  Raman lasers based on bulk diamond are a valuable resource for generating
coherent light in wavelength regimes where no common laser diodes are
available. Nevertheless, the widespread use of such lasers is limited by their
high threshold power requirements on the order of several Watts. Using on-chip
microresonators, a significant reduction of the lasing threshold by more than
two orders of magnitude has been shown. However, these resonators lack a
continuous tuning mechanism and, mainly due to fabrication limitations, their
implementation in the visible remains elusive. Here, we propose a platform for
a diamond Raman laser in the visible. The device is based on a miniaturized,
open-access Fabry-Perot cavity. Our microcavity provides widely-tunable
doubly-resonant enhancement of Raman scattering from high quality
single-crystalline diamond. We demonstrate a $>$THz continuous tuning range of
doubly-resonant Raman scattering, a range limited only by the reflective
stopband of the mirrors. Based on the experimentally determined quality factors
exceeding $300\,000$, our theoretical analysis suggests that, with realistic
improvements, a sub-mW threshold is readily within reach. Our findings pave the
way to the creation of a universal low-power frequency shifter, a potentially
valuable addition to the nonlinear optics toolbox.
