JPH05341231A - Optical isolator - Google Patents

Abstract

PURPOSE:To obtain the optical isolator having a structure which can form a non-reciprocal part and a reciprocal part on the same substrate. CONSTITUTION:On the same substrate 1, a non-reciprocal part 11 and a reciprocal part 12 are connected in series and integrated. The non-reciprocal part 11 consists of a magnetic semiconductor having a Faraday effect, and converts a horizontal polarized wave non-reciprocally to a linearly polarized wave of 45 degrees by a grating 13 formed in the upper part of its waveguide. The reciprocal part 12 consists of a non-magnetic compound semiconductor, and in the upper part of its waveguide, an asymmetrical grating 14 is formed so that anisotropic axes are inclined alternately in the opposite direction against a surface normal, and a linearly polarized wave of 45 degrees is returned reciprocally to the horizontal polarized wave.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to optical communication, optical information processing,
The present invention relates to an optical isolator used for blocking return light and not inducing noise in an optical integrated circuit or the like used in an optical measuring instrument or the like.

[0002]

2. Description of the Related Art In recent years, speeding up information transmission,
Development of optical fiber communication and optical disk memory is progressing with the increase in capacity of information recording.

Optical communication systems include medium-to-long distance transmission systems for trunk lines that use wavelengths of 1.3 μm or 1.5 μm and short-distance transmission systems such as LANs (local area networks) that use the 0.8 μm band. .. In either case, the light emitted from the semiconductor laser is focused on the end face of the fiber and guided in the fiber. At this time, the light reflected by the end faces of the fiber and other optical components returns to the active layer of the semiconductor laser. As a result, the oscillation of the semiconductor laser becomes unstable, causing power fluctuation and wavelength fluctuation. Especially D
A BR (distributed reflection type) laser or a DFB (distributed feedback type) laser is greatly affected by a single mode becoming multimode. Further, the coherent optical communication, which is attracting attention as a future communication system, only changes the phase without turning on / off the light, and therefore the influence of the returning light is particularly great.

Further, in the optical disk memory, in addition to the reflected light 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 simplified optical isolator that combines a quarter-wave plate and a polarization beam splitter may be used in the wavelength range, but the isolation ratio is 20 at maximum.
Only about dB. Also, if the polarization plane of the return light changes, the isolation ratio becomes smaller, so in systems that detect the rotation of the polarization plane of the reflected light from the disc as a signal, such as a magneto-optical disc, The impact is more serious. Therefore, the semiconductor laser has a few
A symptomatic treatment is adopted in which a high-frequency signal of 00 MHz is superimposed and the laser is oscillated in multiple modes so that the laser is not affected by the return light.

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

The only element that removes the returning light to the semiconductor laser is an optical isolator. The optical isolator has an irreversible transmission characteristic and completely blocks the returning light to prevent the returning light from re-incident on the active layer of the semiconductor laser.

An optical isolator is an essential component in an optical integrated circuit in which a light emitting element and a light receiving element are formed on the same substrate. However, the isolator for an optical integrated circuit has the following major problems.

In an optical integrated circuit, each device is connected by a channel type optical waveguide, and in this waveguide, a polarization plane is parallel or perpendicular to the waveguide layer.
Only the E wave or TM wave becomes the guided mode. Therefore, it is impossible to use a bulk type optical isolator that rotates the plane of polarization non-reciprocally by 45 degrees. As a result, the isolator for an optical integrated circuit further requires a reciprocal mode converter for returning the polarized light rotated at 45 degrees in the nonreciprocal part. Moreover, 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 the Faraday effect in the non-reciprocal portion 71 and the Cotton Mouton effect (using magnetic birefringence) in the reciprocal portion 72 is used. Is proposed. The magneto-optical film 73, which is a magnetic garnet film (ferromagnetic material), is GG
It is grown epitaxially on the G substrate 74. The magnetization M is laid in the light traveling direction in the Faraday region 71, and the magnetization M is made θ in the plane perpendicular to the light traveling direction in the Cotton Mouton region 72 from the film surface perpendicular direction. _m = 22.5 degrees tilted. A laser annealing method has been proposed in which a magnetic field is applied to anneal with laser light in order to form such two magnetized regions 71 and 72, but it 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 the optical isolator on the compound semiconductor substrate. Magnetic garnet film 73
Can be grown by liquid phase epitaxy or sputtering. However, this film 73 is
Since a lattice constant and a thermal expansion coefficient are different from those of InP substrate and InP substrate, a good quality film cannot be epitaxially grown. Therefore, it is considered difficult to integrate the optical isolator with other optical devices as long as the magnetic garnet film 73 is used as the Faraday material.

