# Microsphere-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System

^{1}

^{2}

^{3}

^{4}

^{*}

## Abstract

**:**

^{−30}, providing a total of 40 Gbit/s of transmission speed on four channels.

## 1. Introduction

^{10}[25] and routinely reached values are 10

^{7}–10

^{8}[26,27,28]. Silica microspheres can be produced with controllable characteristics, one of which is the free spectral range (FSR). FSR is an important characteristic because in OFC generators for WDM applications, FSR (the distance between optical carrier frequencies) should satisfy ITU-T G. 694.1 recommendation [29].

## 2. Methods

#### 2.1. Calculation of Microresonator Characteristics

_{eff}), where c is the speed of light, R is the radius of a microresonator, and n

_{eff}is the effective refractive index for an operating family of WGMs, so, it is essential to choose the appropriate size and take into account that not only the material but also the waveguide component gives a contribution to n

_{eff}. To find an optimal size of a silica microsphere for obtaining FSR = 200 GHz, we solve the characteristic equation for different radii [34]:

_{l}

_{+1/2}is the Bessel function of the order of (l + 1/2) with the azimuthal index l; H

_{l}

_{+ 1/2}

^{(1)}is the Hankel function of the 1st kind of the order of (l + 1/2); k

_{0}= 2πν/c is the propagation constant in vacuum; ν is a frequency; k = n·k

_{0}and n is the refractive index of the silica glass given by the Sellmeier formula [35]:

_{1}= 0.6961663, C

_{2}= 0.4079426, C

_{3}= 0.8974794; λ

_{1}= 0.0684043 μm, λ

_{2}= 0.1162414 μm, λ

_{3}= 9.896161 μm [35], here λ

_{m}= 2πc/ω

_{m}.

_{l}, we use home-made software. We consider only a fundamental mode family corresponding to the first roots. The roots are localized by using approximation formulas for the eigenfrequencies ν

_{l}

^{approx}given, for example, in [34]:

_{l}

^{approx}are used as the initiators of the algorithm for searching the roots of Equation (1) by the modified Powell method [36]. The iterative algorithm is implemented with allowance for silica glass dispersion given by Equation (2).

#### 2.2. Simulation of OFC Generation

_{R}·t

_{R}are the fast and slow times; N

_{R}is the number of a microresonator roundtrip; t

_{R}= 2πRn

_{eff}/c is the roundtrip time; δ

_{p}is the phase detuning of the continuous wave (CW) pump field E

_{p}from the nearest resonance; θ is the coupling coefficient; β

_{k}= d

^{k}β/dω taken at the pump frequency ω

_{0}(we set β

_{k}= 0 for k ≥ 4); β(ω) = n

_{eff}·k

_{0}is the propagation constant; γ

_{0}is the nonlinear Kerr coefficient at ω

_{0}; α = (2π)

^{2}R/(Qλ

_{p}) is the loss coefficient including intrinsic and coupling losses; and λ

_{p}is a pump wavelength. The response function is approximated by

_{R}= 0.18 is the fractional contribution of the delayed Raman response, and

_{1}= 12.2 fs and τ

_{2}= 32 fs [35].

#### 2.3. Simulation of Silica Microsphere OFC Generator-Based 4-Channel 200 GHz Spaced IM/DD WDM-PON Transmission System

## 3. Results

#### 3.1. Microresonator Characteristics

^{−1}at 193.1 THz, and its frequency dependence can be neglected in the C-band.

