A single-layer optical filter made from thin film gallium phosphide (GaP) is envisioned and a fabrication flow is outlined, with current progress on process development reported. Ion-implantation is simulated and performed on bulk GaP with He⁺, followed by a field-assisted thermal bonding technique that simultaneously bonds a thin GaP film onto a borofloat glass substrate and removes the GaP substrate. The resulting thin films have consistent thickness, both within and between runs, and RMS surface roughness of < 10 nm. Dry-etch processes that further reduce the thin film material are characterized and designs for etching gratings into them are developed. This process is shown to be a reliable means of creating thin films of consistent thickness and smoothness in GaP, for the purpose of establishing visible wavelength filters for spectroscopic applications.