Arsenic chalcogenide filled photonic crystal fibers

Journal of Non-Crystalline Solids 377 (2013) 231–235

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Arsenic chalcogenide filled photonic crystal fibers R. Spittel ⁎, J. Kobelke, D. Hoh, F. Just, K. Schuster, A. Schwuchow, F. Jahn, J. Kirchhof, M. Jäger, H. Bartelt Institute of Photonic Technology Jena, Albert-Einstein-Str. 9, 07745 Jena, Germany

a r t i c l e

i n f o

Article history: Received 14 October 2012 Received in revised form 22 January 2013 Available online 28 February 2013 Keywords: Fiber optics; Photonic crystal fibers; Chalcogenide; Filling; Thermal stress

a b s t r a c t We prepared hybrid fibers by filling all-silica capillaries and photonic crystal fibers (PCFs) with arsenic chalcogenide glasses. The filling technology is described in detail and a theoretical model for the prediction of the filling length in combination with a simple approximation is presented. The prepared fiber samples are analyzed in terms of optical properties by measuring their transmission spectra which exhibit band-gap guidance and are compared with numerical simulations. Finally, we compare calculations and measurements of thermal stress in such fibers. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Chalcogenide glasses are interesting due to their extreme optical properties. They show a transmission window from VIS up to MIR range. Arsenic sulfide fibers are used for signal transmission in a wavelength range between 0.6 µm and 8 µm [1]. Arsenic sulfide single mode fibers with a measured minimum loss of about 40 dB/km (λ=2 µm) were described in [2]. The nonlinearity is about a factor of 260 higher than in silica [3]. The glass chemical properties of chalcogenides are strongly different from silica and other oxide glasses. The glass transition temperature (Tg) is low, whereas the thermal expansion is extremely high. At higher temperatures (>400 °C) chalcogenide glasses tend to evaporate due to thermal dissociation reactions [4]. Typically pure chalcogenide glass fibers show important disadvantages. Their tensile and bending strengths are low [5]. By combination with a mechanical resistant over-cladding (silica) this drawback can be overcome. It also allows the insertion of chalcogenide glasses into conventional silica PCF matrices for the preparation of all solid band gap fibers without inner surface corrosion effects [6–9]. The refractive index contrast is considerably higher than in silica-air band-gap fibers. Also, supercontinuum generation in chalcogenide-filled capillaries has been presented [10].

force Fc, gravity Fg and friction Ff, as well as the forces Fp resulting from the internal and external pressures pint and pext. F ¼ m⋅

d2 Lðt Þ ¼ Fc þ Fg þ Ff þ Fp dt 2

ð1Þ

The capillary force is given by F c ¼ 2πaσ cos θ;

ð2Þ

where a is the capillary radius, σ is the surface tension and θ is the contact angle which we measured to be 95°. The literature values of the surface tension are 0.059 to 0.168 N/m for arsenic sulfide in Melnichenko, 2009. The mass of the liquid column causes a gravitational force of 2

F g ¼ −πa ρgðL0 þ Lðt ÞÞ;

ð3Þ

where g = 9.81 m/s2, ρ is the density (ρAs2 S3 ¼ 3204 kg=m3 [12]) and L0 is the filling length at t = 0. Because of the small hole radii in our investigations, which are in the order of 1 µm and the quite high viscosity of 1 … 10 Pas, a laminar flow can be assumed. With η being the viscosity of the liquid, the friction force is then given by

2. Theory In order to calculate the filling length L(t) of a liquid in a capillary, a force equilibrium model is applied [11]. As illustrated in Fig. 1A, the sum of the forces in the capillary is given as an interaction of capillary ⁎ Corresponding author. Tel.: +49 3641 206326. E-mail address: [email protected] (R. Spittel). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.01.043

F f ¼ −8πηðL0 þ Lðt ÞÞ

d ðL þ Lðt ÞÞ: dt 0

ð4Þ

The last force in the equation corresponds to the pressure difference between the outside and the inside of the capillary. The external pressure is assumed to be constant. The internal pressure will be time

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A

B filling length [mm]

pint(t)

L(t)

4. Fiber preparation

70 60 50 40 30 20 10 0 0

Fc Fg Ff pext

20

40

60

80

100

120

Fig. 1. A) Resulting forces inside the capillary. B) Calculated filling length over time for filling of As2S3 into a capillary of radius a = 1 µm at different external pressurizations.

dependent since the PCF holes are blocked at the upper side of the fiber. By introducing the capillary length Lc and assuming an ideal gas, the force due to the pressure difference can be expressed as 2

