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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 4 — Aug. 1, 2011
  • pp: 686–699
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Acoustic coefficients of P2O5-doped silica fiber: acoustic velocity, acoustic attenuation, and thermo-acoustic coefficient

Pi-Cheng Law, Yuh-Shiuan Liu, André Croteau, and Peter D. Dragic  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 4, pp. 686-699 (2011)
http://dx.doi.org/10.1364/OME.1.000686


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Abstract

We present measurements and modeling of the effect of P2O5 doping on the acoustic damping and temperature sensitivity coefficients of silica fibers. In particular, the Brillouin gain spectrum of a highly P-doped fiber is measured and investigated at different temperatures. It is found that the acoustic damping coefficient (proportional to the Brillouin spectral width) of phosphorus pentoxide (1.41 × 105 m−1 for bulk P2O5 at 11 GHz) is similar to, but larger than, that of germanium dioxide. Additionally, the acoustic velocity (and thereby the Stokes’ shift) is found to be much less dependent on temperature in P2O5 ( + 0.12 m/s/°C) than in SiO2 ( + 0.56 m/s/°C). Using these coefficients (the thermo-acoustic coefficients), the modeled and unique slopes of the Stokes’-shift-versus-temperature curves for the four observed acoustic modes each lie within 3% of the measured values. Finally, utilizing both the thermo-optic and thermo-acoustic coefficients, a design example is presented where a composition is determined for which the dependence of the Brillouin frequency shift on temperature is minimized. In this example, the calculated temperature sensitivity is less than 5 kHz/°C over the temperature range −100 °C < T < 100 °C for the molar composition 0.54P2O5:0.46SiO2.

© 2011 OSA

1. Introduction

Recently, novel optical fibers with tailored acoustic profiles have been designed specifically for applications where Brillouin scattering may be encountered, whether requiring its suppression [1

1. P. D. Dragic, C.-H. Liu, G. C. Papen, and A. Galvanauskas, “Optical fiber with an acoustic guiding layer for stimulated Brillouin scattering suppression,” in CLEO/QELS2005, Vol. 3 of 2005 Conference on Lasers and Electro-Optics, Paper CThZ3.

,2

2. M. J. Li, X. Chen, J. Wang, S. Gray, A. Liu, J. A. Demeritt, A. B. Ruffin, A. M. Crowley, D. T. Walton, and L. A. Zenteno, “Al/Ge co-doped large mode area fiber with high SBS threshold,” Opt. Express 15(13), 8290–8299 (2007). [CrossRef] [PubMed]

] or utilization [3

3. P. D. Dragic, “Novel dual-Brillouin-frequency optical fiber for distributed temperature sensing,” Proc. SPIE 7197, 719710 (2009). [CrossRef]

]. Clearly imperative in the process of designing an acoustic profile is the knowledge of how potential dopants influence the relevant acoustic parameters in a ‘host’ material, such as silica. While there can be found publications in the literature that show how some common dopants influence the acoustic velocity in SiO2 [4

4. C.-K. Jen, C. Neron, A. Shang, K. Abe, L. Bonnell, and J. Kushibiki, “Acoustic characterization of silica glasses,” J. Am. Ceram. Soc. 76(3), 712–716 (1993). [CrossRef]

], much less has been done to characterize how the dynamic viscosity (Brillouin spectral width) and both the fiber thermo-acoustic and thermo-optic coefficients (TAC and TOC, respectively) are influenced by these dopants [5

5. Y. Koyamada, S. Sato, S. Nakamura, H. Sotobayashi, and W. Chujo, “Simulating and designing Brillouin gain spectrum in single-mode fibers,” J. Lightwave Technol. 22(2), 631–639 (2004). [CrossRef]

]. This data is particularly important, for example, when the large acoustic damping coefficients associated with some dopants can be used as a degree-of-freedom in the design of a specialty fiber. Furthermore, understanding the TACs and TOCs of the various dopants can lead to the development of novel optical fibers for distributed sensor applications.

Recently [6

6. P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]

], we presented an analysis that began with the measurement of the Brillouin gain spectrum (BGS) of a heavily germanium-doped optical fiber. From the measured BGS, and a fit to a simple materials model [7

7. P. D. Dragic, “Simplified model for the effect of Ge doping on silica fibre acoustic properties,” Electron. Lett. 45(5), 256–257 (2009). [CrossRef]

] for Ge-doped fibers, we were able to extract an acoustic attenuation coefficient of about 1.38 × 105 m−1 for bulk GeO2 at an acoustic frequency of 11 GHz [7

7. P. D. Dragic, “Simplified model for the effect of Ge doping on silica fibre acoustic properties,” Electron. Lett. 45(5), 256–257 (2009). [CrossRef]

]. This value was later revised downward to 1.11 × 105 m−1 after a more accurate compositional profile was obtained [8

