JPH0720359A - Optical device - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to an optical device having an optical waveguide having an end face from which light is emitted or incident, and an optical circuit element optically coupled to the optical waveguide. The object of the present invention is to sufficiently suppress fluctuations in operating characteristics due to reflection of light. [Structure] An optical waveguide is formed on a substrate whose end face has an optical axis inclined and whose cross-sectional area is gradually enlarged or reduced, and an optical circuit element optically coupled to the optical waveguide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device having an optical waveguide having an end face from which light is emitted or incident and an optical circuit element optically connected to the optical waveguide.

[0002]

2. Description of the Related Art An optical integrated circuit constructed by forming an optical waveguide on a single substrate and connecting and arranging various optical circuit elements through the optical waveguide is subject to vibration and flexure in response to reduction in size and weight. Optical instability due to mechanical disturbances such as the above is suppressed, operation characteristics are improved, and mass productivity and economical efficiency are further improved, and therefore, they are being used in recent years. In particular, the optical waveguide type modulator has a small required power for modulation by confining and transmitting a light beam in the optical waveguide,
In addition, since it has a wide band characteristic and has a structure suitable for coupling and integration with other elements on the same substrate, it is often used in an optical device formed into a monolithic IC in combination with a semiconductor laser.

FIG. 5 is a diagram showing a configuration example of a conventional optical device. In the figure, an optical waveguide 52 is formed between a clad layer 51 and a semiconductor substrate (not shown) disposed below the clad layer 51, and the DFB semiconductor laser 53 and the external light modulation are provided along the optical waveguide. And a container 54 are arranged. The end portion of the optical waveguide 52 corresponding to the emission port of the external optical modulator 54 is optically exposed to the outside through the antireflection film 55 formed in a flat plate shape along the cleavage surface of the semiconductor substrate and the clad layer 51 described above. Be combined with. The DFB semiconductor laser 53 has an electrode 56 formed along the optical waveguide 52.
And the external light modulator 54 also has an electrode 57 formed along the optical waveguide 52.

In the optical device having such a structure, the DFB
The semiconductor laser 53 emits laser light according to a predetermined electric signal given to the electrode 56. Such laser light is guided to the external optical modulator 54 via the optical waveguide 52,
The light is modulated according to the modulation signal given to the electrode 57 while propagating in the optical waveguide and is emitted through the antireflection film 55.

Further, the antireflection film 55 is formed by individually forming thin films on both surfaces by coating or the like, and by presetting the optical path difference of the reflected light from these thin film surfaces to the above-mentioned half wavelength of the laser light, A part of the reflected light to the external optical modulator 54 side is suppressed by the optical path difference. Therefore, the antireflection film 55 suppresses the incidence of the reflected light on the DFB semiconductor laser 53, and changes the intensity and the phase of the reflected light according to the modulation depth of the external light modulator 54 to the DFB semiconductor laser 53.
The oscillation frequency of the semiconductor laser 53 and the emission intensity of the laser light are stabilized.

Further, in the communication system for performing coherent optical transmission, the suppression of such reflected light is such that the frequency, phase and amplitude of the optical signal are modulated at the transmitting end according to the transmission information, and further at the receiving end. Since the demodulation process is performed after mixing the optical signal and the locally oscillated optical signal, it has to be performed particularly sufficiently.

[0007]

By the way, in such a conventional optical device, since the spot size of the optical waveguide 52 is as small as about 1 μm, a high manufacturing technique is required to obtain the above-mentioned optical path difference with high accuracy. In addition, even if such an optical path difference is realized, it is extremely difficult to actually obtain an antireflection film having the desired characteristics because the suppression characteristic of reflected light is deteriorated by the diffraction effect, and the DFB semiconductor laser 5
The stability of the operating characteristics of No. 3 could not be kept high.

It is an object of the present invention to provide an optical device capable of sufficiently suppressing fluctuations in operating characteristics due to reflection on the end face of an optical waveguide.

[0009]

SUMMARY OF THE INVENTION The present invention is directed to an optical waveguide formed on a substrate with an optical axis inclined at the end face and gradually expanding or contracting the cross-sectional area, and an optical waveguide optically coupled to the optical waveguide. And a circuit element.

[0010]

In the optical device according to the present invention, a part of the light emitted from the optical circuit element or passed through the optical circuit element and guided to the optical waveguide is reflected by the end face of the optical waveguide and is again guided. Travel in the opposite direction in the waveguide. However, the light reflected in this way becomes more difficult to be guided because the incident angle increases as the inclination angle of the optical axis of the optical waveguide on the above-mentioned end face increases, and the optical waveguide becomes larger as the spot size on the end face increases. It has a characteristic that it becomes difficult for guided light with a large diffraction angle to be guided.

In the present invention, the end surface is formed by gradually expanding or reducing the cross-sectional area according to the distance from the end surface based on such characteristics and forming the optical waveguide by inclining the optical axis with respect to the end surface. The spot size at and the tilt angle described above are set, and a desired return loss is obtained according to the combination of these values.

