Showing papers on "Diffraction grating published in 2005"

Liquid-crystal diffraction gratings using polarization holography alignment techniques

Abstract: A method of patterning surfaces for liquid-crystal alignment using a polarization holography exposure on a linear photopolymerizable polymer alignment layer is demonstrated. Three configurations are demonstrated which include registered planar-periodic surface boundary conditions on both surfaces (true polarization gratings), planar-periodic and uniform planary surface boundary conditions, and planar-periodic and homeotropic boundary conditions. Two-dimensional polarization gratings are also demonstrated by orientating planar-periodic alignment layers orthogonally. Passive polarization gratings are also demonstrated using reactive mesogens to capture the periodic order indefinitely. The underlying structure of the configuration is discussed, including the nature of their switching transition (threshold or thresholdless), for all three configurations. A simple phenomenological model is presented to describe the Freedericksz transition for the registered planar-periodic boundary condition case.

Nonreciprocal waveguide Bragg gratings.

Abstract: The use of a complex short-period (Bragg) grating which combines matched periodic modulations of refractive index and loss/gain allows asymmetrical mode coupling within a contra-directional waveguide coupler. Such a complex Bragg grating exhibits a different behavior (e.g. in terms of the reflection and transmission spectra) when probed from opposite ends. More specifically, the grating has a single reflection peak when used from one end, but it is transparent (zero reflection) when used from the opposite end. In this paper, we conduct a systematic analytical and numerical analysis of this new class of Bragg gratings. The spectral performance of these, so-called nonreciprocal gratings, is first investigated in detail and the influence of device parameters on the transmission spectra of these devices is also analyzed. Our studies reveal that in addition to the nonreciprocal behavior, a nonreciprocal Bragg grating exhibits a strong amplification at the resonance wavelength (even with zero net-gain level in the waveguide) while simultaneously providing higher wavelength selectivity than the equivalent index Bragg grating. However, it is also shown that in order to achieve nonreciprocity in the device, a very careful adjustment of the parameters corresponding to the index and gain/loss gratings is required.

All-optical switching and routing based on an electromagnetically induced absorption grating

Abstract: An electromagnetically induced absorption grating is formed in a three-level atomic vapor under the condition of electromagnetically induced transparency in which the strong coupling beam is replaced by a standing wave The transmission and reflection behaviors of the weak probe beam are greatly modified at certain frequencies near the two-photon resonance An all-optical two-port signal router–all-optical switch is demonstrated

X-ray wavefront analysis and optics characterization with a grating interferometer

Abstract: We present an interferometric method to measure the shape of a hard-x-ray wavefront. The interferometer consists of a phase grating as a beam splitter and an absorption grating as a transmission mask for the detector. The device can be used to measure wavefront shape gradients corresponding to radii of curvature as large as several dozens of meters, with a lateral resolution of a few microns. This corresponds to detected wavefront distortions of approximately 10−12m or λ∕100. The device was used with 12.4 keV x rays to measure the slope error and height profile of an x-ray mirror. Surface slope variations with periods ranging from less than 1 mm to more than 1 m can be detected with an accuracy better than 0.1μrad.

Rich information format surface plasmon resonance biosensor based on array of diffraction gratings

Abstract: We report a new optical biosensor platform for high throughput label-free monitoring of biomolecular interactions. This biosensor platform is based on spectroscopy of surface plasmons on an array of miniature diffraction gratings. Each miniature grating serves as an independent sensing element and allows the monitoring of refractive index changes induced by biomolecular interactions on the grating surface. The presented sensor concept shows potential for real-time observation of thousands of interactions on a single sensor chip. In this paper, we demonstrate a prototype device consisting of an SPR sensor chip with 216 sensing channels, a fluidic system for sample distribution on the sensor chip and an optical chip readout system.

An intelligible explanation of highly-efficient diffraction in deep dielectric rectangular transmission gratings.

Abstract: This paper describes in a very easy and intelligible way, how the diffraction efficiencies of binary dielectric transmission gratings depend on the geometrical groove parameters and how a high efficiency can be obtained. The phenomenological explanation is based on the modal method. The mechanism of excitation of modes by the incident wave, their propagation constants and how they couple into the diffraction orders helps to understand the diffraction process of such gratings and enables a grating design without complicated numerical calculations.

