JPH10133155A - Broadband optical signal processor and design method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an operating wavelength range wide by using a directional coupler which has wavelength-nondependent characteristics as well amplitude characteristics and phase characteristics or a directional coupler which has a variable amplitude value wavelength-nondependent characteristics as well as amplitude characteristics and phase characteristics. SOLUTION: N+1 two-input, two-output wavelength-nondependent variable directional couplers 3-n (n=1, 2...N+1) which have constant amplitude characteristics and phase characteristics in their operating wavelength ranges are arrayed in a line on a silicon substrate 5 and mutually adjacent wavelength- nondependent variable directional couplers at N places are each coupled by two optical waveguides 1-n and 2-n (n=1, 2...N) having a different optical path length difference ΔL (corresponding to a delay time Δτ). The optical path length difference between every two of the optical waveguides at the N places is set equally to ΔL. Then phase controllers 4-(2n-1) and 4-2n (n=1, 2...N) for controlling phases are installed on every two optical waveguides.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal processor for collectively processing a plurality of multiplexed optical signals in the field of optical wavelength division multiplexing communication.
The present invention relates to a broadband (programmable) optical signal processor for performing wideband advanced information processing and a design method thereof.

[0002]

2. Description of the Related Art As is well known, optical signal processors having two inputs and two outputs are classified into frequency signal processors and wavelength signal processors. Both of the circuit configurations have a configuration in which a Mach-Zehnder interferometer circuit including two directional couplers and two optical waveguides connecting between them is used as a basic circuit and cascaded in a line.

A frequency signal processor has an operating wavelength range of 10n.
m or less. The optical path length difference between the two optical waveguides of the Mach-Zehnder interferometer, which is a basic circuit, is usually 2 to 20 millimeters, and is periodically repeated at a wavelength of 0.04 to 4 nm (optical frequency of 5 to 500 GHz) in the operating wavelength range. Filter characteristics. In addition, since the operating wavelength range is as narrow as 10 nm or less, the wavelength dependence of the coupling ratio of the directional coupler constituting the circuit can be neglected. Therefore, a design theory for realizing an arbitrary filter characteristic is also known. Ori (Journ
al of Lightwave Technology
y, Vol. 13, No. 1, pp. 73-82, 19
95), high-performance optical signal processors such as a group delay equalizer and a high-order dispersion equalizer have been developed.

On the other hand, the operating wavelength range of the wavelength signal processor is 10
The bandwidth is as wide as 0 nm or more. The optical path length difference between two optical waveguides of a Mach-Zehnder interferometer, which is a basic circuit, is usually on the order of μm. Although the operating wavelength range is as wide as 10 nm or more, the wavelength dependence of the coupling ratio of the directional coupler that constitutes the circuit cannot be ignored, making it difficult to design filters, and using wavelength-independent couplers and band-flat optical demultiplexers. Only simple functions such as filters have been developed.

For this reason, conventionally, a (programmable) optical signal processor realizing an arbitrary filter characteristic has been constituted by an optical frequency signal processor (Japanese Patent Application No. 6-136187). FIG. 1 shows a circuit configuration (N-stage configuration) of a conventional programmable optical frequency signal processor. In this circuit configuration, N + 1 variable directional couplers 9-n (n = 1, 2,..., N + 1) with variable coupling ratios and two inputs and two outputs are arranged in a line, and N variable directions adjacent to each other are arranged. Between the sex couplers,
The configuration is such that two optical waveguides 1-n, 2-n (n = 1, 2,..., N) having different optical path length differences (corresponding to the delay time Δτ) are coupled. Two input ports are 14 and 15, and two output ports are 16 and 17.

On each of the two optical waveguides, there are provided phase controllers 4- (2n-1) and 4-2n (n =
1, 2,..., N) are installed. The variable directional coupler 9-n includes two 3 dB couplers 11-
(2n-1), 11-2n, and two optical waveguides 12-n, having the same optical path length connecting the two 3 dB couplers,
13-n, at least one phase controller 10- (2n-1) and 10-2n are installed in the two optical waveguides. The variable directional coupler 9-n can set an arbitrary power coupling ratio by changing the phase amount of the phase controllers 4- (2n-1) and 4-2n. In this programmable optical frequency signal processor, the phase controllers 10- (2n-1), 10-
2n (n = 1, 2,..., N) and phase controllers 4- (2n-1) and 4-2n on two optical waveguides having different optical path lengths.
(N = 1, 2,..., N) realizes an arbitrary filter characteristic.

[0007]

Since the conventional programmable optical signal processor is designed without considering the wavelength dependence of the variable directional coupler, the wavelength dependence of the variable directional coupler is apparent. In the operating wavelength range of 10 nm or more, there is a problem that filter characteristics as designed cannot be realized. The wavelength band used in the optical wavelength division multiplexing communication is determined by the bandwidth of the optical fiber amplifier. The amplification band of the present optical fiber amplifier is about 30 nm, and the development of a fiber amplifier having a wider band is being advanced.
Broadband wavelength multiplexing communication of 0 nm or more is no longer a dream. Along with the progress of the broadband of the optical wavelength division multiplexing communication, development of a wideband (programmable) optical signal processor capable of realizing a designed filter characteristic even when the operating wavelength range is 10 nm or more is desired.

What limits the operating wavelength range of the conventional programmable optical signal processor is the wavelength dependence of the variable directional coupler. The variable directional coupler has a wavelength dependence because the directional coupler constituting the directional coupler has a wavelength dependence. Above 10 nm, at which the wavelength dependence of the variable directional coupler becomes apparent, the amplitude characteristics and the phase characteristics deviate from the design values, and the characteristics of the entire optical signal processor deviate from the designed characteristics. In order to solve this problem, it is necessary to provide a wavelength-independent (variable) directional coupler whose amplitude characteristics and phase characteristics are constant in the operating wavelength range.

