Consider a cube of Neodymium doped Yttrium Vanadate (Nd:VO4) with a length of the edge of 1 mm to which a KTP frequency doubler crystal with the same cross section but a length of 2 mm is bonded. Both opposite sides of the compound are are coated with a mirror forming an optical cavity for the radiation of 1064 nm. When pumped with a small laser diode in a ø3.6 mm housing green radiation is produced. With a typical pump power of 200-300 milliwatt at 808 nm green output power of around 10 milliwatt is obtained. Such a crystal compound (also termed as Green Laser Microchip GLM) allows the design of really small micro laser mainly used for laser imaging, green laser pointers, laser show, spectroscopy, medical diagnostics and a lot of other applications where the size of the laser source is of great importance.
Within this experiment we are using such a GLM and pumping it with the same setup as for the Nd:YAG laser (LE-0700). The GLM is mounted inside a 4 axes kinematic mount in order to align it to the focussed 808 nm pump radiation. As soon as the pump radiation hits the crystal green emission is produced. By aligning the crystal the optimum of green power will be observed. Due to the possibility of tuning the temperature and output power of the pump diode laser a series of measurements are carried out to demonstrate the typical quadratic relation between the green output and the fundamental power. The modulation capability of the diode laser driver allows the periodic switching on and off of the pump laser diode and to observe the "spiking" of the green radiation. Placing the provided RG1000 filter in front of the photodetector only the 1064 nm radiation will be observed. This facilitates the separate characterisation of the fundamental laser as well.
The Green Laser Microchip consists of the Nd:YVO4 crystal and the attached KTP crystal. The mirrors M1 and M2 form the cavity for the fundamental radiation at 1064 nm and are coated for highest reflectivity for this wavelength. In addition, M1 has a high transmission for the pump radiation at 808 nm and a high reflectivity for the second harmonic at 532 nm, while M2 has a high transmission for the second harmonic.
The filter is used to suppress any residual radiation either at 808 nm or 1064 nm or to suppress the green emission to study the behaviour of the fundamental laser. The GLM is pumped by a separate laser diode. The pump laser radiation is collimated by a collimating lens (C) and focussed into the GLM by the lens (L). Considering the index of refraction for YVO4 and KTP the optical cavity length is 5.4 mm, resulting in a spectral mode distance of the fundamental mode of about 28 GHz. The gain bandwidth of the Nd:YVO4 crystal is 0.96 nm at 1064 nm or 254 GHz, allowing about 9 modes to oscillate. In principle each fundamental mode may create a second harmonic. However, due to the mainly homogeneously broadened gain profile, single frequency emission for the 1064 nm and 532 nm is expected. In case the Nd:YVO4 is operating on more than one longitudinal mode, the KTP crystal also creates the sum-frequency of the longitudinal modes. This in turn couples the competing longitudinal modes and gives rise to chaotic fluctuation of the green emission. This phenomenon is also termed as “green problem” and also will subject of the experimental observations. By changing the injection current and / or temperature of the pump laser, the Nd:YVO4 laser can be brought back to single mode operation which is indicated by the disappearance of the “green problem”.