Fiber lasers are everywhere in the modern world. Due to the different wavelengths they can generate, they are widely used in industrial environments to perform cutting, marking, welding, cleaning, texturing, drilling and a lot more. They are also used in other fields such as telecommunication and medicine.
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Fiber lasers use an optical fiber cable made of silica glass to guide light. The resulting laser beam is more precise than with other types of lasers because it is straighter and smaller. They also have a small footprint, good electrical efficiency, low maintenance and low operating costs.
If you want to learn everything you need to know about fiber lasers, keep reading.
Elias Snitzer invented the fiber laser in and demonstrated its use in . Serious commercial applications only emerged in the s, however.
Why did it take so long? The main reason is that fiber laser technology was still in its infancy. For example, fiber lasers could only emit a few tens of milliwatts whereas most applications require at least 20 watts. There was also no means of generating high-quality pump light, as laser diodes did not perform as well as today.
Here are some of the key moments in the history of fiber laser technology, going back as early as when Albert Einstein established its foundations.
Today, important advances are still being made in fiber laser technology, making it more efficient, powerful and accessible. Some of the most upcoming applications include laser cleaning and laser texturing, which can replace polluting technologies and help make the world greener.
Generally speaking, fiber lasers can be categorized using the following criteria:
Fiber lasers can be categorized in many other ways, but the categories mentioned here are the most common. Follow these links if you want to see examples of fiber lasers integrated into products:
The main difference between fiber and CO2 lasers is the source where the laser beam is created. In fiber lasers, the laser source is silica glass mixed with a rare-earth element. In CO2 lasers, the laser source is a mixture of gases which includes carbon dioxide.
Due to the state of their source, fiber lasers are considered solid-state lasers, and CO2 lasers are considered gas-state lasers.
These laser sources also produce different wavelengths. Fiber lasers, for example, produce shorter wavelengths, with some examples ranging between 780 nm and nm. CO2 lasers, on the other hand, produce longer wavelengths that typically range between 9,600 nm and 10,600 nm.
They are used for different applications due to their different wavelengths. For example, nm fiber lasers are usually preferred for metal processing applications. Laser cutting is a notable exception, where CO2 lasers are often preferred to cut metals. CO2 lasers also react well with organic materials.
If you're debating between the two, read our post on choosing between a CO2 and a fiber laser.
When a fiber laser system is engineered into a solution that is ready to be used, that solution is called a fiber laser machine. Whereas the OEM laser system is the tool that performs the operation, the laser machine is the framework in which the tool is integrated.
Laser machines can make sure that:
For example, the fiber laser machine shown here includes a rotary table, a rotary indexer, a Class-1 laser safety enclosure, a fume extractor, a vision camera and an HMI.
Follow these links if you want to see more examples of fiber laser machines:
Most online sources claim that fiber lasers last 100,000 hours whereas CO2 lasers last 30,000 hours. This is not entirely true. These numbers refer to a value called 'mean time between failures' (MTBF), which isn't the same for all fiber lasers. In reality, you will see different numbers for different types of fiber lasers.
The MTBF measures the reliability of a laser by indicating how many hours the laser is expected to function before a failure occurs. It is obtained by testing multiple laser units, and then dividing the total number of operational hours by the total number of failures.
Although this value does not exactly tell you how long a fiber laser can last, it still provides a good idea of the laser's reliability.
If you really want to know the exact lifespan of a fiber laser, you'll be disappointed as there's no real answer. In truth, fiber lasers have critical points in their lifetime when they can fail.
Here's what you need to know if your laser experiences failures at any of these moments:
Fiber lasers use pump light from what is called laser diodes. These diodes emit light that is sent into the fiber-optic cable. Optical components located in the cable are then used to generate a specific wavelength and amplify it. Finally, the resulting laser beam is shaped and released.
Here's how each component is used to perform this operation.
Laser diodes transform electricity into photons'or light' to be pumped into the fiber-optic cable. For this reason, they are also known as the 'pump source'
To generate light, diodes use two semiconductors charged differently:
When the positive and negative charges meet, they try to combine. But to do so, the free electron must be released as a photon. As current flows through the semiconductors, the quantity of photons quickly increases.
The resulting light is pumped into the fiber-optic cable and will be used to generate the laser beam.
In nature, light goes in all directions. To focus light into a single direction and obtain a laser beam, fiber-optic cables use two basic components: the fiber core and the cladding.
Total internal reflection occurs because the cladding has a lower refractive index than the core. You can see similar effects in nature. For example, if you look at submerged objects, they appear deformed. This is because when light travels from air to water, it hits a different refractive index and changes direction. The same applies when light travels from the core to the cladding, except that the change in direction produces a reflection.
Without the cladding, light would go in all directions and exit the core. But thanks to the cladding's refractive index, light remains in the core and continues its path.
