1. Introduction

In many fields of photonics, the light state of polarization (SOP) remains so far one of the most elusive uncontrolled variable, which can dramatically degrade the performances of current optical systems through polarization mode dispersion (PMD), polarization depending loss (PDL) or simply polarization depend components [1–7]. Therefore these deleterious polarization effects represent a serious issue for the development of future all-transparent networks, mainly because all-optical circuits, that will be inserted in those systems via silicon chip technologies [8–10], presently suffer from large polarization dependent performances.

At the same time, with the development of very high bit-rate telecommunication systems, strong intensity degradations occur during the propagation due to linear and nonlinear effects. These degradations originate from intra- and inter-channel nonlinear effects including self-phase modulation (SPM), cross-phase modulation, four-wave mixing (FWM) as well as spontaneous noise emission from optical amplifiers and linear effects such as chromatic dispersion and PMD [11]. If chromatic dispersion could be theoretically perfectively compensated, interactions between linear and nonlinear effects irreversibly lead to pulse profile and amplitude noise degradations, providing jitter accumulation (in time and amplitude), depolarization or ghost-pulse generation in the zero bit-slot [11–15].

In order to combat the cumulative impairments occurring during propagation, both electronic and all-optical data processing approaches are currently considered. Electronic-based amplitude regenerators require an electrical/optical data conversion and are hence limited by the current bandwidth of electronic, making them incompatible with high bit-rate applications. In the same way, despite recent impressive progress [6, 16–18], active optoelectronic polarization controllers may be limited by the electronic response time of their feedback loop and are not fast enough to master strong pulse-to-pulse polarization variations occurring in optical fiber. Consequently, developing new all-optical tools, capable to control or regenerate any properties of a light beam will find a great interest for the communication area but will also bring many benefits in all fields of photonics [8, 19]. Thus designing new devices able to all-optically regenerate both SOP and intensity profile of light still remains an exciting challenge. Thanks to its instantaneous response time, nonlinear Kerr effect in optical fibers has received much consideration as an efficient way to overcome the electronic bottleneck [19–24]. Among the nonlinear reshaping methods, the approach reported by Mamyshev and based on SPM and offset spectral filtering appears as one of the most promising technique, mainly because of its simplicity and its potential for high bit rate applications [20]. Let us recall that, thanks to the intensity-dependence spectral broadening occurring in the optical fiber and its associated offset spectral slicing, the system proposed by Mamyshev acts as a nonlinear optical thresholder which allows an efficient signal regeneration process for both “zero” and “one” symbols [20–24].

Meanwhile, nonlinear effects have also emerged as a possible way to all-optically control the light SOP. So far, Raman effect [25–27], stimulated Brillouin scattering [28–31], photorefractive two beam coupling [32] and four-wave mixing have been proved to be attractive solutions to control light SOP [33–35]. Among them, the last method seems to emerge as the most convincing technique for practical applications. Indeed, very recently, the first demonstration of an all-fibered polarization attractor for telecommunication applications was reported at 10 Gbit/s in a 20-km long standard optical fiber through the injection of a counterpropagating pump wave [36].

In this work, we experimentally demonstrate that it is possible, using a unique segment of single mode fiber, to all-optically regenerate the state of polarization of a 40-Gbit/s Return-to-Zero (RZ) telecommunication signal as well as its intensity profile and average power thanks to an easy-to-implement, all-fibered device. In other words, by combining three critical telecommunication functions, we report in this paper what we believe is the first demonstration of a new type of 3R regenerator, namely Repolarization, Reamplification and Reshaping.

2. Principle of operation

Our approach is schematically illustrated in Fig. 1 . Here, we consider an initial signal carrying Return-to-Zero optical data and exhibiting large intensity profile and polarization state degradations such as peak-to-peak fluctuations, presence of energy in the zero bit-slots and fast variations of the signal SOP. This initial signal is then injected into the SOP/Intensity regenerator box, which is basically made of a low PMD, normally dispersive, nonlinear optical fiber, a counterpropagating continuous pump wave and a bandpass optical filter.

 

Fig. 1 Principle of operation for the simultaneous polarization and intensity regeneration process.

