JPH05100254A - Optical frequency branching and inserting circuit - Google Patents

Abstract

PURPOSE:To obtain high frequency resolving power by using an optical filter whose transmission characteristics are periodic to a frequency. CONSTITUTION:This optical frequency branching and inserting circuit is equipped with an optical filter 12 which branches a light signal inputted to a 1st input terminal 11 into a 1st frequency group and a 2nd frequency group and an optical filter 16 as a frequency inserting means for multiplexing with the light signal of the 2nd frequency group branched by the optical filter 12. In this case, the light signal inputted to the input terminal 11 is a signal group which is multiplexed at substantially equal frequency intervals DELTAf/2 and the optical filter 12 varies in transmissivity from one port to two output parts complementarily at a period DELTAf with the optical frequency. Further, the optical filter 16 multiplexes the input light to a 2nd input terminal 15 with the output light of a port 13 which is not connected to a 1st output terminal 14 between the two output ports of the optical filter 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is used for optical frequency multiplex communication (also called optical wavelength multiplex communication). In particular, it relates to dropping and adding optical frequencies at individual nodes in a communication network.

[0002]

2. Description of the Related Art FIG. 7 shows an example of an optical communication network. In this optical communication network, one upper node 71 and a plurality of lower nodes 72-1 to 72-p are connected in a loop via an optical fiber transmission line 73. Lower nodes 72-1 to 72-
Different optical frequencies are assigned to p, and the upper node 71 and lower nodes 72-1 to
Communication with each of the 72-p is performed. That is, the upper node 71 multiplexes the optical frequencies to make the optical fiber transmission line 7
3, and the lower nodes 72-1 to 72-p take out the information addressed to themselves from the respective assigned optical frequencies. The lower nodes 72-1 to 72-p also send the information addressed to the upper node 71 to the optical fiber transmission line 73 at the respective assigned optical frequencies. Lower node 72-
Signals with different optical frequencies from 1 to 72-p are
The signals are multiplexed on the optical fiber transmission line 73 and transmitted to the upper node 71.

In such an optical communication network, each of the lower nodes 72-1 to 72-p requires an optical circuit for branching and inserting one or more desired optical frequencies among the multiplexed optical frequencies. Become. Such an optical circuit is referred to as an "optical frequency add / drop circuit" in the present specification.

FIG. 8 shows an example of a conventionally proposed optical frequency add / drop circuit. In this conventional example, the interference film filters 81 and 82 that reflect only a specific optical frequency on the optical path 80.
One of the interference film filters 81 branches a specific optical frequency from a plurality of optical frequency-multiplexed signals, and the other interference film filter 82 inserts a specific optical frequency.

[0005]

However, the frequency resolution of the interference film filter is about 500 GHz at most, and it is necessary to set the optical frequency interval for multiplexing to that extent. For this reason, the frequency utilization efficiency is poor and the number of nodes that can be connected is reduced. For example, when using a band of 12,500 GHz having a wavelength of 1.5 to 1.6 μm, which is the lowest loss region of an optical fiber, even if one optical frequency is assigned to each subordinate node, the number of subordinate nodes that can be connected is There are only about 25.

An object of the present invention is to solve the above problems and to provide an optical frequency add / drop circuit having a high frequency resolution capable of adding / dropping densely multiplexed optical frequencies.

[0007]

SUMMARY OF THE INVENTION An optical frequency add / drop circuit of the present invention comprises a first input terminal for inputting a frequency-multiplexed optical signal and an optical signal input to the first input terminal. A frequency branching unit that branches into one frequency group and a second frequency group, a first output terminal that outputs an optical signal of the first frequency group that is branched by this frequency branching unit, and a first frequency group A second input terminal for inputting an optical signal of a frequency included in, and an optical signal input to the second input terminal to an optical signal of a second frequency group branched by the first optical means. In an optical frequency add / drop circuit having a frequency inserting means for oscillating and a second output terminal for outputting the frequency multiplexed optical signal multiplexed by the frequency inserting means, the frequency multiplexed optical signals are substantially equal. Frequency interval Δf / k
The frequency branching means is an optical demultiplexer in which the respective transmittances from one input port to two output ports are complementary and change with a period Δf with respect to the optical frequency. The frequency inserting means includes means for multiplexing the input light of the second input terminal with the output light of the other output port of the two output ports of the optical demultiplexer and the side connected to the first output terminal. It is characterized by including. However, k is an integer of 2 or more.