Creation of optical vortices in femtosecond pulses

Abstract: We experimentally created a femtosecond optical vortex using a pair of computer-synthesized holographic gratings arranged in a 2f - 2f optical setup. We present measurements showing that the resulting donut mode is free of spatial chirp, and support this finding with an analysis of the optical wave propagation through our system based on the Kirchhoff-Fresnel diffraction integral. An interferogram confirms that our ultrashort vortex has topological charge 1, and a conservative experimental estimation of its duration is 280 fs. We used 25-fs radiation pulses (bandwidth approximately 40 nm) produced by a Ti:sapphire laser oscillator.

Dispersion-tailored few-mode fibers: a versatile platform for in-fiber photonic devices

Abstract: In-fiber devices enable a vast array of critical photonic functions ranging from signal conditioning (amplification, dispersion control) to network management (add/drop multiplexers, optical monitoring). These devices have become mainstays of fiber-optic communication systems because they provide the advantages of low loss, polarization insensitivity, high reliability, and compatibility with the transmission line. The majority of fiber devices reported to date are obtained by doping, designing, or writing gratings in the core of a single-mode fiber (SMF). Thus, these devices use the fiber only as a platform for propagating light-the device effect itself is due to some extraneously introduced material or structure (dopants for amplification, gratings for phase matching, etc.) There exists another, relatively less explored degree of freedom afforded by fibers-the ability to copropagate more than one mode. Each mode may have a uniquely defined modal dispersion and propagation characteristic. In this paper, we will describe the variety of fiber devices enabled by few-mode fibers-fibers that typically support two to four modes with suitably tailored dispersive properties. We will show that the unique dispersive properties of various modes, in conjunction with the ability to couple between them with gratings, leads to devices that offer novel solutions for dispersion compensation, spectral shaping, and polarization control, to name a few.

Long-period fiber gratings as ultrafast optical differentiators

Abstract: It is demonstrated that a single, uniform long-period fiber grating (LPFG) working in the linear regime inherently behaves as an ultrafast optical temporal differentiator. Specifically, we show that the output temporal waveform in the core mode of a LPFG providing full energy coupling into the cladding mode is proportional to the first derivative of the optical temporal signal (e.g., optical pulse) launched at the input of the LPFG. Moreover, a LPFG providing full energy recoupling back from the cladding mode into the core mode inherently implements second-order temporal differentiation. Our numerical results have confirmed the feasibility of this simple, all-fiber approach to processing optical signals with temporal features in the picosecond and subpicosecond ranges.

Multiple-wave lateral shearing interferometry for wave-front sensing

Abstract: Multiple-wave achromatic interferometric techniques are used to measure, with high accuracy and high transverse resolution, wave fronts of polychromatic light sources. The wave fronts to be measured are replicated by a diffraction grating into several copies interfering together, leading to an interference pattern. A CCD detector located in the vicinity of the grating records this interference pattern. Some of these wave-front sensors are able to resolve wave-front spatial frequencies 3 to 4 times higher than a conventional Shack–Hartmann technique using an equivalent CCD detector. Its dynamic is also much higher, 2 to 3 orders of magnitude.

Deposition of overlays by electrostatic self-assembly in long-period fiber gratings

Abstract: It was proved that the deposition of an overlay material onto a long-period fiber grating causes important shifts in the wavelengths of the typical attenuation bands that are caused by coupling between cladding and core modes [Opt. Lett.27, 682 (2002) ]. A theoretical model for analyzing a multilayer cylindrical waveguide is presented that permits the phenomenon to be understood and predicted. An overlay of higher refractive index than the cladding starts to guide a mode if a certain thickness value is exceeded. This causes large shifts in the resonance wavelength induced by the grating. One important application of this phenomenon to sensors is enhancement of the sensitivity of a long-period fiber grating to ambient conditions. Theoretical results are corroborated with experimental ones obtained by electrostatic self-assembly.

