JPH079524B2 - Optical demultiplexer - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical multiplex demultiplexer, and more particularly to an optical multiplex demultiplexer used in an optical fiber wavelength multiplex demultiplexing transmission system.

[Conventional technology]

Since the transmission capacity of the optical fiber wavelength division multiplexing / demultiplexing transmission has dramatically increased, research and development have recently been actively conducted. In particular, an optical multiplexing / demultiplexing device that multiplexes and demultiplexes optical signals of different wavelengths is one of important devices for realizing optical fiber wavelength multiplexing / demultiplexing transmission.

Conventionally, as this type of optical multiplexing / demultiplexing element, there is one that multiplexes and demultiplexes an optical signal by using a dielectric multilayer film or a diffraction grating. For details, refer to Optical Fiber Communication (Toshio Soejima et al.), Pages 300 to 303, published by the first edition of December 1981, Telecommunications Technology News Co., Ltd., or Handbook, "Optical Communication Manual," August 1984. The first edition of the month, published from Kagaku Shimbun Co., Ltd., pp. 573 to 583.

This type of optical multiplexing / demultiplexing device has a small number of wavelengths that can be transmitted (about 10 waves) because it is difficult to narrow the wavelength interval of the optical signals that are multiplexed and demultiplexed, which makes it difficult to transmit a large capacity. There is a drawback that

Therefore, recently, by utilizing optical interference, it is possible to multiplex and demultiplex optical signals with a narrow wavelength interval, and a Mach-Zehnder interferometer type of Mach-Zehnder interference type capable of multiplexing and demultiplexing about 100 wavelengths or more. Optical multiplex demultiplexers have been considered. This optical multiplexing / demultiplexing device inputs optical signals of different wavelengths to input terminals of a 2-input / 2-output input optical coupler having a branching ratio of 1: 1 and branches them into two optical waveguides. A phase difference is given between the branched optical signals due to the difference in the optical path lengths, and the two input / output two output optical couplers having a branching ratio of 1: 1 combine again to cause interference. As a result, when the optical demultiplexing element is used as the optical multiplexing element, when the optical signals of different wavelengths multiplexed at one output end of the output optical coupler are also used as the optical demultiplexing element. Output optical signals of different wavelengths, which are respectively demultiplexed, to the two output ends of the output optical coupler.

For details of this optical demultiplexing device, refer to the paper “Proceedings of the 1985 IEICE General Conference” (Lecture No. 26).
46), pp. 10-359.

[Problems to be solved by the invention]

In general, the Mach-Zehnder interference type optical demultiplexing device, as described above, outputs an optical signal branched in a phase of 1: 1 and
The waves are recombined, and due to the interference effect at that time, they are multiplexed or demultiplexed. Therefore, it is necessary that the propagation losses of the two optical waveguides that give the phase difference be the same. Here, if there is a difference in the propagation loss, when used as an optical multiplexing element, the multiplexed optical signals of different wavelengths cause a difference in the propagation loss to the other output end that is not the output end to be originally output. As much as that is output, the loss of the multiplexed optical signal at the original output end increases. When used as an optical demultiplexing element, optical signals having different wavelengths are mixed in the demultiplexed optical signal, crosstalk to the demultiplexed optical signal increases, and the SN ratio deteriorates.

If there is a difference in the propagation loss of the optical waveguide as described above, a great problem occurs when the optical multiplex demultiplexing element is used as the optical multiplex element or the optical multiplex demultiplexing element. However, it is not always easy to fabricate two optical waveguides having no difference in propagation loss, and as a result, there is a major drawback that it is difficult to improve optical signal loss and crosstalk as described above.

The object of the present invention is to eliminate the above-mentioned drawbacks, when used as an optical multiplexing element, it is possible to multiplex an optical signal with the least loss, and when used as an optical demultiplexing element, it is the smallest. An object of the present invention is to provide an optical multiplex demultiplexer capable of demultiplexing an optical signal by crosstalk.

[Means for solving problems]

The present invention, an input branching unit for inputting optical signals of different wavelengths, propagates each output optical signal of this input branching unit,
A Mach-Zehnder interference type optical demultiplexing device having an optical waveguide for giving a phase difference between these output optical signals and an output branching unit for inputting each output optical signal of this optical waveguide and outputting the optical signals of different wavelengths. In at least one of the two branching parts, a branching ratio adjusting part for changing the branching ratio of the branching part is provided.

[Action]

The optical multiplexing and demultiplexing device of the present invention will be described separately for the case where it is used as an optical multiplexing device and the case where it is used as an optical demultiplexing device.

