I noticed that ASDC level changes depending on the angle of ITMY when trying to take some data for loss map of YARM. We finally found that ASDC level behaves strangely when the angle of ITMY in yaw direction is varied, as you can see in Attachment 1. Now, AS port recieved only the reflection of ITMY.
NOTE: This behavior indicates that angular motion could couple to length signal in AS port.
Koji suggested that this behavior might be caused by interference at SR2 or SR3 between main path light and the light reflected by the AR surface. By rough estimation, we confirmed that this scenario would be possible. So it would be better to measure AR reflection of the same mirror to ones used for SR2 and SR3 in term of incident angle.
Ed by KA: This senario could be true if the AR reflection of teh G&H mirrors have several % due to large angle of incidence. But then we still need think about the overlap between the ghost beam and the main beam. It's not so trivial.
Due to the strange behavior (elog 11815) of ASDC level, we checked if it is possible to use POYDC instead of ASDC to measure the power of reflected light of YARM. Attached below is the spectrum of them when the arm is locked. This spectrum shows that it is not bad to use POYDC, in terms of noise. The spectrum of them when ETMY is misaligned looked similar.
So I am going to use POYDC instead of ASDC to measure arm loss of YARM.
Ed by KA:
The spectra of POYDC and ASDC were measured. We foudn that they have coherence at around 1Hz (good).
It told us that POYDC is about 1/50 smaller than ASDC. Therefore in the attached plot, POYDC x50 is shown.
That's the meaning of the vertical axis unit "ASDC".
Tonight I measured "loss map" of ETMY. The method to calculate round trip loss is same as written in elog 11810, except that I used POYDC instead of ASDC this time.
How I changed beam spot on ETMY is: elog 11779.
I measured round trip loss for 5 x 5 points. The result is below.
494.9 +/- 7.6 356.8 +/- 6.0 253.9 +/- 7.9 250.3 +/- 8.2 290.6 +/- 5.1
215.7 +/- 4.8 225.6 +/- 5.7 235.1 +/- 7.0 284.4 +/- 5.4 294.7 +/- 4.5
205.2 +/- 6.0 227.9 +/- 5.8 229.4 +/- 7.2 280.5 +/- 6.3 320.9 +/- 4.3
227.9 +/- 5.7 230.5 +/- 5.5 262.1 +/- 5.9 315.3 +/- 4.7 346.8 +/- 4.2
239.7 +/- 4.5 260.7 +/- 5.3 281.2 +/- 5.8 333.7 +/- 5.0 373.8 +/- 4.9
The correspondence between the loss shown above and the beam spot on ETMY is shown in the following figure. In the figure, "downward" and "left" indicate direction of shift of the beam spot when you watch it via the camera (ex. 494.9 ppm corresponds to the lowest and rightest point).
Edited below on 28th Nov.
To shift the beam spot on ETMY, I added offset in YARM dither loop. The offset was [-30,-15,0,15,30]x[-10,-5,0,5,10] for pitch and yaw, respectively. How I calibrated the beam spot is basically based on elog 11779, but I multiplied 5.3922 for vertical direction and 4.6205 for horizontal direction which I had obtained by caliblation of oplev (elog 11785).
Edited above on 28 th Nov.
I will report the detail later.
Here, I upload data I took last night, including the power of reflected power (locked/misaligned) and transmitted power for each point (attachement 1).
And I would like to write about possible reason why the loss I measured with POYDC and the loss I measured with ASDC are different by about 60 - 70 ppm (elog 11810 and 11818). The conclusion I have reached is:
It could be due to the strange bahavior of ASDC level.
This difference corresponds to the error of ~2% in the value of P_L/P_M. As reported in elog 11815, ASDC level changes when angle of the light reflected by ITMY changes, and 2% change of ASDC level corresponds to 10 urad change of the angle of the light according to my rough estimation with the figure shown in elog 11815 and attachment 2. This means that 2% error in P_L/P_M could occur if the angle of the light incident to YARM and that of resonant light in YARM differ by 10 urad. Since the waist width of the beam is ~3 mm, with the 10 urad difference, the ratio of the power of TEM10 mode is , where . This value is reasonable; in elog 11743 Gautam reported that the ratio of the power of TEM10 was ~ 0.03, from the result of cavity scan. Therefore it is possible that the angle of the light incident to YARM and that of resonant light in YARM differ by 10 urad and this difference causes the error of ~2% in P_L/P_M, which could exlain the 60 - 70 ppm difference.
