9. Project Lasers
An excellent description of the basic properties of lasers, from electrons to NLOs and beyond, is given in Chapter 1 of [O59]: user manual for Quanta Ray / Spectra Physics GCR series of lasers.
The project utilises several lasers, as follows:
# Manf Part Type Class Col λ Mode Beam Energy Power Purpose
1 Chinese - Diode II Red 632.0nm CW 1.0mm - 1mW Alignment
2 Axcel CMA64 Diode IV NIR 1064.0nm CW 100µm - 2W Optics setup
3 Synergy - Diode IIIA VIS 532.00nm CW ? - 200mW Optics setup
4 Kigre MK367 Nd:YAG IV NIR 1064.0nm Q-swt 3.0mm 25mJ 6MW @ 4ns Prototype
5 Sunrise M580KK Nd:YAG IV NIR 1064.0nm Q-swt 5.0mm 1J est. 140MW @ 8ns LHC
6 Sunrise unknown Ho:YAG IIIa NIR 2100.0nm CW 4.0mm 260mW Future experiment
7 DIY Lumenis Nd:YAG IV NIR 1064.0nm CW 4.0mm Experimental
8 DIY rod Ruby IV Red 694.3nm CW 8.0mm Future experiment
9 DIY Lumenis Alexandrite IV Red 700.0nmT Q-swt 4.0mm J est. Future experiment
10 DIY rod Ti:Sapphire IV Red 700.0nmT Q-swt 8.5mm J est. Future experiment
11 DIY rod CrTmHo:YAG IV SWIR 2081.0nm Q-swt 4.0mm J est. Future experiment
12 Lp Phys 50S Argon IIIa Blue 488.0nm CW 0.7mm - 5-50mW LHC
13 Lp Phys 500M Argon IIIa Multiline CW 0.7mm - 30-300mW LHC
14 Rofin 6329A HeNe II Red 632.9nm CW 1.0mm - 1mW Optics setup
15 DIY - TEA CO2 IV LWIR 10640nm CW TBD J est. 100W Future experiment
Key:
T - Tuned
Class - [O2] Class I <1mW, Class II 1-5mW, Class IIIa 5-500mW, Class IV >500mW
CW - Continuous Wave
Q-swt - Q-switched
LHC - Main project
Axcel - Axcel Photonics
Lp Phys - Laser Physics
More information
The following webpage is an excellent primer on lasers:
Chapter 5, pages 183,184 of 'The electro-optics handbook 2nd Edition, 2000' by R.Waynant, M.Ediger [O29] features tables showing the most likely excitation sources for several laser media, including:
Alexandrite 700nm - 820nm flash lamp
CTH:YAG 2081nm flash lamp
Nd:YAG 1064nm flash lamp, LD
Nd:YLF 1047nm flash lamp, LD
Ti:Sappire 660nm -1200nm flash lamp + Nd:YAG -> 2HG, Argon laser
1 Chinese laser diode, generic cheap LD module, cost about $5.
This will be used as an optics alignment laser, shone through the laser rods.
Add 20801...JPG photo of LD
2 Axcel Photonics CM-A64-2000-150.
Bought 4 on eBay from Chinese seller laserdirect who said they were new from Laser Components USA, but they are just a distributor, although the seller seems genuine. The Axcel Photonics CM-A64-2000-150 (C-Mount 1064nm ±5nm 2W 100µm aperture) on their website was the closest match I could find and it seems likely they are this or a very similar part. The seller provided power curves and confirmed VF 1.5V to 2.0V. The lasing threshold of the Axcel parts is around 1.1 to 1.2V. I bought these to experiment with 1064nm optics and test the viability of beam attenuation before the liquid cooled flash lamp pumped Nd:YAG is operational.
I built my own custom driver using a Wavelength Electronics WLD3343 hybrid LD driver chip [D16], a heatsink and fan coupled to two TECs, and a NTC thermistor, driven by a simple LM339 window comparator feedback loop to control the temperature with a trim pot. The LD worked for a short while then failed, but not before I had verified it lased at 1061nm, see below. I'm not sure if it failed due to a localised hotspot, or simply ESD. Despite taking precautions at the time, I'm inclined to think the latter and I have since upgraded my lab's ESD facilities (the literally shocking nylon carpet now has a grounded ESD mat over it) and invested in Lasorbs [D4] to protect the remaining LDs for future experiments, for which I later acquired dedicated TEC [I18] and laser [I19] driver instruments. At $100 a LD, I'd rather not blow any more.
Below left, spec sheet for the 2W 1064nm CW LDs Below right, TEC comparator pcb
Above left, testing the prototype TEC comparator circuit (LD is OFF!) using a thermocouple-capable DMM before building it on the blue pcb (top right photo) located on top of the small fan that cools the heatsink fins. The comparator feedback loop regulates power to the two Marlow RC6-4 TECs [D17] (see photo below left) connected in parallel to the main +8V supply. The thermistor (twisted black & white wires) is located on one corner of the LD heatsink that is secured to the top (cool side) of the TECs.
In the above right photo the 1064nm LD can be seen to the left and like the metal top surface of the WLD3343 driver chip to its right, is bolted to the top of the heatsink.
Below left: the white gunge-looking material around the female crimp contact mated with the LD lead, is in fact solid polymorph smart plastic material [G7], that I moulded into shape after melting it in water heated to 62°C.
The pins on the bottom (red/green) side of the WLD3343 LD driver plug into a DIL socket on the pcb above it that also has the assembly power switch and its LED. This pcb contains the minimal ancillary components for the WLD3343 driver, such as its large 1Ω 5W sense resistor (top right: cermet, white), and white adjustment trimpots on the pcn underside below the LED, middle of the photo below, left.
The large vertical pcb on the left side is an Astec 5V 2A dc converter for the logic and laser driver.
Below far right: my Stellarnet Comet SR spectrometer confirmed LD lasing at 1061.45nm at 25°C.
Below left: The small pcb with two electrolytic cans each side of a blue trimpot, is an eBay China +24V step-up converter to drive the fan which was in my parts box. Centre, a strip of fluorescent paper helps reveal the 1061nm beam to my Panasonic FT4 camera. Finally, the diecast aluminium box housing the entire assembly.