8. P. D. Dragic and B. G. Ward, “Accurate modeling of the intrinsic Brillouin linewidth via finite element analysis,” IEEE Photon. Technol. Lett. 22(22), 1698–1700 (2010). [CrossRef]

]. This measurement was partly enabled by a large variation in the viscosity profile due to a central burn-out characteristic of fibers manufactured via the modified chemical vapor deposition (MCVD) process. This acoustic attenuation value increases with acoustic-frequency-squared for Ge-doped fibers [9

9. C. Krischer, “Optical measurements of ultrasonic attenuation and reflection losses in fused silica,” J. Acoust. Soc. Am. 48(5B), 1086–1092 (1970). [CrossRef]

]. This data point, in addition to other basic materials parameters (including the acoustic velocity and mass density) for the bulk material, can then be used to calculate and design sets of relevant acoustic profiles for arbitrary compositional profiles, and can be extended to include systems of multiple dopants.

Here, we present a similar analysis for a P2O5-doped optical fiber, and refer the reader to [6

6. P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]

] for details such as the model and experimental setup. The present fiber preform is characterized by a similar central index dip, and has fairly sizeable doping (~14.3 mol% oxide at the peak). However, in contrast to the Ge-doped fiber studied previously, it will be shown that the central burn-out becomes ‘filled-in’ due to phosphorus diffusion during the fiber draw process. Thus, the test fiber does not contain a significant index dip. It is found that, for the purposes of modeling P-doped fibers, including P-doped inner cladding layers, bulk phosphorus oxide has an acoustic attenuation coefficient (αP) of about 1.411 × 105 m−1 at an acoustic frequency of 11 GHz. This value is similar to, but larger than, that of bulk GeO2. Both of these values are approximately 10 times larger than of silica [8

8. P. D. Dragic and B. G. Ward, “Accurate modeling of the intrinsic Brillouin linewidth via finite element analysis,” IEEE Photon. Technol. Lett. 22(22), 1698–1700 (2010). [CrossRef]

]. The spectral width of the L01 acoustic mode is found to decrease at a rate of ~−77.7 kHz/°C.

2. Experimental details and optical fiber

A. Optical Fiber

The optical fiber used in this set of experiments is one with a P2O5-doped silica core fabricated by drawing the corresponding optical preform. The fiber was then coated on-line by standard UV-cured acrylate. The silica fiber preform was fabricated by the MCVD process. A combination of a small MCVD substrate tube, some matched cladding layers, and a large preform core size helped lower the final collapse temperature which maximized the P2O5 content in the silica core to 14.3 mol %.

The tube outside diameter was 18 mm and the inside diameter was 15 mm. The matched cladding consisted of 21 P2O5 and F co-doped silica layers. The core consisted of 12 P2O5 doped silica layers. These layers are visible in the refractive index profile (RIP) provided in Fig. 1
Fig. 1 Refractive index profile of the mother preform.
, which was measured using a York P102 preform analyzer.

The preform was drawn into a cane for reducing the core size. The chemical compositional profiles of the cane were measured by the electron probe microanalysis (EPMA) technique. The resulting data is provided in Fig. 2
Fig. 2 Measured compositional profile of the cane precursor to the P2O5-doped silica optical fiber. Fluorine (F) is present only in the inner cladding with a small amount of phosphorus (P) to create an index-matched inner cladding. The MCVD layers are visible in the P2O5 data.
. The EPMA data is expressed in units of measurement X-ray counts, and for P corresponds to a peak P2O5 concentration of roughly 14.3 mol% in the core. The final preform was obtained by over-cladding the cane with a silica tube. The final preform was then drawn into a fiber.

Figures 1 and 2 reveal a significant index dip in the central region of the fiber perform core, which can have a significant impact on the analysis presented in this paper. Therefore, the RIP of the final fiber is measured to determine if this feature was preserved through the draw process. The refractive index profile was measured using two methods: 1) the well-known refracted near field (RNF) technique and 2) a high-resolution spatially-resolved Fourier transform technique [12

12. A. D. Yablon, “Multi-wavelength optical fiber refractive index profiling by spatially resolved Fourier transform spectroscopy,” J. Lightwave Technol. 28(4), 360–364 (2010). [CrossRef]

], with measurement wavelengths at 670 nm and 1000 nm, respectively. The results are shown in Fig. 3
Fig. 3 Refractive index profile of the final P-doped silica fiber measured at 670 nm (dashed line) and 1000 nm (solid line).
. EPMA was not used on the small fiber due to the limited spatial resolution of the available system (~1 μm).

The RIP measurements suggest that the P2O5 dopant experienced significant diffusion during the fiber drawing stage. While a slight central dip can be seen in the 1000 nm high-resolution data, the large burnout observed in both Figs. 1 and 2 appears to have been completely filled-in. In addition, a slight decrease in the refractive index difference is observed in going from 670 nm to 1000 nm, which can be attributed to either 1) chromatic dispersion of the P2O5 dopant or 2) a slight variation in the dopant concentration between the two measurement samples.

