Low dispersion notch filter for multi-GHz frequencies using fiber optics delays

Although their designs appear simple and straightforward, notch filters for application in stochastic cooling systems are frequently difficult to construct with acceptable characteristics. Problems arise because the components are less than perfect, coaxial cables have loss plus dispersion and other necessary hardware has limitations. Usually it is the delay line, typically a high quality coaxial line, which is the source of most problems. We report in this paper on the development of a notch filter which uses a laser diode/fiber optic delay line instead of a coaxial cable. Measurements of its properties indicate it to be a possible alternative to other designs. Bandwidth, dispersion, notch depth, dynamic range and intermodulation are each within the bounds of typical cooling system requirements.