JP2004112711A - Bidirectional optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bidirectional optical communication system in which an optical source is not provided in a user terminal, and the terminal has an exclusive use optical fiber for the terminal. <P>SOLUTION: An office 1 is provided with an optical source for transmission 11 per a user 1, a photodetector 12 per the user 1, an optical resource for a transmission signal 13 for the user 1, a multiplexing and demultiplexing unit 14 in the office, and an optical frequency multiplexing and demultiplexing unit 17 in the office. The system comprises the office 1, a common use optical fiber 3 connecting with the office 1, a birefringence optical frequency filter 4 of 2 frequency light transmission out of sending lights, and a polarization maintaining optical fiber 6 which leads the transmission light to the user terminal 2. The user terminal 2 is composed of a polarization beam splitter, a light detector which receives one light of the separated lights, and a means which modulates the other light by a modulator following a signal to be sent, and then makes the light inputted to the polarized beam splitter again, and makes it proceed in the reverse direction and reach the birefringence optical frequency filter 4. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a bidirectional optical communication system using an optical fiber.
[0002]
[Prior art]
2. Description of the Related Art In recent years, optical fiber communication technology has entered the research stage from the practical stage, and has already been introduced in large quantities into trunk lines such as toll telephone networks. Further, in response to the rapid spread of the Internet and the like, it is expected that the realization of the so-called FTTH (Fiber To The Home), which extends a communication network using an optical fiber to homes, is near. The problem here is the configuration of the two-way communication system.
[0003]
FIG. 1 is a conceptual diagram of a two-way communication system, in which a partner facing a user terminal 2 is indicated as a station 1. A signal such as a home page of the Internet arrives at the station 1 via a path not shown in FIG. 1 and is transmitted from the station 1 to the user terminal 2 via the optical fiber 3. For convenience, this signal is called a downstream signal according to custom. On the other hand, a connection request signal to the homepage is transmitted from the user terminal 2 to the station 1. This will be referred to as an up signal according to a custom.
[0004]
FIG. 8 shows one configuration diagram of a bidirectional optical communication system using the conventional technique. In this example, the user terminal 2 is configured to include a light receiver 22 for downstream signal light via the optical fiber 3 and a light source 26 for upstream transmission for transmitting an upstream signal. There are various light sources for optical communication depending on applications. For example, recording audio from a CD (compact disk) to an MD (minidisk) using an optical code can be considered as one form of optical fiber communication.
[0005]
If the transmission distance and the transmission capacity are as low as the recording of audio from a CD to an MD, a sufficiently reliable consumer product (without worrying about failure) can be obtained, but for large-capacity communication between telephone offices and homes, Special considerations such as high-precision temperature control are required, and the configuration of FIG. 8 has technical disadvantages, but has a disadvantage of high cost.
[0006]
Optical frequency multiplexing transmission, in which optical signals of many different frequencies coexist on a single optical fiber for transmission, is expected to be effective in supporting a wide variety of services and coping with an increase in transmission capacity. It has already been introduced to the trunk system. Further, when the number of user terminals connected to one station increases, it is desired to partially share a transmission line in order to reduce the construction cost of an optical fiber communication network.
[0007]
FIG. 9 shows an embodiment responding to such a request, in which a frequency division multiplexing transmission is combined with a branching / optical frequency filter 5 composed of a branching optical line network called a passive double star type, and a plurality of user terminals (1, 2, 2,. ..N). In order to realize bidirectional optical communication with such a system configuration, as shown in FIG. 10, in the section of the dedicated optical fiber 7 after the branch / optical frequency filter 5, two optical fibers per user terminal are used. A configuration is conceivable in which one is used for downstream signals and the other is used for upstream signals.
[0008]
In this configuration, the necessity of two optical fibers per user terminal is a clear disadvantage from the viewpoint of economy (first disadvantage). In general, the performance (crosstalk, insertion loss, etc.) of the optical frequency filter decreases as the number of output ports increases, so that two output ports of the branching / optical frequency filter 5 may be required for each user terminal. There is also a drawback (second drawback). Further, as described with reference to FIG. 8, the need for the uplink transmission light source 26 in the user terminal is a major drawback (third drawback).
