Intracavity harmonic generation using layered nonlinear optics

Abstract

The present invention proposes the use of specially designed layered nonlinear optics (LNO) for intracavity harmonic generation. LNO generates harmonics and directs the generated harmonic beam to a path different from the path of the fundamental beam by total internal reflection, a phenomenon where all light is reflected when light in one ("inner") optic strikes the interface of a second ("outer") optic with a lower refractive index than the inner optic at a sufficient angle. No coating is required for the harmonics in the fundamental beam laser cavity. The generated harmonic beam also does not propagate through any surface in the fundamental beam cavity. Thus, the present invention improves the reliability of intracavity harmonic generation lasers, especially when the harmonics are in the ultraviolet range.

Description

Technical Field

[0001] Laser applications exist in many fields, from biomedicine, semiconductors to defense industry. Different applications require different wavelengths. Some of the required wavelengths are difficult to obtain by direct laser emission. One way to extend the wavelength range of lasers is through harmonic generation. There are two ways to generate harmonics, namely intracavity harmonic generation and extracavity harmonic generation. The present invention is mainly related to intracavity harmonic generation. Background Art

[0002] Figure 1 A known scheme for intracavity second harmonic generation (SHG) is shown. Item 107 is a pump beam that pumps a gain medium 102. Item 101 is an optical device that reflects a fundamental beam 108 while transmitting the pump beam 107. The laser cavity is between item 101 and item 105, and item 105 is an optical device that reflects the fundamental beam 108 and the second harmonic 106. The fundamental beam 108 oscillates in the cavity along a beam path defined by item 101, item 103, and item 105. Item 103 is an optical device that reflects the fundamental beam 108 but transmits the second harmonic beam 106 generated by the SHG nonlinear optical device 104, and both surfaces of item 104 are coated with anti-reflection (AR) films for the fundamental beam 108 and the second harmonic 106. The coatings of item 103, item 104, and item 105 are exposed to the second harmonic beam 106 and may cause performance degradation, especially when the generated second harmonic is in the UV range. Slight coating degradation can result in significant reductions in output power since they are optics within the fundamental beam cavity.

[0003] Figure 2A known scheme for intracavity third harmonic generation (THG) is shown. Item 217 is a pump beam that pumps the gain medium 209. Item 208 is an optical device that reflects the fundamental beam 218 while transmitting the pump beam 217. The laser cavity is between item 208 and item 214, and item 214 is an optical device that reflects the fundamental beam 218 and the second harmonic 216 simultaneously. Item 210 is an optical device that reflects the fundamental beam 218 and transmits the remaining second harmonic beam 216. The fundamental beam 218 oscillates in the cavity along the beam path defined by item 208, item 210, and item 214. Item 213 is a SHG nonlinear optical device that generates the second harmonic beam 216. Both the fundamental beam 218 and the second harmonic 216 enter the THG optical device 212 and generate the third harmonic 215. The third harmonic 215 is reflected out of the cavity by item 211, which also transmits the fundamental beam 218 and the remaining second harmonic 216. The coatings of item 211 and item 212 are exposed to the third harmonic beam 215 and may cause performance degradation, especially when the generated third harmonic is in the UV range. Slight coating performance degradation may result in a significant reduction in output power because they are optical devices inside the fundamental beam cavity. Summary of the invention

[0004] The present invention proposes the use of specially designed layered nonlinear optics (LNO) for intracavity harmonic generation. LNO generates harmonics and directs the generated harmonic beam to a path different from the path of the fundamental beam by total internal reflection, a phenomenon where all light is reflected when light in one ("inner") optic strikes the interface of a second ("outer") optic with a lower refractive index than the inner optic at a sufficient angle. No coating is required for the harmonics in the fundamental beam laser cavity. The generated harmonic beam also does not propagate through any surface in the fundamental beam cavity. Thus, the present invention improves the reliability of intracavity harmonic generation lasers, especially when the harmonics are in the UV range.

