We demonstrate integrated optical continuous-travelling-wave parametric amplifiers that significantly surpass the amplification bandwidth of traditional Erbium-Doped Fiber Amplifiers. Using a 5.55-cm-long integrated gallium phosphide waveguide, we achieve up to 35 dB of parametric gain in the small-signal regime, and more than 10 dB of off-chip net gain in the wavelength window spanning approximately 140 nm and centered at 1550 nm, with the maximum value of net gain reaching 25 dB. This is, to our knowledge, the first demonstration of such a large and broadband continuous-wave net gain in a photonic integrated waveguide.
Integrated optics has shown itself very convenient for exploiting nonlinear processes as it results in high confinement factor, freedom of dispersion engineering and compactness. However, the choice of materials is crucial for the development of nonlinear systems. Ideally, one looks for a platform that offers high second and/or third order nonlinearities, low loss and ease of fabrication. Silicon nitride (Si3N4) is now proven to be a good platform for frequency conversion based on third order nonlinearity. Supercontinuum generation (SCG) was obtained in the near-IR and mid-IR regions by pumping waveguides with common fiber lasers. It resulted in broadband coherent combs extending in the mid-IR thanks to dispersive wave generation. Yet, Si3N4 does not exhibit any second order nonlinearity desirable for comb self-referencing via second-harmonic generation (SHG). On the other hand, lithium niobate (LiNbO3) is widely used in integrated photonics for second order nonlinear processes. In our work, we exploit a hybrid Si3N4-LiNbO3 photonic integrated platform that combines maturity and dispersion engineering capabilities of Si3N4 integrated photonics with second-order nonlinear properties of LiNbO3 bypassing challenging lithium niobate etching. We study numerically and experimentally the potential of SCG and SHG for frequency comb self-referencing on this platform when pumping with a fiber laser operating at 2 μm for mid-IR operation, a window useful for sensing as it contains many molecular signatures.
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