40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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ID Date Author Type Categoryup Subject
  1854   Wed Nov 4 01:25:37 2020 KojiSummaryOpticsAspheric Lenses and Grating Plates bonded on their mounts

Attachment 1: Black Diamond (GeSbSe) Lens was mounted on the flexure mount. The flat surface should face to the gain chip. It was aligned on the wipe to be flush with the protrusion.

Attachment 2: Applied glue on the four grooves of each flexure mount.

Attachment 3: The grating was bonded on the mount. The arrow marks were arranged as Paco directed. The mount could not stand by itself. And the screws were placed to stop the grating skating on the mount.

Attachment 1: IMG_0188.jpg
IMG_0188.jpg
Attachment 2: IMG_0208.jpg
IMG_0208.jpg
Attachment 3: IMG_0202.jpg
IMG_0202.jpg
  1864   Wed Nov 18 17:49:05 2020 PacoDailyProgressOpticsbeam profiles

Used BeamR and WinCamD to profile the two light sources (ECDL and OPO pump) 

(1) ECDL; profile 19** nm beam after the aspheric lens. I guess we want this beam to be nominally collimated for optimal feedback with the Littrow-configured grating, so I recorded the 1/e^2 waists (x, y) as a function of longitudinal displacement. The result is attached below. Linear fits provide rough estimates for the beam divergences, giving 2.0 mrad along x (parallel to the table) and 1.2 mrad along y (normal to the table) suggesting some astigmatism which is common in high NA aspheric lenses. I inspected the distance from the aspheric lens to the SAF gain chip and measured ~ 2.0 mm (compared to the 1.99 mm working distance specified for this lens with NA=0.71). The SAF1900S specifies a beam divergence angle of 35 deg  (corresponding to NA=0.57), so there is room for improvement by tweaking the aspheric flexure alignment.

(2) NPRO; profile 1064 nm beam at low power (~10 mW) right after the head output. Having 10x more power made things way easier for this as compared to the ECDL, but the method was the same (record 1/e^2 waists as a function of longitudinal displacement). The result is attached below. Linear fits provide rough estimates for the beam divergences, giving 2.1 mrad along x, and 2.1 mrad along y. Here I grabbed the specified divergence of 2.3 mrad from a relatively old manual, and even drew the displaced waist profile (w0 = 160 um) which seemingly fit the profile, but the actual values may be different.

Attachment 1: 2020_11_18_ecdlprofile.png
2020_11_18_ecdlprofile.png
Attachment 2: 2020_11_18_mephistoprofile.png
2020_11_18_mephistoprofile.png
  1870   Tue Dec 1 18:37:09 2020 PacoDailyProgressOpticsOPO pump steering

Enter lab ~09:20. Today I spent a while looking at the broadband EOM drivers used in CTN (presently optimized for 37 MHz) and installed the preceding steering and power control (half waveplate + pbs) optics. The beam path for the OPO pump beam is now set to 3 inches (note the NPRO head is nominally 4 inch above the table).

  1874   Fri Dec 11 16:04:36 2020 PacoLab InfrastructureOpticsPPKTP crystals

Two crystals from Raicol arrived. Picked them up from Downs today and inspected them (see photos below). The lengths are nominal (20 mm), they are serialized as 123 and 124, and the ends look like they have the specified (AR) coating. I reached out for Covesion two days ago to track the ovens so we can mount these guys, but have yet to hear back from them.

Attachment 1: raicol_124.jpg
raicol_124.jpg
Attachment 2: raicol_123.jpg
raicol_123.jpg
  1880   Thu Dec 17 12:01:54 2020 PacoLab InfrastructureOpticscrystal ovens, clips and controllers

Covesion order arrived, containing 2x

  • Crystal oven (20 mm long) (below)
  • Clips (for mounting crystals) (below)
  • Blank crystal (to press on the ppktp crystal) (below)
  • OC2 oven controller
  • Controller cable and power cable
Attachment 1: covesion_oven_clip_blank.jpg
covesion_oven_clip_blank.jpg
  1881   Mon Dec 21 16:35:14 2020 AnchalDailyProgressOpticsTook beam profile of laser right off the head
  • This is a repetition of SUS/1864.
  • Used Data Ray Beam'R2-DD.
  • Took 50 averages and recorded beam "diameters" at 10 different points after the laser head.
  • Configuration file is BeamProfileConfiguration2um.ojf.
  • Used fitBeamWidth function of ala mode to fit X and Y beams separately and then their geometric mean.
  • We'll use the geometric mean as the seed profile for future calculations.

Data

Attachment 1: LaserHeadBeamProfileAvg.pdf
LaserHeadBeamProfileAvg.pdf
Attachment 2: LaserHeadBeamProfileX.pdf
LaserHeadBeamProfileX.pdf
Attachment 3: LaserHeadBeamProfileY.pdf
LaserHeadBeamProfileY.pdf
  1882   Tue Dec 22 15:54:03 2020 AnchalDailyProgressOpticsTook beam profile of near EOM area
  • After installing a 400mm focal length plano-convex lens at 24" from the laser head at (20, 12), we found that higher-order modes are present in the beam.
  • We installed an iris at 34" from laser head at (17, 5)
  • Configuration file is BeamProfileConfiguration2um.ojf.
  • Used fitBeamWidth function of ala mode to fit X and Y beams separately and then their geometric mean.
  • We'll use the geometric mean as the seed profile for future calculations.
  • Found a beam waist of 306 um at 58" from the laser head.
  • Installing the EOM between 49" and 52" from the laser head where the beam waist is between 1 mm and 740 um.

Data

 

Attachment 1: BeamProfileNearEOMAvg.pdf
BeamProfileNearEOMAvg.pdf
  1884   Mon Dec 28 15:51:51 2020 AnchalDailyProgressOpticsMode matching solution for Cavity

Goals and restrictions:

  • Use the fewest lenses as possible.
  • The beam widths in both onward and reflection direction should be such that there is a 5-inch space somewhere where we can put in the faraday isolator which has an aperture size of 3 mm and intensity limit of 500 W/cm2.
  • The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
  • Choose a lens from a list of focal lengths available in west bridge labs.
  • Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.

Analysis & Results

  • CavityLens.m is run to try all possible lens combinations for 2-lens or 3-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
  • Then save solutions with more than 70% overlap in CavityModeMatchingSolutions.mat.
  • findBestSolutions.m increases the overlap threshold to 0.9, calculates reflected beam profile for the sideband reflection from the cavity (blue curves in the figures), and discards solution which does not have a 5-inch long area where we can place a faraday isolator with aperture of 3 mm.
  • Black lines show the region where Thorlabs IO-3-1064-HP can be placed safely without clipping or exceeding the intensity limit with 1W power.
  • All solutions in order of sensitivity are plotted here with details of lens choice and positions. In total 7 solutions were found which are stored in BestSolutions.mat.

