Visible to infrared high-speed WDM transmission over PCF Koji Ieda a), Kenji Kurokawa, Katsusuke Tajima, and Kazuhide Nakajima NTT Access Network Service Systems Laboratories, NTT Corporation, 1 7 1 Hanabatake, Tsukuba, Ibaraki 305 0805, Japan a) ieda@ansl.ntt.co.jp Abstract: We achieved high-speed wavelength division multiplexing (WDM) transmission in the visible to infrared region over a 1 km photonic crystal fiber (PCF). We realized ultra-wide and high-speed WDM transmission using six wavelengths in the 658 to 1550 nm wavelength range, which corresponds to a frequency bandwidth of 263 THz. We demonstrated WDM transmission of 1 Gbit/s at 658 nm and 10 Gbit/s at 780 nm and four infrared wavelengths. Our experimental results show that low loss PCF is very attractive for use in future high capacity WDM systems with an ultra wide bandwidth. Keywords: photonic crystal fiber (PCF), visible wavelength, WDM transmission, high-speed transmission Classification: Photonics devices, circuits, and systems References [1] J. C. Knight, T. A. Birks, D. M. Atkin, and P. St. J. Russell, Pure silica single-mode fibre with hexagonal photonic crystal cladding, Proc. OFC 1996, San Jose, USA, PD3, 1996. [2] T. A. Birks, J. C. Knight, and P. St. J. Russell, Endlessly single-mode photonic crystal fiber, Opt. Lett., vol. 22, no. 13, pp. 961 963, 1997. [3] D. Mogilevtsev, T. A. Birks, and P. St. J. Russell, Group-velocity dispersion in photonic crystal fibers, Opt. Lett., vol. 23, no. 23, pp. 1662 1664, 1998. [4] K. Nakajima, J. Zhou, K. Tajima, K. Kurokawa, C. Fukai, and I. Sankawa, Ultra wide band 190 Gbit/s WDM transmission over a long length and low loss PCF, Proc. OFC 2004, Los Angeles, USA, PDP23, 2004. [5] K. Kurokawa, K. Tajima, J. Zhou, K. Nakajima, T. Matsui, and I. Sankawa, Penalty-free dispersion-managed soliton transmission over 100 km low loss PCF, Proc. OFC 2005, Anaheim, USA, PDP21, 2005. [6] K. Tsujikawa, K. Kurokawa, K. Tajima, K. Nakajima, T. Matsui, I. Sankawa, and K. Shiraki, Application of a prechirp technique to 10-Gb/s transmission at 1064 nm through 24 km of photonic crystal fiber, IEEE Photon. Technol. Lett., vol. 18, no. 19, pp. 2026 2028, 2006. [7] W. Li, G. Khoe, H. van den Boom, G. Yabre, H. de Waardt, Y. Koike, S. Yamazaki, K. Nakamura, and Y. Kawaharada, 2.5 Gbit/s transmission over 200 m PMMA graded index polymer optical fibre using a 645 nm narrow spectrum laser, Proc. LEOS 1998, Orland, USA, FB3, 1998. 375
[8] K. Ieda, K. Kurokawa, T. Shimizu, K. Tajima, K. Nakajima, T. Matsui, K. Tsujikawa, K. Shiraki, and I. Sankawa, Visible to infrared WDM transmission over PCF, Proc. ECOC 2006, Cannes, France, Tu3.3.4, 2006. [9] K. Kurokawa, K. Nakajima, K. Tsujikawa, K. Tajima, T. Matsui, and I. Sankawa, Penalty-free 40 Gbit/s transmission in 1000 nm band over low loss PCF, Proc. OFC 2006, Anaheim, USA, OThH2, 2006. 1 Introduction Photonic crystal fibers (PCF) are very attractive transmission media since they have unique features; they can be endlessly single-mode, have a low bending loss, are capable of dispersion tailoring, and can have a large mode field diameter [1, 2, 3]. These features are not available with conventional single-mode fibers. The ultra-wide single-mode region of PCF has provided the possibility of building communication systems with a bandwidth of over 160 THz [4]. Progress on fabrication techniques has significantly reduced fiber loss. This has led to several reports on transmission experiments using PCFs[4,5,6]. On the other hand, the construction of high-speed and high-capacity short-range networks is attracting a lot of attention with the rapidly increasing demand for broadband services. This makes it important to increase the optical communication bandwidth by developing an additional transmission window, for example, in the visible region. There have already been some reports on transmission experiments that used graded index polymer optical fibers (POF) at visible wavelengths. However, the transmission distance was limited to a few hundred meters due to the large optical loss of more than 100 db/km [7]. Recently, we reported visible to infrared wavelength division multiplexing (WDM) transmission over PCF, where we obtained 1 Gbit/s visible and 10 Gbit/s infrared transmission [8]. This paper reports ultra-wide band and higher speed WDM transmission over a 1 km PCF using the 658 to 1556 nm wavelength range, which corresponds to a bandwidth of 263 THz. We obtained WDM transmission of 1 Gbit/s at 658 nm and of 10 Gbit/s at 780 nm and four infrared wavelengths of 853, 1064, 1309 and 1556 nm. Our experimental results show that low loss PCF is very attractive for use in future high-capacity WDM systems with an ultra-wide bandwidth. 2 Experiment The PCF had 60 holes and the structural parameter d/λ was 0.5. Here, d and Λ denote the hole diameter and pitch, respectively, and Λ was 7.5 µm. The outer diameter of the PCF was 125 µm. The mode field diameters (MFDs) of the PCF were 7.6 and 7.8 µm at 658 and 1556 nm, respectively. Figure 1 shows the optical loss (left axis) and the chromatic dispersion (right axis). The optical losses at wavelengths of 658, 780, 853, 1064, 1309 and 1556 nm 376
