I looked into the design of the P2 interface board. The main difficulty here is geometric - we have to somehow accommodate sufficient number of D-sub connectors in the tight space between the two P-type connectors.
I think the least painful option is to stick with Johannes' design for the P1 connector. For the CM board, the P2 connector only uses 6 pairs of conductors for signals. So we can use a D-15 connector instead of 2 D-37 connectors. Then we can change the PCB shape such that the P1 connector can be accommodated (see Attachment #1). The other alternative would be to have 2 P-type connectors and 3 D-subs on the same PCB, but then we have to be extra careful about the relative positioning of the P-type connectors (otherwise they wont fit onto the Eurocrate). So I opted to still have two separate PCBs.
I took a first pass at the design, the files may be found here. I just auto-routed the connections, this is just an electrical feedthrough so I don't think we need to be too concerned about the PCB trace routing? If this looks okay, we should send out the piece for fab ASAP.
I will work on putting together the EPICS server machine (SuperMicro) this afternoon.
2. D040180 / D1500308 Common Mode Board
CM servo board itself doesn't need any modification. The CM board uses P1 and P2. So we need to manufacture a special connector for CM Board P2. (cf The adapter board for P1 T1800260). See also D1700058.
It's nice and compact, and the cost of new 15-pin DSUB cables shouldn't be a factor here. What does the 15p cable connect to?
it will connect to a 15 pin breakout board in the Acromag chassis
The boards arrived. I soldered on a DIN96 connector, and tested that the goemetry will work. It does . The only constraint is that the P2 interface board has to be installed before the P1 interface is installed. Next step is to confirm that the pin-mapping is correct. The pin mapping from the DIN96 connector to the DB15 was also verified.
*Maybe it isn't obvious from the picture, but there shouldn't be any space constraint even with the DB37/DB15 cables connected to the respective adapter boards.
I added a PA current limiter.
It is only a voltage devider (composed with 3.09k and 1.02k resiste) between DAC and PA current adjustment input.
The output range of DAC is +/- 10[V] and the conversion factor of PA current adjustment is 0.84[A/V] (measured value), so the PA current adjustment is limited +/- 2.1[A] ( 10[V]*1.02k/(1.02k+3.09k)*0.84[A/V] ).
Actually, the manual of the PA tells that the conversion factor is 0.25[A/V].
There is 3 possibility.
1) There are some mistakes in channels of digital system.
2) The PA manual is wrong.
2-1) The conversion factor of current adjustment is wrong.
2-2) The conversion factor of current monitor is wrong.
I measured the signal of current adjustment and current monitor directly, and confirm that they are consistent to the value monitord from MEDM.
Hence the PA manual must be wrong, but I don't know which factor is wrong (or both?).
If the suspect 2-2) is guilty, it means we adjust PA current with very small range.
This is a completly safety way, but a wast of resource.
Now, the slider to control current adjustment indicate the output of DAC.
I will improve this to indicate current adjustment input, but it takes some time for me to learn about EPICS.
[Koji and Kevin]
Since there was still a lot of power being reflected from the PBS before the Faraday rotator, I placed another PBS at the reflection from the first PBS to investigate the problem. If everything was ideal, we would expect the PBS to transmit P polarization and reflect S polarization. Thus, if the laser was entirely in the TEM00 mode, with the quarter and half wave plates we should be able to rotate the polarizations so that all of the power is transmitted through the PBS. In reality, some amount of P is reflected in addition to S reducing the power transmitted. (We are not sure what the PBS is since there are no markings on it but CVI says that their cubes should have less than 5% P reflection).
For the following measurements, the laser crystal temperature was 31.8° C, the current was 2.1 A, the half wave plate was at 267° and the quarter wave plate was at 330°. I first measured the power reflected from the first PBS then added the second PBS to this reflected light and measured the transmitted and reflected powers from this PBS with the following results:
This shows that approximately 81 mW of P polarization was being reflected from the first PBS and that there is approximately 48 mW of S polarization that could not be rotated into P with the two wave plates. Attachment 1 shows the shape of the reflected (S polarization) beam from the second PBS. This shows that the S polarization is not in TEM00 and can not be rotated by the wave plates. The transmitted P polarization is in TEM00.
