Coherent light generators – Particular beam control device – Tuning
Patent
1992-04-21
1993-10-19
Bovernick, Rodney B.
Coherent light generators
Particular beam control device
Tuning
372102, H01S 310
Patent
active
052552730
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention refers to a method and an apparatus to wavelength tune an optical resonator without mode hops of the type comprising an adjustable optical grating, a partially reflecting mirror and transmission components, e.g. an amplifying optical element and an optical element to collimate the radiation which oscillates in the cavity. An important application of the invention is that of a laser cavity. The method of the invention is applicable to UV, light, IR and mm-waves.
The conditions for tuning an optical grating - mirror resonator, the relations (2) and (3), have been known for a long time. A method used in practice to perform this tuning is to have two from each other independent movements, one rotation of the grating according to (2) and one displacement of the grating at right angles to the mirror plane according (3):
If one, however, wishes to achieve a continuous change of the tuning without mode hopping, that is without changing the longitudinal ordinal number N in relation (3), both movements have to be automatically coordinated. There are in the litterature some proposed methods to establish mechanically a connection between the movements so that the oridinal number N of the resonance mode is maintained with the resonance frequency of the resonator is continuously changed.
FIG. 3 shows earlier practical designs of tunable grating-mirror resonators.
In reference 1., F. Favre et.al., Electronics Letters, 17th Jul. 1986, Vol.1 22, No 15, pp.795-6, FIG. 3a, the grating is fixedly mounted on an arm. The arm has two bearing arrangements for rotation, one at the grating end and the other at the opposite end. The one at the grating end is moved along a rail in the direction of the resonator axis. The other is moved along a rail parallel to the plane of the fixed mirror. This mechanical device restricts the movement of the grating in such a way that the relations (2) and (3) are satisfied.
Ref. 1 also shows that within a small wavelength interval it is possible to satisfy the tuning conditions by rigid rotation of the grating about the fixed point P in FIG. 3a. As can be seen, P is chosen in such a way that a small rotation around this point displaces the grating in the direction of the resonator axis.
The solution given in ref. 1 refers to mode hop free continuous tuning of a resonator with a fixed mirror and a movable reflective grating. A characteristic feature of the solution is that it meets s demand to use the same portion of the grating surface for reflection in the resonating, that is limited by the fixed mirror. This implies that the number of nodal planes in the standing wave of the resonator is kept unchanged in tuning to a new wavelength. In a tunable laser constructed according to this principle the laser beam remains fixed with respect to the grating grooves when the grating is moved.
The solution from ref. 1 in FIG. 3a in principle an exact solution, which simulates equations (2) and (3) over the whole range of angles .alpha. in the dispersion free resonator. It is stated in reference 1, that one can make a slight geometric modification, which compensates for chromatic dispersion. The means for doing this have not been specified.
The result in ref. 1 shows that in practice the tuning range is limited to 15-30 nm in a solid state laser at 1550 nm. The solution with rigid rotation around the axis P in FIG. 3a gives both theoretically and experimentally a tunable range of 2-3 nm in the same solid state laser.
There are also tunable solid state lasers, reference 2, J. Mellis et. al., Electronics Letters, 4th Aug. 1988, vol. 24, No 16, pp. 988-9, see FIG. 3b, where the grating is adjusted with a screw to a starting position and fine tuning is accomplished by piezo electric means which tilt, rotate and translate the grating to achieve mode hop free tuning within an interval of 0.5 nm. With the adjusting screws the wavelength range 1515-1555 nm is covered.
DETAILED DESCRIPTION OF THE INVENTION
A conventional optical resonator, eg. a laser resonator, see FIG. 1, con
REFERENCES:
patent: 4873692 (1989-10-01), Johnson et al.
Nilsson Olle B.
Vilhelmsson Kennet J.
Bovernick Rodney B.
Radians Innova AB
Wise Robert E.
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