JPH0527042B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Technical Field The present invention relates to an optical integrated circuit, and particularly to a non-reciprocal optical phase shifter constituting an optical integrated circuit of an optical system for an optical fiber pilot.

(a) Principle of optical fiber coil An optical fiber coil is a device that measures rotational angular velocity and consists of a light emitting element, a light receiving element, a single mode optical fiber coil, a beam splitter, etc.

A laser, a superluminescent diode, or the like is used as the light emitting element. The light emitted from the light-emitting element is split into two by a beam splitter, and the light is made to enter both ends of an optical fiber core made by winding a single-mode optical fiber many times. Each light propagates through the optical fiber coil as left-handed light and right-handed light, and is emitted from the other end. This light is combined by a beam splitter, and the intensity of the interference light is detected by a light receiving element.

When the optical fiber coil rotates at an angular velocity Ω, there is a difference in the propagation time of the counterclockwise light and the clockwise light, so that a phase difference Δθ occurs between the left and right lights when they reach the light receiving element. The phase difference is proportional to the angular velocity Ω, the total length L of the optical fiber coil, and the radius a of the coil, and is given by Δθ=4πLaΩ/cλ (1). c is the speed of light in vacuum, and λ is the wavelength of light in vacuum. This is called the Sagnac effect.

Therefore, in the light-receiving element, the light is represented by a wave function: right-handed light Ersin (ωt + Δθ/2) (2) counterclockwise light Elsin (ωt-Δθ/2) (3). ω is the angular frequency of light, and Er and El are the amplitudes.

Since these lights interfere on the light receiving element, the output I (Δθ) is the square of the sum of (2) and (3), I (Δθ) = 1/2 (Er ² + El ² ) + ErElcosΔθ (4) It is expressed as Since Er and El are known in advance, the phase difference Δθ can be determined by measuring the output I(Δθ) of the light receiving element. This determines the rotational angular velocity Ω.

This is the principle of optical fiber gyros.

(C) Phase-biased optical fiber coil As shown in the basic equation (4), in the basic configuration of the optical fiber coil, the output I (Δθ) includes a phase difference in the form of the cosine (cosΔθ) of Δθ. Since the rate of change of the output I (Δθ) with respect to Δθ is small in a region where Δθ is small, there is a problem that sensitivity is poor in a low speed rotation region. Also, since cosine is an even function, Δθ
The disadvantage is that it is not possible to distinguish between the positive and negative values, that is, the direction of rotation.

In order to overcome this, the optical paths of the counterclockwise light and the clockwise light are partially separated, and the optical paths of the counterclockwise light and the clockwise light are made to differ by π/2.

There are several ways to apply a phase bias. For example, there is a method in which the wavefront of the emitted light is curved by using a defocus between a lens or a fiber.

Another method is to separate the optical paths of left-handed light and right-handed light using four beam splitters, insert a phase plate in one of the optical paths, and apply a phase bias of π/2.

If we can give a phase difference of π/2,
The output I(Δθ) of the light-receiving element is replaced by equation (4) as follows: I(Δθ)=1/2(Er ² +El ² )+ErElsinΔθ (5). In this case, the alternating current component of I(Δθ) becomes an odd function of Δθ, and even if the phase difference is small,
Ω can be calculated with high accuracy. It is also possible to identify the direction of rotation.

However, since this method creates an optical path difference between the left-handed light and the right-handed light, it is difficult to accurately apply a phase bias of π/2.

Furthermore, since the optical path difference changes depending on the wavelength fluctuation of the light emitting element, there is a problem that this appears as a drift in the output.

(iv) Purpose of the Invention The present invention is an optical integrated circuit used in an optical fiber gyro, and is capable of applying a phase bias between left-handed light and right-handed light without creating an optical path difference. The purpose is to provide a small, robust, and stable phase shifter.

(E) Slab waveguide eigenmodes Let us consider the eigenmodes propagating in slab waveguides.

Set up the coordinate system as shown in Figure 5. Consider a slab waveguide in which a thin plate waveguide Q with a width d is sandwiched between semi-infinite blocks R and R on both sides.

The refractive index of the thin plate waveguide Q is higher than the refractive index of R and R, and light is confined within the waveguide Q. Therefore, we tentatively name Q as core and R as clad.

The traveling direction of light is the x-axis direction, and the y-axis is the direction perpendicular to the interface between the core and the cladding. In the z-axis direction,
It is assumed that both the core and the cladding are uniform. As shown in Figure 5, a slab with one core sandwiched between claddings on both sides is called a three-layer slab.

There are two cores and three layers of cladding separate and cover them, that is, R, Q, R, Q, R.
Let's call it a five-layer slab.

