| 1. |
- Dong, Lin, 1983-
(författare)
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Optical Properties of Nanoparticles in Composite Materials
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanoparticles are synthetic structures with dimension from 1 to 100 nanometers and are various in types. Some favorable properties peculiar to the nanoparticles (generally owing to size effects) make them prevailing and beneficial for applications in different scientific and engineering fields. A large portion of these properties find their connection to optics and photonics. In the context of optics, the thesis is devoted to study of two specific categories of nanoparticles, gold nanoparticles and CdSe-CdS core-shell quantum dots, aiming at investigating the influence and potential of the particles in applications of lasing and medical diagnosis/treatment.Gold nanoparticles have been widely exploited in radiative decay engineering to achieve fluorescence enhancement or quenching of fluorophores, with the help of a localized surface plasmon resonance band in visible range. As the technique is recently introduced to lasing applications, the influence of the gold nanoparticles on the photostability of the gain medium needs more attention. In this work, the effect of size and concentration of gold nanoparticles on altering the photostability of aqueous solution of Rhodamine 6G in lasing process is demonstrated and analyzed. Energy transfer and nanoparticle induced heat are found to be responsible for the acceleration of photobleaching. It is shown that coating the gold nanoparticles with a 15 nm thick silica layer can effectively diminish the photostability degradation of the gain medium.Gold nanorods are popular for in vivo diagnostic and therapeutic applications due to their strong absorption of near-infrared light. A novel type of multimodal nanoparticles based on gold nanorods is synthesized here and optically characterized. The coating of silica and gadolinium oxide carbonate hydrate renders the nanoparticles superior performance as MRI/CT contrast agents than commercially available products. Meanwhile, the precise temperature control of bio-tissues using the particles under laser irradiation makes them promising for photothermal treatment of cancer cells.The thesis also addresses several open questions with respect to CdSe-CdS core-shell quantum dots. A numerical model is built to study the spatial separation of electrons and holes in the dots with different core/shell sizes. QDs in different geometrical shapes are investigated. It is found that the spherical core-shell QDs can be flexibly tuned between the type-I and the type-II regime by varying the dimensions of the core and the shell. The feature is confirmed by time-resolved photoluminescence measurements, in which the carrier recombinations from different spatial paths can be distinguished. A sign of amplified spontaneous emission is observed with spherical dots of an appropriate combination of core radius and shell thickness, indicating the potential of the QDs for lasing applications.
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| 2. |
- Kakkar, Aditya, 1989-
(författare)
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Frequency Noise in Coherent Optical Systems: Impact and Mitigation Methods
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increase in capacity demand along with the advancement in digital signal processing (DSP) have recently revived the interest in coherent optical communications and led to its commercialization. However, design and development of robust DSP algorithms for example for carrier phase recovery (CPR) becomes complex as we opt for high order modulation formats such as 16QAM and beyond. Further, electrical-domain dispersion compensation (EDC), while providing many advantages, makes the system more susceptible to laser frequency noise (FN). For instance, in coherent optical links with post-reception EDC, while the transmitter frequency noise causes only phase impairment, the local oscillator (LO) FN in these systems results in a noise enhancement in both amplitude and phase. This noise is commonly known as equalization enhanced phase noise (EEPN). It results in asymmetric requirements for transmitter laser and LO laser. Further, the system design in the presence of lasers with non-white frequency noise becomes increasingly challenging for increased capacity-distance product.The main contributions of this thesis are, firstly, an experimentally validated theory of coherent optical links with lasers having general non-white frequency noise spectrum and corresponding system/laser design criteria and mitigation technique. Secondly, low complexity and high phase noise tolerant CPR for high order modulation formats.The general theory propounded in this thesis elucidates the origin of the laser frequency noise induced noise enhancement in coherent optical links with different DSP configurations. The thesis establishes the existence of multiple frequency noise regimes and shows that each regime results in different set of impairments. The influence of the impairments due to some regimes can ideally be reduced by optimizing the corresponding mitigation algorithms, while other regimes cause irretrievable impairments. Experimentally validated theoretical boundaries of these regimes and corresponding criteria applicable to system/laser design are provided. Further, an EEPN mitigation method and its two possible implementations are proposed and discussed.The thesis also demonstrates an intrinsic limitation of the conventional Blind Phase Search (BPS) algorithm due to angular quantization and provides methods to overcome it. Finally, this thesis proposes and demonstrates single stage and multi-stage carrier phase recovery algorithms for compensation of phase impairments due to the two lasers for higher order circular and square modulations. The proposed methods outperform the state of art algorithms both in performance and in complexity.
