Multipass palladium hydride optical cavity for hydrogen detection.

Abstract

According to the present disclosure, an apparatus for measuring the hydrogen concentration in a gas sample is provided. The apparatus of the present disclosure includes a multi-pass optical cavity 103 with optical elements, a laser device 101, a photodetector 108, and a signal processor 110. The optical elements include mirrors 104 and a transmissive window 105 that support multi-pass optical paths within the optical cavity. A laser beam 102 generated by the laser device enters the optical cavity, reflects multiple times off the mirrors, and propagates along the multi-pass optical path. The intensity of the laser beam exiting the optical cavity is measured by a photodetector, and the signal processor determines the hydrogen concentration in the gas sample within the optical cavity from the intensity. One or more optical elements of the optical cavity have a Pd-containing surface layer on their surfaces that functions as a hydrogen-sensitive element. The Pd-containing surface layer changes its optical properties when hydrogenated by hydrogen in the gas sample.

Description

[Technical Field]

[0001] The present invention relates generally to methods and apparatus for detecting hydrogen gas, and more particularly to detecting hydrogen gas using optical methods. [Background technology]

[0002] Hydrogen is one of the most important fuels in a fossil-free energy future. One of the biggest challenges in developing hydrogen-based infrastructure is the fact that hydrogen gas is both explosive and prone to leaks. To address this issue, there has been active research into hydrogen gas leak detection.

[0003] One active approach to hydrogen detection uses palladium hydride (PdHx) materials. When exposed to hydrogen, palladium absorbs hydrogen, causing changes in various physical properties, including its optical properties. The amount of change in palladium's optical properties depends on the concentration of hydrogen present. This makes it possible to measure hydrogen concentration, for example, by measuring the decrease in the intensity of light reflected from a palladium-coated mirror or transmitted through a palladium-coated tapered optical fiber. However, these existing optical methods are limited in their hydrogen detection sensitivity to the ppm level. Therefore, although efforts have been made to develop improved palladium materials, greater detection sensitivity is still needed to detect trace levels of hydrogen. Summary of the Invention [Means for solving the problem]

[0004] The present invention provides a hydrogen sensor capable of detecting hydrogen at sub-ppb concentration levels. xA multi-pass optical cavity was used to dramatically improve the sensitivity of optical detection. A laser beam passes through free space within the cavity and is reflected many times. As the beam propagates along an optical path within the cavity, it interacts multiple times with palladium-containing coatings applied to reflective and / or transmissive optical elements within the cavity. The palladium-containing coated optical elements are exposed to gas in the free space within the cavity.

[0005] Multi-path optical cavities have been used in gas detectors based on laser absorption spectroscopy. In such devices, a laser beam interacts directly with the gas along its free-space path and repeatedly reflects between two mirrors. The mirrors serve only to increase the free-space path length and play no active role in gas detection. In contrast to the multi-path optical cavities in laser absorption spectroscopy sensors, the multi-path optical cavity of the present invention includes optical elements that play an active role in gas detection. Specifically, palladium material coated on the surfaces of the optical elements within the cavity optically interacts with the laser beam and actively participates in the detection of hydrogen gas within the cavity via palladium hydriding. Therefore, in this design of the hydrogen sensor of the present invention, it is not the free-space optical path length that enhances the sensor's sensitivity, but rather the number of interactions between the laser beam and the palladium-coated optical elements. Therefore, in the hydrogen sensor of the present invention, the mirrors can be placed closer together without reducing sensitivity, allowing for a compact design. The sensitivity of the hydrogen sensor of the present invention can be further improved by placing a palladium-coated optically transparent material within the cavity across the multi-pass path of the laser.

