JP2004138873A - Waveguide optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently carry out fast modulation with simple control while reducing insertion loss. <P>SOLUTION: A semi-insulative InGaAsP guide layer 12 and a semi-insulative InP cladding layer 13 are successively laminated on a semi-insulative InP substrate 11. A semi-insulative InP protrusion layer 14 worked in a curved shape is formed on the semi-insulative InP cladding layer 13 and an AuZnNi/Au layer 15 is formed so as to be along with an outer peripheral part of the semi-insulative InP protrusion layer 14. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a waveguide type optical modulator, and is particularly suitable for application to a radiation type optical modulator using a bent optical waveguide.
[0002]
[Prior art]
As a conventional optical modulator for optical communication, an electric field absorption effect element and an electro-optic effect element have been proposed for a long time.
Among them, the electric field absorption effect element changes the band gap energy (absorption wavelength of light) by applying an electric field to the semiconductor material, and absorbs light of a specific wavelength passing through the semiconductor material, thereby increasing the light intensity. Modulation is performed, and can be divided into Franz-Keldysh effect type and quantum Stark effect type.
[0003]
Here, the technology of the electroabsorption element using a semiconductor material has been developed since the latter half of the 1980's, and the technology for improving characteristics (for example, Patent Document 1) and the technology for integrating with a semiconductor laser (for example, Patent Document 2) Has been developed, and in the 1990's, an optical modulator circuit applying these basic technologies has been developed (for example, Patent Document 3).
On the other hand, as for the electro-optic effect element, the optical waveguide circuit can be classified into a branch interference type and a directional coupling type. Recently, a technique for increasing the speed by devising an electrode such as a traveling wave electrode has been attracting attention.
[0004]
The technical development of the optical modulator using the electro-optic effect started with the technology related to the basic structure in the 1970s (for example, Patent Document 4). In the 1980s, the technology development for speeding up the optical modulation became active, and the light wave and the micro wave were developed. A traveling wave electrode structure that modulates waves at high speed by reducing the phase shift of waves has been developed (for example, Patent Document 5).
Furthermore, in the 1990's, the technology for broadening the modulation characteristics was developed, and the detailed structure of the traveling wave electrode was improved (for example, Patent Documents 6 and 7).
[0005]
Here, in an optical waveguide circuit using an electro-optic effect element, a branch interference type composed of a Y-branch optical waveguide, two phase modulation waveguides, and a multiplexing Y-branch waveguide is generally used. There is an advantage that the structure is not limited by the wavelength band of light to be used or the absorption current.
However, in the branching interference type optical modulator, in order to modulate the light intensity, it is necessary to give a refractive index change in the phase modulator unit to change the phase of light by about 90 degrees, and the modulation voltage tends to be high.
[0006]
In order to reduce the driving voltage even slightly, the length of the phase modulation waveguide generally tends to be long, and the length reaches several cm.
For this reason, the insertion loss of the entire device tends to be large, and particularly, when the branch interference optical modulator is formed of a semiconductor material, when both the insertion loss and the device length or the device length are reduced, There has been a problem that the operating voltage increases (for example, Non-Patent Document 1).
[0007]
Therefore, in the conventional semiconductor-based branch interference modulator, the insertion loss of the entire device is reduced by replacing a part of the p-type InP cladding layer formed on the InGaAsP guide layer with an intrinsic InP cladding layer. (For example, Non-Patent Document 2).
FIG. 10 is a cross-sectional view showing an example of a phase modulation waveguide structure of a conventional branch interference modulator.
[0008]
In FIG. 10, an intrinsic InGaAsP guide layer 102 and an intrinsic InP cladding layer 103 are formed on an n-type InP substrate 101. On the intrinsic InP cladding layer 103, a protruding p-type InP for forming a PN junction surface is formed. The clad layer 104 is formed, and a strip-loaded waveguide is formed.
On the p-type InP cladding layer 104, an AuZnNi / Au layer 105 is formed as a p-side electrode, and on the back surface of the n-type InP substrate 101, an AuGeNi / Au layer 106 is formed as an n-side electrode.
[0009]
Here, the loss of the strip-loaded waveguide structure is reduced by providing the undoped intrinsic InP cladding layer 103 between the intrinsic InGaAsP guide layer 102 and the p-type InP cladding layer 104.
FIG. 11 is a cross-sectional view showing another example of the phase modulation waveguide structure of the conventional branching interference modulator.
[0010]
11, an n-type InP projection layer 202 is formed on an n-type InP substrate 201. On the n-type InP projection layer 202, an intrinsic InGaAsP guide layer 203, an intrinsic InP cladding layer 204, and a PN junction surface are formed. P-type InP cladding layer 205 is formed to form a high-mesa optical waveguide structure.
On the p-type InP cladding layer 205, an AuZnNi / Au layer 206 is formed as a p-side electrode, and on the back surface of the n-type InP substrate 201, an AuGeNi / Au layer 207 is formed as an n-side electrode.
[0011]
Here, the loss of the high-mesa optical waveguide structure is achieved by providing an impurity-free intrinsic InP clad layer 204 between the intrinsic InGaAsP guide layer 203 and the p-type InP clad layer 205.
In addition, the mesa processing is not stopped in the intrinsic InP cladding layer 204, and the high mesa optical waveguide structure is dug down to the n-type InP substrate 201, so that the applied volume of the electric field can be concentrated on the light propagation portion, and the modulation can be performed. Efficiency can be improved.
[0012]
In recent years, low-cost optical modulation methods for short-distance metro networks, access systems, and Ethernet communications, which are being developed in recent years, use semiconductor lasers directly instead of the above-described modulation methods using the electric field absorption effect or the electro-optic effect. A modulation method is often used.
In the method of directly modulating the semiconductor laser, since the drive current of the semiconductor laser is directly modulated, an external modulator is not required, and the cost of the optical communication system can be reduced.
