1. Introduction

In fiber-to-the-customer premises networks, cost effective solutions for both the optical network units (ONUs) at customer premises and the optical access feeder are of crucial importance. The use of carrier re-modulation at the customer terminal rather than a local light source has been suggested as a way of reducing installation and maintenance costs by avoiding stabilization and provisioning issues associated with a local laser [1–4]. Although different technological approaches have been explored for remote carrier re-modulators, those based on a single reflective semiconductor optical amplifier (RSOA) have proved to be the most promising solution, achieving re-modulation at bit-rates up to 1.25 Gb/s of intensity modulated (IM) downstream signals with bit-rates up to 2.5 Gb/s [4]. RSOA devices are commercially available. Future access networks will need to accommodate the constantly growing demand for capacity, and data speeds in the order of 10 Gb/s are foreseen, e.g. due to the introduction of Gigabit Ethernet solutions. Therefore, solutions for ONUs supporting high bit-rates will be needed while still satisfying the stringent cost-effective requirements.

In this letter we propose and experimentally demonstrate a carrier re-modulator based on a reflective monolithically integrated semiconductor optical amplifier and electro-absorber (R-SOA-EA). Although operation at 5 Gb/s is demonstrated for IM signals, the proposed R-SOA-EA approach has the potential to operate at higher bit-rate by proper device optimization.

2. R-SOA-EA design and fabrication

A photograph and schematic layout of the fabricated component are shown in Fig. 1. The device consists of a waveguide separated in two sections defined by the top electrodes shown in the figure. The SOA and EA sections are 300 μm and 45 μm long, respectively. The waveguide is bent in order to have one facet with high and one with low reflection. The two facets are also coated with high and anti-reflection coatings, respectively. The active material consists of eight, compressively strained, 7.0 nm thick InGaAsP quantum wells in a strain compensated structure and is the same for both sections.

 

Fig. 1. Photograph and schematic layout of the reflective semiconductor electro-absorption (R-SOA-EA) re-modulator.

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3. Experiment and results

The experimental setup is shown in Fig. 2. The downstream signal, at 1550 nm wavelength, is intensity modulated and the extinction ratio is set by adjusting the driving bias voltage to the LiNbO₃ Mach-Zehnder interferometer (MZI) intensity modulator. An EDFA followed by an optical bandpass filter (OBPF), to reject part of the amplified spontaneous emission (ASE) noise, is used to amplify the downstream signal before sending it to the ONU through an optical circulator. The variable optical attenuator (VOA₁) and polarization controller PC2 regulate the incoming power level and polarization state of the signal entering the R-SOA-EA device. Further efforts are pursued to reduce the performance dependence of the state of polarization of the incoming signal. Coupling to the R-SOA-EA chip is realized by using a tapered fiber. The coupling loss is estimated to be 3 dB. The average input power to the pigtail that connects to the R-SOA-EA was set to -2.0 dBm and kept fixed through the experiments. The average output power from the R-SOA-EA device was measured to be 0 dBm for a SOA bias current of 100 mA, and the bias of the EA section set to zero Volt. Re-modulation is achieved by direct modulation of the bias voltage applied to the EA section of the R-SOA-EA device. The electrical signals for the downstream and upstream links are pseudorandom binary sequences (PRBS) derived from the data and inverted data outputs of a pulse pattern generator (PPG). The 2⁷-1 PRBS was chosen as the test pattern as it gives the closest match to the maximum run-length of a Gigabit Ethernet 8B/10B line code [4].

 

Fig. 2. Experimental setup for the bit-error rate (BER) measurements. PPG: pulse pattern generator, MZI: Mach-Zehnder Interferometer intensity modulator, BERT: bit-error rate test-set, Rx: receiver, OBPF: optical band-pass filter, PC: polarization controller, VOA: optical variable attenuator, IM: intensity modulation.

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Firstly, we assessed the intensity modulation capability of the EA section when the downstream signal was continuous wave (CW). The eye diagrams of the IM signal modulated by the EA section are shown in Fig. 3(a) and Fig. 3(b) for a bit rate of 4 Gb/s and 5 Gbs/, respectively. Wide-open eye diagrams are obtained with a measured extinction ratio (ratio of the average optical power in the logical one to logical zero level) of 10 dB and 7 dB, respectively. For bit-rates above 5 Gb/s, the rise and fall times of the impulse response of the device increase significantly due to a large capacitance of the current EA design. The bias current to the SOA section is fixed to 100 mA. In Fig. 3(d) we show the eye diagram for the case of signal re-modulation at 5 Gb/s. The bias voltage of the EA section is set to 0.45 V and the peak-to-peak modulation voltage is set to 0.9 V. For comparison purpose, Fig. 3(c) shows the original IM downstream signal with an extinction ratio of 6 dB. As observed in Fig. 3(d), the re-modulated signal shows amplitude overshoots due to the carrier dynamics in the SOA section. However, the zero level is reduced by the noise suppression feature of the EA section [6], yielding an improved extinction ratio compared to the incoming 6 dB extinction ratio of the downstream signal.

Secondly, the performance of carrier re-modulation is assessed by bit-error rate (BER) measurements. Figure 4 shows the results of BER measurements as a function of the average input optical power to the receiver. As it can be observed in Fig. 4, the receiver sensitivity at a BER of 10^-9 is -34.8 dBm and -29.5 dBm for CW modulation and re-modulation at a bit rate of 4 Gb/s, respectively. For a bit rate of 5 Gb/s, the corresponding receiver sensitivity is -28.4 dBm and -24.7 dBm, respectively. The observed power penalty for the receiver sensitivity for the case of re-modulation is attributed to the residual downstream signal (after signal erasure), overshoot and patterning effect in the SOA section. Moreover, the value of the IM extinction ratio of the downstream signal is a parameter for system design as it influences the efficiency of the SOA to erase the downstream signal and therefore it has impact on the performance of the upstream signal.

 

Fig. 3. Eye diagrams. (a) Modulated CW signal at 4 Gb/s and (b) at 5 Gb/s bit rate by using direct voltage modulation of the EA section of the R-SOA-EA device. (c) Eye diagram of downstream signal at 5 Gb/s with an extinction ratio of 6 dB, and (d) corresponding re-modulated signal at 5 Gb/s. Horizontal scale 100 ps/div. for all eye-diagrams.

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Fig. 4. BER measurements as a function of the average input optical power to the receiver. ● Modulation of CW signal at 4 Gb/s. ○ IM re-modulation at 4 Gb/s. ▼ Modulation of CW signal at 5 Gb/s. ▽ IM re-modulation at 5 Gb/s

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4. Conclusion

The above results show the capability of IM erasure and re-modulation of a R-SOA-EA reflective modulator up to 5 Gb/s. The operation speed of an R-SOA-EA can be improved by optimized design of the chip parameters, and additionally by proper circuit design to avoid parasitic capacitances. Single electro-absorption modulators with similar structures have thus been operated at bit rates up to 40 Gb/s [7]. Moreover, due to the prospect of high-speed modulation, a reflective R-SOA-EA re-modulator is an attractive solution for compact and potentially low cost devices for broadband access nodes.