Abstract: External cavity laser (ECL) systems and methods for measuring the wavelength of the ECL by using a portion of the positional light received by the position sensitive detector (PSD) to determine the position of a wavelength tuning element (such as a diffraction grating or an etalon), for determining the longitudinal laser mode or power output of the laser from a portion of the laser light received by a beam-shearing mode sensor, and by using a non-output beam(s) from a transmissive diffraction grating in the ECL to monitor the external cavity laser.

Abstract: Apparatus and methods for altering one or more spectral, spatial, or temporal characteristics of a light-emitting device are disclosed. Generally, such apparatus may include a volume Bragg grating (VBG) element that receives input light generated by a light-emitting device, conditions one or more characteristics of the input light, and causes the light-emitting device to generate light having the one or more characteristics of the conditioned light.

Abstract: A two-dimensional photonic crystal laser light is provided. The two-dimensional photonic crystal laser includes a two-dimensional photonic crystal made of a plate-shaped member provided with a periodic arrangement of identically-shaped modified refractive index areas having a refractive index different from that of the plate-shaped member; and an active layer provided on one side of the two-dimensional photonic crystal. The modified refractive index areas are arranged at lattice points of a lattice with a same period at least in two directions; each modified refractive index area is shaped so that a feedback strength is different with respect to directions of two primitive lattice vectors of the lattice; the two-dimensional photonic crystal has a periodic structure of a supercell, which contains a plurality of lattice points; and the sum of the feedback strengths by all modified refractive index areas in the supercell is identical in each direction of the two primitive lattice vectors.

Abstract: The present invention concerns tunable distributed Bragg reflector (DBR) semiconductor lasers, in particular a DBR laser with a branched optical waveguide 5 within which a plurality of differently shaped lasing cavities may be formed, and a method of operation of such a laser. The laser may comprise a phase control section (418), gain section (420, 422), a sampled grating DBR (412) giving a comb-line spectrum and two tunable, chirped DBRs (414, 416) for broadband frequency training and a coupling section (410).

Abstract: A light source including a light emitting unit, a nonlinear medium, and a resonant grating. The light emitting unit is arranged to emit a first light into the nonlinear medium. The nonlinear medium is arranged to generate a second light such that an optical frequency of the second light is higher than an optical frequency of the first light. The resonant grating is arranged to stabilize an optical frequency of the first light by providing optical feedback to the light emitting unit.

Abstract: A method and apparatus is disclosed for operating a laser output light beam pulse line narrowing mechanism that may comprise a nominal center wavelength and bandwidth selection optic; a static wavefront compensation mechanism shaping the curvature of the selection optic; an active wavefront compensation mechanism shaping the curvature of the selection optic and operating independently of the static wavefront compensation mechanism. The method and apparatus may comprise the nominal center wavelength and bandwidth selection optic comprises a grating; the static wavefront compensation mechanism applies a pre-selected bending moment to the grating; the active wavefront compensation mechanism applies a separate selected bending moment to the grating responsive to the control of a bending moment controller based on bandwidth feedback from a bandwidth monitor monitoring the bandwidth of the laser output light beam pulses. The active wavefront compensation mechanism may comprise a pneumatic drive mechanism.

Abstract: It is an object of the present invention to provide a compact and low-cost system for generating ultra short high energy laser light pulses that does not use the known CPA scheme. The proposed approach is based on spectral filtering of laser light pulses of a mode locked laser source. The used mode locked laser source can be of any kind as long as its filtered residual pulse bandwidth supports pulse durations longer than 20 ps and up to 1 ns. After filtering the laser light pulses are directly amplified in an amplification element consisting of one or more amplification stages. At the same time or just after amplification the light pulses are spectrally broadened through a spectral broadening element, for example a fiber of suitably chosen core diameter and length to achieve a spectral bandwidth that is wide enough to support sub 10 ps pulses. Finally, the spectrally broadened laser light pulses are compressed by a compact dispersive element consisting of one or more Bragg gratings or a grating pair.

Abstract: Apparatus and methods of controlling a frequency-converted diode laser system are disclosed. The diode laser systems can include embodiments of thermally coupled elements facilitating temperature stabilization. Aspects of some methods include monitoring the output of a stabilized diode laser system to reduce noise of the output laser beam. Other aspects of some methods include adjusting the temperature of a frequency converter based on noise in the output beam, and/or the current provided to drive the diode laser. Systems incorporating such control aspects, and others, are also disclosed.

