1,721,093 research outputs found
Hollow core fibre based Fabry-Perot interferometers with high finesse
No full textA typical Fabry-Perot interferometer (FP) is formed by enclosing an optical path with two highly reflective mirrors, forcing light to travel many times in between these two mirrors. This gives rise to many resonant peaks in the transmission spectrum, making the FP element useful in many applications such as sensing and stabilised lasers. In many of these applications, the performance improves when the transmission peaks spectral width is reduced. To achieve this, FP needs to have long optical length or highly-reflective mirrors. Currently, there are two main FPs implementations. The first one is based on free-space light propagation in which the distance between the mirrors (and thus the FP optical lengths) are usually limited to less than 50 cm. To achieve narrow transmission spectral peak width, this requires extremely high mirrors reflectivity and associated finesse (>105 ), which makes such FPs highly sensitive to alignment. An alternative FP implementation uses single-mode optical fibre (SMF-FPs). SMF-FPs can have long lengths (e.g., 100’s of meters) and can be very compact and lightweight. Although the finesse of SMF-FPs does not reach that achievable in free-space FPs (due to the fibre transmission loss), their long length enables achieving narrow transmission peaks with similar to that achieved in the high-finesse free-space FPs. Unfortunately, SMF-FPs have several drawbacks that make free-space FPs the preferred approach for many applications, despite free-space FPs larger size and the alignment challenges. The two primary drawbacks are the large sensitivity to temperature variations and unwanted nonlinear effects like stimulated Brillion scattering. The nonlinear effect can be relatively prominent in FPs, where the intra-cavity power is strongly enhanced by the resonant effect. Both of these parasitic effects are mainly due to the interaction of light with the silica glass material in SMFs. For example, the thermal sensitivity of optical length is dominated (95%) by thermally-induced changes in the refractive index of silica glass (thermooptic effect) with the thermally-induced fibre length (thermal expansion effect) change providing the other 5%. As light in hollow core fibres (HCFs) travels in air, the unwanted light-glass interaction observed in SMFs is strongly supressed, making HCFs ideal medium for FPs. However, up to date, high finesse HCF based FPs (HCF-FPs) were studied in the literature only with very short HCFs lengths (several cm), limiting their use for many applications. Besides this, all reports have dealt with open (unsealed) HCFs with no discussion on their long-term applications, which is expected to be limited due to HCF degradation when opened to atmosphere. Finally, the performance limitation of HCFFPs, e.g., what is the maximum achievable finesse, have not been studied yet. In this Thesis, I focus on long length and high finesse
HCF-FPs and their applications. Firstly, we developed a new method based on an
incoherent source and RF spectrum analysis to characterise long-length FPs. The
method characterises the beating signal of the incoherent optical comb obtained
by transmitting incoherent light through the tested FP. It enabled us to monitor
finesse and free spectral range during alignment of HCF-FPs, including
measurement of long-length FPs. Secondly, we built HCF-FPs with free-space
coupling. We characterized several HCF-FPs of different length to explore
experimentally finesse limits. The results showed that the coupling loss
between the forward and backward propagating light in HCFs reflected off the
mirror can be as low as 0.0028 dB. In turn this enables a finesse of up to 5
000. Experimentally, we I achieved a finesse in excess of 2 500 (limited by the
available mirrors), which is a value 20 times larger than reported before for
HCF-FP. Thirdly, we fabricated two all-fibred HCF-FPs in collaboration with our
colleagues in the Czech Technical University in Prague. The two components had length
of 5 m and 23 m, respectively, and finesse in excess of 120 over the entire C
band. This represented the first demonstration of all fibred long (>1 m)
HCF-FPs with such a high finesse. We have tested the 23 m HCF-FP almost 3 years
later after it was fabricated, with no observable degradation in its
performance. This shows the first HCF-FP that can be used in long term
applications. Subsequently, we demonstrated two applications based on the
fabricated HCF-FPs. The first one uses the HCF-FP to characterise HCF
attenuation. The other uses the all-fibred HCF-FP for microwave photonics
filtering. Finally, we further improve already-low HCF’s thermal stability by
proposing and demonstrating a new method. It is based on coiling the HCF on a
temperature insensitive spool. In our proof-of-principle demonstration, we
achieved HCF thermal sensitivity reduction 3 times, achieving thermal
sensitivity as low as 0.13 ppm/oC. This up to date demonstrates the most
thermally stable fibre FP at room temperature.<p class="MsoNormal"/
Czech prepositional phrases with static local meaning and their equivalents in Chinese