The optical demultiplexer may be a Mach-Zehnder type filter in which two optical paths having different lengths are coupled by two optical directional couplers, or a ring resonator. The means for multiplexing is also the same. Further, a plurality of Mach-Zehnder type filters having different transmittance cycles stepwise may be connected in series to form an optical demultiplexer. At this time, an equivalent Mach-Zehnder type filter can be used in the reverse direction as the multiplexing means, and this can be used by being connected symmetrically with the optical demultiplexer.

[0009]

The optical demultiplexer whose transmittance changes periodically is used to add and drop the optical frequency. At this time, by combining an optical demultiplexer having a large period and an optical demultiplexer having a small period, the resolution of the entire frequency branching unit can be determined by the smallest period.

Here, the ratio between the cycle of the optical demultiplexer and the half-value width of the transmittance peak is defined as "finesse". The finesse value determines the resolution of the transmittance with respect to the cycle of the optical demultiplexer, and even an optical demultiplexer having the same finesse can increase the resolution of the transmittance by reducing the cycle. Also, even if finesses of the same value are configured in multiple stages with different periods, the finesse as a whole improves,
The frequency resolution of transmittance can be improved.

For example, the finesse of a Mach-Zehnder type filter is "2", but if finesses each having a doubled cycle are connected in multiple stages, the overall finesse increases. The ring resonator itself has a large finesse. Therefore, when these are used as an optical demultiplexer, a very large frequency resolution can be obtained. Further, the frequency width or the number of multiplexed signals that can be added / dropped can be set quite freely by combining optical demultiplexers.

Further, since the transmittance changes periodically,
In principle, the number of optical frequencies that can be branched by one frequency branching means also increases infinitely. As the frequency inserting means, the remaining optical signals branched by the respective demultiplexers of the frequency branching means and the optical signals to be inserted may be simply multiplexed, but an optical demultiplexer similar to the frequency branching means Can be used in the reverse direction to increase the selectivity of the optical frequency to be multiplexed.

Therefore, when the optical frequency add / drop circuit of the present invention is used in the loop optical communication network as described above,
The resolution of the optical frequency that can be added / dropped by one node is increased, and the number of nodes that can be connected to the communication network can be increased. In particular, when a Mach-Zehnder type filter or a ring resonator is used as the optical demultiplexer, the frequency interval of the multiplexed optical signal can be narrowed to about 5 GHz,
Using the band of 12,500 GHz with a wavelength of 1.5 to 1.6 μm, lower nodes can be connected to 2500 nodes.

Further, regarding the frequency to be added / dropped at each node, a series of frequency groups densely multiplexed can be added / dropped by giving a width in a certain frequency band, and the frequency dividing means as a whole can It is also possible to add / drop a plurality of frequencies in a cycle. In the case of adding / dropping densely multiplexed frequency groups, the number of optical frequencies included in the frequency groups can be set separately by the node.

[0015]

FIG. 1 is a block diagram showing the first embodiment of the present invention.

This optical frequency add / drop circuit has a first input terminal 11 for inputting a frequency-multiplexed optical signal, and an optical signal input to the first input terminal 11 for a first frequency group and a first frequency group. An optical filter 12 as a frequency branching unit for branching into two frequency groups, a first output terminal 14 for outputting an optical signal of the first frequency group branched by the optical filter 12, and a first frequency group A second input terminal 15 for inputting an optical signal having a frequency included in, and the second input terminal 1
5, an optical filter 16 as frequency inserting means for multiplexing the optical signal input to the optical signal 5 into an optical signal of the second frequency group branched by the optical filter 12, and a frequency-multiplexed optical signal multiplexed by this frequency inserting means. Second output terminal 1 for outputting
7 and 7.

