JPH04291306A - Branching ratio changeable coupler - Google Patents

Abstract

PURPOSE:To offer a branching ratio changeable coupler enabling to control branching ratio without causing increase of insertion loss by an optical coupler. CONSTITUTION:Two optical waveguides 11 and 12 of which propagation coefficients are different each other in accordance with polarizing angle mutually approach to form an optical coupling device 13 having L coupling length. An input ports 14 and 15 are installed at an optical waveguide 11 and 12 of an input side of the device 13 and output ports 16 and 17 are installed at the optical waveguides 11 and 12 of an output side of the optical coupling device 13. And a polarization control device 18 using electrooptic effect are installed between the input port 15 and the optical coupling device 13 of the optical waveguide 11.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable branching ratio optical coupler, and more particularly to a variable branching ratio optical coupler used in an optical receiver for coherent optical communications.

0002

2. Description of the Related Art In an optical receiver used for coherent optical communication, a balanced optical receiver 41 and a 3 dB coupler 42 for splitting light incident thereon are used as shown in FIG. 3 in order to improve reception sensitivity. . The 3dB coupler 42 is
It has two waveguides 43 and 44 into which signal light and local light are incident, and these waveguides 43 and 44 are connected to two photodiodes 46 and 4 of the balanced light receiver 41 via a coupling part 45.
7 is optically bonded. In addition, the balanced light receiver 41 is
These two photodiodes 46 and 47 are connected in series so that a reverse bias is applied to both. The connection point between the photodiodes 46 and 47 is connected to a resistor 48.
It is connected to the amplifier 49 as well as to ground via the amplifier 49 .

[0003] Now, the two waveguides 4 of the 3 dB coupler 42
When signal light and local light are incident on 3 and 44, respectively, these signal light and local light are mixed in a coupling part 45,
The light is branched and incident on photodiodes 46 and 47. Then, mixed detection of the signal light and the local light is performed, and an electric signal is extracted from the connection point between the photodiodes 46 and 47, and amplified by the amplifier 49.

0004

[Problems to be Solved by the Invention] By the way, the main purpose of this balanced photoreceiver is to reduce the intensity noise of the laser light source that serves as local light, and therefore improves the degree of in-phase signal suppression that cancels the intensity noise by bringing it into the same phase. It is required to do so. For this purpose, it is necessary to equalize the quantum efficiency of the light receiving element, to set the branching ratio of the optical coupler to 1:1, and to reduce the delay time difference from the branching point to the light receiving element.

A conventional method is to use an optical fiber coupler formed by fusion welding two optical fibers, but in this case, it is difficult to suppress the variation in the branching ratio of 1:1 to less than ±1%. It was also difficult to match the distance from the branch point to the light receiving element. Therefore, a method has been adopted in which an optical attenuator is inserted between the light receiving element and the coupler to attenuate the optical power, and the optical power is adjusted to match the optical power incident on the light receiving element.

However, this method of inserting an optical attenuator causes an increase in insertion loss, and there remains the problem that it is difficult to adjust the fiber length to several mm or less. Furthermore, even in optical couplers using glass waveguides, it is difficult to suppress variations in the branching ratio of 1:1 to less than ±1%. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a variable branching ratio optical coupler that can control the branching ratio without causing an increase in insertion loss in the optical coupler.

[0007]

[Means for Solving the Problems] FIG. 1 is a diagram illustrating the principle of the present invention. FIG. 1(a) is a schematic diagram illustrating a variable branching ratio optical coupler according to the present invention. Two optical waveguides 11, 1 whose propagation constants differ depending on the polarization angle
2 form an optical coupling portion 13 having a coupling length L in close proximity to each other. The optical waveguides 11 and 12 on the input side of the optical coupling section 13 are provided with input ports 14 and 15, and the optical waveguides 11 and 12 on the output side are provided with output ports 16 and 17. Then, the input port 15 of the optical waveguide 11 and the optical coupling section 13
A polarization control section 18 using an electro-optic effect is provided between the two.

[0008]

[Operation] FIGS. 1(b) and 1(c) show the relationship between the coupling length L, the polarization angle θ, and the branching ratio in the variable branching ratio optical coupler of FIG. 1(a). In Figure 1(b), the polarization angle θ is 0° and 90°.
The relationship between the bond length L and the branching ratio represented by tan-1 (P1 /P2) is shown by a solid line and a broken line, respectively. At this time, P1 and P2 are the optical powers output from the output ports 16 and 17, and as shown by the dashed line, t
When an-1 (P1 /P2) = 45°, the branching ratio is 1
:1. Further, FIG. 1(c) shows the relationship between the polarization angle θ and the branching ratio represented by tan-1 (P1/P2) when the bond length L is constant.

Now, in FIG. 1(b), the polarization angle θ=0
The bond length L that gives a branching ratio of 1:1 when the angle is 90° is L1,
When L2 is assumed, the coupling length L of the optical coupling section 13 is set to a predetermined value between L1 and L2. Next, in FIG. 1(c), the polarization angle θ at which the branching ratio is 1:1 is determined. That is, light is input from the input port 15, its polarization angle θ is controlled by the polarization controller 18 using the electro-optic effect, and the light is input to the output port 1.
The branching ratio of the optical powers P1 and P2 outputted from 6 and 17 is set to 1:1.

In this way, according to the variable branching ratio optical coupler shown in FIG. By changing the output port 16,
The output ratio from 17 can be controlled to 1:1. Therefore, for example, if the light incident from port 15 is local light from a laser light source, the output ports 16 and 1 of the local light are
Since the output ratio from 7 is 1:1, the degree of in-phase signal suppression is improved and intensity noise can be significantly reduced. Therefore, by controlling the polarization angle of the signal light to match the polarization angle of the local light and inputting it from the input port 14, the optical power P1 of the signal light and the local light, which have the same polarization plane.
, P2 are input to the balanced photoreceiver at a ratio of 1:1.