We then rotated the first PBS (in yaw) to minimize the amount of P being reflected. Repeating the above measurement with the current alignment gives
Thus by rotating the cube to minimize the amount of P reflected, ~70 mW more power is transmitted through the cube. This adjustment moved the beam path slightly so Koji realigned the Faraday rotator and EOM. The PMC was then locked and the beam was realigned on the PMC. At 2.1 A, the transmission through the PMC is 6.55 V and the reflection is 178 mV. With the PMC unlocked, the reflection is 312 mV. This gives a visibility of 0.43.
Note by KA:
We realigned the beam toward the PMC at 1.0A at first so that we don't cook any parts. Once we get the TEM00 resonance, the steering mirrors were aligned to maximize the PMC transmission. Then the pumping current was increased to 2.1A.
We installed a Half Wave Plate -> Polarized Beam Splitter -> Half Wave Plate in the PSL beam line, immediately after the EOM, to be used for attenuating the beam when we vent, as in Entry 6892.
It was illuminating to discover that the optics labeled QWP0-1064-10-2 are indeed half wave plates, instead of quarter wave plates as QWP suggests.
The PBS transmits "P"/Horizontal polarization, but the beam coming from the EOM is "S"/Vertically polarized, and we want to keep that, since we do not want the beam attenuated quite yet.
So, we use the HWP to rotate the P from the EOM to S, so that the majority of the power passes through the PBS. The second HWP then rotates the transmitted S back into P, which continues to the mode cleaner. When we want to attenuate, we will simply rotate the first HWP to change the proportion of S polarized light that will pass straight through the PBS and towards the mode cleaner.
After setting the proper HWP angles, we aligned the PBS via minimizing the MC reflection.
Since we have not yet attenuated the power, we have not yet changed the BS for the MC reflection, since this would damage the PD. The beam splitter will be changed out for a 100% reflectivity mirror to increase the power to the PD when we do.
Before we did this, I centered PSL POS and ANG, which gives us a reference of where the PSL beam was good when the MC spots were ~centered. There had been a beam dump blocking them, possibly from the last time we put in the power attenuator optics. This beam dump was moved a little to be out of the way of the PSL QPDs, and the PBS placed closer to the lens after the EOM, so that the PBS reflected beam is dumped. However, we should not remove that razor dump when we remove the attenuation optics, since it is also dumping a stray IR beam from the PSL QPD pickoff windowd.
Jenne and I were musing the other night that the PC drive RMS may have a "favorite" laser temperature, as controlled by the FSS Slow servo; maybe around 0.2.
I downloaded the past 30 days of mean minute trend data for MC Trans, FSS Slow and PC Drive, and took the subset of data points where transmission was more than 15k, and the FSS slow output was within 1 count of zero. (This was to exclude some outliers when it ran away to 3 for some days). This was about 76% of the data. I then made some 2D histograms, to try and suss out any correlations.
Indeed, the FSS slow servo does like to hang out around 0.2, but this does not seem to correlate with better MC transmission nor lower PC drive.
In the following grid of plots, the diagonal plots are the 1D histograms of each variable in the selected time period. The off diagnoal elements are the 2D histograms. They're all pretty blob-y, with no clear correlation.
Since the RFM on the new CDS is not working, we had to test it by using some softwares.
I installed a driver for PCI-5565 on C1SUS and ran a test script wich is one of the packaged test scripts in the driver.
So far the RFM card on C1SUS looked correctly mounted, but I didn't check the memory location and the sending/ receiving functions.
This test will continue sometime on August because right now the RFM test is not higher priority.
Alex suggested to use a driver package for PIC-5565 called "RFM2g Linux 32/64-bit PCIE/PCI/PMC driver for x86 kernels R7.03" , which is available on this web site.
And the package contains some useful test scripts which exactly we want to run for RFM test.
I downloaded the driver and put it on C1SUS.
After doing usual "unzip", "tar" and "make" things, I ran one of the test script called "rfm2g_util".
Currently it lives under /home/controls/Desktop/162-RFM2G-DRV-LNX-R07_03-000/rfm2g/diags/rfm2g_util on C1SUS.
It invokes an interactive shell and firstly it asks the mount point of the RFM card.
I eventually found the card was mounted on #1 which means the card is correctly mounted.
Some detail procedures will be summarized on the wiki later.
We now have the DC signal from three PDs available in the ADC channels 14,15 and 16. The signals are from REFL55, AS55 and POY photodiodes respectively. As the DC signals on all the other PDs of the same port (REFL, AS and PO) have the same information we do not need to monitor more than one DC PD at each port.
