JPH02185084A - Semiconductor laser type optical amplifier - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser type amplifier having small incident polarization dependency of a gain by using a present device manufacturing technique by specifying the difference between band gap energy of first and those of second, third semiconductor materials. CONSTITUTION:An active layer 12 made of a first semiconductor material is interposed between two clad layers 11 and 13 made of different conductivity type second and third semiconductor materials having wider band gap than that of a first semiconductor material in a double hetero structure. The difference of the band gap energy between the first semiconductor material and the second, third semiconductor materials is set to 0.2-0.3eV. Thus, a semiconductor laser type optical amplifier which has small incident polarization dependency of a gain and can be easily expressed by a present technique is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser type optical amplifier for amplifying optical signals.

[Conventional technology]

Conventionally, optical amplifiers have been indispensable devices for purposes such as increasing the distance and capacity of optical communications and increasing the scale of optical switching systems. Although it is possible to use such an optical amplifier that utilizes nonlinear scattering within an optical fiber, it is also possible to use a small, highly efficient,
A semiconductor laser (LD) type is used because of its advantages such as the possibility of integration with other semiconductor optical devices. This LD type optical amplifier has an internal gain of 20 to 30 dB, and a gain of 20 d between optical fibers when optical fibers are connected to the input and output ends.
A value of about B is obtained. Furthermore, in recent years, advances in anti-reflection (AR) coating technology on the input and output end faces have significantly expanded the saturated optical output and gain wavelength band, making the device close to practical.

[Problem to be solved by the invention]

However, conventional LD optical amplifiers have a problem in that their characteristics largely depend on the polarization state of incident light. In other words, in a long-distance single mode optical fiber (SMF) under normal usage conditions, the polarization state of the incident light is not preserved;
Furthermore, the polarization state of the propagating light changes greatly depending on external temperature, pressure, and the like. Therefore, when inserting an LD optical amplifier in the middle of an SMF, the output light intensity will fluctuate greatly unless some kind of polarization control means is also used.

There are three possible reasons why the characteristics of such an LD optical amplifier depend on incident polarization.

(1) Polarization dependence of gain itself (2) Difference in confinement coefficient in the active layer depending on polarization (3)
Polarization dependence of end face reflectance In a normal double heterostructure LD optical amplifier, (1)
There is no polarization dependence in the gain itself, and in principle, problems (2) and (3) can be solved by making the waveguide structure of the active layer isotropic and reducing the end face reflectance. It is. However, such problems have been discussed, for example, at the First Opto-Electronics Conference held in Tokyo in July 1986.
``Post Deadline Vapors Technical Digest'' (Post Deadline PapersTec) at onics Conference)
hnical Digest) B 11-2. 12
-It is also stated on page 13, and according to this published paper,
Even in a traveling wave LD optical amplifier in which an extremely high-quality AR coating with a reflectance R=0.04% is applied to both end faces of an LD with an isotropic buried heterostructure in which the waveguide structure is made isotropic, horizontal polarization ( up to 1 between both polarizations (TE) and vertical polarization (TM>).
Gain differences of 0 dB or more have been observed.

FIG. 4 shows the TE of the above-mentioned normal traveling wave type LD optical amplifier.
It is a gain characteristic diagram for 7MM polarized light.

As shown in FIG. 4, the signal gain of the conventional LD optical amplifier is higher in TB than in TM by more than 10 dB at most.

That is, it is difficult to reduce the polarization dependence of the characteristics of the LD optical amplifier only by reducing the end face reflectance.

Regarding the waveguide structure of the active layer, the thickness of the active layer is generally set to about 0.2 μm or less in order to increase the carrier density relative to the number of injected carriers. On the other hand, the width of the active layer is limited by etching technology and buried growth technology, and it is difficult to make it less than 1 μm. Therefore, it is impossible to make the cross-sectional shape of the active layer waveguide completely isotropic.

Also, the magazine ``Electronics Letters''
onics Letters) J Volume 23, 1987
As described on pages 1387 to 1388 of 2006, a system in which two semiconductor laser optical amplifiers are used to amplify the TE and TM acid components independently can be considered, but this method has the problem of complicating the configuration.

