JP3063131B2 - Integrated optical interferometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[Industrial application field] The present invention uses position, displacement,
More specifically, the interferometer for measuring a speed or the like relates to an integrated optical interferometer in which an interferometer is configured using an optical waveguide. [Prior Art] Conventionally, a heterodyne interferometer for measuring a position, displacement and velocity with high accuracy has 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 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. One of the two beams moves through the polarizing beam splitter 78 and the λ / 4 plate 79
Irradiated at 82. It reflected light received Doppler shift f _S This movement DUT 82, i.e. signal light, again lambda / 4 plate 79
, The polarization plane is rotated by 90 °, becomes a direction perpendicular to the paper surface, and is reflected by the polarization beam splitter 78. On the other hand, the other beam has its polarization plane rotated by 90 ° by the λ / 2 plate 74, becomes perpendicular to the paper surface, is reflected by the mirror 75, and enters the Bragg cell 76. Bragg cell 76
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, f _R by the frequency shift is given the reference beam. When the signal light and the reference light are combined by the half mirror 77 and subjected to heterodyne detection by causing interference, a signal oscillating at the Doppler beat frequency f _R ± f _S is detected by the photodetector 80. This is amplified by the amplifier 86, and the Doppler beat frequency f _R ± f _S is measured by the spectrum analyzer 88, whereby the speed of the moving object as the device under test 82 can be measured simultaneously with the moving directions. In addition, the displacement of the device under test 82 can be measured by integrating the speed with time. [Problems to be Solved by the Invention] However, these heterodyne interference optical systems are
Usually, necessary optical components are placed and fixed on an earthquake-proof table such as an optical bench.Slight adjustment of the optical axis between the components is required, and productivity and reliability are high because the optical system is large and heavy. There is a problem that the property is low. The present invention has been made to solve the above-described problems, and an object of the present invention is to integrate a main part of an interference optical system on a single substrate by using a thin-film waveguide technique. An object of the present invention is to provide a compact, highly reliable, and inexpensive optical integrated interferometer that does not require axis adjustment. Means for Solving the Problems In order to achieve this object, an integrated optical interferometer of the present invention comprises a substrate and a first optical waveguide formed on the substrate and splitting light from a light source into two. Applying a magnetic field that changes the frequency of one of the lights demultiplexed by the first optical waveguide to the first optical waveguide through which the other light passes, by applying a magnetic field that changes in a sawtooth shape by a predetermined amount; A frequency shifter to be illuminated, and a second light beam formed by irradiating the object to be measured with the other light of the light demultiplexed by the first optical waveguide and reflecting light reflected from the object to be measured on the substrate. A light irradiating means for guiding to the optical waveguide, a third optical waveguide formed on the substrate for multiplexing interference between the reflected light and light whose frequency has been shifted by a certain amount by the frequency shifter, and a third optical waveguide formed on the substrate. Light detecting means for detecting light that has been combined and interfered by the wave path. It is characterized by that.

[Action]

