Experimental and theoretical characterisation of tunable fibre gratings

This thesis presents details on both theoretical and experimental aspects of UV written fibre gratings. The main body of the thesis deals with the design, fabrication and testing of telecommunication optical fibre grating devices, but also an accurate theoretical analysis of intra-core fibre gratings is presented. Since more than a decade, fibre gratings have been extensively used in the telecommunication field (as filters, dispersion compensators, and add/drop multiplexers for instance). Gratings for telecommunication should conform to very high fabrication standards as the presence of any imperfection raises the noise level in the transmission system compromising its ability of transmitting intelligible sequence of bits to the receiver. Strong side lobes suppression and high and sharp reflection profile are then necessary characteristics. A fundamental part of the theoretical and experimental work reported in this thesis is about apodisation. The physical principle of apodisation is introduced and a number of apodisation techniques, experimental results and numerical optimisation of the shading functions and all the practical parameters involved in the fabrication are detailed. The measurement of chromatic dispersion in fibres and FBGs is detailed and an estimation of its accuracy is given. An overview on the possible methods that can be implemented for the fabrication of tunable fibre gratings is given before detailing a new dispersion compensator device based on the action of a distributed strain onto a linearly chirped FBG. It is shown that tuning of second and third order dispersion of the grating can be obtained by the use of a specially designed multipoint bending rig. Experiments on the recompression of optical pulses travelling long distances are detailed for 10 Gb/s and 40 Gb/s. The characterisation of a new kind of double section LPG fabricated on a metal-clad coated fibre is reported. The fabrication of the device is made easier by directly writing the grating through the metal coating. This device may be used to overcome the recoating problems associated with standard LPGs written in step-index fibre. Also, it can be used as a sensor for simultaneous measurements of temperature and surrounding medium refractive index.

Dispersion aspects of periodically amplified soliton transmission systems

This thesis presents improvements to optical transmission systems through the use of optical solitons as a digital transmission format, both theoretically and experimentally. An introduction to the main concepts and impairments of optical fibre on pulse transmission is included before introducing the concept of solitons in optically amplified communications and the problems of soliton system design. The theoretical work studies two fibre dispersion profiling schemes and a soliton launch improvement. The first provides superior pulse transmission by optimally tailoring the fibre dispersion to better follow the power, and hence nonlinearity, decay and thus allow soliton transmission for longer amplifier spacings and shorter pulse widths than normally possible. The second profiling scheme examines the use of dispersion compensating fibre in the context of soliton transmission over existing, standard fibre systems. The limits for solitons in uncompensated standard fibre are assessed, before the potential benefits of dispersion compensating fibre included as part of each amplifier are shown. The third theoretical investigation provides a simple improvement to the propagation of solitons in a highly perturbed system. By introducing a section of fibre of the correct length prior to the first system amplifier span, the soliton shape can be better coupled into the system thus providing an improved "average soliton" propagation model. The experimental work covers two areas. An important issue for soliton systems is pulse sources. Three potential lasers are studied, two ring laser configurations and one semiconductor device with external pulse shaping. The second area studies soliton transmission using a recalculating loop, reviewing the advantages and draw-backs of such an experiment in system testing and design. One particular example of employing the recirculating loop is also examined, using a novel method of pulse shape stabilisation over long distances with low jitter. The future for nonlinear optical communications is considered with the thesis conclusions.

Fiber optical parametric amplifier performance in a 1-Tb/s DWDM communication system

We have reduced signal-signal four-wave mixing crosstalk in a fiber optical parametric amplifier (OPA) by using a short nonlinear fiber for the gain medium and a high-power pump. This allowed us to obtain less than 1 dB penalty for amplification of 26 dense wavelength-division multiplexed (WDM) channels modulated at 43.7Gb/s return to zero-differential phase-shift keying, with the OPA placed between transmitter and receiver. We then used the same OPA in several different roles for a long-haul transmission system. We did not insert the OPA within the loop, but investigated this role indirectly by using equivalent results for small numbers of loop recirculations. We found that standard erbium-doped fiber amplifiers currently hold an advantage over this OPA, which becomes negligible for long distances. This paper shows that at this time OPAs can handle amplification of WDM traffic in excess of 1 Tb/s with little degradation. It also indicates that with further improvements, fiber OPAs could be a contender for wideband amplification in future optical communication networks.

Autosoliton transmission in dispersion-managed systems guided by in-line nonlinear optical loop mirrors

We examine the feasibility of optical pulse transmission in dispersion-managed fiber systems with in-line nonlinear optical loop mirrors. Applying numerical analysis, we find regimes of stable propagation over long distances in such lines, with a significant increase in the signal-to-noise ratio. © 2000 Optical Society of America.

Evaluating the capacity of long haul soliton transmission

We review recent developments in the use of optical solitons for communication systems spanning transoceanic distances. The implementation of "soliton control" to alleviate the detrimental impact of effects such as amplifier noise is shown to be critical for obtaining advantages over competing technologies. The potential performance of two control strategies, namely straight line filtering and synchronous phase modulation, is examined in detail. Design diagrams are used to determine the maximum permissible amplifier spacing, which is a key determinant of system economics. To focus the enquiry, two example system spans are taken, representing transatlantic and transpacific distances. It is concluded that straight line filtering provides very little improvement over a basic design without control. However synchronous phase modulation, which may be implemented using a handful of actively driven components, provides very substantial benefits. These may be used either to extend the overall bit-rate-distance product of the system or to increase the amplifier spacing at more moderate capacities.

