I took a picture of the setup in PSL lab, and drew a line for laser path. I omit the mode cleaning part since it's not in use now.
we had several shutdowns of the laser within the last days. A couple of times the well known "HT error", today we had an "PS error" for the first time. When does this happen? The other error is related to a malfunction of the chiller as we found out by luck. The chiller temp readout jumps from 26 down to 15 or so within a fraction of a second (so it's not real). This causes the PS to start heating even if the temp is high enough. This screws up the stabiliy of the laser and sometimes causes a chiller error as well. But the "PS error" ? Any idea?
does anyone know the typical operating current for the 100mW lightwave laser model ? (M126N-1064-100) It's typically ~1.1A for the 200mW model. I've set up everything and it starts to lase around 0.44A, so at least its not dead but i don't know how high up i can go. My guess is that it is something around 0.8A but i have no datasheet which tells me...
so please bring it back - we are waiting for it and can't go on without those... For the future: if you take it, bring it back and don't wait until someone is missing it...
Peter gave me the hint that the default values are stored in an eeprom in the laser head. So connecting the head to a driver not used before shows the default values for the head. For this head it's 0.86A. So i measured the slope of the NPRO up to that value, reaching the 100mW at the default value stored in the head without tuning the diode temperature. So the head seems to be refurbished and not dead...
the laser stopped working while beeing in the lab but not touching the laser or table or anything. I was looking for an EOM mount and suddenly the chiller beeped. The power supply stopped working with a "PS error" message, but the more interesting fact is that the setpoint of the chiller was changed to 15degC (my guess is from the PS), but it should be at 26.5degC. So maybe the PS is also causing the freezing of the chiller, simply sending the wrong setpoint values when having a fault...
Last time, the calculated beam waist I got was based on two identical mirrors with no reflecting coating layers on the back mirror. When that layers are taken into account, I got the same waist size.
The new plot is attached below. Two sidebands are 21.5 and 35.5 MHz. The peaks, from left to right, are 9th, 6th, 3rd, 28th, 25th, 22nd, and 19th order. Choosing 35.5 MHz for frequency modulation should be fine.
The layout for the new PSL setup ( lenses, and their positions are to be calculated.)
A Lightwave 100mW NPRO laser will be the source. AOM will be in the ACav path.
Two cavities will be covered by a box of insulation/ heater.
1) From the laser to the PMC,
1/4 waveplate, to linearly polarize the elliptical polarized beam from NPRO
1/2 wave plates and PBS, to adj the power of the beam
lens, to focus the beam to the EOM
two lens, two mirrors, to mode match the beam to the PMC
a photodiode, a lens, two mirrors, (one for steering the beam, another one for attenuating the beam), for PDH locking
a photodiode and a ccd camera, for the beam behind the PMC
* there will be a Faraday Isolator somewhere here. I forgot to add it.
2) From PMC to PBS
a lens to focus the beam to 35.5 Mhz EOM
1/2 wave plate, to adjust the power between two beams for ACav and RefCav
PBS, to split the beam into two paths
3) AOM path
1 PBS, for reflected beam from the AOM
a lens, an AOM, 1/4 waveplate, two irises, 1 curve mirror; to double pass the beam and shift the frequency
another iris and a mirror, to select only the 1st order beam and send it to ACav.
4) RefCav/ACav path
1/2 wave plate, to correct the polatization
two lens and a set of periscope, to mode match the beam to the cavity
a pbs with 1/4 wave plate, a lens, a mirror, a photodiode, to PDH lock the beam
5) Transmitted beam
1/4 waveplate, to linearly polarize the transmitted beam
a photodiode/ a ccd camera to monitor the transmitted beam with necessary mirrors
lenses, to focus the beam to the PD that measures the beat signal
a beam splitter, to mix two beams together
I measured the beam waist of Lightwave NPRO 1064nm 100mW with WinCamD.
The nominal beam waist are 380 um and 500 um, 5cm from the center. the number I got from the measurement are 237 um (major) and 187.3 um (minor) which are quite different from the nominal values.
I'll check it again tomorrow to see if the data are still the same.
I measured the beam waist again. The laser was operated at full power ~100mW. A mirror attenuated the beam to 60 uW and ND 4.0 was on the CCD.
The fits give
Wx = 155 um, 3.45 cm in front of the opening.
Wy = 201 um, 2.8 cm in front of the opening.
Details for Mode Matching
1) Laser to PMC
The laser has
Wx = 155 um, 3.45 cm in front of the opening.