Full text linkDIPLOMOVÉPRÁCE Ústav bohemistiky pro cizince a komunikace neslyšících Title of thesis: Czech prepositional phrases with static local meaning and their equivalents in Chinese Author: Ding Meng Study program and grade: Third year of master program Thesis advisor: Mgr. Andrea Hudousková, Ph.D. Keywords: prepositional phrases, static localization, spatial expressions, Czech, Chinese, contrastive linguistics Abstract The thesis focuses on Czech prepositional phrases with static local meaning and their equivalents in Chinese. The aim of the thesis is to investigate the ways of expressing static localization in both languages by comparing Czech prepositional phrases with their translated equivalents in Chinese. The research was based on the analysis of linguistic material excerpted from the literary works Povídky and Novépovídky by Zdeněk Svěrák and their translations into Chinese. Prepositional phrases with static spatial meaning were identified in the Czech text and then compared with the Chinese translations. In the theoretical part, the basic characteristics of local prepositions in both languages are presented. Then, Czech prepositional phrases expressing static spatial meanings (na + locative, v + locative, nad + instrumental, pod + instrumental, za + instrumental, před + instrumental, u +...DIPLOMOVÉPRÁCE Ústav bohemistiky pro cizince a komunikace neslyšících Název práce: Česká předložková spojení se statickým lokálním významem a jejich ekvivalenty v čínštině Jméno diplomantky: Ding Meng Studijní Program a ročník: Navazující magisterský program 3 ročníku Vedoucípráce: Mgr. Andrea Hudousková, Ph.D. Klíčová slova: předložková spojení, statická lokalizace, místní určení, čeština, čínština, kontrastivnílingvistika Abstrakt Diplomová práce se zaměřuje na česká předložková spojení se statickým místním významem a jejich ekvivalenty v čínštině. Cílem práce je prozkoumat způsoby vyjádření statické lokalizace v obou jazycích na základě porovnání českých předložkových spojení s jejich překladovými ekvivalenty v čínštině. Výzkum byl založen na analýze jazykového materiálu excerpovaného z literárních děl Povídky a Novépovídky od Zdeňka Svěráka a jejich překladů do čínštiny. V českém textu byla identifikována předložková spojení se statickým místním významem, která byla následně porovnána s překlady do čínštiny. V teoretické části jsou představeny základnícharakteristiky místních předložek v obou jazycích. Dále jsou popsána česká předložková spojení vyjadřující statické prostorové významy (na + 6. pád, v + 6. pád, nad + 7. pád, pod + 7. pád, za + 7. pád, před + 7. pád, u + 2. pád) a vysvětleny...Institute of Czech and Deaf StudiesÚstav bohemistiky pro cizince a komunikaci neslyšícíchFaculty of ArtsFilozofická fakult
Czech prepositional phrases with static local meaning and their equivalents in Chinese
No full textDIPLOMOVÉPRÁCE Ústav bohemistiky pro cizince a komunikace neslyšících Title of thesis: Czech prepositional phrases with static local meaning and their equivalents in Chinese Author: Ding Meng Study program and grade: Third year of master program Thesis advisor: Mgr. Andrea Hudousková, Ph.D. Keywords: prepositional phrases, static localization, spatial expressions, Czech, Chinese, contrastive linguistics Abstract The thesis focuses on Czech prepositional phrases with static local meaning and their equivalents in Chinese. The aim of the thesis is to investigate the ways of expressing static localization in both languages by comparing Czech prepositional phrases with their translated equivalents in Chinese. The research was based on the analysis of linguistic material excerpted from the literary works Povídky and Novépovídky by Zdeněk Svěrák and their translations into Chinese. Prepositional phrases with static spatial meaning were identified in the Czech text and then compared with the Chinese translations. In the theoretical part, the basic characteristics of local prepositions in both languages are presented. Then, Czech prepositional phrases expressing static spatial meanings (na + locative, v + locative, nad + instrumental, pod + instrumental, za + instrumental, před + instrumental, u +..
Tunable lasers phaselocked to Optical Frequency Comb
No full textThe demand for stable and tunable laser sources is steadily growing across a wide range of applications. Optical Frequency Combs (OFCs) have emerged as a powerful reference standard, offering stable frequency spacing between the comb tones or high optical frequency stability. To obtain low-cost tunable lasers with their carriers or relative frequencies locked, we phaselocked a commercial Telecom grade Integrable-Tunable-Laser-Assembly (ITLA) to an OFC. We demonstrate such phaselock within the telecom C-band (1527 nm - 1565 nm), using sub-mW OFC power corresponding to per-tone power down to nW regime. This enables phaselocking of large number of tunable lasers to the same OFC via passive splitting of the OFC power. We achieved short-term integrated phase noise of 10 mrad and long-term frequency stability measured over 10 hours below ± 0.01 Hz
Ultrashort pulse generation in modeless laser cavity
No full textWe demonstrate experimentally that random phase modulation of an erbium-doped fiber ring-laser by an intra-cavity electro-optic phase modulator did not inhibit ultrashort-pulse operation. Stable and self-starting ultrashort-pulse operation with a single pulse circulating in the cavity was achieved even when the phase modulator was driven with random sequences sufficiently fast and strong to render the laser cavity modeless, in the sense that heterodyning of the laser output did not show any spectral lines corresponding to a mode spectrum. No significant change in measured pulse characteristics was observed, compared to conventional mode-locking in the unmodulated cavity. The insensitivity to the random phase modulation is expected, given the lack of phase-sensitive elements in the cavity
Phase locking of a compact tuneable laser to optical frequency comb with low frequency error
No full textWe demonstrate a practical system for OFC tones extractions that is tuneable within the telecom C-Band. By employing two double-integrators and proportional gain (PI4 )as a controller, we achieved long-term frequency stability below ±0.01 Hz.<br/
Long length fibre Fabry-Perot interferometers and their applications in fibre characterization and temperature sensing
No full textSince the free spectral range (FSR) of a fibre Fabry-Perot (f-FP) is the reciprocal of the time delay inside the interferometer, it can be used for the precise measurement of fibre properties (chromatic dispersion when measured as a function of wavelength, birefringence, etc.) or for the measurement of environmental changes (such as temperature, pressure, stress, etc.) which alter the fibre's parameters
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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