Method and system for high-bandwidth immersion grids

Abstract

The immersion grating includes a dielectric substrate having an incident light surface and a second surface opposite the incident light surface. The dielectric substrate is characterized by a substrate refractive index. The immersion grating also includes at least one dielectric layer bonded to the second surface of the dielectric substrate. The at least one dielectric layer is characterized by a layer refractive index greater than the substrate refractive index. The immersion grating further includes a periodic structure formed within the at least one dielectric layer. The immersion grating is characterized by a diffraction efficiency greater than 99% over the wavelength range of 1041 nm to 1066 nm.

Description

Technical Field

[0001] Cross - reference to Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 062,773, filed Aug. 7, 2020, the content of which is hereby incorporated by reference in its entirety for all purposes.

Background Art

[0002]

[0002] Diffraction gratings are typically periodic structures that exist at the interface between two materials, often one of which is air. When a single beam is incident on the structure, the grating produces multiple diffracted beams (referred to as orders). The angular radiation of the orders is given by the grating equation (Equation 1) shown below

Number

[0003]

[0003] In relation to Equation 1, it should be noted that the angle of incidence and the angle of diffraction order are related to each other with respect to a given wavelength and lattice periodicity. The diffraction order can be either inside or outside the medium, corresponding to the reflection order and transmission order, respectively. Accordingly, the diffraction orders inside and outside the medium are coupled by nsin(θ). In embodiments where the transmission order is suppressed, the reflection order is governed by the total internal reflection (TIR) ​​condition.

[0004]

[0004] Figure 1A is a simplified diagram showing a periodic grating between air and glass. Light incident from within the glass 810 generates reflection and transmission orders governed by Snell's law (Equation 1). The angles in air are larger with respect to the surface normal than the angles within the glass. Also, since sin(θ0) is greater than 1, the transmission angle cannot be generated by Equation (1), so it should be noted that the order m = -3 exists only within the glass. This is an indication of total internal reflection. For higher dispersion gratings, all angles within the glass can be subjected to total internal reflection, as depicted in Figure 1B. In this way, the transmission order is thought to be suppressed via total internal reflection (TIR).

[0005]

[0005] Conventional diffraction gratings require a surface on which a periodic structure is to be fabricated. A common method for fabricating a grating is to select a substrate, such as a glass plate, and fabricate a periodic structure on it by etching, deposition, replication, or other methods known to those skilled in the art. When a metal is used as a coating to provide high-efficiency diffraction from the grating, light is incident from the air, diffracted reflectively from the metal grating structure, and does not interact with the glass substrate. This is one of the most common configurations of diffraction gratings used, for example, in spectrometers. However, in photolithography, for example, it is often useful to fabricate a transmission grating on which light incident on the grating is diffracted to a transmission order. In a transmission grating, light must pass through the substrate either before or after being diffracted by the transmission grating. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006]

[0006] Despite the advances made in the manufacture and use of diffraction gratings, there is a need in the art for improved methods and systems related to diffraction gratings. [Means for solving the problem]

[0007]

[0007] This disclosure generally relates to methods and systems related to diffraction gratings. More specifically, embodiments of the present invention provide methods and systems for immersion gratings characterized by a high spectral bandwidth. The immersion gratings described herein can be implemented in a prism configuration to provide an immersion grating prism. This disclosure is applicable to a variety of applications in optics and optoelectronics.

[0008]

[0008] According to one embodiment of the present invention, an immersion grating is provided. The immersion grating includes a substrate having an incident light surface and an optical surface facing a support surface. The substrate, which may be fused silica, has a refractive index greater than 1.45 over a wavelength range of 1020 nm to 1100 nm. The immersion grating also includes a diffraction grating formed within the optical surface. The diffraction grating is configured to receive light incident on the incident light surface. The immersion grating further includes an ambient environment adjacent to the diffraction grating.

[0009]

[0009] In one embodiment, the surrounding environment does not contain metal. For example, no material can be present between the diffraction grating formed in the optical surface and the surrounding environment. The diffraction grating can be configured to diffract the received light only to the m=0 and m=-1 order. The diffraction grating may be a one-dimensional periodic structure, for example, a one-dimensional periodic structure having a period of 2000 lines / mm or more. In some embodiments, the incident light surface and the optical surface are optical surfaces defined by PV with a flatness of 400 nm or less and a surface roughness of 100 ÅRMS or less. For example, the refractive index of the substrate may be 1.8 or more over a wavelength range of 1020 nm to 1100 nm. The diffraction grating can be characterized by a dispersion of 1.0 radians / μm or less, for example, a dispersion of 0.7 radians / μm or less. The center of the optical spectrum can be diffracted within 5° of the Littrow condition, for example, within 3° of the Littrow condition.

[0010]

[0010] According to another embodiment of the present invention, an immersion grid is provided. The immersion grid includes a dielectric substrate having an incident light plane and a second plane, the second plane being parallel to or opposite to the incident light plane. The dielectric substrate is characterized by a substrate refractive index. The immersion grid also includes at least one dielectric layer coupled to the second plane of the dielectric substrate. The at least one dielectric layer is characterized by a layer refractive index greater than the substrate refractive index. The immersion grid further includes a periodic structure formed within the at least one dielectric layer.

[0011]

[0011] In one embodiment, at least one dielectric layer is characterized by a thickness of 100 nm or more, for example, a thickness of 250 nm to 750 nm. The periodic structure can extend at least partially through at least one dielectric layer. Alternatively, the periodic structure can extend within the dielectric substrate through at least one dielectric layer. The periodic structure can have a period of 2000 lines / mm or more. The lattice dispersion in Littrow may be 2.0 radians / μm or less, for example, 1.4 radians / μm or less. The center of the optical spectrum can be diffracted within 5° of the Littrow condition, for example, within 3° of the Littrow condition.

[0012]

[0012] According to a specific embodiment of the present invention, an immersion grating prism is provided. The immersion grating prism includes a substrate having an incident light surface, an optical surface, and a third surface. The prism is characterized by a prism refractive index. The immersion grating prism also includes at least one dielectric layer coupled to the optical surface. The at least one dielectric layer is characterized by a layer refractive index greater than the prism refractive index. The immersion grating prism further includes a periodic structure formed within the at least one dielectric layer.

