Vertical-external-cavity surface-emitting lasers (VECSELs) have emerged as practical contenders for applications that require high continuous-wave output power and high-quality, circularly symmetric output beams. VECSELs combine the power scalability and near-diffraction-limited output of a diode-pumped, solid-state laser with the spectral versatility of a semiconductor quantum-well gain medium. High-power VECSELs are usually pumped optically, exploiting the mature diode pump-laser technology developed for solid-state systems. As frequency-doubled sources that generate blue radiation, VECSELs offer a compact and rugged alternative to the argon-ion laser for the excitation of fluorophores in bio-analytical applications.1
The architecture of this laser can make particularly good use of the broad quantum-well gain bandwidth for the generation of ultrashort pulses, and can easily achieve gigahertz-level repetition rates. In mid-2003, our group reported a 1030-nm device that emitted near-transform-limited 486-fs pulses at a repetition rate of 10.014 GHz.2 This passively mode-locked VECSEL showed no tendency toward the Q-switching instabilities suffered by solid-state lasers that are mode-locked at high repetition rates. The 30 mW average power of this device compares favorably with the typical output power from a mode-locked edge-emitting diode laser. Even better, the large emitting aperture of the VECSEL offers potential for power scaling that the edge-emitter does not. The pulses from this device, at 1.04 X transform-limited, were therefore also "cleaner" than the chirped pulses characteristic of mode-locked edge-emitting lasers.
Our device is a GaAs-based gain structure grown by metal-organic chemical vapor deposition (MOCVD). It contains six compressively strained InGaAs quantum wells, sandwiched between GaAsP, strain-balancing barrier layers and spaced at half-wavelength intervals by layers of GaAs. The active region is enclosed between a 27-repeat GaAs/AlAs Bragg mirror and a 1.45 X λ/4 AlAs window layer, finished by a GaAs capping layer to prevent oxidation of the exposed surface (see figure 1). The mirror can be pumped by 1 W of 830-nm radiation from a fiber-coupled diode, imaged onto a spot of 60-µm radius. The barrier and spacer layers absorb this pump radiation, creating carriers that are then trapped in the wells, to generate gain in the region of 1 µm. The gain structure forms the fold mirror in the asymmetric V-cavity, so that circulating light is amplified twice in each round trip.
Figure 1. A passively mode-locked VECSEL, operating at 10.014 GHz, consists of a GaAs-based gain structure and a saturable absorber mirror.
The laser exhibited spontaneous mode-locking under the influence of the semiconductor saturable absorber mirror (SESAM) that forms one end of the cavity. In collaboration with researchers from the Swiss Federal Institute of Technology (Zurich, Switzerland), our group has established that VECSELs readily exhibit passive mode-locking when a suitable VECSEL cavity incorporates a SESAM of the type used as solid-state laser mode-locking elements. The Zurich group developed a low-temperature molecular-beam epitaxy (MBE) technique to create quantum-well SESAMs with the fast absorption recovery essential for strong pulse shaping and high repetition rates. Low-temperature MBE-grown SESAMs have been used in VECSEL cavities to generate mode-locked pulses of durations as short as a few picoseconds. It is essential in such a device that the cavity mode is more tightly focused on the SESAM than on the gain structure, because both gain and absorption arise from quantum wells, with similar saturation fluences.
Our SESAM design uses a different growth technique to make a fast saturable absorber. We grew it by conventional MOCVD at high temperature. We achieve fast carrier recombination by placing the single InGaAs quantum well within 2 nm of the air surface of the device. Band-bending effects at the surface assist the carriers in tunneling out of the well to the defect-rich surface region, where they recombine in about 20 ps. By choosing a subcavity thickness of 0.68 X λ/4, we set the modulation depth at the desired value of 1%.
We designed the structure to operate on the long-wavelength side of the quantum well exciton resonance. In this regime, the nonlinear response is augmented by the optical Stark effect, a nonlinearity that is fast because it does not involve the generation of real carriers. Moreover, the high-temperature MOCVD growth creates a SESAM free from the neutral As anti-site defects that appear in low-temperature MBE growth, with associated nonsaturable losses. The VECSEL is an inherently low-gain laser, and its efficiency is correspondingly sensitive to cavity loss.
These gain structure and SESAM designs initially generated pulses lasting less than 500 fs in a relatively long cavity, operating at a repetition rate of 1.2 GHz.3 In order to operate in this regime, the laser requires careful control of cavity dispersion. VECSELs designed primarily for high-power operation are very often operated near a high-finesse resonance of the optical sub-cavity around the active region, so as to benefit from enhanced effective gain. The drawback of this design for mode-locked operation is that in the region of such a resonance, the group delay dispersion (GDD) of the active mirror switches rapidly with wavelength from a negative to a positive value, typically changing by some thousands of square femtoseconds over a few nanometers. This large, high-order dispersion imposes nonlinear chirp on the pulses, which may become many times transform-limited. The gain and SESAM structures described here are therefore designed close to antiresonance, so that the cavity has a small overall GDD, varying from 0 to +1000 fs2 over a design wavelength range from 1020 to 1040 nm. Fast and Stable
A striking contrast between a laser of this type and a SESAM-mode-locked solid-state laser appears in the behavior at high repetition rates. A broadband, solid-state laser gain medium such as ytterbium-doped yttrium aluminum garnet or erbium-doped glass has a low emission cross-section, with correspondingly high saturation fluence. At higher repetition rates, with less fluence in the intracavity pulse, the mode-locking of such lasers becomes unstable and Q-switching can occur, with possibly damaging consequences.
