I. I NTRODUCTION
Erbium-doped fiberamplifiers (EDFA) could be used in high-output power fiber laser sources operating in space, as those required in optical inter-satellite links (OISL) or remote sensing (embedded LIDAR). They have been flying around the Earth already, in satellite fiber optic gyroscopes (FOG). Mechanisms of the radiation-induced degradation of EDFA remain nevertheless unclear despite the significant amount of studies that have been published for more than 20 years . Degradation consists in “darkening”, an excess optical loss resulting from the formation of color centers upon ionization effects. It is characterized by the so- called radiation-induced attenuation (RIA), most often at pump or signal wavelengths. Unfortunately, guided radiations do not act as probes only. It is well known, notably, that light at pump wavelength (typically 980 nm) can mitigates the RIA. This effect being most probably due to photo-bleaching (PB), its efficiency depends on pump power. Pump-induced PB and its interplay with darkening have therefore to be properly characterized and understood to elucidate basic mechanisms and design physical explanatory/predictive models. To date, published works have generally not paid in the pump effect the attention it deserves: the pump power is not always specified (see, e.g. [2,3]), not taken into account in the interpretation , and EDFA lengths have been taken in the 1-3 m range . These lengths are chosen to comply with operation conditions, but they result in an inhomogeneous distribution of pump power along the fiber (Fig. 1) and do not offer convenient conditions to elucidate basic mechanisms. In what follows, we use short fiber pieces to demonstrate new remarkable features revealing mechanisms that are included in a physical degradation model.
1. Ecole Polytechnique, Route de Saclay, 91128 Palaiseau Cedex, France
2. Thales Research & Technology, 1 avenue Augustin Fresnel, 91767 Palaiseau Cedex, France 3. Thales LAS France SAS, 2 avenue Gay Lussac, 78995 Elancourt Cedex, France
For the last few years, coherent beam combining (CBC) has been drastically increasing the performances of ytterbium-doped femtosecond fiberamplifiers, up to more than 10 mJ output energy and to the multi-kilowatt level [1,2]. CBC consists in coherently adding the output beams of several independent amplifiers seeded by a common source. This method involves both an efficient combination process along with a phase detection and control technique applied on all the beams to combine. However, for femtosecond pulses to reach the Joule level and address applications such as particles acceleration, several thousands of fibers need to be combined. Thus, highly scalable CBC architectures along with adapted phase measurements techniques need to be investigated.
We report on the first coherent beam combining of 60 fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beams recombination in the far field, this technique appears well suited for the combination of a large number of fiberamplifiers. The 60 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal microlens array. The measured residual errors within the fiber array yields standard deviations of 4.2 μm for the fiber pitch and 3.1 mrad for the beam-to-beam pointing, allowing a combining efficiency of 50 %. The phasing of 60 fiberamplifiers demonstrates both pulse synchronization and phase stabilization with a residual phase error as low as /100 RMS.
pumping at the wavelength p1 in the 800 nm band, and
pumping in the fiber core at p2 in the 1400 band, see Fig.
1b. The method was proposed in  but to our knowledge it was studied neither experimentally nor theoretically. Since the effective absorption cross section can be varied by proper ratio of the core and cladding cross section areas, this method offers one more degree of freedom in the TDF design. In addition, relatively inexpensive, multimode and high power pump diodes at 800 nm can be used. The TDF amplifier performance under this pumping scheme is compared with the upconversion pumping at single
of 3.4 om and numerical aperture of 0.2.
