Frequency Doubling

Lasers which emit light in the short wavelength spectral range are expensive and not sufficiently reliable for many applications. A more economic 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 replaced more and more by frequency doubled diode pumped Nd:YAG lasers. The principles of the generation of frequency doubled light and the possibilities of non-linear optics learnt in this experiment will be explained. The understanding of non-linear effects is very important for laser technology, since the processes of the 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 a certain manner by a practical experiment. The theoretical understanding of the non-linear optics grows by practical verification to increased know how. Incidentally the understanding of birefringent crystals grows by experience of phase matching. The fundamental wave is generated by a diode laser pumped Nd:YAG laser with an open resonator structure. The non-linear crystal is placed into the resonator and intra-cavity SHG is carried out. The reflectivity of the output coupler of the Nd:YAG laser is chosen as high as possible to obtain several watts of power of the fundamental wave inside the cavity.
 

Examples of investigation and measurement

Basics
Non-linear optics is introduced by reference to a curved characteristic line as it is done in communication technology. From the distorted output signal, the fundamental wave as well as the harmonics of higher frequency are received by means of a Fourier transformation. In optics, frequency doubling is introduced as a parametric process for which the parameters like phase matching and high non-linearity of the crystal are discussed under the aspect of getting a high conversion ratio. Using the birefringent properties of the KTP - crystal, it is shown how the phase matching condition can be fulfilled. Since the generation of the fundamental wave is done by a Nd:YAG laser, this one is also described.

Characterisation of the diodelaser
The graphic representation on the right side can be plotted either point by point measurement of the power of the diodelaser and the injection current or by the use of the low frequency modulator (Module M). In the latter case an oscilloscope is used in the XY-mode. The X-channel is connected to the output of the injection current of the controller LDC-01 and the Y-channel to the output of the photo amplifier at the controller LDC-01. A standing picture of the function can be observed on the screen of the oscilloscope. The threshold current and the slope efficiency are, for instance, measured for different temperatures. By variation of the off-set different parts of the characteristic line can be selected to define the criteria for linear modulation. This gives the link to communication technology.

Output power of the Nd:YAG laser
Although a high reflectivity has been chosen for the resonator mirror (R>99.98%) about 10 to 20 mW of fundamental wave power (1064 nm) are transmitted to the outside of the resonator. If 0,5 % is accounted for the total losses, there will be 4 Watt as the fundamental wave power within the resonator. The function shown on the right can be plotted either point by point or by the use of the modulator. In the latter case, the photo detector is connected to the Y-channel and the injection current (respectively the power of the diodelaser) to the X-channel of the oscilloscope. Each modification of the parameters like temperature (wavelength of the diodelaser) or state of adjustment of the resonator mirror can directly be studied.

Power of the frequency-doubled radiation
By means of the filter BG39, which is placed in front of the photo detector, the frequency-doubled radiation is measured. A standing picture of the measured values is received on the oscilloscope in the same way as in the previous example. Filter BG39 is used instead of filter RG1000. In an illustrative way, the quadratic relation between the power of the harmonic wave and the power of the fundamental wave becomes evident. Depending on the state of adjustment and the cleanliness of the optical components, up to 3 mW (multimode) of „green“ power can be generated. By use of the option „1 Watt laserdiode“, the power can be increased to 10-15 mW.