DIY Laser Power Meter Using Ophir 20C-A-1-Y Head

       Since my laser "collection" is growing and I need to know how much power output my lasers have. I needed a decent laser power meter.

       I was previously using a crude DIY Laser power meter found in the 'net using readily available components but it seems to lack accuracy at the top range.

       Reading about possible options, I ended up using an Ophir 20C-A-1-Y laser head which is very simple to use. Just supply it with a bipolar 5-18V DC supply and it outputs an analog voltage proportional to the laser power measured. In this case, 1V = 1W of laser power. This head can measure up to 5Watts limited by the head heatsinking and output voltage swing.

       I needed it to be self contained, small and portable so I can bring it along when shopping for a laser in the mall. I also added a BNC output port so that I can connect it to my digital storage oscilloscope for data logging. Since the meter will already be configured to measure voltage, using a simple 3 position toggle switch allowed it to work as a basic voltmeter that can measure up to 20VDC (useful for checking batteries) using the connector as an input jack, and the third position allows to measure the internal battery voltage to know its charge state while the jack becomes a direct connection to the internal battery which can be used for powering/testing other circuits or charging the internal battery.

       To provide the voltages necessary for the various blocks, I used a simple DC-DC converter board that converts 1-5V input to 5V output (removed the USB connector) to supply the meter and another DC-DC converter module that takes in 5V and puts out plus and minus 12V. A few inductors and caps to filter the supplies and we're all set.

       All these are packed into a DIY aluminum case about 5.5" wide, 2" deep and 1.5" high.

       I can add the power supply schematic later if someone is interested, let me know.

Laser power meter opened.

Front of the Laser Power Meter.

A detail of the meter and power supply circuitry:
(battery is a Sony 18650)

Label behind the Ophir sensor head.

Inset side panel for the multi-function BNC connector, mode and power switches.

Other side panel also inset to expose as much of the Ophir head to ambient air.

Aluminum cover to protect absorber surface of the sensor head during storage or transport.

My pen style dual parallel 10440 build using an Osram PL450B diode doing over 3.9W
(actually, the LPM is displaying the internal battery voltage.)

My 1watt laser with the lens off (diode current is 1.5A)

Same laser as above but with the 3 element glass lens focused to infinity.

Here's a 52 second power output plot of the above laser.
You can see it peaked at 1.48W then slowly droops to 1.42W as the diode and driver heats up.

My 450nm dual 10440 pen style pointer using the Osram PL450B at 100mA.
(It was 100mA before I glued the pot and pressed it into the host - not sure if it shifted during pressing.)

An old pen style greenie I had years ago but no longer use because of the scratched lens.
I never thought it went that high. I always thought it went to only 30mW -
although I was using a dual parallel 10440 in it during the test

The compact 405nm laser keychain using a module rated at 150mW from ePay.

532nm lab style laser rated at 50-70mW.

150mW 405nm lab style laser from Aixiz.

My old 532nm pointer whose output suddenly degraded to a fuzzy dot.

       26 May 2013:

       I changed the Ophir sensor cover to make it swing out instead of having to unscrew to remove.
A spring washer under the screw gives it constant pressure for a smooth action when opening and closing the cover plate.

Power meter with test lead connector.

Sensor cover closed and opened.

       09 Aug 2013:

       I bought an ePay 3 pack red/green/violet pointer set to add to my laser collection and made measurements using this LPM and posted the results in Laser Pointer Forums, check out the link for more information.

       25 Dec 2013:

       The meter display least significant digits always jumps a few points and wouldn't settle to zero at no input so I was wondering what could be the cause.

       Turns out that the DPM connector is causing a miniscule noise due to dirty contacts.

The DPM I use is a 4.5 digit one available online.

       The cause of the problem is the connector. Since it's a mechanical connection, over time (or many plug/unplug cycles later) the contacts get dirty and cause noise. Since this meter is very sensitive due to the 5digit display, the noise shows up as jumpy last two digits.

I removed the connector and cleaned the solder pads on the PCB.

Then cut off the plug on the wires and strip the wire ends.

Solder them directly to the PCB for a solid, reliable connection.

Put it back in and test.
If you noticed, I also changed the battery to a cellphone pack since it already has a built in protection PCB for safety.

A solid zero at no input. Before, it would jump around at a few mW display and no matter what filtering and modification I did to the power supply, the noise remained. After this, the reading became quite stable.

       15 Mar 2014:

       The way I had my LPM wired is that there's a BNC jack that can double as a signal output for external data logging, or as an input to use as a voltmeter.

Well, long story short, the switch was in LPM mode and I applied 13V into the jack. Ended up blowing the Ophir head circuit inside.

Here it is, output is stuck to one voltage rail.

Decided to poke around and see what went poof. Found out it was only one op amp that got damaged since the other one (the expensive one) had a response when a laser is pointed on the thermopile.

Desoldered the thermopile leads and the board is free.

Using the flood-and-pull method, I was able to easily remove the broken IC.

Holes cleaned up without any damaged pads.

I know I will blow it up again in the future so I bought some spares and IC sockets for future use.

And it's back in. op amp is now in a socket so if I ever blow it up again, I just pull it out and slap another in.

And it works. I didn't even have to touch the calibration and offset pots.

While I was at it, I rewired the inside and made a new powersupply.

And added labels so I won't forget which switch does what.

And some warning labels too.

Later, I did a bit more work. I had never been able to eliminate noise coming from the old PSU so made this new one and rearranged the wiring, added inductors and ferrite beads in places and was able to lower the noise floor considerably. Readings are much more stable now.

Page updated and copyright R.Quan © 25 Jan 2011.