In this communication, we report for the first time on a homemade 55 µm core VLMA “Yb-free” Er-doped aluminosilicate double-clad fiber manufactured by the REPUSIL powder sintering technology and its implementation within two different laser configurations emitting around 1560 nm, both pumped at 976 nm. First, a free-running free-space CW oscillator delivers up to 40 W of average power with optical-to-optical efficiency of 30 % and near-diffraction-limited beam, despite the large core size. In a second experiment, the fiber is used as the main amplifier of a MOPA system delivering up to 10 nJ pulses at GHz repetition rate.
We report on mid-infrared supercontinuum generation from 4 to 9 µm in orientation-patterned gallium-arsenide waveguides pumped by nanojoule-class ultrafast fiber lasers. The QPM waveguide and the laser source are optimized in tandem to pump the waveguides close to the degeneracy by means of sub-picosecond pulses at 2760 nm. The use of a waveguide geometry drastically reduces the required energy to the nanojoule level, thereby opening supercontinuum generation in GaAs platforms to fiber lasers.
High-harmonics generation (HHG) in solids require high-energy few-cycle laser drivers at near- to mid-infrared wavelengths with excellent beam quality to reach fluences of ~1 TW/cm2. Along this line, soliton sources based on large mode area silica-core singlemode fibers produce ultrashort (70 fs) pulses at remote wavelengths with hundreds of nJ, thus providing a new platform for driving HHG in solids. In this communication, we explore the potential of such soliton-based fiber driver for HHG in thin-films of zinc oxide. The laser delivers 41 nJ 70 fs solitonic pulses at 1764 nm and drives harmonics generation up to H7.
Many applications such as nonlinear microscopy and strong field optoelectonics require high-energy (> 100 nJ) ultrashort (< 100 fs) pulses above 1.55 µm out of a singlemode fiber. Here, we report on high-energy amplification in tapered Er-doped fiber fabricated by the powder technique. The system based on direct amplification is free from stretcher and compressor units. We generate 90 fs MW-class pulses at 1600 nm by amplification and management of nonlinear effects in the tapered fiber. Despite the output 100 µm core diameter, the emitted beam is near-diffraction limited.
All-optical poling was demonstrated for the first time in 1986 in single mode fibers: such nonlinear optical process enabled the introduction of a second-order susceptibility (χ(2)) in a doped silica fiber. By simply using an intense laser source, alloptical poling, later theoretically described by Stolen and coworkers, permitted the generation of a second harmonic (SH) signal in an otherwise centrosymmetric doped material. More recently, similar experiments have been carried out by exploiting complex beam propagation in multimode fibers. In this work we reveal, for the first time to our knowledge, the 3D spatial distribution of a χ(2) nonlinearity written in a graded-index (GRIN) multimode (MM) fiber. In particular, the presence of a doubly-periodic distribution of χ(2) is unveiled by means of multiphoton microscopy. The shortest period (tens of micrometers) is due to the beating between the fundamental and the SH beams, and it is responsible for their quasi-phase matching (QPM). Whereas the longest period (hundreds of micrometers) is associated with the periodic evolution, or self-imaging, of the power density of the MM beam along the GRIN MM fiber. The complex modal beating, leading to spatial self-cleaning of the fundamental beam, is thus printed inside the fiber core, and revealed by our measurements. We considered two fibers of similar composition and opto-geometric parameters, and we compared the evolution of the optical poling process with time. Despite the rather similar fiber characteristics, we observed a striking difference in the poling efficiency between the two fibers. Such observation led us to point out the importance of considering the complete fiber fabrication process (both the preform elaboration and the drawing steps) on the final structure and microstructure of optical fibers.
We report on the design of OP-GaAs rib waveguides for frequency conversion in the mid-infrared and explore their performances for parametric generation. The samples used are between 10 and 25 mm long and exhibit quasi-phasematched (QPM) periods from 85 to 100 μm. The waveguides are pumped by a femtosecond erbium-doped fluoride fiber laser combined with a soliton self-frequency shift converter delivering sub-300 fs pulses at a wavelength tunable between 2.8 and 3.3 μm. By adjusting the pump wavelength, our OP-GaAs platform can produce ultrashort pulses widely tunable around 4 and 12 μm for the signal and idler, respectively. These results fit quite well our calculations of QPM curves.
