Optical module

Abstract

An optical module equipped with a high-performance optical filter is provided. [Solution] The optical module comprises a plurality of optical filters and an infrared light receiving element, the plurality of optical filters are installed in an optical path formed by infrared radiation emitted from a light source, and have a common transmission wavelength range within the wavelength range to which the infrared light receiving element is sensitive, the maximum transmittance in the common transmission wavelength range is 20% or more of the maximum transmittance of the first optical filter and the second optical filter respectively and less than the maximum transmittance of each, and the full width at half maximum of the first optical filter is less than the full width at half maximum of the second optical filter.

Description

[Technical Field]

[0001] This disclosure relates to an optical module. [Background technology]

[0002] Conventionally, non-dispersive infrared absorption (NDIR) gas concentration measuring devices are known as gas concentration measuring devices for measuring the concentration of target gases in the atmosphere. Non-dispersive infrared absorption gas concentration measuring devices utilize the fact that different types of gases absorb different wavelengths of infrared light, and measure the gas concentration by detecting the amount of absorption. For example, a non-dispersive infrared absorption gas concentration measuring device is composed of an infrared optical element and an optical filter that transmits infrared light of a specific wavelength depending on the target gas. For example, Patent Document 1 discloses a gas sensor equipped with multiple optical filters. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] U.S. Patent No. 11499914 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In gas sensors, a characteristic of optical filters is that if the slope on the cutoff or cuton side is gentle, the filter becomes susceptible to interference from non-detectable gases, potentially reducing the accuracy of gas concentration measurements. Therefore, high-performance optical filters with steep slopes are required.

[0005] In view of these circumstances, the purpose of this disclosure is to provide an optical module equipped with a high-performance optical filter. Here, the optical module comprises an infrared optical element and an optical filter, and is a module used in, for example, a concentration measuring device, an infrared radiation thermometer (non-contact thermometer), an infrared spectroscopic imaging device, a human body detection sensor, etc. The optical module is an optical component in which the infrared optical element and the optical filter are arranged while maintaining the positional relationship between them in order to obtain desired characteristics, and which is then packaged, for example. [Means for solving the problem]

[0006] (1) An optical module according to one embodiment of the present disclosure is It comprises multiple optical filters and an infrared light receiving element, The aforementioned plurality of optical filters are installed in the optical path formed by infrared radiation emitted from the light source, The plurality of optical filters include at least a first optical filter installed to reflect infrared light in the optical path and a second optical filter installed to transmit infrared light in the optical path. The first optical filter and the second optical filter have a common transmission wavelength range within the wavelength range to which the infrared light receiving element is sensitive. The maximum transmittance in the common transmission wavelength range is 20% or more of the maximum transmittance of the first optical filter and the second optical filter, and less than the maximum transmittance of the first optical filter and the second optical filter, The full width at half maximum of the first optical filter is smaller than the full width at half maximum of the second optical filter.

[0007] (2) As one embodiment of the present disclosure, in (1), The wavelength corresponding to the maximum transmittance in the common transmission wavelength range is located on the cut-on side of the first optical filter and on the cut-off side of the second optical filter.

[0008] (3) In one embodiment of the present disclosure, in (1) or (2), The maximum transmittance in the aforementioned common transmission wavelength range is 90% or less.

[0009] (4) In one embodiment of the present disclosure, in any of (1) to (3), The second optical filter has a transmission wavelength range that is shorter than that of the first optical filter. The wavelength corresponding to a 20% transmittance at the cutoff side of the second optical filter is smaller than the wavelength corresponding to the maximum transmittance of the first optical filter.

[0010] (5) In one embodiment of the present disclosure, in any of (1) to (4), The wavelength corresponding to a 20% transmittance on the cut-on side of the first optical filter is greater than the wavelength corresponding to the maximum transmittance of the second optical filter.

[0011] (6) In one embodiment of the present disclosure, in any of (1) to (5), The wavelength corresponding to a 20% transmittance on the cutoff side of the second optical filter is greater than the wavelength corresponding to a 20% transmittance on the cuton side of the first optical filter.

[0012] (7) In one embodiment of the present disclosure, in any of (1) to (6), The transmission spectrum of infrared light reflected by the first optical filter and transmitted through the second optical filter has a larger slope on the cutoff side than the transmission spectrum of infrared light transmitted through the second optical filter alone.

[0013] (8) In one embodiment of the present disclosure, in any of (1) to (7), In the optical path formed by infrared light reflected by the first optical filter and transmitted through the second optical filter, the ratio of the slope on the cut-on side of the first optical filter to the slope on the cut-off side of the second optical filter is 0.7 or greater.

