W. P. Pallmann

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.

Electrically Pumped Vertical External Cavity Surface Emitting Lasers Suitable for Passive Modelocking

Modelocked optically pumped vertical external cavity surface emitting lasers (VECSELs) have generated up to 6.4-W average power, which is higher than for any other semiconductor lasers. Electrical pumping of modelocked VECSELs is the next step toward a higher level of integration. With continuous wave (cw) electrically pumped (EP) VECSELs, an average output power of 900 mW has been demonstrated from the undisclosed proprietary novalux extended cavity surface emitting laser (NECSEL) design. In contrast, modelocked NECSELs have only been demonstrated at 40 mW. Recently, we presented a numerical study of EP-VECSELs suitable for modelocked operation; here, we demonstrate the first realization of this design. Power scaling is achieved with a lateral mode size increase. The competing electrical and optical requirements are, on the electrical side, low ohmic resistance, and on the optical side, low optical losses and low dispersion. Additionally, the device needs to operate in a fundamental mode for stable modelocking. We have fabricated and characterized 60 EP-VECSELs with varying dimensions and compared their lasing performance with our numerical simulations. The tradeoff between good beam quality and output power is discussed with an outlook to the modelocking of these EP-VECSELs. Initial EP-VECSEL devices have generated >;100 mW of cw output power.

Low repetition rate SESAM modelocked VECSEL using an extendable active multipass-cavity approach

Ultrafast VECSELs are compact pulsed laser sources with more flexibility in the emission wavelength compared to diode-pumped solid-state lasers. Typically, the reduction of the pulse repetition rate is a straightforward method to increase both pulse energy and peak power. However, the relatively short carrier lifetime of semiconductor gain materials of a few nanoseconds sets a lower limit to the repetition rate of passively modelocked VECSELs. This fast gain recovery combined with low pulse repetition rates leads to the buildup of multiple pulses in the cavity. Therefore, we applied an active multipass approach with which demonstrate fundamental modelocking at a repetition rate of 253 MHz with 400 mW average output power in 11.3 ps pulses.

Gain characterization and passive modelocking of electrically pumped VECSELs

Linear and nonlinear gain characterization of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) is presented with spectrally resolved measurements of the gain and with gain saturation measurements of two EP-VECSEL samples with different field enhancement in the quantum-well gain layers. The spectral bandwidth, small-signal gain and saturation fluence of the devices are compared. Using the sample with the larger bandwidth, we have demonstrated the shortest pulses generated from a passively modelocked EP-VECSEL to date. With a low-saturation-fluence SESAM for passive modelocking we have achieved 9.5-ps pulses with 7.6 mW average output power at a repetition rate of 1.4 GHz. With a higher output coupler transmission the pulse duration was increased to 31 ps with an average output power of 13.6 mW. The pulses were chirped mainly due to the group delay dispersion (GDD) introduced by the intermediate DBR, which compensates the optical loss in the structure.

Absorber and gain chip optimization to improve performance from a passively modelocked electrically pumped vertical external cavity surface emitting laser

We present an electrically pumped vertical-external-cavity surface-emitting laser (EP-VECSEL) modelocked with a semiconductor saturable absorber mirror (SESAM) with significantly improved performance. In different cavity configurations, we present the shortest pulses (2.5 ps), highest average output power (53.2 mW), highest repetition rate (18.2 GHz), and highest peak power (4.7 W) to date. The simple and low-cost concept of EP-VECSELs is very attractive for mass-market applications such as optical communication and clocking. The improvements result from an optimized gain chip from Philips Technologie GmbH and a SESAM, specifically designed for EP-VECSELs. For the gain chip, we found a better trade-off between electrical and optical losses with an optimized doping scheme in the substrate to increase the average output power. Furthermore, the devices bottom contact diameter (60 μm) is smaller than the oxide aperture diameter (100 μm), which favors electro-optical conversion into a TEM00 mode. Compared to ...

