A gas detector that is arranged to sense the concentration levels of a plurality of target gases within a gas sample from an environment surrounding the detector, comprising: a laser source or sources (12a-12d) that are arranged to transmit radiation through the gas sample at target wavelengths that correspond approximately to the optimum absorption wavelengths of each of the target gases; an optical detector or detectors that are arranged to sense the intensity of the radiation transmitted through the gas sample at each of the target wavelengths and generate representative output intensity signals for each of the target wavelengths; and a control system that is arranged to operate the laser source(s) (12a-12d) and optical detector(s), the control system comprising: a lock-in amplifier (20, 112); and one or more current drivers that are arranged to drive the laser source(s) with drive current signals (18a-18d) that are modulated with a sine wave and a triangle wave such that the signals are simultaneously triangularly ramped according to a triangle wave and sinusoidally modulated according to a sine wave to generate radiation at or about each of the target wavelengths that is wavelength scanned across a small range about its respective target wavelength to allow for long term drifts in the laser source(s) by virtue of the triangular ramping of the drive current signals and frequency modulated at the sine wave frequency so that small signals can be extracted from the background noise using the lock-in amplifier (20, 112), and wherein the control system generates representative concentration level information relating to each of the target gases within the gas sample based on the level of absorption of the radiation transmitted into the gas sample at each of the target wavelengths with reference to the representative output intensity signals from the optical detector(s) for each of the target wavelengths. a laser source or sources (12a-12d) that are arranged to transmit radiation through the gas sample at target wavelengths that correspond approximately to the optimum absorption wavelengths of each of the target gases; an optical detector or detectors that are arranged to sense the intensity of the radiation transmitted through the gas sample at each of the target wavelengths and generate representative output intensity signals for each of the target wavelengths; and a control system that is arranged to operate the laser source(s) (12a-12d) and optical detector(s), the control system comprising: a lock-in amplifier (20, 112); and one or more current drivers that are arranged to drive the laser source(s) with drive current signals (18a-18d) that are modulated with a sine wave and a triangle wave such that the signals are simultaneously triangularly ramped according to a triangle wave and sinusoidally modulated according to a sine wave to generate radiation at or about each of the target wavelengths that is wavelength scanned across a small range about its respective target wavelength to allow for long term drifts in the laser source(s) by virtue of the triangular ramping of the drive current signals and frequency modulated at the sine wave frequency so that small signals can be extracted from the background noise using the lock-in amplifier (20, 112), and wherein the control system generates representative concentration level information relating to each of the target gases within the gas sample based on the level of absorption of the radiation transmitted into the gas sample at each of the target wavelengths with reference to the representative output intensity signals from the optical detector(s) for each of the target wavelengths. a lock-in amplifier (20, 112); and one or more current drivers that are arranged to drive the laser source(s) with drive current signals (18a-18d) that are modulated with a sine wave and a triangle wave such that the signals are simultaneously triangularly ramped according to a triangle wave and sinusoidally modulated according to a sine wave to generate radiation at or about each of the target wavelengths that is wavelength scanned across a small range about its respective target wavelength to allow for long term drifts in the laser source(s) by virtue of the triangular ramping of the drive current signals and frequency modulated at the sine wave frequency so that small signals can be extracted from the background noise using the lock-in amplifier (20, 112), and wherein the control system generates representative concentration level information relating to each of the target gases within the gas sample based on the level of absorption of the radiation transmitted into the gas sample at each of the target wavelengths with reference to the representative output intensity signals from the optical detector(s) for each of the target wavelengths.