Kiwamu and I noticed that there is a ghost beam on the green beam going into the ETM. What we see is some interference fringes on the edge of the transmission of the green beam through the dichroic beam splitter (DCBS). If we look at the reflection from the dichroic beam splitter these are much more pronounced.
The spacing of the fringes (about 2 per 10mm) indicates an angle between the fields of around 0.1 mrad.
We were able to cause significant motion of the fringes by pushing on the knobs of the steering mirrors that steer the beam into the DCBS. A rough calculation of the derivative of optical path difference between the ghost and the primary beam as a function of input angle gives about 15 microns per mrad. What filtering the effect the arm cavity will have on the ghost beam is not immediately clear, but the numbers shouldn't be too difficult to determine.
I somehow screwed up the PDH box at the X end station.
Right now it's not working, so I am going to check and fix it today.
In the last evening I found that one of the gain stages on the PDH box wasn't fully functional.
So I started investigating it and I though it was going to finish soon, but actually it wasn't so easy.
The PDH box has several gain stages. So an input signal goes through a buffer, a filter, a boost and an output buffer stages sequentially.
The boost stage is supposed to have gain of 10, but I found it didn't have such gain.
In fact the gain was something like -30dB which is pretty small. Plus this boost stage was imposing an wired bump on the transfer function around 50 kHz.
I checked the voltages on some components around the boost stage and confirmed there were no strange voltage.
Then I suspected that the op-amp : LF356 had been broken for some reason. So I replaced it by LT1792 to see if it fixes the issue.
Indeed it did make it functional. However after few minutes of the replacement, it went back to the same bad condition.
I have no idea about what was going on at that time. Anyway it needs more careful investigations.
I temporarily put a jumper cable on the board to skip this stage, but now the PDH lock is not healthy at all.
I made a noise budget for the ALS noise measurement that I did a week ago (see #4352).
I am going to post some details about this plot later because I am now too sleepy.
Here I explain how I estimate the contribution from the differential noise shown in the plot on my last entry (#4376) .
According to the measurement done about a week ago, there is a broadband noise in the green beatnote path when both Green and IR are locked to the X arm.
The noise can be found on the first plot on this entry (#4352) drawn in purple. We call it differential noise.
However, remember, the thing we care is the noise appearing in the IR PDH port when the ALS standard configuration is applied (i.e. taking the beatnote and feeding it back to ETMX).
So we have to somehow convert the noise to that in terms of the ALS configuration.
In the ALS configuration, since the loop topology is slightly different from that when the differential noise was measured, we have to apply a transfer function to properly estimate the contribution.
(How to estimate)
It's not so difficult to calculate the contribution from the differential noise under some reasonable assumptions.
Let us assume that the MC servo and the end PDH servo have a higher UGF than the ALS, and assume their gains are sufficiently big.
Then those assumptions allow us to simplify the control loop to like the diagram below:
Since we saw the differential noise from the beatnote path, I inject the noise after the frequency comparison in this model.
Eventually the noise is going to propagate to the f_IR_PDH port by multiplying by G/(1+G), where G is the open loop transfer function of the ALS.
The plot below shows the open loop transfer function which I used and the resultant G/(1+G).
In the curve of G/(1+G), you can see there is a broad bump with the gain of more than 1, approximately from 20 Hz to 60 Hz.
Because of this bump, the resultant contribution from the differential noise at this region is now prominent as shown in the plot on the last entry (#4376).
I am going to post some details about this plot later
[Jenne, Chris, Kiwamu]
A photo diode and an AOM driver have been newly setup on the PSL table to measure the intensity noise coupling to the beat note signal.
We tried taking a transfer function from the PD to the beat, but the SNR wasn't sufficient on the PD. So we didn't get reasonable data.
(what we did)
- put a DCPD after the doubling crystal on the PSL table. The PD is sitting after the Y1 mirror, which has been used for picking off the undesired IR beam.
- installed the AOM driver (the AOM itself had been already in place)
- injected some signals onto the AOM to see if we can see an intensity fluctuation on the PD as well as the beat signal
In order to have better SNR for the intensity measurement, we put an AC coupled SR560 with the gain of 100 just before the ADCs.
When a single frequency signal was applied from a Stanford Research's function generator to the AOM, we could clearly see a peak at the doubled frequency of the injected signal.
Also a peak at the same frequency was found on the beat note signal as well.
