Fundamentals of lasers

Question 1

What is a laser?

Answer 1

A device that emits light (EM radiation) through a process of optical amplification based on the simulated emission of radiation.

Question 2

What are the three useful properties of lasers?

Answer 2

1. It is monochromatic. A single wavelength of light. 2. Coherent. All the waves are in phase. 3. All the waves go in a single direction.

Question 3

What is stimulated absorption?

Answer 3

When an electron absorbs a photon of light and goes to a high energy level.

Question 4

What is spontaneous emission?

Answer 4

When an electron falls in energy levels and emits a photon of light in the process. It does this on its own without a trigger.

Question 5

What is stimulated emission?

Answer 5

When a photon of light interacts with a high energy electron. The electron falls in energy and emits a photon identical to the first photon, creating a pair. The two photons are identical.

Question 6

What is population inversion?

Answer 6

When there are more electrons in the high metastable state than in the ground state.

Question 7

What is the trend for lasers?

Answer 7

Shorter wavelengths and shorter durations. Materials absorb shorter wavelengths better.

Question 8

How could you produce shorter wavelengths of light?

Answer 8

Use 2nd or 3rd harmonics of light. This uses more power for less light though.

Question 9

What are some of the factors affecting laser material interactions?

Answer 9

Question 10

What is Absorptivity?

Answer 10

The fraction of incedent radiation absorbed by the material.

Question 11

What is the beer lambert law?

Answer 11

Question 12

What is the rough reflectivity of metals?

Answer 12

80-90%

Question 13

What is a rough graph descriding the absorptivity of different materials?

Answer 13

Question 14

What is the equation for beam divergence?

Answer 14

Question 15

What are diffraction limited beams?

Answer 15

• beams with a minimum possible beam divergence for a given wavelength

Question 16

What is brightness?

Answer 16

Question 17

What are affecting the absoption of metals?

Answer 17

incident angle

surface roughness (⇧ with roughness ⇧)

surface temperature (⇧ with temperature ⇧)

sufficiently high laser intensity (0.1 – 1MW)

Question 18

What are some of the different laser beam profiles?

Answer 18

Question 19

What is the rayleigh length?

Answer 19

Question 20

What is the equation for total angular spread?

Answer 20

Zr is the rayleigh length

Question 21

What is the M² factor?

Answer 21

a common measure of propagation quality of a laser beam. For an ideal beam it is 1.

Higher M² values might be wanted for welding metal.

Question 22

What is a diffraction limited beam?

Answer 22

An ideal beam with the minimum possible divergence for a given wavelength.

Question 23

What are gaussian beams?

Answer 23

Gaussian beams are selfconsistent and their amplitude profiles (shapes) are only scaled in the transverse direction.

Question 24

What is the equation for fluence?

Answer 24

Question 25

What is the ablation threshold?

Answer 25

Question 26

What is laser ablation?

Answer 26

a collective phenomenon involving the excitation of a large number of atomic sites accompanied by the formation of a dense plume of gas (plasma, ionized vaporized material) resulting in the removal of material from a surface

Question 27

What are ablation regions?

Answer 27

laser-material interactions (sublimation regime) with a tight temporal control to contain excitation energy to the absorption region with minimised thermal damage and melting.

Question 28

What are the different prefixes for small durations of time?

Answer 28

Milli 10^-3

Micro 10^-6

Nano 10^-9

Pico 10^-12

Femto 10^-15

Question 29

What are some of the material dependent time constants?

Answer 29

Question 31

How long are long and short laser pulses?

Answer 31

Long pulses are of the ms or ns duration.

Short pulses are less than this.