WO 2006/067788 PCT/IL2005/001364 1 Method of controlling optical amplifier located along an optical link Field of the invention The invention relates to a technology of shutting down and restarting power optical amplifiers (more particularly, Raman amplifiers) in optical links. Background of the invention Various methods for safe connection and disconnection of high power optic amplifiers in optical links have been discussed in the prior art. , It is understood that the basic reason for shutting down lasers of the optical amplifiers is a fiber cable cut (break) or disconnection in the line; the fiber cut leads to emission of high energy light at the break point, which may cause hazardous consequences to human eyes. The most natural and widely used classic criterion indicating a fiber break is a Loss Of Signal (LOS) in the wavelengths band intended for transmitting the informational optical channels (i.e., in the C-band), being detected at the receiving point of the optical line. In optical lines deploying power Raman amplifiers, a number of non- linear effects usually develop; therefore, the above-mentioned criterion of LOS cannot be used as a universal one for detecting fiber faults of various natures. US patent 6,373,621 to Nortel Networks corporation describes a method of operating Raman amplification pump lasers, especially in telecommunication systems, in which the Raman pump laser output powers are modulated in characteristic fashions. Parameters of these characteristic modulations may be detected at remote locations (i.e. locations along the communications fibres) even in the presence of large amounts of noise. Thus, by detecting losses of signals indicative of the characteristic modulations, breaks in the communication fibres can be detected more reliably. The disappearance of these signals can then be used to shut down the typically high WO 2006/067788 PCT/IL2005/001364 2 power Raman pump lasers, thereby reducing the possibility of high laser powers escaping from the broken fibres. The above solution also describes an improved method of detecting the disappearance of a data signal at an amplifier unit, using a periodic filter to split the incoming signal into two streams, the first stream comprising data and noise, and the second comprising noise only. These two streams are then monitored to provide an indication of the presence or absence of the data signal alone. This form of signal detection can be combined with the technique of modulating the Raman pump laser output power to provide even further improved break detection and subsequent safe shut down of pump lasers. An ITU-T Standard Recommendation G.664 (dated March 2003) named "Optical safety procedures and requirements for optical transport systems" proposes some examples of Automatic Power Reduction (APR) and Automatic Laser Shutdown (ALS) procedures for systems including those based upon Raman amplifiers. Appendixes 1 and 2 of the G.664 proposes using an Optical Supervisory Channel (OSC), when present in the system, to verify link connectivity for performing a restart. Since an OSC is usually operating at a safe optical power level, it can be kept "alive" on the fibre after the power has been reduced to a safe level. Restoration of OSC communication indicates full restoration of the link connectivity, after which the system can be brought back to its full operational power. In this way, it is ensured that the full operational power is only present in a fully enclosed configuration guaranteeing optical safety. The ALS procedure concerns restart of communication in an optical link, and is based on the use of repetitive pulses to restart the system. The Standard Recommendation specifies the minimum delay between the restart pulses for various conditions (for example, between 100s - 300 s), wherein their duration (i.e., the time period when the transmitter Tx is on) can be of about (2± 0.25)s. US patent No. 6,626,587 B1 to ECI Telecom Ltd. describes different modes of ALS procedure in optical telecommunication systems: a manual WO 2006/067788 PCT/IL2005/001364 3 restart in a test mode, a manual restart and an automatic restart. The description is hereby incorporated by reference. There is presently no conclusion in the prior art, which combination of criteria for shutting down and/or restarting the Raman amplifiers in the optical link could be used as the only efficient and reliable combination. Summary of the invention Table of key terms to be used in the following description Auxiliary optical channel - an optical channel not belonging to informational optical channels; for example, can be an OSC - Optical Supervisory Channel, an SVC - Supervisory Channel, or another additional channel. Transmitting side (power) amplifier - an optical amplifier (usually a power Raman amplifier) situated at a transmitting side of an optical span or link. Receiving side (power) amplifier - an optical amplifier (usually a power Raman amplifier) situated at a receiving side of an optical span or link. Optical Span - usually, a set of two amplifiers separated by an optical fiber normally with length of a few tens of km. Optical link - concatenation of a number (1 to n) of spans. FWD (forward direction) Raman amplifier - Raman amplifier that pumps its high output power in the direction of the information carrying signals BWD (backward direction) Raman amplifier - Raman amplifier characterized in that it pumps high output power in the opposite direction to the information carrying signals. LOS-Loss Of Signal. Main band (e.g., C-band, 1530-1565nm) - spectrum of wavelengths utilized for informational