Third-order Gratings Research Articles

Parity-time symmetric gratings in 1550 nm distributed-feedback laser diodes: insight on device design rules

We build on former results from our work on parity-time symmetric gratings implemented in 1550 nm distributed-feedback laser diodes to address the design issues raised by the first trends we observed. These laser diodes are of a complex-coupled nature, with modulations of both real and imaginary parts of the effective index, with a relative phase related to the parity-time symmetry. The unidirectionality of the photonic behavior in the reflection mode is dependent on the level of extra losses incurred by the metallic grating used to implement fixed loss modulation onto a nearly uniform gain, either as a first-order or a third-order grating. The observed behavior suggests that facets play a large role in setting the desired unidirectional lasing operation points, with preferential emission on one side. We explore this issue, which is of generic interest for the coupling of parity-time symmetric structures to open space.

Annealing of High Temperature Stable Hydrogen Loaded Fiber Bragg Gratings

We compare the isothermal and isochronal annealing response of ultrafast laser-induced first-order gratings to that of the third-order gratings up to 1000 °C. Both devices are written in $H_{2}$ -loaded SMF-28 fiber with low-energy femtosecond laser pulses. With the first-order gratings, we observed a regeneration process that allows for a final peak index change of roughly 4% of the original grating. A high-temperature stable peak reflectivity of 39% is obtained (−2.14-dB transmission loss). The third-order gratings do not anneal out completely (they do not regenerate) and roughly 26% of the device persists at 1000 °C corresponding to a final peak reflectivity of 24% (−1.17-dB transmission loss). Due to their similar fabrication conditions, these findings suggest a strong link between the regenerated first-order gratings and the stable third-order devices. We estimate that both first- and third-order gratings of $\sim 4.5$ mm in length are good candidates for the development of high-temperature stable fiber sensors.

POWERFUL, ROTATING DISK WINDS FROM STELLAR-MASS BLACK HOLES

We present an analysis of ionized X-ray disk winds observed in the Fe K band of four stellar-mass black holes observed with Chandra, including 4U 1630-47, GRO J1655-40, H 1743-322, and GRS 1915+105. High-resolution photoionization grids were generated in order to model the data. Third-order gratings spectra were used to resolve complex absorption profiles into atomic effects and multiple velocity components. The Fe XXV line is found to be shaped by contributions from the intercombination line (in absorption), and the Fe XXVI line is detected as a spin-orbit doublet. The data require 2-3 absorption zones, depending on the source. The fastest components have velocities approaching or exceeding 0.01c, increasing mass outflow rates and wind kinetic power by orders of magnitude over prior single-zone models. The first-order spectra require re-emission from the wind, broadened by a degree that is loosely consistent with Keplerian orbital velocities at the photoionization radius. This suggests that disk winds are rotating with the orbital velocity of the underlying disk, and provides a new means of estimating launching radii -- crucial to understanding wind driving mechanisms. Some aspects of the wind velocities and radii correspond well to the broad-line region (BLR) in active galactic nuclei, suggesting a physical connection. We discuss these results in terms of prevalent models for disk wind production and disk accretion itself, and implications for massive black holes in active galactic nuclei.

24‐wavelength distributed Bragg reflector laser array with surface gratings

A small footprint diode laser array of 24 individually addressable distributed Bragg reflector lasers is presented. Third-order gratings etched into the surface of an AlGaAs vertical waveguide structure act as Bragg reflectors. The ridge waveguides have an 87 µm-wide spacing and the emission wavelengths are spread on a 0.5 nm-wide grid around a centre wavelength of 905 nm. All 24 lasers on the array emit single mode with side mode suppression ratios > 50 dB and output powers >17 mW at 40 mA pump current.

