An ancient, lens-free imaging technique provides the basis for a new mid-infrared (MIR) camera that can capture clear, wide-depth images over long distances, even under low light. The camera uses pinhole imaging, a method first described in the 4th Century BC.

The MIR pinhole camera, invented by a team at East China Normal University, overcomes some of the limitations of conventional lens-based systems, especially in the areas of depth of field, field of view, and optical aberrations. It could provide an effective solution for high-sensitivity MIR imaging for night vision, as well as for industrial inspection.

The researchers devised a pinhole imaging system at the MIR wavelength of 3.07 µm. Instead of using a physical aperture, they formed the pinhole optically by using an NIR pump at a 1.03 µm wavelength within a nonlinear crystal. They used a crystal with a chirped-period structure that allowed broadband phase matching over a range of MIR wavelengths up to 5 µm. This approach gave the researchers precise, yet flexible spatiotemporal control over the aperture size, allowing them to optimize imaging performance and achieve a broad field of view.

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Researchers used intense laser light to form an optical pinhole inside a nonlinear crystal, which turns the infrared image into a visible one that can be detected by a traditional, silicon-based camera sensor. With this setup, the researchers were able to capture clear, wide-depth images without using any lenses, even in very low light. Si: Silicon. Courtesy of  East China Normal University/Kun Huang.

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The nonlinear optical pinhole serves both as a spatial filter for image formation and a pump source for frequency upconversion. MIR lightwaves passing through the pinhole are spectrally upconverted to turn the infrared image into a visible image that can be detected by a traditional, silicon-based camera sensor.

Compared to conventional, lens-based MIR upconverters, the nonlinear pinhole imager provides broader imaging coverage, supporting a large depth of field >35 cm and a field of view  >6 cm. The camera demonstrates depth-resolving imaging capabilities across a large depth range in both the reflection and transmission modes, based on time-of-flight and trigonometric techniques, respectively. The synchronized pump pulse in the nonlinear process enables an ultrashort optical gate in time-of-flight imaging, where the noise can be suppressed within a narrow temporal window to support a detection sensitivity of about 1.5 photons per pulse.

“Lensless, nonlinear pinhole imaging is a practical way to achieve distortion-free, large-depth, wide field of view, mid-infrared imaging with high sensitivity,” professor Kun Huang said. “The ultrashort synchronized laser pulses also provide a built-in, ultrafast optical time gate that can be used for sensitive, time-of-flight depth imaging, even with very few photons.”

The system’s transmissive depth-resolving imaging capabilities enable rapid scene reconstruction even under passive illumination, broadening the possible scenarios for MIR sensing and imaging.

The researchers used an aperture size of about 0.2 mm to image targets at distances of 11 cm, 15 cm, and 19 cm. They achieved sharp imaging at 3.07 μm across all three distances, confirming the large depth range of the pinhole camera. They were also able to obtain sharp images of objects placed up to 35 cm away.

To demonstrate time-of-flight imaging in 3D, the researchers used synchronized, ultrafast pulses as an optical gate and reconstructed the 3D shape of the object with μm-level axial precision. When the input was reduced to simulate low-light conditions, after correlation-based denoising, the pinhole camera continued to produce 3D images.

The researchers also demonstrated two-snapshot depth imaging. Using this method, they measured the depth of objects over a range of about 6 cm, without the need for complex pulsed timing techniques.

According to the team, the MIR wavelength used to demonstrate the system’s effectiveness could be extended to cover a wide spectral window of 3-5 µm, thanks to the broadband nonlinear upconverter.

The MIR nonlinear pinhole imaging system could be useful for numerous applications, including industrial process control, infrared (IR) machine vision, and night autopilot.

“This approach can enhance night-time safety, industrial quality control, and environmental monitoring, and because it uses simpler optics and standard silicon sensors, it could eventually make infrared imaging systems more affordable, portable, and energy-efficient,” Huang said. “It can even be applied with other spectral bands such as the far-infrared or terahertz wavelengths, where lenses are hard to make or perform poorly.”

Although the proof-of-concept system currently requires a complex, bulky laser setup, the researchers believe that, as new nonlinear materials and integrated light sources are developed, the technology can become more compact and easier to deploy. The team is now working to increase the speed and sensitivity of the system and its capacity to adapt to different imaging scenarios. Plans include boosting conversion efficiency, adding dynamic control to reshape the optical pinhole for different scenes, and extending the camera’s operation across a wider MIR range.

“Many useful signals are in the mid-infrared, such as heat and molecular fingerprints, but cameras working at these wavelengths are often noisy, expensive, or require cooling,” professor Heping Zeng, who led the research, said. “Moreover, traditional lens-based setups have a limited depth of field and need careful design to minimize optical distortions.

“We developed a high-sensitivity, lens-free approach that delivers a much larger depth of field and field of view than other systems.”

The research was published in Optica (www.doi.org/10.1364/OPTICA.566042).

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