P. Franco

Self-induced modulational-instability laser

Summary form only given. We report a novel scheme of passive modelocking in fiber lasers that, based on modulational instability, provides continuous trains of solitons with large duty cycle and rep rates up to 130 GHz. The basic difference with the standard modelocking technique is that in configuration we are describing continuous wave operation is not inhibited as for modelocking. The train of pulses therefore originates from the forcing action of the modulational instability sidebands that lock the cavity modes. The self-induced modulational instability laser can be achieved either by including in the cavity a polarization beam splitter, which, combined with nonlinear polarization, evolution provides a power dependent transmission, or even without it. In the latter case we show that the whole cavity acts as a nonlinear gain element.

Time-domain Fourier optics for polarization-mode dispersion compensation

We report on a novel technique to compensate for all-order polarization-mode dispersion. By means of this technique, based on a suitable combination of phase modulation and group-velocity dispersion, we compensated for as much as 60 ps of differential group delay that affected a 10-Gbit/s return-to-zero data stream.

Tunable erbium–ytterbium fiber sliding-frequency soliton laser

We characterize the soliton-train emission from an Er–Yb-doped fiber loop laser. We discuss the self-starting dynamics and pulse-repetition-rate control in this sliding-frequency soliton laser. We show that the laser truly self-starts after only one cavity round trip. In the steady state the laser emits a closely spaced train of solitons. We also show that the output pulse width may be controlled by the interplay of continuous frequency shifting, bandwidth-limited amplification, and nonlinear polarization rotation of the circulating solitons. The repetition rate is fixed by means of a weak intracavity feedback. The laser is tunable by shifting of the filter wavelength through the whole spectral band of the active fiber.

Field demonstration of 40 Gb/s soliton transmission with alternate polarizations

A 40-Gb/s single-channel soliton transmission was performed on a dispersion-shifted fiber (DSF) embedded in a link connecting Rome to Pomezia. The soliton system was based on the transmission of pulses with orthogonal states of polarization (alternate polarizations) to limit the impairments due to the soliton interaction. The performance of the system operating in the field has been compared with results obtained in the laboratory. While in the laboratory the system showed a nearly-stable error-free transmission over 700 km, strong temporal fluctuations of the performance have been observed in the field trial. Numerical simulations have shown that such a degradation, and in particular the fluctuation of the Q factor, is mainly due to a polarization mode dispersion of the installed cables larger than that of the fibers used in the laboratory. However, 500 km error-free transmission was observed in the field for time interval longer than one hour, demonstrating the potentiality of a 40 Gb/s soliton system with alternate polarizations operating with long amplifier spacing (100 km) without in-line control.

Characterization and optimization criteria for filterless erbium-doped fiber lasers

The emission wavelength of a unidirectional ring erbium-doped fiber laser (EDFL) with no filters has been characterized. A measurement of the emission and the absorption erbium-doped fiber cross sections together with a theoretical model based on the solution of the rate equations for the ring laser shows that the laser emission wavelength depends on the cold-cavity losses or on the active fiber length and dopant concentration. The threshold pump power, the output power, and the slope efficiency of the EDFL have been measured and theoretically evaluated versus cold-cavity loss and active fiber length. A simple relation among loss, active fiber length, and dopant concentration that permits the design of the cavity with the maximum slope efficiency has been found.

Role of dispersion in pulse emission from a sliding-frequency fiber laser

We present an experimental and theoretical investigation of the role of group-velocity dispersion in the generation of picosecond pulses from a sliding-frequency fiber loop laser.

Relaxation of guiding center solitons in optical fibers.

We present a perturbative study of the mutual coupling between solitons and dispersive waves in periodically amplified links. Our analysis describes the limits of soliton transmissions operating beyond the average soliton regime.

Wavelength division multiplexed soliton transmission with filtering

Abstract We present a theoretically study on the timing jitter induced by the interaction between solitons propagating in two wavelength separated channels of a long-haul fiber transmission system. We show that the inclusion of in-line filters almost suppresses the arrival time jitter even when the collision length is on the order of the amplifier spacing. Numerical results obtained by simulating a real N ×5 Gb/s wavelength division multiplexed system confirm the theoretical predictions. The benefit deriving from the inclusion of in-line filters is maximum for transmission distances on the order of 4000 km, whereas on the transoceanic distance scale (9000 km) the improvement is limited by the incomplete suppression of the soliton frequency shift occurring during collisions.

Maximum soliton-train duty cycle in harmonically mode-locked fiber lasers

We analyze the main methods for high-repetition-rate soliton generation in fiber lasers in view of the applications to fiber transmission. Particular emphasis is given to the problem of intracavity soliton interactions and stabilization methods. Because of the interactions we find that the maximum duty cycle achievable in a soliton source is 0.22. We also find that the relative phase difference between adjacent solitons may be controlled by means of an intracavity etalon, whereas without the etalon the solitons are stable only when they are out of phase. Moreover the introduction of the intracavity etalon has led to a strong reduction in the fluctuations of the soliton parameters induced by quantum noise.

Complete statistical characterization of signal and noise in optically amplified fiber channels

A very simple model is found for phase insensitive laser amplification. Signal and noise along the active fiber are modeled as marked Poisson processes (corresponding to flows of photon bunches of random size) and are dealt with as independent processes. A probabilistic approach, founded on the theory of birth-and-death processes, enables us to give a simple characterization of the amplifier in terms of the statistics of the random gain, of the time intensity of the ASE (amplified spontaneous emission) noise bunches, and finally of the statistics of their sizes. The theory is limited to the linear amplification range, while is valid also for nonhomogeneous inversion along the active fiber. The model can be easily applied for the evaluation of the statistics of the global gain and of the accumulated ASE noise in optically amplified links. >