Lasers Flashcards
Electron volt
The amount of energy that an electron gains while moving through a potential difference of 1V
E=hc/wavelength
Hc=1239nm-eV
Energy carried by a single photon
What si the energy and wavelgnth of a photon emitted when an electron in hydrogen jumps from E2-E1
Cox
Transition rate
Transition rate between different energy levels varies and is determined by the electromagnetic properties of the individual atom.
Atoms that emit strongly at a certain wavelgnth have a high transition rate between the appropriate energy levels.
Atoms that emit weakly at a certain wavelgnth have a low transition rate between the appropriate energy levels
When the pressure is increased in a gas discharge tube, the atoms bump into each other more, the transitions are disrupted, and the ______ broaden the _____
Line spectra broaden the band spectra
If the pressure is increased still more, the band spectra broaden into a continuous spectra
Fluorescent colors
Stand out sharply because the luminaries fluz emitted at the fluorescent wavelengths by a fluorescent substance may be far greater than the luminance flux incident at those wavelengths
In most cases, the energy that drives the fluorescent radiation comes from an incidence higher frequency radiation (such as UV)
In terms os Frequency in fluorescents
The incident radiation has a higher frequency than the emitted or fluorescent radiation
In terms of wavelgnth, the incident radiation has a shorter wavelength (UV) then the emitted or fluorescent radiation (visible)
The strength of the fluorescence depends on the transition rates between the different states
Fluorescent dyes
- NaFL
- intact corneal epithelial cells do not absorb the dye because its a water soluble molecule that will not penetrate the lipid membranes of the cell
- then these membranes are damaged (perhaps by the CL), the dye is absorbed, and eyes areas then fluoresce at a wavelgnth of about 522nm when irradiated by near UV
- flurexon, a higher molecular weight dye, is used with those hydrophilic lenses that absorbed NaFL
Metastabel state
- the electron in an atomic excited state has a certain probability to decay or jump to a lower energy level. Usually these probabilities are such that the jump occurs within 10(-8) of excitation
- however, there are some excited states, called metastable states which ha very low probability of decay
- electrons may stay in the metastable excited states for milliseconds, seconds, minutes, or even hours
when a metastable state with a long lifetime is populated by incident radiation, the material may continue to glow or emit radiation long after the original source is removed. This is the phenomenon called
Phosphofluorescnec
The distinction between fluorescence and phosphorescence is a matter of
Time
According to the quantum theory, all Santa’s have some natural fluctautions or resonant frequency, f01. If an excited state happens to fluctuate too much, a downward jump results with the emission of a photon. This results in __________ and is responsible for the instability of the excited states
Spontaneous emission
Stimulated absorption
- not consider absorption by an atom that has a resonance frequency f01 connecting the ground state E0 and the first excited state E1
- when a photon of frequency is f is incident on this atom, it perturbs the atom, causing the electrical charge density to oscillate with the frequency of the photon
- when f equals f01, the perturbation induces the electron to absorb the photon and jump to the first excited state E1. This is referred to as stimulated absorption
Stimulated emission
- no suppose a photon Frequency f is incident on the excited atom in E1 where f equals the resonance frequency f01
- this photon again perturbs the atom causing the electric charge density to oscillate with the frewuncy f of the photon, but since this frequency if the resonance frequency f01 it induces the electron to emit a photon and jump down to E0
- the emitted photon is coherent with and travels in the same direction as the incident photon (in other words, the mitten photon is indistinguishable from the stimulating photon). This is called stimulated emission
LASER
Light amplification by stimulated emission of radiation
Light amplification
- in a laser, a photon is used to trigger a stimulated emission resulting in two coherent photons that have the same wavelength and are traveling in the same direction
- these two photons each trigger another stimulated emission resulting in four coherent photons al traveling in the same direction
- the four trigger more stimulated emissions resulting in eight coherent photons
- if the process continues, an avalanche of coherent photons is produced (light amplification)
Population inversion
- most atomic electrons are located in the ground sate E0 followed by the first excited state Ei, and a decreasing number are in each successively higher excited state
- to generate an avalanche of coherent photons, we need to move the “most” electrons in the ground state E0 to the spited state E1 but absorbing photons. This method of achieving an avalanche is called population inversion
Pumping
- population inversions can be obtained by using metastable excited states, since on an atomic scale the metastable state live a very long time
- the metastable states are populated by correctly pumping the medium. The pumping consists of supplying energy to the leasing medium
- this can be accomplished by a variety of ways: UV radiation, white light illumination (flashlamps), radiation from another laser, electrical currents, atom-atom collision, chemical reactions, etc
Three level lasers
RUBY laser
- the xenon flash lamp PUMP induces the transition from ground state E0 to the excited state E2
