10 Gb/s transmission over 5 km at 850 nm using single-mode photonic crystal fiber, single-mode VCSEL, and Si-APD

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1 10 Gb/s transmission over 5 km at 850 nm using single-mode photonic crystal fiber, single-mode VCSEL, and Si-APD Hideaki Hasegawa a), Yosuke Oikawa, Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa Research Institute of Electrical Communication, Tohoku University Katahira, Aoba-ku, Sendai , Japan a) hasehide@riec.tohoku.ac.jp Abstract: We describe a 5 km 10 Gb/s transmission at 850 nm using a single-mode photonic crystal fiber (PCF), a vertical cavity surface emitting laser (VCSEL), and a Si-avalanche photodiode (APD). The fabricated PCF has an endlessly single-mode property. The transmission loss and dispersion of the fabricated PCF were 5.2 db/km and 62.8 ps/nm/km, respectively. We also fabricate erbium-doped fluoride fiber amplifier (EDFFA), which operates at 850 nm and report the gain characteristics of EDFFA. 10 Gb/s NRZ signals were successfully transmitted over 5 km. Keywords: photonic crystal fiber, VCSEL, Si-avalanche photodiode, EDFFA Classification: Photonics devices, circuits, and systems References [1] J. Broeng, D. Mogilevstev, S. E. Barkou, and A. Bjarklev, Photonic crystal fibers, Opt. Fiber Technol., vol. 5, pp , [2] T. A. Birks, J. C. Knight, B. J. Mangan, and P. St. J. Russell, Photonic crystal fibers: An endless variety, IEICE Trans. Electron., vol. E84-C, pp , [3] T. A. Birks, J. C. Knight, and P. St. J. Russell, Endlessly single-mode photonic crystal fiber, Opt. Lett., vol. 22, pp , [4] W. J. Wadsworth, J. C. Knight, A. Ortigosa-Blanch, J. Arriaga, E. Silvestre, and P. St. J. Russell, Soliton effects in photonic crystal fibers at 850 nm, Electron. Lett., vol. 36, pp , [5] J. K. Ranka, R. S. Windeler, and A. J. Stentz, Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm, Opt. Lett., vol. 25, pp , [6] K. Iga, F. Koyama, and S. Kinoshita, Surface emitting semiconductor lasers, IEEE J. Quantum Electron., vol. 24, pp , [7] D. G. Deppe, D. L. Huffaker, T. Oh, H. Deng, and Q. Deng, Low-threshold vertical cavity surface emitting lasers based on oxidec IEICE

2 confinement and high contrast distributed Bragg reflectors, IEEE J. Select. Quantum Electron., vol. 3, pp , [8] 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, Optical Fiber Communication Conference (OFC2004), PD23, [9] Y. Oikawa, H. Hasegawa, M. Yoshida, T. Hirooka, and M. Nakazawa, Ultra-broadband dispersion measurement of photonic crystal fiber with pico-second streak camera and group-delay-free supercontinuum, Pacific Rim Conference on Lasers and Electro-Optics (CLEO-PR 2005), CWE2-1, [10] C. Millar, M. Brierley, M. Hunt, and S. Carter, Efficient up-conversion pumping at 800 nm of an erbium-doped fluoride fiber laser operating at 850 nm, Electron. Lett., vol. 26, pp , Introduction Recently, photonic crystal fiber (PCF), which has a core region surrounded by multiple air holes, has received a lot of attention because of its new characteristics [1, 2]. PCF has attractive features including endlessly single-mode operation [3], high nonlinearlity [4, 5], and arbitrary dispersion control [1]. The structure, which is defined by the hole pitch Λ and the hole diameter d, allows much greater flexibility as regards designing the dispersion to suit new applications. A zero dispersion shift toward the shorter wavelength region is interesting in terms of realizing high-speed optical communication in a new wavelength region. For example, the 800 nm region is attractive, since inexpensive and high-speed optical devices such as the AlGaAs VCSEL and Si-APD are available. PCF is also an intrinsically inexpensive transmission medium, because there is no need for expensive doping materials such as GeO 2. In particular, VCSELs have a lot of advantages over edge emitting lasers including low threshold current, a single longitudinal mode and high density integration, due to their two dimensional structure and short cavity length [6, 7]. AlGaAs quantum well based VCSELs at 850 nm have achieved higher efficiency, higher output power and larger modulation bandwidth, compared with InP based VCSELs at longer wavelength. The demand of AlGaAs VCSELs has increased for Local Area Network (LAN) and interconnect. However, since conventional fibers are multi-mode and have large normal dispersion in the 800 nm region, transmission length is limited to m. On the other hand, in PCFs, it is possible to control dispersion characteristics and transverse mode more flexibly. By combining AlGaAs VCSEL and PCF, we can construct inexpensive and high-speed communication systems such as a few km Gb/s Ethernet. Recently Nakajima et al. have reported an ultra wide band transmission using a PCF in the 850 to 1550 nm region [8]. They used a distributed feedback (DFB) laser as a light source and a PCF with a d/λ of 0.50 that was multi-mode at 850 nm. In this paper, we report 10 Gb/s data transmission in the 800 nm region 110