But when random noise was injected from the same function generator, the random noise looked below the ADC noise.
Jenne adjusted the output voltage from the PD to about 1 V to avoid a saturation in the analog path, but later we realized that the ADC counts was marely ~ 20 counts.
So we will check the ADC tomorrow. Hopefully we will get a good SNR.
Noise below 10 Hz became larger again compared with the data before (see here #4352)
Note that the Y-axis is in MHz.
Here is a diagram for our intensity noise coupling measurement.
The below is a plot for the intensity noise on the DCPD. (I forgot to take a spectra of the PD dark noise)
For some reason, the RIN spectrum becomes sometimes noisier and sometimes quieter. Note that after 10 pm it's been in the quiet state for most of the time.
An interesting thing is that the structure below 3 Hz looks like excited by motion of the MC when it's in the louder state.
A photo diode and an AOM driver have been newly setup on the PSL table to measure the intensity noise coupling to the beat note signal.
Here is a new plot for the differential noise measurement. I plot a noise contribution from the intensity noise (brown curve).
If we believe this data, the differential noise is NOT dominated by the intensity noise of the PSL.
(intensity noise coupling measurement)
Here is a plot for the transfer functions (TFs) from the intensity noise DCPD to the beat signal.
In principle these TFs tell us how much intensity noise are contributed into the differential noise.
When I measured the spectra shown above, the frequency offset of the beatnote was at about 8 MHz from the zero cross point.
Keeping the same lock, I measured the transfer function (red curve) by using the swept sine technique on DTT. The setup for this measurement is depicted on the last entry (#4389).
Then I made the spectra above by multiplying the intensity spectrum by this TF.
Later I measured another transfer function when the beatnote was at about 2 MHz from the zero cross point.
According to this measurement, our MFD gets insensitive to the intensity noise as the beat offset goes close to the zero cross point. This is consistent with what we expected.
We are limited by the intensity noise of the X arm transmitted green light.
Since the intensity noise from the PSL wasn't big enough to explain the differential noise (#4392), so this time I measured the noise contribution from the X arm transmitted light.
I performed the same intensity noise coupling measurement, but this time between the DC signal of the beatnote RFPD and the beatnote signal.
While measuring it, I excited the intensity of the PSL laser by using the same AOM like I did yesterday. This AM cause the observable intensity noise on the beatnote RFPD.
With the excited AM, we can pretend to have an excited AM on the green transmitted light from the X arm, of course assuming the intensity noise coupling from the PSL is less.
The next steps we should do are :
We can modify the freq divider circuit to make it a comparator.
There are 3 standard techniques to reduce this effect:
1) Stabilize the end laser by sensing the green light coming into the PSL before recombination and feeding back with SR560 (this is the only one that you should try at first).
2) Moving to the center of the MFD fringe via ETM steps.
3) Auto-alignment of the beam to the arm.
Aidan: Joe and I have added a channel that takes the DC output from the vertex beatnote PD and sends it, via RFM, to a DAC at the ETMX end. Immediately before the output is a filter C1GCX_AMP_CTRL. The output of the DAC is connected to the CURRENT LASER DIODE modulation input on the back of the Innolight driver. This will modulate the current by 0.1A/V input.
We should be able to modulate the green laser on the end now and stabilize the intensity of the amplitude on the beatnote PD at the vertex. (In this configuration, the ampltiude noise of the PSL laser will be injected onto the end laser - we may want to revisit that).
Joe's comments on model change:
I added a RFM connection at the output of the C1:GCV-XARM_BEAT_DC filter in the c1gcv model. The RFM connection is called: C1:GCV-SCX_ETMX_AMP_CTRL.
This RFM connection goes to the c1scx model and into Kiwamu's GCX box, which uses top_names. There's a filter inside called AMP_CTRL, so the full filter name is C1:GCX-AMP_CTRL. The output then goes to the 7th DAC output.
The reason that I chose this PD is that, apparently, the green light coming from the cavity is clipped when it is picked off for its DC PD.
Ridiculous and hacky. Digital stabilization removed as well as the old "leave a pile of equipment on a stool" strategy.
We used a a BNC cable to send a pickoff of the beam before the recombination to the end via an SR560.
Prior to the works on the Y end setup I propose to perform the temperature scan business like Koji and Suresh did before (see this entry).
This business will allow us to easily find a beatnote at 532nm after the installation on the Y end.
