Experimental investigation of the polarization

Experimental investigation of the polarization properties of a hollow core photonic bandgap fiber for 1550 nm Mark Wegmuller, Matthieu Legré, Nicolas Gisin Group of Applied Physics, University of Geneva, 20 Ecole de Médecine, CH-1211 Genève, Switzerland [email protected]

Theis P. Hansen, Christian Jakobsen, Jes Broeng Crystal Fibre A/S, Blokken 84, DK-3460 Birkerod, Denmark

Abstract: The properties of a hollow core photonic bandgap fiber designed for 1.55 um transmission are investigated with special emphasis on polarization issues. Large and strongly wavelength dependent phase and group delays are found. At the same time the principle states of polarization move strongly and erratically as a function of wavelength, leading to strong mode coupling. Wavelength regions with high polarization dependent loss coincide with depolarization due to a polarization dependent coupling to surface modes at these wavelengths. ©2005 Optical Society of America OCIS codes: (060.2310) Fiber optics; (999.9999) Photonic bandgap fiber; (060.2400) Fiber properties; (260.1440) Birefringence; (260.5430) Polarization.

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Received 31 January 2005; revised 18 February 2005; accepted 20 February 2005

7 March 2005 / Vol. 13, No. 5 / OPTICS EXPRESS 1457

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1. Introduction The feasibility of optical fibers based on photonic bandgap guidance has opened the door for a completely new class of fibers [1] with even more interesting properties than the (effective) index guiding photonic crystal fibers (PCF). Most intriguing, it is no longer necessary for the #6504 - $15.00 US

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core to be denser than the cladding material, and therefore light guidance in an air-core can be achieved [2]. One obvious application for such hollow-core photonic bandgap fibers (HCPBGF) is in high power pulse delivery and shaping [3-5] due to their low nonlinearity and tailorable dispersion. For transmission, in addition to the two previous properties, it is the potentially very low loss of HC-PBGF that is of interest as well. By optimizing the cladding design and better mastering technological issues allowing for more regular and homogeneous fiber structures, the losses of HC-PBGF was dramatically lowered from ~1000 dB/km to 13 dB/km [6,7], to 1.7 dB/km [8], to the actual record number of 1.2 dB/km [9] in just 3 years. At the same time, practical fiber lengths have increased to the km range [8,10]. Although the losses are still large compared to standard SMF and therefore continue to be an important research topic [11-13], fibers are now sufficiently advanced so that other practical topics such as mode structures, chromatic dispersion, bending loss, fiber splicing, etc, become important. Whereas the modes and group velocity dispersion have been investigated both theoretically and experimentally in some detail [5,7,8,11,13-16], less effort has been put to investigate the other mentioned parameters of practical interest [10,17,18]. Especially, only a few papers report on the polarization properties of HC-PBGF [18-21], and almost no experimental data are available [18,19,21]. It is therefore timely to expand our previous study [21] on the subject. The paper is structured as follows: Section 2 gives a short description of our HC-PBGF and its basic properties. In Section 3, we report on different investigations of polarization issues such as differential phase and group delay and their wavelength and temperature dependence, polarization dependent loss and degree of polarization of the transmitted light, and polarization mode coupling. Section 4 finally summarizes the paper 2. Description of the investigated fiber and basic properties The HC-PBGF investigated in this paper is Crystal Fibers AIR-10-1550. It is designed to operate, single-mode, in the 1550 nm window. Fig. 1 shows scanning electro-micrographs (SEM) pictures of typical cross-sections of the fiber. From Fig. 1 (right), the diameter of the central air core is determined to 8.6 to 9 um, with a ratio of major to minor axis of ~1.05. The physical length of the fiber sample is 50 m. If not stated otherwise, standard SMF pigtails (G.652, MFD 9.3 um at 1550 nm) of ~1m length, spliced to both the launch and receive end of the HC-PBGF, were employed in the measurements.

