Photonic crystal fibers allow the guidance of an advisedly chosen number of modes and a simultaneous realization of large
mode areas that are essential for high power fiber lasers. The occurring multi-mode beams have spatially inhomogeneous
polarization (e.g. radially polarized light), which is significant for several fiber laser applications. We report on a measurement
technique to determine this local property using a modal polarization analysis based on a rapid Stokes parameter
measurement combined with a computer-generated hologram. We use this method to obtain and visualize spatially varying
polarization ellipses as well as modal polarization states depending on the input polarization.
We investigate the beam emerging from a 3m-long passive large mode area multi-core fiber (MCF) at two different
wavelengths to demonstrate the analysis of differently complex mode mixtures. Our method uses a computer-generated
hologram (CGH) to modally decompose the beam exiting the fiber end facet. At 1064 nm the MCF is
measured to be single-mode whereas at 633nm ten modes are guided. The excitation of distinct mode mixtures
at 633nm is done by changing the input coupling situation enabled by a nano-positioning device. Furthermore
it is demonstrated how bending of the fiber alters the modal spectrum.
We report on a fast measurement procedure for the widespread beam propagation ratio of light emerging from
LMA fibers. The investigated beam is decomposed into its eigenmodes using a computer generated holographic
filter. The modally resolved measurement of amplitudes and phases enables the reconstruction of the optical
field. With the full field information, the propagation of the beam through free space is simulated and a virtual
caustic measurement is realized. After comparing the presented method with ISO-standard measurements it is
applied to conventional step-index fibers as well as to a multicore fiber.
We present a method to measure the complete field distribution emerging from photonic crystal fibers (PCFs). Assuming an
invariant fiber cross-section, the eigenmodes of a microstructured optical fiber can be calculated numerically. These spatial
modes build a complete set of orthogonal eigenfunctions. The modal decomposition of an arbitrary wave field guided
by the fiber is therefore unique. We use an adapted
computer-generated hologram to determine experimentally a single
complex-valued mode coefficient describing the amplitude and the phase of a specific eigenmode. Angular multiplexing
enables us to obtain simultaneously all mode coefficients for one polarization state. A second measurement with the
orthogonal polarization allows the determination of the complete field information described by a coherent superposition of
eigenmodes. Such a reconstructed near field distribution is compared to the measured intensity distribution and conformity
is obtained. Applying this method to a multimode effective index guiding fiber, we investigate how bending of the PCF
affects the modal composition at the fiber output for a wavelength of 1064 nm. Knowing the complete field in the fiber
output plane, the field distribution in every free space plane can be calculated by numerical propagation techniques. Thus,
the determination of the beam propagation ratio M2 can be virtually realized according to the ISO standard 11146 and
allows the comparison of the beam quality for different bending radii.
Modal decomposition by means of correlation filters has been proved as a key for real online laser beam analysis.
To compare that method with the "standard" M2 method, we generated series of different laser beams (1064 nm),
applied both methods to one and the same beam and evaluated achieved results. An adjustable Nd:YAG laser
served as transversal mode generator, delivering diverse "pure" Gauss-Hermite modes and superpositions of
modes, respectively. In the case of incoherent superposition of modes, their particular contribution to the
general M2 value should be proportional to their relative strength, whereas in the case of coherent superposition
M2 is distinctly influenced by phase differences between the discrete modal components. Achieved experimental
findings are well confirmed by computer simulation.
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