JPH10185523A - Optical waveguide type displacement sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical waveguide type displacement sensor which can stably measure the displacement of an object over a wide measuring range by providing a wide-band light source, a light receiving means equipped with a waveguide for extending reference light having an optical path difference adjusted to the interference length or shorter between reference light and measuring light and a narrow-band selecting means having a transmissive wavelength which is different from that of a luminous flux separating means. SOLUTION: A light wave emitted from a wide-band light source 3 is made incident to a polarization plane maintaining optical fiber F1 and led to a port 12 for light incidence. The optical wave led to an optical guide 14 is made incident to the waveguide 14' for extending reference light and becomes reference light after passing through a 3-dB coupler 13. The optical wave introduced to another optical waveguide is led to the coupler 13 as measuring light. The coupler 13 distributes the reference light and measuring light to an optical wave input port 12 and a signal output port 18 in accordance with the phase difference between the two kinds of light. The optical wave made incident on a light receiving device 21 is made incident on light receiving elements 26 and 22 through a half mirror 24. A level difference is measured from the phase information obtained from the interference signals of the elements 26 and 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical waveguide type displacement sensor for measuring a displacement of an object to be measured by optical interference.

[0002]

2. Description of the Related Art In a conventional optical interferometer, there is a method utilizing a principle of a Michelson interferometer, a Mach-Zehnder interferometer or the like. If these interferometers are configured on an electro-optic crystal substrate such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ), a compact displacement gauge that does not require complicated optical system alignment can be realized. . However, a normal interferometer outputs a periodic phase signal according to the displacement of a non-measurement object. That is, when the displacement of the non-measurement object exceeds λ / 2, the phase signal has an indeterminacy of an integral multiple of λ / 2, and the displacement cannot be specified. As a method for solving this problem and expanding the measurement range, a method is known in which an indefinite integer value is obtained at two or more wavelengths having different wavelengths.

FIG. 2 is an overall configuration diagram of a conventional two-wavelength displacement sensor using an optical waveguide. In FIG. 2, 1 is a light source module, 2 is a semiconductor laser for irradiating a laser beam, 1
1 is an optical waveguide element, 12 is a light source input port for inputting a light wave guided from a light source to the optical waveguide element 11, 1
Reference numeral 3 denotes a 3 dB coupler that bisects light, reference numeral 14 denotes an optical waveguide through which reference light passes, reference numeral 15 denotes a mirror that reflects reference light, reference numeral 16 denotes an optical waveguide through which measurement light passes, and reference numeral 17 focuses the measurement light on an object to be measured. Lens, 18 is a signal output port to which the measuring light and the reference light are directed toward the light receiving device, 19 is a modulator, 21 is a light receiving device that receives the measuring light and the reference light, 22 is a light receiving element, F1
Is a polarization maintaining optical fiber, and G1 is a multimode optical fiber. Note that single-digit reference numerals indicate constituent elements of the light source module 1, 10-th reference numerals indicate constituent elements of the optical waveguide element 11, and 20-th reference numerals indicate constituent elements of the light receiver 21. Used for

The principle of operation of a conventional two-wavelength displacement sensor using an optical waveguide will be described below with reference to FIG. FIG.
In the above, the light wave emitted from the semiconductor laser 2 is guided to the light source incidence port 12 formed in the optical waveguide element 11 by the polarization maintaining optical fiber F1, and is split into two by the 3 dB coupler 13 formed in the optical waveguide element 11. You. Optical waveguide 1
The light guided to 4 is reflected by the mirror 15 and again 3d.
The light becomes reference light via the B coupler 13. The light wave guided to the optical waveguide 16 is condensed on the object to be measured by the lens 17 as measurement light, and the reflected light from the object to be measured returns again to the end of the optical waveguide element via the lens 17 and reaches the 3 dB coupler 13. .
At this time, the 3 dB coupler 13 distributes the light wave to the light source input port 12 and the signal output port 18 according to the phase difference between the reference light and the measurement light.

