What type of laser to cut brass?

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]

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Applications

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

See also

References

One of the most commonly used metals for various applications across a range of industries is brass.

Providing manufacturers with a dark gold colouring and shiny appearance, brass is a great choice for applications ranging from decorative items to plumbing parts, household appliances, electrical equipment, and various types of machinery.

Brass is not only practical and decorative; it is also incredibly versatile. A blend of varying levels of zinc and copper, the material brass can produce different mechanical and chemical properties, which steel metalworks use to benefit all applications.

Sheet metal fabrication involves a series of varied processes, one of which is laser cutting and engraving.

Laser cutting is a process suitable for brass; however, brass is a reflective material, making laser cutting challenging for those with little to no experience.

In this post, we look at laser-cutting brass sheets and why working with a professional fabrication team is the best way to ensure a high-quality finished product.

What is brass laser cutting?

Brass laser cutting involves using laser technology where a laser beam is emitted from the equipment using energy to heat the brass and provide the cut.

A gas is then used alongside the laser to blow out and remove the laser-cut metal, providing a clean and precise cut.

Brass laser cutting has no cutting force to the brass, so there is no deformation of the material, allowing a professional sheet metal contractor the ability to achieve precise processing without any burrs, as well as the ability to handle large sheet sizes and varying thickness levels.

Fibre lasers tend to be used over C02 lasers as they can avoid the high reflectivity of the brass, allowing for a clearer cut. Fibre lasers also offer a higher power output and a shorter wavelength (meaning less reflectivity), which is required for brass to avoid burrs from occurring.

Fibre lasers can also melt brass quicker due to their greater energy output, and brass must be in a molten state for a clean cut.

Fibre laser cutting cuts quickly and precisely, with the cutting slit narrow and smooth. This laser technology can also handle a variety of complex patterns, making it suitable for various design applications and specifications.

(Check out one of our other posts on `what is aluminium laser cutting` to find out more about the various materials and fabrication processes required to achieve a quality cut outcome.)

Brass engraving is also popular, as we see it used on brass plates and trophy engravings. Brass engraving is the process of removing the upper surface of the brass material only, to create distinct patterns and etchings.

Laser-cutting brass sheet

Brass is a non-ferrous metal alloy that offers good abrasion resistance and a shiny surface.

However, cutting brass requires a high level of efficiency and precision, and you must be aware that:

Brass is highly reflective of infrared light, and if not managed or handled appropriately, it can reflect the laser beam, causing problems for the laser optics and even the person managing the equipment.

The laser energy used to cut brass is not absorbed well, as most of the laser is reflected. This makes laser cutting brass extremely challenging and a process that should only be carried out by experienced sheet metal fabricators.

The brass must be molten to lower its reflectivity and enable the cut. The more you can lower the reflectivity, the more you can improve the laser energy absorption, which leads to a cleaner cut.

Using laser for brass cutting

For a successful brass cut:

Use the correct power setting &#; this should typically be set high, i.e., the maximum the machine can provide, as this reduces the time for the brass to become molten, reducing the time the brass is at its highest reflectivity. Ultimately, the higher the laser power, the faster the cut.

Opt for the right cutting speed &#; this is often set at a low speed; we recommend 10 to 15% less than the machine&#;s maximum. The slower speed makes brass cutting much easier.

Position the point of focus as close to the top of the brass sheet as possible without affecting the material&#;s quality.

Choose the right cutting gas&#;for brass, nitrogen is the best choice, as this gas can mechanically remove the cut metal, once it is in its molten state effectively. Nitrogen also helps prevent the metal from forming back again after the laser has passed through.

Material size&#;You need to be aware of the material size you&#;re working with, as you will need to use the right tools and equipment, such as a machine with the right bed size to accommodate the sheet metal in question.

Sheet metal fabricators

Offering you precision, speed, efficiency, and a cost-effective brass cutting solution, the team at Morfabrication are experts in the field of laser cutting sheet metal.

We work with your design specifications and can handle even the most intricate and complex cuts.

Providing you with a finished project that is clean, has smooth edges, minimal burring, and a production process that causes minimal waste.

If you have a project coming up that requires precision fabrication, contact the team at Morfabrication today to see how we can help.

If you are looking for more details, kindly visit Fiber Laser Uncoiler Production Line.