Compact Calibration and Testing System For High Power Lasers and Optics

Abstract

A compact high power laser calibration and testing system includes an active intracavity laser system that amplifies the laser power by recycling photons through a thin disk gain medium that is positioned between two or more highly reflective mirrors. The system is configured for calibration and testing of the high power lasers and optics that can be inserted into or positioned at the end of the intracavity. In another embodiment, the system is configured for characterization of high power laser beam propagation in operation-relevant atmospheres. The intracavity high power laser beam is configured to simulate high power laser beams with orders-of-magnitude reduced size, weight and operation power for calibrating laser powers and testing optical components. In applications that require an extra small footing or high portability, thermal management systems are configured to absorb large amounts of heat from the system for fixed time durations with the use of exchangeable cartridges made of phase change materials. The portability of the invention can be further increased and the system footing can be decreased by powering the system with disposable or rechargeable battery cartridges that can be rapidly replaced.

Claims

1. An optical calibration and testing system comprising: (a) an intracavity laser beam in which a laser beam is recycled plural times; and (b) a thin disk laser head to generate and maintain the intracavity laser beam comprising: a thin disk gain medium that generates laser power, and a high reflectance mirror either coated or attached on the gain medium, and a heat sink comprising: a solid metal block comprising aluminum, copper, beryllium or their alloys, or a semiconductor wafer that is attached to a solid or hollowed-out metal block, or a diamond wafer that is attached to a solid or hollowed-out metal block, wherein the hollowed-out metal block filled with a phase change material comprising paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, wherein the heat sink is attached to a common base plate comprising: a solid metal bock, or a hollowed-out metal block filled with a phase change material comprising: paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, or a hollowed-out metal block filled with flow of liquid coolant comprising: water, Freon, or ammonia, or a hollowed-out metal block filled with replacement cartridges comprising; hollowed-out metal block filled with a phase change material comprising paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials; and (c) a radiation pressure meter system comprising: a high reflectance mirror positioned at an angle between 0 and 90 degrees to the direction of the intracavity laser beam, wherein the reflection of the intracavity laser beam creates a repulsive force on the mirror, wherein the high reflectance mirror is attached to a heat sink comprising: a solid metal bock, or a hollowed-out metal block filled with a phase change material comprising: paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, or a hollowed-out metal block filled with flow of liquid coolant comprising water, Freon, or ammonia, or a hollowed-out metal block filled with replacement cartridges comprising; hollowed-out metal block filled with a phase change material comprising paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, wherein the heat sink is attached to a radiation pressure meter that reads the repulsive force from the reflection of the intracavity laser beam on the mirror, which is transmitted through the heat sink to the meter, wherein the radiation pressure meter is attached to the base plate; and (d) an outcoupler high reflectance mirror that terminates the intracavity laser beam and is mounted on a heat sink comprising: a solid metal bock, or a hollowed-out metal block filled with a phase change material comprising: paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, or a hollowed-out metal block filled with flow liquid coolant comprising water, Freon, or ammonia, or a hollowed-out metal block filled with replacement cartridges comprising; hollowed metal block filled with a phase change material comprising paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, wherein the heat sink is attached the base plate; and (e) a tested or calibrated optical element that is positioned in the intracavity beam, and mounted on a heat sink comprising: a solid metal bock, or a hollowed-out metal block filled with a phase change material comprising: paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, or a hollowed-out metal block filled with flow liquid coolant comprising water, Freon, or ammonia, or a hollowed-out metal block filled with replacement cartridges comprising; hollowed metal block filled with a phase change material comprising paraffin, fatty acids, salt hydrates, eutectics or any combination of these materials, wherein the heat sink is attached the base plate.

2. The optical calibration and testing system of claim 1, wherein the intracavity laser power is from 1 milliwatts (mW) to 1,000 gigawatts (GW).

3. The optical calibration and testing system of claim 1, wherein the intracavity laser wavelength is from 100 nm to 100 m.

4. The optical calibration and testing system of claim 1, wherein the intracavity laser force on the pressure meter is greater than or equal to 1 nano-Newtons (nN).

5. The optical calibration and testing system of claim 1, wherein the cross sectional diameter of the intracavity laser beam is greater than or equal to 0.1 mm.

6. The optical calibration and testing system of claim 1, wherein the testing optical element is one of any mirrors comprising the outcoupler mirror, the high reflectance mirror attached on the radiation pressure meter or the thin disk laser head.

7. The optical testing and calibration system in claim 1, wherein the tested element is any of heat sinks in claim 1.

8. The optical testing and calibration system in claim 1, wherein its powering system comprising: a conventional wall-plug power, or a portable power generator, or disposable or rechargeable batteries, or supercapacitor cartridges, or any combinations of the above elements.

