10 Gbit/s All-Optical NRZ-to-RZ Data Format Conversion Based on a Backward
3.3 Results and Discussion
The eye-diagram of NRZ PRBS data-stream and the pulse-train of dark-optical-comb with duty-cycle of 70% are shown in Figs. 3.2(a) and 3.2(b), respectively. Under such an intense injection with a dark-optical-comb of relatively large duty-cycle, the gain of SOA can be fully depleted, and only a greatly narrowed gain window with a full-width at half-maximum of 30 ps is left within most of the modulating period. This results in a pulsating operation as well as NRZ-to-RZ data format conversion to the incoming optical NRZ data. The NRZ PRBS data encounters the pulsed gain in the SOA and is format converted to a RZ data pattern, as shown in Fig. 3.2(c).
By contrast, a normal optical pulse injection only causes finite gain-depletion and leaves a broader gain window in the SOA, which inevitably results in a distorted NRZ shape instead of a RZ pattern. The converted RZ PRBS data is remaining at the identical wavelength and the same data polarity to that of the incoming NRZ PRBS data.
Such a feature is important for a format converter used at interface of WDM and TDM link. As the NRZ-to-RZ conversion of multi-channel WDM data-streams onto a given wavelength is required for transmission across the TDM network [3.10, 3.11], it is highly desirable to preserve the polarity (non-inversion on the phase of data format) of data-stream between the input WDM channels and the output TDM channel when performing data format conversion. Otherwise, the addition of additional overhead prior to the TDM packet header is necessary in order to correct its data polarity.
The extinction ratio is determined by accounting into both the gain depletion and gain-spectral shifting effects occurring in the SOA under high power injection. The peak wavelength of the SOA gain can be shifted from 1530 to 1550 nm under the strong backward injection at 1550 nm. Such a spectral shift in the SOA can be realized by the
expression of λN =λ0-κ0(N-N0) [3.12], where
λ
N is the peak wavelength at carrier density N,λ
0 is the peak wavelength at transparency,κ
0 is a constant characterizing the gain-peak shift, N is the carrier concentration, and N0 is the carrier concentration at transparency. The shift of SOA gain peak to a longer backward injecting wavelength is mainly attributed due to the depletion of carriers in the SOA by the intense dark-optical-comb injection. It is preferred to locate the injected dark-optical-comb at wavelengths near but slightly longer than the gain peak, as the gain SOA can be depleted more severely at larger gain region and thus red-shifted to a longer wavelength.This eventually leads to a higher extinction ratio of the input data-stream at shorter wavelengths as the gain of SOA is changing from saturation to depletion condition.
However, the depletion of carriers as well as gain of the SOA becomes less significant if a backward injecting wavelength is too long to exceed the original gain peak, which in turn causes a reduction in extinction ratio of the input data-stream at shorter wavelengths. As shown in Fig. 3.3, the extinction ratio of the converted RZ PRBS data-stream is dependent on the wavelength of backward dark-optical-comb injection, which reveals a largest extinction ratio with a backward injection at 1550 nm when setting the NRZ PRBS data at shorter wavelengths. The extinction ratio (ER) (i.e. power ratio of the transmitted “1” data to the transmitted “0” data) of the incoming NRZ PBRS data is about 7.13 dB. After conversion, the measured eye diagram for the RZ PRBS data in Fig. 3.2(c) reveals a greatly improved extinction ratio of 13.6 dB. To investigate the minimum ER of the input NRZ in which the ER can still be greatly improved at the output RZ, the input ER versus output ER is measured, as shown in Fig.
3.4. The input ER larger than 6.24 dB can be converted into RZ format with ER of 11.92. The output ER will increase with the increase of the input ER. Note that when the input ER is larger than 11 dB the output ER will saturate.
The BER of the back-to-back (BtB) NRZ PRBS and converted RZ PRBS data-streams at 10 Gbit/s are measured as function of received data power, as shown in Fig. 3.5. The nearly error-free (BER < 10-12) BtB NRZ and RZ PRBS data-streams can be detected at received optical power of larger than -15 and -18.7 dBm, respectively.
The BER of the format-converted RZ PRBS data inevitably increases with reduction of the incoming NRZ PRBS data power. Previously, it has been elucidated that high levels of electrical pumping of SOA accompanied by intense optical injection to suppress lasing and enhance carrier/photon interactions, can significantly result in a faster conversion speed than the carrier recovery rate [3.13]. In our case, the increase on both of the data and backward injection power also result in the shortened rising and falling time of signals, respectively. The BER of the converted RZ PRBS can still be 10-9 when the NRZ PRBS data power is <-6 dBm, which further reduces to 10-12 at data power of -1 dBm.
Note that the upper limitation on operational bit-rate is determined by the gain recovery time of the SOA given by
τ
-1= τ
nr-1+aS [3.14], where τ
nr is the non-radiative
recombination time, a denotes the stimulated emission rate, and S is the internal photon density in the SOA. When the backward injected dark-optical-comb pulse-train turns off, low and high levels of the optical NRZ PRBS provide different photon density and encounter different gain recovery time in the SOA. Such a mechanism further improves the BER performance of the converted RZ PRBS. A negative power penalty [3.15] of -3.7 dB is measured at a BER of 10-12, which has already exceeded the performance of a previous SOA-based fiber-loop-mirror configuration [3.5]. The negative power penalty is attributed to the noise reduction of the data-stream by both the gain-saturated SOA [3.16] and the data format conversion. To date, the proposed data format converter requires only a dark-optical-comb generator and a SOA incomparison with the complicated FWM and XPM techniques.
3.4 Conclusion
In conclusion, an all-optical NRZ-to-RZ data format converter at bit-rate up to 10 Gbit/s is primarily demonstrated by using a backward dark-optical-comb injected SOA.
The wavelength and polarity of the converted RZ PRBS data is preserved with the incoming NRZ PRBS data. The extinction ratio of the incoming NRZ PRBS data and the format-converted RZ PRBS data are 7.13 dB and 13.6 dB, respectively. At 10 Gbit/s, the bit-error rate can be as low as 10-12 at the received optical power of about -18.3 dBm. It is noted that a negative power penalty of about -3.7 dB at a BER of 10-12 is measured, which implies that such a configuration has re-timing, re-shaping, and re-amplifying (3R) capabilities.
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