JP3130131B2 - Optical isolator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to optical communication, optical information processing,
The present invention relates to an optical isolator that is used for blocking return light and not inducing noise in an optical integrated circuit or the like used for an optical measurement device or the like.

[0002]

2. Description of the Related Art In recent years, information transmission has been accelerated,
With the increase in the capacity of information recording, development of optical fiber communication and optical disk memory has been promoted.

[0003] In optical communication systems, there are a medium- and long-distance transmission system using a 1.3 μm or 1.5 μm wavelength and a short-distance transmission system such as a LAN (local area network) using a 0.8 μm band. . In any case, the light emitted from the semiconductor laser is focused on the end face of the fiber and guided through the fiber. At this time, the light reflected on the end face of the fiber or other optical component returns to the active layer of the semiconductor laser. As a result, oscillation of the semiconductor laser becomes unstable, and power fluctuation and wavelength fluctuation occur. Especially D
A BR (distribution reflection type) laser or a DFB (distribution feedback type) laser has a large effect such as a single mode becoming multimode. Further, coherent optical communication, which has attracted attention as a future communication method, only changes the phase without turning on / off the light, and therefore the effect of the return light is particularly large.

In the optical disk memory, in addition to the light reflected from the optical components, the reflected light from the disk substrate returns to the semiconductor laser, and induces the instability and noise described above. In this case, a simple optical isolator combining a quarter-wave plate and a polarizing beam splitter is sometimes used, but the isolation ratio is at most 20.
It is only about dB. In addition, if the polarization plane of the return light changes, the isolation ratio becomes even smaller.Therefore, in systems such as a magneto-optical disk which detect the rotation of the polarization plane of the reflected light from the disk as a signal, the return ratio of the return light is reduced. The consequences are even more severe. Therefore, the number 1
A symptomatic treatment is performed in which a high-frequency signal of 00 MHz is superimposed, the laser is oscillated in multiple modes, and the laser is not affected by the return light.

As described above, the generation of noise due to return light increases the error rate of a transmission signal in optical communication, and causes deterioration of a reproduction signal in an optical disk memory.

[0006] The only element that removes the return light to the semiconductor laser is the optical isolator. The optical isolator has an irreversible transmission characteristic, and completely blocks return light to prevent re-injection into the active layer of the semiconductor laser.

In an optical integrated circuit in which a light emitting element and a light receiving element are formed on the same substrate, an optical isolator is an essential component. However, the optical isolator has a major problem as described below.

In an optical integrated circuit, each device is connected by a channel-type optical waveguide. In this waveguide, the T plane where the plane of polarization is parallel or perpendicular to the waveguide layer is provided.
Only the E wave or the TM wave is in the guided mode. Therefore, it is not possible to use a bulk-type optical isolator that rotates the plane of polarization 45 degrees nonreciprocally. As a result, the isolator for an optical integrated circuit further requires a reciprocal mode converter for returning the polarized light rotated by 45 degrees at the non-reciprocal portion. In addition, there is a demand that these reciprocal / non-reciprocal circuits must be formed on the same substrate.

[0009]

As shown in FIG. 7, an integrated optical isolator having a structure using a Faraday effect in a non-reciprocal portion 71 and a Cotton Mouton effect (using magnetic birefringence) in a reciprocal portion 72, as shown in FIG. Has been proposed. The magneto-optical film 73, which is a magnetic garnet film (ferromagnetic material), is GG
In the Faraday region 71, the magnetization M is laid down in the light traveling direction, and in the cotton Mouton region 72, the magnetization M is set to θ from a direction perpendicular to the film surface in a plane perpendicular to the light traveling direction. _m = 22.5 degrees. A laser annealing method of annealing with a laser beam while applying a magnetic field in order to form such two magnetized regions 71 and 72 has been proposed, but has not been realized yet.

On the other hand, an optical element represented by a semiconductor laser is formed on a compound semiconductor substrate such as GaAs or InP.
Therefore, it is desired to integrate an optical isolator on a compound semiconductor substrate. Magnetic garnet film 73
Can be grown by liquid phase epitaxy or sputtering. However, this film 73 is made of GaAs
A high-quality film cannot be epitaxially grown due to a difference in lattice constant and coefficient of thermal expansion from the InP substrate and the InP substrate. Therefore, it is considered that it is difficult to integrate the optical isolator with other optical elements as long as the magnetic garnet film 73 is used as the Faraday material.

