# Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs

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## Abstract

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## 1. Introduction

## 2. Chirp Measurements

## 3. Simulations

- a directly-modulated VCSEL with chirp parameters as the short-cavity device
- a directly-modulated VCSEL with chirp parameters as the tunable device
- a CW 500-kHz laser followed by an electro-absorption modulator (EAM) with $\alpha =0.4$

^{−3}(corresponding to 7% overhead hard decision FEC limit) for single-channel, single-polarization transmission. Figure 4a–f shows an example of the results obtained for DSB DMT modulation of the tunable VCSEL (α = 2.75 κ = 8.3 × 10

^{12}) in case of one WSS filtering and 40-dB OSNR. Figure 4a–f reports the received electrical spectrum, the bit loading, the BER per subcarrier and examples of received constellations.

## 4. Discussion and Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 2.**Channel transfer functions obtained after 10-km (blue), 24-km (red) and 34-km (yellow) SSMF propagation for: (

**a**) 8-GHz tunable VCSEL and (

**b**) 20-GHz short-cavity VCSEL.

**Figure 3.**DMT SSB optical spectrum (red), obtained by a 9.5-GHz detuned WSS (orange) filtering of a DMT DSB signal with 20-GHz electrical bandwidth and 40-GHz optical bandwidth (blue).

**Figure 4.**(

**a**–

**f**) Tunable VCSEL: DSB DMT (10 GHz) for single WSS filtering and 40-dB OSNR. (

**a**) Received electrical spectrum; (

**b**) Bit loading and corresponding EVM; (

**c**) BER per subcarrier; (

**d**) Bit-loading SNR; (

**e**,

**f**) Examples of received constellations.

**Figure 5.**(

**a**–

**f**) Tunable VCSEL: SSB DMT for single WSS filtering (9.5-GHz detuned) and 40-dB OSNR. (

**a**) Received electrical spectrum; (

**b**) Bit loading and corresponding EVM; (

**c**) BER per subcarrier; (

**d**) Bit-loading SNR; (

**e**,

**f**) Examples of received constellations.

**Figure 6.**Transmission capacities vs number of crossed WSS for: 40-dB OSNR (blue square), 35-dB OSNR (red circle), 30-dB OSNR (green triangle), and short-cavity VCSEL (continuous line open symbols), tunable VCSEL (dashed line full symbol) (

**a**) DSB DMT modulation; (

**b**) SSB DMT modulation.

**Figure 7.**Transmission capacities vs number of crossed WSS for EML: 40-dB OSNR (blue square), 35-dB OSNR (red circle), 30-dB OSNR (green triangle). (

**a**) DSB DMT modulation. (

**b**) SSB DMT modulation.

Optical Source | α | κ |
---|---|---|

Tunable VCSEL | 2.75 | 8.3 × 10^{12} |

Short-cavity VCSEL | 3.7 | 1.52 × 10^{13} |

**Table 2.**Transmitted capacities and SSMF reach for SSB DMT as a function of: span length, VCSEL type/chirp, and number of crossed WSS.

OSNR | 40 dB | 35 dB | 30 dB | |
---|---|---|---|---|

Reach | 35-km span | 70 km | 210 km | 735 km |

65-km span | --- | 65 km | 260 km | |

Short-cavity VCSEL | Capacity w 1 WSS | 78.6 Gb/s | 72.6 Gb/s | 57.3 Gb/s |

Capacity w 3 WSS | 68.6 Gb/s | 63.1 Gb/s | 54.5 Gb/s | |

Capacity w 5 WSS | 61 Gb/s | 58.4 Gb/s | 50.5 Gb/s | |

Tunable VCSEL | Capacity w 1 WSS | 76.2 Gb/s | 70 Gb/s | 56.2 Gb/s |

Capacity w 3 WSS | 73.3 Gb/s | 67 Gb/s | 55 Gb/s | |

Capacity w 5 WSS | 70.4 Gb/s | 63.8 Gb/s | 54 Gb/s |

**Table 3.**Transmitted capacities and SSMF reach for DSB DMT as a function of: span length, VCSEL type/chirp, and number of crossed WSS.

OSNR | 40 dB | 35 dB | 30 dB | |
---|---|---|---|---|

Reach | 35-km span | 70 km | 210 km | 735 km |

65-km span | --- | 65 km | 260 km | |

Short-cavity VCSEL | Capacity w 1 WSS | 45.5 Gb/s | 30.9 Gb/s | 22.8 Gb/s |

Capacity w 3 WSS | 35.3 Gb/s | 27.5 Gb/s | 19.9 Gb/s | |

Capacity w 5 WSS | 31.1 Gb/s | 25.2 Gb/s | 19.2 Gb/s | |

Tunable VCSEL | Capacity w 1 WSS | 61.2 Gb/s | 51 Gb/s | 34 Gb/s |

Capacity w 3 WSS | 52.6 Gb/s | 47.8 Gb/s | 32.4 Gb/s | |

Capacity w 5 WSS | 48 Gb/s | 43.75 Gb/s | 30 Gb/s |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Rapisarda, M.; Gatto, A.; Martelli, P.; Parolari, P.; Neumeyr, C.; Svaluto Moreolo, M.; Fabrega, J.M.; Nadal, L.; Boffi, P.
Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs. *Photonics* **2018**, *5*, 51.
https://doi.org/10.3390/photonics5040051

**AMA Style**

Rapisarda M, Gatto A, Martelli P, Parolari P, Neumeyr C, Svaluto Moreolo M, Fabrega JM, Nadal L, Boffi P.
Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs. *Photonics*. 2018; 5(4):51.
https://doi.org/10.3390/photonics5040051

**Chicago/Turabian Style**

Rapisarda, Mariangela, Alberto Gatto, Paolo Martelli, Paola Parolari, Christian Neumeyr, Michela Svaluto Moreolo, Josep M. Fabrega, Laia Nadal, and Pierpaolo Boffi.
2018. "Impact of Chirp in High-Capacity Optical Metro Networks Employing Directly-Modulated VCSELs" *Photonics* 5, no. 4: 51.
https://doi.org/10.3390/photonics5040051