I found that TRY level degraded and the beam shape seen with CCD camera at AS port was splitted when the beam spot on ETMY was not close to the center. This was because dither started not working well. I suspect so because in such a case TRY level went up when I did iteration with TT1 and TT2 after freezing dither. Splitted beam shape indicates that incident light did not match well with the cavity mode.
TRY level for each point was this:
[[ 0.6573 0.8301 0.8983 0.8684 0.6773 ]
[ 0.7555 0.8904 0.9394 0.8521 0.6779 ]
[ 0.6844 0.8438 0.9318 0.8834 0.6593 ]
[ 0.7429 0.8688 0.9254 0.8427 0.6474 ]
[ 0.7034 0.8447 0.8834 0.8147 0.6966 ]]
In the worst case, TRY level was 70 % of the maximum level. Assuming that this degrade was totally due to the mode mismatch, this corresponds to ~50 urad difference between the angle of incident light and resonant lighe in the arm (see elog 11819).
It might have, so I think I need to estimate shift of beam spot more preciely.
According to Steve's drawing, radius of the hole of the baffle is 19.8 mm.
Intensity distribution of fundamental mode in x axis direction is this (y is integrated out):
With the radius of curvature of ETMY of 60 m and the arm length of 37.78 m, the beam width on ETMY is estimated to be 5.14 mm. From this expression of the intensity, , for example. If round trip loss is considered, these values are doubled.
Although maximum shift of beam spot from the ideal spot on ETMY is estimated to be sqrt(6.0^2+(1.7+1.7)^2)=6.9 mm, this value could have error of several tens of % because I am not sure to what exten the calibration is precise, which means that the maximum shift could be ~10 mm and seperation between the baffle and the beam could be ~10 mm.
Therefore, I need to check how much the beam spot shifts with another way, maybe with captured image of the CCD camera.
On VIDEO.adl, Image Capture and Video Capture did not seem to work and gave me some errors, so I fixed following two things:
1. just put one side of a USB cable to Pianosa the other side of which was connected to Sensoray; I don't know why but this was unconnected.
2. slightly fixed /users/sensoray/sdk_2253_1.2.2_linux/imsub/display-image.py as fpllows
L52: pix[j, i] = R, G, B -> pix[j, i] = int(R), int(G), int(B)
It seems to work, at least for some cameras including ETMYF and ITMYF.
With captured images of ETMYF, I measured the shift of the beam spot on ETMY.
The conclusions are:
the baffle would have almost no effect on loss map measurement and
the calibration of beam spot shift is confirmed to be not so bad.
What I did:
I captured ETMYF images in the cases that (i)beam spot is centered on ETMY, beam spot is at the rightest and lowest point of my loss map measurement (corresponding to [0,0] component of the matrix shown in elog 11818), and beam spot is at the leftest and highest point of my loss map measurement ([4,4] component). Each captured image is attached.
Then using ImageJ, I measured the shift of the beam spot. I calibrated lengh in horizontal direction and vertical direction with the diameter of the mirror.
The amount of the beam shift was 7.2 mm and 8.0 mm for each case.
These values indicate that clapping loss due to the baffle is less than 10 ppm in a round trip.
Today's results support the previous calibration with oplev, which says the amount of the beam shift is 7.0 mm. Two values derived by different calibrations coincide within ~10 % though they are totally different methods. This also support the calibration of the oplev for ETMY (elog 11785) indirectly.
With the same method as reported in elog 11785, I calibrated oplevs for ITMX/ETMX.
According to this measurement, ratio of the calibration factor derived with this measurement (NEW) and the calibration factor for now (OLD), i.e. NEW/OLD was:
The calibration factors of the oplevs for ETMY/ITMY are NOT UPDATED YET. I updated on Dec 11, 2015
I disconnected the cable that was connected to CH6 of the whitening filter in 1Y2, then connected POXDC cable to there (CH6). This channel is where POXDC used to connect.