Further, the phase matching at the non-reciprocal portion 71 is further performed.
The problem also exists. FIG. 8 shows a conventional phase matching method.
Show. As shown in FIG. 8A, a laser beam 81 is guided into a film 82.
Then, as shown in FIG. 8B, the finite thickness h₀At
Is the refractive index n for TE waves due to shape birefringence._TEIs T
That for the M wave (n_TM). With TE waves
Assuming that the propagation constant difference from the TM wave is Δβ, Δβ = 2π / λ (n_TE-N_TM(1) and the mode conversion efficiency R from TE wave to TM wave is R = θ _F ^Two/ [Θ _F ^Two+ (Δβ / 2)^Two] ・ Sin^Two｛[Θ _F ^Two+ (Δβ / 2)^Two]^1/2l｝ (2). Where θ_FIs Faraday per unit length
-The rotation angle, l is the guided distance.

The state of polarization expressed by the above equation is expressed by Poincare sphere.
Is shown in FIG. A point represented by point A
The linearly polarized wave (TE wave) is a point on longitude 0 degrees as it is guided.
Turn on the sphere with the line connecting M and the center O of the sphere as the central axis
You. The angle (θ) from the equatorial plane to the point M is tan θ = θ _F/ (Δβ / 2) (3). As is clear from FIG. 9, Δβ is finite.
As long as you follow the same path, no matter how much
It is impossible to reach the point E corresponding to the linear polarization of 5 degrees.
I understand.

As described above, forming the reciprocal / non-reciprocal regions on the same compound semiconductor substrate and how to match the phases are the most important issues for realizing the integrated optical isolator.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical isolator comprising a non-reciprocal part and a reciprocal part which can be integrated on the same substrate together with an optical element and the like.

[0015]

According to the present invention, a magnetic semiconductor waveguide layer (paramagnetic) having a Faraday effect is used in a non-reciprocal portion to provide a non-magnetic compound semiconductor. It has a structure that uses a wave guide layer as a reciprocal part.

[0016] That is, the non-reciprocal unit and reciprocal unit is a optical isolator formed by connecting in series, the non-reciprocal unit is made of a magnetic semiconductor having a Faraday effect, the non-reciprocal
Due to the grating formed above the waveguide in the section
Ri has a configuration of converting the non-reciprocal horizontally polarized into a linearly polarized wave of 45 degrees, the reciprocal unit is made of a nonmagnetic compound semiconductor, 45
Have a configuration back to horizontal polarization linearly polarized degrees reciprocity, and
The number of periods N of the grating and TE / TM propagation constant difference Δ
β ⁺ , Δβ ⁻ (± are the convex and concave portions of the grating, respectively.
Corresponding to part) and a Faraday rotation angle theta _F lifting the relationship:
One possible wherein the: 2NΔθ = (θ + Δθ / 2) -π / 2 ^{where, θ = (θ + + θ} -) / 2, Δθ = θ + -θ -, tanθ + = θ F / (Δβ + / 2), tan θ ⁻ = θ _F /
(Δβ ⁻ / 2).

[0017] More specifically, the on the same substrate or the non-reciprocal unit and the reciprocal unit is being integrated are connected in series, the non-reciprocal unit, the upper portion of the waveguide in the formed on grating An electrode for applying an electric field is provided, or the non-reciprocal portion generates horizontal polarization by simultaneously applying an electric field and a magnetic field to the waveguide and causing mode conversion by an electro-optic effect and a magneto-optic effect in the same region. A non-reciprocal conversion into a linear polarization of 45 degrees, and the reciprocal portion is formed with an asymmetric grating at the upper part of the waveguide such that the anisotropic axis is alternately inclined in the opposite direction to the surface normal. , 45
The linear polarization of the degree is reciprocally returned to horizontal polarization, or the reciprocal portion is alternately on both sides of the waveguide, such that the anisotropic axis is alternately inclined in the opposite direction to the surface normal, A concave portion is formed to return the 45-degree linearly polarized wave to horizontal polarization in a reciprocal manner. In the reciprocal portion of the optical isolator, the asymmetrical grating period number is set to N and the anisotropic axis is set. Let ρ be the angle that the line makes with the waveguide normal, or have the following relationship: ρ = π / 16N.

