J. P. von der Weid

Experimental and theoretical investigations of coherent OFDR with semiconductor laser sources

Experimental and theoretical investigations of coherent optical-frequency-domain reflectometry using semiconductor laser sources are presented. Good agreement was found between the analysis of the signal-to-noise ratio due to the phase noise and the experimental results. The sensitivity limit due to the quantum noise is also described. Limitations due to the nonlinearity in the optical frequency sweep produced by the thermal-response time of the laser and mode hopping are investigated and compared with experimental results. Two interferometric methods to characterize the thermal-response time of the laser and their implementations are described. The effects of mode hopping in the optical-frequency sweep are compared to numerical simulations. A simple formula to predict the position of spurious peaks due to mode hopping are presented. A spatial resolution of 400 /spl mu/m over 10 cm was obtained by correcting the nonlinearity in the optical-frequency sweep by using an auxiliary interferometer. The Rayleigh backscattering was observed for the first time over more than 400 m of fiber using a DFB laser coupled to an external cavity. >

On the characterization of optical fiber network components with optical frequency domain reflectometry

The basic features of coherent optical frequency domain reflectometry are presented. The ultimate limits of range and sensitivity were discussed, as well as polarization effects. It is shown that this technique is very suitable for optical network components characterization, and accurate measurements of a number of such components are presented.

Local birefringence measurements in single-mode fibers with coherent optical frequency-domain reflectometry

Measurements of intrinsic and induced birefringence of optical fibers are performed at 1550 nm using the optical frequency-domain reflectometry technique. The experiment confirms the theoretical analysis, which predicts the appearance of oscillations on the detected Rayleigh backscattering intensity, with periods equal to the polarization beat length L/sub b/ and to L/sub b//2. Polarization mode-coupling length values are obtained from local birefringence and polarization mode dispersion measurements.

Polarization mode dispersion of short and long single-mode fibers

Polarization mode dispersion (PMD) in short and long single-mode fibers was measured by a polarization-maintaining Michelson interferometer. A nonnegligible PMD was found in some standard fibers. The sensitivity enables PMD to measure the bend-induced PMD of a fiber rolled on a 28-cm diameter drum. A theoretical model for PMD with random mode coupling is developed, and an explicit equation for the time-of-flight distribution is presented. Comparison between measurements on short and long fibers with residual birefringence leads to an estimation of the coupling length on the order of 20-30 m. >

How accurately can one measure a statistical quantity like polarization-mode dispersion?

The differential group delay (DGD) due to varying local birefringence and random polarization mode coupling is a statistical quantity. Its value at any wavelength fluctuates in time. At any given moment it is wavelength sensitive. Hence, polarization-mode dispersion can only be defined as a mean or rms value of the DGD. All measurements of polarization-mode dispersion involve thus an averaging over a finite sample set. Consequently, all measurements are intrinsically of limited accuracy. We present analytical formulae and numerical data characterizing this intrinsic limitation common to all measurement techniques.

Proof-of-principle demonstration of measurement-device-independent quantum key distribution using polarization qubits

We perform a proof-of-principle demonstration of the measurement-device-independent quantum key distribution (MDI-QKD) protocol using weak coherent states and polarization-encoded qubits over two optical fiber links of 8.5 km each. Each link was independently stabilized against polarization drifts using a full-polarization control system employing two wavelength-multiplexed control channels. A linear-optics-based polarization Bell-state analyzer was built into the intermediate station, Charlie, which is connected to both Alice and Bob via the optical fiber links. Using decoy-states, a lower bound for the secret-key generation rate of 1.04x10^-6 bits/pulse is computed.

Experimental and theoretical modeling of polarization-mode dispersion in single-mode fibers

The random coupling-length basis was used in the numerical simulation of single-mode fibers (SMFs) with the coarse step method. We show that a single set of coupling lengths, angles and phases accounts for both, the Maxwellian statistics and the nonperiodical differential group delay spectral dependence in agreement with experimental observations. An SMF and a polarization mode dispersion (PR ID) emulator were both measured and simulated. The comparison between the experimental results and numerical simulations shows that the random coupling-lengths mathematical model and the emulator device provide good descriptions either for the first-order PMD statistics or second-order PMD, being powerful tools for the simulation of signal distortion.

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.

Experimental polarization encoded quantum key distribution over optical fibres with real-time continuous birefringence compensation

In this paper we demonstrate an active polarization drift compensation scheme for optical fibres employed in a quantum key distribution experiment with polarization encoded qubits. The quantum signals are wavelength multiplexed in one fibre along with two classical optical side channels that provide the control information for the polarization compensation scheme. This set-up allows us to continuously track any polarization change without the need to interrupt the key exchange. The results obtained show that fast polarization rotations of the order of 40??rad?s?1 are effectively compensated for. We demonstrate that our set-up allows continuous quantum key distribution even in a fibre stressed by random polarization fluctuations. Our results pave the way for Bell-state measurements using only linear optics with parties separated by long-distance optical fibres.

All-fiber interferometer for chromatic dispersion measurements

An all-fiber interferometric method for chromatic dispersion measurements in meter-length single-mode fibers is presented. In a Michelson setup the physical length of a reference fiber was varied so as to obtain adjustable optical delay. Time resolution, ease of manipulation, and mechanical isolation are considerably improved with respect to conventional interferometers. Resolution of group delay measurement and chromatic dispersion over the full 1100-1700-nm spectral range are better than 5 fs and 0.1 ps/nm-km, respectively. >