NAOJ GW Elog Logbook 3.2
after setting the PM100D badwidth to 150kHz, I repeatd the measurements reported in entry 861
The plot is normalized on the gain and on the DC value.
Then I measured the transfer function of the laser modulation actuator + the integrating sphere photodiode PM100D
Partecipants: Marco, Eleonora, Yuhang, Matteo
We mount in a NIM box the PLL board described in the logbook entry 847. The photo of the box are 'pllboxfrontend.png' and 'pllboxtop.png'.
Description of the front-end:
- 2 SMA connector for the two input beat note and RF channel from DDS board not amplified
- Lemo connector (output) label MUX is a chnnel digitally configurable from the software use for diagnostic purposes (RF monitor, Beat monitor, Lock detect, etc)
- Lemo connector (output) label Fast is the correction singnal sent to lazer PZT, it can be activated or not using a switch mounted on the front panel
- Lemo connector (output) label Slow is the correction signal sent to laser PLT, it can be activated or not using a switch mounted on the front panel
Test of the board using AUX2 as slave laser (fig 'aux2lasrslave.pdf')
We tested the long term stability during the night ant the day later we found the PLL still locked
We measure the PLL phase noise in three different condition (charge pump current: 4.375mA):
- MLfree running (rampe auto swithced off) rms phase noise: 4.9mrad
- ML free running (rampe auto switched on) rms phase noise: 10.6mrad
- ML non free running (filter cavity locked) rms phase noise: 16.7mrad
Test of the board using AUX1 as slave laser (fig 'aux1lasrslave.pdf')
We measure the PLL phase noise in three different condition (charge pump current: 3.75mA):
- MLfree running (rampe auto swithced off) rms phase noise: 5.5mrad
- ML free running (rampe auto switched on) rms phase noise: 14.9mrad
- ML non free running (filter cavity locked) rms phase noise: 15mrad
For both the servo loop we noticed that the output voltage of the rampe auto is high enough to increase the ML frequency noise.
Moreover the noise reduction due to the filter cavity locking between 100 Hz and 12-15 kHz is visible in both the servo loop.
The rampe auto noise is predominant at frequencies above the unitary gain bandwidth of the filter cavity servo loop
I did on the PM100D integrating sphere PD the same noise checks as I did on the InGaAs PD and reported in http://www2.nao.ac.jp/~gw-elog/osl/?r=846 elog entry
the measurements though are not reliable because after making them I found that the output bandwidth was set on 15Hz.
Participant: Eleonora, Marco and Yuhang.
1. Open the loop if you want to change the object you want to lock. Because we have only one board for locking up to now.
2. Reconnect the photodiode, PZT(in the back of red laser head) and pietie(int the back of white laser box and the middle one) from the previous one to the one you will control.
3. Check the photodiode output, and try to adjust steering mirrors to see if you can improve the fiber coupling or not. Sometimes if someone touch the collimator or mirror, the coupling will be changed.
4. Then connect this beatnote between these two lasers to spectrum analyzer. Remember to choose range from 0Hz to 1.3GHz. And change the temperature from the laser box. You will see from the spectrum analyzer the beatnote moves with your changing. Move it close to 20MHz, the frequency we want to use for demodulation. Note here that sometimes if the spectrum analyzer doesn't work, press the preset button.
5. Then look at it more closely and check the level of the peak you want to lock, it should be larger than -16dB. If not, you can check again the fiber coupling. Usually try to change the polarization.
6. Move the peak as close as 20MHz. Here is a splitter(10:90), 10 percent is used to monitor while 90 percent is used to lock the PLL. Then firstly put on the fast control and then slow.
7. If you want to measure the phase noise, check firstly the level of this signal. Change the demodulation phase to make it close to zero, means fluctuating around several hundred microV to 1mV.
8. Change the demodualtion frequency and use oscilloscope to see this frequency component to get the calibration factor.
9. Use DC couple and put the close to zero signal to the network analyzer. Then you will get noise spectrum.
For the 7 of step, first thing is to demodulate this signal with the frequency of beat note. Then by chaning the phase of this demodulation signal, we can make the demodulation output close to zero. This is crucial for the measurement of phase noise with DC coupling.
We used the similar manner to entry 830 characterize the main laser PZT.
