High power DPSS lasers use lasing crystals in the form of rods pumped by radially arranged arrays of high power laser diodes. Since just one of the diode arrays alone would likely break the budget of an individual, what we consider primarily in this chapter are much more modest - typically a chip of Nd:YVO4 a few mm on a side and a mm or so in thickness pumped by a single bare (on heatsink), packaged, or fiber-coupled laser diode. Intracavity frequency doubling is done with a small block of KTP with the various mirrors either combined with, or separate from these. This basic configuration can easily generate over 100 mW of green light. The Nd:YVO4 and KTP are available individually or in 'hybrid' modules (also called composite crystals or composite crystal assemblies) from suppliers such as CASIX. Their "DPM" series combine the Nd:YVO4, KTP, and mirrors into a single unit for as little as $49 (quantity 1). See the sections starting with: Green DPSS Lasers using Hybrid Crystals.) Just add a pump diode and collimating lens and you will have the equivalent of a green laser pointer. Components may also be obtained from various laser surplus outfits and also show up on eBay from time-to-time.
Depending on sophistication, the pump laser diode may be mounted directly on the lasing crystal or have its beam passed through correction and collimating optics first. Thermo Electric (TE) devices may be used to regulate the temperature of the pump diode for general cooling and to temperature tune its wavelength to the optimum for the absorption band of the Nd in the lasing crystal (around 808 nm). Temperature control of the vanadate to keep it cool and happy) and KTP to keep it warm and happy to further optimize output power may also be desirable. If you are determined to generate more than a few mW of green light, all of these will be needed.
However, note that like the home-built pulsed solid state laser, constructing a DPSS laser truly from scratch is way out of the question onless you work for laser diode and laser crystal companies simultaneously. :) So, you may feel that a DPSS 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 unless you can salvage crystals and optics from a broken laser - the major components are still expensive (though slowly coming down in price). Having said all that, I do consider a DPSS laser to be worthy of home-built laser status - else I wouldn't have created this chapter!
Avoid eye contact with the direct or reflected beam. This is particularly critical for the 808 nm (Nd) pump diode wavelength which is almost totally invisible and the output of the YAG or vanadate crystal (1064 nm) which is totally invisible. Neither of these is eye-safe and at the power levels involved, can cause instant and permanent damage to vision. Higher power (e.g., above 5 mW) green output at 532 nm is also dangerous but at least you can see it!
808 nm: The typical pump diode will have a maximum output of between 0.5 and several WATTs. The light appears deep red but only 1/20,000th or less bright than an equivalent visible wavelength light source.
WARNING: The output beam of high power laser diodes with an attached microlens (or other collimating optics) is much better collimated than we are used to for laser diodes - closer to that of a "real" laser. The divergence is typically 10x4 degrees as opposed to 10x40 degrees for a bare laser diode. What this means is that both the direct beam and any specular reflections are MUCH more dangerous to vision even several feet away from the source. Even the reflection from a shiny IR detector card can be dangerous. This is especially scary for people who have become complacent working with laser diodes being used to beams that spread out to safe levels in a few inches.
1064 nm: As noted, this is totally invisible but for small DPSS lasers, is safely tucked away inside the cavity. With proper mirrors, very little 1064 nm light exits either mirror - they are both HR at 1064 nm. However, this can still be several mW or more even with high quality HR coatings. And, if you cobble together a cavity from mirrors that aren't optimal, much more collimated 1064 nm light could exit either end.
532 nm: Most of the green light will exit the OC-end of the laser but where the mirror on the vanadate crystal isn't coated HR for 532 nm, a fair amount will also exit the back of the cavity and bounce around the laser diode beam correction optics. However, this isn't much of a risk since it isn't in the form of a collimated beam anywhere. The output beam will be fairly well collimated and could be quite powerful. As the OC mirror or KTP is adjusted, power can go from zero to high enough to damage vision almost instantaneously.
Provide proper warning signs for both the laser radiation and high voltage. Keep pets and small children out of the area and make sure everyone present is instructed as to the dangers. The use of proper laser safety goggles for the specific wavelength(s) of your laser or indirect viewing methods such as the use of a CCD camera are highly recommended. Relatively inexpensive goggles designed specifically for the 3 relevant wavelengths of the green DPSS laser are available from many companies. One possible source is Spectronika. Ltd.. However, they are likely to be more expensive laser safety goggles for many other lasers. The primary reason is probably that they do have to be triple coated to cover the 3 relevant wavelengths - 808 nm, 1,064 nm, and 532 nm. The best ones will have high quality narrow band coatings to maximize the transmission of non-laser wavelengths and thus brightness. Simple dye-absorption type materials would not work well. You get what you pay for!
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 a comparison, in Version 1.85 or higher, there are also photos of a pair of high quality green DPSS lasers from Coherent, Inc., under "Coherent Diode Pumped Solid State Lasers". These are the Compass 315M rated 100 mW and the Compass 532-200 rated 200 mW.
See Organization of Basic Green DPSS Lasers for examples of the major components of typical green laser pointers and a medium power (50 to 100 mW) green DPSS laser system similar to the 80 mW unit shown in the Laser Equipment Gallery (Version 1.74 or higher) under: "Assorted Diode Pumped Solid State Lasers". Either of these would be a suitable home-built DPSS laser. The second generation pointer (upper right diagram) is not much more than a CASIX DPM hybrid crystal positioned next to an 808 nm pump diode with a microlens to improve the pump beam shape (but that could be left out for the a first version). An IR filter and collimating lens completes the unit. Such an approach may be the best choice for your first green DPSS laser. See the section: Super Simple Green DPSS Laser using Hybrid Crystal.
The medium power green DPSS laser uses closed loop TEC control of both pump diode and cavity temperature and a possibly overly complex set of pump beam correction optics (actual lenses shown are basically just guesses!) which could be simplified considerably. See the section: A HREF="lasercds.htm#cdsrd801">Description of the 80 mW Green DPSS Laser for a listing of the specifications for the major components.
For an example of a higher power and more challenging laser, see the very preliminary diagram of a Typical Home-Built DPSS Laser Assembly. This should be capable of between 100 mW and 2 W of green output. However, any of the higher power versions, at least, would be quite complex and expensive projects. In addition to all the precision machining, the cost of the pump diode, large vanadate and KTP crystals, and optics, there would need to be considerable electronics: The driver for the pump diode, the pump and vanadate TECs with their controllers, and the KTP heater and its controller. Note that while the beam correction optics shown in this diagram use anamorphic prisms, that approach will only work for up to perhaps a 2 to 4 W pump diode. For higher power using a wide stripe pump diode, bar, or array of bars, either a fiber-coupled or fiber bundled diode laser source with output focusing optics, or the use of a lens duct and HR mirror on the vanadate crystal would be needed.
To see how the "big boys" do it, see the Ring Cavity Resonator of Coherent, Inc. Verdi Green DPSS Laser. A photo of the inside of a Verdi cavity can be found at Coherent Verdi DPSS Laser Cavity with Components Labeled. The ring cavity provides robust single mode operation at high power levels as well as nearly 100 percent efficient extraction of the doubled (green) output due to the unidirectional beam circulation. Verdi lasers now exist up to at least 18 WATTs!!! I want one. :)
Photos of the interior of a Coherent Compass 532-200, a lower power green DPSS laser than the Verdi also using a ring cavity can be found in the Laser Equipment Gallery (Version 1.86 or higher) under "Coherent Diode Pumped Solid State Lasers". The 532-200 is rated 200 mW, but can produce at least 400 mW by increasing current to the pump diode (at the expense of diode life expectancy). The last photo in the sequence is a closeup of the cavity and output optics showing the actual beam path.
