Case Study: Sputtered Optical Filters for Satellite Laser Communications
Overview
Chroma Technology worked with a satellite communications developer to manufacture optical filters for free-space optical (FSO) laser communication including sputtered bandpass filters, beamsplitters, and a solar filter to reflect broadband thermal energy while maintaining C-band transmission. To address the solar filter requirement, Chroma developed a gold induced-transmission filter design and delivered optics that passed adhesion and abrasion testing per ISO 9211-4 and MIL-C-48497A.
Industry: Aerospace / Satellite Communications
Application Area: Free-space optical (FSO) laser communications
Technology Focus: Sputtered bandpass filters, beam-splitters, and gold induced-transmission filters
Challenge
The client required bandpass filters isolating Tx and Rx channels near 1545 nm and 1560 nm with low phase retardance, beamsplitters with tight cut-on tolerances, and a solar filter transmitting in C-band while reflecting thermal radiation from the Sun and Earth through the deep IR. No off-the-shelf solution could meet this combination.
The filters had to pass laser signals at precise wavelengths while rejecting thermal energy across the full IR spectrum.
Solution
Rather than quoting against the initial specification alone, Chroma provided modeled spectral curves with its first proposal. This let the customer’s engineers run realistic system simulations early, revealing spectral overlaps that could have degraded dye separation had they gone unaddressed.
Over multiple design iterations, the teams refined the full filter set. Key adaptations included:
Redistributing performance requirements across two filter elements when single-filter designs exceeded manufacturing margins.
Replacing dicing with precision milling for thick substrates, improving dimensional accuracy and enabling recovery of partially damaged components.
Multiple coating iterations on multi-notch filters to achieve the required blocking depth across large substrates.
Sputtered coatings addressed each filter requirement — from narrow channel isolation and polarization control to broadband solar rejection.
Key Features
High C-band transmission
Greater than 92%T in C-band, exceeding theoretical predictions.
Broadband thermal rejection
Ultra-thin gold layer reflects from visible wavelengths through the deep IR, managing solar and Earth thermal loading.
Narrow bandpass precision
Steep edges and tight CWL tolerances for clean Tx/Rx channel isolation.
Coating durability
Sputtered construction with demonstrated adhesion and abrasion resistance.
Implementation
Define spectral requirements for each filter type
Develop etch-back process for continuous ultra-thin gold layers
Characterize thin metal optical constants and iterate design via optical monitoring
Validate filter spectral performance and coating durability to strict ISO & MIL standards
Conclusion
This project demonstrates how FSO laser communications can demand multiple filter types with distinct engineering requirements. Achieving precise channel isolation, low phase retardance, and broadband solar rejection each called for a different design approach — with the gold induced-transmission filter requiring custom process engineering to produce consistent ultra-thin gold layers. Together, the filters met the spectral precision and durability space-based laser communications demand.
Designing optical systems for demanding environments?
Custom filter design starts with understanding your spectral and environmental requirements. Our optical engineering team can help you find the right approach for your application.
