Photo detecting device

Abstract: A photo detecting device (5) has a photoelectric converting element (7) formed on a semiconductor chip with the photoelectric converting element (7) being divided into first and second light receiving regions (7a, 7b), and the photo detecting device further has a signal processing circuit formed on the same semiconductor chip, with the signal processing circuit including subtracting means (A₁) for subtracting an output signal from the second light receiving region (7b) from an output signal from the first light receiving region (7a).

Description: The present invention relates to a photodetecting device comprising a photoelectric converting element formed on a semiconductor chip, said photoelectric converting element divided into first and second light receiving regions and a signal processing circuit including subtracting means for subtracting an output signal from said second light receiving region from an output signal of said first receiving region. Said photo detecting device can be used in an optical pickup for an optical disk apparatus and the like. Such a photo detecting device is known from EP-A-0201917. Description of the Background Art An optical pickup used in a conventional optical disk apparatus comprises a photoelectric conversion device formed on a semiconductor chip for receiving light reflected from a recording medium to convert the same into electric signals. The electric signals outputted from such a photoelectric conversion device are subjected to processes such as arithmetic amplification in a signal processing circuit out of the semiconductor chip to be converted into information reproducing signals, focus error signals and tracking error signals. Recent technique of integrated circuits has developed an OPIC (Opticals Integrated Circuit) photo detecting device (JP-A-1-271918), in which the signal processing circuit as well as the photoelectric conversion device are formed on one semiconductor chip, which has made simple the design of the signal processing circuit. Fig. 4 shows one example of the conventional OPIC photo detecting device. The photo detecting device comprises three photoelectric converting elements 11, 12 and 13. The central photoelectric converting element 11 has four divided light receiving regions 11a, 11b, 11c and 11d. The output signals from the light receiving regions 11a to 11b are connected to minus input terminals of differential amplifiers A₁₁, A₁₂, A₁₃ and A₁₄, respectively, and to an input terminal of an amplifier A₁₅ through coupling capacitors C₁₁, C₁₂, C₁₃ and C₁₄. The differential amplifiers A₁₁ to A₁₄ have their plus input terminals all grounded. The photo electric converting elements 12 and 13 on both ends are connected to minus input terminals of the differential amplifiers A₁₆ and A₁₇, respectively. The differential amplifiers A₁₆ and A₁₇ have their plus input terminals grounded. In the photo detecting device of Fig. 4, signals Sa, Sb, Sc and Sd outputted from the light receiving regions 11a to 11d are amplified by the differential amplifiers A₁₁ to A₁₄, respectively, to be externally outputted. In an external circuit (not shown), the following arithmetic operation is carried on the signals Sa to Sd to provide a focus error signal FES. FES = (Sa + SC) - (Sb + Sd) The signals Sa to Sd outputted from the light receiving regions 11a to 11d have their fluctuation of low frequency components removed through coupling capacitors C₁₁ to C₁₄, and thereafter they are added to each other. A reproduction signal RF is provided by amplifying the summed signal (Sa + Sb + Sc + Sd) by the amplifier A₁₅. Further, signals Se and Sf outputted from the photoelectric converting elements 12 and 13 on both ends are amplified by the differential amplifiers A₁₆ and A₁₇, respectively, to be externally outputted, and the tracking error signal TES is provided by a subtracting operation by an external circuit. In the photo detecting device of Fig. 4, a plurality of operational amplifiers must be provided in the external circuit for obtaining the focus error signal FES. It is difficult to provide the same characteristics to all of these plurality of operational amplifiers in the external circuit. Accordingly, when the ambient temperature changes, an offset voltage tends to be generated in the focus error signal FES obtained from the external circuit at the in-focus state. The offset voltage prevents accurate focus control. In addition, since the photoelectric converting element 11 at the center comprises four light receiving regions 11a to 11d, four output terminals are necessary to take out signals Sa to Sd to the outside from the light receiving regions. This increases the number of the output terminals of the photo detecting device itself, which makes it difficult to reduce the size of the photo detecting device. In addition, when the circuit elements are to be formed on an