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Laser locking (control loops with two feedback paths.)

Started by George Herold January 17, 2013
Laser locking (control loops with two feedback paths.)

So I finally had a user ask about side locking our diode laser.
(That=92s where you lock the frequency to the side of an absorption
feature.)
Now, you can change the laser frequency in two ways.  There=92s a piezo
stack that changes the angle of a diffraction grating.  And you can
change the laser current.
The electronics is all set up to lock the laser with the piezo.
Signal chain looks like,

Photodiode->low pass (1 pole, tc =3D 100ms)->
DC offset->gain->modulation input of piezo control.

With some other bits of gain adjustment sprinkled in there.  (The low
pass is working as both integrator and gain (PI), you crank up the
overall loop gain till it oscillates and then back off a bit.)
This works fine, up to ~3kHz the oscillation frequency.

Now I=92ve heard tell of a trick where I break the error signal into a
low frequency and high frequency part.  And then send the high
frequency part into the laser current modulation input.
It seems I should pick off the error signal before the lowpass (P/I
part of signal chain).(?)
But I'm wondering how to deal with the 'break frequency'
What frequency for the HP?
And do I roll off the 'DC' part at the break frequency too?
(1 pole each)
Or can I leave the rest of the 'DC' signal chain the same if I pick
the right frequency?

Thanks,
George H.
On Thu, 17 Jan 2013 15:45:57 -0800, George Herold wrote:

> Laser locking (control loops with two feedback paths.) > > So I finally had a user ask about side locking our diode laser. (That’s > where you lock the frequency to the side of an absorption feature.) > Now, you can change the laser frequency in two ways. There’s a piezo > stack that changes the angle of a diffraction grating. And you can > change the laser current. > The electronics is all set up to lock the laser with the piezo. > Signal chain looks like, > > Photodiode->low pass (1 pole, tc = 100ms)-> > DC offset->gain->modulation input of piezo control. > > With some other bits of gain adjustment sprinkled in there. (The low > pass is working as both integrator and gain (PI), you crank up the > overall loop gain till it oscillates and then back off a bit.) > This works fine, up to ~3kHz the oscillation frequency. > > Now I’ve heard tell of a trick where I break the error signal into a low > frequency and high frequency part. And then send the high frequency > part into the laser current modulation input. > It seems I should pick off the error signal before the lowpass (P/I part > of signal chain).(?) > But I'm wondering how to deal with the 'break frequency' > What frequency for the HP? > And do I roll off the 'DC' part at the break frequency too? > (1 pole each) > Or can I leave the rest of the 'DC' signal chain the same if I pick the > right frequency?
Too many variables. What are the characteristics of the modulation you get from the piezo vs. modulating the laser current? Why does your piezo loop tend to oscillate at around 3kHz? Why can't you just control the laser current? Do you want to have closed-loop control using the laser current, with increased loop bandwidth, or do you just want to push the laser around open loop at those high frequencies? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On Jan 17, 11:35=A0pm, Tim Wescott <t...@seemywebsite.please> wrote:
> On Thu, 17 Jan 2013 15:45:57 -0800, George Herold wrote: > > Laser locking (control loops with two feedback paths.) > > > So I finally had a user ask about side locking our diode laser. (That=
=92s
> > where you lock the frequency to the side of an absorption feature.) > > Now, you can change the laser frequency in two ways. =A0There=92s a pie=
zo
> > stack that changes the angle of a diffraction grating. =A0And you can > > change the laser current. > > The electronics is all set up to lock the laser with the piezo. > > Signal chain looks like, > > > Photodiode->low pass (1 pole, tc =3D 100ms)-> > > DC offset->gain->modulation input of piezo control. > > > With some other bits of gain adjustment sprinkled in there. =A0(The low > > pass is working as both integrator and gain (PI), you crank up the > > overall loop gain till it oscillates and then back off a bit.) > > This works fine, up to ~3kHz the oscillation frequency. > > > Now I=92ve heard tell of a trick where I break the error signal into a =
low
> > frequency and high frequency part. =A0And then send the high frequency > > part into the laser current modulation input. > > It seems I should pick off the error signal before the lowpass (P/I par=
t
> > of signal chain).(?) > > But I'm wondering how to deal with the 'break frequency' > > What frequency for the HP? > > And do I roll off the 'DC' part at the break frequency too? > > (1 pole each) > > Or can I leave the rest of the 'DC' signal chain the same if I pick the > > right frequency? > > Too many variables. > > What are the characteristics of the modulation you get from the piezo vs. > modulating the laser current?
As long as the change is small they are both approximately linear. Modulating the current also changes the amplitude... but I actually take the difference of two photodiode signals to get the error signal... so to first order the amplitude change caused by current modulation shouldn't be that much of an issue. (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo and the current.)
> > Why does your piezo loop tend to oscillate at around 3kHz?
