1. Introduction

Optical frequency combs (OFCs) have a high potential to replace tens of tunable continuous-wave (CW) lasers with a single laser source and a whispering-gallery-mode microresonator (WGMR) in modern wavelength-division multiplexing (WDM) optical communication systems to upgrade the architecture of the service provider’s central office (CO). OFC can be generated in a controllable manner using different kinds of WGMRs – integrated microring resonators [1], microdisk resonators [2], and microtoroid resonators [3], spatial microdisk resonators [4], and microsphere resonators [5,6]. Even though integrated microresonators, providing Q-factor ∼10⁶, have been studied extensively, silica microspheres [7] have many advantages as more easily and quickly to manufacture by melting the end of standard optical fiber [8]. By using arc discharge of a fusion splicer, it is possible to quickly fabricate microspheres with repeatable diameter, and it is easy to control the free spectral range (FSR), which is inversely proportional to the diameter. Also, routinely reached Q-factor values in silica microspheres are higher 10⁷-10⁸ [9,10] compared to integrated resonators resulting in optical carriers with narrower linewidth [11]. The tapered fiber method of microsphere excitation allows to fine-tune the coupling conditions which is not possible for chip-based resonators with integrated waveguides. Considering these aspects of microsphere resonator advantages, we have chosen them for OFC generation in optical communication systems. While optical data transmission has been well studied and demonstrated employing OFC generated in integrated resonators showing even terabit communications [12–14], the experimental demonstration of data transmission based on OFC generated in microsphere resonator, to the best of our knowledge, has not been demonstrated until now. This research reports the first successful demonstration of experimental 2.5 and 10 Gbps non-return to zero (NRZ) on-off keying (OOK) WDM system based on optical carriers generated in fabricated silica microsphere resonator. We also integrated the experimentally obtained WGMR-OFC into the simulation model corresponding to the experimental system to support and verify the experimental results regarding the overall system operation and signal waveform quality in terms of the bit-error-ratio (BER) values. As a result, it enables the simulation environment to develop more complex WDM transmission systems with advanced modulation schemes.

The remainder of the manuscript is structured as follows: Section 2 describes the fabrication process of a microsphere resonator and an experimental process of coupling the light inside of a Kerr OFC in the produced WGMR. Section 3 presents the designed experimental setup of silica microsphere WGMR-OFC as a C-band light source providing data transmission of up to 10 Gbps/λ NRZ-OOK modulated signals over 20 km the standard ITU-T G.652 telecom fiber using 400 GHz spaced carriers. This section also describes the performed setup for assessing the impact of pump source light polarization on the WGMR-OFC. Experimentally obtained results, spectra, received signal bit pattern, eye diagram, and system performance analysis are provided in section 4. Section 5 presents the integration of the experimental generated OFC comb into the simulation environment to verify the obtained experimental results regarding the overall system operation. The simulation model reproduces the data transmission part of the experimental setup as close as possible and enables the further development of more complex WDM transmission systems with advanced modulation schemes. Finally, section 6 gives a brief summary of the experimental and simulative results and concludes the paper.

2. Fabrication and characteristics of a designed silica WGMR-OFC microresonator

Microspheres with diameters of about 170 µm used in this research were fabricated from a standard ITU-T G.652 single-mode telecommunication fiber by a two-stage method using a specially developed program for a commercially available specialty optical fusion splicer. At the first stage, the fiber was tapered with a decrease in the diameter of the thinnest part by ∼3 times. Then, as the arc discharge current increased, the fiber was divided into two parts. Then one of the parts was removed from the fiber splicer, and the remaining fiber end was melted several times by arc discharge. After that, a microresonator was formed under the action of surface tension. With each subsequent melting, the shape of the microresonator more and more approached a spherical one. Due to fiber tapering at the first stage, the fiber stem was obtained sufficiently thin at the point of contact with the microsphere, making it possible to minimize the effect of the stem on the microsphere’s properties. This method of microsphere fabrication has been previously demonstrated by [9]. By varying the diameter of the microsphere, it is possible to control the distance between the eigenfrequencies and change the dispersion [5,8]. In our case, the calculated dispersion of the microsphere was slightly anomalous at 1550 nm (the zero dispersion wavelength was about 1540 nm). We experimentally optimized the number of fiber end melting cycles to form microspheres with 400 GHz spacing between fundamental eigenfrequencies for OFC generation suitable for WDM applications following the ITU-T frequency grid. It should be noted that the developed program for the fiber splicer provides the manufacturing of microspheres with reproducible parameters. The Q factor of produced microspheres is relatively persistent over at least 2 months, as we experimentally observed. A tapered fiber was used for coupling the light inside the microsphere resonator, please see Fig. 1(a). The jacket and coating were stripped from ITU-T G.652 single-mode optical fiber (900 μm jacket) in the region of 1-2 cm where it was heated. A hydrogen flame was used to soften the fiber, and submicron microstepping motors were used to pull it in both directions with a constant speed of 80 μm/s. The burning temperature of the hydrogen flame is lower than the hydrogen-oxygen flame. It melted and softened the optical fiber slowly, which was more desirable as the time period for tapered fiber fabrication steps was more flexible. The optical fiber transmission spectrum was monitored during the tapering process to determine when it will return to the single-mode operation.

