Laser pumping involves transferring energy from an external source into a laser's gain medium, producing excited states in the gain medium's atoms. When there are more excited atoms than unexcited atoms, population inversion occurs, allowing stimulated emission and laser amplification or lasing. Population inversion is achieved after adequate pumping. Continuous wave lasers emit light continuously while pulsed lasers emit light in optical pulses, most commonly nanosecond pulses from Q-switched lasers. Laser-tissue interaction occurs mainly through absorption, with the light energy being transformed into heat. This results in photothermal, photoablative, or photochemical effects depending on laser parameters like intensity and pulse duration.

1. Dr. Hossam Eldin Sayed Ali
Lecturer of medical Biophysics
The Research Institute of Ophthalmology

2.  Important Points to remember:
 Laser pumping is the act of energy transfer from an
external source into the gain medium of a laser. The
energy is absorbed in the medium, producing excited
states in its atoms. When the number of particles in
one excited state exceeds the number of particles in
the ground state or a less-excited state, a process
known as population inversion is achieved. In this
condition, the mechanism of stimulated emission can
take place and the medium can act as a laser or an
optical amplifier.
 The pump power must be higher than the lasing
threshold of the laser.
 The pump energy is usually provided in the form of
light or electric current, but more exotic sources have
been used, such as chemical or nuclear reactions.

3.  Population Inversion
 A state of the gain medium where a higher-lying
electronic level has a higher population than a lower-
lying level, this is achieved after adequate pumping
 In simple cases, a laser transition, on which optical gain
occurs as a result of stimulated emission, can occur only
when the population of the upper laser level is higher
than that of the lower level (i.e. more laser-active atoms
or ions are in the higher level). This condition of the
laser medium is called population inversion.

4.  Continuous wave (CW) Laser
Continuous-wave (cw) operation of a laser means that
the laser is continuously pumped and continuously
emits light. The emission can occur in a single resonator
mode (single-frequency operation) or on multiple
modes.
Pulsed lasers:
Pulsed lasers are lasers which emit light not in a
continuous mode, but rather in the form of optical
pulses. The term is most commonly used for Q-switched
lasers emitting nanosecond pulses. Versatile types of PL
can be produced depending on: the pulse duration,
pulse energy, pulse repetition rate and wavelength

5.  The term Pulsed Laser is most commonly used for Q-
switched lasers emitting nanosecond pulses. VIDEO
 Nanosecond pulses in the ultraviolet spectral region are
generated with excimer lasers.
 The picosecond or femtosecond pulses are usually
generated with mode-locked lasers. VIDEO
 milli = 1E-3 micro = 1E-6 nano = 1E-9 pico = 1E-12 femto = 1E-15
atto = 1E-18.

6. Excimer Laser
 Excimer stands for excited di meris, a family of lasers that have
repetitively pulsed output, emit powerful pulses lasting for
nanoseconds or tens of nanoseconds, at wavelengths in or near
the ultraviolet, and the lasing medium is a diatomic molecule,
or dimer, in which the component atoms are bound in the
excited state but not in the ground state.
 Sometimes more correctly called an exciplex laser
 The most important excimer molecules are rare gas halides
such as XeF and XeCl.
 Average output power (in watts) is the product of the pulse
energy (in joules) multiplied by the number of pulses per
second (repetition rate). Typical average powers range from
under a watt to over 100W.


7.  Semi conductor (diode) lasers
 An electrically pumped semiconductor laser in which
the active laser medium is formed by a p-n junction of
a semiconductor diode similar to that found in a light-
emitting diode.
 The laser diode is the most common type of laser
produced with a wide range of uses that include fiber
optic communications, barcode readers, laser pointers,
CD/DVD/Blu-ray Disc reading and recording, laser
printing, laser scanning and increasingly directional
lighting sources.

8.  The LED consists of a chip of semiconducting material
doped with impurities to create a p-n junction. As in
other diodes, current flows easily from the p-side, or
anode, to the n-side, or cathode, but not in the reverse
direction. Charge-carriers—electrons and holes—flow
into the junction from electrodes with different
voltages. When an electron meets a hole, it falls into a
lower energy level and releases energy in the form of a
photon.

9. Holography

10.  How do Laser React With Tissue
 Laser as one of the electromagnetic waves, reacts in one of
the following ways when it strikes a surface:
 Reflection,
 Absorption
 Scattering
 Reflection occurs as a consequence of the change of the
refractive index between air and tissue, a small fraction of
the radiation is reflected back.
 Absorption and scattering of the remaining fraction occurs
when the beam is propagated into the tissue.

