building a laser power meter for high peak power pulses - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: July 12, 2013, 05:55:57 pm
content: I want to be able to test the output from a IPG fiber laser, 1064nm output 10 watts average power, 5kW peak pulse power (0.5mJ in 100nS pulse @ 20khz rep rate). I just need to determine if it is close to spec.I have a Coherent lasercheck (only good to 1 watt I think and not really meant for this) and a Synrad powerwizard 250 watt which is meant for CO2 laser thermal measurments. The latter might somewhat work if I could know the reflectivitiy of the black annodized target for 1064nm vs 10,600nmI looked through my junkbox and found some large area photodiode power detectors from IIRC a 1990 vintage medical yag laser with fiber delivery. The two smaller mounts in the photo contain one of these diodes with some sort of white diffuser which is pointed at a optical element at 45 degrees...I assume some sort of power takeoff beamsplitter arrangement. The larger mount seems to have the same photodiode mounted behind a diffuser and was intended to take the direct output from the 100 watt fiber (CW though).I have an Agilent DSO which I could use to measure pulse length, rep rate, etc.How to calibrate one of these sensors when I don't have a known 1064nm source? I think I have an old dpss laser pointer....could remove the filter, run it through a prism to separate out the 532 and 808nm and hope that some 1064nm is leaking through? Then I could measure this 1064nm with the Coherent lasercheck and get a rough baseline for power when I shine the same beam onto my photodiode?Ideas? Thanks. Note in the following photos of the sensors, the little blob seems to be a thermister, as I measure 10.9k at room temp across it and 6k when I wave a heat gun near it. To be honest, I do not have a lot of experience with this type of large area photodiode...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 12, 2013, 06:19:56 pm
content: That power would probably vapourise a photodiode. May be feasible to do some sort of thermal test if you can find something that absorbs well enough - compare temperature rise with laser vs. temp rise when heated with a known resistive heating power.However it may be hard to know what proportion of the beam is being absorbed,and what energy is lost through vapourisation.

author: KTP
author_rank: Frequent Contributor
timestamp: July 12, 2013, 07:34:23 pm
content: Hmmm, I agree that perhaps the best way might be to do the resistive heating thing. I remember doing something similar before I got the Synrad power meter so I could measure my CO2 lasers. This is only going to give you average power, but I could possibly use reflections into the photodiode and my oscilloscope to measure the pulse length and rep rate....some math would then give an estimate of peak power.Or I could just see how long it takes a focused beam to burn through a razor blade and do an estimate :-)

author: KTP
author_rank: Frequent Contributor
timestamp: July 12, 2013, 10:26:32 pm
content: I just read up on my Synrad powerwizard 250 watt wand type meter (thermal) and it has an absorption of better than 96% at 1064nm and can handle a 1J 50us pulse (20kW) over 1 square cm. So that will work for average power and I can just use the photodiode and scope to measure pulse length from some reflected photons. I don't really need to know the actual laser power level per current in the photodiode but rather just the shape of the pulse and the length. This will tell me peak power as long as I know the repetition rate.