That's too restrictive a definition. For instance, I've built coherent detection systems with offset frequencies as high as 8 GHz. (That one was a homodyne interferometer for detecting submicron tin droplets moving at up to 3 km/s [Mach 9].)
The basic definition of optical coherence is the ability to form fringes. The fringes don't have to stay still. I took Joe Goodman's statistical optics class long ago, and his book is an excellent read: .
Cheers
Phil Hobbs
Have you ever used a delay line discriminatory to measure oscillator phase noise ? It can only be used to see phase noise a fair distance from the carrier
Yup. The first engineering thing I ever built for hire was a 12-GHz cavity discriminator, which I still have. It was built for measuring close-in phase noise. Eventually we got an HP 8566A, so we didn't need the discriminator any more.
Cheers
Phil Hobbs
On Feb 11, 2019, Phil Hobbs wrote (in article ):
We do this kind of thing all the time in radar, and we would consider this a coherent radar system.
But we would not claim that the doppler was coherent in the same sense as we use for reference signal generation. Even though the whole radar is
frequency terms, although phase is nonetheless lurking below. There is a implied distinction in there somewhere.
Forming fringes is exactly what happens when interference leads to cancellation, so that part is the same. That definition sounds awfully like a
Joe Gwinn
Was the cavity used to provide the delay?
ng
need
Sort of. It used the cavity to make a 90-degree phase shift. IIRC it used a circulator and a coax mixer, so we tuned it to null the DC, which also get s rid of most of the AM noise as well.
A high-Q resonator has a higher phase slope (d phi/df) than any feasible le ngth of coax.
An unbalanced Mach-Zehnder interferometer is also a delay discriminator.
Cheers
Phil Hobbs
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