JPH0511226A - Optical signal processor - Google Patents

Abstract

PURPOSE:To realize the optical signal processor which has a high transmittivity characteristic even when the number of branch optical waveguides increases. CONSTITUTION:The optical signal processor is provided with an optical branching device 10 which inputs a light signal and has a specific branching ratio, an optical coupling device 12 which has a specific coupling ratio, plural optical waveguides 5-1-5-M which connect respective output terminals of the device 10 and respective input terminals of the device 12, and phase controllers 11-1-11-M which are arranged in the respective optical waveguides 5-1-5-M and shift the phases of light phases of light propagated in the optical waveguides by specific quantities, and the light intensity branching ratio of the device 10 is equalized to the light intensity coupling ratio of the device 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal coherent signal processor using an optical waveguide.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a first conventional example of an optical signal processor (IEEE Transactions on Microwave T).
heory and Techniques MTT, Vol. 33, No. 30, "Optical fiber delay-line signal processing").

In FIG. 2, 1 is a light source, 2 is a light intensity modulator, 3 is an optical splitter for splitting an optical signal into equal power, 4 is an optical coupler for coupling optical signals with an equal coupling ratio, and 5 is an optical coupler. A plurality of (M) optical waveguides 5 for connecting the optical branching device 3 and the optical coupler 4
Optical waveguide group consisting of 1 to 5-M, 6 (6-1 to 6-M)
Is an optical intensity for controlling the intensity of an optical signal which is arranged in each of the optical waveguides 5-1 to 5-M of the optical waveguide group 5 and branched onto each of the optical waveguides 5-1 to 5-M by the optical branching device 3. Variable attenuator (hereinafter referred to as "optical attenuator"), 7 is a photodetector for detecting light emitted from the optical coupler 4, 8 is an optical signal incident point of the optical branching device 3, and 9
Is an optical signal emitting point of the optical coupler 4. In this example, an optical fiber is used as the optical waveguide, and this optical signal processor changes the attenuation factor of the optical attenuator to
It performs various optical signal processing.

In such a configuration, the optical signal emitted from the light source 1 and modulated by the light intensity modulator 2 is
The light is incident on the optical branching device 3 and is branched to each of the optical waveguides 5-1 to 5-M with an equal branching ratio. Each branched optical signal is attenuated to an arbitrary optical intensity by an optical attenuator 6, then combined through an optical combiner 4 having an equal coupling ratio, and detected by an optical detector 7 in the form of an electric signal.

However, since the coherence length of the light source used here is much smaller than the optical path length of each optical path, the phase information of the optical signal on each optical waveguide is uncertain, and the optical coupler. The binding by 4 was incoherent. Therefore, the optical signal processor of FIG. 2 cannot handle the phase information, and there is a limitation in terms of signal processing. For example, it was not possible to obtain high-pass filtering characteristics in frequency characteristics, and it was not possible to perform differential calculation on the time axis.

To solve this problem, a technique of coherently coupling light has been proposed. FIG. 3 is a configuration diagram showing a second conventional example of the optical signal processor, and shows a configuration to which the technique of coherently coupling light is applied (Elec.
"Coherent optical tran submitted in tronics Letters
sversal filter using silica-based single-mode wave
See guides ").

The second conventional example is different from the above-mentioned first conventional example in that a light source having a coherent length longer than the optical path length of each optical waveguide is used as the light source 1 and an optical branching device having an equal branching ratio. , An optical branching device (hereinafter referred to as a variable optical branching device) 10 for branching the input optical signal to an arbitrary branching ratio is arranged, and each optical waveguide 5-1 is used instead of the optical attenuator 6.
˜5-M are provided with phase controllers 11-1 to 11-M for controlling the phase of the branched optical signal so that the phase information of the optical signal can be handled in a fixed form. Especially. In this example, a planar optical waveguide circuit is used instead of an optical fiber as the optical waveguide.

In this optical signal processor, the variable optical splitter 10 is used.
The optical intensity of the optical signal branched on each of the optical waveguides 5-1 to 5-M is controlled by the branching ratio of
By controlling the phase information of the optical signals branched by the phase controllers 11-1 to 11-M provided above, the optical coupler 4 collects the phase information in a fixed form.

