Optical Branching Equipment and Optical Network Using the Same

The present invention relates to optical branching equipment which is employed in an optical network using an optical fiber cable, for switching transmission lines. The invention also pertains to such an optical network.

A bus type optical network calls for optical branching equipment which includes an optical repeater for amplifying an attenuated optical signal and a branching equipment for switching optical signal transmission lines.

FIG. 1 illustrates in block form a prior art example of optical branching equipment.

In FIG. 1 optical fiber cables 61.sub.1 and 61.sub.3 which form a trunk line is made up of optical fibers 52.sub.1 and 52.sub.4 and feed lines 59.sub.1 and 59.sub.2. Optical branching equipment 60 is provided with optical/electrical converters 53 and 55 for converting optical signals from the optical fibers 52.sub.1 and 52.sub.3 into electrical signals, electrical/optical converters 54 and 56 for converting electrical signals into optical signals for input into optical fibers 52.sub.2 and 52.sub.4, an electric circuit 58 which switches between branching circuits for connecting the optical fibers 52.sub.2 and 52.sub.3 to the optical fibers 51.sub.1 and 52.sub.4, respectively, and a short circuit for connecting the optical fibers 52.sub.1 and 54.sub.4 to each other and which performs amplification and other signal processing, and a power circuit 57 for supplying power from the feed line 59.sub.1 to each circuit.

The optical branching equipment 60 is usually employed as optical signal branching means in an optical network which forms a bus type optical transmission line by connecting a branch line to a trunk line. That is, the electric circuit 58 normally connects the trunk line optical fiber cables 61.sub.1 and 61.sub.3 to a branch line optical fiber cable 61.sub.2 and disconnects the short circuit in terms of a signal. The branch line optical fiber cable 61.sub.2 has connected thereto branch line terminal equipment 62, which converts an optical signal from the trunk line optical fiber cable 61.sub.1 into an electrical signal by an optical/electrical converter 63, adds and/or drops required information by a signal add/drop circuit 64 and converts the electrical signal into an optical signal by an electrical/optical converter 65, thereafter sending the optical signal to the trunk line optical fiber 61.sub.3.

Upon occurrence of breakage or some other fault of the optical fiber cable 61.sub.2 which forms a branch line, the electric circuit 58 switches the transmission line to the short circuit which interconnects the optical fibers 52.sub.1 and 52.sub.4, thus disconnecting the optical fiber cable 61.sub.2.

For example, in a submarine repeatered transmission network employing submarine optical fiber cables, optical branching equipment for use in a bus type or star type optical network is required to be highly reliable for its long-term, maintenance-free operation. In the conventional optical branching equipment, however, each circuit employs many parts including semiconductor devices, which must be made highly reliable to meet the reliability requirement for the optical branching equipment--this inevitably raises the manufacturing costs of optical branching equipment.

Moreover, since the transmission line is supervised from a shore terminal, it is necessary to provide an independent transmission line for sending transmission line switching control information.

It is therefore an object of the present invention to provide a low-cost optical branching equipment which permits switching of optical signal transmission lines without involving the conversion of an optical signal into an electrical signal, and hence is highly reliable and free from the necessity of maintenance for a long period of time.

Another object of the present invention is to provide an optical network which employs the above-mentioned optical branching equipment.

According to an aspect of the present invention, the optical branching equipment comprises: an optical divider means for branching an optical signal from a trunk line optical signal input terminal to a branching circuit and a short circuit; an optical coupler means for providing an optical signal from either the branching circuit or the short circuit to a trunk line optical signal output terminal; pump light supply means for supplying pump light of a predetermined wavelength; and an optical limiter amplifier means which is inserted in the short circuit and is held in an excited state by the incidence thereto of the pump light and which responds to the incidence of external control light to control the gain of amplification of an optical signal, thereby permitting or inhibiting the passage therethrough of the optical signal.

The optical network of the present invention, which employs the above-mentioned optical branching equipment, includes: a bus type trunk line including an optical fiber for transmitting an optical signal of a first wavelength; trunk line terminal equipment provided at one end of the trunk line, for transmission and reception of transmission information; the above-mentioned optical branching equipment inserted in the bus type trunk line; a branch line branched from the trunk line via the optical branching equipment and including an optical fiber for transmitting an optical signal; and branch line terminal equipment connected to the branch line, for adding and/or dropping transmission information, the branch line terminal equipment including a control light source for generating control light of a second wavelength and the branch line including means for supplying the control light to the optical branching equipment.

With the optical branching equipment of the above construction, the optical limiter amplifier means inserted in the short circuit is excited by the pump light to amplify an optical signal, and in this instance, its gain is controlled by the control light. That is, when the branch line is normally connected to the optical limiter amplifier means and the control light is input thereinto, the optical limiter amplifier means is put into an output saturation state by the control light and has substantially no gain for the optical signal, and a transmission line is formed via the branching circuit. When the input of the control light stops due to breakage of the branch line, the optical limiter amplifier means amplifies the optical signal and a transmission line is formed via the short circuit.

Thus, the optical branching equipment of the present invention permits automatic transmission line switching and optical signal amplification while maintaining the optical signal intact.

By employing the above optical branching equipment as optical signal branching means in the bus type optical network and supplying the control light to the optical branching equipment via the optical fiber of the branch line from the control light source provided in the branch line terminal equipment, it is possible to perform automatic switching to the short circuit, for example, upon breakage of the branch line. Moreover, the optical signal can be switched and amplified intact on the transmission line.

According to another aspect of the present invention, in the optical network the trunk line terminal equipment provides an optical signal of a first wavelength band on a trunk line cable and the optical branching equipment inserted in the trunk line cable divides the optical signal into a plurality of optical signals and applies them to optical amplifying elements, respectively. Only when excited by the pump light, the optical amplifying elements each provide a gain for the optical signal, and the optical amplifying element is supplied with the pump light via a branch line cable from the pump light source provided in the branch line terminal equipment. When excited by the pump light, the optical amplifying element amplifies the optical signal and sends it via the optical signal transmission fiber to the branch line terminal equipment.

Also in the above optical network, since the pump light source is provided in the branch line terminal equipment, the optical branching equipment does not call for any particular electronic devices for the optical signal transmission. This ensures the implementation of a highly reliable, highly economical optical network which does not require maintenance for a long period of time.

