Demonstration of a CEO-locked Frequency Comb at Telecommunications Wavelengths with Low Pulse Energy
Atsushi Ishizawa, Tadashi Nishikawa, and Hidetoshi Nakano
Optical Science Laboratory
The carrier-envelope offset (CEO) is an absolute phase slip between pulses of a mode-locked laser. Recently, progress in the mode-locked laser techniques has made it possible to generate a CEO-locked frequency comb from a mode-locked laser pulse and contains various frequency components in the hundreds of terahertz region which are regularly spaced. The CEO-locked frequency comb can be used as an "optical frequency ruler" with a cesium atomic clock for controlling the spacing. Optical frequency measurement is one of applications of the "ruler". Lasers for applications, such as precision spectroscopy and telecommunications, should be small and have a high repetition rate.
However, CEO frequency detection needs an octave bandwidth spectrum because self-referencing is employed. As a result, a fiber laser needs amplification, which makes it difficult to reduce the size of the laser system. Furthermore, telecommunications and precision spectroscopy applications require a CEO-locked frequency comb with a high repetition rate. The problem in this case is that the pulse energy becomes smaller as the repetition rate increases. We therefore need to achieve CEO locking with a small and high-repetition rate device with low pulse energy.
Our new method employs a tellurite photonic crystal fiber (PCF) for supercontinuum (SC) generation with low pulse energy, and a periodically poled lithium niobate (PPLN) ridge waveguide for efficient second harmonic generation. We found the optimum condition for generating the SC spectrum by changing the polarization of the laser, the PCF length, and the core size. We found that a SC spanning more than an octave can be generated with 80 pJ fiber-coupling pulse energy (Fig. 1). The CEO frequency is measured with an Mach-Zehnder interferometer by combing the second harmonic into a PPLN ridge waveguide with the short wavelength components of the SC light. From the SC shape, we found the optimum wavelength in the 965 nm region in order to observe the CEO frequency. Hartl et al. succeeded in locking the CEO with a 600 pJ fiber coupling pulse energy . We have demonstrated a CEO-locked frequency comb at the telecommunications wavelengths with a 230 pJ fiber coupling pulse energy, which, to the best of our knowledge, is the lowest fiber coupling pulse energy ever achieved (Fig. 2) . Due to the improvement of the coupling efficiency to the PCF, it would be possible to lock the CEO by using a fiber laser oscillator with a low pulse energy.
 I. Hartl, et al., Opt. Express 13 (2005) 6490.
 A. Ishizawa, et al., Opt. Express 16 (2008) 4706.
Fig. 1. Spectrum spanning more than an octave (black) and the spectrum of fiber laser (gray).
Fig. 2. CEO frequency signal (black) and the phase difference between the CEO signal and a local oscillator (gray).
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