JP2002071978A - Optical multiplexer/demultiplexer improved in group delay characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize a group delay dispersion without changing the multiplexing/demultiplexing characteristics of an optical multiplexer/ demultiplexer when the number of stages of a lattice filter is increased and when the number of combination stages is increased, in the highly functional optical multiplexer/demultiplexer of M-input and M-output using the lattice filter as an optical multiplexing/demultiplexing element of two-input and two- output. SOLUTION: The values of circuit parameters of respective stage lattice filters are designed so that group delay dispersions of multistage connected lattice filters 111-113 equale in amplitude characteristic but different in phase characteristic have inverse characteristics in the forward stage and in the backward stage. Thus, group delay dispersion is negated, consequently, group delay dispersion corresponding to second order derivative to a phase frequency is improved without changing the multiplexing/demultiplexing characteristics of the optical multiplexer/demultiplexer, and group delay dispersion is minimized as the whole of multistage lattice filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to optical communication, optical switching,
The present invention relates to an optical multiplexer / demultiplexer used in the field of optical computing and the like, and more particularly, to an optical multiplexer / demultiplexer in which a Lattice filter in which Mach-Zehnder filters are cascaded in multiple stages is configured in multiple stages.

[0002]

2. Description of the Related Art In recent years, in the fields of optical communication, optical switching, optical computing, and the like, in particular, in the field of wavelength multiplexing optical communication in which a plurality of optical signals having different wavelengths are multiplexed and communicated, wavelength-multiplexed signal light is different. An optical demultiplexer that demultiplexes to a different output port for each wavelength or an optical multiplexer that outputs an optical signal in which a wavelength is multiplexed to one output port when a different wavelength is input to a different input port is important. Parts.

[0003] As a component that can meet such a demand, there is an arrayed waveguide grating (Reference 1: H. T.
akahashi, S. Suzuki, and I. Nishi, "Wavelength mu
ltiplexer based SiO ₂ -Ta ₂ O ₅ arryed-waveguide gratin
g ", J. Lightwave Technol., vol.12, no.6, pp. 989-
995, 1994). The arrayed waveguide grating has already been introduced into a working system as a device suitable for wavelength division multiplexing communication since a very large number of signals can be multiplexed / demultiplexed with one element.

On the other hand, a Mach-Zehnder filter is used as a high-performance optical multiplexer / demultiplexer that has a relatively low number of multiplexes that can be handled as about 16 waves or less, yet has a low loss and also has a switching function in addition to an optical multiplexer / demultiplexer function. There is a combined optical multiplexer / demultiplexer.

FIG. 3 shows a configuration of a conventional optical demultiplexer which handles four waves. A Mach-Zehnder filter is used as a 2-input 2-output optical multiplexing / demultiplexing element, and three of them are combined. First stage Mach-Zehnder filter 10 close to input port
The optical path length difference of 1 is 2ΔL (Δ represents a minute value), and the optical path length differences of the two Mach-Zehnder filters 102 and 103 in the second stage are each ΔL. Conversely, the periodic wavelength is opposite, and the first-stage Mach-Zehnder filter 101 is Δλ, and the second-stage Mach-Zehnder filters 102 and 103 are each 2Δ.
λ. Second stage two Mach-Zehnder filters 10
2 and 103 have their center wavelengths shifted by Δλ.

[0006] Four waves λ1, λ2, λ3, λ4 having different wavelengths input to the optical demultiplexer are divided into λ1, λ3 and λ2, λ4 by a first stage Mach-Zehnder filter 101, and further divided into two stages. Eye Mach-Zehnder filter 102,
103 divides them into λ1, λ3, λ2, and λ4. Further, by driving the phase shifters 41 to 43 provided on the waveguides of the respective Mach-Zehnder filters, it is possible to freely switch the wavelength demultiplexed to the output port.

Recently, instead of the above-described Mach-Zehnder filter, a more sophisticated light combining device having excellent wavelength flat transmission characteristics using a lattice filter in which two or more stages of Mach-Zehnder filters are cascaded. Corrugations are being made.