#### 3.2. OFC Generation in DKS Regime

_{p}/α) [33,41,42,43]. If the solution in the form of DKS exists in this case, one particular value of DKS peak power corresponds to each admissible detuning [41,43]. For a pump power less than a threshold for this detuning, DKS cannot exist, but CW is a solution. We set pump power slightly higher than this threshold and study properties of generated DKS for different values of Δ. We set pump frequency at 193.1 THz assuming that the nearest resonant WGM can be shifted to 193.1 THz due to the thermal effects caused by partially dissipated pump power and/or external heating of a microsphere. Figure 8a shows the DKS spectrum calculated for Δ = 50. This spectrum is asymmetric with respect to the pump frequency, which is explained by the influence of the Raman nonlinearity and agrees with the results presented in [43,44]. Figure 8b demonstrates the spectral envelopes of stable DKSes simulated for different Δ. The larger the normalized detuning, the broader the spectrum is. For example, for Δ = 10, the spectral width at the level of -30 dB is 3.8 THz, but for Δ = 70, the spectral width is 8.8 THz. Next, we count a quantity of spectral lines (harmonics) in OFC spectra with intensity higher than -30 dB (see Figure 8c). The quantity of lines satisfying this condition increases from 19 for Δ = 10 up to 44 for Δ = 70. For Δ > 70, DKS is unstable and we do not consider intracavity nonlinear dynamics for this case. Note that due to higher-order dispersion, the range of DKS stability is slightly wider than with allowance for only the second-order dispersion presented in [43]. We also find DKS duration (full width at half maximum, FWHM) in the time domain as a function of Δ (see Figure 8d). For larger Δ (when the spectrum is wider), the duration is shorter according to the Fourier-transform limitation (379 fs for Δ = 10 and 169 fs for Δ = 70).

#### 3.3. Architecture and Simulation of 4-Channel 200 GHz Spaced IM/DD WDM-PON Transmission System

_{1}(t) [45].

^{−12}, and responsivity of 0.65 A/W [46]. Afterward, the received, modulated signal is filtered by an electrical low-pass filter (LPF) with 7.5 GHz 3-dB electrical bandwidth. The electrical signal analyzer is used to measure the received signal, e.g., showing bit pattern and BER.

^{−30}. The drop in the BER performance is mainly affected by the power and noise floor variation between comb lines and phase noise. In such a case, the comb source for the data transmission system must ensure minimal optical carrier-to-noise power ratio (OCNR) that a comb line must have to be useful for data transmission.

^{−4}, please see Figure 12d. Therefore, our investigated 200 GHz spaced OFC-WGMR light source-based IM/DD WDM-PON transmission system is fully capable of providing 10 Gbit/s of NRZ-OOK modulated signal transmission according to NG-PON2 recommendation specified valid optical link distances of 40 km. That means it is technically challenging to ensure such transmission stability for longer distances by the use of OFC-WGMR as an optical light source for telecommunication applications. More comprehensive future research on the limits of the OFCs parameters for implementing the PON transmission systems segment of such a long transmission distance (60 km) is desirable.

## 4. Discussion and Conclusions

^{2}/km). Both material and waveguide contributions are important here. The zero-dispersion frequency is about 209 THz. Dispersion curves for TE and TM fundamental modes differ slightly. The nonlinear Kerr coefficient is about 3.5 (W·km)

^{−1}. It was also verified that the change in dispersion with temperature increasing was negligible for the considered heat powers of a few mW.

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

OFC | optical frequency comb |

WGM | whispering gallery mode |

WDM | wavelength-division multiplexing |

PON | passive optical network |

FSR | free spectral range |

TM | transverse magnetic |

TE | transverse electric |

CW | continuous wave |

DKS | dissipative Kerr soliton |

FWHM | full width at half maximum |

BER | bit error rate |

NRZ | non-return-to-zero |

OOK | on-off keying |

IM/DD | intensity modulation direct detection |

NG-PON2 | Next-generation PON |

ASE | amplified spontaneous emission |

ASI | amplified spontaneous emission light source |

AWG | arrayed-waveguide-grating |

LPF | low-pass filter |

MZM | Mach–Zehnder modulator |

OBPF | optical band-pass filter |

WGMR | whispering gallery mode resonator |

MUX | multiplexed |

de-MUX | demultiplexer |

PIN | photodiode |

SMF | single-mode fiber |

B2B | back-to-back |

WR | wavelength-routed |

QoT | quality of transmission |

ITU-T | International Telecommunication Union–telecommunication standardization sector |

