Diane's Laser Page

Deutsche Version

Childhood dreams came to an end. No, to be honest, they only slept until I was in the final High School classes. It was the beginning 80ies, and being a frequent reader of the german issue of Scientific American, one day I found the famous description of the mercury laser in it.
Huh? A guy somewhere out there built his OWN laser?
The feaver came back again. When he could do it, I also can do it, I decided. However, I knew a lot more about physics than before, also liked to spent time with the school's little HeNe laser (on rare occasions). Got a basic knowledge of what is important for lasers to become working.
A few telephone calls made me quite unhappy. I had learned you can get all the stuff you need if you really want - and can pay the prices. HIGH prices. Again there was the "ruby problem": lasers are *expensive*.
"Silly, one cannot have one's own laser", a fellow laughed about me, "that's only for laboratories." I better didn't tell him about my desire.
So I thought: "One cannot? Let's see about that."

Don't ask me today why I started to study mathematics. We had a quite famous Department of Quantum Optics on our university, perhaps I should have gone to the laser business. But those days computers were still more exciting than lasers to me, and that's it.
Now, being a student, I had access to real scientific literature, and I learned a lot about the theoretics of lasers (and, reading the business laser magazines, also about technical realisations). I studied various descriptions of laser types and decided to try about a flaslamp pumped dye laser. This one at least needed no expensive ruby. :)
To make a sad story short, I learned a lot about how to blast stroboscope tubes using a voltage doubler and really BIG capacitors, and I guess the carpet in my room at the student's community will still have these nice pink rhodamine spots. :(
I gave up on dyes.
But I still wanted a laser, and some more telephone calls brought a fine small Siemens HeNe tube to me (it was a new 1.5 mW LGR7621). Shall I tell I heard some of the wellknown "brzzzz's" from self-wound transformers until I looked for a used neon transformer? Still a descent of my grandpa I was...
So I spent my time to built a casing for the transformer and a recitifier out of a box of 1N4007's (never tell this a engineer student. I did, the poor boy almost got a heart attack).
By the way, the LGR7621 is quite robust. One day I caused a short which *detonated* the anode resistor (another lesson about BIG capacitors). After my eyes had recovered from the resulting supernova inside the tube, I anxiously looked if there's still anything alive. A visual inspection showed up a lot of very small cracks along the inside of the bore. Replaced the mortal remains of the blown resistor by a fresh one, it still started and lased! I used it for years after this accident without significant drop. Brave little HeNe - see it in "Diane's Laser Museum". :)

After some time, HeNe became boring to me. Using an internal mirror tube is one task, to built a device like Scientific's mercury completely on your own another. So I started some prepearing experiments in that direction for the next time.
I got a simple vacuum pump and an unsealed neon tube and began to work with glow discharges. To keep it short, I had to perform lots of experiments to learn about vacuum, outgassing and purity of gasses. Over the years, my little laboratory grew: a selfmade voltage doubler for the neon transformer, a selfmade mercury vacuummeter, a better pump, noble gases in liter bottles, a hand-held spectroscope, longer discharge tubes. To work, all this took years of learning by doing.
Then the next strike came, of course again by Scientific American. It was the copper vapor laser. How exciting... for I knew very well I had no experiences about laser optics until now - but the superradiant copper lines would need none. In words, I could start immediately.
But unlike the old days, I decided to study a bit about superradiant laser before blindly begin to "hammer and saw". And by this I found out about the still simpler N2 laser.
I decided to have one.
And in 91, a selfmade 10cm test tube of acrylic with quartz windows lased! I danced in my room, for after 10 years I finally got it!

