Demonstration of opposing thermal sensitivities in hollow-core fibers with open and sealed ends.

The output phase and propagation time of an optical signal propagating through a hollow-core optical fiber (HCF) drift with changes in environmental temperature significantly less than in conventional optical fibers. In all earlier experimental studies, however, the simplifying assumption was made that the thermo-optic effect of air was negligible. In this Letter, we present, to the best of our knowledge, the first experimental demonstration that the air inside a HCF core can make an appreciable contribution to the fiber's thermal sensitivity with the performance depending on whether the fiber is open to the atmosphere or sealed at both ends (e.g., spliced to solid fiber pigtails). We measure both the sensitivity of the accumulated phase as well as the signal propagation time for both open and sealed HCF and show that these are opposite in sign. Most importantly, we show that the thermal sensitivity contribution from the air inside an open HCF has the sign opposite to the effect of fiber elongation (which is otherwise the dominant effect responsible for the overall thermal sensitivity of HCF). We then go on to show that these two effects can be used to balance each other out in order to achieve zero thermal sensitivity for both accumulated phase and propagation time. We demonstrate this property experimentally over a large spectral range.

Hollow-core fibres for temperature-insensitive fibre optics and its demonstration in an Optoelectronic oscillator.

Many scientific and practical applications require the propagation time through cables to be well defined and known, e.g., an error in the evaluation of signal propagation time in the OPERA experiment in 2011 initially erroneously concluded that Neutrinos are faster than light. In fact, there are many other physical infrastructures such as synchrotrons, particle accelerators, telescope arrays and phase arrayed antennae that also rely on precise time synchronization. Time synchronization is also of importance in new practical applications like autonomous manufacturing (e.g., synchronization of assembly line robots) and upcoming 5G networks. Even when the propagation time through a coaxial cable or optical fibre is carefully calibrated, it is affected by changes in the ambient temperature, posing a serious technological challenge. We show how hollow-core optical fibres can address this issue.

Low-loss Kagome hollow-core fibers operating from the near- to the mid-IR.

We report the fabrication and characterization of Kagome hollow-core antiresonant fibers, which combine low attenuation (as measured at ∼30 cm bend diameter) with a wide operating bandwidth and high modal purity. Record low attenuation values are reported: 12.3 dB/km, 13.9 dB/km, and 9.6 dB/km in three different fibers optimized for operation at 1 µm, 1.55 µm, and 2.5 µm, respectively. These fibers are excellent candidates for ultra-high power delivery at key laser wavelengths including 1.064 µm and 2.94 µm, as well as for applications in gas-based sensing and nonlinear optics.

Optoelectronic oscillator incorporating hollow-core photonic bandgap fiber.

We demonstrate, to the best of our knowledge, the first optoelectronic oscillator that uses hollow-core photonic bandgap fiber (HC-PBGF) as a delay element of a sufficient length to allow for low-noise operation. We show experimentally that HC-PBGF can improve the temperature stability of the oscillator by a factor of more than 15, as compared to standard optical fiber. We also measured the oscillator's phase noise, allowing evaluation of the suitability of HC-PBGF for this application. Additionally, this Letter also provides, to the best of our knowledge, the first characterization of the temperature stability of a long length (>800 m in our Letter) of low-thermal sensitivity (2 ps/km/K) HC-PBGF wound on a spool.

Optical side scattering radiometry for high resolution, wide dynamic range longitudinal assessment of optical fibers.

Current optical reflectometric techniques used to characterize optical fibers have to trade-off longitudinal range with spatial resolution and therefore struggle to provide simultaneously wide dynamic range (>20dB) and high resolution (<10cm). In this work, we develop and present a technique we refer to as Optical Side Scattering Radiometry (OSSR) capable of resolving discrete and distributed scattering properties of fibers along their length with up to 60dB dynamic range and 5cm spatial resolution. Our setup is first validated on a standard single mode telecoms fiber. Then we apply it to a record-length 11km hollow core photonic band-gap fiber (HC-PBGF) the characterization requirements of which lie far beyond the capability of standard optical reflectometric instruments. We next demonstrate use of the technique to investigate and explain the unusually high loss observed in another HC-PBGF and finally demonstrate its flexibility by measuring a HC-PBGF operating at a wavelength of 2µm. In all of these examples, good agreement between the OSSR measurements and other well-established (but more limited) characterization methods, i.e. cutback loss and OTDR, was obtained.

