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Abstract
A high-power optical transmitter module based on an SOA-assisted extended-reach EADFB laser (AXEL) chip was developed at a wavelength of 1.49 µm to improve the loss budget of current passive optical access networks. The fiber-coupled output power of the modulated signal is 10 dBm owing to the post-amplification effect of the integrated SOA with an extinction ratio of 10 dB. Fiber transmission was demonstrated at data rates of 1.25 and 2.5 Gbit/s, assuming GE-PON and G-PON downstream signals, respectively. Error-free transmission over 100-km fiber was achieved with loss budgets of 46 and 43 dB at 1.25 and 2.5 Gbit/s, respectively.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Access networks based on passive optical networks (PONs) are widely used around the world. A PON is a cost-effective solution for point-to-multipoint data distribution because a large number of uses can share fiber cable [1,2]. The cost of access networks is a critical issue because the access span still accounts for a large fraction of network infrastructure cost. In recent years, data traffic has increased drastically because of the popularity of internet video and streaming services, pointing to the need for higher access network speeds. As the next generation of access networks, 10G-EPON, NG-PON2, and XGS-PON were standardized in 2009, 2015, and 2016, respectively. These systems are expected to be cost-effective because their wide communications bandwidths can accommodate multiple services [2]. During the migration from legacy systems, it is important that 1G-class systems coexist with the next generation of systems [3–5]. On the other hand, such high-end systems require high reliability, for which backup schemes based on redundancy are being considered [2,6]. For the backup system, using N:1 switches is a cost-effective approach because a minimum number of transceivers is required. This method requires high-performance transceivers, in terms of transmitter output power and receiver sensitivity, because of the additional link loss due to the optical switches, which exceeds 3.5 dB [2].
Another prospect for reducing access network cost is central office (CO) aggregation [2,7–9]. The key to this solution is to extend the reach of optical transceivers. Extending the accommodation area of each office can reduce the total number of COs and thus CAPEX and OPEX can be reduced. To increase the loss budget, several methods using optical amplifiers have been proposed [10–13]. However, it is preferable to increase the budget without using external amplifiers because the need for additional optical components increases costs. Improving the output power of the lasers used in the optical transceivers is a promising method for increasing loss budgets, but there are several power-limiting factors for commercially available lasers. For example, the power of the directly modulated distributed feedback lasers (DMLs) used in PONs is around 5 dBm [4,5]. Increasing the power of DMLs without degrading signal quality in terms of the extinction ratio requires a large current modulation amplitude, which is difficult to achieve with commercially available LD drivers. On the other hand, electro-absorption (EA) modulator integrated DFB (EADFB) lasers can be modulated with conventional 50-Ω drivers, and the modulation swing voltage for EA modulators is independent of optical power. EADFB lasers are generally used in high-speed systems because they offer a high extinction ratio and low chirping and thus suffer less from the dispersion penalty at high modulation speeds. However, their output power is low compared with that of DMLs because of the loss due to the absorption-based modulation. At gigabit-class data rates, for which the transmission distance is limited mostly by the lack of optical power, not by dispersion, DMLs have merits in cost and their capability of uncooled operation; therefore, they have been used in G-EPON [14] and G-PON [15] systems so far. However, a high extinction ratio compared with that of DMLs is still beneficial for high receiver sensitivity. To overcome the low output power of EADFB lasers, we proposed AXELs (SOA-assisted extended reach EA-DFB lasers) [16–19]. Owing to the power assist from the gain of the monolithically integrated post-SOA, we achieved very high output power from transmitter modules in the C- and L-bands. In [19], 9 dBm at the wavelength of 1570 nm was obtained, and error-free transmission of 10 Gbit/s signal over 80-km fiber was demonstrated, showing the high potential for budget extension in the next-generation of access networks.
In this paper, we report a high-power optical transmitter module based on an AXEL chip for improving the power budget of gigabit-class PON systems. The AXEL was designed for operation at 1490 nm, which is the wavelength of GE-PON and GPON downstream signal, and for extracting high output power. Section 2 describes the details of the AXEL device structure and the module’s output properties. Section 3 describes fiber transmission experiments at data rates of 1.25 and 2.5 Gbit/s. The results show excellent transmission properties, confirming the laser’s potential to extend the power budget of conventional PON systems.
2. Device structure and optical properties
2.1 AXEL structure
The AXEL chip structure is shown in Fig. 1(a). It mainly comprises three integrated sections, which are the DFB laser, EA modulator, and booster SOA sections. In the fabricated AXEL, the lengths of each section are 300, 200, and 400 µm, respectively. The long SOA was employed for the higher power amplification. The InGaAsP-based active multi-quantum wells (MQWs) were formed by epitaxial growth by metal organic chemical vapor deposition (MOCVD). The DFB and SOA sections were formed in the first epitaxial growth. Then, the EA absorber MQW was formed in a second epitaxial growth using the butt-joint regrowth technique. The bandgap wavelengths of the DFB and SOA MQWs were optimized for 1490-nm operation. The EA absorption wavelength was optimized to reduce insertion loss. Because the same MQWs can be used in the DFB and SOA sections, the fabrication method, entailing only two epitaxial growths, is the same as that of conventional EADFB lasers.