ADD CCT IN PLACE OF 20809
3 Synergy S500
Type: LD module
Pump: 808nm LD
Dimensions: Total 160mmx120x50mm
Extras: Controller & PSU
Mode: CW
Wavelength: 532nm via NLOs
output power: 200mW (2W LD fitted)
Polarity: unknown
Bought non-working for £45. The label says 500mW max. Replaced the original C-mount LD with a $17 1W Chinese one. Without altering the electronics max power is 200mW which is fine = extended life.
4 Kigre MK367
Type: Nd:YAG
Pump: Flash lamp
Dimensions: Total 98mm x 4mm dia
Mode: Passive Q-switch
Wavelength: 1064nm
Output energy: 25mJ
Output power: 6MW @ 4ns
Max frequency: 0.3Hz
Polarity: S polarised
This is the prototype LIBS laser.
Meredith Nd:YAG with FO focus lens
5 Sunrise Technology? M580KK
Type Nd:YAG
Pump: Flash lamp
Dimensions: Rod 78mm x 5mm dia
Mode: CW
Wavelength: 1064nm
Output energy: 1J (Q-swt Est.)
Output power: 140MW @ 8ns (Est.)
Max frequency: 20Hz (Est.)
Polarity: TBD
M580KK Nd:YAG with MK367 at front
Bought on eBay from a Russian sounding seller based in the USA who provided photographs of it firing. No manufacturer name save for a label Kigre M580KK. The seller said it had come from a German medical company. Kigre confirmed: 'The pump chamber is model FC-M580KK and was not shipped with a laser rod or flash lamp so we don't have any energy specs. The resonator bench isn't ours. The recommended flash lamp was our K300. The filter tube is Kigre's KK-1 glass'. A later auction for a laser assembly made for Sunrise Lasers looked surprisingly similar and Alma GmbH makes lasers for them, so I am assuming this is an Alma laser. The flash lamp manufacturer and model is unknown. The seller got it running off 1750V using a Kaiser LS1000 power supply he had modified, but I think this voltage is too high.
I bought a very similar one from Meredith Instruments eBay as a spare, but I have no idea if it works. This one seems to have a high power FO focusing lens on its output (see left end of top right photo). Although not obvious in the photo, there is a gold plated trigger pin protruding out of the left moulding at the back confirming it has a flash lamp, not an arc lamp.
A FO is ideal for my experimental mechanical Q-switch idea as I hope to use the same rotating disk to gate the laser from the PCM401 MCP image intensifier.
6 Sunrise Technology
Type: Ho:YAG
Pump: Arc lamp
Dimensions: Rod 88mm x 4mm dia
Mode: CW
Wavelength: 2100nm
Output energy:
Output power: 260mW
Max frequency: N/A
Polarity: TBD
This is for a future experiment
Bought on eBay for $100 with the description 'unknown industrial laser' because although it was enclosed in a metal case, its exterior looked strikingly similar to the Meredith MK580KK. I was curious to learn where the large number of wires entering its enclosed body went as it might explain what the extra machining holes in the MK580KK chassis were for, although they were mostly what I guessed.
I discovered the HR mirror end has a beam detector. the output end has a Coherent 111670 635nm 1mW red laser & detector on an electromagnetic shutter. This LD may be an aiming point for surgery, because the shutter has a slotted opto switch.
I mistakenly assumed it was another Nd:Yag and it only occurred to me to ask the seller if they knew what it came out of after I bought it. Its originating equipment was a Sunrise Hyperion LTK HoYAG eye laser system, and the rod is indeed Holmium green. LTK = Laser thermal Keratoplasty = 'A surgical procedure in which the tissue in the clear front part of the eye (cornea) is re-shaped by a holmium YAG laser in a predetermined manner to correct farsightedness'.
There is no flash lamp trigger; I conclude this is an arc lamp CW laser. Whether the arc lamp still works is moot (manf date Oct 2000), but SFM 2100nm + 213nm = 193nm so there is a potential future experiment here. This combination closely matches the only 193nm SFM NLO I've found so far, although impractically expensive:
Lithuania ESKMA Optics BBO SFM 213nm + 2130nm to 193nm, part# BBO-0423-03H €1135 (Mar 2019):
http://eksmaoptics.com/nonlinear-and-laser-crystals/nonlinear-crystals/beta-barium-borate-bbo-crystals
7 Lumenis water-cooled block with 97mm x 4mm dia Nd:YAG laser rod but no lamp, which was believed to be an arc lamp providing CW operation. If this is true, the characteristics of the rod will not be suitable for a pumped flash lamp so I acquired some arc lamps. This is for a future experiment to build a Nd:YAG to complement the experimental Alexandrite and Ti:Sapphire lasers.
1 x Lumenis Sharplan AA2636700 QCW201 Nd:YAG 154mm x 4mm bore x 50mm arc 20A max krypton ARC lamp
3 x ILC Tech / PE L-1733 quartz arc lamp 192mm x 4mm bore x 51mm arc 110V 22A max krypton ARC lamp
ALE YD-4 Arc Lamp PSU, 0-30A 0-150V with ignitor & boost & simmer
8 Ruby laser built from scratch.
This laser was for a future experiment to explore the possibility of sum frequency mixing Ruby 694.3nm with Nd:YAG 1064nm 4th harmonic 266nm to produce 193n, notwithstanding the non-availability of SFM NLO.
Ruby lasers rarely come up on eBay and when they do they are usually expensive, probably because nowadays they are of historical interest compared to the more mundane Nd:YAG.
A particular feature of ruby is it needs all light that can be thrown upon it and even minor shadows can cause lasing to cease. The helix flash lamp idolised after Theodore Maiman invented the first ruby laser using one in 1960, is a poor choice for this reason (more below).
In 2014 I bought one of the then few affordable ($100) used ruby rods from eBay Israel, advertised as 8mm dia but turning out to be 6mm, for which I got a 25% refund. This rod was pink along its entire length, complicating any mechanism holding it in place that would inevitably restrict light to the rod.
Around 2015, used ex-USSR ruby rods began to appear on eBay with bubble defects that would have caused them to be swapped out for new replacements, together with poorly stored new rods with chips rendering them equally useless. Not expecting new rods to appear, I bought one with minimal defects. Unlike the Israel rod, this had a ruby red centre section and transparent ends, satisfying ruby's lust for light: the transparent section allows opaque clamps, O-rings, etc. to be fitted without obstructing flash lamp light from reaching the chromium doped lasing section.