Here is the general description covering low, medium, and high power DPSS lasers. See the section: Component Selection Chart for Home-Built DPSS Lasers for an idea of what is required based on output power.
Frequency doubling crystal of KTP inside cavity. Typical size between 2x2x2 mm and 3x3x10 mm.
I recommend wiring a 0.01 uF capacitor, 100 ohm resistor and 1N4148 or fast recovery diode (in reverse polarity, if there isn't one present already) in parallel across the pump diode permanently. These components will help to protect it from ESD and stupid mistakes.
For something to rival the pros, see Typical High Power Green DPSS Laser Optical Path, a side-pumped YAG based Green DPSS laser capable of truly show stopping power. Finally, information on cavity construction of high power DPSS YAG laser with a linear resonator is provided by Bob and hosted by LaserFX in Laser Construction - Pump Cavity. I expect this page to be added to on a regular basis.
However, perhaps, building one of the more modest designs first would be a good idea - this would set you back the price of a nice used car just for the pump diode arrays, crystals, and optics. Throw in the resonator machining, electronics, and what else is required to turn it into a useful laser and you could be motoring around in style. :)
However, Melles Griot's low-to-medium power high quality green DPSS lasers now use composite crystals similar to CASIX's but of their own design optically contacted without any glue between the crystals (the DPM010X crystals are glued), so there is nothing to degrade. Unfortunately, there is no chance of getting these crystals for home use. :( VLOC also manufactures composite crystals that may be capable of very high output power - over 1 WATT of green - but you don't want to ask the price. :) So, what about somehow separating the two crystals by dissolving or softening the glue to eliminate that problem? Even if this could be done, just handling the individual pieces would be a challenge. The larger of the two CASIX composite crystals is 2x2x2.5 mm total dimensions; the smaller, 1x1x1.5 mm. I'm not about to try separating my CASIX crystal but attempting this with other types of composite crystals using an acetone soak was not promising. By the time the glue softened enough for the crystals to come apart, the mirrors had deteriorated to the point of being useless. Also, assuming you could get the two pieces apart without damaging them, what remains are two surfaces which aren't AR coated. Without index matching of some kind, there would be excessive loss and instability from the reflections at the two uncoated surfaces. See the section: Joachim's Comments on Lasers Using Composite Crystals, below.
(Note: Should you acquire an optically contacted crystal (possible sources: Cristal Laser and VLOC), while there is no glue in between the crystals to be damaged by excessive intracavity power, there can still be problems with handling and excessive pump power popping the crystals. Since only the edges are glued, very small optically contacted crystals tend to fall apart either on their own or when pumping causes the central parts to expand. Thus, there still may be an upper limit below the damage threshold of the vanadate. The only way to determine it non-destructively is to check the specifications.)
CASIX has now started selling composite crystals that may be optically contacted rather than glued at about the same prices as they were previously charging for the glued crystals. The DPM1101 and DPM1102 ($99 and $129, respectively, but these prices will no doubt drop quickly) should not have the stability problems and power limitations caused by the face glue of the DPM010X composite crystals. Go to CASIX, then "Products", "Crystal Products", "DPM", "High Power DPM Crystals".
Note that originally, the CASIX Web site stated that the DMP110X composite crystals were diffusion bonded but it is not known if this is true of all of these crystals. Some have been shipped with "splints" on two sides, presumably to hold them together more securely, something that shouldn't be needed with diffusion bonding. Other information has also suggested that they are optically contacted.
See Super Simple Green DPSS Laser. (The magnifications - 1X, 3X, 8X - assume a 100 dpi display or printer.) The mounting shown for the pump diode and crystal are just suggestions - you don't have to use the same really tiny plates and screws that are shown. Note the size of this laser at 1X! Almost anything that puts the output facet of the pump diode almost in contact (but not touching) the vanadate will work.
For experimentation, a very simple power supply is sufficient for the laser diode driver (though a proper driver should be added to make them more user friendly and fool-proof if put to actual use, even if for demonstrations or as that green laser pointer with visible guts. :) My test power supply consists of a low voltage power transformer, rectifier, and filter capacitor, controlled by a Variac with an 8 ohm power resistor in series with the laser diode and a meter to monitor current. With care (always turn the current down before powering up/down, make sure all connections are secure), this can safely drive these diodes with no possibility of overshoot/reverse polarity on power cycling and good immunity to power spikes. Compared to our "killer laser diode driver", a very expensive commercial unit that has obliterated more than one very expensive diode laser due to some intermittent circuit problem, this bare bones driver is quite reliable!
My initial tests of the DPM0101 and DPM0102 were using a fiber-coupled 808 nm pump. In a nutshell, they work great! With the fiber's 100 um core diameter and no beam correction, the thresholds for both were similar and slightly higher than with my discrete (green laser pointer guts) setup using a GRIN lens. However, it was possible to obtain much higher output power - probably 10 or 15 mW - at a pump power of around perhaps 300 or 400 mW (this with the non-polarized output of the multimode fiber which reduced efficiency by about 50 percent compared to a polarized pump beam that matches the preferred axis of the vanadate). The beam shape was actually quite decent even with the planar-planar resonator of the hybrid crystals. (More below.)
One interesting characteristic for the DPM0102 was that as the pump power was increased, output power climbed slowly for awhile and then increased after a time delay rapidly to a much higher level. I suspect that this may be due to heating effects - either thermal lensing of the crystal or improved phase matching at higher temperature - but don't know for sure. The DPM0101 didn't exhibit this behavior - output power increased smoothly with pump power. But it didn't sustain the highest power for more than a couple seconds - the power decayed to a lower level. However, this was reversible by shutting down and letting the crystal cool off. Both these effects may have been related to alignment - my setup, or lack thereof, couldn't really be adjusted precisely.
I expect that even simple pump beam shaping to reduce its diameter in the vanadate will result in higher efficiency and greater output for the same pump power. I haven't done any precise measurements as yet but will do so soon. However, I don't intend to push my luck on output power though knowing the problems that others have experienced with damage to the glue used to bond the two crystals together.
Even Simpler Instant Green DPSS Laser is a photograph of this unit lasing with a power output of about the 10 mW. (For an idea of size, the threaded holes of the optical table are 1 inch apart.) The closeup shows the major components. The pump diode is at the left wired with a reverse polarity protection diode and bypass capacitor. The DPM0102 hybrid crystal is mounted on a miniature three screw adjustable platform which provides for fine control of pitch, roll, and height. It is held snugly, but gently, under the aluminum bracket amidships with its rear face positioned as close to the pump diode as possible without touching. The "pitch" (I.e., tilt) adjustment is for vertical alignment; loosening the bracket and/or the screws that fasten the pump diode to the baseplate allows horizontal alignment of the pump beam with the crystal. The screws marked "roll" are really only to allow the entire crystal/platform to be raised or lowered - slight rotation about the beam axis has no effect of any consequence (but may come in handy for future applications). Although the laser cavity is formed by the faces of the crystal and thus there are no adjustments (!!) for actual mirror alignment, it's still important for the crystal to be fairly well aligned with the diode's output beam (though a huge $500 Newport mount isn't needed). Due to reflection of the backward going green beam from the front facet of the pump diode, there will be ghost spots when alignment isn't correct. The orientation and distance between the spots provide a clear indication of which direction to move. In the case of my diode, the chip appears not be mounted particularly perpendicular to the plate's centerline so it needs to be skewed to make the output beam come out straight when the ghosts spots merge!
