IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 5, SEPTEMBER/OCTOBER 2007 1267 High-Power and Broadband Quantum Dot Superluminescent Diodes Centered at 1250 nm for Optical Coherence Tomography Sumon K. Ray, Tin Lun Choi, Kristian M. Groom, Member, IEEE, Benjamin J. Stevens, Huiyun Liu, Mark Hopkinson, and Richard A. Hogg, Member, IEEE Abstract—Quantum dot (QD) superluminescent diodes (SLDs) exhibiting 8 mW and 95 nm full-width at half-maximum centered at 1270 nm are demonstrated with a flat-topped spectral profile. This is achieved using 3 × 2 dots in compositionally modulated wells technique. Furthermore, techniques for realization of high-power SLDs are also demonstrated. A continuous-wave output power of 42 mW is achieved for narrowband devices centered at 1250 nm. Index Terms—Semiconductor lasers, superluminescent diodes (SLDs), quantum dot (QD). I. INTRODUCTION S UPERLUMINESCENT diodes (SLDs) provide broadband emission for a wide range of applications such as wave- length division multiplexing (WDM) system testing, fiber-optic gyroscopes, and optical coherence tomography (OCT). Recent interest has focused upon applications in OCT where cheap, compact, high-power broadband optical sources are required in order to realize low cost point of care screening and diag- nostics [1]. Due to strong multiple scattering in skin tissue, operating wavelengths in the region of 1050 and 1250 nm are required, corresponding to attenuation minima in ocular me- dia and skin tissue, respectively. Ultrahigh resolution (2-D or 3-D) cross-sectional images of tissue may be obtained noninva- sively and in situ [2]. OCT is a low-coherence technique based on the Michelson interferometer, where the axial resolution is governed by the coherence length, hence the requirement for a broadband light source (l coh ∝ 1/Δλ). Techniques for broadening the optical spectrum of semicon- ductor SLDs typically rely upon chirped [3] or intermixed [4] quantum wells. Recently, quantum dot (QD) materials have at- tracted attention due to their naturally broad-emission spec- trum [5]. We have recently demonstrated novel techniques for further broadening the emission bandwidth of a QD SLD oper- ating around 1250 nm, and tailoring of the shape of the emission spectrum using multiple dots-in-a-well (DWELL) layers [7]. A smooth increase in wavelength is demonstrated as the indium composition in a DWELL is increased from 0% to 20% [6]. Manuscript received October 25, 2006; revised June 18, 2007. This work was supported in part by the Royal Academy of Engineering, in part by the EPSRC, and in part by the EU IST Nano-UB Sources Programme. The authors are with the Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K. (e-mail: s.k.ray@sheffield.ac.uk; elp05tlc@sheffield.ac.uk; k.m.groom@sheffield.ac. uk; b.stevens@sheffield.ac.uk; h.liu@sheffield.ac.uk; m.hopkinson@sheffield. ac.uk; r.hogg@sheffield.ac.uk). Digital Object Identifier 10.1109/JSTQE.2007.902997 Our broadband SLD devices rely upon a multi-DWELL struc- ture, with each well containing a different indium composition. These structures were previously termed as dots in composi- tionally modulated well (DCMWELL) structures [7]. In the present work, we attempt both to tailor the spectral shape of the SLD emission and to increase the output power. We demonstrate 95-nm full-width at half-maximum (FWHM) broadband SLDs centered at 1270 nm, and narrowband SLDs with 42-mW output power. II. SAMPLE GROWTH AND F ABRICATION The QD SLD structures were grown by solid source molecular beam epitaxy upon n + GaAs substrates. DWELL layers consisted of 3.0 monolayers of InAs grown upon 2 nm of In x Ga 1−x As and covered by 6 nm of In x Ga 1−x As. Two indium cells were utilized, with operating conditions optimized for QDs and quantum wells (QWs) individually [6]. Five InAs/InGaAs DWELLs were separated by 50-nm GaAs barriers and em- bedded between 150-nm separate confinement heterostructure GaAs layers. The active region was sandwiched between 1500-nm Al 0. 4 Ga 0. 6 As cladding layers, and 300-nm GaAs contact layer completed the growth. Growth temperatures were 620 ◦ C for the AlGaAs and 510 ◦ C for the indium containing layers. Following the deposition of each DWELL, the initial 15 nm of the GaAs spacer layer was deposited at 510 ◦ C, following which the temperature was increased to 580 ◦ C for the remaining 35 nm. This technique is referred to as high growth temperature spacer layer (HGTSL) [8]. Extremely low-threshold current density J th of 17 A·cm −2 has previously been reported for HR/HR coated three-layer DWELL lasers using the HTGSL [9]. The temperature was then decreased back to 510 ◦ C for the growth of the next DWELL. For a standard DWELL structure, the indium composition x in each In x Ga 1−x As well was optimized as 15%. For the “standard” DCMWELL structure, the five wells contain 12%, 13%, 13.5%, 14%, and 15% indium, respectively, with the gain spectrum resulting from the 13%, 13.5%, and 14% DWELLs filling in the spectral range between the 12% and 15% DWELLs. These indium compositions were chosen such that the resulting spec- trum is Gaussian in shape. The change in indium composition was achieved by changing the temperature of the cell while the barrier layers were grown. The SLDs were fabricated via a shal- low ridge etch, with etching stopped after removal of the upper p-doped GaAs and AlGaAs layers at a depth of 1.8 μm. Wide 1077-260X/$25.00 © 2007 IEEE