3 using a pure single-mode PCF with zero dispersion at 1098 nm, an 850 nm AlGaAs VCSEL and a Si-APD. We also describe the gain characteristics of an erbium-doped fluoride fiber amplifier (EDFFA) that operates at 850 nm. 2 Dispersion characteristics of single-mode PCF for high-speed data transmission in the 800 nm region In the transmission experiment, we used a PCF fabricated by the capillary method. In PCFs, the transmission loss depends strongly on the roughness of the air-hole surface. Therefore, with the capillary method, we need capillary tubes whose diameters are uniform along their length and whose surface is smooth. When fabricating the PCF, we used polishing and etching techniques to reduce the roughness of the air-holes. Figure 1 (a) shows a scanning electron microscope (SEM) image of the cross-section of the present PCF. It is clearly seen that the air-holes are neatly and symmetrically located around the pure silica core. The measured transmission loss was 5.2 db/km at 850 nm. On the other hand, that of the PCF reported in Ref. [8] was 2.7 db/km at 850 nm. The air-hole pitch Λ, the air-hole diameter d, d/λ, the core diameter 2Λ d were 3.4 µm, 1.2 µm, 0.35 and 5.6 µm, respectively. The core diameter of the previously reported PCF was 8.4 µm. The transmission loss increases as the core diameter decreases. Therefore, the present PCF has a higher transmission loss because of the small core diameter. The measured mode field diameter (MFD) at 850 nm was 5.3 µm. Since the MFD of the fabricated PCF is close to that of single-mode fiber (SMF) in the 800 nm region, the measured splicing loss between PCF and SMF was 0.6 db. PCF with a d/λ of 0.35 has an endlessly single-mode property [3], while the PCF with a d/λ of 0.50 in Ref. [8] is multi-mode at 850 nm. Figure 1 (b) shows the dispersion characteristics of the fabricated PCF and SMF. The dispersion characteristics were measured by the time of flight method using supercontinuum generation in highly nonlinear PCF [9]. The zero dispersion wavelength was 1098 nm and the dispersion value was 62.8 ps/nm/km at 850 nm. Compared with a conventional SMF, whose Fig. 1. SEM image of a PCF cross-section (a) and the measured dispersion characteristics (b). 111