Wy = 201 um, 2.8 cm in front of the opening.
The average number for calculation is w = 180 um, 3cm in front of the opening.
First, we focus the beam to the EOM, w can be 250 – 500 um.
We pick 350 um.
F= 200 mm,
W1= 180 um
Distance from w1 to the lens, d1, = 9.36”.
Distance from w1 to w2, L, = 22.8 “.
Then we mode match this beam to PMC
Distance from w1 to first lens, d1, =157mm =6.2”
Distance from first lens to 2nd lens, d2 =120mm = 4.7”
Distance from 2nd lens to PMC, d3 = 358mm
Distance from w1 to w2, L, = 63.5 cm = 25”
I'll check if I can find f =200mm, 63mm, 50mm in the lab or not.
we still have serious problems with the 100mW laser head. I traced it down to the connection between the PCB and the hermetically sealed optical part. There is a really loose connection somewhere. The connector seems to be OK, the PCB and all solder points are OK too (visually). But if you slightly touch the PCB you can see the yellow LED flickering, if you touch it a bit more the laser goes off and on. This happens too if you touch the D-SUB cable on the back. I added some little stress to the PCB when putting the thing back together, now the situation seems to be better, but is far away from being gone. In order to get started we are using it now as long as we can and think about a solution in the meantime. One option would be to fix the busted NPRO Peter has. This would probably take about a week or so. We have to align the laser diode and focusing lens and solder (!) the focusing lens in place. The problem is that you can't align in vertical direction, so you have to remove the laser diode, put some more or less indium foil below and start again from zero. But it's an option. Peter had it already back to 550mW (out of 700mW) or so (for a couple of minutes, simply holding all parts in place. So the difficult part is to keep it permanent in the right place...
I'm trying to align the PMC. The transducer is connected to the PMC servo card's HV out.
HV in is driven by a function generator with triangular function. The output signal looks weird. It is not a nice triangular form, it's more like a u shape waveform connecting to each others with a plateau on top.
I'll check if the transducer on the PMC and the HV out from the card are working correctly or not. Right now, there is not a glimpse of signal coming out of the PMC on CCD.
don't the words HV IN ring a bell? look into the schematic. there are only a couple of inputs at the front so it's not too hard to figure out why there was already a BNC cable connected to that input. Did you check the monitor signal? If you would you would have seen that your HV supply is missing now
After checking the PMC servo card,
ramp signal goes to ext DC
HV in is applied properly,
HV out is fixed.
Now the PMC is scanning and working fine.
The alignment is done. Although optimization is still needed, I can work on the rest of the setup.
*note on PMC servo card
Ratio between Voltage input (Vin), V monitor (Vmon), and High Voltage output (HVout),
HVout = 24 Vin
Vmon = 1/50 HVout
Vmon = 24/50 Vin
see the attachment
We still need two SMA cables. One connects between 21.5Mhz EOM and PMC servo card, another one connects between PMC PD and the servo card.
The 21.5 and 35.5 local oscillator were in the wrong slots, we fixed them.
The 21.5 MHz photo diode that detects the reflected beam from PMC saturates at 15 mW.
Now I'm trying to optimize the PMC setup so that we have maximum transmittedd power.
I don't know why I can't lock the PMC. After changing the 21.5 MHz card because of the loosen SMA connector, the 21.5 MHz EOM is working.
The error signal looks good. I adjusted the gain, flipped the phase by 180 degree, and still cannot lock the cavity.
(The medm was frozen this morning, Peter helped reset it back to work this after noon.) It might be insufficient amount of power coupling into the cavity.
The minimum reflected beam I could get is only ~1/2 of the total DC power, I'll try to align the beam and move the lens a little bit more to see if I can optimize it better.
So I skip this PMC part for now, and pre align the path to mode cleaner. The mode matching is ok. Two lens, f1=114.5 and f2=286.3 mm are good. A CCD behind the cavity is set in place.
After adding another 1/2 plate to have P wave into the cavity, I can lock the PMC cavity. It's been 30 minutes so far.
There is only one transmitted beam now (there were two when I used S wave.)
When I work or knock on the table, sometime the beam switches to another mode (might be it's side band.)
It's very close to the main TEM00 mode. I need to adjust the DC offset a little bit to get back.
The incoming power is 9 mW, and the Transmitted beam has ~ 6 mW.
