Response Of Gratings Research Articles

From parametric gap solitons to chaos by means of second-harmonic generation in Bragg gratings.

We review the theory of light localization due to the combined action of single or double Bragg coupling between dichromatic counterpropagating envelopes and parametric mixing nonlinearities. We discuss existence, stability, and excitation of such localized envelopes. We also investigate the link between stationary gap solitons and input-output response of nonlinear quadratic Bragg gratings. Frustrated transmission and multistable switching is expected to occur under suitable integrable (cascading) limits. Substantial deviations from these conditions lead to the onset of spatial chaos. (c) 2000 American Institute of Physics.

Resonant modes of a bottle-shaped cavity and their effects in the response of finite and infinite gratings

The resonant frequencies of a one-dimensional bottle-shaped cavity embedded in a ground plane are calculated using a modal approach for s and p polarizations. The same formalism is used to solve the problem of scattering from a surface with a finite number of cavities and from an infinite periodic grating. We show numerical results where the resonant behavior is evidenced as dips in the curve of intensity specularly reflected from a surface with one or several bottle-shaped grooves. The surface shape resonances of a single cavity are also shown to have a great influence on the efficiency distribution of the diffracted orders from infinite gratings made of bottle-shaped cavities. The excitation of even and odd modes is analyzed for both polarizations.

Synthesis of grating lattice circuits

The realization of high-performance components based on optical infinite impulse response (IIR) filter design theory is desirable for next-generation global optical networks. Previously proposed IIR filter synthesis methods are matrix factorization techniques for a lattice circuit using ring resonators. The size of ring resonator limits the bandwidth of the lattice filters. In this paper, two configurations of grating lattice filters are synthesized by using a scattering matrix representation for the grating. The grating is one of the most powerful optical elements both in fiber optics and photonic integrated circuits. One configuration is a serial grating lattice filter configuration and the other is a parallel grating lattice filter configuration. The actual frequency response of the synthesized grating lattice filter is calculated to show the design limitation due to the frequency response of the element gratings.

Pupil responses to stimulus color, structure and light flux increments in the rhesus monkey

Visual stimuli that isolate pupil color and pupil grating responses in human vision have been used to investigate the properties of stimulus-specific pupil responses in the rhesus monkey. We measured and compared pupil responses to light flux increments, isoluminant chromatic stimuli, and gratings of equal and lower space-averaged luminance. The parameters investigated were luminance contrast and chromatic saturation. The results demonstrate clearly the existence of pupil color, pupil grating and pupil light reflex responses in the rhesus monkey. Comparison of pupil color and pupil grating responses of equivalent amplitude reveals similar onset response latencies. However, both are approximately 40 ms longer than the corresponding pupil light reflex latency. In general these pupil responses are qualitatively similar to those observed in humans. However, when compared to equivalent human data, pupil onset response latencies are some 80–100 ms shorter and the pupil shows more rapid recovery from constriction.

Complementary roles of two excitatory pathways in retinal directional selectivity.

The two major excitatory synapses onto ON-OFF directionally selective (DS) ganglion cells of the rabbit retina appear to be nicotinic cholinergic and NMDA glutamatergic. Blockade of either of these synapses with antagonists does not eliminate directional selectivity. This suggests that these synapses may have complementary roles in the computation of the direction of motion. To test this hypothesis, quantitative features of the DS cell excitatory pathways were determined by collecting responses, under nicotinic and/or NMDA blockade, to a sweeping bar, hyperacute apparent motions, or a drifting sinusoidal grating. Sweeping bar responses were reduced, but directional selectivity not eliminated, by blockade of either excitatory path, as previously shown (Cohen & Miller, 1995; Kittila & Massey, 1997). However, residual responses under combined blockades were not statistically significantly DS. NMDA blockade reduced responses more than nicotinic blockade for each protocol, and shifted hyperacute motion thresholds to higher values. This supported the notion that glutamate provides the main excitatory drive to DS cells, that is, the one responsible for contrast sensitivity. In turn, nicotinic, but not NMDA blockade eliminated directional selectivity to a drifting low spatial-frequency sinusoidal grating in these cells. This suggested that acetylcholine (ACh) is the main excitatory input with regards to directional selectivity for some textured stimuli, that is, those with multiple peaks in their spatial luminance profile. Moreover, nicotinic blockade raised the low temporal-frequency cutoff of the grating responses, consistent with the proposal that preferred-direction facilitation, which is temporally sustained, is dependent on the cholinergic input. These different properties of the NMDA and nicotinic pathways are consistent with a recently proposed two-asymmetric-pathways model of directional selectivity.