Well back in the dim past I did a back of the envelope calculation and figured this was the self resonant frequency of the piezo stack and the piece of Aluminum that it is pushing around. (Ratio of mass of aluminum vs mass of piezo to the one half power times the unloaded SRF of the piezo.) The Piezo is part number AE0203D04F made by Tokin and a rather long link to a data sheet, http://store.bravoelectro.com/redirect.php?action=3Durl&goto=3Dwww.bravoele= ctro.com%2Fpdf%2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5= g8b0rq258qm SRF ~ 261 kHz. I have no idea if the simple mass scaling is correct.. but about the right number came out the far side of the calculation. The aluminum and grating are part of a flexure... I sorta wondered if the spring constant is different too.... But I'm not sure how I get the spring constant for either the piezo or the flexure, and the mass was easy to measure. (I did try and do some measuments of the flexure spring constant using the piezo as the sensor, very 'squishy' measuments IIRC)
> > Why can't you just control the laser current?
Hmm... OK that's a good question. I'll have to try it! But for long term DC drifts it's better to change the piezo (grating angle.)
> > Do you want to have closed-loop control using the laser current, with > increased loop bandwidth, or do you just want to push the laser around > open loop at those high frequencies?
Oh for sure closed loop control with higher bandwidth. It'd be cool to be able to really bang on the table and have the thing stay locked! I think I've got a paper describing how someone else did this...(Carl Weiman and Leo Hollberg?) it might be in here, (another long link... to a RSI paper) http://www.google.com/url?sa=3Dt&rct=3Dj&q=3D&esrc=3Ds&frm=3D1&source=3Dweb= &cd=3D1&cad=3Drja&ved=3D0CDIQFjAA&url=3Dhttp%3A%2F%2Ftf.nist.gov%2Ftimefreq= %2Fgeneral%2Fpdf%2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2= Z4ocil_Sq6Fm7JQ4ypXg&sig2=3D5lnQLOJC42fKCF_VCTMF8Q But sometimes it's more fun to 'invent' your own method and then see what someone else did. George H.
> > -- > Tim Wescott > Control system and signal processing consultingwww.wescottdesign.com- Hid=
e quoted text -
> > - Show quoted text -
On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote:

>> Too many variables. >> >> What are the characteristics of the modulation you get from the piezo >> vs. modulating the laser current? > > As long as the change is small they are both approximately linear. > Modulating the current also changes the amplitude... but I actually take > the difference of two photodiode signals to get the error signal... so > to first order the amplitude change caused by current modulation > shouldn't be that much of an issue. > > (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo and > the current.) >> >> Why does your piezo loop tend to oscillate at around 3kHz? > Well back in the dim past I did a back of the envelope calculation and > figured this was the self resonant frequency of the piezo stack and the > piece of Aluminum that it is pushing around. (Ratio of mass of aluminum > vs mass of piezo to the one half power times the unloaded SRF of the > piezo.) > The Piezo is part number AE0203D04F made by Tokin and a rather long link > to a data sheet, > > http://store.bravoelectro.com/redirect.php?
action=url&goto=www.bravoelectro.com%2Fpdf% 2Fpiezoelectric_actuators.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm
> > SRF ~ 261 kHz. I have no idea if the simple mass scaling is correct.. > but about the right number came out the far side of the calculation. The > aluminum and grating are part of a flexure... I sorta wondered if the > spring constant is different too.... But I'm not sure how I get the > spring constant for either the piezo or the flexure, and the mass was > easy to measure. (I did try and do some measuments of the flexure > spring constant using the piezo as the sensor, very 'squishy' measuments > IIRC) > > >> Why can't you just control the laser current? > > Hmm... OK that's a good question. I'll have to try it! But for long > term DC drifts it's better to change the piezo (grating angle.) >> >> Do you want to have closed-loop control using the laser current, with >> increased loop bandwidth, or do you just want to push the laser around >> open loop at those high frequencies? > > Oh for sure closed loop control with higher bandwidth. It'd be cool to > be able to really bang on the table and have the thing stay locked! > > I think I've got a paper describing how someone else did this...(Carl > Weiman and Leo Hollberg?) it might be in here, (another long link... to > a RSI paper) > http://www.google.com/url?
sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http% 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% 2F739.pdf&ei=NWH5ULCdFMfg0gHQioDICA&usg=AFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ4ypXg&sig2=5lnQLOJC42fKCF_VCTMF8Q
> > But sometimes it's more fun to 'invent' your own method and then see > what someone else did.
