Philipp Metz

Compact, transmissive two-dimensional spatial disperser design with application in simultaneous endoscopic imaging and laser microsurgery

Minimally invasive surgery procedures benefit from a reduced size of endoscopic devices. A prospective path to implement miniaturized endoscopy is single optical-fiber-based spectrally encoded imaging. While simultaneous spectrally encoded inertial-scan-free imaging and laser microsurgery have been successfully demonstrated in a large table setup, a highly miniaturized optical design would promote the development of multipurpose endoscope heads. This paper presents a highly scalable, entirely transmissive axial design for a spectral 2D spatial disperser. The proposed design employs a grating prism and a virtual imaged phased array (VIPA). Based on semi-analytical device modeling, we performed a systematic parameter analysis to assess the spectral dispersers manufacturability and to obtain an optimum application-specific design. We found that, in particular, a low grating period combined with a high optical input bandwidth and low VIPA tilt showed favorable results in terms of a high spatial resolution, a small device diameter, and a large field of view. Our calculations reveal that a reasonable imaging performance can be achieved with system diameters of below 5 mm, which renders the proposed 2D spatial disperser design highly suitable for use in future endoscope heads that combine mechanical-scan-free imaging and laser microsurgery.

Tunable elastomer-based virtually imaged phased array

Virtually imaged phased arrays (VIPAs) offer a high potential for wafer-level integration and superior optical properties compared to conventional gratings. We introduce an elastomer-based tunable VIPA enabling fine tuning of the dispersion characteristics. It consists of a poly-dimethylsiloxane (PDMS) layer sandwiched between silver bottom and top coatings, which form the VIPAs high reflective and semi-transparent mirror, respectively. The latter also acts as an electrode for Joule heating, such that the optical PDMS resonator cavity tuning is carried out via a combination of thermal expansion and the thermo-optic effect. Analogous to the free spectral range (FSR), based on a VIPA specific dispersion law, we introduce a new characteristic VIPA performance measure, namely the free angular range (FAR). We report a tuning span of one FAR achieved by a 7.2K temperature increase of a 170μm PDMS VIPA. Both resonance quality and tunability are analyzed in numerical simulations and experiments.

Optical beam positioning with PDMS thin film resonators for thermal drift compensation in free-space optical interconnects

Resonance enhanced beam displacements of ∼40 µm are measured using a thin film resonator thermally activated by Joule heating. Using this for thermal drift and placement error compensation in free space optical interconnects is suggested.

Elastomer-based tunable virtually imaged phased array for reconfigurable optical interconnects

Virtually imaged phased arrays (VIPA) offer high dispersion compared to conventional gratings and have been proposed as buildings blocks for several photonic devices, including wavelength multipliers, chromatic dispersion compensators, waveformgenerators and pulse shapers. We introduce an elastomer-based tunable VIPA, providing an additional degree of freedom for these devices. In particular, we investigate its capability to implement reconfigurable optical interconnects. In a wavelength demultiplexing setup it allows for both compensation of misalignment as well as reconfiguration of a source wavelength to a target channel. It consists of an elastomer layer sandwiched between two structured silver coatings on a glass substrate forming the resonator cavity. Using Joule heating of the top silver layer a thermal expansion and a thermo-optic effect of the elastomer cavity is induced allowing for tuning the effective optical resonator cavity. We report a tuning span of one free angular range by a temperature increase of less than 10K induced by a power change in the low mW regime. Both resonance quality and tunability of the device are investigated.

Wavefront and polarization effects in actively tunable thin-film resonators for beam alignment in optical interconnects

Thin-film resonators operated at an oblique incidence angle produce significant spatial shifts of incident low divergence coherent beams based on the resonance-dependent effective group propagation angle. We investigate a single-layer resonator tuned by Joule heating and thermal expansion of the resonator cavity to allow for active beam position control. Operated in reflection on a right-angle glass prism, it is suitable for integration in optical interconnect networks and couplers to enable active beam alignment. The effects of polarization, beam divergence and wavefront aberrations are analyzed both in simulations and experiments.

Wavelength and polarization effects in actively tunable thin film resonators for thermal drift compensation

Thin film resonators produce significant beam shifts of incident light based on incidence angle and wavelength dependent changes of the effective group propagation angle. We investigate these shifts in actively tunable resonators for thermal drift compensation in transparent free-space optical interconnect networks for future inter- and intra-chip applications. The effects of VCSEL wavelength variations, WDM tuning range, and polarization fluctuations on system performance are analyzed.

Integrated refractometer design with directional light source

Integrated chip-refractometers with nanostructured organic light-emitting diodes (OLEDs) as light sources are suggested. In simulations a sensitivity increase by a factor of 16 is demonstrated assuming 0.5% directed power emission compared to purely isotropic emission.

Optical transmission measurements for in-line monitoring of turbid oil-water emulsions

For absorbing media the concentration may be calculated directly from the optical transmission following the logarithmic dependence given in the Lambert-Beer law. Due to multiple scattering events in oil-water emulsions (e.g. milk, cream, etc.), these exhibit a nonlinear relationship between the attenuation and the oil concentration. We demonstrate that for increasing oil content in oil-water emulsions the attenuation first increases, then levels out, and finally even decreases for a fat content of 60%. Single-wavelength optical transmission measurements are found to be well suited for the in-line monitoring of oil-water emulsions of fat contents below 20%, e.g., for the in-line fat content monitoring of milk. Using experiments and ray-tracing simulations we evaluate system optimization.

Thermally tunable optical aperture based on a segmented thin-film resonator

Apertures are basic elements which can be found in many optical systems. Since optical systems are continuously being miniaturized and integrated, there is a need for small and inexpensive apertures to control beam shape and light intensity. Current aperture concepts for the micrometer regime rely on moving MEMS lamella or controlling fluids by capillary or electrostatic forces. We demonstrate an aperture concept for single-wavelength operation based on thermal tuning of a segmented thin film resonator. Thermal tuning changes the optical thickness of the elastomer cavity. This allows for adjusting the intensity to any level between constructive and destructive interference in a specific aperture segment. In order to demonstrate aperture operation we simulate thermal, mechanical and optical properties using finite element method and transfer-matrix method. We confirm our simulation results by experimental beam shape measurements and spatially-resolved spectral transmission and light intensity measurements.

Miniaturized optical fiber endoscope without inertial scan for simultaneous imaging and laser microsurgery

The current autostereoscopic projection system is accomplished by array projectors. It is easy to realize optically but has a drawback with size. Another type is to place the shutter on the screen. It saves the volume but reduces the efficiency depending on how many views are produced. The shutter in the lens aperture has the same efficiency problem, too. To overcome these problems, a full HD autostereoscopic projector based on the lens aperture switching type is proposed. It has RGB laser sources and can produce 16-views or even higher stereoscopic images. This system removes the shutter in the lens aperture by the opti-mechanism itself. The specific light on the lens aperture coming from the point on the DMD is reflected to different angles. The proper angle of light is generated in the object side by the relay and folding system. The UHP lamps or the LED rays are difficult to constrain in a relative small cone angle. For this reason, the laser is applied to the design. The very small etendue of the laser is good for this architecture. The rays are combined by dichroic filter from RGB laser sources then forming and expanding to the mirror. The mirror is synchronized with DMD by the DSP control system. The images of different views are generated by DMD and specific position of the mirror. By the double lenticular screen, the lens aperture is imaged to the observer’s viewing zone and the 3D scene is created.