Note here the resonance begin before 70kHz.
Participant: Eleonora, Marco, Matteo and Yuhang
After realize the 7mm collimator is more suitable, we replace the 11mm one. Then we finalize the optical layout of PLL. The main task we did is the new telescope design and fiber alignment.
The new telescope is shown in attached picture. However, the actual case is a little bit different because of the Faraday influence of optical length. But we made sure the beam is very collimated with a size of 2mm in diameter.
For the fiber coupling, we develop a procedure. We assume you have already a coupled fiber.
1. Put the output of this already coupled fiber to collimator. Now, you have both light going in and back through this collimator. Then use the steering mirror to make them overlap. Always make sure the light is a good round shape after you take off the fiber from collimator.
2. Put the multimode fiber to do pre-alignment. If you did very well the first step, you will have a very large coupling directly after you put multimode fiber. Then use steering mirror to align until get 100% coupling. If you cannot, remember to check the shape after collimator is round.
3. Put the single mode fiber. If you did step 2 as we suggest, you will have very good coupling now even for this single mode fiber. Then just use the steering mirror to do standard alignment. You will get a good coupling result.(we got 70 percent)
According to this procedure, we coupled the fiber for the second main laser pick off and AUX 2 laser(p pol). We got 70% coupling for both of them.
The final layout is attached as picture two.
Participant: Yuhang and Eleonora
Green power measurement
| before EOM | after EOM | before AOM | before PR | before MZ |
| 74mW | 71mW | 13.7mW | 8.8mW | 33.6mW |
Infrared power mesurement
| before PR | after pick off | before pick off |
| 9.4mW | 10.6mW | 17mW |
After rearange the control devices, we recover the green and infrared lock.
| FC green transmission | FC infrared transmission |
| 1.3V | 1.6V |
To transfer data from the Yokogawa SA2400, the only way is through the GPIB port. (otherwise, there is an oldfashioned paper plotter)
So, I wrote a labview program to read the spectra and save it on a file, based on a library I found here: http://sine.ni.com/apps/utf8/niid_web_display.download_page?p_id_guid=E3B19B3E936A659CE034080020E74861
The list of commands for the GPIB is in the (in Japanese sory...) manual I uploaded to the wiki: http://gwpo.nao.ac.jp:8989/wiki/Documents?action=AttachFile&do=view&target=SA2400+GP-IB+manual.pdf
I did some spectrum acquisitions of the photodiode signal. Average number 64. in several conditions.
Plot 1. spectrum in dbV (not normalized per rtHz)
20180625-unplugged (noise without anything connected)
20180625-unplugged400khz
20180625-50ohm (noise with the 50ohm terminator
20180625-50ohm400khz
20180625-darkT50ohm (PD dark noise with a T and 50ohm terminator)
20180625-darkdirect (PD dark noise
20180625-darkAC (PD dark noise after the high pass filter box)
20180625-darkACT50ohm (PD dark noise after the high pass filter box with a T and 50ohm terminator)
20180625-darkDC (PD dark noise after the high pass filter box)
Plot 2. spectrum in dbV/rtHz ( normalized per rtHz)
noise floor
Plot 3. 50ohm terminator as input of the SR560. Icreased the gain of the preamplifier to check where is the noise floor of the SR560.
Plot 4. Comparison of the noise floors of SR560 and PD dark noise
Plot 5. Laser on / off
to be compared with the noise on the HeNe PD
On the other hand there is a strange "large" peak in the middle of the FSR. Where is that coming from? Is there a polarization problem? In the entry it is written that the polarization is OK.
Question: did somebody already check that the input and output mirrors are mounted with the HR side facing the inside of the cavity?
Partecipants: Marco, Eleonora , Yuhang
We match the light of the ML into the fiber.