F p ¼ πa Δp ¼ pext −pint ð0Þ

1 : ðt Þ 1−L L−L c

ð5Þ

0

Eq. (5) already implies that there has to be a maximum filling length Lmax b Lc − L0 because otherwise the internal pressure would result in a singularity. Inserting Eqs. (2)–(5) into (1) and substituting y = (L0 + L(t)) 2 leads to the nonlinear ordinary differential equation pffiffiffi 2p ð0Þ 1−L0 =LC d2 y 8η dy 4σcosΘ 2pext þ 2g y þ int þ þ 2gL0 ; þ ¼ pffiffiffi ρ ρa 1− y=LC ρ dt 2 a2 ρ dt

The simulations in Section 2 point out the necessity for a viscosity below 10 Pas. In order to achieve this condition, the chalcogenide glass is heated up to a temperature of 450 °C using a tube furnace. Fig. 2 shows the experimental setup. It consists of a silica pressure cell which is positioned inside the tube furnace. A small amount of the chalcogenide glass is placed in a silica crucible at the bottom of the pressure cell and the PCF (sealed at its top) is put onto this reservoir. In the next step a vacuum is applied for several minutes and the process volume is purged with high-purity argon in order to avoid atmospheric contaminations. To start the actual filling procedure, a pressure of 100–150 bar is applied for a few minutes up to some hours, depending on the diameter of the fiber holes. For example, according to Eq. (7), 5 min is needed to fill 50 mm of the capillary shown in Fig. 4A for Δp = 120 bar and T = 450 ∘C, while almost 4 h is needed to fill the same length of the PCF shown in Fig. 4B using the same parameters. In order to prove the significance of the evacuation procedure Fig. 3A shows an arsenic sulfide filled PCF which was prepared without evacuation and argon-purging. A large number of inclusions is visible, which are attributed to reaction products (e.g. SO2) between the arsenic sulfide and oxygen. Fig. 3B shows a filled capillary fiber (d = 1 µm) after a filling process which included multiple passes of evacuation and subsequent purging with argon. No inclusions are present due to the inert argon atmosphere at the chalcogenide glass reservoir as well as inside the fiber holes. Filling lengths of several centimeters at hole diameters of about 1 µm have been achieved. Single capillary fibers with diameters from 80 µm down to 1 µm, as well as photonic crystal fibers with

evacuation

Argon

ð6Þ which can be solved numerically. However, for practical purposes it is convenient to have an approximate solution of Eq. (6) in order to calculate the time which is needed to fill a certain length of fiber. For micron-sized hole diameters the gravity can be neglected. By assuming L0 = 0 and Lc ≫ L(t) it is possible to find a very simple equation for the estimation of the filling time: 2

t fill ≈

PCF furnace

furnace

2

4η Lfill 4πηLfill ¼ ; 2σacosθ þ Δp a2 F c þ F p

ð7Þ

which is equal to the approximation found in [8]. Using the material data of arsenic sulfide, pressurizations greater than 100 bar and hole radii smaller than 5 µm, the deviation between (7) and the exact solution (6) is below1%. Fig. 1B shows the results of arsenic sulfide filling simulation over a time of 2 h using the typical experimental parameters a = 1 µm, L0 = 0, Lc = 400 mm, Δp = 100 bar and a viscosity of 6 Pas at T = 450 ∘C. 3. Chalcogenide glass preparation and thermo-chemical behavior The arsenic sulfide was prepared via melting technique in silica ampoules at 900 °C in scale of 25 g from the elements. The purity of arsenic was 99.99999%, and the purity of sulfur was 99.999% [13]. The thermal expansion coefficient is 21⋅ 10−6 K −1, about 40 times higher than silica. A further aspect of the filling process is the evaporation of chalcogenide glass components at high filling temperature. The solidification at high cooling rates or unfavorable pressures can result in the formation of voids inside the chalcogenide, which also was reported by [8,9,14]. The vaporization of arsenic sulfide is discussed in [15]. The filling of capillaries with micrometer-scaled diameters at feasible duration requires a viscosity of about three orders of magnitude lower than typical fiber drawing.

chalcogenide glass Fig. 2. Experimental setup for filling microstructured fibers with soft glasses.

R. Spittel et al. / Journal of Non-Crystalline Solids 377 (2013) 231–235

Fig. 3. Microscope images with lateral view on (A) an As2S3-filled MOF (d= 7.5 µm) with gas inclusions and (B) a homogeneous As2S3-filled capillary fiber (d = 1 μm).