An interesting characteristic of this setup is that above some minimum power level, not very much green light seems to come out the back of the crystal (unlike my green DPSS laser using discrete crystals and optics where everything behind the vanadate literally lights up like a Christmas tree). At low power, there appears to be significant back scattered green light. This actually appears to reduce in intensity as the pump power is increased - but only if the crystal is oriented so that the polarization of the vanadate matches the pump diode. Otherwise, the back scattered green light continues to increase in brightness. In reality, the back scattered light is probably just not increasing in brightness as fast as the main beam so it appears to decrease. Got that? :) I don't know whether this is some non-linear effect with respect to the relative amount of forward/backward conversion in the KTP, simply changes in mode structure resulting in more of the backward green hitting the front facet of the pump diode and being reflected forward, or something else.
Even without any pump beam shaping, the output beam profile is still TEM00 and reasonably circular at low power. At higher power it may split into 2 or three modes though I haven't really determined the conditions for this to occur. Since my pump diode is quite high power (probably good for 2 W at least with a large stripe width), its threshold is much higher (about 700 mA) than the 100 to 250 mW diode recommended for use with these composite crystals, so I don't really know how the efficiency of this rig compares to using the fiber-coupled pump but it seems to lase green with very little pump power (checked by the brightness of the pump light) and output power increases quite rapidly with increasing pump power.
Another incredibly super simple instant design is shown in Green DPSS Demo Laser using CASIX DPM0101 Composite Crystal. The pump diode is a bare chip soldered to a brass plate with the top contact attached using silver conductive Epoxy. The DPM0101 is glued into a slot in its own brass plate with 4 screws and lock-washers for adjustment. I have built a unit similar to this for somone who needs to show non-laser types how a basic diode pumped microchip laser works. It will be mounted in a Plexiglas box with a magnifier. For the power supply, I used the Roithner EU38 laser diode driver with a filtered 4 V linear voltage regulator preceeding it. Input power is from a 6-9 V wall adapter. See: Green Demo Laser Power Supply Using EU38. The complete system is shown in Green Demo Laser With Power Supply. The dimensions aren't quite identical to the diagram, above, but close enough for government work. :) The reason for the ghost spot is that the laser diode chip is mounted a bit skewed on its heatsink so the reflection of backward going green beam from the KTP doesn't reflect on-axis.
There are some disadvantages to this approach. With the batch fabrication technique (see below), both mirrors are planar which means there is little control of transverse mode structure (though with a well shaped pump beam, this doesn't seem to be a problem). And there is that problematic glue used to bond the vanadate and KTP together which is one of the prime limitations on extended operation at high output power. (This only applies to the DPM010X; the new physically similar DPM110X hybrid crystals are optically contacted which should eliminate this problem and allow for much higher continuous output power operation.) However, these deficiencies are more than made up for by the immediate gratification of getting bright green light with without the hassles of handling, mounting, and aligning individual fragile crystals and optics.
DPM crystal specifications:
These are based on my expectations and observations. I have not actually made precise measurements:
Maximum output power:
This is one of those things where there are apparently no hard answers (or at least no one is talking) so much of the following is just my opinion. According to CASIX, the maximum recommended input pump power is around 300 mW resulting in a lifetime of at least 5,000 hours (whatever that really means). The first person I asked at CASIX didn't specify output power but someone else suggested that 10 mW max was a good number. Given that both of these DPM crystals are intended for green DPSS laser pointers, I would assume that a continuous output power of 2 or 3 mW for the DPM0101 and 5 mW for the DPM0102 will be safe for long term reliability. I know it's tempting to drive these things much harder - I've seen over 35 mW (limited only because I didn't wont to fry my samples) and others have achieved 100 mW or more. However, there are both thermal considerations as well as damage to the cement used to glue the vanadate and KTP which may limit use of such high power for more than short periods of time. From conversations with other crystal venders (but not CASIX) using similar adhesives, operating at a 30 mW level may result in damage after a few hours while at 5 mW, there may not be any damage after thousands of hours. Note that since intracavity power density also depends on factors like pump spot size and shape, there won't be a single number for maximum output power. Until more is known, just assume your mileage may vary and don't press your luck too much. :) Please send me mail via the Sci.Electronics.Repair FAQ Email Links Page if you have any additional information or personal experiences using these or similar crystals.
Operational considerations:
Note that with the non-polarized multimode fiber, these lasers run with multiple longitudinal modes even just above threshold since the light that isn't polarized in the proper direction isn't absorbed as quickly in the vanadate resulting in excitation throughout its length. This probably doesn't matter for most applications but may be a consideration if the intent is to build a single frequency laser.
Beam characteristics:
Which one to get? While the DPM0102 is more expensive than the DPM0101 ($49 versus $99 in November, 2002) it is easier to handle and may possibly be more robust, and is potentially easier to cool and thus may be capable of higher output power. And, should one spot of the glue become damaged from overzealous experiments in pushing the envelope, there are plenty of alternative areas to use just by shifting the pump beam location. However, with the introduction of the DPM110X optically bonded hybrid crystals at reasonable prices (see below), the use of glued crystals will only be justified in extremely cost sensitive applications. So, go for the DPM1101 or DPM1102 instead.
When the DMP110X crystals were introduced, the following (slightly paraphrased) appeared on the CASIX product announcement page:
"Optical bonding is based on diffusion. The crystals are permanently bonded by heating to a high temperature under pressure resulting in a chemical exchange of molecules at the interface. The resulting monolithic unit should be stable with respect to high and low temperature storage, temperature cycling, impact and shearing, solvents and other chemical attack. They can work in high vacuum and under intense radiation."
Since these hybrid crystals can be fabricated in large pieces and then diced up into their final size, costs should be much closer to those of glued crystals. The cost of optically contacted crystals will never be very low since they must be assembled individually. In fact, the initial prices for the DPM1101 and DPM1102 ($99 and $129) are similar to the previous prices of the DPM0101 and DPM0102 (which have now dropped to $49 and $99). Given the advantages of the optically bonded approach, there is little reason to even consider glued crystals unless price is the absolutely overriding concern.
However, it now appears as though the DPM110X crystals are indeed optically contacted and not diffusion bonded. This is somewhat confirmed by the existence of the reinforcing strips. If so, most of the advantages are still present, but how they can be fabricated at low cost is a mystery. And, even with the reinforcing strips, the robustness of optically contacted composite crystals is questionable since delamination from the stresses induced by thermal stress (in particular, thermal lensing) can be a problem with this approach.
See the section: Some Tests of the DPM1101 and DPM1102 for some more info.
Note that the output of the multimode fiber is non-polarized. Thus, the portion of the pump light that matches the preferred polarization axis of the vanadate will be absorbed in a shorter distance (100 or 200 um assuming 3 percent doping for the DPM0101) of the vanadate while the other light will have a much deeper penetration - possibly resulting in a significantly higher threshold and lower output power (for a given pump power) if a substantial portion doesn't get absorbed at all.
Here are some observations using the fiber-coupled pump, far from complete:
I later tested the same crystal with a Spectra-Physics SCT100-808-Z1-01 open heatsink laser diode (1 W, 100 um stripe width) temperature tuned for 808 nm. The diode was positioned just short of touching the DPM0101 with the crystal oriented for optimal absorption. The lasing threshold came down to about 100 mW with good beam quality. I did not look at the spectral behavior but assume it to be similar.
Diane has built a nice little laser around the DPM1101 and performed more quantitative tests which are documented on her Web site at Diane's Xenotim Laser. She comments that this isn't a true home-built laser, which is certainly true compared to her "Querflvte" ("German Flute") pulsed multiple gas laser, but it's still nice and worthy of the home-built classification! :)
I would have expected the diffusion bonding technique (which is what I assumed these to be based on the CASIX Web site) to yield a very robust composite crystal. However, the samples I received had pieces of glass or other material glued on two sides apparently to provide reinforcement so they don't fall apart. This is why the size specification has a smaller clear aperture than that of the glued crystals. So, the top view looks something like this:
________________________
_|________________________|_
| | |
| | |
| Van- | |
Pump --> | adate | KTP | --> Green Output
| | |
| | |
|_______|____________________|
|________________________|
Apparently, some samples of the DPM1101 and DPM1102 received by other people do not have the splints. Perhaps CASIX was being overly careful on the first few batches. I did order mine just after the product announcement. However, since they are actually optically contacted, this makes sense. The CASIX Web site no longer lists the method used. If this is so, whether for technical reasons or something else, I don't know. I have my suspicions though.
