Deeper Dive into Ultrafast Lasers
Ultrafast Laser Applications
The high peak power, high energy pulses, and short pulse widths of femtosecond lasers are ideal for a wide range of applications especially for multi-photon microscopy, non-linear spectroscopy, second harmonic generation (SHG), and micromachining.
Peak Power and Average Power in ns and Sub-ns Lasers
A significant and well-recognized difference between lasers and conventional, incoherent light sources, is the ability to concentrate laser emission in short pulses, with durations going down to a few femtoseconds, containing potentially only a few optical cycles. Technically, you can drive an incoherent LED source using current pulses, allowing the emission of light pulses down in the nanosecond range. However, each pulse would have a maximum power (i.e. a peak power) equal to the average power of the same device if a continuous bias were applied. Only laser cavities can concentrate the stored energy within active materials in such a way to achieve peak powers orders of magnitude higher than their average power, up to the exceptional PW-level recently reported in research publications. The extremely high peak power levels achievable by pulsed laser sources are among the main reason for their success in many of the applications which have emerged in the last decades. Therefore, a precise estimation of the laser peak power, given other operational parameters such as average power, pulse duration, and repetition rate, is fundamental to select the best option for a particular application among the different commercial alternatives. In principle, it is quite simple to calculate the peak power, considering the actual temporal profile of the laser pulse. By assuming a train of continuously repeated, periodical, square pulses with repetition rate fR, pulse duration tP and average power PAV, the pulse energy EP and peak power PP calculations are trivial, with pulse energy provided by the ratio between average power and repetition rate and peak power provided by the ratio between energy and pulse duration:
Of course, this simple relationship holds for laser pulses with a square or flat-top temporal profile, which is rather uncommon in practice. Usually, laser pulse temporal profiles are approximated more accurately with bell-shape functions, such as Gaussian profile or Sech2 profile, the latter being relevant mainly for ultrashort pulses obtained by the passive mode-locking regime. In this case, one should redefine pulse duration as being full-width half-maximum (FWHM), a commonly accepted parameter. Since the energy concentration in a Gaussian pulse, with FWHM duration tP, is slightly different from a square pulse with equal pulse duration, one needs to adjust the formula above
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Raman Spectroscopy: Why are Picosecond Pulses Superior to Femtosecond?
Raman-based spectroscopy, on the other hand, is an entirely different nonlinear technique, relying on the frequency shift experienced by laser radiation incident on a molecule, related to its rotational and vibrational modes. Not being related to electronic transitions, the Raman shift is relative concerning the irradiating wavelength, and therefore, unless pursuing coherent excitation, laser source tunability is not required. Raman-based spectroscopy, being a nonlinear process, usually requires ultrashort pulse generation (e.g., practical TPA set-ups usually require < 300 fs pulse duration). On the other hand, since the Raman gain is generally higher than TPA cross-sections, cheaper and simpler picosecond lasers could be efficiently employed in incoherent Raman spectroscopy. Furthermore, in Raman spectroscopy, the spectral resolution is related to the laser source bandwidth. Therefore, the narrower bandwidths of picosecond lasers represent a remarkable advantage over their femtosecond counterparts. For pulsed laser sources, spectral bandwidths and pulse durations are related by a Fourier-transform relationship, descending from the time-energy Heisenberg uncertainty principle. More precisely, the minimum spectral bandwidth is inversely proportional to the pulse duration. The shorter the pulse duration, the larger the bandwidth for a given temporal pulse profile. For instance, for a pulse duration of ~1 ps at the wavelength of ~1 μm, the minimum FWHM spectral bandwidth is ~1 nm. For pulses ten times longer at the same wavelength (~10 ps), the minimum spectral bandwidth is 10 times narrower (~0.1 nm).
Since the spectral resolution in Raman spectroscopy is related to the spectral bandwidth of the illuminating source, 10-ps-long pulses would potentially provide better spectral resolution than 1-ps-long pulses. On the other hand, longer pulses provide lower peak power for a given average power and repetition rate, and therefore a lower signal and a worse signal-to-noise ratio. An optimal pulse duration of a few picoseconds is generally accepted for typical set-ups as a good trade-off between the different requirements. Moreover, it is important to note that the FWHM spectral bandwidth mentioned above is a minimal value. It is not uncommon for practical lasers to emit pulses with a broader spectrum than the narrowest (transform-limited) theoretical profile due to residual, uncompensated, linear, or nonlinear phase shift.
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Femtosecond Lasers for Material Processing and Micromachining Applications
The primary application for femtosecond lasers is micromachining, which can include consumer, medical or R&D applications. Typically, these applications require femtosecond pulses for their cold-ablation affects. Femtosecond lasers are ideal tools for applications where micro-cracks, HAZ, or recasts are detrimental to the integrity or lifetime of the material being processed.
A growing application for femtosecond lasers is in the field of handheld electronics…more specifically cell phones and tablets. Manufactures are experimenting with different materials to make a more robust product. The popularity of ultrafast femtosecond lasers continues to grow in these challenging industrial applications that require cold ablation.
Our femtosecond micromachining lasers can be applied to a variety of industrial processes, including cold ablation, semiconductor processing, stent cutting, laser marking, TFT repair, thin film patterning, marking, dicing, scribing, solar cell cutting, edge isolation, laser deposition, surface patterning, processing volatile materials, or machining hard materials. Femtosecond lasers are also used in various medical procedures within the ophthalmological and dermatological fields
Femtosecond lasers microstructure (groove, cut, drill, micromill, etc.) virtually any material with:
- no thermal side-effects such as microcracks, burrs, or recast
- lateral features as small as a few um
- pulses-on-demand for easy integration into delivery systems
- high average power, pulse energy and rep rate for increased ablation rate per pulse
Examples for industrial femtosecond laser micromachining can include:
- IMD – medical and biomedical devices (PMMA, PLLA, catheters, stents, pacemakers, guidewires, cutting, drilling, de-coating, etc.)
- FPD- flat panel display (AMOLED, OLED, Quantum Dots panels)
- Micro and nano-processing of complex materials (glass, ceramics, quartz, sapphire, organic tissues, etc.)
- Surface texturing
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