The light wave distributed to the signal output port 18 is guided to a multi-mode optical fiber G1, and
1. On the other hand, the light wave distributed to the light source incidence port 12 returns to the semiconductor laser 2 via the optical fiber F1. The optical waveguide 14 is provided with a modulator 19 for accurately measuring the phase difference between the reference light and the measurement light. To increase the measurement stability of the interferometer,
The wavelength stabilization of the semiconductor laser 2 is an important factor, and therefore, an isolator for cutting the return light is provided between the polarization plane maintaining optical fiber F1 and the semiconductor laser 2. The oscillation wavelength of the semiconductor laser 2 fluctuates due to a slight driving current and a small change in temperature. This depends on the thermal expansion of the resonator and the fluctuation of the refractive index of the semiconductor material.
Further, when the drive current and the temperature are greatly changed, the resonance mode of the oscillation is changed, and the oscillation wavelength is largely changed. This is called mode hopping. If this mode hopping is used, it is possible to switch the wavelength temporally with one semiconductor laser and perform two-wavelength measurement.

To switch the wavelength at high speed, a method of switching the drive current value is effective. In other words, specific drive current values I ₁ and I ₂ that are different oscillation modes are selected, the semiconductor laser 2 is driven by the current I ₁ , and the oscillation wavelength λ ₁
Drives the modulator 19 to measure the phase, and then the current I ₂
Drives the semiconductor laser 2 with the oscillation wavelength λ ₂ and the modulator 19.
May be driven to perform phase measurement in the same manner. Using the phases Ψ ₁ and Ψ ₂ measured at each wavelength, the distance to the object to be measured is d = λ ₁ · λ ₂ / ｛(λ ₁ -λ ₂ ) · [(ΔΨ / 4π)
+ 1 / (2 · n)]｝. Here, ΔΨ = (Ψ ₁ + Ψ ₂ ), and n is an integer indicating measurement indefiniteness. Here, ΔΨ changes in a range of ± π. Therefore, the measurement range is λ ₁ · λ ₂ / ｛2
(Λ ₂ −λ ₁ )｝. When measured at a wavelength lambda ₁ of the light source, because the measurement range of the wave is lambda _1/2, the range becomes possible to enlarge the _{λ 2 / (λ 2 -λ 1} ) times in two wavelength measurement, dual-wavelength It can be seen that the range becomes larger as the measurement difference is smaller.

[0007]

However, in order to realize the above-described two-wavelength measuring method, there are the following problems. First, the characteristics of the resonance modes of the semiconductor laser are different one by one, and a stable oscillation state cannot be obtained even when driven by the same current value. In addition, the conditions under which a normal semiconductor laser oscillates in a single mode are limited, and when the semiconductor laser is operated while largely switching current values, it is not possible to reliably perform single mode oscillation at two current values. Furthermore, although the wavelength stability of the semiconductor laser directly affects the displacement measurement accuracy, the oscillation wavelength of the semiconductor laser fluctuates according to changes in the drive current and the temperature, so that strict current and temperature control is required.

Second, when the oscillation wavelength of the semiconductor laser is switched by greatly changing the current value, the power also changes greatly at the same time. An optical waveguide fabricated on a lithium niobate crystal substrate has the property that the refractive index changes according to the light intensity. Immediately after wavelength switching, the reference light and the measurement light are changed by the refractive index change of the optical waveguide itself due to the light intensity fluctuation. Since the phase difference fluctuates with time, it is necessary to wait for the phase difference to stabilize before performing the phase measurement. For this reason, there is a problem that the measurement time becomes long. In addition, since the phase measurement at the two wavelengths is different in time, there is a disadvantage that the measurement error increases as the sample moves faster. The present invention has been made in view of the above problems, and an object of the present invention is to realize an optical waveguide type displacement sensor capable of obtaining a stable and wide measurement range.

[0009]

In order to achieve the above object, an optical waveguide type displacement sensor according to the present invention is an optical waveguide type displacement sensor using a directional coupling type optical waveguide element. A light receiving unit including a plurality of narrow band selecting units having different transmission wavelengths from the separating unit; and adjusting the length such that a path difference between the reference light and the measuring light is equal to or less than an interference length determined by a bandwidth of the narrow band selecting unit. And a reference light extension waveguide.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing embodiments of the present invention, the operation of the optical waveguide displacement sensor according to the present invention will be described with reference to FIG. FIG. 1 is an overall configuration diagram of an optical waveguide type displacement sensor according to the present invention. In FIG. 1, 3 is a broadband light source, 14 'is a reference light extension waveguide, 23 is a lens for collimating light, 24 is a half mirror, 25 is a narrow band filter, 26 is a light receiving element, and 27 is a narrow band filter. is there. Note that the same elements as those described with reference to FIG.