[0018]

The first embodiment shown in FIG. 1 will be described below. The manufacturing method will be described. GaAs on GaAs substrate 1
s buffer layer 3, AlGaAs lower cladding layer 5, Al
GaAs / GaAs superlattice core layer 7 and AlGaAs
The optical waveguide layer of the reciprocal portion 12 composed of the upper clad layer 9 is formed by M
Films are sequentially formed by BE or MOCVD. Then, after selectively etching a part of the GaAs waveguide layer,
CdTe buffer layer 2, CdMnTe lower cladding layer 4, CdTe / CdMnTe superlattice core layer 6, CdMn
The optical waveguide layer of the non-reciprocal portion 11 composed of the Te upper cladding layer 8 is sequentially formed by MBE or MOCVD. After forming the ridge waveguide by ion milling, the gratings 14 and 1 of the reciprocal portion 12 and the non-reciprocal portion 11 are formed.
3 is manufactured. CdMnTe which is a magnetic semiconductor is II-
CdTe, which is a group VI compound semiconductor, in which the Cd position is replaced with Mn, is transparent in the visible region, and has a large Faraday rotation angle. By making the Mn composition of the core 6 smaller than that of the upper and lower claddings 4 and 8, the core 6
Has a refractive index larger than those of the cladding portions 4 and 8 to confine light. The non-reciprocal part 11 /
The operation principle of each reciprocal part 12 will be described.

Operation of Non-Reciprocal Portion In the non-reciprocal portion 11 shown in FIG. 1, a grating 13 having a constant period is formed on the upper cladding 8 in the light waveguide direction. Here, since the thickness of the upper cladding layer 8 is different between the convex portion and the concave portion, the propagation constant is different. TE at the protrusion
/ TM propagation constant difference [Delta] [beta] ^+, it [Delta] [beta] where the recess ^-
Then, as shown in FIG. 2, the Poincare sphere corresponding to each part (the center axis on the spherical surface when finding the polarization path by the Faraday effect is a line connecting the center O and the pole, and the birefringence Is the line connecting the center O and the point A on the equator, and when both are mixed, the center axis is the line connecting the center O and the point between the point A and the pole). point M ⁺ and M ^- to become. Further, the lengths of the convex portions and the concave portions are set to the lengths of rotating by π radians on the Poincare sphere as shown in FIG.

[0020] As apparent from FIG. 3, as the light is guided through the first convex portion, A point moves to π radians rotates around the O-M ⁺ to _point A. Next, when light enters the recess,
A ₁ point is O-M ^- to move to A ₂ points around the axis. As the light is guided, the polarized light at the end of each segment of the grating 13 jumps over points at a longitude of 0 degrees one after another. Therefore, in order to obtain a polarization of 45 degrees (point E in FIGS. 2 and 3) at the exit end at the non-reciprocal portion 11, the rotation angle of the last segment of the grating 13 must be π / 2 radians. It can be seen that, for this purpose, the length of the last segment should be half that of the other segments.

The following relationship is required for the above operation. First, the number of periods of the grating 13 (one period between the convex portion and the concave portion) is set to N (excluding the last segment). Further, O-M ⁺ shaft and O-M ^- axes O-A
The angles formed by the axes are θ + Δθ / 2 and θ−Δθ /
Let it be 2. To get a 45 degree linear polarization (point E) by guiding the last half length segment,
Point (A _2N ) at 0 degree longitude and origin 0 when guided N cycles
Since the angle formed by the line connecting to the O-M ⁺ axis must be π / 2, the following equation holds. 2NΔθ = (θ + Δθ / 2) −π / 2 (4) By designing Δθ by the period number N and the depth of the grating 13 so as to satisfy the relationship of the above expression, the horizontal polarization is non-reciprocal (irreversible). It is possible to obtain a 45 degree linearly polarized wave. For the returning light, since the points M ⁺ and M ⁻ in FIGS. 2 and 3 move from the northern hemisphere to the southern hemisphere, it is clear that non-reciprocal operation is performed.

