Signal filter designs and techniques use a signal filter of a given order to achieve a higher order filter operation than what the filter is designed for. In various implementations, the present signal filter designs and techniques allow achieving desired high order filter operation without using a high order filter. Specific examples are provided to use a double pass configuration to feed a once filtered signal by a filter back to the filter to filter the feedback signal for the second time and a Sagnac interferometer configuration to cause interference between two once filtered signals.