WO2016042285A1 - Solar pumped laser - Google Patents

Solar pumped laser Download PDF

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Publication number
WO2016042285A1
WO2016042285A1 PCT/GB2015/000268 GB2015000268W WO2016042285A1 WO 2016042285 A1 WO2016042285 A1 WO 2016042285A1 GB 2015000268 W GB2015000268 W GB 2015000268W WO 2016042285 A1 WO2016042285 A1 WO 2016042285A1
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Prior art keywords
solar
laser
concentrator
pumped laser
gain medium
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PCT/GB2015/000268
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French (fr)
Inventor
Adrian QUARTERMAN
Keith WILCOX
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The University Of Dundee
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
    • H01S5/041—Optical pumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/14—External cavity lasers
    • H01S5/141—External cavity lasers using a wavelength selective device, e.g. a grating or etalon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/10—Construction or shape of the optical resonator, e.g. extended or external cavity, coupled cavities, bent-guide, varying width, thickness or composition of the active region
    • H01S5/18—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities
    • H01S5/183—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL]
    • H01S5/18383—Surface-emitting [SE] lasers, e.g. having both horizontal and vertical cavities having only vertical cavities, e.g. vertical cavity surface-emitting lasers [VCSEL] with periodic active regions at nodes or maxima of light intensity
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/0206—Substrates, e.g. growth, shape, material, removal or bonding
    • H01S5/0213—Sapphire, quartz or diamond based substrates
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/0206—Substrates, e.g. growth, shape, material, removal or bonding
    • H01S5/0215—Bonding to the substrate
    • H01S5/0216—Bonding to the substrate using an intermediate compound, e.g. a glue or solder
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/0206—Substrates, e.g. growth, shape, material, removal or bonding
    • H01S5/0217—Removal of the substrate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/02—Structural details or components not essential to laser action
    • H01S5/024—Arrangements for thermal management
    • H01S5/02476—Heat spreaders, i.e. improving heat flow between laser chip and heat dissipating elements
    • H01S5/02484—Sapphire or diamond heat spreaders
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
    • H01S5/34313—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser with a well layer having only As as V-compound, e.g. AlGaAs, InGaAs
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/34—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers
    • H01S5/343—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
    • H01S5/34346—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser characterised by the materials of the barrier layers
    • H01S5/34386—Structure or shape of the active region; Materials used for the active region comprising quantum well or superlattice structures, e.g. single quantum well [SQW] lasers, multiple quantum well [MQW] lasers or graded index separate confinement heterostructure [GRINSCH] lasers in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser characterised by the materials of the barrier layers explicitly Al-free

Definitions

  • the present invention relates to a solar pumped laser and in particular to a solar pumped semiconductor laser which is suitable for use for renewable energy generation and storage.
  • MAGIC MAGnesium Injection Cycle
  • solar-powered lasers would be used to thermally reduce magnesium oxide obtained from seawater.
  • the magnesium could then be transported efficiently to the point of use where it would be reacted with steam to produce heat and hydrogen gas. It was proposed that this scheme would enable a renewable hydrogen economy by solving the problems of storage and transport of hydrogen: magnesium has an energy density ten times greater than hydrogen and, as a solid (albeit flammable) powder rather than an explosive gas or cryogenic liquid it is both easier and safer to store and transport.
  • Figure 1 The chemical equations which describe the MAGIC Cycle are shown in Figure 1.
  • the MAGIC cycle was conceived to offer capabilities that complement those of photovoltaic electricity generation and would therefore represent one facet of a robust solar energy generation, transport and storage scheme.
  • the problems associated with the creation of a solar laser include poor solar pump absorption in the gain medium and poor output beam quality and problems associated with providing a sufficiently concentrated beam of sunlight onto the gain medium.
  • a solar pumped laser comprising:
  • a solar concentrator which provides a concentrated output beam of sunlight which acts as a broadband pump source
  • the gain region absorbs the concentrated output beam over a large surface area and to a shallow cross sectional depth below the surface of the gain medium.
  • the broadband semiconductor gain medium is a Vertical External Cavity Surface Emitting Laser VECSEL or semiconductor disk laser (SDL) gain medium.
  • VECSEL Vertical External Cavity Surface Emitting Laser
  • SDL semiconductor disk laser
  • the broadband semiconductor gain medium is a thin disk semiconductor.
  • the gain medium has a thin disk geometry where the region of overlap between the sunlight and the laser light takes the form of a disk. This gives the solar-pumped VECSEL scope for better laser beam quality, as its overlap region only needs to be correct in two dimensions rather than three, the third dimension being too thin to allow significant divergence.