[0008]

However, in an optical multiplexer / demultiplexer using a lattice filter in which Mach-Zehnder filters are cascaded in multiple stages as described above, it is better to increase the number of lattice filters in order to improve the function. However, there is a problem to be solved that increasing the number of stages increases the group delay dispersion.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to improve the function of an M-input M-output using a lattice filter as a 2-input 2-output optical multiplexer / demultiplexer. In an optical multiplexer / demultiplexer, when the number of lattice filter stages is increased and when the number of combination stages is increased, the group delay dispersion is minimized without changing the multiplexing / demultiplexing characteristics of the optical multiplexer / demultiplexer.

[0010]

In order to achieve the above object, according to the present invention, there are provided two optical waveguides and a plurality of N + 1 locations (N is an integer of 2 or more) at different positions. A plurality of different M-waves (M is 2) constituted by combining a plurality of 2-input / 2-output optical multiplexing / demultiplexing elements having a configuration of N + 1 optical couplers coupled in a plurality of stages.
In the optical multiplexer / demultiplexer for multiplexing / demultiplexing the wavelengths of the above integers, the two-input / two-output optical multiplexing / demultiplexing element can have different phase characteristics even if the amplitude characteristics are the same by changing circuit parameters. Utilizing this fact, a plurality of the aforementioned two-input / two-output optical multiplexing / demultiplexing elements having different phase characteristics are combined, and the group delay dispersion having the opposite characteristic to the group delay dispersion of the preceding stage is given to the latter stage. It is characterized in that the group delay dispersion of all channels of the wave filter is minimized.

Here, by flattening the transmission characteristics in the transmission region / rejection region of the two-input two-output optical multiplexer / demultiplexer,
It can be characterized by having a flat transmission area / blocking area.

Further, the two-input two-output optical multiplexing / demultiplexing device can be characterized in that the phase characteristic has a minimum phase or a maximum phase.

The two-input two-output optical multiplexing / demultiplexing device may be a lattice filter in which a plurality of Mach-Zehnder filters each having a phase shifter for phase control are cascaded.

Further, the phase control amount of the phase shifter is adjusted so that the group delay dispersion has inverse characteristics between the optical multiplexing / demultiplexing element (or the optical multiplexing / demultiplexing element group) at the preceding stage and the optical multiplexing / demultiplexing element group at the subsequent stage. It can be characterized by setting.

(Function) 2 used as an optical multiplexing / demultiplexing element
It is known that a lattice filter having two inputs and two outputs has a plurality of phase characteristics with respect to one amplitude characteristic (Reference 2: K. Jinguji and M. Kawachi, "Synthesis of coh
erent two-port lattice-form optical delay-line cir
cuit ", J. Lightwave Technol., vol. 13. no. 1, p
p. 73-82, 1995). Taking advantage of this, in the present invention,
As in the above configuration, the group delay dispersion of the multi-stage connected lattice filters having the same amplitude characteristic and different phase characteristics is combined so that the group delay dispersion has an inverse characteristic, so that the group delay dispersion is canceled out. It is possible to improve the group delay dispersion corresponding to the second derivative of the phase with respect to the frequency without changing the multiplexing / demultiplexing characteristics of the filter, and to minimize the group delay dispersion as a whole multistage lattice filter.

[0016]

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

(First Embodiment) FIG. 1A shows a configuration of a four-wavelength optical multiplexer / demultiplexer having a flat wavelength transmission / rejection band according to a first embodiment of the present invention. FIG. 1B shows a light intensity transmittance characteristic (broken line) and a group delay dispersion characteristic (solid line) with respect to a relative optical frequency of the first-stage lattice filter 111 of the optical multiplexer / demultiplexer, and FIG. ) Shows the light intensity transmittance characteristic (dashed line) and the group delay dispersion characteristic (solid line) with respect to the relative optical frequency of the second-stage lattice filters 112 and 113 of the optical multiplexer / demultiplexer. In the present embodiment, as shown in FIG. 1A, the first stage on the input port side has a lattice filter 111 configured by cascading three stages of Mach-Zehnder filters, and the second stage has Mach-Zehnder filters respectively. A lattice filter 112 constituted by cascading four filters,
113 are used. Further, in the present embodiment, in order to realize a characteristic in which the transmission band / rejection band has a flat wavelength, the optical path length difference of the first-stage lattice filter 111 is 4ΔL, 4Δ.
L, 2ΔL, second stage lattice filters 112, 11
3 are 2ΔL, 2ΔL, 2ΔL, and ΔL, respectively.
And

