JP3254053B2 - Optical integrated circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical integrated circuit, and more particularly, to an integrated optical device comprising a plurality of optical functional elements formed on a semiconductor substrate, and more particularly to an electrode structure of an optical integrated circuit having a plurality of electrodes. It is about.

[0002]

2. Description of the Related Art Interaction between elements is most problematic when integrating optical functional elements. In general, the interaction is based on crosstalk between optical signals between a plurality of optical functional elements and crosstalk between electric signals. Above all, it is difficult to reduce the crosstalk of the electric signal because the conflicting condition of improving the optical coupling efficiency between the elements must be satisfied at the same time. For example,
1989 IEICE Autumn National Convention C-179 Furutsu, Sojita et al. "High-resistance embedded optical modulator / DFB laser integrated light source" In this example, an attempt is made to secure electrical isolation by a method of embedding a semi-insulating semiconductor such as Fe-InP. However, even with this method, it is difficult to completely remove the leakage current through the remaining doping layer, that is, the crosstalk of the electric signal. It is generally said that an integrated light source requires a separation resistance of several megaohms or more in order to completely eliminate electrical interaction, and sufficient isolation between elements is obtained in the above-mentioned conventional example. It can not be said. In addition, since the manufacturing process becomes complicated with the introduction of the semi-insulating semiconductor, the manufacturing cost increases and the yield decreases.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art. That is, it is an object of the present invention to realize an optical integrated circuit in which isolation between elements is secured without complicating the element manufacturing process and reducing the optical coupling efficiency between the elements.

[0004]

According to the present invention, there is provided an integrated circuit comprising a plurality of functional elements formed on a semiconductor substrate, wherein at least one set of optical elements among the plurality of functional elements is provided. A new isolator with a device-to-element isolation structure with a predetermined isolation resistance value
An electrode was provided. The isolation electrode is grounded inside the element or connected to a predetermined low impedance circuit (or ground) outside the element. The separation resistance required for the element isolation structure is set to be sufficiently large as compared with the impedance of the isolation electrode portion. However, the value may be smaller by one or two orders of magnitude than the case where the isolation is secured only by the conventional separation resistor. One set of optical functional elements in a preferred embodiment of the present invention is:
A semiconductor light source such as a laser diode and a light modulation unit described in the following embodiments, but are not limited thereto.

[0005]

Since the isolation electrode according to the present invention is always kept at a constant potential, even if a leakage current occurs between each functional element and the isolation electrode, no leakage current occurs between the elements. Therefore, even when the insulation resistance between elements is relatively small, no electrical interaction occurs between the elements. The isolation electrode newly installed in the present invention can be manufactured at the same time as the electrode forming step of the functional element, so that the same manufacturing process as that of the conventional element can be used. Also,
As described above, the resistance value required for the separation resistance portion of the present invention may be smaller by one to two digits as compared with the conventional structure. Therefore, the structure is also simple, and there is no need to introduce a burying or the like with a semi-insulating semiconductor as in the related art. Therefore, the manufacturing process can be greatly simplified. That is, according to the present invention, complete isolation between devices can be realized by simple steps, and the yield of device fabrication can be greatly improved, and device performance can be stabilized.

[0006]

FIG. 1 shows the structure of a first embodiment of an optical integrated circuit according to the present invention. This embodiment is a semiconductor optical integrated circuit in which an electro-absorption type optical modulator section 14 and a semiconductor laser diode (LD) 13 are monolithically integrated, (a) is a top view, and (b) is b- in (a). It is a fragmentary sectional view of b 'part.
The LD section 13 and the optical modulator section 14 are formed on the same n-type InP substrate 1. The layer structure of both layers is an optical waveguide layer (0.2 μm thick, absorption edge wavelength 1.15 μm) 2, a p-cladding layer 4 (2.0 μm thick), and a p-cap layer (0.2 μm thick).
(Thickness) 5 formed of an undoped multiple quantum well layer. However, the LD section 13 is formed on the region where the grating 7 is formed on the substrate 1 and the undoped multiple quantum well layer has an operating wavelength (1.55 μm).
m), the active layer 3 having an absorption edge wavelength substantially equal to that of the multi-quantum well layer having an absorption edge wavelength several tens nm shorter than the operating wavelength formed on the flat substrate portion. The structure is different from that of the optical modulator section 14.

The wavelength 1.55 μ radiated from the LD unit 13
The m guided light 9 is guided to the optical modulator section 14 via the isolation region 12, where it is modulated according to a predetermined electric signal. At this time, the electric signal applied to the optical modulator section 14 is transmitted through the p-clad layer 4 which is a common conductive layer to the LD.
Leakage into the portion 13 is a main cause of electrical crosstalk. To suppress this, in the present embodiment, a new electrode 6-1 is arranged in the isolation region 12 and electrodes 6-3 and 6- 1 and between the electrodes 6-1 and 6-2 are formed isolation grooves 8 in the p-cladding layer 4. Here, the depth of the separation groove 8 was set to about 1.2 μm in consideration of leakage of the guided light 9 into the p-cladding layer. in this case,
Since about 1 μm of the p-cladding layer 4 remains under the groove,
Scattering of the guided light 9 in the isolation region 12 is sufficiently suppressed.