In order to obtain the dopant concentration in the core, we rely on the RIP measurements of the fiber and the results found in [13

13. M. M. Bubnov, E. M. Dianov, O. N. Egorova, S. L. Semjonov, A. N. Guryanov, V. F. Khopin, and E. M. DeLiso, “Fabrication and investigation of single-mode highly phosphorus-doped fibers for Raman lasers,” Proc. SPIE 4083, 12–22 (2000). [CrossRef]

]. In [13

13. M. M. Bubnov, E. M. Dianov, O. N. Egorova, S. L. Semjonov, A. N. Guryanov, V. F. Khopin, and E. M. DeLiso, “Fabrication and investigation of single-mode highly phosphorus-doped fibers for Raman lasers,” Proc. SPIE 4083, 12–22 (2000). [CrossRef]

], preform RIPs were measured using a York P102 preform analyzer leading to a molar refractivity determined to be 0.88 × 10−3 per mol%. Due to the proximity of the wavelength of the York system (632 nm) to that of the RNF method employed here (670 nm), we use the RNF data to determine the P2O5 concentration (represented here as [P2O5] in mol%). Therefore, based on the measured RIP, we determine that [P2O5] in the center of the fiber is 12.2 mol%.

Integrating the compositional profile in Fig. 2 and that derived from the RNF RIP across the core region give rise to average P2O5 concentrations of 10.5 mol% and 10.3 mol%, respectively (note that this is also a representation of the total number of P atoms in the core). This finding supports the conclusion that the fiber of the present study does not possess a central dip since P2O5 diffused into this region during the drawing process.

The additive model assumes that the glass constituents are independent of each other, and thus requires modification in cases where dopants interact in a non-additive way. A well-known example is the P2O5-Al2O3 co-doped silica system. While doping P or Al alone into silica increases the refractive index (and mass density), adding P to an Al-doped fiber (or Al to a P-doped fiber) decreases the refractive index (and mass density) relative to the binary system [14

14. D. J. DiGiovanni, J. B. MacChesney, and T. Y. Kometani, “Structure and properties of silica containing aluminum and phosphorus near the AlPO4 join,” J. Non-Cryst. Solids 113(1), 58–64 (1989). [CrossRef]

], suggesting an interaction that leads to structural modifications. In this case, one would treat the AlPO4 unit cell as an independent species within the additive model.

As a side note, an interesting special case arises in the additive model when the molar volumes of the dopant and host oxides are identical ((ρs/Ms)(Mpp) ≈1). In that case, the refractive index difference reduces to a linear function of the dopant concentration (in mol%). Finally, since our Brillouin scattering measurements are performed at 1534 nm (λo), we assume that the RIP measurement at 1000 nm is a good approximation to the RIP at 1534 nm, and thus from this data the optical mode will be calculated.

B. Experimental Details

The experimental configuration used to acquire the BGS is identical to that described in [6

6. P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]

]. In short, a continuous-wave, narrow-linewidth (< 100 kHz) external-cavity laser diode signal is amplified and passed through a circulator to a 3.5-meter test fiber. The back-scattered Stokes’ signal and a small quantity of pump signal (courtesy of a carefully-controlled angled cleave at the test fiber output) then propagate back through the third port of the circulator, are optically pre-amplified, and finally heterodyned onto a fast detector and the resulting spectrum is captured with a signal analyzer. Caution was exercised to ensure that Brillouin gain did not narrow the spectra and obscure the measurements [15

15. R. W. Boyd, K. Rzaewski, and P. Narum, “Noise initiation of stimulated Brillouin scattering,” Phys. Rev. A 42(9), 5514–5521 (1990). [CrossRef] [PubMed]

].

3. Room temperature measurements and the acoustic attenuation coefficient of P2O5

The measured BGS of the P-doped fiber is shown in Fig. 5
Fig. 5 Brillouin spectrum of a 3.5 m segment of the P2O5-doped optical fiber at 1534 nm and at two different temperatures, 21.5°C and 118.3°C. Four acoustic modes primarily located in the core are observed, while five reside mainly in the inner cladding. The two peaks near 11 GHz are due to the measurement apparatus.
. A total of eleven acoustic modes is observed. Four peaks (L01 to L04) can be attributed to modes that reside primarily in the core of the test fiber. Two (near 11 GHz) are from the optical circulator in the test apparatus (which includes a Corning SMF-28TM leader attached to the circulator fiber). The remaining five peaks (L05 to L09) result from acoustic modes that reside primarily in the matched-index deposited inner cladding. A best-fit consisting of a summation of eleven Lorentzian profiles was performed, with the results of the measured values summarized in Table 1

Table 1. Measured Parameters for All Observed Acoustic Modes m

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(columns 2 and 4).

