JPH04234733A - Directional coupler type optical function element - Google Patents

Abstract

PURPOSE:To provide the optical function element of a directional coupler type having a high extinction ratio characteristic. CONSTITUTION:The coupling part C0 of the 1-input 2-output directional coupler consisting of a semiconductor or dielectrics is formed by optically connecting a front partial coupling part C3, a front partial coupling part C4 with electrodes, central partial coupling part C5, rear partial coupling part C6 with electrodes, and rear partial coupling part C7 of respectively prescribed lengths in this order form the input side. The coupling state in the front partial coupling part and the coupling state in the rear coupling part and an exit side lead part C2 optically connected thereto are offset and coupling symmetry is equivalently attained, by which the deterioration in the extinction ratio in the cross state is eliminated. In addition, the central partial coupling part of an adequate length is formed, by which the extinction ratio in the through state is maintained at a high level. The high extinction ratio characteristic is thus attained in both of the cross state and the through state.

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 more specifically, it has an extremely high extinction ratio characteristic and is used for optical switches, optical polarization splitters, optical modulators, and optical multiplexers. The present invention relates to a directional coupler type optical functional device suitable for use in wave devices and the like.

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. 12 and 13, respectively. Figure 1
The element indicated by 2 is a 2-input/2-output element, and is shown in Figure 13.
The element shown is a one-input/two-output element.

In FIG. 12, 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 Co of length L. is configured. Joint part C
0 of the optical waveguides A and B, respectively, at the input ends A1,
Curved optical waveguides D1, D2, D3, and D4 having a path width of W and a radius of curvature R are optically connected to B1 and the output ends A2 and B2, respectively, to form an input side lead portion C1 and an output side lead portion C2. There is. Furthermore, the curved optical waveguides D1, D2, D3, and D4 are replaced with straight optical waveguides E1, E2, E3, and E, each having a path width of W.
4 are optically connected at a distance G0 between the centers of their respective path widths. Then, the optical waveguides A and B at the coupling part Co
Electrodes F1, F2, F3, F4 are loaded on top of
From here, electrical signals can be introduced into the optical waveguide. Note that the distance between the electrode F1 and the electrode F3, and between the electrode F2 and the electrode F4 is almost zero.

[0004] If the straight optical waveguide E1 is used as an input port, the straight optical waveguides E3 and E4 become a through port and a cross port, respectively. In the case of the 1-input, 2-output device shown in FIG. 13, in the 2-input, 2-output device shown in FIG. 12, one linear optical waveguide E0 is directly optically connected only to the input end A1 of the optical waveguide A. It has become. In this device, the straight optical waveguide E0 is an input port, and the straight optical waveguides E3 and E4 are a through port and a cross port, respectively.

[0005]

By the way, in order to incorporate these above-mentioned elements into the fiber communication system which is currently in the stage of implementation, it is necessary to prevent the occurrence of errors due to crosstalk. Therefore, a device with low crosstalk, ie, high extinction ratio characteristics, is required. However, in the case of the element shown in FIG. 12, the electrodes F1, F2, F3,
By introducing an electrical signal from F4, complete coupling can be realized and the extinction ratio in the cross state can be increased infinitely, but in the through state, the extinction ratio is theoretically about 20 to 30 dB at most. In addition, in the case of the device shown in FIG. 13, the extinction ratio in the through state is 1
Although it is possible to increase the extinction ratio by about 0 dB, in the cross state, an extinction ratio of only about 10 to 20 dB can be obtained.

[0006] 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.

[0007] The extinction ratio referred to here is expressed by the following formula: 10log10(|r|2 /| s|2). Among the optical functional elements having the above-mentioned structure, examples of those having relatively high extinction ratio characteristics include P. Granestrand et al.
g. The optical switch with an extinction ratio of about 27 dB announced at IGWO '86, and the H. M. The extinction ratio of 28 was presented by Mak et al. at the 1990 Fall National Conference of the Institute of Electronics, Information and Communication Engineers C-216.
Optical polarization splitters of about dB are known.

On the other hand, optical functional elements used in optical communication systems are required to have an extinction ratio of 15 dB or more. Therefore, research into the cause of the low extinction ratio of directional coupler type optical functional devices is being actively conducted. Among these studies, for example, Jean
- Pierre Weber et al., IEEE.
J. of Quantum Electronics
(Vol 24. March. 1988)
It has been announced that the reason for this low extinction ratio is that it is difficult to control the refractive index to produce the necessary switching state in a directional coupler.

