30 Questions
What is the relationship between the probability of absorption P12 and the number of atoms N1 and the spectral energy density u(v) of radiation?
P12 α N1 u(v) or P12 = B12N1 u(v)
What is the lifetime of atoms in the excited state during spontaneous emission?
10^(-8) s
What is the condition for stimulated emission to occur?
Atoms must be in the excited state and a photon of energy hv must be applied.
What is the relationship between the probability of spontaneous emission P21 and the number of atoms N2 in the excited state?
P21 α N2 or P21 = A21 N2
What is the energy of the photons emitted during spontaneous emission?
hv = E1 - E2
What is the result of stimulated emission in terms of the number of photons?
One photon of energy hv produces two photons of energy hv in the same phase and direction.
What is the intensity ∆λ of a spectral line called?
Line width
What is the relationship between wavelength and frequency, according to the equation λ = c/ν?
λ and ν are inversely proportional
What is the condition for two light waves to be coherent?
They must start with the same phase at the same position and have the same wavelength
What is temporal coherence, and what does it measure?
Temporal coherence is a measure of the correlation between the phases of a light wave at different points along the direction of propagation, and it measures how monochromatic a source is.
What is the coherence time, τc, and what does it represent?
The coherence time, τc, is the maximum value of the time interval in which waves maintain constant and have predictable phase difference.
What is the characteristic of laser light that makes it have a higher degree of coherence?
Laser light is a resultant of a number of identical photons which are in phase
What is the wavelength of radiation emitted by CO2 lasers?
10.6μm
What is the typical power per unit length of a CO2 laser?
200-600 W/m
What is the role of neodymium ions in the lasing medium of an Nd: YAG laser?
They provide the energy levels for both pumping and lasing transitions.
What is the significance of the crystal field in an Nd: YAG laser?
It affects the energy levels of the neodymium ions.
What is the typical power range of sealed tube CO2 lasers?
Up to 50W
What is the purpose of the non-radiative and collision-induced transitions in the CO2 laser?
To decay to the lower level 1, which in turn decays to the ground state.
What is the spatial coherence length of the source, and when does it occur?
The spatial coherence length of the source is the separation lw between sources S1 and S2, and it occurs when interference occurs for lmax=lw.
How do conventional light sources and laser beams differ in terms of directionality?
Conventional light sources emit light in all directions, whereas laser beams emit light in only one direction, along the cavity axis.
What is the Rayleigh range, and what happens to the laser beam beyond it?
The Rayleigh range is the distance from the laser over which the light rays remain parallel, and beyond it, the laser beam diverges.
How is the divergence angle of a laser beam measured?
The divergence angle is measured from the center of the beam to the edge of the beam, where the intensity decreases to 1/e2 of that at the center.
What are the two parameters that cause beam divergence in a laser?
The two parameters that cause beam divergence are the size of the beam waist and the wavelength of the laser.
What is the full angle beam divergence, and how is it related to the divergence angle?
The full angle beam divergence is twice the angle of divergence, and it is a measure of the beam's spread as it travels through space.
What is the effect of the crystal field on the transition probabilities between the energy levels of the Nd3+ ion?
The crystal field modifies the transition probabilities to make some forbidden transitions allowed.
What is the typical length and diameter of the Nd: YAG laser rod?
The length varies from 5cm to 10cm, and the diameter is generally 6 to 9 mm.
What is the mechanism used to pump the Nd: YAG laser rod?
Optical pumping mechanism is employed.
What is the wavelength of the laser transition in the Nd: YAG laser system?
1.06 mm
What type of lamps are used to operate the Nd: YAG laser in pulsed and CW modes?
Xenon flash lamps for pulsed output and tungsten-halide incandescent lamps for CW mode.
What are some of the applications of Nd: YAG lasers?
Generation of other wavelengths of light in various scientific applications.
Study Notes
Spectral Line Width and Coherence
- The intensity ∆λ of a spectral line is called the line width.
- The line width can be calculated using the formula: Δλ = -2c Δν / ν^2.
- Coherence is a measure of the correlation between the phases of a light wave.
- Laser light is a resultant of a number of identical photons that are in phase, exhibiting a higher degree of coherence.
- Two conditions necessary for coherence: same phase at the same position and same wavelength.
Types of Coherence
- Temporal coherence: a measure of the correlation between the phases of a light wave at different points along the direction of propagation.
- Temporal coherence tells us how monochromatic a source is.
- The maximum value of the time interval in which waves maintain constant and predictable phase difference is known as coherence time, τc.
Spontaneous and Stimulated Emission
- Spontaneous emission: a random downward transition of atoms with the emission of photons, resulting in incoherent radiation.
- Stimulated emission: a downward transition of atoms with the emission of photons, depending on energy density u(v) and the number of atoms in the excited state N2.
- Stimulated emission produces coherent radiation.
CO2 Laser
- The power output of a CO2 laser increases linearly with length.
- Low power (up to 50W) continuous wave CO2 lasers are available in sealed tube configurations.
- Typical power per unit length is 200-600 W/m.
- The wavelength of radiation from these lasers is 10.6 μm.
Nd: YAG Laser
- The host medium for this laser is Yttrium Aluminum Garnet (YAG) with 1.5% trivalent neodymium ions (Nd3+) present as impurities.
- The laser rod is optically pumped using a xenon flash lamp or a tungsten-halide incandescent lamp.
- The laser output is in the form of pulses with a higher repetition rate or continuous output.
- Powers of over 1 kW are possible.
- Nd: YAG laser applications include generation of other wavelengths of light.
Directionality and Beam Divergence
- Laser emit light only in one direction (along the cavity axis).
- Directionality of a laser beam is expressed in terms of beam divergence.
- Beam divergence: the angle at which the light beam diverges from the center of the beam.
- The Rayleigh range is the distance from the laser over which the light rays remain parallel.
- The laser beam diverges beyond the Rayleigh range.
Test your understanding of laser light and its properties, including coherence and intensity. This quiz covers the mathematical relationships between wavelength, frequency, and coherence in laser light.
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