Then I turned on the whitening filter for POXDC and POYDC (C1:LSC-POXDC FM1, C1:LSC-POYDC FM1) and changed the gain of analog whitening filter for POXDC and POYDC from 0 dB to 45 dB and from 0 dB to 39 dB, respectively (C1:LSC-POXDC_WhiteGain, C1:LSC-POYDC_WhiteGain).
I checked how POXDC level changes when the angle of ITMX is varied. ETMX was misaligned.
Then I found that in YAW direction the POXDC level is maximized but it doesnt have plateau, and in PIT direction it is not maximized so that it is at the slope and it doesnt have plateau, as shown in attached figures. These results indicate that the beam size on POX11 is not small enough compared to the size of the diode and it is not centered well.
I updated the figures. I think it's easier to read now.
To avoid the strange kicking of ETMX, I locked XARM with ITMX actuated instead of ETMX so that I changed elements of C1LSC_OUTPUT_MTRX; before: XARM=ETMX, after: XARM=ITMX.
And I change C1:LSC-XARM_GAIN from 0.007 to 0.022.
With this setup, I ran dither but then error signals of dithering oscillated as shown in the figure below.
Then I found that if C1:ASS-XARM_ETM_PIT_L_DEMOD_SIG_GAIN / C1:ASS-XARM_ETM_YAW_L_DEMOD_SIG_GAIN in C1ASS_LOCKINS_XARM.adl are changed as 0.200 -> 0.100 and 0.200 -> 0.100, respectively, the dithering works well.
But the burt file that is loaded when you let dithering "ON" is not changed, because now I don't know how to update a burt file. So, if you let dithering "ON", the dithering will run with the condition that C1:ASS-XARM_ETM_PIT_L_DEMOD_SIG_GAIN / C1:ASS-XARM_ETM_YAW_L_DEMOD_SIG_GAIN are not 0.100 but 0.200.
As I did for YARM (elog 11779), I measured the relation between offsets added just after the demodulation of the dithering loop of XARM and beam spot shift on ETMX. Defferent from YARM, the beam spot on ITMX DOES change because only BS is used as a steering mirror (TT1&2 are used for the dithering of YARM). Instead, the beam spot on BS DOES NOT change.
This time, I measured by oplevs the angles of both ETMX and ITMX for each value of offset, and using these angles I calculated the shift of the beam spot on ETMX so that I got two independent estimations (one from ETMX oplev, and the other from ITMX oplev) as shown below. The calibration of the oplevs reported in elog 11831 is taken into account.
The difference of two estimations comes from the error of calibration of oplevs and/or imperfect alignment, I think.
To focus POX beam on POX11 PD, I added an iris and a lens before POX11 PD as you can see in Attachment 1.
It seemed that the beam is well focused, but the behavior of POXDC has not changed, as shown in Attachments 2 & 3.
Now, the beam on POX11 PD is well centered and well focused.
We found out why POXDC had behaved as reported in elog 11839. There were a few reasons: the beam was not focused enough, hight of a mirror was not matched to the beam well, path of the light reflected by misaligned SRM was occasionally close to the path of POX beam.
Then, What we did is following:
- changed orientation of SRM slightly
- changed the hight of the mirror whose hight had not matched well, by changing the pedestal (hight of which mirror was changed is shown in Attachment 1.)
- put a lens with f=250 mm (where the lens is located is shown in Attachment 1.)
- refined alignment for the POX beam to hit on the center of POX11 PD.
As a result, POX DC level behaved as shown in Attachment 2&3 when the orientation of ITMX was varied (Attachment 2: POX DC vs ITMX PIT, Attachment 3: POX DC vs ITMX YAW).
You can see broad plateau when varied in both PIT and YAW directions, and the beam is at the center of the plateau if ITMX is aligned ideally.
Today image capture did not work again, though it had worked 3 days before. I also found that red indicator light on the front pannel of SENSORAY was not turned on, which had been turned on 3 days before (you can find SENSORAY on the floor near Pianosa). Possible reason that it did not work again was I restarted Pianosa last night. Anyway, it works now. Here I report what I did to make it work.
