JPH0549963B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is applied to an optical communication device. The present invention relates to an optical multiplexer/demultiplexer suitable for optical multiplexing and optical demultiplexing in optical frequency multiplexing transmission, and relates to an apparatus for multiplexing or demultiplexing optical signals of different wavelengths.

[Conventional technology]

FIG. 11 is a block diagram of a conventional device. In this figure, numerals 1, 5, and 9 are analyzers composed of Wollaston prisms, and numerals 2, 6, and 10
is a quarter-wave plate; numerals 3, 7, and 11 are Fabry-Perot interferometers; and numerals 4, 8, and 12 are reflecting mirrors.

The Fabry-Perot interferometer 3 has a characteristic of transmitting light with a frequency f ₁ and reflecting light with frequencies f ₂ and f ₃ , the Fabry-Perot interferometer 7 transmits a signal with a center frequency f ₂ , and the Fabry-Perot interferometer 11 has a characteristic of transmitting a signal with a center frequency f 2. It has the property of transmitting f ₃ light.

Now, an optical signal in which lights of frequencies f ₁ , f ₂ and f ₃ are multiplexed is polarized in the direction of the transmission axis and is passed through the analyzer 1.
When the light is incident on the optical fiber, it becomes circularly polarized light by the quarter-wave plate 2 and enters the Fabry-Perot interferometer 3. Here, the light of frequency f ₁ passes through the Fabry-Perot interferometer 3 .
However, the light at frequencies f ₂ and f ₃ is reflected here and returns to the quarter-wave plate 2 . The light at frequencies f ₂ and f ₃ that has passed through the quarter-wave plate 2 becomes linearly polarized light again and enters the analyzer 1, but the polarization direction is 90 degrees from the polarization direction at the time of incidence. Since it is shifted, the light is emitted in a direction different from the direction of arrival, and the reflected mirror 4
It is reflected by the beam and enters the analyzer 5. By repeating such operations, the lights of frequencies f ₂ and f ₃ are separated.

[Problem that the invention seeks to solve]

Since this device uses a Fabry-Perot interferometer as an optical filter, the device configuration becomes large. Furthermore, the analyzer is made of calcite, and the quarter-wave plate is made of mica, etc., which are expensive and not suitable for mass production of uniform products. Furthermore, there is a drawback that the optical loss required for separating multiplexed optical signals is large, and the device configuration cannot be stabilized.

The present invention is an improvement on this, and aims to provide an optical multiplexer/demultiplexer that has a simple configuration, low loss, is small and stable, and can be configured as a device that can be mass-produced.

[Means for solving problems]

The present invention includes two optical directional couplers each having four terminals, and the optical directional coupler is configured such that an optical signal inputted to the first terminal does not appear at the second terminal, but only at the third and third terminals. The optical signal appearing at the fourth terminal and input to the third terminal is configured so that it does not appear at the fourth terminal but appears at the first and second terminals, and a first optical waveguide connecting a third terminal of the first optical directional coupler and a first terminal of the second optical directional coupler; a second optical waveguide connecting the fourth terminal of the first optical directional coupler and the second terminal of the second optical directional coupler; a changing means for changing the relative effective length of the waveguide; a detecting means for detecting the optical signal at the second terminal of the first optical directional coupler; control means for automatically controlling the change means based on the detection output of the detection means so as to set the relative effective length of the wave path to a desired value; When the third and fourth terminals of the second optical directional coupler are outputs, and the third and fourth terminals of the second optical directional coupler are inputs. It is characterized in that the first terminal of the first optical directional coupler is used as an output.

The above invention can be used to configure one device by cascading the configurations.

[Effect]

If the terminals of two optical directional couplers are coupled using two optical waveguides and the relative lengths of the optical waveguides are different, then when the same signal passes through these two optical waveguides, there will be a phase conflict. Signals with frequencies that match each other cancel each other out, and signals with frequencies that match the phase are emphasized. Therefore, by matching this frequency to the frequency of the input optical signal, an optical multiplexer/demultiplexer can be constructed. At this time, by making the relative length of the optical waveguide variable and automatically controlling it, it is possible to always operate it in the best condition.

