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We demonstrated high-speed transmission at visible wavelengths over a 1 km photonic crystal fiber (PCF). We achieved a 1 Gbit/s transmission at 783 nm by using the direct modulation of a cost-effective Fabry-Perot laser diode (FP-LD). By employing the external modulation of the longitudinally single-mode grating-stabilized LD, we obtained the first penalty free 10 Gbit/s transmission at 780 nm.
©2007 Optical Society of America
1. Introduction
Photonic crystal fibers (PCF) are very attractive transmission media since they have certain unique features; they can be endlessly single-mode, have a low bending loss, are capable of dispersion tailoring, and offer a large mode field diameter [1-3]. 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 [5]. This has led to several reports on transmission experiments using PCFs [4, 6-8].
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 [9, 10]. Recently, we reported visible to infrared wavelength division multiplexing (WDM) transmission over PCF, where 1 Gbit/s visible and 10 Gbit/s infrared transmission was obtained [11].
This paper reports high-speed transmission at visible wavelengths over a 1 km PCF. First, we describe the properties of the fabricated PCF. Second, we report 1 Gbit/s transmission at 783 nm achieved by using the direct modulation of a cost-effective FP-LD. Third, we describe penalty free 10 Gbit/s transmission at 780 nm that employs the external modulation of a longitudinally single-mode grating-stabilized LD. Then, we also discuss the limitation imposed on the bit rate by the fiber dispersion. Our experimental results show that low loss PCF is very attractive for use in future high-speed short-range networks.
2. Properties of PCF
The fabricated 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 core diameter was 11 μm and the fiber length was 1 km. The effective area at 783 nm was 47 μm2. Figure 1 shows the optical loss (left axis) and the chromatic dispersion (right axis) of the PCF. The optical loss was 4.9 dB/km at 783 nm. We did not apply a dehydration process to this PCF, and the OH absorption loss was as large as 9 dB/km at 1385 nm. The chromatic dispersion was measured with a pulse delay method using a supercontinuum light. The zero dispersion wavelength was 1190 nm. The chromatic dispersion was -107 ps/(nm.km) at 783 nm. We also confirmed from the output field pattern that the 1 km PCF was single-mode at 658 nm. A macrobending loss of the PCF at 783 nm was less than 0.1 dB/km for ten turns of winding at a 35 mm diameter.
3. 1 Gbit/s transmission using direct modulation of a cost-effective Fabry-Perot LD
Figure 2 shows our experimental setup for 1 Gbit/s transmission over the PCF. The light source was a visible FP-LD whose wavelength was 783 nm. The FP-LD was directly modulated at 1 Gbit/s with a non-return to zero (NRZ) format. The pseudorandom binary sequence (PRBS) length was 231-1, and the mark ratio was 1/2. The root mean square (RMS) width of the input signal spectrum was 0.5 nm. The optical signal was guided into a 1.0 km PCF. The input power was -1.0 dBm. The transmitted optical signal was detected with a silicon avalanche photodiode (APD).
Figure 3 shows the BER curves before and after a 1 km transmission. 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 BER of less than 10-9 was successfully obtained with a received power of about -30 dBm. The power penalty at a BER of 10-9 was 0.9 dB. This power penalty was considered to be caused by the dispersion induced power penalty and the mode-partition noise [12], because the FP-LD was longitudinally multimode and the PCF had a large chromatic dispersion of -107 ps/(nm.km) at 783 nm. Thus, we achieved a 1 Gbit/s transmission in the visible region over a 1 km PCF by using the direct modulation of a cost-effective FP-LD.
4. Penalty free 10 Gbit/s transmission using external modulation of longitudinally single-mode grating-stabilized LD
Figure 4 shows our experimental setup for 10 Gbit/s transmission over the PCF. The light source was a high power grating-stabilized LD whose wavelength was 780 nm. The linewidth of this external cavity LD was about 1 MHz and the fiber coupled output power of the grating-stabilized LD was set at 19 mW. The longitudinally single-mode continuous-wave (CW) light from the grating-stabilized LD was modulated at 10 Gbit/s with the NRZ format using a commercially available LN intensity modulator which was optimized for transmission at 780 nm (EOSPACE Inc.). The PRBS length was 211-1 and the mark ratio was 1/2. The input power to the PCF was +1.3 dBm. The transmitted optical signal was detected with a photodiode (PD) which had a 3-dB bandwidth of 12 GHz.
Figure 5 shows the BER curves before and after the 1 km transmission at 10 Gbit/s. 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. 5, a BER of less than 10-11 was obtained with a negligible power penalty. Thus, we achieved the first penalty free 10 Gbit/s visible transmission over a 1 km PCF by using the external modulation of a longitudinally single-mode grating-stabilized LD.
Here we consider the limitation imposed on the transmission distance by the fiber dispersion. We assume that the limiting transmission distance for a 10 Gbit/s intensity modulation with a direct detection (IM/DD) system is given by
where β2 is the group velocity dispersion. Equation (1) corresponds to |D|L < 1000
(ps/(nm.km)) at 1550 nm. Here D is the chromatic dispersion and
Since the limitation of |β 2|L is proportional to (bit rate)-2 [13], the limiting transmission distance for a 40 Gbit/s IM/DD system is given by
The group velocity dispersion (β2) of the PCF was 34.9 ps2/km at 780 nm from D=-108 ps/(nm.km) and Eq. (2). So the limiting transmission distance of the present PCF at 780 nm is given by L<2.3 km for a 40 Gbit/s IM/DD system from Eq. (3). Thus, we can expect to achieve 40 Gbit/s transmission at 780 nm over a 1 km PCF in terms of the fiber dispersion [14].
5. Conclusion
We achieved a 1 Gbit/s transmission at 783 nm over a 1 km PCF by using the direct modulation of a cost-effective FP-LD. By externally modulating a longitudinally single-mode grating-stabilized LD, we obtained the first penalty free 10 Gbit/s visible transmission over a 1 km PCF. In terms of fiber dispersion we can expect to realize a 40 Gbit/s transmission at 780 nm over a 1 km PCF. Our results show that low loss PCF is very attractive for use in future high-speed short-range networks.
Acknowledgments
The authors thank T. Haibara and H. Shinohara for their continuous encouragement.
References and links
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