RF Amp: 7V
Phase shift: 2.87 + 180
Gain: 27.91 dB
DC offset: -2.3 V
I tried to pre align the ref cavity. It's harder than I thought, can't see the beam that well. I'll have to check the manual for the laser controller, so that I can
scan the RefCav when I align the beam into the cavity.
I add the broad band EOM in the beam path. After adjusting the periscope, I can steer the beam into the RefCav and see the reflected light. It's not aligned yet.
The 35.5 MHz is set on the table with a lens to focus the beam on the PD.
I'm not sure if I have to use the laser controller for 126 model or I could use the 10W laser to scan the beam, I'll consult Peter tomorrow.
Right now we are using the 10W laser controller to power the 100mW laser. The connector had been unstable, but now it's working fine.
It will be better if I can use the 10W controller to dither the laser frequency because I won't have to switch the cable, and avoid the risk of having to deal with the cable again.
Forgot to log this yesterday:
The PMC servo in medm's command window is correct.
I need to make SMA cables (properly insulated kind) too.
From 21.5 MHz PD to servo, 25 feet,
from 35.5 MHz PD to servo, 25 feet,
from 35.5 MHz EOM to signal box, 5 feet,
from 35.5 MHz LO to signal box, 20 feet.
This is the schematic of PSL fss servo.
I have to make sure that the modulation voltage will not exceed the controller's limit (0-100V.)
I switched the cable from the 10W controller to the original controller for 100 mW laser. It is working well now, the cables are tied properly.
For now, I don't need to use the FSS servo card to scan the laser frequency.
I'm using a function generator for fast channel (PZT), and a voltage calibrator for slow channel (thermal control.)
The alignment is in progress. With the aid of a CCD camera and a macroscopic lens, looking for the beam position on the mirror is getting easier.
Currently I see some light at the back of the cavity.
Since the frequency of the laser going into RefCav is determined by PMC ,I decided to temporarily remove the PMC for now, so I can scan the laser frequency while aligning RefCav.
It might not be a good idea since the PMC might alter the beam path a little bit, but I just want to align the cavity first.
The plan is after RefCav is aligned, I'll bring back the PMC and fine tune the beam going to RefCav again.
I still got many higher order modes coming out of the cavity.
the PMC is locked to the laser, so it follows the NPRO frequency when you scan the frequency
I see TEM00 transmitted beam out of RefCav. I think I have to fine tune the FSS gain a bit more becasue the spot still oscillates a bit.
The left monitor shows the spot from PMC. The right monitor shows the spot from RefCav. it looks distorted becasue of the filter.
The common gain is 16.1 dB
slow actuator is 9 V
Fast Gain 15dB
Optimization is yet to be done. There is still plenty of reflected power.
This is the schematic for PSL setup.
At this point, Pre Mode Cleaner (PMC) and Reference Cavity (RefCav) are locked. The rest will be locking Analyzer Cavity (ACav) and setting up for beat noise measurement.
ACav's beam path will have double pass AOM [Crystaltech 3080 194]. We'll use +1st order beam. When hook up the VCO, make sure that the power is on only when the VCO and the AOM are connected, otherwise the VCO dies.
Next is aligning the AOM. A good alignment will maximize the power of the +1st order beam. The beam should get close to the AOM's transducer as much as possible to minimize time delay.
The beam at the AOM will be focused to 75 um.
The mirror that reflects the beam back to the AOM is a 0.3m concave mirror, which will be placed 0.3 m away from the AOM. The reflected beam should completely overlap on itself. This will neutralize
the pointing instability when the modulating frequency shifts.
Then we can align ACav, this time I'll try not to remove the PMC when I scan the beam frequency (at~3-10Hz.) If the PMC cannot catch up with the laser, increase the gain of the PMC, sideband power.
ACav should be locked before Monday June 8.
I'm aligning the AOM. The R=0.5m mirror's position crosses the insulation border by 1.5" (see attached picture.) The black line on the table shows the border of the insulator. The mirror is on a translational stage.
I'm thinking of 2 choices to solve this,
1)using a mirror to turn the beam to the side of the table. The mirror will be placed after the AOM, around the edge the border.
2) using 2 mirrors (after the beam is split to RefCav and ACav's paths) to shift the beam path to the side of the table.
The first choice will be better, since I won't have to recalculate the mode matching, but there might be unexpected problems.
The VCO is working fine, I can see +/- 1st order beams coming out.
I'm aligning the AOM and maximizing the diffracted beam's power by positioning the AOM and adjusting the beam size by moving the lens.
