Advanced Manufacturing Processes: Laser

Semiconductor Lasers 5

Semiconductor lasers are based upon radiative recombination of charge carriers . Under certain

circumstances, instead of a simple LED, lasing action can occur: beyond certain threshold current, the

population inversion becomes high enough such that gain

by stimulated emission overcomes the losses, resulting in

lasing action.

To increase laser power, the increase of active layer width

is required (wide stripe laser diodes). Several wide stripe

active layers can be stacked up to further increase power.

The laser diode

stack is modular

so it is possible

to do

maintenance

operation to one

stripe without

stopping the

others.

The output of

the diode should

be managed with optical instruments in order to achieve high collimation. Mirror, lenses and optical fibers

(active and passive) are used for this purpose.

Laser optics

Innovative solid state lasers made with active fiber optic (e.g. Yb:glass, 1080 nm) were introduced to exploit

a great number of advantages with respect to traditional lasers. In fact, high quality of the beam is achieved,

indicated by a Gaussian power distribution. These kind of sources are extremely compact and easy to move;

furthermore, thermal modulus is particularly favourable and cooling by air flux is possible. It is also possible

to create a modular system with different parallel optical fibers: they singularly produce limited power (300

– 1000 W) but they can be coupled in order to obtain a single laser beam with a higher value, up to 40 kW.

This solution, although providing lower quality by increasing the number of moduli, is quite safe: if one sub-

system breaks, all the other continue to work.

The first fiber laser and its potential advantages were demonstrated as early as 1961, but only in the late

1980’s a boom of optic fibers for telecommunication allowed the development of all necessary components

for high performance lasers of this kind, including fiber Bragg gratings (FBG), the cavity mirrors inscribed into

the core which require no adjustment. High brightness laser diodes with fiber pigtails are used as pump

sources. Thus, a fiber laser is a compact structure easy to operate.

In recent years, great advances have been made in the technology of high-power Yb fiber lasers, nowadays

widely spread in material processing (welding, cutting and drilling). At a given power, fiber lasers offer higher

quality and faster processing rates in comparison to high power CO2 lasers.

5 When the external potential (V) is applied such that the p-type region is made positive and the n-type region is made

negative, it reduces the potential barrier. The electrons (holes) now face the reduced potential barrier, which they can

easily overcome while diffusing from the n-type (p-type) region to the p-type (n-type) region: this is the forward bias.

The battery in the circuit can replenish the charges and establish the current flow through the junction. The electrons

and holes injected into the p-type and n-type regions, respectively, act as minority charge carriers and undergo

recombination, resulting into spontaneous emission of photons. 7

Passive fiber lasers are used to transmission of radiation

from the diodes to the workpiece, without losses, acting

as a dielectric waveguide. They are essentially always

based either on glass (fused silica - amorphous silicon

dioxide) or on polymers (plastic optical fibers). Total

Internal Reflection is the physical phenomenon governing

passive fiber lasers, and it is a critical condition of the

Snell’s law: remembering that high index of refraction

causes low speed of propagation of radiation, variation of

propagation direction could happen at the interface

between two different materials. The amount by which

the ray swerves and hence the new direction is

determined by the relative refractive indices of the

materials and the angle at which the ray approaches the

boundary. The angles of the incoming and outgoing rays

are called the angles of incidence and refraction

respectively. Snell’s law: sin =

TIR happens when the region of transmission has a

lower index of refraction (so in this particular case of

fibres, n > n ) and a critical angle is matched:

core cladding

= =

(

sin )

2

To protect the optical fiber from surface scratches, a layer

of soft plastic (primary buffer: jacket) coats the cladding,

providing mechanical protection.

When the light is transmitted along a fiber, it shines out

of the far end with an angle given by Snell’s law

(considering core and air interface). Since light direction is

reversible, this angle is also the angle of entrance: the

cone of acceptance is defined by this acceptance angle.

The numerical aperture of a fiber represents its ability to

gather light, so it is strictly related to refraction indexes

6

and acceptance angle, in particular its max value .

Modes are methods of transmission and are always an

integer number: rays take different paths and their quantity depends on the diameter and on the numerical

6 θ is the max angle able to realize the propagation in term of total reflection inside the fibre - a measure of the

max

capability of the fiber to collect the rays of the beam. 8

aperture of the fiber. Single-mode / monomode fiber presents a core diameter much smaller (5-10 μm) than

the cladding (125 μm), whereas multimode fiber allows bigger core diameter, keeping constant the cladding.