Ultrafast Electrically Pumped VECSELs

We present an improved design for electrically pumped vertical external-cavity surface-emitting lasers (EP-VECSELs) optimized for passive modelocking with a semiconductor saturable absorber mirror (SESAM). In continuous-wave (cw) multimode operation, up to 170 mW of output power are demonstrated at a wavelength of 967 nm. Overcoming our prior limitation, we demonstrate fundamental transverse mode operation with 26 mW of average output power. Passively modelocking one of the fabricated EP-VECSELs with a quantum-well SESAM, we have achieved 6.3-ps pulses with an average output power of 6.2 mW at 1.46-GHz repetition rate, the shortest pulse duration from this type of laser to date. With slightly longer pulses of 7.3 ps, an average output power of 13 mW is obtained. Tradeoffs for further output power scaling and reduced pulse duration are discussed.

Novel ultrafast vertically emitting semiconductor lasers

We demonstrate the highest average power of any modelocked semiconductor laser: the optically-pumped MIXSEL generates 6.4-W in 28.1-ps pulses at 2.5-GHz repetition rate. Furthermore, we discuss electrically-pumped VECSELs optimized for passive modelocking and present initial cw-lasing with >100-mW.

Sub-100 fs high average power directly blue-diode-laser-pumped Ti:sapphire oscillator

Ti:sapphire oscillators are a proven technology to generate sub-100 fs (even sub-10 fs) pulses in the near infrared and are widely used in many high impact scientific fields. However, the need for a bulky, expensive and complex pump source, typically a frequency-doubled multi-watt neodymium or optically pumped semiconductor laser, represents the main obstacle to more widespread use. The recent development of blue diodes emitting over 1 W has opened up the possibility of directly diode-laser-pumped Ti:sapphire oscillators. Beside the lower cost and footprint, a direct diode pumping provides better reliability, higher efficiency and better pointing stability to name a few. The challenges that it poses are lower absorption of Ti:sapphire at available diode wavelengths and lower brightness compared to typical green pump lasers. For practical applications such as bio-medicine and nano-structuring, output powers in excess of 100 mW and sub-100 fs pulses are required. In this paper, we demonstrate a high average power directly blue-diode-laser-pumped Ti:sapphire oscillator without active cooling. The SESAM modelocking ensures reliable self-starting and robust operation. We will present two configurations emitting 460 mW in 82 fs pulses and 350 mW in 65 fs pulses, both operating at 92 MHz. The maximum obtained pulse energy reaches 5 nJ. A double-sided pumping scheme with two high power blue diode lasers was used for the output power scaling. The cavity design and the experimental results will be discussed in more details.

Current confinement in EP-VECSELs for high power single-mode operation suitable for passive mode-locking

One of the main advantages of using VECSEL lasers for mode-locked operation is their power scalability. Best performance data available for mode-locked semiconductor lasers have been achieved with optically pumped VECSELs, reaching pulses in the femtosecond regime and average powers in the watt regime.1 This advantage is challenging for electrically pumped VECSELs, where a homogeneous carrier injection into the center must be provided in order to maintain a single-mode operation for large diameter devices. In this paper we investigate the current injection from the bottom contact of a VECSEL design, and estimate the leakage of the hole current. Then we introduce two designs that can reduce the leakage current and enhance the injection into the center of the device, thereby increasing the simulated output power by more than 20% in CW-mode while maintaining an optimal gain profile suitable for single-mode operation.

Design and simulation of electrically pumped mode-locked VECSELs

Vertical External Cavity Surface Emitting Lasers (VECSELs) feature scaling to large active areas and combine surface emission with high optical power output. In principle, they can be designed with electrical pumping operating in continuous wave or passively mode-locked operation. In this paper, our design and modeling activities towards a high power passively mode-locked VECSEL are described. In particular, design towards single mode high-power CW operation is discussed as prerequisite for passive mode-locking.