I guess the right persons for this work are Bryan and Suresh.
Bryan will have a safety guidance from Steve in this after noon. So after that they can start working on it.
/* - - - coarse plan - - - */
* remove Alberto's laser from the AS table
* setup Alberto's laser on the PSL table
* put some stuff such as lenses, mirrors and etc. (Use the IR beam picked off after the doubling crystal for the main laser source)
* mode matching
Which laser are we going to use, Alberto's laser or MOPA laser ?
We use Alberto's laser for the Y end Green Locking.
Which laser are we going to use, Alberto's laser or MOPA laser ?
The reason for using Alberto's laser is that some amount of work has already gone into characterising its phase noise. Ref elog entry 2788
A rough time-table and the various tasks are given below:
Note: 700mW NPRO sitting on AP table (Model No: 126-1064-700, Sl No. 415) = Alberto's laser
Temperature dependence of frequency of Alberto's laser:
a) Shifting Alberto's Laser (AL) to the PSL table and setting up a beat frequency measurement between AL and PSL
b) Determining the frequency vs Temperature curve for the AL
Repositioning the optics on the Y-end table and relocating Alberto's laser ( at this point it will be rechiristened as Y-End-NPRO )
- Plan for this week
* Intensity stabilization for the end green laser (Matt / Kiwamu)
* Hand off the servo from Green to Red (Matt / Kiwamu)
* Y end green locking (Suresh / Bryan) (rough schedule)
* Reconnect the X end mechanical shutter to 1X9 (Kiwamu)
* Connect the end DCPD signal to a DAC (done)
* Make a LPF in a Pomona box for the temperature (Larisa)
* Clean up and finalize the X end setup (Kiwamu)
* Make a item lists for electronics. Order the electronics. (Aidan / Kiwamu)
I added a new ADC channel for a DC signal from the X end green PD.
It is called C1:GCX-REFL_DC and connected to adc_0_1, which is the second channel of ADC_0.
By the way, when I tried connecting it to an ADC I found that most of the channels on the AA board on 1X9 were not working.
Since the outputs form the board are too small the circuits may have benn broken. See the picture below.
In addition to that I realized that the signal from the PDH box for the temperature actuation is limited by +/- 2V due to the range of this AA board.
In fact the signal is frequently saturated due to this small voltage range.
We have to enlarge the range of this AA board like Valera did before for the suspensions (see this entry).
Some tasks for the daytime tomorrow.
* Beam profile measurements of the Y end laser (Suresh / Bryan)
* Taking care of CDS and the simulated plant (Jamie / Joe)
* Reconnect the X end mechanical shutter to 1X9 (Kiwamu)
* LPF for the X end temperature feedback (Larisa)
OK. Today we did the same type of measurement for the Y arm laser as was done for the X arm laser here: http://nodus.ligo.caltech.edu:8080/40m/3759
And attached here is a preliminary plot of the outcome - oddities with adding on the fitted equations, but they go as follows
(Red) T_yarm = 1.4435*T_PSL - 14.6222
(Blue) T_yarm = 1.4223*T_PSL - 10.9818
(Green) T_yarm = 1.3719*T_PSL - 6.3917
It's a bit of a messy plot - should tidy it up later...
It's a bit of a messy plot - should tidy it up later...
I'm going to take the easy question - What are the pink data points??
A comparator has been installed before the MFDs (mixer-based frequency discriminator) to eliminate the effect from the amplitude fluctuation (i.e. intensity noise).
As a result we reached an rms displacement of 580 Hz or 80 pm.
As a result we reached an rms displacement of 580 Hz or 80 pm.
(differential noise measurement)
Here is the resultant plot of the usual differential noise measurement.
The measurement has been done when the both green and red lasers were locked to the X arm.
In the blue curve I used only MFD. In the black curve I used the combination of the comparator and the MFD.
Noise below 3 Hz become lower by a factor of about 4, resulting in a better rms integrated from 40 Hz.
Note that the blue and the black curve were taken while I kept the same lock.
A calibration was done by injecting a peak at 311 Hz with an amplitude of 200 cnt on the ETMX_SUS_POS path.
Yesterday Koji modified his comparator circuit such that we can take a signal after it goes thorough the comparator.
The function of this comparator is to convert a sinusoidal signal to a square wave signal so that the amplitude fluctuation doesn't affect the frequency detection in the MFD.
