IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 3, FEBRUARY 1, 2010 179 Polarization Characteristics of Quantum-Dot Vertical-Cavity Surface-Emitting Laser With Light Injection Peng-Chun Peng, Ruei-Long Lan, Fang-Ming Wu, Gray Lin, Chun-Ting Lin, Jason (Jyehong) Chen, Gong-Ru Lin, Sien Chi, Hao-Chung Kuo, and Jim Y. Chi Abstract—This investigation explores experimentally the optical characteristics of long-wavelength quantum-dot vertical-cavity surface-emitting lasers (QD VCSELs). The InAs QD VCSEL, fabricated on a GaAs substrate, is grown by molecular beam epitaxy with fully doped distributed Bragg reflectors. The optical characteristics of QD VCSEL without and with light injection are studied in detail. The QD VCSEL has the potential to be used in all-optical signal processing systems. Index Terms—Quantum dots (QDs), vertical-cavity sur- face-emitting lasers (VCSELs). I. INTRODUCTION V ERTICAL-CAVITY surface-emitting lasers (VCSELs) have attracted substantial attention in recent years because they provide various advantages in optical communi- cation systems, such as low power consumption, high-speed modulation, high beam quality, low manufacturing cost, and low threshold current [1]–[4]. The particular advantages of quantum-dot (QD) structures include high thermal stability, low threshold current, low chirp, high material gain, and high differential gain, making QD VCSELs potentially favored for use in optical communication systems [5]–[9]. Recently, sub- stantial progress has been made in the development of 1.3- m Manuscript received July 29, 2009; revised November 09, 2009; accepted November 16, 2009. Current version published January 15, 2010. This work was supported by the National Science Council of the Republic of China, Taiwan, under Contract NSC 97-2221-E-027-114 and Contract NSC 98-2221-E-027- 007-MY3. P.-C. Peng and R.-L. Lan are with the Department of Electro-Optical En- gineering, National Taipei University of Technology, Taipei, Taiwan (e-mail: pcpeng@ntut.edu.tw). F.-M. Wu, J. Chen, and H.-C. Kuo are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan. G. Lin is with the Department of Electronics Engineering, National Chiao Tung University, Hsinchu, Taiwan. C.-T. Lin is with the Institute of Photonic Systems, National Chiao Tung Uni- versity, Tainan 711, Taiwan. G.-R. Lin is with the Graduate Institute of Photonics and Optoelectronics, and the Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan. S. Chi is with the Department of Electro-Optical Engineering, Yuan Ze Uni- versity, Chung Li, Taiwan. J. Y. Chi is with the Institute of Opto-Electronic Engineering, National Dong Hwa University, Hualien, Taiwan, and also with the Electronics and Op- toelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2009.2037332 monolithic QD VCSELs [10], [11]. The eye diagram, frequency response, and intermodulation distortion of 1.3- m monolithic QD VCSEL without and with external light injection have been described [11]. However, the optical polarization characteristics of 1.3- m QD VCSELs with and without light injection have not yet been elucidated. Moreover, optical signal processing based on a QD VCSEL also has not been addressed. This investigation experimentally explores the optical polar- ization characteristics of QD VCSEL with and without external light injection. The 1.3- m InAs–InGaAs QD VCSELs are grown by molecular beam epitaxy (MBE) with fully doped p- and n-doped AlGaAs distributed Bragg reflectors and an AlAs layer to form a current and waveguide aperture. The optical injection power is adjusted to control the polarization of QD VCSEL. The polarization-mode suppression ratio is 29.6 dB at an injection power of 4 dBm. II. EXPERIMENTAL SETUP AND RESULTS Fig. 1 depicts the structure of InAs QD VCSEL. The 1.3- m InAs–InGaAs QD VCSEL is grown on an (100) n -GaAs sub- strate using MBE. The circular mesa with a diameter of 26 m was defined by dry-etched circular trench for subsequent water vapor oxidation. The diameter of the oxide aperture was about 6 m. No intentional asymmetry was imposed during fab- rication. The method of fabrication has been described in our earlier works [11]. The InAs QD VCSEL is hermetically sealed using a TO-Can laser package. The InAs QD VCSEL TO-Can package and the single-mode fiber are assembled by laser welding. The inset in Fig. 1 displays the light–current charac- teristics of QD VCSEL at room temperature. The QD VCSEL has a threshold current of approximately 1 mA. The lasing wavelength of the QD VCSEL is around 1277.2 nm. Fig. 2 presents the experimental setup for measuring the polarization characteristics of QD VCSEL. The polarization of the QD VCSEL output is linear and is polarization. polarization is orthogonal to the polarization. The QD VCSEL without external light injection exhibits no polarization switching. It is believed that stable linear polarization is attributed to the optical gain anisotropy of self-assembled QDs [12]. Fig. 3 displays the experimental setup for measuring the QD VCSEL with external light injection. The injection power is controlled by a variable optical attenuator (VA) at the output of the tunable laser. The polarization of the tunable laser is adjusted using a polarization controller (PC). An optical circulator (C) is adopted to couple the laser light in the QD 1041-1135/$26.00 © 2010 IEEE