JPH05102928A - Optical communication system - Google Patents

Abstract

PURPOSE:To provide an optical communication system which can easily assign the frequency even in a communication network having many nodes with application of the multiplication of optical wavelengths by assigning the optical frequency to each of circuits which secure the connection among the nodes. CONSTITUTION:Each of nodes 11-1 to 11-4 connected together via a common optical transmission line 12 separates (n) pieces of continuous frequency or the frequency set every (n) cycles from the path 12 and the other one of both types of frequency to the path 12 among those optical frequency which are arranged almost at an equidistance on a frequency axis. In such a way, both types of frequency are assigned to the nodes for transmission and reception respectively. Thus the nodes 11-1 to 11-4 can be connected together in a mesh form via the line 12. Furthermore the multiplication of frequency is possible with high density by use of a Mach-Zehnder type optical filter having the cycles different by double from each other or a ring resonator. Thus the number of nodes can be increased without increasing the number of optical fibers.

Description

Detailed Description of the Invention

[0001]

The present invention is used in relay transmission networks and local transfer networks. In particular, it relates to allocation of mutual paths between nodes.

[0002]

2. Description of the Related Art FIG. 12 shows a basic configuration example of a conventional optical communication network. In this conventional example, four nodes 121-1 to 121-1
121-4 are mutually connected by the optical fiber wired in the shape of a mesh. The optical fiber is housed in a cable 122 that is wired in a loop between the four nodes 121-1 to 121-4.

[0003]

As described above, when a plurality of nodes try to communicate using different optical fibers, the number of fibers wired between the nodes increases as the number of nodes increases. The fiber, the cable, and the conduit for accommodating the cable cause congestion, which further complicates maintenance and monitoring equipment for the optical fiber.

It is also possible to perform optical frequency multiplexing using a common optical fiber, but when the number of nodes is large, the number of multiplexes may increase and frequency allocation may become complicated.

An object of the present invention is to solve the above problems and to provide an optical communication system using optical wavelength division multiplexing, which facilitates frequency allocation even when the number of nodes is large.

[0006]

The optical communication system of the present invention comprises:
A plurality of n nodes are connected to a common optical transmission line, and each node has n continuous frequencies from the optical frequencies arranged at substantially equal intervals on the frequency axis or a cycle for every n cycles. A means for branching one of the frequencies from the optical transmission path, and a means for sending out the other of the n consecutive frequencies or the frequency of every n cycles to the optical transmission path. To do.

The branching means or the sending means has n
In order to select the optical frequency of each period, a Mach-Zehnder type filter or a multiple ring resonator in which two or more optical paths having different lengths are coupled by two optical directional couplers is used. It is desirable to include.

[0008]

When performing communication between a plurality of nodes coupled to a ring-shaped or bus-shaped optical transmission line, different optical frequencies (wavelengths) are assigned to the communication lines of each node.
At that time, regarding branching and insertion at each node, adjacent signal groups on the frequency axis (hereinafter referred to as “first type frequency group”)
And a signal group existing at a constant frequency interval and with a constant number of channels (hereinafter referred to as a “second type frequency group”), respectively, as one unit.

Here, a case where each node transmits an optical signal by the first type frequency group and receives an optical signal by the second type frequency group will be described. A certain node (referred to as “node A”) uses the optical frequency included in the first type frequency group assigned to the node A to transmit an optical signal to each of the other nodes. Here, there are n optical frequencies included in the first type frequency group, and the node of the transmission partner is n-1. This is because the optical frequency included in the second type frequency group addressed to itself is also included in the first type frequency group. This frequency can be used for monitoring or other purposes addressed to itself, but need not be used in particular. In another node (this is referred to as a node “B”), the optical signal is similarly transmitted to each of the other nodes by using the first type frequency group adjacent to the frequency group used by the node A for transmission.
In this way, each node transmits a signal using the first type frequency group different from each other. Further, each node branches a frequency at a predetermined position from a plurality of first type frequency groups from other nodes and receives it as a second type frequency group.

The same can be applied to the case where an optical signal is transmitted by the second type frequency group and is received by the first type frequency group.

