Common examples of MacLeod coatings

This article shows common, thin-film components manufactured by MacLeod. As the manufacturer indicates, the components should not be considered the best that can be produced. For these coatings, the incident medium is air unless otherwise noted.
These MacLeod coatings are available in the standard COATING.DAT file that’s included in the OpticStudio installation files.
 

Angus MacLeod, Sandrine Auriol, Thomas Pickering
08/24/2017
OpticStudio
Polarization and Thin Film Coatings

Anti-reflection coatings

For the anti-reflection coatings described in this article the substrate is assumed to be a crown similar to N-BK7 glass unless otherwise noted.

The only coating that uses magnesium fluoride is the single-layer anti-reflection coating AR_400-700_1L. In the other anti-reflection coatings, the low-index material is SiO2. Although MgF2 has a slightly lower index, which improves performance, its environmental resistance is inferior to SiO2. SiO2 is also readily sputtered, whereas MgF2 presents significant sputtering problems.
 
OpticStudio File: For an example of these anti-reflection coatings, refer to the AR_Conic interconnect.zar file in the attachments.
The file shows the coupling of two single mode fibers with a conic lens. The front and back surfaces of the lens have an anti-reflective coating. Each configuration is a different coating. The merit function shows a comparison of the coupling between the different configurations and coatings.

Antireflection coatings plot

This file contains the following:
 
  • AR_400-700_1L:

This well-known, single-layer anti-reflection coating consists of a quarter wave of magnesium fluoride. The coating is simple to apply by thermal evaporation and is extremely tolerant to errors. Although it’s one of the oldest anti-reflection coatings, it’s still produced in large volumes. It usually has a characteristic magenta color in reflection.

 
  • AR_400-700_3L:

This famous and widely used three-layer coating was first conceived by SCHOTT in Jena, Germany during World War II. It was later described by Luther B. Lockhart and Peter King in "Three-Layered Reflection-Reducing Coatings." Journal of the Optical Society of America. Volume 37 (9), 1947, 689-694).

 
  • AR_400-700_4L:
This coating uses three materials. The layer closest to the substrate is of intermediate index. Manufacturers prefer to use only two materials when possible. In this coating, the intermediate index quarter-wave layer is replaced by a two-layer equivalent of the same materials used in the other two layers. The design is then gently refined. This coating provides slightly better performance than the three-layer coating.
 
  • AR_400-900_12L:
This is a 12-layer anti-reflection coating for the visible and near-infrared regions.
 

Cold mirror coatings

A cold mirror coating reflects the visible region and transmits the infrared region.

OpticStudio File: For an example of a cold mirror coating, refer to the Cold_mirror.zar file in the attachments.
The file contains a collimated source ellipse (black body spectrum 6000K 0.4-1.0 µm) and a window that is coated on the front face. The coating and angle of the window have different configurations. Two flux versus wavelength analyses are used to compare the reflected and transmitted sides. 

This file contains the following:
 
 
  • COLD_MIRROR_0DEG
This cold mirror is designed for angles of incidence at or near normal. The design has 31 layers.


 
  • COLD_MIRROR_45DEG_A
This cold mirror is designed for use at 45° incidence in air. It has 29 layers. The materials used have no significant absorption in the visible region.


 
  • COLD_MIRROR_45DEG_B
This cold mirror is designed for use at 45° incidence with slightly higher reflectance than COLD_MIRROR_45DEG_A. It uses titania, rather than niobia, because titania has a slightly higher index. However, titania suffers from absorption at the blue end of the spectrum, hence the ripple for p-polarization. The design has 47 layers.
 

 

Beamsplitter coatings

A beamsplitter coating is placed between two right angle prisms to form a cube beamsplitter. It splits input light into two separate parts.

OpticStudio File: For an example of these beam splitter coatings, refer to the Cube_BS.zar file in the attachments.
The file contains a collimated source ellipse (system wavelengths) and a coated beam splitter. The merit function calculates the reflected and transmitted flux.

