InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructuresquare ring lasers

The processing and optical characteristics of an InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructure square ring laser (4 μm width and 50-μm/side length) with two output waveguides and two narrow physical gap couplers are described. The ridges and the total internal reflection (TIR) mirrors of the square ring lasers are fabricated by two photolithographic steps with a single SiO₂ mask and planarized with polyimide such that the TIR mirrors are etched through the active region while the ridge waveguides are not. Single longitudinal mode operation is observed with a side mode suppression ratio of 20 dB. Asymmetric characteristics of emission spectra from the two output waveguides demonstrates that the square ring lasers operate in traveling wave modes     

Modal Analysis of Large Spot Size, Low Output Beam Divergence Quantum-Dot Lasers

Large spot size ridge waveguide lasers utilizing a low modal gain single quantum dot layer emitting at 925 nm were designed and fabricated. Ridge waveguides with width <3 mum emit in a single transverse mode with a low transverse full-width at half-maximum divergence of 20deg. Wider ridges initially lase in the first-order transverse mode before collapsing to the fundamental mode. This characteristic is explained by a thermally induced increase in the refractive index of the waveguide core. All lasers operate in a single lateral mode     

高斯镜平凸非稳腔本征模场的有限元数值计算

为了研究激光在高斯镜平凸非稳腔中的模式和相位特征,在SIEGMAN分析多元件非稳腔的理论基础上,采用有限元数值方法计算耦合输出镜镜面反射率为高斯分布的圆镜平凸非稳腔本征模场分布。将光场在腔内一次往返的ABCD系数代入惠更斯-基尔霍夫衍射积分方程,可以写出光场往返一次的衍射积分方程。将镜面分割成若干个等宽圆环,把衍射积分方程转化成矩阵相乘形式,经过数值计算,得到理想空腔和非共轴空腔优先起振的光场本征模式分布结果与相位特性。计算结果表明,当合适选择输出镜半径与高斯反射率分布,可以得到光斑半径为0.3cm的基模高斯光束输出,此计算结果与实验结果相符。同时也讨论了耦合输出镜对光束质量的影响。     

Mode coupling in angled facet semiconductor optical amplifiers andsuperluminescent diodes

The modal reflectivity of an angled facet has been calculated for index-guided devices when the fundamental mode excites various higher order modes at the facet. The calculations were carried out for waveguide widths ranging from 2 to 20 μm for various lateral index steps and at wavelengths of 834, 1300, and 1550 nm. Plots from these calculations can be used to determine the appropriate facet angles to obtain a given modal reflectivity and optimize the design of optical amplifiers and superluminescent diodes     

High brightness single-mode ridge laser utilizing buried heterostructure defined by quantum-well intermixing

The authors demonstrate a novel high brightness single-lateral mode ridge laser using quantum well intermixing to form a buried heterostructure. Increased discrimination between the fundamental and higher order modes can be achieved using the buried heterostructure to reduce the width of the gain section, enhancing fundamental mode operation. This allows the ridge width to be increased while maintaining fundamental mode operation, hence reducing the optical intensity at the facet and increasing the optical power before mirror degradation. Standard and novel buried heterostructure ridge lasers of 5-/spl mu/m width are compared; far-field beam profiles clearly show improved modal stability for the novel structure.     

Mode Converters for Coupling to High Aspect Ratio Silicon-on-Insulator Channel Waveguides

The design, simulation, and experimental performance of mode converters for coupling from single-mode silicon-on-insulator ridge waveguides to high aspect ratio channel waveguides are described. The converters consist of a two-level adiabatic taper structure. The final channel waveguide is 1.5 mum high by 0.8 mum wide. Simulations predict that for total coupler lengths longer than 20 mum, the coupling loss from the fundamental ridge mode to the slit mode is better than -0.2 dB. The couplers and waveguides were fabricated using a two-step self-aligned process. The measured coupling loss for fabricated mode converters is -0.4 dB     

Spectral characteristics of coupled-waveguide Fabry-Perotresonators and filters

Conventional Fabry-Perot resonators provide a narrow spectral width but lack the capability of longitudinal mode discrimination. A coupled-waveguide Fabry-Perot structure made of two parallel waveguides with reflecting mirrors at the ends is proposed for applications as mode selective resonator in single-mode diode lasers and as narrow-band wavelength filters. The interference of counter propagating waves from reflection by end mirrors and the coupling of waves between the two parallel waveguides contribute to the operation of this resonator structure. Thus, the device exhibits the attributes of both Fabry-Perot resonator and directional coupler. The coupled-mode theory of parallel waveguides is employed to analyze the structure. Both cases of identical and unidentical waveguides are examined. Resonance conditions and spectral characteristics are determined. It is shown that the coupled-waveguide Fabry-Perot resonator provides significant improvement in mode discrimination capability and longitudinal mode spacing over the conventional Fabry-Perot resonator     

