Valentin J. Wittwer

High-power MIXSEL: an integrated ultrafast semiconductor laser with 6.4 W average power

High-power ultrafast lasers are important for numerous industrial and scientific applications. Current multi-watt systems, however, are based on relatively complex laser concepts, for example using additional intracavity elements for pulse formation. Moving towards a higher level of integration would reduce complexity, packaging, and manufacturing cost, which are important requirements for mass production. Semiconductor lasers are well established for such applications, and optically-pumped vertical external cavity surface emitting lasers (VECSELs) are most promising for higher power applications, generating the highest power in fundamental transverse mode (>20 W) to date. Ultrashort pulses have been demonstrated using passive modelocking with a semiconductor saturable absorber mirror (SESAM), achieving for example 2.1-W average power, sub-100-fs pulse duration, and 50-GHz pulse repetition rate. Previously the integration of both the gain and absorber elements into a single wafer was demonstrated with the MIXSEL (modelocked integrated external-cavity surface emitting laser) but with limited average output power (<200 mW). We have demonstrated the power scaling concept of the MIXSEL using optimized quantum dot saturable absorbers in an antiresonant structure design combined with an improved thermal management by wafer removal and mounting of the 8-µm thick MIXSEL structure directly onto a CVD-diamond heat spreader. The simple straight cavity with only two components has generated 28-ps pulses at 2.5-GHz repetition rate and an average output power of 6.4 W, which is higher than for any other modelocked semiconductor laser.

Femtosecond high-power quantum dot vertical external cavity surface emitting laser

We report on the first femtosecond vertical external cavity surface emitting laser (VECSEL) exceeding 1 W of average output power. The VECSEL is optically pumped, based on self-assembled InAs quantum dot (QD) gain layers, cooled efficiently using a thin disk geometry and passively modelocked with a fast quantum dot semiconductor saturable absorber mirror (SESAM). We developed a novel gain structure with a flat group delay dispersion (GDD) of ± 10 fs2 over a range of 30 nm around the designed operation wavelength of 960 nm. This amount of GDD is several orders of magnitude lower compared to standard designs. Furthermore, we used an optimized positioning scheme of 63 QD gain layers to broaden and flatten the spectral gain. For stable and self-starting pulse formation, we have employed a QD-SESAM with a fast absorption recovery time of around 500 fs. We have achieved 1 W of average output power with 784-fs pulse duration at a repetition rate of 5.4 GHz. The QD-SESAM and the QD-VECSEL are operated with similar cavity mode areas, which is beneficial for higher repetition rates and the integration of both elements into a modelocked integrated external-cavity surface emitting laser (MIXSEL).

Femtosecond pulses from a modelocked integrated external-cavity surface emitting laser (MIXSEL).

Novel surface-emitting optically pumped semiconductor lasers have demonstrated >1 W modelocked and >100 W continuous wave (cw) average output power. The modelocked integrated external-cavity surface emitting laser (MIXSEL) combines the gain of vertical-external-cavity surface-emitting lasers (VECSELs) with the saturable absorber of a semiconductor saturable absorber mirror (SESAM) in one single semiconductor structure. This unique concept allows for stable and self-starting passive modelocking in a simple straight cavity. With quantum-dot based absorbers, record-high average output power was demonstrated previously, however the pulse duration was limited to 17 ps so far. Here, we present the first femtosecond MIXSEL emitting pulses with a duration as short as 620 fs at 4.8 GHz repetition rate and 101 mW average output power. The novel MIXSEL structure relies on a single low temperature grown quantum-well saturable absorber with a low saturation fluence and fast recovery dynamics. A detailed characterization of the key modelocking parameters of the absorber and the challenges for absorber integration into the MIXSEL structure are discussed.

Experimental verification of soliton-like pulse-shaping mechanisms in passively mode-locked VECSELs.

During the less than ten years since the first demonstration of modelocked vertical external cavity surface emitting lasers (VECSELs), their performance strongly improved and starts to become comparable to standard modelocked lasers based on ion-doped glasses or crystals. Moreover, the semiconductor gain material has important advantages such as cost-efficient mass-production, emission wavelength and bandwidth control by bandgap engineering. Picosecond pulses with average output powers ≫2 W were achieved and the repetition rate was increased up to 50 GHz [1]. Pulse durations as short as 260 fs were obtained, but only at low power levels of 15 mW [2]. Despite this impressive progress, so far, femtosecond operation could not be combined with high power levels. Previously, the most relevant parameters for the temporal pulse shaping were identified and a qualitative theory on a quasi-soliton pulse shaping mechanism was developed [3]. Here we demonstrate for the first time the detailed experimental verification of this quasi-soliton pulse formation theory. We show that the achievable pulse duration strongly depends on the group delay dispersion (GDD), and that it is important to provide positive overall GDD for achieving short pulse durations.

VECSEL gain characterization.

