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Message ID: 1196     Entry time: Fri Feb 6 05:31:43 2015
Author: Zach 
Type: Laser 
Category: SiFi 
Subject: Phase modulator characterization 

After rebuilding the PMC setup (see CRYO:1195), I was finally able to move on to characterizing the Photline fiber-coupled phase modulators we will be using (MPX-LN-0.1 --- datasheet attached nope google it yourself). I measured a couple things:

Insertion loss

As with the amplitude modulators (see CRYO:1187), I determined this simply by measusing the power straight out of the laser, then quickly connecting each phase modulator (one at a time) between the laser and the output coupler and measuring again. As I mentioned in the linked post, this is not an exact science due to the somewhat unpredictable behavior from connector to connector. Nevertheless, one can be confident at the one-to-few-percent level.

S/N 10:

2.66 mW out / 5.00 mW in --> loss ~ 2.74 dB

S/N 2:

2.88 mW out / 5.38 mW in --> loss ~ 2.71 dB

Supposedly, we had these two units hand selected for loss < 2.5 dB (for free, after we paid for the $500 low-loss selection of the amplitude modulators), while the standard typical loss from the datasheet is closer to what we have at 2.7 dB. An extra 0.2 dB isn't going to break the bank, but it's a bit disappointing that they didn't give us what they said. Probably too late to say anything anyway...

 

Response

My plan was to use the modulators to pump light into RF sidebands, then use the frequency selectivity of the PMC to measure the SB power and back out the actuation strength (Vpi). I was able to do this, to a degree, but I was thwarted by an unexpected issue: the modulators and the fibers coupling to/from them appear to change the output mode emerging from the collimator. What's worse, the mode seems highly sensitive to any touching of the fiber whatsoever. This was most egregious with S/N 10, with which my new cavity coupling maxed out at 83%(!), even after slight empirical MMT tweaking. S/N 2 wasn't as nasty; I got ~91.5% with it.

Given this, my new plan was to make a quick-and-dirty measurement in the following way:

  1. Optimize the mode matching and record the contrast defect (17% and 8.5%, for S/N 10 and S/N 2, respectively, as mentioned above)
  2. Drive the modulator at a chosen RF frequency (I chose 30 MHz since this is near where we'll be using them), and determine the amplitude necessary to double the reflected power.
  3. The measured amplitude is associated with the modulation depth necessary to pump the same fractional power as the contrast defect out of the carrier (really, you could use any SB power level additively distinguishable from the contrast defect, but doubling it seemed the easiest thing)
  4. Use the bessel function to infer that modulation depth, then scale the measured amplitude up to infer Vpi.

S/N 10:

Measured amplitude to double REFL power: 0.78 Vpp --> 0.39 Vpk.

2*J1^2 = 17% --> gamma = 0.611

Vpi = 0.39 * (pi / 0.611) ~ 2.00 V

S/N 2:

Measured amplitude to double REFL power: 0.52 Vpp --> 0.26 Vpk.

2*J1^2 = 8.5% --> gamma = 0.422

Vpi = 0.26 * (pi / 0.422) ~ 1.93 V

The datasheet claims 3.5 V typical, so this seems pretty good (though the spec is only officially at 50 kHz drive). Holding the amplitudes constant, I also swept the frequency down from 30 MHz to 10 MHz, and the reflected power was stable to around 5%.

Again, this is only really a quick-and-dirty measurement. Unfortunately, the only real way to get a good measurement is to reprofile the beam again with each modulator in place. Then, the contrast defect can presumably be brought down closer to 2% or better again, and the measurement can be made more cleanly. I'm hesitant to waste time doing so, though, given the observed mode dependence on the fiber resting position.

 

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