OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 4 — Feb. 19, 2007
  • pp: 1811–1816
« Show journal navigation

Spun elliptically birefringent photonic crystal fibre

Andrew Michie, John Canning, Ian Bassett, John Haywood, Katja Digweed, Mattias Åslund, Brian Ashton, Michael Stevenson, Justin Digweed, Alfred Lau, and Daniel Scandurra  »View Author Affiliations


Optics Express, Vol. 15, Issue 4, pp. 1811-1816 (2007)
http://dx.doi.org/10.1364/OE.15.001811


View Full Text Article

Acrobat PDF (160 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

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 intrinsic sensitivity to magnetic fields through the Faraday effect without the high inherent temperature sensitivities suffered by conventional spun stress birefringence fibres. The ellipticity of the birefringence has been measured and temperature independence has been demonstrated.

© 2007 Optical Society of America

1. Introduction

Electric current sensing using Spun Highly Birefringent (SHiBi) optical fibres is an idea that has been around since the late 1980s [3

3. R.I. Laming and D.N. Payne, “Electric current sensors employing spun highly birefringent optical fibers,” J. Lightwave Technol. 5,2084–2094, (1989). [CrossRef]

]. Linear birefringence in the sensing fibre quenches the Faraday-induced rotation effectively reducing 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 [3

3. R.I. Laming and D.N. Payne, “Electric current sensors employing spun highly birefringent optical fibers,” J. Lightwave Technol. 5,2084–2094, (1989). [CrossRef]

,4

4. I.M. Bassett, “Design principle for a circularly birefringent optical fiber,” Opt. Lett. 13,844–846, (1988). [CrossRef] [PubMed]

]. The resulting elliptical birefringence partially restores the sensitivity to magnetic fields and retains the desirable polarisation maintaining properties normally associated with HiBi fibres.

Fig. 1. Spun Highly Birefringent Photonic Crystal Fibre (SHiBi-PCF) cross section.

2. Measuring elliptical birefringence

2.1 Background

To understand the effect of spinning the fibre preform on the birefringence of the fibre it is convenient to use the Poincaré sphere. Figure 2 shows elliptical polarisation eigenmodes P+ and P- that are the preserved polarisation 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 ρ̄ passing through these preserved states in Poincaré space. The length of the vector ρ̄ represents the magnitude of the phase shift per metre induced by the birefringence. Similarly τ̄ represents the spin induced circular birefringence and η̄ represents the local linear birefringence of the equivalent unspun fibre. For convenience the vector ρ̄ has been assigned unit length. The terms η̄, τ̄ and ρ̄ all have the units radians per metre (rad/m). The magnitude of these phase shift terms can be expressed as

η¯=2πLBτ¯=4πLTρ¯=2πLB
(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

7. Eric Udd, “Poincaré sphere,” in Fiber optic sensors, Wiley series in pure & app optics,191,375, (1991).

,8

8. I.M. Bassett, I. Clarke, and J. Haywood, “Polarization dependence in a Sagnac loop optical fibre current sensor employing a 3×3 optical coupler,” Optical Fibre Sensors OFS-10 Glasgow, Scotland, (1994).

]. In Poincaré space a 2π rotation of any polarisation 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 Poincaré space. Hence the factor 4π that appears in Eq (1) for the term If |τ̄| = 4π/LT.

Fig. 2. Representation of polarisation states and birefringence in Poincaré space. Current sensitivity

Current sensitivity

The strength of the Faraday effect in a material is represented by V the materials Verdet constant where V describes the phase shift per unit length between orthogonal circular polarisation states when exposed to a magnetic field parallel to the direction of propagation of light. The sensitivity to current of the SHiBi due to the Faraday effect can be represented by an effective Verdet constant Vs and the phase shift per unit length between the orthogonal elliptical polarisation states P+ and P- is

Vs=Vsinθ
(2)

where V is the Verdet constant of straight non birefringent fibre and θ is the ellipticity shown in Fig 2. From Eq (2) it can be seen that fibre with only circular birefringence, where θ = π/2, has the same sensitivity as straight non birefringent fibre [8

8. I.M. Bassett, I. Clarke, and J. Haywood, “Polarization dependence in a Sagnac loop optical fibre current sensor employing a 3×3 optical coupler,” Optical Fibre Sensors OFS-10 Glasgow, Scotland, (1994).

].

2.2 Measurement approach

A modified version of the crossed polariser method was used to analyse the elliptical birefringent SHiBi PCF [1

1. A. Michie, J. Canning, K. Lyytikäinen, M. Åslund, and J. Digweed, “Temperature independent highly birefringent photonic crystal fibre,” Opt. Express. 12,5160–5165, (2004). [CrossRef] [PubMed]

]. Broadband light from an erbium doped fibre amplified spontaneous emission (EDF-ASE) light source was passed through a fibre pigtailed polarising beam splitter (PBS) spliced to a section of SHiBi PCF. The fibre ends were rotated and aligned to excite roughly equal amounts of power into each elliptical polarisation mode of the SHiBi PCF. The light emerging from the SHiBi PCF end was then analysed with a linear polariser and the wavelength dependent modulation, or interferogram, was observed. In the case of linearly birefringent fibre the output spectrum from a single linear polarisation state can be observed by simply rotating the polariser at the output until the modulation is completely extinguished. The orthogonal state can then be observed simply by rotating the output polariser through +/- 90 degrees.