optical channels in WDM transmission. Short band (S-band, 1460-1530nm) - spectrum of wavelengths shorter than those of the C-band. Low band (L-band, 1565 - 1625nm) - spectrum of wavelength longer than those of C-band WO 2006/067788 PCT/IL2005/001364 4 APR - Automatic Power Reduction, one of safety measures of Laser safety standards (IEC) in telecommunication systems. ALS - Automatic Laser Shutdown, one of safety measures of Laser safety standards in telecommunication systems. EDFA -Erbium Doped Fiber Amplifier. The automatic restart procedure discussed in the prior art still leaves a possibility that in some cases, the power optical amplifiers can be switched on (even for a short time) when the fiber failure in the link is not yet repaired. The practice says it is insufficient and not reliable to base the decision of shutting down pumps of the Raman amplifier only on one criterion, for example only on the absence of a signal in a supervisory channel (OSC, SVC, an auxiliary channel, etc.). In other words, a random fault of the supervisory channel transmitter must not lead to the loss of a great amount of information carried by the communication link. The concept of the invention is providing an improved method of controlling (shutting down and/or restarting) of a power optical amplifier in an optical link; since the optical link usually comprises a transmitting side optical amplifier and a receiving side optical amplifier, the method in its various aspects relates to any of the amplifiers and both of them together on the link. According to a first aspect of the invention, there is provided a method of controlling an optical amplifier in an optical link comprising a transmitting side of said link, an optical fiber span and a receiving side of said link, wherein said receiving side comprises said optical amplifier being a receiving side power amplifier; the method comprises transmitting at least a first diagnostic signal via an auxiliary optical channel (SVC, OSC or the like) from the transmitting side towards the receiving side of the link; the method performs shut down of the receiving side power amplifier in case of essentially simultaneously detecting, at said receiving side of the link, absence of the diagnostic signal and at least one of the following events: WO 2006/067788 PCT/IL2005/001364 4 APR - Automatic Power Reduction, one of safety measures of Laser safety standards (IEC) in telecommunication systems. ALS - Automatic Laser Shutdown, one of safety measures of Laser safety standards in telecommunication systems. EDFA -Erbium Doped Fiber Amplifier. The automatic restart procedure discussed in the prior art still leaves a possibility that in some cases, the power optical amplifiers can be switched on (even for a short time) when the fiber failure in the link is not yet repaired. The practice says it is insufficient and not reliable to base the decision of shutting down pumps of the Raman amplifier only on one criterion, for example only on the absence of a signal in a supervisory channel (OSC, SVC, an auxiliary channel, etc.). In other words, a random fault of the supervisory channel transmitter must not lead to the loss of a great amount of information carried by the communication Link. The concept of the invention is providing an improved method of controlling (shutting down and/or restarting) of a power optical amplifier in an optical link; since the optical link usually comprises a transmitting side optical amplifier and a receiving side optical amplifier, the method in its various aspects relates to any of the amplifiers and both of them together on the link. According to a first aspect of the invention, there is provided a method of controlling an optical amplifier in an optical link comprising a transmitting side of said link, an optical fiber span and a receiving side of said link, wherein said receiving side comprises said optical amplifier being a receiving side power amplifier; the method comprises transmitting at least a first diagnostic signal via an auxiliary optical channel (SVC, OSC or the like) from the transmitting side towards the receiving side of the link; the method performs shut down of the receiving side power amplifier in case of essentially simultaneously detecting, at said receiving side of the link, absence of the diagnostic signal and at least one of the following events: WO 2006/067788 PCT/IL2005/001364 5 drop of input signal in a main band carrying information optical channels (preferably, the main band is C-band; for example, the drop of power can be considered when the change is for about 0.5 dB); - power reduction of input signals in both the main band and a short band; (for example, power reduction in both the C-band and S-band for more than about 0.5dB at each of them) - disappearance of a second diagnostic signal, in case it is transmitted from the transmitting side towards the receiving side of the link. Preferably, the transmitting side of the link also comprises an optical amplifier (a transmitting side amplifier) which in turn may be a power amplifier. The term "essentially simultaneously" should be understood as detecting said events within a specified time period (or time window) between one another. Owing to different transient processes which take place at various components of the link, only approximate examples can be presented, such as from a number microseconds to 3 seconds. The time window can be more exactly specified taking into account characteristics and parameters of the power amplifiers, of the fiber connectors, of effective length of the fiber span (see explanation