Silica based air-core photonic crystal fibres for mid-IR
gas sensing applications
J. D. Shephard, N. Gayraud, W. N. MacPherson, R. R. J. Maier, J. D. C.
Jones, M. Mohebbi#, J. Stone#, A. K. George#, J. C. Knight# and
D. P. Hand.
Applied Optics and Photonics Group, Heriot-Watt University, Edinburgh EH14
4AS, United Kingdom.
#
Centre for Photonics and Photonic Materials, University of Bath, Claverton
Down, Bath, BA2 7AY, United Kingdom.
j.d.shephard@hw.ac.uk
Abstract. There is an urgent technological requirement for optical fibres that can
transmit laser energy in the infrared (IR) wavelength region (above 2 μm) driven by
demands of exciting new applications. Recently we demonstrated for the first time that
the practical wavelength range of silica fibres need not be limited by the intrinsic
material absorption, which paves the way to integrate silica fibre technology with IR
applications. We recently reported single-mode bandgap guidance in the region of 3.0
to 3.2 μm in a silica-based air-core photonic crystal fibre. The position of the bandgap,
and the fibre attenuation, is dependent on the micro-structure. The bandgap is shifted
due to changes in core size and distortion of the photonic cladding. By optimisation of
the fibre drawing parameters we have now produced a silica hollow core fibre with sub
1 dBm-1 attenuation in a wavelength region above 3.0 μm demonstrating that these
fibres can now be considered for practical applications in the mid-IR. Two fibres which
exhibit bandgaps at different positions, but both above the conventional cut-off for
standard silica fibres, are reported together with a demonstration of their application to
methane gas sensing.
1. Introduction
Recently, there has been significant progress in the development of laser sources that
operate in the mid-infrared (mid-IR) spectral region spanning the wavelength range from 3 to
5 μm. These advances promise to open this spectral window for a wide range of applications
in the near future. Naturally, development of these laser-based applications would be
facilitated if reliable, low-loss optical fibres were available for these wavelengths. To date the
most successful fibres for the mid-IR operation are based on novel optical materials, such as
chalcogenide glasses, which have been developed and demonstrated in both bulk and fibre
form [1]. However, these chalcogenide fibres can exhibit high toxicity, reduced chemical
durability and lower mechanical strength compared to standard silica fibres requiring
complex processing routes for purification and fibre drawing [1]. Unfortunately, in bulk silica
the material loss above 3 μm is greater than 60 dBm-1 [2] and would be prohibitively high in a
traditional single-mode fibre. Consequently, for all practical purposes the standard silica
fibres are unusable within this mid-IR wavelength range.
One application where mid-IR fibres would be particularly useful is in distributed gas
sensing. Many gases exhibit strong molecular absorption at these wavelengths, especially in
the wavelength range of 3 to 3.5 μm (e.g. CH4 has a strong absorption band around 3.3 μm).
Optical fibres offer a number of advantages relevant to gas sensing: remote access to
hazardous sites; they are non-electrical hence eliminating the risk of electrical spark
discharge (for combustible gas detection), and distributed sensing topologies become
feasible [3]. However, gas sensing based on conventional silica optical fibres are unable to
access the fundamental absorption vibrations of most gases as these lie outside the
transmission window of silica.
Hollow core (or air-core) photonic crystal fibres [4] (HC-PCF’s) are a new form of optical
fibre waveguide with unique properties, which are currently being investigated for a variety of
applications such as high power and ultrashort-pulse delivery [5-8] and gas sensing [9]. The
guided mode in these fibres is strongly confined within a hollow core, greatly reducing the
effect of the solid material on the fibre’s optical properties and liberating its performance from
the material constraints. For example, the damage thresholds of single-mode hollow-core
fibre are above those of their conventional solid-core counterparts [5,6]. In another possible
application area, light-gas interactions, hollow-core fibres provide an outstanding interaction
volume (the entire core) when compared to more traditional evanescent field configurations
[9,10]. It has been predicted that low-loss guidance in a hollow-core fibre is possible if the
fibre is formed from IR-transparent glasses [11,12]. The material losses of chalcogenide
glasses in the region around 3 μm are routinely less than 1 dB m-1, far lower than for silica
(see Fig. 1), providing careful purification techniques are employed. However, the precursor
materials are relatively expensive and toxic compared to silica, making cost and handling an
issue. As an alternative, we demonstrate in this paper that silica based hollow core photonic
crystal fibres could be useful for many mid-IR applications. Due to the low overlap of the
guided light with glass, which can be less than 1% [13], the effect of the relatively high
material attenuation of silica at these wavelengths is minimized. Initial trials demonstrated a
fibre giving a loss of 2.6 dB m-1 in the wavelength range 3.1 to 3.2 μm in an effectively singlemode fibre [14]. However, with improvements in fibre design we have achieved sub 1 dBm-1
losses above 3 μm. Finally, we demonstrate, using methane, that these fibres have the
potential for use in distributed gas sensors operating in the mid-IR.
