LE-0300 HeNe-Laser advanced

LE-0300 HeNe-Laser, advanced

A glass tube (LT) is terminated on both sides with Brewster windows and contains an optimized mixture of Helium and Neon gas. The adjustable mirror M1 and M2 are forming the optical cavity or resonator. 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, defined by the iris diaphragm (IR), a green emitting laser pointer (PL) is provided. In addition we designed the coating of the laser mirror in such a way that a certain reflectivity 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.
To select different lines of a laser a Littrow prism is used (LP). 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.
A double refractive or birefringent plate is mounted in a dual rotational stage (BF). For the intra-cavity operation the birefringent plate 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 is done turning the rotary plate . In addition the birefringent plate can be rotated around its optical axis by tilting the lever. Rotating the plate 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 fiber coupled photodetector (PD), photodetector box 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. To obtain single mode operation, the etalon (ET) is used. Its efficacy is checked either with the electronical or Fabry-Perot spectrum analyzer.
The photodetector (PD) along with the photodetector box is also used to measure optical power which is displayed on a digital voltmeter.
The screen (SC) is used to display the transverse modes.