We report on a fiber laser setup optimized to generate and propagate high energy solitons with megawatt peak power around 1700 nm. Picosecond pulses from a chirped-pulse amplifier system at 1560 nm trigger the formation of sub-100 fs solitons with approximately 70 nJ energy in an all-solid photonic bandgap Bragg fiber with ultra-large mode area. Upon propagation in the same fiber the soliton self-frequency shift effect allows for tuning the central wavelength up to 1680 nm in a 1.5 m long piece of fiber. This work paves the way to miniaturized endomicroscopes in the biologically relevant window around 1700 nm and, thanks to the 20-cm critical bend radius of the delivery fiber, opens the way to deep in vivo imaging of freely moving animals.
Recent studies showed that the excitation spectral window lying between 1.6 and 1.8 μm is optimal for in-depth three-photon microscopy of intact tissues due to the reduced scattering and absorption in this wavelength range. Hence, millimeter penetration depth imaging in a living mouse brain has been demonstrated, demonstrating a major potential for neurosciences.
Further improvements of this approach, towards much higher imaging frame rates (up to 15-20 s/frame in previous achievements) requires the development of advanced molecular optical probes specifically designed for three-photon excited fluorescence in the 1.6 -1.8 μm spectral range.
In order to achieve large three-photon brightness at 1700 nm, novel molecular-based fluorescent nanoparticles which combine strong absorption in the green-yellow region, remarkable stability and photostability in aqueous and biological conditions have been designed using a bottom-up route. Due to the multipolar nature of the dedicated dyes subunits, these nanoparticles show large nonlinear absorption in the NIR region.
These new dyes have been experimentally characterized through the measurement of their three-photon action cross-section, fluorescence spectra and lifetimes using a monolithically integrated high repetition rate all-fiber femtosecond laser based on soliton self-frequency shift providing 9 nJ, 75 fs pulses at 1700 nm. The main result is that their brightness could be several orders of magnitude larger than the one of Texas Red in the 1700 nm excitation window.
Ongoing experiments involving the use of these new dyes for in vivo cerebral angiography on a mouse model will be presented and the route towards three-photon endomicroscopy will be discussed.
Temperature-independent strain measurement is achieved resorting to a taper fabricated on a Bragg fibre using a CO2 laser. The characteristic bimodal interference of an untapered Bragg fibre is rendered multimode after taper fabrication and the resulting transmission spectra are analysed as temperature and strain change. The intrinsic strain sensitivity exhibited by the Bragg fibre is increased 15 fold after tapering and reaches 22.68 pm/μepsilon. The difference in wavelength shift promoted by variations in temperature and strain for the two fringes studied is examined and strain sensing with little temperature sensitivity is achieved, presenting a sensitivity of 2.86 pm/μepsilon, for strain values up to 400 μepsilon.
We present an all-fiber integrated master oscillator power amplifier operating at 1940 nm. The source delivers 422-nJ chirped pulses at a repetition rate of 10.18 MHz corresponding to 4.3 W of average power. The pulses were recompressed down to 900 fs yielding 220 kW of peak power. Stretching the pulse to 200 ps allows further energy scaling beyond the microjoule barrier at low repetition rate (Ep = 4 μJ at 92 kHz, Δτp =1.6 ps).
The use of double-clad fibers for short pulses amplification requires high active ions concentration in order to keep the active fiber length short. In the case of Er-doped fibers an increase of concentration leads to a significant drop of efficiency due to Er ions clustering. We have demonstrated through numerical simulation that efficiency of amplifiers based on double-clad P2O5-Al2O3-SiO2 (PAS) Er-doped fibers decreases slower with Er-concentration growth if compared with standard Al2O3-SiO2 fibers. In this paper, we present single-mode large-mode-area heavily Er-doped double-clad fiber based on PAS glass matrix for short pulses amplification. The developed PAS fiber has a 36 μm singlemode core and a small signal cladding absorption of 3 dB/m at 980 nm leading to an optimal fiber length in range of 5-8 m depending on the central wavelength. At first, an all-fiber nanosecond MOPA at 1560 nm was built using our PAS fiber as the final amplifier. We obtained 28 W of average output power (efficiency of 25 % with respect to the launched pump power at 976) limited by amplified spontaneous emission. Pulse energy of 1.5 mJ was achieved at pump power level of ~120 W. We believe that it is the first demonstration of mJ-energy level single-mode nanosecond fiber system. Then, direct amplification of 100-fs source was performed using this fiber. We obtained 12 nJ pulse energy and 100 kW of peak power from the fiber which is close to the record value for Er-doped fiber amplifiers.
