Photonic-Chip Supercontinuum with Tailored Spectra for Counting Optical Frequencies

Abstract

We explore a photonic-integrated-circuit platform that implements optical-frequency measurements and timekeeping with a perspective towards next-generation portable and space-borne frequency references and optical-clock networks. The stoichiometric-silicon-nitride waveguides we create provide an efficient and low-noise medium for nonlinear spectral broadening and supercontinuum generation with fiber-based optical frequency combs. In particular, we demonstrate detailed control over supercontinuum emission to target specific atomic-transition wavelengths and perform the first optical-clock comparison using on-chip supercontinuum sources. We report a clock-limited relative frequency instability of 3.8 × 10 −15 at τ = 2 seconds between a 1550 nm cavity-stabilized reference laser and NIST's calcium atomic-clock laser at 657 nm using a two-octave waveguide-supercontinuum frequency comb.

Self-organized nonlinear gratings for ultrafast nanophotonics

“…Details regarding the 600-nm thickness waveguides are provided in Ref. 11, while details of the 700-nm thickness waveguides are provided in Ref. 12.…”

“…Here we show that dispersion-engineered SiN photonic waveguides, which are currently enabling breakthroughs for ultrafast χ (3) nonlinear optics [11][12][13][14][15] , can also serve as a versatile platform for χ (2) nonlinear optics with femtosecond pulses. Leveraging the high effective-indexcontrast of air-top-clad SiN waveguides 12 , we achieve group-velocity matching for SHG by engineering the waveguide cross section.…”

Quasi-Phase-Matched Supercontinuum Generation in Photonic Waveguides

“…SCG has numerous applications, including self-referencing frequency combs [4][5][6], microscopy [7], spectroscopy [8], and tomography [9]. SCG is traditionally accomplished using bulk crystals or nonlinear fiber, but recently, "photonic waveguides" (on-chip waveguides produced using nanofabrication techniques) have proven themselves as a versatile platform for SCG, offering small size, high nonlinearity, and increased control over the generated spectrum [10][11][12][13][14][15][16][17][18][19]. The spectral shape and efficiency of SCG is determined by the input pulse parameters, the nonlinearity of the material, and the refractive index of the waveguide, which determines the phasematching conditions.…”

“…These spectral peaks are often referred to as a dispersive waves (DWs) [2,[20][21][22], and they are often crucial for providing sufficient spectral brightness for many applications. The soliton-DW phase-matching condition is typically satisfied by selecting a material with a favorable refractive index profile and engineering the dimensions of the waveguide to provide DWs at the desired wavelengths [15,16]. However, there are limitations to the refractive index profile that can be achieved by adjusting only the waveguide cross-section.…”

Optical-Frequency Measurements with a Kerr Microcomb and Photonic-Chip Supercontinuum

“…Then, amplification in an erbium-doped fiber amplifier (EDFA) and pulse compression in normal-dispersion highly nonlinear fiber (HNLF) is utilized to achieve pulses with energies up to 180 pJ and durations of 100 to 120 fs. These pulses are used to generate coherent supercontinuua in silicon-nitride waveguides, which have recently been demonstrated as suitable platforms for allowing frequency comb self-referencing and precision metrology with low-pulse-energy lasers [22][23][24].…”