JP3666729B2 - Semiconductor optical amplifier and method for manufacturing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor optical amplifier used for optical communication and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, the development of information communication has been remarkable, and it is possible to send and receive large-capacity data such as images by connecting to the Internet not only at companies but also at home.
For this reason, the amount of information to be transmitted is rapidly increasing, and there is a strong demand for an ultrahigh-speed, large-capacity transmission network that can sufficiently transmit and process this information.
Among them, optical transmission networks using optical transmission and processing have a wide THz-class bandwidth and ultra-high-speed transmission capability of several tens of Gbit / s, so they are currently used in communication network trunk systems and submarine cable systems. in use.
[0003]
The optical transmission network is mainly composed of a semiconductor laser as a light emitter, a photodiode as a light receiver, and an optical fiber as a transmission path.
Loss experienced by an optical signal during processing such as transmission through an optical fiber or switching of an optical path degrades a signal / noise (S / N) ratio and the like, and greatly affects transmission characteristics such as reception sensitivity.
Therefore, an optical amplifier capable of amplifying an optical signal is indispensable and important as a component of the optical transmission network.
[0004]
There are two types of optical amplifiers: optical fiber amplifiers using rare earth-doped optical fibers and semiconductor optical amplifiers (SOA) made of compound semiconductors. SOAs are very small, with element sizes of several millimeters or less. In addition, since a large number of semiconductor wafers can be manufactured at once, cost reduction and arraying are easy.
In addition, active research and development to significantly improve mounting tolerance by monolithically integrating spot size converters at both ends of the SOA to expand the spot size of SOA input / output light to 3-4 μm, which is equivalent to that of optical fibers. Has been done.
[0005]
This spot size converter has a structure in which the layer thickness or width of the waveguide is gradually reduced, thereby weakening confinement of light in the waveguide and gradually increasing the spot size.
Using an SOA with an integrated spot size converter, an optical waveguide circuit (PLC) in which a glass waveguide is fabricated on a silicon (Si) substrate, or directly on a Si substrate can be easily mounted, and is inexpensive. Realization of modules can be expected.
[0006]
By the way, if the amplification factor from the input light to the output light, that is, the gain is large in the SOA, the reception sensitivity can be improved or the number of optical amplifiers in the transmission path can be reduced. Therefore, the SOA is required to have a high gain. .
In order to increase the gain, it is conceivable to 1) increase the gain coefficient of the active layer and 2) increase the injection current.
However, in the conventional SOA, even if the active layer is improved as in 1) to increase the gain coefficient and the injection current is increased as in 2), the following oscillation problem occurs. It becomes difficult.
[0007]
The principle structure of the SOA is shown in FIG.
As shown in FIG. 6A, the light a input to the gain medium 01 from the left is output as amplified light c from the right due to the gain generated by the current injection b.
Ideally, if the injection current b is increased, a gain increase corresponding to the injection current b can be obtained from the gain medium 01.
However, actually, as shown in FIG. 6 (b), reflecting mirrors 02 (R ≠ 0) having a certain reflectance on both end faces of the gain medium 01 due to a difference in refractive index at the interface between the gain medium 01 and air. ) Is formed.
[0008]
For this reason, a resonator structure is formed by the gain medium 01 and the reflecting mirror 02. Since the oscillation light d is generated by the oscillation phenomenon when the gain becomes large, most of the gain is consumed by the oscillation phenomenon. It becomes an essential cause that cannot be enlarged.
Therefore, in the conventional SOA, in order to reduce the reflectance at the end face, as shown in FIG. 7A, an antireflection (AR) film composed of dielectric multilayer films such as SiO ₂ and TiO _{2 on} both end faces as shown in FIG. 03 is provided.
[0009]
However, in order to realize the reflectivity (R <0.1%) necessary to obtain a practical high gain only by this method, it is necessary to suppress the film thickness production error to several percent or less. There are difficulties in productivity and productivity.
As shown in FIG. 7A, the conventional SOA has a structure in which the reflected light e is easily coupled to the active layer 04 again because the waveguide is formed perpendicular to the cleavage end face.
Therefore, as a measure for further reducing the reflectivity, the reflected light e from the end face is not coupled to the waveguide by adopting a structure in which the waveguide is inclined with respect to the wall open end face as shown in FIG. 7B. A method to make it is proposed.
[0010]
[Problems to be solved by the invention]
FIGS. 8A and 8B show a conventional configuration in which a waveguide structure oblique to the end face is realized.
Here, an SOA in which a spot size converter is integrated is considered, and a region having an active layer 010 in which gain is generated by current injection is defined as an active region, and a region having a spot size conversion (SS) layer 011 that expands the spot size of guided light is defined. This is called the SS area.
