Bragg Order Research Articles

Narrowband Mg/SiC multilayer mirror working as high-harmonic selector at 30.4 nm wavelength

Multilayers (MLs) are a good choice for high-harmonic generation (HHG) sources for selecting single-wavelength radiation. MLs working around 30.4 nm with a significantly reduced bandwidth based on high Bragg order reflection have been designed and fabricated. The narrowband MLs were characterized using grazing incidence x-ray reflectometry, extreme ultraviolet reflectivity, and intrinsic stress measurements. The results indicate that Mg/SiC MLs designed with 3rd Bragg order have narrowest bandwidth of 0.71 nm (full width half maximum) with a spectral resolution (λ/Δλ) of 42, and reflectance of 30% under near normal incidence geometry. Based on these results, the simulation of narrowband MLs working at large incident angles demonstrates that 3rd Bragg order ML has a better spectral selectivity compared with 1st Bragg order ML for HHG sources.

Distributed feedback lasers up to the 400th Bragg order with an organic active layer

The output characteristics and lasing threshold behavior of higher order Bragg lasers are explored using an organic active layer spin-cast over substrate-defined fused-silica gratings. Gratings ranging from 1st to the 400th Bragg order of varying duty cycle are fabricated with standard e-beam lithography. Distinct diffraction orders are observed at lower Bragg orders but smear out toward higher orders due to overlapping diffracted orders. Significant variation in thresholds is observed with duty cycle for most Bragg orders. A dramatic reduction in threshold is observed with increasing cavity length. The lowest lasing thresholds obtained for 4th and 400th order distributed feedback lasers are ∼1.4 and 4 μJ cm−2, respectively, using F80.9BT0.1 as an active layer. 400th order Bragg lasers are fabricated with direct-write photolithography using a UV laser diode, with comparable thresholds to e-beam lithography fabricated devices.

Enhanced light output from deep ultraviolet light-emitting diodes enabled by high-order modes on a photonic crystal surface.

The authors demonstrate the enhanced light output from 275-nm AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED) structures via the in-plane modulation of shallow photonic crystal (PC) patterns that were fabricated on the p-AlGaN contact layer surface. The employed PC lattice constants are in the range of 270-780 nm, much larger than the fundamental Bragg order lattice constant (∼95 nm). As compared to the unpatterned sample, the intensity of the top (or bottom) emission can be enhanced by up to 331% (or 246%), attributed to the high-order coherent diffraction of the internal trapped light and also the Purcell enhancement of spontaneous emission. The findings in this Letter suggest an easier way for the realization of more energy-efficient DUV LEDs which offer the advantage of high emission for various applications in disinfection and sterilization.

Laser-induced periodic surface structures as optical resonators for organic thin-film distributed feedback lasers

Laser-induced periodic surface structures (LIPSS) have received considerable attention due to their potential for micro-and nanostructuring and surface functionalization of various materials. We present a novel application of LIPSS on a glass substrate as distributed feedback (DFB) laser resonators, capable of providing sufficient positive optical feedback to achieve lasing in thin-film waveguides with organic small molecules as a gain medium. The direct femtosecond micromachining allows for easy variation of the periodicity across a broad range of values, including those required to reach 1st Bragg order DFB operation. We investigate several small molecule organic thin-film systems and observe lasing in strong accordance with the underlying periodicities of all photonic structures involved. These results demonstrate the effectiveness of LIPSS as DFB laser resonators and suggest that they could facilitate the integration of organic thin-film media-based lasers and other photonic devices into various integrated photonic systems.

Bragg order effect on ultrashort pulse generation by distributed feedback dye lasers under subnanosecond excitation: Numerical study

