OPTICAL SWITCHING UNIT, PARTICULARLY FOR SWITCHING TO STANDBY COMPONENTS IN OPTICAL TRANSMISSION SYSTEMS

Abstract

An optical switching unit is disclosed which permits switching from faulty components to standby components in optical transmission systems. A component in this connection may be a transmit laser or an optical fiber, for example. The optical switching unit (OSE) according to the invention contains an n×m optical space switch (ORS) whose ports (P1A . . . PNA; P1B . . . PMB) are monitored by photodiodes (M1A, M1B). The photodiodes provide a control unit (SE) with information as to whether and, if so, what signals are arriving at the ports. The control unit (SE) controls the position of the optical switch (ORS) in accordance with program instructions. The switching unit controls itself, i.e., incorporation of a higher-level control center is not necessary. It thus allows very short switching times and has many applications; it is particularly suitable for use in modular optical transmission systems.

Description

[0001] This invention relates to an optical switching unit, particularly for switching from faulty transmission facilities to standby transmission facilities in optical transmission systems. Such a transmission facility may be a transmit laser, a receiver photodiode, or an optical fiber, for example.

[0002] In optical transmission systems, optical fibers interconnect a transmitter and a receiver. Because of the wide transmission bandwidths of optical fibers, a single such fiber generally suffices to transmit the usual amounts of data. To be able to continue to transmit data if the optical fiber should be damaged, one or more spare fibers are laid parallel to that optical fiber. If any damage to the optical fiber is detected, a control unit will cause the transmitter and receiver to switch to a spare fiber.

[0003] To increase the reliability of optical transmission systems, a standby laser and/or a standby photodiode, which are used in case of failure of the main device, are frequently incorporated into the system. Since communication is interrupted while changeover to a standby transmission facility is effected, the dead time of the transmission system, i.e., the time required for the changeover, should be as short as possible.

[0004] European Patent Application EP-A1-519 712 discloses an optical bus network in which the two end nodes of the bus are connected by optical fibers to an additional monitoring node (“terminal node”). The monitoring node, which is thus connected directly between the two end nodes, establishes a direct connection between the two end nodes with the aid of a switch if there is a discontinuity in the bus. As a result, the nodes in the immediate vicinity of the discontinuity become the new end nodes of the bus. To be able to detect discontinuities (e.g., a break in a fiber), the monitoring node includes optical-to-electrical transducers with which it monitors the signals from the end nodes.

[0005] offenlegungsschrift DE-A1-44 33 031 discloses a method of switching to one or more spare lines in unidirectional or bidirectional optical transmission systems (see FIG. 6). If the receiver portion of a transceiver unit SE6A at one end of a bidirectional optical transmission system does not receive the data stream transmitted over an optical fiber FA6 or receives the data stream only in greatly disturbed form, it will communicate this to a control unit ST6A associated with this end. The control unit causes the transmitter and receiver at this end to be switched to a spare line FAE6. The receiver portion of a transceiver unit SE6B at the other end of the transmission link then determines that no signal is now present on the originally used optical fiber FA6. In response to this, the control unit ST6B initiates a changeover to the spare line FAE6 at that end of the transmission link as well.

[0006] Under certain circumstances, for example in the case of very long transmission links, the probability that not only the optical fiber actually intended for transmission but also the spare fiber will be damaged is relatively high. In that case, the data transmission will collapse. To prevent this, more than one spare fiber may be laid, as is known-from the above-cited DE-A1-44 33 031. Particularly in the case of long transmission links, however, the additional optical fibers to be laid represent a considerable cost factor.

[0007] It is therefore an object of the invention to provide an optical switching unit with which changeover can be effected from faulty transmission facilities to standby transmission facilities in optical transmission systems. The optical switching unit is to be simple in construction and universally applicable. In particular, the optical switching unit is to permit the construction of an optical transmission system which allows reliable data transmission even in the event of frequent damage to optical fibers but requires as few spare optical fiber as possible.

[0008] The invention attains the object with the aid of the features recited in claim 1. One, a number, or all of the ports on both sides of an n×m optical space switch are monitored by photodiodes which are connected to a control unit incorporated into the optical switching unit. The control unit controls the position of the optical space switch in accordance with changes in the signals applied at the ports. Depending on how the control unit is programmed, the optical switching unit according to the invention can be used to advantage for different functions.

