We discuss a novel whole-field optical strain sensor termed the moiré interferometric strain sensor (MISS) for simultaneous measuring of multipoint strains and whole-field contours of in-plane displacement. A high-frequency grating, attached to the surface of a specimen, is used as the displacement and strain-sensing unit. When illuminated by two collimated beams at a prescribed angle, the interference of the diffracted beams gives the whole-field deformation contours. If, on the other hand, each of the individual beams is separately imaged using a multilens CCD sensor similar to a wavefront sensor, the separation between the spot centroids for each microlens is directly proportional to the normal or shear strain component at the corresponding position on the specimen. Applications are demonstrated for uniform rotation and simulated in-plane strains.
Optical adhesives serve a means of structural attachment as well as provide an optical path between connecting elements in an optical modules. The aim of this study was to determine the displacement and normal strain induced in an optical fiber as a result of moisture absorption within the optical adhesive of the module. The displacement and the strain of single mode fiber-adhesive joint on silicon optical bench (SiOB) were measured by Micro Moire interferometry (MMI). The experiments were performed on a module consisting of SiOB and single-mode-fiber attached in a V-groove with the help of a UV-curable adhesive. Moisture saturation of the optical adhesive within the optical package was achieved via the devised moisture uptake by the capillary effect setup and specimens were placed in the set up for one week. Strain in the adhesive was measured by MMI during the moisture desorption, at 60oC. The maximum V and U field displacement demonstrated by the fiber was 0.66μm and 0.59μm respectively. The V and U field induced recovery strain of 0.00488 and 0.00438 respectively as a result of the saturation process grounds the postulation that moisture does indeed affect the optical fiber's relative position within the optical module.
Optical adhesives (index-matching adhesive) serve a means of structural attachment as well as provide an optical path between connecting elements. Optical properties (such as refractive index and birefringence) of these polymeric adhesives are sensitive to environmental factors, which affects optical device performance. Thus this work attempts to develop a methodology to evaluate the impact of moisture on the refractive index of an index-matching adhesive.
The return loss method was employed for monitoring the moisture dependence of refractive index of an acylate-based optical adhesive. The test was performed at a wavelength of 1.55 mm. At fixed time intervals, the return loss of the sample were measured with an optical time-domain reflectometer (OTDR) during moisture desorption. The amount of moisture present in the optical adhesive at different time was calculated with the help of desorption data. Thus, a correlation between time, the amount of moisture present and refractive index was obtained. At a constant temperature of 120°C, the refractive index of the adhesive decreases as moisture content increases. However, this decrease was more drastic for small amount of moisture absorbed at -1 x 10-4 (% moisture content)-1 compared to -7 x 10-6 (% moisture content)-1 when moisture absorption exceeds 10% of the dry sample weight.
The important design factors that have to be considered for packaging of modules, which have single mode fiber (SMF), are mechanical failure and misalignment of the optical components (such as fiber, lens, laser diode and photo diode) during thermal loading. In this paper, the misalignment of single mode fiber and the strain in the optical module was measured for high temperature loading by Micro Moire interferometry. The experiments were performed on a module consisting of SiOB and single-mode-fiber attached in a V-groove with the help of a UV-curable adhesive. Temperature load ranging from room temperature (23°C) to 100°C was applied to the module. Maximum displacement of fiber in the vertical and horizontal directions relative to the SiOB was determined to be 0.69 - 1.21 μm and 0.36 - 0.42 μm, respectively at 100°C. The variation of the displacement for different samples at a given temperature was observed due to different initial position of fiber in the V-groove and the amount of adhesive around the fiber. Normal strain induced in the fiber was also calculated when the module was subjected to a temperature increment of ~75°C. Finally, a qualitative analysis was carried out for shear strain in different parts of the module and the maximum shear stress was observed at the silicon-adhesive-fiber joint region.