9. A process for testing and calibrating optical components, the process comprising: subjecting an optical element to an intracavity laser beam; comparing the characteristics of the tested and calibrated optical elements with reference optical elements.

10. A process of measuring damage threshold of optical components by observing a decrease of the radiation force while increasing the intracavity laser beam power.

11. A process of characterizing laser beam propagation in atmosphere, the process comprising: exposing the system in claim 1 directly to the operational atmosphere; or passing the intracavity through a jet or flow of a sample of the operational atmosphere; or passing the intracavity through a tube that contains a sample of the operational atmosphere, wherein the tube is open, fully or partially closed with one or more optical windows with anti-reflectance (AR) coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Elements in the figures have not necessarily been drawn to scale in order to enhance their clarity and improve understanding of these various elements and embodiments of the invention. Furthermore, elements that are known to be common and well understood to those in the industry are not depicted in order to provide a clear view of the various embodiments of the invention.

[0029] FIG. 1 illustrates schematically an overall cross sectional view of the present invention.

[0030] FIG. 2 illustrates schematically detailed cross sectional views of three examples of the base plate shown in FIG. 1.

[0031] FIG. 3 illustrates a detailed cross sectional view of the thin disk laser components in FIG. 1.

[0032] FIG. 4 illustrates schematically detailed front and side planar views of the holders of optical components in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033] In the following discussion that addresses a number of embodiments and applications of the present invention, reference is made to the accompanying drawings that form a part hereof, in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and changes may be made without departing from the scope of the present invention.

[0034] Recently, the use of solid state lasers with wavelengths of near infrared, on the order of one micrometer, has become popular because of their high energy efficiencies and low mechanical complexities. As the usage of such high power lasers expands, there is increasing need of calibrating the laser power, characterizing the beam propagation and testing the associated optics. The laser powers now routinely exceed 10 kilowatts and often 100 kilowatts. In the near future, the required laser powers are projected to exceed one megawatts. Generation of such laser powers requires high power consumption, complex and large facilities and safety controls, thus is highly costly. In an active intracavity that is employed in the present invention, the optical cavity is formed between two highly reflective mirrors and a gain medium positioned between the two mirrors. The intracavity laser power, P.sub.int, which produced by recycling photons between the mirrors, is mathematically described in a monograph entitled, Lasers, University science Books, Sausalito, Calif., 1986 by Siegman and is given by:

[00001]$\begin{array}{cc}{P}_{\mathrm{int}}=\frac{P_{\mathrm{ext}}}{T^{}}& \left(1\right)\end{array}$

where P.sub.ext is the maximum extractable laser power through an outcoupler mirror, when the outcoupler mirror reflectivity is optimized to maximize P.sub.ext. Typically, P.sub.ext is very close to the maximum deliverable power of lasers from the optical cavity. The P.sub.ext can be estimated by

[00002]$\begin{array}{cc}{P}_{\mathrm{ext}}=\frac{G.Math..Math.I_{\mathrm{sat}}.Math.A}{T^{}}& \left(2\right)\end{array}$

where G is the unsaturated round-trip gain factor, I.sub.sat is the saturation intensity of the gain medium, A is the effective lasing area in the gain medium, and T is given by:

T=T+a+s(3)

where a is the roundtrip absorption coefficient and s is the roundtrip scattering coefficient through the gain medium, optical elements and the surfaces of mirrors. By reducing T the intracavity laser power P.sub.int can be increased by orders of magnitude. For example, if T=0.001, P.sub.int is 1,000 times larger than P.sub.ext. With the state-of-art high power laser mirrors, T, which is smaller than 0.001 can be achieved. Examples of the maximum theoretical intracavity laser power as a function of the cross sectional area correlating with various laser powers with Yb:YAG crystals of I.sub.sat24 kW/cm.sup.2, G1, T0.001, are summarized in Table 1. In principle, a 1-MW intracavity laser power can be achieved by 1 kW of input power on a 0.1 cm.sup.2 Yb:YAG gain medium crystal with a very small foot print. A factor of 1,000 or more reduction in power requirement and foot print permits a much smaller, safer and lighter system than the actual high power laser system for calibration and characterization. Furthermore, the high power laser is confined within the cavity, thus the leaked or scattered laser power on optical components can be greatly reduced resulting in increased operation safety.