A comparative numerical study of the 1st- and 2nd-order distributed feedback (DFB) dye lasers pumped with 0.5 ns, 532 nm laser pulses is performed. A dynamic model of the gain-coupled DFB dye laser based on a coupled-wave theory and taking into account Bragg diffraction order is used. Due to the periodical distribution of the pump field and stimulated emission intensities along the excited dye volume, the rate equations of the population densities are turned into the equations of the Fourier coefficients. To simulate the DFB lasing, coupled-wave equations of optical fields are used. Effect of the pumping intensity on the temporal and energy characteristics of DFB dye lasers is studied depending on the dye concentration and the length of the excited dye volume. Influence of the lasing seed type (constant or noise-like) on the simulation results is also analyzed. A special emphasis is on the characteristics of single ultrashort (US) picosecond pulses generated by the relaxation oscillations mechanism. The results obtained indicate that for both types of DFB dye lasers the temporal course of the output intensity is governed by the pumping strength. Depending on the excess above the threshold (high or low), either US pulse trains or single US pulses are generated. The influence of the active medium parameters on their characteristics has been studied in detail. In addition, it was established that unlike the 1st-order DFB lasing, where the simulation results are independent of the seed type and smooth US pulses are always obtained, for the 2nd-order regime, to get smooth rather than structural single US pulses, a noise-like seed should be used. However, the most important finding is that the changeover from the 1st to the 2nd order leads to ∼1.1–3.2 (∼1.13–1.46) times reduction in duration, ∼1.4–3.4 (∼1.4–3.0)-fold increase in energy and ∼2.8–9.7 (∼1.5–4.8)-fold growth in peak power of single US pulses for the case of constant (noise-like) lasing seed. Hence, the use of the 2nd-order regime represents a simple and convenient way to improve the energy characteristics of single US pulses of DFB dye lasers generated by the relaxation oscillations mechanism.

Helical Bragg Gratings: Experimental Verification of Light Orbital Angular Momentum Conversion

Structured light, in particular light possessing orbital angular momentum (OAM), has been actively studied in recent decades. Helical Bragg grating (HBG) is a reflecting optical element, which predicted to be able to convert the OAM of light in waveguides and fibers. However, the HBG has not been demonstrated experimentally yet. Here we report the first experimental manifestation of HBG created in the form of a waveguide with depressed-index cladding. Few-mode channel helical-depressed-cladding waveguides (HDCWs) have been written in a YAG crystal by a femtosecond laser beam. The HDCW pitches were comparable to near-IR wavelengths. The spectral and polarization characteristics of the transmitted and reflected light have been studied. It was shown that HDCWs behave like HBGs providing narrowband resonance coupling of the counter propagating modes, the OAM of the modes differing by the Bragg order or, what is the same, the topological charge of the helical cladding structure. Adjusting the HDCW parameters allows one to excite a reflected wave at the predetermined wavelength and with the predetermined OAM change with respect to the light coupled to the waveguide. Thus, for the first time, the HBGs suitable for the generation and filtration of the vortex light have been experimentally demonstrated. The experimental results obtained agree well with the coupled mode theory calculations.

Internally wavelength stabilized 910 nm diode lasers with epitaxially stacked multiple active regions and tunnel junctions

We present a multi-active-region bipolar-cascade distributed-Bragg reflector (DBR) laser emitting around 910 nm. The three active regions and two tunnel junctions are located in a single waveguide core sharing the same third-order vertical mode. A slope efficiency of 2.25 W/A was measured with a threshold current density of 400 A/cm2. Due to the implemented 7th Bragg order surface grating, the laser exhibits a narrow optical spectrum with a width of 0.3 nm and a thermal detuning of 60 pm/K.

Strong and Short Bragg Waveguide Gratings With Trapezoidal-Shaped Grooves

In this study, we evaluate strong Bragg waveguide gratings formed of trapezoidal-shaped grooves, as typically produced in surface-etched structures, and we propose optimal groove shapes. We begin by validating our modelling approach, which is based on the finite-difference time-domain method, through comparisons with coupled mode theory applied to rectangular gratings. We then compute the reflectance, transmittance and loss of trapezoidal gratings as a function of Bragg order, and of the top and bottom widths of the grooves. Simulations reveal that the highest reflectance peaks are achieved for widths that are about λ/8, regardless of the Bragg order. We find that the characteristics of trapezoidal-grooved gratings are strongly dependent on the bottom width of the grooves, and that a very high reflectance can be achieved by carefully tailoring this width. We also calculate the coupling coefficients of infinitely long optimized structures, finding that they decrease with increasing Bragg order. Our results are of interest in the design of strong short gratings etched in the surface of semiconductor waveguides.

Высокоточная характеризация сверхмногопериодных AlGaAs/GaAs-сверхрешеток с помощью рентгеновской рефлектометрии на синхротронном источнике

The morphology of Al0.3Ga0.7As / GaAs superlattices grown by molecular beam epitaxy was determined by X-ray reflectometry (including a synchrotron radiation source) and photoluminescence. The thicknesses of the superlattice layers with 100 periods, found using laboratory and synchrotron studies, correlate with an accuracy of ~ 1%. At the synchrotron, beginning with high (> 4−5) Bragg orders, reflection peaks were found that are not observed in measurements with a diffractometer and are apparently associated with the technological features of the growth of such structures. It follows from the analysis that the peaks correspond to modulation in the superlattice with a period 3–5 times greater and characterize the scatter of the thicknesses over the structure depth by several percent.

