In a weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) based DWDM-PON system with an array-waveguide-grating (AWG) channelized amplified spontaneous emission (ASE) source located at remote node, we study the effect of AWG filter bandwidth on the transmission performances of the 1.25-Gbit/s directly modulated WRC-FPLD transmitter under the AWG channelized ASE injection-locking. With AWG filters of two different channel spacings at 50 and 200 GHz, several characteristic parameters such as interfered reflection, relatively intensity noise, crosstalk reduction, side-mode-suppressing ratio and power penalty of BER effect of the WRC-FPLD transmitted data are compared. The 200-GHz AWG filtered ASE injection minimizes the noises of WRC-FPLD based ONU transmitter, improving the power penalty of upstream data by −1.6 dB at BER of 10−12. In contrast, the 50-GHz AWG channelized ASE injection fails to promote better BER but increases the power penalty by + 1.5 dB under back-to-back transmission. A theoretical modeling elucidates that the BER degradation up to 4 orders of magnitude between two injection cases is mainly attributed to the reduction on ASE injection linewidth, since which concurrently degrades the signal-to-noise and extinction ratios of the transmitted data stream.
©2009 Optical Society of America
Amplified-spontaneous-emission (ASE) injection-locked semiconductor optical amplifiers or laser diodes are promising sources for potential application in future wavelength division multiplexed passive optical network (WDM-PON) technology. To construct the WDM-PON, the user terminals or optical network units (ONUs) require the universal light source with broadband gain spectrum which can be employed to all channels. The desired light source must be remote-controlled or injection-locked at specific channel wavelength given by the central office. Several cost-effective issues based on long-cavity Fabry-Perot laser diodes (FPLDs) and reflective semiconductor optical amplifiers (RSOAs) [1–8] have been proposed to meet such a colorless demand for being the universal light source, which can be applicable to each channel under external injection-locking with amplified-spontaneous-emission (ASE) based incoherent broadband light source (BLS). Later on, the ASE injection-locked RSOAs or FPLDs are rapidly emerging to replace the distributed-feedback lasers at particularly selected wavelengths for WDM-PON. To achieve wavelength independent operation and enhance the channel compatibility in DWDM-PON, a new class of FPLD with weak-resonant-cavity (WRC) design has been introduced recently [9,10]. The channelized ASE injection-locked WRC-FPLD with front-facet reflectivity of only 1% exhibits intriguing features such as the much broader spectrum when comparing with the conventional FPLDs, and the preserved longitudinal modes to facilitate the SNR and ER. In fact, the conventional FPLD injected by ASE source filtered with DWDM-PON at 50 GHz AWG channel spacing has ever been achieved, however, which exhibits a difficulty in practical applications resulting from the increasing relative intensity noise (RIN) with such narrow channel spacing . Recently, a wavelength-locked FPLD achieved by injecting the low-noise BLS instead of the erbium-doped fiber amplifier (EDFA) is demonstrated for increasing channel capability of DWDM-PON . Nonetheless, a major reason leading to the constrain on using array waveguide grating (AWG) in such DWDM-PONs is due to intra-band crosstalk, which occurs from the inevitable interference of ASE reflection from AWG facet and the up-stream transmitted data under high-power injection case . By using an AWG filtered ASE as the injection-locking source in this work, we investigate the up-stream transmission performances of the WRC-FPLD based DWDM-PON architecture with AWG channel spacings of 200 GHz and 50 GHz. The error-free transmission at bit-rate of 1.25 Gbit/s can easily be achieved by using the spectrum-sliced ASE injecting-locked FPLD transmitter in the DWDM-PON with AWG channel spacing of 50 GHz. The injection-locked WRC-FPLD spectra within the AWG transmission window, the signal-to-noise ratio and the on/off extinction ratio of the up-stream transmitted data, the receiving power penalty for the back-to-back and the 25-km transmission BER performances at AWG of 200 GHz and 50 GHz cases are compared. In addition, the correlation between the suppressed reflection of AWG filtered ASE source and the corresponding BER performance in these DWDM-PON systems are discussed.
2. Experimental setup
A typical DWDM-PON architecture with an EDFA based broadband ASE source for injection locking the WRC-FPLD based up-stream transmitter at the localized optical network unit (ONU) is shown in Fig. 1 . The output power of the ASE source after passing through each channel of the AWG based DWDM multiplexer must be sufficiently high for achieving better injection-locking and up-stream transmission of the WRC-FPLD. Such a high-power consumption inevitably raises an unexpected broadband reflection along the transmission path in the DWDM-PON. In this case, the crosstalk between the broadband reflection of the injected ASE and the up-stream transmitted data from the ASE injection-locked WRC-FPLD has left as a serious problem to strongly affect the signal performance and network capacity.
