It is shown how to determine the focal point of a small-power infrared laser using the IR indicator IRI 4400. This experiment can also be made with other lasers in the wavelength range of about 830 nm to about 1100 nm.

Required equipment

• IR source: diode laser (830 nm/30 mW) with current and focussed optic
• IR indicator: IRI 4400
• Laser safety goggles
• Support for diode laser head

Safety measures

• Never realize the experiment without laser safety goggles, danger of irreversible eye damage!
• Do not direct the focussed laser beam onto black surface of indicator card (heat damage).
• Do not expose the active card material to temperatures above 50 °C  (heat damage); the phosphor chip supports locally and for a short time 200 °C. Important for power output above  30 mW.

Preparation: Positionning

Realization: Determing the focal point

1. Put the laser safety goggles on.
2. Switch on the diode laser.
3. Move the indicator card in beam direction until the active surface emits green light spots. The focal spot of lasers being very small, the green emission may be subject to strong local fluctuations. The reason is: the degree of efficiency of the conversion depends on how a crystal corn of the phosphor (approx. 100 µm) is hit by the focal point (<10 µm).