Investigation of Long Period Fiber Gratings Sensitivity to Liquid Crystal External Medium

In this work, we study the response of Long Period Fiber Gratings (LPG) to an external Nematic Liquid Crystal medium. By a coupling between the fundamental mode of the core and the cladding modes of the fiber, losses occure at different wavelengths. The spectral position and the intensity of those losses depending on the index of the external medium, one can expect that different orientations of the NLC will induce a modification of the response of LPG. To check this assumption, we have realised two types of experiments: In a first time, we have studied the response of LPG to different treatments of the fiber. In particular, we present for the first time a method to obtain a planar alignment directly on the fiber. It will then be demonstrated that the response of LPG can be modified by Fredericksz reorientation of NLC, creating new opportunities in telecommunications networks.

Spectral characterization of fiber gratings with high resolution

We demonstrate a high-resolution technique to measure the optical magnitude and phase responses of fiber gratings. The technique employs single-sideband modulation of an optical source and has spectral resolution in the hertz regime. The technique is demonstrated by measurement of the phase ripples of unapodized and apodized chirped gratings as well as the transmission spectrum of a pi-phase-shifted grating. Although it is demonstrated on fiber gratings, the technique is applicable to any optical device.

Phase conjugation in the low reflectivity regime by nearly degenerate four-wave mixing in a homogeneous plasma

Accounting for both the long- and short-wavelength density gratings by considering the polarizations of all four electromagnetic waves parallel to each other and a finite tilt angle between the signal and pump waves, the steady-state phase conjugation in the low reflectivity regime by nearly degenerate four-wave mixing in a homogeneous plasma is investigated. The response of the density gratings caused by the beating of the signal and pump waves is considered from the theory of stimulated Brillouin scattering in a two-component plasma. The expression for the power reflectivity of the conjugate wave for the arbitrary difference frequency between the signal and pump waves and that for resonant four-wave mixing are obtained. Numerical results have been discussed in light of the experiment reported in the literature. It is noted that the measured values of the difference frequency corresponding to either density grating resonance, and the corresponding maximum reflectivity of the conjugate wave, provide a diagnostic tool for the electron temperature and the electron–ion temperature ratio, respectively.

Long-phase error-free fiber Bragg gratings

Stitching errors in the phase mask degrade the spectral response of fiber Bragg gratings fabricated with the phase mask technique. We successfully fabricated a stitching error-free phase mask and applied it to the fabrication of fiber Bragg gratings (FBGs). The gratings have no observable excess sidelobes in the reflectivity and show excellent wavelength selectivity.

Polarisation-independent strong Bragg gratings inplanar lightwave circuits

High intensity ArF laser irradiation produces Bragg gratings with index modulations >10/sup -3/ in standard germanium-doped silica waveguides without hydrogen loading. A UV-induced birefringence of 2/spl times/10/sup -4/ is used to compensate for the inherent birefringence of the silica waveguides and makes the grating response identical for TE and TM modes.

Optical storage in an azobenzene-polyimide film with high glass transition temperature

A polyimide polymer containing azobenzene side groups was used to demonstrate the ability of optical storage. Writing and reading processes in the polyimide film were performed using degenerate four-wave mixing. The time response of the light-induced gratings was characterized by three components, a fastest, a faster and a slower, which were attributed to the excited-state population, the trans–cis isomerization and the reorientation of azobenzene molecules, respectively. The dependence of this dynamic behavior on the sample temperature was investigated, and was found that with the increase of the sample temperature the contribution of the orientation component to the light-induced gratings became more and more dominant.

Analysis of the response of long period fiber gratings to external index of refraction

This paper demonstrates that the change in wavelength of a long period fiber grating attenuation band with changes in external index of refraction can be enhanced by proper selection of the grating period. We calculate and experimentally verify that the wavelength shift caused by changing the external index from n=1 to n=1.44 of the attenuation band which appears in the 1400-1600 nm region in a 200-/spl mu/m period grating is four times that in a 350-/spl mu/m period grating. Changes in the spectrum over a wavelength range from 1100 to 1600 nm and 1<n<1.72 index range are also presented, and implications for grating design when enhanced or reduced index sensitivity is desired are discussed. Finally, we demonstrate the use of a highly index-sensitive long period fiber grating as a chemical concentration sensor.

Ionising radiation response of photosensitive germanosilicatethin films: A new radiation sensor strategy

The effect of X-ray irradiation on the absorption behaviour of highly photosensitive germanosilicate thin films is evaluated. The corresponding impact on the optical response of photosensitive Bragg gratings written into these materials indicates that these structures map be useful as ionising radiation sensor elements.