Then out of the blue in 2020 cheap new USSR ruby rods arrived accompanied by compatible USSR flash lamps See bottom of [Flash Lamps]. Inevitably, over time prices began to rise but ever more exotic rods appeared, perhaps culminating in 2021 when I saw a new 640mm x 45mm Nd:YAG rod for a mere $300 that sat there for several weeks. Temptingly beautiful, but who could use such a behemoth?
The sudden influx of affordable new USSR ruby rods meant I could replace my less than ideal earlier purchases, which is why I ended up with a third rod.
However a downside of USSR rods is a low Chromium doping level of around 0.02% to 0.03% vs the 'normal' 0.05%. This affects the lasing threshold, meaning the threshold is reached sooner and more light input produces less laser power output. Even so, I still hope to produce 1J of 694.3nm from rod #3.
DRIVE ELECTRONICS
Amongst my ruby rod driver research I came across Ben Krasnov's home built ruby laser blog and asked him how he chose his rod: http://benkrasnow.blogspot.co.uk/2014/01/ruby-laser-design-process.html
Ben said he'd used an Excel spreadsheet created by Doug Little and sent me a copy, also available at the Sam's excellent laser website: http://www.repairfaq.org/sam/laser/pfncalc1.xls
[O28], Mass Spectrometry Basics, 'Chromium [Cr] ions produce the red colour of ruby because they absorb blue/green light from white light, leaving unabsorbed red light to be transmitted.'
RUBY ROD STOCK
Unlike #2 and #3, the Israel rod is red throughout. Sold with the description 102mm x 8mm but on arrival found to be only 6mm diameter.
Rod #1 Cr area = 102mm x 6mm dia, πxDxL = 1923mm² [Israel]
#2: USSR used rod, total length 120mm, with a 75mm red section in the middle of two transparent end sections that lack Chromium, and which are for mounting the rod:
Rod #2 Cr area = 75mm x 8mm dia, πxDxL = 1885mm² [ex-USSR] ( 2% less area than rod #1)
#3: USSR new rod
Perfectly matches EG&G flash lamp arc length. The total length is 180mm and as with #2, there are two transparent end sections:
Rod #3 Cr area = 120mm x 8mm dia, πxDxL = 3016mm² [ex-USSR] (36% more area than rod #1)
The rods below are totally unsuitable due to their various defects, however the round ones are a similar construction to rods #2 and #3 above.
All of these poorly stored ex-USSR rods are 10mm dia x 180mm long with a 120mm long Chromium section:
Several of the USSR ruby rods have unconventional shapes although all have clear ends; some have a rectangular section and others have rings of bevelled edges; some a combination of both. I have seen it mentioned these were designed to work with helical flash lamps but I am not convinced due to the likelihood of shadows between the bevels. I suspect instead the bevels are intended to project more light into the rod from a conventional linear flash lamp.
These rods are unsuitable for lasers due to damage from poor storage but they were relatively inexpensive and I bought a few to investigate deep UV absorption [See FUTURE EXPERIMENT *TBD*]. They came from eBay seller rusgems based in Thailand, both on eBay and their website rusgems.com, and I highly recommend them. They also supplied these rod photos.
Helical (spiral) flash lamps are NOT a good idea
The first ruby laser was created by Theodore Maiman at Hughes Research laboratories on 16 May 1960,
using a 10mm dia x 15mm long ruby rod in a helical flashlamp: https://en.wikipedia.org/wiki/Ruby_laser
http://www.repairfaq.org/sam/lasercps.htm
'Note that while a helical flash lamp may look "cool", it's generally easier to deal with linear flash lamps for pumping laser rods - too much light energy is lost to reflections from the coils and out the ends of the cavity even if a perfect reflector surrounds them. Additionally, the long discharge path means helical flash lamps will tend to have larger Ko values and thus require higher voltage to achieve the same pulse width. If the helix can't get close to the Ruby rod (touching), it might require close to 1000 joules for the Ruby to lase compared to few hundred joules for a linear flash lamp.'
Applied Photonics, C.Yeh:
'The repulsive force between adjacent coil turns of a helical flash lamp may mechanically flex the glass tube and crack it, limiting both pulse width and input energy.'
EXCITING THE ROD
A useful reference for ruby laser design is [O17]: Design and Optimisation of Ruby Lasers in the USSR.
http://en.wikipedia.org/wiki/Laser_pumping
'The rod and the lamp are relatively long to minimize the effect of losses at the end faces and to provide a sufficient length of gain medium. Longer flash lamps are also more efficient at transferring electrical energy into light, due to higher impedance.[2] However, if the rod is too long in relation to its diameter a condition called "pre-lasing" can occur, depleting the rod's energy before it can properly build up.[3] Rod ends are often antireflection coated or cut at Brewster's angle to minimize this effect.[4] Flat mirrors are also often used at the ends of the pump cavity to reduce loss
Another configuration uses a rod and a flash lamp in a cavity made of a diffuse reflecting material, such as Spectralon or powdered barium sulfate. These cavities are often circular or oblong, as focusing the light is not a primary objective. This doesn't couple the light as well into the lasing medium, since the light makes many reflections before reaching the rod, but often requires less maintenance than metalized reflectors.[6] The increased number of reflections is compensated for by the diffuse medium's higher reflectivity: 99% compared to 97% for a gold mirror.[7] This approach is more compatible with unpolished rods or multiple lamps.'
RUBY ABSORPTION PEAKS AT 400nm & 560nm
Examining the graph below, where:
|| = incident light parallel to c-axis
T = incident light perpendicular to c-axis
AU = arbitrary unit
<=200nm intensity AU >> 3.6 regardless of T or ||
~260nm intensity AU = 0.85 T, 1.0 ||
404nm intensity AU = 3.2 T, 2.8 ||
554nm intensity AU = 2.8 T, 1.4 ||
From this graph appears ruby rod is best irradiated into the rod end T at 404nm and along the rod || at 554nm, but 532nm intensity is only 1.0 T so there is no real advantage to end pumping by 2HG 532nm Nd:YAG.
Below left, Northrup-Grumman's ruby absorption plot Below right, [O48] Page 6, Fig.2
UV ABSORPTION
Interestingly, absorption below 200nm goes off the scale. At first I wondered if it might have been a 'run-in' where the spectrometer began to work, rather than real data. Eventually after much fruitless searching, I emailed Northrop-Grumman not expecting to receive a reply, but instead one came almost immediately:
'The spectrum we have online is typical of Ruby. It shows a significant rise in the 200nm region, which is normal. The band-edge for sapphire is ~190nm or so and by doping sapphire with chromium (thus making Ruby) there will be an increase in the near band-edge absorption.'
