# Imbalanced Mach-Zehnder Modulator for Fading Suppression in Dispersion-Uncompensated Direct Detection System

^{*}

## Abstract

**:**

## 1. Introduction

^{−3}and 1.5 × 10

^{−2}after fiber transmission, respectively [32]. In addition, the received optical power (ROP) sensitivities at back-to-back (BTB) and after transmission are measured and compared with respect to bias, amplitude mismatch, and time skew, respectively.

## 2. Principle

#### 2.1. Power Fading with Differential Driven Mode

#### 2.2. Bias Deviation with Single-Arm Driven Mode

#### 2.3. Amplitude Mismatch in Differential Driven Mode

#### 2.4. Time Skew in Differential Driven Mode

#### 2.5. Perspective of Chirp

_{1}can be calculated by substituting Equation (5) into Equation (16).

_{2}of amplitude mismatch in differential driven mode is shown in Equation (18).

## 3. Numerical Visualization

#### 3.1. Bias Deviation with Single-Arm Driven Mode

#### 3.2. Amplitude Mismatch with Differential Driven Mode

#### 3.3. Time Skew with Differential Driven Mode

## 4. Experimental Setup and DSP Stack

#### 4.1. Experimental Setup

#### 4.2. DSP Stack

^{5}bit samples.

## 5. Experimental Results and Discussion

#### 5.1. Transmission Performance with Single-Arm Driven Mode

^{−2}and 6.08 × 10

^{−2}with FFE only, at the optimal bias voltage of 5.7 V. Further improvement can be obtained by applying VNE for SSBI mitigation. In this case, the BERs of PAM-6 and PAM-8 were 1.41 × 10

^{−3}and 1.37 × 10

^{−2}, which were lower than the 7% and 20% HD-FEC thresholds, respectively. The optimal bias voltage with VNE was smaller than with FFE, because a higher carrier component was needed to suppress the influence of SSBI for FFE.

#### 5.2. Amplitude Mismatch with Differential Driven Mode

^{−3}and 1.37 × 10

^{−2}to 5.87 × 10

^{−4}and 8.84 × 10

^{−3}compared with single-arm driven mode. Therefore, the amplitude ratio was kept as −15% in Figure 9c–f.

#### 5.3. Time Skew with Differential Driven Mode

^{−4}and 5.39 × 10

^{−3}with 26 ps skew for PAM-6 and PAM-8 signals, respectively. Figure 11c–f measures the ROP sensitivity for PAM-6/8 signals after 20 km SSMF transmission and at BTB, respectively. The skew was controlled as 20 ps considering both BTB and transmission performance, suitable for practical implementation with a fixed delay line. After 20 km SSMF transmission, the ROP sensitivities were promoted to −5.1 dBm and −3.9 dBm at the 7% and 20% HD-FEC thresholds, which was the best result among three imbalances. For the BTB case, the penalties were 1.3 dB and 1.2 dB after introducing time skew for PAM-6 and PAM-8 signals. It can be explained as the low pass filtering effect.

#### 5.4. Eye-Diagrams

#### 5.5. Impact of Transmission Distance

#### 5.6. Comparison of Preivous Work

## 6. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 1.**Transmitter configuration of (

**a**) standard differential driven mode, (

**b**) single-arm driven mode, (

**c**) differential driven mode with amplitude mismatch, and (

**d**) differential driven mode with time skew.

**Figure 2.**Calculated transfer function of (

**a**) upper-arm single driven mode with bias around π/4, (

**b**) upper-arm single driven mode with bias around 3π/4, (

**c**) lower-arm single driven mode with bias around π/4, and (

**d**) lower-arm single driven mode with bias around 3π/4. Diff.: standard differential mode.

**Figure 3.**(

**a**) Calculated transfer function of differential driven mode with various amplitude ratios k. (

**b**) Calculated transfer function versus different bias phases and amplitude ratios.

**Figure 4.**Calculated transfer function of differential driven mode with (

**a**) positive and (

**b**) negative skew values τ after 20 km SSMF transmission and with (

**c**) positive and (

**d**) negative skew values τ at BTB.

**Figure 5.**(

**a**) Experimental setup. ECL: external cavity laser; AWG: arbitrary waveform generator; EA: electrical amplifier; SSMF: standard single-mode fiber; EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; PD: photodiode; DSO: digital storage oscilloscope; Tx: transmitter; Rx: receiver. (

**b**) transmitter- and (

**c**) receiver-side DSP stacks. RRC: root raised cosine; FFE: feed-forward equalizer; VNE: Volterra nonlinear equalizer.