[0014] (9) In one embodiment of the present disclosure, in any of (1) to (8), The wavelength corresponding to the maximum transmittance in the common transmission wavelength range is present on the cut-off side of the first optical filter and on the cut-on side of the second optical filter. The transmission spectrum of the infrared light reflected by the first optical filter and transmitted through the second optical filter has a steeper slope on the cut-on side than the transmission spectrum of the infrared light transmitted through the single second optical filter.

[0015] (10) As one embodiment of the present disclosure, in (9), In the optical path formed by the infrared light reflected by the first optical filter and transmitted through the second optical filter, the ratio of the slope on the cut-off side of the first optical filter to the slope on the cut-on side of the second optical filter is 0.7 or more.

[0016] (11) As one embodiment of the present disclosure, in any of (1) to (10), At least one of the plurality of optical filters has an area 1.4 times or more that of the other optical filters.

[0017] (12) As one embodiment of the present disclosure, in any of (1) to (11), At least one of the plurality of optical filters is a band-pass filter having a transmission band with a maximum transmittance of 60% or more in the wavelength range of 2000 nm to 10000 nm.

[0018] (13) As one embodiment of the present disclosure, in any of (1) to (12), At least one of the plurality of optical filters is a reflective filter.

[0019] (14) As one embodiment of the present disclosure, in any of (1) to (13), It is used in a NDIR type gas sensor or a photoacoustic type gas sensor.

[0020] (15) As one embodiment of the present disclosure, in any of (1) to (14), The infrared light receiving element is an infrared photodiode.

[0021] (16) An optical module according to one embodiment of the present disclosure is It comprises multiple optical filters and an infrared light receiving element, The aforementioned plurality of optical filters are installed in the optical path formed by infrared radiation emitted from the light source, The plurality of optical filters include at least a first optical filter installed to transmit infrared light in the optical path and a second optical filter installed to transmit infrared light in the optical path, The first optical filter is a bandstop filter, and its cutoff band includes the transmission wavelength range of the second optical filter. The transmission spectrum of infrared light that has passed through the first optical filter and then through the second optical filter has a larger slope on the cut-on or cut-off side than the transmission spectrum of infrared light that has passed through the second optical filter alone.

[0022] (17) In one embodiment of the present disclosure, in any of (1) to (16), The cut-on side refers to the wavelength shorter than the wavelength corresponding to the maximum transmittance. The slope of the cut-on side is calculated by dividing "0.9 - 0.1" by "the magnitude of the change in wavelength from 10% transmittance to 90% transmittance," when the maximum transmittance is normalized to 1.

[0023] (18) In one embodiment of the present disclosure, in any of (1) to (16), The cut-on side refers to the wavelength shorter than the wavelength corresponding to the maximum transmittance. The slope of the cut-on side is calculated by dividing "0.8 - 0.1" by "the magnitude of the change in wavelength from 10% transmittance to 80% transmittance," when the maximum transmittance is normalized to 1.

[0024] (19) In one embodiment of the present disclosure, in any of (1) to (18), The cutoff side refers to wavelengths longer than the wavelength corresponding to the maximum transmittance. The slope of the cutoff side is calculated by dividing "0.9 - 0.1" by "the magnitude of the change in wavelength from 90% transmittance to 10% transmittance," when the maximum transmittance is normalized to 1.

[0025] (20) In one embodiment of the present disclosure, in any of (1) to (18), The cutoff side refers to wavelengths longer than the wavelength corresponding to the maximum transmittance. The slope of the cutoff side is calculated by dividing "0.8 - 0.1" by "the magnitude of the change in wavelength from 80% transmittance to 10% transmittance," when the maximum transmittance is normalized to 1. [Effects of the Invention]

[0026] According to this disclosure, it is possible to provide an optical module equipped with a high-performance optical filter. [Brief explanation of the drawing]

[0027] [Figure 1] Figure 1 shows an example of a cross-section of an optical filter. [Figure 2] Figure 2 shows an example of a concentration measuring device equipped with an optical module according to one embodiment of the present disclosure. [Figure 3] Figure 3 is a diagram illustrating the configuration of optical filters and infrared optical elements in an optical module. [Figure 4] Figure 4 shows an example of the effective transmission characteristics of a combination of optical filters. [Figure 5] Figure 5 illustrates the change in effective slope size when a set of optical filters is used, with the individual slope sizes being varied and combined. [Figure 6] Figure 6 shows an example of a stacked structure for infrared optical elements. [Figure 7] Figure 7 shows the transmission characteristics when the transmission wavelength range of the first optical filter is shorter than the transmission wavelength range of the second optical filter. [Figure 8] Figure 8 shows that the slope becomes steeper as the full width at half maximum of the optical filter narrows. [Figure 9] Figure 9 shows the transmission characteristics when both the first and second optical filters are allowed to pass through, and the first optical filter is used as a bandstop filter. [Figure 10] Figure 10 shows the configuration when both the first and second optical filters are transmitted. [Modes for carrying out the invention]

[0028] An optical module according to one embodiment of this disclosure will be described below with reference to the drawings.

