Octave Spanning Tunable Frequency Comb from a Microresonator

We report the generation of an octave-spanning optical frequency comb in a continuous wave laser pumped microresonator. The generated comb spectrum covers the wavelength range from 990 to 2170 nm without relying on additional external broadening. Continuous tunability of the generated frequency comb over more than an entire free spectral range is demonstrated. Moreover, the linewidth of individual optical comb components and its relation to the pump laser phase noise is studied. The ability to derive octave-spanning spectra from microresonator comb generators represents a key step towards $f\mathrm{\text{−}}2f$ self-referencing of microresonator-based optical frequency combs.

Figure 1

Octave-spanning frequency comb generation in a microresonator. (a) Experimental setup with external cavity diode laser (ECDL), erbium doped fiber amplifier (EDFA), photo diode (PD), and optical spectrum analyzers (IR-A OSA, $0.6–1.7\mu \mathrm{m}$; IR-B OSA, $1.2–2.4\mu \mathrm{m}$). (b) Octave-spanning optical frequency comb, generated in a $40\mu \mathrm{m}$-radius microresonator with a mode spacing of 850 GHz. The denser lines on the low frequency side are a replica of the high frequency end of the spectrum at half the frequency due to the grating spectrometer’s second order diffraction. This artifact allows us to determine the carrier envelope offset frequency from the optical spectrum within the resolution of the spectrum analyzer (inset). (c),(d) Microscope images of the fused silica microresonator. The $1\mu \mathrm{m}$ light can be detected by the CMOS camera.

Figure 2

Tunable octave-spanning microresonator-based frequency comb. The horizontal axis shows the measured frequency comb at different pump laser frequencies (vertical axis). The brightest line corresponds to the pump laser (with a power of 1 W in this measurement). A magnified part of the spectrum in the right panel shows that the comb lines can be tuned more than a full free spectral range.

Figure 3

Microresonator comb tuning parameters. (a) Measured frequency shift of the $\pm 25$th and $\pm 50$th sideband with respect to the pump laser at different pump laser detuning ($m$ denotes the mode number of the comb line). The black line has a slope of 1 and reflects the shift of the pump laser itself. The line with $m=1$ shows the shift of the carrier envelope offset frequency $f_{\mathrm{ceo}}$. (b),(c) Effect of the pump frequency detuning on $f_{\mathrm{ceo}}$ and comb spacing.

Figure 4

Simulation of mode area and dispersion. (a) Simulated mode profile superimposed on a scanning electron microscope image of a cleaved microresonator. (b) Simulated effective mode area for different wavelengths. (c),(d) Simulated group delay dispersion (GDD) and mismatch $\Delta \nu$ between positions of equidistant comb modes and resonator modes (for zero mismatch at the pump laser frequency of 193 THz). The larger radii show a reduced influence on the resonator dispersion (traces for $3.5$ and $4\mu \mathrm{m}$ radius are nearly identical).

Figure 5

Linewidth of frequency comb modes. (a),(b) Octave-spanning comb with pump laser at 1550 nm and corresponding radio frequency beat note with a fiber laser at 1050 nm (arrow in spectrum). (c),(d) Comb in different resonator with narrowband beat note (measured with 3 MHz resolution bandwidth). Comb and beat note in (e),(f) show the transition to a broader beat note at different detunings between pump laser and microresonator resonance. The beat notes in (d),(f) are generated with an additional external cavity diode laser at the positions of the arrows in the spectra in (c),(e).