JPH05288509A - Gauge interferometer - Google Patents

Abstract

PURPOSE:To provide a microminiature gauge interferometer having high resolution and being capable of discriminating a moving direction. CONSTITUTION:A light source 2, optical wave-guiding channels 51-63 transmitting light outgoing from the light source 2, dividing means 40, 41 dividing the outgoing light from the light source 2 into measuring light and reference light, reflecting means 3, 4 reflecting the reference light, interference optical units 45, 46 which make the measuring light be reflected outside the optical wave- guiding channels and thereafter introduce the measuring light again into the optical wave-guiding channels so as to interfere with the reference light reflected through the reflecting means 3, 4, and light receiving means 5, 6 measuring interference light produced through interference are integrated on a same board.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a length measuring device for accurately measuring a distance between objects by using interference of laser light, and particularly to a laser light source, a light receiving means and various optical parts on the same substrate. The present invention relates to a small interferometric length measuring device integrated and integrated in.

[0002]

2. Description of the Related Art An optical interferometer is known as a device for measuring the position and distance with the wavelength accuracy of light by utilizing the interference of light waves. FIG. 4 is a principle diagram of a Twyman-Green interferometric length measuring device, which is an example of such a length measuring device. The laser light A emitted from the laser light source 81 is split by the half mirror 82 into the measurement light B that passes through the mirror 82 and the reference light C that is reflected. The measurement light B is reflected by a moving mirror (generally a corner cube is used) 84 installed on a measurement object 87 that can move in the direction of the arrow in the figure, and then is incident on the half mirror 82 again in the perpendicular direction. Is reflected. On the other hand, the reference light C is reflected by a fixed mirror (generally a corner cube is used) 83, then enters the half mirror 82 again, passes through it, and travels on the same optical axis as the measurement light B. However, it interferes with the measurement light B. A phase difference occurs between the measurement light B and the reference light C according to the optical path length of each of these lights. Since this phase difference sequentially changes with the movement of the measurement target, the position of the measurement target 87 can be known from the interference signal obtained by the interference.

By the way, in such an interferometer, a gas laser such as a He--Ne laser is generally used as a light source. However, since the size of the gas laser tube is large, there is a problem in that the entire length measuring system becomes large. Further, as shown in FIG. 4, since the components constituting the optical system are individually separated (so-called bulk type), in order to cause the measurement light and the reference light to interfere with each other, these optical components must be accurately aligned on the same optical axis. Had to be aligned. However, this alignment adjustment must be performed precisely, which requires a lot of labor and time.

Therefore, use of a semiconductor laser as a light source
In addition to reducing the size of the light source, a Peltier device was used.
The oscillation wavelength of this semiconductor laser is controlled by precise temperature control.
A proposal was made to standardize and improve the accuracy of length measurement. Also,
Recursive optics to eliminate alignment of optics
An optical integrated circuit interferometer using a system has been proposed.
56-15522). FIG. 5 shows the basic configuration of this optical integrated circuit interferometer.
FIG. In FIG. 5, an optical integrated circuit interferometer
Is a thin film optical waveguide 98 and laser light end face-coupled to the thin film optical waveguide 98.
It consists of source 91. The thin film optical waveguide 98 has a collimator
Lens 97, half mirror 92, reference mirror 93, and
A condenser lens 95 is formed. Half mirror 92
The reference mirror 93 and the reference mirror 93 both use Bragg reflection.
Is. The laser light P emitted from the light source 91 is collimated.
Is converted into a parallel light beam by the
Propagate. The reference light transmitted through the half mirror 92
P ₁Measurement light P reflected by₂Divided into and. Reference light P₁
Is half reflected again after being reflected by the reference mirror 93.
The light enters the mirror 92 and is reflected in the right-angled direction so that the condenser lens 9
Go to 5. On the other hand, the measuring light P₂Configured to be moveable
Half mirror after being reflected by the moving mirror 94
-92, it is transmitted, and it is directed to the condenser lens 95.
U Reference light P traveling toward the condenser lens 95₁And measuring light P
₂Is a coupler prism 9 for emission which travels on the same optical axis.
It is extracted as the interference light combined by 6. Move
The reference light P is moved by the movement of the mirror 94.₁And measuring light P₂With
When the phase difference between them changes, it is caused by the interference.
Since the number of interference fringes also changes, the number of interference fringes can be counted.
You can measure the length. in this case,
If the wavelength of the laser light is λ, it can be measured with a resolution of λ / 4.
It

[0005]

However, when a bulk type interferometer as shown in FIG. 4 is used,
Although a small and highly accurate length measuring device can be realized by using a semiconductor laser, precise and complicated work that requires time and skill is still required for alignment between individual optical components. In particular, when the size of the interferometer is reduced, the size of each optical component is reduced accordingly, and the adjustment work during alignment becomes more difficult.

