Optical waveguide isolator

An optical waveguide isolator for use in an optical communication system is disclosed. The optical waveguide isolator comprises a semiconductive light amplifier structure including a semiconductor substrate of first conductivity type having a surface of a layer to be formed thereon, a first cladding layer of first conductivity type formed on the substrate, an active layer formed on the first cladding layer, a second cladding layer of the second conductivity type opposite to first conductivity type, formed on active layer, a first electrode formed on the surface of the semiconductor substrate opposite to the surface to be formed as a layer, and a second electrode formed on the second cladding layer; the first and the second cladding layers and the active layer form an optical waveguide in which the light wave propagates. The semiconductive light amplifier structure further comprises a light absorptive magnetic material layer having light absorption function for the light wave propagating through the optical waveguide. The magnetic material layer is magnetized so as to have the magnetic-field component in the direction which corresponds to the direction where a magnetic vector of the light wave vibrates, the waveguide structure body has a nonreciprocity optical characteristic that effective refractive index changes into the light wave to which the optical waveguide is propagated according to the magnet-optical effect of the light absorptive magnetic material layer according to the direction of propagation, by the effective refractive index change in the nonreciprocity, the attenuation of the first light wave that the optical waveguide is propagated in the first direction caused when the said waveguide is propagated, becomes small more than the attenuation of the second light wave propagated in the second direction opposite to the first direction caused when the said waveguide is propagated.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical isolator, especially an optical
 wave guide capable of being integrated on a substrate together with a
 semiconductor laser and an optical wave guide.
 2. Related Art Statement
 In the optical communication system, an optical isolator is used to prevent
 reflected light and scattered light in the end face of the optical fiber
 from returning to a light source side. An optical isolator using a
 rotation of polarization plane according to the optical-magnet material,
 has been put to practical use as an optical isolator now. For example, the
 optical isolator of Farady rotation type is constituted by a polarizer, a
 Faraday rotator of optically transparent material and an analyzer, so that
 polarized components corresponding to the plane of polarization of
 polarizer out of advanced circular polarization of light to the forward
 direction, pass through the polarizer, the plane of polarization rotates
 by Faraday rotator by 45.degree., and emanate by passing the analyzer
 which is inclined at the polarizer by 45.degree.. On the other hand, the
 return light propagated in the direction opposite to the forward direction
 is obstructed with a polarizer, since after passing the analyzer, the
 plane of polarization of the return light receives the rotation of
 45.degree. by the Faraday rotator and returns to the polarizer. The
 polarized beam splitter and the double refraction prism are utilized as
 polarizer and analyzer used by conventional optical waveguide isolator.
 The above well known optical waveguide isolator was not able to be
 manufactured with a semiconductor optical element such as semiconductor
 lasers and optical modulators as one body, since the well known optical
 waveguide isolator does not have the semiconductor device structure.
 Therefore, the well known optical waveguide isolator must be made as
 another discrete structure with the substrate of the optical circuit to
 which the semiconductor device such as semiconductor lasers is integrated,
 so that the manufacturing step becomes complicated and the manufacturing
 cost becomes expensive, too. Particularly, since the phase matching is
 necessary, and a precise processing is necessary, so that the
 manufacturing process becomes complex. On the other hand, if the optical
 waveguide isolator can be formed on the substrate by using the same
 semiconductor manufacturing technology as semiconductor devices such as
 semiconductor lasers and photo diodes, it can manufactured with precise
 manufacturing step and thus the manufacturing cost can be made cheap
 greatly.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to eliminate the above described
 disadvantages of the conventional.
 It is another object of the present invention to provide an optical
 waveguide isolator capable of integrating it on the semiconductor
 substrate by using the semiconductor manufacturing technology.
 It is another object of the present invention to provide an optical
 waveguide isolator capable of being manufactured without becoming
 necessary of phase matching and using complicated manufacturing step.
