Drying oven for nonlinear optical crystal and use method

Abstract

A system for wavelength conversion. In some example, an oven for a non-linear optical (NLO) crystal includes a thermally conductive enclosure configured to define an opening for holding the NLO crystal and to conduct heat between a heating element and the NLO crystal, the thermally conductive enclosure being in thermal contact with at least a portion of the NLO crystal, and the heating element is configured to be heated to a temperature including an end value of at least 250 DEG C; a support structure configured to support the thermally conductive housing, the support structure being thermally isolated from the thermally conductive housing; and an expandable attachment assembly configured such that the NLO crystal is secured within the opening of the thermally conductive housing by a spring force applied by the expandable attachment assembly.

Description

[0001] The technical field relates generally to frequency conversion using nonlinear optical (NLO) crystals, and more particularly to ovens and their use for heating NLO crystals during a frequency conversion process. Background Art

[0002] Most laser sources emit only at wavelengths in the infrared, and therefore nonlinear optical effects are used to achieve wavelengths in the visible or ultraviolet (UV). For example, an infrared laser beam (pump beam) generated by a laser source can be doubled in frequency by interacting with an NLO crystal (second harmonic generation), or two separate pump beams can be combined in an NLO crystal to produce a third beam whose frequency is equal to the sum of the frequencies of the incident beams (sum frequency generation). The range of wavelengths that can be created using these techniques spans from 150 nanometers (nm) to 20 micrometers (μm), and the created lasers can be continuous wave (CW) or pulsed, and are used in a wide range of applications including those targeting materials processing, biomedical, scientific, and consumer markets.

[0003] Currently available NLO crystal materials are designed to operate below 220°C, and for most applications this upper limit is sufficient. However, for some specific applications, it is required that the NLO crystal material operate at a temperature higher than this maximum. Summary of the invention

[0004] Aspects and embodiments relate to a method and system for nonlinear frequency conversion.

[0005] According to one example, an oven for a nonlinear optical (NLO) crystal is provided. The oven may include: a thermally conductive housing configured to define an opening for holding the NLO crystal and to conduct heat between a heating element and the NLO crystal, the thermally conductive housing being in thermal contact with at least a portion of the NLO crystal, and the heating element being configured to heat to a temperature of at least 250°C, inclusive; a support structure configured to support the thermally conductive housing, the support structure being thermally isolated from the thermally conductive housing; and an expandable attachment assembly configured to secure the NLO crystal within the opening of the thermally conductive housing by a spring force applied by the expandable attachment assembly.

[0006] In an example, an air space extends at least partially between the thermally conductive housing and the support structure. In another example, the support structure includes a recess for a spring of the expandable attachment assembly. In another example, the oven further includes at least one thermal insulation device positioned between the support structure and the thermally conductive housing. In another example, the thermally conductive housing is configured to thermally conduct heat between the NLO crystal and two heating elements, and the oven further includes a heat sink extending between the thermally conductive housing and the support structure.

[0007] In an example, the heating element is configured to heat to a temperature of at least 400° C., inclusive.

[0008] In an example, the heating element is configured to heat to a temperature in the range of 250° C. to 500° C., inclusive.

[0009] In an example, the thermally conductive housing is constructed of a material having a coefficient of thermal expansion different from that of the NLO crystal, and the expandable attachment assembly is configured to reduce stress on the NLO crystal that occurs due to the difference in coefficient of thermal expansion when the heating element is heated to a temperature within a range of 250° C. to 500° C., inclusive.

[0010] In an example, the oven further includes a controller coupled to the heating element and configured to control the heat generated by the heating element and to receive temperature measurements from a temperature sensor positioned adjacent to the NLO crystal.

[0011] In an example, the thermally conductive housing is constructed of aluminum.

[0012] In an example, the thermally conductive housing includes at least two components and is configured such that a gap exists between a first component and a second component adjacent to each other.

[0013] In examples, the oven is configured to operate without a thermal enclosure.

[0014] In an example, the NLO crystal is configured for non-critical phase matching at temperatures within a range of 250° C. to 500° C., inclusive.

[0015] In an example, a method includes providing an oven according to claim 1 .

[0016] According to another example, a wavelength conversion method is provided. The method may include: providing a laser light source, the laser light source being configured to generate a laser beam having a first wavelength; providing an oven for a nonlinear optical (NLO) crystal, the oven comprising: a thermally conductive housing, the thermally conductive housing being configured to define an opening for holding the NLO crystal and thermally conducting heat between a heating element and the NLO crystal; and a support structure, the support structure being configured to support the thermally conductive housing, the support structure being thermally isolated from the thermally conductive housing; positioning the NLO crystal within the opening of the oven; heating the nonlinear optical (NLO) crystal to a temperature of at least 250° C., inclusive, the NLO crystal being configured to convert the first wavelength into at least one second wavelength; and guiding the laser beam through the NLO crystal.

[0017] In an example, heating includes heating the NLO crystal to a temperature within a range of 250° C. to 500° C., inclusive.

[0018] In an example, the method further includes providing the NLO crystal.

[0019] In another example, nonlinear frequency mixing occurs between different modes of the laser beam having the first wavelength within the NLO crystal, and the nonlinear frequency mixing is a mixing operation selected from the group consisting of harmonic generation, sum frequency generation, difference frequency generation, optical parametric generation, optical parametric amplification, and optical parametric oscillation.

