13. Optics and Layout
This section describes the criteria for use and location of optical components. Refer to References, Optical Components, for a description of each component.
The optical components to build a LIBS system are simple: A laser, focusing lens and spectrometer.
The optical components for the LHC project are more complex:
1. Nd:YAg laser 1J, 5mm beam
2. Q-switch (experimental: mechanical shutter or Pockels cell, in which case customised resonator too)
3. Continually variable beamsplitter to control Nd:YAG beam power
4. Visible diode laser to align optics
5. Optional faraday isolator
6. 1064nm interference filter to isolate flash lamp wavelengths
7. 2nd harmonic crystal to convert 1064nm to 532nm
8. 90° silver steering mirrors (2) to direct 532nm around optical breadboard
9. Half wave plate(s) to align 532nm to 266nm harmonic crystal
10. 4th harmonic crystal to convert 532nm to 266nm
11. Optional 3rd harmonic crystal to convert 1064nm + 532nm to 355nm
12. Optional 5th harmonic crystal to convert 532nm + 355nm to 213nm
13. Optional UV pellin broca prism to split wavelengths / for power measurement
14. Optional UV tophat beam profiling lens
15. Optional UV beam expander to produce smallest focus
16. UV to VIS converter Metrolux K7 20x20x3mm, 110nm-350nm to 612nm peak (= Cohu 4815 0.75 peak)
17. Frosted glass / HE opal to capture VIS image of UV beam for beam profiling camera
18. Attenuation optics & CCD cameras for laser beam profiling
19. UV focusing lens
20. Motor controlled horizontal X stage for focus lens
21. Semiconductor focus PDA to a) align and b) pinpoint focus to 50µm (or less)
22. Motor controlled rotary stage for target
23. Sealed target chamber for pressurised argon gas
24. Project optical wavelength AR windows for sealed target chamber
25. Accessible conduit to hide all beams
26. Beam dumps
27. Argon laser to ionise argon gas in target chamber
28. Optional focusing lens for argon laser
29. PT and/or APD to detect 1064nm laser initial edge for timing
30. High speed ablation observation camera
31. Ablation pit inspection camera
32. Spectrometer for debug
33. Spectrometer with plasma chemistry element detection software
14. UV-NIR FO cables
Add to this, the infrastructure to make it work:
1. Variable high voltage PSU and high speed variable risetime pulse generator to develop system
2. Flash lamp liquid cooling system (reservoir, tubing, pump, chiller, flowrate, level, additives)
3. System safety interlocks
4. Flash lamp PFN & protection components
5. Flash lamp capacitor charging PSU
6. Simmer supply for multi-pulse operation
7. Flash lamp trigger PSU and trigger interface electronics
8. Optical chopper to facilitate experimental Q-switch(es)
9. Electronics to synchronise the Q-switch(es)
10. Polarised reference laser for optical setup
11. CW 1064nm LD for optical experimentation & setup
12. LD driver & TEC driver for CW LD
13. Anti-static workshop
14. Safety goggles for all project wavelengths
15. Pulse generators to control system timing
16. Optical breadboard 4' x 2'
17. Optical mounting hardware including XYZ and precision rotary, and round & square lens holders
18. PMTs / PTs/ APDs and associated electronics to detect lasing parameters
19. Oscilloscopes to observe and measure PSUs and optical parameters (including kV / picosecond probes)
20. Optical energy measurement equipment (probes & meters)
21. PIC microcontrollers and PICC software to control the system
22. VC# or VC++ GUI to control the microcontrollers
23. Hardware controls and displays for GUI to initiate LIBS sequence and display results
Iron Man never had these issues when he built his fusion generator - I must be doing something wrong!
The preliminary optical layout for the LHC project (from my logbook 20 Dec 2015) is shown below:
THE LHC PROJECT
Having finally decided the direction the project was to go in, I thought it would be a good idea to give it a name.
The CERN Large Hadron Collider (LHC), is a huge scientific experiment that accelerates two proton beams around a 27km circuit before smashing them together to release sub atomic particles in a bid to reveal the origin of matter: http://home.cern/topics/large-hadron-collider
My LIBS project is also now a huge experiment and functionally similar to the real LHC, as it smashes a laser beam into an object to release atomic spectra that reveal the elemental composition of matter.
With a twist of humour, as my name is Adrian, LHC stands for the Little Hadrian Collider.
The LHC project is an expansion of my original LIBS prototype with many extra features found generally only in separate research papers.
PROJECT SUMMARY
The goal is to marry fast LIBS atomic spectral release to an S.C.Technology PCM401 Plasma Chemistry Monitor system that was designed to respond to slow spectral release in a semiconductor foundry.
