Photonic detectors for distance measurement

Jon Lawson

Distance measurement using light is important in diverse applications such as handy rangefinders, sports equipment, robot vacuum cleaners, factory automation, surveying/mapping and even autonomous vehicles.

These applications require photodetectors that detect the presence of objects through the light they reflect.

Hamamatsu offers a variety of cutting edge, high-sensitivity photodetectors that can be used for short-, mid-, and long-range detection. These include silicon PIN photodiodes, silicon and InGaAs avalanche photodiodes (APD), silicon photomultipliers (SiPM), and CMOS distance image sensors. In particular a new near infrared (NIR) enhanced Silicon Photomultiplier is a key technology for industrial distance measurement applications.

A LIDAR device works with a light source such as a pulsed IR laser diode, which sends out short light pulses.

Some systems are scanning continuously with a mirror and other systems illuminate the whole area at once.

The emitted light is reflected from every surface of the obstacles in its path, with a reflection ratio depending on the material and colour.

A sensor array detects the returning light, determining the time of flight needed to travel from the light source to the object, and back to the detector.

Knowing that the speed of light is 3*108 m/s, this gives the distance of objects. For example in the automotive sector then the sensor array in civil vehicles is usually a Silicon Photodiode or APD array, Silicon is mainly used for cost reasons, operating in the near infrared range between 800 – 950nm. InGaAs can also be used for the sensor, in combination with a laser diode at higher IR wavelength, for example at 1550nm.

For short range measurements, we also offer a time-of-flight (TOF) CMOS sensor, which works with an indirect TOF measurement as a 3D camera. In this case an infrared laser diode or LED, or an array of these, is used as active illumination and sends out short light pulses.

The CMOS sensor determines the time of flight of the reflected, incident light with each of its pixels, creating a 3D picture. This CMOS sensor is a special, high-speed charge transfer device, which is suitable for distances up to about 10m.

Silicon PIN photodiodes are best for short-range detection. They offer high quantum efficiency (QE) in the NIR region (905 nm and 1000 nm). Our Si PIN photodiodes are available in various packages, and we offer both single element and array configurations.

In addition, Hamamatsu has the capabilities to customise products such as adding an ASIC, changing the number of channels in an array or adding lenses.

The company also offers the possibility to integrate a Trans Impedance Amplifier (TIA) array on chip in the same package with an APD array. This offers great advantages in noise characteristics and saves space and cost.

A key product today is Hamamatsu’s new NIR-enhanced Silicon Photomultiplier. This has a new structure to significantly enhance sensitivity in the NIR, making them suitable for industrial LIDAR applications.

The key feature is photon detection efficiency (PDE), which is increased to 7% at 905nm (up from 2% for conventional devices), which combined with their gain of 1x106, offers the benefit of longer range for distance measurements or a better quality signal (better SNR).