*2.1. Microspectrometers*

An FP filter interferometer is defined as a transparent resonance cavity layer embedded in between two highly reflective mirrors. Within the high reflective spectral range (stopband) defined by the DBRs, only one or several filter lines are allowed to transmit with almost 100% transmission. Each filter transmission line is characterized by its FWHM and its spectral transmission peak (maximum of the filter line). The position within the spectral stopband can be adjusted by a distinct choice of the cavity layer thickness. Please note that the term cavity height or cavity length are used as well in the literature. The longer the cavity, the longer the wavelength of the filter line. For the highly reflective DBRs, dielectric materials of low absorption loss are chosen. For each DBR, thin films are stacked with low and high optical refractive indices alternating in sequence. The higher the width of the stopband, the larger the contrast of refractive indices of the two chosen materials.

By combining an FP filter array and a detector array matching in size, a microspectrometer sensor is obtained. Miniaturizing these sensors does not reduce the spectral resolution as opposed to conventional grating-based spectrometers. These microspectrometers based on FP filter array [28–31] may require complicated fabrication steps for defining all the different heights of the filter cavities. Various methodologies were reported to generate the

unequal cavity heights. Digital etching was used by Correia et al. [28,29] to fabricate 16 different FP cavity heights using four lithography and four corresponding etching steps (4 steps for 16 pixels). Digital deposition was applied by Wang et al. in 2007 [31] to define 128 different FP cavity heights with nine lithography and nine corresponding deposition steps (9 steps for 128 pixels). Digital masking [11] allows (i) digital etching if it is combined with a sequence of etching steps [28,29] and (ii) digital deposition if it is combined with a sequence of deposition steps [31]. The higher the number of different cavity heights (being fabricated) is, the more filter transmission lines (pixels) can be included in an FP filter array. However, these high number of steps might not be cost-efficient in industrial fabrication.
