FREE ESSAY ON EXCIMER LASER

EXCIMER LASER

Excimer laser and its applications
Introduction 
The first report of a new type of excimer system came from Golde and Thrush at
Cambridge who observed characteristic bound-free transitions in ArCl produced by
reacting argon metstables with chlorine in a flowing afterglow system. 
At present the rare-gas halidel asers are undoubtedly the most important excimer
lasers and are being actively developed for applications in laser-induced fusion and
isotope
separation.
A large amount of energy, ranging from 8 eV in Xe to 20 eV in He is required to
produce the first excited state because of the closed shell nature of the normal state of
the
rare gases. 
Application on Water Pollution
A typical spectrum of polluted sea water will contain the intense water Raman
signal at 344 nm, the gelbstoff fluorescence from organic and biological waste, which is
peaked between 400 nm and 420 nm, the fluorescence of light and heavy oils peaked
between 400 nm and 500 nm, and possibly some chlorophyll from phytoplancton peaked
around 685 nm.
The LIF spectra of the crude oil samples (Fig. 2d) show that, at variance with
refined oil samples emitting mostly in the near UV, their fluorescence emission covers
most of the visible spectral range. Although the total emission intensity decreases
dramatically at increasing intensity, measured spectral shapes are quite similar
throughout
this region, where three maxima can be identified, roughly peaked at 460, 490, and 540
nm. Higher resolution measurements were attempted, however did not reveal the
presence of any sharper feature. The general trend is a broadening of the fluorescence
spectra towards longer wavelengths with increasing oil density. The presence of crude
oils on water surface can be recognized from their typical emission spectra, but the
direct
identification of the specific oil seems to be rather difficult if no additional
information is
available.
Measurements of the water Raman signal have been performed at high resolution
in the range 330 nm to 365 nm in order to discriminate both from the intense tail of the
backscattered laser radiation and the rise of oil fluorescence band. Measurements in the
same wavelength range have been performed after adding fixed amounts of different oils
on the surface above a certain water column. The spectra of Kirkuk and Saharan Blend
oil are shown in Fig. 4 and it is noticeable that the water Raman peak intensity is
progressively reduced by the oil absorption of 308 nm laser radiation which thus cannot
effectively penetrate in the water column. In addition, the first peak of the oil
fluorescence
spectrum was detected in this range ( ~360 nm) which is especially intense in the case
of
the lightest oil. the dependence of oil fluorescence intensity and water Raman intensity
upon oil (quantity) thickness has been checked in order to use the lidar fluorosensor
for
field measurements of oil film thickness on sea water. However the integrated oil
fluorescence in the range 360 to 364 nm, after proper background subtraction, vs the
quantity (drops) of oil spilled upon the water surface followed a linear behavior only
at
very small quantities and quickly reached saturation, especially for the heaviest oils.
This
demonstartes that absolute fluorescence measurements, which also require the knowledge
of the kind of oil detected, are not suitable to determine the thickness of the pollutant
film.
Time decays curves for the four crude oil samples have been measured through all
the visible range and the excimer laser pulse profile has been measured as well. In Fig
6
(a) the typical laser profile showing at least two well resolved cavity modes and in
(b)-(e)
crude oils appear distinguishable according to their density, in fact lighter oils are
characterized by longer time constants. the observed trend in lifetime is significant to
the
identification of the crude oil sample. Therefore in conclusion, measuring accurate time
decay constants should allow for the unambiguous identification of pollutant oils in
remote
sensing experiments together with fluorescence. In addition, according to the result, it
comes out that an UV laser source with shorter pulses would permit more accurate time
resloved oil fluorescence measurements. A more complete data base for oils recognition
can be built by increasing the number of parameters in a multiexponential fit.
Application on Optometry
The Argon-Fluoride Excimer Laser is a revolutionary innovation and advanced
treatment modality in an attempt to correct myopia, hyperopia and astigmatism, as well
as
superficial keratectomy to erase corneal scars and irregular corneal surfaces. When the
Argon-Fluoride Excimer Laser is used in corneal reshaping to correct refractive errors,
it
breaks the carbon-to-carbon molecular bonds of the corneal tissue by the ultraviolet
193-nm wavelenghth of emission photochemical effect called photoablation. This
photoablation effect is extremely superficial. Minimal thermal damage is created by the
ultraviolet excimer laser, unlike traditional lasers in which the produced heat causes
damaging effects to surrounding tissue. The pulsing excimer laser removes the tissue in
microscopic layers, leaving virtually no underlying thermal trauma. The carbon-to-carbon
bond holding most of the tissue together has an energy requirement of 3 electron volts.
If
an excimer laser photon is introduced, it can literally crack that bond. The
photon-energy,
or energy per photon, of the excimer photon is 6.4 electron volts, or 10-15 mj per
photon.
One laser pulse contains many photons. One excimer laser pulse contains 2.5 x 1016
photons. Therefore, the energy per pulse at the eye is equal to the 10-15 millijoules
(single
photon energy) times 2.5 x 1016 (number of photons in one pulse), which equals 25
millijoules (mj). (2.5 x 1016 = 25 billion million.) These excimer photons are like
photon
scissors, breaking the carbon-to-carbon bonds of the corneal tissue. Hence, the excimer
photon is incredibly energetic, having 3 times as much energy as the YAG laser photon
and more than twice the energy as the Argon laser photon. The term that has been coined
for the effect of the excimer laser on the tissue is photoablation. The key to the
excimer
laser is the short pulse duration (10 ns or 10 x 10-9 s) with high energy photons
(energy
per pulse is 25 mj at the eye) with the possibility of concentrating large numbers of
these
photons on tissue to crack the carbon-to-carbon bonding that holds tissue together. For
the first time, a no-touch system, or no-touch scalpel, with the ultimate resolution of
a
fraction of a micron, is available to surgeons. (One micron equals one one-thousandth of
a
millimeter.) So, without touching the eye, the excimer can change and sculpt the cornea
(photon scissors) incredibly accurately with virtually no collateral damage conducted
into
the edges of the tissue affected. There is no significant mechanical effect to the
surrounding tissues; and no crushing of tissue