Apparatus and method for protecting an optical amplifier receiving as input a radiation having a plurality of components at different frequency, in particular if the generation of such components is interrupted

Abstract

Optical amplification apparatus comprising a protection device (1) for protecting an optical amplifier receiving as input radiations to be amplified having a plurality of components at different frequency, comprising: - generation means (2) for generating a fundamental radiation at a fundamental frequency ω0; - at least one phase modulator (M) at frequency ωM, separate with respect to the generation means (2) or forming part of the same generation means (2), configured to receive as input said fundamental radiation and to generate as output a modulated radiation to be amplified with respect to the fundamental radiation and comprising superimposed components having frequencies ω0, ω0 + ωM and ω0 - ωM; - generation means (3) for generating a reference radiation at a reference frequency ω1 configured to generate said reference radiation at a reference frequency ω1 other than ω0 or at a frequency ω1 equal to ω0 but controlling the relative phase; - superposition means (V) for superimposing the modulated radiation on said reference radiation in order to generate a superimposed radiation; an electronic detection and control system (4) configured to detect, in said superimposed radiation, an abnormal condition corresponding to the absence of the components at frequency ω0 - ωM - ω1 or ωM + ω1.

Description

[0001] "APPARATUS AND METHOD FOR PROTECTING AN OPTICAL AMPLIFIER RECEIVING AS INPUT A RADIATION HAVING A

[0002] PLURALITY OF COMPONENTS AT DIFFERENT FREQUENCY, IN PARTICULAR IF THE GENERATION OF SUCH COMPONENTS IS INTERRUPTED"

[0003] ★★★★★★★

[0004] DESCRIPTION

[0005] Technical field

[0006] The invention concerns an apparatus and a method for generating laser light at multiple frequency components obtained through the frequency translation of the fundamental radiation and including an automatic switchoff mechanism activated in the event of non-generation of the secondary frequency components.

[0007] In particular, the invention lies in the field of the optical amplification of the radiations composed of multiple components at different frequencies.

[0008] Background of the invention

[0009] Frequency translation of a single-frequency laser represents an important technique in many fields such as, for example, atomic and molecular physics, optical telecommunications systems, generation of beacon stars for scientific / industrial applications, frequency or phase stabilization of lasers etc. More generally, it is about a fundamental technique for those systems in which it is necessary to work simultaneously with multiple frequency components [Rachel Houtz et al, Wideband, Efficient Optical Serrodyne Frequency Shifting with a Phase Modulator and a Nonlinear Transmission Line, 12 October 20091 Vol. 17, No. 21 I OPTICS EXPRESS], Although the generation of sidebands can be obtained by direct modulation of the laser source, this route is not often taken as it often introduces an unwanted widening of the emission line. The best alternative is instead to modulate the beam generated by the optical oscillator using acousto-optic (AOM) or electro-optic (EOM) modulators. This allows the desired sidebands to be introduced while maintaining unchanged the spectral purity of the various frequency components. Both AOM and EOM are commercially available in the form of fiberized or free-space devices.

[0010] Generally, a single-frequency laser (hereinafter referred to as LM, laser master) can provide at most a few tens of mW of radiation. If the final application requires higher power levels, it is necessary to use amplification systems. The ideal amplifier is represented by an object capable of increasing the level of available power without deteriorating the spectral characteristics of the radiations to be amplified. A class of amplifiers often used in fields where the required powers are high, up to a few hundred watts, and the spectral purity must be maintained, are the Raman gain optical amplifiers or Raman Optical Amplifiers.

[0011] The Raman gain amplifiers have some characteristics that make them very interesting for both industrial (telecommunications) and scientific applications [Luigi Sirleto et al., Fiber Amplifiers and Fiber Lasers Based on Stimulated Raman Scattering: A Review, Micromachines 2020, 1 1 , 247], Raman gain amplifiers allow to:

[0012] • Amplify signals at any wavelength, provided that this can be effectively transmitted by the optical fibre used and that it is possible to use a pump laser suitable for the chosen wavelength. This is possible thanks to the fact that the process does not rely on resonant states.

[0013] • Amplify signals by introducing a very low amplification noise level as the amplified spontaneous emission (ASE) is low.

[0014] • Use the same transmission optical fibre as a support for the amplification process.