[0029] (Optical module) The optical module according to this embodiment comprises a plurality of optical filters and an infrared optical element. In this embodiment, each of the plurality of optical filters has a configuration comprising a substrate and a multilayer film having a plurality of layers with different refractive indices formed on at least one surface of the substrate. However, it is sufficient if at least one of the plurality of optical filters has a configuration comprising a substrate and a multilayer film. The infrared optical element is an infrared light receiving element or an infrared light emitting element, and is a collective name for these. Furthermore, in the following, light receiving and light emitting means having at least one of the functions of light receiving and light emitting. The infrared optical element is composed of, for example, a first conductivity type semiconductor layer, an active layer and a second conductivity type semiconductor layer, and performs infrared light receiving and light emitting. That is, the optical module according to this embodiment is an infrared module. For example, an infrared light emitting element is realized with the structure shown in Figure 6, and an infrared light receiving element is realized with the same structure. Specifically, a light-emitting diode (LED) is preferred as the infrared light emitting element. As another example, the infrared light emitting element may be a lamp, a laser (Light Amplification by Stimulated Emission of Radiation), an organic light emitting element, or a MEMS (Micro Electro Mechanical Systems) heater, etc. Furthermore, the infrared light receiving element is preferably a photodiode (PD). As another example, the infrared light receiving element may be a phototransistor, thermopile, pyroelectric sensor, bolometer, or photoacoustic detector.

[0030] Hereinafter, the optical module according to this embodiment will be described as being used in a concentration measuring device. As described above, the optical module is an optical component in which infrared optical elements and optical filters are arranged while maintaining the positional relationship between them in order to obtain desired characteristics, and then packaged, for example. Furthermore, the optical module is not limited to a concentration measuring device, but may be used in infrared radiation thermometers and the like.

[0031] In this embodiment, the concentration measuring device is a gas sensor that measures the concentration of the target gas. The concentration measuring device may be, for example, a non-dispersive infrared absorption (NDIR) gas sensor equipped with a light-receiving unit that receives infrared light transmitted through the gas. Alternatively, the concentration measuring device may be a photoacoustic gas sensor that measures the gas concentration by, for example, picking up the vibrations of gas molecules that have absorbed light as sound with a high-performance microphone. In other words, the optical module may be used in an NDIR gas sensor or a photoacoustic gas sensor.

[0032] As will be described in detail later, the multiple optical filters are installed in the optical path formed by infrared radiation emitted from a light source (infrared light-emitting element). The multiple optical filters include at least a first optical filter installed to reflect infrared radiation in the optical path and a second optical filter installed to transmit infrared radiation in the optical path. Furthermore, the first and second optical filters have a common transmission wavelength range within the wavelength range to which the infrared photodetector is sensitive. The maximum transmittance in the common transmission wavelength range is 20% or more of the maximum transmittance of the first and second optical filters respectively, and less than the maximum transmittance of the first and second optical filters respectively. Also, the full width at half maximum of the first optical filter is less than the full width at half maximum of the second optical filter. In another embodiment, the first and second optical filters can be used together in a transmission mode. In that case, the first optical filter can be a bandstop filter.

[0033] According to the optical module of this embodiment, by combining multiple optical filters, the slope of the combined transmission spectrum can be made larger than the slope of the transmission spectrum of a single optical filter. Here, "making the slope larger" means "making it sharper" or "making it steeper." When the optical module is used, for example, in a gas sensor, if the slope is gentle, it becomes susceptible to interference from interfering gases, which can reduce the accuracy of gas concentration measurement. The optical module of this embodiment has high-performance optical filters that, when used, for example, in a gas sensor, do not reduce the accuracy of gas concentration measurement.

[0034] Figure 1 shows an example of a cross-section of an optical filter. In this embodiment, the optical filter is formed by alternately laminating a layer made of a low refractive index material (L) and a layer made of a high refractive index material (H) on both sides of a Si substrate. The layer made of the low refractive index material (L) is made of silicon monoxide (SiO), silicon dioxide (SiO2), titanium dioxide (TiO2), zinc sulfide (ZnS), or aluminum oxide (Al2O3), etc. The layer made of the high refractive index material (H) is made of Si or Ge, etc. It is preferable that the low refractive index material (L) is selected to be a material with a refractive index of 1.2 to 2.5. It is also preferable that the high refractive index material (H) is selected to be a material with a refractive index that is 0.5 or more greater than that of the low refractive index material (L). The alternately laminated multilayer film is formed such that the layer directly provided on the Si substrate is the high refractive index material (H). However, the optical filter is not limited to the configuration in Figure 1, and for example, the high refractive index material (H) does not have to be directly provided on the substrate.

