JP2993082B2 - Integrated optical interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a position, displacement,
The present invention relates to an interferometer for measuring a speed or the like, and more particularly, to an integrated optical interferometer in which an interference optical system is configured using an optical waveguide.

[Prior Art] Conventionally, a heterodyne interferometer for measuring a position, displacement and velocity with high accuracy is a light source He-dyne as shown in FIG.
Ne laser 71, half mirror 72, λ / 2 plate 74, mirror 75, Bragg cell 76, half mirror 77, polarization beam splitter
78, a λ / 4 plate 79, and a photodetector 80. He-
The laser light emitted from the Ne laser 71 is linearly polarized light polarized in a direction parallel to the paper surface, and is split into two beams by the half mirror 72. The first of the two split beams is applied to the surface of the moving object under test 82 through the polarizing beam splitter 78 and the λ / 4 plate 79. Reflected light or signal light received Doppler shift f _S according to the displacement of the laser beam irradiated portion of the surface of the moving object to be measured 82, the polarization plane is rotated 90 ° by passing through the lambda / 4 plate 79 again, the paper The beam becomes a beam polarized in the vertical direction, and is reflected by the polarization beam splitter 78. On the other hand, the second beam of the two divided beams is rotated by 90 ° by the λ / 2 plate 74 so as to be perpendicular to the plane of the drawing, and the mirror 75
And enters the Bragg cell 76. Bragg cell 76
Is an optical modulator using an acousto-optic effect, first-order light diffracted by an acoustic wave of a frequency f _R, which is excited by the drive circuit 84 becomes the reference light is given by the frequency shift f _R. When heterodyne detection is performed by multiplexing and interfering the signal light and the reference light with the half mirror 77, a signal oscillating at the Doppler beat frequency f _R ± f _S is detected by the photodetector 80. This signal is amplified by the amplifier 86, and the Doppler beat frequency f _R ± f _S is measured by the spectrum analyzer 88. Speed can be measured. That is, assuming that the wavelength of the laser light is λ, the velocity v can be obtained by using the Doppler shift f _S as follows: v = f _S λ / 2 (1) In addition, by integrating the speed with time, the displacement of the surface due to the movement of the measured object 82 can be measured. That is, the displacement Δz is given by Δz = λ / 2 · ∫f _S dt = λ / (4π) · ∫ (δφ / δt) dt = λ / (4π) · (φ _D −φ ₀ ) (2) However, phi represents the phase, phi _D represents a measured phase, phi ₀ is an initial phase.

[Problems to be Solved by the Invention] However, this heterodyne interference optical system is usually configured by arranging and fixing necessary optical components on an anti-vibration table such as an optical bench, and a delicate optical axis between the components. There is a problem that adjustment is necessary, reliability and productivity are low, and the optical system is large and heavy.

The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to integrate a main part of an interference optical system on a single substrate using a thin-film waveguide technique. Another object of the present invention is to provide a small, stable, and highly reliable optical integrated interferometer that does not require optical axis adjustment.

Means for Solving the Problems In order to achieve this object, an integrated optical interferometer according to the present invention includes a light source and a demultiplexer for demultiplexing light from the light source into two light beams, a first light beam and a second light beam. An optical waveguide for use, a mode converter for rotating the polarization plane of the split first light by approximately 90 °, a frequency shifter for giving a constant frequency shift to the first light whose polarization plane is rotated by approximately 90 °, A light irradiating means for irradiating the object to be measured with the demultiplexed second light, guiding the reflected light from the object to be measured to the optical waveguide by rotating its polarization plane by approximately 90 °, and a light guiding means guided to the optical waveguide; The reflected light from the object to be measured and the output light from the frequency shifter are combined,
It comprises a multiplexing optical waveguide for causing interference and a photodetector for detecting output light from the multiplexing optical waveguide. Furthermore, a mode splitter that separates the light whose polarization plane and the substantially 90 ° polarization plane are rotated between the light source and the light irradiation means and guides the light to the photodetector, or a polarization plane of the light source and the substantially 90 ° polarization plane Has a mode filter that attenuates the rotating light.

