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We have developed a new 8-chip, 320 Gchip/s encoder/decoder with eight input/output ports, that can be used in 40-Gb/s PON networks. The device has been to multiplex four asynchronous 40Gb/s users, using DPSK modulation. The transmission over 50 km has been successfully demonstrated for the first time.
©2011 Optical Society of America
1. Introduction
Code division multiple access (CDMA) is the access method mostly used in radio communication systems, that allows several transmitters to send information simultaneously over a single channel. The users share the same physical resources by using the spread-spectrum technique, a coding scheme where a unique code is assigned to each transmitter. With the introduction of the third generation technologies based on CDMA, wireless operators have been able to offer high quality voice services as well as broadband internet access and multimedia services. In December 2010, 314 operators have launched CDMA2000 in 120 countries and territories with 528.1 million subscribers worldwide.
The adoption of the CDMA technique in fiber optics dates back to more than twenty years ago, with the pioneering papers of Salehi et al. [1]. During the last two decades, researchers have tried to adapt this technology to optical transmission systems and networks, to enhance the data capacity over a single fiber and a single wavelength. Many different optical encoding techniques have been proposed, where data are encrypted in the time, frequency domains or both, and many different optical CDMA (OCDMA) systems have been experimentally demonstrated [2,3].
However, since from its early development, OCDMA has been looked upon skeptically by many experts in the industry and, so far, most of the activity surrounding OCDMA has been carried out only in research laboratories; furthermore, in recent times, this topic is attracting less research interest among the optical fiber community, mainly because it is seen as complex, expensive and unpractical technique for access systems.
OCDMA has the main advantage of using all-optical processing to perform networking applications, like addressing and routing and, it is an attractive candidate for the next generation optical access systems. The basic scheme of an OCDMA-based passive optical network (PON) is shown in Fig. 1(a) , where a set of different optical encoders/decoders (E/D) are used in the optical line terminal (OLT) and in the optical network units (ONU) to generate and process the optical codes (OC). However, this system does not satisfy the key requirement of colorless (non-user specific) ONUs, since a different optical device should be provided to each user to receive and transmit data. To overcome this limitation, we have developed and fabricated an innovative optical multiport E/D that is able to simultaneously generate and process a set of different OCs. By using the multiport E/D in the OLT and in the remote node, we can simplify the PON architecture as indicated in Fig. 1(b), using identical ONUs without E/D. In addition, the device is passive and it can process optical signals at very high data rate, without any optical to electrical to optical (O-E-O) conversion.
We observe that the OCDMA-based PON architecture is quite similar to a wavelength division multiplexing (WDM)-based system, where the multiport E/D replaces a standard wavelength multiplexer/demultiplexer. Although WDM-based PONs are considered by most carriers and service providers as the natural evolution of existing fiber to the home (FTTH) systems, we have demonstrated that the use of OCs enhances the system flexibility and the bandwidth granularity, adding an extra physical resource, as shown in Fig. 2(a) .
Fig. 2 (a) Time, wavelength and code dimensions that can be used simultaneously in an OCDMA-based PON.(b) Capacity of a CDMA system: red and blue lines correspond to user bit rates B = 10 Gb/s and B = 40 Gb/s, respectively; the squares represent the results from our experiments, and their references are indicated aside.
The asynchronous access is one of the unique features of OCDMA transmission, that makes this technology a suitable candidate for next generation FTTH systems, because it can simplify network management and control. However, it is also possible to make a combined use of OCDMA and time division multiplexing (TDM) techniques to partition the bandwidth corresponding to each code among different users [4,5].
On the other hand, we have demonstrated in many field-trial experiments that the simultaneous use of WDM and OCDMA can largely enhance the OCDMA system capacity [6–11], that can be evaluated as [12]
Here N is the number of users, B the single user bandwidth and Λ the number of wavelengths. The signal to noise ratio (SNR) depends only on the multiple access interference (MAI) noise.
Figure 2(b) shows the system capacity for a WDM-OCDMA-based system, for different numbers of wavelengths that have been demonstrated [6–11, 13]; the slope of the curves can be enhanced by increasing the users bit rate B. However, the devices that we have used in all the previous experiments generate OCs with duration of about 80 ps, and they can be used in systems with maximum bit rates of 10 Gb/s.
