Temperature dependence of an optical narrow-bandpass filter at 1.5  μm

For superconducting nanowire single-photon detectors, we designed and fabricated an on-chip narrow-bandpass filter at the central wavelength of 1.5 μm to reduce the influence of blackbody radiation from the fiber. The bandpass filter was prepared on a thermally oxidized Si substrate, and it provided a peak transmittance of 88% at the central passband wavelength and 30 dB suppression at the stop bands. We investigate the temperature dependence of the filter from room temperature to 2.9 K. The central wavelength of the passband blue-shifted from 1545±2  nm (295 K) to 1526±2  nm (2.9 K). This effect is explained by the different indices of refraction of Si at different temperatures.