Optics & Photonics News

Northrop Grumman

techniques such as spatially pumping directly into the resonator fundamental transverse mode, but it has also simplified the design of solid-state lasers.

Diode laser designs can be accurately modeled to verify the performance of new solid-state laser configurations. Today’s computer models can unravel a parasitic effect such as up-conversion and heat generation due to the relative simplicity of a small-bandwidth quasi-monochromatic diode laser. This theoretical understanding was far more difficult to model with flashlamp-pumped devices. Diode-pumped solid-state-laser designs are not without their challenges, such as the pump diode output wavelength dependence on drive current and temperature. Recently, researchers have made much progress by inserting a Bragg grating into the diode structure, or by locking to an external volume Bragg grating, and thereby significantly reducing the sensitivity of diode lasers and bars to changes in ambient operating conditions.

The first CW kilowatt average power solid-state laser was demonstrated at the General Electric Corporate Research and Development Center by Joseph Chernoch, the inventor of the slab laser, and Mark Kukula. The device used an Nd:YAG zig-zag slab and was flashlamp-pumped. Since then, development of bright high-power diode laser pump sources has enabled the creation of high-average-power bulk solid-state lasers like JHPSSL with output powers measured in tens of kilowatts. Powers in excess of 100 k W are expected from a multiplexed system later this year. Diode sources are widely credited with enabling the rapid development of high-average-power fiber lasers. A decade ago, the output of fiber lasers was typically at the watt level. Today, a commercial 10 k W version is available.

lasers has steadily increased to the kilowatt level and beyond.

Fiber lasers have attracted a great deal of attention because of their potential to provide highly efficient solid-state laser sources with excellent beam-quality and reduced thermal-management problems. In addition, they are attractive platforms to amplify picosecond to nanosecond pulses generated by an external oscil-

IMRA America, Inc.

lator. However, it is difficult to
Q-switch fiber lasers to produce
those types of pulses as an oscilla-
tor. Fiber lasers have been used to
great profit to produce mode-
locked pulses and today produce
watt-level average powers with
pulse durations in the hundreds
of femtoseconds regime down
to less than 37 fs. Producing
successful fiber ultrafast lasers
involves a complex balancing of
dispersion and nonlinear effects
such as self-phase modulation,
Brillouin scattering and Raman
scattering. These lasers have
found niche applications in micromachining and two- and
three-photon microscopy.

Northrop Grumman JHPSSL demonstrated the highest-power to-date DPSS laser. It was developed for military applications.

The IMRA ultrafast fiber laser system is used in biomedical and micromachining applications.

Fiber laser technology

Major advances in fiber lasers began in earnest more than a decade ago. A paper describing a 110-W CW fiber laser was presented by Dominic et al. at the 1999 CLEO Conference in Baltimore, and this work announced that a major scaling in the power output of fiber lasers was under way. This advance resulted from the development of better fibers, particularly double-clad designs into which one could couple increased amounts of pump light from diode sources that were also growing in output power and brightness. Since that time, the scaling of CW fiber

Ultrafast bulk laser technology

Like fiber laser technology, ultrafast bulk laser technology has become a major sub-field of solid-state lasers. It is at the cutting-edge of laser physics, and its applications have become very important. The development of mode-locked solid-state laser oscillators capable of producing femtosecond pulses, down to less than 5 fs, happened over a period of decades, with each advance building on previous ones. Of critical importance was the development of the laser material Ti:sapphire, with its extraordinarily wide bandwidth, and recognition of the importance of high-order dispersion compensation to achieve near-transform-limited pulsewidths.

Another development of enormous importance was the invention of chirped-pulse-amplification (CPA) by the Mourou group in the late 1980s at the Laboratory for Laser Energetics at the University of Rochester. Using this technique, researchers could “stretch” femtosecond pulses using a diffraction grating to the picosecond or nanosecond regime amplified in a series of cascaded Ti:sapphire amplifiers while avoiding nonlinear effects and laser-induced-damage, and then compress them close to the original pulsewidth using a second diffraction grating.