Showing papers on "Optical filter published in 1996"

Long-period fiber gratings as band-rejection filters

Abstract: We present a new class of long-period fiber gratings that can be used as in-fiber, low-loss, band-rejection filters. Photoinduced periodic structures written in the core of standard communication-grade fibers couple light from the fundamental guided mode to forward propagating cladding modes and act as spectrally selective loss elements with insertion losses act as backreflections <-80 dB, polarization-mode-dispersions <0.01 ps and polarization-dependent-losses <0.02 dB.

All-optical wavelength conversion by semiconductor optical amplifiers

Abstract: Following a brief introduction to the applications for wavelength conversion and the different available conversion techniques, the paper gives an in depth analysis of cross gain and cross phase wavelength conversion in semiconductor optical amplifiers. The influence of saturation filtering on the bandwidth of the converters is explained and conditions for conversion at 20 Gb/s or more are identified. The cross gain modulation scheme shows extinction ratio degradation for conversion to longer wavelengths. This can be overcome using cross phase modulation in semiconductor optical amplifiers that are integrated into interferometric structures. The first results for monolithic integrated interferometric wavelength converters are reviewed, and the quality of the converted signals is demonstrated by transmission of 10 Gb/s converted signals over 60 km of nondispersion shifted single mode fiber.

Blood glucose monitoring system

Abstract: A blood glucose monitoring system includes a broadband light source and a specially fabricated optical filter for modulating optical radiation to be transmitted through a fleshy medium. Optical radiation which passes through the fleshy medium is detected by an optical detector which generates an electrical signal indicative of the intensity of the detected light. Digital signal processing is performed on the electrical signal to extract those optical characteristics of the electrical signal due to the optical characteristics of the filter and constituents of the fleshy May 1, 1995 medium other than blood glucose concentration. The monitoring system employs a unique "double-log" transformation to minimize errors due to indeterminate path length variations of the optical radiation through the fleshy medium. The monitoring system further employs specialized signal processing to avoid inaccuracies due to the previously unidentified solvent effect which arises when glucose is dissolved into water.

Optical filter for spectroscopic measurement and method of producing the optical filter

Abstract: An optical filter used in applications involving spectroscopic measurements is fabricated by depositing layers of optical coatings onto a substrate. The layers are deposited so as to have a substantially constant thickness in a first direction along the surface of the substrate, and a gradually increasing thickness along a direction perpendicular to the first direction. The structure of the optical filter allows for large scale production of the filter so that costs in producing the filter are greatly reduced. The filter may be used in a variety of applications including, but not limited to chemical analysis, blood glucose monitoring, and the like.

Electrically tunable optical filter utilizing a deformable multi-layer mirror

Abstract: An electrically tunable optical filter utilizing a deformable multi-layer mirror construction.

Optical scanner with fixed focus optics

Abstract: Methods are provided to assist in selection of appropriate optical components for optimizing performance of the optical scanner for reading one-, two- and three-dimensional optically encoded information at a wide range of field depths and angles. The optical components are fixed in position relative to each other so that no position adjustments are required to obtain a reading at a desired distance once the optical components are optimized. Among the parameters that are considered are LEDs for emitting light within specific wavelength ranges, optical filters, lens focal lengths, magnification, etc. The Fourier transform of an input square wave signal generated by light reflected from a bar code is used to generate the appropriate resolution requirements. Selection of appropriate optical components is dependent upon the intended operation of the scanner. For example, depth of field is considered, as is scan angle and density of the code to be read. The performance models are based upon the modulation transfer function (MTF) of the optical system, comprising the encoded information, the lens system and the CCD array.

Optical filter including a sub-wavelength periodic structure and method of making

Abstract: An optical filter includes a dielectric layer formed within a resonant optical cavity, with the dielectric layer having formed therein a sub-wavelength periodic structure to define, at least in part, a wavelength for transmission of light through the resonant optical cavity. The sub-wavelength periodic structure can be formed either by removing material from the dielectric layer (e.g. by etching through an electron-beam defined mask), or by altering the composition of the layer (e.g. by ion implantation). Different portions of the dielectric layer can be patterned to form one or more optical interference filter elements having different light transmission wavelengths so that the optical filter can filter incident light according to wavelength and/or polarization. For some embodiments, the optical filter can include a detector element in optical alignment with each optical interference filter element to quantify or measure the filtered light for analysis thereof. The optical filter has applications to spectrometry, colorimetry, and chemical sensing.

