317 research outputs found

    Continuous wave sub-THz photonic generation with ultra-narrow linewidth, ultra-high resolution, full frequency range coverage and high long-term frequency stability

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    We report on a photonic system for generation of high quality continuous-wave (CW) sub-THz signals. The system consists on a gain-switching-based optical frequency comb generator (GS-OFCG), a two-optical-modes selection mechanism and a n-i-pn-i-p superlattice photomixer. As mode selection mechanism, both selective tunable optical filtering using Fabry&-Pérot tunable filters (FPTFs) and Optical Injection Locking (OIL) are evaluated. The performance of the reported system surpasses in orders of magnitude the performance of any commercially available optical mm-wave and sub-THz generation system in a great number of parameters. It matches and even overcomes those of the best commercially available electronic THz generation systems. The performance parameters featured by our system are: linewidth <<10 Hz at 120 GHz, complete frequency range coverage (60&-140 GHz) with a resolution in the order of 0.1 Hz at 120 GHz ({hbox{10}} -12} of generated frequency), high long term frequency stability (5 Hz deviation over one hour). Most of these values are limited by the measurement instrumentation accuracy and resolution, thus the actual values of the system could be better than the reported ones. The frequency can be extended straightforwardly up to 1 THz extending the OFCG frequency span. This system is compact, robust, reliable, offers a very high performance, especially suited for sub-THz photonic local oscillators and high resolution spectroscopy.This work was supported by the Spanish Ministry of Science and Technology through the Project TEC2009-14525-C02-02. The work of Á. R. Criado has been supported by the Spanish Ministry of Science and Technology under the FPI Program, Grant BES2010-030290

    Analog Radio-over-Fiber for 5G/6G Millimeter-Wave Communications

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    Next-generation optical access networks based on Orthogonal Frequency Division Multiplexing

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    Orthogonal Frequency Division Multiplexing (OFDM) is a robust modulation and multiplexing format which is at the base of many present communication standards. The interest of the OFDM application in optical fiber deployments is quite recent. As the next generation of Passive Optical Networks (NG-PONs) is envisioned, targeting greater capacity and user counts, the limitations of TDMA (Time Division Multiplexing Access) approaches to meet the expected increase in requirements becomes evident and therefore new technologies are being explored. Optical OFDMA is an emerging technology which can be a promising candidate. The main goal of this Master Thesis is to study the problem of users multiplexing in access networks, using OFDM as a technology to transmit the user information data. This work has focused in the uplink study of the network, because it is the most challenging part of the network to design. The studies have been conducted both in a theoretical way and also by simulating the targeted environments by means of a fiber optics transmission simulation tool. Virtual Photonics Integrated (VPI) is the software selected for the simulations. This tool is specially designed to simulate optical transmission system environments. The analysis of the Optical Beat Interference, which is a critical impairment in optical carrier multiplexing schemes, is the most important part of the user multiplexing study

    Laser Pulses

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    This book discusses aspects of laser pulses generation, characterization, and practical applications. Some new achievements in theory, experiments, and design are demonstrated. The introductive chapter shortly overviews the physical principles of pulsed lasers operation with pulse durations from seconds to yoctoseconds. A theory of mode-locking, based on the optical noise concept, is discussed. With this approximation, all paradoxes of ultrashort laser pulse formation have been explained. The book includes examples of very delicate laser operation in biomedical areas and extremely high power systems used for material processing and water purification. We hope this book will be useful for engineers and managers, for professors and students, and for those who are interested in laser science and technologies

    Diode-pumped TmÂłâș-doped sesquioxide lasers for ultrashort pulse applications in the 2ÎŒm region