In FIG. 1, a light wave emitted from a broadband light source 3 enters a polarization plane maintaining optical fiber F1 and is guided to a light source incidence port 12 formed in an optical waveguide element 11 while maintaining a polarization plane. Is divided into two parts by a 3 dB coupler 13 built in. The light guided to the optical waveguide 14 is
The light enters the reference light extension waveguide 14 ′, is reflected by the mirror 15, and becomes reference light again through the 3 dB coupler 13.
The light wave guided to the optical waveguide 16 exits from the end of the optical waveguide element as measurement light, and is condensed on the object to be measured by the lens 17.
The reflected light from the object to be measured returns to the end of the optical waveguide element via the lens 17 again, and reaches the 3 dB coupler 13. At this time, the 3 dB coupler 13 distributes the light wave to the light source input port 14 and the signal output port 18 according to the phase difference between the reference light and the measurement light. The light wave distributed to the signal output port 18 is incident on the light receiver as signal light via the multi-mode optical fiber G1. The optical waveguide 14 is provided with a modulator 19 for accurately measuring the phase difference between the reference light and the measurement light.

In the light receiver 21, the light emitted from the multi-mode optical fiber G1 is collimated by the lens 23 and enters the half mirror 24. The light wave reflected by the half mirror 24 enters the light receiving element 26 via the narrow band filter 25. The light wave transmitted through the half mirror 24 enters the light receiving element 22 through the narrow band filter 27. Light receiving element 2
From the light waves incident on 6 and 22, an interference signal that changes according to the phase difference between the reference light and the measurement light is obtained, and the displacement of the object to be measured is measured. At this time, a phase signal measured at the transmission wavelength of the narrow band filter 25 is obtained from the interference signal of the light receiving element 26, and the narrow band filter 2 is obtained from the interference signal of the light receiving element 22.
Thus, a phase signal measured at a transmission wavelength of 7 is obtained. Therefore,
Assuming that the transmission wavelengths of the two narrow-band filters are λ ₁ and λ ₂ , a measurement result similar to that obtained when the oscillation wavelengths of the light source are switched is measured.

In this system, since the light source 3 is driven by a single current, a stable operation is possible. Light source 3
Even if the wavelength fluctuates, the displacement measurement wavelength is substantially fixed by the narrow-band filters 25 and 27, which are passive elements, so that strict current and temperature control is not required. Further, since the power of the light wave incident on the optical waveguide does not change at all, the problem that the refractive index of the optical waveguide changes according to the intensity of light can be avoided. In addition, since two systems of phase measurement can be performed simultaneously, the measurement time is shortened. Further, for the same reason, since the phase measurement at the two wavelengths is performed at the same time, the problem that the measurement error increases as the sample moves faster can be avoided.

However, in this method, the coherence length is restricted by the bandwidth of the narrow band filter, and the coherence length is about several hundred μm even with a filter having a bandwidth of 1 nm. Therefore, in order to perform measurement by causing the reference light and the measurement light to interfere with each other, it is necessary to adjust the optical path length difference between the two light waves to within several hundred μm. Therefore, the reference light extension waveguide 14 ′ needs to be made of a material having a higher refractive index than the lens. If ordinary optical glass is used as a lens material and lithium niobate is used as a reference light extension waveguide substrate, the refractive index of the latter is sufficiently higher than that of the former, and an optical path length difference of 0 can be achieved. As described above, the broadband light source 3 and the reference light extending waveguide 14 ′ whose length is adjusted such that the path difference between the reference light and the measurement light is equal to or less than the coherence length determined by the bandwidth of the narrow band filter are provided. By using an optical waveguide interferometer and a plurality of narrow band selecting means 25 and 27 having different transmission wavelengths, it is possible to provide an optical waveguide displacement sensor capable of stably measuring a step having a size exceeding the wavelength of the light source. It becomes possible.

Embodiment An embodiment of an optical waveguide type displacement sensor according to the present invention will be specifically described below with reference to FIG. In FIG. 1, a superluminescent diode (SL) having a center wavelength of 840 nm and a spectral width of 20 nm is used as the broadband light source 3.
D) was used. The light wave emitted from the SLD enters the polarization-maintaining optical fiber F1, is guided to the light source incidence port 12 formed in the optical waveguide element 11 while maintaining the polarization plane, and is provided with a 3 dB coupler provided in the optical waveguide element 11. 1
Divided by three. The light guided to the optical waveguide 14 enters the reference light extension waveguide 14 ′, is reflected by the mirror 15,
Again, the light becomes reference light via the 3 dB coupler 13.

