2.11.5 Mirror Substrate Glasses

Properties of Mirror Substrate Glasses

* For a list of suppliers, see the section on fused silica.

2.11.6 Athermal Glasses

Athermal glass compositions are selected such that the optical path length, defined as the refractive index times the actual geometric distance the light traverses in the glass, is independent of temperature. The change in optical path length ∆W with temperature is

∆W = s[α(n – 1) + dn/dT]∆T = sGT,

where s is the actual distance in the glass, α is the coefficient of thermal expansion, n is the refractive index, and T is the temperature. G is the thermo-optical coefficient. For ∆w to approach zero, the gradient of the refractive index as a function of temperature must be negative. Examples of glasses with this property can be found in the FK, PK, PSK, SSK, BaLF, F, TiF, and BaSF families on the glass map. Data for several representative athermal optical and laser glasses are given in the table (see, also, sections 2.2.2 and 2.9.2).

Properties of Athermal Glasses

* –30 – +70°C; ** +20 – +40°C

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2.11.7 Acoustooptic Glasses

Acoustic waves create a time-varying refractive index grating in a material via the photoelastic effect. The grating spacing is equal to the acoustic wavelength; the grating depth is determined by the drive power of the transducer. A light beam traversing the medium is deflected by the grating at the Bragg angle ΘB from the normal to the sound propagation direction given by

sin ΘB = (1/2)λ/ Λ,

where λ and Λ are the wavelengths of the light and sound beams. The diffraction efficiency for a transducer of height H and interaction length L is

I/I0 = (π2/2)(L/H)(n6p2/νn3)Pa/λ2

where Pa is the acoustic power, p is the photoelastic constant, ρ is the density, and ν is the sound velocity. Thus an acoustooptic material, in addition to having low losses at the acoustic and optical wavelengths, should also have a large index of refraction and small sound velocity.

A figure of merit for an acoustooptic material is M = n6p2/ρv3. Properties and figures of merit for several glasses are compared below.

Properties of Acoustooptic Glasses

(a) Figure of merit relative to that of SiO2.

Data from Gottlieb, M., Elastooptic materials, Handbook of Laser Science and Technology, Vol. 4 (CRC Press, Boca Raton, FL, 1986), p. 319.

© 2003 by CRC Press LLC

2.11.8 Abnormal Dispersion Glass

Various relative partial dispersions

Px,y = (nx – ny)/(nF – nC)

are defined for other wavelengths x and y. The relative partial dispersion of most glasses obeyed a linear relationship on νd of the form

Px,y ≈ axy + bxy νd ,

where a and b are constants. It is not possible to correct for second-order chromatic aberrations using so-called “normal” glasses that satisfy this equation. Because of the linear relationship between the relative partial dispersions and Abbe number, the difference in partial dispersions will always be the same for normal glasses.

Correction for second-order chromatic aberration (secondary spectrum) is accomplished using glasses with equal partial dispersions for different Abbe values (the corrected systems are called apochromats). These abnormal dispersion glasses depart from the “normal line” and the linear relationship above. The relative dispersion (ng – nF)/(nF – nC) of optical glasses is plotted in the figure below and shows the magnitude of the deviations from the normal line that are possible. The deviations can be either positive or negative. Optical glass catalogs list deviations of the relative partial dispersions from the normal for glasses covering a wide range of νd values.

0.65

0.60

0.55

Deviation of the relative partial dispersion Pg,f of optical glasses from the normal line (Schott Optical Glass Catalog).

© 2003 by CRC Press LLC

Section 3: Polymeric Materials

3.1Optical Plastics

3.2Index of Refraction

3.3Nonlinear Optical Properties

3.4Thermal Properties

3.5Engineering Data

© 2003 by CRC Press LLC

Section 3

POLYMERIC MATERIALS

Of the large number of known polymers, several exhibit useful optical properties. Various properties of optical plastics are compared with those of glasses below. The documentation of optical properties and the accuracy of data on plastics are generally not comparable to that of optical glasses. In addition, mechanical and chemical resistance properties should be checked with the material supplier because they may vary widely within a polymer group. Numerous caveats about the use and application of plastics in optical systems are noted in reference 1.

Optical

From Cook, L. M. and Stokowski, S. E., Filter materials, Handbook of Laser Science and Technology, Volume IV: Optical Materials, Part 2 (CRC Press, Boca Raton, FL, 1995), p. 151.

Common optical plastics include:

polymethyl methacrylate (PMMA) (acrylic) polystyrene (styrene) (PS)

methyl methacrylate styrene copolymer (NAS) stryrene acrylonitrile (SAN), acrylic/styrene copolymer polycarbonate (PC)

polymethylpentene (TPX)

acrylonitrile, butadienne, and styrene terpolymer (ABS) nylon, amorphous polyamide

polyetherimide (PEI) polysulfone

allyl diglycol carbonate (CR-39)

Telfon (Telfon AF® ) (TPFE), fluorinated-(ethylenic-cyclo oxyaliphatic substituted ethylenic) copolymer

In the following tables properties of these and other optical plastics are given in order of decreasing index of refraction.

© 2003 by CRC Press LLC