[0020] In an example, the method further includes measuring a temperature of the NLO crystal and controlling the heating element based on the temperature measurement.

[0021] In an example, the oven further includes an expandable attachment assembly configured such that the NLO crystal is secured within the opening of the thermally conductive housing by a spring force applied by the expandable attachment assembly.

[0022] Other aspects, embodiments and advantages of these exemplary aspects and embodiments are discussed in detail below. In addition, it should be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and characteristics of the claimed aspects and embodiments. The embodiments disclosed herein may be combined with other embodiments, and references to "embodiment", "example", "some embodiments", "some examples", "alternative embodiments", "various embodiments", "one embodiment", "at least one embodiment", "the embodiment and other embodiments", "certain embodiments" or the like are not necessarily mutually exclusive, and are intended to indicate that the specific features, structures or characteristics described may be included in at least one embodiment. The appearance of such terms herein does not necessarily all refer to the same embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Various aspects of at least one embodiment are discussed below with reference to the accompanying drawings, which are not intended to be drawn to scale. The accompanying drawings are included to provide illustration and further understanding of the various aspects and embodiments, and are incorporated into and constitute a part of this specification, but are not intended as a definition of limitations to any particular embodiment. The accompanying drawings, together with the rest of the specification, are used to explain the principles and operation of the described and claimed aspects and embodiments. In the drawings, each identical or nearly identical component illustrated in the various figures is represented by the same reference numeral. For the purpose of clarity, not every component may be labeled in every figure. In the drawings:

[0024] Figure 1A and Figure 1B is a simplified schematic representation of one example of an NLO oven housing and heater taken along the optical axis and the longitudinal axis of the NLO crystal, respectively, according to one or more aspects of the present invention;

[0025] Figure 1C is a perspective view of an NLO crystal according to one or more aspects of the present invention;

[0026] Figure 2A and Figure 2B is a simplified schematic representation of another example of an NLO oven housing and heater taken along the optical axis and the longitudinal axis of the NLO crystal, respectively, according to one or more aspects of the present invention;

[0027] Figure 3 is a simplified schematic representation of one example of a support plate and expandable attachment assembly that may be used with an NLO oven configuration according to aspects of the present invention;

[0028] Figure 4is a simplified schematic representation of an example of a thermal separation device that may be used with an NLO oven configuration according to aspects of the present invention;

[0029] Figure 5 is a simplified schematic representation of another example of a support plate and expandable attachment assembly that may be used with an NLO oven configuration according to aspects of the present invention;

[0030] Figure 6 is a block diagram of an NLO oven system according to aspects of the present invention;

[0031] Figure 7 is a simplified block diagram of a wavelength conversion system according to aspects of the present invention;

[0032] Figure 8 is a graph showing an example of a relationship between UV laser pulse energy and a temperature gradient across an NLO crystal according to aspects of the present invention; and

[0033] Fig. 9 is a graph illustrating an example of the relationship between green laser power, conversion efficiency, and temperature gradient across an NLO crystal according to aspects of the present invention. DETAILED DESCRIPTION

[0034] As mentioned previously, currently available NLO crystal materials are designed to operate at temperatures below 220° C. However, for some specific applications, it is required that the NLO crystal material operate at temperatures above 220° C.

[0035] One non-limiting example of such an application includes the generation of wavelengths shorter than 600 nm. As the average or peak power of the incident laser beam increases, multiple photons can be absorbed by the NLO crystal material, which damages the crystal material and introduces defects that lead to degradation, such as color centers. Furthermore, as the wavelength is further reduced to the ultraviolet (UV) and deep ultraviolet (DUV) wavelength ranges, two-photon absorption becomes a critical issue for conversion efficiency and power degradation.

[0036] Traditional methods for addressing these issues include increasing the temperature of the NLO crystal, which reduces two-photon absorption and improves the operating lifetime of the system. In addition, higher temperatures destroy contaminants that could potentially create surface damage.

[0037] A second non-limiting example of an application that can benefit from NLO crystal materials operating at temperatures above 220°C is in applications where a variable or tunable time delay is introduced between different wavelengths. Frequency conversion of pulsed light requires that the mixed light overlaps spatially and temporally in the crystal. When the two wavelengths of light are polarized orthogonally within the NLO crystal, the temporal overlap can be adjusted. The crystal can be designed to have the correct cut angle and length to overlap the pulses in time. However, as the optical power changes, the pulse overlap may also change due to thermal changes in the refractive index, which reduces the conversion efficiency. The temporal overlap can be corrected by operating the crystal at different temperatures. Having the ability to operate NLO crystals over a wide temperature range permits temporal overlap over a wide output power range.

[0038] Yet another non-limiting example of an application that can benefit from the higher operating temperature of NLO crystals involves phase matching. NLO crystals are cut at specific angles so that there is phase matching between different wavelengths for a specific temperature. Without this phase matching, the nonlinear conversion is quite weak and in some instances so low that the device cannot be used. In addition, it is possible that phase matching conditions do not exist in some crystals. Increasing the upper temperature range of the crystal increases the feasibility of using crystal materials that are difficult to phase match.

[0039] One or more aspects of the present invention address the foregoing and other potential applications by providing a system and method of wavelength conversion that includes heating a NLO crystal to a temperature of at least 250°C, inclusive, and encompassing embodiments where the temperature is as high as 500°C.