below) and the like. It should be noted that if the first diagnostic signal is lost and no one of the mentioned events is detected within the specified time period, the power amplifiers may remain in the working condition but are to be considered under alarm. If, at any time upon failure of the first diagnostic signal, the mentioned at least one event takes place, the power amplifier at the receiving side must be shut down since no risk can be taken any more. This situation is also in the scope of the above-defined method. If any of the "additional" listed events takes place but the first diagnostic signal does not fail within the specified time period, no shut down is performed and no alarm is required. WO 2006/067788 PCT/IL2005/001364 6 Therefore, a risk of a false shut down is minimized to a negligible level. It should be noted that a double fault scenario is not considered in the ITU-T standardization documents when protection mechanisms are analyzed. One specific version of the above method comprises a step of transmitting the second diagnostic signal via the optical link in the same direction. This solution, by using a double overhead approach, actually resolves the problem of a possible fault of the diagnostic signal transmitter. It should be noted that the main band signal comprises a multiplexed signal of information optical channels transmitted at their corresponding wavelengths; the S-band signal comprises wavelengths shorter than the C-band. The diagnostic signal is preferably a low frequency dithering signal transmitted via an auxiliary optical channel, for example as a modulating signal of the optical carrier. Typically, the power optical amplifiers are Raman amplifiers; preferably, the transmitting side power amplifier is a Forward Direction (FWD) Raman amplifier, and the receiving side power amplifier is a Backward Direction (BWD) Raman amplifier. Shutting down of the power amplifiers can be performed by shutting down their pumps, but can also be done by reducing the amplifiers to a very low power level which cannot lead to transmitting hazardous power signals. The proposed list of events which can be detected almost simultaneously with disappearance of the first diagnostic signal in an optical link comprising power amplifiers can be described as follows. As known to those skilled in the art, optical amplifiers (and especially power amplifiers) are to be shut down in case of a fiber failure/disconnection in the optical link. It should be explained that one of the most important non-linear effects created by high power Raman amplifiers is Rayligh back scattering. This phenomena causes creation of noise in both the C-band and S-band, which is amplified not only in the direction of the Raman pump, but more importantly in this case, counter pump wise. The Inventor generally classified the possible fiber failures into three types: WO 2006/067788 PCT/IL2005/001364 7 1.fiber cut remote from the receiving power amplifier; it is characterized by essential drop of power in the band of optical channels, (preferably, in the C- band); 2.fiber cut close to the receiving amplifier (within a so-called effective length, for example it can be of about 20 km); this distance is characterized by noise in C-band and S-band caused by the known effect of back pumping of Raman amplifiers. This noise makes it impossible to detect LOS which would normally be discovered after a fiber cut or disconnecting along the span. A fiber cut within the effective distance leads to a reduction of noise signals in the C-band and S-band 3.open connector of the receiving power amplifier causes the phenomena characteristic for type 2; in case of a flat surface of the fiber cross-section in the open connector, such a fault may also cause sharp increase in a so-called Back Reflection, up to reaching extremely high Back Reflection (HBR) exceeding a predetermined threshold. The method may optionally comprise determining the type of fault that has caused shut down of the receiving side power amplifier; the determination (the diagnosis) can be based on specific combination of the events detected essentially simultaneously with the absence of said at least one diagnostic signal. The diagnosis preferably comprises indication of the type of fault. Further, the method may comprise detailed analysis of the type of fault. For example, the amount of power returned as the High Back Reflection effect can be indicative of the distance from the receiving side power amplifier. In other words, knowing the returning power, the type of the amplifier and characteristics of the fiber, the distance can be calculated. The method according to the first aspect of the invention may be utilized also in a case when said optical link is one unidirectional link from a pair of two unidirectional links forming together a bi-directional optical link. There is also proposed a method of controlling an optical amplifier in an optical link comprising a transmitting side of said link, an optical fiber span WO 2006/067788 PCT/IL2005/001364 8 and a receiving side of said link, wherein said transmitting side comprises said optical amplifier being a transmitting side power amplifier; the method comprises shutting down the transmitting side power amplifier of the optical link upon detecting at least one of the following events: -reduction of power simultaneously in the main band and in the S-band at the output of the transmitting side power amplifier, -High Back Reflection exceeding a predetermined threshold. For the optical link that belongs to a bi-directional optical link, the above method includes