Abstract:
A fast means of detecting that a fiber has been disconnected at one end of a bidirectional optical amplifier by monitoring for several signature effects simultaneously. Upon detection of a loss of signal, the pump laser in the corresponding direction is shut off quickly to prevent oscillation from occurring should the other end of the amplifier be subsequently disconnected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is directed to methods and devices for detecting a loss of signal in an optical transmission system, and more particularly, to detection of a loss of signal and the inputs of bidirectional optical amplifiers. 
     2. Background Art 
     In an optical transmission system, a loss of input signal alarm is an important tool for determining that an optical connector has been disconnected, a cable has been broken, removed, or introduces a high loss. 
     Moreover, in optical amplified systems, the reflection of a significant portion of the light leaving via a given fiber may cause problems with detection of the loss of the input signal on that fiber. The outgoing light, that is then reflected, could be amplified signal and amplified spontaneous emission (ASE), in the case of a bidirectional system, or could be just ASE, in the case of a unidirectional system. Or, the outgoing light could be a combination of signals and ASE from both directions in the case where there are more complex optical path reflections. If the reflected outgoing light could be distinguished from the desired input signal, then appropriate alarms or control actions could be initiated. 
     In bidirectional optical amplifier applications, whenever both connectors are open with the pump laser in both directions being “on”, reflections can cause Q-switching and oscillations, despite the optical isolators that may be present. The oscillation path can involve more than one optical amplifier in the system and be quite complex. Prevention of this condition is desirable, as this phenomena can cause damage to the amplifier. 
     If a fiber is accidentally disconnected from one particular input of the amplifier, the amplifier should be shut off in that direction to minimize the risk of self-oscillation and Q-switching. In addition, shutting off the laser pump must occur very fast, before the other fiber can be disconnected. 
     For this reason, it is advantageous for a bidirectional amplifier to have a fast method of detecting a loss of signal (LOS) due to a disconnected fiber, in order that the amplifier can be shut off quickly in the corresponding direction. 
     Measurement of the strength of reflections is presently done with an optical time domain reflectometer (OTDR) that sends strong short pluses of light down a fiber and measures the signal returned to determined LOS conditions from sudden increases in reflected power levels. This is an accurate method, but the OTDR is a relatively large and expensive piece of test equipment that can not be easily used while there is traffic on the fiber. Also, as an OTDR is a separate device from the amplifier, there is no direct means of shutting off the amplifier in a particular direction when the reflection condition is detected. 
     Another prior art method for detecting reflections is to measure the amount of DC light reflected back via a four port coupler. However, this method cannot be used in bidirectional systems. 
     Techniques such as disclosed in U.S. patent application Ser. No. 08/588,776 (O&#39;Sullivan et al.) filed Jan. 18, 1996 and assigned to Northern Telecom Limited) can be used to measure reflected power on an in-service link at the amplifier site. This method however, can take in the order of a few seconds to provide accurate results following a change in conditions at an input port, leaving enough time for the second input port to be disconnected following the first, resulting in above mentioned undesired mode of operation. 
     There is a need to provide a fast means for detecting LOS in a transmission system equipped with bidirectional optical amplifiers, irrespective if a data signal is present or absent. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a fast means for detecting a LOS at the input of a bidirectional optical amplifier, which alleviates totally or in part of the drawbacks of the prior art. 
     It is another object of the invention to detect when one connector of a bidirectional optical amplifier has been disconnected and to shut-off quickly the pump laser in the corresponding direction for preventing Q-switching and self oscillation, should the other connector be disconnected. 
     Accordingly there is provided a detector fast signalling a loss of signal (LOS) condition at a forward input port of a bidirectional optical amplifier of a bidirectional optical transmission system with optical fiber amplifiers, comprising means for monitoring the forward input port, corresponding to a forward transmission channel, to obtain a first alarm signal, means for monitoring a forward output port of the amplifier, corresponding to the forward transmission channel, to obtain a second alarm signal, means for monitoring a reverse output port of the amplifier, corresponding to a reverse transmission channel, to obtain a third alarm signal, and means for processing the first, second and third alarm signals for generating the LOS signal. 
     The invention further comprises a fast loss of signal (LOS) detector for a bidirectional optical transmission system with an optical amplifier, an add/drop bidirectional optical service channel, and an optical band-pass filter connected at a reverse service output port of the amplifier, comprising means for monitoring the reverse service output port, to produce a service channel alarm whenever a forward service signal is detected at the reverse service output port, and means for processing the service channel alarm for generating a LOS signal. 