FIG. 1 is a block diagram showing conventional optical branching equipment;

FIG. 2 is a block diagram illustrating an embodiment of the optical branching equipment of the present invention, together with branch line terminal equipment connected thereto;

FIG. 3 is a graph showing a characteristic of an optical amplifying element formed by an erbium-doped fiber;

FIG. 4 is a block diagram illustrating an embodiment of the optical network of the present invention;

FIG. 5 is a block diagram illustrating another embodiment of each of the optical branching equipment and the optical network according to the present invention;

FIG. 6 is a block diagram illustrating another embodiment of each of the optical branching equipment and the optical network according to the present invention;

FIG. 7 is a block diagram illustrating another embodiment of the optical branching equipment of the present invention, together with branch line terminal equipment connected thereto;

FIG. 8 is a block diagram illustrating another embodiment of the optical network of the present invention;

FIG. 9 is a block diagram showing a modified form of the optical network depicted in FIG. 8;

FIG. 10 is a graph showing another characteristic of the optical amplifying element formed by the erbium-doped fiber;

FIG. 11 is a block diagram illustrating a modification of the optical network shown in FIG. 9;

FIG. 12 is a block diagram illustrating a modified form of the optical network shown in FIG. 11; and

FIG. 13 is a block diagram illustrating a modified form of the optical network shown in FIG. 8.

With reference to the drawings, embodiments of the present invention will hereinafter be described in detail.

FIG. 2 illustrates in block form an embodiment of the optical branching equipment according to the present invention. For convenience of description, branch line terminal equipment 40.sub.1 connected to the optical branching equipment 44.sub.1 is also depicted.

In FIG. 2 the optical branching equipment 44.sub.1 of this embodiment has an optical signal input terminal 11 and an optical signal output terminal 15 respectively connected to optical signal transmission fibers 33.sub.0 and 33.sub.1 which form a trunk line, and an optical signal output terminal 13 and an optical signal input terminal 14 which are respectively connected to optical signal transmission fibers 35.sub.0 and 35.sub.1 which form a branch line. The optical branching equipment switches an optical signal transmission line to a branching circuit and a short circuit as required.

An optical signal S of a wavelength 1.55 .mu.m, which is input via the optical signal input terminal 11.sub.2, is provided in two directions via an optical divider 21. The one optical signal S is applied to the optical signal output terminal 13 via a pump light coupler 22, an erbium-doped fiber 23 and a 1.55 .mu.m wavelength band-pass optical filter 24.sub.1. An optical path OP.sub.1 from the optical divider 21 to the optical signal output terminal 13 constitutes a branching circuit. The optical signal S is sent via the optical signal transmission fiber 35.sub.0 of a branch line cable 61.sub.1 to the branch line terminal equipment 40.sub.1. In the branch line terminal equipment 40.sub.1, as is the case with the prior art example shown in FIG. 1, the optical signal S is converted by an optical/electrical converter 46 into an electrical signal, which is applied to a signal add/drop circuit 47 for adding and/or dropping of required information, and the electrical signal is reconverted by an electrical/optical converter 48 into the optical signal S, which is then provided via the optical fiber 35.sub.1 to the input terminal 14 of the optical branching equipment 44.sub.1.

The optical signal S thus applied to the optical signal input terminal 14 is applied via a 1.55 .mu.m wavelength band-pass optical filter 24.sub.2, an erbium-doped fiber 25 and a pump light coupler 26 to an optical coupler 27, from which it is provided to the optical signal output terminal 15. An optical path OP.sub.2 from the optical signal input terminal 14 to the optical coupler 27 constitutes a branching circuit. The erbium-doped fiber has a length of, for example, tens to a hundred meters. It is well-known in the art that the erbium-doped fiber is excited by pump light of a wavelength band ranging from 1.46 to 1.49 .mu.m to amplify signal light of a wavelength ranging from 1.53 to 1.56 .mu.m and has peaks of gain at wavelengths of 1.535 and 1.55 .mu.m (see OFC '89 POSTDEADLINE PAPER THURSDAY, FEB. 9, 1989, PD15, "A 212 km NON-REPEATED Er+3 DOPED FIBER AMPLIFIERS IN AN IM/DIRECT-DETECTION REPEATER SYSTEM", K. HAGIMOTO, et al). The erbium-doped fiber will hereinafter be referred to as a fiber optical amplifying element or simply as an optical amplifying element.

The other optical signal output from the optical divider 21 is applied to an eribum-doped fiber 29 via a 1.55 .mu.m wavelength band-pass optical fiber 24.sub.3 and a control light coupler 28. The optical signal emitted from the erbium-doped fiber 29 is applied to the optical coupler 27 via pump light coupler 30 and a band-rejection optical filter 31 which cuts off control light C having a wavelength of 1.535 .mu.m. The optical signal thus applied to the optical coupler 27 is provided therefrom to the optical signal output terminal 15. An optical path OP.sub.3 from the optical divider 21 to the optical coupler 27 constitutes a short circuit.

A laser diode 37, which is connected to power feed terminals 12 and 16 connected to feed lines 34.sub.0 and 34.sub.1 of the trunk line, respectively, and is supplied with power Ep, yields pump light Lp of a wavelength 1.48 .mu.m. The pump light Lp is applied via an optical divider 38 to the pump light couplers 22, 26 and 30, from which it is provided to the erbium-doped fibers 23, 25 and 29.

The control light C of a wavelength 1.535 .mu.m, which is input via a control light input terminal 17 connected to a control light transmission fiber 36 accommodated in the branch line cable 61.sub.1, is combined with the optical signal S by the control coupler 28 for incidence to the erbium-doped fiber 29. The control light C is generated by a control light source 45 provided in the branch line terminal equipment 40.sub.1 and is supplied via the control light transmission fiber 36 of the branch line cable 61.sub.1.

The optical divider 38, the pump light couplers 22, 26 and 30, and the control light coupler 28 are provided to efficiently input the optical signal S and the pump light Lp or the control light C into the erbium-doped fibers 23, 25 and 29. The 1.55 .mu.m band-pass optical filters 24.sub.1 through 24.sub.3 reject optical noises emitted from the erbium-doped fibers 23, 25 and 29, whereas the 1.535 .mu.m band-rejection optical filter 31 rejects the control light C emitted from the erbium-doped fiber 29. The optical filter 31 may also be a band-pass optical filter which passes therethrough the optical signal S but rejects the control light C and other optical noises, and in such an instance, the SN ratio of the optical signal S is improved.

