Passive optical repeater bypass for optical fiber systems
This application is a continuation of application Ser. No. 08/236059, filed on May 2, 1994, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to optical communications systems and more specifically to repeatered remote powered systems. 
     BACKGROUND OF THE INVENTION 
     In certain optical transmission systems which require a relatively large number of serially-connected optical repeaters, each repeater constitutes a potential point of failure for the entire system. A common approach to safeguarding against system failure due to failure of a single repeater is to use ultra-high reliability components. These can be extremely costly, however, and by themselves do not provide for a failure-mode recovery. 
     Some digital optical transmission systems, such as underwater surveillance systems, have relatively close repeater spacing as compared to long-haul systems where as few repeaters as possible are used. With close repeater spacing, there often is ample optical signal strength entering each successive repeater to add an optical repeater bypass path around each repeater to provide some tolerance to repeater fault. 
     While repeater bypasses are generally known in the art, it is not always easy to implement a bypass. One problem is that the bypass signal output must not interfere with the next-in-line digital repeater&#39;s ability to accurately lock onto and amplify the bit stream. At the same time, the bypass signal output must be great enough that the next-repeater can detect the bit stream to be amplified. A further problem relates to detecting what type of malfunction has occurred, and whether it necessitates the shutting down of the local repeater. 
     SUMMARY OF THE INVENTION 
     The invention provides a bypass signal having a strength relative to the recovered and locally amplified signal which is held to within a certain range. The range at one extreme is set on the susceptibility of the next-repeater to degradation from a strong, out-of-phase bit stream replica of the system signal; and at the other extreme on the sensitivity of the next toward-shore repeater to low level bypass signals in the absence of an amplified system signal from the faulted repeater. Settings of a splitter in combination with, optionally, setting of a bypass loop attenuator, provide the requisite relative signal strength. 
     The invention also involves detecting several operating conditions in a repeater monitor unit, and selectively disabling the local repeater in response to detecting one or more of the conditions. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a diagram of a typical ocean surveillance system served by the invention; 
     FIG. 2 is a schematic block diagram of an optical repeater disposed in such a system, with bypass and control elements; 
     FIG. 3 is a state diagram portrayal of repeater operation as controlled by an internal monitor. 
    
    
     DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT 
     Referring to FIG. 1, there is shown an exemplary surveillance system 10 consisting of repeaters 11, 12, 13, 14, connected serially in a system optical fiber 15. It is understood that the system 10 may have multiple further repeaters in its outward-bound path denoted 16. Each repeater includes a sensor 17 designed to detect, for example, acoustic energy present within the beam 18 of its sensor element. The system connects to an information collection facility 19 which may be located on shore, on an undersea platform, or on-board a vessel. While the invention is illustrated through a surveillance system, it is also applicable to optical fiber telecommunications systems. 
     Repeaters 11-14 and others not shown are powered by power circuit 6 provided from facility 19 which includes power drops 7 to each repeater which supply +5 volts to operate the electronics in each repeater. 
     Referring to FIG. 2, each repeater such as repeaters 11-14 consists of an optical receiver 20, a clock recovery 21, a line interface unit 22, and a laser transmitter 23. Receiver 20 converts the digital incoming optical data stream to an electrical signal and amplifies it to a level suitable for inputting to clock recovery 21. Clock recovery 21 conventionally recovers the clock pulse from the incoming data stream and employs it to re-time the digital data which removes timing jitter. Line interface unit 22 synchronously multiplexes the sensor data acquired by local sensor 17 onto the data stream of the system fiber 15. 
     Laser transmitter 23 operates in a band of from about 1.3 μm to about 1.55 μm, the precise wavelength being unimportant to the invention. Laser transmitter 23 typically has a fixed output power level sufficient to assure that the multiplexed system signal arrives at the next repeater at a power level that enables ready detection and reamplification. The output of laser transmitter 23 is fed to coupler 26. 
     The local sensor element 17 of each repeater 11-14 is connected to the line interface unit 22 by conventional multiplexer electronics 28, which provides desired signal pre-processing to extract only selected signal content from the collected sensor input. 
     As noted earlier, in systems with close repeater spacing such as in the present example, there usually is ample power received at the input of each successive repeater to allow some of the incoming signal to be routed via a bypass link around the local repeater and carried to the next repeater. Therefore, in accordance with the invention, as seen in FIG. 2, an optical fiber bypass link consisting of a signal splitter 24, a bypass fiber 25 and a signal coupler 26 is connected in a parallel configuration completely around the local repeater. The bypass link may optionally contain an optical attenuator 27. 
     Coupler 26 combines the signal output denoted &#34;a&#34; of the bypass link, and the output of laser transmitter 23, denoted &#34;b.&#34; The output of coupler 26 is a system signal, denoted &#34;c&#34; which is transmitted to the next repeater, and also a monitor signal, denoted &#34;d&#34; which is fed to monitor 30. 
