Tara got me the information about the adjustable collimator tube he ordered (http://www.thorlabs.us/thorproduct.cfm?partnumber=SM1L30C). I built a mount in Solidworks and added it to the assembly. I also contacted Thorlabs and am discussing how easy it will be to shorten the tube, since we don't need it nearly as long and the length gets in the way a bit. This should be doable.
I decided that we would go for a box where everything is screwed onto a baseplate, and a lid is screwed to the sides of the baseplate. The reason for this is that the base plate will be much easier to build on than building on the bottom of a box. The screws are on the side instead of the top because this will be easier to have machined, and the design is more compact leaving less room for noise when the lid is disturbed.
I'm currently looking at a few tasks that I will try and complete soon:
Attached is a picture of the current setup, built in Solidworks and the lid built separately. I'm not going to bother attaching the Solidworks files until things are more finalized.
Grating mount: I examined different ways to attach the grating to the grating mount. Our options are epoxy or some sort of actual mount the grating fits into. I finally decided that we should use epoxy for the following reasons:
Change of PZT: Our PZT choice relies on how much the PZT will need to be able to move. This changes the length of the cavity as well as the angle of the diffraction grating, and the screw on the PZT will be used to tune the angle. I calculated we will have a 400 nm change in wavelength per mm of the screw length changed, meaning we will only be making changes of less than a mm in the screw length. It made the choice of PZT from before seem a bit excessive.
Instead, I was thinking of having a very short micrometer screw (http://eksmaoptics.com/opto-mechanical-components/adjustment-screws-870/micrometer-screws-870-0040/) with a chip piezoactuator (http://www.physikinstrumente.com/en/products/prdetail.php?sortnr=100800). I'm not sure how to build the threading into Solidworks or if this will be possible to mount, though. Need to keep looking into this...
Shortening collimator tube: I have been corresponding with Thorlabs today about their collimator tube. It is made out of aluminum, and therefore we can probably saw off half of it and leave part of it threaded to mount in the collimator mount. Thorlabs also offers custom modifications, but this will likely take awhile and cost a lot more money.
All of the changes I discuss above were implemented into the Solidworks figures. I just need to figure out the PZT and the parts should all be ready to be machined. I will also try to update the Wiki page this weekend since I haven't for a long time...
Sorry, I've been out for awhile since I had an extremely bad reaction to some medication. Only just starting to recover but I'll work during the weekend to make up for it.
Today, I spent awhile looking at possible current drivers online. Dmass said that the Thorlabs one is not being used for any frequency sensitive measurements. After looking online, nothing seems to beat the Thorlabs driver in terms of noise level (<1 uA RMS), so maybe we will need to look into building our own current driver or buying the one online based on Libbrecht and Hall. That one is quoted at $4000-5000.
I also spent awhile trying to figure out a new box design. I think we will want to purchase an AR coated window for the output beam, much like Birmingham. 1" diameter should be sufficient based on the size of the diode/collimator lens. Thorlabs and Newport have comparable products with regards to this (http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1117). It is also important that the box is airtight so no dust can accumulate on the grating or diode. I was thinking of having a 2 part box, with the metal folded, but I'm not sure how easily the machine shop can do this (fold thicker metal sheets, create rounded edges). Waiting to hear back from Tara about this.
Tara and I received the parts we ordered from Thorlabs. I will be working in the ATF lab on the corner of one of the optical tables. Tara showed me around a little; we will likely work there more tomorrow.
I met with Tara and discussed the final mechanical design. In particular,
The changes have been implemented in Solidworks. The finalized pieces I want to have machined are on the 40m SVN in the ECDL folder. I've also attached a couple of pictures for a quick overview.
Since we are trying to find a good enough current driver to use, Tara thinks I can start by configuring the TEC on a piece of copper to make sure it works. I will try to do this tomorrow now that the design is ready to be sent into the machine shop. I will also figure out a good time to go over to the machine shop and discuss the design with them.
Tara heard back from the machine shop, and they can do 1/4-80 threading. I finalized the design this morning. I was sure to check whether the frequency could be sufficiently tuned, and whether the screw on the grating mount would fit into the box. After making some changes, I printed out the designs and went to the machine shop with Tara to talk about our design. It should be ready sometime next week between Wednesday and Friday. Finalized designs are on the SVN.
Frank (from the Birmingham group) emailed us back about the ECDL. He said they used the $1000 current driver from Thorlabs (http://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=10), although he recommended the $4800 Vescent current driver (http://www.vescent.com/products/electronics/d2-105-laser-controller/). He also said that there was not a good way of predicting the noise budget beyond knowing what contributes to noise at different frequency ranges:
Frank also said that he would be leaving the Birmingham in September and has discontinued the ECDL project, so they haven't gotten past making a working prototype. He was otherwise very willing to help.
I'm currently talking to Thorlabs to see if they can give us the current noise density instead of an RMS noise on their current driver. It seems that if we use the Thorlabs 100 mA driver (instead of the Thorlabs 200 mA driver we had been planning on), the noise is reduced a lot. At 100 mA, we should get an output power of ~75 mW from the Thorlabs diode and ~50 mW from the QPhotonics diode. This actually is probably sufficient for what we need, so the lower current input should not be a huge problem. From a range of 10 Hz to 10 MHz we have the following values:
Vescent current driver does much better at lower frequency ranges, and has RMS noise of 0.05 uA between 10 Hz and 100 kHz, but is comparable over a larger range with higher frequencies. While the RMS values are promising, we aren't sure how the noise density compares over the entire frequency range... I'm hoping to hear back from Thorlabs soon about this. It seems like the Thorlabs driver is an actual possibility though.
Tomorrow Tara and I are going to get started in the lab. Tara will show me around, and I'll try to get the TEC working.
Rana wants for me to build my own low noise driver, since anything on the market isn't sufficient or is terribly expensive. Dmass does not have an extra driver in his lab, and Eric Gustafson doesn't have a spare since his SURF student is using it. Tara found a spare driver in the 40m. It is the LDX-3412 from Newport and operates up to 200 mA. Tomorrow I am going to start trying to make it work with our diode. The two diodes that we ordered operate at 200 mA (QPhotonics) and 280 mA (Thorlabs). However, both vendors provide a plot of power output vs. injected current, and it seems like at 100 mA, they will both still provide 50-100 mW of power.
In the meantime, I will work on putting together the current driver from the Erickson paper (attached). This is an improvement on the Libbrecht and Hall design by over an order of magnitude in noise spectral density. I spent the afternoon trying to compile a list of the parts we will need. The paper is kind of complicated so I'm still trying to understand some of the parts, but I have a pretty comprehensive list of the parts we will want to get. We will likely be able to find at least some parts in the 40m lab. I may go scrounging around.
I also attended the talk that Rana gave today about basic electronics and noise calculations. I will try and get liso on my computer before next week's talk, since it would be useful to have.