[0009] In a first embodiment, the optical switching unit is used as a switch in an optical transmission link. The optical link consists of a first optical fiber and a parallel, second optical fiber as a spare fiber. Optical switching units according to the invention are integrated into the link at given intervals. If the first optical fiber is broken, the optical switching units adjacent to the break will detect this and switch to the existing spare fiber. Since the switching processes are initiated and controlled in the transmission link itself and not centrally at the transmitter end, the dead time of the transmission system, i.e., the time in which no data transmission takes place, is very short.

[0010] The division of the transmission link into several sections ensures reliable data transmission even if both optical fibers have one or even more breaks, since, according to the invention, only the respective faulty section is bypassed by the spare fiber. Another advantage is that the positions of the optical switches can also be controlled from the transmitter directly via the optical fibers if the optical switching unit is programmed in such a way as to detect particular patterns in the data stream and initiate corresponding switching processes in response thereto.

[0011] In a second embodiment, the optical switching unit is used in an arrangement of transmit lasers to switch to a standby laser if one of the transmit lasers fails. In that case, too, the transmission capability of the system can be restored in the shortest time.

[0012] In a third embodiment, the optical switching unit is used as an optical selector switch. In packet-switched transmission systems, the optical switching unit is capable of detecting the address field in the packet header and of establishing connections in accordance with this information. Thus, the switching unit according to the invention can be used to advantage in future optical ATM exchanges, for example.

[0013] Further advantageous features of the invention are defined in the subclaims.

[0014] The invention will become more apparent from the following description of embodiments when taken in conjunction with the accompanying drawings, in which:

[0015] FIG. 1 is a schematic block diagram of an optical switching unit as claimed in claim 1;

[0016] FIG. 2 is a schematic block diagram of an optical switching unit as claimed in claim 3;

[0017] FIG. 3 is a schematic block diagram of an optical switching unit as claimed in claim 2;

[0018] FIG. 4 a is a schematic block diagram of a circuit for switching to a spare fiber in a bidirectional optical transmission system, optical fiber unbroken;

[0019] FIG. 4 b is a schematic block diagram of a circuit for switching to a spare fiber in a bidirectional optical transmission system, optical fiber broken;

[0020] FIG. 5 is a schematic block diagram of a circuit for switching to a spare fiber in a unidirectional optical transmission system, optical fiber broken;

[0021] FIG. 6 is a schematic block diagram of a prior art circuit for switching to a spare fiber in an optical transmission system;

[0022] FIG. 7 a is a schematic block diagram of a circuit for switching to a standby transmitter in an optical transmission system in which all transmitters are operating correctly;

[0023] FIG. 7 b is a schematic block diagram of a circuit for switching to a standby transmitter in an optical transmission system in which one transmitter is faulty;

[0024] FIG. 8 a shows an example of the use of the optical switching unit according to the invention as an optical selector switch in an expansion unit of an optical switching network; and

[0025] FIG. 8 b shows an example of the use of the optical switching unit according to the invention as an optical selector switch in a distribution unit of an optical switching network.

[0026] FIG. 1 shows an optical switching unit according to the invention. The switching unit OSE contains an optical space switch ORS which has n ports P1A . . . PNA on the left-hand side and m ports P1B . . . PMB on the right-hand side. At each port, light can enter or leave or both enter and leave the optical space switch ORS depending on whether unidirectional or bidirectional transmission is taking place. The optical space switch enables each of the left-hand ports P1A . . . PNA to be connected to every right-hand port P1B . . . PMB. In most applications, n will be chosen equal to m, i.e., the space switch will have equal numbers of ports on both sides. Applications are also conceivable, however, in which n is not equal to m. This variant is dealt with in an embodiment explained below.

[0027] In FIG. 1, the port P1A on the left-hand side of the space switch ORS is monitored by a monitoring unit M1A. On the right-hand side of the space switch ORS, a monitoring unit M1B monitors the port P1B. The monitoring units are preferably photodiodes, but it is also possible to use other components, such as photomultipliers. “To monitor the ports” as used herein means that the monitoring units M1A and M1B at least determine whether light is entering the optical space switch ORS at the respective port or not. In a broader sense, “to monitor” may also mean that the monitoring units M1A and M1B are capable of checking signals applied at the respective port for their information content. In that case, these monitoring units also include demultiplexing and/or decoding devices, for example, depending on the transmission technique used. The information thus obtained is transferred from the monitoring units M1A and M1B over links V1A and V1B to a control unit SE. Based on this information, the control unit then controls the position of the optical space switch ORS via the optical signals at the ports. The control unit preferably comprises a microprocessor and a memory unit. The relationship between the switch position and the signals applied at the ports being monitored is determined by programming or by hardwired logic and depends on the application of the optical switching unit.