For the SHiBi PCF the modulation could not be completely extinguished by rotating the output polariser due to elliptical birefringence. The addition of a Babinet-Soleil phase (BSP) compensator between the output end of the SHiBi and the linear polariser allows for some retardance to be added effectively turning the linear output polariser into an elliptical polariser. 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.

Fig. 3 Experimental set up for measuring the ellipticity of the birefringence.

2.3 Birefringence measurement

Bm=nxny,
(3)

where nx and ny are the effective refractive indices for each polarisation mode. The birefringence is often presented in the form of a beat length given by LB = λ/Bm [1

1. A. Michie, J. Canning, K. Lyytikäinen, M. Åslund, and J. Digweed, “Temperature independent highly birefringent photonic crystal fibre,” Opt. Express. 12,5160–5165, (2004). [CrossRef] [PubMed]

], 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 [1

1. A. Michie, J. Canning, K. Lyytikäinen, M. Åslund, and J. Digweed, “Temperature independent highly birefringent photonic crystal fibre,” Opt. Express. 12,5160–5165, (2004). [CrossRef] [PubMed]

]

Bg=(λdBm(λ)dλBm(λ))=λ2LΔλ
(4)

ρ¯=τ¯sinθ=2πLTsinθ
(5)

Similarly the value of η̄ = 2π/LB can be calculated using

η¯=τ¯tanθ
(6)

2.4. Temperature dependence measurements.

The group birefringence, Bg, was calculated using Eq (4) from the recorded interferograms obtained using the crossed polariser method configured in reflection. The EDF-ASE source provided enough power 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.

5. Results and Discussion

Given that the fibre (fig 1) was drawn from the preform phase at 10m/min with a rotation rate of 1300 rpm, a spin-pitch of 7.7mm is determined. A 630mm 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 (fig 4) was recorded using an optical spectrum analyser and the period and depth of the modulation were determined. The linear polariser at the output was then rotated and the modulation depth minimised but not fully extinguished. The BSP compensator was initially set to zero retardance (Ri=0°) and was arbitrarily orientated relative to the linear polariser. The BSP was then re-orientated so that its fast and slow axes were aligned at 45° to the linear polariser. The retardance was then adjusted to fully extinguish the periodic modulation. 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 radians in Poincaré space. From the ratios of the two, the ellipticity of the SHiBi birefringence reduces to θ = [R1/(R2-R1)]. π rad.

Fig. 4. Interferograms with the periodic modulation both extinguished and maximised.

For this section of SHiBi an ellipticity of approx θ = (0.23 ± 0.02) rad (or approx 13.2°) was observed. Using Eq (2) above the SHiBi PCF would retain approx (22 ± 4) % of the sensitivity to current of an entirely circularly birefringent fibre.

The same section of SHiBi PCF was then configured in reflection and placed inside the environmental chamber. The group birefringence was recorded every 10 degrees 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 70ppm/°C [9

9. P.A. Williams, A.H. Rose, G.W. Day, T.E. Milner, and M.N. Deeter, “Temperature dependence of the Verdet constant in several diamagnetic glasses,” Appl. Opt. 30, (1991). [CrossRef] [PubMed]

], for silica based fibres this effect is still an order of magnitude smaller than the high temperature dependence seen in spun Stress HiBi fibres.

Fig. 5. Group birefringence versus temperature for both a conventional stress birefringence Panda fibre and the SHiBi -PCF

6. Conclusion

The first spun highly birefringent photonic crystal optical fibre has been characterised for ellipticity and temperature dependence. This fibre offers a stable effective Verdet constant with low temperature dependence and low packaging sensitivity.

Further refinements in design will potentially provide broad band polarising properties [10

10. M. Szpulak, J. Olszewski, T. Martynkien, W. Urbanczyk, and J. Wojcik, “Polarizing photonic crystal fibers with wide operation range, Opt. Commun. 239,91–97, (2004). [CrossRef]

] 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 of temperature independent control of polarisation that may be of benefit for new areas such as quantum communications based on polarisation degeneracy between entangled light paths.

Acknowledgments

A. Michie acknowledges the financial support of ABB and Transgrid.

References and links

1.

A. Michie, J. Canning, K. Lyytikäinen, M. Åslund, and J. Digweed, “Temperature independent highly birefringent photonic crystal fibre,” Opt. Express. 12,5160–5165, (2004). [CrossRef] [PubMed]

2.

T. Martynkien, M. Szpulak, and W. Urbanczyk, “Modeling and measurement of temperature sensitivity in birefringent photonic crystal holey fibers,” Appl. Opt. 44,7780–7788, (2005). [CrossRef] [PubMed]

3.

