JP3484053B2 - Semiconductor laser type optical amplifier - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser type optical amplification device having a waveguide.

[0002]

2. Description of the Related Art Considering the construction of an optical transmission processing system using light such as optical communication, optical switching, and optical information processing, optical loss in an optical fiber or an optical switch becomes a serious problem. Then, it becomes indispensable to compensate the attenuated optical signal by the optical amplification element. In particular, a semiconductor laser type optical amplification element is very promising because it has a small size and high efficiency, and can be integrated with a semiconductor optical device such as an optical switch.

However, the conventional semiconductor laser type optical amplifying device has a problem that the gain characteristic greatly varies depending on the polarization state of the input signal light. This is generally
This is because the width of the active layer is several μm, whereas the thickness is on the order of sub-μm, and the active layer is not isotropic, so that the gain characteristics of the active layer differ depending on the polarization state.
Therefore, the cross-sectional shape of the active layer is approximated to a square shape and isotropic, for example, a semiconductor laser type in which the gain characteristic difference between the TE polarized light input light and the TM polarized light input light is 1 dB or less. It has been reported that the optical amplifying device of the above was realized (reference: S. Kitamura, K. Komatsu and M. Kitam
ura, 'Very low power consumption semiconductor opti
cal optical amplfier arry ', IEEE Photonics Tecnolog
y Letters, 1995, 7, (2), pp.147-148).

An outline of this semiconductor laser type optical amplifying device will be described. As shown in the sectional view of FIG.
An active layer 403 having a substantially square cross section is arranged on a lower clad layer 402 formed by processing the surface of a substrate 401 made of n-type InP into a ridge shape. Further, a p-type buried layer 404 made of p-type InP and an n-type buried layer 405 made of n-type InP are formed on the side surfaces of the lower clad layer 402 and the active layer 403 so as to fill them, and a pn buried structure is formed. Are configured.

An upper clad layer 406 made of p-type InP is formed on them. Then, an n-side electrode 407 is formed on the back surface of the substrate 401, and a p-side electrode 408 is formed on the upper clad layer 406.
Further, as shown in FIG. 4B which is a cross section perpendicular to the waveguide direction, the antireflection films 410a and 411a are provided on the input end face 410 of the input light and the output end face 411 of the output light, respectively.
Are formed. The active layer 403 has a width of 0.4
The thickness of 0.4 μm makes it possible to reduce the polarization dependence. The semiconductor laser type optical amplification element has a total length of 600 μm.

[0006]

However, it is difficult to actually form the active layer 403 so that its sectional shape is square. The change in the difference in gain characteristics between the TE polarized light input light and the TM polarized light input light with respect to the width of the active layer when the thickness of the active layer is 0.4 μm changes as shown by the black dots in FIG. Therefore, if the difference in the gain characteristics is to be set within the range of 0.5 dB shown by the satin finish, the width of the active layer must be set within the range of about 0.35 to 0.5 μm. That is, an etching technique having vertical or near-vertical anisotropy such that the upper end of the active layer and the load are within this range is required. The stripe-shaped ridge structure in the active layer is formed by using the well-known photolithography technique and etching technique, but in this fine processing, it is very difficult to keep the processing dimension within an error of 0.15 μm. Further, it is not easy to form the stripe width to 0.4 μm.

The present invention has been made in order to solve the above problems, and provides a semiconductor laser type optical amplifying element in which the gain characteristic is hard to change even if the polarization state of the input signal light changes. The purpose is to enable easy formation.

