after a lot of test it turned out that the optically contacted QWP/PBS combinations used for the reference cavity so far are very bad alligned.
We tested two out of three we have in the PSL lab and both are bad, meaning about 10% of the linear polarized light entering the PBS are not converted into circular polarized light and so not reflected when comming in the reverse direction. By replacing the optically contacted version by individual PBS and QWP the amount of wrong light dropped by a factor of 100 or so.

Replacing the bad optics should reduce the effect from backscattered/reflected light, which increases the RIN a lot at low frequencies. It doesn't seem to be the laser itself, as with none, one or two FI the spectrum seems to be the same bad level when light is reflected back into the PMC. It looks like the source is the PMC itself or it's control loop.

So, Tara is replacing the bad ones and re-aligning everything. Temp is good, both cavities are resonant at the same time so i hope we can get new/better data tomorrow.

workers finished the piping stuff for today but have to come back to connect it to the stuff one floor above. It's not the sprinkler stuff, it's heating water.
So some other guys will show up in the future to drill holes for the sprinkler pipes and installation of the sprinklers.

turned the laser back on

The current 1064nm, 100 mW laser in our setup, NPRO lightwave 126 is broken. We are looking for a new one to replace it.

The laser stopped working when I tried to lock the cavity and saw that RCTRANSPD fluctuated a bit even after I adjust the gain setup.

So I turned the HEPA filter above the table off to see if the signal would be more stable, it was not. When I turned it back on

the laser was off. I don't think the laser and the HEPA filter are associated, but that's what happened.

The power output, as indicated on the laser driver is 8 mW. When I turned the laser off for 5 mins and turned it on.

The temperature of  the crystal started from ~40 C and there was power out, then, in ~10 seconds, the temperature went up to  94 C, and the power dropped to 8 mW again.

The voltage supply for TEC went up to 4V which is the maximum V for cooling.

I switched to the 10W laser driver, the same symptom happened again, so the problem might be the head, not the driver.

Frank opened up the laser to find any burnt mark, but found nothing and put it back, and now the laser is working.

We don't know for sure yet, what's wrong with the laser. But I'll use this opportunity to work on modification of PMC servo.

I measured the TF between the current actuator of the laser and PZT on NPRO to see how much the current actuator changes the frequency.

The result, if the current act on the laser mostly adj Freq or Intensity, is yet to be determined

The current actuator on the laser driver changes both frequency and intensity of the out going beam.

This experiment aims to learn how much the current actuator drives the frequency of the laser compared to the intensity.

1) TF between current actuator (Vact) and NPRO PZT (Vpzt)

The source is split and sent to 1)the current actuator, 2) ref ch A on SR785

The response is picked up at fast mon on FSS loop, the voltage between Vmon is the actual voltage sent to the PZT, see the schematic.

The calibration for NPRO PZT is 3.07 MHz/V

Thus response/ref is chB/chA = Vpzt/Vact. I correct the unit to be Hz/Vact by multiplying the Vpzt by the calibration, 3.07MHz/V. or add the original result (in dB) by 20log(3.07 e6) dB.

 2)The TF between the current actuator (ref) and PMC trans PD (resp) was plotted in this entry.

The magnitude on the Y axis is Vpmc_transPD / Vactuator. To correct it to RIN, divide Vpmc by DC value of PMC_trans PD [1.37 V] or subtract 20log[1.37] from the result in (dB).

3) TF between  pmc trans PD (ref chA) and PZT (resp chB), see fig 2.

   The unit after the measurement is [Vpzt / Vpmc_pd]. To correct it to Hz/RIN, multiply Vpzt by the calibration and divided Vpmc_pd by its DC level,

or add 20log(3.07e6) + 20log(1.37) 

 To sum up,

1) TF between current act and PZT tells us how much  frequency changes when we modulate through current actuator,

   2) TF between current act and PMC trans PD tells us how much RIN changes when we modulate through current actuator,

3) TF between PMC trans and PZT tells us what changes more after current act is modulated.

This will be compared with frequency change (calculation) due to RIN-> thermo optic.

Then we can decide if current adj mainly change frequency or intensity.

I re-aligned the PMC, same for the electro-optic amplitude modulator (EOAM). I also adjusted the power levels everywhere up to the PMC for maximum performance.

I also had to re-adjust the PMC phase. Startup-setting was 2.29V but had to be 3.29V! Error signal is 2.46Vpp at mixer mon.

Power is set to 20mW transmitted through the PMC when modulation is off. Power can be adjusted behind the PMC as it was setup before.
Power modulation is +/-1.5mW around 20mW with max on function generator (~15%).

Jenne Laser  (thanks!)

pulling the switch for one of the HEPA fans triggered the laser and put it in standby mode - turned laser back on

I borrow an NPRO from TNI. The model is similar to what we have, but I think the maximum power is 500mW (what we have is 100mW). I'll check the beam waist position tomorrow.

     Since we might switch to 2-laser setup, it is a good idea to have the second one ready. I checked a few companies for 1064 laser diode, but I could not find the information about how to control the frequency and its performance on the webpages. So I asked Eric Black and got the laser from TNI. Plus, the TTFSS is proved to be sufficient to suppress NPRO free running noise below coating noise level, the NPRO laser should be good for our setup.  We might need to think about preparing the rest of the equipments (resonant EOM for 2nd PMC, resonant and Broadband for refcav, PMC, and the servo circuits). For now, if the borrowed laser works fine and has more power, we might just switch it with the current one to gain more power.

Don't forget to write an SOP for that laser before using it. A few things changed recently if you want to use the old SOP as an template. The hazard zone and emergency laser shutdown procedures, table layout etc. have slightly changed since the 10W laser has been moved. The map outside the lab is also not up-to-date anymore. The main kill switch at the door is still active but the second one which was attached to the 10W laser is gone now, but also not required for the NPRO. The required OD for safety glasses has to be recalculated for the document, but the ones we have are all good. The rest of the old SOP should be the same.

You also have to redo the mode matching to the PMC as the beam waist changes if you switch to the higher power laser.

For a two-laser setup we don't have all the parts at the moment to do so. There is no EOM for the PMC nor the refcav sidebands, no servo for the PMC, and no PMC. We don't have all parts to build a new PMC, especially no PZTs. Mirrors exist, spacer parts too but have to be assembled and we have to ask P.K. if we can have them, but shouldn't be a problem.

Duly noted, I'll write the sop for this one and put it on svn and wiki.

I'm looking into how to make external cavity diode laser (ecd).

Here is the list of what we need.

• Laser diode 1064nm, diode (+AR coating)
• Grating
• Current driver for locking the laser (home made)

I ask Akihisa who works in Kimble's group about their home made 850 nm ecl. The performance is not as good as NPRO yet (300kHz line width when locked to CS cell), but it is certainly interesting.

• They use Littrow configuration ( 1 grating to form a cavity). The mirror behind the grating is for adjusting the output beam.
• The PD is not AR coated, a slight power (10 uW) that transmits inside the diode is enough for the feedback.
• The linewidth is ~ 300kHz when locked to CS cell, the free running noise is not measured.
• They use both PZT and current to actuate on the frequency stabilization, the bandwidth is 50kHz.
• Temperature feedback is employed to keep the stability of the laser
• Time for putting everything in the box (once all components are ready) is ~ 1wk

 (

I just asked Aki a few more questions about the ecd laser. If we do not require the performance to be rival to that of the NPRO, making one is possible in a few weeks time scale.