Selection of a two-dimensional image edge using polarisation-independent acousto-optic diffraction

A method for filtering two-dimensional distributions of light-wave spatial frequencies, based on the use of polarisation-independent acousto-optic (AO) Bragg diffraction, is proposed. This method provides a maximally possible transmission bandwidth of spatial frequencies and a minimum size of the resolved AO element. A selection of a two-dimensional edge in the zero Bragg order is demonstrated. The main theoretical conclusions are experimentally confirmed using a paratellurite AO filter.

Red‐emitting distributed‐feedback ridge‐waveguide laser based on high‐order surface grating

Distributed-feedback ridge-waveguide lasers emitting around 670 nm have been fabricated and experimentally characterised. Wavelength stabilisation is provided by 40th Bragg order surface gratings dry-etched into the p-GaAs–AlGaAs–AlGaInP layer structure. The large grating period of 4.2 µm allows the injection of the current between the grooves of the grating without sacrificing the electrical and optical properties noticeably. The 1.5 mm long devices achieve an output power of 50 mW at an injection current of 180 mA. The side mode suppression ratio is 50 dB at 150 mA.

Peculiarities of the acousto-optical field formed during diffraction of dual-mode radiation in uniaxial crystals

This paper discusses acousto-optic Bragg diffraction in uniaxial crystals when two intrinsic optical modes of the crystal participate in the diffraction. The calculations are based on two crystals widely used in acousto-optics: lithium niobate and paratellurite. It is shown that the character of the acousto-optical field formed in the diffraction orders when optical rays propagate close to the optic axis while a sound wave propagates orthogonally to the optic axis makes it possible to use the resulting field for two-dimensional processing of optical images. The image can be filtered in this case by using both the zeroth and the first diffraction orders. Acousto-optic filters are fabricated from a paratellurite crystal and are used to experimentally show that a two-dimensional contour is discriminated when the image is subjected to optical Fourier processing (fast Fourier transform), using the zeroth and first Bragg orders.

A high-order external distributed feedback polymer laser with low working threshold

In this paper, we report a high-order distributed feedback (DFB) polymer laser with low working threshold. Using the high-order grating increases the lithographic tolerances, providing coherent light sources that are more amenable to mass-manufacturing techniques, such as laser direct writing lithography and roll-to-roll processing. To enable high-order DFB lasing, an unconventional working configuration is designed in which the grating is situated on top of the uniform conjugated polymer film. In addition, a novel Forster energy transfer blend of two conjugated polymers is used as the gain medium. Upon pumping, the device emits lasing around 603.6 nm with a bandwidth of 0.5 nm. The threshold is around 20.5 μJ cm−2 (~2.56 kW cm−2), about to enter the regime of inexpensive LED pumping. A further increase in pump energy results in simultaneous oscillations at the 29th and 30th Bragg orders. Operating principles of the high-order DFB polymer laser, including spectral performance and threshold dependence on pump length, are investigated. This approach represents a step towards low-cost, even ‘disposable’ polymer lasers.

Analysis of the Metric in Quasicrystals—Linear Response in Logarithmically Periodic Solids

The metric, that enables measurement of structural data from diffraction in quasicrystals, is analyzed. A modified compromise spacing effect is the consequence of scattering of periodic electromagnetic or electron waves by atoms arranged on a geometric grid in an ideal hierarchic structure. This structure is infinitely extensive, uniquely aligned and uniquely icosahedral. The approximate analytic factor that converts the geometric terms base τ, into periodic terms modulo 2π, is . It matches the simulated metric cs=0.947, consistently used in second (Bragg) order, over a wide scale from atomic dimensions to sixth order superclusters.

