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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B283–B288
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Measured phase and group birefringence in elliptical core fibers with systematically varied ellipticities

Tommy Geisler and Søren Herstrøm  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B283-B288 (2011)
http://dx.doi.org/10.1364/OE.19.00B283


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Abstract

This paper presents the first systematic phase and group birefringence measurements of elliptical core fibers covering an unprecedented ellipticity range from 0.1 to 0.9. Experimental results at 1550 nm are compared with simulations, and the birefringence ratio is shown to depend on both ellipticity and core area. Also the dispersion of the group birefringence is discussed.

© 2011 OSA

1. Introduction

This paper presents an extensive study of Bphase and Bgroup for ECFs at telecom wavelengths around 1550 nm: The semi-minor, b, and semi-major, a, axis ratios, b/a ranging from 1 down to below 0.1, or ellipticity,ε=1b/a, in the range from 0 to 0.9. This is the first report on both types of birefringence and birefringence dispersion for ECFs covering such an extensive range of ellipticities. The core area is shown to impact the ratio between the group and phase birefringence, and the dispersion of the group birefringence or DGD slope is shown to be very dependent on the ellipticity of the core.

2. Phase and Group Birefringence

The phase (modal) birefringence
Bphase(λ)=nxny=λ2π(βx(λ)βy(λ)),
(1)
where nx,yand βx,yare the refractive indices and propagation constants, is connected to the polarization beat-length as Bphase(λ)=λ/LB,phase(λ). In an ECF the birefringence consists mainly of two parts, a thermal stress-induced birefringence part, Bstress, and a part due to form-induced birefringence, Bform [6

6. S. C. Rashleigh, “Measurement of fiber birefringence by wavelength scanning: effect of dispersion,” Opt. Lett. 8(6), 336–338 (1983). [CrossRef] [PubMed]

8

8. J. Noda, K. Okamoto, and T. Sasaki, “Polarization-maintaining fibers and their applications,” J. Lightwave Technol. 4(8), 1071–1089 (1986). [CrossRef]

]. The total phase birefringence is then given byBphase=Bform+Bstress. The Bform part is found by finite element calculations on the measured 2D refractive index profiles [12

12. S. Herstrøm, “Measured and simulated cutoff wavelengths as a function of core ellipticity for higher order modes in elliptical optical fibers,” Opt. Eng. 49(12), 125002 (2010). [CrossRef]

]. The stress birefringence for an ECF can be expressed as [8

8. J. Noda, K. Okamoto, and T. Sasaki, “Polarization-maintaining fibers and their applications,” J. Lightwave Technol. 4(8), 1071–1089 (1986). [CrossRef]

,13

13. W. Eickhoff, “Stress-induced single-polarization single-mode fiber,” Opt. Lett. 7(12), 629–631 (1982). [CrossRef] [PubMed]

,14

14. M. P. Varnham, D. N. Payne, A. J. Barlow, and R. D. Birch, “Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers,” J. Lightwave Technol. 1(2), 332–339 (1983). [CrossRef]

],
Bstress(λ)=M(λ)CEΔαΔT1νaba+b
(2)
where C = −3.36 mm2/kg is the stress-optic coefficient, E = 7830 kg/mm2 is the Young’s modulus, ν = 0.186 is the Poisson’s ratio, Δα the difference in thermal expansion coefficients of the core and the cladding, and ΔT is the difference between the operating temperature and softening temperature of the core [8

8. J. Noda, K. Okamoto, and T. Sasaki, “Polarization-maintaining fibers and their applications,” J. Lightwave Technol. 4(8), 1071–1089 (1986). [CrossRef]

]. The factor M(λ) describes the portion of the mode propagating in the core and is approximated with (neffncl)/(nconcl), where nco, ncl and neff are the core, the cladding and the effective refractive indices, respectively [4

4. P. Hlubina, T. Martynkien, and W. Urbańczyk, “Dispersion of group and phase modal birefringence in elliptical-core fiber measured by white-light spectral interferometry,” Opt. Express 11(22), 2793–2798 (2003). [PubMed]