The erbium-doped fiber 29 is excited by the incidence thereto of the pump light Lp (having a wavelength of 1.48 .mu.m), in which state it is able to amplify the optical signal S having a wavelength of 1.55 .mu.m and the control light C having a wavelength of 1.535 .mu.m within its linear operation range. However, when supplied with the control light C of high power, the erbium-doped fiber 29 is put in an output saturation state, in which case it provides substantially no gain for the optical signal S. FIG. 3 shows measured values of the amplified optical signal level derived from the erbium-doped fiber when an optical signal of the above-mentioned wavelength having a level of 24.68 dBm and control light of the above-mentioned wavelength were input into the erbium-doped fiber excited by pump light of a constant level and the level of the input control light was changed. The level of the amplified optical signal was about -2.5 dBm when no control light was input. As will be appreciated from FIG. 3, an increase in the input level of the control light causes a decrease in the level of the output optical signal and the output level of the optical signal can be suppressed to a very small value by use of control light of a sufficiently high input level. In other words, the erbium-doped fiber 29 in FIG. 2 operates as a limiter amplifier, and by inputting control light of a high level into the erbium-doped fiber 29, the short circuit OP.sub.3 between the optical signal transmission fibers 30.sub.0 and 30.sub.1 is substantially disconnected in connection with the optical signal. 0n the other hand, the control light amplified to its saturation is cut off by the filter 31, and hence is not provided to the output terminal 15. Incidentally, the effect of suppressing a low-power optical signal by a high-power one in the optical amplifying element can be produced between a plurality of optical signals of given wavelengths in the wavelength band over which the optical amplifying element provides its gain, and embodiments of the optical network of the present invention described later utilize the effect of suppressing a low-power optical signal by a high-power optical signal of the same wavelength.

On the other hand, the erbium-doped fibers 23 and 25, which are always excited by the pump light Lp of the aforementioned wavelength, amplify the optical signal S and connect the trunk line optical signal transmission fibers 33.sub.0 and 33.sub.1 and the branch line optical signal transmission fibers 35.sub.0 and 35.sub.1 to each other, forming a bus type transmission line.

When the input of the control light C from the control light transmission fiber 36 stops, the optical limiter amplifier, i.e. the erbium-doped fiber 29 gets out of the saturated state, and consequently, it amplifies the optical signal S applied via the control light coupler 28 and outputs the amplified optical signal S. The optical signal S is applied via the pump light coupler 30, the 1.535 .mu.m wavelength band-rejection optical filter 31, the optical coupler 27 and the optical signal output terminal 15 to the optical signal transmission fiber 33.sub.1, thus interconnecting the optical signal transmission fibers 33.sub.0 and 33.sub.1 of the trunk line to form a short circuit. For instance, when the branch line cable 61.sub.1 accommodating the optical signal transmission fibers 35.sub.0 and 35.sub.1 and the control light transmission fiber 36 is broken by some cause, the supply of the control light C is stopped, and consequently, the optical limiter amplifier 29 amplifies and outputs the optical signal S applied thereto from the optical signal input terminal 11, and the amplified optical signal S is provided via the optical filter 31 and the optical coupler 27 to the optical signal output terminal 15. In the embodiment shown in FIG. 2 the couplers 28 and 30 for the incidence of the control light C and the pump light Lp may be exchanged in position, in which case the optical filter 31 may be left out. This applies to the embodiments described later.

It has been described previously in respect of FIG. 3 that the output signal light of the wavelength 1.55 .mu.m can be suppressed by applying light of the wavelength 1.535 .mu.m as control light to the erbium-doped fiber. Conversely, light of the wavelength 1.55 .mu.m may be used as the control light to suppress the output signal light of the wavelength 1.535 .mu.m. Moreover, in the embodiment of FIG. 2, when the branch line terminal equipment 40.sub.1 is disposed at a location near the optical branching equipment 44.sub.1, it is possible to connect the optical divider 21 directly to the output terminal 13 and the input terminal 14 directly to the optical coupler 27, omitting the couplers 22 and 26, the optical amplifying elements 33 and 25 and the optical filters 24.sub.1 and 24.sub.2.

FIG. 4 illustrates in block form an embodiment of the optical network according to the present invention. This embodiment is a bus type optical network in which an optical signal having a wavelength of 1.55 .mu.m is transmitted, and optical branching equipment used is identical in construction with that shown in FIG. 2.

In FIG. 4 trunk line terminal equipments 40.sub.0 and 40.sub.3 are interconnected via optical signal transmission fibers 33.sub.0 to 33.sub.3 and feed lines 34.sub.0 to 34.sub.2, which form trunk line cables 30.sub.0 to 30.sub.2, and via optical branching equipments 44.sub.1 and 44.sub.2. The optical branching equipments 44.sub.1 and 44.sub.2 have connected thereto branch line terminal equipments 40.sub.1 and 40.sub.2 via optical signal transmission fibers 35.sub.0 to 35.sub.3 and control light transmission fibers 36.sub.1 and 36.sub.2 which form branch line cables 61.sub.1 and 61.sub.2. Now, let it be assumed, for the sake of brevity, that the optical signal S is transmitted in a direction from the trunk line terminal equipment 40.sub.0 to 40.sub.3.

The trunk line terminal equipment 40.sub.0 is provided with a matching circuit 41 which outputs an electrical signal Se produced by converting transmission information D into a line code, an electrical/optical converter 42 which converts the electrical signal Se into an optical signal S having a wavelength of 1.55 .mu.m and provides it to the optical signal transmission fiber 33.sub.0, and a power circuit 43 connected to the feed line 34.sub.0.

The trunk line terminal equipment 40.sub.3 is provided with an optical/electrical converter 49 whereby the optical signal S input thereinto from the optical signal transmission fiber 33.sub.2 is converted into the electrical signal Se, a matching circuit 50 which extracts the transmitted information D from the electrical signal Se, and a power circuit 51 connected to the feed line 34.sub.2.