Our previous study indicated that there is a power penalty up to 2 dB at BER of 10−9 occurred for such a 1.25-Gbit/s directly modulated WRC-FPLD when injection-locking by a 200-GHz AWG channelized ASE source. To promote the error-free transmission with a better sensitivity, a mandatory solution relies strictly on removing such a broadband reflection from the transmission path in the DWDM-PON system. In contrast, a modified DWDM-PON system in Fig. 2 constructed by the WRC-FPLDs based ONUs and an AWG spectrally sliced ASE injection-locker located prior to all ONUs is demonstrated. The EDFA based broadband ASE source passes through an AWG with channel spacing of 50 GHz or 200 GHz to injection-lock the WRC-FPLD via an optical circulator in each ONU.
Such an arrangement of the ASE source at remote node diminishes the broadband ASE reflection as the optical circulator separate the injection and up-stream transmitting paths. In each ONU, the WRC-FPLD exhibits a threshold current of about 25 mA, a longitudinal mode spacing of 0.6 nm, the back and front facet reflectivity of 100% and 1%. The maximum injection power of WRC-FPLD is limited at −3 dBm to avoid the damage on its end-face anti-reflection coating. A long-cavity design enables the WRC-FPLD lasing at least one mode within 50-GHz AWG channel during injection-locking condition. In experiment, the biased current of FPLD directly modulated at 1.25 Gbit/s with pattern length of 223-1 is maintained as 35 mA corresponding to 1.4 Ith for transmission performance diagnosis. I particular, we set the WRC-FPLD temperature at 21°C, 23°C and 25°C to provide the different injection-locked mode numbers within one AWG channel for optimizing the transmission performance.
3. Results and discussions
The spectral characteristics of the WRC-FPLD injected by AWG channelized ASE with different 3dB spectral linewidths (Δλ = 0.35 nm for 50-GHz AWG and Δλ = 1.1 nm for 200-GHz AWG) are also shown in Fig. 3 . The conventional DWDM-PON is based on the ASE injection-locked mode-extinction-free reflective semiconductor optical amplifier, which easily causes transmission error by the ASE source dependent strong intensity noise. Alternatively, the FPLD based transmitted without temperature control usually leads to an injection-locking failure by its thermally drifting wavelength. In comparison, the long and weak resonant-cavity design of the WRC-FPLD concurrently solves the drawbacks happened in conventional DWDM-PON transmitters, which introduces a sufficiently broadband gain spectrum with narrow longitudinal mode spacing, such that the injection-locking can always be maintained and the weak-mode lasing scheme efficiently improves the stimulated to spontaneous power ratio for better noise suppression.
After passing through AWG channel, the filtered spectrum of the directly PRBS-modulated WRC-FPLD differs significantly from that of a free-running WRC-FPLD, in which the signal-to-noise ratio is greatly improved. At least two lasing WRC-FPLD modes can be ensured within the spectral window when using the 200-GHz AWG and Mux/DeMux filters. The injection-locking mode number periodically changes between 2 and 3 within a temperature increment of 5°C, the corresponding mode spectra measured at temperature of 21°C, 23°C, and 25°C are shown in Figs. 3(a), 3(b) and 3(c), respectively . Similar injection-locking behaviour can also be observed if the channel spacing of the AWG changes from 200 GHz to 50 GHz, as shown from Fig. 3(e) to Fig. 3(f). In comparison, the side-mode suppressing ratio (SMSR) of the WRC-FPLD injection-locked spectra also exhibits an inverse proportionality with the spectral linewidth of the AWG channelized ASE source (see Fig. 4 ). At same injecting power of −3 dBm, the WRC-FPLD injection-locked by the 50-GHz AWG-sliced ASE source provides a better SMSR than that by the 200-GHz one (see Fig. 5 ). A maximum deviation on the SMSR up to 3 dB for two different cases is observed, however, which seems to play a trivial role on the transmission performance as compared to the influence by other parameters of the WRC-FPLD as discussed below.
