Broadband all-fiber optical phase modulator based on photo-thermal effect in a gas-filled hollow-core fiber

We report broadband all-fiber optical phase modulation based on the photo-thermal effect in a gas-filled hollow-core fiber.The phase modulation dynamics are studied by multi-physics simulation.A phase modulator is fabricated using a 5.6-cm-long anti-resonant hollow-core fiber with pure acetylene filling.It has a half-wave optical power of 289 mW at 100 kHz and an average insertion loss 0.6 dB over a broad wavelength range from 1450 to 1650 nm.The rise and fall time constants are 3.5 and 3.7μs,respectively,2–3 orders of magnitude better than the previously reported microfiber-based photo-thermal phase modulators.The gas-filled hollow-core waveguide configuration is promising for optical phase modulation from ultraviolet to mid-infrared which is challenging to achieve with solid optical fibers.

Analysis of SF6 decomposed products by fibre-enhanced Raman spectroscopy for gas-insulated switchgear diagnosis

Sulphur hexafluoride(SF6)decomposed products analysis is highly critical in the earlystage fault diagnosis of gas-insulated switchgear(GIS).Spectrum technology outperforms traditional methods on non-invasiveness,no sample preparation,and no consumption.Here,the authors present an improved fibre-enhanced Raman spectroscopy(FERS)as a comprehensive analytical tool to detect a suite of SF_(6)decomposed products(SO_(2)F_(2),SOF_(2),SO_(2),H_(2)S,CF_(4),OCS,CO_(2),and CO).The FERS approach is combined with two iris diaphragms for spatial filtering and a rear-end reflector for additional Raman signal enhancement.Limits of detection down to 1×10^(−6)–8×10^(−6)are achieved for different SF6 decompositions,and quantification of an undefined multigas,sampled from an 800 kV GIS in service,is realised utilising SF6 as the internal standard gas and with a maximum error of 5.5%.The GIS is diagnosed according to the results and confirmed by an on-site check.The authors foresee that this technique will provide a route for trace gas analysis in the power industry.

Towards high-power mid-IR light source tunable from 3.8 to 4.5 µm by HBr-filled hollow-core silica fibres

Fibre lasers operating at the mid-IR have attracted enormous interest due to the plethora of applications in defence,security,medicine,and so on.However,no continuous-wave(CW)fibre lasers beyond 4μm based on rare-earth-doped fibres have been demonstrated thus far.Here,we report efficient mid-IR laser emission from HBr-filled silica hollow-core fibres(HCFs)for the first time.By pumping with a self-developed thulium-doped fibre amplifier seeded by several diode lasers over the range of 1940–1983 nm,narrow linewidth mid-IR emission from 3810 to 4496 nm has been achieved with a maximum laser power of about 500 mW and a slope efficiency of approximately 18%.To the best of our knowledge,the wavelength of 4496 nm with strong absorption in silica-based fibres is the longest emission wavelength from a CW fibre laser,and the span of 686 nm is also the largest tuning range achieved to date for any CW fibre laser.By further reducing the HCF transmission loss,increasing the pump power,improving the coupling efficiency,and optimizing the fibre length together with the pressure,the laser efficiency and output power are expected to increase significantly.This work opens new opportunities for broadly tunable high-power mid-IR fibre lasers,especially beyond 4μm.

Design of large mode area all-solid anti-resonant fiber for high-power lasers

High-power fiber lasers have experienced a dramatic development over the last decade.Further increasing the output power needs an upscaling of the fiber mode area,while maintaining a single-mode output.Here,we propose an all-solid anti-resonant fiber(ARF)structure,which ensures single-mode operation in broadband by resonantly coupling higherorder modes into the cladding.A series of fibers with core sizes ranging from 40 to 100µm are proposed exhibiting maximum mode area exceeding 5000µm2.Numerical simulations show this resonant coupling scheme provides a higher-order mode(mainly TE01,TM01,and HE21)suppression ratio of more than 20 dB,while keeping the fundamental mode loss lower than 1 dB/m.The proposed structure also exhibits high tolerance for core index depression.

Mid-infrared all-optical modulators based on an acetylene-filled hollow-core fiber

We report all-optical mid-infrared phase and intensity modulators based on the photo-thermal effect in an acetylene-filled anti-resonant hollow-core fiber.Optical absorption of the control beam promotes the gas molecules to a higher energy level,which induces localized heating through non-radiative relaxation and modulates the refractive index of the gas material and hence the accumulated phase of the signal beam propagating through the hollow-core fiber.By modulating the intensity of the control beam,the phase of the signal beam is modulated accordingly.By use of a 1.53μm near-infrared control beam,all-optical phase modulation up to 2.2πrad is experimentally demonstrated at the signal wavelength of 3.35μm.With the phase modulator placed in one arm of a Mach-Zehnder interferometer,intensity modulation with on-off ratio of 25 dB is achieved.The gas-filled hollow-core-fiber modulators could operate over an ultra-broad wavelength band from near-to mid-infrared and have promising application in mid-infrared photonic systems.

Freestanding metal nanohole array for high-performance applications

Plasmonic devices using periodic metallic nanostructures have recently gained tremendous interest for color filters,sensing,surface enhanced spectroscopy,and enhanced photoluminescence,etc.However,the performance of such plasmonic devices is severely hampered by the solid substrates supporting the metallic nanostructures.Here,a strategy for freestanding metallic nanomembranes is introduced by taking advantages of hollow substrate structures.Large-area and highly uniform gold nanomembranes with nanohole array are fabricated via a flexible and simple replication-releasing method.The hollow structures include a hollow core fiber with 30μm core diameter and two ferrules with their hole diameter as 125 and 500μm,respectively.As a proof-of-concept demonstration,2 times higher sensitivity of the bulk refractive index is obtained with this platform compared to that of a counterpart on a solid silica substrate.Such a portable and compact configuration provides unique opportunities to explore the intrinsic properties of the metal nanomembranes and paves a new way to fabricate high-performance plasmonic devices for biomolecule sensing and color filter.