[0013]
[Patent Document 1]
Japanese Patent No. 2670061
[Patent Document 2]
Japanese Patent Publication No. 5-8878
[Patent Document 3]
Japanese Patent No. 2817602
[Patent Document 4]
JP-B-61-8411
[Patent Document 5]
Japanese Patent No. 261896
[Patent Document 6]
Japanese Patent No. 2606774
[Patent Document 7]
Japanese Patent No. 2871645
[Non-patent document 1]
1989 IEICE Autumn National Convention C-260
[Non-patent document 2]
H. Takeuchi IEEE. Photonic Technology. Lett. Vol. no. 8 pp227-228, N.P. Yoshimoto. IEEE EL vol. 32 No. 15. 1996 pp1366-1369
[0014]
[Problems to be solved by the invention]
However, in an optical modulator utilizing the electric field absorption effect, the intensity of light is modulated by changing the amount of absorption of light of a specific wavelength passing through the semiconductor material. It is necessary to set the gap energy (wavelength) very close.
[0015]
For this reason, when an optical modulator utilizing the electric field absorption effect is used for wavelength division multiplexing optical communication, it is necessary to prepare an optical modulator having a dedicated band gap energy (absorption wavelength) for each of many optical wavelength channels used for communication. There is a problem that the laser and the absorption modulator must be manufactured and designed as a pair.
Further, in the electroabsorption type element, the absorbed light must be discharged as a photocurrent, so that in the case of a high output or a high speed exceeding 10 GHz, the generation of the photocurrent causes impedance matching and tolerance of a voltage driving source. There is a problem that the design such as the amount of current is limited.
[0016]
In the optical modulator using the electro-optic effect shown in FIGS. 10 and 11, since the absorption coefficient of the p-type Inp cladding layers 104 and 205 is large, when the element is long to about 2 mm, an insertion loss of 3 dB or more occurs.
Of course, if the thickness of the intrinsic Inp cladding layers 103 and 204 is 0.6 μm or more, the insertion loss of the optical modulator can be further reduced.
[0017]
However, when the thickness of the intrinsic Inp cladding layers 103 and 204 is increased, the electric field intensity that can be applied to the intrinsic InGsAsP guide layers 102 and 203 is also reduced. Therefore, there is a problem that the operating voltage increases and easily reaches about 4V. Was.
The p-side electrode of the optical modulator shown in FIGS. 10 and 11 is usually formed by laminating a Pt (platinum) and Au (gold) layer with a thickness of several microns on an AuZnNi layer so that the AuZnNi / Au layer 105, 206 is formed.
[0018]
However, since the electrical resistance value of the p-type Inp cladding layers 104 and 205 is larger than that of the AuZnNi / Au layers 105 and 206, a traveling wave electrode as shown in Japanese Patent No. 2606774 and Japanese Patent No. 2871645 is designed. And the speed of the optical modulator due to the electro-optic effect using a semiconductor material is hindered.
[0019]
Further, in the method of directly modulating the semiconductor laser, since the driving current of the semiconductor laser is directly modulated, in addition to the modulation energy being increased, as the modulation speed is increased, the relaxation determined by the carrier lifetime inside the semiconductor laser is increased. Light modulation output vibration called vibration occurs, which greatly affects light transmission characteristics.
[0020]
For this reason, in the current 10 GHz optical transmission, when a received optical signal is converted into electricity, an electrical filter is applied to correct the disturbance of the optical transmission waveform. However, the communication speed is higher than 10 GHz. Then, there is a problem that the relaxation oscillation frequency and the modulation speed are close to each other, and it is difficult to perform correction by this filter.
Also, in the direct modulation method which seems simple at first glance, the frequency band that can be driven by the problem of the relaxation oscillation frequency fluctuates depending on the driving conditions, so it is necessary to respond individually to the setting, which makes the setting difficult. was there.
SUMMARY OF THE INVENTION It is an object of the present invention to provide a waveguide type optical modulator which can easily control and efficiently perform high-speed modulation while reducing insertion loss.
[0021]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, according to the waveguide type optical modulator of the first aspect, a bent optical waveguide is provided on an outer peripheral portion of the bent optical waveguide, and a refractive index of the outer peripheral portion is changed. And a modulation electrode for controlling the light confinement effect of the outer peripheral portion.
[0022]
Thereby, based on the applied voltage applied to the outer peripheral portion, the guided light confined in the bent optical waveguide can be radiated from the outer peripheral portion, and light modulation is performed by providing an electrode beside the optical waveguide. It becomes possible.
For this reason, it is possible to dispose the electrode very close to the distribution of the guided light while suppressing metal absorption, and it is necessary to provide a p-type cladding layer below the electrode to increase the distance between the guided light and the electrode. Disappears.
[0023]
As a result, it is possible to reduce the absorption loss of the guided light and reduce the insertion loss of the optical modulator, and it is possible to efficiently change the relative refractive index difference of the bent optical waveguide structure, thereby achieving an operating voltage. Can be reduced.
In addition, by removing the p-type cladding layer under the electrode, the application end of the electric field can be made to be the metal electrode surface, and the traveling wave electrode with low electric loss can be easily designed. Thus, it is possible to easily cope with an increase in the speed of the optical modulator.
[0024]
Furthermore, by performing light modulation using the radiation characteristics from the bent optical waveguide and the light confinement characteristics due to the relative refractive index difference, the light is absorbed more than a specific wavelength and compared with an electric field absorption effect element that performs light modulation. It is possible to reduce the wavelength dependence at the time of modulation.
For this reason, it is possible to easily expand the wavelength band that can be covered by the same element, and to easily design an optical modulator, and to prepare one type of optical modulator. It is possible to easily cope with a wavelength division multiplexing optical communication system.
[0025]
According to the waveguide type optical modulator of the second aspect, the bent optical waveguide includes an optical guide layer, an intrinsic or semi-insulating semiconductor clad layer laminated on the optical guide layer, and the intrinsic or semi-insulating semiconductor clad layer. An intrinsic or semi-insulating semiconductor projection layer provided on a semi-insulating semiconductor cladding layer, wherein the modulation electrode is provided along the outer periphery of the intrinsic or semi-insulating semiconductor projection layer. And
[0026]
This makes it possible to guide light along the semiconductor protrusion layer while suppressing absorption loss by the semiconductor cladding layer and metal absorption by the modulation electrode, and through the intrinsic or semi-insulating semiconductor cladding layer. Thus, an electric field can be efficiently applied to the light guide layer.
