Laser in Medicine

For

Medical Physics Students

By

Dr.Taha. M.Alfawwal

الفوال. أحمد طه محمد طه دمساعد أستاذالفيزياء قسم

التطبيقية العلوم كلية

الطب فى الليزر مقرر درجات توزيع

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• 1- Basic Principles

Contents

2 -Energy Levels in lasting medium , Boltzman constant

3- Stimulated emission:

1- Laser definition, Coherence

5- Laser pumping:

Chapter 1. Basic Principle of a Laser

Chapterr1 : LASERS:

•Theory of Operation and Basic Principles

•It is an acronym for Light Amplification by Stimulated Emission of Radiation.

•Coherence: Two waves are said to be coherent if they have a constant amplitude se (السعة) and a constant frequency (Time).

•The first working laser was demonstrated in May 1960 by Theodore Maiman at Hughes Research Laboratories,USA.

•A laser is a coherent light radiation ( in visible, UV, or infra red, IR emitted from an electronic-optical device.

Energy Levels in lasting medium and Boltzman Constant

Lasting medium: a material that can be excited to a metastable state by the introduction of energy of an approprate form

Processess : When energy is pumped into the cavity (e.g ., an intense light source that contains light of the appropriate wavelength to excite the atoms in the cavity ,)

Electrons in the atoms of the lasting medium are excited to higher energy levels, and then de-excite to their orginal states, releasing light of a particular wavelength.The well-known Boltzmann principle states that the number of atoms in a given excited state (Ni) , relative to those in a given by ground energy state (N0) , in a system at temperature T, is given by

Boltzman , constant

Ni/N0 = e –Ei/ Kt

Where Ei is the difference in energy between the excited and ground state and k is Boltzmann ,s constant (1.38 E-23 J/K)

By pumping energy into the cavity , one can induce a condition in when the majority of the atoms are in the excited ; this is called a population inversion . Atoms that are excited by radiation waves already in the lasting medium will emit radiation that in phase with the existing radiation of the appropirate the signal (rate wavelength and thus will add constructively to the radiation. These photons may induce other atoms to emit radiation. , thus continuing to amply (Fig. 3-5)

Stimulated emission:

•In optics, stimulated emission is the process by which, when perturbed (disturbed) by a photon, matter may lose energy resulting in the production of another photon.

•The process can be thought of as "optical amplification“.

•The perturbing photon is not destroyed in the process, and the second photon is created with the same phase, frequency, polarization, and direction of travel as the original.

•It is the act of energy transfer from an external source into the gain medium of a laser.

Laser pumping: ( Population inversion)

•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, population inversion is achieved.

•It is the state when a group of atoms or molecules exists with more members in an excited state than in lower energy states.

Population inversion:

The lasing medium must have at least one excited state in which electrons become trapped for a short period before dropping back to the ground state, If the trapping time is not sufficiently long, the popularion inversion can not

maintened and lasing action will not occur .

Within the optical cavity , the radiation is reflected back and forth between the mirrors on either end, so that even more stimulation and amplification occurs. On the partially mirrored end, however, some of the highly intense, highly coherent radiation is emitted; this is the useful beam of radiation (*Fig.13.6)

•After population inversion and stimulated emission, 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.

•After population inversion, the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, light is amplified.

•The optical cavity, contains a coherent beam of light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption.

Amplification of light:

LASER production in an optical cavity:(Laser production in lasting material)

diagram of basic structure of lasting medium 1-

2 -When laser is pumped , photons are emitted as excited electrons return to lower energy levels

3 -Photons refelect off mirrors and start to stimulate other electrons to emit their photons

4 -A chain reaction of photons begins and the laser begins to lase

5 -Full operation will occur

LASER production in an optical cavity:(Laser production in lasting material)

1

2

3

4

5

Illustration of optical pumping and the generation of laser light.

1 -Define the following items:

A- LASERB- Population inversionC- Stimulated EmissionD- Lasting Medium

2 -Write short notes on the following-:A- Bolttzman constant

Amplification of LASER B-