Mode-synchronized solid-state laser

Details Mode-synchronized solid-state laser Mode-synchronized solid-state laser

2000-07-24

2003-08-12

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

Folded cavity

C372S092000

Reexamination Certificate

active

06606338

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a mode-synchronized solid-state laser comprising a laser medium inside of a laser cavity or laser resonator which is formed by a cavity mirror or resonator mirror and an output coupling mirror and which is folded by means of at least one concave folding mirror, with a saturable absorber inside the laser resonator and with at least one laser pump source whose pump beam pumps the laser medium, wherein the elongated resonator length L is a function of the pulse repetition frequency and the latter is determined by a distance between the resonator mirror and the output coupling mirror.
b) Description of the Related Art
Mode-synchronized solid-state lasers generally require long resonators. The resonator length L determines the repetition frequency v according to the known formula:
v
=
c
2
·
n
·
L
,
where c is the speed of light and n is the index of refraction. Typical frequencies are 50 MHZ to 300 MHZ, resulting in resonator lengths of approximately 3 m to 0.5 m, for example. In order to reduce the constructional length and to achieve sufficient stability, these resonators must usually be folded. When folding, astigmatism results with oblique incident light on spherical surfaces, so that the quality of the laser beam is appreciably worsened.
Further, the quality of the laser beam is worsened by the thermal lens, as it is called. The effect of the thermal lens and the conventional compensation in laser resonators is described in W. Koechner, “Solid-State Laser Engineering”, Springer, 1996. The compensation of the thermal lens requires the use of spherical optics or a corresponding dimensioning of the active medium.
A beam with TEM
00
and M
2
~1 is required for mode locking. Typical resonators for mode-synchronized lasers are described in Keller, U., “Ultrafast All-Solid-State Laser Technology”, Appl. Physics B, 58, 347-363 (1994) or in Keller, U., T. Heng Chiu, “Resonant Passive Mode-Locked Nd:YLF Laser”, IEEE J. of QE vol. 28, 1710-1720 (1992). For the resonators described therein, the passive saturable absorber is typically located in an intracavity focus with high intensity. This is achieved by means of short-focal-length focusing mirrors or by beam-bundling lenses. The active spot is kept small (Ø50-200 &mgr;m). This is required in order to achieve increasingly shorter pulse widths in the femtosecond range.
In practice, it has also been shown that the homogeneity of commercially available saturable absorbers is poor and therefore only a relatively small portion of the surface of a saturable absorber component ever fulfills its function in an optimal manner. Accordingly, a small focus (under 200 &mgr;m) is also advantageous on the saturable absorber; however, this limits the output power of the laser.
Due to the astigmatism of the folded resonator and the influence of the thermal lens in the laser medium, only a small beam cross section (e.g., 50 &mgr;m) is possible in order to achieve an approximately homogeneous beam bundle in the laser resonator. Therefore, the passive mode synchronization with a saturable absorber based on semiconductors was formerly limited to relatively small average outputs (<1 W). The laser destruction threshold of semiconductor materials of this kind is typically in the range of 1 MW/cm
2
to 10 MW/cm
2
(CW). Due to the saturable absorption and a remaining proportion of residual absorption, this value is again substantially reduced and must be taken into account in the design of the resonator. This led to various suggestions for reducing the output density and/or the output distribution in the saturable absorber.
DE 196 80 508 (Nighan) discloses diode-pumped solid-state lasers for higher average outputs in which the beam guidance in the laser resonator is adjusted in such a way that the beam bundle from the resonator mirror into the laser medium is neither confocal nor concentric (page 5, lines 20 ff: “between confocal and concentric” or “confocal-to-concentric” or “almost confocal” (page 12, line 25)). In this case, a beam deformation occurs due to the folding in the confocal resonator as well as due to the influence of the thermal lens. This arrangement is not suitable for passive mode locking since the geometry of the resonator is essentially determined by the thermal lens.
U.S. Pat. No. 5,812,308 (Kafka) discloses a mode-synchronized laser with amplifier. The distinguishing aspect of this system consists in that the laser crystal is located directly in front of the folding mirror and is pumped by the latter. The optical axis of the resonator traverses the crystal twice in the zone excited by the pump optics. The angle between the two parts of the optical axis is relatively small due to the limited dimensions of the laser medium. However, with this small angle, the astigmatism is kept comparatively slight, but severe restrictions result for the design of resonator geometry. The average output is limited by the pump geometry. A passive saturable absorber based on semiconductors with a quantum well is indicated as mode locker and simultaneously serves as a resonator mirror. It is disadvantageous that this arrangement allows only relatively small folding angles because of the laser crystal geometry.
A similar arrangement, although with a plurality of crystals, is described in U.S. Pat. No. 5,237,584 (Shannon). In this case also, the beam quality is far removed from an intracavity beam bundle with TEM
00
and M
2
~1 because of the plurality of folds via the laser crystals and the plurality of intracavity surfaces and is therefore not suitable for mode synchronization with high output.
WO 98/02945 (Nighan) discloses a Nd-Vanadate laser with Q-switching. A single-folded resonator is used for this solution. The folding is carried out via a plane mirror and the pump light is radiated along the optical axis of the resonator. In order to minimize astigmatism, the folding mirror is flat. This arrangement is not suitable for mode synchronization because there is no possibility in this very short resonator (18 cm) for the arrangement of the mode locker or for the corresponding adjustment of the pulse energy for saturation of the passive absorber.
WO 95/21479 (Keirstaed) shows how the ellipticity of the thermal lens can be reduced principally for the Nd:YVO
4
crystal. This is effected by purposeful dimensioning of the heat sink and use of the anisotropy of this crystal. It is disadvantageous that the heat of the pump light which is not converted into laser radiation is preferably carried off only along two surfaces in the crystal. Accordingly, the effective cooling of the laser crystal is reduced, which leads to loss of effectiveness especially with.high laser outputs. The radial symmetry of the thermal lens is suitable for linear resonators in which there is no astigmatism. However, it has been determined that the effect of the pump light distribution on the beam quality is appreciably greater than that of the heat sink.
OBJECT AND SUMMARY OF THE INVENTION
The primary object of the invention is to provide a mode-synchronized solid-state laser whose laser resonator is constructed in a particularly simple manner and which achieves the beam parameters TEM
00
and M
2
=1 with the closest possible approximation more easily or with simpler steps and whose pulsed output radiation is as close as possible to being 100% stable. The invention has the further aim of providing a mode-synchronized solid-state laser and to provide its laser resonator in particular with a pulse repetition frequency between 50 MHZ and 300 MHZ and especially with a pulse duration between 0.1 and 100 picoseconds with higher amplitude stability, especially with an output power greater than 1 Watt and with an almost diffraction-limited laser beam.
The invention is directed to a mode-synchronized solid-state laser comprising a laser medium inside a laser resonator which is formed by a resonator mirror and an output coupling mirror and which is folded by means of at least one concave fo

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