JP2927879B2 - WDM optical communication network and communication method used therein - Google Patents

Description

The present invention relates to a wavelength division multiplexing communication network such as an optical LAN (local area network) and a communication method used therefor.

[Prior art] In recent years, optical communication networks such as optical LANs have been rapidly developing, but in the future it is indispensable to respond to multimedia, and as a countermeasure, high functionality by wavelength multiplexing is expected. ing. However, there are many problems to be solved to realize a wavelength division multiplexing optical LAN compatible with multimedia.

[Problems to be Solved by the Invention] First, when a system is configured with terminal stations that are passive with respect to light, the number of terminal stations connected to the system is limited because each terminal station repeats attenuation. , The system is not developable.

Therefore, the necessity of providing a repeater in the system increases. However, performing amplification by O / E (optical-electrical) and E / O (electrical-optical) conversion for each channel increases as the degree of wavelength multiplexing increases. It becomes difficult. In order to solve this, it is effective to use a broadband optical amplifier that amplifies a plurality of channels at once.

However, the wavelength multiplexing LAN has a problem that it becomes difficult to set the transmission level to a desired level because the input level of such an optical amplifier greatly varies depending on the number of channels used due to multi-access.

Second, the semiconductor laser used as a light source has poor temperature stability of the oscillation wavelength, and it is necessary to widen the channel separation in order to prevent a reduction in the error rate due to crosstalk between channels. As a result, as light sources, semiconductor lasers of the same number as the number of channels, narrow-band and stable wavelength filters (for example, dielectric thin film interference filters or diffraction gratings)
Is required, the cost is high, and the wavelength multiplexing degree is limited, so that it does not satisfy the user's requirements.

Thirdly, although related to the second problem, there has been proposed a method of using a tunable laser as a light source and using a semiconductor tunable filter as a filter in order to increase the wavelength multiplicity.
In order to secure a common number of channels in each terminal station, both the semiconductor laser and the semiconductor tunable filter require high-precision temperature control, and there is a problem that the system becomes extremely expensive in terms of cost.

Therefore, an object of the present invention, in view of the above problems, is that it is possible to increase the wavelength multiplexing degree while increasing the reliability at a relatively low price, and to further eliminate the limitation on the number of terminals. An object of the present invention is to provide a high-density wavelength division multiplexing optical communication network and a communication method used therein.

[Means for Solving the Problems] To achieve the above object, a wavelength division multiplexing optical communication network according to the present invention comprises: an optical transmission line for wavelength multiplexing and transmitting light of a plurality of wavelengths; And a plurality of terminal stations coupled to the optical transmission line. Each terminal station transmits light of a plurality of wavelengths transmitted through the optical transmission line during communication. First in the terminal station
An optical switch, separation means for selecting and separating light of an arbitrary wavelength from the plurality of wavelength lights taken in by the first optical switch, and a data signal for the light of the wavelength selected by the separation means. An optical modulator for superimposing, an optical amplifier for amplifying the light on which the data signal is superimposed by the optical modulator and the light not selected by the demultiplexing unit, and transmitting the light amplified by the optical amplifier to the optical transmission line again. And a second optical switch.

The terminal station may further include a unit for recognizing a superimposed state of the data signal on the light of the plurality of wavelengths transmitted through the optical transmission line.

Further, the communication method according to the present invention for achieving the above object,
An optical transmission line for wavelength-multiplexing and transmitting light of a plurality of wavelengths, a unit for constantly transmitting light of a plurality of wavelengths to the optical transmission line, and a plurality of terminal stations coupled to the optical transmission line. A communication method in a wavelength division multiplexing optical communication network, wherein each terminal selects and separates light of a plurality of wavelengths transmitted through the optical transmission line when performing communication, and Communication is performed by a step of superimposing a data signal on light, a step of amplifying light on which the data signal is superimposed and light not selected in the separation step, and a step of transmitting the amplified light again to the optical transmission line. It is characterized by doing.

The separation step performed in the terminal station recognizes a superimposed state of a data signal on the plurality of wavelengths of light transmitted through the optical transmission line, and determines a wavelength on which data is not superimposed according to the recognized content. May be selected and separated.

