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Spun elliptically birefringent photonic crystal fibre for current sensing
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2007 Meas. Sci. Technol. 18 3070 (http://iopscience.iop.org/0957-0233/18/10/S04) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
MEASUREMENT SCIENCE AND TECHNOLOGY
doi:10.1088/0957-0233/18/10/S04
Meas. Sci. Technol. 18 (2007) 3070–3074
Spun elliptically birefringent photonic crystal fibre for current sensing Andrew Michie1, John Canning1, Ian Bassett1, John Haywood1, Katja Digweed1, Brian Ashton1, Michael Stevenson1, Justin Digweed1, Alfred Lau2 and Daniel Scandurra2 1
Optical Fibre Technology Centre (OFTC), University of Sydney, 206, National Innovation Centre ATP, Eveleigh, Sydney, Australia 2 Department of Physics, Macquarie University, Sydney, NSW 2109, Australia E-mail:
[email protected]
Received 3 January 2007, in final form 30 April 2007 Published 12 September 2007 Online at stacks.iop.org/MST/18/3070 Abstract Spun elliptically birefringent fibre has been fabricated by spinning the preform of a highly linearly birefringent photonic crystal fibre (PCF) during the drawing process. The resulting spun highly birefringent (SHi-Bi) PCF offers sensitivity to magnetic fields for current measurements with greatly reduced temperature dependence in comparison with conventional spun stress birefringence fibres. The ellipticity of the birefringence has been measured and temperature independence has been demonstrated. Keywords: fibre design and fabrication, polarization maintaining fibre, photonic crystal fibre, fibre optics sensors
1. Introduction Since the first PCF was reported, PCFs have been expected to take on major importance in shaping the future of optical fibre sensing technology [1–3]. PCFs utilize an internal structure that offers alternative guidance mechanisms, such as bandgap structures, a single material construction and the possibility of optical interaction with the contents of the holes. This single material construction potentially removes any residual thermal stresses in the fibre due to differential thermal expansion coefficients during the drawing process. This internal structure can also be designed to produce significant form birefringence and it has been demonstrated that these fibres have negligible temperature dependence over extended temperature ranges [4, 5]. Electric current sensing using spun highly birefringent (HiBi) optical fibres was studied extensively in the late 1980s. Linear birefringence in the sensing fibre quenches the Faradayinduced rotation, effectively extinguishing the sensitivity to magnetic fields. Spinning a HiBi fibre preform during the fibre draw process adds circular birefringence to a structure that would otherwise have very large linear birefringence [6, 7]. The resulting fibre has elliptical birefringence that is both sensitive to magnetic fields and is strongly polarization maintaining. This fibre allows for flexible and ruggedized 0957-0233/07/103070+05$30.00
© 2007 IOP Publishing Ltd
packaging through being largely insensitive to bending birefringence and other environmental perturbations. Conventional stress-induced hi-birefringence optical fibres and their spun counterparts suffer from significant changes in linear birefringence with temperature. Overcoming this problem is seen by many as the single largest impediment to significant commercial uptake of novel devices such as low cost optical fibre gyroscopes and current sensors, for example. The active stabilization of temperature and other environmental perturbations pushes the cost of these devices to prohibitive levels. The clearest solution to this problem is the development of low cost temperature insensitive optical fibres. Fibre optic gyroscopes (FOG) have been demonstrated using bandgap fibres, where the light is guided in a hollow core region surrounded by a periodic structure made up of the host material and rings of holes [8]. These fibres offer reduced temperature dependence, and low nonlinearity, since the light is guided mostly in the air core region. Although these fibres largely remove the temperature dependence in the effective refractive index of the material the temperature performance was limited by changes in the physical path length due to the thermal expansion of the host material. These fibres are generally far more expensive to make than solid photonic crystal fibres and have another set of technical
Printed in the UK
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Spun elliptically birefringent photonic crystal fibre for current sensing
