Timothy A. Birks

Anti‐resonant hollow core fibres have significant advantages derived from the guidance of light in air, and have been rapidly developed and employed in the near and mid‐infrared. Despite comparable or greater benefits in the visible and ultraviolet, their development and uptake has been slow due to inherent challenges in their fabrication. In this perspective, we provide a brief look at these fabrication challenges, possible solutions and potential applications for hollow core fibres in the short‐wavelength limit.

We report on a tapered three-core optical fibre that can be used as a tip-tilt wavefront sensor. In this device, a coupled region of a few millimetres at the sensing tip of the fibre converts fragile phase information from an incoming wavefront into robust intensity information within each of the cores. The intensity information can be easily converted to linear wavefront error over small ranges, making it ideal for closed loop systems. The sensor uses minimal information to infer tip-tilt and is compatible with remote detector arrays. We explore its application within adaptive optics and present a validation case to show its applicability to astronomy.

We report on a tapered three-core optical fibre that can be used as a tip–tilt wavefront sensor. In this device, a coupled region of a few millimetres at the sensing tip of the fibre converts fragile phase information from an incoming wavefront into robust intensity information within each of the cores. The intensity information can be easily converted to linear wavefront error over small ranges, making it ideal for closed loop systems. The sensor uses minimal information to infer tip–tilt and is compatible with remote detector arrays. We explore its application within adaptive optics and present a validation case to show its applicability to astronomy.

A novel method of manufacturing silica fibre preforms is presented which exploits ultrafast laser induced selective etching to fabricate cross-sectional disks that are subsequently stacked and bonded via ultrafast laser welding.

Direct imaging instruments have the spatial resolution to resolve exoplanets from their host star. This enables direct characterization of the exoplanets atmosphere, but most direct imaging instruments do not have spectrographs with high enough resolving power for detailed atmospheric characterization. We investigate the use of a single-mode diffraction-limited integral-field unit that is compact and easy to integrate into current and future direct imaging instruments for exoplanet characterization. This achieved by making use of recent progress in photonic manufacturing to create a single-mode fiber-fed image reformatter. The fiber link is created with three-dimensional printed lenses on top of a single-mode multicore fiber that feeds an ultrafast laser inscribed photonic chip that reformats the fiber into a pseudoslit. We then couple it to a first-order spectrograph with a triple stacked volume phase holographic grating for a high efficiency over a large bandwidth. The prototype system has had a successful first-light observing run at the 4.2-m William Herschel Telescope. The measured on-sky resolving power is between 2500 and 3000, depending on the wavelength. With our observations, we show that single-mode integral-field spectroscopy is a viable option for current and future exoplanet imaging instruments.

Optical fibers with a logarithmic index profile can provide invariant mode field diameters along a tapered fiber, which enables adiabatic mode transitions for higher-order mode (HOM) microfibers. A microfiber with a waist diameter of ∼2 µm is fabricated with an insertion loss lower than 0.03 dB for the LP₀₁ and 0.11 dB for the LP₁₁ mode. The concept of the low loss HOM microfibers can be further extended to include more than one fiber and a 2×2 few mode microfiber coupler is fabricated/characterized in our experiments. These single or multiple spatial channel HOM microfibers are beneficial for various applications, including in particle propulsion, atom trapping, optical sensing and space division multiplexed data transmission systems.

Wavelength-to-time mapping (WTM)—stretching ultrashort optical pulses in a dispersive medium such that the instantaneous frequency becomes time-dependent—is usually performed using a single-mode fiber. In a number of applications, such as time-stretch imaging (TSI), the use of this single-mode fiber during WTM limits the achievable sampling rate and the imaging quality. Multimode fiber based WTM is a potential route to overcome this challenge and project a more diverse range of light patterns. Here, we demonstrate the use of a two-dimensional single-photon avalanche diode (SPAD) array to image, in a time-correlated single-photon counting (TCSPC) manner, the time- and wavelength-dependent arrival of different spatial modes in a few-mode fiber. We then use a TCSPC spectrometer with a one-dimensional SPAD array to record and calibrate the wavelength-dependent and mode-dependent WTM processes. The direct measurement of the WTM of the spatial modes opens a convenient route to estimate group velocity dispersion, differential mode delay, and the effective refractive index of different spatial modes. This is applicable to TSI and ultrafast optical imaging, as well as broader areas such as telecommunications.

We observe clear evidence of adiabatic passage between photon populations via a four-wave mixing process, implemented through a dispersion sweep arranged by a core diameter taper of an optical fiber. Photonic rapid adiabatic passage through the cubic electric susceptibility thus opens precise control of frequency translation between broadband light fields to all common optical media. Areas of potential impact include optical fiber and on-chip waveguide platforms for quantum information, ultrafast spectroscopy and metrology, and extreme light-matter interaction science.

We study the mid-infrared attenuation of antiresonant hollow-core fiber made of fused silica glass. The role of absorptive losses increases with wavelength but can be minimized by reducing the overlap of the trapped light with the silica. We show that this overlap is least at the lowest-order antiresonance condition, corresponding to the thinnest core wall, and for higher resonances scales with the core wall thickness. A record-low minimum attenuation of 18 dB/km measured in our fiber at 3.1 µm wavelength is not limited by silica absorption. We measured 40 dB/km attenuation at 4 µm wavelength, where the attenuation of bulk silica is 860 dB/m. We show that this corresponds to a modal overlap of 2.81 × 10⁻⁵ which is in good agreement with simulations, suggesting that at this wavelength, attenuation is limited by silica absorption. This enables us to predict the achievable attenuation at longer wavelengths as well. Extrinsic losses due to gaseous molecular absorption may make demonstration of such losses difficult in some spectral bands. In contrast to shorter wavelengths, where leakage loss is the primary attenuation mechanism, introducing additional elements into the cladding design is unlikely to reduce the attenuation further, and further loss reduction would require a larger core size.

Mode-selective multicore fibre photonic lanterns are used to create a 6-mode multimode optical fibre link that is considerably more resistant to focal ratio degradation (FRD) than a 6-mode step-index optical fibre.