Analysis & Data


Wed Jan 6 10:00:35 2021: This analysis was wrong. See SUS_Lab/1887.

Attachment 1: Solutions1-7merged.pdf
Solutions1-7merged.pdf Solutions1-7merged.pdf Solutions1-7merged.pdf Solutions1-7merged.pdf Solutions1-7merged.pdf Solutions1-7merged.pdf Solutions1-7merged.pdf
  1885   Wed Dec 30 09:57:56 2020 PacoDailyProgressOpticsDOPO crystal oven

Assembled first DOPO oven with the crystal. The components (shown below) are:

  • Oven clip
  • Oven
  • ITO crystal spacer
  • PPKTP crystal

The NL crystal sits in the (brass?) clip directly, with the ITO (dummy) crystal pressing it uniformly down. There are no placement references to align the crystal with the oven axis, so this was done very carefully by hand. Once this is roughly straight, the copper arms are fastened in place tight enough to hold everything in place but without excess strain on the NL crystal. The assembly (shown below) is then mounted enclosed in the oven. I put some kapton in place to shield from dust until operation.

Attachment 1: ppktp2.jpg
ppktp2.jpg
Attachment 2: ppktp1.jpg
ppktp1.jpg
  1886   Thu Dec 31 16:41:59 2020 AnchalDailyProgressOpticsMode matching solution for Cavity

Goals and restrictions:

  • Use the fewest lenses as possible.
  • The beam widths in both onward and reflection direction should be such that there is a 5-inch space somewhere where we can put in the faraday isolator which has an aperture size of 3 mm and intensity limit of 500 W/cm^2.
  • The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
  • Choose a lens from a list of focal lengths available in west bridge labs.
  • Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.

Analysis & Results


Analysis and Data


Wed Jan 6 10:00:35 2021: This analysis was wrong. See SUS_Lab/1887.

Attachment 1: Solutions.pdf
Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf
  1887   Wed Jan 6 09:34:34 2021 AnchalDailyProgressOpticsCorrected analysis and found better solutions.

Errors in the previous analysis

  • Previously, we wrongly assumed. that the reflected light from the cavity would be as if a reflection is happening from a flat mirror. It actually follows the same paths as incident light in the reflection path.
  • Lens were restricted to non-overlapping regions but that meant that solutions where lens are close to each other can only happen near the boundaries of these regions. Removing this condition widens the search for a good solution.
  • We collimated the beam to near 0.5 mm radius with a 229.1 mm focal length lens at 67" from laser head and put faraday isolator in front. So now the problem only remained to match the mode after this point to the cavity mode.

Goals and restrictions:

  • Use the fewest lenses as possible after having used a fixed lens at 67" point before the faraday isolator.
  • Choose a lens from a list of focal lengths available in west bridge labs.
  • Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.
  • The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
  • The beam widths should not exceed 4mm in diameter anywhere to ensure small areas of lenses are used.

Analysis & Results


Analysis & Data

Attachment 1: Solutions.pdf
Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf
  1888   Thu Jan 7 09:44:52 2021 PacoDailyProgressOpticsShaping the OPO cavity mode

Summary of solution number 2 (from previous post). 

After installing the lenses, mirrors and some minor alignment, took the beam profile around the expected minimum waist position (~102" from laser head). The beam profile is astigmatic as can be seen from the plot below (red / blue = x / y), so the mode matching will be suboptimal from the start. 

Taking the geometric mean of the waists (w = sqrt(wx * wy)) we represent our nominal mode and find a min waist of 36.8 um (shaded region in the plot).

The OPO cavity model targets a min waist of 35.5 um (for an optimal Boyd--Klein parameter of ~2.7), but solutions exist with slightly shorter cavities and slightly larger waists which would only compromise the optimal Boyd--Klein parameter to ~2.55 for the sake of better mode matching. I think this is a good place to move out of calculation-land and see how well we can make the cavity work in reality.

Attachment 1: profile_20210106.jpg
profile_20210106.jpg
  1889   Thu Jan 7 17:09:16 2021 Paco & AnchalDailyProgressOpticsMode matching OPO

Fresh attempt at mode matching. For this,

  1. Installed the oven, plugged it to the controller and went to the nominal temperature setpoint (40 C) to match the expected path length inside the NL crystal
  2. Placed the output coupler (roc = 15 mm) and roughly align so that the retroreflection is overlapped with the input beam.
  3. Set up a PD (Newfocus 2001) and scope, operating the laser at relatively low power (current ~ 840 mA), and optimize the FI rejected power.
  4. The output coupler is mounted on a three axis mirror mount (Polaris, hoping to get low drift) such that we have some knobs to tune the mode matching initially.

After a couple of iterations moving the mirror X,Y and then scanning all knobs (X,Y, and XY) to effectively translate along Z, the optimized FI rejection is ~(2.15 mW /2.95 mW) 75% of the input beam power. Looking closely at the backreflection from the output coupler, I can clearly see multiple scattered spots, which could definitely account for the defficiency. The most likely culprit is the crystal itself, which is mounted between brass and glass surfaces with no respect for anti-reflection measures. The waist is small enough that no clipping should be happening, so it looks like the NL crystal placement may have to be revisited. Other than that, this procedure should be fine.

  1890   Fri Jan 8 17:00:26 2021 AnchalDailyProgressOpticsIncluded lens made by cavity input mirror and distrotion due to crystal

Quote:

Errors in the previous analysis

  • Previously, we wrongly assumed. that the reflected light from the cavity would be as if a reflection is happening from a flat mirror. It actually follows the same paths as incident light in the reflection path.
  • Lens were restricted to non-overlapping regions but that meant that solutions where lens are close to each other can only happen near the boundaries of these regions. Removing this condition widens the search for a good solution.
  • We collimated the beam to near 0.5 mm radius with a 229.1 mm focal length lens at 67" from laser head and put faraday isolator in front. So now the problem only remained to match the mode after this point to the cavity mode.

Goals and restrictions:

  • Use the fewest lenses as possible after having used a fixed lens at 67" point before the faraday isolator.
  • Choose a lens from a list of focal lengths available in west bridge labs.
  • Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.
  • The lens should not be closer than 1.5 inches from each other or to the EOM mount or cavity edge.
  • The beam widths should not exceed 4mm in diameter anywhere to ensure small areas of lenses are used.

Analysis & Results


Analysis & Data

 

Attachment 1: Solutions.pdf
Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf
  1891   Fri Jan 8 17:17:19 2021 AnchalDailyProgressOpticsIncluded lens made by cavity input mirror and distrotion due to crystal

Error in previous calculations:

  • We did not take into account the effect of cavity input mirror on the coupled light. It would act as a thick concave lens for the coupled light into the cavity.
  • We did not take into account the divergence due to refraction at the crystal surface.

