TRANSMITTING SUPERVISORY MESSAGES FROM A FIBER AMPLIFIER

Abstract: The present invention relates to data transmission between such amplifiers that use optical fiber as their data transmission channel. Because a fiber amplifier normally only amplifies a signal with a certain factor, without adding information to the signal, signalling associated with the supervision and control of the amplifier generally has to be carried out with separate systems. In accordance with the invention, an electrically controllable optical band-pass filter (22) is employed in addition to its normal function for modulating a signal passing through the fiber amplifier on such a low frequency and amplitude that this modulation does not disturb the actual data transmission. A controller (26) is controlled by a processor (30) to which a read/write memory (34) and a program memory (36) are connected via a system bus (32). A block (40) comprises all the information the amplifier is to forward as supervisory messages.

Description: Transmitting supervisory messages from fiber amplifier The invention relates to data transmission between such amplifiers that use optical fiber as their data transmission channel . A fiber amplifier is a device with which an optical signal can be amplified without converting the signal into an electrical signal in between. These amplifiers are used with very high transmission rates. A typical value of transmission rate may be 1 GHz, for example The operation of a prior art fiber amplifier is illustrated m Figure 1. In the figure the conductors transmitting optical signals are indicated with αouble lines and the conductors transmitting electrical signals are indicated with single lines. An input signal 1 of the amplifier is conducted via an optical isolator 12 to the second input of a wavelength combiner 14. The function of the optical isolator 12 is to attenuate reflections, for exarple An input signal detector 16 after the optical -so-a !- - v 1 -1 αetects the level of tne input signa-. and generates ar. electrical signal corresponding to tnis level. A pump -.aser unit 18 supplies the second input of tne wavelength combiner 14. The amplification of a signal operates m the following way. The pump laser 18 produces light whose wavelength is e.g. 980 or 1480 nm, wnereas the wavelength of the signal light is 1,550 nm, for example. The photons of the pump laser 18 are conduced to an erbium-doped fiber 20 where they excite erbium atoms of the fiber 20. Some of the erbium atoms return to the ground state via spontaneous emission. When the photons of the signal light is directed to the erbium atom tuned by the photons of the pump laser, the erbium atom emits a photon corresponding to the photon of the signal light. The erbium-doped fiber 20 is followed by a band-pass filter 22 passing through the photons displaced by the photons of the signal light, but prevents the travel of the photons which are generated mostly by their spontaneous emission and whose wavelength is not similar to the wavelength of the signal light . An output signal 2 of the band-pass filter is an amplified optical signal transmitted to a (not shown) receiving station. Fiber amplifiers are discussed e.g. m Opti cal Ampl ifiers and their Appl i ca ti ons, edited by S. Shimada and H. Ishido, ^John Wiley S Sons, 1992. Signal wavelength and characteristics of the band-pass filter 22 vary e.g. according to temperature or change considerably if some component of the system has to be replaced. In that case, the amplifier should be able to be adjust the pass wavelength of the band¬ pass filter by external, e.g. electrical control. Prior art arrangements comprise a controller 26 for this purpose. The controller 26 monitors the detector 16 of the input signal and a detector 24 of the output signal and amplifies tne control signal of the oanα-Dass filter 22 for maximizing the amplification of the amplifier or the correlation between the output and input signals. As a fiber amplifier normally only amplifies a signal with a certain factor, without adding information to the signal, signalling associated with supervision and control of the amplifier generally has to be carried out with separate systems. This is a particular problem when using a so-called intermediate amplifier which is far (e.g. over 100 km) from both the transmitter and the receiver. European Patent Application 415 438 discloses an art where supervisory messages are transmitted by modulating the control signal of a pump laser unit in such a manner that the excess amount of pumping light from the amplifier contains a supervisory signal to be transmitted. This prior art solution has, however, several limitations. First, the prior art solution is only applicable on a wavelength of 1,480 nm as the excess pumping light on a wavelength of 980 nm will be absorbed on a long data link by an information transmitting fiber. At the moment, it seems that the use of the wavelength of 1,480 nm is being abandoned as the wavelength of 980 nm produces better noise characteristics to the fiber amplifier and the total efficiency ratio of the amplifier will oe better. Secondly, the prior art solution requires two separate optical systems, one for receiving a payload signal and one for receiving a supervisory signal The reception of supervisory signals according to the prior art requires a separate WDM component (Wavelength Division Multiplexer) . Thirdly, the