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Reflective diffraction gratings consist of a periodic three-dimensional structure that produces appropriate constructive and destructive interference in order to redirect an incoming wave as a function of frequency. Depending upon the angle of incidence, the thickness required for this structure can range from a fraction of a wavelength to several wavelengths. While such a thickness does not present a problem for optical applications, it can make the diffraction grating very bulky for radio applications, notably in the lower frequency bands. This communication presents a new thin diffraction grating, employing resonant structures, whose thickness is only 1/34th of a wavelength. Simulations and measurements are presented for a thin diffraction grating, designed to operate in the VHF band in unfavorable outdoor conditions.

Dielectric high-contrast sub-wavelength grating (SWG) structures have received much attention in recent years, offering a new paradigm for the integration of optical systems. Their nanoscale resonant properties can result in a complex and unintuitive far-field behavior that, if carefully crafted, allows the full control of the optical phase front from a thin sub-wavelength planar layer. To date, experimental demonstrations of these new devices have only been realized with polarized light in a reflective mode, greatly limiting their use for practical systems. In this letter, we demonstrate a highly efficient, sub-wavelength thick, transmissive grating lens configuration using symmetrical resonant posts to achieve polarization-independent operation. Our transmissive SWG lenses are easily fabricated using low-cost scalable semiconductor process technology. To illustrate their performance, we demonstrate the generation of high-order orbital angular momentum beams and their use in an optical mode-isolator application that achieves a suppression ratio of over 25 dB.

We present a novel multiwavelength laser array with a sampled grating formed by periodically loaded n-doped injection blocking layer on top of the uniform base grating. With an optimized design of the sampled grating as well as the laser operation conditions, all the even order, including the zeroth-order reflections, can be effectively suppressed, leaving only the first-order reflection wavelength to lase. Unlike in the conventional sampled grating where part of the base grating will have to be removed, the whole base grating is retained in the proposed structure; hence, the side-mode suppression ratio will not be jeopardized due to the weakened grating coupling strength. As an example, an array of 12 lasers has been designed to cover a wavelength range from 1542 to 1560 nm.

Material dependence of line-and-space metal grating among gold, silver, and aluminum, is experimentally investigated for a 100-nm-thick silicon-on-insulator p-n junction photodiode in terms of enhanced light sensitivity. It is found that light sensitivity in the visible long-wavelength region is enhanced with any grating material, and the peak wavelengths, which are determined by the grating period, are not much affected by the material. The relationship between the peak wavelength and the grating period is explained theoretically. The results indicate that the grating material can be selected from these materials by taking into account the short-wavelength sensitivity and compatibility with applications.

A novel reflection-reduced two-layer grating is described with nearly 100% diffraction efficiency. The efficiency of the conventional grating should be improved, or some reported high-efficiency grating can work only for one polarization. The reflection-reduced grating can achieve high efficiency with a covering layer on the surface-relief grating. In addition, the two-layer grating can have wideband property compared with the single-layer grating. The presented novel grating can have merits of high efficiency of reflection-reduced grating and wideband property of two-layer grating. The modal method is applied to analyze the physical mechanism of high efficiency for both TE and TM polarizations. With the optimized grating parameters, efficiencies of 99.69% and 99.64% can be diffracted into the first order for the TE and TM polarizations, respectively. The moderate tolerance should make it possible to be fabricated easily for such a reflection-reduced two-layer grating, which can achieve nearly 100% diffraction efficiency for both the TE and TM polarizations.

The gallium nitride (GaN) light-emitting-diode (LED) top-bottom (or transmission-reflection) grating simulation results with error grating model are presented. The microstructure GaN bottom hole and top pillar gratings are calculated and compared with the non-grating (flat) case. Grating shapes simulated are either conical or cylindrical. A direct comparison of 181 different combined transmission-reflection grating cases using the finite difference time domain method is presented. The simulation results show that simple or direct combinations of the optimized top grating with the optimized bottom grating only produce a 42% light extraction improvement compared to the non-grating case, which is much lower than that of an optimized single grating case. This is due to the mismatch of grating parameters with the direct addition of the second grating structure, which changes the optical field distribution in the LEDs. Therefore, it is very important to optimize both top and bottom gratings simultaneously for the double-grating design. We also show the optimization of a double grating structure can achieve better performance than a single grating. Finally, transmission-reflection error gratings are also presented. It is also the first time to present randomization in GaN LED grating design and its effects in fabrication. Our data shows that the favorable light extraction improvement is at approximately 10-15% randomization. The randomization can achieve 230% improvement over the original grating at a randomization intensity factor of 12.8%.

