Femtosecond laser

High-energy femtosecond laser

List of Researchers

Seong-Hoon Kwon

국문요약

1991년 커렌즈 모드잠금 기술이 개발됨에 따라 펨토초 레이저는 물리, 생명, 화학 등 여러분야에서 널리 사용되고 있으며, 초고속 이미징, 광통신, 가공, 양자광학 분야에 활용되면서 높은 에너지를 가진 펄스가 요구되고 있다. 일반적인 펨토초 레이저는 반복률이 70-100MHz 에 sub-20 fs 펄스폭을 지닌 수 nJ의 에너지를 발진시킨다. 이에 높은 에너지를 가진 펄스를 발진시키기 위해서는 높은 출력을 지닌 레이저로 펌핑하거나 증폭단을 통해 증폭함으로써 달성할 수 있으며 최근에는 공진기 내에 광변조기를 삽입함으로써 반복률이 1MHz 에서 1μJ의 에너지를 발진하였다. 그러나 이러한 방법등은 고가의 레이저, 증폭단, 광변조기를 필요로 한다. 또 다른 방법으로는 공진기의 길이를 늘림으로써 펄스 에너지를 높일 수 있다.

Self-focusing 효과에 의해 발생되는 커(Kerr) 모드잠김 방법을 이용한 Herriott multipass cavity(HMC)는 펨토초 레이저의 공진기 구조를 간단하게 하면서, 전체길이를 수십미터까지 늘일 수 있으므로 첨두 출력을 기존의 펨토초 레이저보다 현저히 높일 수 있는 장점을 가지고 있다. 본 연구에서는 홀이 뚫린 두 개의 거울을 이용한 HMC 기반의 soft-aperture 펨토초 레이저 오실레이터를 제작하였다. 제작된 HMC 펨토초 레이저는 반복률이 13.57 MHz 에서 162 nJ, 26 fs 펄스를 발생시킨다.

Time-scale

1 picosecond= 1 ps= 10^-12s

1 femtosecond= 1 fs= 10^-15s

1 attosecond= 1 as= 10^-18s

In vacuum, light travels 1 mm in 3.3 ps. To capture the motion of light, it requires a camera with a shutter opening time of less than the pulse duration.

Our research

Since the advent of Kerr-lens mode locking, femtosecond lasers have become widely used for basic research and industrial fields. In particular, high energy pulses at MHz level of the repetition rates are recently desirable for applications such as high harmonic generation, ionization, micromachining, etc. However, standard femtosecond laser oscillators generate pulses at tens of MHz repetition rate, and their pulse energies are below 10 nJ level. Generally in order to increase the output pulse energy, amplifiers and cavity dumpers are utilized. But these introduce complex configuration, operate at kHz repetition rates, and increase overall cost considerably. Accordingly, a cavity extension represents a straightforward procedure that addresses such problems, simultaneously reducing a laser repetition rate and increasing pulse energy.

The technique has recently been widely used to obtain high pulse energy directly from femtosecond laser oscillators, which provides zero effective length via the q-preserving after a given number of complete cavity transits. In addition, if an HMPC consists of notched mirrors for ease of beam entrance and exit without compensators, the laser cavity configuration can be simplified.

The development and measurement of a soft-aperture Kerr-lens mode-locked, high-pulse-energy, femtosecond Ti:sapphire laser oscillator in the negative dispersion regime, based on the HMPC consisting of two notched mirrors, advantageously possessing simplicity and easy alignment are shown respectively, which are in more detail in Refs.

Fig. The schematic of x-folded femtosecond oscillator laser derived from the HMPC consisting of two notched mirrors. The laser comprises a standard cavity with a short arm of 69.6 cm and a long arm of 105.9 cm, and additional HMPC created by use of a standard cavity-generated beam. CMs, curved mirrors; Ms, dielectric mirrors; Ps, prisms, OCs, output coupler on a fused silica wedge; L, pump lens; Ti:S, Ti:sapphire gain medium; L0, separation of M1and M2; n, roundtrip number.

Fig. (a) Mode-locked pulse train with a period of 76 ns, corresponding to the repetition rate of 13.16 MHz, (b) interferometric autocorrelation measurement with 26 fs pulse duration, and (c) associated spectrum with 39 nm bandwidth at the negative dispersion (– 600 fs2).

Publication

[1] Dong Hoon Song, Sung In Hwang, Yong Ho Cha, Kyung Nam Kim, Young Uk Jeong, and Do-Kyeong Ko, "Herriott multipass-type Kerr-lens mode-locked femtosecond laser oscillator", J. Korean Phys. Soc., 59, 2241 (2011).

[2] Dong Hoon Song, Sung In Hwang, and Do-Kyeong Ko, "Compensation of incomplete round trip in an Herriott multipass-based Kerr-lens mode-locked Ti:sapphire oscillator via an output coupler position", Jpn, J. Appl. Phys.. 50, 032705 (2011).

[3] Dong Hoon Song, Sung In Hwang, Do-Kyeong Ko “Dynamics of Sub-microjoule Femtosecond Pulse Formation in a Negative Dispersion Regime” Journal of the Korean Physical Society Vol. 61, No. 5 September (2012).

[4] D H Song, W B Cho, H W Lee, D H Shin, D-K Ko and M Y Jung “A compact Kerr-lens mode-locked Ti:sapphire oscillator with 330 nJ soliton-like pulses” Laser Phys. Lett. 10(6), 065003, April (2013).