First, let T _{1 be} the transmission coefficient of the optical intensity of the 2-input / 2-output input branch section that inputs the optical signals of different wavelengths, and the 2-input / 2-output output branch section that outputs the optical signals of different wavelengths. Let T _{2 be} the transmission coefficient of intensity, α _{1 be} the loss coefficient of the optical waveguide in the transmission direction of the input branch portion, and α _{2 be} the loss coefficient of the optical waveguide in the reflection direction.

When used as an optical multiplexing device, the multiplexed optical signal power of each wavelength at one output end of the output branch section
P ₁ and P ₂ are Becomes Here, P ₀ is the power of the input optical signal. The transmission coefficient T ₂ of the output branch when the optical signal powers P ₁ and P ₂ are maximum is To be satisfied.

Therefore, when the maximum optical signal power is obtained, the optimum transmission coefficient T ₂ of the output branch section is T ₁ α ₁ / {α ₂ + T ₁ (α ₁ −α ₂ )} for the optical signal power P ₁ . Also, the optical signal power P ₂ is (1-T ₁ ) α ₁ / {α ₁ + T ₁ (α ₂ −α ₁ )}. These transmission coefficients are equal when the transmission coefficient T ₁ of the input branch section is 0.5 (branch ratio is 1: 1), and therefore the optimum transmission coefficient T ₂ of the output branch section where the maximum optical signal power is obtained is Becomes From this, the loss factors α ₁ and α _{2 of the} two optical waveguides
Is different, that is, when there is a propagation loss difference between the optical waveguides, the transmission coefficient T _{2 of} the output branching section, that is, the branching ratio of the output branching section is changed according to the loss coefficient of the optical waveguide to obtain the maximum power multiplexing. It can be seen that the obtained optical signal is obtained.

Next, the case of the optical demultiplexer will be described. Let k ₁ and k ₂ be the crosstalks to the optical signals of the respective wavelengths demultiplexed at the two output ends of the output branching section. Becomes The meaning of each symbol is the same as in the case of the optical multiplex element. Here, in order to make the stroke to the demultiplexed optical signal zero, it is only necessary that each numerator of the equation, becomes zero, that is, when k ₁ = 0, the transmission coefficient of the input branching portion.
T ₁ becomes (1-T ₂ ) α ₂ / {T ₂ α ₁ + T ₁ (1-T ₂ ) α ₂ },
Further, when k ₂ = 0, T ₁ may be T ₂ α ₂ / {T ₂ α ₂ + (1-T ₂ ) α ₁ }. These transmission coefficients are equal when the transmission coefficient T ₂ of the output branch section is 0.5 (branch ratio of 1: 1), and therefore the transmission coefficient T ₁ of the input branch section where the crosstalk becomes zero is Becomes From this, as in the case of the optical multiplex element, when the loss coefficients α ₁ and α _{2 of the two} optical waveguides are different, that is, when there is a propagation loss difference between the optical waveguides, the transmission coefficient T ₁ of the input branch portion, That is, it can be seen that a branched optical signal with zero crosstalk can be obtained by changing the branching ratio of the input branching section according to the loss coefficient of the optical waveguide.

As described above, in both cases of the optical multiplexing element and the optical demultiplexing element, each defect can be eliminated by changing the branching ratio of the branching section.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view showing the structure of the optical multiplexing and demultiplexing device according to the first embodiment of the present invention.

Two input optical waveguides 2 and two output optical waveguides 3 each having a width of about 8 μm and a relative refractive index of about 0.3% diffused with Ti are formed on the surface of the substrate 1 of LiNbO ₃ . By forming the input optical waveguide 2 and the output optical waveguide 3 in close proximity to each other, a directional coupler type input coupler (input branch portion) 4 and an output coupler having a branching ratio of about 1: 1 for light with a wavelength of 1.55 μm An (output branch section) 5 is configured. Two optical waveguides 6a and 6b having different optical path lengths are formed between the input coupler 4 and the output coupler 5 in order to give a phase difference between the optical signals. The optical path difference is about 5 mm so that the wavelength interval of the optical signal is about 0.1 nm. And the output optical coupler 5
Electrodes 7 are formed as branching ratio adjusting portions on the surfaces of the two adjacent output optical waveguides 3 constituting the above. In this branching ratio adjusting section, when voltage is applied to the electrode 7, Li
Due to the electro-optic effect of NbO ₃ , the refractive index of the two adjacent output optical waveguides 3 changes, and the propagation constant difference between these optical waveguides changes. As a result, in the output optical coupler 5, it is possible to change the amount of power at which the optical signal is branched to the output terminals 10 and 11, and thus it is possible to change the branching ratio of the output optical coupler.