The branch line terminal equipments 40.sub.1 and 40.sub.2 are respectively provided with optical/electrical converters 46.sub.1 and 46.sub.2 whereby the optical signal S of the wavelength 1.55 .mu.m input from the optical signal transmission fibers 35.sub.0 and 35.sub.2 is converted into an electrical signal Se, signal add/drop circuits 47.sub.1 and 47.sub.2 which drop desired information Dd from the electrical signal Se and add thereto information Da to be transmitted, and electrical/optical converters 48.sub.1 and 48.sub.2 whereby the electrical signal Se having the information dropped therefrom and added thereto is converted into the optical signal S of the wavelength 1.55 .mu.m for output to the optical signal transmission fibers 35.sub.1 and 35.sub.3. Furthermore, the branch line terminal equipments 40.sub.1 and 40.sub.2 include control light sources 45.sub.1 and 45.sub.2 which generate the control light C of the wavelength 1.535 .mu.m and apply it to the optical branching equipments 44.sub.1 and 44.sub.2. This is characteristic of the present invention.

By the input of the control light C of the wavelength 1.535 .mu.m into the optical branching equipments 44.sub.1 and 44.sub.2, the short circuit between the optical signal transmission fibers 33.sub.0 and 33.sub.1 and the short circuit between the optical signal transmission fibers 33.sub.1 and 33.sub.2 are disconnected apparently, and branching circuits are formed, respectively.

The optical signal S delivered to the optical signal transmission fiber 33.sub.0 from the trunk line terminal equipment 40.sub.0 at the transmitting side is provided via the optical branching equipment 44.sub.1 to the optical signal transmission fiber 35.sub.0 for transmission to the branch line terminal equipment 40.sub.1. In the branch line terminal equipment 40.sub.1 the optical signal S is converted by the optical/electrical converter 46.sub.1 into the electrical signal Se, which is applied to the signal add/drop circuit 47.sub.1, in which the information Dd and Da are dropped from and added to the electrical signal Se. Thereafter, the electrical signal Se is converted again by the electrical/optical converter 48.sub.1 into the optical signal S of the wavelength 1.55 .mu.m for transmission to the optical branching equipment 44.sub.1 via the optical signal transmission fiber 35.sub.1. The optical signal S is provided on the optical signal transmission fiber 33.sub.1 from the optical branching equipment 44.sub.1.

Similarly, the optical signal S thus transmitted to the optical branching equipment 44.sub.2 is provided via the branch line cable 61.sub.2 to the branch line terminal equipment 40.sub.2 and thence to the trunk line terminal equipment 40.sub.3 of the receiving station via the optical signal transmission fiber 33.sub.2.

For example, in the event that the branch line cable 61.sub.1 including the optical signal transmission fibers 35.sub.0 and 35.sub.1 and the control light transmission fiber 36.sub.1 is broken between the optical branching equipment 44.sub.1 and the branch line terminal equipment 40.sub.1, the supply of the control light C to the optical branching equipment 44.sub.1 is stopped. As a result of this, the optical branching equipment 44.sub.1 forms the above-mentioned short circuit between the optical signal transmission fibers 33.sub.0 and 33.sub.1, disconnecting the branch line terminal equipment 40.sub.1. Thus, the transmission line can automatically be switched, if the branch line cable is broken.

While in the above the present invention has been described as being applied to the bus type optical network, the invention is applicable as well to the star type network. Further, the above embodiments have been described in connection with the case where the erbium-doped fiber is used as the optical amplifying element and the optical limiter amplifier and the first and second wavelength bands are 1.55 and 1.535 .mu.m, respectively, but other wavelengths can be employed as the first and second wavelength bands in the case of using a fiber doped with other rare earth elements.

FIG. 5 illustrates the principal part of a modified form of the optical branching equipment 44.sub.1 shown in FIG. 2. In this example, the pump light Lp is generated by a pump light source 37.sub.1 provided in the trunk line terminal equipment 40.sub.0 and is provided to the optical branching equipment 44.sub.1 via the pump light transmission fiber 34.sub.0 of the trunk line optical cable 30.sub.0. In the optical branching equipment 44.sub.1 the pump light Lp is divided by an optical divider 38 into two, one of which is applied to the optical limiter amplifier 29 via the coupler 30. The other pump light Lp is sent to the next stage via the pump light transmission fiber 34.sub.1. In this embodiment the pump light Lp to the optical amplifying elements 23 and 25 is supplied from the branch line terminal equipment 40.sub.1. To this end, a pump light source 37.sub.2 is provided in the branch line terminal equipment 40.sub.1 and the pump light from the pump light source 37.sub.2 is supplied to the optical amplifying elements 23 and 25 from couplers 39.sub.0 and 39.sub.1 via the optical signal transmission fibers 35.sub.0 and 35.sub.1, respectively. As is the case with the embodiment depicted in FIG. 2, the branch line terminal equipment 40.sub.1 has the control light source 45, from which the control light C is supplied via the optical fiber 36 to the optical branching equipment 44.sub.1. The principle of operation of the optical branching equipment 44.sub.1 is exactly the same as in the embodiment of FIG. 2, and hence no description will be repeated in this respect. As will be seen from the above, no pump light source is incorporated in the optical branching equipment 44.sub.1 depicted in FIG. 5 unlike in the case of FIG. 2; so that no electronic devices are employed. Accordingly, this optical branching equipment can be expected to perform a highly reliable operation for a long period of time. Also in this embodiment, if the branch line terminal equipment 40.sub.1 is located near the optical branching equipment 44.sub.1, the optical amplifying elements 23 and 25 could be omitted, and consequently, the pump light supply means 37.sub.2, 39.sub.0 and 39.sub.1 would not be needed. While in the above the pump light source 37.sub.1 is described to be provided in the trunk line terminal equipment 40.sub.0 of the transmitting side, it is apparent that the pump light source 37.sub.1 may be provided in the trunk line terminal equipment of the receiving side or in either of them.