  • the fact that the gain region is substantially two dimensional allows improved better beam quality when compared with crystal gain media which have greater depth.
  • the ratio of gain region surface length.breadth or diameter to gain region cross sectional depth is at least 10:1.
  • the ratio of gain region surface length.breadth or diameter to gain region cross sectional depth is at least 100:1.
  • the semiconductor gain medium has a band gap which allows absorption of sunlight.
  • the semiconductor gain medium has a band gap in the range 740 to
  • the semiconductor gain medium comprises Gallium Arsenide.
  • the semiconductor gain medium comprises Indium Gallium Arsenide.
  • the solar pumped laser has a threshold pump power which scales approximately linearly with pump spot diameter.
  • the solar concentrator produces a region of highly concentrated sunlight at an exit face of the secondary concentrator.
  • the power/intensity achieved at the output of the secondary concentrator in the useful part of the solar spectrum can be optimised by design up to the thermodynamic limit of solar concentration so as to exceed the laser threshold value by a large enough factor to provide useful laser performance.
  • the solar concentrator comprises a primary solar concentrator for producing a magnified solar image with a focal point on the gain region of the VECSEL and a secondary solar concentrator for matching the size and angular divergence of the solar image to the focal point.
  • the solar concentrator further comprises a tracking mechanism for aligning the orientation of the concentrator to optimise the amount of sunlight captured from the sun.
  • the primary solar concentrator is a mirror.
  • the primary solar concentrator is a lens.
  • the solar concentrator further comprises an aperture which allows the laser mode to pass through the solar concentrator.
  • the secondary solar concentrator is a non-imaging optical concentrator based on a Compound Parabolic Concentrator (CPC).
  • the secondary solar concentrator is a non-imaging optical concentrator based on a Dielectric Total Internal Reflection Concentrator (DTIRC).
  • DTIRC Dielectric Total Internal Reflection Concentrator
  • the solar laser has a B Factor of at least 7W.
  • the solar laser has a Collection Efficiency of at least 100 W/m 2 .
  • Figure 1 is a set of three chemical equations which describe the Magnesium injection Cycle
  • FIG. 2 is a schematic diagram which shows an example of a solar pumped VECSEL in accordance with the present invention
  • Figure 3 is a graph illustrating the dimensions and properties of an example of a gain medium in accordance with the present invention which shows the bandgap as a function of depth within the sample and the electric field penetration;
  • Figure -4a is a graph which plots emitted photons against increasing Neutral Density filter thickness for a number of long pass filters
  • figure 4b is a graph which emitted photons against wavelength and shows calculated pump spectra for different long pass filters;
  • Figure 5 is a graph which plots quantum efficiency as a function of wavelength for different longpass filter cut-off values
  • Figure 6 is a graph which shows a solar irradiance spectrum and VECSEL pumping efficiency as functions of wavelength
  • FIG. 7 is a graph which plots collection efficiency against B factor for known solar lasers and for solar lasers in accordance with the present invention. Detailed Description of the Drawings
  • the present invention comprises a high power, large mode area VECSEL and high concentration ratio solar concentrators which provides a high power, high slope efficiency, solar-pumped semiconductor lasers which is suitable for use in solar energy generation, distribution and other applications.
  • FIG. 2 is a schematic diagram which shows components of an embodiment of the present invention.
  • the primary concentrator 1 comprises either a lens or mirror which is capable of producing a magnified solar image on the input face of a secondary concentrator 2. It may also include a hole to allow passage of the laser mode if the laser mode is required to pass through the concentration optics.
  • the secondary concentrator 2 is a non-imaging optical concentrator based on a Compound Parabolic Concentrator (CPC) or a Dielectric Total Internal Reflection Concentrator (DTIRC). Input face dimensions and concentrator geometry are designed to optimally match the size and angular divergence of the solar image at the primary concentrator focus.
  • CPC Compound Parabolic Concentrator
  • DTIRC Dielectric Total Internal Reflection Concentrator
  • This and other embodiments of the present invention may also comprise a mechanism for solar tracking so as to maintain the geometrical orientation of the laser relative to the sun. This would be achieved either by using a heliostat or by rotating the entire laser. Optical filters may also be used to prevent unwanted parts of the solar spectrum from reaching the laser gain medium.
  • the solar concentrator (solar tracker, optical filters, primary concentrator and secondary concentrator) produces a region of highly concentrated sunlight at the exit face of the secondary concentrator.