     During normal operation, both the amplified signal &#34;b&#34; and the bypass signal &#34;a&#34; enter coupler 26. In order that the bypass signal &#34;a&#34; not interfere with the operation of the next regenerator, signal &#34;a&#34; must be at a power level maintained substantially below the signal strength of signal &#34;b&#34;--on the order of 3 dB, for example. Interference between &#34;a&#34; and &#34;b&#34; can occur if &#34;a&#34; is not substantially below &#34;b&#34;, which will cause the next repeater during normal operation to be unable to recover the system signal. 
     One option to assure attenuation of signal &#34;a&#34; is to permanently set the split ratios in splitter 24 and coupler 26 to a fixed value to within a range of from 10% to 50%. A typical split setting is, for example, 50/50. This will provide a nominal attenuation of the bypass signal &#34;a&#34; of about 7 dB which will assure recovery of the system signal in the next regenerator. Other split ratios may be used (such as 60/40) depending on system requirements. 
     However, because of system variables a given set of splitter and coupler ratios may not always provide enough attenuation of the bypass signal to avoid causing malfunction of the next regenerator. Accordingly it is useful to serially include an attenuator 27 in the optical fiber bypass link, which advantageously is adjustable either in the field or at the factory. The extent of added attenuation required will depend on how much optical fiber cable is installed between given repeaters, and other factors. 
     The fiber bypass of the present invention is intended to operate basically passively. However, certain control functions associated with the bypass link are active. While these functions are active, and while the repeater monitor requires DC power to operate, in the event of a complete power loss to a local repeater, laser transmitter 23 will automatically be turned off thereby enabling bypass 25 to propagate system transmissions around the disabled repeater and to the next unit. Specifically, the bypass repeater has four operating modes, as depicted in FIG. 3. The modes are: 
     normal repeater operation; 
     transmitter disable (bypass mode); 
     system start-up; and 
     permanent transmitter DISABLE (bypass mode). 
     These modes are determined by operation of repeater monitor 30, which detects conditions signifying various types of malfunctions that can occur in the local repeater. In its normal repeater operating mode, incoming data on fiber 15 to splitter 24 is recovered, baseband data acquired from local sensor 17 as processed by multiplexer 28 is multiplexer into the recovered data stream, and the data is re-transmitted. Monitor 30 recognizes normal operating conditions, in accordance with the logic diagram of FIG. 3. Specifically, monitor 30 obtains an indication over path 31 of the fiber system optical power received by optical receiver 20 from splitter 24. The indication might be derived, for example, by monitoring the photocurrent of the photo diode in receiver 20. Further, over path 32 monitor 30 obtains an indication of the output of clock recovery 21 to calculate whether the unit 21 is recovering a valid clock signal. Next, monitor 30 receives over path 36 a signal from the interface unit 22, which is used to indicate its proper operation (i.e., proper frame synchronization). 
     Still further, monitor 30 samples over path 34 the backface PIN photocurrent of the laser transmitter 23, to determine whether the laser transmitter 23 is putting out its rated power and whether it is operating correctly under feedback control. Also, over a path 35 from the associated power drop 7 seen in FIG. 1, monitor 30 detects a condition of low power supply voltage in the repeater. 
     Finally, repeater monitor 30 monitors the optical output power of the repeater over path 33. Monitoring the optical power level of &#34;d&#34; provides a determination of whether a laser transmitter failure has occurred or whether the repeater bypass signal is present. 
     The local repeater enters the transmitter DISABLE mode if monitor 30 detects any one or more of the following: 
     optical power received at receiver 20 dropping below a set sensitivity threshold; 
     line interface unit 22 not synchronizing to the incoming frame; 
     DC power supply voltage dropping below a threshold value of about 4.5 volts; or 
     incoming data transitions not being detected by clock recovery 21. 
     Disabling occurs by monitor 30 turning laser transmitter 23 OFF by a signal in path 37. This mode allows the bypass signal &#34;a&#34; coming from the next seaward repeater to be received at the next shoreward repeater. In this mode, the optical power at output coupler 26, measured by power of signal &#34;d&#34; is tracked and therefore indicates the presence of a bypass signal &#34;a&#34;. 
     If the local repeater is in the transmitter DISABLE mode and the optical power at coupler 26 drops below a specified level, indicating loss of the bypass signal &#34;a&#34;, monitor 30 switches the repeater into system start-up mode. In this mode, the local repeater generates and transmits a system startup frame using a local clock source in the clock recovery unit 21. By providing system start-up, this mode allows the remaining shoreward repeaters to operate in the event of a serious system failure such as a cut in the system cable. 
     The fourth mode, permanent transmitter DISABLE mode, is entered either from system start-up mode or from the normal repeat operation mode, when the local transmitter output drops below a specified level as measured by the power of signal &#34;d&#34; from coupler 26 or via path 34 from laser transmitter 23. This mode operates on the assumption that the laser transmitter is degraded and that permanent transmitter DISABLE is the only viable operating mode. System signal energy will be muted around the failed repeater via the bypass link to the next shoreward repeater. To exit this mode, the repeater power is cycled off and on.