Here's one leading candidate in the list of things that I'd try, then: Make a block, call it "laser", with a frequency-steering signal in, and a frequency out. Inside of that block, take the frequency-steering signal and run it through matched low- and high-pass filters. Make the cutoff frequency lower than the piezo resonance. Take the low-pass filter, run it through a notch at the piezo resonance frequency, and feed it to the piezo. Take the high-pass filter, and feed it to the diode current. Jigger gains around so that your GHz/whatever (I'm not assuming digital or analog at this point -- GHz/volt, GHz/ADC count, whatever). If your laser current response stays flat up to a much higher frequency than the piezo does, then the overall response of your "laser" block should also be flat out that high, probably with a hiccup around the frequency where you transition from piezo to laser current, and possibly around the piezo resonance, too (although you can damp that one out by playing with your notch and the cutoff frequency). Now wrap a loop around that. Figure that when you knock on the table you'll see it in amplitude -- your cavity will be changing, and you'll be fixing it by changing the laser current, so you'll see it in amplitude. Have fun. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Jan 18, 2:31=A0pm, Tim Wescott <t...@seemywebsite.com> wrote:
> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: > >> Too many variables. > > >> What are the characteristics of the modulation you get from the piezo > >> vs. modulating the laser current? > > > As long as the change is small they are both approximately linear. > > Modulating the current also changes the amplitude... but I actually tak=
e
> > the difference of two photodiode signals to get the error signal... so > > to first order the amplitude change caused by current modulation > > shouldn't be that much of an issue. > > > (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo an=
d
> > the current.) > > >> Why does your piezo loop tend to oscillate at around 3kHz? > > Well back in the dim past I did a back of the envelope calculation and > > figured this was the self resonant frequency of the piezo stack and the > > piece of Aluminum that it is pushing around. =A0(Ratio of mass of alumi=
num
> > vs mass of piezo to the one half power times the unloaded SRF of the > > piezo.) > > The Piezo is part number AE0203D04F made by Tokin and a rather long lin=
k
> > to a data sheet, > > >http://store.bravoelectro.com/redirect.php? > > action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b0rq258qm > > > > > > > SRF ~ 261 kHz. =A0I have no idea if the simple mass scaling is correct.=
.
> > but about the right number came out the far side of the calculation. Th=
e
> > aluminum and grating are part of a flexure... I sorta wondered if the > > spring constant is different too.... But I'm not sure how I get the > > spring constant for either the piezo or the flexure, and the mass was > > easy to measure. =A0(I did try and do some measuments of the flexure > > spring constant using the piezo as the sensor, very 'squishy' measument=
s
> > IIRC) > > >> Why can't you just control the laser current? > > > Hmm... OK that's a good question. =A0I'll have to try it! But for long > > term DC drifts it's better to change the piezo (grating angle.) > > >> Do you want to have closed-loop control using the laser current, with > >> increased loop bandwidth, or do you just want to push the laser around > >> open loop at those high frequencies? > > > Oh for sure closed loop control with higher bandwidth. It'd be cool to > > be able to really bang on the table and have the thing stay locked! > > > I think I've got a paper describing how someone else did this...(Carl > > Weiman and Leo Hollberg?) it might be in here, =A0(another long link...=
to
> > a RSI paper) > >http://www.google.com/url? > > sa=3Dt&rct=3Dj&q=3D&esrc=3Ds&frm=3D1&source=3Dweb&cd=3D1&cad=3Drja&ved=3D=
0CDIQFjAA&url=3Dhttp%
> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ=
4ypXg&=ADsig2=3D5lnQLOJC42fKCF_VCTMF8Q
> > > > > =A0 But sometimes it's more fun to 'invent' your own method and then se=
e
> > what someone else did. > > Here's one leading candidate in the list of things that I'd try, then: > > Make a block, call it "laser", with a frequency-steering signal in, and a > frequency out. > > Inside of that block, take the frequency-steering signal and run it > through matched low- and high-pass filters. =A0Make the cutoff frequency > lower than the piezo resonance. =A0Take the low-pass filter, run it throu=
gh
> a notch at the piezo resonance frequency, and feed it to the piezo. =A0Ta=
ke
> the high-pass filter, and feed it to the diode current. =A0Jigger gains > around so that your GHz/whatever (I'm not assuming digital or analog at > this point -- GHz/volt, GHz/ADC count, whatever). > > If your laser current response stays flat up to a much higher frequency > than the piezo does, then the overall response of your "laser" block > should also be flat out that high, probably with a hiccup around the > frequency where you transition from piezo to laser current, and possibly > around the piezo resonance, too (although you can damp that one out by > playing with your notch and the cutoff frequency). > > Now wrap a loop around that. =A0Figure that when you knock on the table > you'll see it in amplitude -- your cavity will be changing, and you'll be > fixing it by changing the laser current, so you'll see it in amplitude. > > Have fun. > > -- > My liberal friends think I'm a conservative kook. > My conservative friends think I'm a liberal kook. > Why am I not happy that they have found common ground? > > Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wesco=
ttdesign.com- Hide quoted text -
> > - Show quoted text -- Hide quoted text - > > - Show quoted text -
Hi Tim, Thanks for that! I logged in to report that I tried locking with just current modulation... one peice at a time so to speak. And that worked fine, I could bang a bit more on the table. But the current loop oscillates at ~20kHz when I crank up the gain. I don't understand that at all! The current modulation electronics has a bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' physics reason. Modulating the current changes the wavlength through thermal effects. I have no idea what the thermal time of the laser diode is. Would 50us be a reasonable time? (retorical question no answer expected.) I'm going to try measuring the current to frequency modulation parameter as a function of frequency. Hey I might learn someting today! If I get around to closing the 'double loop', I may have more questions.... It's not clear to me where I should put the integrator. Having friday fun, George H.