Let the thickness of the core be d. This is also called width. Since it is assumed to be uniform in the z direction, the only parameters are d, the wavelength λ of light, and the refractive index of the core and cladding. The optical axis is the z-axis.

Also, the mode of light whose electric field vector is in the y direction is
The mode of light in the z direction is called TM mode.
Let's call it TE mode.

While the core width d is small, only the zero-order mode propagates. As d becomes wider, the first mode,
The secondary mode also propagates. Here, the mode order refers to how many 0 points there are in the wave function in the y direction, and as is well known,
In the case of TM _mode , m ⁵ γd＝KdtanKd (6) (Kd) ² + (γd) ² =V ² ₍ 7) V＝kd√ ₁ ² − ₂ ² (8 ) n ₁ ² K ² −β ² =K ² (9) The propagation constant β can be found from (9).

However, n ₁ and n ₂ are the ordinary refractive indices of the core and the cladding, and k is the wave number (2π/λ) in vacuum. V
is the normalized frequency. m=n ₁ /n ₂ .

For odd-order modes such as TM ₁ , TM ₃ , . . . , tan in equation (6) changes to −cot.

For the TE mode, m=1 and the extraordinary refractive index is used.

Figure 2 shows the dispersion curve of the TE mode of the LiTaO ₃ three-layer slab waveguide. On the horizontal axis, the core width d is the normalized waveguide width (d/λ) divided by the wavelength λ. The vertical axis is the normalized propagation constant (β/k) obtained by dividing the propagation constant β by the wave number k in vacuum. The refractive index n ₂ of the cladding is 2.1878, the refractive index n ₁ of the core is 2.1900,
The ratio of refractive index difference is (n ₁ −n ₂ )/n ₁ =0.001.

When the normalized waveguide width is 3 or 4, only the TE ₀ mode exists. It is a single mode slab waveguide. When the waveguide width is 7, the TE ₁ mode can also exist.

Figure 3 shows the dispersion curve of the TM mode of the three-layer slab waveguide. The horizontal axis is the normalized waveguide width (d/
λ). The vertical axis is the normalized propagation constant (β/k). The refractive index n ₂ of the cladding is 2.1834, the refractive index n ₁ of the core is 2.1856, and the ratio of the refractive index difference is (n ₁ − _{n 2} )/
n ₁ =0.001.

In FIG. 2 or 3, the propagation constant β can be uniquely defined when the core width d is constant and the waveguide continues infinitely in the x direction.

The width d of the core is not uniform in the traveling direction x,
In the case of gradual expansion or narrowing, β cannot be defined. However, if the change in width d is small, it is possible to locally consider the propagation constant β(x).

Therefore, when the width d changes in the x direction as a function of x, that is, it can be written as d(x), we can consider the local propagation constant β(d(x)) at the x point. .

If β changes adiabatically, the power of the TE ₀ mode is conserved in the x direction. In the figure, β(x) is considered to vary along the curve of TE ₀ . It is important here that power is conserved.

(f) Mode conversion in a branch Let us consider mode conversion in a Y-shaped branch as shown in FIG.

The traveling direction is the x direction, but there are branches in this direction. Assume that the refractive index is uniform in the z direction.

Assume that a slab waveguide with width W ₀ is divided into waveguides with widths W ₁ and W ₂ . The width W here refers to the normalized waveguide width (d/λ). It was designated as W to distinguish it from d. W etc. may also represent a waveguide.

W ₀ =W ₁ +W ₂ (10). W ₀ is a 2-mode waveguide, W ₁ and W ₂ are 1
Assume that it is a mode waveguide. From Figures 2 and 3
If W ₀ is about 5 to 8, such W ₁ , W ₂
exists.

Since mode conversion occurs in exactly the same way in TE mode and TM mode, there is no need to distinguish between the two modes. Therefore, in any mode, 0
Let T ₀ represent the next mode and T ₁ the first mode. E and M are omitted.

FIG. 6 shows a mode dispersion curve similar to FIGS. 2 and 3.

The horizontal axis is the normalized waveguide width W (d/λ), and the vertical axis is the (normalized) phase constant Σ (β/k). On the horizontal axis,
Take the waveguide widths W ₀ , W ₁ , and W ₂ of the Y branch.

Draw a straight line parallel to the vertical axis from these points and find T ₀
Let A, B, C, and D be the intersection points of the curve and the _T1 curve.

In the wide waveguide W _{0 ,} T ₀ and T ₁ modes exist, and their phase constants are Σa and Σb, respectively. distributed waveguide
In W ₁ and W ₂ , only T ₀ mode exists, and the phase constants are Σc and Σd, respectively.

