Mode-locked microchip lasers for millimeter-wave generation for optically fed wireless systems

Optically fed wireless systems have been investigated for applications like picocellular telephony, for broadband distribution of interactive multimedia services (video, voice, data, etc.), and for remoting and phased array antennas. The principal advantages of the transmission of high frequency signals over fiber is low attenuation and cost when compared to the conventional coaxial cable or radio transmission, and the large aggregated information capacity even when only part of the bandwidth is exploited. An additional advantage that makes millimeter-wave desirable for fiber radio systems is that these frequencies are highly attenuated by the water molecules and oxygen in the atmosphere. This property limits signal propagation within the picocell, allowing for frequency reuse and making the communication system secure. Various methods of optically generating millimeter wave subcarriers have been demonstrated, including direct modulation of semiconductor lasers, external electro-optic modulation, laser heterodyning, optical injection locking, and semiconductor laser mode-locking. The examination of these techniques suggests that conventional solid-state laser sources present lower noise levels, but are expensive and complex. Laser diode based transmitters, in general, have higher phase noise. Laser diode mode-locking offers high modulation efficiency but with higher noise levels. Mode-locked microchip laser combines the pure spectral quality of solid-state lasers with the efficient high frequency and low noise modulation provided by active mode-locking