OCNR | optical carrier-to-noise power ratio |

CO | central office |

## References

- Pasquazi, A.; Peccianti, M.; Razzari, L.; Moss, D.J.; Coen, S.; Erkintalo, M.; Chembo, Y.K.; Hansson, T.; Wabnitz, S.; Del’Haye, P.; et al. Micro-combs: A novel generation of optical sources. Phys. Rep.
**2018**, 729, 1–81. [Google Scholar] [CrossRef] - Strekalov, D.V.; Marquardt, C.; Matsko, A.B.; Schwefel, H.G.; Leuchs, G. Nonlinear and quantum optics with whispering gallery resonators. J. Opt.
**2016**, 18, 123002. [Google Scholar] [CrossRef] [Green Version] - Suh, M.G.; Yang, Q.F.; Yang, K.Y.; Yi, X.; Vahala, K.J. Microresonator soliton dual-comb spectroscopy. Science
**2016**, 354, 600–603. [Google Scholar] [CrossRef] [Green Version] - Yu, M.; Okawachi, Y.; Griffith, A.G.; Picqué, N.; Lipson, M.; Gaeta, A.L. Silicon-chip-based mid-infrared dual-comb spectroscopy. Nat. Commun.
**2018**, 9, 1869. [Google Scholar] [CrossRef] [Green Version] - Xue, X.; Xuan, Y.; Kim, H.J.; Wang, J.; Leaird, D.E.; Qi, M.; Weiner, A.M. Programmable single-bandpass photonic RF filter based on Kerr comb from a microring. J. Lightw. Technol.
**2014**, 4, 3557–3565. [Google Scholar] [CrossRef] - Suh, M.G.; Yi, X.; Lai, Y.H.; Leifer, S.; Grudinin, I.S.; Vasisht, G.; Martin, E.C.; Fitzgerald, M.P.; Doppmann, G.; Wang, J.; et al. Searching for exoplanets using a microresonator astrocomb. Nat. Photonics
**2019**, 13, 25–30. [Google Scholar] [CrossRef] [Green Version] - Kues, M.; Reimer, C.; Lukens, J.M.; Munro, W.J.; Weiner, A.M.; Moss, D.J.; Morandotti, R. Quantum optical microcombs. Nat. Photonics
**2019**, 13, 170–179. [Google Scholar] [CrossRef] [Green Version] - Engin, E.; Bonneau, D.; Natarajan, C.M.; Clark, A.S.; Tanner, M.G.; Hadfield, R.H.; Dorenbos, S.N.; Zwiller, V.; Ohira, K.; Suzuki, N.; et al. Photon pair generation in a silicon micro-ring resonator with reverse bias enhancement. Opt. Express
**2013**, 21, 27826–27834. [Google Scholar] [CrossRef] - Monteiro, F.; Martin, A.; Sanguinetti, B.; Zbinden, H.; Thew, R.T. Narrowband photon pair source for quantum networks. Opt. Express
**2014**, 22, 4371–4378. [Google Scholar] [CrossRef] - Kumar, R.; Ong, J.R.; Recchio, J.; Srinivasan, K.; Mookherjea, S. Spectrally multiplexed and tunable wavelength photon pairs at 1.55 μm from a silicon coupled-resonator optical waveguide. Opt. Lett.
**2013**, 38, 2969–2971. [Google Scholar] [CrossRef] - Raussendorf, R.; Briegel, H.J. A one-way quantum computer. Phys. Rev. Lett.
**2001**, 86, 5188–5191. [Google Scholar] [CrossRef] - Walther, P.; Resch, K.J.; Rudolph, T.; Schenck, E.; Weinfurter, H.; Vedral, V.; Aspelmeyer, M.; Zeilinger, A. Experimental one-way quantum computing. Nature
**2005**, 434, 169–176. [Google Scholar] [CrossRef] [PubMed] - Company, V.T.; Scroder, J.; Fulop, A.; Mazur, M.; Lundberg, L.; Helhason, O.B.; Karlsson, M.; Andrekson, P.A. Laser Frequency Combs for Coherent Optical Communications. J. Lightw. Technol.
