Nonimaging and Imaging Optical Concentrators

One of the obvious methods to increase the optical input to the detector is to collect optical signal from a larger area and to concentrate it to the smaller electrically active area. This can be done by some kind of optical concentrator, i.e., focusing optics integrated with the detector unit. 

A schematic presentation of an optical concentrator is given in Fig. 2.3. The shape of the system can be arbitrary, as well as its mechanism of focusing. The important factor of the system is its concentration ratio, defined as a ratio between the input (collector) area and the exit area. In an ideal concentration system, this ratio should be equal to the ratio of incident optical flux to the exit flux. In a real system, possible absorption losses (for instance in metal parts) will have to be deducted. Thus the concentration ratio of a light concentrator can be defined either as the geometric concentration ratio C, the ratio of the entry aperture area (AcoUector) to the exit aperture area (Aexit), or as the irradiance gain C the ratio of the irra-diance on the collector (fr ) to that on the entry aperture (Ir ). The two concentration terms are related by the fraction of total incident power entering the module that reaches the concentrator exit aperture. 

Different strategies can be used to concentrate the incident optical power. There are three main groups of optical concentrators. One of them is based on refractive optics (conventional lenses). The second one is reflective concentrators (mirrors), while the third group is diffractive optical elements. Obviously, a system may simultaneously incorporate two or even aU three of the mentioned structures. 

From the point of view of coupling efficiency (minimization of reflection losses), the best solution is monolithic or hybrid integration however, in practical situations one can encounter all of the mentioned approaches. Two main types of focusing lenses may be used—either refractive lenses fabricated in material with high real part of refractive index and low absorption coefficient at IR wavelengths, or diffractive lenses. Any of them may be either discrete or arrayed. Reflective optical concentrators may be used, also reflective holographic optical elements or any of their combinations. 

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