Which is closest to the responsivity (current per 1 W of light) of a photodiode with 80% quantum efficiency at a wavelength of 1.55 µm? (R [A/W] ≈ quantum efficiency × λ [µm] / 1.24)

Answer: C. About 1.0 A/W

Responsivity is R ≈ η × λ / 1.24, so 0.8 × 1.55 / 1.24 ≈ 1.0 A/W. The energy per photon is smaller at longer wavelengths, so the same 1 W contains more photons at longer wavelengths, and with equal quantum efficiency more current flows. For example, at 1.31 µm with 80% quantum efficiency it is about 0.85 A/W. Even at 100% quantum efficiency the upper limit at 1.55 µm is about 1.25 A/W, so about 1.5 A/W (multiplying by 1.24 instead of dividing) is impossible. About 0.6 A/W is the value obtained by swapping the wavelength and 1.24.