A laser detection system comprising: a plurality of lasers wherein each laser is configured to produce a respective laser beam for absorption by at least one compound, the plurality of lasers thereby producing a plurality of laser beams; a sample cell for containing a volume of sample gas, wherein the sample cell comprises a reflective multi-pass absorption cell; at least one directing device configured to direct the laser beams to the sample cell, wherein the at least one directing device is configured to direct the laser beams along a common optical path to the sample cell, and a detector apparatus for detecting light output from the cell, wherein the at least one directing device comprises a plurality of optical components arranged such that, for each laser beam a respective at least one of the optical components is arranged to direct said laser beam along the common optical path, and wherein the plurality of optical components are arranged in series substantially along a straight line and are configured such that in operation each optical component directs the laser beam from its corresponding laser in a same direction of propagation along said straight line to join said common optical path, and wherein at least one optical component of the plurality of optical components directs or allows passage of at least one laser beam of the plurality of laser beams from at least one preceding optical component in the series, in the same direction of propagation along said straight line and wherein the laser detection system further comprises a controller configured to control the plurality of lasers to sequentially switch between the lasers and to pulse the plurality of lasers, wherein the controller is configured to select the pulse lengths and frequency of sequentially switching between the lasers such that subsequent laser pulses take into account the time taken by light to traverse an optical path through the sample cell such that light from only one laser is present inside the sample cell over a sample time interval. a plurality of lasers wherein each laser is configured to produce a respective laser beam for absorption by at least one compound, the plurality of lasers thereby producing a plurality of laser beams; a sample cell for containing a volume of sample gas, wherein the sample cell comprises a reflective multi-pass absorption cell; at least one directing device configured to direct the laser beams to the sample cell, wherein the at least one directing device is configured to direct the laser beams along a common optical path to the sample cell, and a detector apparatus for detecting light output from the cell, wherein the at least one directing device comprises a plurality of optical components arranged such that, for each laser beam a respective at least one of the optical components is arranged to direct said laser beam along the common optical path, and wherein the plurality of optical components are arranged in series substantially along a straight line and are configured such that in operation each optical component directs the laser beam from its corresponding laser in a same direction of propagation along said straight line to join said common optical path, and wherein at least one optical component of the plurality of optical components directs or allows passage of at least one laser beam of the plurality of laser beams from at least one preceding optical component in the series, in the same direction of propagation along said straight line and wherein the laser detection system further comprises a controller configured to control the plurality of lasers to sequentially switch between the lasers and to pulse the plurality of lasers, wherein the controller is configured to select the pulse lengths and frequency of sequentially switching between the lasers such that subsequent laser pulses take into account the time taken by light to traverse an optical path through the sample cell such that light from only one laser is present inside the sample cell over a sample time interval.