The model TIS-SF-08 makes use of a linear resonator configuration, with additional beam waist for the non-linear crystal. This is why the second-harmonic (SH) radiation is emitted from both ends of the non-linear crystal.
In order to collect all the SH radiation on one side of the non-linear crystal a dichroic mirror (M4) is used that reflects both the fundamental and the SH radiation. On the other side of the crystal another dichroic mirror (M5) is installed, but this one is totally reflective only for the fundamental radiation, having large transmittance (T>80-85%) for the SH radiation. Through this mirror the SH output is extracted from the cavity, which "locks" the fundamental radiation inside.