(KOJI / ATTACHMENT REMOVED: DO NOT PUT COPYRIGHTED PAPERS ON THE PUBLICLY ACCESSIBLE WEB PAGE)
Current driver: the plan for the summer is to use the current driver that Tara found, the LDX-3412. Dmass has a low noise current driver that he has developed that eventually (after the summer, since it is unfeasible to finish over the summer) will be implemented in the ECDL. I believe he will give me and Tara the schematic for it at some point and we may build it later.
I configured the TEC. We have 2 Peltier elements, the Thorlabs TEC3-2.5 and the TEC1.4-6. I used the TEC3-2.5 in the TEC, since its operating current is lower and works better with the controller we bought. I was stupid and burned myself pretty badly on the Peltier element because I had the wrong settings, and will remember that it heats up easily (be CAREFUL before touching). The TEC is working! It can adjust temperature, although the readings it gives is in resistance (which can be converted to temperature using the attached Excel file). I'm going to play with it more tomorrow to see if I can tweak the settings and understand what every button does.
Today I also figured out how to make the laser diode wire to the 9 pin D-sub connector from the LDX-3412 current source that we found. I built the entire thing in the electrical shop, but haven't put in the reverse bias power supply. I will try to do this tomorrow to see if I can get the laser working. Since the TEC is working, I may try and tune the wavelength of the laser output and see what the noise looks like.
I figured out how to calibrate the potentiometers for the TEC. The three potentiometers (labeled P, I, and D) control how quickly the TEC can reach the desired temperature, and how much the temperature oscillates. I have all on their minimum setting, because increasing any of the potentiometers causes the actual temperature to overshoot the desired temperature many times before reaching an equilibrium. It is a very finicky system, and they don't have that much adjustability so it took me a long time to get right. I can adjust the temperature (resistance) to be within a few ohms of the desired temperature right now, which translates to within 0.01 degrees Celsius. This settles in 3-4 minutes. The reason my accuracy isn't as good as it could be is I have a makeshift mount I'm using to simulate the ECDL. When it is attached to the aluminum box, the temperature variations should be even smaller. Everything is in place for TEC to be implemented quickly into the actual ECDL. I will likely need to calibrate the potentiometers again with the ECDL.
I need a banana cable connector to add to the circuit so I can see if I can get the current driver up and running with the laser diode. I will do this Monday.
This afternoon I worked on writing up a draft of my second progress report and abstract so that I have time to edit it before it's due next Friday. The draft right now is on the SVN in the ECDL folder. I still need a few pictures, and I left some space for stuff I plan to do next week.
I worked on my SURF progress report and abstract (due Friday, August 2). The most revised version is on the SVN.
Tara and I went down to the lab to set up and test the current driver. In order to do this, we are using a function generator as our power supply for the reverse bias on the laser diode at a frequency of 10e-6 sec (for now, sufficient for temporary testing). We were able to see an output beam, so the current driver works! We noticed that the beam diverges VERY quickly. We will need a collimating lens with a very small focal length as a result. We played around with some lenses sitting around the lab, but they had relatively large focal lengths (we are looking at ~2-4mm this is commonly used in the literature...).
Tara found some problems with my soldering so I fixed it. I learned how to use heat shrink to make sure the solder joints don't touch, and I will be careful to do this in the future. I may go and put heat shrink on the TEC solders in order to eliminate the possibility of a short circuit.
Tomorrow I will look at different lenses in the setup and see if I can find a small focal length lens to use.
I fixed the solder joints on the TEC by using heat shrink to make sure nothing short circuits. I figured it would be cautious to go ahead and do to avoid future problems.
Tara worked with me for a bit trying to improve our temporary setup. We trimmed the legs on the laser diode so it will fit snugly into the socket. Then, we used a mirror mount adapter to fix the laser diode securely so there is less movement from this part.
We found a collimator and used it in combination with another lens (planar/convex). We were able to line this up so that the beam spot is comparable to a handheld laser output, but this has a large cavity. Instead, we need to find (or order?) a very short focal length lens (something on the order of several mm). This is because the beam diverges way too fast to use any of the lenses we have been trying in the lab. From Thorlabs and Newport, it seems like it's difficult to find lenses with focal length <10mm.
I also used a visible handheld laser pointer to determine the orientation of the blazing on the grating. There is an arrow marked, which is standard and points in the direction from the normal of the grating surface to the next trough in the blazing. This means that there are 2 possible orientations the grating will operate at - it is better to have the blazing point away from the laser diode. See: http://gratings.newport.com/library/technotes/technote7.asp This is good to know so that we won't have to reattach the grating to the diode mount if we do it wrong the first time.
The machined parts should be done tomorrow so we can begin some construction. Today's setup is pictured, with the output beam on the IR card.
Today I worked on updating my progress report and abstract. Posted to the SVN.
Our machined parts were finished by the machine shop. I picked them up, and Tara and I washed them in a sonicator for an hour to get the oil and metal shavings off. I tried assembling things to see how things look. It seems like the laser diode mount will have enough adjustability with the diode that we will not need to have vertical adjustment ability on the grating mount. We will need to make modifications on the plate with the D-sub and BNC holes because we will need 2 D-sub connectors, and there needs to be a better way to mount the male-to-solder connectors on the plate so they don't move.
I went to Rana's electronics talk. I'm trying to get LISO on my own computer but encountering some problems with Linux.
Tara found a 1/4-80 screw from a mirror mount to put into the grating mount. It was long enough that we'll have adjustability. We may need to get springs to put in the grating mount slit to offset the force from the screw.
Tara and I took apart a 5 mm focal length lens from a fiber optic and added it to our temporary setup from yesterday to test if a shorter focal length lens helps with collimating the beam. It works very well - we can get the beam to be essentially parallel at up to at least 50 cm with the right adjustments.
I put together a shopping list tonight of things we need to get checking Thorlabs and Newport:
Today I tried to set up the TEC on the actual assembly. When doing so, Tara pointed out that I needed to have a separate temperature sensor to monitor the TEC, and to use to calibrate the PID gain on the TEC controller.
I built a simple temperature sensor with a 10k thermistor. The temperature can be determined by measuring Vout and determining RT. Once RT is determined, this can be converted into a temperature using the information on the data sheet for the 10k thermistor. The schematic is attached. I chose the value for R0 based on what would maximize the difference in Vout for a 1 degree C fluctuation about room temperature (25 C) which is what will be used to tune the PID gain. I chose Vin based on what would make the signal have fluctuations of about 500 mV, which is what is needed to be readable on an oscilloscope. Once I built this circuit, I tested it. It is sensitive to temperature changes, since the output voltage changed when I covered the thermistor with my hand.
Tonight I am going to incorporate changes Tara suggested for my progress report. The updated version will be put on the SVN. Tomorrow I will try to the temperature sensor I built today to calibrate the PID gain on the TEC controller.
Today I calibrated the PID gain on the TEC. In order to do this, I used a silicone heat sink compound to help the thermal conductivity between the Peltier element/thermistors and the TEC. Then, I held things together using aluminum tape.
