JPH0738485B2 - Optical integrated circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of an optical integrated circuit in which an active optical element such as a semiconductor laser, a light emitting diode, or a photodiode and an optical waveguide are integrated.

2. Description of the Related Art Conventionally, various structures have been proposed for an optical integrated circuit in which an active optical element and an optical waveguide are integrated. Among these, as a structure in which a semiconductor laser and an optical waveguide are integrated, for example, Y.TOHMORI et al. “Low-threshold-current C
W operation of 1.5 μm GaInAsP / InP bundle-integrat
ed-guide distributed-Bragg-reflector (BIG-DB
R) lasers, "Electron.Lett., Vo
l.21, pp.743-745 (1985). In this case, the semiconductor laser 6 including the InGaAsP active layer 2, the n-type InP isolation layer 3, the n-type InGaAsP optical waveguide layer 4, and the n-type InP confinement layer 5 on the p-type InP substrate 1 and the optical waveguide layer 4, the confinement layer Layer 5
Although the optical waveguide 7 is formed of only the both, both are buried by the n-type InP layer 8 and the p-type InP layer 9. A grating 10 for forming a DBR structure is formed on the substrate 1 in contact with the optical waveguide layer 4 of the optical waveguide 7, an Au-Zn electrode 11 is provided on the back surface of the substrate 1, and an Au is provided on the crystal growth surface. -Sn electrode 12 is formed. The surface of the other crystal growth layer is covered with an insulating film 13 so that the Au—Sn electrode 12 contacts only the confinement layer 5 of the semiconductor laser 6.

Problems to be Solved by the Invention In such a conventional structure, (1) the optical coupling efficiency between the semiconductor laser and the optical waveguide is very high (theoretically 99%),
(2) Although both the semiconductor laser and the optical waveguide have the advantage of having a lateral optical confinement mechanism,
(3) The waveguide loss of the optical waveguide is large, (4) the position where the optical waveguide is formed is limited to the extension line of the active layer stripe of the semiconductor laser, (5) for forming the semiconductor laser and the optical waveguide In addition to crystal growth, it has a drawback that crystal growth is required to make both of them a buried structure.

The present invention retains the advantages of (1) and (2) above,
The present invention is intended to solve the problems (3) to (5), and to provide an integrated structure of an active optical element and an optical waveguide having the following features.

(1) The coupling efficiency between the active optical element and the optical waveguide is high.

(2) The optical waveguide also has an optical confinement mechanism in the lateral direction.

(3) The waveguide loss of the optical waveguide is small.

(4) The active optical element and the optical waveguide can be arranged at arbitrary positions.

(5) Manufacture is easy.

Means for Solving the Problems In order to solve the above problems, the present invention provides a compound semiconductor substrate, an active region including an active layer formed in a partial region on the substrate, and the active region on the substrate. An optical waveguide layer having a band gap larger than that of the active layer formed over the region other than the region and the active region, and a band gap larger than that of the optical waveguide layer formed on the optical waveguide layer. An optical integrated circuit is configured with a structure including the striped loading layer.

The structure of the present invention is obtained by forming the optical waveguide layer and the loading layer on both the active region and the other region and then etching the loading layer into a stripe shape, and the active region is formed on the etching region. Together with the optical waveguide layer and the loading layer, it becomes an active optical element (for example, a semiconductor laser), and the optical waveguide layer and the loading layer formed on the region other than the active region become the optical waveguide. In this structure, since the optical waveguide layer is also formed on the active region, the coupling efficiency between the active optical element and the optical waveguide is increased. Next, since the optical waveguide becomes a loading type optical waveguide, optical confinement is performed in the lateral direction and the waveguide loss is small. Furthermore, since the positions of the active optical element and the optical waveguide are determined by the mask shape when the loading layer is etched, the positional relationship between the two can be set freely and the manufacturing process thereof is simple.