[0005] The proposed LNO for intracavity harmonic generation is a layered structure comprising: a nonlinear optical device layer for generating harmonics, and at least one other optical layer for guiding the generated harmonics out of the fundamental beam cavity by total internal reflection. These optical devices are bonded together without any adhesive, for example using optical contacts. No coating is required for the generated harmonics in the fundamental beam cavity. The generated harmonics will not propagate through any surface in the fundamental beam cavity. The refractive index of the fundamental light of all layers can be selected to match to minimize the loss of the fundamental beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention will be described with reference to the accompanying drawings.

[0007] Figure 1A known scheme for intracavity second harmonic generation (SHG) is shown.

[0008] Figure 2 A known scheme for intracavity third harmonic generation (THG) is shown.

[0009] Figure 3 A layered nonlinear optical device or LNO for intracavity SHG is shown.

[0010] Figure 4 An example of LNO application in intracavity SHG is shown.

[0011] Figure 5 Reflective optics are shown added in a first manner for retro-reflecting the second harmonic.

[0012] Figure 6 Reflective optics added in a second manner for retro-reflecting the second harmonic are shown.

[0013] Figure 7 A detailed example of LNO for ooe SHG is shown.

[0014] Figure 8 An example of LNO application in intracavity THG is shown.

[0015] Figure 9a The third harmonic emitted at the side is shown.

[0016] Figure 9b The third harmonic emitted at the bottom surface is shown.

[0017] Fig.10 Shows the replacement Figure 8 The two-layer LNO of the three-layer LNO.

[0018] Fig.11 An example of LNO for ooe THG is shown.

[0019] Fig.12 Shown is the LNO used for intracavity fourth harmonic generation (FHG).

[0020] Fig.13 An example of intracavity SHG in a unidirectional laser using two layers of LNO is shown. DETAILED DESCRIPTION

[0021] Figure 3An LNO for intracavity SHG is shown. Item 319 is a nonlinear optical device for SHG sandwiched between items 318 and 320, which are selected so that their refractive index for the second harmonic is lower than the refractive index of item 319 for the second harmonic. Angle α is selected so that the second harmonics 322 and 323 generated inside item 319 experience total internal reflection at the interface between items 318 and 319 and at the interface between items 319 and 320, while the fundamental beam 321 does not experience total internal reflection at either interface. One way to understand the structure is to consider items 318 and 319 as cladding material that creates a light guide to confine the second harmonic beams 322 and 323 within item 319 until they reach the exit surface located on the side of the structure. The exit surface can be selected as a Brewster surface to avoid the need for AR coating. From the perspective of fundamental beam 321 , the refractive indices of item 318 , item 319 , and item 320 may be selected to be the same or nearly the same as one another to minimize the loss of fundamental beam 321 .

[0022] The materials used in nonlinear optical devices can be: beta-barium borate (β-BBO), lithium triborate (LBO), cesium lithium borate (CLBO), potassium fluoroborate beryllium oxide crystal (KBe2BO3F2, KBBF), bismuth borate (BiBO), potassium titanium phosphate (KTP), potassium dideuterium phosphate (KD*P), periodically poled lithium niobate (PPLN), periodically poled stoichiometric lithium tantalate (PPSLT) or periodically poled LaBGeO5 (PP-LBGO).

[0023] The materials applied to cladding 318 and cladding 320 (lower refractive index materials) can be: α-barium borate (α-BBO), β-barium borate (β-BBO), lithium triborate (LBO), cesium lithium borate (CLBO), KBBF (KBe2BO3F2), bismuth borate (BiBO), potassium titanyl phosphate (KTP), potassium deuterium phosphate (KD*P), yttrium vanadate YVO4, lithium niobate LiNbO3, lithium tantalate LiTaO3, LBGO, quartz crystal, fused quartz, BK7 and CaF2.

[0024] exist Figure 4 An example of LNO application in intracavity SHG is shown in FIG. 421 and 424 are the two end optical devices that form the fundamental beam laser cavity. Item 422 is the gain medium. Because the present invention is applicable to all pumping methods, such as optical, electrical or other pumping methods, the pump source is omitted in this figure for clarity. The fundamental beam 427 oscillates in the cavity defined by items 421 and 424. Item 423 is Figure 3 Item 425 and Item 426 are two second harmonic beams generated by the LNO.

[0025] If you want only one output beam instead of two, you can add Figure 5 or Figure 6 The optics 428 are shown to reflect the second harmonic back so that there is only one output second harmonic beam. The optics 428 can be designed in such a way that the spatial pattern of the back-reflected second harmonic beam matches the spatial pattern of the other second harmonic beam.

