Light wave has been regarded as another useful carrier of information. As a carrier of information it has many advantages over radio-wave, but several difficulties inherent to the optical frequency prevented it from being used in practice. However, with the advent of the laser some of the aforementioned difficulties are overcome and the optical communication has made a great advance.
Among the many methods of modulating the light wave, the polarization modulation by Faraday rotation effect has some basic advantages, particularly in the audio-frequency range, providing the adequate Faraday rotation material can be found.
In this work, the theory of the method of small angle polarization modulation is developed together with the theory of corresponding demodulation. In accordance with the theory, both the optical system and the electronic instrumentation are completed.
The Faraday rotation coil is 100cm long and 2cm in diameter melt for the present work and $CS_2$ in glass tube whose Verdet constant is 0.04 min. of arc / gauss cm.
With Hg-5461 Å line from a conventional mercury-arc lamp the Verdet constant of the flint glass used ($Pb_{3}0_{4}$ 40 weight %) is found to be 0.17 min. of arc / gauss. cm.
The collimated Hg-5461 light, traversing through the Faraday rotation medium, is modulated and its phase compensated with Babinet-Soleil compensator. The demodulated signal is detected by a PM tube. But it is found to be insufficient to overcome the noise as the consequence of using Hg-5461 Å light instead of laser beam, namely the intensity of light used is too weak and at the same time the Faraday rotation materials show insufficient rotation effect.
It is found by photometric calculation that use of He-Ne laser of several mW or the Faraday rotation material with the Verdet constant, approximately 10 times that of the flint glass used should give the demodulated signal sufficient for voice communication.