Next, the operation of this embodiment will be described. First, in the above configuration, without applying voltage to the electrode 7, the wavelength is about 1.55μ.
Two optical signals λ ₁ and λ ₂ having a wavelength m of about 0.1 nm are input to the input ends 8 and 9 of the input optical coupler 4, respectively. The optical signals λ ₁ and λ ₂ are branched into about 1: 1 by the input optical coupler 4 and propagated to the two optical waveguides 6a and 6b having different optical path lengths. Here, since the two optical waveguides 6a and 6b have different optical path lengths, curvature radii, etc., their propagation losses are different. In this embodiment, as can be seen from FIG. 1, since the optical waveguide 6b has a longer optical path length and a smaller radius of curvature than the optical waveguide 6a, the propagation loss is about 5 dB, which is larger than that of the optical waveguide 6a.
The power of the optical signals λ ₁ and λ ₂ propagating through the optical waveguide 6b is higher.
It is about 5 dB smaller.

Then, the optical signal lambda ₁ propagated through the optical waveguide 6a, lambda ₂ and the optical signal lambda ₁ power than it has propagated through 5dB about small light waveguides 6b, lambda ₂ are multiplexed by the optical coupler 5, The light signals λ ₁ and λ ₂ which interfere with each other and are multiplexed are output to the output terminal 10. However, here, the branching ratio of the output optical coupler 5 is about 1: 1.
Therefore, the optical signal λ ₁ , λ ₂ of maximum power is output to the output terminal 10.
Is not output.

Therefore, by applying a voltage to the electrode 7 and changing the propagation constant difference in the output optical coupler 5, the branching ratio of the output optical coupler 5 is changed and the intensity of the optical signal is maximized at the output end 10. That is, a voltage is applied about 20V to the electrodes 7, the optical signal lambda ₁ outputted from the optical waveguide 6a to the output 10, the optical signal lambda ₁ of the lambda ₂ of the power output from the optical waveguide 6b to the output 10,
The branching ratio of the output optical coupler 5 is changed to about 1: 0.3 so as to be about 70% higher than the power of λ ₂ , and the optical waveguides 6a and 6b are
To approximately the maximum power of the optical signals λ ₁ and λ ₂ output to the output terminal 10. The optical signal λ multiplexed by this
The loss of ₁ and λ ₂ was about 2.2 dB when the branching ratio was about 1 to 1, but improved by about 0.4 dB to about 1.8 dB.

In the first embodiment, only the output optical coupler 5 is provided with an electrode for changing the branching ratio. However, due to manufacturing problems, the input optical coupler 4 is
If it is difficult to make the branching ratio of 1 to about 1: 1, the input optical coupler 4 may be provided with an electrode for changing the branching ratio.
Fig. 2 shows the case where electrodes are provided on both optical couplers.
Will be shown as an example. Electrodes 12 are formed on the surfaces of two adjacent input optical waveguides 2 that form the input optical coupler 4. Since other configurations are similar to those of the optical multiplex demultiplexer of FIG. 1, the same elements are denoted by the same reference numerals. According to this embodiment, the voltage can be applied to the electrode 12 and the branching ratio of the input optical coupler 4 can be adjusted to about 1: 1. The powers of the optical signals λ ₁ and λ ₂ are almost maximum.

FIG. 3 is a plan view showing the configuration of an optical demultiplexing element that is the third embodiment of the present invention. The difference from the configuration of the optical multiplexing / demultiplexing device in FIG. 1 is that the electrodes as the branching ratio adjusting units are formed not on the output optical coupler 5 but on the input optical coupler 4, that is, the proximity optical fibers forming the input optical coupler 4. That is, the electrodes 13 are formed on the surfaces of the two input optical waveguides 2.
Therefore, the same elements as those of the optical multiplex demultiplexer of FIG. 1 are designated by the same reference numerals. With such a configuration, when a voltage is applied to the electrode 13, the refractive index of the two adjacent input optical waveguides 2 changes due to the electro-optic effect of LiNbO ₃ , and the difference in the propagation constant between these optical waveguides changes. To do. Thereby, in the input optical coupler 4, the input optical signal is transmitted to the optical waveguide 6b.
The amount of power that shifts to can be changed, and thus the branching ratio of the input optical coupler 4 can be changed.

