Propagation time through standard optical fibres changes with temperature at a rate of 40 ps/km/K. This can pose significant challenges in many diverse application areas of optical fibres in physics and engineering. Primary examples lie in applications in which very precise timing signals need to be disseminated for synchronization purposes in large experimental infrastructures such as synchrotrons, linear particle accelerators, large telescope arrays, and in phase arrayed antennae. A value of 40 ps/km/K equates to a phase temperature sensitivity of about 48 rad/m/K. This can adversely affect many applications relying on fibre interferometers (e.g. fibre optic sensors, quantum-optics, interferometric measurement techniques, and so on), in which maintaining stable interference would require temperature stabilization below mK level. Similarly, a few key optical metrology applications require the dissemination of optical signals at a precise frequency, for example to compare distant ultra-precise clocks (e.g., national standard clocks) with a precision (fractional stability) at/below the 10-18 level. Such a level of precision is easily compromised by thermally-induced changes in optical path length (temperature drift) with time that unavoidably result in a Doppler frequency shift.
Here, we review our recent results in which we show why and how Hollow-Core Fibres (HCF) are significantly better than solid-core fibres in terms of their sensitivity of propagation time and accumulated phase change to temperature and thus are a better alternative to standard fibres in the above-mentioned fields.
In this paper we present an exploration of the stability and repeatability of a hollow core microstructured fibre (HCMOF) Raman gas sensor. Raman gas detection using HC-MOFs is an exciting technique as it enables high sensitivity, multi-species detection using a small gas volume and within a small physical space. Several previous works have demonstrated the utility of HC-MOF fibres as Raman gas cells for the detection of a wide range of gas species such as methane and hydrogen. Here we take a first look at the Raman signal stability (in a single fibre) and signal reproducibility (from fibre-to-fibre). We show that a HC-MOF Raman system can achieve low within-day variability of 0.3 %CV and fibre-to-fibre variability of 7.6 %CV. Understanding the error within systems such as the one presented is critical in the development of HC-MOF-based gas sensors for practical applications.
The high performance of fibre lasers is largely due to the outstanding characteristics of fibres as an active medium. However, there is a need to overcome some limits at high optical powers which are imposed by the fibre design. We report on a design and fabrication of a stimulated Raman scattering (SRS) filtering fibre for high average or high peak optical power delivery applications. The fibre geometry is based on the series of circularly arranged high index rod resonators embedded in the silica cladding. The operation principle relies on the resonant coupling of the core and resonators modes. The fabricated fibre demonstrated wide transmission window and filtering of SRS from the output spectra (with the extinction which exceeds 20dB at the Raman Stokes wavelength), robustness for bending and high output beam quality. The fibre has been tested as a beam delivery fibre of a commercial pulsed fibre laser system in order to identify filtering performance and its limitations.
While hollow core-photonic crystal fibres are now a well-established fibre technology, the majority of work on these speciality fibres has been on designs with a single core for optical guidance. In this paper we present the first dual hollow-core anti-resonant fibres (DHC-ARFs). The fibres have high structural uniformity and low loss (minimum loss of 0.5 dB/m in the low loss guidance window) and demonstrate regimes of both inter-core coupling and zero coupling, dependent on the wavelength of operation, input polarisation, core separation and bend radius. In a DHC-ARF with a core separation of 4.3 μm, we find that with an optimised input polarisation up to 65% of the light guided in the launch core can be coupled into the second core, opening up applications in power delivery, gas sensing and quantum optics.
We study in detail the macrobending effects in a wide transmission bandwidth (~200nm) 19 cell hollow-core photonic bandgap fiber operating at 1550nm. Our results indicate low bend sensitivity over a ~130nm wide interval within the transmission window, with negligible loss (<0.1dB) for bending radii down to 5mm. The “red shift” and “blue shift” of the bandgap edge have been observed at the short and long wavelength edges, respectively. The cutoff wavelengths where air-guiding modes stop guiding can be extracted from the bending loss spectra, which matches well with the simulated effective refractive index map of such fiber.