[0013]

[0013] According to a particular embodiment of the present invention, a method for forming a diffraction order is provided. The method includes providing an immersion grid having a dielectric substrate having an incident light plane and a second plane facing the incident plane. The dielectric substrate is characterized by a substrate refractive index. The immersion grid also includes at least one dielectric layer coupled to the second plane of the dielectric substrate. The at least one dielectric layer is characterized by a layer refractive index greater than the substrate refractive index. The immersion grid further includes a periodic structure formed within the at least one dielectric layer.

[0014]

[0014] The method also includes guiding a light beam so that it is incident on the incident light plane of a dielectric substrate, and propagating the light beam through at least one dielectric layer. The method further includes diffracting the light beam to form a reflection order, and propagating the reflection order through at least one dielectric layer.

[0015]

[0015] In one embodiment, the method also includes propagating the reflection order through the incident light plane of the dielectric substrate. The reflection order may be m = -1. The method may also include placing the immersion grating in the surrounding environment. The periodic structure may be a one-dimensional diffraction grating. In one embodiment, the reflection order is the sole diffraction order.

[0016] Compared with conventional techniques, numerous advantages are realized by the present disclosure. For example, embodiments of the present disclosure provide a high spectral bandwidth compared to conventional approaches. In one embodiment, a lattice is formed within a high refractive index material coupled to a substrate, enabling the fabrication of an immersion grating characterized by reduced dispersion and increased spectral bandwidth. Optical materials characterized by low optical loss, high transparency, and the ability to support high fluence are suitable for use in conjunction with embodiments of the present invention. These and other embodiments of the present disclosure, along with many of their advantages and features, are described in more detail below with the following text and corresponding figures.

[0017]

[0017] Here, aspects of the present disclosure are described more fully below with reference to the accompanying drawings, which are intended to be read in conjunction with both this summary, the detailed description, and any preferred and / or specific embodiments disclosed either specifically or otherwise. However, the various aspects may be embodied in many different forms and should not be construed as limited to the embodiments described herein. Rather, these embodiments are provided by way of example only, so that this disclosure will be complete and full, and will convey the full scope to those skilled in the art.

Brief Description of the Drawings

[0018] [Figure 1A] A schematic diagram showing a periodic lattice existing between air and glass. [Figure 1B] A schematic diagram showing total internal reflection when a lattice exists. [Figure 1C] A schematic cross-sectional view of an immersion grating according to an embodiment of the present invention. [Figure 2A] A plot of the spectral diffraction efficiency for TE polarization and TM polarization for an immersion grating having a first dispersion. [Figure 2B] A plot of the spectral diffraction efficiency for TE polarization and TM polarization for an immersion grating having a second dispersion. [Figure 2C]Plots of spectral diffraction efficiency for TE polarization and TM polarization with respect to an immersion grating having a third dispersion. [Figure 3A] A plot showing the spectral diffraction efficiency for an immersion grating using a substrate having a first refractive index. [Figure 3B] A plot showing the spectral diffraction efficiency for an immersion grating using a substrate having a second refractive index. [Figure 4] A schematic cross-sectional view showing an immersion grating according to an embodiment of the present invention. [Figure 5A] A plot showing the spectral diffraction efficiency for the immersion grating shown in FIG. 4. [Figure 5B] A plot showing the grating characteristics for an immersion grating having dielectric layers with different refractive indices according to an embodiment of the present invention. [Figure 5C] A plot showing the spectral diffraction efficiency for an immersion grating having dielectric layers with different refractive indices according to an embodiment of the present invention. [Figure 5D] A plot showing a spectral diffraction efficiency higher than 99% for the immersion grating shown in FIG. 5C. [Figure 5E] A plot showing the spectral diffraction efficiency for an immersion grating having dielectric layers with different refractive indices over a limited wavelength range according to an embodiment of the present invention. [Figure 5F] A plot showing a spectral diffraction efficiency higher than 99% for the immersion grating shown in FIG. 5E. [Figure 6] A schematic cross-sectional view showing an immersion grating prism according to an embodiment of the present invention. [Figure 7] A schematic flowchart showing a method of operating an immersion grating according to an embodiment of the present invention. [Figure 8] A schematic overview showing a high-power laser system using an immersion grating according to an embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0019]

[0036] Embodiments of the present invention relate to methods and systems related to diffraction gratings. More specifically, embodiments of the present invention provide methods and systems for immersion gratings characterized by a high spectral bandwidth. The immersion gratings described herein can be implemented in a prism configuration to provide immersion grating prisms. This disclosure is applicable to a variety of applications in optics and optoelectronics.

[0020]

[0037] Immersion gratings are a special class of gratings in which light is incident from within the substrate onto the grating, typically for the purpose of using the reflected order of diffraction. In this case, both the incident order and the desired reflected order are contained within the substrate material. In some implementations, a metal coating is used on the air side of the grating to increase the efficiency of reflection to the reflected order. However, any metal coating absorbs some of the light incident on it. Even with the thinnest metal layer, the absorbed light power can be sufficient, if the incident light power is sufficiently high, to cause undesirable thermally induced changes in grating performance or even catastrophically destroy the grating.

[0021]

[0038] Certain types of immersion gratings are made solely from dielectric materials and do not necessarily utilize metal coatings; rather, they suppress transmission orders based on the material of the immersion grating as described by the lattice equations. Specifically, if the lattice dispersion is sufficiently high, all transmission orders can be suppressed. In this case, the incident angle associated with the suppression of transmission orders is calculated using Equation 1.

[0022]

[0039] Figure 1C is a simplified cross-sectional view of an immersion grid according to one embodiment of the present invention. In the immersion grid shown in Figure 1C, the substrate medium 110 is silica glass, and the grid tooth profile 120 is binary. Those skilled in the art will understand that neither this particular substrate material nor this particular grid tooth profile is required for immersion grids that are reflective only. Conversely, various substrate materials and grid tooth profiles can be used, and they are within the scope of the present invention.