Also shown in Fig. 5 is a spectrum of the fiber when heated from room temperature (21.5 °C) to a temperature of 118.3 °C (only the P-doped fiber was heated). Much like is observed with germanium-doped fibers [16

16. M. Niklès, L. Thévenaz, and P. A. Robert, “Brillouin gain spectrum characterization in single-mode optical fibers,” J. Lightwave Technol. 15(10), 1842–1851 (1997). [CrossRef]

], the spectrum narrows and the Stokes’ frequency increases with increasing temperature. The fibers associated with the measurement apparatus were not heated, as indicated by the data. Therefore, these peaks are fixed in frequency while the test fiber was heated. In addition, the L04 mode almost merges into the peak of the test apparatus at 21.5 °C due to the relative strength and position of this peak. At 118.3 °C the L04 mode becomes hidden underneath the peaks of the test apparatus due to an increasing frequency shift with increasing temperature. Even though the peaks of the test apparatus get smaller, the L04 mode is essentially invisible under them. This introduces increased measurement error for the L04 mode at elevated temperatures.

The L01 parameters provide for a point-of-reference from which the spectral widths of the other modes can be calculated by invoking the frequency-squared dependence of the Brillouin spectral width. Since the Brillouin spectral width of the fiber of this study decreases with increasing temperature, we have made the assumption that the spectral width of P-doped silica fibers obeys the simple frequency-squared law, in contrast to some materials such as boron doped germanosilicate optical fibers [17

17. P.-C. Law and P. D. Dragic, “Wavelength dependence of the Brillouin spectral width of boron doped germanosilicate optical fibers,” Opt. Express 18(18), 18852–18865 (2010). [CrossRef] [PubMed]

]. These effects were taken into consideration for the final fit. We point out that the best-fit spectral width for phosphorus oxide (177 MHz at 11 GHz) in Table 2 is the bulk-material value. This corresponds to an acoustic attenuation coefficient of about 1.411 × 105 m−1 at 11 GHz. This frequency was previously taken as a convenient reference since it is the Brillouin frequency shift of the L01 acoustic mode of Corning SMF-28TM at 1534 nm.

We point out that as with the Ge-doped fiber of [6

6. P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]

], the spectral width decreases with increasing mode number. In [6

6. P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]

], this resulted from the fact that as the mode number increases, the proportion of acoustic power occupying the inner dip region of the core increases. The fact that the central dip region in the Ge fiber contained a lower GeO2 concentration (which has a larger acoustic attenuation coefficient than SiO2) leads to narrower Brillouin spectra for modes occupying a greater proportion of that region. In the present case, however, the P concentration is decreasing radially-outward in the region between 1 and 3 μm. Since the inner-core region has more P and less Si than the outer-core region, and since SiO2 has a much lower attenuation coefficient than P2O5, the spectral widths of the higher-order acoustic modes (HOAMs) are smaller than that of the fundamental mode. This results from the increasing mode diameter of the HOAMs with increasing mode number (i.e. the fundamental acoustic mode is most tightly confined to the center of the fiber where the acoustic attenuation is the largest). To illustrate this point, Fig. 7
Fig. 7 Normalized longitudinal acoustic modes L01 and L04 with the measured RIP. The red-dashed curve is the RIP. The green curve is the spatial distribution of L01 and the brown curve is the spatial distribution of L04. The spatial distribution of L04 has a bigger mode diameter than that of L01 and occupies more of the outer-core region.
shows a plot of normalized longitudinal acoustic modes L01 and L04 together with the measured RIP.

4. Temperature-dependent measurements and analysis

Meanwhile, we use the temperature-dependent Sellmeier model [18

18. G. Ghosh, M. Endo, and T. Iwasaki, “Temperature-dependent Sellimeier coefficients and chromatic dispersions for some optical fiber glasses,” J. Lightwave Technol. 12(8), 1338–1342 (1994). [CrossRef]

] for the refractive index of bulk pure silica and offset the refractive index in [18

18. G. Ghosh, M. Endo, and T. Iwasaki, “Temperature-dependent Sellimeier coefficients and chromatic dispersions for some optical fiber glasses,” J. Lightwave Technol. 12(8), 1338–1342 (1994). [CrossRef]

] at 21.5 °C to 1.443 in accordance with the Table 2 assumption. With the refractive index of bulk P2O5 found in Table 2 and using the TOC (−9.22 × 10−5/°C) found in [19

19. S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J.-P. Meunier, “Strain and temperature sensing characteristics of single-mode–multimode–single-mode structures,” J. Lightwave Technol. 27(13), 2348–2356 (2009). [CrossRef]

,20

20. E. T. Y. Lee and E. R. M. Taylor, “Thermo-optic coefficients of potassium alumino-metaphosphate glasses,” J. Phys. Chem. Solids 65(6), 1187–1192 (2004). [CrossRef]

], one can gain the refractive index of P2O5 at different temperatures and different concentrations for simulating the P2O5-SiO2 mixture via the additive model. In addition, we assume the thermo-optic effect of phosphorus dominates fluorine in the fifth layer in Table 2 at higher temperatures due to the insignificant thermo-optic effect of fluorine in a silica glass with very low concentration as in the present case [21

21. A. Koike and N. Sugimoto, “Temperature dependences of optical path length in fluorine-doped silica glass and bismuthate glass,” Proc. SPIE 6116, 176–183 (2006).

