FIELD OF INVENTION

The presently disclosed subject matter relates to an active-pixel sensor array, in general, and in particular to an active-pixel sensor array for an image having a high resolution in one of the dimensions thereof.

BACKGROUND

Active-pixel sensors (APS) include an array of pixel sensors, each pixel containing a photodetector, implanting various technologies such as CMOS, CCD, etc.

In general, it is desired to improve the pixel sensitivity, such that each pixel can generate a sufficient amount of electrons in response to photon strikes on the active surface of the pixel. Thus, increasing the active area of each pixel sensor results in an enhanced sensitivity of the pixel. In some applications, the electrons generated by each pixel are utilized to form a single pixel in a detected image, increasing the area of each pixel requires to either reduce the number of pixels in one of the dimensions of the detected image, or to increase the size of the entire active-pixel sensors. While reducing the number of pixels in the detected image directly reduces the resolution of the image, increasing the size of the array of pixel sensors, requires changing the physical dimensions of the detector.

In some applications, it is required that the pixel array includes a large number of pixels in one dimension of the array (for example 2048), while fewer pixels in the other dimension of the array. Such an array, which provides an image having a high resolution in one of the dimensions thereof, is commonly utilized in line scanners and laser LIDARs.

Accordingly, as shown inFIGS. 1A and 1B, for these applications, it is possible to arrange the photodiodes11aon the array10aalong a first dimension12a(FIG. 1A). In order to increase the sensitivity of each pixel the active region of each pixel sensor11bcan be expanded in a second dimension12b(FIG. 1B). This way the sensitivity of each pixel sensor is increased, while maintaining the resolution of the entire array along the first dimension.

Increasing the dimension of each pixel sensor11bin the second dimension12b, is however limited to a certain dimension by the semiconductor technology, due to technical difficulties related to the readout of pixels, i.e. the so-called diffusion length of charge carriers in a semiconductor.

As shown inFIG. 2, one known solution for increasing the sensitivity of each pixel sensor is binning technology. A pixel array20can include a large number of photodiodes22disposed along one-dimension24athereof. Each pixel sensor22includes an active region which is increased along a second dimension24bof the array, as in the array10bofFIG. 1b. In addition, the photodiodes22in the pixel array20are arranged in a first row25aand a second row25b. The first and second row are extended along the first dimension, and are disposed with respect to one another along the second dimension24b. Each one of the pixels in the first row25ais coupled to the corresponding pixels of the second row25bsuch that the two coupled photodiodes22aand22bare configured for binning operation and the readout of the two coupled photodiodes22aand22bis carried out through a common floating diffusion region (not shown). This way, the active region of each pixel sensor is the sum of the active region of the two adjacent pixels sensors22aand22bin the first row25aand the second row25b, respectively. The arrangement of the photodiodes in two adjacent rows thus allows increasing the sensitivity of the array however without compromising on the resolution of the image obtained by the number of pixel along the first dimension of the array20.

As shown inFIG. 3, grouping of photodiodes in a pixel array30can be carried out by grouping four adjacent photodiodes32a,32b,32c, and32dto pixel sensors41,42,43, etc., disposed along a first dimension38aof the pixel array30.

The pixel array30includes two rows35aand35b, such that pixel sensors41,42,43, etc., include two photodiodes on the first row35aand two photodiodes on the second row35b. Each of the pixel sensors41,42,43, etc. includes a floating diffusion region45configured to allow a common readout of all the photodiodes32a,32b,32c, and32din each of the sensor pixels. It is appreciated that the resolution of the array30of this example is lower than that of array20ofFIG. 2, this is due to the fact that each sensor pixel includes two columns of photodiodes and that the columns are arranged along the same dimension as the dimension along which the array is arranged, i.e. the first dimension38a. In other words, if the number of photodiodes along the first dimension38ais limited, for example, due to space or structural constrains, the resolution of the array will be compromised, if each pixel sensor includes two photodiodes along the first dimension38a. Alternatively, the array must include double the number of photodiodes along the first dimension of the array.

The pixel arrays of the above examples can be utilized in many detector applications, such as Light Detection and Ranging systems (LiDAR) which are used in autonomous vehicles for assessing distances. In the LiDAR systems a target is illuminated with a pulsed laser light, and the reflected pulse is measured.

As shown inFIG. 4, The LiDAR system is configured to send out light pulse, having a pulse time slot40such as a 50 nm pulse, the readout of the detector is carried out at the end of the 50 nm pulse time slot40. In addition, the detector can be configured to control the exposure time of the detector, for example by using a shutter. This way, the detector can be configured to allow capturing light within a capturing time slot14, which can be synchronized with the pulse time slot40, such that the pulse is sent and at the very same time the detector is available to capture the light reflecting from distance objects.

Due to the time of flight of the pulse, however, the reflected pulse46reaches the detector with a delay with respect to the pulse time slot40and capturing time slot44. Consequently, only a front section46aof the reflected pulse46reaches the detector, before the end of the capturing time slot44, and the back section46bof the reflected pulse46, is not read by the detector, or at least is not read within the capturing time slot44.

Accordingly, the distance can be calculated using the equation,

D=12⁢c⁢⁢t0⁢S2S1+S2
for an ideal detector. Where c is the speed of light; t0is the pulse width; i.e. the pulse duration; S1is the amount of the light of the front section46aof the reflected pulse46, i.e. the amount of light which is received by the detector at the pulse timeslot40, and capturing time slot44; and S2is the amount of the light within the back section46bof the reflected pulse46, i.e. the amount of light which is not read by the detector, or at least is not read within the capturing time slot64.

