We present the tomographic study of the refractive index distribution in polymer bridges between two optical fibers. Detailed refractive index maps are needed in order to improve the technological process for manufacturing those bridges and to achieve a lower return loss. At first, the technological process of the fabrication of bridges through photopolymerization is presented. The interferometric measurements of reference fibers used to produce those bridges and two series of microbridges are performed experimentally in the visible (VIS; 632.8 nm) and infrared (IR; 1550 nm) wavelength regions. The relation between the VIS and IR results is determined, which allows performing tomographic measurements in more accurate conditions in the VIS spectrum. The experimentally obtained refractive index distributions in the microbridges are used for modeling the insertion and return losses, which are compared with the real loss obtained for the produced microbridges. This knowledge will be used for better understanding the manufacturing process and its further optimization.
In this paper we present the tomographic studies of refractive index distribution in polymer bridges between two optical fibers. Detailed refractive index maps are needed in order to improve the technological process of manufacturing of those bridges and to achieve lower return losses. At first the technological process of bridges fabrication through photopolymerization is presented. The interferometric measurements of reference fibers used to produce those bridges and two series of microbridges are performed in visible (632.8 nm) and infrared (1550 nm) experimental systems. The relations between vis and IR results are determined, which allows for performing tomographic measurements in more accurate conditions secured in visible spectrum. The experimentally obtained refractive index distributions in microbridges are used for modeling the insertion and return losses, which are compared with the real losses obtained for the produced microbridges. This knowledge will be used for better understanding of the manufacturing process and its further optimization.
Digital Holographic Microscopes (DHMs) have unique features especially relevant for static and dynamical MEMS characterization. They provide both 3D topography with interferometric resolution and intensity image in a single acquisition at camera rate, without any lateral or vertical scanning. In this presentation, DHM is operated in conjunction with a laser pulsed stroboscopic module providing synchronization of camera, laser pulses, and MEMS excitation signal up to 25 MHz. Three methods for DHM analysis of resonant frequencies are presented with concrete examples. The first method, gives the more general, precise and complete information. Sine wave excitation with increasing (or decreasing) frequencies is used. For each frequency, laser pulses are used to “freeze” the movement of the MEMS. Using the stroboscopic synchronization, each period of the excitation signal is sampled at high resolution, and the topography of the MEMS can be measured at each of those samples points. As implemented, the change of frequency is a continuous sweep: quantitative values in term of displacement amplitude and Bode diagrams can be measured for nonlinear resonances as well. The second method uses sine wave excitation with increasing (or decreasing) frequencies. For each frequency, the optical signal is integrated over an entire number of periods of the MEMS. At resonance, constructive and destructive interference build up on the intensity images. It enables fast frequency scan over large ranges. But it provides neither quantitative values of displacement amplitude, nor Bode diagrams. The third method is to measure the system response to an impulse or chirp excitation signal for instance, and to make a Fourier analysis of this response to determine resonant frequencies. This method is less sensitive as it spreads the excitation energy in many frequencies.
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