Optical demultiplexer and optical transmission system

It is an object of the present invention to provide an optical transmission system in which multiple optical frequencies can be efficiently used and the change of the system can be easily made, and to provide an optical demultiplexer suitable for such optical transmission system. In an optical demultiplexer according to the present invention, signal light having a plurality of wavelength components arranged on a grid having predetermined frequency intervals is input from an input port thereof to be demultiplexed. The demultiplexed signal lights output from the output ports thereof have a plurality of wavelength components, respectively, and any three wavelength components, fa, fb, and fd, that satisfy the following conditions: fa<fb<fd; and fd−fa≦NΔf have the following relationship: 2fb≠fa+fd where N represents an integer, and Δf represents each of the predetermined frequency intervals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical demultiplexer that demultiplexes signal light having a plurality of wavelength components and outputs from a plurality of output ports thereof, and to an optical transmission system including the optical demultiplexer.

2. Description of the Background Art

In an optical transmission system, by transmitting signal light through a transmission line made of an optical fiber, a large volume of information can be transmitted and received at high speed. Above all, in a wavelength division multiplexing (WDM) optical transmission system, signals (wavelength components) which have different wavelengths are multiplexed and transmitted through an optical fiber transmission line, whereby a greater capacity of information can be transmitted.

In this WDM optical transmission system, it is important to suppress the deterioration of the waveform of each signal transmitted through the optical fiber transmission line. The signal waveform deterioration in the WDM optical transmission system is mainly caused by the accumulated chromatic dispersion and nonlinear optical phenomena. In particular, when high density multiplexing is performed, it is essential to suppress signal-waveform deterioration caused by four-wave mixing, which is one of the nonlinear optical phenomena.

Japanese Patent Application Publication No. 9-247091 (corresponding to U.S. Pat. No. 6,366,376) discloses an invention intended to suppress the occurrence of four-wave mixing. In the disclosed optical transmission system, the arrangement of optical frequencies of the signals transmitted through the optical fiber transmission line is contrived so as to prevent the wavelength of four-wave mixed light generated on an optical fiber transmission line from being superimposed on the wavelengths of signals.

The above invention is suitable for use in a system including an optical demultiplexer. Specifically, in the case of optical transmission lines (e.g., optical transmission lines for an access system) connected to output ports of an optical demultiplexer, signal-waveform deterioration due to four-wave mixing can be suppressed by arranging the optical frequencies of signals as described in the above publication. In addition, even if a low-dispersion optical fiber is used for an optical transmission line, signal-waveform deterioration caused by four-wave mixing is suppressed, and in this case, the need for a dispersion compensator is eliminated, thus reducing the cost of the system.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical transmission system in which multiple optical frequencies can be efficiently used and the change of the system can be easily made, and to provide an optical demultiplexer suitable for such optical transmission system.

In an optical demultiplexer according to the present invention, signal light having a plurality of wavelength components arranged on a grid having predetermined frequency intervals is input from an input port thereof to be demultiplexed, and demultiplexed light is output from a plurality of output ports thereof. In the optical demultiplexer, the signal lights output from the output ports have a plurality of wavelength components, respectively, and any three wavelength components that satisfy the following conditions:
fa<fb<fd; and
fd−fa≦NΔf
have the following relationship:
2fb≠fa+fd
where fa, fb, and fdrepresent the optical frequencies of the three wavelength components, respectively, N represents an integer, and Δf represents each of the predetermined frequency intervals.

Alternatively, the signal lights output from the output ports have a plurality of wavelength components, respectively, and any four wavelength components that satisfy the following conditions:
fa<fb<fc<fd; and
fd−fa<NΔf
have the following relationship:
fb+fc≠fa+fd
where fa, fb, fc, and fdrepresent the optical frequencies of the four wavelength components, respectively, N represents an integer, and Δf represents each of the predetermined frequency intervals. The optical frequencies of the wavelength components in each of the output signal lights may be arranged periodically with a cycle of NΔf, where N represents an integer equal to or greater than 2. Among wavelength components in the input signal light, the polarization of any two wavelength components having adjacent optical frequencies is orthogonal to each other and among the wavelength components in the output signal lights, the polarization of any two wavelength components having adjacent optical frequencies is orthogonal to each other.

