Hybrid squeezing of solitonic resonant radiation in photonic crystal fibers

We report the existence of a kind of squeezing in photonic crystal fibers which is conceptually intermediate between four-wave-mixing-induced squeezing in which all the participant waves are monochromatic waves, and self-phase-modulation-induced squeezing for a single pulse in a coherent state. This hybrid squeezing occurs when an arbitrary short soliton emits quasimonochromatic resonant radiation near a zero-group-velocity-dispersion point of the fiber. Photons around the resonant frequency become strongly correlated due to the presence of the classical soliton, and a reduction of the quantum noise below the shot-noise level is predicted. © 2011 American Physical Society.

Full-vectorial modeling of femtosecond pulses for laser inscription of photonic structures

During the last decade, microfabrication of photonic devices by means of intense femtosecond (fs) laser pulses has emerged as a novel technology. A common requirement for the production of these devices is that the refractive index modification pitch size should be smaller than the inscribing wavelength. This can be achieved by making use of the nonlinear propagation of intense fs laser pulses. Nonlinear propagation of intense fs laser pulses is an extremely complicated phenomenon featuring complex multiscale spatiotemporal dynamics of the laser pulses. We have utilized a principal approach based on finite difference time domain (FDTD) modeling of the full set of Maxwell's equations coupled to the conventional Drude model for generated plasma. Nonlinear effects are included, such as self-phase modulation and multiphoton absorption. Such an approach resolves most problems related to the inscription of subwavelength structures, when the paraxial approximation is not applicable to correctly describe the creation of and scattering on the structures. In a representative simulation of the inscription process, the signature of degenerate four wave mixing has been found. © 2012 Optical Society of America.

Polarization multiplexed 16QAM transmission employing modified digital back-propagation

We experimentally demonstrate performance enhancements enabled by weighted digital back propagation method for 28 Gbaud PM-16QAM transmission systems, over a 250 km ultra-large area fibre, using only one back-propagation step for the entire link, enabling up to 3 dB improvement in power tolerance with respect to linear compensation only. We observe that this is roughly the same improvement that can be obtained with the conventional, computationally heavy, non-weighted digital back propagation compensation with one step per span. As a further benchmark, we analyze performance improvement as a function of number of steps, and show that the performance improvement saturates at approximately 20 steps per span, at which a 5 dB improvement in power tolerance is obtained with respect to linear compensation only. Furthermore, we show that coarse-step self-phase modulation compensation is inefficient in wavelength division multiplexed transmission.

Nonlinear pulse distortion in few-mode fiber

Nonlinear pulse propagation in a few mode fiber is experimentally investigated, by measuring temporal and phase responses of the output pulses by use of a frequency discriminator technique, showing that self-phase modulation, dispersion and linear mode-coupling are the dominant effects.

Virtual Gires-Tournois etalons realized with phase-modulated wideband chirped fiber gratings

We propose and demonstrate novel virtual Gires–Tournois (GT) etalons based on fiber gratings. By introducing an additional phase modulation in wideband linearly chirped fiber Bragg gratings, we have successfully generated GT resonance with only one grating. This technique can simplify the fabrication procedure while retaining the normal advantages of distributed etalons, including their full compatibility with optical fiber, low insertion loss, and low cost. Such etalons can be used as dispersion compensation devices in optical transmission systems.

Virtual Gires-Tournois etalons realized with phase-modulated wideband chirped fiber gratings

We propose and demonstrate novel virtual Gires-Tournois (GT) etalons based on fiber gratings. By introducing an additional phase modulation in wideband linearly chirped fiber Bragg gratings, we have successfully generated GT resonance with only one grating. This technique can simplify the fabrication procedure while retaining the normal advantages of distributed etalons, including their full compatibility with optical fiber, low insertion loss, and low cost. Such etalons can be used as dispersion compensation devices in optical transmission systems. © 2007 Optical Society of America.

