Femtosecond Lasers: what they are and how they are used

There are several laser systems currently available, and each is based on early studies on light amplification through accelerated discharge of laser radiation. This revolutionary technology currently has two separate operating modes: continuous wave (CW) and pulsed method.

The refractive power of a CW laser remains constant throughout time, but pulsed lasers changes at a pre-defined pulse repetition frequency (PRF and emit pulses of varying durations. These frequencies correspond to the time domain of the pulsed lasers, which can have shorter pulse durations as short as a picosecond (10-12 seconds).

Companies are becoming capable of developing femtosecond lasers, that are pulsed method equipment with pulse durations in the femtosecond range (10-15). In this article, we will discuss femtosecond lasers, how it functions and how they’re used.

What is Femtosecond Laser?

A femtosecond laser is an instrument that creates light pulses of infrared (IR) laser light beam with femtosecond lengths. It generates incredibly quick pulse energy of laser power at an incredibly higher speed. Ultrashort pulses are the short surge of electromagnetic energy with a length of picoseconds or even less, with maximum energies of megawatts (MW) or greater and similarly significant PRF.

To generate femtosecond laser beams with high peak powers, light amplification techniques such as chirped pulse amplification must be used (CPA). It is a technique for magnifying ultrashort laser beams and achieving maximum energies in the range of petawatts (PW). To produce large peak output ultrashort lights with higher pulse repetition frequencies, every large power femtosecond laser use some kind of CPA.

History of Femtosecond Lasers

In the 1960s and 1970s, groundbreaking studies and development exploring lasers with gradually shorter pulses produced the most practical breakthroughs. Although mode-locked dye lasers could create laser beams in the femtosecond region, equipment restrictions made them unsuitable for most anticipated femtosecond laser uses.

The titanium-sapphire laser, invented in 1982, was the first genuine milestone for commercializing femtosecond laser innovation. Soon afterward, a mode-locked titanium-sapphire laser was combined with a CPA system, winning the scientists involved the Nobel Prize.

In the years afterward, femtosecond lasers have been accessible employing a variety of gain sources, with more amplifying techniques joining the mix to enhance the possible pulse wavelengths and rates of ultrashort pulses.

Types of Femtosecond Lasers

Controlling the various factors of femtosecond lasers like amplification, bandwidth, emission range, and so forth requires CPA. Mode-locking methods manage those laser characteristics and provide high-quality powerful laser energy pulses. Mode-locking ones that have the ability to generate significant femtosecond light pulses are:

Solid-state Bulk Lasers: It can generate significant ultrashort pulses having average lengths ranging from 30 fs to 30 ps. In this domain, several diode-pumped lasers active laser medium and function with normal usual output levels ranging from 100 mW to 1 W.

Although there are lower repetition frequency variants with a little megahertz for high pulse energy, as well as small lasers with tens of GHz frequency range, the PRF in solid state lasers is typically around 50 MHz and 500 MHz.

Fiber Lasers: Superfast fiber lasers are generally complacently mode-locked, have pulse durations ranging from 50 to 500 fs, repetition frequency ranging from 10 to 100 MHz, and also mean powers ranging from a few mW to tens of times.

Even though the initiative needed to produce a device with better efficiency and higher reliability can be significant because of several technical hurdles, every fiber option can be relatively cost-effective in large-scale manufacturing. Highly nonlinear impacts necessitate rather difficult operating principles for superior efficiency, implying that its management is far more complex than for solid state lasers.

Dye Lasers: Before the introduction of titanium-sapphire lasers, dye lasers reigned the area of ultrashort pulse production. Its gain frequency enables pulse periods in the range of 10 fs, and several lasers are appropriate for output at varying wavelengths, frequently in the visible portion of the spectrum. Because of the drawbacks of managing it and the short dye lifespan, they aren’t any more widely employed – especially in spectral areas.

Semiconductor Lasers: Certain mode-locked semiconductor lasers can create femtosecond pulses. The pulse lengths of such lasers are normally a few 100 femtoseconds, although with additional pulse compressing, considerably lower pulse lengths can be obtained. Semiconductor lasers can also attain high pulse repetition rates in the 10s or 100s of GHz range.

Nevertheless, throughout most circumstances, the pulse energy is significantly confined to the picojoule range. Due to the obvious wide mode surface and short propagation distance in the semiconductor, their pulse energy can be significantly greater, although much less than for solid state lasers in general, owing to the modest gain threshold value.

Several femtosecond laser machines are not technically speaking femtosecond lasers since they include necessary extra features including an operational amplifier or equipment for the nonlinear frequency converter required to function in many other wavelength ranges. Color focus lasers and free-electron lasers are far more extraordinary versions of femtosecond lasers. It is possible to make the latter generate femtosecond pulses in the manner of X-rays.

How do Lasers generate Femtosecond Laser Pulses?

Several kinds of lasers can generate femtosecond pulses. Now, how might you have a laser generate pulses rather than continuous wave discharges? It almost always occurs in the laser’s resonator. The light caught within an optic resonator produces a given value of surface waves with varying wavelengths, following the resonance frequency.

The higher standing waves or phases present within a resonator, the wider the range of the laser beam. The amount of these phases is also determined by the spectrum range of the lasers’ lasing medium.