Hereinafter, Σ ₁ will be referred to as a phase constant to distinguish it from the propagation constant β.

When W on the T ₀ curve is W ₀ , W ₁ , W ₂ , the phase constants Σa, Σc, Σd are the values of Σ at points A, C, and D.

The wide waveguide is called an integrated waveguide W ₀ , and W ₁ , W ₂
are called branch waveguides W ₁ and W ₂ .

In the integrated waveguide W ₀ , the modes T ₀ and T ₁ at points A and B
exists. It is a two-mode waveguide.

In branch waveguides W ₁ and W ₂ , mode T ₀ at points C and D
Only exists. It is a one mode waveguide.

Modes such as A, B, C, and D can be defined when these waveguides are sufficiently long. The Y branch in FIG. 7 cannot define such a mode.

It is assumed that the waveform travels from the integrated waveguide W ₀ to the branch waveguides W ₁ and W ₂ . First, consider what goes to W ₁ . Assume that W ₁ > W ₂ .

At this branch, there is a transition from point A to point C, and
A transition from point to point C may occur. In other words, there are cases in which the phase constant is converted from Σa to Σc, and in others from Σb to Σc.

If we write the transition probabilities as P(a→c) and P(b→c), then from perturbation theory, the ratio of these is P(a→c)/P(b→c)=|f((Σa− Σc)
X)/f((Σb−Σc)X) | ² (11) is given by. X is a normalized coordinate in the traveling direction. The function f(x) is f(x)=sinx/x (12). Since X is a normalized coordinate, there is a relationship with the x coordinate as kx=X (13).

Similarly, consider the integrated waveguide traveling from W ₀ to W ₂ . There may be some that proceed from point A to point D, and some that proceed from point B to point D. The ratio of transition probabilities is P(b→d)/P(a→d)=|f((Σb−Σd)
X)/f((Σa−Σd)X) | ² (14)

In equation (11), the difference in phase constant between points A and C (Σa
-Σc) is smaller than the difference between points B and C (Σb - Σc), so as X becomes larger, the value of the ratio in equation (11) increases. The same applies to (14).

Eventually, all the modes at point A will be transferred to point C, and all the modes at point B will be transferred to point D.

The transition from Σa to Σc and the transition from Σb to Σd are
It can be expressed as shown in FIG. 4 using (g/λ) as a coordinate. Figure 4 shows the separation g of the waveguide with λ
The horizontal axis is the value divided by Σa, which is the propagation constant after the Y branch.
It shows how Σb changes to Σd, and Σb changes to Σd.

Let the angle formed by the branch be α, (α≪1) g=xα (15) Therefore, the horizontal axis g/λ is g/λ=α/2πkx (16) =α／2πX (17) It is.

After all, the T ₀ mode (point _A ) of the W ₀ waveguide is
The T ₀ mode (point C) is set, and the T ₁ mode (point B) is
This results in the T ₀ mode (point D) of W ₂ .

Here are the characteristics of asymmetric bifurcation. The two modes in the W ₀ waveguide can be separated. This is important.

Next, consider symmetric bifurcation.

Suppose that the same branch of Wm such that Wm=W ₀ /2 (18) is provided instead of W ₁ and W ₂ .

In the branch waveguide, only the T ₀ mode with a phase constant of Σe exists. Σe is an intermediate value between Σd and Σc.

Then, transition from point A and point B to point E can occur in the two branch waveguides.

In other words, degeneration occurs. Since only one phase constant can exist, T ₀ and T ₁ of the W ₀ waveguide
Both modes are converted into both branches Wm and Wm.

In the case of an asymmetric bifurcation (W ₁ ≠ W ₂ ), the degeneracy is always solved. Proceed from point A to point C, and from point B to point D.

(G) Mode conversion at the integration point Conversely, consider the case where the waveguide propagates from the branches W ₁ and W ₂ to the integration waveguide W ₀ . These phenomena are reversible.

There is a mode at point C in the branch waveguide W ₁ and a mode at point D in W ₂ . When this progresses to W ₀ , the mode at point C is converted to point A, and the mode at point D is converted to point B. In other words, the thicker mode (point C) of the branches
is the thinner mode (point D) in the T ₀ mode of W ₀ .
becomes the T ₁ mode of W ₀ .

The same applies to symmetric branches. The mode conversion of the light traveling from the branch waveguides Wm and Wm to the integrated waveguide W ₀ is performed from point E to point A and from point E to point B. Since the values of (Σa−Σe) and (Σe−Σb) are almost equal, both the T ₀ mode and the T ₁ mode are excited in approximately the same amount in W ₀ .