I ran thes commands in shell, following the instruction of the manual of SENSORAY 2253 (Page 5; link or you can find the manual in /users/sensoray; I put it there).
> cd /users/sensoray/sdk_2253_1.2.2_linux
> make all
> sudo make install
> modprobe s2253
Then the red light got turned on, and image capture worked.
If you recieve an error like "No such file or directory: /dev/video0" at the beginning of the error message when you run image capture scripts from the medm screen, or if you notice that the red indicator light of SENSORAY is not on, this procedure could help you.
I don't know if just running "modprobe" will work or not, because I didn't try it... When the same problem happens again, we can try just running "modprobe" first.
I added 1 line to one of the ASS scripts, UNFREEZE_DITHER.py like this:
L29> ez.cawrite('C1:ASS-'+dof+'_GAIN', 0)
The reason why I added this is: without this line, C1:ASS-'+dof+'_GAIN become larger that 1.0, which is nomial value, if you UNFREEZE DITHER when the dither is already running or C1:ASS-'+dof+'_GAIN is not 0.0.
On the day before yesterday and in this morning, I measured loss map of ETMX. I reported the method I used to change the beam spot on ETMX below.
Round trip loss was measured for 5 x 5 points. The result is below.
455.4 +/- 21.1 437.1 +/- 21.8 482.3 +/- 21.8 461.6 +/- 22.5 507.9 +/- 20.1
448.4 +/- 20.7 457.3 +/- 21.2 495.6 +/- 20.2 483.1 +/- 20.8 472.2 +/- 19.8
436.9 +/- 19.3 444.6 +/- 19.7 483.0 +/- 19.5 474.9 +/- 20.9 498.3 +/- 18.7
454.4 +/- 18.7 474.4 +/- 20.6 487.7 +/- 21.4 482.6 +/- 20.7 487.0 +/- 19.9
443.7 +/- 18.6 469.9 +/- 20.2 482.8 +/- 18.7 480.9 +/- 19.5 486.1 +/- 19.2
The correspondence between the loss shown above and the beam spot on ETMX is shown in the attached figure. In the figure, "up" and "right" indicate direction of shift of the beam spot when you watch it via the camera (ex. 455.4 ppm corresponds to the highest and rightest point in the view via the camera).
This result is consistent withe previous result of 561.19 +/- 14.57 ppm ericq got with ASDC and reported in elog 10248 if the discussion I reported in 11819 is taken into account. Elog 11819 says in short that the strange behavior of ASDC could give us 60-70 ppm error.
The reason why the error is larger than that of the measurement for ETMY is that the noise of POX is larger than that of POY. But I am not sure to what extent the statistical error needs to be reduced.
How I shifted the beam spot on ETMX:
Basically, the method was same as one used for Y arm. Different point is: for Y arm we have two steering mirrors TT1&2, but for X arm we have only one steering mirror BS. Then in order to shift incident beam so that the beam spot on ITMX does not change, I ran the dithering of X arm as well as that of Y arm and added offsets to both dither loops that caused same amount of shift on ETMX and ETMX. Thanks to the symmetry between X arm and Y arm, the dithering of Y arm ensured that the beam spot on ITMX was unchanged as well as that of ITMY. The idea of this method is schematically shown in Attachment 2.
The calibration of how much the beam spot shifted is based on the results of elog 11846 . The offset was [-15,-7.5,0,7.5,15]x[-5,-2.5,0,2.5,5] for pitch and yaw, respectively.
I changed the snapshot file for ASS, /opt/rtcds/caltech/c1/scripts/ASS_DITHER_ON.snap as follows:
L124 > C1:ASS-XARM_ETM_PIT_GAIN 1 -5.000000000000000e-02
=> C1:ASS-XARM_ETM_PIT_GAIN 1 -1.500000000000000e-02
L128> C1:ASS-XARM_ETM_YAW_GAIN 1 5.000000000000000e-02
=> C1:ASS-XARM_ETM_YAW_GAIN 1 1.500000000000000e-02
The purpose of this change is to avoid the oscillation when the dithering of X arm is running.
