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Tuesday, May 17, 2016

Laser Power Meter

Background

After your first mirror alignment on your K40 you realize how nice it would be to measure the optical power and have a reference for when things stop working. 
A store bought power meter is more than 1/2 the price of your K40. Here is an example Laser power meter that is employed by many K40 owners to measure absolute power. 
I wondered if I could construct a power meter that would be:
  • Lower cost
  • Fit in front of mirrors
  • Digital readout
  • Low power
  • Anti-reflective
After some research I stumbled across a Peltier module and was thinking about using this technology to build a cooling system for my K40.

Research 

Today I tried an experiment with a Peltier module connected to a voltmeter and exposed it to a K40 laser pulse. Peltier modules are typically used to convert electric power to heat/cold. Inversely when heated a Peltier module will output a voltage. 

Minor burn on the surface where laser impacted (just below mid line on left)
In the video below the Peltier module is on the left with a sticky note on it. Watch as the laser burns through the note and then notice that the voltmeter quickly raised to .05 VDC. It then slowly tapers back to 0 as the heat in the module dissipates.
It seems feasible that a relative and proportional laser power reading is possible.

Approach to power measurement

I suspect that each Peltier module would have a different voltage vs power characteristic. Therefore it would be difficult to get actual power readings unless each Peltier module was calibrated using a known power meter and source. The cost of calibration defeats the purpose for someone that wants an occasional use low cost meter.
The concept could be to make relative power measurements once the K40 is set up. These measurements would be made before and after each mirror and logged as relative values to be used as a reference if performance drops.
It can also be used to measure relative performance at different machine settings. Example: measure relative power at a specific laser current setting.

Use-case for Peltier Laser Power Meter (PLPM):

  • Align the K40 for the best power output
  • Take and record measurements at each mirror with the PLPM then compare with reference values 

Implementation/features

Peltier module connected to stand alone controller with buttons and display
  • Reference mode: stores pre-mirror values 
  • Troubleshooting mode: take measurement from a specific pre-mirror and compare to reference
  • Bluetooth interface
  • A bracket that would allow the PLPM to interpose the optical path between mirrors

Next steps:

Decide if this project is worth pursuing? Leave a comment with your opinion.

Maker Don