Basic / advanced experiment
Intended institutions and users:
Physics education in Medicine
How it works ...
The humble Helium Neon (HeNe) laser still has many applications, due to its superior beam quality and coherence. In all physics text books this laser represents the class of the gas laser und was the first gas laser invented by Ali Javan in 1960 right after Theodore Maiman demonstrated the first operation of the ruby laser. Since the HeNe laser was continuously operating and easy to build in a laboratory it served as specimen for a lot of scientific work and proof for theoretical predictions. It starts with the theory of optical resonator, Doppler broadened laser active material in a cavity, spectral hole burning (Lamb dip), single mode operation, coherence and intra-cavity absorption (inverse Lamb dip) just to name a few. For technical applications the HeNe laser is still in use due to its outstanding beam quality and coherence as secondary meter standard and is present in each aero plane or ships as laser gyroscope for navigation.
This experiment is designed as an open frame setup in such a way that all components can be arranged freely on a stable optical rail. A Helium Neon tube with Brewster windows on both ends is used to perform a variety of fundamental experiments. Verification of 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.
For the visualisation of the mode structure a „Fabry Perot“ extension is available or an electronic spectrum analyser is used to measure the modes beat frequency. The optical resonator is formed by two precision adjustment holders for common 1/2 “ exchangeable mirrors having different radii of curvature. For ease of adjustment, at the beginning a "green" pilot laser is attached as an alignment aid. The laser tube is mounted into XY-adjustments to align the tube with respect to the pilot laser.
Another way to select different lines of a laser is to use a Littrow prism. Within this experiment we are using such a module to tune the Helium Neon laser. The Littrow prism is made from fused silica which is the required substrate for IBS coating. The spectral range of the coating covers 580..720 nm with a reflectivity >99.98 %. The prism is mounted into a precise adjustment holder where it can be smoothly tilted in vertical or horizontal direction.
The double refractive or birefringent plate (P) is mounted in a dual rotational stage. For the intra-cavity operation the birefringent plate (P) needs to be aligned in such a way that the laser beam hits the plate under the Brewster angle to minimize the reflection losses. This can be accomplished by turning the rotary plate (B). In addition the birefringent plate can be rotated around its optical axis by tilting the lever (L). By rotating the plate (P) its optical retardation δ is changed. If the retardation of two passes is a multiple integer of the wavelength λ, this wavelength undergoes no losses at the Brewster window and will oscillate.
Due to the in-homogeneously broadened gain profile of the Neon laser transition, a multitude of laser modes are oscillating simultaneously. These modes have a frequency difference to each other which depends on the length of the Helium Neon laser cavity given by the distance between mirror M1 and M2. By means of the provided photodetector and an electronic spectrum analyser this beat frequency can be measured as a function of the cavity length. The value of the beat frequency for a 750 mm long cavity is150 MHz for instance.
The main difficulty to operate the HeNe laser experiment is the first alignment of the optical cavity formed by the mirror M1 and M2. To line up the laser tube and the mirror with respect to the mechanical axis of the optical bench, a green emitting laser pointer is provided. In addition we designed the coating of the laser mirror in such a way that a certain reflectivity for the green laser pointer emission is obtained. In case both mirrors are aligned parallel to each other they form a Fabry Perot and the transmitted beam shows the typical interference fringes indicating the perfect alignment.
A glass tube terminated on both sides with Brewster windows contains an optimized mixture of Helium and Neon gas. The mirror M1 and M2 are forming the optical cavity or resonator.