challenges (including splicing and coupling) that have yet to be dealt with. Ideally, the low loss properties of conventional effective index guiding (commonly known as modified total internal reflection guiding) photonic crystal fibres combined with low temperature dependence are desired. Loss in PCF fibres has become more than acceptable with improvements in fabrication techniques, and PCFs are now beginning to challenge the more conventional step index fibres with loss as low as 0.37 dB km−1 at 1550 nm [9]. To this end we have recently reported temperature insensitivity of a hi-birefringence photonic crystal optical fibre over the range −25 ◦ C to 800 ◦ C and others have reported similar temperature sensitivities over a much narrower temperature range [4, 5]. The beat length of HiBi PCF at 1550 nm has been made as low as 0.27 mm, an order of magnitude shorter than conventional bow tie fibres (typically >2 mm at 1550 nm). The next major step in this technology is the development of a spun version of this optical fibre: that is a spun hi-birefringence (SHi-Bi) photonic crystal fibre. Spinning PCF preforms during the drawing process will produce a permanent twist that is frozen into the fibre. The longitudinal array of holes will now be effectively arranged helically along the length of the fibre. This helical structure will induce circular birefringence and when combined with the large inherent linear birefringence will give a resulting elliptical birefringence. Elliptically birefringent fibres are very useful for current sensing where Faraday rotation is used to monitor electric current [6, 7]. When using this fibre for current sensing, only the temperature dependence of the Verdet constant remains. For our SHi-Bi photonic crystal fibre the initial preform was fabricated by conventional capillary stacking method. Three rings are employed. A proprietary method for achieving an elliptical core whilst maintaining a more or less regular lattice around the fibre was used to create a preform with the desired aspect ratios, similar to that reported in [4]. In order to spin the optical fibre, a custom preform spinning unit capable 4000 revs min−1 was designed and installed into a commercial grade draw tower. The preform was first drawn into an unspun optical cane which was then inserted into a second tube for a second stage fibre draw to reduce the final fibre core diameter from ∼10 µm to ∼3 × 2 µm fibre whilst spinning at ∼1300 revs min−1 and drawing at 10 m s−1. This generates a spin pitch ∼7.7 mm. Figure 1 shows the final fibre cross section achieved. Current sensitivity increases with spin rate and the 7.7 mm spin-pitch was chosen as a compromise between previously determined optimum fibre draw and preform rotation rates that would provide a good yield.
2. Measuring elliptical birefringence 2.1. Background The SHiBi PCF’s elliptical birefringence has been studied using the Poincar´e sphere representation [10, 11]. This
Figure 1. SHi-Bi photonic crystal optical fibre cross-section. (This figure is in colour only in the electronic version)
P+
ρ θ
τ
η
P-
Figure 2. Representation of polarization states and birefringence on the Poincar´e sphere.
method allowed for both the elliptical birefringence of the spun structure and the local linear birefringence of the equivalent unspun structure to be determined using a simple and easy to implement experimental method. Figure 2 shows elliptical polarization eigenmodes P+ and P− that are the preserved polarization states (relative to local axes, which keep up with the spin) of the elliptically birefringent SHiBi PCF. The elliptical birefringence can be represented by the vector ρ passing through these preserved states in Stokes space. The length of the vector ρ represents the magnitude of the phase shift per metre induced by the birefringence. Similarly τ represents the spin-induced circular birefringence, and η represents the local linear birefringence of the equivalent unspun fibre. For convenience, the vector ρ has been assigned unit length. The terms η, τ and ρ all have the units radians per metre (rad m−1). The magnitude of these phase shift terms can be expressed as |η| = 2π/LB
|τ | = 4π/LT
|ρ| = 2π/LB
(1)
where LB is the familiar beatlength of the linear birefringence term, LT is the spin pitch and LB is the elliptical beatlength in metres [7, 8]. In Stokes space, a 2π rotation of any polarization state is equivalent to a π rotation in the laboratory frame. As a result spinning the fibre preform through 2π radians is equivalent to a 4π rotation in Stokes space. Hence the factor 4π that appears in equation (1) for the term |τ | = 4π/LT [6, 11]. 3071