2. 40 Gb/s multiport encoder/decoder
To enhance the overall capacity of a OCDMA-based PON systems, we have designed and fabricated a new multiport E/D with eight input/output ports for 40Gb/s transmission that generates phase shift keying (PSK) codes of 25 ps duration. If a short light pulse is sent into one of the device input ports, it generates eight orthogonal codes at its outputs, as shown in Fig. 3(a) ; the time interval between two consecutive chips is 3.139 ps, so that the code chip rate is 318.5 Gchip/s. On the other hand, if we forward one of the OCs to one of the device inputs, at the matched output port we measure the autocorrelation signal (see Fig. 3(b)) and at all the other output ports, we measure very small cross-correlations. Figure 4(a) shows a photograph of the device: the module size, except fibers and connectors, is W100mmxH12mmxD50mm. The spectra of 8-port measured at the device output ports are shown in Fig. 4(b) (the color indicates each port); the free spectral range (FSR) is equal to 318.5 GHz, and therefore, the device can be used not only as an E/D but also as a wavelength multiplexer/demultiplexer. In addition, the device presents good polarization dependent loss (PDL) that is less than 0.1 dB.
Fig. 3 (a) PSK optical code generated by the multiport encoder/decoder. (b) Autocorrelation signal measured at the matched port.
Fig. 4 (a) Photography of the 8X8 multiport encoder/decoder. (b) Spectra measured at the output ports.
3. Transmission experiment
Figure 5 shows the experimental setup. A mode-locked laser diode (MLLD) generates a 2.4 ps optical pulse sequence, at 9.95328 GHz repetition rate and 1550 nm central wavelength, that is shown in Fig. 5(a). This signal is multiplexed to 39.81312 GHz by a 10G-to-40G optical time-division-multiplexer (OTDM-MUX) (shown in Fig. 4(b)) and then phase modulated by a pseudo random bit sequence (PRBS) 231-1 stream data obtained from a pulse pattern generator (PPG), by using a LiNbO3 phase modulator (LN-PM). The signal is split into 4 branches and sent to the odd ports of the multiport E/D, to generate four different encoded signals. In our experiment, we demonstrate a fully asynchronous OCDMA transmission in the worst-case scenario, so that the four OCDMA encoded signals have equal power, random delays, random bit phases, and the same polarization. Figure 5(c) shows the noise-like waveform and spectrum of the 4-user OCDMA-multiplexed signal, that is transmitted into a single mode fiber (SMF), followed by a dispersion compensation fiber (DCF). The total transmission length is 50 km.
At the receiver side, the signal is decoded by the multiport decoder, which is identical to the encoder, as shown in Fig. 5(d). The decoded signal is DPSK detected by using a fiber-based interferometer and a balanced photodiode (PD). Figure 5(e) shows the eye opening of the signal detected at the output of the balanced PD. We employ a clock-and data-recovery circuit (CDR), a 40G-to-10G electrical demultiplexer, and a 40G error detector (ED), which measures the 40 Gbps signal (10 Gbps x 4 channels) in parallel, for the bit error rate (BER) measurements.
Figure 6 shows the bit error rate (BER) performances for the back-to-back (B-to-B) and the 50 km transmissions, in the case of a single user and four simultaneous users. In the case of a single user, error free (BER<10−9) operation has been achieved for all the different OCs both at B-to-B and 50 km transmission. After a transmission over 50 km, BER less than 10−4 has been achieved. This confirms that the system can realize error-free operation, by using forward error correction (FEC). The power penalties that we observe between the single user and multiple users cases are due to the MAI noise.
Fig. 6 Measured BERs: (a) B-to-B and (b) after 50 km transmission, for a single user and four users asynchronously transmitting
4. Summary
We have design and fabricated a new 8-chip, 320 Gchip/s multiport E/D for 40-Gb/s PON networks. A complete characterization of the device has been given, showing that it is able to generate and process simultaneously eight orthogonal OCs. We have also demonstrated asynchronous, 40Gb/s DPSK-OCDMA transmission over 50 km for four simultaneous users, so that the total capacity of the system is 160 Gb/s, as shown in Fig. 2(b). The record capacity of 2.56 Tb/s has been also demonstrated, using the same device to multiplex four OCs over eight wavelengths and two polarizations [13] (see Fig. 2(b)).
