IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 7, APRIL 1, 2008 505 Full Characterization of Low-Power Picosecond Pulses From a Gain-Switched Diode Laser Using Electrooptic Modulation-Based Linear FROG K. T. Vu, A. Malinowski, M. A. F. Roelens, M. Ibsen, P. Petropoulos, and D. J. Richardson, Member, IEEE Abstract—We use a linear frequency-resolved optical gating technique based on electrooptic modulation to fully characterize for the first time the highly chirped pulses from a 1.06- m Fabry–Pérot laser diode and design a chirped fiber Bragg grating to provide high-quality pulse compression. With this grating, we achieved 18-ps pulses at a repetition rate of 1.35 GHz with a time-bandwidth product of 0.7. Index Terms—Fiber Bragg grating, frequency-resolved optical gating (FROG), optical pulse generation, semiconductor lasers. G AIN-SWITCHING of laser diodes represents a very con- venient and practical method to generate high repetition rate picosecond pulses. Gain-switched lasers were primarily de- veloped for telecommunications [1]–[3] providing picosecond pulses of limited peak power (typically 10–100 mW) at wave- lengths around 1.55 m. More recently they have been used as seed lasers for high-power cladding-pumped rare-earth doped fiber amplifiers emitting at 1.55 m [4]–[6], and at 1.06 m [7]–[10], allowing the development of picosecond pulse sources of unprecedented average power levels with peak powers and durations commensurate with both highly efficient frequency conversion, and materials processing requirements. The power scaling possibilities at 1.06 m are particularly interesting: with power levels of 320 W already reported for a diode seeded pi- cosecond system [9], and up to 2.5 kW in the continuous-wave regime. However, both the diodes and associated technology are far less developed in this wavelength regime than at 1.55 m. An inherent characteristic of pulses produced by gain- switching is that they are highly chirped. It is important to be able to characterize this chirp in order to design components, such as fiber Bragg gratings, which can efficiently compress the seed pulses, allowing the shortest, highest peak power pulses to be obtained. Frequency-resolved optical gating (FROG) techniques based on nonlinear processes (e.g., second-har- monic generation FROG)[16] as routinely used for the full Manuscript received October 22, 2007; revised December 14, 2007. K. T. Vu was with the Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, U.K. He is now with the Laser Physics Centre, Australian National University, Canberra ACT 0200, Australia. A. Malinowski, M. Ibsen, P. Petropoulos, and D. J. Richardson are with the Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, U.K. (e-mail: djr@orc.soton.ac.uk). M. A. F. Roelens was with the Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, U.K. He is now with the School of Physics, University of Sydney, NSW 2006, Australia. Digital Object Identifier 10.1109/LPT.2008.918879 characterization (amplitude and phase) of ultrashort short pulses are poorly suited to the characterization of picosecond pulses from these diodes because of their relatively low peak powers (10–100 mW) and long durations (10–100 ps). Recently, however, a linear-gating FROG based on an elec- troabsorption modulator (EAM) was reported, which allowed full characterization of telecommunication signals at 1550 nm [12]. The technique is highly sensitive, and was shown to allow the complete characterization of relatively long duration (10–100 ps), low energy pulses [12], as well as to femtosecond pulses [13]. EAMs are currently not available at 1.06 m. However, electrooptic modulators (EOMs) offer an alternative that enables such measurement in this wavelength range. (Note that the use of EOMs for such measurements has just recently been demonstrated at a wavelength of 1.55 m [14], [15].) A technique using a pair of phase modulators to characterize pulses using spectral shearing interferometry at 1053 nm has been presented [16]. This technique also makes use of linear processes only and hence is sensitive to low energy pulses, but is more complex than the scheme presented here, since it still splits the pulse into separate optical paths. In this letter, we demonstrate for the first time to the best of our knowledge the use of linear FROG for the complete char- acterization of long, low intensity pulses at the technologically important wavelength of 1.06 m. The technique is highly sensitive. It has the advantages of simplicity and convenience, being fully fiberized, and using only electronic delays in the measurement of the spectrogram. We demonstrate application of the measurement technique to develop a fiber-grating-based pulsed compressor and show that high-quality compression can be achieved. We consider that linear FROG will prove a very useful and important tool for the future development of high performance, diode seeded fiber-based picosecond master oscillator power amplifier systems. The setup of the gain-switched diode is shown in Fig. 1. The commercial fiberized single-mode InGaAsP Fabry–Pérot (FP) laser diode manufactured by Bookham (model LC92) op- erated at around 1060 nm, with a cavity length of 0.8 mm and a front facet reflectivity of 1%. It was driven by a sinu- soidal electrical signal to produce gain-switched pulses. The de- vice had a modulation bandwidth much greater than 1.35 GHz. It is temperature-stabilized via the built in Peltier cooler. It is mounted on a home-made printed circuit board. This was de- signed for impedance matching. Gain-switching was realized by driving the laser with a 1.35-GHz sinusoidal drive signal super- imposed on a dc bias current. The diode was biased a little below 1041-1135/$25.00 © 2008 IEEE