 According to the present invention, there is provided an optical isolator
 comprising: a semiconductor optical amplifier structure including a
 semiconductor substrate of first conductivity type having a surface of a
 layer to be formed thereon, a first cladding layer of first conductivity
 type formed on the substrate, an active layer formed on the first cladding
 layer, a second cladding layer of the second conductivity type opposite to
 first conductivity type, formed on active layer, a first electrode formed
 on the surface of the semiconductor substrate opposite to the surface to
 be formed as a layer, and a second electrode formed on the second cladding
 layer; the first and the second cladding layers and the active layer form
 an optical waveguide in which the light wave propagates, the semiconductor
 optical amplifier structure further comprising a light absorptive magnetic
 material layer having light absorption function for the light wave
 propagating through the optical waveguide, the magnetic material layer is
 magnetized so as to have the magnetic-field component in the direction
 which corresponds to the direction where a magnetic vector of the light
 wave vibrates, the waveguide structure body has a nonreciprocity optical
 characteristic that effective refractive index changes into the light wave
 to which the optical waveguide is propagated according to the
 magneto-optical effect of the light absorptive magnetic material layer
 according to the direction of propagation, by the effective refractive
 index change in the nonreciprocity, the attenuation of the first light
 wave that the optical waveguide is propagated in the first direction
 caused when the said waveguide is propagated, becomes small more than the
 attenuation of the second light wave propagated in the second direction
 opposite to the first direction caused when the said waveguide is
 propagated.
 The present invention is based on the recognition that the refractive
 index, that is, the equivalent refractive index of the light wave
 propagating in the optical waveguide can be changed in the entire
 waveguide according to the propagating direction of the light wave by
 using the magnet-optical effect. If the equivalent refractive index can be
 changed according to the propagating direction of the light wave, the
 attenuation amount of the light wave propagated to one direction and the
 attenuation amount of the light wave propagated to the opposite direction
 can be made different, and thus optical waveguide isolator can be achieved
 by using the difference of the attenuation amount in case of propagating
 the light wave in the optical waveguide.
 The present invention is based on the above described recognition, to
 obtain nonreciprocal refractive index change by magnet-optical effect, an
 optical absorptive magnetic material layer is formed on the optical
 waveguide, the optical absorptive magnetic material layer is magnetized in
 the direction corresponding to vibrating direction of a magnetic vector of
 the light wave propagating the optical waveguide. The light wave
 propagates the optical absorptive magnetic material layer with the optical
 waveguide, so that the propagated light wave receives the magnet-optical
 effect of the optical absorptive magnetic material layer as the entire
 waveguide structure, and thus not only real part but also imaginary part
 of refractive index becomes nonreciprocal in the case of the optical
 absorption magnetic material layer, thereby being capable of making the
 amount of attenuation different in case of propagating the light wave in
 the optical waveguide in accordance with the propagating direction. In
 this case, the amount of the attenuation can differ only by appropriately
 setting magnetizable direction of the magnetic material layer according to
 the direction of propagation. As a result, a complicated structure for the
 phase matching and precise processing step become unnecessary.
 Next, the nonreciprocity of the optical waveguide structure according to
 the present invention is explained theoretically due to the magnet-optical
 effect thereof. Here, the xyz coordinate system described later and shown
 in FIG. 1 (the propagating direction of the light wave is assumed to be a
 z direction and the directions orthogonal to (the propagating direction of
 the light wave are assumed to be x and y directions) is assumed. In
 general, the character of the optical-magnet material is shown by the
 dielectric tensor, and when magnetic material formed on the waveguide is
 not magnetized, the permittivity .epsilon. is shown by the following
 expression.
 ##EQU1##
 Herein, .epsilon..sub.0 shows the permittivity in the vacuum. As is seen
 from the expression (1), when optical absorption magnetic material is not
 magnetized, the dielectric tensor has symmetric property, and thus does
 not exhibit the nonreciprocity of the light wave into which the equivalent
 refractive index therefore changes according to the propagating direction
 of the light wave.
 On the other hand, when optical absorption magnetic material is magnetized
 in the y direction, the permittivity .epsilon. is shown by the following
 expression.
 ##EQU2##
 As is seen from the expression (1), when the optical absorptive magnetic
 material layer is magnetized, the dielectric tensor has non-diagonal
 component, so that The nonreciprocity, into which the effective refractive
 index changes by the magnet-optical effect according to the existence of
 the non-diagonal component according to the propagating direction of light
 wave, is caused.
 The changed portion of the equivalent refractive index to the light wave
 propagated in the transverse magnetic mode in the z direction between the
 case of magnetizing optical absorption magnetic material and the case of
 not magnetizing the material in the y direction, can be shown by the
 following expression.
 ##EQU3##
 Herein, .DELTA.n+.DELTA.k shows the changed portion of the equivalent
 refractive index, K0 is a wave number of light in the vacuum, n is
 refractive index of the each layer of the waveguide, and Hy is a
 magnetic-field component of the transverse magnetic mode of magnetic
 material formed on the waveguide.