Fig. 1. SEM pictures of a typical cross-section of the investigated HC-PBGF.

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The overall transmission loss is shown in Fig. 2. It was obtained using a depolarized LED source and an optical spectrum analyzer. The data was verified with some point measures of the transmission using a tunable laser source and a polarization scrambler at the launch end along with a PDL meter capable of giving the mean transmitted power. As can be seen from comparison to the corresponding data for a 2.2 m sample of the same type of HC-PBGF, the transmission window (FWHM) reduces from 179 nm to 116 nm for the 50 m long sample, indicating that there might be some small inhomogeneity in the micro-structure along the fiber. However, a distributed measurement of the reflectivity along the fiber (Fig. 3) using a coherent optical frequency domain technique [22] shows no indication of isolated point defects, and the loss is found to be homogeneously distributed along the fiber. It amounts to 88 dB/km (1550 nm). 0 -60

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Another interesting feature is revealed by Fig. 3: after the peak from the end-reflection of the HC-PBGF, the reflectivity does not immediately drop to the Rayleigh level of the following SMF pigtail, indicating the presence of some light traveling along the fiber in slower higher order or surface modes. The fact that there is a continuous drop rather than different separate peaks suggests that there might be a continuous forth- and back coupling of light between these higher order modes and the fundamental mode occurring along the fiber (rather than a coupling into different modes directly at the launch end). For 1550nm, the overall energy of these slower moving modes is very small (~2%) compared to the #6504 - $15.00 US

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fundamental mode, so that the HC-PBGF can be considered single-moded at this wavelength. This is however not true for all wavelengths in the transmission window as will be shown in the next section. Finally, Fig. 3 also shows that the backscattering level in the HC-PBGF is larger than the Rayleigh backscattering (RBS) in the preceding SMF fiber. Assuming symmetric splice losses of ~2 dB (8.8 dB overall loss minus 4.5 dB fiber loss, all divided by 2), a ratio of ~3.5 is found. Note that mode-field measurements show that most of the energy (~97 %) evolves in the center air hole, and only a very small fraction of light ( =

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Fig. 8. Interferometric PMD traces for 50 m long HC-PBGF, using an LED centered at 1542.5 nm. Green, no HC-PBGF; blue, depolarized LED; red, polarized LED.

3.2 Differential phase and group delay at 1550 nm The phase delay at 1550 nm is measured using a P-OFDR technique described in [23]. As Fig. 7 demonstrates, there are no clearly distinct frequency components in the backscattered signal as would be the case for a fiber with low polarization mode coupling [23, 24], but rather a distribution up to several hundreds of inverse meters. This is a typical signature for large polarization mode coupling [23, 25], and a corresponding model [25] can be used to extract the mean beatlength characterizing the mean phase birefringence of the 50 m long HCPBGF. Averaging over different launch polarizations, a value of =1.1 cm is found. Note that it is very unusual for a fiber with such high birefringence (compared to a SMF fiber where Lb ~ 30 m) to exhibit strong mode coupling - we come back to this point in more detail in section 3.5. Analyzing the P-OFDR data for different locations along the HC-PBGF, no significant difference in the beatlength is found, and one can therefore reasonably assume that the birefringence is quite homogeneously distributed along the fiber. We then investigated the differential group delay at ~1550 nm of the HC-PBGF using the interferometric technique [26]. The employed source was an LED centered at 1542.5 nm with #6504 - $15.00 US

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a bandwidth (3dB) of 50.6 nm. The corresponding temporal coherence of the source gives a system response of ~0.07 ps (green curve in Fig. 8). The interferometric trace of the HCPBGF is shown in Fig. 8 (red trace). Again, the curve is a signature of large polarization mode coupling, and averaging over different measurements, a mean group delay of 38±2.6 ps is found. This is a very large delay for a fiber with a length of just 50 m, and the corresponding PMD assuming a square-root length dependence amounts to 170 ps/√km (PMD