Grating Sections Research Articles

Widely tunable room temperature semiconductor terahertz source

We present a widely tunable, monolithic terahertz source based on intracavity difference frequency generation within a mid-infrared quantum cascade laser at room temperature. A three-section ridge waveguide laser design with two sampled grating sections and a distributed-Bragg section is used to achieve the terahertz (THz) frequency tuning. Room temperature single mode THz emission with a wide tunable frequency range of 2.6–4.2 THz (∼47% of the central frequency) and THz power up to 0.1 mW is demonstrated, making such device an ideal candidate for THz spectroscopy and sensing.

Synthesis of Fiber Bragg Gratings With Arbitrary Stationary Power/Field Distribution

A method to synthesize a fiber Bragg grating (FBG) providing a desired, arbitrary stationary power/field distribution along the grating length is proposed and numerically demonstrated. In the proposed method, starting from the desired stationary power/field distribution or its differential at the Bragg wavelength, the forward and the backward propagation modes are derived for a uniform-period FBG consisting of a prescribed number of grating sections. Using the transfer matrix method, the local reflection coefficient (i.e., ρ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ) is calculated from the two derived propagation modes, and this information is subsequently employed to obtain the corresponding local coupling coefficient (i.e., q <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</sub> ) and the associated refractive index modulation (i.e., FBG profile) along the grating length. The proposed synthesis method is first numerically verified when two stationary power distributions from a uniform and a Gaussian-apodized FBGs are chosen as the target, showing an excellent agreement between the reconstructed refractive index modulations and those of the original FBGs. Next, several FBG profiles providing user-defined stationary power distributions, including a flat-top shape, a sharp peak, a saddle shape, or multiple peaks in the power distribution differential, are successfully synthesized. In addition, the proposed method is also shown to be suitable for the synthesis of integrated-waveguide Bragg gratings with arbitrary stationary power/field distributions.

Numerical Study of Dual Mode Generation Using a Sampled-Grating High-Order Quantum-Dot Based Laterally-Coupled DFB Laser

We propose a fabrication-friendly dual-mode laser source based on a sampled surface-grating, quantum-dot (QD), third-order, and laterally-coupled distributed feedback (LC-DFB) laser composed of alternating grating and Fabry-Perot sections. The dynamic behavior of this device is investigated through numerical modelling, and mode spacing in the millimeter-wave domain (60 GHz) was achieved. We extended a time-domain travelling-wave algorithm, including Streifer's terms, to numerically study the dynamic behavior of the modified high-order LC-DFB lasers. We also incorporated an active QD region via a set of rate equations that considers both in homogeneous broadening because of spatial distribution of QD and homogeneous broadening because of the scattering or polarization dephasing rate. It was found that stable dual mode operation in the millimeter-wave range can be achieved with a dual-side-mode-suppression-ratio as high as ~ 50 dB.

Flexible Fabrication of Long-Period Fiber Grating Devices Based on Erasing Effect by Controlled Co2 Laser Pulse Exposure

An easy way to flexibly fabricate special long-period fiber grating (LPFG) devices by controlled CO2 laser pulses are presented in this paper. By controlling the pulse power and duration, consecutive exposures were made on an optical fiber to fabricate an LPFG. Existing LPFGs can also be erased or modified by applying CO2 laser exposures on specific grating sections. The erasing effect has been studied to fabricate special grating devices for various applications. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1735–1738, 2013

Tunable mid-infrared optical parametric oscillator with intracavity parametric amplification based on a dual-grating PPLN crystal

Intracavity amplification of the idler radiation of a nanosecond optical parametric oscillator (OPO) is achieved within a single dual-grating PPLN crystal com- posed of a uniform-grating section followed by a fan-out grating section. While all the OPO wavelength-tuning capabilities are preserved, this configuration leads to a 60 % increase of the 4-lm idler power and a significant beam quality improvement when compared to a conven- tional singly resonant OPO.

Dual-wavelength distributed Bragg reflector semiconductor laser based on a composite resonant cavity

We report a monolithic integrated dual-wavelength laser diode based on a distributed Bragg reflector (DBR) composite resonant cavity. The device consists of three sections, a DBR grating section, a passive phase section, and an active gain section. The gain section facet is cleaved to work as a laser cavity mirror. The other laser mirror is the DBR grating, which also functions as a wavelength filter and can control the number of wavelengths involved in the laser action. The reflection bandwidth of the DBR grating is fabricated to have an appropriate value to make the device work at the dual-wavelength lasing state. We adopt the quantum well intermixing (QWI) technique to provide low-absorption loss grating and passive phase section in the fabrication process. By tuning the injection currents on the DBR and the gain sections, the device can generate 0.596 nm-spaced dual-wavelength lasing at room temperature.

