Does calibration of an optical frequency comb require that the speed of light be known, either directly or indirectly? By indirectly I mean the case where something you need to use to calibrate your comb depends, directly or indirectly, on knowing the speed of light.
I'm curious because of something the professor did early on in the introduction to optics class I took in college. He picked up a metal ruler and said we were going to measure the speed of light. Everyone laughed (which was fine because he intended it as a joke).
He then set the ruler on a table, directed a laser to reflect at a shallow angle off the ruler onto the blackboard.
The ruler's lines were raised which made it act like a diffraction grating and there was a visible interference pattern on the blackboard.
He then traced the pattern on the blackboard with chalk, turned off the laser, and used the ruler to (1) measure the distance from where it had been to the blackboard, and (2) the spacing of the lines in the diffraction pattern.
From this and the known frequency of the laser and the known spacing of the lines on the ruler the speed of light is an easy calculation.
This was meant as a joke because usually the frequency of light is calculated using methods that depend on knowing the speed of light, so all that was really happening was the he used a rule to very that the frequency calculation of the laser had been done correctly.
But if you could accurately get the frequency without that depending on knowing the speed of light then you could actually measure the speed of light with a ruler.
Frequency combs are optical synthesizers. This means that the frequency of the nth comb mode is exactly n times a radio frequency (+ another radio frequency), where n is a (very large) integer. The speed of light does not matter at all. This is very important since the speed of light (and hence the wavelength) depends on environmental factors such as air pressure and humidity. One way to determine the speed of light (in air) would indeed be to measure the wavelength of a laser whose frequency is calibrated using a frequency comb.
The speed of light in vacuum cannot be measured in SI units since it is the fundamental constants that defines the unit "meter".
I'm curious because of something the professor did early on in the introduction to optics class I took in college. He picked up a metal ruler and said we were going to measure the speed of light. Everyone laughed (which was fine because he intended it as a joke).
He then set the ruler on a table, directed a laser to reflect at a shallow angle off the ruler onto the blackboard.
The ruler's lines were raised which made it act like a diffraction grating and there was a visible interference pattern on the blackboard.
He then traced the pattern on the blackboard with chalk, turned off the laser, and used the ruler to (1) measure the distance from where it had been to the blackboard, and (2) the spacing of the lines in the diffraction pattern.
From this and the known frequency of the laser and the known spacing of the lines on the ruler the speed of light is an easy calculation.
This was meant as a joke because usually the frequency of light is calculated using methods that depend on knowing the speed of light, so all that was really happening was the he used a rule to very that the frequency calculation of the laser had been done correctly.
But if you could accurately get the frequency without that depending on knowing the speed of light then you could actually measure the speed of light with a ruler.