In addition, an optical transmission system is provided which includes an optical demultiplexer according to the present invention, an input-side optical transmission line connected to an input port of the optical demultiplexer; and output-side optical transmission lines respectively connected to output ports of the optical demultiplexer.

Preferably, the absolute value of chromatic dispersion in signal light wavelengths on each of the output-side optical transmission lines is equal to or less than 5 ps/nm/km. The optical transmission system may further include at least one optical amplifier for performing optical amplification on one of signal light which is input to the input port and signal light which is output from each of the output ports.

The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are explained below by referring to the accompanying drawings. In the drawings, the same number refers to the same part to avoid duplicate explanation. The ratios of the dimensions in the drawings do not necessarily coincide with the explanation.

FIG. 1is a block diagram showing an optical transmission system1according to an embodiment of the present invention. The optical transmission system1shown inFIG. 1is a downlink system from a branch optical fiber line2up to user-side apparatuses6(61to6M). The optical transmission system1includes the optical fiber line2, an optical demultiplexer3, optical amplifiers41to4M, access-system optical-fiber transmission lines51to5M, and the user-side apparatuses61to6M. Here, the subscript M represents an integer equal to or greater than 2.

The optical fiber transmission line2transmits signal light having a plurality of wavelengths from a trunk line node (not shown) to branch nodes. The optical demultiplexer3and optical amplifiers41to4Mare provided at the branch nodes. The optical demultiplexer3has an input port. Signal light which has wavelength components and which is transmitted through the optical fiber transmission line2is input to the input port. The input signal light is demultiplexed and signal lights obtained by demultiplexing are output from any one output port to the optical amplifier4m. The subscript m represents an integer which is not less than 1 and not greater than M. The optical amplifier4moptically amplifies signal light which is output from the optical demultiplexer3, and sends the amplified signal light to an optical fiber transmission line5m. The optical fiber transmission line5mis used to transmit, to the user-side apparatus6m, the signal light sent from the optical amplifier4m.

FIGS. 2A to 2Dare graphs illustrating the optical frequencies of wavelength components in signal light which is transmitted through each of optical fiber transmission lines in the optical transmission system1shown inFIG. 1.FIG. 2Ashows an arrangement of optical frequencies of signal light which is transmitted through the optical fiber transmission line2.FIGS. 2B to 2Dshow arrangements of optical frequencies of wavelength components in signal light which is transmitted through each of the optical fiber transmission lines51to53. In each ofFIGS. 2A to 2D, the axis of abscissa indicates the optical frequency, and each line extending upwardly with respect to the axis of abscissa indicates a position in which each wavelength component of signal light is arranged.

As shown inFIG. 2A, the optical frequencies of wavelength components in each signal light which is input to the input port of the optical demultiplexer3through the optical fiber transmission line2are arranged on a grid having interval Δf. As shown inFIG. 2B, the optical frequencies of wavelength components in a demultiplexed signal light which is output from a first output port of the optical demultiplexer3to the optical amplifier41are arranged having a constant period NΔf, where N represents an integer equal to 2 or greater.

Among the demultiplexed signal light which is output from the first output port of the optical demultiplexer3to the optical amplifier41, any three wavelength components that satisfy the following conditions:
fa<fb<fd(1a);and
fdfa≦NΔf(1b)
have the following relationship:
2fb≠fa+fd(2)
where fa, fb, and fdrepresent the optical frequencies of the three wavelength components, respectively. In other words, optical frequencies faand fdare not symmetrically positioned with respect to optical frequency fb. This applies to signal lights which are output from the other output ports of the optical demultiplexer3to the optical amplifiers42to4M(SeeFIGS. 2C and 2D).