Therefore, in the optical circuit element to which the present invention is applied, the intensity of the light that is re-incident through the optical waveguide is suppressed by the above-mentioned reflection, so that the deterioration or fluctuation of the characteristics caused by the light is suppressed. It will be reduced.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a diagram showing a first embodiment of the present invention.

In the figure, (a) is a horizontal cross-sectional view (hereinafter referred to as "top view") along the optical axis of the optical waveguide as in FIG. 5, and (b) is along the optical axis. FIG. 3 is a vertical sectional view. Note that components having the same configurations as those shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted here.

In the present embodiment, the characteristic feature of the present invention is that the optical axis is tilted by a predetermined angle θ with respect to the plane of the antireflection film 55, and the area of the external optical modulator 54 approaches the emitting end surface. Accordingly, the optical waveguide 11 formed in a tapered shape by linearly reducing the cross-sectional area of the core is provided in place of the conventional optical waveguide 52. Further, reference numeral “12” indicates a semiconductor substrate that forms the waveguide 11 together with the cladding layer 51.

Regarding the correspondence relationship between this embodiment and the invention described in claim 1, the optical waveguide 11 corresponds to the optical waveguide, and the DFB semiconductor laser 53 and the external optical modulator 54 correspond to the optical circuit element. .

FIG. 2 is a diagram for explaining the operating principle of this embodiment. The operation of this embodiment will be described below with reference to FIGS. In the region of the optical waveguide 11 formed into a tapered shape as described above, the emitting end face (antireflection film 55)
The value of the spot size is continuously increased as the distance approaches. In the optical waveguide 11, in general, the larger the spot size, the easier the light with a smaller diffraction angle is guided. further,
The reflected light generated on the emission end face of the optical waveguide 11 becomes difficult to be guided in the optical waveguide 11 because the incident angle increases as the above-mentioned angle θ is set larger.

Therefore, the return loss R _L defined by the relative level of the reflected light on the basis of the Fresnel reflection amount at the exit end face when the angle θ is 0 ° is a theoretical analysis of the above two characteristics. According to the approximate calculation based on the result,
As shown in FIGS. 2 to 2 described above, the larger the spot size W on the emission end face, the larger the value, and the greater the rate of change with respect to the angle θ.

Therefore, in this embodiment, the spot size W is set to about 5 μm and the angle θ is set to 10 degrees so that the characteristic of the DFB semiconductor laser 53 can be adjusted as shown by the dotted line in FIG. And an allowable return loss α (> 50 dB) is achieved, and the oscillation frequency of the DFB semiconductor laser 53 and the emission intensity of the laser light are stabilized.

In this embodiment, the "spot size conversion method using a tapered optical waveguide" is not described in detail, but the present invention is not limited to, for example, IEEE (Photo).
nicsTechnology Letters, Vol.5, no.3, pp.345-347.199
Any of the various single-mode tapered waveguide structures described in the paper of 3.) can be applied, and by setting the taper length to an appropriate value, the radiation loss can be suppressed sufficiently. It is also possible to do so.

FIG. 3 is a diagram showing a second embodiment of the present invention. This figure shows a top view corresponding to FIG. 5 (a). In the figure, the same components as those shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted here.

The structural difference between this embodiment and the embodiment shown in FIG. 1 is that the antireflection film 55 is not arranged on the emission end face and the conventional example shown in FIG. 5 is used in the region corresponding to the DFB semiconductor laser 53. Similarly to the above, the tapered optical waveguide is formed perpendicularly to the emission end face, and is further formed in a region corresponding to the external optical modulator 54 by being bent to the emission end face by a curved waveguide or a curved waveguide using a total reflection mirror. The waveguide 31 is provided in place of the optical waveguide 11.

Regarding the correspondence between this embodiment and the invention described in claim 1, the optical waveguide 31 corresponds to the optical waveguide, and the DFB semiconductor laser 53 and the external optical modulator 54 correspond to the optical circuit element. .

In the optical device having such a configuration, the optical axis of the optical waveguide 31 is a region corresponding to the DFB semiconductor laser 53 and is mirrored with respect to the crystal of the semiconductor substrate in the same manner as in the conventional example and the embodiment shown in FIG. It is formed in the direction of the plane of the index [110], and further in a region corresponding to the external optical modulator 54,
It is adjusted in the direction of the emission end face formed on the cleavage plane indicated by the Miller index [-110].

That is, the region of the optical waveguide 31 corresponding to the DFB semiconductor laser 53 is formed without being inclined with respect to the emission end face, and the optical waveguide 31 is tapered on the emission end face formed on the desired cleavage plane. Since the light is guided through the waveguide, the return loss _RL is set to a large value and the oscillation frequency of the DFB semiconductor laser 53 and the DFB semiconductor laser 53 are maintained in the same manner as in the embodiment shown in FIG. The emission intensity of the laser light is stabilized.