We present the first full gain characterization of two vertical external cavity surface emitting laser (VECSEL) gain chips with similar designs operating in the 960-nm wavelength regime. We optically pump the structures with continuous-wave (cw) 808-nm radiation and measure the nonlinear reflectivity for 130-fs and 1.4-ps probe pulses as function of probe pulse fluence, pump power, and heat sink temperature. With this technique we are able to measure the saturation behavior for VECSEL gain chips for the first time. The characterization with 1.4-ps pulses resulted in saturation fluences of 40-80 μJ/cm2, while probing with 130-fs pulses yields reduced saturation fluences of 30-50 μJ/cm2 for both structures. For both pulse durations this is lower than previously assumed. A small-signal gain of up to 5% is obtained with this technique. Furthermore, in a second measurement setup, we characterize the spectral dependence of the gain using a tunable cw probe beam. We measure a gain bandwidth of over 26 nm for both structures, full width at half maximum.

Timing Jitter Characterization of a Free-Running SESAM Mode-locked VECSEL

We present timing jitter measurements of an InGaAs quantum well vertical external cavity surface emitting laser (VECSEL) passively mode locked with a quantum dot semiconductor saturable absorber mirror (SESAM) at 2-GHz repetition rate. It generates 53-mW average output power in 4.6-ps pulses at 953 nm. The laser housing was optimized for high mechanical stability to reduce acoustic noise. We use a fiber-coupled multimode 808-nm pump diode, which is mounted inside the laser housing. No active cavity length stabilization is employed. The phase noise of the free-running laser integrated over a bandwidth from 100 Hz to 1 MHz corresponds to an RMS timing jitter of ≈212 fs, which is lower than previously obtained for mode-locked VECSELs. This clearly confirms the superior noise performance expected from a high-Q-cavity semiconductor laser. In contrast to edge-emitting semiconductor diode lasers, the cavity mode is perpendicular to the quantum well gain layers, which minimizes complex dispersion and nonlinear dynamics.

Femtosecond VECSEL with tunable multi-gigahertz repetition rate.

We present a femtosecond vertical external cavity surface emitting laser (VECSEL) that is continuously tunable in repetition rate from 6.5 GHz up to 11.3 GHz. The use of a low-saturation fluence semiconductor saturable absorber mirror (SESAM) enables stable cw modelocking with a simple cavity design, for which the laser mode area on SESAM and VECSEL are similar and do not significantly change for a variation in cavity length. Without any realignment of the cavity for the full tuning range, the pulse duration remained nearly constant around 625 fs with less than 3.5% standard deviation. The center wavelength only changed ±0.2 nm around 963.8 nm, while the output power was 169 mW with less than 6% standard deviation. Such a tunable repetition rate is interesting for various metrology applications such as optical sampling by laser cavity tuning (OSCAT).

Sub-60-fs Timing Jitter of a SESAM Modelocked VECSEL

We present noise measurements of a pulse train emitted from an actively stabilized semiconductor-saturable-absorber-mirror (SESAM) modelocked vertical external cavity surface emitting laser (VECSEL). The laser generated 6-ps pulses with 2-GHz pulse-repetition rate and 40-mW average output power. The repetition rate was phase locked to a reference source using a piezo actuator. The timing phase noise power spectral density of the laser output was detected with a highly linear photodiode and measured with a signal source analyzer. The resulting RMS timing jitter integrated over an offset frequency range from 1 Hz to 100 MHz gives a value of below 60 fs, lower than previous modelocked VECSELs and comparable with the noise performance of ion-doped solid-state lasers. The RMS amplitude noise was below 0.4% (1 Hz to 40 MHz) and not influenced by the timing phase stabilization.

Green-diode-pumped femtosecond Ti:Sapphire laser with up to 450 mW average power.

We investigate power-scaling of green-diode-pumped Ti:Sapphire lasers in continuous-wave (CW) and mode-locked operation. In a first configuration with a total pump power of up to 2 W incident onto the crystal, we achieved a CW power of up to 440 mW and self-starting mode-locking with up to 200 mW average power in 68-fs pulses using semiconductor saturable absorber mirror (SESAM) as saturable absorber. In a second configuration with up to 3 W of pump power incident onto the crystal, we achieved up to 650 mW in CW operation and up to 450 mW in 58-fs pulses using Kerr-lens mode-locking (KLM). The shortest pulse duration was 39 fs, which was achieved at 350 mW average power using KLM. The mode-locked laser generates a pulse train at repetition rates around 400 MHz. No complex cooling system is required: neither the SESAM nor the Ti:Sapphire crystal is actively cooled, only air cooling is applied to the pump diodes using a small fan. Because of mass production for laser displays, we expect that prices for green laser diodes will become very favorable in the near future, opening the door for low-cost Ti:Sapphire lasers. This will be highly attractive for potential mass applications such as biomedical imaging and sensing.

Few-cycle pulses from a graphene mode-locked all-fiber laser

We combine a graphene mode-locked oscillator with an external compressor and achieve~29fs pulses with~52mW average power. This is a simple, low-cost, and robust setup, entirely fiber based, with no free-space optics, for applications requiring high temporal resolution.