In this case, if an electrode for applying an electric field is provided on the uneven grating 13 along the waveguide direction, the structure can be finely adjusted as desired even if a slight error occurs in the above setting.

Also, no grating is formed on the ridge waveguide, and electrodes are provided on both sides of the ridge waveguide to form a CdM having a zinc blende structure.
An appropriate electric field is applied in the [110] direction (in-plane direction of the cladding layer 8 perpendicular to the waveguide direction) in the nTe upper cladding layer 8 and an appropriate magnetic field is applied in the waveguide direction to obtain an electro-optical effect and a magneto-optical effect. A structure in which mode conversion by the effect is caused in the same region may be adopted. Thereby, TE / TM
Even without phase matching (Δβ = 0), the polarization plane can be rotated 45 degrees nonreciprocally. If shown on the Poincare sphere in FIG. 2, at this time, 0 degrees and 4 degrees
Follow the path of elliptically polarized light at a place other than the linearly polarized light of 5 degrees. This is described in Japanese Patent Application No. 2-2 filed by the same applicant.
No. 07096.

Structure of Reciprocal Section The reciprocal section 12 shown in FIG. 2 is made of a nonmagnetic compound semiconductor waveguide. The upper cladding layer 9 is etched in a checkered pattern. The cross section of the grating 14 is shown in FIG.
As shown in (a) and (b), the waveguide has an asymmetric shape with respect to the center of the waveguide. As a result, the main axis 20 of the birefringence is also inclined by an angle ± ρ from the normal direction. Its magnitude is determined by the degree of asymmetry. The length of each segment of the grating 14 is set so that the phase difference between the two orthogonal modes is π radian.

The waveguide is placed at 45 degrees (E
FIG. 5 shows how the polarization changes when linearly polarized light (point) is incident.
Shown in As light propagates, A at longitude 0 degrees on the equatorial plane
The rotation is performed by π radians around two points P ⁺ and P ⁻ forming an angle of ± 2ρ from the point, respectively. In FIG. 5, when four segments (two periods) are guided, horizontal orthogonal polarization (A
Point).

Assuming that the number of periods is N, a 45 ° reciprocal mode converter can be formed by satisfying the relationship ρ = π / 16N (5) as is clear from FIG. This configuration is described in the literature (Appl. Phys. Lett. 59 (11), p.
p. 1278-1280, 9 Sep. 1991).

A configuration as shown in FIG. 6 may be used to perform the same function. In FIG. 6, concave portions 28a and 28b are alternately formed on both sides of the buried waveguide 27, and the main axis 20 of birefringence is inclined by an angle ± ρ from the normal direction.

As shown in FIG. 1, when the TE wave is incident on the non-reciprocal portion 11 having the above structure, this light becomes linearly polarized light rotated by 45 degrees at the output end of the non-reciprocal portion 11, and then the reciprocal portion 12
When the light passes through the reciprocal portion 12, the light is again turned into TE light and transmitted to the next stage. on the other hand,
When the TE wave enters the reciprocal part 12, this is the reciprocal part 1
At 2, it is rotated by 45 degrees in the opposite direction to the rotation at the previous reciprocal part 12 (the same direction as the rotation at the previous non-reciprocal part 11). Rotated 45 degrees. Therefore, when exiting the non-reciprocal portion 11, a TM wave is generated. If this TM wave is removed by means such as a polarizing plate provided in a stage preceding the non-reciprocal portion 11, the function of the optical isolator is achieved.

[0029]

As described above, according to the present invention, C
Since a high-quality dTe film can be grown on a GaAs substrate or the like by MBE or MOCVD, a nonreciprocal portion and a reciprocal portion can be formed on the same substrate. Therefore, it has become possible to integrate a DBR / DFB laser or an optical amplifier, which has conventionally been considered difficult to integrate due to the presence of return light, on a single substrate, and an optical integrated circuit or the like can be realized.