I installed it and put the beat-note signal to it. Then the output signal from the comparator box is connected to the MFDs.
The input power for the comparator circuit has been reduced to -5 dBm so that it doesn't exceeds the maximum power rate.
And I'm going to answer the easy question - they're additional beat frequency temperature pair positions which seem to correspond to additional lines of beat frequencies other than the three highlighted, but that we didn't feel we had enough data points to make it worthwhile fitting a curve.
It's still not entirely clear where the multiple lines come from though - we think they're due to the lasers starting to run multi-mode, but still need a bit of thought on that one to be sure...
- Plan for tomorrow
* Video cable session (I need ETMY_TRNAS) (team)
* Characterization of the Y end laser (Bryan / Suresh)
* LPF for the X end laser temperature control (Larisa)
* Frequency Divider (Matt)
* X end mechanical shutter (Kiwamu)
Succeeded in handing off the servo from the green to the red.
This time we found that the fluctuation in the IR signals became lesser as the gain of the ALS servo increased.
Therefore I increased the UGF from 40 Hz to 180 Hz to have less noise in the IR PDH signal.
Here is a preliminary plot for today's noise spectra.
The blue curve is the ALS in-loop spectrum, that corresponds to the beat fluctuation.
The red curve is an out-of-loop spectrum taken by measuring the IR PDH signal.
Since the UGF is at about 180 Hz the rms is integrated from 200 Hz.
The residual displacement noise in the IR PDH signal is now 1.2 kHz in rms.
I am going to analyze this residual noise by comparing with the differential noise that I took yesterday (see the last entry ).
With the exception of a 2" mirror mount, I've confirmed that we have everything for the Y-end green production and mode-matching.
We need to calculate a mode-matching solution for the Lightwave laser so that it gives the correct beam size in the doubling crystal.
Additionally, Rana has suggested that we change the pedestals from the normal 1" diameter pedestal+fork combo to the 3/4" diameter posts and wider bases that are used on the PSL table (as shown in the attached image).
There was a 2" mirror mount among the spares on the PSL table. It has a window LW-3-2050 UV mounted in it. I
have moved it to the Y-end table. We seem to have run out of 2" mirror mounts ...
Just a quick update... the Lightwave laser has now been moved up to the end of the Y arm. It's also been mounted on the new mounting block and heatsinks attached with indium as the heat transfer medium.
A couple of nice piccies...
The good news is that we seem to be running in a linear region of the PSL laser with a degree or so of range before the PSL Innolight laser starts to run multi-mode. On the attached graph we are currently running the PSL at 32.26degrees (measured) which puts us in the lower left corner of the plot. The blue data is the Lightwave set temperature (taken from the display on the laser controller) and the red data is the Lightwave laser crystal measured temperature (taken from the 10V/degC calibrated diagnostic output on the back of the laser controller - between pins 2 and 4).
The other good news is that we can see the transition between the PSL laser running in one mode and running in the next mode along. The transition region has no data points because the PMC has trouble locking on the multi-mode laser output - you can tell when this is happening because, as we approach the transition the PMC transmitted power starts to drop off and comes back up again once we're into the next mode region (top left portion of the plot).
The fitted lines for the region we're operating in are:
Y_arm_Temp_meas = 0.95152*T_PSL + 3.8672
Y_arm_Temp_set = 0.87326*T_PSL + 6.9825
X_arm and Y_arm vs PSL comparison.
Just a quick check of the performance of the X arm and Y arm lasers in comparison to the PSL. Plotting the data from the X arm vs PSL and Y arm vs PSL on the same plot shows that the X arm vs the PSL has no observable trending of mode-hopping in the laser, while the Y arm vs the PSL does. Suspect this is due to the fact that the X arm and PSL are both Innolight lasers with essentially identical geometry and crystals and they'll tend to mode-hop at roughly the same temperatures - note that the Xarm data is rough grained resolution so it's likely that any mode-hop transitions have been skipped over. The Lightwave on the other hand is a very different beast and has a different response, so won't hop modes at the same temperatures.
Given how close the PSL is to one of the mode-transition regions where it's currently operating (32.26 degC) it might be worth considering shifting the operating temperature down one degree or so to around 31 degC? Just to give a bit more headroom. Certainly worth bearing in mind if problems are noticed in the future.
