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.
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 (E<sub>p</sub> = 4 μJ at 92 kHz, Δτp =1.6 ps).
New wavelengths of laser radiation are of interest for material processing. Results of application of the all-fiber ultrashort pulsed laser emitting in 2 µm range, manufactured by Novae, are presented. Average output power was 4.35 W in a single-spatial-mode beam centered at the 1950 nm wavelength. Pulses duration was 40 ps, and laser operated at 4.2 MHz pulse repetition rate. This performance corresponded to 25 kW of pulse peak power and almost 1 µJ in pulse energy. Material processing was performed using three different focusing lenses (100, 30 and 18 mm) and mechanical stages for the workpiece translation. 2 µm laser radiation is strongly absorbed by some polymers. Swelling of PMMA surface was observed for scanning speed above 5 mm/s using the average power of 3.45 W focused with the 30 mm lens. When scanning speed was reduced below 4 mm/s, ablation of PMMA took place. The swelling of PMMA is a consequence of its melting due to absorbed laser power. Therefore, experiments on butt welding of PMMA and overlapping welding of PMMA with other polymers were performed. Stable joint was achieved for the butt welding of two PMMA blocks with thickness of 5 mm. The laser was used to cut a Kapton film on a paper carrier with the same set-up as previous. The cut width depended on the cutting speed and focusing optics. A perfect cut with a width of 11 µm was achieved at the translation speed of 60 mm/s.
Photonic bandgap Bragg fibers are promising for designing
large-mode-area structures owing to their high bend
immunity. However, at a large core diameter, filtering of high-order modes (mainly, the LP<sub>11</sub> mode) becomes difficult,
because the propagation constant of such modes is close to that of the fundamental LP<sub>01</sub> mode.
In this paper, we demonstrate the possibility to suppress high-order modes in Bragg fibers by introducing low-index
inclusions into the Bragg fiber core. Numerical analysis shows that an appropriate choice of the position and types of
such inclusions allows one to increase the LP<sub>11</sub> mode radiation loss without increasing the optical loss of the fundamental
LP<sub>01</sub> mode. The Bragg fiber with two B-doped and two
F-doped rods in the core was fabricated and studied. The
fundamental LP01 mode at 1064 nm had a mode-field area of about 340μm<sup>2</sup> and an optical loss below 0.2 dB/m at a
bending radius of 15 cm. The LP<sub>11</sub> mode was not observed in both bent and straight fibers at this wavelength. Only the
LP<sub>21</sub> mode was detected in a straight fiber; however, it was completely suppressed after propagating a length of 60 cm in
a fiber bent with a radius <50 cm.
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 μm<sup>1</sup> (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 μm<sup>3</sup>) 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) design<sup>4</sup>. 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 configuration<sup>5</sup>, chromatic dispersion<sup>6</sup>, polarization maintaining<sup>7</sup>, low bend sensitivity<sup>8</sup>). 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 reached<sup>9</sup>. As an example, an Yb-doped LMA BF with 20 μm
core diameter was realized and single transverse mode operation in continuous wave (cw)<sup>9</sup> and mode-locking<sup>10</sup>
oscillation regimes was demonstrated. Moreover, operation of two dimensional all-solid PBG fibers in laser and
amplifier regimes was recently demonstrated<sup>11-13</sup>.
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.
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.