We present sub-ppm sensitivity, broadband gas absorption measurements using improved hollow-core photonic-bandgap
fibers with low loss in the range 3-3.6 μm. The sensitivity levels, unprecedented for a fiber-based sensor system, were
achieved by addressing the strong mid-IR vibrational bands through use of a high-intensity supercontinuum source and
by exploiting long interaction lengths in the HC-PBGFs. We measured mixtures of methane (1034 ppm) and ethane
(50.4 ppm) and attained 7 and 0.9 ppm sensitivities for 0.925 and 5.69 m long HC-PBGF samples, respectively, from
high-resolution (0.2 nm) spectra. Significant scope for further sensitivity improvement exists through use of longer fibers
in combination with more sophisticated sensing schemes.
Recent years have seen the development of a range of promising optical fibre technologies emerge, enabled by advances
in materials and fabrication techniques. We describe 3 emerging areas in optical fibre developments: nanomechanical
optical fibres, microstructured hollow core silica fibres for high peak optical power and/or extended infrared
transmission, and chalcogenide glasses and fibres for mid-IR applications.
We demonstrate a technique for tapering periodically an all-solid soft glass fiber consisting of two types of lead
silicate glasses by the use of a CO2 laser and investigate the bend sensing applications of the periodically-tapered
soft glass fiber. Such a soft glass fiber with periodic microtapers could be used to develop a promising bend sensor
with a sensitivity of -27.75 μW/m-1 by means of measuring the bend-induced change of light intensity. The proposed
bend sensor exhibits a very low measurement error of down to ± 1%.
We experimentally and numerically demonstrate the possibility of generating parabolic pulses by propagating
Gaussian pulses in 1.8 m-long normally dispersive tapered microstructured optical fibre (MOF). The modelling
of the MOF and the procedure for the determination of the taper's parameters is presented. The proposed taper
is fabricated and experimentally characterised using linear frequency resolved optical gating (l-FROG) technique,
to measure the output pulse intensity. Numerical simulations are in a good agreement with the experimental
results.
We report recent advances in the development of fibers for the delivery and generation of both single-mode and heavily
multimode laser beams as well as recent progress in fibers for supercontinuum generation in spectral regimes spanning
the visible to mid-IR.
Air/silica Microstructured Optical Fibers (MOFs) offer new prospects for fiber based sensor devices. In this paper, two
topics of particular significance for gas sensing using air guiding Photonic Bandgap Fibers (PBGFs) are discussed. First,
we address the issue of controlling the modal properties of PBGFs and demonstrate a single mode, polarization
maintaining air guiding PBGF. Secondly, we present recent improvements of a femtosecond laser machining technique
for fabricating fluidic channels in PBGFs, which allowed us to achieve cells with multiple side access channels and low
additional loss.
Norberto Chiodini, Anna Vedda, Mauro Fasoli, Federico Moretti, Alessandro Lauria, Marie Claire Cantone, Ivan Veronese, Giampiero Tosi, Marco Brambilla, Barbara Cannillo, Eleonora Mones, Gilberto Brambilla, Marco Petrovich
Scintillating materials, able to convert energy of ionizing radiation into light in the visible-UV interval, are presently
used in a wide class of applications such as medical imaging, industrial inspection, security controls and high energy
physics detectors.
In the last few years we studied and developed a new radiation sensor based on silica-glass fiber-optic technology. In its
simplest configuration such device is composed by a short portion (about 10 mm) of scintillating fiber coupled to a
photomultiplier through a suitably long passive silica fiber.
In this work, we present new results concerning the characterization of silica based Ce and Eu doped fibers glasses
obtained by a modified sol-gel method and drawn by a conventional drawing tower for optical fibers. The radio-luminescence
of Eu doped fibers is rather weak; moreover it displays a marked sensitivity increase during subsequent
irradiations, preventing the use of such fibers in dosimetry. On the other hand Ce-doped fibers show very high radiation
hardness, signal stability and reproducibility, and high sensitivity to radiations with energies from 10 keV to several
tens of MeV. Numerous tests with photons (X and gamma rays), electrons, and protons have already been successfully
performed.
At the early stage of its market introduction it is the smallest radiation sensor, also compared to MOSFET and diode
technology and it appears to be the ideal choice for in vivo measurements in medical field or remote sensing.