〔Example〕

FIG. 1 is a block diagram of an embodiment of the device which is the basis of the present invention. In this example, six optical directional couplers C ₁ to C ₆ and eight optical fibers L ₁ to _{L 8} are used to connect as shown in FIG. Optical fiber Li is an optical fiber for input terminal, and optical fiber L ₀₁ to L ₀₄
is an optical fiber for the output terminal. When an optical signal with four frequencies f ₁ , f ₂ , f ₃ , f ₄ multiplexed is input to the optical fiber Li for the input terminal, the optical directional coupler
At the output of C ₂ , optical signals with frequencies f ₁ and f ₃ appear in optical fiber L ₃ , and signals with frequencies f ₂ and f ₄ appear in optical fiber L ₄ . Furthermore, there is an optical fiber at the output terminal.
Optical signals of four frequencies f ₁ to _{f 4} appear in L ₀₁ to _{L 04} , respectively.

This will be explained further. FIG. 2 is a diagram showing a portion of the first optical directional coupler C ₁ and second optical directional coupler C ₂ extracted from the above device. That is, this configuration includes optical directional couplers C ₁ and C ₂ with four terminals. In these optical directional couplers C ₁ and C ₂ , the optical signal input to the first terminal (Li) does not appear at the second terminal (Li′), and the optical signal input to the third terminal (L ₁ ) and The optical signal that appears at the fourth terminal (L ₂ ) and is input to the third terminal (L ₁ ) does not appear at the fourth terminal (L ₂ ) and is input to the first terminal (Li).
and a known optical directional coupler configured to appear at the second terminal (Li'). The third terminal of the first optical directional coupler C ₁ and the second optical directional coupler among these two optical directional couplers C ₁ and C ₂
Connect the first terminal of C ₂ with optical fiber L ₁ .
Also, of these two optical directional couplers, the fourth terminal of the first optical directional coupler _C1 and the second terminal of the second optical directional coupler _C2 are connected by an optical fiber _L2 . Connecting. Moreover, the optical fibers L ₁ and L ₂ are set to have different relative effective lengths, and the first terminal of the first optical directional coupler C ₁ is input,
It is configured to output the third and fourth terminals of the second optical directional coupler _C2 .

In a device with such a configuration, an optical signal input to the optical fiber Li of the input terminal is branched into two optical fibers L ₁ and L ₂ by the first optical directional coupler C ₁ .
They are combined again in a second optical directional coupler _C2 . At this time, if the relative effective lengths of the two optical fibers L ₁ and L ₂ are different, optical signals of a certain frequency will be added with the same phase in the optical directional coupler C ₂ , and optical signals of a certain frequency will be added with the same phase. The optical signals have opposite phases at the optical directional coupler _C2 and are canceled.

FIG. 3 shows this obtained by simulation. The horizontal axis in FIG. 3 is the frequency (GHz) of the optical signal, and the vertical axis is the transmittance (dB) of the optical signal. _t12 shown by the solid line is the optical signal transmittance characteristic from terminal Li to _L3 in Figure 2, and is shown by the broken line.
_t13 is the optical signal transmission characteristic from terminal Li to _L4 .
That is, a frequency that is added at one output terminal will be canceled at the other output terminal, and a frequency that is added at the other output terminal will be canceled at one output terminal.

This constitutes a Matsuhatsu Ender interferometer, and the transmittance characteristic is expressed mathematically as t ₁₂ = (1-T) ² +T ² -2 (1-T) Tcos2πnf/C(l ₁ - l ₂ ) t ₁₃ =2(1-T)T {1+cos2πnf/C(l ₁ -l ₂ )} Where, f is the frequency of the light wave, n is the refractive index of the optical waveguide, T is the coupling efficiency of the directional coupler, l ₁ is the length of the optical waveguide L ₁ , l ₂ is the length of the optical waveguide L ₂ , and C is the speed of light in vacuum. The transmittance here is the ratio of the output power and input power of the optical signal.

