I'm reading "Silicon nitride passive and active photonic integrated circuits: trends and prospects" and one of discussed application of $\rm SiN$ is "true delay line". All the references I checked also just discuss "true delay line" without saying exactly why "true" is needed to specify.
I know what a delay line is. But what is true delay line? I checked "false delay line" in google scholar and it returns no hits. Why is then a "true" distinction needed?
A true delay line is a pair of wires, or a waveguide or a transmission line with on which a single mode is propagating that is used to communicate and the geometric length of the line is proportional to its delay. A "false delay line" is a lumped element circuit, baseband or bandpass that is a filter and whose amplitude - frequency response is flat in the frequency range of interest but in the same range of frequencies the phase - frequency relationship is nearly linear, that is $\tau(\omega)=-\frac{d\phi}{d\omega}$ is approximately a constant, the effective delay. The term "false delay line" is not used by engineers that design such filters.
