A system for measuring the thermal properties of materials using frequency domain thermoreflectance.

Abstract

The present invention relates to a frequency-domain thermal reflectance (FDTR) system 10 for measuring the thermal properties of materials having thermal conductivity and / or optical roughness. The system 10 comprises a pump device 12a configured to emit a continuous-wave laser beam (the laser beam is referred to as the pump beam 41), a signal generator 17 configured to produce a modulation signal 49 for causing the intensity of the pump beam 41 emitted from the pump device 12a to vibrate in time, and an optical system 20 configured to receive and shape the pump beam 41 and guide the pump beam 41 toward a sample of material. This causes the pump beam 41 to interact with the sample 46 during use. The system 10 further comprises a measuring device configured to monitor the thermal response of the sample 46 resulting from the interaction with the pump beam 41. When the pump beam 41 is incident on the sample, it has a spot diameter and temporal intensity vibrations sufficient to induce measurable temperature vibrations that penetrate to a required depth within the sample 46.

Description

Technical Field

[0001] The present invention relates to a frequency-domain thermosreflectance (FDTR) system and a method for measuring the thermal properties of thermally conductive materials, optically rough materials, and further materials that are laterally inhomogeneous and / or have a curved surface.

Background Art

[0002] In many industrial fields, it is essential to design and implement an effective heat transfer system to prevent overheating and damage to equipment. For example, in the electronic device industry, with the miniaturization of components and the increase in power consumption, a large amount of heat accumulates inside the device, which may lead to performance degradation and shortened lifespan due to overheating. In the aerospace industry, accurately understanding heat transfer is crucial for the success of launching and re-entering a spacecraft. In particular, it is essential to safely protect the spacecraft and its crew from extreme temperature environments. Also, in an aircraft engine, it is necessary to appropriately manage heat so that the engine operates at an optimal temperature to prevent overheating and component damage. Furthermore, in the nuclear industry, since a power generation plant generates a large amount of heat, it is essential to efficiently transfer heat to prevent overheating of fuel rods and maintain safe and stable operation. In addition, heat management is also important to sufficiently generate the steam required for power generation and drive the turbine to maximize power generation efficiency. <8000011> Therefore, understanding the thermal properties, particularly the thermal conductivity, of various materials leads to selecting the most suitable materials for transferring heat generated in various applications, and can prevent excessive heat accumulation and heating.

[0004] A common method for measuring the thermal properties of materials is the transient-thermoreflectance method (TTR), which has since evolved into the time-domain thermoreflectance method (TDTR) and the frequency-domain thermoreflectance method (FDTR). These methods are based on the principle that a material's reflectivity depends on its temperature. Typically, a powerful pulsed or modulated laser beam is used to induce a periodic temperature change on the optically smooth surface of the material being measured. This periodic temperature change can be monitored by measuring the change in reflectivity of another laser beam reflected from the material's surface. Thermal properties can be calculated from the phase, amplitude, or time evolution of the measured periodic surface temperature change.

[0005] This invention further improves upon FDTR and provides a method for investigating the thermal properties of a material. [Overview of the project]

[0006] According to a first aspect of the present invention, a frequency-domain thermoreflectance (FDTR) system is provided for measuring the thermal properties of a material having thermal conductivity and / or optical roughness, the system comprising: a pump device having adjustable output power and configured to emit a continuous-wave laser beam, the laser beam being referred to as the pump beam (also called pump light); a signal generator configured to generate a modulation signal for temporally modulating the intensity of the pump beam emitted from the pump device; an optical system configured to receive and shape the pump beam and guide the pump beam toward a sample such that the pump beam interacts with the sample of the material when in use; and a measuring device configured to monitor the thermal response of the sample resulting from the interaction with the pump beam, wherein the pump beam has a spot diameter and temporal intensity modulation sufficient to induce measurable temperature oscillations within the sample to a predetermined depth when irradiated onto the sample.

[0007] In one embodiment, the spot diameter of the pump beam when irradiated onto the sample may be at least 50 μm, at least 75 μm, further at least 100 μm (0.1 mm), at least 400 μm, or at least 500 μm.

[0008] In another embodiment, the spot diameter of the pump beam may be 100 μm to 1000 μm (1 mm) at the time of irradiation, or 100 μm (0.1 mm) to 5000 μm (5 mm), or 400 μm to 700 μm, or 500 μm to 1000 μm, or greater than 500 μm, or greater than 1000 μm (1 mm).

[0009] The pump device may be electrically connected to a driver. The driver may be electrically connected to a signal generator.

[0010] In one embodiment, the pump device may include a high-power solid-state laser. In another embodiment, the pump device may include a high-power laser diode.

[0011] During use, the signal generator may be configured to supply a modulated signal to a driver, which modulates the current supplied to the pump device, thereby modulating the intensity of the radiated pump beam and inducing temperature fluctuations within the sample.

[0012] In one embodiment, the frequency of the modulated signal (modulation frequency) may be variable within a predetermined range. For example, the predetermined frequency range may be up to 50 kHz, arbitrarily up to 100 kHz, arbitrarily from 10 Hz to 10 kHz, or arbitrarily from 10 Hz to 30 kHz. The frequency may be variable from high to low, or arbitrarily from low to high, or arbitrarily in any desired order and direction within the predetermined range.

[0013] The measuring device comprises a probe device configured to emit a further continuous-wave laser beam, the further laser beam being referred to as a probe beam, wherein the probe beam is configured to reflect from a sample, the surface reflection characteristics of the sample being dependent on the sample's temperature, and thereby, temperature fluctuations induced within the sample by the pump beam can cause the intensity of the reflected probe beam to fluctuate over time.

[0014] Therefore, the thermal response of the sample can be monitored by detecting the intensity of the reflected probe beam and measuring the phase lag between the intensity vibrations of the probe beam and the intensity vibrations of the pump beam.

[0015] By changing the modulation frequency within a predetermined range, the pump beams are generated with different temporal intensity modulations, which can result in the induction of different temperature fluctuations within the sample.

[0016] The measuring device can be configured to monitor the thermal response at multiple modulation frequencies within a predetermined range by detecting the intensity of the probe beam reflected from the sample and measuring the phase delay of that intensity relative to the pump beam intensity.

[0017] By analyzing multiple thermal responses within a predetermined modulation frequency range, the thermal properties of the sample can be derived.

[0018] The probe device (apparatus) may be configured to emit a probe beam with a smaller spot diameter than the pump beam when irradiated onto the sample. Alternatively, the probe device may be configured to emit a probe beam having the same or similar spot diameter as the pump beam when irradiated onto the sample.

[0019] In one embodiment, the spot diameter of the probe beam may be up to 10 μm when irradiated onto the sample.

[0020] In other embodiments, the probe beam may be configured such that the spot diameter when incident on the sample is at least 50 μm, optionally at least 75 μm, optionally at least 100 μm (0.1 mm), optionally at least 400 μm, or optionally at least 500 μm.

[0021] In a further embodiment, when incident on a sample, the probe beam may have a spot diameter ranging from 100 μm to 1000 μm (1 mm), and may have a spot diameter of any choice from 100 μm (0.1 mm) to 5000 μm (5 mm), any choice from 400 μm to 700 μm, any choice from 500 μm to 1000 μm, any choice greater than 500 μm, or any choice greater than 1000 μm (1 mm).

[0022] Embodiments of the present invention can generate a pump beam with a greater thermal penetration depth, capable of reaching depths of several tens of micrometers or more, or even millimeters, from the sample surface. The thermal penetration depth is generally proportional to the pump spot diameter; therefore, a larger pump spot diameter results in a greater thermal penetration depth. Furthermore, the thermal penetration depth is inversely proportional to the modulation frequency. That is, a pump beam with temporal intensity modulation at a low modulation frequency (e.g., 50 kHz or less) is suitable for measuring the thermal properties of regions / layers / interfaces of several tens to several hundred micrometers, or millimeters, below the sample surface. On the other hand, at high modulation frequencies, temperature oscillations rapidly attenuate in the depth direction, so temperature oscillations can only be observed near the sample surface, and other regions remain at a nearly constant temperature, making it impossible to measure thermal properties using temperature oscillations. Furthermore, for materials with high thermal conductivity, a higher-power pump beam is required to maintain temperature oscillations that can be measured at the desired depth.

[0023] Furthermore, embodiments of the present invention make it possible to measure samples having optical roughness. Roughness is typically quantified by the root mean square (RMS) value, which represents the average height deviation of feature parts from the mean surface level of the sample. A low RMS value indicates a smoother surface, while a high RMS value indicates a rougher surface. A surface having optical roughness is defined as having an RMS value that is equal to or greater than the wavelength of the probe beam, thereby causing the probe beam to scatter or diffuse more upon reflection. A surface having optical smoothness is defined as having an RMS value that is significantly smaller than the wavelength of the probe beam (e.g., an RMS value greater than one-eighth or one-quarter of the wavelength), thereby causing the probe beam to reflect more specularly.

