Step 3: Inserting FI and un-eliptical-ification of the beam
The FI set up on it's mount and the beam passes through it - centrally through the apertures on each side. Need to make sure it doesn't clip and also make sure we get 93% through (datasheet specs say this is what we should achieve). We will not achieve this, but anything close should be acceptable.
Setting up for minimum power through the FI is HWP @125deg.
Max is therefore @ 80deg
Power before FI = 544 mW
Power after FI = 496 mW (after optimising input polarisation)
Power dumped at input crystal = 8.6mW
Power dumped at input crystal from internal reflections etc = 3.5mW
Power dumped at output crystal on 1st pass = approx 8mW
OK. that gives us a 90.625% transmission and a 20.1mW absorption/unexplained loss.
Well - OK. The important part about isolators isn't their transmission, it's about how well they isolate. Let's see how much power gets ejected on returning through the isolator…
Using a beam splitter to pick off light going into and returning from the FI. A 50/50 BS1-1064-50-1025-45P. And using a mirror near the waist after the FI to send the beam back through. There are better ways to test the isolation performance of FI's but this will suffice for now - really only want to know if there's any reasonable isolation at all or if all of the beam is passing backwards through the device.
Power before BS = 536 mW (hmmn - it's gone down a bit)
Power through BS = (can't access ejected on first pass)
Power through FI = 164 mW (BS at odd angle to minimise refractive effect so less power gets through)
Power lost through mirror = 8.3mW (mirror is at normal incidence so a bit transmissive)
Using earlier 90.6% measurement as reference, power into FI = 170.83 mW
So BS transmission = 170.83/536 = 0.3187
BS reflectivity therefore = 1 - 0.3187 = 0.6813
Power back into FI = Thru FI - Thru mirror = 155.7 mW
Power reflected at BS after returning through FI = 2.2mW
Baseline power at BS reflection from assorted internal reflections in FI (blocked return beam) = 1.9mW
Note - these reflections don't appear to be back along the input beam, but they *are* detectable on the power meter.
Actual power returning into FI that gets reflected by BS = 0.3 mW
(note that this is in the fluctuating noise level of measurement so treat as an upper limit)
Accounting for BS reflectivity at this angle, this gives a return power = 0.3/0.6813 = 0.4403 mW
Reduction ratio (extinction ratio) of FI = 0.4403/155.7 = 0.00282
Again - note that this upper limit measurement is as rough and ready as it gets. It's easy to optimise this sort of thing later, preferable on a nice open bench with plenty of space and a well-calibrated photodiode. It's just to give an idea that the isolator is actually isolating at all and not spewing light back into the NPRO.
Next up… checking the mode-matching again now that the FI is in place. The beam profile was scanned after the FI and the vertical and horizontal waists are different...