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    This thesis presents the development of TmÂłâș-doped sesquioxide laser sources in the 2–2.1 ÎŒm spectral region. The primary focus of this development has been aimed towards high power diode-pumped mode-locked laser sources capable of femtosecond pulse generation. In addition to this, the early development of a compact and low threshold ultrafast laser inscribed waveguide laser has also been realised. Continuous wave characterisation of bulk solid-state crystalline Tm:LuScO₃ and ceramic Tm:Lu₂O₃ lasers has been completed using ~795 nm multimode single emitter laser diode pump sources. Average output powers of 660 mW and 901 mW, and emission wavelengths of 2.1 ÎŒm and 2.06 ÎŒm were achieved from the Tm:LuScO₃ and Tm:Lu₂O₃ lasers, respectively. In addition, both lasers demonstrated smooth and continuous tuning ranges spanning more than 160 nm in the ~2–2.1 ÎŒm spectral region. In the mode-locked regime, pulse durations as short as 170 fs were recorded at an average output power of 113 mW and an emission wavelength of 2094 nm from a diode-pumped mode-locked Tm:LuScO₃ laser through the use of an ion-implanted InGaAsSb quantum-well-based semiconductor saturable absorber mirror. A diode-pumped Tm:Lu₂O₃ laser, utilising the same semiconductor saturable absorber mirror, was able to generate pulses as short as 278 fs at an average output power of 555 mW and a wavelength of 2081 nm through the use of a steeply diving optic axis birefringent filter. This same filter was also used to demonstrate broadly tunable femtosecond pulses in both laser configurations. Subsequent amplification of the ultrashort pulse laser sources realised maximum amplified average output powers of 540 mW and 855 mW, respectively. The results presented in this thesis demonstrate the potential for diode-pumped TmÂłâș-doped sesquioxide laser sources to be developed into an enabler technology for the advancement of a number of photonics applications and techniques in the mid-infrared region."The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) [grant number EP/L01596X/1] and Fraunhofer UK Research Limited studentship funding." -- Acknowledgement

    Rare-earth ion doped chalcogenide waveguide amplifiers

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    Chalcogenide glass waveguide devices have received a great deal of attention worldwide in the last few years on account of their excellent properties and potential applications in mid-infrared (MIR) sensing and all-optical signal processing. Waveguide propagation losses, however, currently limit the potential for low power nonlinear optical processing, and the lack of suitable on chip integrated MIR sources is one of the major barriers to integrated optics based MIR sensing. One approach to overcome the losses is to employ rare-earth ion doped waveguides in which the optical gain can compensate the loss, in such a way that the conversion efficiency of nonlinear effects is increased significantly. For infrared applications, the long wavelengths potentially attainable from rare-earth ion transitions in chalcogenide hosts are unique amongst glass hosts. New rare-earth ion doped chalcogenide sources in the MIR range could benefit molecular sensing, medical laser surgery, defence etc. Despite these promising applications, until now, no one has succeeded in fabricating rare-earth ion doped chalcogenide amplifiers or lasers in planar devices. This work develops high quality erbium ion doped chalcogenide waveguides for amplifier and laser applications. Erbium ion doped As2S3 films were fabricated using co-thermal evaporation. Planar waveguides with 0.35 dB/cm propagation loss were patterned using photolithography and plasma etching on an erbium ion doped As2S3 film with an optimised erbium ion concentration of 0.45x1020 ions/cm3. The first demonstration of internal gain in an erbium ion doped As2S3 planar waveguide was performed using these waveguides. With different film deposition approaches, promising results on intrinsic lifetime of the Er3+ 4I13/2 state were achieved in both ErCl3 doped As2S3 films (2.6 ms) and radio frenquency sputtered Er3+:As2S3 films (2.1 ms), however, no waveguide was fabricated on these films due to film quality issues and photopumped water absorption issues. The low rare-earth ion solubility of As2S3 is considered the main factor limiting its performance as a host. Gallium-containing chalcogenide glasses are known to have good rare-earth ion solubility. Therefore, a new glass host material, the Ge-Ga-Se system, was investigated. Emission properties of the bulk glasses were studied as a function of erbium ion doping. A region between approximately 0.5 and 0.8 at% of Er3+ ion was shown to provide sufficient doping, good photoluminescence and adequate lifetime to envisage practical planar waveguide amplifier devices. Ridge waveguides based on high quality erbium ion doped Ge-Ga-Se films were patterned. Significant signal enhancement at 1540 nm was observed and 50 % erbium ion population inversion was obtained, in waveguides with Er3+ concentration of 1.5x1020 ion/cm3. To the Author's knowledge, this is the highest level of inversion ever demonstrated for erbium ions in a chalcogenide glass host and is an important step towards future devices operating at 1550 nm and on the MIR transitions of erbium ions in chalcogenide glass hosts. Photoinduced absorption loss caused by upconversion products in the waveguides is the remaining hurdle to achieving net gain. Further research is needed to find suitable compositions that possess high rare-earth ion solubility whilst avoiding the detrimental photoinduced losses

