Talk:Frequency comb

This article could use some cleanup and some references, particularly journal articles.Patrick Berry 02:39, 11 September 2006 (UTC)

[Definition not clear] Despite two science degrees and engineering experience, I cannot tell from this what a "frequency comb" actually is. Is it a laser with a piezoelectric mirror with a phase correcting feedback loop? Is it a technique for mapping radio frequecies into the optical frequency range? Is it the the graphic representation?67.142.130.25 00:56, 5 April 2007 (UTC)

I would like to support this comment. In fact the term frequency comb is, in my humble opinion, misleading and a misnomer (it is a very very general and broad term) if one talks about frequency comb _lasers_ or _optical_ frequency combs. A square wave or microwave pulse generator is an electromagnetic (thus ultra low frequency "optical") frequency comb as well. In the past the term frequency comb has been used for such devices (e.g. in Electrical Spectrum & Network Analyzers: A Practical Approach By Albert D. Helfrick, section 8.2.2 and Ch 8 questions) although comb generator, harmonic generator or like terms are more widely spread.

It might be that a frequency comb is solely defined by the relation f_m=f_0 * mf_rep, where f_m is the frequency of comb mode m, f_0 is the offset frequency of the comb from 0 and f_rep is the repetition frequency. In this case one could for example define acoustic combs. Another pressing question currently not answered in the community is that of the mode phase offsets (the phi_m in E_m=A_m*cos(omega_m*t + phi_m). If the phase offsets are randomly chosen over the comb spectrum, one does for example not get pulsed output, but all the same, the frequency content of the frequency comb is very well defined. The same hold for the freely assignable parameter A_m.

With regard to frequency comb lasers, I have heard the line: "If it is not locked (what is locked? f_0 and f_rep were meant in this case) it is not a comb", on the other hand, one could argue, if it is not locked, it is a lousy comb (and, on average, not well defined for certain high m, since modes can smear out over each other, although the relation f_m(t)=f_0(t) + m*f_rep(t) still holds on each moment in time.) and not very usefull for accurate metrology purposes, especially not in the optical.

Frequency comb widening to one octave
Could we have an intro to this section saying the main reasons why one would want to widen the comb to at least an octave, eg. to tune one frequency to double another in the same comb, and for what purposes. Rod57 (talk) 14:23, 27 March 2009 (UTC)

I think that the main reason for widening the spectrum to one octave is to allow performing the f-2f technique in order to measure (and stabilize) the Carrier Envelope Offset frequency. Maybe the expression "To be usable..." is not appropriate here. There are many uses for less-than-one-octave mode-locked lasers.

The f-2f technique means that a frequency comb can be self referenced, in other words it can measure it's own f_0. One can always measure f_0 with external means, also if the frequency comb is not an octave wide. Thus a self referenced comb can be used "in itself"/alone to measure frequencies with as high accuracy as the primary frequency reference allows. For optical frequency measurements this means that things like (calibrated) iodine references are not needed anymore to determine "absolute" optical frequencies. This is exactly the reason why the technical realisation of octave wide spectra were persued very hard by several research groups around the year 2000.

One thing that I don't understand is why the supercontinuum generated in a fiber keeps the comb offset of the original pulse. Can anyone clarify this? — Preceding unsigned comment added by 138.131.232.42 (talk) 14:40, 29 February 2012 (UTC)

The supercontinuum generation is a so called chi^(3) nonlinear optical process, which enables four wave mixing (note, the glasses used in the fibres do typically not posess (high/observable) chi^(2) nonlinearity). The phase relation between the photons absorbed and generated is preserved to a high degree in such processes, and therefore the mode structure of the original frequency comb spectrum is also present in the nonlinearly broadened comb spectrum. It must be noted however that, in order to preserve "full" (or at least good enough for typical applications) coherence over the entire spectrum the pulses must be short enough. For fs frequency comb lasers, a pulse typically needs to be less than 100 fs for rather good, to less than 80 fs for high spectral coherence.

IMHO the offset frequency is preserved because the mode spacing is preserved in the nonlinear process (to very high degree, tested too). Then the collective f-2f beat (_all_ frequency doubled modes of the low f side) coherently bear with the modes of the 2f side of the broadened spectrum. f=f_ceo+m*f_rep and f=f_ceo+2m*f_rep, frequency doubling: 2f=2f_ceo+2m*f_rep (note: 2f_ceo) The difference frequency becomes then f_ceo+2m*f_rep - 2f_ceo+2m*f_rep = f_ceo. This makes that f_ceo is preserved as long as the mode spacing f_rep is preserved in the broadening. (I think I am correct here)

What is preserved for certain is the repetition rate, although the pulse deformes in a fully coherent way. This means new frequency content is created which "keeps" the f_rep spacing. At this point I would not dare to answer if f_0 can shift or not in the continuum generation.

What accuracy or uncertainty
Article says they (optical frequency combs) improve precision of frequency measurements but doesn't give figures. What can we say ? Rod57 (talk) 01:31, 28 March 2011 (UTC)

I should look up the references (no time now) but, if a comb is locked to an RF source, the optical modes are as stable as the RF source (order 10^-14 for cs clock at 100.000 seconds). If a comb is locked to an optical reference (like a stable ultra high finesse optical cavity) it can surpass the stability of RF sources on short time scales (<10^-15 at 1 - 10 seconds).

If a frequency comb is locked to the spectroscopy laser of an optical clock, which is locked to the clock transition, it obtains the long term stability of the optical clock (order 10^-17 at 1000 - 10000 seconds or so)

An important parameter here is the time scale on which the stability measure is wanted. e.g. combinations of frequency comb lasers and cavity loaded sapphire oscilators are made to obtain favourable properties of both — Preceding unsigned comment added by 145.99.194.134 (talk) 22:12, 21 December 2013 (UTC)

Uses
I think it would be useful for the non-technical reader to indicate in the introduction or a 'uses' section what are some of the uses of a frequency comb. --User:Ceyockey ( talk to me ) 01:22, 19 February 2015 (UTC)

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