  We present an integrated hybrid semiconductor-dielectric (InP-Si$_3$N$_4$)
waveguide laser that generates frequency combs at a wavelength around 1.5
$\mu$m with a record-low intrinsic optical linewidth of 34 kHz. This is
achieved by extending the cavity photon lifetime using a low-loss dielectric
waveguide circuit. In our experimental demonstration, the on-chip, effective
optical path length of the laser cavity is extended to 6 cm. The resulting
linewidth narrowing shows the high potential of on-chip, highly coherent
frequency combs with direct electrical pumping, based on hybrid and
heterogeneous integrated circuits making use of low-loss dielectric waveguides.