For the fact the 10cm tube lased quite weak I studied more about N2 laser design. The major problem was, I could reach only 10 kV with my equipment thus having poor energy densities in the discharge. Longer selfmade tubes also lased weaker than expected (I tried several), even if attaching a metal mirror to one end. I had to pinch the discharge somewhat, but doing this in a tube made of acrylic would easily overheat and smoke the walls.
Those days I already worked for my Ph.D. thesis, and the research center where I did it had the most precious thing I ever saw in a library: the COMPLETE Review of Scientific Instruments. Complete means complete: from issue #1 of 1929. And after some hours with it, I found the most useful paper about N2 type gas lasers I know. It is about the "strip line" type laser using a segmented discharge bore originally designed for the UV lines of the hydrogen laser [1].
I adopted the design a bit to my power supply (shorter striplines, shorter discharge segments) and pinched the discharge even more by adding short pyrex capillaries inside the segments (see "The Querflöte"). Believe me, it was a *lot* of work to drill holes for 56 electrodes and fill in the pyrex tubes successivly from the end of the outer acrylic tube. But finally I had a 3 mm x 98 cm bore with a nice high energy density. For its strange appearance, I baptised it the "Querflöte" - english "German Flute".
The first thing I noticed after the first tests was, this baby would lase with every gas containing a bit nitrogen, even dirty air. :) Mirrors were good for nothing, and I easily got all three UV lines on a fluorescent screen using a "water prism" (triangle pot glued together from thin pyrex pieces and filled with water, which absorbs UV much less than a massive pyrex prism).
Of course I also tried other gases, and the green superradiant Ne line at 540.1 nm was strong in this tube, too. Over the time it became my favourite. And on few occasions, after long times of green Ne, it was also possible to get the much weaker orange line at 614.3 nm at a lower pressure. But normally it dissappeared after some time from outgassing impurities. The still weaker yellow line I never catched.

Impurities were what finally drove me tired. It was common for my "German Flute" to be run with gas throughflow. Otherwise lasing stayed only for seconds. On one occasion I tried a bore cleaning via He bombardement which took several hours. But after finish, what I call the "dirt spectrum" (N2 band, H alpha line plus Hg lines -- mercury from the vacuummeter) reappeared in half an hour.
Whenever I liked to start my laser, I had to spent days and hours in front just for basic cleaning the vacuum devices. And, the cost for the needed constant flow of Ne burned a hole in my pocket. Pure lab-grade neon isn't that cheap.
And then, a few years later, I got a pen-sized red diode laser which made roughly twice the output power of my homebuilt in the green. It was depressing.

Sometimes in life one has crises and has to seperate from several things. So it was for me, and it was such a crisis which made me giving away a lot of things - including all my lasers. The university didn't take them (security reasons of course), but I found a physicist collecting strange devices, and I hoped my baby still has a home there (even if it doesn't lase any more). So, off I were.

Yes, I *was*. Some day I visited some friends, and I wondered to see a yellow HeNe in their rooms. They told me to use it for illumination of large naturally grown quartz crystals, for they like the "golden shattered glow" in them doing so.
But they knew less to nothing about lasers, and I told about for an hour or so. And thought about I also would like illuminated crystals. But not red or yellow - green and blue, perhaps from an argon ion laser it would have to be. In the days of internet it is an easy task to feed "argon laser" to google.de and see what happens. So I found "Sam's Laser FAQ's", where a version of this story now also is online.
The dream of an argon ion laser faded away - these things are expensive, even if used. DPSS lasers are much more simpler (amateur designs also)... meanwhile I built one (2005). See Xenotim for details.

My first baby 2003 was 11 years old and its tube not any more in a good condition, partly due to the storage at the physicist. Somewhere it outgassed or leaked.
But in the end, I could not leave it alone at the shelf - it was my biggest and most successfull laser. So in the beginning of 2006 I decided to restore it completely (means rebuild of the bore, exchange of all seals and the blumlein dielectric, cleaning and test of all else parts).
Finally...
"There's no such thing like a gas laser!"

And all else I try to build... I'll write about it from time to time here. :)

~D. N.

The pictures show the laser before the 2006 restoration; however, the changes wouldn't be visible in any way.
Added to the vacuum supply was a second vacuum gauge (-1 to +1.5 bar) so the higher pressure range can be measured in future experiments.

download
diagram of laser (GIF, 348 k)

Picture 1 - Overall view of the "Querflöte" laser (composed from three seperate exposures). Besides and behind the long laser head at the right end the rotary vacuum pump, in the back high voltage transformer and voltage doubler / rectifier, at the left end low-pressure gas can and mercury vacuum gauge. The laser head has a plexiglass front cover, so it is possible to watch the tube in operation. Hires version of picture (207 k)

Picture 2 - Another total view of the laser, front side. As visible, I prefer for vacuum lines glas tubes connected by short pieces of vacuum hose. This design to my experience causes quite low outgassing rates. High voltage cables are spark plug cables from a car spare parts market, it stands up to 40 kV.