With such a configuration, since the reference station is the only light emitting source, stable carriers (wavelengths λ ₁ , λ ₂ ,... Λ _n ) can be always transmitted by stabilizing only the reference station. Ordinarily, the terminal station branches the light on the transmission path, performs a wavelength sweep, and recognizes the data superimposed on the carrier, thereby determining whether or not the channel is in use. Some when recognizing a channel (wavelength and lambda ₁₎ is empty, switch the full carrier to the terminal station side by the optical path changeover switch on the transmission path, the wavelength lambda ₁ of the carrier by the wavelength transmission-separation filter separating the other carriers, multiplexes the only modulating again another carrier wavelength lambda ₁ of the carrier. This keeps the total light level almost constant. In this way, the input levels of the optical amplifiers are made uniform in each channel, so that the output level can be kept constant regardless of the access state by simply performing simple gain control on carriers of all wavelengths.

Embodiment FIG. 1 is a configuration diagram of an embodiment of the present invention. In the figure, 1 is an optical fiber transmission line having a U-shaped bus shape, 1a is a transmission line, 1b is a reception line, 1c is a transmission line turning point,
Reference numeral 2 denotes a reference station disposed at one end of the transmission line 1, reference numerals 3 to 6 denote a plurality of ordinary terminal stations disposed on the transmission line 1, and reference numerals 7 to 10 are connected to the respective terminal stations 3 to 6 to transmit and receive signals. Terminal device for performing the operation, 11 is an optical branching element in the receiving path 1b, and 12, 13 are the transmitting paths 1a.
The optical switch 14 is located at the end of the transmission line 1.

First, the reference station 2 will be described. The reference station 2 is the only light emitting source in the optical communication system, and simultaneously transmits continuous light of each wavelength to be subjected to wavelength multiplexing to the optical fiber transmission path 1a and the reception path 1b. As a light source (not shown) of the reference station 2, for example, a broadband multimode semiconductor laser (hereinafter, referred to as a broadband LD) is suitable.

More specifically, a semiconductor laser having an asymmetric quantum well structure (a quantum well structure having well layers with different band gaps) in an active layer or a light emitting diode having the same structure as the light emitting source has a gain spectrum and a bulk active structure. It is possible to have a significantly broader band than with layers. Therefore, by selecting the end face reflectance of the device, Fabry-Perot modes having a wavelength interval corresponding to the cavity length appear in the emission spectrum, and each mode can be used as a wavelength multiplexed light source. For example, if the resonator length is 30
If the wavelength is 0 μm and the center wavelength is 0.83 μm, the wavelength interval, that is, the channel interval is about 0.3 nm. Therefore, by appropriately biasing the broadband LD, continuous waves of several tens wavelengths (here, the wavelengths are λ ₁ , λ ₂ ,.
_n ) at the same time.

FIG. 2 shows an example of the emission spectrum of this broadband LD. 21 is an axis (vertical) mode, and 22 is an envelope of the axis mode 21. The number of channels is changed by changing the asymmetric quantum well structure parameters (well depth, thickness, barrier height, thickness, etc.), the wavelength interval is changed by changing the resonator length, and the output level is changed by the bias applied to the device. It can be selected by changing the level and the end face reflectivity.

In the above, a method of outputting a continuous wave of a plurality of wavelengths with one element has been described. However, a light emitting element may be individually prepared for each channel (wavelength). In any case, a device for controlling the temperature of the light emitting element is required to stabilize the wavelength and light output of each channel. In the former case (one element only)
It is sufficient to control the temperature of about ± 0.1 ° C.
The cost of the light source unit can be reduced. on the other hand. In the latter case (when each emitting element is used for each channel), the cost is disadvantageous compared to the former, but there is an advantage that the channel interval and the optical output can be set freely.

Next, the ordinary terminals 3 to 6 will be described. FIG. 3 shows a configuration example of a normal terminal station (described as a terminal station 3) in this embodiment. In the figure, 31 is an optical branching element, 32 is an optical converging element, and 12 and 13 described above respectively transmit light on the transmission path 1a between the transmission path 1a (optical path 1) and the terminal side (optical path 15). An optical switch for switching between the optical switch and an optical switch for switching the light on the terminal station side or the optical path 1 to the transmission path 1a, 33 and 34 are tunable filters, 35 is an optical modulator, 36 and 37 are light receivers, Numeral 38 is an optical amplifier, and numeral 39 is a control unit for controlling the optical path selection of the optical switches 12 and 13, the wavelength selection of the variable wavelength filters 33 and 34, the gain control of the optical amplifier 38, and the overall communication procedure.