Alternatively, among the demultiplexed signal light which is output from the first output port to the optical amplifier41, any four wavelength components that satisfy the following conditions:
fa<fb<fc<fd(3a);and
fd−fa≦NΔf(3b)
have the following relationship:
fb+fc≠fa+fd(4)
where fa, fb, fc, and fdrepresent the optical frequencies of the four wavelength components, respectively. In other words, a pair of the optical frequencies faand fband a pair of the optical frequencies fcand fdhave no mirror-image relationship to each other.

FIG. 3illustrates the optical frequencies of each wavelength component in signal light and four-wave mixed light on an optical fiber transmission line51in the optical transmission system1shown inFIG. 1. InFIG. 3, the axis of abscissa indicates the optical frequency, long lines extending upward with respect to the axis of abscissa indicate optical frequencies of wavelength components in signal light, and short lines extending upward indicate optical frequencies at which four-wave mixed light appears. Since the optical frequencies of the demultiplexed wavelength components in the signal light output from the first output port of the optical demultiplexer3are arranged so as to satisfy Expressions (1a) to (2), or so as to satisfy Expressions (3a) to (4), the optical frequency of light caused by four-wave mixing does not coincide with the optical frequency of any wavelength component in signal light on the optical fiber transmission line51, as shown inFIG. 3. This applies to signal light output from the other output ports of the optical demultiplexer3to the optical amplifiers42to4M.

As described above, according to the optical transmission system1, the optical frequencies of wavelength components in signal lights are arranged so as to satisfy Expressions (1a) to (2), or so as to satisfy Expressions (3a) to (4), whereby, on each optical fiber transmission line5mfrom each output port of the optical demultiplexer3to each user-side apparatus6m, the effect of four-wave mixing is reduced and a plurality of wavelength components in signal light can be transmitted to each user-side apparatus6m. In addition, because the optical frequencies of the demultiplexed signal light which is output from the each output port of the optical demultiplexer3to the optical amplifiers4mare arranged having a constant period NΔf, in each user-side apparatus6m, all signal components appearing in the period NΔf can be demultiplexed by using a filter having periodic characteristics, so the apparatus structure is simplified. The optical demultiplexer3can be constituted only by passive optical parts. Thus, its structure is simplified and it easily corresponds to a system change. As described above, the optical transmission system1according to this embodiment has good usability of optical frequencies and ease in system change.

It is preferable that, in each optical fiber transmission line5mconnected to each output port of the optical demultiplexer3, the absolute value of chromatic dispersion in signal light wavelength be equal to or less than 5 ps/nm/km. The absolute value of chromatic dispersion in signal light wavelength in the optical fiber transmission line5mthat is equal to or less than 5 ps/nm/km eliminates the need for providing the user-side apparatus6mwith a dispersion compensator, thus forming an inexpensive system.

FIGS. 4A to 4Cillustrate another arrangement of optical frequencies of wavelength components in signal light transmitted on the optical fiber transmission lines in the optical transmission system1shown inFIG. 1.FIG. 4Ashows an arrangement of optical frequencies of signal light which has a plurality of wavelength components and which is transmitted through the optical fiber transmission line2.FIGS. 4B and 4Cshow arrangements of optical frequencies of wavelength components in signal light transmitted through the optical fibers51and52. In each ofFIGS. 4A to 4C, the axis of abscissa indicates the optical frequency, and each line which extends upward or downward indicates a position in which an optical frequency of wavelength component in signal light is positioned. The signal light indicated by each upwardly extending line and the signal light indicated by each downwardly extending line have orthogonal polarization to each other.

As shown inFIG. 4A, the optical frequencies of wavelength components in signal light which are transmitted through the optical fiber transmission line2and are input to the input port of the optical demultiplexer3are arranged on a grid having each regular frequency interval Δf. Among the wavelength components in the signal light, any two wavelength components having adjacent optical frequencies have orthogonal polarization to each other.

