Lasers (Unit 2)

Question 1

What is stimulated emission?

Answer 1

-the emission of a photon from an excited atom, TRIGGERED by a PASSING photon of energy equal to the energy gap between the excited state and a state of lower energy in the atom.

Question 2

How does stimulated emission lead to light that is coherent?

Answer 2

-The emitted photon has the same frequency, phase, direction of travel and polarisation direction as the passing photon. (light emitted is therefore coherent)

Question 3

What is a LASER?

Answer 3

-Light Amplification by the Stimulated Emission of Radiation.
-A LASER is a coherent light source.

Question 4

What is a population inversion?

Answer 4

-A population inversion is a situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state

Question 5

Why do we need population inversion?

Answer 5

• N2 > N1
  -we need more electrons in an excited state
  -more will drop and emit photons.
  -We need stimulated emission to be more likely than absorption so that the laser beam increases in INTENSITY (i.e., the light is amplified).
  -EXTRA PHOTON PRODUCED

Question 6

What happens if you don’t use a population inversion? (more electrons at ground state)

Answer 6

-when we produce a lot of LIGHT
– the very photons that we produce are the actual photons that can be absorbed (they have the correct energy to produce both effects).
- If photons are being absorbed all the time then our laser beam isn’t getting any stronger.

Question 7

How do you create a population inversion?

Answer 7

-Pumping
- feeding energy into the amplifying medium of a laser to produce a population inversion.
(energy given to electrons)

Question 8

What is spontaneous emission?

Answer 8

• when an electron drops spontaneously (and randomly) from an excited state to a lower energy state and a photon is emitted.

Question 9

What the problem with a two level energy system?

Answer 9

• population inversion is not usually possible
• the best that can be achieved here is N2 = N1
  -the incoming FLOOD OF PHOTONS is just as likely to cause an electron to drop (stimulated emission) as it is to cause an electron to rise (absorption).
• as well as spontaneous emission rapidly depleting the excited state

Question 10

How is population inversion obtained in a 3 level energy system?

Answer 10

• 1) Pumping. Electrons are promoted from the ground state, E1, to a higher energy level, E3.
• 2)Electrons drop quickly ( E3 has a short lifetime ) through SPONTANEOUS EMMISION to the metastable energy level, E2.
• 3)Electrons stay in this level long enough for a photon of the LASER FREQUENCY to cause stimulated emission.
  -4) This is the transition that produces the laser photons so we must have N2 > N1 (more likely to be emitted than absorbed)

Question 11

What is meant by a metastable energy level?

Answer 11

it has a long lifetime and electrons stay there for a relatively long time

Question 12

How is population inversion possible in a 3 and 4 level energy system?

Answer 12

-the pumping frequency is higher than the laser frequency

Question 13

Problem with a 3 level energy system?

Answer 13

-at least half of the electrons in the lower level must be promoted to ensure there is a higher number in level E2 (so stimulated emission occurs). This takes a lot of energy

Question 14

How is population inversion obtained in a 4 level energy system?

Answer 14

1 ) -Pumping. Electrons are promoted from the ground state, E1, to a higher energy level, E4.
2) Fast drop to the metastable state, E3.
3) This is the LASER LIGHT PRODUCING TRANSITION so this time N3 > N2. Because E1 is the ground state, E2 is practically empty initially so obtaining population inversion is far, far easier. (There is no need to pump half of the electrons than with a 3 level energy system)
4)Another fast transition, so E2 has a short lifetime. This is because we want E2 to be empty so that we have a population inversion (if N2 is small it’s easier for N3 to be larger than N2).

Question 15

How is a 4 level energy system better than a 3 level energy system?

Answer 15

-LASER LIGHT PRODUCING TRANSITION is E3 so N3 > N2 There is no need to pump half of the electrons as E1 is the ground state, E2 is practically empty initially so obtaining population inversion is far, far easier (therefore more efficient) as less pumping is required

Question 16

How do electrons fall from N3 to N2 in a 4 level system and from N2 to N1 in a 3 level system?

Answer 16

-stimulated emission

Question 17

How do electrons fall from the highest energy level to the metastable energy level in a 3 or 4 energy level system)

Answer 17

-spontaneous emission

Question 18

How does light from a laser differ from normal light?

Answer 18

• polarised.
• coherent.
  -monochromatic
• has a parallel beam

Question 19

What is the structure of a typical laser?

Answer 19

• an amplifying medium between two mirrors, one of which reflects only 99% of the photons, the other 1% of the photons are transmitted. This is the laser light.
  -The amplifying medium is the region where the population inversion exists. (middle)

Question 20

How is increase in output energy reached in a laser?

Answer 20

-The photons are reflected backwards and forwards, causing further stimulated emission. Eventually they do escape through the partially reflective mirror.
-Under these conditions, one photon has the potential to ‘become’ two photons and these can ‘become’ four photons, then eight photons, etc.
-Like a chain reaction, this process will lead to an exponential increase in output energy.

Question 21

How many times will the beam pass through the amplifying medium in a laser?

Answer 21

Because only 1% of the light exits each time it reflects back and forth between the mirrors, on average, the beam will pass through the amplifying medium a hundred times before it exits.

Question 22

Why must the mirrors in a laser be parallel?

Answer 22

The mirrors must be parallel to one another to a high degree of accuracy in order to ensure that photons can be reflected multiple times without being ‘lost’ at the sides of the amplifying medium

Question 23

Advantages of a semi conductor laser to HIGH POWERED LASERS?

Answer 23

-Cheaper
-Smaller
-More efficient
-Easy to mass produce

Question 24

Uses of semi conductor lasers?

Answer 24

-Inside DVD and CD players
-Barcode readers
-Telecommunications (via optical fibres)
-Image scanning
-Laser surgery

Question 25

What is a semi conductor laser?

Answer 25

-use electrical current to pump the electrons to produce a high population inversion. -The mirrors used in a semiconductor laser TRANSMIT up to 60% of the photons.

Question 26

Disadvantages of high power light beams?

Answer 26

- power loss -inefficient - only travels in straight lines