  • the power/intensity achieved at the output of the secondary concentrator in the useful part of the solar spectrum can be optimised by design up to the thermodynamic limit of solar concentration and will depend on the power/intensity required to exceed laser threshold by a large enough factor to provide useful laser performance.
  • the concentrator system allows a large degree of concentration factors and is scalable in terms of areas, allowing a wide range of powers and intensities to be achieved for pumping solar lasers.
  • the VECSEL gain sample 3 is positioned in optical contact with the exit face of the secondary concentrator 2 so that the concentrated solar power will be transmitted into the VECSEL gain sample.
  • the VECSEL gain sample 3 is a thin-disk semiconductor medium in the form of a disk with a thickness of 1-2 microns and a diameter of several hundred microns. Accordingly, if the area of the semiconductor disk is 200 x 200 microns, the ratio of area to thickness for a 2 micron thickness is 20000:1. It is helpful to consider the diameter of a circular disk, or the surface length and breadth dimensions of a non circular disk where the ratio may be preferably 100:1 or 200:1. The incident light from the solar concentrator is absorbed very strongly by the semiconductor.
  • the thin-disk geometry of the semiconductor gain medium provides a region of overlap between the sunlight and the gain medium which has a large surface area and little depth.
  • a crystal-based laser would have an overlap region with a roughly cylindrical shape, typically with centimetre-scale dimensions.
  • the substantially two-dimensional geometry of the gain region and its small size improve beam quality with respect to known systems.
  • the bandgap of the pump absorbing layers in the gain medium is at an energy level which is suitable for absorbing the desired portion of the solar spectrum, with laser emission at a slightly lower energy.
  • a gallium arsenide semiconductor with a bandgap of 850-870 nm has been selected for the sunlight absorbing layers in the VECSEL gain medium; other semiconductors with similar bandgaps could be used.
  • the VECSEL gain sample also provides high reflectivity at the laser wavelength so as to act as a cavity mirror.
  • An example of an external laser cavity 4 is shown in figure 2, though other cavity geometries are possible. Matching of the laser cavity mode size to the secondary concentrator exit size by suitable laser cavity design allows high laser beam quality to be achieved. Optical components for frequency conversion, modelocking or other mechanisms may be included in the cavity depending on the intended application. A laser mode 5 formed between the VECSEL gain sample and the other components forming the laser cavity is also shown.
  • the solar concentrator 1 is positioned to collect sun light.
  • the primary concentrator focusses ligh into the secondary concentrator 2, whose output is a highly divergent region of concentrated sunlight.
  • the sample must be placed in optical contact with the secondary concentrator 2 because the light from the secondary concentrator is not in the form of a beam which can be refocused onto the sample.
  • the coherent light is reflected and amplified and an output laser beam 5 is created.
  • the gain medium comprised 10 InGaAs quantum wells in a strain-balanced GaAsP active region of thickness 1.7 pm. It was grown upside-down and bonded using indium to a 300-mm thick diamond before having its substrate removed by chemical etching to leave a GaAsP cap layer, whose thickness made the sample resonant at the laser wavelength.
  • Figure 3 is a graph 51 which plots the bandgap as a function of depth within the sample and the calculated electric field penetration for a gain medium in accordance with the present invention.
  • the Graph has an X-axis 53 which plots the depth below the surface of the gain medium in nanometres.
  • the left hand side Y-axis 55 plots the bandgap energy in electron volts and the right hand side Y-axis 57 plots the normalized electric field.
  • the band gap curve 59 shows an initially higher value which falls to ten minimum values 61 at the position of the quantum wells.
  • the E-field curve 63 shows an oscillating E-field and the quantum wells are spaced one per E-field antinode.
  • Figure 4a is a graph 71 which plots emitted photons 73 against increasing Neutral Density filter thickness 75 and shows the number of laser-wavelength photons emitted by the gain sample calculated from the photodiode signal as a function of the neutral density thickness blocking the pump light. This is known as the integrated
  • FIG. 4b is a graph 81 which plots the estimated numbers of absorbed photons 83 against wavelength 85 and shows calculated pump spectra for different long pass filters
  • the diagram key 87 labels the filter types with letters A to F with corresponding letters attached to the measurement curves 89.
  • the calculated spectra of the light for the different longpass filters shown in figure 5b up to the bandgap of Gallium Arsenide and based in an AM .5 standard solar spectrum. Integrating these pump light spectra allow the total number of pump photons incident on the sample and thus the quantum efficiency to be calculated.
  • Figure 5 is a graph 91 which plots quantum efficiency 93 as a function of wavelength 95 for different longpass filter cut-off values. These values are constant to within measurement error across the spectrum, which is consistent with data from lasers pumped with 808 nm and 532 nm laser light.
