Thermal waveguide

Abstract

In a high power optical system, a thermal waveguide including an optical material having an index of refraction sensitive to changes in temperature, the rectangular optical material having a first dimension and a second dimension in a horizontal plane and a third dimension in a vertical plane, the third dimension being approximately ten times smaller than the first and second dimension, at least one heat sink thermally coupled to the optical material to establish a one-dimensional thermal gradient across the third dimension of the optical material, the thermal gradient having a parabolic profile across the rectangular optical material, and wherein the optical material is configured to act as a waveguide when a laser beam having a power of greater than one watt is incident upon the optical material.

Claims

1. An apparatus for thermally inducing an optical waveguide in a nonlinear optical system, the apparatus comprising: a nonlinear optical material without cladding and having an index of refraction that increases with temperature, the nonlinear optical material having a first dimension and a second dimension in a horizontal plane and a third dimension in a vertical plane, the third dimension being approximately ten times smaller than the first and second dimension; at least one heat sink thermally coupled to the nonlinear optical material to establish a one-dimensional thermal gradient across the third dimension of the nonlinear optical material when the nonlinear optical material is heated, the thermal gradient having an approximately parabolic profile across the nonlinear optical material; and a laser having a power of greater than one watt, the laser configured to provide a laser beam incident upon the nonlinear optical material to heat the nonlinear optical material and induce a waveguide to be formed in the nonlinear optical material due to said one-dimensional thermal gradient established by said at least one heat sink.

2. The system of claim 1 wherein the nonlinear optical material has a planar geometry.

3. The system of claim 1 wherein the nonlinear optical material is a semiconductor material.

4. The system of claim 1 wherein the index of refraction of the nonlinear optical material changes by a factor of approximately 1104 for every degree Kelvin change in temperature.

5. The system of claim 1 wherein the nonlinear optical material has a stress fracture limit that permits waveguiding of the laser beam.

6. The system of claim 1 wherein the nonlinear optical material is configured to act as a thermal lens along the third dimension when the laser beam having a power of greater than one watt is incident upon the optical material.

7. The system of claim 1 wherein the nonlinear optical material has a thermal shock resistance and a lensing strength, the product of the shock resistance and the lensing strength being greater than approximately 2000.

8. The system of claim 1 wherein the nonlinear optical material is at least one of ZnGeP2, CdGeAs2, CdSiP2, GaP, GaN, AITGaAs, ZnSe, ZnTe and CdTe.

9. An apparatus for thermally inducing an optical waveguide in a nonlinear optical system, the apparatus comprising: a nonlinear optical material without a cladding and having an index of refraction that increases with an increase in temperature; cooling means thermally coupled to the nonlinear optical material to establish a one dimensional thermal gradient across a smallest dimension of the nonlinear optical material when the nonlinear optical material is heated; and a laser configured to provide a laser beam incident upon the nonlinear optical material to heat the nonlinear optical material and induce a planar waveguide to be formed in the nonlinear optical material due to said thermal gradient established by said cooling means.

10. The system of claim 9 wherein the nonlinear optical material has a planar geometry.

11. The system of claim 9 wherein the nonlinear optical material is a semiconductor material.

12. The system of claim 9 wherein the index of refraction of the nonlinear optical material changes by a factor of 1104 for every degree Kelvin change in temperature.

13. The system of claim 9 wherein the nonlinear optical material has a thermal stress fracture limit that permits the formation of a thermally induced optical waveguide resulting from the incident laser beam.

14. The system of claim 9 wherein the nonlinear optical material is configured to act as a thermal lens across the smallest dimension of the optical material when a laser beam having a power of greater than one watt is incident upon the optical material.

15. The system of claim 9 wherein the nonlinear optical material has a thermal shock resistance and a lensing strength, the product of the shock resistance and the lensing strength being greater than approximately 2000.

16. The system of claim 9 wherein the nonlinear optical material is at least one of ZnGeP2, CdGeAs2, CdSiP2, GaP, GaN, AliGaAs, ZnSe, ZnTe and CdTe.

