Patent Application
20100265975
The present invention relates to an extended cavity semiconductor laser device comprising an array of at least two semiconductor gain elements, each of said semiconductor gain elements comprising a layer structure forming a first end mirror and an active medium.
The lack of integrated laser sources in the green wavelength region has until now hindered the widespread use of lasers for display or illumination applications. Nowadays used laser sources for the green wavelength region rely on frequency conversion either by upconversion or by second harmonic generation (SHG) of an infrared laser source. For the efficiency of the frequency conversion process the intensity of the pump source is of utmost importance. While the conversion efficiency of upconversion processes in most cases depends linearly on the pump intensity, second harmonic generation depends even quadratically on the pump intensity. Therefore pump sources that deliver a high intensity beam are highly desired for efficient frequency conversion.
Quite compact setups that rely on upconversion or second harmonic generation can be realized with optically and especially with electrically pumped surface emitting semiconductor lasers. An example for such an electrically pumped, intracavity frequency doubled laser is based on a vertical cavity surface emitting laser (VCSEL) with an extended cavity (VECSEL: vertical extended cavity surface emitting laser) as shown in
While upconversion in glass-hosts exhibits a relatively broad absorption profile and has accordingly only low requirements on the spectral width of the pump diode, upconversion in crystals and especially second harmonic generation are most efficient for narrow-band pump sources. Therefore, some quite complicated approaches for second harmonic generation lasers have been suggested where the bandwidth of the pump diode is narrowed by additional optical elements like filters or volume Bragg gratings in the pump laser resonator.
Due to thermal constrains the output power of a single device is in many cases limited to some hundred milliwatts. For applications, which require higher output powers, several single devices are coupled to an array of lasers. In this case the wall-plug efficiency of the total device is the same as the wall plug efficiency of the single laser source.
Another approach is described in U.S. Pat. No. 5,131,002 A. This document discloses an extended cavity semiconductor laser system wherein multiple segments of semiconductor material separated from one another are excited with an array of pump sources. This pumping system spreads out the thermal load while providing a high power laser beam. The series operation of the multiple segments of the active medium inside of the extended laser cavity also results in an improved beam quality of the outcoupled laser beam.
It is an object of the present invention to provide a semiconductor laser device, in particular a VECSEL device, generating high power fundamental laser radiation with significantly reduced spectral bandwidth.
The object is achieved with the extended cavity semiconductor laser device, in particular a VECSEL device, according to claim 1. Advantageous embodiments of the device are subject matter of the dependent claims or are described in the subsequent portions of the description.
The extended cavity semiconductor laser device comprises an array of at least two semiconductor gain elements, each of these semiconductor gain elements comprising a layer structure forming a first end mirror and an active medium. A coupling component combining fundamental laser radiation emitted by said array of semiconductor gain elements to a single combined laser beam is arranged between the array of semiconductor gain elements and a second end mirror which reflects at least part of the single combined laser beam back to said coupling components to form extended cavities with said first end mirrors. In a most preferred embodiment the extended cavity semiconductor laser device is a VECSEL device based on an array of VCSEL components representing the semiconductor gain elements. Therefore the proposed laser device and advantageous embodiments of this device are described in the following using the example of a VECSEL. Nevertheless the invention and preferred embodiments also apply to other extended cavity laser devices like e.g. edge emitting lasers.
The VECSEL components of the array preferably have the same construction as common VCSELs with the difference that one of the DBR's forming the end mirrors of these VCSELs is partially transmissive to such an extend that lasing is not achieved without an additional external end mirror. The array of such VECSEL components, which may be a one dimensional array or a two dimensional array, can be formed of a single substrate common to all of the VECSEL components. Furthermore, the array or each single VECSEL component may be arranged on an appropriate heat sink for heat dissipation during operation.
The invention is based on the coherent coupling of laser beams of several VECSELs via constructive interference. The beams of the different VECSEL components are overlaid via the coupling component which acts like an interferometric beam combiner. The beam combiner preferably provides one or several beam splitting regions for appropriately combining the different laser beams to one single laser beam. At such a beam splitting region, for example when combining two laser beams, a portion of one of the laser beams is reflected or transmitted outside of the extended cavity. Since the laser will always tend to operate in a mode which minimizes losses, the interference between both laser beams will adjust in such a way, that the beams in the loss channel will interfere destructively, while the beams in the extended cavity will interfere constructively. Therefore, even though in principle substantial losses would be expected in such a type of cavity, the losses are avoided by destructive interference and the two laser beams are constructively added resulting in a coherent emission of both beams. The same applies to the coupling of more than two laser beams when using an array of more than two VECSEL components. Due to this coherent coupling a significant spectral narrowing is achieved, since all coupled lasers have to share a common longitudinal cavity mode while operating in laser resonators of different length. The same argument also holds true for the transverse modes. Therefore the coherent coupling of the different VECSEL components also results in a substantial improvement of the beam quality. The output of the proposed VECSEL device is correspondingly increased with the number of VECSEL components included in the device. Due to the significant spectral narrowing, the improved beam quality and the higher intensity of the fundamental radiation, preferably infrared radiation, the proposed laser device can be very advantageously used for intracavity or extra cavity frequency conversion, in particular when using frequency converting crystals as for example crystals for second harmonic generation.