High-sensitivity optical chemosensor based on coated long-period gratings for sub-ppm chemical detection in water

Abstract: In this work, the use of fiber long-period gratings (LPGs)—coated with nanoscale overlays of Syndiotactic Polystyrene (sPS) in the nanoporous crystalline δ form as specific and highly sensitive chemical sensors for in water monitoring—is proposed The approach presented here, combines the excellent sorption properties of δ form sPS as a chemosensitive layer with the excellent refractive index sensitivity of LPG-based sensors as ideal transducers In particular, when overlays with a high refractive index compared with the cladding one are deposited along the grating region, as in this case, the refraction-reflection regime at the cladding-overlay interface occurs As result of this mechanism, the attenuation bands of coated LPGs would respond to the optical changes induced in the sensitive overlay due to chemical sorption by a significant modification of the peak central wavelength and intensity The sensitivity depends strongly on the overlay thickness and the grating coupled cladding mode Here, sensor probes were prepared by using a dip coating technique and a proprietary procedure to obtain the δ form sPS An experimental demonstration of the sensor capability to perform sub-ppm detection of chloroform in water at room temperature is reported here Also, the effects of the overlay thickness and the cladding mode order on sensor sensitivity and response time have been numerically and experimentally investigated

Wire-grid diffraction gratings used as polarizing beam splitter for visible light and applied in liquid crystal on silicon

Abstract: The application of wire grid polarizers as efficient polarizing beam splitters for visible light is studied. The large differences between the transmissivity for different polarizations are explained qualitatively by using the theory of metallic wave guides. The results of rigorous calculations obtained by using the finite element method are compared with experiments for both classical and conical mount. Furthermore the application of wire-grid polarizers in liquid crystal on silicon display systems is considered.

Refractive-diffractive spectrometer

Abstract: A diffraction grating and a prism with the appropriate characteristics are employed to provide a combined dispersive characteristic that is substantially linear over the visible spectrum. Radiation from the grating and prism is collimated by a lens towards a detector array. The grating or a telecentric stop between the grating and prism is placed at a focal point of the lens in a telecentric arrangement so that equal magnification is achieved at the detector array. If the detector array is replaced by a plurality of optical channels, a multiplexer/demultiplexer is obtained.

Sandwiched long-period gratings for simultaneous measurement of refractive index and temperature

Abstract: A simultaneous measurement of external refractive-index (RI) and temperature is presented by using a sandwiched structure of long-period gratings (LPGs). An LPG pair with codirectional coupling between the guided mode and a low-order cladding mode acts as a temperature sensor with low RI sensitivity, whereas another in-between LPG which couples the guided mode with a higher order cladding mode performs as an RI sensing element. The experimental results show such an all-LPG structure is not only suitable for use as a compact temperature-compensated RI sensor, but also convenient for fabrication.

Simultaneous measurement of refractive index and temperature based on a reflection-mode long-period grating and an intrinsic Fabry–Perot interferometer sensor

Abstract: We have demonstrated a single fiber probe for simultaneous measurement of external refractive index and temperature based on two interferometers: a reflection-mode long-period grating (LPG) for refractometry and an intrinsic Fabry-Perot interferometer (IFPI) for temperature measurement. Since the output signal of the combined structure is a superposition of signals produced by both sensors, which have different spatial frequencies, the original signals needed for the index and temperature measurements can be separated and recovered through digital filters.

Highly efficient Er,Yb-doped fiber laser with 188W free-running and>100W tunable output power

Abstract: Efficient high-power operation of an erbium-ytterbium co-doped fiber laser cladding-pumped by two spatially-multiplexed and polarization combined 975 nm diode-stacks is reported. Up to 188 W of continuous-wave output at 1.57 µm was generated with a beam-quality factor (M2) of 1.9 and an overall slope efficiency with respect to launched pump power of 41% (and 43% for output powers Tunable operation was demonstrated by use of an external cavity containing a diffraction grating and a maximum output power of 108 W at 1538 nm was generated for a launched pump power of ~ 336 W. The operating wavelength was tunable from 1531 to 1571 nm, with >100W output power over a tuning range of 36 nm from 1532 nm to 1568 nm.