,5

5. T. Ritari, H. Ludvigsen, M. Wegmuller, M. Legré, N. Gisin, J. R. Folkenberg, and M. D. Nielsen, “Experimental study of polarization properties of highly birefringent photonic crystal fibers,” Opt. Express 12(24), 5931–5939 (2004). [CrossRef] [PubMed]

,7

7. S. C. Rashleigh, “Origins and control of polarization effects in single-mode fibers,” J. Lightwave Technol. 1(2), 312–331 (1983). [CrossRef]

,9

9. A.T. Nguyen, E. Lazzeri, P. Ghelfi, A. Bogoni, and L. Poli, “Precise low-cost optical time multiplexer based on the birefringence of polarization maintaining fibers,” Proceedings ECOC2009, paper P.1.20 (2009).

].

3. Experiments and Results

Fibers with elliptical core drawn from a preform with semi-minor to semi-major ratio b/a ranging from close to 1 down to below 0.1 were investigated. Fabrication details and cut-off properties have previously been reported [12

12. S. Herstrøm, “Measured and simulated cutoff wavelengths as a function of core ellipticity for higher order modes in elliptical optical fibers,” Opt. Eng. 49(12), 125002 (2010). [CrossRef]

]. The area of the elliptical core was on average 12.7 μm2 (preform 1) and a maximum index difference of 0.026. In addition, ECFs originating from two other preforms with somewhat lower core areas, 10.8 μm2 (preform 2) and 9.4 μm2 (preform 3) respectively, and ellipticities up to 0.5 were investigated. The normalized cutoff frequencies, Vb=(2πb/λ)(nco2ncl2)1/2of the measured fibers were in the range 0.77-2.23 (Table 1

Table 1. Elliptical core fibers investigated

table-icon
View This Table
).

The phase birefringence was measured by a slightly modified version of the external stress method originally proposed by Calvani et al [15

15. R. Calvani, R. Caponi, F. Cisternino, and G. Coppa, “Fiber Birefringence Measurements with an External Stress Method and Heterodyne Polarization Detection,” J. Lightwave Technol. 5(9), 1176–1182 (1987). [CrossRef]

]. The fiber is laid flat on a hard smooth surface. By applying a lateral force at a point that moves along the fiber axis and simultaneous measuring the output state of polarization (SOP), the LB,phase can be obtained by counting the periodic variation in the output SOP with a polarimeter (PAN9300 FIR). Unlike the classical lateral force method proposed by Takada et al [16

16. K. Takada, J. Noda, and R. Ulrich, “Precision measurement of modal birefringence of highly birefringent fibers by periodic lateral force,” Appl. Opt. 24(24), 4387–4391 (1985). [CrossRef] [PubMed]

], this method does not require a specific or even known input polarisation, but it relies only on the measurement of the output SOP as function of the displacement of the point like stress. The technique is non-destructive, and by measuring the output SOP, the beat-length could be determined at a number of wavelengths in the range from 1520 – 1590 nm provided by a tuneable laser diode source. An example of SOP variation and associated beat-length is illustrated in Fig. 1
Fig. 1 a) Example of SOP evolution on the Poincare sphere. b) The three normalized Stokes vector components as function of displacement length from which the polarization beat-length is determined.
. For short beat-lengths the main uncertainty comes from the reading of the displacement which in the applied setup had an accuracy of 0.5 mm. In order to reduce the uncertainty, the LB,phase(λ) was determined by averaging over 2-20 periods depending on beat-length (4.5 – 110 mm). This reduced the uncertainty to less than 1%. Using the relation LB,phase(λ)=λ/Bphase(λ), the Bphase was calculated.

