# Tunable Fabry-Perot interferometer

1. Jun 24, 2009

### Niles

Hi all.

I am trying to understand how a tunable Fabry-Perot interferometer works. This is what I have understood so far:

An etalon is placed inside a pressure vessel, and the pressure inside this chamber is changed. Apparently, this changes the "effective optical path length" between the two mirrors of the etalon, and thus scans the light over a frequency interval. The change in pressure should alter the index of refraction, but I cannot see how this changes the frequency of the outputted light.

Last edited: Jun 24, 2009
2. Jun 24, 2009

### Niles

Ok I just read this from Wikipedia (http://en.wikipedia.org/wiki/Optical_path_length): [Broken]

"Optical path length is important because it determines the phase of the light and governs interference and diffraction of light as it propagates.".

So by changing the pressure, one can change the phase of the light. Can this be used to alter the frequency?

Last edited by a moderator: May 4, 2017
3. Jun 24, 2009

### Andy Resnick

It's used to alter the frequency of light that is efficiently transmitted. In order for this device to work, there has to be a broadband source somewhere. Perhaps it can, like a distributed bragg reflector laser, work via feedback mechanisms as well.

4. Jun 24, 2009

### Niles

I'm supposed to give a very short presentation on a topic, which includes this. Is it correct if I say that a change in the phase is used to alter the frequency?

Thanks for responding. I really appreciate it.

5. Jun 24, 2009

### Niles

"It is impossible to change the phase of a sinewave without changing its frequency. It is impossible to change the frequency of a sinewave without changing its phase."

6. Jun 24, 2009

### Andy Resnick

That's fine for coherent generation and detection, when the absolute phase can be measured. That's not the case (yet) for optical frequencies, so in practical terms one does not control the absolute phase to adjust the frequency.

7. Jun 24, 2009

### Bob S

The Fabry-Perot interferometer works on constructive interference of the light reflecting between mirrors. If the mirrors are in a pressurized gas, then the gas pressure can be used to change the light velocity, and this will change the number of wavelengths and the interference between reflections. The index of refraction of air is about 1.0003 at -15 centigrade, and 1.0024 at 60 centigrade, for example.

8. Jun 25, 2009

### Andy Resnick

Minor quibble- the primary effect is to change the refractive index, this changes the path length (and propogation velocity).

9. Jun 30, 2009

### onlyengg

hi ...

can any one tell me which kind of interference between incident light waves and reflected light waves at first mirror and then between two mirrors means in cavity and then at last mirror .

Last edited: Jun 30, 2009
10. Jun 30, 2009

### Bob S

11. Jul 1, 2009

### onlyengg

can anyone tell in simple manner what type of interference is occured between the incident waves and the reflected waves at the first mirror ,in cavity and at 2nd mirror of fabry perot filter.......

with regards

12. Jul 1, 2009

### Bob S

The mirrors are partially silvered, and the interference occurs outside the mirrors. See
http://hyperphysics.phy-astr.gsu.edu...opt/fabry.html [Broken]

Last edited by a moderator: May 4, 2017