[0040] It is expected that for certain UV generation applications, the values ​​of the two-photon absorption coefficient β (cm / GW) at 250 to 500°C will be at least three-quarters lower than the value at room temperature. In addition, the long-term power stability for the generation is expected to be improved at these higher temperatures compared to room temperature.

[0041] According to at least one embodiment, Figure 1A and Figure 1B 1 is a simplified schematic representation of one non-limiting example of an NLO oven (generally designated 100 and also referred to herein simply as an “oven”) showing an end view of the optical axis and a side view of the longitudinal axis of the NLO crystal 115, respectively. Figure 1C 1 shows a perspective view of an NLO crystal 115, wherein a longitudinal face 116 (corresponding to the longitudinal axis) and an end face 118 (corresponding to the optical axis) are illustrated. The oven 100 includes a thermally conductive housing 105 for housing the NLO crystal 115, a support structure 120 ( Figure 3 and Figure 4 An example is shown in FIG. 1 ) and an expandable attachment assembly 140 ( Figure 3 and Figure 4 An example is shown in ).

[0042] Thermally conductive housing 105 is configured to define an opening 135 for holding NLO crystal 115 and is in thermal contact with at least a portion of NLO crystal 115. Thermally conductive housing 105 is configured to thermally conduct heat between heating element 110 and NLO crystal 115. Heating element 110 is configured to be heated to a temperature of at least 250° C., inclusive, as discussed in further detail below.

[0043] In accordance with at least one embodiment, thermally conductive housing 105 (also referred to herein simply as "housing") is constructed of a thermally conductive material (e.g., metal). In one embodiment, the thermally conductive material is aluminum, such as 6061 T6 aluminum. The thermally conductive material can be any material that can perform the structural requirements of holding crystal 115 without damage and can withstand or otherwise not be adversely affected by the desired operating temperatures (e.g., temperatures in the range of 250 to 500° C.). In some embodiments, housing 105 is constructed of stainless steel. In accordance with one embodiment, support structure 120 (described in more detail below) can also be constructed of the same material as thermally conductive housing 105.

[0044] In certain embodiments, thermally conductive housing 105 includes at least two components. Figure 1A and Figure 1B The example shown in has two L-shaped components that form an opening 135 for the NLO crystal 115. It should be understood that other configurations are also within the scope of the present disclosure. In some embodiments, the thermally conductive housing 105 is configured such that a gap 109 exists between the first and second components that are adjacent to each other (e.g., see Figure 3 ). The gap 109 is used to allow the NLO crystal 115 to expand when heated by the laser beam. In some embodiments, the gap 109 and the spring force configuration provided by the expandable attachment assembly 140 allow the components of the thermally conductive housing 105 to expand (and contract) in a direction that runs perpendicular to the optical axis. Figure 1A , Figure 3 and Figure 4 shown in (and also in Figure 2A ), components of thermally conductive housing 105 (e.g., length, height, and / or width (interior) dimensions) are sized smaller than corresponding dimensions of NLO crystal 115, which allows for gap 109. It should be appreciated that in some embodiments, thermally conductive housing 105 may include an adapter for holding crystal 115 for a smaller or differently shaped crystal.

[0045] The NLO crystal 115 can be a single crystal of any of a variety of nonlinear optical materials and can have various sizes and orientations. It should be understood that these selections are application specific. As will be understood, the NLO crystal 115 is configured to convert a first wavelength, such as from a laser light source, into at least one second wavelength. According to some embodiments, the NLO crystal 115 is one of a frequency doubling crystal, a frequency mixing crystal, and an optical parametric crystal.

[0046] Non-limiting examples of crystalline materials include lithium triborate (LBO), beta-barium borate (BBO), lithium cesium borate (CLBO), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium dihydrogen phosphate (KDP), and potassium titanyl phosphate (KTP). It should be understood that one or more components of the thermally conductive housing 105 can be sized to accommodate one or more sizes of crystals. The NLO oven design described herein can also accommodate or be modified to accommodate cross-sections of nearly any size of crystalline materials. Although the examples shown and described assume crystals with rectangular cross-sections, aspects of the present disclosure include embodiments in which this is not the case. The design of the thermally conductive housing 105 (and / or other components, such as the expandable attachment assembly 140 described in further detail below) can be modified for different crystal cross-sections (e.g., non-square rectangular cross-sections, cylindrical crystals, or multi-faceted crystals).

[0047] According to some embodiments, thermally conductive housing 105 is configured such that it is in thermal contact with at least a portion of one longitudinal face 116 of NLO crystal 115, such as Figure 1A. As will be appreciated, the end faces 118 of the NLO crystal 115 are not covered or otherwise encapsulated by the thermally conductive enclosure 105 because these portions of the crystal are optically engaged with the laser radiation used in the nonlinear optical process. In addition, according to at least one embodiment, the NLO oven 100 is not configured to be sealed or otherwise encapsulated or thermally sealed around the NLO crystal for heating purposes, as is the case with conventional crystal ovens. In some instances, there may be an overall housing that includes several components (including the oven), such as optical fibers and optical components and structural supports, but it should be understood that even in such an environment, the oven 100 is open to "free space" (or open air) including the incident light beam. According to at least one aspect, the oven 100 is configured to minimize heat losses around the crystal, and a conventional enclosure for thermal packaging is not necessary at all. The oven described herein is configured to operate without a thermal enclosure, i.e., the oven does not require thermal insulation around it or in its immediate vicinity to maintain the temperature of the crystal at or above 250°C. This property reduces costs and allows more design freedom when implementing ovens in frequency conversion applications. However, in some instances, the thermal enclosure may still be used for various purposes, for example, to reduce power consumption and / or to stay within the power limits of the heating element.

