1. Introduction

A conventional duobinary transmitter has been typically implemented by using a Mach-Zehnder (MZ) modulator driven with three-level signals generated by using electrical lowpass filters (LPFs). However, the three-level signals can experience significant distortions due to the imperfect response of LPFs and modulator drivers operating in the saturation region [1, 2].

To overcome these problems, duobinary transmitters based on electrical binary signals have been proposed and demonstrated [1, 2]. Kim, et al. have recently demonstrated an optical duobinary transmitter using a phase modulator and a narrow optical filter [1]. In this scheme, phase-modulated optical binary signals are converted into the duobinary signals by passing through a narrow optical filter. The major drawback of this scheme, however, includes the reduced dispersion tolerance compared with the conventional duobinary transmitter based on LPFs as well as high frequency-stability requirements of the optical filter.

Another way to implement a duobinary transmitter using electrical binary signals is to use a dual-arm MZ modulator driven with two complementary signals relatively delayed by one bit [hereinafter referred to as one-bit delay (OBD) duobinary] [2]. Thanks to low intersymbol interference of the transmitter, the experimental demonstration shows comparable receiver sensitivity to conventional on-off-keying systems at back-to-back (BTB) transmission. However, since the OBD duobinary signals have frequency chirps at the rising and falling edges of the signals, they don’t have as large dispersion tolerance as conventional duobinary signals based on electrical LPFs.

In this paper, we present a way to implement an electrical-binary-signal-based duobinary transmitter without a significant sacrifice of the dispersion tolerance. The aim of this work is to improve the dispersion tolerance of an electrical-binary-signal-based duobinary transmitter rather than to reduce the number of modulators for return-to-zero generation as previously reported [3, 4]. Therefore, the proposed method is different from the previous works in either bias condition [3] or driving configuration [4] of a dual-arm MZ modulator.

We improve the dispersion tolerance of the OBD duobinary transmitter by optimizing 1) relative time delay (Δτ) between two complementary signals and 2) driving voltage (V_d) to the modulator. Through this optimization procedure, we successfully transmit the duobinary signals over 200 km of standard single-mode fiber (SSMF) without dispersion compensation.

2. Experiment

 

Fig. 1. Schematic diagram of the duobinary transmitter and in/out signals of 0.5-bit delay duobinary (MZM: MZ modulator) (a) schematic diagram of the duobinary transmitter, (b) 0.5-bit delay electrical signal and the inverted electrical signal, (c) 0.5-bit delay duobinary signal.

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Figure 1(a) shows a schematic diagram of the proposed duobinary transmitter. A dual-arm MZ modulator is driven with two complementary binary signals, in which one of the signals is delayed by 0~1 bit duration with respect to the other signal [3]. The bias of the modulator is located at its transmission null. As an example, two complementary binary signals driving the MZ modulator and the output of the modulator are illustrated in Fig. 1(b) and (c), respectively.

In our experiment, we utilized 10-Gbit/s non-return-to-zero signals with a pseudorandom bit sequence (PRBS) length of 2³¹-1. The transmission link was composed of two spans of 100-km SSMF with an in-line amplifier between them. The fiber launch power was set to be less than 3 dBm to avoid the effects of fiber nonlinearities. The dispersion of SSMF was ~16.8 ps/nm/km at the operating wavelength of 1550.8 nm. For a receiver, we employed an optically preamplified receiver with an optical bandwidth of 0.3 nm. System performance was assessed by using receiver sensitivities measured at a bit-error ratio of 10^-9. It should be noted that we didn’t use a precoder in our experiment since a precoded PRBS is a time-delayed replica of the original PRBS.

3. Principle

 

Fig. 2. Polar diagrams and optical eye diagrams at back-to-back transmission (a) and (b) 1-bit delay and 100% driving voltage, (c) and (d) 0.5-bit delay and 100% driving voltage, (e) and (f) 0.5-bit delay and 25% driving voltage.

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Figure 2 shows the polar diagrams and optical eye diagrams for duobinary signals with various time delays and driving voltages. Figures 2(a) and 2(b) are for the OBD duobinary signals (i.e., Δτ=1 bit and V_d=100% of its full swing). In this polar diagram, the locus is composed of two circles, indicating bit-alternating chirps as well as large frequency chirps at the rising and falling edges. These chirp characteristics adversely affect the dispersion tolerance of the OBD duobinary signals [5] (see Fig. 5 for measured dispersion tolerance of this signal). However, thanks to wide eye margin as shown in Fig. 2(b), this signal has a good receiver sensitivity of -35.2 dBm at BTB transmission.