I calibrated the TEC so it reaches the correct resistance after only overshooting the value once. It is usually able to reach the correct temperature within about 30 seconds. I had the temperature sensor I built yesterday hooked up to an oscilloscope so that I can monitor the fluctuations in voltage across the thermistor (directly related to resistance). However, my flash drive doesn't work and I didn't have a spare on me today so I will try and record the oscilloscope output either this weekend or on Monday morning. This will be used to estimate the transfer function of the TEC controller.
Important: there is a directionality to the TEC element. There is a hot side and a cold side. The cold side is attached to the laser diode mount, and the hot side is attached to a piece of aluminum we found around the lab to act as a temporary heat sink. Because of this we need to rework some of the design to thermally isolate the diode mount from the box, and let the box act as a heat sink. My proposed design is attached (I made a quick sketch of it in Solidworks). I'm still thinking about the best way to incorporate the Peltier element.
Tara will order the collimator lens, window, and PZT this weekend. Still trying to figure out if it's possible to build a collimator mount that will be sufficient to serve our purposes.
I brought in a different USB drive to get data off of the oscilloscope. It took awhile to figure out how to capture the data with the best settings. I have a sample graph of the heating and cooling of the diode mount attached (converted to temperature using datasheet for 10k thermistor). Notice that I took data over about 4 degrees, so that it was possible to see the change in voltage as the temperature changed. Even then, it would be nice to have more resolution on this data. I cannot make the voltage increments smaller than 500 mV because the offset of the oscilloscope isn't enough to still see the data (I tried). I will talk to Tara tomorrow about if I can get better data on this to analyze, since this data has poor resolution.
Tara asked me to try to calculate the free running noise of the laser diode to have an estimate for when we actually collect this data. We will be using a Michelson interferometer with different arm lengths. I used Erica's past elog entry as a starting point (1241) and wrote a bit more explanation into my own calculations so it will be clear to me in the future and to make sure I understood everything. However, I'm unsure of how to incorporate the noise levels after calculating the power received by the photodiode, and I need to talk to Tara about how to do this tomorrow if he's around. The calculations that I have done are attached.
I calculated a way to convert our spectrum measurement of voltage from the photodiode to the frequency noise of the laser in the Michelson interferometer setup. I still need to check this calculation to make sure it works, and determine the ideal differential arm length to use tomorrow.
Today I also took a measurement of the relationship between power and voltage of the photodiode at 20dB gain. The result for that is also included in the attached file. I will clean all of the calculations up tomorrow; I suspect I've made a mistake or 2.
I fixed my calculations from last time and wrote it up in LaTex. It seems that we can use a differential arm length of somewhere around 10cm and it should work well for our purposes.
Tara: I removed the pdf file, as I have warned you about this for several times.
Chloe: I put the PDF on the SVN. I won't make this mistake again.
Today I designed a better circuit to measure the TEC's response with the oscilloscope. It is called a bridge circuit, and allows for the output voltage to be centered around 0 instead. This type of circuit is often used for different sensors, and seems to fit our purposes well here. The schematic is attached here.
After I built this circuit (modified the circuit I was previously using), I tested it with the TEC to see how the PID gain calibration looked. This took awhile to get a signal, because it seems like the oscilloscope I was using had some problems. I took data of heating and cooling shown below (didn't bother converting to temperature since we're mostly interested in how the temperature or voltage settles right now).
A lot of the data I tried to take today had the same sort of oscillations as for the cooling data shown above (about 0.04 Hz). However, I didn't see such oscillations when I hooked the circuit up to a multimeter and monitored the voltage changes over time. In fact, the voltmeter suggested that the voltage stabilized much more quickly. I'm going to look at this again tomorrow to see if I can figure out the cause of these oscillations, and perhaps tune the PID gain on the TEC better now that I can see how the temperature settles much more easily and quantitatively.
Today, I also finalized the Solidworks drawings for the insulator that will be used to thermally isolate the laser diode from the rest of the setup, as well as the heat sink that will be in contact with the Peltier element. These files are on the SVN, and I will try to go to the machine shop with these soon. I should have done this earlier.
I will be presenting my project at the end of August, so Tara wants me to put together a talk so we can rehearse next week. I am going to start doing this in my free time.
I spent awhile reading about PID controllers in order to understand how to tune the TEC. P represents proportional gains, and deals with the present error from the set value. I represents integral gains, and deals with past errors. D represents derivative values, and uses the current data to predict future errors. They each affect how the TEC overshoots/oscillates about the correct temperature in different ways. I figured out that the oscillations that I saw yesterday in the heating and cooling data were due to improper tuning of the PID gain. I decreased the integral gain and it seemed to fix the problem.
I also discovered that the oscilloscope was on the wrong setting, with 10x attenuation. I noticed this when converting the data from output voltage to temperature. I changed the settings to 1x attenuation and took data for heating and cooling, shown below. There only seems to be one slight overshoot when changing the temperature by about 1 degree, which is entirely reasonable. The correct temperature settles after about 1 minute.
While these measurements were useful in tuning the PID gain so that the temperature settles quickly, there was a discrepancy in the measured resistance across the thermistor and the resistance calculated from the measurement of Vout. Using the TEC controller, I brought the resistance of the feedback thermistor to 10k, but this resulted in a Vout that predicted a thermistor resistance of 9.91k (0.2 degrees K difference). In order to zero Vout, I had to bring the thermistor resistance down to 9.892k. I'm trying to think of a way to calibrate this difference, but I'm not sure which thermistor is reading more accurately right now. I'm going to read more about using thermistors as temperature sensors to see if there is anything I can try to do for this.
I'm also still trying to think if there's a way to adjust the P, I, and D controls so that I can actually go back to previous values. The controls are unlabeled on the TEC controller we have, so they cannot be accurately returned to specific settings. It seems well calibrated for the moment, though.
Tara would like me to present at the SURF Seminar Day in August (either on the LIGO field trip to the Livingston Observatory or at Caltech), so I spent yesterday and today putting together my presentation and trying to organize the work I have done/plan out what to say. The entire presentation will have to be focused on the noise calculations and design, since we are still waiting on parts to arrive (namely, the collimating lens so we can focus the beam to make a free running noise measurement). The presentation for right now is on the SVN: https://nodus.ligo.caltech.edu:30889/svn/trunk/ecdl/documents
Made some modifications to the Solidworks design. All of these have been changed on the SVN.
Tomorrow morning I will go to the machine shop to get the base plate and left plate modified, and get them to machine a heat sink and plastic insulator.
Today I got the newly machined parts. I put together the TEC element and stuff again and will calibrate the next time I get a chance.
Erica and I practiced our presentations in front of Tara. I got a lot of feedback and I'm going to edit my presentation in my free time outside of lab. It was also useful to see someone else's work to get an idea of how to present.