[0048] According to one embodiment, the thermally conductive housing 105 is configured with a recess 107 for the heating element 110, such as Figure 1A and Figure 1B In this example, the recess 107 is outlined by a dotted line because it is respectively Figure 1A and Figure 1B As shown, the heating element 110 can be disposed on a component of the thermally conductive housing 105 (e.g., Figure 1A and Figure 1B 105). In some embodiments, the heating element 110 can be held in place by one or more components of the expandable attachment assembly 140. The recess 107 is sized to accommodate one or more dimensions (e.g., length and width) of the heating element. The depth dimension of the recess 107 can be configured to accommodate the heating element 110 and prevent short circuits between electrical leads (not shown) and one or more surfaces of the thermally conductive housing 105. The recess 107 is configured to allow the heating element 110 to be "nested" within the recess 107.

[0049] In accordance with at least one embodiment, the heating element 110 contains or otherwise includes a ceramic material. One non-limiting example of a suitable ceramic material is silicon nitride (SiN), and it should be understood that other ceramic materials are also within the scope of the present disclosure. The heating element 110 may also include one or more electrical leads that connect the heating element to a power source and / or a controller (e.g., controller 170, described in further detail below). In accordance with one embodiment, the heating element 110 is a ceramic heating element that includes an internal electrical conductor encased in a thermally conductive ceramic material.

[0050] According to at least one embodiment, the heating element 110 is configured to heat to a temperature of at least 250°C (inclusive). In some embodiments, the heating element 110 is configured to heat to a temperature of at least 400°C (inclusive), in other embodiments, the heating element 110 is configured to heat to a temperature of at least 450°C (inclusive), and in yet other embodiments, the heating element 110 is heated to a temperature of 500°C (inclusive). According to certain embodiments, the heating element 110 is configured to heat to a temperature in the range of 250°C to 500°C (inclusive). Although higher temperatures, such as a temperature of at least 250°C, are described herein, it should be understood that the heating element 110 may also be heated to lower temperatures and has the ability to heat the NLO crystal 115 to any temperature between room temperature (20°C) and 500°C (inclusive).

[0051] In addition to being capable of heating to temperatures above 220°C, the heater is also sufficiently sized to heat the crystal 115 via thermal conduction through one or more components of the thermally conductive housing 105. The heat generated by the heating element 110 is transferred to the crystal 115 through one or more components of the thermally conductive housing 105 to the opening 135 where the crystal 115 resides. In some embodiments, the heating element 110 has a length dimension or overall length L in the longitudinal direction that is less than the length of the longitudinal length of the crystal 115 (i.e., the length of the longitudinal face 116). In some instances, the length L of the heating element 110 can be equal to or greater than the length of the crystal 115. In some embodiments, the heating element is sized to have a length dimension (L) that is less than 10 mm, and in one embodiment, the heating element has a length of about 7 mm.

[0052] In accordance with at least one embodiment, the NLO crystal is configured for non-critical phase matching at temperatures in the range of 250° C. to 500° C., inclusive. As mentioned above, phase matching is an additional benefit of operating at these higher temperatures.

[0053] According to some embodiments, the oven may include two heating elements. Figure 2A and Figure 2Bis a simplified schematic representation of one non-limiting example of an NLO oven (generally designated 200) showing an end view of the optical axis and a side view of the longitudinal axis, respectively, of an NLO crystal 215 heated by two heating elements 210, one located adjacent each end of the crystal 215. As will be appreciated by those skilled in the art, this configuration allows for a temperature gradient to be established along the longitudinal axis of the crystal, i.e., a thermal gradient oven configuration. It should be appreciated that oven 200 also includes other features described herein, including a thermally conductive housing 205, an expandable attachment assembly, and a support structure.

[0054] According to some embodiments, and as Figure 3 and Figure 4 As shown in an end view (optical axis) of FIG. 1 , the NLO oven 100 further includes a support structure 120. The support structure 120 is configured to support the thermally conductive housing 105. In some embodiments, the support structure 120 includes at least one plate or platform and is configured to provide a structural base for other components of the oven and to hold the oven in a fixed position. According to some embodiments, the support structure 120 is thermally isolated from the thermally conductive housing 105. The thermal isolation assists in keeping the heat generated by the heating element 110 in a more localized area of ​​the thermally conductive housing 105 around the crystal 115, thereby making it easier for the crystal 115 to maintain a higher temperature. This allows the thermally conductive housing 105 to have a relatively small thermal gradient for temperature control purposes. The term thermal isolation (or substantially isolation) does not mean that there is no thermal conduction or complete insulation at all, but rather indicates that any thermal conduction that does occur is relatively inefficient (compared to thermally conductive materials) and may not substantially reduce the temperature of the thermally conductive housing 105 by redirecting the heat generated (from the heating element 110) to the support structure 120.