The feature of this embodiment is that the optical signal input to the input terminal 11 is a signal group multiplexed at substantially equal frequency intervals Δf / 2, and the optical filter 12 is The transmittances from one input port to two output ports are complementary and have a period Δ with respect to the optical frequency.
The optical filter 16 has a configuration that changes with f, and the optical filter 16 outputs the output light of the port 13 connected to the first output terminal 14 of the two output ports of the optical filter 12 and the output light of the second input terminal 15 It is configured to combine the input light.

Therefore, the optical frequencies of the signal group input to the input terminal 11 are f ₁ , f ₂ , f ₁ + Δf, and f ₂ + Δ.
If f, ..., F ₁ + nΔf, f ₂ + nΔf, then the optical signals f ₁ , f ₁ + Δf, ..., f ₁ + nΔf are output to the output terminal 14, and f ₂ , f ₂ + Δf, to the port 13. …
Each optical signal of f ₂ + nΔf is output. However, f ₂ =
There is a relationship of f ₁ + (2k + 1) Δf / 2, and k is an integer including a negative value. Also, the input terminals are f ₁ , f ₁ + Δ
An optical signal using one or more optical frequencies of f, ..., F ₁ + nΔf is input, and the optical signal from the port 13 and the optical signal from the input terminal 15 are combined and output from the output terminal 17. However, n is an integer.

FIG. 2 shows an example of the optical filters 12 and 16, and FIG. 3 shows the transmission characteristics thereof. Here, an example of a Mach-Zehnder filter is shown.

In this Mach-Zehnder type filter, two optical waveguides 33 and 34 having different lengths are coupled by two optical directional couplers 32 and 36, and frequencies f ₁ and f ₁ input to an input port 31 are input. , F ₁ + nΔf optical signals are output to the output port 37 at frequencies f ₂ , f ₂ + Δf ,.
The optical signal of f ₂ + nΔf is coupled to the output port 38. However, k is an integer. A phase adjuster 35 using a Cr thin film heater is provided on one of the optical waveguides 34 to adjust the wavelength of light that passes therethrough.

The transmission characteristic of the Mach-Zehnder filter is sinusoidal with respect to frequency, and its finesse is "2". By using this filter as the optical filters 12 and 16 in FIG. 1, it is possible to construct a loop communication network in which the number of lower nodes is "2". For example, if the Mach-Zehnder filter is composed of a quartz optical waveguide and the optical path length difference ΔL = 2 cm between the two optical waveguides 33 and 34, the transmittance cycle Δf of the filter is 10 GHz. Therefore, the optical frequency f ₁ is transferred from the input node 31 to the output node 37.
When the optical frequency f ₂ is coupled to the output node 38, f ₁ + nΔf can be added and dropped at _one node, and f ₂ + nΔf can be added and dropped at another node.

FIG. 4 is a block diagram showing a second embodiment of the present invention, showing a configuration in which Mach-Zehnder filters are connected in cascade.

In this embodiment, Mach-Zehnder filters 41 to 43 having different transmittance cycles by two are connected in cascade to form an optical demultiplexer, and equivalent Mach-Zehnder filters 44, 45 and 46 are used. It is used in the opposite direction and is connected symmetrically with the optical demultiplexer. Here, an example in which three stages are connected is shown. Generally, if m stages are connected in series, the period of the entire optical demultiplexer can be extended by 2 ^m-1 times, and the number of subordinate nodes that can be connected to the loop communication network is 2 ^m. Can be

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

FIG. 5 is a diagram showing a third embodiment of the present invention, showing an example in which a ring resonator is used for optical multiplexing / demultiplexing.
Further, FIG. 6 shows the transmission characteristics.

