Microwave circuit theory is utilized to describe three devices which control coherent optical beams using birefringent crystals. A frequency demodulator uses the difference of the index of refraction along two axes of a birefringent crystal together with a Fabry‐Perot interferometer to obtain an ``s'' type frequency discriminator. An optical isolator consists of a crystal and a polarizer that function in such a way that the input of the device is isolated from the reflection which can occur at its output. An optical switch is based on the filter characteristics of a Fabry‐Perot interferometer which can be adjusted with an electrically activated birefringent crystal.