prior art solution is only useable on sucn frequencies whose period is snorter than the lifetime of the fluorescence state. The object of the invention s thus to accomplish signalling related to the super v ision of a fiber amplifier in such a manner that the problems associated with prior art can be solved. The object of the invention is achieved with the methods and systems which are characterized by what is stated m the independent claims . The invention is based on the idea that the optical band-pass filter 22 is employed in addition to its normal function for modulating a signal passing through the fiber amplifier on such a low frequency and amplitude that this modulation does not disturb the actual data transmission. This is realized m such a manner that the control signal of the band-pass filter 22, which is used in prior art fiber amplifiers only for optimizing the signal-noise ratio, for example, is used in the amplifier of the invention for modulating the output signal of the fiber amplifier in the rate of the supervisory message. A primary advantage of tne method and system of the invention is that no separate signalling has to be arranged for supervising the amplifier. Another advantage is that changes required for the amplifier for realizing the invention are small For example, the same optical components used for receiving a payload signal can be employeα for receiving supervisory messages . The invention will now be explained m more detail by means of preferreα embodiments with reference to the accompanying drawings, where Figure 1 shows a block diagram of a fiber amplifier of prior art, Figure 2 shows a fiber amplifier completed m accordance with a basic embodiment cf the -rvert on, Figure 3 shows an ampl-.f-.tr station comprising two fiber amplifiers completed -- accordance .ιt^ the invention, Figure 4 shows ampl-.tuαe modulation of an output signal of the fiber amplifier, and Figure 5 shows one method for detecting supervisory messages Figure 2 shows a fiber amplifier completed in accordance with a basic embodiment of the invention All parts of Figure 1 are also included m the embodiment shown in Figure 2 although Figure 2 only shows of them the controller 26 and the band-pass filter 22. In accordance with the invention, the controller 26 is controlled by a processor 30 whose structure can be a general-purpose computer, or a controller or the like integrated into one printed board or even into one chip. Figure 2 is assumed to comprise all the accessories required by the processor, such as a system clock, bus buffers, etc. Of these accessories Figure 2 shows only a read/write memory 34 and a program memory 36. The program memory 36 can be any memory m which the program is maintained during power failures, e.g. read only memory (ROM) , flash memory or a hard disk. Figure 2 shows a solution where the processor is connected to the controller 26 and the memories 34 to 36 via a system bus 32. A block 40 indicated in Figure 2 comprises all the information the amplifier is to forward as supervisory messages. The two-way arrows connected to the system bus 32 describe an arrangement where the processor 30 has m use all the information of the controller 26 and the processor 30 may have an effect on the operation of the controller 26 and through it, on the operation of the band-pass filter 22. Similarly, information on the characteristics of the band-pass filter can be stored in the read/write memory 34 wnicr. can be accessed both from the processor 30 and tne controller 26. The D ocK 40 comprises all the information that the amplifier is to forward as supervisory messages. This information may contain information on the opera-tion of the fiber amplifier, on conditions, the video signal obtained from a surveillance camera, etc. It is not essential for the invention what the supervisory messages contain but how the messages are transmitted. In accordance with the invention, the processor 30 controls the controller 26 via the system bus 32 in such a manner that the controller 26 adjusts the control signal of the band-pass filter 22 for modulating the optical signal passing through the fiber amplifier on a low frequency and amplitude. A low amplitude here denotes that the modulation level is typically a couple of percentages and a low frequency denotes a frequency that is considerably lower than the frequency used by the fiber amplifier for the actual traffic . When using a band-pass filter 22 for modulating an optical signal, particular attention should be paid to the accuracy of a control signal 22s. There are at least two reasons for this. First, the way the band-pass filter 22 allows an information transmitting optical signal to pass through is not a monotonous function for the control signal 22s, but this function has its peak at a specific value of the control signal. Secondly, the spontaneous emission explained m Figure 1, which is noise for the receiver, is withm the same band as the wavelength of the signal light . The following shows one arrangement for calibrating amplitude modulation in a finer amplifier of Figure 2. It s assumed that tne fioer amplifier transmits and amplifies t e optical signal continuously, (i The control signa- 22s cf tne oand- pass filter is set to its minimum value. m 1 The control signal 22s of the band-pass filter is magnified in small steps until the ratio of the output signal of the fiber amplifier to the input