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2.2. Current sensitivity The sensitivity to current is related to the effective Verdet constant Vs of the elliptically birefringent SHiBi PCF and Vs can be expressed as Vs = V sin θ
(2)
where V is the Verdet constant of straight non-birefringent fibre. From equation (2) it can be seen that fibre with only circular birefringence, where θ = π/2, has the same sensitivity as straight non-birefringent fibre [11]. 2.3. Measurement approach A modified version of the crossed polarizer method was used to analyse the elliptical birefringent SHiBi PCF [4]. Broadband light from an erbium doped fibre amplified spontaneous emission (EDF-ASE) light source was passed through a fibre pigtailed polarizing beam splitter (PBS) spliced to a section of SHiBi PCF. The fibre end was rotated and aligned to excite roughly equal amounts of power into each elliptical polarization mode of the SHiBi PCF. The light emerging from the SHiBi PCF end was then analysed with a linear polarizer and the wavelength dependent modulation, or interferogram, was observed. In the case of linearly birefringent fibre the output spectrum from a single linear polarization state can be observed by simply rotating the polarizer at the output until the modulation is completely extinguished. The orthogonal state can then be observed simply by rotating the output polarizer through ±90◦ . For the SHiBi PCF, the modulation could not be completely extinguished by rotating the output polarizer due to elliptical birefringence. With the linear output polarizer aligned with one of the preserved polarization states of the linear birefringence vector η (i.e. with the minor axis of the ellipse of the elliptical polarization state), the modulation was minimized. The addition of a Babinet–Soleil phase (BSP) compensator between the output end of the SHiBi and the linear polarizer allows for some retardance to be added effectively turning the linear output polarizer into an elliptical polarizer. Alignment and adjustment of the BSP allowed the output modulation to be completely extinguished. The BSP was then calibrated at the operating wavelength to determine the retardance and the ellipticity (θ ) of the birefringence was then calculated.
However, to determine the modal birefringence from the interferogram the wavelength dependence of the birefringence must be known. For HiBi PCF a power law dependence has been widely used and requires measuring the interferogram over a very broad wavelength range [4]. In the case of SHiBi, figure 2 shows that if the spininduced circular birefringence and the ellipticity are known then either the elliptical birefringence of the SHiBi or the local linear birefringence of the equivalent unspun HiBi PCF can be calculated using simple trigonometry. As a result this method allows the modal birefringence of the SHiBi PCF to be determined without the need for characterization over an extended wavelength range. The spin-induced circular birefringence τ can be calculated using equation (1). The elliptical modal birefringence ρ can then be calculated once the ellipticity θ has been determined using the following simple trigonometric relationship: ρ = τ / sin θ = 4π/LT sin θ.
(5)
Similarly the value of η = 2π/LB can be calculated using
η = τ cot θ.
(6)
2.5. Temperature dependence measurements The group birefringence, Bg, was calculated using equation (4) from the recorded interferograms obtained using the crossed polarizer method configured in reflection. The EDF-ASE source provided enough power and the fibre was sufficiently low in loss that the Fresnel reflection from the cleaved end of the SHiBi-PCF produced a clear interferogram at the output. The entire SHiBi-PCF was subsequently placed in an environmental chamber and the group birefringence was recorded from −25 ◦ C to +100 ◦ C. Below 20 ◦ C condensation on the cleaved fibre end reduced the Fresnel back reflection and increased the experimental noise. The modal birefringence was not calculated at each temperature and only the group birefringence versus temperature is presented. Wavelength dependence in the modal birefringence, arising mostly through polarization dependent waveguide dispersion, can result in group birefringence that is sometimes substantially different from the phase birefringence. Since the air silica structure is very stable with temperature it is assumed that the relationship between the group and phase birefringence in this fibre is also largely temperature independent.
