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Polarization division multiplexing (PDM) and wavelength division multiplexing (WDM) are essential techniques for enhancing the capacity of photonic networks and facilitating the efficient use of optical frequency resources. 2 PDM × 2 WDM × 10 Gbps error-free simultaneous transmissions in the 1.0-µm waveband and C-waveband are successfully demonstrated for the first time using an ultra-broadband photonic transport system over a 14.4-km-long holey fiber transmission line.
©2012 Optical Society of America
1. Introduction
The ever-increasing demand for high data transmission capacities has necessitated the use of alternative wavebands and the development of methods for enhancing the transmission capacities of existing photonic networks. Photonic transport systems in the C-waveband (1530–1565nm) and the L-waveband (1565–1625nm) have been used extensively in conventional photonic networks [1]. Recently, we have focused on the development of a novel alternative 1.0-μm photonic waveband (1000–1260nm), which is shorter than the O-waveband (1260–1360nm) [2]. This short waveband is referred to as the thousand-waveband (T-waveband). The T-waveband is considered an attractive alternative waveband for future photonic transport systems on the basis of the assumption that optical frequency resources greater than approximately 50~60 THz may be employed in this waveband [2–4]. Moreover, the currently available ytterbium-doped fiber amplifier (YDFA) can be used as a 1R repeater in the 1.0-μm waveband [5]. In addition, high-performance, ultra-broadband and environmentally friendly photonic devices such as high-power lasers, broad-band optical gain devices using a novel nano-structured material such as a quantum dot, and group-IV-semiconductor-based high-speed photonic receivers are compatible with this waveband [6–10].
The capacity of existing photonic networks can be significantly enhanced by using a photonic transport system that employs wavelength division multiplexing (WDM) technique in novel and conventional wavebands [11–13]. Moreover, a polarization division multiplexing (PDM) technique has also been investigated intensively by R. Noe et al., and K. Suzuki et al. to enhance the photonic network capacity [14, 15]. We therefore proposed an ultra-broadband PDM and WDM photonic transport system that is compatible with the 1.0-μm waveband and the C-waveband to create a high capacity photonic network system. In the proposed system, a 14.4-km-long holey fiber (HF) is used as a novel photonic transmission line. In this study, we successfully demonstrated for the first time 2 PDM × 2 WDM × 10 Gbps error-free simultaneous transmissions in the 1.0-μm waveband and the C-waveband using the proposed ultra-broadband photonic transport system.
2. Ultra-broadband photonic transport system with PDM and WDM in the T- and C-wavebands
PDM and WDM are essential techniques for enhancing the capacity of photonic networks and facilitating the efficient use of optical frequency resources. We constructed and demonstrated the operation of an ultra-broadband PDM and WDM photonic transport system in the T- and C-wavebands. Figure 1 shows the experimental setup used for the demonstration. A wavelength-tunable GaAs-based semiconductor laser diode was used as the narrow-line width light source (approximately 200 kHz) for the T-waveband. The carrier wavelength was tuned to 1063.7nm. The optical output of the T-waveband light source was amplified to approximately 10 dBm using an InGaAs/GaAs-based semiconductor optical amplifier (SOA). A distributed feedback (DFB) semiconductor laser diode with a wavelength of 1550.0nm was used as the light source for the C-waveband. Two LiNbO3 (LN) intensity modulators were used to generate data streams in the T- and C-wavebands. In this experiment, a 9.953 Gbps (OC-192, STM-64) pseudo-random binary sequence (PRBS), which was 27-1 digits in length was used to generate non-return zero (NRZ) on-off keying (OOK) data signals in each channel. A polarization beam splitter (PBS) and a polarization beam combiner (PBC) connected to polarization-maintaining (PM) optical fibers were used to multiplex orthogonal linear polarized signals in the T- and C-wavebands. Two polarization rotators placed before the PBSs were used to control the polarization axis. Two orthogonal linear polarizations directed toward the X- and Y-axis were used as shown in Fig. 1. Two PM optical fiber delay lines were used at the two input ports for inducing Y-axis polarization (In-Py) and for generating two different data streams with orthogonal polarizations in the T- and C-wavebands. The optical signals in the T- and C-wavebands were amplified using a YDFA and an erbium-doped fiber amplifier (EDFA), respectively. WDM coupler and splitter were used at both ends of the transmission line for combining and separating the optical signals in the T- and C-wavebands.