11.  The absorption of light energy, and the transformation
of the light into heat, is essential for the tissue
reactions to occur.
 The optical response of the tissue depends mainly on
the effective absorption coefficient which takes
account of absorption and scattering of the tissue at
the used wavelength

12. Mechanisms
 Laser beam intensity is an important parameter for
the study of the effects of macroscopic and
microscopic interaction of laser with matter and is
defined as: The ratio of the emitted beam power (in
watts) to the unit of irradiated area in square cm.
 The laser intensities can be classified into:
 High ( >1016 W/cm2),
 Medium ( ~1010 W/cm2) and
 Low ( <106 W/cm2). The latter in particular are used in
medicine for diagnostic use, those intermediate for
surgical use and those of high power mainly for
research purposes.

13.  An equal flow of energy can be supplied by changing
the exposure time and wavelength (λ) of the laser
radiation so that we can reach the high intensities like
1016 W/cm2.
 There are 4 types of interactions of the laser beam
with tissues:
 Photochemical.
 Photothermal.
 Photoablative.
 Photomechanical
interactions.

14.  PHOTO-CHEMICAL laser causes chemical change or
response; the light has a chemical impact on the
tissue being treated. Examples include: Photo
Dynamic Therapy (PDT) Cancer treatment,
ophthalmic treatments, and Laser therapy in pain
relief.
 PHOTO-THERMAL occurs when we use long pulses,
biological effect due to heating such as hair removal,
and most surgical lasers.
 PHOTO-MECHANICAL: Short-pulsed (q-switched)
lasers cause pressure waves that can stimulate the
lymph draining system leading to dissolution of
inflammatory mediators.
 Photoablation such as in tattoo removal.

15.  The interactions are based on the absorption of
radiation by :
 Water contained in the tissues.
 Hemoglobin in the blood
 Pigments or chromophores in some tissues normally
present (or externally administered).

16.  The photochemical (Photodynamic) interaction:
 occurs when the energy of photons is greater than that of
chemical energy bond, typically greater than 5 eV;
 Occurs at very low levels of intensity or irradiance (I ~ 1
W/cm2)) and for high exposure times (duration time
greater than a second).
 The field of action is that of ultraviolet radiation that
generates chemical fragmentation effects.
 (If the energy density deposited is very high it is possible to
achieve an ablation with high spatial control) (KrF laser,
ArF). In this case the contours of the spots must be well-
defined for the prevention of thermal spreads.
 The energy absorbed in the tissue is used for structural
modifications of the existing molecules, in fact the
reaction: hν + A + B →(AB) …. a molecule A and another
one B receives energy hν, a new product AB will be
produced.

17.  Fluence rates below the hyperthermia threshold can be
used for PDT (PhotoDynamic Therapy) , a two-step
modality in which the delivery of a light activated and
lesion-localizing photosensitizer is followed by a low,
non-thermal dose of light irradiation to kill cancer cells.
 Almost all photochemical reactions of biological
relevance are dependent on generation of reactive
oxygen species (ROS). The most important is 1O2
 1O2 (Singlet oxygen) is a high energy form of oxygen
 It is the lowest excited state of the dioxygen molecule.
Its lifetime in solutions is in the microsecond range (3
µsec in water).
 It undergoes several reactions with organic molecules.
 It is believed to be the major cytotoxic agent involved in
PDT.

18.  It is not possible to generate 1O2 by light of higher
wavelengths than about 800 nm. Thus, for PDT one
has to use radiation in the UV or visible range.
 Because of the optical window of tissue ‘between’ 620
and 1100 nm, yielding optimal penetration depths (∼1–
3mmat 630 nm), red light is most frequently used for
PDT.
 The red band of heme proteins is at 620 nm, so
chemicals (sensitizers) such as Porphyrins, chlorines
and phthalocyanins that absorb beyond 620 nm are
being used for laser PDT.
 Several lasers can be applied, notably dye lasers and
diode laser.

19.  Absorption of 10 light quanta can give as many as six 1O2
molecules, so the process is extremely efficient, and sub-
hyperthermal fluence rates can be satisfactory.
 Singlet oxygen has a short lifetime of 10–40 ns in cells and
tissues, and its radius of action is only about 10–20 nm.
 This means that PDT acts selectively on targets with high
concentrations of sensitizer . PDT Tumor selectivity is based
on this principle.
 The tumor selectivity of sensitizer accumulation can be
related to a number of factors:
 Low tumor pH (several sensitizers protonate and get more
lipophilic below pH 7),
 High concentration of lipoprotein receptors in tumours
(many sensitizers are bound to lipoprotein in the blood),
 Presence of leaky microvessels with low lymphatic drainage
in tumors, and a high concentration of macrophages (taking
up aggregated sensitizers) in tumors