Therefore, this optical signal processor can process an optical signal containing phase information. Therefore, in the second conventional example, it is possible to realize a differentiating unit and an optical frequency filter having a high-pass filtering characteristic, which could not be realized in the first conventional example.

[0010]

In the field of optical signal processing, a physical quantity of a complex number called complex amplitude is used to represent an optical signal. The complex amplitude expresses amplitude by its absolute value and phase information by its phase angle.

Generally, in optical signal processing, there is no simple adder related to complex amplitude due to the law of conservation of energy. That is, when the optical signals on the M optical waveguides are combined into one optical waveguide, the complex amplitude of the combined optical signals is not a simple addition of the complex amplitudes of the optical signals on the respective optical waveguides.

In the second conventional example, the optical coupler 4 having an equal coupling ratio is used as the optical coupler. In the optical coupler 4 having this equal coupling ratio, (1 / M) ^1/2 of the complex amplitude of the optical signal on each of the M branched optical waveguides is taken out and combined as one optical signal. The remaining optical signal is emitted to the outside.

Therefore, in the optical coupler 4 having the equal coupling ratio, as the number of waveguides M of the branched optical waveguides increases, only a small amount of the optical signals on each optical waveguide can be coupled,
Most of the optical signals have a drawback that they are emitted to the outside. That is, the conventional optical signal processor has a problem that high transmittance characteristics cannot be obtained because an optical coupler having an equal branching ratio is used.

The present invention has been made in view of such circumstances, and an object thereof is to provide an optical signal processor capable of obtaining a high transmittance characteristic even if the number of waveguides of a branch optical waveguide is increased. is there.

[0015]

In order to achieve the above object, in claim 1, an optical splitter for splitting an optical signal at a predetermined splitting ratio and an optical coupler for coupling an optical signal at a predetermined splitting ratio. A plurality of optical waveguides that connect each output end of the optical branching device and each input end of the optical coupler, and a predetermined amount of phase of light propagating through the optical waveguides arranged in each of the plurality of optical waveguides. A phase controller for shifting is provided, and the light intensity splitting ratio of the optical splitter and the light intensity splitting ratio of the optical coupler are matched.

According to a second aspect, in the optical path group from the optical signal incident point of the optical branching device to the optical branching device and each optical waveguide, and further to the optical signal emitting point of the optical coupler,
The optical path length differences between the optical paths are equal to each other, and the optical path length differences between the optical paths are set to have a ratio of 1: 2: ...: M-1 (M is the number of optical waveguides) as a whole.

Further, in claim 3, an optical signal emitting port for emitting a part of an optical signal is provided in either one of the optical branching device and the optical coupler.

[0018]

According to the first aspect, the optical signal is incident on the optical branching device and is branched into a plurality of optical waveguides. The optical intensity of each branched optical signal is controlled by the branching ratio of the optical branching device, and a predetermined amount of phase shift action is performed by the phase controller provided on each optical waveguide. It is incident on an optical coupler having a coupling ratio. In the optical coupler, a plurality of optical signals are combined and output.

According to the second aspect, when the desired function is a periodic function, a light amplitude transmission characteristic that is more closely approximated to the desired function can be obtained.

Further, according to the third aspect, a part of the optical signal is emitted from the optical signal emitting port, whereby the optical amplitude transmission characteristic which is well approximated to the desired function can be obtained for a certain desired function form.

[0021]

[Embodiment 1] FIG. 1 shows a first embodiment of an optical signal processor according to the present invention.
FIG. 4 is a configuration diagram showing the embodiment of FIG. 3, and the same components as those in FIG.

That is, 1 is a light source having a long coherence length, 2 is a light intensity modulator for performing intensity modulation on the light emitted from the light source 1, and 5 is M optical waveguides 5-1 to 5-M. Optical waveguide group, 7 photodetector, 8 optical signal incident point, 9 optical signal emitting point, 10 one input port (optical signal incident point 8)
A variable optical branching device for branching an optical signal incident from a plurality of (M) output ports by branching it with an arbitrary branching ratio, 11-
1 to 11-M are phase controllers for performing a phase shift of about 1 wavelength on an optical signal propagating through an optical waveguide, and 12 are a plurality of (M
This is a variable optical coupler that combines optical signals incident from (individual) input ports with an arbitrary coupling ratio and outputs the combined signals to one output port (optical signal emission point 9).