FIG. 4 is a diagram showing a third embodiment of the present invention. In the figure, the same components as those shown in FIG. 5 are designated by the same reference numerals, and the description thereof will be omitted here.

The characteristic feature of the present embodiment is that the optical waveguide 41 is formed in a region corresponding to the DFB semiconductor laser 53 with the emitting end side tapered and has a linear optical axis.
₁ , an optical waveguide 41 ₂ formed in a region corresponding to the external optical modulator 54, having both an incident end and an outgoing end tapered so as to have a linear optical axis, and optical waveguides 41 ₁ , 4
A groove 42 ₁ formed at the coupling portion between 1 and ₂ by the above-described angle θ with respect to the optical axis of these optical waveguides and formed by physical chemical etching or mechanical processing, and an emission end of the optical waveguide 41 ₂ . And a groove 42 ₂ formed in the same manner with an angle θ inclined with respect to the optical axis of the optical waveguide.

[0028] Note that the spot size at the entrance end and exit end of the optical waveguide 41 ₁ exit end and the optical waveguide 41 _2, for simplicity, set to the same value as the embodiment shown in FIG. 1 (= 5 [mu] m) In addition, the angle θ is similarly set to 10 degrees.

Regarding the correspondence relationship between this embodiment and the invention described in claim 1, the optical waveguide 41 ₁ and the groove 42
₁ (optical waveguide 41 ₂ and groove 42 ₂ ) corresponds to the optical waveguide,
The DFB semiconductor laser 53 (external optical modulator 54) corresponds to an optical circuit element.

In the optical device having such a configuration, the optical axes of the optical waveguides 41 ₁ and 41 ₂ are parallel to each other on the cladding layer 51, and the width of the groove 42 ₁ and the width of the groove 42 ₁ are described above. It is arranged so as to be optically connected according to the angle θ. Therefore, in the groove 42 ₁ , the loss due to the reflection is suppressed and the optical coupling between the optical waveguides 41 ₁ and 41 ₂ is efficiently performed.

Further, the groove 42 ₂ has a plane inclined at an angle θ with respect to the optical axis at the exit end of the optical waveguide 41 ₂ to form an exit end face similar to that of the embodiment shown in FIG. Therefore, the optical waveguides 41 ₁ and 41 ₂ are formed on the semiconductor substrate 12 in substantially the same arrangement relationship as the conventional example, and are guided to the emission end face through the tapered waveguides in the same manner as in the above-described respective examples. Therefore, while maintaining the characteristics of the DFB semiconductor laser 53 and the external optical modulator as in the conventional example, the return loss _RL is set to a large value and the oscillation frequency of the DFB semiconductor laser 53 and the emission intensity of the laser light are stabilized. To be done.

In each of the embodiments described above, the monolithic optical integrated circuit in which the DFB semiconductor laser 53 and the external optical modulator 54 are combined and arranged on a single substrate has been described. Not limited to such an optical integrated circuit, if the optical waveguide is used for any of the incident end face, the emitting end face, and the coupling portion between the steps for each optical circuit element, the type of the optical circuit element and the It is applicable regardless of the combination.

In each of the above-mentioned embodiments, an optical device using an optical waveguide having a rectangular cross section is shown. However, the present invention is not limited to such an optical waveguide, and desired light can be guided. Further, as long as a desired return loss can be obtained for the reflected wave generated at the end face, it is applicable to an optical waveguide having a cross section of any shape such as a circle.

Further, in each of the above-mentioned embodiments, the tapered optical waveguide is formed by linearly changing the cross-sectional area in order to set the spot size on the end face to a desired value. If the desired return loss is obtained while keeping the waveguide characteristics of the light within the optical waveguide within the allowable limit, the cross-sectional area is not linearly changed. You may use the formed optical waveguide.

[0035]

As described above, according to the present invention, of the light emitted from the optical circuit element or passed through the element and guided to the optical waveguide, the light is reflected by the end face of the optical waveguide and passed through the optical waveguide. The intensity of the light re-incident on the optical circuit element is suppressed to a smaller value than that of the conventional example. Therefore, in the optical device to which the present invention is applied, the deterioration or variation of the characteristics caused by the light re-incident on the optical circuit element is reduced as described above, and the stability of the operation of the optical circuit element is improved. Performance is improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram illustrating the operating principle of the present embodiment.

FIG. 3 is a diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram showing a third embodiment of the present invention.

FIG. 5 is a diagram showing a configuration example of a conventional optical device.

[Explanation of symbols]

 11, 31, 41, 52 Optical waveguide 12 Semiconductor substrate 42 Groove 51 Cladding layer 53 DFB semiconductor laser 54 External optical modulator 55 Antireflection film 56, 57 Electrode

Claims (1)

[Claims] 1. An optical waveguide formed on a substrate with an optical axis inclined at an end face and a cross-sectional area gradually enlarged or reduced, and an optical circuit element optically coupled to the optical waveguide. Optical device characterized by.