The method utilized to create a laser pulse inside the resonator is known as mode-locking. Usually, each of these resonator phases oscillates separately and arbitrarily. Mode locking causes resonator mechanisms to oscillate in sequence, resulting in mode intervention and, as a result, steep pulses that move in between the resonators before exiting in one path to generate a solitary laser pulse.

In general, the smaller the pulse length which can be obtained, hence more phases that are mode-locked in their rhythm. The PRFs of femtosecond lasers vary both in length and frequency. They may vary spanning a few MHz to many GHz.

Advantages of Femtosecond Laser Technology

Inside one light beam, femtosecond lasers concentrate power in a rather small timeframe. This results in increased maximum energies that much exceed the energy peaks possible by CW lasers. Providing rapid, high-intensity laser emission raises the potential for all types of laser usage.

Ultrashort pulses reduce heat injuries to materials in industrial settings like the production process. Essentially, the laser pulse functions as a relatively limited, powerful heating element, quickly evaporating materials in the focused zone without causing significant heat loss in the external environment.

An additional benefit for modern innovations is the incredibly short length of the laser pulse, which allows femtosecond lasers to be used to monitor and influence ultrashort operations, like in biology or chemistry.

Applications of Femtosecond Laser Technology

Femtosecond laser technology offers a very broad variety of applications that use relatively distinct features of the pulses. Here are some common instances:

Laser Material Processing

Femtosecond laser technology is largely used in laser material production. Maximum energies of picosecond and femtosecond pulses are much greater than those of nanosecond pulses of similar pulse energy. As a result, the component can get vaporized much faster, potentially improving production-grade in a variety of conditions.

Nevertheless, femtosecond lasers may not always be more appropriate than picosecond lasers, especially if their pulse duration is already less than the electron-photon bonding period. An additional consideration of femtosecond technology is that the extraordinarily high light energies attained with femtosecond pulses produce nonlinear characteristics that can be harnessed.

As multiphoton absorbing accompanied by avalanche ionizing gets particularly powerful, this laser energy can be collected including in truly transparent media like glasses or crystals. These substances aren’t any more transpicuous to laser energy. In this sector, femtosecond pulse lengths could be advantageous, if not required.

Femtosecond lasers can be used to laser cuts a broad variety of materials, notably metals, polymeric materials (plastics), ceramic materials, glasses, semiconductors, and crystallized dielectric materials (including diamonds). Sometimes, a similar laser system may be utilized to treat distinctly diverse materials.

Medical Uses

Femtosecond technology is often utilized in medicine, mostly in optical system for femtosecond laser surgery. For instance, femtosecond pulses are now often used in effective lens position, ophthalmic surgery (sight correction), such as Femto-LASIK or Lens replacement surgery. It’s another application in which the incredibly tiny pulse durations come in handy. In plenty of other medical uses, femtosecond lasers are used for medical diagnostics. Laser microscopy techniques are very useful in this context.

After laser technology was used to remove excess corneal surrounding tissue, femtosecond laser technology was utilized to allow greater precision intrastromal tissue control and elimination. The femtosecond laser platforms offer several therapeutic benefits based on corneal surgery. The femtosecond laser has indeed enabled precise positioning refractive procedures in ophthalmology, but many are eagerly anticipating the growth of new clinical outcomes.

To begin with, femtosecond laser assisted cataract surgery is nearing acceptability, with numerous surgeons now studying its safety and efficacy. Diagnostic tools are also being incorporated onto the femtosecond laser platform to assess the patient’s eye immediately under the instrument. This refractive procedure collects critical corneal characteristics, such as corneal thickness, whereas the eye is docked, ensuring that eye surgery is limited to the specified region.

Laser Microscopy

Femtosecond laser assisted applications are also becoming extremely significant in laser microscopy, such as fluorescence imaging. In this case, multiphoton stimulation (depending on multiphoton absorbing) is widely used, and quite small pulse periods are preferable. Stimulated Raman spectroscopy can also be used.

Measurements

Femtosecond lasers can be used for a broad variety of measurements. They are crucial in current optical timepieces, for instance, functioning as an extremely reliable frequency standard as well as a visual clockwork that creates a phase-coherent connection among several distinct optical and microwave frequencies.

There are several dimension uses, like LIDAR distance estimation, interferometry, and pump-probe dimensions in femtosecond laser assisted usage. The latter technique enables the investigation of ultrafast activities using femtosecond laser platforms, such as those seen in chemistry and biochemistry.

Telecommunications

Femtosecond lasers can be employed in a variety of methods in optical fiber communication systems. By spectrum splitting of bandwidth femtosecond pulses, for instance, it is feasible to create dense wavelength division multiplexing with a rather huge channel capacity (often >1000). Furthermore, using time-division multiplexing, one may attain extraordinarily large data rates of >1 Tbit/s.

Conclusion

The evolution of femtosecond laser technology has enhanced its capability for use in a variety of industries. As a cutting-edge technique, femtosecond laser technology provides several benefits in a variety of medical settings. With the continued development and enhancement of industrial femtosecond lasers with excellent dependability, they’ll be employed in a wider range of applications.