Optics

ULTRAFAST LASERS: Chirped dielectric mirror yields high-energy femtosecond pulses

Nov. 1, 1996

FIGURE 1. Improved Ti:sapphire oscillator produces 120-mW second-harmonic violet-blue pulses, seen by their glow on the screen behind the optical table on which is mounted a Coherent (Santa Clara, CA) argon-ion laser pump.

Researchers of the Abteilung Quantenelektronik und Lasertechnik, Technische Universität (Vienna, Austria) have generated 1-MW peak-power, 7.5-fs pulses from a CW modelocked Ti:sapphire laser for the first time. Mode locked output power is 640 mW at a pulse-repetition rate of 80 MHz. When these pulses are extra cavity frequency doubled, the resulting violet-blue (390-nm) pulses have an average power of 120 mW and a bandwidth of 14 nm (see Fig. 1). The spectrum is sufficient to support generation of sub-15-fs blue pulses.

This performance is due to an improved cavity design for the chirped-dielectric-mirror dispersion-controlled oscillator (see Laser Focus World, Dec. 1995, p. 55). A thin-wedge output coupler and four broadband (two flat and two of 50-mm radius of curvature) chirped mirrors de signed and made by the Research Institute for Solid State Physics (Budapest, Hungary), form the oscillator (see Fig. 2). And a 30-mm focal-length lens re places a 40-mm lens used in previous versions. The pump beam is thus focused more tightly, and threshold pump power is lowered, producing 10% greater output power for the same pump power. Another key is a novel crystal geometry that provides significantly improved cooling efficiency, resulting in increased lifetime of the upper lasing energy level and reduced thermal lensing effect at high pump powers.

FIGURE 2. Thin small-angle output coupler prisms reduce material dispersion in the 1-MW oscillator beam path and keep spurious reflections from the backside of the coupler substrate from returning colinearly into the oscillator.

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.