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Robust mirrors take on demanding applications

15 May 2006

Selecting the right mirror for the right job is no trivial task. However, help is at hand as Michael Case and Mark Dykstra guide you through the process.

Ultraviolet (UV), visible (VIS) and infrared (IR) applications all have something in common; they require optics to control, shape, split, filter, attenuate or focus light. The type of application and spectral region usually dictate the manufacturing methods, materials and thin-film coatings used to produce finished optics.

There are numerous applications for mirrors and thin-film coatings. One of the more common applications for mirrors is the control of laser radiation. Other typical applications include spectroscopy, image projection, military systems, telecommunications, automotive and medical. Some applications for mirrors are wavelength specific. For example, in the UV, typical applications include analytical chemistry, inductively coupled plasma spectroscopy, and excimer laser microlithography, micromachining and LASIK vision correction. Important recent advancements of high laser damage threshold mirrors are making more demanding applications feasible.

Typical VIS applications include projection display optics, analytical instrumentation, architectural lighting, military sighting systems and automotive systems. In the IR spectral region, typical applications include Fourier transform infrared spectroscopy as well as environmental monitoring, defence and aerospace applications.

Substrate materials

There are few limitations on the optical materials used for mirror substrates, which give the designer great flexibility in terms of cost, strength, stiffness and aspect ratio. Materials for transmitting optics on the other hand must be carefully selected to ensure optimum light transfer through a system.

Depending on the application, mirrors can be produced from plastic, glass, metal or ceramic substrates. Mirror substrates for laser applications are normally manufactured using BK7, fused silica or CaF2. Substrate cost, surface figure, surface finish, thermal stability and structural integrity are all drivers for substrate choice.

Surface quality

Along with selection of the proper substrate material, actual damage threshold and long lifetime can greatly depend on the surface preparation of the optics. Most laser optics are specified by surface figure and surface finish.

Surface figure is a measurement of the deviation from an ideal surface, in terms of peak-to-valley waves. A typical specification of 1/10-wave or better at 633 nm is common for surfaces of excimer laser optics, although for some special applications, surface figure specifications can be in the range of 1/20 or better. Surface finish is defined as the number of flaws in an optical surface in terms of the scratches and digs, based on documented military specifications. Surface finish of an excimer mirror is typically specified as 20-10 or better for optimum, low-scatter performance. In some critical applications, a surface finish of 10-5 or better is available. Precise fabrication techniques used to produce these types of surfaces, especially on crystalline materials such as MgF2 and CaF2, can contribute greatly to the overall optical performance of the excimer laser.

Types of mirror

To meet the requirements of specific applications, mirrors can be manufactured flat, spherical or aspherical. Each type of mirror provides unique optical properties for reflection. A flat mirror will reflect light at the same angle as the light that enters the mirror. A concave spherical mirror will focus or collimate incoming light but may introduce spherical aberrations. A parabola is one example of an aspheric mirror and therefore does not create spherical aberrations. Aspheric mirrors also include ellipsoids and toroids, each with its own unique optical properties. Since aspheric mirrors are harder to manufacture, they are generally more expensive than spherical mirrors.

Thin-film coatings

The majority of optical thin-film coatings produced today consist of alternating layers of high and low index dielectric materials, or a combination of dielectric and metallic layers. Common dielectric materials include fluorides such as MgF2, LaF3 and AlF3 and oxides such as SiO2, TiO2, Al2O3 and Ta2O5. Metals typically include aluminium for UV and VIS, silver for VIS and NIR and gold for NIR and IR wavelengths.

The arrangement of material layers within a coating "stack" allows production of efficient antireflection (AR) coatings, near total reflectors, broadband mirrors, optical filters, beam-splitters and a host of other light control coatings. Careful selection of coating materials is an important step in the manufacturing process to yield high efficiency thin-film coatings. Choosing the best coating materials for a specific application is dictated by several factors, including the desired optical properties, wavelength region of interest, refractive index and absorption, inherent stress introduced by the materials, and environmental requirements for the finished part. Candidate materials are routinely characterized by producing a test coating, which is measured to determine refractive index and absorption properties over the wavelength region of interest. Once the data is collected, it can be incorporated into a thin-film design programme for computer modelling of coatings.

