Frequency Comb Generation


Nonlinear silicon nanophotonics

Silicon photonics promises a technological leap forward through the seamless integration of photonic elements with electronics. From a nonlinear optical perspective, the large index contrast of silicon-based nanowaveguides, such as silicon and silicon nitride, combined with the large Kerr nonlinearity allows for large effective nonlinearities that are more than several orders of magnitude larger than those achievable in silica glass fibers. We investigate nonlinear optical devices based on this extremely power efficient platform including devices based on self-phase modulation, stimulated Raman scattering, and in particular four-wave mixing [Opt. Express 16, 1300-1320 (2008)]. Four-wave mixing (FWM) involves the third-order Kerr nonlinearity. This process has been shown to be efficient and broadband provided that the conditions of phase-matching are met through proper choice of the group-velocity dispersion (GVD). The dispersion of the waveguide can be readily tailored to allow for efficient and broadband SCG. For a high index contrast between the core and cladding of the waveguide, strong tailoring of the dispersion is achieved when the cross-sectional size of the waveguide core is comparable to the wavelength. For example, by engineering the waveguide cross section, the normal GVD introduced by the material can be compensated by the waveguide dispersion so that the total chromatic dispersion can be kept near zero or slightly anomalous.


Frequency comb generation in high-Q silicon nitride microresonators

High-Q optical microcavities allow for a resonance enhancement of the nonlinear effect in materials. Combining this enhancement with the high transverse confinement achieved in nanowaveguides, FWM parametric amplification and oscillation are achievable with modest pump powers. Silicon nitride is a CMOS-compatible material that shows promise for nonlinear optics due to its large nonlinear refractive index, low loss, and the absence of two photon absorption at communication wavelengths. Recently, parametric gain has been demonstrated in nitride microrings, enabling optical parametric oscillation [Nature Photonics 4, 37 (2009)] and frequency comb generation with spectral ranges spanning an octave [Opt. Lett. 36, 3398 (2011)]. This is the first demonstration of a CMOS-compatible monolithically integrated source.The full spectro-temporal dynamics of microresonator-based combs can be modeled utilizing the Lugiato-Lefever model, including higher-order dispersion and self-steepening [Opt. Lett. 38, 3478 (2013)]. The generated frequency combs have been observed to transition to a mode-locked state that produces coherent pulses in the time domain [Opt. Express 21, 1335 (2013)], and single-soliton formation has been demonstrated [Opt. Lett. 41, 2565 (2016)]. Furthermore, tailoring the dispersion of the waveguide allows for flexibility in operating wavelengths in the near- and mid-infrared (mid-IR) [Opt. Express 20, 26935 (2012); Opt. Lett. 40, 4823 (2015)].


Silicon-chip mid-infrared sources for spectroscopy

Molecules are uniquely identifiable through their absorption spectra in the mid-infrared (Mid-IR). Doing optical measurements in the mid-IR are fast and noninvasive with high sensitivity. While mid-IR sources still remains in full development, we believe microresonator-based frequency combs in the mid-IR present a novel direction for spectroscopy research. Our goal is to build an on-chip mid-IR spectrometer, which will find a broad range of applications including molecular spectroscopy, chemical/biological sensing, free-space communications, and astronomy.

While many useful all-optical devices based on FWM in silicon have been demonstrated in the telecommunications band, the parametric gain and frequency conversion efficiency are fundamentally limited by the nonlinear optical loss mechanisms of two-photon absorption (TPA) and the resulting free-carrier absorption (FCA). By extending the operation of these devices to the mid-IR wavelength region where TPA is suppressed, the advantages of silicon photonics technology can be further leveraged to create chip-scale devices [Opt. Lett. 40, 2778 (2015); Nature Commun. 6, 6299 (2015)]. We have achieved the first demonstration of a modelocked Mid-IR frequency comb in a silicon microresonator [Optica 3, 854 (2016)]. Broadband spectrum spanning 2.4 – 4.3 μm is achieved via dispersion engineering of the waveguide shape. High optical confinement and high Kerr nonlinearity, the key features of silicon nanophotonics, allows for a low optical power consumption of tens of mW for modelocking. We also achieved the highest pump-to-comb conversion efficiency, in which 40% of the pump power is converted to the output comb power. A novel technique of controlling free carriers via an integrated PIN structure is demonstrated for tuning the silicon microresonator and controlling cavity soliton formation. Our results significantly advance microresonator-based comb technology towards a portable and robust mid-IR nonlinear optical device that operates at low pump powers.


