abstract: We report the fabrication of a wavelength tunable optical filter using surface micromachining technology. The center wavelength is 1.517 /spl mu/m and the transmission bandwidth is 5 nm. The device with a 50-/spl mu/m diameter aperture has an optical loss of about 5 dB. A continuous wavelength tuning of 60 nm has been demonstrated. This device may find applications in optical sensing and wavelength division multiplexing systems, and can be readily integrated with surface emitting lasers, modulators, and detectors.

Tunable add/drop optical filter

Abstract: In all-optical networks, optical switching and routing become the most important issues for interconnecting the transport network layers. This invention describes a novel tunable optical add/drop filter for the all-optical wavelength-division-multiplexing (WDM) network applications. This filter can add or drop part of the high transmission capacity signals of a WDM link. It can be used to decentralized access point in the access network or as small core network node to realizing branching points in the network topology. It works in both wavelength and space domains. It has the advantages of: 1) High throughput and low voltage operation; 2) Wide tuning range and therefore, high channel capacity; 3) High isolation and high directivity between input and output ports; 4) Compact device packaging is possible as compares to the conventional grating and mechanical switching type of add/drop filter; 5) Multiple ports add/drop tunable filters can be realized with this invention to interconnect multiple WDM networks. This novel add/drop filter can be used in various WDM topologies. It enhances the performance of the conventional tunable filter by re-routing the rejected wavelengths back to network, which not only save the precious optical energy, but also cut down the return loss of the device.

Infrared touch panel with improved sunlight rejection

Abstract: Ambient light can saturate the light detectors of a touch screen and cause unreliable operation. To address this concern, one or more filters may be placed proximate the detectors. A first filter may be a spatial filter, such as a microlouvre filter. A microlouvre filter is arranged so that it passes light transmitted by the light detectors and rejects off-axis light. Another filter may be a narrow band optical filter, such as a dichroic notch filter. Such an optical filter passes light having wavelengths within a narrow band selected to correspond to the spectral characteristics of the light emitters, while it rejects light of other wavelengths. The spatial filter and the narrow band optical filter may be used separately or in combination.

A novel high-Q optical microwave processor using hybrid delay-line filters

Abstract: A new hybrid active-passive photonic signal processor, which achieves high-Q microwave bandpass filtering, is presented. It overcomes the problem of achieving very high-Q values, while still operating the active stage with a large gain margin. This enables a significant increase in Q to be obtained, higher filter frequencies, and robust operation. The general synthesis procedure for the hybrid filter is described. The filter response demonstrates very high-resolution microwave signal filtering with a measured Q of 801.

Method and apparatus for monitoring and control of laser emission wavelength

Abstract: An apparatus for monitoring and controlling the wavelength of laser radiation includes at least one optical filter for receiving laser radiation and for transmitting and reflecting first and second filtered beams, respectively. Alternatively, the first and second beams may be transmitted by separate filters (32, 34). The beams are filtered according to respective first and second spectral filter functions that cross at least one crossing wavelength. A beam comparison element (20) compares the first and second filtered beams (36,38) and produces an error signal (22) representative of the deviation of the wavelength of the laser radiation from a set-point wavelength. The beam comparison element (20) can include first and second optical detectors (40, 42) and error signal (22) by taking a ratio or the difference of the signals detected by the detectors. Varying the angle of incidence of the laser radiation upon at least one optical filter varies the spectral filter function of that filter for selecting or varying the operating wavelength of the laser (12). A laser wavelength controller (24) can receive the error signal (22) for stabilizing or tuning the wavelength of the laser radiation. Several embodiments of the invention are disclosed.

Methods and apparatus for filtering an optical fiber

Abstract: Filtering of optical fibers and other related devices. High-energy methods for depositing thin films directly onto the ends of optical fibers can be used to produce high-quality, high-performance filters in quantity at a reasonable cost. These high-quality filters provide the high performance needed for many demanding applications and often eliminate the need for filters applied to wafers or expanded-beam filtering techniques. Having high-quality filters applied directly to optical fiber and faces permits production of high-performance, micro-sized devices that incorporate optical filters. Devices in which these filters may be used include spectroscopic applications including those using fiber optic probes, wavelength division multiplexing, telecommunications, general fiber optic sensor usage, photonic computing, photonic amplifiers, pump blocking and a variety of laser devices.

GaAs microcavity channel-dropping filter based on a race-track resonator

Abstract: We have demonstrated add-drop filters using racetrack-shaped resonators coupled to straight waveguides across gaps which are larger compared with the conventional microcavity ring resonators. The finesse and the maximum transmission are characterized and are shown to be determined uniquely by the round-trip loss and the coupling factor of the resonator.