Power scaling of Yb-free Er-doped fiber lasers is extremely challenging due to low Er ion absorption cross-section and growth of unbleachable loss at high Er concentrations because of clustering effects. Hence, usual double-clad Er-doped fibers suffer from low efficiency. We present an efficient high power all-fiber amplifier based on our newly developed Yb-free Er-doped fiber. Proper core composition and relatively low Er3+ concentration mitigates clustering effect. Furthermore, large single-mode core diameter of 34 um increases the pump absorption and decreases the fiber length. Our amplifier consists in the specialty Er fiber pumped through a commercially available pump combiner by means of 6 pigtailed multimode diodes (D=105 um, NA=0.15, input pump power of 275W). The signal source is a low power continuous wave fiber laser spliced to the amplifier. Therefore we built truly all-fiber laser without any free space coupling. We obtained 103 W of amplified signal limited only by the available pump power. Pump conversion efficiency is as high as 37 %. To the best of our knowledge this is the highest power ever demonstrated for Yb-free Er-doped lasers pumped at 976 nm. This power level is similar to that obtained in resonantly pumped Er-doped fiber lasers.
A highly-birefringent photonic bandgap Bragg fiber loop mirror sensor is proposed. Thanks to the Bragg
fiber geometry, one can observe the group birefringence and the bandgap fiber in the transfer function.
The sensing head presented different sensitivities for strain and temperature measurements. Using the
matrix method, both the physical parameters can be discriminated. It is important to highlight that this
Bragg fiber presents sensitivity to temperature of ~5.75 nm/ºC, for the group birefringence measurand.
In this work it is investigated the strain and temperature sensing characteristics of modal interferometers supported by
two Bragg fibers with different cross-section cladding geometries. It is shown that the sensitivity to these measurands is
different for the two fibers, which turns feasible the conception of several sensing configurations based on the
combination of these two fiber types for simultaneous measurement of strain and temperature.
UV-induced Bragg gratings are written into the three concentric GeO2-doped rings of an Yb3+-doped-core Cantor fractal
photonic-bandgap fibre. These rings can support several modes and the effective indices of these modes are derived
experimentally from the grating peaks. They are found to be in excellent agreement with numerical simulation.
We demonstrate the inscription of Bragg gratings in each of the three, concentric, germanium-doped rings of an
ytterbium-doped-core photonic-bandgap fibre. These rings can support several modes and the effective indices of these
modes are derived experimentally from the grating peaks. They are found to be in excellent agreement with numerical
simulation.
Hollow-core microstructured fibres are designed for the short wavelength domains, either visible or ultra-violet
ones. The experimental results confirm that kagomé-lattice antiresonant fibres are good candidate for this
purpose. Thorough numerical modelling is carried out in order to determine the physical causes responsible for
the loss level observed. From these computations the following conclusions are drawn: (i) the sole antiresonant
core surround dictates the location of the transmission windows and (ii) the cladding bridges are sources of extra
leakage from the core to the surrounding solid cladding. A straightforward model is therefore devised to
determine accurately the loss level in this kind of structure by quasi-analytical calculus.
