The classical specifications of lasers gives us useful data like CW power and beam characteristics. Most of this data is collected under static conditions, i.e. operation at full power after warm-up. For most of us, the lasers in our projectors are typically not operated under static conditions - they are modulated. This modulation can significantly change the characteristics of the system, especially when DPSS lasers are involved.
Now that I finally have a nice power meter at work I'm able to do some measurements on my lasers. Here I present some preliminary results that I hope we can discuss.
First I will show what a typical result using the method described later looks like. Because the lasers used are of different different powers I'm showing the relative power instead, which is simply the momentary power divided by the full power of the laser.
Not surprisingly the diode laser handles the modulation best. The two DPSS lasers are similar, but both have rather low power compared to the diode laser.
To better be able to compare the results you can look at the area under the graph, which is simply the emitted energy (as energy = power*time). It is easily obtained by integration.
140 mW red diode: 22.1 J
430 mW green laserwave: 40.3 J
130 mW blue viasho: 9.16 J
We see that for example the blue viasho, which at full power reaches nearly the same power as the diode laser manages to squeeze out less than half of the energy of the diode. If respect is taken to the the lasers' respective full power output and assume that the diode is ideal (100% efficient) it is possible to calculate a "modulation efficiency". For the green laser that efficiency is 59% and the blue is merely 45% efficient. Interesting!
What practical implications does it have though? Well, I don't want to jump to conclusions, because there are some more measurements I'd like to do. However, two immediate thoughts that occur to me are:
- The ~20% loss in PCAOMs might not be so bad after all
- An explanation to why people get away with using less red (and 405 nm) power from diode lasers than predicted by color models
Looking forward to some comments now...
Equipment used:
Ophir Nova II power meter with a 3A-P-FS thermopile head
Pangolin Flashback 3 SE
Laserwave 532nm 400mW DPSS laser
Viasho 473nm 100mW DPSS laser
Opnext HL6385DG 642nm direct injection diode with FlexMod2 driver
DZ's color correction board
Method:
Rising and falling edge delay were set to 0 and gain to maximum on the color correction board. The offset was then adjusted to just below the lasing threshold. Each laser was allowed to warm up at full power and the output power recorded. After the warm-up was completed the LAStudio show "Show must go on" was played by the FB3 at default settings (single color) through DZ's color correction board and the power logged.
The (radiant) energy Q is given by integrating the radiant flux Φ over time. The modulation efficiency is calculated like:
Q*Φ[ref,CW]/(Q[ref]*Φ[CW])
Where Φ[ref,CW] is the full power radiant flux under and Q[ref] the radiant energy of the reference diode and Φ[CW] the full power radiant flux of the tested laser.