Fiber Lasers: Following their Rise from Invention to Today

Fibers lasers are a very commonly used type of laser. Whether they are being used for communication purposes, materials processing, or medicine, they have come a long way from their humble beginning. This article covers the invention, development, and ultimately, the rise of fiber lasers.

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Invention of the Fiber Laser

The inventor of the fiber laser was Elias Snitzer. When he started at American Optics (AO), despite the company&#;s desire to officially establish themselves as a prominent force in fiber optics through more publications, Snitzer was not initially working towards creating a fiber-based laser. He was hired to work on projects dealing with exploring waveguide-mode propagation. Snitzer built a setup at AO to create his own fibers for his wave-guide mode studies. Creating thin enough fibers, he realized, made them act as single-mode waveguides. Snitzer thought that these supported waveguide modes in the glass fiber could make a good host for a maser, since the waveguide structure already existed.

At that time, most colleagues Snitzer encountered maintained the stance that lasing could not (and would not) occur using glass. Undeterred, Snitzer and his group at AO pressed on using different types of doped glasses. Through their attempts, they realized that by cladding the fiber and having a good interface between the core and cladding made the quality of the end reflectors less critical to the fiber&#;s success.

One of the other keys to creating the fiber laser was using the right material to dope the glass. They realized that visible fluoresces were not going to work, so they moved into the infrared. In collecting the data, he realized neodymium gave very high signals when fluorescing. From that, the research continued using different compositions of glasses doped with rare earth elements. In , they were finally able to observe lasing using the neodymium glass rods. The work was published in Physical Review Letters in .

Fiber Laser Progression

While the fiber laser was invented in , it took a good twenty years before it was developed and integrated into the commercial market. Snitzer had a hand here as well, with the invention of the double-clad fiber laser in .  Double-clad fiber lasers facilitate a higher power output. In double-clad fibers, the laser light propagates through the core, but the pump light travels through the inner cladding that surrounds the core. The inner core is then encased in an outer cladding. The refractive index of the inner cladding is lower than that of the core but higher than that of the outer cladding.

Double-clad fiber lasers are still used today. There are double-clad fiber lasers available with different geometries for the inner core. The different geometries make it easier to match properties of the pump source to the fiber for improved coupling. Snitzer contributed a substantial amount to the field of fiber optics, which, among other things and the lasers mentioned, also included inventing fiber amplifier.

In , work by IPG Photonics in Moscow improved ytterbium-doped fiber lasers to output power of 1 W. By , they recorded a single-mode fiber output of 10-kW. IPG has continued to innovate fiber lasers since that then. As fiber grew more popular and for telecommunications industry was on the rise, unsurprisingly, fiber laser research grew too, though keeping a lower profile than fiber communications.

Fiber Lasers Today

Fiber lasers today are available in multiple configurations. They can be continuous wave (CW), Q-switched, mode-locked, gain-switched, and quasi-continuous wave (QCW) to list a few. Most single-mode fiber lasers tend to be CW. QCW lasers are one of the newer types of fiber lasers. When compared to CW lasers, QCW lasers have both a higher peak power and lower average power. QCW lasers come in both the single-mode and multi-mode variety.

Most fiber lasers have erbium, neodymium, and ytterbium as their gain medium and are diode-pumped. There are different methods to pumping fiber lasers. One method of pumping is side pumping. In side pumping, the pump light is directed into the fiber from an angle close to perpendicular to the laser. Side pumping also distributes absorbed pump power smoothly. This method makes it possible to use pump sources with low spatial coherence allowing to combine different light sources to use as a pump. The other commonly used method of pumping is end pumping. End pumping usually results in a better-quality beam and higher gain than side pumping. A downside of end pumping is that unwanted higher order resonant modes can be present. While double-clad fiber lasers are usually end pumped, they can be side pumped.

Advantages of Fiber Lasers

Fiber lasers have some advantages over other laser types, dependent upon their intended use. One of the benefits of fiber lasers is their sensitivity. Since they are meant to be coupled into fiber, it is difficult to move them out of alignment, unlike other lasers. They are typically more compact than your general laser. Comparatively, fiber lasers also have a lower heat output and lower power consumption with better beam quality than their non-fiber counterparts.

Conclusion

Fiber lasers have gone from milliwatt output powers at the time of their invention to now being capable of outputting power on the order of kilowatts. For instance, the US Navy was in the process of developing a of a 30-kW laser and intends to build a weapon system housing a 150-kW fiber laser. Through the years, it is clear that innovation has improved fiber laser versatility and has rendered them invaluable to a range of industries.

Do you know any interesting or odd facts about fiber lasers? Please comment below.

If you liked this article you might also like our comprehensive guide on gas lasers.

Laser using an optical fiber as the active gain medium

A fiber laser (or fibre laser in Commonwealth English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing.

Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.[citation needed]

Characteristics

An advantage of fiber lasers over other types of lasers is that the laser light is both generated and delivered by an inherently flexible medium, which allows easier delivery to the focusing location and target. This can be important for laser cutting, welding, and folding of metals and polymers. Another advantage is high output power compared to other types of laser. Fiber lasers can have active regions several kilometers long, and so can provide very high optical gain. They can support kilowatt levels of continuous output power because of the fiber's high surface area to volume ratio, which allows efficient cooling. The fiber's waveguide properties reduce or eliminate thermal distortion of the optical path, typically producing a diffraction-limited, high-quality optical beam. Fiber lasers are compact compared to solid-state or gas lasers of comparable power, because the fiber can be bent and coiled, except in the case of thicker rod-type designs, to save space. They have lower cost of ownership.[1][2][3] Fiber lasers are reliable and exhibit high temperature and vibrational stability and extended lifetime. High peak power and nanosecond pulses improve marking and engraving. The additional power and better beam quality provide cleaner cut edges and faster cutting speeds.[4][5]

Design and manufacture

Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing different types of fiber; fiber Bragg gratings replace conventional dielectric mirrors to provide optical feedback. They may also be designed for single longitudinal mode operation of ultra-narrow distributed feedback lasers (DFB) where a phase-shifted Bragg grating overlaps the gain medium. Fiber lasers are pumped by semiconductor laser diodes or by other fiber lasers.