Q1) The setup of the Littrow style laser you showed me had one mirror
behind the grating. Is the setup similar to this
<http://rsi.aip.org/resource/1/rsinak/v72/i12/p4477_s1>? Where the
mirror is used so that the alignment does not change when the laser is
tuned.
A1) Actually, Yes, for the laser you took the photo. But, most of the lasers we use in the lab don't have a mirror, only diode and grating in the box. In our case, we only scan the frequency by ~1GHz and the pointing vector drift is negligible.

Q2) Did you remove the glass window of the diode laser when you assemble
 the laser? If so, how do you keep the diode clean, or it does not matter
for your requirement.
A2) We didn't remove the glass window. What we did is very simple. We mounted the bare diode on the thorlabs mount: http://www.thorlabs.com/thorProduct.cfm?partNumber=LT230220P-B

 Q3) You mentioned that the line width of the laser when locked to CS cell
is ~300kHz. Is it because of gain limited of the servo or the CS cell's
intrinsic noise?
A3) We haven't measure the linewidth with locking and without locking independently. It's possible that our laser linewidth (without frequency lock) might be ~300kHz. So I don't know what limits our linewidth.

One thing you may want to consider is that a diode laser is infamous for the broad background incoherent light, compared to the solid state lasers. We typically observe ~30nm-wide incoherent light around the carrier with 30-40dB suppression compared to the carrier. If your experiment is sensitive to the spectral purity, this might be an issue.

Aki

 So the question is do we want to try to build one similar to what they have? We know that with the time scale and experience we have it will not be as good as the performance of the ecd laser reported in Numata etal paper, but it might be a fun project for the SURF student.

Tara and I measured the power of the currently used NPRO Laser to be 51mW. Previously the power of this laser was measured at 100mW, so this measurement indicates an irregular performance.

The beam is attenuated by 53% in between the laser output and pre-mode cleaner input. For the Pre-mode Cleaner, we tried to optimize the beam alignment by turning the mirrors in front of the Pre-mode cleaner to get maximum visibility. The input power was 26.9 mW and transmitted power was 21.2mW, yielding a 79% visibility.

Using the following setup with the higher power NPRO laser previously used in the TNI experiment we conducted the following measurements:

Directly from the laser, we measured a maximum power of 380mW. We were worried that the high power might burn through the power stick, so we decided to use a mirror to attenuate the beam. We placed the power stick away from the laser in order to get a larger beam size to avoid high intensities, which might destroy the diode as well.

With the mirror we measured the incident and transmitted power of the transmitted beam to calculate 47% power transmitted.

Then we increased the power by turning up the knob on the control box, until maximum power was reached. With the mirror the maximum power read 219mW on the power stick (on the control box it read ~700mW), which corresponds to 466mW directly from the laser. With the maximum power of 466mW, and an attenuation of 53%, we would have 247mW for the pre-mode cleaner input. With a visibility of 79% we would have 195mW from the output of the pre-mode cleaner, assuming the lasers have similar profiles, spot size, and waist position. In reality the power would be lower than 195mW.

We then replaced the currently used lower power NPRO laser with the higher power NPRO laser, using the current setup, to prepare to measure beam properties. We added a silicon heat sink between the laser and its aluminum heat block in order to avoid overheating. Additionally, we still need to finish aligning the beam so that it passes through all elements of the setup.

Today Tara and I aligned the beam of the new NPRO laser, so that it passed through all elements of the setup.

The hole patterns of the laser and aluminum block did not match up: 

Two screws were used in front, and a clamp in back to hold down the laser:

Before locking the Pre-mode cleaner, we measured the photodiode saturation at 684mV, corresponding to an input power to the pre-mode cleaner of 50mW. Thus, a voltage from the photodiode above 684mV would yield incorrect results.

We locked the Pre-mode Cleaner to the TM00 mode and optimized visibility by adjusting the mirrors. The maximized visibility through the pre-mode cleaner was 70%. The beam was attenuated between the laser and the input of the pre-mode cleaner to 28% of its power. So with the laser operating at its maximum power of 466mW, the output power from the pre-mode cleaner would be 91mW.

In an attempt to measure the effect of changing DC power in one cavity to the beat signal, we shifted the DC power for the Rcavity only, adjusting the half wave plate to keep the Acavity power constant, and measured the resulting change in the beat frequency.

 Yesterday Tara and I measured the relative intensity noise behind the cavity. We placed two pda 10Cs behind the cavity, one for the in loop sensor, and one for the out of loop sensor. Since the same source was split on the two photodiodes, the two signals were similar.

After centering the beams on the photodiodes, we measured the noise from the photodiode itself, the noise from the SR758, as well as the intensity noise. The noise from the pd was measured at a -50dB range, while the noise from the sr758 was measured at a -20dB range. At the -50dB range, the sr758 would have 7nN of noise, which is much lower than the pd noise level. Thus, the measurement of the pd noise is valid.The results were processed in MATLAB and are plotted below. 

The following graph shows the relative intensity noise of the laser as measured behind the cavity, obtained by dividing the intensity noise by the DC voltage:

With the above information, the next step will be to measure the transfer function between the power fluctuations and the beat signal.

Today Tara and I measured the coherence between the intensity noise from RCAV and ACAV, as well as the transfer function between the power fluctuations and frequency. We used the following setup:

With a pda 100a photodiode behind ACAV, we looked at the intensity fluctuations, as well as the noise level of the pd. The noise of the sr758 in this case was low enough to consider negligible. The results for pd noise and intensity noise are plotted with respect to the left y-axis. The Relative Intensity Noise for ACAV, the intensity fluctuations divided by the DC voltage, is plotted with respect to the right y-axis.

We also measured the coherence between RCAV_TRANSPD and ACAV_TRANSPD, where we used about 2000 averages:

Next we measured the transfer function between power coupling and frequency. In order to do this, we reduced the power of RCAV to a minimum of 50uW and increased the power in ACAV to a maximum of 2.5mW. The gain in ACAV was too high, and we had to add an attenuator of 30dB to the servo input.

The transfer function is found by dividing the response function by the reference function. In this case, we let the response function be the AOM feedback signal, and the reference signal be the ACAV power fluctuations. The use of ACAV power fluctuations is optimal because ACAV is being operated a maximum power. Additionally, there was a 1kHZ input range to ACAV which will be used for calibration to absolute frequency later.

Using the swept sine measurement group, we measured the following transfer functions:

The blue transfer function was the first obtained. In order to confirm the accuracy of the transfer function, we changed both the excitation and the power level to ACAV. If the transfer function is accurate, we expect it to remain relatively constant in these cases.

Green Plot: After changing the DC power in ACAV to half its current value, to 1.25mW, the green transfer function was obtained. Because the power was decreased, the gain decreased, and we adjusted the attenuation to ACAV from 30dB to 13dB. 

Red Plot: Next, we changed the excitation amplitude down to 1V, and the transfer function remained the same. However, after changing the attenuation back to 30dB, the transfer function did change slightly as shown in the red curve.

Black Plot: Increasing the power to ACAV to 1.9mW, the transfer function is portrayed as the black curve.

Yellow Plot: In an attempt to look for the common mode effect, we increased the power to RCAV to 1mW, a value relatively close to that of ACAV. The resulting transfer function, the yellow curve, does not indicate the presence of any common mode effect. One possible reason for this is that the coupling for the two cavities might not be the same. 

