German laser project gives vital results

This is the second of two articles on the extensive German study of femtosecond lasers. The first part was published in the December 2003 issue, p. 34.

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This is the second of two articles on the extensive German study of femtosecond lasers. The first part was published in the December 2003 issue, p. 34.

A German project on femtosecond-laser technology and applications—established in 1999 and presented to the public in September 2003—is a joint effort by companies and research institutes, and is supported by the German Federal Ministry for Education and Research, which provided about €27 million (US$31.98 million), with industry donating another €15 million (US$17.77 million). The goal was to explore the potential of femtosecond lasers in manufacturing and medicine. Results are highly encouraging. Further public funding is foreseen, to be invested into a broad application field termed "femtonics" by the representative of the main sponsor. The project was divided into six "packages."

Laser-plasma-generated x-ray

One package was devoted to laser-plasma-generated x-ray emission from small spots to be used in analytics, medicine, and nuclear physics. Two Ti:sapphire systems with different cavity designs were set up, both based on oscillator-chirped pulse amplification, and optimized to allow small focal spots so that up to 1020 W/cm2 could be reached. A third system was a diode-pumped colquirite-crystal laser with no pulse stretcher. The Ti:sapphire systems supplied 50-fs pulses of 5 to 10 mJ at 1 kHz; the colquirite produced 75-fs pulses of 115 µJ at 2.5 kHz and an M2 of better than 2.

Both solid and liquid targets were used for x-ray generation. The liquid target was a liquid-gallium jet that emitted x-rays nearly monochromatically in the gallium K-α lines at 9.251 and 9.225 keV and a smaller contribution at K-β of 10.3 keV. This source would allow megahertz laser repetition rates and is suitable for x-ray diffraction, absorption, and fluorescence spectroscopy. The photon flux in the K-α lines amounted to 3 × 1010/s. The solid targets were fast-moving ribbons, 20 µm thick, made of copper, titanium, and chromium that produced the K-α lines. At a repetition rate of 1 kHz, a train 4 × 106 pulses long could be generated, with the photon flux reaching 1012/s. Reflective x-ray imaging optics were developed. Applications included analysis of silicon wafers for surface contamination, x-ray diffraction on short time scales, photoemission spectroscopy, and medical uses such as angiography and mammography. Last, transmutation of long-lived nuclear fission products to short-lived ones induced by gamma-neutron reactions was studied (see Laser Focus World, November 2003, p. 13).

Better vision correction

Two medical packages were included. The first dealt with the development of femtosecond-laser systems for ophthalmology, otology, and dentistry. Different laser systems were built, based on fiber and disk technology, to supply pulses in the range of 150 to 200 fs for the treatment of soft tissue and in the range of 600 to 800 fs for hard tissue. In addition, scanning optics and hand-held beam suppliers were developed. In existing excimer-laser-based LASIK surgery for myopia treatment, a surface flap is cut mechanically and cornea ablation performed by excimer-laser radiation. Using femtosecond-laser radiation, the flap can be cut and the cornea shaped by the same laser, with very high precision and optimal shape. Healing was found (in animals) to be better than with the existing method. Femtosecond-laser treatment of presbyopia (in which the eye loses ability to focus as it ages) is also being studied; here, aged and hardened tissue is loosened by minute laser cuts so that the focal-length control of the ocular lens can be regained.

In dentistry, hard tissue was cut using 700-fs, 25-pJ pulses. Spectroscopy distinguished between sound and bad tissue due to caries. An almost gentle tooth treatment seems possible. This holds similarly for the applications in otology where both hard and soft tissue were treated.

The second medical package was also devoted to ophthalmology, as well as to neurosurgery, especially in the brain. All-solid-state subpicosecond laser sources were developed to perform noninvasive surgery. Intracorneal (or intrastromal) tissue can be ablated for myopia correction by focusing into the interior of the cornea and evaporation of the tissue (no cutting of flaps is required); afterward, the gas diffuses outside within a few minutes and the tissue settles (see figure). Corrections of two diopters have been achieved. While such surgery may not be sufficient for a full correction in one step, several noninvasive steps could be done, or a treatment could adjust the results of a less-than-optimal excimer-laser-based focus correction. Because full control over the shape of the removed tissue is obtained, any type of correction could be applied; for example, coma, cylindrical and spherical aberrations could be corrected.

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Unlike in conventional excimer laser surgery for vision correction, ultrafast laser pulses ablate material within the cornea without the need to cut an external flap.
Click here to enlarge image

Maximal precision is required for neurosurgery in the brain, both with respect to positioning and operational volume. The first is provided by a precise (nonoptical) scan of the brain and the second by computer-controlled femtosecond surgery. A method was developed that allows to feed in the beam via a steel cannula, a method potentially useful in treating diseases such as Parkinson's disease.

A separate package was entirely devoted to risks and safety with respect to femtosecond radiation. Due to the broad spectral bandwidth and high peak power, existing norms do not necessarily hold for femtosecond pulses. Proposals for improvements were made.

Information on the German femtosecond-laser project can be found at

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