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.

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.

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.

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.

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.

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.