Long distance dispersion managed soliton transmission experiments

This thesis presents results of transmission experiments using optical solitons in a dispersion managed optical fibre recirculating loop. The basic concepts of pulse propagation in optical fibre are introduced before optical solitons and their use in optically amplified fibre systems are discussed. The role of dispersion management in such systems is then considered. The design, operation and limitations of the recirculating loop and soliton sources which were used and the experimental techniques are described before the experimental work is presented. The experimental work covers a number of areas all of which used dispersion management of the transmission line. A novel ultra-long distance propagation scheme which achieved low timing jitter by suppression of the amplifier noise and by working close to the zero dispersion wavelength has been discovered. The use of fibre Bragg gratings as wavelength filters to suppress noise and reduce timing jitter has been investigated. The performance of the fibre grating cornpared favourably with that of a bulk device and was in good agreement with theoretical predictions. The upgrade of existing standard fibre systems to higher bit rates is currently an important issue. The possibility of using solitons with dispersion compensation to allow an increase in data rate of existing standard fibre systems to 10Gbit/s over 5000km has been demonstrated. The applicability of this technique to longer distances, higher bit rates or longer amplifier spans is also investigated by optimisation of the dispersion management scheme. The use of fibre Bragg gratings as the dispersion compensating elements in such standard fibre transmission experiments has been examined and the main problem that these devices currently have, high polarisation mode dispersion, is discussed. The likely future direction of optical communications and what part solitons and dispersion management will play in this development is discussed in the thesis conclusions

Nonlinearly regenerated long haul data transmission

This thesis experimentally examines the use of different techniques for optical fibre transmission over ultra long haul distances. Its format firstly examines the use of dispersion management as a means of achieving long haul communications. Secondly, examining the use concatenated NOLMs for DM autosoliton ultra long haul propagation, by comparing their performance with a generic system without NOLMs. Thirdly, timing jitter in concatenated NOLM system is examined and compared to the generic system and lastly issues of OTDM amplitude non-uniformity from channel to channel in a saturable absorber, specifically a NOLM, are raised. Transmission at a rate of 40Gbit/s is studied in an all-Raman amplified standard fibre link with amplifier spacing of the order of 80km. We demonstrate in this thesis that the detrimental effects associated with high power Raman amplification can be minimized by dispersion map optimization. As a result, a transmission distance of 1600 km (2000km including dispersion compensating fibre) has been achieved in standard single mode fibre. The use of concatenated NOLMs to provide a stable propagation regime has been proposed theoretically. In this thesis, the observation experimentally of autosoliton propagation is shown for the first time in a dispersion managed optical transmission system. The system is based on a strong dispersion map with large amplifier spacing. Operation at transmission rates of 10, 40 and 80Gbit/s is demonstrated. With an insertion of a stabilizing element to the NOLM, the transmission of a 10 and 20Gbit/s data stream was extended and demonstrated experimentally. Error-free propagation over 100 and 20 thousand kilometres has been achieved at 10 and 20Gbit/s respectively, with terrestrial amplifier spacing. The monitor of timing jitter is of importance to all optical systems. Evolution of timing jitter in a DM autosoliton system has been studied in this thesis and analyzed at bit ranges from 10Gbit/s to 80Gbit/s. Non-linear guiding by in-line regenerators considerably changes the dynamics of jitter accumulation. As transmission systems require higher data rates, the use of OTDM will become more prolific. The dynamics of switching and transmission of an optical signal comprising individual OTDM channels of unequal amplitudes in a dispersion-managed link with in-line non-linear fibre loop mirrors is investigated.