We propose a method to coherently generate uniform and phase-shifted complex-coupled (CC) semiconductor distributed feedback (DFB) lasers based on intersubband transitions in n-doped quantum-well structures. This is done by utilizing infrared-induced coherent optical processes in these structures including resonant enhancement of refractive index of the conduction intersubband transitions and generation of laser-induced transparency and gain without inversion. We show that these coherent phenomena can generate electromagnetically induced gratings where the index and gain/loss perturbations and their relative phases can be manipulated using an infrared laser beam. This allows us to coherently control optical feedback in a waveguide structure, switching from a case where there is no feedback in the absence of the infrared laser to the case where different types of CC optical feedbacks are generated as this field is properly adjusted. These include generation of gain and index perturbations (partly gain-coupled DFB laser), pure index corrugation (index-coupled DFB laser), and loss and index perturbations (loss-coupled DFB laser). We study these feedback mechanisms in the cases where the optically induced gratings are uniform along the cavity or have a /spl pi//2 phase shift. We discuss mode characteristics of such electromagnetically induced DFB intersubband lasers and find out how here the gain- and index-coupled DFB lasers are associated, respectively, with gain without inversion and laser-induced transparency in the conduction intersubband transitions of quantum-well structures.

A method that can spatially characterize gratings with grating strength |/spl kappa/|L up to 10 is presented. The grating is thermally chirped, which increases the transmissivity through the grating. The complex reflectivity spectrum is measured using optical-frequency-domain reflectometry, and the spatial profile is reconstructed using the time-domain layer-peeling algorithm. The spatial profiles of a uniform grating with grating strength |/spl kappa/|L = 8.25 (-66-dB minimum transmissivity) and a distributed-feedback fiber laser grating with grating strength |/spl kappa/|L = 7.5 are accurately reconstructed from the measured complex reflection spectrum.

In this paper, we carry out a systematic study on the interaction between ultrashort optical pulses and fiber Bragg grating structures operating in the linear regime. Our study is based on the joint time-frequency representation (spectrogram) of the reflection impulse response from the grating structures under analysis. By means of such a representation we get to visualize in a single image all the relevant information concerning the optical and dispersive behavior of the grating structures and more importantly, we obtain information on the pulse-grating interaction process, which otherwise is not accessible from any other method. Here, we analyze uniform and nonuniform fiber Bragg gratings. The effects of the apodization of the coupling coefficient and chirp of the grating period on the macroscopic optical properties of the considered gratings are investigated. Furthermore, we extend our analysis to more complicated in-fiber grating structures such as concatenated gratings, Fabry-Perot-like grating structures, and superimposed gratings. The results of our study indicate that the time-frequency methods constitute a powerful tool for the analysis and design of fiber Bragg grating structures.

We describe some interesting linear and nonlinear optical properties of three-level electromagnetically induced transparency (EIT) systems, such as absorption reduction, sharp dispersion change, and enhanced Kerr nonlinearity. These novel optical properties are very useful in enhancing efficient nonlinear optical processes, which can find applications in optoelectronic devices. We present some experiments done in our group in the past few years with three-level atomic systems, especially more recent experiments with EIT medium inside an optical cavity.

Over the past years, a number of groups have developed phase gratings as local oscillator beam multiplexers for array receivers in the submillimeter wavelength domain. The most popular type of grating is the Dammann (1977) grating, a simple and versatile binary phase grating. We introduce a new type of grating, the Fourier grating, which, in contrast to the binary structure of Dammann gratings, uses a smooth grating structure. Due to the lack of sharp edges these gratings can be manufactured with high precision as reflection gratings even for two-dimensional (2-D) dispersion. Fourier gratings are designed using a simple optimization procedure that only involves a small number of parameters. Their diffraction efficiency is very high. We have produced a number of gratings and tested them at a frequency of 490 GHz, verifying the theoretical results.