On Jan 18, 3:30=A0pm, George Herold <gher...@teachspin.com> wrote:
> On Jan 18, 2:31=A0pm, Tim Wescott <t...@seemywebsite.com> wrote: > > > > > > > On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: > > >> Too many variables. > > > >> What are the characteristics of the modulation you get from the piez=
o
> > >> vs. modulating the laser current? > > > > As long as the change is small they are both approximately linear. > > > Modulating the current also changes the amplitude... but I actually t=
ake
> > > the difference of two photodiode signals to get the error signal... s=
o
> > > to first order the amplitude change caused by current modulation > > > shouldn't be that much of an issue. > > > > (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo =
and
> > > the current.) > > > >> Why does your piezo loop tend to oscillate at around 3kHz? > > > Well back in the dim past I did a back of the envelope calculation an=
d
> > > figured this was the self resonant frequency of the piezo stack and t=
he
> > > piece of Aluminum that it is pushing around. =A0(Ratio of mass of alu=
minum
> > > vs mass of piezo to the one half power times the unloaded SRF of the > > > piezo.) > > > The Piezo is part number AE0203D04F made by Tokin and a rather long l=
ink
> > > to a data sheet, > > > >http://store.bravoelectro.com/redirect.php? > > > action=3Durl&goto=3Dwww.bravoelectro.com%2Fpdf% > > 2Fpiezoelectric_actuators.pdf&osCsid=3Dcgek9fio38jfi297es1j5g8b0rq258qm > > > > SRF ~ 261 kHz. =A0I have no idea if the simple mass scaling is correc=
t..
> > > but about the right number came out the far side of the calculation. =
The
> > > aluminum and grating are part of a flexure... I sorta wondered if the > > > spring constant is different too.... But I'm not sure how I get the > > > spring constant for either the piezo or the flexure, and the mass was > > > easy to measure. =A0(I did try and do some measuments of the flexure > > > spring constant using the piezo as the sensor, very 'squishy' measume=
nts
> > > IIRC) > > > >> Why can't you just control the laser current? > > > > Hmm... OK that's a good question. =A0I'll have to try it! But for lon=
g
> > > term DC drifts it's better to change the piezo (grating angle.) > > > >> Do you want to have closed-loop control using the laser current, wit=
h
> > >> increased loop bandwidth, or do you just want to push the laser arou=
nd
> > >> open loop at those high frequencies? > > > > Oh for sure closed loop control with higher bandwidth. It'd be cool t=
o
> > > be able to really bang on the table and have the thing stay locked! > > > > I think I've got a paper describing how someone else did this...(Carl > > > Weiman and Leo Hollberg?) it might be in here, =A0(another long link.=
.. to
> > > a RSI paper) > > >http://www.google.com/url? > > > sa=3Dt&rct=3Dj&q=3D&esrc=3Ds&frm=3D1&source=3Dweb&cd=3D1&cad=3Drja&ved=
=3D0CDIQFjAA&url=3Dhttp%
> > 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% > > 2F739.pdf&ei=3DNWH5ULCdFMfg0gHQioDICA&usg=3DAFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7=
JQ4ypXg&=AD=ADsig2=3D5lnQLOJC42fKCF_VCTMF8Q
> > > > =A0 But sometimes it's more fun to 'invent' your own method and then =
see
> > > what someone else did. > > > Here's one leading candidate in the list of things that I'd try, then: > > > Make a block, call it "laser", with a frequency-steering signal in, and=
a
> > frequency out. > > > Inside of that block, take the frequency-steering signal and run it > > through matched low- and high-pass filters. =A0Make the cutoff frequenc=
y
> > lower than the piezo resonance. =A0Take the low-pass filter, run it thr=
ough
> > a notch at the piezo resonance frequency, and feed it to the piezo. =A0=
Take
> > the high-pass filter, and feed it to the diode current. =A0Jigger gains > > around so that your GHz/whatever (I'm not assuming digital or analog at > > this point -- GHz/volt, GHz/ADC count, whatever). > > > If your laser current response stays flat up to a much higher frequency > > than the piezo does, then the overall response of your "laser" block > > should also be flat out that high, probably with a hiccup around the > > frequency where you transition from piezo to laser current, and possibl=
y
> > around the piezo resonance, too (although you can damp that one out by > > playing with your notch and the cutoff frequency). > > > Now wrap a loop around that. =A0Figure that when you knock on the table > > you'll see it in amplitude -- your cavity will be changing, and you'll =
be
> > fixing it by changing the laser current, so you'll see it in amplitude. > > > Have fun. > > > -- > > My liberal friends think I'm a conservative kook. > > My conservative friends think I'm a liberal kook. > > Why am I not happy that they have found common ground? > > > Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wes=
cottdesign.com-Hide quoted text -
> > > - Show quoted text -- Hide quoted text - > > > - Show quoted text - > > Hi Tim, =A0Thanks for that! =A0I logged in to report that I tried locking > with just current modulation... one peice at a time so to speak. =A0And > that worked fine, I could bang a bit more on the table. =A0But the > current loop oscillates at ~20kHz when I crank up the gain. =A0I don't > understand that at all! =A0The current modulation electronics has a > bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' > physics reason. =A0Modulating the current changes the wavlength through > thermal effects. =A0I have no idea what the thermal time of the laser > diode is. =A0Would 50us be a reasonable time? (retorical question no > answer expected.) > I'm going to try measuring the current to frequency modulation > parameter as a function of frequency. =A0Hey I might learn someting > today! > > If I get around to closing the 'double loop', I may have more > questions.... > It's not clear to me where I should put the integrator. > > Having friday fun, > > George H.- Hide quoted text - > > - Show quoted text -
Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! George H.