As a result, it is possible to efficiently control the characteristics of light emission from the bent optical waveguide, to reduce the operating voltage, and to efficiently perform high-speed modulation while reducing insertion loss. It becomes.
[0027]
According to the waveguide type optical modulator of the third aspect, an intrinsic or semi-insulating semiconductor substrate, and an intrinsic or semi-insulating semiconductor optical guide layer laminated on the intrinsic or semi-insulating semiconductor substrate, An intrinsic or semi-insulating semiconductor cladding layer laminated on the intrinsic or semi-insulating semiconductor optical guide layer, and an intrinsic or semi-insulating semiconductor protrusion layer provided on the intrinsic or semi-insulating semiconductor cladding layer, Along with the periphery of the intrinsic or semi-insulating semiconductor protrusion layer, a signal electrode provided on the intrinsic or semi-insulating semiconductor cladding layer, and disposed outside and inside the signal electrode. And a ground electrode provided on the intrinsic or semi-insulating semiconductor cladding layer.
[0028]
This makes it possible to control the light emission characteristics at the outer periphery of the bent optical waveguide while adopting a GSG (ground signal ground) type electrode structure, thereby facilitating impedance design while suppressing insertion loss, and achieving high speed operation. Modulation can be performed efficiently.
According to the waveguide type optical modulator of the fourth aspect, the semiconductor substrate, the ridge waveguide type semiconductor laser formed in a partial region on the semiconductor substrate, and the ridge waveguide type semiconductor laser. And a strip-loaded bending optical waveguide formed on the semiconductor substrate and joined to the ridge waveguide-type semiconductor laser, provided on the outer periphery of the strip-loading bending optical waveguide, A modulation electrode for controlling a light confinement effect of the outer peripheral portion by changing a refractive index of the outer peripheral portion.
[0029]
This makes it possible to match the optical path of the semiconductor laser with the optical path of the optical modulator by using lithography technology, eliminating the need for alignment work for aligning the optical path of the semiconductor laser with the optical path of the optical modulator. And the semiconductor laser and the optical modulator can be integrated on the same substrate, and the construction of the optical communication system can be facilitated.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a waveguide type optical modulator according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view illustrating a schematic configuration of a waveguide optical modulator according to a first embodiment of the present invention, and FIG. 2 is a schematic configuration of the waveguide optical modulator according to the first embodiment of the present invention. FIG.
[0031]
1 and 2, an intrinsic InGaAsP guide layer 2 and an intrinsic InP cladding layer 3 are formed on an n-type InP substrate 1, and the intrinsic InP cladding layer 3 is formed on the intrinsic InP cladding layer 3 so as to be curved with a predetermined radius of curvature. The protruding layer 4 is formed to form a strip-loaded bending waveguide.
The intrinsic InGaAsP guide layer 2 may have a multiple quantum well structure using InGaAsP / InGaAs.
[0032]
Further, the intrinsic InP cladding layer 3 and the intrinsic InP protrusion layer 4 can be formed by processing the intrinsic InP layer laminated on the intrinsic InGaAsP guide layer 2 into a mesa shape.
Here, the intrinsic InP protrusion layer 4 can be configured to be bent by 90 degrees on the intrinsic InP cladding layer 3, for example, and the light incident end and the light exit end can be provided on end faces perpendicular to each other. .
[0033]
An AuZnNi / Au layer 5 is formed on the intrinsic InP cladding layer 3 along the outer periphery of the intrinsic InP protrusion layer 4, and an AuGeNi / Au layer 6 is formed on the back surface of the n-type InP substrate 1. Is formed.
The AuZnNi / Au layer 5 can be used as a signal electrode, and the AuGeNi / Au layer 6 can be used as a ground electrode.
[0034]
Then, the light input to the waveguide type optical modulator is guided along the intrinsic InP protrusion layer 4 through the intrinsic InGaAsP guide layer 2, turned 90 degrees, and guided to the light emission side. You.
Here, a part of the light guided through the intrinsic InGaAsP guide layer 2 is radiated from the outer peripheral portion of the light confinement structure by the intrinsic InP protrusion layer 4 when the direction of the light is changed along the intrinsic InP protrusion layer 4. The characteristics can be controlled by the relative refractive index difference between the region R1 and the region R2.
[0035]
The relative refractive index difference between the region R1 and the region R2 can be expressed by the following equation (1), where the relative refractive index of the region R1 is Neq1 and the relative refractive index of the region R2 is Neq2.
Relative refractive index difference = (Neq1-Neq2) / Neq1
A general definition of the relative refractive index difference is disclosed in Kona Corporation's "Basics of Optical Waveguides" by Katsuyuki Okamoto, and its application to semiconductor optical waveguides is disclosed in Japanese Patent Application No. 7-105727.
[0036]
By applying a voltage to the intrinsic InGaAsP guide layer 2 via the AuZnNi / Au layer 5, the relative refractive index of the region R2 can be controlled, and the relative refractive index difference between the region R1 and the region R2 can be reduced. By changing this, it becomes possible to control the radiation characteristics of light emitted from the outer peripheral portion of the intrinsic InP protrusion layer 4.
Also, by using the radiation type modulation principle based on the bent waveguide structure, it becomes possible to remove the p-type cladding layer which absorbs a large amount of light, and to place the AuZnNi / Au layer 5 on the side of the intrinsic InP protrusion layer 4. It is possible to provide a semiconductor optical modulator structure that can be arranged, improves the insertion loss of the entire device and the problem of the design of the traveling-wave electrode, and is simple in control and capable of high-speed modulation.