When a Mach-Zehnder type filter including one or more stages of optical circuits or ring resonators is used for branching or transmitting an optical frequency, the transmittance is periodic, so that the second type frequency group can be efficiently used. Can be branched. Further, these optical circuits have high optical frequency resolution, and when the optical frequencies are arranged at equal intervals, the frequency interval can be set to about 5 GHz. Therefore, the wavelength of 1.5 to 1.6 μm is 125
Up to 2500 channels can be allowed in the 00 GHz band. Therefore, if optical frequencies are assigned to the communication lines between the nodes including the directivity, 50 nodes can be connected to one optical fiber. In the conventional configuration in which one optical fiber is assigned to each communication line, the number of nodes is 5
2500 optical fibers are required to achieve 0,
It can be seen that the invention makes the connection much easier.

[0012]

1 is a block diagram showing an optical communication network according to an embodiment of the present invention. This optical communication network includes a plurality of four nodes 11- that communicate with each other using different optical frequencies.
1 to 11-4 are provided. Here, the feature of this embodiment is that the nodes 11-1 to 11-4 are connected to a common optical transmission line 12, and each of them has an optical frequency that is substantially evenly spaced on the frequency axis. From the optical transmission line 12, one of the n continuous frequencies or the frequency of every n cycles is branched, and the other of the n frequencies continuous from the optical frequency or the frequency of every n cycles is This is because the optical add / drop circuit 13 for sending to the optical transmission line 12 is provided.

FIG. 2 is a block diagram showing an example of the optical add / drop circuit 13. The optical add / drop circuit 13 includes an optical signal add filter 21 and an optical signal add filter 22. The optical signal branching filter 21 outputs the optical frequencies f _i , f _j , f _k, ... Assigned to the node for reception from the optical signal input to the port A to the port B.
And the remaining optical frequency is connected to the port C ′ of the optical signal inserting filter 22 via the port C. The port B ′ of the optical signal inserting filter 22 has optical frequencies f _p , f _q , and f assigned to the node for transmission.
_r ... Is input, they are multiplexed and output to port A '.

FIG. 3 shows frequency allocation to a communication line connecting each node, and FIG. 4 shows the frequency allocation.

In this frequency allocation, for example, node 1
The optical frequency of the signal transmitted from 1-1 to another node is f _1j
(J = 2, 3, 4), f ₁₁ is virtually included, and they are arranged next to each other at a constant frequency interval Δf to form an F ₁ group. Similarly, f _{21 to} f ₂₄ are F ₂ groups, and f _{31 to} f ₃₄ are F ₃ groups.
The group, f _{41 to} f ₄₄ are F ₄ groups, and these are in order at equal intervals ΔF
Line up with. However, f ₁₁ , f ₂₂ , f ₃₃ , and f ₄₄ are assumed to be virtual. Since the adjacent optical frequencies are transmitted by the respective nodes, the optical signal inserting filter 22 having a wide transmission area as shown by a broken line in FIG. 4 is used. For example, an interference film filter can be used.

On the other hand, the optical frequency received by the node 11-1 from another node is f _i1 (i = 2, 3, 4). These frequency intervals are constant at ΔF. To branch such an optical frequency, the optical signal branching filter 21 is used.
In this case, the one having the periodicity shown by the solid line in FIG. 4 is used.

Here, the case has been described in which the optical frequencies adjacent to each other on the frequency axis are transmitted, and the optical frequencies periodically separated are received, but the reverse is also true, and the optical signal branching filter 2 is also provided.
1 and the optical signal insertion filter 22 have the characteristics opposite to each other, the same operation can be performed.

The optical frequency f _ii is a signal which is transmitted from the i-th node and goes around the ring and returns to itself. Therefore, although it is not actually used for communication between nodes, it can be used as a monitoring signal such as disconnection of the ring network, monitoring of the operating states of filters for branching and insertion in each node.

In the present embodiment, one optical frequency is assigned as a path connecting each node, but the present invention can be similarly implemented by assigning a plurality of optical frequencies. Further, although an example in which each node is connected in a ring shape by an optical transmission line has been described, the present invention can be similarly implemented in a communication network connected in a bus shape. Further, as the optical transmission line, only one optical fiber may be used, but it is possible to increase the reliability of the transmission line by using two optical fibers of clockwise and counterclockwise. In that case, a changeover switch between the two optical fibers can be provided for each node to increase the fault tolerance.