    Detection efficiency and bandwidth optimized electro-optic sampling of mid-infrared waves

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    Electro-optic sampling (EOS) is a powerful method for the characterization of electric fields with frequencies in the range of ~ 1-300 THz. For mid-infrared (MIR) radiation (2-20 ”m), it can be understood as a two-step process: first, a sub-MIR-cycle visible/infrared gate pulse generates sum and/or difference frequency radiation with the light field under investigation in a nonlinear medium. Second, the newly generated frequencies are detected in a heterodyne scheme with the transmitted gate pulse serving as the local oscillator. Scanning the delay of the gate pulse with respect to the MIR waveform results in a signal proportional to the incident MIR field, with the spectral response depending on the gate pulse duration and phasematching in the detection crystal. The nonlinear frequency conversion on the one hand transfers the detection to the near-infrared spectral range, affording the use of low-noise photodetectors. On the other hand, it limits the detection efficiency and subjects it to a trade-off against bandwidth. Our research group develops high-power ultrashort-pulsed laser sources for field-resolved infrared spectroscopy. Explicitly, nonlinearly post-compressed femtosecond lasers are used both to drive the generation of waveform-stable MIR light for molecular-sample excitation, as well as for obtaining gate pulses for EOS of the full macroscopic sample response. To maximize the sensitivity of our field-resolved spectrometers, this thesis studied the photon detection efficiency of EOS for MIR radiation with wavelengths in the 6-18-ÎŒm range in experiment and theory. Three different types of gate pulses were investigated experimentally: first, the EOS detection efficiency was characterized for gate pulses with 1030-nm central wavelength, generated by an Yb-thin-disk oscillator. Limited by multi-photon-absorption-caused damage of the GaSe crystal, with an average gate-pulse power of 450mW, a conversion efficiency of 2% from the MIR into sum-frequency photons was achieved in a 500-ÎŒm-thick detection crystal. Accounting for Fresnel reflections at the crystal and losses in the heterodyne detection, up to 0.76% of the incident MIR photons arrived at the balanced diodes. Together with mW-level MIR average powers, this resulted in 13 orders of magnitude frequency-domain intensity dynamic range at 9-ÎŒm wavelength for a measurement time of 16 s and a scan range of 3.3 ps. However, phase-mismatch limited the −20 dB spectral width to 1.2 ”m. Using a 85-ÎŒm-thick GaSe crystal, the full MIR spectrum of the source, spanning from 6.6 to 10.7 ”m at −20 dB, was detected, while trading in two orders of magnitude in peak dynamic range. In our research group, the prototype field-sensitive spectrometer with this record detection efficiency and dynamic range is currently being used for fingerprinting real-world biomedical samples, with up to 40 times higher molecular detection sensitivity than commercial Fourier-transform infrared spectrometers. Due to the dispersion of GaSe, the trade-off between detection efficiency and spectral coverage is mitigated for longer-wavelength gate pulses. Using gate pulses centered at 1550 nm wavelength from an Er-fiber laser and a 300-ÎŒm-thick crystal, a comparable detection dynamic range and bandwidth as