In coherent optical transmission systems using conventional single mode fibers, the received state of polarization (SOP) is unknown and changing [1]. Polarization scrambling of the transmitted signal [2] or the local oscillator [3] overcomes signal fading by forcing a signal to undergo very large polarization changes during each bit cycle. The power penalty is ideally 3 dB because on average the polarization match between signal and local oscillator is only one-half. If the transmitted signal is scrambled, only one scrambler is needed even if several subscribing receivers are used. One reported scrambler [2] has an integrated optical phase shifter as the active element and switches a certain class of input SOPs alternately between themselves and their orthogonal states. This particular scheme suffers from two notable deficiencies. First, some input SOPs are not scrambled at all. Second, a receiver bandwidth of many times the scrambling frequency is required to capture all spectral components produced by the scrambler, which amounts to an additional sensitivity penalty in a practical system.
In coherent optical transmission systems using conventional single mode fibers, the received state of polarization (SOP) is unknown and changing [1]. Polarization scrambling of the transmitted signal [2] or the local oscillator [3] overcomes signal fading by forcing a signal to undergo very large polarization changes during each bit cycle. The power penalty is ideally 3 dB because on average the polarization match between signal and local oscillator is only one-half. If the transmitted signal is scrambled, only one scrambler is needed even if several subscribing receivers are used. One reported scrambler [2] has an integrated optical phase shifter as the active element and switches a certain class of input SOPs alternately between themselves and their orthogonal states. This particular scheme suffers from two notable deficiencies. First, some input SOPs are not scrambled at all. Second, a receiver bandwidth of many times the scrambling frequency is required to capture all spectral components produced by the scrambler, which amounts to an additional sensitivity penalty in a practical system.
Optical-fiber-based polarization scramblers can reduce the impact of polarization sensitive performance of various optical fiber systems. Here, we propose a simple and efficient polarization scrambler based on an all-optical Mach-Zehnder structure by combining a polarization beam splitter and an amplified fiber ring. To totally decoherence one polarization split beam, a fiber ring together with an amplifier is incorporated. The ratio of two orthogonal beams can be controlled by varying the amplification factor, and we observe different evolution trajectories of the output state of polarizations on the Poincaré sphere. When the amplification factor exceeds a certain threshold, the scrambler system exhibits nearly ideal polarization scrambling behavior. A commercial single wavelength laser with a linewidth of 3 MHz is utilized to characterize the scrambling performance. We found that when the sampling rate is 1.6 MSa/s, a scrambling speed up to 2000krad/s can be obtained for the average degree of polarization being less than 0.1. We also exploit these random polarization fluctuations to generate random binary numbers, indicating that the proposed technique is a good candidate for a random bit generator.
We propose and analyze a novel all-optical fiber polarization scrambler based on the transfer (via the Kerr effect) of the intensity fluctuations of an incoherent pump beam into polarization fluctuations of a frequency-shifted signal beam, copropagating in a randomly birefringent telecom fiber. Optimal signal polarization scrambling results whenever the input signal and pump beams have nearly orthogonal states of polarization. The nonlinear polarization scrambler may operate on either cw or high-bit-rate pulsed signals.
In recent years, the dissemination of optical frequency signals along phase-stabilized fibers has allowed the comparison of optical clocks at unprecedented levels of accuracy. This has a strong relevance in frequency metrology and boosted the measurement capabilities also in other research fields such as radioastronomy [1], geodesy [2] and fundamental physics [3]. Fiber-based frequency dissemination relies on the transmission of a continuous-wave, narrow-linewidth laser whose frequency is accurately measured with an atomic clock at the National Metrology Institute (NMI) where it is generated. Specific techniques are used to compensate for the acoustic and seismic noise and temperature variations, which degrade the phase stability of the transmitted signal. However, in existing implementations, it is not possible to detect and compensate for the birefringence variations of the optical fiber. These can induce variations in the state of polarization (SOP) of the transmitted signal, affecting the optical beatnotes at the receiving end. This not only degrades the achievable performances in terms of phase stability of the metrological signal, but can also lead to beatnote fading, with consequent failure of the entire system. This problem currently represents one of the strongest limitations to the reliability of fiber-based frequency dissemination.
