JPH03261802A - Photosensor - Google Patents

Abstract

PURPOSE:To obtain a photosensor which is resistant to electromagnetic noise and safe in the explosion-proof structure by providing means for combining output lights from each laser source, and a signal processing means for measuring the frequency of a beat signal of the combined light thereby to detect the change of the physical amount. CONSTITUTION:An output light from a semiconductor laser 11 enters a first optical resonator 30 through an optical system comprised of a lens 15, a beam splitter 17, a lens 18, an optical fiber 9 and a lens 27. An output light from a semiconductor laser 13 is incident upon a second optical resonator 34 through an optical system comprised of a lens 16, a beam splitter 22, a lens 23, an optical fiber 24 and a lens 31. The condition for resonance at the optical resonator is determined by the optical path length inside the optical resonator, that is, the change of the index of refraction inside the optical resonator or the change of the distance between the resonators. Accordingly, since the change of the physical amount is detected by using only the light for the sensor part, this photosensor is resistant to electromagnetic noise and high in reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical sensor, and more particularly to an optical sensor that measures minute changes in physical quantities of an object to be measured using laser light.

<Conventional technology> In the fields of mechatronics and precision machinery, it is becoming increasingly important to capture and analyze minute displacements (changes in physical quantities) as equipment becomes more sophisticated and precise.

As a sensor for measuring such minute displacements, for example, an interferometer type sensor as shown in FIG. 6 has been developed. This sensor attaches a mirror (target target) to the object to be measured, irradiates the mirror with laser light, and detects positional changes using the reflected light. In this case, a change in the intensity of the interference light caused by a change in the light wavelength of the two optical paths due to a change in position is determined.

In FIG. 6, a laser beam is emitted from a semiconductor laser 1, reflected by a beam splitter 2 and a mirror 3, and incident on a light receiving element 4, and another beam passes through the beam splitter 2 and is attached to an object to be measured. The interference intensity with the light that is reflected by the mirror 5, passes through the beam blur and the ivy 2, and enters the light receiving element 4 again.

That is, it detects the phase difference of light and measures displacement, etc.

<Problems to be Solved by the Invention> The conventional optical sensor described above has problems such as accumulation of errors due to noise in the light receiving element 4 and miscounting of changes in interference fringes, the spectral life of the semiconductor laser, mode hops, etc. If a phase jump occurs due to this, there is a risk that the displacement will become unstable and measurement will become impossible. The latter issue is important and will be discussed in more detail.

FIG. 7 is a diagram showing a phase change when the oscillation mode of the semiconductor laser 1 changes due to Motohop.

Conventional sensors use a relative measurement method that uses a certain phase point as a reference and measures the number of interference fringes (phase) from this phase point, so the light source itself changes as shown in Figure 7. In this case, the reference point itself becomes unstable, making it almost impossible to measure minute displacements.

The object of the present invention is to eliminate the above-mentioned problems of the prior art and to provide a novel optical sensor that is highly reliable and is not affected by noise or instability of the laser light source.

Means for Solving the Problems> The present invention for solving the above problems is an optical sensor that measures changes in minute physical quantities of an object to be measured, which comprises at least two laser light sources capable of changing the oscillation frequency. and a first optical resonator configured such that a change in the physical quantity of the object to be measured results in a change in the length of the resonator or a change in the refractive index inside the resonator; at least one second optical resonator realizing resonance conditions different from those of the optical resonator; and output light of each optical resonator obtained by inputting the output light of each of the laser light sources into the corresponding optical resonator. means for feeding back the output light to each corresponding laser light source to lock the output light frequency of each laser light source to the resonant frequency of each corresponding optical resonator; means for combining the output light of each of the laser light sources; The apparatus is characterized by comprising a signal processing means for detecting a change in the physical quantity by measuring a beat signal frequency of light.

Effect> In the optical sensor of the present invention, an optical resonator such as a Fabry-Perot etalon is used as a measurement system instead of a conventional reflecting mirror. For example, when detecting displacement as a physical quantity, the displacement of the object to be measured becomes the displacement of the end face of the optical resonator, and as a result, it is understood as the displacement of the resonator length.