The lasing tests on the DPM1102 were initially performed using a Sony SLD322V pump diode (808 nm, 500 mW max, 50 um stripe width) with a pair of 12 mm f/1 lenses for beam focusing. The pump assembly was mounted on a Newport XYZ micropositioner.
The lasing threshold was relatively low (about 200 mA). However, a nice TEM00 beam was produced only near the top of the crystal - the classic "sweet spot problem".
Increasing pump power didn't produce proportionately more output power as would be expected (or at least desired). The power seemed to remain steady at a few mW. Granted, I'm not doing anything yet to cool the crystal.
I next tried a DPM1101 (1x1 clear aperture) with a fiber-coupled 808 nm pump (100 um core) whose output fiber was mounted on a Newport XYZ micropositioner. The performance of this estup was much better than that of the DPM1102 producing a nice TEM00 beam from lasing threshold to 50 mW or so which was the maximum I have pumped it so far. (I do not know the exact pump power but expect it to be around 500 mW at the fiber tip.) There was only a slight variation in output power with respect to pump position on the crystal (minimal "sweet spot problem"). The output power increased smoothly with respect to pump power (which is not something I've seen in most of Casix's glued crystals). When pumped to produce about 20 to 30 mW, the output remained quite constant over the course of an hour (eyeballed).
After better success with the DPM1101, I replaced it with the DPM1102 but used the same fiber-coupled pump setup. In this case, the results were somewhat better than with the Sony diode. While there was still some non-uniformity of power output with respect to pump position, it didn't seem nearly as severe. The output power increased smoothly with pump power and was of similar intensity to that from the DPM1101. I've run this setup now for at least 10 hours at a 50 mW output power level with no significant change in behavior. The only cooling is from being in contact with an aluminum plate on which it is loosely clamped.
So, my opinion is that the two crystals do behave in a somewhat similar manner when the pump is the same.
More to follow.
The simplest non-GRIN lens option for a pump diode in a can or other package with a window is to use a pair of short focal length positive lenses. The first lens collimates the beam while the second lens focuses it into the crystal. For the wide divergence of the bare diode, these typically have to be f/2 or f/1 lenses. In essence, this is a relay lens system to permit an image of the inaccessible diode stripe to be placed in the crystal with low losses. By selecting the focal lengths of the two lenses and possibly allowing the beam between them to be not quite parallel, the spot size can be changed (usually it will be desirable to reduce it). For example, using a 12 mm diameter, 12 mm focal length lens (12/12) feeding a 6/6 lens (the beam between them would have to be slightly converging), the spot size of a 100 um stripe width 1 W diode can be reduced to around 50 um.
For more, see the next two sections.
Therefore, attempting to swap in some undocumented high power diode found on eBay to boost power of a DPSS laser is at best a crap shoot even if its wavelength and electrical parameters are known. :)
The typical stripe width of a LD is roughly proportional to its maximum output power. The beam from a 2 W LD will be 4 times as bright as one from a 500 mW unit only if their stripe widths are the same. However, the typical stripe width for a 500 mW LD is 50 um while that of a 2 W LD is 200 um or even 400 um. The stripe width can be determined experimentally by projecting the LD beam onto a screen through a short focal length positive lens producing a focused image of the stripe. The stripe width of the LD will be the ratio of the distance from the lens to the LD and from the lens to the screen multiplied by the image size on the screen.
(From: "Joachim Mueller" (JoachimMueller@swol.de).)
I have tried to work with the parts from CASIX. I ordered some of the DPM-crystals from casix separated (not glued together) and did some experiments with them. The middle surfaces, which normally are bonded together, are only polished - there are no coatings. When only sticking them together, the optical loss resulting from reflections is too high. Only a very weak green beam was visible. One solution is using index matching gel (used in fiber optic technology). This material is able to operate under very high power-density with no damage. The problem is handling the 2 small crystals and keeping the matching gel free from air-bubbles. Also the 2 parts have to be aligned to the proper angle to match the polarization. It was a a lot of work putting the 2 parts together. Another problem is that CASIX can not test the crystals like they do with complete modules. If the polish is not perfect to form a parallel resonator, the result in green light is bad. After spending a lot of time on this approach, I came to the conclusion, that it is not a good way to save money. I think using discrete standard crystals is the better way to build a good working laser.
In fact, I worked for 2 years trying all kinds of microchip modules from $99 to $1,000 each. The result is, that I stopped all work in this direction. Nearly all manufacturers (and users) of microchip lasers had trouble with it. Most of the laser manufacturers went back to using traditional spherical resonators. One of the problems of the modules is, that the beam quality is never quite as good and that there is no second source for such a part. Getting a good module is like playing the lottery. The chance to get the next module with the same characteristic as one you've tested is low.
And, except for the special case of mass produced green laser pointers, microchip technology is expensive. For the same amount of money you can get the parts for a laser made from discrete parts. The vanadate + KTP + OC mirror will cost below $200. Beam quality of a discrete is better. Modules work with a very short cavity, a few millimeters. This increases divergence. It is nearly impossible to get a beam of diameter of 1 mm and less than 1 mR using a microchip module. Pumping a CASIX module with 1 W or more gives a divergence of 10 to 15 mR. Reducing this to 1 to 1.5 mR requires a 10X telescope which enlarges the beam to 2 mm or more. There are some modules with better beams, like the LSB-modules from Russia. They have about half the divergence of a CASIX module but cost at least double the price. (More below.) The only advantage of a module is getting relative high output power the easy way. But what is better: 100 mW of a 2 mm thick 1.5 mR beam or 50 mW of a 1 mm at 0.5 mR? When I see a thin low-divergence DPSS-laser beam, I know that this is NOT microchip technology. If make the effort and spend the time making a stable mechanics for adjusting the optics the results with using discrete parts will be much better.
I´m currently working on a discrete plane-parallel resonator (using HR@1064-coated KTP). It is like using a separated crystal module with the inner surfaces AR coated. My cooperation with a Chinese crystal manufacturer allows me to optimize coatings, materials and other characteristics. First results: 100 mW at 5 mR divergence without any beam optics. This configuration will lower the number of parts and makes me free from adjusting the orientation of the KTP inside the cavity. Moving the pump spot in XY direction does not affect the cavity alignment (This is a big problem when using a spherical resonator). Once adjusted, the cavity is a kind of "big module" and is replaceable inside the laser without the need of readjusting all parts. I'm now thinking about a simple and reliable mechanical adjustment-stage. Then it could be possible to offer the cavity without diode ready-to-use, just mount the thing to a TE-cooler, pump it and add output optics. Like a module! I think this cavity will cost a user around $350 and will allow building a powerful DPSS-laser without the trouble of mounting crystals and glued modules.
There are 2 companies I know making LSB based microchip lasers. One is in Germany and the other is in Russia. The Russian version (from the "Science and Technology Center FIRN" is in a metal housing constructed for low pump power (1 to 2 W). The German version (I forget who it is from) is 3x3x2.5 mm unmounted and better to cool. I tested both versions, but was not happy with the quality and the price ($400 to $500). The LSB itself has a very high efficiency. With the Russian parts, 200 mW at 1 W of pump power was possible. It could be more, if they would use higher quality KTP. The manufacturers told me a maximum pump power of about 3 to 5 W, so 1 W output is theoretically possible. But in reality, trouble starts at pump power greater than about 1.5 W. It had some coating defects and thermal management is also critical. LSB is a very bad thermal conductor. I know that a group of scientists reached 1 W green with a module under optimum conditions but they didn't say anything about the lifetime of their microchips. I think LSB is a good solution for a maximum output at low pump power (1 W). It would be interesting to use LSB-chips in a standard setup with discrete crystals. Unfortunately, I have no single piece of coated Nd:LSB available. :-(
(From: Joe Farina (nospam@newsranger.com).)
