Gain switching for the optical generation of modulated millimetre waves

Abstract

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. 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. 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 processing 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. Several photonic techniques have been reported in the last few years to generate and transmit millimeter (mm-) waves for broadband data distribution. 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 or external modulation. 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. 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. These two optical carriers can be produced by using either a dual mode DFB laser or optical phase locked loop (OPLL). However, these techniques require very narrow linewidth optical sources and optical phase locking to reduce the phase noise in the generated electrical signal, which increases the cost and complexity of the system. Other techniques used to generate two optical carriers involve direct modulation of a narrow band semiconductor laser by driving the laser with half of the desired mm-wave frequency and filtering the carrier. However, this technique is limited to doubling the frequency of the driving signal and also suffers from bandwidth limitations. The use of the non-linearity of external intensity modulators has also been widely used for generating frequency doubling and quadrupling of the RF sinusoidal drive signal. Although the technique is simple and a low drive frequency signal is required, this technique can not generate more than the fourth harmonic of the modulator drive frequency, and requires two external modulators, one to generate the optical carriers with the required frequency separation, and one for the data modulation. This results in an increase in both loss and cost of the whole system and in addition the system suffers from bias drifting and polarization dependence of the modulator which can affect long term stability and performance. In this paper, we propose and demonstrate two configurations for optical mm-wave generation and transmission of 3 Gbps downstream data based on a gain switched laser (GSL)As illustrated in Fig. 1, the first configuration generates an optical comb spectrum from a GSL that can be appropriately filtered to generate two optical sidebands with 60 GHz separation. These sidebands are modulated with baseband data by using an external intensity modulator and then transmitted via optical fiber to the RAU. The second configuration, as in Fig. 2, produces a modulated optical frequency comb by driving the laser with both RF LO and data streams coupled together and...