JP6714894B2 - Array type optical waveguide and semiconductor optical integrated device - Google Patents

Description

The present invention relates to a semiconductor optical integrated device used in the field of optical communication and the like, and particularly to an array type optical waveguide used in the semiconductor optical integrated device.

Along with the dramatic increase in communication demand, a wavelength division multiplexing system (WDM) that enables a large capacity transmission with a single optical fiber by multiplexing a plurality of signal lights with different wavelengths has been developed. Has been realized. In a wavelength division multiplexing communication system, a variable wavelength light source that can cover the entire wavelength band used is required. As such a wavelength tunable light source, there is known a wavelength tunable laser capable of covering a wide wavelength band by forming an array of distributed feedback semiconductor lasers (hereinafter referred to as DFB lasers) and combining the selection of the array and the wavelength tuning by temperature. (Also called array type wavelength selective light source).

Patent Documents 1 and 2 disclose a wavelength tunable laser in which a plurality of DFB lasers, an S-shaped curve type optical waveguide, an MMI type (Multi Mode Interference type) optical multiplexer, and an optical amplifier are monolithically integrated on an InP substrate. It is disclosed.
Patent Document 3 discloses, as an S-curve type optical waveguide, an embedded optical waveguide in which both sides of a ridge type optical waveguide section are embedded with semiconductor layers.

JP, 2014-41889, A (abstract, Drawing 1) JP, 2009-109704, A (paragraph 0021, Drawing 1). Japanese Unexamined Patent Publication No. 2009-71067 (Summary, FIG. 1)

As the wavelength tunable laser is miniaturized, it is necessary to reduce the distance between the optical waveguides. When the embedded optical waveguide is used, there is a problem that a rabbit ear- shaped abnormal abnormal growth occurs in the S-shaped curved portion and it is difficult to embed if the interval between the S-shaped curved portions is narrow.
The present invention has been made to solve the above-mentioned problems, and in an embedded optical waveguide having an S-shaped curved portion, it is possible to perform embedded growth without causing abnormal abnormal growth like a rabbit ear. An object is to provide an optical waveguide.

An array type optical waveguide of the present invention is formed on a semiconductor substrate having a (100) plane as a main surface, and includes a plurality of optical waveguides in which both sides of a ridge portion are embedded with an embedding layer. A semiconductor crystal orientation <0-11> direction or a <011> direction as a longitudinal direction and the other one as a lateral direction, and the linear portion is connected to the linear portion at one end in the longitudinal direction, And a curved line portion which starts the inclination with the inclination of the tangent line of the bending start point being θ, the straight line portion has a space in the lateral direction between adjacent straight line portions as Tw, and the bending start points are adjacent to each other. It is formed by sequentially shifting the length L in the longitudinal direction with respect to the bending start point, and Tw and θ are 3 μm≦Tw≦3.5 μm and 2°≦θ≦5°, respectively, and the bending start point is The distance (Tw+L·tan θ) between the curved portion adjacent to the bending start point in the lateral direction satisfies 4 μm≦Tw+L·tan θ≦7.9 μm.

In the present invention, since the distance between the bending start point and the curved portion adjacent to the bending start point is large, it is possible to carry out the buried growth without causing the abnormal abnormal growth in the form of a rabbit ear. ..

FIG. 3 is a diagram schematically showing the structure of the wavelength tunable laser according to the first embodiment. FIG. 3 is a diagram showing a connection region 7 between the optical waveguide region 3 and the optical multiplexer 5 of the first embodiment. It is a figure which shows the connection area|region 7 of the optical waveguide area|region 3 and the optical multiplexer 5 in a comparative example. FIG. 6 is a diagram showing a method of manufacturing the optical waveguide 4 of the first embodiment. In this invention, it is a figure which shows the range of L which satisfy|fills Formula (5). In this invention, it is a figure which shows the range of L which satisfy|fills Formula (1)=5 micrometers. In this invention, it is a figure which shows the range of L which satisfy|fills Formula (1)=8 micrometers.

Embodiments of the present invention will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repeated description may be omitted.

Embodiment 1
FIG. 1 is a diagram schematically showing a wavelength tunable laser according to a first embodiment of the present invention. Reference numeral 1 is a laser region including a plurality of DFB lasers 2 and 3 is a plurality of S-curve optical waveguides 4. The optical waveguide region 5 is an MMI (Multi Mode Interference type) optical multiplexer that multiplexes a plurality of laser beams, and 6 is an optical amplifier (SOA: Semiconductor Optical Amplifier) that amplifies the laser beams.
In this tunable laser, one DFB laser 2 having a required oscillation wavelength is selected and driven from the laser region 1, and its output light is guided to an optical waveguide 4 and an optical multiplexer 5 connected to the DFB laser 2. The wave enters the optical amplifier 6, is amplified, and is emitted.

