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Ultrafast Lasers

Ultrafast Lasers are lasers emitting optical pulses of ultrashort duration, i.e., picosecond  (10−12 sec) or femtosecond (10−15 sec) time scale. Two important parameters used to describe ultrafast lasers are pulse duration (width) and pulse repetition rate (PRR). Few other parameters used when discussing ultrafast lasers are: peak power (Ppeak) and average power (Pavg).

  • Peak power = pulse energy / pulse duration.
  • Average power = pulse energy x pulse repetition rate

After more than couple of deacdes of development, Ti:sapphire lasers are the gold standard for cutting-edge femtosecond lasers, generating the shortest and highest-energy pulses, with the best spectral quality available. Nonetheless, everybody wants an alternative to Ti:sapphire as it is a free-space laser that has to be aligned and  they are pretty expensive.

Fiber laser technology has made dramatic progress in the past decade regarding output power and ultrashort-pulse achievements. Pulse energies of >100 μJ are now available for commercial ultrafast fiber lasers. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. From a practical advantage-point, fiber lasers offer ease of use, simplicity, durability, high efficiency, compact size, and a relatively modest price. Femtosecond fiber heads can be as small as a Laptop.

Common fiber laser products on the market are Yb-doped fiber lasers, which use ytterbium-doped silica single-mode fibers (or a large effective area fiber) as the gain medium and have the broad gain bandwidth needed to generate femtosecond pulses . The advantages of fiber lasers are their compact size, good beam quality, and high mechanical stability.

Current modelocked fiber-laser oscillators have repetition rates from a KHz to Megahertz t, with pulse durations ranging from 100 fs to around a few picosecond. Pulse energies are modest, but external amplification can boost power upto 100W and more. Fibe Amplifier is based on chirped-pulse amplification, which passes the input pulse through a pulse stretcher that multiplies its duration by inducing strong chromatic dispersion, thus reducing the peak power over the length of the pulse and greatly reducing the impact of nonlinear effects. The amplified output is then passed through a pulse compressor, which reduces the pulse-stretching effect to reduce pulse length to the femtosecond regime. 

Femtosecond fiber lasers are already used in a variety of applications, including nonlinear and fluorescence Spectroscopy, metrology, time-resolved measurements, and Micromachining.

Ultrafast Spectroscopy

Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales (attoseconds to nanoseconds) in a number of scientific fields (physics, chemistry, molecular biology, engineering etc.). Direct observation of the wave packet dynamics of molecular states, the transfer of charge between a donor and acceptor molecule, the coupling of vibrational states which allows for 2d IR, are just a few examples of the new world that can be explored with ultra-fast laser spectroscopy.

Pump Probe Spectroscopy
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Fluoroscence Upconversion 

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Femtosecond Micromachining

It is a process where interaction of the laser pulses with the matter creates various micron or sub-micron features (cutting, drilling, texturing or other modification) and changes in the properties of the matter.

Nowadays, ultrafast laser micromachining is by far the most advanced laser processing technique. There are several new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Unlike material processing with longer laser pulses, ultrashort laser pulses have a negligible thermal impact on the material, which in turn reduces or even eliminates the development of heat-affected zones near the processed areas. Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation.
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