F. Causa

1.3-W ripple-free superluminescent diode

This letter presents the experimental characterization of tapered and stripe superluminescent diodes fabricated from 980-nm high-power triple quantum-well InGaAs-AlGaAs semiconductor material. Record output powers in excess of 1.3 W pulsed have been measured, with 0.1-dB spectral modulation and maximum wall-plug efficiency 16%. Almost 1-W optical power into multimode optical fibers has been achieved with preliminary measurements of coupling efficiency.

Ripple-Free High-Power Super-Luminescent Diode Arrays

This paper presents the experimental and theoretical characterisation of arrays of tapered super-luminescent diodes to achieve very high, broad-band, ripple-free optical output. These arrays were fabricated from 980-nm high-power triple-quantum-well InGaAs-AlGaAs double heterostructures. Output powers of 2.7 W pulsed and of 280-mW quasi-CW were obtained from antireflection-coated arrays, with 10% maximum wall-plug efficiency per pulse at 0.75-W pulsed output power. Experimental and theoretical results for the optical-electrical characteristics and the near-and far-field radiation characteristics of the arrays are discussed and compared with those of corresponding individual super-luminescent diodes.

A versatile method for analyzing paraxial optical propagation in dielectric structures

This paper presents a fast and accurate quasi-analytic model for studying optical field propagation in weakly guiding dielectric structures. The proposed efficient and versatile computational scheme is obtained by merging the Hermite-Gauss (HG) total field expansion with the numerical collocation method and is particularly suited for longitudinally nonuniform structures. By means of a quasilinearization scheme, the same procedure has also been successfully applied to the analysis of field propagation in Kerr-nonlinear media. The latter achievement gives an indication of the great potentialities offered by this straight forward method. Several examples are discussed in the paper and in all cases the results computed by the proposed method favorably compare with those from alternative methods.

Optical guiding properties of high-brightness parabolic bow-tie laser arrays

This paper presents the characteristics of parabolic bow-tie laser arrays (PBTLAs) which are a novel category of laser diodes specially designed to achieve high power with high brightness at 980 nm. Output powers in excess of 2.8 W/facet have been measured from five-element PBTLAs with output beam less than twice the diffraction limit, achieving high brightness of 275 MWcm/sup -2/srad/sup -1/ at 3 A (pulsed) injection current (=22 times the threshold). Changes in the achievable brightness due to changes in the optical cavity geometry and in the lateral optical guiding strength are discussed in detail, using the coupled-mode theory to interpret the experimental results. At threshold all devices operate in the highest (double lobed) array mode. At higher currents the arrays of tapered lasers change to quasi-in-phase operation when the modal gain of the fundamental array mode dominates because of the combined effect of carrier hole burning and spatial filtering from the narrow stripe central section of the device. Similar trends have been observed under continuous-wave operation. The reduction of lateral optical guiding strength is deleterious for the operational characteristics of PBTLAs and linear bow-tie arrays, and it leads to filamentation in gain-guided devices even at low currents. Theoretical results presented in this paper show that scalability is in principle possible; however, changes in the lateral gain profile due to hole-burning can significantly increase the modal gain of higher order modes and, therefore, strongly influence the optical output profile.

A new method for computing nonlinear carrier diffusion in semiconductor optical devices

The solution of carrier diffusion equations typically associated with semiconductor optical devices has been achieved by combining a function expansion scheme, using the Hermite-Gauss functions as the basis set, with the collocation numerical procedure. Results for a wide range of cases obtained by this new scheme compare very favorably with those calculated with other methods. Not only is the present process computationally fast and efficient, but it has the added attraction of providing the basis for conveniently solving also the nonlinear electromagnetic wave equation for the self-consistent modeling of such devices.