On the other hand, at least one optical resonator serving as a reference is prepared separately from this optical resonator. The optical resonant frequency of this optical resonator is different from the optical resonant frequency of the optical resonator of the measurement system.

These optical resonators have a frequency characteristic in which the reflected intensity of manually applied light periodically decreases at predetermined frequency intervals, and this frequency characteristic (frequency interval between resonance points) is determined by the resonant wavelength (or refractive index of the internal material).

Therefore, the driving current of each laser light source is controlled so that the frequency of the laser light input to each of these optical resonators satisfies the optical resonance condition, and the output optical frequency of each laser light source is adjusted to each corresponding optical resonator. to lock to the resonant frequency of.

Here, the difference in resonance frequency of each of these optical resonators is determined by measuring the beat signal frequency of light obtained by combining the output lights of each laser light source.

If the object to be measured is displaced in this state, the resonant frequency of the optical resonator of the measurement system changes. Therefore, it is possible to measure minute changes in the physical quantities of the object to be measured as changes in the optical beat signal frequency obtained by combining the output lights of each laser light source, creating an extremely sensitive sensor. can.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention.

In the figure, a semiconductor laser 11 is driven by a drive circuit 12 having a feedback circuit, and a semiconductor laser 13 is driven by a drive circuit 14 having a feedback circuit. The semiconductor lasers 11 and 13 are capable of single longitudinal mode oscillation and whose oscillation frequency can be changed by driving current.

Lenses 15, 16 are provided to convert the laser beams of each semiconductor laser 11, 13 into parallel beams. The beam splitter 17 condenses the laser beam output through the lens 15 with a lens 18 and splits it into light that is applied to an optical fiber 19 and light that is reflected by a 100% reflective mirror 20 and applied to a beam splitter 21. established for
The beam splitter 22 is provided for condensing the laser light outputted through the lens 16 with the lens 23 and splitting it into light that is applied to the optical fiber 24 and light that is applied to the beam splitter 21 . The beam splitter 21 combines the light split by the beam splitter 17 and reflected by the mirror 20 with the light split by the beam splitter 22 and applies it to the photoelectric conversion element 25. The output signal of the photoelectric conversion element 25 is applied to a frequency measurement circuit 26.

The light that is split by the beam splitter 17 and applied to one optical fiber via the lens 18 is made parallel by the lens 27, and then passed through the two partially transmitting mirrors 28, which transmit a few percent of the light.
Fabry-Perot type first optical resonator 3 composed of 29
0, split by the beam splitter 22 and applied to the optical fiber 24 via the lens 23, the light is applied to the optical fiber 24 through the lens 3.
] and then applied to a second Fapley-Hero type optical resonator 34 composed of two partially transmitting mirrors 32 and 33. Here, the resonator length of the first optical resonator 30 is configured to change in accordance with the displacement of the object to be measured,
The resonator length of the second optical resonator 34 is configured to be constant regardless of the displacement of the object to be measured. That is, the first
The optical resonator 30 functions as a sensor optical resonator, and the second optical resonator 34 functions as a reference optical resonator.

The output light of the first optical resonator 30 is transmitted through the lens 27. One optical fiber passes through an optical system consisting of a lens 18 and a beam splitter 17, is applied to a photoelectric conversion element 35, is converted into an electrical signal, and is applied to the feedback circuit of the drive circuit 12. Second
The output light from the optical resonator 34 is transmitted through the lens 31. optical fiber 24
．． The light passes through an optical system consisting of a lens 23 and a beam splitter 22, is applied to a photoelectric conversion element 36, is converted into an electric signal, and is applied to a feedback circuit of the drive circuit 14.

The optical sensor configured in this way includes a first optical resonator 3
A sensor part A consisting of a second optical resonator 34 and a signal processing part B consisting of other parts such as a laser light source 11.13 and a frequency measurement circuit 26 are connected to two optical fibers 19.24. It will be connected with.

Next, the specific operation will be explained.

