Avalanche photodiode array detector for eye-safe 3D imaging

Many civilian and military applications require the ability to imaging a scene in very low visible light level conditions. It basically provides 2-dimensional (2D) information for the objects of interest. It is used in situations such as security and surveillance during the nights, fire fighting in smoke-filled buildings during a fire, as well as rescue operations in foggy mountainous areas. In operation, a 2D imaging system uses a laser to send out light pulses towards objects that the user wants to image. Some of the light is reflected by these objects. The imaging system has an array of photodiodes made of semiconductor materials to detect the reflected light, by converting light into electrical currents. The term photodiode derives from the facts that they are diodes and that they detect photons, which make up light. The photodiode arrays (each pixel is a photodiode) are similar to those used in digital cameras. A specially designed readout circuit then reads the electrical current in each pixel to generate an image with intensity scales relating to the light intensity falling on each pixel. A 2D imaging system becomes a 3D imaging system when the range (distance between the imaging system and the objects of interest) is also measured. The readout circuit is modified to measure the time elapsed from sending out the light pulse to detecting the reflected light. Since the speed of light travelling in air is known, we can obtain the range. Such system is far more useful than one with only 2D capability. Clutters outside the area of interest can be eliminated easily, presenting a simpler and easier to interpret image to the users. This leads to better detection of concealed or camouflaged objects. With the range information, the system can also be modified to measure concentration of greenhouse gases at different altitudes in the atmosphere. The photodiode array performance often determines the overall performance of an imaging system. Using more advance photodiodes, namely the avalanche photodiodes (APDs), which provides amplification of the electrical currents through avalanche multiplication in the semiconductor materials, improves the imaging system. Hence APD arrays are found in high performance 3D imaging systems. However presently available APD arrays must operate with light with wavelengths of 760 and 1400 nm. These wavelengths present particular hazards because they are invisible to human (visible wavelength range is approximately from 400nm to 700 nm), but are still focused on the retina so can cause severe damage to eyes. The wavelength of 1.5micron is much more suitable for eye-safe operation because its threshold for causing eye damage is several orders of magnitude higher than those of shorter wavelengths. In addition the 1.5micron wavelength light penetrates fog and smoke well therefore operations of imaging systems using 1.5micron wavelength light is less affected by varying weather conditions. There is however no APD arrays that can detect 1.5mm wavelength light available, preventing realisation of eye-safe 3D imaging systems. The University of Sheffield and Lidar Technologies Limited (LT) are therefore working together to develop 1.5mm wavelength APD arrays which will be compatible with eye-safe high performance 3D imaging systems. LT, with strong interest and experience in 3D imaging systems, and Sheffield will first determine the key requirements on APD performance to ensure technology compatibility. Sheffield will use their APD knowledge on design and semiconductor parameters to produce optimised APD designs within the specifications. APD wafer growths and performance validation of the APD wafers will be carried out in Sheffield. LT and Sheffield will work together to determine the required array specifications and have the arrays produced commercially in the UK using the APD wafers validated by Sheffield. LT will test the APD arrays with assistance from Sheffield.