Apparatus and method for recording and reproducing information from a plurality of recording medium having different thicknesses and recording densities

Abstract

A laser diode applies a laser beam through a grating, a beam splitter, and an objective to a compact disc. A laser beam reflected by the compact disc is applied through the objective and the beam splitter to a photodiode. The objective has a large numerical aperture of 0.6 in order to be able of play back a digital video disc that has a thin substrate and contains information recorded at a high density. To prevent the photodiode from suffering aberrations caused due to the large numerical aperture of the objective, the photodiode has a relatively small photodetector unit having a normalized detector size ranging from 3 μm to 16 μm, for detecting only returning light up to a numerical aperture of 0.3 and not detecting light of greater numerical aperture values.

What is claimed is:  
     
       1. An apparatus for recording information selectively on and reproducing information selectively from a first recording medium having an information recording layer on a substrate having a first thickness or a second recording medium having an information recording layer on a substrate having a second thickness, comprising: 
       a light source which generates light to be applied to the information recording layer of the first or second recording medium;  
       an optical system which converges the light generated by said light source and applies the converged light to said information recording layer of the first or second recording medium; and  
       a photodetector for detecting light returning from said information recording layer of the first or second recording mediums;  
       wherein said photodetector has a size of at least  10 mm μm  and at most  14 mm μm , which is normalized based on a magnification of said optical system; and  
       wherein output from said photodetector is supplied to a focusing servo circuit which outputs in response thereto a focusing servo signal, said output from said photodetector also being supplied to a circuit which generates therefrom a signal representative of information recorded on said information recording layer of the first or second recording mediums.  
     
     
       2. An apparatus according to  claim 1 , wherein output of said photodetector is supplied to a circuit which produces therefrom a signal representative of information recorded on said recording layer of the first or second recording mediums. 
     
     
       3. An apparatus for recording information selectively on and reproducing information selectively from a first recording medium having an information recording layer on a substrate having a first thickness or a second recording medium having an information recording layer on a substrate having a second thickness, comprising: 
       a light source which generates light to be applied to the information recording layer of the first or second recording medium;  
       an optical system which converges the light generated by said light source and applies the converged light to said information recording layer of the first or second recording medium; and  
       a photodetector for detecting light returning from said information recording layer of the first or second recording mediums;  
       said photodetector having a normalized detector size of at least  3 mm μm  and at most  16 mm μm ;  
       wherein output from said photodetector is supplied to a focusing servo circuit which outputs in response thereto a focusing servo signal, said output from said photodetector also being supplied to a circuit which generates therefrom a signal representative of information recorded on said information recording layer of the first or second recording mediums.  
     
     
       4. An apparatus according to  claim 1 , wherein said first recording medium has a recording density greater than said second recording medium. 
     
     
       5. An apparatus according to  claim 1 , further comprising: 
       a comparator which receives an output signal of said photodetector and distinguishes between said first recording medium and said second recording medium using said output signal of said photodetector; and  
       an equalizer which receives an output signal from said comparator and equalizes an output signal from said photodetector in response to whether said first recording medium or said second recording medium is being read.  
     
     
       6. An apparatus for recording information selectively on and reproducing information selectively from a first recording medium having an information recording layer on a substrate having a first thickness or a second recording medium having an information recording layer on a substrate having a second thickness, comprising: 
       a light source which generates light to be applied to the information recording layer of the first or second recording medium;  
       an optical system which converges the light generated by said light source and applies the converged light to said information recording layer of the first or second recording medium;  
       a photodetector for detecting light returning from said information recording layer of the first or second recording mediums, said photodetector having a normalized detector size of at least  3 mm μm  and at most  16 mm μm ;  
       a comparator which receives an output signal of said photodetector and distinguishes between said first recording medium and said second recording medium using said output signal of said photodetector; and  
       an equalizer which receives an output signal from said comparator and equalizes an output signal from said photodetector in response to whether said first recording medium or said second recording medium is being read;  
       wherein said comparator distinguishes between said first recording medium and said second recording medium by comparing a level of the output signal from said photodetector with a predetermined value.  
     
     
       7. An apparatus according to  claim 1 , further comprising: 
       a comparator which receives an output signal of said photodetector and distinguishes between said first recording medium and said second recording medium using said output signal of said photodetector; and  
       a control circuit which receives an output signal of said comparator and which controls said optical system according to different tracking control processes when said first recording medium is read and when said second recording medium is read, as determined by said comparator.  
     
     
       8. An apparatus for recording information selectively on and reproducing information selectively from a first recording medium having an information recording layer on a substrate having a first thickness or a second recording medium having an information recording layer on a substrate having a second thickness, comprising: 
       a light source which generates light to be applied to the information recording layer of the first or second recording medium;  
       an optical system which converges the light generated by said light source and applies the converged light to said information recording layer of the first or second recording medium;  
       a photodetector for detecting light returning from said information recording layer of the first or second recording mediums, said photodetector having a normalized detector size of at least  3 mm μm  and at most  16 mm μm ;  
       a comparator which receives an output signal of said photodetector and distinguishes between said first recording medium and said second recording medium using said output signal of said photodetector;  
       an equalizer which receives an output signal from said comparator and equalizes an output signal from said photodetector in response to whether said first recording medium or said second recording medium is being read;  
       a binary signal generator which receives an output signal from said equalizer and which produces therefrom a binary signal representative of information recorded on said first or second recording medium;  
       a demodulator which receives and demodulates an output signal from said binary signal generator; and  
       a phase locked loop circuit which receives said output signal from said comparator and which provides a first clock signal to said demodulator when said first recording medium is being read and a second clock signal, different from said first clock signal, when said second recording medium is being read.  
     
     
       9. An apparatus according to  claim 1 , further comprising a processing circuit which processes an output signal of said photodetector, said processing circuit comprising: 
       a comparator which receives said output signal of said photodetector and determines therefrom whether said first or said second recording medium is being used.  
     
     
       10. An apparatus according to  claim 9 , further comprising: 
       a clock circuit which receives an output signal from said comparator and which provides a first clock signal to said processing circuit when said first recording medium is being used and a second clock signal, different from said first clock signal, when said second recording medium is being used.  
     
     
       11. An apparatus according to  claim 9 , further comprising: 
       a smoothing circuit which receives said output signal of said photodetector, smoothes said output signal of said photodetector and provides said output signal of said photodetector to said comparator.  
     
     
       12. An apparatus for recording information selectively on and reproducing information selectively from a first recording medium having an information recording layer on a substrate having a first thickness or a second recording medium having an information recording layer on a substrate having a second thickness, comprising: 
       a light source which generates light to be applied to the information recording layer of the first or second recording medium;  
       an optical system which converges the light generated by said light source and applies the converged light to said information recording layer of the first or second recording medium;  
       a photodetector for detecting light returning from said information recording layer of the first or second recording mediums, said photodetector having a normalized detector size of at least  3 mm μm  and at most  16 mm μm ;  
       a comparator which receives an output signal of said photodetector and distinguishes between said first recording medium and said second recording medium using said output signal of said photodetector; and  
       a control circuit which receives an output signal of said comparator and which controls said optical system according to different tracking control processes when said first recording medium is read and when said second recording medium is read, as determined by said comparator;  
       wherein:  
       said first thickness is greater than said second thickness;  
       said tracking control process is a three beam tracking process when said first recording medium is being used; and  
       said tracking control process is a differential push-pull tracking process when said second recording medium is being used.