JP2000347148A - Half-band optical signal processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a half-band optical signal processing uint whose circuit constitution is not redundant and also in which the degree of freedom of a design is large. SOLUTION: In this optical signal processing unit, two lines of optical waveguides 21, 22 are coupled with N pieces of optical couplers 23-0 to 23-(N-1) or 23-1 to 23-N while having the difference 2L of optixal path lengths and also emitting sides of optical signals or incident sides of the optical singnals of the optical waveguides 21, 22 are connected with a 3 dB optical coupler 25 while having the difference L of optical path lengths and more-over, phase shifters 24-1 to 24-N are provided at least one side of N places on the optical waveguides 21, 22 held among these optical couplers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal processing device for performing advanced optical signal processing in fields such as image processing, audio processing, optical communication, and optical computing.

[0002]

2. Description of the Related Art In recent years, in the fields of optical communication, optical switching, optical computing, and the like, optical signal processing that does not convert an optical signal into an electric signal and that can perform filtering processing in a wide band and at a high speed with light as it is. The vessel is attracting attention. In particular, in optical frequency multiplexing communication in which optical signals are multiplexed and transmitted, an optical frequency filter that performs filtering processing for each frequency with respect to propagated frequency multiplexed signal light is an important component.

Conventionally, as an optical signal processor used for such a purpose, an optical delay line signal processor using an optical waveguide as a delay line has been widely used. The optical delay line signal processor includes an optical waveguide as a delay line, an optical coupler, and a phase shifter. The optical delay line signal processor has received much attention in recent years because its characteristics are equivalent to those of an electric digital filter, and various filter processes similar to those of a digital filter can be realized. Since the transmission characteristic of the optical delay line signal processor is equivalent to that of an electric digital filter, its frequency response is periodic with respect to frequency.

Optical delay line signal processors can be broadly classified into optical signal processors including a return path such as a ring waveguide and a Fabry-Perot interferometer, and optical signal processors not including a return path. The present invention relates to the latter optical signal processor that does not include the return path.

An optical signal processor not including a feedback path has advantages that an optical signal processor including a feedback path does not have, such as an optical signal processor having zero group delay dispersion, in other words, a linear phase optical signal processor. In the present invention, only the optical signal processor not including the return path will be described below.

[0006] In the present invention, the half-band characteristics, as shown in FIG. 1, the optical power transmission | G (ω) | ² is

[0007]

(Equation 1)

[0008] The filter characteristic is defined as follows.
Here, ω ₀ is a periodic frequency. The filter characteristic of FIG. 1 further has an optical power transmittance | G
(Ω) | ² = | G (−ω) | ² , but this symmetry is not required as a definition of the half-band characteristic.

FIG. 2 shows a typical example of a conventional optical signal processor (not including a feedback path) (Japanese Patent Application No. 5-1490).
No. 81 in the figure, 1 and 2 are optical waveguides,
1a and 2a are input ports, 1b and 2b are output ports, 3
−0,3-1,3-2,3-3,..., 3- (M−
1), 3-M is an optical coupler, 4-1, 4-2, 4-3, ...
.., 4-M are phase shifters.

In this circuit configuration, the optical couplers 3-0 to 3-0
The optical path length differences of the two optical waveguides 1 and 2 at M (M is a natural number) sandwiched between M are all equal L (L is a real number equal to or greater than 0).
It is. With this circuit configuration, the digital filter has a filter characteristic that is periodic with respect to frequency, and is an FIR type (Finite Impulse Response) digital filter.
An optical signal processing function equivalent to the above is realized. In this example, an arbitrary FIR characteristic can be realized by controlling the optical couplers 3-0 to 3-M and the phase shifters 4-1 to 4-M.

However, this circuit configuration has a problem that it is too redundant to realize a special filter characteristic such as a half-band characteristic aimed at by the present invention.