Dense WDM transmission at 2 µm enabled by an arrayed waveguide grating.

We show, for the first time, dense WDM (8×20 Gbit/s) transmission at 2 µm enabled by advanced modulation formats (4-ASK Fast-OFDM) and the development of key components, including a new arrayed waveguide grating (AWGr) at 2 µm. The AWGr shows -12.8±1.78 dB of excess loss with an 18-dB extinction ratio and a thermal tunability of 0.108 nm/°C.

Accurate calibration of S(2) and interferometry based multimode fiber characterization techniques.

We present a novel method to validate the relative amount of power carried by high order modes in a multimode fiber using a Spatial and Spectral (S(2)) imaging technique. The method can be utilized to calibrate the S(2) set-up and uses Fresnel reflections from a thin glass plate to compare theoretical values with experimental results. We have found that, in the most general case, spectral leakage and sampling errors can lead S(2) to underestimate the multipath interference (MPI) of high order modes by several decibels, thus significantly impairing the result of the measurement. On the other hand, by applying suitable corrections as described in this work, we demonstrate that the S(2) produces MPI estimates that are accurate to within 1dB or better.

100 Gbit/s WDM transmission at 2 µm: transmission studies in both low-loss hollow core photonic bandgap fiber and solid core fiber.

We show for the first time 100 Gbit/s total capacity at 2 µm waveband, using 4 × 9.3 Gbit/s 4-ASK Fast-OFDM direct modulation and 4 × 15.7 Gbit/s NRZ-OOK external modulation, spanning a 36.3 nm wide wavelength range. WDM transmission was successfully demonstrated over 1.15 km of low-loss hollow core photonic bandgap fiber (HC-PBGF) and over 1 km of solid core fiber (SCF). We conclude that the OSNR penalty associated with the SCF is minimal, while a ~1-2 dB penalty was observed after the HC-PBGF probably due to mode coupling to higher-order modes.

X-ray tomography for structural analysis of microstructured and multimaterial optical fibers and preforms.

Specialty optical fibers, in particular microstructured and multi-material optical fibers, have complex geometry in terms of structure and/or material composition. Their fabrication, although rapidly developing, is still at a very early stage of development compared with conventional optical fibers. Structural characterization of these fibers during every step of their multi-stage fabrication process is paramount to optimize the fiber-drawing process. The complexity of these fibers restricts the use of conventional refractometry and microscopy techniques to determine their structural and material composition. Here we present, to the best of our knowledge, the first nondestructive structural and material investigation of specialty optical fibers using X-ray computed tomography (CT) methods, not achievable using other techniques. Recent advances in X-ray CT techniques allow the examination of optical fibers and their preforms with sub-micron resolution while preserving the specimen for onward processing and use. In this work, we study some of the most challenging specialty optical fibers and their preforms. We analyze a hollow core photonic band gap fiber and its preforms, and bond quality at the joint between two fusion-spliced hollow core fibers. Additionally, we studied a multi-element optical fiber and a metal incorporated dual suspended-core optical fiber. The application of X-ray CT can be extended to almost all optical fiber types, preforms and devices.

Picometer-scale surface roughness measurements inside hollow glass fibres.

A differential profilometry technique is adapted to the problem of measuring the roughness of hollow glass fibres by use of immersion objectives and index-matching liquid. The technique can achieve picometer level sensitivity. Cross validation with AFM measurements is obtained through use of vitreous silica step calibration samples. Measurements on the inner surfaces of fibre-sized glass capillaries drawn from high purity suprasil F300 tubes show a sub-nanometer roughness, and the roughness power spectrum measured in the range [5 · 10(-3) m(-1) 10(-1) m(-1)] is consistent with the description of the glass surface as a superposition of frozen capillary waves. The surface roughness spectrum of two capillary tubes of differing compositions can be quantitatively distinguished.