[0009]
As far as the first drawback is concerned, as shown in FIG. 11, two optical fibers connected to the branching / optical frequency filter 5 are connected to the multiplexer / demultiplexer, and thereafter, the user is connected by one optical fiber. It is solved by leading to the terminal. However, even in this case, the third disadvantage is still not solved. There is also a bidirectional optical communication method in which signals having different modulation formats are transmitted bidirectionally using the same transmission line optical fiber (see Patent Document 1). However, this method also does not solve the third disadvantage. In addition, there are many problems to be solved in response to the rapid spread of the Internet, especially when two-way communication is extended to homes.
[0010]
[Patent Document 1]
JP-A-9-98137
[Problems to be solved by the invention]
As described above, in the bidirectional optical communication system using the conventional technology, the user terminal is configured to include the light receiver for transmitting the downstream signal light and the light source for transmitting the upstream signal, but to utilize the frequency multiplexing technology. In addition, the temperature control of this light source requires high accuracy (about 1/1000 degrees). In order to avoid this, there is a method using two optical fibers per user terminal. However, two optical fibers are required per user terminal, and two output ports of the optical frequency filter are required per user terminal. Problems have arisen in terms of performance as well as high cost. Therefore, an object of the present invention is to realize an inexpensive and highly reliable two-way optical communication system without installing a light source in a user terminal and using one dedicated optical fiber per user terminal.
[0012]
[Means for Solving the Problems]
To achieve the above object, a two-way optical communication system according to claim 1 of the present invention is a two-way optical communication system for connecting a station and a user terminal with an optical fiber, wherein one optical fiber connected to the station is provided. And a birefringent optical frequency filter that transmits light of two specific frequencies whose polarization directions are orthogonal to each other, among the light transmitted from the station via the one optical fiber, and the birefringent optical frequency filter It comprises a polarization maintaining optical fiber that guides transmitted light to a user terminal while maintaining its polarization state, and a user terminal.
[0013]
Thus, in the birefringent frequency filter, attention is paid to the fact that the polarization directions of transmitted light having adjacent frequencies are orthogonal to each other, and a means for minimizing the number of ports of the optical frequency filter which occupies a major cost of the frequency multiplex transmission system. Inexpensive and highly reliable bi-directional optical communication without using a light source in the user terminal and using one dedicated optical fiber per user terminal by actively utilizing the birefringence The system can be realized.
[0014]
A two-way optical communication system according to a second aspect of the present invention is the two-way optical communication system according to the first aspect, wherein the station is a light source for a user, a light receiver for a user, a light source for a user transmission signal, a multiplexing / demultiplexing device. A plurality of light sources and light receivers for the user, which are devices, and one optical frequency multiplexer / demultiplexer.
[0015]
Thus, two signal lights for uplink and downlink are required to realize two-way communication. To supply these with one or two light sources installed in the station, unmodulated signals for user transmission signals are used. It is necessary to deliver light from the station to the user, but by configuring the station in this way, it is possible to use a dedicated optical fiber per user terminal without installing a light source in the user terminal, and to reduce the cost. Thus, a highly reliable two-way optical communication system can be realized.
[0016]
The bidirectional optical communication system according to claim 3 of the present invention is the bidirectional optical communication system according to claim 1, wherein the user terminal includes a polarization beam splitter that separates the polarization-maintaining optical fiber output light according to its polarization direction. A light detector for receiving one of the lights separated by the polarization beam splitter, and a modulator for modulating the other light according to a signal to be transmitted by a user. Means for reaching the refraction light frequency filter.
[0017]
The bidirectional optical communication system according to a fourth aspect of the present invention is the bidirectional optical communication system according to the third aspect, wherein the modulator comprises a reflection type modulator.