 

Fig. 1. (a) Silica microsphere and tapered fiber position state captured by microscopes with zoom cameras: side view and top view during the experiment where OFC carrier generation was observed. (b) Experimentally observed Q-factor degradation in terms of a 2-month life cycle (freshly fabricated and after 2 months) while it resided inside the cover box. (c) Experimental setup of the designed silica microsphere WGMR-OFC as a light source where 400 GHz spaced carriers provide NRZ-OOK modulated 2.5 and 10 Gbps data transmission over 20 km SMF fiber. Insets show tapered fiber and silica microsphere resonator positions of coupling conditions and WGMR-OFC reduced humidity and dust prevention cover box.

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3. Experimental setup of the employed WGMR-OFC light source for a fiber optical communication system

The measured quality factor (Q) of the fresh microsphere resonator used for the comb generation in this research is 3.7×10⁷ (5.2 MHz FWHM of the WGM resonance). After a two-month life cycle as the resonator ages, it is slightly reduced to 2.0×10⁷ (9.8 MHz FWHM of the WGM resonance), please see Fig. 1(b). The setup for the generation of WGMR-OFC and the realized WDM communication system is shown in Fig. 1(c).

First, we search for stable combs on an optical spectrum analyzer by tuning external cavity CW semiconductor laser in wavelength steps of 10 pm and bumping light source output fiber. Such a kick method could possibly lead to soliton comb formation [15,16]. We found that the most appropriate wavelength, where a CW laser with a linewidth of about 100 kHz and +6 dBm optical output power can be used as an OFC comb pump source is 1552 nm. The light coming from the pump source is further amplified up to +23 dBm by the erbium-doped fiber amplifier (EDFA). The polarization state of the amplified signal is adjusted using the polarization controller (PC1) before coupling the signal into the microsphere through a tapered fiber. The isolator on the EDFA output is used to prevent back-scattered light from entering the output port of the CW laser, destabilizing its central frequency, therefore influencing the stability of the comb. Silica microsphere and tapered fiber are enclosed in a separate box for dust and air flow prevention, providing even further stability to the resulting OFC. The X, Y, and Z micro-translation stage is used to position the microsphere to touch the tapered fiber at a place slightly thicker than the taper waist, which changes such coupling conditions as coupled power and the Q factor of the resonances that we have demonstrated in our previous work [17].

The taper and microsphere are touching to improve WGMR-OFC stability, as maintaining a constant gap is challenging and minuscule changes considerably affect the intensity of the generated optical carriers. As shown in Fig. 1, an optical spectrum analyzer (OSA) with 0.01 nm resolution is used to measure OFC performance over 10 hours period (see Fig. 2(b)) and power stability and power distribution stability over the wavelength of the OFC carriers. After OFC generation the carriers (-1) and (+1) are similar but one can be a few dBm more intense than the other if multiple solitons are circling inside the resonator. We have observed that comb lines (-1) and (+1) have a different performance where power instability over a 10-hour period is within about a 3 dB margin due to the impact of resonator ability to support multiple spatial modes. As our experimental setup is located on an optical table with an active pneumatic vibration isolation system, the disturbance like small vibrations is not affecting the generated OFC and the rest of the transmission system. Setup temperature is not actively stabilized, while humidity inside the cover box is reduced using silica gel desiccant.