Further, the phase matching in the non-reciprocal portion 71 is made.
There is also the problem of. Figure 8 shows the conventional phase matching method.
Show. As shown in FIG. 8A, the laser light 81 is guided into the film 82.
Then, as shown in FIG. 8B, the finite film thickness h₀At
Is a refractive index n for TE waves due to shape birefringence._TEIs T
That for M waves (n_TM) Will be larger than. TE wave
Δβ = 2π / λ (n_TE-N_TM) (1), and the mode conversion efficiency R from TE wave to TM wave is R = θ _F ²/ [Θ _F ²+ (Δβ / 2)²] ・ Sin²{[Θ _F ²+ (Δβ / 2)²]^1/2l} (2) is given. Where θ_FIs Farade per unit length
-The rotation angle, l is the waveguide distance.

The state of the polarization represented by the above equation is represented by the Poincare sphere.
Is used as shown in FIG. Straight represented by point A
As the linearly polarized wave (TE wave) is guided, the point on the longitude 0 degree
It rotates on a sphere with the line connecting M and the center O of the sphere as the central axis.
It The angle (θ) from the equatorial plane to the point M is tan θ = θ _F/ (Δβ / 2) (3) is given. As is clear from FIG. 9, Δβ is finite
As long as you guide, no matter how much you guide, just follow the same path.
It may not be possible to reach point E, which corresponds to 5 degree linear polarization.
I understand.

As described above, how to form 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.

Therefore, an object of the present invention is to provide an optical isolator comprising a non-reciprocal part and a reciprocal part, which can be integrated with an optical element or the like on the same substrate.

[0015]

In order to solve the above problems, the present invention uses a magnetic semiconductor waveguide layer (which is paramagnetic) having a Faraday effect in the non-reciprocal portion and uses a non-magnetic compound semiconductor conductor. It has a structure in which the waveguide layer is used as the reciprocal portion.

That is, 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 the non-reciprocal horizontal polarization is converted into a 45-degree linear polarization. In addition, the reciprocal part is made of a non-magnetic compound semiconductor and has a structure in which linear polarization of 45 degrees is reciprocally converted into horizontal polarization.

More specifically, the non-reciprocal part and the reciprocal part are connected in series and integrated on the same substrate, or the non-reciprocal part is horizontally formed by a grating formed on the waveguide. Polarization is converted to 45 degree linear polarization in a non-reciprocal manner, the non-reciprocal portion is provided with an electrode for applying an electric field on a grating formed above the waveguide, or the non-reciprocal portion is , By applying an electric field and a magnetic field to the waveguide at the same time to cause mode conversion due to the electro-optic effect and the magneto-optic effect in the same region, the horizontal polarization is non-reciprocally converted into a 45-degree linear polarization, In the section, an asymmetric grating is formed in the upper part of the waveguide so that the anisotropic axes alternately incline in opposite directions with respect to the surface normal, and linear polarization of 45 degrees is reciprocally converted to horizontal polarization. ,
The reciprocal portion is formed with concave portions on both sides of the waveguide alternately so that the anisotropic axes are alternately inclined in opposite directions with respect to the surface normal, and linearly polarized waves of 45 degrees are contradictory to each other. Return to horizontal polarization, or in the non-reciprocal part of the optical isolator, the grating period number N and the TE / TM propagation constant difference Δβ ⁺ , Δ
β ⁻ (± corresponds to the convex portion and the concave portion of the grating, respectively) and the Faraday rotation angle θ _F have the following relationship: 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)), in the reciprocal part of the optical isolator, the number of periods of the grating is N, and the axis of anisotropy is the waveguide normal. If the angle is ρ, then the following relationship may be established: ρ = π / 16N.

[0018]

EXAMPLE A first example shown in FIG. 1 will be described below. The manufacturing method will be described. GaA 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 M
Films are sequentially formed by BE or MOCVD. After that, 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 layers of the non-reciprocal portion 11 made of the Te upper cladding layer 8 are 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 produced. CdMnTe, which is a magnetic semiconductor, is II-
The Cd position of CdTe, which is a group VI compound semiconductor, is replaced with Mn, which is transparent in the visible range and has a large Farade rotation angle. Further, by making the Mn composition of the core portion 6 smaller than that of the upper and lower clad portions 4 and 8, the core portion 6
Has a refractive index larger than that of the clad portions 4 and 8 to confine light. Below, the non-reciprocal part 11 /
The operation principle of each reciprocal part 12 will be described.

