In the rapidly evolving domain of satellite communication technologies, optical filters play an indispensable role in ensuring efficient, secure, and high-fidelity data transmission. With the increasing reliance on satellite constellations and the advent of free-space optical (FSO) laser communications, these filters have become critical for overcoming environmental challenges, enhancing communication speed, and improving security.
In this interview, Chris Conca and Brian Manning delve into the multifaceted contributions of optical filters in satellite communications. They discuss their role in enhancing performance, addressing design and environmental challenges, and exploring cutting-edge technologies such as sputter coating and gold-induced transmission filters.
Could you provide an overview of the role of optical filters in enhancing satellite communication technologies?
Chris: Optical filters in satellite communications allow for the line-of-sight use of lasers in satellite-to-satellite or satellite-to-ground communications without the interference of solar and Earth radiations, which have a variety of energies from the UV to the IR.
Brian: Modern optical filters are required in satellite communications applications since they are the primary optics in these systems that can transmit extremely selective information from satellite to satellite or ground. These filters must have very narrow transmission bands to pass only those laser wavelengths being used in the communications. Their transmission efficiency must also be very high (in terms of transmission percentage, %T) in order to maintain high signal fidelity.
Almost more importantly than what they transmit, these filters must also provide a wide range and high optical density (OD) of out-of-band blocking to mitigate extraneous interference from other celestial presences or communications. That blocking must range into the infrared spectrum to limit solar light interference and maintain high signal-to-noise ratios for the transmitted and received signals.
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Can you explain the significance of Satcom, particularly free-space optical (FSO) laser communications, in modern satellite constellations?
C: In Free Space, Optical communications lasers are used, typically in the NIR at 830, 1310, or 1550nm. The lasers are used in line of sight with high data density within satellite constellations, from satellite to satellite or satellite to ground. Optical communications allow for very high data density transfer.
B: Inter-satellite and satellite-to-ground communications are carried through laser signals that are transmitted from the satellite to the target of interest. It is of paramount importance that signal fidelity is maintained through the transmission and reception to ensure accurate communication. In the case of satellite-to-ground connections, accurate laser-based communication between satellites and the ground ensures proper geolocation and orbital placement of satellites.
How does free-space optical communication compare with traditional radio frequency systems in terms of speed, security, and cost?
C: Optical communications have several advantages over RF in terms of speed, security, and costs. Speed can be considered not only as how fast information is traveling but also with regard to optics, as the data being transferred is much quicker and has a much higher bandwidth or density than radio communications.
Data transfers are more secure than RF for multiple reasons. The system as a whole is safer since the optical path from satellite to satellite is significantly narrower than when using RF signals that are only somewhat directional. Optical modes are also safer due to the ability of lasers to create encryption keys that are repeated sequences of polarized photons, also known as quantum key distribution [QKD].
When considering costs, optical communications can be more expensive up front but less expensive over time. Initial costs of optical units are expensive, but as solutions evolve, costs are expected to be reduced. Optical systems are considered a game changer regardless of upfront costs when you couple in the significant increase in data that can be transferred in a given timeframe.
B: Optical communications have higher data transfer rates than RF systems, better signal fidelity (lower interference), and higher security than RF-based systems. This narrow-band nature of the optics also allows for more rigorous monitoring of incoming and outgoing signals to maintain closer security of communicated information than RF-based systems, which use information across a wider “band” of frequencies. All of this also results in lower power consumption for optical communications, keeping costs lower than those of RF.
What are the key challenges faced when designing optical filters for Satcom applications, and how do these challenges impact performance?
C: The challenges associated with designing and manufacturing optical filters for SATCOM are similar to those of other applications in that we want high transmissions at specific wavelengths while also having high optical density at other wavelengths. In the case of SATCOM, the high T% is at the laser wavelengths of interest and the blocking from the deep UV to the IR outside of the laser center wavelengths. Optics may need to be larger in size than for many other applications, and maintaining tolerances over the entire optical surface area can be difficult in the coating phase and critical to the application.
Another important aspect of the coatings for laser optics, which need to have excellent transmitted wavefronts [TWD] and reflected wavefronts [RWD], is the requirement to be as thin as possible. The coating materials we use must be able to have stable properties since the optics are made on Earth under vacuum and are expected to be used in space where the temperatures are not unexpectedly very cold or very hot and under vacuum. Also, any materials other than the substrate and coating materials must be able to withstand UV and IR radiation while not off-gassing.