Nanowire Array Gratings with ZnO Combs

Abstract: Diffraction gratings are mainly manufactured by mechanical ruling, interference lithography, or resin replication, which generally require expensive equipment, complicated procedures, and a stable environment. We describe the controlled growth of self-organized microscale ZnO comb gratings by a simple one-step thermal evaporation and condensation method. The ZnO combs consist of an array of very uniform, perfectly aligned, evenly spaced and long single-crystalline ZnO nanowires or nanobelts with periods in the range of 0.2 to 2 μm. Diffraction experiments show that the ZnO combs can function as a tiny three-beam divider that may find applications in miniaturized integrated optics such as three-beam optical pickup systems.

Phenomenological representation of deep and high contrast lamellar gratings by means of the modal method

Abstract: Some remarkable effects are analyzed at the light of the ‘vertical mode’ representation of diffraction on a deep lamellar grating such as total diffraction in transmission and resonant total reflection. The general methodology of this phenomenological representation is given together with rigorous numerical examples.

Miniature lamellar grating interferometer based on silicon technology

Abstract: A lamellar grating interferometer is described, in which the light beams are collimated and focused onto the grating by means of mirror 9, which at the same time serves for collecting the light reflected from the grating. In this case, the light beam of a white light source 1 is first collimated by means of first lens 2, and sunsequently passed through a sample cuvette 3. The transmitted light beam is subsequentlyy focused and coupled by another lens 2 into a fibre 17. The light to this fibre 17 is subsequentlyy directed towards a mirror 9, reflected from this mirror 9 onto a grating 11, which forms part of a lamellar grating interferometer which is realised by means of a micro electro mechanical device MEMS 7, which is mounted on a MEMS holder 6, as is the fibre 17. The light reflected from this grating 11 is reflected onto the same mirror 9, and focused and coupled by this same mirror 9 into a second multimode fibre 18, which is also fastened to the holder 6. The light guided by this second multimode fibre 18 is subsequently fed into a detection device 4.

Reliable polarization control of VCSELs through monolithically integrated surface gratings: a comparative theoretical and experimental study

Abstract: Vertical cavity surface-emitting lasers (VCSELs) with a well-defined and predictable polarization of the emitted light are sought for a number of applications. In this paper, we show that one can define and stabilize the polarization of single- and multimode oxide-confined VCSELs with a monolithically integrated dielectric surface grating. In recent years, we have developed a three-dimensional, fully vectorial model for VCSELs, which proved to nicely reproduce the experimental results of quite complex structures, such as noncircular devices and phase-coupled VCSEL arrays. This software allows for the first time to analyze the effects of a dielectric grating in the output facet cap layer and its capability to fix the polarization of the emitted light. It is here employed as a design tool, yielding excellent agreement with the experimental data. Since the simulations predict the polarization behavior to be sensitively dependent on the grating parameters, hundreds of VCSELs with 99 different parameter sets, two grating orientations and active diameters of 4 and 7 /spl mu/m have been analyzed. Even VCSELs with eight or more coexisting modes turned out to be linearly polarized with an orthogonal polarization suppression ratio in excess of 15 dB. Theoretical and experimental emission far-fields are compared, and it is shown that diffraction side lobes can be prevented with properly chosen grating parameters which simultaneously ensure full polarization stability.

Embedded slanted grating for vertical coupling between fibers and silicon-on-insulator planar waveguides

Abstract: We propose a compact and efficient grating coupler for vertical coupling between optical fibers and planar waveguides. A grating with a parallelogram shape is designed to be etched through the entire high-index waveguide core. The coupler is optimized using a microgenetic algorithm coupled with a two-dimensional finite-difference time-domain method. Simulations show that up to 75.8% coupling efficiency can be obtained between a single-mode fiber and a 240-nm-thick silicon-on-insulator planar waveguide.

High-performance operation of single-mode terahertz quantum cascade lasers with metallic gratings

Abstract: A periodic array of thin slits opened on a metallic surface can act as a one-dimensional photonic crystal for the propagation of surface-plasmon waves. We have used such structure for the implementation of distributed feedback resonators in quantum cascade lasers emitting near 2.5THz. Single-mode emission, stable at all injection currents and operating temperatures, was achieved both in pulsed and continuous wave. The devices exhibited output powers of several milliwatts with low threshold current densities of ∼100A∕cm2.