[0035] Figure 2 shows an example of a concentration measuring device using the optical module according to this embodiment. In the optical module according to this embodiment, an infrared light receiving element (IR) is placed in the optical path formed by infrared radiation emitted from an infrared light-emitting element (light source), and a plurality of optical filters that selectively transmit the absorption wavelength of the gas to be detected are placed in front of the infrared light receiving element. In other words, the plurality of optical filters are placed in the optical path formed by infrared radiation emitted from an infrared light-emitting element (light source).

[0036] Here, the gas measured by the concentration measuring device is, for example, carbon dioxide (CO2), but is not limited to this. For example, the gas measured could be water vapor, carbon monoxide, nitric oxide, ammonia, sulfur dioxide, alcohol, formaldehyde, methane, propane, etc.

[0037] Figure 3 is a diagram illustrating the configuration of optical filters and infrared optical elements in an optical module. The optical filter is placed in the optical path after the infrared optical element, which is a light-emitting element. The optical filter is also placed in the optical path before the infrared optical element, which is a light-receiving element. The optical filters are placed with a gap between them. Here, da is the distance between the infrared light-emitting element and the nearest optical filter. Also, db is the distance between the infrared light-receiving element and the nearest optical filter. Both da and db should be set to zero or greater. Furthermore, the optical module is not limited to the configuration shown in Figure 3, and may have other optical elements such as lenses or mirrors further arranged in the optical path.

[0038] The components of the optical module according to this embodiment are described below in detail. Here, the optical module comprises at least one infrared light receiving element and an infrared light-emitting element, but the light receiving sensitivity of the infrared light receiving element and the light emission intensity of the infrared light-emitting element are described as "sensitivity". That is, sensitivity can be read as light receiving sensitivity if the optical module is configured to include an infrared light receiving element, and can be read as light emission intensity if the optical module is configured to include an infrared light-emitting element.

[0039] (Optical filter) As described above, the optical filter comprises a substrate and a multilayer film formed on the substrate, having multiple layers with different refractive indices. The multilayer film may be formed on only one surface of the substrate or on both surfaces. In a concentration measuring device, the optical filter is placed in the optical path from an infrared light-emitting element to an infrared light-receiving element. The optical filter can be fabricated by depositing the first and second layers onto the substrate by a vapor deposition method.

[0040] One pair of optical filters among several optical filters has a common transmission wavelength range within the wavelength range to which the infrared optical element is sensitive. That is, the transmission wavelength ranges of the pair of optical filters overlap, and the overlapping portion (common transmission wavelength range) is within the wavelength range to which the infrared optical element is sensitive. Furthermore, the maximum transmittance in the common transmission wavelength range is 20% or more and is smaller than the maximum transmittance of each individual optical filter in the pair. When one of the pair of optical filters is used for reflection and the other for transmission, it is preferable to reduce the full width at half maximum of the optical filter used for reflection. When both of the pair of optical filters are used for transmission, it is preferable to make one of them a bandstop filter.

[0041] Here, Figure 4 shows an example of the effective transmission characteristics of a combination of optical filters. The combination of optical filters is described as consisting of a first optical filter and a second optical filter having a transmission wavelength range shorter than that of the first optical filter. As shown in the left diagram of Figure 4, the combination of optical filters is installed in the optical path, which is the path through which infrared light emitted from a light source (infrared light-emitting element) enters the infrared light-receiving element. In the example of Figure 4, the first optical filter is a reflective filter and the second optical filter is a transmissive filter. However, the types of optical filters, including the combination of optical filters, are not limited; for example, at least one of the optical filters may be a reflective filter. Alternatively, the configuration of Figure 7, consisting of a first optical filter and a second optical filter having a transmission wavelength range longer than that of the first optical filter, may be adopted. As shown in Figures 4 and 7, in both cases, it is preferable that the full width at half maximum of the first optical filter is smaller than the full width at half maximum of the second optical filter. As shown in Figure 8, when the full width at half maximum of an optical filter becomes narrower, the slope tends to become steeper. By making the first optical filter steeper, the effective transmission characteristics of the second optical filter can be made steeper.

