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Materials

• Glass• History • Properties

• Crystalline Materials• Plastic• Glues and Cements• Coatings

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Brief History of Glass

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Brief History of Glass

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Brief History of Glass

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What is Glass?

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What is Glass ?

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How Glass is MadeProduction includes the following steps

Batching: The individual components are measured and combined. Particle size of the components should be matched. Raw materials must be of high purity.

Melting: During melting, the initial batch reactions occur and a glass melt is formed. There are two types of melting, POT and TANK melting.

Refining: The step is carried out at high temperatures. High quality glass must be free from bubbles. To remove bubbles, refining agents such as Arsenic Trioxide are added to the batch.

Stirring: The stirring process is the second step in the homogenizing process. Continuous stirring provides thorough distribution of all components. Stirring eliminates striae and produces a uniform refractive index over the melt.

Forming: During the forming stage the conditioned plate is processed into the desired shape.

Annealing: There are two annealing processes: course and fine – during course thermal stresses are removed.

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Index for various glasses

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Fused Silica Data

See Schott technical note on class website

RefractionReflectionTransmission

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Index Variation in Production Run

See Schott technical note on class website

For IR materials every batch must be measured

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Schott Glass Catalog

K = Kron (german) for CrownF = FlintS = schwer – heavy/dense

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Most Common Glasses

1.5771.581N-BALF41.5611.565N-SK111.6051.609N-SK21.6161.623F21.4571.459Fused Silica

1.5151.518BK7

Index @632nm

Index @550nm

Glass Name

At 20C and 1 atm

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Example N-BK7With the K and L coefficients

Dispersion equations

Many others -> See Smith page 208

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Definitions of Quality

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Definitions of Quality

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List of Major Glass Manufacturers

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Glass to Lenses

From Smith Ch 20. Centering

Generating

• Blanking – heat glass and press it into the desired shaped mold. A lens blank might be 3mm thicker and 2 mm larger diameter than the final lens.

• Rough shaping – diamond coated grinding wheels. For spherical element called generating. Blank rotated and grinder rotated – angle determines R.

– Blocking allows multiple elements to be processed at once. It means to stick multiple elements on tool

• Grinding – loose abrasive in a water slurry and cast iron grinding tools. Different graded (sizes) of slurry particles are used from course to finer to quickly get to desired shape.

• Polishing – polish with slurry of water and iron oxide or cerium oxide (very fine particles).

• Testing surface – typically placing again test plates and counting fringes.

• Centering – grind the rim of the lens so the mechanical and optical axises coincide. Uses tool that finds the optical axis and then grind off the outer rim.

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Crystalline Optical Media

(Near IR and IR)

(Si and Ge in IR are most common and for deep UV Calcium Fluorite)

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Crystalline Optical Media

From Smith page 220

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Plastic Materials• Advantages

– Very inexpensive material– Can be molded very inexpensively in high volume

• Including aspheric surfaces• Hybrid optics possible (glass with plastic cap)• Can mold mounting features as well as optic

– Very useful for replication of diffractive optics– Light and shatter proof

• Disadvantages– Soft and easy to scratch, potentially susceptible to cleaning solvents– Higher scatter – micro-bubbles, etc– Besides molding not easy to manufacture– Limited indices available

• Lucite (1.49), APO (1.5), Polystrene (1.60), Polycarbonate (1.59) F line• Some change index slightly over time or yellow

– Low melting temperature, coating must be lower temp process– Absorb water which changes dimensions– Large change in index and volume with temperature - 10x -100x times glass

• Negative change in index with temperature

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Liquid Lenses• Typically use oils of different refractive index.• Can make dynamic lenses:

– vary the focal lens by micro-pumping liquid in and out of lenses. – based on the electrowetting phenomenon:

-a water drop is deposited on a substrate made of metal, covered by a thin insulating layer. The voltage applied to the substrate modifies the contact angle of the liquid drop. The liquid lens uses two isodensity liquids, one is an insulator while the other is a conductor. The variation of voltage leads to a change of curvature of the liquid-liquid interface

– People looking at this for cell phones with small lens.

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Plastic Materials and Manufacturers

• Materials– Bayer & GE (PC), – DuPont (Lucite)– Zeon Chemicals (APO),

– Dow & BASF & Taita Chemical & NOVA Chemicals (PS)

• Manufacturers of lenses– Philips Optical– GS plastics– JenOptik– Lightpath– Fujinon

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Cements and Glues• Two main types Thermal or UV• UV required for high volume

– Setup times very fast, UV sets up cement• Thermal – piece put in jig and into oven• Plastic cements/glues index -> 1.47 to 1.61• Taking them apart depends on glue or cement

used – solvents, heat/cold, shock• Issues – shrinkage during cure, and out-gassing• Optical contact is possible bonding mechanism

but surfaces must be VERY clean. Can not get apart.

