Abstract: A laser system comprises a laser (500) whose output beam is delivered to a workpiece (504) by an optical fiber (502). A transducer (506) detects the power of light propagating in the cladding layer of fiber (502) at a position near its input end. The transducer (506) also strips out the light travelling in the cladding layer. The output of transducer (506) is divided by the output of a transducer (507), which detects the power of the output beam of laser (500), to produce a signal indicative of misalignment between the laser beam and the core of fiber (502) at the input face. A transducer (505) comprises a second fiber (508) optically coupled with the cladding layer of fiber (502). Light emitted from the ends of fiber (508) is detected by a pair of detectors (509, 510), whose outputs are indicated on displays (512, 514). Display (512) provides information on the integrity of transmissions of the laser beam through fiber (502).