[0024] The inventors have found that conventional systems and methods for performing frequency-domain thermoreflectance (FDTR) are not suitable for measuring the thermal properties of optically rough (uneven) surfaces. The spot diameter of commonly used existing probe beams is approximately 10-12 μm, and when reflected by an optically rough surface, scattering occurs, resulting in an extremely small amount of reflected light returning to the detector. Therefore, conventional systems have the problem of being applicable only to optically smooth samples.

[0025] In embodiments of the present invention, by providing a sufficiently large probe beam diameter, the amount of reflected light increases, and more light is received by the detector, making it possible to measure the thermal response even for optically rough surfaces. By increasing the probe beam size, the influence of surface roughness is averaged out, and sensitivity to surface shape can be reduced. Furthermore, a larger spot diameter also provides an averaging effect on lateral heterogeneity (compositional unevenness) within the sample, resulting in more consistent measurement results. This enables measurements that more accurately reflect the thermal response of the sample.

[0026] This system is designed to measure the average thermal response of materials having rough / irregular and / or laterally non-uniform surfaces, particularly surfaces with optical roughness, and to determine their average thermal properties. However, it is also possible to analyze many other types of materials. This includes surfaces with optical smoothness or optical roughness, and / or bulk (i.e., homogeneous) materials with lateral non-uniformities, thin films (e.g., the thickness of the transducer layer applied to some samples), multi-layer materials, and thick materials (e.g., materials larger than tens of μm). Also, curved materials such as those having cylindrical, hemispherical, or spherical surfaces can be analyzed. The curved materials may have a radius of curvature of at least 0.5 mm, optionally at least 1 mm. In particular, this system can measure the thermal response and properties of particles, such as nuclear fuel particles like triple-structured isotropic particle fuel. The system can be configured to analyze structurally complete nuclear fuel particles. That is, the particles may be unchanged prior to analysis. The nuclear fuel particles may have a diameter of about 1000 μm (1 mm).

[0027] Samples of some materials may include a metallic transducer layer extending over at least a portion of their surface. An adhesive layer may be provided between the transducer and the sample, and the adhesive layer may be composed of a different metal. This adhesive layer improves the adhesion of the transducer and enhances the heat transfer to the sample. The transducer may be configured to absorb the heat given by the pump beam and transfer that heat to the sample.

[0028] Thereby, the pump beam can induce temperature vibrations inside the sample through the thin film. Furthermore, the transducer may be configured to have a high thermo-optical coefficient with respect to the wavelength of the probe beam. This makes the reflectivity temperature-sensitive and enables highly sensitive measurement of temperature changes.

[0029] The amplitude of the thermal response of the sample (the detected intensity vibration of the reflected probe beam) depends on the power density of the pump beam defined by the power / spot area (at the point where the pump beam is incident on the sample), and changes in proportion to the square of the spot diameter. In order to measure thermal properties at a greater depth of heat penetration, a larger pump spot diameter is required, and thus a higher-power pump beam is required to maintain the power density that provides measurable temperature vibrations within the sample. The pump spot diameter when incident on the sample can be determined by the configuration of the optical system.

[0030] The pump device can have an output power that is adjustable from low power to high power. The output power of the pump device can be adjustable from about several hundred milliwatts to several tens of watts. The pump device can be configured to operate at a high enough power to induce measurable temperature vibrations within the sample at a larger pump spot diameter.

[0031] The system can include a pump beam homogenizer optically coupled to the pump device. The pump beam homogenizer can be configured to receive the pump beam emitted from the pump device. The pump beam can be shaped to form a smooth, homogeneous, and circular pump beam profile by being guided through the optical assembly of the pump beam homogenizer.

[0032] In one embodiment, the pump beam homogenizer a first end optically coupled to the pump device and configured to receive the pump beam emitted from the pump device, a second end optically coupled to the optical system and configured to direct the homogenized pump beam towards the optical system. The present invention comprises an optical assembly positioned between the first end and the second end, configured to interact with the pump beam radiated from the pump device and to form a homogenized pump beam having a substantially rotationally symmetric energy intensity distribution when it exits the pump beam homogenizer through the second end.

[0033] When using the present invention, the pump beam interacts with the optical assembly of the pump beam homogenizer, allowing the distribution of modes present in the pump beam to be averaged. As a result, the pump beam approaches a substantially rotationally symmetric energy intensity distribution.

[0034] In one embodiment, the optical assembly of the pump beam homogenizer may include a fiber optic cable. The fiber optic cable may include multimode fiber. During use, the fiber optic cable can homogenize and circularize the pump beam as it propagates through the fiber optic cable.

[0035] The optical fiber cable may include a corrugated shape (undulation) arranged over at least a portion of its length. Furthermore, the optical assembly may include at least one member having a corrugated surface pattern. The member is positioned to receive and interact with at least a portion of the optical fiber cable, thereby imparting a corrugated profile to the shape of the optical fiber cable. The corrugated shape may be configured to interact with the received pump beam to homogenize the distribution of laser beam modes, thereby forming a homogenized laser beam with a substantially rotationally symmetric energy intensity distribution when emitted from the second end of the pump beam homogenizer.

[0036] The optical fiber cable can be arranged to form at least one predetermined loop. The optical fiber cable can be arranged to form at least two predetermined loops, or more than two, for example, three predetermined loops, optionally four predetermined loops, or more than four predetermined loops.

[0037] In other embodiments, the optical assembly of the pump beam homogenizer may include one or more free-space optical devices arranged to interact with the pump beam. These optical devices may be configured to form a homogenized pump beam having a substantially rotationally symmetric energy intensity distribution as it exits the second end of the pump beam homogenizer.

[0038] In one embodiment, the measuring device may include a probe beam homogenizer optically coupled to a probe device. The probe beam homogenizer is configured to receive the probe beam emitted from the probe device and guide the probe beam toward the optical system.

[0039] The probe beam homogenizer is A first end optically coupled to the probe device, configured to receive the probe beam emitted from the probe device, A second end optically coupled to the optical system, configured to guide the homogenized probe beam toward the optical system, The optical assembly is positioned between the first end and the second end and is configured to interact with the probe beam emitted from the probe device to form a homogenized probe beam.

[0040] In one embodiment, the optical assembly of the probe beam homogenizer may include an optical fiber cable. The optical fiber cable may include a multimode fiber. During use, the optical fiber cable can homogenize and circularize the probe beam as it propagates through the optical fiber cable.

[0041] The optical fiber cable may include a wavy shape (undulation) in at least a portion of its length. The optical assembly of the probe beam homogenizer may further include at least one member having a wavy surface pattern. The member is positioned to receive at least a portion of the optical fiber cable and can impart a wavy profile to the shape of the optical fiber cable. This wavy shape can be configured to interact with the received probe beam to homogenize the distribution of laser beam modes, so that a homogenized probe beam with a substantially rotationally symmetric energy intensity distribution is formed when it exits from the second end of the probe beam homogenizer.

[0042] The optical fiber cable may be arranged to form at least one defined loop. The optical fiber cable may be arranged to form at least two defined loops, or more than two defined loops, for example, three defined loops, optionally four defined loops, or optionally more than four defined loops.

[0043] In another embodiment, the optical system of the probe beam homogenizer may comprise one or more free-space optical devices, which are arranged to interact with the probe beam and form a homogenized probe beam having a substantially rotationally symmetric energy intensity distribution when emitted from the probe beam homogenizer through the second end.

[0044] In one embodiment, after being emitted from the pump beam homogenizer and / or probe beam homogenizer, the pump beam and / or probe beam may each have a substantially Gaussian energy intensity distribution, or a flat-top energy intensity distribution, and may also optionally have any other suitable intensity distribution.

[0045] The optical system may include a plurality of free-space optical devices, which may include mirrors, lenses, and / or beam splitters. Each free-space optical device may be configured to receive a pump beam and / or probe beam from a pump beam homogenizer and / or probe beam homogenizer. The optical system can shape the pump beam and / or probe beam and guide it toward the sample. The beam shaping may include expanding and aligning the pump beam and / or probe beam.

[0046] In some embodiments, particularly those involving small-diameter probe beams (e.g., about 10 μm or less), the measuring device may not include a probe beam homogenizer. Therefore, the optical system can be configured to receive the probe beam from the probe device and guide the probe beam toward the sample.

[0047] The system may further include an objective tube configured to receive a pump beam and / or probe beam from the optical system and to guide the pump beam and / or probe beam toward a sample. The objective tube includes at least one objective lens configured to focus the pump beam and / or probe beam onto the sample with a desired pump and / or probe spot diameter.