Goals and restrictions:

  • Use the fewest lenses as possible after having used a fixed lens at 67" point before the faraday isolator.
  • Choose a lens from a list of focal lengths available in west bridge labs.
  • Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.
  • The lens should not be closer than 1.5 inches from each other.
  • The beam widths should not exceed 4mm in diameter anywhere to ensure small areas of lenses are used.
  • Take into account the concave lens due to the input mirror.
  • Take into account the refraction due to crystal surface.

Analysis & Results

  • CavityLens.m is run to try all possible lens combinations for 1-lens or 2-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
  • The cavity input mirror is modeled as two refracting surfaces separated by 6.5mm. The first surface is flat while the second has ROC of -25 mm.
  • The crystal is modeled as two refracting flat surfaces separated by 20 mm.
  • The target beam waist is kept at the center of the crystal with 35.578 um diameter.
  • Then save all possible solutions with more than 90% overlap and where lenses are atleast 1.5" away from each other in AllPossibleSolutionsAbove90.mat using findPossibleSolutions.m.
  • findBestSolutions.m increases the overlap threshold to 0.995, allows maximum beam radius of 2mm anywhere and plots the best solutions in order of positional sensitivity of the lens. These are stored in BestSolutions.mat.

Analysis & Data

Attachment 1: Solutions.pdf
Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf Solutions.pdf
  1892   Wed Jan 13 17:52:35 2021 PacoDailyProgressOpticsOPO cavity resonance

Observed first resonant transmitted (& reflected) light from the DOPO cavity; the PZT scan was centered at 31 V, at 2 Hz, with an amp. of 1.5 Vpp. To get there, revisited the path's alignment upstream to the last mirror (before the last lens), removing, inspecting, and reinstalling each component. After this, I used the camera at the end of the optical path as a "pinhole" (beam center placeholder) and after inserting each element (mirrors / crystal) checked carefully that the beam was landing straight. Then, patiently scanned various knobs (mirror mounts X/Y/XY, crystal manually) until HOM started resonating. After a bit of further alignment managed to see transmission dips in the FI pickoff. Below are two photos illustrating the current state (way more optimization is needed), as well as the setup viewed from one side (for the scope picture, purple is the ramp, yellow is cavity reflection, green is cavity transmission). Will keep optimizing in the couple next days, all at low power first, and then start cranking the power up to factor in any thermal effects into the optimized cavity.

Attachment 1: dopo_first_resonance.jpg
dopo_first_resonance.jpg
Attachment 2: dopo_sideview.jpg
dopo_sideview.jpg
  1895   Tue Jan 26 11:33:32 2021 ranaDailyProgressOpticsIncluded lens made by cavity input mirror and distrotion due to crystal

Quote:

Error in previous calculations:

  • We did not take into account the effect of cavity input mirror on the coupled light. It would act as a thick concave lens for the coupled light into the cavity.
  • We did not take into account the divergence due to refraction at the crystal surface.

Goals and restrictions:

  • Use the fewest lenses as possible after having used a fixed lens at 67" point before the faraday isolator.
  • Choose a lens from a list of focal lengths available in west bridge labs.
  • Find the best overlap with the target beam of 18 um at the cavity waist with the most sensitivity with respect to lens positions.
  • The lens should not be closer than 1.5 inches from each other.
  • The beam widths should not exceed 4mm in diameter anywhere to ensure small areas of lenses are used.
  • Take into account the concave lens due to the input mirror.
  • Take into account the refraction due to crystal surface.

Analysis & Results

  • CavityLens.m is run to try all possible lens combinations for 1-lens or 2-lens solutions using ../20201222_BeamProfileNeatEOM/SeedBeam.mat as the seed beam.
  • The cavity input mirror is modeled as two refracting surfaces separated by 6.5mm. The first surface is flat while the second has ROC of -25 mm.
  • The crystal is modeled as two refracting flat surfaces separated by 20 mm.
  • The target beam waist is kept at the center of the crystal with 35.578 um diameter.
  • Then save all possible solutions with more than 90% overlap and where lenses are atleast 1.5" away from each other in AllPossibleSolutionsAbove90.mat using findPossibleSolutions.m.
  • findBestSolutions.m increases the overlap threshold to 0.995, allows maximum beam radius of 2mm anywhere and plots the best solutions in order of positional sensitivity of the lens. These are stored in BestSolutions.mat.

Analysis & Data

 

  1896   Tue Jan 26 11:34:51 2021 ranaDailyProgressOpticsIncluded lens made by cavity input mirror and distrotion due to crystal

would be good if you could find a solution that is not very sensitive to precise lens placement

 

  1898   Tue Feb 2 10:32:25 2021 AnchalSummaryOpticsFiguring out how much astigmatism is hurting us

Methods

  • Use the actual measured beam profile in X and Y directions.
  • Propagate them with the current position of lens.
  • Assume the position of cavity mirror and crystal as given by the second solution in BestSolutions.mat in the Jan 8th analysis which is implemented currently.
  • Calculate the overlap with target and position of waists in X and Y direction.

Conclusions

  • Astigmatism should not be hurting us significantly.
  • The mode matching in principle can be improved in the experiment.

Analysis

Attachment 1: BeamProfilePropagation.pdf
BeamProfilePropagation.pdf
  1899   Tue Feb 2 17:39:52 2021 PacoDailyProgressOpticsre: Figuring out how much astigmatism is hurting us

Motivated in part by the conclusions below, improved estimated mode matching efficiency from a poor 13% at the beginning of day to 48% (estimated using the reflection signal levels from the rfpd). What helped was walking the beam with the last two mirrors, and then scanning the cavity output coupler around to center the resonant mode which at this point seems optimal. This process was tedious, but effective apparently.

The distance between the two mirrors is ~ 45 mm which slightly undershoots the planned 47.5 mm which could limit the achievable 100% in simulation-land, but I'm moving on for now, hoping the lock will bump it up enough for the OPO threshold to be within our pump power range.

Quote:
  • Astigmatism should not be hurting us significantly.
  • The mode matching in principle can be improved in the experiment
  1900   Thu Feb 4 16:46:06 2021 PacoDailyProgressOpticsOPO cavity lock

Demodulation stage

Update on demod. for OPO cavity lock. Last related elog entry described prevalence of <= -77 dBm of odd line noise harmonics (60, 180...) Hz, along with poor SNR PDH error signal. First attachment is a drawing of the current RF connections. Upon completing list of suggested actions from this post, the difference was mostly made by looking at RFPD RF out power before mixer < -40 dBm. This was no good, so after realizing that the OD = 3 nd filter before RFPD was only allowing 80 uW of a nominally reflected ~25 mW, swapped the ND filter with HWP + PBS for adjustable power splitting. Then, a healthier  -10dBm made it into the mixer and SNR improved considerably (see second attachment). Upon closer examination of err signal, low freq. sinusoidal modulation sat on top of it suggesting slightly off-resonant demodulation so finely adjusted the (Marconi) LO frequency from 36.000 MHz --> 35.999828 MHz until the error signal had a good enough shape (see third attachment below).

