An ultra-broadband photonic transport system has been developed to expand the usable wavelength bandwidth for optical communication. Simultaneous 3 × 10-Gbps error-free photonic transmissions are demonstrated in the 1-μm, C-, and L-wavebands by using the ultra-broadband photonic transport system over a 5.4-km-long holey fiber transmission line.
© 2010 OSA
The ever-growing 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- and L-bands (C-band: 1530–1565 nm and L-band: 1565–1625 nm) have been extensively employed in conventional photonic networks . We have recently focused on the development of a wavelength band having a wavelength range shorter than that of the O-band (1260–1360 nm), such as the 1-μm waveband. This 1-μm waveband is a novel and attractive waveband that can be used in future photonic transport systems assuming that optical frequency resources greater than approximately 10-THz can be employed in this waveband [2–4]. Moreover, the existing ytterbium-doped fiber amplifiers (YDFAs) can be used as 1R repeaters in the 1-μm waveband . High-performance and green photonic devices such as high-power lasers, quantum dot lasers [6–9], YDFAs, and group-IV semiconductor-based high-speed photonic receivers  are compatible with this waveband.
The expansion of the usable bandwidth of optical frequency resources used in future photonic network systems employing wavelength division multiplexing (WDM) can be achieved by combining the 1-μm waveband with conventional wavebands such as the C- and L-bands. Therefore, we propose an ultra-broadband photonic transport system that is compatible with both the novel and conventional wavebands. In the proposed system, a holeyfiber (HF) has been developed that is capable of data transmissions over a few kilometers-long (typical length: 5.4 km) and is used as the photonic transmission line for the ultra-broadband system [11, 12]. In this study, we successfully demonstrated simultaneous 10-Gbps optical data transmissions in the 1-μm, C-, and L-wavebands by using the ultra-broadband photonic transport system.
2. Development of ultra-broadband photonic transport system
Figure 1 shows the experimental setup for demonstrating the working of the ultra-broadband photonic transport system. Distributed-feedback (DFB) semiconductor laser diodes with wavelengths of 1059.18 nm, 1550.44 nm, and 1569.80 nm were used as the light sources for the 1-μm, C-, and L- wavebands, respectively. Two LiNbO3 modulators were used for producing a data stream in the 1-μm, C-, and L-wavebands. In this experiment, a pseudo-random binary sequence (PRBS, lengths: 27-1 and 215-1) data of a non-return-to-zero (NRZ) on-off-keying (OOK) signal at 9.953 Gbps (OC-192, STM-64) was generated in each waveband. The modulated optical signal in the 1-μm waveband was amplified by using a YDFA after the optical signal was passed through an arrayed-waveguide grating (AWG) device. The C- and L-bands were combined using the AWG device and the modulated optical signal in these bands was amplified by using an Er-doped fiber amplifier (EDFA). These AWG devices act as wavelength multiplexers (MUX) for the multi-wavelength optical signals in the 1-μm, C-, and L-wavebands. A number of channels, channel spacing, and insertion loss of the AWG device for the 1-μm waveband are confirmed to be respectively 40-channels, 100 GHz, and < 3 dB. In this experiment, a channel No. 2 was used for the 1-μm waveband transmission. Additionally, the AWG device for the C- and L-wavebands is characterized as a 128-channels, 100 GHz channel spacing, and < 6 dB of an insertion loss. The AWG device covers a bandwidth of the both of C- and L-wavebands. WDM couplers were used at the ends of the transmission line to combine and separate the optical signals of the 1-μm, C-, and L-wavebands. The WDM coupler is also characterized as a < 0.16 dB of a low insertion loss, and > 28 dB of isolation in the both of 1-μm, and C-wavebands. Specific center wavelengths of the WDM coupler are 1045- and 1550-nm. Naturally, we confirmed that three wavelengths used in the experiment can pass through the WDM couplers with the low insertion loss.
A definition of the optical fiber capable for data transmission is crucial for realizing the ultra-broadband photonic transport system. Therefore, we use an HF—a type of a microstructured optical fiber (MOF)—for data transmission in the ultra-broadband photonic transport system. In our previous studies, we had demonstrated the 1-μm-waveband photonic transmission over HF or hole-assisted fibers [2,3,8]. Owing to its endlessly single mode (ESM) characteristic, it is expected that the MOF is suitable for ultra-broadband transmissions . Furthermore, the dispersion characteristics and the mode-field diameter of the HF can be optimized by controlling the hole size and the distance of the hole from the fiber core . Figure 2 shows the typical optical characteristics of the HF developed in this study. Figure 2(a) shows simulated optical-modes of the fabricated HF in both of the 1-μm- and C-wavebands. It is found that the single mode operations in the ultra-broadband are achieved using the holey fiber. As a Fig. 2(b), the transmission losses of the HF in the 1-μm, C-, and L-wavebands are 1.42, 1.07, and 1.08 dB/km, respectively. Additionally, the dispersion values shown in Fig. 2(c) of the 1-μm, C-, and L-wavebands are –20.1 (estimated), 30.7, and 32.2 ps/nm/km, respectively. The estimated zero-dispersion wavelength is approximately 1200 nm. The input power to the WDM coupler of each waveband was fixed at 11.6 dBm (1-μm waveband), 9.0 dBm (C-band), and 4.6 dBm (L-band) in this experiment. A 5.4 km-long HF was used as the transmission line. The optical signals in the 1-μm, C-, and L-wavebands were separated by the WDM coupler after the transmission. Additionally, the AWG devices of the 1-μm, C-, and L-wavebands were used for a de-multiplexing (DEMUX) operation; it was expected that the optical signals of each wavelength were clearly separated. The optical signal in the 1-μm waveband was further amplified by using a YDFA after the transmission. The optical signals in the C- and L-bands were also amplified by using C- and L-EDFAs, respectively. Optical band-pass filters connected after the YDFAs and EDFAs were used for filtering the amplified spontaneous emission (ASE) noise of the fiber amplifiers. The optical signals of each wavelength were detected by using a photonic receiver, which was developed by using a wide-waveband photodetector with an electrical clock-data recovery (CDR) circuit.
