There are many applications involving measuring power out of a fiber, where the power is in the range of tens of uW to a few mW. A typical measurement solution will use a 3A sensor with a suitable fiber adapter.

We recently came across an interesting customer problem, in which every time he disconnected the Fiber Optics connector from the adapter (that is mounted on the sensor) and then reconnected it, the power read about 50-100 uW higher than it did (nothing else changed). It then took about 10 minutes to slowly come back down to what it had been. After an investigation, we found that the increase in reading when disconnecting/reconnecting the fiber connector is a thermal effect, and not a technical flaw in the unit. If you experience something like this, the most likely direct cause is one of the following 2 possibilities:


  • As the Fiber Optics connector is disconnected from the adapter (that is mounted on the sensor), and the connector is moved away from the sensor, for a brief moment the beam is incident on the body of the adapter itself; that adapter is black metal, and the heat created in it by absorbing a beam of 1mW for a second or two might be very small – but it could, possibly, be enough to be detected by the sensor. It also seems reasonable that it would take quite some time for this heat to dissipate and the reading to come back down to the correct level, since there is very little thermal contact area between the adapter and the sensor body.
  • It could also be that the heat from the hand unscrewing the connector could have the same effect as above. (As an experiment, you can try touching the body of the 3A sensor itself – the reading will go up, and then down again. That will happen much faster than the above problem, since a touch right to the sensor’s body will cause a much more immediate heat flow than touching the adapter out at the end of the tube).


The best way to solve/avoid this problem is to try disconnecting/ reconnecting the fiber (when you need to do so) at some location than the fiber adapter on the sensor (either at the laser end, or any other connections along the way between the laser and the sensor if there are any). That way there is no heating of anything near the sensor (and no artifacts that could be caused by turning the laser itself off and on).