[0015] One of the most limiting aspects in increasing the power generated (> 100W) is the stimulated Brillouin scattering (SBS). SBS is a non-linear process that, in the presence of high pump laser power and a very narrow band input signal (single frequency), leads to the generation of a counterpropagating wave called the Stokes wave. This wave is at a wavelength greater than the pump wave since, during the process, energy is transferred to the medium in the form of phonons. This process almost instantly (times of the order of milliseconds) leads to damaging the amplifier. So far, the known techniques to limit this problem are using short fibres (this leads to the reduction of the amplification gain) and using "periodically strained fibres" [Jinyong Leng et al., Investigation on the suppression of stimulated Brillouin scattering in single-frequency Raman fiber amplifiers, Optik 123 (2012) 659-664],

[0016] For the purposes of this document, the key point is represented by the fact that the SBS limits the power generated in a Raman gain amplifier to a defined value for each single frequency component of which the input signal is composed. If it is therefore true that for today's technology the SBS limits the power generated per single frequency component, it is also true that this does not place a limit on the total power generated as the sum of powers for each single frequency component. Therefore, by injecting the amplifier with a beam composed of multiple frequency components, it is possible to circumvent the limit imposed by the SBS. However, this strategy is not without risks: if the subsystem adapted to generate further frequency components starting from that of the laser master (EOM or AOM) were to be interrupted, this would very quickly lead to damaging the Raman gain amplifier. This would in fact be working with a very high gain concentrated on a single frequency component that would lead to the generation of high SBS. It is therefore necessary to introduce a protection system. This document therefore proposes an invention capable of reacting in short times (much less than those necessary to damage the amplifier via SBS) to the non-generation of sidebands by switching off the laser master. For all intents and purposes, this system protects the Raman gain amplifier that can operate safely. Objects of the present invention

[0017] It is an object of the present invention to propose an apparatus and a method for protecting an optical amplifier sensitive to the power generated for each single frequency component present in the beam with which it is injected.

[0018] These objects are achieved by means of a device that provides for the generation of an electromagnetic field at frequency &)Q by using a singlefrequency laser, also called laser master LM. This electromagnetic field is propagated through an electro-optic (EOM) or acousto-optic (AOM) phase modulator, fiberized or not, driven at frequency COM in order to add the secondary frequency components to frequencies also called sidebands. The laser beam is sampled before and after the addition of the sidebands. By analysis through appropriate mixing electronics, the beat notes at frequencies relative integer harmonics of are generated. The signal, now electronic, is filtered through a low-pass filter so as to isolate the component at frequency COM, i.e. a component solely dependent on the presence of the modulation. The signal is now used to control the state of activation of the electronics that drive the laser master, actually creating an interlock capable of automatically switching off the laser source in case of lack of the component at frequency thus protecting the optical amplifier.

[0019] It should be specified that the protection action is preferably implemented over a time range of less than 100 msec.

[0020] Alternatively, a broad spectrum auxiliary radiation source is used whose radiation is superimposed on the radiation in input to the amplifier. This auxiliary radiation is controlled by the control signal so as to maintain the continuity of the spectral multiplicity in input to the Raman amplifier even in the event of a malfunction on the main branch. Si precisa che l’azione di protezione viene preferibilmente attuata su un range temporale inferiore ai 100 msec. In alternativa, viene utilizzata una sorgente di radiazione ausiliaria a largo spettro la cui radiazione viene sovrapposta alla radiazione in ingresso all'amplificatore. Tale radiazione ausiliaria viene controllata dal segnale di controllo in modo da mantenere la continuità della molteplicità spettrale in ingresso all’amplificatore Raman anche in caso di un malfunzionamento sul ramo principale. L’utilizzo di un tale dispositivo permette di iniettare l’amplificatore con più componenti di frequenza in sicurezza. Il dispositivo eviterebbe infatti che l’amplificatore venga iniettato con una sola componente di frequenza, situazione che porterebbe alla generazione di livelli pericolosi di SBS che porterebbero al danneggiamento dell’amplificatore stesso. Infatti, l’SBS è un processo non lin zione di una singola componente di frequenza amplificata dall’am plificatore. Questo perché il guadagno Brillouin ha una forma lorenziana piccata alla freque ampo elettromagnetico che va a stimolare il processo di scattering. I dettagli riguardo a soglia, guadagno e potenza convertita nel processo SBS sono molto complessi e possono v et al. Stimulated Brillouin scattering in optical fibers, Advances in Optics and Photonics 2, 1–59 (2010)]. Ai fini di questa trattazione è importante notare che la potenza retroriflessa^^^^^per ogni componente di frequenza i deve essere minore di un valore di sicur e a^^^^. D’altra parte la somma di tutte le potenze retroriflesse tramite SBS può eccedere ^^^^^^^^. Questo è il motivo per il quale è possibile pilotare un amplificatore ottico a guadagno Raman con un guadagno talmente elevato da generare una potenza complessiva eccedente quella ritenuta di sicurezza per ogni singola frequenza: ^^^^,^^^= ^^^^,^^^^^^^^^ > ^^ ^^^. Come riportato precedentemente, sorge però un problema di robustezza: qualora la generazione di bande laterali si interrompesse, l’amplificatore ottico si troverebbe a lavorare ad un sidebands introduced by the phase modulator and a reference beam is analysed. The reference beam can be generated in the following ways:

[0021] • By using a second laser at single frequency suitably chosen in such a way as to generate a beat note at a frequency compatible with the bandwidth of the electronic instrumentation used;

[0022] • By using the same laser at single frequency &)Q and at a phase control. In fact, in this case it is important to remind how the components at frequencies &)Q — a)mand &)Q + a)mgenerated by means of a phase modulator are out of phase by +n with respect to the carrier. Two beat notes will then be generated at the same frequency a)mand out of phase, in the general case, by a further phase (p with respect to the reference signal (carrier at frequency The signal measured with the optical sensor will therefore have an intensity / (t, < / >) function of (p such that / (t, 2Tini) = 0 with m as an integer. It will therefore be necessary to introduce a phase control so as to limit signal intensity variations introduced by variations in the phase itself;

[0023] • By using the same single-frequency laser &)Q and a frequency translation system operating at frequency a)s. This frequency is suitably chosen in such a way as to generate a beat note at a frequency compatible with the bandwidth of the electronic instrumentation used.

[0024] Regardless of the strategy used to generate the reference signal, the analysis of the beat note involves the use of appropriate electronic filters to isolate the component at the frequency of interest. In the event that ODS« ODQ the usage of a further oscillator (LO) oscillating at frequency can be used to further lower the frequency of the beat signal by an amount equal This allows to ease the technical specifications of the electronics that control the state of activation of the source LM based on the presence or absence of the beat signal. This is particularly important when the modulation frequency a)mis in the order of GHz. The analysis of the beat signal is adapted to generate a further signal whose intensity is not zero only in the event that all the components concurrent with the generation of the beat note are present. The signal thus obtained is used to control the state of activation of the electronics that drives the laser master, actually creating an interlock capable of automatically interrupting the injection of the amplifier in case of lack of the component at frequency a)m. .

[0025] It should be noted that, in case of malfunction of the components of the interlock subsystem (shift AOM, non-superposition, sensor...) the state of the system is automatically switched into protection mode.

[0026] The above discussion also applies in the event that more than one phase modulation is applied to the field oscillating at frequency &)Q . Taking as an example the case in which two modulations at frequencies a)r are imparted to the carrier, the beat frequencies with the carrier itself turn out to be, after filtering the high frequency and zero frequency components, ■ The beat notes at frequencies + a)rand regardless of the value of the additional phase (p between the carrier and the sidebands.

[0027] Brief description of the figures

[0028] Figure 1 represents, in schematic view, an amplification apparatus comprising a protection device according to a first embodiment of the present invention wherein the reference radiation is generated by a second optical oscillator.

[0029] Figure 2 represents, in schematic view, a variant of the device represented in the embodiment of figure 1 , wherein the generation of the reference radiation derives from a phase control unit that picks up the fundamental radiation and controls the relative phase thereof with respect to the fundamental radiation.

[0030] Figure 3 represents, in schematic view, a protection device for protecting an optical amplifier according to a second embodiment of the present invention wherein the generation of the reference radiation is carried out by an optical acoustic modulator that performs a frequency translation of the fundamental radiation.

[0031] Figures 4 and 5 represent, in schematic view, variants of the second embodiment of figure 3.

[0032] Figure 6 represents, in schematic view, a third embodiment of a protection device for protecting an amplifier in which a control is performed on the optical acoustic modulator.

[0033] Figures 7, 8 and 9 represent, in schematic view, embodiment variants defined by a combination of the previous embodiments and embodiment variants.

[0034] Figure 10 represents, in schematic view, a fourth embodiment in which the sampling of the modulated beam is performed downstream of the optical amplifier G.

[0035] Figure 1 1 represents, in schematic view, an embodiment variant defined by a combination of the previous embodiments and embodiment variants in which a control is performed on the optical amplifier G.