Polarization selective spectroscopy experiments: methodology and pitfalls

Abstract: We analyze the effectiveness of various practical implementations of time-dependent pump-probe and transient grating polarization-selective experiments. A variety of optical arrangements are analyzed in the Jones matrix calculus framework. The optical arrangements that permit the correct determination of the time-dependent orientational and excited-state population dynamics are delineated. It is shown that magic angle transient grating experiments yield pure population dynamics under certain conditions only. The effectiveness of spectrally resolved magic angle pump-probe and transient grating experiments that use a monochromator are shown to be dependent on the position of an analyzing polarizer along the experimental beam path relative to various other optical elements. The spectrally resolved experiments will measure pure population dynamics only if a polarizer is placed immediately after the sample. The effectiveness of experiments measuring orientational dynamics by separately measuring the probe signal with its polarization parallel and perpendicular to the pump polarization is not constrained by the conditions imposed on the magic angle experiments.

Angular tolerant resonant grating filters under oblique incidence.

Abstract: Resonant grating filters have been proposed as a promising alternative to multilayer stacks for narrowband free-space filtering. The efficiency of such filters under normal incidence has been demonstrated. Unfortunately, under oblique incidence, the limited angular tolerance of the resonance forbids any filtering applications with use of standard collimated incident beams. Using a multimode planar waveguide and a bi-atom grating, we show how to increase the angular tolerance up to the divergence of standard beams (0.2 deg) without modifying the spectral bandwidth (0.1 nm), under any oblique angle of incidence.

All-optical switching of the transmission of electromagnetic radiation through subwavelength apertures.

Abstract: Unprecedented optical control of the surface plasmon polariton assisted transmission of terahertz radiation through subwavelength apertures is rendered possible by carrier-induced changes to the dielectric properties of a semiconductor grating. Although the study presented is static, the extension of our approach to dynamic switching and tuning is deemed straightforward, opening the way for the realization of ultrafast surface plasmon based devices.

Tunable magnetic fluid grating by applying a magnetic field

Abstract: A kind of tunable magnetic fluid grating is developed in this letter. The operating principle of the tunable magnetic fluid grating is analyzed theoretically. When the absorption coefficient modulation of the grating is not too large, the energy of the zeroth-order diffracted light can be transferred to that of the higher-order completely and vice versa. Experiments are done to investigate the tunable diffraction properties of the magnetic fluid grating, and the transfer of the energy of the zeroth-order diffracted light to that of the higher-order is apparent.

Tunable fiber laser light source

Abstract: An optical fiber loop has a gain medium having a gain at an oscillation wavelength and optical circulators 13 and 14. Collimate lenses 22 and 24 enlarge light bean taken from the optical circulators 13 and 14. A polygon mirror 25 is provided on the light axis, and is rotated. A diffraction grating 27 is provided at the receiving position of the light reflected by the polygon mirror 25, and is of a Littrow configuration which reflects the light in the same direction as the incident light. A selected wavelength changes according to an incident angle to the diffraction grating 27, resulting in increase of selectivity owing to twice incident, thereby permitting to change an oscillation wavelength with narrow band even when changing the selected wavelength by rotating the polygon mirror 25 at high speed.

Strain and temperature characterization of photonic crystal fiber Bragg gratings

Abstract: A Bragg grating in a photonic crystal fiber was written and its dependence with temperature and strain analyzed. The two observed Bragg wavelengths correspond to a fundamental and a higher-order mode in the optical fiber. The temperature and strain calibration curves for both modes are measured and found to be distinct. The general properties of gratings in these fibers, and their implications, are enunciated.

Narrowband bulk Bragg grating optical parametric oscillator

Abstract: A narrowband, pulsed optical parametric oscillator (OPO) whose output coupler is a bulk glass Bragg grating is demonstrated. The OPO is based on periodically poled KTiOPO4 and is pumped by a pulsed, frequency-doubled Nd:YAG laser at 532 nm, generating a signal at 975 nm with a total efficiency of 35%. This novel and compact device shows a spectral bandwidth of 0.16 nm (50 GHz), a decrease by a factor of 20 compared with that when a conventional mirror is used. By using a folded cavity, we demonstrate tunable radiation with a tuning range of 60 nm and maintained spectral bandwidth.