[0028] FIG. 2 shows a specific embodiment of the optical switching unit according to the invention. Here the optical space switch ORS is a 2×2 space switch as is currently obtainable as an integrated optical component. The space switch ORS is connected to optical waveguides, e.g., optical fibers or integrated optical waveguides. The ports of the optical space switch ORS are designated P1A, P2A and P1B, P2B. Connected ahead of the port P1A is an optical directional coupler K1A. The directional coupler K1A, for example a fused fiber coupler, taps off a portion of the optical signal applied at the port P1A and feeds it to the photodiode PD1A. The photodiode is connected to the control unit SE. In analogous fashion, the other three ports are monitored by photodiodes PD2A, PD1B, PD2B. The control unit comprises a microprocessor and controls the position of the optical space switch.

[0029] FIG. 3 shows a particularly advantageous embodiment. Unlike the embodiment illustrated in FIG. 2, this embodiment requires only two directional couplers K1 and K2 to tap off light from all four ports P1A, P2A, P1B, P2B. As in the above embodiment, the port P1A is monitored by the photodiode PD1A. The port P1B on the other side of the optical space switch ORS, however, is monitored by the photodiode PD3 or PD4, depending on the position of the optical space switch ORS. If the optical space switch ORS is set at through-connection (continuous line in FIG. 3), the signal applied at port P1B will pass through the optical space switch and the directional coupler K1 to the photodiode PD3. If the optical space switch ORS is set at cross connection (dashed line), the signal applied at port P1B will pass through the optical space switch and the directional coupler K2 to the photodiode PD4. Since the control unit SE knows the position of the optical space switch ORS, it can unambiguously assign the information provided by the four photodiodes PD1A, PD2A, PD3, PD4 to the individual ports. Since this embodiment requires only two directional couplers, the optical switching unit can be of a very compact design.

[0030] If the optical space switch ORS is an n×m space switch with n≠m, the directional couplers must be located on that side of the optical space switch which has the greater number of ports. In a 4×8 space switch, for example, this eliminates the need for four directional couplers.

[0031] In the embodiment shown in FIGS. 4a and 4b, the optical switching unit according to the invention is used to switch from an optical fiber F to a spare fiber EF in a bidirectional optical transmission system. In FIG. 4a, the path between the communication end points PA and PB is divided into four sections by three optical switching units OSE1, OSE2, and OSE3 according to the invention. The optical switching units are set at through-connection and thus connect the communication end point PA to the communication end point PB via the optical fiber F.

[0032] If, as sketched in FIG. 4b, an interruption in transmission occurs between the two optical switching units OSE2 and OSE3, for example because the fiber has broken or has been bent at too small a radius, the optical switching units OSE2 and OSE3 adjacent to the break BS will detect the absence of input signals on the respective sides facing the break BS. They respond to this immediately by switching to the spare fiber EF. Thus, changeover to the spare fiber is effected only in the faulty section of the transmission path. If the spare fiber EF should be damaged in the section between the optical switching units OSE1 and OSE2, this division of the transmission path according to the invention still permits undisturbed data transmission.

[0033] The principal advantage of this solution over prior art solutions is that the switching actions are initiated at decentralized locations, i.e., by the switching units themselves and not by a central control station associated with the end points of the transmission path. As a result, very short dead times are possible; in addition, the transmission path can be modular in construction and be supplemented without the need to make any changes at the communication end points.

[0034] In the embodiment illustrated in FIG. 5, the principle shown in FIG. 4 for bidirectional transmission is adapted to unidirectional transmission. The unidirectional transmission takes place from the communication end point PA to the communication end point PB. If a break BS occurs in the section between the optical switching units OSE2 and OSE3, the optical switching unit OSE3 will detect no signal at the associated port and then switch to the spare fiber EF. To also enable the optical switching unit OSE2, located to the left of the break in FIG. 5, to detect the disconinuity in the optical fiber F, an optical check signal generated by a check-signal transmitter KS is coupled into the optical fiber F from the receiver end PB. The check-signal transmitter may be a semiconductor laser, for example; coupling to the optical fiber F is preferably effected via a fused fiber coupler VK.

[0035] This signal, which is injected in the direction of arrow P, passes through the optical fiber F in a direction opposite to the direction of transmission. Since after occurrence of the break the optical switching unit OSE3 has already switched (see continuous lines in OSE3), the check signal passes along the spare fiber EF and thus reaches the associated port of the optical switching unit OSE2. Without a break in the optical fiber F, the check signal would have been routed to the adjacent port, i.e., the upper port in FIG. 5. Since, in the event of a break in the optical fiber F, the check signal is automatically routed to the other port of the optical switching unit OSE2, this optical switching unit OSE2 detects the break immediately and switches to the spare fiber EF without delay, so that the signal to be transmitted now passes along the spare fiber EF.