Space-time profiles of shaped ultrafast optical waveforms

Abstract: A derivation of the space-time profiles of ultrafast optical waveforms shaped by filtering of spatially separated frequency components is presented. Closed form expressions for the space-time impulse response functions are given for the cases of single and double passes through a pulse shaping apparatus. For a single pass and a short unshaped pulse, diffraction by the mask filter gives rise to a translational spatial shift in the desired electric field profile that varies linearly with time along the shaped waveform. This result is completely general, and applies to frequency-domain pulse shaping with either continuous or discrete mask filters. It is also shown that double passing the apparatus does not generally reverse this effect but rather introduces further space-time coupling such as a time-varying spotsize. Examples of specific mask patterns are presented and implications for the generation of high-fidelity shaped optical waveforms are discussed.

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.

Crosstalk in multiwavelength optical cross-connect networks

Abstract: Optical crosstalk in multiwavelength optical cross-connect (multi-/spl lambda/ OXC) networks is studied. Two crosstalk mechanisms, interband and intraband crosstalk, caused by nonideal wavelength demultiplexing and space switching, are identified in this work. Their nature and accumulation behaviors are studied in detail. It has been established that the intraband crosstalk, where the crosstalk elements fall within the signal wavelength carrier band, is much more deleterious to the network end-end transparency performance. Three multi-/spl lambda/ OXC node architectures realising similar optical cross-connecting functions have been analyzed. Improvements in the most damaging intraband crosstalk can be realized by introducing a wavelength selective filter between the switch outputs and the wavelength multiplexer inputs. System penalty analyses associated with the crosstalk are also carried out using realistic system and device parameters, Both simulation and statistical analysis based approaches are presented.

High-speed continuously tunable liquid crystal filter for WDM networks

Abstract: We describe a high-speed wavelength tunable liquid crystal filter which can be utilized as the tuning element at the receiving end of wavelength division multiaccess (WDMA) optical networks. The filter uses chiral smectic A electroclinic liquid crystals as the active cavity material in a Fabry-Perot etalon in order to obtain a microsecond switching speed. Using the commercially available BDH764E liquid crystal material, we demonstrate a tunable optical filter both in a bulk Fabry-Perot and a fiber Fabry-Perot (FFP) configuration. A bandwidth of about 0.7 mm and effective finesse of 70 were obtained in the FFP configuration. A FFP tuning range of 13 nm with a switching time of less than 10 /spl mu/s were measured at the operating wavelength of 1.55 /spl mu/m. A theoretical analysis of the expected filter performance in the FFP configuration is given. Diffraction in the Fabry-Perot cavity is identified as the dominant loss factor, resulting in reduced throughput and finesse broadening. It is calculated theoretically that an effective finesse of 130 and a throughput loss of 2.2 dB are achievable for a mirror finesse of 200 and a liquid crystal cavity thickness of 5 /spl mu/m. A short waveguide piece is assumed to be included in the cavity. Other expected loss sources for the filter in the FFP configuration have been calculated, showing negligible effect on the filter performance.

Microwave optical filters using in-fiber Bragg grating arrays

Abstract: A novel multitap optical transversal signal processor based on wavelength multiplexed Bragg grating arrays is presented. This structure enables the realization of a large number of taps for obtaining sharp bandpass filtering with high resolution and also enables arbitrary tap weight profiles to be obtained. Results on a 4-GHz, 29-tap filter demonstrates a sharp bandpass filter response, and windowing techniques on the tap profiles are demonstrated to enhance the sidelobe suppression characteristics.

An 18-channel multifrequency laser

Abstract: A multifrequency laser (MFL) is demonstrated that runs simultaneously CW on 18 channels spaced by 103 GHz. The laser emits -14.6-dBm power per wavelength channel into single-mode fiber. Each wavelength channel can be modulated at 1.24 Gb/s. The MFL exhibits a stable and reproducible optical channel spacing owing to the reproducibility of the waveguide grating router that serves as the intracavity filter element.

