Molecular Spectroscopy
Laser pointer like excitation
Molecular energy level
Dunham Coefficients
Franck Condon Principle
Unidirectional Ring Laser Operation
Optical stability range
Raman Gain
Density Matrix Formalism
Multi-line Laser
Single Line Laser
Single Mode Ring Laser
Hyper Fine Structure
LE-1300 Iodine Raman Laser
Topics:
LE-1300 Iodine Raman Laser
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 favoring 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 (LD) 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 stabilized 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 ring laser cavity of the LE-1300 Iodine Raman Laser consists of four mirrors (M1...M4) forming a so called bow-tied or folded cavity. This configuration is necessary to avoid any back reflections into the single mode pump laser (LD). The mirror M2 and M3 are curved while M1 and M2 are flat ones. The Iodine cell (IC) is placed in the center between mirrors M2 and M3.
The pump radiation passes the mirror M1 and is reflected by the mirror M4 onto the curved mirror M2. Due to the curvature of M2 the pump beam is focused into the Iodine cell. The mirror M3 makes the pump beam parallel again and reflects the beam onto M1, closing the ring.
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 emitting 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.