For single pass, the maximum efficiency I could get is only ~60%, so for double pass, the power will be down to 36%, but for now I'll settle with this number.
I could not find the manual for Crystal technology AOM 3080-194. The closest one is model 3080-197 which is attached below.
I'm not sure what is the difference between the two model, but 3080-197 has 70% diffraction efficiency.
Because of adjusting the lens, the RefCav's beam path also changes, now I have to realign RefCav again.
Another step for AOM alignment is adjusting the mirror that reflects the transmitted beam back to the AOM again.
The distance between the mirror and the center of the AOM should be the same as ROC of the mirror.
After this I should be able to start locking ACav.
Because of adjusting the lens, the RefCav's beam path also changes, now I have to realign RefCav again.
did you measure the power of the vco? How much is it if you tune it to maximum?
Here a copy of a general datasheet for the 3080-194. maximum efficiency is ~80% @2W RF power. You should ask peter about the detailed datasheet which comes with each AOM and contains measured values for the one you are using. Measured values depend on the beam size and RF power. Typical values are 87% in reality.
Oh, I see, the beam diameter is 1100 um, I use 150um. I'll try changing the beam size and see what happens. Thanks Frank. I'll measure the power of the VCO too.
have a look into the datasheet which came with the AOM. Don't make it too large. Clear aperture is about 1.7mm max. You can also have a look into the manual of the 35W laser (ATF lab). It contains a copy of one of these datasheets as well (with the graph of efficiency vs beam size). You don't need more than 60%, but you should try to get around 50% for the double-passed beam as we don't have so much laser power in total available. Assuming the original 15mW on the RF detector you need about 45mW for the acav now and 15mW for the refcav, so 60mW total after the PMC. With the current 95mW out of the laser it should be no problem( in principle). After the isolator and EOM you might have something about 85mW upstream of the PMC which means you need 70% transmission through the PMC. Anyway, a larger beam size gives you better eff. If you make it 500um or so you should get 50% in the double-passed configuration.
I manage to get 70% efficiency from P wave. When I try S wave, I get 78% which is close to the specified value. So for double pass, the efficiency should be upto 50%. The beam size is ~550 um. I redo the mode matching calculation for the AOM (and also RefCav and ACav) and move the beam a bit to the side of the table so that the insulation box won't get in the way.
I'm aligning double pass AOM. After maximizing the power of the 1st order of the transmitted beam, I place the R=0.3m mirror to reflect the beam back to the AOM.
The mirror is mounted on a translational stage for a fine adjustment.
At the right distance L away from the AOM(L = ROC), the size of the reflected beam at the AOM should be the same as the incoming beam.
Thus, there are 3 things to adjust.
First is the angle of the quarter wave plate that rotates the polarization of the beam after 2 passes by 90 degrees.
Second, the angle of the mirror, and
third, the distance of the mirror. At right position the power of the 1st order beam should be maximized.
I might have to change the position of the PBS that reflected the AOM double passed beam. Currently, the PBS is placed before 2 mirrors that move the
beam to the side of the table to avoid the insulation box. The problem is the double passed beam might clip on the mirror. So now I put the PBS after the steering mirrors, just in front of the AOM, but this limits the space for mode matching. I'll have to check which one will be better. From the attached picture, two PBS's are placed on two possible locations. On the bottom right the, and down at the middle next to the AOM.
The maximum power after AOM double pass is 37%, worse than the expected 50% efficiency, but it should be enough.
The good news is, a new mode matching (RefCav and ACav) is calculated, and all positions for the lenses are clear.
I got all the lenses, and borrow one plcx-24.5-51.5-c-1064 from 58C
The problem about the position of the PBS is solved. It will be at the original place, since the clipped beam is the 0th order of the reflected beam which we do not use.
I'll put the lenses in their places and try to lock RefCav again.
When I try to lock the cavity, it's not very stable yet. The transmitted power fluctuates alot.
I try changing the gains, but still could not stabilize the lock . The transmitted beam power is about 60% during the stable lock (I got it nicely locked for 5- 10 mins.)
After RefCav is locked, I'll try to optimize the transmitted power, by adjusting the lenses' positions before moving on to work on ACav
RefCav is locked , the beam is more stable than yesterday setup. I'll write down the values of the setting for a quick reference.
I'm not sure what universal names for all these channels are. I just explain them in more details for my future reference and next generation archeologists.