[0006] Thus, in one aspect, the present invention provides a laser-based hydrogen sensor, including: (a) a laser configured to generate a laser beam at an operating wavelength; (b) a multi-pass optical cavity comprising a reflective optical element with a Pd-containing coating, optionally further comprising a transmissive optical element with a Pd-containing coating; (c) optionally a sealed gas chamber for containing a gas sample, the sealed gas chamber having fittings to facilitate gas flow; (d) a laser beam detector configured to detect the laser beam passing through the cavity; and (e) a signal processor configured to quantify the hydrogen concentration in the gas sample within the cavity based on measurements at the laser beam detector. In some embodiments, the cell has porous walls configured to facilitate diffusion-based sampling and reduce particle entrainment. In some embodiments, the sealed gas chamber includes a particle filter or an interference gas reduction catalyst to prevent degradation of optical surfaces and reduce cross-species interference.

[0007] In another aspect, the present invention provides an apparatus for measuring the concentration of hydrogen in a gas sample, the apparatus comprising: a multi-pass optical cavity with an optical element, the optical element including mirrors supporting multi-pass optical paths within the multi-pass optical cavity; a laser device configured to generate a laser beam, the laser beam entering the multi-pass optical cavity, reflecting multiple times off of the mirrors of the multi-pass optical cavity, and propagating along multi-pass optical paths within the multi-pass optical cavity; a detector configured to measure the intensity of the laser beam exiting the multi-pass optical cavity; and a signal processor configured to determine the concentration of hydrogen in the gas sample within the multi-pass optical cavity from the intensity of the laser beam measured by the detector, wherein a surface of the optical element of the multi-pass optical cavity is provided with a Pd-containing surface layer containing palladium (Pd), and hydrogenation of the Pd-containing surface layer by hydrogen in the gas sample changes the optical properties of the Pd-containing surface layer, thereby enabling optical detection of hydrogen.

[0008] The Pd-containing surface layer may contain a Pd alloy with Au, Co, Ta, Ti, or Hf. The Pd-containing surface layer may include nanoparticles or a polymer coating. The Pd-containing surface layer may have a thickness of 30 nm or 20 nm. The Pd-containing surface layer may have a thickness ranging from 5 nm to 200 nm. The Pd-containing surface layer may have a 160 nm layer of a Pd alloy consisting of 67% Pd and 33% Co or Au.

[0009] The Pd-containing surface layer may be provided on the surface of a mirror. The Pd-containing surface layer may also be provided on the surface of a transmission window (optical element) placed between the mirrors across the multi-pass optical path. The transmission window may be, for example, a contrast enhancement slide. The transmission window may have a 2 nm thick layer of Pd or a Pd alloy. The transmission window may have a Pd x Co 100-x(x is in the range of 50 to 100). The composition of the Pd alloy may have a graded structure to improve responsiveness and faster response times. The transmission window may be a semi-transparent glass slide coated with a nanoparticle layer containing Pd or a Pd alloy with Au, Ti, Co, Ta, Hf, or W. The transmission window may have nanoparticles or nanostructures with diameters in the range of 50 nm to 1000 nm, or 200 nm to 500 nm. The transmission window may have an anti-reflection coating. The transmission window may have a polymer coating of PMMA and / or PTFE. The transmission window may have a polymer coating less than 100 nm thick.

[0010] The mirrors may be configured so that the laser beam reflects off the mirrors of the multi-pass optical cavity at least 5 times, preferably at least 10 times, and more preferably at least 66 times.

[0011] The multi-pass optical cavity may be housed in a cell with porous walls configured to facilitate diffusion-based sampling and reduce particle entrainment, and may be housed in a sealed gas chamber that may have a particle filter or interference gas reduction catalyst to prevent degradation of optical surfaces and reduce cross-species interference.