[0048] The optical system can be configured to guide a pump beam to the sample at a first angle of incidence. The optical system can further be configured to guide a probe beam to the sample at a second angle of incidence. In one embodiment, the first and second angles of incidence are set to be the same (i.e., coaxial configuration), and in other embodiments, the second angle of incidence is set to be greater than the first angle of incidence (i.e., split-axis configuration).

[0049] In a coaxial configuration, the optical system may be arranged such that the optical paths of the pump beam and the probe beam are coupled before they are guided toward the stage.

[0050] The measuring device may further include a photodetector configured to receive a reflected probe beam and detect the intensity of the reflected probe beam.

[0051] The measuring device may further include a phase-sensitive detector. The phase-sensitive detector may be electrically coupled to the signal generator. The phase-sensitive detector may be electrically coupled to the photodetector. The phase-sensitive detector may be configured to determine the phase lag between the intensity of the reflected probe beam and the intensity of the pump beam.

[0052] The phase-sensitive detector may be a multi-channel lock-in amplifier. The phase-sensitive detector may include a signal generator. In this case, the signal generator may be an internal reference oscillator. Alternatively, the phase-sensitive detector and the signal generator may be arranged as separate components. In such embodiments, the signal generator may take the form of a function generator.

[0053] The sample and the objective barrel are movable relative to each other in the X, Y, and Z axes to position the sample at the focal point of the pump beam and / or probe beam. In some embodiments, at least the objective barrel is movable relative to the sample (in other embodiments, the entire system is movable relative to the sample), and in other embodiments, the sample may be moved relative to the objective barrel.

[0054] In one embodiment, the system further includes a stage. The stage may include a holder configured to receive a sample. The holder may be disc-shaped. The system may further include a bridge structure extending above the stage. In one embodiment, the optical system is mounted on the bridge structure, and the objective barrel is positioned above the holder.

[0055] In one embodiment, the holder can be motor-driven, and the movement of the holder is controlled by the motor. The motor is configured to receive commands from a computer program and move the holder in the X, Y, and Z axes to position the sample at the focal point of the pump beam. In one embodiment, this focal point is approximately 20 mm from the end of the objective barrel.

[0056] The computer program may be installed on a computing device (e.g., a desktop personal computer (PC) or laptop) capable of communicating with the system. The operator can interact with the computer program through a graphical user interface displayed on the computing device. The computer program may be configured to receive sample parameters input from the operator, instruct the motor to move the holder in the X, Y, and Z axes to position the sample at the focus of the pump beam, receive the thermal response measurement results of the sample, determine one or more thermal properties of the sample, and further display the thermal properties on a display device (e.g., a screen) for the operator to review. The computer program may also be configured to instruct a signal generator to change the frequency of the modulated signal within a predetermined range.

[0057] The computer program may be configured to instruct a motor, during use (or in relation to the pump beam incident on the sample), to move the holder laterally (in the xy plane) relative to the objective cylinder, thereby creating a two-dimensional map of one or more thermal properties of the sample. The two-dimensional map may be displayed on a display device for viewing by the operator.

[0058] In one embodiment, the system may comprise at least one pumping device, or at least two pumping devices, or more than two pumping devices (e.g., at least three or at least four pumping devices), each configured to emit a pump beam. Each pumping device may be connected to either a signal generator or a signal generator via an associated driver to receive a modulated signal when in use. Furthermore, the system may also comprise at least one probe device, or at least two probe devices, or more than two probe devices (e.g., at least three or at least four probe devices), each configured to emit a probe beam.

[0059] Each pump beam and probe beam may have different wavelengths. As a result, since different materials respond differently to different wavelengths, the system can be designed to analyze samples of different compositions or different transducer layers (if any). Depending on the material being analyzed or the transducer layer applied (if required), the appropriate pump and probe devices can be activated.

[0060] In one embodiment, at least a portion of the system may be mounted on an optical baseplate. The optical baseplate may be coupled to a further baseplate via vibration-damping feet. The further baseplate may communicate with the environment and a cooling fan present in the system. Such a baseplate arrangement helps to minimize the effects of vibrations originating from the environment and / or the cooling fan, thereby minimizing the transmission of vibrations to the optical system, and consequently helping to prevent distortion of measurements due to such vibrations.

[0061] According to a second aspect of the present invention, a self-contained frequency-domain thermoreflectance unit is provided, the unit comprising the following:

[0062] - A housing configured to be selectively movable between an operating position and a non-operating position. - The housing that houses the system according to the first aspect of the present invention, In the non-operating position, the housing is openable, allowing at least a portion of the system to be exposed for placing a sample in the holder, and in the non-operating position, the housing completely encloses the entire system.

[0063] The enclosure may include wall panels and gaps between the panels. The gaps between the panels may be configured to supply cooling to the system when in use. The enclosure may further include a door configured to be selectively movable between an operating position and a non-operating position.

[0064] The enclosure may be a Class 1 interlocked enclosure, which ensures that the system does not operate while the door is open and that the door cannot be opened while the system is operating. Furthermore, if the door is opened during use, the system may shut down. Class 1 refers to the fact that the interlocked enclosure reduces the risk of access to the laser beam during normal use, thereby reducing the risk of eye damage to the operator.

[0065] Self-contained units with such Class 1 interlocked enclosures are recognized as offering improved safety and potentially allowing operators to use the system without safety glasses, as the laser device is completely isolated from the operator during use. This improved safety could potentially allow the system to be used under less restrictive conditions.

[0066] A third aspect of the present invention provides a method for performing frequency domain thermoreflectance measurement using the system according to the first or second aspect. The method includes the following steps:

[0067] - The process of receiving the sample to be measured, - A process for generating continuous wave laser light, the laser light is called a pump beam. - A process of modulating, homogenizing, and shaping the intensity of the pump beam over time, and then irradiating the sample. Here, the pump beam irradiated onto the sample has a spot diameter and intensity modulation sufficient to induce measurable temperature fluctuations that reach a desired depth inside the sample.

[0068] - A step of generating another continuous-wave laser beam, the laser beam being called a probe beam, - A process to monitor the thermal response of a sample by reflecting a probe beam off the sample and detecting the temporal variation in the intensity of the reflected light. - A process for measuring the phase difference between the intensity vibration of the probe beam and the intensity vibration of the pump beam. The method described above may further comprise changing the frequency of the modulation signal within a predetermined range. For example, the predetermined frequency range may be up to 50 kHz, or arbitrarily from 10 Hz to 10 kHz. When the modulation frequency is changed within the predetermined range, a pump beam with different temporal intensity oscillations can be generated, and therefore different temperature oscillations can be induced within the sample.

[0069] The method may further include monitoring the thermal response of the sample by detecting the intensity of the reflected probe beam at a number of different modulation frequencies within a predetermined range, and by measuring the phase delay of the probe beam intensity relative to the pump beam intensity. By analyzing the thermal response at a number of different modulation frequencies within a predetermined range, the thermal properties of the sample can be derived.

[0070] The sample may comprise bulk materials (i.e., homogeneous), thin films (e.g., the thickness of a transducer layer applied to several samples), multilayer materials, and thick materials (e.g., materials greater than several tens of micrometers) having optically smooth or optically rough surfaces and / or lateral non-uniformity. It may also comprise curved materials, such as materials having cylindrical, hemispherical, or spherical surfaces. For example, the sample may comprise an entire nuclear fuel particle, that is, the nuclear fuel particle may be complete / intact and unaltered before analysis. According to a fourth aspect of the present invention, a computer system configured to determine the thermal properties of a material is provided.

[0071] The computer system includes at least one processor and is configured to execute the following program instructions. - To receive the characteristics of a sample of material. - Perform the method according to the third aspect of the present invention. - Receiving the thermal response of the sample (the thermal response includes the temporal intensity variation of the reflected probe beam). - Determine at least one thermal property of the sample by comparing the phase difference between the pump beam intensity fluctuation and the probe beam intensity fluctuation. - The determined thermal characteristics should be displayed on a display device that allows the operator to confirm them. Furthermore, the present invention is not limited to the above-described content, but also extends to any combination shown in the above or subsequent descriptions and drawings.

[0072] Specific embodiments of the present invention will be described in detail, only by reference to the following drawings. [Brief explanation of the drawing]

[0073] [Figure 1] A perspective view of a system according to an embodiment of the present invention is shown. [Figure 2] Figure 1 shows a rear perspective view of the system. [Figure 3] Figure 1 shows a top view of the system. [Figure 4] Figure 1 shows an enlarged perspective view of the top of the system. [Figure 5] Figure 1 shows a side view of the system. [Figure 6] This shows a front view of a disk housing a loop-shaped optical fiber cable according to an embodiment of the present invention. [Figure 7] Figure 1 shows a perspective view of the disks in the system. [Figure 8] This shows a close-up view of some of the components for inducing undulation in an optical fiber cable according to an embodiment of the present invention. [Figure 9] Figure 8 shows a perspective view of the components in use. [Figure 10] Figure 1 shows a schematic diagram of the system. [Figure 11] Figure 1 shows a schematic diagram of the optical system of the system. [Figure 12] Figure 11 shows a schematic diagram of the optical system from the side. [Figure 13a] This figure shows a housing according to one aspect of the present invention. [Figure 13b] This figure shows a housing according to one aspect of the present invention. [Figure 14] Figures 13a and 13b show the system shown in Figure 1 housed within the enclosure (with the door removed). [Figure 15] This is a schematic diagram of a system according to another embodiment of the present invention. [Figure 16] This is an example of a graphical user interface according to an embodiment of the present invention.

