Diffractive Optics Target Mid-IR Imaging

Richard Gaughan

Advances in detector technology and improvements in signal processing electronics have led to reductions in the mass and volume of infrared imaging systems. But these dramatic changes had not been matched by an equivalent reduction in the size and weight of the optics. Optics finally is catching up, as designers turn to diffractive optical elements to replace traditional lenses for infrared imaging.

In the last few years, scientists have demonstrated silicon diffractive optics for 3- to 5-µm, near- to mid-IR imaging. Now researchers at the Technical Research and Development Institute's Second Research Center of the Japan Defense Agency in Tokyo have demonstrated a germanium diffractive optical element for mid-IR applications. The lens replaces a zinc selenide achromat in the imaging optics of a forward-looking infrared system.

The team designed the germanium optical element to minimize chromatic aberration in the imaging lens unit of the multielement system. The five-zone diffractive lens was optimized for a design wavelength of 9.25 µm, which the group predicted would offer good performance over the 8- to 10-µm spectral range of the imaging system.

The researchers verified the performance of the lens by comparing the minimum resolvable temperature difference measured using the ZnSe achromat and the Ge diffractive optical element. With a four-bar target at the detector's Nyquist frequency, the minimum resolvable temperature difference through the ZnSe system was 0.24 K, and through the diffractive optical element system, 0.25 K.

By adopting the diffractive optical element, the researchers reduced the mass of the imaging system by 95 g and the production cost by 30 percent. The approach may enable the development of compact, wearable thermal imaging systems for military, law enforcement and search-and-rescue applications.