[0055] According to some embodiments, and as Figure 3 and Figure 4 As shown in FIG. 1 , there is an air space (or gas space) or gap 130 or spacing extending at least partially between the thermally conductive housing 105 and the support structure 120, providing thermal separation between the two structures. The air gap 130 can extend between at least a portion of the bottom surface of the thermally conductive housing 105 and the top surface of the support structure 120. According to some embodiments, a lower or bottom portion (e.g., a leg or other extension) of the thermally conductive housing 105 can extend downwardly and attach to the support structure 120, which interrupts the air gap 130 (e.g., see below with respect to FIG. 1 ). Figure 5 for discussion).

[0056] According to another embodiment, and as Figure 4 As shown in , thermal isolation or thermal separation between the thermally conductive housing 105 and the support structure 120 is provided by at least one thermal insulation device 125. Figure 4As shown in FIG. 1 , at least one thermal insulation device 125 is positioned between the support structure 120 and the thermally conductive housing 105. According to one embodiment, the thermal insulation device 125 is a ceramic ball, and some embodiments include two or more, for example, three ceramic balls are provided. The thermal insulation device 125 is provided from Figure 4 The end view of is hidden, as indicated by the dashed lines. In this example, the ceramic balls fit into recesses (eg, circular, conical, or V-shaped grooves) of the thermally conductive housing 105 and the support structure 120 .

[0057] According to at least one aspect, the thermal insulation device 125 (e.g., a ceramic ball) and the spring force applied by the expandable attachment assembly 140 work in conjunction with each other to fix the thermally conductive housing 105 to the support structure 120 in a manner that is kinematically on the thermal insulation device 125. This kinematic mounting or connection is a technique for aligning different components, and as used herein, is intended to refer to any of a variety of such techniques that are used to mechanically constrain the relative position of the thermally conductive housing 105 and the support structure 120 and only allow movement in certain directions. For example, a component of the thermally conductive housing 105 can expand at a different rate than the support structure 120, but the two structures can remain aligned relative to each other. According to some embodiments, a ball and groove configuration is used. As will be appreciated, the theory of kinematic design requires completely rigid bodies that are in contact only at one or more points (e.g., point contacts).

[0058] According to certain embodiments, the NLO oven further includes an expandable attachment assembly 140, an example of which is shown in FIG. Figure 3 (shows a similar Figure 1A 1 . The expandable attachment assembly 140 is configured such that the NLO crystal 115 is secured within the opening 135 of the thermally conductive housing 105 by a spring force applied by the expandable attachment assembly 140. According to one aspect, the expandable attachment assembly 140 allows the NLO crystal 115 to thermally expand along one or more axes when heated, which prevents damage to the crystal 115, such as cracking or other mechanical instabilities. The thermally conductive housing 105 is constructed of a material having a coefficient of thermal expansion (CTE) that is different from the CTE of the NLO crystal 115, and the expandable attachment assembly 140 is configured to reduce stress on the NLO crystal 115 that occurs due to the difference in thermal expansion coefficients when the heating element 110 is heated to a temperature in the range of 250° C. to 500° C. (inclusive). The expandable attachment assembly 140 can also hold the NLO crystal 115 stable so that it maintains optical alignment during the heating process.

[0059] As described in further detail below, expandable attachment assembly 140 includes one or more springs 142 that absorb tension applied by NLO crystal 115 as NLO crystal 115 expands when heated. Additionally, as explained above, the material of thermally conductive housing 105 and the material of NLO crystal 115 expand by different amounts with temperature, and the components of expandable attachment assembly 140 allow for these expansion differences without inducing mechanical stress on the crystal. According to one aspect, NLO crystal 115 is secured within opening 135 by at least one spring 142 of expandable attachment assembly 140. One or more longitudinal faces 116 of NLO crystal 115 are maintained in contact with a surface of thermally conductive housing 105 under the spring force applied by expandable attachment assembly 140.

[0060] Expandable attachment assembly 140 includes at least one spring 142 . Figure 3 A non-limiting example of a spring 142 configuration is shown in FIG, wherein the spring 142 is hidden from the end view shown in the figure. According to one embodiment, the support structure 120 includes a recess 122 for the spring 142 of the expandable attachment assembly 140. Figure 3 , recess 122 is hidden, as indicated by the dashed line. In addition to at least one spring 142, expandable attachment assembly 140 may also include one or more screws or pins (non-limiting examples are shown in FIG. 1 ) that attach or otherwise engage with one or more features (recesses, holes, threads, surfaces, etc.) of thermally conductive housing 105 and support structure 120 via friction and spring forces. Figure 3 Shown as 144), clamps, rods, locating pins, etc.

[0061] Figure 3 Two non-limiting examples of how components of expandable attachment assembly 140 can engage spring 142 and housing 105 are shown. Figure 3 The left side of FIG. 1 shows an arrangement in which at least a portion (eg, the head) of a pin or screw 144 engages with the spring 142 and the tip of the pin or screw 144 engages with the lower portion of the thermally conductive housing 105. Figure 3 , an arrangement is shown in which the head of a pin or screw 144 engages with the upper or outer surface of the thermally conductive housing 105 (and in this example resides at least partially within a recess in that surface), and the tip of the pin or screw 144 engages with the spring 142. In some embodiments, the recess 122 of the support structure 120 may also include threads with which the pin or screw may also engage, and / or may also include an O-ring, a gasket, etc. Figure 3 In both examples shown in FIG, the pin or screw 144 is hidden from the end view, as indicated by the dashed line. It should be understood that Figure 3The examples of components of the expandable attachment assembly 140 shown in are non-limiting, and other configurations are within the scope of the present disclosure, including the use of a mechanical plunger, such as a spring or ball plunger.