[0026] It will be helpful to define some terms for the discussion that follows. The defined terms will allow for efficient discussion of certain aspects of the invention as it relates to nonlinear optics. "Birefringent optics" refers to optics whose refractive index varies depending on the polarization and propagation direction of light that strikes or passes through the optic. We define the term "ordinary light (o)" as light whose polarization is perpendicular to the optical axis of the birefringent optic. We define the term "extraordinary light (e)" as light whose polarization and propagation direction form a plane parallel to the optical axis of the birefringent optic and whose polarization is not perpendicular to the optical axis. This allows us to define another term, "ooeSHG," which means that the fundamental light used for SHG is ordinary light, and the second harmonic generated is extraordinary light.

[0027] exist Figure 7A detailed example of LNO for ooe SHG is shown in . Nonlinear β-BBO is sandwiched between two layers of α-BBO. Items 528, 529, and 530 are the optical axes of the top α-BBO, bottom α-BBO, and β-BBO, respectively. The optical axes of the two α-BBO layers (items 528 and 529) are perpendicular to the interface with β-BBO. The optical axis of β-BBO (item 530) is parallel to the paper plane (the plane in which the figure is located), and the orientation within the paper plane is determined by SHG phase matching. The polarization of the fundamental beam 531 is perpendicular to the paper plane. Ordinary light is present in all three layers. Since the refractive index is essentially the same in α-BBO and β-BBO, the fundamental beam passes through the entire LNO in a substantially straight line and lossless manner. The refractive index of the second harmonic 532 and the second harmonic 533, which are extraordinary beams, is higher inside the β-BBO. Therefore, if the incident angle is greater than the critical angle, total internal reflection can be achieved. For example, for ooe SHG at 698 nm, α is chosen to be 15.2°. This not only achieves total internal reflection, but also allows the second harmonic 532 and the second harmonic 533 to leave the LNO at the Brewster angle when the sides of the LNO are cut perpendicular to the fundamental beam 531. The astute reader will appreciate that while the examples given here use ooe SHG, the benefits of this LNO structure also apply to other types of SHG, including quasi-phase-matched SHG.

[0028] exist Figure 8 An example of LNO application in intracavity THG is shown in . Item 631 and item 636 are two end optical devices that form the fundamental beam laser cavity. Item 632 is the gain medium. Because the present invention is applicable to all pumping methods, such as optical, electrical or other pumping methods, the pump source is omitted for clarity. Item 633 is an optical device that reflects the fundamental beam 639 and transmits the remaining second harmonic beam 638. The fundamental beam 639 oscillates in the cavity along the beam path defined by items 631, 633 and 636. Item 634 is the LNO. The fundamental beam 639 and the second harmonic 638 generated by the second harmonic nonlinear optical device 635 pass through the LNO, while the third harmonic 637 is reflected out of the cavity. No coating for the third harmonic is required in the laser cavity. It is appreciated that the third harmonic 637 does not propagate through any surface in the laser cavity. The exit surface can be selected as a Brewster surface so that AR coating is not required. From the perspective of the fundamental beam 639, the refractive index of the materials can be selected to be the same or nearly the same as each other in all layers to minimize the loss of the fundamental beam 639. After the third harmonic is generated, the second harmonic 638 does not have to be collinear with the fundamental beam 639. The second harmonic 638 does not have to propagate through the LNO. The second harmonic 638 can be totally reflected by the LNO like the third harmonic 637, or can be partially reflected.

