Order Gratings Research Articles

Effects of grating order and tooth rounding in laterally coupled distributed feedback lasers

We demonstrate the results of an analysis of laterally coupled distributed feedback (LC-DFB) lasers with higher order gratings, including the effects of radiating partial waves. For a given fabrication resolution, first-order gratings, if they can be reliably manufactured, always provide the strongest coupling. However, at resolutions requiring higher order gratings, if duty cycles of >0.5 are used, the lowest grating order is not always the one with the strongest coupling. An analysis of the rounding of the grating teeth showed that the required threshold gain was increased by nearly 20% when the rectangular grating was rounded in fabrication.

Higher diffraction order photorefractive optical beam splitter

A higher diffraction order photorefractive (PR) optical beam splitter has been realized based on a PR higher diffraction order grating. In the experiments, the beam splitter was produced by two-wave coupling at a small incident angle in Fe:LiNbO 3 using the output from a He–Ne laser at 632.8 nm. An incident signal beam containing three different wavelengths (632.8, 532.0 and 488.0 nm) was split into multi-output beams by the beam splitter. The size of higher diffraction order is given and the influence of crystal thickness is discussed. Results show that the higher diffraction order PR optical beam splitter provides a practical method to split a multi-wavelength signal beam.

Integrated self‐aligned conjugated polymer fiber laser devices

AbstractIn this letter we present a fully integrated self‐aligned distributed feedback (DFB) fiber laser device, which was directly imprinted in the conjugated polymer poly[2‐methoxy‐5 (2‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) on the top of optical fibers by using the soft lithographic technique of “liquid imprinting”. For this process master gratings (360 and 380 nm) for the feedback structure were fabricated via e‐beam lithography, transferred to an elastomeric stamp and used to imprint the grating into the highly luminescent conjugated polymer. Such second order gratings were photo‐pumped with a frequency doubled Nd:YAG laser, the laser emission (around 640 nm, depending on the used grating) was directly coupled into the waveguide and detected via a CCD spectrometer at the end of the fiber. The threshold of the laser devices was found to be in the range of 4.3 mJ/cm2 and exhibited a line width of approximately 2 nm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

External Cavity Lasers Composed of Higher Order Gratings and SLDs Integrated on PLC Platform

Very compact 4-channel 200-GHz-spacing external cavity lasers (ECLs) were fabricated by hybrid integration of reflection gratings and superluminescent laser diodes on a planar lightwave circuit chip. The fifth-order gratings as reflection gratings were formed using a conventional contact-mask photo-lithography process to achieve low-cost fabrication. The lasing wavelength of the fabricated ECLs matched the ITU grid with an accuracy of ×0.1 nm, and optical powers were more than 0.4 mW at the injection current of 80 mA for all channels. The ECLs showed single mode operations with more than 30 dB side lobe suppression.

High-power 894 nm monolithic distributed-feedback laser

A ridge-waveguide InGaAs/GaAsP laser, emitting up to 250 mW in a single lateral and longitudinal mode at a wavelength of 894 nm, is presented. The distributed feedback is provided by a second order grating, formed into an InGaP/GaAs/InGaP multilayer structure. Owing to the stable lasing frequency, the large side mode suppression ratio (> 40 dB) and small spectral line width (< 200 kHz) the diode laser is well suited for caesium D1 spectroscopy. This was verified by the measurement of the hyperfine structure of the D1 line.

Mechanically tunable optofluidic distributed feedback dye laser

A continuously tunable optofluidic distributed feedback (DFB) dye laser was demonstrated on a monolithic replica molded poly(dimethylsiloxane) (PDMS) chip. The optical feedback was provided by a phase-shifted higher order Bragg grating embedded in the liquid core of a single mode buried channel waveguide. Due to the soft elastomeric nature of PDMS, the laser frequency could be tuned by mechanically stretching the grating period. In principle, the mechanical tuning range is only limited by the gain bandwidth. A tuning range of nearly 60 nm was demonstrated from a single dye laser chip by combining two common dye molecules Rhodamine 6G and Rhodamine 101. Single-mode operation was maintained with less than 0.1 nm linewidth. Because of the higher order grating, a single laser, when operated with different dye solutions, can provide tunable light output covering the entire spectrum from near UV to near IR in which efficient laser dyes are available. An array of five DFB dye lasers with different grating periods was also demonstrated on a chip. Such tunable integrated laser arrays are expected to become key components in inexpensive advanced spectroscopy chips.

