Properties of Laser Beams, Optics, and Beam Matter Interaction

EM Wave Description:

• Electric and Magnetic field vectors propogate perpendicular to each other

What is laser wavelength determined by

Lasing medium (e.g. stimulated emission transitions)

Laser Intenstiy Eq and Units

W/cm²

What happens in Transverse Electromagnetic Mode: (TEM)

• Photons travelling at right angles to optical axis will not be amplified

• Photons travelling along the optical axis will be amplified

• Photons travelling zigzag between the mirros will produce complicated patterns, known as transverse electromagnetic mode

What do m and n denote

What is Laser Intensity Distribution

How Laser Energy is distributed

What is a Gaussian Beam

The most desirable beam shape for focusing a laser A

Advantages of Gaussian Beams: (2)

• The intensity distribution is maintained during propogation (ie same distribution at front and back of laser (front and far field)

• Constant phase across the whole wave-front

What is TEM01

What is it good for

• Doughnut Mode: Made from an oscillation between two orthogonal TEM01 modes

• Good for surface treatment and high quality cutting

Modes higher than first order are called

High-order modes

Intensity distribution of high order modes………

Vary with distance and time for real lasers

Transverse mode affects: (3)

• Beam divergence

• Focus spot size

• Beam distribution at focus

Higher order TEM often leads to

Non uniform intensity distribution, thus less stable processing

What is M² and what is it used for

Beam Quality Factor

Used to describe high-order modes of power

what is b? and eq?

A constant called the Rayleigh Length

What is W0?

• The beam waist

• Smallest radius of converging laser beam

M² = 1 means

Gaussian Mode

M² > 1 means

Other modes

What happens at higher modes to b?

Rayleigh Length is reduced

The smaller the M² means

The better the beam quality

So in short, M² measures

Beam quality in space - it tells you how tighlty the beam can be focused, and how fast the beam diverges after focusing

What is the definition of Beam Diameter for Gaussian Mode:

• Diameter at which Intensity drops to I0 / e²

• r = w (where r = distance form beam axis, w = beam radius)

Definition of beam diameter for High-Order Modes:

Diameter within which 1- 1/e² of total power exists (86% of total power)

Beam radius eq (for Gaussian Mode)

What is W(Z)

Beam radius at distance z from the beam waist

What is z

Distance from beam waist

How does wavelength affect beam divergence

• Longer beams lead to higher beam divergence

• Greater lambda = Greater W0 = Greater W(z)

What does Rayleigh Length define:

How is the optical cavity of a laser defined?

With curved mirros so the diameter reaches a minimum in the cavity (Beam Waist)

Near and Far FIelds Definintions

• Near Field: z << b, W(z) ≈ W0

• Far Field: z >> b, W(z) ≈ W0 * z/b

What is Divergence Angle

• The radius increase per unit beam lengthEq

W far field eq (expanded with b)

Divergence angle at ff

Thus Divergence Angle is:

• Proportional to wavelength and M²

  • Inversley proportional to Beam Waist

Average Power for Pulsed Lasers

Average Power = Pulse Energy x Frequency

Peak Power for Pulsed Lasers

Peak Power = Energy per Pulse / pulse duration

Power Density Eq

What is Polarisation

The orientation of the Electric Field Vector

What is Linear Polarisation

Direction of E oscillation is in one direction only, and is not changing with time

What does P-Polarised mean

• If the beam polarisation is parallel to the plane of incidence

• Note it is NOT parallel to the surface, (in fact perp to surface), bc it is parallel to plane of incidence

• Still a form of linear polarisation

What does S-Polarised mean

• If the beam polarisation is perpendicular to the plane of incidence (ie parallel to surface)

• Still a form of linear polarisation

Circular Polarisation:

• Direction of E oscillation rotates circularly with constant angular velocity

• Amplitude remains constant

What are non-standard focusing optics:

Designed for specific processes and applications

What are lenses used for

Power densities of < 10kW/cm²

What is a Singlet

A lens that consists of a single piece

What is a Plano Convex Lens

A spherical single lens that has single positive focal length, and converging incident light, creating real images

What is a Meniscus Lens:

A lens which has one convex surface and one concave surface, which cancels some of the spherical distortions

What is a Doublet (Achromat):

• Two Lenses cemented together to cancel spherical abberation

• One lens is Positive (Converging) and one is Negative (Diverging)