Common Gain( for both fast and PC paths): 23.6 dB (fast path controls the PZT which changes the length of the NPRO, PC path controls the phase
shift of the beam)
Fast Gain ( for PC path only): 12.5 dB
Phase shift: 0 + 180 degree. ("+180 degree" means phase flip)
RF Amplifier Adj (power for 35.5 MHz sidebands): 6.28 V.
Thermal control ADj (a voltage calibrator connected to slow channel of the laser controller): -0.010V
Servo gain Adj (over all gain of the demodulated signal): 27.75 dB
Output DC offset (offset voltage that governs the length of the PMC): -3.12 V
Phase shift: 2.87 V + 180 degree
RF Amp Adj (power for 21.5 MHz sidebands): 5.36V.
Now I'm working on ACav path. I made a cable for a photo diode.
I'm not sure if the last PD is a working 35.5 MHz PD, I'll see if it works or not.
Now I'm using two 2-channel monitors to simultaneously see the beams after PMC and RefCav. It will be more convenient if I use a 4 channel monitor, I'll clear some space for it.
nice good job!
you can get the names for the channels if you click on the corresponding object (slider, button, textbox, ...) using the center mouse button. You will get a green on black box containing the full channel name...
both 35.5MHz photodetectors should be ok as we used them in the previous setup already...
Now I'm working on aligning the beam into ACav. I got the reflected light on the PD, and I'll scan the cavity soon.
My plan on connecting the servo:
I'll use a power splitter to split 35.5 MHz signal from "LO to SERVO" channel on the crystal frequency reference card, which is driving the 35.5 MHz EOM, to beat with the PD's signal.
If the power is too low, I might use a Marconi to beat the signal for ACav, with appropriate power level.
I also need to check which power splitters and mixers are suitable for our power output.
The demodulated signal will be filtered by a 50 Ohms low pass filter before sent to "Servo Input" channel of the Universal PDH Servo box (D0901351.)
The box has two knobs that allow us to change gain and LO phase manually.
From the PDH box, the "Piezo Drive Out" will be connected to the VCO's External Modulator channel.
I try to adjust the voltage of the VCO that maximize the 1st order beam from AOM. I use 5 V which is maximum on the medm control screen, but I'm not sure if it's the best or not because,
the power in the 1st order still goes up even though I reach 5V (see the attached plot.) There is an attenuator on the AOM which Frank left for me. I'll check the power that goes into the AOM and check the manual again how much power it can take. If it can take more power, I'll remove the attenuator and see if I can get more efficiency. But I'll do that after aligning ACav.
There is no attenuator connected to the AOM. The device on the AOM (the white attenuator looking like thing) is a DC-blocker which protects the AOM. The high-power attenuators which have to be used to attenuate the high RF power in order to measure it are the black, radial heatsinked parts.
I'm scanning the laser to align ACav. It's a long day of adjusting 4 knobs and 1 lens (and one periscope for a while.)
I see higher order TEM modes at the back of the cavity, but still cannot see TEM 00 yet.
Thu Jun 10 00:20:39 2010
I saw TEM 00 and trying to minimize the reflected power on the PD.
I just realize that the beam path is very close to the edge of the hole (see attached.) Part of the beam might be clipped.
I'll check that with IR viewer tomorrow.
The value for Voltage Calibrator is 6.17 V.
Thu Jun 10 00:58:56 2010
From yesterday, after getting TEM 00 out of the cavity, I checked the beam if it's clipped on the edge of the hole or not. There is small light on as seen by an IR viewer. Since it seems to be very small, I'll leave it as it is for now.
There was one problem. The beam was almost on the edge of the periscope's top mirror, I decided to change the height and move the periscope , and other optics in the row, side way, since the beam was really close to the edge of the opening ( I set the beam path to the center of the hole before, so it's bit off ( 5mm, 0.2") from the cavity's natural axis), and made sure that the beam is on the center of every mirror. Then, it's 4 knob adjustment which takes me a whole day again .
As of now, I got TEM00, out of the cavity. I still have to adjust the lens' position to minimize the reflected beam. Before doing so, I'll prepare a mixer, a power splitter for locking the cavity.
One thing about the AOM, the beam after double pass is quite elliptic. I'm not sure how to correct it, and whether it's going to be a problem or not. I'll find something to read about this.
Fri Jun 11 01:00:17 2010
The new driver for AOM is working, the maximum power is ~1.3w. There's a switch for int/ext signal.
We use internal signal to drive the AOM for alignment purpose.