As shown inFIG. 4B, the optical frequencies of the demultiplexed signal light which are output from the first output port of the optical demultiplexer3to the optical amplifier41are arranged having a constant period NΔf. Among the demultiplexed signal light output from the first output port of the optical demultiplexer3to the optical amplifier41, when the optical frequencies of any three wavelength components that satisfy Expression (1) are represented by fa, fb, and fd, respectively, these optical frequencies satisfy Expression (2), and the optical frequencies faand fdare not symmetrical in position with respect to the optical frequency fb. Among the demultiplexed signal light output from the first output port of the optical demultiplexer3to the optical amplifier41, when the optical frequencies of any four wavelength components that satisfy Expression (3) are represented by fa, fb, fc, and fd, respectively, these optical frequencies satisfy Expression (4), and a pair of the optical frequencies faand fband a pair of the optical frequencies fcand fddo not have any mirror relationship to each other. In addition, among the demultiplexed signal light which are output from each output port of the optical demultiplexer3, any two wavelength components having adjacent optical frequencies have orthogonal polarization to each other. This applies to the signal lights output from the other output ports of the optical demultiplexer3to the optical amplifiers42to4M, as shown inFIG. 4C.

When the arrangement of optical frequencies of wavelength components in signal light and polarization states thereof are set in a state such as described above, the optical transmission system1not only produces the above-described advantages, but also prevents four-wave mixing itself from occurring. Therefore, in the optical transmission system1, multiplexing can be performed at higher density.

FIG. 5shows the specific structure of an optical demultiplexer31for use in Embodiment 1. The optical demultiplexer31comprises four arrayed waveguide gratings (AWGs)70to73for multiplexing and demultiplexing light each of whose wavelength intervals is Δf and optical fibers for connecting them.

Twelve wavelength (λ1to λ12) components arranged at each frequency interval Δf and constituting signal light transmitted through an optical fiber transmission line2are demultiplexed by the AWG70into twelve signal lights of wavelengths λ1to λ12. Four signal lights of wavelengths λ1, λ2, λ4, and λ8are multiplexed by the AWG71into signal light having four wavelength components, and the multiplexed signal light is transmitted to an optical fiber transmission line51. Similarly, signal lights of wavelengths λ3, λ7, λ10, and λ12are multiplexed by the AWG72into signal light to be transmitted to an optical fiber transmission line52. Signal lights of wavelengths λ5, λ6, λ9, and λ11are multiplexed by the AWG73to be transmitted to an optical fiber transmission line53. The signal lights having four wavelength components and transmitted to the optical fiber transmission lines51,52, and53satisfy the conditions represented by Expressions (1a) to (4), respectively.

In the case of controlling wavelength assignment, the portion denoted by reference numeral8inFIG. 5may be replaced by a 12×12 optical cross connector.

FIG. 6shows the specific structure of an optical demultiplexer32for use in Embodiment 2. The optical demultiplexer32includes four AWGs90to93each having a period 9×Δf. Eighteen wavelength (λ1to λ18) components constituting signal light transmitted through an optical fiber2and sequentially arranged at each frequency interval Δf are demultiplexed by an AWG90into nine signal lights having pairs of wavelengths λ1and λ10, λ2and λ11, . . . , λ9and λ18. Six signal light wavelength components of λ1, λ2, λ4, λ10, λ11, and λ13are multiplexed by an AWG91into signal light having six wavelength components, and the multiplexed signal light is transmitted to an optical fiber transmission line51. Similarly, signal light wavelength components of λ3, λ5, λ8, λ12, λ14, and λ17are multiplexed by an AWG92to be transmitted to an optical fiber52. Signal light wavelength components of λ6, λ7, λ9, λ15, λ16, and λ18are multiplexed by an AWG93to be transmitted to an optical fiber53.

The signal lights which each have six wavelength components and which are transmitted to the optical fibers51,52, and53satisfy the conditions represented by Expressions (1a) to (4). Each signal light has the period 9×Δf.

The present invention is not limited to the above embodiments, but can be modified variously. For example, instead of providing an optical amplifier after the stage of the optical demultiplexer3, the optical amplifier may be provided before the stage of the optical demultiplexer3.

The entire disclosure of Japanese Patent Application No. 2002-105365 filed on Apr. 8, 2002 including a specification, claims, drawings, and a summary are incorporated herein by reference in its entirety.