Accordingly, an object of the present invention is to broaden the operating wavelength range to 10 nm or more in a programmable optical signal processor, and to provide such a broadband optical signal processor. It is an object of the present invention to provide a design method for reliably realizing such a broadband optical signal processor.

[0010]

According to the present invention, a variable directional coupler 9-n (n = 1, 2,..., N) causing wavelength dependency in a conventional programmable optical signal processor.
Instead of +1), a directional coupler having both wavelength-independent and amplitude-dependent characteristics and a directional coupler 3- having a variable amplitude value having both wavelength-independent and amplitude-dependent characteristics. n (n = 1, 2,..., N
By using +1), the operating wavelength range is broadened to 10 nm or more.

That is, in the broadband optical signal processor according to the first aspect of the present invention, N + 1 (N is 1 or more) 2-input / 2-output optical couplers are arranged in a line, and the optical couplers adjacent to each other are arranged. The two optical waveguides are connected by two optical waveguides each having a difference in optical path length, and the optical path length difference between the two optical waveguides is equal to each other between the N optical couplers. Has a circuit configuration in which at least one phase controller is provided, and when the optical coupler receives an optical signal from any one of the two input ports, Optical signals output from the output ports have amplitude and phase transmission characteristics that are constant in the operating wavelength range of the optical signal processor.

[0012] broadband optical signal processor according to claim 2 of the present invention, the claims in a broadband optical signal processor of claim 1, wherein the second amplitude coupling ratio of the input two-output optical coupler theta _n and the two optical The phase amount φ _n of the phase controller provided on the wave path is
For the desired filter characteristics, the following equation:

[0013]

(Equation 3)

Here, a _k ^[n] and b _k ^[n] are complex numbers obtained by the following recurrence formula.

[0015]

(Equation 4)

Further, φ _an and φ _bn are phase amounts corresponding to two output ports having two outputs of the designed n-th optical coupler, and take values calculated based on the values. .

According to a third aspect of the present invention, in the broadband optical signal processor according to the second aspect, the optical coupler includes two optical waveguides at (m + 1) locations (where m is 2
Above), there are (m + 1) directional couplers arranged close to each other, and at m places sandwiched between the directional couplers, the two optical waveguides have an optical path length difference of 5 μm or less. It is characterized by doing.

According to a fourth aspect of the present invention, in the broadband optical signal processor according to the second aspect, the optical coupler includes two optical waveguides at (m + 1) locations (where m is 2
Above), there are (m + 1) directional couplers arranged close to each other, and at m places sandwiched between the directional couplers, the two optical waveguides have an equal optical path length difference; At least one optical path length adjusting unit is provided on each of the optical waveguides.

A broadband optical signal processor according to a fifth aspect of the present invention is the wideband optical signal processor according to the second aspect, wherein the optical coupler has a constant wavelength coupling ratio value in the operating wavelength range. Is a variable optical coupler that is variable.

According to a sixth aspect of the present invention, in the broadband optical signal processor of the fifth aspect, the variable optical coupler includes two optical waveguides at (n + 1) locations (where n is 3 or more) (n + 1)
Directional couplers, and at n positions sandwiched between the directional couplers, the two optical waveguides have the same optical path length difference, or have an optical path length difference of 5 μm or less. ,
At least one optical path length adjusting unit is provided on each of the optical waveguides.

The method for designing a broadband optical signal processor according to claim 7 of the present invention is characterized in that N + 1 (N is 1 or more) 2-input 2
The output optical couplers are arranged in a line, and the optical couplers adjacent to each other are connected by two optical waveguides each having an optical path length difference, and the optical path length difference between the two optical waveguides is N places. A circuit configuration in which at least one phase controller is provided on each of the two optical waveguides, wherein the two optical waveguides are further equal to each other. When an optical signal is input from any of the input ports, the amplitude and phase transmission characteristics of the optical signal output from the two output ports are constant in the operating wavelength range of the optical signal processor. A method for designing a broadband optical signal processor capable of realizing a desired filter characteristic in a broadband optical signal processor having: (a) an amplitude coupling ratio of the optical coupler with respect to the desired filter characteristic and a phase of the phase controller; The quantity below the design By determining on the basis of the forward 1 to 4, to confirm the phase of the amplitude coupling ratio and the phase controller of optical couplers, these by, characterized in that to achieve the desired filter characteristics.

(Design Procedure 1) The desired filter characteristic is N
Approximate by Fourier series expansion of the following complex coefficients.

(Design Procedure 2) The obtained Fourier series function is normalized so that the maximum of the absolute value is 1 or less.

(Design Procedure 3) Using a standardized Fourier series function, all the transmission characteristics of the optical signal processor are represented by a 2-by-2 transmission characteristic matrix.

(Design Procedure 4) The obtained transmission characteristic matrix of the entire optical signal processor is converted into a matrix corresponding to each unit configuration composed of the two optical waveguides having an optical path length difference and the optical coupler. The transmission characteristic matrix is decomposed into a product of the transmission characteristic matrices in the row 2 column, and the transmission characteristic matrix in 2 rows and 2 columns representing the unit configuration is expressed using the amplitude coupling ratio of the optical coupler and the phase amount of the phase controller as parameters. Based on this, the amplitude coupling ratio of the optical coupler and the phase amount of the phase controller are obtained.

[0026]

Embodiments of the present invention will be described below in detail.

(Embodiment 1) FIG. 2 shows a circuit configuration (N-stage configuration) of a wideband programmable optical signal processor of the present invention. In the drawing, the same components as those in FIG. The description will be simplified. In this circuit configuration, the N + 1 amplitude and phase characteristics are constant in the operating wavelength range, and the two-input two-output wavelength-independent variable directional coupler 3-
n (n = 1, 2,..., N + 1) are arranged in a line, and there are different optical path length differences ΔL (corresponding to delay time Δτ) between N mutually adjacent wavelength-independent variable directional couplers. 2
The optical waveguides 1-n and 2-n (n1, 2,..., N) are combined. The optical path length differences of each of the two optical waveguides at N locations are all set equal to ΔL (corresponding to the delay time Δτ). A phase controller 4- (2n-1), 4-2n for controlling the phase is provided on each of the two optical waveguides.
(N = 1, 2,..., N) are at least one.