Bgroup was determined from Eq. (3), where dBphase/dλ was found by making a linear fit to Bphase(λ) over the wavelength range. As illustrated in Fig. 2a
Fig. 2 a) Group birefringence determined from the phase birefringence dependence on wavelength (in blue) and from DGD measurements (in green). b) Examples of DGD as function of wavelength, all well approximated by a linear dependence in the range covered.
, the results were in excellent agreement with the Bgroup determined from DGD measurements in the wavelength range from 1440 – 1590 nm obtained by the Jones-Matrix eigenanalysis (JME) measurement technique with an HP-8509 polarization analyzer. The slope of the DGD was found assuming a linear dependence on wavelength. Justification of this is evident from Fig. 2b which shows examples of DGD as function of wavelength.

The measured Bphase and Bgroup at 1550 nm are displayed as function of ellipticity in Fig. 3
Fig. 3 a) Measured phase (blue) and group (black) birefringence for fiber 1. Calculated stress (green) and form birefringence (red) and the calculated total phase birefringence (magenta). b) Calculated stress, form, total phase and group birefringence for a generic elliptic step index profile with equivalent flat top distribution [6].
. Both are increasing with ellipticity going through a maximum and finally decreasing at very large ellipticities. They show a maximum of approximately 3.5×104 and 6.5×104corresponding to beat-lengths of 4.5 mm and 2.4 mm, respectively. Both Bstress and Bform and their sum, the total birefringence, Bphase, are also included in Fig. 3. The form-induced birefringence is calculated using the actual measured 2D-index profiles. In order to estimate the stress-induced birefringence, we have assumed a Δα of 2×106K−1 due to the Al doping and ΔT = 1000 °C. These are somewhat speculative. However, an excellent agreement is observed between the experimentally measured phase birefringence and the calculated total birefringence. For comparison the Fig. 3b shows the birefringence for a generic step index profile with equivalent flat top distribution [12

12. S. Herstrøm, “Measured and simulated cutoff wavelengths as a function of core ellipticity for higher order modes in elliptical optical fibers,” Opt. Eng. 49(12), 125002 (2010). [CrossRef]

]. There is close agreement with the experimental data even though the group birefringence is a little underestimated. This may be due to the fact that only the dispersion of the stress optic coefficient was included by assuming (λ/C)(dC/dλ)=0.1 [6

6. S. C. Rashleigh, “Measurement of fiber birefringence by wavelength scanning: effect of dispersion,” Opt. Lett. 8(6), 336–338 (1983). [CrossRef] [PubMed]

,17

17. N. Shibata, K. Okamoto, M. Tateda, S. Seikai, and Y. Sasaki, “Modal Birefringence and Polarization Mode Dispersion in Single-Mode Fibers with Stress-Induced Anisotropy,” IEEE J. Quantum Electron. 19(6), 1110–1115 (1983). [CrossRef]

], and that the wavelength dependence of the factor describing the portion of the mode propagating in the core in the stress term had been neglected.

The ratio Bgroup/Bphase between the group and phase birefringence is shown in Fig. 4
Fig. 4 a) Ratio of measured group and phase birefringence for fiber from preform 1, 2 and 3 with decreasing core area. b) Calculated ratio from simulated generic elliptic step index profiles with similar core areas and equivalent flat top distribution.
. For preform 1 it is observed that the ratio is less than 1 at moderate ellipticities (~0.2) and then it increases to values larger than 2 with increasing ellipticities. The birefringence ratios for the fibers with smaller core areas are also included in Fig. 4. It is observed that the ratio increases with decreasing core area and thus effective index, neff, for the same ellipticity. This confirms that the stress birefringence (phase and group) varies considerably with the portion propagating in the core [7

7. S. C. Rashleigh, “Origins and control of polarization effects in single-mode fibers,” J. Lightwave Technol. 1(2), 312–331 (1983). [CrossRef]

,8

8. J. Noda, K. Okamoto, and T. Sasaki, “Polarization-maintaining fibers and their applications,” J. Lightwave Technol. 4(8), 1071–1089 (1986). [CrossRef]

,13

13. W. Eickhoff, “Stress-induced single-polarization single-mode fiber,” Opt. Lett. 7(12), 629–631 (1982). [CrossRef] [PubMed]

,18

18. K. Okamoto, T. Edahiro, and N. Shibata, “Polarization properties of single-polarization fibers,” Opt. Lett. 7(11), 569–571 (1982). [CrossRef] [PubMed]

].

