RM9-PD Femtowatt Measurement

First, clean the absorber surface with a tissue, using Umicore #2 Substrate Cleaner, acetone or methanol. Then dry the surface with another tissue. Please note that a few absorbers (Pyro-BB, 10K-W, 15K-W, 16K-W and 30K-W) cannot be cleaned with this method. Instead, simply blow off the dust with clean air or nitrogen. Don't touch these absorbers. Also, HE sensors (such as the 30(150)A-HE-17) should not be cleaned with acetone.
 
Note: These suggestions are made without guarantee. The cleaning process may result in scratching or staining of the surface in some cases and may also change the calibration.

It is fully compatible with these meters/interfaces:

• Vega / Nova II (firmware vs. 2.44 or higher)
• Juno (1.31 or higher)
• StarLite (1.26 or higher)
• StarBright (1.18 or higher)
• Centauri
• Juno+
• EA-1

It is partially compatible with Ophir’s other meters (Nova, LaserStar, Pulsar, and Quasar). It will function properly with these devices, except with an upper power limit of ~1 mW instead of 100 mW and with reduced accuracy, see specs for more details.

The RM9 is only sensitive to signals chopped at 18 Hz, so placing the chopper as close to the laser source as possible will minimize stray light entering the chopper and being read as part of the signal.
The noise specification is based on a 10 second moving average. Set the power meter to average the measurements for optimal performance.
It is also recommended to zero the sensor before use. This is done by disconnecting the BNC cable between the RM9 sensor and the chopper or turning off the chopper. Then follow the regular instructions for zeroing that can found in your power meter or PC interface manual.

Yes, but keep in mind that the RM9 will measure average power, not energy. Also, pulse rates below ~50 Hz may generate additional noise. Pulse rates close to 18 Hz may cause beat frequency issues.

Yes, but it must be set to a chopping frequency of 18 Hz.
If your chopper has high emissivity (black) surfaces, it should be located as far from the sensor as possible, at least 200 to 300 mm.
If your chopper has low emissivity (bare metal) surfaces, care should be taken to ensure that when it blocks the laser beam it does not generate stray reflections that can reach the sensor

If your source happens to be pulsed at 18 Hz, you cannot use the chopper, since this will generate very low frequency beat signals. However, it might be possible to use the RM9 directly with your laser source, as long as you can connect a BNC sync to the RM9 sensor. Contact us about your particular application to be sure this is the right solution for you.

Unless otherwise indicated, Ophir sensors and meters should be recalibrated within 18 months after initial purchase, and then once a year after that.

In theory, if a beam is completely parallel and fits within the aperture of a sensor, then it should make no difference at all what the distance is; it will be the same number of photons (ignoring absorption by the air, which is negligible except in the UV below 250nm). If, nevertheless, you do see such a distance dependence, there could be one of the following effects happening:

• If you are using a thermal type power sensor, you might actually be measuring heat from the laser itself; when very close to the laser, the thermal sensor might be “feeling” the laser’s own heat. That would not, however, continue to have an effect at more than a few cm distance unless the light source is weak and the heat source is strong.
• Beam geometry – The beam may not be parallel and may be diverging. Often, the lower intensity wings of the beam have greater divergence rate than the main portion of the beam. These may be missing the sensor's aperture as the distance increases. To check that you'd need to use a profiler, or perhaps a BeamTrack PPS (Power/Position/Size) sensor.
• If you are measuring pulse energies with a diffuser-based pyroelectric sensor: Some users find that when they start with the sensor right up close to the laser and move it away, the readings drop sharply (typically by some 6%) over the first few cm. This is likely caused by multiple reflections between the diffuser and the laser device, which at the closest distance might be causing an incorrectly high reading. You should back off from the source by at least some 5cm, more if the beam is not too divergent.

Needless to say, it’s also important to be sure to have a steady setup; a sensor held by hand could easily be moved around involuntarily, which could cause partial or complete missing of the sensor’s aperture at increasing distance, particularly for an invisible beam.

Typical choppers have the same type of surface on both faces: either metallic, low emissivity or black, high emissivity.
When blocking the laser beam, metallic surfaces will reflect or scatter a significant portion of the laser power which may result in stray reflections reaching the sensor.
Black surfaces solve this issue, but if the chopper is positioned close to the RM9 or RM9-THz sensor, they will pick up a thermal signal from the chopper blades.
Stray reflections and thermal signal from the blades can impair measurement accuracy.
Our chopper enjoys the best of both worlds. It has a black surface that should face the laser beam and a low emissivity surface that should face the sensor.