The measured back-to-back optical eye diagrams are shown in Figs. 6 and 7 . Increasing the injection-locking ASE spectral linewidth from 0.35 to 1.1 nm (by changing the channel spacing of the AWG filter from 50 GHz to 200 GHz) could effectively improve the signal-to-noise ratio (SNR) of the WRC-FPLD up-transmitted data from 7.5 dB to 9.7 dB at same injecting power level. The spectrally sliced ASE source increases its intensity noise when reducing the AWG channels bandwidth, which eventually leads to the degradation on up-stream transmitted signal quality by narrowing the injection-locked WRC-FPLD linewidth. In principle, the SNR of the ASE injection-locked WRC-FPLD is inverse proportional to the spontaneous-spontaneous beating noise given by 2I2 ASEBe/mΔλ, where IASE is the ASE injecting power, Be is the electrical bandwidth, m is the polarization ratio, and Δλ is the spectral linewidth. This explains why the SNR is degraded by shrinking the spectral linewidth of the AWG channelized ASE source.
Lower SNR on the up-stream transmitted data from the WRC-FPLD injection-locked by the AWG-sliced ASE with narrower channel spacing is observed, which reveals the difficulty in raising the network capacities in ASE injection-locked WRC-FPLD based WDM-PON. In Figs. 8 and 9 , the dynamic frequency chirps of the up-stream transmitted data from WRC-FPLD injection-locked by 200-GHz and 50-GHz AWG-slice ASE sources at same power level of −3 dBm are compared. As shown in Fig. 8, a broader-linewidth injection (Δλ = 1.1 nm) could gives rise to a larger dynamic chirp (varied from + 5.5 to −3 GHz), as the linewidth enhancement factor (α) in the formula of dynamic frequency chirp (Δνc) is strongly inverse proportional to Δλ,
With increasing power of the AWG-sliced ASE injection, the off-state power level of the injection-locked WRC-FPLD transmitter is also enlarged due to the reduction on its threshold current under external injecting operation. This inevitably causes a decreased on/off extinction ratio (ER, as defined by Ion/Ioff) to dynamically suppress the negative frequency chirp of the up-stream transmitted data, which could further contribute to different power penalty level with replacing AWG channel bandwidth and with lengthening propagating distance. Apparently, the shrinkage of ER on the WRC-FPLD under 50-GHz AWG-sliced ASE injection is more significant than that under 200-GHz AWG-sliced ASE injection, since the injecting power is more concentrated within a narrower linewidth for the former case. As a result, the dynamic range of WRC-FPLD output power shrinks to result in a reduced ER as well as a small chirp, as shown in Fig. 9. However, in the case of the ASE injection-locked WRC-FPLD transmitter, there is an increasing power penalty on the BER receiving sensitivity with reducing AWG channel bandwidth. Although the dynamic chirp of the WRC-FPLD transmitted data is slightly reduced by decreasing ER, the effect of improving ER is more pronounced than the decreasing chirp on the Q parameter as well as the BER of the WRC-FPLD transmitted up-stream data.
To investigate the correlation between AWG-sliced ASE linewidth and transmission performance in more detail, the BER analysis of the back-to-back and 25-km transmitted data from the WRC-FPLD injection-locked by the AWG-channelized ASE source with changing spectral linewidth are compared in Fig. 10 . Under the injection power of −3 dBm, the 200-GHz AWG-sliced ASE injection results in a WRC-FPLD data stream with a requested receiving power as low as −31.6 dBm for BER of <10−9. A receiving power penalty of about 1.3 dB is obtained after 25-km propagation. In contrast, the 50-GHz AWG-sliced ASE injection provides same BER performance at larger receiving power of −30.1 dBm. After 25-km SMF transmission, the power penalty in the 50-GHz AWG based WDM-PON is approximately 1 dB, however, changing the AWG to 200-GHz makes the power penalty increased to 1.5 dB. To verify the nearly wavelength-independent operation of the WRC-FPLD, we detune the operating temperature to make 50-GHz AWG-sliced ASE spectrum injected either on the peak or on the valley between longitudinal modes. There is a negative power penalty of only 0.5 dB observed between two conditions. The contribution of the injected ASE linewidth to the BER is straightforward, since the 200-GHz AWG-sliced ASE with a broader linewidth leads to a BER of only 4 × 10−11 at receiving power of −29.5 dBm even after 25-km propagation, which is at least one order of magnitude lower than the BER of the back-to-back transmitted data generated by the WRC-FPLD under 50-GHz AWG-sliced ASE injection. This result correlates well with the positive contribution of ASE linewidth to the SNR as discussed in above section.
