  Cost-efficient and easily integrable broadband mid-infrared (mid-IR) sources
would significantly enhance the application space of photonic integrated
circuits (PICs). Thermal incandescent sources are superior to other common
mid-IR emitters based on semiconductor materials in terms of PIC compatibility,
manufacturing costs, and bandwidth. Ideal thermal emitters would radiate
directly into the desired modes of the PIC waveguides via near-field coupling
and would be stable at very high temperatures. Graphene is a semi-metallic
two-dimensional material with comparable emissivity to thin metallic thermal
emitters. It allows maximum coupling into waveguides by placing it directly
into their evanescent fields. Here, we demonstrate graphene mid-IR emitters
integrated with photonic waveguides that couple directly into the fundamental
mode of silicon waveguides designed for a wavelength of 4,2 {\mu}m relevant for
CO${_2}$ sensing. High broadband emission intensity is observed at the
waveguide-integrated graphene emitter. The emission at the output grating
couplers confirms successful coupling into the waveguide mode. Thermal
simulations predict emitter temperatures up to 1000{\deg}C, where the blackbody
radiation covers the mid-IR region. A coupling efficiency {\eta}, defined as
the light emitted into the waveguide divided by the total emission, of up to
68% is estimated, superior to data published for other waveguide-integrated
emitters.
