Optical memory system providing improved focusing control and improved beam combining and separating apparatus

In an optical storage system including an optical storage medium, the combination comprising:

• first means for providing a writing radiation beam for producing optically detectable changes in said medium;
• second means for providing a reading radiation beam for reading said optically detectable changes as a result of changes produced in the reading beam after reflection from said medium;
• means providing for relative movement between said beams and said medium;
• optical detection means for detecting changes in a reflected beam applied thereto;
• focus detection means for detecting the quality of focusing;
• and
• third means to which said beams are applied for combining said recording and reading beams, focusing the combined beams onto predetermined areas of said medium, separating the reflected beams after reflection from said medium, and directing at least a portion of the separated reflected reading beam to said optical detection means and to said focusing means;
• said third means including a spatial combining and separating device comprising a thin transparent element having a reflective coating on one side with an opening in the reflective coating extending to said transparent element, the directions of said beams being chosen in conjunction with the positioning of said element so that the original and reflected writing beams pass through said device in opposite directions via said element and said opening while the original and reflected reading beams impinge on said reflective coating and are reflected thereby;
• and
• said focus detection means including means for separating the beam directed thereto into first and second beams;
• said focus detector also including a first optical detector to which said first beam is directed and a second optical detector to which said second beam is directed, each detector providing an electrical output signal having a magnitude indicative of the diameter of the beam applied thereto;
• said focus detection means further including circuit means to which the outputs of said detectors are applied for producing a focus error signal having a value dependent on the relative values of the detector outputs applied to said circuit means;
• said detectors being constructed, arranged and located so that when said reading beam is in proper focus on said medium said focus error signal is substantially zero and when said reading beam is out of focus in a first direction said focus error has one polarity and when said reading beam is out of focus in the opposite direction said focus error signal has a second polarity;
• and
• control means coupled to said directing and focusing means for varying the focusing of said beams on said medium in response to said focus error signal.