Download Full Size | PDF

On the one hand, during propagation in the optical fiber, the polarization of the transmitted signal is step-by-step attracted and settled down to a unique output SOP without any PDL thanks to the FWM-based polarization attraction process induced by the counter-propagating pump wave [33–36]. More precisely, it was demonstrated that a low-PMD standard optical fiber can be considered as a concatenation of short isotropic elements for which a pump wave, injected in a contra-propagation way with a fixed SOP constitutes a polarization attractor or funnel of that system [35]. Indeed, in such a configuration, the FWM process induces a unidirectional exchange of energy between the two polarization components of the signal wave all along the fiber length. Consequently, the polarization of the signal asymptotically converges towards a fixed value at the fiber output, independently of its initial state, while all the polarization fluctuations are vanished from the system through the pump wave in such a way that the entire entropy of the system remains conserved [35, 36].

On the other hand, due to the strongly nonlinear regime of propagation involved in the polarization attractor, the initially degraded RZ signal experiences large self-phase modulation, leading to a wide intensity-dependent spectral broadening. Combination with the normal chromatic dispersion of the fiber also affects the temporal intensity profile, leading to a broadening and reshaping of the pulses that should make them completely unsuitable for further propagation. However, these two intrinsic seemingly degradations can be tuned in assets by inserting an optical bandpass filter (OBPF) at the output of the fiber. Indeed, carving into the expanded spectrum enables then to recover clean temporal and spectral profiles corresponding in first approximation to the properties of the filter under use. Additional benefits can be gained if the central frequency of this filter is shifted compared to the signal frequency, leading to the so-called Mamyshev regenerator [20]. Indeed only the information carried by the ones level will achieve a sufficient spectral broadening to get through the OBPF. Conversely, small amounts of energy contained in the zero bit slot will be annihilated, leading to an efficient regeneration of the “zeros” [20–24]. Moreover, if the regenerator is suitably designed, one can also obtain a saturation of the output peak power beyond a certain threshold leading to a decrease of peak-to-peak fluctuations and consequently to a complete regeneration of the RZ intensity profile [22].

At this point, it is important to note that the combination in a single segment of fiber of the polarization attraction and temporal reshaping is not as straightforward as it could appear. Indeed, the two independent functions are linked by the cumulative nonlinearity undergone by the optical signal during its propagation. In other words, the combination of these two functions in a same unique segment of fiber needs a drastic choice in the fiber parameters so as to carefully manage nonlinearity, chromatic dispersion and polarization evolution.

3. Experimental set-up

The simultaneous regeneration process of both SOP and intensity profile of a 40-Gbit/s RZ signal was experimentally achieved thanks to the set-up depicted in Fig. 2 .

 

Fig. 2 Experimental set-up.

Download Full Size | PDF

The 40-Gbit/s RZ signal is generated by means of a 10-GHz mode-locked fiber laser delivering 7.5-ps asymmetric pulses at 1564 nm and intensity modulated thanks to a LiNbO3 modulator through a 2³¹-1 pseudo-random bit sequence (Modbox Photline technologies). The bias of the modulator is voluntary detuned in order to degrade the extinction ratio of the optical data and to simulate ghost pulses into the blank level. A 2-stage bit-rate multiplier is then used to generate the initial RZ 40-Gbit/s bit stream. Finally, a second intensity modulator, driven by a 100-MHz sinusoidal RF signal, is inserted so as to simulate the presence of noise on the mark level. A polarization scrambler was then used to introduce wide polarization fluctuations at a rate of 0.65 kHz. Before injection into the optical fiber, the 40-Gbit/s signal is finally amplified by means of an Erbium doped fiber amplifier at an average power of 26.5 dBm. The optical fiber which acts as the nonlinear Kerr medium in the polarization attraction process and Mamyshev regenerator is a 6.2-km long Non-Zero Dispersion-Shifted Fiber characterized by a chromatic dispersion D = −1.5 ps/nm/km at 1550 nm, a Kerr coefficient of 1.7 W⁻¹.km⁻¹ and a polarization mode dispersion of 0.05 ps/km^1/2. The choice of this fiber has been motivated by the design scaling rules proposed in [22] taking into account the 7.5 ps initial pulse duration. Note that taking advantage of a multiple segment architecture such as proposed in [37], a combination of highly nonlinear fibers and highly dispersive normal fibers could also have been used to lower the power level involved in the device.