   ==comments==

    To sum up, the transfer function measurements we have might not be the real coupling from RIN to frequency noise because:

• It does not stay at the same value when we change the power to ACAV. (Blue -> Green).
• TF changes with the attenuator we use from mixer out to ACAV servo in order to keep the loop stable (Green-> RED)
• TF does not change when the power to RCAV is changed, and power to ACAV is kept constant. It should become smaller as the power between the two cavities are about the same (assuming similar absorption) due to common mode effect (black and yellow) and become larger if the power difference between the two cavities increase.

   == How does it show up in the noise budget?==

    However, as a rough check, if we assume that the measured TF is valid, the coupling from RIN to frequency noise in beat signal is plotted below.

 The measured TF (feedback to AOM/ ACAV_TRANSPD) used in this plot is taken from the red TF from the above plot which gives the maximum coupling.  I assume that the TF at low frequency has the same slope (1/f^0.15) from DC to around 1kHz and roll off with the same measured slope

.

[above, TF plotted with fit curve]

Then

Estimated Frequency Noise = [ACAV_TRANSPD_DC [v] ] x [measured RIN_ACAV [1/sqrtHz]] x [10  ^(TF/20)] x 2 x 710 [Hz/V] ,where

• TF is measured in dB, [V/V]
• a factor of 2 at the end is for double passed AOM,
• a factor of 710 [Hz/V] is the calibration for 1kHz input range on Marconi for AOM 
• no common mode rejection is assumed (to get the upper bound). This should be valid, since the coherence between ACAV and RCAV_TRANS_PD is very tiny at low frequency, where the coupling and RIN are supposed to be large.
• assume 2mW input for ACAV

As it turns out, the measured TF is not totally wrong since the estimated effect is still below the measured beat signal. It is interesting to see that, from 10 to 100Hz, the shape of the RIN noise follows the trend of the measurement nicely.  Thus, developing ISS might improve our signal to certain level. 

I'll add the contribution from RIN in the noise budget code and see if the sum total noise match the measurement or not.

Today I plotted the transfer function measured last week against a calculated transfer function. First, I plotted the transfer function obtained last week with the physical units of Hz/RIN [Hz/sqrt(Hz)]. Then I used the code from elog entry PSL:372 to calculate a theoretical transfer function. The code calculates delta_f using a 1D heat equation solution, due to a change in power. I modified the code to find delta_f at each frequency, and divide it by the RIN at that frequency. The results are plotted below, with the calculated curve in red and the measured curve in blue:

The slope of the calculated curve appears to be somewhat correct, at ~1/f. The observed differences could be due to non idealities in the actual experiment, or using an imperfect model when calculating the delta_f.

Tara and I performed additional measurements of the transfer function for the coupling between power fluctuations and frequency.

Setup:

This time we altered the setup and instead of attenuating the signal input to the ACAV servo, we added an optical attenuator before the pd. We attenuated before the pd this time because of the possibility of pd saturation in the previous setup. The setup we used today was:

Measurement and Data Analysis:

Consistent with the last measurements, the response function for the transfer function was the AOM Feedback and the reference function was the ACAV power fluctuations. We used the sine swept measurement group in the SR785 to get the data. We checked the TF by changing the power from 2.2mW to .22mW, and the data remained the same. 

We calibrated the raw data from dB to the units of Hz/RIN using the following relationship:

TF [Hz/RIN] = 10^(TF[dB]/20) *710[Hz/V] * 3.6[V] *2

where the factor of 2 accounts for the double passed AOM, the factor of 710 is the calibration for the Marconi, and the 3.6 is the DC level.

The results for the TF are plotted below:

For the magnitude, the data taken at different frequency intervals overlapped perfectly. For the phase, the data did not overlap perfectly, so the data was concatenated with a slight jump at 100Hz.

Application to Noise Budget: 

 Using the previously measured RIN, I estimated the frequency noise with the transfer function just measured. First I fit the transfer function as shown below:

Next I applied the following relationship to obtain the estimated frequency noise, and plotted it with the noise budget:

Frequency Noise: RIN * 3.6 [V] * 2 * 710 [Hz/V] * 10^(TF[dB]/20)

Compared to the RIN induced frequency noise with the previously measured TF (refer to elog entry attached to this one), the noise is lower at higher frequencies, drops off with a slightly steeper slope in the center, and slightly higher at lower frequencies (this could be due to error in fit though). Note that this RIN induced noise is only an estimate, and the calculation may be slightly incorrect due to the fact that the DC level was slightly different (by ~ a factor of 2) for the measurement of the intensity noise (previous elog) and the current transfer function measurement.

Comparing the calculated TF from elog 989 (using RIN from previous setup), the following plot is obtained:

Thus, the transfer function still does not match the calculated curve perfectly, but its a better match shapewise than the previous TF. Additionally, there still might be an error in the calculated curve.

Conclusion:

 The TF looks considerably better than before, perhaps because we optimized the phase for the EAOM two days ago, so we are driving the amplitude with more modulation.

== Note on the measurement ==

•       RCAV is locked with 100 uW input power, Common/Fast gains on the TTFSS are 950/750. The reason to use low power is that we want to make sure that the effect due to power modulation will not be common in both paths. However, we need enough power so that RCAV loop gain is still high enough so that the frequency noise in the laser is suppressed and does not cover the RIN induced noise in ACAV loop.
•       ACAV input power is 2.2 mW. To avoid having too much gain and unstable loop, we use an ND filter to reduce the power on RFPD down to the level it usually is at 1mW power (~200mV).
•       To check if the TF is real or not, we tried to lower the power down to 0.2 mW and see no change in the measured TF. This is good, since the TF should be independent of the power input.
•       The DC level from PDA100A @gain 20 is 3.6 V (the reference signal of our TF measurement). The modulation is 380mV pk-pk in sine wave form, so the TRANSPD_DC is ~ 3.6 +/- 0.19 V. We make sure that the PD is not saturated.
•  ***** The measurement is made possible because the drift is very small. For the past 5 Hrs, it has been less than 1kHz in 5 minutes. **

I have been thinking that the TF measurement (coupling from RIN to frequency noise) we plotted before has unusable unit [Frequency noise /RIN]. We have to correct it by converting it to [frequency noise/ watt]. With that we can compare the result to other experiment/ calculation or use it in the noise budget.  As an example, I plot the result calculated by Cerdonio etal 2003 here as well.

==we use wrong unit==

  I am editing my noise budget code to incorporate the effect from RIN induced noise and find out that, the unit we have in Fnoise/RIN is not making sense. In order to get the noise due to RIN I have to multiply measured RIN to the measured TF, that is    Freq noise due to RIN = [measured TF] x [measured RIN behind CAV] = [Hz/RIN] x [RIN] = Hz. As we can see, it does not change with the input power level. In reality, it should depend on the power level as well.  That is why I think Hz/RIN unit is unusable.