[0074] [Detailed description of the embodiment] The present invention relates to a system for performing frequency-domain thermoreflectance measurements on a sample of a thermally conductive material, or on a material having lateral non-uniformity or irregularity in its surface (e.g., an optically rough surface) or a curved surface.

[0075] Furthermore, the sample may include bulk materials, thin films, multilayer materials, and thick film materials (e.g., several tens of microns or more).

[0076] As an example, the entire nuclear fuel particle, such as tristructural isotropic particle fuel (TRISO fuel), is also included in the measurement targets.

[0077] Figures 1 to 5 show a system 10 for measuring the thermal properties of thermally conductive materials and / or optical roughness and / or multilayer materials. System 10 comprises two devices, high-power laser diodes 12a and 13a, each emitting a continuous-wave laser beam when in use. Laser diode 12a is typically a high-power laser diode with adjustable output power. Laser diode 12a may also be referred to as a pump diode and emits a laser beam referred to as a pump beam 41. Laser diode 13a may also be referred to as a probe diode and emits a laser beam referred to as a probe beam 43. The optical paths of the pump beam 41 and the probe beam 43 are shown in Figures 10 to 12.

[0078] The pump diode 12a and probe diode 13a are electrically connected to the pump diode driver 12b and probe diode driver 13b, respectively. The drivers 12b and 13b are connected to the power supply 65. The pump diode driver 12b is electrically connected to the signal generator 17.

[0079] The signal generator 17 is configured to generate a modulated signal 49 (shown in Figure 10) and supply it to the pump diode driver 12b, which modulates the current supplied to the pump diode 12a, thereby changing the intensity of the pump beam 41 radiated from the pump diode 12a. The signal generator 17 generates a sine wave, but in some embodiments, it may instead generate a square wave or other waveform shape. The pump diode 12a operates at high power so that the output power is capable of generating measurable temperature fluctuations within the sample during use. For example, the output power may be up to several tens of watts (W).

[0080] For example, the signal generator 17 may be limited to outputting a frequency range from 10 Hz to 30 kHz. During use, the frequency of the modulated signal can be varied within this range, for example, from low frequencies to high frequencies.

[0081] The signal generator 17 constitutes part of the phase-sensitive detector 70, which is configured as a lock-in amplifier. Therefore, the signal generator 17 also functions as an internal reference oscillator.

[0082] Conventional systems typically require very high modulation frequencies (MHz band), necessitating either an electro-optic modulator (EOM) or an acousto-optic modulator (AOM). Such high modulation frequencies are necessary when measuring very thin films with submicron thicknesses. However, such high modulation frequencies are unsuitable for measuring materials with thicknesses of tens of microns or more. This is because the temporal temperature fluctuations within the sample induced by the temporal intensity modulation of the pump beam attenuate rapidly in the depth direction, and measurable temperature fluctuations exist only near the surface of the sample. As a result, the temperature in the interior region of the sample becomes almost constant, making it impossible to evaluate its thermal properties by measuring temperature fluctuations.

[0083] Therefore, the present invention provides a high-power pump beam with a lower modulation frequency (kHz band), which allows for deeper penetration into the sample (because low-frequency intensity vibrations decay more slowly). As a result, the present invention does not require an EOM (electro-optic modulator) or aOM (acousto-optic modulator) and provides a more cost-effective solution than known systems. However, in some embodiments of the present invention, an EOM or aOM may still be used to generate the modulated signal.

[0084] System 10 further includes two beam homogenizers 30a and 30b to independently homogenize the pump beam 41 and the probe beam 43. Beam homogenizer 30a is optically coupled to laser diode 12a and configured to receive the pump beam 41 and guide the pump beam 41 from laser diode 12a to optical system 20 (described later). Beam homogenizer 30b is optically coupled to laser diode 13a and configured to receive the probe beam 43 and guide the probe beam 43 from laser diode 13a to optical system 20.

[0085] Each beam homogenizer 30a, 30b is equipped with first ends 31a, 31b and second ends 32a, 32b. The first end 31a is optically coupled to the laser diode 12a and receives an intensity-modulated pump beam 41 emitted from the laser diode 12a when in use. The first end 31b is optically coupled to the laser diode 13a and receives a constant-intensity probe beam 43 emitted from the laser diode 13a when in use.

[0086] The second end 32a is optically coupled to the optical system 20 and directs the pump beam 41 toward the optical system 20 when in use. Similarly, the second end 32b is optically coupled to the optical system 20 and directs the probe beam 43 toward the optical system 20 when in use.

[0087] Optical assemblies are positioned between the first end 31a and the second end 32a, and between the first end 31b and the second end 32b. Each optical assembly is configured as a thick-walled multimode optical fiber cable 39 having two predetermined loops. As is most clearly shown in Figures 5 to 9, a member 38 having a corrugated surface shape is pressed against a portion of the cable 39, thereby imparting a corrugated profile along a portion of the cable length. The looped optical fiber cable 39 is housed in a large disk 15 placed on a heat sink 16, thereby transferring heat generated by the laser diodes 12a and 13a to the heat sink 16 and preventing heat buildup. This contributes to improving the lifespan of the system 10.

[0088] As shown in Figure 6, the disk 15 of each beam homogenizer 30a, 30b consists of three separate parts (labeled 34, 35, and 36). Parts 34 and 35 are shown in a spaced configuration and are configured to slide along a rod 37 to change the gap width, thereby allowing the beam homogenizers 30a, 30b to accommodate fiber optic cables of different lengths. Once the appropriate size is set, parts 34, 35 can be fixed in place.

[0089] Component 36 is inserted into component 35 and secured in place. Component 36 further comprises a component 38 fixed to the outer circumference of disk 15. Component 38 is shown in detail in Figures 8 and 9. Component 38 comprises a series of projections 38a and a cylinder 38b. Each projection 38a has a gap 38c, which is configured to align with the interior of disk 15, so that the gap 38c can receive the loop of the optical fiber cable 39.

[0090] The projections 38a are positioned on both sides of the gap 38c and are configured to fix member 38 to the outside of member 36 via teeth 38d provided on the outside of disk 15. Member 38 further has teeth 38d on both sides of disk 15 to engage with the projections 38a during assembly. Each tooth 38d includes a cylindrical portion 38b', which may extend axially from one side of disk 15 to the other.

[0091] During assembly, the loop of the optical fiber cable 39 is positioned circumferentially along the teeth 38d and the cylinder 38b'. Then, the projection 38a is inserted between the teeth 38d and secured. After assembly, the optical fiber cable 39 is located in the gap 38c and compressed between the cylinders 38b and 38b'. At this time, the cylinders 38b and 38b' impart a wavy profile to the optical fiber cable 39.

[0092] Thus, the optical fiber cable 39 is configured to interact with the pump beam 41 and probe beam 43 via a corrugated shape during use, averaging the distribution of modes present in the beams. Therefore, the pump beam 41 and probe beam 43 emitted from the second ends 32a, 32b of the beam homogenizers 30a, 30b are more homogenized and approach a substantially rotationally symmetric energy intensity distribution compared to those emitted directly from the laser diodes 12a, 13a. During use, each optical fiber cable 39 can homogenize and circularize the pump beam 41 and probe beam 43 passing through the corresponding optical fiber cable, respectively.

[0093] The pump beam 41 and / or probe beam 43 emitted from the laser diodes 12a and 13a typically have non-circular (e.g., elliptical) spot profiles; therefore, it is necessary to shape the beams to form a smooth, homogeneous, and circular beam profile. This improves beam quality and focus.

[0094] The optical system 20 is mounted on a bridge structure 23 located above the stage section 40, which includes a holder 42. The optical system 20 is configured to receive the pump beam 41 and the probe beam 43 from the second ends 32a and 32b of the beam homogenizers 30a and 30b. As will be described later, the optical system 20 includes a series of free-space optical devices arranged to collimate the pump beam 41 and the probe beam 43 and guide them toward the holder 42, and also performs filtering to remove unwanted wavelengths from the beams.

[0095] As shown in Figure 11, the optical system 20 further comprises an optical device D arranged to couple the optical paths of the pump beam and the probe beam so that the pump beam 41 and the probe beam 43 are guided toward the holder 42 along the same (coaxial) optical path.