Due to the spectral narrowing of the coherently coupled laser beams, there is no need for any additional spectral selectivity inside of the cavity. Therefore, much simpler outcoupling mirrors than volume Bragg gratings can be employed. For example, cheap broadband dielectric mirrors can be used for outcoupling, or a dielectric coating can be directly applied to the exit surface of the optical coupling component, in which the laser beams are coupled. The proposed laser device allows a very compact construction for generating the desired laser radiation.
In order to generate frequency doubled radiation, for example in the green wavelength region, a frequency converting medium generating the upconverted laser radiation can be arranged outside of the external cavities of the coupled laser components in the beam path of the outcoupled fundamental laser beam. In this case, the second end mirror of the device is designed to form an outcoupling mirror for said fundamental laser radiation, i.e. it is partially transmissive for said fundamental laser radiation on the one hand but still allows the laser device to operate above the laser threshold. It goes without saying, that the outcoupled fundamental laser beam may be focused by appropriate optical elements like one or several lenses into the frequency converting medium.
Another possibility to generate the frequency doubled radiation is to arrange a frequency converting medium between the coupling component and the second end mirror of the device. In this case, the second end mirror is designed to form an outcoupling mirror for the converted laser radiation and to be highly reflective for the fundamental laser radiation.
In both cases the frequency converting medium may be a doped host material for frequency upconversion or a second harmonic generation crystal, as already described in the introductory portion of this description.
The coupling component preferably comprises two opposing reflective surfaces for beam coupling. One of these surfaces is highly reflective (reflectivity≧95%) for the fundamental laser radiation, whereas the other surface has a reflectivity of between 40 and 60%, preferably 50%, and a transmittance of between 40 and 60%, preferably 50%, for the fundamental laser radiation. With such a coupling component a large number of laser beams can be combined using multiple internal reflections between the two opposing surfaces. In one advantageous embodiment, in which frequency upconversion is performed outside of the extended cavities, the coupling component is directly attached to the second end mirror, i.e. the outcoupling mirror for the fundamental radiation, or this mirror is formed by an appropriate coating on an outcoupling surface of the coupling component. This results in a very compact construction of the whole device.
In a further advantageous embodiment, the second end mirror is attached to a translation stage with which the second end mirror may be displaced to vary the length of the extended cavities. Such a translation stage may be formed of an appropriate actuator, for example a piezo-actuator. When using an optical detector measuring the intensity of outcoupled laser radiation, the length of the laser cavities can be varied through an appropriate control unit based on the measured intensity. With such an arrangement, the operation of the laser device can be optimized to have desired properties, for example a maximum output intensity and/or a stable operation. This allows for compensating any cavity length detuning, which can appear for example when the temperature of the laser device varies during operation and the optical path lengths of the external cavities change due to the resulting refractive index changes within the semiconductor material.
When using the proposed laser device with a frequency converting medium, green laser radiation can be generated with high power. Therefore, such a device can advantageously be used as one of the components of a RGB laser source or as one of the light sources in a laser projection device.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after.
The proposed semiconductor laser device is described in the following by way of examples in connection with the accompanying Figures without limiting the scope of protection as defined by the claims. The Figures show:
The laser power of such a VECSEL device is limited due to the heat generation in this device. Furthermore several measures have to be taken to spectrally narrow the fundamental laser radiation to enable efficient second harmonic generation.
The laser radiation emitted by these semiconductor gain elements 20, 21 is combined by a coupling component 22 to form a single laser beam 25. The coupling component 22 is an optical element which is coated with a high reflection coating on one side and with a coating with approximately 50% reflection on the opposite side (each for the fundamental infrared radiation). The optical element is made of a material transparent for the fundamental radiation, for example made of glass or of an appropriate plastic material. The radiation from the semiconductor gain elements 20, 21, also referred to as pump diodes, enters the optical element as depicted in
As can be seen from
The resulting beam coupled out of this laser cavity has nearly twice the power of a single VECSEL. This leads to a nearly fourfold power increase in the second harmonic radiation. Furthermore, the output coupling mirror may be formed of cheap broadband dielectric mirrors since no need for additional spectral narrowing of the coherently coupled laser beams is needed.
