Optical heads wherein a laser diode (LD) is optically coupled through a small aperture to a recording medium acting as an external mirror are described. A flying head and a fiber- coupled head are presented, and their high-sensitivity, wavelength-free detection characteristics are demonstrated. A micro-lens head is also investigated. Interference undulations of the coupled-cavity laser with various spatial frequencies due to variations in the external-cavity length are completely explained. Anti-reflection (AR) coatings on the LD facet with reflectivities of better than -40 dB greatly suppress these undulations. To achieve practical heads, fundamental fabrication techniques including AR coating and monolithic integration on laser wafers are briefly mentioned.