I later found the following paper citing specific ruby absorption DUV wavelengths in terms of eV:
https://aip.scitation.org/doi/10.1063/1.1726965
'The absorption spectrum of thin samples of ruby with 0.5 to 0.005 weight % of Cr2O3 has been measured up to 8.9eV at room temperature with linearly polarized light. The spectrum shows an absorption peak at 7eV [177nm] followed by humps around 8eV [155nm] with absorption cross section and oscillator strength at the 7eV [177nm] peak ~7×10-17 cm² and ~0.3, respectively. The absorption with E [perpendicular to] c is generally larger than E||c in the vacuum-uv region. The absorption between 6.8eV [182nm] and 8.6eV [144nm] is interpreted as a charge-transfer transition around the chromium ion because of its large oscillator strength and some similarity to the ultraviolet reflectance spectrum of Al2O3 crystal.
The excitation spectrum of ruby cubes with several chromium concentrations has been measured to 11eV at room temperature with linearly polarized light. Structure and polarization effects corresponding to those of the absorption spectrum are observed. The quantum efficiency in the region 7eV [177nm] to 9eV [138nm] is comparable to that in the blue absorption band of ruby and is essentially concentration independent. Above 9eV the relative quantum efficiency drops about two orders of magnitude and is concentration dependent, with low quantum efficiency for samples of low chromium concentration.'
I appended the nm equivalent of the eV above using the following app that converts eV to nm:
http://halas.rice.edu/conversions
9.0eV = 138nm, 8.9eV = 139nm, 8.6eV = 144nm, 8.0eV = 155nm, 7.0eV = 177nm, 6.8eV = 182nm
Furthermore, the XLS calculated %PI [Population Inversion] threshold equation is greatly affected by wavelength. I'm guessing the value of 475nm for ruby absorption may be derived from the area of the curve for ruby absorption, so takes into account ALL of the above peaks. The flash lamp optical characteristics are also included - read the embedded comments: top RHS red triangles. The graph implies there is great absorption ~200nm and low level 200nm is generated by the flash lamp.
The deep UV absorption band is of interest because it is far more sensitive than any of the other wavelengths, and my EG&G Xenon flash lamps have a quartz wall which passes UV. Whether they produce the right UV wavelengths is yet to be determined.
DOES UV RADIATION DAMAGE RUBY?
I needed to determine how destructive UV radiation is to ruby. If the answer is it can survive a thousand shots, that is more than enough to complete my experiments.
US National Bureau of Standards, Electron Paramagnetic Resonance Intensity Standard SRM-2601, [O27]
http://archive.org/details/standardreferenc2605chan, Page 45:
'5.5 Precautions Concerning Visible, UV, X-Ray and Nuclear Radiations
It is well known that ruby suffers damage from intense radiation [8], Usually nuclear radiations are not encountered in EPR [Electron Paramagnetic Resonance] experiments. However, it is possible that x-ray, UV, or strong visible light may be used in certain applications. This may cause damage or may induce undesirable fluorescence.
Damage to ruby can be detected by a change of color, from pink to orange. This can be best observed against a dark background, preferably by comparison with an undamaged ruby SRM [Standard Reference Material]. Exposure to room light at room temperature for several days will usually bleach the discoloration. The effects of low levels of irradiation on the EPR signal is small and reversible [8].
In optical EPR experiments using UV irradiation, the ruby will usually fluoresce, giving off the prominent R-lines (at 694nm). Some of the common varnishes or household cements used to attach the SRM to the cavity or holder, also fluoresce when excited by UV radiation.'
Page 47: '8. R.Wenzel, J.Halpin, F.Campbell, Report NRL-7063 (AD-704069), Naval Res. Lab., 20-Feb-70'
Unfortunately I can find no online copy of NRL-7063.
Laser Induced Damage in Optical Materials, 1978:
http://books.google.co.uk/books?id=tQXxIOc1QEkC&pg=PR9&lpg=PR9&dq=is+ruby+damaged+by+uv?
Laser Induced Damage in Optical Materials: Proceedings of a Symposium Sponsored by the American Society for Testing and Materials and by the National Bureau of Standards: 1979
http://books.google.co.uk/books?id=9OqjlgG2oE8C&pg=PA167&dq=is+ruby+damaged+by+uv?
The overall impression is that yes, ruby is damaged by UV radiation, but I have not been able to quantify how much. It seems I shall have to experiment myself, which is why I bought those old speckled ex-USSR ruby rods. In addition, I'll use them to determine the relative fluorescence intensity of the rod over its absorption bands.
However http://en.wikipedia.org/wiki/Laser_pumping: 'laser host materials are chosen to have a low absorption; only the dopant absorbs. Therefore any light at frequencies not absorbed by the doping will go back into the lamp and reheat the plasma, shortening lamp life.'
So there is a compromise between UV radiation damaging the rod as well as being re-admitted back into the lamp and damaging that too.
DUAL RUBY + Nd:YAG FROM SAME FLASHLAMP
Could I increase ruby absorption by adding a 532nm NLO in the optical path from the Nd:YAG to the ruby?
Paper [N32] compares intracavity vs extracavity NLO generated 532nm from a Q-switched Nd:YVO4 laser with the aim of producing 266nm and concludes extracavity produces more power vs intracavity due to diminished optical timing of the fromer. Although not tried with ruby, I expect similar results.
FLOW TUBES
In order to protect both the rod and flash lamp from breakage, they are usually mounted inside separate glass flow tubes that are sufficiently large to allow coolant to flow through them and over the rod and flash lamp.
A flow tube is often made of glass or quartz doped with a material that absorbs and blocks the transmission of UV light which is generally damaging to lasing media and as we discovered above, this includes ruby. The dopant converts the UV by fluorescence, to wavelengths best suited to the absorption bands of the lasing material.
Cerium is the doping agent best suited for the ruby rod:
Spectroscopic Properties of Cerium-Doped (Ce3+) Aluminosilicate Glasses':
http://www.osapublishing.org/ome/fulltext.cfm?uri=ome-5-4-720&id=312757
From this paper it is evident cerium blocks below 350nm with absorption ~350nm to 400nm and fluoresces ~440nm to 540nm depending on concentration.
Below left - quartz doped with Samarium and Cerium modifies the wavelengths of transmitted light:
a) Cerium is best suited to ruby,
b) Samarium is best suited to Nd:YAG.
