The present invention relates to an optical amplifier with polarization control and a method for controlling polarization in an optical amplifier.

Particularly, the present invention concerns polarization-sensitive optical amplification occurring in the presence of optical amplification medium with polarization-dependent gain (PDG) and polarization mixing caused by the propagation of the optical signal in a medium that has non-negligible polarization mode dispersion

(PMD) values.

In optical amplification, the gain "g" is known to be dependent on the polarization of the signal to be amplified and the pump signal. Such dependency of gain is observed in the presence of both co-propagating amplification schemes (where the dependency coefficient flips from a 0 value for signal with polarization orthogonal to the pump to the 1 value for signals with polarization aligned with the pump) and counter-propagating schemes (where the dependence coefficient flips from 1/3 to 2/3) [Oskar van Deventer M., Boot A.J., Polarization properties of stimulated Brillouin scattering in single-mode fibers, IEEE J. Light. Tech., 585, 12, (1994)]. This phenomenon is so relevant that these amplifiers are defined in the art as polarization- dependent gain (PDG) amplifiers [Lin Q, Agrawal G.P., Statistics of polarization- dependent gain in fiber-based Raman amplifier, Opt. Lett.227, 28, (2003)]. In real fiber-optic propagation, the polarization of the transmitted signal generally has a different evolution from the pump: the two fields may also have significantly different frequencies and hence, due to PMD, their polarization evolves to different polarization states, although they initially had identical states.

PMD is caused by random birefringence which randomly changes the polarization state of the signal to be amplified with respect to the pump to an extent proportional to the frequency difference between the signal and the pump and to the PMD value of the optical amplification medium, such as an optical fiber or a waveguide. In propagations of sufficient length to allow statistical PMD modeling, a D parameter is defined, which is measured in P ^S ^ .

In Thevenaz L., Zadok A., Eyal A., Tur M., All-optical polarization control through Brillouin amplification, Proc. OFC 2008, paper 0ML7, San Diego (CA) ₅ 2008, a polarization pulling device has been proposed for pulling the polarization of the signal to be amplified towards the pump signal polarization in a Brillouin fiberoptic amplifier in counter-propagating configuration, i.e. with the pump introduced from a side opposite to the signal to be amplified.

The stimulated Brillouin phenomenon may be used to amplify an optical signal at a frequency distance of about 10-11 GHz from the counter-propagating pump signal. If the counter-propagating signal has a polarization state that is not completely orthogonal to the pump, it has a higher gain in the direction parallel to the pump signal, which leads to little but non-negligible alignment of the amplified signal with respect to the pump signal. As described in Thevenaz et al., repeated application of this principle leads to progressive pulling of the signal towards the pump, to substantial alignment of the polarization state of the amplified optical signal with the polarization state of the optical pump signal. Due to birefringence and hence PMD of the optical amplification medium, whatever the initial polarization state of the signal to be amplified, such signal will never have, along its path, a state completely orthogonal to that of the pump signal. At the output of the amplifier, the polarization state of the amplified signal will be aligned with the polarization state of the pump signal at the input of the amplifier. This will provide both control and stabilization of the polarization state of the amplified signal, regardless of the birefringence and PMD conditions of the optical fiber.

However, the solution suggested in Thevenaz et al. is not applicable to amplification of telecommunication signals due to the very narrow bandwidth allowed by Brillouin amplification, which is typically of the order of a few tens of MHz.

The object of the present invention is to provide an optical amplifier for use in amplification and polarization control of optical signals used in fiber-optic telecommunications.

This object is fulfilled by an optical amplifier with polarization control as defined in claim 1.

The above object is further fulfilled by a method for controlling polarization in an optical amplifier as defined in claim 11.

Further characteristics and advantages of the optical amplifier with polarization control and the method for controlling polarization in an optical amplifier according to the present invention will be apparent from the following description of one preferred embodiment thereof, which is given by way of illustration and without limitation with reference to the accompanying figures, in which:

- Figure 1 shows a block diagram of a first embodiment of an optical amplifier of the present invention;

- Figure 2 shows a block diagram of a second embodiment of an optical amplifier of the present invention; - Figure 3 shows a block diagram of a third embodiment of an optical amplifier of the present invention;

- Figure 4 shows the results obtained using a 1486 nm Raman pump with DOP=0.1 and P _pump =2.68W according to the block diagram of Figure 1;

- Figure 5 shows a block diagram of a measuring apparatus used to perform tests with the optical amplifier of Figure 2;

- Figure 6 shows the Poincare sphere obtained using a 1486 nm Raman pump with P _pump =5 W according to the block diagram of Figure 2.

