Ultrashort laser pulses deliver broadband sources for excitation of multiple fluorophores at the same time, therefore providing to medical imaging systems an advance tool for imaging deeper into samples. Furthermore, due to their short pulse duration, and smaller average power they also allow to extend the life time of in vivo samples. Ultrashort laser pulses can also be used in surgery for removing damaged tissues in very difficult areas with reduced access, therefore being excellent tools in medical applications.
We present a monolithic fiber optic configuration for generating temporally coherent supercontinuum (SC) pulsed emission with the shortest pulse duration presented to date, to our knowledge, by an all-fiber source. Few-cycle pulses as short as 14.8 fs are obtained, with central emission wavelength of 1060 nm, repetition rate of 75 MHz and average power of 250 mW. The SC generation is obtained by pumping an all-normal dispersion (ANDi) photonic crystal fiber (PCF) with a mode-locked Yb fiber laser. Spectral broadening by self-phase modulation preserves compressible pulses in the temporal domain. Compared to previously reported configurations exploiting ANDi PCFs, all stages of our source are fiber based and fiber coupled between them. Avoidance of free-space propagation between stages confers unequalled robustness, efficiency and cost-effectiveness to this novel configuration. The ANDi PCF was designed and produced to provide a convex, flat-top dispersion curve with group velocity dispersion comprised between -20 and 0 ps/nm/km in the wavelength range from 900 to 1200 nm. A d-scan system was designed and built to compress and characterize the pulses. The spectrum, wider than 150 nm, supports a Fourier limit pulse duration of 13.7 fs, and pulses have been actually compressed down to 14.8 fs, which demonstrates a high level of temporal coherence in the achieved supercontinuum; second- and third-order dispersion of the pulses are measured as low as -145 fs2 and 875 fs3, respectively. The source has been integrated in a twophoton fluorescence and second-harmonic generation microscopy setup, where 3D images of biological samples have been successfully obtained.
The generation of configurable multipulses by advanced MOPA fiber lasers is opening new possibilities in materials processing, showing improvements in efficiency and quality while benefiting from the advantages of competitively priced fiber lasers. We show examples of the configuration capability and control of multipulses of a 20W MOPA pulsed fiber laser at 1.06μm, having and M2 of 1.3. The multipulses consist of user-defined bursts of short pulses, with burst durations ranging from 10ns up to 1000ns, frequencies up to tens of MHz within the burst, with individual pulses in the 10ns to 200ns range and with up to 12kW peak power. Multipulse repetition frequency is controlled externally from single shot up to few MHz, with the possibility of real-time switching between different multipulses.
Dynamic Pulsing is demonstrated using a pulsed MOPA fiber laser at 1064nm. The output of the MOPA laser is a pulsed
profile consisting of a burst of closely spaced pulses. Tests were performed under several materials with pulse bursts
ranging from 10ns to 1μs and operating from 500kHz down to single shot. In particular, percussion drilling in stainless
steel is demonstrated showing improvements in quality and speed of the process. These profiles allow high flexibility
and optimization of the process addressing the specificity of the end application. Dynamic Pulsing allows the same
MOPA fiber laser to be used in diverse materials as well as different processes such us marking, drilling, scribing and
engraving.
The pulsed fiber laser used in this study is a MOPA-DY by Multiwave Photonics. It is based on a modulated seed laser
followed by a series of fiber amplifiers and ending with an optically isolated collimator. This pulsed laser model has an
output in such a way that each trigger produces a fast burst of pulses, with a repetition frequency within the burst of the
order of tens of MHz. Within the burst it is possible to change the number of pulses, the individual pulse profile, burst
pulse period and even to generate non-periodic burst pulse separations. The laser allows full freedom for all these
combinations. The study here reported compares the impact of pulse peak power, number of pulses within a burst and the
pulse burst period, on process quality (heat affected zone, debris, hole uniformity) and drilling yield.
Pulse bursting is demonstrated using a pulsed MOPA fiber laser at 1064 nm for percussion drilling of stainless steel.
Bursts are configured as fast pulse sequences at tens of MHz, with a temporal envelope in the range from 10 to 420 ns.
Their use rapidly enhances the efficiency of material removal by enhancing pulse energy deposition, and quality
improvement. Results are shown for various pulse conditions by changing number of pulses, spacing and peak powers.
Each pulse burst is collectively triggered and amplified as a single pulse group, at repetition frequencies from singleshot
to hundreds of kHz.
A multi-point fibre optic temperature sensor network integrated inside a power transformer for continuous monitoring of
hot-spots on windings, cellulose insulations and oil, is demonstrated and tested. The temperature sensors are based on
proprietary encapsulated fibre Bragg grating (FBG) sensors and the optical interrogation unit uses a special designed
narrowband high power broadband fibre source. The fibre optic sensing network is integrated into a 440 MVA power
transformer having 12 temperature sensing points, distributed over several physical locations inside de transformer
(windings, cellulose insulators, magnetic circuit and cooling oil entrance and exit).
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