were 11, 4.9, 3.7, 1.8, 1.0 and 0.6 db/km, respectively. The chromatic dispersion was measured with a pulse delay method using a supercontinuum light. The dispersion values were 205, 108, 74, 18, +13 and +33 ps/nm/km at wavelengths of 658, 780, 853, 1064, 1309 and 1556 nm, respectively. The zero dispersion wavelength was 1190 nm. We also confirmed that the PCF was single-mode at 658 nm from the output field pattern after a 1 km transmission. Moreover, the bending loss of the PCF at 658 nm with a bending radius r of 15 mm and 10 turns was 0.1 db. Fig. 1. Optical loss and chromatic dispersion of PCF. Figure 2 shows our experimental setup. The light source was a visible Fabry-Perot laser diode (FP-LD) operating at a wavelength of 658 nm, and a 780 nm LD with an external cavity, and four distributed feedback laser diodes (DFB-LD) whose wavelengths were 853, 1064, 1309 and 1556 nm. The linewidth of the 780 nm grating-stabilized LD was about 1 MHz. The FP-LD was directly modulated at 1 Gbit/s with a non-return to zero (NRZ) format. The continuous-wave (CW) lights from the four DFB-LDs and the LD with an external cavity were modulated at 10 Gbit/s with the NRZ format using a lithium niobate (LN) intensity modulator. The pseudorandom binary sequence (PRBS) length was 2 31 1 at every wavelength except 780 nm. At 780 nm, we used a 2 11 1 PRBS. The optical signals were simultaneously multiplexed with a WDM filter module and guided into a 1.0 km PCF. The WDM module consisted of dielectric filters. The input powers were 1.9, +1.3, 5.6, 4.5, 2.6 and 12.3 dbm at wavelengths of 658, 780, 853, 1064, 1309 and 1556 nm, respectively. The transmitted optical signals were demultiplexed with a WDM filter module. The optical signal at a wavelength of 658 nm was detected with a silicon avalanche photodiode (APD). The optical signal at 853 nm was amplified with an erbium-doped fluoride fiber amplifier and detected with a photodiode (PD). Since we adopted the 377
relatively high input power by using the high power 780 nm LD, we were able to detect the optical signal at 780 nm solely with a PD. The optical signals at wavelengths of 1064, 1309 and 1556 nm were also detected solely with a PD. We did not need to use an optical amplifier because the PD had sufficient sensitivity to measure the bit error rates (BER) of the transmitted signals at these wavelengths. Fig. 2. Experimental setup. 3 Results and discussion Figure 3 (a), (b), (c), (d), (e) and (f) show the BER curves before and after 1 km transmissions at 658, 780, 853, 1064, 1309 and 1556 nm, respectively. The solid lines with filled circles and the dashed lines with open circles show the back-to-back BER and the BER after the transmission, respectively. As shown in Fig. 3 (a), a BER of less than 10 9 was successfully obtained with a received power of about 30 dbm at the visible 658 nm wavelength. The power penalties at a BER of 10 9 at 658 nm were 0.4 db. We believe this power penalty was the result of the dispersion-induced penalty, which was caused by the multi longitudinal modes in the FP-LD. As shown in Fig. 3 (b), (c), (d), (e) and (f), a BER of less than 10 11 was obtained with a negligible power penalty at wavelengths of 780, 853, 1064, 1309 and 1556 nm, respectively. Thus, we achieved the high speed WDM transmission in the visible to infrared wavelength region over a 1 km PCF. All the signals were transmitted with a power penalty of less than 0.4 db. Thus, we successfully achieved high-speed WDM transmission over a 1 km 378
PCF in the 658 to 1556 nm wavelength range, which corresponds to an ultrawide bandwidth of 263 THz. This ultra-wide PCF bandwidth is very attractive for future high-speed and high-capacity optical communication networks. Much higher capacity can be realized by using 40 Gbit/s modulators, for example, at 1064, 1310 and 1550 nm [9]. Fig. 3. BER performance for high-speed WDM transmission. 4 Conclusions We achieved ultra-wide band WDM transmission over a PCF in the 658 to 1556 nm wavelength range, which corresponds to a bandwidth of 263 THz. A 1 Gbit/s signal at 658 nm and 10 Gbit/s signals at 780, 853, 1064, 1309 and 1556 nm were successfully transmitted over a 1 km PCF. Our results show that low loss PCF is very attractive for use in future high-capacity WDM systems with an ultra-wide bandwidth. 379