Although the resistance of each isolation groove 8 is about several tens of kilo-ohms (using a 10 μm isolation groove), the isolation electrode 6-1 is connected to a constant voltage circuit having a sufficiently smaller impedance (grounding). (Also possible), it is possible to cut off the electrical connection between the optical modulator unit 14 and the LD unit 13. In this case, a leakage current of about several tens of μA occurs in the LD unit 13 and the optical modulator unit 14, but these are current components flowing between the constant voltage circuit and the individual elements. No signal flows into the LD unit 13. In a normal element configuration not using the isolation electrode 6-1, the LD 13
It is necessary to completely remove the p-clad layer 4 below the separation groove in order to secure the electrical isolation required when the optical modulator 14 is integrated with the optical modulator 14. In this case, scattering of the guided light The optical coupling efficiency between the optical modulator section 14 and the LD section 13 is greatly reduced. In this embodiment, electrical isolation between the two can be realized without causing such difficulties. The isolation electrode 6-1 is connected to the LD unit 13, the electrode 6-3 of the optical modulator unit 14,
Since it can be created at the same time as the step of forming 6-2, there is no need to change the element creation process. Therefore, isolation between elements is greatly improved without causing difficulty in producing an optical integrated circuit element.

FIG. 2 shows the structure of a second embodiment of the optical integrated circuit according to the present invention. In this embodiment, the isolation structure is applied to a Mach-Zehnder optical modulator.
(A) is a top view, and (b) is a partial cross-sectional view taken along line aa ′ of (a). As shown in the figure, the Mach-Zehnder optical modulator has two phase modulators 20 and two Y modulators arranged in parallel.
It has a structure in which the branch circuits 21 are combined, and ensuring electrical isolation between them is important for stabilizing element characteristics. In this embodiment,
Application of the present invention to a device having a Pin structure in which an undoped multiple quantum well structure 3 produced via an optical waveguide layer 2 on a semiconductor substrate 1 is used as an optical waveguide layer and this is laminated with p and n-type semiconductor layers. Although an example is shown, the application example of the present invention is not necessarily limited to the Pin structure, and can be similarly applied to a structure in which an electric field is applied by a Schottky structure or a current injection type structure. It is. In this embodiment, the isolation electrodes 6-1 are arranged at both ends of the phase modulation area. This is because a main leakage current between the phase modulation regions flows through the p conductive layer on the optical waveguide of the Y branch portion 21. By adopting such an arrangement, an electric field is applied to the Y branch portion 21. It is also possible to prevent itself, and it becomes possible to reduce the absorption loss of the guided light in the Y branch portion 21.

FIG. 3 shows the structure of a third embodiment of the optical integrated circuit according to the present invention. In this embodiment, the isolation electrode 6-1 of the embodiment shown in FIG. 2 is forcibly grounded by making contact with the n-doped substrate 2 in the device.
An example is shown. 2, the same parts as those in FIG. 2 are given the same numbers. By grounding the electrode 6-1 in the element as described above, the number of connection points with the outside of the element can be reduced, and the mounting of the element is simplified. In this case, there is no problem as long as the contact resistance value at the contact surface is sufficiently smaller than the resistance value at the separation groove 8, so that a normal electrode material (for example, Cr /
Even when an electrode material such as Au or Ti / Au) is used for the isolation electrode 6-1, electrical contact with the substrate 1 can be obtained without any problem.

FIG. 4 is a top view showing a fourth embodiment of the optical integrated circuit according to the present invention. The feature of the present embodiment lies in that the isolation electrode 6-1 is arranged not only in the inter-element isolation part but also in the entire area around the functional element. This allows
It is possible to reduce not only the crosstalk due to the leakage current between the elements but also the crosstalk due to other electromagnetic interaction, and the isolation characteristic is particularly improved in the high frequency region above gigahertz. The configuration other than the isolation electrode 6-1 is the same as that of the embodiment shown in FIG.