[0023]

[0040] Referring to Figure 1C, several embodiments of the present invention utilize a reflection-only immersion grating having only two orders: specular reflection (order m=0) and the first diffraction order (m=-1). The specular reflection order, as can be understood from Equation 1 by setting m=0, does not contain spectral dispersion; in this case, it is simply wavelength-independent and therefore lacks the desired spectral dispersion. In other words, the m=0 (specular) order functions as a conventional mirror. Therefore, to utilize dispersion from the grating, a diffraction order with m≠0 must be used. Optimization of the efficiency of the desired diffraction order is achieved by the appropriate selection of grating parameters. This includes the grating period and incident angle in Equation 1, but it also includes additional parameters of the grating tooth profile. Many grating tooth profiles can be parameterized by their shape type, e.g., binary, sinusoidal, or trapezoidal. In the binary grating tooth shape shown in Figure 1C, the grating tooth profile parameters are depth and duty cycle. In contrast, the sinusoidal shape is characterized by depth only. A trapezoid is characterized by its depth, duty cycle, and sidewall slope. Those skilled in the art will recognize numerous variations, modifications, and alternative forms.

[0024]

[0041] As shown in Figure 1C, the interface between the immersion grating material, such as fused silica, and the surrounding environment, such as air, is metal-free. That is, the grating tooth profile 120 is not metallized and is formed by the immersion grating material. Therefore, the embodiment shown in Figure 1C can be called a metal-free grating because there is no material between the diffraction grating formed on the optical surface and the surrounding environment.

[0025]

[0042] In many applications, the spectral bandwidth of high-efficiency diffraction is crucial. One of the challenges in implementing conventional reflection-only immersion gratings is achieving the necessary optical bandwidth to obtain high efficiency. The dispersion of the grating needs to be high enough to maintain the reflection-only condition, and high-dispersion gratings typically have a lower bandwidth, which the inventors determined is generally applicable to immersion gratings.

[0026]

[0043] Figure 2A is a plot of spectral diffraction efficiency for TE-polarized and TM-polarized light for immersion gratings with a first dispersion. Figure 2B is a plot of spectral diffraction efficiency for TE-polarized and TM-polarized light for immersion gratings with a second dispersion. Figure 2C is a plot of spectral diffraction efficiency for TE-polarized and TM-polarized light for immersion gratings with a third dispersion. As shown in Figures 2A to 2C, the diffraction efficiency of reflect-only immersion gratings characterized by different dispersions is plotted as a function of wavelength for both TE-polarized and TM-polarized light.

[0027]

[0044] As can be determined by the analysis in Figures 2A and 2C, the immersion grating is characterized by an inverse relationship between the spectral bandwidth and dispersion of diffraction efficiency, i.e., the bandwidth increases as the dispersion decreases. Therefore, this data indicates that the lowest possible dispersion should be used to maximize the bandwidth. However, if high-efficiency reflection-only operation is desired, the lowest dispersion is limited by Equation 1. Specifically, the physics employed to achieve high-efficiency reflection-only operation imposes a lower limit on the usable dispersion range and, therefore, an upper limit on the achievable spectral bandwidth.

[0028]

[0045] The variance D of the lattice can be derived from Equation 1. Under the Littrow condition, defined as the condition where the output angle of order m = -1 is equal to the incident angle, the variance is:

number

[0029]

[0046] Obtaining a wider spectral bandwidth from a reflection-only immersion grating means lower dispersion, as demonstrated by Figures 2A-2C. For a given optical spectrum, tan(θ) is a monotonically increasing function of θ between 0 and 90 degrees, so reducing dispersion means reducing the angle of incidence (via Equation 2).

[0030]

[0047] However, the angle cannot be simply reduced as desired. The conditions for a reflection-only immersion grating are given by Equation 1.

number

[0031]

[0048] Note that sin(θ) is also a monotonically increasing function of θ between 0 and 90 degrees. Therefore, Equation 3 imposes a limit on how small the incident angle can be, and thus an upper limit on the spectral bandwidth achievable via Equation 2. In other words, achieving a wide spectral bandwidth is physically limited by the requirements imposed to operate with highly efficient reflection-only diffraction.

[0032]

[0049] The inventors determined that insights from the analysis of Equation 3 could be used to develop a solution for increasing the optical bandwidth. Conventional free-space optical systems utilizing immersion gratings typically use glass substrates with refractive indices in the range of 1.44 to 1.58. However, the inventors determined that dispersion can be reduced if the immersion material (i.e., the material through which the incident light propagates) has an increased refractive index.

[0033]

[0050] Figure 3A is a plot showing the spectral diffraction efficiency for an immersion grating using a substrate with a first refractive index. Figure 3B is a plot showing the spectral diffraction efficiency for an immersion grating using a substrate with a second refractive index. In Figures 3A and 3B, the diffraction efficiency of the reflection-only immersion grating is plotted as a function of wavelength for both TE and TM polarization. For these immersion gratings, the substrate on which the diffracting elements (e.g., diffraction grating lines) are formed is characterized by its refractive index. The dispersions for the immersion gratings corresponding to the plots in Figures 3A and 3B were equal to 2.7 radians / μm and 1.2 radians / μm, respectively.

[0034]

[0051] Figure 3A is a plot corresponding to an immersion grating where the substrate has a refractive index n=1.45 (similar to fused silica glass), and Figure 3B is a plot corresponding to an immersion grating where the substrate has a refractive index n=2.00. A refractive index of n=1.45 is characteristic of an immersion grating over the wavelength ranges of 1040nm–1080nm, 1030nm–1090nm, or 1020nm–1100nm. These plots show that increasing the refractive index of the substrate, for a given grating periodicity, results in increased dispersion and therefore a wider spectral bandwidth. Thus, for a substrate with a refractive index of 1.45, a dispersion of 2.7 radians / μm is obtained, resulting in a polarization-average spectral bandwidth of only 12 nm with high efficiency exceeding 98%. When the substrate refractive index is increased to a refractive index of 2.00, the dispersion decreases to 1.2 radians / μm, and the polarization-average spectral bandwidth increases to 36 nm.