 

Fig. 2. Measured OFC performance over a 10-hour period: (a) optical spectrum with inset representing captured power stability, and (b) power distribution stability over the wavelength. (c) The experimental measurement setup, where one WGMR-OFC optical carrier (+1) is fed into an MZM modulator driven by a 2 GHz sinusoidal tone to assess the impact of optical signal polarization on the spectral purity of generated optical carriers

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The optical carriers λ = 1549 nm depicted as (-1) and λ = 1555 nm depicted as (+1) are used further to demonstrate NRZ-OOK modulated 2.5 Gbps and 10 Gbps data transmission, please see Fig. 2(a). For a fair comparison, in this paper, we focus on newly generated carriers (-1) and (+1) having the highest achieved peak power levels. As shown in Fig. 1(a), the generated OFC signal is sent to the wavelength selective switch (WSS) used to filter out one optical carrier (-1) or (+1) at a time. The realization of an experimental setup corresponds to wavelength-selective (WS) fiber optical communication system architecture. Fully ITU flexible grid G.694.1 compliant WSS with dynamic channel width control provided by liquid crystal on silicon (LCoS) switching technology is set for 400 GHz OFCs channel spacing, where output optical channels bandwidth is 25 GHz (4 × 6.25 GHz).

The filtered output (-1) or (+1) carrier is amplified with EDFA2 pre-amplifier to the necessary optical power level. Polarization controller (PC2) is placed after EDFA2 to reduce the polarization-dependent loss. Bitrates of 2.5 Gbps and 10 Gbps by a 2¹⁵ long pseudo-random bit sequence (PRBS15) NRZ signal from 65 GSa/s and 25 GHz analog bandwidth arbitrary waveform generator (AWG) are modulated onto the optical carrier signal (-1) or (+1) using the Mach-Zehnder intensity modulator (MZM) with an extinction ratio of 20 dB. Then the modulated signal is transmitted over 20 km of standard single-mode optical fiber (SMF), variable optical attenuator (VOA), and detected with 10 GHz photoreceiver (PIN), which sensitivity is equal to -18 dBm at BER of 10⁻¹⁰ and overload power 0 dBm for BER of 10⁻¹³. An optical coupler (coupling ratio 10/90) and a power meter are used before PIN to monitor the optical power level falling on the PIN receiver. An 80 GSa/s and 33 GHz analog bandwidth digital storage oscilloscope (DSO) is used to sample the received electrical signal, filter it with an 8 GHz low-pass filter (LPF) in case of 10 Gbps and 2.3 GHz LPF in the case of 2.5 Gbps data transmission to measure the quality of the received signal (e.g., eye diagrams, waveform), and estimate its BER. Please note that the represented BER values are calculated from the DSO’s estimated Q-factor values of the captured NRZ signal real-time eye diagrams.

We have also assessed the impact of the pump signal polarization state on the spectral purity of generated OFC carriers. An additional experimental measurement scheme is built for this purpose, as shown in Fig. 2(c). For this measurement, the optical heterodyning process of two optical carriers, where the frequency difference or intermediate frequency (IF) between them generates the desired carrier on the output of high-speed photodiode, is used [18]. RF peaks around DC can be masked by a photodetector flicker noise and low-frequency technical noise. For that reason, we are focusing on higher frequencies, which allows us to better evaluate the spectral purity of generated OFC carriers. For the polarization measurement, to prevent the IF frequency instability caused by laser phase noise we use (+1) WGMR-OFC optical carrier which is fed into an optical input of MZM modulator and modulated by the 2 GHz sinusoidal signal originating from a high-stability microwave signal generator, please see Fig. 2(c). Bias voltage Vb₁ of MZM is set to 3.33 VDC, which is near to its null-bias point. In such a way three optical carriers spaced by 2 GHz are obtained on the output of the 40 GHz analog bandwidth MZM.

The frequency spacing between the first and third carriers is 4 GHz. By changing the power of the 2 GHz sinusoidal signal applied to the S₁(t) RF input of the MZM, an optimal RF drive voltage (V_eff = 1.2 V) ensuring almost equal peak powers of generated RF carriers is found. The first RF peak located at DC is not seen in Fig. 3 as the DC blocker was attached to the electrical output of the PIN receiver. Accordingly, after directly receiving such an optical signal by the 10 GHz high-speed PIN photoreceiver, the optical heterodyne up-conversion process occurs. As a result, on the RF output of this PIN receiver, the electrical signal containing the DC-filtered baseband part and two RF carriers (2 GHz and 4 GHz) is generated. Here the 4 GHz RF tone originates from the heterodyning process of two outer optical carriers, while the 2 GHz RF tone originates from two neighboring optical carriers. An effect of polarization on signal quality is further discussed in the experimental results section of this paper.