The output light of the semiconductor laser 11 is transmitted through the lens 15. Beam splitter 17. Lens 18. 1 optical fiber, 27 lenses
The output light of the semiconductor laser 13 enters the first optical resonator 30 through an optical system consisting of a lens]6. Beam splitter 22. Lens 23. Optical fiber 24. lenth 3
1 and enters the second optical resonator 34.

Here, the resonance condition of light in the optical resonator is determined by the optical path length inside the optical resonator, that is, the change in the refractive index inside the optical resonator or the change in the interval between the resonators.

In FIG. 2, the signal intensity characteristic of reflected light X of an optical resonator having a certain optical path length is shown by a dotted line, and the signal intensity characteristic of transmitted light Y is shown by a solid line. As is clear from the figure, the reflected light X has a specific dip with respect to the optical frequency, and the transmitted light Y has a specific peak with respect to the optical frequency. Third
The figure is a side view of the received light intensity characteristics of the photoelectric conversion element 35 of FIG. 1. Since the photoelectric conversion element 35 receives the reflected light X, the intensity of the light has a dip characteristic as shown in FIG. Further, the solid line curve and the dotted line curve indicate that the resonant frequency changes due to a change in the optical path length of the first optical resonator 3 degrees, and the intensity of light with respect to the light frequency changes.

Incidentally, the frequency of the device e-light composed of a semiconductor laser can be changed by controlling the drive current. Therefore, the system of the semiconductor laser 11 is designed so that the output signal of the photoelectric conversion element 35 is minimized, that is, the frequency of the laser light output from the semiconductor laser 11 is equal to the resonant frequency of the eleventh optical resonator 30. The drive circuit 12 changes the drive current of the semiconductor laser 11 in this manner. Thereby, the oscillation frequency of the semiconductor laser 11 can be locked to the resonant frequency of the first optical resonator 30. On the other hand, in the semiconductor laser 13 system, the output signal of the photoelectric conversion element 36 is minimized, that is, the frequency of the laser light output from the semiconductor laser 13 is made equal to the resonant frequency of the second optical resonator 34. Then, the drive circuit 14 changes the drive current of the semiconductor laser 13. Thereby, the oscillation frequency of the semiconductor laser 13 can be locked to the resonant frequency of the second optical resonator 34. The optical path length of the second optical resonator 34 is fixed to a constant value, and mainly serves to compensate for the temperature of the sensor section A.

Part of the output light from the semiconductor lasers 11.14 is split by beam splitters 17.22 and combined by beam splitter 21. The optical frequencies of the output lights of the semiconductor lasers 11.14 combined by the beam splitter 21 are locked to the resonant frequencies of the optical resonators 30.34 of each system, as described above. Therefore, the photoelectric conversion element 25 detects the beat signal frequency of the optical frequency of the output light of these semiconductor lasers 11 and 14, and measures a minute change in the physical quantity of the object to be measured as a change in the optical beat signal frequency. can do. At this time, the optical frequency itself of the output light of the semiconductor laser 11.14 is several 1007 Hz.
is so high that it is not detected by the photoelectric conversion element 25.

Note that the laser light source used in the present invention may be of any type as long as it can sweep the frequency, and may be other than a semiconductor laser, such as a dye laser.

Further, the structure of the optical resonator constituting the sensor part A is not limited to the embodiment. For example, a ring resonator as shown in FIG. 4 may be used. In FIG. 4, two parallel leading waveguides 42.43 are formed on the surface of a sensor chip 40 serving as a waveguide substrate, with an optical fiber 19.24 connected to one end and a mirror 41 coupled to the other end. At the same time, two ring-shaped leading wave paths 44 and 45 are formed. A part of the back surface of one of the ring-shaped leading waveguides 44 is cut away to form a diaphragm 46. By using a portion of such a sensor, pressure can be measured. In any case, it is sufficient that the optical resonator forming part of the sensor has a reflected light intensity that changes according to the optical frequency.

