Which laser is best for cutting?

Laser using an optical fiber as the active gain medium

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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

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

This article will discuss the 4 types of laser cutters, how they work, and their applications.

1. Fiber Lasers

Fiber lasers are used principally for cutting and engraving metallic parts. They offer several advantages over other types of lasers, making them a logical choice in industrial applications.

Fiber lasers get their name from the chemically doped optical fiber used to induce the lasing and deliver the energy to the cutting point. The laser source starts with a primer laser, usually a diode type, which injects a low-power beam into the fiber. This beam is then amplified within the optical fiber, which is doped with rare earth elements such as ytterbium (Yb) or erbium (Er). The doping process induces the fiber to act as a gain medium, amplifying the laser beam by cascading excitations/emissions.

Fiber lasers emit a wavelength in the near-infrared spectrum, around 1.06 μm. This wavelength is thoroughly absorbed by metals, making fiber lasers particularly well suited to cutting and engraving this class of materials, even the &#;problem&#; reflective metals. 

One of the particular advantages of fiber lasers is their exceptional beam quality. This beam quality determines the laser's ability to produce a highly focused application of radiation and therefore a smaller and more precise cut path and higher specific energy (energy per unit area). This also entails lower beam divergence, allowing cuts that open less with increased target thickness.

Fiber lasers are renowned for offering higher cutting speeds and productivity. This also contributes to lower power consumption, compared to other types of lasers. Fiber lasers are generally optimized for cutting metals, including stainless steel, carbon steel, aluminum, copper, brass, and various alloys. They are not as effective for cutting non-metallic materials like wood, acrylic, or plastics, which are more effectively cut with CO2 lasers. Fiber lasers with higher power levels can also process thicker metals effectively.

Fiber lasers possess an elegant, simple, and robust construction and a near-solid state characteristic. This results in suppressed maintenance requirements, relative to other laser classifications. The absence of mirrors and some of the more delicate focal components minimizes alignment issues, improves beam quality, and elevates life span. Some models are capable of providing tens of thousands of hours of use, before requiring significant maintenance.

Fiber lasers are, in many regards, the optimal choice for metal cutting/ablation and engraving tasks. Pivotal factors cementing their commercial viability include: delivering high throughput, outstanding precision, operational and power efficiency, and low maintenance. Their capabilities render them a preferred tool in diverse industries, including: automotive, aerospace, electronics, and manufacturing, in which precise and efficient metal processing is crucial.

For more information, see our guide on What is a Fiber Laser.

2. CO2 Lasers

Despite being the earliest commercially exploited devices, CO2 lasers remain very widely used in the sector. They benefit from lower CAPEX (though higher OPEX) and a high degree of material versatility/applicability. They&#;re particularly suited to processing non-metallic materials with moderate precision and efficiency. They are also considered viable in many metal-cutting applications. For metal processing, the absorption spectrum is adverse but various, widely used workarounds can facilitate better functionality.

CO2 lasers are gas excitation devices that use a mixture of carbon dioxide (CO2), nitrogen (N2), and helium (He) to produce the laser beam in an energy cascade sequence. The laser source typically consists of a xenon flash tube or similar, which is excited by an electric discharge to initiate the stimulated emission process. This process is characterized by three distinct energy transitions, only the last of which involves a photon emission. N2 molecules are raised to a higher energy state that they then transmit to the CO2 molecules, which emit photons as they lose their excision energy by impacting He atoms.

This class emits at around 10.6 μm, in the far-infrared spectrum. This wavelength is strongly absorbed by organic materials like wood, plastics, leather, various fabrics, paper, and some non-metallic composites, resulting in highly efficient, clean, and precise cutting.

They have a lower beam quality in comparison to fiber lasers, which means the laser beam is less focused. This is a byproduct of the relative optical complexity of the devices and is also intrinsic to the gas emission system. However, advancements in CO2 laser technology have improved beam quality over the long service lifetime of the technology. The beam typically generates a larger spot size and higher divergence than other systems, which can markedly affect the precision of cuts.