 

Fig. 3. Measured effect of polarization, by received electrical signal IF after optical heterodyne up-conversion on photodiode in different polarization states: (a) polarization state with large amplitude modulation, (b) intermediate state, and (c) the optimized polarization state for lowest amplitude modulation showing the highest spectral purity of the generated OFC optical carrier. (d) Part of the received and normalized 10 Gbps NRZ-OOK signal waveform after 20 km transmission with applied upper envelope function, highlighting the periodic waveform fluctuations, (e) 15 ns insight of the normalized waveform showing received bit sequence, and (f) power spectral density of the calculated envelope signal.

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4. Experimental results

The setup displayed in Fig. 2(c) is used to determine the impact of light polarization of the input CW pump laser on generated OFC optical carriers. The effect of polarization is observed at three different scenarios; (1) polarization state with large amplitude modulation, (2) intermediate state, (3) polarization optimized for lowest amplitude modulation by adjusting PC1 before WGMR-OFC, see Figs. 3(a), 3(b) and 3(c).

Microsphere pump light polarization is adjusted by PC1 to minimize 27.92 MHz amplitude self-modulation of comb lines. Fast photodiode measure (+1) comb line to offset spectrum towards higher frequencies. The implemented heterodyne beating method enables to observe the effect of OFC input light polarization on the spectral purity of generated carriers. By the pump source first, and second polarization states, the generation of higher-order sub-carrier signals for 2 GHz and 4 GHz IF frequencies are observed, which lead to deterioration of the quality of our data signal, see Figs. 3(a) and 3(b).

An optimized polarization state, when the CW pump laser polarization coincides with the principal axes of the polarization, shows the best result and guarantees that equal amounts of optical power split to both the X/Y polarization states, see Fig. 3(c) [19]. We have set an optimized polarization state for the lowest amplitude modulation of PC1 during the data transmission of 2.5 and 10 Gbps NRZ-OOK modulated data over B2B and 20 km SMF transmission in both cases using (-1) or (+1) optical carriers. As shown in Fig. 3(d), the waveform fluctuations with periodic nature are observed on the captured data signal waveforms. After 2.5 and 10 Gbps data transmission, we estimate that the period of these fluctuations for (+1) OFC optical carrier is 35.82 ns or 27.92 MHz in frequency, as it is confirmed by the spectral analysis of the applied upper envelope signal, see Figs. 3(e) and 3(f).

It should be noted that the fluctuations on the lowest part of the waveform are optically suppressed by intentionally adjusting the bias point of MZM closer to its null-point. For (-1) harmonic, the same fluctuations period is observed. As observed during the experimental measurements, a periodic intensity modulation often appears on the comb lines, always having the same frequency for the particular resonator.

This process has been previously studied in [20] where it was shown that mechanically driven oscillations occur due to radiation pressure force which subsequently causes microsphere deformation. By using COMSOL Multiphysics we calculate that for a silica microsphere with a diameter of 170 μm this resonance frequency corresponds to the strongest mechanical resonance of 27 ± 1 MHz, see Fig. 4.

 

Fig. 4. COMSOL simulation of mechanical oscillations at nm scale for silica microsphere of 170 μm in diameter indicating the strongest eigenfrequency at about 27 ± 1 MHz that can be excited by a radiation pressure causing periodic detuning from resonance and back manifesting as an amplitude modulation of comb lines, where (a) represents microsphere at is diameter maximal expansion, and (b) represents the diameter maximal contraction. For a better visual representation, the deformation in simulation is exaggerated 100 000 times. The black surrounding line indicates the unperturbed microsphere size. Color scale represents the absolute value of deformation in micrometers.

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We observe that an amplitude modulation depth of less than 30% is deteriorating telecom data demodulation. We can reduce this amplitude modulation by adjusting the polarization of the CW pump light, see Figs. 3(a) to 3(c). This reduction in amplitude modulation could be explained by the sideband cooling/heating effect of the mechanical resonance [21].

