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AR coatings increase the transmittance through an optic by reducing the naturally occurring Fresnel reflection losses always present at any interface separating different indices of refraction. AR coatings are most often applied to lenses, windows, prisms, and rods. TYDEX supplies the following types of AR coatings: single layer MgF2 coating, V-type single wavelength coating, broadband anti-reflection (BBAR) coating, dual band coating, and V-type infrared single wavelength coating.
1. Single layer MgF2 coatings
Perhaps the simplest, most common and least expensive anti-reflective coating consists of a single layer of Magnesium Fluoride (MgF2). Because MgF2 has a very low refractive index (approx. 1.38 at 550 nm), it follows that a single "quarter-wave-thick" layer deposited upon typical optical glasses (with typical refractive indices ranging from 1.45 to 1.85) has attractive broad-band anti-reflective properties. The single layer MgF2 coating will reduce a Fresnel reflection loss from typically 4% (uncoated) to less than 1% (coated). MgF2 coatings can also be optimized for a specific wavelength. For example, in the case of a MgF2 coating designed to reduce Fresnel reflections across the wide band of 400-700 nm, the coating will usually show the least losses at the center wavelength 550 nm. Naturally, if Fresnel losses need to be reduced down to the level of 0.5% or less, then the multi-layer coating options, described below, should be used.
Fig. 1 Single layer MgF2 AR coating optimized at 600-1000 nm, AOI=0°.
2. V-type single wavelength AR coating
V-type AR multi-layer coatings generate extremely low Fresnel losses for a particular wavelength, usually generated by lasers. The designation "V" derives from the shape of the residual reflectance curve, which is characterized by a steep V-shape (as opposed to a single layer MgF2, which looks more like a very shallow "U."
TYDEX' standard reflection loss per surface for a V-coating is less than 0.25%. Performance as good as <=0.1% per surface is available on request.
Fig. 2-2 AR coating at 1550 nm, AOI=0°.
3. Broad-band anti-reflective (BBAR) coating
BBAR coating utilizes multiple layers usually consisting of alternating layers of two materials with different refractive indices, which provides much better performance over a wide wavelength range comparing to single layer AR coating.
Fig. 3-1 AR coating at 250-420 nm, AOI=0°.
Fig. 3-2 AR coating at 425-675 nm, AOI=0°.
Fig. 3-3 AR coating at 1000-1400 nm, AOI=0°.
4. Dual- and triple-band AR coating
Dual-band AR multi-layer coatings are required when an optical component has to simultaneously exhibit extremely high transmission at two different wavelengths.
Fig. 4-1 AR coating at 355 and 532 nm, AOI=0°.
Fig. 4-2 AR coating at 532 and 1064 nm, AOI=0°.
Similarly, triple-band AR coatings optimize an optic's transmission simultaneously at three different wavelengths. A common application is for YAG lasers where the fundamental wavelength, second harmonic, and third harmonic must all pass through the same optic with high efficiency.
Fig. 4-3 AR coating at 355, 532, and 1064 nm, AOI=0°.
5. V-type infrared single wavelength AR coating
Although TYDEX can deliver single-wavelength AR coatings at nearly any infrared wavelength, the most requested wavelength is the 10.6 µm of CO2 laser, the workhorse of the industrial laser industry. TYDEX offers low-loss performance, down to 0.25%, which is suitable for use in focusing lenses and windows used in relatively low-power industrial CO2 lasers.
Fig. 5-1 AR coating at 10.6 µm on ZnSe, 0°.
6. Broadband infrared AR coating
TYDEX is able to offer a very wide range of broad-band coatings for infrared applications such as thermal imaging in the popular 3-5 µm and 7-14 µm bands, which can be deposited upon a very wide variety of substrates. TYDEX broad-band infrared AR coatings are also widely used in FTIR spectroscopy applications. Transmission values below are given for complete window with thickness 2.0 mm. Damage thresholds for these coatings are not listed because the vast majority of applications do not involve high intensities.
Fig. 6-1 AR coating at 2-6 µm on Si, AOI=0°.
Fig. I-6-2 AR coating at 3-5 µm on ZnSe, AOI=0°.
7. Dual band infrared AR coating
Very common for industrial CO2 laser applications is the use of visible laser light traveling along the same optical path as the infrared beam, usually for beam aiming purposes. For this application and especially since the Fresnel losses from uncoated ZnSe are extremely high, a coating that is transmissive both for a visible laser and the CO2 laser is highly advantageous. TYDEX' standard coatings for this application are specified below.
Fig.7-1 AR coating for 532 nm and 10.6 µm on ZnSe, AOI=0°.