[0035]

[0052] The critical angle of TIR decreases as the substrate refractive index increases (given a substrate / air interface); therefore, increasing the substrate / air refractive index ratio results in a smaller TIR angle. A wider bandwidth arises from using smaller incident angles, which are allowed by the smaller TIR angle. Thus, this represents a lower dispersion lattice, as determined by equation (2).

[0036]

[0053] Figure 3B shows that increasing the refractive index of the substrate used in an immersion grating increases the spectral bandwidth; however, materials with higher refractive indices are not always readily available with the correct optical properties. For example, bulk materials with higher refractive indices may not have high optical quality, resulting in abnormal beams passing through them. Furthermore, bulk materials with higher refractive indices may not have sufficiently low absorption, resulting in power loss and thermal aberrations in beams passing through them. As an example, silicon has low absorption in the telecommunications band (1550 nm) and is therefore a suitable candidate for substrate materials for immersion gratings, but it cannot be used in the visible or IR (1000 nm) band due to its extremely high absorption at these light wavelengths. Moreover, while a thin layer of silicon is a high-optical-quality layer, immersion gratings typically require a considerable amount of bulk material (thickness ranging from 1 to 10 mm) for the immersion substrate. Such bulk materials may not have sufficient optical quality for free-space beam propagation.

[0037]

[0054] For reflective-only immersion gratings, these requirements are stringent because they require indirect optical penetration into the substrate medium through interfaces that are not parallel to the grating planes. This can be understood by analyzing Equation 3, which states that any angle of incidence satisfying Equation 3 cannot penetrate air. Those skilled in the art will recognize the converse as well: a beam incident from air cannot penetrate the medium through an interface or a plane parallel to the interface. For this reason, reflective-only immersion gratings are often formed on the surface of a prism, thereby allowing light to be incident on one side of the prism and diffracted through the grating formed on the other side of the prism. The fact that incident light passes through a considerable amount of material significantly limits the number of materials that can be used to increase the spectral bandwidth of an immersion grating by increasing the refractive index of the grating material used for the substrate of the immersion grating.

[0038]

[0055] Figure 4 is a simplified cross-sectional view showing an immersion grid according to one embodiment of the present invention. In the immersion grid 400 shown in Figure 4, the substrate 410 supports a dielectric layer 420 on which a grid tooth profile 422 is formed. The immersion grid is surrounded on one or more sides by an ambient environment 430 shown in Figure 4, which is air. This ambient environment, which may be called a cover layer, provides a lower refractive index than the dielectric layer and can be air, and brings the grid tooth profile, or other material including a low refractive index solid material, to a dielectric / air interface, as will be described in more detail herein.

[0039]

[0056] As shown in Figure 4, the substrate 410 supports the propagation of the incident beam by utilizing a material characterized by low optical loss and high optical quality. As an example, the substrate 410 can be manufactured using fused silica, borosilicate glass, multi-component silicate glass, or other materials characterized by high optical quality at the target wavelength. As described above, the design of the immersion grating parameters is such that only orders m=0 and m=-1 are allowed, resulting in a prohibited transmission zone within the ambient atmosphere 430. Thus, the immersion grating 400 produces only the specular reflection order (m=0) and the single diffraction order (m=-1) for the given incident angle. Therefore, in some embodiments, the dispersion is selected (e.g., at a low level) so that only a single order (i.e., m=-1) is supported by the immersion grating, and the m=+1 and m=-2 orders are not produced by the immersion grating. Figure 4 shows the use of a single degree (i.e., m = -1), but the present invention is not limited to the use of a single degree, and other embodiments may use higher degrees, including the use of a higher degree, such as m = -2. For example, in some embodiments utilizing m = -2, the m = -1 degree can be suppressed. Those skilled in the art will recognize many variations, modifications, and alternative forms.

[0040]

[0057] As will be apparent to those skilled in the art, reducing the number of orders supported by the immersion grating allows for the use of grating design techniques that preferentially direct light to one of the reduced number of orders. Referring to Figure 4 as an example, since the immersion grating supports only two orders, grating parameters, including depth and duty cycle, can be used to diffract most of the power present in the diffracted light to the m=-1 order, with less power present at the m=0 order. For example, the grating tooth profile 422 can be designed such that more than 90% of the diffracted power is present at the m=-1 order. In other embodiments, the percentage of diffracted power at the m=-1 order may be greater than 91%, greater than 92%, greater than 93%, greater than 94%, greater than 95%, greater than 96%, greater than 97%, greater than 98%, or greater than 99%.

[0041]

[0058] To provide a high refractive index material at the immersion grating / air interface shown in Figure 4, the dielectric layer 420 is characterized by high optical quality and a higher refractive index than the substrate 410. Thus, embodiments of the present invention utilize a high refractive index placed between the substrate 410 and the surrounding environment surrounding the immersion grating 400. As an example, the dielectric layer 420 can be manufactured using tantalum pentoxide, hafnium oxide, scandium oxide, titania oxide, and other materials characterized by a higher refractive index than the substrate at the wavelength of interest, and can have a thickness D in the range of 50 to 1,000 nm. In the wavelength range of 1052 nm, the refractive index of tantalum pentoxide is approximately 2.1, and the refractive index of hafnium oxide is approximately 2.1. In the embodiment shown in Figure 4, the surrounding environment is air. As shown in Figure 4, the high refractive index material utilized for the dielectric layer 420 bends the incident light ray closer to perpendicular incidence (via Snell's law), allowing the grating to be examined at lower angles of incidence. As explained in relation to Equation 2, smaller angles of incidence result in reduced dispersion of the immersion grating. Next, as explained in relation to Figures 2A-2C, the decrease in variance leads to an increase in spectral bandwidth.

[0042]

[0059] Accordingly, embodiments of the present invention enable low-dispersion immersion lattices by using a high refractive index material in the form of a dielectric layer 420 while maintaining the low-loss characteristics inherent to the material used for the substrate 410 through which incident and diffracted light propagate. Figure 4 shows a binary lattice of a given depth and period, and a dielectric layer having a refractive index of 2.0, but it should be noted that embodiments of the present invention are not limited to this shown example. In fact, embodiments of the present invention are not limited by any particular requirements regarding the lattice tooth profile or depth, the refractive index of the dielectric layer 420, the thickness of the dielectric layer 420, etc. Those skilled in the art will recognize that the design of a particular lattice tooth profile and / or dielectric layer 420 can be optimized for efficiency, bandwidth, etc.