     The invention also comprises a fast loss of signal (LOS) detector for a bidirectional optical transmission system with an optical amplifier connected to a forward and a reverse transmission channels, comprising an optical tap arranged at an input port of the amplifier corresponding to the forward channel, for diverting a fraction of a forward optical signal traveling over the forward channel, an optical band-pass filter connected to the optical tap for retaining a reflection component present in the fraction based on the frequency of the reverse channel, means for converting the reflected component into a reflected voltage, and means for producing a LOS signal whenever the reflected voltage is higher than a threshold. 
     Further there is provided a method for detecting a loss of signal in an optically amplified transmission system, comprising, at an optical amplifier site, the steps of monitoring an input forward optical signal on a forward channel to produce a first alarm signal whenever the input forward optical signal increases over a first threshold, monitoring an output forward optical signal on the forward channel to produce a second alarm signal whenever the output forward optical signal drops under a second threshold, monitoring an output reverse optical signal on a reverse channel to produce a third alarm signal whenever the output reverse optical signal drops under a third threshold, and processing the first, second and third alarm signals and accordingly declaring a loss of signal (LOS) condition. 
     Advantageously, the present invention provides a method and apparatus which is an inexpensive addition to an optical amplifier module and gives a good accuracy in identifying the LOS. Being built into the equipment, it does not significantly disturb the traffic, and can be continuous or remotely monitored. 
     In addition, some of the components necessary for detecting LOS may be already present at the amplifier site, so that minimal HW may be needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments, as illustrated in the appended drawings, where: 
     FIG. 1 shows how reflections occur when one of the connectors is disconnected, while the amplifier is operational; 
     FIG. 2 shows a block diagram used for monitoring the input port of a bidirectional optical amplifier in the forward direction of transmission; 
     FIG. 3 illustrates means for monitoring the output port of a bidirectional optical amplifier in the forward direction of transmission; 
     FIG. 4 shows means for monitoring the input port of a bidirectional optical amplifier in the reverse direction of transmission; 
     FIG. 5 is a LOS generation circuit; 
     FIG. 6 illustrates means for monitoring the service channel for detecting LOS; and 
     FIG. 7 shows a further block diagram used for monitoring the input port of a bidirectional optical amplifier in the forward direction for detecting LOS. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a bidirectional amplifier  10  connected to a fiber span  12  by a first connector  16  and connected to a fiber span  14  by a second connector  18 . Connector  16  is shown in an open state, with halves  16   a  and  16   b  spaced apart and line  16   c  illustrating the reflection plane. Fibers  12  and  14  are shown intuitively with an oversized cross-section, for better illustrating the optical signals travelling over them. 
     Bidirectional amplifier  10  amplifies a forward optical signal S F  to an amplified forward signal S′ F  and a reverse optical signal S R  to an amplified reverse optical signal S′ R . In this disclosure, the term “forward” is used to indicate the direction from West to East, and the term “reverse” indicates the direction from East to West. It is to be understood that these are relative terms and they are interchangeable, without impacting on the mode of operation of the invention. 
     For the forward direction, the signal at the input port  11  of amplifier  10  is denoted with S IN1  and the amplified forward signal at the corresponding output port  15  is denoted with S OUT1 . For the reverse direction, the signal at input port  13  is denoted with S IN2  and the amplified reverse signal at the corresponding output port  17  is denoted with S OUT2 . It is to be noted that, under normal conditions of operation, S IN1  is less than S F  by the attenuation introduced by connector  16 , and S OUT1  is higher than S′ F  by the attenuation introduced by connector  18 . Similar considerations apply for the reverse direction. Port  11  is also defined as “forward input port”, port  15  is the “forward output port”, port  13  is the “reverse input port”, and port  17  is the “reverse output port”. 
     FIG. 1 also shows the reflections at plane  16   c , when connector  16  is disconnected while the amplifier  10  is operational, i.e. while the laser pumps for both directions are operational. The reflected signals are shown in fine dotted lines and the transmitted signals are shown in course dotted lines. The angle of incidence and reflection are not shown equal on FIG. 1, this drawings is intended to show the reflected and transmitted components in general. 
     When connector  16  is disconnected, forward signal S F  is reflected at  16  as signal S FRef  into reverse direction, and only a small part of S F , namely S FTrans , arrives at input port  11 . Therefore, the power level of signal S F  arriving at the input port  11  drops to essentially 0 mWatts. 