Preferably, the frequency shifter comprises an optical waveguide having a magneto-optical effect, and means for applying a sawtooth-shaped magnetic field to the optical waveguide.

[Operation] In the optical integrated interferometer of the present invention having the above-described configuration, the laser light of the frequency f ₀ emitted from the laser, which is optical, becomes the TE mode in which the direction of the electric field oscillation is parallel to the thin film surface, and propagates through the optical waveguide. The light is split into two lights by the splitting optical waveguide.
The demultiplexed first light is converted into a TM mode by a mode converter and modulated by a modulator using a phase shift due to a magneto-optical effect, that is, by a frequency shifter, so that the frequency is shifted by a fixed amount f _R , The reference light has a frequency of f ₀ + f _R. The demultiplexed second light is applied to the object to be measured by the light irradiation means. Then, it is reflected from the measured object,
signal light frequency becomes f ₀ + f _S receives only the Doppler shift f _S is, the plane of polarization to propagate becomes light waveguide and TM mode by rotating 90 °, the optical multiplexing together with the reference beam is the output light from the frequency shifter The waves are multiplexed by the wave optical waveguide. When this is detected by the light detector, the light detector measurement signal that oscillates at a Doppler beat frequency f _R -f _S by the interference of the reference light and the signal light is detected. By comparing this measurement signal with the reference signal from the frequency shifter, the Doppler shift
f _S is determined, the direction and velocity of displacement of the surface with the movement of the object can be measured. At this time, the light source, which is the light returning from the light irradiation means to the light source, is TM whose polarization plane is rotated by 90 °.
Since the mode is attenuated by the mode filter or separated by the mode splitter and guided to the photodetector, the mode does not reach the light source.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. The integrated optical interferometer is, for example, as shown in FIG.
Bi: YIG (Bi _x Y) formed on a substrate 11 such as G (Gd ₃ Ga ₅ O ₁₂ )
_{3-x Fe 5 O 12)} , Bi: GdIG (Bi x Gd 3-x Fe 5 O 12) magnetic thin film 12 having a magneto-optical effect, such as, a semiconductor laser 13, polarization maintaining optical fiber 15, a SELFOC lens 16 , Λ / 4 plate 17, and a photodetector 18 such as a Si photodiode.

The magnetic thin film 12 is provided with a ridge portion 21 whose film thickness is larger than other portions, forming a ridge-type waveguide 21. The laser light propagates along the ridge waveguide 21. The ridge-type waveguide 21 is manufactured using well-known photolithography. That is, the substrate 11
A magnetic thin film 12 is formed thereon by a thin film forming means such as a liquid phase growth method or a sputtering method, and a photoresist is applied thereon. Then, after patterning into a predetermined waveguide shape, portions other than the portion to be the ridge 21 are etched by a predetermined thickness by plasma etching, sputter etching, wet etching using hot phosphoric acid, or the like. Finally,
Ridge type waveguide by removing photoresist
21 is formed.

The laser light emitted from the semiconductor laser 13 enters the ridge waveguide 21. At this time, the laser light propagating through the ridge-type waveguide 21 emits TE light whose vibration direction of the electric field is parallel to the film surface.
The semiconductor laser 13 is mounted so as to be in the mode. The laser light that has propagated through the ridge-type waveguide 21 is composed of a ridge-type waveguide having one end divided into two branch waveguides.
The light is split into two by the branch 23, and the branch waveguide 24,
Propagate 25.

On the branch waveguide 24, a buffer layer 27 made of SiO ₂ or the like is provided.
Further, electrodes 28 and 30 made of a metal such as Al are provided, and constitute a mode converter 32 and a frequency shifter 33, respectively.

When a direct current is passed through the electrode 28, a magnetic field parallel or antiparallel to the propagation direction of the laser light is periodically applied to the branch waveguide 24 immediately below the electrode 28, and mode conversion by the Faraday effect occurs. By this mode conversion, the TE mode is converted into a TM mode in which the vibration direction of the magnetic field is parallel to the thin film surface.
Generally, since the propagation constants of the TE and TM modes propagating in the branch waveguide 24 are not equal, 100% mode conversion cannot be obtained when a magnetic field in a certain direction is applied. However, since the mode conversion can be added by periodically inverting the direction of application of the magnetic field, the mode converter 32 can apply 100% inversion by applying an inverting magnetic field at an appropriate period as shown in FIG. Is obtained.