In the integrated optical interferometer of the present invention having the above configuration, the light generated from the light source is split into two by the first optical waveguide formed on the substrate, and one of the split lights is used. Is shifted by a certain amount by a frequency shifter that applies a magnetic field that changes in a sawtooth shape. On the other hand, the other light of the demultiplexed light is reflected after irradiating the object to be measured by the light irradiating means, and is guided as reflected light to the second optical waveguide formed on the same substrate. Then, the reflected light and the light whose frequency is shifted by a certain amount are multiplexed and interfered by the third optical waveguide, and the multiplexed interference light is detected by the light detecting means, and the position and displacement of the object to be measured are changed. , Speed, etc. are detected. [Embodiment] An embodiment of the present invention will be described below with reference to the drawings. The integrated optical interferometer uses GGG (Gd ₃ Ga ₅ O ₁₂ ) as shown in FIG.
Bi is formed on the substrate 11 _{equal: YIG (Bi x Y 3-} x Fe 5 O 12), B
_{i: 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, an optical isolator 14, an integrated lens 15, the optical fiber 17, Si photodiode or the like of the optical detection means And a photodetector 18 as a light source. 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, a magnetic thin film 12 is formed on a substrate 11 by a thin film forming means such as a liquid phase growth method or a sputtering method, and a photoresist is applied thereon. And
After patterning into a predetermined waveguide shape, a portion other than the portion to be the ridge 21 is etched by a predetermined thickness by plasma etching, sputter etching, wet etching using hot phosphoric acid, or the like. Finally, the ridge type waveguide 21 is formed by removing the photoresist. The laser light emitted from the semiconductor laser 13 propagates through an optical isolator 14 that passes the light only in one direction, and a focusing lens 15
And is incident on the ridge-type waveguide 21. At this time, the semiconductor laser 13 is mounted so that the laser beam propagating through the ridge-type waveguide 21 has a TM mode in which the direction of the magnetic field oscillation is parallel to the thin film surface. The laser light that has propagated through the ridge-type waveguide 21 is split into two by a Y-branch 23 composed of a ridge-type waveguide having one end divided into two branch waveguides.
The light propagates through the branch waveguides 24 and 25, respectively. The Y branch 23
And the branch waveguides 24 and 25 constitute the first optical waveguide of the present invention. On the branch waveguide 24, a buffer layer 31 made of SiO ₂ or the like is provided.
And an electrode 32 made of metal such as Al
33. When a current is caused to flow through the electrode 32 by the oscillator 34, a magnetic field perpendicular to the direction of propagation of the laser light and parallel to the thin film surface is applied to the branch waveguide 24. 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, and the propagation constant when a magnetic field is applied is β. Then Is represented by Here, γ is the phase shift amount due to the magneto-optical effect. As shown in FIG. 1, the Z axis is in the light propagation direction,
Taking the X axis perpendicular to the thin film surface, γ is Given by Here, ω ₀ is the angular frequency of the laser beam, ε ₀
Is the dielectric constant of vacuum, _x and _z are the x and z components of the electric field of the TM mode, ε _xz is one of the off-diagonal components of the relative permittivity tensor of the magnetic thin film 12, * is the complex conjugate, and Re is the complex number. Represents the real part. ε _xz has a relationship of Faraday rotation coefficient θ _F and ε _xz = j (2n ₁ / k) θ _F (3). Here, n ₁ is the refractive index of the magnetic thin film 12, k
Is given by k = 2π / λ (4) where λ is the wavelength of the laser light in the vacuum and the wavelength of the laser light in vacuum. 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,
Since it is proportional to the current I flowing through 32, if the proportionality constant is V, then θ _F = VI (5). Substituting equation (3) into equation (2) gives Becomes Here, A is a constant. Assuming that the length of the portion where the magnetic field is applied by the electrode 32 in the branch waveguide 24 is l, the phase-shifted TM
The electric field of the mode _{is, x = E xsin [ω 0} t + (β + γ) l] = E xsin [ω 0 t + Aθ F l + βl] becomes (7). _{Here, θ F = ω R t /} (Al) (8) i.e., (5) from equation _{I = ω R t / (AlV} ) (9) and so as to flow the current I to the electrodes 32 when _x = E _xsin [(ω ₀ + ω _R ) t + βl] (10) and the angular frequency of the laser beam is from ω ₀ = 2πf ₀ to ω ₀ +
ω _R = 2π (f ₀ + f _R ), that is, the frequency of the laser light is
The shift from f ₀ by f _R results in f ₀ + F _R. However, electrode 32
As shown in FIG. 2, the reference signal current flowing through the
V), the same frequency shift can be obtained as a sawtooth wave of period 2π / ω _R. In this way, the laser light having a frequency f ₀ as shown in FIG. 3A incident on the branch waveguide 24 is shifted in frequency by f _R by the frequency shifter 33 as shown in FIG. 3B. , The reference light having a frequency of f ₀ + f _R. The laser light that has propagated through the branch waveguide 25 enters the optical fiber 17, is converted into parallel light by a selfoc lens 41 provided at the tip of the optical fiber 17, and is irradiated on the moving object 43 to be measured. Note that the optical fiber 17 and the selfoc lens 41 constitute the light irradiation means of the present invention. DUT
The laser light reflected from 43, as receiving only the Doppler shift frequency f _S according to the speed of the displacement of the laser beam irradiation portion in the surface of the object 43, frequency Figure 3 (c)
f ₀ + f _S , and again enters the optical fiber 17 through the Selfoc lens 41. The signal light having undergone the Doppler shift by f _S propagates from the optical fiber 17 to the branch waveguide 45 as the second optical waveguide, and is shifted by the frequency shifter 33 to f _R by the Y branch 46 as the third optical waveguide. Is multiplexed with the reference light that has been frequency-shifted. This combined laser light is
The amplitude oscillates at the Doppler beat frequency f _R −f _S due to the interference, and heterodyne detection is performed by detecting this at the photodetector 18, and as shown in FIG. 3 (d), 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 f _S
, The velocity can be measured simultaneously with the direction of displacement of the surface of the moving object 43. When the direction of the surface displacement of the object 43 is reversed, a Doppler beat occurs at a position of f _R + f _S with respect to the frequency shift f _R of the reference light as shown in FIG. 3 (e). Furthermore, it is possible to measure also the displacement amount by integrating the rate of change of displacement of the surface of the Doppler shift f measured object obtained from _S 43 times. The laser light returned from the optical fiber 17 to the branch waveguide 25 propagates through the ridge-type waveguide 21 and the focusing lens 15, and
However, since the optical isolator 14 transmits light only in one direction from the semiconductor laser 13 to the focusing lens 15,
The return light from the optical fiber 17 cannot pass through and does not reach the semiconductor laser 13. Accordingly, since there is no generation of noise due to the return light in the semiconductor laser 13, a good S / N measurement can be performed. Although one embodiment of the present invention has been described in detail with reference to the drawings, 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 on the substrate,
Glass and other magnetic thin films other than rare earth iron garnet, CdMn
Semi-magnetic semiconductors such as Te, Faraday rotating glass, TAG (T
A paramagnetic material such as b ₃ Al ₅ O ₁₂ ), an oxide or a nitride such as ZnO or AlN for the buffer layer, and Au or the like for the electrode may be used. Further, the waveguide does not need to be a ridge type, and may be, for example, a buried type, a dielectric loading type, a diffusion type, and the like, particularly if the laser beam can be confined not only in the thickness direction but also in the lateral direction. Not limited. Further, a magnetic thin film may be used only for the frequency shifter, 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 is not limited. Further, a magnetic field may be applied by providing an external electromagnet or coil. Further, the light may be separated by the directional coupler 51 having two waveguides formed in parallel as shown in FIG. 4 instead of the Y-branch waveguide. At this time, the branching ratio can be changed by adjusting the length of the parallel portion of the two optical waveguides. Further, a buffer layer 61 may be provided on the entire surface of the magnetic thin film 12 as shown in FIG. The semiconductor laser 13 is directly coupled to the optical waveguide, a magnetic field applying means 62 such as ferrite is provided in the buffer layer 61, and a magnetic field inclined from the film surface in a plane perpendicular to the propagation direction of the laser light is applied. Transducer 65
May be formed. That is, the light emitted from the semiconductor laser 13 is set in the TE mode in which the direction of the electric field oscillation is parallel to the thin film surface. This TE mode passes through a metal clad 67 of Al or the like and is converted to a TM mode by a unidirectional mode converter. The unidirectional mode converter 65 has a characteristic that mode conversion occurs only for laser light propagating from the semiconductor laser 13 to the Y branch 23 and no mode conversion occurs for laser light propagating in the opposite direction. have. The following operation is exactly the same as the embodiment shown in FIG.
The light traveling from the optical fiber 17 to the semiconductor laser 13 returns to the TM mode without being mode-converted by the unidirectional mode converter 65, but the cladding layer 61 below the metal cladding 67 is thicker than other portions. Since it is smaller, it is absorbed and attenuated by the metal clad 67, so that it does not return to the semiconductor laser 13,
Good and stable measurement can be performed. [Effects of the Invention] As is clear from the above description, according to the integrated optical interferometer of the present invention, light generated from the light source is transmitted to the first optical waveguide and the second optical waveguide formed on the same substrate. The optical waveguide and the third optical waveguide are demultiplexed and interfered with each other, so that adjustment of the optical axis is not required, and a compact and highly reliable optical integrated interferometer can be provided. Further, since the frequency shifter shifts the frequency of light by a certain amount by applying a sawtooth magnetic field, the frequency shifter can be reduced in size and can be provided at a low cost.

