<p> In this dissertation, the development of alternative fiber based polishing tools for the finishing of precision freeform and aspherical optics is introduced. Freeform and aspherical optics, i.e. optics with varying radius of curvatures (ROC), are the current forefront for solutions for modern optical systems, enabling more compact and higher performance designs than possible with systems composed of classical optical components, i.e. planar and spherical optics. The technological challenges associated with the fabrication of freeform optics include; (1) identification of suitable tooling capable of accommodating the different ROC&rsquo;s, and (2) achieving surfaces with low mid spatial frequency (MSF) error content. The mainstream fabrication techniques for freeform optics heavily rely on a process called sub-aperture polishing, whereby a tool (down to 1/10^th of the optic size), is programmatically traversed across the surface to finish the part. This technique, due to the remnant tool path marks, results in MSF errors on the optic&rsquo;s surface. Removal of MSF errors is an ongoing challenge in the optics industry. As the main contribution of this work, it will be shown that fiber based tools, which have the flexibility to conform to varying ROCs, have the potential to remove preexisting MSF errors. The other contributions of this work are as follows: (1) Favorable fiber characteristics for use in the proposed fiber based tools are isolated. (2) The material removal mechanism associated with fiber based tools is described. (3) Finite element models (FEM) are created to gain insights on the fundamental fiber-workpiece interaction, and on the fiber properties and geometries most suited for MSF reduction. (4) The initial steps of implementing fiber based tools in commercial, multi axis optic finishing machines are completed and the associated challenges identified.</p><p> In summary, this work lays the foundation for using fiber based tools to reduce MSF&rsquo;s on freeform surfaces, and in doing so, the work addresses a critical need as identified by the optical fabrication community.</p><p>

<p> An in-process, in-situ cure monitoring technique utilizing a guided wave-based concept for carbon fiber reinforced polymer (CFRP) composites was investigated. Two automated cure monitoring systems using guided-wave ultrasonics were developed for characterizing the state of the cure. In the first system, surface mounted high-temperature piezoelectric transducer arrays were employed for actuation and sensing. The second system motivated by the success of the first system includes a single piezoelectric disc, bonded onto the surface of the composite for excitation; fiber Bragg gratings (FBGs) and/or phase-shifted fiber Bragg gratings (PSFBGs) were embedded in the composite for distributed cure sensing. </p><p> Composite material properties (viscosity and degree of cure) evolved during cure of the panels fabricated from Hexcel^&reg; IM7/8552 prepreg correlated well to the amplitude, time of arrival, and group velocity of the guided wave-based measurements during the cure cycle. In addition, key phase transitions (gelation and vitrification) were clearly identified from the experimental data during the same cure cycle. The material properties and phase transitions were validated using cure process modeling software (e.g., RAVEN^&reg;).</p><p> The high-temperature piezoelectric transducer array system demonstrated the feasibility of a guided wave-based, in-process, cure monitoring and provided the framework for defect detection during cure. Ultimately, this system could provide a traceable data stream for non-compliance investigations during serial production and perform closed-loop process control to maximize composite panel quality and consistency. In addition, this system could be deployed as a &ldquo;smart&rdquo; caul/tool plate to existing production lines without changing the design of the aircraft/structure.</p><p> With the second system, strain in low frequency (quasi-static) and the guided wavebased signals in several hundred kilohertz range were measured almost simultaneously using the same FBG or PS-FBG throughout the cure cycle. Also, the residual strain can be readily determined at the end of the cure. This system demonstrated a real-time, in-situ, cure monitoring system using embedded multiplexed FBG/PS-FBG sensors to record both guided wave-based signals and strain. The distinct advantages of a fiber optic-based system include multiplexing, small size, embedding, utilization in harsh environments, electrically passive operation, and electromagnetic interference (EMI) immunity. The embedded multiplexed FBG/PS-FBG fiber optic sensor can monitor the entire life-cycle of the composite structure from curing, post-cure/assembly, and in-service for creating &ldquo;smart structures&rdquo;.</p><p>