Case Study: Sputtered Optical Filters for Satellite Laser Communications

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.

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.

Implementation

1. Define spectral requirements for each filter type

2. Develop etch-back process for continuous ultra-thin gold layers

3. Characterize thin metal optical constants and iterate design via optical monitoring

4. 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.