signal 16s reaches its maximum point and the value of this control signal 22s is stored in the read/write memory (RAM) 34. This value of the control signal 22s is indicated by S . (in) The value of the control signal 22s is altered until the ratio of the output signal of the fiber amplifier to the input signal 16s is reduced from the peak value measured in step di) to a desired modulation level, for example 2 to 3%, and the obtained value of the control signal 22s is stored in the RAM 34. This value of the control signal 22s is indicated by S . When the value of the control signal 22s is altered in step (in) , the one that best attenuates the power of spontaneous emission is to be selected as the direction (up/down) of the alteration. It was assumed above that the fiber amplifier transmits an optical signal continuously. If the optical conductor passing through the fiber amplifier does not transmit a payload signal, it can transmit null characters. If these null characters are not used, the calioration program of the processor is to be contain a step wnere it is detected that the optical signal is not present at all and then calibration is not attempted. When the equipment of the invention is formed in the fiber amplifier for transmitting supervisory messages, this equipment can be used for adjusting the band-pass filter 22 essential even m the normal operation of tne fiber amplifier Tne calibration program stored m the program memory 36 can ice mace responsive to condition information, especially to temperature information, obtainable from a data acquisition blocκ 40 It depends on the characteristics and the expected installation site of the band-pass filter if condition information is to be supplemented by air pressure and/or relative numidity Also, the calibration program stored m the program memory 36 can be activated by an external command transmitted via the optical fiber. In that case, the fiber amplifier is to be connected to a detector to be described below m Figure 5 and also, the signals have to be assembled into frames and in this case, the fiber amplifier whose band-pass filter is to be calibrated is to be indicated as the receiver. Calibration can alternatively be carried out continuously in such a manner that calibration is performed for example once every minute or whenever the fiber amplifier is not transmitting supervisory messages. Calibration does not prevent or disturb the normal operation of the fiber amplifier. Then null characters, such as frames whose number of data bytes is zero, have to be transmitted for calibration. The other parts of the fiber amplifier of the invention are considered to be known to persons skilled m the art. The critical component is the electrically controllable band-pass filter 22 whose essential parameters for the invention are stability and settling time. Suitable band-pass filters are manufactured e.g. by Queensgate Instruments Ltd in Berkshire, England. When the band-pass filter 22 is calibrated as described above, it can be used for modulating an optical signal m the following way, wherein reference is made to Figure . For the sake of simplicity, it is assumed that supervisory messages are transmitted in binary 0 and 1 states, m which case the presence of amplitude modulation corresponds to state 1 and its absence to state 0. Time T corresponding tc one bit of a supervisory message is divided into four periods Tl to T4. A binary 1 state pulse s transmitted m the following way: period Tl : the value of the control signal 22s is SO, period T2 : the value of the control signal 22s is Sl, period T3 : the value of the control signal 22s is SO, period T4 : the value of the control signal 22s is Sl . Binary 0 state pulse is transmitted otherwise in the same way as the 1 state pulse but in period T2 the value of the control signal 22s is also SO, that is, the amplitude is the same as in periods Tl and T3. It should be noted that the frequency of the optical signal m Figure 4 is drawn considerably smaller than in reality in relation to the frequency of amplitude modulation. In reality thousands cr millions of optical signal 2 periods fit into one amplitude modulation sequence. Furthermore, the modulation level is drawn considerably larger than the optimal value for the sake of illustration. The above modulation method explained m connection with Figure 4 is known per se and it is called Frequency Shift Keying (FSK In short, in the FSK clock pulses are continuously sent at specific intervals (m periods T4) . If there is a data pulse between two clock pulses (m period T2) , this is interpreted as binary 1, otherwise as binary 0 An advantage of the FSK is that it is simple to implement, but a disadvantage is its slow trar.sr ss on rate as eacn 1 state data requires fci c.anαes to tne amplitude modulation state It s ..so evident that on the basis cf the above exp anat-t" tne _nventιon ear- also be used with more advanced modulation methods Figure 5 shows as a DlocK diagram how supervisory messages can be indicated in a receiving station, of which other parts are not shown. The output signal 2 of the fiber amplifier according to Figure 2 is received at a receiving station The optical signal is processed after an optical isolator located there for normal telecommunication in an ordinary way. For detecting supervisory messages of the invention, the optical signal is converted into an electrical signal by a detector 