By flowing a current from an oscillator (not shown) to the electrode 30 of the frequency shifter 33, the branch waveguide 24 immediately below the electrode 30 has:
A magnetic field perpendicular to the propagation direction of the laser light and parallel to the thin film surface is applied. This magnetic field magnetizes the branch waveguide 24 and causes a phase shift due to the magneto-optical effect in the TM mode. That is, the propagation constant of the TM mode when no magnetic field is applied is β ₀ , and the propagation constant when a magnetic field is applied is β
Then, β = β ₀ + γ (3) Here, γ is the amount of phase shift due to the magneto-optical effect. As shown in FIG. 1, z-axis in the propagation direction of the laser light, taking the x-axis in the direction perpendicular to the film plane gamma _{is, γ = 2ω 0 ε 0 ∫Re} [ε xz e x * e z] dx ( 4) given by Where ω ₀ is the angular frequency of the laser beam, ε
₀ is the vacuum dielectric constant, one of the non-diagonal elements of e _x and e _z are the electric field of the x component and the z component of the TM mode, epsilon _xz dielectric constant tensor of the magnetic thin film 12, * the complex conjugate, Re Represents the real part of a complex number. ε _xz has a relationship with the Faraday rotation coefficient θ _F and ε _xz = j (2n ₁ / k) θ _F (5). Where n ₁ is the refractive index of the magnetic thin film 12,
k is the wave number of the laser light in a vacuum, and the wavelength of the laser light is λ
Then, k = 2π / λ (6) Further, in a range where the magnetization of the magnetic thin film 12 is not saturated, the magnetization of the magnetic thin film 12 is applied magnetic field H, that is,
Since this proportionality factor is proportional to the current I flowing through the electrode 30,
Then, θ _F = VI (7)

(5) Substituting expression (4) below, and _{λ = 2ω 0 ε 0 (2n} 1 / k) θ F ∫Re [je x * e z] dx = Aθ F (8). Here, A is a constant.

Assuming that the length of the portion where the magnetic field is applied by the electrode 30 in the branch waveguide 24 is l, the phase-shifted TM
Field mode, the _{e x = E x sin [ω} 0 t + (β 0 + γ) l] = E x sin [ω 0 t + Aθ F l + β 0 l] (9). _{Here, θ F = ω R t /} (Al) (10) i.e., (7) than _{I = ω R t / (AlV} ) (11) become as when the electrode 30 supplying a current I e _x = _{E x sin [(ω 0 +} ω R) t + βl] (12) next, the angular frequency of the laser beam is ω ₀ = 2πf ₀ ω ₀ +
omega _R = to _{2π (f 0 + f R)} , i.e., the frequency of the laser light is shifted from f ₀ by f _R, a f ₀ + f _R. However, the electrode
The current of the reference signal flowing through 30 has an amplitude of 2π / as shown in FIG.
(AlV), similar frequency shift can be obtained as a sawtooth wave having a frequency 2π / ω _R. Thus, the branch waveguide 24
The laser light having a frequency f ₀ as shown in FIG. 3A incident on the laser beam is shifted in frequency by f _R by the frequency shifter 33 as shown in FIG. 3B, and the reference light having a frequency of f ₀ + f _R. Becomes

On the other hand, the laser light that has propagated through the branch waveguide 25 enters the polarization plane preserving optical fiber 15 via the mode splitter 34,
The object to be measured 35 that is moving via the self-fox lens 16 and the λ / 4 plate 17 is irradiated. The laser beam reflected from the measurement object 35, as receiving only the Doppler shift frequency f _S in accordance with the changing speed of the displacement of the irradiation portion of the light at the surface of the object 35, frequency Figure 3 (c) f ₀ + f _S. The signal light, which is the reflected light having undergone the Doppler shift, passes through the λ / 4 plate 17 again, so that its polarization plane is rotated by 90 °, guided by the Selfoc lens 16 to the polarization plane preserving optical fiber 15, and TM The mode enters the mode splitter 34.

