Optical power meter

An optical power meter pairs a detector head with electronics that convert its signal into watts or dBm, where $P_\text{dBm} = 10\log_{10}(P/1\ \text{mW})$. It is the most-used instrument on an optics bench: it measures laser output for LIV curves, insertion loss of components, coupling efficiency during alignment, and link power in the field.

Two families of head cover different ranges. Photodiode heads, silicon for roughly 400 to 1100 nm and InGaAs or germanium for the near infrared, measure from picowatts to tens of milliwatts, or higher behind a calibrated attenuator, and respond quickly. Thermal heads absorb the light on a blackened surface and measure the resulting temperature rise with a thermopile; they are nearly flat across wavelength and handle watts to kilowatts, but respond in seconds and have a noise floor in the microwatt range. Pyroelectric heads measure pulse energy rather than average power.

A photodiode head measures current, and converting current to power requires the responsivity at the light's wavelength, which is why every photodiode meter has a wavelength setting. Responsivity rises in proportion to wavelength at constant quantum efficiency, so an InGaAs head set to 1310 nm but measuring light at 1550 nm reads about 0.73 dB high, by the ratio 1550/1310; the error is larger near the edges of the detector's range, where responsivity changes quickly. Calibration, traceable to national standards and typically quoted at a few percent, is valid only at the calibrated wavelengths and power range.

Most measurement errors come from the geometry rather than the meter. Light reflected from the detector surface or a window, a beam larger than the active area, or a divergent beam from a bare fiber can all lose power; integrating-sphere heads avoid most of these for divergent or large beams. Fiber adapters must match the connector type, and an angled (APC) connector in a flat adapter misdirects the light. Detector surfaces can be slightly polarization-dependent and can produce interference fringes with coherent light, so readings of narrow-linewidth lasers can shift with small changes in angle. At high power a photodiode saturates, reading low with no warning; the usual check is to add a known attenuation and confirm that the reading falls by the same amount.

References: IEC 61315, Calibration of fibre-optic power meters; S. M. Sze, K. K. Ng, Physics of Semiconductor Devices, 3rd ed. (Wiley, 2007).