6 - Lasers Flashcards
What properties do the stimulating and stimulated photons share?
Frequency/wavelength, direction of travel and phase.
What is population inversion?
The cascade movement of electrons from their excited state to the lower state resulting in the laser light emission.
What are the three components of a laser?
Active medium, Pump, Optical resonator.
What is an active medium?
A luminescent material which stores the excited population and can be stimulated to emit it in a cascade.
What is an pump?
The source of energy used to excite the molecules of the active medium.
What kinds of optical pump exist?
Optical pump - light such as a flash lamp that uses photon absorption to excite the active medium.
Electrical pump - energises the active medium through means such as electrical discharge.
What is an optical resonator?
The housing of the active medium which essentially comprises of two highly reflective mirrors at either end, one of which is partially non-reflecting.
Why must the length of the optical resonator/laser cavity be carefully controlled?
It must be an integer multiple of the wavelength to ensure that all waves stay in phase (coherence) so as not to cause destructive interference.
Which of the stimulated photons contribute to the lasing?
Those that propagate along the axis between the mirrors. Others are absorbed by the unmirrored surfaces.
What are the two kinds of laser?
3-level and 4-level.
Describe the sequence of events in a 3-level laser.
Optical pump irradiates the active medium with photons whose energy corresponds to an excitation to an unpopulated level.
Electrons partially relaxed non-radiatively to a slightly lower metastable state.
Spontaneous emission from the metastable state creates a cascade.
What is the main disadvantage of a 3-level laser?
The spontaneous and stimulated emission from the metastable state is to the ground state, hence the photons compete to be re-absorbed to re-populate the state. This makes them very inefficient.
What is the requirement for population inversion in a 3-level laser?
At least half the molecules in the active medium must be excited to the metastable state.
How do 4-level lasers solve the re-absorption competition problem?
The metastable state electron emits a photon to move down to an unstable state (second lasing level) that rapidly decays non-radiatively to the ground state, so the photons emitted are no longer the right energy for re-absorption.
Why is the metastable state stable?
Because the difference in energy between it and the first excited state mean that emission cannot be stimulated by the pumping.
