Ultrasensitive Image Sensor Counts Individual Photons within Picoseconds

When it comes to imaging, every single photon counts if there is barely any available light. This is the point where the latest technologies often reach their limits. Researchers have now developed a diode that can read photons faster than ever before.

Fast and ultrasensitive optical systems are gaining increasing significance and are being used in a diverse range of applications, for example, in imaging procedures in the fields of medicine and biology, in astronomy and in safety engineering for the automotive industry. Frequently the challenge lies in being able to record high-quality images under extremely low light conditions. Modern photo detectors for image capture typically reach their limits here. They frequently work with light-sensitive electronic components that are based on CMOS (Complementary Metal Oxide Semiconductor) or CCD (Charge-Coupled Device) image sensors. The problem is that neither the latest CMOS nor CCD systems can simultaneously guarantee a swift and highly-sensitive high quality image recording if there is a paucity of photons to read.

In cooperation with the partners of the MiSPiA project consortium the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg has now advanced the development of CMOS technology and introduced an ultrasensitive image sensor with this technology, based on Single Photon Avalanche Photodiodes (SPAD). Its pixel structure can count individual photons within a few picoseconds, and is therefore a thousand times faster than comparable models. Since each individual photon is taken into consideration camera images are also possible with extremely weak light sources.

Camera installed directly on chip
To achieve this the new image sensor uses the “internal avalanche breakdown effect”– a photoelectric amplification effect. The number of “avalanche breakdowns” corresponds to the number of photons that the pixels hit. In order to count these events, each of the sensor’s pixels comes with very precise digital counters. At the same time, the scientists have applied microlenses to each sensor chip, which focus the incoming beam in each pixel onto the photoactive surface. Another advantage is that processing the digital image signals is already possible directly on the microchip; therefore, additional analogue signal processing is no longer needed.

“The image sensor is a major step toward digital image generation and image processing. It allows us to have the capability to use even very weak light sources for photography. The new technology installs the camera directly on the semiconductor, and is capable of turning the information from the light into images at a significantly faster pace,” states Dr. Daniel Durini, group manager for optical components at the Fraunhofer Institute IMS.

IMS engineered the sensor under the European research project MiSPiA (Microelectronic Single-Photon 3D Imaging Arrays for low-light high-speed Safety and Security Applications). Altogether, seven partners throughout Europe from the fields of research and business are involved in the project. In the next stage, the scientists from Duisburg are working on a process to produce sensors that are back-lighted, and in this regard, even more powerful. At the same time, the new technology is already being utilized in tests for traffic. Chip-based mini-cameras protect vehicles, bicycles and pedestrians from collisions and chip-based accidents, or assist in the reliable functioning of safety belts and airbags.

The key benefits of rain sensors used in irrigation system are as follows:

·        Inexpensive

·        Less installation time

·        Improved performance and reliability.

Rain Sensors for Automotive

The windshield is mounted with rain sensor to detect the movement of wiper and rain without disturbing the view of the driver. This sensor device can control the wiping actions at variable speeds when the system is switched on. Generally, the following two different sensor systems are used:

·        Rain sensors in a network: These sensors are linked to a bus system through which all information and commands can be sent and received.

·        Stand-alone rain sensors: These rain sensors are directly connected to the wiper motor park signal, wiper motor relays and wiper column switch.

Evaluation of Analog Signal

The following section will discuss in detail about the components used for evaluating analog signal.

Transmitter

The transmitter comprises a current source, a digital-analog converter (DAC) and LEDs. It is necessary to regulate the transmitter current as the light conversion efficiency is variable. In an attempt to increase the current and reduce power loss at LEDs, light from the LEDs is pulsed.

Receiver

The receiver is provided with an amplifier, a current-voltage converter, LRDs, a filter to avoid low frequency or dc offsets and an analog-digital converter (ADC), which is included in a microcontroller. The microcontroller switches on or off the LRD to control the optical path. A variable current- voltage converter is used for wide transmission ranges. The signal is filtered after conversion and then amplified. The amplified signal is converted using ADC.

Microcontroller

The entire system is controlled by the microcontroller, which also performs evaluation of the signal. The best operating point is determined before performing the measurements. A signal is generated by the microcontroller between the upper and lower limits at the receiver. At this point, the sensor starts functioning. The signal and disturbance is evaluated repeatedly. Further, an extra sensor measures ambient light for identifying day/night conditions.