1. Field of the Invention
The present invention relates to a frequency division multiplexing system for the optical transmission of broadband signals.
2. Description of the Prior Art
It has been known in the past to employ frequency division multiplexing to transmit a plurality of signals down a single optical fiber. However, these signals have all been modulated onto a single light source. Those light sources for optical communications systems have nonlinear response characteristics which produce harmonic cross modulation and intermodulation distortion which limit the quality of a received signal. This degradation can be reduced by lightly modulating the light source so that it operates in a more linear fashion. Unfortunately, when an attempt is made to lightly modulate the single light source with a plurality of RF signals, the effective signal power per signal is small and a good signal-to-noise ratio cannot be obtained over transmission distances where there is any great signal attenuation.
This poor signal-to-noise ratio is further aggravated when a plurality of television channels are to be transmitted from a single modulated light source. These broadband signals share the available total optical output of the light source. As a result, the optical power per unit bandwidth is very low for a plurality of broadband signals.
This problem could be solved if a single light source were to be maintained by improving the linear response characteristics of the light source so that a large modulation factor could be used. Unfortunately, if an attempt is made to improve the linear response characteristics of a typical light source such as a light emitting diode (LED) or double heterostructure (DH) laser by suitable alteration of the chemical composition or physical structure of the light source, the optical output power is usually adversely affected. If an attempt to remedy this situation is made by further modification of the chemical composition or physical structure of the light source in order to increase the optical output power, the frequency response of the light source is in turn adversely affected and renders the light source unsuitable for use in a broadband system required in the transmission of a plurality of television signals. In other words, the simultaneous optimization of linear response characteristics, optical output power and frequency response is extremely difficult to achieve in light sources such as the LED or DH laser.