In a symmetrical branch, from both branches, the integrated waveguide
The process of proceeding to W ₀ is as described above, but the same applies to the case of proceeding to the integrated waveguide W ₀ from only one branch. There is light only in one branch, and this
When progressing to W ₀ , the modes at points A and B are excited in equal amounts.

The above properties are the same in both TE and TM modes.

(H) TE→TM converter When the slab waveguide is made of electro-optic crystal with birefringence as shown in Figure 5, the TE/TM converter
Transducers can be configured.

RCAlferness “Efficient waveguide electro
-optic T ET M converter/
wavelength filter ”Appl.Phys.Lett. 36 (7)P.513
-515, 1980 (published April 1, 1980),
TE/TM converters have been proposed.

This uses a LiNbO ₃ crystal as a substrate, diffuses Ti on the surface to form a linear waveguide, deposits comb-shaped electrodes on both sides of the waveguide, and applies a DC voltage between the electrodes. This is what I did.

The operating principle, according to this paper, is as follows.

LiNbO ₃ (Lithium Niobate) has strong birefringence, and for light with a wavelength of 0.6328 μm, no−ne=
It is 0.085.

In Figure 5, the TE mode has an electric field component in the z direction, and an extraordinary refractive index ne is felt. TM mode has an electric field component in the y direction, and the ordinary refractive index no
I feel it.

Let A be one pitch of the comb-tooth electrode, and make it satisfy 2π/λ ₀ |N _TE −N _TM |=2π/Λ (19).

Here, N _TE and N _TM are effective refractive indices in the waveguide for TE mode and TM mode.

Here, A=7 μm. This periodic condition is
While moving the electrode comb teeth one pitch, TM,
The condition is that the phase difference of light in TE mode is 2π. Therefore, it is a device with extremely strong wavelength selectivity, and can also be used as a filter.

The transformation of TETM is reversible, and the transformation coefficient η
is expressed as η=sin ² KL. L is the total length of the electrode section. Since it is a z-cut LiNbO ₃ , the voltage is applied in the y-axis direction in FIG.

K=αkns ³ r ₅₁ V/de (21) where α is the shape constant between 0 and 1, ns is the refractive index of the core, V is the voltage, de is the distance between the electrodes, and r ₅₁ is the electro-optic coefficient. . k is the wave number in vacuum (2π/λ)
It is.

If KL = π/2, η = 1, and the TE mode becomes almost completely the TM mode, and the TM
The mode can be almost completely converted to TE mode. According to the above paper, a conversion efficiency of 99% was obtained.
That's what it means.

TM and TE mode converters can be easily constructed for waveguides made of LiTaO ₃ as well.

(k) TM mode and TE mode When light in TM and TE modes travels through a birefringent medium, the effective refractive index of each mode is different. However, as shown in Figures 2 and 3, if the ratio of the refractive index difference between the core and the cladding (n ₁ - n ₂ )/n ₂ is the same, and the anisotropy of the medium is not very pronounced, then The relationship between the differences in phase constants in the zero-order mode and the one-sheet mode is the same.

In other words, for the phase constants of TEi mode and TMi mode, Σ(TEi(W))−Σ′(TMi(W))=constant (22) (i=0, 1, 2). Here, TMi mode is distinguished by adding a dash.

(J) Structure of the invention FIG. 1 is a plan view of a non-reciprocal optical phase shifter of the invention.

This is because LiTaO ₃ consists of a waveguide system consisting of a diffused channel waveguide formed on the z-cut plane of LiNbO ₃ .

The coordinate system has an x-axis in the direction in which the light travels. Among the directions perpendicular to this, the direction perpendicular to the plane is taken as the z-axis.
The direction perpendicular to the waveguide and in the plane is the y-axis.

In the case of LiTaO ₃ , which is a birefringent medium, (1) extraordinary refractive index ne 2.1878 (2) ordinary refractive index no 2.1834.

In this way, the substrate material must have birefringence. In the example above, the refractive index difference is 0.0044.

Waveguides are fabricated on a substrate by diffusing Ti to locally increase the refractive index. This corresponds to the core.

A waveguide is formed by Ti diffusion. This waveguide is formed only in a shallow portion from the surface. The waveguides described up to the previous section are infinitely wide in the z direction, but such a waveguide cannot be manufactured. In this way, the waveguides have different cross-sectional shapes. However, regarding changes in the phase constant, etc., mode conversion of the waveguide can be inferred by analyzing the three-layer slab and five-layer slab shown in FIGS. 5 and 7.

This phase shifter has a symmetrical branch and an asymmetrical branch formed on a substrate. From the side closest to the light source and receiver to the fiber optic coil, S1,
This will be explained separately as S2, S3, and S4.