In this optical signal processor, the variable optical splitter 10 is used.
And the coupling ratio of the variable optical coupler 12 can be arbitrarily changed, and the light intensity branching ratio of the optical splitter 10 and the light intensity coupling ratio of the optical coupler 12 are set to be equal to each other. Has been done. Specifically, the plurality of optical waveguides 5-1 to 5-are made incident by the optical signal incident point 8 by the variable optical branching device 10.
The light intensity of the optical signal branched to M and the light when it is incident from the optical signal emitting point 9 in the opposite direction, passes through the variable optical coupler 12 in the opposite direction, and is branched to the optical waveguides 5-1 to 5-M. The light intensity splitting ratio of the variable optical splitter 10 and the light intensity splitting ratio of the variable optical coupler 12 are set so that the intensities match.

Next, the operation and the principle of the above configuration will be described.

An optical signal emitted from the light source 1 and modulated in intensity by the optical intensity modulator 2 is incident on the variable optical branching device 10 and branched into M optical waveguides 5-1 to 5-M. The optical intensity of each branched optical signal is controlled by the branching ratio of the variable optical branching device 10, and further, each optical waveguide 5-1 to 5
After being subjected to a predetermined amount of phase shift action by the phase controllers 11-1 to 11-M provided on the -M, the light is incident on the variable optical coupler 12 having an arbitrary coupling ratio. Variable optical coupler 1
In 2, a plurality of optical signals are combined into one. And
The combined optical signal is detected by the photodetector 7 in the form of an electrical signal.

Here, as the light source 1, a light source having a long coherence length is used as in the second conventional example. Further, the variable optical branching device 10 can freely change the branching ratio as described above, and by changing the branching ratio, the optical intensity of the optical signal branched on each of the optical waveguides 5-1 to 5-M. Control the ratio of.

Let P ₁ , P ₂ , ..., P _M be the optical intensities of the optical signals on the respective optical waveguides 5-1 to 5-M, and let A _{1 be} the complex amplitude of each optical signal immediately before the variable optical coupler 12. , A ₂ , ... A _M , each complex amplitude is expressed by the following equation.

A ₁ = (P ₁ ) ^1/2 exp (−jθ ₁ ) exp (−jβΔL ₁ ) A ₂ = (P ₂ ) ^1/2 exp (−jθ ₂ ) exp (−jβΔL ₂ ): A _M = (P _M), where _{^{1/2 exp (-jθ M) exp (}} -jβΔL M), the optical signal phase, the optical path length difference [Delta] L _K between the waveguides
Of the exp (-jβΔL _K ) determined by the phase controller 1
Exp (-jθ _K ) determined by the phase shift amount θ _K due to 1
It is expressed in the form of the product of terms. The former is about tens of thousands of wavelengths, and the latter is about one wavelength. The amplitudes P ₁ ^1/2 , P ₂ ^1/2 , ... P _M ^1/2 of each optical signal can be arbitrarily controlled by changing the branching ratio of the optical branching device.

The phase of each optical signal is largely dependent on the optical path length difference between the optical paths, and fine adjustment of about one wavelength is required for each optical waveguide 5.
Phase controllers 11-1 to 11 provided on -1 to 5-M
It can be controlled by -M.

Next, assuming that the light intensity coupling ratio of the variable optical coupler 12 is P ₁ ′, P ₂ ′, ..., P _M ′, the variable optical coupler 1
2 of the complex amplitude of each optical signal propagating on each of the optical waveguides 5-1 to 5-M, (P ₁ ′) ^1/2 , (P
^{_{2 ') 1/2, ..., (}} P M' is taken out by ^1/2), 1
Are combined into one signal. That is, the complex amplitude of the combined optical signal is Becomes

As can be seen from this equation, the complex amplitude of the combined optical signal does not tend to approach zero even if the number M of optical waveguides is increased, and high transmittance characteristics can be obtained.