FIG. 6 illustrates a modification of the embodiment shown in FIG. 5. In FIG. 5 the pump light Lp for the optical limiter amplifier 29 is supplied via the pump light transmission fiber 34.sub.0 accommodated in the trunk line optical cable 30.sub.0, whereas in FIG. 6 the pump light Lp is supplied to the optical limiter amplifier 20 of the optical branching equipment 44.sub.1 via the optical signal transmission fiber 33.sub.0 of the trunk line optical fiber cable 30.sub.0. That is, the pump light source 37.sub.1 is provided in the trunk line terminal equipment 40.sub.0 and the pump light Lp generated by the pump light source 37.sub.1 is wavelength multiplexed with the optical signal S by a coupler 32 and then provided on the optical signal transmission fiber 33.sub.0. In the optical branching equipment 44.sub.1 the optical signal S and the pump light Lp supplied thereto are divided by the optical divider 21 into two, one of which is applied via the coupler 28 to the optical limiter amplifier 29. The other pump light Lp and optical signal S are applied to the optical amplifying element 23, by which the optical signal S is amplified, thereafter being sent to the branch line terminal equipment 40.sub.1. An important feature of the optical network shown in FIG. 6 resides in the following arrangement. Namely, the pump light source 37.sub.2 is provided in each of the branch line terminal equipments 40.sub.1, 40.sub.2, . . . (only two of which are shown). The pump light Lp from the pump light source 37.sub.2 in the branch line terminal equipment 40.sub.1, for instance, is provided via the coupler 39.sub.0 to the optical signal transmission fiber 35.sub.0 of the branch line optical cable 61.sub.1, over which it is transmitted to the optical branching equipment 44.sub.1, exciting the optical amplifying element 23 therein. At the same time, the pump light Lp from the pump light source 37.sub.2 is wavelength multiplexed with the optical signal S by the coupler 39.sub.1 and thence transmitted via the optical signal transmission fiber 35.sub.1 of the branch line optical cable 61.sub.1 to the optical branching equipment 44.sub.1, exciting the optical amplifying element 25 therein. The remaining pump light Lp from the optical amplifying element 25 is input into the coupler 27, from which it is transmitted via the optical signal transmission fiber 33.sub.1 of the trunk line optical cable 30.sub.1 to the optical limiter amplifier 29 of the next-stage optical branching equipment 44.sub.2 (partly shown) of the same construction as the optical branching equipment 44.sub.1.

As will be understood from the description given previously of FIG. 3, the state of operation of each optical limiter amplifier 29 is dependent on the ON/OFF state of the pump light Lp supplied from the preceding stage and the control light C from the branch line terminal equipment. For instance, in the optical branching equipment of FIG. 6, when the pump light Lp supplied from the trunk line terminal equipment 40.sub.0 and the control light C from the branch line terminal equipment 40.sub.1 are both ON, the operation of the optical limiter amplifier 29 reaches a saturated state, substantially cutting off the optical signal S. Where the pump light Lp is ON and the control light C is OFF, the optical limiter amplifier 29 is a linear operating state and amplifies and outputs the optical signal S. When the pump light Lp is OFF, the optical limiter amplifier 29 operates as an attenuator for the optical signal S, and hence cuts it off regardless of the ON/OFF state of the control light C. Thus, in the optical network of the construction shown in FIG. 6, as is the case with the embodiment of FIG. 5, when the branch line optical cable 61.sub.1 is broken, or when the branch line terminal equipment 40.sub.1 becomes inoperable, the control light C is not supplied to the optical branching equipment 44.sub.1, and consequently, the optical limiter amplifier 29 enters the linear operating state, in which it amplifies the signal light S from the optical divider 21 and outputs it to the coupler 27 (short circuit connection). At this time, the supply of the pump light Lp from the branch line terminal equipment 40.sub.1 to the input terminal 14 of the optical branching equipment 44.sub.1 is also stopped, that is, the pump light supply to the optical limiter amplifier 29 of the next-stage optical branching equipment 44.sub.2 is stopped. In the optical branching equipment 44.sub.2 of the next stage, however, its optical limiter amplifier 29 need not be operative as long as the optical signal S is transmitted via the normal branching circuit; hence the suspension of the pump light supply does not matter. Further, the optical amplifying element 23 of the optical branching equipment 44.sub.2 is supplied with the pump light from the corresponding branch line terminal equipment 40.sub.2, and hence normally operates. In other words, during normal operation in which the optical branching equipment 44.sub.1 of FIG. 6 receives the optical signal S from the preceding stage (the trunk line terminal equipment 40.sub.0, in this case) and sends it to the succeeding stage via the branching circuit passing through the branch line terminal equipment 40.sub.1, the transmission of the optical signal S is normally kept on via the branching circuit, even if the supply of the pump light Lp from the preceding stage is suspended. When not excited by the pump light, the optical limiter amplifier 29 acts as an attenuator on the optical signal, and the level of the optical signal output via the short circuit is negligibly low. That is, the linear amplifying operation of the optical limiter amplifier 29 for the optical signal is needed only when trouble arises in the branch line optical cable 61.sub.1 or branch line terminal equipment 40.sub.1 connected to the optical branching equipment 44.sub.1. In the adjoining two optical branching equipments 44.sub.1 and 44.sub.2, when the supply of the pump light Lp from the branch line optical cables 61.sub.1 and 61.sub.2 connected thereto is suspended, the optical signal transmission via the trunk line is impossible, of course, but the probability of occurrence of such a situation is very small. As mentioned above, when not excited by the pump light, the erbium-doped fiber forming the optical limiter amplifier 29 or optical amplifying element 23 or 25 greatly attenuates the signal light.

As described above, each optical branching equipment in FIG. 6 has an arrangement in which the optical limiter amplifier 29 and the optical amplifying element 23 are excited by the pump light supplied via the optical signal transmission fiber from the preceding stage (at the side of the transmitting trunk line terminal equipment 40.sub.0), the optical amplifying element 25 is excited by one portion of the pump light supplied via the optical signal transmission fiber from the corresponding branch line terminal equipment, and the remaining pump light is supplied via the optical signal transmission fiber to the next stage (the receiving trunk line terminal equipment 40.sub.3 side in FIG. 4). However, each optical branching equipment may be arranged so that the pump light supplied from the branch line terminal equipment is applied to the preceding stage (the transmitting trunk line terminal equipment 40.sub.0 side). That is, for example, in the optical branching equipment 44.sub.2 in FIG. 6, the remaining portion of the pump light supplied to the optical amplifying element 23 from the branch line terminal equipment 40.sub.2 is applied from the optical divider 21 via the optical signal transmission fiber 33.sub.1 to the optical branching equipment 44.sub.1 for exciting its optical limiter amplifier 29 and optical amplifying element 25.