     On the other hand, in the event that the input signal power level P Trans  is small enough in comparison to the output power P OUT2  of the reverse direction, and the change in return loss P RRef  is large enough, a net increase in total input power S IN1  is observed at input port  11 . 
     This is because the amplified output signal S OUT2  is also reflected at  16   c  as S RRef , which travels in the forward direction towards input port  11 . In other words, the input power level P IN1  at input port  11  will not drop and may in fact rise due to the reflected power P RRef  from the reverse direction. The forward signal S IN1  now comprises component S RRef  is S FTrans , and amplified forward signal S′ F  is not in fact the amplified version of forward signal S F . The power level P IN1  of the input signal S IN1  in this case could be generally higher than the power level of the input signal under normal conditions of operation. We will note this power level is denoted herein with P Break1 . 
     As a result, monitoring for drops in total input power is not suitable LOS detection scheme for a bidirectional amplifier. Monitoring for reflected power increases at amplifier inputs provides more appropriate means of LOS detection. 
     The amount of the reflected power may be determined knowing the reflection coefficient of the faulty element at the reflection site. Reflection coefficient ‘r’ is defined as the ratio of reflected power over the incident power, which is for connector  16   r   Con =S RRef /S OUT2  and is known to be −14 dB or 0.04 in linear terms. 
     Knowing the reflection coefficient r con , the total input power after fiber  12  is disconnected P Break1  can be predetermined from the knowledge of the output power level in the opposite direction P OUT2 . For given operating conditions, a threshold TH 1  can be defined between the measured input power P IN1  and the total input power P Break1 , calculated for the case when connector  16  is disconnected. 
     The present invention provides means for monitoring the total power levers at the input and output of the amplifier in order to detect a LOS in bidirectional optical amplifier. For the input port in a particular direction, three of the four ports are monitored for obtaining an indication pertinent so that port and the information so gathered is processed for generating the LOS signal. 
     For example, for detecting a reflection at port  11 , the power at each of input port  11 , output port  15 , and output port  17  is monitored to generate three alarm signals, as shown in FIGS. 2-5. The LOS is then determined by adequately processing these alarm signals. 
     FIG. 2 shows how the forward input signal S IN1  of bidirectional optical amplifier  10  is monitored in the forward direction of transmission for generating a first alarm signal A 1 . 
     The forward signal S IN1  is measured using an optical tap  21  using an optical-to-electrical (O/E) converter  30  comprising PIN diode  22  and transimpedance amplifier  23 , in the known matter. A first voltage V 1  at the output of O/E converter  30  is proportional to the power level at the input port  11 . 
     First voltage V 1  is then filtered in band-pass filter  50  to remove the DC component and provide deglitching. The first band-pass filtered voltage v 1  comprising information on any input power change, is then compared in comparator  24  to a first threshold v 1  comprising by a threshold generator  40 . 
     In this measurement, signal v 1  is applied to the non-inverting input of comparator  24 , while TH 1  is applied to the inverting input. Comparator  24  issues the first alarm signal A 1  whenever v 1  is greater than TH 1 , i.e. v 1  crosses the threshold. The first alarm signal A 1  is then used in the LOS generation circuit of FIG.  5 . 
     The threshold TH 1  is determined by a microprocessor  26  to be approximately half the voltage peak that will be at the non inverting input of the comparator, with connector  16  disconnected, namely ½v 1Break . The output of microprocessor  26  is converted to TH 1  using a digital-to-analog converter (DAC)  25 . First threshold TH 1  can be determined experimentally, or from a knowledge of the output power level in the reverse direction, the responsivity of PIN  22 , and the gain of transimpedance amplifier  23 . This information can be pre-stored in a memory  27 . Alternatively, the value of this threshold can be adjusted closer to the voltage break v 1Break  for better false detection prevention, or closer to zero, for improved detection probability. Selection of the threshold is a compromise between these two requirements. 
     The high pass cutoff frequency of filter  50  is chosen such that if the threshold TH 1  is crossed, v 1  remains above threshold long enough to allow the LOS detection logic (to be described shortly in connection with FIG. 5) to capture the event. It should also be low enough such that the finite rise time of the total input power level can be detected accurately. The low pass cutoff frequency is chosen as a trade-off between activation speed and noise reduction. A nominal value would be of the order of 160 kHz. 
     FIG. 3 illustrates how the forward output signal S OUT1  of bidirectional optical amplifier  10  is measured at output port  15 . When fiber  12  is suddenly disconnected at connector  16 , a temporary drop in output power P OUT1  is observed. This is due to the fact that the amplifier gain cannot change instantaneously. The drop in output power P OUT1  is captured by the circuit shown in FIG.  3 . 