]. Then, from the data, we will deduce an analogous TAC for the acoustic velocity.

VaSiO2(T)=0.555(T21.5C)+5968.65m/s.
(5)

However, we indicate that the P2O5-doped fiber and the Z-fiberTM are produced by different manufacturers with different conditions. It is therefore not surprising that the room temperature acoustic velocity of pure bulk silica derived from the Sumitomo fiber (5969 m/s) differs (although only slightly) from that provided in Table 2 (5970 m/s). This may be due to different thermal histories [22

22. R. Le Parc, C. Levelut, J. Pelous, V. Martinez, and B. Champagnon, “Influence of fictive temperature and composition of silica glass on anomalous elastic behaviour,” J. Phys. Condens. Matter 18(32), 7507–7527 (2006). [CrossRef] [PubMed]

] or draw tensions [23

23. W. Zou, Z. He, A. D. Yablon, and K. Hotate, “Dependence of Brillouin frequency shift in optical fibers on draw-induced residual elastic and inelastic strains,” IEEE Photon. Technol. Lett. 19(18), 1389–1391 (2007). [CrossRef]

]. For consistency with the present analysis, we offset the interception in Eq. (5) of the linear equation of the acoustic velocity to 5970 m/s for the temperature-dependent acoustic velocity of SiO2 (~1.4 m/s or 0.02%).

Interestingly, the slope of the temperature curves is increasing with increasing mode number. As with the spectral width, since the HOAMs occupy proportionally more space in the outer region of the core than the fundamental mode, where there is a lesser content of P2O5, the slope is expected to increase with increasing mode number. This is because the acoustic velocity of silica has a larger dependence on temperature than P2O5 (comparing Eq. (4) given that Cp = + 0.119 m/sec/°C and Eq. (5)), and P2O5 has a relatively large negative thermo-optic coefficient.

Finally, a design example is presented where a composition is determined for which the dependence of the Brillouin frequency shift on temperature is minimized. Given the large negative TOC of P2O5 and its smaller TAC than silica, a composition may be found that will minimize the slope of the frequency shift versus temperature curve. The acoustic velocity as a function of temperature may be written

νa(T)=2n(T)VL(T)λo.
(6)

Taking the derivative with respect to T and setting it to zero yields the following condition for a temperature-independent νa (T)

dn(T)dT1n(T)=dVL(T)dT1VL(T).
(7)

In the present case, P2O5 has negative TOC, dn(T)/dT < 0, but positive TAC, dVL(T)/dT > 0. Assuming that the derivatives are approximately constant over a small temperature range, Eq. (7) is simplified to

VL(T)n(T)=Constant.
(8)

In general, this expression is difficult to satisfy for all T due to the dissimilarity in the functional forms of n and VL. However, a range of temperatures can be found where the slope, dν(T)/dT, is minimized. First, we assume the temperature-dependent forms of the refractive index [18

18. G. Ghosh, M. Endo, and T. Iwasaki, “Temperature-dependent Sellimeier coefficients and chromatic dispersions for some optical fiber glasses,” J. Lightwave Technol. 12(8), 1338–1342 (1994). [CrossRef]

,19

19. S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J.-P. Meunier, “Strain and temperature sensing characteristics of single-mode–multimode–single-mode structures,” J. Lightwave Technol. 27(13), 2348–2356 (2009). [CrossRef]

] and acoustic velocity (Eq. (4) and Eq. (5)) as described previously in this paper. We also assume that over a small temperature range, dn(T)/dT for pure P2O5 is a constant. The dν(T)/dT of P2O5-doped silica fiber (Table 3) is lower than that of pure silica (Fig. 9), therefore, this suggests that pure P2O5 possesses a ν(T) that has a negative slope through Eq. (6), while that of pure silica has a positive slope. Thus, one can conclude that there is a composition of P2O5-SiO2 mixture where the slope is balanced to some minimized value and the acoustic frequency of the composition is independent of temperature.

