Lets take some of the emphasis of this thread off thermal control and move it to where it needs to be, having a TEM00 (Gaussian) transverse mode and a limiting the gain bandwidth of a laser to a single longitudinal mode. Going to a modest thermal mass makes the laser easier to stabilize, and lessens the need to hit some un-practical number like 0.001'C. Russian saying " Best is the Enemy of Good Enough!" Also, "Do not look gift pig in the snout" I used to work with ex Soviet Block laser guys, who often achieved very good lasers with very simple technology, by spending a little longer on the design, so they could use modest, obtainable parts.
!
When you look at those pictures of a stacked TEC, your looking at a unidirectional ring laser cavity with course mode selection by intra-cavity thin etalons and final mode selection by a temperature tuned birefringent mode selection in the doubling crystal. It has an optical isolator in the cavity so the beam is unidirectional. That design is based on a ring cavity (math based design) first developed for dye lasers by Kolgonik (spelling?) and Lee. The cavity is designed to compensate for astigmatism in the beam from the folds and come out circular, while having a small diameter , but long, active region in the lasing crystal and in the KTP. Its the result of say thirty or forty years of intense laser research and development, Its designed by a bunch of Skilled PhDs in physics and optics. The tooling to make it probably costs upwards of a Million Dollars.
!
No matter how much you heat or cool the crystals, you can only reduce thermal lensing somewhat, so lets USE IT TO OUR ADVANTAGE when designing the laser. The Same goes for Polarization, its there, its needed, its inherent to the KTP's functioning as a doubler, so lets live with it and use it, especially if we can use the Brewster polarizing plate as a thin etalon to also select a single mode.
!
The above RING LASER is Not easily cloned. Great for a design where a replacement laser needs to drop in with zero adjustment to system alignment, but NOT a beginner's build.
!
Lets move off of that design to something with three mirrors or less and much more practical with a small ND:YVO4 and a short doubler, shall we?
!
Some tasks:
!
First task, Have a short cavity that has the correct intra-cavity polarization for Type II SHG and lases TEM00. Shorter cavities can have fewer longitudinal modes, making it easier to select just one mode. Lets use the KTP as the selective element if we can.
!
Second task, Have just enough gain bandwidth that we can place a loss element in the cavity to select a single mode.
!
Third task: Have a long, thin, beam waist located in the Vandate and another short , thin diameter beam waist located in the KTP. As conversion to green is driven by a very high electric field in the crystal, we need a short, tiny diameter waist , but not so high a field that we "gray-track" the KTP crystal with permanent damage.
!
Forth task, Match the 808 nm light from the pump diode to a beam waist that is roughly the same length and diameter as the 1064 nm beam waist in the Vandate. Vandate lases with self polarized 1064 nm light, it is also sensitive to the polarization of the pump light, so you not only need to focus the pump light in the crystal, but you may need a waveplate to rotate it to match the alignment of the Vandate crystal. You may find a fiber coupled pump laser makes this easier at first.
!
Fifth task, Minimize "Green Noise" effects generated by non-linear effects in the KTP. The KTP is birefringent, meaning it has a different index of refraction for each of the two possible incoming 1064 nm polarization orientations. Hence the supplier asking about desired crystal cut with respect to the optical axis of the raw crystal and Phi/Theta angles.. (Not going to cover circular polarization in this thread if I don't have to, I may have to...) Really don't like to TRY TO explain "green noise" in a beginning thread, but suffice to say the KTP crystal can make the laser very unstable in amplitude and mode, two things you do not want in a holography laser.
There are ways around it, but ask the laser show guys around here how they like cheap DPSS greens that mode hop for no reason and change divergence on the fly.
!
Sixth task, stabilize the whole thing optically, and thermally. Temperature tune the pump diode so that its lasing bandwidth is centered on the vandate's maximum adsorption range. Heat / cool (depends on design and ultimate power) the KTP so that it is temperature tuned, but do not adjust the KTP angle/temperature so that the tuning is so critically sensitive to temperature that it will only lase in a very narrow range after warming up. Note this is way down on the list. It is not that difficult.
!
Your looking for a copy, preferably second edition of "Laser Fundamentals" by Silfvast. It is a very readable book with the mathematics simplified mostly to high school algebra. It shows up cheap on Abebooks as a used book, because it is often used as a classroom text. If you get the right printing, PL's own Laserman532 is on the cover, or a picture of a whitelight Helium Selenium laser hitting a grating is on the cover. Dr. Silvast discovered a class of metal vapor lasers, including the HE-CAD, so he's writing from a experience perspective. It has really good explanations of the basics. If you want to spend an informative few months reading, "Lasers" by the late Tony Siegman really covers cavity design.
!
There is a reason I suggested that paper, its very similar to the easily stabilized JDSU/Lumentum Microgreen aka "uGreen" commercial laser, a laser I know that you can make a version of with decent tools, and without breaking the bank. It also has the advantage of needing a modest TEC or two without super thermal control.
!
If you want the legendary performance and power of a Compass or Verdi, buy a Compass or Verdi. If you want to learn, build a two or three mirror, two crystal laser first.
Because the setup needed to make sure it is SLM is something you need to plan for as well. With what you have already, design a 200 mW TEM00 laser and be very happy if it does 75-100 mW of SLM. Be happy if it does 50, because most skilled hobbyists barely break 50 with their first DPSS greenie.
!
Now tell me about yourself, and do you have access to a lathe or milling machine?, so I can customize this thread for you. BTW, I'm the one who did some of the library research for Planters, so I know roughly where he started off with the design of Big Green...
!
The exclamation points are because my browser does not play well with the forum software, sorry about that.
!
Steve
Last edited by mixedgas; 05-10-2017 at 14:10.
Qui habet Christos, habet Vitam!
I should have rented the space under my name for advertising.
When I still could have...