Lock

First attempt at cavity lock was done with ~46% mode matching efficiency and max. modulation depth (estimated ~0.21) on the EOM. The loop is achieved using UPDH box (v3) which I stole from CTN lab. Upon connecting all the inputs, scanning the phase shifter without making much of a difference, and enabling the lock, saw a stabler higher order mode on the cavity transmission which is nice. The natural follow up of scanning the PZT driver (i.e. as an offset) and re-engaging the lock resulted in what I can only describe as a "visit to the dentist", where the cavity PZT (on the output coupler) was resonating quite loudly (!!). After looking at the output monitor of UPDH box with engaged lock on SR785 an ~ 8 kHz peak explains the noise as an audible mechanical resonance. Adjusting the servo gain finely tunes it out a bit, and adding an SR560 in line before the PZT driver unit greatly helps, but changes the overall loop gain and the lock becomes unstable...  Current efforts are therefore geared towards improving the pdh loop, for which an option is to bypass the thorlabs MDT694 HV piezo driver and directly connect the UPDH output to PZT (which it may be meant to directly drive) and use slow temp. control on pump laser to approach the lock point. Another option, involving way more time, would be to *not* use UPDH box at all and implement a digital feedback loop + filter with the Red Pitaya. Perhaps the pragmatic action is to get the analog solution working and develop digital solution on the side.

Attachment 1: rf_diagram.png
rf_diagram.png
Attachment 2: offresonantpdherr.jpg
offresonantpdherr.jpg
Attachment 3: pdherrorsig.png
pdherrorsig.png
  1901   Fri Feb 5 14:15:22 2021 ranaDailyProgressOpticsOPO cavity lock

For the splitting, I recommend not to use a splitter.

Instead, you can use a -10 or -20 dBm bi-directional coupler. You send the -10 dBm signal to the EOM amp, and you can fill up the needed power for the LO mixer. Also the "bi" nature of the coupler means that you can check for reflected power to diagnose if you are having impedance mis-match. Since you don't have an isolation amplifier in your setup, its important to make sure that reflections from one leg don't go back into the oscillator and disturb the other leg. Or maybe your oscillator box has an isolation amplifier between the oscillator and the splitter?

  1905   Thu Feb 25 10:28:07 2021 PacoDailyProgressOpticsDOPO lock endurance

Test long term stability of the DOPO cavity lock; The cavity remained resonant overnight (start ~ 8 PM yesterday) and lost around 11 AM today. It might be good enough to approach lock point manually using laser temp. control and then engage the fast loop. In any case, today will set up an acromag channel for this. Configured "XT1541-2um-SlowDAC" to 10.0.1.47

  1906   Tue Mar 9 19:21:38 2021 PacoMiscOpticsDOPO cavity pole

- Noticed that the cavity transmission peaks @ 1064 nm were much wider than originally estimated by the dopo cavity design notebook suggesting a lower Finesse. So using the PDH error signal, and knowing the EOM sidebands are at 36 MHz estimated the current DOPO cavity linewidth to be 19.5 MHz, well in excess of the target 10.4 MHz.

- Updated the crystal AR coating specs from Raicol (R < 0.3% @ 1064/2128), but more importantly, I included the absorption coefficient of KTP, alpha=0.005/cm (often quoted as < 0.01 / cm) into the roundtrip loss and the design now gives 17.97 MHz. So, given the uncertainty in the absorption coefficient of the NL crystal, and all the coatings in the experiment, this adjustment might be enough to explain this observation.

  1908   Wed Apr 14 16:49:30 2021 PacoMiscOptomechanicsDOPO mount v2

Drew some new mounting scheme for the DOPO cavity; main revisions with respect to the current mount are -->

  • Side mounts for both mirrors (instead of vertical)
  • Both mirror mounts are the same (3-axis polaris K1) so both mirrors need to be attached properly
  • Improved access to align crystal using newport 9031 (6 axis displacement mount), which is crucial to make the DOPO fields co-resonant

Attachment 1 illustrates the design; shows three views of the same assembly.

Concerns: mechanical noise from side mounted mirrors ... for this, there could be a solid piece which makes a rigid connection between the two mirrors (that's why they are upside down) and perhaps between the two tall posts (so S-shaped as viewed from the top)? Still working on this.

Attachment 1: dopo_mount_v2.pdf
dopo_mount_v2.pdf dopo_mount_v2.pdf dopo_mount_v2.pdf
  706   Tue Aug 13 13:37:12 2013 Ed Taylor and Nic SmithDailyProgressRingdownRoom Temperature Ring Down Measurement

Yesterday we successfully measured a ringdown with our new apparatus. The HV connector that we would have needed to supply voltage to the esd was not able to arrive on time. Due to time constraints we decided that achieving cryogenic temperatures would not be possible. We decided to move the clamp to Alasair's vacuum chamber where we would be able to perform a ring down at room temperature. A laser was passed through the side of the silicon cantilever and was read by a split photodetector that acted as a shadow sensor. The data was then run through matlab where I converted the signals to power spectral densities and analyzed the decaying peak of the resonant frequency. The characteristic ring down time of the 1st mode was determined to be 1284 +/- 24 seconds which gave a mechanical loss on the order of 10e-6. This is in good agreement with the theoretical result that I previously calculated using the expression for the quality factor of a longitudinally oscillating bar with thermoelastic loss (Landau and Lifshitz).

 

Attachment 1: RingDownMode1.jpg
RingDownMode1.jpg
  707   Tue Aug 13 16:41:51 2013 Ed and NicDailyProgressRingdown4 more resonant modes recorded

A room temperature ringdown was performed for 4 additional resonant frequencies whose ringdown times were on the order of minutes. The high voltage used was rougly  3.4 kV and 1 V for the AC voltage.

  717   Fri Aug 23 14:09:09 2013 Ed Taylor and Nic SmithSummaryRingdownComsol Frequencies and Observed Resonant Frequencies

The observed resonant modes of the oscillating cantilever differed substantially from frequencies computed through comsol (more so at higher frequencies)

Attachment 1: Comsol_Frequencies.pdf
Comsol_Frequencies.pdf
  718   Fri Aug 23 15:13:26 2013 Ed Taylor and Nic SmithSummaryRingdownExponential Fits to Ring Down Data

When performing the ring down, we obtained sets of data which contained time varying votlage of the signal recorded from the split photodiode. We utilized the pwelch function in Matlab and obtained a PSD for each data set. The decaying peak of the resonant frequency was plotted seperately which then formed a decaying exponential. I then applied a fit to each set of data.

Attachment 1: Tao.pdf
Tao.pdf Tao.pdf Tao.pdf
  719   Fri Aug 23 16:25:06 2013 Ed Taylor and Nic SmithSummaryRingdownLog plot and log fit of decaying ringdown for the 3rd mode

I created a log plot for the decaying data and fitted linearly to the data.

