Hollow-core fiber (HCF) has found plenty of interdisciplinary applications in areas ranging from ultra-intense pulse delivery, single-cycle pulse generation, low latency optical communication, UV light sources, mid-IR gas lasers to biochemical sensing, quantum optics and mid-IR to Terahertz waveguides. These applications calls for better performance HCF, especially in loss, bandwidth and mode quality. Here we present a new hollow-core fiber with conjoined-tubes in the cladding and a negative-curvature core shape. It exhibits a minimum transmission loss of 2 dB/km at 1512 nm and a <16 dB/km bandwidth covering the O, E, S, C, L telecom bands. The debut of this conjoined-tube hollow-core fiber, with combined merits of ultralow loss, broad bandwidth, low bending loss, high mode quality and simple structure heralds a new opportunity to fully unleash the potential of hollow-core fiber in laser and telecommunication related applications.
Hollow-core photonic crystal fiber (HC-PCF) has been proved to be a versatile platform for lab-on-a-fiber applications. By filling the fiber with various gases, liquids or solid materials, the light-matter interaction could be greatly enhanced. Many novel optical phenomena that are unattainable in free-space could be easily identified inside the fiber. Such a platform offers a promising route for creating compact, integrable and biocompatible all-fiber multifunctional optical devices. Here, we review our recent progress in developing a novel HC-PCF coined "hollow-core negative curvature fiber" (NCF) that could provide light guidance at spectral ranges covering from UV, visible, NIR to MIR. These NCFs show attributes of low transmission loss, octave-spanning transmission bandwidth, high damage threshold and single modeness. As a proof-of-concept demonstration for lab-on-a-fiber applications, we filled one of the fibers with ethanol (refractive index 1.36) to form a liquid-core anti-resonant fiber. At a low volume of 1 μL, Raman signal from ethanol was observed at a pump power of 2 mW. Such a high performance NCF opens a window for applications in fiber-enhanced spectroscopy, biochemical sensing and nano-plasmonics.
We report on an ultraviolet-enhanced supercontinuum generation in a uniform photonic crystal fiber pumped by a giant-chirped mode-locked Yb-doped fiber laser. We find experimentally that the initial pluses with giant chirp leads more initial energy transferred to the dispersive waves in visible and ultraviolet wavelength. An extremely wide optical spectrum spanning from 370 nm to beyond 2400 nm with a broad 3 dB spectral bandwidth of 367 nm (from 431 nm to 798 nm) is obtained. Over 36% (350 mW) of the total output power locates in the visible and ultraviolet regime between 370 nm and 850 nm with a maximum spectral power density of 1.6 mW/nm at 550 nm. In addition, a blue-enhanced supercontinuum generation pumped by a giant-chirped SESAM mode-locked ytterbium-doped fiber laser is studied. An extremely wide optical spectrum spanning from 380 nm to 2400 nm with total power of 3 W is obtained.
The damage mechanism and test technology of space radiation environment to space equipment was classified and the radiation protection demand of active fiber for space application was analyzed. The radiation hardening techniques of Ce doping, hydrogen loading and pre-radiation exposure and thermal annealing for Er:Yb co-doped fiber was surveyed.
We report on recent design and fabrication of Kagome type hollow-core photonic crystal fiber (HC-PCF) for the
purpose of high power fast laser beam transportation. The fabricated seven-cell three-ring hypocycloid-shaped large
core fiber exhibits an up-to-date lowest attenuation (among all Kagome fibers) of 40dB/km over a broadband
transmission centered at 1500nm. We show that the large core size, low attenuation, broadband transmission, single
modedness, low dispersion and relatively low banding loss makes it an ideal host for high power laser beam
transportation. By filling the fiber with helium gas, a 74μJ, 850fs and 40kHz repetition rate ultra-short pulse at
1550nm has been faithfully delivered with little propagation pulse distortion. Compression of a 105μJ laser pulse
from 850fs to 300fs has been achieved by operating the fiber in ambient air.