980-nm DBR lasers using higher order gratings defined by i-line lithography

We report on the simultaneous definition and fabrication of Bragg gratings and ridge waveguides using wafer stepper lithography and reactive ion etching, respectively. Single-longitudinal mode emission from two-section ridge-waveguide distributed feedback lasers with sixth and seventh order gratings will be reported. This technology enables a cheap fabrication of wavelength-stabilized lasers and a simple variation of the parameters of the gratings on the wafer using optical lithography.

Fabrication methods for a quantum cascade photonic crystal surface emitting laser

Conventional quantum cascade (QC) lasers are intrinsically edge-emitting devices with mode confinement achieved via a standard mesa stripe configuration. Surface emission in edge emitting QC lasers has therefore necessitated redirecting the waveguided laser emission using a second order grating. This paper describes the methods used to fabricate a 2D photonic crystal (PC) structure with or without a central defect superimposed on an electrically pumped QC laser structure with the goal of achieving direct surface emission. A successful systematic study of PC hole radius and spacing was performed using e-beam lithography. This PC method offers the promise of a number of interesting applications, including miniaturization and integration of QC lasers.

Angular responses of the first and second diffracted orders in transmission diffraction grating recorded on photopolymer material

Some of the theoretical models in the literature describing the mechanism of hologram formation in photopolymer materials predict the existence of higher harmonics in the Fourier expansion of the recorded refractive index. Nevertheless, quantitative information is only obtained for the first harmonic of the refractive index using Kogelnik's Coupled Wave Theory. In this work we apply the Rigorous Coupled Wave Theory to demonstrate that when recording phase diffraction gratings in PVA/acrylamide photopolymer materials, a second order grating is also recorded in the hologram even when the material is exposed to a sinusoidal interference pattern. The influence of this second order grating on the efficiency of the first order for replay at the first on-Bragg angular replay condition is studied and the size of the 2nd harmonic examined.

Achromatic Four Quadrant Phase Mask Coronagraph using the Dispersion of Form Birefringence

We describe an alternative design for the 4-quadrant phase mask coronagraph described recently by Rouan et al. (2000). Based on the same principle, i.e. producing a very efficient nulling by mutually destructive interferences of the coherent light from the main source, our mask realises the π phase shift using some properties of ZOGs (Zeroth Order Gratings) and according to an original scheme respecting the 4-quadrant symmetry. When the period of the one-dimension grating structure is smaller than the wavelength of the incident light, the structure becomes birefringent. The effective refractive indices depend on the wavelength. Using this feature, we can design a mask whose nulling efficiency is maintained within a wide wavelength range. Numerical simulations were made according to the RCWA (Rigorous Coupled Wave Analyses).

Compact diffractive interferometric displacement sensor in reflection

A displacement measurement technology of diffractive interferometry has been developed in the objective of miniaturization. The interference principle has been implemented in the form of a miniature read head prototype according to a novel device configuration after a discussion on the fabrication and mouting tolerances has been made conducting to a high resolution micro-optical encoder. A read-out grating acting simultaneously as a subwavelength and a two order grating enables the miniaturization of a diffractive interferometric optical displacement sensor of the reflection type.

Investigation of Polarization-Induced Orientational Order and Surface Gratings in Liquid Crystalline AZO-Polymers

Polarization-induced orientational order in three different azopolymers is investigated and discussed with respect to the formation of surface relief gratings induced by holographic exposure. For this purpose the birefringence and dichroism induced by different light polarization, namely linear, elliptical or circular, has been studied. To determine the symmetry and the induced amount of orientational order the illuminated region is characterized by the scalar Maier-Saupe order parameter S. Tensor gratings were induced in these materials by holographic exposure with different kinds of polarization patterns and were investigated with polarization and atomic force microscopy. The gratings are characterized by diffraction efficiencies as well as by depth and shape of the surface modulation. Large surface modulations (>2,5μm) and diffraction efficiencies (>90%) for a 5μm thick film were obtained.

