Abstract: A laser detection system comprises a sample chamber configured to receive and contain a volume of sample gas, one or more lasers within at least one laser housing, wherein each laser is configured to produce a respective laser beam for excitation of one or more different materials in the sample gas and the one or more lasers are outside the sample chamber, a detector apparatus for detecting light output from the sample chamber, a first optical interface to the sample chamber having at least one window that is at least partially transparent to the laser beams from the one or more lasers, wherein the at least one laser housing is positioned in a close-coupling arrangement relative to the at least one window of the first optical interface such that, in use, the laser beams are substantially unmodified by passage between the laser housing and the at least one window.