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

Abstract

Vertical-external-cavity surface-emitting lasers (VECSELs) yield an excellent beam quality in conjunction with a scalable output power. This paper presents a detailed numerical analysis of electrically pumped VECSEL (EP-VECSEL) structures. Electrical pumping is a key element for compact laser devices. We consider the optical loss, current confinement, and device resistance. The main focus of our investigation is on the achievement of an adequate radial carrier distribution for fundamental transverse mode operation. It will be shown that a trade off between the conflicting optical and electrical optimization has to be found and we derive an optimized design resulting in guidelines for the design of EP-VECSELs which are compatible with passive mode locking. Vertical-external-cavity surface-emitting lasers (VECSELs) [1] are of high scientific and industrial interest due to their large fundamental transverse mode output power, scaling with the device radius, the near diffraction limited output beam, and the suitability for intracavity frequency conversion [2] and passive mode locking [3,4]. Passive mode locking of an optically pumped VECSEL has been demonstrated with a semiconductor saturable absorber mirror (SESAM) [5] in a folded external cavity. After the first passively mode locked VECSEL was demonstrated in the year 2000 [6], the milestone of nearly 1 W average output power was achieved in 2002 with improved thermal management [7]. The pulses in the early experiments were often strongly chirped, but mode-locking dynamics in VECSELs revealed that a soliton-like pulse-shaping mechanism in the positive-dispersion regime can help to generate short pulses with low chirp. With the aid of intracavity dispersion control, it then became possible to obtain nearly transform limited picosecond pulses with record high output powers of 2.1 W at 4 GHz [4] and 1.4 W at 10 GHz [8]. The pulse width could be u Fax: +41-44-633-10-59, E-mail: keller@phys.ethz.ch significantly shortened to the sub-500-fs regime using faster SESAMs based on the ac Stark effect [9]. 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. Thus, an important step towards waferscale fabrication of compact ultrafast VECSELs is the design of electrically pumped VECSEL (EP-VECSEL) structures. This is challenging because of optical losses and Joule heating in the doped layers. Nevertheless, continuous-wave (cw) EP-VECSELs have been demonstrated [11,12] and output powers of up to 900 mW have been obtained with a 150-µm device diameter in multimode operation [13]. Also, mode locking with down to 15-ps FWHM pulse width [14] and a wafer-scale EP-VECSEL w...

Gain characterization and passive modelocking of electrically pumped VECSELs

“…The two chips exhibit a thickness of 59.8 μm (9 pairs) and 148.3 μm (13 pairs). This difference in thickness is due to an optimization in the processing scheme of the devices, the influence of the thickness has already been discussed based on numerical simulations in [20]. This means that the 13-pair device has a higher loss due to free carrier absorption.…”

“…Current is injected into the device through a disk-shaped bottom contact (140 μm in diameter) underneath the p-doped, highly reflecting distributed Bragg reflector (DBR) and through a top ring electrode (opening of 200 μm in diameter). It is assumed that the holes in the p-doped region have a lower mobility compared to the electrons in the n-doped region and remain in the center of the device above the bottom contact, whereas the highly mobile electrons move through the current spreading layer to the center of the structure with the quantum well (QW) gain layers where they recombine with the holes [20]. Thus, the current spreading layer together with the bottom disk contact helps to create a confined inversion profile to support a fundamental lasing mode.…”

“…The structure is mounted with the p-side down on an AlN heatsink tile, which is soldered on a TO-can mount for easy electrical testing. More details on the design and fabrication of EPVECSELs are discussed in [13,20,22].…”

Semiconductor Disk Lasers: Recent Advances in Generation of Yellow-Orange and Mid-IR Radiation

“…If the power level is not the main target, electrically pumping of SDLs, more often termed as EP-VECSELs, can be realized conveniently [116]. The essential challenges of electrical pumping relate to nonuniform current spreading and optical losses in doped semiconductor material [117][118][119]. Doping is necessary for achieving low electrical resistance, but at the same time it does increase absorption.…”

Ultrafast solid-state laser oscillators: a success story for the last 20 years with no end in sight

“…We are currently exploring the possibilities to extend these concepts towards electrical pumping [35] and femtosecond pulse generation. The compactness and the simplicity of the MIXSEL platform appears well-suited for cost-efficient mass production [31] and has the potential to provide ultrafast lasers for applications where the current ultrafast laser technology is still considered to be too expensiveeven though we made tremendous progress in this regard with SESAM-modelocked diode-pumped solid-state lasers, which ultimately enabled modelocked lasers in industrial mass production.…”