The optical multiplexer / demultiplexer is a quartz-based planar lightwave circuit (Reference 3: M. Kawachi, "Silica waveguides on silicon").
and their application to integrated-optic componen
ts ", Optical and Quantum Electronics. Vol. 22. pp.
391-416, 1990). Phase control heaters 41 to 51 were formed on the optical waveguides of the respective Mach-Zehnder filters as phase shifters. In the present embodiment, as an example, each channel interval of four waves is 100 GHz.
z.

Correspondingly, the periodic frequency of the first-stage lattice filter 111 is set to 100 GHz, and the periodic frequencies of the second-stage lattice filters 112 and 113 are each set to 200 GHz. Optical path length difference ΔL determined by periodic frequency
Was 1.03 mm.

Two 4-stage lattice filters 1 at the second stage
12 and 113 have center frequencies of 100 GHz
In contrast, as shown in FIGS. 1B and 1C, the transmission characteristics near the center frequency (wavelengths λ1, λ2, λ3, λ4) of each channel are flattened, and the group near the center frequency of each channel is set. Lattice filter 1 with first stage delay dispersion
11 is designed to have the opposite characteristic.

Although there are a plurality of methods for realizing the wavelength flat characteristic, in the present embodiment, as shown in FIGS. 1B and 1C, the first-stage lattice filter 111 has the maximum flat characteristic, The second-stage lattice filters 112 and 113 have achieved wavelength flattening by Chebyshev characteristics, which are characterized by ripples (pulsations).

By changing circuit parameters with the same circuit configuration, a lattice filter having two or more stages can design filters having the same transmission characteristics and different phase characteristics, in other words, different group delay characteristics. In the present embodiment, the circuit parameters of the lattice filter are the coupling ratio of the directional couplers 21 to 34 and the phase control amounts of the phase shifters (phase control heaters) 41 to 51. In the case of the first-stage three-stage lattice filter 111, FIG.
There are 16 types of filters having maximum flatness characteristics as shown in (1) and different group delay dispersion characteristics. In the case of the lattice filters 112 and 113 having a four-stage configuration of the second stage, FIG.
There are 2 10 powers of filters having Chebyshev characteristics as shown in (1) and different group delay dispersion characteristics.

In this embodiment, as shown in FIG. 2 described later, the center frequencies (wavelengths λ1, λ2, λ
(3, λ4), the group delay dispersion becomes 0, and the first-stage lattices are set so that the group delay dispersions of the first-stage and second-stage lattice filters have mutually opposite characteristics near the channel center frequency. The characteristics of the two output ports of the filter 111 indicate the minimum phase characteristics, and the two lattice filters 11
The design was made so that the characteristics of all four output ports 2, 113 had the maximum phase characteristics.

Here, when the transfer function z of the lattice filter is represented by a polynomial of z = exp (jωΔτ) (1), where ω is a frequency and Δτ is a unit delay time corresponding to the optical path length difference ΔL. The minimum phase characteristic is a filter characteristic in which the zero of the transfer function z of the filter is within a circle of | z | = 1, and the maximum phase characteristic is a zero of the transfer function z of the filter is outside the circle of | z | = 1. It is defined as a certain filter characteristic.

FIG. 2 shows the measurement results of the optical multiplexer / demultiplexer according to the first embodiment of the present invention manufactured as described above, in the vicinity of the channel center frequency of the light intensity transmittance and the group delay dispersion. The solid line A represents the group delay dispersion of the present embodiment, and the dashed line B represents the group delay dispersion of a conventional optical multiplexer / demultiplexer manufactured for reference. This conventional optical multiplexer / demultiplexer has the same circuit configuration as that of the present embodiment, but differs in circuit parameters.

Table 1 below shows the design values of the circuit parameters used in the first embodiment of the present invention and the design values of the circuit parameters used in the conventional example. In Table 1, θn represents the angle of the amplitude coupling of the directional couplers 21 to 34 (the power coupling ratio corresponds to sin ² (θn)), and Δn represents the phase shifter 4.
The phase control amounts of 1 to 51 are displayed, and n defines the numbering in order from the input port of each lattice filter.