[0018]
In this way, by utilizing the birefringence characteristic in which the polarization directions of the transmitted light whose frequencies of the birefringent frequency filter are adjacent to each other are orthogonal to each other, a simple configuration that does not require the installation of a light source in the user terminal can be used. To realize an inexpensive and highly reliable two-way optical communication system using the dedicated optical fiber.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
To realize bidirectional communication, two signal lights for uplink and downlink are required. In order to supply these with one or two light sources installed in the station, it is necessary to deliver unmodulated light from the station to the user for a user transmission signal. Further, in order to configure a system using one optical fiber per user, two light sources having different frequencies are inevitably required, so that a frequency multiplex transmission system is indispensable.
[0020]
The present invention focuses on the fact that, in the birefringent frequency filter, the polarization directions of transmitted light whose frequencies are adjacent to each other are orthogonal to each other, and minimizes the number of ports of the optical frequency filter that occupies a major cost of the frequency multiplex transmission system. It is characterized in that the birefringence is positively used as a means for performing this.
[0021]
Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 2 shows an example of an embodiment of the present invention, and is an overall configuration diagram of a system. This system comprises a station 1, a shared fiber 3, a birefringent optical frequency filter 4, a polarization maintaining optical fiber 6 dedicated to each user, and a user terminal 2 for each user.
[0022]
As shown in FIG. 3, the configuration of the station 1 includes an optical frequency multiplexer / demultiplexer 17, which is a common component for all users, and a user N corresponding to each user connected thereto by two optical fibers per user. The light source / receiver unit (N: 1, 2, ...) 10 is configured. For example, the light source / photodetector 10 for user 1 includes a light source 11 for transmission to user 1, a light receiver 12 for user 1, and a light source 13 for transmission signal for user 1. The optical fiber 15 is directly connected to the optical frequency multiplexer / demultiplexer 17.
[0023]
The user 1 light receiver 12 and the user 1 transmission signal light source 13 are connected to an optical frequency multiplexer / demultiplexer 17 by a second input optical fiber 16 via a multiplexer / demultiplexer 14 connected by an optical fiber. ing. The optical frequency multiplexer / demultiplexer 17 mixes all the input different frequency lights and outputs the mixed light to the shared optical fiber 3 connected to the junction port. FIG. 3 illustrates only the configuration of the light source / light receiver 10 for user 1 in detail, and the light sources / light receivers for other user N are abbreviated.
[0024]
FIG. 4 shows the internal structure of the user terminal 2, which comprises a polarization beam splitter 21, a light receiver 22, a multiplexer / demultiplexer 23, and a modulator 24. The transmission signal 8 to be transmitted by the user is supplied from outside the user terminal 2 here, and is also shown in FIG.
[0025]
Next, the operation of the present invention will be described in detail with reference to FIGS. First, downlink signal light transmitted from the station 1 to the user 1 terminal 2 will be described. As shown in FIG. 3, in the station 1, the light emitted from the transmission light source 11 for the user 1 having the frequency ν ₁ is guided to the optical frequency multiplexer / demultiplexer 17 via one optical fiber 15.
[0026]
The optical frequency multiplexer / demultiplexer 17 transmits only light having a predetermined frequency, ν _k (k = 1, 2,...), To one shared optical fiber 3 connected to only one junction port. Element. The frequency ν ₁ of the light source for transmission to the user 1 is _one of such transmission frequencies. The light transmitted from the optical frequency multiplexer / demultiplexer 17 is guided by the shared optical fiber 3 to the birefringent optical frequency filter 4 in FIG.
[0027]
Before describing the operation and specific examples of the birefringent optical wavenumber filter 4, an optical frequency filter having no birefringence will be described first. The optical frequency filter having no birefringence is substantially the same as the optical frequency multiplexer / demultiplexer 17 installed in the above-mentioned station, and is used by replacing the input port and the merge port. That is, when there are lights of a plurality of frequencies incident from one junction port, only light of a predetermined specific frequency is output from the predetermined branching port.
[0028]
As an element having such a function, an arrayed wave guide grating (AWG) is on sale. Also, as shown in FIG. 5, a similar function is provided to an optical branching element that distributes light ν ₁ , ν ₂ , ν ₃ ... It can also be realized by using an FPE (Fabry-Perot etalon) group which is connected one by one and transmits only light of a specific frequency.