[0009] Also, L. McCanghan et al. also published IEEE. J. of Quantum Ele
ctromics (Vol. QE-22. No.
6. June. (1986) announced that the cause is that the refractive index of the waveguide becomes non-uniform along the extending direction of the optical waveguide (light propagation direction). Furthermore, T. K.
Findakly et al. of light
ve Technology (Vol 6. No.
1. January. 1988), there is a slight amount of coupling at the input and output side leads for connecting these elements with optical fibers, and this makes it impossible to obtain a switching state in the through state. It is pointed out that a decrease in the extinction ratio is inevitable.

By the way, among these three announced causes, Jean-Pierre Weber
It is not impossible to eliminate the causes announced by et al. by appropriately selecting the electrode driving method and the constituent material of the optical waveguide. Also, L. McCangha
The cause announced by N et al. can be solved by improving control of film growth during the formation of the optical waveguide. However, T. K. The cause pointed out by Findakly et al. will remain an unsolvable problem until the outer diameter of the optical fiber to be connected is reduced to about several μm and the lead portion of the element becomes unnecessary.

The present invention solves the above-mentioned causes of the low extinction ratio of the directional coupler type optical functional element and redesigns its configuration, thereby creating a novel device with an extinction ratio characteristic of 30 dB or more. The purpose of the present invention is to provide a directional coupler type optical functional device having a structure.

0012

[Means for Solving the Problems] In order to achieve the above-mentioned object, the present invention provides a uniform width material made of a material that exhibits an electro-optical effect or a material that has a structure that allows control of the refractive index by introducing an electric signal. It has a coupling part of length L in which two optical waveguides are arranged in parallel, and only one of the two optical waveguides of the coupling part connects one straight optical waveguide with light at its input end. In a 1-input/2-output directional coupler in which the output ends of the two optical waveguides of the coupling part are optically connected to curved or straight optical waveguides to form an output side lead part, p1 , p2, p3, are p1 + p2
+p3 < 1 (however, p1≠0) When it is a decimal or zero that satisfies, the above-mentioned joint part is a front part joint part of length p1 × L, length (1-p1 - p2 - p3 ) × L/
2, a central partial joint portion with length p2×L, a partial joint portion with rear electrodes having the same length as the front electrode partial joint portion, and a rear partial joint portion with length p3×L. There is provided a directional coupler type optical functional device characterized by optically connecting the following in this order.

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 similar to the conventional one-input/two-output directional coupling shown in FIG. It is no different from a container-type optical functional device. First, at the joint C0, the width is equal (
Two optical waveguides A and B with a path width W) are arranged in parallel with a minute interval G, and a straight optical waveguide E0 with a path width W is optically connected to the input end A1 of one of the optical waveguides A. Configuring ports. Curved optical waveguides D3 and D4 having a radius of curvature R and a path width W are optically connected to the output ends A2 and B2 of the optical waveguides A and B, respectively, to form an output side lead part C2, and these curved optical waveguides Wave paths D3, D4
In this case, straight optical waveguides E3 and E4 with a path width W are optically connected with a distance G0 between the centers of the path widths, and a through port (
E3) and 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, for example, a straight optical waveguide having a minute taper angle θ may be interposed from the output ends A2, B2 to the straight optical waveguides E3, E4. may be configured. 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. For example, it can be formed by laminating multiple layers of semiconductor materials such as GaAs/AlGaAs using the MOCVD method.

The coupling portion C0 extends from its incident end A1 to its output ends A2 and B2, and includes a front coupling portion C3, a partial coupling portion C4 with a front electrode, a central coupling portion C5, a partial coupling portion C6 with a rear electrode, and a rear portion. The coupling portion C7 is optically connected in this order. The length of each partial joint is now the total length of the joint C0 as L, p1, p2,
p3 is expressed as: p1 +p2 +p3 <1 (however, p
1 ≠ 0), the front partial joint C3 is p1 × L, the front electrode partial joint C4 is (1 - p1 - p2 - p3 ) × L/2, the central partial joint C5 is p2×L, partial coupling part C6 with rear electrode is (1-p1-p2-p3)×L/2,
The latter part connecting portion C7 has p3×L.