104. The signal of the detector 104 is conducted to a transimpedance amplifier 106. This utilizes the fact that the frequency used by the fiber amplifier for the actual traffic is approximately 100,000 times greater than the frequency of supervisory messages. The detector 104 and/or transimpedance amplifier 106 realized with ordinary components allow only the frequency band of supervisory message (typically some tens of kilohertz) to pass through. A DC component is removed from the input signal of the transimpedance amplifier 106 by a capacitor 108 after which the signal is coupled to an automatically adjustable amplifier 110 and therefrom to a threshold value detector 112 The output signal of the threshold value detector 112 is "1" when a pulse is present m the signal transmitting the supervisory message and otherwise "0" The output signal of the detector 112 can be converted m parallel either by such serieε-to- parallel converters that are used m modems and disk drives or, alternatively, by a computer program. The circuit of Figure 2 can transmit supervisory messages to the receiver of an optical signal but not tc its sender. Figure 3 shows how the arrangement of the invention is applied to tne two-way transmission of supervisory messages In view of Figire 2, the circuit of Figure 3 is supplemented by anotner fiber amplifier of Figure 1 for operation m the opposite direction. Operation in one way is called outgoing direction and m the other way return direction. The components m the outgoing direction in Figure 3 are similar to those in Figure 2. The components in the return direction are indicated with apostrophes. The supervisory messages to be returned are in this case obtained from the detector 16 ' of the input signal m the return direction, which is not shown separately. In the transmission of supervisory messages of fiber amplifiers of the invention, special steps should be taken to ensure that the information to be transmitted is error-free. There are several reasons for this; for example, the fact that especially fiber amplifiers used as intermediate amplifiers can be a long distance from the preceding or next stations, whereby an unnecessary maintenance call caused by an erroneous message can be very costly. Another reason is that a compromise has to be made between noise-free messages and the actual operation of the amplifier. The error- ree transmission of supervisory messages is improved by magnifying tne modulation level On the other hand, a high modulation level causes problems to the actual operation of the fiber amplifier An advantageous compromise is brought about m such a manner that the modulation level used m transmitting supervisory messages is relatively low, less than 10 % and an effective error-correcting code is used for transmitting supervisory messages. The excess information required by error correction is not generally a problem as the -.nrcr ation content of supervisory messages is general-v sma-.l An exception to this is a situation wnere the transmission ot supervisory messages of the invention is used e.g. for transmitting a video signal of a surveillance camera. It is generally possible to transmit signals of this kind without error correction. When there is only one intermediate amplifier between a transmitter and a receiver, it is clear that supervisory messages are sent from this intermediate amplifier. If there are several successive intermediate amplifiers, the low modulation level of supervisory messages can disappear under the great amplification of the next intermediate amplifier. In that case, the safest way to operate is to assemble the supervisory messages into frames containing the identifiers of the sender and the receiver of the message. These frames are detected in the next intermediate amplifier and they are forwarded, as well as the specified signals of the next intermediate amplifier, whereby the original sender of the message is naturally set as the sender of the frame. In this case, the intermediate amplifier operates m the case of supervisory messages m the way that is largely analogous to bridges used m local area networks. The frames of supervisory messages can also be simplified m such a manner that the receiver is not specifically indicated but the receiver s always the last station in the chain. An embodiment of the invention was explained m connection with Figures 2 and 3 where a program for transmitting supervisory messages is stored m a program memory 36. It is also possible, of course, to store m the program memory 36 a certain basic software that can be converted and complemented by means cf data transmitted as supervisory messages Ir. that case, a detector explained in connection with Figure Ξ snc lα be added to tne fiber amplifier fer tne ιece-.vec super¬ visory messages. The program memory Ξ- sno ^ -α c~ com¬ plemented so that the fiber amplifier detects e.g. from the header fields of the frame that the receiver is the fiber amplifier m question and its program memory. The altered parameters or the parts of the program are stored m the read/write memory 34 and they replace the corresponding parts in the program memory 36. It is evident for those skilled m the art that as the art develops, the basic idea of the invention can be realized in many ways. The invention and its embodiments are thus not restricted to the examples described above, but they can vary within the scope of the claims.