The characteristics in Figure 3 are the effective optical path length difference (l ₁ − _{l 2} )
1 cm, and the coupling efficiency T of the two optical directional couplers C ₁ and C ₂ was 0.5. In this case, a pass frequency or a stop frequency appears at approximately every 20 GHz. Therefore, 4
The three frequencies f ₁ , f ₂ , f ₃ , f ₄ are combined to 10 GHz.
If the optical fiber Li is arranged at intervals, when an optical signal in which four frequencies f ₁ , f ₂ , f ₃ , f ₄ are multiplexed is input to the optical fiber Li, the output of the optical directional coupler C ₂ is connected to the optical fiber L ₃
Signals with frequencies f ₁ and f ₃ appear on the optical fiber L ₄
Signals with frequencies f ₂ and f ₄ appear at . When a device equivalent to that shown in FIG. 2 is used in a device including optical directional couplers C ₃ and C ₄ and optical directional couplers C ₅ and C ₆ ,
A device as shown in FIG. 1 can now be obtained.

FIG _. 4 shows the effective optical path length _{difference (l 1 −}
l ₂ ) is 1 cm (solid line) and 0.5 cm (broken line). In this way, _the effective optical path length _{difference (l 1 −}
By changing l ₂ ), it is possible to change the arrangement of pass frequencies and stop frequencies.

In the apparatus shown in FIG. 1, simply setting accurately the effective optical path length difference between these two optical fibers requires quite precise machining in practice. Therefore, embodiments of the apparatus of the present invention will be described below.

FIG. 5 is a block diagram of an apparatus according to a first embodiment of the present invention. In this example, compared to the device shown in FIG. 1, the input end and output end are reversed. This is not a problem since the device of FIG. 1 is reversible with respect to optical signals. Furthermore, this example utilizes the fact that when mechanical pressure is applied to an optical fiber, its optical refractive index changes, which changes the effective optical path length of the optical fiber. That is,
Fiber optic L ₂ , Fiber optic L ₅ and Fiber optic
A piezoelectric element P is arranged in each _L8 so as to compress the optical fiber, and a voltage is applied to this from the control circuit CONT to apply mechanical pressure to the optical fiber. Moreover, in the optical directional coupler on the output side of the optical fiber provided with this piezoelectric element P, the optical signal is detected by the detector D at the free terminal on the output side,
Automatic control is performed so that the output optical signal of this vacant terminal is minimized.

For example, the rightmost optical directional coupler in Figure 5
Considering C ₁ , when the effective optical path length difference is optimally adjusted, its output end optical fiber
The multiplexed optical signal is output at the maximum level to Lo, but theoretically there is no output light from optical fiber Lo'. Therefore, the piezoelectric element P is adjusted so that the output light level leaking to this optical fiber Lo' is minimized.
The optimum conditions can be automatically controlled by controlling the voltage applied to. The same applies to the other two optical fibers L _n1 and L _n2 shown in FIG.

FIG. 6 is a simulation diagram showing the effect of applying mechanical pressure to an optical fiber. This simulation assumes that the optical fiber L ₁ is 9.5 cm and the optical fiber L 2 is 10 cm, and that the optical fiber L ₁ is 9.5 cm and the optical fiber L ₂ is 10 cm. This figure shows the change in transmittance of this device when a pressure of 0.1 Kg/mm ² is applied over the length of 2. The photoelastic coefficient of quartz constituting the optical fiber is approximately 3×10 ⁻⁵ mm ² /Kg. The transmission frequency is shown to have shifted by an amount indicated by f (approximately 8 GHz) in FIG. 6 by applying pressure.

It can be seen that a mechanical pressure of 0.1 Kg/mm ² is a value that can be sufficiently applied by the piezoelectric element, and that the transmission frequency can be automatically controlled by the method shown in FIG.

FIG. 7 is a block diagram of an apparatus according to a second embodiment of the present invention. The basic configuration of this example is the same as that of the first embodiment device explained in FIG. 5, but the feature is that the optical waveguide and optical directional coupler are formed on a LiNbO ₃ crystal substrate using an integrated circuit. There is.
That is, each optical waveguide L ₁ to _{L 8} , input/output terminal Li, L ₀ ,
Lm, etc. are all constructed not from optical fibers but by forming regions with different refractive indexes on a LiNbO ₃ crystal substrate by a known method. Also, the optical directional couplers C ₁ to _{C 6} are not separate components, but
Similarly, it is formed on a LiNbO ₃ crystal substrate by a known method. Furthermore, in this example, in order to change the effective optical path length of the optical waveguide, instead of applying mechanical pressure, electrodes M ₁ to _{M 6} are provided near each optical waveguide and a voltage is applied to them. Apply an electric field to each optical waveguide. This structure also allows the effective optical refractive index of the optical waveguide to be changed, so the present invention can be implemented with the same effect as in the previous example. The voltage applied to each electrode is the same as in the previous example.
The leaking optical signal is detected by a photodetector D, and the control circuit CONT is configured to control the output so as to minimize it.

