LASERS, LASER TECHNOLOGY, TERMS,PHYSICS, and information to help targeted individuals understand LASER/MASER technology
INTRODUCTION to LASERS and LASER technology
The term “Laser” is an acronym which stands for “Light Amplification by Stimulated Emission of Radiation“ and is basically a device that converts energy into light.
A laser typically consists of a laser cavity or resonator. In its simplest form a laser cavity is made up of two mirrors: a total reflector and a partial reflector (also called output coupler) between which light bounces back and forth. Some of this light escapes the cavity through the output coupler producing an output beam.
This laser beam, as opposed to ordinary light sources, is monochromatic, directional and coherent. These properties allow for lasers to be projected over long distances thanks to their extreme brightness without spreading significantly and to be focused to a very small spot, which results in a very large power density, acting almost as a true point source.
Inside the laser cavity light gains intensity by oscillating through the gain medium (also called the lasing medium). In most lasers this medium consists of atoms which have been “excited” by means of an electrical current or light (e.g. by a flash lamp). Gain medium can be of any state: gas, liquid, solid, or plasma.
This process of transforming atoms into their excited states is called “pumping”. Once excited, atoms convert this stored energy into light in a process known as “stimulated emission”. Light then travels back and forth between the two reflecting mirrors and is reinforced by constructive interference only at specific wavelengths, called longitudinal modes of the cavity.
The exact wavelength(s) at which emission occurs are determined by the gain bandwidth of the gain medium, the spectral characteristics of the cavity mirrors and the longitudinal modes of the cavity at which constructive interference occurs. The cavity also controls the so-called transverse modes, i.e. the electromagnetic field pattern of radiation measured in a plane perpendicular to the propagation direction of the beam.
Lasers can emit any number of transverse modes, of which TEM00 is usually the most desirable.
Laser output is, in fact, usually described as what percentage of the total beam intensity is in the form of TEM00.
TYPE OF LASERS
Lasers can be classified according to the type of gain medium used to generate and deliver the laser beam:
- Gas Lasers
Gas lasers use, as the name suggests, gases as the gain medium. Basically plasma discharge technology is used to amplify light and produce a laser beam. Common types of gain media are HeNe, Argon-ion, Krypton-ion, carbon dioxide (which emits in the mid infrared). A subclass of gas lasers are excimer lasers where the lasing medium is an excimer
- Solid-state Lasers
Solid-state lasers use a crystalline or glass rod as the gain medium which is “doped” with ions that provide the required energy states. Neodymium is a common “dopant” in various solid-state laser crystals. The family of neodymium-doped crystals (Nd:YAG) emits in the near IR when pumped with an intense light source
- Laser diodes (or semiconductor Laser)
A laser diode is a laser whose gain medium is a p-n junction formed in a semiconductor which is electrically pumped. Recombination of electrons and holes created by the injected current introduces optical gain. Laser diodes can be extremely efficient. This type of lasers is usually employed as the energy source for other lasers, in fact the emission wavelength of a laser diode can also be precisely adjusted to meet the absorption wavelength of a certain gain medium (e.g. the Nd:YAG crystal)
- Fiber Lasers
Fiber lasers are lasers which use the core of an optical fiber as the gain medium to generate and deliver a laser beam. Gain medium consists of a fiber inner core doped with rare earth ions such as erbium, neodymium, ytterbium, thulium or praseodymium. Most fiber lasers use diode bars as energy sources. Typically radiation produced by diode bars is injected into a cladding that surrounds the “active” inner core. To contain this radiation, the fiber (inner core plus cladding) is covered with an outer sheath that has a lower refractive index, thus preventing radiation to escape due to total internal reflection. This way, the confined radiation produced by diode bars bounces around inside the fiber and every time it crosses the core, energy builds up and excited atoms emit photons, producing the laser beam.
In solid-state lasers, the beam propagates in free space through the gain medium with standard free space optics such as flat mirrors, lenses, and diffraction gratings used to control the beam. On the contrary, fibers waveguide the light, confining it in a small core. Advantages of fiber lasers include improvement of many alignment, thermal, contamination, and maintenance issues. Moreover beam quality is very high. This type of lasers is beginning to supplant the traditional solid state laser in many areas such as drilling, welding, foil cutting, laser marking, and precise micro-machining applications.
- Lasers can also be classified according to their behaviour with respect to time: continous wave (CW) lasers produce a constant output over time while pulsed mode lasers produce light pulses at a certain frequency. Pulsed lasers are commonly operated by a Q-switch, a device positioned inside the cavity that operates as a switch: when Q-switch is closed energy builds up in the gain medium and is then released as a very short pulse when switch is opened.
