The term sensitivity is used quite frequently in the area of photonics, and this with different meanings – mostly with qualitative ones. Some cases are discussed in the following sections.
Sensitivity of Photodetectors
In some cases, what is meant is actually the responsivity of a detector. This is essentially the ratio of output signal (e.g. a photocurrent) and the strength of the optical input (as an optical power).
In other cases, sensitivity is understood as the ability to detect weak signals. That a ability is usually limited by detector noise. A quantitative measure for that aspect is the noise-equivalent power, which is essentially the optical input required to obtain the same output signal strength is that which results from detector noise. A related quantity – just the inverse of the noise-equivalent power – is the detectivity.
Note that by adding an electronic amplifier to the output of a photodetector one can effectively increase the responsivity, but not the detectivity. Similarly, some photodetectors have particularly higher responsivity, while others have a higher detectivity.
In the case of the human eye, there is no clearly defined output signal, nor a well defined noise level, but sensitivity can again mean the ability to detect weak light signals. One may say, for example, that the human eye is more sensitive to green light than to red or blue light.
Sensitivity of Devices to Certain Parameters
Frequently, the term sensitivity is used in the discussion of how certain devices react to disturbing influences such as changes of temperature, humidity, mechanical influences like vibrations or bending, input wavelengths, signal powers, back reflections etc. The wise designs may be optimized to minimize unwanted sensitivities.
In the context of optical resonators, the alignment sensitivity is often of interest. It essentially quantifies how much the position of resonator modes shift as a result of misalignments. That depends substantially on the chosen resonator design.
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