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.

The effective medium method is used to analyze an unchirped fiber grating in which the coupling strength between the light field and grating is monotonically increasing with penetration depth into the grating. The potential use of this type of grating for dispersion compensation in intensity modulated optical systems is discussed. It is shown that, when this type of grating is used in reflection, appropriately designed linearly ramped gratings can be used to compensate fiber dispersion over a wide range of bandwidth-distance product values. In contrast, when the grating is used in transmission, there are strong limits on the bandwidth-distance product.

A novel design of wavelength-division multiplexer based on two-mode interference assisted by a highly dispersive grating structure is two-dimensionally simulated and analyzed using finite-difference time-domain method. We show that the designed grating waveguide provides a mode-dependent reflection spectrum which can be used to modify the beat lengths of the two selected wavelengths 1342 and 1560 nm, so that they can be divided in a very short distance about 48 /spl mu/m. Simulation results show that contrasts /spl sim/12 dB and insertion losses about 2 dB could be achieved.

A focusing grating coupler (FGC) for use with a blue laser of 400-nm wavelength as a light source was fabricated for the first time. The FGC was designed to have a numerical aperture of 0.48 and a minimum period of 0.2 /spl mu/m when the focal length and the grating area were 900 /spl mu/m and 1/spl times/1 mm/sup 2/, respectively. Grating pattern of minimum line of 0.1 /spl mu/m was fabricated on a waveguide based on the boron phosphor-silicate glass material by electron-beam lithography process using the vector scan method. The spot size at the full width (1/e/sup 2/) was measured as 0.92 and 0.85/spl mu/m in x and y directions and was nearly the same as the diffraction limit.

By using the magnetic lithography technique previously reported, an optical grating can be easily formed not only on a plane, but also on a convex or concave surface. since the optical characteristics of this relief-type grating depend on its cross-sectional shape, it is important to investigate the factors determining the shape of the grating in magnetic lithography so that this technology can be applied to an encoder plate. In order to improve the encoder's performance, it is necessary to control the crosssectional shape of the grating and to try to produce a grating that has no Fraunhofer zeroth-order diffraction light spot by means of magnetic lithography. Results show that the characteristics of the gratings have good repeatability in production. Conditions for developement of such gratings are investigated.

A new method, previously reported, for fabricating optical diffraction gratings involves writing a periodic magnetic pattern in a magnetic memory medium and using a magnetic fluid as magnetic toner to render the pattern visible. Using this method, an optical grating can easily be prepared even on convex and concave surfaces. Because the optical characteristics of such relief-type gratings are determined by the cross-sectional shape, it is important that the cross-sectional shape be studied prior to application of the method in encoder plates or optical components. When the grating constant is small, the positions at which the magnetic fluid is deposited change from domain walls to magnetic domains. By analyzing the Fraunhofer diffraction pattern of such gratings, the relationship between cross-sectional shape and developing conditions was investigated. According to the results, the shape is that of a sinusoidal or circular arc when the grating constant is less than 10 ?m.

This letter reports and provides an explanation for the growth behavior of long-period gratings in H/sub 2/-loaded fiber immediately after 193-nm UV inscription. Growth of grating resonance peak by as much as 14 dB was measured. Impact of temperature and grating strength, immediately after UV inscription, on the growth behavior are also discussed.

We report the first successful experimental evaluation of the effect of grating height upon the output efficiency of grating couplers as predicted using perturbation theory. In addition, we have also produced a grating with an output efficiency of 70%, which is the highest efficiency yet reported for a silicon-on-insulator waveguide grating coupler.

The shape of the grooves of a diffraction grating can be designed to take advantage of total internal reflection. We demonstrate the effectiveness of the total internal reflection (TIR) grating numerically for the bulk optic case. The results indicate that the efficiency of the retrodiffracted order is enhanced by as much as 11 dB when the TIR grating design is used in place of a blazed grating with no facet metallization.

Experimental results have shown that the peak loss wavelength of long period gratings can shift in the direction of either shorter or longer wavelengths as the induced index change of the grating increases. An expression, which the authors believe to be novel, is derived and verified experimentally, to explain this phenomena. It is shown that the waveguide dispersion characteristics of the core and clad effective indices play a key role in the evolution of the transmission characteristics of the grating.

