To predict the behavior of high power pulsed lasers, fine numerical simulations of nonlinear propagation of short chirped pulses in fiber amplifiers are needed. In particular, the gain used in the ...nonlinear Schrödinger equation (NLSE) should take into account the simultaneous dependence of the gain with frequency (i.e. gain dispersion) and time (i.e. gain saturation). We propose here a method using a time-frequency representation (TFR) of the pulse to compute the gain that can be used in the NLSE. Basic TFRs can be computed by estimating the group delay or the instantaneous frequency of the pulse but the low numerical complexity of these methods introduce some limitations in term of cases that can be properly simulated. We show that the more time-consuming computation of the reassigned spectrogram (RS) of the pulse allows to obtain a gain more suitable for general cases than can be encountered in high energy ultrafast lasers.
Rigid endoscopes like graded-index (GRIN) lenses are known tools in biological imaging, but it is conceptually difficult to miniaturize them. In this letter, we demonstrate an ultra-thin rigid ...endoscope with a diameter of only 125 μm. In addition, we identify a domain where two-photon endoscopic imaging with fs-pulse excitation is possible. We validate the ultra-thin rigid endoscope consisting of a few cm of graded-index multi-mode fiber by using it to acquire optically sectioned two-photon fluorescence endoscopic images of three-dimensional samples.
We report a first demonstration of two-photon endoscopic imaging with a lensless endoscope. The endoscope probe is a double-clad bundle of single-mode fibers; point excitation and scanning is ...achieved by coherent combining of femtosecond light pulses propagating in the single-mode fibers; and back-scattered two-photon signal is collected through the multi-mode inner cladding. We demonstrate the two-photon endoscope on a test sample of rhodamine 6G crystals.
A calibration method is presented for temperature measurements using fiber Bragg grating transducers written with the point-by-point technique and femtosecond laser impulsions in silica optical ...fibers. This method is based on the determination of the wavelength-independent thermal sensitivity coefficients of the waveguide over the temperature range of interest. The calibration law is calculated using the wavelength comb-like spectrum of high order fiber Bragg gratings, allowing measurement accuracy comparable to type N thermocouples up to 900°C. The effects of thermal cycling and annealing on measurement errors are also discussed.
Cerium-doped-silica glasses are widely used as ionizing radiation sensing materials. However, their response needs to be characterized as a function of measurement temperature for application in ...various environments, such as in vivo dosimetry, space and particle accelerators. In this paper, the temperature effect on the radioluminescence (RL) response of Cerium-doped glassy rods was investigated in the 193-353 K range under different X-ray dose rates. The doped silica rods were prepared using the sol-gel technique and spliced into an optical fiber to guide the RL signal to a detector. Then, the experimental RL levels and kinetics measurements during and after irradiation were compared with their simulation counterparts. This simulation is based on a standard system of coupled non-linear differential equations to describe the processes of electron-hole pairs generation, trapping-detrapping and recombination in order to shed light on the temperature effect on the RL signal dynamics and intensity.
In this Letter, we report a high-efficiency, miniaturized, ultra-fast coherent beam, combined with 3D-printed micro-optics directly on the tip of a multicore fiber bundle. The highly compact device ...footprint (180 µm in diameter) facilitates its incorporation into a minimally invasive ultra-thin nonlinear endoscope to perform two-photon imaging.
The lensless endoscope represents the ultimate limit in miniaturization of imaging tools: an image can be transmitted through a (multi-mode or multi-core) fiber by numerical or physical inversion of ...the fiber's pre-measured transmission matrix. However, the transmission matrix changes completely with only minute conformational changes in the fiber, which has so far limited lensless endoscopes to fibers that must be kept static. In this paper, we report for the first time, to the best of our knowledge, a lensless endoscope that is exempt from the requirement of static fiber by designing and employing a custom-designed conformationally invariant fiber. We give experimental and theoretical validations and determine the parameter space over which the invariance is maintained.
In this work, we demonstrate and evaluate a new design of micro-structured core erbium-doped few-mode fiber to be used as optical amplifier in the context of mode-division multiplexing. This concept ...is proposed so as to better control the distribution of the Er
ions in the core area, thus permitting to adjust the overall differential modal gains between the different signal modes. The design presented here consists of 19 erbium-doped inclusions embedded in a pedestal geometry guiding 10 modes in the C-band. It has been optimized numerically so as to reach the equalized amplification of all the signal modes. The fiber has been realized and combined with custom-made dual-wavelength mode multiplexers based on multi-plane light conversion to shape the signal and pump beams. Amplification properties have finally been evaluated experimentally.
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