with the 85-ÎŒm-thick crystal at 1030 nm was achieved, despite the lower gate pulse power of 120mW. This performance enables high sensitivity spectroscopic measurements, when employing the Er-laser in a dual-oscillator fast-scanning mode (~ 1 kHz scan rate), avoiding low-frequency noise sources. In addition to the broader phasematching bandwidth, choosing a longer gate-pulse wavelength also increases the detection-crystal damage threshold due to reduced multi-photon absorption, allowing for the use of higher gate pulse powers and, consequently, enhancing the nonlinear interaction. This benefit was harnessed in the investigation of limitations to the detection efficiency with 1.9-W gate pulses from a Tm-fiber laser at 1965 nm central wavelength, comparing several EOS crystal thicknesses with respect to detection efficiency and spectral coverage. Traces measured with 100 to 300-ÎŒm-thin crystals closely resemble the incident field, spanning from 8.1 ”m to 14.2 ”m at −10 dB, with a conversion efficiency from the MIR into sum-frequency photons of up to ~ 10%. Using a 500-ÎŒm-thick GaSe crystal, more than 20% of the MIR photons from a 3-ÎŒm spectral band around 9.3 ”m were upconverted. Further increasing the crystal thickness resulted in saturation of the depletion, explained by temporal walk-off and reduced peak powers due to dispersion. The overall number of detectable MIR photons of ~ 6.4% from within the detection crystal an interaction time window, together with mW-level MIR powers, lead to a peak intensity dynamic range > 10^14, with twice the detection bandwidth as for the 1030-nm gate pulses in the efficiency-optimized configuration, thus spanning ~ 5 ÎŒm at −20 dB. Despite the MIR depletion upon detection, the EOS signal scaled linearly with the field strength for average photon numbers between 10^3 and 10^17 per second within our measurement accuracy, because enough MIR photons stay available for nonlinear interaction. The multi-percent-level conversion efficiency allows for characterization of waveforms with an average of 22 photons inside the detection crystal in a 2.2-ms-long integration time window per temporal element. The combination of sensitivity, dynamic range and spectral coverage finds application e.g., in broadband vibrational spectroscopy, where the minimum detectable concentration is only a factor of ~ 4 higher than what would be possible when detecting all incident MIR photons. Furthermore, the detection bandwidth allows for the simultaneous measurement of multiple molecular species with spectrally wide-spread absorption lines. Employing the high detection dynamic range, a further study in the frame of this thesis concerned the use of EOS as a highly sensitive characterization technique for the stability and reproducibility of the MIR waveform and, therefore, for the control over optical fields. These capabilities were demonstrated by measuring the temporal fluctuations of the EOS trace, resulting in a record-low timing jitter of < 10 as over billions of pulses. A theoretical model simulating the chain of nonlinear processes from the laser frontend to EOS detection confirmed the measured values, identifying intensity noise of the modelocked oscillator front-end as the main source of the remaining MIR waveform instabilities. These jitter values were 3 orders of magnitude above the field fluctuations expected from a shot noise- limited driving pulse train

