M. Belmonte

Measurement of superluminal optical tunneling times in double-barrier photonic band gaps

Tunneling of optical pulses at 1.5 microm wavelength through double-barrier periodic fiber Bragg gratings is experimentally investigated in this paper. Tunneling time measurements as a function of the barrier distance show that, far from resonances of the structure, the transit time is paradoxically short--implying superluminal propagation--and almost independent of the barrier distance. This result is in agreement with theoretical predictions based on phase-time analysis and provides, in the optical context, an experimental evidence of the analogous phenomenon in quantum mechanics of nonresonant superluminal tunneling of particles across two successive potential barriers.

40GHz pulse train generation at 1.5µm by using a chirped fiber grating as frequency multiplier

Pulse-train multiplication based on the temporal Talbot effect in a linearly chirped fiber Bragg grating has been experimentally demonstrated. A 40-GHz repetition-rate, nearly transform-limited 10-ps duration optical pulse train at 1.533 mum has been obtained from a 2.5-GHz mode-locked Er- Yb:glass laser by use of a 100-cm-long linearly chirped apodized fiber grating.

Very low voltage single drive domain inverted LiNbO 3 integrated electro-optic modulator

Domain inversion is used in a simple fashion to improve significantly the performance of a waveguide electro-optic modulator in z-cut LiNbO(3). The waveguide arms of the Mach-Zehnder interferometer are placed in opposite domain-oriented regions under the same, narrower and more efficient electrode, so that opposite phase shifts (push-pull effect) can still be achieved despite the arms being subjected to the same electric field. Switching voltages close to 2 V are obtained, which allow 10Gb/s modulation with inexpensive drivers, such as those used for electro-absorption modulators, which deliver driving voltages well below 3V.

Transoceanic PM-QPSK Terabit superchannel transmission experiments at Baud-rate subcarrier spacing

We show experimental results on the transmission of a Terabit superchannel, consisting of 10×120-Gb/s PM-QPSK densely-packed subcarriers. We reached 10000km with 1.1×Baud-rate subcarrier spacing and 9000km with Baud-rate subcarrier spacing. We also reached 8000km with 3 superchannels at 1.1 spacing.

Propagation, manipulation, and control of picosecond optical pulses at 1.5 µm in fiber Bragg gratings

We review the main techniques and the most recent experimental achievements obtained by our group on manipulation and control of picosecond optical pulse trains at 1.5-µm based on fiber Bragg grating (FBG) devices of interest in ultrafast optoelectronics. In particular, all-optical techniques for repetition-rate multiplication of picosecond mode-locked pulse trains that use linearly chirped FBGs and pulse-shaping techniques based on Fourier spectral filtering in a novel class of dispersive FBGs are presented. Theoretical and experimental results on the control of the light speed in FBG structures, including superluminal transmission and reflection of picosecond pulses, are also discussed.

All-optical square-pulse generation and multiplication at 1.5 μm by use of a novel class of fiber Bragg gratings

A new class of linearly chirped fiber Bragg gratings, suitable for all-fiber optical multiplication and reshaping of picosecond pulse trains, is proposed and experimentally demonstrated. Repetition-rate multiplication, based on the temporal Talbot effect, is achieved by accurate control of the grating chirp, and a suitable design of the index-modulation depth profile allows for simultaneous square-pulse shaping. A 50-ps squarelike optical pulse train at 10-GHz repetition rate is obtained, starting from a 2.5-GHz Gaussian pulse train emitted by a mode-locked Er-Yb laser.