The performance indicators as eye diagrams and BER values of the received signal verify the feasibility of the fiber optical WGMR-OFC based transmission system. The worst-performing data channel based on the BER value is with the wavelength of 1549 nm, depicted as a carrier (-1), see Fig. 2(a). The BER of received 2.5 Gbps and 10 Gbps NRZ-OOK signals after 20 km transmission is 5.58×10⁻⁷ and 1.58×10⁻⁶, please see Figs. 5(b) and 5(d). The best performing data channel based on the BER is the one with a wavelength of 1555 nm, denoted as a carrier (+1). In this case, the BER of received 2.5 Gbps and 10 Gbps NRZ-OOK signals after 20 km transmission is 1.39×10⁻¹⁵ and 3.64×10⁻¹⁰, please see Figs. 5(a) and 5(c).

 

Fig. 5. Eye diagrams of the received signal after 20 km transmission over SMF fiber at a data rate of 2.5 Gbps for (a) carrier “+1” and (b) carrier “-1”, and at a data rate of 10 Gbps for (c) carrier “+1” and (d) carrier “-1”, and (e) the plots of BER vs. average received optical power in B2B and after 20 km transmission of the NRZ-OOK modulated signal with bitrates of 2.5 and 10 Gbps for “+1” and “-1” carriers.

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The error-free transmission is established during the experiment in both cases of 2.5 and 10 Gbps data rates for OFC comb pump source operating at 1552 nm wavelength. It allows to use WGMR-OFC as a light source where 400 GHz spaced carriers provide 2.5 and 10 Gbps NRZ modulated data transmission over 20 km SMF fiber. Please see the captured eye diagrams in Fig. 5(a) to 5(d). The BER versus received average optical power measurements are preferred for WGMR-OFC carrier signals (-1) and (+1) at 2.5 and 10 Gbps data rates to fully evaluate the quality of transmission characteristics, see Fig. 5(e).

We have chosen B2B transmission and a distance of 20 km corresponding to the NGPON2 requirements to measure the BER correlation diagrams. Figure 5(e) shows that the optical carrier denoted as (+1) provided the highest system performance, where the received optical power varies from -4 to -17.5 dBm, and the BER of 2.5 Gbps NRZ-OOK signal is in the range from 1.39×10⁻¹⁵ to 7.76×10⁻³.

As one can see in Fig. 5, BER curves bending upwards are observed at relatively high detected power levels for WGMR-OFC comb carrier (+1) at 10 Gbps B2B and 10 Gbps 20 km transmission, as well as for (-1) carrier at 2.5 Gbps B2B transmission. It could be explained by detector saturation, nonlinear optical processes in the resonator or transmission fiber.

5. Results of mathematical modeling and discussion

The experimentally generated OFC is integrated into the simulation environment [22] to verify the obtained experimental results regarding the overall system operation, signal waveform quality in terms of eye diagrams, and the BER values. For this purpose, the VPI Photonics Transmission maker software is used, where the target is to reproduce the data transmission part of the experimental setup as close as possible, please see Fig. 6.

 

Fig. 6. Structure of simulation scheme corresponding to the experimental fiber optical data transmission system for data transmission of 2.5 Gbps and 10 Gbps NRZ-OOK modulated signals by 400 GHz spaced carriers generated in the experimental silica microsphere WGMR-OFC.

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The developed simulation model (see Fig. 6) supports future research regarding fiber optical data transmission employing WGMR-OFC generated in silica microsphere, as the simulation environment enables the development of more complex WDM transmission systems with advanced modulation schemes, e.g., higher-order pulse amplitude modulation (M-PAM) or quadrature amplitude modulation (M-QAM) providing higher data rates per carrier. Figure 6 shows the developed simulation setup of a 400 GHz spaced WDM system used for the designed silica microsphere-based OFC comb spectrum implementation and performance assessment. The experimentally obtained silica microsphere-based OFC comb spectrum is implemented as a multiple laser source into the simulation model, shown in Fig. 6. Afterward, spectral lines from WGMR-OFC are filtered and de-multiplexed utilizing WSS, where the bandwidth of each WSS channel for 400 GHz channel spacing is set to 25 GHz, and central frequencies correspond to the optical carriers (-1), (0), and (+1) shown in Fig. 2(a). Implemented OFC comb spectral lines are amplified by EDFA having 15.6 dB gain and fed to the optical input of the first MZM named MZM1. The circuit block, which consists of the electrical sinusoidal signal generator operating at 27.92 MHz frequency, is directly connected to the first MZM modulator (MZM1) to implement experimentally observed periodic oscillations.

The output of the MZM1 is connected to the output of MZM2, where the optical carriers are modulated by the 2.5 or 10 Gbps NRZ encoded bit sequence through the NRZ driver with a length of 2¹⁵–1 (PRBS15) originating from logical pseudo-random bit sequences (PRBS) source. Both MZMs have a 3-dB bandwidth of 12 GHz and a 20 dB extinction ratio. The modulated optical signals are transmitted over 20 km SMF spans.

