LiDAR Optical Filters Explained: Wavelengths, Angles, and Coating Considerations
What does a LiDAR filter do in a satellite system? A LiDAR optical filter in a satellite system isolates the laser return wavelength while rejecting background solar radiation and any unwanted spectral noise. It enables the detector to measure only the specific laser signal, improving signal-to-noise ratio, atmospheric accuracy, and ranging precision in Earth observation or defense applications.
LiDAR for LEO (low earth orbit) occurs just 200-2000 km above Earth and allows for today’s satellites to monitor and review data used for atmospheric sensing, topographic and space mapping, ice sheet and ocean elevation monitoring, and for ground-to-space and space-to-ground communications.
LEO-based instruments are more demanding than typical terrestrial systems, as the functional conditions are incredibly harsh. Optical filters created for space applications must be lightweight, dimensionally small, able to withstand large g-forces and stress, and that’s just getting the system up into space! While in orbit, systems need to withstand radiation, extreme temperature cycling (-100-200 deg C), and passing debris and contaminants.
Omega’s space-qualified TRL9 coatings are designed to provide high durability and optimal performance under demanding conditions for not just LEO, but we provide coated optical products for all Earth orbits – VLEO, MEO, GEO, and HEO.
What Role Do Optical Filters Play in Space Mission LiDAR Systems?
The functional role of an optical filter in this situation is to separate, transmit and receive specific wavelengths, suppress all solar background radiation and isolate signals at high altitudes. Protection of the sensitive detectors is paramount, so optical filters that are proven and tested to function over a lengthy period of time must be used in a space mission-type application.
Filters in this position must function to match the specific transmitted laser wavelength, allow very narrowband transmission (<1-2 nm), function with high out-of-band blocking (OD4-OD6+) and maintain spectral stability at all times.
Omega Space Qualified Optical Coatings and Components
| PRODUCT TYPE | COMMON USE RANGE | KEY CONSIDERATION |
| C-band / SWIR | 1550 nm, 1064 nm | LEO Communications |
| Blocking filters | 905 nm, 1550 nm, 1064 nm, 532 nm | Solar Rejection |
| High wavefront off-axis parabolas | Beam expansion/ reduction | |
| Fast Steering Mirrors | UV – FIR range | Laser Communications |
| Linear Variable Filters | UV – Mid-IR | |
| Gold Coatings | IR | |
| LFV | UV – NIR | |
| GRISM coatings | NIR and IR |
Solar Background Rejection in Orbit
Unlike our daily driver automotive systems, LEO-type LiDAR operates in direct, unfiltered solar exposure. Continuous solar loading on LiDAR systems creates 1000W/m2 of heat, which contributes to signal noise, reduced accuracy and failure. The sun’s radiation reduces the SNR by 3-5 dB, leading to data errors.
High dynamic range is also something to be taken into consideration. The rapid platform movements and/or high-speed vibrations combined with fast moving targets create complexity between the signal return and background, resulting in data degradation.
These compelling issues create a need for strategies to combat data inaccuracy. Typically, we see the following:
- Very narrow FWHM (often 1–3 nm or tighter)
- High optical density outside passband (OD5–OD6+)
- Dynamic target filtering with specialized algorithms to remove noise for increased accuracy and classification.
Solar rejection is more demanding in space than for terrestrial applications as the systems do not require dealing with the filtering and scattering of the Earth’s atmosphere. High intensity and unfiltered light, high albedo surfaces (ice, clouds or other white surfaces) and larger panels requiring management of solar load are responsible for blocking needs.
Angle of Incidence in Spaceborne LiDAR
Space systems introduce unique angular challenges in order for LiDAR systems to function correctly.
Off-nadir pointing in space is a technique to angle the camera’s sensor away from the vertical (the nadir). It is known to cause “building-lean” distortion in images, however; it can be helpful for imaging building facades or ships at odd angles.
Fast steering mirrors (FSMs) are able to move the mirror directionally while counteracting vibration and maintaining alignment. A FSM system allows for high-speed, high-resolution scanning in space at all Earth orbits. Omega creates the mirrored optics for FSM systems with the required reflectivity and durability.
Satellite attitude adjustments – needed for docking, operations by allowing the ADCS to calculate relative orientation and distance measurements in real-time.
Key Discussion:
Interference filters will blue-shift at increasing angles. When using optical filters for a LEO-based application, a small wavelength shift can reduce your return detection and may reject part of the signal. Similarly, off-axis signal returns may miss peak transmission.
How Does Omega Reduce This for End-Users?
Omega filters for LEO / LiDAR applications must be designed for expected angular distribution. Our coating designers take these critical AOI requirements and modeling into early conversations with our customers.
The Effects of Thermal Cycling on Optical Coatings in LEO Orbit
The ability of Omega filters to handle thermal cycling when in LEO orbit is a major differentiator for our products. LEO satellites experience rapid transitions from sunlight to eclipse, with measured temperature swings often exceeding the ±100°C range. Not only does this mean increased pressures on optic longevity, but it is increased even more by the repeated thermal cycling across the space mission’s lifetime.
Optical coating stress accumulation over time will create bandpass shift, reduced blocking capability and overall degrade the filter’s performance. Our engineers mitigate this known with correct material selection, layer thickness design, deposition process controls, and occasionally, stress compensation layers in the coatings.
For large optics used in hyperspectral instrumentation, stress accumulation from thermal cycling increases the risk of:
- Microcracking
- Delamination
- Spectral drift
All of these reasons are why our experts spend time modeling optical performance and mechanical stress, along with prototyping testing and open communication with the LiDAR instrument team.
Designing for Radiation Effects on Optical Coatings
The LEO space environment introduces UV exposure, proton and electron radiation and atomic oxygen (for lower LEO altitudes at 160-500 km). Because of the residual atmosphere, our TRL9 coatings must be able to withstand additional forces.
Potential filter coating darkening from UV radiation, which results in reduced transmission function. Refractive index change over time of even 0.01 can matter quite a bit from a LiDAR or hyperspectral standpoint.
Surface erosion due to the environment is caused by atomic oxygen (AO) sources. These extremely reactive atoms chemically react with the coating materials and remove atoms from the surface, etching the coating. This is similar to the plasma etching issue found in orbit as well.
Looking at long-term transmission stability in Omega’s space-qualified coatings, this is one of the main reasons our customers come back to us, time and time again.
To compensate appropriately and increase the coating longevity, Omega engineers take into account the following items when creating a prototype for LiDAR use.
- Radiation-stable materials
- Dense coating processes
- Controlled porosity
Manufacturability and Repeatability for Space Missions
We already know that our space customers often require:
- Extremely tight center wavelength tolerances
- High lot-to-lot repeatability
- Full traceability
- Environmental testing documentation
To mitigate those concerns, Omega offers:
- Spectral metrology and QA
- Environmental qualification
- MIL or aerospace qualification standards
- ISO 9001:2015 certification and registration, Six Sigma methodologies, and lean manufacturing practices
Designing LiDAR Filters for LEO Mission Success
Our design and engineering teams focus on delivering to you, precise center wavelength targeting, angular robustness, stress-balanced multilayer stacks, select radiation-resistant materials, thermal durability, and high optical density blocking.
LiDAR filter success in LEO is defined not only by wavelength accuracy, but by our ability to create coatings that will finish the mission timetable in orbit. Understanding correct material selection, proper coating options for your specific mission requirements, and years of UV-VIS-NIR-IR understanding and manufacturing set the tone for any conversation around our space-qualified, TRL9 optics.
Designing a LiDAR System for Orbit?
Save headaches and project costs with our optical coating engineers before finalizing your wavelength and angular specifications.
