Generation of Femtosecond Laser Pulses at 1-GHz Repetition Rate Derived from Continuous Wave Laser Diode
Tadashi Nishikawa and Atsushi Ishizawa
Optical Science Laboratory
Femtosecond pulse lasers with a gigahertz repetition rate are attractive for a variety of applications, including the analysis of nanomechanical and optomechanical systems, as a multiphoton tool for nanomedicine and nanobiotechnology, for high-speed asynchronous optical sampling, and as a carrier-envelope-offset-locked frequency comb with a wide mode spacing . Conventional methods for generating a femtosecond pulse train are based on a passive mode-locking technique. However, since the cavity length must correspond to the repetition rate, it is difficult to obtain femtosecond pulses with a high repetition rate exceeding 1GHz. Moreover, the repetition rate and wavelength have limited tunability. We propose a simple method to generate GHz repetition rate optical pulses with a widely selectable repetition rate and wavelength by using commercial optical phase-modulators (PMs), a commercial intensity-modulator (IM), and a standard single-mode fiber (SMF).
The phase of the light from a CW laser diode (LD) with a center wavelength of 1552 nm are modulated with PMs driven by an external RF synthesizer at modulation frequency of 25 GHz. This process causes repetitive up- and down-chirping at 25 GHz. The linear part of the down-chirping is selectively gated with the IM, resulting in a flat optical frequency comb with a 24-nm bandwidth. After chirping compensation with a SMF, we obtained a 230-fs pulse at 25 GHz. After an optical gate, which reduces the repetition rate from 25 GHz to 1 GHz (or 250 MHz), the optical pulse train is amplified up to an average power of 1 W in an erbiumdoped fiber amplifier (EDFA). Since the laser peak intensity is estimated to be more than several kilowatts in the EDFA, spectral broadening occurs by self-phase modulation. Then, the chirped pulse is compressed by 1-m-long glass block. We succeeded in the generation of a 1-W, 120-fs optical pulse at 1 GHz and a 1-W, 80-fs optical pulse at 250 MHz . Figure 1 shows the spectrum and autocorrelation trace for the latter case. As far as we know, these are the shortest pulses ever achieved for these repetition rates using a CW LD as a seed light source. Our scheme could provide a sub-100-fs laser with widely tunable repetition rate and wavelength by changing the modulation frequency of the RF synthesizer and the wavelength of a tunable laser diode. These features mean that our proposed laser system has the potential to become a powerful tool for a variety of applications.
 A. Ishizawa, T. Nishikawa et al., Electron. Lett. 46 (2010) 1343.
 A. Ishizawa, T. Nishikawa et al., Optics Express 23 (2011) 22402.
Fig. 1. (a) Spectrum and (b) autocorrelation trace obtained using the optical gate at 250 MHz.
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