Michael Duparré

Complete modal decomposition for optical fibers using CGH-based correlation filters

The description of optical fields in terms of their eigenmodes is an intuitive approach for beam characterization. However, there is a lack of unambiguous, pure experimental methods in contrast to numerical phase-retrieval routines, mainly because of the difficulty to characterize the phase structure properly, e.g. if it contains singularities. This paper presents novel results for the complete modal decomposition of optical fields by using computer-generated holographic filters. The suitability of this method is proven by reconstructing various fields emerging from a weakly multi-mode fiber (V approximately 5) with arbitrary mode contents. Advantages of this approach are its mathematical uniqueness and its experimental simplicity. The method constitutes a promising technique for real-time beam characterization, even for singular beam profiles.

Mode analysis with a spatial light modulator as a correlation filter

A procedure for the real-time analysis of laser modes using a phase-only spatial light modulator is outlined. The procedure involves encoding into digital holograms by complex amplitude modulation a set of orthonormal basis functions into which the initial field is decomposed. This approach allows any function to be encoded and refreshed in real time (60 Hz). We implement a decomposition of guided modes propagating in optical fibers and show that we can successfully reconstruct the observed field with very high fidelity.

Investigation of computer-generated diffractive beam shapers for flattening of single-modal CO 2 laser beams

A full cycle was realized of the photolithographic development and detailed testing of a diffractive optical element that transforms the diverging Gaussian beams of CO(2) lasers into a uniformly filled-in rectangle. The zone feature size of the beam shaper, the diffractive efficiency and accuracy, the focus depth, and the stability with respect to the size and the divergence of incident Gaussian beams are studied by computer modeling. Calculated flattop intensity distributions are presented in the same form of gray-level pictures and three-dimensional plots as the corresponding results measured by an IR camera.

Mode resolved bend loss in few-mode optical fibers.

We present a novel approach to directly measure the bend loss of individual modes in few-mode fibers based on the correlation filter technique. This technique benefits from a computer-generated hologram performing a modal decomposition, yielding the optical power of all propagating modes in the bent fiber. Results are compared with rigorous loss simulations and with common loss formulas for step-index fibers revealing high measurement fidelity. To the best of our knowledge, we demonstrate for the first time an experimental loss discrimination between index-degenerated modes.

All-Digital Holographic Tool for Mode Excitation and Analysis in Optical Fibers

A procedure for the multiplexing and demultiplexing of modes in optical fibers with digital holograms is presented. By using a spatial light modulator (SLM) to encode a digital hologram, the desired complex field is shaped and injected into the fiber. The SLMs ability to rapidly refresh the encoded transmission function enables one to excite pure single modes, as well as arbitrary coherent mode superpositions, in real-time. The modes from the output of the fiber are subsequently demultipexed by applying a correlation filter for modal decomposition, thus allowing for an all-digital-hologram approach to modal analysis of fibers. The working principle is tested using conventional step-index large mode area fibers being excited with higher-order single modes and superpositions.

Wavefront reconstruction by modal decomposition

We propose a new method to determine the wavefront of a laser beam based on modal decomposition by computer-generated holograms. The hologram is encoded with a transmission function suitable for measuring the amplitudes and phases of the modes in real-time. This yields the complete information about the optical field, from which the Poynting vector and the wavefront are deduced. Two different wavefront reconstruction options are outlined: reconstruction from the phase for scalar beams, and reconstruction from the Poynting vector for inhomogeneously polarized beams. Results are compared to Shack-Hartmann measurements that serve as a reference and are shown to reproduce the wavefront and phase with very high fidelity.

Real-time determination of laser beam quality by modal decomposition

We present a real-time method to determine the beam propagation ratio M2 of laser beams. The all-optical measurement of modal amplitudes yields M2 parameters conform to the ISO standard method. The experimental technique is simple and fast, which allows to investigate laser beams under conditions inaccessible to other methods.

Measuring the spatial polarization distribution of multimode beams emerging from passive step-index large-mode-area fibers

We measure the polarization state of each guided transversal mode propagating in step-index large-mode-area fibers (V≈4) using a correlation-filter based measurement technique in combination with a Stokes parameter measurement. The entire emerging beam, expressed in terms of a phase-dependent superposition of linearly polarized modes, demonstrates spatially varying polarization properties. By knowing the information about modal amplitudes and phase differences, full information about the optical field is available.

Measurement of the orbital angular momentum density of light by modal decomposition

We demonstrate a versatile method for the measurement of the orbital angular momentum (OAM) density of an optical field. By performing a modal decomposition with digital holograms, we reconstruct the full optical field from a small set of single-point intensity measurements, from which optical vortices, global OAM and OAM density can be derived. We validate the method on defined OAM-carrying beams yielding fidelities in the OAM density measurement of up to 99%, and subsequently apply the technique to unknown fields from optical fibers.

Fast M 2 measurement for fiber beams based on modal analysis

We report on a fast and experimentally easy technique for measuring the beam propagation ratio M(2) of light guided by optical fibers. A holographic filter enables us to determine amplitudes and phases of the excited fiber eigenmodes. The coherent superposition of modes allows the reconstruction of the optical field. With this information at hand, we are able to simulate the free-space propagation of the beam and to perform a virtual caustic measurement. Associated beam propagation ratios M(2) accurately agree with ISO-standard measurements.