[0042] Generally, the size of the reflective surface of a reflective filter is larger than the size of the light-receiving surface of a transmissive filter. The sizes of multiple optical filters are not limited to a specific size or range, but at least one (for example, a reflective filter) may have an area at least 1.4 times that of the other optical filters (for example, transmissive filters). Furthermore, the multiple optical filters do not need to be composed of filters of a specific type, and may be a combination of, for example, bandpass filters, short-pass filters, and long-pass filters. As one example configuration, at least one of the multiple optical filters may be a bandpass filter having a transmission band that provides a maximum transmittance of 60% or more in the wavelength range of 2000 nm to 10000 nm.

[0043] Referring again to Figure 4, the effective transmission characteristics of a combination of optical filters are explained. The right-hand figure in Figure 4 shows the transmission characteristics of the first optical filter alone, the transmission characteristics of the second optical filter alone, and the effective transmission characteristics of the second optical filter when combined. The vertical axis represents transmittance. On the vertical axis, 1 corresponds to 100%, 0.6 to 60%, and 0.2 to 20% transmittance. The horizontal axis represents the wavelength of light (infrared). The interpretation of Figure 7 is the same as in Figure 4.

[0044] Regarding the transmission characteristics of an optical filter, the cut-on slope and the cut-off slope are defined as follows. First, the cut-on side refers to the wavelength shorter than the wavelength corresponding to the maximum transmittance. The cut-off side refers to the wavelength longer than the wavelength corresponding to the maximum transmittance. The cut-on slope is calculated by dividing "0.9-0.1" by "the magnitude of the bandwidth (wavelength change) from 10% transmittance to 90% transmittance" when the maximum transmittance is normalized to 1. Alternatively, the cut-on slope may be calculated by dividing "0.8-0.1" by "the magnitude of the bandwidth (wavelength change) from 10% transmittance to 80% transmittance" when the maximum transmittance is normalized to 1. The cut-off slope is calculated by dividing "0.9-0.1" by "the magnitude of the bandwidth (wavelength change) from 90% transmittance to 10% transmittance" when the maximum transmittance is normalized to 1. Alternatively, the slope on the cutoff side may be calculated by dividing "0.8-0.1" by "the magnitude of the bandwidth (wavelength change) from 80% transmittance to 10% transmittance," when the maximum transmittance is normalized to 1. A larger slope value indicates a steeper change in the transmittance (transmission characteristics) of the optical filter. Conversely, a smaller slope value indicates a gentler change in the transmittance of the optical filter. In this embodiment, since the unit of bandwidth (wavelength change) is nm, the unit of slope is "% / nm." Here, when the peak shape is irregular, the transmittance at the center of the half-width may be normalized to 1, and the above calculation formula may be applied.

[0045] In the example in Figure 4, the cut-on slope of the first optical filter alone was 1.40% / nm, and the cut-off slope was 1.17% / nm. The cut-on and cut-off slopes of the second optical filter alone were gentler than those of the first optical filter alone, with the cut-on slope at 0.70% / nm and the cut-off slope at 0.67% / nm. In contrast, the effective transmission characteristic of the second optical filter as a combination was a cut-off slope of 1.27% / nm. Here, the cut-on slope of the second optical filter, which does not overlap in transmission wavelength ranges, remains at 0.70% / nm. In the example in Figure 7, the cut-on slope of the second optical filter is larger due to the effect of the first optical filter.

[0046] In the examples shown in Figures 4 and 7, a pair of optical filters have a common transmission wavelength range within the wavelength range to which the infrared optical element is sensitive. The maximum transmittance in the common transmission wavelength range is 20% or more (approximately 70%) and is smaller than the maximum transmittance of each individual optical filter in the pair (100% each). In other words, the peaks of the transmission characteristics of each optical filter in the pair do not completely overlap, but rather a common transmission wavelength range exists between the peaks of each peak (the point indicating maximum transmittance). In this case, the effective transmission characteristic (cutoff slope) of the second optical filter formed by the combination can be made steeper than the slope of each individual filter. The optical module according to this embodiment utilizes the property that the effective slope can be made steeper by combining a pair of optical filters to realize an optical module equipped with high-performance optical filters.

[0047] Figure 5 illustrates the change in effective slope when a pair of optical filters are combined while varying the individual slopes. The vertical axis represents the effective cutoff slope value of the second optical filter. The horizontal axis represents the maximum transmittance (transmittance at the intersection) in the common transmission wavelength range. Each characteristic curve is shown as "(cutoff slope of the second optical filter)_(cut-on slope of the first optical filter)". In the example in Figure 5, the cutoff slope of the second optical filter is fixed at 1% / nm. Here, a similar trend was observed when the vertical axis was changed to the cut-on slope of the first optical filter, the cut-on slope of the first optical filter was fixed, and the cutoff slope of the second optical filter was varied. As shown in Figure 5, when the maximum transmittance in the common transmission wavelength range (transmittance at the intersection) is 20% or more, the effective slope can be made larger (steeper) than the cutoff slope of the second optical filter alone. However, it was confirmed that the effective slope does not increase if the cuton slope of the first optical filter is made extremely gentle. Also, the maximum transmittance in the common transmission wavelength range may be 90% or less. This is because, as shown in Figure 5, no significant increase is observed even if it is greater than 90%.