FIG. 8A shows a structure in which the active and SS regions are straight and inclined with respect to the end face, and the entire active layer 010 is inclined with respect to the end face.
In FIG. 8B, the active layer 010 is composed of a straight waveguide 010a perpendicular to the cleavage plane and a curved waveguide 010b having a radius of curvature, and the SS layer 011 is a straight or curved waveguide oblique to the cleavage plane. It is a structure with
[0011]
In this structure, in order to connect the active layer 010 and the SS layer 011, a part of the active layer 010 is a curved waveguide (BEND-WG) 010 b having a radius of curvature R.
Here, the end face is formed by a method peculiar to a compound semiconductor called cleavage.
In a compound semiconductor such as InP, there is a weakly bonded portion on a specific crystal plane due to its crystal structure.
Cleavage is a process of applying a predetermined stress to a semiconductor to obtain a smooth surface in the atomic layer order along this weakly bonded crystal plane. The end face is always parallel or perpendicular to a predetermined crystal plane orientation.
By the way, in the SOA, a buried structure that narrows the lateral path of the injection current is widely used so that the current is efficiently injected into the active layer.
[0012]
For example, in a buried structure called pn-BH, when an n-InP substrate is used, both sides of the active layer are buried with a p-InP / n-InP / p-InP layer by crystal growth, and the pn junction is reversed at the time of current injection. No current is supplied to the buried region due to the bias.
For this reason, the current is injected only into the active layer, the efficiency is improved, and the operation current is reduced.
[0013]
As shown in FIG. 9A, the fabrication method is performed after processing the active layer into a mesa stripe shape parallel to a predetermined crystal plane orientation (here, the [011] direction of the n-InP substrate with the (100) plane as the surface). Then, p-InP and n-InP layers are successively formed on the side of the mesa stripe using the selective growth mask on the top.
Thereafter, the selective growth mask is removed and a p-InP layer is formed on the entire surface.
On the other hand, it is widely known that the growth mechanism of a GaInAsP-based crystal growth technique generally differs depending on the crystal plane orientation.
[0014]
For example, when a mesa stripe parallel to the [01-1] direction is formed as shown in FIG. 9B, the selectivity is inferior under the growth conditions optimized for the crystal plane orientation, so that the crystal is grown on the selection mask. It is easy to be done.
Therefore, when crystal growth is selectively performed only on both sides of the active layer as in a buried structure, it is necessary to use a growth process that takes into account the crystal plane orientation.
[0015]
As described above, when the waveguide is perpendicular to the end face, only one crystal plane orientation can be obtained, so that the selective growth conditions can be obtained relatively easily. ), When the whole or a part of the active layer 010 is inclined with respect to the end face, the mesa stripe always includes several crystal plane orientations, and the crystal growth conditions are complicated.
As a result, for example, a sufficient selective growth ratio cannot be obtained as shown in FIG. 9C, and crystals are grown on the selective growth mask, so that uniform current injection into the active layer cannot be performed.
Therefore, problems such as deterioration of element characteristics and deterioration of fabrication reproducibility occur.
[0016]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a semiconductor optical amplifying device having a first optical waveguide having an active region, wherein the optical waveguide direction of the first optical waveguide is the same as that of the semiconductor substrate. A second optical waveguide that is perpendicular or parallel to the crystal plane orientation and has a spot size conversion region is connected to the first optical waveguide, the second optical waveguide has a curved waveguide portion, and The end face of the semiconductor substrate is skewed , the radius of curvature of the curved waveguide portion is 125 to 300 μm, and the inclination angle between the second optical waveguide and the end face is larger than 5 °. .
The method of manufacturing a semiconductor optical amplifier device according to claim 2 of the present invention to achieve the above SL problem, after forming the first optical waveguide in claim 1, that is butt joint of said second optical waveguide Features.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
A conceptual diagram of the SOA in the present invention is shown in FIG.
The SOA is mainly composed of an active layer 10 and an SS layer 11. A gain is generated by injecting current into the active layer 10, and a spot size is enlarged by the SS layer 11 to be coupled to an optical fiber. In the figure, 3 is an AR layer.
The active layer 10 is formed so as to be parallel to a predetermined crystal plane orientation.
What is important here is that the active layer 10 is composed only of a straight waveguide parallel to the crystal plane orientation.
[0018]
For this reason, there is no problem in the above-mentioned crystal growth.
The SS layer 11 includes a straight waveguide 11a inclined by θ with respect to the crystal orientation, and a curved waveguide 11b having a radius of curvature R that smoothly connects the active layer 10 and the active layer 10.
Here, the length of the curved waveguide 11b is uniquely determined by the radius of curvature R and the inclination θ.
Further, the SS layer 11 has a taper shape in the layer thickness direction or the width direction in order to increase the spot size.
