Lamp Light And Laser Light
Pulsed lasers, like this, use an intense burst of white light (lasting fractions of a second) to stimulate the electrons in the laser’s gain medium. In the above photo you can see the lamp running parallel to the glass tube that contains R6G dye. There’s about a millimetre distance between the two and they’re wrapped in aluminium kitchen foil to focus the flash lamp energy into the dye.
A popular misconception of such pulsed lasers is that the light from the lamp is the laser’s light. It’s not. The gain medium makes its own light. We can easily demonstrate this with the photo below:
You can see that the lamp is brightly lit. Here it’s wrapped in printer photo paper, optically coupling it to the dye cell. You can also see the spark gap in action – more on that later. The dye is certainly fluorescing a bright yellow, the electrons may or may not be in a ‘population inversion’, but the feedback mirrors are not aligned. Even with that intense an amount of light no laser light is produced. In fact, very little light exits the laser. Compare this image to the ‘featured image’ accompanying this article. There’s a stark difference between the amount of flash lamp light and laser light.
Laser Cell Construction
Flash pumped lasers are quite simple things but can be very tricky to engineer and construct. Dye lasers are a pain in the …
Things to note in the above photo:
Marked with a letter A and accompanying arrows. The one on the left is the ‘highly reflective’ (HR) mirror. It’s spring mounted and made from a front coated mirror that was removed from an old flat bed photo scanner. If you look closely at the disassembled end mount you can see what looks like a tiny ‘chainsaw’ (near the pencil tip). This piece of metal holds the mirror in place and makes it easy to remove, aiding optical alignment of the two mirrors. You can just see it in place in the right side photo.
The second mirror (or Output Coupler) is actually a piece of “dichoric” glass from a pond/fountain ornament light! I save all types of optical junk. It’s not optimal but it does return at least fifty percent of the light at the wavelength that we are interested in. Lasers can be quite lenient where mirrors are concerned, especially a high gain laser like this. This mirror is not removable and is fixed in place with silicone ‘glue’.
The Glass Dye Laser Tube
This is just a piece of ‘soda’ glass from an old chemistry set. Quartz would be better as it will pass more of the shorter wavelengths from the flash lamp. The bore diameter (for this small laser) should be between one and two millimetres.
To make the laser work we need to align the two mirrors so that they are exactly parallel to each other. The easiest way do this (oh, the irony) is with a laser! I use my trusty He-Ne gas laser, but shinning it down the bore of a one millimetre glass tube is not that easy without it catching the sides. A little thought demonstrates that it would “try the patience of a saint” without the removable HR mirror.
The Epoxy End Mounts
The plastic ‘end mounts’, which hold everything together, are made from clear epoxy resin – which you can buy off of the internet. I recommend at least 72 hours curing time before drilling the stuff. The moulds for the mounts were made from ‘Yakult’ yogurt pots!
I’m not going to go into heavy detail about their construction as I think that the photos are pretty much self explanatory – and in any case I don’t think that any puffins here will be replicating this contraption. Two things to note though, the end glass windows which are made from microscope slides. They are fixed in place with silicone. Also, the rubber ‘O’ ring seals (just visible) around the glass tube. These are, again, fixed in place with clear silicone.
Unlike humans, epoxy resin does not take kindly to being exposed to strong alcohol, I’ve found that it tends to make it brittle over time (perhaps like humans). Because of the peristaltic pump it makes it easy to fill/empty the laser cell. Emptying the dye cell after use will obviously prolong the life of the resin and the O-ring seals. That’s one advantage of having a liquid pump, the other reason I’ll explain (and demonstrate) later.
Finally, the three screws/springs. These hold the end mounts in place and allow a subtle adjustment to get each mirror parallel to the other. Only a fraction of a millimetre of movement is required. The ‘O’ rings mechanically give just enough but keep the system sealed. Simple and effective.
Of Lamps, Sparks And Gaps
The key to making this laser work is literally ‘light speed’. Remember, if we don’t excite (or pump) the electrons in the R6G dye fast enough it won’t lase. To do that we need a very quick and intense white light source. The simplest solution is to use a photographic flash lamp. The problem is that they tend to be slow. Fortunately, if we use a narrow bore flash tube (less than two millimetres) and a high enough voltage, it can be made to work.
In the above photo the green arrow is pointing at a small coil of wire that’s ‘cemented’ to the glass flash tube. When used inside of a camera it’s connected to a tiny high voltage “pulse transformer”. Xenon gas doesn’t conduct electricity easily (a bit of an understatement!) but if a high enough voltage is applied it will. That is the point of this wire, it’s to ‘ionise’ the xenon to make it conduct at a much lower voltage and that lights the lamp for quite a long period of time. Good for a family snapshot but very bad for this laser. We don’t need this wire and (luckily) it’s easy to remove.
What we ideally want is a peak light pulse from the lamp that lasts less than a microsecond. To achieve this we can apply a very high voltage directly across the terminals of the flash lamp, thus ionising the xenon gas and, therefore, producing a fast, bright white, flash of light. Hopefully with enough energy to excite the R6G dye to its lasing threshold while avoiding most of the triplet state energy trap.
I was about to write ‘problem’ here but I think ‘hurdle’ is more appropriate – especially for the electrons. And that hurdle is ‘switching’ or delivering ten thousand volts into that small flash tube, very quickly. Here’s the electron’s hurdle:
…and having waffled on for far too long I’ll leave that little thing for the next article.
In part 3 I’ll write about the spark gap circuit (don’t worry! I’ve gone with a fun explanation) and show you some examples of this little laser in action. And its problems…
© text & images Doc Mike Finnley 2021
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