Frosted flow tubes increase gain. http://en.wikipedia.org/wiki/Laser_pumping: 'Parasitic modes occur when reflections are generated in directions other than along the length of the rod, which can use up energy that would otherwise be available to the beam. This can be a particular problem if the barrel of the rod is polished. Cylindrical laser rods support whispering gallery modes due to total internal reflection between the rod and the cooling water, which reflect continuously around the circumference of the rod. Light pipe modes can reflect down the length of the rod in a zig-zag path.
If the rod has an anti-reflection coating or is immersed in a fluid that matches its refractive index, it can dramatically reduce these parasitic reflections. Likewise, if the barrel of the rod is rough ground (frosted) or grooved, internal reflections can be dispersed.[8]'
I need to determine how close water is to the refractive index of ruby
'Pumping with a single lamp tends to focus most of the energy on one side, worsening the beam profile. It is common for rods to have a frosted barrel to diffuse the light, providing a more even distribution of light throughout the rod. This allows more energy absorption throughout the gain medium for a better transverse mode. A frosted flow tube or diffuse reflector, while leading to lowered transfer efficiency, helps increase this effect, improving the gain.[9]'
THE COLDER THE BETTER
Ruby benefits from a low operating temperature: 'Internal thermal limits of ruby laser performance' [O26] Page 4: 'The lower the operating temperature of the ruby the higher the efficiency of the device.'
Cooling a Ruby laser below room temperature can increase output energy; cooling to 0°C vs 20°C can increase it by perhaps 50%: http://en.wikipedia.org/wiki/Ruby_laser
See also [O26], Internal Thermal Limits of Ruby Laser Performance, Page 4:
'The lower the operating temperature of the ruby the higher the efficiency of the device.'
The flow tube would therefore be best placed around the rod and the cooling liquid directed through it first before cooling the flash lamp which will naturally raise the coolant temperature.
Reflector
http://en.wikipedia.org/wiki/Ruby_laser
'The rod and the lamp are relatively long to minimize the effect of losses at the end faces and to provide a sufficient length of gain medium. Longer flash lamps are also more efficient at transferring electrical energy into light, due to higher impedance.[2] However, if the rod is too long in relation to its diameter a condition called "pre-lasing" can occur, depleting the rod's energy before it can properly build up.[3] Rod ends are often antireflection coated or cut at Brewster's angle to minimize this effect.[4] Flat mirrors are also often used at the ends of the pump cavity to reduce loss
Another configuration uses a rod and a flash lamp in a cavity made of a diffuse reflecting material, such as Spectralon or powdered barium sulfate. These cavities are often circular or oblong, as focusing the light is not a primary objective. This doesn't couple the light as well into the lasing medium, since the light makes many reflections before reaching the rod, but often requires less maintenance than metalized reflectors.[6] The increased number of reflections is compensated for by the diffuse medium's higher reflectivity: 99% compared to 97% for a gold mirror.[7] This approach is more compatible with unpolished rods or multiple lamps.
Aluminium is a far better reflector of UV than gold and will reflect deep UV to the cerium doped quartz flow tube around the rod; silver does not extend down to UV but is otherwise similar:
The curve above left shows the the reflection efficiency of several materials over optical bandwidth; the curve above right shows the effective bandwidth of aluminium vs silver reflector for a Xe flashlamp.
EXCITING THE RUBY LASER WITH A DIODE LASER
From the ruby curve, of some interest is the option to pump it from laser diode 808nm second harmonic 404nm to match its peak absorption around 400nm. However the graph from the Giant Pulse thesis (above) indicates 400nm pumping is more efficient at the rod end and 560nm is better pumped into the rod along its length.
The 13 May 2009 edition of Laser Focus World revealed the Klastech GmbH LD pumped Crescendo ruby laser, although since being acquired by Power Technology Inc in 2013, I've found no further mention of it:
The LHC project uses flashlamps primarily due to their lower cost.
EXCITING THE RUBY LASER WITH A FLASHLSMP
Of interest below are my hand drawn curves derived from several papers, comparing the emission bands of krypton and xenon flashlamps with the absorption bands for ruby, Alexandrite, Ti:Sapphire and Nd:YAg lasers; all have different requirements.
However the following patent reveals a Xenon Krypton flash lamp is best suited to ruby:
https://patents.google.com/patent/US3440559
'...a xenon filled gas tube is not as effectively efficient a light stimulator for a ruby laser as is desirable. This is because the emission spectrum of xenon does not match the absorption spectrum of ruby, which has two peak absorption regions in the visible spectrum at approximately 570nm and 420nm whereas xenon has emission peaks at approximately 467.1nm, 462.4nm and 450nm....a gas filling of xenon and krypton [is better]. A tube with a filling of 50% krypton and 50% xenon has been found to be a substantial improvement over a known xenon filled tube and experiment indicates that still better results follow reduction of the krypton content substantially below 50%. Krypton has emission peaks at approximately 587nm and 557nm, the 587nm line peak coinciding with the second absorption peak of chromium doped ruby...a reduction in the neighbourhood of 25% of the energy required for ruby excitation is obtainable with XeKr vs a purely Xe filled tube.'[text modified for readability]
Something I shall clearly have to do, is determine if my EG&G flash lamps are Xenon (most likely), krypton, or Xenon Krypton.
Xenon and Krypton spectral curves are readily available but I have not yet found a XeKr curve. If I can't find a XeKr flash lamp, could I run both a Xe and a Kr lamp in series for the same rod?
Above left, Flash lamp output at visible wavelengths best suited to ruby significantly increases when operated at high currents (graphs from Perkin and Elmer flash lamp catalogue [O11]), but this in turn limits their life; UV absorption is high for ruby and quartz admits UV, but UV eventually damages rods.
Below, Perkin Elmer flash lamp Xe and Kr flash lamp spectral emission graphs:
The reason why I eventually settled for a rod with a 120mm long ruby section is because towards the start of the project I bought two unknown EG&G flashlamps that look very similar to the Perkin Elmer QXF series: 'liquid cooled xenon filled high average power medium peak power':
6mm bore, Ko 26.7, Ke 184000W, 16kV trigger, Iavg1k1A, Pavg 4713W, 1ms, 800V-2800V.
However as noted above, Xenon krypton flash lamps may be being optimum for ruby.