On the other hand, when the optical integrated circuit interferometer is used, there is a problem that the moving direction of the moving mirror cannot be determined. Also, the resolution was limited to λ / 4. Further, since the thin film optical waveguide and the light source end face-coupled to the thin film optical waveguide are used, it is necessary to adjust the optical axis as described above at this coupling portion. Furthermore, since the interference light is taken out by the coupler prism, there is a problem of lack of reliability.

The present invention aims to solve such a problem.

[0008]

To achieve the above object, in the present invention, a light source, an optical waveguide through which the light emitted from the light source propagates, and the emitted light from the light source are divided into measurement light and reference light. Dividing means, and a reflecting means for reflecting the reference light,
An interference optical system that reflects the measurement light outside the optical waveguide and then introduces it again into the optical waveguide to interfere with the reference light reflected by the reflection means, and a light receiving means that measures the interference light generated by the interference, Were integrated on the same substrate made of semiconductor.

[0009]

According to the present invention, various optical elements (components) forming the interferometer are integrated and integrated on the semiconductor substrate. Therefore, the conventional work of aligning the individual optical components on the same axis becomes unnecessary. As a result, downsizing of the length measuring system can be realized without lowering the stability and reliability. Further, since the reference optical system is composed of two optical systems and the active type λ / 4 phase shifter is provided in one of the optical systems, it is possible to discriminate the moving direction of the interference fringes.

[0010]

1 is a schematic plan view showing an embodiment of the present invention. In this embodiment, an optical waveguide and various optical elements that constitute an interferometer are integrated on a substrate 1 made of a compound semiconductor such as GaAs. These form the main body of the length measuring device of the present invention. A corner cube (not shown) is installed as a reflecting mirror on the object to be measured with respect to this main body part, so that the measuring light emitted from the main body part is reflected by this corner cube and returns to the main body part again. Is becoming Further, in the present embodiment, in order to enable direction discrimination of the interference signal obtained by the interference between the reference light and the measurement light,
The optical path of the reference light optical system is divided into two. The light wave propagating in one of the divided optical paths is set to have a phase shift of λ / 4 (λ is the wavelength of the laser light) with respect to the light wave propagating in the other optical path.

As the optical waveguide formed on the substrate 1, Al
A GaAs / GaAs heterojunction type can be used, but when short-wavelength light is used, a multiple quantum well structure or the like can also be used. A semiconductor laser 2 was used as a light source. The drive circuit for driving the semiconductor laser 2 and the laser power monitor circuit can all be integrated on the substrate 1. As a semiconductor laser, a distributed feedback type (DFB: Distributed Free) in which a desired diffraction grating is formed in the peripheral portion on the active layer.
dback) It is desirable to use a laser. Further, a metal-loaded polarizer 7 is provided to select the TE mode of the laser light (the electric field component in the waveguide in the axial direction becomes 0). Then, after the laser light emitted from the semiconductor laser 2 is converted to the TE mode by the polarizer 7, the laser light is propagated to various optical elements by the single mode waveguide.
The optical waveguide has a Y-shaped branch that splits and combines the laser lights, and an X-shaped cross that crosses the laser lights having different traveling directions. The reference mirrors 3 and 4 that reflect the reference light are configured by Bragg reflectors. Further, the substrate 1 includes a rod lens 11 for emitting the measurement light from the optical waveguide of the substrate 1 toward a corner cube provided on the object to be measured, and a rod for guiding the measurement light reflected by the corner cube to the optical waveguide again. And a lens 12. At the position where the measurement light reflected by the corner cube and the reference light interfere with each other, the light receiving elements 5 and 6 for detecting the interference light generated by this interference.
Is provided.

Further, in this embodiment, in order to equalize the intensity ratio of the reference light and the measurement light to improve the definition of the interference light, the Y-shaped branches 40 and 41 are provided with active refractive indexes of the optical waveguides, respectively. The active double mode beam splitters 13 and 14 for changing to Further, an active phase shifter 8 is provided on one of the two divided reference light optical systems to adjust the phase difference between the reference light and the measurement light. The active double mode beam splitter and the phase shifter both utilize the electro-optic effect.

