The present invention relates to an upconversion laser system comprising at least a semiconductor laser having a gain structure arranged between a first mirror and a second mirror, said first and said second mirror forming a laser cavity of the semiconductor laser.
Highly efficient semiconductor laser typically emit fundamental radiation in the infrared (IR) wavelength range. Many applications however require optical radiation in the visible or ultraviolet wavelength range. In order to use IR semiconductor lasers for such applications it is known to couple the output of the semiconductor laser into the gain medium of an upconversion laser, typically a special waveguide or fiber laser, which generates the desired laser wavelength in the visible wavelength range.
In the upconversion process, a high-lying electronic state of an atom is populated by the absorption of two or more pump photons via intermediate resonances. From this high-lying electronic state a photon of higher energy and accordingly shorter wavelength than the pump radiation is emitted. Using this upconversion process it is possible to convert infrared laser radiation to radiation in the visible wavelength range. A prominent example is the upconversion laser based on Er-doped ZBLAN-glass, where two photons of 970 nm wavelength are absorbed by Er3+-ions and radiation around 550 nm is emitted. Currently, there is an increased interest in this process, since it provides the opportunity to realize an integrated green laser source.
However, due to the required absorption of two photons for the upconversion process, high pump power densities have to be provided. Today this is achieved by confining the pump light to a waveguide, as in the case of the upconversion fiber laser. In this laser the pump radiation from e.g. a laser diode is focused into the Er-doped core of a glass-fiber. The fiber facets are coated with dielectric coatings that transmit the pump radiation and have a certain reflectivity for the upconversion laser radiation, so that a resonator is formed. Typically, the core of these fibers has diameters in the range 2-40 μm. Such small diameters make the coupling of the pump radiation a difficult task. Coupling losses of the pump radiation to the fiber limit the efficiency of the upconversion laser and lead to relatively high laser thresholds.
An example of an upconversion laser system improving the coupling efficiency is disclosed in WO 2005/022708 A1 and shown in
It is an object of the present invention, to provide an upconversion laser system having compact dimensions at the same or even higher output power than the above described upconversion laser system.
The object is achieved with the upconversion laser system according to claim 1. Advantageous embodiments of this laser system are subject matter of the dependent claims or described in the following description and embodiments.
The proposed upconversion laser system comprises at least a semiconductor laser having a gain structure arranged between a first mirror and a second mirror, said first and said second mirror forming a laser cavity of the semiconductor laser, and an upconversion laser for upconverting a fundamental radiation of said semiconductor laser. The upconversion laser system of the present invention is characterized in that said upconversion laser is arranged in the laser cavity of the semiconductor laser, which serves as pump laser for the upconversion laser.
This means that the upconverting material is placed inside the pump laser cavity. Inside the pump laser cavity the pump power density is highest and losses to this cavity are ideally only given by the absorption in the upconverting material. Furthermore, the pump radiation is absorbed in multiple passes through the upconverting material, so that the single pass absorption of this material can be kept much lower than for example with a fiber laser. Therefore the length of the upconverting material can be in the order of a few millimeters. This is a drastic size reduction even without any changes in the doping concentration of the upconverting material. Therefore, the proposed upconversion laser system can be designed in very compact dimensions.
No waveguide or fiber is needed in this intracavity pumping scheme for the upconversion laser. The gain region of the upconversion laser is defined by the pump beam. This makes the alignment of such an upconversion laser an easy task and coupling losses are reduced to a minimum.
The upconverting material is pumped much more homogenously when placed inside the cavity of the pump laser. In fiber lasers, the first part of the fiber is always pumped much stronger than the last, due to absorption of the pump radiation in the fiber. Since in the present upconversion laser system the interaction length is drastically reduced, as explained above, the pump absorption along the upconverting material is much more homogeneous than in fiber lasers.
The compact size at relative high output power makes the proposed upconversion laser a good candidate to replace nowadays UHP lamps as the light source for projection systems or to serve as light source in fiber optic illumination units, for example in endoscopes or display systems. The laser system allows easy power scaling and mass manufacturing. The upconverting material and one of the resonator mirrors of the semiconductor laser can be made in a single element. This element can be placed in front of a single stripe edge emitting laser as well as in front of a laser bar or even a laser diode stack. It can be placed in front of a single VECSEL (Vertical External Cavity Surface Emitting Laser) for a single upconversion laser system or in front of an array of VECSELs. Given the proper optical cavity layout, in all these cases, the only critical parameters are the angles under which the upconverting material with the mirror coated on it has to be positioned. This means simplicity for laser alignment.