Yechh!!  JPG, no error bars, no details of fit ??!!!

Attachment 1: LogofRingDownMode3.jpg
LogofRingDownMode3.jpg
  771   Tue Feb 11 10:53:31 2014 nicolasDailyProgressRingdownRingdown of 70Hz cantilever mode started

at gps time 1076180130.

The amplitude is recorded in X1:SCQ-CANTILEVER_MODERINGER_PWRCTRL_INMON .

  106   Mon Nov 26 16:05:43 2007 Norna RobertsonMiscSUSMass of OMC silca bench (measured 16 Nov 2007)
For the record here is a note sent round OMC-SUS colleagues on 16th November.

--------------------------------

A team of Chris, Chub, Helena and me ably led by Sam (who did the scary lifting) got the OMC bench dismounted from its suspension and weighed today. It is now sitting on its custom plate with dark side down (minus the preamp boxes) in the centre of the optics table in room 56. It will remain there for the forseeable future. The metal parts of the suspension will be disassembled by Chris with Ken Mailand's help starting Monday.

Results of weighing

1) Optical bench including preamps and the wire bundle to the heater, pzt etc = 6.514 kg
2)Preamp cable (mock-up) = 24.4 g (without the connectors to the preamp)
3) The two leaning towers of counterweights (which were situated close to the far corners of the mass - NOT above the holes where they need to be for fixing in place) = 200.2 g and 187.7 g

I remind you all that there are 2 more tombstones ( fused silica 8 mm thick x 30 mm tall x 32? mm wide with 18 mm diam hole- size to be checked from CAD drawing) and two pieces of black glass 1 inch by 1 inch by 3 mm ( I noted at our telecon that these weigh 8.5 gm total) to be added to the bench - situated close to the DC photodiodes which sit on bright side under the preamps.
  119   Wed Jan 23 22:52:21 2008 Norna RobertsonMiscSUSOMC SUS assembly adjustments made and lessons learnt
Calum and I have been working in the 40 m annex re-assembling and testing the OMC SUS after the parts have been cleaned and baked and various parts modified. We have had several problems which needed to be addressed and made several observations of items which need attention for the next OMC. Also we note several points which need to be followed when the OMC is being prepared at LLO for input into the vacuum chamber. The following are in no particular order.

1) Problem with one of the blades
When we got the double pendulum hanging today we noticed that one of the lower blades was interfering with part of the top mass - hitting a block which holds one of the screws for adjusting the position of the clamp for the upper wire ( for adjusting pitch). The blade was 13S - one of those which had an angled clamp of 3.5 degrees (the larger of the angles - the blades at the other end of the bench have 3 degree angles). These large angles are being used to counteract the fact that the bench weighs more than these blades were designed for. The blades thus have a convex curvature (looking from above) and the crown of the bend was where the interference was occuring. The puzzling bit is why only one blade showed this, and not its partner. We tried another 3.5 degree clamp ( in case it was a fault with the clamp) but saw the same effect. We then tried reducing to 3 degrees for that blade - still almost touching. We then reduced the angle for that blade to 2.5 degrees. This worked Ok - and when the masses were hanging again all the blade tips appeared to be at approximately the same height (measured using a steel rule). So the dynamics should be OK. Puzzle remains why only one blade showed this behaviour. Note that for the LHO OMC we will be getting new blades designed for the mass they will be taking, and so we should not need to use angled clamps as large as for the LLO suspension Hence this interference problem should not arise again.
Lesson for the future - need to characterise all blades with the full range of angled clamps to be used. (We did not have time to do this for this suspension).

2) Test of new method of hanging silica bench (using metal bench)
Yesterday with Ken Mailand's help we tried out the new way of suspending the bench using Ken's bench holder with one of the metal benches and the two lab jacks. We learnt that to get the right range of height adjustment to lift the bench to put the discs on the lower wire clamps and then lower the bench again the mating pieces on the lab jacks had to mate with the central portion of the handles at each end rather than the top part of the handles. Also it looks like the lower EQ stop holder (which sits under the bench) should be put in place before the bench is brought in - otherwise difficult to get into place. Finally we note that this job is a three person job - need one on each end to raise and lower the lab jacks and one to fit the discs and then guide the discs and clamps into the holes and check they are seated correctly as the bench is lowered.

3) Height of bench.
We measured the height of the lower surface of the metal bench above the optics table to be (160.5 - 38.75) = 121.75 mm. Requirement is 101.6 + 20 = 121.6 (+/- 2mm) so we are OK. This was without any extra mass added to the top mass. We note that the tabelcloth is close to lower end of its range, but should be OK. However the final alignment needs to be done when the OSEMs are in place.

3) EQ stops and parts which should be removed before putting into vacuum chamber.
For sending the OMC to LLO we put on double nuts on all the EQ stops. This limits the range of the stops and in particular the ones under the bench were not long enough to reach the bench when double nuts are used. So we had to improvise the position of the EQ holder by putting ~ 1/2 inch "shims" to raise the holder ( the shims were three of the masses made for attaching to the metal bench to increase its mass). We also removed all "loose" parts - the adjustment mechanisms for the top blade yaw positions, the magnets and flags, the set screws for locking the larger vertical stops used to hold the top mass, and the screws used to alter the pitch adjustment clamp positions.
When the OMC is reassembled at LLO and made ready for installation, all the second nuts on the EQ stops should be removed. Also once the pitch adjustment using the moveable clamps is set correctly, the screws for doing those adjustments should be removed again. Basically all loose screws should be removed.

4) Parts needing modifications
i) EQ cross pieces holding the plate which goes over the bench need to be reduced in length to fit between the structure legs.
ii) Blind holes in EQ corner brackets.

5) Point to note for next OMC.
i) The slots in the structure which take the dowel pins for alignment need to be lengthened to allow the tableloth the full range of movement which the slots for the attachment screws would allow.
ii) The targets for aligning OSEMS need a hole in the centre for the flag.

More to follow tomorrow
Also we took lots of pictures today. will put relevant ones into installation document.

OMC is due to be crated tomorrow for pick-up on Friday am.
  122   Tue Nov 17 10:33:37 2009 ZachMiscSUSAOSEM test progress

 It's dusty in here...

--------------------------------

 

I was recently commissioned to do some noise measurements on the new  AOSEMS. I set up a humble experiment in the LIGO e-lab to do some preliminary measurements:

 

I made a simple current-to-voltage converter out of an OP27E (using a 100-kohm feedback resistor) to use as the transimpedance amplifier for the readout. This results in a transimpedance of 0.1 V / uA. A simple schematic of the important elements is attached below.

 

DC power was provided without regulators directly from the laboratory DC supply in the lab. The value of 1.7 V across the LED was set such that the current through it was ~35 mA.

