JPH1188299A - Time multiplexed optical circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a time multiplexed optical circuit of a small loss and a small size by inserting a TM/TE mode conversion circuit into at least one route among routes from a branch circuit to a wave synthesizing circuit so as to constitute a time delay circuit. SOLUTION: For example, a light input pulse train 31 is incident by a TM mode, divided into first and second routes 18 and 19 at the Y branching part of a Y branch circuit 13 and the light pulse trains are modulated by the modulation part of each. A pulse train 33 is transformed from a TM mode to a TE mode at the first route 18. A pulse train 34 is wave-synthesized by keeping the TM mode at the second route 19 on the other hand. At each mode propagating a delay circuit 15, time delay at the time of emitting varies depending on the propagating constant difference between them. At the time of properly setting the length of the waveguide of the circuit 15 between the pulse trains including a time delay difference at a Y branch wave synthesizing circuit 16 so as to overlap the other pulse train, the light pulse train 35 (TM&TE coexisting) of a light output 20 provided with a double transmission rate is made.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a time multiplexing optical circuit for increasing transmission capacity by time multiplexing optical pulses.

[0002]

2. Description of the Related Art FIG. 7 shows an example of a method for increasing the transmission capacity of an optical transmission system. As shown in FIG.
A branch portion 03 composed of the optical waveguide 02 formed above,
An optical transmission system is configured by a time multiplexing optical modulation circuit including a modulation unit 04, a delay unit 05, and a multiplexing unit 06. The light input pulse train 011 from the light input 07 is split into two by the branching unit 03 and input to the modulation unit 04. The optical pulse trains modulated by different signals (the optical pulse train 012 in the first path 08 and the optical pulse train 012 in the second path 09) have a time delay corresponding to the optical path length difference in the delay unit 05, and the multiplexing unit 0
6 and are output as an optical output pulse train 014 from the optical output 010 without interference. As a result, the interval between the pulse trains is halved, and the transmission speed is effectively doubled.

[0003]

In practice, the optical path length difference required for time delay is, for example, 400 Gbit / s.
If an optical pulse train of 80 Gbit / s is to be duplicated by using an optical pulse train of the above-mentioned method, about 3.8 mm is required in vacuum. For example, as in the first path 08 of the delay unit 05 in FIG. 7, an Ω-shaped waveguide is often used to cause an optical path length difference. Therefore, when the size of the substrate is constant, a curved waveguide having a small radius of curvature is required, and there is a problem that excess loss increases. Further, when the radius of curvature is large, there is a problem that the substrate becomes large.

[0004] In view of the above-mentioned conventional problems, an object of the present invention is to provide a small time multiplexed optical circuit with a small loss.

[0005]

According to a first aspect of the present invention, there is provided a signal processing apparatus comprising: a circuit for branching an input optical pulse train into two or more circuits; and a circuit for combining the optical pulse trains. In the multiplexed optical circuit, a TM / TE mode conversion circuit is inserted in at least one of two or more paths from the branch circuit to the multiplexing circuit, and the difference between the propagation constants of the TM mode and the TE mode is determined. Is used to constitute a time delay circuit.

A second aspect of the present invention is characterized in that, in the first aspect, the branch circuit is constituted by a TM / TE mode splitter.

A third aspect of the present invention is characterized in that, in the first aspect, the multiplexing circuit is constituted by a TM / TE mode combiner.

According to a fourth aspect of the present invention, in the first to third aspects, a circuit capable of modulating an optical pulse train is inserted in a path from the branch circuit to the multiplexing circuit.

The invention of claim 5 is characterized in that, in any of claims 1 to 4, a delay adjusting section is provided in the time delay circuit.

[0010]

Embodiments of the present invention will be described below. The present invention is not limited to this.

(First Embodiment) FIG. 1 shows a time multiplexed optical circuit according to a first embodiment of the present invention. As shown in FIG. 1, the present embodiment employs, for example, a LiNbO ₃ substrate 1
A Y-branching circuit 13 that divides an input optical pulse train 31 formed on the Y-axis into two or more, a modulation circuit 14 that modulates the optical pulse, and a Y-branching multiplexing circuit 16 that multiplexes the optical pulse train
And a delay circuit 15 for delaying the time between the optical pulses, a TM / TE mode conversion circuit 32 is inserted in the first path 18 of the Y-branch multiplexing circuit 16. Things.

In the above configuration, for example, the optical input pulse train 31 is incident in the TM mode. The Y-branch of the Y-branch circuit 13 is divided into a first path 18 and a second path 19, and the optical pulse trains are modulated by the respective modulators. In the first path 18, the pulse train 33 converts the TM mode into the TE mode,
On the other hand, in the second path 19, the pulse train 34 is multiplexed in the TM mode. Each mode propagating through the delay circuit 15 is
The time delay at the time of emission differs depending on the difference between the propagation constants. If the length of the waveguide of the delay circuit 15 including the time delay difference in the multiplexing circuit 16 is appropriately set so that the other pulse train overlaps between the pulse trains, the optical output 20 having a double transmission speed can be obtained. The light output pulse train 35 (TM & TE mode mixture) is obtained.

(Second Embodiment) FIG. 2 shows a second embodiment of the present invention.
Is shown. In the present embodiment shown in FIG. 2, in a circuit similar to that shown in FIG.
In the example shown in FIG. In the case of the circuits shown in FIGS. 1 and 7 described above, the optical pulse of each path suffers a loss of about 3 dB in the Y multiplexing unit, while the Y
By configuring the multiplexing unit with the TM / TE mode combiner 41, it can be configured with almost no loss.

