Robert Brüning

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.

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.

Fiber propagation of vector modes

Here we employ both dynamic and geometric phase control of light to produce radially modulated vector-vortex modes, the natural modes of optical fibers. We then measure these modes using a vector modal decomposition set-up as well as a tomography measurement, the latter providing a degree of the non-separability of the vector states, akin to an entanglement measure for quantum states. We demonstrate the versatility of the approach by creating the natural modes of a step-index fiber, which are known to exhibit strong mode coupling, and measure the modal cross-talk and non-separability decay during propagation. Our approach will be useful in mode division multiplexing schemes for transport of classical and quantum states.

Comparative analysis of numerical methods for the mode analysis of laser beams

We present a comparative study of four numerical methods to detect the mode content of a laser beam from, at most, two intensity images. The techniques are compared regarding temporal effort, stability, and accuracy, using the example of three multimode optical fibers that differ in the number of supported modes.

Mode Coupling in Few-Mode Fibers Induced by Mechanical Stress

We investigate mode coupling in few-mode fibers induced by homogeneously and periodically applied mechanical stress. To view the power transfer between individual modes, a modal decomposition is performed at the end of the fiber using computer-generated holograms. Coupling between polarization and angular degenerated modes as well as between non-degenerated modes is confirmed experimentally and coupling parameters are inferred. The presented studies pave the way to detailed investigations of mode coupling in mode-multiplexed telecommunication systems and high-power high beam quality fiber lasers.

Measurement of effective refractive index differences in multimode optical fibers based on modal decomposition.

We demonstrate the nondestructive measurement of the effective refractive index difference of two arbitrary modes within a multimode optical fiber by utilizing a tunable fiber grating. We use a mechanical grating of variable period to couple the respective modes and measure the mode content at the fiber output based on the correlation filter technique. From the dependence of the coupling efficiency on the grating period, the effective index difference of the modes can be extracted with high accuracy.

Measurement of the orbital angular momentum density of Bessel beams by projection into a Laguerre–Gaussian basis

We present the measurement of the orbital angular momentum (OAM) density of Bessel beams and superpositions thereof by projection into a Laguerre-Gaussian basis. This projection is performed by an all-optical inner product measurement performed by correlation filters, from which the optical field can be retrieved in amplitude and phase. The derived OAM densities are compared to those obtained from previously stated azimuthal decomposition yielding consistent results.

Selective higher order fiber mode excitation using a monolithic setup of a phase plate at a fiber facet

Controlling the modal content coupled into an optical fiber can be desirable in many situations, e.g. for adjusting the sensitivity of the guided field distribution to external perturbations1. For this purpose we used a monolithic setup of a phase plate at a fiber input facet to excite selectively higher order modes, which theoretically can provide a mode purity of more than 99%. We investigated the capabilities of this approach by complete modal decomposition of the fiber output signals, considering the achievable mode purity with respect to several possible imperfections of the setup. The experiments are compared with detailed numerical simulations and show a high agreement. Additionally a comparison with a well known setup with free space phase plates2–4 was undertaken. This showed the monolithic setup to be energetically twice as efficient.

Direct fiber excitation with a digitally controlled solid state laser source.

Mode division multiplexing has been mooted as a future technology to address the impending data crunch of existing fiber networks. Present demonstrations delineate the light source from the mode creation steps, potentially inhibiting integrated solutions. Here we create fiber modes on demand with a digital laser and couple them directly into a few-mode fiber, where after transmission they are decoupled by modal decomposition. This is the first demonstration of an integrated source for encoding information into the spatial modes of light.

Overlap relation between free-space Laguerre Gaussian modes and step-index fiber modes

We investigated the overlap relation of the free-space Laguerre-Gaussian modes to the corresponding linearly polarized modes of a step-index fiber. To maximize the overlap for an efficient coupling of the free-space modes into a fiber, the scale-dependent overlap was theoretically and experimentally determined. The presented studies pave the way for further improvement of free-space to fiber optical connections.