In the above description, the output ports 16 and 17 are used for the purpose of improving the degree of in-phase signal suppression in a balanced optical receiver used for coherent optical communication.
The branching ratio of the optical powers P1 and P2 output from the is set to 1:1.
Although the case has been described, the present invention is not limited to this case. That is, a desired branching ratio can be achieved by controlling the coupling length L of the optical coupling section 13 and the polarization angle θ by the polarization controller 18.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on illustrative embodiments. FIG. 2(a) is a plan view showing a variable branching ratio optical coupler according to an embodiment of the present invention, and FIG.
It is an enlarged cross-sectional view taken along line A'. An I layer is formed on the semi-insulating InP substrate 21 via the undoped InP lower cladding layer 22.
Formed by stacking 25 nP layers and InGaAsP layers, thickness 3.6 μm, band gap wavelength λg = 1.13 μm
m MQW core layers 23 are provided. On the MQW core layer 23, undoped In is patterned into stripes with a ridge width of 7.6 μm and a ridge height of 6.5 μm.
P upper cladding layers 24 and 25 are provided, and two ridge-type waveguides 26 and 27 are formed. At this time, the waveguide loss of the ridge waveguides 26 and 27 is 0.5 dB/cm, and the junction loss with the optical fiber is 0.9 dB/plane.

Further, the interval between the ridge-type waveguides 26 and 27 in the coupling portion 28 was set to 5 μm, and the coupling length L was set to 15 mm. A polarization control section 29 is provided in the ridge-shaped waveguide 26 on the incident side of the coupling section 28 . That is, on both sides of the InP upper cladding layer 24, A with a length of 5 mm is formed.
uGe electrodes 30 and 31 are provided, and a voltage of 0 to 5 V is applied to control the polarization angle θ of light passing through the ridge waveguide 26 from 0° to 180°.

Next, the operation will be explained. Now, light is incident on the ridge-type waveguide 26. And the Au of the polarization control section 29
A predetermined voltage in the range of 0 to 5 V is applied to the Ge electrodes 30 and 31. As a result, an electric field is applied to the MQW core layer 23 below the InP upper cladding layer 24 sandwiched between the AuGe electrodes 30 and 31. Therefore, in the polarization control section 29, MQ
The W core layer 23 produces an electro-optic effect. Therefore, the polarization angle θ of the light passing through the MQW core layer 23 of the polarization control section 29 changes during the passage. By changing the polarization angle θ in this way, it is possible to change the coupling coefficient of the coupling unit 28 and control the branching ratio of the optical powers P1 and P2 output from the coupling unit 28.

Note that when this variable branching ratio optical coupler is used in a balanced optical receiver for coherent optical communication, the branching ratio of the optical powers P1 and P2 output from the coupling section 28 is adjusted to improve the degree of in-phase signal suppression. Although it is desirable to control the polarization angle θ at 1:1, the coupling portion 28 is used to control the polarization angle θ.
The polarization controller 2 applies a bias voltage to zero the DC output voltage across the resistor of the balanced photoreceptor provided on the output side of the polarization controller 2.
The method of applying voltage to the AuGe electrodes 30 and 31 in No. 9 may be used.

Furthermore, in the above embodiment, a case was described in which the variable splitting ratio optical coupler according to the present invention was manufactured monolithically. Even if this is done, the effects of the present invention can still be achieved.

[0017]

As described above, according to the present invention, two optical waveguides are arranged in parallel, and the propagation constant between them differs depending on the polarization angle of the incident light. A polarization controller is provided on at least one of the two optical waveguides on the incident side, and by changing the polarization of the incident light with the polarization controller, the coupling coefficient in the directional coupler is changed, and the directionality is changed. Since the branching ratio of the two output lights output from the coupler is controlled, the branching ratio can be controlled without causing an increase in insertion loss in the optical coupler.

[0018] As a result, in coherent optical communication, it is possible to control the branching ratio of the optical coupler to 1:1 and input the signal into a balanced optical receiver, thereby improving the degree of in-phase signal suppression and reducing intensity noise. be able to.

[Brief explanation of the drawing]

FIG. 1 is a diagram illustrating the principle of a variable branching ratio optical coupler according to the present invention.

FIG. 2 is a plan view and an enlarged cross-sectional view of a variable branching ratio optical coupler according to an embodiment of the present invention.

FIG. 3 is a schematic diagram showing a 3 dB coupler used in a conventional balanced optical receiver.

[Explanation of symbols]

11, 12... Optical waveguide 1 whose propagation constant varies depending on the polarization angle
3... Optical coupling parts 14, 15... Input ports 16, 17... Output port 18... Polarization control part 21... Semi-insulating InP substrate 22... InP lower cladding layer 23... MQW core layer 24, 25... InP upper cladding layer 26, 27... Ridge waveguide 28... Coupling section 29... Polarization control section 30, 31... AuGe electrode 41... Balanced photodetector 42... 3 dB coupler 43, 44... Waveguide 45... Coupling section 46, 47... Photodiode 48... Resistor 49...Amplifier

Claims (1)

[Claims] 1. A directional coupler in which two optical waveguides are arranged in parallel, the propagation constant between the two being different depending on the polarization angle of the incident light, and the two optical waveguides on the input side of the directional coupler. and a polarization controller provided on at least one of the directional couplers, and the coupling coefficient in the directional coupler is changed by changing the polarization of the incident light by the polarization controller,
A variable branching ratio optical coupler, characterized in that the branching ratio of two output lights output from the directional coupler is controlled.