Spatial light modulators (SLM) offer a broad range of opportunities in optics. Especially liquid crystal on silicon (LCOS) devices are in common use, due to their extreme high spatial resolution and up-scalable production capabilities compared to other technologies. Still, the architectural complexity of these displays causes well known phase errors, such as the inherent backplane curvature, crosstalk between adjacent pixels or spatial varying phase response (SVPR). In our latest work we presented a robust method to characterize the 2D resolved phase response of a SLM by means of a Twyman- Green interferometer and how to calculate a compensation phase mask. Based on the non-linear deviation of the pixel retardation measurements, it was indicated that a pure LC layer thickness variation is not the only contributor to the SVPR. In this work we want to propose a more representative model by taking interfering reflections of different layers, the socalled Etalon effect, into account. The 2D pixel phase response was simulated via Ericksen-Leslie equation and included in the transfer matrix method. The introduced Etalon model is validated by complementary measurements for different center wavelengths. A white-light monochromator was added to the setup to adjust these properties accordingly. First implications of systematic error compensation will be discussed.
The particular capabilities of selected combined adaptive optical systems for laser beam shaping in near infrared were studied. Fast switching sequences were obtained by combining an LCoS-SLM with angular-tuning, piezo-driven MEMS-axicons. By flexibly programming highly localized beams into SLM phase maps, the illumination of MEMS microaxicons, tunable spiral phase plates and Fresnel mirrors was optimized to enable for fast, variable and nondiffracting shaping performance. The approach can be applied to advanced types of optical processors like adaptive autocorrelators, or in micromachining. By varying the divergence of an illuminating beam with a liquid lens, spatio-temporal self-imaging of array patterns known as nondiffracting Talbot effect was demonstrated in adaptive mode with tunable Talbot distances.
Previously we studied the spectral Gouy rotation as a specific rotational phenomenon of conical polychromatic light fields shaped by spiral gratings. The rotation of spectral anomalies around singularities results from accumulated spectrally dependent Gouy phase shift. We proposed to apply radially chirped spiral structures to obtain an axial modulation of the rotational characteristics. Here we present related experimental results with non-uniform spiral gratings which were programmed into a 10-Megapixel, phase-only, liquid-crystal-on-silicon (LCoS) spatial light modulator (SLM). A propagation-dependent variation of the Gouy rotation was indicated. More complex non-uniform geometries are considered.
Spatial light modulators (SLM) based on liquid crystal on silicon (LCOS) are widely used for phase-related applications. The inherent aberrations such as curvature and liquid crystal thickness variation that are caused by the fabrication process may need to be compensated. Measuring the pixel dependent phase response, by means of a Twyman-Green interferometer at 640 nm, enables high precision calculation of compensation functions, leading to a wavefront flatness in the order of λ/11 (peak-to-peak). In this work we present the performance of the measurement setup, the 2D phase response of multiple LCOS panels and the results of active compensation of major aberration effects.
The large bandwidth and high intensity of ultrafast vortex pulses, i.e. pulses with orbital angular momentum (OAM), open new prospects for applications in communication, imaging or nonlinear photonics. In previous experiments, we demonstrated the peculiar spatio-spectral behavior of pulsed polychromatic vortex beams in the vicinity of phase singularities. It was shown that the rotation of characteristic, so-called “spectral eyes” and the spectral dependent Gouy phase are closely connected. For practical applications, a controlled variation of spatio-spectral distributions is required. Here we report on our most recent studies concerning the dependence of time-integrated spectral maps on key optical parameters. It is shown that the speed of rotation of spectral eyes during the propagation is essentially determined by the angular and spectral profiles. This enables to modify the spectral rotation characteristics by applying low-dispersion, adaptive optical components. The performance of reflective liquid-crystal-on silicon spatial light modulators (LCoSSLMs) is compared to diffractive spiral gratings with variable illumination. Moreover, the generation of wavepackets with a time-dependent orbital angular momentum (self-torque) by superimposing multiple tailored vortex pulses is proposed. This allows for extending the capabilities vortex pulses by defined non-stationary spatio-spectral and topological characteristics.
Recently it was reported that free-space propagating, ultrashort-pulsed polychromatic beams with orbital angular momentum (OAM) show a spectral Gouy rotation (SGR) of red- and blue-shifted areas around singularities. In femtosecond laser experiments with different types of spiral phase gratings, pulse propagation in spectral domain was studied with high resolution and sensitivity. By analyzing maps of spectral moments it was found that the interference of multiple OAM beams leads to a periodical revival of SGR by diffractive Talbot self-imaging. If the wavefront twist of the sub-beams is synchronized (co-rotating vortices), an optimum performance is found. In contrast, SGR echoes of counter-rotating beams are periodically distorted by destructive interference. Thus, the fine structure of self-imaged spectral maps enables to sort partial beams from interference patterns by even extremely weak imprinted vorticity information. It may further have implications for highly nonlinear processes and opens new prospects for applications in metrology, optical computing, or interferometry.
Spatially resolved spectroscopy of vortex beams is able to test the state of optical systems, to decode specific information or to sensitively indicate light-matter interactions. Spectral maps of ultrashort vortex pulses generated by hybrid diffractive-reflective spiral phase plates were studied experimentally and theoretically. Local spectral maps were detected by high-resolution scanning with a fiber-coupled spectrometer. Distributions of spectral centers of gravity and second moments were analyzed for femtosecond pulses. Gouy rotation of characteristic spectral features in the proximity of a phase singularity as a function of propagation distance was indicated in the spectral domain. Angular rotation was found to be modulated by weak oscillations. Analysis of spectral meta-moments indicates a fast switching and twisting behavior of spatial chirp.
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