A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its laser medium. Ruby laser is the first successful laser developed by Maiman in 1960.
Ruby laser is one of the few solid-state lasers that produce visible light. It emits deep red light of wavelength 694.3 nm.
Construction of Ruby Laser
A ruby laser consists of three important elements: laser medium, the pump source, and the optical resonator.
Laser Medium or Gain Medium in Ruby Laser
In a ruby laser, a single crystal of ruby (Al2O3 : Cr3+) in the form of a cylinder acts as a laser medium or active medium. The laser medium (ruby) in the ruby laser is made of the host of sapphire (Al2O3) which is doped with small amounts of chromium ions (Cr3+). The ruby has good thermal properties.
Pump Source or Energy Source in Ruby Laser
The pump source is the element of a ruby laser system that provides energy to the laser medium. In a ruby laser, population inversion is required to achieve laser emission. Population inversion is the process of achieving the greater population of higher energy state than the lower energy state. In order to achieve population inversion, we need to supply energy to the laser medium (ruby).
In a ruby laser, we use flashtube as the energy source or pump source. The flashtube supplies energy to the laser medium (ruby). When lower energy state electrons in the laser medium gain sufficient energy from the flashtube, they jump into the higher energy state or excited state.
The ends of the cylindrical ruby rod are flat and parallel. The cylindrical ruby rod is placed between two mirrors. The optical coating is applied to both the mirrors. The process of depositing thin layers of metals on glass substrates to make mirror surfaces is called silvering. Each mirror is coated or silvered differently.
At one end of the rod, the mirror is fully silvered whereas, at another end, the mirror is partially silvered.
The fully silvered mirror will completely reflect the light whereas the partially silvered mirror will reflect most part of the light but allows a small portion of light through it to produce output laser light.
Working of Ruby Laser
The ruby laser is a three level solid-state laser. In a ruby laser, optical pumping technique is used to supply energy to the laser medium. Optical pumping is a technique in which light is used as energy source to raise electrons from lower energy level to the higher energy level.
Consider a ruby laser medium consisting of three energy levels E1, E2, E3 with N number of electrons.
We assume that the energy levels will be E1 < E2 < E3. The energy level E1 is known as ground state or lower energy state, the energy level E2 is known as metastable state, and the energy level E3 is known as pump state.
Let us assume that initially most of the electrons are in the lower energy state (E1) and only a tiny number of electrons are in the excited states (E2 and E3)
When light energy is supplied to the laser medium (ruby), the electrons in the lower energy state or ground state (E1) gains enough energy and jumps into the pump state (E3).
The lifetime of pump state E3 is very small (10-8 sec) so the electrons in the pump state do not stay for long period. After a short period, they fall into the metastable state E2 by releasing radiationless energy. The lifetime of metastable state E2 is 10-3 sec which is much greater than the lifetime of pump state E3. Therefore, the electrons reach E2 much faster than they leave E2. This results in an increase in the number of electrons in the metastable state E2 and hence population inversion is achieved.
After some period, the electrons in the metastable state E2 fall into the lower energy state E1 by releasing energy in the form of photons. This is called spontaneous emission of radiation.
When the emitted photon interacts with the electron in the metastable state, it forcefully makes that electron fall into the ground state E1. As a result, two photons are emitted. This is called stimulated emission of radiation.
When these emitted photons again interacted with the metastable state electrons, then 4 photons are produced. Because of this continuous interaction with the electrons, millions of photons are produced.
In an active medium (ruby), a process called spontaneous emission produces light. The light produced within the laser medium will bounce back and forth between the two mirrors. This stimulates other electrons to fall into the ground state by releasing light energy. This is called stimulated emission. Likewise, millions of electrons are stimulated to emit light. Thus, the light gain is achieved.
The amplified light escapes through the partially reflecting mirror to produce laser light.
Applications of Ruby Laser
Ruby lasers have a variety of applications in diverse fields such as:
- Holography: Ruby lasers are used for producing holograms as they produce a single wavelength of coherent light.
- Pulsed Laser Deposition: Ruby lasers are employed in pulsed laser deposition for depositing thin films on substrates.
- Rangefinding: Ruby lasers have been used in rangefinders for distance measurement in military and surveying applications.
- Material Processing: Ruby lasers are used in various material processing applications, such as drilling, cutting, and welding.
- Laser spectroscopy: Ruby lasers are utilized in laser spectroscopy for the study of atomic and molecular structures.
- Medicine: Ruby lasers have applications in medicine, such as dermatology for the removal of tattoos and pigmented lesions.