George Herold wrote:
> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: >> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: >> >> >> >> >> >>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: >>>>> Too many variables. >>>>> What are the characteristics of the modulation you get from the piezo >>>>> vs. modulating the laser current? >>>> As long as the change is small they are both approximately linear. >>>> Modulating the current also changes the amplitude... but I actually take >>>> the difference of two photodiode signals to get the error signal... so >>>> to first order the amplitude change caused by current modulation >>>> shouldn't be that much of an issue. >>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo and >>>> the current.) >>>>> Why does your piezo loop tend to oscillate at around 3kHz? >>>> Well back in the dim past I did a back of the envelope calculation and >>>> figured this was the self resonant frequency of the piezo stack and the >>>> piece of Aluminum that it is pushing around. (Ratio of mass of aluminum >>>> vs mass of piezo to the one half power times the unloaded SRF of the >>>> piezo.) >>>> The Piezo is part number AE0203D04F made by Tokin and a rather long link >>>> to a data sheet, >>>> http://store.bravoelectro.com/redirect.php? >>> action=url&goto=www.bravoelectro.com%2Fpdf% >>> 2Fpiezoelectric_actuators.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm >>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is correct.. >>>> but about the right number came out the far side of the calculation. The >>>> aluminum and grating are part of a flexure... I sorta wondered if the >>>> spring constant is different too.... But I'm not sure how I get the >>>> spring constant for either the piezo or the flexure, and the mass was >>>> easy to measure. (I did try and do some measuments of the flexure >>>> spring constant using the piezo as the sensor, very 'squishy' measuments >>>> IIRC) >>>>> Why can't you just control the laser current? >>>> Hmm... OK that's a good question. I'll have to try it! But for long >>>> term DC drifts it's better to change the piezo (grating angle.) >>>>> Do you want to have closed-loop control using the laser current, with >>>>> increased loop bandwidth, or do you just want to push the laser around >>>>> open loop at those high frequencies? >>>> Oh for sure closed loop control with higher bandwidth. It'd be cool to >>>> be able to really bang on the table and have the thing stay locked! >>>> I think I've got a paper describing how someone else did this...(Carl >>>> Weiman and Leo Hollberg?) it might be in here, (another long link... to >>>> a RSI paper) >>>> http://www.google.com/url? >>> sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http% >>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% >>> 2F739.pdf&ei=NWH5ULCdFMfg0gHQioDICA&usg=AFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ4ypXg&&#2013266093;&#2013266093;sig2=5lnQLOJC42fKCF_VCTMF8Q >>>> But sometimes it's more fun to 'invent' your own method and then see >>>> what someone else did. >>> Here's one leading candidate in the list of things that I'd try, then: >>> Make a block, call it "laser", with a frequency-steering signal in, and a >>> frequency out. >>> Inside of that block, take the frequency-steering signal and run it >>> through matched low- and high-pass filters. Make the cutoff frequency >>> lower than the piezo resonance. Take the low-pass filter, run it through >>> a notch at the piezo resonance frequency, and feed it to the piezo. Take >>> the high-pass filter, and feed it to the diode current. Jigger gains >>> around so that your GHz/whatever (I'm not assuming digital or analog at >>> this point -- GHz/volt, GHz/ADC count, whatever). >>> If your laser current response stays flat up to a much higher frequency >>> than the piezo does, then the overall response of your "laser" block >>> should also be flat out that high, probably with a hiccup around the >>> frequency where you transition from piezo to laser current, and possibly >>> around the piezo resonance, too (although you can damp that one out by >>> playing with your notch and the cutoff frequency). >>> Now wrap a loop around that. Figure that when you knock on the table >>> you'll see it in amplitude -- your cavity will be changing, and you'll be >>> fixing it by changing the laser current, so you'll see it in amplitude. >>> Have fun. >>> -- >>> My liberal friends think I'm a conservative kook. >>> My conservative friends think I'm a liberal kook. >>> Why am I not happy that they have found common ground? >>> Tim Wescott, Communications, Control, Circuits & Softwarehttp://www.wescottdesign.com-Hide quoted text - >>> - Show quoted text -- Hide quoted text - >>> - Show quoted text - >> Hi Tim, Thanks for that! I logged in to report that I tried locking >> with just current modulation... one peice at a time so to speak. And >> that worked fine, I could bang a bit more on the table. But the >> current loop oscillates at ~20kHz when I crank up the gain. I don't >> understand that at all! The current modulation electronics has a >> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' >> physics reason. Modulating the current changes the wavlength through >> thermal effects. I have no idea what the thermal time of the laser >> diode is. Would 50us be a reasonable time? (retorical question no >> answer expected.) >> I'm going to try measuring the current to frequency modulation >> parameter as a function of frequency. Hey I might learn someting >> today! >> >> If I get around to closing the 'double loop', I may have more >> questions.... >> It's not clear to me where I should put the integrator. >> >> Having friday fun, >> >> George H.- Hide quoted text - >> >> - Show quoted text - > > Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! >
20kHz? That's like molasses. Why so low? And it should not cause it to oscillate. -- Regards, Joerg http://www.analogconsultants.com/
On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote:

