Archive for the ‘Optics’ Category

New optics

April 14, 2010


The new optics arrived. Their quality is great. The old optics are shown on the left, and you can see the new parts are a lot less bulky than my previous set. The vernier adjust is going to make height adjustment a lot easier.

Collimation. The first thing I did was adjust the collimator. The beam on lasers like mine spread out like a flashlight. The purpose of the collimator is to reduce beam divergence and to control the beam into a nice cylindrical beam, and is also useful because it allows you to expand the size of the beam (see: link). My collimator has two lenses and the precise distance of the two lenses influences overall expansion. To adjust these collimator, I rotated the laser sideways and pointed the beam at burn paper at a distance of 6 inches. Then I put a beam stop about 80 inches away and tested the diameter of the beam. A nice collimated beam should have the same diameter from a distance away. The collimator has some graduated lines on the side, and my burn card shows the beam size for each line. The graduated line for .5 inches seemed pretty good so I locked that position into place.

Centering the beam. Once the collimator was ready the laser was reoriented so the beam was shot downwards. The vernier adjust portion and cutting head was threaded on to the collimator. The optics assembly has a elbow bend with adjusting screws that allow you to center the beam. It was also useful to put some rusty carbon steel on the table, and used a cheap USB microscope to look at the position of the beam. I also used a bit of tape in the retaining ring of the cutting head, and hit the laser at low power for 0.04 sec duration to put a little hole in the tape. The beam was centered after a six or so iterations.

Height adjustment. After the beam was centered I started working on adjusting the height of the beam. This picture shows the goal of height adjustment. The issue is that the beam forms a waist and the most power of the laser occurs at the minimum waist diameter. The sweet spot of the beam waist can be placed in path of the beam by adjusting the height of the cutting nozzle.

To find the best height for minimum beam diameter, I used the thermally sensitive paper and looked at the beam diameter as a function of height. The first pass I took at this was by crudely changing the height while the shot a short duration laser pulse at burn paper. The smaller the dot on this burn test the better. This was repeated again to by starting at the general height from round one and using relative small turns on the vernier adjust. Up until this point the height adjustment was done without the copper cutting nozzle on the cutting head. I put the nozzle on the system and did more tests with the burn paper. If you look at this pic, you can see very odd things happen to the beam with nozzle on the cutting head. The beam is shifted around at the elbow using the adjusting screws until the crescent shapes around the central portion of the spot made by the beam is removed.

Laser beam path length

April 8, 2010


Finalizing my plans for purchase of new optics. According to the information from the sales guy at Laser Mechanisms the ideal distance for my new cutting head and collimator will be 350mm. The the cut quality enhancer and circular polarizer are a series of optics and its very hard to measure the length of the entire beam path. The sales rep at Laser Mechanisms gave me this drawing that shows a total length of 8.4 inches. The pic inventories all the distances in the optics chain which includes front brackets, the circular polarizer/cut quality enhancer, and a beam bend. The total is 13.45 inches or 341mm. This is passable because I think I can add another 10mm by adjusting the barrel of the extension out another centimeter.

Additional beam size information

April 6, 2010

We final[ly] had time to ray trace your configuration. We have determined that using a 2.0x beam expander collimator along with a 2.5″ focusing lens will indeed produce a spot size ~150um with a DOF (depth of focus) of 2.2mm. Please note DOF is defined as 2x the Rayleigh Length. This only works however when the collimator is placed 350mm from the laser. It is also important to note that our 2.5x beam expander will not work at this distance. The full expanded beam diameter exceeds our 15mm clear aperture. Let me know if you have any questions.

Best Regards,
Gary, Sales Engineer, Laser Mechanisms

Optics recalculation.

March 15, 2010

Its good that the laser is cutting but I am still interested in tweaking the optics so it will cut thicker metal.

As I stated in this post that has a table which suggested that at a beam diameter of 10mm, an a focal length of 3 inches I could obtain a reasonable power density and still cut thicker metal.

Purchasing optics is an expensive enterprize so I thought it would be a good idea to reality check the diameter of my beam. The original guess I had from back in the day was that I had a beam diameter of 10.5mm. This was based on a table that someone gave me which used a formula to determine spot size based on the total distance between the laser and the focal point.

There are three problems with this plan. I had no idea where the chart for spot sizes came from, the distance the beam travels in my optics chain is difficult to determine, and my optics chain has a colimator which is unlabeled – I had no idea how much that thing expanded of the beam. I went with the calculated sizes on the chart, took a guess at how much the colimator expanded the beam, hooked up the optics and eventually it cut metal. That was fine but in order to maximize the potential of the laser I need more information about the real dimensions of beam size.

I performed some tests with the laser set at low power. I made 11 spots with thermal paper set at different distances from the laser. The spots at each distance are shown in the picture below.

Each row is labeled with the distance from the laser in centimeters. Also shown in the picture is a test shot of spots after it comes from my optics chain. The picture was loaded into a CAD program and the max width of each spot was measured. The totals for each row shown for the spot sizes made from distance of 400, 350 and 300 cm – the totals for the test shot is also shown. It looks like from this analysis that the length of my optics chain is very close to 350 cm.

I also used a micrometer and by eyeball took a couple measurements of each spot, and they correspond _reasonably_ well with what is predicted from the chart. You might wonder why I dont measure the spot size directly from the thermal paper and just leave it at that. If you inspect the spots closely you can see they really vary. I think this is because the laser control software does not do short pulses very well and the total amount of time the beam is on seems to vary. Another issue is that laser beams are a gaussian distribution where the center is a lot hotter than the outer portion and the beam’s “edge” is really not obvious.

Okay, but by look at the chart of expected sizes and by directly measuring the spots on the thermal paper, I think my beam size is somewhere around 3.6mm.

