Dark field illumination (DFI) is an elegant inspection technique sometimes used to detect particles on a specular surface. However, traditional DFI struggles with repeatability, limiting its applications in automated inspection. We present an improvement to DFI by introducing a modulated dark field illumination (MDFI) that utilizes the phase rather than the intensity in the detection of defects. For modulated dark field illumination (MDFI), the phase-based information is independent from the reflectance of the surface, but has a higher sensitivity to the light scattered from a defect than DFI. As a result, we obtain a robust computational image process method that is insensitive to the environment and provides clearly defined defect information. In order to extend the application to industry, the instantaneous MDFI systems were developed and validated.
Dark-field illumination is a simple yet elegant imaging technique that can be used to detect the presence of particles on a specular surface. However, the sensitivity of dark-field illumination to initial conditions affects its repeatability. This is problematic in cases where automation is desired. We present an improvement to the current method of using a modulation field that relies on phase calculations rather than intensity. As a result, we obtain a computational method that is insensitive to noise and provides clearly defined particle information, allowing a global threshold to be set for autonomous measurement purposes. After introducing the theory behind our method, we present experimental results for various scenarios and compare them to those obtained using the dark-field approach.
Bidirectional, high data rate, low size, weight, and power (SWaP), and low cost free space optical links are needed for space and aeronautic communication applications to send and receive large volumes of data. We are exploring design strategies for optical transceivers to reduce SWaP and cost through increased misalignment tolerance (pointing requirement reduction) and sharing the optical transmit and receive paths (imposing optical symmetry). In applications where the detector is fiber coupled, the receive fiber numerical aperture is the main driver of the pointing accuracy requirement. Increasing the numerical aperture of the receive fiber reduces the pointing requirement. In an optically symmetric design, the fibers both transmit and receive the light. Hence, increasing the receive fiber numerical aperture requires a similar increase of the transmit fiber. Unfortunately, increasing the transmit fiber numerical aperture causes instability in received power over small misalignments. Double clad fibers offer a solution. These fibers transmit from a single mode core and receive light in a larger numerical aperture. Results show that as transceiver fibers, double clad fibers have an improved misalignment tolerance and a higher stability for small changes in misalignment when compared to single mode fibers and multimode fibers. Also, double clad fibers are shown to match the performance of an asymmetrical link design with a single mode transmit fiber and a multimode receive fiber.
A 20-meter free-space optical link (FSOL) is proposed for data transmission between external ISS payload sites and the main cabin at a target rate of 10 Gbps (gigabits per second). Motion between a payload site and the main cabin is predicted to cause up to 5 cm in lateral misalignment and 0.2 degrees of angular misalignment. Due to the harsh environment of space it is advantageous to locate the optical transceivers inside the spacecraft or in a controlled environment. With the optical components and transceivers in separate locations, a fiber optic cable will be required to carry light between the two. In our past work we found that the use of large-core fibers provide an increased misalignment tolerance for such systems and could eliminate the need for active control of the optics. In that work, it was shown that a 105 μm core diameter fiber optic cable offered a viable low SWaP (size, weight, and power) solution for the ISS application; however, the effects of modal dispersion were not investigated. This paper will present bit error rate performance of the FSOL using these large-core fibers.