Early Technology Breakthroughs

In 1878, two years after perfecting his speaking telegraph (which became the telephone), Alexander Graham Bell created a device that transmitted the human voice through the air for distances up to 200 meters. The device, which he called the Photophone, used carefully angled mirrors to reflect sunlight onto a diaphragm that was attached to a mouthpiece, as shown in Figure 4-1. At the receiving end (Figure 4-2), the light was concentrated by a parabolic mirror onto a selenium resistor, which was connected to a battery and speaker. The diaphragm vibrated when struck by the human voice, causing the intensity of the light striking the resistor to vary. The selenium resistor, in turn, caused the current flow to vary in concert with the varying sunlight, causing the received sound to come out of the speaker with remarkable fidelity. This represented the birth of optical transmission.

Figure 4-1: Photophone transmitter.

Figure 4-2: Photophone receiver.

Earlier demonstrations of optical transmissions were done before this. Swiss physicist Daniel Colladon (in 1841), and later in 1870, physicist John Tyndall (well known for his work on the properties of gases) demonstrated that a beam...