Measuring pulsed laser average power. - Page 1
Forum section: electronics / projects

author: PbFoot
author_rank: Contributor
timestamp: January 15, 2014, 02:23:07 pm
content: Greetings!I was hoping someone might be able to shed some light on this for me. I have a device that uses a pulsed IR (905nm) laser, and I would like to measure the average power it produces. The pulses are 60ns (FWHM) and are 140 watts peak power. I am using 125 pulses per second. I calculate that I should get about 1.05 mW average power from this arrangement.I am looking for a laser average power meter that will give me an accurate result. What I can't figure out is: Do I need a device that can measure from below 1mW to over 140 watts, or will a device that can measure, for example 40uW to 4 watts be sufficient? In other words, to I need a device that can "see" the peak power without saturating, or can I just use a device that only measures in the range of the average power that I am expecting?I'm hoping that something like this will work:http://overseas.sanwa-meter.co.jp/items/detail.php?id=161Thanks for your help!- PbFoor

author: mathias
author_rank: Regular Contributor
timestamp: January 16, 2014, 03:02:48 pm
content: It depends on the bandwidth... each pulse will generate a huge amount of carriers. When these arrive and are amplified fast enough, you've got a problem. Normal power detectors don't get up to 50ns pulse widths.Let's say you use a fast photodiode and an oscilloscope, than you need to be able to measure the full peak power without saturation.For DC measurements, bandwidth requirement is zero. Therefore, you only need a basic photodiode detector. The one you showed should be up to the job.

author: muvideo
author_rank: Frequent Contributor
timestamp: January 16, 2014, 04:51:22 pm
content: Hello, the power meter has 5% accuracy, but you dont know it's bandwidth,so you dont know how it behaves with a pulsed source.Maybe 140W peak could even damage the sensor.I would consider measuring the heat transferred by the laser to a target.If your target absorbes almost 100% of incoming radiation (is "black" for905nm) and has a time constant long enough, and is little enuogh, you shouldhave no problems measuring 1mW of power.For example you could use a PT1000 as target and as sensor at the same time,a variable current source and measure the current and the voltage on the PT1000.I've never done it before but I would have fun trying this way:If you feed the PT1000 with I0=100uA you make it dissipate a little more than 10uW,and the voltage V0will be a little more than 100mV. Resistance will be R0=V0/I0.Switch the laser on and after few seconds take note of the new voltage reading V1,the PT1000 new resistance will be R1=V1/I0Switch the laser off and raise the current to obtain the same resistance as before,calculating it as R2=V2/I2, when R2=R1it will be somewhere more than 1V.The power absorbed by the PT1000 from the laser is V2*I2-V0*I0Notes:- You need a decent meter, here resolution is important more than precision,but 200mV, 2V, 200uA, 2mA ranges on a dmm should be more than enough.- can do the same thing using an NTC, it will be trickier, but more sensitive.- Make sure the coating on the sensor is absorbing all the radiation, or at leastthat you know how much is reflected back.- The variable current source can be a simple variable voltage source with aresistance in series, it needs only to be stable for few minutes.- The faces of the sensor that are not exposed to the laser can be insulated witha little foam, this will slow down the response.Ciao!