There is a paper which might be worth mentioning. It was published in Optics Letters, Volume 19, Number 18, September 15, 1994: "Intracavity frequency doubling of a continuous-wave, diode-laser-pumped neodymium lanthanum scandium borate laser." Here is part of the abstract:
"...A simple plane-plane 3-mm-long resonator is formed by a coated Nd(10%):LaSc3(BO3)4 crystal and a coated potassium titanyl phosphate (KTP) crystal. The second-harmonic output power at 531 nm is 522 mW at 2.05 W incident pump power of the diode laser... The well-known chaotic power fluctuations of intracavity frequency-doubled lasers (green problem) are avoided by use of a short KTP crystal, between 0.5 and 2 mm in length."
Power Output: 5 mW 20 mW 100 mW 1 W 5 W
-------------------------------------------------------------------------------
Pump Diode (1)
Maximum Power 100 mW 500 mW 1 W 10 W 50 W
TEC Thermal Power -- 1 W 3 W 30 W 150 W
Beam Correction <-- uLens or None --> Prisms <-- Fiber-coupled -->
Resonator (2)
Type <-------- Hemispherical -------> <--- Long Radius --->
Length 7 mm 15 mm 40 mm 80 mm 150 mm
HR Mirror (3)
Diameter --- --- --- 10 mm 15 mm
RoC --- --- --- 250 mm 500 mm
Vanadate:
Overall Size 2x2x0.5mm 3x3x1.2 mm 3x3x1.2 mm 3x3x3 mm 4x4x4 mm
Doping Level 3% 1% 1% 0.7% 0.7%
Side 1 Coatings HT@808nm HT@808nm HT@808nm HT@808mn HT@808nm
HR@1064nm HR@1064nm HR@1064nm AR@1064nm AR@1064nm
Side 2 Coatings AR@1064nm AR@1064nm AR@1064nm AR@1064nm AR@1064nm
TEC Thermal Power --- 0.5 W 1 W 4 W 20 W
KTP (4)
Overall Size 2x2x3 mm 2x2x5 mm 2x2x5 mm 3x3x7 mm 3x3x7 mm
Type <------------ Flux Grown --------> <-- Hydrothermal -->
Side 1 Coating AR@1064nm AR@1064nm AR@1064nm AR@1064nm AR@1064nm
Side 2 Coating AR@1064nm AR@1064nm AR@1064nm AR@1064nm AR@1064nm
Heater Power --- 0.5 W 0.5 W 1 W 2 W
OC Mirror (5)
Diameter 5 mm 5 mm 5 mm 10 mm 15 mm
RoC 10 mm 20 mm 50 mm 100 mm 200 mm
Side 1 Coatings HR@1064nm HR@1064nm HR@1064nm HR@1064nm HR@1064nm
AR@532nm AR@532nm AR@532nm AR@532nm AR@532nm
Side 2 Coatings AR@532nm AR@532nm AR@532nm AR@532nm AR@532nm
Notes:
For up to 10 mW (possibly even a bit higher), the use of a single mode pump diode with a GRIN lens or conventional beam correction optics may result in the highest efficiency. In one test I did, the same vanadate microchip laser (similar to a green laser pointer) that required 50 to 75 mW to reach threshold with a bare or fiber-coupled 1 W multimode laser diode would lase at something like 10 mW of pump power using a focused single mode diode! However, single mode diodes are only available up to 100 or 150 mW and are very expensive.
Diodes like this are also available with a fiber (cylindrical) lens attached to collimate the fast axis. This improves the spot shape if not using beam shaping optics and makes focusing with a GRIN lens more efficient. Spectra-Physics won't sell these in less than 5-packs and their price is somewhat higher) but they may be available from other sources.
The effective cavity length should be just a bit smaller than the RoC of the OC (output) mirror to assure stability in this hemispherical resonator. Since the intracavity beam is diffracted when entering and leaving the vanadate and KTP crystals (and these have a high index of refraction of around 2.2 and 1.9, respectively) the actual cavity length will be slightly longer than if they weren't present, about 2 or 3 mm in this case. So, start with the distance from the pump-side of the vanadate to the OC mirror of about 48 to 50 mm for a 50 mm RoC OC. The KTP should be close to the vanadate to take advantage of the small beam waist (and maximum power density for best doubling efficiency).
No, you can't get these out of a CD player (even 1,000 CD players). :)
Laser surplus places like MWK Laser Products may sell high power laser diodes. However, in many cases, they are pulls from equipment near end-of-life with output power that is way down from new part specifications. For example, a diode spec'd at 20 W may only produce 0.5 W at some ridiculous maximum current. These are probably not terribly useful for incorporation into a serious DPSS laser. (A 20 W diode also has a very long emitting aperture which would make beam shaping very difficult especially if all you can get out of it is a half watt!)
High power laser diodes also turn up on eBay from time-to-time (some are from MWK who goes by the eBay ID hene1). Without a guarantee, there is really no way to know for sure about the output power, remaining life, or if the device works at all. Search for "laser" and "diode".
The companies below are all manufacturers or suppliers of high power laser diodes, laser diode arrays, laser diode bars, etc. But the chances of a private individual getting any free samples or diodes at a discount from them are slim to none (probably none).
It appears as though Thorlabs sells some IMC high power laser diode bars (20 and 40 W at least). What's interesting are the prices. Boston Laser, Roithner, and others also list some prices. The prices are all somewhat depressing. :(
Here is info on some manufacturers/suppliers known to be willing to sell in small quantities:
When ordering laser diodes, if possible, specify a center wavelength range which results in optimal absorption in your laser crystal (typically 808 to 810 nm for vanadate and YAG). Otherwise, they may just send you diodes that are 804 nm at full power - even shorter wavelength for lower power. They may cost a bit more but is well worth not having to heat them significantly (which also reduces their life expectancy) to center the wavelength at 808 nm for maximum absorption. A longer wavelength is almost always better since cooling to shift it 2 or 3 nm is no problem (0.3 nm/°C). Also keep in mind that the wavelength spec is usually listed for rated power. If run at reduced power, it will be somewhat shorter.
Also see: K3PGP's Laser Diode Manufacturers and K3PGP's Laser Diode Specifications maintained by K3PGP (Email: k3pgp@qsl.net). (This is a listing of the database that used to be on the Thorlabs Web site.)
Note that more and more laser diodes are showing surplus and on eBay. The key to not being disappointed is to very carefully check out the seller both in terms of previous sales (e.g., Feedback rating on eBay) and their knowledge of the devices and where they came from. I also have a few laser diodes for sale. See Sam's Classified Page.
Laser diode drivers:
While it's possible to build your own laser diode driver, given that the laser diode (or array or bar or bars) probably represents the single most expensive component of your laser, buying a reliable driver is probably the best option. Laser diodes are very finicky and unless you can afford to blow a few in developing a home-built driver, a commercial unit is really best.
However, I plan to provide a design for a bare bones but reliable (I hope) constant current driver (2.5 A max) as well as a complete power supply using it in the near future. Stay tuned to this FAQ. :)
Various laser resellers and surplus dealers have low cost no-frill driver boards capable of 1 to 1.5 A at prices from around $10 to $50. These are almost certainly far-East imports which may or may not meet advertized specs. I probably wouldn't trust them to drive a $1,000 fiber-coupled laser diode without some testing to make sure they work properly.