The DGD slope at 1550 nm, found by a linear fit to the measured DGD, is shown in Fig. 5
Fig. 5 a) Measured DGD slope at 1550 nm for fiber from preform 1 (blue points) together with the calculated slope (bright blue). Also shown are the DGD slopes for fiber from preforms 2 and 3. b) Calculated DGD slope from simulated generic elliptic step index profiles with similar core areas and equivalent flat top distribution.
. The slope starts at almost zero going through a maximum of approximately 1.5103ps/nm-m at an ellipticity of 0.7 and then it drops to negative values at very high ellipticity. The calculated slope values are found assuming (λ2/C)(d2C/dλ2)=0.06 at 1550 nm [19

19. K. Okamoto and T. Hosaka, “Polarization-dependent chromatic dispersion in birefringent optical fibers,” Opt. Lett. 12(4), 290–292 (1987). [CrossRef] [PubMed]

]. Again an excellent agreement is found. The DGD slope of 1.5103ps/nm-m corresponds to a group birefringence dispersion of 4.5×10−7 nm−1. This is more than one order of magnitude less than reported for highly birefringent microfiber [20

20. Y. Jung, G. Brambilla, K. Oh, and D. J. Richardson, “Highly birefringent silica microfiber,” Opt. Lett. 35(3), 378–380 (2010). [CrossRef] [PubMed]

].

4. Conclusion

Experimental measurements of phase and group birefringence and their wavelength slopes in elliptical core fibers, with ellipticities systematically varying from 0 to more than 0.9, were made at 1550 nm. Phase and group beat-lengths down to approximately 4.5 mm and 2.5 mm were obtained, respectively. However, the minimum beat-length was obtained at intermediate axis ratios for both the phase and group beat-length. The ratioBgroup/Bphasewas shown to be strongly dependent on both the ellipticity and the core area, and the DGD slope was shown to be positive and increasing to about 1.5103ps/nm-m at an ellipticity of 0.7 and then change to negative values at very high ellipticities.

References and links

1.

R. B. Dyott, J. R. Cozens, and D. G. Morris, “Preservation of polarization in optical-fibre waveguides with elliptical cores,” Electron. Lett. 15(13), 380–382 (1979). [CrossRef]

2.

R. B. Dyott, Elliptical Fiber Waveguides (Artech House, 1995).

3.

M. Legre, M. Wegmuller, and N. Gisin, “Investigation of the ratio between phase and group birefringence in optical single-mode fibers,” J. Lightwave Technol. 21(12), 3374–3378 (2003). [CrossRef]

4.

P. Hlubina, T. Martynkien, and W. Urbańczyk, “Dispersion of group and phase modal birefringence in elliptical-core fiber measured by white-light spectral interferometry,” Opt. Express 11(22), 2793–2798 (2003). [PubMed]

5.

T. Ritari, H. Ludvigsen, M. Wegmuller, M. Legré, N. Gisin, J. R. Folkenberg, and M. D. Nielsen, “Experimental study of polarization properties of highly birefringent photonic crystal fibers,” Opt. Express 12(24), 5931–5939 (2004). [CrossRef] [PubMed]

6.

S. C. Rashleigh, “Measurement of fiber birefringence by wavelength scanning: effect of dispersion,” Opt. Lett. 8(6), 336–338 (1983). [CrossRef] [PubMed]

7.

S. C. Rashleigh, “Origins and control of polarization effects in single-mode fibers,” J. Lightwave Technol. 1(2), 312–331 (1983). [CrossRef]

8.

J. Noda, K. Okamoto, and T. Sasaki, “Polarization-maintaining fibers and their applications,” J. Lightwave Technol. 4(8), 1071–1089 (1986). [CrossRef]

9.