     An output tap  31  is connected at output port  15  for diverting a fraction of S OUT1  to a second O/E converter  30 ′. O/E converter provides a second voltage V 2 , corresponding to the output power P OUT2  using a PIN diode  22 ′, and a transimpedance amplifier  23 ′. While components  22 ′ and  23 ′ are nor shown for simplifying the drawing, they are connected as in the case of O/E converter  30  shown in FIG.  2 . 
     A band-pass filter  52  filters V 2  to remove the DC component and for deglitching and provides a second band-pass filtered voltage v 2 , comprising information on any change in output power P OUT2  This filter preferably has a low pass cutoff frequency of 300 kHz and a high pass cutoff frequency of 16 Hz. 
     Threshold generator  40  is used to generate a second threshold TH 2 . This generator is similar in structure to the generator  40 , in fact it may use the same components. As the event to be detected represents a drop in power, voltage v 2  will have a negative peak value when fiber  12  is disconnected. Therefore, threshold TH 2  has a negative value and is applied to the non inverting input of a comparator  24 , while the second band-pass filtered voltage v 2  is applied on the inverting input. The resulting alarm is denoted with A 2 . 
     As the temporary drop in output power varies with amplifier output power, the threshold value TH 2  should ideally be set from a look up table in memory  27 , with a knowledge of the output power, PIN  22 ′ responsivity, and the gain of transimpedance amplifier  23 ′. This threshold should be set at approximately half the voltage of the peak that would be observed at the bandpass filter output when the fiber is disconnected, which we note with ½v 2Break . Alternatively, TH 2  could again be biased for better false detection prevention or improved detection probability. 
     FIG. 4 shows how the reverse output signal S OUT2  is measured, the circuit being similar to that of FIG. 3, with the difference that a tap  41  is connected at output port  17 . When connector  16  is disconnected, a drop in power of signal S OUT2  is observed at the output  17 . This is because the signal reflected in the reverse direction (S RRef  in FIG. 1) takes away some of the amplifier gain from signal S OUT2 . 
     This effect can be captured by the same type of circuit used to detect the power drop at the other output port. The alarm A 3  at the output of comparator  24 ″ is generated whenever the negative peak of the third band-pass filtered voltage v 3  goes under a third threshold TH 3 . 
     In this case, the drop in output power level is in general dependent on the change in reflection coefficient that is observed when the fiber is disconnected and the threshold TH 3  should be set by the microprocessor from a look up table indexed by output power level P OUT2 , PIN responsivity and gain of the transimpedance amplifier. This threshold should be set at approximately half the peak that would be observed at the bandpass filter output when the fiber is disconnected, namely ½v 3Break . Alternatively, it cold be biased for better false detection prevention or improved detection probability. 
     The initial reflection coefficient can be determined by any known methods (e.g. that disclosed in the above identified U.S. patent application Ser. No. 08/588,776 (O&#39;Sullivan et al.). 
     When each characteristic power change alarm A 1  to A 3  is observed on the three ports simultaneously, the LOS condition is asserted as illustrated in FIG. 5, showing a LOS generating circuit. The three alarms A 1 , A 3  and A 3  are added in AND circuit  54 , and the resulting alarm signal A is used to clock a logic 1 to the output of a pre-cleared D flip-flop  55 . LOS is then used to shut-down the laser pump of amplifier  10  for the forward direction. 
     Another method of fast LOS detecting is to use the optical service channel OSC or the bidirectional OSC (Bi-OSC) that is provided in many SONET transmission systems. 
     Bi-OSC is a service channel that is transmitted and terminated at each optical amplifier site and is provided with a transmitter/receiver pair, to give the user access to the service information. The average optical power of the Bi-OSC is accounted for in the link budget, in order to make the average output control in the forward direction of transmission and in the reverse direction of transmission more accurate by subtracting the power contribution from the respective OSC channels. As Bi-OSC is transmitted on the same fiber with the information channels, a break in the transmission link can be determined on this channel. 
     For an amplified link provided with a Bi-OSC channel, LOS detection according to this invention provides for monitoring the reflected power level at the optical service channel wavelength, and AND-ing the event of a reflected power increase with the loss of frame (LOF) condition for the add/drop channel. 
     A circuit to implement this method is shown in FIG.  6 . An optical band-pass filter  32  is provided between connector  16  and Rx/Tx terminal  35  to prevent any other signal from being present at the input of receiver Rx. 
