Abstract
We discuss the potential of photonic crystal fibers (PCFs) for transmission and optical signal processing in future optical communication systems. Transmission of a 40 Gbit/s signal over 5.6 km PCF using dispersion compensation by mid-span spectral inversion realized in 50 m highly nonlinear PCF is reported.

I. Introduction
Photonic crystal fibers (PCFs) are attractive to realize a wide range of functionalities in future optical networks. Compared to conventional step-index fibers, PCFs offer increased flexibility in terms of controlling the dispersive, nonlinear and polarization-related properties. Until now, research has primarily focused on the exploitation of the large nonlinearity of dispersion tailored PCFs designed with small effective areas for the realization of all-optical signal processing functions such as regeneration, wavelength conversion or time division demultiplexing.
PCFs also present great potential as transmission fibers. One of the benefits is the possibility to design single-mode pure-silica core fibers with large effective areas [1], hence alleviating nonlinear impairments. Recent progress in fabrication technology has allowed for the reduction of the transmission loss down to a current record value of 0.28 dB/km at 1550 nm [2], as well as the drawing of longer fibers with uniform structural properties. Therefore, the time has come to explore the use of PCFs for transmission applications.
In this paper, we present the results of a recent experiment, in which we combined transmission and signal processing applications of PCFs by demonstrating 40 Gbit/s transmission over PCFs, including dispersion compensation by mid-span spectral inversion realized in a short piece of highly nonlinear PCF.

Figure 1. Microscope pictures and properties of the two types of PCFs used in the experiment.

II. Photonic Crystal Fibers Properties
The two types of PCFs used in this transmission experiment are represented in Figure 1. For the transmission link, where the nonlinear effects such as self-phase modulation are considered detrimental, two spools of PCF with 72 mm² effective area, 1.7 dB/km loss and 32 ps/(nm·km) dispersion were used. On the other hand, enhanced nonlinearities are required for signal processing applications. A highly nonlinear PCF (HNL-PCF) with zero dispersion at 1552 nm and a nonlinear coefficient of 18 W^-1·km^-1 was used for this purpose [3]. Both types of PCFs are spliced to standard single mode fiber pigtails with less than 0.5 dB loss.

III. Transmission over PCF
We have earlier demonstrated transmission at 10 Gbit/s over 5.6 km of the transmission PCF described above [4]. No penalty was measured after transmission of a non-return to zero (NRZ) signal over the link. However, the dispersion accumulated over such a link is beyond the dispersion limit at a bit-rate of 40 Gbit/s and, consequently, dispersion compensation is required. One possible technique to achieve dispersion compensation is known as mid-span spectral inversion. It consists of an optical phase conjugator (OPC) placed in a transmission link so that the amounts of dispersion accumulated before and after the OPC are equal. We have achieved dispersion compensation by mid-span spectral inversion realized by four-wave mixing (FWM) in 50 m of HNL-PCF [5].
The experimental set-up is shown in Figure 2. A Mach-Zehnder modulator is used to modulate continuous wave (CW) light at 40 Gbit/s in the NRZ format with a 2³¹-1 pseudo-random binary sequence. After transmission through 2.6 km PCF, the signal is phase conjugated using FWM realized with a CW pump tuned to the zero-dispersion wavelength of the HNL-PCF. The pump and signal are combined via a 3-dB coupler and their states of polarization are adjusted in order to maximize the conversion efficiency. The converted signal is selected at the HNL-PCF output using an optical bandpass filter. A FWM conversion efficiency (defined as the ratio of the power of the converted signal at the HNL-PCF output to the power of the signal at the input) of -20 dB (as seen in the spectrum shown as an inset in Figure 3) and a 3 dB conversion bandwidth of 15 nm are obtained with 25 dBm pump power. The signal is then propagated over the remaining 3 km of the link, before being detected in a preamplified receiver. Eye diagrams recorded in a 50 GHz bandwidth at various points of the link are also represented in Figure 2. After transmission over 2.6 km PCF, the eye diagram of the 40 Gbit/s signal is already completely distorted (Figure 2-c) compared to the signal at the transmitter output (Figure 2-b). In case no dispersion compensation is implemented, the signal is totally unrecoverable after the 5.6 km link (Figure 2-d). On the other hand, we observe a clear and open eye when the signal is transmitted over 2.6 km, phase conjugated, and then transmitted through the remaining 3 km PCF (Figure 2-e). Bit-error-rate measurements showed a total power penalty of 0.7 dB for transmission over the entire link, including optical phase conjugation (Figure 3). This penalty is attributed to the different amount of dispersion accumulated before and after the OPC (of the order of 14 ps/nm), mostly due to the length difference between the two spools of transmission PCF. It is therefore expected that tailoring the fiber lengths should result in even lower penalty.

Figure 2. Experimental set-up for transmission over 5.6 km PCF and eye diagrams recorded at various points in the link.

Figure 3. BER curves in the back-to-back configuration and after transmission through the 5.6 km PCF link including OPC. Inset: FWM conversion efficiency as a function of pump-signal detuning.

IV. Conclusion
In conclusion, we have demonstrated the first optical link (including both transmission and dispersion compensation) based entirely on photonic crystal fibers. A 40 Gbit/s signal was successfully transmitted over 5.6 km PCF using dispersion compensation by mid-span spectral inversion in 50 m highly nonlinear PCF. It is expected that further progress in fabrication technology will allow for the drawing of longer fibers with larger effective areas and reduced loss, hence enabling longer transmission distance and resilience to nonlinear impairments to be demonstrated. The experiment reported here constitutes the first step towards the implementation of a promising technology in optical communication systems.

References
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