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Lasers generating short pulses -referred to as ultrafast lasers -enable many applications in science and technology. Numerous laboratory experiments have confirmed that ultrafast lasers can significantly increase telecommunication data rates [1], improve computer interconnects, and optically clock microprocessors [2,3]. New applications in metrology [4], supercontinuum generation [5], and life sciences with two-photon microscopy [6] only work with ultrashort pulses but have relied on bulky and complex ultrafast solid-state lasers. Semiconductor lasers are ideally suited for mass production and widespread applications, because they are based on a waferscale technology with a high level of integration. Not surprisingly, the first lasers entering virtually every household were semiconductor lasers in compact disk players. Here we introduce a new concept and make the first feasibility demonstration of a new class of ultrafast semiconductor lasers which are power scalable, support both optical and electrical pumping and allow for wafer-scale fabrication. The laser beam propagates vertically (perpendicularly) through the epitaxial layer structure which has both gain and absorber layers integrated. In contrast to edge-emitters, these lasers have semiconductor layers that can be optimized separately by using different growth parameters and with no regrowth. This is especially important to integrate the gain and absorber layers, which require different quantum confinement. A saturable absorber is required for pulse generation and we optimized its parameters with a single selfassembled InAs quantum dot layer at low growth temperatures. We refer to this class of devices as modelocked integrated external-cavity surface emitting lasers (MIXSEL). Vertical integration supports a diffraction-limited circular output beam, transformlimited pulses, lower timing jitter, and synchronization to an external electronic clock. The pulse repetition rate scales from 1-GHz to 100-GHz by simply changing the laser cavity length. This result holds promise for semiconductor-based high-volume wafer-scale fabrication of compact, ultrafast lasers.

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We demonstrate an optically pumped passively mode-locked external-cavity semiconductor laser generating 4.7-ps pulses at 957 nm with as much as 2.1 W of average output power and a 4-GHz repetition rate. Compared with earlier results, the chirp of the pulses has been greatly reduced by use of an intracavity etalon. Apart from restricting the bandwidth, the etalon also helps optimize wavelength-dependent gain parameters and dispersion.

The effect of mode-profile specific etching of the top layer in selectively oxidized VCSEL structures at 850 nm emission wavelength is examined. For high reproducibility, a selfaligned etching technique is used which aligns surface etch and oxide aperture by only one additional photoresist step. By optimizing layer structure and etch spot size, completely single-mode devices with aperture diameters up to 16 µm are obtained. Maximum singlefundamental mode output power of 3.4 mW at room temperature and over 4 mW at 0• C is obtained with a maximum far-field angle of 5.5• . Using parameters for etch spot height and diameter, Gaussian beam spot size and phase curvature, the measured diffracted farfield distribution is fitted well over a 20 dB intensity range. The chosen fit parameters therefore enable to estimate the amount of phase curvature within the VCSEL for different operation currents, which cannot be obtained with available measurement methods.

One of the most application-relevant milestones that remain to be achieved in the field of passively mode-locked surface-emitting semiconductor lasers is the integration of the semiconductor absorber into the gain structure, enabling the realization of ultra-compact high-repetition-rate laser devices suitable for wafer-scale integration. We have recently succeeded in fabricating the key element in this concept, a quantumdot-based saturable absorber with a very low saturation fluence, which for the first time allows stable mode locking of surface-emitting semiconductor lasers with the same mode areas on gain and absorber. Experimental results at high repetition rates of up to 30 GHz are shown.PACS 42.55.Px; 42.60.Fc; 42.82.Gw

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