Double-clad fiber

Many high-power fiber lasers are based on double-clad fiber. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer. The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it, and allows the conversion of pump light with relatively low brightness into a much higher-brightness signal. There is an important question about the shape of the double-clad fiber; a fiber with circular symmetry seems to be the worst possible design.[6][7][8][9][10][11] The design should allow the core to be small enough to support only a few (or even one) modes. It should provide sufficient cladding to confine the core and optical pump section over a relatively short piece of the fiber.

Tapered double-clad fiber (T-DCF) has tapered core and cladding which enables power scaling of amplifiers and lasers without thermal lensing mode instability.[12][13]

Power scaling

Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area (LMA) fibers as well as continuing advances in high power and high brightness diodes, continuous-wave single-transverse-mode powers from Yb-doped fiber lasers have increased from 100 W in to a combined beam fiber laser demonstrated power of 30 kW in .[14]

High average power fiber lasers generally consist of a relatively low-power master oscillator, or seed laser, and power amplifier (MOPA) scheme. In amplifiers for ultrashort optical pulses, the optical peak intensities can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or other optical elements may occur. This is generally avoided by employing chirped-pulse amplification (CPA). State of the art high-power fiber laser technologies using rod-type amplifiers have reached 1 kW with 260 fs pulses [15] and made outstanding progress and delivered practical solutions for the most of these problems.

However, despite the attractive characteristics of fiber lasers, several problems arise when power scaling. The most significant are thermal lensing and material resistance, nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), mode instabilities, and poor output beam quality.

The main approach to solving the problems related to increasing the output power of pulses has been to increase the core diameter of the fiber. Special active fibers with large modes were developed to increase the surface-to-active-volume ratio of active fibers and, hence, improve heat dissipation enabling power scaling.

Moreover, specially developed double cladding structures have been used to reduce the brightness requirements of the high-power pump diodes by controlling pump propagation and absorption between the inner cladding and the core.

Several types of active fibers with a large effective mode area (LMA) have been developed for high power scaling including LMA fibers with a low-aperture core,[16] micro-structured rod-type fiber [15][17] helical core [18] or chirally-coupled fibers,[19] and tapered double-clad fibers (T-DCF).[12] The mode field diameter (MFD) achieved with these low aperture technologies [15][16][17][18][19] usually does not exceed 20&#;30 μm. The micro-structured rod-type fiber has much larger MFD (up to 65 μm [20]) and good performance. An impressive 2.2 mJ pulse energy was demonstrated by a femtosecond MOPA [21] containing large-pitch fibers (LPF). However, the shortcoming of amplification systems with LPF is their relatively long (up to 1.2 m) unbendable rod-type fibers meaning a rather bulky and cumbersome optical scheme.[21] LPF fabrication is highly complex requiring significant processing such as precision drilling of the fiber pre-forms.  The LPF fibers are highly sensitive to bending meaning robustness and portability is compromised.

Mode locking

In addition to the types of mode locking used with other lasers, fiber lasers can be passively mode locked by using the birefringence of the fiber itself.[22] The non-linear optical Kerr effect causes a change in polarization that varies with the light's intensity. This allows a polarizer in the laser cavity to act as a saturable absorber, blocking low-intensity light but allowing high intensity light to pass with little attenuation. This allows the laser to form mode-locked pulses, and then the non-linearity of the fiber further shapes each pulse into an ultra-short optical soliton pulse.

Semiconductor saturable-absorber mirrors (SESAMs) can also be used to mode lock fiber lasers. A major advantage SESAMs have over other saturable absorber techniques is that absorber parameters can be easily tailored to meet the needs of a particular laser design. For example, saturation fluence can be controlled by varying the reflectivity of the top reflector while modulation depth and recovery time can be tailored by changing the low temperature growing conditions for the absorber layers. This freedom of design has further extended the application of SESAMs into modelocking of fiber lasers where a relatively high modulation depth is needed to ensure self-starting and operation stability. Fiber lasers working at 1 μm and 1.5 μm were successfully demonstrated.[23][24][25][26]

Graphene saturable absorbers have also been used for mode locking fiber lasers.[27][28][29] Graphene's saturable absorption is not very sensitive to wavelength, making it useful for mode locking tunable lasers.

Dark solitons

In the non-mode locking regime, a dark soliton fiber laser was successfully created using an all-normal dispersion erbium-doped fiber laser with a polarizer in-cavity. Experimental findings indicate that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations the dark pulse formation in the laser may be a result of dark soliton shaping.[30]

Multi-wavelength emission

Multi-wavelength emission in a fiber laser demonstrated simultaneous blue and green coherent light using ZBLAN optical fiber. The end-pumped laser was based on an upconversion optical gain media using a longer wavelength semiconductor laser to pump a Pr3+/Yb3+ doped fluoride fiber that used coated dielectric mirrors on each end of the fiber to form the cavity.[31]

Fiber disk lasers

Another type of fiber laser is the fiber disk laser. In such lasers, the pump is not confined within the cladding of the fiber, but instead pump light is delivered across the core multiple times because it is coiled in on itself. This configuration is suitable for power scaling in which many pump sources are used around the periphery of the coil.[32][33][34][35]

Applications

Applications of fiber lasers include material processing, telecommunications, spectroscopy, medicine, and directed energy weapons.[36]

See also

References

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