Mushroom Stripe Lasers Research Articles

Multiple air-gap filters and constricted mesa lasers – material processing meets the front of optical device technology

Real three-dimensional material structures enable enormous perspectives in the functionality of advanced electronic and optoelectronic III/V semiconductor devices. We report on the technological implementation of surface-micromachined III/V semiconductor devices for optoelectronic applications. Considering fabrication technology, the general principles can be reduced to three fundamental process steps: deposition of a layered heterostructure on a substrate, vertical structurization and horizontal undercutting by selectively removing sacrificial layers. Very useful quality-control elements for precise process control are presented. The basic principles are applied and illustrated in detail by presenting two selected optoelectronic examples. (i) The fabrication technology of buried mushroom stripe lasers is shown. Bent waveguides on homogeneous grating fields are used to obtain chirped gratings, enabling a high potential to tailor specific performances. Excellent optical properties are obtained. (ii) The fabrication technology of vertical optical cavity based tunable single- or multi-membrane devices including air gaps is shown. Record optical tuning characteristics for vertical cavity Fabry–Perot filters are presented. Single parametric wavelength tuning over 142 nm with an actuation voltage of only 3.2 V is demonstrated.

Enhanced modulation bandwidth for strain-compensated InGaAlAs-InGaAsP MQW lasers

Strain-compensated (SC) multiple-quantum-well (MQW) lasers were designed using tensile-strained InGaAlAs barrier layers in order to enhance the modulation bandwidth of MQW lasers at 1.55 /spl mu/m. The design scheme simultaneously ensures the pseudomorphic growth of a large stack of highly strained wells, a uniform hole injection into large number of wells, a large conduction band discontinuity to suppress the carrier overflow effect, and a large differential gain by suppressing the band mixing effect. The SC-MQW structures were processed into a mushroom stripe laser structure to obtain a low parasitic capacitance. The number of wells and the cavity length were optimized to maximize the modulation bandwidth. Both the relaxation oscillation and RC cutoff frequencies increased with reducing the cavity length, and a maximum 3-dB modulation bandwidth of 30 GHz was obtained at a short cavity length of 120 /spl mu/m for 20-well SC-MQW lasers. Moreover, a high internal quantum efficiency and large differential gain were obtained for the SC-MQW lasers with well numbers of up to 20 as a result of the reduced carrier transport and overflow effects. The differential gain, gain compression factor, and K factor were evaluated experimentally from the modulation characteristics and compared to the theoretical calculation based on the spectral hole burning theory. The observed experimental results were well explained by the model using the identical intraband relaxation times typically used for 1.55-/spl mu/m bulk lasers.

Monolithic integration of InGaAsP-laser with transferred-electron device as fast laser driver

A monolithically integrated laser driver circuit has been fabricated incorporating an InGaAs-transferred electron device (TED) and an InGaAsP mushroom stripe laser emitting at 1.3-/spl mu/m wavelength. The TED provides a large-signal modulation of the laser diode along with a pulse shaping of the optical output signal. A threshold current of 18 mA of the integrated laser has been obtained. The modulation of the optical output signal by the TED has been demonstrated. For practical application of the circuit under dc-bias conditions the thermal resistance of the TED's has been calculated and an optimum device structure is proposed. The possibility of triggering the circuit optically will be discussed.

Reduction of degradation in vapor phase transported InP/InGaAsP mushroom stripe lasers

The rapid degradation rate generally observed in InP/InGaAsP mushroom stripe lasers can be considerably decreased by regrowing the open sidewalls of the active stripe with low-doped InP in a second epitaxial step using the hydride vapor phase transport technique. This technique does not change the fundamental laser parameters like light-current and current-voltage characteristics. Because of this drastic reduction in degradation, the vapor phase epitaxy regrown InP/InGaAsP mushroom laser seems to be an interesting candidate for application in optical communication.

Waveguiding analysis of mushroom stripe laser diodes

The waveguide modes of λ=1.3 μm InGaAsP-InP mushroom stripe laser diodes are analysed rigorously by means of the mode matching technique. The propagation constant and confinement factor of the fundamental TE- and TM-modes are calculated for wide ranges of the relevant laser parameters. Thereby, the cutoff condition for both modes is studied, revealing that for very narrow stripe devices (W>1 μm) first the TE-mode becomes cutoff due to radiation losses into the transverse direction. The cutoff-condition for higher order lateral modes is discussed, showing that the maximum stripe width for single lateral mode guiding is higher than for comparable buried heterostructure lasers. Finally, approximate results, as obtained by using the effective refractive index approximation, are compared with the rigorous solutions, showing a satisfactory agreement only for rather large stripe widths far above cutoff.

Localization of degradation in InP/InGaAsP mushroom stripe lasers

The rapid degradation observed in InP/InGaAsP mushroom stripe lasers covered with phosphosilicate glass (PSG) was investigated by comparing the light-current characteristics as a function of the preparation technique. We were able to show that the PSG-covering layer is not the reason for the rapid degradation. By inspecting the light-current characteristics before and after degradation and by additional underetching the laser structure after degradation we were able to localize the degraded regions on the open side walls of the InGaAsP active layer.

Three- and four-layer LPE InGaAs(P) mushroom stripe lasers for λ = 1.30, 1.54, and 1.66 µm

Mushroom stripe (MS) InGaAsP/InP and InGaAs/InP lasers have been realized emitting at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 1.3, 1.54,</tex> and 1.66 μm, respectively. The main advantage of these MS lasers is their technological simplicity, because only one epitaxial growth step consisting of three or four layers, respectively, is required. No contact layer and no filling layers are needed. In our phosphorus silicate glass (PSG) passivated MS lasers, current spreading is completely inhibited. The devices have very low CW threshold currents and high values of output power, external differential efficiency, and T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> . All these properties are equivalent to those of the much more complicated buried heterostructure lasers. CW operation in up-side-up mounted MS lasers on p-type substrates is easily achieved, because their series resistance is very low. The devices oscillate in the fundamental lateral mode for easily achievable width and thickness combinations, and tend to longitudinal monomode behavior at moderate output powers. The modulation capability is more than 1 Gbit/s RZ due to the low capacitance of the mushrooms. The commonly used antimeltback layer for lasers with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda &gt; 1.5 \mu</tex> m on

Low Threshold CW Operation GaInAsP/InP Mushroom Stripe Laser (MSL) Emitting at 1.27 µm

Low Threshold CW Operation GaInAsP/InP Mushroom Stripe Laser (MSL) Emitting at 1.27 µm