Unit 2.8 - Lasers Flashcards
What does the word laser stand for?
Light
Amplification (by)
Stimulated
Emission (of)
Radiation
Stimulated emission of light
The electron is stimulated to fall from the excited state by an incident (incoming) photon
Describe the stimulated photon released during the stimulated emission of light
Same direction, phase and wavelength
(Completely coherent)
Why does stimulated emission of light occur?
The frequency (energy( of the stimulating photons exerts a force on the excited electron, causing it to fall to the ground state
Since the stimulating photon is not absorbed, both photons leave the atom with the same frequency, phase and direction
What is necessary for a laser system to operate
Population inversion
What do we need in order to obtain as much light as possible from a system?
As many atoms in the excited state as possible
Why will we obtain more light from a system if we have more atoms in their excited states?
More to spontaneously fall to the lower state
More that we can stimulate to fall down
What’s the issue when attempting to getan increased amount of light from a system via stimulating electrons in higher energy levels to fall down?
The photons that cause stimulation have exactly the same energy as the ones absorbed in order to excite ground state atoms
How does the value of absorbed photons compare to the emitted photons and why?
Total absorbed = total emitted
The photons that cause stimulation have exactly the same energy as the ones in order to excite the ground state atoms
What fact is true due to total absorbed photons = total emitted photons?
There’s no net gain in the intensity (number) of emitted photons
What can happen when a photon reaches an atom (ignoring spontaneous emission)?
Can pass through with no effect
Can be absorbed (if the electron is in the ground state)
Can cause stimulated emission (if the electron is in the excited state)
What affect does a photon being absorbed have on the intensity of the laser beam?
Net loss of a photon from the beam
What affect does a photon undergoing stimulate emission have on the intensity of the lase beam?
A net gain of a photon from the beam
What’s the ideal situation from the viewpoint of trying to produce a laser beam with high intensity?
Stimulated emission is more likely than absorption
What do we want to be more likely than what to produce a laser beam with high intensity?
Stimulated emission to be more likely than absorption
What will stimulated emission being more likely than absorption lead to?
A net gain in photons and an increase in intensity of the laser beam
What do we need to be true for stimulated emission to be more likely than absorption?
Need more electrons in the excited state than in the ground state
What’s the issue with producing a laser beam with high intensity?
We need more electrons in the excited state than the ground state to make stimulated emission more likely than absorption
However, electrons prefer to be in the ground state (the lowest energy state) as they’re unstable in the excited state
What’s the solution to achieving a high intensity laser beam by getting more electrons in the excited state than in the ground state to make stimulated emission more likely than absorption?
Pumping
What dos pumping energy into a system do?
Forces the electrons to move to the higher energy level
What does pumping do?
Supply energy (light in this case)
What maintains population inversion?
Pumping
What does pumping maintain?
Population inversion
Population inversion
A situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state of the same system
In what situation is population inversion not possible?
With 2 energy levels
What is pumping usually done with?
Light
Why is population inversion not possible with 2 energy levels?
A large number of electrons will be raised to the higher energy level - great!
But, when half the electrons are in the lower state and half in the higher, the incoming photons are just as likely to cause an electron to fall (stimulated emission) as they are of raising an electron from the ground state (absorption)
So, the best we can achieve is N1 = N2
This isn’t good enough for a laser
What’s the names of the states in the 3 energy level laser system?
Excited state
Metastable state
Ground state
What’s the most likely state of an electron?
Ground state
What’s the second most likely state of an electron?
Metastable state
What’s the least likely state of an electron?
Excited state
Describe the stages of the 3 energy level laser system
- The electrons are pumped by light from E1 to E3
E3 has a short lifetime of a matter of nanoseconds - The electrons drop quickly to E2, the Metastable state
Electrons stay in this level for some milliseconds (a long time for an electron). This is long enough for a photon of laser frequency to cause stimulated emission. - The transition that produces the laser light (need the population inversion N2>N1 here)
The electrons are stimulated to fall from E2 to E1 by stimulated emission, giving our beam of laser light - all photons are in phase and in the same direction
Describe the photos released by stimulate mission?
In phase and travelling in the same direction
Why is population inversion possible in a 3 level system?
The pumping frequency and the laser frequency are different
Which electrons can’t pumping photons interact with? Why?
An electron in the E2 state as it doesn’t have the same energy
Which ones are the only electrons that a pumping photons can interact with in a 3 level system?
In E1 and E3
Why are photons unlikely to interact with electrons in E3?
As its so short lived
Why is a population inversion caused in a 3 energy level laser system?
Photons can only interact with electrons in E1 and E3
As E3 is so short lived,they tend to interact with atoms in state E1,which gives a population inversion, which causes stimulated emission
What must the pumping rate be at in a 3 energy level laser system?
High enough to maintain the population inversion N2>N1
Why is the transition from E3 to E2 so short lived in a three energy level laser system?
E3 must be emptied as quickly as possible or we end up in a situation similar to the 2 level system
What must be true to cause a population inversion in a 3 energy level laser system?
Over half of the electrons must be in state E2
Where are there losses in the 3 energy level laser system?