I'm working on putting together a Michelson interferometer to measure the laser diode free running noise. I don't have the actual collimating lens, so I'm using a f=5mm lens from a fiber optic. I have mirrors and I borrowed a beam splitter from the GYRO experiment. Picture below. I'm working on getting the beams to combine by adjusting the mirrors. Will continue doing this tomorrow.
Today Tara and I worked on getting a noise measurement for the bare laser diode using a Michelson interferometer with different arm lengths. The setup is attached. However, at a differential arm length of 20 cm we were unable to see interference because it was too difficult to focus the beams. Tara suggested I use a symmetric Michelson interferometer to see if I can get interference, since the noise levels might be too high for such a large arm length. I then tried much smaller differential arm lengths and I was able to get interference at 1 cm and 5 cm.
I took background measurements of the noise from the SR785 (about 20 nV/rtHz) and from the blocked photodiode (electrical noise, about 50 nV/rtHz). Since these were both small, we can be confident that the measurements we took are mostly the noise from the frequency of the laser diode.
The results from the 1 cm and 5 cm measurements are attached. We seem to have noise levels close to what we predicted (1 MHz/rtHz), which seems odd since there will be extra noise from mechanical components, temperature fluctuations, and a worse current driver than we planned to use. In addition, this doesn't explain why we weren't able to get interference at a differential arm length of 20 cm. The 5 cm measurements have even lower noise levels for some reason. I'm not sure if I'm doing something wrong with factoring in the gain, so I'm going to check my math. Gain still confuses me a little since there's a different gain on each machine I used. Overall, the measurements seem suspiciously low noise.
I'm going to check these calculations again this weekend to make sure I didn't mess up. I will also revise my presentation so that I will be ready to present on the LLO SURF field trip.
Newest version on the SVN with the latest data and Tara's commentary from my practice presentation. I'll probably end up working on this while in Louisiana if I hear back from Tara about whether I did something wrong with the noise measurements.
Today I tried to calibrate the PID gain for the TEC controller. I noticed some connections needed repairing to I resoldered them, and checked every single connection.
However, the TEC controller still couldn't turn the Peltier element on, citing a "OPEN" problem (I believe according to the manual this means that something about the TEC connections are wrong). I checked these several times with my past notes and the instruction manual, but could not fix the problem. Then I tried cleaning the silicone thermal paste off of the Peltier element and was able to briefly make the Peltier element turn on. As soon as I tried reinstalling this in the ECDL setup, it stopped working. I was able to get the element working again briefly, but it was never stable (would stop working after a minute). I believe that I can use isopropyl alcohol without damaging any parts, but I want to do more reading online before I try this so that I am sure. It seems that trying to wipe the silicone paste out is insufficient, as I spent awhile trying this to recreate my results.
Today Tara showed me where to find isopropanol in order to clean the Peltier element. After cleaning it carefully, the TEC controller worked fine again! I am going to avoid using the silicone thermal paste for now in order to avoid this same problem, but if it becomes necessary I will add small amounts very carefully. I'm not sure how safe it is to clean the Peltier element often. The thermistor is being held onto the diode mount with aluminum tape.
I worked on tuning the PID gain on the TEC controller. It seems a lot less stable than before, having a hard time settling on one temperature. Perhaps this is because I am not using the silicone thermal paste. I want to continue tweaking these settings, although I have them at something reasonably workable right now. It takes a minute or two for temperatures to settle, but they seem stable once a temperature has been reached.
I cleared off a shelf in the ATF lab to keep my things. The collimating lens and lens adapter arrived, and Tara and I had to search for awhile to figure out where he put it (since they arrived while I was away). I put the lens into the lens adapter, and put this into the lens mount. Immediately, I noticed 2 problems which need to be fixed immediately:
Tara mentioned that the TEC may not work as well without some of the silicone thermal paste so I added some and returned the PID gains. Sure enough, this helped the temperature stabilize (and now I know if it stops working to clean out the Peltier element with isopropanol).
I emailed DMass about finding a PCB for building the low noise current controller. (I was supposed to do this last week but it slipped my mind)
I moved the laser diode and socket to the actual laser diode mount (from the Michelson setup used in August). Since the laser diode extrudes, we do not have the problem I mentioned last time with the base plate since the hole for mounting the collimating lens is now close enough that we should have enough adjustability to focus the beam. I searched the ATF lab and found a piece of metal about 3mm high, which will fix the difference in height of the diode mount and the collimating lens mount. However, this piece needs to be trimmed down, which I will try to discuss with Tara. Not sure if we have those capabilities here or if I need to take it to the machine shop?
I wasn't able to find Tara today but I need to talk to him about:
Finally, I'm still working on editing my SURF paper. I'm new to LaTex so it's taking me awhile to perform the edits Rana suggested.
Today, I met with Tara and discussed the delay line, which will be used to tune the wavelength of the bare laser diode and measure the free running noise of the ECDL. I will write up notes of what we talked about and how the delay line will work and post these soon, along with a list of items I will need.
Collecting materials for delay line: I will be using an RF photodiode and the series of lenses from the Gyro setup, which Evan is not using right now. I'm in the process of disassembling and reassembling this setup on the table that I'm using. Evan said the mode matching was already messed up, so I will be working on focusing the beam. I will be using NPRO light from the CTN lab via a fiber cable.
Tara helped me modify the piece I found last time, which goes under the collimating lens mount to fix the optical height. I learned how to tap a hole in the metal. I moved the ECDL setup and got the current driver back up and running, and was able to focus the beam using the collimating lens we purchased. The setup so far is attached.
Tara and I decided on a modification to make to the grating mount which will allow for us to make vertical tilt adjustments (we will have 2 holes with adjustment screws, not one). I am going to draw this in Solidworks so that I can get it machined tomorrow at the Caltech machine shop.
I got the modified grating design into the Caltech machine shop; this part should be done by tomorrow. We decided to use 2 vertically placed 1/4-80 holes which will have adjustment screws. This will allow for tilt adjustment.
I found a mixer and a splitter in the CTN lab plus the appropriate adapters to use. I'm still working out how the cable length difference will affect the sensitivity of our measurement.
I have the PD removed from the Gyro table along with the lenses that were used to focus the beam to go into the PD. This was more difficult than I expected to remove these pieces since I'm short and didn't want to disturb the other setup. We are still need several things:
I'll go pick up the modified grating mount from the Caltech machine shop tomorrow so that I can wash it tomorrow afternoon (I don't have much time tomorrow) and do more on Wednesday.
Note: last week I picked up the modified diffraction grating mount. I forgot to bring it in today but I'll put it back in the ATF lab on Thursday.
I've spent the last week reading a few papers Tara sent me about mode matching/old elog entries by various people. I couldn't find Tara around the lab today, so I'll try and talk to him this week to figure out exactly what I'm doing. I'm still a bit confused about how to do the setup, although I've been starting to sketch what I'm planning on doing. I'm also messing around with a few mode matching programs to help me plan my setup.
