Gain switching for the optical generation of modulated millimetre waves

The ever growing demand for high bandwidth allowing broadband applications to be delivered to end-users forces the system operators to seek new ways to increase the bandwidth and capacity of telecommunication systems. It is expected that radio over fiber may be a solution to many problems associated with bandwidth issues. The combination of the two technologies enables use of both their merits: fiber provides a high capacity medium with electromagnetic interference immunity and low attenuation, while radio solves the problem of "the last mile" thereby enabling broadband data to be delivered to the end-users in a quick and cheap manner [1]. The architecture of the radio part of the system is likely to be realised in a similar manner to that used in mobile systems, which means that the terrain over which the system operates is going to be divided into a number of cells. This organisation ensures the best usage of the available spectrum. The radio/fiber systems are likely to use frequencies ranging from around 2.5 GHz up to 300 GHz. Frequencies from 30 GHz and above are especially attractive for high capacity networks due to the large bandwidth available for data transfer. Furthermore the high oxygen absorption in this range of frequencies gives us a large frequency reuse factor thereby implying a small cell size [2]. Subsequently a large number of Remote Antenna Units (RAUs) are required to transmit the signals to users in each cell. Therefore the deployment of microwave wireless networks strongly depends on the cost and complexity of the RAU. Future millimetre wave access networks are likely to employ an architecture in which signals are generated at a central location and then distributed to remote base stations using optical fibre, before being transmitted over small areas using millimetre wave antennas. Optical feeding of RAUs in such systems is an attractive approach because it enables a large number of RAUs to share the transmitting and processin- - g equipment (expensive and power hungry components) remotely located from the customer serving area. Such architectures should prove to be extremely attractive and cost effective for the provision of future broadband services to a large density of customers [3]. Several photonic techniques have been reported in the last few years to generate and transmit millimeter (mm-) waves for broadband data distribution [4, 5]. The simplest and easiest way for optical mm-wave generation for downstream data is to modulate the intensity of the laser output either by using direct [6] or external modulation [7]. After transmission through the optical fiber, the mm-wave can be recovered by direct detection on a photodiode. The main limitation of the use of direct modulation is the limited laser modulation bandwidth. With these techniques the data signal is carried in side-bands on both sides of the optical carrier which is known as double side band (DSB) operation. Transmission of such a signal through a fiber will cause a phase shift between the two sidebands due to the chromatic dispersion effect. This can cause fading in the received power as a result of destructive interference as the two side bands add vectorially. However, it is also possible to suppress one sideband to give single side band (SSB) modulation scheme which reduces the power fading effect, but this scheme has a lower receiver sensitivity than DSB due to the large dc power component at the optical carrier [8]. A different technique for optical mm-wave generation can be realized by using a remote heterodyne receiver where two phase correlated optical carriers are generated at the CS with a frequency offset equal to the desired mm-wave. The generated carriers are then transmitted over the fiber and beat together at a high speed photodetector. The use of this technique can greatly reduce the bandwidth of the optical components required, and can also eliminate the power fading effect due to fibre transmission [8, 9]