To understand star formation and its evolution, one needs to solve
the problem of ejection and collimation mechanisms in jets from young
stars. By measuring the jet opening angle at the ejection region we
can test models for jet origin. However this very small region cannot
be investigated with current monolithic telescopes. Only the AMBER
near-IR instrument coupled with the VLTI provides enough resolution
to reach the ejection region and enough spectral resolution/coverage
to properly separate the emission line jet from the continuum. In
this proposal we show that expected visibilities from jets can
constrain the jet opening angle.