A.T. Nguyen, E. Lazzeri, P. Ghelfi, A. Bogoni, and L. Poli, “Precise low-cost optical time multiplexer based on the birefringence of polarization maintaining fibers,” Proceedings ECOC2009, paper P.1.20 (2009).

10.

F. Zhang and J. W. Y. Lit, “Temperature and strain sensitivity measurements of high-birefringent polarization-maintaining fibers,” Appl. Opt. 32(13), 2213–2218 (1993). [CrossRef] [PubMed]

11.

W. Urbanczyk, T. Martynkien, and W. J. Bock, “Dispersion effects in elliptical-core highly birefringent fibers,” Appl. Opt. 40(12), 1911–1920 (2001). [CrossRef] [PubMed]

12.

S. Herstrøm, “Measured and simulated cutoff wavelengths as a function of core ellipticity for higher order modes in elliptical optical fibers,” Opt. Eng. 49(12), 125002 (2010). [CrossRef]

13.

W. Eickhoff, “Stress-induced single-polarization single-mode fiber,” Opt. Lett. 7(12), 629–631 (1982). [CrossRef] [PubMed]

14.

M. P. Varnham, D. N. Payne, A. J. Barlow, and R. D. Birch, “Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers,” J. Lightwave Technol. 1(2), 332–339 (1983). [CrossRef]

15.

R. Calvani, R. Caponi, F. Cisternino, and G. Coppa, “Fiber Birefringence Measurements with an External Stress Method and Heterodyne Polarization Detection,” J. Lightwave Technol. 5(9), 1176–1182 (1987). [CrossRef]

16.

K. Takada, J. Noda, and R. Ulrich, “Precision measurement of modal birefringence of highly birefringent fibers by periodic lateral force,” Appl. Opt. 24(24), 4387–4391 (1985). [CrossRef] [PubMed]

17.

N. Shibata, K. Okamoto, M. Tateda, S. Seikai, and Y. Sasaki, “Modal Birefringence and Polarization Mode Dispersion in Single-Mode Fibers with Stress-Induced Anisotropy,” IEEE J. Quantum Electron. 19(6), 1110–1115 (1983). [CrossRef]

18.

K. Okamoto, T. Edahiro, and N. Shibata, “Polarization properties of single-polarization fibers,” Opt. Lett. 7(11), 569–571 (1982). [CrossRef] [PubMed]

19.

K. Okamoto and T. Hosaka, “Polarization-dependent chromatic dispersion in birefringent optical fibers,” Opt. Lett. 12(4), 290–292 (1987). [CrossRef] [PubMed]

20.

Y. Jung, G. Brambilla, K. Oh, and D. J. Richardson, “Highly birefringent silica microfiber,” Opt. Lett. 35(3), 378–380 (2010). [CrossRef] [PubMed]

OCIS Codes
(060.2270) Fiber optics and optical communications : Fiber characterization
(060.2420) Fiber optics and optical communications : Fibers, polarization-maintaining

ToC Category:
Fibers, Fiber Devices, and Amplifiers

History
Original Manuscript: September 30, 2011
Revised Manuscript: November 7, 2011
Manuscript Accepted: November 9, 2011
Published: November 18, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Tommy Geisler and Søren Herstrøm, "Measured phase and group birefringence in elliptical core fibers with systematically varied ellipticities," Opt. Express 19, B283-B288 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B283


