1. Introduction

Broadband light source (BLS) based wavelength-division multiplexed passive optical network (WDM-PON) architecture has been considered as a cost-effective way to realize a high-capacity PON system [1–3]. This is because in the BLS based WDM-PON schemes we can inexpensively implement a number of WDM sources using only one BLS and a pair of wavelength demultiplexer/multiplexer for multichannel WDM applications. Two different types of BLS based optical sources, such as a spectrum-sliced source and a BLS seeded source, have been proposed and demonstrated for the implementation of cost-effective WDM-PONs [1–3]. Moreover, BLS seeded optical sources, such as a wavelength-locked Fabry-Perot laser diode (FP-LD) and a reflective semiconductor optical amplifier (RSOA), have been successfully used to implement a commercially-available high-capacity WDM-PONs. Recently, a variety of extended-reach PON architectures based on time-division multiplexed (TDM) or WDM technology have been also demonstrated in order to reduce the number of equipment interfaces, network elements and central offices (CO) [4–7]. We believe that the BLS based extended-reach WDM-PON with a high-speed (>1.25 Gb/s) signal transmission is an attractive solution for the cost-effective high-capacity PON system. However, in BLS based WDM-PONs, the 3-dB bandwidth of optical sources should be wide enough to suppress the effect of relative intensity noise (RIN) for a high-speed signal transmission [1,8]. Therefore, due to the wide source bandwidth for a high-speed signal transmission, the chromatic dispersion of transmission fiber would be one of the main limiting factors in the BLS based extended-reach WDM-PONs [9]. Another noise source in the BLS based WDM-PONs might be an in-band crosstalk generated with a double Rayleigh backscattering within a transmission fiber. Especially, in WDM-PON based on BLS seeded sources, the end facet reflectance and the gain of FP-LD and/or RSOA could increase drastically the effect of in-band crosstalk on system’s performance when both downstream and upstream signals transmit through a single fiber [10].

In this paper, we investigate the system’s performance of BLS based extended-reach WDM-PONs. The maximum achievable reach of BLS based WDM-PON is calculated by taking into account the effects of RIN, chromatic dispersion of transmission fiber, and in-band crosstalk. In our calculation, we assume that the power budget problem can be solved by using optical amplifiers properly. From the results, we find that the maximum reach in BLS based WDM-PONs operating at 1.25 Gb/s could be increased to be ~70 km of conventional single-mode fiber (SMF) by reducing the chirp parameter of BLS based optical sources.

2. Noise sources in BLS based WDM-PON architectures

Figure 1 shows the typical WDM-PON architectures based on a spectrum-sliced optical source and a BLS seeded optical source. In order to obtain a number of WDM sources, an output from the BLS located at CO (central office) was spectrally sliced with an arrayed waveguide grating (AWG) in both architectures. In Fig. 1(a), a number of spectrum-sliced sources were modulated with their external modulators. The modulated downstream signals were multiplexed with another AWG, and then transmitted through a fiber. For the upstream signals transmission, each ONU (optical network unit) should have its own optical source. However, in this spectrum-sliced sources based architecture, the transmission length would be mainly limited by the optical channel power due to the high spectrum-slicing loss and the insertion loss of external modulator. To resolve this power budget problem, the BLS seeded sources, such as a wavelength-locked FP-LD and a RSOA, were proposed and demonstrated for a high-capacity WDM-PON. As shown in Fig. 1(b), each spectrum-sliced source generated with a BLS 2 was amplified and modulated by using a single device (Tx). The upstream signals were also generated with a BLS 1 located at CO and the Tx located at each ONU. In principle, the BLS seeded optical sources could increase the transmission length of WDM-PONs due to the gain of FP-LD and/or RSOA. However, the power budget problem still limited the maximum transmission length of a BLS based high-speed (beyond 1.25 Gb/s) WDM-PON even with a gain of FP-LD or RSOA [11]. We believe that various optical amplifiers or extender boxes could be used to increase the channel output power in the BLS based extended-reach WDM-PON, like other extended-reach TDM-PON architectures [4–6]. Therefore, in our analysis on the maximum reach in the BLS based WDM-PONs, we assumed that we could solve the power budget problem using an optical amplifier properly.