The mode splitter 34 has two parallel optical waveguides 37 and
38, a buffer layer 39 made of SiO ₂ or the like, and a metal clad 40 made of a metal such as Al. However, the buffer layer 39 is made thinner than the other buffer layers 27.
The two optical waveguides 37 and 38 are manufactured so that the TE mode propagation constants are substantially equal to each other, and operate as a directional coupler. Here, since the metal clad 40 hardly affects the propagation constant of the TE mode, the TE mode incident on the optical waveguide 37 shifts to the optical waveguide 38, and the polarization plane preserving fiber 15 connected to the optical waveguide 38. Incident on. Since the buffer layer 39 is thin, the metal clad 40 has a large effect on the propagation constant of the TM mode, and the propagation constant of the TM mode in the optical waveguide 37 loaded with the metal clad 40 is
It is smaller than the TM mode propagation constant at 38. That is, since the propagation constants of the TM mode are different from each other in the optical waveguides 37 and 38, no light wave transition occurs in the TM mode, and as a result, the TM mode does not operate as a directional coupler. Therefore, the TM mode incident on the optical waveguide 38 from the polarization plane preserving optical fiber 15 does not shift to the optical waveguide 37, but propagates through the optical waveguide 38 and the branch waveguide 42. As a result, the signal light that has undergone the Doppler shift does not return to the semiconductor laser 13 that is the light source, so that the semiconductor laser 13 does not generate noise induced by the return light, and can perform good measurement of S / N. .

Further, the frequency shifter 33 receives a frequency shift by f _R ,
The reference light propagating through the branch waveguide 24 is combined with the signal light propagating through the branch waveguide 42 by the Y branch 44.
The combined laser beam, the amplitude due to interference has vibrated in Doppler beat frequency f _R -f _S, heterodyne detection is performed by detecting this by the light detector 18, FIG. 3 (d ) As in the Doppler beat frequency f _R −
A signal oscillating at f _s is obtained.

The output signal of the photodetector 18 is compared with the fundamental frequency f _{R of the} current applied to the electrode 30 of the frequency shifter 33, and the Doppler shift is performed.
By obtaining f _s , it is possible to measure the direction of the displacement of the surface of the moving object 35 and the speed thereof at the same time. In addition,
When the direction of the displacement of the surface of the object 35 is reversed, as shown in FIG. 3 (e), the frequency shift f _R of the reference light becomes f _R + f _S
Doppler beat occurs at the position. Furthermore, it is possible to measure also the displacement amount by integrating the rate of change of displacement of the surface of the object to be measured 35 as determined from the Doppler shift f _S time.

As described above, one embodiment of the present invention has been described in detail with reference to the drawings. However, the present invention is not limited to this embodiment, and various modifications can be made.

That is, the materials of the substrate, the magnetic thin film, the buffer layer, and the electrode are not particularly limited. For example, sapphire, glass, etc. are used for the substrate, rare-earth iron garnet is used for the magnetic thin film, a dilute magnetic semiconductor such as CdMnTe, a paramagnetic material such as Faraday rotating glass, and TAG (Tb ₃ Al ₅ O ₁₂ ) are buffered. An oxide or a nitride such as ZnO or AlN may be used for the layer, and gold or the like may be used for the electrode. Further, a buffer layer may be provided on the entire surface of the magnetic thin film.

The optical waveguide does not need to be a ridge type. For example, a rectangular waveguide as shown in FIG. 4 (a), a buried waveguide as shown in FIG. 4 (b), a dielectric loaded waveguide, A diffused waveguide or the like may be used, and the shape is not particularly limited as long as the laser beam can be confined not only in the thickness direction but also in the lateral direction.