To determine the temperature dependencies of the P2O5-SiO2 mixture, the temperature-dependent physical properties of the individual pure constituents are used in the additive model as described above, and the composition is adjusted until the frequency shift variation is minimized over the region −100 °C < T < 100 °C. The result is shown in Fig. 11
Fig. 11 Brillouin frequency shift versus temperature for the molar composition 0.54P2O5:0.46SiO2. In the range, −100 °C < T < 100 °C, the Stokes’ shift changes by less than 5 kHz per °C. An absolute minimum frequency shift variation is around 10°C where the constant ratio of VL to n is about 2946 m/sec. The inset shows the derivative of the frequency-versus-temperature curve.
for the molar composition 0.54P2O5:0.46SiO2. The inset shows the derivative of the frequency-versus-temperature curve. In the range −100 °C < T < 100 °C, the Brillouin frequency shift changes by less than 5 kHz per °C. Therefore, this composition is calculated to be more than 200 times less sensitive to temperature changes than the pure silica core fiber (see Fig. 9). An absolute minimum in the slope of frequency shift-versus-temperature curve is observed near 10°C. While this result is for the bulk material, wave-guiding properties should be taken into consideration when designing a fiber.

5. Conclusion

From BGS measurements of a fiber with a large P content in the core, we have provided an acoustic damping coefficient for bulk P2O5 that is suitable for modeling purposes. We find that this coefficient is about 1.411 × 105 m−1, which is similar to, but larger than, that of GeO2. Furthermore, our results for the effect of phosphorus oxide on the longitudinal velocity (~−0.90%/mol% up to 10mol %) are similar to those (~−0.71%/mol% up to 5.5mol%) found in [4

4. C.-K. Jen, C. Neron, A. Shang, K. Abe, L. Bonnell, and J. Kushibiki, “Acoustic characterization of silica glasses,” J. Am. Ceram. Soc. 76(3), 712–716 (1993). [CrossRef]

]. We have found that the temperature coefficients for the Stokes’ frequency shift and Brillouin spectral width for the L01 acoustic mode of the fiber are ~ + 0.74 MHz/°C and ~−77.7 kHz/°C, respectively. In addition, we also determine the thermo-acoustic coefficient of bulk P2O5 to be about + 0.119 m/sec/°C, which is much lower than that of pure silica ( + 0.555 m/sec/°C). We have shown that the use of these bulk coefficients can be used to predict the temperature-dependent Stokes’s shift of the higher-order acoustic modes with a high degree of accuracy. This data is also useful for the design of acoustic profiles of optical fiber for applications where Brillouin scattering is significant. To illustrate this point, a design example was presented where the slope of the Stokes’ shift versus temperature curve was compositionally minimized to be less than 5 kHz per °C over the temperature range −100 °C < T < 100 °C.

Acknowledgments

This work was supported in part by the Joint Technology Office (JTO) through their High Energy Laser Multidisciplinary Research Initiative (HEL-MRI) program entitled “Novel Large-Mode-Area (LMA) Fiber Technologies for High Power Fiber Laser Arrays” under ARO subcontract # F014252. P.-C. Law would like to acknowledge Professor Gary R. Swenson for his full support. The RIP measurements at 1000nm were performed by A. Yablon at Interfiber Analysis.

References and links

1.

P. D. Dragic, C.-H. Liu, G. C. Papen, and A. Galvanauskas, “Optical fiber with an acoustic guiding layer for stimulated Brillouin scattering suppression,” in CLEO/QELS2005, Vol. 3 of 2005 Conference on Lasers and Electro-Optics, Paper CThZ3.

2.

M. J. Li, X. Chen, J. Wang, S. Gray, A. Liu, J. A. Demeritt, A. B. Ruffin, A. M. Crowley, D. T. Walton, and L. A. Zenteno, “Al/Ge co-doped large mode area fiber with high SBS threshold,” Opt. Express 15(13), 8290–8299 (2007). [CrossRef] [PubMed]

3.

P. D. Dragic, “Novel dual-Brillouin-frequency optical fiber for distributed temperature sensing,” Proc. SPIE 7197, 719710 (2009). [CrossRef]

4.

C.-K. Jen, C. Neron, A. Shang, K. Abe, L. Bonnell, and J. Kushibiki, “Acoustic characterization of silica glasses,” J. Am. Ceram. Soc. 76(3), 712–716 (1993). [CrossRef]

5.

Y. Koyamada, S. Sato, S. Nakamura, H. Sotobayashi, and W. Chujo, “Simulating and designing Brillouin gain spectrum in single-mode fibers,” J. Lightwave Technol. 22(2), 631–639 (2004). [CrossRef]

6.

P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]

7.

P. D. Dragic, “Simplified model for the effect of Ge doping on silica fibre acoustic properties,” Electron. Lett. 45(5), 256–257 (2009). [CrossRef]

8.

P. D. Dragic and B. G. Ward, “Accurate modeling of the intrinsic Brillouin linewidth via finite element analysis,” IEEE Photon. Technol. Lett. 22(22), 1698–1700 (2010). [CrossRef]

9.

C. Krischer, “Optical measurements of ultrasonic attenuation and reflection losses in fused silica,” J. Acoust. Soc. Am. 48(5B), 1086–1092 (1970). [CrossRef]

10.

A. S. Pine, “Brillouin scattering study of acoustic attenuation in fused quartz,” Phys. Rev. 185(3), 1187–1193 (1969). [CrossRef]

11.