Evaluating the Capacity of Phase Modulator-Controlled Long-Haul Soliton Transmission

We review recent developments in the use of optical solitons for communication systems spanning transoceanic distances. The implementation of "soliton control" to alleviate the detrimental impact of effects such as amplifier noise is shown to be critical for obtaining advantages over competing technologies. The potential performance of two control strategies, namely straight line filtering and synchronous phase modulation, is examined in detail. Design diagrams are used to determine the maximum permissible amplifier spacing, which is a key determinant of system economics. To focus the enquiry, two example system spans are taken, representing transatlantic and transpacific distances. It is concluded that straight line filtering provides very little improvement over a basic design without control. However synchronous phase modulation, which may be implemented using a handful of actively driven components, provides very substantial benefits. These may be used either to extend the overall bit-rate-distance product of the system or to increase the amplifier spacing at more moderate capacities. © 1995 Academic Press. All rights reserved.

Bandpass switching in a nonlinear optical loop mirror

A novel device configuration is used to demonstrate wavelength-confined, a bandpass, switching in a nonlinear-optical loop mirror (WOLM). Demonstrated is a self-switching in the soliton regime using a partially reflecting Bragg grating as a wavelength-dependent loss element. Two wavelength operation in which a signal is switched through the use of cross phase modulation, are demonstrated. Observed is the operation of the device confined to wavelengths defined by the grating reflection band.

Injection locking-based pump recovery for phase-sensitive amplified links

An injection locking-based pump recovery system for phase-sensitive amplified links, capable of handling 40 dB effective span loss, is demonstrated. Measurements with 10 GBd DQPSK signals show penalty-free recovery of a pump wave, phase modulated with two sinusoidal RF-tones at 0.1 GHz and 0.3 GHz, with 64 dB amplification. The operating power limit for the pump recovery system is experimentally investigated and is governed by the noise transfer and phase modulation transfer characteristics of the injection-locked laser. The corresponding link penalties are explained and quantified. This system enables, for the first time, WDM compatible phase-sensitive amplified links over significant lengths. © 2013 Optical Society of America.

Study of EEPN mitigation using modified RF pilot and Viterbi-Viterbi based phase noise compensation

We propose - as a modification of the optical (RF) pilot scheme -a balanced phase modulation between two polarizations of the optical signal in order to generate correlated equalization enhanced phase noise (EEPN) contributions in the two polarizations. The method is applicable for n-level PSK system. The EEPN can be compensated, the carrier phase extracted and the nPSK signal regenerated by complex conjugation and multiplication in the receiver. The method is tested by system simulations in a single channel QPSK system at 56 Gb/s system rate. It is found that the conjugation and multiplication scheme in the Rx can mitigate the EEPN to within 1/2 orders of magnitude. Results are compared to using the Viterbi-Viterbi algorithm to mitigate the EEPN. The latter method improves the sensitivity more than two orders of magnitude. Important novel insight into the statistical properties of EEPN is identified and discussed in the paper. © 2013 Optical Society of America.

Linear dispersive pre-defined peak amplitude modulation of spectrally modulated airy-based pulses

Spectrally modulated Airy-based pulses peak amplitude modulation (PAM) in linear dispersive media is investigated, designed, and numerically simulated. As it is shown here, it is possible to design the spectral modulation of the initial Airy-based pulses to obtain a pre-defined PAM profile as the pulse propagates. Although optical pulses self-amplitude modulation is a well-known effect under non-linear propagation, the designed Airy-based pulses exhibit PAM under linear dispersive propagation. This extraordinary linear propagation property can be applied in many kinds of dispersive media, enabling its use in a broad range of experiments and applications. © 2013 Optical Society of America.