Two optical circulators were inserted at both ends of the fiber so as to inject and collect the pump and signal waves. At the opposite end of the fiber, the counter-propagating pump beam, which acts as the polarization attractor wave, consists of a 1-W polarized incoherent wave having a fixed arbitrary SOP, a spectral linewidth of 100-GHz and a central wavelength of 1545 nm. After propagation, in order to regenerate the intensity profile of the 40-Gbit/s data, the resulting broadened spectrum is then partially sliced thanks to a 50-GHz Gaussian shape OBPF shifted to 260 GHz from the initial signal central frequency. Finally, at the receiver, in order to validate the polarization attraction efficiency into the time domain, a polarizer is inserted to simulate the presence of a polarization dependent component and to translate the polarization fluctuations into intensity fluctuations. Behind the polarizer, the 40-Gbit/s eye diagram is monitored thanks to an optical sampling oscilloscope (OSO EXFO picosolve) while the data are detected by means of a 70-GHz photodiode and electrically demultiplexed at 10 Gbit/s before assessment of the bit-error-rate (BER). The 40-Gbit/s signal SOP was also analyzed using the usual Stokes vectors formalism and is monitored onto the Poincaré sphere by means of a commercially available polarization analyzer.

4. Experimental results

The intensity regeneration capability was first characterized by measuring the transfer function of the device.

Figure 3a shows the experimental transfer function relating the output peak-power of the 40-Gbit/s optical pulses as a function of their input. A significant extinction of low input powers can be observed, leading to an efficient regeneration of the blank levels. On the other hand, beyond 2.5 W and thanks to a careful choice of fiber parameters, a clear plateau area is achieved, which could provide a significant reduction of peak-to-peak fluctuations. Based on these measurements, the optimum working peak-power was then fixed to 2.7 W, corresponding to an average power of 26.5 dBm at 40 Gbit/s. The transfer function of the polarization regenerator was also experimentally measured by evaluating the degree of polarization (DOP) as a function of pump power. DOP is classically defined as $DOP=\sqrt{{〈S_1〉}^2+{〈S_2〉}^2+{〈S_3〉}^2}/S_0$where S_i are the Stokes parameters of the signal wave and < > denotes an averaging over 256 runs of input polarizations. As can be seen in Fig. 3b, the DOP of the signal wave, which is initially at a low level due to its initial scrambling, strongly increases when the counter-propagating pump power is injected into the fiber so as to reach asymptotically a plateau close to unity for pump power above 800 mW. Based on these results, a pump power of 1 W was chosen to ensure a maximum efficiency of the polarization attraction process.

 

Fig. 3 (a) Experimental transfer function of the regenerator: Output peak power as a function of input peak power. (b) Experimental transfer function of the polarization regenerator: Degree of polarization (DOP) as a function of pump power.

Download Full Size | PDF

The performances of our polarization and intensity regenerator were then quantified by means of the eye-diagram visualization, SOP monitoring and BER measurements. At the input of the device, the eye-diagram of the initial 0-dBm 40-Gbit/s signal (Fig. 4a ) shows a non negligible amount of degradations including large intensity fluctuations, asymmetric pulse profile and presence of ghost pulses in the zero bit slots. In a second time, and due to the polarization scrambler, the initial SOP was uniformly spread onto the whole Poincaré sphere (Inset of Fig. 4b), leading to a complete closure of the eye-diagram and loss of the information if a polarization dependant component is inserted in reception, here simulated by the presence of a polarizer (Fig. 4b).

 

Fig. 4 (a) Initial eye diagram of the intensity degraded 40-Gbit/s signal, inset shows the state of polarization (b) Eye diagram behind a polarizer and SOP of the intensity degraded 40-Gbit/s signal after polarization scrambling. All the polarization fluctuations are transformed into intensity fluctuations through the polarizer.