Picture 3 - Closeup view of the laser tube. The segmented design is visible, in an outer tube of plexiglass there are short pieces of pyrex tube with 3mm inner diameter. Between the pyrex tubes the electrodes enter the tube alternating. Both transmission lines of the Blümlein are the top and bottom of a capacitor stack located behind the tube. The common line is between them (not visible) - so the Blümlein is folded. The upper line is slotted allowing a variable length of the discharge; the spark gap is connected to the bottom transmission line.

Picture 4 - Rear end of the laser tube with external aluminum mirror. It is located on an insulating wooden rod and can be adjusted by three screws outside the box - so high voltage cannot escape this way. The mirror has to be as close as possible to the glow discharge inside the tube. Due to this, the reflected laser pulse has a good chance to travel down the bore as long as the discharge is on (from this idea a quite simple method of pulse duration measurement can be derived). Also visible the quartz window and vacuum line of the tube, connected by flanges. This allows a more easy exchange if necessary.

Picture 5 - Enhanced contrast. Laser action in neon. It is not an easy task to photograph a nanosecond pulse on a white screen. The picture is the one hit out of 39 exposures (!). The laser spot is enlarged to give more detail, unfortunally it is too pale in this reproduction. Originally it should look like this. At the right end a tube segment with the salmon red neon glow discharge. download short movie of neon laser pulse burst (MPG, 524 k)

Picture 6 - The violet glow discharge when running the tube with nitrogen as an UV laser. Entire tube shown.

Picture 7 - Farfield spot (10m) of fluorescence of the UV laser beam on white printer paper (the white toner in it shows a blue fluorescence when exposed to UV). Long time exposure, several pulses superimposed. Note the nearly circular beam cross section which is uncommon for nitrogen lasers.

Technical data of the "Querflöte" laser
type	transverse excited pulsed gas laser driven by Blümlein generator, free-running air spark gap
length of transmission lines	10+10	cm
capacity of transmission lines	10+6	nF
charge resistors	29x 1	MOhm
operation voltage	10	kV
stored energy p. p.	0.8	J
electrode separation	14	mm
electrical field strength	6700	V/cm
pulse frequency (roughly adjustable)	0-50	Hz (erratic)
pulse duration	ca. 3	nsec
length of discharge	98	cm
diameter of discharge	3	mm
discharge volume	6.9	cm3
beam divergence	ca. 3.0	mrad (measured)
possible pressure range	0-900	Torr [c]
gas consumption	ca. 10	l/h

Observed laser lines
nitrogen -	35 Torr	337 nm	4-5 mW (mean [a])
air -	35 Torr	337 nm	1.5 mW (mean [a])
neon -	58 Torr	540 nm	0.6-1 mW (mean [a])
neon -	7 Torr	614 nm	(quite weak)
helium-nitrogen (?) -	10 Torr N2 + 760 (?) Torr He -	428 nm ?	(only at one occasion, not reproduced until now. Beam went straight through an UV block filter used to kill the also present 337 nm UV from N2. So it cannot be just a fluorescence effect!)

Examined gases without laser action [b]
helium -	0 - >100	Torr (flow through mode)
argon -	0 - >>100	Torr (flow through mode)

[a]
Measured by silicium-PIN-photodiode. I have no idea if it is suitable for measurement of nanosecond pulse chains. So the real output power could be higher.

[b]
In visible range of the spectrum 400 - 700 nm and near- IR and UV which can easily be detected by means of simple indicators (overall 300 nm - 1.4 µm). Argon was of welding grade.