B: Satcom applications face the same filter specification concerns as practically any other light-based application. Maintaining high transmission efficiencies and controlling optical density properties are constant concerns, as are bandpass specifications (center wavelength (CWL); full-width half-maximal transmission (FWHM or band “width”)). Care must also be taken to maintain the thermal stability of the optical properties of the coatings and ensure that there is no change in the spectral properties of the optics at a wide range of temperatures (launch to vacuum). Environmental susceptibility of the optics in a vacuum must also be accounted for, such as the possibility of damage created by intense visible-range sunlight, UV, or other “cosmic” rays.
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Could you elaborate on the role of the sputter coating process in developing high-precision optical filters and why it is critical for Satcom applications?
C: Sputter coating has proven to be an exceptional method of deposition for the materials used for SATCOM filters. Bulk indices are experienced with sputtering, allowing for very accurate predictive models. Sputtering is very controlled compared to older technologies, and therefore, in most cases, coatings are on single substrates, creating filters that have little need for off-gassing adhesives and paints.
B: Sputter-coating technologies produce interference coatings that possess a near-imperviousness to environmental presences, both physical and photonic. As well as providing the narrow-band, high-transmission efficiencies, and high out-of-band OD required for effective photonic communications, the materials used during the coating process result in an optic that isn’t affected by conditions present either during launch or floating in a vacuum, such as pressure, temperature or even the possibility of physical damage by debris.
How do gold-induced transmission filters enhance the performance of Satcom systems, and what are the trade-offs associated with using these materials?
C: Materials like Au can be sputtered, giving maximum throughput while near-perfect reflectivity in the UV and IR spectral regions with minimum coating thickness. While satellites can and do transmit and receive positionally such that Earth's thermal emissions and the Sun radiations can enter the optical element at extreme oblique angles compared to normal, the Au coatings are relatively insensitive to angles, allowing the coatings to maintain high reflectivity of those unwanted radiations. Au, when sputtered, passes ISO 9211 and MIL-C-48497 durability tests, unlike Au deposited with traditional thermal coating methods. Au deposited with thermal methods must be protected in various ways, affecting their longevity even if they pass the same durability tests.
B: Gold-based coatings can maintain a higher %T than other metals, such as silver (Ag). However, coating deposition thicknesses have to be modeled prior to and maintained during the production of these optics since this thickness can contribute to the absorptive properties of the optics. Gold coatings that are “too thick” create a larger amount of absorption of relevant wavelengths, decreasing their transmission efficiency.
What environmental challenges must optical filters overcome in space-based applications, and how are they addressed during development?
C: I have mentioned a number of challenges already, but the biggest is maintaining optical characteristics like CWL, TWD, and RWD over the entire surface area for larger SATCOM optics, rejection of the sun’s radiation and Earth’s thermal radiation from the UV to the IR without absorptive materials, a filter that performs in a vacuum and at extreme temperatures, and no outgassing. We address a number of these by using single high-quality fused silica substrates.
This mode of manufacturing does not include any adhesives needed for multiple substrate filters. If the filters are required to be mounted, we use a NASA-approved, extremely low-outgassing RTV. RWD can go awry with thicker coatings, so we can use transmission-induced gold designs that reflect all wavelengths but are induced to transmit, utilizing very thin layers compared to other materials. These designs are also insensitive to the angle of incidence, which can be quite high when dealing with the Sun and Earth’s radiation.
B:
- Heat (heat-resistant construction and coatings)
- Radiation (coating materials and substrates that aren’t absorptive to background radiation)
- Chipping of coatings w/physical motion of satellite (impact-resistant sputter coatings)
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Why is polarization and phase control critical in Satcom optical filters, and what techniques are employed to ensure these parameters are optimized?
C: Lasers are polarized, and it is important to be able to control the phase shift of the light, especially in filters designed for redirection, such as a dichroic. Minimizing the phase shift that may occur is called low-phase retardation. Achieving low retardation is in the design process of the coating, where the designer can control the thickness of layers such that the internal refracted rays are in the same phase as the reflected rays from the surface of the coating. Depending on the design necessary, a zero phase shift may not occur for all wavelengths, but the designer can optimize the design for a zero shift at specific wavelengths.