This file contains the following:
 
 
  • CUBE_BS_500-600_99L
This 50/50 dielectric (or lossless) beam splitter has an incident medium and substrate of N-BK7 and is designed for 45° incidence in the incident medium. The component is in the form of a cube with the coating on the diagonal. The problem with such components is the low contrast in p-polarized layer properties (45° incident is not far from the Brewster angle between the two-layer materials). With only 99 layers, it’s also difficult to achieve the required reflectance and transmittance while  eliminating the ripple. The 500-600 nm range is near the edge of possibility.
 

 
  • CUBE_BS_500-600_148L
This is another 50/50 dielectric beam splitter with N-BK7 as an incident medium and substrate. The specification is similar to CUBE_BS_500-600_99L, but with 148 layers (rather than 99), it enables better ripple elimination at the expense of a more demanding manufacture.
 

 
  • CUBE_BS_460-640_316L
This 50/50 beam splitter has a 45° angle of incidence and N-BK7 as an incident medium and substrate, making it a cube beam splitter with coating on the diagonal. The range has been increased to 460 nm to 640 nm, but even with 316 layers there’s some residual ripple. Further ripple reduction indicates the need for more layers, but that would be challenging to manufacture and not guaranteed to be successful.
 

 
  • CUBE_POL_45DEG_21L
This is a simple 21-layer cube polarizer for the visible region. The design is a quarter-wave stack. The interface between the high and low index layers is arranged at the Brewster angle, resulting in low p-polarized reflectance.
 

 

Protected and Enhanced Reflectivity coatings

Protected and enhanced reflectivity coatings offer mirrors a protection to environment and expand the reflectance over the wavelength range.
 
OpticStudio File: For an example of these protected and enhanced reflectivity coatings, refer to the Mirror_enhanced-reflectivity.zar file in the attachments.

The file contains a collimated source ellipse (blackbody spectrum 6000K) and a coated mirror. There are six configurations for the different coatings. A detector viewer displays the true color of the reflection.
 
This file contains the following:
 
 
  • PROTECTED_AL
This is a simple aluminized reflector with a protecting layer of silica that is designed to provide maximum luminous reflectance in a simple coating. The reflectance falls on either side of the visible region.
 
  • ENHANCED_AL

This four-layer quarter-wave overcoat provides high luminous reflectance at near-normal incidence. With more complex designs, a wider range of high reflectance is possible.
 

  • PROTECTED_SILVER

In the visible and infrared regions, silver has the highest reflectance of all metals but offers poor environmental resistance. Protection is afforded by a two-layer system. The thin Al2O3 layer next to the silver provides strong adhesion, while the outer SiO2 layer offers solid protection. The thicknesses were taken from a technique published by G. Hass, J.B. Heaney, H. Herzig, J.F. Osantowski, and J.J. Triolo ("Reflectance and durability of Ag mirrors coated with thin layers of Al2O3 plus reactively deposited silicon oxide." Applied Optics. Volume 14 (11), 1975, 2639-2644), but they can be varied to modify the shape of the reflectance curve.


 
  • HR_400-700_81L
The quarter-wave stack reflector doesn’t cover the entire visible region. This is an 81-layer extended zone coating that provides high reflectance over the entire visible region.
 
  • HR_405-840_101L
Extending the zone of high reflectance over a region that is greater than the visible region requires more than the 81 layers in the previous coating. This coating uses 101 layers.
 
  • HR_633_21L
This is a simple quarter-wave stack of 21 layers that provides high reflectance at 632.8 nm.
 

 

Near-Infrared Blocker coatings

 A  near-infrared blocker coating transmits the visible region but blocks the near infrared.

OpticStudio File: For an example of these near-infrared blocker coatings, refer to the IR_blocker.zar file in the attachments.

The file contains a source (black body spectrum 6000K) and a lens. The front face is coated with the near-infrared blocker coating.

This file contains the following: 

 
  • IR_BLOCKER_45L
There are many applications for this coating . This 45-layer filter offers better infrared blocking than is required for a simple heat-reflecting filter.

References

How to Add Coating and Scattering Functions to Non-Sequential Objects
How to Model a Dichroic Beam Splitter
Thin Film Center