Information-efficient spectral imaging sensor

Abstract: A programmable optical filter for use in multispectral and hyperspectral imaging. The filter splits the light collected by an optical telescope into two channels for each of the pixels in a row in a scanned image, one channel to handle the positive elements of a spectral basis filter and one for the negative elements of the spectral basis filter. Each channel for each pixel disperses its light into n spectral bins, with the light in each bin being attenuated in accordance with the value of the associated positive or negative element of the spectral basis vector. The spectral basis vector is constructed so that its positive elements emphasize the presence of a target and its negative elements emphasize the presence of the constituents of the background of the imaged scene. The attenuated light in the channels is re-imaged onto separate detectors for each pixel and then the signals from the detectors are combined to give an indication of the presence or not of the target in each pixel of the scanned scene. This system provides for a very efficient optical determination of the presence of the target, as opposed to the very data intensive data manipulations that are required in conventional hyperspectral imaging systems.

Cascaded microring resonators for crosstalk reduction and spectrum cleanup in add-drop filters

Abstract: The role of cascaded filters for spectrum cleanup and crosstalk reduction in add-drop filters is addressed experimentally for microring resonator cross-grid technology. The fabricated devices consist of glass microring resonators 10 /spl mu/m in radius vertically integrated above a cross-grid waveguide array. Sharper linewidth rolloff and flattened passband responses are observed. The ratio of 15 to 3 dB bandwidth of a cascaded double filter is reduced by a factor of 3 compared with the single-filter case. Because of the very small dimensions of the resonators, the use of multiple filters to improve the spectral response consumes negligible chip space.

Laser light source apparatus

Abstract: A wavelength locker 4 is provided at one light beam exit end of a laser diode 1. The wavelength locker 4 incorporates an interference optical filter 3, and photo diodes PD1, PD2 for detecting its transmitted light and reflected light. A thermistor 15 and a temperature detector 14 for detecting the temperature of the interference optical filter 3 are provided. Calculating the output ratio of the photo diodes PD1, PD2, the wavelength is controlled so that the correction value by the temperature of the output ratio may be a specified value. The laser diode 1 and wavelength locker 4 are sealed in an optical module 11, and mounted on a substrate together with other blocks. Thus is obtained a laser light source apparatus composed in a simple and small constitution within a emission possible wavelength range of laser light source and capable of emitting stably.

Device and method for performing spectral measurements in flow cells with spatial resolution

Abstract: A device and method for performing spectral measurements in flow cells with spatial resolution using a variable transmission optical filter having at least two areas with different optical properties. Light from a light source passes through the variable transmission filter to a flow cell containing a sample to be analyzed. The resultant light pattern is sensed by a detecting means, which analyzes the spectral properties of the sample within the flow cell.

All-pass optical filters

Abstract: An all-pass optical filter which reduces the dispersion of optical pulses transmitted therethrough is disclosed. The all-pass optical filter reduces the dispersion of optical pulses by applying a desired phase response to optical pulses transmitted therethrough. The all-pass optical filter also has a frequency independent amplitude response. The all-pass optical filter has a structure which includes at least one feedback path, a splitter/combiner, an input port, and an output port.

Narrow-band resonant grating waveguide filters constructed with azobenzene polymers

Abstract: Resonant grating waveguide structures were used to fabricate narrow-bandwidth optical filters. Azopolymer films were deposited on top of slab waveguides, and surface relief gratings were optically inscribed on them to be used as couplers. This technique is a simple one-step process and produces efficient gratings with high accuracy. Sharp resonant peaks are observed in the transmission and the reflection spectra of these structures. The thickness and the index of refraction of the waveguide can be accurately determined from these resonances by use of modal theory. These parameters are then used in the design of an optical filter. Bandwidths of less than 1 nm and a decrease in transmitted signal of 60% are reported. Measurement of these values was limited by the divergence of the probe beam.

Optical system for a scanning fluorometer

Abstract: A method and apparatus for determining the fluorescence, luminescence, or absorption of a sample is provided. The sample may either be contained within a cuvette or within one or more sample wells within a multi-assay plate. A combination of a broadband source, a monochromator, and a series of optical filters are used to tune the excitation wavelength to a predetermined value within a relatively wide wavelength band. A similar optical configuration is used to tune the detection wavelength. An optical scanning head assembly is used that includes mirrored optics for coupling the excitation source to the sample and the emitted light to the detector. An elliptical focussing mirror is used to magnify and focus the light projected from an optical fiber coupled to the source subassembly onto the sample. A portion of the source light is reflected by a beamsplitter onto a reference detector used to monitor the output of the source. The light from the elliptical mirror passes through an aperture in a second elliptical mirror prior to impinging upon the sample. The light emitted by the sample within the sample well is reflected by the second elliptical mirror and imaged onto the entrance aperture of an optical fiber coupled to the detector subassembly. The optical axes of both mirrors are slightly offset from the sample well normal. The mirror offset minimizes the amount of light reflected from the meniscus of the sample or the bottom surface of the sample well that enters the detection subassembly.

