Showing papers on "Diffraction grating published in 1997"

Fiber grating sensors

Abstract: We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors.

Fiber grating spectra

Abstract: In this paper, we describe the spectral characteristics that can be achieved in fiber reflection (Bragg) and transmission gratings. Both principles for understanding and tools for designing fiber gratings are emphasized. Examples are given to illustrate the wide variety of optical properties that are possible in fiber gratings. The types of gratings considered include uniform, apodized, chirped, discrete phase-shifted, and superstructure gratings; short-period and long-period gratings; symmetric and tilted gratings; and cladding-mode and radiation-mode coupling gratings.

Fiber Bragg grating technology fundamentals and overview

Abstract: The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.

Resonant grating waveguide structures

Abstract: Under certain conditions, a resonance phenomenon can occur in waveguide grating structures. Such structures have multilayer configuration, the most basic of which is comprised of a substrate, a thin dielectric layer or semiconductor waveguide layer, and an additional transparent layer in which a grating is etched. When such a structure is illuminated with an incident light beam, part of the beam is directly transmitted and part is diffracted and subsequently trapped in the waveguide layer. Some of the trapped light is then rediffracted outwards, so that it interferes destructively with the transmitted part of the light beam. At a specific wavelength and angular orientation of the incident beam, the structure "resonates"; namely, complete interference occurs and no light is transmitted. This paper reviews previous investigations on the resonance phenomena and presents analytic and numerical models for evaluating the resonance as a function of the geometric and optical parameters of the structures and incident radiation.

Diffraction gratings and applications

Abstract: A brief history of spectral analysis fundamental properties of gratings the types of diffraction gratings efficiency behaviour of plane reflection gratings transmission gratings icily gratings concave gratings surfacewaves and grating anomalies waveguide, fibber, an acousto-optic gratings review of electromagnetic theories of grating efficiencies testing of grating performance instrumental systems grating damage and control mechanical ruling of gratings holographic recording of gratings alternative methods of grating manufacture replication of gratings.

Lightwave applications of fiber Bragg gratings

Abstract: Fiber Bragg gratings (FBGs) have emerged as important components in a variety of lightwave applications. Their unique filtering properties and versatility as in-fiber devices is illustrated by their use in wavelength-stabilized lasers, fiber lasers, remotely pump amplifiers. Raman amplifiers, phase conjugators, wavelength converters, passive optical networks, wavelength division multiplexers (WDMs) demultiplexers, add/drop multiplexers, dispersion compensators, and gain equalizers.

Broad-band erbium-doped fiber amplifier flattened beyond 40 nm using long-period grating filter

Abstract: Broad-bandwidth amplification is essential for the construction of high-capacity multichannel communication systems. We describe a silica-based erbium doped fiber amplifier (EDFA) with a flat gain bandwidth exceeding 40 nm. The dual-stage EDFA includes a precisely designed inter-stage long-period fiber grating filter with more than 14-dB peak attenuation. By careful choice of the filter spectrum and fiber lengths, this EDFA is flat to within 1 dB over 40 nm while producing a noise figure below 4.0 dB and nearly +15-dBm output power.

Improved formulation of the coupled-wave method for two-dimensional gratings

Abstract: A new formulation of the coupled-wave method for two-dimensional gratings is proposed. It is based on mathematical and physical results recently obtained for one-dimensional gratings. Numerical evidence obtained for many different diffraction problems, including dielectric, metallic, volume, and surface-relief gratings, shows that the new formulation outperforms the conventional one in terms of convergence rates. The specific case of gratings with very small thickness, for which opposite conclusions on the convergence performance are obtained, is studied and explained. The methodology can be applied to other numerical techniques that rely on Fourier expansions of the electromagnetic fields and on grating parameters such as the permittivity and the permeability.

Photonic beamformer for phased array antennas using a fiber grating prism

Abstract: This letter presents, for the first time, measured data on a Bragg reflection grating based fiber-optic prism true time delay processor for transmit/receive phased array beamforming. Measurements taken over a 3.5-GHz bandwidth demonstrates high resolution beamsteering and highly linear low-noise phase data. The system takes maximum advantage of component reuse and fully integrates the transmit and receive modes in one efficient hardware compressive topology.

