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.
Beam Delivery Systems and Beam Diagnostics
After the laser beam has left the amplifier, its beam parameters — diameter and divergence — have to be matched to the subsequent optical system and aligned to its optical axis. This is where ultra-precision standard components such as laser mirrors, mirror mounts, and beam expanders with an adjustable expansion ratio fit perfectly. All optical components in the laser beam path — Pockels cells, mirrors, lenses, beam splitters, polarizers — must meet the highest standards of surface accuracy and conformity. A mirror with a surface irregularity of just one micron can cause a wavefront distortion of the laser beam destroying the diffractionlimited focus, which in turn implies a significant reduction in the focal laser power density. Only diffraction-limited focusing of the laser throughout the entire treatment volume guarantees homogeneous cutting performance.
As in all medical products, safety plays a key role, meaning that all functions of the system, including the laser operation, have to be monitored continuously. Before the laser beam is sent through the deflection and focusing unit, small proportions of the laser power are directed via splitter mirrors to dedicated detectors to control, for example, the laser power, the pulse-to-pulse stability of the laser, or the beam direction. In case of a malfunction, the monitoring electronics immediately stops the laser emission, and in addition, a mechanical shutter blocks the beam path in order to guarantee the safety of the patient.
Beam Deflection and Cutting Optics
On the beam path as described above, the laser beam was guided only on an optical axis. For the processing of a tissue volume in the eye, the beam has to be deflected from the axis and focused to different depths in the tissue.
For the deflection in the x- and y-direction, a deflection unit is responsible, which may use various types of scanners, depending on whether the system should cut along any arbitrary trajectory in x and y, or whether the scan across the surface is done in a line scan mode with a fast and a slow axis. The scan devices can be galvanometric scanners, polygon scanners, piezo-scanners, MEMS, and a few others.
The optics behind the beam deflection system is the most challenging part: the angular position of the laser beam must be converted with the utmost precision to the transverse position of the focus in the tissue, while also maintaining the focal quality from the center to the edges. This cannot be achieved with standard optics. The optical design of the “post-scan” system has to be optimized specifically for the requirements of the application. The input parameters from the beam deflection system — aperture and maximum deflection angle — are translated by the optical system to field size and numerical aperture. In the design of the cutting lens, which is closest to the eye, the human anatomy must be considered. In order to avoid collisions with the nose or the forehead of the patient, the lens needs to be tapered toward the eye. The required large working distance can be achieved only with a longer focal length. In order to achieve high scanning speeds the mechanical dimensions of moving optical elements have to be small. Therefore the aperture of the beam deflection system should not be larger than some 15 mm and a further beam expansion between the scanning system and the cutting lens is needed.
To address different working depths in the eye tissue while keeping the numerical aperture and focus quality constant throughout the working volume, the cutting lens has to be designed as a zoom lens system. Single lenses or lens groups move along the system’s optical axis. For a safe procedure, it is mandatory to track the exact location of the laser focus in three dimensions at any time. This requires highly accurate control and monitoring of the lens movement.
Of course the surgeon must have full continuous control over the whole process. This starts with the positioning of the patient relative to the optical system, when the patient's eye is docked to the cutting lens. For the rapprochement between the optics and the eye, the surgeon needs full visual control, for example with a live camera that looks through the cutting lens onto the patient's eye. The location of the various ocular media (anterior chamber, crystalline lens), which must be known precisely for the course of the treatment, are measured for example by an optical coherence tomography system. The calibration of the focal position of the laser system is generally done using a confocal detection of the interface between the optical system and the eye.
The end user of the system, the ophthalmologist, expects a device that fits ergonomically as a compact unit in the environment of his or her operating theater. The above-described long beam path from the laser source through the amplifier, the beam shaping, and beam deflection optics, and finally through the “post-scan” optics to the eye of the patient must fit in a compact and robust medical device.
The optical sub-systems along the laser beam path have to be firmly mounted on a highly rigid but lightweight support structure guaranteeing stable alignment between one another. The control electronics and the above-mentioned measurement systems (OCT, confocal detection, live camera) must be accommodated in this same support structure, which finally has to be rigidly connected to the base unit, which also comprises the laser itself. Even small displacements of the laser beam at the entrance of the optical system or along its way are unacceptable because they would immediately affect the performance of the whole system.
Companies like Qioptiq can offer a wealth of experience in the concept development, design, and manufacture of complex customized optical systems, as well as a wide range of standard components perfectly suitable for the setup of femtosecond-laser-based surgery systems. The full control of the tolerances in the system is crucial for the development of a system that can be reproducibly assembled and aligned in serial production and which is expected to provide excellent day-to-day performance under real working conditions in the ophthalmic practice.
From the microscopic shape of each optical surface through the lens centering accuracy, to the relative positioning of the subsystems to each other, all tolerances are first analyzed in the optical design and proven in the simulation of manufacturing conditions, in order to have optimum efficiency in the manufacturing process and to guarantee reliable and failure-free operation in the field.