The LSC PD Interface Card, D990543 - Rev B, can take 4 PDs and provides the DC signals of the PDs on the connector P2 (the lower of the two) on the back plane of the chassis. An adaptor card, D010005-00, plugs into the back plane from the rear of the Eurorack and provides the four DC signals on two-pin lemo sockets.
I have connected the three DC signals from the relevant RF PDs (above) to a DC whitening filter, D990694-B-1 which is associated with the channels 9 to 16 of the ADC card.
The cables are in a bit of a mess right now as some of the PD power supply lines are too short to reach up the the Interface card in the top Eurocart. Steve and I plan to redo some of the cabling later today
Our RF Switch arrived today, and we mounted it in rack 1Y1 (1st attachment).
We connect our input fiber and all of our output fibers to our 1x16 optical splitter (2nd attachment). Note that the 75 meter fiber we are using for the splitter's input is in a very temporary position (3rd attachment - it's the spool).
We successfully turned our laser on and tested the optical splitter by measuring output power at each fiber using our Thorlabs PM20 power meter. Data was taken with the laser running at 67.5 mA and 24 degrees Celsius:
Detector name Power
I and Eric Gustafson inspected the automated PD frequency response measurement system which Alex Cole built last summer. We just lifted the tops off the tables [AS table, POY table and ITMX table] and looked at the alignment checking to see if the correct optical fibers from the fiber splitter were illuminating the correct photodiodes. We did not change anything at all and put the covers back on the tables.
The PDF attached shows the state of each PD fiber pair. The fibers labeled REFL11 and REFL55 were reversed and illuminating the wrong photodiodes.
We will do a manual measurement of REFL33 tomorrow using the network analyzer and the modulatable laser but not the RF switch. Afterward we will check to make sure the RF cables are connected to the correct channels of the RF switch according to the switch list (/users/alex.cole/switchList).
Here is the logic that I have been using to calculate the transimpedence of PDs. Please let me know if you think anything is wrong.
Summary: Routing Fibers on AP table for Photo Diode Frequency Response Measurement System
Objective: We are to set-up one simultaneous transfer-function measurement system for all the RF-PDs present in 40m lab. A diode laser output is to be divided by 1x16 fiber splitter and to be sent to all the PDs through single-mode fiber. The transfer function of the PDs will be measured using network analyzer. The output of the PDs will be fed to network analyzer via one RF-switch.
Work Done So Far: We routed the fibers on AP table. Fibers from RF PDS - namely MC REFL PD, AS55, REFL11, REFL33, REFL55, REFL165, have been connected to the 1x16 fiber splitter. All the cables are lying on the table now, so they are not blocking any beam.
We will soon upload the schematic diagram of the set up.
Missing Component: Digital Fiber Power Meter, Thorlab PM20C
Here I am attaching the first schematic diagram of the PD frequency response set-up, I will keep updating it with relevant informations with the progress of the work.
Description: Our objective is to set-up one simultaneous transfer-function measurement system for all the RF-PDs present in 40m lab. A diode laser will be used to illuminate the PDs. The diode laser output will be divided by 1x16 fiber splitter and will be sent to all the PDs through single-mode fiber. The transfer function of the PDs will be measured using network analyzer(Agilent 4395A). The output of the PDs will be fed to network analyzer via one RF-switch. The diode laser will be controlled by the controller ILX LDC 3744C. The scanning frequency signal will be fed to this controller from network analyzer through its external modulation port. The output of the controller will be splitted into two parts: one will go to laser diode and the other will be used as reference signal for network analyzer.
I think you have the splitter that splits the RF signal from the network analyzer in the wrong place.
Usually you split the signal immediately after the RF Out, so that half of the signal goes to the A-input of the Analyzer, and the other half goes to your controller (here, the laser diode controller). Then you would take the output of your controller and go straight to the actual laser diode, with no splitting in this path.
The fibers should be routed beneath the electrical cables.
They should be fixed on the table for strain relieving.
The slack of the fibers should be nicely rolled and put together at the splitter side.
These are expected to be done next time when the fiber team work around the table.
We also expect to have the table photo every time the work of the day is finished.
Here our device under test is the photodiode. So for the reference I wanted to retain the response of the laser diode controller. Otherwise I have to consider the transfer function of that LDC too. I may check both the options at the time of experiment.