[0029] like Figure 9a and Figure 9b As shown, a simpler two-layer LNO can also be used for THG. Item 739 is a nonlinear THG optical layer. For the generated third harmonic 741, the refractive index of optical layer 740 is lower than the refractive index of item 739. Angle α is selected so that the third harmonic 741 generated inside item 739 undergoes total internal reflection at the interface between item 739 and item 740, while the fundamental wave beam 742 does not undergo total internal reflection at the interface. The third harmonic 741 is generated at the interface between item 739 and item 740. Figure 9a The side S1 shown leaves item 739, which can be designed as a Brewster surface to avoid AR coating. Alternatively, the third harmonic 741 can be Figure 9b The bottom surface S2 shown leaves item 739, which can also be designed as a Brewster surface to avoid AR coating. From the perspective of the fundamental beam 742, the refractive index of the material can be selected to be the same or almost the same in both layers to minimize the loss of the fundamental beam. After the third harmonic 741 is generated, the second harmonic 743 does not have to be collinear with the fundamental beam 742. The second harmonic 743 does not have to propagate through the upper layer 740. The second harmonic 743 can be totally reflected at the interface between layer 739 and layer 740, or it can be partially reflected there. This double layer LNO can replace Figure 8 Item 634 in . The entire setup is as follows Fig.10 As shown, item 640 is a double layer LNO.

[0030] exist Fig.11 An example of LNO for ooe THG is shown in . Item 842 and Item 843 are the optical axes of α-BBO and β-BBO, respectively. The optical axis item of α-BBO (item 842) is perpendicular to the interface of β-BBO. The optical axis item of β-BBO (item 843) is parallel to the paper plane, and the orientation in the paper plane is determined by THG phase matching.

[0031] Here we demonstrate the use of Fig.11 An example of intracavity THG of LNO is shown in FIG. Fig.10Same scheme as shown. The polarization of the fundamental beam 639 is perpendicular to the paper plane. The SHG optics 635 is chosen to be PPLN (Periodically Poled Lithium Niobate). The polarization of the second harmonic 638 generated by item 635 is also perpendicular to the paper plane. The fundamental beam 639 and the second harmonic 638 are both ordinary light in both layers of item 640. The fundamental beam 639 and the second harmonic 638 pass through the entire LNO in an essentially straight line and lossless manner because the refractive indices of α-BBO and β-BBO are essentially the same for those beams. The refractive index of the third harmonic 637, which is an extraordinary beam, is higher in the β-BBO layer. Therefore, if the angle of incidence is greater than the critical angle, total internal reflection of the third harmonic 637 can be achieved. The astute reader will again realize that although the example given is an ooe THG, the advantages of the LNO structure also apply to other types of THG, including quasi-phase matched THG.

[0032] LNO can also be used for intracavity fourth harmonic generation (FHG). The scheme can be based on Figure 4 The intracavity SHG method shown in FIG. Fig.12 Such a scheme is shown. Items 431 and 432 are two end optics that create a cavity for the second harmonic 430 generated by the fundamental beam 427. One of the end optics 431 and 432 is controlled by a feedback loop to lock the phase of the second harmonic beam 430. The other LNO 429 is designed for SHG of the second harmonic 430. Items 433 and 434 are dual output fourth harmonic beams. Similarly, the exit surface of the LNO 429 can be designed as a Brewster surface for the fourth harmonic, so that AR coating is not required.

[0033] The same principle can be applied to the cavity 2 n Subharmonic generation, where n>2.

[0034] LNO can also be used in unidirectional lasers. Fig.13 An example of intracavity SHG using two layers of LNO in a unidirectional laser is shown in FIG. Optics 901, 902, 903, and 904 form a cavity for fundamental beam 908. Item 905 is the gain medium. Because the present invention is applicable to all pumping methods, such as optical, electrical, or other pumping methods, the pump source is omitted for clarity. Item 906 is a unidirectional optical shutter that forces unidirectional operation. Item 907 is two layers of LNO that produce second harmonics 909.

[0035] If the harmonic generation is critically phase matched, there is a walk-off between the ordinary and extraordinary beams. The acceptance angle is also different in two directions, i.e., perpendicular to the optical axis and orthogonal to it. The generated harmonic beam becomes elliptical. Many laser manufacturers use beam shaping optics to convert harmonics to a circular output beam.

[0036] However, in some applications, elliptical focusing is preferred. A circular or nearly circular beam is usually focused to an elliptical focus using beam shaping optics. Here, we present a method to directly obtain elliptical focusing without beam shaping optics. When the elliptical harmonic beam mentioned in the above paragraph is focused without beam shaping, the size and position of the beam waist are different in two directions, namely, the direction perpendicular to the optical axis and its orthogonal direction. Elliptical focal lengths with different major-minor axis ratios can be obtained at different positions around the two beam waist positions. Increasing the harmonic crystal length will enhance the walk-off and make the harmonic beam more elliptical. Therefore, with longer harmonic crystals, the maximum major-minor axis ratio that can be achieved around the focus is larger. In our SHG experiments using 3 mm and 5 mm long BBO, elliptical focuses with major-minor axis ratios of 3:1 and 4:1 for the second harmonic have been obtained, respectively, using a single focusing lens and no beam shaping optics.