The above distance calculating equation requires however also the S2component of the reflected pulse46, i.e. the back portion46bof the light pulse which did not reach the detector within the pulse timeslot40and the capturing time slot44. Thus, following the initial readout of the detector at the end of the pulse timeslot, a second readout can be carried out after a predetermined time period, i.e. timeslot which can be 50 nm time slot, following the end of the capturing time slot44. The second readout provides the data related to the back portion46bof the light pulse, i.e. the portion of the light which did not make it to the detector during the 50 nm time slot. This way, the two readouts provide both components of the reflected pulse46S1and S2, and the distance of the object can be calculated with the above equation.

Since the above calculation, depends on accurate detection of light reflected from the pulse sent by the laser, the detector can include an array of pixels such as illustrated in one of the examples ofFIGS. 1-3, such that the sensitivity of each pixel in the array is increased while the resolution of the entire array is maintained. It is in particular important to maintain the sensitivity of the array when the vehicle autonomy systems relies on the ability to detect objects at various distances with respect to the vehicle, to determine the distance between the vehicle and the object, and to identify the objects such as identifying pedestrians and cyclists, motor vehicles, side strips, bridge abutments, and road margins, etc.

SUMMARY OF INVENTION

There is provided in accordance with an aspect of the presently disclosed subject matter a range detector for detecting distance of an object including: a light source configured to emit a first light pulse and a second light pulse towards a distant object, the first light pulse being configured for short-range object detection and the second light pulse being configured for long-range object detection; an active pixel sensor having a plurality of pixel elements each of which including at least one photodiode and at least one floating diffusion region configured to receive photoelectric charge from the at least one photodiode, the at least one photodiode being disposed with respect to the light source, such that the first and second pulses are reflected back from the object towards the at least one photodiode; and a controller configured to actuate the light source to selectively emit the first and second light pulses and to determine distance between the object and the active pixel sensor in accordance with photoelectric charge received from the photodiode in response to the first and second light pulses.

The controller can be configured to select between the first and second light pulses in accordance with detection and accuracy requirements.

The controller can be configured to alternately select between the first and second light pulses in predetermined pattern.

The controller can be configured to assess results of detection of an initial light pulse and to determine successive pulse by selecting between the first and second light pulses in accordance with the results.

The initial light pulse can be a short-range pulse and wherein the controller can be configured to select the second light pulse wherein the results indicate that no object is located in the short-range.

The readout time of the floating diffusion region can be synchronized with a time duration of one of the first and second light pulses such that the photodiode detects short-range objects during time duration of the first pulse and detects long-range objects during time duration of the second pulse.

The photodiode can include at least one first photodiode and at least one second photodiode and wherein the at least one floating diffusion region includes a first floating diffusion region configured to receive photoelectric charge from the first photodiode and a second floating diffusion region configured to receive photoelectric charge from the second photodiode.

The controller can be configured to determine distance of a long-range object in accordance with photoelectric charge received from the first photodiode in response to the first light pulse and to determine distance of a short-range object in accordance with photoelectric charge received from the second photodiode in response to the second light pulse. Each the at least one first photodiode can include at least two first photodiodes configured for readout by sad first floating diffusion region, and wherein the at least one second photodiode includes one second photodiode, such that an amount of charges received in the at least two first photodiodes is larger than an amount of charges received in the second photodiode.

There is provided in accordance with an aspect of the presently disclosed subject matter an array of pixel elements arranged in columns along a first dimension of the array. Each of the pixel elements includes at least one first photodiode and at least one second photodiode, the first and second photodiodes having an active area for receiving light radiation; a first floating diffusion region configured to receive photoelectric charge generated in the active area of the first photodiode and a second floating diffusion region configured to receive photoelectric charge generated in the active area of the second photodiode; wherein active area of the second photodiode is larger than active area of the first photodiode.

The at least one first photodiode can include at least two photodiodes and wherein the at least one second photodiode includes one second photodiode.

The array can include at least a first row and a second row of photodiodes and wherein at least one first photodiode includes one first photodiode disposed on the first row and two first photodiodes disposed on the second row, and wherein the at least one second photodiode includes one second photodiode disposed on the first row.

The total active area of the first photodiodes is larger than the active area of the one second photodiode, such that sensitivity of each pixel with respect to light detected by the first photodiode is higher than sensitivity of same pixel with respect to light detected by the second photodiode.

The first floating diffusion region can be configured for readout of the first photodiode at a first frequency and wherein the second floating diffusion region is configured for readout of the second photodiode at a second frequency, and wherein the first and second frequencies are configured such that the first photodiode detects a first light signal and the second photodiode detects a second light signal.

The second photodiode can be coupled to the second floating diffusion region and wherein the two photodiodes includes a selectable photodiode which is selectively coupled to the first floating diffusion region and second floating diffusion region. The selectable photodiode includes a switching element for selecting between the first and second floating diffusion regions.

The array can further include a controller for controlling the switching elements, wherein the controller is configured to configure the switching elements to couple the selectable photodiode to the second floating diffusion region in response to a low readout of the second floating diffusion region increasing thereby an active area of photodiodes coupled to the second floating diffusion region.

