The first ruby laser is a three-level system, and there are three energy levels in the ruby matrix: E0, E1, and E2, where E0 is the steady state, E2 is the excited state, and E1 is the metastable state.
Initially, all the atoms in the laser material are at the lowest energy level E1, and when these materials are excited by radiation at certain frequencies, the particles of energy level E1 absorb radiation and transition to broadband energy level E3. In this way, the pump lamp causes the atoms to rise from the base energy level to the "pump band", which is the energy level E3.
Usually the "pump band" (energy level E3) is made up of many bands, so optical pumping can be done over a wide spectral range. Most of the excited particles are transferred to the intermediate energy level E2 by a fast radiation-free transition. In this process, the energy lost by the electrons is transferred to the crystal lattice, and finally the electrons are returned to the ground state by the radiation of the photons, and it is this final transition that produces the laser action.
If the pumping intensity is less than the laser threshold, the atoms of energy level E2 will return to the ground state in the form of spontaneous radiation, and the ordinary fluorescence effect is the consumption of the number of particles in energy level E2. When the pump radiation stops, the energy level E2 fluoresces at a rate until the number of particles is exhausted, which varies depending on the material.
At room temperature, the lifetime of ruby energy level E2 is 3ms, and when the pumping intensity exceeds the threshold, the decay of the fluorescence energy level includes stimulated radiation and spontaneous radiation. Stimulated radiation produces a laser output beam, and because the terminal energy level of the laser transition is the ground state with a large number of particles, the particles of the E2 energy level must reach a very high density before the 2-1 transition is reversed.
Generally speaking, in a three-level laser, the radiation-free transition rate from the highest energy level to the energy level generated by the laser must be faster than the other spontaneous transition rates. Therefore, the lifetime of the E2 energy state is longer than the relaxation time of the 3-2 transition, and the number of atoms N3 in the E3 energy level is negligible compared to the number of atoms in the other two energy states.
N3 << N1、N2 N1+N2 ≈ Ntot
One aspect that is important in the three-level system is that the atom is actually pumped directly from level E1 to the metastable level E2, where it only has a short residence time at level E3. Based on these conditions, we can calculate as if there were only two levels. In order for the number of atoms between the E2 and E1 levels to be equal, half of all atoms must be excited to the E2 level.
In order to maintain a particular amplification, the number of particles in the second level must be greater than the number of particles in the first level. However, in the vast majority of cases of practical significance, the necessary inversion (N2 - N1) is small compared to the total number of all ions. The power of the pump required to maintain this reversal is much smaller than the power required to achieve the same magnitude.
Disadvantages of three-level systems
- More than half of the atoms in the ground state must rise to the metastable energy level E2. Hence there are many ions that help in spontaneous radiation.
- Each ion involved in the pump cycle transfers energy from the E3 → E2 transitions into the crystal lattice, which is usually radiation-free and the energy is carried into the crystal lattice by phonons.
Be the first to comment