On the other hand, in the second conventional example shown in FIG. 3, unlike the present embodiment, an optical coupler having an equal branching ratio is used as the optical coupler instead of an optical coupler having an arbitrary coupling ratio. It was This corresponds to a constant light intensity coupling ratio of P _K ′ = 1 / M (k = 1, ..., M), and the complex amplitude of the coupled optical signal is Is expressed as At this time, the complex amplitude of the combined optical signal tends to approach zero as the number of optical waveguides M increases, because of the term of 1 / M. Therefore, the optical signal processor of FIG. 3 cannot obtain a high transmittance characteristic.

As described above, according to this embodiment,
A variable optical coupler 12 having a variable coupling ratio is used instead of an optical coupler having an equal coupling ratio.
The light intensity coupling ratio of the optical coupler is also changed, and the light intensity branching ratio of the optical branching device and the light intensity coupling ratio of the optical coupler are not independent of each other, but they have the same relationship, that is, P _K = P _K ′ (k = 1, ... M), it is possible to realize a high transmittance optical signal processor having an arbitrary complex amplitude transmittance.

[0034]

Second Embodiment FIG. 4 shows a second embodiment of the optical signal processor according to the present invention.
It is a block diagram which shows the Example of.

The present embodiment is different from the first embodiment in that the variable optical branching device 1 is provided from the optical signal incident point 8 of the variable optical branching device 10.
0, through the optical waveguides 5-1 to 5-M, and further in the optical path group reaching the optical signal emission point 9 of the variable optical coupler 12, the optical path length differences between the optical paths are equal to each other, and as a whole, between the optical paths. The optical path length difference is set to have a ratio of 1: 2: ...: M-1.

As described above, the optical path length difference between the optical signal entrance points 8 and the optical signal exit points 9 is 1: 2: ...: M-.
When there is a relationship of 1, the complex amplitude of the optical signal combined by the variable optical coupler 12 is Is expressed as In this equation, the complex amplitude transmittance is expressed in the form of the sum of expansion terms corresponding to each optical path. Here, the expansion coefficient is (P _K · P _K ′) ^1/2 exp (−jθ _K ),
Consider the expansion function as exp (-jkβΔL). It can be seen that this expansion function forms a periodic orthogonal function system.

In general, when a periodic desired function is expanded by a certain periodic functional system and it is an orthogonal functional system,
It is known that the degree of approximation to the desired function is the best. For this reason, when the desired function is a periodic function when the optical path length difference between the optical paths is 1: 2: ...: M−1, as compared with the case where the optical path length difference between the optical paths is irregular, It is possible to obtain a light amplitude transmission characteristic that closely approximates the desired function.

Therefore, in the optical signal processor according to the present embodiment, even when a large number of optical waveguides are used in order to increase the degree of approximation of the complex amplitude transmittance characteristic to the desired function form, it can be seen from the second conventional example. Such a phenomenon that the amplitude transmission characteristic approaches zero is not seen, and the obtained complex amplitude characteristic can be approximated to the desired function. This means that in the optical signal processor of this embodiment, high functionality and high transmittance can be simultaneously realized without contradiction.

In FIG. 5, the optical path length difference between the optical paths is 1: 2 :.
...: shows a specific circuit example of an optical frequency filter having a rectangular periodic transmission characteristic used in optical frequency multiplex communication, which has a relationship of M-1. In this example, this circuit is realized by a silica-based optical waveguide on a Si substrate, and has a so-called ladder structure in which bridges are bridged at equal intervals.
ΔL / 2 between each bridge (optical waveguides 5-1 to 5-M)
Are evenly spaced. Further, the variable optical branching device 10 and the variable optical coupler 12 are composed of an optical component called a variable directional coupler 101 whose coupling rate can be freely set.

The value of the waveguide length difference ΔL is the period f of the optical frequency.
It is obtained from c. In this example, the cycle of the optical frequency is 10 GH
z and the distance ΔL / 2 between the bridges was 1.0372 cm. The number of bridges was 17. Furthermore, in this example,
The coupling ratio Cm of the variable directional coupler 101 forming the variable optical branching device 10 and the coupling ratio Cm 'of the variable directional coupler 101 forming the variable optical coupler 12 are equal, that is, Cm = C.
It is set to m '(m = 1, ..., M-1).

