VECSEL (semiconductor disk laser)

A VECSEL (vertical-external-cavity surface-emitting laser), also called a semiconductor disk laser or optically pumped semiconductor laser (OPSL), is a surface-emitting semiconductor laser whose cavity is completed by an external mirror millimetres to centimetres from the chip. The gain chip is a stack of quantum wells grown on top of a high-reflectivity distributed Bragg reflector; a fiber-coupled 808 nm diode pumps it at an angle, and the external mirror, usually curved, defines a stable fundamental mode. The emission wavelength is set by the quantum well composition, so the same technology covers roughly 0.9–1.2 µm on GaAs, with other material systems extending it into the red and the 2 µm region. Continuous-wave output of 1–20 W in a near-TEM00 beam is typical of well-developed InGaAs devices around 1 µm, and intracavity second-harmonic generation gives visible lines such as 488 nm and 577 nm that are hard to reach with other compact lasers.

Chip structure

The distinction from a VCSEL is the cavity: a VCSEL has two monolithic Bragg mirrors a wavelength or so apart and a mode diameter of a few micrometres, which limits single-mode power to milliwatts. In a VECSEL the mode size is set by the external cavity and can be 100–500 µm wide on the chip, matching the pump spot, so power scales with area while the beam stays single transverse mode.

The quantum wells are placed at the antinodes of the standing wave, a scheme called resonant periodic gain. The antinode spacing is $\lambda/(2n)$; at 1000 nm in GaAs-based material with $n \approx 3.5$, that is

$$\frac{1000\ \text{nm}}{2 \times 3.5} \approx 143\ \text{nm}.$$

Typical chips have 6–14 wells, with barrier and spacer layers that absorb the pump. The DBR under the gain region, commonly 20–30 AlGaAs/GaAs pairs, reflects above 99.9% at the laser wavelength. Because the gain per pass is only a few percent, the output coupler transmission is also a few percent at most.

Heat and power scaling

Heat removal limits power, as in the thin-disk laser, whose geometry the VECSEL resembles. Two approaches are common: the substrate is removed and the chip is bonded DBR-side down onto a copper or diamond heat sink, or a transparent diamond heat spreader is bonded to the top surface. The quantum defect alone is a significant heat load: pumping at 808 nm for emission at 1060 nm,

$$1 - \frac{808\ \text{nm}}{1060\ \text{nm}} = 0.24,$$

so 24% of the absorbed pump power becomes heat before any other losses are counted, and roughly 2.4 W of every 10 W absorbed. Barrier absorption, non-radiative recombination, and DBR absorption add to it. The gain peak shifts with temperature faster than the microcavity resonance, so chips are designed with a deliberate detuning that aligns them at the operating temperature. Thermal lensing in the thin chip is weak compared with bulk solid-state lasers, which helps maintain the TEM00 mode at high pump power.

Mode locking and single frequency

The short carrier lifetime, about a nanosecond, means a VECSEL does not store energy for Q-switching; it is well suited to passive mode locking with a SESAM in the external cavity, producing picosecond and femtosecond pulse trains at repetition rates from about 1 GHz to tens of GHz. Integrating the absorber into the gain chip gives the MIXSEL, a mode-locked laser with a single semiconductor element. With an intracavity birefringent filter and etalon, VECSELs run single-frequency with narrow linewidths, which suits laser cooling and spectroscopy after doubling.

Pitfalls

Pump absorption, gain peak, and microcavity resonance must all align at the working temperature, so output can fall steeply if the heat sink runs hotter than designed. The DBR stop band, typically 50–100 nm wide, limits tuning. Surface contamination or bonding voids on the chip produce hot spots, and the external cavity needs mechanical stability comparable to other free-space lasers.

Common questions

Is a VECSEL the same as an OPSL?

Yes, for practical purposes. OPSL is the name often used for commercial optically pumped devices; VECSEL and semiconductor disk laser are the terms common in the research literature. Electrically pumped VECSELs exist but are less developed.

Why is optical pumping used instead of current?

Injecting current uniformly across a mode several hundred micrometres wide is difficult, and doped contact layers add optical loss. An 808 nm pump beam absorbed in the barrier layers distributes carriers evenly over the pumped area.

References: M. Kuznetsov, F. Hakimi, R. Sprague, A. Mooradian, IEEE Photon. Technol. Lett. 9, 1063 (1997); A. C. Tropper et al., "Vertical-external-cavity semiconductor lasers," J. Phys. D 37, R75 (2004); Siegman, Lasers 1986.