Optical Waveguide Element 11 and Reference Light Extension Waveguide 1
4 ′ are both produced on a lithium niobate crystal substrate by a proton exchange method. The reference light extension waveguide 14 'was set to 8.15 mm in accordance with the optical length of the measurement light. The refractive index of lithium niobate is 2.175 at a wavelength of 840 nm, and the optical path length is 17.73 mm. Optical waveguide element 11
Is polished to 8 °, the former is 0.611 mm longer at the emission end of the reference light and the measurement light, and the optical path length is 18.34 mm based on the measurement light emission end surface. The angle between the waveguide and the end face of the waveguide of both the optical waveguide element 11 and the reference light extension waveguide 14 'is 8 °, and both are bonded with an ultraviolet curing resin.

On the other hand, the light wave guided to the optical waveguide 16 exits from the end of the optical waveguide element as measurement light, is condensed on the object to be measured by the lens 17, and the reflected light passes through the lens 17 again to the end of the optical waveguide element. Return to the 3dB coupler 13. As the lens 17, a gradient index condenser lens whose end face was polished to 8 ° was used. The distance from the lens end surface to the focal point of the measurement light is 1.3 mm. Therefore, the optical path length of the measurement light is 18.
35 mm, which matches the optical path length of the reference light.

At this time, the 3 dB coupler 13 transmits the light wave according to the phase difference between the reference light and the measurement light.
2 and the signal output port 18. The light wave distributed to the signal output port 18 is incident on the light receiver 21 as signal light via the multi-mode optical fiber G1. The optical waveguide 14 is provided with a modulator 19 for accurately measuring the phase difference between the reference light and the measurement light. In the light receiving section, the light emitted from the multi-mode optical fiber G1 is collimated by the lens 23,
Incident on. The light wave reflected by the half mirror 24 has a center wavelength of 842 nm and a transmission band width of 1 nm.
After that, the light enters the light receiving element 26. The light wave transmitted through the half mirror 24 has a center wavelength of 840 nm and a transmission bandwidth of 1 n.
The light enters the light receiving element 22 through the narrow band filter 27 of m. Therefore, the signal obtained by the light receiving element 26 has the same result as that obtained by measurement with a light source having a wavelength of 842 nm, while the signal obtained by the light receiving element 22 has a wavelength of 8
The same results are obtained as when measured with a 40 nm light source.
From the phase information obtained from the interference signals of the respective light receiving elements, a step of 177 μm (１ ／ of the combined wavelength of the two wavelengths) can be measured.

In general, a narrow band filter made of a dielectric multilayer film has an incident angle dependence of a transmission center wavelength. Therefore, by utilizing this property, the narrow band filter is used to form the dielectric multilayer film on a transparent substrate. If formed and manufactured, a narrow band filter having exactly the same characteristics as the narrow band filter 25 can be installed in the light receiver 21 of the above-described embodiment in place of the narrow band filter 27 at an angle.

[0020]

The optical waveguide type displacement sensor according to the present invention comprises:
As described above, the broadband light source, the reference light and the reference light and the reference light extension waveguide, the length of which is adjusted so as to be equal to or less than the interference length determined by the bandwidth of the narrowband selection means,
Since the light receiving means including the light beam separating means and a plurality of narrow band selecting means having mutually different transmission wavelengths is used, there is an advantage that the displacement can be measured in a stable and wide measuring range.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of an optical waveguide type displacement sensor according to the present invention.

FIG. 2 is a schematic configuration diagram showing an embodiment of a conventional optical waveguide type interference measuring device.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 light source module 2 semiconductor laser 3 broadband light source 11 optical waveguide element 12 light source incident port 13 3 dB coupler 14 optical waveguide 14 ′ reference light extension waveguide 15 mirror 16 optical waveguide 17 condensing lens 18 emission port 19 modulator 21 light reception Device 22 light receiving element 23 lens 24 half mirror 25 narrow band filter 26 light receiving element 27 narrow band filter F1 polarization maintaining optical fiber G1 multimode optical fiber

Claims (1)

[Claims] 1. An optical waveguide displacement sensor using a directional coupling type optical waveguide element, comprising: a broadband light source; a light receiving unit including a plurality of narrow band selecting units having different transmission wavelengths from a light beam separating unit; An optical waveguide type displacement sensor comprising a reference light extension waveguide whose length is adjusted so that the path difference of the measurement light is equal to or less than an interference length determined by the bandwidth of the narrow band selecting means.