The receiver consists of a PIN photodiode with 12 GHz of 3-dB bandwidth, -18 dBm sensitivity for BER of 10⁻¹⁰, and responsivity of 0.65 A/W, electrical low-pass filter LPF, and electrical scope. In the case of 10 Gbps bitrate, the received, modulated signal is filtered by an LPF with 8 GHz 3-dB electrical bandwidth, while for 2.5 Gbps, the filter bandwidth of 2.3 GHz is used. The electrical signal analyzer is used to measure the received signal, e.g., showing a bit pattern for BER and eye diagram measurements. The parameters for all used elements are set according to the real equipment specifications used in the experiment. The same fluctuation period of 35.82 ns for 2.5 and 10 Gbps waveform fluctuations can be successfully emulated also in the simulation environment providing an experimentally observed effect of 27.92 MHz periodic oscillations, see Figs. 7(a) and 7(b). Received eye diagrams of the 2.5 and 10 Gbps NRZ-OOK modulated signals after transmission over 20 km SMF fiber span for implemented 400 GHz spaced (+1) and (-1) OFC carriers are displayed in Fig. 7.

 

Fig. 7. Simulated and captured waveforms of (a) 10 Gbps NRZ-OOK signal waveform after 20 km transmission, and (b) 20 ns insight of the waveform showing received bit sequence. Simulative eye diagrams of the received signal after 20 km transmission over SMF fiber at a data rate of 2.5 Gbps for (c) carrier “+1” and (d) carrier “-1”, and data rate of 10 Gbps for (e) carrier “+1” and (f) carrier “-1”.

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The BER values corresponding to received eye diagrams in Fig. 7 are as follows. In the case of 2.5 Gbps data transmission over 20 km of SMF fiber, the BER value of simulated carrier (+1) is 2.39×10⁻¹⁶ (Fig. 7(c)), and the corresponding experimental value is 1.39×10⁻¹⁵.

For simulative carrier (-1), the BER value is 3.7×10⁻¹⁰ (Fig. 7(d)), and the corresponding experimental value is 5.58×10⁻⁷. In the case of 10 Gbps data transmission over 20 km of SMF fiber, the BER value for the simulated carrier (+1) is 1×10⁻⁹ (Fig. 7(e)), and the corresponding experimental value is 3.64×10⁻¹⁰. For simulative carrier (-1), the BER value is 1.8×10⁻⁹ (Fig. 7(f)), and the corresponding experimental value is 1.58×10⁻⁶. There is a strong correlation between the obtained BER results and eye diagrams of the realized experimental and simulative setups, indicating that the mathematical modeling following the experiment is performed correctly.

6. Conclusions

We have designed and demonstrated that the WGMR-OFC generated in silica microsphere with an FSR of 400 GHz can provide the stable operation of 2.5 and 10 Gbps NRZ-OOK modulated WDM data transmission over 20 km SMF link and have the potential to replace individual laser arrays as a multiple laser source. Modeling of the WDM data transmission system based on the experimentally measured parameters shows a high coincidence of results. The developed mathematical model reproduces the integration of generated OFC into the simulation environment and will support future research as the simulation platform. Note that lower spacing between comb carriers can be achieved by using a WGMR with a larger diameter. To obtain 200 GHz spacing, one can use ∼330 µm diameter silica microsphere [22]. To obtain standard 100 GHz spacing, one can use ∼660 µm diameter silica resonator. For operating with 100 GHz mode spacing, microspheres may be not a very optimal choice due to excitation of WGMs from not the fundamental mode family, so using silica microrod or microdisks [8] may be beneficial for this purpose. Moreover, when using larger WGMRs compared to our samples, the effective field areas of the fundamental modes will be larger, so the nonlinear Kerr coefficient γ will be smaller. The nonlinear processes leading to OFC generation depend on γ×P_pump, so the pump power P_pump should be increased, which means that the power in each harmonic will grow too. This may be also beneficial for demonstrating advanced systems (with higher baud rate, coherent modulation formats, etc.). In addition, careful system optimization can help increase the efficiency of harmonic generation. Further, the application of WGMR-OFC is considered as potential to be used in more complex optical communication systems with advanced modulation formats, namely, M-PAM or M-QAM.