FIG. 2A is an enlarged top view of the connection region 7 between the optical waveguide region 3 and the optical multiplexer 5 in FIG. 1, showing the lower half of the connection region 7, FIG. It is a sectional view of the -A' part. Reference numeral 12 denotes a ridge portion that will be the optical waveguide 4, 13 is a buried portion that fills both sides of the ridge portion, and the ridge portion 12 is laminated on the n-InP substrate 14 in that order. The waveguide layer 16, the first p-InP clad layer 17, and the second p-InP clad layer 18 are included. The buried portion 13 is composed of a p-InP buried layer 19, an n-InP buried layer 20, and a p-InP buried layer 21.

The n-InP substrate 14 has the (100) plane as the main surface. As shown in FIG. 2A, the optical waveguide 4 has a linear portion 8 in a connection region 7 with the optical multiplexer 5. The longitudinal direction of the linear portion 8 of the optical waveguide 4 is parallel to the resonator direction of the DFB laser 2 and the traveling direction of light in the optical multiplexer 5, and when the ridge portion 12 is a forward mesa, the semiconductor crystal orientation < It is formed in the 0-11> direction, and when the ridge portion 4 has an inverted mesa, it is formed in the <011> direction of the semiconductor crystal orientation.
Note that the above crystal orientation represents a typical orientation and includes equivalent orientations.

Looking at the optical waveguide region 3 side from the optical multiplexer 5 side, it can be said that the connection region 7 is a region where the array-type optical waveguide 4 moves from a straight portion 8 having a small mutual spacing to a curved portion having a wide mutual spacing. it can. The shape of the optical waveguide 4 in such a region will be described.
As shown in FIG. 2A, the distance between one optical waveguide 4 and the linear portion 8 of the optical waveguide 4 adjacent to it is Tw. When the point at which the optical waveguide 4 shifts from the straight line portion 8 to the curved portion is referred to as a bending start point 9, the curved portion is inclined with the inclination in the tangential direction being θ at the bending start point 9.
One bending start point 9 is sequentially formed in the longitudinal direction of the straight line portion 8 by a shift amount L with respect to the adjacent bending start points 9.
The distance between one bending start point 9 and an adjacent curved line portion: Tx (short side direction of straight line portion) is
Tx=Tw+L·tan θ (1)
Therefore, in the first embodiment, the optical waveguides are not adjacent to each other at a narrow interval,
4 μm ≦ Tw+L·tan θ (2)
And

The curved portion of the optical waveguide 4 in a region apart from the connection region 7 may be a straight line that extends the tangent line at the bending start point 9 as it is, and preferably has a certain radius of curvature and is adjacent to the conductive line as shown in FIG. The distance to the waveguide may increase.
In order to suppress the loss of light in the curved portion, it is necessary to set the radius of curvature to 100 μm or more. Therefore, the inclination θ of the bending start point 9 is
θ≦5°
And need to.
Further, as the inclination θ of the bending start point 9 becomes smaller, the length of the optical waveguide region 3 becomes longer. Therefore, the lower limit of the inclination θ of the bending start point 9 needs to be about 2°, and in the present embodiment,
2°≦θ≦5° (3)
Is.

In order to achieve the function of the optical multiplexer 5, when the linear portion 8 is connected to the optical multiplexer 5,
3 μm≦Tw≦3.5 μm (4)
And need to.

FIG. 5 shows the lower limit of the left side of Expression (2), that is,
Tw+L·tan θ=4 μm (5)
It is a figure which shows the range of L which satisfy|fills.
L=(4-Tw)/tan θ
Therefore, when the interval Tw of the straight line portion is 3 μm which is the narrowest and the inclination θ of the bending start point is 2° which is the smallest, it is necessary to make L the longest, and the value in that case is 28.6 μm. I understand. Further, when Tw is wide and the inclination θ is large (θ=5°, Tw=3.5 μm), L is the shortest, and the value in that case is 5.7 μm.
Applying the upper and lower limits of equations (3) and (4) to equation (5) gives
When θ=5° and Tw=3.5 μm, L=5.7 μm
When θ=2° and Tw=3.5 μm, L=14.3 μm
When θ=5° and Tw=3.0 μm, L=11.4 μm
When θ=2° and Tw=3.0 μm, L=28.6 μm
Becomes

When the right side of Expression (5), that is, the distance Tx between one bending start point 9 and an adjacent curved portion is increased from the lower limit of 4 μm, L increases. Figure 6
Tw+L·tan θ=5 μm
It is a figure which shows the range of L which satisfy|fills.
Applying the upper and lower limits of equations (3) and (4) to the equation,
When θ=5° and Tw=3.5 μm, L=17.2 μm
When θ=2° and Tw=3.5 μm, L=43.0 μm
When θ=5° and Tw=3.0 μm, L=22.9 μm
When θ=2° and Tw=3.0 μm, L=57.3 μm
Becomes
It should be noted that the lower limit of L is 5.7, which is a value when θ=5° at Tx=4 μm and Tw=3.5 μm within a range satisfying the formulas (3) and (4). Under the condition (2),
L≧5.7 μm (6)
Holds.