Groove depth dependence of the anchoring strength of a zero order grating-aligned liquid crystal

The influence of in-plane electric fields on the twist profile of a liquid crystal aligned by nondiffractive (zero order) gratings has been explored using an optical guided mode technique. The liquid crystal is studied in a novel cell geometry comprising one rubbed polyimide alignment layer and one zero order diffraction grating. Comparison of the director-distorted profiles with predictions from elastic continuum theory allow both the twist elastic constant of the bulk liquid crystal and the azimuthal anchoring strengths at the two surfaces to be determined. Experiments have been performed on different depth gratings. Good agreement with the Berreman topological alignment model has been achieved for fairly shallow gratings, but not for gratings with deeper groove profiles. This implies that the theory needs modification to account for deep gratings.

Complex-coupled DFB lasers: advantages and drawbacks of gain and radiation loss grating

Two types of InGaAs-GaAs-AlGaAs complex-coupled distributed feedback laser structures grown by molecular beam epitaxy are compared. The first structure uses interrupted quantum wells (QW's) grown over a grating. Its properties strongly depend on the interface cleaning resulting in nonreproducible threshold current densities. The second structure includes three uniform QW's, the gain coupling being induced by the vertical radiation of the second order grating. This last structure has reproducible, low continuous wave threshold current densities (jth=300 A/cm/sup 2/). The coupling strength and its origin are described for the two structures. Both structures have high mono-mode yield (>75%) and side-mode suppression ratios larger than 40 dB.

Fabrication of long-period fiber gratings with no harmonics

We demonstrate a method to fabricate spectrally clean long-period gratings in optical fibers. Because the photosensitivity of glass is nonlinear, even a sinusoidal light pattern will generate harmonic gratings, and these higher order gratings can cause unwanted resonances. We solve this problem by first measuring the photosensitivity of the glass and then scanning the fiber across an ultraviolet light beam at a variable speed under computer control. The resulting spectra are free from extraneous resonances caused by harmonics of the fundamental grating period.

First order gain-coupled GaInAs/GaAs distributed feedback laser diodes patterned by focused ion beam implantation

Electrically pumped gain-coupled distributed feedback lasers with first order gratings have been fabricated on a (Ga,In)As/(Al,Ga)As separate confinement heterostructure by focused ion beam implantation and epitaxial overgrowth. The lasers are operating at room temperature at wavelengths around 950 nm. All investigated devices show single longitudinal mode emission at all operating conditions.

High efficiency surface emitting lasers using blazed grating outcouplers

Grating coupled surface emitting lasers with rectangular as well as blazed grating outcouplers were investigated. Horizontal cavity InGaAs/AlGaAs laser oscillators, with two rectangular first order surface gratings as feedback elements, were integrated with single detuned rectangular or blazed second order gratings as surface output couplers, using electron beam lithography and chemically assisted ion beam etching. Superior differential quantum and surface emission efficiencies of 56% and 84%, respectively, were measured for lasers with optimized blazed output couplers.

The influence of aluminum concentration on photoelectrochemical etching of first order gratings in GaAs/AlGaAs

We present a set of experiments which systematically clarifies the enhancement of photoelectrochemical (PEC) etching due to the mole fraction of aluminum in AlxGa1−xAs. The spatial resolution of gratings etched in Al0.3Ga0.7As is as much as three times greater than the spatial resolution of gratings etched in GaAs, so that the smallest practical grating period is about 0.3 μm, as compared with about 0.7 μm using previous techniques. This technique enabled PEC fabrication of first order gratings for waveguide outcouplers. The lower hole mobility of AlxGa1−xAs is proposed as a possible explanation for this grating resolution improvement.

Second order grating formation in dry holographic photopolymers

The first and second order grating strengths in Du Pont holographic photopolymers were measured through the diffraction efficiencies at the first and second Bragg angles, for various recording irradiances. Grating profiles up to the second order were constructed. It was found that high recording irradiance results in large nonlinearity.

Artificial refractive index gratings manufacturing using electron beam lithography

A new physical principle for creation of optical elements which allows the fabrication of structures with an artificial refractive index was developed. It needs the manufacturing of structures with feature sizes much less than the wavelength of the incident light. The technique for the experimental determination of the proximity function parameters in electron lithography eliminating the influence of the resist development process is presented. The correction of proximity effects was made fora number of zero order gratings and artificial index gratings fabrication. The Fresnel zone plate was manufactured with an artificially varying phase retardation.