Phase-controlled, Long-pass, Cold Mirror, Dielectric Beam Splitter

Client: Medical Photonics Equipment Manufacturer

Application Requirements:

  • A pair of thin film coatings applied to each surface of a custom, octagonal, borosilicate substrate that work in tandem to reflect > 70% of radiation across the visible spectrum and transmit > 95% of light at a discrete infrared wavelength at a 45-degree angle of incidence.
  • While maintaining the specifications detailed above, these coatings must also transmit the infrared light without changing the polarization state of the incident beam.

Critical Issues:

  • Due to the non-normal angle of incidence, polarization effects, that is, separation of phase and intensity between the orthogonal planes of light (traditionally referred to as “S” and “P” polarization), were inherent to any traditional thin film structure.
  • The infrared laser beam’s circular polarization state necessary for measuring fine topological differences was distorted to a severe elliptical shape upon transmittance through traditional thin film design structures.
  • Measurement of the phase of the transmitted beam required the acquisition of a polarimeter to verify our design improvements.

Design Basis:

  • Starting from a traditional structure with spectral intensity parameters specified, Blue Ridge Optics’ team of engineers developed a custom function using the numerical analysis algorithms within our design software to find a solution that balanced the intensity and phase requirements.
  • Traditional designs minimize the number of “optimized” layer thicknesses within the thin film stack for repeatability and ease of manufacturing—preferring periodic layer thicknesses to create reflectance bands where needed. However, due to the multi-faceted requirements of this project, we allowed all layers to optimize freely. Each layer was tailored to an exact thickness and then deposited to an individual accuracy of within several nanometers of our theoretical model.


Blue Ridge Optics designed, prototyped, and now manufactures a proprietary set of coatings at production quantities that achieve all customer specifications. The elliptical polarization distortion of the infrared laser beam upon transmittance has been eliminated and is verified in-house before delivery to the customer. 

The targeted engineering expertise Blue Ridge Optics brought to the table allowed for unprecedented improvement in our customer’s final product that will be used to diagnose, prevent, and treat disease.