I sent up the power supply for the PD and confirmed it works. I'm going to try to talk to Tara tomorrow or Friday so I know what I need to do in the next week. My midterms are starting so I may have a hard time being around the lab much until afterwards.
Edit (awade, Sun Mar 19 21:42:11 2017) -- Fixed linking to plots. Under inserted figure url need to add ?thumb=1 to end to make display and clickable.
Edit(awade, Sun Mar 19 22:01:20 2017) -- Linking didn't work. To make pictures clickable need to wrap images within a link in the html source, that is <a href="dirfilebucket/filename.pdf">Image markup here </a>. Seem to remember this used to be automatic, there is probably some clicking gymnastics with the toolbar that gets it done, but easier to just edit source.
Andrew and I have been investigating the South Path PDH error signal ringing the past few days. The South Path becomes unstable if we try to raise the common or fast gain too much, but not so unstable that it unlocks the cavity. There are a couple of different problems which have manifested themselves, but the biggest one is the overall noisiness of the South Path error signal, shown here in red in comparison with the North Path error signal:
We took also measurements of the North and South OLGTFs:
Finally we took a measurement of the North and South Crossover TFs:
A couple of weird things:
1) The noise level of the south path error signal
2) The 180 degree phase shift in the crossovers of both paths.
3) The flattening of the South OLG at low frequency vs the North OLG high suppression
I guess our goal is to figure out why these things are different between the paths, and what could be causing the excessive high noise spectra in the South Path.
The DC Power Supply in the EE shop had a broken negative banana plug terminal, so I replaced it.
Next is to take a transfer function of the EOM driver with -/+18 Volts.
I tried to calibrate our ASD spectrum into units Hz/rtHz.
To do this, I retook our demodulated beatnote spectrum in Vrms/rtHz from ~12Hz to ~12kHz , retook an open loop gain measurement of the PLL, and tested the actuation of the Marconi in kHz/Vpeak.
I'm pretty sure I did it wrong since our ASD reports a noise level of 1e-3 Hz/rtHz at 100 Hz.
The way I calibrated was by the following:
Hz/rtHz ASD = Vrms/rtHz ASD * PLL CLG * Hz/Vrms Actuation
where Vrms/rtHz is the original spectrum (plot 3 below), PLL CLG is the closed loop gain of the PLL equal to 1/(1+ OLG), and Hz/Vrms is read straight off the Marconi in 400 kHz/Vpeak and multiplied by 1e3/rt(2) to get 2.8e5 Hz/Vrms.
The PLL OLG is plot 2 below. The Agilent is unable to measure lower than 10 Hz. I feel like the solution is simple here... just make an analytic single pole and delay TF and extend it down to DC. In reality this control system is a bit weird since we are actuating on frequency but detecting phase, so we get a 1/f fall off which extends I don't know how far down to DC. Don't trust anything below 10 Hz.
Next step is to diagnose our North and South cavity FSS. Both are susceptible to "ringing", or high amplitude modulations at 10 to 20 kHz. "Ringing" is mitigated by turning off and on the Boost and dumping the integrator on the FSS boxes, but the ringing always comes back after a while. We suspect ringing is a result of the PZT railing from poor control loop design. Turning down the Common and Fast gains reduces the occurances of ringing, but eventually we want to turn up the gains as much as possible to get a high bandwidth and a lot of laser frequency noise suppression at low frequency. One theory Andrew has is one of our power supplies may not actually be supplying any current (see elog 1850).
I fixed the beatnote calibration script. I thought about it more, and figured out the math was wrong. Now I have implemented
Hz/rtHz ASD = Vrms/rtHz ASD * Hz/Vrms Actuation / (PLL OLG / 1 + PLL OLG)
This gives more sensible results, as seen in the plot. We'll take better measurements shortly.
The script is in
This was great... thanks a lot "Andrew".
These directions work ONLY if you register your ssh key with your github.com account and not your git.ligo.org account. You'll have to generate TWO ssh keys on a single compypyie, and put one key on your github.com account and another on your git.ligo.org account.
I have done this for my git accounts on acromag1 and 2, so now we can git clone via ssh from both git.ligo.org and github.com. V convenient.
I took 5 spectra, 4 with the Agilent at high freq from 1kHz to 5 MHz and 1 with the SR785 from 100 mHz to ~10kHz, stitched them together, and calibrated them.
The original spectra were all in Vrms/rtHz, and the two spectrum analyzers were not in perfect agreement when it comes to calibration. Their levels were about a factor of 2 off from one another, see the last plot... the SR785 is the pink line that reaches low frequency. I put them together anyways.
The Marconi I tuned down to a relatively low (for us) 3 kHz/Vpeak FM demod. This was a mistake, the temp fluctuations are still off the charts and I had to babysit the PLL all dang night.
I took a PLL OLG measurement by doing the CLG = OUT2/EXC measurement, opening the loop, doing it again to get the excitation gain (should be the same as the preamp gain which was x10), then combining it to get the OLG between 100 Hz and 10 MHz. I need to fix our PLL OLG, whatever I'm measuring this is not right b/c it flattens out at 10 kHz. Also I'm still using the physical measurement to calibrate the spectrum instead of some 1/f fit. This limits our trusted spectrum calibration to between 100 Hz and 10 MHz... at very low frequencies the gain of the PLL should be higher.
When I talk about PLL OUT2/EXC, I mean that I put in EXC into the B input port on the SR560 preamp, and read OUT2 from the 50Ω output. My OUT2/EXC measurement confirms the leveling out at 10 kHz. I am probably saturating something.
Despite these glaring issues I calibrated the spectrum anyways, recalling the 3 kHz/Vpeak marconi settings, and got plot one. I'm posting this because this is not a completely crazy spectrum, even if the PLL OLG is weird. I'll iron this out. We need a good Start of September Spectrum so that when we get to the End of September and we're done venting and putting in temp controls and PMCs we gotta know that it wasn't for nothing.
For some reason the new workstation we have in the lab was not communication with our acromag cards, leading to hours of confusion about why pressing buttons did nothing. We have restarted the acromag communications and everything works now, but we aren't sure what went wrong in the first place. We proceed characterizing the FSS box with this spectre haunting us.
The noise budget posted in the previous noisebudget elog has been described by experts as "bunk". I am here to right the ship with a more correct noise budget. I have committed the corrected noisebudget code in the CTN_noisebudget git under /CTN_noisebudget/noisebudget/noisebudget.ipynb.
In particular, the pink curve in the new noise budget is the real COATING THERMO-OPTIC NOISE. It is far lower than before. This is why the optimized coating layers were created in the first place: to strategically cancel coating thermoelastic and coating thermorefractive noise. Recall that thermo-optic = thermoelastic + thermorefractive, and they can cancel if their phases are carefully controlled.
With lowered COATING THERMO-OPTIC NOISE, we can now think about measuring COATING BROWNIAN NOISE for the AlGaAs coatings.
