We have demonstrated a pulsed 1064 nm PM Yb:fiber laser system incorporating a seed source with a tunable pulse repetition rate and pulse duration and a multistage fiber amplifier, ending in a large core (>650 μm<sup>2</sup> mode field area), tapered fiber amplifier. The amplifier chain is all-fiber, with the exception of the final amplifier’s pump combiner, allowing robust, compact packaging. The air-cooled laser system is rated for >60 W of average power and beam quality of M<sup>2</sup> < 1.3 at repetition rates below 100 kHz to 10’s of MHz, with pulses discretely tunable over a range spanning 50 ps to greater than 1.5 ns. Maximum pulse energies, limited by the onset of self phase modulation and stimulated Raman scattering, are greater than 12.5 μJ at 50 ps and 375 μJ at 1.5 ns , corresponding to >250 kW peak power across the pulse tuning range. We present frequency conversion to 532 nm with efficiency greater than 70% and conversion to UV via frequency tripling, with initial feasibility experiments showing >30% UV conversion efficiency. Application results of the laser in scribing, thin film removal and micro-machining will be discussed.
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 M<sup>2</sup> of 1.06. These results represent a significant advance in high brightness, spectrally beam combined laser systems.
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 M<sup>2</sup> 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.
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.
A design, as well as verification measurements, are presented for an end pumped, 20 Watt output power, single frequency, Tm:YAG laser driver for pumping a HBr mid-IR laser. Efficient end pumping of the Tm:YAG is achieved by `close lens coupling' 15 Watt average power, room temperature, 785 nm diode bars to several Tm:YAG rods. The Tm:YAG laser is operated single frequency (injection seeded) in order to couple efficiently its output to the narrow absorption band of HBr. A 2 micron laser operating multi-line, but with a bandwidth less than 1 - 2 GHz, is also under consideration using a HBr laser with increased pump absorption characteristics obtained by increasing the pressure or by placing the HBr laser inside the 2 micron laser using intra-cavity 2 micron pumping.
One way of achieving high-power diode-end pumped lasers is to angularly multiplex several diodes on each end of the laser rod. We have successfully multiplexed four 15 W diode arrays on each end of a 6.3 mm diameter X 7.5 mm long Nd:YAG rod to produce an approximately equals 2 mm diameter pump spot. Higher laser power was achieved by adding a second laser rod pumped at both ends. The addition of the second rod facilitates thermally induced birefringence compensation by introducing a quartz polarization rotator between the rods. In addition, it was necessary to add an aspheric lens to compensate the thermal aberration induced at these high, nonuniform pump powers. With this arrangement, > 90 W has been extracted multimode, and > 60 W in a near-diffraction-limited beam.
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.