Referring to the annexed figures, numeral 1 generally designates an optical amplifier with polymerization control of the present invention. The optical amplifier 1 comprises a signal input 2 for receiving an optical signal to be amplified Sj _n at a wavelength λs, a Raman optical pump source 5 for supplying an optical pump signal P at a pump wavelength λp and an optical amplification medium 4. The optical amplification medium 4 may be, for instance, an optical fiber, a waveguide or a photonic crystal structure.

The optical amplification medium 4 has a first port 4a and a second port 4b and is coupled to the signal input 2 and to the Raman optical pump source 5 for amplifying the optical signal to be amplified Sj _n by co-propagation of the optical signal to be amplified S; _n and the optical pump signal P in the optical amplification medium 4 and produce an amplified optical signal S ₀ Ut-

The amplifier 1 further has a signal output 3 to the optical amplification medium 4 for supplying the optical amplified signal S _out - The optical signal to be amplified Sj _n is, for instance, a signal that has propagated in a fiber-optic transmission line and is thus degraded due to losses, chromatic dispersion and polarization mode dispersion.

The Raman optical pump source comprises a laser adapted to generate a Raman optical pump signal P at a pump wavelength λp. As the optical pump signal P propagates in the Raman optical amplification medium 4, it produces an amplification of the input signal Sj _n by Raman effect.

Particularly, the Raman optical pump source 5 and the optical amplification medium 4 are designed to cause polarization pulling, in the optical amplification medium 4, of the optical signal to be amplified S; _n towards the optical pump signal P, by co-propagation of the optical signal to be amplified Sj _n and the optical pump signal P in the optical amplification medium 4 so that the polarization state of the amplified optical signal S _out corresponds to the polarization state of the optical pump signal P at the output of the optical amplification medium 4. By virtue of the broad amplification band allowed by Raman amplification, which is broader than 2 THz, the amplifier 1 is suitable for use for optical signal amplification in the field of fiber-optic telecommunications. Furthermore, the co-propagating configuration of the optical signals to be amplified Sj _n and the pump signals P in the optical amplification medium 4 allows maximized utilization of the Raman gain, which is lower than Brillouin gain, and provides polarization pulling, allowing to control and stabilize the polarization state of the amplified optical signal S _out -

According to one embodiment, polarization pulling of the optical signal to be amplified Sj _n towards the optical pump signal P occurs under the following conditions: a) g _R aman*Ipump*max(LpMD,L _D wo) ^> l and b) L _p rop ^>:> LpMD5 wherein

- gRaman is the Raman gain of the optical amplification medium 4,

- Ip _u mp is the intensity of the optical pump signal P, which is defined as the ratio of the power Ppump of the optical pump signal and the effective area A _eff ,

- Lp _M D is a length value which is a function of the polarization mode dispersion (PMD) of the optical amplification medium 4 and signal wavelength λs and pump wavelength λp,

- L _D W _O is the walk-off length between the optical signal to be amplified Sj _n and the optical pump signal P in the optical medium 4, and

- Lp _ro p is the distance of propagation of the optical signal to amplified Sj _n and the pump signal P in the optical amplification medium (4).

Advantageously, the following condition must also be fulfilled c) LDWO ^> LPMD- In this case, the condition a) becomes gRaman Ipump LpMD^l -

Assuming that the optical medium 4 has a PMD value, as measured by a

parameter D, (P ^SQC ' ^V &m ^ ^ ₆ folloΛving quantity will define a differential group delay (DGD): where the quantity LPM _D has been introduced, which represents the distance of propagation at which the two polarization states that delimit the frequency width ^{15 p} of the two signals Sj _n and P accumulate a phase delay of ^π ' 4 , that is:

2π(ω _x -ω ₂ )Aτ = π/4 whence

The walk-off length L _D W _O represents the walk-off between the pump signal P and the signal to be amplified Sj _n , which is caused by the presence of a non-zero dispersion D^ of the optical medium 4 and is defined by the following relation:

T

^ ¹ DWO K, — 1 τDW0 τ DWO ^ where

B is the equivalent band of the signal Sj _n ,

and is determined from the dispersion parameter Ox of the optical medium 4 as expressed in [psec/nm Km] , whereby ^T w ^{= D} *. ^{' Aλ} \P ^sec/ -^ ^m ] .

Concerning the above conditions, it shall be noted that, with equal Raman gain values gRaman, the length LPM _D shall be sufficiently large to avoid the need of too high a pump intensity I _P ump- Also, the effective area A _eff must be selected to be as small as possible to ensure high pump intensity I _pump .

The condition b) must be fulfilled for PMD to be effective in mixing the polarization state of signal light.

