  Photonic integrated circuits (PICs) are today acknowledged as an effective
solution to fulfill the demanding requirements of many practical applications
in both classical and quantum optics. Phase shifters integrated in the photonic
circuit offer the possibility to dynamically reconfigure its properties in
order to fine tune its operation or to produce adaptive circuits, thus greatly
extending the quality and the applicability of these devices. In this paper, we
provide a thorough discussion of the main problems that one can encounter when
using thermal shifters to reconfigure photonic circuits. We then show how all
these issues can be solved by a careful design of the thermal shifters and by
choosing the most appropriate way to drive them. Such performance improvement
is demonstrated by manufacturing thermal phase shifters in femtosecond laser
written PICs (FLW-PICs), and by characterizing their operation in detail. The
unprecedented results in terms of power dissipation, miniaturization and
stability, enable the scalable implementation of reconfigurable FLW-PICs that
can be easily calibrated and exploited in the applications.