Further, the optical system may have a leading waveguide structure as shown in FIG. In the figure, 50 is a waveguide substrate on which the optical system of the signal processing section B is formed, 60 is a waveguide substrate on which the optical system of the sensor part A is formed, and these substrates 50.6
0 are connected by optical fibers 19 and 24. Board 5
0 has a semiconductor laser 11.1 coupled to one end.
Two parallel leading wavepaths 51.52 are formed which transmit the output light of 3 to the optical fiber 19.24 connected to the other end. A leading wave path 53 including a Y-shaped branch coupler is formed between these leading wave paths 51 and 52. A photoelectric conversion element 35, 36 is coupled to each end face of the branch path of the Y-shaped branch coupler of the optical waveguide 53, and a directional coupler DC is connected between the intermediate portion of the branch path and the leading waveguide 51, 52, respectively. The photoelectric conversion element 25 is coupled to the end face of the coupling path. On the surface of the substrate 60, two parallel waveguides 61, 62 are formed, to which an optical fiber 19, 24 is connected at one end, and partially transmitting mirrors 63, 64 are formed on both end surfaces of the substrate 60. ing. A portion of the back surface of one of the leading waveguides 61 is cut away to form a diaphragm 65. With this configuration, pressure can be measured in the same way as the part of the sensor shown in FIG. 4.

Furthermore, the object to be measured is not limited to only displacement. It is also effective for measuring stress, electro-optic effects, and changes in refractive index.

It is also possible to combine three or more systems of laser light sources and optical resonators.

<Effects of the Invention> As described above in detail, the present invention detects changes in physical quantities using only light in a part of the sensor, so it is resistant to electromagnetic noise and has an intrinsically safe explosion-proof structure.

Further, according to the present invention, remote measurement using an optical fiber is possible, and furthermore, temperature compensation for a part of the sensor can be performed using two optical fibers. Since only two optical fibers are required, the connection between the optical system and the optical fibers is simplified, reliability is increased, and costs are reduced.

Additionally, since the resonant frequency of an optical resonator changes significantly even with small changes in physical quantities, it is possible to create a highly sensitive sensor.

Furthermore, since there is no mechanically driven part in the signal processing section, reliability is increased.

Furthermore, since it is sufficient to use light with an oscillation frequency that matches the resonant frequency of the optical resonator as a light source, a general multi-mode laser can also be used, and a low-cost sensor can be realized.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of the optical signal intensity of the optical resonator, FIG. 3 is an explanatory diagram of the received light intensity in the photoelectric conversion element 35 of FIG. FIG. 4 is an explanatory diagram of another specific example of an optical resonator, FIG. 5 is a configuration diagram of main parts of another embodiment of the present invention, FIG. 6 is a configuration diagram showing an example of a conventional optical sensor, and FIG. The figure is an explanatory diagram of a phase change of laser light when mode hopping occurs in a semiconductor laser. 11.13...Semiconductor laser 12.14...Drive circuit 15.16, 18, 23, 27.31...Lens 17
．． 21.22...Beam splitter 1924...Optical fiber 20...Mirror (100% reflection) 25.35.36...Photoelectric conversion element 26...Frequency measurement circuit 28.29, 32.33. Transmission mirror 30, first optical resonator (for sensor) 34...second optical resonator (for reference) Figure 2 x (reflected light) Optical frequency diagram Optical 1711 wave number diagram Motohop bond

Claims (1)

[Scope of claims] An optical sensor that measures minute changes in physical quantities of an object to be measured, comprising at least two laser light sources whose oscillation frequencies can be changed, their driving circuits, and a physical quantity of the object to be measured. a first optical resonator configured such that a change in the length of the resonator results in a change in the length of the resonator or a change in the refractive index inside the resonator; one second optical resonator; and the output light of each optical resonator obtained by inputting the output light of each of the laser light sources into the corresponding each optical resonator is returned to the corresponding each laser light source to generate each laser. means for locking the output optical frequency of the light source to the resonant frequency of each corresponding optical resonator; means for combining the output lights of the respective laser light sources; and measuring the beat signal frequency of the combined light to determine the physical quantity. and a signal processing means for detecting a change in the light sensor.