The temperature sensitivity of modal birefringence (dB/dT) of different types of polarization-maintaining fibers with side holes was measured using a Sagnac loop interferometer. The thermal expansion coefficient can be varied by controlling the amount of germanium doped in the core region. Using this method, dB/dT could be made higher (~10-7/°C) than that of standard PMFs (~10-8/°C) or comparable to that of standard PMFs (~10-9/°C).
©2007 Optical Society of America
Several techniques of fiber optic temperature sensors have been proposed for many industrial applications. Among them, the use of Fiber Bragg gratings [1–2] is particularly attractive for distributed sensing, but they suffer from limited temperature-induced spectral displacements (~ 0.01 nm/K) and need isolators to prevent back reflections. The use of long-period fiber gratings for a temperature sensor, which exhibit higher sensitivity (~0.1 nm/K) and low back reflections, may cause undesirable changes of the spectral response due to the high bending sensitivity . A polarization maintaining fiber (PMF) Sagnac loop interferometer, where two different polarization modes exhibit different responses to temperature, has attracted much attention owing to its advantages such as easy manufacture, great flexibility, and good stability [4–6].
The birefringence of stress-induced PMFs is in general more sensitive to temperature variation than that of standard PMFs. Indeed, high temperature sensitivity in commercial PMFs has been demonstrated by several temperature sensors [7–8]. On the other hand, when the PMF is used to sense other physical parameters such as strain/bending/pressure, the temperature effect must be minimized [9–11]. This requires the fiber with birefringence that is insensitive to temperature variation. Thus, the control of temperature sensitivity of the modal birefringence in PMFs is important for fiber optic sensing. In this paper, we investigate the temperature sensitivities of modal birefringence of different types of PM side-hole fibers using a Sagnac loop interferometer.
2. The birefringence measurement of the fabricated PM side-hole fiber
To fabricate the PM side-hole fiber, we used the MCVD process. Figure 1 shows the cross sections of the fabricated PMF with an elliptical core and side holes. The elliptical-shape fiber with elliptical core [Fig. 1(a)] was fabricated by over-jacketing (19 × 25 tube) and collapsing after cutting both sides of the preform. In this preform, the Ge/B was co-doped in the core region . To fabricate the circular-shape fibers with elliptical core [Figs. 1(b), 1(c)], two holes with the diameter of 5 mm were drilled on both sides of the core in the preform . To make different Δn, the different amounts of germanium were doped in the core region. The core with an elliptical shape was made by partially collapsing the holes during fiber drawing at 2000 °C. Table 1 shows specification of fabricated PM side-hole fibers.
Generally, the wavelength separation between two transmission peaks of the Sagnac loop interferometer’s output is given by Δλ=λ2/BL (where B is the modal birefringence, L is the length of the PM side-hole fiber, and λ is the operation wavelength) . From the wavelength separation between adjacent transmission peaks of the Sagnac loop interferometer’s output as shown in Fig. 2, the birefringences (B) of the fabricated elliptical core side-hole fibers were 1.7×10-4 (elliptical shape, Δn=0.02), 1.16×10-4 (circular shape I, Δn=0.018), and 1.6×10-5 (circular shape II, Δn=0.0046), respectively.
3. Measurement of temperature sensitivities of modal birefringence (dB/dT) of fabricated PM side-hole fibers using Sagnac loop interferometer
In general, the modal birefringence found in conventional PM optical fibers can be attributed to either geometric- and stress-modal birefringence [15–16]. Geometric modal birefringence is usually induced by an elliptical shaped core and/or cladding region. Stress modal birefringence arises through the stress-optic effect. By placing regions with different thermal expansion coefficients from the host material on either side of the core, differential stress for the x and y polarization modes is introduced. Because dB/dT due to the fiber geometry is much less than that due to the stress-induced modal birefringence, we can effectively control dB/dT by varying the amount of germanium in the core region and thus adjusting the thermal expansion coefficient. In case of PM-PCF, because the core and the cladding are made of the same pure-silica, it has a very small change in the birefringence due to the thermal expansion .
A broadband light source was used as the light input, and the two ends of fiber coupler were fusion-spliced to the PM side-hole fiber forming the Sagnac loop. The PM side-hole fiber was placed in a temperature-controlled oven as shown in Fig. 3.
The phase-shift sensitivity is given by the simple formula :
where, is the phase difference, T is temperature, λ is the wavelength, and L is the length of PM side-hole fiber.
As the temperature is increased, the peak wavelengths shift to shorter wavelength due to the decrease of the birefringence of the PM side-hole fiber in the Sagnac loop interferometer as shown in Fig. 4. From the fitting slope, we can obtain the phase-shift sensitivity of fabricated PM side-hole fibers as shown in Table 2. As Δn increases, the phase-shift sensitivity also increases due to the increase of the thermal expansion coefficient. In temperatures below 45 °C, in case of the PM side-hole fiber with a circular shape II (Δn=0.0046), the peak wavelength shift to longer wavelength because the fiber elongation effect is more dominant than the decrease of birefringence. The relationship between and is given by formula :
From Eq. (2), we could calculate dB/dT as shown in Table 2. The dB/dT was measured to be in the range of ~10-7/oC to ~10-9/°C. In standard PMF and PM-PCF, dB/dT is approximately ~10-8/°C and ~10-9/°C, respectively . Compared with them, the data shows that we can obtain dB/dT higher (~10-7/°C) as well as much less (~10-9/°C, comparable to that of PM-PCF) than that of standard PMF.
4. The peak separation versus temperature
Figure 5 shows the peak separation versus the temperature. The peak separation value at reference temperature was subtracted from each measured data point for offset adjustment. As the temperature is increased, the peak separation increases due to the decrease of the birefringence of the PM side-hole fibers in the Sagnac loop. The measured temperature sensitivity of the peak separation was ranged between 0.013 rad/°C and 0.0063 rad/°C. In terms of the wavelength, the sensitivity was 140 pm/°C (an elliptic shape, 2π rad =67.85 nm), 50 pm/°C (a circular shape I (Δn=0.018), 2π rad =50.29 nm), and 124 pm/°C (a circular shape II (Δn=0.0046), 2π rad =111.64 nm), respectively.
In this work, we have experimentally investigated the temperature sensitivities of modal birefringence of different types of fabricated PM side-hole fiber using Sagnac loop interferometer. By controlling the amount of germanium in the core region, which affects the thermal expansion coefficient, we could make dB/dT either higher (~10-7/°C) than that of standard PMFs (~10-8/°C) or comparable to that of standard PCFs (~10-9/°C). Because the Sagnac loop interferometer has many advantages such as good flexibility and stability, compared with a conventional fiber Mach-Zenhder interferometer, the experimental results in this work will be very useful for applications in optical communication and sensing.
This work was performed under the partial support from the Second-Phase of the Brain Korea-21 Project and the Basic Program Project of KOSEF (Grant No. R01-2006-000-11088-0).
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