[0043]

[0060] The angle at which m=-1 order diffraction occurs is the same within the substrate, regardless of whether the diffraction originates at the substrate / air interface shown in Figure 1C or at the dielectric layer / air interface shown in Figure 4. For a given lattice periodicity, this same substrate diffraction angle arises from the refraction of light at the substrate / dielectric layer interface and the resulting decrease in the incident angle at the dielectric layer / air interface. Those skilled in the art will recognize many variations, modifications, and alternative forms.

[0044]

[0061] Figure 4 shows a single dielectric layer 420, but embodiments of the present invention are not limited to a single layer. In other embodiments, multiple dielectric layers are bonded to a substrate 410, and the refractive index of each layer increases with distance from the substrate. As an example, a first dielectric layer located proximal to the substrate and having a refractive index of 1.8 can be used together with a second dielectric layer located distal to the substrate and having a refractive index of 2.1. In this example, a lattice tooth profile is then to be formed within the second dielectric layer. Three or more dielectric layers can be bonded to the substrate. Embodiments are described herein in relation to layers, but it will be understood that the term layer can include sublayers, gradient composition layers, etc. Those skilled in the art will recognize many variations, modified forms, and alternative forms. Furthermore, although a lattice tooth profile with a binary profile without brazing is shown, it will be understood that any suitable lattice profile can be used, including multilevel lattices, sinusoidal lattices, sawtooth lattices, trapezoidal lattices, blazed lattices, nanostructures, metaplanes, etc. The advantage offered by the binary grids shown in Figure 4 (and several other grid shapes) is that the depth of the grid teeth can be modified independently of the grid period (i.e., duty cycle). As will be apparent to those skilled in the art, binary grids characterized by non-vertical grid teeth resulting from a manufacturing process are included within the scope of the present invention.

[0045]

[0062] Figure 5A is a plot showing the spectral diffraction efficiency for the immersion grating shown in Figure 4. Figure 5A presents data for one specific configuration of the parameters for the immersion grating shown in Figure 4, but it should be understood that variations in the immersion grating parameters may result in changes to the plot shown in Figure 5A.

[0046]

[0063] Referring to Figure 5A, the high diffraction efficiencies for both the TE and TM modes are substantially equal over the wavelength range of 1040 nm to 1080 nm shown in the figure. In particular, the diffraction efficiencies for both the TE and TM modes are greater than 99% over the wavelength range of 1040 nm to 1080 nm. Although Figure 5A shows the wavelength range of 1040 nm to 1080 nm, embodiments of the present invention are applicable to the wavelength ranges of 1030 nm to 1090 nm and 1020 nm to 1100 nm. Furthermore, although infrared wavelengths are shown in Figure 5A, the present invention is not limited to infrared wavelengths, and embodiments of the present invention are suitable for use over a broad wavelength range covering ultraviolet wavelengths, including visible wavelengths, to infrared wavelengths. Compared to Figure 3A, which is a plot showing the spectral diffraction efficiency for an immersion grating using a substrate with a refractive index n=1.45 similar to substrate 410 manufactured using silica glass (i.e., fused silica), as shown in Figure 4, using a high refractive index material as the dielectric layer 420 at the immersion grating / air interface significantly improves the spectral bandwidth because the diffraction efficiency increases over many wavelengths from 1040 nm to 1080 nm. Note that although light is incident on substrate 410 (e.g., silica glass with a refractive index n=1.45), the spectral bandwidth is also approximately the same width as that of the bulk material with a refractive index n=2.00, as shown in the plot in Figure 3B. In high-power laser applications, high diffraction efficiency (i.e., diffraction efficiency greater than 99% at the operating wavelength) enables operations that would not be possible with low diffraction efficiency (i.e., diffraction efficiency less than 99%). In some embodiments, high efficiencies (i.e., diffraction efficiencies greater than 98% for both TE and TM modes) are present over wavelength ranges of 1040–1080 nm, for example, 1041–1066 nm.

[0047]

[0064] In high-power laser applications, diffraction efficiency is crucial, and the transparency of the material used for the immersion grating enables high-power operation without damaging optical components.

[0048]

[0065] Comparing Figure 1C and Figure 4, the same substrate and lattice parameters are used, and the immersion lattice shown in Figure 4 incorporates a high refractive index dielectric layer 420. Comparing the dispersion of 2.7 radians / μm corresponding to Figure 3A with the dispersion of 1.2 radians / μm corresponding to Figure 5A, the addition of the high refractive index dielectric layer on which the lattice is formed increased the spectral bandwidth with an average polarization efficiency of 98% from 12 nm in Figure 3A to 44 nm in Figure 5A. Therefore, by increasing the refractive index at the lattice / environment interface, a reduction in dispersion and an increase in spectral bandwidth are achieved while the majority of the optical element remains unchanged.

[0049]

[0066] Figure 5A demonstrates the increase in spectral bandwidth provided by embodiments of the present invention compared to conventional methods; however, this specific data does not limit the scope of the present invention. Rather, the increase in spectral bandwidth shown in Figure 5A indicates that the structure provided according to embodiments of the present invention offers similar advantages to the structure shown in Figure 5A, and Figure 5A is merely an example of the increase in spectral bandwidth that can be achieved by embodiments of the present invention.

