A wideband and fast tunable chromatic dispersion compensator is one of the key components for the future high-speed optical transmissions. We have so far proposed and demonstrated a new tunable dispersion compensation scheme called parametric tunable dispersion compensator (P-TDC), which is based on the combination of parametric frequency conversion and frequency dependent dispersive media. The P-TDC has many attractive features such as a seamlessly wideband operation, wide tunable range and fast dispersion tuning. In fact, with appropriate configurations of dispersive media, the P-TDC can compensate the dispersion slope of transmission fibers even though the second-order dispersion is small. In this paper, we use such a P-TDC scheme and successfully achieve high-speed optical transmissions over a second- and third-order dispersion managed dispersion shifted fiber (DSF) span. The transmission experiments show low-penalty 172 Gbit/s return-to-zero on-off-keying transmissions over 126-km DSF.
©2010 Optical Society of America
In future high-symbol rate optical transmissions over 160 Gsymbol/s, a wideband and fast tunable operation of tunable chromatic dispersion (CD) compensators is essential . One of the authors recently proposed a new tunable CD compensation scheme called parametric tunable dispersion compensator (P-TDC), that is comprising a tunable parametric frequency shifter and dispersive media with frequency dependent group velocity dispersion (GVD) [2,3].
The P-TDC scheme has many attractive features that conventional TDCs for 40Gbit/s transmission  do not have, such as a wide seamless operating band while providing a large tunable range, intrinsically fast response only limited by the response speed of the tunable pump lasers and tunable bandpass filters used, and capability of dispersion slope compensation. The P-TDC is modulation-format and bit-rate independent and should be applicable to the simultaneous dispersion compensation of wavelength-division multiplexing (WDM) signals. On the other hand, at the expense for these unique features, the power consumption of the P-TDC is higher compared with optical passive compensations because the tunable frequency shifter requires active components such as a tunable light source and an optical amplifier. The total power consumption of a single wavelength conversion is estimated less than 25 W from the specifications of commercial products. If the P-TDC were applied to WDM signals, the power consumption per WDM channel would be smaller for large number of channels.
The operating principle of P-TDC was applied to a parametric tunable delay to overcome the delay limitations due to dispersion  and thereby microsecond optical delay was experimentally demonstrated at 10 and 40 Gbit/s transmissions, which in fact achieved the record delay-bandwidth products [6,7]. While the transmission characteristics seem promising, the transmission characteristics of the P-TDC in which the dispersion slope has a significant impact have not been extensively investigated. We have so far proposed P-TDC configurations with dispersion slope compensation for the transmission over dispersion shifted fiber (DSF) having a large relative dispersion slope (RDS), which consist of a parametric wavelength converter based on degenerate four-wave mixing (FWM) in highly nonlinear fiber (HNLF) and two dispersion compensating fiber (DCF) spans as dispersive media . The preliminary experiments reported an operating bandwidth of more than 1 THz  and dispersion-managed 1.8 ps pulse 43 Gbit/s return-to-zero (RZ) on-off-keying (OOK) transmissions over 126-km DSF .
In this paper, we report performances of our proposed P-TDC with dispersion slope compensation and demonstrate second- and third-order dispersion-managed 43 and 172 Gbit/s RZ-OOK transmissions over 126-km DSF. The P-TDC almost completely restored the pulse shape. In the transmission experiments at 43 and 172 Gbit/s, the power penalty after compensation is 1 and 3 dB, respectively. These low-penalty transmissions cannot be achieved without dispersion slope compensation for high-speed transmissions over such a long DSF.
2. P-TDC with slope compensation for DSF transmission
2.1 Principle of P-TDC employing frequency conversion with spectral inversion
Figure 1 shows the configuration and principle of the P-TDC in the case that we adopt the phase preserving frequency shifter with spectral inversion (SI). The P-TDC input optical signal at a center frequency of ω0 passes through the first dispersive medium (DMa) then the frequency is converted to ω1 by the parametric frequency shifter with SI. The frequency conversion is, for example, achieved through degenerate four-wave mixing (FWM) process in optical fiber . Finally, the converted signal is launched into the second dispersive medium (DMb). For in-line use, another frequency shifter may follow, converting the signal wavelength back to the original. The total effective second-order dispersion Deff is determined from the difference of the GVD between the two dispersive media, DMa and DMb, as
In the P-TDC, the optical signal also experiences the third-order dispersion of the respective media because the tunability of the second-order dispersion is attributed to the third-order dispersion. The effective third-order dispersion Seff is determined as the sum of the third-order dispersion of DMa and DMb.
