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New optical power meter instrument offers 100,000-readings/s

Posted: 24 Nov 2005 ?? ?Print Version ?Bookmark and Share

Keywords:optical test equipment? Model 4100? Optical Power Meter? dBm Optics?

As Internet paths get faster, optical test equipment is increasingly called upon to measure both passive and active lightwave components. The Model 4100 optical power meter instrument from dBm Optics Inc. provides both speed and repeatability when measuring fiber optic systems. User can also view the acquired power meter readings numerically or in graphical or tabular formats.

The multi-channel Model 4100 can be cost-effective. As a modular DSP-equipped instrument, users can buy only what they need. The box can also be used on your bench, or installed in a rack in an automated system.

Base price is about $9,000, but may range as high as $20,000, depending on options. This includes adjuncts such as a precision power meter module, a DC voltage measurement module and single-channel or 8-channel photodiode measurement modules.

In terms of performance, the TFT LCD-equipped 4100 breaks new ground, especially when compared to competing instruments, many of which take about 20ms or so to change range. Also, many of them use logarithmic amplifiers, but that approach usually compromises low-level measurement accuracy and linearity. Other techniques compromise speed. Not so the 4100.

The electrometer technique

dBm Optics's approach uses an electrometer to measure low power (about 200fA or -95dBm) at high speed with little drift. The electrometer technique compensates for the capacitance of the photodiode and provides low equivalent input resistance. That means the measurement is less affected by noise and users get a faster response.

All of this is especially significant when testing deep-well devices where the goal is to test at narrowly spaced optical wavelengths using a continuously sweeping tunable laser. Measurements include the nature of the device's passband and structure of its stop band.

If users use the log-amp approach, wide-ranging signals will be compressed with respect to dynamic range. These systems also impose undesirable non-linearity over time and temperature, and noise. As such, some vendors use slow sweeping techniques, and slow measurement with linear ranges, auto-ranging a measurement for each point.

Still other measurement systems run multiple sweeps, one on each of two or three different ranges, then they stitch them together. This is okay but it requires more time to run the additional sweeps (and time for the laser to return to its home wavelength). This so-called stitched approach is also susceptible to non-linearity between ranges.

In contrast, the 4100's dynamic range of greater than 65dB at full speed usually cases eliminates the need to change range. Operating at 100,000-readings/s (most power meters drop to 50-readings/s to change ranges), the Model 4100 auto-ranges across three ranges, spanning that 65dB at full speed.

Another feature of the Model 4100 is its low-level capability. One of the limits to making low-level measurements is the dark-current of the system's internal photodetector.

This product uses a reduced dark-current detector and special temperature control circuitry; the photodiode is run at -200C to lower dark-current. The cooling subsystem is also driven at high current, supporting rapid stabilization. That lets the 4100 adapt to environmental changes without transient errors. Warm-up time, to full specs, is one hour.

- Alex Mendelsohn

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