Meeting Measurement Challenges For Low-Power, Pulsed, Or ...

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Recent developments in power meter technology have now enabled decent temporal signalmeasurements, eliminating the need for multiple instruments. One of the most advanced power metersin the market has been designed by Newport Corporation, Model 2936-C. This state-of-the-art, dualchanneloptical power meter has several sophisticated features that allow the user to achieve variousmeasurements, including frequency measurements of pulses up to 250 kHz (see figure 1).Figure 1: Newport Corporation’s optical power meter Model 2936-C with the 918D Series Photodiodes allowsvarious measurements for pulsed or modulated light input as well as continuous wave.Analog And Digital FilteringThe analog filter is an electronic, low-pass filter implemented on an optical power meter while thedigital filter averages the most recent 10, 100, 1,000, or 10,000 samples. The 2936-C optical powermeter has selectable 5 Hz, 1 kHz, 12.5 kHz, and 250 kHz low-pass filters. An optical power meter isessentially an amp meter with very low noise and high accuracy. Therefore, the proper filter settingwould make a huge impact on the signal response of the optical meter. For the best CW measurementor average power measurement of a modulated/pulsed light source, the heaviest filtering would beutilized, i.e., the 10,000 point digital filter with the 5 Hz analog filter. For a modulated or pulsed signal,an appropriate analog filter must be selected to obtain the best waveform, while the digital filter mustnot be used at all. Moreover, applying the digital and the analog filters on the pulsed or modulated inputsignal effectively averages out the signal.As seen in the 11/16/11 edition of the Photonics Online (www.photonicsonline.com) newsletter.
Statistics And Time PlotRobust statistical measurement capability, such as average, minimum, maximum, and standarddeviation is a standard feature in many optical power meters nowadays. With a photodiode detector,Newport’s 2936-C power meter can log a data point every 0.1 msec, up to 250,000 data samples perchannel, which can be easily stored on a USB thumb drive for further analysis using a spreadsheetprogram like Excel. In addition, the collected data can be viewed in a scope-like time plot directly fromthe meter display, as shown in figure 2. One can easily see whether the input signal is the expected formor if it is distorted due to overly heavy analog filtering, excessive noise, or any other external influences.Figure 2: The time plot of the data stored by the internal sample storage.Peak-To-Peak And RMS MeasurementWith the fast 250 kHz sampling rate of the 2936-C, modulated and pulsed signals can be measured in thePeak-to-Peak Mode. The power meter will determine the difference between the maximum and theminimum value within the given time window, which can be defined by the user. For differentmeasurement ranges of the power meter, there are trade-offs between bandwidth and noise level.Therefore, when using large bandwidth ranges, it is important that the signal being measured has largeenough power to ensure a good signal-to-noise ratio. The RMS (root mean square) measurement isanother measurement mode available in the 2936-C optical power meter.As seen in the 11/16/11 edition of the Photonics Online (www.photonicsonline.com) newsletter.
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Meeting Measurement Challenges For Low-Power, Pulsed, Or ...

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