1 November 1998 Ž . Optics Communications 156 1998 53–57 Broadly tunable self-starting passively mode-locked Ti:sapphire laser with triple-strained quantum-well saturable Bragg reflector Jia-Min Shieh a , T.C. Huang b , K.F. Huang b , Chi-Luen Wang c , Ci-Ling Pan a, ) a Institute of Electro-Optical Engineering, National Chiao Tung UniÕersity, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan b Department of Electrophysics, National Chiao Tung UniÕersity, 1001 Ta-Hsueh Road, Hsinchu 30010, Taiwan c Precision Instrument DeÕelopment Center, Science-Based Industrial Park, Hsinchu 300, Taiwan Received 22 April 1998; accepted 22 June 1998 Abstract We demonstrate broadband mode-locking of femtosecond Ti:sapphire lasers with a new type of saturable Bragg reflector Ž . SBR . Triple-strained quantum wells with separate and sequential bandgaps were used as the absorbing layer. The saturation fluence was as low as 7 m Jrcm 2 . Self-starting sub-100 fs pulses tunable from 768 to 804 nm were generated in a standard X-folded cavity without intracavity tight focusing on the SBR or temperature tuning. The threshold fluence for self-starting mode-locking was 1 m Jrcm 2 . q 1998 Elsevier Science B.V. All rights reserved. PACS: 42.55.Rz; 60.Fc; 42.65.Rc; 85.30.De In the past few years, wide-bandwidth solid-state gain media such as Ti:sapphire, Cr:LiSAF, Nd:glass, and 4q w x Cr :YAG 1–4 have been successfully utilized to gener- ate mode-locked femtosecond pulses. Non-linear elements commonly used to establish passive mode-locking in these lasers include saturable dye, Kerr-lens medium, and semi- conductor saturable absorbers. Kerr-lens effect, in particu- lar, has been extensively studied as an equivalent saturable absorber. It is, however, relatively difficult to self-start a Ž . Kerr-lens mode-locked KLM solid-state laser. The KLM cavity has to be especially designed and aligned as the operation point is usually very close to the limit of the wx stability regime 5 . A more recent development is the Ž . semiconductor saturable-absorber mirror SESAM . These have also been successfully employed in mode-locked solid-state laser systems for generating femtosecond pulses w x from visible to the infrared 6–8 . There are two major types of SESAMs: a semiconductor multiple-quantum-well Ž . MQW saturable absorber monolithically integrated be- Ž tween two reflecting mirrors the antiresonant Fabry–Perot ) Corresponding author. E-mail: clpan@cc.nctu.edu.tw . wx saturable absorber, A-FPSA 6 ; and the Bragg reflector Ž . with a single quantum well QW buried in the last growth Ž . wx layer saturable Bragg reflector, SBR 7 . Pulses as short wx as 6.5 fs were recently achieved using the SESAM 9 . The starting mechanism of femtosecond lasers with SESAM was based on nonlinear reflectivity from resonant excitonic transitions of the QW absorber. However, the absorption band of the SESAM usually limits the tuning range. Previ- wx ously, Tsuda et al. 7 showed that it is possible to enlarge Ž . the self-starting mode-locking range D l of a Cr:LiSAF Ž . Ž y2 . laser l s840 nm to 30 nm D lrl s3.7 =10 by heating the SBR to 1508C. With two identical quantum wells in a lr2-thick layer on a distributed Bragg reflector Ž . Ž DBR mirror as a SBR, a tuning range of 47 nm D lrl y2 . 4q s3.0 =10 for the passively mode-locked Cr :YAG w x laser at l s1500 nm has been demonstrated 10 . This use of two QWs instead of one on the DBR enhances the absorption strength and allows the laser to mode-lock near the tail of the absorption band. Recently, Kopf et al. demonstrated broadband mode-locking of a Cr:LiSAF laser with a SESAM device using broadened absorption edge of Ž . low-temperature molecular beam epitaxially MBE grown w x GaAs quantum wells 11 . Sub-200 fs pulses with a tuning 0030-4018r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved. Ž . PII: S0030-4018 98 00350-2