A further example of the proposed laser device is depicted in
As already described, the proposed laser device is of course not limited to extracavity frequency conversion, but can also be used with the frequency converting medium inside of the laser cavity. Such a setup for intracavity frequency conversion is sketched in
Let us now consider in detail two VECSEL lasers, which shall be coherently coupled in a set-up as described above. As already mentioned the extended cavities have different lengths for both devices. Let L1 be the optical length of the laser cavity for the VECSEL with the shorter extended cavity. The extended cavity of the second laser may then have the length L2=L1+D. The additional length D is mainly given by the geometrical distance between both semiconductor gain elements on the array, but may also take into account optical path differences between the two beams in the coupling device. For optimum coupling of the two lasers the frequency overlap of their longitudinal cavity modes should be safeguarded. The frequency spacing of longitudinal modes in a laser cavity, sometimes also referred to as the Free Spectral Range (FSR), is given by
(c: light velocity, L: cavity length).
The (half) width of the longitudinal modes ΔvFWHM depends on the finesse F of the resonator via the relation
and the finesse is determined by the reflectivities R1 and R2 of the two mirrors, which define the respective extended cavity:
In general it cannot be expected, that two longitudinal modes from the two coupled cavities overlap perfectly within the spectral width of the laser gain profile, as the two cavities have different lengths, and the linewidths ΔvFWHM of the longitudinal modes are usually very small.
Consider as an example a typical design with R1=99.8 and R2=99.5% for both cavities and cavity lengths of L1=8 mm and L2=13 mm, respectively. The finesse of both cavities is then F≈896, and the free spectral ranges are ΔvFSR1≈18.7 GHz for the shorter and ΔvFSR2≈11.5 GHz for the longer cavity. The linewidths are with ΔvFWHM1≈20.9 MHz and ΔvFWHM2≈12.9 MHz considerably smaller, and additional measures are preferably taken to make sure that at least two longitudinal modes of the two coupled cavities mutually match.
The quite high finesse in the example above applies to devices, in which the infrared radiation generated in the semiconductor lasers is not directly used but converted to other, preferably visible, wavelengths within the extended cavity by means of e.g. SHG or upconversion. Here the reflectivity of the outcoupling mirror is chosen as high as possible in order to achieve a highest possible infrared intensity within the extended cavity and a lowest possible laser threshold. In applications, in which the generated infrared laser light shall be used directly, the outcoupling degree for infrared radiation will be higher, and a lower value for the reflectivity R2 of the outcoupling mirror will be selected resulting in a lower finesse. But even with a reflectivity of say R2=80% and a resulting finesse of F≈28 additional measures to optimize the frequency overlap of the longitudinal modes would be advantageous.
The following considerations shall yield the minimum longitudinal tuning range of the translation stage 28, which is necessary to make sure that an optimum overlap of the two involved longitudinal modes can be achieved. Consider the case, when two longitudinal modes of the two coupled laser devices ideally match. The wavelengths of the longitudinal cavity modes are given by the boundary condition that the resonator length is an integer multiple of the half wavelength, i.e. for both cavities:
L
1
=m
1·λ/2 and L2=L1+D=m2·λ/2
The integer numbers m1 and m2 represent the so called order of the longitudinal modes, and the orders of the matching modes fulfil the relation
L
1
/m
1=(L1+D)/m2 (1)
which also shows that the shorter cavity has a lower mode order than the longer one (m1<m2).
Now consider a movement of the outcoupling mirror resulting in a length variation ΔL of the two coupled cavities. The resulting wavelength shift for each longitudinal mode is
Δλ=2/m·ΔL. (2)
As the wavelength shift is inverse proportional to the mode order the longitudinal modes of the shorter cavity will shift faster in wavelength than the longitudinal modes of the longer cavity. The next matching of adjacent longitudinal modes after increasing the cavity lengths by ΔL will thus be achieved when mode number m1 from the first cavity coincides with mode number (m2−1) of the second cavity, yielding the condition
(L1+ΔL)/m1=(L1+D+ΔL)/(m2−1). (3)
Inserting Eq. (1) into Eq. (3) yields after some calculations the maximum necessary longitudinal tuning range for the translation stage:
ΔL=L1/(m2−m1−1)≈L1/Δm with Δm=m2−m1.
Using the typical numbers of the above given example (L1=8 mm, L2=13 mm, D=5 mm) and assuming a typical infrared wavelength of λ=1 μm one gets m1=16000, m2=26000, Δm=10000 and finally ΔL≈800 nm. The resonance wavelength, at which the longitudinal modes of the two coherent cavities overlap, shifts according to Eq. (2) only by Δλ≈0.1 nm, which is usually way below the gain width of the semiconductor laser material.
While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments. The different embodiments described above and in the claims can also be combined. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. For example, the proposed setup is not limited to linear arrays of semiconductor gain elements or to only two semiconductor gain elements but may also be used with two-dimensional arrays or arrays having a higher number of semiconductor gain elements. Furthermore, not only VCSEL based structures but also other structures like edge emitting laser structures can be used to achieve similar advantages. The exact construction of the layer structure is not critical in order to achieve the disclosed advantages, therefore different layer structures forming the first end mirror and the active gain medium may be used as known in the art.
In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. The reference signs in the claims should not be construed as limiting the scope of these claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07120175.0 | Nov 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB08/54552 | 11/3/2008 | WO | 00 | 5/3/2010 |