[0036] Figures 12 and 13 represent, in schematic view, further alternative embodiments of the present invention in which there is a generation of a broad spectrum auxiliary radiation (JJBB to be superimposed on the radiation to be amplified in input to the amplifier, and wherein the control is performed by activating or not such auxiliary radiation.

[0037] Description of one or more preferred embodiments according to the present invention

[0038] With reference to the aforementioned figures, a protection device for protecting an optical amplifier G receiving as input radiations to be amplified having a plurality of components at different frequency has been globally indicated with reference numeral 1 .

[0039] Preferably, the protection device 1 is configured to protect the amplifier G from a sudden increase of SBS in case the frequency sidebands are missing.

[0040] In general, the protection device 1 comprises: generation means 2 for generating a fundamental radiation at a fundamental frequency &)Q (preferably an optical oscillator LM or Laser Master driven by a current source IS); at least one phase modulator M at frequency separate with respect to the generation means 2 or forming part of the same generation means 2, and configured to receive as input said fundamental radiation and to generate in output a modulated radiation to be amplified with respect to the fundamental radiation and comprising superimposed components having frequencies &)Q , &)Q + ayMand &)Q - coy . It should be noted that for modulation, there are two alternative embodiment variants:

[0041] - a first variant in which said at least one modulation stage at frequency COM or said at least one phase modulator M driven at frequency are separate units with respect to the generation means 2;

[0042] - a second embodiment variant in which said at least one modulation stage at frequency coy is included in said generation means 2 so as to directly generate a fundamental radiation modulated.

[0043] The device 1 also comprises generation means 3 for generating a reference radiation (which, as we will see below, can take different embodiments) at a reference frequency configured to generate said reference radiation at a reference frequency other than &)Q or at a frequency equal to O)Q but controlling the relative phase thereof; superposition means V for superimposing the modulated radiation on said reference radiation in order to generate a superimposed radiation.

[0044] In particular, the reference frequency is chosen so that one of the frequency components &)Q - a>M - is compatible with the bandwidth of the electronic detection and control system 4. In other words, as will be better explained below, the detection and control system 4 comprises an electronic filter of the low-pass type in which the cut-off frequency is selected so that the frequency of at least one of the signals relating to the initial frequency components &)Q - COM - - is compatible with the bandwidth of the low-pass filter. With particular reference to figure 1 , this apparatus therefore includes an optical oscillator, also called laser master LM, adapted to provide the fundamental radiation at frequency &)Q. This laser master is driven by a current source IS whose activation state, switched on or off, is controlled by the actuator A which is part of an electronic detection and control system 4 explained below.

[0045] The radiation generated by the LM is propagated through a phase modulator M, or through a generic phase modulation stage M, in order to add to the radiation one, or both, of the oscillating components E+m(t), E_m(t) , respectively, at frequencies ODQ + ODM and ODQ — ODM superimposed on each other and superimposed on the fundamental radiation at frequency &)Q. The electromagnetic field Emoa(t) = E-m(t) + Eo(0 + Em(t), present at the output of the modulator M and composed of several single-frequency components, can be conveniently used to inject a high gain driven optical amplifier such that PSBS,tot = PsBs.tot(Ptot) > P1SBS-

[0046] A further laser master LM (defining said generation means 3 for generating a reference radiation) is configured to generate a single-frequency electromagnetic field Erefoscillating at frequency (1)^. The field Eref, also called a reference field, is used to demodulate the field Emod by defining a superimposed radiation. For this purpose, the two fields Emod and Erefare analysed through an appropriate optical sensor D (detector). The output signal of the optical sensor D is proportional to the power at the input thereof, that is, to the square of the electromagnetic field Ioutoc Pin=

[0047] 2

[0048] Ein— (EQ + E-m+ E+m and therefore comprises components at zero frequency and at frequencies given by the linear combination of the frequencies of the fields Emodand Eref.

[0049] By using a suitably configured electronic unit C it is possible to isolate, preferably by filtering, the component of interest I A of the beat signal, i.e. that generated by the beat of one of the two fields E+m(t), E_m(t) , generated by the phase modulator M, with the reference field Eref. The signal thus obtained is used to drive the actuator A in order to define the state (switched on / off) of the current source supplying LM. The actuator A is configured so as to keep the current source S active only in the presence of the signal I A- The electromagnetic field Emodwi|1be present in U only if the modulator M is operating correctly, thus protecting the optical amplifier placed downstream from possible malfunctions of M. In practice, the protection device 1 defines a self-switching off system of the generation means for generating the fundamental radiation, according to the present invention.