Alternatively, a magnetic thin film may be used only for the frequency shifter and the mode converter, and a dielectric material having no magneto-optical effect may be used for other portions. Further, the shape of the electrode for applying a magnetic field to the frequency shifter and the mode converter is not particularly limited. Furthermore, a magnetic field may be applied by providing an external electromagnet, coil, or the like. Further, the branch waveguide 24 is made of TE and TM.
When the propagation constants of the modes are formed to be equal to each other, there is no need to apply a periodically reversing magnetic field, and the electrodes 46 and 46 shown in FIG.
The constant magnetic field parallel to the direction of propagation of the laser light may be applied by passing a current I in a direction parallel to the film surface and perpendicular to the direction of propagation of the laser light using the 47.

Further, the demultiplexing of the laser beam is performed not by the Y-branch waveguide but by the sixth
As shown in the top view of the figure, this may be performed by directional couplers 52 and 53 in which two optical waveguides are formed in parallel. At this time, the wavelength division ratio can be changed by adjusting the length of the parallel portion of the two optical waveguides. Furthermore, as shown in the figure, a laser beam from the reference light and the light source subjected to frequency shift by f _R by the frequency shifter multiplexing, may put the frequency f _R of the reference signals to interfere. That is, a part of the TE mode emitted from the semiconductor laser 13 and propagating through the optical waveguide 54 is split by the directional coupler 52 into the optical waveguide 55. The laser light that has propagated through the optical waveguide 55 enters the polarization plane preserving fiber 15 via the mode splitter 34 as described in detail, and is irradiated to the device under test 35 through the Selfoc lens 16 and the λ / 4 plate 17. . The reflected light from the DUT 35 whose frequency is f ₀ + f _S undergoes a Doppler shift by the frequency f _S in accordance with the change in the displacement of the surface of the DUT 35, and is λ / 4
After the polarization plane is rotated by 90 ° by the plate 17, it is guided to the branch waveguide 56 as a TM mode by the Selfoc lens 16, the polarization plane preserving optical fiber 15, and the mode splitter 39. Optical waveguide
The TE mode propagating in 54 is further split by the directional coupler 53 into the optical waveguide 58. The remaining TE modes propagating in the optical waveguide 54 are converted to the mode converter 32 as described in detail above.
To the TM mode, and the frequency is shifted by f _R by the frequency shifter 33, so that the reference light has a frequency of f ₀ + f _R. This reference light is split into two by the Y branch 60 and propagates through the branch waveguides 61 and 62, respectively. The reference light propagating through the branch waveguide 61 and the signal light propagating through the branch waveguide 56 are combined by the Y branch 63. The amplitude of the multiplexed laser light is changed by Doppler beat frequency f _R due to interference.
-F _S is vibrated, the heterodyne detection is performed by detecting this by a photodetector 64, a signal that oscillates at the top color beat frequency f _R -f _S is obtained. On the other hand, when the reference light that has propagated through the branch waveguide 62 and the laser light that has not undergone the frequency shift that has propagated through the optical waveguide 58 are multiplexed by the Y-branch 65, the amplitude of the multiplexed laser light becomes larger due to interference. There has been vibrated at a frequency f _R, a signal that oscillates at a frequency f _R is obtained by detecting this in the photodetector 66. By determining the Doppler shift f _S by comparing the output signal of the photodetector 64 and 66 can simultaneously measure the speed and direction of the displacement of the surface of the moving object to be measured 35. At this time, even if the frequency shift f _R of the reference light fluctuates due to fluctuations in the temperature or the like of the frequency shifter 33, the Doppler beat frequency f _R −f _S of the output of the photodetector 64 and the reference signal of the output of the photodetector 66 the difference f _S of the frequency f _R of the can be accurately determined Doppler shift f _S becomes constant.

As shown in FIG. 7, a Y-branch 67 is used instead of the mode splitter 34, and the Y-branch 67 and the semiconductor laser 13 as a light source are used.
For example, a mode filter 70 including a buffer layer 68 and a metal clad 69 may be provided on the ridge waveguide 21. The mode filter 70 allows only the TE mode to pass by the function of the metal clad 69 and attenuates the TM mode. That is, the laser light reflected from the device under test 35 has its polarization plane rotated by 90 ° by the λ / 4 plate, enters the TM mode, is branched by the Y branch 67, and propagates through the branch waveguides 25 and 42. here,
Since the TM mode propagating through the ridge waveguide 21 through the branch waveguide 25 is attenuated by the mode filter 69, the TM mode does not reach the semiconductor laser 13. Therefore, the S / S
Good measurement of N can be performed. On the other hand, the TE mode emitted from the semiconductor laser is not affected by the metal cladding and propagates through the ridge waveguide 21 without being attenuated.

