**4.1 Operating principle and measurement results for a four-channel demultiplexer**

By extending the channel count to four, it was possible to investigate two different demultiplexer configurations. Those were:


The principle of operation of a four-channel demultiplexer with serial configuration is shown in **Figure 8**. In this configuration the dichroic mirrors were cascaded such that each mirror (except the last one) demultiplexed a single-wavelength channel while passing all other wavelengths.

For practical realization 425, 505, and 567 nm cutoff long-pass dichroic mirrors were cascaded so that the interference filters centered at 405, 450, 525, and 650 nm could be implemented in the output ports 1–4, respectively. The corresponding transfer function and the basic parameters of the demultiplexer are shown in **Figure 9** and **Table 3**, respectively. The factors contributing to high IL in the output ports 3 and 4 are discussed in the next subchapter.

The principle of operation of a four-channel demultiplexer with two-stage configuration is shown in **Figure 10**. The first stage of the demultiplexer, represented by a dichroic mirror that follows directly after the collimating lens, splits the
