* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download LASER IN Medicine
Survey
Document related concepts
Particle in a box wikipedia , lookup
Bremsstrahlung wikipedia , lookup
Tight binding wikipedia , lookup
Double-slit experiment wikipedia , lookup
X-ray photoelectron spectroscopy wikipedia , lookup
Matter wave wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Franck–Condon principle wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Atomic theory wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Wave–particle duality wikipedia , lookup
Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup
X-ray fluorescence wikipedia , lookup
Ultrafast laser spectroscopy wikipedia , lookup
Transcript
Light Amplification by Stimulated Emission of Radiation (LASER) Is a mechanism for emitting light within the electromagnetic radiation region of the spectrum, via the process of stimulated emission. light broadly denotes electromagnetic radiation of any frequency, not only the visible spectrum; hence infrared laser, ultraviolet laser, X-ray laser, and microwave laser (maser). Properties of laser light: 1. The laser light is (usually) narrow low-divergence beam, that can be manipulated with lenses. 2. coherent light waves of identical frequency and phase. 3. the laser light is a narrow-wavelength electromagnetic spectrum monochromatic light. Basic Principles of Lasers To explain the process of light amplification in a laser requires an understanding of the energy transition phenomena in the atoms of its active medium. They include: spontaneous emission, stimulated emission/absorption and non-radiative decay. The theory of quantum mechanics states that the electrons of atoms can take different energy states, E1, E2, E3, for example, with E1<E2<E3. Spontaneous Emission By quantum mechanics the lower energy level is more stable than higher energy levels, so electrons tend to occupy the lower level. Those electrons in higher energy levels decay into lower levels, with the emission of EM radiation. This process is called spontaneous emission. The radiation emitted is equal to the energy difference between the two levels. E2 - E1 = hn0 Where E2 is the upper energy level E1 is the lower energy level h is Plank’s constant n0 is frequency of the radiated EM wave. Stimulated Emission Suppose the atoms of the active medium are initially in E2. If external EM waves with frequency n0 that is near the transition frequency between E2 and E1 is incident on the medium, then there is a finite probability that the incident waves will force the atoms to undergo a transition E2 to E1. Every E2-E1 transition gives out an EM wave in the form of a photon. We call this stimulated emission since the process is caused by an external excitation. The emitted photon is in phase with the incident photon, has the same wavelength as it and travels in the same direction as the incident photon. Stimulated Absorption If the atom is initially in the ground level E1, the atom will remain in this level until it gets excited. When an EM wave of frequency n0 is incident on the material, there is a finite probability that the atom will absorb the incident energy and jump to energy level E2. This process is called Stimulated Absorption. Non-Radiative Decay Note that the energy difference between the two levels can decay by nonradiative decay. The energy difference can change into kinetic energy or internal energy through collisions with surrounding atoms, molecules or walls. Population Inversion Normally the population of the lower energy levels is larger than that of the higher levels. The processes of stimulated radiation/absorption and spontaneous emission are going on in the same time, yet even if we ignore the decay factors, stimulated absorption still dominates over stimulated radiation. This means that the incident EM wave cannot be amplified in this case. Amplification of incident wave is only possible when the population of the upper level is greater than that of the lower level. This case is called Population Inversion. This is a mechanism by which we can add more atoms to the metastable level and hold them there long enough for them to store energy, thereby allowing the production of great numbers of stimulated photons. To do this, we pump atoms into the metastable level at a rate that exceeds the rate at which they leave. A large number of atoms are therefore excited to and held in this level, leaving an almost empty level below it. The atoms stay in this metastable level without de-exciting while the population builds up, giving rise to a population inversion. In practice laser action cannot be achieved for only two levels, as described above. Three and four level systems work however. An analysis of these systems follows, followed by a description of the pumping schemes for each system. (Note: A metastable level is one that has a long lifetime and the for which the probability of spontaneous emission is low. This favors conditions for stimulated emission. If an atom is excited to a metastable state it can remain there long enough for a photon of the correct frequency to arrive. This photon will then stimulate the emission of a second photon.) Principal components of laser system: 1. Gain medium(active medium) 2. Laser pumping energy 3. High reflector 4. Output coupler