[c]
In helium, a stable glow discharge can be sustained up to atmoshere pressure or higher, possibly even higher than I did - but the vacuum hose pops off the tube at approx. 1.5 atm total pressure. :(

In 2004 I restarted the project for a "divided by two" tube - now it was a common longitudinal pumped laser. Nevertheless I kept the name of it, even if there remains not much of the original plan.
Janus II was planned more or less as a copy of the Erikson and Lidholt laser [2] which was used to check for laser action in various gases. The bore of it is much longer than for Janus I so a much higher operation voltage became a necessity. Hum, to be honest, I always dreamed to have a Marx generator - here was the reason to built one.
Pumping of longitudinal nanosecond lasers is done in a certain way. A network of two capacitors is coupled by a resistor and a spark gap. The storage capacitor can be replaced by a marx bank, the peaking capacitor is set coaxially direct onto the discharge bore to have the lowest possible inductive load. This basic principle already worked for Janus I. However, for the much longer bore of the Janus II laser I needed a voltage of 60kV - which meant a six-stage marx connected to my power supply.

Picture 1+2 - The tube of Janus II, the marx generator, vacuum supply and HV power supply. The picture below shows the discharge tube in some more detail. Easy to see the silvery cylinder of the peaking capacitor (500pF / 100kV) inside the laser head. It is located coaxially on to bore, connected to the electrodes by metal pieces as short and thick as possible. The dielectric, a long piece of wrapped vinyl, is oversized by about 10 cm to prevent arcing over. Left of it centered the thin discharge bore. In the background the box of the Marx generator. Note the heavily insulated 60-kilovolts connection line.

Diagram 1 - The charge transfer circuit schematic. Both capacitors are seperated by a low ohm resistor R (ca. 10 Ohm) and a spark gap SG. The storage capacitor C2 should have at least three times the value of the peaking capacitor C1 [3]. On principle, the combination of SG and C2 can be replaced by a n-stage Marx bank as is suggested by the red dots. In this case the series capacity of the marx bank capacitors should be at least three times C1.

Janus II had some surprises for me when the initial tests began. Already the marx bank driven alone shoots some quite impressive 6 cm long sparks. Connected to the peaking capacitor it forms a system of coupled oscillators which can produce essential overvoltages [3] - I observed up to 10 cm sparks if the tube was plugged in but not pumped down (E/p measurements of laser action confirmed that the tube voltage is about 90kV in operation).
First experiments showed up the 337 nm UV nitrogen laser line and the 614 nm orange neon laserline also work in this longitudinal pumped laser device. Nevertheless, the Janus II design has some serious disadvantages:
(1) The extreme voltage is too unhandy for an amateur's laboratory. All corresponding modules need to have large volume - due to the neccesary insulation. Common spark plug cable isn't sufficiently insulated, too.
(2) The long thin bore is hard to drive in flow-through mode at very low pressures. Usually, laser action is present just for a few single pulses. Then the HV has to be switched off until the bore has "flushed". For neon (~1.5 Torr), this is quite pronounced.
Thus, I abandoned the Janus project the second time now.

Picture 1 - Total view of the Xenotim laser - somewhat smaller than my other homebuilt stuff (see above)! In the background the power supply with diode current regulator and supply for the thermoelectric coolers (TECs). Lines are ordered by color. Red/blue diode feed, yellow/black for the TECs and green/green for a heater.

Picture 2 - Detail view of laser head interior, cover removed. At the left end the laser diode, a collimator lens is held in front of the output window by brass clamps. In the middle the focus lens (vertical brass plate) and the heat sink on which is mounted the DPM crystal with cooler and heater. At the right end the telescope which collimates the green output beam. As well the diode module as the crystal module can be removed from the base plate to be used for seperate experiments. (The collimated pump laser beam is a very good tool to manufacture optical pinholes from black cardboard... ;)

I decided to have a 500 mW pump diode (808 nm) and a Casix hybrid crystal DPM 1101. From a fincancial point of view, it was an investment in the order of one of the better green laser pointers - I did it driven by the hope to get much more power out of it than from an ordinary pointer. The latter became true. :) All other are common electronic parts or scrapped optics.