TABLE-US-00001 TABLE 1 The maximum theoretical intracavity laser power based on Yb:YAG with I.sub.sat ~24 kW/cm.sup.2, G~1, and T~0.001. The actual achievable intracavity laser power also depends on other parameters, such as thermal management capability. The large cross sectional area of gain media can be achievable either with a single crystal or by multiplexing numbers of smaller gain media. Power Minimum Cross Required Sectional Area of Maximum Maintaining Gain Medium Intracavity the Operation (Yb:YAG) Power 100 W 0.01 cm.sup.2 100 kW 1 kW 0.1 cm.sup.2 1 MW 10 kW 10 cm.sup.2 10 MW

[0035] FIG. 1 illustrates schematically a fundamental aspect of the present invention in a cross sectional view. The intracavity laser beam, 101, produced by recycling photons generated in a thin disk gain medium, 102, between highly reflective mirrors, 103, which is mounted on a heat sink, 104. The highly reflective mirrors are dielectric mirrors. The dielectric mirror, 103, is constructed on the gain medium, 102, by coating thin dielectric films on the gain medium, 102. The gain medium, 102 and the dielectric mirror, 103, are attached to a heat sink, 104, which comprises a metal block, a semiconductor wafer, or a diamond wafer. The metal block heat sink, 104, consists of a solid block of metal, such as aluminum, beryllium or copper, or a hollow metal block, which is hollowed out and filled with a phase-change material. The semiconductor or diamond heat sink can be attached to a metal block that is hollowed out and filled with a phase change material. The phase change material comprises a material made of a single component or a combination of materials that have high heat capacities, such as paraffin, fatty acids, salt hydrates and eutectics. The heat sink block, 104, is fixed on a base plate. The intracavity laser beam, 101, is reflected on a high reflectivity mirror, 106, at an angle between 0 and 90 degrees to be directed on the outcoupler dielectric mirror, 105. The reflection of the intracavity laser beam, 101, creates a repulsive force on the mirror, 106. The repulsive force is transmitted through a heat sink, 110 and a pedestal, 111, to and measured by a radiation pressure meter, 112. The heat sink, 110, consists of a solid block of metal, such as aluminum or copper, or a hollow metal block, which is filled with a phase change material. The phase change material comprises a material of a single component or a combination of materials that have high heat capacities, such as paraffin, fatty acids, salt hydrates and eutectics. All these elements are mounted on the base plate that is detailed in the later section.

[0036] By measuring the power, W, of the laser beam, 109, transmitted through the outcoupler mirror, 105, on a laser power meter, 113, and with a known transmittance, T, of the outcoupler mirror, 105, the intracavity laser power P.sub.int is accurately determined with:

[00003]$\begin{array}{cc}{P}_{\mathrm{int}}=\frac{W}{T}.& \left(4\right)\end{array}$

If the transmittance, T, of the outcoupler, 105, is 0.0005, for example, and P.sub.int is 500 kW, the registered laser power on the radiation power meter, 113 should be 250 W. Once P.sub.int is determined, calibration of the radiation pressure meter, 112, can performed. For example, if the angle between the front surface of the dielectric mirror, 106, and the intracavity beam, 101, is positioned at 45 degree and if the intracavity laser beam power is 500 kW, the force registered on the radiation pressure meter, 112, should be 4.71 mN or 0.481 gram in weight. Conversely, if the calibration of the radiation pressure meter, 112, is well established, it can be used to calibrate and test the transmittance of the outcoupler mirror, 105, accurately. Similar calibration can be performed on an optical component, 107 or 108, which can be positioned within the intracavity. The component includes a mirror, a lens, and a beam splitter. The insertion of the optical component, 107 or 108, changes the intracavity laser power registered on the radiation pressure meter, 112, and the power of the transmitted laser beam, 109, registered on a laser power meter, 113. With the use of these changes, the transmittance and reflectance of the optical component, 107 or 108, can be calibrated with the use of Eqn. 2 and Eqn. 3 to high precision.

[0037] The present invention can be also used for characterizing atmospheric propagation of the high power laser beam by exposing the system directly to an operation atmosphere or to a sample of the operation atmosphere in a form of flow or jet into the intracavity laser beam, 101, or in a tube, 114, with or without end windows that have anti-reflectance coating. By measuring the change in intracavity laser power between without and with the operational atmosphere or its sample, the absorption and scattering through the operation atmosphere are measured. The importance and the art of such measurements can be found in a presentation entitled, Absorption and Scattering of an HEL Beam by Atmospheric Aerosols, HPLA and DE Symposium, Santa Fe, N. Mex., 2016 by Fischer et al. The present art, however, fail to disclose the use of intracavity amplification systems as disclosed in the present invention.