[0037]
Further, by arranging the AuZnNi / Au layer 5 beside the intrinsic InP protrusion layer 4, even when the height of the intrinsic InP protrusion layer 4 is increased in order to reduce the absorption loss, the AuZnNi / Au layer 5 is kept in an intrinsic state. It becomes possible to approach the InGaAsP guide layer 2, and an electric field can be efficiently applied to the intrinsic InGaAsP guide layer 2.
[0038]
For this reason, the operating voltage can be reduced while suppressing the absorption loss, and high-speed modulation can be performed efficiently.
In addition, as the bending curvature of the intrinsic InP protrusion layer 4 in the bent waveguide structure, a critical curvature that can keep the bending loss at 1 dB or less can be selected. Here, the critical curvature can be adjusted by the layer thickness d of the intrinsic InGaAsP guide layer 2 and the layer thickness t of the intrinsic InP cladding layer 3, which are the structural parameters of the optical waveguide.
[0039]
FIG. 3 is a diagram showing the relationship between the cladding layer thickness t, the relative refractive index difference, and the critical curvature of the waveguide type optical modulator according to one embodiment of the present invention. The critical curvature was defined as the critical curvature at which the propagating light attenuated by 3 dB.
3, the horizontal axis represents the thickness t of the intrinsic InP cladding layer 3, and the vertical axis represents the relative refractive index difference between the region R1 and the region R2.
[0040]
Here, the critical curvature of the intrinsic InP protrusion layer 4 in the bent waveguide structure can be determined by the relative refractive index difference, and it is understood that the larger the relative refractive index difference, the smaller the critical radius of curvature.
Then, when a reverse voltage is applied to the AuZnNi / Au layer 5 provided along the outer peripheral portion of the intrinsic InP protruding layer 4, for example, as described in the “1989 IEICE Autumn Meeting C-260” In the case of a guide layer of a bulk material, the AuZnNi / Au layer 5 is formed by a quantum confined Stark effect (QCSE) when a multiple quantum well structure is used instead of the primary electro-optic effect, the Franz-Keldysh effect, and the bulk material. The refractive index of the lower intrinsic InGaAsP guide layer 2 increases by about 5 / l0000 per volt.
[0041]
Due to this change in the refractive index, the bent waveguide structure loses the effect of confining the light in the lateral direction, and the propagation light is radiated from the outer periphery of the bent waveguide, so that the loss of the propagation light increases.
Therefore, by controlling the voltage applied to the AuZnNi / Au layer 5 to change the refractive index of the intrinsic InGaAsP guide layer 2, light intensity can be modulated.
[0042]
In the method of controlling the light confinement effect at the outer peripheral portion of the bent optical waveguide, propagation light is radiated from the outer peripheral portion of the bent waveguide, so that it is possible to suppress generation of an absorption current.
For this reason, in the case of high output and high speed exceeding 10 GHz, it is possible to relax restrictions on design such as impedance matching and allowable current amount of a voltage driving source, and to easily cope with high output and high speed. Becomes possible.
[0043]
Further, in this modulation method and element structure, the AuZnNi / Au layer 5 is provided beside the light confinement region, so that it is possible to arrange an electrode very close to the distribution of the guided light, and to set the Pd from the bent optical waveguide. The mold clad layer can be removed.
That is, of the materials constituting the optical modulator, the InGaAsP guide layer and the upper InP cladding layer are made of intrinsic or semi-insulating semiconductor, so that the p-type InP cladding layer 104 is provided above the PIN structure of FIG. , The insertion loss of the optical modulator can be reduced to about 1/5.
[0044]
This is because the absorption loss of the p-type InP cladding layer 104 of FIG. ¹⁸ cm ^-3 When the intrinsic InP cladding layer 3 and the intrinsic InP protrusion layer 4 are used, the absorption loss becomes almost zero, while the AuZnNi / Au layer 5 has a propagation light distribution of about 80 dB / cm. However, since the optical modulator has a strip-loaded structure, the light distribution is confined in the intrinsic InGaAsP guide layer 2 and is hardly affected by metal absorption.
[0045]
Further, assuming that the structure having a small relative refractive index difference, for example, the layer thickness d of the intrinsic InGaAsP guide layer 2 is 0.4 μm and the layer thickness t of the intrinsic InP cladding layer 3 is 0.4 μm, the ratio between the region R1 and the region R2 is The difference in refractive index is about 0.06%.
For this reason, if the critical radius of curvature of the intrinsic InP protrusion layer 4 in the bent waveguide structure is set to 15000 μm, an extinction ratio of about 20 dB or more can be obtained at an operating voltage of about 2 V, and an increase in the operating voltage can be suppressed. In addition, the modulation efficiency can be improved.
[0046]
Further, in the bent waveguide structure shown in FIGS. 1 and 2, the p-type InP cladding layer can be removed, so that the electric field application end can be the metal electrode surface.
For this reason, it is possible to reduce the electric loss of the traveling wave, to facilitate the traveling wave design, and to increase the speed to 20 GHz or more by providing an electrode structure with speed matching. Become.
[0047]
In addition, by performing light modulation using the radiation characteristics from the bent optical waveguide and the light confinement characteristics based on the relative refractive index, the light is absorbed more than a specific wavelength and compared with the electric field absorption effect element that performs light modulation. It is possible to reduce the wavelength dependence at the time of modulation.
For this reason, it is possible to cover the wavelength band of light of about several tens of nm with the same structure, and when an optical modulator is used for wavelength division multiplexing optical communication, a dedicated optical modulation is performed for each of many optical wavelength channels used for communication. It is not necessary to prepare a device, and it is possible to easily perform the manufacturing design of the optical modulator.
[0048]
Also, regarding the characteristic change due to the temperature change, even if the room temperature changes, the relative refractive index difference in the horizontal direction hardly changes, so that the light modulation characteristic is hardly affected.
For this reason, the optical modulator can be used in various use environments, and maintenance and inspection of the optical transmission system can be facilitated.