In either of the cases where the pump light is supplied from the optical branching equipment 44.sub.1 to 44.sub.2 in the same direction as is supplied the optical signal and the case where the pump light is supplied from the optical branching equipment 44.sub.2 to 44.sub.1 in the direction opposite from tee optical signal, the optical branching equipments 44.sub.1 and 44.sub.2 can each utilize the pump light supplied thereto when it is arranged so that the pump light supplied from the preceding or succeeding stage is used to excite the optical limiter amplifier 29 alone. For instance, in FIG. 6, when the pump light Lp is transmitted in the same direction as the optical signal S, such an optical divider of a wavelength characteristic that divides light in the wavelength band of the optical signal S and merely passes light in the wavelength band of the pump light Lp is formed, as the optical divider 21 of each optical branching equipment, by a directional coupler, for instance. In the case of transmitting the pump light Lp in the direction reverse from the direction of transmission of the optical signal S, such an optical coupler of a wavelength characteristic that couples two kinds of light in the wavelength band of the optical signal S and merely passes light in the wavelength band of the pump light Lp is formed, as the optical coupler 27 of each optical branching equipment, by a directional coupler, for example.

In either of the above-described cases, when a given branch line terminal equipment is installed near the corresponding optical branching equipment, it is possible, as referred to previously, to employ an arrangement in which either or both of the optical amplifying elements 25 and 23 of the optical branching equipment are omitted and the pump light Lp is entirely delivered from the coupler 27 or optical divider 21 to the optical branching equipment of the next or preceding stage. In such a case, the optical coupler 39.sub.0 or 39.sub.1 in the branch line terminal equipment is unnecessary.

FIG. 7 illustrates a modified form of the embodiment shown in FIG. 6. In this embodiment the control light transmission fiber is not provided in the branch line optical cable 61.sub.1 and the control light is transmitted using either one of the optical signal transmission fibers 35.sub.0 and 35.sub.1, the latter 35.sub.1 in this example. Accordingly, in the branch line terminal equipment 40.sub.1 the output optical signal S from the electrical/optical converter 48, the pump light Lp and the control light C are sequentially wavelength multiplexed in couplers 39.sub.1 and 39.sub.2 and then provided on the optical signal transmission fiber 35.sub.1. In the optical branching equipment 44.sub.1 the control light C is separated by an optical demultiplexing coupler 28.sub.0 from the wavelength-multiplexed input applied to the input terminal 14 and is provided to a coupler 28.sub.1, the remaining optical signal S and the pump light Lp are being input into the optical amplifying element 25. This embodiment is identical in construction and operation with the FIG. 6 embodiment except for the above, and hence no further description will be given.

FIG. 8 illustrates an embodiment of the optical network according to the second aspect of the present invention. In the trunk line terminal equipment 40.sub.0 the electrical signal Se from the matching circuit 41 is converted by the electrical/optical converter 42 into the optical signal S, which is provided on the optical signal transmission fiber 33.sub.0 accommodated in the trunk line optical cable 30.sub.0. In the optical branching equipment 44.sub.1 the signal light S transmitted thereto is divided by an optical divider 21.sub.1 into two, one of which is applied to the optical fiber 33.sub.1 of the trunk line optical cable 30.sub.1 and the other of which is provided to an optical divider 21.sub.2, by which it is subdivided into a plurality of optical signals, three in this example. The three optical signals S are amplified by optical amplifying elements 23.sub.1, 23.sub.2 and 23.sub.3, respectively, and the thus amplified optical signals S are supplied to branch line terminal equipments 40.sub.1, 40.sub.2 and 40.sub.3 via optical signal transmission fibers 35.sub.1, 35.sub.2 and 35.sub.3 of branch line optical fiber cables 90.sub.1, 90.sub.2 and 90.sub.3. In this example the branch line terminal equipments 40.sub.1, 40.sub.2 and 40.sub.3 are shown only to receive information, and the supplied optical signals S are converted by receivers 47.sub.1, 47.sub.2 and 47.sub.3 into electrical signals, from which desired information P is extracted. Also in this embodiment the optical amplifying elements 23.sub.1, 23.sub.2 and 23.sub.3 of the optical branching equipment 44.sub.1 are each formed by an erbium-doped fiber, for example, and the wavelength band of each optical signal is selected to be 1.55 .mu.m, for instance. The pump light Lp as of a wavelength band 1.48 .mu.m for exciting the optical amplifying elements 23.sub.1, 23.sub.2 and 23.sub.3 is produced by each of pump light sources 37.sub.1, 37.sub.2 and 37.sub.3 in the branch line terminal equipments 40.sub.1, 40.sub.2 and 40.sub.3 and supplied via optical couplers 39.sub.1, 39.sub.2 and 39.sub.3 and optical signal transmission fibers 35.sub.1, 35.sub.2 and 35.sub.3, respectively. It is therefore possible to constitute optical branching equipment which has capabilities of branching, amplifying and transmitting an optical signal to branch line terminal equipment without employing electrically active devices; hence the optical branching equipment is highly reliable in operation and maintenance-free for a long period of time.

Incidentally, the optical network shown in FIG. 8 is applicable to an optical fiber video delivery system. FIG. 9 illustrates the construction of the optical network in such a case. For instance, the trunk line terminal equipment 40.sub.0 is an information providing station and delivers a program P as an optical signal S, and the branch line terminal equipments 40.sub.1, 40.sub.2 and 40.sub.3 are subscriber-furnished terminal equipments and receive the program P. In this case, at a desired time of receiving the program P, the respective subscriber-furnished terminal equipments start the pump light sources 37.sub.1, 37.sub.2 and 37.sub.3 by start switches 71.sub.1, 71.sub.2 and 71.sub.3 to supply the pump light Lp to the corresponding optical amplifying elements 23.sub.1, 23.sub.2 and 23.sub.3 of the optical branching equipment 44.sub.1. In the optical branching equipment 44.sub.1 there are provided optical couplers 72.sub.1, 72.sub.2 and 72.sub.3 loosely coupled with the optical fibers 35.sub.1, 35.sub.2 and 35.sub.3, respectively, in the wavelength band of the pump light Lp and monitoring equipments 73.sub.1, 73.sub.2 and 73.sub.3 respectively connected to the optical couplers 72.sub.1, 72.sub.2 and 72.sub.3. When supplied with the pump light Lp from the subscriber-furnished terminal equipments via the optical fibers 35.sub.1, 35.sub.2 and 35.sub.3 of branch line optical cables 90.sub.1, 90.sub.2 and 90.sub.3, the monitoring equipments 73.sub.1, 73.sub.2 and 73.sub.3 detect the pump light Lp to thereby learn the state of utilization of the program P by the subscriber-furnished terminal equipments.

