Power Meters FAQ's

Laser Power/Energy Sensors

Choosing a Sensor

05/26/15

The biggest issue is picking out the appropriate sensor that will measure the laser light (most will measure other types of broadband light as well). Since it is not trivial to find the best sensor for a given application, we recommend using our Sensor Finder available on our website that automatically calculates the best sensor for the measurement conditions you input. However, let us list below the main criteria:

§  First of all, do you want to measure average power or pulse energy? If power, then choose a thermal or photodiode sensor. If energy, then if the repetition rate is less than one pulse every 5s, then you can use a thermal or pyroelectric sensor. If faster than that, then a pyroelectric.

§  After you have chosen the type of sensor, then look at the dynamic range and choose a sensor that will be able to measure the highest and lowest power/energy you want to measure.

§  Check that the sensor covers the wavelength region you want to measure.

§  Check that the aperture of the sensor is large enough for the beam size.

§  Now check the damage threshold. One needs to accurately know how much power or energy density one has in order to select a sensor that will not be damaged. To do that one needs to know the beam spot size, and what the energy distribution is, since for example a Gaussian beam has much higher density in the peak of the beam than a flat top or other modal beam. If the laser is pulsed, one also has to know the pulse length in time as most sensors have a different damage threshold value based on peak power; a shorter pulse with the same energy per pulse will give a much high peak power and will more readily damage the sensor. If one cannot seem to find a sensor that will not damage, then one needs to look attenuation options including beam splitters, diffusers, ND filters and possibly measuring just the leakage through a mirror.

In the above analysis there are some trade-offs.

§  Dynamic range that one needs to measure. Typically one can get roughly 3 ½ decades of range from a single sensor. ND filters and/ or other attenuation options can extend that by any number of decades. Of course with each attenuation option the uncertainty of the measurement increase, so the trade-off here would be dynamic range with a single sensor and extending with attenuators but having to sacrifice uncertainty, therefore accuracy. One may find that it may be better to have 2 sensors, one more sensitive and one able to measure higher power or energy.

§  Multiple lasers. Many end-users try to measure as many lasers as they can with as few sensors as they can in order to reduce their cost. To do this there may be some trade-offs. Maybe one won't be able to cover their full dynamic range of each laser measurement, or they have to sacrifice accuracy to cover their full range. In some cases they'll just have to get more sensors to cover all their lasers. The trade-off here is cost.

§  . Physical size of the sensors. Most end-users usually want the smallest physical size sensor possible, or maybe a large aperture with large sensing area, but the housing small. There may be some trade-offs there. If it's high power, there may an issue in cooling the sensor; either through convection, conduction, forced-air, or water. Each comes with its own trade-off. If attenuators are used, the space they require may be an issue.

A more detailed guide to selecting the optimal sensor for a given application can be found in our online Tutorials at https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/tutorial/properly-select and https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/tutorial/select-sensors. And again, use the sensor finder to point you in the right direction. 

05/26/15

One has to be careful about this since many vendors quote the damage threshold at a power much lower than the maximum power of the particular sensor being advertized. As an extreme case, a meter that can measure up to 1000W, may be quoted a certain damage threshold and in a footnote it may be noted that this damage threshold is for 10W. The damage threshold can go down dramatically with power and the damage threshold at 1000W may be 4 or 5 times less than at very low powers. Ophir always quotes damage threshold for a particular sensor at the highest power the sensor can measure.

05/26/15

Very often, a laser spec gives a range of values for various parameters – for example, pulse width might be specified as "Up to 30 msec", or pulse frequency might be given as ""10 – 200 Hz". If we enter the highest numbers for all the parameters into the Sensor Finder – in this example, 30msec pulse width and 200Hz frequency – then clearly the math may not line up, since these 2 values do not match each other. In such a case, it is important to check what the actual "worst case combination" (or perhaps combinations) of conditions of measurement will be.

General Specifications

09/05/16

We have had customers in the past who measured electron beams with our thermal sensors (with the BB absorber), and it seems the absorber absorbs close to 100% of the radiation. So it can be done. Presumably this is in a vacuum, so that has to be taken into consideration.

01/18/17

Each sensor from Ophir includes a performance specification datasheet but not a user manual. This datasheet lists operational parameters for the sensor, including items such as spectral range, power range, aperture size, accuracy specification, etc. Copies of these datasheets for current sensors are available on the website by clicking on the “Specifications” tab of the sensor's product page.
 
As for the user manual, a hard copy of the user manual is included with every meter sold. The manual also has a section explaining the operation of the various types of sensors; thermopile, photodiode, and pyroelectric. Copies of the meter user manuals are available on the web site at: https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/services/manuals.
 