[0050] In other words, the sensor D, the actuator A and the electronics C together define an electronic detection and control system 4 configured to detect, in said superimposed radiation, an abnormal condition corresponding to the absence of one or both of the frequency components &)Q - COM - ANCL&)Q + -d>i, and therefore of the signal I A, and to command:

[0051] - a deactivation of the generation means for generating the fundamental radiation; and / or

[0052] - an interruption of the propagation of the radiation to be amplified; and / or

[0053] - a non-generation of the modulated radiation to be amplified; and / or

[0054] - a generation of a gain adjustment signal for the optical amplifier; and / or

[0055] - a generation of a wide spectrum auxiliary radiation &)BB to be superimposed on the radiation to be amplified in input to the amplifier (G).

[0056] In practice, in figure 1 the generation means 2 for generating a fundamental radiation at a fundamental frequency &)Q comprises the optical oscillator LM which is configured to generate a fundamental radiation at a fundamental frequency &)Q , and a current source IS configured to drive said optical oscillator LM. The electronic detection and control system 4 is configured to command, when said abnormal condition occurs, a deactivation of said current source IS so as to deactivate the generation means for generating the fundamental radiation. The generation means 3 for generating a reference radiation comprises a second optical oscillator LM configured to generate said reference radiation at the reference frequency 0)^.

[0057] As already stated, the detection and control system 4 comprises the sensor D, the actuator A and the electronics C.

[0058] By way of example, the electronics C comprise a low-pass filter (already mentioned above) in which the cut-off frequency thereof is selected so that the frequency of at least one of the signals relating to the initial frequency components OJ0- a)M- a)1or OJ0+ a>M- a)1is compatible with the bandwidth of the low-pass filter.

[0059] Figure 2 shows a variant of the embodiment 1 in which said generation means 3 for generating a reference radiation at a reference frequency 60^ is not defined by a further optical oscillator as in figure 1 , but it comprises a fundamental radiation sampling unit F (defining a phase control) configured to realize said reference radiation (1)^= &)Q deriving it from the fundamental radiation and controlling the relative phase thereof with respect to the fundamental radiation itself. This phase control is performed in such a way that the beat notes, at the same frequency^, do not cancel each other out. In other words, the phase control is performed by varying the parameters of the unit F so as to vary the relative phase between the electric field of the fundamental radiation and that of the reference radiation. The two electric fields have the same frequency and it is therefore possible to define a phase between the two sine waves. To control the phase it is possible, for example, to use a suitably configured AOM (Acoustic Optical Modulator) or EOM (Electro-Optical Modulator) or an opto-mechanical actuator.

[0060] In addition, the reference field Erefis generated by sampling the field EQ before the phase modulator M. The relative phase (p between the reference field (sampled field EQ ) and the sidebands introduced by the modulator M is stabilized by the unit F.

[0061] Preferably, said unit F is present on a by-pass branch arranged upstream of the modulator M according to the direction of propagation of the fundamental radiation. The reference radiation generated by the unit F is then superimposed on the radiation in output from the modulator M to define said superimposed radiation.

[0062] Figure 3 shows an embodiment in which the generation means 3 for generating a reference radiation at a reference frequency comprises a further modulator S configured to generate a translation of said reference radiation at the frequency = &)Q - a)swherein a)scorresponds to said modulation frequency.

[0063] Said further modulator S is arranged on a branch parallel with respect to said phase modulator M with respect to the direction of propagation of the reference radiation.

[0064] In this case, the reference field Erefis generated starting from the field EQ sampled before the phase modulator M and by introducing a frequency translation through the further modulator S. Figure 4 shows an embodiment variant of figure 3 in which said further modulator S is arranged in series with said phase modulator M, with respect to the direction of propagation of the reference radiation downstream of an extractor device, arranged between the modulator M and the further phase modulator S, and in charge of generating the radiation to be amplified which is forwarded to the amplifier. In this case, the reference field Eref is generated by sampling the field EQ . The further modulator S is used to introduce a frequency translation on the field Emod.

[0065] Figure 5 shows a variant of figure 4 in which the further modulator S is arranged in series with said phase modulator M, and the extractor device is arranged downstream of the modulator M and of the further modulator S. In this case, the reference field Erefis generated by sampling the field EQ. The further modulator S is used to introduce a field frequency translation on the field EQ before, or equivalently after, the passage through the phase modulator M .

