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By Leslie Wright
For my very first dye laser attachment for the nitrogen laser, I used home-made cuvettes. These were made of either glass tubing, or microscope slides glued together. Some af my first efforts are pictured below.
The one on the left is made from tubing, with windows installed, the next is made from `window glass` ! and the next two are made from microscope slides.
The first ones were glued using 5 minute epoxy, which rapidly failed as the dye solvent (methanol) dissolved it! The subsequent ones were glued using silicone sealant, applied with a stout wire. Whilst these cells functioned reasonably well, they were prone to leaking, and very difficult to clean between dye changes.
I purchased a new cuvette, from an optics company, that was made from SOG (special optical glass) , but that set me back about £70 , but it was far more superior, although the path length was a little short (10mm). However I kept seeing fluorimeter cells, or cuvettes on e-Bay, at knock down prices. The only problem is that most cells on the surplus market, are only transparent on two sides (the rest are frosted). For a dye laser we need cells that are transparent on three sides, one for the pump beam, and the other two for our outputs. I figured it should be possible to polish up one of the frosted sides, so that they could be used in a dye laser system, so I picked up this little lot for £20 !
I also bought for a few pounds, a kit for polishing scratches out of glass. It contained four polishing heads, comprised of compressed cotton, a bag of powdered pumice, and a bag of Cerium oxide.
As the glass is only frosted, I decided that pumice powder was much too abrasive, so I just used the cerium oxide. The Cerium oxide was mixed with a little water, and applied to the ground glass surface, and some was also applied to the polishing head.
The polishing head was mounted in a pillar drill and the workpiece held in the hand as shown below.
The Cerium oxide paste drips into the small tray, at the bottom of the picture. When the polishing head removed the Cerium oxide from the work, it is simply re applied.
The polishing head begins to dry out as polishing takes place, due to friction. This is made evident by the little puffs of Cerium oxide dust that are blown off, as the work progresses. When this happened, the polishing head was simply re-wetted.
It is important to keep the work piece moving back and forth during polishing to prevent overheating!
The polishing takes a little over 15 minutes. Once the glass was looking transparent, the piece was further polished for 5 minutes or so, without adding further Cerium oxide to the work, or the polishing head. Instead, both the work piece and the polishing head were repeatedly wetted, until the water ran clear, this ensures a high final polish.
The finished cuvette is pictured below:
The one on the left, is unpolished, and the one on the right is. (the `X` is drawn on the remaining frosted surface)
Whilst the polished surface cannot be claimed to be opticaly perfect, it is certainly sufficient for a nitrogen laser to effectively pump the dye.
Here is the modified cuvette, containing Rhodamine 6G in methanol, being pumped with a nitrogen laser:
As you can see, the uncollimated output beam is quite intense, so intense in fact, that the small piece of aluminiumised mirror I used as a high reflector, had its coating blown off , after the first run!! **see note at bottom of page regarding power**
The modified cuvette, gives much better results than my original £70 cuvette, as less of the pump beam is wasted due to the 20mm pathlength. The pump beam is 20mm across when it hits the cell.
** Regarding power. Some time ago when I first built the dye laser, I was photographing it in the old lab. I was running it in a 10mm long cuvette, which was less than optimal, as this article shows. However at some point the beam entered the lens of the digital camera, and instanly destroyed a portion of the CCD! It actually blew a hole in the element. To look at, the beam appears to be 20mW or so, but its peak power is enormous! If you figure that the peak power of the pump laser is around the 100kW mark, and then assume 30% efficiency, of the dye laser pumped in this manner, then peak power out is 30kW !! The picture below, was taken with the camera after the damage had occured:
As you can see the laser obliterated the pixels! I turned on the camera, and pointed it at a diffuse light, and gave it a shake, and sure enough, there are little bits of silicon rattling around in there!
Although this laser would be classified IIIB, I would be inclined to treat it as class IV, especially if the output is invisible, and after this incident, I re labelled the Nitrogen laser to reflect this concern.