Efficient and Robust Kerr-Lens Mode-Locking of Cr:LiSAF Lasers Using Gain-Matched Output Couplers

Abstract

Abstract:We have demonstrated robust and efficient Kerr-lens mode-locked operation of a lowcost, diode-pumped Cr:LiSAF laser using a gain-matched output coupler. Sub-30-fs pulses with 80-pJ pulse energy have been demonstrated with 80 mW of pump power. IntroductionKerr-lens mode-locking (KLM) is a very well-known and versatile technique that is widely used for the generation of ultrashort pulses from broadband gain media such as Ti:Sapphire [1, 2], Cr:ZnSe [3], Cr:Forsterite, and Cr:LiSAF [4][5][6][7][8]. KLM generates an artificial saturable absorber with an instantaneous lensing effect that shortens the circulating intracavity pulsewidth via self-amplitude modulation (SAM). Hence, unlike saturable Bragg reflectors, KLM does not impose any bandwidth limitation effect and the obtainable pulsewidth is limited only by the gain bandwidth of the lasing material and dispersion bandwidth of the laser cavity. To achieve the shortest pulses: (i) flat dispersion profile over the whole gain bandwidth should be utilized, and (ii) strong gain filtering effect of the active medium should be overcome. To fulfill the second requirement, a large SAM is desired, which necessitates the use of low output coupling, high pump powers, critical cavity alignment, and, in some cases, operation near the edge of the stability range. This may result in reduced output beam quality, lower laser efficiency, high mode-locking threshold, and stability problems in long-term operation. These issues are even more significant in gain media with a low nonlineer refractive index (n 2 ) such as Cr:LiSAF (n 2 =0.8 x10 -16 cm 2 /W, four times lower than that for Ti:Sapphire [9]). Even though pulses as short as 10 fs have been demonstrated from low-cost diode-pumped Cr:LiSAF lasers [4], the ultrafast technology based on KLM Cr:LiSAF lasers could not be developed beyond laboratory demonstrations due to the above mentioned problems,.To overcome the limitations imposed by KLM, the gain filtering effect should be minimized. In that respect, Chen et al. recently suggested the use of gain matched output couplers (GMOC) with a transmission profile that matches the gain spectrum of the laser active medium [10]. This creates a loss profile that has the same shape as the cavity gain, which results in a broadband flat net gain profile. Then, this eliminates the need for strong SAM, and a low level of KLM action is sufficient to sustain ultrashort pulses. By employing a 4% GMOC in a Ti:Sapphire laser, Chen et al. demonstrated robust mode-locking at greatly-reduced Kerr nonlinearity requirements [10], and the laser produced sub-8-fs pulses with excellent beam quality.In this study, we employed a gain matched output coupler to demonstrate robust KLM operation of a diodepumped, low-cost Cr:LiSAF laser with long-term stability. The laser is pumped with a 130 mW diode having a diffraction-limited output beam at the wavelength of around 660 nm. KLM operation did not require careful cavity alignment and could be initiated at incident pump powers as low as 50 mW. Once initi...