 

Fig. 1 Typical WDM-PON architectures based on (a) a spectrum-sliced optical source and (b) BLS seeded optical source

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In order to estimate the maximum reach in the BLS based WDM-PONs, we first took into account the effect of intrinsic RIN in the BLS based optical source. It has been reported that the RIN in a spectrum-sliced source was inversely proportional to the equivalent optical bandwidth B_o [1,8]. Thus, the spectrum-sliced source based system has a signal-to-noise ratio of SNR ~B_o/B_e = Q² (for the unpolarized spectrum-sliced source), where B_e is the equivalent electrical bandwidth and Q is related to bit-error rate [1,8]. From this equation of SNR, we could estimate that the equivalent optical bandwidth B_o of a spectrum-sliced source should be ~0.48 nm (i. e. 60 GHz) to guarantee the performance of BER = 10⁻¹⁴ (Q = 7.65) for a 1.25 Gb/s signal transmission with a receiver having a B_e of 1 GHz. The intrinsic RIN in a spectrum-sliced optical source could be suppressed by using the gain saturation characteristics of optical amplifiers [12]. The amount of RIN suppression was strongly dependent on the input power into SOA (deep saturation of gain with a high input power). The 3-dB SNR improvement was observed when the input power of SOA was less than −5 dBm [12]. Figure 2 shows the typical RIN spectra of a spectrum-sliced amplified spontaneous emission (ASE) source and a wavelength-locked FP-LD (with −7 dBm of input power into FP-LD) [9]. Both RIN and optical spectra were measured after passing through the AWG for WDM multiplexing to take into account the effect of optical filtering at transmitter side [13]. In these measurements, we used an AWG having a 3-dB bandwidth of 0.4 nm and a channel spacing of 100 GHz to spectrally slice the wide output spectrum of a BLS.

 

Fig. 2 Measured RIN spectra of a spectrum-sliced ASE 3 and a wavelength-locked FP-LD.

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As it can be seen in Fig. 2, the RIN components of a wavelength-locked FP-LD were suppressed especially at below 1 GHz. For a 1.25 Gb/s signal transmission with these BLS based sources and a B_e of 1 GHz, the total RIN of a wavelength-locked FP-LD was calculated to be a 3-dB less than the one of a spectrum-sliced ASE source. Due to this 3-dB SNR improvement, the B_o of a wavelength-locked FP-LD could be reduced by a factor of 2 to guarantee the same performance as the spectrum-sliced source based system. Thus, for example, the B_o of a wavelength-locked FP-LD could be decreased to be ~0.24 nm for a 1.25 Gb/s signal transmission with a BER = 10⁻¹⁴ and input power of −7 dBm into FP-LD. From these results, we confirmed that we could decrease the required B_o of BLS seeded optical sources due to the effect of RIN suppression in FP-LD and RSOA [12] for the high-speed signal transmission, as compared to the B_o of a spectrum-slice optical source.

Next, we calculated the power penalties induced by the chromatic dispersion of transmission fiber and the in-band crosstalk in the BLS based WDM-PONs using the following two equations [9,10];

 

Fig. 3 Dispersion- and in-band crosstalk-induced power penalties calculated as a function of equivalent optical bandwidth, crosstalk-to-signal ratio and transmission length of SMF.

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3. Results and discussion

In a spectrum-sliced source based WDM-PON as shown in Fig. 1(a), the signal could be modulated with a chirp-free external modulator. Therefore, we didn’t need to take into account the effect of additional chirp-induced pulse broadening when we calculated the dispersion-induced penalty with Eq. (1). In addition, the in-band crosstalk-induced penalty was negligible in the spectrum-sliced source based WDM-POM, since there was no gain medium within a transmission fiber. Therefore, we could estimate the system’s penalty with only including the effect of chromatic dispersion due to the wide B_o for the suppression of RIN. Figure 4 shows the contour plot of calculated system’s penalty in a spectrum-sliced source based WDM-PON as a function of B_o and the transmission length of SMF. In this calculation, the system’s penalty at BER = 10⁻⁹ was calculated by simply adding both the dispersion-induced penalty and the in-band crosstalk-induced penalty. We also assumed that the chirp parameter α of source was 0 and the crosstalk-to-signal ratio was −30 dB. From the results, we confirmed that the system’s penalty increased with increasing the B_o and the transmission length. Thus, the dispersion-induced penalty due to the wide B_o was a main limiting factor in a spectrum-sliced source based WDM-PON. For comparison, the measured dispersion-induced penalties (reported in [9]) were also represented by using the closed square symbols in Fig. 4. The calculated results agreed well with the previously measured results. Using Fig. 4 with a 1-dB penalty (@BER = 10⁻⁹) guideline, the maximum reach in a spectrum-sliced source based WDM-PON could be estimated to be ~45 km for a 1.25 Gb/s signal transmission, since the B_o of optical source should be ~0.48 nm for an error-free transmission.