In addition to the SNR, there is another factor to affect the BER performance of the ASE injection-locked WRC-FPLD is the on/off extinction. If we define the BER of WRC-FPLD transmitted up-stream data as a function of Q parameter with optimum setting of the decision threshold at the receiver part by 14], and the bandwidth of AWG is decisive to transmission performance in WDM-PON transmitter . Furthermore, only when the injection increases extremely high, which inevitably causes a decreased on/off extinction ratio (ER, as defined by Ion/Ioff) to degrade the BER performance of the up-stream transmitted data. When comparing with the SNR in general case, the ER and chirp parameter are not the dominant factor to affect the back-to-back BER performance of the injection-locked WRC-FPLD up-stream transmitter. Under the AWG-sliced ASE injection, the WRC-FPLD transmitted data with relatively high ER exhibits Q = IASE/Isp-sp = (SNR)0.5 = (mΔλ/2Be)0.5∝ (Δλ/Be)0.5.
In Fig. 11 , it is observed in experiment that the SNR of the WRC-FPDL transmitted data linearly increases by 2 dB when enlarging ASE injecting power from −12 to −3 dBm, whereas the ER oppositely degrades due to the reduction of threshold current associated with the shifted power-current response at higher ASE injecting condition. In our case, the linewidths of the WRC-FPLD injection-locked by 200-GHz and 50-GHz AWG-sliced ASE are 1.1 nm and 0.35 nm, respectively. The difference on Q parameters by a factor of 1.76 calculated from the measured SNR is almost identical with that evaluated from the spectral linewidth deviation (Q200GHz/Q50GHz = 1.77). By enlarging the 50-GHz and 200-GHz AWG-sliced ASE injecting power from −12 to −3 dBm, the ER is oppositely decreased from 9 to 8 dB and from 11.5 to 9 dB, corresponding to the decreasing of the (ER-1)/(ER + 1) factor from 0.86 to 0.77 and from 0.77 to 0.72, respectively. A more significant degradation on ER by 2.5 dB at higher injecting level has been observed when injection-locking the WRC-FPLD with ASE of larger linewidth. Under high injection, the ER starts to play more important role on the BER than that under low injection case. Even though, the ER of the WRC-FPLD transmitted data can still meet the demand of data communication standards (ER >8 dB) no matter the injection ASE source is sliced by 50-GHz or 200-GHz AWG. In summary, the 200-GHz AWG-sliced ASE injection provides an increment on Q parameter by 1.75 times than the 50-GHz AWG-sliced ASE injection case. This clearly elucidates the BER deviation between two injecting conditions up to four orders of magnitude at same receiving power obtained.
In a weak-resonant-cavity Fabry-Perot laser diode (WRC-FPLD) based DWDM-PON system with channelized AWG DWDM multiplexer/de-multiplexer, the injecting-linewidth dependent transmission performances of a channelized ASE injection-locked WRC-FPLD directly modulated at 1.25Gbit/s is characterized. To effectively raise the network capacity of the DWDM-PON system with injection-locked transmitter based ONU, the location of 50-GHz or 200-GHz AWG-sliced ASE source at remote-node for reducing the interfered crosstalk induced by broadband ASE reflection at transmission path is proposed. With the DWDM AWG filters of two different channel spacings at 50 and 200 GHz, several characteristic parameters such as interfered reflection, relatively intensity noise, crosstalk reduction, side-mode-suppressing ratio and power penalty of BER effect of the WRC-FPLD transmitted data are compared. The ideal WDM-PON structure with 200-GHz channel bandwidth significantly improves the receiving power of BER at 10−9 from −30 to −31.6 dBm. The 200-GHz AWG filtered ASE injection minimizes the noises of WRC-FPLD based ONU transmitter, thus improving the power penalty of upstream data by −1.6 dB even at BER of 10−12. In contrast, there is a power penalty of 1.5 dB if the AWG channel bandwidth 200-GHz is replaced by 50-GHz at same ASE injection power. The 50-GHz AWG channelized ASE injection fails to promote better BER under back-to-back transmission due to its narrow spectral linewidth. Nevertheless, the 50-GHz AWG exhibits a lower negative frequency chirp as well as extinction ratio compared to 200-GHz in the WDM-PON. Furthermore, the effects of signal-noise ratio and on/off extinction-ratio on the BER and power penalty are experimentally demonstrated and theoretically elucidated. The BER degradation up to 4 orders of magnitude is mainly attributed to the reduction of injection-locked mode number and slightly increasing RIN noise, which concurrently degrade the signal-to-noise and extinction ratios of the transmitted data stream.
This work is partially supported by the National Science Council of Republic of China under grants NSC97-2221-E-002-055.
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