FIG. 5 is a block diagram showing a fifth embodiment of the optical integrated circuit according to the present invention. The integrated optical device in the figure is the same as the optical integrated circuit shown in FIG. 1 or FIG.
3, an isolation unit 12 and a modulation unit 14. LD
The LD unit drive circuit 15 is connected to the electrode 6-1 of the unit 13, the modulation unit drive circuit 16 is connected to the electrode 6-2 of the modulation unit 14, and the isolation electrode 6-1 is connected to the bias T circuit (DC bias). A DC bias circuit (control circuit) 18 is connected to a DC bias circuit (control circuit) 18 through a multiplexing circuit 17 and a high-frequency signal demultiplexing circuit, and the demultiplexed high-frequency component is grounded. When the absorption edge wavelength of the active layer in the isolation region 12 is set close to the operating light wavelength, a current is injected into the isolation region 12 by applying a positive bias to cause an optical amplification effect. That is, the isolation region 12 has a function of amplifying light as well as a function of reducing electrical interaction between the functional elements, and can compensate for attenuation of an optical signal in an integrated circuit. it can.

The absorption edge wavelength of the active layer in the isolation region 12 is set to a value close to the operating light or several tens of nm.
When the wavelength is set to the shorter wavelength side by about m, the region functions as a light absorption region. In particular, when the absorption edge wavelength is set to a shorter wavelength side than the operation light, the absorption coefficient can be adjusted by the applied voltage. As a result, a part of the guided light emitted from the LD section 13 can be detected as the photocurrent 19 in the isolation region 12 and the optical output can be monitored. Conventionally, an LD output monitor has a photodetector disposed outside the LD. However, by using the method of this embodiment, the photodetection function can be integrated in the device, and the optical system can be simplified. And stabilization. Since the leakage current from the optical modulator 14 flowing in the isolation region 12 can always be regarded as an AC signal having a signal component of a predetermined frequency or more, the leakage current can be completely removed in the bias T circuit 17 and the light detection can be performed. There is no mixing in the signal.

When the absorption edge wavelength of the active layer in the isolation region 12 is set to a wavelength shorter than the operating light wavelength by several tens to hundreds of nm, the region operates as an optical modulator for the intensity or phase of the guided light. I do. Also in this case, the adjustment of the modulation degree can be adjusted by the bias circuit 17, which effectively works for enhancing the function of the optical integrated circuit.

[0015]

According to the present invention, electrical isolation between elements in an integrated optical circuit can be realized by a simple structure, and the problem of crosstalk between elements due to integration can be solved. It becomes possible.

[Brief description of the drawings]

FIGS. 1A and 1B show the configuration of a first embodiment of an optical integrated circuit according to the present invention, wherein FIG. 1A is a top view and FIG.

FIGS. 2A and 2B show a configuration of an optical integrated circuit according to a second embodiment of the present invention, wherein FIG. 2A is a top view and FIG.

3A and 3B show the configuration of a third embodiment of the optical integrated circuit according to the present invention, wherein FIG. 3A is a top view and FIG. 3B is a partial sectional view.

FIGS. 4A and 4B show the configuration of a fourth embodiment of the optical integrated circuit according to the present invention, wherein FIG. 4A is a top view and FIG.

FIG. 5 is a fifth embodiment of the present invention, and shows a configuration of an optical transmitter using the optical integrated device of the present invention.

[Explanation of symbols]

 1: semiconductor substrate, 2: optical waveguide layer, 3: active layer, 3 ′: light absorbing layer, 4: clad layer, 5: cap layer, 6: p electrode, 7: grating, 8: separation groove, 9: conductive Wave light, 10: optical waveguide, 11: passivation, 12: isolation section, 13: LD section, 14: modulator section, 15: LD section drive circuit, 16: optical modulator drive circuit, 17: bias T circuit, 18 : Control circuit, 19: photocurrent, 20: phase modulator, 21: Y branch circuit.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-55877 (JP, A) JP-A-5-63179 (JP, A) JP-A-1-192168 (JP, A) 1992 Electronic Information Communication Conference Autumn Meeting C-146 p. 4-168 (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 27/15

Claims (2)

(57) [Claims] A first optical element and a second optical element on a semiconductor substrate;
The elements are monolithically integrated, and the first optical element and the second optical element are an optical waveguide layer, a first conductivity type.
A first electrode of the first optical element and a second electrode of the second optical element
The first optical element and the second
The third electrode is provided in the isolation region with the optical element
Between the third electrode and the first electrode and the
A separation groove is provided between the third electrode and the second electrode.
Conductive layer is provided so that the grooves bottom remains, the third electrode, characterized in that it is connected either to ground inside the device or external, or outside of the element a predetermined low impedance circuit
Optical integrated circuit. 2. A semiconductor laser diode and an InP substrate.
And an electro-absorption type optical modulator are monolithically integrated, and the laser and the modulator are an optical waveguide layer, a p-cladding layer.
And a first electrode on the laser and a second electrode on the modulator.
At the same height and at the connection between the laser and the modulator
A third electrode is provided in the isolation region, which is located between the third electrode and the first electrode and the third electrode.
A separation groove is provided between the electrode and the second electrode.
The third electrode is provided so that the lower part of the groove is left in the gate layer.
Is grounded inside or outside the device, or
It is characterized by being connected to a predetermined low impedance circuit
Optical integrated circuit to be.