[0048] Furthermore, with respect to the superposition of a pair of optical filters, the wavelength corresponding to 20% transmittance on the cutoff side of the second optical filter may be smaller than the wavelength corresponding to the maximum transmittance of the first optical filter. Also, the wavelength corresponding to 20% transmittance on the cut-on side of the first optical filter may be larger than the wavelength corresponding to the maximum transmittance of the second optical filter (see Figure 4). Also, the wavelength corresponding to 20% transmittance on the cutoff side of the second optical filter may be larger than the wavelength corresponding to 20% transmittance on the cut-on side of the first optical filter (see Figure 4).

[0049] Furthermore, the transmission spectrum of infrared light reflected by the first optical filter and transmitted through the second optical filter can have a larger slope on the cutoff side than the transmission spectrum of infrared light transmitted through the second optical filter alone. In the optical path formed by infrared light reflected by the first optical filter and transmitted through the second optical filter, the ratio of the cut-on slope of the first optical filter to the cut-off slope of the second optical filter may be 0.7 or greater. In other words, the calculated value of "(cut-on slope of the first optical filter) / (cut-off slope of the second optical filter)" may be 0.7 or greater. For example, in Figure 5, "1%_1.52%", "1%_2%", and "1%_2.5%" satisfy the above ratio condition, and the effective slope is larger (steeper) compared to other combinations.

[0050] As mentioned above, the effect of making the effective slope steeper is more pronounced the steeper the slope of the first optical filter. Also, as shown in Figure 8, the slope of an optical filter tends to be steeper when the half-width is small. Therefore, by making the half-width of the first optical filter smaller than that of the second optical filter, the effect of making the effective slope steeper through the combination becomes greater.

[0051] Furthermore, there may be a configuration in which the transmission wavelength range of the first optical filter is located on the shorter wavelength side than the transmission wavelength range of the second optical filter. In this case, the transmission spectrum of infrared light reflected by the first optical filter and transmitted through the second optical filter can have a larger cut-on slope than the transmission spectrum of infrared light transmitted through the second optical filter alone. In the optical path formed by infrared light reflected by the first optical filter and transmitted through the second optical filter, the ratio of the cut-off slope of the first optical filter to the cut-on slope of the second optical filter may be 0.7 or greater. In other words, the calculated value of "(cut-off slope of the first optical filter) / (cut-on slope of the second optical filter)" may be 0.7 or greater.

[0052] Furthermore, as shown in Figure 10, both the first and second optical filters may be used in transmission mode. In this case, by making the first optical filter a bandstop filter, the effective transmission spectrum of the second optical filter can be made steeper. Figure 9 shows the transmission characteristics when both the first and second optical filters are used in transmission mode, and the first optical filter is a bandstop filter. It can be seen that the effective transmission spectrum on the cutoff side of the second optical filter is made steeper. Also, by designing the cutoff band of the first optical filter to be on the shorter wavelength side of the second optical filter, it is possible to make the cut-on side steeper.

[0053] Here, transmittance varies depending on the measurement conditions. Specifically, it varies depending on the temperature and the angle of incidence of light. In this embodiment, the temperature is assumed to be 25°C. The angle of incidence of light may be, for example, 0°, 10°, 20°, 30°, 40°, 45°, etc., depending on the design of the concentration measuring device, but it is sufficient that the above characteristics are satisfied at any of the angles of incidence. For example, the optical filter only needs to satisfy the above characteristics at at least one of the angles of incidence that can be set in the design (30° as an example). However, it is even more preferable that the optical filter satisfies the above characteristics at all of the angles of incidence that can be set in the design.

[0054] (substrate) The substrate can be any material suitable for forming each layer that makes up the multilayer film. Examples include, but are not limited to, silicon substrates, germanium substrates, sapphire substrates, or glass substrates.

[0055] (Multilayer film) A multilayer film is a film having multiple layers with different refractive indices. In this embodiment, the multilayer film includes a structure in which a first layer having a refractive index of 1.2 to 2.5 in the wavelength range of 6 μm to 10 μm and a second layer having a refractive index of 3.2 to 4.3 in the wavelength range of 6 μm to 10 μm are alternately stacked. The first layer is composed of the low refractive index material (L) described above. The second layer is composed of the high refractive index material (H) described above.