[0012] The laser beam may have a wavelength in the range of 400 to 1000 nm. The laser beam may have a wavelength in the range of 1000 to 8000 nm. The laser beam may have a wavelength of 5.051 μm. The laser beam may have a wavelength of 1300 nm or 1550 nm. The laser device may be a mid-infrared laser device. The laser device may be a diode laser device, a quantum cascade laser device, or an interband cascade laser device. [Brief explanation of the drawings]

[0013] [Figure 1A] FIG. 1A is a schematic diagram illustrating a hydrogen detection system according to one embodiment of the present invention. [Figure 1B]FIG. 1B is a cross-sectional view of a multi-pass palladium hydride optical cavity for use in a hydrogen sensor, according to one embodiment of the present invention. [Figure 2] FIG. 2 is a graph showing the transmittance and reflectance of a Pd—Au alloy versus wavelength in the presence and absence of 4% hydrogen. [Figure 3] FIG. 3 is a cross-sectional view of a multi-pass palladium hydrogen optical cavity with multiple transmissive windows with Pd-containing coatings, according to one embodiment of the present invention. [Figure 4] FIG. 4 shows a perspective view of a mirror and cross-sectional views of two variations of a Pd-containing coating on the surface of the mirror, according to one embodiment of the present invention. [Figure 5] FIG. 5 is a perspective view of a multi-pass palladium hydride optical cavity with two mirrors, showing the multi-pass propagation path between the mirrors, according to one embodiment of the present invention. [Figure 6] FIG. 6 shows a perspective view of a multi-pass palladium hydride optical cavity with two transmissive windows with Pd-containing coatings, according to one embodiment of the present invention. [Figure 7] Figure 7(A) is a graph showing the reflectance of Pd versus wavelength, demonstrating that the reflectance of Pd increases significantly in the infrared region. Figure 7(B) is a graph showing the absorbance versus wavelength for various thin film compatible polymers. Figure 7(C) is a graph showing the absorbance versus wavelength showing simulated infrared spectra of H2O and CO2. [Figure 8] FIG. 8 is a graph of water absorbance versus wavelength showing selected operating wavelengths for hydrogen detection to avoid water interference. DETAILED DESCRIPTION OF THE INVENTION

[0014] 1A shows a hydrogen detection system according to one embodiment of the present disclosure. The hydrogen detection system of the present disclosure includes a laser device 101 that emits a beam 102. The beam 102 enters an optical cavity 103 and exits the optical cavity 103 after being repeatedly reflected between mirrors 104a and 104b that have a special palladium (Pd) coating. The intensity of the beam 102 exiting the optical cavity 103 is measured by a photodetector 108. A sample gas enters the optical cavity 103 through a gas inlet 106, interacts with the Pd coatings on the surfaces of various reflective optical elements 104a and 104b, and transmissive optical elements 105a, 105b, and 105c, and then exits the optical cavity 103 through a gas outlet 107.

[0015] The hydrogen detection system of the present disclosure also includes a laser controller 109 that controls the operation of the laser device 101. In addition to controlling the laser device 101, the laser controller 109 can control the external temperature of the optical cavity 103 via a resistive heater or a thermoelectric cooler. Because the absorption and desorption rates of hydrogen are significantly faster at high temperatures, actively controlling the temperature of the optical cavity 103 can further improve the time response of the hydrogen sensor of the present disclosure (the hydrogen detection system of the present disclosure).

[0016] The hydrogen detection system of the present disclosure also includes a signal processor 110 that determines the hydrogen concentration in the sample gas from the beam intensity measured by the photodetector 108. A sampling system, such as a pump, may be provided at the gas outlet 107. A compressor may also be provided at the gas inlet 106. The sampling system may be replaced by an open-path system that operates the hydrogen sensor at ambient pressure.