FIG. 5 shows an example of an optical filter having a periodic transmission characteristic used as the optical signal branching filter 21 or the optical signal inserting filter 22, and FIG. 6 shows the transmission characteristic.

This optical filter is a filter having an asymmetric Mach-Zehnder interferometer structure, that is, a Mach
It is a Zener type optical filter and has a structure in which two optical waveguides 52 and 53 having different lengths are coupled by two optical directional couplers 51 and 54. A Cr thin film heater 55 is provided on one of the optical waveguides 52 to adjust the transmission center frequency.

The transmission characteristic of this optical filter is sinusoidal with respect to frequency. If this optical filter is used as the optical signal branching filter 21 shown in FIG. 2, for example, frequencies can be separated in a communication network having "2" nodes. That is, for two frequencies f ₁₂ and f _{21 with} a frequency interval Δf, when f ₁₂ is assigned to the communication line from the node “1” to the node “2” and f ₂₁ is assigned to the communication line from the node “2” to the node “1”. , F ₂₁ can be branched at the node “1”, and f ₁₂ can be branched at the node “2”.

The transmittance cycle of the Mach-Zehnder type optical filter is determined by the optical waveguide length difference and its refractive index. For example, if a quartz optical waveguide is used, the optical waveguide length difference ΔL = 2c
At m, the cycle is 2Δf = 10 GHz.

FIG. 7 shows another example of the optical filter having a periodic transmission characteristic, and FIG. 8 shows its transmission characteristic.

This optical filter comprises two Mach-Zehnder type optical filters 71 and 73 having a period of 2Δf and a period of 4Δf.
And one Mach-Zehnder type optical filter 72. In general, if m types of Mach-Zehnder type optical filters with doubling the cycle are connected in cascade, the cycle of the entire filter can be extended by 2 ^m-1 times and the number of nodes that can be connected to the communication network should be 2 ^m. You can

Since the optical filter shown in FIG. 7 has m = 2,
Four nodes can be connected to the communication network. At this time, the frequency arrangement of the optical frequency-multiplexed signals is such that the frequency interval within the F _i group is Δf and the frequency interval between the F _i groups is ΔF = 8Δf. When the multiplexed signal group Σf _ij enters the port A which is the input terminal of the Mach-Zehnder type optical filter 71, the Mach-Zehnder type optical filter 71 changes the optical frequencies f ₁₂ , f ₁₄ , f ₂₄ and f. ₃₂ , f ₃₄ , f ₄₂ and optical frequencies f ₁₃ , f ₂₁ , f ₂₃ , f ₃₁ , f ₄₁ , f ₄₃ are divided into the Mach-Zehnder optical filter 73 for the former and the Mach-Zehnder type for the latter. Output to the filter 72. The Mach-Zehnder type optical filter 72 further divides the inputted optical frequency into f ₂₁ , f ₃₁ , f ₄₁ and f ₁₃ , f ₂₃ , f ₄₃ , the former as port B and the latter as Mach-Zehnder type. It outputs to the optical filter 73. Mach-Zehnder type optical filter 73
Outputs the two input lights from the port C to the optical transmission line.

FIG. 9 shows another frequency arrangement when the optical filter shown in FIG. 7 is used.

When the optical filter shown in FIG. 7 is used, the value of ΔF may be set to an integral multiple of 4Δf. Therefore, if ΔF = 8Δf is set as described above, there is a frequency margin between the F _i groups. Therefore, in the frequency arrangement shown in FIG. 9, the F ₁ ′ group including f ₁₂ ′, f ₁₃ ′ and f ₁₄ ′ between the F ₁ group and the F ₂ group, and f between the F ₂ group and the F ₃ group are included. By arranging frequency groups such as F ₂ ′ group including ₂₁ ′, f ₂₃ ′ and f ₂₄ ′, two optical frequencies can be assigned to the path connecting each node. Also, three or more optical frequencies can be assigned to one path by the same method.

