Home Laser Experiments LE-0700 "Green" SHG with Diode pumped Nd:YAG Laser

Keywords:

  • Interaction of Light and Matter
  • Crystal Optics
  • Non-linear Optics
  • Second Harmonic Generation
  • Phase Matching Condition
  • KTP Crystal
  • Nd:YAG Laser

Basic experiment

Intended institutions and users:

Physics Laboratory

Engineering department

Electronic department

Biophotonics department

Physics education in Medicine

 

Introduction

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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

The KTP crystal is located inside the cavity formed by the mirror M1 and M2, close to the smallest beam waist of the fundamental wave. The Nd:YAG rod (M1) as well as the mirror M2 are coated for the operation at 1064 nm. The pump radiation at 808 nm is focused by the lens L1 into the Nd:YAG rod. In addition the mirror M1 of the Nd:YAG rod has also a high reflectivity at 532 nm, thus the green radiation will leave only via the MIrror M2 the Nd:YAG laser cavity.

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

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