In this embodiment, a circuit is formed by the quartz planar waveguide formed on the silicon substrate 5. As the waveguide, a quartz thin film was formed on the substrate 5 using a flame deposition method, and the waveguide structure was formed by a photolithography process. The optical waveguide had a square cross section of 7 × 7 μm, and the refractive index of the optical waveguide was made 0.7% higher than the surrounding quartz glass.

The programmable optical signal processor exhibits periodic characteristics at certain wavelength intervals in the operating wavelength range. This frequency Δλ is

[0030]

(Equation 5)

## EQU1 ## Here, ΔL is the optical path length difference between the two optical waveguides, n is the refractive index of the optical waveguide, and λ is the center wavelength of the operating wavelength range.

As can be seen from this equation, the period Δλ of the filter characteristic of the programmable optical signal processor is equal to the optical path length difference ΔL.
Is determined by For example, when the optical path length difference ΔL = 2 mm, the periodic wavelength of the filter characteristic is 0.8 nm (corresponding to a frequency interval of 100 GHz when converted in frequency). In this embodiment, the periodic wavelength is 1.6 nm (frequency interval 2).
(Corresponding to 00 GHz).

FIG. 3 shows a circuit configuration (three-stage configuration) of the directional coupler 3 used in this embodiment, which has a variable coupling ratio and whose amplitude characteristics and phase characteristics are simultaneously independent of wavelength. The circuit has a configuration in which three Mach-Zehnder interferometers are arranged in multiple stages.
The circuit comprises four directional couplers 7-1 to 7-4 and two optical waveguides 6-1 to 6-6 connecting them, two input ports 18, 19 and two output ports 20, 21. Having. In the present embodiment, the optical path lengths of these two optical waveguides are set equal. The optical path length adjusting units 8-1 to 8-6 are arranged on each of the two optical waveguides. The variable directional coupler used in the present embodiment, in which the amplitude characteristic and the phase characteristic are simultaneously wavelength-independent, adjusts the change in the optical path length of the optical path length adjusting units 8-1 to 8-6 to adjust the amplitude characteristic. It is possible to arbitrarily set the amplitude coupling ratio while keeping the phase characteristics flat with respect to the wavelength.

In this embodiment, the phase controller 4- (2n
-1), 4-2n (n = 1, 2,..., N) and the optical path length adjusting units 8-1 to 8-6 of the variable directional couplers whose amplitude characteristics and phase characteristics are simultaneously independent of wavelength. Used a heater utilizing the thermo-optic effect. By heating the heater,
Utilizing the fact that the refractive index of the glass in the heated part is high,
Phase control and optical path length adjustment are performed.

The transmission characteristics of a general two-input two-output optical coupler are expressed by the following transmission matrix of two rows and two columns.

[0036]

(Equation 6)

Here, θ is the angle display of the amplitude coupling ratio sin θ (hereinafter, not only sin θ, but also the angle display θ).
And also represents called the amplitude coupling ratio), φ _1, φ ₂ represents the output light of the phase from the two output ports. Amplitude coupling rate θ
Φ ₁ and φ ₂ are determined by designing the optical coupling ratio with respect to.

Here, it is assumed that the circuit has no loss.
The transmission matrix is a unitary matrix. The (1, 1) element of the matrix indicates the transmission characteristic from the input port 18 to the output port 20,
The (2,1) element represents a transmission characteristic from the input port 18 to the output port 21.

In the variable directional coupler used in the conventional programmable optical signal processor, the two directional couplers 11- (2n-1) and 11-2n forming the circuit are all operating wavelengths. The power coupling ratio was designed to be 3 dB. In the variable directional coupler used in the present embodiment in which the amplitude characteristic and the phase characteristic are wavelength-independent at the same time, the operating wavelength range is as wide as 10 nm or more, and the four directional couplers 7- Since the power coupling ratio values of 1 to 7-4 vary depending on the wavelength, the power coupling ratio values cannot be used to define the directional coupler. For this reason, here, the directional coupler is represented by a portion where the two optical waveguides are close to each other, that is, a length of the coupling portion. The lengths of the coupling portions of the four directional couplers 7-1 to 7-4 are 0.78 mm, 0.01 mm, 0.01 mm, and 0.
It was 78 mm. FIGS. 4 and 5 show the power coupling ratio and the wavelength dependence of the phase characteristics φ ₁ and φ ₂ of the variable directional coupler in which the optical path length adjustment units 8-1 to 8-6 are adjusted so as to achieve 50% coupling. It is a measurement result. Here, the amplitude characteristic 2
The power becomes the power coupling ratio. The operating wavelength range is 1.5 to 1.6
μm. From these figures, it can be seen that both the power coupling ratio characteristic (amplitude characteristic) and the phase characteristic of the variable directional coupler achieve a constant value with respect to wavelength in the operating wavelength range.

For reference, FIGS. 6 and 7 show a power coupling ratio characteristic (amplitude characteristic) and a phase characteristic of a conventional variable directional coupler. In this figure, at a wavelength of 1.55 μm, 5
It is designed to have 0% coupling. As can be seen, the power coupling ratio has a large wavelength dependence. The phase term φ ₁ also has wavelength dependence.
However, the phase term φ ₂ is completely constant with respect to the wavelength. This can be proved from the symmetry of the circuit configuration of the conventional variable directional coupler. As described above, since both the amplitude characteristic (power coupling ratio characteristic) and the phase characteristic have large wavelength dependence, the conventional programmable optical signal processor using the variable directional coupler has a wavelength of 10 nm or more from the design wavelength. In the shifted wavelength region, there is a problem that the filter characteristics are largely shifted from desired filter characteristics.