[0062] In accordance with at least one embodiment, placement of components of expandable attachment assembly 120 , including spring 142 , positioned within support structure 120 assists in retaining heat generated by heating element 110 in a more localized area of ​​thermally conductive housing 105 , which allows crystal 115 to achieve and maintain higher temperatures.

[0063] As part of the functionality of securing the crystal 115 in the opening 135 created by the components of the thermally conductive housing 105, the expandable attachment assembly 140 is configured to hold these components to each other. In some embodiments, the expandable attachment assembly 140 is also configured to hold the heating element 110 to the thermally conductive housing 105. At least one of the thermally conductive housing 105 and the support structure 120 includes a component or feature (e.g., a recess, notch, hole, slot, tab) that assists the expandable clamp assembly 140 in securely holding the NLO crystal 115.

[0064] Now go to Figure 5 , showing a support structure 220 with two heating elements (e.g., as in Figure 2A and Figure 2B 215 . As previously mentioned, positioning two heating elements adjacent to each end 118 of the crystal allows a temperature gradient to be established along the longitudinal axis of the crystal 215. In accordance with at least one embodiment, a heat sink 224 extends between the thermally conductive housing 205 and the support structure 220. This feature allows heat to be deposited in an area adjacent to one end of the crystal 215, which assists in establishing or otherwise forming a thermal gradient. An air or gas space 230 extends at least partially between the thermally conductive housing 205 and the support structure 220.

[0065] Figure 5 Also shown is an expandable attachment assembly 240. Figure 3As with the embodiments discussed above, the expandable attachment assembly 240 is configured such that the NLO crystal 215 is secured within the opening of the thermally conductive housing 205 by a spring force applied by the expandable attachment assembly 240, and includes one or more springs 242 that absorb tension applied by the NLO crystal as the NLO crystal expands when heated. As also mentioned above, positioning the springs 242 in the support structure 220 assists in maintaining the heat generated by the heating element 210 in a more localized area of ​​the thermally conductive housing 205. The expandable attachment assembly 240 functions in a similar manner as previously described, for example, it is configured to reduce stress on the NLO crystal 215 that occurs due to differences in CTE between the housing 205 and the crystal 215. Figure 5 In the side view shown in , spring 242 is hidden from view, as indicated by the dashed lines, and is positioned within recess 222 (recess 222 may include threads, o-rings, washers, as previously mentioned), which is also hidden in this view and indicated by the dashed lines.

[0066] The expandable attachment assembly 240 may also include one or more screws or pins (non-limiting examples are shown in FIG. 1 ) that attach or otherwise engage with one or more features (recesses, holes, threads, surfaces) of the thermally conductive housing 205 and the support structure 120 via friction and spring forces. Figure 5 Shown as 244), clamps, rods, locating pins, etc. Figure 5 244a and the tip of the pin or screw 244a engages with the lower portion of the thermally conductive housing 205. Figure 5 It is not explicitly shown, but according to other embodiments, the head of the pin or screw can engage with the upper surface or outer surface of the thermally conductive housing 205 and the tip of the pin or screw engages with the spring 242.

[0067] According to some embodiments, and as Figure 5 , the expandable attachment assembly 240 is also configured to hold the heating element 210 to the thermally conductive housing 205. In this example, the head of the pin or screw 244b engages the spring 242, and the tip of the pin or screw 244b engages the surface of the heating element 210. In some embodiments, a plate or other structure can be positioned between the tip of the pin or screw 244b and the heating element 210, wherein the spring force is applied through the structure.

[0068] According to various aspects, configuring the oven with two heaters and establishing a temperature gradient across the NLO crystal enhances the performance of the frequency conversion system. Figure 8 and Fig. 9 is shown in the graph. Figure 8is a graph showing the relationship between UV pulse energy and the temperature gradient across an NLO crystal. As indicated, the UV pulse energy increases from about 100 microjoules (μJ) at zero (no temperature gradient) to about 150 μJ at a temperature gradient of about 6°C, after which the pulse energy is reduced. The data was obtained using an ytterbium picosecond fiber laser. Fig. 9 is a graph showing the relationship between green laser power output (left y-axis), conversion efficiency (right y-axis), and temperature gradient across the NLO crystal. As shown, the laser power and conversion efficiency increase from about 265 Watts (W) and about 44% at zero (no temperature gradient) to about 280 W and about 47% at a temperature gradient of 13 to 14°C, respectively. The conversion efficiency is improved because the introduction of the temperature gradient increases the spectral acceptance of the NLO crystal in applications where a spectrally broadband laser beam is used.

[0069] According to at least one embodiment, the temperature sensor 160 (see Figure 3 , as indicated by the dashed lines and hidden from the end view) is used to make temperature measurements and can be used in combination with a controller 170 (described in more detail below) to control the heating element 110. One or more components of the thermally conductive housing 105 can be configured to accommodate the temperature sensor 160, for example by having a recess for the temperature sensor to nest therein. In some embodiments, the temperature sensor 160 is positioned adjacent to the NLO crystal 115. The heat generated by the heating element 110 is conducted to the NLO crystal 115 through one or more components of the thermally conductive housing 105 and is measured by the temperature sensor 160.

