Crystals Materials
Measure Emission, Absorption & Optical Pumping of crystals?


Optical pumping is a process in which an electron of an atom ore molecule is excited (or “pumped”) from a lower to a higher energy level. We are using as pump light source as diode laser and Neodymium (Nd) atoms which are hosted in a crystal lattice formed by an Yttrium Aluminum Garnet (YAG). This material (abbreviated Nd:YAG crystal) is one of the most important laser materials.

At the beginning Nd:YAG has been pumped by flash lamps. To improve the efficiency diode laser are used which emission wavelength is almost completely absorbed whereby the light of the flash lamps to an extend of only about 5%.

The experiment provides a diode laser which is mounted onto a Peltier element. Changing the temperature tunes the wavelength of the diode laser with 0.25 nm / °C. Within the temperature range of 10 – 60°C a spectral range of 12.5 nm is covered allowing the measurement of the spectral absorption of the Nd:YAG crystal. By using the well known absorption peaks of the Nd:YAG the emission wavelength of the diode laser is determined. Furthermore the injection current of the diode laser can periodically be switched on and off allowing the recording of the timely decay of the fluorescence of the Nd:YAG crystal with an oscilloscope. The inverse value of the measured lifetime is the Einstein coefficient for spontaneous emission. By means of the optional spectrometer the fluorescence spectrum of the Nd:YAG crystal as well of the diode laser can be recorded and printed.

How to measure the output power

The laser diode emits a wavelength of 808 ±3 nm and is mounted onto a Peltier element allowing the temperature to be varied from 10 to 50°C to study the thermal effect on the laser properties. A change of the temperature affects the emitted wavelength as well as the output power. The setup as shown in Fig. 1 measures the relative output power of the laser diode versus the injection current with the temperature as parameter. The beam divergence is controlled by the collimating lens (C) and is set to a suitable intensity without saturating the photodetector.

Fig. 1 Measuring the output power

How to measure the absorption coefficient

Fig. 2 Absorption Measurement

The setup according to Fig. 2 is used to measure the spectral property of the laser diode using the well known absorption lines of a Nd:YAG crystal. In this arrangement a focusing lens is added to create a tight focus inside the Nd:YAG crystal. The photodetector sees the unabsorbed pump light as well as the created fluorescence. However, its intensity is comparably small and will not affect the measurement.

How to measure the fluorescence

Fig. 3 Fluorescence Measurement

In the setup of Fig. 3 a filter is added which blocks the pump radiation and only the fluorescence will be seen by the photodetector. Furthermore, the injection current of the pump laser is modulated and allows the measurement of the temporal build up and decay of the fluorescence using an oscilloscope. From the results the important lifetime of the excited state is determined. The inverse value is also defined as Einstein coefficient for spontaneous emission.

For more information on the relationship between absorption coefficient and concentration of laser crystal materials, please consult

Key words: lasers crystals; absorption coefficient; Nd:YVO4 crystal; Nd:yag crystal; Yb:YAG crystal; Pr:YLF crystal