In order to clarify this fact, for an eight-stage programmable optical signal processor, the amplitude characteristic and the phase characteristic of the programmable optical signal processor using the conventional variable directional coupler and the wavelength characteristic are simultaneously measured. A wideband programmable optical signal processor using the independent directional coupler of the present invention was fabricated, and the characteristics of both were compared. The programmed characteristic is a periodic wavelength of 1.6 μm (frequency interval 200 GHz
) Of the group delay equalizer. The group delay equalizer is
This is an equalizer for compensating for a linear group delay that occurs during long-distance transmission of an optical fiber. 8 to 11 show the measurement results of the group delay equalizer characteristics programmed by the broadband programmable optical signal processor of the present invention. FIGS. 12 to 15 show the group delay equalizer programmed by the conventional programmable optical signal processor. It is a measurement result of a container characteristic. 8 to 11, the operating wavelength range of the broadband programmable optical signal processor is 1.5 to 1.6 μm, and FIGS. 8 and 9 show the group delay equalizer characteristics at the short wavelength end of the operating wavelength range. Figure 1
0, FIG. 11 shows the central wavelength range of the operating wavelength range (1.55 μm)
And has a group delay equalizer characteristic. 8 and 10
Indicates light transmission intensity characteristics, and FIGS. 9 and 11 illustrate group delay characteristics. The light transmission intensity characteristic is the amplitude characteristic 2
Expressed as a power. The group delay equalization is performed using a linear group delay portion having a slope of the group delay characteristic shown in FIG. The slope of the group delay is designed to have an inverse value with respect to a linear fiber group delay caused by fiber dispersion. As can be seen from the figure, it is understood that the group delay characteristics at the center and the end of the operating wavelength range are substantially equal. This is an effect of the variable directional coupler used in the present invention in which the amplitude characteristic and the phase characteristic are simultaneously independent of wavelength. 12 to 15, the conventional programmable optical signal processor is 1.55
It was designed to achieve the desired group delay equalization characteristics at μm. 12 and 13 show the group delay equalizer characteristics in the 1.5 μm band, and FIGS. 14 and 15 show the group delay equalizer characteristics in the 1.55 μm band (design wavelength band). . FIG.
14 and FIG. 14 show light transmission intensity characteristics, and FIGS. 13 and 15 show group delay characteristics. As can be seen from the figure, the group delay characteristic as designed is realized in the design wavelength region, but it is significantly deviated from the design group delay characteristic in the 1.5 μm band 50 nm away from the design wavelength. This is due to the wavelength dependency of the conventional variable directional coupler.

The details of the design of the broadband programmable optical signal processor will be described below. The unknown circuit parameters to be designed are N, whose amplitude characteristics and phase characteristics are simultaneously wavelength-independent.
+1 variable directional couplers 3-n (n = 1, 2,..., N
Amplitude coupling rate _{+1) θ n (n = 0,1} , ..., N) and, phase controller 4- (2n-1), 4-2n (n = 1,
2,..., N) is the phase amount φ _n (n = 1, 2,..., N).

In FIG. 2, the signal light incident from the input port 14 is divided into two optical waveguides 1 having different optical path lengths at each stage.
−n, 2-n (n = 1, 2,..., N), and are output to two output ports 16 and 17. At this time, the signal light is transmitted to each of the variable directional couplers 3-n (n = 1, 2,..., N + 1).
Is branched. For example, the signal light traveling the shortest optical path passes through the waveguides 2-1, 2-2, ..., 2-N. The signal light traveling the longest optical path is composed of waveguides 1-1, 1-2,.
1-N. As described above, the output light from the output port is expressed as a sum of signal lights passing through various optical paths. There are 2 ^N physical optical paths through which an optical signal passes. If one having the same optical path length is counted as one optical path, the signal light is 0, ΔL, 2ΔL,..., NΔL (ΔL is 2 Pass through any one of the N + 1 optical paths having a waveguide length of (wavelength difference of the optical waveguides). Therefore, the complex amplitude of the output light from port 16 is

[0044]

(Equation 7)

## EQU5 ##

Here, the expansion coefficient a _k in the equation (3-1) is N
+1 represents the complex amplitude of the signal light passing through each optical path of +1.
β is the propagation constant of the optical waveguides 1 and 2, X is the optical frequency in the operating wavelength range, and X ₀ is the periodic frequency of the frequency characteristic.

Here, since the variable directional coupler whose amplitude characteristic and phase characteristic are simultaneously independent of wavelength is used,
The wavelength dependence of the variable directional coupler is ignored. Also,
In the equation (3-1), as is often done in the electric digital filter theory,

[0048]

[Outside 1]

Is extended to a function h (z) on a complex plane. On this z complex plane, the frequency function h (X) is | z
It is regarded as a function on the circumference of the radius 1 of | = 1.

In the equation (3-1), the through characteristic h
(Z) is represented by a polynomial of z ^-1 . This equation is used in the field of digital filters in FIR (Finite
It can be seen that the transmission characteristics are equal to those of a digital filter of a type without feedback called an “Impulse Response” type. This fact plays an important role in the synthesis of the optical filter described below.

Similarly, light enters from port 14 and
The output light f (z) emitted from 7 is represented by the following equation.

[0052]

(Equation 8)

Since the present signal processor has two inputs and two outputs, the transmission matrix S of the entire signal processor is represented by a 2 × 2 matrix expressed by the following equation.

[0054]

(Equation 9)

Here, the suffix * represents paraconjugate (h ^* (z) = h ^* (1 / z ^* )). Assuming that the circuit is lossless, between h (z) and f (z), the determinant of the matrix of equation (4) is 1,

[0056]

(Equation 10)

A unitary relationship represented by the following is established.

More specifically, circuit parameters for satisfying the desired filter characteristics of the cross characteristics (the transmission characteristics from the input port 14 to the output port 17) were obtained by the following procedure. The following design procedure is summarized in FIG. 16 and FIG.

