Smart Sensor Systems

4

Optical Sensors Basedon Photon Detection

Reinoud F. Wolffenbuttel

4.1 Introduction

Optical sensors are used for the measurement of radiation properties, such as intensity, polarization and spectral distribution (color when restricted to the visible spectral range). The two types of optical sensors available are: thermal detectors (=absorbing detectors) and photon detectors (=quantum detectors). The emphasis in this chapter is on silicon photon detectors, because of their high performance in applications in the visible spectral range and their compatibility with integrated circuit (IC) fabrication [1, 2].

A thermal detector is basically a tandem transducer in which the energy associated with the impinging radiation is first absorbed and the temperature rise due to the heat generated is measured, using one of the contact temperature sensors described in Chapter 7. The main advantage of the thermal detector is the spectral uniformity, which depends on the absorbing layer only. The photon detector enables the conversion of photons into electron–hole pairs, the concentration of which can be directly electrically measured.

Figure 4.1 shows the typical spectral response curves of a thermal detector (bolometer) and a silicon photon detector. The responsivity is basically the sensitivity specification. The spectral response of the thermal detector can be designed for wideband operation, since this property is determined by the absorber and can be engineered independently from the thermal ...