[0049]
In the above-described first embodiment, a method has been described in which the intrinsic InP protruding layer 4 is bent by 90 degrees to form a bent waveguide, and the light incident end and the light output end are respectively provided on end surfaces perpendicular to each other. However, for example, the curved waveguide may be formed in a U-shape, and the light incident end and the light emitting end may be provided on the same end surface. The end and the light emitting end may be provided on the opposing surfaces, respectively.
[0050]
FIG. 4 is a plan view showing a schematic configuration of a waveguide optical modulator according to the second embodiment of the present invention. Note that the second embodiment is an example in which the semiconductor material constituting the bent optical waveguide, including the substrate, is entirely made of a semi-insulating material, and the traveling wave electrode structure is a GSG planar structure.
In FIG. 4, a semi-insulating InGaAsP guide layer 12 and a semi-insulating InP cladding layer 13 are formed on a semi-insulating InP substrate 11, and the semi-insulating InP cladding layer 13 is curved with a predetermined radius of curvature. As a result, the semi-insulating InP protrusion layer 14 is formed to form a strip-loaded bending waveguide.
[0051]
The semi-insulating InGaAsP guide layer 12 may have a multiple quantum well structure using InGaAsP / InGaAs.
The semi-insulating InP clad layer 13 and the semi-insulating InP protrusion layer 14 can be formed by processing the semi-insulating InP layer laminated on the semi-insulating InGaAsP guide layer into a mesa shape.
[0052]
Here, the semi-insulating InP projecting layer 14 is configured to bend by 90 degrees on the semi-insulating InP cladding layer 13, for example, and the light incident end and the light emitting end are provided on end faces orthogonal to each other. it can.
On the semi-insulating InP cladding layer 13, an AuZnNi / Au layer 15 is formed as a signal electrode along the outer periphery of the semi-insulating InP protrusion layer 14, and both sides of the AuZnNi / Au layer 15 are formed. AuGeNi / Au layers 16a and 16b are respectively formed as ground electrodes on the semi-insulating InP clad layer 13 to form a GSG type traveling wave electrode structure.
[0053]
Further, anti-reflection films 17a and 17b are formed on end faces of the optical modulator where the semi-insulating InP protrusion layers 14 intersect.
Then, the light input to the waveguide type optical modulator is guided along the semi-insulating InGaAsP guide layer 12 along the semi-insulating InP protrusion layer 14 and changes its direction by 90 degrees. It is guided to.
[0054]
Here, when a part of the light guided through the semi-insulating InGaAsP guide layer 12 changes its direction along the semi-insulating InP projection layer 14, an outer peripheral portion of the light confinement structure by the semi-insulating InP projection layer 14 is formed. And the radiation characteristics can be controlled by the relative refractive index difference of the light confinement structure by the semi-insulating InP protrusion layer 14.
The bent optical waveguide is made of a semi-insulating material, and the signal electrode and the ground electrode are formed on the semi-insulating InP clad layer 13 to form a GSG type traveling wave electrode structure, thereby suppressing insertion loss. In addition, the impedance between the signal electrode and the ground electrode can be reduced, impedance design can be easily performed, and high-speed modulation of 20 Hz or more can be efficiently performed.
[0055]
FIG. 5 is a diagram illustrating the relationship between the applied voltage and the normalized optical output of the waveguide optical modulator according to the second embodiment of the present invention.
In FIG. 5, when the substrate temperature was 25 ° C., the wavelength was 1550 nm, the optical input was 10 mW, and TE polarized light was incident, good extinction characteristics of about 20 dB and an insertion loss of about 1.5 dB were observed at an operating voltage of 2 V.
[0056]
Also, when the wavelength is changed from 1530 nm to 1570 nm, almost the same extinction characteristics as those in FIG. 5 can be observed, and the same structure can cover the wavelength band of light of about several tens nm. A modulator could be obtained.
FIG. 6 is a cross-sectional view illustrating a manufacturing process of the waveguide-type optical modulator according to the second embodiment of the present invention.
[0057]
In FIG. 6A, for example, a semi-insulating InGaAsP guide layer 12 is grown on a semi-insulating InP substrate 11 by using MOVPE (metal organic chemical vapor deposition).
The band gap energy of the semi-insulating InGaAsP guide layer 12 can be, for example, about 0.95 eV, and the layer thickness of the semi-insulating InGaAsP guide layer 12 can be, for example, about 0.4 μm.
[0058]
Here, by growing an InGaAsP quaternary mixed crystal while adding an impurity such as iron, the InGaAsP quaternary mixed crystal can be made semi-insulating.
Subsequently, a semi-insulating InP layer 13 ′ in which iron ions are mixed is grown on the semi-insulating InGaAsP guide layer 12 by using MOVPE.
The thickness of the semi-insulating InP layer 13 ′ can be, for example, about 1.5 μm.
[0059]
Then, a silicon oxide film, which is a mask material for dry etching, is deposited on the surface of the semi-insulating InP layer 13 ′ by CVD or the like, and then silicon oxide is formed by using photolithography technology and dry etching technology. The film is bent and patterned into an optical waveguide shape.
The thickness of the silicon oxide film may be, for example, about 0.3 μm, the width of the bent optical waveguide pattern may be, for example, about 2.5 μm, and the radius of curvature of the bent optical waveguide pattern may be, for example, 15 mm. .
[0060]
Then, the semi-insulating InP layer 13 'is etched to a depth of about 1.1 [mu] m using the silicon oxide film patterned in the optical waveguide shape as a mask, thereby obtaining a semi-insulating InP layer 13' as shown in FIG. The semi-insulating InP clad layer 13 provided with the InP protrusion layer 14 is formed on the semi-insulating InGaAsP guide layer 12.
By this etching process, a strip-loaded waveguide structure in which the semi-insulating InP protrusion layer 14 is provided on the semi-insulating InP cladding layer 13 can be formed.
[0061]
Subsequently, as shown in FIG. 6C, the AuZnNi / Au layer 15 is placed on the insulating InP clad layer 13 along the outer periphery of the semi-insulating InP protrusion layer 14 so as to have a thickness of 0.05 μm, 1.. The AuGeNi / Au layers 16a and 16b are formed on the insulating InP clad layer 13 so as to have a thickness of 0.05 μm and 1.2 μm, respectively, so as to be formed on both sides of the AuZnNi / Au layer 15. Formed.