Soliton generation from randomly modulated return-to-zero pulses

We consider return-to-zero (RZ) pulses with random phase modulation propagating in a nonlinear channel (modelled by the integrable nonlinear Schrödinger equation, NLSE). We suggest two different models for the phase fluctuations of the optical field: (i) Gaussian short-correlated fluctuations and (ii) generalized telegraph process. Using the rectangular-shaped pulse form we demonstrate that the presence of phase fluctuations of both types strongly influences the number of solitons generated in the channel. It is also shown that increasing the correlation time for the random phase fluctuations affects the coherent content of a pulse in a non-trivial way. The result obtained has potential consequences for all-optical processing and design of optical decision elements.

Evaluating the capacity of long haul soliton transmission

We review recent developments in the use of optical solitons for communication systems spanning transoceanic distances. The implementation of "soliton control" to alleviate the detrimental impact of effects such as amplifier noise is shown to be critical for obtaining advantages over competing technologies. The potential performance of two control strategies, namely straight line filtering and synchronous phase modulation, is examined in detail. Design diagrams are used to determine the maximum permissible amplifier spacing, which is a key determinant of system economics. To focus the enquiry, two example system spans are taken, representing transatlantic and transpacific distances. It is concluded that straight line filtering provides very little improvement over a basic design without control. However synchronous phase modulation, which may be implemented using a handful of actively driven components, provides very substantial benefits. These may be used either to extend the overall bit-rate-distance product of the system or to increase the amplifier spacing at more moderate capacities.

Optical frequency conversion, pulse compression and signal copying using triangular pulses

We propose techniques of optical frequency conversion, pulse compression and signal copying based on a combination of cross-phase modulation using triangular pump pulses and subsequent propagation in a dispersive medium.

Ultra-long haul optical fibre transmission systems

This thesis examines experimentally options for optical fibre transmission over oceanic distances. Its format follows the chronological evolution of ultra-long haul optical systems, commencing with opto-electronic regenerators as repeaters, progressing to optically amplified NRZ systems and finally solitonic propagation. In each case recirculating loop techniques are deployed to simplify the transmission experiments. Advances in high speed electronics have allowed regenerators operating at 10 Gbit/s to become a practical reality. By augmenting such devices with optical amplifiers it is possible to greatly enhance the repeater spacing. Work detailed in this thesis has culminated in the propagation of 10 Gbit/s data over 400,000 km with a repeater spacing of 160 km. System reliability and robustness are enhanced by the use of a directly modulated DFB laser transmitter and total insensitivity of the system to the signal state of polarisation. Optically amplified ultra-long haul NRZ systems have taken on particular importance with the impending deployment of TAT 12/13 and TPC 5. The performance of these systems is demonstrated to be primarily limited by analogue impairments such as the accumulation of amplifier noise, polarisation effects and optical non-linearities. These degradations may be reduced by the use of appropriate dispersion maps and by scrambling the transmitted state of signal polarisation. A novel high speed optically passive polarisation scrambler is detailed for the first time. At bit rates in excess of 10 Gbit/s it is shown that these systems are severely limited and do not offer the advantages that might be expected over regenerated links. Propagation using solitons as the data bits appears particularly attractive since the dispersive and non-linear effects of the fibre allow distortion free transmission. However, the generation of pure solitons is difficult but must be achieved if the uncontrolled transmission distance is to be maximised. This thesis presents a new technique for the stabilisation of an erbium fibre ring laser that has aUowed propagation of 2.5 Gbit/s solitons to the theoretical limit of ~ 18,000 km. At higher bit rates temporal jitter becomes a significant impairment and to aUow an increase in the aggregate line rate multiplexing in both time and polarisation domains has been proposed. These techniques are shown to be of only limited benefit in practical systems and ultimately some form of soliton transmission control is required. The thesis demonstrates synchronous retiming by amplitude modulation that has allowed 20 Gbit/s data to propagate 125,000 km error free with an amplifier spacing approaching the soliton period. Ultimately the speed of operation of such systems is limited by the electronics used and, thus, a new form of soliton control is demonstrated using all optical techniques to achieve synchronous phase modulation.