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 [1]. 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 [2]. 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.

[1] A. Ishizawa, T. Nishikawa et al., Electron. Lett. 46 (2010) 1343.
[2] 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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