There is provided a gain medium capable of radiating spectral lines differing in intensity as a function of position along a selected, gain-medium axis. A first resonator is arranged along a first optical axis that crosses the gain medium in a first region along the selected, gain-medium axis; the first resonator is adapted to establish a first optical mode that selectively lases one group of the spectral lines of the gain medium, namely a group thereof having high intensity within the volume in which the first mode is established. A second resonator is arranged along a second optical axis that crosses the gain medium in a second region along the selected, gain-medium axis; the second resonator is adapted to establish a second optical mode that lases a second group of spectral lines differing at least in part, from the first group. The second group of spectral lines has high intensity within the volume in which the second mode is established. The laser energy from the first and second resonators is outcoupled onto a target. In the case of a gas transport laser, the first resonator is adapted to lase a group of spectral lines having a higher frequency than the second resonator.

Laser scanning apparatus for the monitoring of gaseous pollutants (e.g. on a chemical plant) in which two laser beams having different wavelengths (one corresponding to an absorption line of the gas to be monitored) and modulated at different frequencies are combined into a single scanning beam. A portion of the scattered radiation is collected, detected and measured to give, for each chosen beam direction, the amount of the gas being monitored. The amount of radiation reaching the detector from the laser source is varied according to a predetermined programme or in response to an external stimulus, and by this means the detector can be protected against severe overload when the beam scans over positions of abnormally high reflectivity.

A diagnostic apparatus comprises a source (1) of probing electromagnetic radiation and means (2) for transmitting the output from the probing radiation source (1) to biological tissue (3) to be examined. The apparatus also comprises means (4, 41) for detecting probing radiation reflected from the tissue (3) and stimulated radiation resulting from excitation of the tissue (3) by the probing radiation. A processing means (5-9) responsive to the reflected and stimulated radiations to produce a signal for diagnosis of the condition of the tissue and means (15, 16) for regulating the intensity of the probing radiation including a feedback circuit (16, 17, 18) for controlling the regulating means (15) and responsive to the intensity of the probing, reflected and/or stimulated radiation are also included.

An external mirror is positioned relative to the output mirror of a laser to form a Fabry-Perot cavity. The light transmitted by this Fabry-Perot cavity is compared to a reference value in order to develop an error signal which in turn is used to adjust the transmission characteristics of the Fabry-Perot cavity. In the embodiment constructed a beam splitter is positioned to deflect light coupled out of the Fabry-Perot cavity to an optical detector. The output of this detector is compared to a reference voltage in a difference amplifier which generates an electrical error signal. The electrical error signal is coupled to a piezoelectric translator which is attached to the external mirror and is capable of moving that mirror in a way so as to change the transmission characteristics of the Fabry-Perot cavity. Specific embodiments using a dye laser and a soliton laser are also disclosed.

A method and apparatus for matching the transmission characteristics of two beam modulators (4,4') comprises radiation generating means (30); and the first and second beam modulators (4,4'), the radiation generating means causing beams of radiation (16,16') to impinge on each modulator. Control means (20) generates first and second control functions for application to the first and second modulators (4,4') respectively, the first control function cyclically varying between first upper and lower values (V.sub.1,V.sub.2) and the second control function cyclically varying between second upper and lower values (V.sub.3,V.sub.4) at the same rate and at the same time as the first control function, the range of the first and second control functions being such that the transmission characteristics of the first and second beam modulators overlap. Detecting means (18,19) detects the phase relationship between the radiation transmitted by the first and second beam modulators and having the higher or lower intensity and the first or second control function; and storage means (32) stores the second upper value (V.sub.3) when the detecting means determines a change in the phase relationship.