Stable multiwatt lasers with gigahertz pulsed outputs have potential application in telecommunications and in optical clocking, but conventional solid-state lasers, which are capable of such performance, are too large and too expensive for these applications. A team of researchers from the Swiss Federal Institute of Technology in Zurich has designed and demonstrated a mode-locked, vertical-cavity external-mirror semiconductor laser capable of producing 2.1 W in a stable, 4-GHz pulse train. This performance is as good as that achievable with a traditional, diode-pumped solid-state laser.Mode-locked vertical external-cavity surface-emitting lasers (VECSELs) have the potential to be smaller, less complex and less expensive than conventional mode-locked solid-state lasers such as, for example, diode-pumped Nd:YAGs and Yb:YAGs. In addition, they provide a wider wavelength tuning range, and because they have a broad gain bandwidth, can produce mode-locked pulses with durations of a few picoseconds or even well below a picosecond. The ultimate advantage, however, is the possibility of integrating gain and saturable absorber into a single monolithic device, as they are fabricated essentially from the same semiconductor materials.The researchers pumped their laser, whose output wavelength was 960 nm, at 45° incidence with an 808-nm, fiber-coupled laser diode (Figure 1). The resonator was defined by an external, 2.5 percent transmitting output mirror and a semiconductor saturable absorber mirror, which comprised an 8.5-nm-thick quantum well and provided a modulation depth of approximately 1 percent, sufficient to mode-lock the laser. With an 808-nm input power of 18.9 W focused to a 175-nm radius on the surface of the semiconductor gain structure, the laser produced 2.1 W in its 4-GHz output pulse train.Figure 1. The mode-locked, surface-emitting semiconductor laser produced 2.1 W in a 4-GHz pulse train. An intracavity etalon (not pictured) served to select a regime of net positive group delay dispersion to achieve minimum pulse chirp.An inherent drawback of using a saturable absorber to mode-lock a laser is that, as its absorption saturates, its refractive index changes, and this time-dependent change in the refractive index imposes a time-dependent phase change on the pulse passing through the absorber. In the case of a mode-locked VECSEL, the pulse also saturates the laser gain, producing another time-dependent phase change. As a consequence, many previous mode-locking results with these lasers suffered from strongly chirped pulses with rather large bandwidth, although considerable output power was obtained. The increased bandwidth is a serious drawback in many applications, because as the pulse propagates through a dispersive medium, such as an optical fiber, it will experience significant stretching.To control the pulse chirp, the researchers inserted a 20-µm-thick, uncoated, fused-silica etalon into the resonator. According to a quasi-soliton pulse-formation mechanism, a net positive group delay dispersion is necessary to obtain transform-limited pulses when mode-locking VECSELs. The main contribution arises from the laser itself, which behaves as a Gires-Tournois interferometer. Figure 2. The measured autocorrelation trace of the mode-locked pulses agreed very closely with the ideal shape. Inset: The spectral width of the pulses was approximately twice the transform limit.Therefore, the net group delay dispersion can be adjusted simply by choosing the lasing wavelength using a wavelength-selective element such as an etalon. Although it is difficult to implement a comprehensive model -- e.g., including temperature effects -- the general prediction of wavelength regimes having large differences in pulse chirp was confirmed using the technique.By tuning the operation wavelength with the etalon, the researchers experimentally found configurations that produced stable output with significantly reduced pulse chirp. When the etalon was at near-normal incidence, they observed the 2.1-W, 4-GHz output with 4.7-ps pulses whose bandwidth was approximately twice the transform limit (Figure 2). This is a significant improvement over previously reported high-power, mode-locked VECSELs whose bandwidths have been 15 to 20 times the transform limit.