Narrow spectral bandwidths with grating waveguide structures

Abstract: Under certain conditions grating/waveguide structures have a resonant behavior with narrow spectral bandwidths. Several such structures were designed, fabricated, and evaluated, demonstrating resonances with narrow spectral bandwidths ranging to as low as 0.035 nm at full width at half maximum. These can be exploited as compact spectral filters having high finesse.

Use of multimode interference couplers to broaden the passband of wavelength-dispersive integrated WDM filters

Abstract: We describe how multimode interference couplers (MMI) may be used to broaden and flatten the passband of integrated wavelength-dispersive filters. We discuss the approach and demonstrate its effectiveness with a passband-broadened InP arrayed waveguide filter operating at 1.5 /spl mu/m.

A modular optical cross-connect architecture with optical wavelenght switching

Abstract: An optical cross-connect node architecture interfaces plural optical fiber input and output links, each link containing plural wavelength channels. In one embodiment, the input links are connected to a single optical coupler, or alternatively, to an associated one of plural optical couplers. Pairs of tunable optical filters and optical wavelength converters are each connected to an output port of the optical coupler, or to each of the plural optical couplers, and perform wavelength channel routing and switching in the wavelength domain, i.e., without the need for any optical space switch. In other embodiments, an additional input wavelength converter is connected to each input fiber link to convert the plural wavelength channels on each link to different, non-interferring wavelengths. This prevents wavelength contention in the optical coupler to which the input wavelength converters are connected. New fiber links may be added in modular fashion without significant impact on the pre-existing optical cross-connect structure. Similarly, new wavelength channels may also be multiplexed onto existing fibers to provide wavelength modularity without having to reconfigure the node.

Rapidly tunable narrowband wavelength filter using LiNbO/sub 3/ unbalanced Mach-Zehnder interferometers

Abstract: We demonstrate a rapidly tunable narrowband wavelength filter using unbalanced Mach-Zehnder interferometers in lithium niobate. It is designed to switch among eight channels spaced 50 GHz apart in optical frequency (approximately 0.4 nm in wavelength) in less than 50 ns. The insertion loss is 19 dB and crosstalk is less than -22 dB. The filter consists of a cascade of three electrooptically tunable unbalanced Mach-Zehnders and a feedback circuit to keep the filter centered on the desired wavelength.

Optical filters from photonic band gap air bridges

Abstract: Surrounding waveguides by air reduces radiation losses so the air bridge geometry can produce optical filters with sharp transmission resonances and very wide stop bands.

Transparent optical networks with time-division multiplexing

Abstract: Most research efforts to date on optical networks have concentrated on wavelength-division multiplexing (WDM) techniques where the information from different channels is routed via separate optical wavelengths. The data corresponding to a particular channel is selected at the destination node by a frequency filter. Optical time-division multiplexing (OTDM) has been considered as an alternative to WDM for future networks operating in excess of 10 Gb/s. Systems based on TDM techniques rely upon a synchronized clock frequency and timing to separate the multiplexed channels. Advances in device technologies have opened new opportunities for implementing OTDM in very high-speed long-haul transmission as well as networking. The multiterahertz bandwidth made available with the advent of optical fibers has spurred investigation and development of transparent all-optical networks that may overcome the bandwidth bottlenecks caused by electro-optic conversion. This paper presents an overview of current OTDM networks and their supporting technologies. A novel network architecture is introduced, aimed at offering both ultra-high speed (up to 100 Gb/s) and maximum parallelism for future terabit data communications. Our network architecture is based on several key state-of-the-art optical technologies that we have demonstrated.

Variable group-delay dispersion equalizer using lattice-form programmable optical filter on planar lightwave circuit

Abstract: The authors report, in detail, an integrated-optic variable group-delay dispersion equalizer based on a lattice-form programmable optical filter. The variable dispersion equalizer consists of an alternating cascade of symmetrical and asymmetrical Mach-Zehnder interferometers. An equalizer with nine symmetrical and eight asymmetrical interferometers is fabricated on a planar lightwave circuit and its dispersion varied step by step from -681 to +786 ps/nm in the operational frequency range of 16.3 GHz. The effectiveness of the equalizer is shown by compensating the dispersion of three different fibers with a single equalizer. The performance of the equalizer is also evaluated and examined by numerical investigations.