In this embodiment, a first input terminal 51 for inputting a frequency-multiplexed optical signal, and an optical signal input to the first input terminal 51 for a first frequency group and a second frequency A ring resonator 52 as frequency branching means for branching into a group, a first output terminal 54 for outputting an optical signal of the first frequency group branched by the ring resonator 52, and a first frequency group A second input terminal 55, which receives an optical signal of the included frequency, and an optical signal input to the second input terminal 15 are combined with an optical signal of the second frequency group branched by the ring resonator 52. A ring resonator 56 as a frequency inserting means for oscillating, and a second output terminal 57 for outputting the frequency-multiplexed optical signal multiplexed by the ring resonator 56 are provided. The optical signals input to the input terminal 51 are signal groups f ₁ and f multiplexed at substantially equal frequency intervals Δ / p.
₂ , ..., f _p , f ₁ + Δf, f ₂ + Δf, ..., f _p + Δ
f, ..., f ₁ + nΔf, f ₂ + nΔf, ... f _p + nΔf
The ring resonator 52 has a configuration in which the respective transmittances from one input port to two output ports are complementary and cyclically change with respect to the optical frequency, and the cycle of the transmittance is Δf. The ring resonator 56 is the output terminal 5 of the two output ports of the ring resonator 52.
The output light from the port 53, which is different from the one connected to port 4, is connected so as to be combined with each other. By this connection, for example, the optical frequencies f ₁ , f ₁ + Δf, ...
An optical signal of f ₁ + nΔf is output, and f from the input terminal 55
Optical signals of ₁ , f ₁ + Δf, ..., F ₁ + nΔf can be input and multiplexed with optical signals of other frequencies.

The finesse of the ring resonator is large, and the optical frequency to be dropped / added can be changed by the phase adjusters 58 and 59 using Cr thin film heaters.

In order to extend the period of the optical demultiplexer, for example, Oda, Takato, Kominato, Toba, "Waveguide Double Ring Resonator", 1988 Proceedings of the IEICE National Convention, B
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 demultiplexer.

[0029]

As described above, the optical frequency add / drop circuit of the present invention uses an optical filter whose transmission characteristic is periodic with respect to frequency, so that the number of optical frequencies added / dropped at a node can be reduced. It can be determined by how many cycles of the optical filter are used. This number is in principle unlimited. Further, by using a Mach-Zehnder type filter or a ring resonator whose cycle is different by twice, high-density frequency multiplexing is possible, and the number of multiplexing within one cycle can be increased. This means that a large number of nodes can be taken when used in a loop communication network. Further, when used in such a communication network, since the number of frequencies can be spared, the optical frequency add / drop circuit of the present invention can be used for communication between lower nodes.

[Brief description of drawings]

FIG. 1 is a block diagram showing the first embodiment of the present invention.

FIG. 2 is a diagram showing an example of an optical filter, showing the structure of a Mach-Zehnder filter.

FIG. 3 is a diagram showing an example of transmission characteristics of a Mach-Zehnder filter.

FIG. 4 is a block diagram showing a second embodiment of the present invention.

FIG. 5 is a block diagram showing a third embodiment of the present invention.

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

FIG. 7 is a diagram showing an example of a loop optical communication network.

FIG. 8 is a diagram showing a configuration of a conventional optical frequency add / drop circuit.

[Explanation of symbols]

 11, 15, 51, 55 Input port 12, 16 Optical filter 13 Output port 14, 17 Output terminal 31 Input port 32, 36 Directional coupler 33, 34 Optical waveguide 35, 58, 59 Phase adjuster 41-46 Mach Zender type filter 52, 56 Ring resonator 71 Upper node 72-1 to 72-p Lower node 73 Optical fiber transmission line 80 Optical path 81, 82 Interference film filter

Claims (1)

[Claims] 1. A first input terminal to which a frequency-multiplexed optical signal is input, and an optical signal input to the first input terminal is branched into a first frequency group and a second frequency group. Frequency branching means, a first output terminal for outputting an optical signal of the first frequency group branched by the frequency branching means, and a first input terminal for inputting an optical signal of a frequency included in the first frequency group Two input terminals, frequency inserting means for combining the optical signal input to the second input terminal with the optical signal of the second frequency group branched by the first optical means, and the frequency inserting means An optical frequency add / drop circuit having a second output terminal for outputting a frequency-multiplexed optical signal multiplexed by the means, wherein the frequency-multiplexed optical signal is multiplexed at substantially equal frequency intervals Δf / k. Signal group, The branching means includes an optical demultiplexer in which the transmittances from one input port to the two output ports are complementary and change with a period Δf with respect to the optical frequency, and the frequency inserting means includes A light including a means for multiplexing the input light of the second input terminal with the output light of the port different from the side connected to the first output terminal of the two output ports of the wave filter. Frequency add / drop circuit. However, k is an integer of 2 or more.