School of Engineering and Digital Arts, University of Kent, Canterbury, United Kingdom, CT2 7NT.
School of Optical and Electronic Information (SOEI), Next Generation Internet Access National Engineering Laboratory (NGIAS), Huazhong University of Science and Technology, Wuhan, 430074, China.
Sci Rep. 2018 Jan 12;8(1):600. doi: 10.1038/s41598-017-18920-8.
Most overlooked challenges in ultrafast optical time stretch imaging (OTSI) are sacrificed spatial resolution and higher optical loss. These challenges are originated from optical diffraction devices used in OTSI, which encode image into spectra of ultrashort optical pulses. Conventional free-space diffraction gratings, as widely used in existing OTSI systems, suffer from several inherent drawbacks: limited diffraction efficiency in a non-Littrow configuration due to inherent zeroth-order reflection, high coupling loss between free-space gratings and optical fibers, bulky footprint, and more importantly, sacrificed imaging resolution due to non-full-aperture illumination for individual wavelengths. Here we report resolution-improved and diffraction-efficient OTSI using in-fiber diffraction for the first time to our knowledge. The key to overcome the existing challenges is a 45° tilted fiber grating (TFG), which serves as a compact in-fiber diffraction device offering improved diffraction efficiency (up to 97%), inherent compatibility with optical fibers, and improved imaging resolution owning to almost full-aperture illumination for all illumination wavelengths. 50 million frames per second imaging of fast moving object at 46 m/s with improved imaging resolution has been demonstrated. This conceptually new in-fiber diffraction design opens the way towards cost-effective, compact and high-resolution OTSI systems for image-based high-throughput detection and measurement.
在超快光时域成像（OTSI）中，最容易被忽视的挑战是牺牲了空间分辨率和更高的光损耗。这些挑战源于 OTSI 中使用的光学衍射器件，它将图像编码成超短光脉冲的光谱。传统的自由空间衍射光栅，如现有 OTSI 系统中广泛使用的衍射光栅，存在几个固有缺陷：由于固有零级反射，在非里特罗配置下衍射效率有限；自由空间光栅与光纤之间的耦合损耗高；体积庞大；更重要的是，由于单个波长的非全孔径照明，牺牲了成像分辨率。在这里，我们首次报道了使用光纤内衍射来提高分辨率和提高衍射效率的 OTSI。克服现有挑战的关键是 45°倾斜光纤光栅（TFG），它作为一种紧凑的光纤内衍射器件，提供了更高的衍射效率（高达 97%），与光纤固有兼容，并且由于几乎对所有照明波长进行全孔径照明，因此提高了成像分辨率。已经证明，以 46m/s 的速度对快速移动的物体进行每秒 5000 万帧的成像，具有更高的成像分辨率。这种全新的光纤内衍射设计为基于图像的高通量检测和测量的经济高效、紧凑和高分辨率 OTSI 系统开辟了道路。