[0027]

[Table 1]

The group delay dispersion of the optical multiplexer / demultiplexer in the present embodiment is such that the group delay dispersion of the first-stage lattice filter 111 and the group delay dispersion of the two second-stage lattice filters 112 and 113 have opposite characteristics. Since the values of the circuit parameters are designed as shown in (A) of Table 1, the channel center frequencies (wavelengths λ1, λ2, λ3, In the vicinity of λ4), the group delay dispersion is minimized over a frequency of 6 GHz or more.

On the other hand, in the conventional optical multiplexer / demultiplexer, the minimization of the group delay dispersion is not taken into consideration, and the phase characteristic of the output port of the lattice filter, in other words, the group The circuit parameters are designed by appropriately selecting the delay dispersion. Therefore, the group delay dispersion of the entire circuit has a large value. Here, the dashed line in FIG.
In the conventional example shown in B, all lattice filters are designed so as to have maximum flatness characteristics and minimum phase characteristics.

In this embodiment, as described above, in a lattice filter having two or more stages, by changing the design value of the circuit parameter, it is possible to design a filter having the same transmission characteristics and different group delay characteristics. Utilizing what can be done, by combining a lattice filter with group delay dispersion with forward characteristic in the preceding stage and a lattice filter with group delay dispersion with inverse characteristic in the latter stage, optical combining that minimizes group delay dispersion of all channels A wave device was realized.

(Other Embodiments) In the above-described first embodiment of the present invention, the two-stage lattice filter 111 of the first stage is used.
The characteristics of the output ports have the minimum phase characteristic, and the characteristics of all four output ports of the two lattice filters 112 and 113 in the second stage have the maximum phase characteristic. However, in addition to this combination, for example, the characteristics of the two output ports of the first-stage lattice filter 111 are set to the minimum phase characteristic and the maximum phase characteristic, respectively, and the corresponding second-stage lattice filters 112 and 113 are set.
In contrast, the output port characteristics of (1) and (2) have maximum phase characteristics and minimum phase characteristics, so that group delay dispersion can be minimized for all channels. Thus, in the present invention, there are a plurality of combinations that minimize the group delay dispersion. For this reason, the present invention is not limited to only the combination of the lattice filters of the first embodiment described above, but includes all combinations that can achieve the object of the present invention.

Also, in the first embodiment of the present invention, a four-wave optical multiplexer / demultiplexer is realized by combining two stages of lattice filters. However, the number of stages is further increased, and for example, three stages of lattice filters are combined. It is also possible to form an optical multiplexer / demultiplexer of 16 waves by combining 8 waves and 4 stages of lattice filters. At this time, in the case of a three-stage eight-wave optical multiplexer / demultiplexer, for example, the first stage, the second stage, and the third stage are minimized by using a lattice filter having inverse group delay dispersion. be able to. In the case of a 4-stage 16-wave optical multiplexer / demultiplexer, for example, the first stage, the second stage, the third stage, and the fourth stage are minimized by using a lattice filter having inverse group delay dispersion. It can be performed.

As described above, the present invention has been described using the specific embodiments. However, the present invention is not limited to the above-described embodiments.

For example, in the first embodiment of the present invention, a quartz-based planar optical circuit is used, but 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.

In the first embodiment of the present invention, a thermo-optical effect by heater heating is used as a phase control method. However, the phase control can be performed by another method such as an electro-optical effect. .

In the first embodiment of the present invention, the directional coupler is used as the optical coupler.
It is also possible to use an ference (MMI) coupler.

As described above, the present invention relates to a method of assembling elements, and is not restricted by a physical realizing means of the circuit. The circuit parameters shown in the first embodiment of the present invention are merely examples of the present invention, and the present invention is not limited to these values.

[0038]

As described above, according to the present invention, in an optical multiplexer / demultiplexer having high functional characteristics such as wavelength flattening, a multistage connected lattice filter having the same amplitude characteristics and different phase characteristics is used. Since the group delay dispersion is canceled by combining them so that the group delay dispersion has the opposite characteristic, the multiplexing / demultiplexing characteristics of the optical multiplexer / demultiplexer are not changed, and the differential with respect to the frequency of the phase is twice differentiated. The group delay dispersion can be improved, and the group delay dispersion can be minimized as a whole multistage lattice filter.