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| 3. |
- Rodrigo Navarro, Jaime, 1987-
(författare)
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Phase Noise Tolerant Modulation Formats and DSP Algorithms for Coherent Optical Systems
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Coherent detection together with multilevel modulation formats has the potential to significantly increase the capacity of existing optical communication systems at no extra cost in signal bandwidth. However, these modulation formats are more susceptible to the impact of different noise sources and distortions as the distance between its constellation points in the complex plane reduces with the modulation index. In this context, digital signal processing (DSP) plays a key role as it allows compensating for the impairments occurring during signal generation, transmission and/or detection relaxing the complexity of the overall system. The transition towards pluggable optical transceivers, offers flexibility for network design/upgrade but sets strict requirements on the power consumption of the DSP thus limiting its complexity. The DSP module complexity however, scales with the modulation order and, in this scenario, low complex yet high performance DSP algorithms are highly desired.In this thesis, we mainly focus on the impact of laser phase noise arising from the transmitter and local oscillator (LO) lasers in coherent optical communication systems employing high order modulation formats. In these systems, the phase noise of the transmitting and LO lasers translate into phase noise in the received constellation impeding the proper recovery of the transmitted data. In order to increase the system phase noise tolerance, we firstly explore the possibility of re-arranging the constellation points in a circularly shaped mQAM (C-mQAM) constellation shape to exploit its inherent phase noise tolerance. Different low-complex carrier phase recovery (CPR) schemes applicable to these constellations are proposed along with a discussion on its performance and implementation complexity. Secondly, the design guidelines of high performance and low complex CPR schemes for conventional square mQAM constellations are presented. We identify the inherent limitation of the state-of-the-art blind phase search (BPS) carrier phase recovery algorithm which hinders its achievable performance and implementation complexity and present a low complex solution to overcome it. The design guidelines of multi-stage CPR schemes for high order modulation formats, where the BPS algorithm is employed at any of the stages, are also provided and discussed. Finally, the interplay between the received dispersed signal and the LO phase noise is analytically investigated to characterize the origin of the equalization enhanced phase noise phenomena.
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| 4. |
- El-Taher, A., et al.
(författare)
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Noise characterization and transmission evaluation of unrepeated Raman amplified DP-16QAM link
- 2015
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Ingår i: Optical Fiber Communication Conference, OFC 2015. - Washington, D.C. : The Optical Society. - 9781557529374
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Konferensbidrag (refereegranskat)abstract
- Impairments characterization and performance evaluation of Raman amplified unrepeated DP-16QAM transmissions are conducted. Experimental results indicate that small gain in forward direction enhance the system signal-to-noise ratio for longer reach without introducing noticeable penalty.
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| 5. |
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| 6. |
- Iglesias Olmedo, Miguel, 1987-
(författare)
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Impairment Mitigation in High Capacity and Cost-efficient Optical Data Links
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis fits within the broader area of fiber optics communications. This is an important area of research as it provides a breeding ground for the present and future technologies supporting the Internet. Due to the ever-increasing bandwidth demands worldwide, the network infrastructures that make up the Internet are continuously being upgraded. This thesis aims to identify key segments of the Internet that are deemed to become the Internet's bottleneck if new technology does not replace the current one. These are datacenter intra and inter-connects, and metropolitan core area networks. In each category, we provide a comprehensive overview of the state of the art, identify key impairments affecting data transmission, and suggest solutions to overcome them. For datacenter intra and inter-connects, the key impairments are lack of bandwidth from electro-optic devices, and dispersion. Solutions attempting to tackle these impairments must be constrained by cost and power consumption. The provided solution is MultiCAP, an alternative advanced modulation format that is more tolerable to dispersion and provides bandwidth management features, while being flexible enough to sacrifice performance in order to gain simplicity. MultiCAP was the first advanced modulation format to achieve over 100~Gb/s in 2013 for a data-center interconnect and set the world record on data transmission over a single VCSEL in 2014 for a short reach data link. On metro-core networks, the challenge is to efficiently mitigate carrier induced frequency noise generated by modern semiconductor lasers. We point out that, when such lasers are employed, the commonly used laser linewidth fails to estimate system performance, and we propose an alternative figure of merit we name "Effective Linewidth". We derive this figure of merit analytically, explore it by numerical simulations and experimentally validate our results by transmitting a 28~Gbaud DP-16QAM over an optical link.