If for any reason you are not able to find the particular information needed or are not able to retrieve the manuals or information from the website, just send us an email request to service@us.ophiropt.com and we’ll get the information for you.
 

06/09/14

If the power is P and the diameter of the beam is D then the power density is P /(.785 * D2) . If it is a pulsed laser and the energy is E, the repetition rate is R and the diameter is D then the power density is E*R/(.785 * D2), The energy density is E/(.785 * D2). The sensor finder will automatically calculate the power and energy density.

05/26/15

Generally, our sensors are calibrated (traceable to NIST) to within ±3% accuracy 2 sigma which means that 95% of the sensors are accurate within ±3%. However, if your application requires very high accuracy, we also offer something called “double calibration” which can bring the error down to ±2%.

05/26/15

The damage threshold refers to the power or energy density at any point in the beam. So if we have a gausssian beam, the damage threshold refers to the power in the center of the beam. For a top hat beam, the damage threshold will be at any point in the beam. The sensor finder (add link) allows you to choose between a top hat and gausssian beam when calculating the damage threshold.
Some beams are not smoothly top hat or Gaussian and may have hot spots. Furthermore the damage threshold is not always an exact level. So the user is recommended to choose a sensor that does not exceed more than 50% of the specified damage threshold.

05/26/15

This depends on whether you are using a thermal sensor or a photodiode sensor. With our most sensitive thermal sensor, model RM9, one can measure down to about 500nW. With our photodiode sensor heads we have a several types, silicon, InGaAs and Germanium. Each has a spec on minimum power, which can be as low as 10pW

05/26/15

In order to get a meaningful power reading on a power meter, the signal being measured must be considerably larger than the noise. In fact there is a measure of this, the signal to noise ratio or SNR that is often used. If the SNR is 1 then the noise is the same size as the signal and this signal is barely distinguishable. There are some that will quote this number as the minimum measurable, but most agree that the signal must be considerably larger than the noise to be measurable. One criterion used by many is 10:1 SNR as the minimum useable measurement. However, when the noise referred to is 1 standard deviation or 1 Sigma, then a certain amount of the time the noise is 2 times this or even 3 times so 10:1 SNR 1 Sigma is still not a very precise measurement. For these above reasons, Ophir has taken a particularly strict definition of minimum measurable power and that is 20 times the 3 Sigma noise value. This is 6 times as strict as the usual 10:1 1 Sigma value often given or implied. The Ophir value means that most of the time, the noise does not exceed more than 2% of the signal.

05/26/15

For pyroelectric energy sensor heads there is no limit on how short the pulse is, as they are integrating devices. As long as one does not exceed the damage threshold expressed in terms of energy density then they will accurately integrate pulses as short as femtoseconds. With thermopile sensors they similarly can be used as integrating devices to measure energy, although one can only measure single pulses every few seconds as they have a much slower response time than pyroelectrics. With repetitive short pulses one can measure the average power with a thermopile with no restriction on how short the pulses are, as long as the maximum energy density is not exceeded. The spec for damage threshold varies on type of absorbing surface of each sensor head type. Consult our damage threshold charts or use the Sensor Finder for detailed information.

03/06/16

It depends on the sensor and on the range the sensor is set to for the measurement. The easiest way to figure out the resolution is to look at the display. For example, a 30 W sensor has three places after the decimal in the 5 W range and two in the 30 W range. So the resolution is 1 mW for the 5 W range and 10 mW in the 30 W range.

Calibration

05/26/15

The Power Accuracy of +/-3% refers to the absolute uncertainty of the measured value. For example, for a 2 Watt reading, the actual "true" value would be between 1.94 W to 2.06 W (with reference to NIST, to which all our calibration is traceable). This assumes the reading is from about 5% of full scale up to full scale. It should be noted that our accuracy specification is in general based on 2 sigma standard deviation.

Repeatability of the measurement (assuming the laser itself is perfectly stable) is limited in the best case by the power noise level of the sensor, and is typically better than  +/- 1%  depending on the thermal stability of the environment. Stability at higher powers from the middle to the top of the range of the sensor head is usually better than the low end. This is due to small temperature variations having less of an effect as they are proportionally a lower percentage of the total power. For more information, refer to our Web tutorial at: https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/tutorial/calibration-procedure

05/26/15

With normal usage we recommend calibrating every 12 months. To accommodate shelf time and shipping time new manufactured product comes with a calibration sticker that shows a recalibration period of 18 months from manufacturing. However this does not negate the recommended 12 month recalibration interval should you receive the product with more than 12 months remaining on the new manufactured calibration sticker.

05/26/15

If the calibrated wavelength is W1 and I want to measure at wavelength W2 then I look at the relative sensitivities on the curve at W1 and W2 and calculate as follows: Sensitivity at W1: s1 Sensitivity at W2: s2 When instrument is set to W1 and I measure W2, then multiply reading at W2 by s1/s2 to get correct reading at W2.