Figure 2(c) illustrates the polar diagram of the duobinary signals with half-bit time delay (i.e., Δτ=0.5 bit and V_d=100%). The polar diagram has four circles: two outer circles for transitions between marks and spaces, and the other two inner circles for space-to-space transitions. The inner circles of the polar diagram imply ripples between spaces as also clearly seen in the eye diagram of Fig. 2(d). It has been recently shown that the ripples between spaces help improve the dispersion tolerance of duobinary signals [6, 7]. It is thought that bit-alternating phase of the ripples compensates for adverse intersymbol interference generated by dispersion-induced pulse broadening. However, the duobinary signal with half-bit time delay still has a large amount of frequency chirps at the rising and falling edges. We substantially reduce the frequency chirps by decreasing the driving voltage to the modulator.

Figure 2(e) and (f) show the polar and eye diagrams of the duobinary signals with half-bit time delay and 25% driving voltage (Δτ=0.5 bit and V_d=25%) [8]. Compared to Fig. 2(c), the locus of Fig. 2(e) shrinks in vertical direction, indicating reduced frequency chirps at the rising and falling edges of the signals.

4. Results

We optimize through experiment the time delay and the driving voltage of the duobinary transmitter to improve the dispersion tolerance. First of all, we optimize the time delay (Δτ) at the transmission distance of 0, 100, and 150 km. In this case, the driving voltage was set to be 100% of its full swing (i.e., twice the switching voltage (V_π) of the modulator). Figure 3 shows the measured receiver sensitivities versus the relative time delay between two complementary electrical binary signals. At BTB transmission, the receiver sensitivities are improved with increasing the time delay. After larger than 100-km transmission, however, the best receiver sensitivity is achieved at half-bit time delay.

 

Fig. 3. Measured receiver sensitivities as a function of the relative time delay of 100% driving voltage.

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We then optimize the driving voltage to the modulator. From the results of previous optimization, we set the time delay of the duobinary transmitter to be half bit (i.e., Δτ=0.5 bit). Figure 4 shows the measured receiver sensitivities as a function of the driving voltage. The receiver sensitivities at BTB transmission are nearly unaffected by the driving voltage. However, the receiver sensitivities are slightly improved at both 100-km and 150-km transmission as the driving voltage decreases. For example, by reducing the driving voltage to 25%, we achieve 1.4-dB and 2.0-dB receiver sensitivity gain after 100-km and 150-km transmission, respectively.

 

Fig. 4. Measured receiver sensitivities as a function of the driving voltage of 0.5-bit delay electrical signal (left), transmitter insertion loss as a function of the driving voltage of 0.5-bit delay electrical signal (right).

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Nonetheless, reducing the driving voltage induces a large insertion loss for the duobinary transmitter. As shown in Fig. 4, the driving voltage of less than 25% induces an insertion loss of larger than 9 dB. It can degrade the optical signal-to-noise ratio of the signals when an optical boost amplifier is employed at the output of the transmitter. Therefore, we set the optimum driving voltage to be 25% to avoid a significant transmitter insertion loss.

Figure 5 shows the measured receiver sensitivities as a function of transmission distance. Due to the large frequency chirps of the signals, it is not possible for the OBD duobinary signals to be transmitted over larger than 50 km of SSMF. However, the dispersion tolerance of this signal can be greatly improved by reducing the time delay by half bit. The measured results show that we can transmit the duobinary signal with half-bit time delay over 175 km of SSMF without dispersion compensation. Reducing the driving voltage to 25% further improves the dispersion-limited transmission distance. We achieve a good receiver sensitivity of -26.0 dBm after 200-km transmission using a duobinary transmitter with half-bit time delay and 25% driving voltage.

 

Fig. 5. Measured receiver sensitivities as a function of a transmission distance.

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Compared to the duobinary transmitter generated by electrical LPFs, the proposed one is expected to have poor performance in terms of dispersion tolerance [9]. This is because although we significantly reduce the chirp of the OBD duobinary signal by reducing the driving voltage the signal still has some chirp at both rising and falling edges. However, since our scheme is based on binary signals and doesn’t require electrical LPFs for the generation of ternary signals, it has numerous benefits including (1) less pattern-length dependence, (2) possible monolithic integration of electrical components [10], and (3) less performance sensitivity to driver and modulator response.

5. Summary

We have proposed and demonstrated a way to greatly improve the dispersion tolerance of an electrical-binary-signal-based duobinary transmitter. The transmitter is composed of a dual-arm MZ modulator driven with two complementary binary signals without LPFs. For the maximum dispersion tolerance of the transmitter, we generate ripples between spaces by decreasing the relative time delay to half bit, and reduce frequency chirps of the signals by decreasing the driving voltage to 25% of a full swing. With the help of this optimization, we successfully transmit the duobinary signals over 200 km of SSMF without dispersion compensation.