17. In a nonlinear optical system, a method for guiding an optical beam and inducing a waveguide in a nonlinear optical material using a laser beam, the method comprising: providing a nonlinear optical material without a cladding and having an index of refraction sensitive to changes in temperature, the nonlinear optical material having a thermal shock resistance and a lensing strength, the product of the shock resistance and the lensing strength being greater than approximately 2000; providing a cooling means thermally coupled to the nonlinear optical material to establish a one dimensional thermal gradient across a smallest dimension of the nonlinear optical material; delivering a first laser beam having a power of greater than one watt to an entrance facet of the nonlinear optical material heating the nonlinear optical material, the laser beam transforming the nonlinear optical material into a waveguide due to said thermal gradient established by said cooling mean; and delivering the first laser beam and additional beams created by nonlinear processes to an exit facet of the nonlinear optical material, wherein the first laser beam and the additional beams are guided to the exit facet by the nonlinear optical material transformed into said waveguide.

18. The method of claim 17 wherein the nonlinear optical material is a semiconductor material or a nonlinear optical material.

19. The method of claim 17 wherein the index of refraction of the nonlinear optical material changes by a factor of approximately 1104 for every degree Kelvin change in temperature.

20. The method of claim 17 wherein the nonlinear optical material is at least one of ZnGeP2, CdGeAs2, CdSiP2, GaP, GaN, AIIGaAs, ZnSe, ZnTe and CdTe.

21. The system of claim 1 where the formation of the nonlinear optical waveguide is combined with an appropriate optical design to minimize the change in output beam properties caused by generation of heat in a nonlinear material resulting from an incident laser beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

(2) FIG. 1 is a diagram of a thermal waveguide system according to an illustrative embodiment of the invention.

(3) FIG. 2 is a graph showing the temperature gradient in the thermal waveguide system of FIG. 1, according to an illustrative embodiment of the invention.

(4) FIG. 3 is a table showing parameters describing an illustrative embodiment of the invention.

(5) FIGS. 4A and 4B show equations used to describe the behavior of the index of refraction of an optical material.

(6) FIG. 5 shows a table of crystals and their associated physical properties.

DETAILED DESCRIPTION

(7) FIG. 1 depicts a thermal waveguide 100 in accordance with an illustrative embodiment of the present invention. The thermal waveguide 100 includes a nonlinear optical or semiconductor material 105 having an entrance facet 106, one or more heat sinks 110, an elliptical pump beam 115 travelling along the y-axis, and an exit beam 116 travelling along the y-axis. The heat sinks 110 are thermally coupled to the nonlinear optical material 105. During operation, the elliptical pump beam 115 is incident upon the entrance facet 106, heating the nonlinear optical material 105. The generated heat flows out of the nonlinear optical material and into the heat sinks 110, generating a one-dimensional spatial temperature variation in the nonlinear optical material (i.e. a thermal gradient) along the z-axis as illustrated by the temperature/index profile 120. The thermal gradient can lead to a one-dimensional spatial variation in the index of refraction along the z-axis, which is also illustrated by the temperature/index profile 120. The temperature/index profile 120 shows that both the temperature and index of refraction can have parabolic profiles in the region of the semiconductor material 105 and non-parabolic profiles in the region of the heat sinks 110. The spatial variation in the index of refraction can lead to a thermal lensing effect. The thermal lensing can have an associated focal length that is short compared to a dimension of the thermal waveguide 100. The nonlinear optical material 105 can act as a waveguide, confining light along the z-axis when the focal length associated with the thermal lensing effect is short compared to a dimension of the thermal waveguide 100.

(8) In some embodiments the nonlinear optical material can convert a portion of the elliptical pump beam 115 into a signal beam and an idler beam. In some embodiments, the exit beam 116 can include a portion of the elliptical pump beam 115, a signal beam, and an idler beam.

(9) As an exemplary calculation for the thermal, mechanical and optical properties of the structure shown in FIG. 1, we show in FIG. 2 the predicted thermal profile along the z axis indicated in FIG. 1. The abscissa in FIG. 2 shows distance along the z-axis as measured from the center of the nonlinear optical material 105. The ordinate shows the temperature, measured in degrees Kelvin. At a distance of 0 mm (at the center of the crystal), the temperature is about 294.7 K. At a distance of 0.2 mm away from the center of the crystal, the temperature is about 294.25 K. A summary of the parameters used in the calculations appear in FIG. 3, and are appropriate for the nonlinear material orientation-patterned GaAs (OP-GaAs). For the results plotted in FIG. 2, we assumed a total optical power in the material of 2 kW, and an absorption coefficient for that power of 0.01 cm.sup.1.