(Design Procedure 1) In determining the complex expansion coefficient b _k of the equation (3-2) so as to satisfy the desired filter characteristic, the transmission characteristic of the optical filter described in the equation (3-2) is obtained. Is FIR (Finite Impulse Re)
It utilizes the same as a digital filter called a “sponse” type. Specifically, the complex expansion coefficient b _k
In order to obtain, various approximation techniques developed in the field of digital filters are used. For example, Fourier expansion method, window function method, frequency sampling method, Remez
The Exchange method and the like are well known (Open
Heim & Schafer, "Digital S
signal processing ”). In this calculation, the minimum number of expansion terms necessary for the realized filter characteristic to sufficiently satisfy the condition of the desired filter characteristic is obtained from the required condition of the desired filter characteristic. Corresponds to the number N of stages of the programmable optical signal processor.

(Design Procedure 2) In the design procedure 1, the complex expansion coefficient of the equation (3-2) was obtained by using the above-mentioned optimal digital filter design technique so as to realize desired filter characteristics. If an optical signal processor is realized using the obtained complex expansion coefficients as they are, the absolute value of the amplitude characteristic may exceed 100%. However, in this signal processor, which is an analog signal processor using a physical quantity called a light wave, the absolute value of the complex amplitude in the equation (3-2) is 1 due to the law of conservation of energy.
It cannot be essentially exceeded 00%. For this reason, it is necessary to normalize the complex expansion coefficient b _k obtained by the synthesis method of the digital filter by the following equation so that the absolute value of the amplitude characteristic does not exceed 100%.

[0061]

[Equation 11]

(Design Procedure 3) The complex expansion characteristic having the normalized complex expansion coefficient obtained in Design Procedure 2 is realized by the cross output (output from port 14 to port 17) of this optical signal processor. An S matrix representing the transmission characteristics of the optical signal processor is obtained. Specifically, the remaining unknown transfer function h (z) of the S matrix is obtained from the unitary relation of Expression (5). h (z) h ^* (z) is expressed by the following equation (7) by using f (z) standardized in the design procedure 2 from the unitary relation, and further developing it into the form of a linear product of z. It is represented by

[0063]

(Equation 12)

Where a _k and a _k ^* are known polynomials f
(Z) The root of f ^* (z), known.

In the equation (7), a ₀ is a known value of f
It is obtained as in equation (8) using the complex expansion coefficient b ₀ of (z).

[0066]

(Equation 13)

In the equation, φ _aN and φ _bN represent a wavelength-independent (variable) directional coupler 3- (N +
1) shows the phase of the optical output from the two output ports. φ _aN and φ _bN can be obtained as follows. | A ₀ | is obtained from equation (8),

[0068]

[Equation 14]

[0069] (wherein theta _N is a amplitude and phase characteristics at the same time the wavelength-independent (amplitude coupling rate of the variable) directional couplers 3- (N + 1)) than theta _N is obtained. Since it is possible to design a wavelength-independent (variable) directional coupler corresponding to the amplitude coupling rate θ _N and having both amplitude characteristics and phase characteristics simultaneously, the phases φ _aN and φ
_bN is required. In this way, the phase of a ₀ is obtained from equation (8), and the calculation of a ₀ can be performed. Using a ₀ determined, h (z) is obtained from the equation (7).

In the equation (7), h (z) and h ^* (z)
, The zeros of the polynomial on the right side are obtained as N pairs of { _ak , 1 / _ak ^* }. When calculating h (z), the polynomial of h (z) is ｛ _ak ,
Since it is necessary to have one of the root pairs of 1 / a _k ^{* * as} a root, h (z) has 2 ^N solutions depending on how this root is selected. These 2 ^N h
The solution (z) has the same amplitude characteristics and different phase characteristics. As a whole, there are 2 ^N S matrices having common f (z) and different h (z), and as a result, 2 ^N types of circuit parameters are obtained. However, the cross characteristic is the same characteristic f (z) regardless of whether the circuit is manufactured using any of the 2 ^N circuit parameters. In the present embodiment, since we are interested in the characteristic of f (z), which is the cross characteristic, 2 ^N h
An appropriate one solution of (z) was selected. of course,
The selection of h (z) can be made by appropriately setting a selection criterion, such as one having low element sensitivity and one having an appropriate phase characteristic.

(Design Procedure 4) In order to obtain the circuit parameters, S representing the transmission characteristics of the entire N-stage optical signal processing circuit
The matrix is developed in the form of a product of S matrices representing the transmission characteristics of a single-stage unit configuration.

[0072]

(Equation 15)

Here, S _n is the unit configuration circuit of the _n- th stage.

In FIG. 2, the unit configuration circuit of the n-th stage is 1-n,
It comprises a 2-n optical waveguide, 4- (2n-1) and 4-2n phase controllers, and 3- (n + 1) amplitude characteristics and phase characteristics simultaneous wavelength-independent (variable) directional coupler. , N = 0
Is constituted by one (variable) directional coupler 3-1 whose amplitude characteristics and phase characteristics are simultaneously independent of wavelength. The 3- (n + 1) variable directional coupler is shown in FIG.
The circuit configuration shown in FIG. The S matrix of the (variable) directional coupler in which the amplitude characteristic and the phase characteristic are simultaneously wavelength-independent (variable) shown in FIGS. 4 and 5 is given by Expression (2). Using this, the S matrix of the n-th unit configuration is given as follows.

[0075]

(Equation 16)

Here, θ _n is a wavelength-independent (variable) directional coupler 3- (n +
1) shows the angle display of the amplitude coupling ratio of 1), and φ _an , φ _bn
Represents the phase of the light output from the two output ports. Φ _n is a phase controller 4- (2n-1), 4-2n on two optical waveguides sandwiched by a (variable) directional coupler whose amplitude characteristics and phase characteristics are simultaneously independent of wavelength. Represents the amount of phase difference. S ₀ is composed of only the (variable) directional coupler 3-1 in which the amplitude characteristic and the phase characteristic are simultaneously wavelength-independent,

[0077]

[Equation 17]

Is expressed as follows.

