  The PCI diagnostic is an internal reference interferometer that creates an
image of absolutely calibrated electron density fluctuations integrated along
the line of sight of the probing light beam. While conventional PCI diagnostics
installed on fusion experiments worldwide employ light of wavelength equal to
10.59 $\mu$~m, the same system using light at 1.55 $\mu$~m wavelength would
extend the spectral response in wave-number and frequency by factors of seven
and over one hundred, respectively, thereby potentially providing quantitative
measurements of the internal structure of density perturbations induced by
either turbulent or radio-frequency waves, simultaneously covering ion to
electron gyro-radius scales up to the GHz frequency region. Based on a
previously developed 1.55 $\mu$~m PCI prototype system, constraints to the
design for such a diagnostic in fusion grade devices are presented and compared
to those faced with the conventional method.