Input power: 3mW
Matched power: 0.71*2mW
Fiber matching: 47%
After that we control the matching of the AUX2 fiber:
Input power: 3.5mW
Matched power: 1.25*2mW
Fiber matching: 71%
Voltage level on photodiode:
ML: 7.8V
AUX1: 5.2V
Total: 2.6V
We found the beat note between the two lasers and we measure the following levels:
| Beat note | Amplified signal | 90% signal | 10 % signal | |
| Carrier | -21dBm | -6.33dBm | -7.67 dBm | -17.60 dBm |
| Sideband 1° order | -33.8dBm | -17.67dBm | -18.33dBm | -29.43 dBm |
| Sideband 2° order | -54dBm | -38.17dBm | -38.67dBm | -49.77 dBm |
| Sideband 3° order | not visible | -65.33dBm | -61.50dBm | not visible |
The AUX1 laser temperature was set at 30.67 °C, at the spectrum analyzer we see both the beat note and the lateral sidebands due to EOM modulation.
The minimum level required for the beat note to lock the PLL is -16 dBm, thus the amplitude of the carrier is enough to perform the PLL locking. Concerning the sideband to perform the lock on them their level must be incresed at least of 2-3 dBm
[ Yuhang, Matteo, Eleonora]
After locking the Green mode cleaner we measured a trasmissivity below 50% which is much lower than what we expected.
An extremely rough power budget gave us:
P_in = 22 mW
P_tra = 8.5 mW
P_ref = 10.5 mW
P_tra from end mirror = 0.5 mW
Missing = 2.5 mW
We investigated some of the following possibilities:
Mirrors transmissitvity
For a triangular cavity, as our modecleaner, the transmission is given by:
T = (t_in*t_out)/1-r_in *r_out *r_end)^2
according to the spec for the mirrors used (see pic 1) :
R_in = R_out = 0.992 and R_end > 0.995 (measured from the producer 0.9993)
= > T = 0.92 (taking R_end 0.9993)
Considering an error of +/- 0.003 in all the three nominal transimissivity, the expected cavity trasmissivity is 0.92 +/- 0.33
Matching and alignment
The optical spectrum of the cavity is shown in pic.1. The alignement seems good. The sidebandes at 78 MHz ( used for the lock of the MC) are not visible while we can see the 15.2 MHz modulation that we know to be high.
[ Note that we are sending at the 78 MHz resonant EOM, a driving RF signal with amplitude 1 V pp which should correspond to a modulation depth of 0.185 rad. this means that the expected power in the sidebads is 0.0086]
input beam polarization
Yuhang used a PBS for 532nm and verified that the light is almost all in s-pol, as it should be.
Conclusions: The origin of the low MC transmissivity is not clear but the most probable hypotesis, among those considered, is that the effective transmission of the mirrors are a bit different from the nominal ones.
On the other hand there is a strange "large" peak in the middle of the FSR. Where is that coming from? Is there a polarization problem? In the entry it is written that the polarization is OK.
Question: did somebody already check that the input and output mirrors are mounted with the HR side facing the inside of the cavity?
I checked the input and output mirror. From the point view of marker on the side of mirror, I am sure the mirror is installed in a correct way.
I checked also the mirror from the same box, this arrow points to the HF side of this mirror.
Are the values given above now confirmed?
P_in = 22 mW
P_tra = 8.5 mW
P_ref = 10.5 mW
P_tra from end mirror = 0.5 mW
Missing = 2.5 mW
Are the 2.5 mW still missing?
Partecipants: Marco, Matteo
In addition to the ADF4002 phase frequency detector evaluation board we design an external board to perform both the fast loop that acts on the Laser PZT and the slow loop that acts on the Laser PLT.
Description of the external circuit
Attached: Fig 'boardblockscheme.png' shows the block scheme of the board and Fig '8.pdf' is the board electrical scheme.
The board mounts the loop filter (see entry 837 ) and 6 Op-Amps:
Loop Filter
C1 = 33nF, C2=680nF, R=27 Ω
Fast loop (PZT) input loop filer output
- IC1 is a 2x non-inverting amplifier (with the aim to amplify the correction signal from 0-5V to 0-10V)
- IC2 is an active notch filter with center frequency 270kHz. Fig 'pllnotchcharacterzation.pdf' represents the notch filter magnitude transfer function.
Slow loop (PLT) input loop filter outout
- IC3 is a difference amplifier in order to center arond zero V the loop filter output. The 2.5V offset can be tuned from 2V to 3V acting on a trimmer,
- IC4 is an integrator with a zero around 2Hz
- IC5 is a variable gain non iverting amplifier (gain from 1/2 to 1/5000)
- IC6 is an inverter to change the slow loop sign
Loop performances
We lock both the loop on PLT and PZT and measure the loop performances. The PLL servo loop was closed acting on AUX2 laser as slave laser.