[0017] FIG. 1B shows a cross-sectional view of a multi-pass palladium hydrogen optical cavity (MHPOC) according to one embodiment of the present invention. The optical cavity (MHPOC) includes two or more mirrors 151a, 151b. The mirrors 151a, 151b are oriented so that a laser beam is reflected multiple times between the reflective surfaces 152a, 152b of each mirror. Each mirror's reflective surface is composed of a thin film containing Pd or a Pd alloy with Au, Co, Ta, Ti, Hf, or W. The composition of the Pd alloy may be graded to increase sensitivity without sacrificing response time. Additionally, each mirror's reflective surface may contain nanoparticles and polymer coatings, as described below. A laser beam 153 enters the optical cavity through an entrance 157 and repeatedly reflects between the reflective surfaces 152a, 152b of each mirror within the free-space region 156 between the mirrors. An attenuated beam 154 then exits the optical cavity through an exit 158. The combination of these components forms one form of optical cavity, designated 155. The optical cavity may be housed in an airtight sampling chamber (sealed gas chamber) to allow for controlled flow of the gas sample, or the optical cavity may remain open path if ambient conditions are suitable to provide sufficient sensitivity.

[0018] The fundamental principle behind MHPOC is the fact that a coating layer composed of a palladium (Pd)-containing material changes in reflectance and transmittance when exposed to various concentrations of hydrogen. This dependence is illustrated in Figure 2, which shows the reflectance and transmittance of a Pd-Au alloy in the 400-1000 nm wavelength range, in the presence and absence of 4% hydrogen (H) in air. The reflectance and transmittance also depend on the thickness, structure, and composition of the coating layer. The reflectance and transmittance data shown in Figure 2 are for coating layer thicknesses of 30 nm and 20 nm. As shown, in the presence of hydrogen, the reflectance decreases and the transmittance increases.

[0019] The hydrogen contrast (HC) in this embodiment is obtained from the following formula (1).

[0020]

number

[0021] where ΔI is the change in light intensity in the beam 154 exiting the optical cavity outlet 158 ​​due to the presence of hydrogen (H), I is the light intensity of the beam 154 exiting the optical cavity outlet 158 ​​in the absence of hydrogen (H), R is the reflectivity of the mirror in the absence of hydrogen (the reflectivity of the two mirrors was assumed to be uniform), and R H is the modified reflectivity of the mirror in the presence of hydrogen (which is a function of the hydrogen concentration), and n is the total number of reflections of the light beam between the two mirrors.

[0022] Another embodiment of the MHPOC is shown in Figure 3. In this embodiment, to increase the sensitivity of the hydrogen sensor to trace amounts of hydrogen, multiple transmission windows 302a, 302b, 302c, and 302d with Pd-containing coatings are provided in the optical path. These transmission windows consist of a substrate made of fused silica or other optically transparent material and a Pd-containing coating 303 applied to one or both sides of the substrate. The mirror coating 301 can be similar to that of the previous embodiment. T In this embodiment in which the transmission windows are provided, the hydrogen contrast (HC) can be expressed by the following formula (2):

[0023]

number

[0024] In the above equation, T is the transmittance of the transmission windows 302a, 302b, 302c, 302d in the absence of hydrogen (assuming all transmission windows have the same coating), and T His the transmittance of the transmission windows 302a, 302b, 302c, and 302d in the presence of hydrogen. Notably, the transmission windows have two surfaces coated with a Pd-containing material, which significantly increases the sensitivity of the hydrogen sensor with fewer passes. However, this comes at the expense of transmission losses through the transmission window substrate, which are typically higher than the reflection losses. This can be compensated for by using a higher-power laser, a more sensitive photodetector, or by applying an anti-reflection (AR) coating.

[0025] In a variation of this embodiment, the mirror coating 301 may be a Pd-free, high-reflectivity mirror coating, in which case the transmission window provides all of the hydrogen signal contrast. In this variation, where a non-hydrogenated, high-reflectivity reflective material is used as the mirror coating, the hydrogen contrast equation simplifies to Equation (3) below:

[0026]

number

[0027] The coating structure is shown in Figure 4. The mirror substrate 401 can be made of fused silica or any other suitable material. The mirror substrate 401, which is an optically transmissive substrate, is optically transparent to the selected wavelength of the laser. The aperture 403 can be a physical hole or an optically transparent region of the mirror. The reflective and transmissive coating 402 can be a thin film and / or nanoparticle layer containing Pd or a Pd alloy. A thin-film-only coating structure is relatively simple and inexpensive. Such a reflective coating is formed by depositing multiple material layers (404, 405, 407) on a substrate 406, such as fused silica. Layer 405 is a thin adhesive layer, such as Ti or Ni, for adhering other layers onto it. Layer 405 is not required for all material combinations. Layer 404 is a layer of Pd or a Pd alloy (Pd alloyed with Au, Co, Ta, Hf, Cu, etc.). Layer 404 serves as the primary hydrogen-sensing element of the MHPOC. Layer 407 is a top layer made of a polymer such as polymethyl methacrylate (PMMA). This layer 407 allows H diffusion while reducing cross-species sensitivity and poisoning of layer 404. Another coating structure is nanoparticle layer 408. This coating structure is preferred for transmission windows because it is more responsive in both reflection and transmission. The nanoparticle layer can be composed of nanopatch-like particle arrays. This layer significantly enhances the hydrogen sensitivity of the MHPOC. This layer is typically covered by a very thin Pd alloy nanoparticle layer 409, with a total thickness in the range of approximately 2-20 nm, forming the primary hydrogen-sensing element. This layer is further covered by a specific layer, typically a polymer layer 412 such as PMMA, which reduces cross-species interference and poisoning. Covering the Pd alloy layer with a 30 nm TAF (Teflon AF2400) layer significantly improved the response time of the hydrogen sensor.

[0028] Figure 5 shows the path of a light beam within an MHPOC according to one embodiment of the present invention. In this embodiment, the MHPOC includes two mirrors 501, 502 facing each other. A laser beam enters the MHPOC through a physical aperture 503. Once inside the MHPOC, the beam interacts with the Pd-containing reflective coating at multiple spots, including three spots 504a, 504b, and 504c, and undergoes 66 reflections within the cavity. The beam then exits the MHPOC along path 505. The hydrogen contrast in this embodiment is given by n=66 in equation (1) above, i.e., equation (4) below:

[0029]

number

[0030] Figure 6 shows another embodiment in which two transmission windows 601a, 601b are placed in the optical cavity shown in Figure 5. Both sides 602 and 603 of the transmission window 601a are coated with nanoparticles to enhance the sensitivity of MHPOC. The hydrogen contrast is calculated by the equation (2) above, where n=66 and n T =2, which is expressed by the following equation (5).

[0031]

number

[0032] To calibrate the system, the optical cell is periodically partially emptied to measure a baseline intensity, I, in the absence of hydrogen. This baseline intensity, I, is then compared to the intensity, I, in the presence of the gas sample. H By comparing with ΔI=II HIt can be obtained. The ratio ΔI / I provides the hydrogen partial pressure. This method takes more time than creating contrast modulation by other methods, but it is very effective. Since the hydrogen absorption amount depends on temperature, this pressure effect can also be replaced by a thermal effect. When the temperature changes, the amount of hydrogen trapped within the palladium lattice changes significantly. Therefore, when the thermal effect does not cause significant optical distortion, adjusting the temperature of the medium results in an intensity change only when hydrogen is present. The temperature of the glass slide can be changed faster than the pressure within the MHPOC. However, for the thermal effect, periodic calibration may be necessary.

