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Mode-locking Techniques and Principles
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
It is relatively simple to realize a picosecond laser by incorporating a suitable saturable absorber (e.g. a SESAM) in the laser resonator (see Figure 20.8), which also contains a usual kind of laser crystal (e.g. Nd3+:YAG or Nd3+:YVO4). Normally, such a device is used as an end mirror, and the resonator design is made such that the fundamental mode size on the absorber is suitable for achieving an appropriate strength of absorber saturation. Additional means such as dispersion compensation are not required in that operation regime. The pulse duration is often determined essentially by the balance of pulse shortening by the absorber and pulse broadening by the limited gain bandwidth. That means that an absorber with higher modulation depth allows the generation of shorter pulses; the pulse duration is inversely proportional to the square root of the modulation depth (Paschotta and Keller 2001), while the absorber recovery time has no significant influence on the pulse duration; it only needs to be short enough to obtain stable mode-locking. The pulse duration is also inversely proportional to the gain bandwidth.
Fibre Lasers
Published in Yu. N. Kulchin, Modern Optics and Photonics of Nano and Microsystems, 2018
The generation of laser radiation in the form of short pulses is an extremely important problem. The significance of this problem is due to two reasons. First, the pulse duration determines the time interval for the interaction of laser radiation with the medium, which is extremely important in the study of fast processes. Secondly, the energy of laser radiation, being concentrated in a short laser pulse, causes a large pulse power and high intensity of the electromagnetic field of its light wave. At present, lasers, including fibre ones, are capable of generating ultrashort pulses with a duration of only a few femtoseconds. During this time, the pulse can pass a very short distance of several micrometers in length. But even small changes in its energy during this time can lead to significant changes in the power and strength of the pulse field.
Nonlinear tissue processing in ophthalmic surgery
Published in Pablo Artal, Handbook of Visual Optics, 2017
Shortening the pulse duration is a basic physical problem, which is related to the maximum possible spectral bandwidth of the laser medium (so called time–bandwidth product). Titanium–sapphire lasers, for example, have the broadest spectrum (>100 nm) and the shortest pulses (≪100 fs); however, they are very complex in their setup and relatively expensive. Ytterbium-doped fiber lasers or solid-state lasers, which emit around 1040 nm wavelength, are today the most reliable systems and also the cheapest way to produce femtosecond pulses. Their pulse duration is typically around 200–800 fs. In this range the energy threshold for optical breakdown increases almost linearly with pulse duration [Noa 99].
Drilling of CFRP plates with adjustable pulse duration fiber laser
Published in Materials and Manufacturing Processes, 2021
Wenyuan Li, Guojun Zhang, Yu Huang, Youmin Rong
The experiment selects CFRP plates with three thicknesses of 0.5 mm, 1 mm, 2 mm to explore the drilling efficiency under the same experimental conditions. As can be seen from figure 4, the pulse energy values have some differences under different pulse duration, the pulse energy increases as the pulse duration increases, and the increasing trend gradually slows down. However, the drilling time at different material thicknesses decreases with the increase of pulse duration, as presented in figure 4, the drilling time and pulse energy have the same changing trend as the pulse duration changes. Therefore, the pulse energy determines the processing efficiency, while the pulse duration itself has little effect. High pulse energy improves the ability of laser ablation of materials, thereby reducing cutting time. It is worth noting that there are obvious differences in the drilling time of materials with different thicknesses. Thick materials seriously increase the shielding effect in laser drilling, making it more difficult to remove the underlying material. Therefore, the laser drilling ability of materials with a thickness of 2 mm is significantly reduced.
Clinical efficiency and safety of the oticon medical neuro cochlear implant system: a multicenter prospective longitudinal study
Published in Expert Review of Medical Devices, 2020
David Schramm, Joseph Chen, David P. Morris, Nael Shoman, Daniel Philippon, Per Cayé-Thomasen, Michel Hoen, Chadlia Karoui, Ariane Laplante-Lévesque, Dan Gnansia
Cochlear implantation is a well-known treatment option for people with severe to profound sensorineural hearing loss. The Oticon Medical Neuro cochlear implant (CI) system consists of the Neuro Zti implant and the Neuro sound processor (first generation: Neuro One; second generation: Neuro 2). The Neuro Zti implant, available since 2015, has a titanium base and a zirconia casing. The surgery involves a minimally invasive pocket technique and two titanium screws secure the implant to the temporal bone [1]. The Neuro Zti implant is compatible with body and head magnetic resonance imagining (MRI) scans up to 1.5 Tesla with the magnet in place [2]. The electrical stimulation that the Neuro Zti implant delivers uses pulses with an anodic active phase and a capacitive discharge [3]. The pulse amplitude is fixed, and the pulse duration is modulated to code loudness [4]. The typical pulse amplitude is around 0.4 milliampere (mA). The pulse duration can be adjusted in steps of 1 microsecond (μs), from 10 and 120 μs. The typical pulse duration is 25 μs for T levels and 55 μs for C levels. The implant is compatible with two 20-channel electrode arrays, the Classic and the EVO. The Classic electrode array [5], stiffer than the EVO electrode array [6], is better suited for challenging anatomic situations. The EVO electrode array, used in the present study, is better suited for a soft surgery approach [7]. The EVO electrode array has an active length of 24 mm, a proximal diameter of 0.5 mm, and a distal diameter of 0.4 mm. It is flexible and has a smooth surface. It consists of 20 micro-machined titanium-iridium full-band electrodes separated by silicone rings.