Nd:Glass is an alternative to Nd:YAG with similar lasing characteristics. Its major benefit is the availability in large sizes at reasonable cost (usually surplus from large Government laser projects). However, the thermal conductivity is much lower than YAG so repetition rates need to be much lower. (This wasn't a problem with those huge lasers that fired once an hour!)
Ruby can also be used (it was, of course, the lasing material in the first laser) and has both advantages and disadvantages. On the plus side, the output is visible (694 nm, deep red) and pumping is a bit easier because the flash duration can be much longer. However, the threshold to get lasing is much higher for ruby (all other factors being equal) and its conversion efficiency is much lower. Being a 3 level rather than 4 level system (like YAG), the entire rod needs to be pumped or else unpumped regions will absorb any laser light which may result in an unusably high threshold and greatly reduced output.
A variety of other laser crystals can be used but probably don't offer significant advantages unless you are interested in the particular wavelength they generate. Three common types are Erbium:YAG (Er:YAG, lases at 2,940 nm), Er:Glass (1,540 nm), and Holmium:YAG (Ho:YAG, 2,100 nm). They are likely to be less common on the surplus market than either Nd:YAG or ruby. But, due to lower popularity, may be available at attractive prices.
The power supply for a pulsed solid state laser consists of a Pulse Forming Network (PFN), capacitor charger, and flashlamp trigger circuit. Detailed information can be found in the chapters starting with: SS Laser Power Supplies.
However, note that like the home-built diode pumped solid state laser, constructing a PSS laser truly from scratch is way out of the question onless you are willing to do the glass work and gas filling of a custom flashlamp lamp and work for a laser crystal company to fabricate, grind, polish the rod. :) So, you may feel that a PSS laser doesn't offer enough of a challenge or reward without being able to say you did everything from the the ground up. Building one for $10 is also probably not realistic although with a bit of ingenuity, the costs can be kept quite low. I do consider a PSS laser to be worthy of home-built laser status as long as we're talking about dealing with the individual components (rod, lamp(s), mirrors, cavity reflector, power supply, etc.). You will need to decide whether just hooking up an M60 rangefinder ruby laser head or SSY1 Nd:YAG laser head to a home-built capacitor charger and PFN qualifes. :)
Much of the information in this chapter also applies to arc lamp and halogen lamp pumped SS Nd:YAG (the material that would be most suitable) lasers. However, with the increasing popularity of high power laser diodes as pump sources, CW lamp pumped lasers are rapidly losing their appeal due to their HUGE power requirements and waste heat, and terrible efficiency. Therefore, there will be little or no specific information on these beasts.
The output beam should be fully enclosed if possible (especially where it is well collimated at the output of the rod) to avoid any possibility of eye contact with the direct or reflected beam as well as damage to property. Access should be restricted to the focal point of any focusing optics. Even if not focused, a small PSS laser can char darker materials and the focused beam will blow holes in many things.
Power supplies should include safety interlocks and automatic bleeders when any cover is removed. All connections should be more than adequately insulated to prevent accidental contact.
If you are using water cooling (probably not likely except for a VERY large system) - either tap water or a closed loop system, make sure that water circulation paths are well insulated from the high voltage and that all fittings are securely grounded. Tap water is a fairly good conductor of electricity - think of it as a very soft wire. :) Unless you have a totally closed system filled with 100 percent distilled de-ionized water, there can be enough current flow to be lethal. Water and electricity do not mix!
For more information, see the chapter: Laser Safety. Sample safety labels which can be edited for this laser can be found in the section: Laser Safety Labels and Signs.
As noted previously, for a YAG laser, the specific sizes/ratings of the components aren't at all critical. A wide variety of designs will result in a working PSS laser, possibly with better performance than the one shown. Thus, don't restrict your search for the YAG rod, flashlamp, and optics to those shown in the diagram or described below. Matching the lengths of the rod and flashlamp will be best, but for YAG, it isn't critical. Using a 2 inch flashlamp with a 3 inch rod should still work. (With ruby, a 3-level laser, unpumped regions of the rod absorb the laser light and if a ruby rod isn't fully pumped, result in a much higher threshold or inability to lase at all). Of course with either, using a flashlamp that is too long is just wasting photons. :)
Refer to Typical Home-Built PSS Laser Assembly for a simplified diagram of a typical laser head.
