An efficient wavelength-conversion-free all-optical 2R regenerator is proposed. The regeneration is based on power-dependent spectrum broadening in normal-dispersion fiber and subsequent off-centered filtering. Twofold regeneration is performed in a bidirectional configuration where the signal is transmitted twice along the fiber in opposite directions. Experiment at 10Gb/s shows no penalty arising from the bidirectional highpower signal transmission in the fiber and demonstrates strong improvement of extinction ratio of the input signal by the regenerator.
© 2006 Optical Society of America
Future high-speed (>100Gb/s) long-distance optical fiber transmission systems will use in-line signal regenerators to extend transmission distance in opposition to severe impairments caused by dispersion, polarization-mode dispersion, and nonlinearity of transmission fibers. All-optical regeneration will be needed because not only of its high-speed operation capability but also of its potentials for low-cost fabrication and low-power consumption operation. Phase-preserving or even phase-regenerating operation may be realizable, which is important in regeneration of phase-modulated signals [1–6]. Although 3R (reamplification, reshaping, and retiming) regenerators are highly desired, 2R (only reamplification and reshaping) regenerators, which do not need clock extraction and local pulse sources, will benefit much in medium distance systems . 2R regenerators can also be used in combination with 3R regenerators to extend 3R- regeneration intervals.
Simple, robust, and high-speed 2R regenerators can be realized by the use of self-phase modulation (SPM) in fibers, where power-dependent spectrum broadening in a normaldispersion fiber and subsequent off-centered filtering give rise to strong discrimination between space and mark levels . In-line use of this kind of regenerators in 40-Gb/s systems  and 160-Gb/s operation as a regenerative wavelength converter  have been demonstrated. One concern regarding their operation is that the center frequency of the output signal is necessarily shifted from that of the input signal. Two stages of regeneration are required for wavelength-shift-free operation needed in in-line regeneration . Multiple stages of regeneration also enhance the strength of reshaping. In this paper, bidirectional configuration is proposed, where the two-stage regeneration is performed by transmitting signals twice along a single nonlinear fiber in opposite directions. The bidirectional two-stage configuration can reduce the number of nonlinear fibers needed from two to one, which is important because it is often difficult to prepare two spools of nonlinear fibers satisfying requirements for the regenerator application. Strong improvement of extinction ratio is experimentally confirmed, where no penalty arising from Rayleigh back scattering  is observed.
It is noted that bidirectional use of nonlinear fibers has already been demonstrated in other types of all-optical signal processing [12, 13]. The present paper shows that the bidirectional configuration is also effective in high-signal-power application such as SPM-based signal regeneration.
2. Wavelength-conversion-free 2R regenerator in bidirectional fiber configuration
Figure1(a) shows the proposed fiber-based 2R regenerator. The incoming return-to-zero signal pulses are fed into a highly nonlinear fiber (HNLF) through an input circulator after being amplified by an EDFA. The HNLF has normal dispersion at the signal wavelength, by which almost flat-top spectral broadening is obtained with its width larger for larger input signal power. After exiting the HNLF the signal is directed through the second circulator to an optical bandpass filter (OBPF) and its spectrum is sliced at an off-centered (by a few nanometers) wavelength. Owing to the power-dependent flat-top spectrum broadening, amplitude of the output pulses is regularized and zero-level noise is rejected . The signal is again amplified and redirected to the HNLF through the circulator. In the return path the signal spectrum is broadened similarly to the forward path but with shifted center wavelength. At the exit of the HNLF, the spectrum is filtered by another OBPF, the center wavelength of which is now the same as that of the signal incoming to the regenerator. Wavelength-shift-free operation is thus obtained with enhanced regeneration strength owing to the repeated use of the HNLF.