[Brief description of the drawings]

FIG. 1 is a perspective view of a first embodiment of an optical isolator according to the present invention.

FIG. 2 is a diagram of a Poincare sphere for explaining the operation principle of the non-reciprocal portion of the first embodiment of FIG.

FIG. 3 is a diagram of a Poincare sphere for explaining a light guiding path of a non-reciprocal portion in the first embodiment of FIG. 1;

FIG. 4 is a partial cross-sectional view for explaining the operation principle of the reciprocal portion of the first embodiment of FIG.

FIG. 5 is a diagram of a Poincare sphere for explaining a light guide path of a reciprocal portion in the first embodiment of FIG. 1;

FIG. 6 is a partial cross-sectional view illustrating the operation principle of a non-reciprocal portion according to another embodiment.

FIG. 7 is a perspective view showing a configuration of a conventional example.

FIG. 8 is a diagram illustrating a TE / TM propagation constant difference due to shape birefringence when a laser beam is guided through a film.

FIG. 9 is a diagram illustrating a state of polarization in the case of the conventional example using a Poincare sphere.

[Explanation of symbols]

 Reference Signs List 1 GaAs substrate 2 CdTe buffer layer 3 GaAs buffer layer 4 CdMnTe lower cladding layer 5 AlGaAs lower cladding layer 6 CdTe / CdMnTe superlattice core layer 7 GaAs / AlGaAs superlattice core layer 8 CdMnTe upper cladding layer 9 AlGaAs upper cladding layer 11 Non-phase Part 12 reciprocal part 13 grating 14 asymmetric grating 20 main axis of birefringence 27 buried waveguide 28a, 28b alternate concave part

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 27/28 G02F 1/09 505

Claims (7)

(57) [Claims] 1. An optical isolator comprising a non-reciprocal part and a reciprocal part connected in series, wherein the non-reciprocal part is made of a magnetic semiconductor having a Faraday effect, and is formed by a grating formed above the waveguide at the non-reciprocal part. It has a configuration for converting horizontal polarization into non-reciprocal 45-degree linear polarization, and the reciprocal portion is made of a non-magnetic compound semiconductor, and has a configuration for reversing 45-degree linear polarization back to horizontal polarization. And the number of periods N of the grating and the TE / TM propagation constant difference Δβ ⁺ , Δβ
⁻ (± corresponds to the convex and concave portions of the grating, respectively)
And an optical isolator and a Faraday rotation angle theta _F characterized as having the following relationship: 2NΔθ = (θ + Δθ / 2) -π / 2 ^{where, θ = (θ + + θ} -) / 2, Δθ = θ + −θ ⁻ , tan θ ⁺ = θ _F / (Δβ ⁺ / 2), tan θ ⁻ = θ _F /
(Δβ ⁻ / 2). 2. The optical isolator according to claim 1, wherein said non-reciprocal part and said reciprocal part are connected in series and integrated on the same substrate. 3. The optical isolator according to claim 1, wherein said non-reciprocal portion is provided with an electrode for applying an electric field on a grating formed above said waveguide. 4. The non-reciprocal part is characterized in that an electric field and a magnetic field are simultaneously applied to the waveguide to cause mode conversion by an electro-optic effect and a magneto-optic effect in the same region, thereby converting a horizontally polarized wave into a linearly polarized wave of 45 degrees. 2. The optical isolator according to claim 1, wherein the optical isolator is converted into a non-reciprocal. 5. The reciprocal portion includes an asymmetrical grating formed on an upper portion of the waveguide such that an anisotropic axis is alternately inclined in the opposite direction with respect to a surface normal. 2. The optical isolator according to claim 1, wherein the polarization is returned to horizontal polarization. 6. A reciprocal portion is formed on both sides of the waveguide such that a concave portion is formed so that an anisotropic axis is alternately inclined in the opposite direction with respect to a surface normal. 2. The optical isolator according to claim 1, wherein the polarization is reversed to the horizontal polarization. 7. The reciprocal part of the optical isolator has the following relationship, where N is the period number of the asymmetric grating, and ρ is the angle between the anisotropic axis and the waveguide normal. The optical isolator according to claim 5, wherein ρ = π / 16N.