Helium-Neon laser is a type of gas laser in which a mixture of helium and neon gas is used as an active or gain medium and electrical pumping is used to achieve population inversion. Helium-Neon laser is also known as He-Ne laser.
Helium Neon Laser Construction and Working
Construction of He Ne Laser
The Helium-neon gas laser system consists of a glass tube of nearly one meter in length. The gain medium in the He-Ne laser is a mixture of helium (90%) at 1 mm of Hg pressure and neon (10%) at 1/10 mm of Hg pressure filled in the glass tube.
The reflecting mirrors M1 and M2 are fixed at the end of the tube. The mirror M2 is partially transparent and the laser light comes out of it. On the other hand, the mirror M1 is totally reflecting mirror.
The glass tube containing the He-Ne gases mixture and has two mirrors at its end is called a resonator or optical cavity.
A high potential difference is applied across the two electrodes of the discharge tube for creating population inversion.
he ne laser diagram
Helium-Neon laser Working
Helium and neon are noble gases having only one atom per molecule. The energy level diagram of helium and neon atom is shown below
he ne laser energy level diagram
When electric discharge occurs in the discharge tube, some of the atoms of the mixture of He-Ne that ionized. The electrons obtained so collide with Helium atom in the ground state or level and hence Helium atom goes to the excited state F2.
The helium atom requires less energy to go to the excited state F2 from its ground level or state than and the Neon atom to go to the excited state E3 from its ground level or state (E3>F2). The Helium atom at the excited state (F2) is Forbidden to make a spontaneous transition to the ground state (F1).
The excited Helium atom ( at level F2 ) collides with unexcited neon atoms at the ground state. The excitation energy (E3) of neon atoms nearly matches the excitation energy (F2) of the Helium atom.
Therefore, there is an exchange of energy among the atom of helium in the excited state F2 and the atoms of neon gas in the ground state.
As a result of this, the Helium atoms fall back to the ground state and the neon gas neon atom goes to the excited state E3 whose lifetime is greater than the lifetime of the energy state E2 and hence called metastable state. Thus, the Helium atom in the excited state act as a pumping source for a neon atom.
Neon atom in the metastable state any E3 jumps to the lower energy state E2 through spontaneous emission and a Proton of wavelength 632.8 nm is emitted due to the transition 3s – 2p, This is the most prominent wavelength in He Ne laser in the red portion of the spectrum.
The other wavelength emitted in Helium-neon laser is 3.39 μm due to the transition 3s – 3p and 1.15 μm due to the transition 2s – 2p.
The Neon atom in the excited state E2 (having a small lifetime) rapidly deexcited to the Neon ground state by the non-radioactive transition. Thus, there are more neon atoms in the metastable state E3 than the atom in the excited state E2.
Hence, population inversion is established between the energy states E3 and E2. The population inversion of a neon atom is achieved through Helium atoms.
The photon of wavelength 632.8 nm emitted due to the transition of a neon atom from the metastable state E3 to the excited state E2 through spontaneous emission travel through the He-Ne gas mixture along the direction of the axis of the discharge in the resonator or optical cavity.
The Photons emitted due to the transition of neon atoms from E3 to E2 energy levels traveling along the axis of the resonator are reflected many times between the two Mirrors placed at the ends of the resonator or cavity.
These reflecting photons ( called stimulating Photon ) interact with other neon atoms in the metastable state E3 and causes them to emit Photons each of wavelength 632.8 nm through stimulated emission process.
The photons created through the stimulated emission process have the same wavelength and polarization as that of the stimulating Photon and travel in the direction of stimulating photons.
The photon multiplication or Light amplification process gives rise to many photons of the same wavelength and polarization. These photons travel to and fro between the two mirrors of the resonator or cavity.
When the photons beam is quite intense, a part of it comes out of the partially transparent mirror M2. Hence, we get a laser beam. He – Ne gas laser operates continuously because the Helium and neon atoms are excited by the collision of electrons which take place of the time.
helium neon laser construction and working
Characteristics Helium Neon Laser
|2.||Active Medium||A mixture of helium and neon gas.|
|3.||Pumping Method||Electrical pumping.|
|4.||Power output||He-Ne laser generates power of less than 10mW.|
|5.||Wavelength of output||The most common wavelength produced by the He-Ne laser is 632.8 nm.|
Advantages of He-Ne Laser.
- It is more directional and monochromatic than a solid-state laser.
- It has high stability of frequency.
- It can operate continuously without the need for cooling as in done in ruby laser.
- Low cost.
- High stability.
Disadvantages of He-Ne laser
- The output power of He-Ne laser is moderate as compared to the solid-state laser.
- Low efficiency.
- Low gain.
Applications of He-Ne laser
- He-Ne laser is used in data processing.
- He-Ne laser used in holography.
- Studying interference and diffraction patterns.
- Used in Telecommunication.
FAQ on helium neon laser construction and working
What are the advantages of He Ne laser?
Why He Ne laser is better than ruby laser?
it can operate continuously without the need for cooling as in done in ruby laser