An object of the present invention is to eliminate the above-mentioned problems and to provide a semiconductor laser type optical amplifier in which the gain is small in dependence on incident polarization, using the only device that can be realized using current device manufacturing technology.

[Means to solve the problem]

The semiconductor laser type optical amplifier according to the present invention includes an active layer made of a first semiconductor material, and a second active layer having a wider bandgap than the first semiconductor material and having different conductivity types. A double heterostructure sandwiched between two cladding layers made of a third semiconductor material, a means for injecting current into the active layer, and an input/output end face for coupling input and output optical signals to the active layer are reduced. In the semiconductor laser type optical amplifier having the first
a semiconductor material; and the second semiconductor material. It is characterized in that the difference in band gap energy between the third semiconductor materials is 0.2 to 0.3 eV.

[Effect]

The present invention is based on the relationship between the refractive index and confinement coefficient in an optical waveguide. Figure 3 shows the optical confinement coefficient r T of TE and TM waves in the active layer in a three-layer slab optical waveguide.
This is the result of calculating the relationship between tl r TM and the thickness of the optical waveguide layer. However, the wavelength of light is 1.55 μm, and the refractive index of the optical waveguide layer and cladding layer is nl+n2 (nt I n3
) is ■(3,5,3,2),■(3,5,3,4)
As is clear from the 0 diagram calculated for the two cases of ■ (3, 5, 3, 4), r tit r 7
The difference in M is getting smaller. In other words, by reducing the refractive index difference between the optical waveguide layer (active layer) and the cladding layer, the polarization dependence of the optical confinement coefficient of the active layer can be reduced. In laser-type optical amplifiers, the composition of the active layer is set to a bandgap wavelength of ~1.6 μm.
The cladding layer is formed of InGaAsP and the cladding layer is formed of InP. Therefore, the relationship between the refractive indexes is close to the state of ■. In the case of a semiconductor laser type optical amplifier, the composition of the active layer is determined according to the wavelength of the light to be amplified, so in order to reduce the refractive index difference between the active layer and the cladding layer, it is necessary to In the case of semiconductors, the refractive index is related to the bandgap energy, and increasing the refractive index effectively allows carriers to enter the active layer, which corresponds to narrowing the bandgap. For confinement, the difference in bandgap energy between the active layer and the cladding layer Δ
E is normally 0.3eV, minimum limit is 0.2
About eV is required. The bandgap wavelength of the active layer is set to 1
．． When 6 μm and ΔE are 0.2 ev, the bandgap wavelength of the cladding layer is ~1.27 μm. At this time, the refractive indices of the active layer and the cladding layer at a wavelength of 1.55 μm are 3.5 and 3.4, respectively, which corresponds to the case (2) shown in FIG. In other words, the composition of the cladding layer is In
InGaA with a bandgap wavelength of 1.27 μm from P
By changing to sP, it is possible to reduce the polarization dependence of the confinement coefficient as shown in FIG. As shown in the calculation results in FIG. 3, this effect is obtained in the slab optical waveguide state and is not dependent on the structure of lateral optical confinement.

In other words, no matter what kind of lateral optical confinement structure (buried structure, ridge structure, etc.), the cladding layer is made of InGa.
By setting AsP (λg=1.27.czm),
Polarization dependence can be reduced compared to the case of an InP cladding layer. It goes without saying that the polarization dependence can be further reduced by introducing a lateral optical confinement structure.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view of an optical amplifier for explaining one embodiment of the present invention.

In this embodiment, the structure of the optical amplifier of this embodiment will be explained by taking as an example a 1,55-series LD optical amplifier made of InGaAsP/InP-based materials. This LD tank structure has a double channel brainer buried hetero (DC-PB) in which the mesa stripe including the active layer is buried with p and n type semiconductors.
H) It is based on structure. In order to fabricate this structure, first, n-InP
Substrate 10. (100 directions) on top of the n-type cladding layer 11
．． A non-doped InGaAsP active layer 12 and a p-type cladding layer 13 are successively grown to fabricate a double heterostructure (DH) wafer. At this time, the n-type cladding 111 and the p
The composition of the molded rad layer 13 is set to a bandgap wavelength of 1.
The key point of the present invention is to use InGaAsP with a thickness of 27 μm. Further, the thickness of the active layer 71112 was 0.2 μm.