[0029]
Next, the birefringent optical frequency filter will be described. Birefringence is a phenomenon in which the same substance has a different refractive index depending on the polarization direction of light, and is particularly remarkable in an optical waveguide formed on a substrate.
[0030]
In each case, the array waveguide grating and the Fabry-Perot etalon group each have a specific output port whose transmittance is equal to the optical path length (product of the refractive index and the actual length) of the light path in the device. Utilizing the property that depends on the ratio of the wavelengths of light, incident light is filtered according to its frequency. The birefringence phenomenon is observed in an actual optical waveguide, and the filter characteristics become complicated. That is, since the optical path length differs depending on the polarization state of the incident light, the transmission frequency of the filter differs even for the same port.
[0031]
FIG. 6 shows the transmission characteristics of a Fabry-Perot etalon having birefringence. Originally, the transmission characteristic of the Fabry-Perot etalon is completely periodic with respect to frequency, and the period is called FSR. In the case of the Fabry-Perot etalon having birefringence, as shown in FIG. The signal component ν _j has two adjacent elements ν _j (1) and ν _j (2).
[0032]
Here, when frequency ν _j (1) <ν _j (2), of the two birefringent principal axes, ν _j (1) is on the axis with a slow light speed, and ν _j (2) is on the axis with a fast light velocity. Yes, it is. Even when the polarization state at the time of incidence of the etalon is elliptically polarized light inclined with respect to the main axis of birefringence, at the time of emission, these transmitted lights are linearly polarized lights whose polarization directions are orthogonal to each other. Further, the frequency difference δ = ν _j (2) −ν _j (1) of the adjacent transmission peaks is proportional to the magnitude of birefringence.
[0033]
The present invention utilizes the fact that two types of frequency light are obtained from one output port and their polarization directions are orthogonal. For example, when the frequency ν ₁ of the transmission light source 11 for the user 1 in FIG. 2 is matched with ν _j (1), this light passes through the birefringent optical frequency filter 4 and is coupled to the polarization maintaining optical fiber 6.
[0034]
The polarization maintaining optical fiber 6 is an extremely large birefringent optical fiber and is commercially available. The birefringence of the ordinary optical fiber is about 10 ⁻⁷ to 10 ⁻⁶ , whereas the birefringence of the polarization maintaining optical fiber 6 is about 10 ^−4. I have. For this reason, the polarization state changes randomly with propagation in a normal optical fiber. However, when linearly polarized light is incident parallel or perpendicular to the main polarization axis of the polarization maintaining optical fiber 6, the polarization state at the time of incidence is changed. Is maintained as it is to the exit end of the polarization maintaining optical fiber 6.
[0035]
In the present invention, the polarization direction of the transmission light addressed to the user 1 at the frequency ν ₁ (= ν _j (1)) that has passed through the birefringent optical frequency filter 4 is made to coincide with the main axis of the polarization maintaining optical fiber 6. This state will be written as ν ₁ ^} . FIG. 4 illustrates a state in which the transmission light ν ₁ ^{} of the} polarization direction fixed to the user 1 having the frequency ν ₁ and directed to the user 1 is incident on the user terminal 2 from the polarization maintaining optical fiber 6.
[0036]
The polarization beam splitter 21 in the user terminal 2 is an element that guides a specific polarization direction component of the incident light to a first port and a polarization component orthogonal thereto to a second port. In the embodiment of the present invention, since the transmission light directed to the user 1 is all linearly polarized light polarized in a specific direction, by setting this direction to the transmission direction of the first port of the polarization beam splitter 21, the It is received without loss by the photodetector connected to one port.
[0037]
Next, a method in which the user 1 sends a signal to the station will be described. As shown in FIG. 3, the light source (light source for the user 1 transmission signal) 13 for this purpose is installed in the station 1 instead of the user terminal. The frequency of the user 1 outgoing signal light source 13 and [nu _2, distinguished from [nu _1.