Embodiment FIG. 1 is a perspective view showing an embodiment of the optical integrated circuit of the present invention. In FIG. 1, a part of the region on the p-type InP substrate 14 is I
nGaAsP active layer 15 (bandgap wavelength λg = 1.3μ
m) and n-InP isolation layer 16 are laminated to form an active region. N-type on the entire surface of the substrate 14 including the active area
An InGaAsP optical waveguide layer 17 (λg = 1.1 μm) is laminated, and a striped n-InP loading layer 18 is further formed thereon. The exposed portions of the loading layer 18 and the optical waveguide layer 17 are covered with an insulating film 19, but there is no insulating film 19 only on the top of the loading layer 18 on the active region.
It is in contact with the electrode 20. In addition, the entire back surface of the substrate 14 is Au-Zn.
The electrode 21 is formed.

In the above structure, the portion including the active region is the semiconductor laser 22, and the other portion is the optical waveguide 23. Semiconductor laser
22 operates by injecting a current from the Au—Sn electrode 20 to the loading layer 18. The injected current flows into the active layer 15 immediately below the loading layer 18.
The light is recombined and emits light in the stripe region of, but since the stripe region has the loading layer 18 thereon, the effective refractive index is high, and the light is also confined in the lateral direction. Here, the optical waveguide layer 17 and the separation layer 16 are interposed between the loading layer 18 and the active layer 15, but the lateral spread of the current between them is very small. On the contrary, this structure has an advantage that the reactive current due to surface recombination is suppressed as compared with the structure in which the active layer is in direct contact with the loading layer. Further, also in the optical waveguide 23, since the effective refractive index of the stripe region of the optical waveguide layer 17 immediately below the loading layer 18 becomes high, light is confined in this stripe region.

The present embodiment has a structure in which the semiconductor laser 22 and the optical waveguide 23 are integrated in the optical axis direction, and can be applied as a monolithic external resonator laser. That is, laser oscillation light with a narrow spectral line width can be obtained by using the optical waveguide as an external resonator. Here, the spectral line width can be narrowed as the optical waveguide is longer than that of the semiconductor laser and the amount of light returned from the optical waveguide to the semiconductor laser is large. Therefore, it is required that the coupling efficiency between the semiconductor laser and the optical waveguide is high and the waveguide loss of the optical waveguide is small. This embodiment is similar to the conventional example in terms of optical coupling between the semiconductor laser and the optical waveguide, and since the active region is composed of three layers, that is, the active layer, the separation layer and the optical waveguide layer, the equivalent refractive index in the stacking direction thereof is obtained. Becomes almost equal to the equivalent refractive index of the optical waveguide layer in the optical waveguide, so that the coupling efficiency becomes high.
On the other hand, regarding the waveguide loss of the optical waveguide, it is 24 cm in the conventional example.
^Although it was very large as ^-1 , it became 1 cm ^-1 or less in the present invention. This is the first point of the present invention, and the reason why the waveguide loss is reduced is explained as follows.

Usually, free carrier absorption is considered as a cause of causing waveguide loss in an optical waveguide made of a semiconductor. However, in the buried waveguide as in the conventional example shown in FIG. 3, the scattering loss due to the unevenness of the side wall of the waveguide layer has a larger effect.

When processing the waveguide layer into a stripe shape, it is unavoidable that unevenness occurs on the side wall, and if there is a large difference in the refractive index between the waveguide layer and the embedded layer that embeds it like a buried waveguide, a large scattering loss occurs. Will occur. On the other hand, in the loaded waveguide as in this example, the lateral confinement of light in the optical waveguide layer is performed by the small difference in the effective refractive index.
Even if the side wall of the loading layer has irregularities, the scattering loss due to it is small.

Further, in the present embodiment, the stripe structure is completed only by etching the loading layer in stripes, whereas in the conventional example, the buried layer needs to be epitaxially grown. From this point, it is clear that this embodiment can be manufactured more easily.

The advantages described above that the waveguide loss is small and the manufacturing is easy are not limited to the case where the present invention is applied to a monolithic external cavity laser as in the present embodiment, and are widely used in active optical elements and optical waveguides. This is common to the waveguide integrated structure in general.

