UK, if the Laserbee Meyers aren't very good for what Hap and I are after, is there anything that would work better without breaking the bank?
Don't get me wrong, a laserbee will work to meter for a general idea of output, but dont hold its measurement to be pure truth. they're too slow to accurately measure finicky lasers most of the time. Alot of their sensors are 10+seconds in response, some of them are half a minute to almost a minute long! unfortunately fast accurate sensors are very expensive. especially new...
I've been wondering why LPMs advertise for a specific way a laser is produced, e.g. DPSS vs direct diode vs Nd:YAG etc. Why does it matter to the LPM?
Depends on the meter/sensor pair. there are different types, each with their own advantages. but generally speaking, most we use for the hobby use thermopile sensors. but there are ones that are diode based too. Thermopiles consist of an absorbant material that when the laser strikes it, it generates heat, which is then sent as a signal to the meter. more heat=more voltage, thus higher output. Thermopiles don't care about wavelength generally, as heat is heat, as long as its of a material that's high absorbance. I dont remember what the laserbee sensor materials are, but i'm sure they're halfway decent. Really good ones use graphite generally. The more absorbance, the more of the light you're actually measuring, and thus more accuracy.
The problem stems from laserbee sensors being passively cooled mostly. They're shaped like a heatsink, and eventually get saturated with heat so they can eventually even over-read if the heat doesn't dissipate fast enough. They also have a response time of 10-40 seconds depending on the model, and a resolution of 1mW. that means that you only get an accurate measurement about every 10s or more! and it only displays in 1mW increments. They have a high-ish threshold, so they only measure accurately (or at all) above that threshold (something like a 3mW iirc but i don't remember.) So it cant catch all the mode hops and peaks and such, only give you a ballpark of its power every so often.
In contrast, a high precision meter such as my Melles Griot PEM for example's sake, consists of a graphite disk with a high sensitivity feedback controlled TEC and heatsink for stability. The higher power incurrs more current to the TEC, cooling the disk to keep it at reading temperature. It has a threshold of 20 microwatts and a response time of about half a second and a resolution of 10uW. meaning it'll measure as low as 20uW accurately in 10uW incraments and give an accurate reading about every half a second digitally, and has analog for in between readings. It also enables it to read extremely short pulsed lasers as well.
If you were to compare something like a diode laser on both meters, it would look similar... but a DPSS graph, would look substantially different on each meter. the laserbee would make the spartan look stable, with a smooth curve. A higher response meter it would look jagged, as you'd see every peak and drop in output all over the place as it was warming up.
I won't get into photodiode ones, but they're even faster to respond, but have limits on wavelength (sensor response to particular wavelengths) and have lower power tolerance. So they're even better in the low power, ultrastable market and pulsed markets. like anything, you buy the tool for the job. The laserbee works fine for general power measurements, but if you're looking for accuracy, such as alignments or stability checks, it simply won't do. It'd be like trying to screw a bolt in with a screwdriver and pliers instead of a wrench.
