IN The Name of GOD

1. IN The Name of GOD Basic of femtosecond laser A . Abolhasani MD A . Shojaee MD BASIR EYE CENTER TERHRAN – IRAN There is not a financial interest in the products or companies mentioned herein www.iranophthalex.com

2. Although laser light is perhaps the purest form of light, it is not of a single, pure frequency or wavelength. All lasers produce light over some natural bandwidth or range of frequencies A typical helium-neon (HeNe) gas laser has a gain bandwidth of approximately 1.5 GHz (a wavelength range of about 0.002 nm at a central wavelength of 633 nm), whereas a titanium-doped sapphire (Ti:Sapphire) solid-state laser has a bandwidth of about 128 THz (a 300 nm wavelength range centred around 800 nm).

3. In a simple laser, each of these modes will oscillate independently, with no fixed relationship between each other, in essence like a set of independent lasers all emitting light at slightly different frequencies. The individual phase of the light waves in each mode is not fixed, and may vary randomly due to such things as thermal changes in materials of the laser. In lasers with only a few oscillating modes, interference between the modes can cause beating effects in the laser output, leading to random fluctuations in intensity; in lasers with many thousands of modes, these interference effects tend to average to a near-constant output intensity, and the laser operation is known as a c.w. or continuous wave.

4. If instead of oscillating independently, each mode operates with a fixed phase between it and the other modes, the laser output behaves quite differently. Instead of a random or constant output intensity, the modes of the laser will periodically all constructively interfere with one another, producing an intense burst or pulse of light. Such a laser is said to be mode-locked or phase-locked Δtpulse duration N modes locked Δν frequency separation the HeNe laser with a 1.5 GHz spectral width, the shortest Gaussian pulse consistent with this spectral width would be around 300 picoseconds; for the 128 THz bandwidth Ti:sapphire laser, this spectral width would be only 3.4 femtoseconds

5. What is a Femtosecond? The number of femtoseconds in a second is far greater than the number of seconds in a human lifespan

6. The shortest directly produced optical pulses are generally produced by Kerr-lens mode-locked Ti-sapphire lasers, and are around 5 femtoseconds long produce optical features with durations as short as 100 attoseconds (10−18 second) in the extreme ultraviolet spectral region (i.e. <30 nm )

7. Applications • Nuclear fusion • Optical Data Storage • Femtosecond laser micromachining – drilling the silicon jet surface of ink jet printers • Two-photon microscopy • Corneal Surgery. Femtosecond lasers can create bubbles in the cornea, if multiple bubbles are created in a planar fashion parallel to the corneal surface then the tissue separates at this plane and a flap like the one in LASIK is formed (Intralase: Intralasik or SBK (Sub Bowman Keratomileusis) if the flap thickness is equal or less than 100 micrometres). If done in multiple layers a piece of corneal tissue between these layers can be removed (Visumax: FLEX Femtosecond Lenticle Extraction). • A laser technique has been developed that cutting steel , tooth enamel to very soft materials like heart tissue • Photonic Sampling, using the high accuracy of lasers over electronic clocks to decrease the sampling error in electronic ADCs • Femtochemistry • Medical imaging

8. laser-matter interaction

9. The course of a photodisruptive process is shown. Due to multiphoton absorption in the focus of the laser beam, plasma develops (A). Depending on the laser parameter, the diameter varies between 0.5 μm to several micrometers. The expanding plasma drives as a shock wave, which transforms after a few microns to an acoustic transient (B).In addition to the shock wave’s generation, the expanding plasma has pushed the surrounding medium away from its center, which results in a cavitation bubble (C). The maximum diameter of the cavitation bubble can reach 10 to 100 μm. Its lifetime is only a few microseconds. After the collapse of the cavitation bubble, a gas bubble is left behind, containing carbon dioxide ,water, nitrogen, and other gas molecules (D).

10. Mechanism of corneal femtodissection

11. Typical applications of photodisruption and their effects as a function of pulse energy.Nd:YAG lasers (10-ns pulse duration) are used to produce posterior capsulotomies at milli- Joule energies (A); femtosecond lasers can cut LASIK flaps with a microJoule of energy (approximately 930 femtoseconds) (B); almost bubbleless LASIK flaps at 100 nanoJoules (200 femtoseconds) (C); and cutting of mitochondria within a living cell with femto-scissors at 1 nanoJoule (90 femtoseconds) with a very high numerical aperture lens is shown(D).

12. dissection of a chromosome within a living cell using 80 MHz femtosecond

13. THz imaging THz radiation provides a means of identification of specific materials, including biomedical materials such as DNA. This is because molecular rotations, vibrations or librations occur in this frequency range

14. Group Velocity Dispersion (GVD) Optical pulse in a transparent medium stretches because of GVD • Because of GVD, red components (longer wavelengths) of the pulse propagate faster than blue components (shorter wavelengths) leading to pulse stretching (“chirp”). • Uncompensated GVD makes fs laser operation impossible • GVD can be compensated by material with abnormal dispersion

15. GVD Compensation

16. The first femtosecond laser approved for bladeless LASIK in the United States was the IntraLaselaser ( AMO ), which gained FDA approval in 2001 Femtec. This femtosecond laser from 20/10 Perfect Vision received FDA clearance in 2004 VisuMax. FDA-approved in 2007, from Carl Zeiss Meditec ZiemerFemtoLDV Ophthalmic Systems received FDA approval in March 2008 for its portable femtosecond laser

17. The threshold for optical breakdown (photodisruption) is inversely related to the laser’s intensity. The shorter the pulse’s duration and the smaller the diameter (and volume) of the spot, the lower the energy needed for photodisruption. Laser pulses of longer duration require greater energy to generate optical breakdown The side effect of greater pulse energy is disruption of the surrounding tissue. The slower the laser pulse, the more excess heat, shock, and acoustic waves can singe, melt, or otherwise alter the In matter around it.

19. Unique spherical Patient Interface of the FEMTEC workstation, Which does not applanate the cornea The laser's cuts are also curved, Following the stromal lamella FEMTEC uses tracker-friendly circular patterns, Patient Interface Suction is computerized and permanently controlled Other femtosecond lasers do applanate the cornea with a flat Contact glass, compressing the cornea and inducing higher Intraocular pressure . Central bubble layer scan interfere with Excimer tracker systems. Suction is typically applied manually without system monitoring and control

20. Laser Intensity

21. Thank you for your attention