FIG. 6 shows an example of measuring the light intensity transmission characteristics of the optical filter when the maximum light intensity value of the desired function is 0.5. In the figure, the horizontal axis represents the relative frequency and the vertical axis represents the transmittance. It represents. The curve indicated by the solid line is the measured value of the light intensity transmittance of the optical frequency filter (product of the present invention) according to this example, and the curve indicated by the broken line is the functional form of the desired light intensity transmittance drawn for reference. The curve indicated by the alternate long and short dash line is the measurement of the light intensity transmittance of the optical frequency filter manufactured so as to have the same characteristics as the embodiment with the configuration of the optical signal processor described in the second conventional example shown in FIG. The results are shown respectively.

As shown in FIG. 6, in the conventional optical filter, the light intensity transmissivity is far from the desired light intensity transmissivity, and the transmissivity is only about 0.07.
On the other hand, the light intensity transmittance of the optical filter of this embodiment is quite close to the desired function.

As described above, according to this embodiment,
Since the optical path length differences of the respective optical paths are configured to have a relationship of 1: 2: ...: M-1, in addition to the effect of the first embodiment, when the desired function is a periodic function, it is well approximated to the desired function. It is effective in obtaining the above-mentioned optical amplitude transmission characteristics.

[0044]

[Third Embodiment] FIG. 7 shows a third embodiment of the optical signal processor according to the present invention.
It is a block diagram which shows the Example of.

The present embodiment is different from the second embodiment in that among the optical signals which are incident from the optical signal incident point 8 and are branched by the variable optical branching device 10 into the respective optical waveguides 5-1 to 5-M. An optical signal emission port (hereinafter referred to as emission port) 13 for emitting a part to the outside is provided in the variable optical branching device 10, and
The variable optical coupler 12 is also provided with an emission port 14 for emitting a part of an optical signal so that an optical amplitude transmission characteristic that closely approximates a desired function can be obtained.

FIG. 8 shows a concrete circuit example of an optical frequency filter designed with the emission ports 13 and 14 and the maximum value of the desired function set to 0.1. In this example, as in the case of the second embodiment, the coupling ratio Cm of the variable directional coupler 101 that constitutes the variable optical branching device 10 and the coupling ratio Cm of the variable directional coupler 101 that constitutes the variable optical coupler 12 are described. 'Is equal, that is,
Cm = Cm '(m = 1, ..., M). Also,
Emission ratio C of the emission port 13 provided in the variable optical branching device 10
M and the emission ratio CM 'of the emission port 14 provided in the variable optical coupler 12 are set equal. The number of bridges was 17 as in the case of Example 2.

FIG. 9 shows the measurement results of the light intensity transmittance of the manufactured optical filter. In the figure, the curve indicated by the solid line is the light intensity transmittance of the optical frequency filter according to this embodiment, the curve indicated by the broken line is the functional form of the desired light intensity transmittance, and the curve indicated by the alternate long and short dash line is the optical signal emitting port 13. , 14 are not provided, the measurement results of the light intensity transmittance of the optical frequency filter according to Example 2 are shown.

As can be seen from FIG. 9, in the case where there is no optical signal emission port, the maximum transmittance is 0.43, which is a transmission characteristic significantly different from the desired function. On the other hand, in the configuration of the present embodiment in which the optical signal emission ports 13 and 14 are provided, it is possible to obtain the optical amplitude transmission characteristic that is close to the desired function.

As is clear from this result, in a certain desired function form, the effect of letting a part of the optical signal escape is extremely large in terms of improving the degree of approximation of the light intensity transmittance to the desired function form.

In particular, in this embodiment, the emission rate of the emission port 13 provided in the variable optical branching device 10 and the emission rate of the emission port 14 provided in the variable optical coupler 12 are set equal. Setting them equal to each other is effective in increasing the function approximation degree of the complex amplitude transmittance to the desired function, as compared with the case where they are not set equal to each other.

Further, in the present embodiment, the variable optical branching device 10 and the variable optical coupler 12 are constituted by using the variable directional coupler 101, but they may be replaced with another branching component such as Y branching. It is possible. It is also possible to provide the optical signal emission port on either the optical branching device or the optical coupler. In this case, the characteristics are slightly worse than in the case where the emission ports are provided on both sides, but there is an advantage that the number of parts can be reduced.