Abstract: A method to form a an LD with the buried mesa type is disclosed, in which the n-type current blocking layer is stably kept with a distance to the active layer in the buried mesa. The method of the invention includes a step to form the mesa by iterating the RIE and the ashing to obtain in a mesa side a steep edge with the (110) surface. A wet-etching process subsequent to the iterative etching and ashing removes residuals left on the mesa side. Then, the growth of the current blocking layer shows two modes of the horizontal growth of the (110) surface and the vertical growth of the (001) surface comparably.

Abstract: A wavelength tunable system including a laser having a lasing cavity and an external cavity, the cavities having a common optical axis; a reflective grating fixed in the external cavity; a collimating lens between the lasing cavity and the reflective grating; and an adjustable reflective mirror defining one end of the external cavity and optically coupled to the fixed reflective grating, the adjustable reflective mirror pivots about the fixed reflective grating, and a method of use as defined herein.

Abstract: Provided is a fiber laser including a mode filter for selectively attenuating, among modes included in laser light propagating through a multi-mode fiber, any mode other than a radially polarized mode. Among the modes included in the laser light propagating through the multi-mode fiber, the fiber laser causes the radially polarized mode to resonate, so as to emit radially polarized laser light. The mode filter includes a long-period fiber grating obtained by writing, to a multi-mode fiber capable of guiding the radially polarized mode, a grating for selectively attenuating any waveguide mode other than the radially polarized mode.

Abstract: A wavelength tunable filter and a wavelength tunable laser module are a codirectional coupler type whose characteristics do not vary significantly with a process error. They are structured so as to include a semiconductor substrate which has a first optical waveguide and a second optical waveguide. The first and the second optical waveguides are extended from a first side of the semiconductor substrate to an opposing second side thereof. The first optical waveguide includes a first core layer, which has a planar layout having periodic convexes and concaves, and a pair of electrodes, which vertically sandwich the first core layer. The second optical waveguide includes a second core layer, which has a lower refractive index than the first core layer. Further, a layer having the same composition and film thickness as the second core layer is placed under the first core layer.

Abstract: In some embodiments, an apparatus includes a first laser cavity including a set of high-reflector gratings coupled in series and a second laser cavity including a set of high-reflector gratings coupled in series. Each high-reflector grating has an associated spectral bandwidth. The first laser cavity is configured to receive at a first end optical energy having a first spectral bandwidth and the second laser cavity is configured to receive at a first end optical energy having a second spectral bandwidth. A fiber coupler is coupled to a second end of the first laser cavity and to a second end of the second laser cavity. The fiber coupler is configured to receive optical energy from the first laser cavity having a third spectral bandwidth greater than the first spectral bandwidth, and receive optical energy from the second laser cavity having a fourth spectral bandwidth greater than the second spectral bandwidth.

Abstract: A Bragg reflector grating comprises a plurality of chirped grating sections (65-72), in which at least a first chirped grating section and a second chirped grating section have differing ranges of grating pitches. The combined range of grating pitches provided by the first and second chirped grating sections includes at least one discontinuity, such that the first and second chirped grating sections have one or more grating pitches in common and/or there are one or more ranges of grating pitches within the combined range of grating pitches that are absent.

Abstract: A single-mode, etched facet distributed Bragg reflector laser includes an AlGaInAs/InP laser cavity, a front mirror stack with multiple Fabry-Perot elements, a rear DBR reflector, and a rear detector. The front mirror stack elements and the rear reflector elements include input and output etched facets, and the laser cavity is an etched ridge cavity, all formed from an epitaxial wafer by a two-step lithography and CAIBE process.

Abstract: A large diameter optical waveguide, grating, and laser includes a waveguide having at least one core surrounded by a cladding, the core propagating light in substantially a few transverse spatial modes; and having an outer waveguide dimension of said waveguide being greater than about 0.3 mm. At least one Bragg grating may be impressed in the waveguide. The waveguide may be axially compressed which causes the length of the waveguide to decrease without buckling. The waveguide may be used for any application where a waveguide needs to be compression tuned. Also, the waveguide exhibits lower mode coupling from the core to the cladding and allows for higher optical power to be used when writing gratings without damaging the waveguide. The waveguide may resemble a short “block” or a longer “cane” type, depending on the application and dimensions used.

