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Tech Notes » Optics Tutorial


Transmitting Optics 

(Windows, Lenses, Beamsplitters, Prisms, etc.)

UV Transmitting Substrates (Below 400nm): UV Fused Silica, CaF2, MgF2, BaF2, Quartz, Sapphire, etc.

The challenge with ultraviolet (UV) optics is that ultraviolet light, having the shortest wavelengths, absorbs and scatters at greater rates than visible or IR light. Even the tiniest scratches and surface flaws can be absorption and/or scatter points for UV light. In general, absorption into an optical part can result in damage and component failure. In some cases, too much UV absorption can actually alter a substrate’s chemical properties, rendering it useless. Scattering can result in energy loss within the optical system, reducing the efficiency. To remedy this, UV substrates must fully transmit the desired wavelength and must be polished to near perfection to avoid surface absorption and scattering. Among the UV transmitting substrates, UV-grade fused silica (UVFS) is the most widely used, and excimer-grade FS is used for applications down to 193nm. UVFS is readily available, affordable and relatively easy to fabricate compared to fluorides. For UV applications lower than 193nm, VUV CaF2 is the next best choice. Although it is delicate, and therefore significantly more difficult to fabricate than UVFS, CaF2 is also readily available and relatively inexpensive.

In some cases where deep UV transmission is required, VUV MgF2 is used because it has the best transmission for the shortest UV wavelengths. Users should be warned that MgF2 is expensive in comparison to UVFS and is cost prohibitive for sizes larger than 2" diameter. Sapphire is another good UV transmitting substrate that is used when high temperature, mechanical shock and vibration within the application are a key consideration. Sapphire is significantly more expensive than UVFS. Due to its physical hardness, sapphire is also one of the most difficult to achieve high surface quality among the UV transmitting substrates.

RMI is one of the few companies that can also fabricate and coat precision BaF2 optics for UV and IR applications. However, BaF2 is not widely used because of high cost, low availability and the many challenges associated with handling the substrate. Typically, BaF2 is only used in very special, custom applications.

VIS/NIR Transmitting Substrates (400nm–2µm): BK7, Fused Silica and all UV materials

Any substrate that is transparent, or “see through,” is transmitting at least part of the visible/near infrared (VIS/NIR) spectrum. In fact, all of the above mentioned UV substrates fully transmit VIS/NIR light as evidenced by their clear appearance. These substrates are used in applications where UV and visible light are used together. For example, UV/Excimer laser applications that use a red beam to align the system. In this case, a visible-only substrate (such as BK7 or fused silica) would not transmit the UV portion of the application.

BK7 and fused silica are the most widely used VIS/NIR substrates because of their superb transmission characteristics, low price, wide availability and ease of processing. Although BK7 is only one of many different types of glass, it is considered the standard substrate for precision VIS/NIR optics.

One drawback to BK7 is its high sensitivity to temperature variance. For example, under high heat (approximately 400˚ C), BK7 starts to soften and its transmission characteristics start to become unstable. For high temperature applications where BK7 is not appropriate, fused silica has all of the same benefits while maintaining stability under changes in temperature. Fused Silica is, however, generally more expensive than BK7.

Other types of glass, such as SF2, offer a higher index of refraction and are ideal for certain applications like broadband polarizers.

IR Transmitting Substrates (2µm–30µm): ZnSe, ZnS, Cleartran, Si, Ge, Sapphire, and fluorides

There are two major far infrared (FIR) applications: imaging applications (FLIR and thermal) and CO2 laser cutting and welding applications. Imaging devices operate at 3-5 and 8-12 µm regions, and CO2 laser devices operate at 10.6 µm. Both of these applications are considered “far IR” applications that require special IR materials. Common IR transmitting materials include Zinc Selenide, Zinc Sulfide, Cleartran, Germanium and Silicon. Sapphire, along with the fluorides discussed in the UV and VIS/NIR substrates sections, also transmit infrared light up to about 5 µm. In general, processing IR substrates requires special fabrication techniques that are different from those of UV and Visible substrates. RMI is one of the few companies in the world that manufactures UV, visible and IR optics in house.

