JPH11112073A - Semiconductor laser type optical amplifying element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily form a semiconductor laser type optical amplifying element, wherein gain characteristics is hard to fluctuate even when a polarization state of input signal light fluctuates. SOLUTION: A taper waveguide channel 103a is provided at both ends in waveguide direction of an active layer 103 on a clad layer 102, while connected to an end part of the active layer 103. The taper waveguide channel 103a comprises, for example, an InGaAsP of band gap wavelength 1.3 μm. The taper waveguide channel 103a is so formed as to be thinner in film thickness as going away from a joint part to the active layer 103.

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 amplifier having a waveguide.

[0002]

2. Description of the Related Art When 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 is indispensable to compensate the attenuated optical signal by the optical amplifier. In particular, a semiconductor laser type optical amplifying element is very promising because it has advantages such as being small in size and high in efficiency, and being capable of being integrated with a semiconductor optical device such as an optical switch.

However, the conventional semiconductor laser type optical amplifying element has a problem that the gain characteristic greatly varies depending on the polarization state of the input signal light. This is generally
While the width of the active layer is several μm, the thickness is on the order of sub-μm and the active layer is not isotropic. This is because the gain characteristics of the active layer differ depending on the polarization state.
For this reason, the cross section of the active layer is made into a shape close to a square and isotropic, for example, a semiconductor laser type in which the difference in gain characteristic between the TE polarization input light and the TM polarization input light is 1 dB or less. There have been reports of realizations of optical amplification devices (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).

The 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 disposed on a lower cladding 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 cladding layer 402 and the active layer 403 so as to bury them. Is composed.

Further, an upper clad layer 406 made of p-type InP is formed thereon. 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 cladding layer 406.
Further, as shown in FIG. 4B, which is a cross section perpendicular to the waveguide direction, the input end face 410 of the input light and the output end face 411 of the output light have antireflection films 410a and 411a, respectively.
Are formed. Then, the active layer 403 has a width of 0.4
By setting the thickness to 0.4 μm, the polarization dependence can be reduced. Note that the semiconductor laser type optical amplifying element has a total length of 600 μm.

[0006]

However, it is difficult to actually form the active layer 403 to have a square cross section. When the thickness of the active layer is 0.4 μm, the change in the difference in gain characteristic between the TE-polarized light and the TM-polarized light with respect to the width of the active layer changes as indicated by the black dots in FIG. Therefore, if the difference in the gain characteristics is to be kept within the range of 0.5 dB indicated by satin, the width of the active layer must be within the range of about 0.35 to 0.5 μm. That is, it is necessary to use a vertical or near-vertical anisotropic etching technique so that the upper end of the active layer and the load fall within this range. The stripe-shaped ridge structure in the active layer is formed by using a known photolithography technique and an etching technique. However, in this fine processing, it is very difficult to keep the processing dimension within an error of 0.15 μm. Then, it is not easy to form the stripe width to 0.4 μm.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a semiconductor laser type optical amplifying element whose gain characteristic is hardly changed even if the polarization state of the input signal light changes. It is intended to be easily formed.

[0008]

SUMMARY OF THE INVENTION A semiconductor laser type optical amplifier according to the present invention amplifies an optical signal by having a cross section perpendicular to a waveguide direction in which a width perpendicular to the current injection direction is larger than a thickness in a current injection direction. And an optical waveguide formed in contact with the active layer in the waveguide direction and having a thin film thickness from the contact end of the active layer to the other end serving as the input end of the optical signal. And a cladding layer formed to sandwich the active layer and the tapered waveguide above and below, and a positive electrode and a negative electrode for injecting current into the active layer. The cross-sectional shape perpendicular to the direction was formed so that the width perpendicular to the current injection direction was larger than the thickness in the current injection direction. With the configuration as described above, more TM polarized input light is guided at the input / output end of the optical signal of the tapered waveguide, and the gain for the TE polarized input light becomes larger in the active layer.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. Embodiment 1 FIG. 1 is a configuration diagram showing a configuration of a semiconductor laser type optical amplifier according to a first embodiment of the present invention. Here, for example, a semiconductor laser type optical amplifying device for a 1.5 μm band will be described as an example. As shown in FIG. 1, the semiconductor laser type optical amplifying device according to the first embodiment first has n
Substrate 101 made of InP in the shape of
By processing the surface, the ridge-shaped lower cladding layer 102 is formed.
Are formed. Then, the lower cladding layer 102
InGaAsP with a bandgap wavelength of 1.55 μm
An active layer 103 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.