Today we have routed the fibers from 1x16 fiber splitter to POX table for POX11 PD and POP55 PD. Also we labeled the fibers on AP table, they have been fixed on the table. The photo of the table after work is attached here. We will do it for POX table tomorrow.
No.... what I told was to put the roll next to the splitter, not on the table.
The table area is more precious than the rack space.
Koji> The slack of the fibers should be nicely rolled and put together at the splitter side.
No.... what I told was to put the roll next to the splitter, not on the table.
The table area is more precious than the rack space.
Ok, will do it on the coming week.
I was sad to see that there wasn't a photo of the POX situation after the fiber work was done on Thursday.
Also, I was out looking at something else, and noticed that the fibers aren't in a very good/safe place from the POX table over to your splitter. Getting to the POX table is certainly more tricky than the AP table, since the fiber splitter is right next to the AP table, but we should go back and try to make sure the fibers to the more distant tables are laid in a nice, safe way.
Is there a reason that we're not using the clear plastic tubing that Eric bought to put the fibers into? It seems like that would help a lot in keeping the fibers safe.
I took a few photos of the things that I'm sad about:
1. We should not be keeping fibers on the floor in an area where they can be stepped on. This will be fixed (I hope) as part of putting the extra coiled length over by the splitter.
2. Again, in an area where we semi-regularly walk, the fibers should not be a tripping hazard. Behind the table legs (rather than under the middle of the table) is safer, and will help tuck them out of the way.
3. It's not obvious when we're pumped down, but we remove the access connector (top right side of this photo), and need to walk in this area. I can pretty much guarantee that within 1 day of the next time we vent, these fibers will be stepped on, tripped over, and broken if they are not moved to a different location. I'm not yet sure what the best way to route these fibers is, but this is not it.
Riju, since Eric will be away next week, please let one of us "40m Regulars" know when you plan to come over (at least a few hours ahead of time), and we can give you a hand in protecting these fibers a little bit better. Thanks!
Today I have rerouted the fibers on AP table to remove the fiber rolls out of the AP table. I removed the fibers one by one from both ends - from the 1x16 splitter and from the AP table - keeping the fiber roll intact, and then connected it in reverse way, i.e. the fiber end which was on AP table now is connected to the splitter (since length of the outside the roll is shorter that side) and the fiber end connected to splitter is now rerouted on AP table.
We need to keep the fibers in such a fashion so that no sharp bending occurs anywhere, and also it does not get strained due to its weight, particularly near the 1x16 splitter. Jenne suggested to use a plastic box over the splitter rack to keep the fiber rolls for time-being. We discussed a lot how this can be done nicely; in future we may use array of hooks, Koji suggested to use cable hangers and to tie the rolls using more than one hanging point, Jenne suggested to use the bottom shelf of the rack or to use one plastic box with holes. We tried to make holes on the plastic box using drill, but it developed crack on the box. So ultimately I used the opened box only and put it over the rack.
The corresponding photographs are attached herewith.
Tomorrow we will reroute the fibers for POX table.
Today I have rerouted the fibers on POX table. The aim was to lay it overhead through the plastic pipe. A pipe ~50ft (~15.5m) long was taken for this purpose. I disconnected the two 25m long fibers for POP55 and POX11 PD (those had been already routed) from both of their ends - i.e. from the POX table and also from 1x16 splitter. Jenne and Koji suggested that we may have another two PDs ( POP22 and POP110) on POX table in future. So we used another 25m long fiber for these two (POP22/POP110). We could not use two fibers for these two since we have only four 25m long fibers and one of them we need for POY11 PD on POY table. Jenne and me put the three fibers inside the pipe using a copper tube. The tube then was put on the overhead rack, Manasa helped me to do it. The fiber ends were finally laid on the POX table at one end and connected to the 1x16 splitter at the other end.
The corresponding photos are attached herewith.
Today I routed fiber from 1x16 splitter to POY table. Manasa helped me doing that. The fiber(25m) was laid on overhead rack through plastic pipe of length ~76ft. We put the fiber inside the pipe using one copper tube, and then tied the plastic pipe on the overhead rack. Finally one end of the fiber was laid on POY table and the other end was connected to the 1x16 splitter. The photographs corresponding are attached. There is no picture of splitter end, cause it was dark that time.
