Mode competition in acousto-optically Q-switched planar waveguide lasers

Multi-mode rate equations have been developed to investigate mode competition in high-power acousto-optically Q-switched planar waveguide lasers. The mode competition arises from coupling effects and temporal losses in the transform between guided modes and free-space propagation. Pulse-to-pulse instability and temporal beam distortions are enlarged by mode competition when the laser works in the multi-mode regime. The influence of parasitic oscillation is also discussed. A Nd:YAG planar waveguide laser has been established with a folded hybrid/unstable resonator. A maximum average power of 83 W with a beam propagation factor M-x(2) x M-y(2) = 1.2 x 1.4 is obtained. The theoretical simulation agrees well with the experimental observation. (c) 2006 Elsevier Ltd. All rights reserved.

Study of crystal structure and phase transition studies in perovskite-type oxides using powder-diffraction techniques and symmetry-mode analysis.

251 p.

Impact of mode localization on the motional resistance of coupled MEMS resonators

This paper investigates the effect of mode-localization that arises from structural asymmetry induced by manufacturing tolerances in mechanically coupled, electrically transduced Si MEMS resonators. We demonstrate that in the case of such mechanically coupled resonators, the achievable series motional resistance (R x) is dependent not only on the quality factor (Q) but also on the variations in the eigenvector of the chosen mode of vibration induced by mode localization due to manufacturing tolerances during the fabrication process. We study this effect of mode-localization both theoretically and experimentally in two pairs of coupled double-ended tuning fork resonators with different levels of initial structural asymmetry. The measured series R x is minimal when the system is close to perfect symmetry and any deviation from structural symmetry induced by fabrication tolerances leads to a degradation in the effective R x. Mechanical tuning experiments of the stiffness of one of the coupled resonators was also conducted to study variations in R x as a function of structural asymmetry within the system, the results of which demonstrated consistent variations in motional resistance with predictions. © 2012 IEEE.

250 MHz repetition rate monolithically integrated modulated mode-locked semiconductor laser

We present the first monolithically integrated semiconductor pulse source consisting of a mode-locked laser diode, Mach-Zehnder pulse picker, and semiconductor optical amplifier. Pairs of 5.6 ps pulses are generated at a 250 MHz repetition rate. © 2012 OSA.

Variable repetition rate monolithically integrated mode-locked-laser- modulator-MOPA device

A monolithically integrated MLLD-modulator-MOPA is presented generating 12.5 ps pulses. The Mach-Zehnder modulator allows tunable repetition rates from 14 GHz to 109 MHz, and the MOPA boosts the peak power by 3.2 dB. © 2012 IEEE.

Analytical formulation of modal frequency split in the elliptical mode of SCS micromechanical disk resonators

This paper presents an analytical formulation of frequency splitting observed in the elliptical modes of single crystal silicon (SCS) micromechanical disk resonators. Taking the anisotropic elasticity of SCS into account, new formulae for computing modal mass and modal stiffness are first derived for accurate prediction of the modal frequency. The derived results are in good agreement with finite element simulation, showing a factor of 10 improvement in the prediction accuracy as compared to using the formula for the isotropic case. In addition, the analysis successfully explains the effect of anisotropy on the modal frequency splitting of primary elliptical modes, for which the maximum modal displacement is aligned with the directions of maximum (1 1 0) and minimum (1 0 0) elasticity respectively on a (1 0 0) SCS wafer. The measured frequency splitting of other degenerate modes is due to the manufacturing imperfections. © 2014 IOP Publishing Ltd.

20GHz colliding pulse mode-locking of ingaas quantum dot lasers

Colliding pulse mode-locking is demonstrated for the first time in quantum-dot lasers. Close to transform limited, 7ps, 20GHz pulses are achieved using an absorber length considerably longer than typically used in similar quantum-well lasers. © 2004 Optical Society of America.

