10W Laser Cutter Calculations
Thank you, Tim.
I suggest that we blog our laser progress and ask the community to provide feedback. Would you mind sending me a doc with images and links to the most pertinent points -
1. Overview of what we are doing with Laser Cutter and Hydrafabber 2. Why is it important? 3. How it fits within the GVCS 4. Cost breakdown. 5. Performance goals 6. Testing procedure to achieve goals - what specifically are we interested in cutting, how precisely, and specifics of our testing procedure 7. Challenges and risks 8. Future directions
Can you do this for Saturday so we can discuss this in the Design Sprint
Laser cutter overview
Initial objective -
Build laser cutter to be incorporated into hydrafabber device for cutting paper and cardboard.
The laser cutter uses a focused diode laser to cut paper and cardboard by thermal ablation, or the vaporization of the paper fibers. This eliminates uncontrolled combustion.
We will scan the laser position in X dimension and the paper will be moved in Y dimension. Ablation requires high focused intensity, therefore high laser power is required.
975 nanometer wavelength 10 watt fiber coupled diode lasers are now commercially available at low cost. The laser power can be delivered to the X scanning stage by a multimode fiber optic. The laser power is then focused on to the paper with a fiber coupled optical assembly. The assembly focusses the power in a 200 micron spot at a distance of a few inches from focusing optics. This arrangement allows smoke and particulates to vacuumed away from the optics.
 A simple calculation regarding paper cutting speed
Ten watts of laser power in a 0.2 mm diameter spot results in an intensity of 30000 watts per square cm or 300000 times the intensity of sunlight.
Assuming the paper absorbs 1/2 of the incident power or 5 joule per second in a 0.2 mm diameter spot and assuming a paper thickness of roughly 0.1 mm results in 5 joules/sec into a volume of roughly 3 millionths of a cubic centimeter.
Given a mass density of paper of 0.7 gram/cc http://wiki.answers.com/Q/What_is_the_density_of_paper ,the mass is roughly 2 micrograms.
The specific heat of paper is 1.4 kilojoules per kilogram kelvin http://www.ehow.com/list_6893512_thermal-properties-paper.html#ixzz2bW4unfsD ,or 1.4 joule per gram for 1 degree centigrade.
All of these factors result in a temperature rise of the volume of paper of roughly 1 million degrees centigrade per second.
Assuming 1000 degrees sufficient for vaporization then a period of one millisecond is required for each spot. This suggests a cutting speed 200 mm per second.
 Requirements for initial test
Before laser module, fiber assembly and focusing optics are procured we must decide whether commercial power supplies for diode current and thermoelectric control should be procured or built by us for test by experienced personnel.
Also laser safety is paramount. The 975 nm wavelength is the wavelength presently favored because high power modules are most easily obtainable at this wavelength. However these are eye hazardous wavelengths so the paper cutting operation must be completely shielded by plastic that is totally opaque at these wavelengths. We should investigate dye impregnated Plexiglas or other transparent plastic for its optical density at 975 nm. Fortunately such material is available for industrial application for the high power Nd:YAG wavelength of 1064 nm.
However we must have minimum of equipment to asses light leakage.
Caustic? The focus of optical power or radiant flux... the envelope of rays where they focus -- the geometrical object is called the caustic of rays. The caustic is the transparent object, red in color. It includes smoke and particulates. Maintain a little vacumn current or blower to clear the spot where work is taking place. Smoke will interrupt the power from the lens. An aquarium pump used in reverse might be sufficient to remove smoke from that area in an air tight enclosure. It has an air bladder that might either push or pull air. A big case might be needed to enclose the fiber and the x-y axis. The enclosure might be folded up, perhaps a roll of vinyl placed on a frame that might be more easily transported. It must be absolutely black to prevent light exposure to anyone who might be standing nearby.