DESCRIPTION OF THE DRAWINGS
In the following the invention will be described in more detail with reference to the appended drawings, in which
FIG. 1 depicts the structure of a laser according to an example embodiment of the present invention as a simplified cross-sectional view,
FIG. 2 depicts the structure of a laser according to an example embodiment of the present invention as a simplified cross-sectional view,
FIG. 3 depicts the structure of a laser according to an example embodiment of the present invention as a simplified cross-sectional view,
FIG. 4 depicts the structure of a laser according to an example embodiment of the present invention as a simplified cross-sectional view.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 a structure of a laser 1 according to an example embodiment of the present invention is depicted as a simplified cross-sectional view. The laser is a single-crystal passively mode-locked semiconductor vertical-external-cavity surface-emitting laser (VECSEL). The laser structure is grown on a substrate (not shown in the figures). The material of the substrate is GaAs, InP or GaSb semiconductor compound. In this embodiment the substrate is only needed to grow the laser 1 and can be removed after forming the layers of the laser 1. In an example embodiment of the present invention the laser 1 is formed in the following way. First, a bottom reflector 4 is grown on the substrate. The bottom reflector 4 is a Distributed Bragg Reflector. Then an active region 3 (gain region) of the laser according to the present invention is sandwiched between a bottom reflector 4 and a partial reflector. Therefore, the gain region 3 is grown on the bottom reflector 4 after which the first partial reflector 5 is grown on the gain region 3 using appropriate semiconductor material compounds. The first partial reflector 5 comprises a number of quantum wells and forms the structure of Distributed Bragg Reflector (DBR). The active region 3 is formed in such a way that it comprises a number of quantum wells (QW) made of GaInAs, GaInAsP, GaInNAs, GaInNAsSb, AlGaAs or GaAsP. The fundamental lasing wavelength of the laser 1 can be chosen by appropriate selection of the material of the gain region (gain material).
The reflectivity of the bottom reflector 4 is selected close to 100% and the reflectivity of the first partial reflector 5 is selected to be less than the reflectivity of the bottom reflector 4. In some embodiments the reflectivity of the first partial reflector 5 is near 0% thus being almost antireflective. For example, the reflectivity of the first partial reflector 5 is between 25-85%.
A spacer layer 6 is formed on the surface of the first partial reflector 5. After that, a semiconductor saturable absorber 7 can be grown on the spacer layer 6. Therefore, the spacer layer 6 separates the gain region 3 and the semiconductor saturable absorber 7. The saturable absorber 7 consists of one or more quantum wells made of GaInAs, GaInAsP, GaInNAs, GaInNAsSb, AlGaAs or GaAsP and a second partial reflector 8. The quantum wells of the saturable absorber 7 can be of undoped, n-doped, p-doped or co-doped of such a semiconductor material that the optical energy emitted by the material of the gain region 3 is partly absorbed by the saturable absorber quantum well material. Also the second partial reflector 8 is a Distributed Bragg Reflector (DBR) having the reflectivity less than the reflectivity of the bottom reflector 4, for example between 25-85%. In an example embodiment of the present invention the reflectivity of the first partial reflector 5 and the second partial reflector 8 is selected to be about the same.
After the layers are grown on the substrate, the substrate can be removed e.g. by etching.
