Critical angle focus error detector

A focus error detector for detecting a focus error of an objective lens which focuses incident collimated light onto an optical disk, said focus error detector comprising:

• a diffraction device for diffracting reflected light reflected from said optical disk and passed through said objective lens into first and second diffracted light rays;
• critical angle reflecting means for reflecting said first and second diffracted light rays at first and second critical angles, respectively;
• first and second bi-segmented photo-detectors for receiving said first and second diffracted light rays reflected from said critical angle reflecting means, respectively, and for detecting signals depending on an amount of light received;
• and
• circuit means including first and second differential amplifiers for differentiating the signals detected by said first and second bi-segmented photodetectors and for generating respective output signals, said circuit means further including a summing amplifier connected to the first and second differential amplifiers for summing the output signals of the differential amplifiers so as to offset a light amount difference due to tilt or shift of the reflected light rays and for outputting a signal representing a degree of focus error.