# Microscope-Based Optical Frequency Comb Generator for 200 GHz Spaced WDM Data Transmission System
###### Abstract  
Optical frequency comb (OFC) generators based on whispering gallery mode (WGM) microresonators have a massive potential to ensure spectral and energy efficiency in wavelength-division multiplexing (WDM) telecommunication systems. The use of silica microspheres for telecommunication applications has hardly been studied but could be promising. We propose, investigate, and optimize numerically a simple design of a silica microsphere-based OFC generator in the C-band with a free spectral range of 200 GHz and simulate its implementation to provide 4-channel 200 GHz spaced WDM data transmission system. We calculate microsphere characteristics such as WGM eigenfrequencies, dispersion, nonlinear Kerr coefficient with allowance for thermo-optical effects, and simulate OFC generation in the regime of a stable dissipative Kerr soliton. We show that by employing generated OFC lines as optical carriers for WDM data transmission, it is possible to ensure error-free data transmission with a bit error rate (BER) of \(4.5\times 10^{-30}\), providing a total of 40 Gbit/s of transmission speed on four channels. optical frequency comb (OFC); silica microsphere; whispering gallery mode resonator (WGM); dissipative Kerr soliton (DKS); wavelength-division multiplexing (WDM); non-return-to-zero (NRZ); passive optical network (PON) +
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OFCs have an enormous potential for the development of telecommunication systems, especially optical communication systems, which is the primary attention of this paper. OFCs generated in WGM resonators (WGMRs) are the ideal candidate to ensure such optical communication systems' requirements like broadband phase coherence, high-relative frequency stability, which cannot be met with typically used expensive individual laser array solution [13]. Microcombs have been demonstrated for data transmission reaching 170.8 Gbit/s [14], 392 Gbit/s [15], 432 Gbit/s [16] and for coherent terabit communications [17; 15]. Error-free data transmission with rates up to 1.44 Tbit/s for distances up to 300 km has been reported in [15; 18]. Then data rate of 19.7 Tbit/s was realized on 94 comb lines modulated with PDM-16-QAM 224 Gbit/s data streams [19]. After that, two interleaved OFCs in the regime of dissipative Kerr solitons (DKS) were used to transmit a data stream with rates \(>\)50 Tbit/s on 179 individual optical carriers [17]. The application of microresonator-based OFCs in wavelength-division multiplexing passive optical networks (WDM-PONs) is a promising and cost-efficient solution. It allows using a miniature single laser source providing an entire grid of equidistant optical references instead of a traditional array of distributed feedback lasers. In telecommunications, ring resonators are typically investigated as OFC generators [15; 17; 20; 21; 22; 23]. Such resonators have several advantages, including the possibility of on-chip integration, although the development cycle of such devices is quite complicated and time-consuming [24]. The use of silica microspheres for WDM-PONs has not been studied extensively but may be promising. Silica spherical microresonators can be quite simply and quickly manufactured by melting the end of standard optical fiber, followed by the formation of a microsphere under the action of surface tension. In principle, photonic devices based on silica microspheres can also be compactly packed, so their size can be comparable with miniature on-chip devices. Silica glass resonators have high Q-factors: record value is \(0.8\times 10^{10}\)[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].