[0066] Figure 6 shows an embodiment with respect to figure 5 in which said electronic detection and control system 4 (comprising the sensor D, the electronic unit C and the actuator A), in the event that it detects said abnormal condition, is configured to deactivate the further modulator S adapted to introduce a frequency translation on the field Emod. This embodiment provides that the injection condition of the amplifier is interrupted in the event that the operation of the further modulator S is suspended following the activation of the actuator A.

[0067] In this embodiment, the reference field Eref is generated by sampling the field EQ . The further modulator S is used to introduce a field frequency translation on the field EQ before, or after, passing through the phase modulator M. In addition, the actuator A is used to activate / deactivate the further modulator S so as to control the state of the electromagnetic field in U.

[0068] Figure 7 shows a variant of the embodiment of figure 6 in which there are two modulators M and a further modulator S in series. The reference field Erefis generated by sampling the field EQ. The further modulator S is used to introduce a field frequency translation on the field EQ before, or equivalently after, passing through the phase modulators M. Two modulators M driven at frequencies 0Dr are used to generate the modulated field Emod. In addition, the actuator A is used to activate / deactivate the further modulator S so as to control the state of the electromagnetic field in U.

[0069] Figure 8 shows a variant of the embodiment of figure 7 in which the reference field Erefis generated by sampling the field Emodafter having crossed two modulators M driven at frequencies a)rand a)d. In addition, the actuator A is used to activate / deactivate the further modulator S so as to control the state of the electromagnetic field in U.

[0070] Figure 9 shows a further variant of the embodiment illustrated in figures 7 and 8 in which said electronic detection and control system 4 (comprising the sensor D, the electronic unit C and the actuator A), in the event that it detects said abnormal condition, is configured to deactivate the current source IS. The reference field Erefis generated by sampling the field Emodafter having crossed two modulators S driven at frequencies ODr

[0071] Figure 10 shows an optical amplification apparatus receiving as input a radiation to be amplified having a plurality of components at different frequency, comprising:

[0072] - a protection device 1 previously described; - an optical amplification device, preferably of the Raman gain type, for the radiation to be amplified at a predefined gain G and configured to generate as output an amplified radiation.

[0073] Such apparatus comprises a superposition device 5 for superimposing the amplified radiation and said reference radiation to define a feedback radiation. The electronic detection and control system 4 is configured to detect said abnormal condition on said feedback radiation and to command a control of the generation means for generating the reference radiation to interrupt it.

[0074] In detail, figure 10 shows an embodiment variant in which the reference field Erefis generated by sampling the field EQ. The further modulator S is used to introduce a field frequency translation on the field EQ before, or after, the passage through the phase modulator M. In addition, the actuator A is used to activate / deactivate the further modulator S so as to control the state of the electromagnetic field in U. The electromagnetic field Emodis sampled after the optical amplifier.

[0075] Finally, figure 11 shows a variant of figure 10 in which the electronic detection and control system 4 is configured to detect said abnormal condition and to adjust the gain of the amplifier G. In detail, the reference field Eref is generated starting from the field EQ sampled before the phase modulator M and by introducing a frequency translation through the further modulator S. The actuator A is used to vary the parameters of the amplifier G.

[0076] Figure 12 shows an alternative embodiment of the present invention in which the apparatus comprises generation means 5 for generating an auxiliary radiation at a broad spectrum frequency Ct)##. The device 1 is configured to superimpose this auxiliary radiation (at the frequency Ct)##) on the radiation to be amplified in input to the amplifier G.

[0077] The electronic detection and control system 4 is therefore configured to command, by means of the appropriate control signal, said generation means 5 (and not the source IS or the further modulator S) in order to generate or not said auxiliary radiation as a function of the detection of said aforementioned abnormal condition.

[0078] In particular, in a first embodiment illustrated in figure 12, the device comprises a wide spectrum auxiliary radiation source (further optical oscillator LD) whose radiation is superimposed on the radiation in input to the amplifier G. The presence or not of such auxiliary radiation is controlled by the control signal in output from the system 4, obtained from any of the configurations already described above. The activation or not of the auxiliary radiation can be achieved by means of any known device capable of switching the propagation of the radiation, or by direct activation of the source on a time scale of the order of milliseconds.

[0079] In this way, the continuity of the spectral multiplicity in input to the Raman amplifier is maintained even in the event of a malfunction on the main branch by activating the broad spectrum auxiliary radiation.