**Figure 6.**Received signal spectra versus bias voltage with (

**a**) differential driven mode and (

**b**) single-arm driven mode after 20 km SSMF transmission.

**Figure 7.**Measured BER versus bias voltage for (

**a**) PAM-6 and (

**b**) PAM-8 signals after 20 km SSMF transmission with differential and single-arm driven mode. Measured ROP sensitivities of (

**c**) PAM-6 and (

**d**) PAM-8 signals after 20 km SSMF transmission and (

**e**) PAM-6 and (

**f**) PAM-8 signals at BTB, respectively.

**Figure 8.**(

**a**) Received signal spectra versus amplitude ratio with 5.1 V bias voltage after 20 km SSMF transmission. (

**b**) Received signal spectra versus bias voltage with amplitude ratio of −15% after 20 km SSMF transmission.

**Figure 9.**Measured BER versus amplitude ratio and bias voltage for (

**a**) PAM-6 and (

**b**) PAM-8 signals after 20 km SSMF transmission. Measured ROP sensitivities with −15% amplitude ratio for (

**c**) PAM-6 and (

**d**) PAM-8 signals after 20 km SSMF transmission and for (

**e**) PAM-6 and (

**f**) PAM-8 signals at BTB, respectively.

**Figure 10.**Received signal spectra versus time skew (

**a**) at BTB and (

**b**) after 20 km SSMF transmission.

**Figure 11.**Measured BER versus time skew for (

**a**) PAM-6 and (

**b**) PAM-8 signals after 20 km SSMF transmission with differential driven mode. Measured ROP sensitivities with 20 ps for (

**c**) PAM-6 and (

**d**) PAM-8 signals after 20 km SSMF transmission and for (

**e**) PAM-6 and (

**f**) PAM-8 signals at BTB, respectively.

**Figure 12.**Typical eye-diagrams and amplitude histograms of PAM-6/8 signals with FFE/VNE at BTB and after 20 km SSMF transmission operating at different modes, respectively. w/diff., conventional IM with differential driven mode; w/bias, with bias deviation in single-arm driven mode; w/amp., with amplitude mismatch in differential driven mode; w/skew, with time skew in differential driven mode.

**Figure 13.**Calculated transfer function operating at different modes after (

**a**) 20 km, (

**b**) 40 km, (

**c**) 60 km, and (

**d**) 80 km SSMF transmission, respectively. IM: conventional IM with differential driven mode; Bias (40°), with 40° bias phase in upper-arm single driven mode; Amp (−20%), with −20% amplitude mismatch in differential driven mode; Skew (20 ps) with 20 ps time skew in differential driven mode.

Year | Transmitter | Wavelength | Bitrate (Gb/s) | Format | Distance (km) | Additional Device | Algorithm |
---|---|---|---|---|---|---|---|

2016 [16] | DDMZM + DAC × 2 | C-band | 105 | DMT | 80 | - | VNE + TCM |

2016 [25] | DDMZM + DAC × 2 | C-band | 128 | PAM-4 | 80 | - | Pre-CDC |

2017 [15] | IQM+ DAC × 2 | C-band | 4 λ × 112 | 16-QAM | 320 | - | Hilbert + KK |

2018 [21] | MZM + DAC × 1 | C-band | 56 | PAM-4 | 80 | - | THP |

2018 [26] | IM + PM + DAC × 2 | C-band | 108 | OFDM 64-QAM | 20 | - | Subcarrier allocation |

2019 [11] | MZM + DAC × 1 | C-band | 8 λ × 50 | PAM-4 | 100 | DCF | FFE |

2019 [13] | DML + DAC × 1 | C-band | 100 | PAM-4 | 45 | OBPF | VNE |

2019 [27] | IQM + DAC × 2 | C-band | 129.6 | OFDM 64-QAM | 20 | - | Subcarrier allocation |

2019 [28] | PolM + DAC × 1 | C-band | 100 | PAM-4 | 5 | - | VNE |

2020 [10] | DML + DAC × 1 | O-band | 100 | PAM-4 | 50 | - | VNE |

2020 [22] | MZM + DAC × 1 | C-band | 64 | OOK | 100 | - | VNE + DFE + MLSD |

2020 [23] | MZM + DAC × 1 | C-band | 112 | PAM-4 | 20 | - | Pre-Equ. + FFE-DFE |

2020 [29] | MZM + DAC × 1 | C-band | 120 | PAM-8 | 10 | - | VNE |

2020 [30] | MZM + DAC × 1 | C-band | 96 | PAM-8 | 80 | - | VNE |

This work | MZM + DAC × 1 | C-band | 104 | PAM-8 | 20 | - | VNE |

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**MDPI and ACS Style**

Zhu, Y.; Miao, X.; Wu, Q.; Yin, L.; Hu, W.
Imbalanced Mach-Zehnder Modulator for Fading Suppression in Dispersion-Uncompensated Direct Detection System. *Electronics* **2021**, *10*, 2866.
https://doi.org/10.3390/electronics10222866

**AMA Style**

Zhu Y, Miao X, Wu Q, Yin L, Hu W.
Imbalanced Mach-Zehnder Modulator for Fading Suppression in Dispersion-Uncompensated Direct Detection System. *Electronics*. 2021; 10(22):2866.
https://doi.org/10.3390/electronics10222866

**Chicago/Turabian Style**

Zhu, Yixiao, Xin Miao, Qi Wu, Longjie Yin, and Weisheng Hu.
2021. "Imbalanced Mach-Zehnder Modulator for Fading Suppression in Dispersion-Uncompensated Direct Detection System" *Electronics* 10, no. 22: 2866.
https://doi.org/10.3390/electronics10222866