Gigahertz frequency comb offset stabilization based on supercontinuum generation in silicon nitride waveguides

Abstract

Silicon nitride (Si₃N₄) waveguides represent a novel photonic platform that is ideally suited for energy efficient and ultrabroadband nonlinear interactions from the visible to the mid-infrared. Chip-based supercontinuum generation in Si₃N₄ offers a path towards a fully-integrated and highly compact comb source for sensing and time-and-frequency metrology applications. We demonstrate the first successful frequency comb offset stabilization that utilizes a Si₃N₄ waveguide for octave-spanning supercontinuum generation and achieve the lowest integrated residual phase noise of any diode-pumped gigahertz laser comb to date. In addition, we perform a direct comparison to a standard silica photonic crystal fiber (PCF) using the same ultrafast solid-state laser oscillator operating at 1 µm. We identify the minimal role of Raman scattering in Si₃N₄ as a key benefit that allows to overcome the fundamental limitations of silica fibers set by Raman-induced self-frequency shift.

Two-octave spanning supercontinuum generation in stoichiometric silicon nitride waveguides pumped at telecom wavelengths

“…We note that the absence of Raman contributions is a main advantage as compared to SC generation in optical fibers where such non-parametric contributions may introduce noticeable shot-to-shot incoherent output. Furthermore, from the absence of Raman scattering a coherent supercontinuum is expected [35]. We have confirmed coherent generation explicitly with calculation of the shot-to-shot coherence for the supercontinua presented in Fig.…”

“…The latter is central for narrowband spectral filtering and optical delay lines as in quantum optical systems [31], in microwave photonic filters [32,33] or in programmable optical processors [34]. Finally, as Si 3 N 4 has negligible Raman gain, highly coherent supercontinua can be generated using low pump energies [35]. So far in this platform SCG has been mainly investigated with a relatively short pump wavelength around 1 µm, yielding spectral coverage mainly at shorter wavelengths from the blue and across the visible into the near infrared [35][36][37][38] .…”

High power sub-200fs pulse generation from a colliding pulse modelocked VECSEL

“…First, the high finesse of the external optical cavity can provide a high power circular beam with a very low divergence, close to the diffraction limit. 1 A high beam quality is particularly important for non linear applications, such as supercontinuum or frequency comb generation 2 where an efficient coupling into a non linear medium (single mode fiber, semiconductor waveguide, etc) is necessary. It is also very important for applications requiring high power density such as multi-photon imaging, 3 or material processing.…”

Structure and cavity geometry optimization for ultrashort and high power pulse generation from a VECSEL

“…However, the continuing improvements of VECSEL performance in term of power and pulse duration open up the possibility of a direct supercontinuum generation without prior amplification, in a highly non-linear silicon nitride or tantalum pentoxide waveguide for example. 3 In this proceeding, we describe the strategy employed to shorten the pulse duration while keeping a high output power from a passively modelocked VECSEL. We present the design of the gain and saturable absorber structure, and we study the influence of the cavity geometry on the pulse duration and peak power achievable, as well as on the robustness of the modelocking regime when the laser parameters are changed.…”