[0038]
The multiplexer / demultiplexer 14 in the station functions as a multiplexer for guiding part or all of the incident light to one output port when used from the two-port side in the forward direction, and serves as a multiplexer when used from the one port side in the reverse direction. It works as a duplexer for splitting and outputting to ports. The light emitted from the user 1 transmission signal light source 13 is coupled to the optical frequency multiplexer / demultiplexer 17 via the multiplexer / demultiplexer 14.
[0039]
By keeping the frequency ν ₂ of the light source 13 for the user 1 transmission signal equal to the second transmission frequency ν _j (2) based on the birefringence of the j-th output port of the optical frequency multiplexer / demultiplexer 17, the frequency ν 2 ₁ (= ν _j (1)) can be emitted from the same j-th output port as the transmission light addressed to the user 1. At this time, the polarization direction is orthogonal to the transmission light addressed to the user 1 at the frequency ν ₁ (= ν _j (1)). This is denoted as ν ₂ 〃.
[0040]
In FIG. 2, the main axis of the polarization maintaining optical fiber 6 connected to the j-th port of the birefringent optical frequency filter 4 coincides with the polarization direction of the transmission light addressed to the user 1, and therefore has a polarization direction orthogonal to this. The polarization state of the light for the user 1 transmission signal also reaches the user terminal 2 while being kept in the linearly polarized light.
[0041]
FIG. 4 illustrates a state in which light 1 for a user 1 transmission signal ν ₂入射 is incident on the user terminal 2 as linearly polarized light orthogonal to transmission light ν ₁ ^宛 destined for user 1. Eventually, light of two frequencies, ν ₁ ^⊥ and ν ₂す^る , whose polarization directions are orthogonal to each other, enters the polarization beam splitter 21 from the polarization maintaining optical fiber 6. The former is output to the first port and input to the optical receiver 22, whereas the latter, that is, the light for the user 1 transmission signal is output to the second port and transmitted to the modulator 24 via the multiplexer / demultiplexer 23. Be guided.
[0042]
Up to this point, the light for the user 1 transmission signal has not been modulated, but when passing through the modulator 24 in the user terminal 2, the light is modulated according to the transmission signal 8 to be transmitted to the station by the user. The modulated user 1 signal light is guided to the second port of the multiplexer / demultiplexer 23 and reaches the polarization beam splitter 21 again.
[0043]
At this time, the frequency of the user 1 outgoing signal light being modulated are not subject only change the order of the modulation band, but treated as remains virtually [nu _2, the polarization direction slight change may occur. Even if there is fluctuation in the polarization direction of the light incident on the polarization beam splitter 21, the polarization beam splitter 21 transmits only the transmission axis component thereof, so that the light coupled to the polarization maintaining optical fiber 6 retains its polarization. .
[0044]
However, since the fluctuation in the polarization direction of the incident light affects the coupling efficiency to the polarization maintaining optical fiber 6, it is desirable that the polarization direction coincides with the second port transmission axis of the polarization beam splitter 21 as much as possible.
[0045]
The user 1 signal light having the frequency ν ₂ that travels backward through the polarization beam splitter 21 is coupled to the polarization maintaining optical fiber 6 again, passes through the birefringent optical frequency filter 4 and the shared optical fiber 3 in FIG. The light passes through the optical frequency multiplexer / demultiplexer 17 in the station 1 and reaches the multiplexer / demultiplexer 14 in the station. Further, the light receiver 12 for user 1 is connected to the second output port of the multiplexer / demultiplexer 14. Received.
[0046]
FIG. 7 shows a second configuration example of the user terminal 2, and the use of the reflection type modulator 25 eliminates the need for a multiplexer / demultiplexer in the user terminal. Specific examples of the reflection type modulator 25, a liquid crystal modulator, or as being capable of high-speed operation than can be achieved in combination with a reflecting mirror a phase modulator using an electro-optical effect such as LiNbO _3.
