We present an experimental implementation of a joint classical/quantum transmission architecture over 10km of SMF with shared hardware at the emission and using a 64-QAM probability shaping modulation format on the quantum channel. We report a mean excess noise measurement of 0.0212 shot-noise units (SNU) on the quantum channel, leading to a secret key rate of 19.5 Mbps when taking into account finite-size effects, while no errors are accounted for on the classical channel. Thus we show that the classical data can provide solid estimates for the phase and frequency recovery of the quantum channel without adding excess-noise above the null key rate threshold. This enables us to perform CV-QKD protocols without dedicated reference signals and paves the way towards hybrid classical and quantum communication protocols.
We present a kW level pulsed laser based on a master oscillator power amplifier (MOPA) configuration. The directly modulated single frequency laser at 1952 nm was pulsed in the nanosecond regime with a repetition rate frequency from 10 kHz to 2 MHz. The MOPA topology was based on a two stage amplifier using single clad Thulium-doped fiber: it consisted of a pre-amplifier stage followed by a booster stage. We investigated the performance of this pulsed laser for two different TDFs with different saturating energies in the booster stage. The direct modulation allowed us to demonstrate more than 1 kW of output peak power over pulse repetition frequencies from 10 kHz to 500 kHz. For a pulse duration of 21 ns, we measured output energy of 13 μJ and 29 μJ for booster fiber saturating energies of 15 μJ and 30 μJ, respectively.
Cladding-pumped rare-earth-doped fiber laser technologies are currently among the best sources for high power applications. Theses extremely compact and robust sources appoint them as good candidate for aeronautical and space applications.
The double-clad (DC) fiber converts the poor beamquality of high-power large-area pump diodes from the 1st cladding to laser light at another wavelength guided in an active single-mode core. High-power coherent MOPA (Master Oscillator Power Amplifier) sources (several 10W CW or several 100W in pulsed regime) will soon be achieved. Unfortunately it also brings nonlinear effects which quickly impairs output signal distortions. Stimulated Brillouin scattering (SBS) and optical parametric amplification (OPA) have been shown to be strong limitations.
Based on amplifier modeling and experiments we discuss the performances of these sources.
Brillouin distributed optical fiber strain sensors are often limited to static events because of the long acquisition time due to the classic frequency sweep technique. Recently, BOTDA sensor based on slope-assisted (SA) method has been proposed for fast strain measurement along the sensing fiber. In this paper, we present a novel dynamic SA-BOTDR for single-ended truly distributed strain measurement. Geometrical displacement and damping time of a 10.4 m instrumented pipe with a 3 glued fibers system. A 7.6 Hz acquisition rate was achieved with a strain error of 40 με and a 1m spatial resolution. From integration of longitudinal strains, local pipe displacement is calculated with a 12 mm precision.
A careful comparison of experiment and theory is important both for basic research and systematic engineering design of Thulium fiber amplifiers operating in the 2 μm region for applications such as LIDAR or spectroscopy (e.g. CO2 atmospheric absorption at 2051.4 nm). In this paper we report the design and performance of a multistage high-power PM Tm-doped fiber amplifier, cladding pumped at 793 nm. The design is the result of a careful comparison of numerical simulation, based on a three level model including ion-ion interactions, and experiment. Our simulation model is based on precise measurements of the cross sections and other parameters for both 6 and 10 μm core diameter fibers. Good agreement for several single and multistage amplifier topologies and operating conditions will be presented. Origins of the difference between theory and experiment are discussed, with emphasis on the accuracy of the cross sections and the cross relaxation parameters. Finally based on our simulation tool, we will demonstrate a design with an output power greater than 10 W for a multistage amplifier with a single-frequency signal at 2050 nm. The power stage was constructed with a 6 μm active fiber showing a 64 % optical slope efficiency. The output power is found to be within 5 % of the simulated results and is limited only by the available launched pump power of ~24 W. No stimulated Brillouin scattering is observed at the highest output power level for an active fiber well thermalized.
In order to meet application requirements, dependence of the Brillouin frequency shift to strain should be enhanced. We evaluated the influence of several optical fiber dopants on Brillouin scattering strain sensitivity. Based on a FEM-2D modeling, we developed a model for Brillouin gain spectrum and its strain sensitivity. Geometrical profile, doping composition and drawing conditions are taken into account. We show alumina is a very interesting dopant to enhance strain sensitivity in optical fibers. This result is experimentally validated: up to 0.0639MHz/μƐ is obtained with a 5.2mol% Al2O3 doped fiber. We expect to reach 0.1MHz/μƐ with 25mol% doping level.