The increase of the output power in fiber lasers and amplifiers is directly related to the scaling of the core diameter. State
of the art high power laser and amplifier setups are based on large mode area (LMA) photonic crystal fibers (PCF)
exhibiting core diameters ranging from 40 μm up to 100 μm1 (rod-type PCF). For instance, a two-stage femtosecond
chirped pulse amplification (CPA) system based on 80 μm core diameter rod-type PCF was demonstrated generating
270 fs 100 μJ pulses2. Although highly suited to reach very large mode areas, this fiber design suffers some drawbacks
such as high bend sensitivity (for core diameter equal to or larger than 40 μm3) and practical handling (cleaving, splicing,
etc.) due to presence of air holes. As an alternative we have recently proposed all-solid photonic bandgap (PBG) Bragg
fiber (BF) design4. Due to their waveguiding mechanism completely different from total internal reflection this type of
fiber offers a very flexible geometry for designing waveguide structures with demanding properties (singlemodedness in
large core configuration5, chromatic dispersion6, polarization maintaining7, low bend sensitivity8). During the last few
years our interest was mainly focused on the realization of an active BF and scaling up the core diameter. We showed
that, in principle, core diameters in excess of 50 μm can be reached9. As an example, an Yb-doped LMA BF with 20 μm
core diameter was realized and single transverse mode operation in continuous wave (cw)9 and mode-locking10
oscillation regimes was demonstrated. Moreover, operation of two dimensional all-solid PBG fibers in laser and
amplifier regimes was recently demonstrated11-13.
In this paper we report on the first demonstration of amplification of femtosecond pulses in LMA PBG BF. A single
transverse mode was obtained and the BF allowed for generating 5 μJ 260 fs pulses in a system with a moderate
stretching of 150 ps.
Chalcogenide or heavy metal oxide glasses are well known for their good transparency in the mid-infrared (MIR)
domain as well as their high nonlinear refractive index (n2) tens to hundreds times higher than that of silica. We have
investigated the nonlinear frequency conversion processes, based upon either stimulated Raman scattering (SRS) or
soliton fission and soliton self-frequency shift (SSFS) in fibres made up with such highly nonlinear infrared transmitting
glasses. First, SRS has been investigated in a chalcogenide As2S3 step index fibre. In the single pass configuration, under
quasi continuous wave 1550 nm pumping, Raman cascade up to the forth Stokes order has been obtained in a 3 m long
piece of fibre. The possibility to build a Raman laser thanks to
in-fibre written Bragg gratings has also been investigated.
A 5 dB Bragg grating has been written successfully in the core. Then, nonlinear frequency conversion in ultra-short pulse
regime has been studied in a heavy metal oxide (lead-bismuth-gallium ternary system) glass photonic crystal fibre.
Broadband radiation, from 800 nm up to 2.8 μm, has been obtained by pumping an 8 cm long piece of fibre at 1600 nm
in sub-picosecond pulsed regime. The nonlinear frequency conversion process was assessed by numerical modelling
taking into account the actual fibre cross-section as well as the measured linear and nonlinear parameters and was found
to be due to soliton fission and Raman-induced SSFS.
In this work, we propose to take advantage of properties of a Bragg fibre for optical sensing. The Bragg fibre exhibits
three concentric high refractive index layers embedded in pure silica and surrounding a 35μm diameter core. A short
(0.3 m long) piece of Bragg fibre slightly multimode, is used to elaborate an intermodal interferometer, the spectral
response of which exhibits a fringe pattern that depends on the operating wavelength, which can therefore be used as a
sensor. The two modes considered were found to be the fundamental LP01 and the high-order mode LP02. The sensor has
been characterized in strain and temperature and presents a sensitivity of - 1.09 pm/με and 14.1 pm/°C respectively. The
sensor demonstrated insensitivity to curvature thanks to well known Bragg fibre properties.
A chalcogenide optical fiber of special design is proposed to convert a short-wavelength IR radiation (around 2 μm) up
to second transparency window of atmospheric air (around 4.5 μm) by degenerate four-wave mixing. The fiber supports
a small core surrounded by three large air holes. The zero-dispersion wavelength is shifted down to 2 μm in this fiber by
properly tailoring geometry of the fiber core. We demonstrate by solving the nonlinear Schrödinger equation that
efficient wavelength-conversion can be obtained by pumping the fiber with a Tm:SiO2 pulsed fiber laser.
Optical fiber sources have experienced a massive growth over the past ten years principally due to the compactness,
robustness and good spatial quality of such systems. Fiber sources now cover a large spectrum from visible to near
infrared helped on this point by the development of microstructured fibers (MOFs). A particular class of MOFs also
called hollow-core photonic crystal fibers (HC-PCFs) offers to get rid of silica's absorption thanks to band gap guidance
and therefore to extend transmission range of silica fibers. We propose here two all-fiber architectures based on HCPCFs
in view to generate mid infrared wavelengths by amplification of spontaneous Raman scattering (SRS) in gaseous
medium. We report on design, fabrication and characterization of two kinds of HC-PCF matching the architecture needs.