[0096] As shown in Figure 12, the system 10 further comprises an objective barrel 22 housing at least one objective lens 33. During use, the coaxial pump beam and probe beam 41, 43 are guided from the optical arrangement 20 into the objective barrel 22. For example, the coaxial beams can be deflected by approximately 90 degrees to guide them into the objective barrel 22. The pump beam and probe beam 41, 43 travel through the objective barrel 22 and the objective lens 33 toward the holder 42. The pump beam and probe beam 41, 43 reach the holder 42 at the same angle of incidence and, in this example, are incident perpendicular to the holder 42.

[0097] The objective barrel 22 is positioned on the side of the bridge structure 23, and the holder 42 can be positioned directly below the objective barrel 22 during use. In some embodiments, the objective barrel 22 may comprise multiple objective lenses having different optical powers. These objective lenses are mounted on a rotating platform, and by rotating the platform, the desired objective lenses can be selectively positioned in the optical paths of the beams 41 and 43.

[0098] The holder 42 is disc-shaped and configured to receive and hold a sample 46 of material during use. In some examples, the sample may consist of a number of particles, each up to 1000 μm or about 1000 μm in diameter, such as entire nuclear fuel particles. The particles can be mounted on the holder 42 by adhering them to a base using a thermally conductive epoxy or wax.

[0099] The holder 42 is operably coupled to a motor (not shown) which moves the holder 42 in the x, y, or z axis direction (i.e., horizontal and vertical) during use to align the sample 46 to the focus of the pump beam 41 and probe beam 43. The holder 42 is mounted on a track 44, which allows the motor to move the holder 42 along the x axis. The stage unit 40 may also include additional tracks (not shown) for moving the holder 42 along the y and z axes. The motor includes a driver 69, which is connected to a power supply 68 shown in Figure 2.

[0100] When a transducer is used, the transducer absorbs heat from the pump beam 41 and transfers that heat to the sample 46. This causes the pump beam 41 to periodically heat (temperature fluctuation) the sample 46 through the thin film. The wavelengths of the emitted pump beam and probe beams 41 and 43 are selected based on commonly used transducers, as each metallic material has different thermo-optic coefficients and absorption spectra. The transducer material is selected to maximize the absorption of the pump beam and to ensure the probe beam's reflectivity is highly sensitive to temperature.

[0101] As will be described in more detail below, and as shown in Figures 10 and 12, during use, the pump beam 41 is irradiated onto the sample 46. The sample 46 (or the transducer, if present) absorbs at least a portion of the pump beam 41, thereby inducing temporal temperature oscillations within the sample. When irradiated onto the sample 46, the pump beam 41 has a spot diameter and temporal intensity oscillations sufficient to induce measurable temperature oscillations that can reach a predetermined depth. The system 10 further includes a measuring device that monitors the thermal response of the sample 46, determines the phase lag of the thermal response relative to the pump beam 41, and calculates at least one thermal characteristic of the sample 46.

[0102] The measuring device will be described in detail below. The measuring device comprises a probe diode 13a, a probe driver 13b, a beam homogenizer 30b, a photodetector 80, and a lock-in amplifier 70.

[0103] Since the surface reflection characteristics of a sample depend on the sample temperature, the thermal response of the sample 46 can be determined by analyzing the characteristics of the reflected probe beam 47 (shown in Figures 10 and 12). Therefore, the photodetector 80 is positioned to detect the reflected probe beam 43 from the sample 46 during use. The photodetector 80 receives the reflected probe beam 47, detects the intensity fluctuations of the reflected probe beam 47, and from this, can detect the thermal response of the sample 46 induced by the temporal intensity fluctuations of the pump beam 41.

[0104] A lock-in amplifier 70 (shown in Figure 10) is electrically connected to the photodetector 80. As shown in Figure 10, the lock-in amplifier 70 receives a signal 48 containing thermal response data from the photodetector 80 during use and then determines the phase lag of the probe beam intensity oscillations relative to the pump beam intensity oscillations. This phase lag data can then be analyzed to determine the thermal properties of the sample 46.

[0105] As shown in Figures 1 and 2, the system 10 further comprises a base plate 60 on which the entire system 10, including a cooling fan 66, is mounted. Above the base plate 60 is another base plate 64, on which all optical elements, including the beam homogenizers 30a and 30b, the optical system 20, the objective lens barrel 22, and the stage section 40, are mounted. Vibration isolation legs 62 are provided between the two base plates 60 and 64. The vibration isolation legs 62 prevent vibrations from being transmitted from the base plate 60, which is in contact with the environment and the cooling fan 66 (a source of vibration), to the optical elements and the stage section 40, thereby preventing disturbances in the thermal response measured by mechanical vibrations during the use of the system 10.

[0106] In this embodiment, two cooling fans 66 are provided: one to cool the electrical system of the system, and the other to cool the area around the optical system where the laser diodes 12a and 12b are located. The cooling fans 66 are driven by the power supply 67 shown in Figure 2.

[0107] As shown in Figures 13a, 13b, and 14, the system 10 can be housed in a Class 1 interlocked enclosure 90. The enclosure 90 comprises wall panels 91 and panel gaps 92. The panel gaps 92 are provided to introduce airflow into the enclosure 90 during use and prevent overheating of the system 10.

[0108] As shown in Figures 13a, 13b, and 14, the system 10 may be housed in a Class 1 interlocked enclosure 90. The enclosure 90 comprises wall panels 91 and a panel gap 92. The panel gap 92 is designed to allow airflow through the enclosure 90 during use, which helps prevent the system 10 from overheating.

[0109] The housing 90 further includes a door 94, as shown in Figures 13a and 13b. The door is removed in Figure 14 to show the system 10 inside the housing 90. The door 94 can move between a closed position (operated position shown in Figures 13a and 13b) and an open position (non-operated position, not shown).

[0110] In some cases, if door 94 is opened during operation, the laser diodes 12a and 13a are automatically switched off. In other cases, door 94 is equipped with a locking mechanism, which is controlled by the FDTR program 86. When the locking mechanism is deactivated, door 94 can be opened and system 10 cannot operate. That is, the FDTR program 86 does not execute program instructions while door 94 is open. When door 94 is closed and the locking mechanism is activated, system 10 can operate as described above. If system 10 is operating, the locking mechanism cannot be deactivated and door 94 cannot be opened. Door 94 can only be opened after system 10 has completed the required measurements or after a command to abort the program has been received by the operator and the power to the laser diodes 12a and 13a has been turned off.

[0111] Such an interlocked housing 90 reduces the risk of access to the laser beams 41, 43 during normal use, and therefore reduces the risk of eye injury to the operator. When the system 10 is housed in the Class 1 interlocked housing 90, it can be operated safely without protective goggles.

[0112] As shown in Figure 10, system 10 is connected to a computing device 85, such as a desktop computer or notebook computer, by wired communication 87. The computing device 85 has a computer program 86 for FDTR installed, and when the program 86 is executed, a graphical user interface (GUI) is displayed on the screen of the computing device 85. An example of the GUI 81 is shown in Figure 16. As shown in box 82 in Figure 16, the interface 81 can receive information about the sample 46 from the operator. The information received may include the types and compositions of the layers constituting the sample 46, the known thermal conductivity of each layer contained in the sample 46, and the thermal properties of each layer.

[0113] If layer parameters such as thermal conductivity or thermal diffusivity are unknown, this is entered as "unknown". These unknown characteristics are determined by the FDTR system and method described herein. It will be understood that other relevant parameters for each layer present may also be entered into interface 81. The operator can then start the measurement process by clicking the "Start new measurement" button 83a in the upper right corner of interface 81.

[0114] In some embodiments, if the measurement process has already started, the operator can choose to stop the process by clicking the "abort" button 83b.

[0115] The FDTR program 86, when executed after receiving sample information from the operator, provides program instructions for performing the required measurements. Once data is collected, a graph 84a of the measured phase delay (y-axis) against the modulation frequency (x-axis) is generated and displayed on the interface 81. The interface 81 can also display a real-time camera feed 88a of the operating system 10, with a progress bar 88b indicating the progress. Once all data has been collected, unknown thermal properties (i.e., unknown thermal conductivity and thermal diffusivity of a particular layer in the sample) are calculated from graph 84a, and the result 84b may be displayed on the interface 81 for the operator to view. The collected data may also be saved and / or downloaded 89.

[0116] A typical (but not limited to) example of the operation of System 10 is described below.

[0117] The operator opens the door 94 of the housing 90, places a sample 46 of the material on the holder 42, and then closes the door 94 again. The sample 46 may include bulk material, thick film material, thin film material, and / or multilayer material. The sample 46 may also have an optically smooth surface, an optically rough surface, and / or in-plane heterogeneity, and may even be curved.

