Substrate Materials

Fused Silica

General Information

Fig.1:Transmittance spectrum

a)Various types of Fused Silica, 6.35 mm thick

b)Close up of Fused Silica absorption band

Fused Silica (SiO₂) is one of the most important materials in the optical industry. For the production of high-performance optics, Fused Silica of particularly high quality is required. Synthetic materials are used instead of natural quartz sand in the production of this high-quality Fused Silica SiO₂ corresponds in its chemical composition to the simplest form of glass and is its most stable modification. Fused Silica has a refractive index of 1.46 (for λ = 500 nm) and an Abbe number of 67.70. It transmits light from 180 nm to about 3 µm. Absorption bands also occur within this transmission range due to the hydroxyl groups it contains. Fused Silica with a high OH content is suitable for UV applications. For transmissive components in the wavelength range of 940 nm, 1390 nm and 2 µm - 3 µm, quartz glass with a low OH content is recommended.

Production

For the production of an amorphous (bubble- and streak-free) Fused Silica without impurities, there are different approaches that influence the final product in terms of optical specifications (see table).

Table 1:Optical specifications of Fused Silica depending on production technology

Manufacturing process	OH	Cl	Cations	UV- Edge 50 % transmission	Examples for trade marks
Electrofusion of quartz sand or rock crystal	< 20 ppm	0 ppm	50 – 300 ppm	220 nm	Infrasil^® Vitreosil-IR^®
Single-stage CVD process (flame hydrolysis) using organic Si compounds	200 – 500 ppm	0 ppm	10 – 50 ppm	210 nm	Homosil^® Optosil^®
Two-stage process CVD deposition and densification, use of org. Si compounds	600 – 1200 ppm	50 – 100 ppm	< 1 ppm	170 nm	Corning 7980^® Suprasil 1^® J-Plasma SQ^®
Silicon tetrachloride in hydrogen/oxygen flame	< 20 ppm	< 200 ppm	1 – 2 ppm	170 nm	Suprasil 3001^®

Properties and Applications

The most important properties of Fused Silica, which are essential for LAYERTEC´s products:

High chemical purity
Durability
Heat resistance
Low coefficient of thermal expansion at high temperatures
High softening temperature
High transparency over a wide spectral range (UV-IR)
High radiation resistance

In the short wavelength and visual range Fused Silica (with higher OH content, e.g. Corning 7980^®) is used. Excimergrade materials are used for transmissive optics for UV high power lasers. Standard Fused Silica is of limited use for transmissive optics in the infrared range. Naturally occurring Fused Silica (e.g. Infrasil^®) and specially manufactured Fused Silica (e.g. SUPRASIL 3001/3002/300^®, …) have an extremely low OH content (< 20 ppm) and can therefore also be used for the infrared wavelength range to approximately 3 µm.

Calcium Fluoride

General Information

Fig.2:Transmittance spectrum calcium fluoride (3 mm thick)

CaF₂ (calcium fluoride, fluorspar, fluoride) is an optically homogeneous crystal material with a cubic structure. The refractive index of 1.43 (for λ = 500 nm) is very homogeneous, the Abbe number is 94.996. The material has a wide transmission range from 0.125 µm to 8 µm. The main application as a UV material requires high purity and crystal quality. The material is ideally suited for correcting chromatic aberration in the spectral range from VIS to NIR.

Production

While natural fluorspar has lost its importance, CaF₂ is now synthesized from the purest raw materials and grown into large-volume crystals. The purity and perfection of the crystals determines the material’s possible applications.

Properties and Applications

The mechanical properties of CaF₂ make machining optics a challenge. The material is very soft (Mohs hardness 4) and is therefore scratched easily. It can be easily cleaved (cleavage level 5 after {111}). Due to its high coefficient of thermal expansion, there is a risk of cracking if machined quickly or at high temperature gradients. In addition, it cannot be cemented to other materials. Uncoated polished surfaces react with atmospheric moisture and become hazy after prolonged exposure as micro-roughness increases. This creates stray light and reduces contrast.

Because of the high coefficient of thermal expansion, fluoride substrates should also be coated with fluoride layers if possible. Furthermore, heating and cooling rates must be observed to prevent cracking of the substrates.

Calcium fluoride is used at LAYERTEC for special applications in the ultraviolet range. It is processed into laser mirrors, output couplers, beam splitters, lenses, windows for excimer lasers and frequency-multiplied solid-state lasers, among others. In fluorine environment, the lifetime of CaF₂ optics is significantly longer than that of other materials.

Due to the mechanical properties, it should be ensured in the application that the products are not subject to temperature fluctuations and temperature gradients. The optics should be stored with desiccant and under constant conditions. Therefore, the packaging should be opened only shortly before use.

Sapphire

General Information

Fig.3:Transmittance spectrum of sapphire (3 mm thick)

The sapphire (Al₂O₃) is a variation of the mineral corundum. For the optical industry, the synthesized form is used. Sapphire is an anisotropic material with a rhombic-hexagonal structure and shows different optical properties depending on the crystal axis and angle of incidence. These must be taken into account for the calculation of the final products. Sapphire is transparent in the range from 180 nm to about 4 µm. The refractive index of the material is 1.77 – 1.78 (for λ = 500 nm).

