Power Meters FAQ's

Photodiode Laser Power Sensors

Using Your Sensor

I am using a PD300 type sensor. I have a broadband light source, but only want to measure the power of a specific wavelength. Can I set my meter to that wavelength and get a measurement?

06/09/14

The PD300 series of photodiode-based sensors are calibrated with a full spectral curve using a scanning monochromator (plus a few laser "anchor points").

 

The wavelength ("Laser") setting tells the meter what wavelength is being used and hence what calibration factor to apply when a measurement is underway. It does not, however, physically limit the possibility of other wavelengths from entering. All light (within the sensor's specified range of course) entering the detector will be measured; the meter will apply the calibration factor meant for the selected wavelength, "thinking" that only that wavelength is present.

 

In other words, these sensors assume a monochromatic light source. Their relative spectral response is not flat and they are therefore not suited for broadband beams.

 

So, if you want to check one wavelength from a broadband source, you will need to use a wavelength filter that only passes that wavelength. Then you should set your meter to the appropriate wavelength to account for the detector's relative sensitivity.

Can I use a sensor from the PD300 family to measure average power of pulsed lasers?

06/03/15

In general yes, but several technical issues need to be kept in mind (most of which are results of the fast physical response time of these sensors):

  • The pulse rate should be more than about 30Hz, otherwise the reading is unstable. At higher pulse frequencies, the sensor will respond as if the beam were CW. 
  • It is possible for a pulsed beam to have average power within the sensor spec and yet have the energy of the pulses themselves be high enough to cause a momentary saturation of the sensor. It is important to be sure that pulse energy is also within sensor spec (the parameter "Max pulse energy" is included in all specs for the PD300 family, for just this reason).
  • The beam diameter should be no less than about 1mm .
  • The average power and power density restriction in the spec should not be exceeded

 

Note: At the maximum pulse energy limit given in the spec, the reading will be saturated by about 5%, i.e. the reading will be about 5% lower than it should be. At 1/3 the maximum, the saturation will be about 1%.

When measuring a fiber output, should I put the fiber tip right up against the PD300 detector? If not, how close should I come?

06/09/14

The PD300 sensors are not designed to measure with the fiber pushed up right against the detector surface. It may be reading lower in such a case due to saturation of the detector from the concentrated beam or higher due to back reflections off the detector and back again from the fiber tip. The optimal reading will be where the beam is expanded to a size of 2-5mm diameter. Therefore, you should back off the fiber to a distance where the beam has expanded somewhat. Do not back off too far, otherwise if the nominal beam size is larger than given above, you may lose some of the beam off the edges of the detector.

When I hook my PD300 photodiode sensor to a Vega I see 5 drop-down wavelengths? Is that all I can use, or is there a way to select the wavelength I need to use it at?

05/26/15

Yes, the 5 default wavelengths are the discrete wavelengths that we have actually calibrated that sensor to NIST-Traceable standards. We have also run a spectrophotometer curve for that sensor and fitted that curve to the 5 discrete wavelengths that you see in the drop down menu. To set it to a wavelength that one needs (within its spectral bandwidth, i.e. 350-1100 nm for the PD300 and 200-1100 nm for the PD300_UV) one highlights one of drop down wavelengths in the drop down using the Down arrow key, and then you use the Right arrow selector to get into the menu for changing it. Then one uses the Up and Down and Right Arrow keys to select the wavelength that one wants to use it at. When done press SAVE and then SAVE again. Now it's stored in the E-PROM of the Smart Sensor connector and available for use. One can repeat the above procedure to store any 5 wavelengths that they use most often.
See the Video on how to change the sensor wavelength: http://www.youtube.com/watch?v=1qWXouQP18U

Why is the PD300-1W specified for only up to 10W/cm² , while the regular PD300 can go up to 50W/cm?

05/26/15

The PD300-1W also has an extra ND filter glass, one with more attenuation. This sensor works at up to 1W, and at this higher power the extra ND glass gets hot; as a result, its absorption can change (becoming less absorbing) so it is not specified for use at above 10W/cm2. Not because it will damage there, but because it will not perform correctly there . 

(It is worth noting that the PD300-3W has a different type of extra filter based on an evaporated reticle. This filter does not change its absorption when it gets hot, so can be used at up to 100W/cm2.)

Can I use a PD300 photodiode sensor to measure power of a scanned beam?

05/26/15

For measuring power of a scanned beam we recommend using the BC20, and not the PD300. Since a scanned beam will spend only a fraction of the time of each scan on the detector, the average power measured by the detector will correspondingly be only a fraction of the actual power of the beam. The BC20 is specially designed for such applications by having a peak-hold circuit integrated in its electronics.

The sensor I need uses water cooling. Can you recommend a water cooling system?

09/06/15

We don’t supply chillers, nor insist on specific models; the only important thing from our point of view is to simply keep to the requirements specified for the cooling water of the specific model of sensor, such as minimum flow rate at full power, water temperature range, and - more important than the actual water temperature - water temperature stability. The temperature of the water should not be changing by more than 1 deg/min (because changes in water temperature could cause heat flow in the sensor which would be detected as if it were laser power, and cause errors in the reading).

We also have a video on our site at https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/knowledge-center/water-cooled-sensors-youtube, which discusses various issues and tips about water cooling. There is a short discussion of coolant pressure requirements in our FAQ section at https://www.ophiropt.com/laser--measurement/knowledge-center/faq/2404

Can I use a PD300 sensor inside a thermal chamber, in which the temperature cycles beyond the recommended operating temperature range?

09/06/15

It should be okay, as long as:

  • the wavelength is not near the long wavelength limit where the PD300 has a large temperature dependence;
  • there is no condensation on the window of the detector which could interfere with the beam and affect the reading.

We suggest the customer does an experiment with a stable laser source (such as a pointer laser) shining in through a window onto the detector while the unit is temperature cycled to see if the reading changes. The final measurement should be back at the original temperature so as to make sure the laser hasn’t changed.

Why do Photodiode-based power sensors like the PD300 have a specified “Max pulse energy”?

02/26/20

Although these sensors measure average power (of both CW and repetitively pulsed beams), not pulse energy, it is possible for a pulsed beam to have average power within the sensor’s rated limits and yet have the energy of the pulses themselves be high enough to cause a momentary saturation of the sensor. It is important to be sure that pulse energy is also within sensor spec – not just the average power. This is explained in detail in this White Paper.

In the datasheets of photodiode-based energy sensors like the PD10-C, the lowest measurable energy is given specifically for one wavelength – 900nm. How do I know the lowest measurable energy at MY wavelength?

08/18/21

Let’s look at an example. In the spec of the PD10-C, it gives the lowest measurable energy as 1nJ at wavelength 900nm. Let’s say the wavelength you work at is 633nm. Looking at the relative spectral sensitivity curve for this sensor (shown below), we see that the sensitivity at 633nm is only about 2/3 of what it is at 900nm (as seen in the attached spectral curve). That means that the lowest measurable energy at 633nm will be (1nJ / ⅔), or about 1.5nJ.