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Frequency Detection Suggestions (corrected)

Started by Bradley Kimbrough March 8, 2017
Hello All, 

I am currently building a stabilized Helium Neon laser that uses a 3 mode tube and stabilizes the optical frequency by monitoring a beat frequency between the modes. The beat frequency is 450-600 kHz and is sensed from the waste beam at the back of the laser tube. For a prototype I used an amplified moderately high speed Si detector that fed into a 12 bit 125 MSPS DAC.  This data was then sent to the computer , bandpass filtered and Fourier transformed to determine the signal frequency. This frequency was then fed into a PID algorithm whose output controlled a heater that would heat the tube to stabilize its length. 

Question:  Can you suggest and analog solution to frequency detection that could work in this scenario?  It does not need to be exceedingly fast since the PID loop time constant is on the order of seconds.  I've noticed that Analog Devices has an IC for Freq to Voltage conversion.  A PLL may be useful as well.  Not looking for specifics, just suggestions as to the best path forward. 

Thank you, 

Brad Kimbrough
take a look at 
http://www.hudler.org/pub/HP/05526-90051%20-%205526A%20Operating%20and%20Service%20Manual%20[Prefix%201828A]%20(Nov%201978).pdf
as inspiration...

seems they control the laser by voltage (rapid reading...)

bye
delo


"Bradley Kimbrough" <bradley.kimbrough@gmail.com> ha scritto nel messaggio 
news:d76f6a4d-6a8b-42fa-b0fa-2513c5c7df22@googlegroups.com...
Hello All,

I am currently building a stabilized Helium Neon laser that uses a 3 mode tube and stabilizes the optical frequency by monitoring a 
beat frequency between the modes. The beat frequency is 450-600 kHz and is sensed from the waste beam at the back of the laser tube. 
For a prototype I used an amplified moderately high speed Si detector that fed into a 12 bit 125 MSPS DAC.  This data was then sent 
to the computer , bandpass filtered and Fourier transformed to determine the signal frequency. This frequency was then fed into a 
PID algorithm whose output controlled a heater that would heat the tube to stabilize its length.

Question:  Can you suggest and analog solution to frequency detection that could work in this scenario?  It does not need to be 
exceedingly fast since the PID loop time constant is on the order of seconds.  I've noticed that Analog Devices has an IC for Freq 
to Voltage conversion.  A PLL may be useful as well.  Not looking for specifics, just suggestions as to the best path forward.

Thank you,

Brad Kimbrough 


Interesting--I wouldn't have thought that a very well-posed problem. How do you correct for frequency pulling due to the natural line width and spectral hole burning?

(The HP one was a single mode Zeeman-split laser where the beat frequency between the two polarizations was phase-locked to a 2-MHz crystal oscillator.)

Cheers

Phil Hobbs
On 09/03/17 02:40, Bradley Kimbrough wrote:
> Hello All, > > I am currently building a stabilized Helium Neon laser that uses a 3 mode tube and stabilizes the optical frequency by monitoring a beat frequency between the modes. The beat frequency is 450-600 kHz and is sensed from the waste beam at the back of the laser tube. For a prototype I used an amplified moderately high speed Si detector that fed into a 12 bit 125 MSPS DAC. This data was then sent to the computer , bandpass filtered and Fourier transformed to determine the signal frequency. This frequency was then fed into a PID algorithm whose output controlled a heater that would heat the tube to stabilize its length. > > Question: Can you suggest and analog solution to frequency detection that could work in this scenario? It does not need to be exceedingly fast since the PID loop time constant is on the order of seconds. I've noticed that Analog Devices has an IC for Freq to Voltage conversion. A PLL may be useful as well. Not looking for specifics, just suggestions as to the best path forward. > > Thank you, > > Brad Kimbrough >
I would definitely use a digital solution. At these frequencies you can just count clock ticks between N zero crossings of your beat signal. Pere
Hall actually used an UP/Down counter, called a "reversible" counter. That counter drove a few latches, a gate circuit,  and a DAC. The Dac controlled the heater or piezo. The direction of the counter was reversed at 10 Hz, and driven by a  one Mhz reference, from which the 10 Hz was derived.  The counter counted the mixed down IF frequency.  Thus he had digital steering to a clock reference  with off the shelf, simple parts, and a small analog loop to add a bias. 

Steve