  Integrated optical filter based on microring resonators plays a critical role
in many applications, ranging from wavelength division multiplexing and
switching to channel routing. Bandwidth tunable filters are capable of meeting
the on-demand flexible operations in complex situations, due to their
advantages of scalability, multi-function, and energy-saving. It has been
investigated recently that parity-time (PT) symmetry coupled-resonant systems
can be applied to the bandwidth-tunable filters. However, due to the trade-off
between the bandwidth-tunable contrast ratio and insertion loss of system, the
bandwidth-tunable contrast ratio of this method is severely limited. Here, the
bandwidth-tunable contrast ratio is defined as the maximum bandwidth divided by
the minimum bandwidth. In this work, we show that high bandwidth-tunable
contrast ratio and low insertion loss of system can be achieved simultaneously
by increasing the coupling strength between the input port and the resonant.
System characterizations under different coupling states reveal that the low
insertion loss can be obtained when the system initially operates at the
over-coupling condition. A high bandwidth-tunable contrast ratio PT-symmetry
band-pass filter with moderate insertion loss is shown on the Silicon platform.
Our scheme provides an effective method to reduce the insertion loss of on-chip
tunable filters, which is also applicable to the high-order cascaded microring
systems.