==result from Cerdonio==

 Braginsky etal calculated the noise from thermal expansion due to heat absorbed from shot noise. The result was later corrected for all frequency span by Cerdoniot etal in 2003. The result tells us the displacement noise due to shot noise. With some modification, we can apply the result to get the displacement noise due to RIN. This will be compared with the measurement later. [add calculation]

fig1: calculation for TF using Cerdonio etal's result.

fig2: calculated TF in unit of [Hz/Watt] (I convert the result from m/watt to Hz/watt). Frequency noise can be calculated by multipling the TF with RIN x Pin. [fig file, code]

==some thing about the measurement setup==

The nice thing from the measurement is that we can see the Cerdonio effect when the thermal diffusion length is comparable to the spot size (around 3Hz) nicely. However, the asymptotic behavior of our measurement does not agree well with the prediction. It has a slope of 1/f^0.75 , while we expect 1/f.  I'll find out what's wrong with the setup (bad alignment on EAOM, PDH signal, etc) or other mechanism that might cause this effect.

I tried to remeasure the coupling from RIN to frequency noise (same setup as in psl:1005).  However I have not been able to reproduce the result yet.

The result we got in psl:1005 does not have the same TF shape as we expected from the calculation. The slope is off by 1/f^0.25. So There might be something wrong with the setup, for example bad alignment in EAOM that adds PM to the beam and effect the PDH signal. So in order to verify the problem I redo the measurement with the similar setup. This time, however, when I change the power into ACAV, the measured TF changes as well (this did not happen before), so I'm checking what's wrong with the setup.

Checking plan:Once I can remeasure the TF (ACAV AOM feedback/ ACAV TRANSPD). I want to check

1. effect of beam EAOM alignment to the TF
2. comparison between TF from (ACAV AOM feedback/ACAV TRANSPD) and (beat signal/ACAV TRANSPD). I tried this already and they are quite similar except some phase shift.
3. To verify that the measurement is valid, I'll make sure that the TF is independent of excitation level and power input to ACAV.

Tara and I measured the correlation between Voltage and Power behind ACAV, and applied the found calibration to get the TF magnitude in the units of Hz/Watt.

Intro:

We wanted to convert the previously measured TF magnitude (elog 1005) into units of Hz/Watt, as opposed to the previously used Hz/RIN. See attached elog (1014) for an explanation of why these units have a more applicable meaning and application.

Setup/Data Obtained:

We measured the power immediately behind ACAV, the power immediately before the pd (there is a beam splitter between the first and second power measurements), and the corresponding voltage after the pd. The pd gain was set to 20dB. I plotted the results and fit each data set:

The fit equations are:

pd fit: y = 5680.4x + 0.036843

cavity fit: y = 2923.3x + 0.027703

Where y is the voltage (Volts) after the pd and x is the power (Watts) in front of the pd for the pd fit, and right behind the cavity for the cavity fit. 

TF Magnitude in Hz/Watt:

Using the slope of the line relating the power behind the cavity to the voltage as the calibration, I was able to convert the previously measured TF magnitude into the units of Hz/Watt, and plot it against the calculated TF (elog 1014) magnitude in units of Hz/Watt (calculated using Cerdonio et al's results):

Application to Noise Budget:

I also plotted the RIN induced noise using the TF in units of Hz/Watt using :

Frequency Noise = TF[Hz/Watt] * RIN * Power_in 

I used an input power of 2mW, and the RIN from 6/13 behind ACAV.

The frequency noise generated is a factor of 1.4 larger than that generated using the TF in units of Hz/RIN. Since the TF in Hz/RIN was measured, it is more reliable. This discrepancy indicates that the TF magnitude is also probably too high, and there is a possible error in the calibration constant used to convert voltage to power, or possibly an error in the power readings. 

I scanned the profile of the laser borrowed from 40m. The avg beam radius is 220um ~ 1 cm in front of the laser opening. This number will be used for a new table layout.

The laser was operated at full power (~700mW as expected). I used a mirror to attenuated the beam and use WINCAM to measure the beam profile (power incident on WINCAM was ~0.7mW). To measure the full power and avoid burning the power meter, I used a polarizing beam splitter with 1/2 wave plate to reduce the beam power by half then measured and summed the power from two sides of the PBS.

The beam shape is looking more like a blob than an oval. This might explain why the fitting does not match the measurement well.

[add fig]

To measure the laser output profile is not actually that easy, although measured values are not so terrible.

Make sure the PBS you used is not BK7, but Fused Silica.

It would be nicer if you don't need to use transmissive optics for the measurement.
i.e. Put a fused silica AR-coated window and use the reflection.

BK7 windows may not work. BK7 has more than x10 CTE compared with Fused Silica. (7.5 ppm/K vs 0.55 ppm/K).
The absorption may also be higher.

Some notes about external cavity diode laser. I investigated in this about a year ago.  I think it might be a good time to work on it, since I'm modifying the  ctn layout and we can use a frequency stabilized laser (although locked to a short cavity) via fiber optic to test with a home made ecdl. 

Yesterday I used the CTN network analyzer to look at the RF spectrum of a 1.0 mW beam from the ATF Lightwave with a New Focus 1811. This beam is picked off from the main ATF laser beam pretty much immediately after the laser head; there are some waveplates, PBSs, and lenses, but no EOMs or modecleaners. The laser was free-running, with nothing plugged into the temperature or frequency BNCs.

In addition to the spectrum of the beam intensity, I took a spectrum with the beam blocked to get a measurement of the dark current. In the plots below, I've referenced everything to the current through the diode. This means taking the W/Hz spectrum from the network analyzer, multiplying by 50 Ω and taking the square root to get the V/rtHz across the analyzer's internal 50 Ω resistor, then multiplying by 2 to get the V/rtHz put out by the 1811 (since its output impedance is 50 Ω), then dividing by the 4×10⁴ V/A figure given in the 1811 manual to get the A/rtHz across the diode. To get the 'expected photocurrent shot noise' given below, I watched the DC output of the 1811, which was at 680 mV with the 1.0 mW beam and 10 mV dark. So I divided 670 mV by the 1 V/mA figure given in the manual to get the DC photocurrent. The shot noise of this photocurrent is then sqrt(2eI). I haven't measured any of these 1811 conversion factors, so I don't have complete confidence in this shot noise value. However, the value for the DC current agrees roughly with what you get if you take the power measured with the ThorLabs meter (1.0 mW) and multiply by the quantum efficiency (0.7).

You can see in the first plot that the dark current is subdominant to the photocurrent all the way out to 100 MHz, and subdominant to the expected shot noise out to maybe 40 MHz or so. In the second plot I've taken the quadrature subtraction of the blue trace from the red trace to get an estimate of the photocurrent noise alone. The spectrum looks approximately white from 10 MHz out to 50 MHz and (if you at all believe the shot noise value) is about 1.7 times the shot noise. If this truly is the level of the excess noise, then to get excess intensity noise whose PSD is equal to 1% of the shot noise PSD at 20 MHz, we'll need a cavity pole at 5.6 MHz. If the calibration is spectacularly off and the total noise is 20 times the shot noise, we'd need at cavity pole at 1.4 MHz to get the excess intensity noise PSD to be 1% of the shot noise PSD at 20 MHz. The way I've arrived at this is as follows: if S(f) denotes the value of the linear spectral density at f relative to shot noise (f = 20 MHz and S(20 MHz) = 1.7 in this case), then

and so the suppression of the excess noise after transmission through a modecleaner is

and from this f_HWHM can be chosen to give the desired amount of suppression. Edit: actually, the right way to do this is to write the excess noise PSD as a relative intensity noise (which scales as P²), and to then compute the desired amount of suppression for the maximum amount of power we're going to send through the PMC (2 W or so). Computing the suppression relative to shot noise for a 1 mW beam is not sufficient, because the suppression requirement gets more stringent as the power increases. The RIN here at 20 MHz is 3×10^-8 /rtHz, and so for 2 W beam we require a cavity pole of 420 kHz to get a factor of 100 suppression below shot noise.