Here, β3(a) and β3(b) are the third-order dispersion of DMa and DMb, respectively. These formulations mean that the P-TDC has a potential to compensate for both the second- and third-order dispersion with the choice of two dispersive media having appropriate dispersion profiles. The detailed principle and formulations of the P-TDC are discussed in .
2.2 P-TDC configuration for 126-km DSF transmissions
A high-speed transmission over DSF requires a compensation of not only the dispersion but also the dispersion slope. Figure 2(a) shows the bit-rates limited due to a third-order dispersion of 0.13 ps3/km (the value of DSF used in the following experiments) versus transmission fiber lengths, which is calculated assuming chirp-free Gaussian input pulses with the criteria that 95% of the pulse energy remains within the bit slot . Figure 2(b) is a shape of a 1.8-ps transform-limited Gaussian pulse and that after experiencing a third-order dispersion of 16.21 ps3, corresponding to the 126-km DSF used in the following transmission experiments. The dispersion slope induces an oscillating tail in the temporal waveform, and hence the slope compensation is required for the high-speed transmissions over 160 Gbit/s.
The dispersive media of the P-TDC should be determined from the dispersion profiles such that the third-order dispersion of the transmission fiber is compensated. The second-order dispersion can be always precisely compensated by tuning the converted wavelength. As expressed in the Eqs. (1) and (2), the second-order dispersion of the first dispersive medium is effectively subtracted from that of the second dispersive medium, while the third-order is just added up. This principle allows us to design arbitrarily large effective RDS as the effective second-order dispersion can be completely suppressed by the subtraction. Therefore, we can compensate both the second- and third-order dispersion of DSF with a large RDS.
Figure 3 shows the schematic configuration of a dispersion managed transmission line with 126-km DSF and P-TDC. We chose for the first and second dispersive media, the DCFs with the lengths of 9.31 and 7.82 km, respectively. These DCFs are the off-the-shelf conventional DCFs that compensate both the dispersion and the slope of the standard single mode fibers (G. 652, or SMF). The wideband frequency conversion with SI is achieved through the degenerate FWM process in HNLF . Table 1 shows the effective second- and third-order dispersion estimated from the measured fiber dispersion profiles when the signal and converted wavelength is 1560 and 1546 nm, respectively. It is confirmed from these results that both the second- and third-order dispersion are almost canceled at this converted wavelength. The RDS is 0.069 nm−1, more than three times as large as the reported largest RDS of 0.02 nm−1 in DCF .
Figure 4 shows the effective dispersion curves for various converted wavelength when the signal passes through the 126-km DSF and the P-TDC. The curves are calculated from the measured fiber dispersion profiles used. The wideband and wide-range tunable dispersion compensation is expected in the transmission over 126-km DSF. We have experimentally demonstrated more than 1 THz wideband dispersion compensation . In this configuration, the coefficient of GVD tuning due to the change of the converted wavelength is approximately 5.75 ps2/nm. The GVD tuning resolution of 0.23 ps2 requires 0.02-nm resolution of the pump-wavelength tuning, which is available with the tunable laser source (TLS) used in the following experiments. This tuning step is approximately one fifth of the dispersion inducing 1-dB penalty at 172 Gbit/s. The calculation results also suggest that the P-TDC configuration achieves infinite RDS as the second-order dispersion can be adjusted to zero with almost constant third-order dispersion.
3. Basic operations of P-TDC in 126-km DSF transmissions
We conducted the experiments of 126-km DSF transmissions in order to evaluate the compensation performances of the P-TDC designed in the previous section. Figure 5 shows the experimental setup. The transmission signal from the mode-locked fiber laser was 2.7 ps pulse train whose repetition rate and center wavelength were 10 GHz and 1560 nm, respectively. Following the 126 km DSF span with moderate amplifications by Erbium doped fiber amplifiers (EDFAs), the P-TDC compensated for the CD of the transmitted signals. We used a CW wavelength TLS for pumping the FWM in a 100-m HNLF with the nonlinear coefficient of 13.2 km−1W−1. The HNLF has a typical low-slope dispersion profile with the zero dispersion wavelength of 1534 nm. The wide range tunable wavelength conversion based on a single-pump degenerate FWM was confirmed in this HNLF . The input pump power limited by stimulated Brillouin scattering was approximately 21 dBm in the experiments.
In the experiment, we confirmed that the P-TDC consistently restored the pulse width. Figure 6 plots the measured and calculated pulse widths after compensation for various converted wavelengths, or amounts of residual dispersion. The theoretical calculation of the pulse width ΔTFWHM was based on the following formulation assuming that the input pulse was transform-limited Gaussian waveform.
