Deran J. Maas

50-GHz passively mode-locked surface-emitting semiconductor laser with 100-mW average output power

We have developed a passively mode-locked optically-pumped vertical-external-cavity surface-emitting semiconductor laser (VECSEL) which delivers up to 100 mW of average output power at a repetition rate of 50 GHz in nearly transform-limited 3.3-ps pulses at a wavelength around 960 nm. The high-repetition-rate passive mode locking was achieved with a low-saturation-fluence semiconductor saturable absorber mirror (SESAM) incorporating a single layer of quantum-dots. The output power within a nearly diffraction-limited beam was maximized using a gain structure with a low thermal impedance soldered to a diamond heat spreader. In addition, we systematically optimized the laser resonator to accommodate for the strong thermal lens caused by the optical pumping. We measured the thermal lens dioptric power and present a numerical model which is in good agreement with the measurements and is useful for optimizing resonator designs. The experimental setup is very versatile and its design and construction are discussed in detail

Highly efficient optically pumped vertical-emitting semiconductor laser with more than 20 W average output power in a fundamental transverse mode

We have demonstrated an optically pumped vertical-external-cavity surface-emitting laser (OP-VECSEL) generating more than 20 W of cw output power in a fundamental transverse mode (M2 approximately 1.1) at 960 nm. The laser is highly efficient with a slope efficiency of 49%, a pump threshold of 4.4 W, and an overall optical-to-optical efficiency of 43%.

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.

Growth parameter optimization for fast quantum dot SESAMs

Semiconductor saturable absorber mirrors (SESAMs) using quantum dot (QD) absorbers exhibit a larger design freedom than standard quantum well absorbers. The additional parameter of the dot density in combination with the field enhancement allows for an independent control of saturation fluence and modulation depth. We present the first detailed study of the effect of QD growth parameters and post growth annealing on the macroscopic optical SESAM parameters, measuring both nonlinear reflectivity and recombination dynamics. We studied a set of self-assembled InAs QD-SESAMs optimized for an operation wavelength around 960 nm with varying dot density and growth temperature. We confirm that the modulation depth is controlled by the dot density. We present design guidelines for QD-SESAMs with low saturation fluence and fast recovery, which are for example important for modelocking of vertical external cavity surface emitting lasers (VECSELs).

Femtosecond thin disk laser oscillator with pulse energy beyond the 10-microjoule level

We report on a passively mode-locked Yb:YAG thin disk laser oscillator that generates 11.3-microJ pulses without the use of any additional external amplification. A repetition rate of 4 MHz is obtained using a 23.4-m-long multiple-pass cavity that extends the resonator length to a total of 37 m. The nearly transform-limited pulses at 45 W of average output power have a duration of 791 fs with a 1.56-nm-broad spectrum centered at 1030 nm. The laser is operated in a helium atmosphere to eliminate the air nonlinearity inside the resonator that previously limited the pulse energy.

High precision optical characterization of semiconductor saturable absorber mirrors

Precise semiconductor saturable absorber mirrors (SESAM) design has enabled modelocked lasers with >100 GHz pulse repetition rate or >10 muJ pulse energy. We discuss a new method for wide dynamic range nonlinear reflection measurements of SESAMs with <0.05% accuracy.

Efficient femtosecond high power Yb:Lu2O3 thin disk laser

We demonstrate the first passively mode-locked thin disk laser based on Yb:Lu(2)O(3). The laser generates 370-fs pulses with 20.5 W of average power in a diffraction-limited beam (M(2) < 1.1). The nearly transform-limited pulses have a spectral bandwidth of 3.4 nm centered near 1034 nm. With slightly longer pulses (523 fs) we obtained 24 W of average power at a pump power of 56 W, resulting in an optical-to-optical efficiency of 43%, which is higher than for any previously mode-locked thin disk laser.

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.

Low saturation fluence antiresonant quantum dot SESAMs for MIXSEL integration

A detailed QD-SESAM growth study enabled the first modelocking of a VECSEL with similar spot size on gain and antiresonant SESAM. Antiresonant designs can strongly improve MIXSELs, a novel type of ultrafast integrated VECSELs.

Dual-comb spectroscopy of water vapor with a free-running semiconductor disk laser

The slight differences between two optical frequency combs from the same laser source capture precise microwave intervals. Two different combs from a single source Combs of light divide the optical frequency spectrum into closely spaced tines that can measure molecular absorption spectra with exceptional precision. One appealing method to extend this precision down into the microwave regime is to simultaneously use two slightly distinct combs that differ in spacing by the magnitude of a microwave frequency. The challenge is ensuring that the combs remain synchronized. Link et al. solve this problem by generating both combs from the same semiconductor laser source. The resultant dual comb delivers highly accurate spectra of water vapor, and the approach could be generalized across the optical spectrum by tuning the semiconductor source. Science, this issue p. 1164 Dual-comb spectroscopy offers the potential for high accuracy combined with fast data acquisition. Applications are often limited, however, by the complexity of optical comb systems. Here we present dual-comb spectroscopy of water vapor using a substantially simplified single-laser system. Very good spectroscopy measurements with fast sampling rates are achieved with a free-running dual-comb mode-locked semiconductor disk laser. The absolute stability of the optical comb modes is characterized both for free-running operation and with simple microwave stabilization. This approach drastically reduces the complexity for dual-comb spectroscopy. Band-gap engineering to tune the center wavelength from the ultraviolet to the mid-infrared could optimize frequency combs for specific gas targets, further enabling dual-comb spectroscopy for a wider range of industrial applications.