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References

  1. R. B. Dyott, J. R. Cozens, and D. G. Morris, “Preservation of polarization in optical-fibre waveguides with elliptical cores,” Electron. Lett.15(13), 380–382 (1979). [CrossRef]
  2. R. B. Dyott, Elliptical Fiber Waveguides (Artech House, 1995).
  3. M. Legre, M. Wegmuller, and N. Gisin, “Investigation of the ratio between phase and group birefringence in optical single-mode fibers,” J. Lightwave Technol.21(12), 3374–3378 (2003). [CrossRef]
  4. P. Hlubina, T. Martynkien, and W. Urbańczyk, “Dispersion of group and phase modal birefringence in elliptical-core fiber measured by white-light spectral interferometry,” Opt. Express11(22), 2793–2798 (2003). [PubMed]
  5. T. Ritari, H. Ludvigsen, M. Wegmuller, M. Legré, N. Gisin, J. R. Folkenberg, and M. D. Nielsen, “Experimental study of polarization properties of highly birefringent photonic crystal fibers,” Opt. Express12(24), 5931–5939 (2004). [CrossRef] [PubMed]
  6. S. C. Rashleigh, “Measurement of fiber birefringence by wavelength scanning: effect of dispersion,” Opt. Lett.8(6), 336–338 (1983). [CrossRef] [PubMed]
  7. S. C. Rashleigh, “Origins and control of polarization effects in single-mode fibers,” J. Lightwave Technol.1(2), 312–331 (1983). [CrossRef]
  8. J. Noda, K. Okamoto, and T. Sasaki, “Polarization-maintaining fibers and their applications,” J. Lightwave Technol.4(8), 1071–1089 (1986). [CrossRef]
  9. A.T. Nguyen, E. Lazzeri, P. Ghelfi, A. Bogoni, and L. Poli, “Precise low-cost optical time multiplexer based on the birefringence of polarization maintaining fibers,” Proceedings ECOC2009, paper P.1.20 (2009).
  10. F. Zhang and J. W. Y. Lit, “Temperature and strain sensitivity measurements of high-birefringent polarization-maintaining fibers,” Appl. Opt.32(13), 2213–2218 (1993). [CrossRef] [PubMed]
  11. W. Urbanczyk, T. Martynkien, and W. J. Bock, “Dispersion effects in elliptical-core highly birefringent fibers,” Appl. Opt.40(12), 1911–1920 (2001). [CrossRef] [PubMed]
  12. S. Herstrøm, “Measured and simulated cutoff wavelengths as a function of core ellipticity for higher order modes in elliptical optical fibers,” Opt. Eng.49(12), 125002 (2010). [CrossRef]
  13. W. Eickhoff, “Stress-induced single-polarization single-mode fiber,” Opt. Lett.7(12), 629–631 (1982). [CrossRef] [PubMed]
  14. M. P. Varnham, D. N. Payne, A. J. Barlow, and R. D. Birch, “Analytic Solution for the Birefringence Produced by Thermal Stress in Polarization-Maintaining Optical Fibers,” J. Lightwave Technol.1(2), 332–339 (1983). [CrossRef]
  15. R. Calvani, R. Caponi, F. Cisternino, and G. Coppa, “Fiber Birefringence Measurements with an External Stress Method and Heterodyne Polarization Detection,” J. Lightwave Technol.5(9), 1176–1182 (1987). [CrossRef]
  16. K. Takada, J. Noda, and R. Ulrich, “Precision measurement of modal birefringence of highly birefringent fibers by periodic lateral force,” Appl. Opt.24(24), 4387–4391 (1985). [CrossRef] [PubMed]
  17. N. Shibata, K. Okamoto, M. Tateda, S. Seikai, and Y. Sasaki, “Modal Birefringence and Polarization Mode Dispersion in Single-Mode Fibers with Stress-Induced Anisotropy,” IEEE J. Quantum Electron.19(6), 1110–1115 (1983). [CrossRef]
  18. K. Okamoto, T. Edahiro, and N. Shibata, “Polarization properties of single-polarization fibers,” Opt. Lett.7(11), 569–571 (1982). [CrossRef] [PubMed]
  19. K. Okamoto and T. Hosaka, “Polarization-dependent chromatic dispersion in birefringent optical fibers,” Opt. Lett.12(4), 290–292 (1987). [CrossRef] [PubMed]
  20. Y. Jung, G. Brambilla, K. Oh, and D. J. Richardson, “Highly birefringent silica microfiber,” Opt. Lett.35(3), 378–380 (2010). [CrossRef] [PubMed]

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