Andreas Hermerschmidt

Wave front generation using a phase-only modulating liquid-crystal-based micro-display with HDTV resolution

Liquid-crystal (LC) based micro-displays can be used to modulate incoming light waves with respect to amplitude, phase and polarization. Twisted-nematic LC displays produce a combined phase-polarization modulation so that it is difficult to achieve pure phase modulation without amplitude modulation. We present a new phase-only modulating LCOS (Liquid Crystal On Silicon) spatial light modulator (SLM) based on an electrically controlled birefringence (ECB) liquid crystal mode. The device has a HDTV (1920x1080) resolution and a small pixel pitch of only 8&mgr;m (87% fill factor) on a digital silicon back plane. The LC molecules are aligned parallel to the electrodes and an applied electric field forces them to tilt towards the direction of the field. This leads to a pure phase modulation with a phase retardation of 2&pgr; for wavelengths between 420 and 1064nm, with negligible polarization change (<1%) if the light is linearly polarized parallel to the director axis of the LC molecules. The shape of the back-plane of the LCOS micro-display was investigated using a Twyman-Green interferometer and the observed deviation from a plane surface was compensated by addressing the inverse spatially resolved phase retardation function. The interferometer was then used to measure wave fronts that were generated with the micro-display, representing optical elements like e.g. single lenses, lens arrays and tilted mirrors.

Narrow-bandwidth high-power output of a laser diode array with a simple external cavity

Abstract Using a simple external cavity consisting of a grating and an etalon, we demonstrate bandwidth narrowing of a high power broad-area 3-stripe laser diode array. At a drive current up to 3.6 times the threshold current, the laser diode array can always be locked. An available output of 2.3 W is obtained with the bandwidth being narrowed from 2.2 nm to no more than 0.07 nm. By tilting the etalon, the narrowed wavelength can be tuned continuously in a range of 1.6 nm.

Holographic optical tweezers with real-time hologram calculation using a phase-only modulating LCOS-based SLM at 1064 nm

We present a method that enables the generation of arbitrary positioned dual-beam traps without additional hardware in a single-beam holographic optical tweezers setup. By this approach stable trapping at low numerical aperture and long working distance is realized with an inverse standard research microscope. Simulations and first experimental results are presented. Additionally we present first steps towards using the method to realize a holographic 4π-microscope. We will also give a detailed analysis of the phase-modulating properties and especially the spatial-frequency dependent diffraction efficiency of holograms reconstructed with the phase-only LCOS spatial light modulator used in our system. Finally, accelerated hologram optimization based on the iterative Fourier transform algorithm is done using the graphics processing unit of a consumer graphics board.

Binary diffractive beam splitters with arbitrary diffraction angles.

Diffractive optical beam splitters designed with iterative Fourier transform type algorithms can produce only certain diffraction angles given by the spatial frequencies used for the computations, which are multiples of a certain base spatial frequency. We have developed a design algorithm that overcomes this limitation and can be used to compute binary diffractive elements with arbitrary diffraction angles. The simulated and experimentally measured properties of optical elements producing beam arrays in circular arrangements are presented and discussed.

Broad-area laser diode with 0.02nm bandwidth and diffraction limited output due to double external cavity feedback

External cavity feedback for a broad-area laser diode (BAL) with antireflection coating on the front facet is investigated experimentally. The feedback is created by a high-reflection mirror and a blazed grating, which reflect two parts of the laser emission back to the BAL, thereby forming an external resonator. When a Fabry–Perot etalon was inserted into the resonator an output beam with a bandwidth of 0.02nm and power of 150mW was obtained. The beam diffraction limited factor amounted to M2=1.16.

High-power narrowed-bandwidth output of a broad-area multiple-stripe diode laser with photorefractive phase-conjugated injection

By using a new mutually pumped phase-conjugation setup consisting of two photorefractive crystals, a broad-area three-stripe diode laser is locked to a single-mode diode laser at a drive current up to 2.25 times the threshold current. The bandwidth is narrowed from about 2 to 0.034 nm, with an available output power of 1.5 W.