The tapered shape is determined by the forming method, and a parabolic shape or exponential shape is often selected in order to reduce the radiation loss.
[0019]
Although it is not a problem with a straight waveguide having a constant layer thickness and width, it is known that a radiation loss due to bending occurs in the curved waveguide 11b.
This is because the propagation mode of the straight waveguide 11a is the same in all regions, but the curved waveguide 11b receives a loss because the propagation mode in the curved portion is slightly different.
This radiation loss is known to increase when the waveguide radius is sharply bent with a small radius of curvature when the layer thickness is constant.
Further, when the waveguide thickness is reduced, light confinement in the waveguide is weakened, and radiation loss tends to increase even with the same radius of curvature.
[0020]
When the radius of curvature is increased in order to reduce radiation loss due to bending in a waveguide having a tapered shape like the SS layer 11 used in the SOA of the present invention, the length of the curved waveguide 11b increases accordingly. To do.
As a result, the thin portion of the layer is also included in the curved waveguide 11b region, so that the radiation loss due to the thinning of the waveguide increases.
Therefore, even if the radius of curvature is increased, the radiation loss is not necessarily reduced, and may increase on the contrary.
[0021]
The calculated value of the radiation loss with respect to the radius of curvature at the inclination angle θ with the end face of the SS layer 11 is shown in FIG.
Here, as an example, the case where the total length of the SS region is 200 μm and 300 μm is shown.
When the value of the inclination angle θ shown in the inset in FIG. 2 is small and the waveguide is close to a straight line, the radiation loss can be reduced by increasing the curvature radius R, but when the inclination angle θ is increased, the curvature radius is increased. The radiation loss can be reduced up to around R 200 μm, but it tends to increase on the contrary.
[0022]
This is an important problem that has not been discussed in the conventional configuration, and shows that there is an optimum radius of curvature R for each inclination angle θ in order to reduce the radiation loss. It is an indispensable condition.
Accordingly, when the curvature radius R is set to 100 to 500 μm at an inclination θ> 5 ° for obtaining a return loss of 30 dB or more necessary for practical use as the reflection attenuation amount at the end face, low loss characteristics can be obtained.
Furthermore, if it says, 125-300 micrometers is preferable and 150-200 micrometers is the most preferable.
[0023]
〔Example〕
The semiconductor optical amplifier according to the present invention will be described below with reference to examples.
A manufacturing procedure of a semiconductor optical amplifier according to an embodiment of the present invention is shown in FIGS.
First, as shown in FIG. 3A, a GaInAsP active layer 22 and an InP thin clad layer 23 are formed on an n-type InP substrate 21 by MOVPE.
[0024]
A SiO ₂ film 24 is formed thereon using a thermal CVD method, and a portion of the SiO ₂ film 24, thin clad layer 23, and a portion where a spot size conversion layer is formed by photolithography, CF ₄ / H ₂ -RIE, and wet etching. The active layer 22 is removed.
Next, as shown in FIG. 3B, a GaInAsP spot size conversion layer 25 is formed by selective MOVPE using the SiO ₂ film 24 as a mask.
At this time, by forming the mask into a predetermined shape, a layer thickness taper shape in which the thickness is the thickest at the boundary with the active layer 22 and the film thickness decreases as the distance from the active layer 22 increases is realized.
[0025]
As described above, after forming the first optical waveguide having the active region, the active layer and the spot size conversion layer are separately formed by using the fabrication method by butt-joining the second optical waveguide having the SS region. Therefore, the active layer can be designed to be optimized only for the optical amplification characteristics.
On the other hand, the spot size conversion layer is not affected at all by the design of the active layer, and can be designed considering only spot size conversion such as low propagation loss and high efficiency coupling with an optical fiber.
[0026]
Therefore, the design freedom for the active layer and the SS layer can be greatly increased, and it is easy to achieve both high optical amplification characteristics, low propagation loss, and high coupling efficiency.
Subsequently, the SiO ₂ film 24 is removed by HF, and as shown in FIG. 3C, an InP cladding layer 26 is formed on the entire surface by the MOVPE method.
Thereafter, the SiO ₂ film 27 is formed, and the SiO ₂ film 27 other than the upper part of the portion where the waveguide is formed is removed by photolithography and CF ₄ / H ₂ -RIE.
[0027]
Then, as shown in FIG. 3D, the InP cladding layer 26, the InP thin cladding layer 23, the GaInAsP active layer 22, the GaInAsP spot size conversion layer are formed by CH ₄ / H ₂ -RIE using the SiO ₂ film 27 as a mask. 25 and a part of the n-InP substrate 21 are removed.
Further, as shown in FIG. 3E, the p-InP layer 28 and the n-InP layer 29 are formed on both sides of the waveguide portion by selective MOVPE again using the SiO ₂ film 27 as a mask.