[0050]

[0067] In some of the figures and descriptions provided herein, the surrounding environment in which the immersion grid is placed is air (see, for example, Figure 4). However, embodiments of the present invention are not limited to this particular ambient atmosphere and are used merely illustratively. Other gases that can be used as the ambient atmosphere include vacuum and / or inert gases, alternative gases such as nitrogen, argon, or other noble gases, low refractive index materials such as low refractive index polymers, aerogels, or low refractive index liquids such as water, which can be used according to Formula 3. Thus, the ambient environment is not limited to gases and may include solids as needed. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0051]

[0068] Figure 5B is a plot showing the lattice properties for an immersion grating having dielectric layers with different refractive indices, according to one embodiment of the present invention. As shown in Figure 5B, wavelength spans with diffraction efficiency greater than 99% (in nanometers) are plotted on the left vertical axis as a function of the refractive index of dielectric layer 420 shown in Figure 4. The diffraction efficiency is calculated as the lower of the diffraction efficiency of the TE mode and the diffraction efficiency of the TM mode. The wavelength spans are plotted only in increments of 0.1 in refractive index values, but plotting in finer increments would likely yield smoother curves. Those skilled in the art will recognize many variations, modifications, and alternative forms.

[0052]

[0069] Referring to Figure 4, as shown in Figure 5B, for an immersion grating with a dielectric layer 420 having a refractive index of 1.7, the wavelength span at which diffraction efficiency is greater than 99% is only 13 nm. As the refractive index of the dielectric layer increases to 1.9, the bandwidth associated with diffraction efficiency greater than 99% increases significantly, resulting in a wavelength range of 25 nm to 26 nm, which is approximately twice the bandwidth associated with the immersion grating with a dielectric layer having a refractive index of 1.7. In the refractive index range of 1.9 to 2.2, the diffraction efficiency remains at this high level, providing a wavelength range of 25 nm to 26 nm at which diffraction efficiency exceeds 99%. As a result, these immersion gratings with high diffraction efficiency are applicable to a variety of high-power laser applications.

[0053]

[0070] Referring again to Figure 5B, as the refractive index of the dielectric layer increases further to 2.6, the wavelength span for diffraction efficiency greater than 99% decreases, dropping to the 18 nm wavelength range for a refractive index of 2.6. Therefore, we determined that a narrow range of refractive indices is associated with a plateau in the large wavelength range for immersion gratings with high diffraction efficiency (i.e., >99%), specifically for dielectric layers with refractive indices of 1.9–2.2.

[0054]

[0071] In addition to the wavelength span variation, which enables high diffraction efficiency as a function of the dielectric layer's refractive index, other parameters of the immersion grating also vary with the dielectric layer's refractive index. As shown on the right vertical axis, the thickness of the dielectric layer 420, called the layer thickness (in μm), and the etching depth associated with the grating defined within the dielectric layer, also measured in microns, vary with the refractive index. The layer thickness varies from approximately 0.8 μm for a refractive index of 1.7 to approximately 0.45 μm for a refractive index of 2.6. Similarly, the etching depth of the periodic structure defining the grating varies from approximately 0.35 μm for a refractive index of 1.7 to approximately 0.25 μm for a refractive index of 2.6. Therefore, to achieve a refractive index of 1.9, a layer thickness of approximately 0.7 μm and an etching depth of approximately 0.3 μm can be used. To achieve a refractive index of 2.2, a layer thickness of approximately 0.58 μm and an etching depth of approximately 0.27 μm can be used. Refractive index values ​​between 1.9 and 2.2 can be achieved by using layer thicknesses and etching depths between these values.

[0055]

[0072] In addition to layer thickness and etching depth, the right vertical axis is used to represent the portion of the dielectric layer etched to form a lattice, called the etching rate, and the duty cycle of the lattice, both measured as percentages. The etching rate varies from approximately 40% for a refractive index of 1.7 to approximately 50% for a refractive index of 2.6. Similarly, the duty cycle varies from approximately 0.35 for a refractive index of 1.7 to approximately 0.2 for a refractive index of 2.6. Therefore, to achieve a refractive index of 1.9, an etching rate of approximately 45% and a duty cycle of approximately 27% can be utilized. To achieve a refractive index of 2.2, an etching rate of approximately 46% and a duty cycle of approximately 25% can be utilized. Refractive indexes between 1.9 and 2.2 can be achieved by using etching rates and duty cycles between these values.

[0056]

[0073] Figure 5C is a plot showing the spectral diffraction efficiency for immersion gratings having dielectric layers with different refractive indices, according to one embodiment of the present invention. In Figure 5C, the diffraction efficiencies of immersion gratings similar to those shown in Figure 4 are plotted for immersion gratings having dielectric layers 420 with refractive indices of 1.7, 1.9, 2.2, and 2.6, respectively. As shown in Figure 5C, using a dielectric layer 420 with a refractive index of 1.7, the diffraction efficiency exceeds 99% over a narrow wavelength range of about 13 nm. As the refractive index of the dielectric layer is increased to 1.9 and 2.2, the bandwidth associated with diffraction efficiencies greater than 99% increases significantly, resulting in a wavelength range of about 25 nm, which is about twice the bandwidth associated with the immersion grating with a dielectric layer with a refractive index of 1.7. As the refractive index of the dielectric layer is further increased to 2.6, the bandwidth decreases to about 18 nm. Thus, as explained in relation to Figure 5B, a narrow range of refractive indices is associated with high diffraction efficiency (i.e., >99%).

[0057]

[0074] Referring to Figure 5C, for an immersion grating with a dielectric layer having a refractive index of 1.7, the diffraction efficiency ranges from approximately 96% at 1030 nm to approximately 98.5% at 1040 nm, and down to approximately 96% at 1075 nm. For this particular immersion grating, the narrow diffraction efficiency above 99% results in a diffraction efficiency below 99% over most of the 1030 nm to 1075 nm range. The low efficiency associated with a dielectric layer having a refractive index of 1.7 leads to insufficient performance in high-power laser applications.

[0058]

[0075] Figure 5D is a plot showing spectral diffraction efficiencies higher than 99% for the immersion grating shown in Figure 5C. Figure 5D shows the same data presented in Figure 5C, but over a narrower diffraction efficiency range of 99% to 1.0. As shown in Figure 5D, for immersion gratings with dielectric layers having refractive indices of 1.9 and 2.2, the diffraction efficiency exceeds 99% over the wavelength range of approximately 1040 nm to approximately 1065 nm.

