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20 October 2016

Canon develops first InP immersion grating

Tokyo-based Canon Inc has developed what it claims is the first indium phosphide (InP) immersion grating (a spectrographic optical element capable of covering the InP transmissive frequency range of the infrared spectrum, i.e. 1.5-8μm). Compared with conventional reflective diffractive gratings, immersion gratings enable higher dispersion relative to the refractive index (about 3.2 for InP).

Strengthening its lineup of immersion gratings, which includes gratings made from germanium (Ge) and cadmium zinc telluride (CdZnTe), Canon aims to contribute to greater progress in cosmological observation by expanding the range of observable infrared frequencies.

To retrieve information contained within light emitted from space, astronomical telescopes and man-made satellites are equipped with spectroscopes that incrementally divide light by its different frequencies and play a vital role in cosmological observation. Compared with typical reflective elements, immersion gratings enable spectrometers that are smaller in size and realize higher levels of performance. With the addition of an InP immersion grating to Canon's range, it is reckoned that spectrometers could be reduced to about 1/27th the volume of those equipped with typical reflective elements that cover the same frequencies. Overcoming restrictions on size and weight (which, until now, made it difficult to launch man-made satellites equipped with high-performance spectrometers) is expected to further expand the possibilities of cosmological observation. Also, application of this grating to next-generation large ground-based telescopes, which face the problem of ever-increasing sizes, could lead to reductions in size without sacrificing performance.

With the addition of an InP immersion grating, Canon's immersion grating lineup now covers light from near infrared to far infrared, enabling observation of almost the entire spectrum of infrared frequencies used in the field of astronomy (1-20μm). Infrared light can be captured from much greater distances than visible light, making detection of matter in space possible on a molecular, and even atomic, level. The new InP immersion grating could hence facilitate research into not only the origin of life and planets, but also the origin of the universe itself, contributing to even greater developments in space science.

While the benefits of fabricating immersion diffractive gratings were realized long ago, because the transmissive semiconductor materials suited to the 1-20μm infrared frequencies used in astronomy are particularly brittle, achieving a surface of virtually perfect regularity with grooves measuring only a few nanometers proved difficult. Canon applied its own ultra-precision processing technology, cultivated through the manufacture of precision components, using only machining processes to develop immersion gratings even with such brittle materials. The resulting InP immersion grating realizes an arrangement of 990 steps at 47μm intervals.

Diffractive elements for use with high-dispersion IR spectra ordinarily have an absolute diffraction efficiency (ratio of diffracted light intensity to incident light intensity at the precise point of strongest diffracted light intensity) of 50–60%. Canon's InP immersion grating, however, achieves an absolute diffraction efficiency of about 75% (compared with a theoretical limit of 85%). Canon reckons that, with its high-efficiency performance enabling superior light capture even amid low light intensity, it will enable small telescopes to achieve high-precision measurement, and large telescopes to measure infrared light from much greater distances in space.

Canon is giving a presentation on its InP immersion grating at the International Conference on Space Optics 2016 in Biarritz, France (18-21 October).

Going forward, Canon intends to develop an immersion grating from materials suitable for frequencies close to visible light (0.8–1.2μm) and, with the development of its InP immersion grating, the firm is a step closer to achieving that goal. By developing a lineup of immersion gratings using a variety of different materials, users can choose the optimal grating based on the frequency range it will be used with to enable a wide range of applications in infrared spectroscopy. Canon anticipates applications in medicine and communication, as well as astronomy.

Tags: InP

Visit: www.global.canon

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