Overview of Commercially Available Femtosecond Lasers in

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					Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery Tammo Ripken

Laser Zentrum Hannover e.V. Biomedical Optics Department Head of Medical Applications Group Hollerithallee 8 D-30419 Hannover Tel.: ++49 511 2788-228 Fax: ++49 511 2788-100 E-mail: t.ripken@lzh.de Internet: www.lasermedizin.uni-hannover.de

The author receives research funds from Surgical Instrument Engineering..

Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

Introduction While the trend in refractive surgery is heading for all-laser-treatments, there are now different fs-laser systems for applications in corneal surgery available. All systems use ultra short laser pulses to dissect corneal tissue, whereas interaction process for cutting is based on photodisruption. However, there are significantly different technical details which have to be considered. This article will give a brief overview of the principal physical differences. Laser Tissue Interaction The interaction process of all laser systems for corneal surgery, e.g. preparation of corneal lenticule in LASIK procedures is based on nonlinear absorption and consecutive disruption of the tissue followed by a cavitation bubble and a residual gas bubble. Nonlinear in contrast to linear absorption means that the tissue is transparent for the infrared laser radiation at moderate intensities and no absorption occurs. Only at very high intensities, which can be achieved by compressing the laser pulse in time ("ultra short") and in space (strong focussing), some photons can be absorbed coinstantaneously by the tissue. The narrowness of the region of photodisruption depends on the degree to which the beam is strongly focused and is never, in practice, a single point in z-axial space. However, nonlinear interaction gives the user the advantage of three dimensional tissue processing. The absorption process is not limited to the surface anymore [Hei 02, Lub 00, Noa 99, Vog 05]. While photodisruption scales with pulse intensity (W/cm²), the unwanted side-effects like thermal heating, stress- and shockwaves, large residual gas bubbles scale with pulse energy (J). It is therefore desirable to minimize pulse energy while keeping the pulse intensity over the photodisruption threshold level (figure 1). Two obvious principles can be used: pulse shortening and strong focussing. Minimizing energy – maximizing cutting precision Shortening the pulse duration is a basic physical challenge, which is related to the spectral bandwidth of the laser medium. The pulse duration of typical fs-laser systems is around 200 to 800 fs (1 fs = 10-15 sec). In this range the energy threshold for optical breakdown increases almost linearly with pulse duration (figure 2). Thus, the shorter pulse duration of Ziemer’s Femto LDV (approx. 250 fs) is related to a lower pulse energy threshold then e.g. the AMO IntraLase with approx. 600 fs. The second way to decrease the energy is to decrease the focal volume of the laser spot. The focal volume of a Gaussian laser beam depends on the axial extension, the so called Rayleigh range (z = w0²/) and the beam waist w0 = f/wL, where f is the focal length of the focusing lens,  the laser wavelength and wL the radius of the beam at the focussing lens. In other words, the focal volume varies inversely with the fourth power of the numerical aperture NA = wL/f of the focussing optics. The larger the NA the smaller is the focal spot and finally, the less pulse energy is needed (figure 3) to cause photodisruption. According to its definition, there are two ways to increase the NA. One possibility is to increase the beam diameter at the focussing optics, which requires large and expensive optical components. As an alternative, one can also decrease the focal length of the focussing objective, which on the other hand reduces the working distance of the laser system (figure 4).

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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

Finally the lasers repetition rate (laser frequency) has an important influence on the pulse energy threshold. The higher the laser frequency the less pulse energy is needed (figure 5). Thus, it is obvious that MHz-Laser take advantage of this [Rip 07]. Technical Realization and clinical outcomes With respect to the interaction process and the beam delivery, femtosecond surgical laser can be classified into two groups: one group characterized by high pulse energy with low pulse frequency and the other group characterized by low pulse energy with high pulse frequency. In the "high pulse energy – low pulse frequency" group amplified laser systems are used to deliver pulse energies in the range of 1 µJ to the cornea. The typical repetition rate is about some kHz (AMO IntraLase, 20/10 Perfect Vision). Contrary to that the Ziemer’s Femto LDV which delivers only nJ pulse energies to the eye and uses MHz repetition rates clearly belongs in the “low pulse energy - high pulse frequency” group. Based on the laser parameters, the physical nature of the cutting processes of the two groups is different. In the "high pulse energy laser group", the cutting process is driven by mechanical forces which are applied by the expanding bubbles and which disrupt the tissue. This cutting process is very efficient because the radius of disrupted tissue is larger than the laser spot itself. Hence, the spot separation of the scanned laser pulses can be larger than the spot diameter (figure 6, left). On the other hand, due to the lower focussing accuracy and the larger remaining gas bubbles, poorer flap qualities, OBL (opaque bubble layer), TLS (transient light sensitivity syndrome) and anterior chamber bubbles may appear. There can be an additional waiting time for the proper use of the Excimer’s eye tracking system as well. Using low pulse energies, the cutting process is confined by the focal spot size of the laser pulse. As a consequence, more pulses are needed to cut the same area. To keep the total operation time at the same level, higher pulse repetition rates are required (figure 6 right). Since a pure fs-laser oscillator is used, it delivers these many pulses as well as very high stability and robustness against environmental influences compared to amplifier systems. The clinical outcome shows no OBL, no TLS, no anterior chamber bubbles and due to the very small amount of gas properly located in between the cutting plane, no gas pocket is needed and no waiting time has to be considered. To document these differences porcine cornea was cut with two different experimental setups. The histological findings are shown in figure 7, where the porcine cornea was cut with 2 µJ pulse energy and with 30 nJ, respectively. Both pictures are chosen as exaggerated examples to illustrate the effect of high pulse energy with disruptive large bubbles compared to the low pulse energy laser with smaller spot sizes and smaller bubbles. Conclusion From the physical point of view, all known parameters for fs-laser induced optical breakdown to cut corneal tissue are beneficial for the low pulse energy high frequency femtosecond surgical laser like Ziemer’s Femto LDV. The used fs-laser oscillator makes the Femto LDV compact and robust, delivers low pulse energy at high frequencies and has an unreached preciseness due to the high focussing optics
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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