90 holes and a hole diameter of 1.2 µm were prepared. The scanning electron microscope (SEM) image in Fig. 4A shows a capillary with an inner diameter of 8 µm filled with As2S3. Unlike the silica end face, the surface of the cleaved arsenic sulfide shows a spontaneous broken shape. This is a result of the mechanical separation of both glasses. The propagation of the glass fracture front is interrupted between silica and chalcogenide glass. Fig. 4B–C shows a prepared micro-structured optical fiber (Λ = 4.4 µm, d = 1.2 µm) with a hexagonal arrangement of arsenic sulfide rods in a silica matrix. Only a few filled holes still exhibit small inclusions which have been exposed during cleaving (see Fig. 4D). 5. Transmission spectrum In order to characterize the prepared fiber samples, their transmission spectrum was measured using a commercial supercontinuum

233

source. The near field of the fiber sample was imaged onto an iris aperture to block the cladding light and the remaining core light was focused into a detector fiber and analyzed with an optical spectrum analyzer. For the experiments the PCF shown in Fig. 4B–D with a hole diameter of d = 1.2 µm was used. The fiber cavities have been filled with arsenic sulfide over a length of a few centimeters. Fig. 5 shows the transmission spectrum of a 10 mm long PCF filled with As2S3. The measurement (Fig. 5, top) exhibits the formation of photonic band gaps, which can be explained by antiresonant scattering at the As2S3-cylinders [16,17]. The resonant wavelengths which correspond to the maximum propagation loss of the core mode can be predicted using [18]

λm ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2d n2As2 S3 −n2SiO2 m þ 1=2

:

ð8Þ

Furthermore, a numerical simulation using a finite element method was implemented in order to obtain the propagation loss of the fiber which is shown in the bottom in Fig. 5. The refractive indices of arsenic sulfide and silica were taken from literature [19,20]. Both, experiment and simulations show a good agreement. The regions of high transmission in the experiment correspond to the low loss regions in the simulations. The bandgaps around 550 nm and 650 nm have a higher attenuation and cannot be seen in the transmission spectrum due to the high overall loss of the fiber sample. A possible reason for the increased loss could be the inclusions (see Fig. 4D) which appear randomly in size and location along the filling length.

Fig. 4. Scanning electron microscope (SEM) images of the end face of (A) a As2S3-filled capillary fiber with a hole diameter of d=8 µm and (B–D) a photonic crystal fiber (Λ=4.4 µm, d=1.2 µm) filled with As2S3.

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Transmission [dB]

450 -10

500 11

550 10

600

9

650

8

700

750

7

800

6

-20 -30 -40 -50

Attenuation [dB/m]

103 102 101 100 10

Fig. 6 shows that all 3 investigated samples have negligible differences in the stress in the cladding region, which means that no thermal stress appears by filling it with the chalcogenide glass. This implies that there is no chemical bonding between the two glasses, which can also be emphasized by the fact, that a protruding arsenic sulfide core over a distance of 1 mm could be observed for a filled capillary fiber with an inner diameter of 50 µm after cleaving and stripping off the silica cladding. Considering the different expansion coefficients, a small air gap with a maximum width of approximately 0.4% of the hole diameter is expected, i.e. 5 nm for d = 1.2 µm or 27 nm for d = 8 µm. However, we believe that these air gaps do not have a significant impact on the propagation properties or scattering losses if they are much smaller than the wavelength. 7. Conclusions

-1

10-2 10-3 450

500

550

600

650

700

750

800

wavelength [nm] Fig. 5. Top: Normalized transmission spectrum of an arsenic sulfide filled PCF. The vertical red lines show the predicted resonance wavelengths of different orders (m=6–11) which correspond to maximum propagation losses. Bottom: Calculated propagation losses using a finite element method.

We successfully demonstrated the fabrication of photonic crystal fibers filled with chalcogenide glass, using the example of arsenic sulfide. A simple equation for the estimation of the filling time was derived from the exact semi-analytical model. The measured transmission spectrum of the prepared fibers exhibited small and steep band gaps, which could be applicable as spectral filters. A thermal stress analysis proved that there is no adhesion between the chalcogenide and silica. Acknowledgements

6. Thermal stress The large difference of the thermal expansion coefficient between silica (0.5⋅ 10−6 K −1) and arsenic sulfide (21 ⋅ 10−6K −1) gives reason to expect large thermal stresses inside these fibers. In order to investigate the intrinsic stress, a measurement setup as described in [21] is used. For the stress analysis, a silica capillary was filled (inner diameter: 80 µm, outer diameter: 500 µm) with arsenic sulfide and compared with a reference measurement on a fresh untreated and a tempered untreated capillary. Fig. 6 shows the axial component of the stress tensor. The dark blue dashed line corresponds to the calculated stress distribution. It shows a compressive stress of − 16 MPa in the cladding region and suggests a large balancing tensile stress of a few hundred MPa in the core. However, due to the large index step between the chalcogenide core and the fused silica cladding (Δn >1.0), it was not possible to realize a clean image of the measurement beam.

10

axial stress [MPa]

5 0

As2S3

-5 -10

theory

silica

silica

-15 -20

SiO2 capillary

-25

As2S3-filled SiO2 capillary

-30 -0.3

tempered SiO2 capillary

-0.2

-0.1

0.0

0.1

0.2

0.3

radius [mm] Fig. 6. Axial stress distribution in an As2S3 filled silica capillary. Positive values correspond to tensile stress, negative values to compressive stress. The blue line represents simulation results.

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