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発表タイトル

Improvement of the zoomable single-shot tomography and profilometry using a low-dense groove spatial phase modulator

概要

NKT Photonics社のスーパーコンティニュームレーザを用いたシングルショットトモグラフィの研究の進捗について5th Asia Pacific Optical Sensor Conferenceで発表しました。下記に発表の概要(英文)をご案内いたします。

Realtime observation of the objects beneath surfaces (tomography) and the three-dimensional step-height (profilometry) is required for various applications of medical engineering, biochemistry and the industrial manufacturing. In the previous research, we introduced a 2D, long-range and single-shot tomography and profilometry using optical frequency comb interferometry combined with the spatial phase modulator (SPM) and CCD camera. By using the high order interference order of combs, the measurement range was effectively expanded by 30 times compare with that of the conventional whitelight interferometry while keeping the same resolution of the whitelight interferometer. In this report, we present another way to expand the measurement range. The effectiveness of the diffraction order of the SPM to the measurement range of the tomography is investigated. It is confirmed that the measurement range can be expanded or shortened by switching the diffraction beams from the SPM. This leads to the first zoom-in/zoom-out tomography and profilometry, to our knowledge.

The super broadband light from a Supercontinuum source passes an optical resonator to create optical frequency combs. Then the beam’s diameter is expanded by a beam expander before entering the Mach-Zehnder interferometer. A diffraction grating, as the SPM, is set in the reference arm of the interferometer. The diffraction beam from the SPM is recombined with the reflection beam from a sample to create interference images on the CCD camera. Because the phase of the diffracted beam at each pixel on the SPM is slightly modulated, line-shape interference fringes are generated and observed by the CCD camera. The modulation depth of the spatial phase modulation depends on the tilt angle of the SPM that determines which diffraction order of the SPM is used for the interferometer. The dynamic measurement range of the proposed system is calculated from the modulation depth. Therefore, by rotating the SPM to which the different diffracted beam propagates toward to the interferometer, the different dynamic measurement range will be selected.

The principle of the optical frequency combs related to the measurement range of the proposed tomography and profilometry will be explained in the manuscript. In addition, the dynamic measurement range calculation from the modulation depth of the SPM is also shown. The design of the SPM, to have more diffraction orders from the super broadband source, in order to increas the magnification of the zoomable tomography and profilometry will also be investigated.

ズーム機能を備えたコム干渉システム
図1:ズーム機能を備えたコム干渉システム

 

 

空間位相変調器で使用する回折次数の選択により深さ方向の2次元断層像を光学的に調整した結果
図2:空間位相変調器で使用する回折次数の選択により深さ方向の2次元断層像を光学的に調整した結果

 

 

段差試料をを用いた場合のSPMの調整による撮像倍率の選択

図3:段差試料をを用いた場合のSPMの調整による撮像倍率の選択

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