Here, ΔT0, λC, and λmin are the input pulse width, the converted wavelength and the wavelength in which the pulse width is minimum, respectively. The coefficient μ is the differential dispersion with respect to the wavelength change. The calculated curve agrees well with the measured plots. The result also indicates that the pulse width was minimized at the converted wavelength of around 1546 nm, which is consistent with the estimation results shown in Table 1.
4. 1.8-ps pulse 43-Gbit/s transmissions over 126-km DSF
In this section, we evaluate the BER performances of 1.8-ps pulse 43 Gbit/s RZ-OOK transmissions over 126-km DSF with the P-TDC. Figure 7 shows the experimental setup. The pulse source based on optical pulse compression  generated a 1.8-ps pulse train whose repetition rate was 43 GHz. The central wavelength was 1560 nm. The pulse train was modulated with PRBS (231-1) signals by a lithium niobate (LN) modulator. We launched the modulated 43-Gbit/s RZ-OOK signals into the 126-km DSF, where the fiber loss was compensated by the EDFAs. The P-TDC configuration was the same as the one mentioned in the previous section. However the length of the HNLF used for the FWM-based wavelength conversion here was 82-m of the same fiber as the one used in the previous experiment. The input CW pump power was about 19 dBm.
We checked the pulse width dependence on the pump wavelength again because we introduced a small change in the experimental setup, and obtained an optimal pump wavelength of 1553.2 nm slightly shifted from 1553 nm as obtained in the previous results. Then we measured BER characteristics of 43 Gbit/s RZ-OOK transmissions (pulse width = 1.8 ps) as the pump wavelength changed from 1553.2 nm at the interval of 0.5 nm. Figure 8 shows the results of the BER characteristics. The wavelength conversion efficiency was kept at −19 dB by adjusting the polarization states of the pump signal. The power penalty from the back-to-back measurement at the optimal converted wavelength was approximately 1 dB, which was attributed to the lowering signal-to-noise (S/N) ratio caused by the transmission. The penalty increases with the pump wavelengths shifted apart from 1553.2 nm because of the residual dispersion.
Figure 9 shows the relationship between the power penalty and the residual dispersion. The power penalty is defined at a BER of 10−9 and the reference is the curve at the optimal wavelength. The residual dispersion is estimated from the converted wavelength and the measured fiber dispersion profiles. The slight difference between the estimated zero dispersion and the measured minimum penalty point is mainly due to the frequency chirping caused by the self phase modulation (SPM). The measured results indicate that 1-dB penalty from the optimal compensation corresponds to the residual dispersion of approximately 6 ps2. Assuming Gaussian waveform as input signals, 1-dB penalty is induced by a dispersion parameter (Bit-rate × Dispersion × Root mean square width of the spectrum) of approximately 0.15 . As the dispersion parameter of 0.15 corresponds to the residual dispersion of 5.6 ps2 in the experimental parameters, the measured penalty agrees well with the theoretical dispersion-induced penalty.
5. 172-Gbit/s RZ-OOK transmission over 126-km DSF
In this section, we demonstrate 126-km DSF transmission at 172 Gbit/s employing P-TDC with slope compensation. Figure 10 shows the setup of transmission experiments. For 172 Gbit/s RZ-OOK signal generations, 43 Gbit/s RZ-OOK optical signals through the LN modulator were multiplexed in time-domain by the 1x4 optical multiplier. The signal after 126-km DSF transmissions was launched into the P-TDC with the pump wavelength of 1553.2 nm. The compensated signal was demultiplexed from 172 to 43 Gbit/s and received with BER measurements. The optical demultiplexing was achieved through parametric sampling technique [15,16], utilizing a FWM process in 100 m HNLF. The pump signal was 2-ps pulse train generated by the mode locked laser diode (MLLD) with a recovered clock signal. The phase-locked loop using an electro-absorption (EA) modulator as a phase comparator extracted the clock signal.
Figure 11 shows the input and compensated signal waveforms measured by the optical sampling oscilloscope (OSO). The waveform is almost completely restored by the dispersion compensation. The BER characteristic is shown in Fig. 12 . In the back-to-back measurement, the MLLD was driven with a reference clock. The power penalty from the back-to-back measurement is approximately 3 dB at a BER of 10−3. The penalty increases by 2 dB compared with 1 dB penalty in the 43 Gbit/s transmission experiment. The degradation of the pump from the MLLD caused by the timing jitter of the recovered clock is the main reason of the penalty increment. The polarization mode dispersion is another potential source to increase the penalty. We confirmed that the P-TDC was effective for the dispersion and dispersion slope compensation in 172 Gbit/s transmission over DSF.
