JPS6275231A - Humidity sensor - Google Patents

Abstract

PURPOSE:To realize a small-sized, stable sensor by utilizing variation in the heat conductivity of air with the amount of vapor in the air. CONSTITUTION:A Mach-Zehnder interferometer 11 is provided on a substrate 1. The interferometer 11 consists of an input optical waveguide 3, parallel optical waveguides 1 and 2 branched from it, and an output optical waveguide 4 where the waveguides 1 and 2 meet each other. A heat radiation layer 12 which covers one optical waveguide 1 is provided on the optical waveguide 1 and a thick heat insulating layer 12 is arranged on the remaining part of the surface of the substrate 10 including the other optical waveguide 2. When the substrate 10 is heated from below by a constant quantity, the temperature of the waveguide 1 where the heat radiation plate 12 is placed varies with humidity, so there is a temperature difference generated between the optical waveguides 1 and 2. Light waves propagated in the optical waveguides 1 and 2 have a phase difference because of the temperature difference and light obtained from the optical waveguide 4 varies in intensity with the phase difference. For the purpose, the output light is detected to sense humidity by an optical integrated circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention This humidity sensor utilizes changes in the thermal conductivity of air due to humidity. Two optical waveguides are formed on the substrate, a heat dissipation layer is provided on one optical waveguide, and a heat insulating layer is provided on the other optical waveguide. The phase change of light propagating through one optical waveguide provided with a heat dissipation layer is measured with respect to the phase of light propagated through the other optical waveguide provided with a heat insulating layer, and humidity is determined based on this phase change. It will be done.

Background of the invention The present invention relates to a humidity sensor using an optical integrated circuit.

Humidity is considered to be one of the physical quantities that is difficult to measure. In the past, psychrometric hygrometers and hair hygrometers were used to measure humidity.More recently, psychrometric hygrometers and hair hygrometers have been used to measure humidity. There is a method using a thermistor that takes advantage of changes in the thermal conductivity of air.

Any method other than the thermistor method mentioned above.

Hysteresis occurs. Poor responsiveness, maintenance required. There are problems such as a complicated structure.

The thermistor type does not have the above drawbacks and can be said to have the highest stability and reliability among current humidity sensors.

On the other hand, optical integrated circuits have the advantage that by integrating optical circuits on a solid substrate, it is possible to construct functional elements that are compact, high-performance, and stable against external disturbances such as vibrations.

Therefore, applying this optical integrated circuit technology to sensors is expected to satisfy the demand for highly accurate and reliable measurements.

In recent years, research on optical sensors including optical fiber sensors has been actively conducted. However, although some optical circuits for sensors have been integrated, there are currently very few reports of sensors in which the sensor itself is constructed from optical integrated circuits.

Summary of the Invention An object of the present invention is to apply the advantages of the thermistor method, which utilizes changes in the thermal conductivity of air due to humidity, to an integrated optical circuit, in order to realize a more compact and stable humidity sensor. The present invention provides a humidity sensor.

The humidity sensor according to the invention includes at least two
The first one. A second optical waveguide and a means for supplying light having a predetermined phase relationship to these optical waveguides are provided, a heat dissipation layer is provided on the first optical waveguide, and a heat insulating layer is provided on the second optical waveguide. It is characterized in that humidity is detected based on the phase difference of light output from two optical waveguides provided respectively. In the simplest case, the input light is split into two, the first. The light is supplied to the second optical waveguide. In this case, the first. The lights respectively supplied to the second optical waveguides are in phase. It is convenient to configure a Michelson interferometer using the first and second optical waveguides and detect humidity based on the output light intensity generated by interfering the two lights. Furthermore, in order to compensate for changes in the output light intensity due to shifts in the optical axis due to changes in humidity, a third optical waveguide is provided on the substrate, and the output light intensity of the Michelson interferometer and the output light of the third optical waveguide are It is preferable to detect humidity based on the comparison result with intensity.

According to this invention, since it utilizes changes in the thermal conductivity of air due to humidity, it inherits the advantages of the thermistor method, such as no hysteresis and good responsiveness. Moreover, since it is in the form of an optical circuit integrated on one substrate, it is small in size and has good stability.

Description of Embodiments [Principle of Humidity Detection] The humidity sensor according to the present invention utilizes the fact that the thermal conductivity of air changes depending on the amount of water vapor contained in the air.

