We report the bi-directional transmission of 8×10Gb/s signals using an in-line SOA over 80km SMF-28 with matching DCF for the first time. Our results suggest very promising applications for bi-directional transmission in metro and access networks with simple configuration and low cost.
©2004 Optical Society of America
The semiconductor optical amplifier (SOA) is a promising candidate for metro and access networks because of its wide amplification bandwidth and potential low cost [1–7]. Bi-directional transmission can reduce the complexity and cost of optical transmission systems and networks by enabling the sharing of optical fiber [2, 3]. The use of SOAs as in-line amplifiers is very suitable for bi-directional transmission systems and networks because SOAs do not need any optical isolators as often used in erbium doped fiber amplifiers (EDFAs). However, gain saturation and cross gain modulation (XGM) in SOAs can degrade the transmission performance when regular modulation format like on-off keying (OOK) modulation is employed [1–4]. Since the intensity of differential phase shift keying (DPSK) signals is constant, cross-gain modulation (XGM) and gain saturation in SOAs can be overcome [6, 7]. Recently, experiments show that DPSK modulation format can realize long distance and high-spectral efficiency signal transmission by using SOA in-line amplification [6, 7]. In this paper we report bi-directional transmission of 8×10Gb/s signals using an in-line SOA over 80km SMF-28 with matching DCF. To our knowledge, this is the first time to realize bi-directional DPSK signal transmission by using SOA in-line amplification.
2. Experimental setup and results
The experimental setup is shown in Fig. 1. The four DFB-LD wavelengths at location A are 1550.2, 1550.6, 1551.0, and 1551.4nm respectively. The output power of each laser is 10dBm. We used a four-output port and double-stage interleaver with a channel spacing of 50GHz to combine the four DFB-LDs. The insertion loss of the interleaver is 1dB. Then we used a phase modulator, which was driven by a 10Gb/s electrical data with a PRBS with a sequence length of 231-1, to generate DPSK signals. The SOA used in the experiment has a polarization dependence of 0.3dB and a peak gain wavelength of 1560nm. The SOA saturation output power and gain for small signal are 5.7dBm and 15dB, respectively, when pumped at 200 mA. The span of the transmission fiber includes two segments of SMF-28. The length of each segment is 40km. After transmission over 40km SMF-28 some matched DCFs were used to compensate for the accumulated dispersion. The total power of the four channels when the signals are transmitted from A to B is labeled in Fig. 1. At location B, the transmitter and receiver are similar to that at location A except that the DFB laser wavelengths are different. Their wavelengths are 1558.1, 1558.5, 1558.8, and 1559.2nm respectively. At each receiver, we used another SOA to provide 10dB gain for the DWDM signals and no EDFA was used; therefore an experimental system with pure SOA amplification was realized. This SOA has the same characteristics as the in-line SOA except that this SOA was pumped at 140mA. Then we used a Mach-Zehnder delay interferometer (MZ-DI) to demodulate the DPSK signals. Due to experimental limitations we used a single-ended detector to receive the demodulated signals. The length difference between the two arms of the MZ-DI is 1.2cm, which corresponds to 60ps in the time domain; therefore a RZ-shaped signal can be obtained. After optical filtering by a tunable filter with a bandwidth of 0.22nm, a 7GHz bandwidth PIN detector was used to realize O/E conversion and clock recovery.
Figure 2 shows the optical spectra from A to B and from B to A with SOA or without SOA. We can see that the in-line SOA has provided a gain of over 15dB for all channel signals. Because the two circulators have different performances, the amplitude of optical signal reflections from A to B and from B to A is different. However, the reflected signals do not affect the transmission signals because the wavelengths from A to B and from B to A are different. The signal to noise ratio (SNR) of all channels in a 0.05nm optical bandwidth is larger than 30 dB after SOA amplification. The BER performances of all channels are measured and they show identical characteristic. Therefore in Fig. 3 we only show the BER performance of one channel. The inset (a) is the eye diagram of DPSK signals at 1550.6nm after transmission. We can see that the penalty at a BER of 10-9 is 0.3dB. For comparison, we changed the phase modulator to an intensity modulator, and removed the DPSK demodulator at the receiver, but the other conditions were maintained. Inset (b) shows the measured OOK eye diagram after transmission. Due to XGM and gain saturation in SOA when the OOK signal is used, very large fluctuation of “1”s can be seen and the BER floor is at 10-5.
In this letter we only demonstrated bi-directional transmission with non-overlapping wavelengths for two reasons: 1) the reflections caused by the circulator do not affect the signals at the receiver; 2) there will be interference between the counter-propagating DPSK signals as the mutual coherence length of the lasers at the same wavelength will likely be comparable with the SOA length. As a result, even though DPSK signal has a constant intensity, the total intensity in the SOA at a particular wavelength will change with time as the phases of the two counter-propagating signals at the same wavelength will convert to intensity modulation due to interference. Therefore, in the real system we should avoid the bi-directional transmission with overlapping wavelengths. We can set the signal wavelengths at bi-directional transmitters to have a small difference. Typically, SOAs have a high amplified spontaneous emission (ASE) noise and a relatively small output power, therefore it is difficult to cascade SOAs over many stages which is required for long-haul transmission systems with a large number of channels. Hence, SOAs are more suitable for metro and access networks with a short transmission distance and a small channel count. Another possible problem with DPSK signal amplification using SOAs is the conversion of intensity noise to phase noise when the SOAs are saturated. Therefore, SOAs should be operated in the unsaturated region. Since the SOAs in our experiment are used for in-line amplification and pre-amplification, the input power is always small and the SOAs are always operated in the unsaturated region.
We have demonstrated error-free 8×10Gbit/s DPSK DWDM bi-directional transmission over 80km SMF fiber combined with a suitable DCF and in-line SOA. The results suggest very promising applications for bi-directional transmission in metro and access networks with simple configuration and low cost.
The authors would like to thank the reviewers for their constructive comments on overlapping and non-overlapping bi-directional transmission using SOAs. The research work presented here is supported in part by a grant from Bellsouth and Georgia Research Alliance.
References and Links
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