A switchable and tunable coherence-free microwave photonic filter

15 Nov 2007

Researchers in Singapore describe a tunable and switchable coherence-free microwave photonic filter.

The large time bandwidth of photonic processors make them attractive for processing microwave and millimeter-wave signals, since they could the limitations of existing electronic processors. However, most conventional incoherent photonic processors cannot realize a negative coefficient, and those that can lack the flexibility to switch between bandpass and low-pass responses.

Now, however, researchers at Nanyang Technological University, Singapore, have fabricated a tunable and switchable coherence-free microwave photonic filter that can convert between bandpass and low-pass responses. The system is based on a Sagnac interferometer consisting of a variable polarization beamsplitter (VPBS) made from a fiber loop, a phase modulator, and a tunable time delay line (J. Opt. A: Pure Appl. Opt. 9 1136).

The VPBS generates a phase difference between the clockwise and counterclockwise waves propagating inside the fiber loop interferometer. By adjusting the time delay line, the free spectral range of the bandpass filter or low-pass filter can be tuned continuously.

The researchers describe the experimental results obtained in bandpass and low-pass modes, which demonstrate that the system exhibits a robust notch filter response and achieves more than 50 dB rejection.