**2019**, 37, 1663–1670. [Google Scholar] [CrossRef] [Green Version] - Pfeifle, J.; Weimann, C.; Bach, F.; Riemensberger, J.; Hartinger, K.; Hillerkuss, D.; Jordan, M.; Holtzwarth, B.; Kippenberg, T.J.; Leuthold, J.; et al. Microresonator-Based Optical Frequency Combs for High-Bitrate WDM Data Transmission. In Proceedings of the Optical Fiber Communication Conference, Los Angeles, CA, USA, 4–8 March 2012. [Google Scholar] [CrossRef]
- Pfeifle, J.; Brasch, V.; Lauermann, M.; Yu, Y.; Wegner, D.; Herr, T.; Hartinger, K.; Schindler, P.; Li, J.; Hillerkuss, D.; et al. Coherent terabit communications with microresonator Kerr frequency combs. Nat. Photonics
**2014**, 8, 375–380. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Pfeifle, J.; Coillet, A.; Henriet, R.; Saleh, K.; Schindler, P.; Weimann, C.; Freude, W.; Balakireva, I.V.; Larger, L.; Koos, C.; et al. Optimally Coherent Kerr Combs Generated with Crystalline Whispering Gallery Mode Resonators for Ultrahigh Capacity Fiber Communications. Phys. Rev. Lett.
**2015**, 114, 093902. [Google Scholar] [CrossRef] - Marin-Palomo, P.; Kemal, J.N.; Karpov, M.; Kordts, A.; Pfeifle, J.; Pfeiffer, M.H.P.; Trocha, P.; Wolf, S.; Brasch, V.; Anderson, M.H.; et al. Microresonator-based solitons for massively parallel coherent optical communications. Nature
**2017**, 546, 274–279. [Google Scholar] [CrossRef] [Green Version] - Pfeifle., J.; Yu, Y.; Schindler, P.C.; Brasch, V.; Weimann, C.; Hartinger, K.; Holzwarth, R.; Freude, W.; Kippenberg, T.J.; Koos, C. Transmission of a 1.44 Tbit/s data stream using a feedback-stabilized SiN Kerr Frequency Comb Source. In Proceedings of the Optical Fiber Communication Conference, San Francisco, CA, USA, 9–13 March 2014. [Google Scholar] [CrossRef]
- Pfeifle, J.; Kordts, A.; Marin, P.; Karpov, M.; Pfeiffer, M.; Brasch, V.; Rosenberger, R.; Kemal, J.; Wolf, S.; Freude, W.; et al. Full C and L-Band Transmission at 20 Tbit/s Using Cavity-Soliton Kerr Frequency Comb Source. In Proceedings of the Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, USA, 10–15 May 2015. [Google Scholar] [CrossRef]
- Fülöp, A.; Mazur, M.; Lorences-Riesgo, A.; Helgason, Ó.B.; Wang, P.H.; Xuan, Y.; Leaird, D.E.; Qi, M.; Andrekson, P.A.; Weiner, A.M.; et al. High-order coherent communications using mode-locked dark-pulse Kerr combs from microresonators. Nat. Commun.
**2018**, 9, 1598. [Google Scholar] [CrossRef] [Green Version] - Addanki, S.; Yupapin, P.; Amiri, I.S. Enhanced NRZ multi-carriers modulation technologies for microresonators in THz technology applications. Results Phys.