B: Maintaining the polarization state through the communication process (transmission and reception) is required, or valuable information may be lost. Maintaining the polarization vector/”direction” and phase (“time delay”) of both outgoing and incoming lasers allows for greater communication efficiency and fidelity over the large distances required. Intentionally designed polarization—and phase-insensitive optics can transmit with maximum efficiency, regardless of the laser's polarization state. These optics are currently being produced with this in mind.
Can you discuss the design and application of solar filters for Satcom systems, particularly their ability to handle wide-band rejection and high transmission requirements?
C: As we have already discussed, SATCOM filters will be out in low, mid, or high orbit and are subject to radiation from both the Earth and the sun. These radiations are in the deep UV out into the FIR. This is where the induced transmission gold coatings come in handy. A gold layer is super thin to help minimize thickness and stress for good RWD outcomes, as mentioned earlier, but because it is an exceptional reflector, it allows for an extraordinarily wide reflection range outside of the induced transmission at the laser wavelength. It is so far-reaching that it covers the spectrum from the deep UV to the FIR as required to keep the Sun and Earth’s radiation out of the satellite’s optical assemblies.
B: Solar filters must block or reflect visible light from the Sun and thermal emanations from the Sun and other IR-reflecting celestial bodies. These optics must also be able to block or reflect at large cone angles since these small optics are constantly exposed to the light and heat from extremely large bodies in relatively close proximity (astronomically speaking). In addition, these solar filters are often required to maintain high transmission efficiency, most commonly at 1550nm for modern communications.
Looking ahead, what are the most promising advancements or areas of research in optical filter technology for satellite communications, and how might they transform the industry?
B: Improvements in coating materials and deposition technology will improve transmission and blocking abilities, leading to ever-increasing signal fidelity in SatComm. Process and material improvements during the production of the optics (choice of substrates, “sealing” of the optics with vacuum-tolerant materials, and so on) will increasingly mitigate physical stresses on those optics in a vacuum. All of this will result in increases in signal fidelity and allow for more secure, less costly transmission of information on a global scale.
Where can readers find more information?
For more information on Chroma's SatCom filters, you can visit our dedicated SatCom applications page at https://www.chroma.com/applications/satcom-applications. This page provides detailed insights into our standard filter offerings, performance specifications, and applications across various satellite communication needs. Additionally, you can explore more about sputtered coatings for space-based optical systems, including gold induced-transmission filters, by reviewing the resource available at https://www.chroma.com/assets/documents/chroma-space-communications-and-navigation.pdf. If you don't find the specific filter information you're looking for, our team is ready to assist with custom solutions tailored to your unique requirements. Feel free to contact us at [email protected], and our experts will work with you to develop the perfect filter for your application.
About Chris Conca
I am a sales engineer with a Mechanical and Biology background. I have worked at Chroma for over 20 years and at an optical thin film company for nearly 30 years in all aspects of designing and manufacturing filters. My focus at Chroma Technology is helping OEM customers understand what we are capable of and finding the best options to bring their concepts to fruition. I am also the point person for the Raman Spectroscopy product line.
About Brian Manning
Brian is a Senior Application Scientist at Chroma, having been with the company for 19 years. Prior to Chroma, Brian completed a post-doctoral fellowship at Massachusetts Eye & Ear Infirmary/Harvard University with Dr. William Sewell focused on cochlear neurobiology and defended his doctorate at the University of Vermont under Dr. Gary Mawe in enteric neurobiology. At Chroma, his responsibilities are both external- and internal-customer-focused and involve all aspects of technical support from making optical recommendations to assisting in the design of custom optics.
About Chroma Technology Corp.
Chroma is a 100% employee-owned leading manufacturer of highly precise optical filters using thin-film coating technology. Our reputation is built on dedicated customer service, including technical and application support. We manufacture high-performance optical filters covering a spectral range from 200-5000nm with superior durability and longevity across many industries and applications. We are an ISO-certified B Corp with U.S., Germany, China, and Japan sales offices. Our filter types include long, short, and multi-bandpass filters, notch rejection filters, neutral density filters, beamsplitters, and reflective metal mirrors. Offering off-the-shelf, custom, and high-volume production products and solutions, we can design and deliver optical filters that precisely meet our customers' needs.
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