482 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 3, MAY/JUNE 2009 Discretely Tunable Semiconductor Lasers Suitable for Photonic Integration Diarmuid C. Byrne, Jan Peter Engelstaedter, Wei-Hua Guo, Qiao Yin Lu, Brian Corbett, Brendan Roycroft, James O’Callaghan, F. H. Peters, and John F. Donegan, Senior Member, IEEE Abstract—A sequence of partially reﬂective slots etched into an active ridge waveguide of a 1.5 μm laser structure is found to provide sufﬁcient reﬂection for lasing. Mirrors based on these re- ﬂectors have strong spectral dependence. Two such active mirrors together with an active central section are combined in a Vernier conﬁguration to demonstrate a tunable laser exhibiting 11 dis- crete modes over a 30 nm tuning range with mode spacing around 400 GHz and side-mode suppression ratio larger than 30 dB. The individual modes can be continuously tuned by up to 1.1 nm by carrier injection and by over 2 nm using thermal effects. These mirrors are suitable as a platform for integration of other optical functions with the laser. This is demonstrated by monolithically integrating a semiconductor optical ampliﬁer with the laser result- ing in a maximum channel power of 14.2 dBm from the discrete modes. Index Terms—Photonic integration, semiconductor lasers, semi- conductor optical ampliﬁers, tunable lasers. I. INTRODUCTION W IDELY tunable semiconductor lasers will play an impor- tant part in next generation optical networks. Tunable lasers are essential in wavelength-agile networks and as a means to reduce costs as sparing lasers in wavelength-division multi- plexing (WDM) systems. New approaches to data transmission such as coherent WDM (CoWDM [1]) require discrete tuning between particular wavelength channels on a grid. There is ad- ditionally an urgent need to integrate semiconductor lasers with other optical components such as ampliﬁers, modulators and detectors [2]–[5] in order to reduce chip cost, system size, and complexity. Tunable lasers are also needed in other important markets such as trace gas detection for environmental emission motoring [6]. Laser operation requires optical feedback, which is conven- tionally obtained in a semiconductor Fabry–P´ erot laser by cleav- ing the ends of the laser waveguide along either (0 1 1) or (0 1 −1) crystallographic planes to form two semireﬂecting facets. However, due to the need for cleavage, it is difﬁcult to integrate these lasers with other optical components on a single chip. Manuscript received October 30, 2008; revised February 3, 2009. First pub- lished May 15, 2009; current version published June 5, 2009. D. C. Byrne, W.-H. Guo, Q. Y. Lu, and J. F. Donegan are with the Semicon- ductor Photonics Group, School of Physics and Centre for Telecommunication Value Driven Research (CTVR), Trinity College, Dublin 2, Ireland (e-mail: byrnedc@tcd.ie). J. P. Engelstaedter, B. Corbett, B. Roycroft, and J. O’Callaghan are with the Tyndall National Institute and Centre for Telecommunication Value Driven Re- search (CTVR), Cork, Ireland. F. H. Peters is with the Physics Department, University College Cork, Cork, Ireland. Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JSTQE.2009.2016981 Distributed-Bragg-reﬂector (DBR) lasers and distributed feedback (DFB) lasers, which employ a series of small refrac- tive index perturbations to provide feedback, do not rely on cleaved facets, and therefore, can be integrated with optical am- pliﬁers and modulators [4], [5]. However, complex processing with multiple epitaxial growth stages is required for fabricating these lasers. Another method to obtain feedback is to etch a facet. However, this approach is limited by difﬁculties in the smoothness and verticality of the etched facet, particularly, for structures based on InP materials. Previously, it was shown that by introducing a shallow slot into the active ridge waveguide of a laser, the longitudinal modes of the Fabry–Perot (FP) cavity were perturbed according to the position of the slot with respect to the cleaved facets [7]–[9]. By judicious placement of a sequence of low-loss slots with respect to the facets, preselected FP modes could be signiﬁ- cantly enhanced leading to robust single-frequency lasing with wide temperature stability [10], [11] as well as tuning with fast switching characteristics [12]. More recently, we have character- ized the properties of slots that are etched more deeply, namely, to the depth of, but not through, the core waveguide containing the quantum wells [13]. In that case, the reﬂection of each slot is of the order of ∼1% with transmission of ∼80% and the slot will strongly perturb the mode spectrum of the FP cavity by creating subcavities. The loss introduced by the presence of the slot is compensated by gain in the laser. An array of such slots can provide the necessary reﬂectivity for the laser operation in- dependent of a cleaved facet where the gain between the slots compensates for the slot loss producing an active slotted mir- ror region. Such a mirror has been used in conjunction with a cleaved facet permitting the integration of a photodetector with the laser [14]. In this paper, we use reﬂective slots and the associated mir- rors as the platform technology for the realization of a facetless laser that can be tuned using differential current injection into different longitudinal sections. Furthermore, the integration of the tunable laser with an optical ampliﬁer is also demonstrated. The electrical isolation between the different sections is made possible by the etched slots. The slots are realized by conven- tional photolithography and dry etching during the deﬁnition of the waveguide. As the technology is based on a generic single epitaxial growth stage and upon standard laser processing steps, it is compatible with implementation in a foundry. II. TUNABLE LASER DESIGN The semiconductor laser employing etched slots as the front and back mirrors is shown schematically in Fig. 1(a). The laser 1077-260X/$25.00 © 2009 IEEE Authorized licensed use limited to: TRINITY COLLEGE LIBRARY DUBLIN. Downloaded on September 2, 2009 at 09:22 from IEEE Xplore. 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