A ruby laser is a solid-state laser that utilizes synthetic ruby crystal as its laser medium. In 1960 Maiman designed the first working ruby laser. Ruby laser is among the few solid-state lasers that emit visible light. It produces wavelength 694.3 nm of deep red light.
Within a ruby laser, a single ruby crystal (Al2O3: Cr3+) serves as a laser media or active medium in the shape of a cylinder. The laser medium in the ruby laser consists of the sapphire host (Al2O3), which is doped with small quantities of chromium ions (Cr3+). The ruby possesses good thermal properties.
The pump source is the component of a ruby laser system that provides energy to the laser medium. Population inversion is required in this kind of laser to achieve laser efficiency To achieve population inversion, the laser medium must be supplied with energy. We use a flash tube as power source ruby laser. The flash tube provides energy for the laser medium. Once lower-energy-state electrons gain enough energy from the flash tube in the laser medium, they will move to an excited state.
The ends of the cylindrical ruby rod are flat and parallel. The cylindrical ruby rod is positioned between two mirrors. Both mirrors are coated with the optical coating. The method of depositing thin layers of metals on glass substrates to create mirror surfaces is known as silvering. Both the mirror is silvered or coated differently.
The mirror is completely silvered at one end of the rod while the mirror is partially silvered at another end. The fully silvered mirror reflects the light meanwhile the partially silvered mirror reflects most of the light but allows a small portion of the light to produce laser light output through it.
Energy level diagram
Consider a ruby laser medium consisting of three levels of energy E1, E2, E3 with N number of electrons. We say the energy levels are E1 < E2 < E3. The energy level E1 is referred to the ground state, the energy level E2 is referred to the metastable state, and the energy level E3 is referred to the high energy state. Let’s assume that most electrons are initially in the lower E1 state and only a limited number of electrons are in the E2 and E3 state.
As the laser medium absorbs light energy, the electrons gain enough energy in the E1 state and transfer to the E3 state. The lifespan of pump state E3 is 10-8 sec which is very short, so the electrons in the E3 do not stay for a long time. After a short time, they fall into the metastable state E2 by releasing radiationless energy. The lifespan of metastable state E2 is 10-3 sec which is much greater than the lifespan of E3 state. Therefore, the electrons reach E2 much faster than they leave E2.
After a certain time, by releasing energy in the form of photons, the electrons in metastable state E2 fall into the lower energy state E1. This is known as spontaneous radiation emission. When the emitted photon interacts with the electron in the metastable state, it forcefully causes that electron to fall into E1 state. As a consequence, it emits two photons. It is called stimulated radiation emission and since the reaction is continuous, millions of photons are produced.
A mechanism called spontaneous emission creates light in an active medium. The light produced inside the laser medium will bounce between the two mirrors. By releasing light energy this causes other electrons to fall into the ground state. It is called stimulated emission. Similarly, it causes millions of electrons to emit light. And the light gain is attained.