[0079] degradation of the actual S matrix, the inverse matrix S _n ^-1 = S _n ⁺ unit configuration S matrix (S _n ⁺ is transposed conjugate matrix of S _n) n
= N, N-1,..., 2, 1, 0 in order.

[0080]

(Equation 18)

[0081] In this _{^{case, S [n] = S n}} S n-1 ... S 2 S 1 S
Represents ₀ . _Sn is obtained by this repetitive operation.

If S ^[n] is known from equation (12), z
^{Under the} condition that the term of ^{(n + 1) / 2} becomes zero, the amplitude characteristic and the phase characteristic are simultaneously wavelength-independent (variable) directional coupler 3- (n
Amplitude coupling ratio θ _n +1) is obtained. In particular,

[0083]

[Equation 19]

From the above, θ _n is obtained. Where a _n ^[n] and b _n ^[n]
Is a known complex expansion coefficient in S ^[n] . Once θ _n is determined, a wavelength-independent (variable) directional coupler whose amplitude characteristics and phase characteristics simultaneously correspond to the amplitude coupling ratio can be designed. As a result, phases φ _an and φ _bn are determined. . Also,

[0085]

[Outside 2]

The phase difference φ _n between the phase controllers 4- (2n-1) and 4-2n is obtained. In particular,

[0087]

(Equation 20)

However,

[0089]

(Equation 21)

Is as follows.

[0091]

[Outside 3]

Θ _n is obtained from equation (13) and is known, and the phases φ _an and φ _bn are also known. Also, a _n-1 ^[n] , a
_{Since n} ^[n] , b _n-1 ^[n] and b _n ^[n] are known with known complex expansion coefficients in S ^[n] ,

[0093]

[Outside 4]

Further, the phases φ _an−1 , φ _{an of the} directional coupler 4-n in which the amplitude characteristic and the phase characteristic are simultaneously independent of wavelength (variable).
_bn-1 is

[0095]

(Equation 22)

[0096] than to obtain the amplitude coupling ratio phi _n-1, the amplitude and phase characteristics and is designed at the same time wavelength-independent (variable) directional couplers 4-n and the phase φ _an-1, φ _{bn -1} was determined.

Thus, at each decomposition stage of the S matrix,
Circuit parameters θ _n and φ _n included in the unit configuration circuit
Is required. By completing the disassembly procedure up to the final stage, all circuit parameters θ _n (n = 0, 1,.
N) and φ _n (n = 1, 2,..., N) are obtained.

As described above, in the present invention, 2N kinds of circuit parameters are obtained for desired cross filter characteristics. Regardless of which of these circuit parameters is employed, the same cross filter characteristics can be obtained. From this, the present invention asserts that the rights of the present invention extend to all the circuit parameters obtained by the above-described circuit synthesis method, without being limited by numerical values relating to the circuit parameters.

(Embodiment 2) In Embodiment 1, the periodic wavelength is set to 1.6 μm (corresponding to a frequency interval of 200 GHz). In the present embodiment, a programmable optical signal processor similar to that of the first embodiment was manufactured by setting the periodic wavelength to 4 μm (corresponding to a frequency interval of 500 GHz). Embodiment 1
The only difference is the optical path length difference ΔL between the two optical waveguides.

FIG. 18 shows group delay equalization characteristics programmed using the wideband programmable optical signal processor of the present invention. The operating wavelength range is 1.5 to 1.6 μm. 100n
It can be seen that equal group delay equalization characteristics are realized over the entire operating wavelength range up to m. FIG. 19 shows group delay equalization characteristics programmed using a conventional programmable optical signal processor. The design wavelength is 1.55 μm. It can be seen that the group delay equalization characteristic greatly deviates from the desired characteristic (the characteristic near 1.55 μm) as the distance from the design wavelength increases. In this example, the wavelength range in which the group delay equalization characteristic almost equal to the design characteristic is maintained is approximately 20 μm.
m. In this group delay equalization characteristic,
It cannot be used for optical wavelength multiplex communication over a wide band of 30 μm or more. In future broadband optical wavelength division multiplexing communications,
It is clear from the present embodiment that the broadband programmable optical signal processor of the present invention plays a large role.

The configuration and operation of the present invention have been described above using the embodiment, but the present invention is not limited to the embodiment. For example, although a quartz planar optical circuit is used in the present invention, a semiconductor such as an InGaAsP-based semiconductor, LiNb
It is also possible to form a planar circuit with another material such as an electro-optical material such as O ₃ or an organic optical material. It is also possible to use an optical fiber instead of a planar circuit. Further, in the present invention, in order to enable arbitrary optical signal processing with one circuit, a directional coupler having a variable coupling ratio and having amplitude and phase characteristics simultaneously independent of wavelength is used. In order to realize a functional optical signal processor, instead of a variable directional coupler in which the amplitude and phase characteristics are simultaneously wavelength-independent, the amplitude and phase characteristics have a fixed coupling ratio at the same time. It is also possible to use dependent directional couplers,
This also has the merit that the number of parts can be reduced. In the present embodiment, the thermo-optical effect is used to perform the phase control and the optical path length control. However, other physical phenomena such as the electro-optical effect and the nonlinear effect represented by the Kerr effect are used as the phase control method. It is also possible to use an optical effect, a magneto-optical effect, and the like.

As described above, the present invention is an invention for realizing an optical signal processor by combining a circuit configuration representing a combination of elements and a circuit synthesis technique for calculating circuit parameters by giving desired filter characteristics. You are not bound by the physical implementation of the circuit. It is also noted that the circuit parameters described in the present embodiment are only one of the design examples and are not limited by the numerical values.