The B&W Tek driver (BWD800) looks identical to the one from Roithner (EU38) so it's probably sourced elsewhere. I have used one of these to power the green demo laser described in the section: Even Simpler Instant Green DPSS Laser.
For higher quality modules and lab-style (and expensive) controllers, see:
The Wavelength Electronics modules have a good reputation (although I do have a blown one, cause unknown!) but are relatively expensive: around $300 for a 2.5 A driver. Lab style instruments from places like ILX Lightwave can run upwards of $10,000! And, they can still blow multi-$K laser diodes! Don't ask me know I know. :(
Laser and non-linear crystals and optics:
The following are some possible sources for laser crystals (e.g., Nd:YAG, Nd:YVO4), non-linear crystals (e.g., KTP, LBO), hybrid modules combining the lasing and doubling crystals, and optics (HR and OC mirrors, filters, etc.). Most probably won't give you the time of day unless you can convince them you are are associated with a company or university with deep pockets but others like CASIX and Roithner have prices for some items on their Web sites. These companies may have many more products than those listed below but these are the most relevant to the home-built DPSS laser. (Legend: LC=laser crystals, CC=composite crystals, NC=non-linear crystals, OP=optics, HM=hybrid modules. An HM is a CC mounted in a case with or without a driver.)
The following are manufacturers:
The following are resellers:
CASIX can provide everything but the pump diode for composite and discrete green DPSS lasers. The authorized distributor in the USA for CASIX is U-Oplaz Technologies, Inc. whose prices are the same as those listed on the CASIX Web site. I ordered the DPM0101 and DPM0102 from U-Oplaz with very responsive courteous service but watch out for the $15 S&H charge. The DPM0102 even came in a cute cloth covered Chinese box. :)
Roithner sells everything needed for a 100 mW-class green DPSS laser including pump beam focusing optics (required for use with their packaged laser diodes).
Optics component companies like Melles Griot, Newport, and others will have the various lenses and prisms needed for pump beam shaping.
Laser (resonator) mirrors:
While there are many sources of laser mirrors including places like CVI and Newport, their cost can run several hundred to over $1,000 - for a single mirror using ion beam sputtered super polished fused silica substrates. However, there may be no choice for use with higher power (e.g., Laserscope class) lasers. For low power DPSS lasers, the only reasonably priced source I know of at the present time for cavity mirrors is CASIX (see contact info, above). They have some of the types of planar and concave small radius mirrors required for low to medium power green and IR DPSS lasers.
Thermal control devices and supplies:
Large electronics distributors may have some of these items, like TECs but their selection is probably limited. And, they certainly won't have supplies like indium foil (for cushioning laser crystals while providing good thermal contact) and low temperature solder (for mounting bare laser diodes, laser diode bars, and submodules). Some of this stuff shows up on eBay from time-to-time (Doesn't almost everyting? :) I recently came across some sheets of indium there but they were way too thick and the quantity was HUGE! Apparently, indium sheets, wire, gaskets, and other forms are common for cryogenic sealing applications but what you need is really just a fraction of a square inch of 0.001" thick indium foil). So, I wouldn't recommend holding your breath while waiting for the perfect deal at an auction. For these specific items, after reading what they would cost if purchased in the micro-sized quantities needed for DPSS lasers, see the special arrangement we have made, a few paragraphs from here.
(From: Richard Everett (reverett@newtonlabs.com).)
"I just got ten new surplus TEC devices in from BG Micro (Spring, 2002). These are NEW Marlow Industries DT12-4 (roughly 12 V at 4 A) 1.2x1.2 inch modules for about $6.75 each. This is really not a bad price, but here is the neat thing: You can buy the same module mounted on a heatsink with a little fan for $9.95. The cold side has a gold anodized metal plate with a prism shaped part sticking out of the middle of it with a hole tapped in on the flat of the metal prism shaped thing. It *almost* looks like this thing was designed for bolting on a c-mount diode. It is kind of funky, but it looks like it could possibly be made to cool a C-mount diode almost as-is."
Here are some sources for TECs, controllers, temperature sensors, accessories, and supplies, as well as application information on-line. However, some of these companies probably have a $100 or so minimum order and small quantities of specialty items like indium foil and low temp solder are bound to be exhorbitantly priced.
However, at least one company, Melcor has an on-line store with relatively reasonably prices for small quantities (linked from their homepage).
TECs are often available surplus. Sometimes these are manufacturer rejects but should work fine, while many are just new excess stock. For example, All Electronics currently has several decent size ones in the $10 to $15 range (Winder 2005). Various types may also be found on eBay.
A couple of notes about benefits of indium over silicone heat sink compound: First, although indium's thermal conductivity is much lower than copper, it is still much higher than silicone or other types of grease-type fillers or pads. Another advantage is that it doesn't make a mess of everything, which is a definite plus in the middle of precision optics. :)
If indium foil isn't readily available but wire or another shape is, transform these non-planar shapes into foil is easy and quick. Dig out a smooth steel plate and rod from your junk drawer. Put a small piece of indium on the plate and use the rod as a rolling pin. In under a minute, this should produce a reasonable facsimile of foil since indium is very soft. It doesn't have to be perfect. You'll also wonder how they can charge so much for so little. :-) I assume tools made from something other than steel will also work but indium does stick to some, especially textured, materials.
Additional suppliers of TEC controllers (both lab style and modules):
For more information and suppliers than you could possibly want, see the Peltier Device Information Directory.
There are also TEC controller ICs if you want to roll your own and would rather not design it from scratch. For example, check out:
National Semiconductor has an app note on using an audio amplifier as a TEC controller. See their: Application Brief 118.
And, it's certainly possible to construct a useful TEC controller with discrete components (transistors, op-amps, etc.). This is particularly true where the drive is low power (as for a bit of KTP) or can be unipolar - only heating (typically for SHG crystals) or cooling (pump diode) is required. For simple examples of each type, see Low Power TEC Controller and Unipolar TEC Controller. However, high power TEC controllers with full functionality can also be constructed using any number of techniques.
(From: joefarina@my-deja.com.)
LakeShore has 2" x 2" sheets of indium foil (apparently a pack of 5) for $93. Go to "Products", "Cryogenic Accessories", "Solder". They also have a very low temperature (70 °C) solder called "Ostaloy 158" for $57. I don't know if there's a minimum order. (These items seem rather difficult to obtain in small quantity from other sources.)
I recently received some low temperature solder (93 °C) from Melcor but there was a $100 minimum order from them. (I couldn't find solder listed on their Web site. --- Sam) I also got a couple of TE coolers and some thermally conductive epoxy. My order didn't quite make it to $100, but they let it slide.
(From: Bob.)
There is a HUGE increase in price when you buy in small quantity. If you spent $1,000 with an indium supplier you may get 1,000 times as much material as if you spent $100, no exaggeration! When I called around for quotes on low temp solder I got back numbers like $50 a foot for a 10 to 50 foot quantity. I then asked for a quantity of 500 feet, and got quotes of around $5 a foot I don't know of any vendor willing to supply small quantities at a fair price. And, while there is a lot of indium floating around on eBay, unfortunately, pure indium has much too high a melting point (157 °C) for use in soldering assembled diodes, as the diodes themselves are often assembled with either indium solder or an alloy with an even lower melting point.
(From: Sam.)
Through an arrangement with Bob and Sterling Resale Optics, we will be able to provide small quantities of indium foil (for cushioning laser crystals with good thermal contact) and low temperature solder (for mounting laser diode bars and bare laser diodes on heatsinks). And, perhaps other similarly hard to find items in the future.