[0062]
Here, by using the AuZnNi / Au layer 15 as a signal electrode and the AuGeNi / Au layers 16a and 16b as ground electrodes, a GSG type traveling wave electrode structure can be formed, and the speed of the optical modulator can be increased. it can.
Next, as shown in FIG. 6D, antireflection films 17a and 17b are formed on the light incident surface and the light emission surface, respectively, by using electron beam evaporation or the like. In addition, as the antireflection films 17a and 17b, for example, SiO 2 ₂ Film and TiO ₂ A laminated structure including two layers of films can be used.
[0063]
In the above-described second embodiment, a method has been described in which, in order to form a bent waveguide, the intrinsic InP protrusion layer 4 is bent by 90 degrees, and the light incident end and the light output end are provided on the end surfaces orthogonal to each other. However, for example, the curved waveguide may be formed in a U shape, and the light incident end and the light emitting end may be provided on the same end surface. Alternatively, the curved waveguide may be formed in an S shape or a wave shape, and the light incident end may be formed. Alternatively, the light emitting end may be provided on the facing surface.
[0064]
FIG. 7 is a plan view showing a schematic configuration of a waveguide optical modulator according to the third embodiment of the present invention. Note that the third embodiment is an example in which a bent waveguide optical modulator and a semiconductor laser are monolithically integrated on the same substrate.
In FIG. 7, a 1.3 μm band distributed feedback semiconductor laser 40 provided with a p-type InP ridge layer 29 is formed in a partial region on the n-type InP substrate 21. An AuZnNi / Au layer 34 is formed on the p-type InP layer 27 and the p-type InP ridge layer 29 provided in the 1.3 μm band distributed feedback semiconductor laser 40 via an insulating film. The Au layer 34 is connected to the p-type InP ridge layer 29 via an opening formed in the insulating film.
[0065]
Here, by providing the AuZnNi / Au layer 34 not only on the p-type InP ridge layer 29 but also on the p-type InP layer 27, the area of the AuZnNi / Au layer 34 is enlarged and the AuZnNi / Au layer 34 is formed on the AuZnNi / Au layer 34. Wire bonding can be easily performed.
An intrinsic InGaAsP guide layer 31 and an intrinsic InP cladding layer 32 are formed on the n-type InP substrate 21 so as to embed the 1.3 μm band distributed feedback semiconductor laser 40, and are formed on the intrinsic InP cladding layer 32. Is formed so as to be bonded to the p-type InP ridge layer 29 so that the intrinsic InP protrusion layer 33 is curved, thereby forming a strip-loaded bending waveguide.
[0066]
The intrinsic InGaAsP guide layer 31 may have a multiple quantum well structure using InGaAsP / InGaAs.
The intrinsic InP cladding layer 32 and the intrinsic InP protrusion layer 33 can be formed by processing the intrinsic InP layer into a mesa, and the p-type InP layer 27 and the p-type InP ridge layer 29 It can be formed by processing the type InP layer into a mesa type.
For this reason, the mesa processing of the intrinsic InP layer of the bent waveguide and the p-type InP layer of the semiconductor laser is performed at once using the photolithography technique and the dry etching technique, so that the optical path of the bent waveguide and the optical path of the semiconductor laser are formed. Can be matched exactly.
[0067]
For this reason, it is not necessary to align the optical axis of the bent waveguide and the semiconductor laser, the optical axis deviation can be eliminated, and the loss at the joint between the bent waveguide and the semiconductor laser can be reduced.
An AuZnNi / Au layer 35 is formed on the intrinsic InP cladding layer 32 along the outer periphery of the intrinsic InP protrusion layer 33, and an AuGeNi / Au layer 36 is formed on the back surface of the n-type InP substrate 21. Is formed.
[0068]
The light emitted from the 1.3 μm band distributed feedback semiconductor laser 40 is guided along the intrinsic InP protrusion layer 33 through the intrinsic InGaAsP guide layer 31, and is turned by 90 degrees. Guided to the side.
Here, a part of the light guided through the intrinsic InGaAsP guide layer 31 is radiated from the outer peripheral portion of the light confinement structure by the intrinsic InP protrusion layer 33 when changing the direction along the intrinsic InP protrusion layer 33, and the radiation thereof is performed. The characteristics can be controlled by the relative refractive index difference of the light confinement structure by the intrinsic InP protrusion layer 33.
[0069]
By using the radiation modulation principle, it becomes possible to bend the p-type cladding layer having large light absorption from the waveguide structure and to place the AuZnNi / Au layer 35 on the side of the intrinsic InP protrusion layer 33. It is possible to provide a semiconductor optical modulator structure that can be arranged, improves the insertion loss of the entire device and the problem of the design of the traveling-wave electrode, and is simple in control and capable of high-speed modulation.
[0070]
In the example shown in FIG. 7, since the distributed feedback semiconductor laser 40 for the 1.3 μm band is monolithically integrated on the n-type InP substrate 21, the traveling wave type electrode is formed in a single strip type instead of a GSG planar type. It is necessary.
For this reason, the capacitance between the signal electrode and the ground electrode increases, and the frequency band in which modulation can be performed is limited. However, high-speed operation of about 10 GHz can be maintained by precisely performing impedance design.
[0071]
For example, in the embodiment of FIG. 7, when the operating current of the laser is set to 120 mA, good characteristics such as an optical output of 20 mW and a modulation band of about 8 GHz can be obtained.
This is probably because the upper part of the intrinsic InGaAsP guide layer 31 of the optical modulator section is an intrinsic InP layer, the absorption loss is almost zero, and the design of the single strip line is somewhat effective.
[0072]
In addition, the extinction characteristic is slightly deteriorated because the radius of curvature is set as small as 1 mm as compared with the configuration in FIG. 4, and an extinction ratio of about 13 dB can be obtained at 2 V.