2

( )

2

=

2

The problem of multimode lasers is that rays with

different path arrive at the end of the fiber in different

time. To achieve coherency, the effect can be

compensated by controlling the refractive index of the

fiber, thus slowing down the straight-path rays and

accelerating the others: the solution is to adopt

variable refractive index. Step-index fiber has a

discontinuity between core and cladding; graded-index

fiber progressively decreases n from the centre of the

core to the sides (the ray will increase in speed as it

moves away from the centre). An amplifier based on an ordinary doped single-mode fiber can generate a

diffraction-limited output, but it restricts the pump sources to those with diffraction-limited beam quality

and thus normally to those with low power. On the other hand, the use of multimode fibers usually leads to

2

poor beam quality (higher divergence, higher M ). This dilemma has been resolved with the invention of

double-clad fiber designs, which allow cladding pumping of fiber devices. Here, the laser light propagates in

a single-mode (or multimode) core, which is surrounded by an inner cladding in which the pump light

propagates. Only the core is rare-earth-doped. The pump light is restricted to the inner cladding by an outer

cladding with lower refractive index, and

also partly propagates in the single-mode

core, where it can be absorbed by the laser-

active ions. The inner cladding has a

significantly larger area (compared with that

of the core) and typically a much higher

numerical aperture, so that it can support a large number of propagation modes, allowing the efficient launch

of the output, e.g. of high-power laser diodes (e.g. beam-shaped high-power diode bars), despite their poor

beam quality.

In active fiber lasers, radiation emitted by diodes passes through the active medium many times, pumping

energy in Yb and generating a high-energy laser beam. The fiber itself is the resonant cavity where the lasing

emission starts. A fiber Bragg grating is

exploited as an optical filter to block

certain wavelengths. Bragg gratings are

distributed Bragg reflectors that reflect

particular wavelengths, transmitting all

the others. This is achieved by creating

a periodic perturbation in the refractive

index of the fiber segment, which

generates a wavelength-specific

dielectric mirror. Interaction between the grating and the optical signal occurs when the pitch of the grating

and the optical wavelength meet the Bragg scattering regime.

The basic configuration of an active fiber system is made up of three regions: 9

 Pump section: laser light from LEDs

pass through the combiner whose

output is a single-mode optical fiber.

 Oscillator section: the laser light from

the combiner propagates through the

double-clad active fiber, getting

amplified by FBG.

 Beam delivery section: it is composed

by a passive optical fiber that

transports the light to the processing

head.

Diode pumped fiber lasers reach efficiency of 75-80%. Other advantages are:

 Monolithic laser source: no ordinary maintenance (lamps, mirror re-alignment);

 Constant quality in time: no warm up and thermal lensing;

 High DC wall-plug efficiency: 35-40%;

 Small cooling, air/water;

 Reliable: 100.000 h diode operating time. 7

Another diffused technique of beam delivery system is made of mirrors and focusing lenses . This kind of

system occupies more space than a fiber laser and also can have

problems of alignment. Lenses must be highly reflective and heat

dissipative – to avoid deformation subsequent to thermal

8

stresses . Anti-oxidant coatings are also used. The optical system

has to guarantee transport function even if there is a relative

motion between beam and workpiece: rotations and translation

of mirrors are possible. This solution is quite simple and is

feasible with every kind of laser source and temporal profile.

High powered

(> 5 kW) CO

2

laser adds water-cooling systems on mirrors. To improve

alignment, a fixed couple of mirrors is generally adopted,

fixing their position: rotation is achieved by moving this

block around its axis.

After the transport, focusing of the beam on the workpiece

is possible through reflective

mirrors (parabolical or

spherical) or transmissive lenses (biconvex, plano-convex or positive

meniscus). The former solution is used with high-power CO lasers for

2

welding; the latter is typical for near-IR lasers. A common characteristic of

optical focusing system is focal length f: it is the distance from the lens plane

(from the incident beam axis in case of mirrors) to the waist. This definition

is given for sources at infinite, so incident beam rays parallel to the axis of the beam. In reality, the source is

7 Fiber is the most flexible system, but cannot be used with every source: CO laser is easily absorbed, generating losses

2

(heat dissipation) and damaging the fiber itself, thus a different delivery system is required.

8 A common used material is copper. 10

at finite distance so rays are divergent

(diffraction): focus is at a distance z 0

(focus position) higher than f. by the

≈

way, divergence is small, leading to 0

. Typica