 

Rana and I took a few important PSDs (one of the DC supply, one of the OP27E with no supplied current, and two with the setup fully connected--one each with and without the PD covered), all from 250 mHz - 200 Hz, AC coupled. Using a sophisticated estimation method (called, by some, the "pick two points and approximate with a power law" method for lack of something fittingly elegant), we obtained a rough estimate of these spectral densities in order to compare them.

 

These were all converted into equivalent PD current noise. For all but the "supply" noise, this was done trivially by dividing by the transimpedance of the OP27E. For "supply", LED voltage noise had to be converted to PD current noise in the following way:

 

Z_LED = 1.7 V / 35 mA ~ 50 ohm

 

equivalent PD current noise = (I_PD / I_LED) * (measured supply voltage noise / 50 ohm)

 

where the PD-LED current ratio was found empirically to be (I_PD / I_LED) ~ 1 / 1000 by measuring the voltage out of the amp with full brightness (i.e. I_LED = 35 mA, no obstruction) and dividing by the transimpedance (see 2nd figure).

 

The third figure below is a plot of these spectral densities in common units. Somewhat expectedly, the noise of the "dark" configuration seems limited by the supply noise. However, the "bright" line seems to be dominated by something else. I'm not sure I see how it could be anything but the LED itself, but it is worthwhile to repeat this "test" with a better setup.

 

On the to-do list:

 

1. Voltage regulator/reference

Rana thinks that the AD587LN is a good choice of reference given its performance on some LISA tests. I am in contact with AD, and there is no longer a 'LN' package, but I am trying to get samples of the currently manufactured one that is most similar (AD587KNZ).

In the meantime, I am going to find some simple regulators downstairs or at the 40m.

 

2. Bandpass filter

I was advised that it is a good idea to build your own high/bandpass filter instead of relying on the spectrum analyzer's AC coupling function. I will be doing just this.

 

3. Switch to a better op amp

        Like the AD743

 

4. Calibration

I need to find a good way to hold the OSEM in place while I stick something in there with a micron drive without it being unreliably shaky.

 

 

 

 

 

 

Attachment 1: schematic.jpg
schematic.jpg
Attachment 2: 2009-11-12_17.34.58.jpg
2009-11-12_17.34.58.jpg
Attachment 3: noise_comparison.png
noise_comparison.png
  123   Tue Nov 17 21:23:08 2009 KojiMiscSUSAOSEM test progress

We have LT1021-7 at the 40m, next to the Alberto's desk. This is the VREF for 7V.

Quote:

1. Voltage regulator/reference

Rana thinks that the AD587LN is a good choice of reference given its performance on some LISA tests. I am in contact with AD, and there is no longer a 'LN' package, but I am trying to get samples of the currently manufactured one that is most similar (AD587KNZ).

In the meantime, I am going to find some simple regulators downstairs or at the 40m. 

 

  124   Thu Nov 19 03:41:12 2009 ZachMiscSUSAOSEM calibration

 Tonight, I calibrated the AOSEM's response in [A/m]. I used a sophisticated rig consisting of:

 

1. One of those anodized Faraday isolator mounts to hold the OSEM

 

2. A translation stage with a screw gauge to jam something into it (gracefully)

 

3. Some DC power supplies and multimeters

 

4. My simple transimpedance amplifier sketched in the previous post (I did not bother upgrading the readout circuit for this measurement since I was just taking a relatively rough DC measurement)

 

---------------

 

I used a 9/64 hex key to simulate the shadowmaking magnet. A picture of the setup is attached below.

 

Some pertinent info:

 

- The current through the LED was maintained at 35 mA throughout the measurement. The measured voltage across it was 1.62 V, giving Z_LED = 46.3 ohm.

 

- The op amp supply voltage was +/- 10 V, and the PD bias was +10 V.

 

- The output voltage of the amplifier with the PD fully lit was 3.04 V (measured before and after the test). Note that this voltage increases slightly as the key is inserted due to reflections. 

 

 

The second attachment is a plot of the photocurrent versus the position of the key (the x axis is shifted such that the key is roughly centered at x = 0, and x < 0 corresponds to the key being further inside). The response of the OSEM in the linear region is roughly 0.05 A/m.

Attachment 1: 100_0362.JPG
100_0362.JPG
Attachment 2: calibration_plot_11_18_09.png
calibration_plot_11_18_09.png
  125   Thu Nov 19 12:00:04 2009 ZachMiscSUSbandpass filter

 Attached is the transfer function for the bandpass filter I built for the AOSEM readout. The schematic is primitively outlined below. The corner frequencies are what they should be (HP: 100 mHz, LP: 1 kHz)

The Johnson noise for the 1 k resistor is roughly 4 nV/rt(Hz). Frank says that it doesn't make sense to use much lower than 1 k if I'm putting it into an SR785.

 

 

           10 uF                  1 k

      -------| |-------------/\/\/\/\/\/------------------

                        | |

        < |

         160 k  >        _|_  160 nF

        <        ---

        > |

        | |

                V                 V

 

Attachment 1: BPF_bode_11_18_09.png
BPF_bode_11_18_09.png
  126   Fri Nov 20 06:18:51 2009 ranaMiscSUSbandpass filter

seems OK, as long as the current noise doesn't get you. To make sure you have to terminate the input to this filter and then look at the resulting noise in the SR785.

  127   Sun Nov 22 16:02:09 2009 ZachMiscSUSAOSEM noise measurement

On Friday, Rana and I discovered that my transimpedance amp was oscillating like whoa at about 100 kHz. A little research showed this to be due to the input capacitance of the AD743 (~20 pF). To fix this, I put a 20-pF cap in parallel with the 100k feedback resistor, and that seemed to do the trick.

 

The relevant circuitry is shown in attachment 1. +/- 12 V DC was provided by voltage regulators (7912, 78M12). The voltage across the LED was measured to be V_LED = 1.61 V, and the current through it was I_LED = 31.6 mA, giving Z_LED = 50.9 ohm. The voltage out of the amp with a fully lit PD was V_out = -2.83 V, giving a photocurrent of I_ph = 28.3 uA. 

 

I was concerned about noise that might be imposed by the bandpass filter, so I compared spectra I took with and without it (that is, AC coupled, no BPF, and DC coupled, with BPF). This comparison is shown in attachment 2. There appears to be no difference apart from the aliasing effects at low frequency.

 

After this, I took the real measurement, extending the range to 800 Hz, averaging 100x and with a linewidth of 1 Hz (I realize now that I should probably have done this with a smaller linewidth, so that I could see below ~1 Hz. I will repeat the measurement this week with better low-frequency resolution). The result can be seen in attachment 3, calibrated to displacement noise in m/rt(Hz) using the measured 0.05-A/m response of the OSEM in the linear region. The four lines are:

 

- Bright: noise in the OSEM with a fully lit PD

- Dark: noise in the OSEM with the LED off

- Amp: noise in the transimpedance amp with the input terminated

- V_LED: noise in the LED voltage

 

The first three spectra were taken at the output of the amplifier and calibrated back to meters using the transimpedance gain and OSEM response in A/m. The last was taken across the LED, and calibrated into meters using the values given in paragraph 2. All measurements were taken with the OSEM under a box and with the lights out.

