High stability laser frequency references at 1.5 μm using molecular lines: state of the art and prospects

The main results obtained with 1.5 μm frequency references using molecular lines are summarised. It is shown how to detect their saturated-absorption and experimentally obtained saturated-dispersion spectra are presented. The main problems that still remain to be solved are pointed out. Multifrequency standard which could precisely and automatically lock to many reference frequencies is also considered.