Production

Various growing processes are used industrially to produce sapphire. In addition to HEM (heat exchanging method) and the Czochralski process (both growing from the melting phase), the Verneuil process (flame melting process) is also used.

Properties and Applications

The most important properties of sapphire for use as a substrate material:

Extreme hardness (Mohs hardness 9), can only be further processed with a few materials (e.g. diamond)
Scratch resistance
High chemical resistance
Very good thermal conductivity
Wide transmission range

Due to its optimum thermal conductivity and particularly good transmission properties in the mid-infrared range, sapphire is used at LAYERTEC primarily for high-performance components in the spectral range of 2 – 3 µm.

Yttrium Aluminum Garnet

General Information

Fig.4:Transmittance spectrum of YAG undoped (3 mm thick)

Yttrium aluminum garnet (Y₃Al₅O₁₂ or YAG) is a crystal material with cubic structure, which is produced synthetically. The refractive index is 1.84 (for λ = 500 nm). It indicates good transmission behavior in the range from 0.25 µm to 4 µm. Undoped YAG is free of absorption in the range of 2 – 3 µm, whereas Fused Silica shows high absorption bands exactly here due to the higher proportion of OH groups.

Production

The YAG crystal is produced mainly by the Czochralsky method. A crystal gruel is brought into contact with the melt and then slowly moved upwards while rotating. The result is a single crystal over 300 mm in length and up to 100 mm in diameter are produced.

Properties and Applications

YAG has several properties which are advantageous for the manufacturing of high performance optics. Its chemical and mechanical resistance are similar to sapphire. However, due to its lower Mohs hardness of 8.5, YAG is easier to machine. The crystal’s high thermal conductivity and low absorption losses allow it to withstand high laser energies. The YAG crystal in doped form is also well suited for use as an active medium for lasers (Yb:YAG 1030 nm, Nd:YAG 1064 nm, Tm:YAG 2.01 µm, Ho:YAG 2.1 µm, Er:YAG 2.94 µm).

At LAYERTEC undoped YAG is used especially in the mid-infrared range (MIR) up to ≈ 4 µm. It has the advantage over sapphire that it lacks birefringence and therefore the crystal orientation can be chosen arbitrarily for many purposes. In the in-house optics production YAG substrates are manufactured in different sizes both as flat parts and as curved substrates or lenses.

Various Substrate Materials for UV, VIS and NIR/IR Optics

Table 2:Specifications of various substrate materials

	Fused Silica (UV)	Infrasil^®¹⁾	YAG (undoped)	Sapphire (C-cut)	CaF₂	N-BK7^®²⁾	Si
Wavelength range free of absorption	190 nm – 2.0 μm ³⁾	300 nm – 3 μm	400 nm – 4 μm	400 nm – 4 μm	130 nm – 7 μm	400 nm – 1.8 μm	1.4 – 6 μm

Refractive Index at
200 nm	1.55051				1.49516
300 nm	1.48779				1.45403
500 nm	1.46243	1.48799	1.8450	1.775	1.43648	1.5214
1 μm	1.45051	1.45042	1.8197	1.756	1.42888	1.5075
3 μm		1.41941	1.7855	1.71	1.41785		3.4381
5 μm				1.624	1.39896		3.4273
9 μm					1.32677

Absorbing in the 3 μm region	yes	yes	no	no	no	yes	no

Absorbing in the 940 nm region	For high power applications at 940 nm the Fused Silica types SUPRASIL 300^®¹⁾ and SUPRASIL 3001/3002^®¹⁾ are recommended.

Birefringence	no	no	no	yes	no ⁴⁾	no	no

Thermal expansion coefficient [10^–6 K^-1] ⁵⁾ (0 – 20°C)	0.5	0.5	7	5	18	7	2.6

Resistance against temperature gradients and thermal shock	high	high	high	high	low	medium	low

GDD fs² per mm
400 nm	98	98	240	150	68	120
800 nm	36	36	97	58	28	45
1064 nm	16	16	61	29	17	22
1500 nm	-22	-22	13	-25	1.9	-19
2000 nm	-100	-100	-59	-120	-21	-99

TOD fs³ per mm
400 nm	30	30	75	47	19	41
800 nm	27	27	57	42	16	32
1064 nm	44	44	71	65	21	49
1500 nm	130	130	140	180	46	140
2000 nm	450	450	360	530	120	460
1) Registered trademark of Heraeus Quarzglas GmbH & Co. KG
2) Registered trademark of SCHOTT AG
3) Absorption band within this wavelength range, please see transmittance curve
4) Measurable effects only in the VUV wavelength range
5) Please note that different authors in the literature are inconsistent. Moreover, the thermal expansion coefficient of crystals may depend also on crystal orientation. Thus, the value given here are approximated.
All values are for informational purposes only. LAYERTEC cannot guarantee the correctness of the values given.