Thermal decay of fiber Bragg gratings written in boron and germanium codoped silica fiber

Abstract: Fiber Bragg gratings (FBGs) have been written in silica fiber codoped with boron and germanium, using a continuous-wave (CW) 244-nm laser. The thermal decay of the gratings written with and without hydrogen loading is examined over a period in excess of 5000 h. It is shown that the thermal decay of the nonhydrogen loaded gratings is well explained by the power law model proposed by Erdogan. Gratings written in hydrogen loaded fiber, however, do not obey the power law model. A new model is presented based on a log time representation which can be used to predict the decay characteristics of gratings written in hydrogen loaded fiber.

Fiber Bragg gratings for dispersion compensation in transmission: theoretical model and design criteria for nearly ideal pulse recompression

Abstract: We propose a transmission-based dispersion compensator employing an apodized, unchirped fiber Bragg grating (FBG). A theoretical model for dispersion compensation in transmission based on the dispersive properties of the periodic structure is developed. A figure of merit is defined for optimization of the grating parameters for maximum recompression of dispersion-broadened optical pulses in long-haul communication systems. Numerical examples confirm that nearly perfect compensation with very low insertion losses can be achieved for many practical cases of interest.

Achromatic quarter-wave plates using the dispersion of form birefringence

Abstract: We propose achromatic quarter-wave plates of a subwavelength grating structure. When the period of the grating structure is smaller than the wavelengths of the incident light, the structure is considered to be an optically anisotropic medium. The effective refractive indices strongly depend on the wavelengths, especially when the period is close to the wavelength. Using this feature, we can design a grating quarter-wave plate whose phase retardation is maintained at pi/2 for a wide wavelength range. A design method using the effective medium theory is described, and the wave plates designed were evaluated by numerical calculation with a rigorous electromagnetic grating theory. The calculation results led to the possibility of an achromatic quarter-wave plate whose retardation errors are smaller than 3 degrees for a +/-10% change in wavelength.

Silicon-on-insulator (SOI) phased-array wavelength multi/demultiplexer with extremely low-polarization sensitivity

Abstract: We demonstrate the first phased-array wavelength multiplexer fabricated in the silicon-on-insulator (SOI) waveguide technology. The four-channel wavelength division multiplexer (WDM) has a channel spacing of 1.9 nm centered at 1550-nm wavelength and a 3-dB channel bandwidth of 0.72 nm. The crosstalk to neighboring channels is less than -22 dB and the on-chip insertion loss is below 6 dB for all channels. The TE-TM shift is less than 0.04 nm which is the smallest attained without compensation techniques in any integrated optic technology.

Coupled-mode theory of resonant-grating filters

Abstract: An approximate closed-form expression for the loss in a planar phase grating is derived by using coupled-mode theory. It is shown that this loss expression can be used to determine the spectral and angular width of a resonant-grating filter. A resonant-grating filter is a free-space optic that takes advantage of grating resonances to create narrow-band reflection peaks. Design characteristics, such as bandwidth, have previously been determined by profiling the resonance in reflectivity with the use of numerically intensive vector-diffraction methods such as rigorous coupled-wave analysis. The coupled-mode approach described here, however, gives the resonant-filter width directly, without the need to profile the resonance. Therefore computation time and hence design time are reduced. In addition, it is shown that the coupled-mode approach provides physical insights into the factors contributing to filter bandwidth.

Optical rearview system for vehicle

Abstract: An optical rearview system for a vehicle includes a reflective element which reflects light incident thereon and at least one diffraction grating in the path of light from an object rearward of the vehicle towards the reflective element. The at least one diffraction grating diffracts light from the object wherein light passing through the diffraction grating is directed towards the reflective element. Such optical rearview system eliminates secondary reflections and flare which have precluded successful commercialization of previously known optical rearview systems.

Design of high-efficiency dielectric reflection gratings

Abstract: We discuss examples of designs for all-dielectric reflection gratings that tolerate high intensity and are potentially capable of placing up to 99% of the incident light into a single diffraction order, such as are needed for contemporary high-power lasers utilizing chirped-pulse amplification. The designs are based on placing a dielectric transmission grating atop a high-reflectivity (HR) multilayer dielectric stack. We comment on the connection between transmission gratings and reflection gratings and note that the grating and the HR stack can, to a degree, be treated independently. Because many combinations of gratings and multilayer stacks offer high efficiency, it is possible to attain secondary objectives in the design. We describe examples of such designs aimed toward improving fabrication and lowering the susceptibility to laser-induced damage. We present examples of the dependence of grating efficiency on grating characteristics. We describe examples of high-efficiency (95%) gratings that we have fabricated by using hafnia and silica multilayers.