FIG. 3 shows another example of a conventional optical signal processor, here an example (K. Ji) designed to solve the redundancy of the circuit configuration of FIG.
nguji et. al. "Two-port opt
ical waveguide circuits c
omposed of cascaded Mach-
Zehnder Interferometers w
is point-symmetrical con
figures "J. Lightwave T"
echnol. , Vol. 14, no. 10,199
6, pp. 2301-2310 (particularly FIG. 12)
Reference). In the figure, 11 and 12 are optical waveguides, 11a and 12a are input ports, 11b and 12
b is an output port, 13-0, 13-1, 13-2, 13
-3 and 13-4 are optical couplers, 14-1, 14-2 and 14
Reference numerals -3 and 14-4 denote phase shifters.

This circuit configuration has five optical couplers, and the overall circuit configuration has point symmetry. That is, the 0th optical coupler 13-0 and the fourth optical coupler 13-4 have the same coupling ratio, and the first optical coupler 13-1 and the third optical coupler 13-3 are equal. It has a binding rate. Also, 1
The fourth phase shifter 14-1 and the fourth phase shifter 14-
4 have the same amount of phase shift, and the second phase shifter 1
4-2 and the third phase shifter 14-3 have the same phase shift amount. It is known that this circuit configuration realizes a half-hand characteristic having a flat transmission region and a blocking region.

However, this circuit configuration has five optical couplers.
There is a drawback that the number of individual half-band characteristics is limited and the degree of freedom in design is small.

[0015]

As described above, the conventional example 1 has a large degree of freedom in design, but has a redundant circuit configuration, and requires extra circuit elements to realize a half-band optical signal processor. Contains. Further, in the second conventional example, the number of circuit elements is fixed, so that there is a problem that the degree of freedom in design is small. It is to provide a signal processor.

[0016]

In order to solve the conventional problems, the present invention employs the circuit configuration shown in FIG. FIG. 1A shows a configuration corresponding to claim 1, and FIG. 1B shows a configuration corresponding to claim 2.

In FIG. 4A, N (N is a natural number) optical couplers 23-0, 23-1, 23-2, 23-3,...
, 23- (N-1), 25, the optical path length difference between the two optical waveguides 21, 22 is 2L, 2 from the optical signal incident side.
L, 2L, ..., 2L, L (... represents all 2L)
(L is a real number equal to or greater than 0), and the optical coupler 25 closest to the light emitting end is a 3 dB coupler. Also, phase shifters 24-1, 24-2, 24-3,...
.., 24-N are provided on each waveguide.

In FIG. 4B, N (N is a natural number) optical couplers 25, 23-1, 23-2, 23-3,...
The optical path length difference between the two optical waveguides 21 and 22 sandwiched between 3-N is L, 2L, 2L,..., 2L, 2 from the optical signal incident side.
L (... represents all 2L) (L is a real number equal to or greater than 0), and the optical coupler 25 closest to the light incident end is a 3 dB coupler. Similarly, phase shifters 24-1, 24-2, 24-3,..., 24-N for controlling the phase are provided on each waveguide.

As described above, according to the circuit configuration of the present invention, the conventional optical signal processor employs only the optical path length difference L having the same circuit configuration, whereas the double optical path length difference such as 2L is used. There is a great feature in adopting

According to a third aspect of the present invention, in the configuration of the first or second aspect, the 3d at the exit end or the entrance end
All optical couplers except for the B coupler are replaced with optical couplers with variable coupling ratios.

A configuration corresponding to claim 1 or 2 of the present invention basically corresponds to a special case in which some elements are reduced from the configuration of the conventional example 1. Specifically, in the circuit configuration of Conventional Example 1, except for the optical couplers of θ ₁ , θ ₃ , θ ₅ ,..., Θ _N-2 , the optical coupler of θ _N is replaced with a 3 dB coupler. The circuit configuration of item 1 is obtained. Similarly, the circuit configuration of claim 2 is the same as the circuit configuration of the conventional example 1, except that the optical coupler of θ ₀ is replaced by a 3 dB coupler, and θ ₂ , θ ₄ , θ ₆ ,.
..., except for the optical coupler of θ _N-1 .