R. E. Youngman, J. Kieffer, J. D. Bass, and L. Duffrène, “Extended structural integrity in network glasses and liquids,” J. Non-Cryst. Solids 222, 190–198 (1997). [CrossRef]

12.

A. D. Yablon, “Multi-wavelength optical fiber refractive index profiling by spatially resolved Fourier transform spectroscopy,” J. Lightwave Technol. 28(4), 360–364 (2010). [CrossRef]

13.

M. M. Bubnov, E. M. Dianov, O. N. Egorova, S. L. Semjonov, A. N. Guryanov, V. F. Khopin, and E. M. DeLiso, “Fabrication and investigation of single-mode highly phosphorus-doped fibers for Raman lasers,” Proc. SPIE 4083, 12–22 (2000). [CrossRef]

14.

D. J. DiGiovanni, J. B. MacChesney, and T. Y. Kometani, “Structure and properties of silica containing aluminum and phosphorus near the AlPO4 join,” J. Non-Cryst. Solids 113(1), 58–64 (1989). [CrossRef]

15.

R. W. Boyd, K. Rzaewski, and P. Narum, “Noise initiation of stimulated Brillouin scattering,” Phys. Rev. A 42(9), 5514–5521 (1990). [CrossRef] [PubMed]

16.

M. Niklès, L. Thévenaz, and P. A. Robert, “Brillouin gain spectrum characterization in single-mode optical fibers,” J. Lightwave Technol. 15(10), 1842–1851 (1997). [CrossRef]

17.

P.-C. Law and P. D. Dragic, “Wavelength dependence of the Brillouin spectral width of boron doped germanosilicate optical fibers,” Opt. Express 18(18), 18852–18865 (2010). [CrossRef] [PubMed]

18.

G. Ghosh, M. Endo, and T. Iwasaki, “Temperature-dependent Sellimeier coefficients and chromatic dispersions for some optical fiber glasses,” J. Lightwave Technol. 12(8), 1338–1342 (1994). [CrossRef]

19.

S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J.-P. Meunier, “Strain and temperature sensing characteristics of single-mode–multimode–single-mode structures,” J. Lightwave Technol. 27(13), 2348–2356 (2009). [CrossRef]

20.

E. T. Y. Lee and E. R. M. Taylor, “Thermo-optic coefficients of potassium alumino-metaphosphate glasses,” J. Phys. Chem. Solids 65(6), 1187–1192 (2004). [CrossRef]

21.

A. Koike and N. Sugimoto, “Temperature dependences of optical path length in fluorine-doped silica glass and bismuthate glass,” Proc. SPIE 6116, 176–183 (2006).

22.

R. Le Parc, C. Levelut, J. Pelous, V. Martinez, and B. Champagnon, “Influence of fictive temperature and composition of silica glass on anomalous elastic behaviour,” J. Phys. Condens. Matter 18(32), 7507–7527 (2006). [CrossRef] [PubMed]

23.

W. Zou, Z. He, A. D. Yablon, and K. Hotate, “Dependence of Brillouin frequency shift in optical fibers on draw-induced residual elastic and inelastic strains,” IEEE Photon. Technol. Lett. 19(18), 1389–1391 (2007). [CrossRef]

OCIS Codes
(060.2290) Fiber optics and optical communications : Fiber materials
(060.2300) Fiber optics and optical communications : Fiber measurements
(290.5830) Scattering : Scattering, Brillouin

ToC Category:
Materials for Fiber Optics

History
Original Manuscript: June 17, 2011
Revised Manuscript: July 15, 2011
Manuscript Accepted: July 16, 2011
Published: July 22, 2011

Citation
Pi-Cheng Law, Yuh-Shiuan Liu, André Croteau, and Peter D. Dragic, "Acoustic coefficients of P2O5-doped silica fiber: acoustic velocity, acoustic attenuation, and thermo-acoustic coefficient," Opt. Mater. Express 1, 686-699 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-4-686


