A laser is a device that emits light at a specific wavelength by stimulating atoms or molecules. A 3 level laser is an electrical system that uses three energy levels and the transition ends on the ground state. A 4 level laser is an electrical system that uses four energy levels where transition ends on a level above the ground state.
It is important to know which is superior to the other when choosing between the two. What would helpfully guide your choice?
3 Point Laser Level Vs 4 Point Laser Level
In 4 level laser, the population inversion is guaranteed regardless of how fast you pump the system. In contrast, with a 3 level laser, you might fail to get population inversion because the atoms tend to remain at the lowest level. The pumping power required in 3 level laser is quite higher.
If the relaxation rate is faster compared to where the laser action should occur, the population is achieved.
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Things to note:-
- To pump means to supply energy in the system.
- Population is the number of electrons per unit volume in an energy state.
- Population inversion is what it takes to achieve a greater population of higher energy state compared to lower energy state.
- The population of higher energy state will never exceed the population of lower energy state.
Laser Transition Rate
The transition rate of atoms in 4 level laser is quite faster because of the small lifetime of the atoms. In 3 level laser, atoms drop to a metastable state hence is the likelihood of atoms crowding at giving a slow transition rate. If the relaxation rate is faster compared to where the laser action should occur, the inversion is achieved
What to note:-
- The transition is radiation-less, it is just energy given away.
- Metastable state is a lower energy state of a long life of atoms.
- Atoms from higher state decay spontaneously to this metastable state.
Mode of Operation
4 Level laser operates in a continuous wave mode ensuring they produce a steady beam, unlike 3 level laser that operates in a pulsed mode since atoms easily absorb the stimulated emissions.
What to note:-
- Stimulated emission is the interaction of an emitted photon with electrons in a metastable state forcing that electron to fall back to the ground state’
- Photon is energy released when electrons fall back to a lower energy state.
Instances of Defects
Defects are present in 3 level laser dues to imperfect crystalline whereas there is no instance of crystalline imperfection in 4 level laser.
What to note:-
Crystalline imperfection is a region of distortion with a thickness of a few atomic diameters.
Based on the above, if you were to choose between the two, it would be advisable to choose 4 level laser. Even though both achieve inversion by optical pumping, 4 level laser will efficiently work in population inversion and will have a faster transition rate.
Also, 4 lever laser produces a continuous beam of light compared to 3 level laser and has no instances of defects on the product of the system.