Abstract: The bandwidth selection mechanism includes a first actuator mounted on a second face of a dispersive optical element, the second face being opposite from a reflective face, the first actuator having a first end coupled to a first end block and a second end coupled to a second end block, the first actuator being operative to apply equal and opposite forces to the first end block and the second end block to bend the body of the dispersive optical element along the longitudinal axis of the body and in a first direction normal to the reflective face of the dispersive optical element. The bandwidth selection mechanism also includes a second actuator being operative to apply equal and opposite forces to bend the body along the longitudinal axis of the body, in a second direction perpendicular to the reflective face of the dispersive optical element.

Abstract: A pulsed light generator of the invention includes: an excitation light source; a fiber grating into which excitation light from the excitation light source enters; a rare-earth doped optical fiber optically coupled with the fiber grating, in which a rare-earth element is doped into a core, serving as an optical transmitting section; an optical switch including a deflection element for causing a Q-switching operation; a first optical fiber that causes light from the rare-earth doped optical fiber to enter into the optical switch; and a second optical fiber for waveguiding pulsed light output from the optical switch. One surface side of the optical switch, into which light enters, is subjected to anti-reflection treatment with a reflectance with respect to a wavelength of the pulsed light output from the optical switch being 0.1% or less.

Abstract: Provided is a two-dimensional photonic crystal surface emitting laser which can suppress light leaking outside in an in-plane direction of the two-dimensional photonic crystal and an absorption loss in an active layer caused by the latter layer serving as an absorbing layer without contributing to light emission, and can improve light use efficiency. The surface emitting laser has a laminated structure in which an active layer and a photonic crystal layer are laminated in a vertical direction, has a resonance mode in an in-plane direction of the photonic crystal, and light is extracted in a vertical direction to a surface of the photonic crystal, wherein the laminated structure has a multi-refractive index layer including a central region made of a high refractive index medium and a peripheral portion made of a low refractive index medium with a lower refractive index than that of the high refractive index medium.

Abstract: This invention relates to optoelectronic devices of improved efficiency. In particular it relates to light emitting diodes, photodiodes and photovoltaics. By careful design of periodic microstructures, e.g. gratings, associated with such devices more efficient light generation or detection is achieved.

Abstract: An apparatus is disclosed which may comprise a grating receiving light, a first prism moveable to coarsely select an angle of incidence of the light on the grating, and a second prism moveable to finely select an angle of incidence of the light on the grating. In one application, the apparatus may be used as a line narrowing module for a laser light source.

Abstract: A gain-coupled distributed feedback (DFB) semiconductor laser includes a grating formed by grooves through at least a part of an active region of a laser cavity. The DFB laser may be configured with a substantially pure gain-coupled grating and may be configured to provide facet power asymmetry. The grating may include at least a first-order grating section and a second-order grating section. A lasing wavelength may be obtained at the Bragg wavelength of the second-order grating section by substantially eliminating index coupling in the grating. The first-order grating section may act as a reflector for the lasing wavelength, thereby producing asymmetric power distribution in the laser cavity.

Abstract: An optical package is provided comprising a laser diode and a wavelength conversion device. The laser diode and the wavelength conversion device define an external laser cavity and the wavelength conversion device is tilted relative to the output face of the laser diode to define a tilt angle ? that is less than approximately 85°. The input face of the wavelength conversion device comprises a pair of tapered facets and a microlens. The pair of tapered facets and the microlens are defined on the input face such that they share respective portions of the facial waveguide region on the input face, with the tapered facets occupying peripheral portions of the facial waveguide region on the input face and the microlens occupying an interior portion of the facial waveguide region on the input face. Each of the pair of tapered facets define a facet angle ? within the facial waveguide region that is less than the facet angle ? and is greater than approximately 45°. Additional embodiments are disclosed and claimed.

Abstract: A tunable laser cavity for selectively emitting laser light having a first wavelength and a second wavelength using pump light emitted by a pump light source.

