UNIT 2 - Lasers Flashcards
What does laser mean/stand for
Light Amplified by the Simulated Emission of Radiation
5 facts about energy levels and electrons
- Electron cannot exist between energy levels
- Electron energies are quantised and have a definite amount of energy
- Electron energies are negative compared to an electron outside of an atom
- Electrons always seek to get to the ground state, lowest possible energy
- If an electron is in a high level and there is no space below it , then we say the electron is in an excited state
Whats spontaneous emission
When an electron is in an excited state the atom is said to be unstable. The electron can jump down to a less excited state by randomly emitting a photon of specific energy. The photon emission could take place any time any direction without any phase of oscillation
What is stimulated emission
The process by which lasers produce light, occurs when an electron is already in an excited state. If a photon has an energy equal to the energy difference of electrons excited level and the level below, it can stimulate the electron to drop down to that lower level, releasing a photon of equal energy to the incomming
photon.
Why is the electron stimulated to drop levels in stimulated emission
Because the incoming photon is an electromagnetic wave and its electromagnetic field will exert an oscillating force on the excited electron. If the incoming photon is of the correct frequency, this oscillating force will cause the excited electron to drop and both photons will exit with same frequency, phase and direction.
What is population inversrion
A process required by laser technology. Population inversion is when there are more electrons in the upper level than the lower level, which causes stimulated emission to have a higher chance of taking place as opposed to absorption.
What is pumping
The process of feeding external energy into a system to create a population inversion where ground state electrons rise to the highest energy level
What are the two types of pumping
1.Optical pumping by an incoming light photon of the correct frequency/energy;
- Electron collision (electrical current)
Why cant lasers ordinarily operate with a 2 level energy system
Because an incoming photon of the correct frequency is just as likely to promote an electron and get absorbed as it is to stimulate an emission of a photon
What is a metastable energy level
Means that electrons in this level can stay there for a long time, typically milliseconds, before decaying to the ground state
How does a 3 level energy system work?
There is an extra 3rd level. Pumping occurs between E₁ and E₃ , and then decay rapidly to E₂ without releasing radiation but instead transferring heat to atoms within the medium. At E₂ many electrons accumulate and a population inversion is attained
How does a 4 level energy system work?
Similar to 3, has 4 levels. Electron is pumped from E₁ to E₄ and then from E₄ it decays rapidly to E₃ without releasing radiation but instead transferring heat to atoms in the medium. Then electrons may fall to E₂ and then rapidly decay to E₁ without releasing radiation. And so very few electrons sit on E₄ and E₂ and a population inversion is achieved much easier between E₃ and E₂
Describe the main structure of a typical laser
- energy input mechanism (pumping method)
- amplifying medium
- 2 mirrors to allow optical feedback
What is the energy input mechanism in the structure of a laser
It is the pumping method which is necessary to create a population inversion in the atoms of the amplifying medium.
What is optical pumping
A flash lamp emitting a wavelength and frequency with enough energy to promote electrons to the higher energy level. Pumping photon frequency is different to laser photon frequency so it doesn’t stimulate emission