RUBY LASER CONSTRUCTION
Having acquired rod #3 to match the EG&G flash lamps, I sketched the assembly (below left), deciding to make the end blocks out of delrin, a hard plastic often used in similar applications. The aluminium reflector would be held within the blocks by four steel threaded rods at each corner of the delrin block, slack taken up by a sheet of neoprene rubber at each end. The assembly was intended to be cooled by my stock of C-74 perfluorocarbon (see Cooling Loop section) which would run through the reflector with no need for flow tubes.
Above: Clockwise,the machining drawings for the two delrin ends to the ruby cavity, the overall dimensions of the completed cavity, and the completed assembly view (from my logbook).
Below, the finished blocks:
RUBY LASER REFLECTOR
Whilst the Delrin blocks were being machined, I began to make the reflector.
Laser flash lamp reflectors can take several forms, largely depending upon the number of flash lamps but an elliptical tube is more efficient than a round tube because it focuses the light onto the rod:
https://en.wikipedia.org/wiki/Laser_pumping
'Smaller ellipses create fewer reflections, (a condition called "close-coupling"), giving higher intensity in the centre of the rod.[1] For a single flash lamp, if the lamp and rod are equal diameter,
an ellipse that is twice as wide as the height is the usually the most efficient at imaging the light into the rod.'
My experimental ruby laser cavity tube is an aluminium pipe intended for an automotive exhaust fitting. Understandably its inside doesn't have the nice chrome finish that was on the outside but this isn't an issue: the reflectivity curves below reveal aluminium is a far better reflector of UV than chrome and indeed gold, normally used for Nd:YAG cavities.
I cut the aluminium tube to size with a simple pipe cutter then I made an electro-polisher using 80% phosphoric acid in a pint beer glass and various pieces of polypropylene block (inert in the acid) as a scaffold to hold the tube in place. With only crude wood cutting tools at my disposal, the finish is poor but usable. I put a smaller aluminium rod in the centre extending the full length of the tube to be polished, and used it for the positive terminal. The ruby tube is the negative terminal and to prevent the outside being etched as well, I covered that in adhesive backed paper masking tape.
The small tube also doubled up as an aerator using a small pump feeding into the top and an airstone at the base, secured by crimping the end of its tube. To get the starting temperature up to 74°C, I placed the acid filled glass on top of a heating block, and initially used an electrophoresis PSU but although it had some effect, much more current was required.
I hooked up my industrial automotive battery charger from the garage and set to 12V, the solution took 22A from the supply, melting the insulation on the feeble croc clips and burning the masking tape:
After rinsing and drying, I finished off the inside of the tube to a mirror like surface by rubbing it down with Autosol aluminium polish and a soft cloth.
Below left: after electro-polishing Below right: after hand polishing
CHANGE OF PLAN
The assembly was intended to be cooled by my stock of C-74 perfluorocarbon but my research into cooling fluids took place after this decision, see [Cooling Loop], as I had previously earmarked the FC-74 for a PC overclocking project when I believed it be be inert. My new research proved otherwise, and discovering ruby benefits from cold also came later. As they say, the benefit of hindsight is wonderful. These new discoveries meat I will now be cooling the laser with water, and I will require flow tubes to protect the flash lamps and rod (although I'll still run the UV absorption experiment). The twin flowtube design allows freshly cooled water to first flow across the ruby rod before passing through the flash lamp flowtube. This in turn means I will have to re-machine the Delrin blocks to accommodate the flowtubes, or perhaps abandon them and start again. However finding affordable flow tubes for the rod and flash lamps is difficult, and this is now compounded by my desire to research the benefits of DUV.
Furthermore initially it was my intention to use the two EG&G flash lamps to build the Nd:YAG before the MK580KK appeared. When I came to consider building the ruby, they seemed the ideal choice and I dimensioned the blocks to accommodate them. Now, much later, I think they are too powerful and operating flash lamps below their rated power is not good for them either, see [O11]: PE flash lamps catalogue, plus I only have two of them.
However like the ruby rods, there is now a glut of very cheap ex-USSR laser flash lamps on eBay, and I bought a few to experiment with. They seem very crude in comparison to the EG&G lamps, but they are so cheap they are expendable whereas the EG&G lamps are not. The USSR flash lamps also come with specifications, lacking from the EG&Gs whose parameters I have only guessed.
It seems more practical to redesign the ruby to accommodate these instead, so the design is now in a state of flux. This is why still in 2020 I haven't got much further with the construction.
MIRRORS
From: http://www.repairfaq.org/sam/lasercps.htm
'The HR can be either a totally reflecting **HE** dielectric mirror or a roof prism or corner reflector, AR coated for the laser wavelength if possible. A high quality first surface aluminized mirror might survive for a few shots or more but with the high intra-cavity power of a PSS laser, possibly not for long. And, probably not at all if Q-switched! The OC mirror really needs to be one designed for PSS laser.Without a Q-switch, figure something like 70 percent reflectivity at the lasing wavelength.With a Q-switch, it can be as low as 30 percent
I'd suggest avoiding "resonant" type mirrors in favor of high energy dielectric types. While resonant optics have no coatings to be damaged, their internal optical element spacing and alignment are absolutely critical (if disassembled for any reason, will likely be ruined). And, their condition or even effective reflectivity can't easily be determined. The reflectivity is also likely to be too low for useful operation without a Q-switch. Although the original ruby laser, circa 1960, used an aluminum output mirror with a hole in the middle, this is probably not a great idea (but it wouldn't hurt to try!).'
Below left: Typical ruby OC reflectivity vs output power; 70% is peak, but 50% is more stable
from [O19], CORD Module 3-5, Pulsed Solid-State Laser Systems:
Chris Hardy (Kigre) gave the following advice which should help me drive my unknown flash lamps: 'Using a good hall-effect current probe (Such as Pearsons), a high voltage probe (Tektronix), and oscilloscope (Tek, etc) you can determine the lamp's impedance at any time in the pump pulse. The most convenient way is to connect the probes to the scope and the scope to a computer to calculate the impedance. You can also measure the input energy as the integration of lamp voltage and current over time (the pump pulse) using a scope and computer. Some fancy scopes can do this without a computer if they have advanced math capability. Published formulas for Ko (lamp impedance) are pretty accurate.'
9 Lumenis water-cooled block with Alexandrite rod 97mm x 4mm dia but no lamp. This is believed to be a CW laser block that used an arc lamp. Initially bought to build a future experimental 680.4nm CW laser as an alternative to the ruby using the same arc lamps as the Lumenis Nd:Yag block. The following paper managed to get 2.6W at 755nm, optical afficiency 24%, out of a 7mm x 3mm dia Alexandrite rod pumped by an 11W 532nm TEM00 source focused to 44µm within the body of the rod in a relatively simple setup, [O20]: High Power Continuous-Wave Alexandrite Laser with Green Pump.
