Pulse Formation and Stability of a SESAM Mode-locked Laser Depending on the SESAM Position
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Pulse Formation and Stability of a SESAM Mode-locked Laser Depending on the SESAM Position Seong-Hoon Kwon
and Do-Kyeong Ko
∗
Department of Physics and Photon Science, Gwangju Institute of Science and Technology, Gwangju 61005, Korea (Received 23 August 2020; revised 13 October 2020; accepted 23 October 2020) By using split-step Fourier method, we conducted the simulation of pulse formation in two SESAM mode-locked lasers. One is a conventional laser with the SESAM at the cavity end. The other is constructed by moving the SESAM of the conventional laser to the middle of the cavity. In the laser with the SESAM in the middle of the cavity, since the pulse meets the SESAM twice during one round-trip, the pulse shaping also occurs twice by the saturable absorption induced at the SESAM. For this reason, in net positive dispersion regime, the chirped-pulse propagating in the laser with the SESAM in the middle of the cavity was shorter than the chirped-pulse that experiences the pulse shaping per one round-trip by the SESAM in the conventional laser. In the operation regime of net negative dispersion, the soliton-like pulse shaping is a more dominant process in pulse shortening than the pulse shaping by the SESAM, therefore, there was no big difference between the steadystate pulse profiles of both lasers regardless of the SESAM position. However, it was found that the pulse contrast is better and the initial pulse changes to a steady-state pulse faster when the SESAM is in the middle of the cavity than at the cavity end. Since the enhanced cavity loss by the SESAM in the middle of the cavity suppresses pulse destabilization more effectively, it is demonstrated that the pulse stability against cw generation is better in the laser with the SESAM in the middle of the cavity than at the cavity end. Keywords: SESAM mode-locked lasers, Mode-locking, Split-step Fourier method DOI: 10.3938/jkps.77.1153
I. INTRODUCTION Over the past few decades, ultrafast solid-state lasers have attracted a great deal of attention due to the high peak-power and extremely short pulse duration. In particular, since the ultrashort pulses are easily achieved by passive mode-locking compared to other methods such as Q-switching and cavity dumping, various mode-locked solid-state lasers have been preferred for many applications including optical frequency combs, micromachining, and ionization [1–3]. The characteristics of passively mode-locked pulses depend on the cavity configuration [4, 5], but the pulse formation and shortening commonly follow two dominant mechanisms which are called self-amplitude modulation (SAM) and soliton-like pulse shaping [6, 7]. The SAM suppresses the build-up of noise-like signals and provides an intensity-dependent net-gain window supporting the pulse formation. Kerr-lens mode-locking (KLM) has been used as an efficient SAM technique [8–10]. But it imposes restrictions on the choice of a gain medium and the cavity structure. Saturable absorbers can strengthen the SAM effect and they have ∗ E-mail:
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pISSN:0374-4
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