As the electron drops from E3 to E2, a photon is emitted, which is of no use to us
Hwy do we use 4 energy level lasers?
Getting over half of the electrons to state E2 for the population inversion requires a significant amount of energy
Why is a 4 energy level laser better than a 3 energy level laser?
A 4 energy level laser allows us to achieve a population inversion with just one excited electron, so less energy is needed
Describe the stages in a 4 energy level laser
- Pumping. Electrons are promote from E1 to E4
- Fast drop to the Metastable state, E3
- Laser light producing transition, so we need N3>N2
It’s much easier to reach this point, since the ground state is very close to E2
E2 is essentially empty, so obtaining population inversion is much easier
There’s no need to pump half the electrons this time - Another fast transition, so E2 has a short lifetime
This is because we want E2 to May so that we have a population inversion (if N2 is small, it’s easier for N3 to be larger than N2) = less pumping is required and the laser is more efficient
In which energy level system for a laser is there the most energy loss and why?
4 energy level
Occurs in stages 1 and 4
which energy level laser required the leat amount of pumping and why?
4
It’s much easier to reach N3>N2, since the ground state is very close to E2
E2 is essentially empty, so obtaining population inversion is much easier
Draw and label the structure of a typical laser
(See notes)
Which state is the Metastable state in the 4 energy level laser?
E3
What are the reflectivity values of the mirrors in a typical laser?
One mirror with 100% reflectivity
Other mirror that reflects 99% of the incident light and allows 1% to be transmitted
What forms laser light in a typical laser?
The 1% that’s transited from the 99% reflective mirror
Where are the atoms in a typical laser?
Amplifying medium
Where does population inversion take place in a typical laser?
In the amplifying medium
Where in a laser are the conditions right for stimulated emission to occur?
In the amplifying medium
Why is the amplifying medium important in a laser?
It’s where the atoms are and where population inversion takes place and where the conditions are right for stimulated emission to occur
How are the conditions in the amplifying medium of a laser right for stimulated emission to occur?
-The photons are reflected backwards and forwards to cause further simulated emissions
-Mirrors must be parallel to one another to a high degree of accuracy to ensure that photons can be reflected multiple times without being “lost” at the sides of the amplifying medium
Why is it important that the mirrors are parallel to one another to a high degree of accuracy in a laser?
To ensure that photons can be reflected multiple times without being “lost” at the sides of the amplifying medium
Do photons eventually scape the laser?
Yes - through the partially reflective mirror
How many photons can each stimulating photon produce in the amplifying medium of a laser?
2
What word best describes how photons are produced in a laser? Explain
The process exponentially produces photons in a laser beam like a chain reaction, leading to an exponential increase in output energy
(2 produce 4 produce 8…)
How does the output energy of a laser increase in the amplifying medium?
Each stimulating photon can produce 2 photons, and the process exponentially produces photons in a laser beam like a chain reaction, leading to an exponential increase in output energy
How many times does the beam of light pass through the amplifying medium before it exits? Explain
On average - 100 times
Only 1% of the amplified light is released at any time when reflecting back and fourth between mirrors
Name some of the properties of laser light
Polarised
Coherent
Monochromatic (single wavelength)
Has a parallel beam
What is the efficiency of a normal laser?
Generally less than 1%
Why is the efficiency of a laser generally less than 1%?
Due to the energy needed to invert the population
Give reasons for why the efficiency of lasers is generally less than 1%
The pumping energy is considerably lager than the output photon energy
High intensity pumping combined with the high intensity of the laser beam means that the amplifying medium gets hot, which leads to large heat losses
The amplifying medium needs to be cooled so that it or its container doesn’t melt - by cooling the system, we jut transfer more heat and increase the losses
Semi-conductor lasers
Use electrical current to pump the electrons to produce a high population inversion
What type of laser uses an electrical current to pump the electrons to produce a high population inversion?
Semi-conductor laser
What’s the advantage or semi-conductor lasers over traditional lasers?
Much lower energy losses
Compare the energy loss of a semi-conductor laser compared to a traditional laser
Semi-conductor laser —> 30%
Traditional laser —> 99%
Advantages of semi-conductor lasers
Cheaper
Smaller
More efficient
Easy to mass produce
Uses of semi-conductor lasers
DVD and CD players
Barcode scanners
Optical fibres
Image scanners
Laser surgery
Laser printers
Disadvantage of semi-conductor lasers
Cannot produce the high power, focused beams that conventional lasers do
Absorption
Photon disappears and the electron gains its energy
Spontaneous emission
Electron drops randomly with stimulation, emitting a photon
What is meant by coherent light?
Monochromatic
Photons in phase
Light in phase
Why’s is stimulated emission needed for light amplification?
Because in each stimulated emission event, 2 photons out for 1 in
Why does E3 stay relatively empty in a three level energy system?
To receive pumped electrons
To keep the population of E2 high to maintain a population inversion
Pros of lasers over electrical wires
No visual pollution
Cheaper as less raw materials used
No danger of electrocution
Cons of lasers over electrical wires
Only travel in straight lines
Intercepted = cuts power
Inefficient
Danger to things in its path
Are laser pointers dangerous? Why?
They’re usually too low powered to be dangerous