[0037] The astute reader will have no difficulty devising numerous obvious modifications and variations of the present invention, all of which are intended to be encompassed by the appended claims.

Claims

1. An intracavity harmonic generation laser, comprising at least one layered nonlinear optical device LNO, wherein the LNO comprises: A first optical layer of a nonlinear optical device, for generating harmonics when excited by a fundamental wave beam, wherein the first optical layer has a first plane and a second plane opposite to each other, and a first side surface which is not parallel to or perpendicular to any one of the first plane and the second plane; a second optical layer having a third plane and a second side surface that is not parallel or perpendicular to the third plane, the third plane being bonded to the first plane in an adhesive-free manner to define a first interface; and a third optical layer having a fourth plane and a third side surface that is not parallel to or perpendicular to the fourth plane, the fourth plane being bonded to the second plane in an adhesive-free manner to define a second interface; The refractive indices of the first optical layer, the second optical layer and the third optical layer are selected such that: The generated harmonics are totally internally reflected at the first interface and the second interface and leave the first optical layer through the first side surface as at least one light beam, and The fundamental wave beam follows a path that enters and exits the LNO through the second optical layer and perpendicular to the second side, passes through the first interface and the second interface at a non-zero angle to the angle between the first interface and the second interface, and enters and exits the LNO through the third optical layer and perpendicular to the third side.

2. The laser according to claim 1, wherein: The bonding is optical contact, fusion bonding or diffusion bonding.

3. The laser according to claim 1, wherein: The third plane and the first plane, and the fourth plane and the second plane are plated with a dielectric film before the combining.

4. The laser according to claim 1, wherein: The intracavity harmonic generation is intracavity second harmonic generation.

5. The laser according to claim 1, wherein The intracavity harmonic generation is intracavity third harmonic generation.

6. The laser according to claim 1, wherein The intracavity harmonic generation is the intracavity fourth harmonic generation, or the intracavity 2 n Subharmonic generation, where n>2.

7. The laser according to claim 1, wherein: The laser is a bidirectional laser or a unidirectional laser.

8. The laser according to claim 1, wherein The laser is a bidirectional laser.

9. The laser according to claim 1, wherein: The LNO includes an optical layer of a nonlinear harmonic optical device that generates desired harmonics, sandwiched between the second optical layer and the third optical layer.

10. The laser according to claim 9, wherein With respect to harmonics generated by the LNO, a respective refractive index of each of the second optical layer and the third optical layer is lower than a refractive index of the first optical layer.

11. The laser according to claim 9, wherein: The first optical layer includes: β-BBO, LBO, CLBO, KBBF, BiBO, KTP, KD*P, PPLN, PPSLT, or PP-LBGO.

12. The laser according to claim 9, wherein: Each of the second optical layer and the third optical layer includes α-BBO, β-BBO, CLBO, KBBF, BiBO, KTP, KD*P, YVO4, LiNbO3, LiTaO3, LBGO, quartz crystal, fused silica, BK7, or CaF2.

13. The laser according to claim 9, wherein: The harmonic beam generated by the LNO leaves the first optical layer at the Brewster angle.

14. An intracavity harmonic generation laser having at least one layered nonlinear optical device LNO, the LNO comprising: a first optical layer of a nonlinear optical device, for generating harmonics when excited by a fundamental wave beam, wherein the first optical layer has a first plane and a second plane opposite to each other, and a first side surface which is not parallel to or perpendicular to any one of the first plane and the second plane; and a second optical layer having a third plane and a second side surface that is not parallel or perpendicular to the third plane, the third plane being bonded to the first plane in an adhesive-free manner to define a first interface; The refractive indexes of the first optical layer and the second optical layer are selected such that: The generated harmonics are totally internally reflected at the first interface and leave the first optical layer through the first side surface as at least one light beam, and The fundamental beam follows a path that passes through the second optical layer and enters and exits the LNO perpendicular to the second side and passes through the first interface at a non-zero angle to the first interface.

