Diffractive Optical Elements (DOEs) are lightweight, thin optical components with many applications in laser beam-shaping.
In this paper we consider the application of DOEs for coupling of high power Nd:YAG laser light to fibre
For the laser system in question intra-cavity DOEs are considered for the generation of a super-Gaussian cavity mode,
while an extra-cavity element is considered for shaping the beam to produce a profile suitable for fibre coupling.
The arrangement to be considered in our application involves coupling a 100mJ, 20ns pulse laser beam of 5mm diameter
into 3 fibres, each with a core diameter of 400μm, positioned in an equilateral triangle formation with a centre to centre
spacing of 2mm. The threshold power density for the fibres is 4.5GW/cm2.
512x512 pixel DOEs with 16 phase levels have been optimized using the iterative Fourier transform algorithm (IFTA).
The optimized element produces spots with a radius of 14 diffraction orders. The modeled efficiency of the element is
91.4% with a peak power of 1.26GW/cm2. Experimental measurements using a low power 633nm source equate to a
peak power of 2.65GW/cm2 for the high power laser, well within the damage threshold.
As the use of DOEs has become ever more popular, there has been a concurrent increase in the development of the
design algorithms used to optimise their phase profiles. The earliest design methods claimed efficiencies of around 75%
and an image spot intensity variation of ±15%. Methods used today can give efficiency percentages in the high 90s and
non-uniformities below 2%. As developments made in the design algorithms continue it increasingly becomes the case
that the major factor contributing to losses in efficiency and increases in non-uniformity are not the ability of the
algorithm to optimise the phase profile but the errors introduced by the fabrication process. In this paper we simulate
the effects of misalignment and feature rounding on the quality of the output intensity of 7 different fan-out gratings.
From these simulations we observe that the affect of misalignment on efficiency is generally greater for masks with
deeper etches, although the extent of the drop in efficiency can be influenced by the direction of misalignment. Nonuniformity
is less consistently affected, in some cases the π level is dominant, in others it is the π/2 level and there is
often strong asymmetry between negative and positive misalignment. Study of feature rounding produces results which,
as one might expect, indicate levels with deeper etches have a greater influence on the drop in efficiency and increased