Nick Brewer

5/21/14

I'm interested in measuring how well the cavity is mode matched. According to Jinlu's thesis, to measure the mode matching coefficient $m$, you can use the formula ${\frac {I_{r}}{I_{in}}}={\frac {m}{R_{in}}}{\frac {(R_{in}-|g|)^{2}}{(1-|g|)^{2}}}+(1+m)R_{in}$

there are three things you have to measure:

• The input coupler reflectivity with the cavity blocked $R_{in}$
• The finesse of the cavity to determine |g| using $F={\frac {\pi {\sqrt {g}}}{1-|g|}}$
• The reflection dip percentage $\eta =1-{\frac {I_{R}}{I_{in}}}$

A couple days ago I shined 84.1 mW of 1055 nm light through the input coupler and measured 4.4 mW transmitted, that corresponds to an input reflectivity of $R_{in}$=0.9476. I tried to measure the reflection off of the flat surface of the input coupler to account for that but I was unable to see anything; the reflected beam was too big and bright. In the future I should try again with less power.

After aligning the cavity today I measured the finesse by just looking at the ratio of the FSR to the FWHM of the cavity peaks and found $F$=65.5

After tuning up the cavity today and locking it, we measured 5.5 mW of green. The input power was ~45 mW. That is an efficiency of ~12%.

5/20/14

We are still using the fiber we borrowed from Saffman's group but we are anxiously awaiting the arrival of the fibers we ordered from Oz Optics.

Zach got the locking circuit working for the cavity without the PPKTP crystal, but there still seems to be a resonance at the piezo frequency (~420 Hz). I rearranged the cavity and put the crystal in it and set up the analyzer for the locking circuit with some cage mount stuff ordered from Thorlabs. It was tougher to set up the cavity with the crystal. The first attempt or two resulted in the input mirror being tilted much more than what the cavity layout we designed. In the end I determined the best way was to optimize a single pass, then put the cavity mirrors in one by one trying to follow the designed beam path as best as I could. The final mirror I put in was the input coupler. I think it's pretty critical to be hitting the center of the curved mirrors so they don't have to be tilted at extreme angles. I looked at the output of the cavity by putting a microscope slide between the two flat mirrors, this is convenient to align the cavity because there isn't enough intracavity power to see a transmission through the HR mirrors. Also it doesn't perturb the cavity a huge amount; only minimal realignment is necessary when it is removed. Once I saw a couple passes of IR with the microscope slide I could see a couple passes of the green output and used that to get a rough alignment. I optimized the cavity then by looking at the cavity peaks of the green light. I touched the two mirrors that walked in the input beam very little because for fear of moving the beam path off of the crystal.

After a playing around with the lock a little bit we eventually got the cavity locked to the laser and at one point saw ~9 mW of green (from 65 mW of IR). Because the crystal is birefringent, the error signal now looks like what they describe in Vainio's paper. The output isn't super stable when we look at it with an oscilloscope. Also, since we put the crystal in it seems like the peaks jump around instead of drifting around more slowly like they did before, so that might be hurting the lock. Zach is also adding in a fast feedback to the laser current.

The polarization that generates the most green is vertical, I determined that with a Glan Taylor polarizer. We determined that in order to properly align the cavity we need to have the temperature far out of tune so we can see the cavity peaks produced from the vertically polarized light. Otherwise there is too much loss at that polarization due to the generation of green light.

5/9/14

The fibers that we were using to clean up the tapered amplifier output were introducing power fluctuations, over the course of this week we tried three fibers:

- The first fiber we tried was single mode but not polarization maintaining (and not angle polished?) and there were power fluctuations >20%.

- The next fiber was single mode and polarization maintaining but not angle polished from Oz Optics. The same power fluctuations were occurring. We determined back reflected light was getting back through the isolators and that was causing the power fluctuations.

- We borrowed a single mode, polarization maintaining, angle polished fiber that had previously been repaired from Saffman's group. This fixed the power fluctuations seen earlier, but the output polarization rotated causing the output power to drift by 50% after a beam cube. I tried to find the correct input polarization by jiggling the fiber and watching the output power fluctuate after a beam cube, but I couldn't find a good polarization. Jared said he's seen output polarization rotate due to the wrong input polarization, but it was not as extreme as this, it only caused the power to fluctuate by ~10%. I suspect this fiber is just damaged and the repair that was previously made did not completely restore the fiber (and possibly caused the current problems?).

Jared ordered some new polarization maintaining, single mode, angle polished fibers from Oz Optics so hopefully that fixes the problems we are seeing. In the mean time I just used the output of the TA to do the single pass texts on the PPKTP crystal. The results were surprisingly close to what was expected:

Single pass efficiency: E_NL=P_out/P_in²=0.0027 (Simulated E_NL=0.0029)

Temperature bandwidth = ~2.4 °C (In Kumar's paper they report a temp bandwidth of 3 °C for a 19 mm PPKTP crystal at 1064 nm)

Temperature tuning coefficient = 0.0591 nm/°C (The salesman from Raicol said they measured 0.053 nm/°C for SHG at 1064 nm)

For the single pass efficiency and temperature tuning bandwidth measurements the TA was set to 1799mA and had a power of 478 mW right after the TA. While determining the temperature coefficient the TA was set at 1500 mA and had a power of 350 mW right after the TA. This power changed from wavelength to wavelength a little bit though.

5/7/14

Re-centered fiber scope lens; avoided forking fiber to the table.