15. The laser according to claim 14, wherein The bonding is optical contact, fusion bonding or diffusion bonding.

16. The laser of claim 14, wherein: The third plane and the first plane are plated with a dielectric film before the combining.

17. The laser of claim 14, wherein: The intracavity harmonic generation is intracavity second harmonic generation.

18. The laser of claim 14, wherein: The intracavity harmonic generation is intracavity third harmonic generation.

19. The laser of claim 14, wherein: The laser is a bidirectional laser or a unidirectional laser.

20. The laser of claim 14, wherein: The laser is a bidirectional laser.

21. The laser of claim 14, wherein: The LNO includes: an optical layer of a nonlinear harmonic optical device for generating desired harmonics; and a second layer.

22. The laser of claim 21, wherein With respect to harmonics generated by the LNO, the refractive index of the second optical layer is lower than that of the first optical layer.

23. The laser of claim 21, wherein: The first optical layer includes: β-BBO, LBO, CLBO, KBBF, BiBO, KTP, KD*P, PPLN, PPSLT, or PP-LBGO.

24. The laser of claim 21, wherein: The second optical layer includes: α-BBO, β-BBO, CLBO, KBBF, BiBO, KTP, KD*P, YVO4, LiNbO3, LiTaO3, LBGO, quartz crystal, fused quartz, BK7, or CaF2.

25. The laser of claim 21, wherein: The harmonic beam generated by the LNO leaves the first optical layer at the Brewster angle.

26. A method for generating a harmonic beam, performed by a layered nonlinear optical device (LNO), the LNO comprising: a first optical layer of a nonlinear optical device, for generating harmonics when excited by a fundamental wave beam, wherein the first optical layer has a first plane and a second plane opposite to each other, and a first side surface which is not parallel to or perpendicular to any one of the first plane and the second plane; and a second optical layer having a third plane and a second side surface that is not parallel or perpendicular to the third plane, the third plane being bonded to the first plane in an adhesive-free manner to define a first interface; The refractive indexes of the first optical layer and the second optical layer are selected such that: The generated harmonics are totally internally reflected at the first interface and leave the first optical layer through the first side surface as at least one light beam, and The fundamental beam follows a path that passes through the second optical layer and enters and exits the LNO perpendicular to the second side and passes through the first interface at a non-zero angle to the first interface; The method comprises the following steps: a. allowing the fundamental wave beam to pass through the first optical layer and the second optical layer, wherein the fundamental wave beam is arranged to follow the path; b. generating harmonics in the first optical layer; c. Allowing the generated harmonic light beam to exit the first optical layer through the first side.

27. A method for generating a harmonic beam, performed by a layered nonlinear optical device (LNO), the LNO comprising: A first optical layer of a nonlinear optical device, for generating harmonics when excited by a fundamental wave beam, wherein the first optical layer has a first plane and a second plane opposite to each other, and a first side surface which is not parallel to or perpendicular to any one of the first plane and the second plane; a second optical layer having a third plane and a second side surface that is not parallel or perpendicular to the third plane, the third plane being bonded to the first plane in an adhesive-free manner to define a first interface; and a third optical layer having a fourth plane and a third side surface that is not parallel to or perpendicular to the fourth plane, the fourth plane being bonded to the second plane in an adhesive-free manner to define a second interface; The refractive indices of the first optical layer, the second optical layer and the third optical layer are selected such that: The generated harmonics are totally internally reflected at the first interface and the second interface and leave the first optical layer through the first side surface as at least one light beam, and the fundamental beam follows a path that enters and exits the LNO through the second optical layer and perpendicular to the second side, passes through the first interface and the second interface at a non-zero angle to the first interface and the second interface, and enters and exits the LNO through the third optical layer and perpendicular to the third side; The method comprises the following steps: passing the fundamental wave beam through the first optical layer, the second optical layer, and the third optical layer, wherein the fundamental wave beam is arranged to follow the path; generating harmonics in the first optical layer; The generated harmonic light beam is allowed to exit the first optical layer through the first side.

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