The best types of flashlamps are those designed for laser pump applications. They are generally of higher quality than the type in that disposable camera flash and are constructed with materials to handle higher energy and pass the wavelengths needed for pumping laser rods. You'll also want a flashlamp with an arc length approximately equal to the length of your laser rod. This will result in highest efficiency for any laser but is particularly critical for ruby. If the lamp is too short, unpumped regions will absorb any laser light which may result in an unusably high threshold and greatly reduced output. Note that while a helical flashlamp may look 'cool', it's generally easier to deal with linear flashlamps for pumping laser rods - too much light energy is lost to reflections from the coils and out the ends of the cavity even if a perfect reflector surrounds them. And, the long discharge path means helical flashlamps will tend to have larger Ko values and thus require higher voltage to achieve the same pulse width.
The "gold standard" for flashlamps has been EG&G (now Perkin-Elmer) but many other companies make similar lamps. (See the section: EG&G 1300 Series Linear Flashlamp Specifications.) Needless to say, these are very expensive if purchased new.
Laser pump flashlamps can often be found at the major laser surplus places as well as on eBay. However, make sure that what is listed is actually a flashlamp and not an arc lamp - the latter won't survive for long when driven in pulsed mode! Sellers frequently don't know the difference. The only sure way to know is from the part number though high power arc lamps tend to have one pointed electrode and better coupling between the electrodes and exterior for improved heat transfer.
For a small PSS laser, common photographic flashlamps available from major electronics distributors like DigiKey, Mouser and electronics surplus places may in fact be adequate, if not ideal. These may be suited for ruby with its long fluorescence lifetime so the pulse width can be long thus reducing stress on the flashlamp and allowing regular electrolytic capacitors to be used in the PFN. However, this is offset somewhat by the much lower threshold inherent with YAG where the flash unit from a disposable camera may be adequate to pump a 1 to 1.5 inch YAG rod.
Ideally, what you want are never used rods with ground, polished, and AR (Anti-Reflection) or mirror coated ends. Except for the AR coatings, this is essential - you aren't going to do optical quality finishing work in your basement! If the seller doesn't explicitly say this, ask. It's possible to use rods without AR coatings. However, efficiency and stability may be slightly lower if they are not cut at the Brewster angle. Getting rods with at least one of the mirrors already on the end of the rod will certainly simplify the laser design but these are not very common.
In particular, avoid those from eBay seller ID ninteach whose "Military Surplus" rods are probably good only to cut up for jewelry or as conversation pieces, but not for lasers. (He may also be selling the same junk under other eBay user IDs selling the same junk.) It's quite amazing what uninformed people will pay for these! $15 for a garbage rod to experiment with or as a curiosity might be reasonable. But I've seen them go for $150 or more! Some of the specific known problems with these include:
Aside from the tint of the material, a high quality rod with polished and AR coated ends should just about disappear when viewed lengthwise. The interior should be crystal-clear with absolutely no distortion.
Realistically, most of the rods available at reasonable cost for home-built PSS lasers are going to be used and in varying states of health but most should be adequate if in good physical condition. However, that may take some careful inpsection to identify. In particular, damage to the rod ends (AR coatings or mirrors) and possibly thermal induced microfractures at the ends or internally. But rods pulled from commercial lasers when their output energy/power falls below spec'd values will likely have a lot of life left. And, sometimes, they will be replaced as a "shotgun" fix along with the mirrors or other optics - which may have been the actual problem. Thus the rods will often be nearly good as new.
Some people claim the existence of such a thing as a depleted rod, and it has nothing to do with nuclear reactors! The idea is that somehow, the Nd in the rod migrates from where it is supposed to be to somewhere else after long use. A senior engineer at Synoptics and a retired engineer from Union Carbide both agree that this is an urban myth despite claims from some others whose information is normally reliable. If anyone has additional comments or references on this topic, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.