Assuming that there is an object to which a constant amount of heat is continuously applied, the surface temperature of that object will reach an equilibrium state at the temperature where the amount of heat applied is equal to the amount of heat lost by heat conduction into the air. The amount of heat released into the air depends on the thermal conductivity of the air, so if the thermal conductivity changes, the amount of heat released changes, and the surface temperature of the object changes to reach a new equilibrium state. Since the thermal conductivity of air depends on the amount of water vapor in the air, the surface temperature of an object to which a certain amount of heat is applied changes depending on the amount of water vapor in the air, that is, the absolute humidity.Therefore, changes in the surface temperature can be detected. This makes it possible to sense absolute temperature.

The heat exchange that occurs when a flat plate and a fluid (in this case air) are in contact is called heat transfer. Now, if a flat plate at temperature θ is in contact with a fluid at temperature θA, the heat transfer is θV - θ
This happens due to the driving force.

In this case, the heat ff1Q flowing out from the flat plate through the surface (area S) of the flat plate is expressed as follows.

Q−hS(θV−θA) −(1) where h is called the average heat transfer coefficient.

Assuming that the surface temperature of the flat plate is constant everywhere, and considering only natural convection on a horizontally placed rectangular flat plate with a long side length of 1, h is expressed as in equation (2).

h -0,54(g#ri 114・[(θ-θ)lθ]
114・(Cρ2/λ3μ)m
WAA p... (2) Here.

λ: Thermal conductivity of air μ: Viscosity coefficient of air ρ: Density of air g: Gravitational acceleration C: Low-pressure specific heat of air Substituting equation (2) into equation (1) and rearranging it, we get the surface temperature of the flat plate. It can be seen that θV is expressed by the following equation.

θ−θ+(Q/ 0.548)”5・(μθ/gλ3ρ
2C) 115WA A
p... (3) Since the thermal conductivity λ of air is a function of absolute humidity, it can be seen from equation (3) that the surface temperature θV of the flat plate depends on the absolute humidity.

FIG. 1 shows the results of numerical calculations of changes in surface temperature with respect to absolute humidity based on equation (3).

The dimensions of the deck are 1 = 40mm, S = 40mm
It was set as 2. Further, it was assumed that a constant amount of heat was applied so that the surface temperature of the flat plate reached an equilibrium state at 200° C. when the absolute humidity was O (g/kg). The numbers shown in this graph, such as 10°C and 20°C, indicate the temperature.

From the graph in Figure 1, it can be seen that the temperature change of the flat plate is not affected much by the air temperature. Also, the amount of temperature change is 0.5
It's about ℃. Although the phase of light propagating through an optical waveguide formed on a substrate changes depending on the substrate temperature, the amount of phase change can be sufficiently detected if there is a temperature difference of about 0.5°C.

For example, if the flat plate is used as a substrate and an interferometer including an optical waveguide is provided on this substrate, this temperature change can be reliably measured.

Therefore, in principle, the humidity sensor can be constructed as shown in FIG.

In FIG. 2, a Matsuhatsu Enda type interferometer 11 is provided on a substrate 1. This interferometer 11 includes an input optical waveguide 3, two parallel optical waveguides 1 and 2 branched from the optical waveguide 3, and an output optical waveguide 4 where these optical waveguides 1 and 2 are combined. It consists of A heat dissipation layer 12 is provided on one parallel optical waveguide 1 so as to cover it, and a thick heat insulating layer 13 is arranged on the remaining portion of the surface of the substrate 10 including the top of the other parallel optical waveguide 2. When a constant amount of heat is applied from below the substrate IO, the temperature of the optical waveguide 1 on which the heat sink 12 is placed changes depending on the humidity, so there is a temperature difference between the two optical waveguides 1 and 2. arise. Due to this temperature difference, a phase difference occurs between the light waves propagating through both optical waveguides 1.2, and the intensity of light obtained from the output optical waveguide 4 changes due to this phase difference. Therefore, by detecting the output light, the optical integrated circuit can sense humidity.

[Humidity Sensor and All Determination Results] FIGS. 3 and 4 show an example of a humidity sensor according to the present invention.

LiN b Oa is used as the substrate 10, and the above-mentioned Matsuhatsu Enda interferometer 11, reference optical waveguide 5, and input optical waveguide 6 are formed on this substrate IO by thermally diffusing Ti. . The input optical waveguide 6 is branched into a figure 7 shape, and an interferometer 11 and an optical waveguide 5 are installed at this branch part.
are directly connected.

The output light of the output optical waveguide 4 of the interferometer 11 is ill constant light.

The output light of the optical waveguide 5 becomes the reference light. Reference optical waveguide 5
This is because the substrate IO is heated to a high temperature, so that the influence of a change in the measurement light intensity due to a shift in the optical axis due to a temperature change or the like is removed.