[0059]

[0076] Figure 5E is a plot showing the spectral diffraction efficiency for an immersion grating having dielectric layers with different refractive indices over a limited wavelength range, according to one embodiment of the present invention. Compared to Figure 5C, which covers the wavelength range of 1030 nm to 1075 nm, Figure 5E covers only the wavelength range of 1041 nm to 1065 nm, allowing for further analysis of the spectral characteristics of the diffraction efficiency. In Figure 5E, the diffraction efficiencies of immersion gratings similar to those shown in Figure 4 are plotted for immersion gratings having dielectric layers 420 with refractive indices of 1.7, 1.9, 2.2, and 2.6, respectively. As shown in Figure 5E, using the dielectric layer 420 with a refractive index of 1.7, the diffraction efficiency exceeds 99% over a narrow wavelength range of approximately 13 nm, extending from approximately 1044 nm to approximately 1057 nm. As the refractive index of the dielectric layer increases to 1.9 and 2.2, the bandwidth associated with diffraction efficiencies greater than 99% increases significantly, resulting in a wavelength range of approximately 25 nm extending substantially across the entire plotted range from 1041 nm to 1066 nm, which is about twice the bandwidth associated with the immersion grating with a dielectric layer having a refractive index of 1.7. Further increasing the refractive index of the dielectric layer to 2.6 reduces the bandwidth to approximately 18 nm, extending from approximately 1043 nm to approximately 1061 nm.

[0060]

[0077] Figure 5F is a plot showing spectral diffraction efficiencies higher than 99% for the immersion gratings shown in Figure 5E. Figure 5F shows the same data presented in Figure 5E, but over a narrower diffraction efficiency range of 99% to 100%. As shown in Figure 5F, for immersion gratings with dielectric layers having refractive indices of 1.9 and 2.2, the diffraction efficiency exceeds 99% over a wavelength range of approximately 1041 nm to approximately 1066 nm.

[0061]

[0078] Therefore, the inventors determined that an immersion grating based on the design shown in Figure 4 and utilizing a material for the dielectric layer 420 having a refractive index of 1.9 to 2.2 provides high diffraction efficiency (i.e., diffraction efficiency greater than 99%) over the wavelength range of 1041 nm to 1066 nm.

[0062]

[0079] The inventors determined that the high refractive index associated with dielectric layers using, for example, 1 μm, i.e., hafnium oxide or tantalum pentoxide in the range of 1041 nm to 1066 nm, combined with the low absorptivity of dielectric layers using, for example, 1 μm, i.e., hafnium oxide or tantalum pentoxide in the range of 1041 nm to 1066 nm, enables both high diffraction efficiency and low loss, which are important for high-power laser applications.

[0063]

[0080] Figure 6 is a simplified cross-sectional view showing an immersion grating prism 600 according to one embodiment of the present invention. As shown in Figure 6, an incident light beam 640 is incident on the incident light surface 605 of a prism 610 which includes an incident light surface 605, an optical surface 607, and a third surface 609. The prism is characterized by its prism refractive index. The incident light beam 640 passes through the incident light surface 605 and propagates toward the optical surface 607. At least one dielectric layer 620 is coupled to the optical surface 607. As described in relation to Figure 4, at least one dielectric layer 620 is characterized by a layer refractive index greater than the prism refractive index.

[0064]

[0081] As shown in Figure 6, a periodic structure 630 is formed within at least one dielectric layer 620. The periodic structure 630 defines the interface between at least one dielectric layer 620 and the surrounding environment 635. As described in relation to Figure 4, the periodic structure, such as a diffraction grating, diffracts the light in the incident light beam 640 into m=0th order 642, which propagates through the third surface 609, and m=-1st order 644, which is diffracted to propagate through the incident light surface 640. Thus, both the incident light and the reflected light (i.e., m=-1st order 644) pass through the incident light surface 605, the incident light beam that passed through the incident light surface as incident light enters the prism, and the reflected light (i.e., m=-1st order 644) that passed through the incident light surface as reflected light exits the prism. In the embodiment shown in Figure 6, a cover 650 is provided to seal the surrounding environment 635, for example, to form an airtight seal. In other embodiments, the cover 650 is optional.

[0065]

[0082] As will be apparent to those skilled in the art, the immersion grid prism 600 shares elements in common with the immersion grid 400 shown in Figure 4, and the description provided in relation to Figure 4 is applicable to the immersion grid prism 600 as appropriate. Those skilled in the art will recognize many variations, modifications, and alternative forms.

[0066]

[0083] Using the immersion grating shown in Figure 6, the collimated input beam represented by the input beam 640 is reflected at order m=0 as shown by the reflected beam 642. A single diffraction order (i.e., order m=-1) is diffracted as shown by the diffracted beam 644. In some embodiments, the input beam 640 is close to the Littrow condition (e.g., Littrow within 5 degrees or Littrow within 3 degrees), and the m=-1 order diffracted beam 644 is substantially parallel to the input beam 640 (e.g., parallel within 5 degrees or parallel within 3 degrees).

[0067]

[0084] Figure 7 is a simplified flowchart showing a method for operating an immersion grating according to one embodiment of the present invention. Method 700 includes providing an immersion grating having a dielectric substrate having an incident light surface and a second surface opposite the incident surface, forming a diffraction order (710). The dielectric substrate is characterized by a substrate refractive index. The immersion grating also includes at least one dielectric layer coupled to the second surface of the dielectric substrate. The at least one dielectric layer is characterized by a layer refractive index greater than the substrate refractive index. The immersion grating further includes a periodic structure formed within the at least one dielectric layer. The periodic structure can be formed as a one-dimensional diffraction grating.

[0068]

[0085] The method also includes guiding a light beam so that it is incident on the incident light plane of a dielectric substrate (712) and propagating the light beam through at least one dielectric layer (714). In the embodiment shown in Figure 4, the incident light propagates through the substrate 410 and the dielectric layer 420. The method further includes diffracting the light beam to form a reflection order (716) and propagating the reflection order through at least one dielectric layer (718). As shown in Figure 4, the reflection order includes an m=-1 order. In some embodiments, the reflection order is a single diffraction order.

[0069]

[0086] In some embodiments, the method further includes propagating the reflection order through the incident light plane of the dielectric substrate, as shown in Figure 4. The immersion grid can be placed in the ambient atmosphere, which can be protected using a cover similar to the cover 650 shown in Figure 6, for example.