[0038] The present invention can be used for testing the damage threshold an optical component, 105, 106, 107, or 108. The tested optical components include a mirror, a lens and a beam splitter. For example, in a damage threshold test, the intracavity laser power, P.sub.int, is ramped up slowly from a low power until the damage on the optical component, 105, 106, 107, or 108, occurs. Any serious damage on the components would immediately lower the intracavity power by orders of magnitude. In particular, a highly reflective mirror optimized to a laser reflection angles between 0 and 90 degrees can be positioned for calibration and testing at the position of 106. The maximum value of this test can be used as a damage threshold. For example, with T of 0.001 and the cross sectional area of the intracavity laser beam, 101, of 0.5 cm.sup.2, the damage threshold of 1 megawatts per square centimeter can be measured with P.sub.int=0.5 megawatts. The extractable laser power, P.sub.ext, is 500 watts. Therefore, the testing laser system operates as a 500 W laser system rather than a 500 kilowatts system.

[0039] FIG. 2 illustrates schematically of cross sectional views of representative examples of the base plate that is used for mounting the laser and optical components in FIG. 1. The base plate, 201, can be made of a solid block of metals that include aluminum, copper or alloys. In an embodiment, the base plate, 201, can be hollowed out to fill phase change materials, 202, that include paraffin, fatty acids, salt hydrates and eutectics. In this embodiment, the base plate can be cooled by air flows that can be provided by various means that includes a fan during or after the laser operation. In another embodiment, the base plate, 203, can be hollowed out to accept removable or exchangeable cartridges, 204, that contain phase change materials, 205, that include paraffin, fatty acids, salt hydrates and eutectics. In this embodiment, once heated up, the cartridges, 204, can be rapidly exchanged with pre-cooled cartridges. Yet in another embodiment, the base plate, 206, can be hollowed out to fill phase change materials that include paraffin, fatty acids, salt hydrates and eutectics, and to have flow of liquid coolants, 208, that include water, Freon and ammonia. In this embodiment, once heated up the whole components of the base plate can be rapidly cooled by the flow of coolants.

[0040] FIG. 3 illustrates schematically the detailed cross sectional view of the thin disk gain medium area that includes components, 102 through 104 of FIG. 1. The thin disk gain medium, 301, the highly reflective mirror or coating, 302, and the heat sink, 303, are attached to a thermally conducting metal block support, 304, that is in turn mounted on the base plate. The metal block support, 304, can be made of a solid block of metals that include aluminum, copper or alloys. In an embodiment, the support, 304, can be hollowed out to fill phase change materials, 310, that include paraffin, fatty acids, salt hydrates and eutectics. In another embodiment, the support, 304, can contain flows or jets of coolants, 310, that include water, Freon and ammonia in separate hollows that are not explicitly shown in FIG. 3. Yet in another embodiment, the support, 304, can contain both phase change materials that include paraffin, fatty acids, salt hydrates and eutectics, and flows or jets of coolants, 310, that include water, Freon and ammonia in separate hollows that are not explicitly shown in FIG. 3 for simplicity. The thin disk is optically pumped by another laser beam, 305, which bounces multiple times between the parabolic mirror, 306, and the end mirror, 307. These optical components. 306, 307, are mounted on a thermally conducting metal support, 308, that is in turn mounted on the base plate. In an embodiment, the support, 308, can be hollowed out to fill phase change materials, 309, that include paraffin, fatty acids, salt hydrates and eutectics. In another embodiment, the support, 308, can contain flows or jets of coolants, 309, that include water, Freon and ammonia. Yet in another embodiment, the support, 308, can contain both phase change materials that include paraffin, fatty acids, salt hydrates and eutectics, and flows or jets of coolants, 309, that include water, Freon and ammonia in separate hollows that are not explicitly shown in FIG. 3 for simplicity.

[0041] FIG. 4 illustrates schematically the front and side planar views of optics mounts, 105, 107 and 108, in FIG. 1, that are in turn mounted on the base plate. The metal block mount, 402, holds and transmits the heat generated on an optical component, 401, that includes, a mirror, a lens, and a beam splitter. The metal block mount, 402, can be made of a solid block of metals that include aluminum, copper or alloys. In an embodiment, the mount, 402, can be hollowed out to fill phase change materials, 403, that include paraffin, fatty acids, salt hydrates and eutectics. In another embodiment, the mount, 402, can contain flows or jets of coolants, 403, that include water, Freon and ammonia. Yet in another embodiment, the mount, 402, can contain both phase change materials that include paraffin, fatty acids, salt hydrates and eutectics, and flows or jets of coolants, 403, that include water, Freon and ammonia in separate hollows that are not explicitly shown in FIG. 4 for simplicity.