[Brief description of the drawings]

1 to 5 show an embodiment of the present invention. FIG. 1 is a block diagram of an optical integrated interferometer according to an embodiment of the present invention, and FIG. 2 is a diagram of a frequency shifter. FIG. 3 is a diagram showing a waveform of a current applied to an electrode, FIG. 3 is an explanatory diagram showing a relationship between frequencies of laser light in an optical integrated interferometer according to an embodiment of the present invention, and FIG. 4 is an optical integrated interferometer FIG. 5 is a configuration diagram showing another embodiment of the optical integrated interferometer, and FIG. 6 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, 18
Is a photodetector, 21 is a ridge waveguide, 23 and 46 are Y branches, 24 and
25 and 45 are branch waveguides, and 33 is a frequency shifter.

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Claims (1)

(57) [Claims] 1. A substrate, a first optical waveguide formed on the substrate and splitting light from a light source into two, and one of light split by the first optical waveguide. A frequency shifter that applies a sawtooth-like changing magnetic field to the first optical waveguide through which one of the lights passes to shift the frequency by a fixed amount; and among the lights split by the first optical waveguide, A light irradiating unit that irradiates the other object to the object to be measured and guides the reflected light reflected from the object to the second optical waveguide formed on the substrate; and the reflected light and the frequency shifter. A third optical waveguide formed on the substrate for multiplexing interference with light whose frequency has been shifted by a fixed amount, and light detecting means for detecting light multiplexed and interfered by the third optical waveguide. Integrated optical interferometer.