[Effects of the Invention] As is clear from the above description, according to the present invention, the main components of the interference optical system such as a laser beam splitter, a multiplexer, and a frequency shifter using the thin-film waveguide technology. Since the units are integrated on one substrate, it is possible to manufacture a small, stable, and highly reliable optical integrated interferometer with high productivity without the need for optical axis adjustment. In addition, a mode splitter and a λ / 4 plate or a Y-branch waveguide and a mode filter are combined to prevent the reflected light from the device under test from returning to the light source. No noise occurs, and good S / N measurement can be performed.

[Brief description of the drawings]

FIGS. 1 to 7 show an embodiment embodying the present invention. FIG. 1 is a configuration diagram of an optical integrated interferometer according to an embodiment of the present invention, and FIG. FIG. 3 is a diagram showing a waveform of a current applied to an electrode of a frequency shifter. FIG. 3 is an explanatory diagram showing a relationship between laser light frequencies in an optical integrated interferometer according to an embodiment of the present invention. FIG. 4 is a partial view showing another embodiment of the integrated optical interferometer, and FIG. 5 is a partial view showing another embodiment of the mode converter in the integrated optical interferometer. FIG. 7 is a top view showing another embodiment of the integrated optical interferometer, and FIG. 8 is a block diagram showing an optical system of a conventional heterodyne interferometer. In the figure, 11 is a substrate, 12 is a magnetic thin film, 13 is a semiconductor laser, 15 is a polarization plane preserving fiber, 16 is a selfoc lens, 17 is λ
/ 4 plate, 18 is a photodetector, 21 is a ridge waveguide, 23, 44, 67
Is a Y-branch, 24, 25, and 42 are branch waveguides, 32 is a mode converter, 33 is a frequency shifter, 34 is a mode splitter, and 70 is a mode filter.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-43704 (JP, A) JP-A-64-32102 (JP, A) JP-A-60-85312 (JP, A) 20107 (JP, U) (58) Field surveyed (Int. Cl. ⁶ , DB name) G01B 9/00-11/30 102

Claims (4)

(57) [Claims] A light source, a demultiplexing optical waveguide for demultiplexing light from the light source into two light beams, a first light beam and a second light beam, and a polarization plane of the demultiplexed first light beam having approximately 90 degrees. A mode converter for rotating, a frequency shifter for giving a constant frequency shift to the first light whose polarization plane has been rotated by approximately 90 °,
A light irradiating means for irradiating the object to be measured with light, guiding the reflected light from the object to be measured to the optical waveguide by rotating its polarization plane by approximately 90 °, and guided from the light irradiating means to the optical waveguide. A multiplexing optical waveguide for multiplexing and interfering reflected light from the device under test and output light from the frequency shifter, and a photodetector for detecting output light from the multiplexing optical waveguide. An optical integrated interferometer characterized by the following. 2. The integrated optical interferometer according to claim 1, wherein:
An optical waveguide in which the frequency shifter has a magneto-optical effect,
Means for applying a magnetic field whose amplitude varies in a sawtooth manner to the optical waveguide having the magneto-optical effect. 3. The integrated optical interferometer according to claim 1, wherein:
Between the light source and the light irradiating means, the polarization plane of the light source and approximately 90
光 An integrated optical interferometer, comprising: a mode splitter that separates light whose polarization plane is rotating and guides the light to a photodetector. 4. The integrated optical interferometer according to claim 1, wherein:
Between the light source and the light irradiating means, the polarization plane of the light source and approximately 90
光 An integrated optical interferometer, comprising: a mode filter for attenuating the light whose polarization plane is rotating.