Fractional Talbot imaging of phase gratings with hard x rays.

Abstract: Fractional Talbot images of optical gratings acting as periodic phase objects have been obtained by use of x rays of 0.069-nm wavelength from a third-generation synchrotron radiation source. Quantitative evaluation of the data obtained as a function of defocusing distance provides information on the lateral coherence of the beam as well as on the phase modulation in the object.

High resolution optical multiplexing and demultiplexing device in optical communication system

Abstract: An optical multiplexing and demultiplexing device of a type using a lens and a diffraction grating, which incorporates a waveguide array having waveguide channels with narrow spacings for facilitating high resolution, where the narrow spacings are realized by forming an etching groove between each adjacent waveguide channels and providing a measure for preventing optical coupling among the waveguide channels in the form of an air gap, or a layer having a refractive index lower than a cladding layer, or a layer for absorbing/reflecting light, which is provided in the etching groove. The device also incorporates a reflection mirror for reflecting the initially diffracted lights from the diffraction grating back to the diffraction grating, such that the diffracting grating diffracts the diffracted lights reflected back from the reflection mirror again to produce wavelength division multiplexed/demultiplexed optical signals in high resolution.

Multiparameter grating metrology using optical scatterometry

Abstract: Scatterometry, the analysis of light diffraction from periodic structures, is shown to be a versatile metrology technique applicable to a number of processes involved in the production of microelectronic devices. We have demonstrated that the scatterometer measurement technique is robust to changes in the thickness of underlying films. Indeed, there is sufficient information in one signature to determine four process parameters at once, namely the linewidth and thickness of the photoresist grating, and the thicknesses of two underlying film layers. Results from determining these dimensions on a 25 wafer study show excellent agreement between the scatterometry measurements and measurements made with other metrology instruments [top-down and cross-section scanning electron microscopy (SEM) and ellipsometer]. In particular, measurements of nominal 0.35 μm lines agree well with cross-section SEM measurements; the average bias is −1.7 nm. Similarly, for nominal 0.25 μm lines, the average bias is −7.3 nm. In ad...

Grism-pair stretcher–compressor system for simultaneous second- and third-order dispersion compensation in chirped-pulse amplification

Abstract: We present a grating pair based on Carpenter prisms whose third-order dispersion is opposite that of a traditional grating pair. A properly designed stretcher–compressor system with these gratings has the unique characteristic that it simultaneously compensates for second- and third-order dispersion as a function of grating separation, as opposed to traditional systems, which require an additional grating angle mismatch. The applicability of this design to 30-fs, millijoule-level chirped-pulse amplification is discussed.

Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber grating

Abstract: In this paper, we present and demonstrate a novel approach of a true time-delay (TTD) optical feeder for phased-array antennas (PAAs). A continuously variable TTD is achieved by employing tunable lasers and a wide-bandwidth chirped fiber Bragg grating (FBG) as the dispersive element. The results show that a very high resolution performance (equivalent to a 6-b microwave phase shifter) is obtained for an L-band PAA employing narrow-tuning bandwidth lasers with a wavelength stability of 0.005 nm and a 4-nm bandwidth chirped grating with a dispersion of 835 ps/nm.

Fiber gratings in lasers and amplifiers

Abstract: The emergence of UV-written fiber gratings, both short and long period, as a reliable fiber technology has revolutionized the field of active fiber devices. The advantageous properties of spectral selectivity, low insertion loss, and component ruggedness have made devices possible that would not be viable without their use. The use of fiber gratings in conjunction with erbium-doped fiber amplifiers (EDFAs), fiber lasers, and with semiconductor diode lasers is reviewed.

Active wavelength selection with resonant devices

Abstract: This invention discloses an electro-optically controlled optical element (300) including a diffraction grating (130); and a planar waveguide (122) associated with the diffraction grating (130), the diffraction grating (130) and the planar waveguide (122) being configured to undergo resonance of at least one of transmitted or reflected light at a wavelength which is selectable by means of an electrical input.

