  Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded
project hosted at KU Leuven and a new visitor instrument for the Very Large
Telescope Interferometer (VLTI). Its primary goal is to image the snow line
region around young stars using nulling interferometry in the L-band (3.5 to
4.0)$\mu$m, where the contrast between exoplanets and their host stars is
advantageous. The breakthrough is the use of a photonic beam combiner, which
only recently allowed the required theoretical raw contrast of $10^{-3}$ in
this spectral range. Nulling interferometry observations of exoplanets also
require a high degree of balancing between the four pupils of the VLTI in terms
of intensity, phase, and polarization. The injection into the beam combiner and
the requirements of nulling interferometry are driving the design of the warm
optics and the injection system. The optical design up to the beam combiner is
presented. It offers a technical solution to efficiently couple the light from
the VLTI into the beam combiner. During the coupling, the objective is to limit
throughput losses to 5% of the best expected efficiency for the injection. To
achieve this, a list of different loss sources is considered with their
respective impact on the injection efficiency. Solutions are also proposed to
meet the requirements on beam balancing for intensity, phase, and polarization.
The different properties of the design are listed, including the optics used,
their alignment and tolerances, and their impact on the instrumental
performances in terms of throughput and null depth. The performance evaluation
gives an expected throughput loss of less than <6.4% of the best efficiency for
the injection and a null depth of $\sim2.10^{-3}$, mainly from optical path
delay errors outside the scope of this work.