Operation of Non-Reciprocal Section In the non-reciprocal section 11 shown in FIG. 1, the grating 13 having a constant period is formed in the upper clad 8 in the light guiding direction. Here, since the thickness of the upper clad layer 8 is different between the convex portion and the concave portion, the propagation constant is different. TE at the convex part
/ TM propagation constant difference [Delta] [beta] ^+, it [Delta] [beta] where the recess ^-
Then, as shown in FIG. 2, the Poincaré spheres corresponding to the respective parts (on this sphere, the central axis for obtaining the polarization path due to the Farade effect is the line connecting the center O and the pole, and the birefringence The central axis when is due to is the line connecting the center O and the point A on the equator, and when they are mixed, the central 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 portion and the concave portion are set to the length of π radian rotation on the Poincare spherical surface as shown in FIG.

As is clear from FIG. 3, as the light is first guided through the convex portion, the point A rotates by π radians about O-M ⁺ and moves to the point A ₁ . Next, when the 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 polarization at each segment end of the grating 13 jumps from point to point on the longitude of 0 degree one after another. Therefore, in order to obtain a polarization of 45 degrees (point E in FIGS. 2 and 3) at the exit end of the non-reciprocal portion 11, the rotation angle of the last segment of the grating 13 must be π / 2 radians. For this purpose, it can be seen that the length of the last segment should be half that of the other segments.

The following relationships are necessary for the above operation. First, the number of periods of the grating 13 (one period for the convex portion and the concave portion) is set to N (excluding the last segment). In addition, the O-M ⁺ axis and the O-M ^- axis are O-A.
The angles formed by the axes are θ + Δθ / 2 and θ−Δθ /
Set to 2. In order to become a 45 degree linearly polarized wave (point E) by guiding the segment of the last half length,
A point (A _2N ) on the longitude 0 degree and the origin 0 when guided for N periods
Since the angle formed by the line connecting the lines and the OM ⁺ axis must be π / 2, the following equation holds. 2N Δθ = (θ + Δθ / 2) −π / 2 (4) By designing Δθ according to the number of periods N and the depth of the grating 13 so as to satisfy the relationship of the above equation, horizontal polarization is non-reciprocal (irreversible). It is possible to make a linearly polarized wave of 45 degrees. With respect to the returning light, it is clear that the M ⁺ and M ⁻ points in FIGS.

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 some error occurs in the above setting.

Further, without forming a grating in the ridge waveguide, electrodes are provided on both sides of the grating to form a CdM of zinc blende structure.
An appropriate electric field is applied in the [110] direction (in-plane direction of the cladding layer 8 orthogonal to the waveguiding direction) in the nTe upper clad layer 8 and an appropriate magnetic field is applied in the waveguiding direction to produce the electro-optic effect and the magneto-optical effect. The structure may be such that mode conversion due to the effect occurs in the same region. This allows TE / TM
Even if the phase matching (Δβ = 0) is not taken, the plane of polarization can be rotated 45 degrees non-reciprocally. If it is shown on the Poincare sphere in FIG. 2, at this time, as indicated by the one-dot chain line,
Follow the path that becomes elliptically polarized light at a position other than 5 ° linearly polarized light. Regarding this, Japanese Patent Application No. 2-2 by the same applicant
See 07096.

Structure of Reciprocal Section The reciprocal section 12 shown in FIG. 2 is composed of a non-magnetic compound semiconductor waveguide. The upper clad layer 9 is etched in a checkered pattern. The cross section of the grating 14 is shown in FIG.
As in (a) and (b), the shape is asymmetric with respect to the center of the waveguide, and as a result, the principal axis 20 of birefringence is also inclined by an angle ± ρ from the normal direction. Its size 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.

From this non-reciprocal portion 11 to 45 degrees (E
Fig. 5 shows how the polarized light changes when linearly polarized light
Shown in. As the light is guided, A with a longitude of 0 degrees on the equatorial plane.
Rotate by π radians about two points P ⁺ and P ⁻ forming an angle of ± 2ρ from the point. In FIG. 5, when four segments (two periods) are guided, horizontal orthogonal polarization (A
Points).