[0028]
Thereafter, the SiO ₂ film 27 is removed by HF, and an InP overcladding layer 30 and a GaInAsP cap layer 31 are formed on the entire surface by MOVPE.
Then, as shown in FIG. 4A, the cap layer 31 other than the active region is removed by photolithography and wet etching.
Subsequently, the SiO ₂ film 32 is formed on the entire surface, and the SiO ₂ film 32 above the active region is removed by photolithography and CF ₄ / H ₂ -RIE.
[0029]
Thereafter, an AuZnNi / Au electrode 33 is formed on the active region and an AuGeNi / Au electrode 34 is formed on the n-InP substrate by EB deposition.
Finally, the element end face is formed by cleaving, and as shown in FIG. 4C, a TiO ₂ / SiO ₂ antireflection film 35 is formed on both end faces by EB sputtering.
The dimensions of the device are as follows: active layer length 600 μm, active layer thickness 0.4 μm, spot size conversion layer length 300 μm, spot size conversion layer thickness 0.4 μm (connection to active layer) to 0 .2 μm (element end face).
[0030]
Further, the radius of curvature R of the curved waveguide portion of the spot size conversion layer was 200 μm, and the other portions were linear waveguides having an inclination θ = 7 ° with respect to the crystal plane orientation.
In the SOA manufactured by such a process, a chip gain of 30 dB, which is larger than the chip gain of 25 dB at the injection current of 200 mA of the SOA constituted only by the waveguide perpendicular to the conventional end face, was obtained.
In addition, the radiation loss was as good as 0.5 dB or less.
Here, an S-shaped configuration in which the SS layers at both ends are parallel to each other is shown. However, as shown in FIG. 5, the SS layer 11 at one end is folded around a straight line passing through the active layer 10. Of course, a letter-shaped configuration is also acceptable.
[0031]
In this embodiment, the active layer and the SS layer are formed of different materials. However, the active layer and the SS layer are formed by selective crystal growth using a selection mask that can change the composition wavelength of the material in different regions. Of course, the layers may be formed together.
Further, although the active layer and the SS layer are each a single layer, it is of course possible to adopt a loading structure in which an optical waveguide layer common to the active region and the SS region and an active layer and an SS layer formed thereon are formed.
Moreover, although the taper shape of the SS layer is formed in the layer thickness direction, the taper shape in the width direction may naturally be formed by using photolithography and etching techniques.
[0032]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a high gain while keeping radiation loss low, and thereby an effect that a high gain semiconductor optical amplifier can be obtained with high reproducibility and productivity. Is obtained.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of SOA in the present invention.
FIG. 2 is a graph showing a calculated value of radiation loss with respect to a radius of curvature at an inclination angle with an end face of an SS layer.
FIG. 3 is a process diagram showing a manufacturing procedure of a semiconductor optical amplifying device according to an embodiment of the present invention.
FIG. 4 is a process diagram showing a manufacturing procedure of a semiconductor optical amplifier according to an embodiment of the present invention.
FIG. 5 is a perspective view of a semiconductor optical amplifier according to another embodiment of the present invention.
FIG. 6 is an explanatory diagram showing a principle structure of an SOA.
FIG. 7 is an explanatory view showing the structure of an SOA provided with an antireflection film.
FIG. 8 is an explanatory diagram showing a structure of a conventional SOA that realizes a waveguide structure that is inclined with respect to an end face;
FIG. 9 is a process diagram showing a conventional SOA manufacturing method.
[Explanation of symbols]
3 AR layer 10 Active layer 11 SS layer 11a Linear waveguide 11b Curved waveguide 21 n-type InP substrate 22 GaInAsP active layer 23 InP thin clad layer 24 SiO ₂ film 25 GaInAsP spot size conversion layer 26 InP clad layer 27 SiO ₂ film 28 p-InP layer 29 n-InP layer 30 InP overclad layer 31 GaInAsP cap layer 32 SiO ₂ film 33 AuZnNi / Au electrode 34 AuGeNi / Au electrode

Claims (2)

In the semiconductor optical amplifier having the first optical waveguide having the active region, the optical waveguide direction of the first optical waveguide is perpendicular or parallel to the crystal plane orientation of the semiconductor substrate, and the second optical waveguide has a spot size conversion region. An optical waveguide is connected to the first optical waveguide, the second optical waveguide has a curved waveguide portion, and is skewed with an end surface of the semiconductor substrate, and a curvature radius of the curved waveguide portion is 125 to 300 μm, and an inclination angle between the second optical waveguide and the end face is larger than 5 ° . Wherein after forming the first optical waveguide, the second manufacturing method of the semiconductor optical amplifier device according to claim 1, characterized in that butt the optical waveguide.

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