Kiwamu and I looked at all the electronics that are currently in place for the green locking on the X-arm and have made a set of block diagrams of the rack mounted units that we should build to replace the existing ... "works of art" that sprawl around out there at the moment.
1. "ETM Green Oscillator/PDH support box". Not a great name but this would provide the local oscillator signal for the end PDH (with a controllable phase rotator) as well as the drive oscillator for the end laser PZT. Since we need to hit a frequency of 216.075kHz with a precision that Kiwamu needs to determine, we'd need to be able to tune the oscillator ... it needs to be a VCO. It'd be nice to be able to measure the output frequency so I've suggested dividing it down by N times to put it into the DAQ - maybe N = 2^7 = 128x to give a measured frequency of around 1.7kHz. Additionally this unit will sum the PDH control signal into the oscillation. This box would support the Universal PDH box that is currently at the X-end.
2. "Vertex X-arm beatnote box" - this basically takes the RF and DC signals from the beatnote PD and amplifies them. It provides a monitor for the RF signal and then converts the RF signal into a square wave in the comparator.
3. "Mixer Frequency Discriminator" - just the standard MFD setup stored in a box. For temperature stability reasons, we want to be careful about where we store this box and what it is made of. That's also the reason that this stage is separated from the X-arm beatnote box with it's high-power amps.
4. RS232 and EPICS control of the doubling ovens
5. Intensity stabilization of the End Laser
P.S. I used Google Diagrams for the pictures.
Today we tried the Schmitt trigger DFD, and while it works it does not improve the noise performance. At least part of our problem is coming from the discrete nature of our DFD algorithm, so I would propose that an industrious day job person codes up a new DFD which avoids switching. We can probably do this by mixing the input signal (after high-passing) with a time-delayed copy of itself... as we do now, but without the comparator. This has the disadvantage of giving an amplitude dependent output, but since we are working in the digital land we can DIVIDE. If we mix the signal with itself (without delay) to get a rectified version, and low-pass it a little, we can use this for normalization. The net result should be something like:
output = LP2[ s(t) * s(t - dt) / LP1[ s(t) * s(t) ]],
where s(t) is the high-passed input and LP is a low-pass filter. Remember not to divide by zero.
I modified the c1gfd.mdl simulink model. I made a backup as c1gfd_20110325.mdl.
The first change was to use a top_names block to put everything in. The block is labeled ALS. So all the channels will now be C1:ALS-GFD_SOMETHING. This means medm channel names will need to be updated. Also, the filter modules need to be updated in foton because of this.
I then proceeded to add the suggested changes made by Matt. To avoid a divide by zero case, I added a saturation part which saturates at 1e-9 (note this is positive) and 1e9.
I measured some laser powers associated with the beat-note detection system on the PSL table.
The diagram below is a summary of the measurement. All the data were taken by the Newport power meter.
The reflection from the beat-note PD is indeed significant as we have seen.
In addition to it the BS has a funny R/T ratio maybe because we are using an unknown BS from the Drever cabinet. I will replace it by a right BS.
During my work for making a noise budget I noticed that we haven't carefully characterize the beat-note detection system.
The final goal of this work is to draw noise curves for all the possible noise sources in one plot.
To draw the shot noise as well as the PD dark noise in the plot, I started collecting the data associated with the beat-note detection system.
* Estimation and measurement of the shot noise
* measurement of the PD electrical noise (dark noise)
* modeling for the PD electrical noise
* measurement of the doubling efficiency
* measurement of an amplitude noise coupling in the frequency discriminators
In the last week Matt and I modified the MFD configuration because the mixer had been illegally used.
Since the output from the comparator is normally about 10 dBm, a 4-way power splitter reduced the power down to 4 dBm in each output port.
In order to reserve a 7 dBm signal to a level-7 mixer, we decided to use an asymmetric power splitter, which is just a combination of 2-way and 3-way splitter shown in the diagram above.
With this configuration we can reserve a 7 dBm signal for a mixer in the fine path.
However on the other hand we sacrificed the coarse path because the power going to the mixer is now 2.2 dBm in each port.
According to the data sheet for the mixer, 1 dB compression point for the RF input is 1dBm. Therefore we put a 1 dB attenuator for the RF port in the coarse system.
In the delay line of the fine path we found that the delay cable was quite lossy and it reduced the power from 2.2 dBm to about 0 dBm.
