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The manufacture and development of optical components for the Internet era poses considerable problems for effectively characterizing, in an integrated way, the optical and topographic properties of these components and the associated integrated devices. Near-field optics is a new tool, which bridges the worlds of atomic force and far-field optical microscopy, to provide unique information on the characteristics important in maximizing the manufacturing and the development of optical components, sub-components and systems.
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The Nanonics MultiView 1000™, as seen in the figure, brings this exciting new technology to the aid of the optical component industry in its quality control and in its development. For this application, a green laser at 532 nm was passed through the cleaved lens fiber via a fiber coupler seen beneath the system in the figure. The light that was transmitted through the fiber and the micro lens at its tip was collected by the Nanonics cantilevered optical fiber that performed both the topographic analysis and the distribution of the light that was being transmitted.
The measurements were made with this cantilevered fiber tip in a non-contact mode of atomic force microscopy that permitted the topography of the lens to be measured while the collected light at each point in the topographic scan was monitored and displayed simultaneously.
Nanonics MultiView 1000™ System. The cantilevered tip fits under the lens, while the 3D Flat Scanning TechnologyTM allows the lensed fiber to be held under the cantilevered tip for inspection and characterization.
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The first images shown are the topography:
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Micro lens topography
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Line Scan through micro lens topography
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The next image is the near-field optical distribution of the light on the lens surface:
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Near-field optical distribution of light on the surface of the micro lens
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Next, a collage of the topography and light distribution in the near- field of the lens is shown.
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A collage of micro lens topography and light distribution
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The fiber tip was then moved in z away from the cantilevered fiber in order to monitor the distribution of the light at various optical planes from the fiber tip until a focal point was reached. This was possible because of the large z range of the Nanonics 3D Flat Scanner which is a unique aspect of the Nanonics MultiView 1000™. These optical sections are shown with micron or submicron z steps in the sequence shown below.
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| Figure 25. Light distribution through different optical planes through the lens focus and beyond |
The focal point was achieved at 5 microns from the top of the lens. Based on our scanner characteristics nanometric resolution could be achieved if this is important. As can be seen various modes propagate from the near- field to the far- field with the intensity of these modes changing till a focal point is reached. This focal point is displaced relative to the center of the lens in this particular case.
From the knowledge of the topography of the lens, the focal point of the same lens and the distribution of light on the surface of the lens, the parameters of the lens can be calculated. From the data we have obtained the focal plane in the sequence of images above is from the surface of the lens. This can be amended from the data we have obtained on the aperture of the lens. This data is taken from the distribution of light on the surface of the lens in the image of the collage of the topography and near-field light distribution.
In addition, from the topography and the geometry of the lens (two images shown below in Figure 26) we can calculate the radius of curvature of the lens, which is found from the topographic image to be 4.4 microns.
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Figure 26. The geometry and the topography of the lens for the radius of curvature measurement
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We can then calculate the numerical aperture of the lens. For this we need the diameter of the light distribution in the focal plane and this is shown below (Figure 27) from our measurements in Figure 24.
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Figure 27. The diameter of the light distribution at the focal region of the micro lensed fiber
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With this data and the focal distance and the size of the aperture of the lens, which our data also have provided above, the numerical aperture of this lens is calculated to be 0.2.
As shown in this application note the unique aspects of the Nanonics system is now available to solve the most difficult problems in optical component, sub- component and system characterization. The Nanonics MultiView 1000™ allows the optical industry to see the previously unseen in the optical system and sub-systems that are an integral part of the wavelength division multiplexing revolution today and will be a critical part of these systems tomorrow.
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