Evaluation of depth distribution and characterization of nanoscale Ta/Si multilayer thin film structures

A multilayer thin film structure of ten alternate Ta and Si layers with approximately 18nm thickness for the combined (Ta+Si) layer, was evaluated to explore the individual layer thickness and the interface mixing behavior using different surface characterization techniques like Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS), X-ray Reflectometry (XRR) and Kelvin Probe Force Microscopy (KPFM). These results were compared with measurements performed earlier using cross section Transmission Electron Microscopy (TEM). The TOFSIMS depth profile results indicate the individual thickness of Si and Ta layers to be 8.1nm and 5.9nm respectively which are less than the corresponding actual thickness measured by cross section TEM as 10.5nm and 7.5nm. The difference in thickness measurement has been explained in the light of ion bombardment induced atomic mixing in the interface during sputter depth profiling. A scanning electron micrograph shows the actual crater and its edges created due to the sputtering including the multilayer for real view of the structure. The XRR observations however reveal better agreement with the cross section TEM data, both being non-destructive in nature. Attempts were made to characterize the multilayer using KPFM technique which clearly elucidated the grating type cross section of the structure.

Polarized GaN-based light-emitting diode with an embedded metallic/dielectric subwavelength grating

Highly polarized light from an InGaN/GaN light emitting diode is proposed using an embedded multi-layer metallic/dielectric sub-wavelength grating and a dielectric transition layer. Transmission of transverse magnetic mode ( TTM), reflection of transverse electric mode, and polarization extinction ratio ( ER) were calculated using commercial “GSOLVER” software, based on a full vector implementation of Rigorous Coupled-Wave Analysis algorithm. TTM and ER were found to be largely enhanced by the presence of the transition layer, made of MgF 2 or SiO 2, placed between GaN and the grating section. TTM > 95% and ER > 34 dB for closely optimized Al/MgF 2 gratings were predicted. These values are significantly higher than those obtained by single-layer metallic gratings.

A Special Sampling Structure with an Arbitrary Equivalent-Phase-Shift for Semiconductor Lasers and Multiwavelength Laser Arrays

A method used to introduce an arbitrary equivalent-phase-shift (EPS) into an asymmetric sampled Bragg grating (SBG) is reported. Under the same structural parameters except for the sampling pattern, the asymmetric SBG offers better performance than that of a normal SBG structure for keeping single-longitudinal-mode (SLM) operation. This is because the proposed sampling pattern can suppress the 0th-order light resonance due to the mismatch between the two different grating sections along the whole SBG. This method can be used to design and fabricate semiconductor lasers and multiwavelength laser arrays (MLAs) with the required EPS at high yield and low cost.

Passively Mode-Locked Lasers With Integrated Chirped Bragg Grating Reflectors

Passively mode-locked laser devices are presented incorporating chirped distributed Bragg reflectors. The Bragg grating sections consist of post-growth, sidewall relief, tapered gratings, offering continuous chirp and modulation of the coupling coefficient. Blue-chirped gratings are found to compensate for the pulse chirp accumulated in the laser gain and absorption sections. Pulses are measured with the largest 3 dB bandwidths, up to 2.3 nm, and smallest pulsewidths, 1.49 ps, achieved in passively mode-locked DBR lasers.

High channel-count comb f ilter based on multi-concatenated sampled chirped fiber Bragg gratings

A high channel-count comb filter based on multi-concatenated sampled chirped fiber Bragg gratings (MC-SCFBGs) is proposed and optimally designed by using several chirped gratings with different fundamental grating periods, instead of non-grating sections of SCFBGs. The numerical simulations of the reflection spectra show that the channel spacing and the channel bandwidth in MC-SCFBGs are smaller than those in multi-concatenated chirped fiber Bragg gratings (MC-CFBGs) and that the spectral bandwidth of MC-SCFBGs can be greatly broadened by increasing the cascade number of the grating sections in each sampling period.

High-Power Pulse Generation in GHz Range With 1064-nm DBR Tapered Laser

For high-power pulse generation at 1060-nm, distributed Bragg reflector tapered lasers were investigated. The lasers consist of a tapered section, a ridge waveguide (RW) absorber section and a passive grating section with a sixth-order surface grating. For the generation of short optical pulses, the tapered section was biased with a dc-current and the RW absorber section was modulated with a 1-GHz sinusoidal current. At a repetition rate of 1 GHz-6.3 W optical pulses with 74-ps full-width at half-maximum and a small spectral widths of 50 pm were generated.

Fabrication of highly reflective gratings in 1.5 μm semiconductor lasers using focused ion beam-based etching

This paper reports on fabrication of semiconductor/air gratings in 1.5 μm double-section semiconductor lasers to achieve a high reflectivity in order to compensate low round-trip gain. Fabrication of the gratings with varying thicknesses and with thicknesses down to 160 nm is carried out at the gain section of the double-section diode laser using focused ion beam etching (FIBE) and inductively coupled plasma (ICP) techniques. Theoretical results of reflectivity are given for 1.5 μm AlGaInAs/InP semiconductor lasers by adding wavelength dependence of the refractive index into the calculations. We also compare our reflectivity results with that of a commercial simulation program and show a good agreement between them. Our results demonstrate that the gratings fabricated consist of only six air/semiconductor layer pairs and achieve theoretical reflectivity higher than 99%. Due to a high index contrast of the both layers, n l = 1, n h ∼ 3.5, a reflectivity bandwidth of >230 nm is obtained in 1.5 μm semiconductor lasers. Finally, lasing operation from AlGaInAs/InP semiconductor lasers with highly reflective grating section is achieved with a low threshold current of ∼8 mA, which is almost three times lower than devices without semiconductor/air gratings.

