Evaluation of Pearson correlation coefficient and Parisi parameter of replica symmetry breaking in a hybrid electronically addressable random fiber laser.

The hybrid electronically addressable random (HEAR) laser is a novel type of random fiber laser that presents the remarkable property of selection of the fiber section with lasing emission. Here we present a joint analysis of the correlations between intensity fluctuations at distinct wavelengths and replica symmetry breaking (RSB) behavior of the HEAR laser. We introduce a modified Pearson coefficient that simultaneously comprises both the Parisi overlap parameter and standard Pearson correlation coefficient. Our results highlight the contrast between the correlations and presence or not of RSB phenomenon in the spontaneous emission behavior well below threshold, replica-symmetric ASE regime slightly below threshold, and RSB phase with random lasing emission above threshold. In particular, in the latter we find that the onset of RSB behavior is accompanied by a stochastic dynamics of the lasing modes, leading to competition for gain intertwined with correlation and anti-correlation between modes in this complex photonic phase.

Hybrid electronically addressable random fiber laser.

We report here a novel architecture for a random fiber laser exploiting the combination of a semiconductor optical amplifier (SOA) and an erbium doped fiber (EDF). The EDF was optically biased by a continuous wave pump laser, whereas the SOA was arranged in a fiber loop-mirror and driven by nanosecond duration current pulses. Laser pulses were obtained by synchronizing the SOA driver to the returning amplified Rayleigh back-scattered light from a selected short section of the EDF. By tuning the SOA pulse rate, random lasing was achieved by addressing selected meter-long sections of the 81-m long EDF, which was open-ended. Laser oscillation can be potentially obtained with SOA modulation frequencies from several kHz to the MHz regime. We discuss the mechanism leading to the hybrid random laser emission, connecting with phase sensitive optical time domain reflectometry and envision potential applications of this electronically addressable random laser.

Quantum random number generation enhanced by weak-coherent states interference.

We propose and demonstrate a technique for quantum random number generation based on the random population of the output spatial modes of a beam splitter when both inputs are simultaneously fed with indistinguishable weak coherent states. We simulate and experimentally validate the probability of generation of random bits as a function of the average photon number per input, and compare it to the traditional approach of a single weak coherent state transmitted through a beam-splitter, showing an improvement of up to 32%. The ensuing interference phenomenon reduces the probability of coincident counts between the detectors associated with bits 0 and 1, thus increasing the probability of occurrence of a valid output. A long bit string is assessed by a standard randomness test suite with good confidence. Our proposal can be easily implemented and opens attractive performance gains without a significant trade-off.

Few-photon heterodyne spectroscopy.

We perform a high-resolution Fourier transform spectroscopy of optical sources in the few-photon regime based on the phenomenon of two-photon interference in a beam splitter. From the heterodyne interferogram, between test and reference sources, it is possible to obtain the spectrum of the test source relative to that of the reference. The method proves to be a useful asset for spectral characterization of faint optical sources below the range covered by classical heterodyne beating techniques.

Investigation of bend loss in single mode fibers with ultra-high-resolution photon-counting optical time domain reflectometer.

An intensity peak associated with fiber bending could be detected thanks to the use of an ultra-high-resolution photon-counting optical time domain reflectometer (OTDR) setup. The peak intensity is shown to be dependent on the curvature radius and angular distance of the bend. To account for such peaks, we propose a model based on modal mismatching and coupling inside the bend region and show that the model is highly consistent with the acquired data. Combining the information of the bend peak and bend loss, and taking advantage of the high dependence of the peak value with the local modal field parameter, the technique could be employed as an optical fiber local-parameter characterization method.

Phase measurements on terahertz waves.

The change in phase of the free space terahertz (THz) electric field as a sample of material introduced into the THz beampath of a CW THz system is measured and used to calculate the index of refraction of materials at 250 GHz.

Stable single-photon interference in a 1 km fiber-optic Mach-Zehnder interferometer with continuous phase adjustment.

We experimentally demonstrate stable and user-adjustable single-photon interference in a 1 km long fiber-optic Mach-Zehnder interferometer, using an active phase control system with the feedback provided by a classical laser. We are able to continuously tune the single-photon phase difference between the interferometer arms using a phase modulator, which is synchronized with the gate window of the single-photon detectors. The phase control system employs a piezoelectric fiber stretcher to stabilize the phase drift in the interferometer. A single-photon net visibility of 0.97 is obtained, yielding future possibilities for experimental realizations of quantum repeaters in optical fibers and violation of Bell's inequalities using genuine energy-time entanglement.

Full polarization control for fiber optical quantum communication systems using polarization encoding.

A real-time polarization control system employing two non-orthogonal reference signals multiplexed in either time or wavelength with the data signal is presented. It is shown, theoretically and experimentally, that complete control of multiple polarization states can be attained employing polarization controllers in closed-loop configuration. Experimental results on the wavelength multiplexing setup show that negligible added penalties, corresponding to an average added optical Quantum Bit Error Rate of 0.044%, can be achieved with response times smaller than 10 ms, without significant introduction of noise counts in the quantum channel.

Low-pump-power, short-fiber copropagating dual-pumped (800 and 1050 nm) thulium-doped fiber amplifier.

Using optical frequency-domain reflectometry to reveal the gain distribution and allow us to optimize a thulium-doped fiber amplifier, we have demonstrated 18-dB gain by employing only 5 m of a 2000-parts-in-10(6)-Tm-doped fiber pumped with 145 mW of power at dual wavelengths of 800 and 1050 nm. The role of the 800-nm pump, which by itself does not permit population inversion, was clearly observed experimentally.

Asymmetric alignment-stabilized external-cavity semiconductor laser.

We have demonstrated an alignment-stabilized asymmetric cavity configuration for a grating-tuned externalcavity semiconductor laser. The grating in the asymmetric cavity has a 16-fold greater angular misalignment tolerance than in a conventional symmetric external-cavity semiconductor laser. The tuning ranges are nearly the same for both cavities (approximately 30 nm).

Phase measurement in frequency-doubling fibers.

Fibers were prepared for second-harmonic generation by injecting a frequency-doubled (seed) signal with the fundamental light from a Nd:YAG laser. The relative phase shift between the seed and the second-harmonic light generated by the fiber was measured to be close to 90 degrees ( approximately 99 +/- 9.2 degrees ). A water cell was used to sweep the relative phase of the waves with interferometric precision. Some physical consequences of a pi/2 phase shift are pointed out.

Polarization-mode interferometry in birefringent single-mode fibers.

An interferometric technique was used to investigate the relative phase delay between polarization modes in birefringent single-mode fibers. Polarization-mode dispersion is directly deduced from the measurements of relative phase delay at different wavelengths. Relative group delays of 20 fsec can be measured in meter-length samples without the need for light-polarizing devices.