We present results obtained from the first all-fiber, lensless, optical correlation spectroscopy gas sensor for acetylene
(C2H2). In the reported sensing configuration, hollow-core photonic bandgap fiber (PBGF) is employed to contain all gas
samples required for optical absorption measurements. This sensor relies upon comparison of the absorption spectrum of
acetylene held in a 'reference gas cell' to that of a gas sample under test, which is contained in the 'measurement gas
cell'. Ingress of the test gas mixture into the measurement cell is achieved via femtosecond laser-machined micro-channels
running from the surface of the PBGF to its hollow core. Stable, lensless optical interrogation of the
measurement cell is guaranteed by means of arc fusion splices to standard (solid-core) single-mode fiber (SMF). The
reference cell is filled with acetylene at atmospheric pressure, and is permanently sealed at both ends by splices to SMF.
Therefore, being constructed entirely from optical fiber, both the reference and measurement gas cells are inherently
compact and coilable, and dispense with the need for lenses or other free-space optics for connection to the correlation
spectroscopy system. We quantify the acetylene concentration of various test gas mixtures and compare our sensor's
measured results with computer simulations.
Gas detection and gas sensing based on hollow core photonic bandgap fiber (HC-PBF) is a very promising technique due
to the long interaction light-gas lengths that are achievable. However, long path-lengths also imply higher gas filling
times of the hollow fiber and higher response times of the detection systems what can constitute a serious practical
inconvenience. In this paper, the high sensitivity is maintained but the sensor response time is reduced by using multiple-coupling
fiber gaps. The results and conclusions extracted from a systematic experimental study (comparing the spectra
and filling time of different HC-PBF lengths and different number of coupling gaps) are presented and discussed.
Finally, the maximum number of gaps allowed in the system is modelled.
In recent years, hollow-core photonic bandgap fibers (HC-PBFs) have been demonstrated to be a promising technology
for gas sensing. In particular, the long interaction path lengths available with these fibers are especially advantageous for
the detection of weakly absorbing gases such as methane. In the near-infrared region, methane has the strongest
absorption band, 2ν3, at 1670 nm. However, HC-PBFs were not available until recently in this wavelength range and gas
sensing devices based on HC-PBFs were previously made in the weaker band of 1300 nm. In this paper, we report the
demonstration of a methane sensor based on a 1670-nm-band HC-PBF. A strong spectral feature, the R(6) manifold
(near 1645 nm), was selected for sensing purposes as it shows a good signal-to-noise ratio. This absorption line is
comprised of six energy transitions, strongly overlapped at our experimental conditions. For that reason, we applied a
multiline algorithm that used information from the six transitions to fit the manifold. The goodness of the fitting was
assessed measuring the concentration of different methane samples. With this method, a minimum detectivity of 10
ppmv for the system configuration was estimated.
We report the generation of white light comprising red, green, and blue spectral bands from a frequency-doubled
fiber laser in submicron-sized cores of microstructured holey fibers. Picosecond pulses of green light are launched
into a single suspended core of a silica holey fiber where energy is transferred by an efficient four-wave mixing
process into a red and blue sideband whose wavelengths are fixed by birefringent phase matching due to a slight
asymmetry of the structure arising during the fiber fabrication. Numerical models of the fiber structure and
of the nonlinear processes confirm our interpretation. Finally, we discuss power scaling and limitations of this
white light source.
In this paper we seek to assess the potential impact of microstructured fibres for security and defence applications. Recent literature has presented results on using microstructured fibre for delivery of high power, high quality radiation and also on the use of microstructured fibre for broadband source generation.
Whilst these two applications may appear contradictory to one another the inherent design flexibility of microstructured fibres allows fibres to be fabricated for the specific application requirements, either minimising (for delivery) or maximising (for broadband source generation) the nonlinear effects.
In platform based laser applications such as infrared counter measures, remote sensing and laser directed-energy weapons, a suitable delivery fibre providing high power, high quality light delivery would allow a laser to be sited remotely from the sensor/device head. This opens up the possibility of several sensor/device types sharing the same multi-functional laser, thus reducing the complexity and hence the cost of such systems.
