# What do I measure with power meter?

I modulate directly a laser diode by setting the pump current periodically from 0 A to $I_{on}$ A. I do it for different duty cycles and different frequencies. But I need to keep my average power of laser diode the same. For this purpose, I use conventional power meter with a semiconductor detector.

If I have duty cycle 100% (CW light) power meter measures the power $P_{100}$, which is average power and peak power at the same time. Now I am going to duty cycle 15% while trying to keep the average power the same by increasing the pump current. I measure $P_{15}=P_{100}$ again without any changes in power meter settings. My question is: is it really the average power of modulated light?

The same time I make the measurement of optical spectrum with OSA (optical spectrum analyzer). OSA calculates total power. What does total power mean? If I consider peak power, I expect that $P_{d.c}^{peak}=P_{100}/d.c.$ independently on frequency, but in my measurement it depends on the frequency and is not as I expect it.

Edit 1: I use PM100D from ThorLabs and S122C as the detector. https://www.thorlabs.de/drawings/8d38cd843521876e-920B9BC1-5056-2306-D9089CE0AC301D49/PM100D-Manual.pdf On page 22 there is information about bandwidth setting. My power meter has BWH/LOW

• It depends on how your power meter works, particularly the sampling time and rate of the electronics compared to the sampling time. It might be helpful if you specified the model of power meter. – Gremlin Aug 25 '16 at 14:27