Theoretical biosensing performance of surface plasmon polariton Bragg gratings

Surface plasmons have raised much interest over the past few decades in part for their potential in biosensing applications. Waveguide Bragg gratings supporting long-range surface plasmon polaritons, constructed by stepping the width of a metal stripe embedded in Cytop, with an etched microfluidic channel to expose the sensing surface, are proposed as biosensors. Several designs are investigated theoretically to evaluate their bulk and surface sensitivities under wavelength interrogation. Detection limits of ∼10−6 RIU and ∼8 pg/mm2 are predicted for bulk and surface sensing, respectively, with corresponding figures of merit of ∼1000 RIU−1 and ∼0.2 nm−1. Third-order gratings are good choices because they have essentially the same sensitivity as first-order gratings but over a narrower bandwidth, and they are easier to fabricate due to their larger pitch.

Narrow linewidth 1560 nm InGaAsP split-contact corrugated ridge waveguide DFB lasers.

We demonstrate a split-contact corrugated ridge waveguide InGaAsP distributed feedback laser at 1560 nm. The laser cavity has been defined with uniform third-order gratings etched along the sidewalls of the ridge waveguide. The gratings were fabricated using a standard I-line stepper lithography technique along with an inductively coupled reactive ion-etching process. Stable single-mode operation has been achieved with side-mode suppression ratios ≥50 dB, output powers ≥7 mW, a wavelength tuning range ≥2.3 nm, and narrow linewidths (≤140 kHz) for different biasing conditions, with a minimum of 70 kHz. The effect of p-contact partition on device performance is also studied.

Fiber Bragg Gratings in the Visible Spectral Range With Ultraviolet Femtosecond Laser Inscription

In this letter, we investigate the inscription of fiber Bragg gratings in the visible spectral range using deep ultraviolet femtosecond laser exposure and two-beam interferometry. The properties of first-order reflection gratings and third-order gratings for use in the visible wavelength range are compared. Stronger gratings have been achieved for first-order reflecting Bragg gratings compared with third-order gratings. We demonstrate a fiber Bragg grating with a grating period of 226 nm and a filtering efficiency of more than 30 dB.

Narrow Linewidth 1550 nm Corrugated Ridge Waveguide DFB Lasers

We report on the design and characterization of InP-based multiple quantum well corrugated ridge waveguide distributed feedback diode lasers operating at 1550 nm. Third-order gratings have been etched along the sidewalls of the ridge waveguide using the standard I-line stepper lithography technique with an inductively coupled reactive ion etching process. An as-cleaved 1500-μm-long laser diode shows stable continuous wave single-mode operation at 1550 nm with high side-mode suppression ratios (>50 dB), a temperature-dependent wavelength shift dλ/dT ~0.095 nm/°C, and output powers ≥7 mW at 25 °C. Linewidth determination has been carried using the delayed self-heterodyne interferometric technique. Narrow linewidths (≤250 kHz) have been observed for a wide range of current injection, with a minimum of 184 kHz at 300 mA.

Time-Domain Analysis of Third-Order Quantum-Dot-Based Laterally-Coupled Distributed Feedback Lasers Using Travelling-Wave Approach

In this paper, we present a time-domain (TD) coupled model suitable to investigate high-order $\lambda/4$ phase-shift grating quantum-dot (QD) based laterally-coupled distributed feedback (LC-DFB) laser. First, by integrating the Streifer's coefficients, we include the effects of radiation and evanescent modes into the TD coupled wave equations, and hence we investigate the effect of the radiation modes. Then, via a well-established set of rate equations coupled with the travelling field propagation equations, population dynamics in the QD region are modeled and the homogeneous and inhomogeneous broadening of, respectively, the whole QD ensemble and each QD inter-band transition are properly taken into account in the model. Finally, we consider the coupling between forward and backward electric fields due to the grating via the travelling-wave approach. This approach advances the impact of radiating partial waves in high-order QD-LC-DFB lasers. It is shown that, in particular for third-order rectangular grating, longitudinal spatial hole-burning is highly reduced, high single mode suppression ration (60 dB) is obtained and larger frequency modulation is achieved thanks to the fine engineering of the grating features and the particular properties of QD. Eventually, such results highlight the beneficial effect of considering high-order grating QD-LC-DFB lasers for better longitudinal-mode discrimination and high device performances.