Ultracompact, narrowband three-dimensional plasmonic waveguide Bragg grating in metal/multi-insulator/metal configuration

Ultracompact three-dimensional (3D) waveguide plasmonic Bragg gratings in a metal/multi-insulator/metal (MMIM) configuration with sinusoidal width modulations are presented. A semi-analytical approach from the eigenvalue problem and finite transmission-line network perspectives is described to facilitate the 3D designs with Bragg wavelength errors being within the range of 0.12%-3.99%. A narrowband design operating in the 1550 nm band with a FWHM bandwidth of 10.8 nm and an extinction ratio of approximately 12 dB is numerically demonstrated within a footprint of <17 μm(2) (10 periods). Unlike other types of plasmonic Bragg gratings, the bandwidth is increased as the MMIM grating length increases. The number of distinct plasmonic z-directed Poynting vector patterns within one period is found to be identical to the corresponding Bragg order. Narrowband characteristics are attributed to delicate, concurrent contra-flow interactions in and between photonic and plasmonic modes occurring simultaneously in multiple places within one period.

Polarization characteristics of a near infrared distributed feedback dye laser operated at the first, second and third orders of Bragg condition

The first, second, and third order distributed feedback lasing characteristics of LDS 759 dye in methanol were investigated. Both intensity modulation and polarization modulation were used to generate laser action. Tunable narrow line-width lasing output was achieved by crossing the pump beams at intersection angles matching the Bragg condition. It was observed that the threshold pump energy increased with the increase of the Bragg order. The ratio of the threshold energy required when using polarization modulation to that required when using intensity modulation increased with increasing Bragg order. The laser output showed different polarization characteristics when the feedback mechanism was changed from intensity modulation to polarization modulation. The degree of polarization of the laser output showed an increasing trend with the Bragg order, which changed from 0.13 at the first Bragg order to 0.67 at the second Bragg order under polarization modulation. Wide-band tuning of the distributed feedback lasing was obtained for first-order operation using both intensity modulation and polarization modulation, from 708nm to 896nm and 712nm to 820nm, respectively.

High-Power Distributed Feedback Lasers With Surface Gratings

We present the results for high-power broad-area distributed feedback lasers with surface gratings of 80th, 135th, and 270th Bragg orders. A maximum output power of 11 W for a laser with 80th order grating, emitting around 976 nm with a spectral width <; 1 nm has been achieved. Similar narrow linewidth operation occurs for devices with 135th Bragg order from 7- to 11-W output power. The technological approach to fabricating these devices based on stepper lithography and reactive ion etching is described.

Properties and fabrication of high-order Bragg gratings for wavelength stabilization of diode lasers

We report further progress in the design and fabrication of high-order Bragg gratings defined by I-line projection lithography that are implemented in a high-power diode laser. Simulations of surface Bragg gratings with large duty cycles predict large reflectivities even for the 25th Bragg order. We implemented such Bragg gratings in high-power diode lasers and compared the spectral and electro-optical properties with diode lasers having seventh-order Bragg gratings. Details of the fabrication and encapsulation of the Bragg gratings will also be presented.

Fabrication of Bragg Gratings on LiNbO3 optical waveguides

We report on three approaches to achieve 1086nm period Bragg Gratings structures (BGs), corresponding to the third Bragg order at 1550nm, on X-cut lithium niobate (LiNbO3) substrates. The first method relies on Reactive Ionic Etching (RIE) with fluorine gases, associated with e-beam lithography and electroplating deposition. BGS with etched angles close to 60^o, an aspect ratio AR=1.4 and a reflectivity of 13% have been fabricated. The second process is based on Focused Ion Beam (FIB) milling. BGs are etched in standard optical waveguides. The process has led to etched angles close to 85^o with AR=6.3 and to a reflectivity of 50%. Finally, we propose an original method based on the use of lateral FIB etching on the edge of deep-etched ridge optical waveguides predefined by ''optical grade dicing''. Etched angles close to verticality with AR>9 have been obtained and optical characterizations are under work.

Why momentum width matters for atom interferometry with Bragg pulses

We theoretically consider the effect of the atomic source's momentum width on the efficiency of Bragg mirrors and beamsplitters and, more generally, on the phase sensitivity of Bragg pulse atom interferometers. By numerical optimization, we show that an atomic cloud's momentum width places a fundamental upper bound on the maximum transfer efficiency of a Bragg mirror pulse, and furthermore limits the phase sensitivity of a Bragg pulse atom interferometer. We quantify these momentum width effects, and precisely compute how mirror efficiencies and interferometer phase sensitivities vary as functions of Bragg order and source type. Our results and methodology allow for an efficient optimization of Bragg pulses and the comparison of different atomic sources, and will help in the design of large momentum transfer Bragg mirrors and beamsplitters for use in atom-based inertial sensors.