Here, the change in the refractive index due to the electro-optical effect will be briefly described. The phenomenon in which the refractive index of a substance changes due to an electric field applied to the substance (electro-optic crystal) is called the electro-optical effect.
And a secondary electro-optic effect in which Δn is proportional to the electric field strength E ² . The change in refractive index due to the first-order electro-optical effect is caused by the voltage (electric field) along a certain crystal axis.
It can be obtained by considering how the refractive index ellipsoid (representing the state of light propagation in an anisotropic crystal) represented by the following mathematical expression is deformed or rotated when is applied.

[0014]

[Equation 1]

When a voltage (electric field) is applied, X in the above equation,
Since Y and Z are not the principal axes, the index ellipsoid is represented by the more general formula below.

[0016]

[Equation 2]

The coefficients of these equations are given by the electro-optic constant tensor and the applied electric field vector E ^e = (Ex ^e , E y ^e , ^E z ^e ).
Further, the primary electro-optical constant tensor of a III-V compound semiconductor belonging to a certain crystal group (Td type) is the first on the right side of the following equation.
It is known to have the components shown in the section.

[0018]

[Equation 3]

[0019] Then, an externally applied electric field component Ei ^e in the equation
By substituting, the general formula of the index ellipsoid when an electric field is applied can be introduced. By converting the quadratic form thus obtained into the standard form of the index ellipsoid by converting the coordinates, the index of refraction depending on the polarization direction sensed by the light wave can be obtained. Here, consider a case where a light wave propagates in the [01] (where 1 is indicated by a minus sign above 1) direction using GaAs whose crystal plane is represented by (100). The refractive index change Δn when the vertical electric field E is applied is | Δn | _TE = 1/2 · n ₀ ³ γ ₄₁ · E TM mode | Δn | _TM = 0. In this embodiment, since the laser light is selected in the TE mode, the effective refractive index of the waveguide can be changed by selectively applying the vertical electric field (voltage) on the waveguide.

Therefore, the active double-mode beam splitters 13 and 14 in this embodiment have a perfect coupling length of the 0th-order mode and the 1st-order mode (propagation in the double-mode region) as the effective refractive index of the double-mode region changes. The length at which the phase difference between the 0th-order mode and the 1st-order mode becomes π changes, so that the branching ratio of the TE mode can be set to a desired value by adjusting the applied voltage. Similarly, in the case of the active phase shifter 8 as well, by adjusting the applied voltage, a desired phase shift can be given to the TE mode propagating in the waveguide. Active double mode beam splitters 13 and 14
Also, as means (circuit) for applying a desired vertical electric field to the active phase shifter 8, known means may be separately provided although not shown. As the electro-optic crystal, in the Td type, in addition to GsAs, InP, CdTe, ZnT
e, CuCl, etc. are known.

Next, the operation associated with the progress of the laser light in this embodiment will be described. Laser light (hereinafter referred to as light wave) emitted from the semiconductor laser 2 is directly transmitted to the optical waveguide 5
0 is coupled with high efficiency and is incident. This light wave is split into two by the active double-mode beam splitter 13 and the Y-type branch 40 after only the TE mode of its own vibration mode is selected by the metal loaded polarizer 7. One of the demultiplexed light waves is used as the first reference light S in the optical waveguide 5
Propagate 2. The other light wave propagates through the optical waveguide 51, is then split again by the active double-mode beam splitter 14 and the Y-shaped branch 41, and propagates through the second reference light T and the optical waveguide 53 that propagate through the optical waveguide 54. It becomes the measurement light U. The measuring light U is transmitted to the optical waveguide 5 by the rod lens 11.
3 is emitted, enters a corner cube installed on the object to be measured, and is reflected here. The reflected light U ′ from the corner cube is incident and condensed by the rod lens 12, guided to the optical waveguide 59, and then branched by the Y-shaped branch 44. Two demultiplexed reflected lights U ₁ 'and U ₂ '
Propagate in the optical waveguides 60 and 61, respectively.

On the other hand, the second reference light T propagating in the optical waveguide 54 is reflected by the Bragg mirror 3, propagates in the optical waveguide 56, and is reflected in the optical waveguide 61 and the reflected light U ₂ 'and the Y-shaped branch 45. And they interfere with each other in the optical waveguide 62. The interference light generated by this interference is received by the light receiving element 5
Detected by. Also, the first propagating through the optical waveguide 52
The reference light S is reflected by the Bragg mirror 4, propagates through the optical waveguide 58, passes through the phase shifter 8, and is then combined with the reflected light U ₁ ′ propagated through the optical waveguide 60 by the Y-shaped branch 46 and is guided. They interfere with each other in the waveguide 63. The interference light here is detected by the light receiving element 6. At this time, in order to maximize the sharpness of each interference light, as described above, the splitting ratios in the active double mode splitters 13 and 14 are set to the first reference light S, the reflected light U ₁ 'and the second reference light S ₁ . The reference light T and the reflected light U ₂ 'are adjusted so that they have the same intensity.