**2019**, 12, 178–189. [Google Scholar] [CrossRef] - Hu, H.; Da Ros, F.; Pu, M.; Ye, F.; Ingerslev, K.; Da Silva, E.P.; Nooruzzaman, M.; Semenova, E.; Guan, P.; Zibar, D.; et al. Single-source chip-based frequency comb enabling extreme parallel data transmission. Nat. Photonics
**2018**, 12, 469–473. [Google Scholar] [CrossRef] - Peichang, L.; Changjing, B.; Almaiman, A.; Kordts, A.; Karpov, M.; Pfeiffer, M.H.P.; Lin, Z.; Alishahi, F.; Yinwen, C.; Kaiheng, Z.; et al. Demonstration of Multiple Kerr-frequency-Comb Generation Using Different Lines From Another Kerr Comb Located Up to 50 km Away. J. Lightw. Technol.
**2019**, 37, 579–584. [Google Scholar] [CrossRef] - Kovach, A.; Chen, D.; He, J.; Choi, H.; Dogan, A.H.; Ghasemkhani, M.; Taheri, H.; Armani, A.M. Emerging material systems for integrated optical Kerr frequency combs. Adv. Opt. Photonics
**2020**, 12, 135–222. [Google Scholar] [CrossRef] [Green Version] - Gorodetsky, M.L.; Savchenkov, A.A.; Ilchenko, V.S. Ultimate Q of optical microsphere resonators. Opt. Lett.
**1996**, 21, 453–455. [Google Scholar] [CrossRef] [PubMed] - Spillane, S.M.; Kippenberg, T.J.; Vahala, K.J. Ultralow-threshold Raman laser using a spherical dielectric microcavity. Nature
**2002**, 415, 621–623. [Google Scholar] [CrossRef] [PubMed] - Webb, K.E.; Erkintalo, M.; Coen, S.; Murdoch, S.G. Experimental observation of coherent cavity soliton frequency combs in silica microspheres. Opt. Lett.
**2016**, 41, 4613–4616. [Google Scholar] [CrossRef] [Green Version] - Andrianov, A.V.; Anashkina, E.A. Single-mode silica microsphere Raman laser tunable in the U-band and beyond. Results Phys.
**2020**, 17, 103084. [Google Scholar] [CrossRef] - ITU-T G.694.1 Recommendation. Spectral Grids for WDM Applications: DWDM Frequency Grid. Available online: https://www.itu.int/itu-t/recommendations/rec.aspx?rec=11482&lang=en (accessed on 10 September 2020).
- Fujii, S.; Tanabe, T. Dispersion engineering and measurement of whispering gallery mode microresonator for Kerr frequency comb generation. Nanophotonics
**2020**, 9, 1087–1104. [Google Scholar] [CrossRef] [Green Version] - Godey, C.; Balakireva, I.V.; Coillet, A.; Chembo, Y.K. Stability analysis of the spatiotemporal Lugiato-Lefever model for Kerr optical frequency combs in the anomalous and normal dispersion regimes. Phys. Rev. A
**2014**, 89, 063814. [Google Scholar] [CrossRef] [Green Version] - Carmon, T.; Yang, L.; Vahala, K.J. Dynamical thermal behavior and thermal self-stability of microcavities. Opt. Express
**2004**, 12, 4742–4750. [Google Scholar] [CrossRef] [Green Version] - Herr, T.; Brasch, V.; Jost, J.D.; Wang, C.Y.; Kondratiev, N.M.; Gorodetsky, M.L.; Kippenberg, T.J. Temporal solitons in optical microresonators. Nat. Photonics
**2014**, 8, 145–152. [Google Scholar] [CrossRef] [Green Version] - Oraevsky, A.N. Whispering-gallery waves. Quantum Electron.
**2020**, 32, 377–400. [Google Scholar] [CrossRef] - Agrawal, G.P. Nonlinear Fiber Optics, 6th ed.; Elsevier: London, UK, 2019. [Google Scholar]
- Powell, M.J.D. A hybrid method for nonlinear equations. In Numerical Methods for Nonlinear Equations; Rabinowitz, P., Ed.; Gordon and Breach: London, UK, 1970. [Google Scholar]