Fig. 1 Ultra-broadband PDM and WDM photonic transport system using a 14.4-km-long holey fiber transmission line for simultaneous transmissions in the T- and C-wavebands. PBS: polarization beam splitter, PBC: polarization beam combiner, and PC: polarization controller. YDFA: ytterbium-doped fiber amplifier for T-band, EDFA: erbium-doped fiber amplifier for C-band, and SOA: semiconductor optical amplifier. ATT: optical attenuator, PD: broadband photo-detector for T- and C-wavebands, and CDR: electrical clock-data recovery circuit.
An optical fiber as a long-distance transmission line is crucial to an ultra-broadband photonic transport system. Therefore, we used a HF, which is a photonic crystal fiber, as the transmission line for the ultra-broadband photonic transport system. HFs are considered suitable for ultra-broadband transmission owing to their endlessly single-mode (ESM) characteristic [16]. Furthermore, the dispersion characteristics and the mode field diameter ofa HF can be optimized by varying the hole size and the distances of the holes from the fiber core [17]. Figure 2 shows the simulated optical modes of the fabricated HF in both of the T- and C-wavebands. This result demonstrates that single mode operations in the ultra-broadband photonic transport system can be achieved using HFs. The zero-dispersion wavelength was estimated to be approximately 1200nm, and the dispersion values were found to be −20.1 ps/nm/km at 1.0 μm and 32.5 ps/nm/km at 1.55 μm. In this transmission experiment, the input powers to the HF transmission line were fixed at 6.6 dBm for the T-waveband and −2.5 dBm for the C-waveband. The length of the HF was 14.4 km. The transmission losses of the HF were estimated to be approximately 0.89 dB/km at 1.0 μm and 0.43 dB/km at 1.55 μm. After simultaneous transmissions, the optical signals in the T- and C-wavebands were separated by the WDM splitter. Then, the optical signals in the T- and C-wavebands were amplified by the YDFA and EDFA, respectively. Optical band pass filters placed after the YDFA and EDFA were used to eliminate the amplified spontaneous emission (ASE) noise generated by the fiber amplifiers. The polarization axes were controlled by using the two polarization rotators placed before the PBS. The PBS, which was connected to the PM optical fibers, was used to de multiplex the orthogonal linear polarized signals in the T- and C-wavebands. The optical signals of each wavelength were detected by a photonic receiver, which consisted of a broadband photodetector and an electrical clock and data recovery (CDR) circuit.
Fig. 2 Simulated optical mode field in the holey fiber at wavelengths of 1050nm and 1550nm used for the estimation of the endlessly single-mode characteristics.
Eye-diagrams and the bit error rate (BER) of the electrical output signal from the photonic receiver were measured using a communications analyzer and a BER tester, respectively. In addition, optical spectra before and after the simultaneous transmission were examined using an optical spectrum analyzer (OSA). We also estimated the polarization cross-talk and wavelength cross-talk in the proposed PDM and WDM photonic transport system in the T- and C-wavebands, respectively, using an OSA.
3. Simultaneous transmission characteristics of the ultra-broadband PDM and WDM photonic transport system for the T- and C-wavebands
Figure 3(a) and 3(b) shows the optical spectra measured before and after the simultaneous transmission, respectively, over a wide wavelength range. A clear peak was observed at a wavelength of 1063.7nm and 1550.0nm in the spectra measured before and after the simultaneous transmission, respectively. The observation of these two peaks indicates that simultaneous transmissions in the T- and C-wavebands can be achieved using the 14.4-km-long HF transmission line. Figure 3(c) and 3(d) shows the optical spectra, which was measured at the output port with Y-axis polarization (Out-Py) in the T- and C-wavebands, respectively, by selecting each of the input ports, In-Px and In-Py, separately. Polarization cross-talk of less than −17 dB in the T-waveband and −22 dB in the C-waveband was measured. Figure 3(e) and 3(f) shows the optical spectra measured at the back of the WDM splitter after transmission. Wavelength cross-talk of less than −20.8 dB in the T-waveband and −40.6 dB in the C-waveband was measured. The polarization and wavelength cross-talk values are summarized in Table 1 . From this table, it is found that the cross-talks in the T-waveband are slightly higher than that in the C-waveband. As a one of the reasons, it is considered that we used optical-components (such as PBC and PBS) optimized at 1045nm and 1550nm for constructing the ultra-broadband photonic transport system.