Power Supply:
Core of the power supply is a lab power supply module from a local electronics shop. It allows to adjust voltage and current seperately, I decided for a max. 3A type to have room for later extentions. Voltage rough adjust is set to a value which allows the fine adjust values between 1.5 and 2.5 volts. This should be compatible to quite a number of laser diodes. The module is feeded by a standard 25W transformer.
Some experiments with a laser diode simulator (2 Si rectifier diodes and 0.2 Ohm resistor in series) showed up some problems. There were substantial voltage spikes when switching on and off - necessary to include some power on delay. Now the diode lines are enabled 1 sec. after power-on, at power-off they are disabled immediately. When not powered, the diode lines are kept shorted (ESD protection). At present, the real diode has 40 hours on it and some 30 on/off cycles and is still happy.
A second transformer in the power supply case feeds the TECs of diode and vanadate. I did it the easy way, a bridge rectifier feeds into constant voltage regulators of sufficient power. Adjust is done via power resistors.
An additional adjustable output is for a KTP heater.

Pump Diode:
The laser diode is a 500mW type L081T500m, manufacturer HTOE. My specimen has a wavelength of 806.7 nm at 25 deg C and puts out 500mW at a current of 570 mA (tells the distributor's test sheet). Its TO-3 case I mounted with a 4W TEC to a well overdimensioned heat sink of 1.1 K/W. Here also is room for extensions. The back of the heat sink contains a reverse protection diode and a RC filter ESD protection. The connections are mounted in a way a shunt can be plugged in as transportation safeguard.

Pump Optics:
The pump optics origin is the scrapped pickup of a dead CD writer. A CD is read/written at a wavelength of 785 nm so the AR coatings are still good for 808 nm. The short-focus (f = ~3.5mm) objective lens, glued to a small piece of brass now is the collimator lens of the pump diode and part of the diode module. Sadly the numerical aperture of the lens allows to collect only 85% of the very divergent diode output, but this is bundeled to a more or less parallel beam (at least for half a meter or so) of changing rectangular cross-section. The focus lens of the pump optic I put to the point where the cross-section is roughly a square.
The focus lens (f = ~8 mm) is glued to a seperate stiffened brass plate. By oversized bolt holes it can be adjusted horizontally, by a stack of washers and rubber "O" rings between brass and base plate it can be adjusted vertically.

DPM Crystal:
As mentioned above, the crystal is an optical contacted hybrid crystal DPM 1101 (not one of the glued DPM 0101's) of the manufacturer Casix. It combines 0.5mm Nd:YVO₄; 2mm KTP; mirror 1: AR @ 808nm, HR @ 1064nm, HR @ 532nm; mirror 2: HR @ 1064nm, AR @ 532nm in one crystal module. The tiny little crystal has to be cooled if pumped that powerful - at least the vanadte part of it which has to absorb most of the pump power. I glued the vanadate end of the crystal to a brass cooler by use of epoxy mixed with silver powder (in liquid state I added silver until it reached a consistency of tooth paste). Of course the mirrored end facet of the vanadate must not be degraded by it, not an easy operation. The brass block is clamped onto a 4W TEC, and by my home-brewed silver epoxy there is a good thermal contact of the crystal (I checked this by glueing a dummy before).
The free-standig KTP end of the crystal is surrounded by a heater coil (NiCr, 0.3 Ohm) without touching it. It can be used to control KTP temperature; of course the control is somewhat restricted for the KTP is in direct contact to the cooled vanadate. See also picture 3 (below).

Telescope:
Purpose of the telescope is to reduce the divergence of the green output laser beam somewhat. For a hybrid crystal has short resonator length (2.5 mm) divergence isn't that well. Both lenses are from an old 24.5mm microscope eyepiece. They are uncoated so causing some reflection losses. The eyepiece tube I sawed in two and lengthened it by some copper plumbings so it became a 2:1 telescope. It can be adjusted with respect to the output beam by some oversized bolt holes and rubber "O" rings.

Picture 3 - Enlarged detail view of the mounted crystal. The DPM crystal itself is hard to see, it is easier to see the brass cooler the crystal's vanadate end is glued to by silver epoxy and the NiCr heater coil which surrounds the KTP end. Beyond the transparent plexiglass clamp the TEC is partially visible.