MEMS applications in optical systems

Abstract: We describe progress in the development of micro-optomechanical components which are compatible with MEMS and integrated circuit processing. Devices which have been developed include polygon and diffraction grating scanners, tunable laser diodes, tunable IR filters, and optical waveguides.

The electrically adjustable optical filter

Abstract: The invention relates to an electrically controllable optical interferometer filter with a layered structure made using silicon micromechanical techniques. The filter comprises an essentially planar substrate (1), a first mirror element (15) deposited on said substrate (1) and a second mirror element (12, 17) superimposed on said first mirror element, and an optical resonator cavity (10) formed between said mirror elements (15, 17) with an optical length of about n.μ/2, where n = 1, 2, 3. According to the invention, above said second mirror element (12, 17) is further made a third mirror element (16), and between said third mirror element (16) and said second mirror element (12, 17) is formed a second optical resonator cavity (10) with an optical length of about n.μ/2, where n = 1, 2, 3.

Performance of multiwavelength simultaneous monitoring circuit employing arrayed-waveguide grating

Abstract: This paper proposes a novel multiwavelength simultaneous monitoring (MSM) circuit that uses the wavelength crossover properties of an arrayed-waveguide grating (AWG). The MSM circuit consists of an AWG, a stabilized semiconductor laser as a reference light, and logarithmic amplifiers. The AWG chip is a simple planar-lightwave-circuit chip. It functions as multiple optical filters, and make it possible to monitor multiple wavelengths simultaneously. The MSM circuit, locked to the reference wavelength produced by a semiconductor laser stabilized to the 1547.49 mn /sup 13/C/sub 2/H/sub 2/ absorption line, achieved 10 MHz resolution and 30 MHz stability for 24 h in the stable polarization state. Measurement accuracy of better than 1.1 GHz can be realized even if the state of polarization of the input light fluctuates at random. Multiwavelength simultaneous monitoring is successfully demonstrated using tunable lasers.

Sensitivity of optically preamplified DPSK receivers with Fabry-Perot filters

Abstract: The error-rate performance of a DPSK lightwave receiver having an optical amplifier followed by a Fabry-Perot filter and delay-line demodulator is analyzed. Receivers with sampling and with integrate-and-dump threshold comparison are compared to the well-known result for a matched optical filter. The Fabry-Perot filter decreases the sensitivity at 10/sup -9/ error-rate from 20 to 24.5 photons/b with optimum optical filter bandwidth and postdetection integration time.

Transmission of directly modulated 2.5-Gb/s signals over 250-km of nondispersion-shifted fiber by using a spectral filtering method

Abstract: We demonstrate 250-km nondispersion-shifted fiber transmission of directly modulated 2.5-Gb/s signals. The chirping-induced power penalty is reduced by using a spectral filter whose transmission peak is tuned to "mark" wavelength of the modulated laser output. A well-known intensity modulation with a sufficiently high extinction ratio is used to suppress the intersymbol interference induced by nonuniform frequency modulation response of semiconductor lasers. The observed results are explained theoretically by using eye closure penalty.

Methods for crosstalk measurement and reduction in dense WDM systems

Abstract: We propose a scheme for the monitoring and reduction of crosstalk arising from the limited stop-band rejection of optical bandpass filters in dense WDM systems. The optical carrier at each wavelength is modulated with a subcarrier tone unique to that wavelength. The level of crosstalk from a given channel can be determined by measuring the power of the corresponding tone. Crosstalk from other channels can be cancelled in a linear fashion by weighting and summing the photocurrents of the desired channel and several adjacent interfering channels. Alternatively, in nonlinear crosstalk cancellation, decisions are made on the interfering signals, and these decision are weighted and summed with the photocurrent of the desired channel. For example, assuming an optical filter having a Gaussian passband, the channel density can be increased from 20 to 30%, depending on the number of adjacent channels detected. The signal-to-interference ratio can be increased by 10-20 dB and the system can achieve a BER<10/sup -9/ under conditions where, without interference cancellation, the signal-to-interference ratio would be less then 10 dB.

