Diode-Pumped Yb:YVO4 Laser Generates Femtosecond Pulses
Scientists at Belarus National Technical University and at Solix Ltd., both in Minsk, Belarus, and at the Swiss Federal Institute of Technology in Zurich have demonstrated -- for what they believe is the first time -- continuous-wave mode-locking of an Yb:YVO4 laser. The result is noteworthy because Yb:YVO4 is a new laser material with several desirable properties -- notably, a high thermal conductivity, a broad emission bandwidth and a very low quantum defect -- and may prove to be the material of choice in commercial mode-locked thin-disk lasers.
Yb:YAG currently is the favored material for such high-power lasers, and very high mode-locked outputs have been obtained with this material. Its drawback is its relatively narrow emission bandwidth, which sets a lower limit for mode-locked pulse duration at many hundreds of femtoseconds. Much shorter pulses, often less than 100 fs, have been reported from ytterbium lasers in crystalline hosts such as KGW and BOYS, and from Yb:glass lasers. But these materials have low thermal conductivity and, hence, are not suitable for high-power operation.
Two new materials, Yb:CaF2 and Yb:YVO4, combine high thermal conductivity with a broad emission bandwidth, offering the potential for high-power, sub-100-fs, mode-locked lasers. Observing that the emission spectrum of Yb:YVO4 is smoother than that of Yb:CaF2, the collaboration focused its investigation on that material.
Figure 1. A drawback of a low quantum defect in an end-pumped laser is that the laser mirror rejects a significant portion of the pump radiation. In this case, only 60 percent of the pump radiation was transmitted through the mirror after the laser diode. Images ©OSA.
A semiconductor saturable absorber mirror (SESAM) replaced one of the end mirrors in the Yb:YVO4 laser's layout (Figure 1). The SESAM, which comprised a single 15-nm InGaAs quantum well and a Bragg reflector at 1040 nm, passively mode-locked the laser. Although a low quantum defect -- in this case, 3.9 percent -- is advantageous because it minimizes the heat dissipated in the laser crystal (see "Good Quantum Defects Make Good Lasers," page 113), there can be drawbacks. In this case, the laser was longitudinally pumped through one of the high-reflectance mirrors, and the proximity of the pump and laser wavelengths dictated that the mirror reject 40 percent of the incident pump power.
Figure 2. Mode-locked with a semiconductor saturable absorber mirror, the Yb:YVO4 laser generated 120-fs pulses with a time-bandwidth product of 0.348, close to the transform limit of 0.315. The plot on the left is the intensity autocorrelation with a sech2 fit, and the plot on the right is the optical spectrum.
More of the unpolarized pump power was lost at the Brewster face of the laser crystal, so that in the end, only about a third of the 7.5 W of 980-nm pump power produced by the laser diode was absorbed in the 2-mm-thick, 3-percent-atomic doped laser crystal. A pair of intracavity prisms separated by 45 cm achieved the negative group dispersion required for soliton mode-locking.
The laser produced 120-fs mode-locked pulses with a peak power of 14.5 kW and a time-bandwidth product of 0.348, close to the transform limit of 0.315 (Figure 2). The mode-locking frequency was 150 MHz, the average power was 300 mW, and the pulses were centered at a wavelength of 1021 nm. The scientists believe that the laser could generate sub-100-fs pulses if its negative group dispersion and the SESAM's modulation depth were optimized.
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