[0036] Another embodiment of the present invention is shown in FIGS. 7a and 7b. In this embodiment, the optical switching unit according to the invention is used in a standby arrangement for optical transmitters. In the undisturbed condition, each of four transmitters S1 . . . S4 is connected to a respective one of four optical fibers F1 . . . F4 via respective optical switching units OSE1 . . . OSE4. The states of the optical switching units are indicated in FIG. 7a by dashed lines. In this normal state, a check-signal transmitter KS is connected to a standby transmitting unit RES. The standby transmitting unit comprises an optical transmitter which is preferably implemented in the same manner as the four transmitters S1. . . S4. In addition, the standby transmitting unit incorporates a photodiode which monitors the presence of a signal. When none of the four transmitters S1 . . . S4 is faulty, this photodiode detects the check signal emitted by the check-signal transmitter KS. As long as this signal is detected, the optical transmitter in the standby transmitting unit RES remains in the standby mode, i.e., it does not emit light.

[0037] If, for example, the transmitter S3 fails, as shown in FIG. 7b, the optical switching unit OSE3 will detect the absence of a signal at the associated port. The optical switching unit will then change its state. The check-signal transmitter KS is now connected to the failed transmitter S3 rather than the standby transmitting unit RES. The two optical switching units OSE2 and OSE1 then detect the absence of the signal emitted by the check-signal transmitter KS and change their states as well. Since the standby transmitting unit RES is no longer receiving a check signal, it will change from the standby mode to the active mode and emit light. Thus, the end state shown in FIG. 7b has been reached. The standby transmitting unit RES and the transmitters S1, S2, and S4 are now connected to the optical fibers F . . . F4. After the failure of the transmitter S3, the connection is thus maintained virtually unchanged.

[0038] Depending on whether all four optical fibers are fed with identical or different signals, electronics of suitable design must ensure that upon changeover to the standby transmitting unit RES, the modulation currents supplied to the optical transmitters S1. . . S4 are diverted appropriately. Furthermore, it may be advantageous to select for the signal emitted by the check-signal transmitter KS a separate wavelength which is different from the transmission wavelength. In this manner, light which does not originate from the check-signal transmitter KS is effectively prevented from being coupled into the standby transmitting unit RES by crosstalk in the optical switching units.

[0039] Here, too, the essential advantage of the use of the optical switching unit according to the invention is that switching of the space switches is effected without the need for a central control unit and, therefore, without delay.

[0040] In a further embodiment, the optical switching unit is used as an optical selector switch. Such optical selector switches are needed for all-optical switching in optical transmission systems. In an optical exchange, like in electronic exchanges, switching networks are to be used for concentrating, distributing, and expanding transmission lines. In packet-switched optical transmission systems, the packet headers include data fields for the address. This address must be recognized by the switching network so that the call can be guided to the correct destination.

[0041] FIG. 8 a shows a simple example of an expansion unit in an optical switching network. By the use of three optical switching units according to the invention, OSE0, OSE11, and OSE12, the input E can reach any of the four outputs A1 . . . A4. At the port of the optical switching unit OSE0, the data packets are monitored by a photodiode. The control unit incorporated in the optical switching unit according to the invention recognizes the address in the data packet and, depending on how the switching unit is programmed, causes the signals to be directed to one of the two outputs of the optical switching unit. The two optical switching units OSE11 and OSE12 in the second switching-network plane operate correspondingly. Thus, in this embodiment, optical space switches as are commonly used in switching networks are not controlled by a central control unit connected to all space switches, but they control themselves, so to speak. As a result, a modular construction of such switching networks becomes much simpler and less costly. It should be noted that in this embodiment, the numbers of ports on the two sides of the optical space switch are not equal (n≠m).

[0042] FIG. 8 b shows a distribution unit in a switching network. Each of the four ports A1 . . . A4 on the left-hand side of the distribution unit is connectable via the five optical switching units OSE1 . . . OSE5 to any one of the four ports B1 . . . B4 on the right-hand side. Each optical switching unit incorporates a 2×2 optical space switch whose four ports are monitored by one photodiode each. This makes it possible to process call requests arriving from both sides of the distribution unit.

[0043] While only a few simple examples of the possible applications of the optical switching unit according to the invention have been described, it is readily apparent from these examples that the optical switching unit according to the invention has many applications in optical transmission and switching systems.

