Apparatus for monitoring the focus condition of a writing beam.

摘要

Apparatus for monitoring the focusing condition of the writing light beam (46) impinging the writing surface (48) of a recording medium (50) which operates on the principal that, although for all practical purposes a light beam from a laser device is collimated, it is in fact very slightly divergent as it emerges from the laser with a focal resonator because of diffraction of the Gaussian intensity profile plane wave that exists between the laser resonator mirrors at the beam waist and from refraction as the output light traverses the thickness of the output mirror. Accordingly, since the beam exciting the laser tube (2) is slightly divergent, the reflected light from the video disc (50) even though small in intensity, is conversely (provided the system is in focus) convergent as it passes in reverse direction through the diverging lens (34) and on toward the output of the laser device and beyond to the conjugate point where it forms a real image of the source plane wave, thus eliminating the need for a relay lens. The present invention makes use of this slight divergence and convergence of the incident and reflected beams, respectively, at the exit end of the modulator, to effect monitoring of the focus condition of the writing beam.