Key considerations in choosing an optical power meter
Optical power meters for testing fiber optic components use semiconductor photodiodes as detectors to generate electrical current proportional to the incident optical power. This photocurrent is then measured, typically with a transimpedance amplifier and analog-to-digital converter, to determine that power. That requires the conversion factor from mA current to mW power, which depends on the wavelength of the light and combines contributions from the properties of the detector as well as any optics used to collect the light. Calibration of the power meter thus involves tracably measuring and recording the wavelength dependent responsivity and including this data with the instrument.
This responsivity is one of the key considerations in choosing an optical power meter for a particular application. First, the instrument must be calibrated at the wavelength of the light to accurately determine the absolute power level. If only relative power change will be measured, as for determining the attenuation of a passive optical component, this calibration factor is not actually needed. However it is still necessary that the detector has sufficient responsivity for this wavelength. For measuring light that is distributed over a range of wavelength or for which the wavelength is not accurately known, it is
also important that the variation of responsivity over wavelength is not too large.
Example responsivity spectra for optical power meters based on three commonly used semiconductor materials are shown in Fig. 1. These are actual calibration data for individual instruments and the curves can vary somewhat from unit to unit, but the spectral shapes are primarily determined by the detector material. The values displayed on the y-axis correspond approximately to conversion efficiency in mA/mW. For wavelengths supported by standard single-mode fiber from about 1250 nm to 1650 nm, the InGaAs detector (like used here in the Keysight 81624B optical head) provides the highest performance with high responsivity and relatively low wavelength dependence. InGaAs (atually a shorthand label for the alloy chemical formula InxGa1-xAs) as a direct-gap semiconductor also typically provides the lowest noise level which permits power measurements over the widest dynamic range.
The germanium detector (81623B Figure 2) is useful over an even wider wavelength range and is less expensive, so these make good general purpose power meters. However the steep wavelength dependence above about 1545 nm makes the measurements more sensitivity to wavelength uncertainty or instability.
optical power meter, responsivity of sensivity to wavelength
For shorter wavelengths, including visible light, the silicon detector (81620B) provides good responsivity. This can be used for the 650 nm red light used with POF (plastic optical fiber), but as discussed in the following is also an attractive alternative to germanium for the widely used 850 nm wavelength range.
850 nm optical power measurement
Fiber links for transmission over short distances, like within buildings and data centers, predominantly use multimode fiber and signals at 850 nm. Another less common wavelength used with this fiber is 1300 nm. The wavelength here is a nominal value and the actual wavelength can be offset substantially. For example the IEEE 802.3 standard requires center wavelength to be between 840 nm and 860 nm. Other applications may tolerate wider wavelength variation. If the actual wavelength of such sources is not used to make the optical power measurement, this variation contributes to the measurement uncertainty. With this in consideration, the silicon detector has clear advantages. The responsivity is about five times stronger than for germanium, which itself is stronger than for the InGaAs detector. But more important for measuring moderate signal levels like 1 mW is the dependence on wavelength, as shown expanded for this wavelength range in Fig. 4.
The germanium has moderate dependence, but a 10 nm wavelength offset will still cause about 0.2 dB measurement error (4.7%), which is large compared to the ±4.0% uncertainty specification or the 81623B when the correct wavelength setting is used. The comparable error for the 81620B with the silicon detector is only 0.05 dB.
This low wavelength dependence can also be convenient if additional wavelengths are used in this region, such as the 4 wavelength channels between 850 nm and 940 nm defined for the SWDM grid.
On the other hand, if a multimode fiber test setup will be used for both 850 nm and 1300 nm wavelengths, then the germanium detector is the best choice since silicon is not useful at the longer wavelengths, where the photon energy is smaller than the semiconductor bandgap.
Finally when considering the requirements for accuracy specifications, the impact of other dependency besides wavelength should be considered. For measuring polarized light, like most laser signals, the polarization dependence can be a significant source of uncertainty because the polarization at the output of most optical fibers is not stable and changes with temperature and movement of the fiber. For measuring coherent light, again like laser signals, the impact of possible multiple reflections between the power meter optics and the fiber connector output leads to measurement instability, so such reflections should be minimized. This is characterized in the Keysight specifications as “spectral ripple” because the coherent interference will vary periodically with wavelength.
Additional functionality of the optical power meters
Keysight optical power meters do support a programming command to read out the wavelength responsivity calibration data (like used for the graphs in this document). This can be used for example in post-processing to get calibrated absolute power values without needing to change the wavelength setting of the power meter each time that the wavelength of the signal is changed. That can be especially helpful when the optical power meter logging function is used to record a series of samples, during which the wavelength setting cannot be changed. When used together with a tunable laser, this can provide the input power to a device under test, while the wavelength is swept. That is important for example to measure O/E conversion devices. It can also be used to normalize a reference measurement made on one optical power meter port for use as reference on other power meter ports connected to the device.
Besides simple optical power measurements, Keysight optical power meters provide higher functionality, especially including the logging function just mentioned and flexible internal and external triggering functions for synchronization with other instruments or the DUT itself. The optical head models mentioned above have memory for up to 20k samples with individual averaging times selectable between 100 µs and 10 s duration. Other models support logging of up to 1M samples and averaging times down to 1 µs. The optical heads also provide an analog output signal with a voltage proportional to the input optical power. Especially combined with the large 5 mm diameter detector area, this supports various automated alignment procedures. The heads can be used to measure open beams and have a selection of fiber connector adapters. As external heads connected to the mainframe with a cable, these can be located conveniently on optical tables or workbenches.