Modal characteristics of bent dual mode planar optical waveguides

Modal characteristics of bent dual-mode planar optical waveguides are obtained. The bending-induced changes in the modal power distribution is found to be quite different for the two modes. Surprisingly, unlike the fundamental mode, bending causes the fractional modal power for the second mode to increase in the inner core-half and to decrease in the outer core-half of the waveguide. Interestingly, this leads to a decrease in effective index of the second mode due to bending at sufficiently high V-values     

Enhancement of the Mode Area and Modal Discrimination of Microchip Lasers Using Angularly Selective Mirrors

Angularly selective mirrors (ASMs) are proposed as a means to expand the mode area and modal discrimination of microchip lasers. ASMs used as output couplers selectively reflect incoming k vectors over a narrow angular range, while they transmit more inclined components. The eigenvalue problem of a microchip resonator equipped with a Gaussian ASM is solved analytically in the paraxial optics approximation using the ABCD matrix formalism. The narrow angular distribution of the reflected beam produces, through the laws of diffraction, a significant increase of the mode size and improved transverse mode discrimination, at the expense of higher oscillation threshold due to larger output coupling losses. Simulations performed using the parameters of Yb³⁺-doped YAG material show that one order of magnitude increase of the mode area can reasonably be achieved without causing overheating and thermal fracture. ASMs can be directly deposited on the active material in the form of a resonant grating mirror. This technology involves only planar batch processes that retain the mass production advantage of microchip lasers. The significant increase of brightness of microchips expected from this innovation will give rise to more effective and more compact devices and new applications.     

Propagation properties in short lengths of rectangular epoxywaveguides

The propagation loss and beam spreading in large-core epoxy ridge waveguides on a glass substrate at wavelengths ranging from 630 to 1550 nm have been measured. At the intermediate wavelengths, a loss floor of 1.2 dB/cm is observed which is caused by inclusions and wall imperfections. The theoretical and experimentally determined equilibrium numerical aperture is 0.65 and 0.29 and the modal dispersion is 5.1 and 1.0 ps/cm, respectively. The discrepancy is due to strong scattering. The scattering does not give rise to any noticeable mode mixing. The epoxy waveguides also provide a rapid filling of the modes which is attractive for modal noise suppression     

Index profiles of planar optical waveguides determined from the angular dependence of reflectivity

The relation between the refractive index distribution and the angular dependence of reflectivity is investigated for planar optical waveguides. Index distributions with rapid variations of the profile function can be determined from the angular dependence of the difference in reflectivityDeltaRbetween the waveguide and the substrate by means of numerical parameter optimization.DeltaRis measured with high precision using a simple lock-in technique. This nondestructive method for profile evaluation is appropriate in particular for monomode waveguides where other methods fall to apply. It is tested with He+-implanted fused silica and results obtained for several ion energies and doses demonstrate the practical use of this method.     

Improved mode properties of unstable resonators with tapered reflectivity mirrors and shaped apertures

The use of tapered reflectivity mirrors and shaped apertures in unstable resonators is shown to be an effective method for improving the mode properties. Mode intensity and phase profiles are smoothed and the mode discrimination ratio is increased. Results are presented illustrating the importance of diffracted waves from sharp mirror edges in determining these mode properties. A simple expression for estimating the diffractive contribution is given and is used to determine optimum mirror designs. Shaped mirrors and mirrors having amplitude and phase reflectivity tapers are studied. The concept of equivalent Fresnel zones is used to gain physical insight into the mode properties.     

Rectangular Ring Lasers Based on Total Reflection Mirrors and Three Waveguide Couplers

Novel rectangular ring lasers containing active and passive sections are fabricated and characterized. The rectangular laser cavity is formed using four low-loss total internal reflection (TIR) mirrors and an output coupler made out of passive three coupled waveguides. The fabrication process is exactly the same as for other active and passive devices except for one deep etch step for TIR mirror fabrication. Two different lasers having active section lengths of 250 and 350 mum and total cavity lengths of 580 and 780 mum are fabricated. For both devices, lasing thresholds of 38mA are obtained at room temperature and under continuous-wave operation. Lasing is predominantly single-mode with a sidemode suppression ratio better than 20 dB. The power loss of a single TIR mirror is also determined to be about 0.5 dB. Such low-loss TIR mirrors enabled lasers with very small footprints     

Coupled-mode formulations for parallel-laser resonators withapplication to vertical-cavity semiconductor-laser arrays

