5808f643   
   From: ggherold@gmail.com   
      
   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?   
      
   Thanks,   
   George H.   
      
   (Posted separatly on SED)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)