Ultrafast laser
A laser that emits pulses shorter than a few picoseconds, usually by mode locking. Typical oscillators give 10–500 fs pulses at 10–100 MHz with nanojoule energies; amplified systems reach millijoules and peak powers of gigawatts or more.
An ultrafast laser produces optical pulses in the femtosecond (10⁻¹⁵ s) to low-picosecond range. In 100 fs light travels 30 µm, so the pulse occupies a slab of light about 30 µm thick along the beam. Most ultrafast sources are mode-locked oscillators, often followed by amplifiers: a Ti:sapphire oscillator gives 10–100 fs pulses near 800 nm at 70–100 MHz with a few nanojoules each, and an amplified Ti:sapphire system delivering 1 mJ in 35 fs at 1 kHz reaches a peak power of about 27 GW (Gaussian shape) from only 1 W of average power. The term overlaps with picosecond laser; usage varies, but "ultrafast" generally covers pulses from a few femtoseconds to a few picoseconds.
Main source types
| Source | λ (nm) | Pulse | Typical use |
|---|---|---|---|
| Ti:sapphire | 700–1000 | 5–100 fs | Science, CPA |
| Yb fiber | 1030–1080 | 0.1–1 ps | Machining |
| Yb thin disk | 1030 | 0.5–2 ps | High power |
| Er fiber | 1560 | 50–500 fs | Combs, sensing |
Ti:sapphire has the broadest gain bandwidth and supports the shortest pulses, with Kerr-lens mode locking as the usual mechanism; it requires a green pump laser and is mostly a laboratory instrument. Ytterbium-doped fiber lasers and thin-disk lasers are diode-pumped, efficient, and scale to high average power; research systems have reached multi-kilowatt average power by coherently combining several fiber amplifiers, while industrial sources typically deliver tens to a few hundred watts. Erbium fiber lasers at 1560 nm use telecom components and are the common basis of compact frequency combs. Wavelengths outside these bands come from harmonic generation and from optical parametric oscillators and amplifiers.
Bandwidth and duration
A short pulse needs a broad spectrum. By the time-bandwidth product, a transform-limited 100 fs sech² pulse needs 3.15 THz of bandwidth, which is 11.1 nm at 1030 nm and 25.6 nm at 1560 nm. Yb fiber oscillators and amplifiers typically deliver 100–300 fs pulses, though Yb oscillators have reached below 50 fs, and Yb:YAG thin disks, with narrower gain, typically around 1 ps; Ti:sapphire supports a few femtoseconds. Because every glass element adds group-delay dispersion, ultrafast systems include dispersion control: prism or grating pulse compressors, or chirped mirrors.
Amplification
Oscillator pulses carry nanojoules. To reach microjoules and millijoules without destroying the amplifier by self-focusing, chirped-pulse amplification stretches the pulse in time by a factor of 10³–10⁴, amplifies it, then compresses it again. A pulse picker first reduces the repetition rate, and the amplifier is often a regenerative amplifier. A Yb fiber amplifier giving 50 W at 1 MHz with 300 fs pulses delivers 50 µJ per pulse and a peak power of about 150 MW. By contrast, an Er fiber oscillator at 100 mW and 100 MHz carries 1 nJ per pulse, and with 100 fs pulses its peak power is about 8.8 kW.
Applications
Multiphoton microscopy uses the high peak intensity to excite fluorescence only at the focus. Time-resolved spectroscopy uses one pulse to excite a sample and a delayed pulse to probe it, with resolution set by the pulse duration. In laser machining, sub-picosecond pulses deposit energy faster than heat diffuses, which gives small heat-affected zones in metals, glass and polymers, and they are used for ophthalmic surgery (LASIK flap cutting). Stabilized ultrafast lasers form frequency combs for optical clocks and spectroscopy, and amplified systems generate high harmonics, attosecond pulses and terahertz radiation.
Practical considerations
The pulse measured at the laser output is not the pulse at the sample: windows, lenses and objectives add dispersion that must be precompensated. Duration is measured by autocorrelation or FROG, because photodiodes are far too slow. Damage thresholds of optics are specified separately for femtosecond pulses, and nonlinear effects in air, fiber and glass become significant at the peak powers these systems reach.
Common questions
What is the difference between femtosecond and picosecond lasers?
Femtosecond lasers need gain media and optics with broad bandwidth and careful dispersion management; picosecond lasers need only about a tenth of a nanometre of bandwidth for 10 ps pulses and tolerate ordinary optics. For machining, both give small heat-affected zones; femtosecond pulses ablate transparent materials more precisely through nonlinear absorption.
Why is Ti:sapphire still used when fiber lasers are cheaper?
It supports shorter pulses, is tunable across roughly 700–1000 nm, and remains the standard driver for high-field physics and many spectroscopy experiments. Yb fiber and disk lasers have taken over most industrial and high-average-power uses.
References: A. M. Weiner, Ultrafast Optics (Wiley, 2009); J.-C. Diels, W. Rudolph, Ultrashort Laser Pulse Phenomena, 2nd ed. (Academic Press, 2006); U. Keller, Nature 424, 831 (2003); D. Strickland, G. Mourou, Opt. Commun. 56, 219 (1985); A. E. Siegman, Lasers (University Science Books, 1986).