Dynamic Analysis of High-Order Laterally Coupled DFB Lasers Using Time-Domain Traveling-Wave Model

This paper describes the application of a time-domain modeling approach for a laterally coupled distributed feedback (LC-DFB) semiconductor laser for the first time. We numerically study the effect of the radiation modes on LC-DFB laser properties. We integrate the Streifer's coefficients, which represent the effects of radiation and evanescent modes into the time-domain coupled-wave equations. High-order corrugated gratings with λ/4 phase-shift are analyzed, where the degree of longitudinal spatial hole burning can be effectively reduced by means of fine tuning of the grating duty cycle. Additionally, we show a remarkably enhanced side-mode suppression ratio (SMSR). For example, for the third-order gratings with a 50% duty cycle, an SMSR as high as 45 dB can be predicted.

InP-Based Midinfrared Quantum Cascade Lasers for Wavelengths Below 4 μm

We review the recent development of high-performance short-wavelength (λ ~ 3.05-3.8 μm) strain-compensated InGaAs/AlAs(Sb)/InP quantum cascade lasers (QCLs). The lasers are demonstrated in which wavelengths as low as 3.05 μm are obtained at temperatures up to 295 K. We also verify that strain-compensated In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> As/AlAs(Sb) QCLs with AlAs barriers in the active region operate with much better performance compared with the lasers having identical design but with AlAsSb barriers throughout the whole core region. λ ~ 3.3-3.7 μm laser emission is observed at temperatures up to at least 400 K and up to 20 W of output optical power at 285 K for the QCLs with various core region designs. Room temperature distributed feedback InGaAs/AlAs(Sb) QCLs with buried third-order gratings have been also developed, displaying single-mode operation in the wavelength range of 3.358-3.380 μm for temperatures between 270 and 360 K.

Optical modeling of laterally-corrugated ridge-waveguide gratings

The paper presents some model improvements for the optical simulation of laterally-corrugated ridge-waveguide distributed feedback lasers. Simulation results are discussed and design principles for achieving single-longitudinal-mode operation are outlined. The effects of the laterally-corrugated ridge geometry both on the coupling coefficient and on the Bragg wavelength of different transverse modes are presented. The improved modeling has been used to design 980 nm distributed feedback lasers with laterally-corrugated ridge-waveguide third-order gratings. The lasers fabricated using nanoimprint lithography exhibited single-mode operation with 50 dB side-mode suppression ratio.

Tunable dispersion slope compensation for 40-Gb/s WDM systems using broadband nonchannelized third-order chirped fiber Bragg gratings

We demonstrate tunable dispersion slope compensation using broadband nonchannelized third-order chirped fiber Bragg gratings (FBGs) for 40-Gb/s wavelength-division-multiplexed (WDM) systems. The slope is tuned by stretching as it has a third-order time delay variation with wavelength, and thus a second-order dispersion-wavelength curve. Calculation results include the examples of the slope tuning in addition to the interchannel dispersion differences and the slope tuning ranges with respect to the wavelength in a single grating. To verify the functionality of the third-order grating, we prepare a third-order grating with a dispersion variation of up to 400 ps/nm over the 3.5-nm usable bandwidth, and a dispersion slope tuning range of /spl sim/65 ps/nm/sup 2/ is achieved. For 40-Gb/s systems, two new gratings are designed and inversely cascaded to eliminate the inherent errors originating from the deleterious higher-order dispersion components in a single grating. The slope tuning range is -20 to +20 ps/nm/sup 2/ over the 5-nm usable bandwidth. A /spl sim/10-dB power penalty improvement is achieved and the BER floor is eliminated for the worst-case channel after transmission through a 170-km link using dispersion shifted fiber (DSF) with a 4/spl times/40-Gb/s RZ data stream.

Quasi phase matching in GaAs–AlAs superlattice waveguides through bandgap tuning by use of quantum-well intermixing

We report the observation of second-harmonic generation by type I quasi phase matching in a GaAs-AlAs superlattice waveguide. Quasi phase matching was achieved through modulation of the nonlinear coefficient chi((2))(zxy), which we realized by periodically tuning the superlattice bandgap. Second-harmonic generation was demonstrated for fundamental wavelengths from 1480 to 1520 nm, from the third-order gratings with periods from 10.5 to 12.4microm . The second-harmonic signal spectra demonstrated narrowing owing to the finite bandwidth of the quasi-phase-matching grating. An average power of ~110 nW was obtained for the second harmonic by use of an average launched pump power of ?2.3mW .