We report a high power, single frequency, linearly polarized master oscillator power amplifier emitting 110 ns, 1 kW
peak power pulses at 2050 nm. A 20 % slope efficiency and a beam quality of M2 = 1.2 are achieved with a three stages
double clad Tm3+-doped fiber architecture. Various pump schemes are compared leading to the conclusion that 793 nm
pump wavelength is the most efficient for amplification at 2050 nm. Based on a numerical simulations, the Brillouin gain
coefficient around 2 μm in Tm3+ highly doped silica fiber is estimated to 1.2x10-11 m/W. Output peak power is limited by Stimulated Brillouin scattering to 535 W without mitigation and to 1 kW with application of a strain distribution
along the doped fiber.
Usually, strain and temperature coefficients of Brillouin frequency shift (BFS) of optical fibers result from experimental
measurements. For the first time, theoretical strain dependence of BFS is analyzed depending on fiber properties. Based
on a FEM-2D modeling, the strain coefficient Cε is determined considering geometrical profile, doping composition and
drawing conditions. Theoretical results showed great accordance with measurements for different types of single-mode
fibers. The Cε coefficient is evaluated with a relative uncertainty better than 8%. Significant Cε magnitude variation has
been observed depending on doping profile. This paves the way towards major improvement of strain optical fiber
sensors.
We investigate the feasibility of pulsed fiber amplifier coherent combining. Therefore, we characterize phase fluctuations
in low-peak-power pulsed fiber amplifiers using two different interferometric techniques. These measurements reveal
that for low peak-powers, phase fluctuations remain moderate during the pulses. Noticeable phase fluctuations occurring
between the pulses can be perfectly controlled using classical continuous-wave-efficient combining techniques. Results
of such realization combining two low-peak-power pulsed fiber amplifiers, using classical frequency-tagging coherent
combining techniques, are presented. Phase difference measurement is performed between pulses using a small signal
leak from the common master oscillator. For the first time to our knowledge, successful coherent combining of two low-peak-
power pulsed fiber amplifiers is thereby demonstrated.
KEYWORDS: Error analysis, Wavefronts, Fiber amplifiers, Near field, Beam analyzers, Fiber lasers, Near field optics, Monte Carlo methods, Data modeling, Laser optics
We perform sensitivity analyses on two different array configurations of coherently combined fiber amplifiers to study
the impact of residual phase errors onto the combining efficiency. The arrays studied are: a square of 16 fibers and a
hexagon of 19 fibers. For the hexagon, two different shaped wavefronts are studied. In this method a global analysisis
performed: we modify simultaneously all the phase errors using numerical space filling designs. Then, the construction
of a metamodel makes it possible to investigate more precisely the role of each fiber and specially the role of interactions
between fibers onto the combination with less runs than classical approaches. Results exhibit different behaviors and
specially interactions between fibers with respect to the array configurations and with respect to the two different shaped
wavefronts. Finally, we demonstrate that we can study arrays of more than 100 fibers.
The emergence of new fibers families induces considerable requirements in terms of characterization and metrology such
as group delay, chromatic dispersion, birefringence, bending losses, etc. Unlike classical characterization techniques
such as the well-known phase shift method, optical low-coherence reflectometry (OLCR) technique requires only short
fiber samples (i.e. <50cm). Characterization results concerning different types of specialty fibers including erbium-doped
fibers, few-modes fibers, photonic crystal fibers will be presented. Unique dispersive properties of higher-order mode
fibers offer novel solutions for dispersion compensation and nonlinear effects management. OLCR can allow each
LP mode characterization without the requirement of mode converters. A new method, called "Time-wavelength
mapping," based on the process of the OLCR interferogram is demonstrated as a versatile method to determine
chromatic dispersion of each guided LP mode whatever their group index. Different characterization results concerning
photonic crystal fibers with guiding based on to the conventional total internal reflection principle - high index guiding - or photonic bandgap effect - low index guiding - will be presented. Finally, we show that the versatility and deep
physical insight of OLCR technique can play a key role in the study of photonic crystal waveguides in terms of structural
disorder, losses, group delay in highly dispersive region and emphasizes the unique role of this technique in the
understanding of their properties.
Coherent beam combining of fiber amplifier arrays is a promising way to increase power of fiber lasers, and overcome
the physical limitations to fiber laser power scaling. We performed the coherent combining of fiber amplifier arrays
using active control of the phase of each amplifier. The phase fluctuations in the fiber amplifiers have been measured and
their effect on the beam combining process stability evaluated. We extended the coherent beam combining technique to
perform wavefront shaping, in order to deliver a high brightness beam after turbulent atmospheric propagation. We
present experimental results exhibiting the capability of the modulation multiplexing technique that we implemented to
compensate phase fluctuations due to turbulent atmospheric propagation on the laser beam path. Moreover, and for the
first time to our knowledge, we demonstrate automatic coherent combining of fiber amplifiers on a diffuse surface, after
propagation through turbulent atmosphere, without any external turbulence measurement subsystem.