Laser surplus places like Meredith Instruments and others may carry ruby, Nd:YAG, and sometimes, other strange laser rods. Since these are usually not continuously stocked items but come and go, checking back from time-to-time will be desirable. In any case, I'd recommend sticking with the common lasing materials even if you find something else cheap. YAG is easiest to make lase but its output is invisible. Ruby produces visible output but has a higher threshold. Optics for both of these are likely to be more readily available than for others. A bargain rod isn't a bargain if everything else costs more!
And if you're determined to have the biggest laser on your block, check out Neodymium Glass. They must have gotten surplus optics from the older Livermore lasers.
Laser rods turn up quite frequently on eBay, both pulls from equipment and never used (NOS, New Old Stock). Monitoring auctions with the words "laser", and "YAG" or "ruby" (as desired) shouldn't take too much time.
The HR can be either a totally reflecting high energy dielectric mirror or a roof prism or corner reflector, AR coated for the laser wavelength if possible. A high quality first surface aluminized mirror might survive for a few shots or more but with the high intra-cavity power of a PSS laser, possibly not for long. And, probably not at all if Q-switched!
The OC mirror really needs to be one designed for PSS laser. Without a Q-switch, figure something like 70 percent reflectivity at the lasing wavelength. With a Q-switch, it can be as low as 30 percent. I'd suggest avoiding "resonant" type mirrors in favor of high energy dielectric types. While resonant optics have no coatings to be damaged, their internal optical element spacing and alignment are absolutely critical (if disassembled for any reason, will likely be ruined). And, their condition or even effective reflectivity can't easily be determined. The reflectivity is also likely to be too low for useful operation without a Q-switch. Although the original ruby laser, circa 1960, used an aluminum output mirror with a hole in the middle, this is probably not a great idea (but it wouldn't hurt to try!).
Laser surplus places and eBay are the likely sources unless you are willing to pay big $$$ for them.
(From: Chris Leubner (firstname.lastname@example.org).)
I have found that some of the IR attenuators out of very old color video cameras work as a half way descent OC for those ruby lasers, You need the type that are made of clear glass with a reddish mirror coating, the blue ones will not work.
So, I built a resonator using some scrap aluminum with a pair of simple adjustable mirror mounts with SSY1 mirrors (from a defunct SSY1 - whereabouts of rod and flashlamp unknown). I mounted a 20 mm focal length focusing lens in front of the OC mirror.
The original flashlamp was apparently supposed to be triggered using a series pulse technique. I prefer external triggering so I added a fine wire running along the side of the flashlamp for the trigger transformer I use with PFN1. Triggering works great. :)
Alignment was the real pain with all the weak reflections (HR, OC, and rod, front and rear, and both mirrors were ground with wedge). But, I was able to smoke black electrical tape when focused on the first try using the SSY1 PFN1 at 800 to 900 V (approximately 10 to 15 J input energy). I'ms sure alignment isn't optimal though. The output energy seems similar to that of SSY1, maybe a bit better on black tape (nice wisps of smoke), not quite as good on aluminum foil. Without a Q-switch, the output pulse is longer and of lower peak power which may be the main factor. (The behavior does appear similar to that of SSY1 with its Q-switch removed.) I will really need to have some sort of energy meter though to optimize alignment. This could possibly be done with just a photodiode centered on the optical axis with the beam spread out (to reduce peak power to the photodiode) monitoring the current on a scope.
Summary of specifications:
This is an experimental dual flashlamp-pumped solid-state laser built quite inexpensively to be used for educational demonstrations as well as the experimental testing of various components to determine their effect on laser performance. See: Chris's High Energy Dual Flashlamp Pumped Nd:YAG Laser (Diagram) for the basic layout of the system. The completed unit (with cavity cover removed) is shown in Chris's High Energy Dual Flashlamp Pumped Nd:YAG Laser (Photo) It really does look just like the diagram! :)
I did get my first coherent photons from it just the other evening. I am in need of some good YAG resonant optics to maximize this laser's performance, as efficiency is not very high right now. The best I've done so far has been a 0.97 joule output pulse with an input of 200 joules. This was read with my questionably calibrated Ophir laser power meter. I will try to get some pictures of this laser soon.