I think to do this measurement properly I'll need to get a better handle on the relative calibration of the DC and RF transimpedance gains of the 1811. It might also be nice to take a measurement both before and after an existing PMC, just to see the expected filtering effect.

Some interesting papers on the intensity noise in NPROs (Ingo Freitag is an author on both):

From Roland Schilling: Suppression of the intensity noise in a diode-pumped neodymium:YAG nonplanar ring laser

From DMC: Intensity-noise dependence of Nd:YAG lasers on their diode-laser pump source

I tried getting the relative gain between the AC and DC paths of the New Focus 1811, essentially repeating the measurement in elog #1152 but (a) taking measurements for both the DC and AC paths and (b) taking measurements in the region 10 kHz to 100 kHz, where the DC and AC bands overlap. According to the manual, the DC path is meant to be used up to 50 kHz, and the AC path is meant to be used down to 25 kHz.

I again had roughly 1 mW of light on the PD. I took spectra with the Agilent 4395A, which has 50 Ω input impedance. For the AC path, I took the spectra in W/Hz and multiplied by 50 Ω, giving a spectrum in V/rtHz referenced to the input of the spectrum analyzer. For the DC path, I took the spectra in W/Hz, multiplied by 50 Ω, took the square root, and then multiplied by 40/(2*0.82) = 24. The factor of 40 is the value given in the manual for the relative gain between the AC and DC paths, the factor of 2 accounts for the 50 Ω output impedance of the AC path, and the factor of 0.82 accounts for the apparent 11 Ω output impedance of the DC path. (The DC voltage out of the PD was 0.66 V when fed directly into the 1 MΩ input of the scope, but dropped to 0.54 V after I inserted a 50 Ω feedthrough; the quotient of these two numbers is 0.82 and this implies that the DC path has an 11 Ω output impedance). If the relative gain value (40) as given in the manual is correct, the DC and AC spectra should overlap so long as they are dominated by intensity noise rather than detector noise.

The first plot shows the AC and DC spectra, both light and dark. The intensity noise is not as dominant over the detector noise as I had hoped. Still, in the second plot I've quadrature subtracted the dark spectra from the light spectra to produce what are in principle the spectra of the photocurrent alone. Even with the caveat that certain bins in these spectra are dominated by detector noise, it appears that the spectra don't even have the same shape. My guess is that we may already be seeing the action of the high-pass rolloff on the AC path.

It might be worth having another go at this measurement with a higher photocurrent, but I'm guessing that this isn't a great way to get the relative gain calibration.

Laser is locked to north cavity, with slow PID loop engaged.

Current north laser slow DC voltage: 6.55 V, with some slow upward drift

TTFSS settings: 634 fast, 888 common (very lucky!)

A list of small tasks and some data points:

As of yesterday, the tank is floated. This required minimal realignment of the input pointing into the cavities.

Adjusted powers so that there is 1.04 mW incident on north and 0.96 mW incident on south.

For the south PDH loop, the highest gain we can get seems to be 590 common and 710 fast. Not great. The south error signal has terrible 270 kHz oscillation as well (~100 mVpp).

For the north PDH loop, the highest gain we can get is 807 common and 908 fast. Not perfect, but better than south. No 270 kHz oscillation here.

South path adjustments

RAM on the south PD was terrible: 259(28) mVpp at the PDH frequency; the uncertainty is dominated by slow breathing of the RAM amplitude. I need the PD rf transimpedance to convert this voltage to an actual RAM.

I tried adjusting the alignment of the south EOM, with little effect. The big effect came from slightly rotating the λ/2 plate immediately preceding the EOM: rotating by less than half a degree takes the RAM from >200 mVpp through zero and back to 200 mVpp again. The λ/2 plate was in one of those no-frills rotational mounts where sub-degree precision can be achieved only by nudging, so I instead put the waveplate into a precision mount with a worm drive and a knob. I then tuned the rotation to null the RAM on a scope. There is still some breathing of the amplitude, so that at times the RAM is 40 mVpp. Not good, but better than before.

I measured the south modulation index both before and after this change. I swept the south laser frequency and watched the transmission on the ISS PD. Before, the carrier and single-sideband transmission peaks were 1.91(1) V and 39.6(1.2) mV, respectively, and after they were 1.72(2) V and 38.2(6) mV, respectively. This means the modulation index actually increased from 0.288(4) to 0.298(3) (using the Γ²/4 approximation).

Beat measurement

I used a function generator to drive the south NPRO PZT with a triangle wave. Then with the 14.75 MHz sidebands on, I used a PDA100A to watch the south cavity transmission.

Looking by eye at the carrier and sideband transmission peaks, I find an actuation coefficient of 4.4(2) MHz/V, which is higher than what Tara measured in 2010 (maybe the coefficient depends on which axial mode the NPRO is operating on?)

From the attached plot, we can also see that the mode splitting for the south cavity is 2.0(4) MHz.

Summary

While I was debugging the "high" intensity noise at the ISSPD north i have noticed some scattering from the FI (north laser). It seems relatively well aligned but I did not want to touch it for now.

However I have measured the power emitted by the north laser and it is 306mW. The current setup provides ~99.3% dumping of the light into it. It means that only 2mW is at the output of the FI

while all the rest is dumped in the way out ---> I want to change it as soon as possible.

TO BE DONE

The laser setup will be changed in a way that the lambda/4 will maximize the linearization of the light (whatever angle is) and lambda/2 will maximize the power in transmition at the FI. A lambda/2 and a PBS

will be placed either before or after the FI in order to send 2mW to the rest of the setup. Now the question is to take care of the type of PBS because the damage threshold can be too low:

I consider the following two options:

1. PBS with damage threshold of 100W/cm^2 @(532nm) --> The minimum radius of the beam at 306mW is r ~ 628um (taking care of a factor 2 of safe margin and a factor 2 for 1064nm)

2. PBSO with damage threshold of 1MW/cm^2 @(1064nm) --> The minimum radius at the same 306mW is r ~ 4um;

I do not know the size of the beam. I do not have the optics to measure it and at moment I am not sure about previous measurements.

I have measured the power of the North laser vs the power on the display.

Some settings:

Laser South

Power Display = 479mW; Measured P = 306mW

DC = 2.12A

ADj +2

T + 34.4965

Laser North

Power Display = 68mW; Measured P=230mW

DC = 2.08A

ADj 0

T + 26.465

A question was raised as to what the beam profiles of the two lasers were (M126N-1064-100 and M126N-1064-500).

Spec sheet says that their output beam profile sould nomninally be (W_v,W_h) = (380,500) um at 5 cm from the laser head.

Tara measured this for the 100 mW laser and found to be (W_v,W_h) = (155,201) at 3.45 cm and 2.8 cm from the opening respectivly. (see: https://nodus.ligo.caltech.edu:8081/CTN/120)

When the 500 mW NPRO was aquired a note was made that the beam profile would be measured (see: https://nodus.ligo.caltech.edu:8081/CTN/934) Looking a month around these dates I can't find a measurment.

