The high-transmission small-angle-prism output coupler broadens bandwidth, reducing excessive nonlinearities arising at high peak powers in the gain medium. The investigators found a 16% coupler to be optimum, because higher-transmission output couplers did not facilitate modelock starting due to lower intracavity power and weaker self-amplitude modulation. To suppress the CW background, the slit aperture was replaced with an elliptical one, closer to the beam profile. The result is improved self-amplitude modulation, which, along with enhanced cooling, eliminates CW background at very high pump powers.
In assessing beam quality, profile measurements made with a SensorPhysics (Oldsmar, FL) CCD camera show a more than 90% fit to a Gaussian shape in both horizontal and vertical directions. The beam profile M2 value is about 1.34.
For the extracavity frequency doubling, a broadband, chirped curved mirror focuses the pulse into a 300-µm-thick lithium borate crystal. Use of a mirror avoids pulse broadening from chromatic aberration in the lens. Power measured before the focusing mirror is approximately 580 mW, resulting in a conversion efficiency of more than 20% to the 120-mW violet-blue output.
A commercial 0.4-MW version of the device is now available. The investigators see ultrafast applications in high-resolution time-resolved spectroscopy and as seed oscillators for 10-fs-range high-power laser systems.