Here are the general specifications:
I do plan on the passive Q-switching of this laser in the future. My 'ever cheap plan' is to construct a small quartz dye chamber placed near, or as a part of, the high reflector and employ an organic dye suitable for YAG Q-switching within this cell.
Well after several weeks of construction I finally got my ruby laser working. See Sinebar's Ruby Laser. I don't know how much power it is putting out but I think it is probably somewhat modest. The ruby rod is 3" x 1/4" and the ends are dielectric coated. The HR end is 99.9% reflective and the OC end is 55% reflective. I am using the PFN that was originally used for the Hughes range finder. Cap is 150 uF, 1250 V. I think that is about 117 joules. The power supply is from a bug zapper. The external trigger consists of a small trigger transformer and a simple circuit that charges a capacitor which dumps voltage across the primary of the trigger transformer when a button is pushed. The laser cavity I robbed off a Hughes range finder as well as the flashlamp. In fact, I used quite a bit of the rangefinder parts. I put everything in a nice wood box that gives it a kind of old timey look.
Detecting the beam and optimizing alignment can be done by checking the beam profile with Zapit(tm) paper, thermal fax or printer paper, a photocopy of a black original, other black paper, or black electrical tape, and maximizing hole-blasting effectiveness. ;-)
Lamp pumped SS lasers often use flat-flat resonators to prevent hot spots inside the YAG rod. The intra and output beams both have a large diameter (almost equal to the rod diameter) and are massively multi (transverse) mode. Just inserting a KTP crystal into the beam (inside or outside the cavity) without taking into these factors and peak power density of the beam will likely result in either poor performance or damage to the KTP. With the multimode beam, it isn't sufficient to just consider the average power density in the beam cross-section - peak power in the hot spots due to the sub-beams must be taken into account.
I know that those hybrid vanadate crystals used in green laser pointers (Nd:YVO4 and KTP) will lase when you let a camera flash go off within a few cm of the pump surface. I really didn't expect anything to happen, thinking that this was about lasers and nothing with lasers is ever easy. I first tried it with the flash of my digital camera, then with the flash unit of a disposable camera. Both worked without any attempt at focusing the flashlight on the crystal, up to about 3 cm from the surface. I didn't dare try playing with any means of focusing, scared that I might ruin the laser crystal or the mirrors.
I did a very rough energy calculation, gross assumptions made here and there. Please correct me if I'm off by more than a galaxy. ;o)
Based on flash cap capacity and voltage drop before and after the flash, there's about 6 Joules going into the flash tube. I have no idea about the efficiency of a small (tiny?) xenon tube, but assuming 50% (probably way too much), there's 3 J available in light. Of that light, maybe 2% has the appropriate wavelength for effectively pumping the vanadate. Also, with the (lack of) construction in this setup, at least 95% of the light misses the vanadate completely, leaving only 3 mJ being used for lasing. With an oscilloscope measured flash duration of about 0.4 ms (FWHM), this results in 7.5 W average pulse power. I still have to put both flash and laser output in the same 'scope shot, to compare input and output. These 7.5 W are spread over the entire 5 square mm (the edges of the crystal are slightly ragged and the mirror coating is a circle just touching the edges) of the vanadate. That's 1.5 W per sq. mm not counting whatever light may enter from the sides of the vanadate. Using diode pumping, Pd = about 150 mW (estimate), focus spot size is about 0.002 square mm. (another estimate), the power density is 75 W/square mm. Even if my calculations are off by a factor 50, the flash pumping is still equal to the usual diode pump. I do suspect that I'm on the low side with my numbers for the flash pumping, or vanadate thresholds even easier than I thought. About the green output, divide your results by 8 or 10 again...
For what it's worth...