Excimer Laser
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.