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References

  1. P. D. Dragic, C.-H. Liu, G. C. Papen, and A. Galvanauskas, “Optical fiber with an acoustic guiding layer for stimulated Brillouin scattering suppression,” in CLEO/QELS2005, Vol. 3 of 2005 Conference on Lasers and Electro-Optics, Paper CThZ3.
  2. M. J. Li, X. Chen, J. Wang, S. Gray, A. Liu, J. A. Demeritt, A. B. Ruffin, A. M. Crowley, D. T. Walton, and L. A. Zenteno, “Al/Ge co-doped large mode area fiber with high SBS threshold,” Opt. Express 15(13), 8290–8299 (2007). [CrossRef] [PubMed]
  3. P. D. Dragic, “Novel dual-Brillouin-frequency optical fiber for distributed temperature sensing,” Proc. SPIE 7197, 719710 (2009). [CrossRef]
  4. C.-K. Jen, C. Neron, A. Shang, K. Abe, L. Bonnell, and J. Kushibiki, “Acoustic characterization of silica glasses,” J. Am. Ceram. Soc. 76(3), 712–716 (1993). [CrossRef]
  5. Y. Koyamada, S. Sato, S. Nakamura, H. Sotobayashi, and W. Chujo, “Simulating and designing Brillouin gain spectrum in single-mode fibers,” J. Lightwave Technol. 22(2), 631–639 (2004). [CrossRef]
  6. P. D. Dragic, “Estimating the effect of Ge doping on the acoustic damping coefficient via a highly Ge-doped MCVD silica fiber,” J. Opt. Soc. Am. B 26(8), 1614–1620 (2009). [CrossRef]
  7. P. D. Dragic, “Simplified model for the effect of Ge doping on silica fibre acoustic properties,” Electron. Lett. 45(5), 256–257 (2009). [CrossRef]
  8. P. D. Dragic and B. G. Ward, “Accurate modeling of the intrinsic Brillouin linewidth via finite element analysis,” IEEE Photon. Technol. Lett. 22(22), 1698–1700 (2010). [CrossRef]
  9. C. Krischer, “Optical measurements of ultrasonic attenuation and reflection losses in fused silica,” J. Acoust. Soc. Am. 48(5B), 1086–1092 (1970). [CrossRef]
  10. A. S. Pine, “Brillouin scattering study of acoustic attenuation in fused quartz,” Phys. Rev. 185(3), 1187–1193 (1969). [CrossRef]
  11. R. E. Youngman, J. Kieffer, J. D. Bass, and L. Duffrène, “Extended structural integrity in network glasses and liquids,” J. Non-Cryst. Solids 222, 190–198 (1997). [CrossRef]
  12. A. D. Yablon, “Multi-wavelength optical fiber refractive index profiling by spatially resolved Fourier transform spectroscopy,” J. Lightwave Technol. 28(4), 360–364 (2010). [CrossRef]
  13. M. M. Bubnov, E. M. Dianov, O. N. Egorova, S. L. Semjonov, A. N. Guryanov, V. F. Khopin, and E. M. DeLiso, “Fabrication and investigation of single-mode highly phosphorus-doped fibers for Raman lasers,” Proc. SPIE 4083, 12–22 (2000). [CrossRef]
  14. D. J. DiGiovanni, J. B. MacChesney, and T. Y. Kometani, “Structure and properties of silica containing aluminum and phosphorus near the AlPO4 join,” J. Non-Cryst. Solids 113(1), 58–64 (1989). [CrossRef]
  15. R. W. Boyd, K. Rzaewski, and P. Narum, “Noise initiation of stimulated Brillouin scattering,” Phys. Rev. A 42(9), 5514–5521 (1990). [CrossRef] [PubMed]
  16. M. Niklès, L. Thévenaz, and P. A. Robert, “Brillouin gain spectrum characterization in single-mode optical fibers,” J. Lightwave Technol. 15(10), 1842–1851 (1997). [CrossRef]
  17. P.-C. Law and P. D. Dragic, “Wavelength dependence of the Brillouin spectral width of boron doped germanosilicate optical fibers,” Opt. Express 18(18), 18852–18865 (2010). [CrossRef] [PubMed]
  18. G. Ghosh, M. Endo, and T. Iwasaki, “Temperature-dependent Sellimeier coefficients and chromatic dispersions for some optical fiber glasses,” J. Lightwave Technol. 12(8), 1338–1342 (1994). [CrossRef]
  19. S. M. Tripathi, A. Kumar, R. K. Varshney, Y. B. P. Kumar, E. Marin, and J.-P. Meunier, “Strain and temperature sensing characteristics of single-mode–multimode–single-mode structures,” J. Lightwave Technol. 27(13), 2348–2356 (2009). [CrossRef]
  20. E. T. Y. Lee and E. R. M. Taylor, “Thermo-optic coefficients of potassium alumino-metaphosphate glasses,” J. Phys. Chem. Solids 65(6), 1187–1192 (2004). [CrossRef]
  21. A. Koike and N. Sugimoto, “Temperature dependences of optical path length in fluorine-doped silica glass and bismuthate glass,” Proc. SPIE 6116, 176–183 (2006).
  22. R. Le Parc, C. Levelut, J. Pelous, V. Martinez, and B. Champagnon, “Influence of fictive temperature and composition of silica glass on anomalous elastic behaviour,” J. Phys. Condens. Matter 18(32), 7507–7527 (2006). [CrossRef] [PubMed]
  23. W. Zou, Z. He, A. D. Yablon, and K. Hotate, “Dependence of Brillouin frequency shift in optical fibers on draw-induced residual elastic and inelastic strains,” IEEE Photon. Technol. Lett. 19(18), 1389–1391 (2007). [CrossRef]

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