- Lasers also differ in the type of energy source employed to excite (or «pump») the atoms of the gain medium. Examples of energy (or pump) sources include electrical discharges, flash-lamps, arc-lamps, diodes and even light from another laser (optically pumped).
The output beam of most lasers can be accurately modeled as a Gaussian beam. A Gaussian beam is a beam of electromagnetic radiation whose transverse electric field and intensity I (irradiance) distributions are well approximated by a Gaussian function:
r = radial coordinate
z = longitudinal coordinate (along the optical axis)
w0 = beam waist, i.e. the semi-diameter of the laser beam at its narrowest width (at the 1/e2 intensity point) I0(0,0) = intensity at the center of the beam at its waist
w(z) = beam width, it is the distance from the beam axis (r = 0) where the intensity drops to 1/e2 (≈ 13.5%) of the maximum value
In a diffraction limited Gaussian beam, beam width w(z) (also called spot size) varies according to the following equation
λ = laser wavelength
z = longitudinal coordinate (along the optical axis)
w0 = beam waist, i.e. the semi-diameter of the laser beam at its narrowest width
Beam divergence θ is an angular measure of the increase in beam width (or radius) with distance from the optical aperture from which the laser beam emerges. In other words it is the angular spreading of light waves as they propagate through space.
A perfectly collimated laser beam would have planar wavefronts, normal to the direction of propagation, with ideally zero beam divergence: this is obviously an ideal situation. In reality beam divergence is limited both by diffraction and optical aberrations. According to the previous equation, perfect collimation can never be achieved since θ = 0 would require an infinite beam width w0. However this equation suggests two ways of lowering divergence: the bigger the waist width w0 and the shorter the wavelength λ, the lower is the divergence θ of the beam.
Keeping λ fixed, one can lower the divergence θ by means of simply increasing beam width. This is precisely why beam expanders are employed in wide variety of laser systems.
Unfortunately, the output from real-life lasers is not truly Gaussian. A quality factor, M2 (called the “M-square” factor), has been defined to measures the difference between an actual beam and a theoretical Gaussian. For a theoretical Gaussian TEM00, M2=1 while for a real laser beam, M2>1. M2 value of a laser may be calculated using the following expression
If M2=1 the focused spot is diffraction limited and the product of width (w0) and divergence (θ) is the smallest possible.
ACTUAL SPOT SIZE AT FOCUS
The equation for calculating the spot size of a laser beam focused by an lens is given by:
λ = laser wavelength
f = lens focal length
w0 = input beam radius at the first lens surface of the objective w = radius of the focused spot
LASER DAMAGE THRESHOLDS (LDT)
Laser beams often contain high energies that might damage sensitive optical components. The most common type of laser damage is physical deterioration at coated optical surfaces (e.g. pitting, erosion, melting, or delamination). Fracture and discoloration of the bulk optical material are also often encountered. The two main mechanisms that cause laser damage to an optical coating are dielectric breakdown and thermal absorption. Usually small defects or impurities in the optics absorb laser radiation preferentially. As they heat up and expand, they may cause melting or thermal stress fracturing of the surrounding material. Alternatively, they may just vaporize violently, physically pitting the surface.
Therefore it is useful to identify a Laser Damage Threshold (LDT) of pulse intensity below which no damage is likely to occur. The most common definition of “threshold ”is the value at which there is zero probability of laser damage.
Reducing impurity levels and controlling surface fabrication defects are key strategies for making high damage threshold optics. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption) while pulsed lasers often strip electrons from the lattice structure of an optic before causing thermal damage. LDT is usually expressed in J/cm2 (conditions such as pulse duration and pulse repetition frequency are also specified). LDT scales with wavelength, so the damage threshold at 532 nm will be half that at 1064 nm.
If the maximum energy density of a laser is less than the LDT maximum energy density, then the optic should be suitable for that application.
LASER POWERS AND APPLICATIONS
Compared to conventional processes, lasers offer advantages due to their speed, accuracy, and ability to handle widely diverse operations including cutting, welding, drilling, engraving and marking. Different applications need lasers with different output powers. Lasers that produce a continuous beam or a series of short pulses can be compared on the basis of their average power.
Regarding laser marking, lasers create marks in materials by either engraving the surface, vaporizing the material to a user-controlled depth or modifying the surface color or texture of the material.
