JPH01214060A - Ccd photodetection device - Google Patents

Abstract

PURPOSE:To obtain a device having a function to separate and select a laser beam which has been intensity-modulated by a pulse even when a background noise beam changing with the passage of time exists by a method wherein a photodetector and a capacitor which accumulates a photodetecting current as an electric charge are arranged separately as separate elements. CONSTITUTION:A photodetector PD and a capacitor (C) which is used to accumulate a photodetecting current as an electric charge are arranged separately as separate elements. For example, a transfer gate TG is in an OFF state and an electric charge of a capacitor (C) is 0; when a clock phiM is at a HIGH level, M1 and M2 become in an ON state and M3 and M4 become in an OFF state; the capacitor (C) is charged with the photoelectric current flowing through a route of +VCC-M1-PD-M2-C and corresponding to a signal beam + a noise beam. Then, when the clock phiM is at a LOW level, M3 and M4 become in the ON state instead of M1 and M2. Because a laser is in the OFF state during this process, the photoelectric current corresponding to the signal noise flows through a route of C-M3-PD-M4-VEE and discharges the capacitor (O). If the duration of this charging operation and that of this discharging operation are equal, the electric charge corresponding to the signal beam remains in the capacitor (C).

Description

[Detailed Description of the Invention] [Industrial Application] The present invention has a function of detecting pulse-modulated laser signal light, and is used to detect signal light in the presence of background noise light. This invention relates to a CCD light receiving device.

[Prior Art] CCD image sensors are frequently used in facsimile transmitters, television cameras, and the like as devices that convert optical information into electrical signals. The CCD is a charge accumulation type device, and the time integral of the received light 1 is extracted as an electrical signal.

One-dimensional CCD for facsimile and two-dimensional CCD for television.
Since the principle is the same for a dimensional CCD, a one-dimensional CCD will be explained here for simplicity.

Figure 1 shows the basic structure of a CCD image sensor.
The light receiving element PD is a reverse biased PN junction or a MOS diode biased above a threshold voltage value, and also serves as a charge storage capacitor. When this portion is irradiated with light, electron-hole pairs are generated, separated into positive and negative charges by a bias voltage, and stored in a capacitor. After a predetermined integration time, the transfer gate TG is turned on to transfer the accumulated charge to the CCD analog shift register. Thereafter, a transfer lock pulse is applied to the shift register to sequentially transfer the charges from each pixel to the output side and read out each pixel.

The method of measuring distance by irradiating an object with laser light and detecting the scattered reflected light from the object is widely used in rangefinders, industrial robot sensors, and for recognizing the shape of objects. In this method, the distance from the laser beam emission point on the reference line to the point where the scattered reflected light intersects with the reference line (incidence point),
The distance is measured based on the principle of triangulation from three quantities: the emission angle of the laser beam with respect to the reference line, and the incident angle of the scattered reflected light with respect to the reference line. Distance information for each of these points is accumulated, and the shape of the object is recognized through mathematical processing.

On the other hand, when performing distance measurement or object shape recognition in the presence of intense noise light with high frequency components, such as in arc welding, it is necessary to separate the laser light that is the signal and the background light that is the noise. A commonly used method is to modulate laser light at a higher frequency than the frequency components contained in noise light and selectively amplify the received light signal.

However, conventional CCD image sensors do not have frequency selectivity for single intensity modulated laser signals because they cannot distinguish changes in light intensity within the integration time.

[Problems to be solved by the invention] The present invention solves the problem even in the presence of temporally changing background noise light.
The present invention provides a CCD light-receiving device having a function of separating and selecting pulse-intensity-modulated laser light.

[Means for Solving the Problems] To simplify the explanation, it is assumed that the intensity of the laser light is modulated to a modulation degree of 100% by the driving pulse. That is, the laser is completely turned off by the modulation pulse. if,
If the laser is not completely turned off, low-level light emission can be considered as background noise light.
Even if we make this assumption, we do not lose the outermost part.

In the present invention, this laser light is irradiated onto an object, and the scattered reflected light is converted into an electric current by a CCD light receiving element.
This light receiving element is responsible only for photoelectric conversion, and a capacitor for charge storage is provided as a separate element.

When the laser is on, the current corresponding to the signal light is ■, 1
．． . , 1 and the current corresponding to the noise light■. . 1. , is obtained, and the charge storage capacitor is charged with this current. Also, the current IOFF obtained when the laser is OFF is a current equivalent to only noise light. . Turtle 1. The above-mentioned charge is discharged with this current. Assuming that the charging period and the discharging period are equal and Ttat.

IoT+fit I. pp-TitmiIs+
. 1·T ta t, and the signal charge corresponding to only the signal light can be taken out. In a conventional CCD image sensor, the light receiving element and the charge storage capacitor are both used, so I 0N-Ttnt and I opp -Tta
Although it is possible to accumulate the sum of t, it is not possible to calculate the difference between the two.