Laser resonators, with laterally patterned end mirrors, are considered in the framework of coupled-mode theory. Two distinct coupled-mode formulations are presented. The first is applicable to cavities having high-reflectivity mirrors surrounded by a significantly lower reflectivity background. The second formulation applies to high-reflectivity mirrors with a slightly lower reflectivity background. Unlike coupling between parallel waveguides, the interaction between neighboring elements of the parallel cavities is achieved via diffraction, and the perturbation is of the boundary conditions rather than of the dielectric constant. These formulations are directly applicable to the case of arrays of coupled vertical-cavity semiconductor lasers     

Properties of an unstable confocal resonator CO2laser system

The results of an experimental and theoretical study of the properties of a confocal unstable resonator used with a conventional CO2-N2-He laser operating at 10.6 μ are presented. Measurements were made of the near- and far-field intensity distribution of the aligned resonator for Fresnel numbers of 3.7, 6.1, and 7.1. Fundamental mode operation was achieved in all cases. Beam steering and mode degradation associated with mirror misalignment were also studied experimentally. Suitable geometric models have been developed to account for 1) the sensitivity of the resonator output coupling and center far-field intensity to variations in the resonator length and mirror curvatures, and 2) the sensitivity of the beam direction and power to angular misalignment of the mirrors.     

System experiments with multimode waveguides and fibers forapplications in computer interconnects

The effect on the modal noise induced bit-error-rate floor when passing light through short lengths (0.5→70 mm) of large core epoxy ridge waveguides and multimode fibers was investigated. It was demonstrated that the bit-error-rate floor declines rapidly with the waveguide transmission distance, whereas for the multimode fiber the floor is independent of the length. The issue of estimating modal noise bit-error-rate floors at realistic levels of the mode selective loss is addressed. The experimental set-up is discussed, and the results of modal noise system experiments performed at 1 Gb/s non-return-to-zero (NRZ) are presented. Worst-case requirements to the waveguide design are discussed     

Self-aligned chemically assisted ion-beam-etched GaAs/(Al,Ga)Asturning mirrors for photonic applications

It is shown that totally reflecting turning mirrors can be fabricated in GaAs/(Al,Ga)As graded-index separate-confinement-heterostructure single-quantum-well (GRIN-SCH SQW) material using Ti metal masking and Cl₂ chemically assisted ion-beam-etching (CAIBE). Combined with ridge waveguides, these techniques have been used to make turning-mirror cleaved-facet lasers that have acceptable external slope efficiencies with a modest estimated increase in threshold current. Scanning electron microscope examination of the mirrors indicates some residual roughness attributed to the edge definition of the deposited titanium film. The mirror loss is estimated to be 68% for these particular turning mirrors. These components, which continue to be developed and improved, are being applied to increasingly sophisticated and demanding photonic programs     

Sol-gel glass waveguide and grating on silicon

This paper reports on the fabrication and characterization of hybrid organic-inorganic glass sol-gel slab and channel waveguides by ultraviolet light imprinting in thin films deposited by a one-step dip-coating process. The adjustment of chemical composition of the materials provides precise selection of refractive index from 1.48 to 1.52 at the wavelength of 632.8 mn. The refractive index of the waveguides at 1.55 μm is similar to that of optical fiber, thus reducing the reflection loss between the two to less than 0.01 dB. The effect of ultraviolet light exposure and heat treatment on waveguide refractive index is studied. Fabrication parameters to produce ridge waveguides are optimized to achieve very smooth side walls. Propagation losses in these waveguides are ~0.1 dB/cm. Single mode buried waveguides, at 1.55 μm wavelength, with circular mode profile are demonstrated     

Modal analysis of step discontinuities in graded-index dielectricslab waveguides

The modal solutions of the TE- and TM-modes of an arbitrary graded-index dielectric slab waveguide have been derived by applying the generalized telegraphist's equations to the equivalent inhomogeneous parallel-plate waveguide model with electric or magnetic walls. These modal solutions have been employed in a mode-matching procedure to calculate the transmission properties of step discontinuities in arbitrarily graded-index dielectric slab waveguides such as diffused optical waveguides having exponential, Gaussian and complementary error-function index profiles. For slab waveguides containing an abrupt offset, the radiated power is found to increase smoothly with increasing displacement. Power loss calculations for an abrupt change in diffusion depth for which the dominant mode is in the vicinity of cutoff, exhibit a sharp transition from almost zero loss to nearly total radiation loss     

Analysis of the modal solutions of rib antiresonant reflectingoptical waveguides

The modal behavior of rib antiresonant reflecting optical waveguides (ARROWs) is theoretically analyzed in this work. Two numerical analysis methods have been used for the analysis of an open rib structure: the spectral index method and a vectorial mode matching method, where the real and the imaginary part of the modal solutions of several structures have been studied. A nearly polarization independent open rib structure and a embedded rib structure have also been studied using the mode matching method. Obtained numerical results will be also compared with experimental data in order to point some observed discrepancies