[0033] One of the important considerations in an embodiment with a transmissive glass slide within an optical cavity is the fact that the total output of the laser beam decreases with each pass. For example, an uncoated BK-7 glass slide transmits only 92% of the incident light at 633 nm. When there are 66 reflections and one transmissive glass slide, the output light is 0.4% compared to not using the transmissive glass slide. The remaining light is reflected within the cavity and becomes stray light, which can cause fringe noise and an etalon effect. To achieve an ultra-low detection limit (LOD), it is necessary to satisfy the condition ΔT << T0. Such a condition is achieved when T0 is close to 1 and ΔT is small. The absorption rates of the PS monolayer and the polymer coating are negligibly small, but the absorption rate of the glass substrate is usually large because the refractive indices of the glass substrate and air are different. To reduce such losses, in a preferred embodiment, a thin anti-reflection layer (e.g., TiO2 / SiO2-TiO2) is provided on the transmissive glass slide. As a result, the transmittance of the glass substrate at a specific wavelength reaches 99%. A Pd coating with a thickness of several nm x Co 100-x By providing a coating, ΔT can be reduced. When a thin Pd layer with a thickness of 2 nm was provided, the transmittance decreased by 7% at a wavelength of 1000 nm. Also, Pd x Co 100-xA thin coating of ensures that T0 is close to 1 and the response of the hydrogen sensor is fast. This can also be achieved by applying a nanotextured surface by dry / wet etching or molding processes.

[0034] Next, nanoparticle-containing coatings are described in more detail. In one embodiment, nanoparticle lithography (NL) and glancing angle co-evaporation (GLACD) techniques are used in combination to deposit Pd nanoparticles onto polystyrene (PS) nanoparticle arrays with diameters D = 500 nm and 200 nm. x Co 100-x It is possible to fabricate nanopatch (NPD) arrays of 10 -7 The evaporation rates of Pd and Co can be independently controlled by two electron beam sources at 1000 rpm. Nanostructures of various shapes and sizes can be obtained by varying the angle of incidence θ and the azimuthal angle φ. The formation and composition of the alloys were experimentally confirmed by elemental mapping using energy dispersive X-ray spectroscopy (EDS). The evaporation angle of incidence θ was set at 50° to avoid film deposition on the glass substrate. The hysteresis-free response ensures that the accuracy of the hydrogen sensor is improved by more than 3%. In general, Pd 80 Co 20 Under the same conditions, Pd 80 Ag 20 and Pd 80 Au 20 It has a faster response time (in the sub-second range) than Pd, but is less sensitive. 67 Co 33 NP200 has improved resistance to humidity. In a preferred embodiment, a hydrogen sensor with low limit of detection (LOD) at ppb level and fast response time using an array of nanopatches is coated with Pd x M 100-xThe structure is fabricated using an alloy (M = Ag or Co). The use of an array of PS nanoparticles as a substrate has several advantages. (1) Such structures are very useful for optical transmission readout because the thickness, shape, and morphology of the structure can be controlled by the size of the PS beads, the incident angle θ (see Figure 4), and the deposition thickness, as shown in Figure 4. (2) Due to the fast diffusion of hydrogen within PS, hydrogen can interact with the sensing composite through the substrate sites. Therefore, this structure nearly doubles the surface-to-volume ratio (SVR) compared to deposition on glass or Si substrates, accelerating the H sorption / desorption process. The unique design of such nanopatch-type hydrogen sensors allows for very high surface coverage (>90%), which allows for very low V H2 Even at low temperatures, a significant optical contrast can be obtained. The loose coupling between the curvature of the patch and the PS beads mitigates substrate effects, allowing for a fast response time to the expansion and contraction of the lattice and better mechanical stability.

[0035] To obtain a faster, more sensitive, and more selective hydrogen sensor, it is preferable to provide a polymer coating, such as PMMA and / or PTFE, on the nanostructure. The PMMA layer is known to filter gases harmful to hydrides and is used as the outer layer. The PTFE layer also modulates the surface energy of the metal alloy, resulting in faster hydrogen absorption kinetics. In principle, layers of PMMA and PTFE coatings less than 0.1 μm thick should not significantly affect the response time. The PMMA layer can be deposited by spin-coating techniques, and the PTFE layer can be deposited in 10 μm thickness. -7 It can be deposited in an evaporation system with a base pressure of 1000 torr.