We present and discuss the results of picosecond transient grating self-diffraction measurements in germanium that can be understood in terms of an orientational grating produced by anisotropic (in k space) state-filling. Although there have been predictions and indirect experimental evidence for isotropic state-filling in germanium, this is the first direct experimental indication of anisotropic state-filling in a semiconductor. We compare the self-diffracted signals from orientational gratings with those from band-filling induced concentration gratings and find several distinctions, all of which can be explained in terms of the theory developed in the preceding paper. In addition, we have performed three-pulse transient grating lifetime measurements which indicate that the concentration grating decay is tens of picoseconds, while the orientational grating decay is too rapid to resolve with our 8 ps pulses.

The picosecond holographic grating generation and detection of ultrasonic waves are described. Two mechanisms, a heating mechanism and a stimulated Brillouin scattering mechanism, are described. These permit the generation of tunable ultrasonic waves in the MHz to GHz range. It is demonstrated that the two mechanisms can be distinguished by the distinct time dependences of the diffraction of a variably delayed probe pulse from the acoustic gratings. In addition, the acoustic waves can be optically manipulated, i.e., amplified, cancelled, and phase shifted. The use of the technique for the detection of weak ground state absorptions and excited state-excited state absorptions is illustrated. Mixed excited state and acoustic gratings are discussed. The phase grating contribution to diffraction from an excited state grating is displayed, and the interference between the acoustic phase grating and the excited state phase grating is described and experimentally illustrated. Contributions to the acoustic grating diffraction from spectral shifts when the probe is near a strong, narrow transition are considered. Grating experiments on various phases of liquid crystal thin films are presented. It is shown that in addition to an acoustic grating, a Kerr effect grating is produced. The Kerr grating can be separated from the acoustic grating using polarization grating excitation. The Kerr grating reveals nonexponential relaxation of orientation order in liquid crystals on a short time scale (

Three four-wave mixing techniques for measuring orientational relaxation of molecules in solution using time-separated pulses are compared both theoretically and experimentally. This comparison is shown to be greatly facilitated by modifying the conventional third-order response functions to incorporate a portion due to the slowly varying reorientation of the transition dipoles. Ordinary Bragg diffraction of a probe field, in which a transient grating formed by excitation pulses of the same polarization is probed with parallel and perpendicular polarizations, has the advantage of yielding both the time dependence and the magnitude of the induced anisotropy. However, the accompanying acoustic grating interferes with the polarization-sensitive excited state concentration grating unless the probe wavelength falls within a very strong absorption band. The acoustic grating can be eliminated by using perpendicularly polarized pumping pulses (the "crossed grating" configuration). Polarization spectroscopy in which the generated field is collinear with the probe gives the same anisotropy decay kinetics as the crossed grating, but is more sensitive to interference from background birefringence. These three methods, together with time-resolved fluorescence polarization, have been used to examine rotational dynamics in the ground and excited states of rhodamine 6G, 9-aminoacridine, and benzophenone.

A transient grating of singlet electronic excited states is produced in a pentacene-doped p-terphenyl molecular crystal by optical absorption from two crossed time-coincident picosecond excitation pulses at 532 nm. The diffraction properties of this volume grating are probed by a weak, variably delayed, Bragg-matched picosecond probe pulse. At high excitation intensities, a strong oscillatory behavior in the time-dependent scattering efficiency is observed superimposed on the exponential decay pattern of the excited-state grating. We attribute the oscillatory behavior to a thermal grating which induces coherent microwave acoustic phonons. These in turn modulate, at the sound frequency, the optical absorption properties of the pentacene molecules in the excited state. An acoustically induced amplitude grating effect is thus obtained, in contrast to conventional acousto-optic phase grating effects.

The basic principles of four-wave mixing are reviewed. Emphasis is on the coupling between light waves and material excitational waves. Only cases where material excitations are excited by optical mixing of input waves are considered. The material excitations appear in the form of a dynamic grating that would coherently scatter the incoming radiation. Probing of the dynamic grating allows the deduction of information about the material excitations.