Further, according to the present invention, two optical waveguides at N locations sandwiched between N + 1 variable directional couplers whose amplitude characteristics and phase characteristics are simultaneously wavelength-independent have a constant optical path length difference. In this case, the two optical waveguides at N locations sandwiched by the variable directional couplers whose amplitude characteristics and phase characteristics are simultaneously independent of wavelength are aligned with one of the longer optical path lengths. The present invention is not limited to the waveguide configuration of whether or not. This is because the amplitude characteristic and the phase characteristic simultaneously change the coupling rate of the variable directional coupler in which the wavelength is independent at the same time, so that the long side of the optical path length of each of the two optical waveguides is alternated. This is because it is also possible to realize a filter having a waveguide configuration.

The present invention relates to a circuit configuration representing a combination of elements and a procedure for designing circuit parameters.
A feature of the claims of the present invention resides in that a request is made by adding a design procedure for obtaining circuit parameters in addition to the circuit configuration. In order to actually achieve the desired characteristics, a design procedure for calculating the circuit parameters for realizing the desired characteristics is required. According to the present invention, the coupling ratio of N + 1 variable directional couplers, in which the amplitude characteristics and the phase characteristics, which are the circuit parameters of the circuit configuration, are simultaneously wavelength-independent, N
The phase shift amount of each of the phase controllers is obtained based on the design procedure described in the embodiment. It is actually impossible to obtain such a large number of circuit parameters by trial and error, and it is impossible to obtain the optimal circuit parameters even if the circuit parameters are determined by a successive approximation method using a computer. Close to. The design procedure used in the present invention is unique to the present invention.
Unlike the successive approximation method, the point is that the optimal circuit parameters can be obtained with a finite number of calculations.

Even if an invention having a circuit configuration similar to that of the present invention exists, the present invention claims differentiation from other patents due to the existence of a unique design procedure of the present invention. Specifically, the group delay equalizer described in the embodiment has characteristics that cannot be obtained without using this design procedure, and even when the circuit configuration is similar,
By comparing the embodiments, it is possible to differentiate the present invention from other inventions.

[0106]

As described above, the broadband optical signal processor of the present invention adopts the circuit configuration of claim 1 using a directional coupler whose amplitude characteristics and phase characteristics are simultaneously independent of wavelength. Thus, an optical signal processor having a wide band of 10 nm or more, which cannot be realized in the past, can be realized. Further, specifically, it is possible to determine the circuit parameters for the desired characteristics by a circuit parameter designing technique for realizing the desired characteristics required in claim 2. Further, by using a directional coupler having the fixed coupling ratio required in claim 3 and having amplitude characteristics and phase characteristics simultaneously independent of wavelength, a wideband programmable optical signal processing in which an arbitrary filter characteristic can be programmed. Vessels are possible. In addition, it has a variable coupling rate required in claim 5 or 6,
By using a directional coupler in which the amplitude characteristic and the phase characteristic are wavelength-independent at the same time, a broadband programmable optical signal processor in which an arbitrary filter characteristic can be programmed by one circuit can be realized. Further, such a broadband optical processor can easily realize desired filter characteristics by the design procedure described in claim 7.

The broadband optical signal processor of the present invention having such excellent characteristics is used as a high-performance optical signal processor for performing advanced information processing of various optical fibers in the field of wideband wavelength division multiplexing communication. It is possible. Further, since the present optical signal processor can realize filter characteristics equivalent to an electric digital filter, it can also be used as an ultra-high-speed optical signal processor.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a conventional programmable optical signal processor.

FIG. 2 is a circuit configuration diagram of a broadband optical signal processor according to the present invention.

FIG. 3 is a circuit configuration diagram (N-stage configuration) of a directional coupler used in Embodiment 1 of the present invention, in which amplitude characteristics and phase characteristics are simultaneously independent of wavelength and whose amplitude coupling ratio is variable.

FIG. 4 is a graph showing a power coupling ratio (amplitude characteristic) of a variable directional coupler used in the first embodiment of the present invention, in which the amplitude characteristic and the phase characteristic are simultaneously wavelength-independent.

FIG. 5 is a graph showing a phase characteristic of a variable directional coupler used in the first embodiment in which the amplitude characteristic and the phase characteristic are simultaneously independent of wavelength.

FIG. 6 is a graph showing a power coupling ratio (amplitude characteristic) of a variable directional coupler used in a conventional programmable optical signal processor.

FIG. 7 is a graph showing a phase characteristic of a variable directional coupler used in a conventional programmable optical signal processor.

FIG. 8 is a circuit configuration (eight-stage configuration) of Embodiment 1 of the present invention.
6 is a graph showing light transmission intensity characteristics in a 1.5 μm wavelength band which is a short wavelength end of an operating wavelength range (periodic wavelength is 1.6 nm) in the group delay equalizer realized in FIG.

FIG. 9 is a circuit configuration (eight-stage configuration) of Embodiment 1 of the present invention.
6 is a graph showing a group delay equalization characteristic in a 1.5 μm wavelength band which is a short wavelength end of an operation wavelength range in the group delay equalizer realized in FIG.

FIG. 10 is a graph showing light transmission intensity characteristics in a 1.55 μm wavelength band which is a center wavelength of an operation wavelength band in the group delay equalizer realized by the circuit configuration (eight-stage configuration) of the first embodiment of the present invention. is there.

FIG. 11 is a graph showing a group delay equalization characteristic of a 1.55 μm wavelength band which is a center wavelength of an operation wavelength band in a group delay equalizer realized by a circuit configuration (eight-stage configuration) of the first embodiment of the present invention. It is.

FIG. 12 shows light transmission in a 1.5 μm wavelength band which is a short wavelength end of an operating wavelength band (periodic wavelength is 1.6 nm) in a group delay equalizer realized by a conventional programmable optical signal processor (eight-stage configuration). 5 is a graph showing strength characteristics.

FIG. 13 is a graph showing a group delay equalization characteristic of a 1.5 μm wavelength band which is a short wavelength end of an operation wavelength region in a group delay equalizer realized by a conventional programmable optical signal processor (eight-stage configuration). .

