IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 12, JUNE 15, 2007 937 980-nm Monolithic Passively Mode-Locked Diode Lasers With 62 pJ of Pulse Energy Juliet T. Gopinath, Member, IEEE, Bien Chann, Robin K. Huang, Member, IEEE, Christopher Harris, Jason J. Plant, Leo Missaggia, Joseph P. Donnelly, Life Fellow, IEEE, Paul W. Juodawlkis, Senior Member, IEEE, and Daniel J. Ripin Abstract—Passively mode-locked 980-nm slab-coupled optical waveguide lasers have been demonstrated with pulse energies as high as 62 pJ and average powers of 489 mW at 7.92 GHz from a 5-mm device with a 300- m absorber. Mode-locking has been observed with devices ranging from 3 to 10 mm in length. Index Terms—High-power diode lasers, mode-locked lasers, quantum-well devices, semiconductor lasers, single-mode semi- conductor lasers. I. INTRODUCTION M ODE-LOCKED semiconductor lasers are attractive sources due to their potential for compactness, effi- ciency, and wavelength flexibility. They are promising for a number of applications including frequency metrology [1], light detection and ranging, Raman spectroscopy [2], and com- munications. In particular, frequency-converted semiconductor lasers show great potential as suitable deep ultraviolet sources for sensors and nonline-of-sight communications [3], [4]. However, to date, mode-locked semiconductor lasers have generally been demonstrated only at low powers and pulse en- ergies. Typical reported pulse energies from sources operating between 800 and 900 nm are 1–10 pJ, with corresponding av- erage powers ranging from 1 to 5 mW [5]. In this work, we report on passively mode-locked electrically pumped 980-nm slab-coupled optical waveguide lasers (SCOWLs) with record- breaking pulse energies and powers. Passive mode-locking has been demonstrated with a large range of device lengths incor- porating an integrated saturable absorber. An average power of 489 mW with 62 pJ of pulse energy has been achieved from a 5-mm-length device, with a 0.3-mm absorber. The power and pulse energy from semiconductor lasers are limited by their saturation energy, , where is the photon energy, is the active region area, is the differential gain, and is the confinement factor. Thus, by increasing the ac- tive-region area, or decreasing the confinement factor, one can increase the average power and pulse energy. The active area of a semiconductor laser can be increased by adding a flare to a Manuscript received November 13, 2006; revised March 6, 2007. This work was supported by the Department of the Air Force under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommenda- tions are those of the authors, and are not necessarily endorsed by the United States Government. The authors are with MIT Lincoln Laboratory, Lexington, MA 02420 USA (e-mail: juliet@ll.mit.edu). 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.2007.898873 Fig. 1. Schematic of a typical monolithic mode-locked 980-nm SCOWL. Sev- eral parameters critical for single spatial mode operation are indicated: m, m, and m. waveguide or using a vertical-cavity surface-emitting geometry. With flared waveguides, including an integrated saturable ab- sorber for passive mode-locking, 6.8 pJ and 9.1 mW of average power has been demonstrated [6] at 940 nm. An inverse bow-tie amplifier, operated in an external cavity with a separate sat- urable absorber, produced 36 pJ with 36 mW of average power at 1 GHz [7]. An optically pumped vertical-cavity surface-emitting laser generated 2.1 W of average power with 0.5 nJ of pulse en- ergy [8]. However, optical pumping is complex and inefficient. Using a combination of two devices, a flared and a straight am- plifier, and through active mode-locking, Goldberg et al. were able to achieve 12-ps pulses at a repetition rate of 1 GHz, with pulse energies of 0.5 nJ [9]. Our approach to this problem is to combine an electrically pumped SCOWL with a low confinement factor, 0.003, and an integrated saturable absorber for passive mode-locking. The SCOWL has been demonstrated CW with diffraction-limited single-mode output of 1 W at 980 nm, and 0.8 W at 1550 nm [10]–[12]. Passive mode-locking has been demonstrated with 250 mW of average power and 58 pJ of pulse energy at 1550 nm [13], and active mode-locking producing 40 mW of average power [14]. This work reported here is focused on passive mode- locking of 980-nm electrically pumped SCOWLs, suitable for frequency conversion to ultraviolet wavelengths. II. DESIGN AND FABRICATION The SCOWL is based on a principle developed by Marcatili [15] in which single-mode operation in a multimode waveguide 1041-1135/$25.00 © 2007 IEEE