As described above, in the array-type optical waveguide 4 of the first embodiment, the adjacent bending start points 9 are formed by shifting the length L in the longitudinal direction of the linear portion 8 in order, and are shown by the formula (1). The distance Tx between one bending start point 9 and the adjacent curved portion is 4 μm or more.

Here, a method of manufacturing the optical waveguide 4 of the present embodiment will be described with reference to FIGS.
First, as shown in FIG. 4A, a wafer in which an n-InP clad layer 15, an optical waveguide layer 16, and a first p-InP clad layer 17 are sequentially grown on the entire surface of the n-InP substrate 14. To prepare. Next, as shown in FIG. 4B, the insulating film 22 is formed and etching is performed using the insulating film 22 as a mask to form the ridge portion 12. Subsequently, as shown in FIG. 4C, a p-InP burying layer 19, an n-InP burying layer 20, and a p-InP burying layer 21 are sequentially grown on both sides of the ridge portion 12 to form the burying portion. 13 is formed. Then, the insulating film 22 is removed, and the second p-InP cladding layer 18 is grown on the entire surface to complete the optical waveguide 4.

Next, the reason for shifting the straight line portion in the longitudinal direction, which is one of the features of this embodiment, will be described.
3A is a top view of the connection region 7 of the optical waveguide 4 in the comparative example, and FIG. 3B is a cross-sectional view of the BB′ portion. As can be seen by comparing FIG. 2A and FIG. 3A, in the comparative example, the lengths of the linear portions in the longitudinal direction are all the same, that is, the shift amount L is 0, and the distance between the linear portions 8 is Tw and the inclination θ of the curved portion are the same as in the first embodiment. As a result, the distance between one bending start point 9 and the adjacent curved portion (short side direction of the straight line portion) is
Tw
Becomes

In the curved portion of the optical waveguide, due to the influence of the crystal plane orientation, abnormal growth like a rabbit ear occurs when the embedded portion 13 is embedded and grown. In particular, when the distance between the curved waveguides is smaller than 4 μm as in the comparative example, the influence of abnormal growth is large as shown in FIG. As a result, the rabbit ear- like irregularities are inherited to the upper surface of the second p-InP cladding layer 18, and irregularities are formed near the upper portion of the ridge portion 12. Due to the influence, the light propagating through the optical waveguide 4 is scattered and a light loss occurs.
On the other hand, in the present embodiment, when the waveguides in the curved portion have a large distance, as shown in FIGS. 4C and 2B, when the embedded portion 13 is embedded and grown, a rabbit ear shape is formed. The gap between the ridges 12 can be flatly embedded without causing abnormal growth. As a result, it is possible to obtain an optical waveguide with little light propagation loss.

Embodiment 2
In the first embodiment, the linear portion 8 of the optical waveguide 4 is shifted in the longitudinal direction by the length L so that the distance Tx represented by the formula (1) is 4 μm or more, thereby improving abnormal growth during the embedded growth. did. The lower limit of Tx is 4 μm, and in terms of improving abnormal growth, the larger Tx is, the better, and there is no upper limit.
However, in order to shift the straight line portion 8 in the longitudinal direction, it is necessary to lengthen the waveguide region 3. The number of array-type optical waveguides is, for example, 20. Since the optical waveguides are arranged in a left-right manner as shown in FIG. 1, if the value of the shift amount L of one waveguide is 30 μm, the whole (10 on one side) ,
30 μm x 20 lines/2 = 300 μm
Therefore, it is necessary to lengthen the waveguide region by 300 μm as compared with the comparative example in which L=0.
In the second embodiment, from the viewpoint of miniaturization of the optical waveguide, the allowable value of the increase amount of the length of the waveguide region is up to 500 μm, and the number of optical waveguides is 20 at maximum (10 on each side). The upper limit of the allowable value of the waveguide shift amount L is within 50 μm. That is,
L ≤ 50 μm
And combined with equation (6),
5.7 μm ≦L ≦50 μm (7)
Is.
In order to satisfy the equations (3), (4), and (7), the upper limit of Tx is limited as follows.