- two electron then makes a transition from E2 down to E1
- E1 is METASTABLE and so the electrons tend to stay there longer providing amply time to built up electrons in E1 more than in E0 by vigorous pumps (the POPULATION INVERSION)
- eventually (on the atomic time scale), one of the metastable states spontaneously decays to the ground state and emits a photon
This photons is incident on another E1 atom and stimulates the emission of a second photon that is coherent with the first
- these two are then incident on two more E1 atoms and stimulate the emission of two more coherent photons. The stimulated emission AVALANCHE is not started
- since the pumping radiation operates at a different wavelength (E0-E2), it does not compete with the avalanche
Four level lasers
- pumping from E0 to E4
- radiationless transition, E4 to E3
- at E3, metastable state allows to accumulate
- laser transition by spontaneous or stimulated emission from E3-E2
- radiationless transition from E2-E1
Cavity oscillator
- an active medium is placed between two reflectors s othat the coherent photons travel back and forth across the medium, thus building the strength of the avalanche. Such a setup is called a cavity oscillator
- the pump feeds energy into the active medium to build and sustain the population inversion. The coherent light sweeping back and forth sets up a standing wave in the cavity between M and G
- the Mirror G may be partially transmitting, in which cases a small percentage of the coherent light in the cavity is steadily drained off resulting in a continuous beam laser
- a laser considered contentiously only if the laser output is longer than 250ms
- typical ophthalmic lasers that operates in the continuous beam mode are Ar, Kr, He-Ne and diode lasers. Out put powers range from mW to Watts
Cavity oscillatory: pulsed lasers
- in a pulse beam laser, there is a “gate” that lets most (or all) of the oscillating coherent light in the tube come out in a pulse
- then the gate closes, and the laser needs to recycle before it is ready to pulse again
- much higher intensities can be achieved with pulsed beam lasers
- many pulsed lasers operate at very fast repetition rates
- in the pulsed mode laser, the power output is not constant with time
- pulsed mode lasers operate in range from fre ms to few femtoseconds
- Nd:YAG
Long pulsed laser
Laser pulse duration as fast as 1ms, is called a long pulsed laser
Ruby laser
Short pulse laser
- a much shorter pulse could be achieved by Q-switching, which lasts from 3-20ns
- ‘Q’ refers to the resonsnat quality of the laser cavity
- a higher Q factor indicates that laser emission will occur. Where a a low Q factor indicates that no emission will occur
Mode locking
-even shorter pulse, an ultra short pulse could be achieved by a process called mode locking
-the pulse lasts from 6 femtoseconds (10^-15)
To 80 picot’s Econ’s (10^-12)
-by synchronizing closely spaced oscillations, the peaks of the waves will occur simultaneously at a given instant and result in brief series of very short pulse
Laser components
In order to construct a laser, there has to be three components present: a laser medium, an excitation mechanism, and a feedback mechanisms
Laser medium
1, gas: He-Ne, Ar, Kr, Co2, Excimer (ArF)
- Liquid: rhodamine dye used in tunable dye laser
- Solid: Nd, diode laser
Excitation mechanisms
- Electrical pumping: electrons can be excited by passing an electric current
- Optical pumping: electrons are excited by absorption of photons from light sources
- Chemical pumping: electrons are excited by absorption of photons caused by breaking or making of chemical bonds
Feedback mechanisms of lasers
Optical cavity oscillator with two mirrors that the photons may be reflected back and forth from one mirror to the other building the strength of avalanche
Laser beam profile
Photons in cavity oscillator traveling back and forth between two Mirrors, achieve constrictive interference and form certain modes, known as transverse electromagnetic modes (TEM)
Types of laser beam profile modes
- Fundamental or single mode
2. Multimode
Fundamental or single mode
- the output of the laser operating in single mode is Gaussian in nature
- the Gaussian beam profile has the smalles spot size, imprtioant for ophthalmic applications
- the smallest spot size achieves the highest power density at the focus
- single mode laser is used for delicate work, such as a posterior capsulotmey as it localized the damage to the smallest area
Mutlimode laser
- the output of the laser operating in multimode generates more than single mode, or higher order modes
- the intensity distrivbution is non-Gaussian, which leads to larger cross-sectional power output
- because of the beam characteristics, the multimode laser is used on thick structures, such a cutting thick vitreous strands
Laser light characteristics
Coherence
Monochromatic
Collimated
Coherence light characteristic
The incidence photon and the photon emitted as result of stimulation are both in phone with each other, and are, hence coherent
Monochromic laser light
-each of the photons has the same energy, and hence, the same wavelgnth. Therefore the radiation is monochromatic. Monochromatic feature is important for OD application, as it eliminates chromic aberration and allows selective tissue damage because of selective tissue absotoyon
Collimated laser light
- the emerging laser beam of the cavity is collimated (parallel to each other)
- the monochromatic and collimated characteristics are most important for the ophthalmic users by providing fantastic focusing potential. These characteristics allow the laser output to be focused down to a diffraction limited spot size, if necessary