- Andrianov, A.V.; Marisova, M.P.; Dorofeev, V.V.; Anashkina, E.A. Thermal shift of whispering gallery modes in tellurite glass microspheres. Results Phys.
**2020**, 17, 103128. [Google Scholar] [CrossRef] - Anashkina, E.A.; Sorokin, A.A.; Marisova, M.P.; Andrianov, A.V. Development and numerical simulation of spherical microresonators based on SiO
_{2}–GeO_{2}germanosilicate glasses for generation of optical frequency combs. Quantum Electron.**2019**, 49, 371–376. [Google Scholar] [CrossRef] - Anashkina, E.A.; Marisova, M.P.; Sorokin, A.A.; Andrianov, A.V. Numerical simulation of mid-infrared optical frequency comb generation in chalcogenide As
_{2}S_{3}microbubble resonators. Photonics**2019**, 6, 55. [Google Scholar] [CrossRef] [Green Version] - ITU-T Recommendation G.989.2. Digital Sections and Digital Line System—Optical Linesystems for Local and Access Networks—40-Gigabit-Capable Passive Optical Networks 2 (NG-PON2): Physical Media Dependent (PMD) Layer Specification; ITU-T: Geneva, Switzerland, 2019; pp. 1–122. [Google Scholar]
- Coen, S.; Erkintalo, M. Universal scaling laws of Kerr frequency combs. Opt. Lett.
**2013**, 38, 1790–1792. [Google Scholar] [CrossRef] [Green Version] - Shen, B.; Chang, L.; Liu, J.; Wang, H.; Yang, Q.F.; Xiang, C.; Wang, R.N.; He, J.; Liu, T.; Xie, W.; et al. Integrated turnkey soliton microcombs. Nature
**2020**, 582, 365–369. [Google Scholar] [CrossRef] [PubMed] - Wang, Y.; Anderson, M.; Coen, S.; Murdoch, S.G.; Erkintalo, M. Stimulated Raman scattering imposes fundamental limits to the duration and bandwidth of temporal cavity solitons. Phys. Rev. Lett.
**2018**, 120, 053902. [Google Scholar] [CrossRef] [Green Version] - Milián, C.; Gorbach, A.V.; Taki, M.; Yulin, A.V.; Skryabin, D.V. Solitons and frequency combs in silica microring resonators: Interplay of the Raman and higher-order dispersion effects. Phys. Rev. A
**2015**, 92, 033851. [Google Scholar] [CrossRef] [Green Version] - IXblue Photonics. MX-LN Series 1550 nm Band Intensity Modulators; Technical Specification; iXblue Photonics: Paris, France, 2019; pp. 1–6. [Google Scholar]
- Amonics. 10G Receiver Module; Technical Specification; Amonics Ltd.: Hong Kong, China, 2008; pp. 1–2. [Google Scholar]
- Lobanov, V.E.; Lihachev, G.V.; Pavlov, N.G.; Cherenkov, A.V.; Kippenberg, T.J.; Gorodetsky, M.L. Harmonization of chaos into a soliton in Kerr frequency combs. Opt. Express
**2016**, 24, 27382–27394. [Google Scholar] [CrossRef] - Suchkov, S.V.; Sumetsky, M.; Sukhorukov, A.A. Frequency comb generation in SNAP bottle resonators. Opt. Lett.
**2017**, 42, 2149–2152. [Google Scholar] [CrossRef] - Oreshnikov, I.; Skryabin, D.V. Multiple nonlinear resonances and frequency combs in bottle microresonators. Opt. Express
**2017**, 25, 10306–10311. [Google Scholar] [CrossRef]