(10) While the temperature difference across the crystal shown in FIG. 2 is only a few degrees, the thermal lens in the small dimension of the crystal (z-axis) can be significant due to the large value of do/dT in GaAs, as indicated in FIG. 3. A temperature difference as shown in FIG. 2 can lead to a thermal lensing effect that can be described by a cylindrical lens with a focal length of about 6.7 mm, close to the length of the crystal and thus clearly outside of the range for a simple lens approximation. The thermal gradient in the x-axis direction as shown in FIG. 1 can be much weaker than the thermal gradient in the z-axis direction, leading to a much weaker lens in the x-axis direction, on the order of meters of focal length. Such a condition suggests we treat the crystal as a planar optical waveguide, with the index gradient produced by the thermal profile in the crystal along the thin dimension.

(11) Based on our calculation, the stresses produced in the model nonlinear optical material OP-GaAs, following the mechanical properties indicated in FIG. 5, we find that the stress in the material is a small fraction of the calculated fracture limit. Our calculations show that, even for absorptions as high as 0.1 cm.sup.1, the OP-GaAs would be only at about 1% of its stress-fracture limit.

(12) In some embodiments, the nonlinear optical material can be ZnGeP.sub.2, CdGeAs.sub.2, or CdSiPz. In some embodiments, the nonlinear optical material 105 can include the III-V materials GaP and GaN as well as ternary III-V compounds such as AlIGaAs, and the II-VI materials ZnSe, ZnTe and CdTe.

(13) FIG. 4A shows an equation describing a parabolic variation in the index of refraction, as a function of position within an optical medium. The treatment of thermally induced planar waveguides has been presented in the context of laser crystals, as an analogy to more conventional step-index planar waveguides formed by sandwiches of different-index materials. In the case of thermal waveguides, the best approximation is to consider the effect as creating a parabolic-index guide.

(14) We assume that the absorbed power is uniformly deposited in a region of height D. The index of refraction, n, can be expressed as a function of the position (x), thermal conductivity (k), heat input per unit volume (Q), and, the change in refractive index per unit change in temperature (dn/dT).

(15) FIG. 4B shows an equation describing a relationship between a numerical parameter, F.sub.m, and the mode number, m (0 and higher), for the waveguide mode. The formula includes a small number, .sub.m, used to correct for deviations from a true parabolic shape to the refractive index distribution. To support a given mode number, m, the value of F.sub.m must be positive.

(16) Our model calculations assume an optical power in the crystal of 2.1 kW, and absorption coefficient of 0.01 cm-1, a pumped region of 0.03 (D)13 cm and the nonlinear optical material thermo-mechanical-optical parameters listed in FIG. 5. The result of such calculations can indicate a volumetric thermal load (Q) of about 700 W/cm.sup.3 and a value of F.sub.m of approximately 2.3 for the lowest order (fundamental) mode (m=0) and 0.33 for the next higher-order mode (m=1), assuming that the mode wavelength is 4000 nm. We estimate that vertical mode radius for the lowest order mode is about 43 m. The results of the calculations can show the nonlinear optical material acting as waveguide, guiding two modes. Any optical design involving the model nonlinear crystal would endeavor to match this fundamental mode, and the result would be a beam that propagated inside the nonlinear crystal with a constant vertical size, and outside of the crystal would be expected to have properties that were relatively insensitive to the actual power absorbed in the nonlinear material. Our calculations show a small change in mode size from our nominal 2.1 kW of optical power, ranging from 46.6 m at 1.5 kW of power to 41.1 m at 2.5 kW of power.

(17) Those skilled in the art will understand that the model calculations can be applied to a wide variety of planar geometries similar in general shape to that in the FIG. 1, but differing in details regarding dimensions, optical powers involved, absorption coefficients and particular nonlinear materials. The waveguide design will be effective for nonlinear optical materials 105 having a relatively large value, (exceeding approximately 2000) of the product of thermal shock resistance and lensing strength, as shown in FIG. 5. In some embodiments, the nonlinear optical material can be ZnGeP.sub.2, CdGeAs.sub.2, or CdSiP.sub.2. In some embodiments, the nonlinear optical material 105 can include the orientation-patterned III-V materials GaP and GaN as well as ternary III-V compounds such as AlIGaAs, and the II-VI materials ZnSe, ZnTe and CdTe.

(18) The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of the inventive concepts. It will be understood that, although the terms first, second, third etc. are used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present application.

(19) While the present inventive concepts have been particularly shown and described above with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art, that various changes in form and detail can be made without departing from the spirit and scope of the present inventive concepts described and defined by the following claims.