very close to the treated area. The special feature of the low energy pulses can reduce bubble formation during the cutting process. The smaller the bubbles, the more precise the cut can be positioned, especially with respect to Sub Bowman Keratomileusis (SBK).

References:
[Hei 02] Heisterkamp A, Ripken T, Mamom T, Drommer W, Welling H, Ertmer W, Lubatschowski H; Nonlinear side effects of fs-pulses inside corneal tissue during photodisruption; Appl.Phys.B 74, 1-7 (2002) [Lub 00] Lubatschowski H, Maatz G, Heisterkamp, Hetzel U, Drommer W, Welling H, Ertmer W; Application of ultrashort laser pulses for intrastromal refractive surgery; Graefe’s Arch Clin Exp Ophthal 238, 33-39 (2000) [Noa 99] Noack J.; Vogel A.; Laser-Induced Plasma Formation in Water at Nanosecond to Femtosecond Time Scales: Calculation of Thresholds, Absorption Coefficients, and Energy Density; IEEE J. Quantum Electron. 35 (8), 1156 (1999) [Vog 05] Vogel A. Noack J.; Mechanism of femtosecond laser nanosurgery of cells and tissue Appl. Phys. B, 81(8) 1015 (2005) [Rip 07] Ripken T; Anwendung von MHz-fs-lasern in der Ophthalmologie und Erarbeitung eines Therapiekonzeptes für die laserassistierte Behandlung der Alterssichtigkeit, Dissertation, Gottfried Wilhelm Leibniz Universität Hannover, 2007

Figures: Fig. 1 Two laser pulses with the same pulse energy. The red labelled laser pulse is compressed in time domain (ultra short) and in space (strongly focussing with high NA). As a result its intensity increases above threshold for photodisruption. Threshold values for photodisruption in water. The black curve shows the intensity threshold and the red curve represents the fluence of a single laser pulse. In the range of 100 fs to 1 ps (red box) the function of energy threshold is almost linear with pulse duration. Calculated data points are from Noack [Noa 99] The focal volume of a Gaussian laser beam scales with the numerical aperture NA = wL/f of the focussing lens. The larger the NA, the smaller is the focal spot volume. At a constant relationship of focal length and lens diameter, certain focal diameter of the laser beam can be achieved by either large lenses and long working distances or by smaller lenses with shorter working distances.

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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

Fig. 5

The threshold energy in water decreases with increasing laser frequency. For MHz laser frequencies the threshold is almost half of kHz laser frequencies. For smaller focussing angles with higher pulse energies (lower laser frequencies) the cutting effect is driven by mechanical forces of the increasing cavitation bubble (right figure). On the other hand, MHz laser frequencies can offer many more pulses that are needed for cutting with lower pulse energies and larger numerical aperture (right figure). In this case, the size of the cut is defined just by the focal spot size. Histological sections of porcine cornea cut with an experimental laser setup with 2 µJ pulse energy, 930 fs (left) and with 30 nJ, 250 fs (right). Both pictures are chosen as exaggerated examples to illustrate the effect of high energy/low frequency and low energy/high frequency laser tissue interaction.

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Fig. 7

Figure 1

Intensity Threshold for disruption

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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

Figure 2
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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

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Ripken: Physical Aspects of Femtosecond Surgical Laser for Refractive Surgery

The Author: Tammo Ripken (33) has studied physics at the University of Hannover, Germany. He finished his Diploma in 2000 about the optimization of full-fs-Lasik with ultrashort laser pulses. Afterwards he worked as a member of the team of Dr. Lubatschowski in the Department of Biomedical Optics at the Laser Center in Hannover. His PhD thesis was about the use of MHz-fs-Laser in Ophthalmology and the presentation of a new treatment of presbyopia with ultrashort laser pulses. Tammo Ripken is still working at the Laser Center Hannover and is a consultant of Surgical Instrument Engineering.

Contact info: Dr. Tammo Ripken Laser Zentrum Hannover e.V. Hollerithallee 8 30419 Hannover Germany t.ripken@lzh.de www.lzh.de

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