One of the powerful spectroscopic techniques based on combs is known as dual-comb spectroscopy which involves interfering 2 combs with near identical comb spacings. It allows for rapid acquisition of spectra with high precision using a single detector, which is particularly important in the mid-IR w here detector arrays are not available. We demonstrate a microresonator-based platform designed for mid-IR dual-comb spectroscopy (DCS) [arXiv:1610.01121 (2017)]. A single continuous-wave (CW) low-power pump source generates two mutually coherent mode-locked frequency combs spanning in two silicon micro-resonators. Thermal control and free-carrier injection control modelocking of each comb and tune the dual-comb parameters. As a proof-of-principle, an experiment of vibrational absorption DCS in the liquid phase is performed with spectra of acetone spanning from 2870 nm to 3170 nm at 127-GHz (4.2-cm-1) resolution. We take a significant step towards a broadband, mid-IR spectroscopy instrument on a chip for liquid/condensed matter phase studies. With further system development, our concept holds promise for real-time and time-resolved spectral acquisition on the nanosecond time scale.


However, OFC's in such miniature devices inherently have large repetition rates typically from 10 to 1000 GHz, which precludes their use for high-spectral-resolution molecular spectroscopy. In order to overcome that, we report the first demonstration of a microresonator-based scanning OFC spectrometer suitable for gas-phase spectroscopy. We demonstrate mode-hop-free tuning of the frequency of a modelocked mid-IR frequency comb in a silicon microreresonator over 16 GHz via simultaneous tuning of temperature and pump laser frequency [Opt. Lett. 42, 4442 (2017)]. The modelocked comb spans 2520 - 4125 cm-1 (2.425 - 3.970 µm) with a comb line spacing of 4.23 cm-1 (127 GHz). The absorption spectrum of acetylene in the gas phase is measured in the ν3 and ν+ (ν+ ν5) bands at a frequency sampling step of 80 MHz despite using a low-resolution FTIR (15 GHz). This technique overcomes the resolution limitation induced by the ultra-small physical size of the integrated device and the instrumental lineshape of the FTIR.


Supercontinuum in silicon-based waveguides

Coherent supercontinuum generation (SCG) has been utilized as a phase-coherent broadband source for biological imaging and molecular detection techniques. A phase-coherent optical spectrum is also critical for applications including pulse compression, frequency metrology, and wavelength division multiplexing. Additionally, coherent supercontinuum (SC) with an octave-spanning bandwidth is highly desirable for the detection of the carrier envelope offset frequency (fceo) of a modelocked laser through a self-referencing scheme using f-2f interferometry, enabling a fully-stabilized frequency comb source. In the f-2f interferometer, the low-frequency portion of the spectrum is frequency-doubled and heterodyned with the high-frequency portion, the generated beatnote allows for extraction of the fceo, which is then referenced to a microwave frequency standard for stabilization. Chip-based SCG offers the potential for cost-efficient, large-scale production, and more importantly, the potential for a completely integrated photonic SC source.


We have demonstrated SCG in both the near-IR using silicon nitride waveguides [Opt. Lett. 37, 1685 (2012)] and the mid-IR using silicon waveguides [Opt. Lett. 39, 4518 (2014)] with octave-spanning bandwidths. However, in order to achieve a fully stabilized frequency comb source, a coherent SC source is necessary. We have investigated the coherence properties of the generated SC spectrum pumped at 1 μm [Opt. Lett. 40, 5117 (2015)] and have characterized and stabilized the fceo of our modelocked pump source using f-2f interferometry [Opt. Express 23, 15440 (2015); Opt. Express 24, 11043 (2016)]. We achieve broadband coherent SC is generated with low pulse energies (~ 36 pJ in the waveguide). This low power requirement stems from the high mode confinement and high nonlinearity at 1 μm which combine to give a high nonlinear coefficient (γ ~ 3 W-1m-1) that allows for efficient SCG.