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

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

As shown in FIG. 1, when a TE wave is incident on the non-reciprocal portion 11 having the above structure, this light becomes a linearly polarized light rotated by 45 degrees at the output end of the non-reciprocal portion 11, and then the reciprocal portion 12 is generated.
When it leaves the reciprocal portion 12, it becomes TE light again and is transmitted to the next stage. on the other hand,
When the TE wave enters the reciprocal part 12, this is the reciprocal part 1
In the step 2, in the direction opposite to the rotation in the previous reciprocal portion 12 (the same direction as the rotation in the previous non-reciprocal portion 11), it is rotated by 45 degrees, and by further entering the non-reciprocal portion 11, in the same direction here as well. It is rotated 45 degrees. Therefore, when it leaves the non-reciprocal portion 11, it becomes a TM wave. If this TM wave is removed by a means such as a polarizing plate provided in front of the non-reciprocal portion 11, the function of the optical isolator will be fulfilled.

[0029]

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

[Brief description of 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 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 guide path of a nonreciprocal portion of the first embodiment of FIG.

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

5 is a diagram of a Poincare sphere for explaining the optical waveguide path of the reciprocal portion of the first embodiment of FIG. 1. FIG.

FIG. 6 is a partial cross-sectional view illustrating the operation principle of a non-reciprocal portion of 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 in a film.

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

[Explanation of symbols]

 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-reciprocal Part 12 Reciprocal part 13 Grating 14 Asymmetric grating 20 Birefringent main axis 27 Embedded waveguides 28a, 28b Alternate recesses

Claims (9)

[Claims] 1. An optical isolator comprising a non-reciprocal portion and a reciprocal portion connected in series, wherein the non-reciprocal portion is made of a magnetic semiconductor having a Faraday effect, and the horizontal polarization is non-reciprocal to a linear polarization of 45 degrees. An optical isolator having a configuration for converting a linearly polarized wave of 45 degrees to a horizontally polarized wave in a reciprocal manner. 2. The optical isolator according to claim 1, wherein the non-reciprocal portion and the reciprocal portion are connected in series and integrated on the same substrate. 3. The optical isolator according to claim 1, wherein the non-reciprocal portion non-reciprocally converts the horizontally polarized wave into a 45 degree linearly polarized wave by a grating formed on an upper portion of the waveguide. 4. The optical isolator according to claim 3, wherein the non-reciprocal portion is provided with an electrode for applying an electric field on a grating formed above the waveguide. 5. The non-reciprocal portion causes horizontal polarization to be 45 degree linear polarization by simultaneously applying an electric field and a magnetic field to the waveguide to cause mode conversion due to electro-optic effect and magneto-optic effect in the same region. The optical isolator according to claim 1, wherein the optical isolator is converted into non-reciprocal. 6. The reciprocal portion is formed with an asymmetrical grating in the upper part of the waveguide so that the anisotropic axes are alternately tilted in opposite directions with respect to the surface normal, and reciprocal 45 degree linearly polarized waves. The optical isolator according to claim 1, wherein the optical polarization is returned to horizontal polarization. 7. The reciprocal portion is formed with concave portions on both sides of the waveguide so that the anisotropic axes are alternately inclined in opposite directions with respect to the surface normal, and a linear line of 45 degrees is formed. The optical isolator according to claim 1, wherein the polarized waves are reciprocally returned to horizontal polarized waves. 8. In the non-reciprocal part of the optical isolator, the grating period number N and TE / TM propagation constant differences Δβ ⁺ , Δβ ⁻ (± corresponds to the convex and concave portions of the grating) and the Faraday rotation angle θ. claim 3 wherein the optical isolator and the _F characterized as having the following relationship: 2NΔθ = (θ + Δθ / 2) -π / 2 ^{where, θ = (θ + + θ} -) / 2, Δθ = θ + + θ ^-, ta
nθ ⁺ = θ _F / (Δβ ⁺ / 2), tan θ ⁻ = θ _F / (Δβ ⁻
/ 2). 9. The reciprocal part of the optical isolator,
The optical isolator according to claim 6, wherein the following relationship is established, where N is the number of periods of the grating and ρ is an angle formed by the axis of anisotropy and the waveguide normal: ρ = π / 16N.