Abstract: Various embodiments described herein relate to a laser source for producing a pulsed laser beam comprising a plurality of ultrashort optical pulses having a variable repetition rate. In one embodiment, the laser source comprises a fiber oscillator, which outputs optical pulses and a pulse stretcher disposed to receive the optical pulses. The optical pulses have an optical pulse width. The pulse stretcher has dispersion that increases the optical pulse width yielding stretched optical pulses. The laser source further comprises a fiber amplifier disposed to receive the stretched optical pulses. The fiber optical amplifier has gain so as to amplify the stretched optical pulses. The laser source includes an automatically adjustable grating compressor having dispersion that reduces the optical pulse width. The grating compressor automatically adjusts this dispersion for different repetition rates.

Abstract: The present invention relates to a laser light source having a structure for narrowing a wavelength bandwidth of output light. The laser light source comprises, at least, a laser resonator, a pumping light source, an optical path switch device, a diffraction grating, and a total reflection mirror. The laser resonator has a light amplifying fiber for output of emission light arranged on a resonance optical path thereof. The optical path switch device has a first port and a second port on the resonance optical path of the laser resonator. The diffraction grating spectrally separates the emission light outputted from the second port. The total reflection mirror reflects a specific wavelength component out of wavelength components spectrally separated by the diffraction grating, so as to feed the specific wavelength component back to the second port.

Abstract: A laser source (10) for emitting a set of sequential, different wavelength output beams (12) includes a gain medium (16), a feedback assembly (26) and a control system (30). The gain medium (16) includes a first facet (16A), and the gain medium (16) generates a beam (12A) that exits the first facet (16A). The feedback assembly (26) includes a feedback device (40) and a device mover (42). The feedback device (40) is positioned in the path of the beam (12A) that exits the first facet (16A) and the feedback device (40) redirects at least a portion of the beam (12A) back to the gain medium (16). The device mover (42) continuously adjusts an angle of incidence (?) of the beam (12A) on the feedback device (40). The control system (30) selectively directs pulses of power to the gain medium (16) as the device mover (42) is continuously adjusting the angle of incidence (?) of the beam (12A).

Abstract: An external-cavity one-dimensional multi-wavelength beam combiner that performs wavelength beam combining along a stacking dimension of a laser stack formed of a plurality of laser arrays, each laser array configured to generate optical radiation having a unique wavelength, and each of the plurality of laser arrays including a plurality of laser emitters arranged along an array dimension of the laser stack. The multi-wavelength beam combiner includes a cylindrical telescope configured to image each of the laser emitters along a slow axis of the laser emitters, a transform lens arranged to intercept the optical radiation from each of the plurality of laser arrays and combine the optical radiation along a stacking dimension of the laser stack to form a multi-wavelength optical beam, and a diffraction element positioned at a region of overlap of the optical radiation to receive and transmit the multi-wavelength optical beam.

Abstract: A means for optically coupling a semiconductor laser to an optical fiber is disclosed. In one embodiment, a volume phase holographic element is disposed on a light-emitting surface of a semiconductor laser. The volume phase holographic element acts as an aberration-corrector for a lens that is disposed between the semiconductor laser and the optical fiber. In this way, an inexpensive lens that is not aberration free can be used. In some embodiments, the volume phase holographic element converts a Gaussian light beam emitted by the semiconductor laser into an annular beam that is more suitable for long distance transmission in multimode fibers.

Abstract: A diffraction grating device includes a substrate with a primary surface having a plurality of grating areas that are periodically arranged with a constant period in a predetermined axis direction, the grating area including a first area and a second area, a diffraction grating structure providing a chirped grating whose pitch monotonically changes along the predetermined axis direction, a core layer that is optically coupled with the diffraction grating structure with a coupling coefficient, a plurality of grating portions including the diffraction grating structure and the core layer, the grating portion including a first portion and second portion that are arranged on the first area and the second area, respectively, of the primary surface and a perturbing layer disposed at the first portion or the second portion. The perturbing layer changes the coupling coefficient between the diffraction grating structure and core layer.

Abstract: A surface emitting laser element that includes a cylindrical mesa post in which a plurality of semiconductor layers including an active layer is grown and that emits a laser light in a direction perpendicular to a substrate surface, the surface emitting laser element including a dielectric multilayer film on a top surface of the mesa post in at least a portion over a current injection area of the active layer; and a dielectric portion that includes layers fewer than layers of the dielectric multilayer film and that is arranged on a portion excluding the portion over the current injection area on the top surface of the mesa post and on at least part of a side surface of the mesa post.

