Advanced Manufacturing Processes: Laser

Laser index laser

• Basics of lasers ............................................................................................................................................. 1
• Laser sources ................................................................................................................................................ 6
• Laser optics................................................................................................................................................... 7
• Laser-material interaction .......................................................................................................................... 12
• Laser cutting ............................................................................................................................................... 13
• Laser welding.............................................................................................................................................. 17
• Safety ......................................................................................................................................................... 18

Basics of lasers

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It is essentially a coherent, convergent, and monochromatic beam of electromagnetic radiation with wavelength ranging from UV to IR.

When the oscillations of the electric field vector are in particular order, the light is polarized. In⃗ particular, in a plane-polarized wave, oscillates in⃗ a single plane as the wave travels. In contrast, in the completely unpolarised light can assume any possible directions (i.e., it oscillates randomly in more than one plane). ⃗ = 2( − ) =

Intensity is defined as the energy per unit area perpendicular to the direction of motion of the wave and is proportional to the square of amplitude of the wave.

In 1960, Maiman came up with the first working ruby laser. The three processes required to produce the high-energy laser beam are population inversion, stimulated emission and amplification.

• Population inversion. Without this process, there will be net absorption of emission instead of stimulated one. According to the 1 Boltzmann law, the higher energy states decreases exponentially with energy. Population inversion corresponds to a non-equilibrium distribution of electrons such that the higher energy states have a large number of them than the lower energy states. The process of achieving that by exciting the electrons to the higher energy states is referred to as pumping. In most of the lasers, population inversion involves three or four levels. Radiation decay is really rapid – thus the population inversion is achieved. −2 11 = exp⁡[− ]2 1 1

Pumping can be:

• Optical: glass or quartz tubes filled with gases such as Xe or Kr where some wavelength of the flash matches with the absorption characteristic of the active laser medium, facilitating population inversion. This is used in solid-state lasers like ruby and Nd:YAG. Recently, diode-pumped solid-state laser have been developed, offering significant advantages over conventional flashlamps such as better match between the output spectrum of the pumping laser and absorption characteristics of laser medium, increased efficiency, and compact and lighter laser systems.
• Electrical: used in gas lasers, is achieved by passing a high-voltage electric current directly through the mixture of active gas medium. The collision of discharge electrons of sufficient kinetic energy excites one of the gases to high energy levels, which subsequently transfer its excitation to the second gas through collision. There is a minimum population inversion, referred to as threshold condition, required for lasing. −  2 1 =

Stimulated emission. The incoming photon of energy interacts with the excited atom of 2ℎ active laser medium with population inversion between the two states. Thus the incoming photon triggers the emission of radiation by bringing the atom to the lower energy state. The resulting radiations have the same frequency, direction and phase as the incoming photon, giving rise to a stream of photons.

• Amplification. Since the stimulated photons are in the same phase and state of polarization, they add constructively to the incoming photon, resulting in an increase in amplitude. The active laser material is placed in a resonant cavity, consisting of a set of well-22 aligned highly reflecting mirrors at the ends, perpendicular to the cavity axis (a). When the laser is off, the optical cavity contains all the laser material in its unexcited state. The excitation of atoms is soon achieved by pumping (b), followed by initiation of stimulated emission (c). The intensity is increased as it travels to the end of the mirrors. Further amplification is accomplished by reflecting the photon into the active medium (d). The photons travel the long path back and forth through the lasing medium stimulating more and more emissions, resulting in a high-intensity laser beam output (e).