According to the embodiment of Figure 1, the first port 4a of the optical amplification medium 4 is coupled to the signal input 2 and the Raman optical pump source 5, whereas the second port 4b is coupled to the signal output 3.

Particularly, the signal to be amplified Sj _n and the optical pump signal P are coupled with the optical amplification medium 4 via an optocoupler 6, such as a wavelength division multiplexer (WDM). Such optocoupler 6 is coupled at one end to the signal input 2 and to the Raman pump spurce 5 and at the other end to the port 4a of the Raman amplification medium 4. In the configuration of Figure 1, the Raman pump source 5 is adapted to generate a pump signal P with fast polarization fluctuations.

In other words, the pump signal P has a very low degree of polarization DOP, e.g. 0.1 or less.

In order to obtain a pump signal P with a low DOP value, the pump signal P may be generated using two high-bandwidth orthogonally polarized signals, i.e. having a bandwidth at least one order of magnitude higher than the optical signal Sj _n to be amplified. There may be other methods to generate low-DOP pump signals.

Once the conditions for generating pulling have been fulfilled in the optical amplification medium 4, the amplified signal S _ou t will have the same degree of polarization or DOP as the pump signal P. In order to obtain a linearly polarized optical signal at the signal output 3 of the amplifier 1, such amplifier 1 comprises an optical polarizing filter 7 located between the amplification medium 4 and the signal output 3. For instance, a polaroid 7 may be used to filter the output signal from the optical amplification medium 4 and supply a linearly polarized amplified signal S _ou t to the signal output 3.

Figure 4 shows the results obtained using a 1486nm Raman pump with DOP=O.1 and P _pump ^.όδW, a 1581nm input signal to be amplified Sj _n with an initial power of lOμW and a maximum gain of 4IdB. This configuration provided an amplified signal S _ou t with a DOP of 0.09 to 0.1.

While the above embodiment provides a linearly polarized amplified output signal, it introduces a 3dB loss due to the use of the optical output filter.

Such loss may be eliminated using an amplifier of a second embodiment, as shown in Figure 2.

According to the embodiment of Figure 2, the optical amplifier 1 comprising polarization rotation and reflection means, generally designated by numeral 8. According to one embodiment, the polarization rotation and reflection means 8 consist of a Faraday rotator mirror.

In this case, the optocoupler WDM 6 couples the signal input 2 and the Faraday rotator mirror 8 to the first port 4a of the optical amplification medium 4. The second port 4b is coupled to a second optocoupler 9, which is in turn coupled to a Raman optical pump source 5 and to the signal output 3 of the amplifier 1.

The Faraday rotator mirror 8 comprises an optical polarization rotation device 8 a which is adapted to rotate signal polarization passing therethrough by 45° and an optical mirror 8b, such as a metal mirror, for reflecting the signal incident thereupon. In this configuration, the Raman optical pump source 5 is adapted to generate a linearly polarized optical pump signal P, e.g. with vertical polarization, and hence with a degree of polarization DOP=I.

As the vertically polarized optical pump signal P propagates in the optical amplification medium 4, it changes its polarization due to the polarization mode dispersion (PMD) and hence the birefringence of the optical amplification medium 4, whereby the vertically polarized optical pump signal P introduced at the input from the port 4b of the optical medium 4 reaches the port 4a with an elliptical polarization, having a predetermined direction of rotation V. This elliptically polarized optical pump signal P with a direction of rotation V is transmitted, via the coupler 6, to the Faraday rotator mirror 8, which reflects such signal and, by passing it twice through the 45° optical rotator, causes a change of the rotation direction V of the elliptical polarization of the pump signal P, whereby the optical pump signal P reflected by the Faraday rotator mirror has an elliptical polarization in the opposite direction -V. Such optical pump signal P is conveyed via the coupler 6 towards the optical amplification medium 4 for co-propagation with the input signal Sj _n that comes from the signal input 2.

Since the elliptical polarization of the optical pump signal at the input of the optical medium 4 has an opposite direction of rotation -V, the propagation of the optical pump signal P in the optical medium 4 causes the optical pump signal P at the output of the optical medium to have a linear polarization orthogonal to the polarization that it had at the output of the Raman optical pump source 5, and is hence horizontally oriented.

Once the conditions to obtain co-propagating pulling in the optical amplification medium 4 are fulfilled, the amplified signal S _out will have the same polarization state as the pump signal P and hence a linear polarization, here a horizontal polarization. The second coupler WDM 9 separates the optical pump signal

P and the amplified signal S _out , for the signal output 2 to only receive the optical amplified signal which, as mentioned above, has a linear polarization. Thus, this configuration avoids the use of an optical output filter and the consequent signal loss introduced thereby.

