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Entry  Wed Feb 11 03:58:52 2015, Zach, Laser, Transfer Functions, Wipf nonlinear temperature actuation proof of principle Wipf_actuation_POP_2_10_15.png
    Reply  Thu Feb 12 15:34:12 2015, Nic, Chris, Laser, Transfer Functions, higher bandwidth frequency readout 
Message ID: 1203     Entry time: Wed Feb 11 03:58:52 2015     Reply to this: 1204
Author: Zach 
Type: Laser 
Category: Transfer Functions 
Subject: Wipf nonlinear temperature actuation proof of principle 

Nic elucidated to me today Chris W.'s idea for getting truly wideband (~500 MHz) actuation out of our diode lasers. In case the reader isn't familiar, the lasers have two parallel linear actuation pathways converting current into frequency: one from current modulating the temperature, which is the strongest effect at DC and then dies off above ~1 MHz due most likely to the thermal response, and another, weaker but much wider-band, flat pathway arising from solid state effects that did not survive the elucidating. At some frequency (around 50 MHz, I believe?), there is a crossover between these paths, but there is a differing sign, which creates a "non-minimal-phase zero", leaving the phase at -180° and making the overall system a difficult actuator to deal with at high frequencies.

As I understand it, Chris's idea involves using the full, nonlinear current-to-temperature response to effectively circumvent the direct linear response at low frequencies. This can be done, for example, by pumping a strong RF carrier current (say, around 1 GHz) into the diode, and then using amplitude modulation on this carrier to produce baseband frequency actuation from the temperature beating. By choosing the phase of the AM correctly, one can make it so this pathway (now dominant at low frequencies) results in a nicer crossover with linear pathway #2 from above.

I performed a very simple proof-of-principle test today by doing the following:

  • Dither lock my temporary diagnostic PMC to one laser using the setup described in CRYO:1195.
  • Set the UGF fairly low (a few 100 Hz)
  • Drive the laser current with a 1-kHz sine wave, strong enough to be clearly present above the noise in the error signal. I found that 200 uVpp (= 2 uApp) gave me a nice SNR around 20.
  • Using a Marconi into the SMA bias tee adapter directly on the diode, inject a fairly strong RF carrier current. I used 600 MHz at ~200 uArms, though the amplitude was determined empirically over the course of the test to see an effect.
  • Engage amplitude modulation at 1 kHz and a pretty strong modulation (I chose "50%").
  • (As I mentioned a couple bullets above, in reality, I removed the direct 1-kHz injection and pumped this RF-with-AM current up until I saw an effect in the error signal)
  • With these two signals on, and adjusting the AM phase, I was clearly able to see modulation of the line in the error signal, indicating that the two drives were interfering as desired.

Trimming the RF amplitude and phase a bit to get a nice result, I was able to take the two spectra shown below. In the first trace, only the direct current line is present at 1 kHz. In the second one, the RF source is engaged and you can see an exact cancellation of the line in the error signal. Increasing or decreasing the RF (or audio) amplitudes led to the reemergence of the line (assuredly with 180º relative phase from one case to the other). To do the wideband actuation, one would simply make sure that the RF power is strong enough that the nonlinear path dominates.

So, it should work! We'll have to change the measurement setup to make a full transfer function showing clean actuation to very high frequencies, but it should be pretty straightforward.

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