Determination of the Jones matrix of an LC cell and derivation of the physical parameters of the LC molecules

The wavelength-dependent Jones matrix representation of a twisted-nematic liquid crystal (TN-LC) cell contains four independent parameters. The absolute values of these parameters and two mutual sign relationships can be determined from comparatively simple transmission measurements of the TN-LC cells sandwiched between two rotatable polarizers. The physical parameters of the cell (twist angle α, director orientation ψ, birefringence β) can be retrieved if the Jones matrix is known for more than one wavelength. We have measured the Jones matrices of the TN-LC cells of a translucent Sony LCX-016 microdisplay for six wavelengths ranging from 488nm to 1064nm and determined the physical parameters of the cell. We have also measured the Jones matrices for one wavelength for a number of applied voltages. These experimental results show that it is not sufficiently exact to calculate the Jones matrix from the known physical parameters of the cell assuming a voltage-dependent birefringence only. We attribute the deviations from the theoretical model to edge effects which are not taken into account. The direct experimental determination of the Jones matrix components is therefore preferable and permits a more accurate simulation of the TN-LC microdisplay in experimental configurations involving other polarization-dependent optical components.

Design of diffractive beam-shaping elements for nonuniform illumination waves

A method for the design of diffractive beam-shaping elements is presented which extends the finite-element-meshes method proposed by Dresel et al. In particular, the modified method can be applied to non-uniform intensity distributions in the input and output planes. This extension is of practical significance due to the non-uniformity of real laser beam intensity distributions. The method is shown to result in so-called smooth phase functions which are desired for beam- shaping applications to overcome the well-known speckle problem. Transmission functions of diffractive phase-only elements have been computed solving beam-shaping problems of practical relevance. We have demonstrated by numerical simulation experiments that the computed elements perform the desired beam-shaping operations in good approximation.

New HDTV (1920x1080) phase-only SLM

With this paper we present a new developed phase-only LCOS (Liquid Crystal On Silicon) spatial light modulator (SLM) based on an electrically controlled birefringence (ECB) liquid crystal mode for dynamic diffractive optics applications, optical tweezing, wave front control, digital holography and beam/pulse shaping. This device is the first phase-only SLM showing HDTV resolution and a small pixel pitch of only 8&mgr;m (87% fill factor) on a digital silicon back plane. Here the LC molecules are aligned parallel to the electrodes and an applied electric field forces them to tilt in the direction of the field. In this way, the refractive index seen by the light is changed for one polarization direction. This leads to a pure phase modulation without any polarization change (<1%) if the incident light is polarized linearly parallel to the director axis of the LC molecules. We have investigated two versions of this new SLM. One version is optimized for the visible wavelength region (420-800nm) and the other one is designed for 2&pgr; phase retardation up to 1064nm. We will discuss the optical modulation and show measurements on reflectivity, diffraction efficiency as well as measurements of the surface quality (flatness). With user software one is able to adapt the electro-optical response of the system to different wavelengths and applications. Furthermore, we discuss the optical effect of different sequence encoding for the phase modulation properties.

Applications of the high-resolution optical reconstruction of digital holograms

Adressable spatial light modulators with as much as possible ideal phase modulation are the precondition for their application in digital holography. An adapted driver electronics for the modulator and a correct knowledge of the modulation behavior can lead to a dynamic phase modulating device with nearly linear characteristic curve and a maximum phase range of 2π. We show a system for recording and reconstruction of digital holograms applying a spatial light modulator for the optical reconstruction and the digital processing of the holograms. The data of a CCD-camera are taken to a PC and sent to a spatial light modulator. In that sense we realised an analog-digital converter for recording and a digital-analog converter for the optical hologram reconstruction. We discuss the resolution of the reconstruction and their applications, especially possibilities for the manipulation with the reconstructed wave field.