Mode coupling and vertical radiation loss for whispering-gallery modes in 3-D microcavities

The characteristics of whispering-gallery modes (WGMs) in 3-D cylindrical, square, and triangular microcavities with vertical optical confinement of semiconductors are numerically investigated by the finite-difference time-domain (FDTD) technique. For a microcylinder with a vertical refractive index 3.17/3.4/3.17 and a center layer thickness 0.2 mu m, Q-factors of transverse electric (TE) WGMs around wavelength 1550 nm are smaller than 10(3), as the radius R < 4 mu m and reach the orders of 10(4) and 10(6) as R = 5 and 6 mu m, respectively. However, the Q-factor of transverse magnetic (TM) WGMs at wavelength 1.659 mu m reaches 7.5 x 10(5) as R = 1 mu m. The mode coupling between the WGMs and vertical radiation modes in the cladding layer results in vertical radiation loss for the WGMs. In the microcylinder, the mode wavelength of TM WGM is larger than the cutoff wavelength of the vertical radiation mode with the same mode numbers, so TM WGMs cannot couple with the vertical radiation mode and have high Q-factor. In contrast, TE WGMs can couple with the corresponding vertical radiation mode in the 3-D microcylinder as R < 5 mu m. However, the mode wavelength of the TE WGM approaches (is larger than) the cutoff wavelength of the corresponding radiation modes at R = 5 mu m (6 mu m), so TE WGMs have high Q-factors in such microcylinders too. The results show that a critical lateral size is required for obtaining high, Q-factor TE WGMs in the 3-D microcylinder. For 3-D square and triangular microcavities, we also find that the Q-factor of TM WGM is larger than that of TE WGM.

Influence of different interlayers on growth mode and properties of InN by MOVPE

We grow InN epilayers on different interlayers by metal organic vapour phase epitaxy (MOVPE) method, and investigate the effect of interlayer on the properties and growth mode of InN films. Three InN samples were deposited on nitrided sapphire, low-temperature InN (LT-InN) and high-temperature GaN (HT-GaN), respectively. The InN layer grown directly on nitrided sapphire owns the narrowest x-ray diffraction rocking curve (XRC) width of 300 arcsec among the three samples, and demonstrates a two-dimensional (2D) step-flow-like lateral growth mode, which is much different from the three-dimensional (3D) pillar-like growth mode of LT-InN and HT-GaN buffered samples. It seems that mismatch tensile strain is helpful for the lateral epitaxy of InN film, whereas compressive strain promotes the vertical growth of InN films.

Design of high polarization and single-mode photonic crystal laser

The dipole mode in triangular photonic crystal single defect cavity is degenerate. By deforming the lattice in photonic crystal we can obtain non-degenerate dipole modes. Lattice deforming in the whole photonic crystal destroys the characteristic of symmetry, so the distribution of the electromagnetic field is affected and the polarization of the electromagnetic field is also changed. Lattice deforming divides the degenerate dipole mode into the x-dipole mode and the y-dipole mode. It is found that the non-degenerate modes have better properties of polarization. So the high polarization and single dipole mode photonic crystal laser can be achieved by deforming the lattice of photonic crystal. In this paper, we simulated the cavity in photonic crystal slab and mainly calculated the quality factor of x-dipole mode under different deforming conditions and with different filling factors. The properties of polarization of x-dipole and y-dipole modes are also calculated. It is found that the ratio of intensities of E-x to E-y in x-dipole mode and that of E-y to E-x in y-dipole mode are 44 and 27, respectively.

Low power 2 x 2 thermo-optic SOI waveguide switch fabricated by anisotropy chemical etching

A 2 x 2 thermo-optic (TO) Mach-Zehnder (MZ) switch based on silicon waveguides with large cross section was designed and fabricated on silicon-on-insulator (SOI) wafer. The multi-mode interferometers (MMI) were used as power splitter and combiner in MZ structure. In order to get smooth interface, anisotropy chemical wet-etching of silicon was used to fabricate the waveguides instead of dry-etching. Additional grooves were introduced to reduce power consumption. The device has a low switching power of 235 mW and a switching speed of 60 mus. (C) 2004 Elsevier B.V. All rights reserved.

Ridge waveguide laser integrated with spot-size converter in a single step epitaxial for high coupling efficiency to single-mode fibres

A 1.55-mum laser diode integrated with a spot-size converter was fabricated in a single step epitaxial by using the conventional photolithography and chemical wet etching process. The device was constructed by a conventional ridge waveguide active layer and a larger passive ridge-waveguide layer. The threshold current was 40 mA together with high slope efficiency of 0.24 W/A. The beam divergence angles in the horizontal and vertical directions were as small as 12.0degrees x 15.0degrees, respectively, resulting in about 3.2-dB coupling losses with a cleaved optical fibre.