FIG. 10 shows the relationship between the gain of the optical amplifying element formed by the erbium-doped fiber and the power of the pump light. As shown in FIG. 10, when the power of the pump light becomes smaller than a certain value, the gain becomes smaller than 0 dB, and as mentioned previously, when no pump light is provided, the optical amplifying element greatly attenuates the input signal light. Accordingly, in the optical network depicted in FIG. 9, the pump light must be supplied to the optical amplifying elements 23.sub.1, 23.sub.2 and 23.sub.3 of the optical branching equipment 44.sub.1 in order for the subscriber-furnished terminal equipments to receive the program. The monitoring equipments 73.sub.1, 73.sub.2 and 73.sub.3 monitor the state of use of the program by the subscriber-furnished terminal equipments through the detection of the pump light.

While in FIG. 9 the monitoring equipments 73.sub.1, 73.sub.2 and 73.sub.3 are shown to be connected to the optical fibers 35.sub.1, 35.sub.2 and 35.sub.3 of the respective branch line optical cables, respectively, they need not always be provided separately. For example, as indicated by the broken line, one monitoring equipment 73 may be provided in association with the optical path between the optical dividers 21.sub.1 and 21.sub.2, or it may be provided, for instance, in association with the optical fiber 33.sub.0 in the trunk line terminal equipment 40.sub.0 so that it detects the pump light remaining after being consumed in the optical amplifying elements 23.sub.1, 23.sub.2 and 23.sub.3. In these cases, the subscriber-furnished terminal equipments (i.e. the branch line terminal equipments) 40.sub.1, 40.sub.2 and 40.sub.3 are each adapted to deliver the pump light after modulating it with a modulation signal of a particular frequency or code of its own and the monitoring equipment 73 demodulates the modulation signal of the pump light to thereby detect which of the subscriber-furnished terminal equipments supplies the pump light.

FIG. 11 illustrates a modified form of the optical network applied to the optical fiber video delivery system shown in FIG. 9. This embodiment is designed to offer two kinds of information to subscribers through utilization of the fact that since the transmission loss by silica fiber and erbium-doped fiber is very small at a wavelength of 1.3 .mu.m and this wavelength is outside the energy transition absorption wavelength band of the erbium-doped fiber, no light of 1.3 .mu.m is absorbed by erbium-doped silica fiber in either of its excited and non-excited states. That is, in the trunk line terminal equipment (i.e. information providing station) 40.sub.0 program information P.sub.1 input to a matching circuit 41.sub.1 is converted by an electrical/optical converter 42.sub. 1 into an optical signal S.sub.1 of a 1.55 .mu.m wavelength and control infOrmation or service information P.sub.2 input to a matching circuit 41.sub.2 is converted by an electrical/optical converter 42.sub.2 into an optical signal S.sub.2 of a 1.3 .mu.m wavelength. These optical signals S.sub.1 and S.sub.2 are wavelength multiplexed by the coupler 32 and then provided on the optical signal transmission fiber 33.sub.0. The branch line terminal equipment (i.e. the subscriber-furnished terminal equipment) 40.sub.1 is always supplied with the optical signal S.sub.2 of the 1.3 .mu.m wavelength regardless of whether or not it delivers the pump light Lp, and the optical signal S.sub.2 is separated by a demultiplexing coupler 74.sub.1 and applied to the receiver 47.sub.1, by which the service information P.sub.2 can be regenerated. On the other hand, the optical signal S.sub.1 of the 1.55 .mu.m wavelength is supplied from the optical branching equipment 44.sub.1 to the branch line terminal equipment 40.sub.1 only when the pump light Lp emanating from the pump light source 37.sub.1 started by the start switch 71.sub.1 is supplied to the optical amplifying element 23.sub.1 of the optical branching equipment 44.sub.1, and the optical signal S.sub.1 is applied to the receiver 47.sub.1, by which the program information P.sub.1 can be regenerated.

Incidentally, if the two optical signals S.sub.1 and S.sub.2 are subjected to different but proper premodulations, then the two electrical signals corresponding to these optical signals could easily be separated from each other, and consequently, the demultiplexing coupler 74.sub.1 would be unnecessary. Assuming, for example, that the first optical signal S.sub.1 is obtained by frequency modulating a carrier F.sub.1 of a frequency f.sub.1 with a first electrical signal to be transmitted and then by converting the modulated carrier into the optical signal S.sub.1 and that the second optical signal S.sub.2 is similarly obtained by frequency modulating a carrier F.sub.2 of a frequency f.sub.2 with a second electrical signal to be transmitted and then by converting the modulated carrier into the optical signal S.sub.2, an electrical signal containing the modulated carriers F.sub.1 and F.sub.2 of the frequencies f.sub.1 and f.sub.2 can be obtained by converting the multiplexed first and second optical signals S.sub.1 and S.sub.2 into electrical form without separating them from each other. Accordingly, by frequency discrimination of the received electrical signals through use of a local oscillation signal of either one of the frequencies f.sub.1 and f.sub.2, the desired one of the first and second electrical signals can be obtained.

With the construction shown in FIG. 11, the trunk line terminal equipment 40.sub.0 delivers, by the optical signal of the 1.55 .mu.m band, for example, charging program information P.sub.1 for which subscribers are charged in accordance with the amount of viewing time, and delivers, by the optical signal S.sub.2 of the 1.3 .mu.m band, basic service information P.sub.2 such as control information of the branch line terminal equipments 40.sub.1, 40.sub.2 and 40.sub.3, introduction of the contents of the charging program information, and an advertisement. The subscribers monitor the information P.sub.2 reproduced from the optical signal S.sub.2 of the 1.3 .mu.m band and, if they find desired information, they send out the pump light Lp by starting the pump light sources 37.sub.1, 37.sub.2 and 37.sub.3 with the start switches 71.sub.1, 71.sub.2 and 71.sub.3, by which they can receive the optical signal S.sub.1 of the 1.55 .mu.m band. Accordingly, the subscribers are allowed to make a choice of information without being charged, and the information provider is allowed to advertise the information he provides; therefore, this optical network is highly beneficial to both of them. Also in this embodiment the monitoring equipment 73 may be provided at the position indicated by the broken line in FIG. 9 instead of using the monitoring equipments 73.sub.1, 73.sub.2 and 73.sub.1.