(Third Embodiment) FIG. 3 shows a third embodiment of the present invention.
Is shown. In the present embodiment shown in FIG. 3, in a circuit similar to that shown in FIG.
8 and the second path 19, the TM / TE mode conversion circuit 3
2 and 32 are provided, the Y branch portion of the branch circuit 13 is configured by a TM / TE mode splitter 42, and the delay circuit 15 is provided with a delay adjustment unit 43. However,
The light input pulse train 17 changes its polarization direction with respect to the substrate.
The light is incident at an angle of about 45 ° to excite the TM mode and the TE mode. Also in this embodiment, TM
When the loss of the / TE mode conversion circuit and the propagation loss difference between the TM mode and the TE mode are large, since the same loss occurs in each path, the magnitude of the pulse train in each path hardly fluctuates. Further, as the delay adjusting unit 43,
For example, a configuration may be employed in which the refractive index of the waveguide is changed using a thermo-optic effect or an electro-optic effect.

(Fourth Embodiment) FIG. 4 shows a fourth embodiment of the present invention.
Is shown. In the present embodiment, an embodiment in which the delay circuit 15 shown in FIG. 2 is constituted by a birefringent polarization maintaining optical fiber will be described. Here, for example, a birefringent polarization maintaining optical fiber 44 is used as the delay unit, and a difference in propagation constant between the x polarization and the y polarization of the birefringence polarization maintaining optical fiber 44 is used. In addition, other waveguide substrates may be coupled in addition to the birefringent polarization maintaining optical fiber.

(Fifth Embodiment) FIG. 5 shows a fifth embodiment of the present invention.
Is shown. In the present embodiment, the TM / TE mode combiner 41 and the TM / TE shown in FIG.
The circuit having the mode splitter 42 has a structure excluding the delay circuit 15, and has a TM path in the second path 19.
Two / TE mode conversion circuits 32, 32 are inserted. In the present embodiment, the TM mode having good modulation efficiency before and after the modulator is used, and a time delay is generated by using the propagation constant difference between the TM mode and the TE mode of the branching unit and the multiplexing unit.

(Sixth Embodiment) FIG. 6 shows a sixth embodiment of the present invention.
Is shown. This embodiment is an embodiment of the present invention in which the optical pulse train is duplicated and the propagation speed is doubled. The configuration is such that the modulation circuit 14 is removed from the circuit shown in FIG. With such a configuration,
An optical pulse train having twice the transmission speed can be produced without a modulation unit.

In the above embodiment, as the TM / TE mode conversion circuit 32, for example, a groove may be formed in the waveguide and a half-wave plate whose main axis is inclined by 45 ° may be inserted. Also, TM / TE mode splitter or TM
As a / TE mode combiner, for example, a configuration in which the refractive index of a part of the Y-shaped waveguide is different in each mode may be adopted. If these circuits have similar functions,
It is not limited to the above configuration at all.

In the above embodiment, the branching and multiplexing circuit has a Y-shaped configuration, but a directional coupler type or an X-shaped configuration may be used. In the above-described embodiment, the modulation efficiency of the TM mode is assumed to be good, but the present invention is not limited to this.

Further, in the above embodiment, the case of two branches is shown, but the same configuration can be applied to the case of three branches or four branches.

Further, the branch circuit, the modulation circuit, the multiplexing circuit, and the delay circuit may be formed on different substrates and connected to each other. Further, not only the waveguide but also individual components may be combined.

The substrate is made of a material or a material capable of forming a waveguide having structural birefringence, for example, LiNbO ₃ or Ga.
Any material such as As, Si, and glass may be used.

[0023]

As described above, the time multiplexing optical circuit according to the present invention includes the TM / TE in at least one of two or more paths from the branch circuit to the multiplexing circuit.
Since a time delay circuit is configured by inserting a mode conversion circuit and using the difference between the propagation constants of the TM mode and the TE mode, the problem of the conventional device can be solved, and a small loss and small time multiplexing optical circuit can be easily realized. realizable.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a first embodiment.

FIG. 2 is a schematic diagram showing a second embodiment.

FIG. 3 is a schematic diagram showing a third embodiment.

FIG. 4 is a schematic diagram showing a fourth embodiment.

FIG. 5 is a schematic view showing a fifth embodiment.

FIG. 6 is a schematic diagram showing a sixth embodiment.

FIG. 7 is an example of a conventional optical transmission system.

[Explanation of symbols]

 Reference Signs List 11 substrate 12 optical waveguide 13 branch circuit 14 modulation circuit 15 delay circuit 16 multiplexing circuit 17 optical input 18 first path 19 second path 20 optical output 31 input optical pulse train 32 TM / TE mode conversion circuit 33 pulse train 34 pulse train 35 light output Pulse train 41 TM / TE mode combiner 42 TM / TE mode splitter 43 Delay adjuster 44 Birefringent polarization maintaining optical fiber

Claims (5)

[Claims] 1. A time multiplexing optical circuit comprising a circuit for branching an input optical pulse train into two or more, and a circuit for multiplexing the optical pulse train, wherein a time multiplexing circuit between the branching circuit and the multiplexing circuit is provided. A time multiplexing optical circuit, wherein a TM / TE mode conversion circuit is inserted in at least one of the above paths, and a time delay circuit is configured using a difference between propagation constants of the TM mode and the TE mode. . 2. The time multiplexing optical circuit according to claim 1, wherein said branch circuit is constituted by a TM / TE mode splitter. 3. The time multiplexing optical circuit according to claim 1, wherein said multiplexing circuit is constituted by a TM / TE mode combiner. 4. The time multiplexing optical circuit according to claim 1, wherein a circuit capable of modulating an optical pulse train is inserted in a path from the branch circuit to the multiplexing circuit. 5. The time multiplexed optical circuit according to claim 1, wherein a delay adjusting unit is provided in the time delay circuit.