By adding a single birefringent plate they were able to tune the wavelength by up to 85nm and indicated higher output power was still available if the rod was heated; I suspect the main reason why they achieved such a high output was due to focussing the 11W 532nm source to a 44µm spot. Nonetheless, if 2HG and 4HG NLOs are available then potentially this is very efficient way of generating 193nm:
193 x 4 = 772nm and is well worth exploring, particularly with the 2kW arc lamps.
This experiment is on hold until an affordable SFM crystal can be located.
10 Ti:Sapphire rod 10.5mm long x 8.5mm dia.
This is for a future experiment to build a tuned laser as an alternative to the ruby, to combine with 266nm to produce 193nm. Unfortunately power will be very low at 700nm, which is on the limit of the tuning range. This will use an as yet unspecified pulsed flash lamp, or a semiconductor laser pump. This experiment is on hold until an affordable SFM crystal can be located.
11 Cr Tm Ho : YAG rod 89mm long x 4.0mm dia.
This is for a future experiment to build an experimental 2081nm laser to combine with 213nm to produce 193nm. I picked this rod up because it was only $30. Its wavelength is very close to [O12] OPO 2074nm. Its peak absorptions seem to be 781nm and 786nm. Pump laser diodes are often used, but a krypton flash lamp might work too. Having said that, there is no way of telling if it was intended for CW or pulsed operation - this wavelength is often found in CW surgical lasers where there is no real advantage of the more expensive pulsed flash lamp design (I suppose the analogy of diesel vs petrol engines).
12 Laser Physics 50S
Model: Reliant 50S-488
Type: Argon gas
Type: CW
Wavelength: 488nm single line
Output power: 5mW-50mW (max 50.5mW measured)
Mode: TEMoo
Polarity: Vertically Polarised
Regulation: Light controlled power regulation
Use: Resonance lIBS in lieu of multiline
Bought: 08/02/12 Oz $400, seller est. ~400 hours use
Status: Working 12/09/23
November 2022:
0 mins: Min power measured: 5.50mw, max power measured: 50.02mW
60mins: Min power measured: 5.85mW, max power measured: 50.57mW
I bought this for $AU400 (Australia) because 50mW+ multiline Ar lasers rarely come up on eBay. I plan to replicate paper [L16] that ionised Ar gas in the target chamber, see [Spectral Enhancement].
13 Laser Physics 500M
Model: Reliant 500M
Type: Argon gas
Type: CW
Wavelength: 457nm-514nm multiline
Output power: 30mW-300mW (max 326mW measured)
Mode: TEMoo
Polarity: Vertically Polarised
Regulation: Light controlled power regulation
Use: Resonance lIBS
Bought: 30/09/19 eBay USA $350, 3824 hours
Status: Failed 05/02/23, total hours 3932
I need a multiline Ar laser to ionise the pressurised Argon gas in the LIBS target chamber for RELIPS experimentation [L16]. http://www.dzlaser.com/Laser_Physics
To avoid over-pressure, I run all gas lasers once a month for at least 1 hour. I had been doing this at minimum intensity (to preserve their life) for several years when one day in 2022 this 500M laser wouldn't start. Eventually it did start and I measured its output at over 500mW. I then ran it for about 4 hours at minimum power, thinking the extra time would de-pressurise it.
However when I tried to start it a month later, it again would not ignite.
Removing its cover I found unlike the 50S, this one has a pressurised Ar canister in it that 'drip-feeds' the resonator, and it is this that leads to over-pressurisation if the laser is left unused. It occurred to me it had been a mistake to run it at minimum power.
I tried to help it by ionising its gas using the 50S but this didn't work. Reading up about Ar lasers, the most common way of ignition is by a high voltage spike on the anode. I also read this is often derived from the mains supply. It occurred to me if I increased the mains supply voltage I would also increase this striking voltage. This is not something I would want to do often as it is overstressing the tube, but it was now my last resort. I then hooked it up to my 13 Amp lab variac which has an output from 0-270Vac.
In [Repairs: Variac Repair & Add Meters] I discovered my UK mains supply is typically ~250Vac instead of the nominal 230Vac. I set the variac to its maximum which I measured at 287Vac and to my relief it started. I reduced the variac to 240Vac and measured max power was now 485mW way above the rated 300mW.
I then ran it for 12 hours at what seemed a reasonable 120mW but it again failed to start the next month.
I had also checked it has a BS1352 compliant 13A fuse fitted, which is capable of carrying 20.8A and should only blow at 24.7A. [U10]: A BS1362 fuse must carry 1.6x its rated current; the fuse must blow at 1.9x its rated current within 30 minutes. This is just as well because when I momentarily put the laser up to max power it drew 20A. It would be catastrophic if the fuse blew and the fan stopped as normally the fan runs for a good couple of minutes after the beam is turned off. I fitted a 15A fuse: carry 24A and blow at 28.5A [U10] and decided to run the laser off the 32A oven socket in my kitchen
I left it for another month before trying to start it normally off UK mains (250Vac) but once again it failed to ignite. I also tried it on the variac and it again failed to start and I wondered if it might be a good idea to leave it for a few days. Up until this point I had been running it in a centrally heated room, although the temperature was only around 20°C.
A fortnight later I tried again with the variac at 287Vac on a cold Spring morning with all windows open when I imagined pressure would be lower [Q10a]: Gay-Lassac's Law, and this time it fired up. I reduced the variac voltage and set power to the maximum 13 amp current my mains variac could carry (higher voltage would be lower current but again I wanted to minimise stress on the tube, so I lowered the voltage).
I measured 270mW at 13A. I then left it running at this level for 12 hours. For the first 30 minutes there were fluctuations and I had to manually adjust the power to stop thermal runaways but eventually it settled into a pattern of slowly decreasing current over an hour or more from ~13A to ~11A, at which point I increased power, noted power was now greater than before, then the current began to slowly increase and I lowered the power, until once again I had equilibrium.
Finally after 12 hours I measured maximum power at 436mW. Maybe it's over-pressure as it's is only rated to 300mW, but this is about the power I got when it first arrived, although it dropped to ~270mW when I ran it for more than a couple of minutes. At the time I was unfamiliar with Ar lasers and was worried it would keep going down the longer I ran it at high power, much the same as some high power LDs I bought from eBay China (which of course don't have a pressurised gas reserve inside them).