The type of laser to be employed in a certain application and for processing a specific material is basically selected according to the delivered output power and the emission wavelength, which has to be appropriate to process the material (i.e. the material should not be highly reflective or transparent to the laser light).
- Two of the most widely used laser types are CO2 (carbon dioxide) and
- Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers.
- CO2 lasers emit in the infrared at about 10.6 μm:
- systems in the 20 to 500 W range are used for marking, engraving and cutting wood and plastics,
- while systems in the 500-4000 W are mostly applied for welding.
- Nd:YAG lasers also emit in the infrared although at shorter wavelength (1.06 μm) with power outputs ranging from a few watts to 4 kW.
- As previously stated, laser application is selected according to the laser output power:
- low-power Nd:YAGs (10-100 W) are used for marking and fine cutting,
- medium-power Nd:YAGs (100-500 W) are used for welding and cutting/drilling of metals and
- higher power Nd:YAGs (500–4000 W) are also used for welding/cutting of metals.
Laser marking/engraving is nowadays a common option to engrave or mark a product traveling along a production line for printing expiry codes, dates and lot numbering of products (e.g. PCB marking).
A two-axis laser marking system (see Fig. 4) consists of a laser source, a beam-positioning system, which is made up of two beam positioning elements in the X and Y directions, two galvanometers (or called galvo scanner motors) driven by a controller (usually a computer) and the beam-shaping optics, commonly called scanning lenses.
In common type laser marking/scanning systems, beam positioning elements such as a set of mirrors or rotating polygons is mounted to high-speed, high-accuracy galvanometers and deployed to move the laser beam over the work-piece surface on both the X and Y axis. This way the laser beam traces patterns onto the work-piece surface exactly the same way a pencil writes onto a paper sheet. Any type of contour may be created by combining the mirror movement with activation and deactivation of the laser beam.
Deflection of the laser beam by these so-called “galvo mirrors” occurs in the beam path in front of the lens, therefore the scanning path lies in a plane perpendicular to the optical axis of the scanning lens.
Laser marking systems require special scanning lenses to focus the laser beam over a flat surface (the focal field), where the work-piece to be marked/scanned is positioned. F-Theta lenses are commonly deployed: these lenses are specifically designed to provide a flat focal field and a linear relationship between the distance traveled by the scanned spot and the scan angle, which is then directly proportional to a voltage applied to the galvo scanner motor. This linear relationship is accomplished by adding a specific amount of barrel distortion to the scanning lens, thus eliminating the need for electronic correction.
F-theta lenses are ideal for most marking, writing, and photoresist exposure applications.
HOW AN F-THETA LENS WORKS
Scanning/engraving systems consist of a laser source, a set of rotating mirrors used to scan across the work- piece surface and a focusing lens system that focuses the laser beam on a flat focal field.
Conventional focusing lens form images on a curved surface (Fig. 5 A) and therefore they cannot be deployed in such scanning systems: specialized flat field scanning lenses are needed in order to form an image on a flat surface.
In addition to a flat focal field, scanning/engraving systems require a uniform scanning speed, especially in those cases where the duration of exposure of material surface to the laser beam is an important parameter. A uniform scanning speed can be accomplished by deflecting the laser beam at a constant angular velocity, provided that identical scan angles (θ) are translated into identical scanning paths, i.e. the position of the spot on the image plane is directly proportional to the scan angle.
- Standard scanning lenses (Fig. 5 B) provide a flat field, however the distance traveled by the laser spot is not a linear function of the scan angle (θ), in fact
Such lenses provide scan length (y’) proportional to the tangent of the scan angle (θ) and not linear to the angle itself, requiring complicated control electronics to obtain a uniform scanning speed.
- Conversely, F-Theta lenses (Fig. 5 C) are designed both to form an image on a flat plane and to provide a linear relationship between the scan length (y’) and the scan angle (θ), in accordance with the following so-called F-Theta condition
𝑦′ = 𝑓 ∗ 𝜗
Scan length (y’) is simply equal to the incident scan angle (θ) multiplied by the focal length (f), i.e. the position of the spot on the image plane is directly proportional to the scan angle. This eliminates the need for complicated electronic correction required with standard scanning lenses. F-Theta lenses satify the previous relationship thanks to a special corrected «built-in» negative distortion (barrel type).
The size of the marking fields depend on the lens’ focal length. For instance, a focal length of 163 mm provides a field to be marked (or scan area) with a size of 115×115 mm, while f = 250 mm provides a 176×176 mm scan area.