Diagram 1 - The orientation of the DPM crystal *does* matter for the vanadate's absorption is polarization dependent. The DPM 1101 has glass splints on its sides so orientation can be displayed well. Shown the optimal orientation I found for my specimen, this way it absorbs roughly two times as much pump light as turned by 90 degress. To measure the pump optics need not to be in place, it is sufficient to watch out the weakening of the collimated pump beam of low power by the help of a solar cell or photo diode.

Temperature management of diode and crystal is done manually. I performed a number of systematic measures to find out the diode temperature in dependence of current, TEC setting and environment temperature. Same I did for the vanadate cooler to find the optimal TEC setting to get maximal green output for a given pump power and environment temperature. So I have now a table which gives the optimal TEC (and heater!) settings for a number of temperatures and diode currents.
The influence of the KTP heater is highly nonlinear. Very often it is claimed a heater cannot be used for the tiny little hybrid crystals. To my experience, this is not true. At very low to medium pump power, by fine adjust of KTP heating the output can be increased by 5-10%.

Low Pump Power:
Here the temperature of the crystal doesn't increase enough by absorbed pump power to bring the KTP to optimal temperature. Cooling of vanadate not neccessary (actually it would decrease output), KTP heating increases output power.

Medium Pump Power:
The absorbed pump power is large enough to require vanadate cooling. However, the optical flux inside the resonator still is too low to heat the KTP enough, at least not against the vanadate cooling. Heating of KTP end (some cases a bit, some cases more) still can increase output power.

High Pump Power:
Pump power and flux inside the resonator now require massive vanadate cooling. But the losses now heat the KTP sufficient so no more heating is needed.

There can be speculation if the KTP heater does only a temperature fine control of the overall DPM crystal (not only the KTP end). I did some experiments concerning this by measuring the ratio of 532nm vs. 1064nm outputs. I had only filters of a relatively low quality so I couldn't measure the output directly but just the ratio of the various wavelengths. Even if the total (532 + 1064 nm) output power doesn't increase much, the ratio changes in favour of the green component which is an argument for an influence of the KTP temperature:

Easy to see that the power ratio (532:1064nm) changes from nearly 1:3 to 1:1.5 which is better. Measured by use of a monocrystalline (blue) Si solar cell, spectral response at the different wavelengths taken into account.
The green laser output power of the Xenotim laser with optimal settings (at least until now) for a number of diode currents is given below. Note that the table is valid only for an optimal position of the pump spot which is very close to the cooled side of the vanadate. At other positions of the pump spot (e.g., at the center of the front facet) the output maximum is lower (approx. 15 mW @ 400 mA) and cannot be increased further by an increase of pump power input. This clearly shows the influence of the low thermal conductibility of the vanadate which is close to that of glass.
The values below are in the order of magnitude to what Casix promises in the DPM 1101 datasheet, at least if the losses in the telescope are taken into account:

Picture 4 - View of the beam in evening dim. Power level approx. 30 mW. Note that the photo was taken *without* the help of any artifical smoke or fog, it's just the scatter on natural dust particles present in air which makes the beam visible.

Picture 5 - Beam reflection at my living room wall, ca. 12 m distance of laser. The core beam is overexposed and appears white at the photo. Also visible at the photo only is the reddish arc below, here the camera has taken an image of the non-absorbed part of the pump laser beam. Invisible to the naked eye, it also can be seen on an IR indicator card. The irregular small green spots are reflections from the non-coated telescope lenses.

Picture 6 - What looks like a view to the "hot spot" of an arc welder is a photo of the scattered pump beam inside the opened laser head. The naked eye sees only the relatively weak scattered green laserlight (here visible at the fins of the crystal module heat sink). The totally overexposed white pump beam scatter is visually just a tiny little red spot at the crystal front facet - a very impressive demonstration how misleading the *visible* laser light from a pump diode can be!

