Nick Brewer

7/22/14 We tried to see some absorption again today. The data is here, and looks unconvincing. There is not a strong peak at 1055.076 nm like we are expecting. I think in order to get a better spectrum we will need to measure fluorescence from the crystal like in Shen's paper instead of trying to directly measure absorption. Zach has been working on the locking circuit and we are now able to get a much less noisy signal. The fast feedback is fed directly to the diode instead of going through the current driver. Before the signal fluctuated ~25% or more when locked but now it is much less and we are able to get a clean signal out. There are still certain wavelengths where the error signal looks more like a peak than a dispersion graph and we are unable to lock, but we haven't yet determined the cause..

7/18/14

We tried to see some absorption yesterday. We saw fluorescence at the wavelengths we expected to, but the numbers from the photodiodes aren't too convincing. Our setup is shown to the right. For each wavelength of light we took the ratio of the photodiode signals. Scanning over the wavelength was not done too systematically. We tried to get as many wavelengths as we could close to where the peaks should have been. We adjusted the wavelength by adjusting the grating angle, current, temperature, and piezo. We saw fluorescence at wavelengths near the peak, but the quantitative data does not look as good. Our results are shown here.

6/27/14

This week we got liquid helium and cooled the cryostat down. Everything seemed to be working. We currently have no way of monitoring how much liquid helium we use though so it might be nice to come up with a way to do that.

After playing with the PID settings on the temperature controller I was able to keep the temperature mostly at 5.00 K +/- 0.01 K, which is what Janis said we should expect for a stability. The PID settings for this were: P=220, I=50, D=0 with the heater range set to medium. Once in a while it jumps out of this range though. We can probably get it better than what it is now though. Hooking the temperature controller up to a computer so we can see a graph of temperature vs time might help us more fine tune the PID settings. The flow rate of the LHe probably is a big factor in how stabilized the temperature is. So far the only way we have to control the flow rate is the flow regulator valve on the transfer line. Once the cryostat is cooled down to ~4.5 K I close the valve until the temperature starts to rise, then slowly open it until it begins to drop again. If it can't make it back to ~4.5 K I open it a smidgen more, etc, until it just reaches 4.5 K. It's not as easy as that though, there seems to be quite a bit of lag in the control, especially when it is at it's minimum temperature. I closed the flow regulator valve and tried to get the same flow rate to see if the temperature stabilized again and it did. I think you can feel the LHe flow while adjusting the valve so you can feel if it is flowing or not. A master sensei could probably use his sense of touch to adjust the flow rate.

5/30/14

The cryostat was pumped down to 1.9E-5 mbar this morning.

After closing the butterfly valve and shutting off the pump, the pressure went down to ~4E-4 mbar almost immediately. After 15 minutes the pressure was at 8.6E-3 mbar. After 20 min it was at 1E-2 mbar.

It might be best to have the pressure gauge directly attached to the 'instrumentation skirt' of the cryostat. There are blank flanges we could have a KF-25 flange put on.

5/29/14

I hooked up the turbo pump to the cryostat today. The pumping station is pretty easy to use. It was running for maybe an hour and got to 1.6E-4 mbar. I left it run overnight. Dan Logan from Janis said the cryostat can hold ~1E-4 torr and if we continually pump it with the turbo pump we can maybe get 1E-5 or 1E-6.

5/23/14

Today the cavity is working great. There were a few small peaks between the 0,0 peaks but they were small and I was able to get rid of them by adjusting the mirrors and mode matching lenses. Yesterday's problems might have been a laser issue due to the weather. This morning the humidity was 30%, I'm not sure what it was yesterday but I should keep an eye on how that affects how well the cavity is behaving.

I must have done the $I_{r}/I_{in}$ measurement wrong on 5/21/14. Today I measured the same finesse that I got two days ago (~65.5%). I looked at the reflected signal before it went through the beam cube and got a similar measurement for $I_{r}/I_{in}$ (5.4/7.02 = 0.769). I think the reflectivity needs to be measured once the temperature is optimized for green generation. When I did this I got $I_{r}/I_{in}=0.239$ (accounting for a 106 mV noise the photodiode was seeing).

I redid the mode matching calculation with g=0.953, $R_{in}=0.0523$, and $I_{r}/I_{in}=0.239$. With this numbers I get $m=0.759$. This is good because it means the cavity is mode matched pretty well, and also that the cavity must be pretty well impedance matched. If the impedance matching is perfect, I think the mode matching coefficient, m, is just $1-I_{r}/I_{in}$. In this case, $1-I_{r}/I_{in}=0.761$.

According to my Matlab program, we should expect 15.9% conversion efficiency. I was getting 10 mW of green for 65.5 mW of IR, which is 15.3%.

Josh has a plastic 1" optics adaptor for the piezo we are using, so we can try that to see if it increases the resonant frequency at all. We replaced the piezo holder and the resonant frequency is almost the same. It is ~450 Hz now and was 420 Hz with the home made mirror mount.

5/22/14

I think the mode matching calculation I did was wrong because the value $I_{R}/I_{in}$ should definitely just be 1.01/1.21. When I looked at the reflected beam I was just using one of the photodiodes from the analyzer so it's possible that messed up the measurement. I will try to look at the cavity dips before it goes through the beamcube and redo the calculation. Regardless of what the calculation says, the amount of green we were generating is promising.

Today there were two extra peaks showing up that I couldn't get right of. One was the same height as the 0,0 mode and the other was less than 25% of the 0,0 mode. The usual alignment procedure didn't solve the problem. Moving around the mode matching lenses didn't help much either. I checked the laser on the spectrum analyzer (which is currently not working great), and the wavemeter. Both seemed to indicate it was single mode.

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.

I also measured the cavity dips from the reflected beam and found that the dip minimum to be 1.01 V and the max value to be 1.21 V. I am confused whether 1.01/1.21 is $\eta$ in the formula above or if it is just $I_{R}/I_{in}$.

Once the cavity was tuned and the temperature was adjusted to be at the optimal value, we measured 5.5 mW of green (I'm almost positive we used the right wavelength settings on the power meter). The input power was ~45 mW. That is an efficiency of ~12%. If I take $\eta =1.01/1.21$ so that $I_{R}/I_{in}=0.165$ and figure out the mode matching coefficient with the other measured values above, I get m=0.84 (very well mode matched...almost too well). If there is 45 mW of power before the cavity, and the mode matching coefficient is 0.84, that means that 36 mW of power is being coupled into the cavity. When I ran the Matlab code I wrote to simulate the cavity enhancement, I find that for and input coupler with T=0.05, we should expect ~12% efficiency. This result is so close to what we measured that I almost certainly calculated something wrong. If I use $I_{R}/I_{in}=1.01/1.21=0.83$, I get a mode matching coefficient of m=0.12, which corresponds to 5.4 mW getting coupled into the fiber, which is less than the green power we got out.

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.