Even
though self-pulsating laser diodes are supplied by a DC current, their output is
characterised by an emission of pulses at a regular frequency, called the
self-pulsation (SP) frequency. This frequency can vary depending on the
structure of the laser and its working conditions.
Frequencies within the range of 300 MHz up to 164 GHz have been reported
in the literature. Self-pulsating lasers are currently used in compact disc
optical data storing systems, due to their low coherence length which reduces
the errors due to interferences. In 1989, these self-modulating lasers became of
interest for optical communication systems when it was demonstrated that SP
lasers synchronise to a return-to-zero optical signal. This is a useful property
for clock recovery applications in which the information can be kept in its
optical format. The signal is split into two parts, one of which is injected
into the SP laser in order to synchronize the pulses emitted to the incoming
data rate. The output of the SP laser is now called the clock signal. There are
a couple of key features of the SP laser that have to be mentioned. Firstly, the
SP lasers are insensitive to optical feedback. Secondly they can be synchronised
over a large frequency range, and so, can be used for data being transmitted
over a large frequency range. This can lead to an implementation of this
component in different scale size networks. Thirdly, the SP lasers can operate
over a wavelength range of more than 13 nm. All of these features are highly
advantageous for any clock recovery application in WDM and OTDM networks. In the
literature different types of SP lasers, Fabry-Perot, distributed feedback or
distributed Bragg reflector lasers exhibit these different features to varying
degrees. It has been demonstrated that the fast SP frequencies (>3 GHz) are
due to the presence of effective differential gain. Based on this theory, a new
type of DBR SP laser has been designed. I have a new type of SP device which is
a multi-section DBR laser allowing control over the SP behaviour, the wavelength
and the SP frequency. The laser is designed to achieve the state of the art,
exhibiting the highest frequency and largest wavelength tunability. The aims of
this project are to study the physics of the hybrid laser and to assess its
performance for potential applications in OTDM or DWDM systems.
Seminconductor optical amplifier:
The all-optical clock division is a key component in optical time division multiple (OTDM) access communication networks. It retrieves the clock signal from the data stream and generates a series of optical pulses as the clock frequency. These pulses can then be exploited to demultiplex the OTDM signal. Mainly two candidates can perform a clock recovery function: i) the self-pulsating laser diode as mentioned above and ii) the semiconductor optical amplifier (SOA). The SOA can be depicted as an active waveguide electrically pumped and surrounded by antireflection-coated mirrors. An injected signal into the SOA is amplified as it propagates along the active waveguide and emerges larger. The gain is provided by the carriers, which emit a photon as they relax from the conduction band to the valence band. The SOA can be used in various functions in all-optical networks. It presents the advantage of being applicable to different clock recovery set-ups: the feedback loop with an optical beam splitter and the mode-locked figure eight. In these configurations, the clock extraction is all-optical. There is also another configuration of interest where the clock division is performed using optical and electronic hybrid systems, such as the phase-locked loop.
The candidates are invited to email their CV including a covering letter at landaisp@eeng.dcu.ie.