Resonator mirror with a saturable absorber

Details Resonator mirror with a saturable absorber Resonator mirror with a saturable absorber

2000-07-24

2003-05-06

Davie, James (Department: 2881)

Coherent light generators

Particular resonant cavity

Specified cavity component

C372S011000

Reexamination Certificate

active

06560268

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a resonator mirror with a saturable absorber which is formed of a plurality of semiconductor layers on a substrate for use in a solid-state laser resonator.
b) Description of the Related Art
WO 96/36906 A1 describes optical components for generating pulsed laser radiation which can be used as a resonator mirror. These resonator mirrors contain a stack or layer construction having a reflector and a saturable absorber. The position of the absorber layer in an ensemble of layers is utilized to compensate for the wavelength dependence based on the structure of the ensemble of coatings with the absorption given by the absorbing material for a given wavelength range (page 8, lines 29-35). It can be gathered from FIG.
3
and the accompanying description that this step should make it possible to maintain a reflectivity of almost 100% over a wavelength range of approximately 50 nm.
Further, a negative dispersion of the group velocity of the radiation waves in the laser resonator is to be achieved with the layer construction (page 11, lines 1-3). The aim of the arrangement of the saturable absorber within the layer construction is to integrate the characteristic of saturable absorption in the layer construction in an optimal manner in addition to the characteristic of a negative dispersion (page 11, lines 19-27). The curve of the intensity inside the ensemble of coatings is shown in
FIG. 4
for four wavelengths within a wavelength range of 40 nm. It can be gathered from FIG.
8
and the accompanying description that the reflectivity of the optical components can be adjusted by means of the position of the saturable absorber in the ensemble of coatings.
Further, claim 4 states that the saturable absorber is arranged at a location where there is a high radiation intensity change and a high absorption change in a wavelength range. Therefore, a predetermined reflection curve is to be provided within a wavelength range in cooperation with the other layers (see claim 5). A uniformly high reflection factor is to be achieved over a wavelength range of 50 nm (
FIGS. 3
,
8
a-e
). An optimization criterion is to maintain an optimal saturably absorbing effect of these optical components. It can be gathered from the description that the optimum (page 11, lines 33 to 37) consists in maintaining laser pulses with extremely short pulse widths (less than 10 fs) and/or (page 15, lines 10 to 16) that the saturable absorber is placed at a location where the desired effect occurs: to maintain the desired broad band with respect to a desired wavelength range.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the present invention to provide a resonator mirror of comparatively simple construction with a saturable absorber for use in a solid-state laser resonator that can be highly loaded with respect to power. The saturable absorber should generate laser pulses with a width in the range of 0.1 to 100 ps, wherein a predetermined peak output should be maintained as constant as possible.
The invention relates to a resonator mirror with a saturable absorber for a laser wavelength &lgr;
L
which is formed of a layer sequence of a plurality of semiconductor layers on a substrate, wherein a Bragg reflector formed of a plurality of alternately arranged layers comprising a first material with an index of refraction n
H
and a second material with a lower index of refraction N
L
is grown on a surface of the substrate. The resonator mirror is provided for use in a mode-synchronized solid-state laser resonator with an output power greater than 1 W, especially greater than 7 W.
According to the invention, the resonator mirror with a saturable absorber comprises the Bragg reflector grown on a substrate and a threefold layer which is grown on the latter and which acts as a saturable absorber for the laser wavelength &lgr;
L
, wherein a single quantum layer is embedded within two layers outside an intensity minimum for the laser radiation &lgr;
L
and the threefold layer has a combined optical thickness of
λ
L
2
.
The refractive indices of the respective materials for the laser wavelength &lgr;
L
are taken into account for determining the layer thickness of the single quantum layer, the two layers between which the latter is embedded, and the first and second layers for the Bragg reflector. The optical thickness is given by the air wavelength divided by the index of refraction n of the corresponding layer for the laser wavelength &lgr;
L
. In this connection, the refractive indices of the two layers enclosing the single quantum layer are not critical and can also be different for each of the two layers. It is important to maintain the total optical thickness
λ
L
2
of the threefold layer. The selection of materials for the threefold layer is therefore governed in particular by the material properties of the Bragg reflector, especially by the grating constants of the utilized materials which should be identical as far as possible. Identical grating constants allow a monocrystalline growth of the layers on a monocrystalline substrate with as few defects as possible. Monocrystalline layer systems are particularly advantageous because they have an especially small absorption. The selection of material for the single quantum layer and its thickness depends, in turn, on its saturably absorbing properties (band gap) for the laser wavelength and is not limited to the materials mentioned herein. Above all, the two layers must have a low absorption for laser wavelength &lgr;
L
and the property that they produce a permanent fixed connection with the single quantum layer and the layer system of the Bragg reflector.
The threefold layer will be referred to hereinafter as the layer having the saturably absorbing effect; only the single quantum layer with its band gap is the actual absorbing layer, but it is only capable of functioning due to the fact that it is embedded within the threefold layer in the manner described in the following.
In the present case, the single quantum layer is not subject to any resonance condition within a laser resonator. Its function is comparable with that of a dye absorber in a dye laser or solid-state laser. It must be stressed that ultra-short pulses in the femtosecond and milliwatt range which are desirable in communications technology should not be generated in this case. In practice, the resonator mirror is designed in such a way that a given high reflection factor for the laser wavelength &lgr;
L
is achieved with the smallest possible number of alternating individual layers, wherein a reflection factor of 98% is generally sufficient for laser operation. Including the threefold layer as saturable absorber, for example, only about 30 individual layers are required for a reflector with a saturably absorbing effect. This comparatively small number of individual layers requires a correspondingly low expenditure on manufacturing. What is more important, however, is that the comparatively small number of individual layers in combination with a corresponding control and management of the coating process leads to a very homogeneous layer construction vertical to the radiation direction of the laser light. This, in turn, enables the use of a comparatively slight focusing of the laser beam on the resonator mirror. The spot diameter on the resonator mirror can be more than 200 &mgr;m in this case and can be expanded to approximately 5 mm, wherein a neat, constant mode synchronization of the laser is effected. These comparatively large spot diameters substantially reduce the power density at the resonator mirror. Typical values range from less than 100 kW/cm
2
to about 2 kW/cm
2
with respect to CW operation of the laser. However, in practice, operation is performed as close as possible to the load limit of the resonator mirror in order to achieve a maximum laser output power over a given lifetime of the laser radiation source.
The invention makes possible a comparatively simple, manageable calcul

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