Optical fibre delay line filter free of limitation imposed by optical coherence

Abstract: The authors propose a novel technique for overcoming the limitation of the frequency response of an optical fibre delay line filter imposed by optical coherence. A high birefringence fibre based filter is demonstrated, giving a response free of optical interference noise and tunable by temperature.

Properties of a novel noncascaded type, easy-to-design, ripple-free optical bandpass filter

Abstract: The chief principle and properties of a novel noncascaded type optical bandpass filter with three unique general characteristics are presented. This device is based on a combination of a Michelson interferometer and a Gires-Tournois resonator (GTR). First, the filter has square-like frequency response with zero ripple, wide flat-top (0.74/spl times/bandwidth), unity contrast, sharp steepness, and infinite rejection ratio when GTR front surface mirror reflectance /spl rho/ is equal to one-third. Second, this filter possesses variable bandwidth and center wavelength tunability features with simple control mechanism and nanosecond temporal response potential. Third, the desired features of the filter can be synthesized easily by mere inspection since only three parameters are required, and their expressions are given in simple analytic forms. Numerical results showing these features are presented and other characteristics are briefly discussed.

Optical add/drop wavelength switch

Abstract: An optical add/drop wavelength switch is controllably changed from a bridge state, in which output is identical to input, e.g. a Wavelength Division Multiplexed (WDM) input, and an add/drop state, In which a signal input to an add port is substituted for a particular wavelength subrange of the WDM input, other wavelengths of the WDM input being unchanged. In one embodiment, the wavelength subrange of the WDM signal is given a polarization different from other wavelengths of the WDM, such as by using a stacked waveplate or other optical filter or polarization discriminator. The differently-polarized wavelengths can the be spatially separated, e.g. by a birefringent element or a polarization beam splitter, preferably In a bit-controlled fashion, such as by using a liquid crystal or other polarization controller. Polarization controllers and discriminators can be used similarly to selectably align or combine the add signal with the portion of the WDM signal outside the subrange. The add/drop wavelength switch can be used, e.g. In an optical token ring network and/or to make-up an optical crossbar for exchanging any arbitrarily designatable channels, e.g. among a plurality of multi-channel optical fibers.

Tunable fiber optic filter

Abstract: The invention provides improved optical devices, systems, and methods for controllably varying an optical characteristic of an optical signal, particularly for filtering and generating optical signals about a narrow tunable central wavelength. Thin film multi-cavity Fabry-Perot bandpass filters have film thicknesses which vary so as to vary the center wavelength of the filter's pass range. Novel micro lenses and lens arrangements avoid transmission of optical signals through a large area of the optical filter, which would widen the effective pass bandwidth and reduce coupling efficiency. These bandpass filtering techniques can be used in an external cavity laser system to provide a tunable laser source.

Ellipsometric characterization of anisotropic porous silicon Fabry–Pérot filters and investigation of temperature effects on capillary condensation efficiency

Abstract: Porous silicon Fabry-Perot optical filters with reflectivity peaks in the visible and near infrared spectral range have been manufactured electrochemically and characterized with variable angle spectroscopic ellipsometry. Generalized ellipsometry and the Bruggeman effective medium approximation were employed to study the anisotropy of the samples including determination of the tilt of the optic axis relative to the sample normal. At a wavelength of 700 nm, the difference between the real parts of the ordinary and extraordinary indexes of refraction of the low and high index sublayers were 0.05 and 0.07, respectively. In addition, the effect of temperature on the spectral redshifts caused by capillary condensation of water and toluene vapors in the porous material are investigated. It is demonstrated that a temperature decrease causes the spectral shifts to occur at lower partial pressures, indicating a more effective vapor capture at lower temperatures. Finally, it is shown that this phenomenon can be use...

Temperature-corrected wavelength monitoring and control apparatus

Abstract: Apparatus and methods for monitoring the wavelength of laser radiation and that provide a temperature-corrected error signal responsive to the deviation of the wavelength of the laser radiation from a nominal wavelength is disclosed. The apparatus includes a least one optical filter for filtering the laser radiation according to at least one spectral filter function to produce filtered laser radiation and at least one optical detector for detecting the filtered laser radiation to produce a first detected signal. The apparatus also includes a temperature sensor for sensing temperature characteristic of at least the optical filter and processing circuitry for providing a temperature-corrected output signal responsive to the deviation of the wavelength of the laser radiation from the nominal wavelength. The processing circuitry includes an error circuit for providing, responsive to at least the first detected signal, an uncorrected error signal responsive the deviation of the wavelength of the laser radiation from the nominal wavelength and a memory for providing offset values corresponding to selected temperatures. The processing circuitry, responsive to the temperature sensor, modifies the uncorrected signal based on at least one offset value to produce the temperature-corrected error signal. The laser wavelength monitoring apparatus can be advantageously combined with a laser and a laser wavelength control to provide a laser having a regulated or tunable wavelength.