1 Introduction
To reduce the influence of noise, nonlinearity, dispersion, and other factors in a high-speed ultra-long haul (ULH) optical fiber communication system, as well as enhance the optical signal quality, many technologies have been adopted. For example, replacing erbium-doped fiber amplifier (EDFA) with Raman amplifier, optimizing system management of dispersion and power, setting an adjustable dispersion compensator at the end of the link, compensating the polarization mode dispersion (PMD) and high-order dispersion, paving a special fiber (such as LEAF, DSF), introducing forward error correction (FEC) and the modulation format (RZ, CSRZ, DPSK, etc.) technologies and so on 1–623456. The performance that profits from various modulation formats is particularly prominent in a ULH system 7–158910111213141515. It had been validated again in a multi-channel transmission platform of 10 Gbps RZ and NRZ in this paper. The platform is based on dispersion compensation chirped fiber Bragg gratings (CFBGs) that compensate over 2500 km-G.652 fiber.
2 RZ signal
If the duty cycle of RZ is 0.5 (half of NRZ), by Fourier transform relations, the corresponding width of the spectrum will have been doubled, as shown in
Fig. 1 16. In the figure, for the unipolar code, if the bit rate of the single channel is B, the required bandwidth of NRZ is not less than 2B (2B is the smallest transmission bandwidth required by Nyquist principles when the filter roll-off coefficient is 1 17); and that of RZ is not less than 4B. Namely, for 10 Gbps systems, at least 0.32 nm (in C-band) channel bandwidth is required. The widening of the spectrum of RZ increases the group dispersion and decreases the spectral efficiency of the system. However, the dispersion tolerance of a 10 Gbps system is great, and the dispersion compensation is not practically difficult. The smallest channel spacing is 0.4 nm (50 GHz) as recommended in the ITU-T, and 10 Gbps RZ does not reduce spectral efficiency in practice. On the other hand, RZ has better nonlinearity tolerance and clock extraction condition, and the power control and clock synchronization are easier. Thus, it can be expected that transmission performance of 10 Gbps RZ would be better in ULH than in NRZ.
RZ code can be generated through code converters (for example, SHF1551 RZ converter) or two-stage modulation method. The latter was adopted in this paper. The two stage modulation methods are classed as the half clock method and the full clock method. In the latter, the pulse modulator is driven by a full clock, in which the modulator work is at the same state as the data modulator (modulation equipment as
Fig. 2(a)); In the former, the pulse modulator is driven by a half clock, in which the DC bias of the modulator is at the highest point of the transmission curve, drive voltage is 2Vπ (Fig. 2(b)). The latter one was chosen in the paper.
3 Dispersion compensation module-linear CFBG
The grating mentioned in this paper is a linear chirped fiber-optic Bragg grating that was researched and manufactured by the Institute of Lightwave Technology of Beijing Jiaotong University 18. Lots of advanced techniques were adopted such as hydrogen carrier, the apodizer and the packaging technique. As a result, the grating has perfect performance: the temperature coefficient is between 0.2 and 0.5 pm/°C; the polarization module dispersion is very small, PMD < 2 ps; the reflectivity spectrum is flat, the power spectrum ripple is less than 1 dB (
Fig. 3); the -3 dB bandwidth is about 0.35 nm, enough for the transmission of the 10 Gbps system (including RZ code and CSRZ code); the group delay ripple is not larger than |± 15| ps; the group delay is large, about 2600 ps/nm (the G.652 optical fiber length that can be compensated by CFBG is about 160 km); the insertion loss is low, about 4–5 dBm (the loss is equal to that of 7–8 km DCF, and the DCF of this length can compensate only about 50 km G.652 optical fiber).
4 Experiment system
The structure of the system is shown in
Fig. 4. This 10 Gbps platform already had been set up, which is about 2500 km in length, and has 16 wavelength channels. The channel space is 0.8 nm (100 GHz), according to the wavelength standard (in C band) of the ITU-T suggestion. The total 2560 km G.652 optical fiber has 27 segments, 22 segments of 100 km, another 5 segments from 60 km to 90 km. This setting is for easier dispersion management.
The structure of the transmitter includes the tunable laser and two LiNbO3-MZ modulators, transmitting 9.953 Gbps pseudo-random bit frame (PRBS = 1023 - 1). The signal-to-noise ratio (SNR) of the RZ is 30.5. The output eye and spectrum (Note: for ease of comparison, two actual results measured of the spectrum were put in the same picture box) of the transmitter are shown in
Fig. 5.
In dispersion management, the dispersion of each channel is compensated by 16 CFBGs. Because the group delay of the CFBG (about 160 km G.652 optical fiber, as shown above) is more than 100 km (the amplification spacing), while less than 200 km (2 times amplification spacing), some segments of 60–90 km was needed fiber to adjust the remains of the total dispersion, making the largest absolute value of the remainder of the total dispersion as small as possible (in this paper it is less than 120 km) at any EDFA. At the same dispersion compensation location, the gratings (16 roots) are connected together in series.
In loss management, the input power of the fiber is 5 dBm. At each segment end, the power can be re-boosted to 5 dBm through a light amplifier (EDFA). According to the above, not all segments are connected to the grating. As a result, the loss of each segment is different; the loss of some segments without CFBG is about 21 dBm, and the loss of some segments with CFBG is about 26 dBm. Therefore, the gain of different EDFAs is also different, but all the input power of every segment is certain (5 dBm). The EDFA is the product of our institute 19, the largest gain in small signal is about 30 dB, the maximum output power is 15 dBm, the noise figure is 4–5 dB, and the gain is flat between 1540 nm and 1560 nm. The EDFA location is adjusted with the noise figure, and the small noise figure is set in front.
The receiver is the APD detector. When various biases are placed in the best state, the back to back sensitivity is -20 dBm.
5 Test results and analysis
The eye patterns and bit-error rate (BER) curves of NRZ and RZ had been tested at 2000 km and 2500 km of the transmission system (
Figs. 6 and
7, taking CH9 for example, the center wavelength is 1557.365 nm, and channel bandwidth is 0.28 nm at 3000 km, slightly narrow to RZ. Other channels are similar to the following results). As shown in the eye patterns (Fig. 6), the two signals have similar shape at 2000 km. The power penalty can also prove this: NRZ's power penalty is about 2 dB and RZ's is about 3 dB (as Fig. 7, bit-rate is 9.953 Gbps, PRBS = 1023 - 1). At 2500 km, RZ's eye patterns do not seem to have changed (Fig. 6(b)), while that of the NRZ's have obvious deterioration (Fig. 6(d)). At the same time, the NRZ's power penalty is already very large, and the RZ's power penalty is about 5 dB. The RZ's transmission result is much better than NRZ.
In the 10 Gbps system, there are two factors influencing the optical pulse transmission performance: chrom-dispersion and nonlinearity. Two modulation format pulses have the same input power to fiber, and the RZ's transmission performance is better, so it is indicated that RZ's nonlinearity tolerance is bigger than NRZ. Based on the dispersion tolerance, the RZ should have poor performance. However, the opposite result shows that the RZ's nonlinearity tolerance plays a more important role.