1.55 µm InAs/GaAs Quantum Dots and High Repetition Rate Quantum Dot SESAM Mode-locked Laser

High pulse repetition rate (≥10 GHz) diode-pumped solid-state lasers, modelocked using semiconductor saturable absorber mirrors (SESAMs) are emerging as an enabling technology for high data rate coherent communication systems owing to their low noise and pulse-to-pulse optical phase-coherence. Quantum dot (QD) based SESAMs offer potential advantages to such laser systems in terms of reduced saturation fluence, broader bandwidth, and wavelength flexibility. Here, we describe the development of an epitaxial process for the realization of high optical quality 1.55 µm In(Ga)As QDs on GaAs substrates, their incorporation into a SESAM, and the realization of the first 10 GHz repetition rate QD-SESAM modelocked laser at 1.55 µm, exhibiting ∼2 ps pulse width from an Er-doped glass oscillator (ERGO). With a high areal dot density and strong light emission, this QD structure is a very promising candidate for many other applications, such as laser diodes, optical amplifiers, non-linear and photonic crystal based devices

First Fully Stabilized Frequency Comb from a SESAM-Modelocked 1.5-μm Solid-State Oscillator

International audienceWe have demonstrated the first fully stabilized frequency comb generated by an ultrafast Er:Vb:glass laser. The SESAM-modelocked, diode-pumped laser uses a high-Q cavity which results in low-noise operation. A coherent, octave-spanning SC is generated by nonlinear spectral broadening in a polarization maintaining highlynonlinear fiber. We observe a more than 10-times improvement in the free-running CEO frequency linewidth compared to free-running femtosecond fiber laser systems operating in this spectral region. The CEO frequency and the repetition rate are stabilized to the same 10-MHz external reference over hours. The Allan deviation of the 20-MHz CEO reaches 10 -8 at 1-s with an H-maser as an external reference and a stable locked operation of more than 11 hours was achieved in a preliminary long-term evaluation. A CEO-beat signal can still be observed at a large reduction of the pump power, resulting in a reduced intracavity pulse energy and in an increased pulse duration up to 260 fs. We expect that this result will have a significant impact for the future development of more compact stable frequency combs as the relatively long pulse duration relaxes the requirements on the modelocked laser. This will become even more important for gigahertz pulse repetition rates

Sub-40-fs pulses with 18-W average power from a passively mode-locked thin disk Yb:YAG laser with nonlinear fiber compression

By combining a passively mode-locked high-power laser with a large mode area holey fiber for nonlinear compression, we generated 33-fs pulses with 18-W average power. The output beam is linearly polarized and close to diffraction-limited. ©2002 Optical Society of America.F. Brunner, T. Siidmeyer, E. Innerhofer, R Paschotta, U. Keller, K. Furusawa, J. C. Baggett, T. M. Monro, and D. J. Richardso

High-power femtosecond nonlinear devices pumped with a mode-locked thin disk laser

We report on experiments with high-power femtosecond pulses from a passively mode-locked thin disk Yb:YAG laser: nonlinear fiber compression to 33 fs with 18 W average power and a fiber-feedback parametric oscillator generating 15 W in the 1.5-μm region

The Australian National Windbreaks Program: overview and summary of results

This overview paper presents a description of the National Windbreaks Program (NWP) — its objectives, the main methods used to achieve these objectives and a summary of the key results. It draws these from the individual papers appearing in this special issue, which provide detailed descriptions and discussion about the specific research sites and research methods used, in addition to interpreting and discussing the results. The key findings were the following: (i) Two broad areas of crop and pasture response can be identified downwind of a porous windbreak: a zone of reduced yield associated with competition with the windbreak trees that extended from 1 H to 3 H, where H is the windbreak height, and a zone of unchanged or slightly increased yield stretching downwind to 10 H or 20 H. (ii) Averaged over the paddock, yield gains due to the effect of shelter on microclimate were smaller than expected — especially for cereals. Yield simulations conducted using the APSIM model and 20 years of historical climate data confirmed this result for longer periods and for other crop growing regions in Australia. Larger yield gains were simulated at locations where the latter part of the growing season was characterised by high atmospheric demand and a depleted soil water store. (iii) Economic analyses that account for the costs of establishing windbreaks, losses due to competition and yield gains as a result of shelter found that windbreaks will either lead to a small financial gain or be cost neutral. (iv) Part of the reason for the relatively small changes in yield measured at the field sites was the variable wind climate which meant that the crop was only sheltered for a small proportion of the growing season. In much of southern Australia, where the day-to-day and seasonal variability in wind direction is large, additional windbreaks planted around the paddock perimeter or as closely-spaced rows within the paddock will be needed to provide more consistent levels of shelter. (v) Protection from infrequent, high magnitude wind events that cause plant damage and soil erosion was observed to lead to the largest yield gains. The main forms of direct damage were sandblasting, which either buries or removes seedlings from the soil or damages the leaves and stems, and direct leaf tearing and stripping. (vi) A corollary to these findings is the differing effect that porous windbreaks have on the air temperature and humidity compared to wind. While winds are reduced in strength in a zone that extends from 5 H upwind to at least 25 H downwind of the windbreak, the effects of shelter on temperature and humidity are smaller and restricted mainly to the quiet zone. This means that fewer windbreaks are required to achieve reductions in wind damage than for altering the microclimate. (vii) The wind tunnel experiments illustrate the important aspects of windbreak structure that determine the airflow downwind, and subsequent microclimate changes, in winds oriented both perpendicular and obliquely to porous windbreaks. These results enable a series of guidelines to be forwarded for designing windbreaks for Australian agricultural systems.H. A. Cleugh, R. Prinsley, P. R. Bird, S. J. Brooks, P. S. Carberry, M. C. Crawford, T. T. Jackson, H. Meinke, S. J. Mylius, I. K. Nuberg, R. A. Sudmeyer and A. J. Wrigh