Zinc Selenide (ZnSe) is widely used in both imaging and CO2 laser applications. ZnSe is one of the most commonly used IR substrates because of wide availability, good mechanical strength and ease of processing. ZnSe also has good transmission characteristics for some visible (red) light often used for alignment purposes. ZnSe is the industry standard for CO2 laser optics because of lower absorption at 10.6 µm.

Zinc Sulfide (ZnS) has good transmission characteristics in the 3-5 / 8-12 µm regions and its mechanical strength makes it a common choice for FLIR and thermal imaging applications. Unlike ZnSe, ZnS has poor transmitting characteristics in the visible region.

Cleartran is a special type of multi-spectral ZnS (chemically identical) that is clear in appearance. This means that it has the same IR transmitting properties as ZnS with the added benefit of transmitting the full VIS/NIR spectrum as well. Cleartran is a more expensive optical material (comparable to ZnSe) and is generally used in special applications requiring multiple wavelength regions.

Silicon (Si) is commonly used for 3-5 µm imaging applications. Widespread use of this material in the semiconductor industry has driven the price of silicon down, making it the least expensive of IR materials. Silicon is readily available, lightweight and possesses very good thermal and mechanical durability. Although silicon has some absorption in the 8-12 µm range, the low power levels of these imaging applications make it a suitable choice. Silicon is not used for CO2 transmitting optics because of high absorption characteristics, however, it is widely used in mirror applications due to its high thermal conductivity and mechanical characteristics.

Germanium (Ge) is also commonly used in 3-5 µm and 8-12 µm imaging applications where a high index is required in the optical design. Disadvantages of using Germanium include higher price, heavier weight and sensitivity to high heat.


Reflecting Optics

(Mirrors, Partial Reflectors, Beamsplitters, Output Couplers)

UV Mirror Substrates

The most important considerations in selecting a UV mirror substrate include the high absorption characteristics of UV light (see UV transmitting optics) and the ease of fabricating the optic. In the case of mirrors, one might assume that substrate selection is irrelevant because it merely serves as a vessel for the coating. Yet, even with high quality, dielectric mirror coatings, there is always some “leakage” into the substrate. Having the leakage pass through (or transmit) with minimal absorption is the key to good UV mirrors. Therefore it is important to select a good UV transmitting substrate even for mirrors, especially in higher power applications (i.e. lasers). RMI recommends UV grade fused silica (UVFS) for most high damage threshold UV mirrors.

In cases where high power is not a critical issue (non-laser or low power laser applications), a cheaper substrate like BK7 or fused silica may be an adequate solution.

UV light is also more susceptible to scattering than visible or IR light because of its shorter wavelength. Under UV light, the effects of surface imperfections (scratches and pits) are amplified. Therefore it is critical to have excellent surface quality to minimize energy loss within the system.

VIS/NIR Mirror Substrates

As in the case of UV mirrors, VIS/NIR mirror substrates must transmit any “leakage” that gets through the coating. In general, both BK7 and fused silica do this well for VIS/NIR light. If extreme thermal conditions and the need for mechanical strength are key considerations, then fused silica is the better choice because of its stability under thermal shock and high mechanical strength (see VIS/NIR transmitting substrates). In cases where these are not an issue, BK7 is the more economical choice.

IR Mirror Substrates

Since IR light is largely made up of heat, it is important to choose a substrate that has good heat dissipating properties. Silicon is widely used for IR mirrors, particularly in low power thermal imaging applications, due to its high durability, wide availability, light weight and low price. Additionally, silicon is a popular IR mirror substrate because of good IR transmission and heat dissipation in cases where heat absorption can lead to component damage.

For higher power laser mirrors (e.g. CO2 laser mirrors), RMI recommends copper mirror substrates. Copper is known for its tremendous heat dissipating properties that will minimize the damaging effects of absorption from high power IR lasers. RMI also produces Molybdenum (also referred to as “Moly”) mirror substrates for more special/higher power CO2 lasers.