S1 is a symmetrical branch, S2 is an integration section, S3 is an asymmetrical branch, and S4 is a TE/TM mode conversion section.

The waveguide is divided into a first waveguide 1 to a ninth waveguide 9.

The symmetrical branch of S1 is provided with a first waveguide 1 that introduces light from a light source and a ninth waveguide 9 that outputs light to a light receiver. Both widths are equal. Wm and
Wm. Therefore, this is called a symmetric bifurcation. The first waveguide 1 has a metal coat 10 .

The integration section of S2 has one second waveguide 2 for integration. The width of this is W ₀ , which is equal to 2Wm.

The asymmetric branch of S3 has a narrow waveguide and a wide waveguide. It doesn't matter which one is wider, but here the third waveguide 3 is narrower (width W ₂ ) and the eighth waveguide is wider (width W ₁ ). Further, electrodes parallel to the xy plane are provided here. The electrode plate has a ground electrode on the back surface of the substrate, and electrodes 11 and 11 on the third waveguide 3 and the eighth waveguide 8, respectively.
12 will be provided. The same function can be achieved even if one of the electrodes 11 and 12 is used as a ground electrode. This method is easier to produce. Even in this case, since the waveguide is thin, an electric field is applied in the z-axis direction.

A DC voltage may be applied, or an AC voltage may be applied.

For example, voltage application is +V to electrode 11, +V to electrode 1
Add -V to 2. Although the grounding electrode of the battery 13 is not shown, in this case, a battery with a total of 2V is used, and the middle is connected to the grounding electrode.

Of course, there may be only one electrode.

In the waveguide, the voltage is in the direction perpendicular to the plane, that is, z
occurs in the direction.

Some of the light propagating through these waveguides has a polarization direction in the z direction and some in the y direction. Although it is no longer a slab waveguide, we will use the same notation. Something with an electric field component in the y-axis direction,
This is called TM mode, and the one that has an electric field component in the z-axis direction is called TE mode.

Since an electric field is applied to the electrodes 11 and 12 in the z-axis direction, only the TE mode light is affected by the electric field and has a different phase constant. In other words, z
The extraordinary refractive index ne changes depending on the electric field Ez in the direction,
Therefore, the phase constant differs only while this electric field exists. The product of the change in the phase constant and the length of the electrode is the change in the phase of the TE mode light passing through it. Letting the phase change be ΔΦ ₁ and ΔΦ ₂ ,
The total variation is given by this sum. TM mode does not change at all.

S ₄ is a narrow fourth waveguide 4 and wide waveguides 6 and 7.
has. The fourth waveguide 4 follows the third waveguide 3. A TE/TM mode converter 14 is provided in the wide waveguide. The waveguides before and after the sixth waveguide,
They are called seventh waveguides 6 and 7.

The TE/TM mode converter 14, which has already been described, rotates the polarization of the light propagating through it by 90 degrees. TE mode becomes TM mode, and TM mode becomes TE mode. It's exactly the same no matter which direction you go.

A single mode optical fiber coil 5 is provided on the end face of _S4 . This is a single mode optical fiber wound into a coil many times.

Since it is a single mode fiber, only the zero order mode is propagated. There are two polarization directions in the zero-order mode: the y direction and the z direction. Moreover, what is important is that the two polarized waves must pass through the coil while maintaining an orthogonal relationship. What enters as a y-polarized wave must come out as a y-polarized wave. What enters with z-polarization must come out with z-polarization. Let's write each wave as Ty and Tz. Both Ty and Tz have the same phase constant.

(S) Effect Monochromatic light from the light source is coupled to the first waveguide 1. The first waveguide 1, the third waveguide 3, the eighth waveguide 8, and the ninth waveguide 9 are single mode waveguides, and only TE ₀ and TM ₀ exist.

The second waveguide 2 is a two-mode waveguide, with TE ₀ ,
In addition to TM ₀ , TM ₁ and TE ₁ modes may also exist.

(1) The light entering the first waveguide 1 has TE ₀ and TM ₀ modes. The metal coat 10 is a surface coated with metal. have an electrode parallel to this
The TM ₀ mode produces eddy currents in the metal and is attenuated.

(2) Only the TE ₀ mode travels from the first waveguide 1 to the second waveguide 2, which is an integrated waveguide.

There was a transition from the W m waveguide to the W ₀ waveguide, which was twice as wide. As explained using FIG. 6, this is a transition from point E to points A and B. Since the difference in phase constants (Σa − Σe) and (Σe − Σb) are almost equal, in the second waveguide 2 (W ₀ )
TE ₀ mode and TE ₁ mode will be excited in equal amounts.