Thin-film modelling

It is possible to accurately model a wide variety of thin-film coatings after you create a database of material properties (n and k) for available coating processes. Commercial thin-film design programmes enable the user to rapidly simulate many different combinations of materials and layers to achieve the desired results before operating the coating chamber.

Coating deposition

There are several physical vapour deposition processes capable of producing thin-film coatings, including electron beam (e-beam), ion-assisted-deposition (IAD) and resistive heating. The e-beam process is popular because it enables deposition of dielectric coating materials and can be used with IAD for the production of extremely durable coatings. In addition, multi-pocket e-beam guns can hold several different materials, making it relatively easy to incorporate more than two coating materials into a design. Resistive heating is commonly used for metallic coatings and can also be used for certain fluoride materials.

Optical monitoring

To facilitate production of more complex designs, a state-of-the-art optical monitoring system (OMS) has been developed, which automates the coating process and enables a wide variety of standard and hybrid coatings to be produced. The system works in conjunction with thin-film design programs to enable a wide variety of coatings options. The OMS uses information from the thin-film programme to calculate exact layer thickness and determines the "cut-point" for each layer. As layers are deposited, the OMS monitors and compares actual data to the computer model. It selects the appropriate coating materials, deposits the layers and makes adjustments as required to follow the computer model. For high-volume production, the automated OMS assures a high degree of lot-to-lot consistency. In addition, any fraction or multiple of a quarter-wave layer thickness can be deposited, creating many new and exciting possibilities for mirror coatings.

Anti-reflection coatings

The purpose of an AR coating is to reduce unwanted reflections and to increase transmission within a desired wavelength region. AR coatings can range from a single layer MgF2 AR to traditional "V" coatings to more complex broadband multi-layer designs (see "Thin-film application"). AR coatings are routinely used on lenses, windows and back surfaces of beam-splitters to reduce unwanted reflections.

Broadband vs multi-layer dielectrics

Mirror coatings generally fall into two categories which include broadband metallic or wavelength specific multi-layer-dielectric (MLD) designs. Common broadband metallic coatings include enhanced (or protected) aluminium, silver and gold. Overcoat layers can include single dielectric layers such as MgF2 or SiO2, or MLD coatings to provide enhanced reflectivity and protection in adverse environmental conditions.

Wavelength-specific MLD mirrors, routinely used in high-energy laser applications, consist of alternating layers of high and low index materials to achieve near total reflect-ance at the design wavelength. Advantages of MLD coatings are greater resistance to laser induced damage, extremely low absorption and near total reflectivity. It should be noted that MLD coatings are relatively narrow reflectors compared to broadband metallic mirrors, with bandwidths ranging from 5 to 20% of the design wavelength.

Advanced optics for high-power semiconductor and medical lasers

Over the past year there have been significant developments in damage resistant coatings for 193 nm medical and semiconductor laser markets. Excimer lasers used in LASIK applications typically operate at relatively high fluence levels (∼300 mJ/cm2) and moderate (200-400 Hz) repetition rates, subjecting optical coatings to high average power during a typical vision-correction procedure. On the other hand, semiconductor lasers operating at moderate fluence levels (∼3-5 mJ/cm2) now achieve repetition rates approaching 3-4 kHz, continuously exposing mirrors to energetic UV radiation for billions of laser pulses.

Taking advantage of the new OMS, hybrid 193 nm mirrors were produced that satisfy both medical and semiconductor requirements. Coatings have been tested for more than 2 million laser pulses at 300 mJ/cm2 (200 Hz) and greater than 8 billion laser pulses at 5 mJ/cm2 (4 kHz) without damage. In addition, the new hybrid coatings pass Mil-Spec adhesion, abrasion and humidity testing, making them some of the most robust UV coatings available.

In summary, mirrors are critical components that precisely control light in countless optical applications. Selecting the right mirror for an application, however, is no trivial task. The key to designing an effective and affordable solution is to partner with a firm that has expertise in your specific application. Several critical factors must be addressed to ensure that the performance objectives will be met and that the optic will survive the operating conditions in your application. These factors include substrate material, surface figure, surface finish, mirror geometry, coating design and deposition method. The optimal solution is the result of a rigorous balance of these important factors.

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