Here I explain usage of my scripts for loss map measurement. There are 7 script files in a same directory /opt/rtcds/caltech/c1/scripts/lossmap_scripts. With these scripts, round trip loss of an arm cavity with the beam spot on one mirror shifted to 5x5 (option: 3x3) points is measured. You can choose on which cavity you measure, the beam spot on which mirror you shift, and maximum shift of the beam spot in vertical and horizontal direction.
To start measurement from the beginning
Run the following command in an arbitrary directory and you will get several text files including the result of loss map measurement:
> python /opt/rtcds/caltech/c1/scripts/lossmap_scripts/lossmap.py [maximum shift in mm (PIT)] [maximum shift in mm (YAW)] [arm name (XorY)] [mirror name (E or I)]
Optionally, you can add "AUTO" at the end of the above command. Without "AUTO", you will be asked if the dithering has already settled down or not after each shift of the beam spot and you can let the scripts wait until the dithering settles down sufficiently. If you add "AUTO", it will be judged if the dithering has settled down or not according to some criteria, and the measurement will continue without your response to the terminal.
The files to be created in the current directory by the scripts are:
- lossmapETMX1-1.txt # [POX power (locked)] / [POX power (misaligned)]
- lossmapETMX1-2.txt # standard deviation of [POX power (locked)] / [POX power (misaligned)]
- lossmapETMX1-3.txt # TRX
- lossmapETMX1-1_converted.txt # round trip loss (ppm) calculated from lossmapETMX1-1.txt
- lossmapETMX1-1_converted_sigma.txt # standard deviation of round trip loss calculated from 1-1.txt and 1-2.txt
- lossmapETMX_result.txt # round trip loss and its error in a clear form.
The name of the files would be "lossmapITMY1-1.txt" etc. depending on which mirror you have chosen.
To restart measurement from a certain point
Run the following command in a directory containing "lossmap(mirror name)1-1.txt", "lossmap(mirror name)1-2.txt" and "lossmap(mirrorname)1-3.txt" which are created by previous not-completed measurement:
> python /opt/rtcds/caltech/c1/scripts/lossmap_scripts/lossmap.py [maximum shift in mm (PIT)] [maximum shift in mm (YAW)] [arm name (XorY)] [mirror name (E or I)] [restart point (PIT)] [restart point (YAW)]
You can also add "AUTO".
How to designate the restart point:
Matrix elements of output of this measurement procedure are characterized by a pair of two numbers as the following shows.
(-1,-1) -> (-1,-0.5) -> (-1,0) -> (-1,0.5) -> (-1,1)
(-0.5,1) <- (-0.5,0.5) <- (-0.5,0) <- (-0.5,-0.5) <- (0.5,-1)
(0,-1) -> (0,-0.5) -> (0,0) -> (0,0.5) -> (0,1)
(0.5,1) <- (0.5,0.5) <- (0.5,0) <- (0.5,-0.5) <- (0.5,-1)
(1,-1) -> (1,-0.5) -> (1,0) -> (1,0.5) -> (1,1)
Please write the numbers that correspond to the matrix element you want to restart at. Arrows show the order of sequence of measurement. About the correspondence between the matrix elements and real position on the ETMY and ETMX, see elog 11818 and 11857, respectively.
This script will overwrite the files (~1-1.txt etc.) so it is safer to make backup of the files before you run this script.
Some notes on the scripts and measurement
- Calibration has been done only for ETMs, i.e. for ITMs unit of [maximum shift] is not mm, but the values written in [maximum shift] equal to the maximum offsets added just after demodulation of ASS loop (ex. C1:ASS-YARM_ITM_PIT_L_DEMOD_I_OFFSET).
- It should be checked before doing measurement if the following parameters are correct or not.
POXzero (L47 in lossmapx.py and L52 in lossmapx_resume.py: the value of C1:LSC-POXDC_OUTPUT when no light injects into POXPD.)