(3) Both modes proceed through waveguide 2 and enter the asymmetric branch. In the S ₃ branch, the waveguide 3 is narrow (W ₂ ) and the waveguide 8 is wide (W ₁ ). However, in both cases, only the zero-order mode propagates.

It is a single mode waveguide. What is important is that it is an asymmetric branch.

The TE ₀ mode of S2 has a phase constant of Σa, and the TE ₁ mode has a phase constant of Σb.

A waveguide with a width of W ₁ and W ₂ can transmit a TE ₀ mode with phase constants of Σc and Σd. So Σa is Σc
and Σb is converted to Σd. This is because Σc＞Σd. It is converted to a mode with a closer phase constant.

The TE ₀ mode of S2 travels to waveguide 8, and the TE ₁ mode of S2 travels to waveguide 3. The reason for the asymmetric branch is that modes do not mix.

If S3 is a symmetric branch, waveguide 3,
8, conversion from both modes of S2 is performed,
There are two phase relationships. The Σd mode entering the waveguide 3 undergoes a phase change ΔΦ ₁ while passing through the electrode 11.

Since this is the TE mode, the phase changes.

(4) The Σd mode travels from waveguide 3 to waveguide 4 and enters the single mode optical fiber. It is important here that the light enters as light with a polarization direction in the z direction.

It propagates clockwise in the single mode optical fiber coil 5. Inside the coil, the phase constant is the same no matter which direction the plane of polarization is.

(5) Single mode optical fiber coil 5 in the clockwise direction
The light that has passed through enters the right end of the sixth waveguide 6. Here, the polarization direction is the z direction. Enters TE ₀ mode.

Since the sixth waveguide 6 has a wide width _W1 , the phase constant is Σc.

(6) It passes through the TE/TM mode converter 14 from right to left. Here, TE ₀ is converted to TM ₀ mode. From then on, it remains in TM mode. The seventh waveguide and the eighth waveguide have a phase constant of Σc'.

(7) Although the TM ₀ mode light passes through the eighth waveguide 8 , the phase is not changed by the electrode 12 . This is because the polarization is in the y direction, which is orthogonal to the direction of the electric field.

(8) When exiting from the eighth waveguide 8 to the thick two-mode waveguide 2, it is an asymmetric branch, so Σc' is converted to Σa'. In other words, the entire amount is converted to the TM ₀ mode of S2.

This is the important point.

(9) This light reaches the symmetric branch of S1. Equal amounts are distributed to the first waveguide 1 and the ninth waveguide 9.
This is the transformation from point A to point E in FIG.

Goes to TM ₀ mode facing left. However, the light that returns to the first waveguide 1 is attenuated by the metal coat 10 and does not return to the light source. The light source is often a semiconductor laser, whose oscillation becomes extremely unstable due to returned light. The fact that there is no return light is very convenient.

(10) On the other hand, the light that enters the thicker waveguide 8 during the transition from S2 to S3 will undergo a transition from point A to point C in FIG. The Σa light in TE ₀ mode of S2 becomes Σc TE ₀ mode in S3. This passes through electrode 12 and undergoes a phase change ΔΦ ₂ .

(11) When entering waveguide 7 from waveguide 8, TE of Σc _{is 0}
The mode light is converted into TM ₀ mode by the TE/TM mode converter 14 . The phase constant is Σc'.

(12) The TM ₀ mode of Σc′ is emitted from the waveguide 6,
The light enters the single mode optical fiber 5. Polarization is in the y direction.

There is only one phase constant in the single mode optical fiber 5. This light passes through the optical fiber coil counterclockwise and enters the fourth waveguide 4 on the opposite side. Polarization is in the y direction.

(13) The fourth waveguide becomes TM ₀ mode. The phase constant is Σd′.

(14) The TM ₀ mode light travels to the third waveguide 3, but does not undergo any phase change even if it passes through the electrode 11 here. This is because the electric field and the polarization direction are orthogonal.

(15) The second waveguide 2 may have TM ₀ and TM ₁ modes. Since the third waveguide 3 is narrow, it was Σd', but this is due to the fact that the second waveguide 2
is converted to TM ₁ mode, and the phase constant becomes Σb′.

(16) The light that becomes TM ₁ mode is a symmetric branch of S1,
Equal amounts are distributed to the first waveguide 1 and the ninth waveguide 9. This is the conversion from point B to point E in FIG.
The phase constant changes from Σb′ to Σe′. TM ₀ mode.

(17) The TM ₀ mode light that returned to the first waveguide 1 is
Attenuated by metal coat.

