752 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 37, NO. 6, JUNE 2001 Deep-Etched Distributed Bragg Reflector Lasers with Curved Mirrors—Experiments and Modeling Peter Modh, Student Member, IEEE, Niklas Eriksson, Manuel Quiroga Teixeiro, Anders Larsson, Member, IEEE, and Toshiaki Suhara Abstract—A semiconductor laser with deep-etched distributed Bragg reflectors (DBRs) supporting a planar Gaussian mode has been experimentally and theoretically studied. A 90- m-long laser with two-groove DBRs has a low threshold current of 7 mA and a maximum side mode suppression of 17.6 dB under continuous op- eration. The laser resonator supports a mode that closely resem- bles the desired planar Gaussian mode. The reflectivities of the deep-etched DBRs were experimentally determined using broad area devices, and the reflection, transmission, and scattering prop- erties of the DBRs were simulated using a finite-difference time-do- main model. The simulations show that deep grooves, covering the full transverse extent of the guided mode, are needed to maximize the reflectivity and to minimize the scattering loss. A beam-prop- agation model was used to simulate the laser resonator. The simu- lations (as well as the experiments) show that the laser is sensitive to thermal effects. Thermal lensing narrows the mode waist, and therefore increases the spatial hole burning in the center of the res- onator where the intensity is at its maximum. At high drive cur- rents, this leads to a degradation of the spatial mode quality. The simulations also indicate that a laser with optimized DBRs (one one- and one two-groove DBRs with an etch depth of 1 m) would have a threshold current less than 2 mA and support a high-quality planar Gaussian mode to an output power of 9 mW under contin- uous operation. Index Terms—Beam-propagation methods, distributed Bragg reflector lasers, dry etching, finite difference time-domain methods, monolithic integration, short-cavity lasers. I. INTRODUCTION C OHERENT light generation in photonic integrated circuits (PICs) requires integration-compatible in-plane lasers. Among the desired features of such devices are a small size, a low-threshold current, a high efficiency, and controlled spatial mode behavior. In addition, the laser design should allow for fabrication techniques of low complexity for low-cost fabrication [1], [2]. Monolithic integration excludes cleaved facets as feedback elements for the laser resonator. Integration-compatible alter- natives are etched facets, distributed feedback (DFB) and dis- tributed Bragg reflectors (DBRs). The etched facet suffers from Manuscript received September 13, 2000; revised January 31, 2001. This work was supported by the Swedish Research Council for Engineering Sciences (TFR) and the Swedish National Board for Industrial and Technological Devel- opment (NUTEK). P. Modh, N. Eriksson, M. Teixeiro, and A. Larsson are with the Microtech- nology Center at Chalmers and the Photonics Laboratory, Department of Mi- croelectronics ED, Chalmers University of Technology, SE-412 96 Göteborg, Sweden. T. Suhara is with the Department of Electronics, Faculty of Engineering, Osaka University, Osaka 565, Japan. Publisher Item Identifier S 0018-9197(01)04301-9. a limited achievable reflectivity and the DFB solution usually requires regrowth which increases the fabrication complexity. The DBR (with surface gratings), on the other hand, is straight- forward to fabricate and offers high reflectivity with only a few number of periods, using deep-etched grooves to form semi- conductor/air DBRs [3]–[10] that act as 1-D photonic bandgap structures. The deep-etched DBRs provide high reflectivity over a wide range of wavelengths and angles of incidence. Short re- flectors with high reflectivity have several advantages. The res- onator length can be reduced, thus decreasing the overall de- vice size and increasing the longitudinal mode selectivity. The latter is important since it compensates for the poor spectral se- lectivity of the high contrast DBR. A high reflectivity also re- duces the drive current which can eliminate problems associ- ated with thermal effects. Additional advantages include a small parasitic capacitance associated with the small device size. This contributes to a large modulation bandwidth. Earlier work on lasers with deep etched DBRs, both exper- imental [3]–[5], [7]–[10] and theoretical [4], [6], have all used straight reflectors where the focus has been on compact designs andlowthresholdcurrent(i.e.,highlyreflectiveDBRs).However, no attention has been paid to the scattering and transmission char- acteristicsoftheDBRsortothespatial-modecharacteristicsofthe resonator. All these characteristics are of utmost importance when integrated with other elements such as amplifiers and grating cou- plers in fully integrated PICs. The use of more elaborate reflector designs can, for instance, tailor the spatial mode behavior and/or control the lateral diffraction loss. This loss has a small effect in broad area lasers [4], [8], but the effect is greater for narrow, spatiallysinglemodelasers[5],[9],[10].Thetransversescattering loss can be reduced by using a waveguide structure with a broader transverse mode profile [11]. We have earlier reported on the first experimental results on a semiconductor laser with curved deep-etched DBRs as reflec- tors [12]. The laser is designed to support a planar Gaussian mode by use of deep-etched DBRs with a curvature matching the wavefront of the desired mode [13]. The curvature of the DBRs reduce the lateral diffraction loss. The desired planar Gaussian mode with a narrow waist produces a highly diverging wave. This is of particular interest in PICs where the laser is fol- lowed by an optical amplifier for high power generation [14]. A diverging wave increases the filamentation threshold in the am- plifier, thereby allowing for a high degree of spatial coherence and good beam quality at high output power. The main conclu- sions from our first experiments were that the transverse mode of the laser closely resembled the desired planar Gaussian mode and that the limited etch depth of the DBRs resulted in lossy re- 0018–9197/01$10.00 © 2001 IEEE