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.

3D-printed polymer antiresonant waveguides for short-reach terahertz applications.

In this work, we present a 3D-printed waveguide that provides effective electromagnetic guidance in the THz regime. The waveguide is printed using low-cost polycarbonate and a conventional fused deposition modeling printer. Light guidance in the hollow core is achieved through antiresonance, and it improves the energy effectively transported to the receiver compared to free space propagation. Our demonstration adds to the field of 3D-printed terahertz components, providing a low-cost way of guiding terahertz radiation.

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.

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.

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.