The optical multiplexer / demultiplexer of the present invention can be combined with an arrayed waveguide grating to form an optical multiplexer / demultiplexer having a very large number of wavelengths that can be handled.

As described above, the optical multiplexer / demultiplexer of the present invention can be expected to have a very large application range as a key device for wavelength division multiplexing communication.

[Brief description of the drawings]

FIG. 1A is a schematic diagram illustrating a circuit configuration of a four-wavelength optical multiplexer / demultiplexer having a flat transmission band / rejection band according to a first embodiment of the present invention, and FIG. Graph showing light intensity transmittance characteristics (broken line) and group delay dispersion characteristics (solid line) with respect to the relative optical frequency of the first stage lattice filter of the filter.
(C) is a graph showing a light intensity transmittance characteristic (broken line) and a group delay dispersion characteristic (solid line) with respect to a relative optical frequency of the second stage lattice filter of the optical multiplexer / demultiplexer.

FIG. 2 is a graph showing measurement results of the optical multiplexer / demultiplexer according to the first embodiment of the present invention and the conventional optical multiplexer / demultiplexer near the channel center frequency of the optical intensity transmittance and the group delay dispersion.

FIG. 3 is a schematic diagram showing a circuit configuration example of an optical multiplexer / demultiplexer formed by combining a conventional Mach-Zehnder filter.

DESCRIPTION OF SYMBOLS 11 to 14 Optical waveguides 21 to 34 Optical coupler 41 to 51 Phase shifter 101 First-stage Mach-Zehnder filter 102, 103 Second-stage Mach-Zehnder filter 111 First-stage three-stage lattice filter 112, 113 Second-stage 4-stage lattice filter

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) The inventor, Manabu Oguma 2-3-1, Otemachi, Chiyoda-ku, Tokyo Within Nippon Telegraph and Telephone Corporation (72) The inventor, Yoshinori Hibino 2-3-3, Otemachi, Chiyoda-ku, Tokyo No. 1 Nippon Telegraph and Telephone Corporation F-term (reference) 2H047 KA03 KA12 KB04 LA18 MA05 RA08 TA13

Claims (5)

[Claims] 1. A two-input, two-output configuration having a configuration comprising two optical waveguides and N + 1 optical couplers for coupling the two optical waveguides at a plurality of N + 1 locations (N is an integer of 2 or more) at different positions. An optical multiplexer / demultiplexer for multiplexing / demultiplexing a plurality of M-waves (M is an integer of 2 or more) composed of a plurality of optical multiplexing / demultiplexing elements in a plurality of stages. Utilizing the fact that the optical multiplexing / demultiplexing element can have different phase characteristics even if the amplitude characteristics are the same by changing circuit parameters, a plurality of the 2-input / 2-output optical multiplexing / demultiplexing devices having different phase characteristics are used. An optical multiplexer / demultiplexer characterized in that the group delay dispersion of all channels of the optical multiplexer / demultiplexer is minimized by giving the group delay dispersion of the opposite characteristic to the group delay dispersion of the preceding stage in the latter stage. 2. A flat transmission / rejection region by flattening transmission characteristics of a transmission / rejection region of the two-input / two-output optical multiplexing / demultiplexing device. 2. The optical multiplexer / demultiplexer according to 1. 3. The optical multiplexer / demultiplexer according to claim 1, wherein a phase characteristic of the two-input two-output optical multiplexer / demultiplexer has a minimum phase or a maximum phase. 4. The optical multiplexer / demultiplexer having two inputs and two outputs is a lattice filter in which a plurality of Mach-Zehnder filters each having a phase shifter for phase control are connected in cascade. An optical multiplexer / demultiplexer according to any of the claims. 5. The phase control amount of the phase shifter is adjusted so that group delay dispersion has inverse characteristics between the optical multiplexing / demultiplexing element (or the optical multiplexing / demultiplexing element group) at the preceding stage and the optical multiplexing / demultiplexing element group at the subsequent stage. The optical multiplexer / demultiplexer according to claim 4, wherein the optical multiplexer / demultiplexer is set.