[0070] According to at least one embodiment, controller 170 is used in combination with one or more components of oven 100. Figure 6 An example is shown in a block diagram of , in which a controller 170 communicates or is otherwise coupled (e.g., via electrical leads) to a temperature sensor 160 and a heating element 110 of an oven 100. For example, the controller 170 is configured to control the amount of heat generated by the heating element 110 and to receive temperature measurements from the temperature sensor 160. A predetermined target temperature may be input to the controller 170 by a user, or in some instances, the controller 170 may determine the predetermined target temperature based on other inputs. The controller 170 may control the heating element 110 so that it continues to increase heat output until the target temperature is measured by the temperature sensor 160 and received by the controller 170. According to one embodiment, the controller 170 and the temperature sensor 160 have the ability to maintain the temperature at the temperature sensor within 0.2°C. As will be appreciated, the controller 170 includes hardware (e.g., a computer) and software that can be used to control components of the system (including the heating element 110) and to receive information from components of the system (including the temperature sensor 160). Although reference is made herein to Figure 6 Oven 100 is described, but it should be understood that reference Figure 2A , Figure 2B and Figure 5 The oven 200 described may also be used in a similar manner in combination with a controller and temperature sensors. Two temperature sensors may be implemented, one for each heater.

[0071] Now go to Figure 7 , shows a simplified block diagram of a wavelength conversion system, generally indicated at 780, which can be used to implement wavelength conversion methods according to various embodiments. A laser light source 782 is provided, which generates a laser beam 784 at a first wavelength (e.g., a fundamental wavelength) and, in some embodiments, propagates through an optical fiber, which then passes through an NLO crystal 715 as previously described, which converts the first wavelength into at least one second wavelength emitted as a beam 786. The system 780 also includes an oven 700 as described herein (e.g., oven 100 or 200) controlled by a controller 770. The controller 770 also controls one or more components of the laser light source 782 (e.g., a power supply, a pump for the laser, such as an SM or MM laser diode or a fiber laser pump).

[0072] As will be appreciated, wavelength conversion system 780 may optionally include more than one NLO crystal (in order of magnitude) for applications requiring more than one NLO crystal to achieve the desired nonlinear frequency conversion. Figure 7 denoted by 715n, with the associated oven indicated by 700n).

[0073] Nonlinear frequency mixing occurs between different modes of the laser beam 784 within the NLO crystal 715 (and optionally 700n depending on the application). According to various embodiments, the nonlinear frequency mixing is a mixing operation selected from the group consisting of harmonic generation, sum frequency generation, difference frequency generation, optical parametric generation, optical parametric amplification, and optical parametric oscillation.

[0074] The laser light source 782 may include any of a number of different laser sources, non-limiting examples of which are solid-state lasers, including fiber lasers, semiconductor lasers, and disk lasers, as well as gas lasers (e.g., excimer lasers). As will be appreciated, the laser light source 782 may include one or more optical devices, including lenses (e.g., collimation, focusing, field, condensing, etc.), reflective elements (e.g., mirrors, beam splitters), and other optical or laser elements, including pump sources, amplifiers (e.g., doped fiber amplifiers), multiplexers, couplers, circulators, filters (e.g., fiber Bragg gratings), optical isolators, power supplies, etc. Depending on the desired application, the light generated may be CW, QCW, or pulsed.

[0075] One or more embodiments of the present invention include wavelength conversion methods. Some methods include: providing a laser light source (e.g., Figure 7 782); providing an oven for NLO crystals 715, 715n (e.g., Figure 7 700, 700n); positioning an NLO crystal 715, 715n within the oven 700, 700n; heating the NLO crystal 715, 715n to a temperature of at least 250°C (inclusive), wherein the NLO crystal 715, 715n is configured to convert a first wavelength into at least one second wavelength; and directing a laser beam through the NLO crystal.

[0076] According to another embodiment, one or more of the NLO oven designs described herein can be used in combination with an airtight enclosure. The primary function of the airtight enclosure is to isolate the crystal from contaminants (e.g., degassed components). Non-limiting examples of degassed components include heating elements 110 and temperature sensors 160. For this purpose, the heating element 110 and / or the temperature sensor 160 can be enclosed within a structure, such as a cylindrical box (e.g., a tubular structure) or other enclosures that surround these components and include seals to seal them therein. The enclosure and seals are made of a material (e.g., metal) that can withstand the high temperatures (e.g., 500° C.) generated by the heating element 110. For example, in one embodiment, the seal is made of copper.

[0077] In some embodiments, the crystal 115 and the portion of the oven surrounding the crystal (eg, thermally conductive housing 105) may be enclosed in a single hermetically sealed optical housing. This serves to protect the crystal from contamination.

[0078] The aspects according to the present invention disclosed herein are not limited to the details of the construction and arrangement of the parts stated in the following description or illustrated in the accompanying drawings in their application. These aspects can assume other embodiments and can be practiced or executed in various ways. The examples of the specific embodiments provided herein are only for illustrative purposes and are not intended to be restrictive. Specifically, the actions, parts, elements and features discussed in conjunction with any one or more embodiments are not intended to be excluded from similar roles in any other embodiments.

