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Optical amplification is a process through which an input pulse undergoes optical gain to increase its energy. The spatial and temporal quality as well as the noise of the incoming pulse train strongly affect the achievable gain factor. Indeed, pulse pedestals and intensity noise subtract the energy available for the pulse amplification process leading to lower amplification factor.
Lithium Six guarantees pedestal-free low-noise femtosecond pulses that are ideal to reach an effective amplification. Moreover, this laser provides high pulse energies up to 125 nJ allowing to simplify the amplification chain and reduce the number of amplifiers
Supercontinuum generation
In supercontinuum light generation an input pulse with narrow optical spectrum is converted into a pulse with wide spectral bandwidth. The broadening process takes place when the laser pulse with high intensity propagate through an optical medium and stimulate nonlinear optical interactions. The higher the pulse intensity, the higher the supercontinuum light generation efficiency.
Lithium Six guarantees pedestal-free low noise femtosecond pulses that are ideal to reach an effective amplification. Moreover, this laser provides high pulse energies up to 125 nJ allowing to simplify the amplification chain and reduce the number of amplifiers.
Harmonic generation
In harmonic generation (also named frequency doubling) two input photons of a given energy pass through a non-linear crystal and are converted into a single output photon having two times the energy of each input photon. Since the second harmonic conversion efficiency linearly increase with the input intensity, efficient harmonic generation requires high optical intensities.
Lithium Six lasers emit ultra-short pedestals-free pulses with time duration down to 100 fs, low noise intensity and perfect spatial profile (TEM00). These properties enable our lasers to reach harmonic conversion efficiencies up to more than 70 %.
Broadband CARS microspectroscopy
Coherent anti-Stokes Raman scattering (CARS) is a nonlinear optical microscopy technique that relies only on molecular vibrations. It provides non-invasive and label-free imaging of specific biomolecules at high-resolution. CARS microspectroscopy has been widely used for cell imaging, tissue imaging and material science. In CARS microscopy two light sources that simultaneously excite the sample are required, the first of which is referred to as the pump and the second one is the Stokes. When a broadband Stokes signal is used in combination with a narrow band pump signal an instantaneous excitation of a broad variety of vibrational modes is achieved and real time characterization of molecular structures can be acquired. This novel method called Broadband CARS (B-CARS) or Multiplex CARS can detect a full range of spectral information in real time, in contrast to other CARS techniques that can only acquire information at a certain frequency at a time. B-CARS has a good imaging speed and spatial resolution and can be used to achieve quantitative analysis.
Requisites of laser systems to generate broadband Stokes signals for B-CARS microscopy
In order to drive an efficient excitation of all vibration modes pump and Stokes signals must be well synchronized. To ensure this, the laser used as pump for the broadband Stokes signal generation must have transform limited, so as to ensure temporally end spatially overlapped longitudinal modes over the whole spectrum. An extra-requisite for efficient detection of small differential CARS signals is that the pump signals should exhibit low intensity noise at high frequencies.
Thanks to the low relative intensity noise and high peak power, Lithium Six 1050 represents a high-quality pump source for broadband spectrum generation through non-linear conversion.
Lithium Six can be used as a pump for very short LMA-PCF fibers to convert the narrow band high peak power pulse into a high-power broadband output. The low intensity noise of the input signal is preserved through the LMA-PCF fiber resulting in low-noise broadband Stokes signal, which ensures effective detection of small CARS differential signals. The multicolor pulses emitted by the system are temporally and spatially overlapped allowing a perfect synchronization of the narrowband pump and the broadband Stokes signal. Lithium Six provides an exceptional pump for the generation of synchronized broadband Stokes signal for B-CARS microscopy.
Optical spectrum obtained with Lithium Six and a 5 cm long LMA 10 fiber
Simultaneous Spatial Temporal Focusing (SSTF)
Simultaneous Spatial and Temporal Focusing (SSTF) is a focusing technique that allows to obtain ultra-short pulse duration only at the focus, reduce detrimental nonlinear interactions out of the focal plane and gain maximum precision. The SSTF makes use of dispersive elements such as gratings or prisms to spatially separate in the horizontal plane the chromatic components of the laser beam and a focusing lens to focus the dispersed laser beam down onto the sample. Thanks to this focusing technique, the pulse reaches its shorter temporal duration at the focal plane whereas outside of the focal region it diverges both spatially and temporally. Thus, in SSTF the non-linear absorption is confined to a much smaller volume than that of traditional focusing schemes based on spatial focusing only allowing an improvement of precision.
Applications
This focusing method provides great advantages to many applications such as non-linear microscopy and material processing. In multiphoton microscopy SSTF of ultra-short pulses allows indeed to reduce the background excitation and improve the signal-to-background ratio. When combined with USPL micromachining, SSTF decreases detrimental pulse-material interactions outside of the focal plane and provides unprecedented precision in processing of transparent materials.
Lithium Six provides a robust source of high spatial and temporal quality broadband pulses for the most advanced SSTF experiments
In order to push the resolution of SSTF based experiments, it is suitable to employ broadband transform-limited pulses as to strongly disperse and recompress them at the focal plane guaranteeing highly confined light packets only at the focal point. Lithium Six provides pulses with Gaussian spectrum with a full width at half-maximum (FWHM) up to 7 nm for Lithium Six 525, 14 nm for Lithium Six 1050 and 260 nm for Lithium Six Broadband. These optical bandwidths are as wide as to guarantee strong spatial-temporal compression at the focal point. Moreover, Lithium Six 525 and Lithium Six 1050 emit transform limited pulses allowing a better control over the pulse front tilt at the focal plane. The broadband pulses of Lithium Six Broadband can be compressed down do the relative transform limited value and can guarantee strong compression factors and extremely confined non-linear interaction.
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