In the last ten years, the development of air-silica microstructured fibers has opened an exciting route to study new type
of optical waveguides, leading to a wide range of applications. Now, the possibility offer by this photonic technology to
incorporate original materials and mix different fabrication processes give a promising way to adapt the optical designs
and extend the applications in a large area from UV to mid IR.
Although singlemode fiber lasers become a mature technology, enhancements, in terms of output power, spatial beam
quality, bend insensitivity are still required. A major trend is to increase the active core area to increase the thresholds of
nonlinear effects while ensuring a transverse singlemode behavior. Actually, increasing the active ions' concentration is
also demanded since it allows a drastic reduction of the fiber length, everything being equal. Two non-exclusive
strategies are laid out to overcome fiber laser limitations. On the one hand, it is demonstrated that surrounding a highly
multimode active core by a properly designed microstructured cladding, exhibiting specific resonant features, allows the
fiber laser to be operated in the singlemode regime. On the other hand, a large mode area photonic bandgap fibre is
shown to lead to a transverse singlemode fiber laser with very good lasing efficiency.
The delivery or generation of high power in optical fibre requires the increase of the core size to increase the threshold of
nonlinear effects and the damage threshold. However the bend loss strongly limits the increase of the effective area
(Aeff). All-solid photonic bandgap fibres are attractive for the delivery of power since they can be made singlemode
whatever the core diameter is. Moreover the silica core can be doped with rare-earth ions. A Bragg fibre is a bandgap
fibre composed of a low index core surrounded by N concentric layers of high and low index. We have fabricated Large
Mode Area Bragg fibres by the MCVD process. These Bragg fibres present a ratio Aeff/λ2 close to 500. A first Bragg
fibre, defined by N = 3 and an index contrast between the cladding layers Δn = 0.01, exhibits a measured critical bend
radius Rc close to 16 cm (bend loss equal to 3 dB/m). Increasing the index contrast Δn leads to a tighter field
confinement. The field distribution of the guided mode strongly decays in the periodic cladding and is thus less sensitive
to bending. We propose here the design of an improved Bragg fibre with a very large index contrast Δn = 0.035 which
leads to a dramatic reduction of the bend loss. The critical bend radius was measured to be lower than 3 cm. This fibre is
less bend sensitive than an equivalent solid core fibre, either a step-index fibre or a photonic crystal fibre.
Photonic bandgap fibers have already proved their huge potential for guiding light in air over kilometric lengths.
Nowadays, solid-core bandgap fibers draw considerable attention due to their unusual properties. For instance, the
bandgap effect may lead to very large mode area operation, management of the chromatic dispersion curve, spectral
filtering or bend loss reduction, all features that could enhance fiber laser performances. Recent results about the design,
fabrication and characterization of large mode area solid-core bandgap fibers are presented. Prospects of further
development of bandgap fiber lasers are discussed.
The propagation properties of microstructured optical fibers useful for sensing applications are reviewed. The interaction between light and sample can reach 95 % in singlemode hollow core fibers and examples of structures exhibiting such large overlap ratios are described. The generation of 300 nm and 700 nm flat continua of visible and IR light in a single highly nonlinear holey fiber, well suited for the detection of biological species by spectroscopy, is reported. The low temperature sensitivity of long period gratings and of the birefringence in holey fibers is attractive for sensors operating in varying environmental conditions.
We have fabricated Long Period Grating filters in two Dual Concentric Core Fibers by an electric arc discharge technique. The gratings have induced coupling between the fundamental modes of the two cores. We have obtained filter with rejected band around 1220 nm and 1559 nm respectively, characterized by insertion loss lower than 0.5 dB. We also have investigated the inteest of using this fiber to implement a highly selective Mach-Zehnder interferometer with a 2.4 nm inter-fringe. Despite the high Ge-doping of the used fiber, thermal characterizations show a temperature sensitivity of the transmitted spectrum similar to that of the same grating written in a standard Single Mode Fiber.
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