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Power and Energy Meters Application Notes

Differences among measurement instruments
Measuring power or energy can be done in many different ways and to obtain either basic or complex information. In the majority of applications it is done by the commonly known power/energy meters, by laser probes or OEM sensors. In each case the finality may the same (e.g. knowing the laser power) but real, final needs may be very different, as well as their consequent economical aspects.
A power/energy meter is made of a detector linked to a monitor via a special connector, storing all relevant data concerning the head. The monitor can work as a stand alone unit displaying the power/energy values and other information, like statistics, or be connected to computers or chart recorders. Some monitors, like LaserPoint's DUO, can read and display from two different heads at the same time, thus so expandening the instrument application range.
Power/energy meters are high end instruments whose performances may vary with the number of features or complexity of the overall setup. Prices will follow accordingly . They are measurement instruments, the which thing means that they have to undergo to classes of precision, must have tight values of repeatability, need to be periodically re-calibrated (at least once a year) to trace their performance.
For powers exceeding 100W the heads need an internal cooling system which could be a fan extracting heat from dissipating fins or water running through channels carved into the head. This is because power/energy meters can work continuously for long periods of time facing the laser beams, even many KWs, without problems. Their reliability and capacity to work indefinitely at the maximum powers or energies makes them the ideal instrument for applications like:
-Measurement of laser or laser system stability
-Alignment of laser cavity
-Alignment of optical setups
-Measurement of losses
-Derive statistical information on laser behavior
-Measure the power/energy ratio between 2 optical paths
Doing all this is only possible by the association of a properly cooled detector with a complex electronics which reads the row data and elaborates/displays the measurements. Once the measurement session is over, the equipment can be easily removed and used elsewhere, with another laser or system.

This is not so with OEM detectors. OEM detectors use the same sensors of power/energy meters embedded in simplified housings; they respond to the need of laser manufactures (in the majority of cases) to have a continuous monitoring of their sources and are permanently fixed into the laser head or the laser machine. But we are now talking of monitoring and not fine measurement: the associated electronics is in general very simple and limited to convert the sensor output into a signal manageable by the laser control unit. The power/energy value is displayed, maybe after basic signal treatment (e.g. the speed-up of response time) but no significant elaboration is made. Measurements are accurate and repeatable but the display of information is used to preset the power/energy levels necessary for an application , verify that set levels remain as fixed and no problems occur on the machine.

Laser Probes only read power. They make a measurement in a fixed time period and are used to make time-to-time controls at the source exit, at the working area or before/after an optical component to check its losses.
Probes simply tell which is the power level at the time of measurement, like a photo camera provides the picture of a shot taken at a certain instant; on the contrary power/energy meters can be imagined as movie or video cameras that show a real time and continuous evolution.
For the above reasons probes cannot be used for alignments, monitoring of stability or provide statistical information which all require a continuous use .
They are low cost devices and, until the introduction of LaserPoint's Cronos and FIT series with their patented measurement and acquisition techniques, the market was only offering poorly repeatable and precise instruments.
FIT and Cronos have been the real breakthrough in this kind of measurement with the reduction of reading time (lowered from 20 sec or more to 4sec and 8sec) , the possibility of making several measurements without cooling and the complete automatism of measurement which makes them independent from errors generated by wrong timings.
All these innovations have now brought those probes to have performances comparable to power meters (e.g. the FIT instruments have the same capacity of a power meter to repeat measurements with a 1% error bar).


Examples of Power/Energy Meters applications

Measurement of long term stability
Long term verification of stability , necessary for example for the final validation of a laser source, is made by directly placing a powermeter head , eg associated to the PLUS monitor, in front of the laser. The main screen of PLUSSOFT shows the long term behavior of up to 12 hours. Plus and PlusSoft also have the possibility to set hi and low alarm thresholds .

Alignment of laser cavity
Alignments require continuous tweaking of cavity mirrors: during these actions values change very quickly and understanding whether tuning is improving or not need a proper tool. Digital displays, with their fast running numbers are very un-easy; on the contrary, analog displays with their moving needle require a lot of attention to catch the value. Bargraphs are of great help to provide an immediate feeling of the direction of alignment, like in LaserPoint PLUS:, to have a fast perception of tuning direction at the same time mantaining the possibility to read the reached value.

Alignment of optical setups
Optical losses can be introduced by a number of reasons, like the non correct angular positioning of elements or by damaged components; more often a sensitive decrease of throughput can be reached by misalignments or simply by summing individual losses of optical components. A power/energy meter positioned along the beam path, permitting correct alignments and verification of losses, optimizes yield and efficiency of the optical system.

Measurement of losses
Verification of reflectivity, transmission and losses of beamsplitters, filters and other optical elements can be done by inserting a power/energy meter in front or behind the component. Dynamic changes, like modification of behavior or performances under the action of temperature variations, can be also monitored.

Simultaneous measurement on different optical chains
Some optical systems are complex and made with more than a single arm. Alignments, monitoring, equalization or keeping the power/energy between the arms at the necessary ratio can be very time consuming. The use of an instrument capable to work with 2 heads contemporarily, like LaserPoint DUO will save a lot of time. Additional features like calculating and showing the RATIO between the measured quantities will help further. The same dual heads configuration can be used for faster, cheaper in-house quality controls of 2 lasers, at the same time.

Statistical measurements of laser or laser machine behaviour
Statistical measurements are used for monitoring quality aspects of a source, a laser machine or setup. Having for references heads bearing a NIST (National Institute for Standard and Technology) and PTB (Physikalisch-Technische Bundesanstalt) traceable calibration , data can be used for internal files, for quality reports, can be added to manuals or given to customers. All relevant statistical information as Current Value, Min &Max Values, Average, RMS Stability, Standard Deviation are supplied by the PLUSoft on measurement periods as long as required.For easiest handling, all statistical data can be transferred and saved on files or printed

 



 

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