A buried heterostructure (BH) is used for better heat dissipation to suppress thermal roll-off and for carrier blocking to enhance effective carrier injection into the MQW regions. Because of the integrated SOA, residual facet reflection can be amplified and then fed back to the oscillator, causing signal distortion and increased noise. Therefore, we employed a slanted output waveguide structure to suppress output facet reflection, in addition to an anti-reflection (AR) coating. The AXEL chip placed on a sub-carrier was mounted in a box-type transmitter optical sub-assembly (TOSA) package [Fig. 1(b)].
The fiber-coupled output power as a function of the current applied to the DFB laser section is shown in Fig. 2(a). The laser chip temperature was controlled at 50°C. The experimental operation condition of this module is 100 mA for the DFB laser section and 200 mA for the SOA section. At the current condition, a CW output power of 33 mW (EA open) was obtained. In Fig. 2(b), the laser spectrum for different SOA currents is shown, and a wide-range spectrum for the aforementioned current condition is shown in the inset. The side-mode suppression ratio (SMSR) is more than 50 dB even for the SOA current as high as 200 mA.
Fig. 2. (a) I-L properties for LD current sweep at different SOA currents; (b) optical spectrum of the TOSA module for different SOA currents. The DFB current was fixed at 100 mA. The inset shows the laser spectrum with amplified spontaneous emission.
The EA attenuation as a function of DC bias voltage is shown in Figs. 3 and 4. Figure 3(a) shows the extinction as a function of EA negative bias voltage at different DFB laser currents from 30 to 100 mA in 10-mA steps. The SOA current was fixed at 200 mA. The extinction ratio calculated from Fig. 3(a) is shown in Fig. 3(b). The modulation swing voltage is assumed to be 1.5 V peak to peak. As shown in the figure, the bias voltage which gives the highest extinction ratio shifts to a higher one when the DFB current increases because of the hole pile-up effect [20,21]. With higher DEB current the extinction ratio is lowered. Therefore, there is a trade-off relation between the output power and extinction ratio. Figure 4(a) shows the EA extinction characteristics for different SOA currents. The DFB current was 100 mA. The bias voltage where the extinction ratio is the maximum is unchanged for different SOA currents [Fig. 4(b)]. The extinction ratio is better at higher SOA currents, which we consider to be due to the higher saturation power of the SOA. As shown in Fig. 4(b), for SOA current higher than 80 mA, the output power at deep bias (more than −1 V) is almost the same as that for the different SOA currents. Because the DFB laser current is fixed, the input power is considered to be the same, which means the SOA gain (small signal gain) is almost the same for different SOA currents in this region. On the other hand, the output power for shallow EA bias is different for different SOA currents because the saturation power of the SOA is higher for high injection current, i.e., high carrier density, for which the carrier lifetime is shorter.
Fig. 3. (a) Measured EA extinction curves for different LD current at SOA current of 200 mA. (b) Estimated extinction ratio from (a) assuming modulation voltage swing of 1.5 V applied to the EA modulator.
Fig. 4. (a) Measured EA extinction curves for different SOA current at LD current of 100 mA. (b) Estimated extinction ratio from (a) assuming modulation voltage swing of 1.5 V applied to the EA modulator.
The E/O response characteristics of the AXEL module at different SOA currents are shown in Fig. 5. The DFB current is 100 mA and the EA bias is −0.5 V. From the response characteristics, the −3 dB bandwidth is found to be 10 GHz, which is sufficient for 1.25 and 2.5 Gbit/s modulation. The dip seen at lower frequency is due to SOA saturation effect. If the modulation period is longer than the carrier lifetime, the gain varies with the intensity-modulated signal. This lowers the amplitude of the output signal from the SOA. In optical communications, this SOA gain saturation causes the pattern effect, which degrades the waveform quality. To avoid the pattern effect, we need to increase the saturation power by increasing injection current to the SOA. As seen in Fig. 5, the dip is shallower for 150 and 200 mA compared with 50 and 100 mA.