VII. C ONCLUSIONS
Using a comprehensive numerical model we have shown the potential of the developed thulium-doped fibers for applications in fiberamplifiers for S-band telecommuni- cation and for fiber lasers around 810 nm. The gain exceeding 20 dB of the S-band TDFA could be obtained with optimized fiber waveguiding parameters and optimized pump wavelength. Although the required pump power levels are relatively high, of the orders of Watts, thanks to the progress in ytterbium-doped fiber lasers even these pump powers can provide a cost effective solution. We have shown that efficient lasing at 810 nm can be achieved using silica-based Tm-doped fiber with enhanced 3 H 4 lifetime for
2. Experimental setup
The experimental setup starts with a femtosecond oscillator delivering 200 fs pulses, centered at 1030 nm, at 55 MHz repetition rate. It is followed by a pulse picker allowing to reduce the pulse repetition rate, a 500 ps Chirped- Fiber Bragg Grating used for chirped-pulse amplification and a pulse-shaper. The beam is then split into eight channels, seven of which are used for the beam combining. These seven sub-beams are seeded through variable optical delay lines in free-space followed by piezo-driven fiber stretchers allowing to match the delay and the phase between the pulses, respectively. A second pulse picker stage allows for a further reduction of the pulse repetition rate down to 2 MHz. The output beams are pre-amplified in large-mode area core-pumped ytterbium-doped fiberamplifiers before seeding seven power amplifiers of 30-µm mode-field diameter generating 25 W average power each. The fiber outputs are then stacked into a hexagonal array and individually collimated by a hexagonal microlens array. The common image focal plane represents the tiled-aperture and defines the near field of the global beam. The combined beam is directly found in the far field, obtained in the image focal plane of a lens. Hence, no combining elements are needed.
Figure 5.3 shows three time series of the differential phase evolution between the two ALS fiberamplifiers in the coherent beam combination setup. We note that during the periods a
marked in the figure the system was left untouched, while some laboratory activities were undertaken during the period b
. Whenever the dynamic range is reached and that the power of the combined beam drops to a certain threshold (∼ 15 % from the maximum), the locking loop stops functioning with the control signal set to zero, and re-locks again when the same threshold is triggered by spontaneous fluctuations. The re-locking process is rather fast, so we can assume that it does not interfere the combination system. We may then estimate the dynamic of the differential phase drift by stitching together the discontinuities at the unlocks, as shown in the lower half of Figure 5.3 .
and I DS =20mA). Topology is different from 1-stage LNA version to optimize
NF and gain of the amplifier. Size of the circuit is 22x7.5 mm².
B. MMIC LNAs
Monolithic circuits have also been designed still based on the electrical and HF noise model of the InAlN/GaN MOS- HEMT device (same biasing conditions), with the GH-25 design kit developed by UMS. Different versions have been designed, with lumped matching networks and with distributed lines. The electrical model of the transistor is tailored to fit to the pattern of the MMIC design by de-embedding the tapers, lines and RF pads from the measured/modelled chip. The circuits have not been realized at this time, and only simulations are presented: however during the design steps, sensitivity and yield analysis have been considered to avoid the problems previously evidenced on hybrid amplifiers. Figure 5 represents the layout of a 3-stage MMIC LNA which makes use of L-C networks for matching the different input-output of the transistors (version #A).
“Put simply, reflection is about maximising deep and minimising surface approaches to learning.” [30, p. 3]. As a strategy to promote deep learning, this study asked learn- ers to use annotations as “reflection amplifiers”, i.e. brief and repeated reflection af- fordances, interspersed in the learning material and activated, through the support of a dedicated widget, in support to the first-order learning task at hand. These stop-and- think episodes could be seen as a tentative instantiation of “split screen learning”  that consists in maintaining a dual focus on the content of the lesson and the acquisi- tion processes that are in play. Overall, the results obtained are disappointing.
i pulse without or with
shelving. Rabi oscillations of the four qubits, measured at the optimal powers indicated in Fig. 4(b) are shown in Fig. 4(c). This data shows that JBA readout is com- patible with qubit driving and simultaneous multiplexed operation. The overall performance of our multiplexed JBA is thus comparable with that achieved using linear dispersive readout and parametric amplifiers , albeit with larger errors not due to the readout method itself.
Key words: Semiconductor optical amplifiers, all-optical regeneration
Until now, signal processing in long distance transmission systems is performed by optoelectronic repeaters. However the higher the bit rate, the more expensive and complex the optoelectronic repeaters. In this circumstance, all-optical devices become attractive solutions. Therefore, an all-optical solution should have criteria such as stability, compactness, simplicity of operation and low-power consumption. The two functionalities which are closest to implementation in real systems are wavelength conversion and regeneration.
The theory of I.F. amplifiers with negative feedback Ferguson, Alex J.
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