Appl Phys B (2009) 96: 287–291 DOI 10.1007/s00340-009-3489-9 Implementation of the direct locking method for long-term carrier-envelope-phase stabilization of a grating-based kHz femtosecond laser J.-H. Lee · Y.S. Lee · J. Park · J.J. Park · D.S. Kim · T.J. Yu · C.H. Nam Received: 5 December 2008 / Revised version: 2 March 2009 / Published online: 2 April 2009 © Springer-Verlag 2009 Abstract We have stabilized the carrier-envelope phase (CEP) of amplified femtosecond laser pulses from a grating- based chirped-pulse amplification femtosecond laser by the direct locking method. Long-term CEP stabilization in the oscillator was achieved by employing a double-feedback loop to control both the pumping power and the cavity dis- persion. Large CEP drift, induced during amplification, was compensated by adjusting the grating separation in the pulse compressor, and the CEP stabilization was maintained for four hours with a phase jitter of about 180 mrad. After pulse compression to 5.5-fs pulses in a filamentation pulse com- pressor, CEP-stabilized laser pulses were applied for high- harmonic generation to confirm the CEP stabilization. PACS 42.65.Tg · 42.65.Re · 42.65.Ky 1 Introduction Advancement of femtosecond laser technology has facili- tated the development of few-cycle high-power lasers. Com- pact high-power femtosecond lasers based on the chirped- pulse amplification (CPA) technique are widely available nowadays. With pulse compression in a gaseous medium, intense laser pulses containing only few optical cycles can J.-H. Lee · Y.S. Lee · J. Park · J.J. Park · D.S. Kim · C.H. Nam ( ) Department of Physics and Coherent X-ray Research Center, KAIST, 305-701 Daejeon, Korea e-mail: chnam@kaist.ac.kr T.J. Yu Advanced Photonics Research Institute, GIST, 500-712 Gwangju, Korea be produced. The electric-field profile of a laser pulse is then a more pertinent physical entity in light–matter inter- actions than the intensity profile. The electric-field profile of few-cycle laser pulses, however, changes sensitively to the carrier-envelope phase (CEP) of the laser pulse. The stabilization of CEP is thus one of critical techniques in investigating few-cycle laser–matter interactions, such as high-harmonic generation (HHG) [1], above-threshold ion- ization [2], and Rabi flopping [3]. The CEP stabilization of a femtosecond oscillator can be realized in a couple of different ways. The CEP stabilization technique based on a phase-locked loop (PLL) was devel- oped for frequency metrology [4–6] and was also widely used for the CEP stabilization of high-power femtosecond lasers. The PLL method makes the carrier-envelope offset frequency (f ceo ), or the rate of CEP change, constant; thus, the laser pulses from a CEP-stabilized oscillator have con- tinuously changing CEP with a constant rate. This is dis- advantageous in applications requiring all laser pulses with identical CEP. On the other hand, the direct locking method is a different kind of a CEP-stabilization scheme operating in the time domain [7]. This scheme eliminates CEP drift between successive pulses by quenching the beating signal from an f -to-2f interferometer generating pulses with iden- tical CEP value, or equivalently zero f ceo . While the conven- tional PLL method electronically detects the RF phase error between the RF reference and the optical beating signal from the interferometer using an electrical phase detector, the di- rect locking method directly detects the CEP variation from the optical beating signal, not necessitating an external ref- erence signal. For precise and long term CEP stabilization a homodyne balanced detection and a double-feedback loop have been devised, which greatly enhanced practicality of the direct locking scheme [8]. Due to simplicity in equip- ment and intuitiveness in data analysis the CEP stabilization