In principle the beam specs for both lasers should be similar but it doesn't appear we have a measurment. Maybe something for me to do in the next few days.

- awade Tue May 31 09:32:58 2016

---

Also first post, hi. 

I put a thorlabs power meter (S130C) to see what the actual output of the south laser (M126N-1064-500) was. I expected 500 mW, but the power was turned down to about 184 mW. The power indicated on the laser controler was 308 mW.  After playing around it seems that you can set the calibration point for the laser head.  Error on ThorLabs heads is ussuallaly about  so not brilliant, but I adjusted the set calibration point down to match what the power meter was actually measuring at the ouput.  Hopefully this means the head better reports the power.

Let me know if there is a reason for this offset and I can change it back.

-AW Wed Jun 1 18:42:29 2016

Its taken me a while to get to posting this.

I took a pick off of the main beam coming out of the 500 mW laser using a coated fused silica window (W1-PW-1037-UV-1064-45 UNP) and used the WinCamD camera to find the profile as a function of distance.  The difficulty with the measurement is reducing the power enough for the CCD and fitting around the already aligned components.  Unfortunately there are already two wave plates and a steering mirror in the path that are already aligned to a mode cleaner etc that can't/shouldn't be moved. The profile measurements are therefore at quite some distance from the laser head.

Set up with lengths is in attached schematic. Fitted data (referenced to the front of the laser head) is in other attached fit. Data is as follows:

z= [0 20 40 75 105 130 160 190 230 270 310 350]*1e-3 + (123e-3+35e-3+52e-3); % Distance from reference point plus distance to laser head
W_horz = [776.6 849.2 915 1048 1140 1218 1333 1427 1556 1704 1828 1905]*1e-6/2; %Horizontal beam radius
W_vert = [1259 1349 1349 1475 1615 1707 1751 1895 2099 2218 2413 2554]*1e-6/2; % Vertical beam radius

--

Fit gave the following values:

Horz. beam waist = 194.7896 um
Horz. beam waist position = 9.07 mm
Vert. beam waist = 177.6142 um
Vert. beam waist position = -97.3669 mm

Note sure about the vertical waist position there. But those are the fitted values.

Other information pertinent to the measurement is that the laser power measured at the output of the M.A.-1064-500 head was 171.3 mW.  This was just the value it was set at, that I assume was chosen for a reason.  Varying the power from this value may change beam characteristics.  

It would be nice to have measured closer to the output of the laser, but this is not possible without disturbing the rest of the ongoing experiment.

Antonio mentioned that he thought the power at the output of the 'south' laser used to be higher.  A measurement was made 3-6 months ago of power verses laser current (and the power reported on the laser head). I think this is the post to refer to: https://nodus.ligo.caltech.edu:8081/CTN/1600 which indicates a power of 306 mW. Missing from the data is the laser current values (Antonio has these on file). The most coming directly out of the head now is ~235 mW. Even with a 10% error on the thorlabs power head (a S130S in this case), this seems like the power has depreciated.

It is good to document the state of the laser so I went back to the lab and stepped the laser through a bunch of currents and measured the power directly out of the laser head (for the south M126N-1064-500). Figure one shows the first take of data; the data has a discontinuous jump at about 1.99 A.

I thought that maybe I had made an error in taking down power or current, so went back to the lab and remeasured in finer increments.  It turns out that results were reproduced exactly, see Figure 2.  It appears that maybe we are going through a mode hop or maybe some other (maybe) temperature related jump as the laser reaches the higher powers.  Interestingly, once I tripped over the 1.99 A mark and then stepped back down the laser stuck at the slightly lower power.  The only way to 'reset' the effect was to dial the current right down, wait 5 minutes and then gradually bring it back up to the 1.99 A point.  This effect was repeatable.

--Also, for good measure, here are the settings as displayed on the south laser controller at the nominal 178 mW operating power

ADJ  = 0
DC = 2.08 A
DPM = 0.00V
Neon/off
LDON/off
Display 5
DT = 28.7 C
DTEC = +0.2V
LT = 34.8 C
LTEC = +0.5V
T = +34.4965
Pwr = 179 mW

Data for plots are in the attached Matlab script along with stuff to plot it.

-awade Tue Jun 14 22:37:55 2016

Here is the data for the power output vs current for the North laser (North M126N-1064-700).  This is the 700 mW model of this range of lasers and not the 100 mW.  The beam profile will therefore have to be remeasured as I'm not sure that is on file.

As the power output was greater than the max for Thorlabs powermeter head (500 mW), I installed a beam splitter to bring the power down. The reflected and transmitted powers were 465.7 mW and 235.4 mW respectivly: the reflection ratio was therefore 0.6642.  Power reported for the north laser are corrected to give the power exiting the laser head. The data and plot  is below.

--

Also, for good measure, here are the settings as displayed on the south laser controller at the nominal 178 mW operating power

ADJ  = 0
DC = 2.08 A
DPM = 0.00V
Neon/off
LDON/off
Display 1
DT = 22.3 C
DTEC = +0.6V
LT = 44.8 C
LTEC = -0.2V
T = +26.4650
Pwr = 68 mW (I think this may just be the controler being way off calibration)

-awade Thu Jun 16 18:00:18 2016

A pickoff of the 700 mW north laser was made with two UV grade fused silica widows to make a beam profile measurement.  Attached figure shows setup.  

Here the data is for the North laser (measured on June 17 2016) operating at 701.1 mW (Adj number = 0).  The laser was allowed time to warm up and a pickoff of the beam was taken by first reflecting off the front surface of a W1-PW1-1025-UV-1064-45UND UV grade fused silica window and then a W2-PW1-1037-UV-45P UV grade fused silica window with AR coating on front and back: the resulting light was ~200 uW. Beam widths as a function of distance were collected using the WinCamD after isolating a single spot with an iris.  Because of the need for two windows, it difficult to sample less than 150 mm from the laser head.

The data is as follows:
z= [0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425]*1e-3 + (72e-3+25e-3+62e-3); % Distance from the laser head. The fixed added on value at the end is the distance to the first measurement point
W_horz = [768 829 866.2 915.7 965.3 1018.0 1072.3 1135.7 1180.0 1231.7 1285.1 1346.9 1402.5 1462.6 1517.0 1568.1 1648.3 1700.1]*1e-6/2; % Horizontal beam radius
W_vert = [760.4 762.8 870.1 971.8 983.2 1119.2 1223.5 1231.2 1353.5 1428.1 1522.5 1558.4 1619.4 1729.9 1813.9 1856.0 1888.6 1969.1]*1e-6/2; % Vertical beam radius

The fitting routine, plot and schematic of setup are attached.

--

The fit to the data gave:
Horz. beam waist = 272.6917 um
Horz. beam waist position = -62.2656 mm
Vert. beam waist = 213.3873 um
Vert. beam waist position = -36.3761 mm

For some reason the horziontal data looks noiser, I'm not sure what is happening there.

-awade Mon Jun 20 12:16:23 2016

Earlier I posted a beam profile of the north laser path that seemed to have much greater uncertainty on the horizontal axis (see: https://nodus.ligo.caltech.edu:8081/CTN/1646). Antonio was a little concerned that maybe the iris or something else was clipping, this might have caused some defraction. 