POYzero (L45 in lossmapy.py and L50 in lossmapy_resume.py: the value of C1:LSC-POYDC_OUTPUT when no light injects into POYPD.)
mmr (L11 in lossmap_convert.py: (mode matching carrier power)/(total power))
Tf (L12 in lossmap_convert.py; transmittivity of ITM)
Tetm (L13 in lossmap_convert.py: transmittivity of ETM in ppm)
- Changing n (L50 in lossmap.py) from 5 to 3, the grid points will be 3x3 changed from the default value of 5x5. If 3x3, the matrix elements are characterized by
(-1,-1) -> (-1,0) -> (-1,1)
(0,1) <- (0,0) <- (0,-1)
(1,-1) -> (1,0) -> (1,1)
similarly to the case of 5x5.
- You can copy the directory lossmap_scripts anywhere in controls and use it. These scripts will work as long as all the 7 scripts exist in a same directory.
I estimated power recycling gain with the results of arm loss measurement.
From elog 11818 and 11857, round trip losses including transmittivity of ETM of Y arm and X arm (let us call them and ) are 229+13.7=243 ppm and 483+13.7=495 ppm, respectively.
How I calculated:
I used the following formula.
Amplitude reflectivity of an arm cavity :
(see elog 11816)
Amplitude reflectivity of FPMI :
With power transmittivity of PRM and amplitude reflectivity of PRM , power recycling gain is
I assumed , , and , and then I got
PRG = 9.8.
Since both round trip losses have relative error of ~ 4 % and PRG is proportional to inverse square of up to the leading order of it, relative error of PRG can be estimated as ~ 8 %, so PRG = 9.8 +/- 0.8.
According to elog 11691, which says TRX and TRY level was ~125 when DRFPMI was locked, power recycling gain was at the last DRFPMI lock.
Measured PRG is lower than PRG estimated here, but it is natural because various causes such as mode mismatch between PRC mode and arm cavity mode, imperfect contrast of FPMI, and so on could decrease PRG, which Eric suggested to me.
Added on Dec 9
If were as small as , PRG would be 16.0. PRC would be still under coupled.
I did additional tests for the strange behavior of ASCD. ETMY, ETMX and ITMY were misaligned so that only light reflected by ITMX went into AS port. I had done similar measurement before with ITMY YAW varied.
Attachment 1 shows how ASDC level changed when ITMX PIT varied.
Attachment 2 shows how ASDC level changed when ITMX YAW varied.
Attachment 3 shows how the power of light measured by a power meter just after the AS view port varied when ITMX YAW varied.
Comparing 1 & 2, we can say that this behavior is not unique to YAW direction.
From 2 & 3, we can say something strange is happening inside the chamber.
To check if the strange behavior of ASDC is caused by SR2/SR3 or not, I did the following measurement:
ASDC measures the power of the light reflected by ITMX. POXDC measures the power of the light reflected by ITMX and SRM successively. Then I varied the angle of ITMX in YAW direction and compared the behaviors of ASDC and POXDC.
The results are shown in Attachments 1-3.
As you can see in these figures, the strange up-and-down behavior appeared ONLY in ASDC. Therefore, the cause of this behavior exists between AS table and SRM (I had confirmed that the angle of SRM did not affect ASDC).
And this behavior is fringe-like, as can be seen in the figures (there seems to be 3 "peaks" and 2 "valleys"), so the cause could be interference between main path and not good AR reflection at a mirror after SRM before AS table (I suspect a mirror is flipped mistakenly).
I took PR3 AR reflectivity and calculated PRG (PR3 is flipped and so AR surface is inside PRC).
As shown in attached figure, which shows AR specification of the LaserOptik mirror (PR3 is this mirror), AR reflectivity of PR3 is ~0.5 %. Since resonant light in PRC goes through AR surface of PR3 4 times per round trip, round trip loss due to this is ~2 %. Then I got
PRG = 7.8.
Attached is the plot of relation between the average arm round trip loss and power recycling gain. 2 % loss due to PR3 AR reflection is taken into account.
Based on calibration measurement I have done (elog 11785, 11831), I updated calibration factors of oplevs on medm screen as follows. Not to change loop gain oplev servo, I also changed oplev servo gain.
(45.1,16) => (200,3.5)
(85.6,8) => (222,3.0)
(26,-16) => (140,-3.0)
(31,-21) => (143,-4.5)
(110,8) => (122,7.2)
(81,-11) => (147,-6)
(159,15) => (239,10)
(174,-21) => (226,-16)