ND:VANDATE has a upper laser level storage time of 800 or more microseconds.. So if you have a blanked DPSS laser, you need to slam in a lot of pump diode to get past the threshold of lasing. As you blank, things go the other way and the 800 uS storage time takes a while to bleed off. This is the upper limit on speed for a SMALL doubled yag laser. Larger lasers with bulk crystals are even slower in CW mode.
The lasing threshold varies all over the place based on the immediate past history of the crystal set and the thermal history of the heatsinking...
Take your left hand, spread your fingers apart and hold in it front of you. This is the output spectrum of a low cost 808 nm pump laser in the dpss, its about 2-3 nanometers wide and has several spikes across its wavelength spectrum. Now factor in that a .1' shift in diode core temp moves the wavelengths up or down about 1 nanometer. The adsorption band for Nd:YAG or ND:VANDATE is also a series of spikes of max adsorption next to gaps of much less adsorption. It is also temperature driven, but not as bad as the diode is. Spread your fingers on your right hand to represent your pump adsorption. Now slide your hands across each other, and see that as you move your hands, there are places that sometimes have a good pump and other times do not. Since the pump diode is so tiny, its very hard (well, pretty much impossible) to have the huge mass of the pump diode chiller react to the tiny cavity size changes from the pump current that shift the wavelength.
The change in cavity length changes the resonant frequency of the pump diodes cavity and moves it across its gain curve... The gain bandwidth of a diode is large, but follows a bell curve, so as you move towards its edges the power can really fall off..
If you want to see a example of this, watch a high power green dpss pointer "ramp up" to power, or "ramp down" right after you hit the button. Or place your pointer in ziplock bag to prevent condensation and toss it in a freezer for about 5 minutes, then hit the button. You'll see all sorts of effects as the pointer trys to make it back to the room temperature equilibrium it was tuned at. Pump diodes are graded by wavelength at the factory under steady state heatsinking.. Often times the diodes that are not near the +/- 5 nanometer wide adsorption band in yag show up on ebay, and are real lemons if you buy them to repair a laser
Professional high wattage DPSS display systems and laser TVs use a EO or AO modulator for this reason.
Modern Laser TVs use vertical cavity surface emitting lasers (vecsels) or similar tiny lasers, in arrays , the vecsels are small enough to quickly reach thermal equilibrium, but are very limited in power. A vecsel is so small that the doubling crystal and host laser crsytal can be vacuum deposited on the laser at the wafer level.. The down side of the is a green laser for a TV emits a 1 or 2 cm square beam with the quality of a flashlight and a huge divergence..
DPSS 473 nm blue is even worse, the temperature of the whole assembly often has to be held to .01 degree C or less for peak power...
Novasel laser arrays would be pretty much useless for a pointer or classical vector laser scanning.
See:
http://www.photonics.com/Article.aspx?AID=32111
Steve