> George Herold wrote: >> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: >>> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: >>> >>> >>> >>> >>> >>>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: >>>>>> Too many variables. >>>>>> What are the characteristics of the modulation you get from the >>>>>> piezo vs. modulating the laser current? >>>>> As long as the change is small they are both approximately linear. >>>>> Modulating the current also changes the amplitude... but I actually >>>>> take the difference of two photodiode signals to get the error >>>>> signal... so to first order the amplitude change caused by current >>>>> modulation shouldn't be that much of an issue. >>>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo >>>>> and the current.) >>>>>> Why does your piezo loop tend to oscillate at around 3kHz? >>>>> Well back in the dim past I did a back of the envelope calculation >>>>> and figured this was the self resonant frequency of the piezo stack >>>>> and the piece of Aluminum that it is pushing around. (Ratio of mass >>>>> of aluminum vs mass of piezo to the one half power times the >>>>> unloaded SRF of the piezo.) >>>>> The Piezo is part number AE0203D04F made by Tokin and a rather long >>>>> link to a data sheet, >>>>> http://store.bravoelectro.com/redirect.php? >>>> action=url&goto=www.bravoelectro.com%2Fpdf% >>>> 2Fpiezoelectric_actuators.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm >>>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is >>>>> correct.. but about the right number came out the far side of the >>>>> calculation. The aluminum and grating are part of a flexure... I >>>>> sorta wondered if the spring constant is different too.... But I'm >>>>> not sure how I get the spring constant for either the piezo or the >>>>> flexure, and the mass was easy to measure. (I did try and do some >>>>> measuments of the flexure spring constant using the piezo as the >>>>> sensor, very 'squishy' measuments IIRC) >>>>>> Why can't you just control the laser current? >>>>> Hmm... OK that's a good question. I'll have to try it! But for long >>>>> term DC drifts it's better to change the piezo (grating angle.) >>>>>> Do you want to have closed-loop control using the laser current, >>>>>> with increased loop bandwidth, or do you just want to push the >>>>>> laser around open loop at those high frequencies? >>>>> Oh for sure closed loop control with higher bandwidth. It'd be cool >>>>> to be able to really bang on the table and have the thing stay >>>>> locked! I think I've got a paper describing how someone else did >>>>> this...(Carl Weiman and Leo Hollberg?) it might be in here, >>>>> (another long link... to a RSI paper) >>>>> http://www.google.com/url? >>>>
sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http%
>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% >>>>
2F739.pdf&ei=NWH5ULCdFMfg0gHQioDICA&usg=AFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ4ypXg&&shy;&shy; sig2=5lnQLOJC42fKCF_VCTMF8Q
>>>>> But sometimes it's more fun to 'invent' your own method and then >>>>> see >>>>> what someone else did. >>>> Here's one leading candidate in the list of things that I'd try, >>>> then: Make a block, call it "laser", with a frequency-steering signal >>>> in, and a frequency out. >>>> Inside of that block, take the frequency-steering signal and run it >>>> through matched low- and high-pass filters. Make the cutoff >>>> frequency lower than the piezo resonance. Take the low-pass filter, >>>> run it through a notch at the piezo resonance frequency, and feed it >>>> to the piezo. Take the high-pass filter, and feed it to the diode >>>> current. Jigger gains around so that your GHz/whatever (I'm not >>>> assuming digital or analog at this point -- GHz/volt, GHz/ADC count, >>>> whatever). If your laser current response stays flat up to a much >>>> higher frequency than the piezo does, then the overall response of >>>> your "laser" block should also be flat out that high, probably with a >>>> hiccup around the frequency where you transition from piezo to laser >>>> current, and possibly around the piezo resonance, too (although you >>>> can damp that one out by playing with your notch and the cutoff >>>> frequency). Now wrap a loop around that. Figure that when you knock >>>> on the table you'll see it in amplitude -- your cavity will be >>>> changing, and you'll be fixing it by changing the laser current, so >>>> you'll see it in amplitude. Have fun. >>>> -- >>>> My liberal friends think I'm a conservative kook. My conservative >>>> friends think I'm a liberal kook. Why am I not happy that they have >>>> found common ground? Tim Wescott, Communications, Control, Circuits & >>>> Softwarehttp://www.wescottdesign.com-Hide quoted text - - Show quoted >>>> text -- Hide quoted text - - Show quoted text - >>> Hi Tim, Thanks for that! I logged in to report that I tried locking >>> with just current modulation... one peice at a time so to speak. And >>> that worked fine, I could bang a bit more on the table. But the >>> current loop oscillates at ~20kHz when I crank up the gain. I don't >>> understand that at all! The current modulation electronics has a >>> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' >>> physics reason. Modulating the current changes the wavlength through >>> thermal effects. I have no idea what the thermal time of the laser >>> diode is. Would 50us be a reasonable time? (retorical question no >>> answer expected.) >>> I'm going to try measuring the current to frequency modulation >>> parameter as a function of frequency. Hey I might learn someting >>> today! >>> >>> If I get around to closing the 'double loop', I may have more >>> questions.... >>> It's not clear to me where I should put the integrator. >>> >>> Having friday fun, >>> >>> George H.- Hide quoted text - >>> >>> - Show quoted text - >> >> Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! >> >> > 20kHz? That's like molasses. Why so low? And it should not cause it to > oscillate.