We have now demonstrated and characterized gas-filled hollow-core fiber lasers based on population inversion from
acetylene (12C2H2) and HCN gas contained within the core of a kagome-structured hollow-core photonic crystal fiber.
The gases are optically pumped via first order rotational-vibrational overtones near 1.5 μm using 1-ns pulses from an
optical parametric amplifier. Transitions from the pumped overtone modes to fundamental C-H stretching modes in both
molecules create narrow-band laser emissions near 3 μm. High gain resulting from tight confinement of the pump and
laser light together with the active gas permits us to operate these lasers in a single pass configuration, without the use of
any external resonator structure. A delay between the emitted laser pulse and the incident pump pulse has been observed
and is shown to vary with pump pulse energy and gas pressure. Furthermore, we have demonstrated lasing beyond 4 μm
from CO and CO2 using silver-coated glass capillaries, since fused silica based fibers do not transmit in this spectral
region and chalcogenide fibers are not yet readily available. Studies of the laser pulse energy as functions of the pump
pulse energy and gas pressure were performed. Efficiencies reaching ~ 20% are observed for both acetylene and CO2.
We report on progress in different hollow core photonic crystal fiber (HC-PCF) design and fabrication for atomic
vapor based applications. We have fabricated a Photonic bandgap (PBG) guiding HC-PCF with a record loss of
107dB/km at 785nm in this class of fiber. A double photonic bandgap (DPBG) guiding HC-PCF with guidance bands
centred at 780nm and 1064nm is reported. A 7-cell 3-ring Kagome HC-PCF with hypocycloid core is reported, the
optical loss at 780nm has been reduced to 70dB/km which to the best of our knowledge is the lowest optical loss
reported at this wavelength using HC-PCF. Details on experimental loading of alkali metal vapours using a far off
red detuned laser are reported. This optical loading has been shown to decrease the necessary loading time for Rb
into the hollow core of a fiber. The quantity of Rb within the fiber core has been enhanced by a maximum of 14%
through this loading procedure.
We report on recent developments on fabrication and optical guidance of Kagome-lattice hollow-core photonic crystal
fiber (HC-PCF). These include the design and fabrication of a hypocycloid-shaped core Kagome HC-PCF that combines
a record optical attenuation with a baseline exhibiting ~180 dB/km over a transmission bandwidth larger than 200 THz.
These results are corroborated with theoretical simulations which show that both the core-shape and the cladding ring
number play role in inhibited coupling, inducing core-mode confinement for the fundamental transmission band. We also
show that the inhibited coupling is weaker for the first higher-order transmission band by theoretically and
experimentally comparing Kagome HC-PCF with a single anti-resonant ring hollow-core fiber.
A photonic microcell (PMC) is a length of gas-filled hollow core-photonic crystal fiber (HC-PCF) which is hermetically
sealed at both ends by splicing to standard single mode fiber. We describe advances in the fabrication technique of PMCs
which enable large core Kagome-lattice HC-PCFs to be integrated into PMC form. The modified fabrication technique
uses fiber-tapering to accommodate the large dimensions of the fiber and enables low loss splices with single mode fiber
by reducing mode field mismatch. Splice losses as low as 0.6 dB are achieved between 1-cell defect Kagome HC-PCF
and single mode fiber. Relative to the previously reported PMCs, which were based on photonic bandgap HC-PCF, the
present Kagome HC-PCF based PMC provides broad optical transmission, surface mode-free guidance and larger core at
the cost of slightly increased fiber attenuation (~0.2 dB/m). Therefore, the integration of this fiber into PMC form opens
up new applications for PMC-based devices. The advantage of the large core dimensions and surface mode free guidance
for quantum optics in gas-filled HC-PCF are demonstrated by generation of narrow sub-Doppler features in an acetylenefilled
large core PMC.