• They have different refactive indexes

What are the materical requirements for lenses

High transparency

Thermally Stable

Laser wavelengths (names) and example lens materials

• CO2 uses Far infrared, often uses Zinc Selenide

• YAG laser and Diode laser use Visible to near infrared, use Glass or Borosilicate Crown Glass

• Excimer uses Ultraviolet, uses saphhire

What are Focusing Mirros used for

Highest power densities (>10kW/cm²)

2 types of mirror

• Parabolic mirror

  • Spherical Mirror

Parabolic mirror adv + disadv

• Good optical properties

• Expensive

Spherical mirror adv + disadv

• Produces more distortions (innacurate)

• Cheaper than Parabolic Mirrors

Requirements for materials to make mirrors

• High Reflectivity

• High thermal conductivity

  • Low thermal expansion coefficient

Diff types of material combos:

• Copper with Silver or Gold Coating, Highest reflection to IR beams (99.4%), High thermal conductivity, Used for most CO2 Laser Beams

• Silicon + Silver, 98.9% reflective, high thermal stability + low weight

• Mo + Ag Coating, 98.9% reflective, Durable and rugged

What is a Collimator + 3 points:

• A Beam expander which increases beam diameter (made of 2 lenses)

  • Reduces power density

  • Reduces beam divergence

  • Improves focusability (reduced focus spot size) (after beam expands it makes it easier to focus)

Equation + what it looks like

2 Types of Beam Scanners:

• x-y scanner (scanning galvanometer)

• Polygon Scanner

How do x-y scanners/ galvanometer scanners work

Uses a pair of mirros on galvo motors

How do Polygon Scanners work

Applications?

For a x-y scanning pattern, one axis is scanned using a standard galvo-mounted mirror, and the other axis is scanned using a polygon mirror

For high-speed applications

What is a Circular Polariser and two types:

• Something used to convert a linearly polarised beam into a circularly polarised beam (useful to remove orientation sensitivity)

• Reflective

• Transmissive

Reflective type:

• Applications

• Setup

• Used for high power

• ¼ dielectic coating on a flat mirror, so that p-polarised component (reflected from mirror) is λ/4 out of phase with the s-polarised component (reflected from dielectric)

Transmissive Type:

• Applications

• Setup

• Low power applications

• Laserbeam is transmitted through a birefringent material, inducing a λ/4 phase shift between horizontal and vertical polarisation components

Optical Fibre Basic Principle (and diff between modes?):

• Cladding has a lower refractive index than the core, causing TIR to occur

Focal spot size is generally limited by

Fibre core diameter

Minimum laser focused spot diameter eq:

F number (focal number) equation:

What is Depth of Focus and Eq:

The length along the beam axis, above and below the focal point, under which the focal spot size changes by <5%

Defocused Beam Spot Size eq

Difference in refactive index for transparent and absorbing materials

• Transparent: Refractive index is a real number

  • nt = n

• Absorbing: Refractive index is a complex number

  • na = n + ik

  • n is the real refractive index

  • k is the extinction coefficient

What is the absorption coefficient and equation

The rate at which light will attenuate (decay) after it propogates below the surface of a absorbing material

Transmitted Intensity Equation

What is Transmitted Intensity proportional to?

Electric field

Reflectivity definition and equation:

Absorptivity definition and equation:

Metal absorption?

Metals strongly absorb light, α is range of 10^5

What is skin depth/absorption length

The depth at which most of the optical power is absorbed

z = 1/α

(Because I = I0 * e^( -α*z), when z = 1/α, I = I0 / e)

3 types of laser absorption mechanisms:

• Fresnel Absorption

• Inter-band Absorption

• Photo-Chemical Absorption

What happens in Fresnel Absorption

Incident photons cause electrons to vibrate, which indusces heat in the body.

It is a pureley thermal interaction, photon energy converts into thermal energy

What is Inter-band Absorption

When the absorption of a single photon causes an electron to transition band

Why is inter-band absorption impossible on metals for the lasers we studies

Metals require photon energy >10eV

Photon energy of CO2, YAG and Excimer are all too small (max being excimer at 4.9eV)

Therefore, not possible, not eneough energy in photon

What happens in Photo-Chemical Absorption:

• Molecule bonds are directly broken by the laser beam, without causing a temperature rise, known as cold machining

• Only happens for short wavelength ie high photon energy lasers such as Excimer lasers, when fired at polymer materials that have low bond energies