A 5iOz<777 layer 9 is provided on the upper surface of such a substrate 10, an AI heat dissipation layer 12 is provided on the upper part of the optical waveguide 1, and a super heat-resistant special polyimide resin P is provided on the other parts.
A heat insulating layer 13 made by IQ (Hitachi Chemical) is provided in each case. The buffer layer 9 is.

This is to prevent loss of light waves in the optical waveguide 1 that occurs when the heat dissipation layer 12 is directly provided.

This humidity sensor is manufactured as follows, for example. First, an interferometer 1 is placed on the L iN b Oa substrate 1.
The optical waveguide pattern 1 and the patterns of optical waveguides 5 and 6 are formed to a thickness of 45 nm by Ti sputtering. 6.5 in an oxygen atmosphere at a temperature of 980°C.
By thermally diffusing the time Ti into the substrate 1, the optical waveguide 1
~ form 6. after that.

A buffer layer 9 is formed by sputtering SiO2 to a thickness of 200 nm, a heat insulating layer 13 is spin-coated to a thickness of 6 μm, and an Ai heat sink is vapor deposited to a thickness of 2 μm.

The experiment was conducted as follows.

The humidity sensor 20 is shown in FIGS. 3 and 4.

As shown in FIG. 5, the back side is tightly attached to a copper jig 21 containing a heater 23 with heat dissipation paste, and the
It was heated at a constant amount of heat so that the temperature reached 00°C. The jig 21 is placed on a heat insulating material 22. Regarding the size of the humidity sensor 20, the length of the portion of the optical waveguide 1 on which the heat dissipation layer 12 is placed is about 18 mm. In the experiment.

Using a He-Ne laser and 0.63 μm light, light wave input and output was performed by end face irradiation. The humidity was changed using a small amount of hot water, and the actual humidity was measured using a thermistor humidity sensor.

The results of measurements made using TM mode at an air temperature of 25°C are shown in FIG. Here the vertical axis (normalized output)
is determined from the ratio of reference light and measurement light, that is, (measurement light)/(reference light). From FIG. 6, it can be seen that the amount of humidity change required to shift the phase of the light wave by half a wavelength is approximately 6.8 g/kg.

As can be seen from the graph in FIG. 6, humidity can be measured using a humidity sensor using an optical integrated circuit.

The interferometer is not limited to the above-mentioned Matsuhatsu Enda type. For example, it is also possible to configure a Michelson interferometer using an asymmetric X-branch optical waveguide. This asymmetrical X-branch optical waveguide was developed as a waveguide optical beam splitter in
8-20240 [i (Japanese Patent Application No. 57-86178). When this optical waveguide is used, the element length can be reduced by approximately half because the light travels back and forth, which has the advantage that the sensor itself can be made more compact.

[Brief explanation of drawings]

Fig. 1 is a graph showing the relationship between absolute humidity and the surface temperature of a flat plate, Fig. 2 is a plan view showing the principle of the humidity sensor according to the invention, and Figs. 3 and 4 show examples of the invention. Figure 3 is a perspective view, and Figure 4 is a perspective view of Figure 3.
A cross-sectional view along the V-IV line, Fig. 5 is a perspective view showing the jig used for the humidity sensor measurement experiment, and Fig. 6 is a 1N
It is a graph showing a -1 constant result. 1.2...The first part of the Michelson interferometer. Second optical waveguide. 3... Optical waveguide for input of Michelson interferometer. 4... Optical waveguide for output of Michelson interferometer. 5... Reference (third) optical waveguide. 6... Input optical waveguide, 10... Substrate. 11... Michelson interferometer, 12... heat dissipation layer. 13...Insulating layer.

Claims (4)

[Claims] (1) At least two first and second optical waveguides and means for supplying light having a predetermined phase relationship to these optical waveguides are provided on the substrate, and a heat dissipation device is provided on the first optical waveguide. layer, and a heat insulating layer is provided on the second optical waveguide, respectively.
A humidity sensor that detects humidity based on the phase difference between the light output from both optical waveguides. (2) The humidity sensor according to claim (1), wherein the lights supplied to the first and second optical waveguides are in phase. (3) The humidity sensor according to claim (1), wherein the first and second optical waveguides constitute a Michelson interferometer, and humidity is detected based on the output light intensity of the first and second optical waveguides. (4) A third optical waveguide is provided on the substrate, and humidity is detected based on a comparison result between the output light intensity of the Michelson interferometer and the output light intensity of the third optical waveguide. The humidity sensor described in section 3).