[0036] Next, we discuss the selection of the operating wavelength. The reflectivity of palladium increases steadily with increasing wavelength. Also, the absorptivity of the polymer layer used to enhance hydrogen selectivity increases with increasing wavelength. Balancing these tradeoffs allows for the selection of a preferred operating wavelength.

[0037] The total optical output at the cavity exit depends exponentially on the mirror reflectivity. The reflectivity of Pd increases significantly in the infrared region, as predicted by the Lorentz-Drude model (Figure 7A), improving the total optical intensity at the cavity exit. We used the wavelength dependence of the reflectivity of various coatings from the mid- to far-infrared. For example, for the 66-reflection Zephyr cavity shown in Figure 5, a reflectivity of 93.26% results in only 1% of the laser intensity at the output. This threshold, indicated by the dotted line labeled >1% signal range in Figure 7A, occurs around 3.42 μm. Applying a polymer coating to the mirror can significantly enhance its selectivity for hydrogen absorption. Figure 7B shows the calculated absorbance of various thin-film compatible polymers for various coating thicknesses achieved so far. As can be seen, the optical attenuation in the polymer layer follows a similar trend to the Pd reflectivity at each wavelength. This implies that a trade-off must be managed between the increased reflectivity and the optical attenuation provided by the selectivity-enhancing polymer coating. Furthermore, the selectivity of hydrogen sensors also depends on the selectivity of the laser path to optical absorption due to direct absorption by atmospheric gases. Figure 7C shows simulated infrared spectra of H2O and CO2 in the wavelength range of 0.25–12 μm using the HITRAN database. Competition with other atmospheric gases like these significantly narrows the wavelength selection options. Additionally, other molecules, such as various hydrocarbons, carbon monoxide, and nitrogen oxides, may also interfere. All these factors lead to the conclusion that a wavelength of 5.051 μm is preferable for this material implementation, as shown in Figure 8. The calculated reflectivity of palladium (Pd) at this wavelength is 0.973, and the absorbance of a 20 nm PMMA overlayer is 1.8 × 10 -4 In a cavity with 66 reflections, a significant amount of light output of 16% is expected at the cavity exit. Preferably, mid-infrared lasers (quantum cascade lasers and interband cascade lasers) close to that wavelength range are used. PdH xThe infrared reflectance properties of the mirror can be further measured using a Fourier transform infrared spectrometer (FTIR). In summary, the reflectance component can benefit from choosing an operating wavelength above 3.4 μm for pure Pd mirrors. However, this can change, for example, for alloys of Pd and Au.

[0038] The transmission of light through ultrathin layers of Pd is much more forgiving with regard to wavelength selection. Therefore, wavelength selection depends primarily on the wavelength sensitivity of various layer thicknesses of the Pd alloy. Mass-produced visible or near-infrared diode lasers for communications (600-1800 nm) can be used for transmission applications.

Claims

1. 1. An apparatus for measuring hydrogen concentration in a gas sample, comprising: a multi-pass optical cavity comprising an optical element, the optical element including a mirror that supports multi-pass optical paths within the multi-pass optical cavity; a laser device configured to generate a laser beam, the laser beam entering the multi-path optical cavity, reflecting multiple times off the mirrors of the multi-path optical cavity, and propagating along the multi-path optical path within the multi-path optical cavity; a detector configured to measure the intensity of the laser beam exiting the multi-pass optical cavity; a signal processor configured to determine a concentration of hydrogen in the gas sample within the multi-pass optical cavity from the intensity of the laser beam measured by the detector; a Pd-containing surface layer containing palladium (Pd) is provided on a surface of the optical element of the multi-pass optical cavity; The device wherein the Pd-containing surface layer is hydrogenated by the hydrogen in the gas sample, thereby changing the optical properties of the Pd-containing surface layer, thereby enabling optical detection of hydrogen.

2. 10. The apparatus of claim 1, The Pd-containing surface layer is provided on a surface of the mirror.

3. 10. The apparatus of claim 1, The apparatus, wherein the laser beam is reflected at least 10 times off the mirrors of the multi-pass optical cavity.