(18) The Σe′ returns to the 9th waveguide and becomes the TM ₀ mode (counterclockwise light). This and the right-handed light that returned to the ninth waveguide in (9) are both in the TM ₀ mode of Σe', so they interfere with each other. The interference wave is detected by a photoreceiver.

(b) Optical path difference In order to find the phase difference between right-handed light and counter-clockwise light, consider the optical path difference in consideration of the phase constant.

In _S1 , since both lights are in the same mode, there is no need to consider them. Since the optical fiber coils have the same phase constant (the polarization differs), there is no need to take this into account.

The problem is the difference in phase constants in S2, S3, and S4. Contrast and compare the phase constants.

The clockwise light becomes TM in waveguides 7, 8, and 2, and TE in waveguides 2, 3, 4, and 6.

The counterclockwise light becomes TE in waveguides 2, 8, and 7, and TM in waveguides 6, 4, 3, and 2.

Let the length of waveguide 2 be L, waveguide 3, 4, or 6,
The total length of the waveguides 7 and 8 is L ₀ , and the length of the waveguides 6 and 7 from the center of the mode converter 14 to the end point of the waveguide 6 is L ₁ .

(1) The clockwise light is generated in each waveguide as follows: Waveguide 2 Σb L Waveguide 3, 4 Σd L ₀ Waveguide 6 Σc L ₁ Waveguide 7, 8 Σc′ (L ₀ −L ₁ ) Waveguide 2 Σa ′ is associated with the phase constant of L.

(2) The counterclockwise light is transmitted in each waveguide as follows: Waveguide 2 Σa L Waveguide 8, 7 Σc (L ₀ −L ₁ ) Waveguide 6 Σc' L ₁ Waveguide 4, 6 Σd' L ₀ Waveguide 2 It corresponds to the phase constant of Σb′ L .

It is assumed that the difference in phase constant between the TEi(W) mode and the TMi(W) mode is a constant value regardless of the number of modes i and the width W. This is the assumption of equation (22). This constant value will be written as Q.

Equation (22) is Σa+Σa′=Q (23) Σb−Σb′=Q (24) …………… Σe−Σe′=Q (25) That's what it means.

The phase difference between clockwise light and counterclockwise light is the phase constant,
The sum of the products of the waveguide lengths is calculated and is given by the difference between the sums.

The phase difference ΔΨ ₀ eventually becomes ΔΨ ₀ =2QL ₁ (26).

Since the TE/TM mode converter 14 is not at the end of the branch, the waveguide 6 (L ₁ ) remains, and for this part,
There is a phase difference offset.

This is an offset due to the fact that the clockwise light is longer by the distance _2L1 in TE mode.

It is assumed that a phase shifter including electrodes 11, 12 and a battery 13 gives a phase difference of ΔΦ ₁ to the clockwise light and ΔΦ ₂ to the counterclockwise light.

The total phase difference ΔΨ is ΔΨ=ΔΦ ₁ −ΔΦ ₂ +2QL ₁ (27).

In Figure ₂ , the _phase constant (β/
k) changes are noted. E ₊ is applied with an electric field of 2 x 10 ⁶ V/m, and E _- is applied with an electric field of -2 x 10 ⁶ V/m.

The product of the length of the electrode and the phase constant gives the phase differences ΔΦ ₁ and ΔΦ ₂ . Here, W ₀ =7, W ₁ =4,
An example of W ₂ =3 is shown. By applying a voltage in the opposite direction, the phase constant changes to the point indicated by the white circle.

If the mode and W are constant, the phase difference ΔΦ ₁ is
It is proportional to the applied voltage.

The voltage of the electrodes 11 and 12 of the waveguides 3 and 8 is +
Letting V ₁ and −V ₂ and the proportionality constant be ξ, it can be written as ΔΦ ₁ = ξV ₁ (28) ΔΦ ₂ = −ξV ₂ (29). The total phase difference ΔΨ is ΔΨ=ξ(U ₁ +V ₂ )+2QL ₁ (30).

Since ΔΨ is a phase, adding or subtracting an integer multiple of 2π has no effect on the wave function. So ΔΨ
subtracts an integer multiple of 2π from equation (30), and the domain is 0.
It can be made to be 2π from .

Hereinafter, the range of the phase difference ΔΨ will be considered in this way.

(S) When DC voltage is applied Here, if ΔΨ is set to π/2, a phase difference of π/2 will be given for clockwise and counterclockwise light, so it can be used as an optical integrated circuit for optical fiber gyros. In this case, as shown in equation (5), a phase difference appears in the photoreceiver output I(Δθ) in the form of sinΔθ.

In this case, even when the rotational speed is low, measurement can be performed with high accuracy, and the rotational direction can be discriminated.