As described above, the present invention can greatly reduce the number of circuit elements as compared with the prior art 1 because unnecessary elements are eliminated by utilizing the symmetry inherent in the half-band characteristic represented by the equation (1). This is because we have succeeded in reducing it. Therefore, the degree of freedom of design is as large as that of the conventional example 1, and the number of elements can be reduced to about half. Therefore, according to the present invention, the problem of the redundancy of the circuit configuration of Conventional Example 1 and the degree of freedom of design of Conventional Example 2 can be solved simultaneously.

According to a third aspect of the present invention,
Since the coupling ratio of the optical coupler and the phase shift amount of the phase shifter can both be freely set, there is a great feature that a programmable optical signal processor having an arbitrary half-band characteristic can be realized by one circuit.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to embodiments.

[Embodiment 1] In Embodiment 1, FIG.
The circuit configuration shown in FIG. In FIG.
2 is an optical waveguide, 21a and 22a are optical input ports, 21
b, 22b are output ports, 23-0, 23-1, 23-
2, 23-3,..., 23- (N-1) are optical couplers,
.., 24-N are phase shifters, and 25 is a 3 dB coupler.

Taking this embodiment as an example, it will be shown that the circuit configuration of the present invention can realize the half-band characteristic shown in equation (1). For the sake of explanation, the number of stages is assumed to be N (N is a natural number).

Now, an optical path length difference 2L surrounded by a broken line in FIG.
Is a 2-by-2 transfer matrix representing the waveguide portion of

[0028]

(Equation 2)

Let us say that

Here,

[0031]

(Equation 3)

Where the subscript “ _* ” is G _* (z) = G
^* (1 / z ^* ) (the superscript “ ^* ” represents a normal complex conjugate).

In general, the periodic frequency of an optical delay line optical signal processor is determined by the minimum optical path length difference of the circuit. In this case, the periodic frequency ω ₀ is calculated from the optical path length difference L as ω ₀ = 2π (c / L) (c is the speed of light).
Each matrix element of the transfer matrix of equation (2) is

[0034]

(Equation 4)

## EQU2 ## Both the transfer functions of G ₁ (z) and H ₁ (z) include only the even-order terms such as z ⁻² , z ⁻⁴ , and z ^−6, because the optical path length difference is 2L and twice as large. This is because they have a long difference.

When the transfer matrix of the entire optical circuit including the waveguide portion having the optical path length difference L at the output end is obtained,

[0037]

(Equation 5)

Is required. here,

[0039]

(Equation 6)

Is as follows.

In equation (5), the phase shift amount ψ _N of the phase shifter 24-N on the optical path length difference L is set to 0, π. G
(Z) and H (z) are given by the unitary property of the transfer matrix.

[0042]

(Equation 7)

The following relationship is established. The half-band characteristic represented by the equation (1) can be rewritten using the z variable.

[0044]

(Equation 8)

## EQU5 ## From this, using equation (6), the half-band characteristic becomes

[0046]

(Equation 9)

Can be rewritten.

In the circuit configuration of the present invention, G (z), H
Since (z) satisfies the relationship of equation (5)

[0049]

(Equation 10)

The following relationship is established. In equation (9), G
_The fact that both ₁ (z) and H ₁ (z) include only even orders is used.

From these results, it is shown that G (z) and H (z) satisfy the half-band characteristics of Expression (8), and the circuit configuration of the present invention has half-hand characteristics.

FIG. 4 shows another circuit configuration of the present invention.
It can be shown that the circuit configuration of FIG.

As can be seen from equation (4), according to the circuit configuration of the present invention, it is understood that the (2N-1) -order filter characteristic can be realized with the N-stage configuration. In other words, in order to realize the (2N-1) -order filter characteristics with the circuit configuration of the first conventional example, a circuit configuration of (2N-1) stages is necessary. Then, N stages are sufficient.

As described above, the circuit configuration of the present invention is the same as that of the conventional example 1.
It can be seen that an equivalent half-band filter can be realized with about half the number of elements as compared with. The reason why the number of elements was successfully reduced in the circuit configuration of the present invention is that the use of the symmetry of the half-band characteristic represented by the equation (1) succeeded in removing extra circuit elements.