[Eric, Riju, Annalisa]
Today we have cleared up the fiber spool near AP table. We have put the 1x16 fiber splitter and a box (we made two openings on it) for fiber spool on a different part of the rack. Also put a plastic tubing or the fibers coming out of AP table. Now the fibers coming out from AP table and also from POX table first enter the box through one opening and the end of the fibers come out of the other opening to get connected to to splitter. Photographs of the work are attached. I don't think enough fiber is there to make a similar loop for fiber coming from POY table.
The PD (pda255) at the AP table, close to the MC refl , which Steve mentioned to be not in use, has been removed from the table for testing.
The PD installed at MC trans to make ringdown measurements has been replaced with the above PDA255.
We discovered that the analog whitening filter of the REFL55_I board is not switching when we operate the button on the user interface. We checked with the Stanford analyzer that the transfer function always correspond to the whitening on.
This turned out to just be a loose connection of the ribbon cable from Contec board in the LSC IO chassis at the BIO break-out box. The DSUB connector at the break-out box was not strain relieved! I reseated the connector and strain relieved it and now everything is switching fine.
I wonder if we'll ever learn to strain relieve...
Since lately the alignment of the input beam to the interferometer has changed, I went checking the alignment of the beam on the photodiodea. They were all fine except for pd9, that is AS DD 199. Here the DC is totally null. The beam seems to go right on the diode but the scope on the PD's DC output shows no power. This is really strange and bad.
After inspecting PD9 with the viewer and the cards, the beam looks like it is aligned to the photodiode althought there is no signal at the DC output of the photodetector. So I checked the spectrum for PD9_i and Q (see attachments) and it seems that those channels are actually seeing the beam. I'm going to check the alignemtn again and see the efefct on the spectra to make sure that the beam is really hitting the PD.
I aligned PD9. Here are the spectra confirming that.
It turned out that the signal was too small with PDA10A to detect the 22 and 110 MHz RF sidebands.
The DC output coming out from it was about half mV or so (corresponding to few uW in laser power) when the PRCL was locked to the carrier.
This is because PDA10A is a silicon detector which is more sensitive to visible light than IR.
The reason we chose PDA10A was that it has relatively a large diode size of 1 mm in diameter.
However according to the data sheet the responsibility at 1064 nm is about 0.05 A/W which is sad.
I will replace it by PDA10CF, which is made from InGaAs and supposed to have 10 times bigger responsibility.
Though the diode size will be half mm in diameter, which may require another strong lens in front of it.
The POP22/110 RFPD has been replaced by PDA10CF. As a result the 22 and 110 MHz signals became detectable.
However the signal level maybe too low according to a quick look with an RF spectrum analyzer.
The level at 22 and 110 MHz were both approximately -70 dBm although these values were measured when the central part was freely swinging.
Perhaps we need to amplify the signals depending on the actual SNR.
Also I have updated the optical tables' wiki page :
Per Yehonathan's request, I removed one PDA10CF from a pickoff of REFL on the AS table (it was being used for the mode spectroscopy project). I placed a razor beam dump where the PD used to be, so that when the PRM is aligned, this pickoff is dumped. This is so that team ringdowns can use a fast PD.
The PDA255 that Koji repaired is still not alright. It seems to be saturating again. I've left it in the PD cabinet where it is marked 'PDA 255'. I've asked Steve to order a fast PD at 150MHz, PDA10A because we don't seem to have any at the 40m.
In a attempt to debug the values of transimpedance generated by the PDFR system, I did a manual measurement for REFL11 PD.
Pinc = 1.12 mW T_dc = 10000 V/A (datasheet)
Vdc = 7.68 V T_rf = 700 V/A (datasheet)
Calculated Responsivity = 0.68 A/W (Which matches perfectly with the datasheet value of 0.68 A/W)
Pinc = 0.87 mV T_dc = 66.2 V/A (schematic)
Vdc = 32.5 mV
Calculated Responsivity = 0.56 A/W
Network analyzer reading at 11 MHz : 0.42
Calculated RF Transimpedance = 460 V/A
40m Wiki : RF Transimpedance = 4 kV/A
I ran the same measurement using PDFR system and got the same results.
Attached: the automatic data and plot obtained.
Conclusion: The PDFR system and manual measurements agree with each other. However the values do not match with 40m Wiki. I have no clue about which measurement is correct or any mistakes I might be making in the calculations.
What is the coupling factor between the RF in and the RF mon of the demodulator?