I cleaned the first window and replaced the second window with another W2-PW1-1037-UV-45P (AR coated on both sides), then the profiling camera was realigned. A couple of screen shots are attached of the beam profile at a couple of points, it doesn't look nicely Gaussian close in, I'm not sure what is causing this, but this is the beam directly from the laser (via two windows).

The profiles were fitted using the D4σ method (standard and default) to find the 1/e^2 drop off of the beam radius (as was done in all the previous posts).

This time the data looks better for the horizontal.

--

The data is as follows:
z= [0 25 100 125 150 175 200 225 250 300 325 350 375 400 425]*1e-3 + (72e-3+24e-3+63e-3); % Distance from the laser head. The fixed added on value at the end is the distance to the first measurment point
W_horz = [752.9 804.0 957.5 1010.9 1077.6 1136.6 1190.7 1244.2 1301.1 1407.0 1461.6 1513.9 1585.5 1622.6 1668.0]*1e-6/2; % Horizonal beam radius
W_vert = [696.0 754.0 995.0 1066.2 1130.2 1210.7 1286.8 1356.9 1414.7 1580.1 1642.9 1697.5 1786.8 1859.7 1951.1]*1e-6/2; % Vertical beam radius

And fitted values are:

Horz. beam waist = 272.0952 um
Horz. beam waist position = -60.2164 mm
Vert. beam waist = 216.5147 um
Vert. beam waist position = -22.9082 mm

These are very similar to the previous measurement. Plots and configuration schematic are attached.

Earlier I posted a beam profile of the north laser path that seemed to have much greater uncertainty on the horizontal axis (see: https://nodus.ligo.caltech.edu:8081/CTN/1646). Antonio was a little concerned that maybe the iris or something else was clipping, this might have caused some defraction. 

I cleaned the first window and replaced the second window with another W2-PW1-1037-UV-45P (AR coated on both sides), then the profiling camera was realigned. A couple of screen shots are attached of the beam profile at a couple of points, it doesn't look nicely Gaussian close in, I'm not sure what is causing this, but this is the beam directly from the laser (via two windows).

The profiles were fitted using the D4σ method (standard and default) to find the 1/e^2 drop off of the beam radius (as was done in all the previous posts).

This time the data looks better for the horizontal.

--

The data is as follows:
z= [0 25 100 125 150 175 200 225 250 300 325 350 375 400 425]*1e-3 + (72e-3+24e-3+63e-3); % Distance from the laser head. The fixed added on value at the end is the distance to the first measurment point
W_horz = [752.9 804.0 957.5 1010.9 1077.6 1136.6 1190.7 1244.2 1301.1 1407.0 1461.6 1513.9 1585.5 1622.6 1668.0]*1e-6/2; % Horizonal beam radius
W_vert = [696.0 754.0 995.0 1066.2 1130.2 1210.7 1286.8 1356.9 1414.7 1580.1 1642.9 1697.5 1786.8 1859.7 1951.1]*1e-6/2; % Vertical beam radius

And fitted values are:

Horz. beam waist = 272.0952 um
Horz. beam waist position = -60.2164 mm
Vert. beam waist = 216.5147 um
Vert. beam waist position = -22.9082 mm

These are very similar to the previous measurement. Plots and configuration schematic are attached.

In a previous post (PSL_Lab/1641) I identified a possible mode hop not far from our typical operating point by tuning the laser diode current down two clicks from the typical Adj# = 0 point. It is likely that the slightly lower temperature/operating point of the laser diode put it closer to the edge.

The experiment was intermittently dropping lock last time it was in operation and it is likely that mode hopping in the south laser was a culprit.

I tried reproducing this by putting a 2 Vpp ramp on the slow temperature control input of the laser controller.  I was able to reproduce some clear mode hopping.  This behavior didn't really set in until the laser had had some time to warm up.  It seemed less prevalent at the default factory temperature set point of 48 C, but our operating point was closer to 34.4965 C (this is to match the north laser). At one point the mode hopping was close to the V_slowcontrols = 0 but after turning the laser off and on again I couldn't find this.

The added difficulty is that both lasers must be kept to within the beat note detection bandwidth and so we must find a sweet spot of no mode hopping between the two lasers.  Tomorrow, after some warm up, I will try some temperatures around 34.5 C to see if there is a sweet spot. Need to confirm with Aidan what the intended slow controls range is.  From the manual we know we should get about 3.1GHz/˚C with a mode hop at intervals of >10 GHz.  We don't need this much range but need to make sure the laser has the stable region appropriately centered.

Pictures all at the 34.4965 C setpoint after turning the laser off for a period and then back on

Attachment 1: What scan looks like shortly after turning on laser (0.01 Hz ramp + PD signal picked off from the laser path)

Attachment 2: Discontinous jump induced by temperature ramp (0.01 Hz ramp + PD signal picked off from the laser path)

Attachment 3: Some hoping, was much more distinctive earlier in the evening (0.01 Hz ramp + PD signal picked off from the laser path)

Sorry these look like they were taken with a potato, I didn't have a USB drive and phone was all I had.

Andrew got hopping mad about laser mode hopping, so we decided to sweep the laser control voltage and look for stable regions.  I wrote a script to automatically step through the laser slow control voltage while measuring the DC power on the precavity transimpedence photodetector.  It's on acromag2 under /home/controls/CTNWS/computing/scripts/LaserDCPowerMeasurement.py.  First block one laser's path, say the South, to the transimpedence photodetector, and then run

python LaserDCPowerMeasurement.py North 10.0

to slowly vary the North slow laser control voltage from whatever it currently is to 10.0 volts

The South path has a fairly linear dependence of slow control voltage and output laser power.  (The more negative the transimpedence photodetector voltage, the more laser power was output.  Also, increasing the slow control voltage decreases the laser temperature.)  Basically this plot is telling us that at the highest laser temperature we get the most power output.  We also see some junk at 0 volts, and nonlinearities at -7.5 volts, 3.5 volts, and 7.5 volts.

The North path is also linear, with weird junk around 0.0 volts.  There's also a strange leveling-off at 2.5 volts before a sharp decline.  The North Laser DC power plot seems noisier, but this is just an artifact of the ADC. 

The range of the North path laser power output is 0.035 volts, while the range of the South path is 0.35 volts, 10 times greater.  This could be related to the laser power caps: the South laser can output up to 2 watts, while the North is limited to 0.5 watts. 

The next step is to choose a slow control voltage region from these plots, find the precavity beatnote, lock each cavity, find the transmission beatnote, demodulate it with a PLL, and take a preliminary cavity length noise measurement. 

Attached is the data from shown in these plots.

This is a brief note on what laser temperature regions Andrew and I found to be bad for mode hopping or otherwise nonlinear behavior.  Laser mode hopping proved to be the main issue we didn't understand for the last three months until Andrew studied the problem carefully (See elogs 1865, 1863, 1858, and 1857). 

After we recalibrated the laser temperatures and increases the slow voltage rails from -2 and 7 volts to the full -10 to 10 volts in elogs 1857 and 1858, we found strong beat notes (precavity value of around -5 dBm) using the slow voltage control at the following voltages:

South: 0.5 V,     North: 8.3 V

South: -7.4 V,    North: 1.9 V

South: -6.73 V,   North: 2.76 V  (This is what we are currently locked at)

Beware of weak precavity beat notes on the order of -40 dBm, we thought these were the main beatnotes for a long time and could never find them on transmission, probably because they were higher order modes beating with a carrier and the HOM didn't make it through the reference cavity.