Concerning the long term stability the PLL remains locked between Friday evening and Monday morning.
Concerning the loop phase noise I measured the PLL output phase noise between 100Hz and 102.k kHz with different charge pump gain. The used gain, the rms phase noise and the approximative loop bandwidth are reported in the following table:
| CP Gain [mA] | r.m.s phase noise [mrad] | Approx. loop bandwidth [kHz] |
| 1.875 | 8.67 | 30 |
| 2.5 | 7.78 | 36 |
| 3.125 | 6.56 | 38 |
| 3.75 | 5.13 | 43 |
| 4.375 | 4.53 | 49 |
| 5 | 3.96 | 53 |
Fig. 'phasenoisevsgain.pdf' shows the ouptput phase noise in the different configurations of the table above.
Fig 'phaenoisecp5mA.pdf' shows the phase noise and its cumulative rms value of the final configuration CP Gain = 5mA.
Considering the space we have, we designed the telescope for main laser pick off.
Participant: Eleonora and Yuhang.
For the locking of main laser with two auxiliary lasers, we need to use part of mian laser.
So we used a 70:30 BS, which is now put after the first BS 25cm. The transmission is 21mW and reflection is 11mW. After put it, we aligned the two steering mirrors so that we can have infrared beam aligned in the filter cavity.
After locking the filter cavity, we can have infrared transmission as 900mV.
The sequence is like this: 1. Mach-Zehnder 2. green mode cleaner 3. OPO
For the purpose of offering a stable light for OPO, we decided to use the transmission of mode cleaner as a feedback signal. Then take this signal to Mach-Zehnder to control MZ's PZT.
We put a beam splitter to get this feedback signal. The transmission is used for feedback. The reflection is used for OPO. The ratio between them is 99.2:0.8 (reflection:7.7mW and transmission:67uW)
The signal we get from transmiaaion photo detector is around 20mV(read from oscilloscope).
The procedure I did:
Firstly, I use SR560 lock green mode cleaner.
Secondly, I make sure the loop for MZ is fine.(check signal is going to control board, move the offset to make error signal close to zero, turn on lock switch, turn on intergrator)
Thirdly, I moved the offset of MZ control, I could see the mode cleaner transmission changed with my adjust.
The noise spectrum is attached.
However, the duration of lock is not good. I tried only once. It is around 10minutes.
Characterization of the minicircuits ZFRSC-4-842-S+ (one to four splitter)
The characterization frequency is 500MHz. (wenzel output)
power splitter Input: 3.07dBm
power splitter output: (1) -4.77dBm (2) -5.10dBm (3) -4.27dBm (4)-4.43dBm
This means we loss around 8dB.
Partecipants: Marco, Eleonora
We repeat the same work reported in the entry 838 but this time we reverted the lasers configuration: AUX1 is the Slave laser and AUX2 is the Master laser of the OPLL.
The circuit scheme is the same used for the other measurement, the selected loop filter was the same of the Config1: C1=680nF R1=27Ω and C2=100nF. The charge pump current was set to 3.75mA and the frequency offset was set to 20MHz.
Picture 'PLL_20MHz.jpg' shows the 20MHz beat note peak once locked. We can see the same structures of the previous configuration.
The phase noise was measured again with the demodulation technique. The mixer calibration factor is measured at 100Hz and is again Vpp=98mV.
The measured phase noise is shown in picture 'pll20mhzpn.pdf' the blue line represents the phase noise of the pll with AUX2 slave laser and the red line represents the phase noise with AUX1 as slave laser.
The rms phase noise between 120 Hz and 102.4kHz is equal to 10.04 mrad.
We try to increase the CP gain in order to decrease the phase noise but the first laser PZT resonance was excited (see fig 'pll20mhzaux2res.jpg'). A notch filter centered at f=160kHz will be added in series to the loop filter.
Participants: Marco, Eleonora
We did the characterization of the AUX1 laser PZT gain with the same procedure reported in entry 830.
The attached plot shows the PZT gain as a function of the frequency from 1 kHz to 300 kHz. A forest of piezo resoances appear above 160 kHz.