    High-Capacity Hybrid Optical Fiber-Wireless Communications Links in Access Networks

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    High Accuracy Relative Luminescence Quantum Yield Measurements of Upconverting Nanoparticles

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    The name Upconverting nanoparticles refers to a novel category of luminescence emitters that are capable of generating visible luminescence upon excitation with noncoherent and longer wavelength monochromatic near-infrared light, at excitation fluence rates as low as 1-103 W/cm2. This unique optical behaviour is of great interest to researchers as it could potentially allow for virtually autofluorescence free luminescence imaging of living tissue. With them also exhibiting other advantageous properties such as a low toxicity to living cells, a high resistance to photobleaching and a small size, these nanocrystals show great potential for replacing or complementing conventional fluorophores in a wide selection of imaging applications. For this thesis, a system based on a conventional fluorometer concept, is designed and assembled which allows for the full characterization of the quantum yield of dilute samples of Upconverting nanoparticles as a function of the excitation radiation fluence rate. In addition to this, a proof-of-concept experiment is carried out with the aim to demonstrate how this dependence of the upconversion process on the excitation power density can be utilized to extract additional spatial information with a nanoparticle tomography measurement that would not be obtainable if conventional fluorophores instead had been probed.I dagens moderna samhĂ€lle har mĂ€nniskan numera tillgĂ„ng till en alltmer kapabel sjukvĂ„rd, med en lĂ€ngre livslĂ€ngd och en ökad levnadskvalitet, vĂ€rlden över, som följd. Det Ă€r inte bara sĂ„ att vĂ„r kunskap om kroppens funktioner och Ă„kommor har förbĂ€ttras med tiden utan det Ă€r Ă€ven sĂ„ att lĂ€kare numera har tillgĂ„ng till avancerad teknik som hjĂ€lper dem att stĂ€lla trĂ€ffsĂ€kra diagnoser och utvĂ€rdera huruvida en behandlingsmetod har önskad verkan. Eftersom kroppens mjuka vĂ€vnad men inte sjĂ€lva benstommen Ă€r mer eller mindre genomskinlig för kortare ljusvĂ„glĂ€ngder Ă€r det till exempel möjligt att avgöra om en arm Ă€r bruten genom att studera hur röntgenstrĂ„lning pĂ„verkas dĂ„ den fĂ€rdas genom kroppen. Å andra sidan innebĂ€r detta att nĂ„gon annan typ av ljus mĂ„ste anvĂ€ndas för att undersöka andra detaljer sĂ„ som blodkĂ€rl och organ. Inom det synliga omrĂ„det Ă€r avbildningssituationen betydligt mer komplicerad. Om du nĂ„gon gĂ„ng hĂ„llit en ficklampa mot din handflata och sett hur handen pĂ„ andra sidan tycks lysa rött kan du förestĂ€lla dig hur svĂ„rt det Ă€r att ta fram en bild av insidan av kroppen. Visst tar det röda ljuset sig igenom till andra sidan men det gĂ„r inte att urskönja nĂ„got spĂ„r frĂ„n handens ben. Situation pĂ„minner vĂ€ldigt mycket om hur ljus beter sig i ett glas mjölk. Oavsett ljusets vĂ„glĂ€ngd sĂ„ Ă€r sannolikheten stor för ljusstrĂ„lar utifrĂ„n att de ska lyckas undkomma vĂ€tskan, nĂ„got som ger upphov till glasets vita fĂ€rg. All information om vad som fanns pĂ„ andra sidan glaset döljs dock av den stora mĂ€ngden riktningsförĂ€ndringar som strĂ„larna utsĂ€tts för under passagen. För att ta sig runt det hĂ€r problemet vid medicinsk avbildning drar anvĂ€ndare nytta av statistik och datorsimuleringar för att Ă€ndĂ„ kunna ta fram en bild av det som Ă€r svĂ„rt att se direkt med blotta ögat. Ett annat knep Ă€r att ocksĂ„ anvĂ€nda sig av nĂ„got som kallas för biomarkörer. Dessa smĂ„ partiklar kan designas sĂ„ att de dĂ„ de belyses med synligt ljus svarar och skickar tillbaka nĂ„got mindre energirik strĂ„lning, nĂ„got som kallas för fluorescens. Genom