Spectral Response of a Photodetector
The spectral response of a photodetector is the range of optical wavelengths or frequencies in which the detector has a significant responsivity. There is no universally defined criterion for the minimum responsivity; it may, for example, be taken as one tenth of the maximum responsivity, or even much less. In other cases, a drop of at most 50% may be acceptable for an application. Due to that uncertainty, given specifications can vary even for the same device.
Note that the spectral range quoted for an optical power meter, for example, may be smaller than the spectral response: there may be wavelength regions where the detector reacts, but not with a calibrated response.
Typical Limiting Factors
Some typical limiting factors for the spectral response of photodetectors are:
- Many types of detectors, for example all photoemissive detectors and all semiconductor-based detectors containing a p–n junction, work only for photon energies above a certain level. That condition translates into some maximum optical wavelength.
- Such a limitation does not occur for thermal detectors, which therefore can exhibit a very broad and smooth spectral response. There, however, the spectral response may be limited by the wavelength-dependent absorption of the used absorber.
- Many detectors have an optical window, e.g. for protecting the light-sensitive area or for preserving a vacuum inside the detector, and that window has a limited wavelength range with high transmissivity.
- Sometimes, the spectral response of a photodetector is intentionally limited with an optical filter, because a response to certain other wavelengths is undesirable for a particular application. In some cases, one wants to avoid degradation effects caused by short-wavelength light (e.g. ultraviolet light).
For a given material, e.g. of a photocathode, the spectral response may substantially vary due to different factors, e.g. the applied thickness of a layer, an additional reflector or details of the material fabrication process.
Measuring the Spectral Response
For measuring the spectral response of a photodetector over some wavelength range, one requires a light source (or a combination of light sources) which can provide quasi-monochromatic light over the whole wavelength range, or at least at certain discrete wavelengths. For each measurement wavelength, one can then simply obtain the responsivity as the ratio of photocurrent and incident optical power. The latter may be measured with a thermal detector, for example, which exhibits only a minimum dependence on the optical wavelength. Of course, any other photodetector with calibrated spectral response could also serve this purpose.
In practice, the main challenge is usually to obtain a light source which can cover the required wavelength range – with additional requirements concerning spatial characteristics and optical power. Exam cases, one can use a broadband light source (e.g. a white light source) in combination with a tunable monochromator, provided that the light throughput in a reasonably small optical bandwidth is sufficient.
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