[0056] (1st layer) Specific materials for the first layer include titanium dioxide, zinc sulfide, silicon monoxide, and silicon dioxide.

[0057] (2nd layer) Specific materials for the second layer include silicon (Si) and germanium (Ge).

[0058] (Method for measuring refractive index) The refractive indices of the first and second layers can be measured using an ellipsometer in accordance with "JIS K7142".

[0059] Here, the materials and film thicknesses of each of the multiple stacked first layers may be the same or different. Similarly, the materials and film thicknesses of each of the multiple stacked second layers may be the same or different. The multilayer film may further include layers different from the first and second layers.

[0060] (Infrared optical element) The infrared optical element may have a configuration comprising a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. Specifically, the infrared optical element is an infrared light-emitting diode or an infrared photodiode.

[0061] The active layer is a light-absorbing layer or a light-emitting layer (see Figure 6). In this embodiment, the active layer is Al y In 1-y Sb(0≦y≦0.14) or InAs y S 1-y It is composed of (0≦y≦0.2). y In 1-y The expression "Sb(0≦y≦0.14)" means that the layer contains Al, In, and Sb, but it also includes the presence of other elements. Specifically, it includes cases where minor changes are made to the composition of this layer, such as adding small amounts of other elements (for example, elements such as As, P, Ga, and N in amounts of a few percent or less). This is also true for other compositional expressions.

[0062] Here, the Al composition or the As composition can be determined by, for example, the secondary ion mass spectrometry (SIMS) method. For measurement, a magnetic field type SIMS device IMS 7f manufactured by CAMECA can be used, for example.

[0063] The second conductivity type is a conductivity type different from the first conductivity type. The first conductivity type and the second conductivity type may each be any of n-type (including n-type impurities), i-type (not including impurities), and p-type (including p-type impurities). The first conductivity type semiconductor layer may be composed of, for example, n-type InSb (see FIG. 6). Also, the second conductivity type semiconductor layer may be composed of, for example, p-type InSb (see FIG. 6). In the present embodiment, the first conductivity type is n-type and the second conductivity type is p-type.

[0064] The first conductivity type semiconductor layer, the active layer, and the second conductivity type semiconductor layer may be formed on a semiconductor substrate such as a gallium arsenide (GaAs) substrate or a silicon substrate. In the present embodiment, in the infrared optical element, the layers are provided in order from the substrate as the first conductivity type semiconductor layer, the active layer, and the second conductivity type semiconductor layer. As another example, in the infrared optical element, the layers may be provided in order from the substrate as the second conductivity type semiconductor layer, the active layer, and the first conductivity type semiconductor layer.

[0065] One or more barrier layers may be provided between the first conductivity type semiconductor layer and the active layer. Also, one or more barrier layers may be provided between the active layer and the second conductivity type semiconductor layer. In the present embodiment, an n-type barrier layer is provided between the first conductivity type semiconductor layer and the active layer, and a p-type barrier layer is provided between the active layer and the second conductivity type semiconductor layer. The n-type barrier layer is composed of, for example, n-type Al x In 1-x Sb (0.13 ≤ x ≤ 0.35) (see FIG. 6). The p-type barrier layer is composed of p-type Al z In 1-z Sb (0.13 ≤ z ≤ 0.35) (see FIG. 6).

[0066] Infrared optical elements preferably have a ratio of 20 or more between maximum and minimum sensitivity in the wavelength range of 2000 nm to 10000 nm.

[0067] As described above, the optical module according to this embodiment can increase the effective slope of the transmission spectrum by combining multiple optical filters compared to the slope of a single optical filter (making it sharper and steeper). Therefore, it is possible to provide an optical module equipped with high-performance optical filters.

[0068] While embodiments of this disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art will find it easy to make various modifications or alterations based on this disclosure. Therefore, it should be noted that these modifications or alterations are included within the scope of this disclosure.

Claims

1. It comprises multiple optical filters and an infrared light receiving element, The aforementioned plurality of optical filters are installed in the optical path formed by infrared radiation emitted from the light source, The plurality of optical filters include at least a first optical filter installed to reflect infrared light in the optical path and a second optical filter installed to transmit infrared light in the optical path. The first optical filter and the second optical filter have a common transmission wavelength range within the wavelength range to which the infrared light receiving element is sensitive. The maximum transmittance in the common transmission wavelength range is 20% or more of the maximum transmittance of the first optical filter and the second optical filter, and less than the maximum transmittance of the first optical filter and the second optical filter, An optical module in which the full width at half maximum of the first optical filter is smaller than the full width at half maximum of the second optical filter.

2. The optical module according to claim 1, wherein the wavelength corresponding to the maximum transmittance in the common transmission wavelength range is located on the cut-on side of the first optical filter and on the cut-off side of the second optical filter.