A paper [O12] achieves 193nm by utilising multiple non-linear crystals (NLOs) in the laser beam path, and ultimately by sum frequency mixing (SFM) the 1064nm Nd:YAG, 213nm fifth harmonic with 2074nm generated from an operational parameter oscillator (OPO) seeded by the Nd:YAG 532nm second harmonic.
My problem with this approach is the authors are accomplished scientists with a budget able to specify and buy the necessary crystals and the expertise to align them on a purpose built optics breadboard with all the requisite hardware and cooling apparatus. Not only that, but the summing crystal is unobtanium, meaning only available purchased new from specification.
Unlike the scientists, I lack the knowledge to formulate the equations necessary to specify crystals or even prove my system before I build it, and I am constrained by budget to crystals of unknown chemistry and properties, although I have done my best to acquire good pedigree, and where possible all optics are new.
Ultimately I would like to ablate the LIBS target with 193nm because papers I have read imply this improves spectral quality. However this is impossible due to the non-availability of affordable summing crystals, and so has been reduced to 213nm accompanied by as many enhancements as possible, see
[Spectral Enhancement] to amplify and extend the atomic spectral signature.
LHC project goals:
1. Produce a 1J 1064nm Nd:YAG output
2. Produce Nd:YAG harmonics 532nm, 266nm, 213nm
3. Focus the beam to at least 50µm
4. Ensure the PCM401 image intensifier is not harmed by laser or Bremsstrahlung radiation
5. Extend and enhance spectra to meet the needs of the PCM401 Plasma Chemistry Monitor
6. Produce basic LIBS functionality
7. Explore the extents of LIBS enhancements
8. Automate the LHC under PC control
9. Expand the original PCM401 library to all accessible elements
10. Replace the original PCM401 PDA with a custom designed faster, gated alternative
LHC project milestones:
1. Back up the PCM401 hard drive installation DONE
2. Build and test the 1064nm 25mJ Nd:YAG prototype DONE
3. Acquire the necessary optics and hardware to build a 1J 1064nm Nd:YAG DONE
4. Acquire NLO crystals to produce subharmonics of 1064nm down to 213nm DONE
5. Acquire, repair and test all measurement instruments IN PROGRESS
6. Design Build and test liquid cooling loop IN PROGRESS
7. Design build and test system safety interlock system IN PROGRESS
8. Build and test a liquid cooled flash lamp based 1J 1064nm Nd:YAG IN PROGRESS
9. Design build and test Q-switch for Nd:YAG IN PROGRESS
10. Confirm production of 532nm harmonic DONE
11. Confirm production of 266nm harmonic
12. Confirm production of 213nm harmonic
13. Ensure Bremsstrahlung and laser radiation cannot harm PCM401 image intensifier
14. Produce tight focus of UV beam to at least 50µm
15. Confirm PCM401 detection of elements from LIBS spectra
15a (if not, design build and test PIC based fast 2MHz PDA sampler / gate the PDA MCP)
16. Design build and test PC-based system controller using Visual C GUI
17. Automate harmonic crystal peak efficiency under GUI/PICC control
18. Automate target focus under GUI/PICC control
19. Automate target table rotation under GUI/PICC control
20. Integrate target image and measurement with GUI
OPTICAL ENVIRONMENT
As the LHC is an optics driven project, its hardware is based on an optics breadboard. These are usually very expensive in the UK and impractical to ship from the USA.
US breadboards generally use imperial screw threads (typ 1/4"-20) and spacing, whilst Europe uses metric threads (typ M6) and spacing. Since most of my optical equipment came from the USA but metric screws are more readily available in the UK, I decided to have a custom breadboard made that could accommodate US equipment with a metric thread.
I ordered a 4' x 2' custom sheet of 2mm thick perforated steel from a local metal merchant. I would have preferred something thicker but the cost would have been prohibitive (this sheet alone was $250),
Instead I decided to have a thin sheet of steel as a mechanical guide and a low cost thicker block of MDF (Medium Density Fibreboard (i.e. imitation wood) behind it, into which I would tap threads as required. It would not surprise me if optics breadboard manufacturers use a similar technique.
To make alignment easier I set the pitch at 0.5" rather than the familiar 1" and the holes 5mm diameter so they could be tapped to 6mm where desired. This also meant they could be tapped to 1/4" if essential. The 2mm thick steel sheet is mounted on a wooden frame. To begin with optics hardware will be mounted finger-tight in M6 holes tapped in the thin sheet. When the design is finalised MDF sheet will be added below and the necessary holes drilled and tapped for a more secure finish.
