1. Introduction

Optical frequency synthesizers with discrete, precisely equidistant frequencies over a large bandwidth can be applied to a wide array of research areas such as spectroscopy, sensing, and optical clocks [1–3]. A highly promising route to such optical comb generation, utilizes ultra-broadband cascaded parametric four-wave mixing (FWM) oscillation in high-Q microresonators [4–12]. While there has been significant experimental [13–19] and theoretical [20–23] development of ultra-broadband, microresonator-based comb generation, most of the emphasis has been on generating frequency combs with a pump in the telecommunication window. Broadband frequency combs pumped at near-IR wavelengths centered at 1 μm can offer several advantages such as the existence of commercially available technology for f–2f stabilization, the potential spectral overlap with optical frequency standards and to generate frequency-doubled comb in the visible spectrum for astronomical applications [24].

It has been established that dispersion engineering of microresonator geometries allows for using pump waves at different wavelengths to generate a frequency combs [25–27] over distinct wavelength ranges, but it still remains a challenge to design a device using the same device platform with the flexibility of pumping at different wavelengths and generating an ultra-broadband frequency comb in the visible or near-IR wavelength range with a desired comb spacing. For example, parametric comb generation was demonstrated in a CaF₂ cylindrical and spheroid resonators with a 794-nm pump but with a relatively narrow ∼2 THz bandwidth [25]. In a microtoroid [26] or microsphere [7] geometry, the dispersion is dependent on the size and shape of the cavity. Since the free spectral range (FSR) is determined by the cavity size, it is difficult to design a structure where the pump wavelength and FSR can be freely determined. In contrast, in a silicon-nitride microring resonator, the dispersion and FSR can be controlled separately, and we have recently shown ultra-broadband (octave-spanning) comb generation with a pump at 1550 nm [14] and have established that this system provides unmatched flexibility for tuning the FSR [28].

Here, we show the multi-wavelength pumping functionality of the silicon-nitride microresonator platform and report the first demonstration of optical parametric oscillation and generation of an ultra-broadband frequency comb at 1-μm in a silicon-nitride ring resonator by dispersion engineering FWM gain in the 0.8–1.5 μm wavelength range.

2. Dispersion engineering for comb generation

In parametric comb generation, the pump power builds up in the resonator, resulting in parametric oscillation of a signal-idler pair. As the pump power is increased, the power in the signal-idler pair builds up further and leads to additional higher-order FWM processes that result in multiple components near the peaks of the cavity modes. Parametric oscillation in a cavity geometry arises from the interplay between the nonlinearity and the group-velocity dispersion (GVD), which is characterized by the phase mismatch for the FWM process and is given by,

The dispersion is determined by the material and for relatively large ring diameters by the waveguide cross-sectionional geometry, whereas the FSR is determined by the circumference of the microring. Figure 1 shows the simulated dispersion curves for silicon-nitride waveguides with varying cross-sections for the fundamental TE-mode. The curves show that decreasing the width of the waveguides shifts the anomalous dispersion region to lower wavelengths. The peak of the anomalous GVD occurs for a width of 1100 nm and then decreases with further reduction in waveguide widths, which is detrimental for wide bandwidth phase-matching and FWM.

 

Fig. 1 Simulated dispersion curves for the fundamental TE-mode of a silicon-nitride waveguide with 600-nm height and with widths of 1000, 1100, and 1200 nm.

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3. Experiment and discussion

In our experiment, we use a single-frequency continuous wave laser centered at 1064 nm and amplified to 2 W using an Ytterbium-doped fiber amplifier (YDFA). The polarization of the laser is adjusted to quasi-TE and sent into a bus waveguide for coupling into the microring resonator. Both the coupling waveguide and the microring are monolithically fabricated. The bus waveguide and the ring have cross-sections of 600 nm by 1150 nm, which gives a broad region of anomalous GVD, centered at the pump wavelength of 1064 nm. The ring has a 115-μm radius which corresponds to a 230-GHz FSR. The loaded-quality factor Q of the resonator is 200,000. The output light is collected using an objective and sent to an optical spectrum analyzer. Figure 2(a–e) shows the optical spectrum associated with the comb generation dynamics as the pump wavelength is tuned into a cavity resonance. As the power oscillating in the microring increases, the cavity modes that are multiple FSRs away from the pump, experience the maximum FWM gain and oscillate at a threshold power of 80 mW. With further increases in power, cascaded FWM occurs resulting in multiple oscillations and minicomb formation [17] [Fig. 2(d)]. The density of comb lines increases as the pump frequency is tuned deep in the cavity resonance and adjacent cavity modes are filled through multiple higher-order degenerate and non-degenerate FWM processes [Fig. 2(e)].

 

Fig. 2 (a–e) Comb generation dynamics (from top to bottom) with a pump laser at 1064 nm. As the power oscillating inside the microring increases and threshold is reached, cavity modes that are maximally phase-matched experience gain and oscillate. When the pump is tuned deeper into resonance and power in the side modes is further increased, cascaded four-wave mixing takes place, leading to multiple cascaded oscillations and development of a wide bandwidth comb. (f) Frequency comb spectrum generated with 2 W of pump power. The comb spans 97.3 THz with a spacing of 230 GHz. (g) A zoomed in spectrum at the longer wavelength end. Comb teeth do not appear at every FSR due to lack of proper phase-matching and power buildup.

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The optical spectrum of the fully formed comb is shown in Fig. 2(f). The 230-GHz-spaced comb lines are generated over a wavelength span of 406 nm which corresponds to 97.3 THz. Figure 2(g) shows the zoomed-in optical spectrum of the higher wavelength side of the frequency comb. We observe that the comb teeth are not formed at every FSR, which can be explained by considering the phase-matching relations for FWM. The relatively low anomalous dispersion of the microring resonator leads to improper phase-matching for wavelengths farther detuned from the original pump wavelength, which is due to the fact that the low anomalous dispersion cannot compensate for the non-linear contribution to the phase mismatch from self- and cross-phase modulation [7]. This results in small FWM mixing gain and insufficient buildup of signal/idler pairs which ultimately must act as pump modes for further frequency generation, thus preventing further comb line formation. To investigate this issue further, we dispersion engineer and fabricate devices with larger anomalous dispersion.

Figure 3(a) shows the dispersion curves for a silicon-nitride waveguide with a cross-section of 725 nm by 1000 nm with TE and TM modes. With the pump at 1064 nm, the generated frequency comb is shown Fig. 3(b). The polarization of the pump was set to quasi-TM in this case due to the lower coupling and waveguide losses as compared to the quasi-TE polarization input for the microring resonator. The loaded-Q is 250,000, which is higher than that of the previous sample and as expected, we observe a lower threshold power of 40 mW for oscillation. The frequency comb spans 55 THz with an FSR of 230 GHz. In comparison to the previous case, the obtained comb bandwidth is smaller, which is due to the insufficient pump power required for efficient cascaded-FWM at the wings of the comb, which can be improved by reducing both the coupling and propagation losses. Moreover, since the same amount of pump power (as compared to previous) gets distributed to the comb lines in this case,for a given pump power, more filled the comb is, the smaller the bandwidth will be. Therefore, the effect of a marginally higher Q which affects both the conversion efficiency as well as the bandwidth, is not drastic and hence is not observed in the data shown. Nonetheless, the zoomed-in viewgraphs [Fig. 3(c–d)] show that larger net anomalous dispersion of the resonator allows for comb lines to be generated at every adjacent FSR of the resonator. These results are consistent with the recent investigation on the dependence of frequency comb on the dispersion of a microresonator [17, 27]. We are currently investigating the dispersion engineering conditions further and working on improving the losses and well as the Q to achieve broader bandwidth comb generation.

 

Fig. 3 (a) Simulated dispersion curves for the fundamental TE-mode (dashed yellow) and TM-mode (solid red) of a silicon-nitride waveguide with a 725-nm height and 1000-nm width. (b) Broadband frequency comb at 1 μm spanning 55 THz and 230-GHz comb spacing. (c–d) Zoomed in spectra of the low and high wavelength regions of the frequency comb showing fully developed comb lines at every cavity mode.

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Recently there have been multiple investigations directed towards testing the coherence properties of frequency combs [12,17,29–31]. We have recently characterized spectral and temporal properties of the microresonator-based combs at 1.5-μm and have observed mode-locking and ultrashort pulse generation [32]. As we use the same silicon-nitride microresonator platform, and observe that the spectral dynamics of the comb generated in the 1-μm region is similar to that of the 1.5-μm comb, we expect to observe similar temporal and coherence properties as well. We are currently working on characterizing the phase noise and temporal dynamics of the frequency comb generated in the 1-μm region.

4. Conclusion

In conclusion, we demonstrate broadband frequency comb generation in a monolithically integrated silicon-nitride microring resonator using a single-frequency laser at 1064 nm. We achieve a 55-THz comb bandwidth with a FSR of 230 GHz. Dispersion engineering a wider anomalous region around 1064-nm, in a microresonator with higher Q-factor will allow for generation of an octave spanning comb for comb stabilization using existing frequency stabilization equipment and techniques. Our results illustrate how the microring geometry in silicon nitride can be readily extended to broadband comb generation over a wide range of wavelengths, which could have significant applications in spectroscopy, metrology, high-speed communications, and on-chip optical clocks.