Next, the operation of this embodiment will be described. First, in the above configuration, without applying voltage to the electrode 13, the wavelength is about 1.55μ.
m, the multiplexed optical signals λ ₁ , λ with a wavelength interval of about 0.1 nm
₂ is input to the input end 8 of the input optical coupler 4. The input optical coupler 4 splits the optical signals λ ₁ and λ ₂ into about 1: 1 and propagates to the two optical waveguides 6a and 6b having different optical path lengths. Where the first
Since the two optical waveguides 6a and 6b have different optical path lengths, radii of curvature, etc., as in the above embodiment, their respective propagation losses are different, and the optical signals λ ₁ and λ ₂ propagating through the optical waveguide 6b have a power 5 dB
Small. Therefore, by applying a voltage to the electrode 13 and changing the propagation constant difference in the input optical coupler 4, the branching ratio of the input optical coupler 4 is changed to cancel the intensity difference of about 5 dB between the optical signals propagating in the optical waveguides 6a and 6b. To do so. That is, the electrode 13
A voltage of about -20 V is applied to the input optical coupler 4 and the branching ratio of the input optical coupler 4 is changed to about 1: 3 so that the power propagated to the optical waveguide 6a is about 70% lower than the propagation power to the optical waveguide 6b. By doing so, the intensities of the optical signals propagating through the optical waveguides 6a and 6b could be made substantially equal. Then, the crosstalk of the optical signals λ ₁ and λ ₂ demultiplexed at the output ends 10 and 11 by being multiplexed and interfered again in the output optical coupler 5 could be improved from −10 dB to −30 dB.

In the third embodiment, only the input optical coupler 4 is provided with an electrode for changing the branching ratio. However, due to manufacturing problems, the output optical coupler 5
If it is difficult to make the branching ratio of 1 to about 1: 1, the output optical coupler 5 may be provided with an electrode for changing the branching ratio. Fig. 4 shows the case where electrodes are provided on both optical couplers.
This will be shown as a fourth embodiment. Electrodes 14 are formed on the surfaces of two adjacent output optical waveguides 3 which constitute the output optical coupler 5. Since other configurations are the same as those of the optical multiplex demultiplexer of FIG. 3, the same elements are denoted by the same reference numerals. According to this embodiment, the voltage can be applied to the electrode 14 to adjust the branching ratio of the output optical coupler 5 to about 1: 1, so that the above-mentioned problems in manufacturing and the like can be solved.

In the above four examples, LiNbO ₃ was used as the material, but the material is not limited to this, and semiconductor materials (for example, GaAs, InP) are used.
Etc.), quartz glass, etc. may be used.

Further, although the electro-optical effect is used as the means for changing the branching ratio in the branching ratio adjusting unit, the present invention is not limited to this. For example, if a semiconductor material is used, the effect of inter-band transition accompanying carrier injection, the plasma effect, etc. However, if quartz glass is used, the photoelastic effect may be used.

Further, although the optical signals of two different wavelengths are used in the above embodiments, the present invention is not limited to this, and basically, even if the optical signals of three or more different wavelengths are used, the optical multiplex components of the above embodiments are basically used. Multiplexing or demultiplexing can be performed by combining wave elements.

〔The invention's effect〕

As described above, according to the present invention, by providing a branching ratio adjusting unit that changes the branching ratio in at least one of the input branching unit and the output branching unit, the loss is the smallest when used as an optical multiplexing device. Optical signals can be multiplexed, and when used as an optical demultiplexing element, there is an effect that the optical signals can be demultiplexed with the smallest crosstalk.

[Brief description of drawings]

1 is a plan view showing a first embodiment of the present invention, FIG. 2 is a plan view showing a second embodiment of the present invention, and FIG. 3 is a plan view showing a third embodiment of the present invention. FIG. 4 is a plan view showing a fourth embodiment of the present invention. 1 …… LiNbO ₃ substrate 2 …… input optical waveguide 3 …… output optical waveguide 4 …… input optical coupler 5 …… output optical coupler 6a, 6b …… optical waveguide 7,12,13,14 …… electrodes 8,9 …… Input end 10,11 …… Output end

Claims (1)

[Claims] 1. An input branch section for inputting optical signals of different wavelengths, and respective output optical signals of the input branch section are propagated,
A Mach-Zehnder interference type optical demultiplexing device having an optical waveguide for giving a phase difference between these output optical signals and an output branching unit for inputting each output optical signal of this optical waveguide and outputting the optical signals of different wavelengths. 2. An optical multiplexing / demultiplexing device according to claim 1, wherein at least one of the two branching sections is provided with a branching ratio adjusting section for changing a branching ratio of the branching section.