Side-mode injection-locked FPLD transmission diagnosis
2.5 Data transmission diagnosis of side-mode injection-locked FPLD
We also evaluate the transmission performance with a BER evaluation method previously demonstrated by Bergano et al., which is approached by measuring the signal-to-noise ratio at the decision circuit of an optical transmission and receiving system.[2.13] The equivalent mean value and standard deviation of the marks and spaces are determined by fitting this data to Gaussian function, and the measured BER of the optical transmitting eye-diagram can be accurately calculated from the recorded Q factor at a desired data rate. The measured Q factor of the slave FPLD with optimized injection-locking condition can be as high as 9.2, providing a reachable BER of 1.8u10-20 at the data rate of 2.5 Gbit/s. By increasing the biased current of the slave FPLD to 20 mA, the wavelength locking at the data rate of >2.5 Gbit/s can be achieved, which is eventually limited by the transient gain contribution to each longitudinal mode. On the other hand, the injection power required to maintain the slave FPLD within the injection-locking range as a function of the biased current, and the corresponding Q factor are also measured and shown in Fig. 2.7. The calculated Q-factor of the injection-locked slave FPLD based transmitter at different driving currents and injection-locking powers was illustrated. The effective transmission is obtainable within
blue-shaped region of an estimated Q > 7 corresponding to a BER of about 10-12. The red-shaped region represents the practical implementation of such an injection-locked slave FPLD at an injecting power below 2 dBm. That is, the optimized operating parameters for concurrently achieving high-Q and low-injection are determined at the driving current for FPLD between 12 and 17 mA.
Fig. 2.7 Measured injected power (hollow markers) and measured Q (solid markers) of the driving current at the principle longitudinal mode.
Later on, the BER analysis at 2.5 Gbit/s is also performed to characterize the data transmitting performance of a simulated multi-channel DWDM fiber-optic network using the directly NRZ-modulated FPLD under side-mode injection-locking regime. The PRBS data pattern length for modulating the injection-locked FPLD is 231-1. Figure 8 shows the measured BER for the channels at 1st, 9th, 13th, 14th, 15th, and 21th orders, which are corresponding to the mode number ('m) of -12, -4, 0, 1, 2, and 8, respectively, away from the central mode.
The power of the downstream signal injected into the slave FPLD was fixed to -3 dBm by using an optical attenuator, and the biased current of the injection-locked FPLD was fixed at 15 mA. The maximum usable channel of the side-mode injection-locking slave FPLD is 22,
covering a wavelength range up to 24 nm. A BER of <10-12 is obtained for the nearest 13 side-modes and a BER of 10-10 can be achieved for all of the 22 injection-locked side-modes.
Without any chirping compensation, the data streams exhibit a power penalty of about 1.1 dB at a BER of 10-12 after 25-km transmission. In particular, there is a measured positive power penalty of 0.7 dB at a BER of 10-12, which is mostly attributed to the reduction of the relaxation oscillation of the slave FPLD from the competition among longitudinal modes as shown in the inset of Fig. 2.8. As the transient situation of the carrier density changing from the bit 0 to bit 1, the photon density of the desired longitudinal mode reaches a stable gain by external optical injection, whereas the carrier density continuously depletes and cannot form relaxation oscillation. Consequently, the rising time and falling time (defined as the duration between 20% and 80% of the on-level amplitude) are 118 ps and 125 ps, respectively, and a well-opened eye pattern can be obtained with a relatively large dynamic range as shown in inset of Fig. 2.8. Both the nearly error-free (BER < 10-12) back-to-back transmissions with and without optical injection can be detected at the received optical power of larger than -24.4 and -23.7 dBm, respectively. Up to 7 dB power penalty is observed at BER of 10-9 when changing the injection-locking from principle to the largest side mode, however, the corresponding receiving power level is still beyond the requirement for data communication.
-28 -26 -24 -22 -20 -18 -16 -14
Fig. 2.8 BER analysis of wavelength injection-locked FPLD at different longitudinal modes and measured eye diagrams (inset) with and without injection
2.6 Summary
In this chapter, we theoretically analyzed the effect of the injection-locking power and side-longitudinal-mode order on the linewidth, SMSR, and BER characteristics of a slave FPLD injected by another spectrally sliced master FPLD. The SMSR and 3-dB linewidth of such a FPLD-FPLD link as a function of the injection-locking power dependent reflectivity change are simulated. The back-to-back and 25km-SMF transmission performances of the 2.5-Gbit/s directly modulated FPLD based WDM-PON transmitter under side-mode injection-locking is demonstrated. Such a wavelength injection-locked FPLD shows a largest SMSR of 35 dB and a Q factor ranging from 9.2 to 7.5 as the injection-locked channel extends to the 12th side-mode with respect to the central carrier. Degradation on the linewidth enhancement factor from 1.5 to 2.1 corresponding to the principle- and side-mode injection-locking conditions of the slave FPLD injection-locked are observed. The
maximum usable channels of the side-mode injection-locking FPLD are 22, covering a wavelength-locking range up to 24 nm. A BER of <10-12 is obtained for the nearest 13 modes and a 10-10 error rate can be achieved for all of the 22 injection-locked modes, providing a negative power penalty of -0.7 dB due to the reduction on relaxation oscillation of the FPLD. These results indicates that the demonstrated side-mode injection-locked FPLD can be a potential candidate of unified WDM-PON transmitter to achieve the cost effective and high-capability 2.5 Gbit/s WDM systems.
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