Goal:
To mix two audio signals using amplitude modulated lasers as the transition medium and a polarizing filter for the selection of the mixing ratio.
Concept:
Two low-cost red laser pointers which normally use 3VDC power supplies are modified with an audio transformer so that for each laser the supply voltage varies about 3VDC proportionally to an audio signal. The fluctuating supply voltages result in fluctuating beam brightnesses. The beams are made colinear with a 50/50 beamsplitter and the lasers are rotated along their beam axes such that the polarization of the light due to either laser in the collinear beam is orthogonalized. The beam then passes through a hand-held polarizing filter which selects out a linear combination of the orthagonal polarizations and so also a linear combination of the audio signals those polarizations encode. The beam post-polarizer is then incident on a (passive) phototranstor attached to a high-gain (741-based) pre-amp and a boom-box, so that the mixed audio signal may be heard in real-time.
Photos:
an annotated beam’s-eye view
(note Steve’s Cell-Detecting Helical Antenna of Death in back left)
Steve hand-mixing some beats
a top-view of the transmitter in the dark
a beam’s-eye view in the dark
(You have to imagine the otherworldly blend of Moondog and eurotrance emanating from the boom-box.)
the development and testing team
(Raj, Langley, Merck, Englehardt, Steve)
DJ Raj demos the POM
Schematics:
Each of the transmitting lasers requires a modulation circuit like the following. Note that you could use many other methods to modulate the laser, but I have found the transformer to be very effective for audio:
the wiring diagram for a single-laser transmitter (click to make bigger)