[0047]
As described above, two signal lights for uplink and downlink are required to realize two-way communication. Therefore, in order to supply these with one or two light sources installed in a station, it is necessary to use a signal for user transmission. Since it is necessary to deliver unmodulated light from the station to the user, and to configure a system using one optical fiber per user, two light sources having different frequencies are inevitably needed. Although a multiplex transmission system is indispensable, the present invention focuses on the fact that, in a birefringent frequency filter, the polarization directions of transmitted light whose frequencies are adjacent to each other are orthogonal to each other. The birefringence is actively used as a means for minimizing the number of ports of the optical frequency filter which occupies a major cost of the transmission system.
[0048]
【The invention's effect】
As described above, the present invention focuses on the fact that, in a birefringent frequency filter, the polarization directions of transmitted light whose frequencies are adjacent to each other are orthogonal to each other, and the number of ports of the optical frequency filter occupying a major cost of the frequency multiplex transmission system. Active use of the birefringence as a means to minimize inconvenience, without installing a light source in the user terminal and using one dedicated optical fiber per user terminal, inexpensive and reliable , It is possible to realize a bidirectional optical communication system with high performance.
[0049]
Further, since two signal lights for uplink and downlink are required to realize bidirectional communication, in order to supply these with one or two light sources installed in the station, unmodulated signals for user transmission signals are used. Although it is necessary to deliver light from the station to the user, as in the present invention, by configuring the station and the user terminal, it is possible to install one dedicated light source per user terminal without installing a light source in the user terminal. By using the fiber, an inexpensive and highly reliable two-way optical communication system can be realized.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of a two-way communication system.
FIG. 2 is an overall configuration diagram of a two-way communication system according to the present invention.
FIG. 3 is a configuration diagram of a station according to the present invention.
FIG. 4 is a first configuration diagram of a user terminal according to the present invention.
FIG. 5 is an embodiment diagram of an optical frequency filter having no birefringence.
FIG. 6 is a transmission characteristic diagram of a birefringent Fabry-Perot etalon.
FIG. 7 is a second configuration diagram of the user terminal of the present invention.
FIG. 8 is a first configuration diagram of a conventional bidirectional optical communication system.
FIG. 9 is a branched optical line type optical frequency multiplex transmission system (one-way transmission)
FIG. 10 is a second specific configuration diagram of a conventional bidirectional optical communication system.
FIG. 11 is a third specific configuration diagram of a conventional bidirectional optical communication system.
[Explanation of symbols]
1 station 11 light source for transmission to user 1 12 light receiver for user 1 13 light source for user 1 transmission signal 14 multiplexer / demultiplexer 15, 16 first and second input optical fiber 17 optical frequency multiplexer / demultiplexer 2 user terminal 21 Polarization beam splitter 22 light receiver 23 multiplexer / demultiplexer 24 modulator 25 reflective modulator 3 shared optical fiber 4 birefringent optical frequency filter 5 branch / optical frequency filter 6 polarization maintaining optical fiber 7 dedicated optical fiber 8 transmission signal 10 Light sources and receivers for users

Claims (4)

In a two-way optical communication system that connects a station and a user terminal with an optical fiber, one optical fiber connected to the station, and a polarization direction of light transmitted from the station via the one optical fiber. A birefringent optical frequency filter that transmits light of two specific frequencies orthogonal to each other, a polarization maintaining optical fiber that guides the birefringent optical frequency filter transmitted light to a user terminal while maintaining its polarization state, and a user terminal A two-way optical communication system comprising: The station is composed of a plurality of light sources / receivers for a user including a light source for transmission to a user, a light receiver for a user, a light source for a user transmission signal, a multiplexer / demultiplexer, and one optical frequency multiplexer / demultiplexer. The two-way optical communication system according to claim 1, wherein: The user terminal transmits a polarization beam splitter that separates the polarization-maintaining optical fiber output light according to its polarization direction, a photodetector that receives one of the lights separated by the polarization beam splitter, and a user that sends out the other. 2. A bidirectional optical communication system according to claim 1, further comprising means for modulating the signal in accordance with a power signal by a modulator, and then re-entering the polarization beam splitter to reach the birefringent optical frequency filter. system. The two-way optical communication system according to claim 3, wherein the modulator is a reflection type modulator.

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