As shown in FIGS. 1B and 1C, the tapered waveguide 1 is connected to both ends of the active layer 103 in the waveguide direction on the cladding layer 102 and connected to the ends of the active layer 103.
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
03a has a composition with a small loss with respect to the optical signal to be amplified. Then, the tapered waveguide 103a
Becomes more distant from the junction with the active layer 103.
It is formed so as to have a small thickness. The light input end and light output end of the tapered waveguide 103a are formed to have a thickness of 0.2 μm.

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

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

As described above, if the selective growth mask is used, the active layer 103 and the tapered waveguide 103a can be formed simultaneously. However, as described above, when the active layer and the tapered waveguide are crystal-grown simultaneously using the selective growth mask, the crystal also grows on the substrate surface outside the selective growth mask. . Also, of course, the selective growth mask is
It is unnecessary as an element.

The lower cladding layer 102 and the active layer 10
On the side surfaces of the 3, 3, and tapered waveguide 103a in the waveguide direction, a p-type I
A buried layer 104 made of nP is formed.
A current block layer 105 for a current block made of n-type InP is formed to a thickness of 0.5 μm. And
Active layer 103, tapered waveguide 103a, buried layer 10
4, and on the current block layer 105, p-type In
An upper cladding layer 106 made of P is formed to a thickness of 2 μm.

The upper cladding layer 106 has Ni
A 1-μm-thick p-side electrode 107 made of / Zn / Au / Ti / Au is formed, and AuGeN
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 of the tapered waveguide 103a where the film thickness is reduced. 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 has a rectangular shape whose cross section perpendicular to the waveguide direction is horizontally long. Then, a tapered waveguide 103a is arranged at an end of the active layer 103 in the waveguide direction. And, with such a configuration, the semiconductor laser type optical amplifying device according to the first embodiment has the following features. First, the active layer 103 has a thickness of 0.4 μm.
m, the gain for the TE-polarized input light is smaller than the gain for the TM-polarized input light, as shown by the black circle on the horizontal axis 0.6 in FIG. 0.7 dB higher.

On the other hand, the tapered waveguide 103a has a thickness of, for example, 0.2 μm at the input end of the input light. Generally, when the cross-sectional dimension of the waveguide is smaller than 0.3 μm,
Light confinement in that direction is weakened. Therefore, in the portion where the thickness of the tapered waveguide 103a is small, the light confinement in the vertical direction is weak, and the light seeps in the vertical direction and becomes long vertically.
For this reason, 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 a white circle in FIG. 2, the coupling efficiency of the TM polarized input light is higher. In particular, the first embodiment
Since the width of the tapered waveguide 103a is 0.6 μm, the coupling efficiency of the input and output light of the TM polarized input light is improved by about 0.7 dB.

That is, at the input / output end of the input light, T
M polarized input light is input and output more, and the active layer 103
In the above, the gain of the TE-polarized input light is increased. Therefore, according to the semiconductor laser type optical amplifying device of the first embodiment, as shown by a 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 zero, and the width can be made relatively wide. That is, the active layer can be relatively easily manufactured. The polarization dependence of the entire device is 0.5 d.
The width of the active layer within B is also 0.3 to 0.7 μm, which increases the allowable amount. In this range, the width of the tapered waveguide 103a may be smaller than the width of the active layer 103.

