IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 1, JANUARY 1998 57 The Monolithic Integration of a Superluminescent Diode with a Power Ampliﬁer Guotong Du, Gregory Devane, Kathleen A. Stair, Shengli Wu, R. P. H. Chang, Yongsheng Zhao, Zhongzhe Sun, Ying Liu, Xiuying Jiang, and Weihua Han Abstract—Monolithic integration of a superluminescent diode with a tapered semiconductor power ampliﬁer is proposed. The basic operation of the integrated optical source is demonstrated under pulse conditions. Output power obtained by the integrated device is one to two orders of magnitude higher than the conven- tional superluminescent diode (SLD) devices. Index Terms—Integrated optoelectronics, laser diodes, optical integrated circuits. S UPERLUMINESCENT diodes (SLD’s) have attracted wide attention as light sources for many applications, such as ﬁber gyroscopes [1], optical time-domain reﬂectors [2], and an optical coherence tomography for medical tissue imaging [3]. For most optical applications, it is desirable to increase the optical output power of the device. But in most cases, the power is limited by cavity resonances appearing at high gain conditions. Several methods have been used to increase the power, such as using a ﬁber power ampliﬁer [4] or a tapered optical semiconductor ampliﬁer seeded by SLD’s [5]. The combination of an edge emitting LED and a single-transverse-mode semiconductor ampliﬁer to achieve both high power and high modulation bandwidth has also been reported [6]. However, in these reports the ampliﬁers and SLD’s (or LED’s) were connected either by ﬁbers or lenses, leading to signiﬁcant coupling losses. Coupling efﬁciency of the lens system reported in [5], for example, is only 16%; likewise, the optical ﬁber coupling in [6] is less than 46%. In addition, in the general condition, the spectral peaks of SLD and ampliﬁer are not matching, it is impossible to obtain maximum optical gain of the ampliﬁer [6]. To overcome these problems and improve the coupling efﬁciency and compactness of the devices, a monolithically integrated SLD is imperative. In this letter, an all semiconductor integrated SLD source with a tapered stripe ampliﬁer is proposed. The basic operation of the integrated device is demonstrated by our experiment, although heat sinking and electrical isolation have not yet been optimized. Integration of an SLD with a tapered semiconductor am- pliﬁer is possible, since both are traveling-wave amplifying devices. Usually, the starting point for making these types of Manuscript received May 9, 1997; revised Sept. 28, 1997. G. Du, Y. Zhao, Z. Sun, Y. Liu, X. Jiang, and W. Han are with the State Key Laboratory on Integrated Optoelectronics, Jilin University, Changchun 130023, China. G. Devane, K. A. Stair, S. Wu, and R. P. H. Chang are with the Materials Research Center, Northwestern University, Evanston, IL 60208-3108 USA. Publisher Item Identiﬁer S 1041-1135(98)00430-3. Fig. 1. Schematic structure of the high-power integrated SLD with tapered ampliﬁer. devices is a semiconductor laser chip. By removing the optical feedback from the laser facets, the laser oscillator is converted to an optical amplifying device. Suppression of lasing is a key point in this case. For the SLD, lasing is suppressed by applying an antireﬂection (AR) coating on the emitting facets [7] or by utilizing a long unpumped region at the back end of the excited stripe [8]. For the tapered traveling-wave ampliﬁer, a dielectric coating is also deposited onto both cleaved facets to reduce the residual reﬂectivity. If we connect the SLD directly to the narrow end of the tapered ampliﬁer, there should not be any reﬂecting facet between them. When the integrated SLD operates, light from the superluminescent region enters directly into optical ampliﬁer region. The injected light is ampliﬁed in a single pass through the ampliﬁer because the AR coating on the output facet of the ampliﬁer region light is not reﬂected. Thus, lasing is suppressed in both sections of the device and superluminescent output is obtained. The coupling efﬁciency of the connection is nearly 100%. In our experiment, the gain-guided, oxide-stripe integrated SLD was fabricated using an AlGaAs SQW heterostructure wafer as shown in Fig. 1. A 2- m-wide SLD with 300–500- m length is aligned with the 2- m-wide input aperture of the tapered ampliﬁer. The ampliﬁer is 1.3–1.5 mm in length and the gain region expands linearly from 2- m wide at the input end to 110–130 m at the output end. The device structure consists of epitaxial layers grown on n-GaAs by molecular beam epitaxy and are as follows: 1- m-thick n(2 10 )-Al Ga As buffer layer stepped from 0 to 0.05 to 0.25 1.4- m-thick n-Al Ga As layer with n stepped from 2 10 to 2 10 cm 9.6-nm quantum well of Al Ga As, bounded by two 0.075- m-thick undoped Al Ga As conﬁnement layers; 1.18- m-thick p-Al Ga As cladding with p stepped from 2 10 to 2 10 cm 1- m-thick p-GaAs cap (8 10 1041–1135/98$10.00  1998 IEEE