[0070]

[0087] Please understand that the specific steps shown in Figure 7 provide a particular method for operating an immersion grid according to one embodiment of the present invention. Other sequences of steps may be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Furthermore, the individual steps shown in Figure 7 may include a plurality of substeps that can be performed in various orders appropriate to the individual steps. In addition, further steps may be added or omitted depending on the particular application. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0071]

[0088] Figure 8 is a simplified schematic diagram showing a high-power laser system utilizing an immersion grating according to one embodiment of the present invention. As shown in Figure 8, the high-power laser system 800 includes a high-power laser 810 that emits light incident on an immersion grating 820. The synchrotron radiation beam is represented by the incident beam 812. Diffraction from the immersion grating 820 of order m=-1 results in a diffracted beam 822. Using embodiments of the present invention as described herein, the diffraction efficiency at which the diffracted beam 822 is diffracted exceeds 99% over the wavelength range of 1041–1066, resulting in 99% of the power present in the incident beam 812 being present in the diffracted beam 822. The reflected beam 824 contains less than 1% of the power lost from the incident beam 812. Diffraction efficiency in both TE and TM polarization is considered in the design of the immersion grating to ensure that reflection losses present at order m=0 do not damage system components, such as components that absorb reflection losses, when the high-power laser is operating in an undesirable polarization state.

[0072]

[0089] In spectral beam coupling applications suitable for immersion gratings described herein, the incident power is approximately 100 kW to 500 kW. In these applications, power that is not diffracted as desired will be lost. This lost power is typically captured and dissipated by one of several methods. As a result, for immersion gratings operating with 99% diffraction efficiency over a given wavelength range described herein, 1% power loss corresponds to 1 kW to 5 kW of laser power that must be captured and dissipated. Due to these high power levels associated with lost power, the high-efficiency immersion gratings provided by embodiments of the present invention enable applications that cannot be satisfied by gratings operating with lower efficiencies, particularly those with efficiencies less than 99%. Those skilled in the art will recognize many variations, modifications, and alternative forms.

[0073]

[0090] Furthermore, it should be understood that the examples and embodiments described herein are for illustrative purposes only, and that various modifications or changes will be suggested to those skilled in the art and should be included within the purpose and scope of this application and the attached claims.

Claims

1. It is an immersion grid, A dielectric substrate having an incident light surface and a second surface facing the incident light surface, wherein the dielectric substrate is characterized by the refractive index of the substrate, At least one dielectric layer bonded to the second surface of the dielectric substrate, wherein the at least one dielectric layer is characterized by a layer refractive index greater than the refractive index of the substrate and a predetermined thickness D, and the at least one dielectric layer is A first portion of the at least one dielectric layer having a first thickness d 1, Including a second portion of the at least one dielectric layer having a second thickness d2, D = d1 + d2, A periodic structure is formed within the second portion of the at least one dielectric layer and An immersion grid comprising, wherein the immersion grid is characterized by having a Littrow dispersion of 2.0 radians / μm or less and a diffraction efficiency greater than 99% over a wavelength range of 1041 nm to 1066 nm.

2. The immersion grid according to claim 1, further comprising an ambient environment, wherein the periodic structure is immersed in the ambient environment.

3. The immersion grid according to claim 2, wherein there is no material between the periodic structure formed in the at least one dielectric layer and the surrounding environment.

4. The immersion grid according to claim 1, wherein a light beam is incident on the periodic structure from at least one dielectric layer.

5. The immersion grid according to claim 1, wherein the at least one dielectric layer does not contain metal.

6. The immersion grid according to claim 1, wherein the immersion grid supports only the order of reflection.

7. The immersion grid according to claim 1, wherein the periodic structure includes a period measured in one dimension.

8. The immersion grating according to claim 7, wherein the periodic structure comprises a one-dimensional diffraction grating.

9. The immersion grid according to claim 8, wherein the immersion grid is configured to generate only diffraction orders m=0 and m=-1.

10. The immersion grid according to claim 1, wherein the dielectric substrate contains fused silica.

11. The immersion grid according to claim 1, wherein the at least one dielectric layer is characterized by a thickness of 0.55 μm to 0.7 μm.

12. The immersion grid according to claim 1, wherein the refractive index of the layer is in the range of 1.9 to 2.2 over a wavelength range of 1030 nm to 1080 nm.

13. The immersion grid according to claim 1, wherein the at least one dielectric layer comprises at least one of tantalum pentoxide or hafnium oxide.

14. The immersion grid according to claim 1, wherein the thickness of at least one dielectric layer is 0.45 μm to 0.85 μm.

15. The immersion grid according to claim 1, wherein the periodic structure is characterized by an etching depth of 0.25 μm to 0.35 μm.

16. The immersion grid according to claim 1, wherein the periodic structure is characterized by a duty cycle of 0.20 to 0.

35.

17. It is an immersion grid prism, A prism having an incident light surface, an optical surface, and a third surface, wherein the prism is characterized by its refractive index, At least one dielectric layer coupled to the optical surface, wherein the at least one dielectric layer is characterized by a layer refractive index greater than the prism refractive index and a predetermined thickness D, and the at least one dielectric layer is A first portion of the at least one dielectric layer having a first thickness d 1, Including a second portion of the at least one dielectric layer having a second thickness d2, D = d1 + d2, A periodic structure is formed within the second portion of the at least one dielectric layer and An immersion grating prism comprising the above, characterized in that the Littrow dispersion is 2.0 radians / μm or less and the diffraction efficiency is greater than 99% over a wavelength range of 1041 nm to 1066 nm.

18. The immersion grating prism according to claim 17, wherein the periodic structure is configured to generate diffraction orders that pass through the incident light plane.

19. The immersion grating prism according to claim 18, wherein the diffraction order is m = -1.

20. The immersion grating prism according to claim 17, wherein the at least one dielectric layer comprises at least one of tantalum pentoxide or hafnium oxide.

21. The immersion grid prism according to claim 20, further comprising an ambient environment adjacent to the periodic structure, wherein no material exists between the periodic structure and the ambient environment.

Images