2. Fabrication of air-core photonic crystal fibres
The HC-PCF was fabricated using the stack and draw technique in which thin walled silica
tubes (Suprasil F300, Heraeus) are drawn down to form capillaries and then stacked to form
a close packed array. Nineteen capillaries were omitted from the centre of the stack to form
the core and the whole stack was jacketed with a silica cladding and drawn down to the final
fibre dimensions. A scanning electron micrograph of two fibres is shown in Figure 2. The first
fibre has noticeable distortion around the hollow core (Figure 2 left) however, the second
fibre, although very similar to the first, shows slightly improved features (Figure 2 right). The
fibre core diameter in both cases is 40 μm and the overall outside diameter is 150 μm. The
nearest-neighbor hole spacing, or cladding structure pitch, is around 7 μm for the distorted
fibre (Figure 2 left) and around 6.4 μm for the improved fibre (Figure 2 right).
The use of a large (19-cell) core and the choice of a relatively thick core wall are the result
of a prior optimization procedure which reduced the overlap of the guided mode with the
silica [12]. Some distortion of the structure around the hollow core is apparent in the fibre
shown in Figure 2 left which had a loss of 2.6 dBm-1. We have shown (see section 3),
however, that the fibre performance improves with refinement of the fabrication parameters
to produce fibres with less distortion around the core (Figure 2 right).
Figure 2. Micrograph of the HC-PCF designed for operation in the mid-IR region. The
fibre on the left exhibited 2.6 dBm-1 loss at 3.2 μm whereas as the improved fibre on
the right (with slightly less distortion around the core) exhibited < 1 dBm-1 loss.
3. Infrared Guidance
All the characterization of the fibres was carried out using a Bentham TM300
Monochromator, with a 300 lines mm-1 grating (dispersion: 10 nm mm-1), a 22 mm
pyroelectric detector. A tungsten halogen bulb was used as a broadband light source. The
experimental set-up for spectroscopy measurements is shown in Fig. 3. The fibre output end
was positioned in the plane of the “wide open” input slit of the monochromator. The output
slit, set at 1.5 mm, determines the wavelength resolution of the scans since the input slit
width is essentially the fibre core diameter in this arrangement.
Figure 3. Experimental set-up for spectroscopy measurements
3.1. Bandgap Guidance
With the arrangement shown in Figure 3, using 2.7 m of the HC-PCF, a series of 20 scans
using a slit width of 1.5 mm (resolution 15 nm) and a step size of 2 nm were performed and
averaged. The wavelength range scanned was from 2.7 to 3.5 μm. The spectra shown in
Figure 4 clearly demonstrates a low-loss transmission window for both fibres with a peak
around 3.14 μm for the distorted fibre and around 3.08 μm for the improved fibre, both well
above the cut-off for transmission in a silica core fibre (considered to be around 2 μm for
practical purposes. The effect of the fibre structure on the bandgap position and shape is
clearly demonstrated with noticeable difference between the two fibres. The attenuation of a
standard silica fibre of this length and at this wavelength is expected to be greater than
180 dB, while the attenuation of the fundamental capillary mode should be far greater [15].
No additional transmission bands were observed outside the regions shown in Figure 4. The
structure within the low-loss window can be attributed to surface-mode anticrossings, as
observed in similar fibres at shorter wavelengths [16].
3.2. Loss measurements
The attenuation of both fibres at the peak transmission wavelength (3.14 μm for the distorted
fibre and 3.08 μm for the improved fibre) was determined by a cut-back technique: light from
the source was coupled into the fibre to obtain the maximum signal while the output end of
the fibre was placed in the monochromator input slit plane. A cut-back was performed on the
end part of the fibre, which was then realigned in the input slit plane. The intensity of the
signal was recorded before and after the cutback. The launch optics at the input end of the
fibre was not disturbed throughout the measurements.
An initial 3 m-long fibre was cut back to 1 m. For each length of fibre, a series of 5 scans
was performed along the bandgap and averaged. To quantify the repeatability associated
with removing the fibre from the V-groove to perform the cut-back and realigning it, 5 short
cleaves of 5 mm each were performed for each length. These results give an overall
attenuation for the distorted HC-PCF at 3.14 μm to be 2.6 ± 0.3 dBm-1 and for the improved
fibre at 3.08 m to be 0.875 ± 0.1 dBm-1. We believe that the improved fibre is first sub
1 dBm-1 attenuation to be reported for an all-silica based air-core photonic bandgap fibre
guiding in the mid-IR.
9
-1
2.6 dB m
m
8
-6
1.5x10
-6
1.0x10
-6
5.0x10
-7
0.88 dBm-1 =3080nm
 = 3.14 m
7
6
Signal (nA)
Signal (a.u.)
(a.u.)
2.0x10
5
4
3
2
1
0
2.90
2.95
3.00
3.05
3.10
3.15
Wavelength ((m)
Wavelength
3.20
3.25
0.0
2.7
2.8
2.9
3.0
3.1
3.2
3.3
3.4
3.5
Wavelength (m)
Figure 4. Average of 20 spectra (normalized against the lamp spectrum)
demonstrating location of the bandgap with a peak at 3.14 μm for the distorted fibre
(left) and a peak at 3.08 μm for the improved fibre.