[0080] The expression "broad spectrum" means a spectral width such that the gain of the amplifier remains below the safety threshold of the Raman amplifier ( PsafeSBS )inthe operating conditions thereof with respect to the phenomenon SBS.

[0081] Alternatively, as represented in figure 13, the generation means 5 in place of the oscillator LD, may comprise a further back-up modulator (B) configured to generate and introduce a further phase shift by + / -(i)gg on the reference radiation before the input to the amplifier G. Obviously, this further back-up modulator (B) is activated only in case of emergency as a function of the detection of said abnormal condition.

[0082] Note that the embodiments just described herein with reference to figures 12 and 13 are combinable with any one of the previously described embodiments and such combinations are believed to be contemplated and described in the present document.

[0083] A further object of the present invention is a process for protecting an optical amplifier receiving as input radiations to be amplified having a plurality of components at different frequency. This process derives directly from what is described above in relation to the apparatus and device that is understood to be fully referred to hereinbelow.

[0084] This process comprises the following operating steps: generating a modulated fundamental radiation comprising superimposed components having frequencies &)Q , &)Q + a)Mand &)Q - (I>M; or generating a fundamental radiation at a fundamental frequency &)Q and modulating said fundamental radiation by means of at least one phase modulator (M) at frequency COM configured to generate as output a modulated radiation to be amplified with respect to the fundamental radiation and comprising superimposed components having frequencies superimposing said modulated radiation on said reference radiation in order to generate a superimposed radiation; electronically detecting and controlling said superimposed radiation, so as to recognize an abnormal condition corresponding to the absence of the components at frequency and to command:

[0085] - a deactivation of the generation means for generating the fundamental radiation; and / or

[0086] - an interruption of the propagation of the radiation to be amplified; and / or

[0087] - the adjustment of the gain of the optical amplifier; and / or

[0088] - a generation of a wide spectrum auxiliary radiation &)BB to be superimposed on the radiation to be amplified in input to the amplifier.

[0089] The present invention achieves the set objects. In fact, the use of such a protection device 1 allows the amplifier to be injected with multiple frequency components safely.

[0090] In fact, a portion of the signal to be amplified is processed so as to isolate the components at frequency &)Q - a>M or &)Q +ODM, that is, dependent on the presence of modulation. The signal is now used to control the state of activation of the electronics that control the generation of the modulated radiation to be amplified or the parameters of the amplifier G, actually creating an interlock capable of automatically switching off the laser source in case of lack of the modulation at frequency &)M , thus protecting the optical amplifier.

Claims

CLAIMS1 . An optical amplification apparatus receiving as input a radiation to be amplified having a plurality of components at different frequency, comprising an optical amplification device (G), of the Raman gain type, for the radiation to be amplified at a predefined gain value and configured to generate in output an amplified radiation; said amplification apparatus comprising a protection device (1 ) for protecting said optical amplification apparatus configured to protect said optical amplification apparatus from the SBS phenomenon in the event that components of the modulated frequencies in the radiation to be amplified are missing, wherein said device (1 ) comprises: generation means (2) for generating a fundamental radiation at a fundamental frequency &)Q ; at least one modulation stage at frequency COM, or at least one phase modulator (M) driven at frequency COM and configured to receive as input said fundamental radiation, which are adapted to generate a modulated radiation to be amplified comprising sidebands with components having frequencies &)Q + a)M, <^>o -superimposed on each other and superimposed on the fundamental radiation at frequency 6o0; generation means (3) for generating a reference radiation at a reference frequencyconfigured to generate said reference radiation at a reference frequencyother than; superposition means (V) for superimposing the modulated radiation on said reference radiation in order to generate a superimposed radiation; an electronic detection and control system (4) configured to detect, in said superimposed radiation, an abnormal condition corresponding to the absence of the components at frequency too -command: a deactivation of the generation means for generatingthe fundamental radiation; and / or an interruption of the propagation of the radiation to be amplified; and / or a generation of a gain adjustment signal for the optical amplification apparatus; and / or a generation of a wide spectrum auxiliary radiation &)BB to be superimposed on the radiation to be amplified in input to the amplifier.

2. The apparatus according to claim 1 wherein said generation means (2) for generating a fundamental radiation at a fundamental frequency &)Q comprises an optical oscillator (LM) configured to generate a fundamental radiation at a fundamental frequency &)Q , and a current source (IS) configured to drive said optical oscillator (LM).