I don't assume you have the same amount RF power at those two points unless you have an RF amplifier in the mon path.
I want to use the Fiber Coupled laser from the PDFR system to characterize the response of the fiber coupled PDs we use in the BeatMouth. The documentation is pretty good: for a first test, I did the following in this order:
Seems like stuff is working as expected. I don't know what the correct setpoint for the TEC is, but once that is figured out, the 1x16 splitter should give me 250 uW from each output for 4mW input. This is well below any damage threshold of the Menlo PDs. Then the plan is to modulate the intensity of the diode laser using the Agilent, and measure the optoelectronic response of the PD in the usual way. I don't know if we have a Fiber coupled Reference Photodiode we can use in the way we use the NF1611 in the Jenne laser setup. If not, the main systematic measurement error will come from the power measurement using a Fiber Power Meter.
The Transimpedance plots of PDFR now have a reference plot or baseline plot along with the current measurement, for easy comparision.
Current Work: Getting Matlab's vectfit3 to work simultaneously on the transimpedance readings and print the zeros and poles alongside the plots.
The PDFR system now has the capability to automatically run vectfit3.mat using a wrapper script named vectorfitzpk.m
This is done via a shell script being called from inside python that inturn runs the matlab script.
The PDFR system's interface and scripts have been updated to include quite a few more features.
On the interface side, there are buttons to open the previous plot for each PD and also a single button to run the scans on all PDs sequentially. The previous plot buttons actually open a softlink that is updated each time a measurement is taken.
Running a scan now pops up a terminal window to show messages that help understand whats going on.
In the background, the script now takes in the transfer function of the demodulator board in ZPK format and calibrates it out of each measurement. The parameters are given .dat files making it easier to replace the transfer function. (Remember my last elog which showed that the fitting of transfer functions were not really great and that I am going to use it anyway to get the script updated.) Also, the script now takes the delay in the RF cables and calibrates out that as well. So we no longer have the huge phase variations and the phase related to transimpedance are visible.
A test run was conducted today. Plots attached.
NOTE: The test can be conducted only on REFL 11,33,55,165 , AS55, and POX11.
POY11 has an optical fiber routed from this system, but there is no space to actually illuminate this PD. So it is currently not included in our system, even though there is a button for this.
POP22 has a fiber illuminating it, but its a unknown broadband PD. I do not know it's DC transimpedance or other values. Its just of matter of updating a few files that feed it's parameters into PDFR.
However, for the above PDs, the demodulator boards have been fit to a transfer function and the script is ready to go as soon as the above problems are fixed.
Conclusion: The plots look noisy. But, the transimpedance now resembles the one on 40-m wiki for all the PDs, both the shape and values.
There will be some errors that are induced because of improper demodulator TF fitting. This has to be taken care of eventually.
Work remaining: Create a canonical set of plots for each PD and set them as the baseline. These canonical plots will be plotted along with each measurement for easy comparison.
A well documented manual for the whole system clearly explaining where and how it takes all the parameters into account so that anybody can easy update just the essential information.
The PDFR system has been documented in the 40m wiki and all the relevant information about making changes and keeping it updated have been mentioned.
This pretty much wraps up my SURF 2014 project at the 40m lab.
AIM: Taking DC output readings with multimeter for each PD to create a database (required for transimpedance calculations), by taking off the table tops. Also, making sure each PD is illuminated properly.
What we did:
REFL11: Pinc = 0.91 mW VDC = 34.9 mV
REFL33: Pinc = 0.83 mW VDC = 33.2 mV
REFL55: Pinc = 1.08 mW VDC = 42.7 mV
REFL165: Pinc = 0.79 mW VDC = 115.3 mV
AS55: Pinc = 0.78 mW VDC = 31.3 mV
POX11: Pinc = 0.83 mW VDC = 34.7 mV
POP22**: Pinc = 1.08 mW VDC = 5.82 mV
POY11: Not illuminated; there was no optical fiber mount. Although, there was a fiber near it with a cap on the end. It also looks like there is no space to put in a new mount near the PD.
REF PD: Pinc = 1.19 mW VDC = 8.2 V (REF PD = New focus 1611)
**Note: The current POP 22 PD does not have 2 different outputs for DC and RF signals. I unplugged the RF cable from the output, took readings with the multimeter and then plugged back the RF cable.
I will calculate the responsivity for each PD and compare it to the expected values.