LASERs and Intense Light Devices Flashcards
What is spontaneous emission?
Emitted photon with energy and wavelength equal to band gap emitted at a random time, direction and phase.
What is stimulated emission?
Emitted photon with energy and wavelength equal to band gap emitted in response to an incoming photon. Close match in energy influences quantum state.
Photon is emitted in the same direction, and with the same phase as the incoming photon.
Why does the stimulated photon always get emitted in the same direction as the incoming one?
Multiple stimulating centres may have symmetry so emitted photons would interact semi-classically resulting in a preferential emission vector.
However, discrete centres may have axial symmetry but no preferential radiation direction.
The key is likely to be the bound nature of the charge centres and the quantum state.
What is necessary for LASEing to occur?
Population inversion. Preferably N2»N1.
What are the different mirror designs for lasing cavities and what are their advantages?
Plane parallel - Easy to manufacture and align for small cavities (e.eg diode LASERs) Concentric spherical (R=L/2 at both ends) - Good for specialist applications. Confocal (R=L at both ends) - Produces minimum possible beam size at each mirror, minimising diffraction loss. Good for large cavities (e.g. gas LASERs). Hemispherical - (R=L, and plane) Concave-Convex = (R_1 = L, R_2=L-R_1) - Good for specialist applications.
What is the equation for the longitudinal modes in a plane parallel LASER?
L=q(lambda/2)
How are transvers mode (TEM) profiles made?
A combination of Gaussian and Laguerre polynomials.
What are the advantages and disadvantages of lower order TEM profiles?
+ Reduced diffraction loss at the periphery.
- Don’t always oscillate in preference due to competition for axial atoms within the medium.
What would be the ideal surgical LASER?
- TEM_00 (Single mode, gaussian profile)
- Plane parallel or confocal design.
What are the practical limitations of LASERs?
- Spontaneous Emission
- Mirror Alignment Issues
- Mechanical Stability
- Particle Interaction
- Number of excited modes
- Particle motion.
- Natural Linewidth.
What causes the natural linewidth?
Broadening of the upper and lower energy levels in the atom.
What are two causes of broadening leading to the natural linewidth?
Collision Broadening - Collisions between excited populations cause atoms to de-excite sooner, interrupting phase and broadening frequency.
Depends on pressure (gas LASERs), temperature, and LASing medium.
Doppler Broadening - Thermal movement of atoms, shifts natural frequencies.
Depends on Temeprature and LASing medium.
What is the difference between T1 collision broadening and T2 collision broadening?
T2 collision broadening has a phase influence.
What are the properties of a CO2 LASER?
Wavelength: 10600nm
Efficiency: 10-20%
Power supply: No special power supply
Lasing medium: CO2 plus N2, Helium. Premixed in a sealed tube. Replenish at service
Pulse modes: CW, pulsed, RF
Delivery system: Articulated arm, or (newer) flexible waveguide
Power: 100 watts typical
Uses: Surgical, aesthetic, Cutting, peeling
Reason: Low thermal impact on surround
What are the properties of a Nd-YAG LASER?
Wavelength: 1068nm
Efficiency: 0.1-1%
Power Supply: Special power supply required for some. Water cooling.
Lasing Medium: Solid state,Flash-lamp pumped 0.1-10ms,
Pulse modes: Q-switched 10-30ns
Delivery System: Normally fibre delivery
Uses: Surgical, aesthetic, Cutting, coagulation