Design and realization of multiple quarter-wave phase-shifts UV-written bandpass filters in optical fibers

Ultraviolet (UV)-written optical fiber bandpass filters based on multiple quarter-wave phase-shifts, up to eight, are presented and modeled. It is demonstrated that the shape of the bandpass peak can be tailored by optimization of the phase-shifts locations. We show that flat top bandpass peak with negligible ripples (<0.01 dB) at the top can be designed and that a tradeoff between nearly rectangular shape and stopband width occurs when gratings containing more than two phase-shifts are to be used. We also present experimental spectral responses of gratings with up to three phase-shifts and comparisons with theoretical results.

Long period two-dimensional gratings for 8–12 μm quantum well infrared photodetectors

By considering all possible high order diffracted waves, the authors investigate the spectral response of two-dimensional gratings for quantum well infrared photodetectors (QWIPs). A new method is proposed that using long period gratings may improve grating quality and reduce the resulting cross talk in grating-coupled QWIPs. A sensitivity analysis indicates that the influence of variation of the grating constant on the coupling efficiency is less sensitive for the long period gratings than for the short ones. A large coupling efficiency has been demonstrated for long period gratings. The calculated wide grating response spectra are in good agreement with the experiment result.

Hybrid fiber Bragg grating/long period fiber grating sensor for strain/temperature discrimination

We demonstrate a novel sensor which uses the difference in strain and temperature response of fiber Bragg gratings and a long period fiber grating to discriminate between strain and temperature induced wavelength shifts. Sensor interrogation is performed entirely on the fiber Bragg grating reflection signals. Strain and temperature were simultaneously measured to /spl plusmn/9 /spl mu/strain and /spl plusmn/1.5/spl deg/C over a wide range of conditions.

The optical response of magnetic gratings

We present calculations, using a modal expansion and scattering matrix approach, for the optical response of magnetic gratings as a function of angle of incidence. The optical rotation for diffracted beams of a magnetised Fe grating are enhanced near grazing conditions.

Conical diffraction from multicoated gratings containing uniaxial materials

The full optical response of multilayer gratings containing anisotropic layers until now has been the domain of the experimentalist with no suitable theoretical model available to investigate such systems. The introduction of such a model would clearly benefit this relatively unexplored area of diffractive optics. To this end we present a differential theory based on the work of Chandezon et al. [ J. Opt. Soc. Am. A72, 839 ( 1982)], extending our previous analysis to explore not only the in-plane diffraction with anisotropic layers but also the twisted grating, or conical diffraction, case. In this case there are now two possible mechanisms for transverse magnetic to transverse electric conversion, those being the twisted grating and the anisotropic, uniaxial layer. To illustrate the modeling, results of the new theory are compared with experimental data for a twisted grating anisotropic liquid-crystal system. Resonance mode position and intensity are in good agreement, showing the validity of the new mathematical procedure.

Magnetic and electrical brain responses to chromatic contrast in human

Differences between magnetic responses to red-green chromatic gratings and yellow-black luminance gratings were: (1) response waveforms differed considerably; (2) at some recording sites the chromatic grating response was considerably greater than the sum of responses to the red and green components of the chromatic grating; (3) the latencies of the successive peaks in the response to the onset of chromatic contrast were greater than the latencies of the corresponding peaks in the response to luminance contrast onset; (4) chromatic grating responses were lowpass with respect to spatial frequency while luminance grating responses were bandpass; (5) chromatic grating responses attenuated more steeply with increasing frequency above 2 c/deg than did luminance grating responses. Items (3)–(5) above are consistent with well-known psychophysical findings that contrast sensitivity is lowpass for chromatic gratings and chromatic responses are more sluggish than luminance responses. In subsidiary experiments we found that magnetic responses to red-green and blue-yellow equiluminant gratings had similar waveforms in all six subjects tested, but the topographical distributions were different in three subjects. The results of comparing magnetic and electrical responses to the onset and offset of contrast can be understood in terms of the considerable intersubject variability in the relation between neuroanatomy and cortical function that has been demonstrated by other techniques.

Development of pupillary responses to grating stimuli*

SummaryPupillary responses to stationary, achromatic sine wave gratings were measured in adults and in 1‐ and 3‐month‐old infants. Measurements of behaviourally determined contrast sensitivity were also obtained from the infant subjects. Contrast sensitivity functions were typical of those previously described: band‐pass in nature with sensitivity increasing markedly with age. The pupil grating response amplitudes also exhibited a spatial frequency and age dependence similar to that observed for contrast sensitivity. These findings provide further evidence that the neural mechanisms which underlie visual sensitivity to contrast also mediate pupillary responses to spatial structure.