If he's already got substantial phase shift elsewhere, then the photodiode rolling off would cause oscillation somewhere around 20kHz. The fact that it happens at _exactly_ 20kHz just means that, sans photodiode, he's got about 45 degrees of margin at 20kHz. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
On Fri, 18 Jan 2013 12:30:54 -0800, George Herold wrote:

> Hi Tim, Thanks for that! I logged in to report that I tried locking > with just current modulation... one peice at a time so to speak. And > that worked fine, I could bang a bit more on the table. But the current > loop oscillates at ~20kHz when I crank up the gain. I don't understand > that at all! The current modulation electronics has a bandwdith that's > near 1 MHz, so the 20kHz might be for some 'real' physics reason. > Modulating the current changes the wavlength through thermal effects. I > have no idea what the thermal time of the laser diode is. Would 50us be > a reasonable time? (retorical question no answer expected.) > I'm going to try measuring the current to frequency modulation parameter > as a function of frequency. Hey I might learn someting today! > > If I get around to closing the 'double loop', I may have more > questions.... > It's not clear to me where I should put the integrator.
freq error -o-> prop. gain - + --> laser | A '-> integrator --' If your loop is crapping out at 20kHz with your 20kHz photodiode, chances are that even with a better photodiode in there you'll need some derivative action to push much above 20-ish kHz: freq error -o-> prop. gain - + --> laser | A o-> integrator --+ | A '-> derivative --' If you're doing this in analog, or if you're sampling good and fast in digital-land, you'll almost certainly want to band-limit the derivative. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Tim Wescott wrote:
> On Fri, 18 Jan 2013 13:36:40 -0800, Joerg wrote: > >> George Herold wrote: >>> On Jan 18, 3:30 pm, George Herold <gher...@teachspin.com> wrote: >>>> On Jan 18, 2:31 pm, Tim Wescott <t...@seemywebsite.com> wrote: >>>> >>>> >>>> >>>> >>>> >>>>> On Fri, 18 Jan 2013 06:59:08 -0800, George Herold wrote: >>>>>>> Too many variables. >>>>>>> What are the characteristics of the modulation you get from the >>>>>>> piezo vs. modulating the laser current? >>>>>> As long as the change is small they are both approximately linear. >>>>>> Modulating the current also changes the amplitude... but I actually >>>>>> take the difference of two photodiode signals to get the error >>>>>> signal... so to first order the amplitude change caused by current >>>>>> modulation shouldn't be that much of an issue. >>>>>> (Hmm maybe I can generate freq vs 'voltage' scans for both the piezo >>>>>> and the current.) >>>>>>> Why does your piezo loop tend to oscillate at around 3kHz? >>>>>> Well back in the dim past I did a back of the envelope calculation >>>>>> and figured this was the self resonant frequency of the piezo stack >>>>>> and the piece of Aluminum that it is pushing around. (Ratio of mass >>>>>> of aluminum vs mass of piezo to the one half power times the >>>>>> unloaded SRF of the piezo.) >>>>>> The Piezo is part number AE0203D04F made by Tokin and a rather long >>>>>> link to a data sheet, >>>>>> http://store.bravoelectro.com/redirect.php? >>>>> action=url&goto=www.bravoelectro.com%2Fpdf% >>>>> 2Fpiezoelectric_actuators.pdf&osCsid=cgek9fio38jfi297es1j5g8b0rq258qm >>>>>> SRF ~ 261 kHz. I have no idea if the simple mass scaling is >>>>>> correct.. but about the right number came out the far side of the >>>>>> calculation. The aluminum and grating are part of a flexure... I >>>>>> sorta wondered if the spring constant is different too.... But I'm >>>>>> not sure how I get the spring constant for either the piezo or the >>>>>> flexure, and the mass was easy to measure. (I did try and do some >>>>>> measuments of the flexure spring constant using the piezo as the >>>>>> sensor, very 'squishy' measuments IIRC) >>>>>>> Why can't you just control the laser current? >>>>>> Hmm... OK that's a good question. I'll have to try it! But for long >>>>>> term DC drifts it's better to change the piezo (grating angle.) >>>>>>> Do you want to have closed-loop control using the laser current, >>>>>>> with increased loop bandwidth, or do you just want to push the >>>>>>> laser around open loop at those high frequencies? >>>>>> Oh for sure closed loop control with higher bandwidth. It'd be cool >>>>>> to be able to really bang on the table and have the thing stay >>>>>> locked! I think I've got a paper describing how someone else did >>>>>> this...