> On Thursday, October 24, 2002 at 11:01:10 AM UTC+2, BS wrote:
> If you need high frequency locking, I suggest you actually lock your fringes
> around a fixed frequency, not around a fixed position. For instance,
> introduce a 100Hz movement into one of the mirrors of your setup via a piezo
> and correct this movement with a second mirror of the setup (also on a
> piezo). The advantage here is that because your mirrors are already
> vibrating at a 'high' frequency (but out of phase so as to cancel their
> movements), you get a much higher bandwith in the correction of random phase
> shift errors. In addition, you don't actually need to use two photodetectors
> (which are not so handy) but only one. A shift of the fringes in one
> direction then results in a decrease (or respectively increase) of the
> 'carrier' frequency and hence direction of the shift is known and can be
> corrected for.
> BS.

Greetings, reviving that interesting thread as I am currently digging into the subject of Fringe lockers.
I have always believed the approach as described above with a carrier frequency should be way better. I remember reading a paper about this approach back in 1995 or so but have been frustrated unsuccessfully trying to find it..
Anyhow, my question is about how the control system would look like with the carrier frequency approach? I am guessing it is not the same design as with two photodiodes but rather a PLL approach? If anyone has more info on a possible design I am interested!

> Anyhow, my question is about how the control system would look like with the carrier frequency approach?

[Edit] sorry, I meant to say "Anyhow, my question is about how the control system would look like with the single photodetector approach? "

On Wednesday, 8 February 2023 at 15:41:42 UTC, HoloLab wrote:
> > On Thursday, October 24, 2002 at 11:01:10 AM UTC+2, BS wrote:
> > If you need high frequency locking, I suggest you actually lock your fringes
> > around a fixed frequency, not around a fixed position. For instance,
> > introduce a 100Hz movement into one of the mirrors of your setup via a piezo
> > and correct this movement with a second mirror of the setup (also on a
> > piezo). The advantage here is that because your mirrors are already
> > vibrating at a 'high' frequency (but out of phase so as to cancel their
> > movements), you get a much higher bandwith in the correction of random phase
> > shift errors...

I'm possibly being thick here, and my hands on holography experience is decades out of date, but I'm struggling to find an intuitive understanding for why vibrating the mirror is better vis-a-vis random phase shifts: surely half the time the mirror is travelling in the wrong direction, so first you have to stop it, and then move it to the right place but you have now less time to do so (owing to time spent halting it)...

Maybe if the vibration was mains-electricity related (motors, etc.) it would make more sense?

Thanks

Henry

On Sunday, February 12, 2023 at 11:43:32 PM UTC+1, Henry Nebrensky wrote:
> On Wednesday, 8 February 2023 at 15:41:42 UTC, HoloLab wrote:
> I'm possibly being thick here, and my hands on holography experience is decades out of date, but I'm struggling to find an intuitive understanding for why vibrating the mirror is better vis-a-vis random phase shifts: surely half the time the mirror is travelling in the wrong direction, so first you have to stop it, and then move it to the right place but you have now less time to do so (owing to time spent halting it)...
>
> Maybe if the vibration was mains-electricity related (motors, etc.) it would make more sense?
>
> Thanks
>
> Henry

My (limited) understanding of it is that in the case of the control loop with a vibrating mirror, the control loop is about making slight adjustments (increasing or decreasing) to the carrier frequency while the control loop without a vibrating mirror has to start from idle to a given frequency (that of the random phase shifts) to compensate them which might be less "efficient"? (serious lack of formalism here, lol)