Monolithic double-grating phase mask for large-period highly coherent grating printing

A monolithic double-grating phase mask comprising three short-pitch grating sections of spatial frequencies k(1) and k(2) collocated at one side of a substrate produces a large-period interferogram without higher harmonics to print in a photoresist film a latent grating of small spatial frequency equal to twice k(2)-k(1). When incorporated in a write-on-the-fly scheme, the elements permit the fabrication of unlimitedly long gratings.

All optical Flip-Flop based on a nonlinear DFB semiconductor laser: Theoretical study

A new design of an all optical Flip-Flop is proposed. It consists of a nonlinear distributed feedback (DFB) laser. The wave guiding layer of the DFB laser consists of linear grating section followed by a nonlinear one, and both sections are separated by a phase shift section. In the OFF-state, the real part of the refractive index in the wave guiding layer forms a Bragg grating. In the ON-state, it forms a Bragg grating with a phase shift section. Optical gain is achieved by current injection in the semiconductor active layer. Nonlinearity in the nonlinear layers of the semiconductor grating was achieved by direct absorption at the edge of the absorption band (Urbach tail). Numerical simulation shows that the device switches in a nanosecond time scale.

Spectral Properties of High-Power Distributed Bragg Reflector Lasers

In this paper, spectral data of distributed Bragg reflector lasers emitting around 974 nm and 1084 nm are presented. The devices are fabricated by a single-growth molecular beam epitaxy step, and the gratings are defined by holographic interferometry. Spectral dependencies on the grating and gain section lengths are systematically investigated in the so-called cleaved-back experiments. Experimental data for the side-mode suppression ratio, mode spacing, and group refractive index are given for devices emitting around 974 nm. In addition, the coupling efficiency between the grating and gain section is derived experimentally for devices emitting around 1084 nm.

Design of all-optical high-order temporal integrators based on multiple-phase-shifted Bragg gratings

In exact analogy with their electronic counterparts, photonic temporal integrators are fundamental building blocks for constructing all-optical circuits for ultrafast information processing and computing. In this work, we introduce a simple and general approach for realizing all-optical arbitrary-order temporal integrators. We demonstrate that the N(th) cumulative time integral of the complex field envelope of an input optical waveform can be obtained by simply propagating this waveform through a single uniform fiber/waveguide Bragg grating (BG) incorporating N pi-phase shifts along its axial profile. We derive here the design specifications of photonic integrators based on multiple-phase-shifted BGs. We show that the phase shifts in the BG structure can be arbitrarily located along the grating length provided that each uniform grating section (sections separated by the phase shifts) is sufficiently long so that its associated peak reflectivity reaches nearly 100%. The resulting designs are demonstrated by numerical simulations assuming all-fiber implementations. Our simulations show that the proposed approach can provide optical operation bandwidths in the tens-of-GHz regime using readily feasible photo-induced fiber BG structures.

Wavelength Shift of Cladding Mode Resonances in a Mechanically Induced LPFG by Twisting the Fiber

The long-period fiber grating is mechanically induced over a twisted fiber and its characteristics are investigated. The amplitude as well as the wavelength shift of the resonance is studied in response to the applied twist and pressures. These resonances decrease in amplitude and shift to shorter wavelength side as the applied twist increases. The spectral responses of a grating assembly formed by two grating sections in series, one section with a twisted fiber and other with an untwisted fiber, are also investigated. Shearing stress and photo-elasticity causes the fiber to be circularly birefringent and the mechanically induced grating formed over the twisted fiber region causes the appearance of two resonances shifted away from the resonances of the untwisted grating section. At higher twist rates, resonant wavelength shift becomes insensitive to applied pressures, showing a reduction in the induced linear birefringence. The wavelength shift is almost symmetric with respect to the applied twist rate in clockwise and counterclockwise directions.

Experimental and theoretical analysis of fiber Bragg gratings under Transverse Force to a Small Grating Section

Fiber Bragg grating under transverse force on a small grating section is studied by numerical simulation and experimentation. A numerical simulation based on the transfer matrix method is used to calculate the consequent changes in reflected spectrum. The reflected spectra of the FBG subjected to the transverse force split into two main peaks, and the split point shifted linearly and periodically versus the applied force. The split point is shifted in the bandwidth with the period of 11N, and the sensitivity of the split point wavelength shift versus the applied force is 0.05 nm/N in one period. The experimental results show good agreement with the simulation analysis.

Optical fiber twist sensor with two orthogonally oriented mechanically induced long-period grating sections

We propose and demonstrate, for the first time, an optical fiber twist sensor consisting of two mechanically induced long-period grating sections, whose birefringent axes are mutually orthogonal. The twist sensor, whose temperature stability is supposedly high and sensitivity is tunable by pressure, is based on the change in the loss peak separation due to the twist-induced polarization rotation.