In this embodiment, an optical signal processor for realizing the Chebyshev characteristic is manufactured using a quartz-based planar optical circuit. As the phase shifter, a heater formed on the waveguide was used. The phase was controlled by changing the optical path length difference of the heater heating unit using the thermo-optic effect in which the refractive index changes with temperature. The number of stages N was four, and the waveguide length difference was 2.07 mm in order to realize a periodic frequency of 100 GHz. Table 1 shows a list of setting values of the coupling ratio of the optical coupler and the phase shift amount of the phase shifter.

[0056]

[Table 1]

FIG. 6 shows the measurement results of the optical power transmittance spectrum of the optical signal processor designed and manufactured based on the set values. The obtained optical power transmission characteristic is obtained by comparing the transmittance of the transmission area by −
0.025 dB, a transmittance of −58 dB in a stop band, a band edge of a transmission band of 0.15ω ₀ , and a band edge of a transmission band of 0.35ω ₀ .

[Second Embodiment] A second embodiment also employs the circuit configuration shown in FIG. In this embodiment,
An optical signal processor realizing the maximum flatness was fabricated using a quartz-based planar optical circuit. The number of stages N is 4 stages, 100 GHz
Waveguide frequency difference of 2.07 m
m. Table 2 shows a list of setting values of the coupling ratio of the optical coupler and the phase shift amount of the phase shifter.

[0059]

[Table 2]

FIG. 7 shows the measurement results of the optical power transmittance spectrum of the optical signal processor designed and manufactured based on the set values. It can be seen that the optical power transmittance of the manufactured optical signal processor achieves the maximum flatness as designed.

[Embodiment 3] In this embodiment, based on claim 3, the optical coupler in the circuit configuration of FIG. 4A is replaced by an optical coupler having a variable coupling ratio. FIG. 8 shows the circuit configuration used this time.

This circuit configuration corresponds to the optical coupler 23 shown in FIG.
, 23- (N-1) are respectively connected to optical couplers 26-0, 26-1,.
-(N-1). Here, as the optical couplers 26-0 to 26- (N-1) whose coupling ratios are variable, the optical waveguides having the same optical path length difference and the two 3 dB couplers 27-
, 27- (N-1) and phase shifters 28-0, 28-1,..., 28- provided on the waveguide.
A variable coupling rate optical coupler having a symmetric Mach-Zehnder interferometer configuration composed of (N-1) was employed.

The variable coupling rate optical couplers 26-0 to 26-
In (N-1), the phase shifters 28-0 to 28- (N-
By changing the phase shift amount of 1), an arbitrary coupling ratio can be realized. By using this variable coupling ratio optical coupler, it is possible to realize the programmability of obtaining an arbitrary half-band characteristic with one circuit.

Also in this embodiment, a programmable optical signal processor was manufactured using a quartz-based planar optical circuit. Number of stages N
Has four steps, and the waveguide length difference is 2.07 mm in order to realize a periodic frequency of 100 GHz. In order to confirm the programmability, the first and second embodiments are actually used.
It was confirmed that the half-band characteristics of the Chebyshev type and the maximum flat type were obtained, respectively.

As described above, the configuration and operation of the present invention have been described with reference to the embodiments. However, the present invention is not limited to these embodiments. For example, although a quartz-based planar optical circuit is used in the present invention, it is also possible to form a planar optical circuit with another material such as a semiconductor or LiNbO ₃ . Further, it is also possible to make an optical delay circuit by using an optical fiber instead of the planar optical circuit. Further, although the thermo-optical effect is used in each embodiment as a phase control method, the phase control can be performed by another method such as an electro-optical effect.