R.I. Laming and D.N. Payne, “Electric current sensors employing spun highly birefringent optical fibers,” J. Lightwave Technol. 5,2084–2094, (1989). [CrossRef]

4.

I.M. Bassett, “Design principle for a circularly birefringent optical fiber,” Opt. Lett. 13,844–846, (1988). [CrossRef] [PubMed]

5.

Katja Digweed, Andrew Michie, John Canning, Brian Ashton, Michael Stevenson, Justin Digweed, Ian Bassett, and John Haywood, “Spun highly birefringent photonic crystal fibre,” Australasian Conference on Optics Lasers and Spectroscopy (ACOLS), Rotorua, New Zealand, (2005).

6.

Andrew Michie, Katja Digweed, John Canning, Brian Ashton, Michael Stevenson, Justin Digweed, Ian Bassett, and John Haywood “Spun Highly Birefringent Photonic Crystal Fibre for Current Sensing,” Optical Fibre Sensors (OFS) - 18,Cancun, Mexico, (2006).

7.

Eric Udd, “Poincaré sphere,” in Fiber optic sensors, Wiley series in pure & app optics,191,375, (1991).

8.

I.M. Bassett, I. Clarke, and J. Haywood, “Polarization dependence in a Sagnac loop optical fibre current sensor employing a 3×3 optical coupler,” Optical Fibre Sensors OFS-10 Glasgow, Scotland, (1994).

9.

P.A. Williams, A.H. Rose, G.W. Day, T.E. Milner, and M.N. Deeter, “Temperature dependence of the Verdet constant in several diamagnetic glasses,” Appl. Opt. 30, (1991). [CrossRef] [PubMed]

10.

M. Szpulak, J. Olszewski, T. Martynkien, W. Urbanczyk, and J. Wojcik, “Polarizing photonic crystal fibers with wide operation range, Opt. Commun. 239,91–97, (2004). [CrossRef]

OCIS Codes
(060.2280) Fiber optics and optical communications : Fiber design and fabrication
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(060.2420) Fiber optics and optical communications : Fibers, polarization-maintaining

ToC Category:
Photonic Crystal Fibers

History
Original Manuscript: December 4, 2006
Revised Manuscript: January 29, 2007
Manuscript Accepted: February 5, 2007
Published: February 19, 2007

Citation
Andrew Michie, John Canning, Ian Bassett, John Haywood, Katja Digweed, Mattias Åslund, Brian Ashton, Michael Stevenson, Justin Digweed, Alfred Lau, and Daniel Scandurra, "Spun elliptically birefringent photonic crystal fibre," Opt. Express 15, 1811-1816 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-4-1811


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. A. Michie, J. Canning, K. Lyytikäinen, M. Åslund, and J. Digweed, "Temperature independent highly birefringent photonic crystal fibre," Opt. Express. 12, 5160-5165, (2004). [CrossRef] [PubMed]
  2. T. Martynkien, M. Szpulak, W. Urbanczyk, "Modeling and measurement of temperature sensitivity in birefringent photonic crystal holey fibers," Appl. Opt. 44, 7780-7788, (2005). [CrossRef] [PubMed]
  3. R.I. Laming and D.N. Payne, "Electric current sensors employing spun highly birefringent optical fibers," J. Lightwave Technol. 5, 2084-2094, (1989). [CrossRef]
  4. I.M. Bassett, "Design principle for a circularly birefringent optical fiber," Opt. Lett. 13, 844-846, (1988). [CrossRef] [PubMed]
  5. Katja Digweed, Andrew Michie, John Canning, Brian Ashton, Michael Stevenson, Justin Digweed, Ian Bassett, and John Haywood, "Spun highly birefringent photonic crystal fibre," Australasian Conference on Optics Lasers and Spectroscopy (ACOLS), Rotorua, New Zealand, (2005).
  6. Andrew Michie, Katja Digweed, John Canning, Brian Ashton, Michael Stevenson, Justin Digweed, Ian Bassett, and John Haywood "Spun Highly Birefringent Photonic Crystal Fibre for Current Sensing," Optical Fibre Sensors (OFS) - 18, Cancun, Mexico, (2006).
  7. Eric Udd, "Poincaré sphere," in Fiber optic sensors, Wiley series in pure & app optics, 191,375, (1991).
  8. I.M. Bassett, I. Clarke and J. Haywood, "Polarization dependence in a Sagnac loop optical fibre current sensor employing a 3x3 optical coupler," Optical Fibre Sensors OFS-10 Glasgow, Scotland, (1994).
  9. P.A. Williams, A.H. Rose, G.W. Day, T.E. Milner, and M.N. Deeter,"Temperature dependence of the Verdet constant in several diamagnetic glasses," Appl. Opt. 30, (1991). [CrossRef] [PubMed]
  10. M. Szpulak, J. Olszewski, T. Martynkien, W. Urbanczyk, and J. Wojcik, "Polarizing photonic crystal fibers with wide operation range," Opt. Commun. 239,91-97, (2004). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited