Frequency tunable laser system comprising - a laser with first and second reflective surfaces defining a laser cavity with a laser gain medium between the first and second reflective surfaces, wherein the second reflective surface is semi-transparent for emitting, in use, a laser beam from the second surface; - frequency varying means arranged for varying an optical frequency output of the laser; - an intensity sensor arranged for receiving light from the laser and providing an intensity signal (Sm) that is indicative of an intensity output of the light from the laser; - a processor arranged for o controlling the frequency varying means for varying the optical frequency output of the laser; and o receiving the intensity signal (Sm) from the intensity sensor for monitoring the intensity output of the laser; wherein - the frequency tunable laser system further comprises an external reflective surface, in use, fixedly arranged in a light path of the laser beam outside the laser cavity at a predefined distance from the second reflective surface along the light path of the laser beam to reflect part of the emitted laser beam back into the laser cavity; and wherein - the processor is further arranged for o processing the intensity signal (Sm) and registering oscillations of the intensity output caused by interference of the reflected part of the laser beam in the cavity and responsive to a change in optical frequency (ΔF) as a result of the varying of the optical frequency output of the laser; and o calculating the change in optical frequency output (ΔF) from the registered oscillations of the intensity output and predefined distance. - a laser with first and second reflective surfaces defining a laser cavity with a laser gain medium between the first and second reflective surfaces, wherein the second reflective surface is semi-transparent for emitting, in use, a laser beam from the second surface; - frequency varying means arranged for varying an optical frequency output of the laser; - an intensity sensor arranged for receiving light from the laser and providing an intensity signal (Sm) that is indicative of an intensity output of the light from the laser; - a processor arranged for o controlling the frequency varying means for varying the optical frequency output of the laser; and o receiving the intensity signal (Sm) from the intensity sensor for monitoring the intensity output of the laser; wherein o controlling the frequency varying means for varying the optical frequency output of the laser; and o receiving the intensity signal (Sm) from the intensity sensor for monitoring the intensity output of the laser; wherein - the frequency tunable laser system further comprises an external reflective surface, in use, fixedly arranged in a light path of the laser beam outside the laser cavity at a predefined distance from the second reflective surface along the light path of the laser beam to reflect part of the emitted laser beam back into the laser cavity; and wherein - the processor is further arranged for o processing the intensity signal (Sm) and registering oscillations of the intensity output caused by interference of the reflected part of the laser beam in the cavity and responsive to a change in optical frequency (ΔF) as a result of the varying of the optical frequency output of the laser; and o calculating the change in optical frequency output (ΔF) from the registered oscillations of the intensity output and predefined distance. o processing the intensity signal (Sm) and registering oscillations of the intensity output caused by interference of the reflected part of the laser beam in the cavity and responsive to a change in optical frequency (ΔF) as a result of the varying of the optical frequency output of the laser; and o calculating the change in optical frequency output (ΔF) from the registered oscillations of the intensity output and predefined distance.