att samla in den hĂ€r signalen kan man med ett sĂ„dant beteende öka kontrasten mellan intressanta partiklar och bakgrunden under avbildningsprocessen, vilket kan förbĂ€ttra bildkvaliteten avsevĂ€rt. Utveckling av tekniken har idag nĂ„tt sĂ„ pass lĂ„ngt att vissa biomarkörer nu kan behandlas pĂ„ ett speciellt sĂ€tt sĂ„ att de efter att ha tagits upp av kroppen automatiskt ansamlas vid ett visst stĂ€lle som forskare Ă€r nyfikna pĂ„. Den hĂ€r egenskapen kan i sin tur till exempel anvĂ€ndas för att upptĂ€cka cancerceller hos en patient eller hos ett försöksdjur, Ă€ven om tumörens utseende ursprungligen inte skiljer sig Ă„t frĂ„n dess omgivning. I dagslĂ€get Ă€r det möjligt att skapa mycket ljusstarka biomarkörer och att avbilda dessa pĂ„ ett sĂ€tt som Ă€r betydligt billigare Ă€n andra alternativ som till exempel magnetröntgen (MRI). Teknikens svaghet, Ă„tminstone för tillfĂ€llet, Ă€r istĂ€llet att de allra flesta partiklar krĂ€ver ljus med relativt hög energi för att kunna generera fluorescens. Som exemplet med ficklampan ovan visar har sĂ„dant blĂ„tt eller ultraviolett ljus dock det svĂ„rt att fĂ€rdas lĂ€ngre strĂ€ckor i levande vĂ€vnad, nĂ„got som gör den fluorescenssignal som kan uppnĂ„s utan att skada celler och organ betydligt svagare. Det Ă€r dessutom sĂ„ att strĂ„lning inom det hĂ€r vĂ„glĂ€ngdsomrĂ„det tenderar att aktivera andra fluorescerande partiklar som förekommer naturligt i vĂ€vnaden sĂ„ att Ă€ven de börjar skicka ut ljus som Ă„tminstone till viss del kan drĂ€nka den intressanta signalen. Den hĂ€r uppsatsen Ă€r inriktad pĂ„ att undersöka en ny grupp av biomarkörer som, dĂ„ de belyses med betydligt rödare ljus, har en förmĂ„ga att generera strĂ„lning som Ă€r mer energirik Ă€n den typ som de tar upp, genom ett komplicerat samarbete mellan de joner som finns inuti varje partikel. Det hĂ€r fenomenet Ă€r möjligt utan att lagen om energins bevarande kringgĂ„s eftersom utsĂ€ndandet av ljus föregĂ„s av en period med ett ansamlande av energi inom partikeln. PĂ„ grund av det hĂ€r beteendet och deras diameter som oftast Ă€r mindre Ă€n 100 nanometer, kallas de hĂ€r biomarkörerna för Uppkonverterande nanopartiklar. De hĂ€r nanopartiklarna har en fördel gentemot mer konventionella biomarkörer eftersom deras optiska egenskaper kan anpassas under tillverkningsprocessen sĂ„ att de Ă€r optimala för avbildning av levande vĂ€vnad. Genom att bĂ„de ta emot och skicka ut rödare vĂ„glĂ€ngder kan man i teorin fĂ„ bĂ€gge signalerna att fĂ€rdas betydligt lĂ€ngre utan att dö ut Ă€n vad som kan uppnĂ„s med andra alternativ. Detta innebĂ€r ocksĂ„ att man inte pĂ„ samma sĂ€tt mĂ„ste ta hĂ€nsyn de fluorescerande partiklar som förekommer naturligt i kroppen eftersom dessa inte aktiveras av rött ljus. Den stora nackdelen med att anvĂ€nda Uppkonverterande nanopartiklar tycks idag vara att de partiklar som i nulĂ€get kan tillverkas Ă€r relativt ineffektiva nĂ€r det gĂ€ller att omvandla energin frĂ„n insignalen till en stark utsignal. Visst Ă€r det lĂ€ttare för det röda ljuset som sĂ€nds ut frĂ„n nanopartikeln att nĂ„ detektorn men om det Ă€r alltför ljussvagt redan frĂ„n början Ă€r de annars gynnsamma optiska egenskaperna inte lĂ€ngre sĂ€rskilt betydelsefulla. PĂ„ grund av detta handlar idag mycket utav forskningen kring Uppkonverterande nanopartiklar om att förbĂ€ttra deras uppkonverteringseffektivitet och ljusstyrka. Trots detta Ă€r det just nu faktiskt inte helt sjĂ€lvklart vilket det bĂ€sta sĂ€ttet Ă€r att mĂ€ta dessa egenskaper. Det finns visserligen redan vĂ€lutvecklade rutiner för att undersöka vanliga konventionella biomarkörer men eftersom det visat sig att nanopartiklarnas effektivitet, till skillnad frĂ„n de andras, beror pĂ„ insignalens energimĂ€ngd per ytenhet, den sĂ„ kallade excitationsintensiteten, Ă€r dessa inte direkt applicerbara. För