Thin films give optical components a boost

30 Jan 2004

Thin-film coatings can be essential to obtaining the desired performance from an optical component. Stuart Allan describes the types available and provides purchasing advice.

From Opto & Laser Europe February 2004

Optical thin-film coatings have now penetrated almost every aspect of everyday life. A little investigation shows that they are present in everything from the lenses in spectacles and cameras to the specialized optics inside lasers.

Of the optical components in use today, the vast majority are made of glass which has been coated. Coatings are usually made from either a single dielectric thin film or a stack of such films.

These films are often made from metal oxides and fluorides, and are typically just a few tens or hundreds of nanometres thick. The purpose of the coating is to modify the transmission and reflection of the optical part, such as minimizing reflections or blocking certain wavelengths.

The need for coatings, especially anti-reflection versions, is easy to understand. Whenever light passes from one medium to another, such as from air to glass, some of the light is transmitted and some reflected. The amount that is reflected is determined by the difference in the refractive index between the materials, and by the angle of incidence of the incoming light.

For a light beam in air striking uncoated glass at normal incidence, the strength of the reflection is typically just a few per cent of incident optical power. For example, in the case of the common glass BK7, which has a refractive index of 1.52, the reflection is 4.25% at each air-glass boundary.

Although this may not seem much, for optical designs using more than a few components, losses in transmitted light can rapidly multiply, especially if high index glasses are used - as in the case of infrared systems. In imaging equipment, these weak reflections can also cause a significant loss in image contrast as ghost images, since the reflections may get superimposed on the primary image.

Fortunately, in many cases these troublesome reflections can be eliminated by the use of a thin-film anti-reflection coating. In a similar fashion it is possible to create highly reflective coatings that give almost perfect mirrors with a reflectivity of almost 100%.

However, the use of thin-film coatings is not limited to these tasks - they can also be employed to create different kinds of wavelength filters and beamsplitters.

The science behind optical coatings So how do thin-film coatings work? Put simply, the optical properties of a coating are governed by optical interference between the reflections from the upper and lower surfaces of the film. With a stack of thin films, reflections from each of the layers need to be considered to understand the coating's optical characteristics.

A simple anti-reflection coating can be made by choosing a film of a certain thickness and refractive index so that the reflections from its upper and lower surfaces are out of phase and will interfere destructively. In reality, this coating is often made from a single film with an optical thickness equal to a quarter of the incident light's wavelength. These "quarter-wave" films are a common building-block in many designs of coating.

For example, a highly reflective coating can be made from an alternating stack of high- and low-index quarter-wave films. All the reflections from the layers are designed to be in-phase so that they interfere constructively. The reflectance of the coating increases with the number of layers. Alternatively, optical elements can be coated with a metal such as aluminium or gold to create a high-performance mirror.

Coating materials In principle, the surface of any optical element can be coated with thin-film layers of various materials to provide the desired transmission and/or reflection characteristics. Potential materials include titanium dioxide, tantalum oxide, zirconium oxide, aluminium oxide, silicon dioxide, magnesium fluoride, zinc sulphide, germanium, silicon and silicon oxide. With the exception of metallic coatings, the optical performance of the coating depends on the nature of the substrate material.

Note that thin-film coatings are designed to work under a precise set of operating conditions. The performance of a coated optical component is likely to vary with the wavelength of incident light, its angle of incidence and the polarization, as well as the ambient humidity and operating temperature.

Clever designs of multilayered films are useful for creating a coating that meets several sets of specific conditions (wavelength and angle of incidence) or a particular range of conditions.

Specifying an optical coating Optical coatings generally fall into three main spectral regions: the ultraviolet (UV), the visible and near-infrared (VIS-NIR) and the infrared (IR). The coating materials used differ depending on the wavelength region of operation, though some specialized coatings are multi-spectral. Different coating houses may specialize in some or all of the wavelength regions.