As the Mach-Zehnder type optical filter,
For example, Oda, Takatou, Toba and Nos "Wideband Guided Wave Periodic Multi / Demultiplexer with a Ring Cavity for Optical FDM Transmission Systems", IEEE Electronics Letters Vol. 24, Vol. 4 Issue 210-212, 1988 (K. Oda,
N.Takato, H.Toba and K.Nosu, "Wideband guided-wave
periodic multi / demultiplexer with a ring cavity fo
r optical FDM transmission systems ", IEEE Electron
ics Letters, Vol.24, No.4, pp.210-212, 1988) with a ring waveguide can also be used.

FIG. 10 shows an example in which a ring resonator is used as an optical filter having a periodic transmission characteristic, and FIG. 11 shows its transmission characteristic and frequency arrangement.

In the ring resonator 101, the ring-shaped optical waveguide operates as a resonator and can selectively branch only the optical frequency of a specific cycle. In the example shown in FIG. 10, the number of nodes is “4”, the ring resonator 101 has an optical frequency f ₁₁ ,
f ₁₃ , f ₁₄ , f ₂₁ , f ₂₃ , f ₂₄ , f ₃₁ , f ₃₂ , f ₃₄ , f
_{From 41} , f ₄₂ , and f ₄₃ , the resonance frequency of the ring is f
₂₁ , f ₃₁ and f ₄₁ are branched. The branched optical frequency can be adjusted by the phase adjuster 103 provided on the ring.

FSR with a constant transmission width of the ring resonator
Can be increased, the number of optical frequencies to be multiplexed can be increased, and the number of nodes connected to the communication network can be further increased.

To extend the period of the optical filter, for example, Oda, Takato, Kominato, Toba, "Waveguide Double Ring Resonator", Proc.
It is also possible to use a multi-ring resonator structure such as shown at -667. A Fabry-Perot interferometer with an optical circulator added to the input section can also be used as the optical filter.

[0034]

As described above, in the optical communication system of the present invention, by assigning an optical frequency to each of the lines connecting the nodes, the nodes can be connected in a mesh by a common optical transmission line. Further, when an optical filter whose transmission characteristic is periodic with respect to frequency is used as the optical add / drop circuit, the number of nodes can be determined depending on how many cycles the optical filter uses. Further, by using a Mach-Zehnder type optical filter or a ring resonator whose cycle is different by twice, high-density frequency multiplexing is possible, and the number of nodes can be increased without increasing the number of optical fibers used.

[Brief description of drawings]

FIG. 1 is a block configuration diagram showing an optical communication network according to an embodiment of the present invention.

FIG. 2 is a block diagram showing an example of an optical add / drop circuit.

FIG. 3 is a diagram showing frequency allocation to a communication line connecting each node.

FIG. 4 is a diagram showing the frequency arrangement.

FIG. 5 is a diagram showing an example of an optical filter having a periodic transmission characteristic.

FIG. 6 is a diagram showing an example of its transmission characteristics.

FIG. 7 is a diagram showing another example of an optical filter having a periodic transmission characteristic.

FIG. 8 is a diagram showing its transmission characteristics.

FIG. 9 is a diagram showing another frequency arrangement.

FIG. 10 is a diagram showing an example in which a ring resonator is used as an optical filter having a periodic transmission characteristic.

FIG. 11 is a diagram showing the transmission characteristics and frequency allocation.

FIG. 12 is a diagram showing a basic configuration example of a conventional optical communication network.

[Explanation of symbols]

 11-1 to 11-4, 121-1 to 121-4 Nodes 12 Optical transmission line 13 Optical add / drop circuit 21 Optical signal add filter 22 Optical signal add filter 51, 54 Optical directional coupler 52, 53 Optical waveguide 55 Cr thin film heater 71, 72, 73 Mach-Zehnder type optical filter 101 Ring resonator 103 Phase adjuster 122 Cable

Claims (1)

[Claims] 1. An optical communication system provided with a plurality of n nodes that communicate with each other using different optical frequencies, wherein the plurality of n nodes are connected to a common optical transmission line, The node includes means for branching from the optical transmission path, one of n frequencies that are continuous from the optical frequencies that are arranged at substantially equal intervals on the frequency axis, or frequencies that have a cycle of n intervals, An optical communication system, comprising: a means for transmitting to the optical transmission path one of the n frequencies that are continuous from the optical frequency or the frequency having a cycle of every n frequencies.