펨토초 레이저의 가간섭 펄스 결합 연구

Abstract

Laser manufacturing is essential for scribing a fine texture, drilling small holes and sharply cutting plates. Even, manufacturing with the aid of femtosecond pulses have become important to obtain high quality products because it can reduce debris which can be generated in the other laser manufacturing processes. In this case, the energy of the single femtosecond pulse is mainly considered and it should be amplified for various application fields. The typical amplification of femtosecond pulses can be achieved with pulse stretching, i.e. chirped pulse amplification (CPA), to avoid the damage of the amplifying medium but the single pulse CPA still has practical limitation when the pulse energy is higher than the damage threshold of the medium. Thus, another amplification method is needed. Divided pulse amplification (DPA), which combines individually amplified pulses after dividing pulses spatially and temporally, can be a good candidate to obtain ultra-high pulse energy. However, one of drawbacks of DPA is the large size of the system because it is based on interferometric technique and the optical path difference should be long enough to separate the pulses temporally. In this investigation, we propose a compact temporal DPA system based on the multi-path interferometric technique for industrial application. The total optical path reaches up to a few meters to divide pulses with the order of ns. time interval, but the size of the system is approximately (0.35 m × 0.35 m) because of the multi-path configuration. For combining the divided pulses, locking of optical coherent via single-detector electronic-frequency tagging (LOCSET) method based on the modulation and demodulation technique was used and the pulses could be coherently combined to generate constructive interference. The LOCSET method was configured with a cpmmercial lock-in-amp, and the modulation frequency used in the modulation was 35 Hz. The control signal was obtained by the lock-in-amplifier and was tuned by a PID controller. When the PID control was off, the interference signal was fluctuated seriously, but the interference signal maintains the constructive interference, which shows the coherent pulse combination, during the control status. Consequently, we obtained the stability of the compact T-DPA module as the Allan deviation of 2.97 × 10-2 (1 s.). As the experimental results, the femtosecond pulses were successfully divided and coherently combined in the cpmpact T-DPA module and it is expected that the proposed system can be applied to achieve the industrial high power femtosecond pulse lasers.

key words: coherent pulse combining, divided pulse amplification, temporal-divided pulse amplification, multi-path interferometric technique, compact T-DPA module