# Generating mechanical frequencies

1. Oct 12, 2005

### androz

Hi there !

I have a question : does a mechanical system generating a lower frequency
when excited with a hig frequency exists ? In this case, would it be possible to start stimulated processes of emission of a lower frequency from a high frequency ? Then, we could imagine a kind of laser not made with photons but with mechanical waves like water waves.....

2. Oct 12, 2005

### pervect

Staff Emeritus
If you have a non-linear mechanical system, you should certainly be able to "mix" frequencies.

I.e. if you have frequency f1 and f2 as inputs, you should generate some components with frequency f1+f2 and some with frequency f1-f2.

However, if you only input a single frequency, I think a non-linear system will generate harmonics (f*2, f*3, etc) but not subharmonics (f/2, f/3 etc).

This is commonly done with non-linear electrical systems rather than non-linear mechanical systems, but the math is the same. A non-linear electrical system that adds frequencies is called a "mixer".

3. Oct 12, 2005

### Claude Bile

Any mechanical oscillater with a high Q factor is analogous to a laser. The defining charactersitic of a laser is not the amplification it can provide, but rather the narrow frequency range that the energy becomes locked in.

Claude.

4. Oct 12, 2005

### androz

I don't entirely agree with you Mr. Bile.
A laser is not only a high Q factor oscillator, it is also an amplifier placed in an oscillator. I can imagine a mechanical oscillator but I have more difficulties to imagine an amplifier.
My idea concerns water waves, they can be reflected by walls, diffracted ... we can do a lot a things with periodic arrays of obstacles like we do in optics. But the question I have is can we find a mechanical system, for water waves, that would absorbe the wave at a given frequency, and "transform" this wave in another wave at another frequency plus something else like heat or whatever ??
I have just read an interesting article this week in Physical Review Letters about refraction of water waves by periodic cylinder arrays. The results is quite similar as what we observe in optics. This is a very interesting article and subject. Imagine we could find an amplifing media for those waves, then we could easy reproduce the equivalent of a laser with water waves !! Water waves are easier to see and the time scale also is interesting. I was wondering how far we could investigate the analogy ??

5. Oct 16, 2005

### Claude Bile

Despite the acronym, lasers do not amplify anything. The correct acroynm for a laser is Light oscillation by stimulated emission of radiation, however this would result in the unfortunate acronym of 'Loser'. Lasers posses gain, but this is not necessarily synonymous with amplification.

You CAN have light amplification, but such devices do not have any feedback Erbium doped fibre amplifiers for example are true amplifiers.

Using an electrical analogue, a laser performs exactly the same function as an electrical oscillator, not an amplifier.

Claude.

P.S.

Essentially you are asking whether it is possible to pump a mechanical 'laser' with another mechanical wave, analogous to an optically pumped laser. Many lasers are not optically pumped, they are pumped by other means, so you don't necessarily need to pump this 'laser' with a mechanical wave.

Last edited: Oct 16, 2005
6. Oct 17, 2005

### androz

Thank you for the exaplanation, but I am aware of the little history .... In fact I do not really agree with you when you say that a laser do not amplify anything. Yes it is an oscillator, but it is also a gain medium as an amplifier. From noise, you amplify the signal coherently at each pass in the oscillator (stimulated emission).
Yes I know, but what you omit is that it is an oscillator PLUS a source term which represents the atoms in the media. You do not amplify the light going through the active media, but you amplify the light generated in the active media. Laser effect occurs however when resonance if achieved, or you generate only ASE. So you need an amplifier AND an oscillator.
Of course yes, but if such a "laser" is not pumped mechanically, my question wouldn't have any sense.

7. Oct 17, 2005

### Claude Bile

I see where you're coming from, I guess it depends on what you refer to when you say amplification. From a signals perspective, the laser, as a source, cannot amplify a signal, it simply emits coherently over a narrow range of frequencies. A true amplifier can amplify a signal, such optical amplifiers are simply lasers without mirrors (such as EDFAs).

I'm not trying to argue that your point of view is invalid, I am just explaining where I was coming from in my earlier comment.

8. Oct 18, 2005

### androz

Thank you Claude for all.
I don't want to argue on lasers which are my research area. All I wanted to know is if it exists an equivalent optically pumped laser in mechanics, a mechanically pumped "mechanical laser" ?

PS : an optical amplifier amplify the signal coherently too.

9. Oct 18, 2005

### pallidin

I am not an expert, although I am well versed in some aspects of physics.
Anyway, if I am understanding your question correctly, I doubt that what you propose is possible if you are referring to photons. If you are referring to creating a mechanical system that creates coherency with sound waves, yes, that is possible.
However, there IS a way to "mechanically" amplify a light source non-coherently, but the technique somewhat depends on "non-use" of the resultant amplification. For example, take a laser light and direct it at a mirror.
Now, from that reflection beam have another mirror that directs it to the same point as the reflection.
Minus reflection losses, one ends up with an almost doubled intensity impact on the first mirror.

10. Oct 18, 2005

### androz

Hi Pallidin,

I'm refering to photons, but to mechanical waves.
Your example of the coherent sound waves interest me a lot. If you can direct me on a web link, an article, or whatever, that's would be appreciated.
I was in fact more interested with water waves, a way to amplify them mechanically an place them in a mechanical oscillator.....