den hĂ€r uppsatsen har ett unikt mĂ€tsystem tagits fram och byggts upp just för att kunna utvĂ€rdera hur uppkonverteringseffektiviteten beror pĂ„ insignalens intensitet hos prover med Uppkonverterande nanopartiklar lösta i genomskinliga vĂ€tskor. Metoden, som gĂ„r ut pĂ„ att jĂ€mföra provets effektivitet med ett prov som innehĂ„ller vanliga nedkonverterande partiklar med en kĂ€nd effektivitet dĂ„ dessa belyses med laserstrĂ„lning, Ă€r visserligen i grunden baserad pĂ„ teknik som redan anvĂ€nds idag, men uppstĂ€llningen har anpassats efter de speciella krav som nanopartiklarna för med sig. Uttryckligen sĂ„ undersöks excitationsenergin för de tvĂ„ laserstrĂ„larna, hur mycket av energin som tas upp av de olika partiklarna, diametern pĂ„ tvĂ€rsnittet av den laserstrĂ„len som trĂ€ffar nanopartiklarna samt storleken pĂ„ de signaler som sĂ€nds ut av de tvĂ„ proverna. Den hĂ€r mĂ€tdatan analyseras sedan i datorprogrammet MatLab frĂ„n företaget Mathworks. Utöver detta har Ă€ven ett avbildningsexperiment designats för att demonstrera hur nanopartiklarnas excitationsintensitetsberoende faktiskt kan anvĂ€ndas till att samla in information som inte skulle vara möjlig att ta fram pĂ„ samma sĂ€tt med hjĂ€lp av vanliga fluorescerande partiklar. För att Ă„stadkomma detta förbereds en sĂ„ kallad avbildningsfantom, en vĂ€tskevolym med optiska egenskaper som Ă€r tĂ€nkta att efterlikna de man kan förvĂ€nta sig i levande vĂ€vnad. Inuti fantomen placeras ett prov med nanopartiklar vars avstĂ„nd till volymens yta, provets sĂ„ kallade djup, kan justeras med hög noggrannhet. Sedan undersöks hur utsignalen frĂ„n nanopartiklarna pĂ„verkas av en förĂ€ndring av excitationsintensiteten vid fantomens yta med hjĂ€lp av samma laser som anvĂ€ndas för att bestĂ€mma partiklarnas uppkonverteringseffektivitet och biofotonikgruppen i Lunds avancerade avbildningskamera. Eftersom det finns ett samband mellan provets avstĂ„nd till ytan och vilken intensitet som den upplever men ocksĂ„ ett mellan intensiteten och partiklarnas ljusgenereringseffektivitet kan provets djup bestĂ€mmas genom att undersöka hur systemets utsignal Ă€ndras dĂ„ energin pĂ„ laserstrĂ„lningen varieras

    Next-generation optical access networks based on Orthogonal Frequency Division Multiplexing

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    Orthogonal Frequency Division Multiplexing (OFDM) is a robust modulation and multiplexing format which is at the base of many present communication standards. The interest of the OFDM application in optical fiber deployments is quite recent. As the next generation of Passive Optical Networks (NG-PONs) is envisioned, targeting greater capacity and user counts, the limitations of TDMA (Time Division Multiplexing Access) approaches to meet the expected increase in requirements becomes evident and therefore new technologies are being explored. Optical OFDMA is an emerging technology which can be a promising candidate. The main goal of this Master Thesis is to study the problem of users multiplexing in access networks, using OFDM as a technology to transmit the user information data. This work has focused in the uplink study of the network, because it is the most challenging part of the network to design. The studies have been conducted both in a theoretical way and also by simulating the targeted environments by means of a fiber optics transmission simulation tool. Virtual Photonics Integrated (VPI) is the software selected for the simulations. This tool is specially designed to simulate optical transmission system environments. The analysis of the Optical Beat Interference, which is a critical impairment in optical carrier multiplexing schemes, is the most important part of the user multiplexing study
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