Next, by photolithography method and chemical etching,
By removing the DH wafer to the middle of the n-InP substrate 1o, two parallel linear grooves close to each other are formed.

The central mesa stripe 31 formed by this etching becomes the active waveguide of this LD optical amplifier. Here, the width of this mesa stripe 31 is 1.5 μm.

Mesa stripes of this width can be easily formed. Next, a p-type first layer is grown on this wafer by liquid phase growth.
current blocking layer 14. n-type second current blocking layer 1
5. A p-InP buried layer 16 and a p-InGaAsP cap layer 17 are sequentially grown. At this time, the n-type second current blocking layer 15 is controlled so as not to grow on the mesa stripe 31.

This, the first. The composition of the second current blocking layer 14.15 is also InGaAsP with a bandgap wavelength of 1.27 μm.

Next, in order to make an ohmic contact with the wafer thus formed, AuZ is deposited on the cap layer 17 as an electrode.
Deposit n18.

Next, the n-InP substrate 1 is polished to a thickness of about 150 μm so that it can be easily cleaved, an n-side ohmic electrode 19 is formed, and the wafer is cleaved. After exposing the input/output end face 20 by this cleavage, a 5iNxAR coating film is formed on the input/output end face 20 by sputtering to complete the LD optical amplifier shown in FIG. The element length is 300 to 50
Approximately 0 μm is appropriate. Note that a simple diagonal AR coating film is not shown in FIG.

FIG. 2 is a diagram for explaining the operation of this embodiment,
2 is a cross-sectional view of the embodiment shown in FIG. 1 taken along the optical axis and perpendicular to the substrate. Figure 2 shows AR coating film 2.
0a and 20b were shown. In this example, the oscillation threshold before and after the formation of the AR coat film is 20 m, respectively.
A, it was 100mA or more. Spherical fibers 21a, 21b are used to couple the incident light into the active layer 12 and to extract the optical signal. When an original bias is applied between the electrodes 18 and 19 and a current below the threshold current is injected, the gain in the active layer increases and an amplification function is obtained. At this time, since the refractive index difference between the active layer 12 and the surrounding medium is kept small, the optical confinement coefficient difference of the fundamental modes of TE and TM 1ike becomes small. Therefore, the dependence of gain on incident polarization is reduced.

[Effects of the Invention] As described in detail above, according to the present invention, a semiconductor laser type optical amplifier can be obtained in which the dependence of the gain on incident polarization is very small and can be easily realized using the current technology.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of an embodiment of the semiconductor laser type optical amplifier according to the present invention, FIG. 2 is a diagram for explaining the operation of the semiconductor laser type optical amplifier according to the present invention, and FIG. 3 is optical confinement in the active layer. FIG. 4, which is a diagram for explaining the calculation results of the coefficients, is a diagram showing the dependence of the gain of a conventional semiconductor laser type optical amplifier on the incident polarization. In the figure, 10, 11, 12, 13, 14, 15, 16
．． 17 is a semiconductor, 18.19 is an electrode, 20 is an end surface, 20
a and 20b are AR coat films, 21a and 21b are tipped fibers, and 31 is a mesa stripe.

Claims (1)

[Claims] A double heterostructure in which an active layer made of a first semiconductor material is sandwiched between two cladding layers made of second and third semiconductor materials having a wider band gap than the first semiconductor material and having different conductivity types. , in a semiconductor laser type optical amplifier having at least a means for injecting a current into the active layer, and an input/output end face for coupling an input/output optical signal to the active layer, the first semiconductor material; The difference in band gap energy between the second and third semiconductor materials is 0.2 to 0.
．． A semiconductor laser type optical amplifier characterized by having a voltage of 3 eV.