8 and 9 are cross-sectional views illustrating the steps of manufacturing a waveguide-type optical modulator according to the third embodiment of the present invention.
In FIG. 8A, in order to integrate the distributed feedback type (DFB) laser 40 for the 1.3 micron band, for example, by using the MOCVD method, the intrinsic property of a 1.15 μm composition (band gap energy about 1.07 eV) is obtained. An InGaAsP buffer layer (1.15Q) 22, an intrinsic InGaAsP active layer (1.3Q) having a composition of 1.3 μm, an intrinsic InGaAsP guide layer (1.15Q) having a composition of 1.15 μm 24 and an intrinsic InP cladding layer 25 are n-type. The layers are sequentially laminated on the Inp substrate 21.
[0073]
Next, as shown in FIG. 8B, the intrinsic InP cladding layer 25 is removed with an etching solution such as a hydrochloric acid: phosphoric acid = 1: 4 aqueous solution, and the period adjusted to the transmission wavelength is obtained by using an electron beam exposure technique. To form a diffraction grating 26 on the intrinsic InGaAsP guide layer 24.
Subsequently, as shown in FIG. 8C, a p-type Inp cladding layer 27 'is grown on the intrinsic InGaAsP guide layer 24 on which the diffraction grating 26 has been formed by using, for example, the MOCVD method. Protect.
[0074]
Then, as shown in FIG. 8D, a nitrogen oxide film 28 is formed on the p-type Inp clad layer 27 'by CVD or the like, and by using photolithography technology and dry etching technology, 1. Patterning is performed so that the nitrogen oxide film 28 remains only in the region of the distributed feedback semiconductor laser 40 for the 3 μm band.
Then, using the patterned nitrogen oxide film 28 as a mask, the intrinsic InGaAsP buffer layer 22, the intrinsic InGaAsP active layer 23, the intrinsic InGaAsP guide layer 24, and the p-type Inp clad layer 27 'are wet-etched to perform a 1.3 μm band. The intrinsic InGaAsP buffer layer 22, the intrinsic InGaAsP active layer 23, the intrinsic InGaAsP guide layer 24, and the p-type Inp cladding layer 27 'other than the area of the distributed feedback semiconductor laser 40 are removed.
[0075]
As an etching solution for wet etching the intrinsic InGaAsP buffer layer 22, the intrinsic InGaAsP active layer 23, the intrinsic InGaAsP guide layer 24, and the p-type Inp cladding layer 27 ', for example, a phosphoric acid aqueous solution of phosphoric acid and sulfuric acid: hydrogen peroxide: A water solution or the like can be used.
Then, as shown in FIG. 9A, while using the MOVPE with the nitrogen oxide film 28 left on the p-type Inp cladding layer 27 ', an intrinsic InGaAsP guide layer (1.15Q ) 31 and the intrinsic InP layer 32 ′ are selectively grown on the n-type Inp substrate 21.
[0076]
The band gap energy of the intrinsic InGaAsP guide layer 31 can be, for example, around 0.95 eV, and the layer thickness of the intrinsic InGaAsP guide layer 12 can be, for example, about 0.4 μm.
The thickness of the intrinsic InP layer 32 'can be, for example, about 1.5 μm.
[0077]
Then, the nitrogen oxide film 28 on the p-type Inp cladding layer 27 'is removed, and the surface of the p-type Inp cladding layer 27' of the laser unit and the surface of the intrinsic InP layer 32 'of the optical modulator unit are formed by CVD or the like. A silicon oxide film as a mask material for dry etching is deposited.
Then, by patterning the silicon oxide film using photolithography technology and dry etching processing technology, the silicon oxide film on the laser portion is patterned into stripes, and the silicon oxide film on the optical modulator portion is bent. It is patterned into an optical waveguide shape.
[0078]
The thickness of the silicon oxide film can be, for example, about 0.3 μm, the width of the bent optical waveguide pattern can be, for example, about 2.5 μm, and the radius of curvature of the bent optical waveguide pattern can be, for example, 1 mm. .
Then, the p-type Inp cladding layer 27 ′ and the intrinsic InP layer 32 ′ are etched to a depth of about 1.1 μm using the silicon oxide film patterned in the stripe shape and the bent optical waveguide shape as a mask.
[0079]
Thereby, as shown in FIG. 9B, the intrinsic InP clad layer 32 provided with the intrinsic InP protrusion layer 33 can be formed on the intrinsic InGaAsP guide layer 31, and the p-type Inp clad layer 27 can be formed. Can be provided with a p-type InP ridge layer 29 above.
By this etching process, a strip-loaded waveguide structure can be formed in the optical modulator section, and a ridge structure bonded to the strip-loaded waveguide structure can be formed in the laser section. Thus, the optical path of the optical modulator section and the optical path of the laser section can be accurately matched without increasing the number of manufacturing steps.
[0080]
Subsequently, as shown in FIG. 9C, on the intrinsic InP clad layer 32 provided with the intrinsic InP protrusion layer 33 and on the p-type Inp clad layer 27 provided with the p-type InP ridge layer 29 by CVD or the like. Then, an insulating film 38 is formed.
Then, an opening is formed on the outer peripheral portion of the intrinsic InP protrusion layer 33 and on the p-type InP ridge layer 29 by patterning the insulating film 38 using a photolithography technique and a dry etching technique.
[0081]
Then, AuZnNi / Au layers are respectively formed on the intrinsic InP cladding layer 32 provided with the intrinsic InP protrusion layer 33 and the p-type Inp clad layer 27 provided with the p-type InP ridge layer 29 by a method such as vapor deposition. It is deposited to a thickness of 0.05 μm or 1.2 μm.
Then, the AuZnNi / Au layer laminated on the entire surface is patterned by using the photolithography technique and the dry etching technique, and the AuZnNi / Au layer is patterned on the p-type Inp clad layer 27 and the p-type InP ridge layer 29 via the insulating film 38. The AuZnNi / Au layer is formed along the outer periphery of the intrinsic InP protrusion layer 33 while forming the AuZnNi / Au layer.