A quasi-analytic model for longitudinally non-uniform semiconductor optical sources

Abstract This paper presents a new, efficient and compact method to solve the self-consistent model for semiconductor optical sources that admit longitudinally slowly varying (adiabatic) field solutions. Both the optical field and the carrier equations are computed by a function expansion technique using the Hermite–Gauss functions as the basis set. Results obtained by the proposed method are shown to agree favourably with those from other numerical methods.

Observation and Analysis of Phase-Locking in Parabolic Bow-Tie Laser Arrays

This paper presents experimental and theoretical results to show that in-phase coherent coupling occurs in high-power arrays of parabolic bow-tie lasers. Such devices require simple and inexpensive device fabrication, thus providing a cost-effective high-power laser for high-brightness operation at 980 nm. The maximum brightness achieved was 318 MWmiddotcm-2 sr-1 at 20 times the threshold with far-field full-width at half-maximum less than 1.5 times the diffraction limit, measured without the use of external lenses under low-duty cycle (0.1%) pulsed operation. Results obtained from double-slit interference experiments show a clear distinction between phase-coherent arrays and corresponding arrays of independent emitters

High brightness index-guided parabolic bow-tie laser arrays

This letter describes a novel 980-nm parabolic bow-tie laser array (PBTLA) that is suitable for high-power and high-brightness operation. Output powers in excess of 2.5 W/facet pulsed in a 1/spl deg/ (lateral) beam, less than twice the diffraction limit, corresponding to 275 MW/spl middot/cm/sup -2//spl middot/sr/sup -1/ brightness, have been measured without the use of external lenses from uncoated PBTLAs fabricated in-house (top metal contact surface area/spl sim/0.1 mm/sup 2/). Experimental results presented in this letter indicate that coherence effects are significant in the operation of such devices. Theoretical models based on the simple diffraction theory and on the coupled-mode theory have been used to interpret the experimental results.

Computation of propagation in adiabatically tapered dielectric structures based on eigenfunction expansions: application to (active) optical devices

An eigenfunction expansion method is presented which uses the complete set of Hermite-Gauss (HG) functions to obtain the required solution of the propagation problems and has certain advantages, as discussed. This method may also be considered as a perturbation method of analysis since the HG functions are the solutions of a longitudinally uniform waveguide with a parabolically varying transverse refractive index distribution. Note that the HG functions form a complete and discrete set for the function space of interest namely that corresponding to square integrable functions. As a proof of its effectiveness the HG function expansion method is applied to analyse the fields in a variety of longitudinally non-uniform passive devices. The extension of this approach to the to the analysis of active optical devices requires a self-consistent solution to be determined to take into account both the non-uniform device geometry and the non-linear interaction of the optical field with the inversion population distribution in the device. Further, compactness of the analysis scheme for the overall model is achieved by demonstrating that the HG method is also very effective in solving the carrier diffusion equation. In addition, the merits of the collocation numerical procedure have been utilised to reduce the complexity of the formalism.

Advances in nano-enabled GaN photonic devices

In this work, the results are presented of a nanorod LED array. If the lateral size of the nanorods is small enough, it is possible to achieve a degree of lateral confinement. If the nanorods are ordered into a suitable photonic lattice, then this will reduce the lateral spontaneous emission and enhance emission along the vertical axis via the Purcell effect. Additionally there is a degree of dislocation filtering that can occur [1]. However, one potential drawback of this device is the large free surface that borders the multi-quantum well active region. Nevertheless, it has been shown that the surface recombination in the nitride materials is the lowest of all III-V semiconductors. Results of SEM, PL, EL, and far field pattern are presented to compare the progressive effect of using photo-assisted electroless and wet etching [2]. It can be seen that over time the photo-assisted electroless method clearly delineates the active MQW region, possibly as a result of the different etch rate of InGaN. Alternatively, a purely chemical etching method was used. With a narrowing of the nanorods, there is a progressive blue shift of the photoluminescence peak. The optical image of the emission shows that there are well-defined lines of enhanced light propagation that match the symmetry of the nanorod array, thus showing there is a photonic crystal effect.