4. 10. The apparatus of claim 1, The apparatus, wherein the optical element includes a transmissive window disposed between the mirrors across the multi-path optical path.

5. 10. The apparatus of claim 1, the optical element includes a transmissive window; The Pd-containing surface layer is provided on at least one surface of the transmission window.

6. 10. The apparatus of claim 1, An apparatus wherein the multi-pass optical cavity is housed within a cell having porous walls configured to facilitate diffusion-based sampling and reduce particle entrainment.

7. 10. The apparatus of claim 1, The apparatus wherein the multi-pass optical cavity is contained within a sealed gas chamber.

8. 8. The apparatus of claim 7, The apparatus, wherein the sealed gas chamber has a particle filter or an interference gas reduction catalyst to prevent degradation of optical surfaces and reduce cross-species interference.

9. 10. The apparatus of claim 1, The device, wherein the Pd-containing surface layer comprises a Pd alloy with Au, Co, Ta, Ti, or Hf.

10. 10. The apparatus of claim 1, The device, wherein the Pd-containing surface layer comprises a nanoparticle coating or a polymer coating.

11. 10. The apparatus of claim 1, The apparatus wherein the laser beam has a wavelength in the range of 400 to 1000 nm.

12. 10. The apparatus of claim 1, The device, wherein the Pd-containing surface layer has a thickness of 30 nm or 20 nm.

13. 10. The apparatus of claim 1, The apparatus wherein the laser beam has a wavelength in the range of 1000 to 8000 nm.

14. 10. The apparatus of claim 1, The device, wherein the Pd-containing surface layer has a thickness in the range of 5 nm to 200 nm.

15. 10. The apparatus of claim 1, The device, wherein the Pd-containing surface layer comprises a Pd alloy with Co or Au.

16. 5. The apparatus of claim 4, The apparatus, wherein the transparent window is a contrast enhancing slide.

17. 10. The apparatus of claim 1, The apparatus, wherein the laser beam is reflected at least five times off the mirrors of the multi-pass optical cavity.

18. 10. The apparatus of claim 1, The apparatus, wherein the laser beam is reflected 66 times off the mirrors of the multi-pass optical cavity.

19. 10. The apparatus of claim 1, The apparatus wherein the laser beam has a wavelength of 5.051 μm.

20. 10. The apparatus of claim 1, The laser device is a mid-infrared laser device.

21. 10. The apparatus of claim 1, The laser device is a diode laser device, a quantum cascade laser device, or an interband cascade laser device.

22. 10. The apparatus of claim 1, The device, wherein the Pd-containing surface layer comprises a 160 nm thick layer containing a Pd alloy with Co or Au.

23. 5. The apparatus of claim 4, The apparatus wherein the transmission window is a semi-transparent glass slide coated with a layer of nanoparticles containing Pd or Pd alloys with Au, Ti, Co, Ta, Hf, or W.

24. 5. The apparatus of claim 4, The apparatus wherein the transmission window has an anti-reflective coating.

25. 5. The apparatus of claim 4, The transmission window is made of Pd ｘ Co １００－ｘ The device has a coating, and x is in the range of 50 to 100.

26. 5. The apparatus of claim 4, The device, wherein the transmission window comprises a 2 nm thick layer containing Pd or an alloy thereof.

27. 5. The apparatus of claim 4, The apparatus, wherein the transmission window comprises nanoparticles or nanostructures having a diameter in the range of 50 nm to 1000 nm.

28. 5. The apparatus of claim 4, The apparatus wherein the transmission window has a polymer coating of PMMA and / or PTFE.

29. 5. The apparatus of claim 4, The apparatus wherein the transmission window has a polymer coating with a thickness of less than 100 nm.

30. 10. The apparatus of claim 1, The apparatus, wherein the laser beam has a wavelength of 1300 nm or 1550 nm.

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