IC chip, a capacitor requires relatively large area. Threfore, when four coupling capacitors C₁₁to C₁₄ are provided, these capacitors occupy 1/8 to 1/4 of the whole area of the IC chip, which naturally make the IC chip larger. This further presents reduction in size of the photo detecting device. SUMMARY OF THE INVENTION In view of the foregoing, an object of the present invention is to provide a photo detecting device which is compact and in which an offset voltage in a focus error signal is hardly generated. The present invention is defined in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing an OPIC photo detecting device in accordance with one embodiment of the present invention; Fig. 2 shows main components of an optical pickup including the photo detecting device of Fig. 1; Figs. 3A, 3B and 3C show a relation between the state of optical beam spot projected on photo detecting elements of the photo detecting device of Fig. 1 and focusing; and Fig. 4 schematically shows a conventional OPIC photo detecting device. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, an OPIC photo detecting device in accordance with one embodiment of the present invention is shown. The photo detecting device 5 comprises four photoelectric converting elements 7, 8, 9 and 10. The photoelectric converting element 7 is divided into two light receiving regions 7a and 7b. An output signal from the light receiving region 7a is connected to a plus input terminal of a differential amplifier A₁ serving as a subtracting means, and an output signal of the other light receiving region 7b is connected to a minus input terminal of the same differential amplifier A₁. An output signal from the photoelectric converting element 8 formed of a single light receiving region is connected to an input terminal of an amplifier A₂. The light receiving region 7a is further connected to an input terminal of the amplifier A₂ through a coupling capacitor C₁ and a resistance R₁ parallel to the coupling capacitor C₁. The other light receiving region 7b is connected to the input terminal of the amplifier A₂ through a coupling capacitor C₂ and a resistance R₂ parallel to the coupling capacitor C₂. Output terminals of the differential amplifier A₁ and of the amplifier A₂ are drawn to the outside as outputs of the photo detecting device 5. Output signals of the remaining photoelectric converting elements 9 and 10 are connected to minus input terminals of differential amplifiers A₃ and A₄. The differential amplifiers A₃ and A₄ have their plus input terminals grounded. The output terminals of the differential amplifiers A₃ and A₄ are drawn to the outside as outputs of the photo detecting device 5. Fig. 2 shows main components of a hologram type optical pickup using the photo detecting device of Fig. 1. The principle of operation of the hologram type optical pickup is disclosed in detail in United States Patent No. 4,835,378, for instance. Referring to Fig. 2, an optical beam emitted from a light source 1 is divided into three sub beams along a line by a diffraction grating 2a below a diffracting apparatus 2. The sub beams are converted into parallel sub beams by a collimator lens 3. The parallel sub beams are focused on an information surface of a disk 6 by an objective lens. The three reflected beams from the disk 6 enter the objective lens 4 and pass through the collimator lens 3. The three reflected beams which passed through the collimator lens 3 are diffracted toward the photo detecting device 5 by a diffraction grating 2b above the diffracting apparatus 2. On this occasion, each of the three reflected beams along one line is divided into two sub beams. Namely, one of the sub beams divided from the central reflected beam, used for reading recorded information and for focusing, enters the photoelectric converting element 7, and the other sub beam enters the photoelectric converting element 8. Two sub beams divided from one of the reflected beams on both ends used for tracking both enter the photoelectric converting element 9. Two sub beams divided from the other one of the reflected beams on both sides both enter the photoelectric converting element 10. Figs. 3A, 3B and 3C show a relation between focusing and the state of the optical beam spot projected on the photoelectric converting element 7. When the information recording surface of the optical disk 6 is at an in-focus position, the optical spot P is a small circle crossing the boundary between two light receiving regions 7a and 7b, as shown in Fig. 3B. When the optical disk 6 moves away from the objective lens 4, the optical spot P becomes an enlarged semicircle on the light receiving region 7b, as shown in Fig. 3A. On the contrary, when the optical disk 6 comes near to the objective lens, the optical spot P become an enlarged semicircle on the light receiving region 7a, as shown in Fig. 3C. Namely, the difference between