  • the threshold pump power of a laser is the value of the pump power at which the laser threshold is just reached, usually assuming steady-state conditions. At this point, the small-signal gain equals the losses of the laser resonator. Threshold pump power was found to scale quadratically with pump spot diameter. On that basis, the output power of an OP-VECSEL can be scaled up quadratically with pump spot diameter.
  • the intensity needed to reach lasing threshold in high power VECSELs is typically in excess of -20 MW/m A 2 , depending on the specifics of the gain sample and cavity loss.
  • the irradiance at the earth's surface has an intensity of -1000 W/m A 2, of which approximately 65 % is at wavelengths which can be used to generate carriers in a GaAs-based VECSEL's active region. It is therefore clear that sunlight must be concentrated by a factor of at least 30000 in order to even reach lasing threshold, and by a larger factor to achieve useful power output.
  • Figure 7 is a graph 11 1 which plots collection efficiency 115 and B factor 113 of existing solar lasers and the calculated performance of an embodiment of the present invention on logarithmic scales.
  • Collection efficiency is defined as the output power per unit of primary sunlight collector area and is a measure of how efficiently the available solar power is used.
  • the B Factor is defined as the output power divided by the square of the M-squared value of the laser output (the conventional measure of laser beam quality).
  • the B Factor is a general figure-of-merit of the potential usefulness of the laser output.
  • a solar-pumped laser in accordance with the present invention may have many applications.
  • One of the principal applications envisaged for the present invention is as a power source for the initiation of the MAGIC cycle where Magnesium Oxide powder is heated to very high temperatures, causing the reduction of the Magnesium Oxide.
  • the Magnesium metal can then be used as an easily transportable, high energy density fuel, either by burning it directly in air or carbon dioxide, or by reacting it with steam to produce hydrogen gas.
  • a means of producing magnesium using sunlight could be a very valuable green energy technology.
  • the temperatures needed for the process are achievable using solar-pumped lasers.

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  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
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Abstract

In a solar pumped laser, a vertical external cavity surface emitting laser (VECSEL) is attached to a two-stage solar concentrator comprising a primary concentrator (1) and a non-imaging secondary concentrator (2). The semiconductor gain medium (3) with broadband absorption absorbs the concentrated output beam over a large surface area and a shallow cross sectional depth below the surface of the gain medium. This gives the solar-pumped VECSEL scope for better laser beam quality, as its overlap region between pump light and laser mode (5) only needs to be matched in two dimensions, the third dimension being too thin to allow significant divergence.

Description

Solar Pumped Laser Introduction The present invention relates to a solar pumped laser and in particular to a solar pumped semiconductor laser which is suitable for use for renewable energy generation and storage.
Background to the Invention
Energy storage is a problem which affects many renewable energy sources: the unpredictability of strong winds or bright sunshine makes matching electricity supply with demand difficult. Electrical transmission presents a further problem for photovoltaic sources. Sunlight can provide a plentiful supply of clean power, a fact which has motivated research into photovoltaic cells and has spurred on the rapid growth of the photovoltaic market over recent decades. Solar power stations are frequently situated in desert or near-desert areas, where bright sunshine is guaranteed and competition for land use is low. Transmission of power from these stations to areas of high demand incurs an unavoidable loss of power in electrical grids.
In 2006, researchers at the Tokyo Institute of Technology proposed a solution to these problems in the form of the MAGnesium Injection Cycle (MAGIC). Here, solar-powered lasers would be used to thermally reduce magnesium oxide obtained from seawater. The magnesium could then be transported efficiently to the point of use where it would be reacted with steam to produce heat and hydrogen gas. It was proposed that this scheme would enable a renewable hydrogen economy by solving the problems of storage and transport of hydrogen: magnesium has an energy density ten times greater than hydrogen and, as a solid (albeit flammable) powder rather than an explosive gas or cryogenic liquid it is both easier and safer to store and transport. The chemical equations which describe the MAGIC Cycle are shown in Figure 1. The MAGIC cycle was conceived to offer capabilities that complement those of photovoltaic electricity generation and would therefore represent one facet of a robust solar energy generation, transport and storage scheme. Despite the promise of the MAGIC scheme it has proved very difficult to create a solar laser device which is fit for purpose. The problems associated with the creation of a solar laser include poor solar pump absorption in the gain medium and poor output beam quality and problems associated with providing a sufficiently concentrated beam of sunlight onto the gain medium. Summary of the Invention
It is an object of the present invention to provide an improved solar pumped laser.
