A heat-assisted magnetic recording (HAMR) head in which the core of the optical waveguide has an end face that abuts the NFT and the write pole lip has a diffusion barrier between the end face of the waveguide core and the write pole lip. The diffusion barrier layer may also be located between the waveguide core end face and the NFT, in which case it is formed of an optically transparent material, like TaNx, TiNx, ZrNx, HfNx, NbNx, CrNx, VNx, TiC, TaC, WC, SiC or SiNx, or a layer of Au, Ru, Rh or Ir with a thickness less than 5 nm. In addition to being located between both the NFT and the write pole lip and the waveguide core end face, the diffusion barrier layer may also be located between the waveguide core and the lower waveguide cladding layer.