The lens-free system not only focusses, but changes the image wavelength to suit a silicon-based sensor.
Key to the proof-of-concept camera is a custom non-linear crystal with a chirped-period structure.
To create the pin-hole, a powerful laser was focussed into the crystal to form a zone of optical mixing around 0.2mm across, and it is this that acts as the image-focussing pin-hole and the wavelength converter from 3.07μm radiation into the visible spectrum.
“The up-conversion detection method naturally suppresses noise, which allows it to work even in very low light conditions,” according to Optica, which published the research findings.
Targets 110, 150 and 190mm were in focus without optical changes, and a sharp image could be obtained of an object 350mm away.
“Lens-less non-linear pinhole imaging is a practical way to achieve distortion-free, large-depth, wide-field-of-view mid-infrared imaging with high sensitivity,” said researcher Kun Huang. “The ultra-short synchronised laser pulses also provide a built-in optical time gate that can be used for sensitive, time-of-flight depth imaging, even with very few photons.”
Time-of-flight capability was used to image a solid object to “micron-level axial precision”, said Optical. “Even when the input was reduced to about 1.5 photons per pulse, the method still produced 3D images after correlation-based de-noising.”
Two-snapshot depth imaging was also used, on two objects at slightly different distances. “With this method, they were able to measure the depth of the objects over a range of about 6cm, without using complex pulsed timing techniques,” said Optica.
While the experimental equipment was bulky, Optical said: “As new non-linear materials and integrated light sources are developed, the technology should become far more compact and easier to deploy.”
The work is to be published in Optical as ‘Mid-infrared nonlinear pinhole imaging‘.
Proably the largest infra-red sensor ever made is being commissioned at the Vera Rubin Observatory
Image credit: Kun Huang, East China Normal University