In accordance with a first aspect of the invention there is provided a solar pumped laser comprising:
a solar concentrator which provides a concentrated output beam of sunlight which acts as a broadband pump source;
and a semiconductor gain medium with broadband absorption which is in optical contact with the concentrated output beam in its gain region; and
an external laser cavity wherein the gain region absorbs the concentrated output beam over a large surface area and to a shallow cross sectional depth below the surface of the gain medium.
Preferably, the broadband semiconductor gain medium is a Vertical External Cavity Surface Emitting Laser VECSEL or semiconductor disk laser (SDL) gain medium.
Preferably, the broadband semiconductor gain medium is a thin disk semiconductor.
The gain medium has a thin disk geometry where the region of overlap between the sunlight and the laser light takes the form of a disk. This gives the solar-pumped VECSEL scope for better laser beam quality, as its overlap region only needs to be correct in two dimensions rather than three, the third dimension being too thin to allow significant divergence. The fact that the gain region is substantially two dimensional allows improved better beam quality when compared with crystal gain media which have greater depth.
Preferably, the ratio of gain region surface length.breadth or diameter to gain region cross sectional depth is at least 10:1.
More preferably, the ratio of gain region surface length.breadth or diameter to gain region cross sectional depth is at least 100:1.
Preferably, the semiconductor gain medium has a band gap which allows absorption of sunlight.
Preferably, the semiconductor gain medium has a band gap in the range 740 to
1000nm. Preferably, the semiconductor gain medium comprises Gallium Arsenide.
Optionally, the semiconductor gain medium comprises Indium Gallium Arsenide.
Preferably, the solar pumped laser has a threshold pump power which scales approximately linearly with pump spot diameter.
Preferably, the solar concentrator produces a region of highly concentrated sunlight at an exit face of the secondary concentrator. Preferably the power/intensity achieved at the output of the secondary concentrator in the useful part of the solar spectrum can be optimised by design up to the thermodynamic limit of solar concentration so as to exceed the laser threshold value by a large enough factor to provide useful laser performance.
Preferably, the solar concentrator comprises a primary solar concentrator for producing a magnified solar image with a focal point on the gain region of the VECSEL and a secondary solar concentrator for matching the size and angular divergence of the solar image to the focal point.
Preferably, the solar concentrator further comprises a tracking mechanism for aligning the orientation of the concentrator to optimise the amount of sunlight captured from the sun.
Preferably, the primary solar concentrator is a mirror. Optionally, the primary solar concentrator is a lens.
Preferably, the solar concentrator further comprises an aperture which allows the laser mode to pass through the solar concentrator. Preferably, the secondary solar concentrator is a non-imaging optical concentrator based on a Compound Parabolic Concentrator (CPC).
Optionally, the secondary solar concentrator is a non-imaging optical concentrator based on a Dielectric Total Internal Reflection Concentrator (DTIRC).
Preferably, the solar laser has a B Factor of at least 7W.
Preferably, the solar laser has a Collection Efficiency of at least 100 W/m2. Brief Description of the Drawings
The present invention will now be described with reference to the accompanying drawings in which:
Figure 1 is a set of three chemical equations which describe the Magnesium injection Cycle;
Figure 2 is a schematic diagram which shows an example of a solar pumped VECSEL in accordance with the present invention;
Figure 3 is a graph illustrating the dimensions and properties of an example of a gain medium in accordance with the present invention which shows the bandgap as a function of depth within the sample and the electric field penetration;
Figure -4a is a graph which plots emitted photons against increasing Neutral Density filter thickness for a number of long pass filters, figure 4b is a graph which emitted photons against wavelength and shows calculated pump spectra for different long pass filters;
Figure 5 is a graph which plots quantum efficiency as a function of wavelength for different longpass filter cut-off values;
Figure 6 is a graph which shows a solar irradiance spectrum and VECSEL pumping efficiency as functions of wavelength; and
Figure 7 is a graph which plots collection efficiency against B factor for known solar lasers and for solar lasers in accordance with the present invention. Detailed Description of the Drawings
The present invention comprises a high power, large mode area VECSEL and high concentration ratio solar concentrators which provides a high power, high slope efficiency, solar-pumped semiconductor lasers which is suitable for use in solar energy generation, distribution and other applications.
Figure 2 is a schematic diagram which shows components of an embodiment of the present invention. The primary concentrator 1 comprises either a lens or mirror which is capable of producing a magnified solar image on the input face of a secondary concentrator 2. It may also include a hole to allow passage of the laser mode if the laser mode is required to pass through the concentration optics. The secondary concentrator 2 is a non-imaging optical concentrator based on a Compound Parabolic Concentrator (CPC) or a Dielectric Total Internal Reflection Concentrator (DTIRC). Input face dimensions and concentrator geometry are designed to optimally match the size and angular divergence of the solar image at the primary concentrator focus.
