The application of femtosecond laser systems for eye surgeries has been a tremendous success story, not only driven by developments in new and improved laser sources, but also due to the continued development of optical systems to deliver the beams to the surgical field.
Femtosecond Lasers: From Basic Research to Industrial and Medical Applications
Around the mid-1990s, the first solidstate femtosecond lasers capable of generating ultra-short laser pulses with durations ranging from a few 100fs down to below 10fs were realized. These relatively robust laser sources quickly replaced the mode-locked dye lasers, which until then were the most important sources of ultra-short laser pulses for the research field.
New laser media, doped optical fibers, and the development of high-power diode lasers as pump sources allowed photonics manufacturers to develop more compact and robust femtosecond lasers while simultaneously increasing their overall efficiency. With the availability of true “turn-key” femtosecond laser sources capable of sufficient performance parameters, many ideas for industrial and medical applications could now be realized.
Femtosecond Lasers in Ophthalmology
One of the most prevalent uses for ultra-fast laser technology in medicine is the practice of refractive and cataract surgery in the field of ophthalmology. A femtosecond laser beam, focused into the ocular tissue by an optical system with high numerical aperture and diffraction- limited performance, allows surgeons to perform ultra-precise incisions in various ocular tissues based on the laser-tissue interaction resulting from the extreme focal power densities, including:
• the creation of the corneal flap as the first step of the LASIK procedure;
• relaxing incisions for the correction of corneal astigmatism;
• precise peripheral incisions in the cornea providing access to the interior of the eye, which in cataract surgery is required for the removal of the clouded lens and the subsequent implantation of an intraocular lens;
• the capsulotomy, the circular cut in the lens capsule which gives access to the lens tissue;
• the fragmentation of the murky lens, allowing the removal of the lens tissue from the eye;
• micro-structuring of lens tissue to partially restore lens elasticity to enhance the accommodating ability of presbyopic eyes.
The cut is created by laser disruption as a result of the nonlinear interaction of laser light with ocular tissue, which leads to a separation of the tissue layers. Every single laser pulse in its focus causes a microscopic cavitation in the tissue, which leaves behind a bubble of a few micrometers in diameter.
The equidistant and slightly overlapping planar juxtaposition of these bubbles, which are created in the focal plane, forms a planar perforation or section. The highly accurate dynamic positioning of the laser pulses in the tissue volume requires an optical system like those manufactured by Qioptiq (Fairport, NY), that can produce diffraction- limited focal spots localized to micrometer precision throughout the entire tissue volume under treatment. For a homogeneous perforation, it is also necessary to precisely synchronize the pulse train and the beam motion.
Pockels Cells and Faraday Isolators for the Laser Source
The timing of laser pulses can be controlled by Pockels cells (Fig. 1). These high-speed electro-optical switches allow the selection of the right laser pulse from the pulse train coming from the laser oscillator and passing it through for further amplification at the highest temporal precision. If further amplification is accomplished, for example by means of a regenerative amplifier, an additional Pockels cell in the resonator of the amplifier is required to inject the laser pulse into the resonator and to eject the amplified laser pulse after a fixed number of passes through the gain medium.
Pockels cells are the only optical switches which provide simultaneous nanosecond range switching times, low transmission losses, a high extinction ratio, and high switching rates. Lasers generally respond with extreme sensitivity to even very-low intensity backscattered light re-entering the oscillator, which occurs in particular if the laser beam is coupled into a subsequent amplifier stage. For these cases, the use of Faraday isolators is mandatory. These magneto-optical elements transmit the laser light in the forward direction and simultaneously block the return direction with an extinction ratio exceeding 1:1000.