FIG. 12 illustrates a modified form of the optical network of FIG. 11, in which another facility is added. This embodiment utilizes the characteristics of the erbium-doped fiber referred to previously in respect of FIG. 3. When the information providing equipment 40.sub.0 is transmnitting the program information P.sub.1 by the first optical signal S.sub.1, supervisory station 80 may couple, by an optical coupler 85, a third optical signal S.sub.3 of high power to the first optical signal S.sub.1 and provides the signals to an optical limiter amplifier 86, by which it is possible to suppress the first optical signal S.sub.1 by the third optical signal S.sub.3, temporarily interrupting the program information P.sub.1 with program information P.sub.3. That is, assume that the information providing equipment 40.sub.0 always delivers the program information P.sub.1 and the service information P.sub.2 by the optical signal S.sub.1 of the 1.55 .mu.m wavelength and the optical signal S.sub.2 of the 1.3 .mu.m. A matching circuit 81 converts emergency information P.sub.3, input thereinto as required, into a drive signal and applies it to an electrical/optical converter 82, which responds thereto to output the high-power optical signal S.sub.3 of the same wavelength 1.55 .mu.m as that of the first optical signal S.sub.1. The optical signal S.sub.3 is coupled with the optical signals S.sub.1 and S.sub.2 from the information providing station 40.sub.0 and input into the optical limiter amplifier 86 via a coupler 84. The optical limiter amplifier 86, which is supplied with the pump light Lp from a pump light source 83 via the coupler 84, normally amplifies the optical signal S.sub.1 and delivers it and the optical signal S.sub.2 to the optical branching equipment 44.sub.1 via the optical signal transmission fiber 33.sub.0. If the optical signal S.sub.3 of high power is input into the optical limiter amplifier 86, the optical signal S.sub.1 is suppressed and the optical signal S.sub.3 is output instead of the optical signal S.sub.1. Accordingly, the optical signal S.sub.1 supplied to the optical branching equipment 44.sub.1 until then is switched to the optical signal S.sub.3. Incidentally, the optical signal S.sub.2 of the 1.3 .mu.m passes through the optical limiter amplifier 86 regardless of its operation state and is always supplied to the optical branching equipment 44.sub.1.

With the structure shown in FIG. 12, the supervisory station 80 is allowed to interrupt, as required, the optical signal S.sub.1 from the information providing station 40.sub.0 by the optical signal S.sub.3 for supply to the optical branching equipment 44.sub.1. Consequently, only those subscriber-furnished terminal equipments 40.sub.1, 40.sub.2 and 40.sub.3 that are supplying the pump light Lp to the optical branching equipment 44.sub.1 and receiving the optical signal S.sub.1 at that time receive the optical signal S.sub.3. This interrupt function can be utilized for an emergency warning, for example.

Although in the embodiments shown in FIGS. 8, 9, 11 and 12 the pump light Lp from the branch line terminal equipments 40.sub.1 to 40.sub.3 is supplied via the optical signal transmission fibers 35.sub.1 to 35.sub.3 to the corresponding optical amplifying elements 23.sub.1 to 23.sub.3 of the optical branching equipment 44.sub.1, it is obvious that the pump light Lp may be supplied to the optical branching equipment 44.sub.1 via pump light transmission fibers which are provided along the optical fibers 35.sub.1 to 35.sub.3. FIG. 13 illustrates an embodiment of this construction as applied to the optical network of FIG. 8. As depicted in FIG. 13, the branch line optical cables 90.sub.1 to 90.sub.3 have accommodated therein pump light transmission fibers 87.sub.1 to 87.sub.3 along the optical signal transmission fibers 35.sub.1 to 35.sub.3. The pump light Lp from the pump light sources 37.sub.1 to 37.sub.3 in the branch line terminal equipments 40.sub.1 to 40.sub.3 is input into the pump light transmission fibers 87.sub.1 to 87.sub.3 and transmitted therethrough to the optical branching equipment 44.sub.1. In the branching circuits of the optical branching equipment 44.sub.1 there are inserted optical couplers 39.sub.1 to 39.sub.3, through which the pump light Lp supplied thereto via the pump light transmission fibers 87.sub.1 to 87.sub.3 is input into the optical amplifying elements 23.sub.1 to 23.sub.3 in the respective branching circuits, exciting them. The optical signals S amplified by the optical amplifying elements 23.sub.1 to 23.sub.3 are transmitted to the branch line terminal equipments 40.sub.1 to 40.sub.3 via the optical couplers 39.sub.1 to 39.sub.3 and the optical signal transmission fibers 35.sub.1 to 35.sub.3, respectively. This construction can be applied as well to the embodiments shown in FIGS. 9, 11 and 12.

While in the embodiments of FIGS. 9, 11, 12 and 13 an optical signal of 1.55 .mu.m wavelength is employed as the optical signal S.sub.1, an optical signal of 1.535 .mu.m may also be used. In the embodiment of FIG. 12 the wavelengths of the optical signals S.sub.1 and S.sub.3 may be different from each other. Also in these embodiments optical signals of wavelengths different from those mentioned above can be employed by using, as the optical amplifying elements and optical limiter amplifiers, optical fibers doped with other rare earth elements.

As described above, according to the second aspect of the present invention, since the optical branching equipment between the trunk line terminal equipment and the branch line terminal equipments is provided with optical amplifying elements which amplify an optical signal when supplied with pump light of a specified wavelength, the optical signal can be distributed to many branch line terminals. Besides, since electronic devices which call for electric power supply are not needed in the optical transmission line from the trunk line terminal equipment to the branch line terminal equipment, no maintenance is required for a long period of time; hence the present invention is highly reliable and highly economical.