It's just a common side effect of DPSS lasers- none of them will have a true linear curve when modulated through the pump diode.
Remember, you've got a separate laser system running inside, with it's own needs. As
Leodahsan said, the crystal assembly (both Nd:YVO4 and KTP) are temperature and wavelength sensitive. Even a small shift in either temperature or wavelength will cause a noticable power fluctuation.
Remember, the heat output of the pump diode will change as it's modulated; this change must be compensated for. Simple modulating of the diode won't do, for example, changing from CW to 5kHz TTL can cause a temperature change of a few degrees within seconds, along with the accompanying wavelength shift.
Before I begin explaining the next bit: get this into your heads. Most <300mW labbies have a TEC on the pump diode, most >300mW units have TWO TECs, each with their own sensor/driver, one on the pump, one on the crystal assembly. Some higher-power assemblies (2+W) even have three TECs, one on the pump, one on the gain medium (Nd:YVO4) and one on the KTP (As a heater, KTP likes heat).
As a result of this, the TECs are modulated alongside the pump diode. You ever seen threads about lasers going 'out of tune'? This is where they go wrong- there's an issue with TEC drive tuning, which in turn, causes instability issues when the laser is modulated. All the DPSS I know out there have a modulated TEC, an unmodulated TEC is asking for mode-hopping and power fluctuations.
On that note, if you want anything even resembling remotely stable power, TEC stabilisation is an absolute must. I've seen 50mW lightshow-style modules with TEC, a 1W
needs a TEC if you don't want your laser to turn into a TEMwtf projection device.
And if you think 532nm's bad? 457nm has an even slower response time, and even less stable.
The only way to get something even remotely linear using the laser only is to use optical feedback, this adjusts output on-the-fly using a power reading taken from an optical pickup.
The other way, that requires complex and fidgety equipment external to the laser is to use an AOM.
So, there. That's it; there's really not much to it. Every DPSS green will be non-linear to an extent, except some are better, much better, than others. There's nothing we can do about it, just be glad the beam specs aren't like our 445s
As for the reds, I have one of these 'flashlight' reds sitting on my desk next to me. It's a CNI unit, OEM, 655nm at 500mW. 5x8mm beam at aperture, with 2.5mRad divergence (measured, confirmed).
It's a 8x8cm blob at 20m, and it's visibly divergent. It's not ideal, of course, but it's the only cheap option. It's either that, or throw down $$$$ for a 640. Yes, it's fine and dandy if you can afford it, but most times, 655 is still the way to go.
Even with a FAC, you won't get any better. These diodes are multi-mode and multi-emitter, you simply can't get any better out of them even with corrective optics. 2.5mRad is good, 4 to 5mRad is not unheard of. At this point, even with the rectangular beam profile, I'm just glad they aren't like the 445nm diodes, with a distinct fast and slow axis.
Inevitably, you'll lose a *bit* (ahem) of power, I'm estimating 100mW of my red to disappear. But $250 for 500+mW of red? It's hard to beat, especially when you're on a tight, tight budget. I'm not complaining.
And on a final note, DHOM is one of the better manufacturers out there, in the same league as CNI or Viasho. I'd definitely go for them, one of my regrets with my build is not going all-DHOM (with a CNI scanset*). But that's kinda hard to do when 3/4s of your budget go flying out the window. Oh well, long story.
I'd definitely go for DHOM. Thumbs up, that's for sure.
*Yes, I know it goes against everything I've said about Sonima being absolute bulldust. But the SS2800s (ScanPro50s) are one of their better galvos, and at 60 degrees at close to 20kpps, it isn't too bad of a beamshow solution. Not to mention it has an angled X-galvo mount, which is essential when working with oversize beams, meaning more of a given beam will fit after your first bounce.
I'd have liked a pair of EMS4000s, but $1500 for galvos is too much for me, even if it does mean getting a pair of indestructible galvos. You can't actually wreck them, even grabbing one of the mirrors and holding it while scanning (which would kill most galvos and amps) won't hurt it. Overspeed, underspeed, it's one tough set of scanners.