[0118] Subsequently, the operator starts the FDTR program 86 on the computing device 85 connected to the system 10, inputs the necessary sample information such as the aforementioned parameters into the GUI interface 81 displayed on the screen, and specifies any unknown parameters. Next, the operator starts the measurement process by clicking button 83a.

[0119] The FDTR program 86 first instructs the locking mechanism to be activated, thereby preventing the door 94 from being opened until the measurement process is complete. The measurement process usually takes less than 10 minutes, but may take longer if the operator requests two-dimensional mapping of the sample 46. The progress of the measurement process is displayed on the progress bar 88b.

[0120] When the locking mechanism is activated, the FDTR program 86 activates the laser diodes 12a and 13a to generate the pump beam and probe beam 41 and 43.

[0121] The signal generator 17 generates and supplies a modulated signal 49 to the pump diode driver 12a, which modulates the current supplied to the pump diode 12a, thereby modulating the intensity of the radiated pump beam 41. An intensity-modulated pump beam 41 is necessary to induce temperature fluctuations within the sample 46.

[0122] On the other hand, the probe diode driver 13b is not connected to the signal generator 17, and therefore a constant current is supplied to the probe diode 13a. Consequently, the probe diode 13a emits laser light 43 of constant intensity.

[0123] Subsequently, the motor moves the holder 42 by predetermined amounts in the x, y, and z axes so that the sample 46 is positioned at the focal points of the pump beam and probe beams 41 and 43. In one embodiment, the focal points of the pump beam and probe beam are located approximately 20 mm away from the end of the objective barrel 22.

[0124] Next, the intensity-modulated pump beam 41 is injected into the loop-shaped optical fiber cable 39 at the first end 31a of the beam homogenizer 30a, and the constant-intensity probe beam 43 is injected into the loop-shaped optical fiber cable 39 at the first end 31b of the beam homogenizer 30b. The pump beam and probe beams 41 and 43 are respectively guided within the corresponding loops of the optical fiber cable 39.

[0125] At least a portion of the loops of each optical fiber cable 39 is given a wavy undulation by the member 38. This configuration homogenizes the mode distribution within the pump beam and probe beams 41, 43, making these beams more homogenized and circular when they exit the beam homogenizers 30a, 30b.

[0126] Next, the second ends 32a and 32b of the beam homogenizers 30a and 30b guide the homogenized pump beam 41 and probe beam 43 toward the optical system 20, where a series of optical devices shape, collimate, and guide the pump beam 41 and probe beam 43, while removing unwanted wavelengths from the beams 41 and 43.

[0127] As shown in Figure 11, the pump beam 41 and the probe beam 43 travel along separate optical paths from the beam homogenizers 30a and 30b to the optical system 20. The probe beam 43 is first received by a beam splitter BS, which is necessary to separate the incident probe beam 43 from the reflected probe beam 47 and guide the incident probe beam 43 toward the dichroic mirror D.

[0128] The pump beam 41 is first received by a short-pass filter F1 to remove unwanted wavelengths from the pump beam 41, and then guided by mirror m towards dichroic mirror D. The dichroic mirror D then guides the pump beam 43 to align with the probe beam 41, coupling the optical paths of beams 41 and 43 so that they travel along the same optical path toward the sample 46.

[0129] Next, the combined (i.e., coaxial) beams 41 and 43 are incident on a mirror "45°m" positioned at a 45-degree angle to their optical paths. The mirror "45°m" changes the direction of propagation of the combined beams 41 and 43 by approximately 90 degrees, so that these beams 41 and 43 travel coaxially within the objective barrel 22. The beams 41 and 43 pass through an objective lens 33 located within the objective barrel 22 and are focused onto the sample 46 with a desired spot diameter by the objective lens 33.

[0130] As shown in Figure 12, the combined beams 41 and 43 are emitted from the objective tube 22 and incident on the sample 46 placed on the holder 42. Since beams 41 and 43 are coaxial, they incident on the same position (spot) on the sample 46.

[0131] The pump beam 41 and the probe beam 43 have the same angle of incidence and, in this embodiment, are incident perpendicular to the surface of the sample 46. After the probe beam 43, or both the pump beam 41 and the probe beam 43, are incident on the sample 46, the motor is instructed to fine-tune the position of the sample 46 relative to the beams so that the sample 46 is precisely positioned at the focal point of the pump beam / probe beam.

[0132] The pump beam 41 induces temperature fluctuations within the sample 46, for example, at the sample surface or the interface within the sample. Meanwhile, the probe beam 43 is reflected from the sample 46, for example, at its surface or interface. If the sample surface is irregular, for example, optically rough or heterogeneous within the surface, the average thermal response can be measured instead of the local response by increasing the spot diameter of the probe beam.

[0133] As shown in Figure 12, the probe beam is reflected 47 along the normal to the surface of the sample and received by the objective tube 22. The reflected probe beam 47 then travels along the objective tube 22 toward the mirror "45°M", which guides the reflected probe beam 47 into the optical system 20. The dichroic mirror D allows the reflected probe beam 47 to pass through and travel toward the beam splitter BS. The beam splitter BS then guides the reflected probe beam 47 toward the photodetector 80 at a 90-degree angle. Before being received by the photodetector 80, the reflected probe beam 47 passes through a focusing lens and a bandpass filter F2, which removes unwanted wavelengths from the beam, such as the reflected wavelength from the pump beam 41.

[0134] System 10 is typically calibrated before measurement to reduce the effects of systematic phase shifts. Before measurement, a phase calibration curve can be recorded by removing the bandpass filter F2 and placing a highly reflective target (e.g., a mirror) in the holder 42 in place of the sample. The target reflects the pump beam, which is detected and measured by the photodetector 80. The pump phase is determined, and this pump phase can then be subtracted from the phase lag of the sample's thermal response.

[0135] The photodetector 80 measures the intensity of the reflected probe beam 47 (thermal response). This intensity changes in response to temperature fluctuations induced within the sample 46. These measurement signals 48 are transmitted to the lock-in amplifier 70, as shown in Figure 5. The lock-in amplifier 70 compares the thermal response signal with the signal of the pump beam 41, calculates their phase delay, and transmits this phase delay data to the FDTR computer program 86. In calculating the phase delay, the phase shift originating from the apparatus (i.e., the phase difference between the pump beam and the modulation signal) is usually corrected.

[0136] The FDTR program 86 then instructs the lock-in amplifier 70 to change the frequency of the modulation signal 49 supplied to the pump driver 12b. This modulates the intensity of the pump beam 41 at different frequencies. The measurement process described above is repeated for several different frequencies within a predetermined frequency range, for example, from 10 Hz to 10 kHz. The order of the measurement frequencies is not particularly limited, but for convenience, a method of sweeping the frequencies from the low-frequency side to the high-frequency side is usually employed. The frequency range swept by this system 10 is lower than the frequencies used in conventional systems. Therefore, this system 10 can measure the thermal response of deeper parts inside the sample, for example, boundary layers or interfaces located inside the material.

[0137] The lock-in amplifier 70 may be multi-channel. By using a multi-channel lock-in amplifier 70, phase delay data at multiple different frequencies can be measured simultaneously, thereby reducing the program execution time.

[0138] Next, the FDTR program 86 plots a graph 84a showing phase lag (y-axis) versus frequency (x-axis). This graph 84a can be displayed on a computer screen for the operator to view. Then, the plotted curve is analyzed to determine calculated values ​​for unknown properties of the sample. The results 84b are displayed on the screen.

[0139] In some embodiments, the measurement process ends here. The determined thermal properties are then displayed on a computer screen for the operator to view. However, in other implementations, if the operator requests a two-dimensional map of the thermal properties, the motor is then instructed to move the holder 42 so that a second point on the surface of the sample 46 is located at the focal point of the beams 41, 43. The system 10 then repeats the measurement process at the second point. This process is repeated for a number of different points on the surface of the sample 46 until a two-dimensional map of the thermal properties is created and then displayed on a computer screen for the operator to view.

[0140] When the operation of system 10 is complete, the FDTR program 86 instructs the laser diodes 12a and 13a to be stopped at the end of the measurement process. The FDTR program 86 also deactivates the locking mechanism, allowing the operator to open the door 94 and remove the sample from the holder 42. If necessary, a new sample can be placed on the holder 42, and the measurement process can be repeated for the new sample using the same procedure.

[0141] The operator may download and / or save thermal characteristic data for future reference. This data may be stored in non-volatile memory, such as a solid-state drive, hard disk drive, optical disc, or flash drive.

[0142] Figure 15 shows a further embodiment of the present invention. Components identical to those shown in the embodiments in Figures 1 to 14 are denoted by reference numerals preceded by "1". Unless otherwise specified, identical components should be interpreted as having the same function as those in the embodiments described above.