FIG. 8 is a diagram showing changes in transmittance frequency characteristics when an electric field is applied to an optical waveguide formed on a LiNbO ₃ crystal substrate. This characteristic shows the result of simulation for the configuration on the right side of the optical directional coupler _C2 in Figure 7.The difference in optical path length between the two optical waveguides _L1 and _L2 is assumed to be 3.5 mm. An example is shown in which an electrode is provided over a length of 3 mm in the optical waveguide L ₁ and an electric field of 10 ⁶ V/m is applied to the optical waveguide. It can be seen from FIG. 8 that the transmission frequency changes by about 20 GHz under the above conditions. In a practical device, the diameter of the optical waveguide formed on the LiNbO ₃ crystal substrate is several μm, and it is possible to apply an electric field of several V to it, so the change in transmission frequency described above is almost practical. is the value of the device.

FIG. 9 is a block diagram of an apparatus according to a third embodiment of the present invention. This example is equivalent to the device explained in Fig. 1, and has a configuration in which the signal propagates in the opposite direction to the device explained in Fig. 5, and each part is reversible with respect to the optical signal. , multiplexed optical signals can be separated. In the configuration shown in FIG. 9, the output ends of the optical fibers L ₀₁ and L ₀₃ are coated so that a portion of the emitted light is reflected back. This returning light wave should return to the input end optical fiber Li along the original optical path when each part of this device constitutes an ideal Matsuhatsu Ender interferometer. However, when there is a misalignment in each Matsuhatsu Ender interferometer, the optical fiber does not return to the original optical path.
It will leak to L _n1 or L _n2 or Li'. Therefore, by observing this leaked light and controlling the voltage applied to each piezoelectric element P so as to minimize it, the reflected light will return to its original optical path, and the device can be operated. It can be adjusted and maintained in the best condition.

FIG. 10 is a block diagram of an apparatus according to a fourth embodiment of the present invention. In this example, a structure similar to that of the embodiment shown in FIG. 9 described above is constructed on a LiNbO ₃ crystal substrate. The optical waveguides L ₀₁ and L ₀₃ at the output end are coated so that a portion of the emitted light is returned. The effect is the same as in the previous example. In this example, in order to control the effective length of each optical waveguide, a method is used in which electrodes M ₁ to _{M 6} are provided and the voltage applied thereto is controlled. This effect is the seventh
This is equivalent to the example explained in FIG. 8 and FIG.

Each of the above examples is a device that performs multiplexing or demultiplexing on four frequencies, but this is just an example, and in general, the device of the present invention can be used to multiplex or demultiplex a plurality of n frequencies as many times as necessary. Similar configurations can be made by combining or repeating.

Furthermore, the structure for integrated circuits is not limited to the above-mentioned LiNbO ₃ crystal substrate, and the present invention can be similarly implemented using other technologies.

〔Test results〕

FIG. 13 is a diagram showing the test results of the apparatus according to the present invention. This is the result of testing the configuration explained in Fig. 2. The locus in the upper row of Fig. ₁₃ shows that when the semiconductor laser light is input to the optical fiber Li and its frequency is swept, The output optical power is expressed as an antilog. Moreover, the locus in the lower part of FIG. 13 shows frequency markers simultaneously measured by a Fabry-Perot interferometer (FSR7.5kHz). It was manufactured so that the difference (l ₁ −l ₂ ) between the two optical path lengths was about 1 cm. From the results shown in FIG. 13, it can be seen that the pitch of the passing frequency or blocking frequency is approximately 22 GHz. This agrees well with the calculated value shown in FIG. 3 described above (the transmittance is on a logarithmic scale).