05/26/15

All absorbers used in power/energy measurement are not entirely flat spectrally, that is, they vary in absorption with wavelength. For this reason, Ophir measuring sensors are usually calibrated at more than one wavelength. If the absorption changes only slightly with wavelength, then we define wavelength regions such as <800nm, >800nm and give a calibration within these regions. In that case, the error in measurement between the wavelength the device was calibrated for and the measurement wavelength is assumed to be within the primary wavelength calibration error.

05/26/15

An explanation of how we can accurately calibrate at a small fraction of the maximum power is given in our catalog introduction and on our website. In addition, in order to be sure of the calibration at higher powers, we have to know if the linearity of our sensors is within specification. For this purpose we have a 1500W sensor calibrated at various powers at a standards lab. Using a beam splitter and a 15,000 Watt laser we periodically check the linearity or our highest power sensors against this secondary standard.

Using Your Sensor

09/06/15

This depends on what information you are trying to discover. If your laser is CW, then you will measure the output in Watts (power). To do this you can use a thermopile sensor (for medium and high power) or photodiode sensor (for low power).
 
If you have a pulsed source you can measure the average power using a thermopile sensor. If you would like to measure energy in individual pulses, then a pyroelectric sensor is required. We have pyroelectric sensors that can measure the energy in individual pulses up to 25 kHz pulse repetition rate.

09/05/16

The basic physics of light tells us that – unless we are dealing with some exotic effect of absorption by the air, such as with extreme UV – a parallel beam contains the same amount of power at long distance as at short distance.

Assuming that the beam really is in fact parallel…The sensor may be measuring something other than just the optical power in the beam. At a close distance, it might be measuring heat coming out of the laser diode as well as light. These laser diodes can produce significant heat; this would explain the decrease in measured power with increasing distance. You can check this by putting a window between the laser diode and the sensor to block the heat (briefly) and seeing if the same effect happens (of course the window will introduce some reflective losses, but this is just to confirm what is happening). It might be a good idea to put the window at a slight angle, so any reflected power won’t go back into the laser diode where it could cause damage.

06/09/14

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 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.

05/26/15

There is a rule of thumb about this for sensors made of aluminum. You can use the sensor without cooling for about 1 minute/watt/cm3 of sensor. So if a sensor has a volume of 300cm3 and you put in 100W, you can use it for 3 minutes. The sensor finder program https://www.ophiropt.com/laser-measurement-instruments/laser-power-energ... has an option for intermittent use of an Ophir sensor and will automatically calculate this for you.

05/26/15

The largest problem we see from equipment that is not working at top performance is contamination on the sensor. Dust or other contamination on the sensor surface can greatly impact the readings the sensor provides. When dust or other contaminations are on the sensor when it is illuminated by laser power/energy it can become "burned onto" the sensor. Simply blowing this contamination off before using the sensor can greatly reduce these problems and make the equipment perform at top performance for a longer period of time. Using canned air or dry nitrogen from a distance of 6 inches or more to lightly blow off the sensors can remove most of the contamination. Turn the sensor upside down so the surface the laser hits on is pointing to the floor. Start a light flow of air while pointed away from the sensor and lightly sweep it across the sensor without increasing the flow. This will lift most of the dust or other contamination from the sensor surface and gravity will continue to pull it to the floor.

06/03/15

Ophir meters and sensors are calibrated independently. Each meter has the same sensitivity as the other within about 2 tenths of a percent. Each sensor is calibrated independently of a particular meter with its calibration information contained in the DB15 plug. When the sensor is connected to the meter, the meter reads and interprets this information. Since the accuracy of our sensors is typically +/-3%, the extra 0.2% error that could come from plugging into a different meter is negligible and therefore it does not matter which calibrated meter we use with a particular calibrated sensor.

Other

06/09/14

All Ophir power meters, including photodiode power meters, have an air gap between the fiber tip and the sensor. Therefore they measure the power emitted by the fiber into the air and do not take into account any reflection losses there are in the fiber. Therefore, if in actual use, the fiber will be coupled with no loss to another element, then the losses should be added to the reading. These losses are usually about 4%. Thus if the reading on the Ophir meter is say 100mW, then in lossless use, the real power will be 104mW.

05/08/17

The spec was designed around the way such lasers are typically used. Since these lasers are normally used with focusing optics, the spec of the 120K-W does not give a maximum power density; rather, it defines the assumed focusing-lens focal length and position such that the beam will end up having a 100mm diameter at the cone, and defines the assumption of a near Gaussian beam under those conditions so we can define a baseline number. This is defined briefly in the spec, and in a bit more detail in the User Note that comes with the sensor.