Figure 5 shows a block diagram of a measuring apparatus 100 used to perform tests with the optical amplifier of Figure 2.

A Raman fiber laser 5 manufactured by Spectra-Physics (model Steramline- RL), emitting radiation at a wavelength of 1486 nm and with an average power of 5

Watt was used for such measuring apparatus 100. Such Raman pump 5 has fast intensity and polarization fluctuations. In order to obtain an optical pump signal P polarized with a single polarization state, a fiber polarizer 5b was used, which was connected to the Raman laser 5 with an optoisolator 5c interposed therebetween. The output of the polarizer 5b was coupled by a WDM fiber coupler 9 to a 2.1 km long DS

(dispersion shifted) optical fiber 4, having zero dispersion at 1536 nm.

The optical signal to be amplified Sj _n was generated by a signal laser 20 adjustable as needed, having at its output an optoisolator 20a, a variable attenuator 20b which attenuates the signal Sj _n to 0.1 μW and a polarization controller 20c that controls signal polarization S; _n and is coupled to the other end of the DS fiber 4 by a WDM fiber coupler 6.

The pump signal P injected into the DS fiber 4 is in counter-propagating arrangement with respect to the input signal Sj _n to be amplified and, after passing through the DS fiber 4 and the WDM coupler 6, it is reflected from the Faraday mirror 8 and runs through the DS fiber 4 in co-propagation with the signal to be amplified Sj _n .

As anticipated above, the action of the Faraday mirror 8 consists in turning the polarization of the pump signal P ₅ after a return path thereof within the DS fiber 4, into a polarization state orthogonal to the initial state, regardless of fiber birefringence. Since Raman-induced polarization pulling occurs in the return path of the pump signal P, i.e. when the pump signal P and the signal to be amplified Sj _n are counter- propagating, the polarization of the signal to be amplified Sj _n is "pulled" by the polarization of the pump signal P towards a state orthogonal to the initial state of the pump P.

Once the amplified signal S _out has passed through the fiber, it is separated from the pump P by the WDM coupler 9, filtered by two cascaded optical filters 30, 30a having 0.5 nm and 0.3 nm bandwidths respectively, attenuated by an attenuator 30b and finally analyzed by a polarimeter 30c. Figure 6 shows the Poincare sphere obtained by the polarimeter 30c, representing the final polarization states of the output signal S _out after propagation in the fiber, in response to a change of the initial polarization state of the optical signal to be amplified Si _n over the whole sphere.

As shown by the Poincare sphere of Figure 6, as the initial polarization of the optical signal to be amplified Sj _n is changed by controlling the fiber polarizer 20c, the polarization of the amplified signal S ₀ Ut ₉ after propagation in the fiber, changes in a limited region of the sphere, due to the polarization pulling effect.

It shall be noted that, due to the greater attenuation of the pump P caused by dual passage thereof through the DS fiber 4, the pump signal P has such an optical power that such region of the Poincare sphere is larger than that shown in Figure 4, which is obtained using a scheme in which the pump signal P undergoes no reflection, i.e. passes once through the optical medium 4. Such region has also been found to exhibit long-time stability (for measuring times of a few hours) because the Faraday mirror leaves the final polarization state of the pump P unchanged after fiber propagation, even in the presence of time-dependent variations of local birefringence along the fiber.

The scheme of Figure 2 may be further improved by interposing an optical amplifier 10 between the optocoupler 6 and the Faraday rotator mirror 8, to amplify the optical pump signal P at the output of the first port 4a, which is directed towards the Faraday rotator mirror 8, and to amplify the optical pump signal P reflected by the Faraday rotator mirror 8 and directed towards said first port 4a of the optical amplification medium (4) (see Figure 3). Particularly, to allow bidirectional amplification of the optical pump signal P, the optical amplifier 10 has no optoisolator. As clearly shown in the above description, the optical amplifier of the present invention fulfills the above mentioned needs and also obviates prior art drawbacks as set out in the introduction of this disclosure.

Particularly, co-propagating polarization pulling in a Raman amplifier allows the polarization state of the amplified optical signal to match the polarization state of the optical pump signal at the output of the optical amplification medium. Furthermore, by virtue of the high amplification bandwidth allowed by Raman amplification, which is higher than 2 THz ₅ the optical amplifier of the present invention is suitable for use for optical signal amplification in the field of fiber-optic telecommunications.

Those skilled in the art will obviously appreciate that a number of changes and variants may be made to the optical amplifier of the invention as described hereinbefore to meet specific needs, without departure from the scope of the invention, as defined in the following claims.