Coherent light generators – Particular beam control device – Q-switch
Patent
1997-08-11
1999-03-30
Davie, James W.
Coherent light generators
Particular beam control device
Q-switch
372 21, 372 37, 372 92, 372 95, 372 99, 372105, H01S 310
Patent
active
058897984
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The invention relates to the field of lasers and in particular actively switched, solid microlasers.
The main advantage of the microlaser is its structure in the form of a stack of multilayers, which constitutes its essential feature. The active laser medium is constituted by a material of limited thickness between 150 and 1000 .mu.m and of small size (a few mm.sup.2), on which are directly deposited dielectric cavity mirrors. This active medium can be pumped by a III-V laser diode, which is either directly hybridized on the microlaser, or is coupled to the latter by an optical fibre. The possibility of a collective production using microelectronics means allows mass production of such microlasers at a very low cost.
Microlasers have numerous applications in fields as varied as the car industry, the environment, scientific instrumentation and telemetry.
2. Prior Art
Known microlasers generally have a continuous emission of a few dozen mW power. However, most of the aforementioned applications require peak powers (instantaneous power) of a few kW supplied for 10.sup.-8 to 10.sup.-9 seconds with a mean power of a few dozen mW.
In solid lasers, it is possible to obtain such high peak power levels by making them operate in the pulsed mode at frequencies between 10 and 10.sup.4 Hz. For this purpose use is made of cavity switching methods, such as Q-switching. Such methods applicable to lasers in general are described by N. Koechner "Solid state laser engineering", Springer Verlag, 1988.
A cavity can be actively or passively switched. In the case of passive switching, variable losses are introduced into the cavity in the form of a saturable absorber material. In the case of active switching, the value of the losses is controlled externally by the user, e.g. by a rotary cavity mirror, by acoustooptical or electrooptical, intracavity means changing either the path of the beam, or its polarization state. The storage time, the opening time of the cavity and the repetition rate can be independently chosen.
U.S. Pat. No. 5,132,977 and U.S. Pat. No. 4,982,405 describe actively switched lasers. In these documents, switching takes place in a configuration of two coupled Fabry-Perot cavities. Such an assembly is illustrated in FIG. 1, where reference 2 designates the active laser medium, 4 a switching material, e.g. an electrooptical material such as LiTaO.sub.3. The active medium 2 of the laser forms with an input mirror 6 and an intermediate mirror 8 a first Fabry Perot cavity. The switching material forms with the intermediate mirror 8 and the output mirror 10 a second Fabry Perot cavity. The switching material 4 can e.g. be bonded to the surface of the intermediate mirror 8. The two cavities are coupled. Switching can take place by modifying the optical length of the switching material 4 by an external action. On designating as L.sub.1, n.sub.1, .lambda..sub.1 (respectively L.sub.2, n.sub.2, .lambda..sub.2) the length, optical index and optical resonance wavelength of the first cavity (respectively the second cavity), the following relation exists: m.sub.1 .lambda..sub.1 =2n.sub.1 L.sub.1 and m.sub.2 .lambda..sub.2 =2n.sub.2 L.sub.2 with m.sub.1 and m.sub.2 being integers.
If the material 4 is a electrooptical material, switching electrodes 12, 14 are placed perpendicular to the axis of the laser beam 16 on either side of the switching material 4. If a voltage V is applied between these electrodes, an electric field E=V/e, where e is the distance between the electrodes (corresponding to the electrooptical material thickness) is the result. The optical index n.sub.2 and consequently the optical length n.sub.2 L.sub.2 of the electrooptical material is modified by the action of the field E. This affects the coupling of the two cavities and modifies the reflectivity of the intermediate mirror 8 seen by the laser medium. Thus, if the resonance wavelengths of the two cavities coincide (.lambda..sub.1 =.lambda..sub.2 or n.sub.1 L.sub.1
.sub.2 L.sub.2 =m.sub.1 /m.sub.2), the reflectivity of t
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Molva Engin
Thony Philippe
Commissariat a l''Energie Atomique
Davie James W.
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