outputs from the light receiving regions 7a and 7b can be used as a focus error signal corresponding to deviation of the information recording surface of the disk 6 from the in-focus position. The reflected sub beam projected on the light receiving regions 7a and 7b are converted into signals of the magnitudes corresponding to the intensity of light received by the light receiving regions 7a and 7b to be outputted, respectively, and thereafter the output signal from the light receiving region 7b is subtracted from the output signal from the light receiving region 7a by the differential amplifier A₁. The focus error signal provided by this subtraction is externally outputted. The output signals from the light receiving regions 7a and 7b with the low frequency component thereof removed by coupling capacitors C₁ and C₂ are added to the output signal from the photoelectric converting element 8. The summed signal is amplified by the amplifier A₂ to be externally outputted as a reproduction signal. In addition, DC component of the output signals from the light receiving regions 7a and 7b are added to the DC component of the output signal from the photoelectric converting element 8 through the resistances R₁ and R₂, and the added DC components are also detected. The DC components are used for distinguishing the information surface and the protection surface of the optical disk. The output signals of the photoelectric converting elements 9 and 10 are amplified by differential amplifiers A₃ and A₄, respectively, to be outputted. The signals amplified by the differential amplifiers A₃ and A₄ are subtracted by an external circuit (not shown), to be used as the tracking error signal. As described above, the focus error signal can be provided by arithmetic operation in the photo detecting device. Therefore, generation of an offset voltage in the focus error signal derived from the change in ambient temperature can be suppressed. In addition, since the number of output terminals for the focus error signal outputted outside of the photo detecting device is 1, and the reduction of the number of the output terminal enables reduction of the photo detecting device in size. In addition, since the focus error signal is calculated in the photo detecting device, a circuit for calculating the focus error signal can be removed from the external circuit. Accordingly, when the photo detecting device is used built in the optical pickup, the design of the external circuit can be simplified. In addition, only two coupling capacitors are provided between the two light receiving regions 7a and 7b and the single light receiving region 8, the ratio of the area occupied by the coupling capacitors to the total area of the photo detecting device becomes smaller, which further reduces the size of the photo detecting device.

Claims: A photo detecting device comprising: a photoelectric converting element (7) formed on a semiconductor chip, said photoelectric converting element (7) divided into first and second light receiving regions (7a, 7b), an additional photoelectric converting element (8) formed on said semiconductor chip a signal processing circuit including: subtracting means (A₁) for subtracting an output signal from said second light receiving region (7b) from an output signal of said first receiving region (7a) to provide a focus error signal FES, characterized in that said signal processing circuit is formed on the same semiconductor chip and further includes: low frequency component removal means (C₁, C₂) coupled between said output signals from said first and second light receiving regions (7a, 7b) respectively, and the output signal from said photoelectric converting element (8), for adding said output signals from said first and second light receiving regions (7a, 7b) with the low frequency components thereof removed, to the output signal of said additional photoelectric converting element (8), to provide a reproduction signal RF, and DC-component removal means including first and second resistors (R₁ and R₂) coupled between said output signals from said first and second light receiving regions (7a, 7b) respectively, and said output signal from said additional photoelectric converting element (8) for adding the DC component of said output signals from said first and second light receiving regions (7a, 7b) to said reproduction signal RF. The photo detecting device of claim 1, said low frequency component removal means comprising first and second capacitors (C₁, C₂) coupled between said output signals from said first and second light receiving regions (7a, 7b) respectively, and said output signal from said additional photelectric converting element (8).

Inventors: NISHIHARA HIKARU, YOKOTA TAIZO, YOSHIFUSA KOJI, NISHIHARA, HIKARU, YOKOTA, TAIZO, YOSHIFUSA, KOJI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1990
Main classification: G11B7
IPC codes: H04N5, G11B7