  We present a new method of DEEM, the direct energy encircling method, for
characterising the performance of fibres in most astronomical spectroscopic
applications. It's a versatile platform to measure focal ratio degradation
(FRD), throughput, and point spread function (PSF). The principle of DEEM and
the relation between the encircled energy (EE) and the spot size were derived
and simulated based on the power distribution model (PDM). We analysed the
errors of DEEM and pointed out the major error source for better understanding
and optimisation. The validation of DEEM has been confirmed by comparing the
results with conventional method which shows that DEEM has good robustness with
high accuracy in both stable and complex experiment environments. Applications
on the integral field unit (IFU) show that the FRD of 50$\mu$m core fibre is
substandard for the requirement which requires the output focal ratio to be
slower than 4.5. The homogeneity of throughput is acceptable and higher than 85
per cent. The prototype IFU of the first generation helps to find out the
imperfections to optimise the new design of the next generation based on the
staggered structure with 35$\mu$m core fibres of $N.A.$=0.12, which can improve
the FRD performance. The FRD dependence on wavelength and core size is revealed
that higher output focal ratio occurs at shorter wavelengths for large core
fibres, which is in agreement with the prediction of PDM. But the dependence of
the observed data is weaker than the prediction.