In the circuit configuration of the embodiment shown in FIG. 4 (including FIG. 5) and FIG. 8, each of the two optical waveguides sandwiched between the optical couplers with an optical path length difference is an optical path. The longer ones are all aligned on one side (in FIGS. 4 and 8, above the drawings). However, the present invention is not limited to a configuration in which all of the optical waveguides having a longer (or shorter) optical path length are aligned on one side, and how the optical waveguides having a longer (or shorter) optical path length are arranged. This is optional. For example, an odd-numbered optical waveguide having a longer optical path length is arranged on one side (upper side in the drawing), and an even-numbered optical waveguide having a longer optical path length is arranged on the other side (lower side in the drawing). ) Can be adopted. Furthermore, the present invention is not limited to the configuration in which the phase shifters are provided in the optical waveguide having the longer optical path length. Each phase shifter is provided in the optical waveguide having the longer optical path length. The arrangement in the shorter optical waveguide is optional.

As described above, the present invention relates to a circuit configuration representing a method of assembling elements, and is not restricted by physical means for realizing the circuit. In addition, it is also noted that the circuit parameters described in each embodiment are only one of the design examples and are not limited by the numerical values.

[0068]

The present invention provides a circuit configuration of a minimum configuration including a minimum necessary circuit element for realizing a typical half band characteristic as an optical signal processing function. According to the present invention, a circuit can be constituted by about half the number of circuit elements of a conventional optical signal processor, and the circuit can be reduced in size. Further, since the number of elements can be reduced, the time required for adjusting circuit parameters can be significantly reduced. As described above, the present invention has a great economic effect in mass-producing the half-band optical signal processor.

[Brief description of the drawings]

FIG. 1 is a diagram showing a half-band characteristic defined by the present invention.

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

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

FIG. 4 is a configuration diagram showing an example of an embodiment of an optical signal processor of the present invention.

FIG. 5 is a diagram illustrating that the optical signal processor of the present invention has a half-band characteristic.

FIG. 6 is a diagram showing a measurement result of a Chebyshev type transmission characteristic of the optical signal processor according to the first embodiment having the circuit configuration of FIG. 4A;

FIG. 7 is a diagram showing a measurement result of a transmission characteristic of a maximum flat type of the optical signal processor according to the second embodiment having the circuit configuration of FIG. 4A;

FIG. 8 is a configuration diagram showing another example of the embodiment of the optical signal processor of the present invention.

[Explanation of symbols]

21, 22: optical waveguide, 21a, 22a: input port,
21b, 22b: output port, 23-0 to 23-N: optical coupler, 24-1 to 24-N: phase shifter, 25: 3d
B coupler, 26-0 to 26- (N-1): Optical coupler with variable coupling ratio, 27-0 to 27- (N-1): 3 dB coupler constituting an optical coupler with variable coupling ratio, 28- 0-28
-(N-1): a coupling ratio adjusting phase shifter constituting an optical coupling device having a variable coupling ratio.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Manabu Oguma F-term (reference) 2H079 AA06 BA03 CA07 EA04 EA05 EB27 in Nippon Telegraph and Telephone Co., Ltd. 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo

Claims (3)

[Claims] 1. An optical system comprising: two optical waveguides; and N + coupling the two optical waveguides at N + 1 (N is a natural number) different positions.
The optical signal output device has a configuration including one optical coupler and a phase shifter provided on at least one of the two optical waveguides at N locations sandwiched between the (N + 1) optical couplers. One of the optical couplers at the end on the side is a 3 dB coupler, and the two optical waveguides at N locations sandwiched between the (N + 1) optical couplers are 2L, 2L, and 2L from the optical signal incident side. 2
A half-band optical signal processor having an optical path length difference of L,..., 2L, L (... represents all 2L) (L is a real number equal to or greater than 0). 2. An optical system comprising: two optical waveguides; and N + coupling the two optical waveguides at N + 1 (N is a natural number) different positions.
The optical signal input device comprises: one optical coupler; and a phase shifter provided on at least one of the two optical waveguides at N locations sandwiched between the (N + 1) optical couplers. One of the optical couplers at the end on the side is a 3 dB coupler, and the two optical waveguides at N locations sandwiched between the (N + 1) optical couplers are L, 2L, 2
L, ..., 2L, 2L (... represents all 2L) (L
A half-band optical signal processor having an optical path length difference of 0 or more. 3. The half-band optical signal processor according to claim 1, wherein all the remaining optical couplers except the 3 dB coupler have variable coupling ratios.