2.4. Birefringence measurement
3. Results and discussion
The modal (or phase) birefringence Bm is defined as [4] Bm = |nx − ny |,
(3)
where nx and ny are the effective refractive indices for each polarization mode. The birefringence is often presented in the form of a beat length given by LB = λ/Bm [4], where λ is the free space wavelength. The group birefringence, Bg, can be determined by measuring the period of the modulation seen in the output spectrum and using the following relationship [4] λdBm(λ) λ2 |Bg | = − Bm(λ) = (4) dλ 2Lλ where L is the length of the SHiBi PCF being tested and λ is the period of the spectral modulation seen in the output. 3072
A 630 mm length of SHiBi PCF was tested using the experimental set-up shown in figure 3 and described above. The periodic modulation seen in the output spectrum (figure 4) was recorded using an optical spectrum analyser and the period and depth of the modulation were determined. The linear polarizer at the output was then rotated and the modulation depth minimized but not fully extinguished. The BSP compensator was initially set to zero retardance (Ri = 0◦ ) and was arbitrarily orientated relative to the linear polarizer. The BSP was then re-orientated so that its fast and slow axes were aligned at 45◦ to the linear polarizer. The retardance was then adjusted to fully extinguish the periodic modulation.
Spun elliptically birefringent photonic crystal fibre for current sensing 45o splice
Polarizer
BSP
ASE Source
OSA PBS
DUT SHiBi PCF Rotating mounts
Figure 3. Experimental set-up for measuring the ellipticity of the birefringence.
10
-32
8
Percentage change (%)
-30
Tx power (dBm)
-34 -36 -38 -40 -42 -44
Crossed polarizer
-46
Aligned polarizer
-48 -50 1525
1540
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Spun HiBi PCF
6
Panda fibre
4 2 0 -2 -4 -6 -8
1585
1600
-30
-10 -10
Wavelength (nm)
10
30
50
70
90
110
Temperature (deg C)
Figure 4. Interferograms recorded both when the periodic modulation is extinguished and also when it is maximized.
Figure 5. Group birefringence versus temperature for both a conventional stress birefringence Panda fibre and the SHiBi-PCF.
The BSP was then calibrated by continuing to add retardance until the periodic modulation was again extinguished (Ri = θ ). The difference in retardance indicated by the BSP settings R1 and R2 for these two positions corresponds to π rad in Stokes space. From the ratios of the two, the ellipticity of the SHiBi birefringence reduces to θ = [R1/(R2 − R1)] π rad. For this section of SHiBi an ellipticity of approx θ = (0.23 ± 0.02) rad (or approx 13.2◦ ) was observed. Using equation (2), the SHiBi PCF would retain approx (22 ± 4)% of the sensitivity to current of an entirely circularly birefringent fibre. Using this measured value for θ = (0.23 ± 0.02) rad and the spin pitch of Lp = 7.7 mm, we can estimate the phase shift per unit length due to the effective modal birefringence of the SHiBi PCF using equations (1) and (5). The twist rate, τ ∼ 1631 rad m−1 gives a phase shift per unit length between elliptical modes P+ and P− of |ρ| = 7159 rad m−1. This corresponds to an elliptical beatlength of (0.88 ± 0.06) mm at 1550 nm. For comparison the elliptical beatlength of LB = 0.83 mm was calculated from the periodic modulation using the methods described by Michie et al [4]. This represents good agreement between the two different methods for determining the modal birefringence of the SHiBi PCF and is supportive of the analysis presented above. The same section of SHiBi PCF was then configured in reflection and placed inside the environmental chamber. The group birefringence was recorded every 10◦ between −25 and 100 ◦ C. The change in group birefringence relative to the value recorded at room temperature is presented in figure 5 along with results for a HiBi Panda fibre. The increased temperature sensitivity for the Panda fibre is clearly evident in the graph, and no change in group birefringence within the experimental error range is observed for the SHiBi PCF. In the case of Spun Panda fibre the ellipticity, and the corresponding sensitivity to
current, is then affected by changes in temperature. Although the spin pitch remains relatively constant the Panda fibre’s birefringence changes significantly with temperature resulting in large changes in ellipticity and sensitivity to current. The use of SHiBi PCF fibre removes this problem since the ellipticity is now largely temperature independent. Although there remains a small temperature dependence in the basic material Verdet constant, of the order of 70 ppm ◦ C−1 [12], for silica based fibres this effect is still an order of magnitude smaller than the high temperature dependence seen in spun Stress HiBi fibres.