A writing technique for long fiber Bragg gratings with complex reflectivity profiles

Abstract: We have developed and demonstrated a novel technique to manufacture very long gratings with a nearly arbitrary reflectivity profile. The technique is based upon writing a set of consecutive subgratings with interferometric control of the relative position of each subgrating.

Thermal stability analysis of UV-induced fiber Bragg gratings

Abstract: The phenomenon of decay in UV-induced Bragg gratings is examined. Thermal stability of gratings with large and small index changes are studied and compared. The role of stabilization in enhancing the thermal stability of gratings is explained. Approaches to analyzing and predicting grating decay are reviewed and compared.

Cholesteric Gratings with Field-Controlled Period

Abstract: Diffractive gratings with period varied by an electric field are developed using a cholesteric liquid crystal confined between two transparent electrodes with unidirectionally treated alignment layers. In the initial state (zero field), the cholesteric layers are parallel to the cell planes. The electric field creates an in-plane modulated structure of variable period. Diffraction regimes of both Raman-Nath and Bragg types are demonstrated.

True time-delay scheme for feeding optically controlled phased-array antennas using chirped-fiber gratings

Abstract: A novel approach of true-time delay (TTD) optical feeder for phased-array antennas is proposed and demonstrated. A compact size continuously variable TTD is achieved by employing tunable lasers and one wide bandwidth chirped-fiber grating as dispersive element. A high-resolution performance (5.3 ps) is obtained for a 26-GHz phased-array antenna employing narrow tuning bandwidth lasers with a wavelength stability of 0.005 nm and a 4 nm bandwidth chirped grating.

Optical semiconductor device with diffraction grating structure

Abstract: Optical semiconductor devices with integrated diffraction gratings with higher quality are realized through the use of Al-free grating layers. AlGaAs/GaAs regime optical semiconductor devices, such as laser diodes or optical filters, conventionally utilize an AlGaAs grating layer that has a strong affinity for oxidation. Instead of a Al-containing layer, a quantenary, InGaAsP grating layer is utilized, lattice matched to the underlying AlGaAs/GaAs structure, substantially eliminating any problem of oxide contamination. Also, an Al-free, ternary InGaP grating layer is utilized in the InGaP/InGaAsP/GaAs material regime. The quantum well active region of these devices may also be modified to extend the gain bandwidth of operation of these devices to insure continued operation over a wider temperature range with the wavelength peak of the grating in that the wavelength peak of the grating more assuredly remains within the wavelength operating range of the device.

Optical wavelength multiplexer/demultiplexer

Abstract: Plurality of channel waveguides of an arrayed waveguide diffraction grating are arranged such that all of intervals thereof are not constant between each two adjacent channel waveguides at a first coupling portion for coupling an input slub waveguide and the arrayed waveguide diffraction grating and a second coupling portion for coupling an output slub waveguide and the arrayed waveguide diffraction grating. Further, the plurality of channel waveguides of the arrayed waveguide diffraction grating are arranged such that all of length differences thereof are not constant between each two adjacent channel waveguides. For example, one of the channel waveguides is deleted, so that a length difference of adjacent channel waveguides is determined to be larger than that of the other two adjacent waveguides. According to this structure, it is possible to provide an optical waveguide multiplexer/demultiplexer in which an insertion loss property becomes flat in all wavelength ranges and the insertion loss characteristics are not fluctuated by the fluctuation of the wavelength.

Depth dependence of the effective properties of subwavelength gratings

Abstract: Effective-medium theory is often helpful for understanding the behavior of subwavelength gratings with small period-to-wavelength ratios. By analytically solving Maxwell’s equations in the small-depth limit, we show that the effective properties of subwavelength gratings strongly depend on the grating depth. Moreover, the effective properties are shown to depend not only on the grating structure but also on the optical indices of the surrounding media. A simple expression for the effective indices of one-dimensional (1-D) and two-dimensional (2-D) gratings with arbitrary depths is proposed. Comparison with rigorous computations shows that for TE polarization of 1-D gratings the depth dependence of the effective index prediction is accurate. For TM polarization of 1-D volume gratings and for 2-D volume gratings a slight deviation between rigorous computation and the prediction is observed for approximately quarter-wave depths. For TM polarization of 1-D surface-relief gratings and for 2-D surface-relief gratings, no closed-form expression for the depth dependence of the effective indices is found, but a very sharp variation of the effective index at small thicknesses is predicted. This prediction is confirmed by rigorous computation.