How low can you measure laser power?

Ophir has very high standards for what is considered “measurable.”  That is why when we delineate a power scale, the minimum power is usually about 20 times the 3-sigma noise rating (or 60 times the RMS noise).  Although it isn’t inherently wrong to say that one can measure power lower than this scale, we believe that it is important to err on the side of caution when it comes to power scales and thresholds.

However, this often leads people to ask: “If Ophir’s standards are so strict, can I measure outside the range stated in the specs?”  What exactly happens as the laser power approaches the noise level?  It’s clear that if the laser power is of the same order as the noise, we won’t get a meaningful reading.  I want to discuss what happens to the accuracy level between the minimum laser power rating and the noise level.

Ophir sensors are typically calibrated to an accuracy of +/- 3%, but as the laser approaches the noise level, the noise will affect this error more and more significantly.  The graph below is a sample curve based on a noise of 2mW and a 3% maximum calibration error.  As the graph suggests, this is not simply a linear effect.  In fact, as the graph is logarithmic and already on a semi-logarithmic scale, one can see that the significance of the noise becomes much more relevant as the power gets lower.

Knowledge is the key.  The specs that Ophir writes down are not magic numbers.  They are in fact extra-cautious, so it is certainly “permissible” to cross these thresholds.  With the above information and an understanding of your application requirements, you should be able to make an educated decision about where to draw your own line.

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