B. Rudin

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).

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.

Nearly quantum-noise-limited timing jitter from miniature Er:Yb:glass lasers.

We havemeasured therelative timing jitter oftwopassively mode-locked 10-GHz Er:Yb:glass lasers tobe190fs(100Hz-1.56 MHz)infree-running and26fs(6Hz-1.56 MHz) insynchronized operation. Wepresent theresults oftiming jitter measurements fortwopassively mode-locked 10-GHzEr:Yb:glass lasers (Time- Bandwidth Products ERGO PGL).Thecavities arebuilt withgreat careformechanical stability andadditionally enclosed inametal case.Thelasers produce ;15mW ofaverage output power(fiber coupled) in1.5-ps Gaussian pulses. Thetiming ofthepulses relative tothat ofanexternal reference oscillator canbestabilized withaphase-locked loop which controls thecavity length bymoving anendmirror mounted onapiezo actuator. Tomeasure therelative timing jitter ofthetwolasers, weuseanindirect phase comparison method(l) that allows precise jitter measurements forfree-running ortiming-stabilized mode-locked lasers. Thefigure shows measured two- sided timing phase noise powerspectra ofthefree-running andthetiming-stabilized lasers. Eachcurve represents the average offoursingle measurements with ameasurement timeof0.17s.Thedashed line showsthelimit given by quantum noise sources inthecavities.(2)

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.

Modelocked Integrated External-Cavity Surface Emitting Laser (MIXSEL)

For the first time we have successfully demonstrated a novel concept of a passively modelocked vertical-external-cavity surface-emitting semiconductor laser with an integrated saturable absorber. This MIXSEL will be ultimately suitable for cost-effective high-volume wafer-scale fabrication.

High precision optical characterization of semiconductor saturable absorber mirrors (SESAMs)

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.

Nearly quantum noise limited timing jitter from miniature Er:Yb:glass lasers

We have measured the relative timing jitter of two passively mode-locked 10-GHz Er:Yb:glass lasers to be 190 fs (100 Hz-1.56 MHz) in free-running and 26 fs (6 Hz-1.56 MHz) in synchronized operation.