In this case, coefficients p1 and p3 have length p
The length of the connection at the front partial connection part C3 of 1×L is p3.
×L rear part connecting part C7 to output side lead part C2
The element is selected so that it matches the coupling of the parts including the elements, and the element as a whole is equivalent to having a completely symmetrical entrance side lead part and output side lead part. Further, the coefficient p2 is selected as a coefficient that achieves the maximum value when the extinction ratio in the through state is measured by varying the length of the central partial coupling portion C5.

Furthermore, the mutual connection state of the electrodes F1, F2, F3, and F4 in the partial coupling portions C4 and C6 varies depending on the material of the optical waveguide used. For example, when the optical waveguide material is a semiconductor, As shown in Figure 2,
An inverted Δβ structure in which electrodes F1 and F4 and electrodes F1 and F4 are connected by leads f1 and f2, respectively, may be used. If the optical waveguide material is a dielectric such as LiNbO3, the structure shown in FIG. Just connect as shown.

[0018]

[Operation] In the case of the directional coupler type optical functional device of the present invention, the coupling state in the front-stage partial coupling portion C3 and the coupling state in the combined portion of the rear-stage partial coupling portion C7 and the output side lead portion C2 match, so that This directional coupler as a whole is in a state equivalent to having a completely symmetrical input lead part and output lead part, with the front-stage electrode-equipped partial coupling part C4, the central part-coupling part C5, and the rear-stage electrode-equipped partial coupling part C6. is expressed, and as a result, extinction ratio deterioration in the cross state is eliminated.

Further, the length of the central partial coupling portion C5 is set to such a length that the extinction ratio in the through state is maximized. Therefore, in both the cross state and the through state, this optical functional element has a significantly improved extinction ratio compared to the conventional structure.

[0020]

EXAMPLE An optical functional device as shown in the plane pattern diagram of FIG. 4 was manufactured. This optical functional element is the element shown in FIG. 1 with p3 = 0, that is, the optical waveguide A
, B are directly connected to the curved optical waveguide D3,
It has a structure in which it is optically connected to D4 and does not have a subsequent partial coupling part.

In the figure, the length of the joint C0 is 7.5
mm, the distance G between the optical waveguides A and B is 3.5 μm, the distance G0 between the path width centers of the through port E3 and the cross port E4 is 250 μm, and the curvature of the curved optical waveguides D3 and D4 that constitute the output side lead portion C2. The radius R is 3 in both cases.
0 μm, and the road width W is 7 μm. The length of the front-stage partial joint C3: p1 ×L is 267 μm (p1 = 0
．． 0356), length of central partial joint C5: p2 ×
L is 540 μm (p2 = 0.072), and the lengths of the front-stage electrode-attached partial coupling portion C4 and the rear-stage electrode-attached partial coupling portion C6 are both 3.3465 mm.

[0022] In this joint C0, partial joint parts C4 and C6 with front and rear stage electrodes, and a central partial joint part C5
FIG. 5 is a cross-sectional view taken along line V-V in FIG. 4, respectively.
, the configuration is as shown in FIG. 6, which is a cross-sectional view taken along line VI-VI. That is, on the lower electrode 1 made of AuGeNi/Au, an 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 a 0.5 μm thick buffer layer 3 made of n+ GaAlAs.
A core layer 5 made of As and having a thickness of 1.0 μm is laminated in this order. Furthermore, on this core layer 5, n-G
Cladding consisting of aAlAs 6a, p- GaAlAs
A cladding 6b made of p+ GaAs and a cap 6c made of p+ GaAs are sequentially laminated by MOCVD to form an upper cladding layer 6, whose upper surface is covered with an insulating film 7 such as a SiO2 film, so that the path width is reduced to W. Two optical waveguides A and B are formed in a ridge shape with a distance G between them.

As shown in FIG. 5, at the location where the electrodes F1, F2, F3, and F4 are loaded, a part of the insulating film 7 is removed in the form of a slit to form a window 7a, from which the cap 6 is inserted.
For example, Ti/Pt/Au is deposited on the upper surface of electrode F2.
, F4. In FIG. 6, a lead f2 connecting the electrode F1 and the electrode F4 is formed on the insulating film 7. In FIG.