We also discovered some regions with nonlinear laser frequency behavior (i.e. our strong beatnote would sink to nothing at certain laser temps).  These regions were:

North: Directly below 3.9 V

North: Directly above 4.6 V

North: Directly above 8.8 V

If you are struggling to locate your beatnote, you may be looking with a mode hopping laser.  Try returning to the good zones above.

I made a script called SlowVoltageToHertzCalibration.py in the ctn_labdata/scripts/ Git repo.  It takes in a cavity scan like those found in ctn_labdata/data/20170817_LaserSlowFreqScan_RefCavResonances, with Laser Slow Control Voltage in the first column and Cavity Transmission DCPD Voltage in the second column.  The cavity scan changes the slow voltage between -10 V and 10 V.  Transmission light peaks with each FSR, and for the higher order modes... The script sorts out which peaks are the carrier peaks, finds the average slow voltage difference between peaks, and compares it to the FSR to generate a calibration.

The North Path laser slow control calibration is 3480.89915798  3480 MHz / SlowVolt

The South Path laser slow control calibration is 3644.56423675  3640 MHz / SlowVolt

The FSR for both cavities is 4069.94919902 4070 MHz for a cavity of length 3.683 cm.  The laser frequency is 281 THz.  (1 THz = 1000 GHz)

Cavity scans are shown below for convenience (except that I used a linear y-scale, so its not actually very convenient).

There has been a distinct drop in laser power in the south path since yesterday.

It looks like the drop in power happened between GPS* 1209950712 s and 1209966050 s (18:24:54 and 22:40:32).  This was in the lab when nobody was there.

I've been tuning the temperature on the cavities to bring the beatnote into range and the current south laser slow voltage is -6.2759 V when the cavity is locked -- as an aside cavity heaters are set to 0.455 W diff heating and 0.56831 W common heating.   This is about where it was before and tuning the laser slow temperature around doesn't seem to get back to the original power levels as reported by the reflection and transmission PDs.  So it seems we are not in a mode hopping region.  I've attached a dataview screen shot showing that the prior to the drop the south trans PD was giving 4.25 V and the reflected 1.094 V (when locked).  After the change in power (after some unlock time for the cavities) the south trans PD was 3.11 V  and south refl PD was 0.488 V (when locked).

The points at which the cavity was unlocked show reflected power dropped from 3.81 V to 2.56 V.  This doesn't look great and its not clear what is going on with the laser.

For reference the current power going into each cavity is 3.1 mW for the north and 2.02 mW for the south.  Visibility is good on the south at 62% (Refl Vmin=0.592, Vmax =2.55  V)

Not clear what happen here, there is no mode cleaner or ISS applied in the south at the moment so seems like the only source of a power change would be a misalignment or a change in the laser.  Nobody has been in the lab since last night and looking around with the IR viewer there is no apparent clipping.

For now I think 3.1 mW for the north is a little bit too high.  I've turned down north power to 2 mW to match the south and we will watch the changes in power over the next few days in both lasers.

* Note: the GPS time on fb4 has now drifted by 9 days, so not clear if this is correct time, it is certainly in the early hours of this morning.

On Oct 11th at 15:04:04, the south laser switched off on its own. I would like to know if anyone entered the lab around this time. Koji did mention that our Laser Safety sign outside was blinking, but I have no more information than that. Attached is the data of south PMC reflection DC, which is the first photodiode that measures the laser. It suddenly went to zero, indicating the laser was switched off and the locks did not drive it to this point. I'm also finding that the laser intensity is reducedasit used to saturate the South PMC reflection photodiode when unlocked but presently shows around 5V. I'm trying to put the experiment back to same parameters as before.

Code and Data

I put laser settings on both North and South Cavities back to default. From this point onwards, all settings about the lasers would be known and kept track of. The red values are the settings that were changed.

NPRO Laser Settings

Property	Display Symbol	North	South	Units	Notes
Laser Model	-	M126N-1064-700, SN 5519, Dec 2006	126N-1064-500, SN 280, Nov 1997	-
Diode Temperature	DT	22.3	28.7		Informational only.
Diode TEC Voltage	DTEC	0.8	0.7	V	Informational only. +ve -> cooling, -ve -> heating.
Measured Laser Crystal Temperature	LT	40.8	55.2		Informational only. Calibration dependent.
Laser TEC Voltage	LTEC	0.0	-0.5	V	Informational only. +ve -> cooling, -ve -> heating. Manual says typically should be 0.0V.
Target Laser Crystal Temperature	T	40.087 -> 40.0010	48.0010		Changed back to factory set value on North Side.
Laser Head Power Level	PWR	66  ->  624	92  ->  101	mW	Calibration dependent. CHanged the calibration to meet the power meter but even then, power meter says a maximum 500 mW, so North Side is not entirely correct. On the south side, it was difficult to mount power meter perpendicular to the beam, so there might be some clipping loss in calibration.
Power Adjustment	ADJ	0	-2  ->  0	-	From -50 (off) to +10. Changed the diode current around set value.
Diode Current	DC	2.06	2.04	A	It can be changed to change power level. Reflects measured value.
Diode Power Monitor	DPM	0.00	0.00	V	Calibration Dependent.
Noise Easter	NE	ON	ON	-	-
Laser Diode Status	LD	ON	ON	-
Nominal Diode Current	-	All the way clockwise	All the way clockwise	-	It can be changed by turning the left potentiometer from the back of the laser head. Factory default is all the way clockwise. I have set both North and South Lasers to this point.

Note:

While turning the nominal diode current of south laser all the way clockwise, I found that the laser power peaks before the maximum diode current is reached. This diode current is about 1.9 A. This is unexpected. Any explanations on this would be helpful.

After the last calibration, South Laser power at Laser head determined by its internal photodiode and show as setting PWR today (noticed today) dropped to 80 mW with none of the other settings changed. The diode current is still the same, so this could be one of the following two things:
1) The internal circuit to measure laser power at head went wrong.

2) Something is wrong with the laser crystal.

The laser power as seen by the reflection photodiode at South PMC captured a glitch on Dec 17th, 2019 8:30 am (might be 7:30 am in PST). The glitch shows that PMC went out of lock due to it but then returned to a laser power lower than before. The voltage level decreased from 2.72 V to 2.3 V after lock, which is equivalent to a drop of ~15.4%. The laser head power level according to PWR monitor dropped from 101 mW to 80 mW, which is equivalent to ~20.8%. While these don't match, PMC reflection is also not a true measure of power level. But I am sure this power has actually reduced and even if the laser head power meter is off a little bit, we have witnessed a big drop in power.

Restarting the laser (Put it on Standby and switch it back on) didn't change the power level.

Restarting the laser with turning the key off and on also didn't change the power level.

Even though 80 mW is enough for our experiment, this sudden decline in power shows there is something happening with the laser that we do not understand.

Data

The north laser power supply display is not working and when the key is turned to ON (1) position, the status yellow light is blinking. I'm able to switch on the south laser though with normal operation. But as soon as the key of the north laser power supply is turned on, the south laser goes back to standby mode. This is happening even when the interlock wiring for the north laser is disconnected which is the only connection between the two laser power supplies (other than them drawing power from the same distribution box). This is weird. if anyone has seen this behavior before, please let me know. I couldn't find any reference to this behavior in the manual.