3.3. Guided mode
The confinement of light inside the core was demonstrated using a knife-edge scanning
technique. Using the same setup as in Figure 3 and setting the wavelength at 3.14 μm for the
distorted fibre and 3.08 µm for the improved fibre, the output end face of the fibre was
scanned across the edge of the monochromator input slit. The data points were normalised
and represented in Figure 5. The signal increases to a maximum over a distance of 40 µm
equivalent to the core diameter of the fibre. The solid line in figure 5 represents a Gaussian
mode shape with an e-2 diameter of 15 µm corresponding to the structure of the fibres used
(figure 2). A similarity between the experimental data and the Gaussian shape is observed
for both fibres and suggests that the light is well confined inside the fibre core.
3.4. Bend loss
Other hollow-core fibre designs suffer from losses when the fibre is bent, which is frequently
a limitation on their applications. In contrast, silica hollow-core photonic bandgap fibres, like
those described here but fabricated for shorter wavelengths within the transparency window
of silica, have no discernable bend loss, even when the fibre is bent to just short of the
fracture point. In our experiments we have no measurable reduction in the transmitted signal
at a bend radius of 12.5 mm, which we discovered to be the fracture point. Smaller bend radii
were not possible partly because of the absence of a protective polymer coating.
Normalized Signal (at =3080nm)
Normalised signal (at =3140nm)
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
35
1.0
0.8
0.6
0.4
0.2
0.0
0
40
5
10
15
20
25
30
35
40
Distance across the fiber end face (m)
Distance translated by slit (mm)
Figure 5. Signal recorded as a knife edge is scanned across the output face of the
distorted fibre (left) and improved fibre (right). The zero position represents one edge
of the air core. The solid line shows a calculated Gaussian mode profile with an e-2
width of 15μm.
Normalised transmitted signal
4. Demonstration of Gas Sensing
The improved fibre (with sub 1 dBm-1 loss) was used as a demonstrator gas cell for sensing
of methane. The calculated absorption spectrum of methane does have a slight spectral
overlap with the long wavelength edge of the improved fibre bandgap in the 3.15-3.5 µm
region (see Figure 6 left) although this fibre is not optimized for measurement at the peak
methane absorption wavelength which occurs around 3.3 μm [17].
Intensity (A.U.)
2.0
1.5
1.0
0.5
0.0
2.8
2.9
3.0
3.1
3.2
3.3
Wavelength ( m)
3.4
3.5
1.0
Decrease of methane
concentration
0.8
0.6
0.4
t = 0h15
t = 1h45
t = 6h
0.2
0.0
3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40
Wavelength (m)
Figure 6. Overlap of methane spectral lines (HITRAN database [17]) and the fibre
bandgap (left) and Intensity of the signal recorded at different times during the
diffusion of methane (right).
The fibre was filled with the test gas (5% methane in nitrogen), cleaved at both ends and
inserted into the arrangement shown in Figure 3. The spectrometer was set to perform 25
nm-step size scans in the 3 to 3.4 µm region. A model of methane diffusion indicates that ~6
hours are necessary for the gas to completely diffuse out of a 1 m long fibre. Each scan
required 5 minutes and 80 scans were recorded in total. The monochromator output slit was
set to 3 mm (giving 30 nm optical resolution) in order to obtain sufficient optical signal,
however this had the detrimental effect of averaging the spectrum such that individual
methane absorption lines would not be resolved. Figure 6 (right) shows three normalised
transmission curves derived from averages of 5 scans at t = 0 h15, t = 1 h45 and t = 6 h. We
can easily see that the transmission spectrum changes as a function of time i.e. as the gas
concentration varies due to diffusion. The data in figure 6 has been normalised with respect
to the transmission spectrum when there is no methane present inside the fibre. The
transmitted power at ~3.15-3.3 μm increases as the gas diffuses out of the fibre, from the
initial ~5% concentration at t = 0 h, to ~0% at t = 6 h. The change in intensity due to gas
absorption is somewhat reduced due to the averaging effect of the spectrometer resolution future experiments are planned to enhance the resolution to achieve <0.1% concentration
resolution using a high brightness source.
5. Conclusions
We have described the demonstration, for the first time, of a silica based air-core photonic
crystal fibre with a sub dBm-1 loss transmission window in the mid-IR. The fibre presented
was then used for methane sensing around 3.2 µm. The spectral resolution of our current
measurement is insufficient to resolve individual gas absorption lines; however it was
sufficient to demonstrate the potential of this fibre for gas sensing. Although it is possible that
hollow core photonic crystal fibres fabricated from novel chalcogenide glass materials may in
the future surpass the losses achieved with silica based fibres, silica-based technology offers
some advantages in terms of cost, ease of fabrication and toxicity. This makes the silica
based HC-PCF’s ideal candidates for many mid-IR applications.
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