Reducing the mechanical-attenuator calibration uncertainty to ≤0.1% in the 1 mW dissemination chain of the State Working Standard of average optical power (dissemination directly from the primary standard GET 170-2011)

The 2016 paper by Glazov, Kozachenko, Tikhomirov, and Khatyrev describes a Russian state working standard of the unit of average power of optical radiation (0.05–5 W, 0.19–12 µm, expanded uncertainty $\le 0.985 \cdot 10^{-2}$) that disseminates the unit at the 1 mW level directly from the primary standard GET 170-2011 through an independently calibrated mechanical attenuator. The dominant component of the working standard's error budget is the relative calibration error of the attenuator's attenuation factor, about 0.32%, and the paper states that work was underway to reduce this error to not more than 0.1% through improved attenuator design and higher-precision dimensional calibration. No publication documents achievement of the ≤0.1% attenuator-related uncertainty in this dissemination chain, so the problem remains open.

Background

In legal metrology, a national measurement institute maintains a primary (state) standard of a unit and transfers ('disseminates') it to working standards, which in turn calibrate measuring instruments in industry. In Russia this hierarchy for optical power is maintained by VNIIOFI (the All-Russian Research Institute for Optical and Physical Measurements, Moscow). Its top for average optical power is GET 170-2011, the State Primary Special Standard of the units of length and propagation time of the signal in an optical cable, and average power, attenuation, and wavelength of optical radiation for fiber-optic communication and information transmission systems. 'Average power' here means the time-averaged power of continuous or repetitively pulsed laser and fiber-optic radiation, and the wavelength range 0.19–12 µm covers ultraviolet through mid-infrared sources used in fiber-optic communication systems and laser technology. A 'state working standard' is a certified reference facility that carries the unit from the primary standard to routine calibrations; its quality is quantified by an uncertainty budget, in which each component (detector calibration, amplifier linearity, attenuator error, etc.) is estimated and combined according to the GUM (the ISO Guide to the Expression of Uncertainty in Measurement, the international convention for such budgets). The 'expanded uncertainty' is the standard combined uncertainty multiplied by a coverage factor (usually $k = 2$), giving the interval within which the transmitted value is asserted to lie with ~95% confidence. To calibrate instruments at power levels far below or above those the primary standard reproduces directly, the transfer chain uses a mechanical attenuator — a physical device (such as a rotating sector disk or a calibrated set of transmissive/reductive elements) that reduces the beam power by a known factor. Its attenuation factor must itself be calibrated. In the 2016 working standard, GET 170-2011 reproduces the unit at levels up to 1 mW, whereas the working standard's calorimetric transducer operates reliably at ≥20–50 mW, so an independently calibrated ~100-fold mechanical attenuator bridges the two: a blackened rotating disk with a narrow angular slot, whose attenuation factor $N = 2\pi R/L$ is computed from dimensional measurements via $R = 0.5D - h + 0.5b$, with the transfer realized at the 1.55 µm calibration wavelength using a 20–200 mW source. The relative calibration error of this attenuation factor — about 0.32% with the caliper- and indicator-grade linear measuring instruments used — was the largest single component of the error budget, and the total expanded uncertainty of the working standard was $\le 0.985 \cdot 10^{-2}$ (about 1%). The authors state that studies were underway to reduce the attenuator error to not more than 0.1% by improving the attenuator's design and its calibration technique, in particular through higher-precision dimensional measurements (the source's stated route is a better attenuator construction together with higher-accuracy linear-dimension measuring instruments for the geometric parameters entering $R = 0.5D - h + 0.5b$ and $N = 2\pi R/L$). The ≤0.1% attenuator uncertainty has not been achieved or documented for this dissemination chain, so it remains the open problem.

Problem Statement

For the state working standard of the unit of average power of optical radiation for fiber-optic systems and lasers (0.05–5 W, 0.19–12 µm, expanded uncertainty $\le 0.985 \cdot 10^{-2}$), which disseminates the unit at the 1 mW level directly from the primary standard GET 170-2011 through an independently calibrated ~100-fold mechanical attenuator (a blackened rotating disk with a narrow angular slot, whose attenuation factor $N = 2\pi R/L$ is computed from dimensional measurements via $R = 0.5D - h + 0.5b$; the transfer is realized at the 1.55 µm calibration wavelength using a 20–200 mW source): improve the design of the mechanical attenuator and the technique of its dimensional calibration — in particular via higher-precision dimensional measurements with higher-accuracy linear-dimension measuring instruments — so that the relative calibration error of the attenuation factor at the 1 mW transfer point is reduced from its present ~0.32% to not more than 0.1%, and document the revised uncertainty budget of the dissemination chain. The revised transfer chain must document the attenuator construction, the calibration method and its achievable accuracy, the resulting attenuation-factor calibration error ≤0.1% at the 1 mW transfer point, and a recomputed uncertainty budget showing the attenuator component ≤0.1%.

References

1. A. I. Glazov, M. L. Kozachenko, S. V. Tikhomirov, N. P. Khatyrev, "State Working Standard of the Unit of Average Power of Optical Radiation for Fiber-Optic Systems and Lasers", Measurement Techniques 59(3), 209–215 (2016). DOI: 10.1007/s11018-016-0944-y, https://doi.org/10.1007/s11018-016-0944-y
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