I'm trying to repair a DPSS green laser with a blown pump diode. The original diode had a FAC so I ordered a 3W 808nm C-mount diode with FAC from LaserLands. I don't know what power the original diode was but after measuring a drive current of 2.2A, a 3W diode seemed like a safe bet.
The diode was easy enough to replace and powers up just fine but the green output is less than stellar. With 440mW of pump output I get 40mW of green and with 2W of pump output I get 150mW of green. Thats 9% conversion efficiency on low power and 7% conversion efficiency in high power.
I played with the pump polarization by rotating the top of the module that holds the crystals and that's the most green output I could get. After measuring the wavelength of the diode its producing 810nm @ 440mW and 813nm @ 2W. Is it safe to assume that the wavelength shift away from 808nm is the main culprit for the terrible conversion efficiency?
For fun put the diode in the fridge and then screw it backmon quick and take reading. If the diode is shifting that should tell you. 2nm is The usually window so 810 is right there. I didn’t say fridge the whole thing as if you cool the shg Crystal you’ll off tune it. Those usually like to be heated. Cool the back heat the front
I’ll give that a shot, that would explain why the laser has better conversion efficiency at 810nm vise 813nm. Kinda bummed it would wavelength shift that much even when being driven at only 2/3rds it claimed rating. Good ol’ cheap Chinese diodes for ya I guess.
the range of phase matching on this stuff can be all over the place so don't be too hard on them. I've seen some between 805-820 You just don't always know. This is usually temp tuned on the doubling crystal to match the pump but on the cheap stuff like this that is what you get. For all you know the match could be closer or father out. you'd have to temp tune to figure it out.
I cooled the diode and its mounting block by spraying them with CO2 for a nice quick and repeatable chill. On low power the wavelength reduced from 810nm to 807nm and on high it went from 813nm down to 809nm. I wasn't able get a power reading on green light conversion before it warmed back up as the diode and block would heat up before my meter would reach steady state.
Visually it did look brighter but that's less than scientific. What I did find is that heating the crystal set granted a measurable increase. Even just leaving the laser on and letting it slowly warm up I could watch the green output climb from 150mW (room temp) up to 210mW (warm to the touch).
One thing I did notice it that the laser seems to be operating in different modes depending on the power. In low power mode the laser produces a pencil thin beam while in high power mode the laser produces a fat beam more akin to a direct diode laser. In both modes it remains focused to infinity and throws a nice small dot.
Using a lens to enlarge the dot really shows the difference between low and high. I'm wondering if the difference in size is representative of an inefficiency in one of the power modes. I have other variable output DPSS green lasers and they don't change beam diameter.
It’s called mode hopping and you can’t prevent it without temp control. I’d suggest try heating the front end and leave the diode alone. You might get lucky. Or….dump it and get something stable. They are cheap these days.
I thought it might be mode hopping but i didn't think it would change the diameter so drastically. Been fiddling with this thing for so long I'm kinda fed up with it, might just direct diode green swap it and call it a day. Thank you for your insight!
The widening is another effect I am going to call Kerr but I think I’m wrong. Idea is as it heats the refractive index changes defocusing the beam. Kerr can do that but that’s not the same principle.