For applications requiring broadband source characteristics, microstructured fibres can also offer advantages over conventional sources. By exploiting the nonlinear effects it is possible to realise a multifunctional source for applications such as active hyperspectral imaging, countermeasures, and biochemical sensing.
These recent results suggest enormous potential for these novel fibre types to influence the next generation of photonic systems for security and defence applications. However, it is important to establish where the fibres can offer the greatest advantages and what research still needs to be done to drive the technology towards real platform solutions.
In this paper we demonstrate how Holey Fibre (HF) technology can positively impact the field of materials processing and fabrication, specifically Direct Write (DW). DW is the large scale, patterned deposition of functional materials onto both flat and conformal surfaces. Currently, DW techniques involve thermal post-processing whereby the entire structure is enclosed inside an oven, so limiting the DW technique to small, heat resistant surfaces.
Selectively laser curing the ink would allow the ink to be brought up to the required temperature without heating the surrounding substrate material. In addition the ability to trim components would allow miniature circuits to be written and devices to be tuned by changing the capacitance or resistance. HF technology enables in-situ curing and trimming of direct write components using the same rig and length of fibre. HF's with mode areas in excess of 450μm2 can be routinely fabricated allowing high power transmission whilst retaining the high beam quality of the radiation source.
We will present results of curing and trimming trials which demonstrate that HF's provide a distinct advantage over standard multimode fibres by allowing both curing and machining to be achieved through a single delivery fibre.
Fiber delivery of intense laser radiation is important for a broad range of application sectors, from medicine through to industrial laser processing of materials, and offers many practical system benefits relative to free space solutions. In recent years, photonic crystal fiber technology has revolutionized the dynamic field of optical fibers, bringing with them a wide range of novel optical properties that make them ideally suited to power delivery with unparalleled control over the beam properties. The DTI funded project: Photonic Fibers for Industrial beam DELivery (PFIDEL), aims to develop novel fiber geometries for use as a delivery system for high power industrial lasers and to assess their potential in a range of "real" industrial applications. In this paper we review, from an industrial laser user perspective, the advantages of each of the fibers studied under PFIDEL. We present results of application demonstrations and discuss how these fibers can positively impact the field of industrial laser systems and processes, in particular for direct write and micromachining applications.
Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silica's transparency window, and in particular in the 3μm wavelength region.
Fiber delivery of intense laser radiation is important for a broad range of application sectors, from medicine through to industrial laser processing of materials, and offers many practical system design and usage benefits relative to free space solutions. Optical fibers for high power transmission applications need to offer low optical nonlinearity and high damage thresholds. Single-mode guidance is also often a fundamental requirement for the many applications in which good beam quality is critical. In recent years, microstructured fiber technology has revolutionized the dynamic field of optical fibers, bringing with them a wide range of novel optical properties. These fibers, in which the cladding region is peppered with many small air holes, are separated into two distinct categories, defined by the way in which they guide light: (1) index-guiding holey fibers (HFs), in which the core is solid and light is guided by a modified form of total internal reflection, and (2) photonic band-gap fibers (PBGFs) in which guidance in a hollow core can be achieved via photonic band-gap effects. Both of these microstructured fiber types offer attractive qualities for beam delivery applications. For example, using HF technology, large-mode-area, pure silica fibers with robust single-mode guidance over broad wavelength ranges can be routinely fabricated. In addition, the ability to guide light in an air-core within PBGFs presents obvious power handling advantages. In this paper we review the fundamentals and current status of high power, high brightness, beam delivery in HFs and PBGFs, and speculate as to future prospects.
The advantages of gallium lanthanum sulphide (GLS) based glass over other competing glasses for active and infrared applications are evident through its low-phonon energy, high rare-earth solubility, high transition temperature and non-toxicity. However this glass often devitrifies during fibre drawing due to a small separation between the crystallisation and fibre thawing temperatures. Improving GLS fabrication technology may hold the key to achieving practical optical waveguide devices. In this paper, we describe the cunent GLS research status, methods ofimproving glass purity and our directions toward alternatives to traditional fibre technology, in particular planar channel waveguides and holey or microstructured fibres.
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