[0008]

The semiconductor laser type optical amplifying element of the first invention has a sectional shape perpendicular to the waveguide direction in which the width perpendicular to it is larger than the thickness in the current injection direction.
An active layer that amplifies an optical signal so that the gain for E polarized light input light is higher than the gain for TM polarized input light;
A tapered waveguide, which is an optical waveguide formed in contact with the active layer in the waveguide direction so as to have a thin film thickness from the active layer contact end to the other end serving as an input end of an optical signal; The taper waveguide is provided with a clad layer formed so as to sandwich the top and bottom thereof, and a positive electrode and a negative electrode for injecting a current into the active layer. The width of the active layer is 1.25 to 1.75 of the thickness, and the width is smaller than the dimension in which light confinement in the direction is weakened.
Formed twice. In the second invention, the thickness of the active layer
Is formed to a thickness of 0.4 μm, and the width of the active layer is 0.5 to
It was formed to 0.7 μm. In the third invention, the active layer
Thickness is 0.4 μm, and the width of the active layer is 0.
It was formed to 6 μm. With the configuration as described above, the TM polarized wave input light is guided more at the input / output end of the optical signal in the tapered waveguide, and the gain for the TE polarized wave input light becomes larger in the active layer.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. First Embodiment FIG. 1 is a configuration diagram showing a configuration of a semiconductor laser type optical amplification element according to a first embodiment of the present invention. Here, for example, a semiconductor laser type optical amplification element for the 1.5 μm band will be described as an example. As shown in FIG. 1, the semiconductor laser type optical amplifying element according to the first embodiment is composed of n
On the surface of the substrate 101 made of InP
A ridge-shaped lower clad layer 102 is formed by processing the surface.
Are formed. Then, the lower clad layer 102
InGaAsP with a bandgap wavelength of 1.55 μm on top
An active layer 103 made of is formed. This active layer 1
03 has a thickness of 0.4 μm, a width of 0.6 μm, and a length of 600 μm in the waveguide direction.

Further, as shown in FIGS. 1B and 1C, on the cladding layer 102, both ends of the active layer 103 in the waveguide direction are connected to the ends of the active layer 103, and the tapered waveguide 1 is formed.
03a is formed. This tapered waveguide 103a
Is, for example, InGa having a band gap wavelength of 1.3 μm.
It is composed of AsP. That is, the tapered waveguide 1
No. 03a has a composition that causes little loss with respect to the optical signal to be amplified. Then, this tapered waveguide 103a
Is farther from the junction with the active layer 103,
It is formed so as to have a thin film thickness. The light input end and the light output end of the tapered waveguide 103a are formed to have a film thickness of 0.2 μm.

The taper waveguide 103a is formed by the active layer 10
3 may be formed after formation, or as shown below, it may be formed simultaneously with the active layer 103. First, a selective growth film made of silicon oxide having stripe-shaped openings having a width of 0.6 μm is formed on a substrate in a region to be an active layer and a waveguide. At this time, the lateral width of the selective growth mask is first set to 5 on one side in the active layer forming portion.
0 μm and continue to a length of 600 μm. Also,
In the tapered waveguide forming portion, the lateral width of the selective growth mask is formed so that one side is gradually narrowed from 50 μm,
Finally, the width is set to 10 μm.

Then, using the selective growth mask, InGaAsP having a bandgap wavelength of 1.55 μm is crystal-grown so as to have a film thickness of 0.4 μm in the active layer forming portion. As a result, in the tapered waveguide formation portion, the film thickness of the selectively grown crystal layer becomes thinner as the distance from the active layer formation portion increases. The composition is also composed of InGaAsP having a bandgap wavelength of 1.3 μm in the portion where the film thickness is as thin as 0.2 μm.

As described above, by using the selective growth mask, the active layer 103 and the tapered waveguide 103a can be formed at the same time. However, when the active layer and the tapered waveguide are simultaneously crystal-grown by using the selective growth mask as described above, the crystal also grows on the substrate surface outside the selective growth mask, so those portions must be removed. . Of course, the selective growth mask also
It is not necessary as an element.

Further, the lower cladding layer 102 and the active layer 10
3, and the side surface of the tapered waveguide 103a in the waveguide direction is embedded in the p-type I on the substrate 101 so as to be embedded therein.
A buried layer 104 made of nP is formed, and on the buried layer 104,
A current block layer 105 made of n-type InP for a current block is formed to a thickness of 0.5 μm. And
Active layer 103, tapered waveguide 103a, buried layer 10
4, and p-type In on the current blocking layer 105.
The upper clad layer 106 made of P is formed to a film thickness of 2 μm.

