Charles E. Hamilton

High-power laser source with spectrally beam-combined diode laser bars

Aculight has demonstrated spectral beam combining of four diode laser bars in a single optical cavity; each 1 cm wide diode bar included 200 individual single mode laser emitters. The beam combining was accomplished in the plane of the diode bar -- slow direction. In earlier work, Aculight has reported near diffraction limited performance from single diode laser bars where we have spectrally beam combined 200 laser emitters while maintaining a beam quality near the diffraction limit. Without spectral beam combination these diode laser bars will have a beam quality, in the plane of the bar, corresponding to an M2 of 1000. In current work, Aculight is extending this technology to demonstrate a spectrally beam combined, diode laser system of 50 Watts, with near diffraction limited beam quality. To accommodate multiple diode laser bars, optical modeling was used to design and complete sensitivity analysis of a unique optical cavity based on the Schmidt telescope principal to remove off-axis aberrations. Error trees have been developed for beam quality and efficiency that illustrates just how the efficiency and beam quality have been maintained within this system.

258 W of spectrally beam combined power with near-diffraction limited beam quality

We report on progress toward power scaling Yb fiber lasers beyond kW levels by an efficient and versatile architecture that maintains near diffraction limited beam quality. For this work, power scaling is performed at two distinct levels. The first utilizes a diffraction grating to spectrally beam combine (SBC) the output from several master-oscillator, poweramplifier (MOPA) fiber lasers with a goal of producing high quality combined beams with > 1 kW of power. The second involves scaling individual MOPA outputs to > 200 W, thereby reducing the number of lasers required for SBC. As a first step toward reaching these goals, we have developed Yb fiber MOPAs producing up to 208 W of polarized, narrow band, and near diffraction limited output and have demonstrated two-channel fiber laser SBC with a power combining efficiency of 93%, a combined beam power of 258 W, and a dispersed axis M2 of 1.06. These results represent a significant advance in high brightness, spectrally beam combined laser systems.

Solid state lasers for 193-nm photolithography

Advances in technology now make possible solid-state 193 nm lasers. Solid-state can operate with pulse repetition rates of > 10 kHz, minimizing peak-power damage to stepper optics. Furthermore, solid-state lasers are potentially more reliable and could have lower operating costs than ArF excimer lasers. Achievement of spectral linewidths < 0.1 pm for use with refractive lens systems is straightforward in solid-state laser systems. Ultraviolet solid-state laser technology is much less mature than excimer laser technology; so while there is far to go, there is much more potential for rapid progress in solid-state lasers than in excimer lasers. Aculight has begun a program to develop a multiwatt, 10 kHz solid-state 193 nm laser. Although efficient conversion from 1064 nm to 193 nm is easier for high peak power pulses, minimization of lens damage requires low peak power. Eventual goals for the technology are to achieve output powers 10 - 20 W at > 20 kHz repetition rate in > 10 ns pulses, limiting peak powers to < 200 kW. High pulse repetition rates will permit excellent dose control, and facilitate decoherence of the high-coherence beam from the solid-state laser system.

Fully stabilized single-frequency Ti:Al2O3 laser oscillator

Single-longitudinal-mode (SLM), pulsed operation is demonstrated on a tunable Ti:Al2O3 oscillator that utilizes a glancing-incidence cavity configuration. The oscillator is tunable over 720-915 nm, and the output has a bandwidth that is near transform-limited at equal to or less than 500 MHz. Beam walk-off and diffraction effects define cavity configurations for which SLM operation is possible. Stable SLM operation, without any active stabilization, is achieved with a 6.5-cm long cavity in which the separation between the tuning mirror and the grating is kept small (1.5 cm). The oscillator is actively stabilized by a feedback mechanism that monitors small changes in the direction of the output beam that result from small wavelength deviations. Single-longitudinal-mode operation over several hours has been demonstrated. The temporal jitter of the oscillator is reduced to + or - 1.2 ns while maintaining SLM operation.

Development of ytterbium doped Sr 5 (PO 4 ) 3 F for high-energy diode-pumped lasers

We describe progress made in the growth of defect-free Yb3+:Sr5(PO4)3F (Yb:S-FAP) and present results of laser characterization of these crystals for high-energy, Q-switched operation. Growth of Yb:S-FAP at 45° from the c-axis yielded what is believed to be the first crystal grown free of any slip dislocation defects. This crystal contains ~50% more active Yb3+ ions than previous crystals grown along the a-axis using identical concentrations of Yb3+ in the melt. Q-switched pulses can be extracted from these crystals at fluence levels over 3 times the saturation fluence (11 J/cm2) without optical damage to the crystals indicating that this material is suitable for high-energy, Q-switched operation.