No abstract

Modern optical measurement technologies such as structured light microscopy or fringe-projection profilometry rely fundamentally on structured illumination of the specimen or probe. Miniaturizing the applied illumination concept enables the availability of these methodologies even in spatial domains that have remained inaccessible so far. Here we introduce a design methodology to realize complex illumination patterns with high diffraction efficiencies in a strongly miniaturized and functional integrated approach. This is achieved by combining the advantages of refractive freeform wavefront tailoring and diffractive beam shaping. This novel concept overcomes classical stray light issues known from conventional diffractive beam shaping and remains valid for micro-optical systems, i.e., beyond the geometric optical regime. Moreover, the design process is in particular optimized to reduce the aspect ratio of the obtained surface features. This strongly improves the manufacturability and as-built performance of the designed optical element, and the feasibility of the approach is demonstrated by the design and realization of monolithic beam shaping units on the tips of optical fibers via two-photon direct laser writing. This provides the means to realize complex illumination patterns in an integrated and mechanically flexible approach.

The optical design and analysis of modern micro-optical elements with high index contrasts and large numerical apertures is still challenging, as fast and accurate wave-optical simulations beyond the thin-element-approximation are required. We introduce a modified formulation of the wave-propagation-method and assess its performance in comparison to different beam-propagation-methods with respect to accuracy, required sampling densities, and computational performance. For typical micro-optical components, the wave-propagation-method is found to be considerably faster and more accurate at even lower sampling densities compared to the different beam-propagation-methods. This enables realistic wave-optical simulations beyond the thin-element-approximation for micro-optical components. As an example, the modified wave-propagation-method is applied for in-line holographic measurements of strongly diffracting objects. From a direct comparison of experimental results and corresponding simulations, the geometric parameters of a test object could be retrieved with high accuracy.

The interaction of a fiber Bragg grating and longitudinal acoustic waves in a three-airholes suspended core fiber is experimentally investigated and employed to mode-lock an ytterbium-doped fiber laser. An optimized design of an acousto-optic modulator based on two piezoelectric transducers and 1 cm grating length is also proposed. For an electrical signal strength of 10 V applied to the modulator, the results indicate output pulses with a width of less than 550 ps at a repetition rate of 10 MHz. The reduction of the grating length and the power consumed by the transducer, when compared to previous studies, points out to more efficient, compact and fast acousto-optic modulators for mode-locked all-fiber lasers.