6. Summary and conclusion
We demonstrated 43 and 172 Gbit/s RZ-OOK transmissions over a 126-km DSF with the P-TDC. The P-TDC configuration was designed to compensate both the second- and third-order dispersion of the DSF having a large RDS. The experimental results showed that the P-TDC achieved wideband and low-penalty tunable second-order dispersion compensations with the dispersion slope compensation. As the P-TDC is insensitive to the modulation format, it is applicable to a wide variety of high-symbol rate transmissions requiring the dispersion slope compensation.
Part of this work was supported by Special Coordination Funds for Promoting Science and Technology of MEXT. Authors thank FURUKAWA ELECTRIC CO. LTD for providing DCFs.
References and links
1. S. Vorbeck and R. Leppla, “Dispersion and Dispersion Slope Tolerance of 160-Gb/s Systems, Considering the Temperature Dependence of Chromatic Dispersion,” IEEE Photon. Technol. Lett. 15(10), 1470–1472 (2003). [CrossRef]
2. S. Namiki, “Wide-Band and -Range Tunable Dispersion Compensation Through Parametric Wavelength Conversion and Dispersive Optical Fibers,” J. Lightwave Technol. 26(1), 28–35 (2008). [CrossRef]
3. S. Namiki, “Tunable Dispersion Compensation Using Parametric Processes,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OWP1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2008-OWP1 http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-14-11958
4. C. Doerr, Optical Compensation of System Impairments,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, Technical Digest (CD) (Optical Society of America, 2006), paper OThL1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2006-OThL1
5. T. Kurosu and S. Namiki, “Continuously tunable 22 ns delay for wideband optical signals using a parametric delay-dispersion tuner,” Opt. Lett. 34(9), 1441–1443 (2009), http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-34-9-1441. [CrossRef] [PubMed]
6. E. Myslivets, N. Alic, S. Moro, B. P. Kuo, R. M. Jopson, C. J. McKinstrie, and S. Radic, “Microsecond Parametric Optical Delays,” J. Lightwave Technol. 28, 448–455 (2010). [CrossRef]
7. E. Myslivets, N. Alic, S. Moro, B. P. Kuo, R. M. Jopson, C. J. McKinstrie, M. Karlsson, and S. Radic, “1.56-micros continuously tunable parametric delay line for a 40-Gb/s signal,” Opt. Express 17(14), 11958–11964 (2009). [CrossRef] [PubMed]
8. S. Namiki, “Wideband Tunable Dispersion Compensation of 126 km zero-DSF Using Parametric Processes,” in 34th European Conference and Exhibition on Optical Communication (ECOC2008), Technical Digest (CD), paper Tu.4.B.3.
9. K. Tanizawa, T. Kurosu, and S. Namiki, “1.8-ps RZ-Pulse 43-Gbps Transmissions over 126-km DSF with Parametric Tunable Dispersion Compensation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper OThJ4. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2010-OThJ4
10. K. Inoue and H. Toba, “Wavelength conversion experiment using fiber four-wave mixing,” IEEE Photon. Technol. Lett. 4(1), 69–72 (1992). [CrossRef]
11. P. Govind, Agrawal, “Fiber-Optic Communication Systems Third Edition,” (John Wiley & Sons, Inc., New York, 2002).
12. M. Takahashi, K. Mukasa, and T. Yagi, “Full C-L Band Tunable Wavelength Conversion by Zero Dispersion and Zero Dispersion Slope HNLF,” in 35th European Conference and Exhibition on Optical Communication (ECOC2009), Technical Digest (CD), paper P1.08.
13. M. Wandel, P. Kristensen, T. Veng, Y. Qian, Q. Le, and L. Grüner-Nielsen, “Dispersion compensating fibers for non zero dispersion fibers,” in Optical Fiber Communications Conference, A. Sawchuk, ed., Vol. 70 of OSA Trends in Optics and Photonics (Optical Society of America, 2002), paper WU1. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-2002-WU1
14. T. Inoue, H. Tobioka, K. Igarashi, and S. Namiki, “Optical Pulse Compression Based on Stationary Rescaled Pulse Propagation in a Comblike Profiled Fiber,” J. Lightwave Technol. 24(7), 2510–2522 (2006). [CrossRef]
15. P. A. Andrekson, “Optical techniques for high-bit-rate systems,” in Optical Fiber Communications Conference, Vol. 8 of 1995 OSA Technical Digest Series (Optical Society of America, 1995), paper ThL6. http://www.opticsinfobase.org/abstract.cfm?URI=OFC-1995-ThL6
16. C.-S. Brès, A. O. J. Wiberg, B. P.-P. Kuo, J. M. Chavez-Boggio, C. F. Marki, N. Alic, and S. Radic, “Optical Demultiplexing of 320 Gb/s to 8 × 40 Gb/s in Single Parametric Gate,” J. Lightwave Technol. 28(4), 434–442 (2010). [CrossRef]