(Carl Weiman and Leo Hollberg?) it might be in here, >>>>>> (another long link... to a RSI paper) >>>>>> http://www.google.com/url? > sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDIQFjAA&url=http% >>>>> 3A%2F%2Ftf.nist.gov%2Ftimefreq%2Fgeneral%2Fpdf% >>>>> > 2F739.pdf&ei=NWH5ULCdFMfg0gHQioDICA&usg=AFQjCNFQ5Nw0h2Z4ocil_Sq6Fm7JQ4ypXg&&shy;&shy; > sig2=5lnQLOJC42fKCF_VCTMF8Q >>>>>> But sometimes it's more fun to 'invent' your own method and then >>>>>> see >>>>>> what someone else did. >>>>> Here's one leading candidate in the list of things that I'd try, >>>>> then: Make a block, call it "laser", with a frequency-steering signal >>>>> in, and a frequency out. >>>>> Inside of that block, take the frequency-steering signal and run it >>>>> through matched low- and high-pass filters. Make the cutoff >>>>> frequency lower than the piezo resonance. Take the low-pass filter, >>>>> run it through a notch at the piezo resonance frequency, and feed it >>>>> to the piezo. Take the high-pass filter, and feed it to the diode >>>>> current. Jigger gains around so that your GHz/whatever (I'm not >>>>> assuming digital or analog at this point -- GHz/volt, GHz/ADC count, >>>>> whatever). If your laser current response stays flat up to a much >>>>> higher frequency than the piezo does, then the overall response of >>>>> your "laser" block should also be flat out that high, probably with a >>>>> hiccup around the frequency where you transition from piezo to laser >>>>> current, and possibly around the piezo resonance, too (although you >>>>> can damp that one out by playing with your notch and the cutoff >>>>> frequency). Now wrap a loop around that. Figure that when you knock >>>>> on the table you'll see it in amplitude -- your cavity will be >>>>> changing, and you'll be fixing it by changing the laser current, so >>>>> you'll see it in amplitude. Have fun. >>>>> -- >>>>> My liberal friends think I'm a conservative kook. My conservative >>>>> friends think I'm a liberal kook. Why am I not happy that they have >>>>> found common ground? Tim Wescott, Communications, Control, Circuits & >>>>> Softwarehttp://www.wescottdesign.com-Hide quoted text - - Show quoted >>>>> text -- Hide quoted text - - Show quoted text - >>>> Hi Tim, Thanks for that! I logged in to report that I tried locking >>>> with just current modulation... one peice at a time so to speak. And >>>> that worked fine, I could bang a bit more on the table. But the >>>> current loop oscillates at ~20kHz when I crank up the gain. I don't >>>> understand that at all! The current modulation electronics has a >>>> bandwdith that's near 1 MHz, so the 20kHz might be for some 'real' >>>> physics reason. Modulating the current changes the wavlength through >>>> thermal effects. I have no idea what the thermal time of the laser >>>> diode is. Would 50us be a reasonable time? (retorical question no >>>> answer expected.) >>>> I'm going to try measuring the current to frequency modulation >>>> parameter as a function of frequency. Hey I might learn someting >>>> today! >>>> >>>> If I get around to closing the 'double loop', I may have more >>>> questions.... >>>> It's not clear to me where I should put the integrator. >>>> >>>> Having friday fun, >>>> >>>> George H.- Hide quoted text - >>>> >>>> - Show quoted text - >>> Oops... dumb dumb dumb, 20kHz is the bandwidth of my photodiode! >>> >>> >> 20kHz? That's like molasses. Why so low? And it should not cause it to >> oscillate. > > If he's already got substantial phase shift elsewhere, then the photodiode > rolling off would cause oscillation somewhere around 20kHz. > > The fact that it happens at _exactly_ 20kHz just means that, sans > photodiode, he's got about 45 degrees of margin at 20kHz. >
But where does all that phase margin fall through the cracks? Unless everything rolls off fast, of course. 20kHz BW for the photodiode sounds really low, unless it is one the size of a dinner plate. -- Regards, Joerg http://www.analogconsultants.com/