Modal decomposition for photonic crystal fibers using computer-generated holograms

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 M-2 can be virtually realized according to the ISO standard 11146 and allows the comparison of the beam quality for different bending radii.