Lasers Characteristics

Question 1

What is the function of the cavity?

Answer 1

Provides feedback for the gain
Consists of two or more mirrors that build a strong laser field
One mirror leaks a small amount for the output

Question 2

Laser cavity forces the emitted radiation into an extremely small solid angl. What is the solid angle equation?

Answer 2

(Beam radius/ cavity length)^2

Question 3

What determines the beam profile and divergence?

Answer 3

The laser cavity. This is due to the electric field of the laser light being a solution to Maxwell’s equations and the cavity mirrors create the boundary conditions.

Question 4

What is the relationship between beam divergence and spot size?

Answer 4

A collimating beam has to be large

A highly focused beam is divergent

Question 5

What can a gain medium be made of?

Answer 5

Atoms/molecules in a gas or liquid or solid
Semiconductor
Electron beam

Question 6

What creates temporal coherence?

Answer 6

Waves emitted from the same point at different times have a fixed phase difference.
Michelson interferometer.

Question 7

What creates spatial coherence?

Answer 7

Waves emitted from different points at the same time have s fixed phase difference.
Double slit experiment.

Question 8

What is the coherence length?

Answer 8

Max path difference between interfering beams that allow the observation of the interference fringes. Measured with a Michelson interferometer.

Question 9

What is the relationship between fringe visibility and coherence length?

Answer 9

Fringe visibility drops as arm length increases beyond coherence length.

Question 10

What process happens in the gain medium that allows stimulated emission to be possible?

Answer 10

Population inversion. This requires at least three atomic levels and occurs when energy is pumped into the system either electrically or optically.

Question 11

When does lasing begin?

Answer 11

When the gain exceeds the loss in the cavity. This is called the threshold point.

Question 12

What is the result of stimulated emission?

Answer 12

A photon is emitted which is an exact replica of the original input photon. They have the same frequency, phase, direction of prolongation, polarisation etc.

Question 13

Is laser light completely homogeneous?

Answer 13

No, the line shape function, g(v), describes the distribution of emitted intensity vs. frequency. g(v)dv is the probability that a spontaneously emitted photon has a frequency between v and v+dv.

Question 14

What is the difference between homogeneous and inhomogeneous broadening?

Answer 14

Homogeneous broadening affects all the atoms in the same way where as with inhomogeneous broadening different atoms see different shifts.

Question 15

Why does natural broadening happen?

Answer 15

Due to the finite lifetime of the state the energy level has a particular width which affects all atoms equally.

Question 16

Why does pressure broadening occur?

Answer 16

As atoms in a gas bump into each other energy levels are shifted and lifetimes shortened. This affects all atoms equally.

Question 17

Why does broadening from phonon interactions occur?

Answer 17

Due to acoustic vibrations in solids which affects all atoms equally.

Question 18

What is Doppler broadening?

Answer 18

Inhomogeneous broadening that occurs when atoms in a gas move and see different Doppler shifts.

Question 19

What effect does the crystal field have on the line width?

Answer 19

Causes inhomogeneous broadening due to the transition frequencies dependence on the immediate crystalline surroundings. Different ions in different positions in the crystal will have a different frequency transition. This is fundamentally due to the stark effect where atomic energy levels shift with the electric field.

Question 20

What determines the gain bandwidth?

Answer 20

The combination homogeneous and inhomogeneous broadening.