HeNe - Laser

The humble Helium Neon laser still has many applications, due to its superior beam quality and coherence.
An experimental laser based on a Helium Neon tube is used to verify mode selection properties, the optical stability range and the ABCD matrix formalism of the cavity used are discussed. A birefringent filter as well as a Littrow prism is used for the wavelength selection and the effect of an etalon used inside the cavity are investigated. A photo detector for measuring the relative output power and an alignment laser are supplied with a 1 metre long optical rail, along with all necessary mounts and adjusters. Within this experiment, a HeNe-laser is set-up.
In the first part of the project, the basics of the resonator will be understood and verified. When the mechanism of excitation due to collision and the relevant energy level scheme are known, the operation of the HeNe-laser at different wavelengths is performed. The selection of the wavelengths will be carried out by means of a dispersion prism ( Littrow set-up ) or by using a birefringent filter, known as a tuning element from Dye-lasers. The reduction of longitudinal modes is done by an intracavity Fabry Perot of low finesse (Etalon).
For the visualisation of the mode structure, the use of some parts of the „Fabry Perot Resonator“ is recommended. The HeNe-laser tube with Brewster’s windows on both sides is connected to the power supply with current read out and regulation. The resonator is formed by precision adjustment holders for common 1/2 exchangeable mirrors having different radii of curvature. For ease of adjustment, at the beginning a HeNe-laser is attached as an alignment aid and the laser tube is mounted into XY-adjustments to align the tube with respect to the pilot
 

Examples of investigation and measurement

Resonator geometry
After the initial adjustment has been done, all components which are not needed are removed from the set-up. Through the modification of the resonator length, the stability criteria is verified. For this reason, the hemispherical arrangement is used with the plane mirror and with the curved mirror of radius 700 mm. The beam diameter within the resonator is measured by means of a simple vernier calliper gauge for different laser mirrors. The optimum position of the laser tube within the resonator is determined anticipating a capillary diameter of 1 mm.

Single mode etalon (H)
The necessity of this element is created by the desire to produce a laser emission of high coherence and narrow bandwidth. The understanding of the etalon‘s function requires the discussion of the inhomogeneous amplification profile of the Neon and its Doppler broadening, the resonator modes, the finesse of a Fabry Perot resonator and their mutual relations. The orders of the single mode etalon can be observed experimentally. By means of an optical spectrum analyser the phenomena can be represented in an impressive way. Here we recommend also the use of some of the components of the experiment „Fabry Perot Resonator“. First the multimode operation is shown and then, after insertion of the etalon, the single mode operation. By a slight tilting of the etalon the mode „is shifted“ under the gain profile and additional information with regard to the gain profile can be obtained.

Line selection
This experiment enhances the understanding of the participating atomic energy levels, population inversion and supplies comprehensive basic knowledge with regard to crystal optics, Brewster‘s angle and amplification of a lasing medium. By means of the provided birefringent filter (C), a total of five visible lines can be selected. The significant influence of the discharge current on the output power is measured and discussed with special accent on the weak lines.

Littrow prism (G)
The classical selection element is tested and the special profile of the Littrow prism in conjunction with the Brewster‘s angle is discussed. It is shown that, contrary to the birefringent filter, only two lines can be selected, the dominant red main line at 632 nm and the orange line at 611 nm. The student experiences that the main line follows an adjustment of the prism in the resonator until its amplification decrease which prevents the oscillation of neighbouring narrow lines.