[0082]
Further, the AuGeNi / Au layer 36 is formed on the back surface of the n-type Inp substrate 21 to a thickness of 0.05 μm and 1.2 μm, respectively, by a method such as vapor deposition.
Here, by using the AuZnNi / Au layer 35 as a signal electrode and the AuGeNi / Au layer 36 as a ground electrode, a single strip line structure can be formed. Can be achieved.
[0083]
Next, as shown in FIG. 9D, an anti-reflection film 37 is formed on the light emitting surface by using electron beam evaporation or the like, and if necessary, a distributed feedback laser for the 1.3-micron band. An anti-reflection film is also formed on the end face of 40. Here, as the antireflection film 37, for example, SiO 2 ₂ Film and TiO ₂ A laminated structure including two layers of films can be used.
[0084]
In the above-described third embodiment, a method has been described in which the intrinsic InP protrusion layer 4 is bent by 90 degrees to form a bent waveguide, and the light incident end and the light output end are respectively provided on end faces perpendicular to each other. However, for example, the curved waveguide may be formed in a U-shape, and the light incident end and the light emitting end may be provided on the same end surface. Alternatively, the light emitting end may be provided on the facing surface.
[0085]
【The invention's effect】
As described above, according to the present invention, by controlling the light emission characteristics of the bent optical waveguide based on the applied voltage applied to the outer peripheral portion of the bent optical waveguide, while suppressing an increase in insertion loss, The p-type cladding layer under the modulation electrode can be removed, and high-speed modulation can be efficiently performed while facilitating the design of the optical modulator.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a schematic configuration of a waveguide type optical modulator according to a first embodiment of the present invention.
FIG. 2 is a plan view showing a schematic configuration of a waveguide optical modulator according to the first embodiment of the present invention.
FIG. 3 is a diagram showing a relationship between a cladding layer thickness t, a relative refractive index difference, and a critical curvature of the waveguide type optical modulator according to one embodiment of the present invention.
FIG. 4 is a plan view illustrating a schematic configuration of a waveguide optical modulator according to a second embodiment of the present invention.
FIG. 5 is a diagram showing a relationship between an applied voltage and a normalized optical output of a waveguide optical modulator according to a second embodiment of the present invention.
FIG. 6 is a cross-sectional view illustrating a manufacturing process of the waveguide-type optical modulator according to the second embodiment of the present invention.
FIG. 7 is a plan view illustrating a schematic configuration of a waveguide optical modulator according to a third embodiment of the present invention.
FIG. 8 is a cross-sectional view illustrating a manufacturing process of the waveguide-type optical modulator according to the third embodiment of the present invention.
FIG. 9 is a cross-sectional view illustrating a manufacturing process of the waveguide-type optical modulator according to the third embodiment of the present invention.
FIG. 10 is a cross-sectional view illustrating an example of a phase modulation waveguide structure of a conventional branching interference modulator.
FIG. 11 is a cross-sectional view showing another example of the phase modulation waveguide structure of the conventional branching interference modulator.
[Explanation of symbols]
1,21 n-type InP substrate
2, 24, 31 Intrinsic InGaAsP guide layer
3,32 Intrinsic InP cladding layer
4,33 Intrinsic InP protrusion layer
5, 15, 34, 35 AuZnNi / Au layer
6, 16a, 16b, 36 AuGeNi / Au layer
11 Semi-insulating InP substrate
12 Semi-insulating InGaAsP guide layer
13 Semi-insulating InP cladding layer
14 Semi-insulating InP protrusion layer
17a, 17b, 37 Anti-reflective coating
22 Intrinsic InGaAsP buffer layer
23 Intrinsic InGaAsP Active Layer
25 Intrinsic InP cladding layer
26 diffraction grating
27 p-type InP layer
28 Nitric oxide film
29 p-type InP ridge layer
38 Insulating film
40 1.3μm Distributed Feedback Semiconductor Laser

Claims (4)

A bent optical waveguide;
A waveguide-type optical modulator, comprising: a modulation electrode provided on an outer peripheral portion of the bent optical waveguide and controlling a light confinement effect of the outer peripheral portion by changing a refractive index of the outer peripheral portion. . The bent optical waveguide includes:
A light guide layer,
An intrinsic or semi-insulating semiconductor clad layer laminated on the light guide layer,
Comprising an intrinsic or semi-insulating semiconductor protrusion layer provided on the intrinsic or semi-insulating semiconductor cladding layer,
The waveguide type optical modulator according to claim 1, wherein the modulation electrode is provided along an outer periphery of the intrinsic or semi-insulating semiconductor protrusion layer. An intrinsic or semi-insulating semiconductor substrate;
An intrinsic or semi-insulating semiconductor optical guide layer laminated on the intrinsic or semi-insulating semiconductor substrate,
An intrinsic or semi-insulating semiconductor cladding layer laminated on the intrinsic or semi-insulating semiconductor optical guide layer,
An intrinsic or semi-insulating semiconductor protrusion layer provided on the intrinsic or semi-insulating semiconductor cladding layer,
Along with the periphery of the intrinsic or semi-insulating semiconductor protrusion layer, a signal electrode provided on the intrinsic or semi-insulating semiconductor cladding layer,
A waveguide type optical modulator, comprising: a grounding electrode provided on the intrinsic or semi-insulating semiconductor cladding layer so as to be disposed outside and inside the signal electrode. A semiconductor substrate;
A ridge waveguide type semiconductor laser formed in a partial region on the semiconductor substrate,
A laser electrode provided on the ridge waveguide type semiconductor laser,
A strip-loaded bending optical waveguide formed on the semiconductor substrate and bonded to the ridge waveguide semiconductor laser;
A waveguide electrode, comprising: a modulation electrode that is provided on an outer peripheral portion of the strip-loaded bent optical waveguide and that controls a light confinement effect of the outer peripheral portion by changing a refractive index of the outer peripheral portion. Light modulator.

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