In the above configuration, consider a case where transmission is performed from the terminal station A3 to the terminal station B4 in FIG. In the terminal station A3, the wavelength division multiplexed light propagating through the receiving path 1b downstream of the turning point 1c is further split by the optical branching element 11 into the downstream transmission path 1 side and the terminal station 3 side. The filter 33 selects only light of an arbitrary wavelength (for example, λ _i ) from the light (wavelengths λ ₁ , λ ₂ ,... Λ _n ) branched to the terminal 3 according to the control state. A specific method of selecting the wavelength is to scan the channel with the wavelength tunable filter 33 capable of continuously sweeping the wavelength, monitor the optical output of the transmitted light at each time with the light receiver 36, and check whether the signal is superimposed. Whether or not each channel is occupied is recognized based on whether or not.

As the wavelength tunable filter 33 having such a function,
Here, a transmission filter of a distributed feedback (DFB) type having a semiconductor structure similar to the wideband LD used for the reference station 2 is used.

Here, when it is recognized as a vacant wavelength lambda ₁ of the channel, the control unit 39 in the terminal station A3 controls the optical switch 12 switches the light on transmission lines 1a to all end stations A3 side. Light switch
Numeral 12 generally uses a fail-safe type connected to the transmission line 1 side. The light switched in this way is
In the wavelength variable filter 34, the wavelength is swept by the above-described wavelength variable filter 33, and the wavelength is separated into the same wavelength as the wavelength (λ ₁ ) recognized as being vacant and light having the other wavelength.

This tunable filter 34 is the tunable filter 33 described above.
It has a structure according to.

Next, the separated light of wavelength λ ₁ is split in two directions by an optical splitter 31, and one light enters a light receiver 37 where the wavelength, light intensity, phase, etc. are monitored, and the other light is converted to the other light. Is
The data from the terminal device 7 is transmitted to the control unit by the optical modulator 35.
Superimposed via 39. The modulation method in this case is preferably a modulation method in which the optical output does not change, for example, frequency modulation (FM) or phase modulation (PM), but may be ordinary intensity modulation (AM). The modulated light (wavelength λ ₁ ) is multiplexed by the optical multiplexing element 32 together with the above-described wavelength-separated light having a wavelength other than λ ₁ .

At this time, when the modulation scheme is FM and PM is the amount of the modulated optical signal input to the optical multiplexing element 32, a wavelength other than the wavelength (lambda ₁ to be input from the wavelength variable filter 34 to the optical multiplexer 32 ), The same level as that of the terminal A3
The intensity of light of each wavelength is kept constant irrespective of the access state of the other terminal stations.

When the modulation method is AM, the instantaneous optical output fluctuates, but by selecting an encoding method (such as Manchester encoding) that sets the mark rate (DC component) to 50%, the time average of the output level at an arbitrary time is selected. Can always be 50% of the case of FM and PM. Therefore, if a gain of 3 dB is given to the optical modulator 35 under the control of the control unit 39, the output level of each wavelength input to the optical amplifier 38 can be kept constant.

Thus, the light of each wavelength simultaneously amplified by the amplifier 38 is transmitted to the transmission path 1a by the optical switch 13 controlled by the control unit 39.

On the other hand, when receiving the signal at the terminal station B4, the light on the receiving path 1b is branched by the optical branching element 11 of the terminal station B4, the wavelength is selected by the wavelength variable filter 33, and the signal is received by the light receiver. At this time, are controlled by the control unit 39 as the light of the wavelength tunable filter 33 at a wavelength lambda ₁ is selected, thus signal superimposed on the wavelength lambda ₁ of the light at the end station A3 is received by the terminal station B4 Will be.

Indicating that the selected wavelength of the wavelength tunable filter 33 as a method for setting the lambda ₁ is, for example, a signal of the terminal station B4 addressed to the head when superimposing a signal on wave signal lambda ₁ light at the end station A3 There is a method in which information is added, the wavelength is swept by the wavelength tunable filter 33 of the terminal station B4 to find out, and the selected wavelength of the filter 33 is fixed at the wavelength where the signal with the found header is placed.

At this time, the optical switch 12 of the terminal station B4 is on the optical path 1 side, and the light on the transmission path 1a only passes through the terminal station B4. That is, the terminal station B4 works only as a receiving unit. However, it is also possible to transmit using another channel while receiving. For this purpose, for example, a carrier, that is, a channel that is free when the wavelength is swept by the wavelength tunable filter 33 is found, and the transmission unit including the optical modulator 35 and the like may transmit using this carrier. Such control is performed by the control unit 39, and the communication procedure is the same as that in the case of the transmission in the terminal station A3 described above.