Optical waveguides A and B thus formed
, the interface between cladding 6a and cladding 6b is pn
A bonding interface 6d is formed. Therefore, electrodes F1, F
When a predetermined electrical signal is introduced from 2, F3, and F4, p
At the n-junction interface, an electro-optic effect, a plasma effect, a band filling effect, etc. occur, and the refractive index of the core layer 5 located directly under the electrode changes, and the coupling state of light between the optical waveguides A and B changes.

In this device, a theoretical characteristic diagram of switching characteristics is shown when TE mode light with a wavelength of 1.3 μm is excited at the input port E0 and a reverse bias voltage is applied to the electrodes to express only the electro-optic effect. It is shown in FIG. When this element was actually driven with a reverse bias voltage, the applied voltage in the cross state was -8.5 due to the measurement system.
When the voltage is V, the extinction ratio is 30 dB or more, and when the applied voltage in the through state is -19 V, it can be evaluated that the extinction ratio is also 30 dB or more.

Next, FIG. 8 shows the variation of the extinction ratio in relation to p2 when the length p2×L of the central partial coupling portion C5 is changed by changing the coefficient p2 in this element. In the figure, the ● mark represents a through state, and the black square mark represents a cross state. As is clear from FIG. 8, even if p2 changes a little, that is, even if the length of the central partial coupling part C5 changes a little, this element still exhibits an extinction of 40 dB or more in both the through state and the cross state. It is possible to obtain the ratio.

Furthermore, considering FIG. 8 and FIG. 9 which will be described later, in order for this element to exhibit the maximum extinction ratio, the coefficient p1 must be
, p2, p3 should be selected to certain values. First, FIG. 9 is a theoretical characteristic diagram showing changes in the extinction ratio at the through port E3 and the cross port E4 when p2 = p3 = 0 and p1 is varied. In the figure, the ● mark represents the extinction ratio of the through port, and the black square mark represents the extinction ratio of the cross port. As is clear from this figure, p
When p1 is 0.032 to 0.039, the extinction ratio of the cross port is 40 dB or more, and p1 is 0.035.
It can be seen that the cross port exhibits the maximum extinction ratio when the front partial coupling portion C3 is formed with a value near 616.

Therefore, for example, in the case of an element in which G0, R, G, W, etc. have the above values, p1 is set to 0.03561.
6, unless p3 is set to 0, deterioration of the extinction ratio in the cross state is unavoidable. Also, p2 is 0.07
When the value deviates from 2, the maximum extinction ratio in the through state does not show 63.06 dB as shown in FIG. However, this device exhibits an extinction ratio of at least 40 dB when p2 is in the range of 0.066 to 0.076, as is clear from FIG.

Next, in the structure of FIG. 4, the length of the joint C0: 6 mm, W: 6 μm, G0: 250 μm, G: 3
．． 0μm, R: 50mm, Length of front part joint part C3: 249μm (p1 = 0.0415), Length of central part joint part C5: 498μm (p2 = 0.083)
, p3 = 0, and an optical functional element was manufactured. Figure 10 shows a theoretical diagram of the switching characteristics of this device when TE mode light with a wavelength of 1.3 μm is excited at the input port E0 and a reverse bias voltage is applied to the electrodes to produce only the electro-optic effect. .

When this element is actually driven with a reverse bias voltage, due to the circumstances of the measurement system, when the applied voltage in the cross state is -8.0 V, the extinction ratio is 30 dB or more, and when the applied voltage in the through state is -8.0 V, the extinction ratio is -8.0 V. At 20.0V, the extinction ratio can be evaluated to be 30dB or more. Next, in this element, when the length p2×L of the central partial coupling portion C5 is changed by changing the coefficient p2, the variation in extinction ratio is shown in FIG. 11 as a relationship with p2. In the figure,●
The mark represents a through state, and the black square mark represents a cross state.

The device of this example also has an extinction ratio of 30 dB or more in both the through state and the cross state.

