Wavelength tunable transmitters for future reconfigurable agile optical networks

Wavelength tuneable transmission is a requirement for future reconfigurable agile optical networks as it enables cost efficient bandwidth distribution and a greater degree of transparency. This thesis focuses on the development and characterisation of wavelength tuneable transmitters for the core, metro and access based WDM networks. The wavelength tuneable RZ transmitter is a fundamental component for the core network as the RZ coding scheme is favoured over the conventional NRZ format as the line rate increases. The combination of a widely tuneable SG DBR laser and an EAM is a propitious technique employed to generate wavelength tuneable pulses at high repetition rates (40 GHz). As the EAM is inherently wavelength dependant an accurate characterisation of the generated pulses is carried out using the linear spectrogram measurement technique. Performance issues associated with the transmitter are investigated by employing the generated pulses in a 1500 km 42.7 Gb/s circulating loop system. It is demonstrated that non-optimisation of the EAM drive conditions at each operating wavelength can lead to a 33 % degradation in system performance. To achieve consistent operation over a wide waveband the drive conditions of the EAM must be altered at each operating wavelength. The metro network spans relatively small distances in comparison to the core and therefore must utilise more cost efficient solutions to transmit data, while also maintaining high reconfigurable functionality. Due to the shorter transmission distances, directly modulated sources can be utilised, as less precise wavelength and chirp control can be tolerated. Therefore a gain-switched FP laser provides an ideal source for wavelength tuneable pulse generation at high data rates (10 Gb/s). A self-seeding scheme that generates single mode pulses with high SMSR (> 30 dB) and small pulse duration is demonstrated. A FBG with a very large group delay disperses the generated pulses and subsequently uses this CW like signal to re-inject the laser diode negating the need to tune the repetition rate for optimum gain-switching operation. The access network provides the last communication link between the customer’s premises and the first switching node in the network. FTTH systems should take advantage of directly modulated sources; therefore the direct modulation of a SG DBR tuneable laser is investigated. Although a directly modulated TL is ideal for reconfigurable access based networks, the modulation itself leads to a drift in operating frequency which may result in cross channel interference in a WDM network. This effect is investigated and also a possible solution to compensate the frequency drift through simultaneous modulation of the lasers phase section is examined

Fabrication and experimental characterisation of multiple quantum well semiconductor laser amplifier with integrated saturable absorber.

The demand for high capacity, long-haul telecommunication systems is increasing at a steady rate, and will be expected to accelerate in the coming decade. At the same time, communication networks that cover long distances and serve large areas with a large information capacity are also in increasing demand. To satisfy the requirements on long distances and low cost, the communication channel must have a very low loss and low component count. A large information capacity can only be achieved with a wide system bandwidth which can support a high data rate (> 40 Gbits/s). Therefore it is important to increase the bandwidth of the communication network whilst also limiting the loss through the system. A way to achieving this is represented by the so-called all-optical signal processing. This thesis investigates the practical realisation of an all-optical 2R (Re-amplification, and Re-shaping) regenerator, employing an InGaAsP/InGaAsP multiple quantum wells (MQW) semiconductor laser amplifier (SLA) integrated with a passive saturable absorber (SA) device with recovery time made short by means of ion- implantation. The saturable absorber should have strong non-linearity to the input signal, to suppress low-power noise accumulated in long-haul optical systems, whereas the waveguide amplifier will provide the signal re-amplification function in the system. This work focuses on the fabrication process and experimental analysis of the semiconductor laser amplifier/saturable absorber (SLA/SA) devices. In particular we investigate the SLA and SA characteristics separately for simplicity. The design and fabrication issues encountered during the fabrication process are discussed, and the key characteristics that determine the SLA behaviour such as the amplified spontaneous emission (ASE), the gain, the output saturation power, and gain recovery time will be covered and for the SA, characteristics such as the non-linearity and recovery time. In addition, the use of ion implantation to create electrically isolated InGaAsP devices is studied. The work presented although not yet demonstrating a fully functional integrated SLA/SA for use as a regenerator in an all-optical system offering channel rates > 40 Gb/s and higher, offers a tool towards the determination of the best device and material design parameters to achieve the desired functionality for the SLA/SA

Advanced raman amplification techniques for high capacity and broadband coherent optical transmission systems

This thesis presents a detailed study of different advanced Raman fibre laser (RFL) based amplification schemes and the development of novel broadband distributed and discrete Raman amplifiers in order to improve the transmission performance of modern high capacity, long-haul coherent optical systems. The numerical modelling of different Raman amplifier techniques including power distribution of signal, pump and noise components, RIN transfer from pump to signal, broadband gain optimization and so on have been described in details.The RIN and noise performances of RFL based distributed Raman amplifiers (DRAs) with different span lengths, forward pump powers and input reflection levels have been characterized experimentally. It has been shown through coherent transmission experiment that, in order to improve pump power efficiency, a low level of input reflection up to ~10% can be allowed without increasing the Q factor penalty > 1dB due to additional signal RIN penalty.A novel broadband (>10nm) first order Raman pump is developed for use as a forward pump in long-haul transmission experiment. Significant signal RIN mitigation up to 10dB compared with conventional low RIN, narrowband sources was obtained for bidirectional DRA schemes. Long-haul coherent transmission experiments with 10×120Gb/s DP-QPSK system were carried out in are circulating loop setup using the proposed broadband pump in bidirectional and backward only pumping configurations. The maximum transmission reach up to ~8330km was reported with first order broadband pumped bidirectional DRA, with transmission reach extensions of 1250km and1667km compared with conventional backward only and first order semiconductor pumped bidirectional pumping scheme respectively.Finally, a novel design of bidirectional broadband distributed DRA is proposed to reduce the noise figure tilt and improve the WDM transmission performances. Furthermore, broadband discrete Raman amplifier schemes in dual stage configuration are also shown for high gain, high output power, low noise and low nonlinear performance

NASA Tech Briefs, June 1997

Topics include: Computer Hardware and Peripherals; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery/Automation; Manufacturing/Fabrication; Mathematics and Information Sciences; Books and Reports

Cumulative index to NASA Tech Briefs, 1986-1990, volumes 10-14

Tech Briefs are short announcements of new technology derived from the R&D activities of the National Aeronautics and Space Administration. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This cumulative index of Tech Briefs contains abstracts and four indexes (subject, personal author, originating center, and Tech Brief number) and covers the period 1986 to 1990. The abstract section is organized by the following subject categories: electronic components and circuits, electronic systems, physical sciences, materials, computer programs, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences

Bibliography of Lewis Research Center technical publications announced in 1988

This bibliography contains abstracts of the technical reports that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1988. Subject, author, and corporate source indexes are also included. All the publications were announced in the 1988 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses

NASA Tech Briefs, February 1992

Topics covered include: New Product Development; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; Life Sciences