Thus, from the user's point of view, the ordinary terminals 3 to
6 has all the functions of a normal passive terminal,
Commonly used communication procedures or access methods (for example, CS
MA / CD (carrier sense multiple access with collisio
n detection) can be used almost as is.

If necessary for system expansion or the like, for example, an optical amplifier may be installed as a repeater at an arbitrary position on the transmission line.

[Effects of the Invention] As described above, according to the present invention, first, since the light emission source is only the reference station, the temperature control basically needs to be provided only to the reference station, and the wavelength multiplexing optical communication network can be configured at extremely low cost. In addition, since the input signal levels of the optical amplifiers of the optical transceiver are aligned in each channel, the level can be easily adjusted by amplifying all the wavelengths with a constant gain, and the system becomes extremely simple. The limitation on the number of terminals, which was a disadvantage of the above, is eliminated.

Further, from the viewpoint of the user of the ordinary terminal station, all of the functions of the passive terminal station are provided, so that the communication system which is usually used can be applied almost as it is, and the system design becomes easy.

Although a specific example of the wavelength band of 0.8 μm is shown in the above embodiment, it is needless to say that other wavelength bands, for example, 1.3 μm band, 1.55 μm band, and the like may be used.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the optical communication system of the present invention, FIG. 2 is a diagram of an example of an emission spectrum of a broadband semiconductor laser of a reference station, and FIG. 3 is a block diagram of an ordinary terminal station. DESCRIPTION OF SYMBOLS 1 ... Optical path, 1a ... Transmission path, 1b ... Reception path, 1c ... Fiber return point, 2 ... Reference station, 3-6 ... Normal terminal station, 7
... 10 Terminal device, 11, 31 Optical branching element, 12, 13
Optical switch, 33, 34 ... Tunable filter, 35 ... Optical modulator, 36, 37 ... Receiver, 38 ... Optical amplifier, 39 ... Control unit

Claims (4)

(57) [Claims] 1. A wavelength division multiplexing communication network for performing communication using a plurality of lights having different wavelengths, comprising: an optical transmission line for wavelength multiplexing and transmitting lights of a plurality of wavelengths; Means for constantly transmitting to the transmission line, and a plurality of terminal stations coupled to the optical transmission line, each terminal station having a plurality of wavelengths transmitted through the optical transmission line when performing communication. A first optical switch for taking light into the terminal station, separating means for selecting and separating light of an arbitrary wavelength from the plurality of wavelength lights taken in by the first optical switch, and selecting by the separating means An optical modulator that superimposes a data signal on the light of the selected wavelength, an optical amplifier that amplifies the light on which the data signal is superimposed by the optical modulator and the light that is not selected by the demultiplexing unit, and a light that is amplified by the optical amplifier. And a second optical switch for transmitting the signal again to the optical transmission line. Characteristic wavelength multiplexing optical communication network. 2. The wavelength division multiplexing optical communication according to claim 1, wherein said terminal station further comprises means for recognizing a superimposed state of a data signal on said plurality of wavelengths of light transmitted through said optical transmission line. network. 3. An optical transmission line for wavelength-multiplexing and transmitting light of a plurality of wavelengths, means for constantly transmitting light of a plurality of wavelengths to the optical transmission line, and a plurality of terminals coupled to the optical transmission line. A communication method in a wavelength division multiplexing optical communication network having a station, wherein each of the terminal stations captures light of a plurality of wavelengths transmitted through the optical transmission line into the terminal station when performing communication. A step of selecting and separating light of an arbitrary wavelength from among the lights of the plurality of wavelengths, a step of superimposing a data signal on the light of the selected wavelength, and a step in which the data signal is superimposed and the separation step. A communication method comprising performing communication by amplifying unselected light and transmitting the amplified light to an optical transmission line again. 4. The terminal station recognizes a superimposed state of a data signal on light of the plurality of wavelengths transmitted through the optical transmission line,
4. The communication according to claim 3, wherein light of a wavelength on which data is not superimposed is selected and separated according to the recognized content, and a data signal is superimposed on the light of the selected wavelength and transmitted again to the optical transmission line. Method.