FIG. 14 is a graph showing light transmission intensity characteristics in a 1.55 μm wavelength band which is a center wavelength of an operation wavelength region in a group delay equalizer realized by a conventional programmable optical signal processor (eight-stage configuration).

FIG. 15 is a graph showing a group delay equalization characteristic in a 1.55 μm wavelength band which is a center wavelength of an operation wavelength band in a group delay equalizer realized by a conventional programmable optical signal processor (eight-stage configuration).

FIG. 16 shows circuit parameters θ _n (n = 0, 1,..., N) and φ _n (n) of the programmable optical signal processor of the present invention.
= 1, 2,..., N).

FIG. 17 shows circuit parameters θ _n (n = 0, 1,..., N) and φ _n (n
= 1, 2,..., N).

FIG. 18 is a graph showing a group delay equalization characteristic (a periodic wavelength is 4 nm) of the group delay equalizer realized in the second embodiment of the present invention.

FIG. 19 is a graph showing group delay equalization characteristics (periodic wavelength: 4 nm) of a group delay equalizer realized by a conventional programmable optical signal processor (eight-stage configuration).

[Explanation of symbols]

Reference numerals 1 and 2 Optical waveguide 3 Amplitude characteristics and phase characteristics Simultaneous wavelength-independent variable directional coupler 4 Phase controller 5 Substrate 6 Optical waveguide 7 Directional coupler 8 Optical path length adjustment unit 9 Conventional variable directional coupler 10 Phase Controller 11 3 dB directional coupler 12, 13 Optical waveguide 14, 15 Input port 16, 17 Output port 18, 19 Input port 20, 21 Output port

Claims (7)

[Claims] 1. N + 1 (N is 1 or more) two-input two-output optical couplers are arranged in a line, and two optical waveguides having an optical path length difference between adjacent optical couplers are used. The two optical waveguides have the same optical path length difference between the N optical couplers, and at least one phase controller is provided on each of the two optical waveguides. In the case where the optical coupler has an optical signal input from any of the two input ports, the optical coupler has a circuit configuration with respect to the optical signal output from the two output ports. A broadband optical signal processor having amplitude and phase transmission characteristics that are constant in an operating wavelength range of the optical signal processor. 2. The amplitude coupling ratio θ _{n of the} two-input two-output optical coupler and the phase amount φ _n of the phase controller provided on the two optical waveguides are as follows with respect to a desired filter characteristic. Equation; Here, a _k ^[n] and b _k ^[n] are complex numbers obtained by the following recurrence formula. (Equation 2) _Also, φ _an, φ _bn is the phase amount corresponding to each output port with two outputs of the n-th of the optical coupler designed; claim 1, characterized in that taking the calculated value based on 4. The broadband optical signal processor according to 1. 3. The optical coupler according to claim 1, wherein the two optical waveguides are arranged in the optical coupler (m
+1) locations (m is 2 or more) and (m + 1) directional couplers arranged close to each other, and at m locations sandwiched between the directional couplers, the two optical waveguides are 5 μm.
3. The optical system according to claim 2, wherein the optical path length difference is as follows.
4. The broadband optical signal processor according to 1. 4. The optical coupler according to claim 1, wherein the two optical waveguides are connected to each other by (m
It has (m + 1) directional couplers arranged close to each other at +1) locations (m is 2 or more), and the two optical waveguides are equal at m locations sandwiched between the directional couplers. The broadband optical signal processor according to claim 2, wherein the optical waveguide has a difference in optical path length, and at least one optical path length adjusting unit is provided on each of the optical waveguides. 5. The wide-band optical signal according to claim 2, wherein said optical coupler is a variable optical coupler whose amplitude coupling ratio value is constant in said operating wavelength range. Processor. 6. The variable optical coupler includes (n + 1) directional couplers each having two optical waveguides arranged close to each other at (n + 1) locations (n is 3 or more), At n positions sandwiched by the directional couplers, the two optical waveguides have the same optical path length difference, or have an optical path length difference of 5 μm or less, and at least one optical waveguide is provided on each of the optical waveguides. The broadband optical signal processor according to claim 5, further comprising: an optical path length adjusting unit. 7. N + 1 (N is 1 or more) 2-input / 2-output optical couplers are arranged in a line, and two optical waveguides having an optical path length difference between adjacent optical couplers are used. The two optical waveguides have the same optical path length difference between the N optical couplers, and at least one phase controller is provided on each of the two optical waveguides. In the case where the optical coupler has an optical signal input from any of the two input ports, the optical coupler has a circuit configuration with respect to the optical signal output from the two output ports. A method for designing a broadband optical signal processor capable of realizing desired filter characteristics in a broadband optical signal processor having amplitude and phase transmission characteristics that are constant in an operating wavelength range of the optical signal processor, wherein the desired filter Characteristics of the optocoupler The phase of the width coupling ratio and the phase controller, the following design procedure 1 4
Based on the above, the amplitude coupling ratio of these optical couplers and the phase amount of the phase controller are determined, and thereby, the desired filter characteristics are realized, thereby designing a broadband optical signal processor. Method. (Design Procedure 1) The desired filter characteristics are approximated by Fourier series expansion of N-order complex coefficients. (Design Procedure 2) Normalize the obtained Fourier series function so that the maximum absolute value is 1 or less. (Design Procedure 3) Using a standardized Fourier series function, all transmission characteristics of the optical signal processor are expressed by a 2-by-2 transmission characteristic matrix. (Design Procedure 4) The obtained transmission characteristic matrix of the entire optical signal processor is converted into two rows and two columns corresponding to each unit configuration including each of the two optical waveguides having an optical path length difference and the optical coupler. Is decomposed into the product of the transmission characteristic matrices, and a 2-by-2 transmission characteristic matrix representing the unit configuration is expressed using the amplitude coupling ratio of the optical coupler and the phase amount of the phase controller as parameters. Based on this, an amplitude coupling ratio of the optical coupler and a phase amount of the phase controller are obtained.