References and links
1. J. A. Salehi, “Emerging optical code-division multiple-access communications systems,” IEEE Netw. Mag. 3(2), 31–39 (1989). [CrossRef]
2. J. A. Salehi, “Emerging OCDMA communication systems and data networks,” J. Opt. Netw. 6(9), 1138–1178 (2007). [CrossRef]
3. P. R. Prucnal, Optical Code Division Multiple Access: Fundamentals and Applications (Taylor & Francis, 2006).
4. S. Yoshima, N. Nakagawa, N. Suzuki, M. Noda, M. Nogami, J. Nakagawa, K. Kitayama, “Demonstration of burst transmission of multiple capacity 10G-PON over OCDMA uplink using hybrid SSFBG encoder/multi-port decoder and 10 Gbps burst-mode receiver,” pdp OFC 2009.
5. S. Yoshima, Y. Tanaka, N. Kataoka, N. Wada, J. Nakagawa, and K. Kitayama, “Full-duplex 10G-TDM-OCDMA-PON system using only a pair of en/decoder,” ECOC 2010.
6. X. Wang, N. Wada, G. Cincotti, T. Miyazaki, and K. Kitayama, “Demonstration of over 128-gb/s-capacity (12-User/spl times/10.71-gb/s/user) asynchronous OCDMA using FEC and AWG-based multiport optical encoder/decoders,” IEEE Photon. Technol. Lett. 18(15), 1603–1605 (2006). [CrossRef]
7. N. Kataoka, N. Wada, X. Wang, G. Cincotti, A. Sakamoto, Y. Terada, T. Miyazaki, and K.- Kitayama, “Field trial of duplex, 10 Gbps x 8-user DPSK-OCDMA system using a single 16x16 multi-port encoder/decoder and 16-level phase-shifted SSFBG encoder/decoders,” J. Lightwave Technol. 27(3), 299–305 (2009). [CrossRef]
8. X. Wang, N. Wada, T. Miyazaki, G. Cincotti, and K. Kitayama, “Field trial of 3-WDM×10-OCDMA×10.71-Gbps, asynchronous, WDM/DPSK-OCDMA using hybrid E/D without FEC and optical thresholding,” J. Lightwave Technol. 25(1), 207–215 (2007). [CrossRef]
9. X. Wang, N. Wada, N. Kataoka, T. Miyazaki, G. Cincotti, K. Kitayama, “100km field trial of 1.24 Tbit/s, spectral efficient, asynchronous 5 WDMX25 DPSK-OCDMA using one set of 50X50 ports large scale en/decoder,” pdp OFC 2007.
10. N. Kataoka, N. Wada, X. Wang, G. Cincotti, T. Miyazaki, and K. Kitayama, “Full-duplex demonstration of asynchronous, 10Gbps x 4-user DPSK-OCDMA system using hybrid multi-port and SSFBG en/decoder,” ECOC Brussels Belgium 2008.
11. N. Kataoka, X. Wang, N. Wada, G. Cincotti, Y. Terada, and K. Kitayama, “8x8 Full-duplex demonstration of asynchronous, 10Gbps, DPSK-OCDMA system using apodized SSFBG and multi-port en/decoder,” ECOC Vienna, Austria 2009.
12. E. Narimanov, “Information capacity of nonlinear fiber optical systems: fundamental limits and OCDMA performance,” in Optical Code Division Multiple Access: Fundamentals and Applications, P. R. Prucnal, ed. (Taylor & Francis, 2005).
13. N. Kataoka, N. Wada, G. Cincotti, K. Kitayama, “2.56 Tbps (40-Gbps x 8-wavelength s 4-OC x 2-POL) asynchronous WDM-OCDMA-PON using a multi-port encoder/decoder,” pdp ECOC 2011.