[0143] The system 110 shown in Figure 15 is for measuring the thermal properties of sample 146. System 110 includes two devices in the form of high-power laser diodes 112a and 114a, each emitting a continuous-wave laser beam 141a and 141b, respectively, when in use. Both emitted laser beams 141a and 141b are pump beams.

[0144] The laser diodes 112a and 114a are electrically connected to the laser diode driver 112b. The laser diode driver 112b is electrically connected to the signal generator 117. In this example, the signal generator 117 is a separate component from the phase-sensitive detector 180, which again takes the form of a lock-in amplifier.

[0145] The signal generator 117 can take the form of a function generator. The function generator 117 supplies a modulated signal 149 to the diode driver 112b, thereby generating intensity-modulated pump beams 141a and 141b. As described above, when the intensity-modulated pump beams 141a and 141b are incident on the sample 146, they can induce temperature fluctuations within the sample 146. For example, the frequency of the modulated signal 149 can be varied in the range of approximately 10 Hz to 10 kHz during use.

[0146] System 110 comprises a series of free-space optical devices that receive pump beams 141a and 141b from laser diodes 112a and 114a, and then guide the beams toward a sample 146 placed on a holder 142 of the stage unit 140. Several related free-space optical systems are described below.

[0147] (a) In the process described above, pump beams 141a and 141b are generated.

[0148] (b) In step (b), a pair of cylindrical lenses are placed following a spherical collimating lens, corresponding to each laser diode 112a, 114a, to circularize the pump beams 141a, 141b. The spherical collimating lenses serve to collimate the fast axis components of the pump beams 141a, 141b.

[0149] Subsequently, the slow-axis component is magnified by a concave cylindrical lens, and then collimated by a convex cylindrical lens.

[0150] In step (c), a polarizing beam splitter "PBS" is used to combine the two pump beams 141a and 141b into a single, higher-power pump beam 141.

[0151] In step (D), the spot size and shape are adjusted. An iris is positioned after the polarizing beam splitter "PBS" in the optical path of the coupled pump beam 141, followed by a beam compressor and a diffuser. During use, after coupling the pump beam, the coupled beam 141 is guided to the iris using two mirrors. The iris further shapes the beam and improves its circularity. The beam compressor adjusts the diameter and shape of the coupled pump beam, and a diffuser positioned at the focal point of the beam compressor homogenizes the coupled pump beam 141.

[0152] Steps (b), (c), and (D) constitute a beam homogenizer 130 that shapes and homogenizes the pump beams 141a and 141b. The beam homogenizer 130 is optically coupled to the laser diodes 112a and 114a and is configured to receive the pump beams 141a and 141b when in use and to guide the pump beams from the laser diodes 112a and 114a to the optical system 120 (described later).

[0153] The beam homogenizer 130 comprises a first end 131 and a second end 132. The first end 131 is optically connected to laser diodes 112a and 114a and, when in use, receives intensity-modulated pump beams 141a and 141b emitted from the laser diodes 112a and 114a. The second end 132 is optically connected to the optical system 120 and, when in use, guides the combined pump beam 141 toward the optical system 120.

[0154] An optical assembly is positioned between the first end 131 and the second end 132. The optical assembly consists of a row of free-space optical elements as described in steps (b), (c), and (D) above. The optical assembly is configured to interact with the pump beams 141a and 141b emitted from the laser diodes 112a and 114a, and to form a composite pump beam 141 having a substantially rotationally symmetric energy intensity distribution when it is emitted from the second end 132 of the beam homogenizer 130.

[0155] During use, the pump beams 141a and 141b interact with the optical assembly to average the distribution of modes present in the coupled pump beam 141, thereby causing the coupled pump beam 141 to approach a substantially rotationally symmetric energy intensity distribution.

[0156] The remaining free-space optical components constitute the optical system 120, which receives the pump beam 141 from the beam homogenizer 130. The remaining free-space optical components constituting the optical system 120 are described below. The coupled pump beam 141 exits from step (D) and is transmitted to a short-pass filter, thereby ensuring that no light other than the coupled pump beam 141 is directed toward the sample 146. Next, the coupled pump beam 141 is guided to pass through an ND filter (neutral density filter), which provides the ability to adjust the power of the coupled pump beam 141, thereby helping to adjust the heating power of the coupled pump beam 141 and avoid overheating of the sample 146.

[0157] (e) In step (e), the synthesized pump beam 141 is received by a beam sampler. The beam sampler is used to split a portion (about 1-2%) of the synthesized pump beam 141 and guide the split beam to a reference detector 150. The reference detector 150 is typically a photodetector. The reference detector 150 measures the light intensity of the sampled pump beam and transmits this information as a reference signal 152 to the lock-in amplifier 170. The reference signal 152 is used to determine the phase difference of other input signals, such as the probe beam signal. In the embodiments shown in Figures 1 to 9, the reference detector is not specifically shown, and the reference signal is provided by an internal reference oscillator of the lock-in amplifier 70.

[0158] The optical system 120 guides a coupled pump beam 141 toward a sample 146 placed on a holder 142 of the stage section 140. When the coupled pump beam 141 is incident on the sample 146, it has an average optical output power of up to several watts and a spot diameter of at least 75 μm, and optionally at least 100 μm.

[0159] System 110 further comprises an objective barrel 122. When in use, the holder 142 is located directly below the objective barrel 122. The holder 142 is configured to receive and hold a sample 146 of material when in use. The holder 142 is operably coupled to a motor (not shown) which moves the holder 142 in the x, y, or z axis direction (i.e., horizontal and vertical) when in use to position the sample 146 at the focal point of the coupled pump beam 141. When in use, the coupled pump beam 141 is received by the sample 146. The sample 146 absorbs at least a portion of the pump beam 141, thereby inducing thermal oscillations within the sample 146. When the coupled pump beam 141 is incident on the sample 146, it has a spot diameter of at least 75 μm.

[0160] System 110 is configured to receive and analyze samples having a thickness (in the z-axis direction) ranging from tens of micrometers to several millimeters, and dimensions up to 200 mm in the x-axis and / or y-axis directions. In some implementations, the sample has a thin-film metal transducer layer, which is provided along the x and y surfaces of the sample.

[0161] System 110 further includes a measuring device that monitors the thermal response of sample 146, calculates the phase lag of the thermal response relative to the pump beam 141, and subsequently calculates one or more thermophysical properties of sample 146. This measuring device will be described in detail below.

[0162] The measuring apparatus includes a probe device 113, which is configured as a laser diode and is configured to irradiate a continuous wave laser beam (probe beam 143). The probe beam 143 is configured to reflect from the sample 146 and is configured to detect the thermal response of the sample caused by temperature fluctuations induced by the composite pump beam 141, by utilizing the fact that the surface reflectance of the sample 146 depends on temperature.

[0163] The measuring device further comprises a photodetector 180 and a lock-in amplifier 170. Since the surface reflection characteristics of the sample 146 depend on the temperature of the sample, the thermal response of the sample can be determined by analyzing the characteristics of the reflected probe beam 147. For this reason, the photodetector 180 is positioned to detect the probe beam after it has been reflected from the sample 146 during use. The photodetector 180 receives the reflected probe beam 147, detects the reflection intensity from the sample, and from this, can determine the thermal response of the sample 146 caused by temperature fluctuations induced by the pump beam 141.

[0164] The lock-in amplifier 170 is electrically connected to the photodetector 180. The lock-in amplifier 170 is configured to receive a signal containing thermal response data from the photodetector 180 and a reference signal 152 from the reference detector 150, and to calculate the phase delay of the thermal response with respect to the composite pump beam 141.

[0165] The optical system 120 further comprises a series of optical devices positioned between the laser diode 113 and the sample 146. These optical devices receive the probe beam 143 from the diode 113, manipulate the characteristics of the probe beam 143, and guide the probe beam 143 toward the sample 146, as will be described later. The probe beam 143 is not identified and homogenized before being guided toward the sample because its spot diameter is much smaller than the pump spot diameter. In this example, the probe beam 143 has a spot diameter of at least 10 μm when incident on the sample 146.

[0166] The optical system 120 further comprises a series of optical devices positioned between the sample 146 and the photodetector 180. This series of optical devices receives the probe beam 143 after reflection from the sample 146 and guides the reflected probe beam 147 toward the photodetector 180. A further beam sampler is positioned in the optical path of the reflected probe beam 147 and splits a portion of the reflected probe beam 147, guiding this portion to the CCD camera shown in step (h). Step (h) can monitor the focus and position of the sample 146 relative to the probe beam 143 and indicate if repositioning of the sample 146 is necessary.