THERMAL EXPANSION
Since the project is being built in my house, I have no control of temperature and humidity other than the central heating thermostat (I never did get around to fitting thermal radiator valves). If thermal expansion proves to be an issue, the plan is to replace the perforated steel sheet with one made from Delrin (thermoplastic). However Delrin is also prohibitively expensive so this will not be attempted unless absolutely essential. Delrin is a proprietary name for PolyOxyMethylene (POM), aka acetal:
http://en.wikipedia.org/wiki/Polyoxymethylene
AIR QUALITY
By far the biggest problem is DUST. My house seems to suffer from this more than most, I suspect more than likely because I loathe house DIY and I haven't painted the lab ceiling and walls for many years:
vacating the lab of its equipment would be a major challenge, particularly as it is in use 24/7.
Dust is a major contribution to laser optics damage and I recognise I have a serious issue. For now, all I can do is enclose the project area and laser beam paths, and filter the air using fans. Fortunately LIBS is a low frequency application, so hopefully I will get away with just this simple remedy.
The LHC project covers a number of disciplines and research has been running on all of these simultaneously:
1. Papers and design research
2. Documentation and website build
3. Safety requirements and acquisition
4. Optical design and acquisition
5. Mechanical design and acquisition
6. Cooling system design and acquisition
7. Instrument research, acquisition and repair
8. Electronics design, acquisition, build and test
9. PC hardware design and acquisition
10. Software design and acquisition
INITIAL OPTICAL LAYOUT
The initial optical layout (taken from my logbook) is shown below : (LIBS2 P.113) ruby LIBS2 P.141
9. Optics and Layout
OPTICAL LAYOUT
TARGET ASSEMBLY
The target assembly consists of a rotatable platform upon which the target sits. Arranged about the assembly are the following facilities:
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Beam for ablation of target
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Argon gas flow across target
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Argon ion laser beam to ionise the gas
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Gas pressure sensor
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Fibre optic pickup of atomic spectra to the spectrometer
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Camera inspection of ablation pit
The assembly will be encased to ensure the gas is pressurised to 3psi. This will take the form of a sealed chamber covering the entire target assembly, with AR-coated windows and access points for electrical and mechanical interfaces. One idea was to acquire a large quartz cuvette instead of windows but this makes target rotation difficult. A quartz bell jar over the rotary stepper is ideal, but these are prohibitively expensive.
TARGET FOCUS
Focus will be achieved by manually moving the target until it appears in focus on the ablation camera, The ablation camera will have been pre-focused on a 250µm reticule '+' target with 5µm divisions.
LASER FOCUS
The laser will be pre-focused to this point also using a 0.5 degree stepper motor on a stage with a 0.488µm accuracy encoder. Focus will be confirmed by a beam splitter and attenuator to a 4-quadrant silicon PDA or pyroelectric detector. The goal is to focus the beam at the target to least 50µm, and ideally to 20µm.
LASER BEAM PROFILING
Final output UV beam profiling will be achieved utilising a **telescope to increase NIR beam diameter and the UV visualised via a Metrolux K7 110nm-350nm to 632nm UV to VIS converter with a 3ms latency onto a diffusing focus screen and a 0.5" CCD camera (**the telescope may degrade the profile).
The target assembly will be mounted on a stage rotated by a NEMA 17 1.8° stepper motor [E25], typically used on 3D printers: http://reprap.org/wiki/NEMA_17_Stepper_motor
A NEMA 17 is a stepper motor with a 1.7 x 1.7 inch (43.2 x 43.2 mm) faceplate [E26]. EBay China sells square NEMA 17 motors for around $7, less for circular types, and A4899 driver modules for $1 that can drive motors rated for 8V to 35V up to ±2A per winding.
360°/1.8° = 200 steps. Circumference of a circle = π x diameter.
If the target = 1"/25mm diameter then circumference = 3.1416"/78.54mm.
3.1416"/78.54mm divided by 200 steps = 0.0157"/0.39mm = 390µm per step.
A camera will be mounted at the side, 90° to the beam. Once the laser has fired, the stage will be rotated 90° for the ablation pit to be viewable by the camera, then rotated 1.8° to the next ablation position. This process will provide 200 permissible target spots for each full rotation of the target.
Initially whilst focus is coarse (it will be hard enough to focus invisible wavelengths to 1mm, let alone 20µmm which like like 193nm, is another Holy Grail), the step distance between ablation pits will be increased and gradually reduced as focus improves. Step distance will be decreased if the target is very small.
Height is an issue since the target needs to be hit by the laser; the final choice of motor may therefore be a half height type with a with body height typically 27mm excluding the spindle.
Without a lathe chuck it's actually quite difficult to make a metal disc with a hole dead centre and my solution to finding an affordable plate on which to rotate the target, was to buy a similar ready made item from eBay China, intended for use as a table corner leg mount. These conveniently come in a large range of diameters: