JPH04355731A - Directional coupler type optical functional element and its driving method - Google Patents

Abstract

PURPOSE:To obtain a >=30dB high extinction ratio in both of a cross state and a through state. CONSTITUTION:The two-input, 2-output directional coupler optical functional element is constituted by providing a coupling part C0 where two optical waveguides A and B which are loaded with propagation constant control means F1, F2, F3, and F4 and equal in propagation constant are arranged in parallel, optically connecting the incidence ends A1 of only one of the two optical waveguides A and B, i.e., optical waveguide A to one straight optical waveguides E1 to form an incidence-side lead part C1, and also optically connecting the projection ends A2 and B2 of the two optical waveguides A and B to curved or straight optical waveguides D3 and D4 to form a projection- side lead part C2 connected to a through port E and a cross port E4; and mode coupling suppressing means F5 and F6 are arranged on the respective optical waveguides D3 and D4 of the projection-side lead part C2.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a directional coupler type optical functional element with a new structure and a method for driving the same. The present invention relates to a directional coupler type optical functional device that can achieve a high extinction ratio and a method for driving the same.

0002

[Background Art] Recently, various optical functional devices having a waveguide-type directional coupler structure have been developed, and optical switches, optical polarization splitters, optical modulators, optical multiplexers/demultiplexers, etc. using them have been proposed. has been done. Planar patterns of examples of conventional directional coupler type optical functional elements are shown in FIGS. 6 and 7, respectively. The element shown in FIG. 6 is a two-input/two-output element, and the element shown in FIG. 7 is a one-input/two-output element.

In FIG. 6, two optical waveguides A and B having the same path width W are arranged close to each other in parallel with an interval G that allows evanescent coupling, thereby forming a coupling portion C0 of length L. is configured. Connecting part C0
The input ends A1 and B of the optical waveguides A and B at
1 and the output ends A2, B2 are curved optical waveguides D1, D2, D3, D4 with a path width W and a radius of curvature R.
are optically connected to form an input side lead portion C1 and an output side lead portion C2. Furthermore, the curved optical waveguide D1
, D2, D3, and D4 each have a road width of W.
The straight optical waveguides E1, E2, E3, and E4 are optically connected at intervals of a distance GF between the centers of their respective path widths. Electrodes F1, F2, F3, and F4 are loaded on the optical waveguides A and B in the coupling portion C0. These electrodes F1 to F4 function as a propagation constant control means for controlling the propagation constant of the optical waveguide located directly below each electrode to a required value by introducing a predetermined electric signal therefrom.

[0004] Here, if the straight optical waveguide E1 is used as an input port, the straight optical waveguides E3 and E4 connected to the output side lead portion C2 become a through port and a cross port, respectively. In the case of the 1-input/2-output element in Fig. 7, Fig. 6
In this two-input/two-output device, one linear optical waveguide E0 is directly optically connected only to the input end A1 of the optical waveguide A to form the input side lead portion C1. In this device, the straight optical waveguide E0 is the input port,
Straight optical waveguide E3 connected to output side lead part C2,
E4 is a through boat and a cross port respectively.

[0005] In order to incorporate these above-mentioned elements into fiber communication systems that are currently being implemented, it is necessary to prevent errors due to crosstalk. Therefore, a device with low crosstalk, ie, high extinction ratio characteristics, is required. By the way, in the case of the element shown in FIG.
By introducing appropriate electrical signals from 1, F2, F3, and F4, it is theoretically possible to achieve a complete cross state.

[0006] However, in the case of a through state, a small amount of coupling occurs in the input side lead part C1 and the output side lead part C2, so a complete through state cannot be realized, and the extinction ratio is about 25 dB at most. It only becomes. In the case of the element shown in Fig. 7, in the through state it is possible to obtain an extinction ratio of about 35 dB, which is higher than that in the case of the element in Fig. 6.However, in the cross state, the symmetry is broken, so the extinction ratio is about 20 dB at most. Only an extinction ratio of .

[0007] As described above, the extinction ratio of the conventional element is low in either the through state or the cross state,
It is not the case that high extinction ratio characteristics are exhibited in both states. Since the extinction ratio characteristic of an optical functional element is defined by the lower extinction ratio of the through state or cross state extinction ratio, only a low value can be obtained as the extinction ratio of the entire element.

[0008] The extinction ratio referred to here is expressed by the following formula: 101og10 ( |r|2 /| s|2). Among the optical functional elements having the above structure, for example, P. Granestrand et al.
．． The optical switch with an extinction ratio of about 27 dB announced at IGWO'86, and the H. M. Extinction ratio 2 presented by Mak et al. at the IEICE 1990 Autumn National Conference C-216
Optical polarization splitters of about 8 dB are known.

[0009] Also, H. M. At the IEICE 1991 Spring National Conference C-224, Mak et al.
An element with a structure as shown in FIG. 8 was proposed. This element has a coupling part C0 of length L, and p1, p2, p3 are p
1 + p2 + p3 < 1 (however, p1 ≠ 0), assuming that it is a small number or zero, the length p1 ×
L's front partial joint C3, length (1-p1-p2
-p3)×L/2 partial coupling part C4 with front stage electrode,
a central partial joint part C5 with length p2×L; a partial joint part C with rear stage electrodes having the same length as the partial joint part with electrodes in the former stage;
6, and a rear partial connecting portion C7 having a length p3×L.

In the case of this element, it is theoretically possible to obtain an extinction ratio of at least about 40 dB.

[0011]

[Problem to be solved by the invention] The element shown in FIG.
It is certainly possible to achieve a high extinction ratio in theory, but
This is limited to the case where the dimensional parameters of each of the above-mentioned partial joints are approximately the same as the theoretical values. However, when actually manufacturing an element, it is not always possible to create each of these partial joints and the like with dimensional accuracy that corresponds to the calculated theoretical value, and there may be slight deviations from the theoretical value.

[0012] When such a problem occurs, the actual coupling state between the optical waveguides at each partial coupling part deviates from the theoretical coupling state obtained by calculation, and the extinction ratio in the cross state or through state becomes A decline in The present invention solves the problems caused by variations in the precision of the dimensional parameters during manufacturing in the device shown in FIG. The purpose of the present invention is to provide a directional coupler type optical functional device.

[0013]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a coupling section in which two optical waveguides are loaded with a propagation constant control means and have propagation constants of the same value and are arranged in parallel. , only one of the two optical waveguides is optically connected to one straight optical waveguide at its input end to form an input side lead part, and the output end of the two optical waveguides is is a 1-input/2-output directional coupler type optical functional element, which forms an output side lead part that is optically connected to a curved or straight optical waveguide and connects to a through port and a cross port, respectively; A directional coupler type optical functional element is provided, characterized in that each optical waveguide is provided with a mode coupling suppressing means, and a driving electric signal necessary for the propagation constant controlling means is introduced, and , a method for driving a directional coupler type optical functional element, characterized in that a mode coupling suppressing means on the cross port side in the output end lead section is operated to realize a through state with a high extinction ratio; A direction characterized in that a necessary driving electric signal is introduced into the constant control means, and the mode coupling suppression means on the through port side of the output side lead section is operated to realize a cross state with a high extinction ratio. A method for driving a sexual coupler type optical functional device is provided.

The basic configuration of the optical functional element of the present invention is shown in FIG. 1 as a plan pattern diagram. As is clear from the figure, the planar pattern of the optical functional element of the present invention is different from that of the conventional one-input device shown in FIG. -No different from a 2-output directional coupler type optical functional device. First, the joint C0
, two optical waveguides A and B of equal width (path width W)
are arranged in parallel with a minute interval G, and a straight optical waveguide E0 having a path width W is optically connected to the input end A1 of one of the optical waveguides A, thereby forming an input side lead portion C1. In addition, each output end A2 of the optical waveguide A and the optical waveguide B
, B2 has a curved optical waveguide D3 with a radius of curvature R and a path width W.
, D4 are optically connected to form the output-side lead part C2, and these curved optical waveguides D3 and D4 are connected to straight optical waveguides E3 and E4 with a path width W, with a distance G between the centers of the path widths.
F is optically connected to each through port (E3)
and a cross port (E4).

Note that in the present invention, the output side lead portion C
2 is not limited to being constructed with a curved optical waveguide as shown in the figure, but may be constructed by interposing a straight optical waveguide with a minute taper angle θ from the output ends A2, B2 to the straight optical waveguides E3, E4. You may. Each of these optical waveguides is made of a material that exhibits an electro-optic effect or a material that has a structure that allows its refractive index to be controlled by introducing an electric signal, and in its natural state, the propagation constants of each other are are equal. For example, GaAs/AlGaAs
It can be formed by stacking semiconductor materials such as in multiple layers using the MOCVD method.

In the coupling portion C0, electrodes F1, F2, F3, and F4 are loaded in each of the optical waveguides A and B in an inverted Δβ structure, thereby forming propagation constant control means. That is, by introducing predetermined electrical signals from these electrodes, it is possible to change the propagation constant of the optical waveguide located directly below them. For example, when the semi-waveguides A and B are made of semiconductor material, as shown in FIG.
, electrode F2 and electrode F3 are respectively connected to leads f1 ,
Just connect with f2.

The optical functional element of the present invention has an output side lead portion C.
2 optical waveguides D3 and D4 are provided with electrodes F5 and F5, respectively.
Loading F6 constitutes a mode coupling suppressing means. That is, when a predetermined forward current is injected from either electrode F5 or F6, a plasma effect or a band filling effect occurs in the optical waveguide D3 or D4 directly under the electrode, causing a decrease in the refractive index. The coupling between the optical waveguides D3 and D4 becomes asymmetric, and as a result, the optical waveguides D3 and D4
Mode coupling of light between the two is suppressed.

When the injection value of the above-mentioned forward current is changed, the suppression state of this mode coupling also changes. For example, when a larger forward current is injected, an optical waveguide equivalently exists at a certain level. It is possible to obtain a state in which the mode coupling becomes zero, that is, a state in which the mode coupling becomes zero. This state is the ultimate state of mode coupling suppression and is called a mode cutoff state.

[0019]

[Operation] In the optical functional element of the present invention, since the input lead part C1 and the output lead part C2 are not symmetrical, the electrodes (inverted Δβ structure) of the coupling part C0 F1 to F4
Even if an appropriate electrical signal is introduced into the circuit, it is not possible to obtain a complete through state or a complete cross state. Therefore, in order to obtain a switching state with a high extinction ratio, it is necessary to remove the slight coupling at the output side lead part C2.
And the entrance side lead part C1 and the output side lead part C2
It is necessary to match the bond with (even if the bond does not exist).

In the optical functional device of the present invention, the coupling portion C
Corresponding to the switching state of 0, for example, in the cross state, the optical waveguide D3 (connected to the through port E3) of the output side lead part C2 is switched, and in the through state, the optical waveguide D4 (connected to the cross port E4) of the output side lead part C2 is ) by operating the respective mode coupling suppressing means F5, F6, the coupling at the output side lead portion C2 can be removed.

Therefore, in this state, since the slight coupling at the output side lead portion C2 is also removed and the cause of extinction ratio deterioration is eliminated, the electrode F of the coupling portion C0 is removed.
By introducing electrical signals to F1 to F4, it becomes possible to realize a cross state and a through state with a high extinction ratio.

[0022]

EXAMPLES Example 1 An optical functional element as shown in the planar pattern diagram of FIG. 3 was manufactured. In the figure, the length of the coupling part C0 is 6.0 mm, the distance G between the optical waveguides A and B is 3.5 μm, and the through port E3
The distance GF between the center of road width and cross port E4 is 2
The radius of curvature R of the curved optical waveguides D3 and D4 constituting the output side lead portion C2 is both 30 μm and the path width W is 7 μm.

The coupling portion C0 and the output side lead portion C2 are as shown in FIG. 4, which is a cross-sectional view taken along line IV--IV in FIG. 3, and FIG. 5, which is a cross-sectional view taken along line V-V in FIG. It is configured. That is, AuGeNi/Au
On the lower electrode 1 consisting of n
A substrate 2 made of + GaAs, a 0.5 μm thick buffer layer 3 made of n+ GaAs, a 3.0 μm thick lower cladding layer 4 made of n+ GaAlAs, and n− Ga.
Core layers 5 made of As and having a thickness of 1.0 μm are laminated in this order. Further, on this core layer 5, n-GaA
A cladding 6a made of lAs, a cladding 6b made of p- GaAlAs, and a cap 6 made of p+ GaAs.
c are sequentially stacked by MOCVD to form an upper cladding layer 6, and its upper surface is covered with an insulating film 7 such as a SiO2 film, thereby forming two optical waveguides A with a path width of W.
, B are formed in a ridge shape with an interval G.

Note that the height h of each optical waveguide was 1.0 μm for each optical waveguide. Further, the gap g between each electrode was set to 3 μm. Electrodes F1, F2, F3
, F4 are loaded as shown in Figure 4.
A part of the insulating film 7 covering the optical waveguides A and B is removed in the form of slits to form windows 7a and 7b, from which the cap 6 is formed.
For example, Ti/Pt/Au is vapor-deposited on the upper surface of electrode F.
3, F4 (F1, F2). The electrode F3 and the electrode F2 are connected by the lead f1, and the electrode F4 and the electrode F1 are connected by the lead f2 to form an inverted Δβ structure, thereby forming a propagation constant control means.

In the output side lead part C2, as shown in FIG. 5, a part of the insulating film 7 covering the optical waveguides D3 and D4 is removed in the form of slits to form windows 7a and 7b. Ti/Pt/Au on the top surface of the cap 6c.
are deposited to form electrodes F5 and F6. Each of these electrodes F5 and F6 is independently connected to an electric signal introduction system, with electrode F5 being a mode coupling suppressing means for optical waveguide D3, and electrode F6 being a means for suppressing mode coupling of optical waveguide D3.
4 as mode coupling suppression means.

In the optical waveguide thus formed, the interface between the cladding 6a and the cladding 6b is a pn junction interface 6d. Therefore, now the electrodes F1, F
When a predetermined electric signal is introduced from 2, F3, and F4, an electro-optic effect, a plasma effect, a band filling effect, etc. occur at the p-n junction interface, and the refractive index of the core layer 5 located directly under the electrode changes, causing light guide. A propagation constant difference Δβ occurs between wave paths A and B, and the coupling state of light changes.

In this device, TE mode light with a wavelength of 1.3 μm is excited at the input port E0, and the electrodes F1 to F
According to theoretical calculations, if a reverse bias voltage is applied to 4 and only the electro-optic effect is produced, 1 in the cross state.
A switching characteristic of about 8 dB is obtained, and about 37 dB in the through state. However, in reality, when the coupling part C0 is driven by applying a reverse bias voltage, the extinction ratio is about 17 dB when the drive voltage in the cross state is -7V, and the drive voltage in the through state is -15V.
In this case, the extinction ratio was about 26 dB, and the deterioration of the extinction ratio was particularly large in the through state.

Therefore, in the crossed state where the driving voltage is -7V, the optical waveguide D3 (
When a suitable value of forward current was injected from electrode F5 on the side (connected to through port E3), the extinction ratio characteristic was 30d.
It was possible to achieve a cross state with an extremely high extinction ratio. In addition, in the through state where the drive voltage is -15V, the optical waveguide D of the output side lead portion C2
4 When an appropriate value of forward current was injected from electrode F6 on the side (connected to cross port E4), the extinction ratio characteristic improved to over 30 dB, making it possible to achieve an extremely high optical extinction ratio through state. .

Example 2 An optical functional element was manufactured in the same manner as in Example 1, except that R was 50 mm. In this device, there is a TE with a wavelength of 1.3 μm at the entrance port E0.
When mode light is excited and a reverse bias voltage is applied to the electrodes F1 to F4 to produce only the electro-optic effect, theoretical calculations show that the radius of curvature of the optical waveguides D3 and D4 is greater than that of Example 1. Because it is large, it is 1 in the cross state.
A switching characteristic of about 8 dB is obtained, and about 33.8 dB in the through state.

However, in reality, when the coupling part C0 is driven by applying a reverse bias voltage, when the drive voltage in the cross state is -8V, the extinction ratio is about 16.5 dB, and the drive voltage in the through state is about 16.5 dB. At −16 V, the extinction ratio was about 24 dB, which was worse than in Example 1. Therefore, in the crossed state where the driving voltage is -8V, the optical waveguide D3 of the output side lead portion C2
When an appropriate value of forward current was injected from electrode F5 on the side (connected to through port E3), the extinction ratio characteristic was 30.
It was possible to realize a cross state with an extremely high extinction ratio.

Furthermore, in the through state where the driving voltage is -16V, the optical waveguide D4 (
When a suitable value of forward current was injected from electrode F6 on the side (connected to cross port E4), the extinction ratio characteristic was 30d.
The optical extinction ratio was improved to B or higher, and a through state with an extremely high optical extinction ratio could be realized.

