Coherent light generators – Particular beam control device – Tuning
Patent
1986-09-04
1989-04-18
Scott, Jr., Leon
Coherent light generators
Particular beam control device
Tuning
372 9, 372 98, 372 23, 372 15, 372102, H01S 310
Patent
active
048233510
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a device for the fast variation of the wavelength of lasers, provided with a movable mirror arrangement in the optical path of the laser resonator, by means of which the beam can be directed to a grating, the desired wavelength being adjustable by varying the angle of incidence of the light on the grating.
2. Description of the Prior Art
Lasers permitting fast tuning between two wavelengths are being used in a wide variety of applications, e.g. for the remote measurement of pollutants by means of pulsed lasers using the differential absorption technique. The spacing of the laser pulses of the two different wavelengths should range between one and ten milliseconds.
For fast tuning, it is known to use a grating which rotates in the resonator of a pulsed laser at constant angular velocity. In the course of one rotation period, the grating passes through all possible angular positions which determine the emission frequency of the laser. The desired laser frequency can thus be adjusted by triggering the laser exactly at the time when the angular position of the rotating grating is correct. This ensures full liberty to select the laser frequency at each laser pulse. In the case of rapidly pulsed lasers, the frequency of successive pulses can be arbitrarily determined. Instead of a rotating grating, a rotating mirror or a rotating polygonal mirror is frequently used to direct the beam to the grating. A scan mirror oscillating at constant amplitude can have the same function.
In the case of pulse repetition frequencies of about 100 to 1000 pulses per second, rather high rotational velocities of the grating or of the reflecting mirror result. Consequently the time of laser triggering has to be observed with very narrow tolerances, typically about 0.1 microseconds. This accuracy necessitates sophisticated control equipment and involves major technological problems in particular in the case of pulsed CO.sub.2 lasers; these problems may reduce the tuning accuracy and slightly vary the direction of the emitted beam.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an easily realizable device which permits fast switching between different wavelengths without affecting the tuning accuracy or the beam adjustment.
According to the invention, this object is reached by alternately deflecting the light by means of a rotating chopper mirror, to the grating or at least to a second grating or at least to a second mirror. According to another embodiment, an oscillating mirror is provided which is driven at constant frequency, but with variable amplitude. The subclaims reciting variations in the rotating or oscillating mirror relate to preferred embodiments of the device according to the invention.
According to the invention, there are two possibilities of avoiding the drawbacks of the known devices for fast tuning. The first embodiment of a simple tuning device consists, for example, of a rotating chopper mirror which alternately transmits and deflects the laser beam in the optical resonator. The two resultant resonator arms contain tuning elements such as gratings, which can individually be adjusted to different wavelengths. As the chopper mirror merely interrupts or reflects the laser beam, there are no critical tolerances for the triggering the laser in this case, either.
The chopper mirror as such may consist of several segments, in order thus to reduce the required rotational velocity. At a desired tuning frequency of, e.g., 1000 Hertz and a chopper mirror with five segments and a corresponding number of cuts, the required rotational frequency of the mirror is 6000 rpm. This rotational velocity can be reached without any difficulties. Appropriate balancing permits vibration-free running, so that this tuning device does not affect the other components of the laser resonator.
The second embodiment consists in an oscillating mirror to which the beam is directed such that the desired wavelengths are adjusted when the mirr
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Endemann Martin
Hoeffgen Hansjuergen
Battelle-Institut e.V.
Jr. Leon Scott
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