A laser detection system comprising: a sample chamber comprising an enclosed volume configured to receive and contain a volume of sample gas; one or more laser modules, wherein each laser module comprises a laser package within a laser housing, wherein each laser package of the one or more laser modules 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 laser modules are outside the sample chamber, wherein the laser package comprises a lower surface and an output aperture on the lower surface for laser light to exit the laser module, wherein the aperture houses a window and wherein the laser module further comprises a cavity positioned below the lower surface of the laser package; a detector apparatus for detecting light output from the sample chamber; wherein the system comprises an interface plate configured to accommodate the laser module, wherein the interface plate provides a first optical interface between the laser module and the sample chamber, the first optical interface having at least one window that is at least partially transparent to the laser beams from the one or more laser modules, wherein the system further comprises coupling means configured to position and hold the at least one laser housing 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, wherein the interface plate comprises a coupling member for a corresponding laser module, wherein the coupling member comprises a protrusion extended from the upper surface of the interface plate and having a rimmed seating at the upper end thereof, wherein the at least one optical window of the first optical interface is fixed in the rimmed seating, wherein the coupling means comprises the coupling member and the cavity of the corresponding laser module, wherein the cavity is dimensioned to complement and conform to the shape of the protrusion and wherein the cavity is configured to be mounted on the protrusion. a sample chamber comprising an enclosed volume configured to receive and contain a volume of sample gas; one or more laser modules, wherein each laser module comprises a laser package within a laser housing, wherein each laser package of the one or more laser modules 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 laser modules are outside the sample chamber, wherein the laser package comprises a lower surface and an output aperture on the lower surface for laser light to exit the laser module, wherein the aperture houses a window and wherein the laser module further comprises a cavity positioned below the lower surface of the laser package; a laser package within a laser housing, wherein each laser package of the one or more laser modules 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 laser modules are outside the sample chamber, wherein the laser package comprises a lower surface and an output aperture on the lower surface for laser light to exit the laser module, wherein the aperture houses a window and wherein the laser module further comprises a cavity positioned below the lower surface of the laser package; a detector apparatus for detecting light output from the sample chamber; wherein the system comprises an interface plate configured to accommodate the laser module, wherein the interface plate provides a first optical interface between the laser module and the sample chamber, the first optical interface having at least one window that is at least partially transparent to the laser beams from the one or more laser modules, wherein the system further comprises coupling means configured to position and hold the at least one laser housing 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, wherein the interface plate comprises a coupling member for a corresponding laser module, wherein the coupling member comprises a protrusion extended from the upper surface of the interface plate and having a rimmed seating at the upper end thereof, wherein the at least one optical window of the first optical interface is fixed in the rimmed seating, wherein the coupling means comprises the coupling member and the cavity of the corresponding laser module, wherein the cavity is dimensioned to complement and conform to the shape of the protrusion and wherein the cavity is configured to be mounted on the protrusion.