There are two types of special flux that can be used with the low temp solder. They can be identified by their color: The stuff for bare copper is lighter than the stuff for gold (which looks more like normal rosin-type flux). The type included with the solder will be the more reactive flux which may be used on either surface.
There is also a small fixed shipping charge. Prices subject to change without notice. Please no complaints, this is barely above cost when purchased in large volume and a teeny tiny fraction of the cost when puchased in the quantities you could reasonably use in a lifetime (if available at all).
IR blocking (and other) filters
The most important is the filter at the output of the laser to block potentially harmful 808 nm and 1,064 nm IR light. You may already have a piece of this in a defunct green pointer. Otherwise, suitable material is readily available.
(From: Kevin Criqui.)
The blue-green filters you typically find in laser pointers and small dpss systems is Schott BG38 or equivalent.
I think BG38 absorbs the IR so it can't be used for high power lasers. You can get dichroic low pass filters that reflect IR, but be careful not to reflect it back into the laser cavity. Check out Edmund Online Catalog of Precision Optical Components for "Hot Mirrors" and "IR Cutoff Filters".
Additional manufacturer links
(From: Robin Bowden (Rob@Radioeng.demon.co.uk).)
Only a few short weeks ago Greenie #1, the 532 nm DPSSFD laser was born. A 1 W Polaroid 9 mm 808 nm laser diode pumping a CASIX DPM0102 hybrid crystal through a GRIN lens.
Using 4 Alkaline AA batteries and switchable dropper - running the 808 nm diode at around 750 mW produced several tens of mW out. - Power dropped off fairly quickly after each battery change - see Voltage/Life curves for Duracell. It worked fine for 20 Hours and a similar number of battery changes.
Power hungry I built a switch mode PSU to run the pump diode at a constant current (Soft start over 50 ms - no overshoot to 1.21 A - current for 1 W out from manufacturers test data at 25C)
Using this PSU gave just under 100 mW out - time to pop balloons/cut black electrician`s tape. However, I was resigned to the fact that the adhesive used to glue the Nd:YVO4 to the KTP may not be able to cope with the high green power (experiments by Joachim Mueller showing black dot in adhesive after 40 Hours operation at 1.2 W pump).
Then last Sunday after 35 hours or so total use I tried to fire up Greenie #1. Output came up, fluctuated, and then went out for good.
The postmortem revealed that it was the pump diode that had failed!!!!!
The pump diode had a 50 ms soft start PSU with no overshoot. The diode had a 4C/watt heatsink which was at 12C-ish when I turned it on. The diode was also protected by 100 nF cap and 10 A Schottky diode for reverse protection while off.
Just one question: WHAT WENT WRONG?
Thoughts, comments, condolences and pearls of wisdom gratefully received.
(From: King Toebie (olx08152@online.be).)
Did you view the output of your switch mode PS on an oscilloscope? Diodes are VERY sensitive to overvoltage, and if you look on a wrong time-scale, you might miss the spikes. Take a simple pointer: The best way to kill it is to use a standard "stabilized" AC to DC converter - guaranteed to kill the pointer, batteries never will.
(From: Robin.)
Yep, did all that to the PSU before it came anywhere near the diode. Used four 1N4001 diodes as a dummy load.
I designed the switch mode PSU for less than 50 mA ripple at 1 A. (200 kHz ripple). It is a buck converter running at 200 kHz into two 47 uF Sanyo OSCON caps and a 10 nF 1206 (SMT) cap, then through 808 nm LD into a 1 ohm sense resistor for current control. Confirmed ripple using 100 MHz analog scope.
(From: Bob.)
A diode lasting only 20 hours sounds like classic diode infant mortality to me. There are tons of reasons why a diode may fail after only a few tens of hours. Improper soldering of the actual chip, a poor cleve during production, and contamination of the facets being the most popular reasons. A lot of vendors these days burn their single chip diodes in for 100 hours. Your source probably didn't do this. Either that or you just got unlucky.
As far as the other possibilities go, 50 milliseconds is fine for soft-start. If your power supply is intended for powering laser diodes and doesn't have any noise issues, then it wouldn't have killed it. A 4C/W heatsink is a little bit on the high side, I presume you were using a passive heatsink, I.e., no TE cooler. The main problem with that is heatsinks are rated when the heat source is evenly distributed over the entire surface. If you have a big heatsink and a small heat source you can get into problems with localized heating very quickly. Have you ever tried measuring the temperature of the diode itself? I once saw a guy build this huge elaborate air cooled heatsink for a custom 3 W, 660 nm laser diode that would cost about $10,000 to buy off the shelf. He was so proud of the design, but didn't think too much about the flow of heat. while he was running it in the lab. He complained about the wavelength of the diode being off, so I looked at his set up, and it was little more than a big block of passively cooled aluminum with a 5 mm laser diode can screwed onto it. The temperature of the heat sink was a nice 21 or 22 °C, but he almost soiled his drawers when I put a miniature thermocouple against the base of the diode package, and the temp readout displayed nearly 60 °C!
If you had reverse polarity protection on your diode, and a crowbar circuit to short the diode when not in operation, then there is next to no chance ESD or anything along that lines killed it.
I personally have never had a problem with back reflected green. Vanadate, especially highly doped vanadate in these small units is a very good green absorber, and this prevents nearly all of the green light from going back towards the diode.
I don't know if any of this can be classified as a pearl of wisdom, but you certainly have my condolences for what seems to be a case of diode SIDDS (sudden infant diode death syndrome).
(From: Sam.)
As far as back-reflected green, I guess you haven't seen the light show the DPSS laser I'm attempting to restore puts on, huh? The diode and all the correction optics light up like a Christmas tree from the green leaking out through the vanadate! You'd think that was the main beam path for the intra-cavity green. :)
(From: Robin.)
I would have preferred it if it was the PSU that caused the diode to blow - that would give more confidence that diode death would not happen again. Design defect I can live with, unknown infant mortality is harder to swallow.
The SMPS can be set for 0 to 1.25 A - set to 1.21 A for this diode. SMPS ripple is less than 50 mA at 1.2 A. I did use a passive heatsink (thick spreader with CPU heatsink and fan). The diode case never went over 35 °C - more like 15 °C when it failed. In the order of 30 to 40 mW at 532 nm going back to pump diode based 30% from rear of CASIX module. The diode is/was Polaroid MLD 808, 1 W, 9 mm - with full test data. Reverse polarity protected but no crowbar - but very unlikely to have ESD damage due to two 22 uF caps and the 10 nF cap directly across the diode.
(From: Dirk Baur (dbaur@medialas.com).)
There are different models of the Polaroid 808 diodes on the market. The first grade quality are quite expensive, the second grade you can get for a few bucks. Are you sure it was a second grade?
We used different Polaroid diodes, first grade, second grade at 660 nm, at 808 nm and so on. Not one of those made more than a few hundred hours. The first diode that died, was the first grade 660 nm with a micro lens inside. Maybe 30 hours, even with TEC cooling and antistatic equipment.
You are not alone!
(From: Joachim Mueller (JoachimMueller@swol.de).)
May it rest in peace.
My personal laser graveyard is full of Polaroids and also some SLIs and other nameless diodes.
An SMPS is not the best solution to drive a diode. SMPSs can produce short spikes in the microsecond range which are enough to kill the diode and are not easy to see with a simple scope. Capacitors across the diode are not enough to protect against spikes and never enough to protect against static discharge. When I buy a whole package (10 or 20) of diodes, these are sealed in an antistatic package, packed by the manufacturer. But when I buy a sample, the sales office or distributor puts it into a separate package. And I can tell you: Most of the problems I had with the samples! Never with the higher-quantity packages! So I think some salesmen don't take enough care when unpacking and handling the devices.