(C) When applying AC voltage Phase modulation can be applied to ΔΨ using the electrodes 11 and 12 forming a phase shifter and an AC power source instead of a battery. Alternating current may be added to the battery.

In this case, the output of the optical receiver contains a signal such as sin(Δθ+εsinωt) (31). It is a phase modulation method. In this case, the amplitude of the modulated wave of sinωt includes Δθ, so by detecting the intensity of the modulated wave, the phase difference Δθ,
In other words, the angular velocity Ω is determined.

(S) Effect (1) Since the right-handed light and the right-handed light pass through the same optical path, there is no fluctuation or drift in the phase difference.

(2) Although it passes through the same optical path, it is possible to apply a phase bias to only one of the lights, resulting in good sensitivity.
It is possible to provide an optical fiber gyroscope that can discriminate the direction of rotation.

(3) Instead of a beam splitter, symmetrical and asymmetrical branches are fabricated on the substrate, so the optical system is extremely compact and robust, and optical path deviations do not occur.

(4) Depending on how the voltage is applied to the phase shifter, it can be made into a DC type optical fiber gyro or a phase modulation type optical fiber gyro.

(5) Since we use an asymmetric branch that combines a 2-mode integrated waveguide and a 1-mode branch waveguide with different widths, the 0th mode of the branched waveguide is either the 0 or 1st mode of the 2-mode integrated waveguide. It is determined uniquely whether it is combined with

[Brief explanation of drawings]

FIG. 1 is a plan view of the non-reciprocal optical phase shifter of the present invention. Figure 2 is a graph showing the change in the phase constant Σ (β/k) of the TE ₀ , TE ₁ , and TE ₂ modes with respect to the normalized width W (d/λ) of the slab waveguide in a three-layer slab. . The refractive index of the core is 2.1900 and the refractive index of the cladding is 2.1878.
It is. Figure 3 shows that in a three-layer slab, TM ₀ , TM ₁ ,
Graph showing changes in phase constant Σ(β/k) of TM ₂ mode. The refractive index of the core is 2.1856, and the refractive index of the cladding is 2.1834. FIG. 4 is a graph showing changes in the phase constant Σ in a five-layer slab waveguide including two slab waveguides with different widths (W ₁ , W ₂ ). The horizontal axis is the distance g between branch waveguides divided by the wavelength λ. The upper branch shows a change from Σa of TE ₀ mode to Σc of TE ₀ mode of the thicker branch waveguide. The lower branch shows a change from TE ₁ mode Σb to TE ₀ mode Σd in the narrower branch waveguide. FIG. 5 is a perspective view showing the correspondence between the three-layer slab waveguide and the coordinate system. Figure 6 shows the symmetric bifurcation (Wm,
Wm) and the mode change between the asymmetric branch (W ₁ , W ₂ ) and the two-mode integrated waveguide.
Graph of phase constant Σ. The horizontal axis is the normalized waveway width W
(d/λ), vertical axis is T ₀ (TE or TM) mode, T ₁
It is the phase constant Σ(β/k) of the mode (TE or TM). FIG. 7 is a needle view showing a Y-shaped branched slab waveguide. 1...First waveguide, 2...Second waveguide, 3...
Third waveguide, 4...Fourth waveguide, 5...Single mode optical fiber coil, 6...Sixth waveguide, 7
...7th waveguide, 8...8th waveguide, 9...9th waveguide
Waveguide, 10... Metal coat, 11, 12... Electrode, 13... Battery, 14... TE/TM mode converter.

Claims (1)

[Claims] 1. Symmetrical branch waveguides 1, 9 of equal width formed by diffusion on a birefringent and electro-optic effect substrate.
, followed by a thick integrated waveguide 2, followed by asymmetrical branch waveguides 3, 4 and 8, 7 with different widths.
6, and electrodes 11 and 12 are provided on both or one side of the asymmetric branch waveguides 3 and 8 so as to sandwich the branch waveguides, and electrodes 11 and 12 are provided on the front and back surfaces or only on the front surface to sandwich the branch waveguides. A TE/TM mode converter 14 is provided on either one of the symmetric branch waveguides 1 and 9, and a metal coating 10 is provided in the planar direction on either of the symmetrical branch waveguides 1 and 9. A non-reciprocal optical phase shifter characterized in that all are single-mode waveguides that pass zero-order mode light, and the intermediate integrated waveguide 2 is a two-mode waveguide that passes zero-order and first-order mode light. 2. The non-reciprocal optical phase shifter according to claim 1, wherein the substrate is LiTaO ₃ or LiNbO ₃ and the waveguide is formed by Ti diffusion.