 

It appears that we are still limited by our setup. The "Dark" noise is coincident with the amplifier noise, while the "Bright" noise is coincident with the LED noise. That said, it is fairly comforting that all this noise is at the level of around 10^-10 m or less, as we can probably expect the true noise of the OSEM to be lower than this. We will know this for sure once we have a truly quiet setup (starting with ultra-low-noise voltage references).

Attachment 1: schematic.png
schematic.png
Attachment 2: BPF_noise_comparison.png
BPF_noise_comparison.png
Attachment 3: 0-800.png
0-800.png
  128   Tue Nov 24 12:28:52 2009 ZachMiscSUSAOSEM measurement update

  I was able to take some better measurements last night. I took data in two bands: 0-100 Hz, 0-1.6 kHz, each with 800 lines. This gives us a decent idea of what's going on at low and high frequency. Attached are four plots, two from each band. All measurements were taken with a box over both the OSEM and the readout circuit and the lights out.

The first two are low- and high- frequency comparisons of the noise in the full (bright) configuration as measured with no BPF and AC coupling vs with the BPF and DC coupling. There appears to be no difference apart from the expected effect above the pole at 1 kHz.

The next two are plots of the noise in various components and the full scheme calibrated into equivalent displacement noise. Everything is below ~10e-10 m/rt(Hz) with the exception of line peaks, and again it would appear that we are limited by our measurement equipment.

Some notes:

- The "dark" noise seems to be coincident with the "amp" noise with the exception of some extra pickup that increases at high frequency (seems to be line-related).

- The "LED" noise is coincident with the "supply" noise up until its 8-Hz corner frequency, after which it falls off as expected until it hits an apparent floor around 100 Hz.

- The "bright" noise seems to be coincident with the "supply" noise, while the "dark" and "amp" are much lower. This could be because the supply noise only shows up when there is an appreciable voltage at the output of the amp.

 

Have to think about this for a bit, but the next logical step is to turn the measurement setup into something solid (i.e. soldering, enclosure, etc.).

Attachment 1: BPF_low.png
BPF_low.png
Attachment 2: BPF_high.png
BPF_high.png
Attachment 3: OSEM_low.png
OSEM_low.png
Attachment 4: OSEM_high.png
OSEM_high.png
  129   Tue Nov 24 23:17:02 2009 ZachMiscSUSAD587KN voltage noise

I got a few AD587KN (high-precision 10V reference) samples today from AD. I hooked them up to see how much quieter my DC supply would be. The results are pretty good, with the voltage noise reduced by a factor of 5-10 throughout. The first two attachments below are comparisons of the noise in

1. The +12V regulator (MC78M12) alone

2. The AD785KN reference with V_in = +12 V provided by the regulator

3. The same as in 2, only now with an additional "noise reduction" capacitor (a 1-uF capacitor from pin 8 to ground forms a LPF with an internal 4-k resistor, giving a corner frequency of 40 Hz to reduce high-frequency noise),

plotted with the same frequency ranges and settings as those in the previous post.

The reference comes very close to its noise spec of 100 nV/rt(Hz) @ 100 Hz. The only issue is that it seems to have much more line pickup than the regulator (which seems almost completely insensitive to line noise), and this is worsened by the extra capacitor. Attachment 3 is a close-up of the low-frequency spectrum around 60 Hz. I suspect that this will be alleviated somewhat when I move away from the breadboard phase.

I want to rig this up so that I can stabilize the supply voltage to the transimpedance amp and LED, but in order to do so I will need to build a higher-current source using a power transistor, like either of those shown in attachment 4 (the AD587LN is only able to provide <10mA). 

Attachment 1: ref_noise_low.png
ref_noise_low.png
Attachment 2: ref_noise_high.png
ref_noise_high.png
Attachment 3: line_noise.png
line_noise.png
Attachment 4: high_current.png
high_current.png
  130   Thu Nov 26 02:59:35 2009 ranaElectronicsSUSLED Driver circuit

We want to have a simple low noise circuit to drive the LED. Our plan is to use the AD587 followed by a filter/buffer.

Requirements:

from 0.1-10 Hz, produce less RIN in the LED light than shot noise by a factor of 3.

With 35 mA of LED drive, we get ~35 uA of photocurrent (no magnet/flag). The shot noise of 35 uA is ~3.5 pA/rHz.

So the RIN from shot noise is 1e-7. So we shoot for a RIN of 3e-8 from the LED.

 

The AD587 voltage reference has a relative noise of 1e-7 at 0.1 under very good conditions (perhaps our vacuum system will be so kind). So we have to get a factor of 3 filtration at 0.1 Hz.

The following circuit should it for us: its a 2nd order Butterworth implemented in a Sallen-Key configuration. The noise is reasonable and the cutoff frequency is so low (0.03 Hz) because of the latest in capacitor technology.

We can buy metal poly caps which are as large as 47uF and have a reasonable physical size and tolerance and noise.

On page 2 of the plot you can see that the noise performance of this filter is limited by the input voltage noise of the FET opamp (op1) (AD743 - soon to be obsolete). The noise of the BUF634 (op2) is insignificant in this configuration. What we really need to make this good is a part with just as good of an input current noise spec as the AD743 but 3x less voltage noise at 0.1 Hz. I offer one cookie to whomever can find an opamp that fits those parameters.

These images show the circuit diagram (left) and the proto setup (right):

IMG_0232.JPGIMG_0226.JPG

update: added a 230 Ohm series resistor between the BUF634 output and the LED to step the voltage down to the 1.7V that the LED wants.

Attachment 3: sallenkey2.pdf
sallenkey2.pdf sallenkey2.pdf
  131   Thu Nov 26 15:17:01 2009 KojiElectronicsSUSLED Driver circuit

I found a Quad Opamp OP497 (neither dual nor single!), but this is not enough to expel AD743.
Dis-continuation of OP497 is also close except for one SMD package.

AD743 (reference):
LF Voltage Noise: 0.38 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 2.9 nV/√Hz @ 10 kHz
Current Noise: 6.9 fA/√Hz @ 1 kHz

OP497:
LF Voltage Noise: 0.3 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 15 nV/√Hz @ 1kHz
Current Noise: 5 fA/√Hz @ 1kHz

Quote:

What we really need to make this good is a part with just as good of an input current noise spec as the AD743 but 3x less voltage noise at 0.1 Hz. I offer one cookie to whomever can find an opamp that fits those parameters.

