Question

1. Why are the thermal detectors slower and less sensitive than the photon detectors?

2. What are the physical origins of the shot noise and Johnson noise in an optical detector?

Answer 1

1) In comparison with photon detectors, thermal detectors have been considerably less exploited in commercial and military systems. The speed of thermal detectors is quite adequate for non-scanned images with two-dimensional detectors. In comparative studies, more attention is paid to a wide family of photon detectors. Progress in infrared (IR) detector technology is connected with semiconductor IR detectors that are included in the class of photon detectors. The photon detectors show a selective wavelength dependence of response per unit incident radiation power. They exhibit both perfect signal-to-noise performance and a very fast response. Thermal detectors operate on a simple principle, that is, when these detectors are heated by incoming IR radiation their temperature increases and the temperature changes are measured by any temperature-dependent mechanism, such as thermoelectric voltage, resistance, pyroelectric voltage. The extrinsic photon detectors require more cooling than intrinsic photon detectors having the same long wavelength limit. The theoretical detectivity value for the thermal detectors is much less temperature dependent than for the photon detectors. The temperature requirements to attain background fluctuation noise performance, in general favor thermal detectors at the higher cryogenic temperatures and photon detectors at the lower cryogenic temperatures.

2) In electronics shot noise originates from the discrete nature of electric charge. Shot noise also occurs in photon counting in optical devices, where shot noise is associated with the particle nature of light.

Johnson noise is generated by thermal fluctuations in conducting materials. It is sometimes called thermal noise. It results from the random motion of electrons in a conductor. The electrons are in constant motion, colliding with each other and with the atoms of the material.