Using 2 dBm for a Level 7 mixer is so bogus, that I will dismantle this as soon as I come over.
PLEASE DO NOT DISMANTLE THE SETUP !
Actually we tried looking for a level-3 or a smaller mixer, but we didn't find them at that moment. That's why we kept the level-7 mixer for the coarse path.
As you pointed out we can try an RF amplifier for it.
Right. I've got a whole load of info and data and assorted musings I've been saving up and cogitating upon before dumping it into these hallowed e-pages. there's so much I'll probably turn it into a threaded entry rather than put everything in one massive page.
An overview of what's coming:
I started out using http://lhocds.ligo-wa.caltech.edu:8000/40m/Advanced_Techniques/Green_Locking?action=AttachFile&do=get&target=modematch_END.png as a reference for roughly what we want to achieve... and from http://nodus.ligo.caltech.edu:8080/40m/100730_093643/efficiency_waist_edit.png we need a waist of about 50um at the green oven. Everything else up to this point is pretty much negotiable and the only defining things that matter are getting the right waist at the doubling oven with enough available power and (after that point) having enough space on the bench to separate off the green beam and match it into the Y arm.
Step 1: Measure the properties of the beam out of the laser. Really just need this for reference later because we'll be using more easily measurable points on the bench.
Step 2: Insert a lens a few cm from the laser to produce a waist of about of a few 100um around the Faraday. Note that there's really quite a lot of freedom here as to where the FI has to be - on the X arm it's around columns 29/30 on the bench, but as long as we get something that works we can get it closer to the laser if we need to.
Step 3: After inserting the FI need to measure the beam after it (there *will* be some distortion and the beam is non-circular to begin with)
Step 3b: If beam is non-circular, make it circular.
Step 4: Insert a lens to produce a 50um waist at the doubling oven position. This is around holes 7/8 on the X arm but again, we're free to change the position of the oven if we find a better solution. The optical set-up is a little bit tight near that side of the bench on the X end so we might want to try aiming for something a bit closer to the middle of the bench? Depends how the lenses work out, but if it fits on the X end it will fit on the Y end.
RIght! Overview out of the way - now comes the trivial first bit
Step 1: Beam out of the laser - this will be tricky, but we'll see what we can actually measure in this set-up. Can't get the Beamscan head any closer to the laser and using a lambda/2 plate + polariser to control power until the Faraday isolator is in place. Using 1 inch separation holes as reference points for now - need better resolution later, but this is fine for now and gives an idea of where things need to go on the bench. The beam is aligned to the 3rd row up (T) for all measurements, the Beamscan spits out diameters (measuring only the 13.5% values) so convert as required to beam radius and the beam is checked to ensure a reasonable Gaussian profile throughout.
Position A1_13.5%_width A2_13.5%_width
(bench) (um mean) (um mean)
32 2166.1 1612.5
31 2283.4 1708.3
30 2416.1 1803.2
29 2547.5 1891.4
27 2860.1 2070.3
26 2930.2 2154.4
25 3074.4 2254.0
24 3207.0 2339.4
OK. As expected, this measurement is in the linear region of the beampath - i.e. not close to the waist position and beyond the Rayleigh length) so it pretty much looks like two straight lines. There's no easy way to get into the path closer to the laser, so reckon we'll just need to infer back from the waist after we get a lens in there. Attached the plot, but about all you really need to get from this is that the beam out of the laser is very astigmatic and that the vertical axis expands faster than the horizontal.
Not terribly exciting, but have to start somewhere.
Step 2: Getting the beam through the Faraday isolator (FI).
Started out with an f=100mm lens at position 32,T on the bench which gave a decent looking waist of order 100 um in the right sort of position for the FI, but after checking the FI specs, it's limited to 500W/cm^2. In other words, if we have full power from the laser passing into it we'd need a beam width of more than 211 um. Solution? Use an f=150mm lens instead and don't put the FI at the waist. I normally don't put a FI at a waist anyway, for assorted reasons - scattering, thermal lensing, non-linear magnetic fields, the sharp changing of the field components in an area where you want as constant a beam as possible. Checked with others to make sure they don't do things differently around these parts… Koji says it doesn't matter as long as it passes cleanly through the aperture. So… next step is inserting the Faraday.
The beam profiles in vertical and horizontal around the FI position with the f=150mm lens in place are attached. Note that the FI will be going in at around 0.56m.