CO2 lasers are widely accepted because of their versatility, relatively low purchase cost, and higher power use per watt of cutting. They can be considerably slower in cutting thick metal materials than fiber lasers. For non-metallic materials, they can offer excellent cutting speed, making them suitable for intricate designs and a wide range of applications. CO2 lasers require more maintenance than fiber lasers, due to the presence of mirrors and other optical components in their design. Additionally, the primary laser source degrades with usage time. They need regular optical-system cleaning and delicate realignment to maintain performance.

For more information, see our guide on CO2 laser cutters.

3. Nd:YAG/Nd:YVO Lasers

Nd:YAG (neodymium-doped yttrium aluminum garnet) and Nd:YVO (neodymium-doped yttrium vanadate) lasers are fundamentally similar solid-state devices. Both emit in the near-infrared spectrum, differentiated by the medium within which the stimulated emission occurs. They are most applicable to cutting and marking of metals and a limited range of non-metals.

Nd:YAG and Nd:YVO lasers are closely related solid-state laser devices doped with neodymium ions. In Nd:YAG lasers, the laser medium is yttrium aluminum garnet crystals doped with neodymium ions. In Nd:YVO lasers, the laser medium is yttrium vanadate crystals similarly doped with neodymium ions. When optically pumped (by a laser or discharge source), the neodymium ions become excited. This leads to the emission of laser light, as they lose the excitation energy.

These lasers emit at a wavelength of 1.064 μm, while Nd:YVO lasers emit at either 1.064 μm or 1.34 μm, differentiated by the crystal orientation. These wavelengths are in the near-infrared range and are well-absorbed by many metals, making these lasers suitable for metal cutting, engraving, and marking applications. Neodymium lasers generally possess high beam quality, low divergence, and a small spot size resulting in high specific energy.

Nd:YAG and Nd:YVO lasers are effective for cutting and processing metals, especially thin sheets and high-precision and lower gauge materials. They are best used for metals, including the more &#;reflective&#; materials: stainless steel, carbon steel, aluminum, brass, and copper. They are also suited to cutting ceramics, plastics, and certain composites&#;but they are poorly adapted to cutting other non-metallic materials. These laser types are well appreciated for their durability and relatively low maintenance requirements, improving up-time and commercial performance. They can provide thousands of hours of use before requiring major maintenance.

4. Direct Diode Lasers

Direct diode (or simply diode) lasers are a type of laser technology that utilizes single semiconductor junctions to generate laser light. They are increasing in market penetration in industrial applications, including: cutting, welding, and surface treatment. A direct diode laser is based on semiconductor junctions, typically made of gallium arsenide (GaAs). When a forward bias current is applied to the diode, it emits light by electroluminescence, without requiring a light source for initiation. The emitted light is then guided and focused into a laser beam by optical elements that make a stimulated emission resonant cavity with a half mirror at one end, through which the laser energy is emitted.

Diode lasers are available in a range of wavelengths, varied by the selection of the semiconductor material, dopants, and resonant cavity design. The most common wavelengths for direct diode lasers used in cutting applications are in the near-infrared spectrum, around 900 to 1,100 nm (0.9 to 1.1 μm). Alternate diode systems can emit in the blue and green wavelength ranges. The beam quality of direct diode lasers can vary considerably, though in general diode beam quality is improving with each device generation. Beam quality often does not match that of fiber lasers or CO2 lasers.

Diode lasers offer excellent energy efficiency by low loss conversion of electrical energy into laser light, reducing operating costs somewhat. However, their cutting speeds are generally lower than for fiber or CO2 laser-based devices, when material thicknesses are larger. Direct diode lasers are suitable for cutting a variety of materials, including metals, plastics, composites, and certain non-metallic materials. They are considered effective for high-speed cutting or welding of thin metal sheets, making them suitable for industries such as automotive, electronics, and sheet metal manufacture. This family of devices is simpler and more robust in construction than most other laser types, resulting in a longer operational life span and lower maintenance requirements. They are also of smaller physical size and require fewer ancillary devices, further enhancing maintenance and suitability for mobile applications.

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