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Coatings

Remember uncoated glass (n=1.5) R is about 4% and AOI=0°. Coatings can reduce this reflectance and improve throughput across AOI and wavelength

Simplest example single quarter wave thick film.

Anti-Reflection (AR) Reflective

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Coatings CalculationAt interface I, tangential components must be continuous

Where we used

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Coatings CalculationAt boundary II

Where with propagation distance h

So,

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Coatings CalculationSolve last 2 equations for EtI and E’rII and substituted into first equation EI and HI

Where

In matrix formulation this is:

M is characteristic matrix

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Coatings CalculationMatrix formulation for multiple boundaries

For our simple one layer coating diagram

In general with p layers

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Coatings CalculationExpanding out EI and EII in terms of their components

Defining (allows us to get to the components needed for reflection coefficient)

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Coatings CalculationUsing Definitions

We can arrange the equations into

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Quarter Wave Single Layer Coating Example

At normal incidence, r1 becomes

To find R need to multiple by complex conjugate, yielding

If koh = π/2 , in other words the distance is odd multiple λf/4, then this is

Which is zero for

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CoatingsExamples from Newport

Single layer

Laser Line <0.25%, 0-15 Broadband ave<0.5%

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Coating Materials

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Thin Film Summary• It is difficult to make polarization independent coatings at

large angles (see form of Y)• Large wavelength bandwidth at particular angle is same

as large angular bandwidth at one wavelength. Both together is difficult.

• Small beams have large divergence, requiring large angular bandwidth.

• Angular bandwidth is more difficult far from normal incidence.

• AR coatings are more difficult for large index contrast.• Phase of reflectance typically varies rapidly over pass-

band = dispersion• S and P phase shifts usually not the same = retardance

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MirrorsMirrors are glass with a reflective coating on the surface.

CoatingsMetals – Al, Au, etc. Most of the time these have coating for protection or better reflection properties. Most common example is enhanced Al.Dielectrics designed for reflection and specific properties

SubstratesPyrex® is a borosilicate glass with a low coefficient of thermal expansion. Zerodur® is a glass ceramic material that has a coefficient of thermal expansion approaching zero, as well as excellent homogeneity of this coefficient throughout the entire piece.Fused Silica is used if light has to go through substrate. More expensive but low CTE and high quality.

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MirrorsSummary of types

Metallic Mirrors represent a good mixture of performance and value. These broadband coatings are relatively insensitive to wavelength, angle of incidence, and polarization. High-energy levels should be avoided, however.

Dielectric Mirrors offer near total reflection, minimizing losses in nearly all optical systems. These coatings are very durable, easily cleaned, and resistant to laser damage. Available either for broadband or narrowband, they operate efficiently over 0–45° angle of incidence.

High-Energy Laser Mirrors are designed and manufactured utilizing meticulous procedures to resist laser damage. They are intended for use with high-power CW lasers and high-energy pulsed lasers. These mirrors operate at a single or dual laser lines at either 0° or 45° angle of incidence.

Ultrafast Mirrors are all dielectric coated optics designed to minimize dispersion effects on ultrashort laser pulses. Specially optimized coatings deliver maximum bandwidth while minimizing pulse broadening.

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Mirrors

Typical Damage ThresholdsAl, 100 W/cm2 CW, 0.3 J/cm2 with 10 nsec pulses at 308 nm, typicalE Al, 100 W/cm2 CW, 0.3 J/cm2 with 10 nsec pulses at 532 nm, typicalProtected Silver, 1000 W/cm2 CW, 1 J/cm2 with 10 nsec pulses at 1064 nm, typicalProtected Au, 200 W/cm2 CW, 1 J/cm2 with 10 nsec pulses at 1064 nm, typical

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MirrorsEnhanced Aluminum

Dashed is Al

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MirrorsDielectric Coatings Examples for Newport

Damage Threshold: 1000 W/cm2 CW, 2 J/cm2 with 10 nsec pulses at 325 nm

Broadband Broadband

Laser Line

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Reading

W. Smith “Modern Optical Engineering”

Chapter 10

Homework out

Zemax Files are under Class Resources on Website