A low-loss and highly birefringent polarization maintaining photonic crystal fiber has been fabricated. The fiber loss and modal birefringence at 1550 nm were 1.3 dB/km and 1.4×10-3, respectively.
© Optical Society of America
Polarization maintaining fibers (PMFs) are expected to play an important role in high bit-rate fiber transmission systems. For example, PMFs can eliminate the influence of polarization mode dispersion (PMD) or stabilize the operation of optical devices. Currently, PMFs such as PANDA  and bow-tie fibers  are used for polarization maintaining applications or transmission lines. Their birefringence is due to stress in the core region, and their modal birefringence is ~5×10-4.
Photonic crystal fibers (PCFs) are of great interest for optical communication in new wavelength regions and for new optical functional devices [3–5]. It is also possible to realize highly birefringent fiber with a photonic crystal structure [6–9]. Polarization maintaining (PM) PCFs have different air-hole diameters along two orthogonal axes near the core region [6–8], which provide an effective index difference between the two orthogonal polarization modes. It has been shown that their birefringence is of the order of 10-3, which is one order of magnitude larger than that of conventional PMFs, and better PM characteristics are expected. However, neither low loss PM-PCF with a length of ~km nor high-speed transmission experiments have been reported.
In this paper, we report for the first time a 1.5 km-long ultra low-loss PM-PCF whose transmission loss and crosstalk at 1550 nm were 1.3 dB/km and less than -22 dB, respectively. The polarization properties, such as birefringence, PMD, and group velocity dispersion (GVD) were measured and analyzed using a numerical simulation.
2. Design of PM-PCF
Figure 1 shows the schematic design and a scanning electron microscope (SEM) photograph of a PM-PCF. To introduce large birefringence into the PCF, we enlarged two of the central air holes. The birefringence arises from the effective refractive index difference between the x and y polarization modes. The diameters of the small and large air holes, (d1) and (d2), respectively, and the air hole pitch (Λ) gives the characteristics of PM-PCF. This is a variation of the structure described in Ref. 7. This fiber was designed to have high birefringence and low loss with a simple structure. The structure in Ref. 8 appears similar to ours, however it is difficult to use that structure to fabricate fiber with high dimensional precision because the large air holes shift towards the core. By contrast, our fiber has a constant Λ, making it easier to fabricate with a conventional multiple-capillary drawing method.
The ratio d1/d2 determines the modal birefringence of the fiber. When d1/d2=1, the fiber is identical to conventional PCF. There is no birefringence in this structure . The modal birefringence increases as d1/d2 decreases. This is because the six-fold symmetry is destroyed with a decrease in the d1/d2 ratio and the effective refractive index for the orthogonal polarization modes changes. In this experiment, d1/d2 was set at 0.40, and the fiber was 1.5 km long. It should be noted that this fiber was single-mode from 1.0 to 1.7 µm, however the cutoff wavelength could not measured by the bending method because higher order modes could not be converted to radiation modes by bending. The near field pattern of the fiber was elliptical and the mode field diameters at 1550 nm for the x and y directions were 3.5 and 6.1 µm, respectively.
The wavelength dependence of the fiber loss is shown in Fig. 2(a). The fiber losses at 1550, 1300, and 850 nm were 1.3, 2.0, and 4.2 dB/km, respectively. The Rayleigh scattering coefficient of the PCF and the structural imperfection loss were 1.9 dB/km/(µm)4 and 1.0 dB/km, respectively. If we can reduce the structural imperfection loss, the fiber loss will be reduced to less than 0.5 dB/km, which is of the same order as that of conventional polarization maintaining fiber. We measured the fiber loss along the fiber length using an optical time domain reflectometer (OTDR) at 1.55 µm as shown in Fig. 2(b). It is clear from the figure that the slope is constant and there is no discontinuity caused by structural imperfections.
The maximum loss difference of the two orthogonal polarization modes (PDL) in the 1.5 µm band, measured using polarization analyzer, was 0.27 dB/km. This shows that the large air holes have little influence on PDL.
3. Polarization characteristics of PM-PCF
We evaluated the birefringence of the PM-PCF by measuring the beat length between the two orthogonal polarization modes. The relationship between the polarization beat length LB and the modal birefringence B is given by 
where βx and βy are the propagation constants of the two modes and nx and ny are the effective refractive index for each polarization mode, λ is the wavelength of the light in vacuum. Direct observation of the intensity change due to the polarization beat between the polarization modes along the fiber was difficult, because the beat length of the fiber was of the order of a few mm at 1550 nm. So, we launched the linearly polarized light at 45 degrees from the principal axis into the PM-PCF and measured the output power as a function of wavelength. The optical sources used for the measurement were edge emitting LEDs operating from 1200 to 1600 nm and a tunable cw Ti: sapphire laser operating from 750 to 950 nm. Figure 3 shows the wavelength dependence of the fiber birefringence. A birefringence of 1.4×10-3 was obtained at 1550 nm. This is three times larger than that of a conventional PANDA fiber. The birefringence decreased as the wavelength decreased. The mode field diameter becomes smaller as the wavelength shortens, and this results in a reduction in modal asymmetry. The blue curve in Fig. 3 shows calculated results obtained using the fiber parameters . The experimental and calculated results agreed very well. If we assume that birefringence B is expressed as , the experimental and calculated k0 values are 2.65 and 2.74, respectively.
The PMD at 1550 nm was 4.7 ps/m, which we measured from the pulse separation due to PMD by using a subpicosecond optical pulse from an optical parametric oscillator operating in the 1.5 µm band. The PMD wavelength dependence is shown in Fig. 4. The PMD decreases monotonously as the wavelength decreases, which is similar to the birefringence. Calculated results are also shown in Fig. 4.
The crosstalk between the polarization modes for 100 m and 1.5 km-long fibers were -35 and -22 dB, respectively. These values are better than that of conventional PANDA fiber. By optimizing the fiber parameters, we can realize higher birefringence and lower crosstalk, which means that we can use this PM-PCF as a polarization maintaining optical transmission line.
Figure 5 shows the GVD wavelength dependence for each (fast and slow) polarization mode. Measurements were performed by launching a linearly polarized signal along the one of the polarization axes of the fiber. The black and blue circles show the measured results for the fast and slow polarization modes, respectively. The GVDs for the two polarization modes at 1550 nm were 59.5 and 66.8 ps/km/nm, and the difference was as much as 7.3 ps/m/nm. The dispersion slopes for each polarization mode were 0.070 and 0.071, respectively. This can be explained as follows. The GVD is given by
where c is the light velocity in a vacuum, and k=2π/λ. GVD is mainly comprised by waveguide dispersion and the material dispersion. As mentioned above, the frequency dependence of the propagation constant β is calculated by using the geometrical parameters of the fiber. And, the material dispersion is given by Sellmeier’s formula. The difference between the GVD values of the two orthogonal polarization modes, which is the same as the difference in waveguide dispersion, is given as
The experimental and calculated values of δσ at 1550 nm were 7.3 and 9.5 ps/km/nm, respectively. The calculated dispersion slopes for both modes were 0.047 and 0.052. These show that there is good agreement between the calculated and experimental results.
A low-loss (1.3 dB/km) and high extinction ratio polarization-maintaining photonic crystal fiber was successfully fabricated. Its birefringence of 1.4×10-3 was higher than that of conventional stress-induced PM fibers. The polarization properties (birefringence, PMD, GVD) were well explained by using a numerical simulation.
The authors thank Dr. M. Kawachi, Dr. K. Sato, and H. Tanaka for their encouragement.
References and Links
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