     If connector  16  is disconnected, the optical output S OUTS  of the respective optical transmitter  35  for the service channel is reflected at  16   c  and the reflected optical signal S RefS  travels on the fiber in the forward direction towards the input of receiver  35 . 
     Between Rx/Tx  35  and filter  32 , the optical reflected signal S RefS  is tapped at  34 , in the same manner as indicated above, and converted to a service voltage V S  in O/E converter  30 . Voltage V S , corresponding to the reflected power at the transmitted wavelength, is then low-pass filtered in filter  56  to provide deglitching. 
     The low-pass filtered voltage v s  proportional to the reflected power change is then compared in comparator  24  to a threshold TH S . produced by threshold generator  40 , v S  being applied on the non-inverting input and TH s , on the inverting input. The service channel alarm signal output by comparator  24  is noted herein with A S . 
     Threshold TH s  is set as in the previous cases using the microprocessor  26 , and is chosen at approximately half the voltage that will be at the non inverting input of the comparator  24 , should the fiber be disconnected, namely ½v SBreak . This voltage can be determined from a knowledge of the add/drop optical service channel transmitted power P OUTS , the PIN responsivity, and the transimpedance gain. Alternatively, threshold TH S . can be biased closer to the voltage after the fiber is disconnected for better false detection prevention, or closer to zero for improved detection probability. 
     The low-pass cutoff frequency is chosen as a trade-off between activation speed and noise reduction. For example, this frequency could be 160 kHz for Nortel&#39;s Bi-OSC. 
     When the fiber is disconnected, the rise in reflected power creates a positive edge A s  to clock input of the previously cleared D flip-flop  55 , which subsequently clocks a “logic 1” to the output. This thereby latches in the event at the output “Q” of latch  55 . 
     Output “Q” is then AND-ed with the LOF condition detected over the optical service channel in the corresponding direction, to provide the LOS at the output of AND gate  57 . 
     Another technique that may be used for fast detection of LOS is illustrated in FIG.  6  and described next. This technique may be used for both systems with and without an optical service channel or other additional wavelength. 
     At the input port to the amplifier  10 , a power monitor tap  34  is typically used for input power monitoring purposes. If connector  16  is open or the fiber is disconnected in any way at input port  11 , the light which has been tapped off of the main path consists of the incoming signal S IN1  and reflected output light S RRef . A WDM filter  32  is used to separate the reflected light from the other input light. The WDM filter acts as a band pass filter in the output wavelength band, hence allowing only reflected light to pass. The WDM filter  32  also directs the light which is not in the output band to another port which can be used to measuring the input power. 
     The reflected light is directed to a O/E converter  30  including a PIN detector  22  and transimpedance amplifier  23 , (shown on FIG. 2) which, as in the previous examples, converts the reflected light to a proportional voltage V R . The voltage corresponding to the reflected power at the transmitted wavelength is then filtered in a low-pass filter  56  to remove glitches and high frequency noise. The low pass cutoff frequency is chosen according to the desired response time. The greater the cutoff frequency, the faster the response time. Preferably, this value is 160 kHz. 
     The filtered voltage v R  corresponding to the reflected power change is then compared to a threshold TH R  set by a threshold generator  40  in a comparator  24 . The filtered voltage v R  is applied on the non-inverting input of the comparator  24 , while the threshold TH R  is applied on the inverting input. 
     The threshold TH R  is chosen at approximately half the voltage level that will be at the non inverting input of the comparator should the filter be disconnected. This voltage can be determined from a knowledge of the output power, the PIN responsivity, and the transimpedance gain. Alternatively, this threshold can be biased closer to the voltage peak for better false detection prevention, or closer to zero for improved detection probability. 
     When the fiber is disconnected, the rise in reflected power creates a positive edge to the clock input of a previously cleared D flip-flop  55 , which subsequently clocks a logic “1” to the output. This thereby latches in the event as the LOS signal at the output Q of the flip-flop. 
     To clear the LOS condition in each of the fast LOS detection methods described above, the CLEAR line on the D flip-flop  55  is asserted by the microprocessor  26 . The condition to clear the LOS can be determined on the microprocessor by detecting the amount of reflection present at the input port where the LOS was declared. The amount of reflection can be determined by any prior art method, such as that of above U.S. patent application Ser. No. 08/588,776 (O&#39;Sullivan et al.) as clearing the LOS condition does not require the same fast response speed as asserting it does.