Superluminal pulse propagation in linear and nonlinear photonic grating structures

Optical pulse propagation in photonic grating structures can show anomalous (i.e., superluminal or negative) group velocities under certain circumstances owing to the anomalous dispersive properties induced by the periodic grating structure. Such phenomena can be observed for either linear pulse propagation in passive dielectric grating structures, such as in fiber Bragg gratings (FBGs), as well as in frequency-conversion processes exploiting second-order cascading effects in quasi-phase-matched (QPM) nonlinear crystals. Engineering of the grating structure can be exploited to observe a wide variety of anomalous pulse transmission and reflection behaviors. In this article, we review the main recent experimental and theoretical achievements obtained by our group in this field. In particular, we report on superluminal propagation of picosecond optical pulses at the 1.5-/spl mu/m wavelength of optical communications in FBGs, both in transmission and reflection configurations, with the observation of group velocities as large as /spl sim/5c/sub 0/. We also show that the phenomenon of transparent pulse propagation at a negative group velocity in a gain doublet atomic amplifier, recently observed in cesium vapor by Wang and co-workers (L. J. Wang, A. Kuzmich, and A. Dogariu, Nature vol.406, p.277-9, 2000), can be achieved as well in a photonic parametric amplifier by exploiting the anomalous dispersive properties of the amplifier induced by a suitably designed QPM grating profile.

Waveguide electro-optic modulation in micro-engineered LiNbO3

In this paper, after describing the basics of LiNbO3 based integrated electro-optic modulators, we will show how techniques such as etching, domain inversion and thin film processing can be used to realize new geometries which can take the performance to unprecedented levels. In particular we will review recent results on the use of domain inversion on a micron scale to improve the electro-optic response of LiNbO3 waveguide modulators in terms of bandwidth and driving voltage. These applications of domain inversion techniques might be even more important and commercially valuable than those in nonlinear optics (e.g. quasi-phase-matched optical parametric devices). With respect to standard single-domain structures, larger bandwidths and lower driving voltages can be obtained, thus achieving figure of merits for the electro-optic response that are up to 50% larger. As a demonstration, a chirp-free modulator, having ∼2 V switching voltage and bandwidth of 15 GHz, was fabricated by placing the waveguide arms of a Mach–Zehnder interferometer in opposite domain oriented regions. The modulator, as indicated by system measurements, could be driven in a single-drive configuration with inexpensive low-voltage drivers, e.g. SiGe based, typically used for electro-absorption devices.

Comb-locked Lamb-dip spectrometer

Overcoming the Doppler broadening limit is a cornerstone of precision spectroscopy. Nevertheless, the achievement of a Doppler-free regime is severely hampered by the need of high field intensities to saturate absorption transitions and of a high signal-to-noise ratio to detect tiny Lamb-dip features. Here we present a novel comb-assisted spectrometer ensuring over a broad range from 1.5 to 1.63 μm intra-cavity field enhancement up to 1.5 kW/cm2, which is suitable for saturation of transitions with extremely weak electric dipole moments. Referencing to an optical frequency comb allows the spectrometer to operate with kHz-level frequency accuracy, while an extremely tight locking of the probe laser to the enhancement cavity enables a 10−11 cm−1 absorption sensitivity to be reached over 200 s in a purely dc direct-detection-mode at the cavity output. The particularly simple and robust detection and operating scheme, together with the wide tunability available, makes the system suitable to explore thousands of lines of several molecules never observed so far in a Doppler-free regime. As a demonstration, Lamb-dip spectroscopy is performed on the P(15) line of the 01120-00000 band of acetylene, featuring a line-strength below 10−23 cm/mol and an Einstein coefficient of 5 mHz, among the weakest ever observed.

Tailoring the Electrooptic Response and Improving the Performance of Integrated

Domain inversion (DI) is applied to traveling-wave integrated electrooptic (EO) LiNbO3 modulators to achieve predefined EO frequency response and improved performance with respect to standard single-domain (not domain-inverted) counterparts, including lower driving voltages and larger modulation bandwidths. In particular, when an appropriate poling (DI) is performed along the interaction length, two novel configurations are envisaged. 1) An apodised longitudinal poling profile is used in non-velocity-matched (NVM) modulators to achieve a desired EO response, such as amplitude modulation with intrinsic third-order Bessel-type optical filtering for duobinary transmission. 2) Two- section longitudinal poling is used to counteract the detrimental microwave loss effect in VM modulators and to increase the bandwidth to driving voltage ratio, achieving a value up to 50% larger than that of single-domain (unpoled) structure.