For supplying the current to the laser a first contact 9 and a second contact are formed to the laser 1 e.g. by metallization. The first contact 9 (n-contact) is in electrical contact with the second partial reflector 8 of the saturable absorber 7. The second contact 10 (p-contact) is in electrical contact with the bottom reflector 4 of the laser 1. Therefore, the first 9 and second contact 10 are metallized on opposite sides of the laser 1 in this example embodiment. The first contact 9 has an annular shape i.e. having a dielectric coated output window 12 in the centre region. The second contact 10 is preferably a circle and it is positioned below the output window 12 of the first contact 9 in the direction of the emitted radiation. Further, the diameter of the second contact 10 is smaller than the outer diameter of the first contact 9. Preferably, the diameter of the second contact 10 is even smaller than the inner diameter of the first contact 9. When an electrical power is connected to the first 9 and second contact 10, a laser diode current (biasing current) begins to flow through the layer structure of the laser 1 thus partially saturating the semiconductor saturable absorber 7. The flow of the current is shown with reference 13 in FIG. 5. The mutual location and structure of the first 9 and the second contact 10 effects that the current flow has a conical shape as can be seen from FIG. 5. This has the effect that a so-called thermal lens (illustrated with an ellipse 19 in FIG. 5) is generated in the laser device focusing the emitted radiation. It should be noted here that in the laser 1 of FIG. 1, the second contact 10 has a larger area than described above, but an insulating layer 11 is formed partly between the second contact layer 10 and the bottom reflector 4 in such a way that only a portion of the second contact 10 is in electrical contact with the bottom reflector 4.
The biasing current density is chosen in such a way that the saturable absorber 7 is partially saturated by the operating current. This lowers the saturation photon intensity. Therefore, the semiconductor saturable absorber 7 modulates the gain in the cavity of the laser 1 as a function of intensity, which mode-locks the laser 1 without any active control.
The thickness of the spacer layer 6 can be selected so that the thickness of the laser cavity is an integer of the wavelength of the radiation generated in the gain region 3.
FIG. 2 depicts a second example embodiment of the laser 1 according to the present invention. In this embodiment the contacts 9, 10 are both formed on the same side of the substrate. In this case the first contact 9 is formed by etching through the layer structure i.e. through the layers from the bottom reflector 9 to the second partial reflector 8 and forming an ohmic contact above the second partial reflector 8. The ohmic contact is formed e.g. by adding a semiconductor contact layer 13 on the second partial reflector 8. There is also a third metallic layer 14 formed on the second partial reflector 8. In this embodiment the etching forms a kind of a groove 22 penetrating through the layers from the bottom reflector 4 to the second partial reflector 8, wherein the first contact layer 9 is formed on the surface of the groove 22 e.g. by metallization.
A third example embodiment of the laser 1 according to the present invention is shown in FIG. 3. In this embodiment the voltage across the saturable absorber 7 can be controlled by a third contact 14, made of a conducting material such as a metal. It is noticeable that the saturable absorber 7 consists of a number of quantum wells, preferably more than one, and a single partial reflector 8. The quantum wells are located in the antinode of the optical field. In case of the three-contact design, the second partial reflector 8 in the saturable absorber 7 is made of variable composition of p-type AlGaAs.
FIG. 4 depicts a device 15 in which the laser according to the present invention is used. The device 15 comprises an output coupler mirror 16 which is aligned with the laser chip 1 to form an external cavity. A nonlinear crystal 17 is placed inside the external cavity to generate visible double frequency 21 of the fundamental infrared laser radiation 20. The reflectivity of the output coupler mirror 16 is chosen to be high for the fundamental wavelength 20 and low for the second harmonic 21. The exit surface 18 of the output coupler mirror 16 is antireflection coated for the second harmonic wavelength. The laser cavity is aligned in such a way that the laser 1 oscillates in TEM00 mode for efficient second harmonic generation. The reflectivity of the partial reflectors 5, 8 is chosen in such a way that the lasing threshold cannot be reached without the output coupler mirror 16.
The device 15 can also be used without the nonlinear crystal 17 to generate mode-locked pulses of the fundamental wavelength radiation. In this case the reflectivity of the output coupler mirror 16 is less than 100%.
The semiconductor saturable absorber is acting as passive mode-locking device causing self-pulsing of the external-cavity laser device 15. The conversion efficiency of the nonlinear crystal 17 depends strongly on the intensity of the light and thus the high-intensity fundamental wavelength pulses with high repetition rate increase the average second harmonic light output. The advantage of the passive mode-locking with integrated saturable absorber 7 is the reduction of the manufacturing costs and the simplicity of the cavity configuration.
In practical implementations there can also be more than one laser 1 on the same chip. For example, a number of lasers 1 can be formed as a laser matrix (not shown) on a semiconductor chip.
Patent Application
20070291799