I also tried some crude experiments and I think my colimator expands the beam to around 10mm (exactly what i predicted in my first analysis. So if I were to use my original chart I would go with my current colimator, get a 10mm spot size and use a cutting head with 3 inch focal length.

Unfortunately there’s a problem. Laser mechanisms doesnt make a cutting head with 3 inch focal length. They have 4 inch and 2.5 inch. This is a chart of power densities and depths of field for 2.5 inches.

This is a new chart that reflects the stats if I went with 2.5 inch focal length. It looks like what’d be useful is to create a beam diameter of 8 mm.

Quote for optics.

March 5, 2010

Configuration of optics

March 5, 2010

I’ve been talking to the guys over at laser mechanisms and it looks like I could get into a new, better quality laser cutting head for around $1200. I’ve been wanting a new laser head for a while because I am pretty sure if I go with a better focal length I’ll be able to cut thicker metal. There is a good chance I may be able to double the thickness of the sheet metal in fact.

Life is all about power density. As in, if you want to blow a hole in metal you need to focus the beam of the laser to pack as much possible power into a really tight spot. There are couple of formulae that govern what happens with the spot size. The formula for the size of the spot is:

spot size = .013 * M^2 * (fl/D)

where M^2 is equal to 1.5 (comes from the laser specs),
fl is the focal length of the cutting head, and
D is diameter of incoming beam.

So there are two things I can change that impact the spot size, focal length and the incoming beam size.

Take a look at this picture. This shows the idea. The spot size (called beam waist in the pic) changes depending on focal length from the lens. The picture shows another important element to the process — focus depth. The focus depth is the distance range that an object can be placed in front of the lens and still get cut. The focus depth is a volume (shaped like an hour glass) that is packing a reasonable amount of energy that can actually get through the metal. The larger the depth, the thicker the metal is that I can cut.

So: small spot size good, big focus depth good.

The focus depth is governed by focal length and beam diameter in this equation, where:

depth = 2.5 x wavelength x ( focal_length / beam_diameter )^2

Have a look at this chart:

It calculates various spots sizes and focus depths – called “DOF” (depth of field) in the chart. The main factors that are varied in the chart are focal length and beam diameter. So my current arrangement is shown on the third line down. I have a 1.5 inch focal length and 10.5mm beam. This creates a tight tight little spot size of 76 micron.

small spot size: good, not big focus depth: not good.

The focus depth is only 0.36 mm. It is this short little number that explains why I can only cut 0.032 inch thick metal.

Lets change this. How much energy do I actually need to pack into a little spot in order to cut metal?

This book has an offhand statement “The power density is raised above 10^6w/cm^2 levels, at which most metals can be vaporized.”

The last column of my table also shows the power density for various configurations. What you can see is for my current set up, I have 61,000 W/mm^2, and in theory I only need 10,000. Wow. Have I been wasting major energy while not even maximizing my laser?

Crazy. I made a graph of showing power densities for different beam size and focal lengths…


what it shows is that its easy to get up over the 10,000 W/mm^2 range with pretty much any focal length.

So going back to the chart, there’s a row marked in red that looks pretty promising. The beam diameter is 10mm which is good for me because that’s the current beam size — I wouldnt have to make any changes to my optics in that case.

Conclusion: If I get a new cutting head with focal length of 3 inches and use my current beam diameter I can be well over beyond my power density and get a depth of field of 1.57 mm. This has the potential of tripling the thickness of metal I am currently cutting.

Quote to Haas LTI

March 4, 2010


Sending this out to Haas LTI

Dear Haas LTI,

I have a Coherent G100 that I use with oxygen assist to cut sheet metal. I use an early model of one of your cutting heads.

There are three items that I would like you to address.:

One – the retention springs on my current cutting head are not able to manage the 40 PSI oxygen that is supplied to the head without it expanding. When I looked at your site I noticed that the latest generation cutting head has some sort of retention clip that is attached to the micrometer and prevents the sliding ortion of the head from expanding. I was wondering if it would be possible to retrofit the new micrometer portion of you latest generation system to my cutting head. I include a picture that shows the new cutting head in contrast to my system. If its possible to retrofit my old cutting head, please supply me with a quote.

Two – my current system has a focal length of 1.5 inches and I would like to convert to 4 inches. If its possible to perform the retrofit, could you please supply me with a quote to convert the cutting head to 4 inch focal length.

Three – if I cant do the retrofit please supply me with a quote for a cutting head with micrometer adjustment and four inch focal length.

thanks much,

xxxx

Repair of optics

March 3, 2010

…was much cheaper than originally quoted.

Hello XXXX,

We have your feedback isolator repaired. The 90 degree phase shifter optic was burned. We do not believe this was caused by operating at 250W or necessarily from contamination. After time this coating can just fail, which is what we believed happened here.

The good news is the rest of the optics are OK. Attached is a copy of our sales order. We can ship this back to you as soon as we get your credit card info. Please call or e-mail me your credit card info including the number, expiration date, and 3 digit code on the back. Thanks.

Best Regards,
Gary M. Cican Jr.
Sales Engineer
Laser Mechanisms, INC
25325 Regency Drive
Novi, MI 48375
(248) 474-9480 Phone
(734) 417-0065 Cell
(248) 474-9277 Fax

The attached quote was for $460.

Quote for optics repair

February 24, 2010

….is around $1600. Dang those mirrors are pricey.

Repairing the optics.

February 14, 2010

I’m sending my Cut Quality Enhancer and Feedback Isolator to Laser Mechanisms for repair. The optics got burnt right before I took the old system down.

I sent the package on Feb 16th, attention to Gary Cican, RMA number: 2408.