In contrast, our VECSEL has a high differential gain and a low saturation fluence, and is free from Q-switching instabilities even with pulses of low intracavity energy. Our group was therefore able to reproduce the generation of sub-500-fs pulses in the short (15-mm-long) cavity with a fundamental repetition rate of better than 10 GHz. The chief challenge in designing such a short cavity was obtaining a mode area on the SESAM sufficiently small relative to the mode area on the gain structure, which is fixed by the available pump source. We therefore designed the cavity close to its stability limit, with a mode area on the SESAM approximately 20 times smaller than that on the gain structure.
The laser generated 486-fs pulses at about 1032 nm (see figure 2). The time-bandwidth product is 0.328, corresponding to a hyperbolic secant pulse at only 1.04 X transform-limited, despite some modulation on the optical spectrum. Estimates of the nonlinear phase shifts in the laser cavity under these operating conditions suggest that a quasi-soliton mechanism contributes to the mode-locking of this laser. In this mechanism, the cavity dispersion is balanced not by a Kerr nonlinearity, but by the phase shifts associated with saturation in the gain and SESAM structures. Rüdiger Paschotta at the Swiss Federal Institute of Technology has used a numerical propagation model to show that mode-locked VECSELs can support stable pulses of this type, and that the quasi-solitons will exhibit some scaling properties analogous to those of conventional solitons.
Figure 2. The VECSEL emitted pulses with an average power of 30.3 mW at 10.014 GHz, with an optical spectrum around 1034 nm (inset).
Although VECSELs have chiefly attracted interest to date as frequency-doubled blue sources, their potential as practical and inexpensive ultrashort pulse sources is equally great. VECSELs have been demonstrated to date in six spectral regions: 660 nm, 850 to 870 nm, 960 to 1030 nm, 1052 nm, and 2100 nm. Although the ultrashort solitons described above have so far only been generated using a GaAs:InGaAs laser, nothing inherent to the technique restricts it to a specific material system.
We have recently demonstrated a picosecond mode-locked VECSEL operating at 1.5 µm.4 The lasers can readily attain the high repetition rates that applications such as high-bandwidth communication, switching, sampling, and clocking require. Moreover, these lasers have proven power-scaling capability: Paschotta and his team have demonstrated a mode-locked VECSEL operating with nearly 2 W of average power, and a CW VECSEL emitting 8 W has recently been reported by researchers at Osram Opto Semiconductors GmbH (Regensburg, Germany). oeReferences
1. E. Wahl, B. Richman, et al., Optics & Photonics News 14, p. 36 (2003).
2. At the 2003 CLEO-US conference.
3. A. Garnache, S. Hoogland, et al., Applied Physics Letters 80, p. 3892 (2002).
4. S. Hoogland, A. Garnache, et al., Electronics Letters 39, p. 846 (2003).
Anne Tropper, Sjoerd Hoogland
Anne Tropper is a professor of physics and Sjoerd Hoogland is a post-doctoral research assistant at the University of Southampton, Southampton, UK.
A Career That Started with a 'Bang!'
Many people wait for a sign when deciding on a careersomething that grabs their attention and captures their imagination. For laser physicist Anne Tropper, a lifetime of scientific pursuit began with an electric explosion and a house in flames. "My parents were both academics at the University of London. My father had founded a high-voltage laboratory at Queen Mary College, and I have early memories of going to the demonstration lectures that he used to give to the general public around Christmas time," Tropper recalls. "Enormous sparks and electrical dischargesutterly thrilling to a child of any age, and slightly terrifying too. I particularly remember a high-voltage 'cloud'actually made of cotton woolpassing safely over a model wooden church that was protected by a lightning conductor, but causing an unprotected model house to burst into flames under an apocalyptic bolt of charge that also set fire to the 'cloud.' It was clear to me from a very young age that science was the most exciting game around."
Tropper completed her undergraduate degree in physics at the University of Oxford (Oxford, UK), followed by a doctorate in physics (1978) under the supervision of Mike Leask. Although Leask's laboratory was not quite as dramatic as her father's high-voltage lab, it did contain a 2-kA solenoid, powered by an immense motor-generator set that, in the early years of the 20th century, had supplied the Manchester tramway system with electricity.
Tropper landed a Lindemann Fellowship that took her to what is now IBM's Almaden Research Laboratory (San Jose, CA), before returning to the University of Southampton (Southampton, UK) in 1983, where she converted from spectroscopist to laser physicist. "The world-class fiber fabrication at Southampton in David Payne's Optical Fiber Group enabled us to make pioneering studies of lanthanide-doped silica fiber lasers," she says. "I was a founding member of the Optoelectronics Research Centre at Southampton, formed in 1989, and later worked on upconversion fluoride fiber lasers, planar crystal waveguide lasers, and, most recently, on semiconductor lasers."
Tropper was appointed personal chair in the University of Southampton's School of Physics and Astronomy in 2000 and currently serves as the head of the department. She is married with three children, ages 17, 14, and 8.