For example with SLI diodes, the sample died after 30 to 40 hours but the following 15 pieces run hundreds of hours without problems. I don't have such experience with Polaroids because I don't use them any more - too many 'samples'! Remember: If a diode gets a very small static discharge, enough to cause a very small defect, death can happen hours later. Morituri te salutant...
(From: Robin.)
They were fairly cheap - may have been second grade. I probably had 45 Hours out of the pump diode. I toasted a few business cards before integrating it into the DPSSL.
I bought 2 of these diodes from the surplus market. No undershoot spikes, designed those out, but did not include a crowbar circuit - could have been a mistake. Will add one for the other diode. I have the feeling that if I had posted here 2 months ago, saying that I intended to make a DPSSL from a Polaroid diode and a CASIX hybrid module I may not have built Greenie #1. However, I think that it was worth it for the few brief hours of 100 mW+. Biggest one before was 10 mW Argon.
I ordered flowers for Greenie #1, crowbar for #2. Oh, and a mount to move the crystal about when I put the 50 um black dots in it (assuming the pump diode holds out).
Fingers crossed.
(From: Joachim.)
When pumping with maximum 1 W and a normal (not microlensed) diode with a GRIN lens, the defect in the CASIX module appears much later. You should have at least 50 to 100 hours of green light (maybe longer).
I used a diode with a microlens and this makes a very small pumping spot, reducing the time to defect dramatically (less than 30 hours).
(From: Sam.)
What was the failure - no lasing, weak lasing, shorted diode, etc.?
(From: Robin.)
The failure indicated by sudden drop in output power - from 1 W to approximately 200 to 250 mW.
I just lit up the diode now as I'm typing this. No lens - straight at wall 1 cm by 9 cm red stripe with 3 black dots 1 mm in diameter randomly spread and a darker stripe centrally along the axis 3 mm thick. When I tried this a month or so ago the stripe was clean and the central strip was if anything brighter than the surrounding area.
The flickering at switch on was: bright, dim, bright, dim, bright, off, over approx 3 seconds from what I can remember. I had previously tested the SMPS in the lab for an hour and it behaved itself. There are no switching spikes on the supply other than 45 mV p-p ripple at 200 kHz with no overshoot on the edges. It has a 1 ohm sense resistor so approximately 45 mA p-p ripple through diode.
The only thing I have not done is to check the loop stability of the current sense feedback. It was not provoked by power on/off in the lab, but I must admit the compensation components were straight off the data sheet. I did not check phase margin myself.
Something to do before Greenie #2
(From: Sam.)
I'd still guess the SMPS did something bad. Once a diode is damaged, it doesn't come back to life, even momentarily. Once it's gone, it's gone. So, perhaps some testing of the SMPS using either the damaged laser diode or 4, 1N400Xs in series-parallel would be useful besides going back through the calculations!
(From: Bob.)
Was it a bare diode??? Laser diodes really DO NOT like crap on the facets. If you toasted anything in it's vicinity and you were using a bare diode, I'd be willing to bet the farm thats what caused your failure
(From: Mike Poulton (tjpoulton@aol.com).)
If I'm real lucky, I may soon have a 15 W fiber-coupled diode to make a serious DPSS unit with. My goal is to make a unit capable of about a watt when the pump is running at 12 W -- not amazing efficiency for a laser that size, but pretty good. I would like an extremely rugged setup, though, that can take some abuse and not loose alignment. Chances are I'll just have to make a really tough sled-type frame for the optics. It would be nice, though, If I could do it with optics that are permanently aligned in one unit, like the CASIX hybrid modules (e.g., the DPM series). Sure, the efficiency isn't great, but with 15 W in, I'm sure to get something impressive out. Yeah, yeah, I know the KTP won't be efficient at low temperature and all that, but it sure would be easy to maintain! Anyways this got me thinking about all kinds of elaborate water-cooling schemes to keep one of those CASIX modules from catching on fire instantly, when I remembered that pesky glue spot problem due to green intracavity power density. This got me thinking. What pump spot size are you using? Perhaps it would be best to defocus the pump beam until the Nd:YVO4 is just slightly above threshold. This would increase the active area inside the cavity and decrease the power density in the glue layer. My idea was that, since the diameter of the fiber on my 15 W laser is 1.2 mm, it could simply be close-coupled to a fluid-cooled CASIX module (2 mm diameter clear aperture) for an average power density of about 1,000 W/cm2 in the center 50% of the aperture. This would probably give me a watt of green for a couple seconds. If, through some miracle, the module could handle the average power (not likely), then it would probably last quite awhile, since the power density is most likely far lower than what is typically achieved with a diode and focusing lens. The two questions are: What is the approximate power density needed to threshold Nd:YVO4, and how severely will decreased power density affect the SHG efficiency? I sure hope I can get that diode.
(From: Bob.)
If you are pumping with around 10 to 15 W you want a 300 um spot size. If you used a 1.2 mm spot size instead you would probably only get milliwatts of power out. Unfortunately, things just ain't that simple! :)
Also, with regard to the fiber, if you are planning on picking up surplus diodes (e.g., from MWK on eBay), keep in mind that while they may be have been spec'd at 15 W, it is likely that they have been run to the end of life and put out maybe 1 or 2 W now.
(From: Mike Poulton (tjpoulton@aol.com).)
It appears, then, that the power density needed for reasonable conversion efficiency is a couple orders of magnitude higher than I expected. That's fine. I expected that simple close coupling wouldn't be very effective, and originally intended to focus it. Does the power density have to be that high to threshold the Nd:YVO4, or it is only necessary to get decent SHG efficiency? It would expect the latter.
(From: Bob.)
Your expectations are mostly correct, but the pump power density needs to be higher to get more efficient lasing in the fundamental as well, which obviously affects the harmonic conversion. So both factors come into play.
(A few months later - the conclusion.)
(From: Robin.)
I eventually figured out what had killed Greenie #1. I originally thought it was infant mortality of the pump diode - in reality it was more embarrassing. I designed a really nice SMPS to drive the diode - buck converter configured for adjustable constant current through the diode and 1 ohm current sense resistor. I made the mistake of having a BNC connector between the output of the constant current SMPS and the diode (with some static protection). But the center pin of the BNC somehow got pushed in. The next time I used the laser the output was open circuit so the SMPS went open loop and charged the output smoothing cap to 12 V. The BNC than made contact causing the 470 uF cap charged to 12 V to be placed in series with the laser diode (1 W Sony 9 mm) and the 1 - shoving 9 A through the diode. :( After 5 cycles of make/break on the BNC the LD turned into a DELD.
(From: Steve Roberts (osteven@akrobiz.com).)
Perhaps you can give me some hints, I have a 2.5 watt 795 nm pump diode hitting a block of Nd:YLF via GRIN lens, from a surplus research laser. The Nd:YLF is coated to reflect the 1,053 nm and pass the 795 nm pump, and is uncoated on the other surface. The Nd:YLF is about 5 mm thick and 6 mm in diameter. There is also a Brewster plate in the cavity. It does 800 mW of 1,053 nm with an IR OC. Can I use a normal KTP cut for 532 nm or do I need one cut for 523 nm?
Right now it's configured as:
What I want is something like this
I can build the pump diode PSU and thermal controllers. What I need to know is if I should keep the magnificent chunk of high grade Nd:YLF or not, and if I can get away with normal 532 nm KTP. If I've got something at 15 degrees to normal in the cavity, that sounds like a lot of Fresnel reflection losses. The pump diode is designed to match the YLF's optimal adsorption so I have a really rocking pump coupling, 2.5 A gets 800 to 900 mW of IR and if the Brewster plate is removed, there is even more output in the IR. The Brewster was there just to improve coupling to a single mode fiber. The pump diode has the TE