Download Full Size | PDF

At the output of the system, when the 40-Gbit/s signal is injected into the 6.2-km long NZ-DSF with an average power of 26.5 dBm and simultaneously counter-propagates with the 1-W polarization control continuous pump wave, we now clearly observe the regeneration of both SOP and intensity profile (Fig. 5 ). As can be seen, the polarization attraction process leads to the convergence of all polarization states towards a small area on the Poincaré sphere (Fig. 5a), indicating an efficient stabilization of the 40-Gbit/s signal SOP. After offset filtering, the 15-dBm output eye-diagram, detected behind the polarizer (Fig. 5a) demonstrates an excellent reshaping of the data as it could have been anticipated from the flat transfer function. Indeed, intensity fluctuations on the ones level as well as ghost pulses in the zero bit slots are significantly annihilated. Moreover, compared to the initial degraded signal in Fig. 4a, the pulse profile is now much more symmetric, confirming the efficiency of the intensity regeneration process. But more strikingly, compared to Fig. 4b and despite the initial scrambling process, the output eye-diagram remains completely opened behind the polarizer, indicating that the polarization attraction process operates in full strength. Let us note here that all the presented records have been obtained without any manual adjustment of the SOP of signal or pump waves.

 

Fig. 5 (a) Eye diagram (detected behind a polarizer) and SOP (inset) of the output regenerated 40-Gbit/s signal (b) BER measurements for the four 10-Gbit/s channels demultiplexed from the 40-Gbit/s signal. Red curves: Initial 40 Gbit/s signal, Blue curves: after intensity degradation and polarization scrambling (the blue dashed-line corresponds to a detection behind a polarizer), Green curves: after regeneration, Black curves: after polarization and intensity regeneration and detected behind a polarizer.

Download Full Size | PDF

Finally, we have measured the bit-error-rate of the 40-Gbit/s bit stream as a function of the incoming power on the receiver for the four demultiplexed 10-Gbit/s channels (Fig. 5b). We can first observe that the intensity regeneration process (green curves) itself leads to around 3 dB of improvement on the receiver sensitivity compared to the initial degraded signal (blue curves). But more importantly, despite the initial dramatic drop of the BER to a threshold around 10⁻⁵ at the input of the system when detected beyond a polarizer and due to the polarization scrambling operation (blue dashed-line), the simultaneous polarization and intensity regeneration process allows to completely recover the initial non-degraded data, even slightly better (red curves), providing small penalties compared to the non-scrambled signal (green curves). These experimental results prove the great efficiency of our new Repolarization, Reshaping and Reamplification 3R regenerator.

5. Conclusion

Future transparent networks will undoubtedly involve direct and all-optical controls of light parameters such as intensity profile, power, phase, modes, wavelength and state of polarization [38–40]. Among all of these parameters, the state of polarization currently remains so far the most uncertain to control. In this paper, we report an all-optical device, which tackles this issue by simultaneously regenerating the state of polarization, intensity profile and average power of a 40-Gbit/s signal in a unique standard single mode optical fiber. These experimental results were achieved in the mid-infrared region and by means of a 40-Gbit/s Return-to-Zero telecommunication signal but could be transposed to any wavelength and signal type. In particular, higher bit-rates and other modulation formats could be employed, but providing a careful management of chromatic dispersion and nonlinear propagation regime occurring in the device, especially for phase modulation formats. Note however, that in this configuration, the proposed technique could not be compatible with polarization multiplexed formats and wavelength division multiplexing signals. Further developments are thus required in order to foresee a trial implementation. The optimum average power could be also decreased by a suitable design of fibers with enhanced nonlinearity such as highly nonlinear silica fiber, photonic crystal fiber or strongly nonlinear glass fiber, tellurite, bismuth or chalcogenide fibers [41–43]. On the other hand, since the concept of polarization attraction is based on a general FWM counter-propagating interaction, the device could also be extended to other physical parameters providing an equivalent system of equations for instance attraction between two spatial propagation modes or wavelengths [33, 44]. The device could include another function such as Raman amplification [45] (also compatible with Mamyshev regenerator [46]), phase regeneration and/or pulse-to-pulse retiming [23, 47]. Finally, based on these observations, we believe that this new type of 3R device, combining three critical telecommunication functions (Repolarization, Reshaping, Reamplification) could open up the path to an additional stage in all-optical ultrafast signal processing and could become an important building block of future exciting applications in photonics.