I assumed it had been slowly rising over the several years I've owned it even though I'd run it every
month, albeit at the minimum power setting. However the next month it again didn't start.
Having tried everything else, I left it for 6 months and in November 2022 tried it again and it fired up first time on UK Mains. I forgot to measure it at the start but at the 20 minute point once I'd taken it up to max power then reduced it to minimum, the minimum output increased from 65mW to 74mW.
20mins: Min power measured: 65mw, max power measured: 463mW
60mins: Min power measured: 78mW, max power measured: 474mW
I left it for another 6 months but it didn't fire up and it is now officially dead.
Below, the Laser Physics Reliant Ar lasers range and their basic specifications:
Other Argon lasers considered
Manufacturer: ILT (Ion Laser Technology)
Model: see table
Type: Argon
Type: CW
Wavelength: 457nm-514nm
Output power: see table
Mode: TEMoo
Polarity: Vertically Polarised
14 Rofin-Sinar
Model: 6329A
Type: Helium Neon gas
Mode: CW
Wavelength: 632.9nm
Output power: unknown but max 1.1mW measured
Polarity: Vertically Polarised
Use: LHC polarisation reference
Bought: 12/12/14 eBay UK £15
Status: stopped working 10/09/23
15 DIY
Type: TEA CO2 (Transverse Electrical discharge at Atmospheric pressure carbon dioxide gas)
Mode: CW / pulse
Wavelength: 10640nm
Output power: CW 100W / pulse 100mJ @ 100ns
Polarity: TBD
This is for a future enhancement to combine with 213nm to produce a dual wavelength LIBS system.
It will be used experimentally to ionise CO2 gas as an alternative to Argon RELIPS.
It will also investigate the benefit of heating the target prior to the Nd:YAG pulse.
Research papers below hint at least 100W [L31] CW or 40mJ [L32] 100ns (1µs tail) pulse is required for a CO2 laser to sufficiently heat the target prior to the Nd:YAG pulse. Due to the expensive of achieving this amount of power commercially, the only viable route is either a cheap but also unreliable
Chinese 100W CO2, or a DIY TEA CO2.
[L29], Thermodynamic and Spectroscopic Properties of Nd:YAG CO2 Double-Pulse Laser-Induced Iron Plasmas,
[Abbreviated] 'Nd:YAG laser 39mJ 5ns pulse, 4.7×10^9W/cm². CO2 75mJ per 100ns pulse with a 1µs long
tail, 30×10^6 W/cm²'.'The bright continuum emission representative of heavy and slow moving particles
extracted and ejected during the CO2 laser interact[sic] with the iron sample. These particles act as fuel for the second pulse, and lead to greatest signal enhancement approximately one microsecond after the beginning of the CO2 laser pulse.'
[L30], Nd:YAG-CO2 Double-Pulse Laser Induced Breakdown Spectroscopy of Organic Films,
[abbreviated] '...two lasers: Nd:YAG 1064µm 5ns 17.5mJ focus 400µm giving 2.8GW/cm²; TEA CO2 10.6µm
energy 63mJ per 100ns pulse with a 1µs long tail, spot diameter 1.2mm, 5MW/cm²; longer wavelengths are more easily absorbed at lower electron densities by inverse Bremsstrahlung process; CO2 laser pulse following the Nd:YAG pulse by 500ns. To reduce the background radiation, the signal was acquired after a 50ns gate delay. To acquire maximum signal, a 100µs acquisition window was used to integrate over the complete decay time of the emitting species; LIBS emission enhancements with a factor of 25-300...have been reported using multi-wavelength 1.064µm/10.6µm'.
[L31], Dual laser LIBS-LIDAR System with Higher SNR Using Simultaneous CW-CO2 and Q-switched Nd: YAG Lasers, 'Dual beam LIBS to increase SNR using 100W CW-CO2 to preheat target prior to 300mJ 10ns Nd:YAG;
20 times stronger signals respect to single shot for the target temperature up to 600K.'
[L32], Enhancement of Nd:YAG LIBS Emission of a Remote Target Using a Simultaneous CO2 Laser Pulse,
[Abbreviated] 'Nd:YAG 50mJ/pulse focused to a 1mm spot. CO2 pulse 40mJ/mm². Timing overlap of the two laser pulses within 1µs was important for enhancement to be observed; CO2 laser pulse had an initial TEA laser spike of 100ns followed by a nitrogen-fed tail about 5μs long. The CO2 laser output beam size was controlled using a lens to have a diameter on target between 3mm to 15mm. For a 6.5mm diameter beam, the energy density was about 40mJ/mm²; enhancement of neutral atomic emission was usually on the order of 25 to 60 times, while enhancement of ionized species tended to be higher, 50 to 300 times'.
[L33], LIBS Plasma Enhancement for Standoff Detection Applications,
[abbreviated] 'Nd:YAG 50mJ 5ns pulse focused to 1mm spot. CO2 pulse 60mJ/mm²; Timing overlap of
the two laser pulses within 1µs was important for enhancement to be observed; CO2 laser pulse 100ns
spike, 5µs tail; Enhancement of neutral atomic emission was usually on the order of 5-20X, while
enhancement of ionized species tended to be higher, 10-200X. We attribute the increase in both the
atmospheric components and the +1 and +2 ionic emission to heating of the Nd:YAG plasma by the
coincident CO2 laser.'
[O31], Construction Details for a DIY 40mJ (10mJ-50mJ / 100kW-300kW) DIY TEA CO2 Laser:
http://laserkids.sourceforge.net/eng_co2teaLaser.html
16 Other lasers bought for experimentation
Manufacturer: Laser Export, Russia
Model: LCS-DTL-374QT
Type: YV04 810nm > 1064nm + 532nm > 355nm
Mode: Q-switched
Wavelength: 355nm (810nm, 532nm, 1064nm)
Output power: 355nm 30µJ 10ns / 355nm avg 50mW / 810nm, 532nm, 1064nm avg 420mW
Mode: unknown
Polarity: unknown
808nm + 1064nm measured at 10.157mW
I opened the PSU & rotated its mains plug so I could rotate the PSU 180 degrees to hide its unused wiring harness inside the case wall cavity. I repaired the chopped mains lead as I didn't have a 90 degree plug.
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