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Towards all-optical label switching nodes with multicastFiber optics has developed so rapidly during the last decades that it has be- come the backbone of our communication systems. Evolved from initially static single-channel point-to-point links, the current advanced optical backbone net- work consists mostly of wavelength-division multiplexed (WDM) networks with optical add/drop multiplexing nodes and optical cross-connects that can switch data in the optical domain. However, the commercially implemented optical net- work nodes are still performing optical circuit switching using wavelength routing. The dedicated use of wavelength and infrequent recon¯guration result in relatively poor bandwidth utilization. The success of electronic packet switching has inspired researchers to improve the °exibility, e±ciency, granularity and network utiliza- tion of optical networks by introducing optical packet switching using short, local optical labels for forwarding decision making at intermediate optical core network nodes, a technique that is referred to as optical label switching (OLS). Various research demonstrations on OLS systems have been reported with transparent optical packet payload forwarding based on electronic packet label processing, taking advantage of the mature technologies of electronic logical cir- cuitry. This approach requires optic-electronic-optic (OEO) conversion of the op- tical labels, a costly and power consuming procedure particularly for high-speed labels. As optical packet payload bit rate increases from gigabit per second (Gb/s) to terabit per second (Tb/s) or higher, the increased speed of the optical labels will eventually face the electronic bottleneck, so that the OEO conversion and the electronic label processing will be no longer e±cient. OLS with label processing in the optical domain, namely, all-optical label switching (AOLS), will become necessary. Di®erent AOLS techniques have been proposed in the last ¯ve years. In this thesis, AOLS node architectures based on optical time-serial label processing are presented for WDM optical packets. The unicast node architecture, where each optical packet is to be sent to only one output port of the node, has been in- vestigated and partially demonstrated in the EU IST-LASAGNE project. This thesis contributes to the multicast aspects of the AOLS nodes, where the optical packets can be forwarded to multiple or all output ports of a node. Multicast capable AOLS nodes are becoming increasingly interesting due to the exponen- tial growth of the emerging multicast Internet and modern data services such as video streaming, high de¯nition TV, multi-party online games, and enterprise ap- plications such as video conferencing and optical storage area networks. Current electronic routers implement multicast in the Internet protocol (IP) layer, which requires not only the OEO conversion of the optical packets, but also exhaus- tive routing table lookup of the globally unique IP addresses. Despite that, there has been no extensive studies on AOLS multicast nodes, technologies and tra±c performance, apart from a few proof-of-principle experimental demonstrations. In this thesis, three aspects of the multicast capable AOLS nodes are addressed: 1. Logical design of the AOLS multicast node architectures, as well as func- tional subsystems and interconnections, based on state-of-the-art literature research of the ¯eld and the subject. 2. Computer simulations of the tra±c performance of di®erent AOLS unicast and multicast node architectures, using a custom-developed AOLS simulator AOLSim. 3. Experimental demonstrations in laboratory and computer simulations using the commercially available simulator VPItransmissionMakerTM, to evaluate the physical layer performance of the required all-optical multicast technolo- gies. A few selected multi-wavelength conversion (MWC) techniques are particularly looked into. MWC is an essential subsystem of the AOLS node for realizing optical packet multicast by making multiple copies of the optical packet all-optically onto di®er- ent wavelengths channels. In this thesis, theMWC techniques based on cross-phase modulation and four-wave mixing are extensively investigated. The former tech- nique o®ers more wavelength °exibility and good conversion e±ciency, but it is only applicable to intensity modulated signals. The latter technique, on the other hand, o®ers strict transparency in data rate and modulation format, but its work- ing wavelengths are limited by the device or component used, and the conversion e±ciency is considerably lower. The proposals and results presented in this thesis show feasibility of all-optical packet switching and multicasting at line speed without any OEO conversion and electronic processing. The scalability and the costly optical components of the AOLS nodes have been so far two of the major obstacles for commercialization of the AOLS concept. This thesis also introduced a novel, scalable optical labeling concept and a label processing scheme for the AOLS multicast nodes. The pro- posed scheme makes use of the spatial positions of each label bit instead of the total absolute value of all the label bits. Thus for an n-bit label, the complexity of the label processor is determined by n instead of 2n.
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Macular edema in Asian Indian premature infants with retinopathy of prematurity: Impact on visual acuity and refractive status after 1-yearPurpose: To report the impact of transient, self-resolving, untreated "macular edema" detected on spectral domain optical coherence tomography in Asian Indian premature infants with retinopathy of prematurity (ROP) on visual acuity (VA) and refraction at 1-year of corrected age. Materials and Methods: Visual acuity and refraction of 11 infants with bilateral macular edema (Group A) was compared with gestational age-matched 16 infants with ROP without edema (Group B) and 17 preterms infants without ROP and without edema (Group C) at 3, 6, 9 and 12 months of corrected age using Teller Acuity Cards and cycloplegic retinoscopy. Sub-group analysis of the previously described pattern A and B macular edema was performed. Results: Visual acuity was lower in infants with macular edema compared with the other two control groups throughout the study period, but statistically significant only at 3 months. Visual improvement in these infants was highest between the 3 rd and 6 th month and plateaued by the end of the 1 st year with acuity comparable to the other two groups. The edema cohort was more hyperopic compared to the other two groups between 3 and 12 months of age. Pattern A edema had worse VA compared to pattern B, although not statistically significant. Conclusion: Macular edema, although transient, caused reduced VA as early as 3 months of corrected age in Asian Indian premature infants weighing <2000 g at birth. The higher hyperopia in these infants is possibly due to visual disturbances caused at a critical time of fovealization. We hypothesize a recovery and feedback mechanism based on the principles of active emmetropization to explain our findings.