[0079] In addition, the words and terms used herein are for descriptive purposes and should not be considered limiting. Any reference to the examples, embodiments, parts, elements or actions of the systems and methods mentioned herein in the singular may also include embodiments including the plural, and any reference to any embodiment, part, element or action herein in the plural may also include embodiments including only the singular. References in singular or plural form are not intended to limit the currently disclosed system or method, its parts, actions or elements. The use of "including", "comprising", "having", "containing", "involving" and its variations herein is intended to encompass the items listed thereafter and their equivalents and additional items. References to "or" may be interpreted as inclusive, so that any term described using "or" may indicate any of a single, more than one and all of the terms described. In addition, in the case of inconsistency in the use of terms between this document and the documents incorporated herein by reference, the use of terms in the incorporated reference is a supplement to the use of terms in this document; for irreconcilable inconsistencies, the use of terms in this document shall prevail. Furthermore, headings or subheadings may be used in the specification for the convenience of the reader and will not affect the scope of the present invention.

[0080] Thus, having described several aspects of at least one example, it will be appreciated that various changes, modifications, and improvements will readily occur to those skilled in the art. For example, the examples disclosed herein may also be used in other contexts. Such changes, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the examples discussed herein. Therefore, the foregoing description and accompanying drawings are by way of example only.

Claims

1. An oven for nonlinear optical (NLO) crystals, comprising: a thermally conductive housing configured to define an opening for holding the NLO crystal and configured to thermally conduct heat between a heating element and the NLO crystal, the thermally conductive housing being in thermal contact with at least a portion of the NLO crystal, and the heating element being configured to heat to a temperature greater than or equal to 250° C.; a support structure configured to support the thermally conductive housing, the support structure being thermally isolated from the thermally conductive housing; and An expandable attachment assembly is configured such that the NLO crystal is secured within the opening of the thermally conductive housing by a spring force applied by the expandable attachment assembly.

2. The oven according to claim 1, wherein: An air space extends at least partially between the thermally conductive housing and the support structure.

3. The oven according to claim 2, wherein: The support structure includes a recess for a spring of the expandable attachment assembly.

4. The oven of claim 2, further comprising at least one thermal insulation device positioned between the support structure and the thermally conductive housing.

5. The oven according to claim 2, wherein: The thermally conductive housing is configured to thermally conduct heat between the NLO crystal and two heating elements, and the oven further includes a heat sink extending between the thermally conductive housing and the support structure.

6. The oven according to claim 1, wherein: The heating element is configured to heat to a temperature greater than or equal to 400°C.

7. The oven according to claim 1, wherein: The heating element is configured to heat to a temperature in the range of 250°C to 500°C, inclusive.

8. The oven according to claim 1, wherein: The thermally conductive housing is constructed of a material having a coefficient of thermal expansion different from that of the NLO crystal, and the expandable attachment assembly is configured to reduce stress on the NLO crystal that occurs due to the difference in the coefficients of thermal expansion when the heating element is heated to a temperature in the range of 250°C to 500°C, inclusive.

9. The oven of claim 1, further comprising a controller coupled to the heating element and configured to control an amount of heat generated by the heating element and to receive temperature measurements from a temperature sensor positioned adjacent the NLO crystal.

10. The oven according to claim 1, wherein: The thermally conductive housing is made of aluminum.

11. The oven according to claim 1, wherein: The thermally conductive housing includes at least two components, and the thermally conductive housing is configured such that a gap exists between a first component and a second component adjacent to each other.

12. The oven of claim 1 configured to operate without a thermal enclosure.

13. The oven according to claim 1, wherein: The NLO crystal is configured for non-critical phase matching at a temperature in the range of 250°C to 500°C, inclusive.

14. A method comprising: There is provided an oven according to claim 1.

15. A wavelength conversion method, comprising: providing a laser light source configured to generate a laser light beam having a first wavelength; An oven for nonlinear optical (NLO) crystals is provided, the oven comprising: a thermally conductive housing configured to define an opening for retaining the NLO crystal and configured to thermally conduct heat between a heating element and the NLO crystal; and a support structure configured to support the thermally conductive housing, the support structure being thermally isolated from the thermally conductive housing; positioning the NLO crystal within the opening of the oven; heating the nonlinear optical (NLO) crystal to a temperature greater than or equal to 250° C., the NLO crystal being configured to convert the first wavelength into at least one second wavelength; and The laser beam is directed through the NLO crystal.

16. The wavelength conversion method according to claim 15, wherein: Heating includes heating the NLO crystal to a temperature in a range of 250°C to 500°C, inclusive.

17. The wavelength conversion method of claim 15, further comprising providing the NLO crystal.

18. The wavelength conversion method according to claim 17, wherein: Nonlinear frequency mixing occurs between different modes of the laser beam having the first wavelength within the NLO crystal, and the nonlinear frequency mixing is a mixing operation selected from the group consisting of harmonic generation, sum frequency generation, difference frequency generation, optical parametric generation, optical parametric amplification, and optical parametric oscillation.

19. The wavelength conversion method according to claim 15, further comprising: The temperature of the NLO crystal is measured, and the heating element is controlled based on the temperature measurement.

20. The wavelength conversion method according to claim 15, wherein: The oven further includes an expandable attachment assembly configured such that the NLO crystal is secured within the opening of the thermally conductive housing by a spring force applied by the expandable attachment assembly.

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