Ni is deposited on the upper cladding layer 106.
A 1 μm-thick p-side electrode 107 made of / Zn / Au / Ti / Au is formed, and AuGeN is formed on the back surface of the substrate 101.
An n-side electrode 108 made of i / Au and having a thickness of 1 μm is formed. Then, an antireflection film 109 is formed so as to be connected to the end portion where the film thickness of the tapered waveguide 103a is thin. This antireflection film 109 is made of TiO ₂ /
It is composed of SiO ₂ .

As described above, according to the first embodiment, first, the active layer 103 is formed in a rectangular shape whose cross section perpendicular to the waveguide direction is horizontally long. Then, the tapered waveguide 103a is arranged at the end of the active layer 103 in the waveguide direction. Since it is configured in this way, the semiconductor laser type optical amplification element according to the first embodiment has the following features. First, the active layer 103 has a thickness of 0.4 μm.
Since the width is formed to be 0.6 μm wide with respect to m, the gain for the TE polarized light input is smaller than that for the TM polarized input light, as indicated by the black circle on the horizontal axis 0.6 in FIG. It becomes 0.7 dB higher.

On the other hand, in the tapered waveguide 103a, the film thickness at the input end of the input light is 0.2 μm, for example. Generally, when the cross-sectional dimension of the waveguide becomes smaller than 0.3 μm,
Light confinement in that direction becomes weak. Therefore, in the portion where the film thickness of the tapered waveguide 103a is thin, the light confinement in the vertical direction becomes weak, and the light leaks out in the vertical direction and becomes vertically long.
Therefore, for example, an optical fiber is arranged at the optical input / output end,
When coupling the input light from the optical fiber and the output light to the optical fiber, as shown by the white circles in FIG. 2, the TM polarization input light has a higher coupling efficiency. In particular, this Embodiment 1
Since the width of the tapered waveguide 103a is set to 0.6 μm, the TM polarization input light has a coupling efficiency of the input / output light of about 0.7 dB.

That is, at the input / output end of the input light, T
More and more M polarization input light is input and output, and the active layer 103
In, the TE polarized light input light is gained more. Therefore, according to the semiconductor laser type optical amplification element of the first embodiment, as shown by the black square in FIG.
When the width of the active layer is 0.6 μm, the gain difference (polarization dependence) of the entire device becomes almost 0, and the width can be made relatively wide. That is, the active layer can be manufactured relatively easily. In addition, the polarization dependence of the entire device is 0.5d.
The width of the active layer within B is 0.3 to 0.7 μm, and the allowable amount is widened. In this range, the lateral width of the tapered waveguide 103a may be smaller than the lateral width of the active layer 103.

Second Embodiment Hereinafter, a second embodiment of the present invention will be described.
In the second embodiment, as shown in FIG. 3, the input end and the output end of the optical signal are configured the same. That is, as shown in FIG. 3, first, the lower clad layer 10 is formed.
2, the tapered waveguide 10 is provided at one end of the active layer 103.
3a is arranged, and the antireflection film 109 is arranged at the tip where the film thickness of the tapered waveguide 103a is thin. Then, on the other end of the active layer 103, a high reflection film 109a made of TiO ₂ / SiO ₂ is arranged. That is, the second embodiment
Then, on the antireflection film 109 side of the tapered waveguide 103a,
The optical signal input end face and output end face are combined. Even with such a configuration, it is the same as in the first embodiment.

Although the n-type InP is used as the substrate in the first and second embodiments, the present invention is not limited to this, and a p-type substrate may be used, and other semiconductor crystals may be used. May be used. Although the bulk InGaAsP is used for the active layer, the present invention is not limited to this, and a quantum well structure or a strained quantum well structure (compressive strain, tensile strain) may be used for the active layer. InGaAlA is used as the material of the active layer.
The same applies when materials such as s series, InAlAs series, and AlGaAs series are used.

