Waveguide Modulators: Single sideband goes optical

June 1, 2000
TOKYO—The Communications Research Laboratory at the Ministry of Posts and Telecommunications and Sumitomo Osaka Cement have jointly developed an optical modulator for single-sideband (SSB) optical transmission.

Incorporating news from O plus E magazine, Tokyo

TOKYO—The Communications Research Laboratory at the Ministry of Posts and Telecommunications and Sumitomo Osaka Cement have jointly developed an optical modulator for single-sideband (SSB) optical transmission. Commonly used in wireless communications, SSB increases frequency-utilization efficiency by approximately 200% while cutting power usage in half. In the past, SSB has not been used for optical communications due to the lack of an appropriate modulator.

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When signals are created using amplitude modulation, two sidebands appear, one on either side of the carrier wave on the frequency axis (top figure). All of the information is contained in just one band, so only one side needs to be transmitted; in addition, the carrier does not need to be sent. When SSB is used in multiplexing applications, twice as many channels can be packed into the same wavelength band. In addition, the power needed for each channel is only half the original power. Because the input power decreases, noise due to the nonlinearity in the optical fiber also decreases.

The newly developed optical modulator uses the phase modulation of an optical waveguide and includes a nonsymmetric crossing waveguide (bottom figure). Frequencies up to 40 GHz can be used for the modulation signal. The device can also be used as an optical modulator for optical subcarrier communications, a fusion of wireless and optical communications.

Courtesy O plus E magazine, Tokyo

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