Molecular spectroscopy is one of the most important technologies to identify molecules in science, chemistry, biology and even in security applications. Precision spectroscopy, optical communication, modern length and frequency measurements using references and secondary frequency standards respectively based on stabilised laser. At present 12 optical frequencies in the visible and near infrared range are proposed by the “Comité International des Poids et Mesures (CIPM)”. Six of them use transitions of Iodine molecules. For the time being the hyper fine transition of the iodine molecule a10 of the R(56)(32-0) is declared as reference with a relative uncertainty of 7∙10-11. In the Fig. 2.42 the transition (32-0) is shown. Hereby, is 32 the vibration quantum number of the excited state and 0 those of the ground state. However, a great variety of transitions to the ground state exist which will be one topic of interest within this experiment. Iodine is ideally suited since it consists out two identical atoms also termed as a diatomic molecule or also as dimer. Remarkably, optical transitions are only allowed between the electronic states of the molecule resulting in a clearer spectrum. Even more, with a suitable narrowband laser only one level is excited from which a series of transitions down into various vibrational levels of the ground state take place. In the past, expensive lasers have been used to study the properties of molecular Iodine. The inexpensive “green laser pointer” provides a wavelength at 532 nm which is ideal to excite the iodine molecule. However, the underlying generation of the green radiation is based on the frequency doubling of a diode pumped Nd:VO4 laser. Such a laser has a gain bandwidth of 1 nm. Due to thermal drift of the cavity, the frequency doubled radiation also drifts in a range of 0.5 nm (530 GHz). However, the absorption width of the Iodine molecule due to the Doppler broadening of 437 MHz at 25°C is much smaller compared to the thermal drift of the excitation laser. Therefore the cavity of the “green laser” must be actively thermally stabilised.