Embodiment 2 Hereinafter, a second embodiment of the present invention will be described.
In the second embodiment, as shown in FIG. 3, the input terminal and the output terminal of the optical signal are configured identically. That is, as shown in FIG.
2, one end of the active layer 103 is connected to the tapered waveguide 10.
3a, and the antireflection film 109 is disposed at the tip of the tapered waveguide 103a where the film thickness is reduced. At 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,
An input end face and an output end face of an optical signal are used. Even with this configuration, it is the same as in the first embodiment.

In the first and second embodiments, n-type InP is used as a substrate. However, the present invention is not limited to this, and a p-type substrate may be used. May be used. Further, although 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 (compression strain, extension strain) may be used for the active layer. As a material for the active layer, InGaAlA
The same applies to the case where a material such as an s-based, InAlAs-based, or AlGaAs-based material is used.

[0021]

As described above, according to the present invention, there is provided an active layer for amplifying an optical signal having a cross-sectional shape perpendicular to the waveguide direction, the width of which is larger than the thickness in the current injection direction. A tapered waveguide which is an optical waveguide formed in contact with the active layer in the waveguide direction and formed so as to have a small thickness from the contact end of the active layer to the other end serving as an input end of the optical signal; The tapered waveguide includes a clad layer formed to sandwich the upper and lower sides of the tapered waveguide, a positive electrode and a negative electrode for injecting current into the active layer, and the tapered waveguide has a cross section perpendicular to the waveguide direction at the other end. The shape was formed so that the width perpendicular to the thickness in the current injection direction was larger than the thickness in the current injection direction. With the configuration as described above, more TM polarized input light is guided at the input / output end of the optical signal of the tapered waveguide, and the gain for the TE polarized input light becomes larger in the active layer. As a result, according to the present invention, it is possible to eliminate the gain difference (polarization dependency) of the entire device even if the active layer does not have a square cross section in the waveguide direction. Therefore, according to the present invention, it is not necessary to form the cross section of the active layer into a square, so that even if the polarization state of the input signal light fluctuates, a semiconductor laser type optical amplifying element whose gain characteristic is hardly fluctuated can be easily obtained. Can be formed.

[Brief description of the drawings]

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

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

FIG. 3 is a configuration diagram showing a configuration of a semiconductor laser type optical amplifier 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 amplifying element.

FIG. 5 is a characteristic diagram showing a change in a gain characteristic difference between a TE polarization input light and a TM polarization input light with respect to the width of an 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 ... anti-reflection film.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Katsuaki Song 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (5)

[Claims] 1. An active layer for amplifying an optical signal having a cross-sectional shape perpendicular to a waveguide direction whose width perpendicular to the current injection direction is larger than a thickness in a current injection direction, and contacting the active layer in the waveguide direction. A tapered waveguide which is an optical waveguide formed so as to have a small thickness from the contact end of the active layer to the other end which is an input end of the optical signal; and a tapered waveguide of the active layer and the tapered waveguide. The tapered waveguide includes a clad layer formed so as to sandwich the upper and lower sides, a positive electrode and a negative electrode for injecting current into the active layer, and the tapered waveguide has a cross section perpendicular to the waveguide direction at the other end. Wherein the width perpendicular to the current injection direction is greater than the thickness in the current injection direction. 2. The semiconductor laser type optical amplifying device according to claim 1, wherein the tapered waveguide is formed so as to have a small thickness from the contact end of the active layer to the other end serving as an output end of the optical signal. A semiconductor laser type optical amplifying element characterized by the above-mentioned. 3. The semiconductor laser type optical amplifying device according to claim 1, wherein the tapered waveguide is formed at both ends of the active layer in the waveguide direction. 4. The semiconductor laser type optical amplifying device according to claim 1, wherein a high reflection film is formed at an end of said active layer where said tapered waveguide is not formed. Laser type optical amplifier. 5. The semiconductor laser type optical amplifying device according to claim 1, wherein said tapered waveguide is made of a composition having a smaller loss than said active layer with respect to said optical signal. Characteristic semiconductor laser type optical amplifying element.