Optical design for a high-etendue imaging Fourier-transform spectrometer

Abstract: Current hyperspectral imaging cameras are typically limited in throughput either by a slit or other optical geometries requiring small solid angle instantaneous fields of view. The high etendue imaging Fourier transform spectrometer optical design is a new optical geometry for the production of spectral autocorrelation fringe modulation over an image plane defined by a large CCD array. A throughput advantage approaching several hundred appears realizable, with an accompanying increase in signal to noise proportional to 'root N'. THe simple optical implementation of this design will be discussed along with initial experimental results.

Temperature-independent narrowband optical filter at 1.3 [micro sign]m wavelength by an athermal waveguide

Abstract: The authors report the realisation of a temperature independent narrowband optical filter at 1.3 µm wavelength. A ring resonator has been designed and fabricated using an athermal waveguide, in which the optical path length is independent of temperature. The temperature coefficient of the central wavelength was successfully reduced to 7 × 10-4 nm/K (7% of that of a conventional filter).

Polarized light transmission through skin using video reflectometry: toward optical tomography of superficial tissue layers

Abstract: The movement of polarized light through the superficial layers of the skin was visualized using a video camera with a polarizing filter. This study constitutes a description of the impulse response to a point source of incident collimated linearly polarized light. Polarization images reject unwanted diffusely backscattered light from deeper in the tissue and the specular reflectance from the air/tissue interface. Two experiments were conducted: (1) Video polarization reflectometry used a polarized HeNe laser (633 nm) pointing perpendicularly down onto a phantom medium (0.900-micrometer dia. polystyrene spheres in water). The video camera was oriented 10 degrees off the vertical axis and viewed the irradiation site where the laser beam met the phantom. Video images were acquired through a polarizing filter that was either parallel or perpendicular with the reference plane defined by the source, camera, and irradiation site on the phantom medium's surface. The source polarization was parallel to the reference plane. The two images (parallel and perpendicular) were used to calculate a polarization image which indicated the attenuation of polarization as a function of distance between the source and point of photon escape from the phantom. Results indicated a strong polarization pattern within approximately 0.35 cm (approximately 2.2 mfp') from source. [mfp' equals 1/(microna plus microns')]. (2) Optical fiber reflectometry using a polarized diode laser (792 nm) coupled to a polarization-maintaining single-mode fiber, and a multi-mode fiber collector to collect regardless of polarization. Reflectance as a function of fiber separation was measured for the source fiber oriented parallel and perpendicular with the reference plane. Results indicated that the strongest polarization propagated within approximately 0.43 cm (2.2 mfp') from source. The polarization survived approximately 2.2 mfp', which for skin at 630 - 800 nm (mfp' approximately equals 0.066 cm) corresponds to 1.5 mm (or 6.4 ps of travel at the speed of light). Using 6.4 ps as a maximum time of survival, classical paths of photon transport (Feynman paths) were calculated to illustrate the expected depth of interrogation by polarized imaging. The expected mean depth of photons is about 0.36 mm at these longer wavelengths. Shorter wavelengths would result in a shorter mfp' and therefore more superficial imaging of the skin. Polarization images offer an inexpensive approach toward 2-D acquisition of time- gated images based on the early light escaping the tissue. Polarization imaging is an opportunity for a new form of optical image especially useful for dermatology.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical transport networks employing all-optical wavelength conversion: limits and features

Abstract: An analysis of transmission performance of optical transport networks employing two different types of wavelength conversion (based on four wave mixing and cross phase modulation) and adopting two different node architectures is reported. The performance is evaluated by means of a system modeling which allows one to consider physical phenomena occurring during transmission and all the important impairments that introduce system penalties. The limits of these kinds of network are investigated and the main features are reported. The analysis reveals that both the types of converters can be employed with different features and limitations, according to the considered architecture. An example of geographical network is discussed and its simulated transmission performance is reported.