A reciprocity-aware GUM assessment of mode-selective back-reflection suppression in mid-infrared laser power measurement using a bulk-Dirac-semimetal photonic spin Hall multilayer

A reciprocity-aware theoretical assessment is presented for using a bulk-Dirac-semimetal (BDS) photonic spin Hall effect (PSHE) multilayer in a mid-infrared laser-power measurement chain. The revision does not treat the passive asymmetric multilayer as a strict two-port optical isolator. Instead, the device is analysed as one element of a mode-selective return-light rejection architecture that also contains relay optics, a centred spatial-mode collector and a rejected-light dump. The distinction is essential: a horizontally polarised beam is decomposed into left- and right-circular components that acquire opposite transverse shifts, while the total beam centroid remains unchanged in the equal-weight model. Lorentz reciprocity further requires zero isolation for a specular time-reversed field in the matched input–output mode. Conditional suppression is therefore evaluated only for mode-converted return fields that populate a different spatial or polarisation state. Using the published shift bounds δ low ≤ 1.20 λ and δ high = 14.36 λ at λ = 8.9 μ m , an illustrative 6 × relay and a Gaussian collector radius of 200 μ m yield a conditional mode-rejection ratio of 63 dB under ideal alignment. The ratio remains 50 dB at a lateral collector offset of ± 0.10 mm . A common angular mismatch of ± 0.20 ° reduces desired-mode coupling by 0.71 dB but does not change the ratio in the symmetric Gaussian-overlap model. Monte Carlo vibration analysis gives a 95% lower rejection bound of 62 dB for 10 μ m rms lateral jitter and 0.02° rms pointing jitter. The Cd 3 As 2 conductivity case at E F = 11.1 meV is explicitly classified as cryogenic; room-temperature operation requires a finite-temperature conductivity model and experimental re-optimisation. A parametric GUM model replaces the unsupported assumption that back-reflection is universally 2%–5% of incident power. For a baseline standard uncertainty of 1.5%, a bounded coherent perturbation exceeding approximately 2.3% makes an expanded-uncertainty design target of 4% unattainable at k = 2 . The results define testable conditions and limitations for a future experiment rather than claiming an experimentally certified isolator.

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