Picosecond laser

Picosecond lasers sit between nanosecond Q-switched lasers and femtosecond ultrafast systems. Their pulses, of order 10⁻¹² to a few times 10⁻¹⁰ s, are produced mainly by passive mode locking with a SESAM in solid-state oscillators (Nd:YVO₄, Nd:YAG and Yb-doped crystals) and in fiber lasers, and by gain-switching laser diodes, which reach tens of picoseconds directly. Oscillators run at tens of megahertz with nanojoule pulses; for machining, a pulse picker and amplifier bring the energy to tens or hundreds of microjoules at repetition rates of hundreds of kilohertz to a few megahertz.

Why the duration matters

In a material, absorbed energy spreads by heat conduction over a distance of about $\sqrt{D\tau}$ during a pulse of duration $\tau$, where $D$ is the thermal diffusivity. For stainless steel, with $D$ about 4 × 10⁻⁶ m²/s, that is roughly 6 nm for a 10 ps pulse and 200 nm for a 10 ns pulse. Picosecond pulses therefore deposit their energy before much of it can conduct into the surrounding material, and ablation leaves a small heat-affected zone, little melt and clean edges; this is the basis of so-called cold ablation for drilling, scribing, cutting glass and thin films, and structuring of solar cells and displays. Compared with femtosecond lasers, picosecond sources are simpler and reach higher average power at lower cost, and for many metals and dielectrics they give much of the same quality benefit.

A transform-limited 10 ps pulse with a sech² shape needs only 31 GHz of bandwidth, about 0.12 nm at 1064 nm, by the time-bandwidth product, so picosecond gain media and optics need not be broadband, and dispersion of ordinary optics is negligible, unlike for femtosecond pulses.

Other uses

In dermatology, picosecond lasers at 532, 755 and 1064 nm are used for tattoo and pigment removal, where the short pulse fragments pigment particles by photomechanical stress. In science they pump optical parametric oscillators and amplifiers, drive time-resolved spectroscopy and fluorescence lifetime measurements, and provide the timing source for time-correlated single-photon counting.

Measurement

Pulse durations from about 1 to 20 ps are measured with an autocorrelator; longer pulses directly with a fast photodiode and a sampling oscilloscope, whose combined rise time must be several times shorter than the pulse, or with a streak camera. Energy comes from a pyroelectric meter, and the peak fluence at the workpiece from that energy and the measured spot size.

The pulsed laser calculator gives the pulse energy, peak power and fluence of a picosecond source from its average power, repetition rate and spot size.

References: U. Keller, Nature 424, 831 (2003); C. Momma et al., Appl. Surf. Sci. 109, 15 (1997); B. N. Chichkov et al., Appl. Phys. A 63, 109 (1996).