Abstract

At present, amplitude modulation is preferred for the distribution of cable television signals, although it places stringent requirements on carrier-to-noise ratio (CNR) and linearity.¹ For example, consider a system consisting of a 200-mW laser with relative intensity noise of −165 dB/Hz, fiber coupling of −35 dB, external interferometric modulator with 6-dB insertion loss, link loss of 5 dB, and receiver with rms noise current of 15 pA/(Hz)^1/2 and operating at an optical modulation depth (OMD) of 1% per channel. In principle this system limits the broadcasting of 40 channels with a desired CNR of 50 dB (4-MHz bandwidth) and composite triple beat of 62 dB below the carrier (dBc) to two locations. We have analyzed the distortion characteristics of a variety of modulators based on coupled waveguides and variable phase mismatch to look for a passive means of reducing the distortion, thus allowing us to broadcast to more sites. Here we show analytically that these structures exhibit intermodulation distortion similar to the interferometric modulator.² Measured 3IM distortion levels for samples of Ti:LiNbO₃ interferometric, uniform Δβ, Δβ reversal, and weighted coupling³ projected to an OMD of 4% are in the range of −74 to −72 dBc and confirm the models.

© 1990 Optical Society of America