Summary form only given. Due to a growing demand of security in transmission systems, recent research have been focus on the ability to manipulate a light beam in such a way to hide, namely to cloak, an event over a finite time or localization in space. The main idea is to create a hole or a gap in the spatial or time domain so as to allow for an object or a data to be kept hidden for a while and then to be restored [1-2]. Here we experimentally demonstrate a new type of temporal cloaking based on the ability for an arbitrary polarized light beam to self-organize and trap its state of polarization (SOP) in an optical fiber and then, to self-scramble in a reverse manner. We analyze the performance of our system by alternatively spying and then cloaking a 10Gbit/s signal propagating in an optical fiber and threaten by an attempt of hacking though an optical parametric based copying process. We successfully show that a quasi-annihilation of the probing operation (cloaking mode) or alternatively an error-free copying process (spying mode) can be achieved, both modes being independent from the incident SOP. The principle is illustrated in Fig. 1a. After travelling through a transmission system, an incident signal reaches the cloaking/spying zone with an arbitrary SOP. At this point, a continuous wave (cw) probe is then used to attempt a copy of the transmitted signal through a fiber optical parametric amplifier (FOPA). Since parametric process in optical fiber is highly polarization dependent, the conversion efficiency towards the copy wave (i.e. the idler) is strongly dependent on the incident signal SOP, which in usual situation where SOP randomly fluctuates, does not allow any kind of stable processing. Nevertheless, a successful cloaking or spying operation could be performed if one can trap the incident signal SOP orthogonal or parallel to the probe state. Here, this operation of polarization trapping is realized by means of an all-optical Omnipolarizer (OP) [3] in which self-organization of light SOP is spontaneously achieved by means of a counter-propagating four-wave mixing between the signal beam and its own backward replica generated through a reflective-loop [3]. As a consequence, in the cloaking zone, if the OP traps the signal SOP orthogonally to the probe state, no idler is generated and the signal remains hidden. At the opposite, if the OP locks the incident signal SOP parallel to the probe state, a maximal idler is then generated and the signal is kept hacked independently of its input SOP. Finally, to keep a transparent operation for the user, polarization randomness of the signal is restored thanks to a second Omnipolarizer, in which a high power backward replica induces the self-polarization process to become chaotic, leading to the unexpected behavior of the device to play the role of all-optical scrambler.Γ
In this paper, a security enhanced physical layer encryption scheme is proposed for coherent optical polarization division multiplexing (PDM) systems. The concept of a digital optical polarization scrambler (DOPS) is introduced to apply high speed rotation of state of polarization (RSOP) to the transmitted signal, which enables encryption based on polarization perturbations and offers superior flexibility in polarization management. By utilizing different combinations of digital polarization device matrices and adjusting their key parameters, four encryption modes are designed. The proposed encryption scheme is successfully implemented in a PDM-QPSK system at the data rate of 32 Gbps. Experimental results demonstrate that authorized users can successfully decrypt the received signal, while the eavesdroppers cannot derive useful information with a bit error rate (BER) at approximately 0.5. To enhance system security, a 5-D chaotic system is introduced with superior properties of high sensitivity to initial values and improved uniform distribution, which guarantees the large entropy and further the system’s security. This scheme can effectively prevent against brute attacks with the expanded key space of 10 60 .
A physical-layer secure key generation and distribution (SKGD) scheme in classical fiber channel is proposed and demonstrated using a bidirectional wide-band polarization scrambler (WBPS), to alter rapidly and symmetrically the state of polarization (SOP) for the two counter-propagating optical signals in fiber. Due to the optical channel reciprocity, highly-consistent key bits can be extracted from the correlated SOP fluctuations, while the key secrecy is ensured by the fiber channel uniqueness. A final key generation rate (KGR) of 2.7 Gb/s is demonstrated over 10 km single mode fiber (SMF), where error-free and true randomness of the generated key are achieved after post-processing. The obtained KGR at Gb/s makes a further step towards the implementation of one-time-pad data encryption with perfect secrecy in fiber networks.