I have included a new trace: the SEPT 2017 BEAT MEAS. These are just some preliminary PLL noise limited spectra to keep you excited for great low noise measurements to come.
I spent a bunch of time making an interactive noise budget bokeh plot. HERE IT IS.
Advantages: You can scroll over and see exact numbers at whatever frequency you want. You can pan and zoom. You can click the legend to remove lines you don't want to see. It looks nice. We can host these on our 40m server soon (I'm hosting this on the ldas cluster, so you have to login).
Disadvantages: File is huge (~20 Mb) and takes a long time to load. Interactivity is choppy. Panning causes lines to disappear. Tick labels are cancerous. The dark background is good only if I can figure out how to make the entire background dark, this means making custom CSS files and I'm no html expert.
This is an overview of the old noise budget made by Evan and Tara. The plot is the result of noisebudgetQWL.ipynb written by Evan. QWL stands for quarter wavelength, referring to the coating layers' thickness (see Fig 2 of Evan's paper).
Each curve is given an brief statement about it's origin. Here is a link to Tara's paper on the PSL Lab setup. Here is a link to Evan's paper. These papers have convenient tables with parameter values for the setup and reference cavities.
The x axis is frequency in Hz, the y axis is the ASD in Hz / rtHz, aka frequency noise. Many of the thermal noises are reported as length noise. To convert from length to frequency noise, use Delta f / Delta L = c / (L * lambda).
COATINGS THERMAL NOISE BUDGET
TOTAL EXPECTED (blue): Sum of all expected noises. This does not equal the actual measurement in red, meaning not all sources of noise are accounted for in this plot. One suspicious missing noise source is scattering. Not much has been done to mitigate scattering in the PSL Lab setup.
MEASUREMENT (red): Actual beatnote measurement measured using a phase locked loop with the cavities' transmission radio-frequency photodetector (See Fig 2). The two lasers are locked to their respective cavities to reduce the free-running laser noise via PDH control loop gain. By suppressing laser noise, we can reveal the residual cavity length noise, hopefully dominated by broadband thermal noise.
COATING BROWNIAN (green): Theory curve of the estimated coating brownian noise. Brownian noise magnitude is governed by a material's mechanical loss. "Mechanical loss" refers to rate at which kinetic energy in a material is "lost" to thermal energy. Unclear why Equation 8 in Tara's paper and Equation 3 in Evan's paper are different, I think it has to do with assumptions about the coating and substrate Young's modulus and Poisson ratio being the same. In the noisebudgetQWL.ipynb, Tara's coating brownian noise equation is used.
COATING THERMO-OPTIC (pink): Theory curve of the estimated thermo-optic noise. Thermo-optic noise comes from temperature fluctuations in a material causing cavity length changes. This seems to be the key curve to all Tara and Evan's work. The idea here, originated by Evans et al., seems to be that thermoelastic noise and thermorefractive noise can cancel one another in thin enough coatings. Given by Equation 9 in Tara's paper and Equation 4 in Evan's.
SUBSTRATE BROWNIAN (yellow): Theory curve of the estimated substrate brownian noise. Like the coating brownian, but refers to noise originating from the fused silica making up most of the mirror. Equation 5 in Tara's paper.
SUBSTRATE THERMOELASTIC (teal): Theory curve of the estimated substrate thermoelastic noise. Thermoelastic noise refers to how temperature fluctuations cause a material to modulate its length. Governed by the coefficient of thermoelasticity alpha = 1/L(dL/dT) Equation 6 in Tara's paper.
Incidentally, I will mention THERMOREFRACTIVE noise here since there is no curve dedicated directly to it, but it is important to thermo-optic noise. Thermorefractive noise comes from temperature fluctuations changing the refractive index n of a material light is passing through. Governed by the coefficient of thermorefractivity beta = dn/dT.
POUND DREVER HALL SHOT NOISE (orange): Theory curve of shot noise. Shot noise refers to the Poisson statistics of fluctuations in the number of photons incident on a photodetector. This noise PSD is flat in frequency, but falls as 1/f^2 in power. To convert from the power PSD to frequency PSD, multiply the power PSD by (1 + f^2/fc^2)/(2 P0 Gamma/fc)^2 where fc is the cavity pole, Gamma is the modulation depth, and P0 is the incident power on the cavity. Equation 20 of Tara's paper.
PHASE LOCKED LOOP OSCILLATOR NOISE (grey): Measured noise from the PLL, presumably originating from the voltage-controlled oscillator (VCO). Figure 5 in Tara's paper shows the PLL and the various noises found in it, including photocurrent shot noise, photodiode amplifier noise, and VCO frequency noise. Unclear what the 707 Hz/V means, probably is the VCO control slope (i.e. if I want to change my VCO freq 707 Hz, I raise the control voltage by 1 V).
PHASE LOCKED LOOP READOUT (purple): Theory curve of the PLL readout noise. The PSD for this noise rises as f^2, due to the fact that the PLL is a phase detector but the noise budget is in units of Hz/rtHz. This curve is poorly documented compared to the rest of them (Evan calls it a "magic number" curve). To convert from phase noise to frequency noise, multiply the phase PSD by f^2.
SEISMIC COUPLING (black): Measured curve of the seismic coupling into the experiment. The raw data taken appears to be seismic velocity in units of m / (s * rtHz) as a function of frequency. Then, seismic acceleration is obtained by multiplying the raw seismic velocity data by 2*pi*f. Then the two stacks (?) and a spring (??) TF are modeled with hard-coded resonant frequencies and Q's and multiplied together to give a final seismic TF falling as 1/f^6. The final seismic PSD is found by squaring the product of seismic acceleration, the 1/f^6 seismic TF, and an additional hard-coded seismic coupling factor dependent on the cavity length with units m/(m s**-2).
PHOTOTHERMAL NOISE, ISS ON (brown): Measured curve of the photothermal noise. Photothermal noise originates from fluctuations in laser intensity causing changes in the amount of laser power absorbed by the coatings, which causes coating temperature fluctuations. Seems to be the expected limiting noise at low frequency. The raw measurement was of relative intensity noise from both lasers. To get the photothermal noise PSDs for each path, the RINs from each path are multiplied by the absolute laser power absorbed by the coatings squared, a "total photothermal TF" squared, and converted from length PSDs in units of m^2/Hz to frequency PSDs Hz^2/Hz. The "total photothermal TF" is the sum of the coatings photothermal thermoelastic TF, coatings photothermal thermorefractive TF, and subtrate photothermal thermoelastic TF. Each of these photothermalTFs are from units of transmitted cavity power to beatnote frequency fluctuations, (i.e. Hz/W). The process of measuring these three TFs is explained in Evan's paper, section VI, subsection A. It seems that the successful cancellation of expected photothermal noise was the main success of Evan's paper.