**Figure 1.**Schematic diagram of the whispering gallery modes (WGMs) for a microsphere at the initial temperature T

_{0}and heated by ΔT.

**Figure 2.**The normalized absolute value of the electric field for transverse electric (TE) mode at the eigenfrequency of about 193.1 THz in the equatorial plane (

**a**) and in a plane perpendicular to equatorial one (

**b**).

**Figure 4.**(

**a**) Dispersion as a function of frequency for microspheres with indicated diameters for TE and TM modes. (

**b**) Dispersion as a function of frequency for a microsphere with a diameter of 328.5 μm for TE modes.

**Figure 5.**WGM shift (

**a**) and free spectral range (FSR) (

**b**) as functions of temperature increase for a microsphere with a diameter of ~328.5 μm.

**Figure 6.**Temperature distributions for heat powers: 1 mW (

**a**,

**b**); 3 mW (

**c**,

**d**); and 5 mW (

**e**,

**f**). The temperature increase as a function of heat power (

**g**). All subfigures are calculated for a microsphere with a diameter of ~328.5 μm.

**Figure 8.**(

**a**) Normalized intracavity spectral intensity of DKS for Δ =50. (

**b**) Envelopes of normalized DKS spectra. (

**c**) Number of spectral lines (harmonics) with spectral intensity higher than −30 dB relative to the maximum as a function of normalized detuning. (

**d**) FWHM DKS duration as a function of normalized detuning.

**Figure 9.**Simulation model of the 4-channel 200 GHz spaced IM/DD WDM-PON system used for the designed silica microsphere-based OFC comb spectrum implementation and performance assessment where ASE—Amplified spontaneous emission light source, AWG—Arrayed-waveguide-grating, LPF—Low-pass filter, MZM—Mach–Zehnder modulator, OBPF—Optical band-pass filter, OFC-WGMR—Optical frequency comb generator based on whispering gallery mode resonator, PIN—photodiode, SMF—single-mode fiber.

**Figure 10.**Optical spectra: (

**a**) after user-defined OBPF with implemented microsphere-based OFC comb source, (

**b**) after OBPF-four optical carriers from microsphere-based OFC comb source, (

**c**) modulated optical carriers after B2B transmission for 4-channel 200 GHz spaced IM/DD WDM-PON system operating at 10 Gbit/s per channel.

**Figure 11.**Quality of transmission (QoT) characteristics for the 10 Gbit/s NRZ-OOK signals in the 4-channel 200 GHz spaced IM/DD WDM-PON system: the fair comparison of BER vs. Optical fiber length for implemented microsphere-based OFC comb carriers’ performance.

**Figure 12.**Eye diagrams of the received signal: (

**a**) after B2B, (

**b**) after 20 km, (

**c**) after 40 km, (

**d**) after 60 km transmission via SMF optical link section for investigated 4-channel 200 GHz spaced IM/DD WDM-PON system operating at 10 Gbit/s per channel.

© 2020 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**

Anashkina, E.A.; Marisova, M.P.; Andrianov, A.V.; Akhmedzhanov, R.A.; Murnieks, R.; Tokman, M.D.; Skladova, L.; Oladyshkin, I.V.; Salgals, T.; Lyashuk, I.;
et al. Microsphere-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System. *Photonics* **2020**, *7*, 72.
https://doi.org/10.3390/photonics7030072

**AMA Style**

Anashkina EA, Marisova MP, Andrianov AV, Akhmedzhanov RA, Murnieks R, Tokman MD, Skladova L, Oladyshkin IV, Salgals T, Lyashuk I,
et al. Microsphere-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System. *Photonics*. 2020; 7(3):72.
https://doi.org/10.3390/photonics7030072

**Chicago/Turabian Style**

Anashkina, Elena A., Maria P. Marisova, Alexey V. Andrianov, Rinat A. Akhmedzhanov, Rihards Murnieks, Mikhail D. Tokman, Laura Skladova, Ivan V. Oladyshkin, Toms Salgals, Ilya Lyashuk,
and et al. 2020. "Microsphere-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System" *Photonics* 7, no. 3: 72.
https://doi.org/10.3390/photonics7030072