Next, FIG. 2 shows a second embodiment applying the fact that the positions of the active optical element and the optical waveguide, which are the second essential points of the present invention, can be freely set. FIG. 2 is a plan view of the second embodiment. The light emitting region 24 and the light receiving region 25 shown in the figure have the same structure as the semiconductor laser 22 of FIG. 1, and function as a light emitting diode and a photodiode, respectively. The other regions are optical waveguides, but the first loading layer
An Au—Sn electrode 28 having the same structure as that of the semiconductor laser 22 of FIG. 1 is formed in a parallel portion where 26 and the second loading layer 27 are close to each other. The parallel portion where the two loading layers are close to each other functions as a directional coupler 29.

This embodiment operates as described below. The light Pi incident on the optical waveguide layer (hereinafter referred to as the second optical waveguide) below the second loading layer is obtained by setting the directional coupler 29 to have an appropriate directional dimension. In this portion, the light is moved to the optical waveguide layer (hereinafter referred to as the first optical waveguide) below the first loading layer, and is output as the output light Po from the first optical waveguide. Here, when current is injected from the Au-Sn electrode 28 of the directional coupler 29, the propagation constant of the optical waveguide in the directional coupler portion changes, and the light traveling from the second optical waveguide to the first optical waveguide changes. The amount changes.
At this time, if an appropriate current value is selected, the light input to the second optical waveguide travels through the second optical waveguide as it is and is incident on the light receiving region 25. At the same time, the light output from the light emitting region 24 is directly output as the output light Po through the first optical waveguide.

Since this embodiment operates as described above, it can be used as an optical repeater in optical fiber communication, for example. In a normal operation state, current is injected into the directional coupler 29, and incident light Pi from the outside is received by the light receiving region 25 and converted into an electric signal. After the electric signal is electrically amplified or signal-processed, the electric signal is converted into light in the light emitting region 24 and output as output light Po. On the other hand, when the light emitting region 24 or the like fails, the incident light Pi can be directly output as the output light Po by setting the injection current of the directional coupler 29 to 0. Since this function operates even when the electric system fails, this embodiment can be used as an optical repeater having a fail-safe function.

In the description of the above embodiments, the material is InP / InGaAs.
Although the P type has been used, the present invention is naturally GaAs / AlGa
It can also be implemented using other compound semiconductor materials such as As series. Further, it is possible to integrate a larger number of light emitting elements, light receiving elements, optical waveguides, and directional couplers as an optical integrated circuit, and in this case as well, the manufacturing process does not become so complicated. Further, the application of the present invention is not limited to optical fiber communication, and an optical head for an optical disc,
It can be widely used for optical measurement and optical information processing.

As described above, according to the present invention, when an active optical element such as a semiconductor laser and an optical waveguide are integrated, the coupling efficiency between the active optical element and the optical waveguide is high, and the optical waveguide is laterally aligned. Also has an optical confinement mechanism, the waveguide loss of the optical waveguide is small, the active optical element and the optical waveguide can be arranged at arbitrary positions, and a structure that can be easily manufactured can be realized.

[Brief description of drawings]

FIG. 1 is a perspective view of an optical integrated circuit according to an embodiment of the present invention, and FIG.
FIG. 3 is a plan view of an optical integrated circuit according to another embodiment of the present invention, and FIG. 3 is a sectional perspective view of a main part of a conventional optical integrated circuit. 14 ... Substrate, 15 ... Active layer, 17 ... Optical waveguide layer, 18 ... Loading layer.

Claims (1)

[Claims] 1. A compound semiconductor substrate, an active region including an active layer formed in a partial region on the substrate, a region other than the active region on the substrate, and the active region formed over the active region. An optical waveguide layer having a band gap larger than that of the active layer, and a stripe-shaped loading layer formed on the optical waveguide layer and having a band gap larger than that of the optical waveguide layer. Optical integrated circuit.