3. Fiber transmission experiment
To test the signal transmission properties of the TOSA module, we performed an optical fiber transmission experiment using standard single-mode fiber (SSMF). The experimental setup is shown in Fig. 6. The modulation signal was generated using a pulse pattern generator (PPG). We tested the transmitter properties at data rates of 1.25 Gbit/s and 2.5 Gbit/s (2.48832 Gbit/s), assuming G-EPON and GPON downstream signals, respectively. The test pattern was a pseudo random binary sequence (PRBS), and the lengths were 27−1 and 223−1, respectively. The signal format was non-return to zero (NRZ). The modulation swing was 1.5 V (peak to peak). The EA bias voltages were −0.5 V for 1.25 Gbit/s and −0.6 V for 2.5-Gbit/s signal. The DFB and SOA currents were 100 and 200 mA, respectively. The chip temperature was 50°C. The fiber coupled modulation output power was measured to be 10 dBm. The transmission experiment was performed for back-to-back (BtoB) and 20, 60, and 100-km fibers. A variable optical attenuator (VOA) was used to adjust the power input to the receiver. A different receiver was used for each data rate. The receiver for 1.25-Gbit/s signal was an in-house one based on an avalanche photodiode (APD) and a trans-impedance amplifier (TIA) with a commercial clock/data recovery (CDR) from Maxim Integrated. For the 2.5-Gbit/s experiment, a different commercial clock/data recovery (HP 83446A) receiver was used. The bit error rate (BER) was measured with an error detector (ED).
The eye patterns for the AXEL transmitter at each data rate are shown in Fig. 7. The patterns were filtered with standard defined reference filters. Each pattern shows clear eye opening. The dynamic extinction ratio measured at the eye center for both signals was 10 dB. The overshoot is slightly larger for the 2.5-Gbit/s signal. The standard compliant masks are shown in the upper BtoB patterns. The darker regions correspond to a 30% mask margin. There were no mask hits in the 30% mask margin region for 1,000 waveforms (1,350 points/waveform). In general, a waveform with a mask margin of more than 30% is required for a transmitter, which is satisfied despite of the overshoot.
For the 1.25-Gbit/s signal, the eye patterns after fiber transmission show no significant waveform distortion even after the signal had passed through the 100-km fiber. For the 2.5-Gbit/s signal, good waveform quality was confirmed for the transmission fiber length of 60 km. Because different optical modules were used for the measurements, the eye pattern of the 100-km transmission for 2.5-Gbit/s signal could not be measured due to the low signal to noise ratio of the equipment. The coupling fiber of the module for the 1.25-Gbit/s signal was a multimode fiber and that for the 2.5-Gbit/s signal was a single-mode fiber. The multimode fiber has better coupling efficiency, and thus a better signal-to-noise ratio was obtained.
Owing to the high optical power of 10 dBm and waveform quality with the dynamic extinction ratio of 10 dB of the transmitter, a successful error-free signal transmission up to a fiber length of 100 km was achieved. The transmission result for the 1.25-Gbit/s signal is shown in Fig. 8. In accordance with the G-EPON standard [14], we measured the receiver sensitivity at a BER of 10−12. The minimum receiver sensitivity was −36 dBm after 100-km fiber transmission. The sensitivity is slightly better for longer fiber lengths because of the chromatic dispersion in the SSMF. The transmission penalty was found to be less than 1 dB. The loss budget was 46 dB. In the GE-PON standard [14], the required highest budget is 34 dB (PX40). The transmitter power of +4 dBm and the receiver sensitivity of −30 dBm are required for the downstream link. For the demonstrated optical link, the budget extension of 12 dB is possible.
The transmission characteristics for the 2.5-Gbit/s signal are shown in Fig. 9. Similar to the result for the 1.25-Gbit/s signal, sensitivity is better for longer fiber length. According to the GPON standard [15], receiver sensitivity is defined at BERs of 10−10 and 10−4. For GPON class C+ and D, the receiver sensitivity is defined at a BER = 10−4 because pre-FEC can be used. For the transmission distance of 100 km, the minimum receiver sensitivities were −33 dBm (at the BER = 10−10) and −36.6 dBm (at the BER = 10−4); therefore, the loss budgets were 43 and 46.6 dB, respectively. The transmission penalty was also less than 1 dB, similar to the result for the 1.25-Gbit/s signal. According to the standard, the required loss budget is 36 dB for class D. The budget extension demonstrated here exceeds this requirement by more than 10 dB. As shown in these results, the transmission penalty is small for these data rates, and the excess fiber loss due to extended fiber length is the main factor limiting the transmission reach. Therefore, the high output power of the transmitter is very important in terms of extending the reach of PONs. Our experimental results show that the loss budget can be extended to more than 10 dB without using external optical amplifiers.
4. Conclusion
An optical transmitter using an AXEL was investigated at the wavelength of 1490 nm to enhance the output power of transmitter and thereby increase the power budget for optical access networks. In the AXEL a 400-um-long SOA was integrated with an EADFB laser. The output power as high as 10 dBm was obtained with a dynamic extinction ratio of 10 dB. In a fiber transmission experiment, the transmitter module showed excellent transmitter properties. Owing to the good waveform quality, successful error-free transmission over 100-km SSMF was achieved at data rates of 1.25 and 2.5 Gbit/s. The loss budgets were 46 and 43 dB, respectively, which is a 10-dB improvement over the current standard specification.
Acknowledgments
Portions of this work were presented at the 24th Microoptics Conference in 2019, “High-power-output (11 dBm) SOA assisted extended reach EADFB laser (AXEL) for 1.25-Gbit/s high-loss budget optical access networks.”
Disclosures
The authors declare no conflicts of interest.
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