Claims

1. An optical switching unit for use in optical transmission systems, comprising an n×m optical space switch (ORS) having n+m ports (P1A . . . PNA; P1B . . . PMB), with n=1, 2, 3, 4, . . . and m=2, 3, 4, . . . ,

2. An optical switching unit as claimed in claim 1 wherein the means monitor all signals entering through the ports (P1A, P2A in FIG. 3) on one side of the optical space switch (ORS) and all signals exiting through said ports.

3. An optical switching unit as claimed in either of the preceding claims wherein the means comprise photodiodes (PD1A, PD2A, PD1B, PD2B in FIG. 2), directional couplers (K1A, K1B, K2A, K2B) and a control unit (SE).

4. An optical transmission system comprising a transmission facility, at least one standby transmission facility, and at least two optical switching units as claimed in any one of the preceding claims wherein, if the transmission facility does not operate correctly, the at least two optical switching units detect this and switch autonomously to one of the standby transmission facilities.

5. A bidirectional optical transmission system comprising: a) a first transceiver unit; b) a second transceiver unit; c) a first optical waveguide (F in FIGS. 4a and 4b) connecting the first transceiver unit with the second transceiver unit; d) at least k further optical waveguides (EF) arranged parallel to the first optical waveguide (F), with k=1, 2, 3, . . . ; and e) at least two optical switching units (OSE1, OSE2, OSE3) as claimed in any one of claims 1 to 3, which are integrated into the optical waveguides (F, EF), and which, if any one of the total of k+1 optical waveguides (F, EF) is damaged in at least one of the sections located between two optical switching units, switch autonomously to another optical waveguide in said section.

6. A unidirectional optical transmission system comprising: a) a first transmitter; b) a receiver; c) a first optical waveguide (F in FIG. 5) which connects the first transmitter with the receiver and in which the signals to be transmitted propagate in a first direction of propagation; d) at least k further optical waveguides (EF) arranged parallel to the first optical waveguide (F), with k=1, 2, 3, . . . ; e) a second transmitter (KS); f) a coupler (VK) which couples the light generated by the second transmitter (KS) into the first optical waveguide such that it propagates in a direction opposite to the first direction of propagation; and g) at least two optical switching units (OSE2, OSE3 in FIG. 5) as claimed in any one of claims 1 to 3which are incorporated in the waveguides and which, if any one of the total of k=1 optical waveguides (F, EF) is damaged in at least one of the sections located between two optical switching units, switch to another optical waveguide in said section.

7. An optical transmitting unit comprising: a) a first transmitter; b) an optical switching unit having 2×2 ports as claimed in any one of claims 1 to 3 which, if the first transmitter operates properly, optically connects the latter to an optical waveguide; c) a standby transmitting unit comprising a standby transmitter and a receiver; and d) a second transmitter which is so connected to the optical switching unit that, if the first transmitter operates properly, the second transmitter is optically connected to the receiver of the standby transmitting unit, and that, if the first transmitter is faulty, and the optical switching unit detects this and operates the optical space switch, the second transmitter is not optically connected to the standby unit, but the standby transmitter of the standby transmitting unit is optically connected to the optical waveguide.

8. An optical transmitting unit comprising: a) n transmitters (S1 . . . S4), with n=2, 3, 4, . . . ; b) n optical waveguides (F1 . . . F4); c) n optical switching units (OSE1 . . . OSE4) having 2×2 ports as claimed in any one of claims 1 to 3which, if all of the n transmitters (S1 . . . S4) operate properly, each connect a respective one of the n transmitters (S1 . . . S4) optically to a respective one of the n optical waveguides (F1 . . . F4), and which are interconnected by respective optical waveguides; d) a standby transmitting unit (RES) comprising a standby transmitter and a receiver; and e) an additional transmitter (KS in FIGS. 7a/7b) which is so connected to one of the n optical switching units (OSE1 . . . OSE4) that, if all of the n transmitters (S1 . . . S4) operate properly, the additional transmitter (KS) is optically connected through all of the optical switching units (OSE1 . . . OSE4) to the receiver of the standby transmitting unit (RES), and, if one (S3) of the n transmitters (S1 . . . S4) is faulty, and the optical switching unit (OSE3) connected to said optical transmitter detects this and operates the optical space switch, the additional transmitter (KS) is not connected to the standby transmitting unit (RES), but the standby transmitter of the standby transmitting unit (RES) is optically connected to that optical waveguide which in the undisturbed condition was connected to the transmitter which is now faulty.

9. An optical switching network comprising a plurality of selector switches, characterized in that the selector switches are optical switching units as claimed in any one of claims 1 to 3.