When you are specifying an optical coating, the thin-film design engineer would benefit from the following information:

• the purpose of the intended optic;

• the type of coating proposed, for example whether it is long- or short-wave pass

• the wavelength range involved;

• the transmission, reflection, absorption and scatter requirements (with tolerances) of the filter;

• the angles of incidence, e.g. 45±5°;

• the polarization state or states of the incident light;

• the type of substrate material;

• the dimensions of the part;

• the clear aperture;

• the cosmetic quality required, i.e. the scratch/dig requirements;

• which side(s) of the optic can be coated;

• the optic's environmental requirements;

• any military (MIL) specifications or similar restraints; and finally,

• the quantity required.

In conclusion, there are many aspects to optical thin-film coatings that need to be taken into account before you place an order. Coatings can be made to meet the most stringent requirements from a technical point of view, but remember that the price depends significantly on the tightness of the specification. So always ask yourself, "Do I really need ±1nm tolerance, or will ±5 or ±10 nm suffice?" The difference in price is remarkable.

Remember too that the optical thin-film design engineer is always willing to discuss your requirements.

Filter types explained Here is a summary of the most common types of optical thin-film filter.

ANTI-REFLECTION COATINGS

Single-layer anti-reflection (SLAR): this is a single-layer coating and is usually specified for one wavelength, one angle of incidence and one set of reflection/transmission requirements.

Broadband anti-reflection (BBAR): this is a multilayer anti-reflection coating that is designed to operate over a wavelength range, e.g. 400-700 nm, at designated angles of incidence and with specified reflection/transmission characteristics.

Extended-band anti-reflection: as above, except the wavelength range is extended to, for example, 400-1100 nm, and the reflection/transmission requirements are not usually as stringent as the broadband coating.

Dual- or triple-band anti-reflection as suggested these are specified over two or three distinct wavelength ranges.

V-Coat anti-reflection: this type of coating is used at a specified wavelength with a very low reflection (R) requirement - usually R < 0.1%, though in some telecoms applications R < 0.0001% is now demanded.

WAVELENGTH FILTERS

Short-wave pass (SWP): this type of coating is specifically designed to transmit shorter wavelengths and to reflect longer wavelengths. A sample specification could be a transmission of T > 85% absolute at 400-520 nm, T = 50% at 530±5 nm, T < 0.1% at 550-690 nm; angle of incidence 0±5°; random polarization. The transmission values can be higher if an anti-reflection coating is applied to the reverse face.

Long-wave pass (LWP): this coating type is designed to transmit longer wavelengths and reflect shorter wavelengths. A sample specification could be T > 85% at 540-700 nm, T = 50% at 530±5 nm, T < 0.1% at 410-510 nm; angle of incidence 0±5°; random polarization. Again, transmission values can be higher if an anti-reflection coating is applied to the reverse face.

Band pass (BP): these allow the transmission of light within a carefully defined wavelength range, while light outside this range is blocked. Available with narrow or wide wavelength windows of transmission, these filters are usually defined through their centre wavelength (l 0), full-width half-maximum (FWHM), peak transmittance (T0) and rejection bands. For all the parameters, tolerances are required.

Notch: a notch filter is used to reflect a specified wavelength, or a narrow wavelength region with high transmission outside that region. A typical performance might be T = 45±2% at 550±3 nm, T > 95% at 420-520 nm and T > 95% at 580-770 nm for a specific notch-depth coating. Sometimes notches are specified with a wavelength window, e.g. Dl < 20 nm at 10% points and T < 0.0001% at 560 nm. An alternative specification would be optical density OD > 5, T > 95% at 420-540 nm and T > 95% at 580-770 nm.

REFLECTIVE FILTERS

Three types are available: commonly high reflectors, metallic reflectors and enhanced metallic reflectors. These are all specified with various levels of reflection, angles of incidence, polarization and wavelength range. For a high reflectance over a restricted wavelength range a multilayer would be best. In contrast, for a wide wavelength range and a lower reflection requirement, the enhanced metallic reflector is suitable, and for a very wide wavelength region, potentially from the visible into the far-infrared, a metallic reflector is probably best.

BEAMSPLITTER COATINGS

Beamsplitters split an incoming light beam into two distinct output beams, and often make use of specialized coatings. When purchasing, the transmission and reflection characteristics, a wavelength range, angles of incidence and polarization must be specified, e.g. T = 70±3%; R = 30±3% at 420-700 nm; angle of incidence 45±5°; random polarization.