 On the other side, the changed component of the equivalent refractive index
 to the light wave propagated in -z direction opposite to the z direction,
 is the same as to residual elements, except that the sign of the term of
 .epsilon.xz of the integration term at the right of the expression (3)
 only reverses. Therefore, the equivalent refractive index will differ
 between the traveling wave and the retrograding wave. By positively using
 the change in the equivalent refractive index of the nonreciprocity due to
 the magnet-optical effect, the energy attenuation amount according to the
 energy transfer caused when the light wave propagates the said waveguide,
 from the optical waveguide to the optical absorptive magnetic material
 layer, changes according to the direction of propagation, so that the
 optical waveguide isolator can be achieved. In this case, the incident
 signal light can be outputted with light amplification by assuming the
 optical waveguide structure to be a semiconductor optical amplifier
 structure so as to compensate the attenuation amount of the signal light
 caused in case of propagating the signal light on the waveguide, so that
 the isolation function can be achieved without attenuating the signal
 light.
 Next, the isolation ratio is explained. Isolation ratio IR can be shown by
 the following expressions.
EQU IR=output strength of backward propagating wave/output strength of forward
 propagating light (4)
 By using the expression (2), the isolation ratio can be shown by difference
 2.DELTA.k between the attenuation of the forward-propagating wave and the
 attenuation of the backward propagating wave. this is shown by expression
 (5).
EQU I.sub.R =.vertline.exp(2.vertline..DELTA.k.vertline.k.sub.0
 L).vertline..sup.2 (5)
 Herein, L is length of the device. It was confirmed to be able to obtain
 the isolation ratio of 40 dB in the length of the device of about 2.0 mm
 since the optical waveguide isolator explained by the embodiment described
 later was simulated.
 In a preferable embodiment of the optical waveguide isolator according to
 the present invention, first light wave is a signal light to be
 transmitted along a transmission system, and the second light wave is the
 return light propagated in the direction opposite to the signal light.
 According to such a construction, the signal light to be transmitted is
 not so attenuated, and the return light such as reflected light and the
 scattered light caused in transmission path can be greatly attenuated.
 In another embodiment of the optical waveguide isolator according to the
 present invention, the optical absorption magnetic material layer is
 constituted by magnetic material magnetized so as to have the
 magnetic-field component in the direction corresponding to the vibrating
 direction of a magnetic vector of the first light wave propagating on the
 waveguide. To obtain the nonreciprocity according to the magnet-optical
 effect, the direction of the external magnetic field should be made to
 correspond to the vibrating direction of a magnetic vector of the light
 wave having various modes propagating on the waveguide, for example, the
 magnetic material layer is formed in the case of the transverse magnetic
 mode light along the plane parallel to the substrate, so that the
 nonreciprocity according to the magnet-optical effect can be achieved by
 magnetizing the magnetic material layer in a direction orthogonal to the
 propagating direction of light wave.
 As optical absorption magnetic material which causes the magnet-optical
 effect, ferromagnetic material of nickels, iron, and cobalt, etc. and
 various magnetic materials such as yttrium iron garnet (YIG) can be used.
 According to the suitable embodiment of the optical waveguide isolator of
 the present invention, this substrate is semiconductor substrate, and the
 core layer and first and second cladding layer are constituted with
 semiconductor material. According to such a constitution, the optical
 waveguide isolator can be formed on the same semiconductor substrate with
 semiconductor laser by using the semiconductor manufacturing technology.
 Another embodiment of the optical waveguide isolator according to the
 present invention, there is provided an optical waveguide isolator
 comprising a semiconductive light amplifier structure including a
 semiconductor substrate of first conductivity type having a surface of a
 layer to be formed, a first cladding layer of first conductivity type
 formed on the substrate, an active layer formed on the first cladding
 layer, a second cladding layer of second conductivity type opposite to
 first conductivity type, formed on the active layer, a first electrode
 formed on the surface of the semiconductor substrate opposite to the
 surface to be formed as a layer, and a second electrode formed on the
 second cladding layer; the first cladding layer, the active layer, and the
 second cladding layer constitute an optical waveguide, the second
 electrode includes a light absorptive magnetic material layer magnetized
 so as to have a magnetic-field component in the direction corresponding to
 the vibrating direction of a magnetic vector of the light wave propagating
 in the waveguide, the optical waveguide and the magnetic material layer of
 the second electrode constitute an optical waveguide structure, the
 waveguide structure body has a nonreciprocity optical characteristic that
 effective refractive index changes into the light wave to which the
 optical waveguide is propagated according to the magneto-optical effect of
 the light absorptive magnetic material layer according to the direction of
 propagation, by the effective refractive index change in the
 nonreciprocity, the attenuation of the first light wave that the optical
 waveguide is propagated in the first direction caused when the said
 waveguide is propagated, becomes small more than the attenuation of the
 second light wave propagated in the second direction opposite to the first
 direction caused when the said waveguide is propagated. Thus, if the
 optical waveguide isolator is constituted as a structure of the
 semiconductor optical amplifier, the incident signal light can be
 amplified by adjusting the output voltage in the DC bias source connected
 between the first electrode and the second electrode and can emanate as a
 signal light with the same energy level as the energy level at the
 incident time.