I fired up some old waistplotter routines, and set the input conditions as the measured waist after the lens and used that to work out what the input waist is at the laser. It may not be entirely accurate, but it /will/ be self consistent later on.
Vertical waist = 105.00 um at 6.282 cm after laser output (approx)
Horizontal waist = 144.63 um at 5.842 cm after laser output (approx)
Step 3: Inserting FI and un-eliptical-ification of the beam
The FI set up on it's mount and the beam passes through it - centrally through the apertures on each side. Need to make sure it doesn't clip and also make sure we get 93% through (datasheet specs say this is what we should achieve). We will not achieve this, but anything close should be acceptable.
Setting up for minimum power through the FI is HWP @125deg.
Max is therefore @ 80deg
Power before FI = 544 mW
Power after FI = 496 mW (after optimising input polarisation)
Power dumped at input crystal = 8.6mW
Power dumped at input crystal from internal reflections etc = 3.5mW
Power dumped at output crystal on 1st pass = approx 8mW
OK. that gives us a 90.625% transmission and a 20.1mW absorption/unexplained loss.
Well - OK. The important part about isolators isn't their transmission, it's about how well they isolate. Let's see how much power gets ejected on returning through the isolator…
Using a beam splitter to pick off light going into and returning from the FI. A 50/50 BS1-1064-50-1025-45P. And using a mirror near the waist after the FI to send the beam back through. There are better ways to test the isolation performance of FI's but this will suffice for now - really only want to know if there's any reasonable isolation at all or if all of the beam is passing backwards through the device.
Power before BS = 536 mW (hmmn - it's gone down a bit)
Power through BS = (can't access ejected on first pass)
Power through FI = 164 mW (BS at odd angle to minimise refractive effect so less power gets through)
Power lost through mirror = 8.3mW (mirror is at normal incidence so a bit transmissive)
Using earlier 90.6% measurement as reference, power into FI = 170.83 mW
So BS transmission = 170.83/536 = 0.3187
BS reflectivity therefore = 1 - 0.3187 = 0.6813
Power back into FI = Thru FI - Thru mirror = 155.7 mW
Power reflected at BS after returning through FI = 2.2mW
Baseline power at BS reflection from assorted internal reflections in FI (blocked return beam) = 1.9mW
Note - these reflections don't appear to be back along the input beam, but they *are* detectable on the power meter.
Actual power returning into FI that gets reflected by BS = 0.3 mW
(note that this is in the fluctuating noise level of measurement so treat as an upper limit)
Accounting for BS reflectivity at this angle, this gives a return power = 0.3/0.6813 = 0.4403 mW
Reduction ratio (extinction ratio) of FI = 0.4403/155.7 = 0.00282
Again - note that this upper limit measurement is as rough and ready as it gets. It's easy to optimise this sort of thing later, preferable on a nice open bench with plenty of space and a well-calibrated photodiode. It's just to give an idea that the isolator is actually isolating at all and not spewing light back into the NPRO.
Next up… checking the mode-matching again now that the FI is in place. The beam profile was scanned after the FI and the vertical and horizontal waists are different...
Step 3b: Non-circular? We can fix that...
A quick Beamscan sweep of the beam after the Faraday:
25.8 503.9 478.8
25 477.5 489.0
24 447.1 512.4
21 441.6 604.5
20 476.3 645.4
19 545.4 704.1
18 620.3 762.8
OK. It looks not too bad - doesn't look too different from what we had. Note that the x axis is in local table units - I found this useful for working out where things were relative to other things (like lenses and the FI) - but it means the beam propagates from right to left in the plot. in other words, the horizontal waist occurs first and is larger than the vertical waist. Also - they're not fitted curves - they're by-eye, best guesses and there's no solution for the vertical that doesn't involve offsets... discussion in a later part of the thread.
Anyway! The wonderful thing about this plot is that the horizontal and vertical widths cross and the horizontal focussing at this crossing point is shallower than the vertical. This means that we can put a lens in at the crossing point and rotate it such that the lens is stronger in the horizontal plane. The lens can be rotated until the effective horizontal focal length is right to fix the astigmatism.
I used a 200mm lens I had handy - a rough check sweeping the Beamscan quickly indicated should be about right though. Adjusting the angle until the beam size at a distant point is approx circular - I then move the profiler and adjust again. Repeat as required. Now… taking some data. with just that lens in:
24 371.7 366.1
21 360.3 342.7
20 447.8 427.8
19 552.4 519.0
18 656.4 599.2
17 780.1 709.9
16 885.9 831.1
Well now. That looks quite OK. Fit's a bit rubbish on vertical but looks like a slight offset on the measurement again.
