Basic / Advanced / Master experiment
Intended institutions and users:
Physics education in Medicine
How it works ...
The Iodine Raman laser belongs to the class of molecular laser. However, the laser transition starts from the same level as the pump laser forming a so called Λ system. Due to this coupling a variety of coherent phenomena occur. One of it is the Raman gain which leads to an asymmetrical gain distribution favouring the direction of the pump laser. This effect causes spontaneous and unidirectional propagation inside a ring laser and has been firstly observed and explained by Wellegehausen et. al. in 1979. So far known experiments have been carried out with expensive pump laser. The invention of inexpensive laser pointer like DPSSL emitting laser radiation at 532 nm which is ideal to excite the iodine molecule allowing new affordable exciting new experiments for education. 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 about 1 nm. Due to thermal drift of the cavity also the frequency doubled radiation is drifting in a range of 0.5 nm. The absorption width of the Iodine molecule is much smaller compared to the thermal drift of the excitation laser. Therefore the cavity of the “green laser” must be thermally stabilised by controlling the temperature of the pump laser with an accuracy of 0.01°C and the injection current of 0.1 mA. In such a way it is possible to tune the pump laser to the resonance of the transition indicated by the appearance of strong fluorescence light of the excited Iodine.
The well known Iodine molecule provides a rich variety of transitions ranging from the green up to the near infrared spectral region. The green line of an ordinary laser pointer fits to the transition from the ground state (v''=0) into the first excited electronic state (v'=32). From here more than 72 transition exist back down to the ground state. Indeed, 30 laser lines in a range from 557.3 nm to 802.7 nm can be observed within this experiment. The range is only limited by the coating of the laser mirror.
The Iodine Raman laser can be operated in a linear cavity consisting out of two mirrors (M1 and M2). A four mirror ring cavity has two main advantages. Firstly in such a ring structure there are no back reflexes into the pump laser. This is very important since the pump laser is operating in a single mode. All back reflections into the cavity of the pump laser destabilises the emission and unwanted chaotic mode hopping would occur. Secondly, a major feature of the Raman effect is that the gain in forward direction with respect to the pump direction is much higher than in the backward direction. This leads to the unidirectional operation in a ring cavity. The pump radiation enters the ring cavity via the mirror M1 which has a high transmission (HT) for the pump and a high reflectivity (HR) for the Iodine Raman laser. The flat mirror M2 has a high reflectivity for the pump as well as for the Iodine Raman Laser radiation and deflects the pump radiation to the mirror curved M3. The radius of curvature of M3 is chosen in such that the pump radiation is focused into the middle of the Iodine cell (IC). M4 has the same properties as M3.