Abstract: A tuning methods of an External Cavity Laser Diode (ECLD), comprises steps of: (1) providing a laser light to a grating so as to tilt the grating to an optical axis of a collimated laser light, (2) adjusting the orientation of the grating relative to the collimated laser light, (3) monitoring singleness of longitudinal mode and optical power of a light emitted from the grating, (4) calculating an orientation of the grating to exhibit intended optical power at which the singleness of longitudinal mode shows at least predetermined value, (5) adjusting the grating orientation or the optical axis of the collimated laser light to the calculated grating orientation.

Abstract: A method and apparatus for two-dimensional wavelength beam combining of laser sources. In one example, an external cavity multi-wavelength laser includes an array of laser emitters each producing an optical beam having a specified wavelength, a grating stack comprising a plurality of first-order diffraction gratings arranged linearly in a first dimension, and a dispersive element. The laser further includes a cylindrical telescope that images the optical beams from the array of laser emitters onto the grating stack. A first cylindrical transform lens spatially overlaps the optical beams in a second dimension forming a first region of overlap at the grating stack. A second cylindrical transform lens spatially overlaps the optical beams from the grating stack in the first dimension forming a second region of overlap at the dispersive element. The dispersive element transmits a multi-wavelength output beam comprising the spatially overlapped optical beams from the array of laser emitters.

Abstract: A Fiber Bragg grating is sliced into small segments (such as 1 mm in length), the sliced fiber Bragg grating segments are used as external cavities for lasers to stabilize their center wavelength. In one embodiment, a semiconductor laser has an anti-reflection coating on the front facet and a high reflectivity coating on the back facet, a sliced fiber Bragg grating is used as a partial reflection mirror to form a lasing cavity. Since the sliced fiber Bragg grating has a very small wavelength drift with temperature change, the semiconductor laser has a stable center wavelength output. In the other embodiment, a solid state laser has an anti-reflection coating on the front facet and a high reflectivity coating on the back facet, a sliced fiber Bragg grating is used as a partial reflection mirror to form a lasing cavity. The solid state laser has a stable center wavelength output.

Abstract: A semiconductor chip has at least two DFB etched facet laser cavities with one set of facets with AR coatings and a second set of etched facets with HR coatings that have a different relative position with respect to the gratings. This creates a difference in the phase between each of the etched facets and the gratings which changes the operational characteristics of the two laser cavities such that at least one of the lasers provides acceptable performance. As a result, the two cavity arrangement greatly improves the yield of the fabricated chips.

Abstract: A device having a light cavity includes, at one end, a plasmonic reflector having a grating surface for coupling incoming light into traverse plasmon waves and for coupling the traverse plasmon wave into broaden light, the surface serving to redistribute light within the cavity, the reflector being well suited for use in laser diodes for redistributing filamental cavity laser light into spatially broaden cavity laser light for translating multimodal laser light into unimodal laser light for improved reliability and uniform laser beam creation.

Abstract: A semiconductor laser includes an active layer, a first GaAs layer formed on the active layer, the first GaAs layer including a plurality of recessed portions periodically arranged, each of the recessed portions including a bottom surface of a (100) crystal surface and a slope including a (111) A crystal surface at least in parts, the recessed portion being disposed in contact with each other or with a minimal gap between each of adjacent ones of the recessed portions, the width of the bottom surface being greater than the minimal gaps, an InGaP layer formed on the recessed portion, and a second GaAs layer formed on the InGaAs layer over the recessed portion.

Abstract: A fiber laser light source is provided with a laser resonator including a pair of fiber gratings optically connected to a fiber in a state that the fiber is interposed between the paired fiber gratings. The reflection center wavelength of a laser-exit side fiber grating, out of the paired fiber gratings, lies in a wavelength range where the reflectance of a fiber grating, out of the paired fiber gratings, closer to the pump laser light source is not smaller than 80% but not larger than 98%.

Abstract: A polarization pinned vertical cavity surface emitting laser (VCSEL). A VCSEL designed to be polarization pinned includes an upper mirror. An active region is connected on the upper mirror. A lower mirror is connected to the active region. A grating layer is deposited to the upper mirror. The grating layer includes a low index of refraction layer formed by deposition on the upper mirror. The grating layer further includes a high index of refraction layer formed by deposition on the low index of refraction layer. A grating is formed into the grating layer.