This and other embodiments of the present invention may also comprise a mechanism for solar tracking so as to maintain the geometrical orientation of the laser relative to the sun. This would be achieved either by using a heliostat or by rotating the entire laser. Optical filters may also be used to prevent unwanted parts of the solar spectrum from reaching the laser gain medium.
The solar concentrator (solar tracker, optical filters, primary concentrator and secondary concentrator) produces a region of highly concentrated sunlight at the exit face of the secondary concentrator. The power/intensity achieved at the output of the secondary concentrator in the useful part of the solar spectrum can be optimised by design up to the thermodynamic limit of solar concentration and will depend on the power/intensity required to exceed laser threshold by a large enough factor to provide useful laser performance. The concentrator system allows a large degree of concentration factors and is scalable in terms of areas, allowing a wide range of powers and intensities to be achieved for pumping solar lasers.
The VECSEL gain sample 3 is positioned in optical contact with the exit face of the secondary concentrator 2 so that the concentrated solar power will be transmitted into the VECSEL gain sample. The VECSEL gain sample 3 is a thin-disk semiconductor medium in the form of a disk with a thickness of 1-2 microns and a diameter of several hundred microns. Accordingly, if the area of the semiconductor disk is 200 x 200 microns, the ratio of area to thickness for a 2 micron thickness is 20000:1. It is helpful to consider the diameter of a circular disk, or the surface length and breadth dimensions of a non circular disk where the ratio may be preferably 100:1 or 200:1. The incident light from the solar concentrator is absorbed very strongly by the semiconductor. The thin-disk geometry of the semiconductor gain medium provides a region of overlap between the sunlight and the gain medium which has a large surface area and little depth. In contrast a crystal-based laser would have an overlap region with a roughly cylindrical shape, typically with centimetre-scale dimensions. This gives the solar-pumped VECSEL scope for better laser beam quality, as its overlap region only needs to be correct in two dimensions rather than three, the third dimension being too thin to allow divergence. The substantially two-dimensional geometry of the gain region and its small size improve beam quality with respect to known systems. The bandgap of the pump absorbing layers in the gain medium is at an energy level which is suitable for absorbing the desired portion of the solar spectrum, with laser emission at a slightly lower energy. Most of the energy in sunlight is at wavelengths shorter than 900 nm, so a semiconductor with a bandgap close to 900 nm was chosen because it allows most of the energy to be absorbed. In this example, a gallium arsenide semiconductor with a bandgap of 850-870 nm has been selected for the sunlight absorbing layers in the VECSEL gain medium; other semiconductors with similar bandgaps could be used.
The VECSEL gain sample also provides high reflectivity at the laser wavelength so as to act as a cavity mirror.
An example of an external laser cavity 4 is shown in figure 2, though other cavity geometries are possible. Matching of the laser cavity mode size to the secondary concentrator exit size by suitable laser cavity design allows high laser beam quality to be achieved. Optical components for frequency conversion, modelocking or other mechanisms may be included in the cavity depending on the intended application. A laser mode 5 formed between the VECSEL gain sample and the other components forming the laser cavity is also shown. In use, the solar concentrator 1 is positioned to collect sun light. The primary concentrator focusses ligh into the secondary concentrator 2, whose output is a highly divergent region of concentrated sunlight. The sample must be placed in optical contact with the secondary concentrator 2 because the light from the secondary concentrator is not in the form of a beam which can be refocused onto the sample. The coherent light is reflected and amplified and an output laser beam 5 is created.
In the example of the invention shown in figure 2, the gain medium comprised 10 InGaAs quantum wells in a strain-balanced GaAsP active region of thickness 1.7 pm. It was grown upside-down and bonded using indium to a 300-mm thick diamond before having its substrate removed by chemical etching to leave a GaAsP cap layer, whose thickness made the sample resonant at the laser wavelength.
Figure 3 is a graph 51 which plots the bandgap as a function of depth within the sample and the calculated electric field penetration for a gain medium in accordance with the present invention. The Graph has an X-axis 53 which plots the depth below the surface of the gain medium in nanometres. The left hand side Y-axis 55 plots the bandgap energy in electron volts and the right hand side Y-axis 57 plots the normalized electric field. The band gap curve 59 shows an initially higher value which falls to ten minimum values 61 at the position of the quantum wells. The E-field curve 63 shows an oscillating E-field and the quantum wells are spaced one per E-field antinode.