As condition 1, Tx satisfying L≦50 μm is obtained for all Tw and θ in the ranges of the expressions (3) and (4). For (Tw+L·tan θ), when the interval Tw of the straight line portion is 3 μm which is the narrowest and the inclination θ of the bending start point is 2° which is the smallest, it is necessary to make L the longest. If L=50 μm,
Tw+L·tan θ=3+50·tan 2°=4.74
Becomes
Therefore, for all Tw and θ in the range of equations (3) and (4),
4 μm≦Tw+L·tan θ≦4.7 μm and L≦50 μm
There exists L that satisfies the above.
For example, FIG. 6 shows an example of Tx=5 μm, and it can be seen that L≦50 μm for almost all Tw and θ.

As condition 2, Tx satisfying L≦50 μm is obtained for a part of Tw and θ in the ranges of the formulas (3) and (4). (Tw+L·tan θ) is 3.5 μm, where the distance Tw of the straight line portion is widest, and 5° where the inclination θ of the bending start point is the largest, L may be the shortest, so the value of L is the upper limit. 50 μm is
Tw+L·tan θ=3.5+50·tan 5°=7.9
Is the case.
Tw+L·tan θ=7.9 μm, and L ≦50 μm
Is within the range of equations (3) and (4),
Tw=3.5 μm and θ=5°
Only in the case of, for other Tw and θ, 50 μm<L.
For example, FIG. 7 shows an example of Tx=8 μm, and it can be seen that 50 μm<L for all Tw and θ.
Therefore, for some Tw and θ in the range of the equations (3) and (4),
4.74 μm<Tw+L·tan θ≦7.9 μm,
And L ≦50 μm
There exists L that satisfies the above.
Further, with respect to Tw and θ in the ranges of Expression (3) and Expression (4),
7.9 μm<Tw+L·tan θ, and L ≦50 μm
There is no L that satisfies.

From the above, in the second embodiment, the upper limit of Tx is set,
4 μm≦Tw+L·tan θ≦7.9 μm,
And 3 μm≦Tw≦3.5 μm,
And 2° ≤ θ ≤ 5°
And at this time,
5.7 μm≦L ≦50 μm
Holds.
Under this condition, the amount of increase in the length of the optical waveguide region can be suppressed within the allowable value of 500 μm, and a small-sized optical element can be obtained.

Embodiment 3
In the above embodiment, an example of a wavelength tunable laser provided with a semiconductor laser, an array type optical waveguide, an optical multiplexer, and an optical amplifier is shown. However, the invention of the present application is the optical waveguide shown in FIG. The effect is obtained if there is a configuration of the region 3 alone, and it does not depend on the individual optical elements connected before and after it.
For example, an optical integrated device in which an optical modulator is added to the configuration of FIG. 1 may be used, and an optical integrated device using an optical modulator instead of the optical amplifier of FIG. 1 or an optical integrated device without the optical amplifier may be used. You can also
In addition, in FIG. 2A, an example in which all θ and L are the same is shown, but the smallest one of the distances between one bending start point and the adjacent curved portion is expressed by the formula (2). ) May be satisfied. At the other bending start points, the distance Tx becomes 4 μm or more, and abnormal growth does not occur.

1 laser area 2 DFB laser 3 optical waveguide area 4 optical waveguide 5 optical multiplexer 6 optical amplifier
7 Connection area 8 Straight part 9 Bending start point
12 Ridge part 13 Embedded part

Claims (5)

Formed on a semiconductor substrate having a (100) plane as a main surface,
Both sides of the ridge portion are equipped with a plurality of optical waveguides embedded with an embedding layer,
The plurality of optical waveguides have a straight line portion having a semiconductor crystal orientation <0-11> direction or a <011> direction as a longitudinal direction and the other one as a lateral direction,
A curve portion that is connected to the straight line portion at one end in the longitudinal direction and starts the inclination with the inclination of the tangent line of the bending start point being θ.
The straight line portion has a space between adjacent straight line portions in the short-side direction as Tw,
The bending start point has a length L in the longitudinal direction with respect to adjacent bending start points.
Formed staggered,
The Tw and the θ are respectively
3 μm≦Tw≦3.5 μm,
2° ≤ θ ≤ 5°,
And
The distance (Tw+L·tan θ) between the bending start point and the curved portion adjacent to the bending start point is (Tw+L·tan θ),
4 μm≦Tw+L·tan θ≦7.9 μm,
An array type optical waveguide characterized by satisfying: The array type optical waveguide according to claim 1, wherein L is 5.7 μm≦L 2 ≦50 μm. The array type optical waveguide according to claim 1, wherein the semiconductor substrate is made of InP. The arrayed optical waveguide according to claim 1, wherein the buried layer is InP. A plurality of semiconductor lasers arranged in an array,
The array-type optical waveguide according to claim 1, which is connected to the plurality of semiconductor lasers,
An optical multiplexer in which the linear portion of the array type optical waveguide is connected,
Equipped with
A semiconductor optical integrated device comprising the semiconductor laser, the arrayed optical waveguide, and the optical multiplexer formed on the semiconductor substrate.

Images