FIG. 12 shows the results of testing the effectiveness of the automatic control described in FIG. 5. The wavelength of the input laser light was kept constant, and the optical power leaking to the vacant terminal was measured. When measurements are taken for several minutes each in the non-controlled and controlled states, it is found that in the non-controlled state, the effective length of the optical fiber changes due to external disturbances, causing imbalance and leakage of optical power from free terminals. By performing control, leakage of optical power to vacant terminals is reduced,
It can be seen that the equilibrium state can be maintained for a long time.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain an optical multiplexer/demultiplexer that has a simple configuration, has low loss, and can be configured as a small and stable device.

The optical multiplexer/demultiplexer of the present invention is capable of demultiplexing even with a wavelength difference of 1 Å or less because the wavelength difference that can be demultiplexed is determined by the effective length difference between the two optical waveguides.The optical multiplexer/demultiplexer of the present invention is connected in multiple stages. By doing this, it is possible to separate N multiplexed optical signals, but at that time, the signal light is divided into halves, so the number of stages that the signal light passes through is log2N for any signal light.
Optical directional coupler By using a vacant terminal, there is an advantage that leakage light can be detected without providing a separate structure for separating input light and leakage light.

Since the device of the present invention is suitable for being configured as an integrated circuit, as exemplified by a LiNbO ₃ crystal substrate, it can achieve practical stability. The apparatus of the present invention is suitable for mass producing uniform products.

Furthermore, automatic control ensures that it always operates at its best.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a device that is the basis of the present invention. FIG. 2 is a partial configuration diagram for explaining the principle of operation. FIG. 3 is a diagram showing transmittance frequency characteristics to two output ends. FIG. 4 is a diagram showing differences in transmittance characteristics due to differences in optical path length of optical fibers. FIG. 5 is a block diagram of the apparatus according to the first embodiment of the present invention. FIG. 6 is a diagram showing changes in transmittance characteristics when mechanical pressure is applied to the optical fiber. FIG. 7 is a block diagram of an apparatus according to a second embodiment of the present invention. FIG. 8 is a diagram showing changes in transmittance characteristics when changing the electric field applied to the optical waveguide. FIG. 9 is a block diagram of a device according to a third embodiment of the present invention. FIG. 10 is a block diagram of an apparatus according to a fourth embodiment of the present invention. FIG. 11 is a block diagram of a conventional device. FIG. 12 is a diagram showing the test results of the automatic control state. FIG. 13 is a diagram showing the test results of the apparatus according to the present invention.

Claims (1)

[Claims] 1. Two optical directional couplers each having four terminals are provided, and the optical directional coupler is configured such that an optical signal input to the first terminal does not appear at the second terminal. The two optical directional couplers are configured such that an optical signal appearing at the third and fourth terminals and input to the third terminal does not appear at the fourth terminal but appears at the first and second terminals. a first optical waveguide connecting the third terminal of the first optical directional coupler and the first terminal of the second optical directional coupler; a second optical waveguide connecting a fourth terminal of the first optical directional coupler and a second terminal of the second optical directional coupler; changing means for changing the relative effective lengths of the optical waveguides; detecting means for detecting the optical signal at the second terminal of the first optical directional coupler; control means for automatically controlling the changing means based on the detection output of the detecting means so that the relative effective length of the optical waveguide of the first optical directional coupler is set to a desired value; When the first terminal is used as an input, the third and fourth terminals of the second optical directional coupler are used as outputs, and the third and fourth terminals of the second optical directional coupler are used as inputs. An optical multiplexer/demultiplexer, wherein the first terminal of the first optical directional coupler is used as an output. 2. A patent claim characterized in that the first and second optical waveguides are optical fibers, and the changing means is a means for applying mechanical pressure to at least one of the first and second optical waveguides. The optical multiplexer/demultiplexer according to item 1. 3. The optical multiplexer/demultiplexer according to claim 2, wherein the means for applying mechanical pressure includes a piezoelectric element, and the control means controls the voltage applied to the piezoelectric element. 4 The first and second optical waveguides are optical waveguides formed on a LiNbO ₃ crystal substrate, the changing means is an electrode that applies an electric field to at least one of the first and second optical waveguides, and the control The optical multiplexer/demultiplexer according to claim 1, wherein the means controls the voltage applied to the electrodes.