Demonstration of amplified data transmission at 2 µm in a low-loss wide bandwidth hollow core photonic bandgap fiber.

The first demonstration of a hollow core photonic bandgap fiber (HC-PBGF) suitable for high-rate data transmission in the 2 µm waveband is presented. The fiber has a record low loss for this wavelength region (4.5 dB/km at 1980 nm) and a >150 nm wide surface-mode-free transmission window at the center of the bandgap. Detailed analysis of the optical modes and their propagation along the fiber, carried out using a time-of-flight technique in conjunction with spatially and spectrally resolved (S2) imaging, provides clear evidence that the HC-PBGF can be operated as quasi-single mode even though it supports up to four mode groups. Through the use of a custom built Thulium doped fiber amplifier with gain bandwidth closely matched to the fiber's low loss window, error-free 8 Gbit/s transmission in an optically amplified data channel at 2008 nm over 290 m of 19 cell HC-PBGF is reported.

Hollow-bottle optical microresonators.

Selective excitation of whispering-gallery and bottle modes in a robust hollow-bottle optical microresonator, fabricated from a silica microcapillary by a pressure-compensated, "soften-and-compress" method, is demonstrated. Characteristic resonance spectra of bottle modes were obtained by using a tapered fiber coupled at different locations along the hollow bottle. The spectral characteristics (Q-factor, excitation efficiency) are shown to have high tolerance to angular misalignment of the tapered fiber. In addition, introduction of localized losses on the outer surface of the resonator results in selective clean-up of the transmission spectrum and superior performance. A theoretical analysis of modal turning points and associated resonant wavelengths is used to explain the mechanism of mode-suppression and the resultant spectral cleaning.

Large mode area silicon microstructured fiber with robust dual mode guidance.

A silicon microstructured fiber has been designed and fabricated using a pure silica photonic bandgap guiding fiber as a 3D template for materials deposition. The resulting silicon fiber has a micron sized core but with a small core-cladding index contrast so that it only supports two guided modes. It will be shown that by using the microstructured template this fiber exhibits a number of similar guiding properties to the more traditional index guiding air-silica structures. The large mode areas and low optical losses measured for the silicon microstructured fiber demonstrate its potential to be integrated with existing fiber infrastructures.

Robustly single mode hollow core photonic bandgap fiber.

We report the fabrication of a novel type of hollow core photonic bandgap fiber (PBGF) with a small core formed by 3 omitted unit cells in a triangular array of holes. The transmission properties of fibers designed for operation at 1500nm wavelength are investigated both experimentally and through extensive modeling. The novel PBGF structure provides robust single mode guidance and is of particular interest for device applications which require low index bandgap guidance and short device lengths.

Efficient white light generation in secondary cores of holey fibers.

We report the generation of white light from a picosecond pump by efficient four-wave mixing processes. A 530 nm green source based on a frequency-doubled Yb-doped fiber laser generates strong red and blue sidebands in the secondary cores of a holey fiber with large air-filling factor. Phase matching is attributed to birefringence within the submicrometer-sized secondary cores induced by non-symmetric deformation during the fiber drawing.

Methane detection at 1670-nm band using a hollow-core photonic bandgap fiber and a multiline algorithm.

The long interaction pathlengths provided by hollow-core photonic bandgap fibers (HC-PBFs) are especially advantageous for the detection of weakly absorbing gases such as methane (CH(4)). In this paper, we demonstrate methane sensing with a 1670-nm band HC-PBF. A multiline algorithm is used to fit the R(6) manifold (near 1645 nm) and, in this way, to measure the gas concentration. With this method, a minimum detectivity of 10 ppmv for the system configuration was estimated.

Design of 7 and 19 cells core air-guiding photonic crystal fibers for low-loss, wide bandwidth and dispersion controlled operation.