Fundamental noise limitations of distributed quantum amplifiers are discussed. For Raman amplifier pumps to signals
noise transfer, Rayleigh backscattering and polarization fluctuations of the pump are additional noise sources, which are
discussed including their impact on system performances.
We have demonstrated error-free operations of slow-light via stimulated Brillouin scattering (SBS) in optical fiber for
10-Gb/s signals with different modulation formats, including
non-return-to-zero (NRZ), phase-shaped binary
transmission (PSBT) and differential phase-shift-keying (DPSK). By directly modulating the pump laser diode (LD)
using current noise source, the SBS gain bandwidth and profile can be simply controlled by the peak-to-peak value and
power density distribution of the current noise. Super-Gaussian noise modulation of the Brillouin pump LD allows a
flat-top and sharp-edge SBS gain spectrum, which can reduce
slow-light induced distortion in case of 10-Gb/s NRZ and
PSBT signals. For the 10-Gb/s NRZ signal, the error-free slow-light operation has been achieved for the fist time and the
corresponding maximal delay-time with error-free operation is 35 ps. Then we propose the PSBT format to minimize
distortions resulting from SBS filtering effect and dispersion accompanied with slow light owing to its high spectral
efficiency and strong dispersion tolerance. The maximal delay of 51 ps with error-free operation has been achieved.
Furthermore, the DPSK format is directly demodulated through a Gaussian-shaped SBS gain, which is achieved using
Gaussian-noise modulation of the Brillouin pump. The maximal
error-free time delay after demodulation of a 10-Gb/s
DPSK signal is as high as 81.5 ps, which is the best demonstrated result for 10-Gb/s slow-light.
Fiber lasers and amplifiers are used in a variety of applications either for scientific (spectroscopy, medicine...) or industrial applications (free space communications, laser marking and drilling ...). The combination of doped double clad fibers (DCF) and high power multimode semiconductors laser diodes technologies allows to achieve very high output power in very compact, robust and maintenance free systems. Yb 3+ doped DCF are well suited for 1μm wavelength amplification. In pulsed regime, achievable peak power can be strongly limited by nonlinear effects such as Kerr effect, Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS). Consequently, the optimisation of optical amplifier architecture is required. In this paper, we demonstrate performances obtained for the generation of 2ns optical pulses up to >1.7kW peak power in a Master Oscillator Power Fiber Amplifier (MOPFA) configuration. The laser seed signal at 1060nm is emitted out of a single longitudinal mode source with spectral linewidth <0.2nm. The pulse repetition rate can be changed between 3 and 30MHz. The high power stage, based on a 2-stages architecture, allows to deliver >10W average output power with a good beam quality (M2<1.2). No significant limitation due to nonlinear effects of the type of the Kerr effect or SRS appears by means of the optimisation of the final stage’s fiber parameters. Results, such as a concentration of more than 80% of the total output power in a 1nm window around the central wavelength and above all an excellent conservation of the spectral properties of the seed source are demonstrated for a peak power of >1.7kW. These high performances are obtained in a fully-integrated device.
Laser phase noise deteriorates the high sensitivity of heterodyne optical receivers. To reduce phase noise influence, the intermediate frequency signal resulting from the coherent detection is filtered by a narrow bandpass filter (BPF). The phase noise at the input of the BPF generates an amplitude and phase noise at the output of the BPF. The joint probability density function of these noises is evaluated in the case of a first order filter by numerical resolution of a Fokker-Planck equation. A finite difference operator splitting scheme is used. The accuracy of the numerical solution is checked comparing numerically and analytically calculated moments. In addition, a new very efficient method for the analytical calculation of moments is developed. Contour plots of the probability density for both a finite time integrator and a first order filter are compared in order to show the impact of different filter types on phase noise filtering. The marginal pdf of the amplitude and phase noise at the output of the above filters are also calculated.
This paper proposes an accurate computer model of the nonuniform FM response of semiconductor lasers, to be used in the computer- aided design of coherent optical communication systems. The model is communications engineer oriented and does not involve the physical insight of the device. The main idea of this approach is that the FM response of the laser can be approximated by a recursive digital filter based directly on measurements of the FM response. The procedure is divided into two steps: First, measurements of the FM response are fitted by a rational interpolant using the theory of multi-point Pade approximants. Then, the impulse invariant transformation is used to calculate digital filter coefficients. The procedure is applied in the case of a conventional single-electrode distributed-feedback laser. The calculated digital filter is used to study the influence of the nonuniform FM response on the performance of a coherent heterodyne CPFSK system with differential receiver operating at 1 Gb/s. The sensitivity penalty is given as a function of SNR, phase noise, and sequence length by a semianalytical technique. Theoretical and experimental results are in excellent agreement.
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