[0021]

As described above, according to the first invention,
Amplifies the optical signal so that the gain for the TE polarized light is higher than that for the TM polarized light by having a cross-sectional shape perpendicular to the waveguiding direction in which the width perpendicular to the current injection direction is larger than the thickness. Taper waveguide that is an optical waveguide that is formed in contact with the active layer in the waveguide direction and is formed so as to have a thin film thickness from the contact end of the active layer to the other end that is the input end of the optical signal. A cladding layer formed so as to sandwich the active layer and the tapered waveguide above and below, and a positive electrode and a negative electrode for injecting a current into the active layer, and a film at the other end of the tapered waveguide. thickness, and less than that dimension thickness direction of the light confinement is weakened, the active
The width of the layer was 1.25 to 1.75 times the thickness. Well
In the second invention, the thickness of the active layer is 0.4 μm.
And the width of the active layer is 0.5 to 0.7 μm.
It was Further, in the third invention, the thickness of the active layer is 0.4 μm.
and the width of the active layer was 0.6 μm.
Since it is configured as described above, more TM polarized input light is guided at the input / output end of the optical signal in the tapered waveguide,
In the active layer, the gain for TE polarized light input light becomes larger. As a result, according to the present invention, it is possible to eliminate the gain difference (polarization dependence) in the entire element even if the cross section of the active layer in the waveguide direction is not square. Therefore, according to the present invention, since it is not necessary to form the cross section of the active layer into a square shape, even if the polarization state of the input signal light changes, the gain characteristics of the semiconductor laser type optical amplification element can be easily changed. Can be formed.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a configuration of a semiconductor laser type optical amplification element according to a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a change in the difference in gain characteristics between TE polarized light input light and TM polarized light input light with respect to the width of the active layer.

FIG. 3 is a configuration diagram showing a configuration of a semiconductor laser type optical amplification element according to a second embodiment of the present invention.

FIG. 4 is a configuration diagram showing a configuration of a conventional semiconductor laser type optical amplification element.

FIG. 5 is a characteristic diagram showing a change in the difference in gain characteristics between TE polarized light input light and TM polarized light input light with respect to the width of the active layer.

[Explanation of symbols]

101 ... Substrate, 102 ... Lower cladding layer, 103 ... Active layer, 103a ... Tapered waveguide, 104 ... Buried layer, 1
05 ... Current blocking layer, 106 ... Upper cladding layer, 10
7 ... P-side electrode, 108 ... N-side electrode, 109 ... Antireflection film.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuaki Katsuaki 3-19-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) Reference JP-A-9-246661 (JP, A) Special Features Kaihei 8-211342 (JP, A) JP-A-4-179179 (JP, A) JP-A 8-330673 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5 / 00-5/50

Claims (3)

(57) [Claims] 1. A device having a cross-sectional shape perpendicular to the waveguide direction in which the width perpendicular to the current injection direction is larger than the thickness in the current injection direction so that the gain for TE polarized light input light is higher than the gain for TM polarized light input light. An active layer for amplifying an optical signal, and a thin film formed in contact with the active layer in the waveguide direction from the contact end of the active layer to the other end serving as the input end of the optical signal. A tapered waveguide which is a formed optical waveguide, a clad layer formed so as to sandwich the active layer and the tapered waveguide, and a positive electrode and a negative electrode for injecting a current into the active layer. provided, the thickness of the portion of said other end of said tapered waveguide, the semiconductive <br/> body laser type optical amplifier device light in the film thickness direction confinement is smaller than the dimension weakened, the active layer The width is 1.25 to 1. Formed 75 times
Semiconductor laser type optical amplifier element characterized in that
Child . 2. The semiconductor laser type optical amplification device according to claim 1, wherein the active layer has a thickness of 0.4 μm, and the active layer has a width of 0.5 to 0.7 μm . A semiconductor laser type optical amplification element characterized by the above. 3. The semiconductor laser type optical amplifying element according to claim 1, wherein the active layer has a thickness of 0.4 μm and the active layer has a width of 0.6 μm. Semiconductor laser type optical amplifier.