  132   Mon Nov 30 18:53:28 2009 ZachElectronicsSUSLED Driver circuit

Not having much luck. I found the LT1028, which has 10x better low-frequency voltage noise (35 nVpp, 0.1 Hz to 10 Hz), but its current noise is worse by a ridiculous factor of 1000:

Screen_shot_2009-11-30_at_6.51.02_PM.png           Screen_shot_2009-11-30_at_6.51.16_PM.png

 

 

Quote:

I found a Quad Opamp OP497 (neither dual nor single!), but this is not enough to expel AD743.
Dis-continuation of OP497 is also close except for one SMD package.

AD743 (reference):
LF Voltage Noise: 0.38 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 2.9 nV/√Hz @ 10 kHz
Current Noise: 6.9 fA/√Hz @ 1 kHz

OP497:
LF Voltage Noise: 0.3 V pp, 0.1 Hz to 10 Hz
Voltage Noise: 15 nV/√Hz @ 1kHz
Current Noise: 5 fA/√Hz @ 1kHz

Quote:

What we really need to make this good is a part with just as good of an input current noise spec as the AD743 but 3x less voltage noise at 0.1 Hz. I offer one cookie to whomever can find an opamp that fits those parameters.

 

Attachment 1: Screen_shot_2009-11-30_at_6.51.02_PM.png
Screen_shot_2009-11-30_at_6.51.02_PM.png
  133   Tue Dec 1 00:49:19 2009 KojiElectronicsSUSLED Driver circuit

The situation is well illustrated in the following application note of Analog Devices:

Low Noise Amplifier Selection Guide for Optimal Noise Performance

Even though the graph is created for 1kHz, it is very clear that AD743
has superb performance for high source impedance purposes:
combination of low current noise and low voltage noise.

If the source impedance of Rana's circuit (400kOhm@DC) are reduced to 20K or so,
OP-27 type OPamp can come into the scope. However this means we need to use 10 times
larger capacitors. This is almost impossible for now, though innovation on the caps
can change the situation.

LT1028 is an AD797 type opamp. This works greatly with the smaller source impedance.

Attachment 1: AN_940_Page_08.png
AN_940_Page_08.png
  134   Wed Dec 2 18:29:07 2009 ZachMiscSUSLow-noise LED driver

 Yesterday, I rebuilt Rana's low-noise LED driver in the Bridge elab. It is based on a 2nd-order active lowpass filter (using the Sallen-Key topology). The schematic is shown below. The circuit is essentially the same as the one Rana posted a few days ago, only the R and C values are all around twice what they are in his schematic. This results in the same corner frequency of fc = 0.03 Hz.

 

schematic.png

 

I hooked it up and measured Vsk,out = 9.76 V. I then used it to drive a 50-ohm resistor, and measured VLED = 1.71 V, then measured the current to be ILED = 33.5 mA.

After ensuring that it was supplying the correct voltage, I hooked it up to the LED and took a spectrum of the voltage noise across it over the two frequency bands I have been using in previous posts. The following are comparison plots of the noise here and the noise with the simple RC filter used before, calibrated to displacement noise.

Low-freq:

noise_comp_low.png

RA: although these plots have Displacement in the y-axis, they are NOT measurements of actual displacement noise. They are estimates for the contribution to the displacement noise made by the LED RIN based on measurements of the voltage noise across the LED.

High-freq:

noise_comp_high.png

Something is clearly wrong: not only is the new configuration worse at lower frequencies, but the rolloff seems to go as 1/f and not 1/f2. Investigating after dinner...

 

  135   Thu Dec 3 02:35:39 2009 ranaMiscSUSLow-noise LED driver

 

 OP27 current noise is too high - use AD743.

  136   Mon Dec 7 02:59:38 2009 ZachMiscSUSLED Driver noise (with AD743)

 I retook the measurement from my last post, this time using an AD743 in place of the OP27 (per Rana's comment). The results are below.

Low-frequency:

noise_comp_low.png

RA: Although it says m/rHz, this is not measured displacement noise, but rather estimated displacement noise due to the LED noise. The previously measured conversion from the LED RIN to apparent displacement is used to convert from the voltage noise of the LED driver to the contribution to the OSEM's displacement readout.

High-frequency:

noise_comp_high.png

Seems better than before, but not quite what expected. I observed that the transfer function of the S-K filter was what it should be up until a decade or two above the corner frequency, after which it appeared to spring zeroes out of nowhere and level off at high frequency. I tried to see what would happen if I changed the resistor values, and the following plot is what I got.

S-K_respons_vs_R.png

This plot seems familiar from my electronics courses, but I haven't put a finger on what is causing this behavior yet. I'm sure that the answer is somewhere in H&H (or in the brain of a kind soul who happens to be reading this--wink wink).

  137   Mon Dec 7 12:01:14 2009 ranaMiscSUSAOSEM LED Driver noise (with AD743)

The low frequency noise looks pretty good now. The funny shape is most likely a thermal transient due to having not enough insulation. You need to droop some Kleenex over the circuit to stop the thermal air currents and then put a second box over the first box. Then its probably best to sit outside of the room when taking the measurement to reduce the human noise.

  138   Tue Dec 8 10:07:39 2009 ZachElectronicsSUSSallen-Key filter attenuation limit

 Last night, I was going to retake the noise measurement with the added elements that Rana suggested, but instead I spent a ridiculous amount of time trying to figure out what is going on with my Sallen-Key filter. I now know a lot more about their limitations, but am still at a loss as to what is happening in this case. The problem is that no one seems to be using a corner frequency anywhere near this low (at least not anyone trying to explain these filters). The following is a comparison of an ideal 2nd-order Butterworth filter and a real one using an LT1464.

butterworth_limitation.png

The filter behaves as expected a ways past the corner frequency, rolling off at 40 dB/decade. As the signal increases in frequency, the capacitors' impedances decrease, until (at point 'b'), they fall below the output impedance of the amp, causing the response to climb at 40 dB/decade. This happens for a short while, until the unity-gain bandwidth frequency (~ 1-10 MHz, 4 MHz for the AD743) of the op amp is reached, and the filter can attenuate no more ('c'), so the response flattens out to 0 dB/decade.

Different component values affect the high-frequency behavior of the filter, as shown below.

S-K_different_components.png

 

This makes sense, since with smaller capacitors it takes a higher frequency to fall below the output impedance of the amp. In any case, though, the final flatline always happens near the UGB frequency. The following is a plot of the transfer functions I measured (also in last post). I did not change the C values--only the R's--so the corner frequency is different in each case. What I observe is some R-dependent maximum attenuation, which sets on well before the 4-MHz UGB frequency of the AD743. The strangest part is that for small enough R this maximum "attenuation" is actually a positive gain.

S-K_respons_vs_R.png

I suppose it is not a huge deal, as I can increase the stopband attenuation as R while only increasing the Johnson noise as 2 * sqrt(R), but it would be nice to get some insight into what 's going on.

NOTE: While it appears that the high-frequency flatline in the other plots occurs for a different reason, it still seems to be R-dependent. I could not find any explanation for what determines this asymptotic behavior.

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