Measuring an Unstable Laser Can Be Frustrating

1. Fluctuations , where the time constant is short,

2.Drift, where the time constant is longer.

To deal with random fluctuations, the obvious solution is to take the average of the power readings over enough time to “average out”, or smooth out, the fluctuations.

Ophir meters have a built-in “Average” function that does this for you automatically. You only need to set the time period over which the meter will calculate the average, and the displayed reading will be the running average over the selected time.

Now let’s look at how to measure power of a laser beam whose power is drifting slowly upward or downward, for whatever reason. If you need to know the beam’s power as you perform some critical step in an experiment, but the reading you take after that step is, let’s say, lower than it was just before you performed that step, which reading do you use?

As before, you’ll first need to map out the situation; you’ll want to take a number of readings at “enough” time points to establish the pattern.

For example, let’s say that critical action during which you need to know the beam’s power takes you 30 seconds, and let’s say the power reading just before it was 1.5W and just after it was 1.3W. If from the “pre-mapping” you did, you saw that that drift rate is reasonably consistent over that time period, you can quite safely interpolate and use 1.4W as the power reading during the critical measurement time.

In summary, do take power measurements of a laser beam whose power is unstable, you need to look at both –

• The standard deviation (so you can suitably average out fluctuations), and -
• The trend over a longer time