Figure 4a is a graph 71 which plots emitted photons 73 against increasing Neutral Density filter thickness 75 and shows the number of laser-wavelength photons emitted by the gain sample calculated from the photodiode signal as a function of the neutral density thickness blocking the pump light. This is known as the integrated
photoluminescence measurement and the measurements were taken over 20 minutes on a cloudless day, with the average insolation over the course of the measurements being 720 W/m2. The diagram key 77, labels the filter types with letters A to F with corresponding letters attached to the measurement curves 79. Figure 4b is a graph 81 which plots the estimated numbers of absorbed photons 83 against wavelength 85 and shows calculated pump spectra for different long pass filters The diagram key 87, labels the filter types with letters A to F with corresponding letters attached to the measurement curves 89. The calculated spectra of the light for the different longpass filters shown in figure 5b up to the bandgap of Gallium Arsenide and based in an AM .5 standard solar spectrum. Integrating these pump light spectra allow the total number of pump photons incident on the sample and thus the quantum efficiency to be calculated.
Figure 5 is a graph 91 which plots quantum efficiency 93 as a function of wavelength 95 for different longpass filter cut-off values. These values are constant to within measurement error across the spectrum, which is consistent with data from lasers pumped with 808 nm and 532 nm laser light.
The threshold pump power of a laser is the value of the pump power at which the laser threshold is just reached, usually assuming steady-state conditions. At this point, the small-signal gain equals the losses of the laser resonator. Threshold pump power was found to scale quadratically with pump spot diameter. On that basis, the output power of an OP-VECSEL can be scaled up quadratically with pump spot diameter.
The intensity needed to reach lasing threshold in high power VECSELs is typically in excess of -20 MW/mA2 , depending on the specifics of the gain sample and cavity loss. The irradiance at the earth's surface has an intensity of -1000 W/mA2, of which approximately 65 % is at wavelengths which can be used to generate carriers in a GaAs-based VECSEL's active region. It is therefore clear that sunlight must be concentrated by a factor of at least 30000 in order to even reach lasing threshold, and by a larger factor to achieve useful power output. Thermodynamic arguments limit the maximum concentration of sunlight, C, in a medium with refractive index n to C = 1/ηΛ2 sinA2 thetaS, where thetaS is the angular subtense of the sun. In air this factor is 48600, meaning that an intensity of 1.6 times the typical VECSEL threshold can be reached at useful pump wavelengths in air. However, to reach this intensity requires nonimaging optics rather than simple imaging of the sun onto the sample surface.
Two figures of merit are used to characterise the performance of solar powered lasers, the output power emitted per unit primary collection area which characterises the laser efficiency, and the output power divided by the square of the ΜΛ2 beam quality factor, a metric of the usefulness of the output. Current records for these factors are 20 W/mA2 and 1.1 W which were achieved in different systems. On the basis of the conservative estimates above an SP-VECSEL with 1 mm pump spot diameter would reach 49 W/mA2 and 0.5 W. Figure 6 is a graph which plots a solar irradiance spectrum 103 and VECSEL pumping efficiency 105 as functions of wavelength 107 and shows at ground level, demonstrating the good overlap between the two. The surprising prediction of these calculations is that the pumping efficiency is only reduced by 17% of its original value when switching from 808 nm laser pumping to solar pumping. This value is crucial as a first-order estimate of the reduction in performance that could be expected with solar pumping, and such a small value is extremely encouraging as it implies that slope efficiencies approaching 50% could be achieved by SPSLs using existing technology.
Figure 7 is a graph 11 1 which plots collection efficiency 115 and B factor 113 of existing solar lasers and the calculated performance of an embodiment of the present invention on logarithmic scales. Collection efficiency is defined as the output power per unit of primary sunlight collector area and is a measure of how efficiently the available solar power is used. The B Factor is defined as the output power divided by the square of the M-squared value of the laser output (the conventional measure of laser beam quality). The B Factor is a general figure-of-merit of the potential usefulness of the laser output.