The mode matching for ACav is on hold. Because the1st order beam coming out of the AOM looks very elliptic.
I'm not sure if it's the result of the large beam size in the AOM or the alignment problem.
the beam might clip on the edge of AOM, the beamsize is quite large, the data sheet give 80% 1st order efficiency for 1100 um diameter spot [AOM] and I adjusted the beam size to maximize the power before. I'll try decrease the beam size and see if this reduce the elliptical shape of the beam. Once the beam size on the AOM is determined, the rest of the mode matching can be calculated.
1) DC ext channel on PMC servo: 32.82 MHz/ V
2) Fast channel on the laser controller: 3.07 MHz/V ( Our Fn generator can provide +/- 20V -> 60 MHz span, this won't let us see all three peaks of the error signal from RefCav which has 35.5x2 = 71 MHz span)
3) Temperature actuator on the laser control: 220 MHz/V. (1FSR= 714 MHz, 2x cavity length = 0.42m)
This measurement is done by scanning the laser frequency (1 and 3)or cavity length (2) and see the error signal.
The two sidebands are 43 MHz apart , divided that by the corresponding Voltage difference of the sidebands to get the calibration. For Temperature actuator we see use the PMC's FSR instead of the sidebands for better accuracy.
I moved the laser driver to the electronic rack and it should stay there for the final setup.
A cable for interlock is made and connected to the laser driver.
A cable for slow actuator is also made and connected. Now we can use the medm FSS screen instead of the voltage calibrator
to adjust the NPRO's temperature (slow actuator.)
I went to 40m lab for a measurement
laser & PMC:
re-characterized the laser and PMC
new laser temp setpoint : 44.1 C - almost centered between mode hops,
also PMC resonance close to that with centered PZT offset
current values for TEM00:
RCAV resonant at 0.0182V
ACAV resonant at 0.0566V
When both cavities are locked, I measured
1) power fluctuation behind ACAV (DC@0.55V) and RCAV (DC@2.2V)
2) power fluctuation behind PMC (DC@3.17V)and RCAV (DC@2.2V)
the HEPA filter was on, the SLOWDC loop was engaged.
SR785 is set to AC couple mode, auto range on both channels.
The level of RIN from RCAV from two measurements are a bit different,see fig2.
RIN from ACAV is not significantly higher than that of RCAV. This is good news, I thought AOM might pose more pointing instability to the system.
Today I investigated which optics reflect most power back to the PMC and the laser.
The back reflection seems to limit our sensitivity.
the power meter is placed to measure the reflected light that comes back and reflects off the PMC's mirror.
The beam path to the ACAV is blocked and ignore for now, since AOM in the path will act as an optical isolator.
The reflected power from this path will be smaller than that of Rcav's path.
periscope [M] 1/4&PBS [L] PLCX [K] Mirror
Blocked PBS [F] PLCX [E] 1/2 [D] EOM [C] PLCX [B] 1/2 [A] \ pmc / <---- beam
A razor blade beam dump was placed at each position (alphabet in square brackets).
The red entries corresponds to second set of data after realignment
B 52.3 uW 49.2 uW
C 0.184 mW 0.184mW
D 0.748 mW 0.745 mW
E 0.748 mW 0.752 mW
F 0.749 mW 0.754 mW
G 0.831 mW
H 0.831 mW
I 0.831 mW
J 0.831 mW
K 0.831 mW
L 0.831 mW 0.88 mW
M 0.831 mW 0.88 mW
reflection from RCAV 1.53 mW 1.96mW
It seems that the reflection from the cavity is giving us a hard time.
The PBS with 1/4 plate might not well adjusted to minimize the back reflection.
After I minimized it by tilting/rotating the PBS, the PBS was tilted around 5 degree (judging by my eyes) and it looked bad.
The beam is almost clipped on the edge, and Frank noticed that the beam height was not right.
I realigned the beam again (the efficiency is ~97%).
Unfortunately, I lost the beam so much I had to remove a few optics ( 2 mirrors adjacent to the RCAV) behind RCAV,
and used a CCD camera to see the beam while adjusting the knobs. I placed the optics back, but
I haven't adjusted the beam path for beat measurement yet.
Now the PBS is quite flat on the table.
I measured the power reflected at each optic again (see red entries in the table above.)
I rotated the PBS to see if the power reflected from RCAV can be reduced. It turns out that
I couldn't do much, only from 1.96 - > 1.93 mW.