 In another embodiment of the optical waveguide isolator having the light
 amplifier structure, the one electrode of the light amplifier structure
 has a nickel layer and a gold layer which are magnetized in the direction
 corresponding to the vibrating direction of a magnetic vector of the first
 light wave.
 In this embodiment, there is an advantage by which the electrode of the
 light amplifier can be co-used as an optical absorptive magnetic material
 layer.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENT
 Now to the drawings, there is shown an embodiment of an optical waveguide
 isolator according to the present invention. FIG. 1 is a diagrammatic
 cross-sectional view showing the constitution of one embodiment of the
 optical waveguide isolator according to the present invention. This
 embodiment explains the optical waveguide isolator having a semiconductor
 optical amplifier structure for amplifying and emanating incident light.
 In FIG. 1, assuming that the light wave is propagated to a z direction,
 and a semiconductor layer structure is formed along an x direction.
 Substrate 1 of InP of n type is prepared, and the semi-conductor layer
 structure is formed on a layer forming surface 1a of the substrate.
 Substrate 1 has the thickness of 100 .mu.m, and the high impurity
 concentration is assumed to be 1.times.1018 atoms/cm.sup.3 for example.
 For example, the sulfur can be used as impurities of n type.
 First cladding layer 2 of n type InP is deposited on the substrate 1 of
 InP. First cladding layer 2 has the thickness of 200 nm and the refractive
 index of 3.16, and the high impurity concentration is assumed to be
 1.times.1017 atoms/cm.sup.3. Undoped first guide layer 3 of InGaAsP is
 formed on the first cladding layer 2. The thickness of first guide layer 3
 is, for example, 120 nm, and the refractive index is 3.37. Undoped active
 layer 4 of InGaAsP is formed on the first guide layer 3. The active layer
 4 has a thickness of 100 nm and refractive index of 3.4132. For xample, in
 this embodiment, the active layer of the monolayer is used, but, a
 multiple quantum well structure of InGaAs and InGaAsP can be used. Undoped
 second guide layer 5 of InGaAsP is formed on the active layer 4. Second
 guide layer 5 has the thickness of 120 nm and refractive index of 3.37.
 A second cladding layer 6 of p type InP is deposited on the second guide
 layer 5. The second cladding layer has a thickness of 300 nm and a
 refractive index of 3.16, and its impurity concentration is 1.times.1017
 atoms/cm.sup.3. A cap layer 7 of p type InGaAs is formed on the second
 cladding layer 6. The cap layer 7 has a thickness of 30 nm, and its
 impurity concentration is 1.times.1019 atoms/cm.sup.3.
 First electrode 8 is formed on the cap layer 7. The first electrode 8 is
 formed by a nickel layer 8a of 50 nm in thickness and a gold layer 8b of
 100 nm in thickness formed on the nickel layer 8a. The nickel layer 8a
 constitutes a first electrode 8 together with the gold layer 8b, and
 functions as an optical absorption magnetic material layer which exhibits
 a magneto-optical effect for the light wave propagating waveguide. A
 second electrode 9 is formed on the other side of substrate 1 opposite to
 the layer forming surface 1a. The second electrode is formed by a titanium
 layer 9a of 50 nm in thickness and a gold layer 9b of 100 nm in thickness.
 A DC bias source 10 is connected between the first electrode 8 and the
 second electrode 9, so that the incident light wave is amplified to the
 level more than the energy level thereof and emanated therefrom.