We study the modal properties of feasible hollow-core photonic bandgap fibers (HC-PBGFs) with cores formed by omitting either 7 or 19 central unit-cells. Firstly, we analyze fibers with thin core surrounds and demonstrate that even for large cores the proposed structures are optimum for broad-band transmission. We compare these optimized structures with fibers which incorporate antiresonant core surrounds which are known to have low-loss. Trade-offs between loss and useful bandwidth are presented. Finally, we study the effects that small modifications to the core surround have on the fiber's group velocity dispersion, showing the possibility of engineering the dispersion in hollow-core photonic bandgap fibers.

Optimizing the usable bandwidth and loss through core design in realistic hollow-core photonic bandgap fibers.

The operational bandwidth of hollow-core photonic bandgap fibers (PBGFs) is drastically affected by interactions between the fundamental core mode and surface modes guided at the core-cladding interface. By systematically studying realistic hollow-core PBGFs we identify a new design regime robust in eliminating the presence of surface modes. We present new fiber designs with a fundamental core mode free of anticrossings with surface modes at all wavelengths within the bandgap, allowing for a low-loss operational bandwidth of ~17% of the central gap wavelength.

Metastability of Si1-yCy epilayers under 2 MeV alpha-particle irradiation

In this work we present some recent results concerning the alpha-particles irradiation of Si1-yCy alloy epitaxially grown on silicon. The study of the damage process is interesting because of the extensive use of backscattering technique as a tool of characterisation of this kind of materials and because of the possibility of adding information about the transformations that this metastable material undergoes. We point out that the irradiation damage process causes a change in the material structure different from that due to the thermal treatments. The irradiation damage occurs at a rate much higher than in Si, however it involves only a silicon atom fraction that appears to be proportional to the substitutional carbon content.

Contributions of adenosine receptor activation to the ocular actions of epinephrine.

PURPOSE: Epinephrine is an effective drug for glaucoma treatment. However, the mechanisms responsible for the ocular hypotensive action of this compound are not completely understood. Adenosine is an autacoid released by all cells. This study evaluated the role of adenosine receptor activation in epinephrine-induced changes in ocular function. METHODS: Rabbits were pretreated topically with the moderately selective adenosine A1 antagonist 8-(p-sulfophenyl)theophylline (8-SPT) or the adenosine A2 antagonist 3,7-dimethyl-l-propargylxanthine (DMPX). Epinephrine (500 microg) was then administered, and intraocular pressures (IOPs), pupil diameters (PDs), or total outflow facility was evaluated. In a separate group of animals, epinephrine or vehicle was administered, and aqueous humor samples obtained to evaluate changes in aqueous humor purine levels by means of high-performance liquid chromatography. RESULTS: In control animals, epinephrine produced a biphasic change in IOP: an initial rise in IOP of approximately 1 mm Hg from 1/2 to 1 hour followed by significant reduction in IOP of 8 to 9 mm Hg from 3 to 5 hours postadministration. These animals also exhibited a significant increase in PD of 2 to 3 mm from 1/2 to 2 hours postadministration. Pretreatment with 8-SPT (1000 microg) enhanced the initial rise in IOP, while significantly inhibiting the ocular hypotensive response. Pretreatment with 8-SPT also significantly enhanced the epinephrine-induced increase in PD. Inhibition of the epinephrine-induced reduction in IOP by 8-SPT was dose-related with an IC50 of 446 microg. Administration of 8-SPT alone did not significantly alter IOP or PD. The A2 antagonist DMPX did not alter the epinephrine-induced change in IOP or PD. In rabbits pretreated with 8-SPT, the epinephrine-induced increase in outflow facility was significantly reduced by 60% when compared with those in rabbits treated with epinephrine alone. In vehicle-treated rabbits, aqueous humor adenosine and inosine levels were 2.7 +/- 0.38 and 29 +/- 4.2 ng/100 microl, respectively. Three hours after epinephrine administration, adenosine and inosine levels had significantly increased to 11 +/- 1.6 and 66 +/- 4.4 ng/100 microl, respectively. CONCLUSIONS: These results support the idea that in rabbits epinephrine administration stimulates adenosine release in the anterior segment. This rise in endogenous levels of adenosine then leads to the activation of ocular adenosine receptors and is in part responsible for the ocular hypotensive action of epinephrine.