shutting down of the transmitting side power amplifier also in case of detecting Loss of Signal (LOS) in the main band of informational optical channels at (or before) the input of the transmitting side power amplifier. The above method may be performed together with the previously described method of shutting down the receiving side power amplifier. The main band is typically C-band, though L-band can sometimes be used for transmitting of informational channels. The mentioned power reduction is, for example, in the range of more than 0.5 dB in the main band and in the S-band. However, it should be noted that the Raman effect occurs for any wavelength band whatsoever - for example the Raman effect can work also in 1300,1400, 1600 nm ranges and fully depends on the lasers used in the Raman amplifier. As noted, the above criteria are suitable for shutting down the transmitting side power amplifier in a bi-directional optical link. These criteria are independent from a method of shutting down the receiving side power amplifier (for example, the latter can be shut down without utilizing a diagnostic signal). The above phenomena to be detected for shutting down the transmitting side power amplifier can be explained by the same three categories of the fiber faults listed with respect to the receiving side power amplifier. WO 2006/067788 PCT/IL2005/001364 9 According to a second aspect of the invention, there is proposed an improved method of controlling an optical amplifier in an optical link comprising a transmitting side of said link, an optical fiber span and a receiving side of said link, wherein said transmitting side comprises said optical amplifier being a transmitting side amplifier; the method comprises transmitting a first and a second diagnostic counter-propagating signal from the receiving side towards the transmitting side of the link; shutting down the transmitting side amplifier in case of detecting, at said transmitting side of the link, disappearance of said first and said second diagnostic counter-propagating signals. In the preferred embodiment, the transmitting side amplifier is a power amplifier. In one embodiment, the second diagnostic counter-propagating signal can be transmitted via an additional auxiliary optical channel. The above-defined solution dramatically simplifies the process of shutting down the power optical amplifier at the transmitting side of the link, and allows performing it without interaction with another optical link usually serving the opposite traffic direction. Using two diagnostic signals transmitted by separate transmitters renders the above method more reliable. The method according to the second aspect of the invention (i.e., shutting down the transmitting side amplifier based on detecting disappearance of diagnostic counter-propagating signals) may be utilized also in a case when said optical link is one unidirectional link from a pair of two unidirectional links forming together a bi-directional optical link. It is independent from a method of shutting down the receiving side amplifier (i.e., the latter can be shut down without utilizing any diagnostic signal(s). It is to be mentioned that to restart the receiving side amplifier (power or not), it is sufficient that presence of said at least first diagnostic signal be detected at the receiving side of the link. WO 2006/067788 PCT/IL2005/001364 10 According to a third aspect of the invention, there is proposed an improved method of controlling an optical amplifier in an optical link, wherein the optical link comprises a transmitting side of said link, an optical fiber span and a receiving side of said link, wherein said transmitting side comprises said optical amplifier being a transmitting side amplifier; the method comprises transmitting a first diagnostic counter-propagating signal and a second diagnostic counter-propagating signal from the receiving side to the transmitting side of the link; restarting the transmitting side amplifier, previously shut down, in case of detecting presence of at least one of said diagnostic counter-propagating signals at the transmitting side of the link. The proposed method is most preferred for a case where the transmitting side optical amplifier is a power amplifier. The above method of restarting the transmitting side power/nonpower amplifier is simple, fast and reliable, especially in comparison with the methods known in the prior art. These methods can be successfully implemented also in a bi-directional link. According to a fourth aspect of the invention, there is further proposed a method for restarting a transmitting side power amplifier in a bi-directional link, improving a standardized ALS procedure described in the ITU-T G.664. There is provided a method of controlling an optical amplifier in a bi- directional optical link consisting of a first and a second unidirectional optical links, each comprising a transmitting side, an optical fiber span, and a receiving side, wherein at least the first unidirectional link at its transmitting side comprises said optical amplifier being a transmitting side power amplifier; the method comprises: WO 2006/067788 PCT/IL2005/001364 11 a) transmitting at least a first diagnostic signal via an auxiliary optical channel from the transmitting side of the first unidirectional link towards the receiving side of the first unidirectional link, and performing the following operations for restarting the transmitting side power amplifier of the first unidirectional link in case said amplifier has been shut down b) applying restart pulses having duration X and periodicity T (X