What is Ruby Laser?

Lasers are devices that emit light using atoms or molecules at a certain wavelength and amplify the light to produce a narrow beam of radiation. It works as per the principle of electromagnetic radiation. Their source of emission contains the same frequency and same phase. It was invented in the year 1960 by the great noble scientist, Theodore Maiman.

Laser is also classified based on the duration of laser emission which is of continuous waves and pulsed waves. A continuous wave frequently emits lights at the continuous interval while the pulsed lasers emit light in the form of optical pulses or light flashes at a certain interval at a certain rate. A helium-neon laser is the first laser made to produce continuous waves. The ruby crystal laser is the first laser made producing pulsed waves. There are mainly four types of ruby lasers: Solid-state, liquid-state, gas, and semiconductor lasers.

A ruby laser is a three-level solid-state laser that emits coherent light using synthetic ruby crystal as its laser medium. It is known to be the first laser developed by Theodore Maiman in 1960. It emits dark red light which lies in the visible region. The wavelength in this visible region is approximately 694.3 nm. Ruby's composition is chromium-doped corundum ( ), where this ruby is used as an active medium. The Ruby laser has extremely short pulses of high power.

Principle of Operation:

A ruby laser works on the principle of population inversion. Population inversion is a phenomenon that is done by optical pumping in which the atoms in the ground state or lower energy level excites to a higher energy level, that is metastable state by absorption of pump light, so there is a greater population in the higher energy level than the lower energy level. But to achieve population inversion, there are a few important conditions; the electrons in higher energy levels should be higher than electrons in lower energy levels or ground state. The electrons should excite to the metastable state. This state lies between the ground state and the excited state.

Construction of Ruby Laser:

A ruby laser is made up of three components: an active laser medium, pump source, and an optical resonator or cavity.

• The pump source provides energy to the active laser medium. This process is known as optical pumping and it works on the principle of population inversion. An active light source such as a flash tube or Xenon flash lamp is used to provide the necessary energy for the excitation of the carries in the ruby crystal. This excitation depends on temperature too and hence has excellent thermal properties.
• The optical resonator or optical cavity is constructed by placing two optical coated mirrors at both ends of the cylinder which are flat and parallel. Each of the mirrors is coated or silvered differently where one end is completely coated and the other end is partially coated, mainly for enhanced reflection of the light beam. Silvering is the process of adding the necessary chemicals which are reflective substances.
• The cylindrical ends of the ruby rods are coated precisely in such a way that the ends are flat to within a quarter of a wavelength of the light and parallel to each other within few seconds of arc.
• These reflective ends act as Fabry-Perot interferometer. 

Working of Ruby Laser:

• The ruby laser is a three-level laser. The first procedure is to apply optical pumping, where the energy is given to the active medium of the ruby laser. This energy raises the electrons in the crystal excites from the ground state to a higher energy level state. Here the active source light such as Xenon flash lamp or flashtube is used. 
• The three energy levels are ground state, metastable state, and pump state (highly excited state). When the active light energizes the ruby, which is considered as the active medium, the electrons in the ground state excite to the pump state.
• As the electrons do not hold longer in the pump state, they fall into the metastable state within a very short duration of time emitting radiation less energy. The electrons hold a much longer time in the metastable state than in the pump state. There is an increase in the number of electrons in the metastable state than in the pump state, hence the principle of population inversion occurs.
•  The electrons in the metastable state transit to the ground state after some duration of time by emitting photons. This process is known as the spontaneous emission of radiation. The photos emitted have a wavelength of 694 nm where this particular wavelength belongs to the red color in the visible region.
• When the emitted photons fall to the metastable state, the electrons in the metastable state transits to the ground state. Again the principle of population inversion occurs between the metastable state and the ground state. So another photon is emitted and hence this is known as stimulated emission of radiation.
• These photons continuously move back and forth in the optical resonator or cavity which keeps on producing more photons of the same wavelength and hence the same coherent red color is produces a high-intensity red laser beam.
  

Application of Ruby Laser:

• Ruby lasers are used in range finding where Q-switching ruby lasers are used in military rangefinders.
• Ruby lasers were the first laser used in optically pumping tunable dye lasers.
• Ruby lasers are also used for industrial purposes for their high-intensity beam. They are also used for penetration of hard surfaces, especially drilling holes through diamonds.
• Ruby lasers are used for tattoo removal, hair removal, and pigmentation removal. 
• Ruby lasers are also used in holography due to their high pulsed power and coherence length.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science Physics
• Masters in Science Physics