[0032]

Effects of the Invention As is clear from the above description, the directional coupler type optical functional device of the present invention is equipped with a propagation constant control means and connects two optical waveguides having the same propagation constant in parallel. only one optical waveguide among the two optical waveguides is optically connected to one straight optical waveguide at its input end to form an input-side lead portion, and the two optical waveguides In the 1-input/2-output directional coupler type optical functional element, the output end of the waveguide is optically connected to a curved or straight optical waveguide to form an output side lead part connected to a through port and a cross port, as described above. Each of the optical waveguides in the output side lead section is equipped with a mode coupling suppression means, so that by operating the mode coupling suppression manual, even a slight coupling in the output side lead section can be removed. Can be done. As a result, the cross state,
A high extinction ratio characteristic of 30 dB or more can be achieved in any through state.

Furthermore, as is clear from the devices of Examples 1 and 2, even if the design parameters of the device change, the extinction ratio does not depend on it, and is a high extinction ratio of 30 dB or more. Therefore, the device of the present invention can obtain high extinction ratio characteristics even if manufactured without strictly controlling the dimensional parameters with high precision, and therefore is easy to manufacture and has great industrial value.

[0034] In the embodiment, a case was explained in which the optical functional element of the present invention is driven as an optical switch. However, the optical functional element of the present invention can be operated, for example, by simultaneously injecting a forward current from an electrode and applying a reverse voltage to generate TE mode light and TM mode light. It can also be used as an optical polarization splitter that separates light. Furthermore, when used as an optical modulator or optical multiplexer/demultiplexer, high extinction ratio characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a plan pattern diagram showing the basic configuration of an optical functional element of the present invention.

FIG. 2 is a plan pattern diagram showing an example of electrode connection.

FIG. 3 is a plan pattern diagram showing an optical functional element of an example.

FIG. 4 is a sectional view taken along line IV-IV in FIG. 3;

FIG. 5 is a sectional view taken along line V-V in FIG. 3;

FIG. 6 is a plan pattern diagram of a conventional 2-input/2-output directional coupler.

FIG. 7 is a plan pattern diagram of a conventional 1-input/2-output directional coupler.

[Figure 8] IEICE 1991 Spring National Conference C-
224 is a planar pattern diagram of an optical functional element announced at 224.

[Explanation of symbols]

1 Lower electrode 2 Substrate 3 Buffer layer 4 Lower cladding layer 5 Core layer 6 Upper cladding layer 6a, 6b Cladding 6c Cap 6d PN junction interface 7 Insulating film 7a, 7b Window A Optical waveguide A1 Incident end A2 of optical waveguide A Optical waveguide A Output end B of optical waveguide B1 Input end B2 of optical waveguide B Output end D1, D2, D3, D4 of optical waveguide B Curved optical waveguide E0
Straight optical waveguide (input port) E1 Straight optical waveguide (input port) E3 Straight optical waveguide (through port) E4 Straight optical waveguide (cross port) C0
Coupling section C1 Incident side lead section C2 Output side lead section F1, F2, F3, F4 Electrodes (propagation constant control means) F5, F6 Electrodes (mode coupling suppressing means) f1, f2 Lead h Optical waveguide height g Gap W Optical guide Wavepath width GF Distance between optical waveguides A and B

Claims (5)

[Claims] 1. A coupling section in which a propagation constant control means is loaded and two optical waveguides having the same propagation constant are arranged in parallel, and only one of the two optical waveguides is connected. The input end is optically connected to one straight optical waveguide to form an input side lead part, and the output ends of the two optical waveguides are optically connected to a curved or straight optical waveguide, respectively, to form a through port and a cross port. In the one-input/two-output directional coupler type optical functional device forming the output side lead section connected to the output side lead section, each optical waveguide of the output side lead section is loaded with mode coupling suppressing means. Characteristic directional coupler type optical functional device. 2. The directional coupler type optical functional device according to claim 1, wherein the optical waveguide is made of a material that exhibits an electro-optic effect or a band filling effect. 3. Introducing a necessary driving electric signal to the propagation constant control means of the optical functional element according to claim 1, and operating the mode coupling suppression means on the cross port side in the output end lead part, A method for driving a directional coupler-type optical functional device characterized by realizing a through state with a high extinction ratio. 4. Introducing a necessary driving electric signal to the propagation constant control means of the optical functional element according to claim 1, and operating the mode coupling suppression means on the through port side of the output side lead part, A method for driving a directional coupler type optical functional device characterized by realizing a cross state with a high extinction ratio. 5. The method of driving a directional coupler type optical functional element according to claim 3, wherein the mode coupling suppressing means is operated in a state where mode coupling becomes zero.