3. The apparatus according to claim 2 wherein said electronic detection and control system (4) is configured to command, when said abnormal condition occurs, a deactivation of said current source (IS) so as to deactivate the generation means for generating the fundamental radiation.

4. The apparatus according to any one of the preceding claims wherein said generation means (3) for generating a reference radiation comprises a second optical oscillator (L1 ) configured to generate said reference radiation at the reference frequency 0)^.

5. The apparatus according to any one of the preceding claims wherein said generation means (3) for generating a reference radiation at a reference frequencycomprises a fundamental radiation sampling unit configured to realize said reference radiation 0)^ by controlling the relative phase with respect to the fundamental radiation.

6. The apparatus according to any one of the preceding claims wherein said generation means (3) for generating a reference radiation at areference frequency comprises a further modulator (S) configured to generate a phase shift of said reference radiation at frequency= O)Q - a)swherein a)scorresponds to said modulation frequency.

7. The apparatus according to any one of the preceding claims wherein said further modulator (S) is arranged in series or parallel with respect to said phase modulator (M) with respect to the direction of propagation of the reference radiation in order to introduce a frequency translation in the modulated field Emodof the superimposed radiation with respect to the reference field Erefof the reference radiation.

8. The apparatus according to any one of claims 6 or 7 wherein said electronic detection and control system (4), in the event that it detects said abnormal condition, is configured to command a control of the propagation means for propagating the radiation to be injected into the optical amplification apparatus in order to interrupt it.

9. The apparatus according to any one of the preceding claims wherein said at least one modulation stage at frequency a)y is part of the same generation means (2) for generating the fundamental radiation so as to generate a fundamental radiation already modulated.

10. The apparatus according to any one of the preceding claims, characterized in that it further comprises generation means (5) for generating auxiliary radiation at a wide spectrum frequency CO##; said device (1 ) being configured to superimpose said auxiliary radiation on the radiation to be amplified in input to the amplifier (G); said electronic detection and control system (4) being configured to command said generation means (5) in order to generate or not said auxiliary radiation as a function of the detection of said abnormal condition.1 1 . The apparatus according to claim 10 wherein said generation means (5) comprises a further optical oscillator (LD) distinct from the opticaloscillator (LM) and configured to generate said broad spectrum auxiliary radiation.

12. The apparatus according to claim 10 wherein said generation means (5) comprises a further back-up modulator (B) configured to generate and introduce a further phase shift by +I- (J)BBonthe reference radiation before the input to the amplifier (G).

13. The apparatus according to any one of the preceding claims characterized in that it comprises a superposition device for superimposing the amplified radiation and said reference radiation to define a feedback radiation; said electronic detection and control system (4) being configured to detect said abnormal condition on said feedback radiation and to command a control of the propagation means for propagating the radiation to be amplified in order to interrupt the generation of the amplified radiation.

14. The apparatus according to any one of the preceding claims wherein said electronic detection and control system (4) is configured to detect said abnormal condition and to adjust said predefined gain through said gain adjustment signal.

15. A process for protecting a Raman-type optical amplification apparatus from the SBS phenomenon in the event that components of the modulated frequencies in the radiation to be amplified are missing, receiving as input radiations to be amplified having a plurality of components at different frequency, comprising the following operating steps: generating a modulated fundamental radiation comprising sidebands with superimposed components having frequencies &)Q , &)Q + COM and &)Q - O)M,' or generating a fundamental radiation at a fundamental frequency &)Q and modulating said fundamental radiation by means of at least one modulation stage at frequency a)y or at least one phase modulator (M) at frequency a>M configured to generate as output a modulated radiation to be amplifiedwith respect to the fundamental radiation and comprising superimposed components having frequencies &)Q + COM and &)Q - COM superimposed on each other and superimposed on the fundamental radiation at frequency <D0; generating a reference radiation at a reference frequencyother than O)Q or 0)^ equal to &)Q but controlling the relative phase thereof; superimposing said modulated radiation on said reference radiation in order to generate a superimposed radiation; electronically detecting and controlling said superimposed radiation, so as to recognize an abnormal condition corresponding to the absence of the components at frequencyand to command:- a deactivation of the generation means for generating the fundamental radiation; and / or- an interruption of the propagation of the radiation to be amplified; and / or- a generation of a gain adjustment signal for the optical amplification apparatus;- a generation of a wide spectrum auxiliary radiation &)BB to be superimposed on the radiation to be amplified in input to the amplifier.

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