The eye-diagrams were obtained using a communication analyzer and the bit error rate (BER) of the output electrical signal from the photonic receiver was measured by using a BER tester. Additionally, the optical spectra before and after the transmission were also observed by using an optical spectrum analyzer (OSA).
3. Transmission characteristics of ultra-broadband photonic transport system
Figures 3(a) and 3(b) show the optical spectra before and after the transmission, respectively, over a wide wavelength range. Three peaks pertaining to the optical signals in the 1-μm, C-, and L-wavebands are clearly observed at the wavelengths of 1059.18 nm, 1550.44 nm, and 1569.80 nm, before and after transmission. The observation of these three peaks indicates that simultaneous photonic transmissions in the 1-μm, C-, and L-wavebands can be achieved over the 5.4-km-long HF transmission line. Figure 3(c) shows an optical spectrum observed at the receiving port A of the 1-μm waveband illustrated in Fig. 1. A peak at 1059.18 nm is also clearly observed in the spectra obtained at port A. On the other hand, in the spectra observed at the receiving port A, peaks were absent at wavelengths within the wavelength range of the C- and L-bands. The cross-talk between the 1-μm waveband and the C- and L-bands is less than –56 dB. It is also confirmed that no cross-talk (<–55 dB) is observed at the receiving ports B and C, as shown in Fig. 1.
Figure 4 shows the eye-diagrams obtained before and after transmission in the 1-μm, C-, and L-wavebands. After the transmission, eye-openings of the 9.953-Gbps PRBS (length: 215-1) signals can be observed in all the three wavebands without the CDR. Additionally, we also observed the clear eye-openings of the optical data transmission obtained using a photonic receiver with the CDR, as shown in Fig. 4. The dependencies of the BER on the received optical power in the 1-μm, C-, and L-wavebands are shown in Figs. 5(a) , 5(b), and 5(c), respectively. A BER of less than 10−9 was clearly observed when the PRBS optical data signal at 9.953 Gbps was transmitted over the 5.4-km-long HF in the 1-μm, C-, and L-wavebands. Power penalties between the 5.4-km-long transmission and the back-to-back (BtoB) are found to be 0.55 dB, 0.33 dB, and 0.66 dB in the 1-μm, C-, and L-wavebands respectively. From these results, simultaneous and error-free 10-Gbps photonic transmissions over the 5.4-km-long HF are successfully demonstrated in the 1-μm, C-, and L-wavebands by employing the ultra-broadband photonic transport system.
Moreover, we also estimated a possible bit rate B of this ultra-broadband photonic transport system with the HF transmission line for the NRZ data signal in the 1-μm, C-, and L-wavebands by using the following equation:14]. The estimated bit rates are high—33.0 Gbps (1-μm waveband) and 18.1 Gbps (C-and L-bands)—when the power penalty is assumed to be less than 1 dB. Therefore, the use of a dispersion compensation technique for the HF in the 1-μm, C-, and L-wavebands is also important for the construction of a high-speed (>10 Gbps) ultra-broadband photonic transport system. From this bit rate estimation and a previous report of the transmission of a photonic crystal fiber , it is also expected that the transmission distance of the proposed photonic transport system can possibly be expanded to approximately > 20 km in the future provided that the HF transmission loss decreases with further development of the HF. In addition, a transmission loss and a dispersion of the HF in the O-band (1310-nm) are estimated to be 3.0 dB/km and 12 ps/nm/km. This transmission loss is approximately 3-times higher than that in the C-band. It is considered that an increasing of the transmission loss in the O-band may be caused by an OH-absorption in the HF. Therefore, it is considered that a low-loss HF with reducing the OH-absorption will be an important optical component for further expanding the usable-waveband, such as the O-band in the ultra broadband photonic transport system.
We developed and proposed the use of the ultra-broadband photonic transport system for expanding the usable bandwidth of the optical frequency resources for enhancing the data transmission capacity of future photonic network systems employing WDM. We also proposed the use of a 1-μm waveband in the photonic transport system since it is expected that the ultra-broadband optical frequency resources in the 1-μm waveband have the potential to realize new-generation photonic networks. Simultaneous 10-Gbps photonic transmissions in the 1-μm, C-, and L-wavebands were successfully demonstrated over a 5.4-km HF transmission line. Clear eye-openings were observed and error-free transmissions were also successfully achieved in the 1-μm, C-, and L-wavebands. Additionally, low cross-talk (<–55 dB) was observed between the 1-μm and the C- and L-wavebands. To construct future photonic network system, we believe that the demonstrated ultra-broadband photonic transport system using the HF transmission line will be a breakthrough in pioneering the use of optical frequency resources in the ultra-broadband.
We are highly grateful to Dr. K. Mukasa, Dr. K. Imamura, Dr. R. Miyabe, Dr. T. Yagi, and Dr. S. Ozawa of Furukawa Electric Co. for manufacturing the novel optical fibers. We also thank the staff of the Photonic Device Laboratory (PDL), Dr. I. Hosako, and Dr. Y. Matsushima of NICT.
References and links
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