Home ► Laser Experiments ► LE-0700 "Green" SHG with Diode pumped Nd:YAG Laser
Keywords:
Basic experiment
Intended institutions and users:
Physics Laboratory
Engineering department
Electronic department
Biophotonics department
Physics education in Medicine
LE-0700 "Green" SHG with Diode pumped Nd:YAG Laser
Lasers which emit light in the short wavelength spectral range are expensive and not sufficiently reliable for many applications. A more economically way to generate such radiation is achieved by frequency doubling. Especially the generation of green laser radiation is an important requirement of the lithographic industry. At present the argon ion laser is being displaced more and more by frequency doubled diode pumped Nd:YAG lasers which deliver tens of watt in TEMoo and are the first choice for pumping the Titanium Sapphire which is of high importance and frequently used in research labs. In biophotonics the green radiation serves as "optical tweezers" and the analysis of living cells. The principles of the generation of frequency doubled light will be explained and simultaneously the possibilities of non linear optics learnt in this experiment. The understanding of non linear optical effects is very important for laser technology, since the processes of generation of short pulses are also based on non linear effects. Within the experiment the phase matching condition will be presented and analysed. The efficiency of frequency doubling will be determined and hints for an optimized conversion rate will be evaluated in the experiment. For the first time the frequency doubling can be followed up in an impressive manner by a practical experiment. The fundamental wave is generated by a diode laser pumped Nd:YAG laser with an open resonator structure. The non - linear crystal is placed simply into the resonator and suddenly green light appears. Furthermore also the mode structure of the Nd:YAG laser becomes visible and shows a great variety of transverse modes. By introducing an adjustable iris into the cavity the number and kind of modes can be controlled and reduced down to TEMoo. By using the "Red 660 nm" option the Nd:YAG laser is operated at 1.3 µm and a special cut KTP crystal allows the frequency doubling into the red part of the spectrum.
LE-0700 "Green" SHG with Diode pumped Nd:YAG Laser
“Green” Second Harmonic Generation (SHG)
Adjustable iris to reduce transverse modes down to TEM00
Tuning different “green” lines from 526 up to 539 nm
This unique experiment uses a birefringent tuner (BFT) to select one of the manifold of lines of the Nd:YAG laser around 1064 nm (see ) resulting in the same number of frequency doubled lines from 526 to 539 nm. The BFT is placed under the Brewster angle along with the KTP crystal inside the cavity. The change of the colour is not sufficient to distinguish the lines, thus the use of a spectrometer is recommended.
One side of the Nd:YAG crystal is coated and forms the first mirror (M1) for the laser cavity. The second mirror (M2) is a curved mirror resulting in a hemispherical cavity. The Nd:YAG crystal is pumped by the radiation of 808 nm emitted from the laser diode. The divergent radiation is collimated (C) to an almost parallel beam and afterwards focused by the lens (L) in such a way that the focus lies within the Nd:YAG crystal. The KTP crystal is inserted into the cavity close to the Nd:YAG crystal where the beam waist is at smallest and thus the intensity of the fundamental radiation (1064 nm) at highest for efficient second harmonic generation (SHG). A filter is used to suppress the residual fundamental and pump radiation and to transmit the “green” SHG only.
Due to the high gain and the cavity design a lot of transverse modes of the fundamental wave exist which become visible due to the SHG process. By means of an adjustable iris which is used inside the cavity close to the spherical mirror the manifold of the these modes can be controlled down to TEM00. However, the iris can be used to set and study controlled higher transverse modes.
LE-0700 "Green" SHG with Diode pumped Nd:YAG Laser