4. Conclusion The first spun highly birefringent photonic crystal optical fibre has been characterized for ellipticity and temperature dependence. This fibre offers a stable effective Verdet constant with low temperature dependence and low packaging sensitivity. In addition what can be observed is that in contrast to bandgap fibres a much lower number of rings are used allowing a significant amount of silica in the cladding. This in turn permits conventional splicing and coupling techniques to be employed in the practical packaging of such fibres. The use of the Poincar´e sphere to represent polarization and elliptical birefringence allows for an alternative method that is simple to implement for determining the modal birefringence of the SHiBi-PCF structure that does not require measurements over a very large wavelength range. Further refinements in design will potentially provide broad band polarising properties [13] that will allow SHiBi to be used effectively for current sensing and other variations will undoubtedly find application in established areas such as fibre optic gyroscopes. Additionally, these fibres may permit a level 3073
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of temperature-independent control of polarization that may be of benefit for new areas such as quantum communications based on polarization degeneracy between entangled light paths.
[6] [7]
Acknowledgment [8]
A Michie acknowledges the financial support of ABB and Transgrid. [9]
References [1] Birks T A, Knight J C and Russell P S J 1997 Endlessly single-mode photonic crystal fiber Opt. Lett. 22 961–3 [2] Jones J D C and Russell P St J 2002 Photonic crystal fibers for sensor applications Invited Proc. 15th Int. Conf. on Optical Fiber Sensors (Portland OR, 6–10 May 2002) IEEE Catalogue Number 02EX533 pp 167–70 [3] Bjarklev A, Broeng J and Bjarklev A S 2003 Photonic Crystal Fibres (Dordrecht: Kluwer) ˚ [4] Michie A, Canning J, Lyytik¨ainen K, Aslund M and Digweed J 2004 Temperature independent highly birefringent photonic crystal fibre Opt. Exp. 12 5160–5 [5] Ortigosa-Blanche A, Diez A, Delgado-Pinar M, Cruz J L and Andres M V 2004 Temperature independence of
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[12]
[13]
birefringence and group velocity dispersion in photonic crystal fibe Electron. Lett. 40 1327–9 Laming R I and Payne D N 1989 Electric current sensors employing spun highly birefringent optical fibers J. Lightwave Technol. 5 2084–94 Bassett I M 1988 Design principle for a circularly birefringent optical fiber Opt. Lett. 13 844–6 Kim H K, Dangui V, Diggonnet M and Kino G 2005 Fiber optic gyroscope using an air-core photonic bandgap fibre Optical Fiber Sensors (OFS 2005) (Bruges, Belgium) Proc. SPIE 5855 198–201 Tajima K, Zhou J, Nakajima K and Sato K 2004 Ultralow loss and long length photonic crystal fiber J. Lightwave Technol. 22 7–10 Udd E 1991 Poincar´e sphere Fiber Optic Sensors (Wiley Series in Pure & Applied Optics) (New York: Wiley) pp 191, 375 Bassett I M, Clarke I and Haywood J 1994 Polarization dependence in a Sagnac loop optical fibre current sensor employing a 3 × 3 optical coupler Optical Fibre Sensors OFS-10 (Glasgow, Scotland) Williams P A, Rose A H, Day G W, Milner T E and Deeter M N 1991 Temperature dependence of the Verdet constant in several diamagnetic glasses Appl. Opt. 30 1176–8 Nasilowski T et al 2005 Temperature and pressure sensitivities of the highly birefringent photonic crystal fiber with core asymmetry Appl. Phys. B 81 325–31
