[ 0035 ] fig1 shows an example of an optical remote - pumped amplifier of the invention . the example involves an optical preamplifier consisting of an erbium - doped fiber 1 and a receiver and pump unit 2 connected to the erbium - doped fiber by a transmission and pump fiber 3 , which may typically have a length of 10 - 50 km . in the receiver and pump unit 2 , the light arriving from the fiber 3 passes via a wavelength multiplexer 4 to a receiver or amplifier circuit 5 , in which the transmission or information signals contained in the light may be received and optionally be passed on for further processing . a pump laser 6 generates optical pump power which is transmitted via the wavelength multiplexer 4 out on the fiber 3 in a direction toward the erbium - doped fiber 1 . typically , the light with the information signals may have a wavelength of 1550 nm , while oppositely directed pump light may have a wavelength of 1480 nm , thereby enabling the wavelength multiplexer 4 to transmit the information signals from the fiber 3 to the receiver circuit 5 and the pump power from the pump laser 6 to the fiber 3 . the mean power of the pump signal will usually be considerably higher than the mean power of the transmission signals , and interruption of the fiber 3 between the unit 2 and the erbium - doped fiber 1 would therefore involve the risk that a harmful quantity of light might hit an eye if no safety measures were taken . the receiver and pump unit 2 is therefore adapted to be able to assume three states . in an operational state assumed when the receiver 5 detects a communications signal , the pump laser gives full nominal pump power , as the received communications signal is a guarantee that the fiber 3 is intact all the way to the erbium - doped fiber 1 . in a safety state assumed when there is no connection from the receiver and pump unit 2 to the erbium - doped fiber 1 , the pump signal is pulsed so that its mean power is below 10 mw , which means i . a . that the equipment may be categorized as safety class 1 according to the iec 825 recommendations . an intermediate state is assumed when connection to an erbium - doped fiber is detected , while the receiver 5 has not yet detected a communication signal . in this state , the pump power is detected so that the mean power constitutes about ⅔ of the nominal pump power . if the fiber 3 is intact when the system is in the safety state or the intermediate state , the pump pulses will reach the erbium - doped fiber 1 , and the optical power contained in the pulses will be absorbed by the erbium - doped fiber , while a spontaneous noise pulse of so - called ase noise ( amplified spontaneous emission ) is generated in response to each pulse in the erbium - doped fiber 1 . these ase noise pulses will then be returned via the fiber 3 to the unit 2 , where , as will be described more fully below , they can be detected to indicate that there is no break on the fiber 3 . if , on the other hand , there is a break on the fiber 3 , the pulses emitted from the pump laser 6 will not reach the erbium - doped fiber 1 , and thus no ase noise pulses will be generated . when the system is started , the receiver and pump unit 2 will first assume the safety state , while it is checked whether connection to an erbium - doped fiber has been established . when this has been found to be the case , the unit 2 switches to the intermediate state until the receiver 5 detects a transmission signal . in the intermediate state where the pump power is about ⅔ of the nominal value , the transmission quality is just slightly inferior relative to normal function , and the rest of the system is therefore capable of performing a normal start - up procedure . when the receiver 5 detects a transmission signal , the unit 2 switches to the normal operational state . if it is detected at any time while the system is in the operational state that the receiver 5 no longer detects a communications signal , the unit 2 immediately switches to the safety state , as the missing communications signal may , for example , be caused by a fiber break between the unit 2 and the erbium - doped fiber fig2 shows in greater detail how the receiver and pump unit 2 may be constructed . as will be seen , the pump laser 6 is controllable partly from a control unit 16 and partly from a clock generator 17 . the control unit 16 decides which of the three above - mentioned states the unit is to assume , while the clock generator 17 determines the pulse frequency in the states where the pump laser is pulsed . the pulse frequency may , for example , be selected at 75 hz . having passed the wavelength multiplexer 4 , the light received from the fiber 3 may optionally be amplified in an optical amplifier 7 , following which it is split into two branches in the optical coupler 8 . the branch having the units 12 - 15 , which will be described more fully below , detects whether the received light includes ase noise pulses with the pulse frequency , while the detector 11 detects whether the light contains communications signals . in the operational state , the pump laser 6 pumps continuously with the nominal pump power , and the communications signals received from the fiber 3 reach the detector 11 via the wavelength multiplexer 4 , the amplifier 7 and the coupler 8 . the detector 11 passes the signals on for further processing and also informs the control unit 16 that communications signals are received at the moment . the control unit 16 therefore ensures that the laser 6 continues to give full pump power . if the detector 11 detects that the communications signals fail to appear , it informs the control unit 16 which immediately sets the pump laser 6 in the safety state via the connector 9 , where the pump power is transmitted in pulses determined by the clock generator 17 . the pulsed pump power may , for example , look as shown on curve a in fig3 . the repetition frequency of the pulses is here selected to be 75 hz , and the duty cycle is selected such that the resulting mean power is below 10 mw . typically , the nominal power will be 110 mw , and the duty cycle will then be { fraction ( 1 / 11 )} or less . when the fiber 3 is intact , the pulses will move along it until they reach the erbium - doped fiber 1 , and part of the pulse will be reflected on the way because of rayleigh scattering , and , therefore , a reflected signal will return to the receiver and pump unit 2 from the fiber 3 . this signal may , for example . look as shown on curve b in fig3 . it is noted that the amplitude of the reflected signal is considerably smaller than the emitted pulses . when the pump pulse reaches the erbium - doped fiber 1 , this will be active and start generating ase noise , which is likewise passed via the fiber 3 back to the receiver and pump unit 2 . the ase noise will be generated as long as the pulse lasts , and will then decrease according to an exponential curve whose time constant is long with respect to the transmission time on the fiber 3 . the ase noise received on the receiver and pump unit 2 may look as shown on curve c in fig3 . these ase noise pulses are used in the receiver and pump unit 2 as an indication that the fiber 3 is intact . since , however , the signal received from the fiber 3 is the sum of curves b and c , the inhibition circuit 12 provides for blocking of the received signal as long as the signal reflected from the fiber 3 lasts ( curve b ) because of the pulse transit time in the fiber , this will be a period after the end of the transmitted pump pulse , which will be about 0 . 5 msec . with a fiber length of 50 km . as mentioned below , a reflected signal will return also if the fiber is broken , but also this signal will at most be of the same duration . the inhibition circuit 12 is also controlled by the clock generator 17 . thus , only the exponential “ tail ” of the ase noise pulse will be present on the output of the inhibition circuit 12 , as shown on curve d in fig3 . this signal , like the emitted pulses , has a repetition frequency of 75 hz , and it is now passed through a bandpass filter having a center frequency of the 75 hz and a bandwidth of e . g . 15 hz to filter out partly components from a possible communications signal partly signals originating from a constant spontaneous emission in the erbium - doped fiber 1 . the bandpass - filtered signal is then fed to a sample - and - hold circuit 14 which samples with the same frequency as the pulsed pump signal so as to provide a sampling value for each pulse . the sampled values are lowpass - filtered in the lowpass filter 15 and are then compared in the control unit 16 with a threshold value to decide whether a sufficiently great value of the ase noise is received . if the control unit 16 detects that the ase noise pulses are above the threshold value , it instructs the pump laser to switch to the intermediate state , which will be described below , as the fiber 3 must be intact . if , on the other hand , the fiber 3 is broken , no ase pulses can come from the erbium - doped fiber 1 , as the pump pulses do no reach it . but then there will be a strong reflection of the emitted pulse from the break . depending on the distance from the break , this reflection will usually have a considerably greater amplitude than both curves b and c in fig3 ; but this reflection will be over at the latest simultaneously with curve b and will therefore be blocked by the inhibition circuit 12 , so that the control unit 16 does not detect any signal . this is an indication of a break on the fiber , and the control unit therefore informs the pump laser 6 to remain in the safety state . when the control unit 16 has established that ase pulses return , the unit switches to the intermediate state , as mentioned , where the pump laser emits a signal , as shown in fig4 . the power level between the pulses is selected at about ⅔ of the nominal pump power , and the peak level of the pulses corresponds to the nominal power . if the fiber 3 is still intact , a signal corresponding completely to the one described above and shown in fig3 will be returned , the amplitude of the signals being merely smaller . the difference is just that the erbium - doped fiber 1 will now receive sufficient pump power to make it capable of passing on communications signals . when these are detected by the detector 11 , the control unit switches to the normal operational state . the intermediate state is necessary , because the operational state can only be maintained when communications signals are received . therefore , in this circuit , it will not be expedient to switch directly from the safety state to the operational state . if the ase pulses disappear in the intermediate state , this indicates that the fiber has been interrupted again , and the control unit 16 will therefore return to the safety state . the repetition frequency of the emitted pulses is here selected at 75 hz ; but may also assume other values of course . it must be sufficiently low so that the next ase pulse is not emitted before the ase pulse caused by the pulse has died away , and the lower limit of the frequency is determined by the maximum time it may take the system to switch from the intermediate state to the safety state .