Research

Broadband Hybrid Radio over Fiber Systems

Generation of Tunable Optical Pulses

Wavelength Packet Switched Networks

Broadband Hybrid Radio over Fiber Systems

As the demand for broadband mobile services such as video-on-demand increases, so does the need to develop high capacity mobile communication networks. Hybrid radio/fiber systems are a very attractive option to realize such broadband networks. In these systems, the microwave data signals are modulated onto an optical carrier at a central location, and then distributed to remote base stations using optical fiber. The base stations then transmit the RF signals over small areas using microwave antennas. Within these future radio networks it is also expected that the available bandwidth will be divided into a number of frequency channels for signal distribution. This use of multiple RF carrier distribution (known as Sub-Carrier Multiplexing (SCM)) is normally required in high capacity multi-path environments in order to overcome multi-path fading effects, and it thus important for simplifying the complexity of the radio links and the management of the available spectral bandwidth. In addition to SCM, it is also expected that hybrid radio/fiber distribution networks may employ Wavelength Division Multiplexing (WDM) to allow different base stations to be fed with a common optical fiber. In such a system, an optical filter at the base station will be required to select one of the wavelength channels carrying a specific SCM data signal. For increased flexibility in these radio/fibre systems it may also be suitable to consider the use of a tuneable laser transmitter at the central station. In such a configuration the tuneable lasers can be used for sending information to specific base stations by transmitting on the wavelength assigned to that station (determined by band-pass filter at the base station). The tuneable lasers will be able to switch quickly between sending information to different base stations by tuning their emission wavelengths, thus allowing very fast reconfiguration of the system and a dynamic allocation of the available bandwidth between the base stations.

Generation of Tunable Optical Pulses

The use of wavelength tunability as a means of providing enhanced flexibility and efficiency in next generation photonic systems, is currently a key research and development area in the optical communications domain. In addition to this development, current trends and technology maturity would seem to suggest that future high capacity WDM systems are likely to operate at line rates of 40 Gb/s and beyond, thereby making it more likely that Return-to-Zero (RZ) coding be used for data transmission (as it is easier to compensate for dispersion and non-linear effects in the fiber by employing soliton like propagation). Taking into account these moves towards tuneable optical systems employing RZ coding, it is likely that the development of a wavelength tuneable source of short optical pulses will become important for future high capacity optical systems. The use of a fast tuneable pulse source may be useful for:

(i) Wavelength-routed burst-mode transmission systems operating at line rates of 40 Gbit/s. In these systems, the RZ format may be required for transmission across the wavelength-routed core network (because distances will be a few hundred km or more).

(ii) High performance switch/interconnects in which the use of the RZ format may be used to simplify the clock recovery technology. Clock recovery is easier to do with RZ signals as there is a strong high-quality clock component in the RF spectrum after detection, that can be filtered electrically (as compared to digital clock recovery required for NRZ which is more complex and has higher latency).

 In addition, the development of tuneable pulse sources will be vital for long haul WDM communications systems that only require static tuning of the pulse sources. Regardless of the application, there are a number of basic requirements on the pulse source that need to be meet. These are predominantly high levels of spectral and temporal purity (including good side mode suppression ratio and temporal pedestal suppression, low levels of timing jitter and amplitude jitter, and close to transform limited pulses). The main purpose of this work is to ensure that the generated pulses have the required levels of spectral and temporal purity over the range of available wavelengths, which will make them suitable for use in numerous applications. This signal purity is vital to prevent interference between the wavelength channels if future WDM systems are to progress to data rates of 40 Gbit/s, with channel spacing of 100 GHz or below. To develop wavelength tuneable RZ transmitters in this work we plan to use fast wavelength tuneable sources coupled with high-speed external modulators.

Wavelength Packet Switched Networks

As optical fibre communication systems become increasingly important for transporting information at very high data rates between different areas, a new generation optical Internet is gradually being developed. Although most of the packets of information transmitted over the Internet are sent in the optical domain, the time-consuming process of determining where the packet must be sent is still undertaken using electronic routers. This electronic routing of the data packets thus requires optical-to-electronic conversion, followed by electronic routing, and then another electrical-to-optical conversion to actually send the information on to its destination over the optical fibre. As the capacity of these networks increases, it is expected that the electro-optic conversions will prove too costly, both in terms of time and money. It will thus become attractive to have one layer of the overall network in which the routing of packets is undertaken in the optical domain. One possible solution for optical packet switching may be to transmit different packets of information on different optical wavelengths, and then use wavelength selective filtering techniques to determine where the packets are sent. The basic idea of wavelength packet switching involves the use of wavelength tuneable transmitters to route optical packets to different destinations.

In such systems each transmitter can tune its emission wavelength to transmit packets of information at a number of different wavelengths. The information from all the transmitters can then be multiplexed together and sent over optical fibre. As we are basically sending information on multiple wavelengths over a single fibre we are essentially using wavelength division multiplexing in the wavelength packet switched network. The composite wavelength signal then passes through an Array Waveguide Grating (AWG), which sends each of the incoming wavelengths to one specific output fibre port. In general it is possible to write the wavelength connectivity in the form of a matrix. Thus by choosing an appropriate wavelength on the transmitter side, the laser selects the output port to which the information packet is sent. The tuneable transmitters, together with the optical coupler and the AWG become a strictly non-blocking switch fabric with a switching speed equal to the tuning speed of the lasers.