Power and Energy Meters: From Sensors to PC

By Ilan Haber, Ophir Optronics Ltd

From the time of its invention, more than 30 years ago, the laser power meter was generally comprised of two parts: a measurement head and a display box. It was always considered better to have such an arrangement with a cable connecting the two because of the hazardous nature of the laser beam. As the display of the results is separated from the measurement head, so are the eyes of the operator separated from the laser beam.

Now that the PC is an indispensable part of the office and the laboratory, it is important to be able to integrate measurement instruments to the PC, particularly instruments that can gather large volumes of data. There is a need for a unified connectivity architecture wherein all measurement heads are compatible with all display boxes and are then easily connected to the PC.

PC Connectivity Options for Power/Energy Measurement

The connection of the Laser Power and Energy sensor with a PC results in a powerful system. It combines the convenience, value, and flexibility of computer-based operation with the choice of a wide array of NIST-traceable, calibrated thermal, pyroelectric, and semiconductor sensors, providing accurate laser, diode, and fiber-optic power measurements from 10 pW to 10 kW. The software application packages make single or multi-channel power monitoring applications easy, with a complete set of statistical analysis, graphics display, and data storage functions.
 
Components for system integrators and LabVIEW solutions are offered to those that want to tailor-make a solution that best integrates into their test and measurement system. Software is available that can turn the PC into a full-blown laser test station.
 
Utilizing smart head technology, the PC sensor interface supports the complete line of thermopile, photodiode, and pyroelectric smart head sensors.
 
Some PC sensor interfaces can be expanded to handle 1, 2, or 4 sensors. These collect data from all sensors simultaneously and transmit to the PC via USB. Some interfaces can also measure pulses at up to 25 KHz and detect missing pulses. Applications include:

• Peak-to-peak stability of energy pulses
• Power drift of CW lasers over time
• Graphical display of dB loss measured in a fiber optic cable
• Logging energy of rapidly pulsing lasers at 1000 Hz or more

In addition to a standard USB cable connection to the PC, there is a new way to connect to
the PC -- with a Bluetooth wireless interface. Just plug any smart sensor into the battery operated wireless interface and it broadcasts to any PC or laptop within 10 meters. No clutter on the workbench and no long cables to worry about. This is a great solution for hard to measure situations, such as a glove box or a high power laser behind barriers. Some extended range versions reach up to 60 meters. The Bluetooth wireless interface is also available mounted directly on the measuring head for maximum portability with no wires.
 
One of the inconveniences of measuring laser power and energy is related to the cables connecting the display to the sensor. The cables are of limited flexibility and clutter the workspace where the measurement is to be done. Sometimes, due to their stiffness, movement of the cable disrupts the sensors and misaligns the set up.
 
Cables are also a problem in areas which are supposed to be sealed, like a glove box or a vacuum chamber. In these cases, the introduction of the cable in the measurement location requires special means to maintain the integrity of the chamber or the box. This might require inclusion of connectors on the wall of the chamber or box, or special fittings to allow for the passage of the cable.
 
Cable length is determined during manufacturing and different set ups call for different cable lengths, a fact which makes the sensor more expensive since it has to be custom-made and custom-calibrated.

The Bluetooth wireless interface solution solves these problems and has been approved by the FCC and other regulating authorities around the world. The PC can run dedicated software supplied with the Bluetooth wireless interface.
 
The Bluetooth wireless interface is powered by a rechargeable battery which powers it and the sensor. Battery life is typically 20 hours with a pyroelectric head and 40 hours with a thermal sensor.

PC Software Options for Power/Energy Measurement

Standard software for power measurement applications is a Windows-based application. The software includes complete data logging, statistics, and trending analysis. Additional features should include:

• Offset selection
• Selection of measurement wavelength
• Extensive logging options
• Unlimited data storage text files for graphical analysis; can also be imported into Microsoft Excel
• Selectable measurement limits
• Displayed power units in watts, dB or dBm

The software can simultaneously perform statistical and trend analysis for power/energy
stability and drift for each channel. Measurements can be recorded and analyzed over a selected period of time for display on a histogram plot for each channel.

The software data logging functions collect user-specified power at selected intervals and durations. The data log files contain all test information (data, time, detector, and settings), and are formatted for easy spreadsheet importing and analysis. Data can also be recorded in real-time mode and viewed at a later date to reproduce the displayed results,

System Integrator Solutions

Besides their use as stand-alone, full-featured laser power/energy meters, the abovementioned devices are easily incorporated into larger end-user applications. This allows system integrators to leverage commercial laser power and energy measurement hardware with legacy analysis packages.
 
These devices can support various communication protocols including RS232, USB, and GPIB. RS232 communication is the simplest to integrate into your OEM application. Integrated Development Environments (IDE’s) such as Microsoft Visual Studio provide functions and methods for accessing the PC’s com port.

Automation Tools: Labview drivers
 
The laser community has long recognized the growing LabVIEW community of developers. For over 10 years, vendors have been providing LabVIEW libraries for all of Ophir’s laser measurement devices. These are full, open-source applications that can be used as is or tailored by the LabVIEW programmer to his specific needs.
 
Many are simple starter applications that allow the LabVIEW programmer to experiment freely to fully feel the strength of our devices’ respective command sets.
 
These applications contain VI’s (Virtual Instruments) to control the instrument. Users can combine VI’s to create successively larger and more versatile larger VI’s by simply connecting them together. Users can create sophisticated, custom applications in minutes.
 
Now that the PC is an indispensable tool in the laboratory and engineering/design, it is important to be able to integrate laser measurement instruments with PCs. A wide variety of connectivity options and software tools have expanded the capabilities and flexibility of today’s laser power meters.