 The optical waveguide isolator functions as a semiconductor optical
 amplifier and functions as a single optical waveguide structure. That is,
 the active layer 4 and the first and second guide layers 3 and 5 formed on
 both sides thereof constitute a core layer of the optical waveguide, The
 first cladding layer 2 constitutes a cladding layer formed on the one side
 of the optical waveguide, the second cladding layer 6 and cap layer 7 form
 a cladding layer formed on the other side of the optical waveguide, and
 the nickel layer 8a constitutes an optical absorption magnetic material
 layer, so that an optical waveguide is constituted by the core layer and
 the first and second cladding layer, and an optical absorptive magnetic
 material layer is constituted by the nickel layer 8a, thereby constituting
 a single optical waveguide structure. In this case, the waveguide type
 optical waveguide isolator having the substantially same structure as the
 semiconductor laser can be integrated on the same semiconductor substrate.
 It is necessary that the optical waveguide isolator according to the
 present invention have an asymmetrical structure to the light wave
 propagating the waveguide. In the embodiment shown in FIG. 1, it can be
 considered that the y direction has extended to infinity. On the other
 hand, the optical absorptive magnetic material layer is formed only on the
 other side of the substrate about the x direction, so that an asymmetrical
 structure is secured in the x direction.
 The second cladding layer 6 has an important meaning to define the distance
 between the core layer and the optical absorptive magnetic material layer
 of the optical waveguide. That is, the thickness of the second cladding
 layer 6 has an important meaning to define the energy amount which begins
 to exude on the light absorptive magnetic material layer 8a side of the
 light wave propagating on the waveguide structure, that is, the amount
 that the evanescent light of the propagated light wave is infiltrated to
 the light absorptive magnetic material layer, and to define the strength
 according to the magnet-optical effect of the propagated light wave to the
 optical absorptive magnetic material layer. Therefore, the thickness of
 second cladding layer 6 is appropriately set in consideration of the wave
 length of the light wave propagating on the waveguide and the usage as the
 optical waveguide isolator.
 Next, the magnetizing direction of the optical absorptive magnetic material
 layer 8a is explained. The transverse magnetic mode wave propagated in the
 z direction of FIG. 1(From the left side of space aiming at the right of)
 is assumed as signal light. In this case, a magnetic vector of the
 transverse magnetic mode wave vibrates in the y direction. Therefore,
 magnetic material layer 8a is formed to be extended along the y direction,
 and magnetized so as to turn from the interior side of space to the
 frontward side. By magnetizing the optical absorptive magnetic material
 layer in such a direction, the attenuation to the signal light propagated
 in the z direction is minimized, the attenuation to the reflected wave and
 the scattered light propagated in the opposite direction (-the z
 direction) can be maximized, and thus a large isolation ratio can be
 obtained. The magnetizing direction of the magnetic material layer need
 not accurately correspond to the direction of a magnetic vector of the
 light wave propagating the waveguide, and thus the desired performance can
 be obtained by only magnetizing the magnetic material layer so as to have
 a magnetic component of the direction corresponding to the direction of a
 magnetic vector.
 Moreover, the vibrating direction of a magnetic vector of the transverse
 electric mode wave becomes x direction in FIG. 1, so that in the case of
 the waveguide propagating the transverse electric mode wave, the optical
 absorptive magnetic material layer extended in a direction which is almost
 orthogonal to layer forming surface 1a of substrate 1 is formed on the
 side of the waveguide structure, the magnet-optical effect of the magnetic
 material layer can be used by magnetizing the optical absorptive magnetic
 material layer in the x direction.
 Next, operation of the optical waveguide isolator when the isolator shown
 in FIG. 1 is used as an optical waveguide isolator having light
 amplification operation, is explained. Assuming that the signal light
 propagates in +z direction (from the left side to the right side on the
 plane), and the return light to be attenuated propagates in the opposite z
 direction. The optical waveguide isolator causes a strong attenuation
 operation for the return light and has the attenuation operation about the
 signal light. Therefore, the optical waveguide isolator performs the
 optical amplification in such a manner that the attenuation level may at
 least become 0 levels for the incident signal light and emanates the thus
 amplified light. The optical amplification rate is controlled by adjusting
 the voltage level of the DC bias source 10. As a result, the energy level
 of the return light can be greatly attenuated without decreasing the
 energy level of the signal light to be propagated.
 The present invention is not limited to the above described embodiment, and
 various modifications and changes can be performed. For example, the
 compound semiconductor materials of the InP system is used in the above
 described embodiment, but, for example, GaAs system semiconductor material
 and GaN system semiconductor material can be used. When GaAs system
 semiconductor material is used, GaAs is used as a material of the core and
 AlGaAs can be used as a material of the cladding layer. Moreover, when GaN
 system semiconductor material is used, InGaN is used as a material of the
 core and GaN can be used as a material of the cladding layer.