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Entry  Thu Sep 12 11:04:17 2013, Evan, DailyProgress, ISS, North EOAM: broken? 
    Reply  Tue Sep 24 11:12:49 2013, Evan, DailyProgress, ISS, North EOAM: broken? 
       Reply  Thu Oct 24 16:44:43 2013, Evan, DailyProgress, ISS, North EOAM: working north_eoam.pdf
Message ID: 1335     Entry time: Thu Sep 12 11:04:17 2013     Reply to this: 1352
Author: Evan 
Type: DailyProgress 
Category: ISS 
Subject: North EOAM: broken? 

Neither Tara nor I can get the north New Focus 4104 to put out a significant amount of power modulation, despite going through (what we think is) the biasing procedure several times. We're getting modulation on the order of 1 µW/V, compared to 30 µW/V when we first installed the south EOAM (PSL:1287).

To review, these New Focus EOAMs consist of two lithium niobate crystals mounted with their fast and slow axes orthogonal to each other. If the crystals are the same length, then with zero applied voltage the EOAM should have no birefringence. Any applied voltage causes the EOAM to become birefringent; the voltage required to produce a λ/4 retardation between the two optical axes is called the quarter- wave voltage, and the voltage required to produce a λ/2 retardation is called the half-wave voltage (Vπ).

To use the EOAM for intensity modulation, we put down a HWP before the EOAM to make sure the input beam is either p- or s-polarized relative to the optics on the table. The EOAM crystals are mounted at 45 deg., so the input beam therefore is projected in equal parts onto the EOAM's two optical axes. After the EOAM there is a QWP with its fast and slow axes aligned to the EOAM's optical axes [Edit: actually the manual says to align the QWP axes to be horizontal and vertical wrt the table, which I don't understand. At any rate, neither configuration makes the EOAM work.], and following that there is a PBS which passes only p-polarized light. The intensity of the light transmitted through the PBS is a linear function of the EOAM birefringence only when the beam entering the PBS is nearly circularly polarized, so the purpose of the QWP is to optically bias the beam so that we can actuate around zero volts on the EOAM. (The alternative is to have no QWP and instead electrically bias the EOAM to its quarter-wave voltage.)

Anywho, the procedure that Tara and I have gone through is

  1. Remove the EOAM and QWP. Adjust the HWP preceding the EOAM so that the intensity through the PBS is at a minimum.
  2. Put down the QWP and adjust it so that the intensity transmitted through the PBS is equal to the intensity reflected from the PBS. This should mean that the beam into the PBS is circularly polarized.
  3. Put down the EOAM and align it so that the beam passes through.

After this, the setup should now give linear intensity modulation when a few volts are applied to the EOAM. For the south EOAM this procedure worked fine—by applying a few volts to the EOAM we could see the power change on the ThorLabs power meter. But with the north EOAM the power changes are much, much smaller.

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