Fig. 3 Optical spectra (a) before and (b) after transmission. Estimation of polarization cross-talk using optical spectra in the (c) T- and (d) C-wavebands. Estimation of wavelength cross-talk using optical spectra in the (e) T- and (f) C-wavebands.
Table 1. Polarization and Wavelength Cross-talk Values in the T- and C-Wavebands
Figure 4 shows eye diagrams for the 10 Gbps data-stream transmission observed at the Out-Px and Out-Py in the T-waveband. Eye openings were clearly observed, when the each input ports and then both In-Px and In-Py were selectively connected for the transmission of data with orthogonal polarization. Moreover, we also confirmed a large BER (>10−1) at the output port for the unused polarization axis. It is also confirmed that eye openings were clearly observed at the Out-Px and Out-Py in the C-waveband. Figure 5 shows the BER dependencies on the received optical power at the Out-Px, Out-Py and back-to-back (BtoB) in both (a) the T- and the (b) C-wavebands, respectively. BERs of <10−9 were observed when the 10 Gbps optical data signals with orthogonal linear polarization were transmitted over the HF in the T- and C-wavebands. Power penalties between the 14.4-km-long transmission and the BtoB were found to be 2.03 dB and 0.95 dB in the T- and C-wavebands, respectively. A high amplification rate of YDFA was needed after a data-transmission, because the transmission loss in the T-waveband is higher than that in C-waveband. That is, it is considered that a performance of the data transmission in T-waveband over the long-distance HF is slightly worsened by the transmission loss and ASE-noise of the YDFA in comparison with a performance of C-waveband. In case of BtoB, we consider that a performance of T-waveband might be improved compared with that of C-waveband, because a high-intensity laser amplified with the SOA was used for an optical data formation. From BER estimations, these results indicate that 2 PDM × 2 WDM × 10 Gbps error-free simultaneous photonic transmissions were realized over a 14.4-km-long HF in the novel T-waveband and the conventional C-waveband.
Fig. 4 Eye diagrams after transmission at each output port Out-Px and Out-Py of the ultra-broadband PDM and WDM photonic transport system for the T-waveband.
Fig. 5 Dependence of the BER on the received power in the ultra-broadband PDM and WDM photonic transport system for the (a) T- and (b) C-wavebands.
4. Conclusion
We propose an ultra-broadband photonic transport system that employs PDM and WDM for transmission over a 14.4-km-long HF. This new system with improved transmission capacity can be used in future photonic networks. This study will open up new avenues for photonic data transmission and will facilitate the effective use of optical frequency resources. 2 PDM × 2 WDM × 10 Gbps simultaneous photonic transmissions in the T- and C-wavebands were successfully demonstrated over a 14.4-km-long HF transmission line. Clear eye openings were observed and error-free simultaneous transmissions were successfully achieved in the T- and C-wavebands. In addition, low cross-talk (−17 dB) was measured at the output ports with X- and Y-axis polarizations. Further, low wavelength cross-talk (−20 dB) was observed between the T- and C-wavebands. These results indicate that the proposed ultra-broadband photonic transport system for the T- and C-wavebands using a 14.4-km-long HF transmission line with PDM and WDM is a breakthrough in the field of ultra-broadband optical frequency resources, and will therefore facilitate the effective use of these optical frequency resources in an access photonic network and/or a data-center as next-generation photonic network systems.
Acknowledgment
The authors would like to thank Dr. K. Mukasa, Dr. K. Imamura, Dr. R. Miyabe, Dr. R. Sugizaki, Dr. T. Yagi, and Dr. S. Ozawa of Furukawa Electric Co. for providing the novel optical fibers. The authors are extremely grateful to Dr. K. Akahane and Dr. I. Hosako of the National Institute of Information and Communications Technology (NICT) for their encouragement. The authors also thank the staff of the Photonic Device Laboratory (PDL) at NICT.
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