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| 7. |
- Leong, Miu Yoong, et al.
(författare)
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Low-complexity BCH codes with optimized interleavers for DQPSK systems with laser phase noise
- 2017
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Ingår i: Photonic network communications. - : Springer Science and Business Media LLC. - 1387-974X .- 1572-8188. ; 33:3, s. 328-333
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Tidskriftsartikel (refereegranskat)abstract
- The presence of high phase noise in addition to additive white Gaussian noise in coherent optical systems affects the performance of forward error correction (FEC) schemes. In this paper, we propose a simple scheme for such systems, using block interleavers and binary Bose-Chaudhuri-Hocquenghem (BCH) codes. The block interleavers are specifically optimized for differential quadrature phase shift keying modulation. We propose a method for selecting BCH codes that, together with the interleavers, achieve a target post-FEC bit error rate (BER). This combination of interleavers and BCH codes has very low implementation complexity. In addition, our approach is straightforward, requiring only short pre-FEC simulations to parameterize a model, based on which we select codes analytically. We aim to correct a pre-FEC BER of around . We evaluate the accuracy of our approach using numerical simulations. For a target post-FEC BER of , codes selected using our method result in BERs around 3 target and achieve the target with around 0.2 dB extra signal-to-noise ratio.
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| 8. |
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| 9. |
- Pang, Xiaodan, Dr., et al.
(författare)
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Experimental Evaluation of Impairments in Unrepeatered DP-16QAM Link with Distributed Raman Amplification
- 2017
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Ingår i: Photonics. - : MDPI AG. - 2304-6732. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- The transmission impairments of a Raman amplified link using dual-polarization 16-quadrature amplitude modulation (DP-16QAM) are experimentally characterized. The impact of amplitude and phase noise on the signal due to relative intensity noise (RIN) transfer from the pump are compared for two pumping configurations: first-order backward pumping and bi-directional pumping. Experimental results indicate that with increased Raman backward pump power, though the optical signal-to-noise ratio (OSNR) is increased, so is the pump-induced amplitude and phase noise. The transmission performance is firstly improved by the enhanced OSNR at a low pump power until an optimum point is reached, and then the impairments due to pump-induced noise start to dominate. However, the introduction of a low pump power in the forward direction can further improve the system's performance.
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| 10. |
- Pang, Xiaodan, et al.
(författare)
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Experimental Study of 1.55-μ m EML-Based Optical IM/DD PAM-4/8 Short Reach Systems
- 2017
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Ingår i: IEEE Photonics Technology Letters. - : Institute of Electrical and Electronics Engineers Inc.. - 1041-1135 .- 1941-0174. ; 29:6, s. 523-526
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally evaluate high-speed intensity modulation/direct detection (IM/DD) transmissions with a 1.55-μ text broadband electro-Absorption modulated laser and pulse amplitude modulations (PAM). We demonstrate 80 Gb/s/ λ PAM-4 and 96 Gb/s/ λ PAM-8 transmissions with low-complexity digital equalizers at the receiver. Performance comparison with different types of equalizers are performed, including linear symbol-spaced feed-forward equalizer (FFE), fractional (half-symbol) spaced FFE and decision feedback equalizer (DFE), with different tap number. It is found that for both cases, a 6-Tap symbol-spaced FFE is sufficient to achieve a stable performance with bit-error-rate below the 7% overhead hard decision forward error correction (7%-OH HD-FEC) threshold over a 4 km standard single mode fiber link. Practical considerations including comparison between adaptive and static equalizer implementation and tolerable fiber chromatic dispersion are discussed.
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