Further, in the present embodiment, the structure of the optical circuit is symmetrical with respect to the plurality of optical waveguides, but this is not important in the present invention. What is important is that the optical signal branching ratio when this optical signal is branched from the optical signal incident point 8 to the optical waveguides 5-1 to 5-M via the variable optical branching device 10 and the reverse from the optical signal emitting point 9. That is, the optical signal branching ratios in which the optical signal is made incident on the optical signal are branched to the optical waveguides 5-1 to 5-M via the variable optical coupler 12 in the same relationship. That is, the symmetry regarding the optical signal branching ratio is important.

[0053]

As described above, according to the first aspect of the present invention, it is possible to realize an optical signal processor having a high transmittance having an arbitrary complex amplitude transmittance which could not be realized conventionally. In particular, in the optical signal processor of the present invention, the transmittance does not deteriorate even if the number of optical waveguides is increased in order to increase the degree of approximation of the complex amplitude transmittance to the desired function. That is, increasing the degree of approximation of the function and maintaining high transmittance can be realized simultaneously without any contradiction.
An optical signal processor with high functionality and high transmittance can be realized.

Further, in the optical signal processor realized according to the present invention, since the complex amplitude transmission characteristic can be freely designed, not only the light intensity characteristic but also the phase characteristic can be designed at the same time, and its use is various. It can be used as various optical frequency filters such as a high-pass optical filter and a low-pass optical filter in optical frequency multiplex communication, or as various signal processors such as an optical signal differentiator in optical digital communication.

Further, in the second aspect, when the desired function is a periodic function, it is possible to obtain a light amplitude transmission characteristic which is well approximated to the desired function.

Further, according to the third aspect, it is possible to obtain the light amplitude transmission characteristic that is close to the desired function with respect to a certain desired function form.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of an optical signal processor according to the present invention.

FIG. 2 is a configuration diagram showing a first conventional example of an optical signal processor.

FIG. 3 is a configuration diagram showing a second conventional example of an optical signal processor.

FIG. 4 is a configuration diagram showing a second embodiment of the optical signal processor according to the present invention.

FIG. 5 is a diagram showing a specific circuit example of an optical frequency filter having a rectangular periodic transmission characteristic using a ladder circuit according to the second embodiment (when there is no optical signal emission port).

6 is a diagram showing a measurement result of light intensity transmittance of the optical frequency filter of FIG.

FIG. 7 is a block diagram showing a third embodiment of the optical signal processor according to the present invention.

FIG. 8 is a diagram showing a specific circuit example of an optical frequency filter having a rectangular periodic transmission characteristic using a ladder circuit according to Example 3 (when an optical signal emission port is provided).

9 is a diagram showing a measurement result of light intensity transmittance of the optical frequency filter of FIG.

[Explanation of symbols]

1 ... Light source, 2 ... Light intensity modulator, 5 ... Optical waveguide group, 7 ... Photodetector, 8 ... Optical signal incident point, 9 ... Optical signal emitting point, 10 ...
Optical branching device having variable branching ratio, 11 ... Phase controller, 12
... Optical couplers having variable coupling ratios, 13, 14 ... Optical signal emitting ports, 101 ... Variable coupling rate directional couplers.

Claims (3)

[Claims] 1. An optical splitter for splitting an optical signal at a predetermined split ratio, an optical coupler for splitting an optical signal at a preset coupling ratio, each output end of the optical splitter and each of the optical couplers. A plurality of optical waveguides that connect the input end and a phase controller that is arranged in each of the plurality of optical waveguides and that shifts the phase of light propagating through the optical waveguides by a predetermined amount. An optical signal processor characterized in that the ratio and the light intensity coupling ratio of the optical coupler are matched. 2. An optical path length difference between each optical path in an optical path group from the optical signal incident point of the optical branching device to the optical branching device and each optical waveguide, and further to the optical signal output point of the optical coupler. Are equal to each other, and the optical path length difference between the optical paths is 1: as a whole.
2. The optical signal processor according to claim 1, wherein the optical signal processor has a ratio of 2: ...: M-1 (M is the number of optical waveguides). 3. The optical signal processing according to claim 1 or 2, wherein one of the optical branching device and the optical coupler is provided with an optical signal emitting port for emitting a part of an optical signal. vessel.