[Example] The light-receiving element may have a structure similar to that of a CCD image sensor according to the prior art; however, since the light-receiving element in the present invention does not require a charge storage function, it is desirable that the parasitic capacitance is small. , - In the following description, it is assumed that the light receiving element is a reverse biased PN junction, but the operating principle is the same for other light receiving elements. When a reverse-biased PN junction is irradiated with light, a photocurrent flows, but in that state the impedance is large and it can be thought of as a nearly constant current source, so if the electrostatic field xC is charged with this photocurrent, the photocurrent The integral is obtained.

Furthermore, the switch that controls the direction of the photocurrent may be any semiconductor element that has a switching function, or may be a bipolar transistor, but in reality, it is more realistic to use a MOS transistor that is compatible with the CCD. Therefore, the following explanation will be made using a MOS transistor. In the description, the polarity of the power supply may be reversed and at the same time, the N-channel MOS transistor and the P-channel MOS transistor may be replaced.

<Example 1> The first example is shown in Fig. 1. In this example, there is one light receiving element and one charge storage capacitor per pixel, and four switches arranged in each pixel. to control the direction of the photocurrent.

Figure 2 shows the equivalent circuit per pixel. Now, the threshold voltages of N-channel and P-channel MOS transistors are VT).
IN, VTI4P, positive and negative power supply voltages
VEE, the HIGH and LOW levels of clock φM are set to φ
Fight, φML.

First, consider the state in which the transfer gate TG in FIG. 1 is OFF, and assume that the charge on C is 0 at this time.

Assume that the clock φ is HIGH, and φMH is φMH>
VT)IN +VCC, Ml and M2 are ON, M3. M4 is in the OFF state, so the IC goes through the path +VCC-M1-PD-M2-C, and the 9th clock φMakoto, which is charged with a photocurrent corresponding to the signal light and the noise light, goes LOW. If φML<(l VTHP l +VCC), then M4. M5 becomes ON state. At this time, the laser is OFF, so the photocurrent corresponding to the noise light is CM3 PD M4 Vl! s.

flows along the path of and discharges C. If the charging and discharging periods are equal, charge corresponding to the signal light will remain in C.

Next, the transfer gate TG is turned ON, the charge of C is transferred to the CCD analog shift register SR, and the shift clock φ1. φ2 is applied, the charge is transferred to the output side, and read out by the output circuit.

<Example 2> A second example is shown in FIG. 3. In this example, the pixels are:
It consists of two light-receiving elements connected in reverse to each other and one charge storage capacitor. FIG. 4 shows the equivalent circuit per pixel, and the explanation of the same symbols as in FIG. 2 will be omitted. now,
Consider the state in which the transfer gate TG in FIG. 3 is OFF, and assume that the charge on C is O at this time. HI of clock φ support
If the GH level is appropriate, M2 can be turned on and M3 can be turned off at the same time. At this time, Yong IC.

If the LOW level of the sixth clock φM, which is charged by the photocurrent corresponding to the signal light + noise light through the path of 10 Vcc M2-PD2-PDI-MIC, is appropriate,
M2 can be turned OFF and M3 can be turned ON. At this time, the laser is OFF, so the photocurrent corresponding only to the noise light is CMI PDI PD2 M3
It flows through the path of Vat and discharges C. If the charging and discharging periods are equal, charge corresponding to the signal light will remain in C.

Next, the transfer gate TG is turned ON, the charge of C is transferred to the CCD analog shift register SR, and the shift clock φ1. φ2 is applied, the charge is transferred to the output side, and read out by the output circuit.

[Effects of the Invention] According to the present invention, when receiving pulse-modulated laser light to separate background noise light and signal light, a single frequency tuning circuit is not used, and the signal is detected using a clock synchronized with the laser modulation pulse. The light-receiving device according to the present invention is capable of extracting components.1 Since it is not possible to integrate a large number of light-receiving elements on one chip like a normal CCD image sensor, an integrated light-receiving device with a large number of pixels can be used for three-dimensional object shape recognition. It is what makes it possible.

[Brief explanation of the drawing]

Fig. 1 shows the first embodiment of the present invention, Fig. 2 shows the equivalent circuit diagram per pixel, Fig. 3 shows the second embodiment of the invention, and Fig. 4 shows the equivalent circuit diagram per pixel.
The figure is an equivalent circuit diagram per pixel. In the figure, C is a valve-shaped capacitor for accumulating the light-receiving current, and M1 to M6 are MOS
) transistor, where N indicates N-channel type and P indicates P-channel type. PD, PDI, PD2: light receiving element TG: transfer gate SR: CCD analog shift register φ,; laser modulation clock φ1. φ2: CCD analog shift register transfer lock

Claims (1)

[Claims] (1) A CCD in which the light-receiving element and the capacitance for storing the light-receiving current as charge are separated and arranged as separate elements.
Light receiving device