4 dispersion is 97.5 ps/nm/km at 850 nm, it is possible to reduce the pulse waveform distortion caused by the group velocity dispersion. The dispersion length L D of the PCF is 37.4 km, when the pulse width T o is 50 ps, roughly corresponding to a bit rate of 10 Gb/s NRZ signal. Here the dispersion length is defined as πcto 2 L D = 2ln2λ 2 (1) D where c is the velocity of light in a vacuum and λ is the wavelength. For a 5 km transmission, which is much shorter than L D, the influence of dispersion is still negligible. 3 Gain characteristics of EDFFA in the 800 nm region We fabricated an optical amplifier that operated in the 800 nm region and measured its gain characteristics. The amplifier employed erbium-doped fluoride fiber (EDFF) as a gain medium in the 800 nm region [10]. Figure 2 (a) shows the Er ion energy diagram. With Er ions, the energy difference between the 4 S 3/2 and 4 I 13/2 levels corresponds to 850 nm. With 980 nm pumping, the electrons in the ground state ( 4 I 15/2 ) are excited to 4 F 7/2 by two photon absorption and transit quickly down to the 4 S 3/2 level. An 850 nm signal is then amplified by the stimulated emission between the 4 S 3/2 and 4 I 13/2 levels. A 550 nm green light, corresponding to the energy difference between the 4 S 3/2 and 4 I 15/2 levels, is also emitted. Figure 2 (b) shows the gain characteristics of the EDFFA with a forward pumping at nm, where the pump power is 500 mw and the input signal power is 25 dbm. The EDFF length is 5 m and the Er ion concentration is 2000 ppm. Although the pump power is 500 mw, the output power is as small as 0 dbm. The reason for such a small energy conversion rate is that the pump power is also used for the green light emission. The 3 db gain bandwidth is about 3 nm. As the bit rate of 10 Gb/s corresponds to a bandwidth of nm, the gain bandwidth of the EDFFA is sufficient to amplify 10 Gb/s signals. Fig. 2. Simplified energy diagram of Er ions (a) and the gain characteristics of EDFFA (b). 112

5 4 10 Gb/s transmission experiment in the 800 nm region Figure 3 (a) shows our experimental setup for a 10 Gb/s data transmission in the 800 nm region. A single-mode AlGaAs VCSEL was used as a light source. The maximum output power was 1 mw and the wavelength was 852 nm. A CW light from the VCSEL was modulated with a LiNbO 3 (LN) intensity Fig Gb/s transmission experiment in the 800 nm region. (a) Experimental setup. (b) and (c) Eye patterns of output signals after 3 km and 5 km transmission, respectively. (d) Measured bit error rate performance. 113

6 modulator and a 10 Gb/s NRZ signal was generated. The sequence length was The input signal power to the PCF was amplified to 4.04 dbm by the EDFFA. The output signal transmitted through a 3 km or 5 km PCF was amplified by the EDFFA. The noise figure of the EDFFA was 6.5 db. The amplified signal was detected with a Si-APD with a conversion gain of 500 V/W, and the bit error rate was measured. Figure 3 (b) and (c) show eye patterns of output signals after 3 km and 5 km transmission, respectively. The eye patterns exhibited wide eye opening, since the EDFFA compensated for the transmission loss successfully. Figure 3 (d) shows the measured BER. shows the BER performance under a back-to-back condition, and and show the BER after 3 km and 5 km transmission, respectively. The power penalty was 0.9 db at a BER of 10 9 for a 3 km transmission. This result indicates that the influence of dispersion and the signal to noise ratio (SNR) degradation due to the transmission loss were negligible. For a 5 km transmission, although there was a power penalty of 3.5 db at a BER of 10 9,aBERof10 12 was obtained at a received power of 15 dbm. This power penalty was caused by a low input signal power of 4.04 dbm and a high transmission loss of 5.2 db/km. Since the transmission length was much shorter than the PCF dispersion length, the influence of dispersion on the power penalty may still be negligible. The transmission distance can be extended by increasing the output power of single-mode VCSEL and reducing the transmission loss of the PCF. 5 Conclusion We described the 10 Gb/s single-mode transmission performance over a 5 kmlong PCF with an AlGaAs VCSEL, EDFFAs and a Si-APD. The gain of the fabricated EDFFA was 25 db at 850 nm and this successfully compensated for the transmission loss of the PCF. A 10 Gb/s NRZ signal was successfully transmitted through the 5 km PCF. This result indicates that inexpensive and high-speed optical transmission over short distances appears to be feasible by combining these devices and PCF. 114