This paper describes for the first time the experimental results of 3-channel simultaneous OFDM television signal modulation by using external optical modulators. In the experiment, carrier suppression was performed to improve the modulation characteristics. Furthermore, this paper describes the modulation characteristics such as the effect of the change in the modulation index of the optical modulator on the signal quality. In this experiment, although the 3-channel simultaneous modulation experiment was successful, in order to succeed in the simultaneous OFDM modulation experiment with a larger number of channels, it is necessary to obtain the maximum extinction ratio with a larger modulation index. At present, the optical modulator with sine curve characteristics used in the experiment increases the nonlinear distortion as the modulation index increases. Therefore, in this experiment, the modulation index was limited to about 10%. In the future, in order to increase the modulation index of the optical modulator, it will be necessary to reduce the non-linear distortion generated by the optical modulator with a predistortion circuit or the like.

A novel super wideband optical FM modulation scheme that employs an optical heterodyne detection technique is proposed and demonstrated. The experimental results show that applying this scheme to 40-channel AM video transmission expands the power budget by approximately 10 dB at a carrier- to-noise ratio (CNR) of 42 dB. The feasibility of the scheme is also discussed from the viewpoints of connector reflectivity and fiber-dispersion.

The possibility of an application of Er-doped fiber amplifiers to AM-FDM (frequency division multiplexing) or FM-FDM video distribution networks is discussed. The measured noise and modulation distortion properties of ER-doped fiber amplifiers are good enough to meet even the severe quality standards set for trunk lines. A carrier-to-noise ratio (CNR) of 57 dB for an AM-FDM transmission was measured. A second-order harmonic distortion (HD2) of less than -56 dB was measured by the monotone method (modulation depth m=50%), and a cross modulation distortion (XM) of less than -63 dB was measured by the two-tone method (m=25%*channel). On the basis of these values, composite second-order distortion (CSO) and XM in a 40-channel transmission were estimated as less than -57 dB and -73 dB, respectively. Significant reduction of noise and modulation distortion is made possible by optimizing the length of Er-doped fiber amplifiers and using input- and output-port isolators.>