Surface-grating distributed Bragg reflector quantum-well lasers fabricated in AlGaAs–GaAs asymmetric epitaxial waveguides

Ridge waveguide lasers, integrated with single deep-surface distributed Bragg reflectors (DBR's) in the passive section, were fabricated with a GaAs-AlGaAs double-quantum-well structure in an asymmetric waveguide. Third-order gratings, with a period of 389 nm and defined by holographic lithography, were formed by low-damage reactive ion etching processes. The grating losses and optical coupling coefficients were estimated, in particular, by use of the relationship between the real and the effective grating lengths that were computed and reexamined by measurements of grating periodicity and mode spacing. By use of two different geometries, we produced guide lines for obtaining high-performance lasing properties for these surface-grating DBR lasers. Additionally, a detailed analysis of lasing wavelength shifts was carried out for this study. It was found that injected-carrier-induced effects shift the lasing wavelength much more than gain-loss competition within an extended DBR laser cavity.

Edge-emitting GaInAs-AlGaAs microlasers

The fabrication and characteristics of edge-emitting microlasers with deeply etched distributed Bragg reflector (DBR) mirrors are presented. Using reactive ion etching, the mirrors are formed at both cavity ends of a ridge waveguide laser with an GaInAs-AlGaAs single-quantum-well active layer. The devices have continuous-wave threshold currents as low as 2 and 6 mA at cavity lengths of 80 and 40 μm, respectively. Lasing operation is achieved down to a length of 28 μm. Diffraction limited reflectivities >75% are obtained for third-order gratings, with two DBR periods.

Structural and optical properties of epitaxially overgrown third-order gratings for InGaN/GaN-based distributed feedback lasers

Laser-diode heterostructures of InGaAlN containing a third-order diffraction grating for distributed optical feedback have been examined with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The grating was defined holographically and etched by chemically assisted ion-beam etching into the upper GaN confinement layer of the laser structure. After the etch step, it was overgrown with an Al0.08Ga0.92N upper cladding layer. Threading dislocations were present that initiated at the sapphire substrate, but no new dislocations were observed at the grating/Al0.08Ga0.92N interface. A comparison of TEM and SEM micrographs reveals that there is a compositional gradient in the AlGaN upper cladding layer; however, calculations show that it did not reduce the optical coupling coefficient of the grating.

Demonstration of an InGaN/GaN-based optically pumped multiquantum well distributed feedback laser using holographically defined third-order gratings

We demonstrate an optically pumped InGaN/GaN-based multiquantum well distributed feedback laser in the blue spectral region. The third-order grating providing feedback was defined holographically and dry etched into the upper waveguiding layer by chemically assisted ion-beam etching. When aligning the stripe-shaped pump beam either parallel or perpendicular to the grating grooves, we found a considerably lower pumping threshold, higher slope efficiency, a slightly longer emission wavelength, and a much narrower linewidth for the geometry with the pump beam orthogonal to the grating lines. A nearly constant emission wavelength of 400.85 nm and a linewidth of 0.7 Å were observed under various pump intensities. To the best of our knowledge, this is the narrowest linewidth ever reported for an optically pumped device in this material system.

Four wavelength distributed feedback ridge waveguide quantum-well heterostructure laser array

A four-wavelength strained-layer InGaAs-GaAs-AlGaAs distributed feedback ridge waveguide quantum-well heterostructure laser array, with a lasing emission wavelength separation of ∼16 Å between adjacent emitters, is described. The four-wavelength array requires only a single MOCVD growth step. Lateral optical confinement is achieved with dry-etched lateral third-order gratings. Each element operates in the fundamental lateral mode with a near-field FWHM of 2.6±0.1 μm and the entire width of the near-field pattern is &amp;lt;80 μm.