[0032]

Effects of the Invention As is clear from the above description, the directional coupler type optical functional element of the present invention is made of a material that exhibits an electro-optic effect or a material that has a structure that allows the refractive index to be controlled by introducing an electric signal. It has a coupling part of length L in which two optical waveguides of equal width are arranged in parallel, and only one of the two optical waveguides of the coupling part has a straight line at its input end. A 1-input/2-output directional coupling that is optically connected to an optical waveguide, and the output ends of the two optical waveguides of the coupling part are optically connected to a curved or straight optical waveguide to form an output side lead part. In the container, p1, p2, p3 are p1 + p2
+ p3 < 1 (however, p1 ≠ 0) When it is a decimal or zero that satisfies, the above-mentioned joint part is a front part joint part of length p1 ×L, length (1-p1 - p2 - p3 )
×L/2 front-stage electrode-attached partial joint part, length p2 ×L central partial joint part, rear-stage electrode-equipped partial joint part of the same length as the front-stage electrode partial joint part, and length p3 ×L rear stage Since the partial coupling portions are optically connected in this order, by appropriately designing the lengths of the front partial coupling portion and the rear partial coupling portion, the input side of the conventional directional coupler can be It is possible to eliminate the asymmetry in the coupling between the lead part and the output side lead part, and as a result, it is possible to eliminate the deterioration of the extinction ratio in the crossed state. Further, since the central partial coupling portion is formed with a length that maximizes the extinction ratio in the through state, a high level of extinction ratio in the through state can also be achieved. That is, the optical functional device of the present invention exhibits a high extinction ratio in both the through state and the cross state.

Therefore, when the element of the present invention is incorporated into an optical communication system, low crosstalk can be achieved and optical signals can be transmitted more accurately. In addition, in the example, the case where it was driven as an optical switch was explained, but
The optical functional element of the present invention can produce TE mode light and TM light by simultaneously injecting a forward current and applying a reverse voltage from an electrode, for example.
It can also be used as an optical polarization splitter that separates mode 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 another example of electrode connection.

FIG. 4 is a plan pattern diagram showing an optical functional element of an example.

FIG. 5 is a sectional view taken along line V-V in FIG. 4;

6 is a cross-sectional view taken along line VI-VI in FIG. 4. FIG.

FIG. 7 is a theoretical characteristic diagram of switching characteristics of the example element.

FIG. 8 is a graph showing the relationship between coefficient p2 and extinction ratio when p3=0.

FIG. 9 is a graph showing the relationship between coefficient p1 and extinction ratio when p2 = p3 =0.

FIG. 10 is a theoretical characteristic diagram of switching characteristics of another example element.

FIG. 11 is a graph showing the relationship between p2 and extinction ratio in other example elements.

FIG. 12 is a plan pattern diagram of a conventional two-input/two-output directional coupler.

FIG. 13 is a plan pattern diagram of a conventional 1-input/2-output directional coupler.

[Explanation of symbols] 1 Lower electrode 2 Base 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 Window A Optical waveguide A1 Coupling part C0 Incident end A2 Output end B of optical waveguide A Optical waveguide B2 Output end D3, D4 of optical waveguide B Curved optical waveguide E0 Straight optical waveguide (input port) E3 Straight optical waveguide (through port) E4 Straight optical waveguide (cross port) C0 Coupling part C2 Output side lead part C3 Front stage partial joint part C4 Front stage partial joint part with electrode C5 Central partial joint part C6 Rear stage partial joint part with electrode C7 Rear stage partial joint part F1, F2, F3, F4 Electrode f1, f2 Lead L Joint part C0 Length p1, p2, p3, p1 +p2 +p3 <1(
However, p1 ≠ 0) is a decimal or zero W Path width of the optical waveguide G Distance between optical waveguides A and B

Claims (1)

[Claims] Claim 1: A length L in which two optical waveguides of equal width are arranged in parallel and are made of a material that exhibits an electro-optical effect or a material that has a structure in which the refractive index can be controlled by introducing an electric signal.
one of the two optical waveguides of the coupling part.
Only the main optical waveguide is optically connected to one straight optical waveguide at its input end, and the output ends of the two optical waveguides of the coupling part are optically connected to a curved or straight optical waveguide, respectively, and the output side lead part In a directional coupler with 1 input and 2 outputs, p1, p2, p3 are p1 + p2 + p3
1 (however, p1 ≠ 0) is a decimal or zero, the joint part is a front part joint part of length p1 × L, a front part joint part of length (1 - p1 - p2 - p3 ) × L/2 A partial joint with electrodes, a central partial joint with length p2×L, a rear partial joint with electrodes having the same length as the front partial joint with electrode, and a rear partial joint with length p3×L in this order. A directional coupler type optical functional device characterized by being formed by optically connecting.