Abstract: Techniques and devices that stabilize a laser to a whispering gallery mode optical resonator.

Abstract: Disclosed are a nitride semiconductor laser element including a light emitting portion made of a nitride semiconductor, and an external-cavity semiconductor laser device using it. In the nitride semiconductor laser element, a coat film made of silicon oxynitride is formed on the light emitting portion, and the reflectance of the coat film to feedback light of laser light emitted from the light emitting portion is 0.5% or less.

Abstract: A tunable diode laser system with external resonator in Littrow or Littman configuration has an optical lattice on which the light beam from a laser diode is diffracted, a support element to hold the lattice or to hold a mirror, which reflects the light diffracted by the lattice, and an actuator to change the position of the lattice or the mirror. The tunable diode laser system enables a reliable mode-hop-free tuning and furthermore is easy and economical to realize by having the support element include a carrier, on which the lattice or the mirror is arranged, and a base body, while the actuator acts on the carrier and rests against the base body. The carrier is connected to the base body via linkages, such that a linear deflection of the actuator is transformed into a rotation of the carrier in the plane of the light beam, while the center of rotation lies outside the base body.

Abstract: An apparatus and source arrangement for filtering an electromagnetic radiation can be provided which may include at least one spectral separating arrangement configured to physically separate one or more components of the electromagnetic radiation based on a frequency of the electromagnetic radiation. The apparatus and source arrangement may also have at least one continuously rotating optical arrangement, e.g., polygonal scanning mirror and spinning reflector disk scanner, which is configured to receive at least one signal that is associated with the one or more components. Further, the apparatus and source arrangement can include at least one beam selecting arrangement configured to receive the signal.

Abstract: A fiber laser having at least one pair of reflectors coupled to an optical fiber, the at least one pair of reflectors defining an optical cavity between the at least one pair of reflectors and being configured to reflect light within the optical cavity. At least one light pump is coupled to the optical fiber and configured to provide pump light into the optical cavity, and at least one medium is positioned within the optical cavity and configured to generate signal light from the pump light in the optical cavity. Further, at least one grating positioned within the optical cavity and configured to couple the signal light out of the optical cavity.

Abstract: A tunable distributed feedback semiconductor laser includes a substrate; an optical waveguide structure disposed on a main surface of the substrate and including an active layer and a diffraction grating, the optical waveguide structure being divided into a first DFB portion, a wavelength-tuning region, and a second DFB portion in that order; a first electrode for injecting carriers into the active layer in the first DFB portion; a second electrode for injecting carriers into the active layer in the second DFB portion; and a third electrode for supplying a wavelength tuning signal to the wavelength-tuning region. The diffraction grating extends over the first DFB portion, the wavelength-tuning region, and the second DFB portion. An optical confinement factor of the wavelength-tuning region is smaller than that of the first and second DFB portions.

Abstract: High-power, phased-locked, laser arrays as disclosed herein utilize a system of optical elements that may be external to the laser oscillator array. Such an external optical system may achieve mutually coherent operation of all the emitters in a laser array, and coherent combination of the output of all the lasers in the array into a single beam. Such an “external gain harness” system may include: an optical lens/mirror system that mixes the output of all the emitters in the array; a holographic optical element that combines the output of all the lasers in the array, and an output coupler that selects a single path for the combined output and also selects a common operating frequency for all the coupled gain regions.

Abstract: The basic gain medium enclosure for laser devices comprises two parallel lateral mirrors which geometrically define the extent of the gain medium enclosure and which allow the formation of lateral stationary sinusoidal waves.

Abstract: Optical resonators and optical devices based on optical resonators that implement diffractive couplers for coupling light with the optical resonators.

Abstract: The invention relates to a highly repetitive laser system operating according to the reproducible amplifier principle. Said system comprises at least one amplified laser medium, a laser resonator provided with at least one resonator mirror and at least one modulator and a pump source, in particular, a laser diode source, which is used to pump the laser medium. The highly repetitive laser system is compact by virtue of the fact that a pulse extensor, having a highly dispersive effect as a result of the structure or material thereof, is integrated into the laser resonator.

Abstract: A laser diode is provided comprising a multiple quantum well structure, a current concentrating layer having an oxide-confined aperture, a grating layer having an index of refraction, and a transparent electrode, wherein the transparent electrode has an index of refraction less than the index of refraction of the grating layer.