2 research outputs found
Advanced optical modulation and fast reconfigurable en/decoding techniques for OCDMA application
With the explosive growth of bandwidth requirement in optical fiber communication
networks, optical code division multiple access (OCDMA) has witnessed tremendous
achievements as one of the promising technologies for optical access networks over the
past decades. In an OCDMA system, optical code processing is one of the key
techniques. Rapid optical code reconfiguration can improve flexibility and security of
the OCDMA system. This thesis focuses on advanced optical modulations and
en/decoding techniques for applications in fast reconfigurable OCDMA systems and
secure optical communications.
A novel time domain spectral phase encoding (SPE) scheme which can rapidly
reconfigure the optical code and is compatible with conventional spectral domain phase
en/decoding by using a pair of dispersive devices and a high speed phase modulator is
proposed. Based on this scheme, a novel advanced modulation technique that can
simultaneously generate both the optical code and the differential-phase-shift-keying
(DPSK) data using a single phase modulator is experimentally demonstrated. A
symmetric time domain spectral phase encoding and decoding (SPE/SPD) scheme using
a similar setup for both the transmitter and receiver is further proposed, based on which
a bit-by-bit optical code scrambling and DPSK data modulation technique for secure
optical communications has been successfully demonstrated. By combining optical
encoding and optical steganography, a novel approach for secure transmission of time
domain spectral phase encoded on-off-keying (OOK)/DPSK-OCDMA signal over
public wavelength-division multiplexing (WDM) network has also been proposed and
demonstrated.
To enable high speed operation of the time domain SPE/SPD scheme and enhance the
system security, a rapid programmable, code-length variable bit-by-bit optical code
shifting technique is proposed. Based on this technique, security improvements for
OOK/DPSK OCDMA systems at data rates of 10Gb/s and 40Gb/s using reconfigurable
optical codes of up to 1024-chip have been achieved.
Finally, a novel tunable two-dimensional coherent optical en/decoder which can
simultaneously perform wavelength hopping and spectral phase encoding based on
coupled micro-ring resonator is proposed and theoretically investigated. The techniques
included in this thesis could be potentially used for future fast reconfigurable and secure
optical code based communication systems
Optical pulse processing towards Tb/s high-speed photonic systems
Due to the continued growth of high-bandwidth services provided by the internet, there is a requirement to operate individual line rates in excess of 100 Gb/s in next generation optical communications systems. Thus, to implement these high-speed optical networks all-optical processing techniques are necessary for pulse shaping and pulse routing. Two sub-systems (pulse generation and wavelength conversion), which exploit optical processing techniques are explored within this thesis.
Future systems will require high-quality pulse sources and this thesis develops the pulse generation technique of gain switching to provide simple and cost efficient pulse sources. The poor pulse quality typically associated with gain switching is enhanced by developing all-optical methods. The main attribute of the first pulse generation scheme presented is its wavelength tunability over 50 nm. The novelty of the second scheme lies in the ability to design a grating which has a nonlinear chirp profile exactly opposite to the gain-switched pulses. This grating used in conjunction with the gain-switched laser generates transform limited pulses suitable for 80 Gb/s systems. Furthermore the use of a vertical microcavity-based saturable absorber to suppress detrimental temporal pulse pedestals of a pulse source is investigated.
Next generation networks will require routing of data in the optical domain, which can be accomplished by high-speed all-optical wavelength converters. A semiconductor optical amplifier (SOA) is an ideal device to carry out wavelength conversion. In this thesis pulses following propagation through an SOA are experimentally characterised to examine the temporal and spectral dynamics due to the nonlinear response of the SOA. High-speed wavelength conversion is presented using SOA-based shifted filtering. For the first time 80 Gb/s error-free performance was obtained using cross phase modulation in conjunction with blue spectral shifted filtering. In addition an important attribute of this work experimentally examines the temporal profile and phase of the SOA-based shifted filtering wavelength converted signals. Thus the contribution and effect of ultrafast carrier dynamics associated with SOAs is presented