3. The optical module according to claim 1 or 2, wherein the maximum transmittance in the common transmission wavelength range is 90% or less.

4. The second optical filter has a transmission wavelength range that is shorter than that of the first optical filter. The optical module according to claim 1 or 2, wherein the wavelength corresponding to a 20% transmittance at the cutoff side of the second optical filter is smaller than the wavelength corresponding to the maximum transmittance of the first optical filter.

5. The optical module according to claim 4, wherein the wavelength corresponding to a 20% transmittance on the cut-on side of the first optical filter is greater than the wavelength corresponding to the maximum transmittance of the second optical filter.

6. The optical module according to claim 4, wherein the wavelength corresponding to a 20% transmittance on the cutoff side of the second optical filter is greater than the wavelength corresponding to a 20% transmittance on the cuton side of the first optical filter.

7. The optical module according to claim 4, wherein the transmission spectrum of infrared light reflected by the first optical filter and transmitted through the second optical filter has a greater slope on the cutoff side than the transmission spectrum of infrared light transmitted through the second optical filter alone.

8. The optical module according to claim 7, wherein in the optical path formed by infrared light reflected by the first optical filter and transmitted through the second optical filter, the ratio of the slope on the cut-on side of the first optical filter to the slope on the cut-off side of the second optical filter is 0.7 or more.

9. The optical module according to claim 1, wherein the wavelength corresponding to the maximum transmittance in the common transmission wavelength range is located on the cutoff side of the first optical filter and on the cut-on side of the second optical filter, and the transmission spectrum of infrared light reflected by the first optical filter and transmitted through the second optical filter has a larger slope on the cut-on side than the transmission spectrum of infrared light transmitted through the second optical filter alone.

10. The optical module according to claim 9, wherein in the optical path formed by infrared light reflected by the first optical filter and transmitted through the second optical filter, the ratio of the slope on the cutoff side of the first optical filter to the slope on the cut-on side of the second optical filter is 0.7 or more.

11. The optical module according to claim 1 or 2, wherein at least one of the plurality of optical filters has an area 1.4 times or more that of the other optical filters.

12. The optical module according to claim 1 or 2, wherein at least one of the plurality of optical filters is a bandpass filter having a transmission band that provides a maximum transmittance of 60% or more in the wavelength range of 2,000 nm to 10,000 nm.

13. The optical module according to claim 1 or 2, wherein at least one of the plurality of optical filters is a reflective filter.

14. The optical module according to claim 1 or 2, used in an NDIR type gas sensor or a photoacoustic type gas sensor.

15. The optical module according to claim 1 or 2, wherein the infrared light receiving element is an infrared photodiode.

16. It comprises multiple optical filters and an infrared light receiving element, The aforementioned plurality of optical filters are installed in the optical path formed by infrared radiation emitted from the light source, The plurality of optical filters include at least a first optical filter installed to transmit infrared light in the optical path and a second optical filter installed to transmit infrared light in the optical path, An optical module in which the first optical filter is a bandstop filter and its cutoff band includes the transmission wavelength range of the second optical filter, and the transmission spectrum of infrared light transmitted through the first optical filter and the second optical filter has a larger slope on the cut-on or cut-off side than the transmission spectrum of infrared light transmitted through the second optical filter alone.

17. The optical module according to claim 1 or 2, wherein the cut-on side refers to a wavelength shorter than the wavelength corresponding to the maximum transmittance, and the slope of the cut-on side is calculated by dividing "0.9 - 0.1" by "the magnitude of the change in wavelength from 10% transmittance to 90% transmittance," when the maximum transmittance is normalized to 1.

18. The optical module according to claim 1 or 2, wherein the cut-on side refers to a wavelength shorter than the wavelength corresponding to the maximum transmittance, and the slope of the cut-on side is calculated by dividing "0.8 - 0.1" by "the magnitude of the change in wavelength from 10% transmittance to 80% transmittance," when the maximum transmittance is normalized to 1.

19. The optical module according to claim 1 or 2, wherein the cutoff side refers to a wavelength longer than the wavelength corresponding to the maximum transmittance, and the slope of the cutoff side is calculated by dividing "0.9 - 0.1" by "the magnitude of the change in wavelength from 90% transmittance to 10% transmittance," when the maximum transmittance is normalized to 1.

20. The optical module according to claim 1 or 2, wherein the cutoff side refers to a wavelength longer than the wavelength corresponding to the maximum transmittance, and the slope of the cutoff side is calculated by dividing "0.8 - 0.1" by "the magnitude of the change in wavelength from 80% transmittance to 10% transmittance," when the maximum transmittance is normalized to 1.

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