[0167] During use, the optical system 120 is configured to guide the combined pump beam 141 and the probe beam 143 toward the stage section 140. The optical system 120 and the objective cylinder 122 are configured to guide the combined pump beam 141 toward the sample 146 at a first angle of incidence. Furthermore, the optical system 120 is configured to guide the probe beam 143 toward the sample 146 at a second angle of incidence that is greater than the first angle of incidence (i.e., a split-axis configuration). Unlike Figures 1 to 9, the probe beam 143 is not incident from the normal direction of the sample 146, but is reflected at an angle to the normal.

[0168] The method for calculating the thermal characteristics of sample 146 based on a graph showing the relationship between phase lag and frequency is the same as described above in relation to Figures 1 to 14.

[0169] Although the principles of the present invention have been described using exemplary embodiments, the present invention is not limited to these embodiments and can be implemented by other modifications as defined in the claims of this application.

Claims

1. A frequency-domain thermoreflectance (FDTR) system for measuring the thermal properties of thermally conductive materials and / or optically rough materials, The pump device has adjustable output power and is configured to emit a continuous-wave laser beam, the laser beam being referred to as the pump beam. A signal generator configured to generate a modulation signal for temporally modulating the intensity of the pump beam ejected from the pump device, An optical system configured to receive and shape the pump beam, guide the pump beam toward a sample of the material, and interact with the sample during use, The system includes a measuring device configured to monitor the thermal response of the sample due to its interaction with the pump beam, A frequency-domain thermoreflectance (FDTR) system in which the pump beam incident on the sample has a spot diameter and temporal intensity oscillations sufficient to induce measurable temperature oscillations in the sample that reach a required depth within the sample.

2. A system according to claim 1, wherein the pump beam has a spot diameter of at least 400 μm when incident on the sample.

3. The system according to claim 1, wherein the pump beam is configured to have a spot diameter of at least 50 μm when incident on the sample.

4. A system described in any one of the preceding paragraphs, The pump device is electrically coupled to a driver, and the driver is electrically coupled to a signal generator. The signal generator is configured to supply a modulated signal to the driver during use, modulating the current supplied to the pump device, thereby modulating the intensity of the radiated pump beam and inducing temperature fluctuations within the sample, and A system in which the frequency of the modulated signal is variable within a predetermined range.

5. A probe device configured to emit a continuous wave laser beam, wherein the laser beam is called a probe beam, the probe beam is configured to reflect from a sample, and the reflectivity characteristics of the sample depend on the temperature of the sample, thereby causing temperature fluctuations induced inside the sample by a pump beam to cause temporal fluctuations in the intensity of the reflected probe beam, as described in any of the above claims.

6. The system according to claim 5 or 6, characterized in that the measuring device monitors the thermal response of a sample by detecting the intensity of the reflected probe beam, and measures the phase delay of the intensity vibration of the probe beam relative to the intensity vibration of the pump beam.

7. The system according to claim 5 or 6, characterized in that the probe beam is configured such that the spot diameter is a maximum of 10 μm when incident on the sample.

8. A system according to claim 5 or claim 6, wherein the probe beam has a spot diameter of at least 50 μm when incident on the sample.

9. A system according to any one of the preceding paragraphs, further comprising a pump beam homogenizer optically coupled to a pump device, wherein the beam homogenizer is configured to receive a pump beam emitted from the pump device, and The pump beam homogenizer, A first end optically coupled to the pump device. The first end is configured to receive the pump beam emitted from the pump device, It has a second end optically coupled to the optical system, the second end being configured to guide the homogenized pump beam toward the optical system, A system having an optical assembly positioned between the first end and the second end, wherein the optical assembly is configured to interact with the pump beam emitted from the pump device and to form a homogenized pump beam having a substantially rotationally symmetric energy intensity distribution when it exits the pump beam homogenizer through the second end.

10. The system according to claim 9, wherein the optical assembly of the pump beam homogenizer includes an optical fiber cable, The optical fiber cable is characterized in that it has a wavy (serpentine) section over at least a portion of its length.

11. The system according to claim 10, wherein the optical assembly further includes a member having a wavy surface pattern, and the member contacts and interacts with at least a portion of the optical fiber cable to impart a wavy shape to the optical fiber cable.

12. The system according to claim 11, wherein the optical assembly of the pump beam homogenizer includes one or more free-space optical elements, and the free-space optical elements interact with the pump beam to form a homogenized pump beam having a substantially rotationally symmetric energy intensity distribution when it exits from the second end of the beam homogenizer.

13. A system according to any one of claims 5 to 8, The measuring device includes a probe beam homogenizer optically connected to the probe device. The probe beam homogenizer is configured to receive the probe beam emitted from the probe device, The probe beam homogenizer is A first end optically connected to the probe device, the first end configured to receive the probe beam emitted from the probe device, A second end optically connected to the optical system, the second end configured to guide the homogenized probe beam toward the optical system, The invention is characterized by including an optical assembly positioned between the first and second ends, which interacts with the probe beam emitted from the probe device to form a homogenized probe beam.

14. A system according to claim 13, wherein the optical assembly of the probe beam homogenizer comprises a fiber optic cable, the fiber optic cable comprising a undulation located along at least a portion of the length of the fiber optic cable.

15. The system according to claim 14, wherein the optical assembly further comprises at least one component having a wavy surface pattern, the component being arranged to receive light from at least a portion of the optical fiber cable and to interact with it to induce a wavy profile in the shape of the optical fiber cable.

16. A system according to claim 13, wherein the optical assembly of the probe beam homogenizer comprises one or more free-space optical devices arranged to interact with the probe beam to form a homogenized probe beam having a substantially rotationally symmetric energy intensity distribution when emitted from the probe beam homogenizer through the second end.

17. A system according to any one of claims 13 to 16, The optical system includes multiple free-space optical elements, The plurality of free-space optical elements are configured to receive, shape, and guide pump beams and / or probe beams from a pump beam homogenizer and / or probe beam homogenizer toward a sample.

18. The system according to claim 17, wherein the system further includes an objective barrel, The objective tube is configured to receive a pump beam and / or probe beam from the optical system and guide the beam toward the sample.

19. The system according to claim 17 or claim 18, wherein the optical system is configured to guide a pump beam to a sample at a first angle of incidence and a probe beam at a second angle of incidence, The system is characterized in that the first angle of incidence and the second angle of incidence may be set to be the same, or the second angle of incidence may be set to be larger than the first angle of incidence.

20. A system according to any one of claims 5 to 19, wherein the measuring device further comprises a photodetector configured to receive a reflected probe beam and to detect the intensity of the reflected probe beam.

21. The system according to claim 20, wherein the measuring device further comprises a phase-sensitive detector electrically coupled to the photodetector and the signal generator, and configured to determine the phase delay of the intensity of the reflected probe beam relative to the intensity of the pump beam.

22. A self-contained frequency-domain thermal reflectance unit, A housing configured to selectively move between an operating position and a non-operating position, A system according to any one of claims 1 to 21, The system is housed within the aforementioned enclosure, In the non-operating position, the housing can be opened to expose at least a portion of the system in order to mount the sample on the holder, and In the non-operating position, the housing is a unit that completely encapsulates the system.

23. A method for performing frequency domain thermoreflectance measurement using the system described in any one of claims 1 to 21, This method - The process of receiving the sample, - In the process of generating a continuous wave laser beam, the laser beam is called a pump beam. The process includes modulating the intensity of the pump beam over time, homogenizing and shaping it, and guiding the pump beam toward the sample. Here, the pump beam, upon impact with the sample, has a spot diameter and intensity vibration sufficient to induce measurable temperature vibrations that reach a desired depth inside the sample. Furthermore, the process includes generating a continuous wave laser beam, the laser beam being called a probe beam, A process to monitor the thermal response of a sample by irradiating it with a probe beam and detecting the temporal fluctuations in the intensity of the reflected probe beam. The method includes a step of measuring the phase lag of the intensity vibration of the probe beam relative to the intensity vibration of the pump beam.

24. The method according to claim 23, comprising: generating different pump beams having different temporal intensity oscillations by changing the temporal frequency of a modulated signal within a predetermined range, thereby inducing different temperature oscillations within the sample; monitoring the thermal response of the sample by detecting the intensity of the reflected probe beam; measuring the phase delay of the intensity of the reflected probe beam with respect to the intensity oscillations of the pump beam at a number of different modulation frequencies within the predetermined range; and deriving the thermal characteristics of the sample from the phase delays measured at each modulated signal frequency.

25. A computer system for determining the thermal properties of a thermally conductive and / or optically rough material, comprising at least one processor, wherein the processor is capable of receiving properties relating to a sample of the material, performing the method according to claim 24, receiving the thermal response of the sample including temporal intensity vibrations of a reflected probe beam, determining at least one thermal property of the sample with respect to intensity vibrations of a pump beam based on the phase delay of the intensity vibrations of the probe beam, and executing program instructions that perform a process of displaying the at least one thermal property on a display device for viewing by an operator.

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