As shown in Figure 7 the B Factor and the Collection Efficiency are 10 W and 180 W/m2 respectively and both exceed the current state-of-the-art by large factors. A solar-pumped laser in accordance with the present invention may have many applications. One of the principal applications envisaged for the present invention is as a power source for the initiation of the MAGIC cycle where Magnesium Oxide powder is heated to very high temperatures, causing the reduction of the Magnesium Oxide. The Magnesium metal can then be used as an easily transportable, high energy density fuel, either by burning it directly in air or carbon dioxide, or by reacting it with steam to produce hydrogen gas. A means of producing magnesium using sunlight could be a very valuable green energy technology. The temperatures needed for the process are achievable using solar-pumped lasers. Other applications of the present include space-based power beaming, solar-pumped semiconductor lasers could be placed on satellites given that sunlight is significantly more powerful in space than at the earth's surface. They would then provide a means of converting solar radiation, which is diffuse, into laser radiation in the form of a collimated beam which could be used to transmit power to other spacecraft or, in principle, to the earth's surface. Improvements and modifications may be incorporated herein without deviating from the scope of the invention.

Claims

Claims
1. A solar pumped laser comprising:
a solar concentrator which provides a concentrated output beam of sunlight which acts as a broadband pump source;
and a semiconductor gain medium with broadband absorption which is in optical contact with the concentrated output beam in its gain region; and
an external laser cavity,
wherein the gain region absorbs the concentrated output beam over a large surface area and to a shallow cross sectional depth below the surface of the gain medium.
2. A solar pumped laser as claimed in claim 1 wherein, the broadband
semiconductor gain medium is a Vertical External Cavity Surface Emitting Laser (VECSEL) or semiconductor disk laser (SDL) gain medium.
3. A solar pumped laser as claimed in claim 2 wherein the gain medium has a thin disk geometry where the region of overlap between the sunlight and the laser light takes the form of a disk.
4. A solar pumped laser as claimed in a claim 3 wherein the region of overlap and the thin disk geometry provides excitation of the gain medium at shallow cross sectional depth acts to minimise beam divergence within the gain medium to provide improved output beam quality.
5. A solar pumped laser as claimed in any preceding claim wherein, the ratio of gain region surface area, as defined by the length, breadth or diameter of the gain region surface, to gain region cross sectional depth is at least 50:1.
6. A solar pumped laser as claimed in any preceding claim wherein, the ratio of gain region surface area, as defined by the length, breadth or diameter of the gain region surface, to gain region cross sectional depth is at least 100:1.
7. A solar pumped laser as claimed in any preceding claim wherein, the
semiconductor gain medium has a band gap in the range 740 to 1000nm.
8. A solar pumped laser as claimed in any preceding claim wherein, the
semiconductor gain medium comprises Gallium Arsenide.
9. A solar pumped laser as claimed in any preceding claim wherein, the
semiconductor gain medium comprises Indium Gallium Arsenide.
10. A solar pumped laser as claimed in any preceding claim wherein, the solar pumped laser has a threshold pump power which scales approximately linearly with pump spot diameter.
11. A solar pumped laser as claimed in any preceding claim wherein, the solar concentrator comprises a primary solar concentrator for producing a magnified solar image with a focal point on the gain region of the VECSEL and a secondary solar concentrator for matching the size and angular divergence of the solar image to the focal point.
12. A solar pumped laser as claimed in claim 10 wherein, the solar concentrator produces a region of highly concentrated sunlight at an exit face of the secondary concentrator.
13. A solar pumped laser as claimed in claim 1 or claim 12 wherein the
power/intensity achieved at the output of the secondary concentrator in the useful part of the solar spectrum can be optimised by design up to the thermodynamic limit of solar concentration so as to exceed the laser threshold value by a large enough factor to provide useful laser performance.
14. A solar pumped laser as claimed in any preceding claim wherein, the solar concentrator further comprises a tracking mechanism for aligning the orientation of the concentrator to optimise the amount of sunlight captured from the sun.
15. A solar pumped laser as claimed in claims 1 to 14 wherein, the primary solar concentrator is a mirror.
16. A solar pumped laser as claimed in claims 1 to 14 wherein, the primary solar concentrator is a lens.
17. A solar pumped laser as claimed in any preceding claim wherein, the solar concentrator further comprises an aperture which allows the laser mode to pass through the solar concentrator.
18. A solar pumped laser as claimed in claims 11 to 17 wherein, the secondary solar concentrator is a non-imaging optical concentrator based on a Compound Parabolic Concentrator (CPC).
19. A solar pumped laser as claimed in claims 1 to 17 wherein, the secondary solar concentrator is a non-imaging optical concentrator based on a Dielectric Total Internal
Reflection Concentrator (DTIRC).
20. A solar pumped laser as claimed in any preceding claim wherein, the solar laser has a B Factor of at least 7W.
21. A solar pumped laser as claimed in any preceding claim wherein, the solar laser has a Collection Efficiency of at least 100 W/m2.
22 A solar pumped laser as claimed in claim 11 wherein the secondary solar concentrator is in physical contact with the semiconductor gain medium.
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