The angle of the lens looks awful, but if it's stupid and it works then it isn't stupid. If necessary, the lens can be tweaked a bit more, but there's always more tweaking possible further down the line and most of the astigmatic behaviour has been removed. It's now just a case of finding a lens that works to give us a 50 um beam at the oven position...
Step 4: Matching into the oven
Now that the astigmatism is substantially reduced, we can work out a lens solution to obtain a 50um waist *anywhere* on the bench as long as there's enough room to work with the beam afterwards. The waist after the Faraday and lens is at position 22.5 on the bench. A 50 mm lens placed 18 cm after this position (position 14.92 on the bench) should give a waist of 50 um at 24.57 cm after the waist (position 12.83 on the bench). This doesn't give much room to measure the beam waist in though - the Beamscan head has a fairly large finite size… wonder if there's a slightly less strong lens I could use…
OK. With a 66 mm lens at 23 cm (position 13.45 on the bench) after the waist we get a 50 um waist at 31.37 cm after the waist (position 10.15 on the bench).
Closest lens I found was 62.9mm which will put the 50um point a bit further towards the wall, but on the X-arm the oven is at position 8.75 ish. So anything around there is fine.
Using this lens and after a bit of manual fiddling and checking with the Beamscan, I figured we needed a close in, fine-grained measurement so set the Beamscan head up on a micrometer stage Took a whoie bunch of data around position 9 on the bench:
(mm) (um mean) (um mean)
-15 226.8 221.9
-14 210.9 208.3
-13 195.5 196.7
-12 181.0 183.2
-11 166.0 168.4
-10 154.0 153.1
-9 139.5 141.0
-8 127.5 130.0
-7 118.0 121.7
-6 110.2 111.6
-5 105.0 104.8
-4 103.1 103.0
-3 105.2 104.7
-2 110.9 110.8
-1 116.8 117.0
0 125.6 125.6
0 125.6 125.1
1 134.8 135.3
2 145.1 145.6
3 155.7 157.2
4 168.0 168.1
5 180.5 180.6
6 197.7 198.6
7 211.4 209.7
8 224.0 222.7
9 238.5 233.7
10 250.9 245.8
11 261.5 256.4
12 274.0 270.4
13 291.3 283.6
14 304.2 296.5
15 317.9 309.5
And at this point the maximum power available at the oven-waist is 298mW. With 663mW available from the laser with a desired power setting of 700mW on the supply. Should make sure we understand where the power is being lost. The beam coming through the FI looks clean and unclipped, but there is some stray light around.
7 868.5 739.9
6 1324 1130
5 1765 1492
4 2214 1862
The plot looks pretty good, but again, there looks to be an offset on the 'fitted' curve. Taking a couple of additional points further on to make sure it all works out as the beam propagates. I took a few extra points at the suggestion of Kiwamu and Koji - see the zoomed out plot. The zoomed in plot has by-eye fit lines - again, because to get the right shape to fit the points there appears to be an offset. Where is that coming from? My suspicion is that the Beamscan doesn't take account of the any background zero offsets when calculating the 13.5% and we've been using low power when doing these measurements - very small focussed beams and didn't want to risk damage to the profiler head.
Decided to take a few measurements to test this theory. Trying different power settings and seeing if it gives different offset and/or a changed width size
7 984.9 824.0 very low power
7 931.9 730.3 low power
7 821.6 730.6 higher power
7 816.4 729.5 as high as I'm comfortable going
Trying this near the waist…
8.75 130.09 132.04 low power
8.75 106.58 105.46 higher power
8.75 102.44 103.20 as high as it can go without making it's saturated
So it looks like offset *is* significant and the Beamscan measurements are more accurate with more power to make the offsets less significant. Additionally, if this is the case then we can do a fit to the previous data (which was all taken with the same power setting) and simply allow the offset to be a free parameter without affecting the accuracy of the waist calculation. This fit and data coming to an e-log near you soon.
Of course, it looks from the plots above (well... the code that produces the plots above) that the waist is actually a little bit small (around 46um) so some adjustment of the last lens back along the beam by about half a cm or so might be required.