RESIDUAL NPRO NOISE (dark green): Theory curve for the residual NonPlanar Ring Oscillator (NPRO) laser noise. The freerunning ASD of the NPRO is reported to by 10**4/f with units of Hz/rtHz. This is then divided by the PDH control loop gain for both paths, squared into a PSD, and summed together into a final NPRO residual PSD.
The rack has been powered.
I made, installed, and powered the +/- 24V and +/- 18V LIGO DC power rails today.
The 24 V rail is powered by the Kepco power supplies. The australian scavenged some Sorensen power supplies for the 18 V rail.
Our Kepco power supplies are 1/4 rack single sided supplies. (Serial number: ATE-25-2M F54459 R21) In picture two, you'll see that two of the power supplies are missing, 'cause I took 'em out because they weren't working.
I took a quick look and found they aren't broken (one rattles when turned on -_-). They just don't have the 50 pin jumper program card in the back (pictures 4, 5, 6). I looked very extensively in our downstairs labs but was unable to find anything like this. I also checked the Kepco website for what this connector is called, they called it a PCA 25-1 but apparently this isn't the real name of the connector according to google. If anyone knows the name of this connector we can buy two and have two additional good variable power supplies, or just buy some from Kepco themselves.
The next step is to create good LIGO DC connectors for our FSS panels. Then we can turn on the lasers, power the PDs, connect the FSS boxes, and lock again.
K and K assembled the temperature sensor box. We want to put this on the rack ASAP, which means they'll have to make 4 long two pin LEMO connectors. The vac can heater is in a rack panel already, all we have to do is attach it. Soon (tomorrow?) we will be able to close the loop on vac can temperature stabilization.
I made a script called SlowVoltageToHertzCalibration.py in the ctn_labdata/scripts/ Git repo. It takes in a cavity scan like those found in ctn_labdata/data/20170817_LaserSlowFreqScan_RefCavResonances, with Laser Slow Control Voltage in the first column and Cavity Transmission DCPD Voltage in the second column. The cavity scan changes the slow voltage between -10 V and 10 V. Transmission light peaks with each FSR, and for the higher order modes... The script sorts out which peaks are the carrier peaks, finds the average slow voltage difference between peaks, and compares it to the FSR to generate a calibration.
The North Path laser slow control calibration is 3480.89915798 3480 MHz / SlowVolt
The South Path laser slow control calibration is 3644.56423675 3640 MHz / SlowVolt
The FSR for both cavities is 4069.94919902 4070 MHz for a cavity of length 3.683 cm. The laser frequency is 281 THz. (1 THz = 1000 GHz)
Cavity scans are shown below for convenience (except that I used a linear y-scale, so its not actually very convenient).
Today I was starting measurements of the North TTFSS, when I noticed that the 100Ω metal film feedback resistor on the Common Path was bent (R3 on the TTFSS Common Path schematic). When I touched it further, the pad it was soldered to came out. I believe this is the cause of awade's poor measurements of the North TTFSS this weekend.
I repaired the North TTFSS by soldering a 100Ω black surface mount resistor that was taped inside the box into the feedback resistor spot, and connecting everything with a hunk of solder. Not the best practice, but it works even though the pad and trace from R5 to R3 is gone.
We have to figure out why our beatnote is flying around if we are going to do science down here. It's preventing us from taking a low actuator noise spectrum off of our PLL, and that is required for measuring our true noise of our cavity beatnote.
Tonight I have set up the Agilent to always be taking spectra and recording them with acromag1. This will allow us to track the beatnote motion and peak strength overnight with a data point coming once every 45 seconds. I'll have to write a peakTracker.py script which takes in a spectrum.txt and finds the peak, should be easy.
I have also written a channelLogger.py directly on acromag1, which is logging the channels for slow control voltages for both cavities, the in-loop and out-of-loop vaccan temp sensors, and an environmental temp sensors overnight.
The point of all of this is to look for obvious correlations between beatnote motion and temperature controls.
One thought I had today was the power difference in the laser cavities. If I recall correctly, the North cavity has about 2.5 times as much power as the South cavity. Also, our slow voltage laser temperature controls change both the laser frequency and the laser power. This means that any large swings in the slow voltage laser control will result in significantly different power resonanting in the cavities, which could result in significant differential temperature changes. Our cavity finesse is like 10000, so this effect is multiplied. Perhaps we should think about evening out the power in the cavities, AKA putting a ND filter in front of the North cav?
I have added an October spectrum to the noise budget, it's the first plot below. Click here for the Bokeh version. Sorry it's not on nodus yet.
The Marconi actuator strength was at 10 kHz/Vpp. The PLL OLG used is shown in a plot below. The phase locked loop open loop gain unity gain frequency (PLL OLG UGF) is ~30 kHz.
Not much has changed in the last two months, as we've been running around like chickens with their heads cut off accomplishing little in the way of science but much in the way of making that future science possible. Still, the hard stuff lies in front of us.
Getting down to the predicted coatings thermal noise must be our top priority for the next two months.
This means we need to understand what limits us, i.e. make additions to this noise budget where the total expected and measured agree in some sense. One curve that is obviously wrong is the PLL osc. noise curve in grey. At 10 kHz/Vpp we are ~15 times worse than that curve indicates on this noise budget.
FSS boxes must be better understood to remove suspected low frequency noise injection and avoid saturation of the high gain EOM paths.
EOMs must be looked at to see if thermally stable and the EOM drivers are okay.
Thermal drift of the cavity MUST be understood and eliminated. Perhaps the different laser power resonating in the cavities is causing this. Measurements are currently underway.
Think about adding PMCs or venting, but we must understand WHY we need to do these time-consuming things before we do them. Also good if we can model our expected improvement. If we can get coatings thermal noise without this, all the better.
Two days ago I set up the Agilent to keep taking spectra overnight while I tracked some relevant channels (elog 1966). Today I plotted them the results.
Plot 1 shows the beatnote frequency [Hz] as a function of time [s]. Plot 2 shows some relevant channels over nearly the same timespan. I have normalized the channels to the range [0,1] and put the min and max values of the channels in the legend.
There is some distinct correlation between the slow control voltages and the beatnote frequency. The vaccan temperature stabilization seems to be working, even according to the out-of-loop sensor.
EDIT: Despite many attempts from several browsers and compressing the plot, I was unable to attach the second plot. It's on the git repo here though if you care to look at it.
We are wondering what noise is limiting our trans beatnote measurements. We suspect the FSS is particularly noisy, so we are taking PZT fastout spectra, calibrating it, and putting it on the noisebudget.
When I did so I ended up with a spectrum which was two orders of magnitude too great. I am not sure what is going on. Maybe the EOM path is working to cancel most of the PZT noise?
Bokeh version of the noisebudget.
Both path's FSS UGF ~ 120 kHz.
South Error Signal RMS ~ 160 mV (read off of oscilloscope)
North Error Signal RMS ~ 60 mV (read off of oscilloscope)
South PZT Vrms to Hz Calibration: 6.55 MHz/Vrms (done to check whether we were crazy for posting this spectrum in the first place)
South Test 2 Excitation digital button doesn't work. The acromag doesn't seem to be responding for that particular button, even though the Ramp Engage and Test 1 Excitation buttons work.