Claims: Claims 1. A method for transmitting supervisory messages from a fiber amplifier transmitting an optical signal, the fiber amplifier comprising a band-pass filter (22) responsive to an external control signal (22s) , c h a r a c t e r i z e d in that the method comprises the steps of : forming a modulation signal corresponding to the supervisory message; combining the modulation signal to a control signal of the band-pass filter m such a manner that the optical signal of the fiber amplifier will be modulated m the rate of the modulation signal 2. A metnod according to claim 1, c h a r a c¬ t e r i z e d in that it further comprises the step Of: defining calibration information which comprises the effect of the control signal (22s) of the band-pass filter on how great a portion of the optical signal will pass through tne band-pass filter ,22) 3 A method according to claim 2, c h a r a c¬ t e r i z e d m that tne calibration information _s defined at predetermined time intervals. 4. A method according to claim 1, c h a r a c¬ t e r i z e d m that amplitude modulation is used m modulating step di) . 5. A method according to claim 4, c h a r a c¬ t e r i z e d m that the modulation level is in the range of 2 to 10 %. 6. A method according to claim 1, c h a r a c¬ t e r i z e d m that the supervisory messages are encoded into a digital form. 7. A method according to claim 6, c h a r a c- t e r i z e d m that error detection coding is used in the digital coding of a supervisory message. 8. A method according to claim 6, c h a r a c¬ t e r i z e d in that error correcting coding is used in the digital coding of a supervisory message. 9. A method according to claim 6, c h a r a c¬ t e r i z e d in that m the coding stage of a supervisory message the supervisory messages are assembled into frames comprising the identifier of the sender. 10. A method according to claim 9, c h a r¬ a c t e r i z e d in that the transmission is a forwarding of the received, codec supervisory message and the method comprises tne steps of : receiving the supervisory message and defining the identifier of the original sender contained therein; and transmitting the supervisory message further, whereby the identifier of the sender is the identifier of the original sender of the supervisory message 11. A method according tc claim Q or 10, c h a r a c t e r i z e d ->n tnat the frames also contain the identifier of the receiver 12 A method according tc any one of claims 1 to 10 m an amplifier station comprising at least two fiber amplifiers, one for the outgoing direction and the other for the return direction, c h a r a c t e r¬ i z e d m that the control signal of the band-pass filter (22 ') in the return direction is controlled m the modulating step. 13. A system for transmitting supervisory messages from a fiber amplifier transmitting an optical signal, the fiber amplifier comprising a band-pass filter (22) responsive to an external control signal (22s) , c h a r a c t e r i z e d m that the method comprises: means (40) for defining the supervisory message; memory means (34, 36) for storing program and supervisory messages; a processor means (30) for processing information; and a first controller (26) responsive to the processor means (30) for controlling the band-pass filter (22) . 14. A system according to claim 13, c h a r¬ a c t e r i z e d in that the system further comprises a second controller (26' ) responsive tc the processor means (30) for controlling the band-pass filter (22' ) in the return direction. 15. A system according to claim 13 , c h a r¬ a c t e r i z e d in that the memory means (34, 36) comprise a read memory (36) for storing the program and a read/write memory (34) for storing the information to be converted. 16. A system according to claim 13, c h a r- a c t e r i z e d in that the system further comprises means (104 to 112) for receiving a supervisory message. 17. A system for receiving supervisory messages superimposed on an optical telecommunication signal of a fiber amplifier, c h a r a c t e r i z e d in that the system comprises : first means (104) for converting an optical signal of the fiber amplifier into an electrical signal ; second means (106) operationally connected to the first means for separating a supervisory message from a telecommunication signal; and third means (108 to 112) operationally connected to the second means for converting the supervisory message into a digital form; whereby the supervisory messages and the telecommunication signal can be converted from an optical form into an electrical form by the same means (104) .

Inventors: TAMMELA SIMO, TAMMELA, SIMO
Applicants: NOKIA TELECOMMUNICATIONS OY, TAMMELA SIMO, TAMMELA, SIMO
Application year: 1996
Main classification: H04B10
IPC codes: H04B10