High-power passively mode-locked semiconductor lasers

Abstract

We have developed optically pumped passively mode-locked vertical-external-cavity surface-emitting lasers. We achieved as much as 950 mW of mode-locked average power in chirped 15-ps pulses, or 530 mW in 3.9-ps pulses with moderate chirp. Both lasers operate at a repetition rate of 6 GHz and have a diffraction-limited output beam near 950 nm. In continuous-wave operation, we demonstrate an average output power as high as 2.2 W. Device designs with a low thermal impedance and a smooth gain spectrum are the key to such performance. We discuss design and fabrication of the gain structures and, particularly, their thermal properties. Index Terms-Mode locking, pulse generation, semiconductor lasers, thermal effects in lasers.

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

“…6a and 7. Thus, a moderately doped thick FIGURE 7 Carrier distribution along the radial axis in the active region. The device exhibits a bottom contact radius r 3 = 25 µm, an aperture radius r 2 = 50 µm, and a bottom p-DBR.…”

“…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.…”

Modelocked quantum dot vertical external cavity surface emitting laser

“…We did not optimize thermal management because this experiment was a proof-of-principle for modelocking a QD-VECSEL. Thus the 450 µm thick GaAs substrate has not been removed, and the average power is limited by thermal heat sinking [4]. The advantage of this structure is that almost no additional processing is required before the device can be used in the laser cavity.…”

“…For our experiment, the QD-VECSEL structure was directly soldered onto a heat sink, and no further steps were taken towards improved thermal management [4]. During modelocked laser operation, the heat sink temperature was set to −20 • C.…”

Vertical-external-cavity semiconductor lasers

“…The thermal conductivity of the ternary alloy can be as low as 11 W m −1 K −1 for compositions close to Al 0.5 Ga 0.5 As. A similar approach has enabled Häring et al [8] to demonstrate 2.2 W in cw operation in a near-diffraction-limited beam. An effective technique that extracts heat directly from the active region, rather than through the DBR, was introduced by Alford et al [9], who put an uncoated sapphire window incontact with the front surface of a VECSEL gain element, where it acted as a heat-spreading plate.…”