Properties of laser radiation

• Monochromaticity. The laser output consists of very closely spaced, discrete and narrow spectral lines, which satisfies the resonance condition given by: = /2 where d is the cavity length. These discrete lines, called laser modes or cavity modes, spread over a range of frequencies /2. separated by Frequencies in this range are amplified if the gain is higher than losses. Δν# = /2Δ is the range of frequency, also called spectral width. The number of axial modes may exceed hundreds of modes: monochromaticity is due to narrow spectral widths of individual modes. A laser can be constructed to operate in only one longitudinal mode to give better results.
• Collimation. It is related with the directional nature of the beam. Highly collimated beams can be focused on a very small area even at longer distances, hence energy can be efficiently collected on a small area without much loss in the intensity. 2 One of the mirrors has some transmission to allow laser output. 3 ≈ ⁡ The divergence angle of a diffraction limited beam can be expressed as: where is the 00 beam waist, i.e. the smallest value of sideways spread of the beam, therefore the minimum spot. Laser beams are characterized by very small divergence angle (0.2-10 mrad).
• Coherency. It is the degree of orderliness of waves. Spatial coherence correlates the phases at different points in space at a single moment in time; temporal coherence correlates the phases at a single point in space over a period of time.
• Brightness or radiance. It is the amount of power emitted per unit area per unit solid angle. Laser-6 beams have divergence angles in the range of 10 steradians, hence they can be focused on a very small area: high brightness is achieved. It is a very important factor in material processing and 3 determines the intensity (power density) or fluency (energy density) of the laser beam. It cannot be increased by the optical system; however, it is possible to operate in Gaussian mode with minimum divergence angle.
• Spatial (transverse) modes. The cross section of laser beams exhibit distinct spatial profiles termed as transverse modes and are represented as the transverse electromagnetic mode, TEM , where mn m and n are small integers representing the number of nodes in direction orthogonal to the beam propagation direction. The fundamental mode TEM has Gaussian spatial 00 distribution and is the most commonly used mode in laser machining applications. The intensity distribution in the Gaussian beam can be expressed as: 22() = exp⁡[− ]0 2 Where r is the radius of the beam, I the intensity at r = 0 and 0 −2 = = 0.135w is the radius of the beam at which . The 0 0 quality of a beam is expressed in terms of the beam quality factor or 2 beam propagation ratio M , comparing the divergence with that of a pure Gaussian beam for 2 which M = 1. 2 = Therefore, we can express every divergence angle as: 3 = ⁡[]; = [ ]; = [ ] Beam energy: fluency: intensity: .2 2 42 = 0 Often, for solid-state lasers, the beam parameter product is preferable: 2 = =0
• Temporal modes. The output can be continuous, constant amplitude (CW mode) or periodic (pulsed beam mode). The former discharges constant energy, uninterruptedly for a long time. The latter stores pumped energy until a threshold is reached. Once it happens, the stored energy is rapidly discharged into short duration 4 pulses of high energy density. Most of the gas lasers, Nd:YAG and semiconductor lasers are operated in CW mode, whereas solid-state lasers such as ruby Nd:glass lasers are primarily operated in pulsed mode. In this case a fundamental parameter is the pulse repetition time. Then, pulsing can be carried out in various ways:

• Normal pulsing/ free running: variation of inductance and capacitance in the circuit of the flashlamp. Properties of the resonator are kept constant. Typical pulses are μs-ms (pulse frequency of 0.01 Hz).
• Q-switching: pulsing is obtained by changing through different methods the Q value of the cavity (i.e. the measure of its ability to store the radiant energy). -9 Short (10 s) and intense pulse of radiation is achieved. Typical pulse frequency is 100 kHz.
• Mode locking: a train of extremely short (10^-12 -10^-15s) and equally spaced pulses is produced. Interaction among the longitudinal modes results in oscillatory behaviour of the laser output. This makes the longitudinal modes maintain fixed phase relationship. Typical pulse frequencies are 1MHz – 1 GHz. 4 = / The peak power of pulsed laser is always higher than CW laser.

Laser sources

Lasers are generally classified into four main types depending on the physical nature of the active medium used: solid-state lasers, gas lasers, semiconductor lasers, and dye lasers.

Solid-state lasers

In solid-state lasers, active medium consists of a small percentage of impurity ions doped in a solid host material. Nd:YAG is the most commonly used laser. Crystalline YAG is the host material and it is a four-level system. Its wavelength is 1.06 μm, with efficiency of 5% and good thermal stability. To increase efficiency, diode pumping is preferred.

Gas lasers

The active laser medium is a gas. With respect to solid-state lasers, gases:

• Act as homogeneous laser medium
• Can be easily transported for cooling and replenishment
• Are relatively inexpensive
• But low density requires huge amounts to achieve population inversion, so the sys