Vertical integration of ultrafast semiconductor lasers

Abstract

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.

On the design of electrically pumped vertical-external-cavity surface-emitting lasers

“…This is required for the integration of the absorber into the VECSEL. Such integration has been recently demonstrated with the optically pumped MIXSEL (OP-MIXSEL) [10]. For stable mode-locking operation, the saturation energy of the absorber has to be lower than for the gain [4,10].…”

“…Such integration has been recently demonstrated with the optically pumped MIXSEL (OP-MIXSEL) [10]. For stable mode-locking operation, the saturation energy of the absorber has to be lower than for the gain [4,10]. The ratio can be controlled by the field enhancement and the intrinsic absorber properties [37].…”

“…The vertical integration of the absorber into one monolithic structure with the gain quantum wells (QWs) is an important step towards higher pulse repetition rates with decreased cavity size and wafer-scale integration. This has been achieved for the first time and is referred to as the mode-locked integrated-externalcavity surface-emitting laser (MIXSEL) [10]. So far, all these devices have been optically pumped, which involves a more complex optical setup.…”

Experimentally verified pulse formation model for high-power femtosecond VECSELs

“…Vertical-external-cavity surface-emitting lasers (VECSELs) [1] passively modelocked [2] with an intra-cavity semiconductor saturable absorber mirror (SESAM) [3][4][5] and Modelocked Integrated eXternal-cavity SurfaceEmitting Lasers (MIXSEL) [6,7] benefit from the advantages of diode-pumped solid-state lasers (DPSSL), such as excellent beam quality and high-average output power at high repetition rates [2] and do not suffer from Q-switching instabilities [8]. The semiconductor gain chip enables bandgap engineering to provide a high degree of flexibility in the operation wavelength [9].…”

“…The SESAM can be integrated into the VECSEL structure, such that the gain layers and the saturable absorber layers are integrated into one single semiconductor chip, referred to as the MIXSEL structure [6]. To date, the highest average output power from a passively modelocked semiconductor laser has been obtained with such a MIXSEL generating up to 6.4 W with 28-ps pulses at a center wavelength of & 960 nm [21].…”