Ultra-long haul optical fibre transmission systems

This thesis examines experimentally options for optical fibre transmission over oceanic distances. Its format follows the chronological evolution of ultra-long haul optical systems, commencing with opto-electronic regenerators as repeaters, progressing to optically amplified NRZ systems and finally solitonic propagation. In each case recirculating loop techniques are deployed to simplify the transmission experiments. Advances in high speed electronics have allowed regenerators operating at 10 Gbit/s to become a practical reality. By augmenting such devices with optical amplifiers it is possible to greatly enhance the repeater spacing. Work detailed in this thesis has culminated in the propagation of 10 Gbit/s data over 400,000 km with a repeater spacing of 160 km. System reliability and robustness are enhanced by the use of a directly modulated DFB laser transmitter and total insensitivity of the system to the signal state of polarisation. Optically amplified ultra-long haul NRZ systems have taken on particular importance with the impending deployment of TAT 12/13 and TPC 5. The performance of these systems is demonstrated to be primarily limited by analogue impairments such as the accumulation of amplifier noise, polarisation effects and optical non-linearities. These degradations may be reduced by the use of appropriate dispersion maps and by scrambling the transmitted state of signal polarisation. A novel high speed optically passive polarisation scrambler is detailed for the first time. At bit rates in excess of 10 Gbit/s it is shown that these systems are severely limited and do not offer the advantages that might be expected over regenerated links. Propagation using solitons as the data bits appears particularly attractive since the dispersive and non-linear effects of the fibre allow distortion free transmission. However, the generation of pure solitons is difficult but must be achieved if the uncontrolled transmission distance is to be maximised. This thesis presents a new technique for the stabilisation of an erbium fibre ring laser that has aUowed propagation of 2.5 Gbit/s solitons to the theoretical limit of ~ 18,000 km. At higher bit rates temporal jitter becomes a significant impairment and to aUow an increase in the aggregate line rate multiplexing in both time and polarisation domains has been proposed. These techniques are shown to be of only limited benefit in practical systems and ultimately some form of soliton transmission control is required. The thesis demonstrates synchronous retiming by amplitude modulation that has allowed 20 Gbit/s data to propagate 125,000 km error free with an amplifier spacing approaching the soliton period. Ultimately the speed of operation of such systems is limited by the electronics used and, thus, a new form of soliton control is demonstrated using all optical techniques to achieve synchronous phase modulation.

Evaluating the Capacity of Phase Modulator-Controlled Long-Haul Soliton Transmission

We review recent developments in the use of optical solitons for communication systems spanning transoceanic distances. The implementation of "soliton control" to alleviate the detrimental impact of effects such as amplifier noise is shown to be critical for obtaining advantages over competing technologies. The potential performance of two control strategies, namely straight line filtering and synchronous phase modulation, is examined in detail. Design diagrams are used to determine the maximum permissible amplifier spacing, which is a key determinant of system economics. To focus the enquiry, two example system spans are taken, representing transatlantic and transpacific distances. It is concluded that straight line filtering provides very little improvement over a basic design without control. However synchronous phase modulation, which may be implemented using a handful of actively driven components, provides very substantial benefits. These may be used either to extend the overall bit-rate-distance product of the system or to increase the amplifier spacing at more moderate capacities. © 1995 Academic Press. All rights reserved.

Performance comparison of SSMF and ultra wave fibers for ultra-long-haul 40-Gb/s WDM transmission

We experimentally compare the performance of standard single-mode fiber (SSMF) and UltraWave fiber (UWF) for ultra-long-haul (ULH) 40-Gb/s wavelength- division- multiplexing transmissions. We used the carrier-suppressed return-to-zero amplitude-shift-keying (CSRZ-ASK) and the carrier-suppressed return-to-zero differential-phase-shift-keying (CSRZ-DPSK) formats, which are particularly well-adapted to 40-Gb/s pulse-overlapped propagation. We demonstrate that transmission distance well beyond 2000 km can be reached on UWF with both the CSRZ-ASK and CSRZ-DPSK formats, or on SSMF with the CSRZ-DPSK format only, thus indicating that SSMF-based infrastructure of incumbent carriers can be upgraded at 40-Gb/s channel rates to ULH distances. © 2007 IEEE.

Accommodation in young adults wearing multifocal soft contact lenses under long- and short- wavelength lighting

Purpose: Several studies have suggested accommodative lags may serve as a stimulus for myopic growth, and while a blurred foveal image is believed to the main stimulus for accommodation, spectral composition of the retinal image is also believed to influence accommodative accuracy. Of particular interest is how altering spectral lighting conditions influences accommodation in the presence of soft multifocal contact lenses, which are currently being used off-label for myopia control. Methods: Accommodative responses were assessed using a Grand Seiko WAM-5500 autorefractor for four target distances: 25, 33, 50, and 100cm for 30 young adult subjects (14 myopic, 16 emmetropic; mean refractive errors (±SD, D) -4.22±2.04 and -0.15±0.67 respectively). Measurements were obtained with four different soft contact lenses, Single vision distance (SVD), Single vision near (SVN), Centre-Near (CN) and Centre-Distance (CD) (+1.50 add), and three different lighting conditions: red (peak λ 632nm), blue (peak λ 460nm), and white (peak λ 560nm). Corrections for chromatic differences in refraction were made prior to calculating accommodative errors. Results: The size of accommodative errors was significantly affected by lens design (p<0.001), lighting (p=0.027), and target distance (p=0.009). Mean accommodative errors were significantly larger with the SV lenses compared to the CD and CN designs (p<0.001). Errors were also significantly larger under blue light compared to white (p=0.004) and a significant interaction noted between lens design and lighting (p<0.001). Blue light generally decreased accommodative lags and increased accommodative leads relative to white and red light, the opposite was true of red light (p≤0.001). Lens design also significantly influenced direction of accommodative error (i.e. lag or lead) (p<0.001). Interactions with or between refractive groups were not found to be statistically significant for either the magnitude or direction of accommodative error (p>0.05 for all). Conclusions: Accuracy of accommodation is affected by both lens design and by wavelength of lighting. These accommodative lag data lend some support to recent speculation about the potential therapeutic value of lighting with a spectral bias towards blue during near work for myopia, although such treatment effects are likely to be more subtle under broad compared to the narrow spectrum lighting conditions used here.