Using EQ's netgpib infrastructure, I was able to communicate with our PLL Marconi remotely. This will enable us to continually lock our PLL despite the crazy MHz-per-hour fluctuations in the trans beatnote frequency we see.
I added the script Marconi2023A_BeatnoteTrack.py to the labutils/netgpibdata/ git repo which tracks our transmission cavity beatnote in real time.
The script adjusts the Marconi carrier frequency as the PLL control voltage approaches too close to the rails. The rails are defined within the script, and are not yet customizable, but it would be a very simple thing to do.
The script adjusts the carrier frequency up or down by the FM deviation value itself. So if your FM deviation value is 10 kHz, like ours, then your carrier frequency of 90 MHz will be adjusted either down to 89.99 MHz or up to 90.01 MHz. This is not robust, and other users will have to adjust the rails and the amount and direction of adjustment based on their personal setups.
I'm going to leave this running in our lab overnight while tracking with stripTool to see if anything crazy happens. If the PLL somehow gets out of range, the script will shut itself off, so nothing bad will happen to our electronics.
With the help of the PLL autolocker, I retook the OLG of the PLL with a frequency modulation level of 10 kHz and 3 kHz.
I did this because we have new beatnote spectra in the can at both levels of frequency modulation and we would like to calibrate and add to the noise budget. The new spectra are in the ctn_labdata/ git repo under ./cit_ctnlab/ctn_labdata/data/20171108_TransBeatnoteSpectrum/
I calculated this PLL OLG the same way I did so previously through the SR560 preamp. This result is plot 2.
I wasn't convinced this was right, so I did it again my calculating the PLL OLG from the mixer error signal through some OUT1/EXC = G/(1 + G) math. This result is plot 1.
The results matched up pretty well:
The 3 kHz FM Deviation PLL OLG had a UGF of ~2.3 kHz in both plots 1 and 2.
The 10 kHz FM Deviation PLL OLG had a UGF of ~5 kHz in plot 1, and ~4.3 kHz in plot 2.
I was doubtful because this is so much less than our previous PLL OLG UGF, which was ~30 kHz at 10 kHz FM Devn. The preamp gain is down to 100 from 200, but not clear otherwise why the PLL UGF is down 6 times as opposed to just 2 times...
With this in hand we should be able to easily calibrate the low-actuator noise spectra we took earlier today.
We need to lock our ISS to the PD output in real time without worrying about DC fluctuations in the PD response.
I made a dual voltage follower using an OP275 and some capacitors.
The schematic is on the box in picture 2.
I checked both voltage follower input/output pairs with an SR785. We see a nice low pass with cutoff frequency ~0.7 Hz.
I'll install this tomorrow.
I was wondering what is causing our huge mountain at 10 kHz. awade said it might be the PZT EOM crossover with gain peaking. I did a quick test in which I turned down the Fast gain sliders to -2V, then incremented them up by 1V while taking spectra at every point.
We start to get diminishing returns from increasing the Fast gain at around 4V. Before then, we can see some FSS gain peaks moving around and breathing at around 2-3 kHz.
We have a broad peak at 350 Hz. This one is weird in that it likes to breath, or come in an out of existence, on a timescale of a few seconds.
Similar descriptions for the 640 Hz and 1040 Hz lines, but to a lesser extent. There is some sort of creepy-crawly noise floor we're hitting between 300-2000 Hz once we're no longer FSS limited. I blame the australian.
Also the mountain at 10 kHz is not affected by any of this.
Next step: FSS error signal spectra, FSS OLGs, and FSS calibration for both cavities. This will help with determining our current limiting noise floor.
Some interesting points:
The North cavity has weird breathing in it's error signal on the order of a few milliseconds.
The South cavity does not like a Fast gain of 4V: the error signal tends to blow up randomly, then be fine for a minute, then do it again. Could be the crossover between the EOM and PZT acting up.
The North cavity and South cavity have significantly different responses to extremely high Fast gain. The South cavity, when the Fast gain is too high, causes the EOM to start fighting it, increasing the EOM response until it rails. The North cavity has no limit to how high you can turn the Fast gain; the EOM response never increases in response to a huge PZT gain. Cause unknown, maybe the North has good phase at the crossover while the South is bad?
The Fast and Common gain sliders are calibrated to 32 dB/V.
The Common gain sliders were set to -3.5V.
The number of averages was 25.
The PLL was locked at FM Devn of 1 kHz/Vrms, Preamp Gain of 2000, and Carrier Freq around 109 MHz.
Edit: Remade plot with more resonable labels and ylabel.
Craig response to RXA: I had forgotten to take into account the reduced power on the reflection PDs when the cavity is locked. What I called "light noise" is in fact just shot noise, no need to confuse things.
Shot noise goes like
where S_e(f) is shot noise in watts/rtHz, h is Planck's constant, c is speed of light, lambda is the laser wavelength, and P is the power on the photodiode.
When we use PDH to lock a Fabry Perot cavity, we modulate the laser light at frequency . This splits the laser power into parts:
where is the original laser frequency, is the modulation sideband frequency, and is higher order modes. When the cavity is locked, all of the carrier light is transmitted through the cavity, and does not reach the reflection PD and does not contribute to shot noise. My previous plot neglected to account for this.
I made some measurements of the DC voltage of the reflection RFPD when the cavity was both unlocked and locked. This way I can estimate the amount of power on the PD when the cavity is locked:
South Power ratio: 0.316
North Power ratio: 0.724
Attached in Plot 2 is a new plot of the old shot noise data scaled by these new ratios.
RXA: Most of this ELOG is bogus. Future readers should ignore. I think Craig was going to fix it...
Yesterday I spent a bunch of time thinking about shot noise.
I found our shot noise should be about 6 mHz/rtHz. This should be correct in order of magnitude... I think this should be multiplied by a factor of root two because we have two cavities. Attached is a zipped Mathematica notebook with my calculations.
I also measured and plotted the REFL PD dark noise and what I call "light noise". The dark noise is just blocking the light on the PD and measuring the electronics noise at the PD error point (Common Path Test Point 1 in the FSS Box). The light noise measurement is measuring the PD error signal without the light blocked, but with the FSS loop open.
The approximate calibration I used in these plots is 6 MHz/Vrms. This should definitely taken as a rough calculation, a full calibrated FSS analysis is on the way.
We are not shot noise limited with our PDs.
With the light noise measurements, we see some of the same creepy-crawly peaks at 300-2000 Hz that we saw in the Transmission Beatnote spectrum for high Fast gain. In particular, the South light noise measurement features the broad 350 Hz peak.
awade found that when we are making the light noise PD measurements, if you tap the optics table, those creepy-crawly peaks jump up. This is definite confirmation of scattering coming from our tabletop optics. We need to buzz all of our optics posthaste!