The change in distance due to the movement of the object to be measured can be obtained by counting the change in brightness of the interference light detected by the light receiving elements 5 and 6 and analyzing it.
At this time, 'the phase difference [Phi ₁ between the second reference beam T, the reflected light U _1' reflected light U ₂ phase difference [Phi ₂ between the first reference beam S phase By the shifter 8, (Φ ₁ −Φ ₂ )
It is adjusted so that = π / 2 (λ / 4). Therefore, the function of interpolating the measured distances obtained by the light-receiving elements 5 and 6 mutually works, and as is well known, it has a resolution of π / 4 (λ / 8) and can discriminate the moving direction of the corner cube. Although not shown here, since the means for counting the change in brightness of the interference light detected by the light receiving means can be configured by a simple digital circuit, when the substrate 1 is a GaAs substrate, such a signal processing circuit is used. It is also possible to integrate it on a substrate.

FIG. 2 shows the D used as the laser light source 2.
FIG. 6 is a schematic cross-sectional view showing a coupled state of an FB laser and an optical waveguide 50. FIG. 3 is a schematic sectional view of the phase shifter 8.
Although these device manufacturing processes are well known and will not be described in detail, in the present embodiment, an optical waveguide portion is formed by a dry etching technique after the DFB laser portion is formed, and then liquid phase epitaxial growth advantageous for buried growth is performed. This waveguide was made into a single mode by (LPE: Liquid Phase Epitaxitial growth). Then, after performing p-type diffusion with Zn at a predetermined position, an electrode was formed with Au / Zn. Here, since the n-type is used as the GaAs substrate, there is no particular need for n-type diffusion from the element surface side as shown in FIG. 3, and a desired element can be formed by a simple process. In addition to the DFB laser, for example, a distributed reflection type (DBR: Distributed Bragg) is used as the light source.
It is also possible to use a Refrector laser. Both lasers are suitable for integration, and it is possible to couple laser light into an optical waveguide with high efficiency.

[0025]

As described above, according to the present invention, λ / 4
It is possible to provide an ultracompact interferometer having the above resolution and capable of discriminating the moving direction. Further, since the alignment of the optical system which is required in the conventional length measuring device is almost unnecessary, the cost required for this adjustment can be significantly reduced and the reliability of the product is enhanced.

Further, the entire interference optical system can be downsized, and the substrate elements themselves can be mass-produced by a batch process. As a result, there is also an advantage that the manufacturing cost of the interferometer can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a schematic plan view showing an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing a coupled state of a DFB laser used as a light source in this example and an optical waveguide.

FIG. 3 is a cross-sectional view of the phase shifter used in this example, taken along the line AA ′ in FIG. 1.

FIG. 4 is a principle diagram of a conventional Twyman-Green interferometer.

FIG. 5 is a schematic diagram showing a basic configuration of a conventional optical integrated circuit interferometer.

[Explanation of symbols]

 1 substrate 2 semiconductor laser (light source) 3 Bragg mirror (reflecting means) 4 Bragg mirror (reflecting means) 5 light receiving element 6 light receiving element 7 metal-loaded polarizer 8 active phase shifter 11 rod lens 12 rod lens 13 active double mode Beam splitter 14 Active double-mode beam splitter 40 Y-type branching (splitting means) 41 Y-type branching (splitting means) 42 Y-type branching 43 Y-type branching 47 X-type crossing 48 X-type crossing

Claims (4)

[Claims] 1. A light source, an optical waveguide through which light emitted from the light source propagates, a dividing means for dividing the emitted light from the light source into measurement light and reference light, and reflection means for reflecting the reference light. An interference optical system that reflects the measurement light outside the optical waveguide and then introduces the measurement light again into the optical waveguide to interfere with the reference light reflected by the reflection means; and a light receiving means that measures the interference light generated by the interference. , Is integrated on the same substrate made of semiconductor, and an interferometer. 2. The interferometer according to claim 1, wherein the light source is a distributed feedback laser or distributed reflection laser. 3. The interferometer according to claim 1, wherein the means for splitting the light emitted from the light source comprises an active double mode beam splitter. 4. The optical system according to claim 1, wherein the reference light is divided into two optical systems and propagates through the optical waveguide, and one optical system has an active phase shifter. Interference measuring device described.