In the proposed upconversion laser system the upconversion laser preferably comprises a solid state medium made of the upconverting material between two mirrors, one highly reflective (preferably T<1%) and the other one partially transmitting (preferably T=1-30%) for upconverted radiation. Nevertheless, even more than 30% transmission may be preferred for the partially transmitting mirror (up to 96% has been proven to work efficiently in fibers for PrYb, up to 70% for Er-ZBLAN). In the preferred embodiment, one of said mirrors of the upconversion laser is the second mirror of the semiconductor laser. This mirror is preferably in direct contact with the upconverting material and also allows for coupling out a portion of the upconverted radiation. In a preferred embodiment the two mirrors of the upconversion laser are formed of dielectric coatings that are directly applied to the surface of the upconverting material.
In a further preferred embodiment, which can also be combined with other embodiments of the proposed upconversion laser system, an optical system generating a beam waist of the fundamental radiation within the upconverting material is arranged inside the semiconductor laser cavity. This optical system can be a single lens or a more complicated arrangement of optical elements. Such an optical system has a twofold advantage. First, the end mirror of the pump laser cavity or resonator can be a flat mirror, which facilitates the laser alignment. Secondly and more important, the beam diameters decrease and therefore the pump power density increases inside the upconverting material, resulting in a further improved efficiency of the upconversion laser. With the beam waist of the pump laser placed at the resonator mirror (second mirror), an optimum situation in view of pump power density is achieved.
In the case of a pump laser having a gain material emitting fundamental radiation in the infrared wavelength range, for example at a central wavelength of 970 nm, the upconverting material of the upconversion laser is preferably an Er3+-doped ZBLAN-glass. Nevertheless, the present upconversion laser system is not restricted to the upconversion of infrared radiation or to the use of doped ZBLAN-glass as the upconverting material. The skilled person is able to use other combinations of gain materials for generating laser output of a desired wavelength. Such materials are for example other rare earth ions or a combination of ions in ZBLAN or other hosts like LiLuF4, YLF, BaY2F8, Y2O3, YAlO3 or tellurite glasses, all characterized by low phonon energies. Although in the following examples two special cavity layouts are described, there are also other possibilities for the cavity layout of the proposed upconversion laser system, which are generally known in the field of laser technique.
In the present description and claims the word “comprising” does not exclude other elements or steps as well as an “a” or “an” does not exclude a plurality. Also any reference signs in the claims shall not be construed as limiting the scope of these claims.
Exemplary embodiments of the proposed upconversion laser system are described in the following in connection with the accompanying figures without limiting the scope of the invention as defined by the patent claims. The figures show:
In the pump laser cavity 7 a lens 12 is placed to achieve a beam waist 13 of the pump laser radiation within the upconverting material 8, for example 3000 ppm-doped Er:ZBLAN. The upconverted radiation is coupled out of this upconversion laser system through the second mirror 6 which is indicated as visible output 14 in
The lens 12 reduces the beam diameter of the pump radiation within the upconverting material 8, leading to improved efficiency of the upconversion process. In
The upconverting material 8 should be made in such a way that the intracavity power is reduced by 1 to 10% due to absorption in the upconverting material. The absorption properties of the upconverting material can be tailored by the dopant concentration and the length of the medium. This consideration should be explained with the example of 3000 ppm Er3+-doped ZBLAN as the upconverting material. This material has an absorption coefficient of about α=0.12 cm−1 at a wavelength of around 970 nm. The absorption through a material of length x and absorption coefficient α is described by the following equation:
I(x)=I0e−αx
The material should have a length L. A roundtrip of the pump radiation through the material then corresponds to an absorption path of 2L. The fraction k of the absorbed pump power should be the roundtrip loss from the pump laser cavity. Therefore the power fed back to the pump laser cavity reads as:
I(2L)=(1−k)I0
Finally the length L(k) of the upconverting material as a function of the absorption k can be calculated according to:
This curve is plotted in
Number | Date | Country | Kind |
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06113175.1 | Apr 2006 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2007/051367 | 4/17/2007 | WO | 00 | 10/10/2008 |