To visualize how light travels in fiber cables, you can watch this video:
As pump light travels through the fiber-optic cable, it eventually enters the laser cavity'a small region of the cable where only light of a specific wavelength is produced. Physical engineers say that the fiber is 'doped' in this region because it has been mixed with a rare-earth element.
As particles from the doped fiber interact with light, their electrons rise to a higher energy level. When they fall back to their basic state, they release energy in the form of photons or light. Physical engineers refer to these phenomena as 'electron excitation' and 'electron relaxation'.
The laser cavity also acts as a resonator where light bounces back and forth between what is called 'fiber Bragg gratings'. This leads to 'Light Amplification by the Stimulated Emission of Radiation', or LASER. Put simply, this is where the laser beam is formed.
There are two types of Bragg gratings:
Here's how amplification takes place: when photons hit other excited particles, these particles also release photons; since the Bragg gratings reflect photons back into the cavity, and more pump light is sent into the cavity, an exponential number photons are released.
As a result of this stimulated emission of radiation, laser light is created.
The wavelength produced by the doped fiber varies according to the doping element of the laser cavity. This is very important, as different wavelengths are used for different applications. The doping element could be erbium, ytterbium, neodymium, thulium, and so on. Ytterbium-doped fiber lasers, for example, generate a wavelength of nm and are used for applications like laser marking and laser cleaning.
Different doping elements produce different wavelengths because specific particles release specific photons. As such, photons generated in the laser cavity all have the same wavelength. This explains why each type of fiber laser generates a specific wavelength'and only that wavelength.
Photons that exit the resonant cavity form a laser beam that is extremely well collimated (or straight) due to the fiber's light guiding properties. In fact, it is too collimated for most laser applications.
To give the laser beam a desirable shape, different components can be used, such as lenses and beam expanders. For example, our fiber lasers are equipped with a 254 mm focal length lens for laser applications that dig into the material (i.e., laser engraving and laser texturing). This is because their short focal length allows us to focus more energy onto an area for a more aggressive form of laser ablation.
Other types of lenses provide different advantages, which is why experts choose them carefully when optimizing a laser for a specific application.
Not all lasers and laser applications use the same parameters. For example, different ones need to be adjusted for laser cutting and laser marking. Some parameters, however, are used for all types of fiber lasers. Here are the ones you are most likely to encounter.
The wavelength produced by a fiber laser corresponds to the level of electromagnetic radiation of the laser light. Typically, fiber lasers produce wavelengths between 780 nm and nm, which is located in the infrared spectrum and is invisible to the human eye. This range of infrared light tends to react well with metals, rubber and plastics, making it useful for a wide range of materials processing applications.
Some fiber lasers such as green lasers produce visible light which can react well with soft materials such as gold, copper, silicone and soft glass. Green fiber lasers are also used for holography, therapy and surgery, among other things.
These lasers require additional components to generate visible light. John Wallace from Laser Focus World explains how this is done:
['] there is actually no fiber laser on the market that produces visible laser light from within the lasing fiber itself. Visible light can, however, be obtained from a near-infrared (IR)-emitting fiber laser by external frequency conversion'for example, Raman-shifting, frequency-doubling, frequency sum-mixing, or combinations of these approaches.
Excerpt from
Photonics Products: Fiber Lasers: Visible fiber lasers do red, green, and now bluishby
Laser Focus World
The mode of operation is the way in which the laser beam is released. Fiber lasers typically operate in the continuous-wave or in the pulsed mode.
In the continuous-wave operation mode, a continuous, uninterrupted laser beam is released, which is ideal for applications like laser welding and laser cutting.
In the pulsed operation mode, short pulses are released at a set repetition rate. Pulsed laser beams reach higher peak powers and are ideal for laser engraving and laser cleaning. This mode includes the following parameters:
The laser power is the amount of energy that can be produced by the laser over one second. It is also known as 'average power' and 'output power'.
Pulsed lasers may also indicate a peak power, which is a different parameter. The peak power is the maximum amount of energy reached by a single pulse. For example, a 100W pulsed fiber laser can easily reach 10,000W of peak power. This is because pulsed lasers do not distribute energy evenly over time as opposed to continuous-wave lasers.
The beam quality indicates how close the beam is to what is called a Gaussian beam. In actual applications, this is relevant because it indicates how well focused the laser beam is.
Mathematically speaking, a perfect beam quality is expressed as M2=1. Laser beams that are well-focused concentrate more energy in a smaller area. High-quality laser beams are required for applications like laser engraving and laser cleaning, whereas lower beam qualities may be more appropriate for applications where ablation is not desired, such as laser welding.
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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
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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
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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
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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
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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
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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
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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
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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
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Three fiber disk lasersAnother 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
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Applications of fiber lasers include material processing, telecommunications, spectroscopy, medicine, and directed energy weapons.[36]
See also
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References
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