3. Experimental results
Reshaping performance of the regenerator is experimentally assessed at 10Gb/s. A short-pulse mode-locked semiconductor laser diode oscillating at 1548.5nm is used as the source of 10GHz pulse trains. After amplitude of the pulses are modulated by a LiNbO3 modulator, the pulses are filtered by an OBPF (bandwidth of 1nm) for broadening the temporal pulse width to 4.3ps. The extinction ratio of the pulse train is varied by controlling the driving voltage to the modulator. The 10Gb/s signal is input into the regenerator shown in Fig. 1(b). The signal is first amplified by an EDFA and then fed into the HNLF. In the experiment an extra OBPF with bandwidth 2nm is inserted just after the EDFA to remove out-of-band noise emitted from the EDFA. Furthermore, a directional coupler with coupling ratio 80:20 is used in place of the input circulator. This is merely because of unavailability of the circulator at the input end of the HNLF. Although the use of directional coupler gives extra losses of about 1 and 7dB, for the forward and backward directions, respectively, this does not influence much on the performance assessment in this experiment. The HNLF has dispersion, dispersion slope, nonlinear coefficient, loss, and length of -0.35ps/nm/km at 1548.5nm, 0.03ps/nm2/km, 16.2/W/km, 0.52dB/km, and 1,800m, respectively. The signal with its spectrum broadened is directed to the OBPF through the circulator. For the OBPF for spectrum slicing, a three-cavity tunable filter with 3dB bandwidth of 1nm is used. The center wavelength of the filter was chosen to be 1550 ~ 1551nm as detailed later. The output signal is again amplified by the second EDFA and returned to the same HNLF. The spectrum of the signal is again broadened by the effect of self-phase modulation during transmission in the return path. A part of the signal is extracted by the input directional coupler and its spectrum is sliced by another threecavity OBPF with 1nm bandwidth. The center wavelength of the OBPF is now the same as that of the input signal.
Figure 2 shows averaged signal power output from the OBPF in the first stage of the regenerator, measured at point B shown in Fig. 1(b), versus input signal power into the HNLF, where no amplitude modulation is applied to the signal pulse train. Wavelength offset of the OBPF was chosen between 1.5 and 2.5nm. It is shown that larger wavelength offset gives stronger suppression of zero-level noise but, on the other hand, needs larger input power. In the experiment below in this paper, the wavelength offset was fixed at 2nm.
Figure 3 shows examples of signal spectra at the exit of HNLF in the forward path measured at point A in Fig.1(b) (solid curve marked with filled circles), at the exit of the first spectrum slicing filter measured at point B (dashed curve marked with filled circles), at the exit of HNLF in the return path measured at point C (solid curve marked with open circles), and at the exit of the second spectrum slicing filter measured at point D (dashed curve marked with open circles). Pulse train is amplitude-modulated with a pseudo-random bit pattern of length 27-1. Signal powers launched into the HNLF are 9.7 and 12.9dBm for the forward and return paths, respectively. Slight asymmetry in the spectra is due to the dispersion slope of the HNLF. Cleanly broadened spectra indicate that there is no serious nonlinear crosstalk between the signals counter-propagating in the HNLF. Pulse widths at the output of spectrum slicing filters are 4.8 and 4.7ps for the first and second regeneration stages, respectively.
Figure 4 shows improvement of extinction ratio (ER) of the modulated pulse train by the regenerator. ER of the input signal is degraded by decreasing the driving RF voltage to the LiNbO3 amplitude modulator. ER is measured from the eye patters recorded on a sampling oscilloscope with electrical bandwidth of 50GHz. The ER improvement by the first stage of the regenerator is shown by a dashed curve in Fig. 4. Input power to the HNLF is adjusted so that the ER improvement becomes maximum. ER improvement by more than 15dB can be obtained by the first regeneration stage for the input ER > 10dB. After the second-stage regeneration, the ER is further improved. Signal input power to the HNLF in the return direction is again adjusted so that the output ER becomes almost maximum. Total ER improvement as large as 24dB is obtained for the input ER of 6dB, indicating strong reshaping ability of the cascaded 2R regeneration. Fig. 5 shows examples of eye patterns of 10Gb/s signals before regenerator (a), after the first stage of the regenerator (b), and after the second stage of the regenerator (c) for the ER of the input signal of 2.5dB. Strong eye-opening improvement by the regeneration can be seen in Fig. 5(c).
By a separate measurement, we confirmed receiver sensitivity improvement for the 10Gb/s signal by passing the signal together with intentionally-added noise through the twostage bidirectional regenerator. Sensitivity degradation caused by Rayleigh back scattering was not observed.
In this paper bidirectional operation of all-optical 2R regenerator based on power-dependent spectrum broadening and off-centered filtering is proposed. We can obtain strong reshaping performance without wavelength conversion by the use of a single spool of highly nonlinear fiber. No linear and nonlinear crosstalk between counterpropagating signals along the fiber is observed.
The crosstalk caused by Rayleigh back scattering may be anticipated for higher signal speeds with larger average signal power. Our preliminary recirculating transmission experiment shows that the crosstalk is still insignificant at least up to 40Gb/s. Details of the in-line regeneration experiment using the bidirectional 2R regenerator will be reported elsewhere.
This work is supported by the Japan Society for the Promotion of Science (JSPS) Grant-in-aid for Scientific Research and by the National Institute of Information and Communications Technology (NICT).
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