 

Fig. 4 Contour plot of the system’s penalty calculated as a function of the equivalent optical bandwidth and the transmission length of SMF in a spectrum-sliced optical source based WDM-PON. We assumed that α = 0 and R = −30 dB in this calculation. For comparison, the measured results reported in [9] also represented by using square symbols.

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Next, the maximum reach in a WDM-PON based on BLS seeded optical sources was also evaluated by using a required B_o for RIN suppression and a contour plot of system’s penalty. In a BLS seeded source based WDM-PON, the in-band crosstalk component was increased with an end facet reflectance and a gain of FP-LD/RSOA. The level of in-band crosstalk could be estimated with the gain of optical amplifier and reflectance in a bidirectional signal transmission over a single fiber. In our calculation, we assumed that the gain of FP-LD and RSOA should be less than 20 dB, which in turn induced the crosstalk-to-signal ratio of <-10 dB with a 100% reflectance in the end facet of FP-LD/RSOA and a Rayleigh backscattering within a transmission fiber (@ reflectance of −30 dB). In addition, the additional chirp induced by the direct modulation of signal should be taken into account for the estimation of dispersion-induced penalty in a WDM-PON based on BLS seeded optical source [9,13].

Figure 5 shows the contour plot of system’s penalties calculated as a function of B_o and transmission length of SMF. In this calculation, the system’s penalties were calculated with a crosstalk-to-signal ratio of −10 dB and three different values of chirp parameter α. As it can be seen in Fig. 5, the system’s penalty increased drastically with a narrow B_o (<0.1nm) due to the in-band crosstalk in BLS seeded source based WDM-PONs. The system’s penalty also increased with a wide B_o and a long transmission length due to the chromatic dispersion of transmission fiber. Moreover, the dispersion-induced penalty also increased drastically with a large values of α. For a 1.25 Gb/s signal transmission with BLS seeded optical source (@ <-5 dBm launch into FP-LD or RSOA), the B_o should be > 0.24 nm for the RIN suppression, as mentioned before. Using this value of B_o as a typical example, we could estimate the maximum reach in BLS based WDM-PONs. For BLS seeded optical sources with α = 1 and 3, the maximum reach were estimated to be ~50 km and 20 km, respectively. However, with a chirp-free optical source, we could increase the maximum reach in BLS based WDM-PONs to be 70 km. From these results, we found that the effect of chromatic dispersion due to the chirp and wide B_o for RIN suppression was a main limiting factor in the WDM-PON based on BLS seeded optical source. The effect of in-band crosstalk with a high R (@ −10 dB) was significant only when the B_o was narrower than 0.1 nm. This is because the BLS based optical source is more tolerant to the in-band crosstalk than the DFB laser diode [10]. However, the narrow B_o (<0.1 nm) was not realistic due to the large RIN in BLS based WDM-PONs.

 

Fig. 5 Contour plot of the system’s penalty calculated as a function of the equivalent optical bandwidth and the transmission length of SMF in a BLS seeded optical source based WDM-PON. We assumed that R = −10 dB and used three different values of α.

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4. Summary

We estimated the maximum reach in BLS based WDM-PONs, taking into account the effects of RIN, chromatic dispersion and in-band crosstalk. We confirmed that the system’s performance was strongly dependent on the equivalent optical bandwidth B_o of BLS based optical source. Due to the wide B_o for RIN suppression, the maximum reach in a spectrum-sliced optical source based system was mainly limited by the chromatic dispersion. In WDM-PONs based on BLS seeded sources, such as a wavelength-locked FP-LD and a RSOA, the B_o could be reduced by the effect of RIN suppression in FP-LD and RSOA. However, due the chirp induced by the direct modulation of FP-LD or RSOA, the chromatic dispersion had a severer impact on the system’s performance than the in-band crosstalk. We found that the maximum reach in a BLS based WDM-PON operating at 1.25 Gb/s could be increased to be ~70 km of SMF with a chirp-free optical source. In our calculation, we did not take into account the filtering effect of receiver optical filter. We believe that we could reduce this receiver filter-induced discrepancy in the maximum reach estimation with a proper choice of optical filter and the design optimization of FP-LD or RSOA [13].