DIY Invisible Infrared Wiring

Infrared lasers are invisible, but behave in many ways just like visible light beams.

Chances are that your television remote control transmits information through infrared radiation. The remote modulates an infrared carrier wave, which is detected at your television, then demodulated. The demodulated signal is interpreted by electronics within your television, and the proper command is carried out. You can use infrared as an invisible information carrier, replacing a physical wire with a directed infrared beam.

Things You'll Need

• IR laser module
• IR detector
• Detector mount
• Adjustable laser mount
• Modulation circuit
• Demodulation circuit
• Infrared power meter
• IR attenuator
• IR viewer or card

Instructions

1. Point the IR laser at the IR power meter. Check the rating on the IR power meter detector, and if the detector cannot handle the laser output, put an IR attenuator in front of the power meter.

2. Increase the drive current to the IR laser from the lasing threshold to the maximum rating. Plot the measured IR power versus the drive current. Use the plot to find the linear range of the laser.

3. Set the modulation circuit so that the minimum and maximum currents fall within the linear range of the laser. You’ll connect your signal source to the modulation circuit.

4. Mount the laser and adjust the pointing to center on the photo-detector. Use the IR viewer or IR card to make the beam visible during your alignment. Adjust the photo-detector gain to maximize the voltage difference between the minimum and maximum signal levels.

5. Connect the IR detector to the demodulation circuit. The demodulation circuit output will connect to the next link in your signal chain.

Tips & Warnings

• There are dozens of different modulation schemes. The simplest method is a straightforward intensity modulation, but it is also the most sensitive to noise. The next step up is to impose an amplitude modulation on a constant-frequency infrared beam. There are other amplitude modulation schemes, along with frequency modulation schemes, and phase-shift schemes as well. Each method has specific advantages, and each allows you to put in a varying signal and encode it on the invisible infrared beam.

• Although IR laser radiation is invisible, it is not harmless. Be sure to set up your beam where it will not be possible for anyone to have their eyes exposed to radiation from the laser.

References

• Fermilab: Wide-Band Free Space Transmission Link...
• NYU: Examples of Modulation Techniques
• State University of New York, Stony Brook: Recipe for Locking an Extended Cavity Diode Laser...