In the output beam there are substantial parts of invisible infrared laser radiation. Some of it is non-absorbed pump power, some of it infrared laser which leaked off the crystal (finally, even not a HR laser mirror is totally perfect). Both of them are in the order of magnitude of the desired green laser beam so they cannot be neglected. For a certain case, I took a number of detailed measures:

Measure at 500 mA diode current
diode temp 24.3 deg C (equals 806.1 nm), vanadate cooler temp 14 deg C, KTP heater "off"
environment temp 18.5 deg C
1064nm radiation detectable at far field ca. 1 diameter off-axis to 532nm beam center.

	wavelength	value
electrical input power, diode:	-	~900 mW
pump power, Diode:	806 nm	460 mW
net power input, crystal:	806 nm	390 mW
not absorbed pump power (divergent):	806 nm	>15 mW
crystal output, infrared:	1064 nm	20 mW
crystal output, green:	532 nm	28 mW
not absorbed pump power, telescope output:	806 nm	13 mW
same, part in main beam (through pinhole):	806 nm	1.5 mW
telescope output, infrared:	1064 nm	17 mW
telescope output, green:	532 nm	24 mW

Diagram 2 - The fact my pump diode is a bit short in wavelength and the cold winter weather in January 2005 made possible to measure the lower end of the vanadate absorption peak. Absorption coefficient here is displayed by laser output of the crystal. Input power is constant low ~160 mW (300 mA diode current), vanadate cooler and KTP heater both switched off. Wavelength varied by temperature control of the diode in the range of +7 deg C to +26 deg C. Note the local maximum at 804 nm which corresponds to a side absorption band inside the 808 nm main peak of vanadate. To be resumed in summer...! :)

Conclusion:
It doesn't need a 1W or 2W pump diode which fries a DPM crystal close to death to get a simple-design respectable-power laser. The former too often was claimed at the hobbyist laser forums I saw. A bit temperature optimization does the same job at medium pump power keeping the crystal happy and long-living.
The Xenotim laser, despite of the bells and whistles added for TEC setting, is the most simple laser I ever built, one of the cheapest, surely it has the highest efficiency and is by far the easiest to transport. Once warmed up and tweaked for optimal power, I can let it run alone for hours if the environment temperature is at least roughly constant. It needs no consumables. And, it puts out laser radiation at a level not seen before.
Nevertheless I don't view the Xenotim laser as a true homebuilt laser. Sure, it is a robust tool for experimentation but its core is a bought hybrid crystal which is a complete little laser like a HeNe tube is. In the end I didn't much more than add a pump source and some temperature control.

Picture 1 - Last trace of my early 1980ies dye laser project is this drawing. Still it has a smell of the "dye cell replaces ruby" philosophy. It was built as drawn, of course it didn't work. Even later modifications made no success so I gave up in 1985.

Picture 2 - When my old Siemens HeNe tube finally died after 17 years, I discovered this one (Melles Girot) while looking for audio amplifier tubes. Of course I had to get it. Measured 1.8 mW linear polarized light at 633 nm. Discharge current ca. 7 mA at 1800 volts across the tube.

Picture 3 - Farfield spot (12 m) of the HeNe beam. Due to overexposure the center of the spot appears too yellow. Compare the photo to the corresponding one of the Xenotim laser -- HeNe's still possess an amazingly low beam divergence...

Picture 4 - A red diode laser pointer, first generation (670 nm wavelength). For a time in my life it was the only functional laser I owned. I still use it to adjust the mirror of the Querflöte and Janus II lasers.

Picture 5 - Once again the laser pointer with its red spot. Due to the exposure time the spot appears larger than normal.

Picture 6 - A Nd:YVO4 micro laser module which I got as a present from a professional laser engineer (thanks, Peter!). I just added the heat sink and the battery pack. A nice laser for travel, it is small and powerful. Depending on temperature, it puts out 5-10 mW at 532 nm.

Picture 7 - The pump diode of the Xenotim project is an impressing laser in its own right. The output of 500 mW at 807 nm is sufficient to show unsuspecting spectators exciting burning of any black stuff in its way... :)

[1] Kirkland, Dogett & Kim:
    Vacuum-UV H2-Laser excited by a travelling-wave discharge,
    Rev. Sci. Inst. 52(1981) p.1338
    
[2] Erikson & Lidholt:
    Superradiant transitions in argon, krypton and xenon
    IEEE J. Quan. Elec. QE-3(1967) p.94
    
[3] Papadopoulos & Serafetinides:
    Investigation of the electrical characteristics of charge transfer circuits used in gas laser excitation,
    J. Appl. Phys. D 24(1991) p.1917

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